How to enhance the transparency of PVC? Heat stabilizer is the key to the problem
The effect of heat stabilizers on PVC transparency is determined by several factors.
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1, the refractive index of the stabilizer
The heat stabilizer and the PVC resin have the same or similar visible light refractive index of 1.52~1.55, so the transparency of the PVC product is better, and vice versa.
2. Linear length of stabilizer molecules (molecular groups)
The thermal stabilizer molecules (or molecular groups) have a linear length less than the visible wavelength of 400-735 nm, less refracted light, higher transparency, and lower transparency.
3. The "solubility" of heat stabilizer in PVC, ie compatibility
By compatibility is meant the mutual affinity of two or more substances when mixed. Good compatibility means that it is possible to achieve molecular-level dispersion. The heat stabilizer has good compatibility with the PVC resin in a molten state. The shape does not form two phases, that is, there is no interface or the interface is not obvious, the refracted light is less, and the transparency of the PVC product is higher. The liquid stabilizer has better compatibility with the corresponding solid metal soap in PVC, and the linear length of the molecule is also smaller, so the transparency of the PVC is higher.
Liquid organotin heat stabilizers have the best transparency because R2SnCl2, which has not participated in the heat stabilization reaction itself, or R2SnCl2 which has been formed after the stabilization reaction, has good compatibility in PVC resin. Ba/Zn, Ba/Cd, Ca/Zn stearic acid soap has certain compatibility in PVC, and the light transmittance is also high. However, due to its limited compatibility, the molecular linearity is long and large. The products after heat stabilization are typical metal salts such as CaCl2, BaCl2, etc., and have poor compatibility with PVC. Therefore, when the amount is large, there is more refracted light, which affects the light transmittance and becomes turbid.
The tribasic lead sulfate and the dibasic lead phosphite with poor compatibility have relatively large molecular groups, so the PVC product is opaque.