Release liners a fresh review | PDF - SlideShare

• 1. Latest Review
• 2. Introduction
• 3. Principles of a Release Liner ● Silicones define base materials for Release Liner ● Papers: the most important bases for Release Liner ● Key applications and value chain for Release Liner ● Labels ● Graphic Arts ● Fibre Composites
• 4. What is a Release Liner A Release Liner is a carrier for self adhesive products; it comprises a substrate in web form with a surface on which adhesives don’t stick. o Substrate in web form o Not sticking surface
• 5. Silicone Properties & Factors Influencing Release Anchorage Low Surface Energies Solid Surface Silicone Factors Influencing Sensitive Cost Polymerisation Effective Physical state / solid/liquid
• 6. Framework for Release Liners Variables All are interlinked Silicone Properties Carrier Coating Technology
• 7. Release Liner base materials: properties ❑ Almost all Release Liners carry a silicone layer ❑ Silicone is by far the most expensive material in such liners ❑ Therefore all efforts are taken to minimize the coat weight necessary to about 0.3 to 1 g/m² = 1 µm ❑ To generate silicone hold-out the substrate (paper) is made with a surface as much tight and as smooth as possible ❑ With various substrates tightness and smoothness are achieved by: Avoid catalyst poisoning by using chemically very clean base papers / films.
• 11. Release liners based on super calendared papers:
• 12. Release liners based on clay coated papers:
• 13. Release liners based on super calendared papers:
• 14. A release liner generally consists of a thin-layer of release coating applied to a carrier substrate as illustrated in the label construction of Figure 1. This design is an essential feature of a typical PSA construction. There are many different PSA constructions available depending on the application. These include pressure sensitive tapes, labels, double backed tape, and so forth. Release agents are substances that control or eliminate the adhesion between two surfaces. Release liners are simply carriers for the PSA that prevent it from sticking to itself and provide a method of dispensing and application. Certain products and even industries could not have been developed to their current level of importance without the availability of modern release coatings and liners. Importance of Release
• 15. PSAs may be constructed as either a one sided or two sided product. With a one sided product such as pressure sensitive tape, the tape backing may also serve as the release mechanism. With two sided products such as transfer film or double sided tape, a separate liner is required. The PSA product construction can vary significantly. Figure 2 represents a complex PSA construction. The multi-functional nature of the liner and its value to the end-user is readily apparent. Construction of a typical pressure sensitive label Figure 1: Construction of a typical pressure sensitive label Importance of Release
• 16. Release liners serve several very useful functions. They are used as a carrier sheet onto which the adhesive can be cast. They protect the adhesive during storage and transit and also during various converting and assembly processes so that unintended blocking does not occur. Liners provide a functional support during die cutting and printing. For example, liners can serve as a printing substrate for advertising, product instructions, product identification, etc. The liner also provides a lay-flat characteristic that prevents curling. This is an important attribute in graphics and other applications. Most importantly liners must perform these functions without damaging the adhesive or compromising its subsequent performance properties. Release liners represent an enabling technology that allows one to deliver and apply a PSAproduct to bond two materials together. Manufacturers and processors use release coatings and liners in a wide variety of general handling operations, such as calendaring, casting, embossing, extrusion, forming, laminating, packaging, and labeling. Arguably the most import applications are PSAtape, transfer film, and label release.
• 17. The optimal selection of a release liner can be a significant factor that impacts greatly on productivity. PSA products need to be designed not only to meet the required end use specifications, but also to maximize speed, efficiency, and reliability of the entire adhesive application process. Poorly chosen release liners(ms,th,tr) can result in expensive downtime due to problems such as the following. o The release liner fails to protect the adhesive o The liner releases prematurely o Machines must be set to a slower speed because of poor liner reliability and performance
• 18. It should provide an adequate release. The release level should be sufficiently low to provide an easy unwind without unduly stretching the adhesive film or it's backing, but it should be high enough to prevent flagging when the PSA product is wound in a roll. The release should be slow at a low peel angle and rapid and easy at 90 degree and higher angles, normally used for unwind. The level of release should be reproducible. This requires that the release level is not very sensitive to the amount of release agent used. The release liner must be specifically designed to perform in a particular product and production process to which it will be subjected. For all applications, liners must stay in place until the appropriate time during assembly. They then must remove easily with the application of a consistent force. The release liner must at least meet the following minimum requirements.
• 19. The release agent should be firmly anchored and should not transfer to the adhesive surface, causing a decrease in tack and inherently poor adhesion when applied. The release coating should be resistant to aging and the unwind should not change much with prolonged aging. There usually are additional requirements for specific application. For example, masking tape requires that the release coating have a good solvent resistance and that the paint adhere reasonably well to the release coating. Otherwise, the paint chips may fall off and remain adhered to the freshly painted surface.
• 20. The selection of a proper liner material will depend on the application and the type of PSA product being constructed. A number of essential questions will require answering before proper selection can be made. Table 1 summarizes the type of forethought that is required in this process. A properly designed liner will function reliable in high speed and automated processes, enabling significant gains in productivity. Release liners on automated assembly lines are usually made of film to avoid tearing during the removal. Film liners are also preferred for most electronics applications, because the fibers introduced with paper liners can cause contamination.
• 21. What is the price vs. performance limit and which properties can be sacrificed for lower price? Is environmentally acceptable disposal of the liner after its use a requirement? Is product identification or color important? Will the liner be removed during the initial assembly procedure or remain intact until removed by the end- user? How important is dimensional stability and the liner having a lay-flat characteristic? Will the liner be die-cut or printed? If so, what are the particulars of these processes (e.g., thermal printing, type of die-cut operation, etc.)? Is a heavyweight liner required to give the product stiffness, or would a lightweight liner be better to allow conformation to the substrate surface? Should the liner offer easy release, or will a higher release strength ensure that it will stay in place during all processing conditions?
• 22. The release of a pressure sensitive adhesive from a liner is a very complex phenomenon. There are a number of factors that contribute to the release level of a liner and to the difficulty that is generally experienced in selection of an appropriate liner product. These are indicated in Table 2. 2. the end user's application process. At a minimum this includes an understating of the types of sub-processes (e.g., dispensing, printing, die-cutting, etc.) that will be employed, the sequence of events, the force and speed of each sub- process, and the environmental conditions that will exist during adhesive storage and application. Nature of the adhesive Chemical type Thickness Modulus Diluents Nature of the base liner Roughness Porosity Sizing or plasticizers used Surface energy Nature of the release coating Chemical composition Coating weight Film continuity Degree of cure Crosslink density Modulus Nature of the PSA product prior to release Age of the liner / coating Thickness and modulus of the product Mode of adhesive applications Conditions of storage (e.g. temperature and humidity) Stripping operation Speed Angle of removal Physical dimensions Ambient conditions (e.g., temperature and humidity) Table 2: Factors Affecting Release Level In order to provide an optimized product with both an adhesive and liner system that acts in concert, the manufacturer of the PSA product must completely understand the 1. the functional requirements of the adhesive itself and
• 23. A variety of release liners are used in PSA products. They range from coated paper to plastic films. They all have certain advantages and disadvantages and must be chosen in concert with the needs of the specific application. The various types of liners that are generally used are described in Table 3. The surface to which the release coating is applied should be reasonably nonporous to hold the release coating on the surface and to prevent the flow of soft pressure sensitive adhesive into the pores and roughness of the surface, causing an increase in the unwind force. Backside coating, which might or might not have the release characteristics, is often used on fabric and paper surfaces to provide a smooth and impermeable surface for release. Types of Release Liner
• 24. A recent requirement placed on all release liners is that they be recyclable once used. It is generally considered that all spent liners (paper, film, polycot, etc.) are recyclable. However, there are issues regarding segregation of material by type, packaging requirements, and quantity that will enter into a successful recycling program. Film liners represent the greatest potential for growth over the next three to five years, especially in high- volume, fast moving applications. However, paper liners will continue to enjoy the majority of the market due to their lower cost, caliper, and technology enhancements via silicone release coatings. Kraft liners can exhibit problems with dimensional stability under changing temperature or humidity pieces. strength than Kraft papers due to the brittle nature of the clay. for all forms of die cutting. high tear strength, high speed rotary cutting and hot-wire cutting. The production of static electricity from the Type Description Characteristics Kraft paper Release coating is applied on one or both sides of densified Kraft paper An economical option, suited for general-purpose applications and rotary die cutting. Not for kiss cutting. conditions. Board Made of paper with a heavy weight and a caliper of 12-14 mils The large caliper maximizes kiss-cutting performance and allows easy removal of small parts and waste Clay-coated paper Paper coated on one or both sides with clay Provides high temperature performance and better humidity resistance (dimensional stability). Lower tear Poly-coated paper Base paper with extruded polyethylene or polypropylene film on one or both sides and sometimes coated with a silicone release agent A versatile option. Resistant to tearing and to wrinkling or "cockling" when exposed to humidity. Can be used Film Produced from low or high density polyethylene, polypropylene, and polyester Most expensive liner. Provides excellent humidity resistance (dimension stability). Liner will provide the smoothest coating of adhesive possible. Do not produce "paper dust" during slitting and conversion. Offers liner is a concern. Table 3: Types of Liners Commonly Employed in PSA Products
• 25. Types of Release Coatings Many types of release coatings have been used on liners over the years. The first release coatings for pressure sensitive tapes were polymers such as shellac, starch, casein, and nitrocellulose. These older release coatings did not exhibit good release characteristics but functioned primarily by preventing the adhesive from penetrating the pores of the liner. In more recent times, PVC resin, polyvinyl butyral, polyvinyl alcohol, vinyl acetate copolymers, and acrylic resins have all been applied as release coatings for PSA products. Several of the more widely used release coatings are summarized in Table 4. When a liner consists of a release coating applied to a substrate (e.g., Kraft liners), the release coating is applied over the substrate from a dilute solution or dispersions. Many modern release liner manufacturers now use UV curing coatings, which have no VOCs or environmental constraints. Light coatings are generally sufficient.
• 26. coating. Could effect performance of the pressure sensitive adhesive if the coating attaches to release characteristics. Generally copolymer of alkyl acrylate and acrylic acid, nitrocellulose, are attached to C14 -C18 fatty acids. subst ra tes. Use d si m i l arly t o t he c hrom i um c om pl e xes. Type C h a r a c t e r i s t i c s Si l i c one s Most widely used release coating of PSA products. Exhibit easy release at low peel rates. Can be applied as solventless, solvent borne, or water borne systems. Low surface energy makes silicone coatings ideal for all types of liners including polymeric film. Wa x e s Generally used as additive to film forming polymer coatings to improve release. Effectiveness is based on their incompatibility and ability to migrate to the surface of the the adhesive. Long chain branched p o l ym e r s Polymers with long side chains are waxy compound exhibiting good coating performance and and vinyl chloride. Polyvinyl carbamates Generally used as a release agent on film backings (e.g., cellophane film). C h r o m i u m c o m p l e x e s Provides good release characteristics and water repellenc y to paper. Chromium complexes Fl uoroc a rbon c o p o l ym e r s Have very low surface tension to provide for easy release characteristics. Expensive. Am i ne s Long chain alkyl substituted amines have good release properties especially on paper Tabl e 4: Com m onl y Appl i e d Re l e a se Coa t i ngs for Pre ssure Se nsi t i ve Produc t s
• 27. Silicone Chemistry Siloxane polymers have many attractive attributes including low surface energy, low glass transition temperature, thermal stability, and a low viscosity at relatively high molecular weights. Silicone polymers used for release liner coatings start with polydimethylsiloxane (PDMS) backbone base polymers. Base polymers have pendant or end-blocked functionalities. In thermally-cured systems, hydroxyl groups (condensation reaction, Figure 1) or vinyl groups (addition reaction, Figure 2) react with silane functionalized PDMS in the presence of heat and an organometallic catalyst (Sn, Pt, or Rh). These reactions may take place in the presence or absence of solvents. In solvent-based systems, common solvents include heptane, methyl ethyl ketone, toluene, and isopropanol. Full cure is achieved in a few seconds up to a few days.
• 29. Radiation-cured silicones incorporate two basic systems: epoxy chemistry with a cationic curing mechanism (Figure 3) and acrylate chemistry with a free radical curing mechanism (Figure 4). Both reaction types occur without the need for a heat source. UV light can be used as an energy source for both types of chemistry, while e-beam is restricted to free radical chemistry.
• 30. Radiation-cured silicones incorporate two basic systems: epoxy chemistry with a cationic curing mechanism (Figure 3) and acrylate chemistry with a free radical curing mechanism (Figure 4). Both reaction types occur without the need for a heat source. UV light can be used as an energy source for both types of chemistry, while e-beam is restricted to free radical chemistry.
• 31. UV-cured systems with cationic chemistry involve a cycloaliphatic epoxide functionalized PDMS and require a photo initiator. Upon irradiation, the cationic photo initiator reveals a strong acid which in turn initiates a ring opening reaction for polymerization of the epoxides to polyethers. A diaryliodonium salt with a metal halide counterion is typically used as a photo initiator . UV light may also be used an energy source for free radical polymerization. This system involves acrylate functionalized PDMS and also requires the use of a photo initiator. UV light irradiates the photo initiator to generate free radicals and curing proceeds via free radical polymerization of the acrylates. E-beam cured systems involve acrylate functionalized PDMS and proceed via a free radical polymerization reaction. E-beam systems provide a higher amount of energy than UV and initiate the free radical reaction without the need for a radical initiator.
• 32. In free radical polymerization of acrylates, the intermediate free radicals may be quenched by oxygen, so an inert atmosphere is required to ensure the reaction drives to completion. Cationic polymerization does not require inerting. However, in cationic polymerization systems cure can be inhibited by moisture, so direct coating onto paper is a limitation. Direct-coated applications utilizing cationic chemistry are sometimes possible with prior treatment of the paper to reduce or eliminate the availability of water.
• 33. Why Radiation Instead of Thermal? The most significant advantage of the use of radiation-cured silicone coatings for release liners is that complete cure is achieved quickly without the use of heat. From a liner manufacturer’s perspective, this means that a radiation curing coating machine may be constructed with a much smaller footprint due to the absence of lengthy ovens needed to provide enough heat for thermally- cured liner coatings. Thermally-cured silicone systems also impose limitations and complications in the use of certain substrates. The heat required for thermally-cured coatings is more than many filmic substrates can withstand while maintaining dimensional stability. Substrates for thermal systems are generally limited to papers, polyester, and some polypropylenes. There are some tin-cured systems on polyethylene, although low oven temperatures and slow line speeds are required.
• 34. The use of radiation-curable coatings opens the door for the use of a much larger variety of substrates. These coatings may be applied to nearly any temperature sensitive substrate such as ✓ lower gauge polyester, ✓ polypropylenes, ✓ polyethylene, ✓ polystyrene, ✓ polyvinyl chloride, ✓ nylon, in addition to the substrates mentioned above. The heat required in thermal systems also drives moisture out of paper. Moisture control in direct coated Papers is crucial for lay-flat and dimensional stability. A certain amount of moisture is required for Control of liner curl and cockling or the development of bagginess over time due to uneven adsorption of moisture in water-starved sheets. Implementation of moisturization units and careful tuning of process parameters are often required where these properties are critical to the end use application of the liner, adding cost and complexity.
• 35. Radiation-curable coatings are notably fast and complete cure can be achieved at very fast line speeds without the need of a catalyst. Line speeds are typically limited by machine constraints or misting at the coater head, instead of minimum oven dwell times, as with thermally-cured coatings. Tin-catalyzed thermally-cured systems sometimes suffer from post cure of several days resulting in dramatic shifts in release force at the point of use compared to that measured directly off the coater. Platinum-cured systems typically do not suffer from the post cure seen with tin systems, but may be subject to catalyst poisoning from plastic additives and cross contamination, which can prevent complete cure. Because of the absence of a catalyst, UV acrylate systems are far less sensitive to contamination. Electron beam vs Ultraviolet Radiation E-beam Radiation curing technology for silicone coatings began with E-beam as an energy source, starting in the s. E-beam curable silicone systems consist of an acrylate functionalized siloxane polymer backbone cured by free radical polymerization (Figure 4
• 36. Energy needed for this reaction is provided from an electron accelerator. Free radical polymerization proceeds extremely fast and does not require the addition of a photo initiator because of the amount of energy provided by the electron beam. E-beam cured coatings can provide a wide range of release force, and very tight release forces can be achieved (400+ g/in-width, measured with rubber-based Tesa test tape). One disadvantage of E-beam coatings is their propensity to develop increasing adhesion to some aggressive acrylic adhesives over time. For this reason, E-beam coatings are generally recommended for rubber-based adhesives and should be evaluated on a case-by-case basis for acrylic adhesives.
• 37. Ultraviolet Since the inception of radiation curable coatings, UV has gradually taken a strong lead as the go-to method of commercialized silicone coating liners cured without heat. Because UV light can be used as the energy source for both types of curing chemistries (Figure 3 and Figure 4), UV curable systems can realize most of the benefits of e-beam cured coatings with few disadvantages. The notable difference in these 2 systems is the requirement of a photo initiator when UV light is used. However, a strong advantage of UV-cured coatings is that they are compatible with a wide variety of adhesives and are not restricted from use with acrylic adhesives. Traditionally, mercury vapor lamps have been used as a UV light source. Mercury lamps emit a broad spectrum of light without the ability for tuning to narrower wavelength bands. For the purposes of free radical initiation, mercury lamps yield 70-75% of radiation in non-useful wavelengths, including enough high energy infrared light to produce a significant amount of heat.
• 38. Recent work has resulted in the development of light emitting diodes as UV light sources. LED light sources benefit from the ability to fine tune their emission spectrum; typically in a 20-40 nm range. This concentration of energy output allows for more efficient machine operation by delivering a sufficient dose of radiation at the target wavelength without the emission of radiation at wavelengths not needed, which in turn requires less power input. A notable benefit of this is the much cooler operating temperatures. LEDs also benefit from a much longer lifetime than mercury vapor lamps (20,000 h vs. 2,000 h).
• 39. Summary Radiation-cured silicone coatings offer several advantages over their thermally-cured counterparts. These types of chemistry provide efficient, fast cure and allow for the use of many substrates not available with the use of traditional thermally-cured coatings.
• 41. Films made from a variety of low surface energy polymers have also been found to be useful as backings as well as release liners. For example, oriented polypropylene film tape can be constructed by corona discharge treating one side to which the adhesive is applied. Asufficiently low release level is obtained on the other side without release coating if a polar acrylic is used. However, if a styrene block copolymer is used a release coating may be required. Other film release liners include polyethylene, polypropylene, fluorocarbon polymers, and polyester.
• 42. Silicones are, by far, the most widely used materials for pressure sensitive release applications. They provide uniform, thin coatings that exhibit easy release at low peel rates. The good release characteristics of silicone are related to its low surface energy (22-24 dyne/cm) compared to most organic adhesives (40-50 dynes/cm). This provides sufficient adhesion between the silicone and the PSA to keep the adhesive in place, yet not enough adhesion to prevent easy parting of the liner from the PSA during application.
• 43. The three basic types of silicone release coatings used in the PSA industry can be classified by the form in which they are applied to the liner: solvent borne, water borne, and solvent-free. These are characterized in Table 5. The major component of a cured silicone release coating is polydimethylsiloxane. Crosslinking of the silicone coating is necessary for it to resist penetration by the PSA. Without crosslinking, the adhesive could diffuse into the flexible siloxane polymer chain. Crosslinking also provides a coating with the physical characteristics required of a release liner (strength, elongation, etc.). Type Solvent Cure Rate Release Comments Fast to slow Easy to tight Dull to glossy, good anchorage Medium to slow Easy to medium Some coatings require post cure Fast to medium Easy to tight Glossy coatings, premium release Table 5: Comparison of Silicone Release Coating Systems
• 44. UV and EB curing of silicone release coatings seem to be the major driver in release coatings today. This is due to their very fast cure, low energy consumption, and ability to be easily modified to meet specific release requirements. These materials can be applied to both paper and polymeric film based liners. A significant benefit of UV and EB cured release coatings is that significant heat is not required for cure and as a result they can be applied to heat sensitive substrates such as plastic film. Significant activity has occurred in the thermally cured silicone release segment of this market. The high cost of platinum catalysts have been addressed by Dow Corning and Wacker who has developed proprietary polymer and crosslinkers that enable lower platinum usage. These new products also feature lower temperature cures and higher speed coating application. Solvent less silicone release coatings are the most important type. These have been developed to address environmental pollution / health problems, solvent cost, and productivity issues related to solvent and water borne systems. This class of coatings includes thermal, ultraviolet (UV), and electron beam (EB) curing.
• 45. Cationic curing can take place without an inert atmosphere in the coating unit. However, the coating will undergo post curing after it leaves the UV unit. Clay coated papers cannot be used as liners with cationic cured coatings as they produce poisoning effects from the alkaline components. The curing of these coating is also negatively affected by high humidity conditions. Each of these systems has its advantages and disadvantages. The radical polymerization of the acrylate groups is much faster than the cationic polymerization, so that faster processing speeds are possible. Their main disadvantage is the need to use nitrogen blanketing to eliminate the presence of oxygen, which will terminate the polymerization process. There are two UV curable silicone release systems widely used: Silicone acrylate (cures by a free radical mechanism) Epoxy silicone (cures in the presence of cationic initiators).
• 46. EB curing is the subject of much activity since it provides certain advantages that offset the higher cost of the curing equipment. UV radiation is usually associated with some heat energy output, but EB is a completely room temperature process that does not require a post cure. Electron beam curing also provides improved anchorage of the release coating to many different substrates. With EB curing there is no need to cure the coatings under an inert atmosphere. The intensity of EB radiation, however, may be so strong that it could affect the base liner properties. Similar to UV curing, EB curing has achieved commercial status. The types of coatings that are used with EB curing processes are acrylate functional, epoxy functional, or thiolene based. Acrylate materials are most widely used and provide very fast cure at dosages in the region of 1-5 Megarads.
• 48. For Self ive A y WaterWater Solvent less SolventSolvent Based onBased on Silicone The Types and the Chemistry Release Coating Adhes Tapes ver crucial component
• 49. Platinum UV Cure Platinum UV Cure Platinum Cure Platinum CureNow Outdated / Banned Platinum CureTin Cure WaterSolvent lessSolvent Based on Silicone The Types and the Chemistry
• 50. Coating Substrate for Transfer Coating Release Liner Double side Release Liner Single side Back sizing / Controlled Release on backside ApplicationsApplications Silicone
• 51. Recommended for Single Generally not Suitable / sided Tapes requiring Higher Self Adhesion Force Not Suitable / Recommended for Transfer Coating LimitationsLimitations Silicone
• 52. A Adhesive Release Coating is Possible inline with Adhesive Coating Faster Coating speeds , do not require curing Recommended for Reverse Printed Tapes / Printing is possible after Release coat 100% Transfer of Coated Recommended for Self Adhesive Tapes with higher SelfAdhesion Values Very Low Release Values Relatively High Release Force Advantagesdvantages Silicone
• 53. Silicone More about Silicone Coating NOW
• 54. he age Transfer coating allows us to produce Adhesive coated with almost any substrate , very Thin or very Thick , Very delicate to very rigid , extremely porous to absolutely non porous Let us see how this is possible ? Transfer Coating is a method of Transferring Dry Adhesive Film from Silicone liner to the Main Substrate. What is Transfer Coating ? Silicone coated Liner : Paper or Film Allows Transfer Coating T MainAdvant of Silicone based Release Coating
• 55. Slot Die Floating Knife Comma Gravure / Gravure + Myer Bar Sketch of Coating and Lamination Machine suitable for Transfer Coating by
• 56. is he 1 st Unwind Coating Zone 2 ed Unwind Rewind GRAVURE COATING & LAMINATION Have a look at the most common four methods of Transfer Coating Below What common in all these methods ? What is t special advantage of these methods ?
• 57. is he Unwind 1 stCoating Zone 2 ed Unwind Rewind COMMA COATING AND LAMINATION Have a look at the most common four methods of Transfer Coating Below What common in all these methods ? What is t special advantage of these methods ?
• 58. is he Zone Coating 2 ed Unwind Rewind1 st Unwind FLOATING KNIFECOATING AND LAMINATION Have a look at the most common four methods of Transfer Coating Below What common in all these methods ? What is t special advantage of these methods ?
• 59. is he SLOT DIE AHESIVE COATING & LAMINATION Coating Zone 1 st Unwind Rewind 2 ed Unwind Have a look at the most common four methods of Transfer Coating Below What common in all these methods ? What is t special advantage of these methods ?
• 60. The on he e ethod Hence the Substrate is not exposed to the wetAdhesive at all. The Substrate comes in contact with theAdhesive Dry Film only at the Lamination Zone . What are th Advantages of this m ? T Substrate to be converted into aAdhesive Coated Product is mounted at Unwind Station no. 2 Comm factor is that The coating is done on Silicone coated Liner : Mounted at Unwind station no. 1 Yes , you are Right
• 61. the od he Minimum snaps or wastage Very ThickDelicateFlexibleRigid Ability to handle number of Main Substrate Thick FoamNon Woven / Film FabricsMetal Foil Very Quick Change overs , in case of change in t Main Substrate Uniform Coating Nip : Hence Uniform Coating Thickness of Adhesive Advantages of this meth ? are
• 62. Non no Silicone coatings w Let us see more about Non Silicone
• 63. What is it s wound Adhesive Tape Transparent Printed Electrical Yellow is called a Adhesion to self Packaging BOPP Brown PET Insulation The Tape adheres to its own backside and hence a force is required to pull or peel of the tape from the roll. This force BOPP Packaging BOPP Packaging All These Tape Are self Wound & they need a very Important Property inbuilt “ Called as Adhesion to self ” This is a very Important property especially of a Single side or self Adhesion to Self ?
• 64. our s e Non Silicone Coating on Backside Silicon Coating on Backside No Coating on Backside We will study 3 variation now as follows Now let us try and understand How coating on backside of the substrate can change the behavi of self wound Tape
• 65. iations s The Tape adheres to its own backside and hence a force is required to pull or peel of the tape from the roll. This force is called a Adhesion to self . In case of Packing or Insulation Tapes this force will be almost 75-80% of the peel adhesion strength of the tape .for example if the peel strength is say 650-700 grams/ 25 mm width ,then The adhesion to self will be nearly 500 to 550 grams/ 25 mm & this forc unwin p off necessary for some tapes like above No Coating on Backside We will study 3 var now as follows
• 66. iations ne the e tape ill t g on Now imagine the backside of the substrate is coated with Silicone ? What will be Adhesion to self . Suppose the peel strength is say 650-700 grams/ 25 mm width ,then The adhesion to self , in this case will drop down nearly 25 to 50 grams/ 25 mm & so what will be the inbuilt problem here ? Th w star unwindin it own the moment the tape end is hold . This could cause lots of problems on the assembly line , packaging line , because you need certain amount of resistance to unwind or peel off Silico Coating on Backside We will study 3 var now as follows
• 67. iations the e ape ill not g on Now imagine the backside of the substrate is coated with non Silicone ? What will be Adhesion to self . Suppose the peel strength is say 650-700 grams/ 25 mm width ,then The adhesion to self , in this case will drop down nearly 150 to 250 grams/ 25 mm & so what will be the inbuilt advantage here ? Th t w unwindin it own the moment the tape end is hold . But at the same time the tape peel off will be much more easier than the first case , where there was no coating at all on the back side . Non Silicone Coating on Backside We will study 3 var now as follows
• 69. They are also used domestically for bake ware, ice cube trays, and easily cleaned surfaces. Abhesive coatings are employed for deicing surfaces such as airplane wings, ship structures, and windshields. Abhesives are formulated into anti-graffiti and "self-cleaning" paint coatings. Abhesives are also used to prevent microorganism growth and salt formation on marine or outdoor seacoast structures. In the medical industry, abhesive's are used for providing non-stick and bio-resistant surfaces for medical devices such as catheters. The applications for abhesive's are nearly as numerous and as commercially important as they are for adhesives. The most recognizable abhesive's are used as mold release surfaces and as liners for pressure sensitive tapes and labels. However, abhesive's are also used to protect general plastic processing equipment such as used for casting, extrusion, laminating, and molding. Introduction "Abhesives" are films or coatings that are used to prevent or greatly decrease adhesion. An abherend is a surface that discourages adhesion. Abhesive materials are also often referred to as mold-release agents, non- stick surface coatings, parting agents, or antistick agents. Abhesion is exactly the opposite of adhesion, and the requirements for a good abhesive are reverse that which is necessary for a good adhesive to function.
• 70. This is an important concept for the polymer scientist or engineer since manufacturers of polymeric products frequently tackle release problems and also because an efficient and profitable solution to a release problem is often a polymer. However, the main purpose of this review will be to explain the material and surface science fundamentals required for an abhesive to function. A variety of materials and processes have been developed to provide surfaces that function as abhesive's. Many of these will be described in this article, and several new abhesive's will be discussed.
• 71. Therefore, surface treatments that enhance adhesion do so by removing weak boundary layers, changing surface topography, changing the chemical nature of the surfaces, and modifying the physical structure of the surface. Important criteria in adhesion include the surface topology, surface tension and energy, wetting, and thermodynamic work of adhesion. Adhesion occurs through a combination of the following mechanisms: mechanical interlocking, interdiffusion, adsorption (surface reaction), and electrostatic attraction In order to understand how abhesive's work, one must first understand the mechanism of adhesion and the factors that affect it. Mechanisms Abhesion
• 72. of Since many of the factors causing adhesion are of a chemical nature, a good abhesive must also be chemically inert toward the two materials whose adhesion are to be prevented. o A barrier to mechanical interlocking o Prevention of interdiffusion o Poor adsorption and surface reaction o Low surface tension and thermodynamic work of separation o Limited or no electrostatic attraction o Incorporation of a weak boundary layer. Abhesion requires just the opposite. For maximum adhesion, or resistance to adhesion, the surface should exhibit the following characteristics. Mechanisms Abhesion
• 73. Sacrificial Abherends require only that the abhesive material fill the pores or smooth-out the roughness of a surface (e.g., an inert powder on a rough metal surface). Sacrificial Abherends generally remain attached to both surfaces after release, and they must be applied to a surface every time it is to be protected. On the other hand, permanent Abherends will last through many cycles of release. Permanent Abherends require that the abhesive material have good spreading tendency on the material to be protected and a surface that exhibits poor spreading tendency to the material which adhesion is to be prevented. The permanent abherend must be a good coating material (i.e., easily applied, uniformly spread over the surface to be protected, and relatively permanent during all expected processes). Thus, there are several ways in which abhesive's can be classified. The most popular classifications are permanent (corresponding to 1 above) and sacrificial (corresponding to 2 above). Several examples of each are readily evident in the household environment. The coating on a non-stick baking pan is an example of permanent abherend. Flour, grease, or oil used for non-stick baking are examples of sacrificial Abherends. Abhesion, therefore, occurs via one of two primary modes: (1) prevention of adhesion to the abhesive coating layer, or (2) an easily separable coating or cohesively weak boundary layer.
• 74. Another factor to be considered in choosing an abhesive is volatility. Water would be a good abhesive, but because it vaporizes at relatively low temperatures, water could not be used as a mold release in many high temperature applications. An important factor in choosing an abhesive is temperature dependence. A material could act as an abhesive at room temperature and as an adhesive at elevated temperatures. Thermoplastic polyethylene coatings are good examples of this. Polyethylene is relatively inert with a low surface energy, non-stick surface in its solid state, but it has good adhesive properties in the molten state.
• 75. The term is the interfacial tension of the solid material in equilibrium with a fluid vapor, is the surface tension of the fluid material in equilibrium with its vapor, and is the interfacial tension between the solid and liquid materials. Complete, spontaneous wetting occurs when = 0 deg, or when the material spreads uniformly over a substrate to form a thin sheet.Acontact angle of 0 deg occurs with a pure water droplet on a clean, glass slide. Therefore, for complete spontaneous wetting, cosine > 1.0 or when: Wetting can be determined by contact angle measurements. It is governed by the Young equation which relates the equilibrium contact angle, , made by the wetting component on the substrate to the appropriate interfacial tensions: Two solid materials generally do not adhere to each other because wetting does not take place and there is no penetration or interdiffusion of one material into the other. When wetting is minimal, the secondary van der Walls bond forces that provide the majority of molecular adhesion are not large, the work of adhesion is minimal, and the surface has the properties of a good abherend. Theory
• 76. A simple view of the relationship of wetting and adhesion is provided by Figure 1. Here the contact angle of a drop of a liquid on a surface of different critical surface tension is shown. The expected bond strengths would increase as the contact angle decreases. Therefore, best abhesive scenario is when the contact angle is greatest. Thus, most common adhesive liquids readily wet clean metal surfaces ( > 100 dyne/cm), ceramic surfaces, and many high-energy polymeric surfaces. However, common adhesives do not wet low energy surfaces such as polyethylene and fluorocarbons. The fact that good wetting requires the adhesive to have a lower surface tension than the substrate explains why organic adhesives, such as epoxies ( about 40 dyne/cm), have excellent adhesion to metals, but offer weak adhesion on many untreated polymeric substrates, such as polyethylene, polypropylene, and the fluorocarbons ( < 30 dyne/cm). Wetting is favored when the substrate surface tension, , or its critical surface energy, , is high, and the surface tension of the wetting liquid, , is low (i.e., wetting is favored when > ). Low energy polymers, therefore, easily wet high-energy substrates such as metals. Conversely, polymeric coatings and polymeric substrates having low surface energies will not be readily wet by other materials and are useful for applications requiring nonstick, passive surfaces. Theory
• 77. end Many commercial abhesive's are of proprietary composition. These products can be formulations of more than one type of abhesive with modifiers or additives, such as emulsifiers, biocides, solvents, etc., incorporated into the final product. There are many materials that can be used as abhesive's. These are generally classified chemically as shown in Table 1. They can be supplied in many different forms such as fluids, waxes, greases, emulsions, dry films, and solid powders. Types ofAbher Materials
• 78. Two solid materials generally do not adhere to each other because wetting does not take place and there is no penetration or interdiffusion of one material into the other. When wetting is minimal, the secondary van der Walls bond forces that provide the majority of molecular adhesion are not large, the work of adhesion is Theory Water would be a good abhesive, but because it vaporizes at relatively low temperatures, water could not be used as a mold release in many high temperature applications. Another factor to be considered in choosing an abhesive is volatility. Polyethylene is relatively inert with a low surface energy, non-stick surface in its solid state, but it has good adhesive properties in the molten state. A material could act as an abhesive at room temperature and as an adhesive at elevated temperatures. Thermoplastic polyethylene coatings are good examples of this. An important factor in choosing an abhesive is temperature dependence.
• 79. oil Chemical Class Chemical Subdivision Specific Examples Waxes Petroleum waxes Paraffin wax, microcrystalline wax Vegetable waxes Carnaubawax Animal waxes Lanolin Synthetic waxes Polyethylene wax Fatty acid metal soaps Metal stearates Magnesium stearate, zinc stearate Others Calcium ricinoleate Long chain alkyl derivatives Fatty ester synthetic waxes Diethylene glycol monostearate, hydrogenated castor Fatty acids Stearic acid, oleic acid Fatty amides Ethylenebis(stearamide), oleyl palmitamide Natural products Cellulose derivatives Cellophane, cellulose acetate Polysaccharides Sodium alginate Inorganic materials Silicates Talc Clay Kaolin, mica Other Silica, graphite Silicones Polydimethylsiloxane, polyalkylmethylsiloxane Fluorocarbons Polytetrafluoroethylene Synthetic polymers Other fluoropolymers Poly(fluoroacrylates), poly(fluoroethers) Polyolefins Polyethylene, polypropylene Other Polyvinyl alcohol Fluorinated compounds Fluorinated fatty acids and alcohols Perfluorolauric acid Table 1: Chemical Classification of Abhesives 2
• 80. This is an added advantage while processing further to adhesive coating. Non silicones generally do not require any Thermal curing , and hence can be coated at lower temperatures. These coatings are therefore coat able inline with adhesive coating. This does not mean that non Silicone coatings is a answer to all self wound tapes. While choosing a most suitable release coat , one has to take into consideration , The peel strength values of the product , as well as the Tensile strength and percentage of elongation of the substrate. Secondly , all most all Silicone coatings are thermally cured at processing temperature above 90 deg C. Most importantly , it is really impossible to coat and cure silicone coatings inline with adhesive coating. The only exception to this could be UV cured silicones , which cured at room temperature. However this option is relatively ruled out owing to the cost of manufacture. They are very expensive. Now on the other hand , Non silicone coatings are meant for controlled release with release values of 50 grams to 200 grams depending upon the requirement , as well as the backing substrate. Silicone release coated film or paper will provide a too easy release while unwinding a self wound Adhesive tape. Many a times this could be a unwanted property , considering the further conversion or application methods. Let us now compare the Release properties of Silicone based Coating and Non Silicone based. Silicone based release coatings are meant for , very easy release , or the release values as low as , 0 to say 70 grams/25 mm width. In other words a
• 81. s lease There are many materials that can be used as abhesive's. These are generally classified chemically as shown in Table1. They can be supplied in many different forms such as fluids, waxes, greases, emulsions, dry films, and solid powders. Many commercial abhesive's are of proprietary composition. These products can be formulations of more than one type of abhesive with modifiers or additives, such as emulsifiers, Type of Non Silicone Re Coatings The unwind force or the adhesion to self values should be lower than the tensile strength values for a particular substrate. This is true in particular , when we consider films like Poly ethylene , PVC , Poly propylene or various paper as backing substrate. The unwind force higher than the tensile strength , could result into elongation of films like PE, PVC , PP , and snapping of paper based tapes. The elongation of films will cause
• 82. dyne/cm Paraffin wax 23 Polyethylene 25-36 Polymethylsiloxane film 24 Polyvinyl fluoride 28 Polytetrafluoroethylene 18.5 Perfluorolauric acid monolayer 6 Petroleum lubricating oil 29 Table 2: Surface Tensions of VariousAbherend Substrates Silicone oil 21 Fluoroethylene propylene 16 Polyvinylidene fluoride 25 Polymethylsiloxane fluid 20 Polypropylene 29-34 Fatty acid monolayer 24 Abherend Surface Tension, Carnauba wax 38 References: 1. Pocius, A. V., Chapter 6, Adhesion and Adhesive Technology, Hanser Publishers, New York, . 2. Encyclopedia of Polymer Science and Engineering, Volume 14, Second Edition, John Wiley & Sons, New York, . 3. "Abherends", Adhesion and Bonding, N.M. Bikales, ed., John Wiley & Sons, New York, .
• 83. Now let’s have a look at the most common method of Non Silicone Release Coating What is special advantage of these methods ? SLOT DIE CASTING SLOT DIE AHESIVE COATING & LAMINATION KISS ROLL + MYER COATING & LAMINATION FLOATING KNIFE COATING AND LAMINATION COMMA COATING AND LAMINATIONGRAVURE COATING & LAMINATION
• 84. a ve in olvents , olve ,Do not dissol S water or Polymers & hence the same do not get transferred to any other The coated Adhesive forms a firm bond with the liner , substrate that come in contact after applying Nip Pressure The coated Adhesive forms a Film , & the same gets transferred to any other substrate that come in contact after applying Nip Pressure The coating once fully cured used in the Adhesive Formulation The coating never cures. Hence the solvent is just evaporated and the solids in the solution remain on the surface. The solids on the surface diss back in the Solvents or Polymers used in theAdhesive Formulation Based on Crosslinking Reaction ( Thermal Process ) Based on simple drying & No Cross linking Silicone The Types and the Chemistry Here is valid reason for the same Why Non Silicone Coated liner is not suitable for Transfer Coating ?
• 85. um Able to coat in line , along with adhesive. cost effective Should not interfere with the PS Adhesive , or lower the peel adhesion , due to unwanted pick up of the release coating by adhesive very low coat weight Non curing type , no chemical reaction or Cross linking is involved. The Non Silicone Release coating should be Quick drying. Just to s up
• 86. The term RC Silicones stands for Radiation Curing Silicones. Release liners made with UV curable silicones are growing in popularity. For the last 25 years, Evonik has been an expert in UV curable silicone release systems for the PSA (pressure sensitive adhesive) market. TEGO® RC Silicones are functional silicone polymers. The functional groups are firmly linked to the silicone backbone. The products are 100% polymeric materials and contain no solvents. UV curing requires the addition of a photoinitiator (PI). There are two UV curable silicone release systems on the market. Both are solventless and produce release coatings without the use of heat, but differ in their underlying chemistry.
• 87. The term RC Silicones stands for Radiation Curing Silicones. Release liners made with UV curable silicones are growing in popularity. For the last 25 years, Evonik has been an expert in UV curable silicone release systems for the PSA (pressure sensitive adhesive) market. TEGO® RC Silicones are functional silicone polymers. The functional groups are firmly linked to the silicone backbone. The products are 100% polymeric materials and contain no solvents. The first is based on silicone acrylate and cures via a free radical mechanism, whilst the other release system uses epoxy silicones and cures in the presence of a cationic photo-catalyst. Evonik offers both free radical and cationic curing silicones. This guide has been prepared as an aid for line supervisors and operators when using RC Silicones from Evonik. It covers
• 89. Mixing of Tego® Rc Silicones It is best to keep silicones separated from all other coating materials. We recommend that the silicones are handled in a specially marked, separate area. Always wear personal protective gear: rubber gloves and eye goggles. In order to avoid spills of silicone on the floor, it is advisable to cover the floor with cardboard or paper. Note: spilled silicone will make any surface extremely slippery. The silicones can be measured by their weight or volume. Variations of 1 – 2% silicone in a blend will not have a big influence on release properties. However, the photoinitiator or photocatalyst should be added more accurately, e. g. 2.0 +/- 0.2%. To use RC Silicones, merely blend the appropriate amounts of RC Silicones needed for the desired release. Add the photoinitiator or photocatalyst as the last component to avoid having this small addition remaining on the bottom of your bucket and not mixed properly. Mix until the blend is homogeneous in color. The recommended mixing equipment is depending on the daily volumes used. For smaller amounts of silicone up to 30 kgs, a drilling machine with a stirrer may be good enough. For higher volumes we would recommend more professional mixing equipment.
• 90. However, there are no special requirements to blend TEGO® RC Silicones since they are easily mixable with each other. The different RC products have different colors. The color does not affect the quality of the RC product. With most recipes, the mixed silicones will turn turbid after blending. The turbidity of the silicone blend will disappear completely during the UV curing process. The silicone coating will become transparent. The turbidity is due to the fact, that some of the RC Silicones are not soluble into each other. They are miscible,but will separate on standing. The time for separation is dependent on temperature, the silicone formulation and component ratio. If the mixture is left undisturbed, separation may occur within a few hours. Therefore, continuous stirring in the holding tanks at the process line is advisable. After longer stand-still or storage, separated silicone formulations can be easily remixed. Also the one-component products as well as the ones containing fillers will separate on standing, although this needs a much longer time. Therefore, these products should be stirred prior to use. The entire content of the drum must be stirred well before removing any material. To facilitate this, for these products extra headspace has been left in the drums. Refer to the corresponding technical data sheet before use of the product.
• 92. Silicones At The Coater Head Blended silicones should be transported to the coater head in a closed container or tank. We recommend that the holding tank is located next to the coater head or coating station and is equipped with a stirrer. This helps to keep the blend mixed during the production period. In order to get air bubbles out of the silicone blend, the RC Silicones should be circulated between the coater head and holding tank. RC Silicone blends range in viscosity from 150 to mPa*s. Some silicone lines are not equipped to handle viscosities at the upper end of this range. High or inconsistent coat weights and poor coverage may result. Lowering the viscosity is possible by heating free radical curing silicone and coating head to 60°C (140°F), which will reduce the viscosity into the range of 150 to mPa*s. At 60°C (140°F) or below, there will be no risk of gelation on the coater head, but the blend will need to remain agitated in order to prevent separation. To reduce the viscosity further, you can add special reactive UV curable diluents to the free radical silicones. These diluents should have a good silicone solubility to avoid fast separation. ▸ ▸ ▸ The addition of 5 to 10% dodecyl acrylate or multifunctional acrylates is possible. At this addition level, the release properties are influenced to a small but noticeable amount only. However the compatibility of these reactive diluents has to be tested thoroughly with the adhesives of the final application.
• 93. Cationic UV silicone formulations often show lower viscosities than comparable silicone acrylates. To avoid gelation on the coater head, they should not be heated to more than 30°C (85°F). Nevertheless it is advisable to control the temperature of the coating head in the range between 20 and 30°C (70 to 85°F). This ensures that the system is not heated up by friction during longer production runs and therefore enables constant coat weights and uniform coating quality. Coating Technologies TEGO® RC Silicones can be applied with all current coating techniques such as 5-roll-coater, offset gravure and flexo. 5-roll-coater For high speed coating and the best coating quality, TEGO® RC Silicones can be applied with a smooth multi roller coating head. Smooth filmic substrates can be siliconized with a silicone coat weight of 0.6 to 0.8 g/m². Rough substrates and paper may require higher silicone coat weights for full coverage. Offset Gravure Coater Another common method to apply silicones is offset gravure coating. Smooth filmic substrates can be siliconized with a silicone coat weight of 0.8 to 1.2 g/m². Again, rough sub strates and paper may need a slightly higher silicone coating weight for a sufficient surface coverage.
• 109. Silicone Coating with smooth application ( Rubber ) Roller Silicone Coating with worn application ( Rubber ) Roller
• 110. Rubber Rollers The most used roller materials are ✓ EPDM Ethylene-Propylene-Diene Rubber ✓ PUR Polyurethane Rubber The preferred hardness of the applicator for silicone coating both on paper is Shore A 55 – 75. Softer rollers have a better recovery compared to harder rollers. The preferred hardness of the dosage roll of 5-roll-coater heads is Shore A 75 – 80. The cleaning solvents must be compatible with the rubber roller material you use. If rubber rollers are cleaned with unsuitable solvents, the rubber will swell. Repeated swelling and shrinking can reduce the diameter and flexibility by extracting plasticizers and fillers. The silicone can migrate into the rubber and become sticky, which decreases the life span of the roller and harms the coating quality. We recommend isopropanol for cleaning. For PUR, however, you should consult with the manufacturer, which solvents are recommended. The surface smoothness of the rubber does have a huge influence on coating quality. Both microscope pictures above (50 times magnification) show silicone coatings made with Shore A 60 applicator EPDM rollers: Left a coating made with a very smooth roller, right a coating employing a roller with a worn surface.
• 111. Silicones And Printing Operations If you keep the workplace clean and the silicones confined, RC Silicones can be used in the same production area, even on the same production machinery as printing inks or overprint varnishes. It is important to avoid silicone contamination of the printing stations or the printing inks. Silicone contamination can cause print failures, such as pinholes or orange peel effects. Silicone Coverage and Release Properties A high-quality silicone coating requires good coverage of the substrate. Uncoated areas, pinholes, and exposed paper fibers will increase the release value and give poor release stability over time. In general, we can expect similar release values on all substrates when coating the same release blend. Air bubbles and foam in the silicone can have an influence on the silicone coverage. This occurs most often with offset gravure coaters, but also happens with 5-roll coaters. We recommend circulating the silicones between the holding tank and coater. This allows the silicones to de aerate. With open pan or 5-roll coaters, it may be useful to discharge these air bubbles by means of a moving bar. This will avoid the formation of areas with high foam/air bubbles, which may transfer to the web and cause stripes in the machine direction. ▸ ▸ ▸
• 112. Prerequisites For Consistent Release Values very good silicone coverage no interfering additives or migratory (“blooming”) components in the substrate good silicone anchorage same surface roughness of the substrates
• 113. 5-roll coating system Extremely slight and precise coating weights are adjustable Coating weight can be increased or reduced, up to 50%, by changing one cylinder velocity during machine run Accurate repeatability through high precision mechanical stops with micro-adjustments Specifications Production speed up to m/min (ft/min) Coating width >mm possible High precision cross profile over the whole web width of 2% which is guaranteed by using a special cylinder technology with a patented deflection compensated impression roll Applications solvent less coatings with 100% solids thermal curing UV curing EB curing
• 114. 5-roll coating head for solventless, 100% solids coatings. The coating head is designed for a production speed of up to m/min., with a coating width of mm and is equipped with a patented roll deflection compensating system. This coating method is especially suitable for low coating weights with high quality requirements in regard to the coverage and coating weight tolerances at high production speed. The precision presetting mechanism ensures highest accuracy in repeatability of production parameters.
• 115. 5-roll coating system Extremely low and precise coating weights are adjustable Coating weight can be increased or decreased, up to 50%, by changing one-cylinder velocity during machine run Accurate repeatability through high precision mechanical stops with micro-adjustments
• 116. Roller No Type Speed R 1 Steel Gravure 1 S 1 Silicone / EPDM Rubber 10 R 2 Stainless Steel 30 S 2 Silicone / EPDM Rubber 99 R 3 Stainless Steel 100 Roller Details
• 117. Corona Treatment In-line corona treatment just before siliconizing is always recommended when using free radical RC Silicones. It can also help to improve the anchorage of cationic curing silicones. Corona treatment forms hydroxyl, carboxyl, and free radical groups, which are important for anchorage of the RC Silicones. Corona treatment also influences the spreading behavior of the silicones. High surface tension is not a guarantee for good silicone anchorage, rather it is the presence of reactive molecules. As they disappear quickly, in-line corona treatment is recommended even on high level pre-treated substrates. With most types of film, in-line corona treatment is recommended for good silicone anchorage. Some PET films generate very high surface tension, and may need only a little or no in-line treatment. On clear films, a good in-line corona treatment will help to coat a transparent silicone coating without blemishes.
• 118. Flame treatment or extremely high corona treatment of a plastic film can cause low molecular weight polymers to be formed on the film surface, which can cause anchorage problems. With rough substrates, such as some clay coated papers, in-line treatment may not be necessary. When using a pre-treated substrate for single side siliconizing, please make sure to in-line treat and siliconize on the pre-treated side. If the non-treated side is silicone coated and wound onto a roll, the silicone coated side of the substrate and the pretreated side (backside) will be in contact. This contact will influence the release properties and may result in undesirable release values. This is a time-dependant effect and may take months to become noticeable.
• 119. Corona Treatment For Double-sided Release Coatings When coating the second pass of a double-sided release coating, it is important to make sure that the in-line corona treatment is not affecting the first silicone coating. Backside corona treatment can happen with some corona treaters. It is generally caused by a thin layer of air trapped between the treater roll and the film. Even a small amount of backside corona treatment can increase the release value of the first pass silicone coating. Usually, back side treatment does not happen uniformly across the web, thus results in stripes or spots. To avoid backside corona treatment, use a corona treater with a lay-on roll, a tight wrap angle, and a clean, smooth treater roll. This will help minimize the amount of air being trapped behind the substrate. When a double-side silicone coating is made with differential release, it is advisable to run the tight release side first.
• 120. UV Power Demand And Cure Speed Speed of cure is one of the essential points of our RC technology. Due to the cold curing there is little heat stress to the substrate. Contrary to the thermal silicone systems, the maximum cure speed does neither depend on the type or the gauge of the substrate to be coated nor on the silicone coat weight. UV lamps are available with outputs of 80 - 240 W/cm depending on the working width. Standard medium pressure mercury lamps are widely used in the UV printing industry. These Hg-lamps have strong emission bands at 250 - 300 nm, which fits best into the absorption spectrum of the commonly used photo initiators and photocatalysts. The cure speed of free radical curing silicones is very fast.
• 121. Their UV power demand for a complete cure is very low. One bank of 120 W/cm (300 W/in) lamps can be sufficient to cure at 200 m/min (660 ft/min). Some of our silicone acrylates cure even much faster. As the UV bulbs and reflectors have a reduced UV output over time, it is recommended to overdose a little, e. g. to 160 W/cm (400 W/in) at 200 m/min. The properties of the release coating will not be affected by a higher UV output applied during the curing process. The reaction speed of cationic curing silicones is mainly influenced by the substrate used and by the silicone formulation and therefore can show strong variations. Hence, cationic curing silicones often require more UV power than free radical curing silicones.
• 122. One bank of UV arc lamps with 120 W/cm should be enough to run line speeds up to 100 m/min. The shape of the reflector has a major influence on cure speed. Trials have shown, that a diffused UV light reaches a higher cure speed compared to focused UV light. Perhaps due to this, we have seen a higher cure speed of both UV silicone systems with standard arc lamps compared to microwave powered lamps. The latter always have a focused reflector system. Best results for free radical curing have been seen with standard medium pressure mercury lamps (so called H-lamps, not doped). For cationic curing, Gallium doped mercury bulbs, so called D-bulbs, may be of some advantage.
• 123. Suitability Tests Before using any new silicone formulation, it is recommended to check whether the final product meets all given requirements. After coating and curing, it is necessary to check whether the cured silicone has the desired performance. This includes o coverage of the silicone coating on the substrate o full cure of the silicone coating o silicone anchorage o compatibility of the release coating against the adhesives via release ageing tests at low and high temperatures Post-irradiation may cause a property change in the final product. If the final product is subject to o electron beam or Gamma irradiation for e. g. sterilization purposes or o a secondary UV exposure e. g. when curing UV printing o inks on label stock with a clear face stock, the influence on the final product should be tested.
• 124. Changing Between Different RC Blends If you use different mixtures of RC Silicones of one and the same system (just free-radical or just cationic curing products), they will not influence each other in curing properties but might have an influence on release properties. If you change from a blend of easy release to a blend of tight release, it will be necessary to clean the coater head carefully. Small amounts of an easy release blend can reduce the release of a tight release blend considerably. If you want to produce low and very high release values on the same production line, we recommend having a separate stirrer, holding tank, and piping for each blend. The coating order for production should be to coat tight release blends first, followed by lower release blends. However, if you are not sure, clean the coater head very carefully in order to avoid any production failures
• 125. Using Tego® Rc Silicones And Other Types Of Silicones On The Same Machine Thermal, cationic and free radical silicone systems are not compatible with each other; thus contamination of one of the other systems will not cure within the main silicone system. Any residuals of different silicone types left on the coater head, in the holding tank, or in the piping, will remain uncured in the final silicone coating. This will cause release and subsequent adhesion values to decrease. Therefore, strictly avoid cross-contamination of different silicone systems as the curing mechanisms are not compatible with each other. When changing between silicone systems, it is important to clean all parts very carefully. There may be silicones, especially in the piping and mixing container, which are not easy to be removed completely. Therefore, we recommend having separate silicone mixing devices available for each silicone system you use.
• 127. Silicone acrylate polymerization is initiated by the formation of free radicals, which are formed by UV irradiation of the photo initiator. Free radicals are chemical species that have a free electron but no charge. They exist everywhere, including in human bodies. They react quickly with other free radicals, acrylates, and oxygen. In fact, they react so quickly with oxygen that they would react with oxygen molecules rather than the acrylate groups of the TEGO® RC Silicone acrylates, preventing the silicones from curing. Therefore, it is imperative to ensure as much as possible the absence of air-borne oxygen in the curing chamber for the reaction in order to achieve the required release characteristics. The proportion of oxygen remaining on the surface of the substrate as well as at any place in the inerted chamber should not exceed 50 ppm. The inert gas recommended for the process is nitrogen as supplied for general technical applications, with a purity level of 99.996 vol. % (quality 4.6 or 5.0). Residues are hydrocarbons, oxygen, argon and other noble gases. For the purging process, the residual oxygen should not exceed 10 ppm. The quality of nitrogen should be discussed well ahead of time with the supplier. An economical way to purchase nitrogen is a liquid nitrogen tank with an evaporator. Both, tank and vaporizer normally are rented on a monthly basis from the nitrogen supplier. The nitrogen tank will be refilled without interrupting production. Nitrogen tanks and evaporators are available in different sizes and will be adapted to your needs and requirements.
• 128. Nitrogen is extremely safe to use. It makes up approximately 80% of the air we breathe (the remaining 20% is oxygen and other gases), and nitrogen is non-toxic. By using nitrogen as the inerting gas, special exhaust systems and safety precautions are not needed. However care must be taken to ensure that outside air is circulated in the working environment. Nitrogen gas is easily available worldwide and inexpensive. Carbon dioxide (CO2) has been proposed as an inert gas as well. Unlike nitrogen, CO2 is heavier than air and would concentrate on the production area floor. Thus, CO2 is not a safe inert gas and is not recommended for this application.
• 129. Nitrogen Consumption Every substrate has a surface boundary layer of air with an oxygen content of approximately 20 vol. %. If the substrate passes through a purged reaction chamber, the oxygen will be lost by diffusion. However, since this diffusion process requires too much time, efficient production would be seriously hampered. It is, therefore, necessary to accelerate the removal of the boundary layer by the use of suitable nitrogen jets or nozzles. The chamber under the UV lamps is specially engineered to remove the boundary layer of air that is carried along by the moving web. This is accomplished by using a nitrogen “knife” at the front of the chamber. The “knife” provides a laminar flow of nitrogen, which effectively removes the boundary layer, like a real knife peeling an apple. The exit side of the chamber is designed to allow the substrate to pass from the chamber and limit the amount of nitrogen used. Good sealing of the chamber is essential to minimize the consumption of nitrogen and prevent air (oxygen) from getting inside the chamber.
• 130. The preconditions for effective inerting and minimum inert gas consumption are as follows: ✓ low reaction chamber volume and good reaction chamber ✓ seals (gaskets) ✓ properly sealed quartz plates ✓ effective barrier nozzle at the entry side ✓ uniform nitrogen distribution in the chamber ✓ carefully conceived oxygen monitoring ✓ good exit-side resistance against nitrogen outflow ✓ certain interlocks and further options The overall nitrogen consumption depends on the width of the line and the number of UV lamps, thus the length of the UV unit.
• 131. In addition, the line speed influences the nitrogen consumption to a great extent. In a UV line with a working width of mm, a well-designed inerting unit will work efficiently on smooth film substrates with the following quantities of nitrogen: ✓ at 100 m/min approx. 45 – 60 m³/h, ✓ at 200 m/min approx. 60 – 80 m³/h ✓ at 300 m/min approx. 75 – 100 m³/h. On rougher substrates, replacing the air boundary layer is more challenging than on smooth surfaces. In such cases, the nitrogen consumption for the inerting of paper substrates is expected to be approx. 20 - 50% higher.
• 132. Siliconizing of Plastic Films The UV radiation necessary to cure RC Silicones will not increase the web temperature during production significantly. Typically a temperature increase of only 5°C (10°F) is seen, depending on the material, thickness and the color of the film. A broad range of plastic films down to 3 µm PET (polyethylene terephthalate) or 15 µm LDPE (low density polyethylene) can be coated. When stopping the line, very thin filmic substrates may get hot due to heat released from the quartz glass windows. This may require the web to run at very low speed through the line until the window is cooled down. Free radical RC Silicones will cure on all types of substrates. However, there are additives in some plastic films that may influence the silicone anchorage to the film, e. g. plasticizer in PVC (polyvinylchloride) or slip additives and anti-stats in PP (polypropylene) or PE (polyethylene) films.
• 133. Silicone anchorage (rub-off) with RC Silicones usually does not depend on time. If the anchorage is good right after coating and curing of the silicones, the rub-off will not change. A bad rub- off usually stays bad. There are only few exceptions from this rule: In some rare occasions (mainly on PET) the rub-off may improve within the first 24 hours after coating. On the other hand, on soft PVC, the plasticizers might be migratable and cause initially good anchorage to get worse over time. Some migratory (“blooming”) components of the film can travel through the silicone coating and reduce the release force or give false subsequent adhesion values. Silicone coated soft PVC therefore often exhibits low subsequent adhesion values, as low as 55%, although the silicone coating is fully cured. If anchorage failure is observed on a particular type of film substrate, a special anchorage additive to the silicone may help. With the addition of an additive, good silicone anchorage can be achieved even on most PVC substrates. Please contact your Evonik technical service team for further details.
• 134. Pot Life / Remaining Silicone Acrylates After The Job The guaranteed pot life of coater-ready free radical curing silicones is 72 hours, however, they can easily be stored for much longer time. Without UV light, free radical TEGO® RC Silicones will not cure. Store the unused portions of the blends in either lined drums or pails, or plastic containers. Keeping the following preconditions in mind pot life can be extended to several months: • Prevent exposure to UV light. Use containers that prevent light from getting through to the silicone. • Do not store the silicone outside where it can be exposed to direct sunlight or high temperatures. • Allow air to get to the silicone blend. The oxygen in the air prevents the silicone from curing. Allow headspace in the container, and reopen the container from time to time. • If the blend will be sitting for a long period, you may want to stir the pail occasionally. This mixes air into the silicone, which helps prevent curing. • Clearly identify the container to avoid confusion.
• 135. direct sunlight reaching the silicones, they will not cure inside the tubes. Flushing will be required if you will be using other materials or silicones after running free radical RC Silicones. Also, silicone acrylates will not cure on the roller for a production break overnight or for the weekend (as long as it is not exposed to direct sunlight). The coating remains liquid and does not plug up the cells. Therefore, there will be no build up on the doctor blade. However, the silicones will attract dust and dirt from the production environment. Therefore the rollers should be protected by a kind of cover against dirt and light during stand still. Of course, before longer stand-still of the coating line and changes between different coating or silicone systems, the coating system needs to be cleaned thoroughly. To clean the coater head, drain the silicone from the coater head and piping into the holding tank. Keep the silicones in the holding tank or put them into a properly identified container to store it for future use. The rollers and the silicone pan or closed chamber system should be cleaned by means of disposable towels and a suitable solvent such as isopropanol. If you need a thorough cleaning, stronger solvents like white spirit may be used to clean the roller. Please make sure that all regulations for the use of solvents are observed and that the solvent would not tend to swell the rubber rollers.
• 137. Siliconizing Of Paper Substrates The standard pulping process for the production of common paper grades is alkaline. Since the mechanism of cationic polymerization is initiated via an acid, most paper substrates are not suitable to be coated with cationic Curing silicones. The residual basic components in the paper will react with the acid formed by the photocatalyst, thus The curing process will not be initiated properly. There are specialty papers manufactured in acidic pulping processes. These papers can be used with cationic curing silicones providing the same advantages as free radical curing UV silicones (excellent paper lay-flat, no re humidification). Polyolefin coated kraft papers (PEK) are coated with a thin layer of plastic film to prevent coatings from penetrating into the paper fibres. Thus the silicone layer will be in contact to the thin filmic layer on top of these papers. Therefore, many PEKs are suitable for cationic siliconizing, too. The polyolefin used for the coating of the kraft paper should be free of poisoning ingredients for the cationic curing silicones as described before.
• 138. However, keeping the following preconditions in mind, even the pot life of cationic curing silicones can be extended to weeks or even months: o Prevent exposure to UV light. Use containers that prevent light from getting through to the silicone. o Store the material below 30°C. Above 30°C polymerization will start resulting in higher viscosities over time. o Do not store the silicone outside where it can be exposed to direct sunlight or high temperatures. o Clearly identify the container to avoid confusion. To make sure that the silicones are still good to be used, check the viscosity before use, e.g. by means of a Ford cup flow time measurement. It is also advisable to run a short trial with the material after longer storage time before starting bigger production campaigns. Cleaning Procedure In contrast to silicone acrylates, epoxy silicones left on the coating equipment, holding tank and pipes must be removed prior to a lengthy downtime (overnight or weekend breaks) in order to avoid silicone build up. Therefore it is recommended to clean the whole equipment after every use.
• 139. To clean the coater head, drain the silicone from the coater head and piping into the holding tank. Keep the silicones in the holding tank or put them into a properly identified container. The silicone blend should be used in the shortest possible time or disposed immediately to avoid gelation in the holding tank or container. The rollers and the silicone pan or closed chamber system should be cleaned by means of disposable towels and a suitable solvent such as isopropanol. If you need a thorough cleaning, stronger solvents like white spirit may be used to clean the roller. Please make sure that all regulations for the use of solvents are observed and that the solvent would not tend to swell the rubber rollers.
• 140. Test Methods QUALITY TESTS Off machine tests are useful to gather first information on the silicone coating quality right after production. These tests should be known to the operators and performed frequently,e. g. every jumbo roll: SMEAR TEST Rub the silicone surface with your finger. There should be no sign of liquid silicone (= smear). The smear test is a good indication for the initial cure of cationic curing silicones. Very slight smear/marks in the silicone layer from the finger might be acceptable and should disappear within one day after production (post curing). If smear is observed on free radical curing silicones, there is something wrong in the curing process. Check the formulation (photo initiator), the UV bulbs and reflectors and the inerting setup. RUB-OFF TEST Rub the silicone surface a bit harder with your finger. There should be no rub-off of any dry particles of silicone. Please note that if you rub hard enough, you may abrade the surface, especially at higher coat weights. This is not a sign of poor cure or anchorage failure. It is the nature of solventless silicones to have a more rubber-like behavior.
• 141. The anchorage of cationic curing silicones may improve within the first 24 hours after production due to post curing of the silicone layer in contact with the substrate. Loop Test Apply an approximately 20 cm (8 inch) piece of TESA tape on the cured silicone coating, peel it off and form a loop by putting the adhesive coated sides together. Opening the loop should require a certain level of force. In case of a transfer of liquid (uncured) silicone, a considerable reduction of the release force will be noticed. Dye Stain Test A test ink of 1% methylene blue in water is applied for one minute onto the silicone coated surface by means of a Cobb tester. The blue ink will stain any uncoated areas of the paper. A certain level of staining may be acceptable. A standard should be defined for each product. This method works only on paper substrates, not with poly-coated papers or films.
• 142. Microscope To evaluate the silicone coverage on filmic substrates, a coaxial illuminated microscope with a 50x magnification can be very helpful. A small, portable and battery operated microscope, can be used on the production line for quality control.
• 145. Release And Subsequent Adhesion Test For a full quality control, release and proper cure tests need to be done. For the European label industry FINAT test methods FTM 3, 4, 10 (release) and FTM 11 (subsequent adhesion) are often used. The European tape industry also employs AFERA test methods. In the US and in Asia, there are various ASTM, TLMI and/or PSTC methods used. For most of the different test methods we have more detailed information at hand. Please consult with our technical service teams for assistance. The subsequent adhesion test (FTM 11) is a good indication if the test tape has been contaminated by liquid silicone residues, i.e. the test can indicate insufficient curing. Due to the post curing of cationic curing silicones, subsequent adhesion tests can improve to a great extent within the first day after production. Therefore, this test is not meaningful in this period of time. After 24 hours, the post curing should finally have taken place, therefore, the subsequent adhesion will normally show very high (good) numbers. Hence, this test method is not suitable as quality control test for cationic curing silicones.
• 146. Silicone Coat Weight Measurement The silicone coat weight can be measured with a precision balance (4 decimal points) by washing off the silicone by means of a solvent. This can only be made on smooth filmic substrates and without corona treatment. A more versatile method is the X-ray analysis that gives a signal proportional to the Si-atoms concentration in the coating. TEGO® RC Silicones are silicone acrylates or epoxy functional silicones. These organic groups must be considered when determining the actual silicone coat weight. Since calibration curves in X-ray units are generally based on 100% polydimethylsiloxanes, The readings observed must be adjusted to calculate the actual silicone coat weight when using TEGO® RC Silicones.
• 147. For this, we have introduced the “RC-Number” which is a measure for the organic content of the silicone (or silicone blend). Extremely high Si blank values (as found with clay coated paper and pigmented films) can cause errors in the coat weight determination. Very porous papers can absorb silicones which results in lower readings. On thin filmic substrates, make sure to measure on the silicone coated side. Thin substrates will give readings even on the uncoated side by scanning through the substrate. This effect will lead to erroneous readings on double side coated thin films, too.
• 148. The quality of a silicone coating is dependent on the conditions of curing. For a good silicone cure, a sufficient amount of UV light and – for free radical curing - good inerting conditions are required. Using too much UV light cannot harm the properties of UV silicone systems. However, using too little UV light, e. g. by reduced transparency of the quartz plates or worn-out UV- bulbs, can reduce the degree of cure. This is an example of a maintenance plan for an inert UV unit. This maintenance schedule can be used as a guideline for operators and the engineering department. For non-inert UV units, just ignore the recommendations for the maintenance of the nitrogen supply system.
• 156. Silicone release liners, problem solving in release applications Introduction Elkem Silicones is one of the foremost fully integrated silicone manufacturer. Elkem Silicones offers a comprehensive range of silicone products in the sectors of release coating, textile, engineering elastomers, healthcare, specialty fluids. The Silcolease® range is unique in its ability to cover all technologies used in silicone release coatings: Solventless thermal, Solventless radiation, Emulsion and Solvent. The problems that are commonly associated with end use of Silcolease® products for release applications against pressure sensitive adhesives are relatively few in number, but each has many contributing factors, and many problems are interlinked. This document tries to relate problems to possible causes or what should be analyzed and gives advice on how to resolve certain problems.
• 157. Problems commonly encountered 1. Release Force 2. Bad curing/Migration 3. Change of subsequent adhesion 4. Printability 5. Rub-Off 6. Slip/Friction 7. Die-cutting 8. Bath life/gelling 9. Gloss Problems 11. Blocking 12. Telescoping 13. Laminate curl 14. Emulsion coating problems 10. Oven dust
• 158. Release Force When you have a release force problem, a question to ask yourself is “Have I got the right combination of raw materials?”. You should always keep in mind that not all silicones and adhesives give the same release and that the chemical and physical nature of your liner substrate can have a significant impact on the release force. Stock Face Adhesive • Chemical nature • Coat weight • Residuals (solvents & component migration) Silicone • Polymer type & stoichiometry • Coat weight and uniformity • Surface energy & release additives Paper (or filmic substrate) • Smoothness • Porosity • Stiffness • pH & residual chemicals
• 159. Here is an example of how the release force can change with silicone polymer type and the speed of peel for any given adhesive system. Polymer A giving similar release force at any speed whilst Polymer C rapidly increases with speed of release.
• 160. Tight Release The most fundamental property of the release coating is the requirement of giving suitable release of the adhesive material that it is protecting. The most common of problems is that release is not easy enough and this fact can be caused by any number of the conditions that follow. When we talk about tight release this can be at: – High speed only – Low speed only – All speeds The following factors have been found to affect the release, and some understanding of these factors will be essential for each case to be investigated:
• 161. • Silicone Release Layer – Polymer type – Uniformity of coating weight – Surface energy – Polymer properties – Smoothness of coating – Extent of cure – Crosslink density – Level of release additive – Modulus – Film continuity – Coating weight – Cure mechanism • Adhesive – Chemical nature – Coating weight – Residual solvent level – Modulus – Uniformity of coating weight – Component migration – Surface energy – Cure (crosslinking) • Substrate – Type of paper or film – Calliper consistency – Surface absorbency – Porosity – Surface energy – Residual chemicals from manufacturing – Smoothness – pH – Stiffness / Flexibility – Internal bond strength
• 162. • Laminate – Type of substrate and face sheet – Mode of adhesive application – Elongation – Stiffness / Flexibility – Moisture content balance • Converting – Stripping speed – Stripping angle – Mode – Physical dimension • Key Paper Properties – Porosity – Surface bond strength – pH – Internal bond strength – Smoothness – Surface absorbency – Stiffness / Flexibility – Caliper consistency • Paper Chemicals – Nature of clay coating – Binder chemistry – Sizing chemistry – Inhibitors of platinum However, many of these factors will be well known when any problem is reported. From the silicone side of the problem it is essential to establish for any complaint of tight release that the following factors are known: – Coat weight – Coverage (pinholes etc....) – Cure – Silicone formulation / composition – Release specificity to particular adhesives By establishing these basics, it should then be possible to decide on which area to concentrate to explain the occurrence of tight release. Only from an understanding of these factors can a solution be proposed with any confidence.
• 163. Release instability/Variation in release values Release values are measured usually after a certain set period of time for either silicone coated papers or for laminates. The variation in the release test results (typically FINAT TM 3, 4 & 10) can vary by at least 10% simply due to experimental error/ variation and the vast number of factors affecting a single measurement. Obviously each construction will have different parameters with varying degrees of effect, and so there will be cases of perfect reproducibility and others of huge variation. These variations are unavoidable, in practice, when measuring a liner release against the most relevant adhesive. The release values vary to differing degrees, but on certain occasions an increase or reduction of release values over time (for either the liner alone or the laminate) can cause problems for end applications.
• 164. In summary, the main effects are as follows: • Lock-up - Release values increase with age of laminate Because of an interaction between “acrylic” adhesives and residual silicone reactive groups, by poor coverage of silicone, poor silicone cure, poor adhesive drying, silicone or adhesive formulation (migration of tackifier or resins in adhesive or silicone). • Drop-off - Release values decrease with age of laminate or paper – Poor cure of silicone (migration) – Hydrolysis of excess crosslinker (SiH) groups – Migration from face or backing substrate (particularly PVC) – General adhesive degradation • Reel to reel - Where inconsistent values are seen from one run to another
• 165. Too Low Release o Silicone formulation (e.g. not enough tight RCA for solventless silicone) o Coat weight too high o Under-cure (migration) Zippy Release – Adhesive problem (drying, formulation, de-wetting…) Two other common release phenomena can occur, namely Zippy release, otherwise known as a Slip-Stick type of release, and Peak release, an initial high release force. Zippy Release This problem manifests itself mainly in low speed release. The average release can be good but the release curve is very spiky with a large variation between minimum and maximum values for a peel test. The reasons for the zippy release are most commonly very similar to those causing peak release problems but when compared to a smooth release profile sample, a slow speed zippy release will also show more “zippy” nature at high speed.
• 166. Most common causes are: – Adhesive modulus, tackifier level and type, etc. – Pinholes / poor coverage of silicone Release Force Trend Line – Chemical or physical interactions – Backing and face substrates stiffness and strength – Silicone modulus Figure 3. Zippy Release Peak Release Release Force trendline Usually appears as a very tight release peak at low to medium peeling speed. Most common causes are : – Adhesive modulus – Chemical or physical interactions – Silicone modulus – Pinholes/poor coverage of – Backing and face substrates – Die-cutting problems silicone stiffness and strength
• 167. Bad Curing/Migration Cure is difficult to define as an absolute factor as the only true effect is whether the laminate performs adequately under the end use. Some laminate constructions are capable of accepting a degree of under-cure that in other more sensitive areas would cause total rejection of the laminate. • Bad cure (under-cure/smear/migration/extractables) can be most commonly attributed to: – Machine speed too fast – Temperature too low (oven air velocities, etc.) – Coat weight – Silicone formulation (SiH/SiVi, catalyst level, etc.) – Substrate - catalyst poisoning – Age of bath – Other potential inhibitors of reaction (from machine, raw materials, etc) – Solvent type & grade - volatility, inhibition (Sulphur free grades)
• 168. • Silicone extractables as a measure of cure A commonly used industry standard but it is important to understand the relevance of this test! o Non-chemically linked materials are extracted from the cured network by an organic solvent and measured analytically o Some silicone materials can be non-chemically linked but form a stable and reliable release performance o Extractable species are not necessarily the same thing as migrating species o Many applications can be unaffected by, or even benefit, from a small amount of migrating silicone: o Bitumen o High adhesive coat weight applications o Zippy release
• 169. Change of subsequent adhesion Sometimes the tack and adhesive performance can be changed after contact with the silicone coated substrate. The single most important factor is likely to be silicone migration caused by poor curing, but this is not the only effect. The most important factors are: • Low subsequent adhesion o Under cure of silicone leading to migration o Other migratory material from backing or top face or from within adhesive itself o Very rough silicone surface can reduce the smoothness of the adhesive o Adhesive deterioration with age, atmospheric exposure, moisture content of paper etc...
• 170. • Higher subsequent adhesion o Very smooth silicone surface can improve the smoothness of the adhesive o There can be adhesive changes due to contact with silicone such as tackifier migration etc... o Printability Problems o These can be either on the back of the silicone paper, or on the top face of the laminate. o Off-line: Silicone under-cure causing migration to the back of the paper (and then potentially on to top face). o In-line: Silicone volatiles or misting. o Substrate problems: penetration of silicone through paper. o Silicone or other sprays used around printing machines. o General contamination from a dirty machine, or another source.
• 171. Rub-Off This can be seen immediately or on ageing of the silicone coated substrate: • Silicone formulation (can be anchorage or adhesive failure, or cohesive failure of the cured silicone): o Too low crosslinker level o Wrong crosslinker type o Poor cure o Crosslink density of the silicone (more relevant for solventless silicone grades) • Substrate (will lead to anchorage or adhesive failure): ✓ Too smooth or too low in porosity ✓ Surface energy (films) ✓ Surface inhibition ✓ Lack of adequate corona or other surface treatment Type of solvent used for dilution • Aged rub-off can be due to: o Post curing o Re-orientation of silicone matrix o Migration to substrate surface of disruptive materials o Surface energy changes
• 172. Slip/Friction The lack of friction or slipperiness of the silicone surface can cause production problems further down the converting line. These can be controlled to some extent by: ✓ Silicone type: solvent Vs solventless, hardness, etc. ✓ Silicone cure: migrating silicone ✓ Coat weight ✓ Additives Figure 6. Slip/F
• 173. Die Cutting Not usually a silicone problem but matrix removal may highlight release problems. Soft silicones can be more easily damaged by die-cutting process and could lead to subsequent problems. Bath Life/Gelling Bath life problems can lead to bad release, bad cure, rub-off or general runnability problems caused by viscosity changes and gelling. The main reasons for bath life problems are: Silicone formulation (too reactive - high catalyst, high crosslinker or low inhibitor levels) ✓ Solvent type and grade (volatility, inhibiting nature) ✓ Ambient temperature ✓ Inhibitor evaporation or reaction with other material ✓ In emulsions - droplet coalescence leading to micro-gelling ✓ Temperature on the machine generated by coating head or bath handling apparatus ✓ Bath size and storage
• 174. Gloss Problems Gloss levels are often used as an indicator of quality. Loss of gloss through a coating run could suggest bath life problems or a general change in process conditions. Gloss can also have some useful properties in terms of release level and subsequent gloss level of the adhesive leading to change in immediate tack. Low gloss on a silicone coated substrate can be due to: o Paper surface (too rough or too porous) o Silicone cure often associated with aged baths o Too much heat on PE Kraft can cause PE to fuse (as can wrong grade of PE) o Micro-gels in silicone emulsions o Adhesive / silicone interaction causing surface damage o Rub-off
• 175. Oven Dust Dust can come from many sources such as paper. When we consider silicone dust, this is usually only residue from tin catalyst in the older style polycondensation systems, or volatile silicones which have been thermally degraded to silica. This degradation normally only occurs when direct flame burners are used to heat the drying / curing air which is then recirculated through the ovens several times. Dust can also be a problem on paper machines using direct contact drying cylinders. Blocking This problem was a severe problem with older tin catalyzed systems. It is mainly seen with two side silicone coated substrates with today’s more commonly used polyaddition silicones. There is a strong bond between the two sides of the paper which leads to the surfaces sticking together under pressure in the reel. This is sometimes only a nuisance in terms of noise (when re-reeling / slitting / converting), but in its worst case can cause damage to the silicone surface or even cause paper tears. Figure 9. Oven Dust
• 176. Blocking is most commonly caused by: o Under-cure of silicone followed by interaction between surfaces o Softness of silicone coating o Static problems • Blocking can be made worse by: ✓ Very tightly wound reels ✓ Certain re-humidification conditions causing swelling of fibres after reeling ✓ Plasticizing materials in the silicone formulation
• 177. Telescoping The telescoping of the reel edges can be caused by several factors: • Silicone formulation: • Coating conditions: • Problem with the machine: ✓ Lack of slip properties ✓ Inappropriate coat weight ✓ Misalignment of incoming webs or winder rolls ✓ Too low coefficient of friction ✓ Nature of the network (too hard or too soft) ✓ Unsymmetrical diameter of paper roll ✓ Variation in thermic properties at curing
• 178. Laminate Curl Excessive drying of paper based substrates can lead to curl in the laminate. To avoid this effect either a low temperature thermal cure silicone, good rehumidification or a less sensitive substrate must be used. This curl problem is even more significant with emulsion based silicones. Emulsion Coating Problems There are several different problems occurring due to the use of emulsion silicones in particular: o Wrinkles/cockling effect of water on paper o Curl due to moisture imbalances caused by excessive drying of papers o Wetting problems due to surface tension or viscosity of emulsion coating bath o Higher energy to dry water can give curing problems o Foaming, micro-gelling in droplets, basic emulsion stability and effects such as coalescence are fairly unique to emulsions o Solids increase due to preferential absorption of water into paper
• 179. Summary The problems listed in this document are those most commonly facing the silicone release liner side of the self-adhesive laminate market. The testing of each variable suggested can be done according to standard test methods either from industry standards (such as FINAT, TAPPI, and TLMI) or from other set methods. Elkem Silicones has available a full list of test methods that are followed to test all parameters discussed. Other test regimes are used, such as standard FINAT methods (for all laminate related tests), or generally accepted tests used within the industry.
• 191. Silicone Benefits for Tape Applications Tim Rummel, Technical Manager, Wacker Chemical Corporation, Adrian MI Despite silicone having a major role in almost all label products, usage in pressure sensitive tape applications is somewhat sporadic. Silicone is nearly essential to some applications, while for others it is a particularly poor choice. This paper will examine some of the positive and negative attributes of silicone as a release coating. The first parts will detail stable release, low release, and adjustable release applications. Pigmenting will follow these, as will overviews of low friction and low contamination properties. Silicones in Stable Release Applications Traditional tape release coatings consist of film forming organic polymers with low surface energy additives. A typical system could consist of acrylic or vinyl acetate polymers with a release material such as surfactants or waxes. The problem with such systems is that release has a tendency to change over time. The most common problem is that release increases with time, and does so more quickly and to a higher degree in the presence of elevated humidity.
• 192. Multiple Phase Release Systems These changes in release can most often be attributed to the mismatch in surface energy that is inherent between organic emulsion polymers and low release materials. There are higher repulsive forces between a PSA and a low surface energy material than there are for a high surface energy material. Therefore, the lower surface energy material will preferentially be displaced away from the surface of the release coating, rendering it inactive. Consequently, the surface concentration of the release agent decreases over time. Generally, there is some moisture sensitivity on the part of the release agent or polymer or both, so high humidity increases the mobility of the system components due to water vapor ingression. Temperature, as it does with virtually all systems, increases the reactivity and mobility as well and serves to increase the aging rate.
• 193. Sensitivity of temperature versus humidity depends on the chemical nature of the components in use, and some will not experience any release-based aging until a threshold of one or the other is reached. As is always the case, a lot depends on the adhesive. Mechanical wet-out of the adhesive with time is always a factor as well. The rate of this wet-out varies greatly with both the hardness of the adhesive and the pressure on the layers. Softer adhesives and higher pressure from firmer winding of rolls both will age faster than the alternatives.
• 194. Single Phase Release Systems Some of the early release systems in tape consisted of low surface energy polymers cast in solvent. Some of these aged extraordinarily well, simply because there was only one component present and no displacement would take place. The same is true with silicones, which can be cast in solvent, as an emulsion, or as a pure silicone oil. 100% silicone systems generally age very well. Many times, silicone resins can be used to affect release, and these often contain some α-olefin such as 1- hexadecene or 1-dodecene to improve release and coat ability. Both the resins and the α-olefins can have an effect on aging. Release modifiers will be discussed in more detail in the next section.
• 195. The following chart shows the aging behavior of three different types of release coatings. Two of the release coatings are from common types of tape. Both the release layer and adhesive layer of these tapes are tested independently. One is a general-purpose natural rubber masking tape, the release coating layer of which will be referred to as Release 1. The other is a painter’s masking tape with a styrene isoprene-styrene (SIS) adhesive and a release layer referred to as Release Both are well recognized market-leading products from two of the industry’s leading manufacturers. The adhesive layer of the two tapes were then tested against their own release coating layer, as well as each other’s release coatings. Both tapes’ adhesives Were also tested against silicone, which serves as a third release coating here. The silicone was coated on SCK liner paper. Initial release data was collected, as well as wet and dry aging data . Aging was done for 10 days at 40°C under weights producing 2.0 lb/in of pressure to simulate in-roll conditions. The wet aging was done at 50% relative humidity. The silicone had 65% of release modifying resin added, which was enough to roughly equal that of the starting release of the two tapes (Release 1 and Release 2).
• 197. Both of the tape release coatings (Release 1 and Release 2) performed fairly well, as would be expected from premium market products. As would be expected, the natural rubber adhesive aged at a faster rate than the SIS adhesive. Silicone release showed almost no impact due to humidity level, while the other two systems aged quite a bit worse with high humidity present. Silicone outperformed both systems in wet conditions. In dry conditions, silicone was comparable to Release 1 while it outperformed Release 2. Both releases are believed to be polymer and surfactant, but the exact compositions are not known. Adjustable Release Using Silicone Resins or Sodium Silicate Silicone polymer alone will generally only make a narrow band of possible release levels available to users. Unless an adhesive is an extremely aggressive variety such as bitumen, the release will generally be very low. In many cases, an adjustment is required to achieve a high enough level of release to suit a particular product or application. There are varieties of technologies available to achieve this increased release effect. The most common three are reactive MQ resin, non-reactive MQ resin, and sodium silicate.
• 198. Reactive MQ Resin MQ resin is a material made from mono-functional and quaternary-functional silane building blocks. The Q units form a rigid 3-dimensional core for the molecule that is essentially quartz, as it is simply a network of silicone and oxygen atoms. The M units give the molecule an organic covering with siloxane type properties, and they also serve as the boundaries of the particles. MQ resins can vary greatly in size. They serve to break up the smooth, uniform surface of a silicone coating and thereby increase its release force. In release systems, the MQ resins will often include functional groups that are the same as one of the reactive components in the silicone coating being used. In the case of platinum or rhodium cured systems, this is an unsaturated terminal hydrocarbon group, or vinyl group. If a tin catalyzed system is in use, then hydroxyl groups will be present the advantage of functional groups is that they lock the MQ resin in place, resulting in more stable aging and more predictable release force. The disadvantage is that they are less efficient than their non-reactive counterparts.
• 199. Non-Reactive Release Modifiers – MQ and Sodium Silicate MQ resin can be made without functional groups. Sodium silicate, also called water glass, functions much the same way as a non-functional MQ resin. Both have partial immiscibility with siloxane polymer, so both concentrate at the surface of a coating between the time that it is coated and cured, even though this can be less than a second. Because of this, they are generally more efficient than functional offsets, sometimes needing only 2% to see a noticeable release effect. The disadvantage for these is that they are significantly harder to control. They can be sensitive to coat weight and line speed. In addition, they often have large swings in release force as the product ages. Process upsets are more likely to alter release with these chemistries. Additionally, they can show up in silicone extractable tests, which is misleading because they are less detrimental to adhesive performance than uncured silicone polymers.
• 200. Release altering resins in general cost a little more than silicone polymers. They also slow down cure a bit, which requires more catalyst, further increasing costs. But, the proper release level is critical to most applications. The following chart shows how much more efficient a non-functional MQ resin can be than a functional one. It also gives some idea of release response to resin content. The chart after this one shows the difference in aging behavior.
• 202. As can be seen in Chart 2, the non-functional resin is about twice as efficient as the functional resin in raising release force values. However, Chart 3 shows the non-functional release nearly doubling with time while the functional one barely changes at all. Both coatings represented in Chart 3 had 40% dry MQ resin in their formulas. All of these were done with a high crosslink density polymer, which does not react very efficiently to resins. Softer polymers will have a more exaggerated response to such resins.
• 203. Process-Based Release Modification Coating weight, silicone cure, and coverage quality can all affect release force if they are too low. While any high level of release can be achieved when these are not ideal, this is not used in practice to control release. The problem is that process controls are never precise enough to consistently give the needed release. Even with coat weight, which would be the easiest to control, this is not possible in a practical sense. 0.50 g/m already gives very low release. Additionally, if 0.55 g/m of coat weight may shred the backing because it is too firm, while 0.60 g/m gives ideal release today, very small changes in the next roll of substrate or other variables can cause a huge shift one way or the other. In addition, these techniques are neither widely used nor recommended.
• 204. The coating technique used to apply adhesive can have a major impact on release values. Adhesive is often applied to a carrier, but it can be applied directly to the silicone-coated liner. This can produce a large differential in release force, even if the same silicone has been applied to both sides of the liner. Adhesive is always applied in some fluid form, whether this be hot melt at elevated temperatures or an adhesive in solvent or emulsion form. The lower viscosity and energy input cause a much tighter bond between the adhesive and silicone than if a finished and cooled adhesive were simply laminated to the liner. In many cases, this is sufficient to create the needed release differential.
• 205. Pigmenting of Silicone There are numerous colors of silicone pigments available on the market. These are most often used in rubber articles, such as silicone bakeware, gloves, gaskets, caulks, and wire casings. The same pigments can be used in silicone release coatings. Because release coatings are very thin, deep colors are not possible in most cases, but strong pastel colors can be achieved.
• 206. Emulsion Pigments When silicone emulsions are in use, pigmenting is easier. Since many pigments and dyes are available as aqueous dispersions and solutions, blending is quite easy. Coarser ground pigments may increase release force, and interference with curing remains a concern. There is also a risk of poor appearance or bleeding if the colorant is not compatible with silicone. Requirements of Silicone Pigments The pigments have to be ground very fine. Since a typical silicone coating on tape is only about a micron thick, pigments have to be ground into the nanometer range. After this, the pigments have to be completely dispersed into a silicone oil. Pigments are often not compatible with silicone and require a silane to make them compatible. Not all types of pigments are compatible either, as there are some that interfere with the hydrolyzation curing mechanism. These pigments are most often sold as a high viscosity paste that must then be mixed with the silicone oil that makes up the bulk of the coating. Settling is possible once mixed, but this takes place very slowly over several days, if it happens at all. Dyes are often polar organic compounds and therefore not remotely compatible with silicone.
• 207. Low Coefficient of Friction Silicone Coatings Low COF silicone coatings have traditionally been made from the tin (Sn) curable systems in solvent, as these naturally have a low COF. Users have slowly gotten away from tin systems in favor of platinum (Pt) curing silicones. Platinum systems’ biggest advantages are that it can be solvent free and can run at much higher speeds, both of which more than offset the cost of the Pt. Another reason is that tin is poisonous and many customers want tin free products. In any case, the gradual reduction of tin-based coatings has made it harder to find low COF silicone coated structures. There are tin-free options available, though none seem to work quite as well as the original. Slip agents and hardeners can be added to rubbery-feeling Pt based coatings to give some improvement. There are also solvent-based options available with Pt catalysts that come close to tin’s COF performance. Friction depends on the material that the silicone is rubbing up against as well. A study was done looking at glass, steel, and more silicone. The details can be found in the following chart.
• 209. As the chart shows, the typical Pt cured silicone coating at the bottom has roughly double the COF of the tin coating at the top. The solvent-based Pt coating comes pretty close to the tin system. The hardener was only effective on glass and the slip agent had a modest effect on all three materials but worked best on a different silicone coated sheet. Low Contamination Silicone Systems All fully cured silicone systems will have some residual uncured silicone oil. While it may be as little as 1% of the total silicone mass, this uncured silicone is free to migrate and contaminate anything that comes into contact with the coated silicone layer. For some applications, significant quantities of uncured silicone is not problematic. For most tape applications, small, normal amounts of silicone do not present a problem for adhesive performance. However, some applications are very sensitive to silicone contamination.
• 210. Non-functional Content Many silicone coatings are made with simple linear silicone polymers. These polymers have two crosslinking points on either end of the chain. As long as either reactive site crosslinks in, the silicone will become covalently bonded to the coating matrix. However, if no part of the chain crosslinks in, that silicone is free to move wherever repulsive or attractive forces push or pull it. Most of this un crosslinked Material is in the form of a cyclic structure which has no functional groups. A small percentage of these structures form whenever any silicone polymer is made. The vast majority of them are small and therefore volatile, so they are stripped from the polymer during production. Inevitably, a small amount remains with the polymer. These ring structures can be of any size, so the larger one are unable to be stripped off. They remain in the polymer after coating and can contaminate adhesives and substrates.
• 211. Extraction Testing The amount of extractable silicone can be measured by any number of methods. These generally use solvents such as toluene, heptane, MIBK, or xylene, to extract free silicone. A coated sample is measured for coat weight using XRF and then placed in the solvent with enough time to allow for extraction. Then, the sample is either re-measured for coat weight to determine how much silicone was taken off, or the solvent is measured to determine its silicon concentration via AAS or ICP. This test will determine the amount of extracted silicone. The extracted silicone will include both silicone that had no reactive groups like cyclic, and silicone that was reactive but was not exposed to enough energy for it to react with an available crosslinking site. The result is generally expressed as a percentage of the total silicone coating mass. Most systems are considered adequately cured when less than 5% is extracted, but lower numbers can be used for sensitive applications. Some applications with heavy adhesives tolerate 10-15% without issue.

Tapes adhering prematurely waste time and cause frustration. 3M release liners deliver a flexible solution designed to work with different adhesive products in different applications.

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Our high-quality release liners are available in a wide range of paper, poly-coated paper, polyester film and HDPE film substrates and feature silicone release coatings on one or both sides of the liner. Polyester film release liners are also available with proprietary non-silicone release coatings.