Release liner/paper

Pressure sensitive adhesives can be coated on a substrate such as a polyester or paper backing to make a tape or label, or on a release liner to make a transfer adhesive, or a special substrate to make a graphic film or retro-reflective sheeting product. In some cases, where precision placement of the PSA is required, one may also coat directly on the substrate using techniques such as screen-printing. 

Silicone release liners are used as a nonadhering surface to which adhesive materials, such as, for example, pressure-sensitive adhesives, can be laminated. UV curable release coatings are based on poly(dimethylsiloxane) oligomers functionalized with acrylate or epoxy groups and are mostly coated onto papers or thin films. 

Tape and film are the most common forms of solid epoxy adhesives. Tape and film are terms that are used rather loosely for adhesives in a thin sheet form. The term tape generally refers to an adhesive that is supported on a web of paper or nonwoven fabric, or on an open-weave scrim of glass, cotton, or nylon. Films, on the other hand, are free of supporting material and consist only of the adhesive. Both tape and film products may be accompanied with a release liner depending on the tack or blocking characteristics of the adhesive. 

The final thickness of epoxy tape or film adhesives is on the order of 5 to 15 mils. These adhesives may be soft and tacky, or stiff and dry, depending on their formulation. The soft and tacky products are valuable in products requiring application to contoured or vertical surfaces. If the product is especially tacky, a release liner (e.g., polyethylene film, coated paper) is generally used to keep the film from blocking. The stiff and dry products are generally used for flat surfaces where speed and ease of application are required. 

The primary mode of bonding for a pressure-sensitive adhesive is not chemical or mechanical but rather a polar attraction to the substrate. This always requires pressure to achieve sufficient wet-out onto the surface thereby providing adequate adhesion. The four main varieties of pressure-sensitive adhesives are derived from rubber-based, acrylic, modified acrylic and silicone formulations. Release liners are used to carry the sticky label and enable it to be printed. The release liners are normally paper, treated with a very thin silicone coating to allow the label to be peeled away easily without tearing. Some transfer of the silicone into the adhesive is inevitable. 

Label stock is the combination of a release liner and a label. The former is the silicone-coated paper or film while the latter is the face stock with the adhesive. The silicone coating on the paper or film normally has a weight of around 1 gram per square meter (g/m ) while that of the adhesive is normally around 30 (g/m ). 

The backing material and release liner can be fabricated from a variety of materials including polyvinylchloride, polyethylene, polypropylene, ethylene-vinyl acetate and aluminium foil. The most important property of these materials is that they are impervious to both drug and formulation excipients. The most useful backing materials conform with the skin and provide a balanced resistance to transepidermal water loss, which will allow some hydration of the stratum corneum, yet maintain a healthy subpatch environment. The release liners are usually films or coated papers and must separate easily from the adhesive layer without lifting off any of the pressure-sensitive adhesive. Silicone release coatings are used with acrylate and rubber-based adhesive systems, and fluorocarbon coatings with silicone adhesives. 

Release liners protect and position the adhesive. Although the liner is discarded during installation, it needs to be right from the beginning for effective processing. Release liners should have good holdout, be heat resistant, and possess a consistent caliper for consistent die cutting. The carrier, which could be in the form of paper, film, foam or another specialty material, is the middle layer that must be compatible with the adhesive to ensure the best product performance. 

Figure 4.42 shows a cross-section of a PVC foam gasket. The gasket (30) comprises a release liner (40) comprising release paper (42) having a release (preferably silicone) coating (38) which provides a release surface. This release liner (40) has an upper surface which is a release surface. The release paper is about 78 lb., clay-coated, long-fiber, super-calendered, flat, heat-resistant paper. Plastic or polymeric film may be substituted for the paper layer (42). A layer of pressure sensitive adhesive (36) is applied to the release surface of the silicone... 

Uses Release coating for use on most common release liner substrates, super calendered and clay coated paper, nonporous substrates such as PE coated kraft, polyester, and PP films, and HDPE films suifable for food contact use, for resinous/polymeric coatings, and as components of paper/cardboard... 

Poly(vinyl alcohol) is widely used on release liners for silicone topcoat holdout. The wide diversity in performance requirements of the various release liners has resulted in recommendation for add-on ranging from 1.5-2.0 (279) to 10 g/m. Ultimately, the characteristics of the base paper itself, as well as the level of release required of the finished sheet, determine the best add-on level of the PVA. 

Class Two. The use of cross-linked PDMS coatings on paper and plastic substrates avoids this thermal variation difficulty. There is also the advantage that such materials are readily available as commercial products primarily to provide release liners for the facile delivery of PSA-coated labels, decals and tapes. Two types of well-understood cross-linking technology are utilized tin-catalyzed systems based on the condensation of silanol and alkoxysilyl functionalities, and platinum-catalyzed systems based on hydrosilylation addition of SiH to vinyl functional siloxanes. 

PSAs are applied to backings in several ways depending on the nature of the adhesive. Solutions or latices are coated on a moving web of backing or release liner, in a continuous operation including drying in an oven. If the adhesive is coated on a release paper or liner, it can be subsequently laminated or transferred to the desired backing and the liner retained or removed. Hot melts, or 100% solid adhesives,... 

Coating of tapes is often directly onto the backing. Label adhesives, in contrast, are always coated on a release liner which is then laminated (nip rolls) to the paper or other backing and used or sold in this construction. 

Silicone coatings represent about one-quarter of tape release coatings (in area) but completely dominate the release liner field. The most common liner substrate is kraft paper or, to a limited extent, polyethylene-coated paper. Silicone vehicles may be either solutions (by far the most common), or emulsions of selfcrosslinking poly(dimethylsiloxane) polymers which are coated and oven dried in a continuous operation. Solventless or 100% solids coatings have also been developed. Electron beam cured coatings, also solventless, may be accepted in the future but at present represent a very small part of the market. In-house production of release liners is conducted by many PSA tape and label manufacturers but most purchase liner stock from paper companies. 

Laminating adhesives An adhesive coated on a release liner intended to be bonded to a film or other material that can be printed (foil, paper, fabric or laminate). 

In the vast majority of tape and label applications which use a release liner, the adhesive is applied to the release liner and then nipped to the backing. This method does not optimize adhesion to the backing, but does prevent its exposure to heat (from hot adhesive or subsequent solvent/water removal). The release liner is normally quite stiff because of its paper or PET support layer and is more resistant to heat than many backings (most liners have already been exposed to the oven cure of the silicone layer). In addition, the silicone layer is mirror smooth which yields a correspondingly smooth exposed adhesive layer for the tape or label. 

Uses Surfactant for waterborne pressure-sensitive and laminating adhesives powerful wetting agent providing exc. wetting of low-vise, adhesives over difficult-fo-wet surfs, like Aims and silicone release liners food-pkg. adhesives, coatings, paper... 

Self-adhesive labels (and decals) are pieces of paper, plastic film or metal foil coated with pressure sensitive adhesives which adhere to any solid surfaces after removal from a release liner. They are typically used to convey various kinds of information -for example, product or package contents, barcodes, price stickers, insfructions, and technical data. 

Foil duct tapes, used in North America for heating, ventilation and air conditioning systems, are constructed of 50 pm aluminum foil coated with 50-100 pm of an acrylic dispersion PSA to provide high adhesion and functionality at extreme use temperatures. Once coated, the adhesive is covered with a release liner to prevent adhesion to the top side of the foil and subsequent blocking of the tape roll. Some foil tapes use strands of fiber reinforcement laminated between foil and kraft paper layers to improve tensile strength [24]. 

Other than the more restricted list of curatives, the formulation components for the one-part epoxy are the same as for the two-part ones. Film adhesives are a special segment of one-part epoxies in that these materials are coated onto release liners and are supplied in roll form to the user. Thus, a typical epoxy film adhesive would be coated onto a paper-backed release liner, dried, and then covered with another release liner, possibly polyethylene. Film adhesives are manufactured in a number of forms including one side tacky or supported by a nonwoven or woven fabric. The adhesive can be placed on the adherend, moved around until it is correctly positioned, and then pressed into place and the other adherend applied. In general, these materials are cured in an autoclave in order to have better control of applied pressure and temperature. 

---