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1、 Comparative analysis of PVC process and indexes between calcium carbide process and ethylene process
1. Technical analysis of two raw material routes for vinyl chloride production
(1) Brief introduction of calcium carbide acetylene process
In this method, vinyl chloride (VCM) is directly synthesized by the addition of acetylene and hydrogen chloride in the presence of mercuric chloride catalyst, and then polyvinyl chloride is prepared by polymerization. Its production process can be divided into acetylene preparation and purification, vinyl chloride synthesis, product refining and vinyl chloride polymerization. This method has the advantages of simple process and equipment, low investment and high yield; However, due to high energy consumption, high raw material cost and high toxicity of catalyst mercury salt, it is restricted by environmental protection.
(2) Brief introduction of ethylene oxychlorination process
At present, the popular ethylene oxychlorination process for VCM production in China consists of 8 units, namely direct chlorination of ethylene, oxychlorination of ethylene, dichloroethane (EDC) rectification, EDC cracking, HCl hydrodealkyne, VCM refining, wastewater treatment and incineration. Ethylene chlorination can be divided into low temperature method (50 ℃), medium temperature method (90 ℃) and high temperature method (120 ℃).
The main advantage of ethylene oxychlorination is that hydrogen chloride produced by the thermal cracking of dichloroethane is used as chlorinating agent, so that chlorine can be fully utilized. Because the calcium carbide acetylene process is relatively simple and the ethylene process is long, the investment is large, but the chlorine of the latter can be fully utilized, and the "three wastes" can be treated without discharge.
2. Quality index comparison and impurity influence analysis of VCM products obtained from two raw material routes
Water content in vinyl chloride produced by calcium carbide process ≤ 500 × 10- 6 ～ 600 × 10 - 6, water content of vinyl chloride by ethylene method ≤ 100 × 10-6 is 4 ~ 5 times higher, and the former is difficult to produce high-quality PVC products.
There are many kinds of impurities in monomer vinyl chloride, and the types of impurities in vinyl chloride obtained from the two raw material routes are different. VCM produced by ethylene method contains a little more impurities than VCM produced by calcium carbide method.
3. Influence analysis of VCM impurities from two raw material routes
For example, there are trace acetylene, vinyl acetylene and other acetylene impurities in VCM, which have a negative impact on the thermal stability of PVC and become a weak link in the degradation and removal of HCl. When the acetylene content is too high, the thermal stability of PVC will be reduced under the synergistic action of acetaldehyde and iron. Therefore, some foreign companies limit the acetylene content to less than 0.5 mg / kg and the iron content to less than 0.2 mg / kg.
The amount of acetylene in vinyl chloride by calcium carbide method is more than that by ethylene oxychlorination method. If acetylene and vinyl acetylene are combined, the latter is slightly more than the former. According to the actual analysis, the acetylene content in the two methods has no effect on the polymerization induction period and polymerization degree.
Generally, when the content of high boiling matter is high, it will significantly affect the polymerization degree and reaction speed of PVC. In addition, high boiling matter will also affect the quality indexes such as adhesive kettle and "fish eye". Therefore, in industrial production, the content of high boiling matter in monomer is generally controlled below 100 mg / kg. The so-called high boiling matter is 1,1-dichloroethane, 1,2-dichloroethane, 1,1,2-trichloroethane, acetaldehyde, etc.
The existence of iron in the monomer will prolong the polymerization induction period and slow down the reaction speed; The second is to make the thermal stability of the Resin worse and the product colored. In addition, divalent iron (Fe2 +) will react with organic peroxide initiator to consume an additional part of initiator and prolong the polymerization time. In order to control the iron content in the monomer, on the one hand, pay attention to the ACID content and moisture content in VCM, on the other hand, the pipes and storage equipment conveying the monomer should be made of stainless steel, enamel and other materials.
Vinyl PVC is generally considered to be superior to calcium carbide PVC in quality.
2、 Performance comparison of PVC materials by calcium carbide method and ethylene method
1. Molecular weight and molecular weight distribution of Pvc Resin by calcium carbide method and ethylene method
The number average and weight average molecular weight and number average and weight average degree of polymerization of PVC Resin by calcium carbide method were lower than those of PVC resin by ethylene method; The molecular weight distribution of PVC resin by calcium carbide method is wider than that by ethylene method. The differences in molecular weight and distribution between calcium carbide and ethylene PVC resins will produce great differences in rheological and mechanical properties.
With the increase of molecular weight of PVC resin, the degree of attraction or entanglement between molecular chains increases accordingly, the glass transition temperature increases, the mechanical strength of products increases accordingly, and the thermal deformation temperature increases. Generally, with the increase of molecular weight or polymerization degree of PVC resin, the apparent viscosity of PVC melt also increases and the fluidity becomes worse. It is necessary to increase the processing temperature accordingly. Increasing the processing temperature too high will cause resin degradation.
The processing and product properties of PVC depend not only on the average molecular weight, but also on the molecular weight distribution of PVC. Generally speaking, PVC resin with narrow molecular weight distribution has good molding processing performance, not only because it is necessary to maintain the uniformity of processing performance and product performance, but also because abnormal structures such as double bonds are mostly concentrated in low molecular weight components. Wide molecular weight distribution will significantly reduce its thermal stability, heat deformation temperature, electrical insulation, mechanical strength and aging resistance; In addition, it can make PVC resin difficult to plasticize evenly under normal processing conditions, so that the products have serious internal quality problems. Therefore, the molecular weight and distribution of PVC resin should be strictly controlled in the actual production process of PVC resin.
2. Rheological properties
The rheological properties of two kinds of PVC dry mixtures were analyzed by torque rheometer. Compared with ethylene PVC dry mixture, calcium carbide PVC dry mixture has larger plasticizing torque, shorter plasticizing time and smaller equilibrium torque, which shows that the plasticizing performance of calcium carbide PVC resin is better than ethylene PVC resin. This is because the molecular weight of calcium carbide PVC resin is smaller than that of ethylene PVC resin, and its molecular weight distribution is wider than that of ethylene PVC resin.
3. Mechanical properties
The difference of mechanical properties between calcium carbide PVC sample and ethylene PVC sample mainly focuses on elongation at break. Due to the high degree of polymerization and molecular weight of Vinyl PVC resin, the elongation at break of its sample is large. Considering comprehensively, the mechanical properties of PVC samples by calcium carbide method and ethylene method are almost the same.
4. Conclusion
For the PVC resin samples investigated in the test, the conclusions are as follows:
(1) The molecular weight of calcium carbide PVC resin is lower than that of ethylene PVC resin, and its molecular weight distribution is wider than that of ethylene PVC resin.
(2) The plasticizing performance of PVC dry mixture by calcium carbide method is better than that of PVC dry mixture by ethylene method.
(3) The mechanical properties of PVC resin by calcium carbide method and ethylene method are almost the same.
3、 Comparison of PVC application fields between calcium carbide process and ethylene process
1. Main uses of PVC:
(1) PVC profile
Profile and profile are the areas with the largest consumption of PVC in China, accounting for about 25% of the total consumption of PVC. They are mainly used to make doors and windows and energy-saving materials. In recent years, their application has still increased significantly throughout the country. In developed countries, the market share of plastic doors and windows is also the highest, such as 50% in Germany, 56% in France and 45% in the United States.
(2) PVC pipe
Among many PVC products, PVC pipeline is the second largest consumption field, accounting for about 20% of its consumption. In China, PVC pipe is developed earlier than PE pipe and PP pipe, with many varieties, excellent performance and wide application range, which occupies an important position in the market.
(3) Polyvinyl chloride film
The consumption of PVC in the field of PVC membrane ranks third, accounting for about 10%. 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.
(4) PVC hard materials and plates
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 for 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 plate can be cut into the required shape, and then welded into various chemical corrosion-resistant storage tanks, air ducts and containers with PVC electrode and hot air.
(5) PVC general soft goods
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.
(6) PVC packaging materials
PVC products are mainly used for packaging various containers, films and hard pieces. PVC containers mainly produce mineral water, beverage and cosmetic bottles, and are also used for the packaging of refined oil. PVC film can be used for CO extrusion with other polymers to produce low-cost laminated products and transparent products with good barrier. PVC film can also be used for stretch or heat shrinkable packaging, and for packaging mattresses, cloth, toys and industrial commodities.
(7) Pvc wall panels and floors
Pvc wall panels are mainly used to replace aluminum wall panels. In addition to a part of PVC resin, the other components of PVC floor tiles are recycled materials, adhesives, fillers and other components, which are mainly used in the ground of airport terminal building and hard ground in other places.
(8) PVC consumer goods
Luggage bag is a traditional product made of PVC. PVC is used to make all kinds of imitation leather, luggage bag and sports products, such as basketball, football and football. It can also be used to make uniforms and belts for special protective equipment. PVC fabrics for clothing are generally absorbent fabrics (without coating), such as ponchos, baby pants, imitation leather jackets and various rain boots. PVC is used in many sports and entertainment products, such as toys, records and sporting goods. At present, PVC toys have a large growth. PVC toys and sporting goods have advantages because of their low production cost and easy molding.
2. Application field of PVC resin by calcium carbide method
(1) PVC building materials
Construction industry is one of the important fields of PVC application. In foreign countries, PVC for building materials accounts for 65% of the total PVC consumption, and the penetration rate of PVC indoor drainage pipes reaches more than 90%. In China, with the development of economy, the market share of PVC doors and windows, PVC building pipes and PVC waterproof materials is gradually increasing. Especially in the north, PVC plastic doors and windows are popular because of their excellent thermal insulation performance, good mechanical strength performance and many colors.
(2) PVC plastic products for industrial use
PVC pipe has excellent properties of acid resistance, alkali resistance and organic solvent resistance, so it is more suitable for conveying chemical media than other materials. At present, PVC pipes are mainly used for medium transportation in chemical, mining, petroleum and other industries in China. In addition, PVC pipe can also be used for slurry transportation of coal mining, mining and underground transportation of oil fields, as well as sewage pipe, ventilation pipe, conductor and cable pipe, etc.
(3) Packaging materials and other PVC films
It is an inevitable trend for plastic products to replace wood and other traditional materials as the main packaging materials in the future. Economic development will drive the rapid development of industrial products, chemical products, electromechanical products, washing supplies, cosmetics, beverages, packaging and transportation materials, clothing and other industries, resulting in a great demand for packaging products such as composite films, packaging films, containers and turnover boxes. It is estimated that the total demand for plastic packaging materials and products will reach about 6 million tons in 2005, and PVC will play a great role in this field and occupy a large share.
3. Application field of Vinyl PVC resin
According to the existing literature and data query: the application fields of ethylene PVC and calcium carbide PVC are not very different, but the process routes are different. Calcium carbide PVC is mainly used for pipes and building materials, while ethylene PVC is mainly used for high-grade profiles, transparent sheets Film and automotive interior products (such as roof liner, carpet, car inner covering, insulating pipe, refueling funnel, door trim strip, bellows, sealing strip, seat, instrument panel skin, etc.). According to the comparative analysis of the quality indexes of VCM products obtained from the two raw material routes of calcium carbide method and ethylene method, vinyl VCM contains slightly more impurities than calcium carbide VCM. However, it is generally recognized in the industry that Vinyl PVC has poor quality It is superior to calcium carbide PVC and affects the application field of calcium carbide PVC in high-end products. According to the data analysis, there are mainly the following reasons:
(1) Effect of impurities in VCM on the quality of PVC
High moisture, acetylene, 1,1-dichloroethane and other impurities in VCM obtained by calcium carbide method significantly reduce the thermal stability, whiteness and aging resistance of PVC resin quality by calcium carbide method, and then affect the color and other quality problems of PVC products.
(2) Influence of environmental protection factors
At present, most PVC produced by domestic enterprises belong to general-purpose type, and the degree of polymerization is basically concentrated in 650 ~ 1300. Mercury chloride catalyst is used in the production of PVC by acetylene method. Shenzhen entry exit inspection and Quarantine Bureau has studied whether there is residual mercury in PVC resin by acetylene method. The residual amount of mercury in PVC resin is determined by hydride generation atomic fluorescence spectrometry. The test result is that the mercury content of 1 sample in 7 samples is 0.6mg (1kg PVC sample) [3], which will affect the application of PVC by acetylene method. Ethylene PVC will maintain great advantages in subsequent application research and development. Production enterprises can develop new products of high polymerization degree PVC, high transparent PVC and Toughened PVC to meet user requirements and improve the added value of products.
(3) Influence of material properties
The molecular weight of calcium carbide PVC resin is lower than that of ethylene PVC resin, and its molecular weight distribution is wider than that of ethylene PVC resin. The processing and product properties of PVC depend not only on the average molecular weight, but also on the molecular weight distribution of PVC. Generally speaking, PVC resin with narrow molecular weight distribution has good molding processability, not only because it is necessary to maintain the uniformity of processability and product performance, but also because abnormal structures such as double bonds are mostly concentrated in low molecular weight components. The wide molecular weight distribution of calcium carbide PVC will significantly reduce its thermal stability, heat deformation temperature, electrical insulation, mechanical strength and aging resistance; In addition, it can also make PVC resin difficult to plasticize evenly under normal processing conditions, so that the products have serious internal quality problems. How to produce PVC resin with narrow PVC molecular weight distribution and eliminate the quality problems caused by wide PVC molecular weight distribution is one of the directions for calcium carbide PVC enterprises to improve product quality, and it is also an important subject we need to research and develop.
(4) Influence of PVC resin processing level
China's PVC resin processing enterprises are mixed, there are many small workshops and small enterprises, and the level of production and processing technology is relatively low. According to incomplete statistics, by 2010, China's total PVC production capacity will reach about 20 million tons, of which calcium carbide process accounts for about 75%. These small enterprises are for the purpose of short-term profit, and there are a large number of PVC enterprises using calcium carbide method with relatively low price. The investment in R & D is very small, and the defects of calcium carbide PVC resin can not be solved in time in the production and processing process, resulting in the low grade of calcium carbide PVC resin products, which affects the application of calcium carbide PVC resin in high-grade products. For example, under the same formula, the transparent sheet processing manufacturers reflect that the products processed by ethylene resin show a blue feeling, and the products processed by acetylene method have some black hair; The profile processing manufacturer reflects that the products processed by acetylene resin are yellow, etc.
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