The base resistance of paint films and coating films can be understood as their ability to maintain their wide mechanical properties at low temperatures. Cold resistance can be represented by a fixed temperature, below which the coating film will become unsuitable for use, that is, the coating film will be damaged by itself or under slight mechanical action (stretching, impact, bending, etc.). These damages are generally manifested in the brittleness of the paint film and start with the formation of small cracks, which then gradually increase to the point of causing complete damage to the paint film.
Kalkin pointed out: "When the temperature is lowered, the western phenomenon may occur in the polymer: the stop of the mutual movement between the macromolecules and the loss of fluidity of the polymer, and the stop of the mutual movement between the molecular halls, which leads to the loss of elasticity. ".
Aleksandrov pointed out that the brittle damage of materials (such as plastics) at low temperatures starts from the smallest unevenness in the specimen (miscellaneous, etc.). In these inhomogeneities, there are obvious excessive tension phenomena. When these excessive tension phenomena gradually become the equal tension state of material strength, the cracking phenomenon that can lead to brittle failure of the specimen begins to occur in these places.
Films of most polymers also gradually lose their elasticity when the temperature decreases. Their deformability (strain) continues to decrease until they become very small. During deformation, all displacements become purely elastic. At this time, the glassy state of the elastic polymer begins. The temperature at which a polymer changes from a highly elastic state to a brittle state is called the glass transition temperature. The glass transition temperature determines the cold resistance of plastic materials and elastic materials.
Well not all polymers vitrify when the temperature is lowered. For example, polymers with a very broad spatial structure, that is, highly (degree) oriented polymers, maintain their mechanical properties even at the lowest temperature and do not become brittle (such as oriented polystyrene, oriented methacrylate, leather, wood, etc.).
Since the quality of a paint film depends on the sum of its physical and mechanical properties, it is very important to keep these properties at low temperatures.
Schedlis once pointed out that the higher the mechanical properties (toughness, stretchability, and flexibility) of the material at room temperature, the better the flexibility (flexibility) and elasticity at low temperatures will be. (Related Instruments: Tensile Tester)
The change in the mechanical properties of the coating film at low temperature is not only caused by the temperature factor, but also depends on the strictly violated test conditions necessary to obtain reliable results. The main conditions include: deformation speed, the storage time of the coating film in the deformed state, and the duration of low temperature action.
The speed at which the coating film deforms during the test is an important condition. As Aleksandrov pointed out, an increase in the deformation rate can lead to a decrease in the material's elasticity, just as it does when the temperature decreases. The temperature at which a material loses its brittleness depends on the rate at which the material deforms when it is used.
Therefore, when tested under rapid deformation, the coatings may become very brittle, but when tested at the same low temperature but with slow deformation, they retain their elasticity.
The factor of the storage time of the coating film at low temperature can not only cause the vitrification process, but also lead to the process of the gradual change of the material structure with time.
Generally, the static test method used in the cold resistance test of the coating film in the laboratory cannot obtain reliable results, because the paint coating film is always used under strained load conditions. Therefore, in order to determine the cold resistance of the coating film, a more reasonable method is to conduct a dynamic test of the coating film, and constantly measure the change of the mechanical properties (elasticity, toughness, flexibility, etc.) of the coating film at low temperature. (Related instrument: impact Tester)