Abstract
Brittle lacquer coating of air-drying type is superior in its applicability for experimental stress analysis to that of baking type, although on the former crazing often appears due to excessive internal tensile stress developed during the curing process at room temperature. Brittleness of the coating is markedly dominated, therefore, by the internal stress which is closely related to the basic ingredients, solvents and plasticizers used as the constituents of lacquer coating.
Although previous studies have given informations about basic ingredients and solvents suitable for lacquer coating which displays high sensitivity and is of low toxicity and non-flammable, few studies have been made on plasticizers. Since it was expected that some neutral ingredients found in crude rosin might be useful as a plasticizer of the coating, a systematic study has been undertaken on the influence of these ingredients on both the brittleness and crazing. The results obtained are as follows.
The lacquer coating employing β-sitosterol, which is most satisfactory among the ingredients tested, is not only free of crazing but also has an improved strain sensitivity of circ. 600 micro strain. The efficacy of β-sitosterol on the coating is attributable to the action that it relieves the overdeveloped internal stress effectively and increases adhesion between the coating and the top surface of the specimen. The glass transition temperature of the coating scarcely changes when a proper amount of β-sitosterol is used, so that the present lacquer coating can be applied even in summer.
