Polyvinyl chloride (PVC) is an important organic synthetic material in China. Its products have good physical and chemical properties and are widely used in industry, construction, agriculture, daily life, packaging, electric power, public utilities and other fields.
From the perspective of product classification, PVC belongs to the synthetic resin category among the three synthetic materials (synthetic resin, synthetic fiber and synthetic rubber), including five general resins, polyethylene PE, polyvinyl chloride PVC, polypropylene PP, polystyrene PS and ABS.
1、 Introduction to PVC
Polyvinyl chloride is a non-toxic, odorless white powder. High chemical stability and good plasticity. Except for a few organic solvents, it can withstand hydrochloric acid of any concentration, sulfuric acid of less than 90%, nitric acid of 50 ~ 60% and caustic soda of less than 20% at room temperature, and is also quite stable for salts; PVC has poor thermal stability and light resistance. It can start to decompose and release hydrogen chloride (HCl) gas above 140 ℃, resulting in discoloration of PVC. It has excellent electrical insulation and generally will not burn. It can burn on the flame and release HCl, but it will self extinguish when leaving the flame. It is a kind of "self extinguishing" and "flame retardant" substance. It is mainly used in the production of transparent sheets, pipe fittings, gold cards, blood transfusion equipment, soft and hard pipes, plates, doors and windows, profiles, films, electrical insulation materials, cable sheaths, blood transfusion materials, etc.
Polyvinyl chloride is formed by free radical polymerization of vinyl chloride monomer, and the degree of polymerization n is generally in the range of 500 ~ 20000
2、 Classification and representation of polyvinyl chloride
1. Classification of polyvinyl chloride
According to different production methods, PVC can be divided into general-purpose PVC resin, high degree of polymerization PVC resin and cross-linked PVC resin. General purpose PVC resin is formed by polymerization of vinyl chloride monomer under the action of initiator; High polymerization degree PVC resin refers to the resin polymerized by adding chain growth agent in vinyl chloride monomer polymerization system; Crosslinked PVC resin is a resin polymerized by adding crosslinking agent containing diene and polyene into vinyl chloride monomer polymerization system.
According to the method of obtaining vinyl chloride monomer, it can be divided into calcium carbide method, ethylene method and imported (EDC, VCM) monomer method (traditionally, ethylene method and imported monomer method are collectively referred to as ethylene method).
According to polymerization methods, PVC can be divided into four categories: suspension PVC, lotion PVC, bulk PVC and solution PVC. Suspension PVC is the largest variety in production at present, accounting for about 80% of the total output of PVC.
2. Nomenclature of polyvinyl chloride
Suspension PVC is divided into six models according to absolute viscosity [1]: XS-1, xs-2... Xs-6; XJ-1、XJ-2……、XJ-6。 The meaning of each letter in the model: x-suspension method; S-loose type; J-tight type; The following chart shows the characteristics of domestic suspension PVC.
Polyvinyl chloride produced by lotion polymerization is called lotion polyvinyl chloride. It is a paste resin with high molecular weight and fine particles. The model of PVC by lotion method is rh-x-y, in which r- lotion method; H-paste resin; Absolute viscosity of x-resin alkene solution; Y-paste viscosity [2]. X is divided into types 1, 2 and 3. The absolute viscosity of type 1 is 2.01-2.4mpa · s, the absolute viscosity of type 2 is 1.81-2.00mpa · s, and the absolute viscosity of type 3 is 1.60-1.80mpa · s. Y is divided into I, II and III. The viscosity of I paste is not greater than 3000mpa · s, that of II paste is 3000 ~ 7000mpa · s, and that of III paste is 7000 ~ 10000mpa · s.
Bulk polyvinyl chloride (bulk polymerization) has been produced in China. Sichuan Yibin Tianyuan and Inner Mongolia Haiji two enterprises. The product transparency and insulation of this method are higher than those of other methods. Solution polymerized polyvinyl chloride resin is mostly used for surface coating. Low temperature polymerization refers to suspension polymerization, lotion polymerization or bulk polymerization at a low temperature of 20 ~ 30 ℃ or below 0 ℃. Low temperature polymerized PVC has high molecular weight, high crystallinity, good structural regularity, high glass transition temperature, good heat resistance and solvent resistance. However, it is more difficult to process than ordinary PVC and has slightly lower impact strength. It is used as fiber and special plastic products.
[1] Refers to the viscosity of 1% 1,2-dichloroethane solution measured at 20 ℃.
[2] It refers to the viscosity measured when PVC and dioctyl phthalate are 1:1 and placed at 25 ℃ for 24 hours.
3、 Comparison of PVC production processes
China's PVC output has developed rapidly since the 1990s, but it still can not catch up with the faster development of PVC product processing demand, and the self-sufficiency rate can only be maintained at about 70%. The vigorous demand, the shortage of domestic ethylene resources, the decline of import volume after the final decision of anti-dumping, and the rising price of international crude oil and petrochemical products have not only increased the production cost of ethylene method, but also made calcium carbide method the preferred process for many enterprises.
With the high international crude oil price in recent five years, the process route of making PVC by oil and gas route will be affected by the limitation of ethylene supply and cost. In recent period, the market competition is not as low as that of calcium carbide PVC. At present, PVC production in China is mainly based on calcium carbide method. According to the analysis of China's existing energy structure and the domestic competitiveness of PVC industry, the construction of calcium carbide PVC project in the western region (including Ningxia) still has competitive advantages in the future, but it is not optimistic.
1. The raw material route of calcium carbide process is different from that of ethylene process. The former consumes about 3000kwh / T of electricity (including electricity for chlorine and hydrogen), about 1.5t/t of calcium carbide, and the latter consumes about 3000kwh / T of electricity and about 0.5t/t of ethylene. Calcium carbide consumes about 3500kwh / T. therefore, calcium carbide PVC actually takes electricity as the main raw material, and the comprehensive power consumption is about 8250kwh / T. Therefore, power supply and electricity price are the key to PVC production by calcium carbide method. From the perspective of self provided thermoelectric balance, the production cost of PVC will be greatly reduced. When the construction period cannot be synchronized, the generator set can be purchased and transformed in advance to supply power and steam for the phase I project. Considering that the power cost of normal power generation profit (about 8%) is about 50% lower than the online power supply price, so as to ensure the goal of controlling the cost of calcium carbide PVC in a lower range, which has considerable market price competitiveness compared with ethylene PVC.
2. Compared with Vinyl PVC, calcium carbide process has a by-product in the former production: calcium carbide slag slurry, that is, the saturated solution containing about 20% Ca (OH) 2, which can be used as the desulfurizer of power generation flue gas, and the rest can be pressure filtered. More than 296% of the solid phase of Ca (OH) is used to produce cement, and all the liquid phase containing about 25% Ca (OH) is recycled. Now there is a dry acetylene production technology worthy of attention, which can better improve the impact of the production process on the social environment, especially save water, reduce the emission of solid and gas waste and reduce the use of electricity.
3. Large scale and joint production are the basic conditions for the realization of clean technology. In fact, the treatment technology of PVC three wastes by calcium carbide process is basically mature. However, in the past, calcium carbide PVC enterprises were mainly distributed in the western region, with small scale, poor strength, backward technology and no economic ability to solve it. Large scale and joint production not only have the conditions for comprehensive utilization, turning waste into treasure and greatly reducing the treatment of three wastes, but also provide products with considerable benefits for the society. If calcium carbide slag is used as cement, the scale of 200000 tons / year can guarantee the principal and the scale of 1 million tons / year can be profitable. The calcium carbide PVC production enterprises in the West are expected to become the pioneers of calcium carbide PVC cleaner production process. In addition, starting from the concept of sustainable development, technologies such as membrane filtration and biochemical treatment are adopted to meet the discharge standards.
PVC production routes, whether calcium carbide process or ethylene process, are finally polymerized from vinyl chloride monomer to produce PVC. There are five main synthetic methods of vinyl chloride monomer: acetylene method, combined method, olefin method, ethylene oxychlorination method and one-step ethane oxychlorination method. At present, acetylene method (calcium carbide method) and ethylene oxychlorination method are widely used in China. At present, most of China's chlor alkali industry uses calcium carbide method to produce PVC to balance chlorine. Although this method consumes high energy, it has the characteristics of low investment, simple equipment, high conversion rate and product purity. Therefore, calcium carbide method accounts for a large proportion in PVC production in China.
4、 Main uses of PVC
（1） PVC general soft products
It can be extruded into hoses, cables, wires, etc. by extruder; Plastic sandals, soles, slippers, toys, auto parts, etc. can be made by injection molding machine with various molds.
（2） PVC film
After PVC is mixed and plasticized with additives, a transparent or colored film with a specified thickness is made by a three roll or four roll calender. The film is processed by this method to become a calendered film. You can also cut and heat seal packaging bags, raincoats, tablecloths, curtains, inflatable toys, etc. The wide transparent film can be used for greenhouse, plastic greenhouse and plastic film. The biaxially stretched film has the characteristics of thermal shrinkage and can be used for shrinkage packaging.
（3） PVC coated products
Artificial leather with substrate is made by applying PVC on cloth or paper and plasticizing it above 100 ℃. PVC and additives can also be calendered into a film, and then pressed with the substrate. The artificial leather without substrate is directly calendered into soft sheets of a certain thickness by calender, and then pressed with patterns. Artificial leather can be used to make suitcases, bags, book covers, sofas and car cushions, as well as floor leather, which can be used as paving materials for buildings.
（4） PVC foam products
When mixing soft PVC, add an appropriate amount of foaming agent to make sheets, which are foamed into foamed plastics, which can be used as foam slippers, sandals, insoles, and shockproof cushioning packaging materials. It can also be formed into low foaming rigid PVC plate and profile based on extruder, which can replace wood for trial. It is a new type of building material.
（5） PVC transparent sheet
PVC is added with impact modifier and organotin stabilizer, which is mixed, plasticized and calendered to become a transparent sheet. Thermoforming can be used to make thin-walled transparent containers or vacuum blister packaging. It is an excellent packaging material and decorative material, such as moon cake packaging box.
（6） PVC paste resin
PVC is dispersed in liquid plasticizer to swell and plasticize to form plasticized sol. Usually, lotion or micro suspension resin is used, and stabilizer, filler, colorant, etc. are added. After full mixing and degassing, PVC paste is prepared, and then processed into various products by feeding, dipping, casting or plastic lining. Such as clothes hanger, tool handle, Christmas tree, etc.
（7） PVC hard board and plate
PVC is added with stabilizer, lubricant and filler. After mixing, hard pipes, special-shaped pipes and corrugated pipes of various diameters can be extruded by extruder, which can be used as downcomer, drinking water pipe, wire sleeve or stair handrail. The calendered sheets can be overlapped and hot pressed to make hard plates of various thicknesses. The PVC welding rod can be used to cut and weld various corrosion-resistant plates into various shapes, and then the PVC welding rod can be used.
（8） PVC others
Doors and windows are assembled with hard special-shaped materials. In some countries, it has occupied the market of doors and windows together with wooden doors, windows and aluminum windows; Imitation wood materials and steel building materials (North and seaside); Hollow container.
