Experimental study of the mechanism of film formation

The temperature above which a polymer latex forms a continuous film, i.e., the minimum film formation temperature (MFT), was studied as a function of particle size, surface tension, and stiffness of the polymer. The MFT was found to occur in the region where the modulus of the polymer G is decreasing very rapidly with increase in temperature. The MFT appears to be independent of particle size but very strongly dependent on polymer stiffness. The random error in MFT measurement, however, is equivalent to a change in polymer modulus of the same order of magnitude as the particle size variation investigated. Thus the ratio Gr /γ, where r is the particle radius and γ the surface tension, may be constant as predicted by Brown for a given chemical composition but the instrumentation used was not sensitive enough to prove it. The value of that ratio, however, depends markedly upon chemical composition going from an average value of 0.58 for a nonpolar copolymer (75% styrene, 25% n ‐butyl acrylate) to an average value of 260 for a more polar copolymer (50% ethyl acrylate, 50% methyl methacrylate). The quantity (MFT‐ T g ), where T g is the glass temperature of the polymer, also depends upon the chemical composition going from a value of 10°C to −3°C. when changing from the nonpolar copolymere to the more polar copolymer mentioned above.