Release coating

Release coatings are important components of pressure sensitive tapes as they enable the formulator to adjust the release force according to the application, for example, the unwinding force of tapes and the transfer of double-sided tapes to substrates. 

The surface energy of a release coating is much lower than that of the adhesive. Thus the degree of wetting and adhesion are reduced. Further factors are the viscoelastic properties of the adhesive, the surface texture, and the number of pores in the release coat. External factors such as contact pressure, contact time and temperature should have as low an influence as possible. 

There are three main types of release coatings  

The key process for silicones is cross-linking, since residues of low molecular mass silicone oil on the surface of the PSA will eliminate tack. 

The first reactions used were tin-catalyzed condensation reactions, which could be applied from solvents only. Since ca. 1980, noble metal catalyzed addition reactions have been introduced to coating technology. The combination with retarders gives systems which cross-link efiiciently at high temperature (e.g., 10 s at 150 °C) but are 

The introduction of a dimethyl-siloxane group between two cycloaliphatic epoxy groups was found to increase drastically, the reactivity of these monomers [58,131]. The cyclo epoxy group can be located at the ends of the polydimethyl-siloxane chain and/or on pendent groups [132], 

UV-cured acrylate functionahsed silicones have been successfully used as release coatings [125,129,134]. They consist of either acrylate end-capped linear polydimethylsiloxanes or of sdicone chains containing pendent acrylate groups. 

UV-curable adhesives have established themselves as a significant part of the overall market for specialty adhesives because they offer a number of advantages over traditional adhesives, as illustrated in Fig. 13. One can expect this part to increase in the near future, as the ever more stringent regulations aiming to lower air pollution and energy consumption will progressively curtail the use of solvent-based adhesives. 

The author would like to thank his coworkers from the Polymer Photochemistry Laboratory, Drs. C. Bianchi, D. Decker, L. Keller, I. Lorinczova, F. Masson, K. Studer, E. Weber-Koehl and K. Zahouily. He also acknowledges the financial support from BASF, Ciba SC, Dupont Performances Coatings and CNRS. 

They consist of either acrylate end-capped linear 

The principal driving force behind the steadily increasing consumption of UV adhesives will be the speed and control of the curing process and the compatibility of radiation products with automated 

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Papermill wastes Paper, photographic Paper pouches Paper preservative Paper processing Paper pulp Paper pulps Paper pulp slurries Paper, recycled Paper recycling mills Paper release coatings Papers Paper sizes Paper sizing Paper wastes... 

Packaging (paper and plastic) packaging adhesives release coatings barrier coatings Photochemical machining (89) micromechanical parts optical waveguides... 

For hquid systems these surface energies expressed in mj/m are numerically equivalent to the surface tensions in mN/m(= dyn/cm). If the adhesive is phase 1 and the release coating is phase 2, then the spreading coefficient, S, of 1 on 2 is as given in equation 2. 

Adhesive Transfer Processes. Many polymers, whether dehberately or accidentally, are adhesives, so that much of the adhesive industry can be regarded as a part of the mbber and plastics industry. However, there are several important material-transfer appHcations involving polymer products that are so critically dependent on controlled adhesion that they merit specific mention in that category. They include hot stamping foils, release coatings for pressure-sensitive adhesive products, photocopier materials, transfer coatings, and transfer printing of textiles. 

Most general-purpose release agents have been developed for this market in part because of their low toxicity and chemical inertness and do not usually present health and safety problems. Some of the solvent dispersions require appropriate care in handling volatile solvents, and many supphers are offering water-based alternatives. Some of the sohds, particularly finely divided hydrophobic sohds, can also present inhalation problems. Some of the metallic soaps are toxic, although there is a trend away from the heavier, more toxic metals such as lead. The reactive type of release coating with monomers, prepolymers, and catalysts often presents specific handling difficulties. The potential user with health and safety questions is advised to consult the manufacturer directly. 

Typical substrates for siUcone release coatings are supercalendered kraft paper, glassines, and thermally sensitive films such as polyethylene and polypropylene. Ideal curing conditions are 150°C or lower, and line speeds are as fast as 460 m /min. Key properties for release coatings are cure speed, integrity of cure, and stable release values. 

R. P. Eckberg, "Chemistry and Technology of Radiation Curable Sihcone Release Coatings," in D. Satas, ed.,Mdpances in Pressure Sensitive A.dhesives, Satas Associates, Warwick, R.I., 1992. 

Dibutyltin and dioctyltin diacetate, dilaurate, and di-(2-ethylhexanoate) are used as catalysts for the curing of room-temperature-vulcanized (RTV) sihcone elastomers to produce flexible siUcone mbbers used as sealing compounds, insulators, and in a wide variety of other appHcations. Diorganotin carboxylates also catalyze the curing of thermosetting siHcone resins, which are widely used in paper-release coatings. 

PSA coated tapes are used for a variety of applications in the electrical industry, but they have to meet additional requirements to be accepted in this industiy The adhesive are typically expected to be non-corrosive, they need to have good dielectric properties, they need to hold out moisture, and they need to show good flame retardaney. Espeeially for eleetronic applieations, little or no outgassing is aeceptable, and the adhesive should not introduce migratory contaminants resulting from exposure to release coatings or the presence of additives. 

To avoid the need for a liner, the industry has also developed linerless labels that have the release coating on the face of the labels allowing them to be stacked without liner and to be peeled one at a time. The release coatings for these types of application are often quite different from those used on a conventional liner. Since the label is typically printed or written upon, ink-receptivity of the release coating becomes critical. As the industry identifies more and more applications requiring this type of performance, specialized release coatings have also been developed. 

Release coatings are important components of pressure sensitive adhesive (PSA) products such as tapes and labels [1]. Release materials are coated onto the backside of PSA tape backings (often called low adhesion backsizes or LABs in this form) to provide the desired tape roll unwind force. They are also coated onto various substrates to form release liners for PSA products such as labels and transfer tapes. Typically the thickness of the release coating is less than 1 p,m, and often times less than 0.1 jLm. Release coatings can be thought of as the PSA delivery system, providing a controlled unwind or release force and protecting the adhesive from contamination and unintentional contact until it is applied. 

Some additional requirements of the release coating may include the ability to write, print, or paint onto it, to provide quiet or smooth unwind, or to exhibit low coefficient of friction. There is also a trend towards the use of solventless release materials in the form of waterbased or 100% solids coatings. 

The types of polymers that are used as release coatings include silicone networks, silicone containing copolymers, polymers with long alkyl or fluoroalkyl side chains, fluoropolymers, and polyolefins. These polymers have surface energies that are less than the surface energies of commonly used PSAs, an important feature of release materials. 

The work of adhesion between a PSA and a release coating can be expressed in terms of the dispersive and polar components of the surface energies of the PSA and release coating [8]. 

Typically, the polar surface energy component of polymers used for release coatings is relatively small and the work of adhesion can be written simply as... 

Based on the arguments presented thus far, it would seem that, for a given PSA, the work of adhesion, and thus the peel force, should decrease systematically as the surface energy of the release coating is decreased. Therefore, fluorochemical containing polymers should provide the lowest release forces. In practice, these generalities often do not hold, due to other factors, such as interfacial dynamics and rheological considerations. 

Surface energies of the chemical constituents commonly found in PSA release coatings... 

Another factor that can contribute to the low release force provided by a release material is the presence of a mechanically weak boundary layer at the surface of the release coating [40,41]. Upon peeling the PSA from the release coating, the locus of failure is within this mechanically weak layer, resulting in transfer of material to the adhesive and a subsequent loss in adhesion of the PSA. Although the use of a weak boundary layer may not be the preferred method of achieving low adhesion for PSA release coatings, it can be useful if the amount of transfer is consistent and kept to a minimum [42]. However, in many cases the unintentional or uncontrolled transfer of a weak boundary layer to a PSA results in an undesirable loss in readhesion. 

Silicone acrylate technology, while known since the 1970s [68], has been applied to release coatings more recently [69]. Both homopolymerization of multifunctional silicone acrylates and copolymerization with organic acrylates is practiced [22,70]. Examples of blended systems will be deferred to the next section, understanding that an increase in the non-silicone component acts to increase the release level, analogous to the epoxy system described above. 

Silicone release coatings are the workhorses of the easy release industry because the very nature of the molecule fulfills most requirements for low adhesion. When well cured, silicone networks are fairly inert and present a very low sur-... 

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