Introduction To Polyvinyl Chloride (PVC) Resin

Polyvinyl chloride (PVC) is a polymer formed by the polymerization of vinyl chloride monomer (VCM) in the presence of initiators such as peroxides and azo compounds or under the action of light or heat according to the free radical polymerization mechanism. Vinyl chloride homopolymers and vinyl chloride copolymers are collectively referred to as vinyl chloride resins.

1. Basic physical properties

PVC is an amorphous white powder with a low degree of branching. It starts to decompose at about 170°C and has poor stability to light and heat. It will decompose to produce hydrogen chloride at temperatures above 100°C or after long-term exposure to sunlight, and further autocatalytic decomposition will cause discoloration. The physical and mechanical properties will also decrease rapidly. In practical applications, stabilizers must be added to improve the stability to heat and light.

The molecular weight of industrially produced PVC is generally in the range of 50,000 to 110,000, with a large polydispersity. The molecular weight increases with the decrease of polymerization temperature. It has no fixed melting point, starts to soften at 80-85°C, becomes viscoelastic at 130°C, and starts to change to viscous flow at 160-180°C. It has good mechanical properties, tensile strength of about 60MPa, impact strength of 5-10kJ/m2, and excellent dielectric properties.

Industrial polyvinyl chloride resin is mainly an amorphous structure, but it also contains some crystalline areas (about 5%), so polyvinyl chloride has no obvious melting point. It begins to soften at about 80°C and has a heat deformation temperature (under a load of 1.82MPa) of 70-71°C. It begins to flow at 150°C under pressure and begins to slowly release hydrogen chloride, causing the polyvinyl chloride to change color (from yellow to red, brown, or even black).

2. Transparency and hardness

The natural color is slightly yellow and translucent, with a glossy appearance. The transparency is better than that of polyethylene and polypropylene, but worse than that of polystyrene. Depending on the amount of additives used, it is divided into soft and hard polyvinyl chloride. The soft product is soft and tough, and feels sticky. The hardness of the hard product is higher than that of low-density polyethylene, but lower than that of polypropylene, and whitening will occur at the bends. It is stable; not easily corroded by acids and alkalis; and relatively resistant to heat.

3. Flame retardancy

Polyvinyl chloride is flame retardant (oxygen index value is above 40), and has good mechanical strength and electrical insulation.

4. Photothermal stability

Polyvinyl chloride has poor stability to light and heat. The softening point is 80℃, and it starts to decompose at 130℃. Without heating stabilizer, polyvinyl chloride starts to decompose at 100℃, and decomposes faster above 130℃. When heated, it releases hydrogen chloride gas (hydrogen chloride gas is toxic) and changes color from white to light yellow to red to brown to black. Ultraviolet rays and oxygen in sunlight can cause polyvinyl chloride to undergo photo-oxidative decomposition, thereby reducing the flexibility of polyvinyl chloride and finally making it brittle. This is why some PVC plastics turn yellow and brittle over time.

5. Corrosion resistance

It has stable physical and chemical properties, is insoluble in water, alcohol, gasoline, and has low gas and water vapor permeability. At room temperature, it can withstand any concentration of hydrochloric acid, sulfuric acid below 90%, nitric acid at 50-60%, and caustic soda solution below 20%, and has a certain resistance to chemical corrosion. It is quite stable to salts, but can be dissolved in organic solvents such as ether, ketones, chlorinated aliphatic hydrocarbons and aromatic hydrocarbons.

6. Main Applications

Rigid polyvinyl chloride (without plasticizer) has good mechanical strength, weather resistance and flame resistance. It can be used alone as a structural material and applied to the manufacture of pipes, plates and injection molded products in the chemical industry. Rigid polyvinyl chloride can be used as a reinforcement material.

PVC is the world's largest-volume general-purpose plastic and is widely used. It is widely used in building materials, industrial products, daily necessities, floor leather, floor tiles, artificial leather, pipes, wires and cables, packaging films, bottles, foaming materials, sealing materials, fibers, etc.

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