Transparent PVC products, if they are impregnated by water, or outdoors, or by bending and stretching, will produce albinism and lose transparency.
1 water whitening
Many types of transparent PVC products exhibit a white, foggy and turbid appearance when they are in contact with water or water vapor for a long time. Soft products are more powerful than hard products.
In most cases, this phenomenon can be reversed, and the cloudy product is restored to its original transparency when it is thoroughly dried. But in some transparent hard products, this phenomenon is irreversible. It is believed that this phenomenon is caused by the presence of additives or auxiliaries that are easy to hydrate in the formula. Because the water soaked, the plasticizer and stabilizer from PVC precipitation and hydration, the formation of hydrate precipitates on the surface (affecting transparency), even if it is not saturated water, plasticizer, stabilizer can not return it, only increase the temperature, can increase the compatibility of plasticizer and stabilizer has been restored to become transparent.
It has been proved that this phenomenon can be caused by stabilizers, plasticizers, lubricants, impact modifiers, and processing aids. The formulation of almost the alkaline earth metal salts in the stabilizers, especially barium and calcium, is prone to this problem in varying degrees. This phenomenon occurs occasionally in materials containing only cadmium or zinc salts. Organic tin generally does not appear to be white in the water.
2 exposure whitening
PVC products are exposed to outdoor exposure because of the effect of water, carbon dioxide, sulfur dioxide and light in the air.
This is related to the compatibility of stabilizers. In metal soap, benzoate with good compatibility with PVC is less than stearate. Organotin is not easy to produce albino, and sulfur containing organotin is the best, followed by lauric acid salts and maleic acid salts. The addition of light stabilizer, phosphite and liquid compound stabilizer can prevent or alleviate the whitening phenomenon of PVC due to exposure to the sun to some extent.
An interesting phenomenon is that hard PVC products will become white after exposure to Florida or other humid areas, while it will remain unchanged when exposed to Anathan state. So, humidity is a condition for the PVC exposure.
3 stress whitening
Stress whitening refers to PVC transparent hard mechanical products in the role of external forces, such as bending, stretching, PVC products such as the local bending crease and tensile parts bleaching. This may be due to the external force acting on the molecular structure, resulting in the orientation of polymer chains, and the change of PVC density. At the same time, there are gaps between some molecules, which form light scattering and make PVC products appear white.
Broutmann believes that adding a stabilizer with high surface energy to make up the gap method produced by the molecular chain orientation is effective. The use of organotin stabilizers can reduce the whitening phenomenon, especially with the use of organotin maleate.
4 other whitening
PVC transparent products in the process, if the use of excess lubricant, lubricant precipitates can also lead to more transparent products have a whitish phenomenon, this phenomenon is sometimes called albino.
This whitening phenomenon, the surface of the product generally has a more obvious wax like substance. The solution is to reduce the amount of lubricant, or to use a better compatibility of the lubricant variety, so that the inside and outside of the formula can reach a balance.