Лако-красочные материалы — производство

As mentioned earlier, the critical characteristic is the correct tackifier and components ratio. Although no rules exist for tackifier selection, there are certain shortcuts, based on chemical compatibility and melt point. Certain classes of tackifiers work well with specific types of elastomers. For example, aliphatic hydrocarbons generally work better with

natural rubbers, and, aromatic types are preferred for SBR. With block copolymers, aliphatic resins of low melt point improve tack and low-temperature flexibility, while high-melting aromatic resins in small quantities stiffen the product, giving improved heat and shock resistance. Rosin derivatives and terpene resins offer good performance with most elastomers, generally at higher cost. In general, resins with solubility parameters close to those of the elastomer selected are most likely to offer good performance.

Tackifiers with melt points substantially above the Tg value of the elastomer can be expected to improve the strength of the adhesive at elevated temperatures but reduce the tack, while low-melting resins will impact greater tack and low-temperature flexibility at the expense of creep resistance and shear strength. The tackifier is responsible primarily for the balance of tack, peel, and shear properties in the finished adhesive. Usually, some of these properties must be traded off to optimize one property. For any given system, increasing tack is generally related to decreasing peel and shear strengths, and similarly, any modification intended to improve shear strength is likely to be at the expense of tack. High peel and shear strengths both require high cohesive strength within the film, but peel strength is dependent on adhesion to a much greater extent than is shear.

Thus most formulations are compromises which will favor the property that is most critical in the application intended. Figure 2 illustrates typical dependence of tack, peel, and shear performance in a given system as the resin/elastomer ratio is increased. The maxima occur at different tackifier percentages. Silicon elastomers for pressure-sensitive adhesives are invariably used in conjunction with silicon gums as tackifiers. Table 3 displays the uses of various elastomers, and Table 4 contains information on tackifiers and plasticizers.

Solvents are selected primarily on the basis of solubility parameters and evaporation rate. Where mixed solvent systems are used to achieve the desired balance at the best cost, the selection should be such that the slowest solvent remains an effective solvent for the system on its own. In addition, consideration should be given to the effect of the solvent on the substrate: too strong a solvent could degrade the substrate, but the right choice can assist in keying the adhesive to the surface.

The use of plasticizers is relatively uncommon in solvent-based pressure-sensitive adhesives, especially for use on tapes. Where plasticizers are included, their compatibility with the substrate should be considered, to ensure that plasticizer migration will not lead to transfer of the adhesive. Where plasticizing of block copolymers is intended, plasticizers should be selected that are compatible with the diene midblocks rather than with the polystyrene domains.

Additives used should include stabilizers or antioxidants, especially in products containing ethylenic unsaturation. Thickeners or thixotropes may be used to modify rheology. Fillers may be used in certain applications, in which case pigment-dispersing aids may be included to reduce settlement. Silane coupling agents may be used to improve adhesion to specific substrates. UV absorbers may be added to improve exterior durability, and pigments or dyes may be added to highlight the adhesive film.

Elastomers used in water-based systems include various rubber latices, especially natural rubber and SBR, and occasionally, polychloroprene. The bulk of the market in water-based adhesives is now held by acrylic dispersions. Although these are designed for use without modification, it is normal to formulate, especially by addition of tackifiers. Commonly used tackifying resins include soft resins, or hard resins in solution, which may

often be emulsified directly into the dispersion, and aqueous resin dispersions, which may be prepared separately and added or may be purchased from resin suppliers.

When using dispersions of tackifying resins, stability tests must be performed to ensure that there are no undesirable reactions between the emulsifier systems used in the resin dispersion and the elastomer dispersion. Resin dispersions often produce lower shear than resin solutions, thus necessitating reduced levels which result in lower tack. Additives used in water-based systems will include defoamers and preservatives as well as UV absorbers if necessary. Antioxidants are normally included only if service conditions require them. Catalysts may be added to cross-linkable grades to improve performance at elevated temperature, or self-cross-linking grades may be selected. Small quantities of solvents may be included to improve adhesion or penetration. Fillers are not generally used, although in applications on vinyl or carpet floor tiles fillers may be included at levels up to 30% to reduce the glue-line shrinkage and the price. Rheology modifiers, including polyacrylates or inorganic thixotropes such as fumed silica, may be added. Table 5 shows are characteristics of some common dispersions.