Pressure sensitive adhesives are frequently utilized in removable protective films for glass and polymer surfaces, especially the displays in portable computers, cell phones, electronics, and their components. Protective films may be applied to and removed from these substrates during manufacturing, assembly, shipping, and final use.
In applications where a protective film is applied temporarily, it is critical that the film can be removed with low peeling force. If the protective film adheres too strongly to the substrate, removal can be difficult or cause damage to delicate surface features, such as printed designs or electronic traces. However, the film must also adhere to the substrate with sufficient strength to prevent accidental removal during normal handling.
Additionally, a removable protective film should not leave any residue upon removal. Transfer of the adhesive to the substrate may degrade the appearance of the surface or interfere with subsequent operations on the surface such as chemical treatment, printing, or bonding. Residues may be visible or invisible to the naked eye, in which case they can be detected by a variety of methods.
Further, a protective film should minimize the entrapment of air bubbles during application. Bubbles trapped between a protective film and substrate produce an unsightly and non-uniform construction that tends to increase the transfer of adhesive residue to the substrate. In order to reduce bubble entrapment, a protective film placed in contact with a substrate should form a PSA-substrate interface that advances across the surface or “wets” rapidly and spontaneously. It is advantageous if any entrapped bubbles can be easily removed from between the film and substrate by gentle application of pressure.
Currently, protective films are made using any one of acrylate, urethane, and silicone pressure sensitive adhesives. Silicone PSAs are generally the most expensive of the three technologies, but have superior surface wetting and high-temperature performance.
Silicone PSAs are provided as liquid mixtures of polydiorganosiloxanes that are blended, coated onto film face-stock, and cured during the production of a protective film construction. Silicone PSAs are normally cured by crosslinking via either organic peroxide initiated radical reaction, “peroxide cure” or metal catalyzed hydrosilylation, “addition cure.” PSAs used in protective films are usually of the addition cure type.
While several silicone PSAs are available on the market, these materials leave detectable silicone residue on glass substrates, require excessive peeling force, or wet too slowly. Each of these technical difficulties causes their own problems in production and end-user quality. For example, silicone residue can be seen in the loss of paintability of glass substrates after hot aging in contact with the available PSAs.