The weather resistance of the paint base depends in many cases on the stability of the film former to high-low temperatures. The aging phenomenon of the coating film is manifested in three forms: physical, chemical and physicochemical, which cause the coating film to deform and shrink due to stress, resulting in coating film damage. The coating film is exposed to the sun in the atmosphere, and ultraviolet radiation is the main reason for the aging of the coating film. The actual aging and damage results of the coating film are manifested in blistering, delamination, wrinkling, cracking, chalking, discoloration, loss of light, peeling, etc. These phenomena are attributed to deformation induced by brittleness or non-brittleness. The aging process of the coating film is listed in the following figure:
The coating film is affected by sunlight, wind, rain, high-low temperature changes, etc., thermal expansion and contraction appear alternately, coupled with external wind and sand abrasion, etc., to accelerate its aging and damage process. Therefore, the following factors should be considered for paint bases with good weather resistance for external use .
1. The condensation temperature of the film-forming substance in the paint base should be lower than the actual use temperature of the coating film. If the condensation temperature of the coating film is higher than the actual use reduction in volume shrinkage tension will be slower, so that gradually disappear. It has adverse effects on the aging of the paint film. Condensation temperature is also known as glass transition temperature or brittleness temperature, but it is not equal to the brittleness concept commonly used in the coatings industry.
2. The influence of the film-forming substance in the paint base on the ultraviolet absorption capacity is mainly related to the chemical structure of the film-forming substance. This is because different chemical bonds require different energy when they are broken, as shown in the table below. According to JS1ong, increasing the content of fatty acid or oil in the alkyd resin will increase the absorption capacity of the coating film against ultraviolet rays. Therefore, it reflects Its weatherability is also improved .
3. The evaporation and movement of the film-forming substances in the paint base in the atmosphere should be less, especially for some paint bases containing plasticizers, this point is more important, otherwise the destruction of the plasticizing effect will lead to the aging of the coating film. This is one of the reasons for the embrittlement of the coating film
4. If the film-forming substance in the paint base contains highly hydrophilic components or has water-soluble particles, the water in the coating film or the water diffused through the coating film will have an osmotic effect at the corresponding position. The resulting pressure will deform the film, cause the film to blister or peel off the substrate.
5. The paint base should have good wetting properties for the pigments. During long-term outdoor exposure, the coating film should maintain a good gloss to prevent the pulverization of the pigments.
6. The film-forming substance in the paint base should have good color retention performance.
With regard to the selection of paint bases for outdoor paints, attention should be paid to the coordination of various indicators and the proper matching with pigments in order to achieve good results. Therefore, when designing the paint formula, it is necessary to consider the environmental conditions of its application. Generally, artificial accelerated aging tests can be used for preliminary selection in the laboratory, but the formal determination of the formula must be based on the results of outdoor natural exposure.
