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The processing principle of PVC resin and the composition of ingredients
The composition and function of materials
Mainly PVC resins, heat stabilizers, plasticizers, lubricants, modifiers, fillers, colorants, and other processing aids. When there are special requirements, it is necessary to add some special additives, each component It has a certain effect in the formulation and ultimately affects the performance of the product.
PVC resin is the main component in the formula, and its molecular weight and molecular weight distribution affect its performance:
1. The influence of molecular weight:
1.1 As the molecular weight increases, the gravitational force and entanglement between molecular chains increase, the glass transition temperature increases, and the mechanical properties of the product increase accordingly;
1.2 As the molecular weight increases, the melt viscosity increases, the fluidity becomes worse, and the processing difficulty increases
2. The influence of molecular weight distribution:
2.1 If the molecular distribution is too wide, it means that there are a certain number of molecules whose molecular weight is low and high. The first will lead to the reduction of the mechanical properties of the product, and the latter will easily lead to processing difficulties and easily lead to poor appearance of the product;
2.2 is an important reason for fish eyes: First, there are some ultra-high molecular weight molecules in PVC resin, which are not easy to absorb plasticizers. Under normal processing conditions, they can only swell with plasticizers but not with other components in the formulation. Points are compatible; secondly, there are some molecules with high molecular weight and linear structure or too tight physical internal structure in PVC resin, which are also difficult to plasticize.
Stabilizer : It is an indispensable part of the formula. PVC resin will gradually decompose and change color during heating or heating due to the characteristics of the material, ranging from light yellow, yellow, yellow orange, red orange, brown, tan to black , So it is necessary to add enough stabilizers to the formula to be able to produce normally. Commonly used stabilizers include basic lead salt stabilizers, metal soap stabilizers, organotin stabilizers, composite stabilizers and other four types of epoxy compounds and so on. Due to the diversity of stabilizers, the following points must be considered when selecting stabilizers: excellent thermal stability, good compounding process performance, and impact on product performance.
1. Basic lead salt stabilizer: It is the most used stabilizer variety at present. It has excellent comprehensive performance such as thermal stability, electrical performance, processing performance and low price advantage, but lead salt is toxic, and The products are not transparent, and quite a few products have been banned.
2. Metal soap stabilizers: usually have good light stability, weather resistance and excellent lubricating properties, but usually two metal soaps or use with basic lead salt stabilizers are required to obtain a good stabilizing effect.
3. Organotin stabilizer: There are mainly fatty acid organotin, maleate organotin and mercaptan organotin. The common features are high stability and efficiency, good transparency, low dosage and resistance to vulcanization pollution. The three are moderately hot. The stability number of fatty acid organotin is poor, but the processing lubricity is the best.
4. Compound stabilizers: The current compound stabilizers mainly include lead salt compound stabilizers, calcium-zinc compound stabilizers, and a small amount of other compound stabilizers.
5. Auxiliary stabilizers: compounds that have relatively weak stability, but can improve the stabilization effect of other stabilizing systems, mainly phosphite esters, epoxy compounds, etc.
Plasticizer : It can make PVC resin produce plasticity, flexibility, lower processing temperature, lower thermal melting temperature, improve fluidity, but reduce modulus, strength, hardness, glass transition temperature, and embrittlement within the plasticizing range Temperature and improve impact performance; increase modulus, strength, hardness and brittleness in the anti-plasticizing range. There are mainly metal soaps, saturated hydrocarbons, fatty acids and their lipids.
Lubricant : an additive that can improve the flow properties of PVC melt and prevent the melt from adhering to equipment.
1. The main function of lubricant:
1.1 Prevent the adhesion of materials to the processing equipment, so that the melt can be transferred or transported well;
1.2 Adjust the melting characteristics, melt viscosity and rheological characteristics of the material;
1.3 Provide proper lubrication, so that the material produces suitable frictional heat generation speed during the processing, obtains a lower equilibrium temperature and better quality melt, and provides relatively wide processing conditions;
1.4 The final impact on the performance of the product: such as transparency, chemical resistance, weather resistance, etc.
2. According to the effect and mechanism, it can be divided into internal lubricant and external lubricant. Internal and external lubricant:
2.1 Internal lubricant: It has good compatibility with PVC, mainly refers to the internal lubrication of the polymer melt. The main function is to reduce the friction between the molecular chains of the polymer melt and improve the fluidity and uniformity of the melt. Features.
2.2 External lubricant: It has little compatibility with PVC and mainly involves the process of phase boundary. The main function is to reduce the friction between the polymer melt and the processing equipment and mold or reduce the friction between the polymer particles.
3. Selection of lubricant:
3.1 It can be well dispersed in PVC resin and does not interfere with other additives.
3.2 Does not hinder the plasticization of PVC resin.
3.3 High lubrication efficiency and long lasting.
3.4 It is better to improve the quality of the product without seriously reducing the quality of the product.
4. Because different processing techniques have different lubrication requirements, the amount of lubricant required for PVC extrusion molding also has a great relationship with the machine type, screw structure, and mold structure. Therefore, the following points should be paid attention to in the specific selection. :
4.1 In the molding process, the faster the shear rate, the better the internal lubrication effect is required, and the more lubricant is required.
4.2 When no plasticizer is added to the formula, more lubricant is required than when plasticizer is added. At the same time, the inherent lubricating properties of certain stabilizers need to be considered.
4.3 In the formula with plasticizer, since the plasticizer already has internal lubricating effect, only a small amount of internal lubricant is needed.
4.4 The internal and external lubricants in the formula should be balanced, otherwise it will cause processing difficulties.
4.5 When there are more fillers in the formula, the amount of lubricant should be appropriately increased.
4.6 Products with a simple and large cross-section structure require less lubricant, and vice versa.
Processing aids : in order to improve the processing performance of the compound
Impact modifier
Filler : Mainly to reduce production costs, but to a certain extent, it can also improve certain properties of the product, such as shrinkage, heat-resistant deformation temperature, and increase the hardness of the product.
Other components
Processing and application of rigid PVC
Rigid PVC solid wall pipe
1. The basic principle of the formula: the performance and processing performance of the product must be taken into account;
2. Problems that often occur in the production of rigid PVC solid-wall pipes:
