Silicone release liner

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. 

A hydrophilic polyurethane prepolymer was made according to the procedure taught by Braatz. - The prepolymer was mixed with water at a ratio of 10 parts water to one part prepolymer. The emulsion was poured immediately onto a silicone release liner and allowed to cure for 30 min. It was free of voids and had a density roughly the same as water. For the contraction experiments, the gel was immersed in excess distilled and sterile water for 24 h, tlicii cut with a steel-ruled die into circles 40 mm in diameter. 

Fig. 2. Schematic picture of a coating machine for silicone release liners.

In order to avoid the sticking together of the two-sided and foamed adhesive tapes when rolling them up, siliconized release liners are required. The specifically formulated release layers enable the residue-free stripping of the adhesive tape. The same release liners serve the temporary fixation of adhesive labels. 

One method occasionally used evaluate the relationship of silicone release liners or other materials of very low surface energy to a pressure sensitive, is to bring a coated pressure sensitive into contact with the liner under pressure, to carry out an adhesion test. This is of little value, as the adhesive is unable to wet out and so come into intimate contact with the release liner. The adhesive must first be coated onto the liner, dried, or cooled in the case of a hot melt, and then a carrier laminated to it, as is standard practice for transfer coating. As low values of adhesion can be expected, the force required to bend the backing may dominate, and so a thin flexible backing should be used, to maintain a constant peel angle, 25 pm polyester being satisfactory. Then a standard 180° peel test can be carried out with the release liner secured to a test panel. 

Silicone elastomer based adhesives cover wide range of service temperatures (-70 to ca. 250 C) and provide excellent aging resistance. Owing to their low surface energy, they are the only adhesives applicable to low-energy substrates like silicone release liners and PTFE. 

Features Provides good coverage over silicone release liners in acrylic, vinyl-acrylic, and S/B systems Regulatory FDA 21CFR 175.105... 

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... 

One of the important breakthroughs in the silicone PSA industry has been the development of new release liners based on fluorosilicones [116]. These new materials significantly improve the release characteristics of the silicone PSAs, something that has been difficult to achieve because of the high adhesion silicone PSAs show to low energy substrates, including silicone rubbers and liners. 

Another important application area for PSAs in the electronic industry focuses on the manufacturing, transport and assembly of electronic components into larger devices, such as computer disk drives. Due to the sensitivity of these components, contamination with adhesive residue, its outgassing products, or residue transferred from any liners used, needs to be avoided. Cleanliness of the whole tape construction becomes very critical, because residuals like metal ions, surfactants, halogens, silicones, and the like can cause product failures of the electronic component or product. Due to their inherent tackiness, acrylic PSAs are very attractive for this type of application. Other PSAs can be used as well, but particular attention has to be given to the choice of tackifier or other additives needed in the PSA formulation. The choice of release liner also becomes very critical because of the concern about silicone transfer to the adhesive, which may eventually contaminate the electronic part. 

A word should be said about the weak boundary layer effect and silicone release [40,41]. Studies have shown that having loose silicone oil that can transfer to the PSA will lower release, however subsequent adhesion will likely suffer as well. In most commercial instances using silicone liners, a weak boundary layer is not intentionally employed. Additionally, many low transfer silicone liners are commercially available which provide premium release and show low to no PDMS transfer to PSAs, indicating that PDMS transfer is not a necessary condition for easy release. 

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. 

Many of the unique properties of silicone oils are associated with the surface effects of dimethylsiloxanes, eg, imparting water repellency to fabrics, antifoaming agents, release liners for adhesive labels, and a variety of polishes and waxes (343). Dimethylsilicone oils can spread onto many solid and liquid surfaces to form films of molecular dimensions (344,345). This phenomenon is gready affected by even small changes in the chemical structure of siloxane in the siloxane polymer. Increasing the size of the alkyl substituent from methyl to ethyl dramatically reduces the film-forming ability of the polymer (346). The phenyl-substituted silicones are spread onto water or solid surfaces more slowly than PDMS (347). 

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... 

---