PVC Resin

PVC resin, with the full name of polyvinyl chloride resin, is a kind of synthetic plastic polymer with large production volume each year. It is widely used in various plastic industrial fields. Polyvinyl chloride resin is a thermoplastic polymer compound formed by polymerization of vinyl chloride monomer(VCM) or dichloroethane(EDC) under the action of an initiator. Its main components is polyvinyl chloride, and other components are added to enhance its heat resistance, toughness and ductility. With the characteristics of good chemical stability, corrosion resistance and water resistance, polyvinyl chloride is the most widely used raw material in plastic production.

Before PVC Resin can be made into finished products, it always need to be converted into compounds by adding additives, such as, heat stabilizers, UV stabilizers, plasticizers, heat modifiers, flame retardants, bactericides, foaming agents, etc.

According to the production method, it can be divided into general-purpose PVC resin, high polymerization PVC resin, cross bonding polyvinyl chloride resin.

The general-purpose type is formed by the polymerization of vinyl chloride monomer under the action of an initiator.

High polymerization type refers to a resin polymerized by adding a chain extender to the vinyl chloride monomer polymerization system.

The cross bonding type is a resin polymerized by adding a cross-linking agent containing diene and polyene into the vinyl chloride monomer polymerization system.

General-purpose type has a simple preparation method and a wide range of uses. But it has some shortcomings that it can’t be adapted to high-end and special processing applications. The high-polymerization polyvinyl chloride resin generally refers to PVC resin with an average degree of polymerization above 1700 or a slight cross-linked structure between the molecules. This type improves the processing performance of general-purpose polyvinyl chloride resin, and is more suitable for processing special soft products, high resilience, high toughness rubber products, as well as matte products. This type can make the products with the characteristics of higher tensile strength, tear strength, elongation at break, abrasion resistance, high temperature and low temperature resistance, lower compression set, excellent plasticizer retention, etc.

According to the content of plasticizer, it can be divided into rigid PVC and flexible PVC. Without plasticizer, it is rigid PVC, and its glass transition temperature is as high as 80℃-85℃, which is not easy to process. But after adding plasticizer, the glass temperature will decreasse, plasticity and flexibility will in crease and make it easy to process even at low temperature. The rigid type has good tensile, bending, compression and impact resistance, can be used alone as a structural material. The flexibility, elongation at break and cold resistance of the flexible type will increase, but the brittleness, hardness and tensile strength will decrease.

The industry generally distinguishes the PVC resin according to the method of obtaining vinyl chloride monomer. Depends on the polymerization method, PVC suspension resin is currently the largest variety, followed it is the PVC paste resin made by emulsion method.

1. It has the characteristics of abundant raw materials, mature manufacturing technology, low price, easy processing and wide application.
   Flame retardancy is its greatest feature, so it is widely used in fire protection applications.
2. It has good electrical insulating properties, which makes it can be used as low-frequency insulating material. In addition, its chemical stability is also good.
3. It has good mechanical properties, tensile strength is about 60MPa, impact strength is 5 ~ 10kJ/m2.
4. The degree of branching is small, the glass transition temperature is 77~90℃, and the decomposition starts at about 170℃.
5. The molecular weight of industrially produced PVC powder is generally in the range of 50,000 to 110,000, which has a large dispersibility.
6. The molecular weight increases with the decrease of the polymerization temperature. There is no fixed melting point. The powder starts to soften at 80-85℃, and becomes viscoelastic at 130℃. And it begins to transform into a viscous flow state when the temperature is 160-180℃.
7. Its stability to light and heat is poor. If it exceeds 100℃ or is exposed to sunlight for a long time, it will decompose to produce hydrogen chloride, and further autocatalytic decomposition will cause fading. Meanwhile, the physical and mechanical properties will also drop sharply. Stabilizers must be added to improve thermal and light stability in practical applications.
8. It can’t be used alone, but can be mixed with other ingredients to produce various products.

According to the raw material, the production methods of polyvinyl chloride are mainly divided into calcium carbide acetylene method, petroleum ethylene method and natural gas acetylene method. The main raw materials commonly used are monomeric vinyl chloride, dispersant, deionized water and initiator.

There are three main methods for the synthesis of vinyl chloride monomer. 1. Ethylene oxychlorination with ethylene method with ethylene as raw material. 2. Synthesis of acetylene and hydrogen chloride from calcium carbide. 3. A method of using dichloroethane as a raw material and then cracking the dichloroethane into vinyl chloride.

Production fundamentals. Using calcium carbide as raw material, through three main chemical reactions of calcium carbide hydrolysis reaction, addition reaction of acetylene and hydrogen chloride, and vinyl chloride polymerization reaction, then the production of PVC resin can be completed. The production device includes reactor, fractionation process, separation process, etc. There are many types of equipment involved, the process control route is long, and the control parameters are more complicated. In actual production, most of them are completed by DCS automatic control.

Quality index of PVC. Many aspects such as raw materials, production process and operation have a great influence on the quality of PVC. Viscosity number, apparent density, oil absorption rate, volatile matter content, sieving rate, number of impurity particles, heat aging whiteness, “fish eye” number, electrical conductivity, residual VCM content are the ten important parameters of general PVC resin powder. In addition to the above indicators, polymerization degree distribution, particle size distribution, particle regularity, initial coloration, aging discoloration rate, dry fluidity, plasticizing rheological properties are all indicators for judging the quality of PVC products.

The K value is an index indicating the polymerization degree and molecular weight of polyvinyl chloride. In addition, the viscosity number is also an indicator of the molecular weight of the polymer. This product is an amorphous linear polymer compound. The larger the relatively molecular weight, the higher the viscosity, the better the strength, stiffness, toughness, heat and low temperature resistance of the material, but the worse the molding processability. The difference in K value of polyvinyl chloride resin will have a certain impact on the processing technology and product performance, Therefore, when purchasing polyvinyl chloride resin, customers need to choose the suitable grade depends on the K value and applications.

K70-75 PVC resin with high K value and higher molecular weight has the best mechanical properties, but is more difficult to process.

PVC resin k65/k66, PVC resin k67/k68 with suitable K value have a good balance between mechanical properties and processability. They are the most popular and can meet the needs of the most PVC applications.

K58-60 polyvinyl chloride with low value has the lowest mechanical properties and the processing is easiest. However, it has many difficulties in the application process.

• From the physical state, polyvinyl chloride paste is in the form of white fine powder, while the PVC resin is in the form of white fine particles.
• In terms of usage method. PVC paste resin should be mixed with appropriate amount of plasticizer and be used in the form of liquid plastisol, which is generally used in the production of flexible plastic products. Polyvinyl chloride resin is used after granulation with an appropriate amount of plasticizer, and is generally used in the production of rigid plastic products.