How a class of materials based on chlorine chemistry becomes part of the landscape.
As the pace of development of synthetic polymers began to accelerate in the 1920s and early 1930s, a class of materials based on chlorine chemistry became part of the landscape. The first evidence of PVC "synthesis" actually dates back to the mid-1830s, when the monomer vinyl chloride used to produce PVC was first synthesized in 1835 by Justus von Liebig and his student Henri Victor Regnault.
Neither of them has any interest in polymers. Von Liebig will continue to make significant contributions to the development of fertilizers using nitrogen and trace mineral chemistry. He is considered one of the founders of modern organic chemistry. Regnault's interest is to study the properties of gases, including vinyl chloride. But like many low molecular weight unsaturated compounds, vinyl chloride has a tendency to spontaneously polymerize. In 1838, Regnault found a white powder in a container containing vinyl chloride. This is the first known PVC product. The polymer was "discovered" for the second time in 1872.
Due to the thermal stability of the polymer, attempts to use it for commercial purposes in the early 20th century were unsuccessful. As we have already mentioned, this problem was solved by Waldo Semon of BF Goodrich, who discovered the plasticizer for PVC. The decrease in polymer softening temperature opens a narrow but feasible window for melt processing. The plasticization process of PVC was later expanded to produce various compounds ranging from rigid to flexible, depending on the amount and type of plasticizer added to the polymer.
In 1930, Elmer Bolton, head of research at DuPont, was interested in acetylene chemistry when he was looking for expanded business opportunities. Acetylene chemistry had produced compounds such as vinyl acetylene. When reacted with hydrogen chloride, vinyl acetylene is converted to chloroprene, a monomer of chloroprene. By 1931, DuPont had purchased a patent from Dr. Julius Nieuwland, a chemical developer at Notre Dame de Paris, and polymerized chloroprene to produce the first real synthetic rubber. Like many elastomers, the properties of neoprene can be adjusted by adding plasticizers. The same chemicals that have been found to plasticize PVC are also suitable for neoprene.
In 1933, the laboratory staff of another company accidentally discovered another chlorine-containing polymer, polyvinylidene chloride (PVDC). The company will have a huge impact in the polymer field, Dow Chemical. Chemically, PVC and PVDC are very similar, as shown in the attached picture. The road to commercialization of PVDC is more tortuous than that of neoprene, but it is still much faster than the journey of PVC in the past 90 years.
Ralph Wiley, the creator of PVDC, is working to produce perchloroethylene, a dry-cleaning product, and discovered that some of his beakers produced a residue that could resist all cleaning attempts. As with many early polymer discoveries, PVDC was first used as a coating to protect other products from moisture and corrosion because it can be easily sprayed onto a variety of materials. It is used in car interiors and fighter jets, just like cellulose acetate was used twenty years ago. Wiley saw the potential of this type of fiber material, but Wiley's boss John Reilly wanted to move the development in the direction of the movie. In six years, this material has undergone a series of improvements to eliminate green and unpleasant odors. By 1942, it was used as a protective film for canvas and rubber in military equipment.
Saran is a hybrid of the names of John Reilly's wife (Sarah) and daughter (Ann), which became synonymous with PVDC.
In 1943, Willard Dow, then president of Dow, promoted the abandonment of PVDC development. But at this time Wiley has obtained multiple patents for the material and persuaded Dow to continue using the product. As we have seen in many examples of the development of new polymers, the key to the long-term success of PVDC is process development. Wilbur Stephenson was praised for developing the famous Saran bubble, which is the key to the production of film products. Saran is a hybrid of the names of John Reilly's wife (Sarah) and daughter (Ann), just as the Kleenex brand name is associated with the organization, it becomes a synonym for PVDC.
It quickly became the material of choice for packaging military equipment shipped overseas to protect it from the corrosive effects of moisture and salt spray. When the war ended and the market dried up, Dow actually sold the product to two of its employees, who established a food packaging company in Midland. The product sold well, and Dow acquired the business in 1948 and formally consolidated the well-known relationship between Dow and Saran Wrap.
Chlorinated polyethylene (CPE) was developed shortly after the commercialization of PVC, neoprene and PVDC. Unlike these other polymers, where chlorine is already part of monomer chemistry, chlorinated polyethylene is produced by reacting polymerized polyethylene with a chlorinated solvent to replace hydrogen atoms with chlorine atoms along the polymer backbone. The properties of chlorinated polyethylene depend on the type of modified polyethylene (LDPE and HDPE) and the amount of chlorine reacted into the material. When the chlorine content is low, CPE is a thermoplastic.
However, as the chlorine content increases, the material first becomes a thermoplastic elastomer, then an elastomer material that is more like rubber, and finally a rigid polymer. The original patent for CPE was filed in 1939 by the same Eric Fawcett, who was a member of the team that made polyethylene for the first time in 1933. Since then, the same general method has been used to make chlorinated polypropylene. Chlorinated polyolefins can be blended with PVC to improve impact resistance. Chlorination after polymerization has also been applied to PVC itself, creating CPVC. Increasing the chlorine content can improve the heat resistance of the material, increasing the glass transition temperature from about 80 C to 110 C.
The presence of chlorine in these materials provides some very desirable properties at a relatively low cost. PVC, neoprene and PVDC all exhibit excellent barrier properties, making them unique. According to reports, neoprene as a mask material can block 99.9% of particles larger than 0.1 microns. The average diameter of the coronavirus is 0.125 microns. Saran, actually a combination of PVDC and acrylonitrile, has an oxygen barrier that is more than 3000 times better than LDPE, and has unparalleled barrier properties to various other ingredients that give food taste and aroma. Chlorine also provides inherent flame retardant properties.
At the same time, the presence of chlorine makes these materials very sensitive to thermal effects, resulting in a narrow processing window and producing corrosive by-products. Special protection must be given to tools and processing equipment. In addition, PVC has become a typical representative of the anti-plastic movement, partly because of the controversy over phthalate plasticizers, and also because some studies have shown that the polymer itself will form dioxins when incinerated.
In 2004, six years after SC Johnson purchased Saran Wrap from Dow Chemical, the composition of the material was changed from PVDC to polyethylene, mainly because of concerns about the environmental impact of such disposal. The new generation of materials is lighter in weight and lower in cost, so it is undeniably easier to produce. But it lacks adhesion, making it adhere to itself and all other objects, and it no longer has the unique barrier properties of the original product.
PVC is the third-ranked material in the global annual consumption, which is mainly due to the characteristics that chlorine imparts to the material.
Concerns about chlorine have extended to all halogen substances, and regulatory pressures have also affected various chlorinated and brominated compounds used as flame retardants. Only neoprene seems to have gotten rid of the public relations nightmare and has become a material widely used in various consumer products, including laptop covers, mouse pads, Halloween masks, tabletop gaming surfaces, yoga mats and high-end fashion from well-known brands. Designers like Vera Wang and Gareth Pugh. This may be one of the best examples of the love-hate relationship between the public and plastics.
Despite many controversies, chlorine-containing polymers have been around for nearly 100 years. Although efforts will almost certainly continue to reduce its use, PVC has been the third-largest material in global annual consumption for many years, mainly due to the properties that chlorine imparts to the material. Another halogen, fluorine, which also plays an important role in the polymer field, will be our next topic.
About the author: Michael Sepe is an independent materials and processing consultant headquartered in Sedona, Arizona, with clients in North America, Europe and Asia. He has more than 45 years of experience in the plastics industry and assists customers in material selection, design for manufacturability, process optimization, troubleshooting and failure analysis. Contact: (928) 203-0408 •mike@thematerialanalyst.com
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