1. Refractive index
If the visible light refractive index of PVC resin is the same or similar to 1.52~1.55, the transparency of PVC products will be better, otherwise the transparency will be lower.
2. Linear length of molecule (molecular group)
The linear length of the molecule (or molecular group) is less than the visible light wavelength of 400~735nm, the refractive index is less, the transparency is higher, and vice versa, the transparency is lower.
3. Solubility is compatibility
The so-called compatibility refers to the mutual affinity when two or more substances are mixed. If the compatibility is good, it is possible to achieve molecular-level dispersion. The heat stabilizer has good compatibility with PVC resin in the molten state, and cannot form two phases, that is, there is no interface or the interface is not obvious, and the refracted light is less. The transparency of PVC products Higher. The liquid stabilizer has better compatibility in PVC than the corresponding solid metal soap, and the molecular linear length is also smaller, so the transparency of PVC is relatively higher. The tribasic lead sulfate and dibasic lead phosphite, which have extremely poor compatibility, have relatively large molecular clusters, so PVC products are opaque. Lead stearate is translucent because of its compatibility.
Brand differentiation:
There are many grades of PVC. The viscosity or K value commonly used in the industry indicates the average molecular weight (or average degree of polymerization) of PVC, that is, the length of the PVC molecular chain, which determines the grade of the resin and the corresponding processing parameters. The molecular weight of the resin is related to the physical and mechanical properties of the product.
PVC resin is an amorphous linear polymer compound. The larger the relative molecular mass, the higher the viscosity, the higher the molecular weight, the higher the tensile strength, impact strength, and the modulus of elasticity of the product. Decline in sex and plasticity. The viscosity number is usually used to express the molecular weight of the polymer and divide the polymer model.
The most common one in our country is SG-5, followed by SG-3, SG-7, SG-8 and so on. Compared with type 5, the production of type 3 PVC requires long reaction time, high polymerization temperature, high requirements for the addition of auxiliary materials, and production costs are 50-100 yuan/ton higher than type 5. Type 3 PVC should have high purity, less fish eyes, easy to blister, high temperature resistance, insulation, soft product texture, etc. It is mainly used to produce soft products such as wires and cables, agricultural films, conveyor belts, and daily plastic products.
The production process of SG-7/SG-8 and SG-5 resin is basically the same, and the production equipment is exactly the same. The main difference lies in the formulation of additives, polymerization temperature, reaction pressure and so on. The reaction temperature and reaction pressure of SG-8 resin have basically reached the working pressure limit of the polymerizer. The reaction requirements are relatively strict, and the requirements on operation, safety, technology, and personnel quality are more stringent, and the resin quality is more difficult to control.
In terms of product quality, the main difference between SG-8 and SG-7 resin lies in the viscosity, while other indicators have little difference. The risk factor in production is greater than that of SG-7 resin. The production cost of SG-8 resin is basically the same as SG-7 resin, which is slightly higher than SG-7, and should not exceed 15 yuan/ton per ton. Most companies said that the cost of SG-8 is slightly higher than that of SG-5, but the cost spreads of different companies are slightly different. Companies that need to switch production with other models said that the cost of SG-8 is slightly higher than that of SG-5 at 100-150 yuan/ton, but companies with dedicated production lines say that the cost of SG-8 and SG-5 is not much different.
In principle, the higher the model, the greater the hardness of the product.
SG5 (K-65, S1000): pipes, profiles, plates, etc.;
SG3 (K-70, S1300): mostly soft products, hoses, wires and cables, films, shoe materials, toys, auto parts;
SG7, SG8 (K-60, S800, S700): pipe fittings, sheets, containers, etc.