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Basic knowledge of PVC (polyvinyl chloride) (two) how much do you know

2019-07-10 11:40:47
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Characteristics of PVC


Glass transition temperature: 85℃. Melting point: 130℃.


Amorphous state density (25℃): 1.385 g/cm3.


Crystal density (25℃): 1.52g/cm3. The molecular weight of PVC produced is generally in the range of 50,000 to 120,000, with a large polydispersity, and the molecular weight increases as the polymerization temperature decreases.


There is no fixed melting point. It starts to soften at 80~85℃, becomes viscoelastic at 130℃, and becomes viscous at 160~180℃.


It has good mechanical properties, the tensile strength is about 60MPa, and the impact strength is 5-10kJ/m2.


Has excellent dielectric properties. PVC is a white powder with an amorphous structure with a small degree of branching. However, the stability to light and heat is poor. It will decompose to produce hydrogen chloride above 100℃ or after long-term sunlight exposure, and further auto-catalytically decompose, causing discoloration, and rapid decline in physical and mechanical properties. It must be stabilized in practical applications. Agent to improve the stability to heat and light.


The stability to heat during processing is relatively poor.


PVC is very hard and has poor solubility. It can only be dissolved in a few solvents such as cyclohexanone, dichloroethane and tetrahydrofuran. Rigid PVC is one of the most widely used plastic materials. PVC material is a non-crystalline material.


In actual use, PVC materials often add stabilizers, lubricants, auxiliary processing agents, pigments, impact resistance agents and other additives.


PVC material has non-flammability, high strength, weather resistance and excellent geometric stability.


PVC has strong resistance to oxidants, reducing agents and strong acids. However, it can be corroded by concentrated oxidizing acids such as concentrated sulfuric acid and concentrated nitric acid and is not suitable for contact with aromatic hydrocarbons and chlorinated hydrocarbons.


The melting temperature of PVC during processing is a very important process parameter. If this parameter is not appropriate, it will cause material decomposition.


The flow characteristics of PVC are quite poor, and its process range is very narrow. In particular, PVC materials with high molecular weight are more difficult to process (this material usually needs to be added with lubricant to improve flow characteristics), so PVC materials with low molecular weight are usually used.


The shrinkage rate of PVC is quite low, generally 0.2~0.6%.


Main varieties of modified PVC


(1) Vinyl chloride-vinyl acetate copolymer: manufacturing plastic floors, coatings, films, compression molded products, records and short fibers.


(2) Vinyl chloride-vinylidene chloride copolymer: The film made by this copolymer is non-toxic, transparent, has extremely low air and moisture permeability, and is an excellent food packaging material. This copolymer is also an excellent anti-corrosion material. The fiber made from it is called metachlorofluorocarbon, which can be used for fishing nets, seat cushion weaves and chemical filter cloths.


(3) Propylene-vinyl chloride or ethylene-vinyl chloride copolymer: a copolymer with a propylene content of about 10%, used for blow molding and injection molding. Compared with vinyl chloride-vinyl acetate copolymer, the processing temperature is lower, and the interval from the thermal decomposition temperature is large, the melt fluidity is good, it is non-toxic, transparent and can be used to make films and containers with high transparency.


(4) Vinyl chloride graft copolymer: Vinyl chloride graft copolymer based on ethylene-vinyl acetate resin, with excellent impact resistance, weather resistance and heat resistance, suitable for outdoor building materials .


(5) Chlorinated polyvinyl chloride: a thermoplastic resin derived from the chlorination of PVC, commonly known as perchlorovinyl, or CPVC for short, with a chlorine content of 61 to 68%, and not all hydrogen atoms are replaced by chlorine. White or light yellow-purple powder, solubility is better than PVC, soluble in acetone, chlorobenzene, dichloroethane and tetrachloroethane, heat resistance is 20-40℃ higher than PVC, cold resistance is better than polyvinyl chloride Vinyl chloride is about 25°C lower, is not easy to burn, and has excellent weather resistance, chemical resistance and water resistance.


Extrusion method can be used to produce pipes, mainly used as hot water supply pipes.


The solution of chlorinated polyvinyl chloride has good adhesion, film-forming and fiber-forming properties, and can be used in adhesives, varnishes and spinning. Adhesives are mainly used to bond PVC boards and their products.


The varnish film is corrosion resistant, soft, abrasion resistant and has high peel strength.


The silk spun from it is called perchlorin, which is stable to acid, alkali and salt, and is suitable for chemical corrosion resistant filter cloth, work clothes, screen mesh, fishing net and conveyor belt, etc.


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