Evaluation of the Resistance to Cracking of Paint and Varnish Coatings

The Chapter contains information about the method of assessing the crack resistance of coatings. The influence of the porosity of the substrate on the resistance of the coatings to cracking has been established. It is proposed to evaluate the resistance of coatings to cracking by the method based on the ratio between the crack length upon indentation of the Vickers indenter and the fracture toughness. It is shown, that in the process of aging protective decorative coatings of external walls of buildings there is a change in the mechanism of their destruction from elastic-plastic to brittle, i.e."embrittlement" of coatings is observed. It was established that in coatings "embrittlement" happens after particular time of humidification. For coatings MA-115 the coating peeling after 2 months of humidification is characteristic, for coatings PF-266 -cracking. With increase of surface porosity of a substrate to a particular limit the decrease of coefficient of intensity of tension is observed.

With aging in PVAC and the polymer-limy coatings on a solution substrate the "embrittlement" occurs after a particular duration of impact of alternate freezing and thawing. Cracks in coatings at cave-in of an indentor of Vikkers appear only after 15-20 testing cycles. At studying a thermoaging it was recorded that the increase of time of thermoaging leads to natural increase of value of coefficient of intensity of tensions. Calculation of stresses in coatings