In the metallic coating for automobile, there are such two typical defective phenomena of the top-coat clear paint film as cracking and flaking. Careful observation have been carried out with the aide of a scanning microscope on cracking and flaking phenomena of the paint film caused by various weathering condition, and the process of such defects occurred were classified into the following four. 1) Because of the degradation of polymer and deterioration of flexibility of the topcoat clear paint film, checking appeared, at first, then it became cracking which spread to all directions to become alligatoring surface. Owing to friction and water permeation, etc, the adhesion between layers degraded down to flaking. 2) Having similar process described in above 1), there came out checking, and water penetrated into both such checking area and its surrounding paint film until lifting up the paint film like blister, then grew to flaking. 3) Because of ultra-violet rays, the surface of the top coat clear film was affected with erosion, on which the shrinking force was acted and the top coat clear film was deteriorated to cracking. 4) Water penetrated into the topcoat clear paint film and caused the blister, and at the bottom part of the blister there broke out cracking which caused round shape flaking. In the next, as a result of the study of accelerated weathering test methods for cracking phenomena, it was found that the undermentioned conditions were the most suitable method ; e. i., Nichiyu Accelerated Weathering condition (NAW method), for cracking are : B. P. T >90°C Spray water temp. : 7°C Spray water cycle. : 6 min. spray 24 mins. no spray Humidity :>93% Under these conditions, it was found that the time of cracking started was 35% of the time required in the test conditions based on Japanese Industry Standard, and also the shape and state of the cracking broke out under the NAW conditions was in good relations with the one occurred in the natural exposure.
The thermal cracking phenomena was studied by the /theological method, when the paint film was applied on the iron plate and cooled rapidly. As a result, it was confirmed that the important factors on these phenomena were as follows ; (i) the thermal contraction stress caused by thermal shrinkage between the glass transition temperature and the temperature studied and (ii) the tensile strength caused by elongation at the speed corresponding to the cooling rate. On the other hand, above observation suggested a possibility that by the measurement of these values, a future occurrence of the thermal cracking phenomena on the paint film, when cooled rapidly, could be estimated.
In recent years, epoxy-tar coatings have become widely used as thick type coatings. However, there are few reports giving adequate results of their long-term durability ; rather, in many cases, they are selected merely on the basis of experience. Using the period of penstock replacement at hydraulic power station, the applicability of eight paints including four types of epoxy-tar paints for the inside surface coatings of the penstocks was determined to make sure the practicability. Evaluation of the durability of the paint films was based on visual appearance and on measurements of the impedances of the films. From the results of the fifteen years test, providing the good surface preparation before painting and the suitable paint thickness depend on the surface roughness, it was clarified that the epoxy-tar paints have excellent durability, and corrosion resistance, with one exception.