PVC is difficult to ignite and can burn on flames, extinguishing immediately after leaving the flame; When burning, the flame is yellow, with a green bottom and white smoke emitting; When burned, plastic becomes soft and emits a pungent sour taste. Under ultraviolet light, hard polyvinyl chloride PVC produces light blue or purple white fluorescence, while soft polyvinyl chloride PVC produces blue white fluorescence. PVC is generally rarely modified with flame retardants, and calcium zinc stabilizers are the main additives added. The six major characteristics and uses of polyvinyl chloride PVC are as follows.
1、 Main characteristics
1. Forming and processing characteristics
Polyvinyl chloride (PVC) can be processed using various molding methods, such as rolling, extrusion, injection, blow molding, etc. Hard product processing often involves adding processing aids (1-5 parts) or impact modifiers (8-15 parts) to improve processing performance and increase impact strength; When processing soft products, plasticizers and other components need to be added.
(1) Firstly, mix Pvc Resin and plasticizers, stabilizers, colorants, and other additives in a certain formula proportion, and then plasticize the mixture. The kneading time is 0.5-1.5h, and the temperature is 125-150 ℃. The plasticized plastic can be directly used for molding and processing, or the plasticized material can be pulled into pieces or cut into particles. Dry mixing technology is the process of vigorously stirring dry Polyvinyl Chloride Resin and absorbing plasticizers at lower temperatures to obtain a dry, completely dispersed powder mixture. The advantage is that the Resin is not easily decomposed when kneaded at lower temperatures.
(2) The rolling temperature is usually 145~170 ℃, and the thickness of the processed film is 0.05~1.00mm. The technological improvement of large-scale production of soft films is to stretch the film several times in both vertical and horizontal directions on a stretching equipment after rolling, which can improve the transparency and strength of the film and produce a wide width film.
(3) Laminate the thin sheets with a thickness of about 0.5mm obtained by rolling, and stack them for lamination. The process conditions are: the hot plate temperature is 140-180 ℃, the pressure is 2-10MPa, and the time is 0.5-1h. The process conditions vary with the thickness of the plate.
(4) It is better to use loose polyvinyl chloride PVC resin for extrusion molding. The extrusion of hard products involves heating the kneaded mixture to 120-180 ℃, extruding and granulating under pressure, and then heating and extruding it into a certain shape of the product; The extrusion temperature of soft products should be appropriately reduced as the content of Plasticizer increases. The heating section temperature starts at 130 ℃ and reaches about 170-180 ℃ at the mold mouth. The screw speed is 10-70r/min, the length to diameter ratio of the screw is (15:1)~(24:1), and the compression ratio is (2:1)~(3:1). Soft products can use single screw extruders, while hard products often use double screw (or multi screw) extruders to improve mixing efficiency.
(5) Injecting hard polyvinyl chloride PVC products is difficult, so loose PVC resin is often used. The pre-treatment temperature is 110 ℃± 10 ℃, and the time is 1-1.5 hours. Injection molding temperature: 149~213 ℃ for hard products; The temperature for soft products is 160~196 ℃. Injection molding pressure: 69-276MPa for hard products; Soft products ranging from 55 to 172MPa.
PVC has the characteristics of good chemical resistance, flame retardancy, wear resistance, noise reduction, shock absorption, excellent electrical insulation performance, good mechanical properties, wide source of monomers, and low product cost. And multiple forming methods can be used for processing and forming. The main drawbacks are poor thermal stability, varying 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
Polyvinyl chloride (PVC) has high mechanical strength, and its wear resistance at room temperature exceeds that of vulcanized rubber. Its hardness and rigidity are superior to those of polyethylene.
3. Electrical performance
The electrical insulation performance of polyvinyl chloride PVC is good, which depends on the type and quantity of various additives in the formula, and is also related to the heating conditions. For example, when PVC decomposes due to heating, the presence of chloride ions reduces the electrical insulation performance.
4. Thermal performance
The embrittlement temperature of rigid polyvinyl chloride PVC is below -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 between 80 and 85 ℃. PVC will degrade above 150 ℃ in the air and release HC1. If exposed to 100 ℃ for a long time, it will also degrade without the addition of alkaline stabilizers; If it exceeds 180 ℃, it rapidly degrades, and PVC molecules contain a large amount of chlorine, making it difficult to ignite. However, the addition of plasticizers will reduce the difficult to ignite.
5. Aging resistance
PVC will degrade under the action of light, and the process is similar to thermal degradation. The most sensitive wavelength of PVC is 270-310nm. Resistant to non polar solvents such as petroleum and mineral oils. Products without plasticizers are resistant to most inorganic acids and alkalis, and are quite stable to salts. They can swell or dissolve in certain ketones, ethers, esters, and chlorinated hydrocarbons. Cyclohexanone and tetrahydrofuran are good solvents and are insoluble in water, alcohol, and gasoline. Usually, the addition of plasticizers makes PVC products susceptible to solvent attack and bacterial attack in hot and humid environments, so it is necessary to add mold inhibitors.
6. Physical performance
The water absorption of PVC is less than 0.5%, and its breathability and moisture permeability are very low.
PVC plastic is a multi-component material. Plasticizers, stabilizers, and other additives can be added according to different purposes. With different compositions, PVC products can exhibit different physical and mechanical properties, such as soft and hard products with or without (or with little) plasticizers.
2、 Purpose
PVC has a wide range of uses, involving various fields of the national economy. Hard polyvinyl chloride PVC is mainly used in the construction, electrical appliances, and packaging industries. Pipe materials account for over 50% of hard products and are commonly used for water pipelines and corrosion-resistant pipelines in the chemical industry. There are also hard boards, bars, storage tanks, doors and windows, etc. Soft polyvinyl chloride (PVC) is mainly coated with film, artificial leather, and wires and cables. Other products include sheet materials, hoses, shoe materials, various soft pipes, plates and other profiles, as well as injection, extrusion, blow molding products, etc.
