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Briefer — Physical Properties of Film
195
Brie% stated, the treatment mentioned consists of applying to one side of the celhilose skin an alcoholic solution of gelatin, called substratum or simply ^^sub" to which the subsequent emulsion coating will adhere. A number of other ^'subbing" methods not involving the use of gelatin have been tried among which is the practice of etching with ether and other solvents.
Whatever the practice may be, it is simple to err on the safe side and create a condition when the gelatin adheres too firmty to the film, in which case, felting between the base and gelatin occurs. It will be shown that this condition favors so called brittle film.
Photomicrograph A. (coated film)
Effect of Adhesion Edge (a) premature fracture from too much adhesion Edge (b) Correct adhesion; shows full resistance to breaking load. Film base break independent of gelatin.
Black spaces are emulsion coating.
Gelatin, when dry, is quite brittle as compared with the film support. If the gelatin coating and film base are felted by reason of too great adhesion, then any fracture of the gelatin carries also through the base. Both gelatin and base crack and the raw stock or finished print breaks in two. On the other hand, when the correct condition of adhesion is obtained, the gelatin, because of its ability to creep along the surface of the film support cracks first, the film base holding its original flexibilit^^ Photomicrograph (A) shows the effect of the break in the tensile strength tester on brittle and nonbrittle film. Edge (b) shows where the gelatin has broken away from the film base. Edge (a) shows the strong adhesion of the gelatin. In other words, the gelatin and base are firml}^ cemented. This is practically the whole story of brittle and non-brittle film, except that