Polyvinyl chloride is a polymer formed by the polymerization of vinyl chloride monomer by free radicals, abbreviated as PVC in English. PVC is one of the earliest resin varieties to achieve industrialization, and it is the largest resin variety before the 60s of the 20th century, but it retreated to second place in the late 60s. In recent years, due to the abundance of PVC synthetic raw materials, the improvement of synthetic routes, the reduction of vinyl chloride monomer content in resins, and low prices, the application in chemical building materials and other fields is expanding day by day, and its demand has increased rapidly and its position has gradually strengthened.
The synthesis of PVC resin has been produced by acetylene calcium carbide method until the 50s of the 20th century, and in the late 50s, it turned to the ethylene oxidation method with sufficient raw materials and low cost. At present, more than 80% of the PVC produced by this method can be produced in the world. The synthesis of PVC resin is mainly based on the suspension method, and the output can account for 80%-85%, followed by the emulsion method. According to the size of the molecular weight, PVC can be divided into general-purpose type and high-polymerization type, and most of the commonly used resins are general-purpose type. According to the form of resin, it can be divided into powder and paste. Powders are often used in the production of calendered, injection molded and extruded products, while emulsions are often used in the production of artificial leather, wallpaper, children's toys and latex gloves. According to the different structures, it can be divided into two types: compact type and loose type, the loose type can absorb a large amount of plasticizer and is often used in the production of soft products, and the compact type has low ability to absorb plasticizer and is mainly used in the production of hard products.
The outstanding properties of PVC are high mechanical strength, high hardness, good chemical corrosion resistance, good electrical insulation, good printing and weldability, flame retardant, low price, adjustable hardness of products, etc., and are often used to replace metal and wood. The disadvantages of PVC are poor thermal stability, poor processability, poor impact resistance, poor aging resistance and cold resistance.
1. Structural properties of PVC plastics
(1) Structure of PVC
The molecular chain of PVC contains strong polar chlorine atoms, and the intermolecular force is large, which improves the mechanical properties such as rigidity and hardness of PVC products, and gives excellent flame retardant properties. Its molecular structure makes PVC easy to degrade under the action of light and heat, and the crystallinity of the product is low, generally only 5%-15%.
(2) Performance of PVC
(1) General properties PVC resin is a white or light yellow powder with a relative density of 1.35-1.45. The softness and hardness of its products can be adjusted by adding the number of plasticizers to make products with great differences in softness and hardness. Pure PVC has very little water absorption and air permeability.
(2) Mechanical properties PVC has high hardness and mechanical properties, and increases with the increase of molecular weight, but decreases with the increase of temperature. The number of plasticizers added to PVC is different, which has a great impact on the mechanical properties, and generally decreases with the increase of plasticizer content. The wear resistance of PVC is average, the static friction coefficient of rigid PVC is 0.4-0.5, and the dynamic friction coefficient is 0.23.
(3) Thermal properties The thermal stability of PVC is very poor, pure PVC resin begins to decompose at 140 °C, and accelerates rapidly to 180 °C, while the melting temperature of PVC is 160 °C, so pure PVC resin is difficult to process by thermoplastic method. PVC has a small coefficient of linear expansion. It is flame retardant, and its oxidation index is as high as 45 or more.
(4) Electrical properties PVC is a polymer with good electrical properties, but due to its large polarity, its electrical insulation is not as good as PE and PP; the electrical properties of PVC are greatly affected by temperature and frequency, and are generally only suitable for medium and low voltage and low frequency insulation materials.
(5) Environmental performance PVC is resistant to most inorganic acids (except oleum acid and concentrated nitric acid), alkali, most organic solvents and inorganic salts. PVC swells or dissolves in esters, ketones, aromatics and halogenated hydrocarbons, the best of which are tetrahydrofuran and cycloethyl ketone. PVC is not well tolerant to light, oxygen, and heat, and is prone to degradation.
2. Molding and processing of PVC plastics
(1) Processing characteristics of PVC
(1) The thermal stability of PVC processing is not good, the melting temperature is higher than the decomposition temperature, and it is difficult to process by melting and plasticizing without modification. The modification method is to add a stabilizer to it to increase its decomposition temperature and make it reach above the melting temperature, and the other is to add a plasticizer to reduce its melting temperature and make it lower than the decomposition temperature.
There are four types of heat stabilizers for PVC, namely lead salts, organotins, metal soaps and rare earths. Lead salts are the most commonly used, but can not be used for non-toxic and transparent products; organotin has good stabilization effect, transparent, toxic or non-toxic, and the use is limited by high price; metal soaps have a general stabilization effect, divided into transparent or opaque, toxic or non-toxic, rarely used alone, often used in compounds, mostly used in soft products; rare earth stabilizers are the latest varieties, with the advantages of transparency and non-toxicity, and have great prospects for development. PVC heat stabilizer can be used for both hard and soft products, and the dosage in hard products is greater than that in soft products.
The most commonly used main plasticizers for PVC are phthalates, such as DOP, DBP, DIDP, etc., and auxiliary plasticizers include DOA, DOS, chlorinated paraffins, epoxy soybean oil, petroleum esters, phosphates, etc. Plasticizers are mainly used for soft products, and more than 25 parts must be added. Only add less than 5 servings of hard products.
The temperature control of PVC processing should be precise and the processing time should be as short as possible.
(2) The fluidity of PVC melt is not good, the melt strength is low, and it is easy to produce melt crushing and rough surface of products, especially hard products, which are more prominent. To improve flowability, additives must be added, the most commonly used being ACR (methyl methacrylate copolymer).
(3) The PVC melt has a large tendency to adhere to metals, and the friction between the melts and between the melts and the processing equipment is large, so lubricants need to be added to overcome the frictional resistance. According to the compatibility between lubricants and PVC resins, they can be divided into internal lubricants (high compatibility) and external lubricants (poor compatibility).
(4) PVC melt is a non-Newtonian fluid, and the viscosity of the melt is sensitive to the shear rate. Therefore, for heat-sensitive PVC resin, the viscosity can be reduced during processing, and the purpose can be achieved by increasing the speed of the screw and adjusting the temperature as little as possible.
(5) PVC needs to be dried before processing, the temperature is 110°C, 1-1.5h.
(6) There are many components in the PVC formula, which should be fully mixed and evenly mixed. At the same time, we should pay attention to the feeding sequence, the filler with large oil absorption should be added later to prevent oil absorption, and the plasticizer should be added last to prevent the dispersion of other components. Second, the mixing temperature should be controlled, generally around 110 °C.
(7) PVC will accelerate the degradation of metal ions, magnetic separation should be carried out before processing, and the equipment should not have rust.
(2) Processing method of PVC
(1) Extrusion can be used to produce films, sheets, plates, tubes, rods, profiles, wires and other products.
(2) Injection molding can be used to produce sandals, shells, pipe fittings, valves, pumps and other products.
(3) Calendering can be used to produce films, sheets, boards, artificial leather, wallpaper and other products.
(4) Pressing The pressing method is mostly used for thermosetting plastics, but PVC plastics are also commonly used for pressing molding, mainly used for the production of simple shape products such as shoe soles, hard boards and turnover boxes.
(5) Plastic paste molding: PVC paste resin is applied to the substrate, and embossed and cooled after full melting. The specific coating methods include scraping method, rotational molding method and dipping method.
3. Modified varieties of polyvinyl chloride plastics
(1) High degree of polymerization PVC
The structure of high-polymerization PVC is basically the same as that of ordinary PVC, but the differences are that its molecular weight is large, the molecular chain length is long, the regularity and crystallinity of the chain are increased, and the winding points between the molecular chains are increased, and it has a similar cross-linked structure. The properties of high polymerization PVC are as follows:
It has a strong ability to absorb plasticizers and can be mixed with up to 150 parts of plasticizers to make soft products.
Excellent mechanical properties, high tensile strength and tear strength.
The permanent compression set is small, only 35%-60%, while ordinary PVC is more than 65%, and the resilience is high, generally up to 40%-50%.
The hardness of the product can be adjusted arbitrarily in the range of Shore A40-95, and it is less affected by temperature, and the use conditions are relatively wide.
Excellent heat, cold and aging resistance.
Good abrasion resistance, twice as high as ordinary PVC.
The processing performance of high-polymerization PVC is poor, the melting temperature is about 10°C higher than that of ordinary PVC, and the melt viscosity is large. Specific processing methods, injection molding can produce high-grade soles, sandals, gaskets, etc., extrusion can produce heat-resistant and cold-resistant cables, pressure-resistant pipes, automobile and refrigerator sealing strips, calendering can produce high-grade artificial leather, waterproof membrane and geomembrane, etc.
(2) Chlorinated polyvinyl chloride
Chlorinated polyvinyl chloride (CPVC) has a larger chlorine content than PVC, enhances the polarity, and restricts the movement of the backbone of macromolecules, so that the heat resistance, electrical insulation, flame retardancy, corrosion resistance, and mechanical properties of the product can be improved. However, thermal stability, melt fluidity and impact deteriorate
CPVC can be processed alone or blended with CPE, EVA, ABS, etc., to improve processability and brittleness of products. CPVC is currently mainly used for flame-retardant and heat-resistant pipes.
(3) PVC alloy
PVC alloy is mainly a blend of elastomer impact modified resins, specifically ACR, CPE, MBS, EVA, ABS, neoprene and nitrile rubber, among which ACR and CPE have the best effect and are the most commonly used, MBS is only used in transparent products.
4. The application range of PVC plastics
(1) Rigid PVC products
Pipes can be used for water pipes, water pipes, gas pipes, threading pipes, etc.
Profiles can be used for doors and windows, decorative panels, wood lines, furniture and stair handrails.
Plates include corrugated boards, dense boards and foam boards. It is used for wall panels, ceilings, shutters, floors, decorative materials, furniture materials and chemical anti-corrosion storage tanks.
Sheets are used for blister products.
Silk is used for window screens, mosquito nets, ropes, etc.
Bottles are used for food, medicine, cosmetics and other packaging.
Injection molded products pipe fittings, valves, various types of housings, electrical housings, etc.
(2) Soft PVC products
Film: Agricultural greenhouse film, packaging film, daily decorative film, raincoat film, skin film, etc.
Cables are used for medium and low voltage insulation and sheathed cable materials.
Footwear Sole and upper material.
Leather: artificial leather, floor leather, etc.
Others: Soft transparent tubes, gaskets, etc.
(3) PVC paste products
Artificial leather, floor leather, wallpaper, latex gloves, toys, gaskets, etc.
