Study on Crack Formation in Brittle Lacquer Coating

Abstract

In his previous report, the author mentions success in preparing an available brittle lacquer coating which always presented strain sensitivity to circ. 800 micro strain under whatever condition of temperature and humidity without resorting to heat treatment. The difficulty in lacquer coating lies in maintaining its thickness perfectly even. It was particularly the case when the body for coating happened to be engineering structures to be analysed and of complicated construction. Since cracking on the lacquer coating develops in consequence of the strain transmitted from the top surface of the structures to the coating, the properties of crack formation on it will be markedly influenced by the thickness of the coating. The author has investigated, therefore, into the effect of the thickness on both the strain sensitivity and the crack density which are very important factors in practical use.As the result of the study, the following facts have been proved.The thickness of the coating need not be precisely even, since there is scarcely any change either in the strain sensitivity or in the crack density so long as it is restricted within 0.08-0.12mm. Moreover, the behavior of the strain sensitivity under curing process which has been hitherto held as inconceivable is now found as has been caused by fine crystallization of abietic acid used as a basic ingredient of the lacquer coating.