1. Viscosity number
The viscosity number reflects the average molecular weight of the resin and is the main characteristic for determining the type of resin. The properties and uses of the resin vary depending on the viscosity. As the degree of polymerization of PVC resin increases, mechanical properties such as tensile strength, impact strength, fracture strength, and elongation at break increase, while yield strength decreases. The research results indicate that as the degree of polymerization of PVC processing aids increases, the basic properties of the resin improve, while the processing performance and rheological behavior deteriorate. It can be seen that the molecular weight distribution of PVC resin has a close relationship with plastic processing and product performance.
2. Impurity particle count (black and yellow dots)
Impurity particles are one of the important indicators for evaluating PVC resin. The main factors affecting this indicator are: firstly, the residual material on the coating wall of the polymerization kettle is not washed thoroughly and the raw material is contaminated with impurities; secondly, mechanical wear mixed with impurities and improper operation bringing in impurities; In the process of plastic processing, if there are too many impurity particles, it will have adverse effects on the performance and consumption of PVC products produced. For example, in the processing and shaping of profiles, there are many impurities and particles, which can cause spots to appear on the surface of the profile, thereby reducing the appearance effect of the product. In addition, due to the non plasticization of impurity particles or the low strength despite plasticization, the mechanical properties of the product are reduced.
3. Volatiles (including water)
This indicator reflects the weight loss of resin after being heated at a certain temperature. The low content of volatile substances can easily generate static electricity, which is not conducive to feeding operations during processing and molding; If the volatile content is too high, the resin is prone to clumping and poor fluidity, and bubbles are easily generated during molding and processing, which has a negative impact on product quality.
4. Apparent density
The apparent density is the weight per unit volume of PVC resin powder that is essentially uncompressed. It is related to the particle morphology, average particle size, and particle size distribution of the resin. Low apparent density, large volume, fast absorption of plasticizers, and easy processing. On the contrary, high average particle size density and small volume lead to the absorption of PVC processing aids. For the production of hard products, the molecular weight requirement is not high, and plasticizers are generally not added during the processing. Therefore, the porosity of resin particles is required to be lower, but there is a requirement for the dry flow of the resin, so the apparent density of the resin is correspondingly higher.
5. Plasticizer absorption of resin
The absorption amount of PVC processing aids reflects the degree of pores inside the resin particles, with high oil absorption rate and large porosity. The resin absorbs plasticizers quickly and has good processing performance. For extrusion molding (such as profiles), although the requirement for resin porosity is not too high, the pores inside the particles have a good adsorption effect on the addition of additives during processing, promoting the effectiveness of additives.
6. Whiteness
The whiteness reflects the appearance and color of the resin, as well as the degradation caused by poor thermal stability or prolonged retention time, resulting in a significant decrease in whiteness. The level of whiteness has a significant impact on the aging resistance of trees and products.
7. Residual vinyl chloride content
VCM residue refers to the portion of the resin that has not been adsorbed or dissolved in the polyethylene monomer, and its adsorption capacity varies depending on the type of resin. In actual VCM residue factors, the main factors include low top temperature of the stripping tower, excessive pressure difference in the tower, and poor resin particle morphology, all of which can affect VCM residue desorption, which is an indicator for measuring the hygiene level of resins. For special products, such as tin foil hard transparent film packaging bags for medical pharmaceuticals, the residual VCM content of resin is not up to standard (less than 5PPM).
8. Thermal stability
If the water content in the monomer is too high, it will produce acidity, corrode the equipment, form an iron polymerization system, and ultimately affect the thermal stability of the product. If hydrogen chloride or free chlorine is present in the monomer, it will have adverse effects on the polymerization reaction. Hydrogen chloride is prone to form in water, which reduces the pH value of the polymerization system and affects the stability of the polymerization system. In addition, the high content of acetylene in the monomer of the product affects the thermal stability of PVC under the synergistic effect of acetaldehyde and iron, which affects the processing performance of the product.
9. Sieve residue
The sieve residue reflects the degree of uneven particle size of the resin, and its main influencing factors are the amount of dispersant in the polymerization formula and the stirring effect. If the resin particles are too coarse or too fine, it will affect the grade of the resin and also have an impact on the subsequent processing of the product.
10. “Fish Eye”
“Fish eye”, also known as crystal point, refers to transparent resin particles that have not been plasticized under normal thermoplastic processing conditions. Impact in actual production. The main factor of “fish eye” is that when the content of high boiling substances in the monomer is high, it dissolves the polymer inside the particles during the polymerization process, reduces the porosity, makes the particles hard, and becomes a temporary “fish eye” during plasticization processing. The initiator is unevenly distributed in the monomer oil droplets. In a polymerization system with uneven heat transfer, the formation of resin with uneven molecular weight, or the uncleanness of the reactor during feeding, residual resin, or excessive sticking of the reactor material can all cause “fisheye”. The formation of “fish eyes” directly affects the quality of PVC products, and in subsequent processing, it will affect the surface aesthetics of the products. It will also greatly reduce the mechanical properties such as tensile strength and elongation of the products, which can easily lead to perforation of plastic films or sheets, especially cable products, which will affect their electrical insulation properties. It is one of the important indicators in resin production and plasticization processing.
Post time: Jun-12-2024
