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Basic understanding and application of PVC polyvinyl chloride

Polyvinyl chloride, referred to as PVC in English, is a polymer polymerized by vinyl chloride monomer (VCM) with initiators such as peroxides and azo compounds or under the action of light and heat according to the free radical polymerization reaction mechanism. . Polyvinyl chloride was discovered by V. Renault of the United States as early as 1835. When vinyl chloride (VCM) is irradiated with sunlight, a white solid, namely polyvinyl chloride, is generated. Polyvinyl chloride (PVC) is a polymer material that uses a chlorine atom to replace a hydrogen atom in polyethylene. Vinyl chloride homopolymer and vinyl chloride copolymer are collectively called vinyl chloride resin.

  0 1  PVC structure

Molecular Structure:

The PVC molecular chain contains highly polar chlorine atoms, and the intermolecular force is large, which improves the rigidity, hardness, and mechanical properties of PVC products, and has excellent flame retardancy (flame retardancy refers to the properties of a substance or after the material has been processed It has the property of obviously delaying the spread of flame); but its dielectric constant and dielectric loss tangent are larger than PE.
PVC resin contains a small amount of double bonds, branch chains and initiator residues remaining in the polymerization reaction. In addition, there are chlorine atoms and hydrogen atoms between two adjacent carbon atoms, which are easy to dehydrochloride, causing PVC to be easily dehydrated under the action of light and heat. Degradation reactions occur.
The spatial arrangement of chlorine and hydrogen atoms on the PVC molecular chain is basically disordered, so the crystallinity of its products is low, generally only 5 to 15%.

02  The main purpose

Polyvinyl chloride plastics come in many different forms, and their processing methods are also diverse, including pressing, extrusion, injection, coating, etc. PVC plastic is often used to make films, artificial leather, insulation layers of wires and cables, hard products, floors, furniture, sports equipment, etc.

PVC polyvinyl chloride products can generally be divided into two categories: hard and soft. No plasticizer is added during the processing of hard products, while a large amount of plasticizer is added during processing of soft products. PVC is originally a rigid plastic with a glass transition temperature of 80 to 85°C. After adding plasticizer, the glass transition temperature can be lowered, which facilitates processing at lower temperatures, increases the flexibility and plasticity of the molecular chain, and can be made into soft products that are elastic at room temperature. Generally, the amount of plasticizer added to soft PVC plastic is 30% to 70% of PVC. ​

PVC polyvinyl chloride can be processed into various profiles and products after adding plasticizers, stabilizers, lubricants, colorants, and fillers during processing. The specific uses of PVC are as follows:

1. PVC profiles and special profiles

Profiles and special-shaped profiles are the largest areas of PVC consumption in my country, accounting for about 25% of the total PVC consumption. They are mainly used to make doors, windows and energy-saving materials. Currently, their application volume is still growing significantly across the country.

2. PVC pipes

PVC pipes are the second largest consumer area of PVC, accounting for about 20% of its consumption. In my country, PVC pipes were developed earlier than polyethylene (PE) pipes and polypropylene (PP) pipes. They have many varieties, excellent performance, and a wide range of uses, and they occupy an important position in the market.

3. PVC film

PVC film is the third largest consumer area of PVC, accounting for about 10% of its consumption. PVC can be made into a transparent or colored film of specified thickness using a calender. The film produced by this method is called calendered film. PVC granular raw materials can also be blown into films using a blow molding machine. The films produced by this method are called blown films. The film has great uses and can be processed into packaging bags, raincoats, tablecloths, curtains, inflatable toys, etc. through cutting and heat sealing methods. Wide sheets of transparent film can be used to build greenhouses and plastic greenhouses, or can be used as mulch.

 

4. PVC hard materials and plates

Stabilizers, lubricants and fillers are added to PVC. After mixing, the extruder can be used to extrude hard pipes, special-shaped pipes and corrugated pipes of various diameters, which can be used as sewer pipes, drinking water pipes, wire casings or stair handrails. . By overlapping and hot-pressing the rolled sheets, hard plates of various thicknesses can be made. The plates can be cut into the required shapes, and then PVC welding rods and hot air are used to weld them into various chemically resistant storage tanks, air ducts and containers.

5. PVC general soft products

The extruder can be used to extrude hoses, cables, wires, etc.; the injection molding machine can be used with various molds to make plastic sandals, soles, slippers, toys, auto parts, etc.

6. PVC packaging materials

PVC products are mainly used for packaging various containers, films and hard sheets. PVC containers are mainly used to produce bottles for mineral water, beverages, and cosmetics, as well as for packaging refined oils.

7. PVC siding and flooring

PVC siding is mainly used to replace aluminum siding. Except for part of the PVC resin, the remaining components of PVC floor tiles are recycled materials, adhesives, fillers and other components. They are mainly used on the hard floors of airport terminals and other places.

8. PVC daily consumer goods

PVC products can be seen everywhere in our daily lives. PVC is used to make various imitation leathers for luggage bags, sports products such as basketballs, footballs and rugby balls, etc. It can also be used to make belts for uniforms and special protective equipment. PVC fabrics for clothing are generally absorbent fabrics (no coating required), such as ponchos, baby pants, imitation leather jackets and various rain boots. PVC is also used in many sports and entertainment products, such as toys, records and sports equipment.

 

 

03  Basic performance

General performance: PVC resin is a white or light yellow powder with a specific gravity of 1.35 to 1.45; the hardness of PVC products can be adjusted by adding plasticizer content, and products with different hardness can be made. Pure PVC has very little water absorption and breathability.
The true color is yellowish translucent and shiny. The transparency is better than polyethylene PE and polystyrene, but worse than polystyrene PS. Depending on the amount of additives, it is divided into soft and hard polyvinyl chloride. Soft products are flexible and tough and feel sticky, while hard products have a higher hardness than low density. Polyethylene, but less than polypropylene, will show whitening at the bends.
Solubility: PVC is resistant to water, concentrated alkali, non-oxidizing acids, hydrocarbons, oil and ozone. Oxidizing acids (such as sulfuric acid, nitric acid, chromic acid) can corrode PVC.
Polyvinyl chloride is a polar polymer with a solubility parameter of approximately 9.5. PVC can be dissolved in tetrahydrofuran, cyclohexanone, methyl ethyl ketone or a mixture of acetone and carbon disulfide, as well as tetrahydrofurfuryl alcohol, dioxane, dichloroethane, o-dichlorobenzene, toluene, etc. The solubility of polyvinyl chloride has a great relationship with its molecular weight. The larger the molecular weight, the worse the solubility. Float resins are generally less soluble than suspension resins.
Mechanical properties: Polyvinyl chloride molecules contain a large number of chlorine atoms. The molecules are highly polar, the intermolecular force is strong, the degree of macromolecule aggregation is high, and the inter-chain distance is 2.8×10-10m, which is much larger than that of polyethylene PE. The degree of convergence (4.3×10-10m) is small, so the tensile strength and compressive strength of PVC are high, the hardness and stiffness are large, but the impact strength and elongation at break are small. It can be seen that PVC has high hardness and mechanical properties, which increase with the increase of molecular weight, but decrease with the increase of temperature. The different plasticizer content added to PVC has a great impact on the mechanical properties. The mechanical properties decrease with the increase of plasticizer content. The wear resistance of PVC is average. The static friction coefficient of hard PVC is 0.4 to 0.5, and the kinetic friction coefficient is 0.23.
Thermal properties: PVC has a relatively small linear expansion coefficient, is flame retardant, and has an oxygen index of over 45%. The biggest feature of PVC is flame retardancy, so it is widely used in fire protection applications. However, polyvinyl chloride releases hydrogen chloride (HCl) and other toxic gases such as dioxins during the combustion process. The combustion of PVC is divided into two steps. First, hydrogen chloride gas and dienes containing double bonds are decomposed by combustion at 240°C-340°C, and then carbon combustion occurs at 400-470°C.
Thermal stability: The thermal stability of PVC is very poor. Pure PVC resin begins to decompose at 140°C, and decomposes immediately at 180°C. The melting temperature of PVC is 160°C, so pure PVC resin is difficult to process using thermoplastic methods. . The softening degree of polyvinyl chloride resin is low, about 75 to 80°C, and the brittleness temperature is lower than -50 to -60°C. The long-term use temperature of most products should not exceed 55°C, and special formulas can reach 90°C. Even if the polyvinyl chloride resin is of high purity, hydrogen chloride (HCI) gas will begin to escape when the temperature is above 100°C for a long time or when exposed to ultraviolet radiation. It shows that there is an unstable structure in its molecular structure. The longer the time, the more degradation, the higher the temperature, the faster the degradation, and the faster the degradation in the presence of oxygen or air.
The degree of branching is small, the glass transition temperature is 77~90℃, and it starts to decompose around 170℃. It has poor stability to light and heat. When it is above 100℃ or exposed to sunlight for a long time, it will decompose to produce hydrogen chloride, and further autocatalyze it. Decomposes, causing discoloration, and the physical and mechanical properties also decrease rapidly. In practical applications, stabilizers must be added to improve the stability to heat and light.
Reason for instability: PVC is polymerized by free radical initiation of vinyl chloride monomer. During the reaction, during the growth of the molecular chain, a chain transfer reaction will occur to generate tertiary carbon atoms. The chlorine atoms and hydrogen atoms connected to the tertiary carbon atoms will become active atoms due to the small electron cloud distribution density and low bond energy. It is easy to remove one part of HCl from adjacent H and Cl. If the PVC resin has purely the above-mentioned linear structure and is composed of secondary carbon atoms and chlorine atoms, then its stability will be relatively good.
But in fact, even very pure PVC resin begins to decompose HCl above 100°C, which shows that there are still unstable factors in its molecular structure. The unstable structure of PVC is believed to be caused by free radical-initiated polymerization of vinyl chloride. The reason may be that part of it is generated by peroxidation reduction of the initiator, and the oxygen-containing structure may be caused by the polymerization reaction in the presence of trace oxygen or by the post-oxidation of the polymer.
The HCl removal reaction occurs when industrially produced PVC is heated to about 100°C. At normal thermoplastic processing temperatures (100-200°C), in addition to HCl removal, the pigment color will continue to deepen until it turns black. The physical and mechanical properties of the products continue to decline until they lose their use value.
Electrical properties: PVC is a polymer with good electrical properties. However, due to its high polarity, its insulation is not as good as PE and PP. Its dielectric constant, dielectric loss tangent, and volume resistivity are large, and it is not resistant to water. Conductive materials such as polyolefins have greater water resistance, so their resistance is smaller than non-polar polyolefins, but they still have higher volume cathode and breakdown voltage.
The electrical properties of PVC are greatly affected by temperature, frequency, and types of additives. Its corona resistance is also not good. It is generally only suitable for low-voltage and low-frequency insulation materials. The polar groups of polyvinyl chloride are directly attached to the main chain. Below the glass transition temperature, the dipole segment is restricted by the atoms of the main chain and cannot move. Therefore, dipolarization does not occur and it can be used as a high-frequency at room temperature. Insulation Materials. When used for wire insulation, the insulation properties of suspended resin are 10 to 100 times higher than that of floating resin. The presence of chloride ions produced by degradation reduces electrical insulation.
Environmental performance: PVC can resist most inorganic acids (except fuming sulfuric acid and concentrated nitric acid), inorganic salts, alkali, and most organic solvents (such as ethanol, gasoline and mineral oil), and is suitable for chemical anti-corrosion materials. PVC will swell or dissolve in esters, ketones, aromatic hydrocarbons, and halocarbons. The best solvents are tetrahydrofuran and cyclohexanone. PVC is not resistant to light, oxygen, and heat, and is easily degraded, causing the color of the product to change (white → pink → light yellow → brown → reddish brown → red black → black).

Company: Henan Hesu Industrial Co., Ltd., a professional plastic supplier

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