Лако-красочные материалы — производство

The majority of standard product formulations for PVA and EVA adhesives use poly(vinyl alcohol) as the main protective colloid and thickener. Poly(vinyl alcohol) can be obtained in a number of grades produced from poly(vinyl acetate) by hydrolysis. As a consequence, almost all products are effectively vinyl acetate/vinyl alcohol copolymers. Lower levels of hydrolysis, 88% most commonly, are readily water soluble in hot or cold water, although in practice care has to be taken in making solutions to avoid clumping of the grains, which can then be difficult to disperse. Eighty-eight percent hydrolyzed material is normally coupled with a ‘‘fully hydrolyzed” grade, in which the hydrolysis has been taken into the range 97 to 99.5% of the theoretical maximum.

These grades are not easily soluble in cold water, but once made into a solution by heating, are usually stable. Because of insolubility in cold water, the higher hydrolysis-grade materials have better water resistance as dried films. Their disadvantage is to give less viscous products for poly(vinyl alcohol) of the same molecular weight. This may be associated with the particle size of the polymer formed—the presence of vinyl acetate groups gives some surfactancy, especially if the acetate groups are in blocks. Finer average particle size—and hence more particles—fill the free space more effectively, increasing viscosity. Particle-particle interactions are also important, and particle size distribution has a profound effect on rheology.

Introduction of surfactants, especially anionic surfactants or in mixtures with nonionics, gives wider distributions that often lose viscosity on shearing. This makes pumping and application easier but may bring in the problem of overspreading. For consistency of application between different machines, a more Newtonian rheology is an advantage. Many commercial adhesives are further compounded by the addition of fillers, thickening agents of various kinds, plasticizers, and solvents, although the range of the latter that are acceptable is diminishing rapidly.