Influence of Pigment on Paint Film Weathering H.IRL1h- A. DEPET .imerican Zinc Sales Cornpan?,
Columbus, Ohio
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The ph? sical characteristics of pigment zinc oxide, including particle size and shape. are of first importance in the choice of a zinc oxide for good-weathering paint. Reactivit? of zinc oxide with the fatty acids in linseed oil is of secondary importance as is also opacity to the short wave lengths of light. The preferred form of zinc oxide particles for developing good-weathering paint is large, coarse needles. The superiority of this kind of zinc oxide in improving the weathering of paint films is explained in part by the development of a reenforcing brush heap structure as shown by microscopic investigation. The superiority is also explained as being due in part to separation of the vehicle from large irregular shaped pigment particles and from acicular particles in particular. On this basis, uring acicular pigments. a paint film is formed t h a t debelops innumerable niicroscopic failures on weathering. The tin) failures relieve stresses and are prevented from becoming visible to a large extent through the brush-heap reenforcement. Rubber ma! be considered as a specialized form of paint vehicle, and inbestigations of pigments in it are most helpful in understanding the behapior of pigments in paint behicles in general.
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pocket's (Figure 1). He ~howed that this s e p a r a t i o n tjecanie more pronounced when largesize pigment particles were investigated and in high pigment loadings. He al.so slion-ed that, of all the pigments investigated,% his sample of zinc oxide showed the l e a s t s e p a r a t i o n . As a m a t t e r of fact, there was 110 separation in the case of zinc oxide until the amount added was more than 100 per cerit by weight calculated on the rubber. This rrork n-as extended hy several other investigators. Depew and Easley ( 5 ) showed that the shape of the particle \)-as of almost as great importance as p a r t i c l e s i z e . They also showed that the nature of thc surface'wa? a factor of secoiitlary importance. Their n-ork, which agreed n-ith that of Bartell (1. 2 ) and his eo-workers, 4o\ved that the atlheRion of organic niaterials to zinc oxide \vas relatively weak antl that the veliicle .
