There are two types of brittle lacquer coating, air-drying and baking. The air-drying type is generally less fit than the baking kind to bring out brittleness, while even the latter leaves the coating to lose its brittleness rapidly after heat treatment. Neither of these types is worth recommendation therefore to analyse stress distribution. In order to cope with the problem, another, and better, method has been suggested of applying the“solvent-cracking”which is often observed in high polymers where crack is occasioned at lower stress than in air. The experimental stress analysis made with this method has won higher credit than what is given to the ordinary method exercised in air. This method is rendered futile, however, when the machines or their parts are studied respecting stress analysis under varying dynamic load such as rotating, running and flying. For materials for stress analysis in general, therefore, lacquer coating of high sensitivity to strain even without heat treatment is required. (Strain sensitivity is in proportion to the brittleness of the coating.)
The principle for improving the brittleness of the coating will be educed through the micro structure of the coating as previously reported. As the result of the study, the author has found an efficient method for lacquer coating using abietic acid and calcium abietate which have displayed high sensitivity at room curing temperature. The cracking properties and the composition of the new lacquer coating are described in the present paper.
