Polyvinyl chloride (PVC) is the world’s third-largest synthetic polymer plastic (after polyethylene and polypropylene), producing about 40 million tons of PVC per year. PVC is a polymer formed by the polymerization of vinyl chloride monomer (VCM) in peroxides, azo compounds and other initiators, or under the action of light and heat according to the mechanism of free radical polymerization. Vinyl chloride homopolymers and vinyl chloride copolymers are collectively known as vinyl chloride resins.
PVC used to be the world’s most produced general-purpose plastic and has a wide range of applications. PVC is available in both hard (sometimes abbreviated as RPVC) and soft. Rigid polyvinyl chloride is used in building pipes, doors and windows. It is also used in the manufacture of plastic bottles, packaging, bank cards, or loyalty cards. Adding plasticizers makes PVC softer and more elastic. It can be used in pipes, cable insulation, flooring, signage, gramophone records, inflatable products, and rubber substitutes.
PVC is a white powder with amorphous structure, the degree of branching is small, the glass transition temperature is 77~90 °C, and the decomposition begins at about 170 °C, the stability of light and heat is poor, above 100 °C or after long-term sun exposure, it will decompose and produce hydrogen chloride, and further automatic catalytic decomposition, causing discoloration, physical and mechanical properties also decline rapidly, stabilizers must be added in practical applications to improve the stability of heat and light.
The molecular weight of industrially produced PVC is generally in the range of 50,000~110,000, with large polydispersity, the molecular weight increases with the decrease of polymerization temperature, there is no fixed melting point, it begins to soften at 80~85 °C, becomes viscoelastic at 130 °C, and begins to transform into viscous flow state at 160~180 °C. It has good mechanical properties, tensile strength of about 60 MPa, impact strength of 5~10kJ/m2; It has excellent dielectric properties. PVC is insoluble in common solvents, but swells in monomers and some chlorinated hydrocarbon solvents.
PVC composition
- Polyvinyl chloride is a polymer material that uses one chlorine atom to replace one hydrogen atom in polyethylene, and is an amorphous polymer with a small amount of crystalline structure. The structure of this material is as follows: -(CH2-CHCl)n-. PVC is a linear polymer in which VCM monomers are mostly linked by a head-tail structure. The carbon atoms are arranged in a zigzag pattern, and all the atoms are connected by σ bonds. All carbon atoms are sp3 hybridized.
- There is a short inter-regular structure on the PVC molecular chain. With the decrease of the polymerization reaction temperature, the regularity of the interstructured structure increases. In the macromolecular structure of polyvinyl chloride, there are shortcomings such as head structure, branched chain, double bond, allyl chloride, tertiary chloride, etc., which make the heat deformation resistance and aging resistance poor. After cross-linking, such shortcomings can be eliminated.
Crosslinking is divided into radiation crosslinking and chemical crosslinking.
- Radiation cross-linking: the use of high-energy rays, generally the rays generated by the cobalt-60 radiation source or the electron rays generated by the acceleration of electrons, mainly using the latter. A cross-linking additive (a monomer with two or more carbon-carbon double bond structures) is then added for cross-linking. However, it is difficult to operate and has high requirements for equipment.
- Chemical cross-linking: Triazole dimercaptoamine salt (FSH) is used for cross-linking, and the cross-linking mechanism is that the amine and sulfhydryl group combine to attack the carbon-chlorine polar bond to carry out the substitution reaction. After cross-linking, the performance of the product will be comprehensively improved in terms of ultraviolet resistance, solvent resistance, temperature resistance, impact toughening, etc.
