PVC resin is the largest component of PVC cable, and its own quality has a great influence on the mechanical and electrical properties of cable materials.
1 Conductive mechanism of PVC
In general, both electron conduction and ion conduction are observed in polymers, but the degree is different. The biggest difference between the two conductive mechanisms is the difference in charge carriers. In polymers, the carrier fluid of electron conduction mechanism is the free electron whose π bond electron is delocalized. The fluid carrier of ion conduction mechanism is generally positive and negative ions. Most of the polymers based on electronic conductivity are conjugated polymers, and PVC main chain is mainly a single bond link, does not have a conjugated system, so it mainly conducts electricity by ion conduction. However, in the presence of current and UV light, PVC will remove HCl and form unsaturated polyolefin fragments, so there are π-bonded electrons, which may drive electrical conduction
2.2.1 molecular weight
The influence of molecular weight on the conductivity of polymers is related to the main conductive mechanism of polymers. For the electron conductance, the conductivity will increase because the molecular weight increases and the intramolecular channel of the electron is prolonged. With the decrease of molecular weight, the ion migration increases and the conductivity increases. At the same time, molecular weight also affects the mechanical properties of cable products. The higher the molecular weight of PVC resin, the better its cold resistance, thermal stability and mechanical strength.
2.2.2 Thermal stability
Thermal stability is one of the most basic and important indexes to evaluate the quality of resin. It directly affects the processing technology of downstream products and the properties of products. With the widespread use of PVC building materials, the demand for thermal stability of PVC resin is getting higher and higher. The aging whiteness is an important index to evaluate the stability of resin, so as to judge the thermal stability of resin.
2.2.3 Ion content
In general, PVC conducts electricity mainly by ion conduction, so ions have a significant impact on the conduction. The metal cations (Na+, K+, Ca2+, Al3+, Zn2+, Mg2+, etc.) in the polymer play a leading role, while the anions (Cl-, SO42-, etc.) have little influence on the electrical conductivity due to their large radius and slow migration rate. In contrast, when PVC can cause the side effect of dechlorination under electric current and UV radiation, Cl- is released, in which case the anion plays the dominant role.
2.2.4 Apparent density
The apparent density and oil absorption of resin affect the post-processing properties of resin, especially the plasticization of resin, and the plasticization directly affects the properties of products. Under the same formulation and processing conditions, the resin has a high apparent density and relatively low porosity, which may affect the transfer of conductive materials in the resin, resulting in a high resistivity of the product.
2.2.5 other
PVC resin in the “fisheye”, impurity ions and other substances in the cable production process become knob-like impurities, so that the cable surface is not smooth, affect the appearance of products, and “knobs” around the formation of a certain electrical gap, destroy the PVC material inherent insulation performance.
Under the same post-processing conditions, the apparent density, plasticizer absorption and other performance indicators directly affect the post-processing effect, and the different degree of plasticization leads to the difference of product performance.
In addition, studies have shown that additives with functional groups can be introduced after polyvinyl chloride polymerization, for example, at the end of synthesis or before final drying. Poly has 1~30% moisture with a total of 0.0002~0.001% polycarboxylic acid, can improve the volume resistivity of products. The INTRODUCTION OF 0.1-2% phosphate ion containing COMPOUNDS (alkyl hydrogen phosphate, ammonium oxyphosphate, C≤20 alkyl phosphate, organic phosphate) in suspension polyvinyl chloride, and the addition of alkaline earth metal compounds containing 0.1-2%, so as to deposit them on the polymer, can effectively improve the volume resistance and dielectric constant of resin.
Post time: Sep-09-2022