PVC floor covering
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Polyvinyl chloride (IUPAC) is a widely used thermoplastic polymer, which is usually abbreviated as PVC. In terms of income, it is one of the most valuable products in the chemical industry. Worldwide, more than 50% of PVC products are used in buildings. As a building material, PVC is cheap, durable and easy to assemble. In recent years, PVC has replaced traditional building materials such as wood, concrete and clay in many fields.
PVC has many uses. As a hard plastic, it is used as a fixing device for vinyl siding, magnetic stripe cards, window profiles, phonograph records (this is the origin of the term vinyl records), pipes, pipes and conduits. This material is usually used in the plastic pressure piping system of water supply and sewage treatment industries because of its cheap characteristics and flexibility. PVC pipes are usually white, while ABS is usually gray, black and white.
By adding plasticizer, it can become softer and more elastic, and phthalate is the most widely used. In this form, it is used in clothing and interior decoration, making hoses and pipes, floor and roof membranes and cable insulation. It is also often used for figurines.
As shown in the figure, PVC is polymerized from monomer vinyl chloride. Because 57% of its mass is chlorine, producing a certain quality of PVC requires less oil than many other polymers.
history
PVC was accidentally discovered on at least two different occasions in the19th century, the first time was in 1835 by Henry Victor Raignault, and the second time was in 1872 by Eugen Bowman. In both cases, the polymer appears as a white solid in a vinyl chloride flask exposed to sunlight. At the beginning of the 20th century, Russian chemist Ivan Ostromislensky and German chemical company Griesheim-Elektron's Fritz Klatte both tried to use PVC (polyvinyl chloride) in commercial products, but the difficulty in processing this hard and sometimes fragile polymer hindered their efforts. In 1926, Waldo Semon and B.F. Goodrich developed a method to plasticize PVC by mixing PVC with various additives. The result is a more flexible and easy-to-process material, which soon gained a wide range of commercial applications.
Polyvinyl chloride is one of our most common synthetic materials. Polyvinyl chloride, usually called "PVC" or "vinyl", is a widely used resin with thousands of different formulations and configurations. In the United States, we produced more than10 billion pounds of PVC resin in 1992. In plastics, its quantity is second only to polyethylene. PVC is the most commonly used plastic in buildings, and 6.3 billion pounds of resin is used in 1992. Polyvinyl chloride compounds (resins combined with various additives) appear in various applications, such as sewer pipes, wire sheaths, floors and weather strips (see table).
Although some vinyl products, such as siding and flooring, have been criticized for a long time, the whole PVC industry has recently been attacked for environmental reasons. The most violent attack recently is not only against PVC, but also against the wider problem of chlorine use in industrial society. As reported in recent issues of EBN, Greenpeace calls for phasing out all chlorine-containing industries, including PVC, for a series of health and environmental reasons. Groups that are not as politicized as Greenpeace also openly oppose the use of chlorine, although they are not particularly strongly opposed to PVC. These include the International Joint Commission on the Great Lakes Region (IJC) and the American Public Health Association (APHA).
After all this publicity, many builders and architects began to question the wisdom of specifying PVC materials and looking for alternatives. However, in order to make an informed choice, especially in such a controversial debate, it is useful to know some background knowledge.
How is it made?
PVC consists of chlorine, carbon and hydrogen. Chlorine usually comes from the saline solution of common rock salt (sodium chloride). Chlorine is separated by electrolysis: a strong current passing through the liquid solution attracts sodium ions to the (negatively charged) cathode, while chlorine accumulates at the anode.
Until recently, electrolysis still needed to use liquid mercury as cathode, and a small amount of toxic mercury often polluted by-products and waste liquid. Most manufacturers no longer use mercury; However, in 1992, only 14% of chlorine production in the United States used mercury.
Today, the most common chlorine separation process used in 77% production in the United States depends on the diaphragm in the electrolytic cell. All new facilities have adopted an updated membrane-based method, because this method is more energy-saving than other systems and the by-products produced are more valuable. This membrane technology accounts for about 7% of chlorine production.
PVC industry is the largest industrial chlorine consumption industry in the world, using about 30% of chlorine production. The rest enter papermaking, pesticides, medicines and a large number of other products and processes. Some chlorine-based refrigerants and propellants (CFCs and HCFCs) were phased out for fear of damaging the ozone layer.
In addition to chlorine, the electrolysis of salt also produces caustic soda (sodium hydroxide), which is used to make soap, paper and man-made fibers and as a neutralizer in many other industries. Although chlorine was an unnecessary by-product in the production of caustic soda at first, the demand for chlorine increased so much that caustic soda became relatively cheap, which led to its use in more and more processes.
PVC resin contains 57% chlorine by weight. The rest is hydrogen and carbon, which come from fossil fuels: mainly natural gas and oil. Today, almost all PVC is made of ethylene, which is the preferred petrochemical product in many industrial processes. In the United States, ethylene is made by cracking ethane at about 800℃ in a reactor. Ethane is a kind of light hydrocarbon extracted in the process of natural gas refining. Many by-products of ethylene production (olefins, dienes and methane) are used in other industries. An old process for producing vinyl chloride by combining chlorine gas with acetylene is still used in one or two existing factories, although it no longer competes with ethylene-based processes, where a large amount of infrastructure investment locks manufacturers in the process.
Ethylene and chlorine combine to form 1, 2- dichloroethane (EDC), which is then converted into vinyl chloride. Vinyl chloride, commonly called VCM in industry (stands for "vinyl chloride monomer"), is a gas at normal temperature and pressure. The by-product of converting EDC into vinyl chloride is hydrochloric acid.
EDC is made from ethylene and chlorine by two processes: direct chlorination, using pure chlorine; And oxychlorination, wherein ethylene is combined with hydrochloric acid (see formula). The oxychlorination process is carried out at higher temperature, which produces much more toxic by-products than direct chlorination. This is mainly to make use of hydrochloric acid by-products produced when EDC is converted into vinyl chloride.
Polyvinyl chloride is made by combining vinyl chloride into chains or polymers. There are several different methods of polymerization, and each method gives the polymer different properties. So far, the most common is the suspension process. Vinyl chloride is stirred into water with a small amount of methyl cellulose and organic peroxide, which initiate polymerization and prevent aggregation of polymeric particles. During 6-8 hours, the contents of the polymerization chamber must be stirred vigorously. In addition, the process must be continuously cooled, because it generates about 660 btu/lb PVC. Cooling is accomplished by passing water through the side of the polymerization tank, and about 30 gallons of water are used for each pound of PVC produced.
When about 90% of vinyl chloride has been polymerized, the process is terminated. The remaining vinyl chloride monomer is pumped out by vacuum, and most of it is recovered. Until recently, traces of vinyl chloride remained in PVC materials. Recognizing that vinyl chloride is easy to leach into food or water from PVC containers, coupled with the discovery that it is carcinogenic, leads to strict control of the amount of residual vinyl chloride in PVC, especially for food and water containers.
In order to meet these requirements, PVC manufacturers added a separate stripping process to remove almost all residual monomers in PVC. After stripping, PVC exists as small particles suspended in water. These are spin-dried in a centrifuge and then air-dried before packaging.
In addition to suspension method, two other polymerization methods can also produce PVC for specific purposes. An emulsification process is used to produce finer PVC particles, which are used in vinyl paste-also known as plastisol-and some vinyl floors are made of plastisol. The large-scale process is simpler, although it is not flexible in adding copolymers (other plastic resins mixed with PVC to enhance certain properties). Bulk polymerization accounts for about 20% of PVC produced in America.
History of PVC industry
PVC was first produced in a laboratory in 1872. It began commercial production in the 1930' s, when people knew the technology of mixing it with plasticizer, and PVC appeared as a substitute for rubber. During World War II, when the shortage of materials limited the supply of traditional pipes, German scientists developed PVC pipes for water supply systems.
In the1950s and1960s, many American companies established facilities to polymerize vinyl chloride into PVC. At that time, polymerization was carried out in open barrels and required relatively little capital. Workers exposed to vinyl chloride in large quantities during processing are not considered dangerous, although they usually have anesthetic effects. "We used to joke about getting cheap pleasure from it," said Rudolph Dinin, a professor at the University of Massachusetts in Lowell who was working in the PVC production department.
In 197 1, a rare angiosarcoma of liver cancer was traced to the exposure of vinyl chloride among PVC workers, and the Occupational Safety and Health Administration (OSHA) established strict workplace exposure limits. These limitations make it necessary to completely change the manufacturing environment-all polymerization barrels must be sealed and controlled. These changing costs and the growth of economies of scale enjoyed by large factories eventually eliminated small producers, who either closed PVC production facilities or were acquired by large manufacturers.
Today, the PVC market in North America is controlled by about a dozen large manufacturers. Some of these companies, such as western oil companies, operate facilities at all stages of the process, from chlorine and ethylene production to final products. However, most companies buy some refined materials from other manufacturers. Dow Chemical Company produces a large amount of vinyl chloride and sells it to other companies, but it does not produce PVC itself.
additive
PVC resin alone is not so useful. It is relatively easy to mix with additives, but it lends PVC compounds with different properties. The actual PVC resin usually accounts for only about 70% of the final PVC product, sometimes as little as 35% or 40%. In the production process, PVC may be mixed with other polymer resins (copolymers) or with various additives mixed later. The most common additives include plasticizer and stabilizer. Plasticizer makes PVC have the flexibility of many vinyl products, and stabilizer reduces the degradation trend of PVC under various conditions. The process of mixing these additives with PVC is called mixing. Mixing can be done by PVC manufacturers, companies specializing in this process or manufacturers of final products.
plasticiser
Plasticizers contain a lot of chemicals, most of which come from fossil fuels. They are used in all PVC products that need flexibility, such as cables, hoses, gaskets and vinyl floors. Plasticizers are also used with other plastic resins, but the PVC industry consumes most of all plasticizers (about 80%). Although PVC has inherent fire resistance because of its high chlorine content, adding plasticizer will reduce this fire resistance, so it is necessary to add flame retardant.
The most common traditional plasticizer is called DOP or DEHP (di -2- ethylhexyl phthalate). The world produces about 9 million tons of DOP every year. DOP was identified as a suspected carcinogen in 1987, and its use in medical blood bags was suspended when it was found to leach into stored blood. People are also worried that DOP will be released into the environment. The US Environmental Protection Agency's Toxic Substances Emission List (TRI) reports that1100,000 pounds of DOP are released into the air in 199 1 year.
stabilizing agent
Stabilizers are added to PVC to reduce degradation, mainly due to heat or ultraviolet rays. The main chemical function of stabilizer is to prevent the formation of hydrochloric acid (or absorb any hydrochloric acid formed) in PVC, because acid will promote the degradation of materials.
Traditionally, heavy metals such as cadmium and lead have been used as stabilizers. Concerned about the toxicity of these elements, the industry has turned to substitutes in many applications. Nevertheless, a recent article in Plastic Engineering reported that 15% of all cadmium in the ash of municipal solid waste incinerator came from PVC products. Lead also continues to be used for insulation of large-diameter pipelines and cables (see page 15). Common substitutes for these metals are calcium zinc and barium zinc formulations. High cost and technical difficulties are the reasons why these alternatives are not used in all applications.
Other additives
There is a long list of other additives for PVC products: processing AIDS, impact modifiers, pigments, chalk and other inert fillers, lubricants that help extrusion, flame retardants, smoke suppressants and biocides. The amount of these additives is usually much less than that of plasticizers and stabilizers, and most of them are also used in compounds based on other (non-PVC) plastic resins.
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Please also refer to:
History of PVC floor coverings (PDF magazine):/static/WMA/PDF/11/6/2/5/wave15 _ uk.pdf.
PVC building products: http://www.vinylbydesign.org/site/page_two_col.asp? Orbit =&cid = 30&did = 6.