Polyvinyl chloride (PVC) is nonflammable and can burn on the flame and extinguish when it is out of fire; When burning, the flame is yellow, and the lower end is green, emitting white smoke; When burning, the plastic becomes soft and emits a pungent sour smell. Under ultraviolet light, hard PVC produces light blue or purple white fluorescence, while soft PVC produces blue white fluorescence. PVC is seldom modified with flame retardant, and calcium zinc stabilizer is the main additive. The six characteristics and uses of PVC are as follows.
1、 Main characteristics
1. Forming processing characteristics
PVC can be processed by a variety of molding methods, such as calendering, extrusion, injection, blow molding, etc. Processing aids (1-5 phr) or impact modifiers (8-15 phr) are often added in hard products processing to improve processing performance and impact strength; Plasticizer and other components shall be added during the processing of soft products.
(1) In kneading, Pvc Resin, plasticizer, stabilizer, colorant and other additives shall be uniformly mixed in a certain proportion, and then the mixture shall be plasticized. The kneading time is 0.5~1.5h, and the temperature is 125~150 ℃. The plasticized plastic can be directly used for forming processing or the plasticized material can be pulled and pelleted. Dry mixing technology is to mix the dry PVC Resin vigorously and absorb plasticizer at a lower temperature to obtain a dry, completely dispersed powder mixture. The advantage is that the Resin is not easy to decompose when kneaded at a lower temperature.
(2) The rolling temperature is usually 145~170 ℃, and the film thickness is 0.05~1.00mm. The technical improvement of large-scale production of soft film is to make the film cold stretched several times in both vertical and horizontal directions at the same time on the stretching equipment after calendering, which can improve the transparency and strength of the film and produce a wide film.
(3) Laminate the thin sheets with a thickness of about 0.5mm obtained by rolling, and then laminated them. The process conditions are: hot plate temperature 140~180 ℃, pressure 2~10MPa, time 0.5~1h. The process conditions vary with the thickness of the plate.
(4) It is better to use loose PVC resin for extrusion molding. For the extrusion of hard products, the kneaded mixture is heated to 120~180 ℃, extruded under pressure and granulated, and then heated to extrude the products into a certain shape; The extrusion temperature of soft products should be appropriately reduced as the content of plasticizer increases. The temperature of the heating section starts from 130 ℃ to about 170~180 ℃ at the die mouth, the screw speed is 10~70r/min, the screw length diameter ratio is (15:1)~(24:1), and the compression ratio is (2:1)~(3:1). Single screw extruder can be used for soft products, and twin screw (or multi screw) extruder can be used for hard products to improve mixing effect.
(5) It is difficult to inject rigid PVC products, so loose PVC resin is often used. The pretreatment temperature is 110 ℃± 10 ℃ and the time is 1~1.5h. Injection molding temperature: 149~213 ℃ for hard products; 160~196 ℃ for soft products. Injection molding pressure: 69~276MPa for hard products; 55~172MPa for soft products.
PVC has the characteristics of good chemical resistance, fire resistance, abrasion resistance, noise elimination, shock absorption, excellent electrical insulation, good mechanical properties, wide source of monomers, low product cost, etc. And can use a variety of molding methods processing molding. The main disadvantages are poor thermal stability, different degrees of degradation caused by heating, and low softening temperature. Soft products also have the disadvantage of plasticizer migration, so they can only be used below 80 ℃.
2. Mechanical properties
PVC has higher mechanical strength, abrasion resistance at room temperature is better than vulcanized rubber, and hardness and rigidity are better than polyethylene.
3. Electrical performance
The electrical insulation performance of PVC is good, and its performance depends on the type and quantity of various additives in the formula, as well as the heating condition. For example, when PVC is decomposed due to heating, the electrical insulation performance is reduced due to the presence of chloride ions.
4. Thermal performance
The embrittlement temperature of rigid PVC is lower than - 50 ℃, and it becomes soft at 75~80 ℃. The glass transition temperature varies with the polymerization temperature. It is generally believed that the glass transition temperature is 80~85 ℃. PVC will degrade and release HC1 when it is higher than 150 ℃ in the air. If it is exposed to 100 ℃ for a long time, it will degrade without adding alkaline stabilizer; If it exceeds 180 ℃, it will degrade rapidly. PVC molecules contain a large amount of chlorine, making it flame retardant, but the addition of plasticizer will reduce the flame retardant.
5. Aging resistance
PVC degrades under the action of light, and its process is similar to that of thermal degradation. The most sensitive light wave length of PVC is 270~310nm. It is resistant to non polar solvents such as petroleum and mineral oil. The products without plasticizer are resistant to most inorganic acids and alkalis, and are quite stable to salts. They can swell or dissolve in some ketones, ethers, esters and chlorinated hydrocarbons. Cyclohexanone and tetrahydrofuran are good solvents, and are insoluble in water, alcohol and gasoline. Generally, the addition of plasticizer makes PVC products vulnerable to corrosion by solvent and bacteria in hot and humid environment, so it is necessary to add mold inhibitor.
6. Physical performance
The water absorption of PVC is less than 0.5%, and the air permeability and moisture permeability are very low.
PVC plastic is a kind of multi-component material. Plasticizer and stabilizer can be added according to different uses. With different compositions, PVC products can show different physical and mechanical properties, such as soft and hard products with or without plasticizer.
2、 Purpose
PVC is widely used in various fields of the national economy. Rigid PVC is mainly used in construction, electrical appliances and packaging industries. Pipes account for more than 50% of hard products, and are usually used as water transmission pipes and corrosion resistant pipes for chemical industry. There are also hard plates, bars, storage tanks, doors and windows, etc. Soft PVC mainly consists of film, artificial leather, wire and cable coating, etc. Other products include sheet, hose, shoe materials, various soft pipes, plates and other profiles, injection, extrusion, blow molding products, etc.
