Лако-красочные материалы — производство

The three major components are an elastomer, which provides the elastic phase, the tackifier, and the carrier. The earliest pressure-sensitive adhesives used natural rubber tackified with wood rosins, or later, zinc oxide. With the advent of synthetic rubbers and other polymers, formulators have a very much larger range of elastomers at their disposal, including butyl rubber, styrene-butadiene rubber (SBR), polyisoprene, and the more recent thermoplastic rubbers, which are block copolymers of styrene with butadiene or isoprene, as well as acrylic polymers. Silicone elastomers are available for specialty applications, especially for use at elevated temperatures. Vinyl ethers and polyisobutylene can be used as both elastomer and tackifier, depending on the grade. Many of these types are not compatible, and where intermediate properties are required, they are generally achieved by blending homologs from the same or related families.

Tackifying resins fall mainly into two classes: wood rosin derivatives and hydrocar­bon resins. Gum rosin is no longer widely used, as heat or aging lead to loss of tack through oxidation. Stable derivatives are produced by hydrogenation or esterification, and these are used extensively as tackifiers. Modern hydrocarbon resins are usually aliphatic, aromatic, or terpenes, although blends of these or certain specialty types may be suitable. There is no universal guide to selection, and a good deal of trial and error may be necessary to arrive at the ideal elastomer-tackifier combination and proportions. In addition, these may differ from one application to another, depending on whether the end use has tack, peel, or shear as the dominant criterion.

The fundamentals of pressure-sensitive hot-melt adhesives are similar to those of solvent — based systems. Most elastomers and tackifiers are suitable, although ethylene-vinyl acetate copolymers are also used and the conventional rubber types are not. Pressure-sensitive hot melts are dominated by thermoplastic rubbers, which are ideal for use in these applica­tions. Their unique properties arise from their essentially two-phase structure, in which thermoplastic regions of styrene end blocks lock the elastomeric midsections of butadiene or isoprene at room temperature but allow the elastomer to move freely at elevated temperatures or in solvent. This gives the polymer properties that are akin to those of vulcanized rubbers at room temperature, while allowinig it to behave as a thermoplastic when heated or dissolved. This structure is illustrated in Fig. 1.

Early pressure-sensitive hot-melt adhesives used ethylene-vinyl acetate copolymers as elastomers, but they are seldom used now. Atactic polypropylene is sometimes used on its own or in admixtures. More recently, vinyl ethers and acrylic resins have become available and will probably play an increasingly important role as the technology is developed, especially on polar surfaces.

The major differences between solvent-based and hot-melt pressure-sensitive adhe­sives is that with hot melts the viscosity can no longer be controlled with solvents, and must, instead, be controlled either by temperature or by formulating. A further limitation is that waxes cannot, in general, be used for reducing viscosity as is the case with conven­tional hot melts, as waxes tend to reduce tack drastically. Hence the major influence on viscosity in the formulation must come from the choice and quantity of tackifier resin.

polystyrene

domains

(regions of

associated

polystyrene

end-blocks)

Low-melting-point resins or even liquid resins may be used to keep application tempera­tures as low as practicable.

Because pressure-sensitive hot melts will be applied typically at temperatures between 120 and 160° C, heat resistance is a critical factor. Double bonds accelerate oxidative degradation, leading to a loss of tack, while cleavage will usually result in increased tack, but a drop in viscosity. Use of antioxidants is essential, and trials should always be undertaken to ensure that enough of the right antioxidant has been incorporated to protect the hot melt adequately at the application temperature even if a machine stoppage leads to the molten adhesive being kept in a heated bath for much longer than usual.