If PVC products are soaked in water, exposed outdoors, bent or stretched, they will whiten and lose transparency. The following three whitening problems are analyzed from PVC stabilizer, which are water whitening, exposure whitening and stress whitening.
1. Water immersion whitening
Many types of transparent PVC products show a whitish, misty and cloudy appearance when they are in contact with water or steam for a long time. Soft products are more powerful than hard products.
It is believed that this phenomenon is caused by the existence of auxiliary agents or decomposition products of auxiliary agents which are easy to hydrate in the formula. If only the PVC stabilizer is analyzed, it is because of the penetration of water that the stabilizer precipitates from PVC and hydrates, forming hydrated precipitates on the surface (affecting transparency).
In this case, even if the saturated water is gone, the stabilizer can't be returned as it is. Only when the temperature is raised, the compatibility of the stabilizer is restored, can it become transparent.
Experiments show that the stabilizer contains almost alkaline earth metal salts, especially barium and calcium, which are prone to such problems to varying degrees. Materials containing cadmium salt or zinc salt will occasionally appear this phenomenon. Generally, organotin stabilizers do not appear this whitening phenomenon when exposed to water.
2. Exposure and whitening
Outdoor exposure of PVC products will also show whitening phenomenon.
This is related to the compatibility of PVC stabilizer. Among metal soap PVC stabilizers, benzoate with good compatibility with PVC has less whitening than stearate.
Organotin stabilizers are not easy to whiten, and organotin containing sulfur is the best, followed by laurate and maleate. Adding light stabilizer, phosphite ester, liquid compound stabilizer, etc. can prevent or alleviate the whitening phenomenon of PVC caused by exposure to sunlight to a certain extent.
3. Stress whitening
Stress whitening refers to the whitening phenomenon of PVChard products under the action of mechanical external force, such as bending and stretching. This may be due to the change of molecular structure, the orientation of polymer molecular chain, the change of PVC density, and the occurrence of gaps between some molecules to form light scattering, which makes PVC products appear white.
Some experts think it is effective to add PVC stabilizer with high surface energy to make up the gap caused by molecular chain orientation. Using organic tin stabilizer can reduce whitening phenomenon, especially using organic tin maleate has better effect.
