Binders barrier effect

In two-layer coating sterns the best choice is to endow both layers with the barrier effect and choose a binder having good wet adhesion. Other ooimbinations are less effective or even not reasonable (Figure 3). 

Ethylene Copolymers. Ethylene-vinyl acetate copolymers differ according to their vinyl acetate content and molecular mass. With increasing vinyl acetate content, compatibility with paraffin waxes decreases, but that with other binders increases. Low molecular mass types containing 25-40% vinyl acetate are readily or sufficiently soluble in solvents. With a 40% vinyl acetate content, they can be combined with polar resins and nitrocellulose. Terpolymers with free carboxyl groups exhibit improved adhesion. Ethylene-vinyl acetate copolymers are primarily added to waxes to improve their properties, but are also used to increase flexibility and adhesion in paints, printing inks and adhesives, and for hot melt coatings. Ethylene-vinyl acetate copolymers have low water vapor and gas permeabilities (barrier effect). 

Binder type and additives are not the same. In accelerated testing, the type of binder is usually the most important factor because of its barrier properties. Only after the binder barrier is breached does effect of pigment become apparent. 

In particular, micaceous iron oxide (MIO) contributes to the outstanding protective properties of barrier pigments through a parasol effect the organic binder beneath the flakes is shielded from harmful ultraviolet (UV) radiation and therefore the flakes remain on the surface although the binder above or around is degraded with time. Figure 1.7 illustrates the UV protection by MIO. 

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