Plastics are made of two types of components. The main component is the polymer, or resin, which makes up the bulk of the plastic material. The second type of component is additives. Although additives are a small part of the overall plastic composition, they are a very important part. Additives are used to give plastics useful properties such as color, resistance to fire, strength and flexibility. To a large extent it is additives which allow the great variety of plastics which are available.
One of the most important classes of plastics additives are plasticizers. Plasticizers give plastics flexibility and durability. Plasticizers hold 65 percent of the 7.5 million ton world market for plastics additives, accounting for 4.9 million tons, with a value of $ 7.6 billion. The majority of this, about ninety percent, is used for polyvinyl chloride (PVC), a polymer used in diverse applications such as coatings, plumbing, construction materials and plastic bottles. This market is likely to experience continued growth, as the market for polyvinyl chloride has grown by of eight percent a year recently 1.
There are a huge variety of chemicals used as plasticizers. The most common type of plasticizers are chemicals called phthalates. In particular, dioctyl phthalate (DOP) is extensively used as a plasticizer for PVC. Several factors make phthalates less than ideal for use as plasticizers. They migrate to the surface of plastics, and can then evaporate or leach into the surrounding environment. This limits the usefulness of phthalate plasticizers, as they eventually migrate out of plastics entirely, and as a result the plastics become brittle 2. The release of phthalates into the environment represents an environmental hazard as well. Because of the widespread use of phthalates, they have become one of the most abundant industrial pollutants in the environment 3. Many phthalates, including DOP, are classified as toxic chemicals by the EPA's Toxic Release Inventory. A class of more environmentally benign plasticizers are derived from vegetable oils. Vegetable oil plasticizers provide about fifteen percent of the total US market for plasticizers, and represent about eight percent of the industrial market for vegetable oils 4.
The most significant vegetable oil plasticizer is epoxidized soybean oil (ESO), which holds 43 percent of the vegetable oil derived plasticizer market. ESO is used as a secondary plasticizer (plasticizers used at levels of one to four percent) in flexible PVC. The Viking Products Division of Elf Atochem (Philadelphia, PA) produces ESO plasticizers under the trade name Vikoflexreg., at about $ 0.70 per pound. This is somewhat high in comparison to plasticizers such as DOP, which costs $ 0.53 per pound. However, ESO is a higher value product because of benefits such as its resistance to migration, evaporation and leaching, its low odor, and the stability to light and heat which it adds to PVC 5. Other epoxidized oils used by in flexible PVC include linseed oil and epoxidized tallates, although these account for less than 9 percent of the vegetable oil plasticizer market. Witco (Greenwich, CT) produces tallates, which are derived from the wood product tall oil, under the Drapexreg. trademark. These plasticizers cost about $ 1.20 per pound 6. Elf Atochem produces linseed based plasticizers, at $ 0.93 per pound, under the Vikoflexreg. trademark. Like ESO, linseed based plasticizers offer the additional benefit of high stability, which makes them competitive in food and medical applications where low levels of metallic stabilizers are required 7. Another class of vegetable based plasticizers are esters. Esters are derived from the reaction of an alcohol with a fatty acid. Fatty acids are the main component of vegetable oils. Esters of fatty acids can be formulated to produce plasticizers with a range of desirable properties. Several fatty acids are important in the plasticizer market. Stearic and oleic acids are available from a variety of vegetable oils, including coconut, canola, and soybean, and are used in plasticizers for polystyrene and cellulosic polymers like celluloid and ethyl cellulose. Myristic and palmitic acids, derived primarily from palm oil, are also used as plasticizers in cellulosic polymers. Sebacic acid, a component of castor oil, is used in plasticizers for PVC. Ester based plasticizers vary considerably in price, ranging from $0.60 to $1.70 per pound, depending on the vegetable oil and alcohol raw materials, and the purity of the plasticizer. Caschem Inc. (Bayonne, NJ), Harwick Chemical Corp. (Akron, OH), and the Stepan Co. (Maywood, NJ) offer vegetable oil derived ester plasticizers 8.
Although vegetable oil derived plasticizers tend to be more expensive on a cost per pound basis than petrochemical plasticizers, they offer performance benefits which can make their overall economics favorable. Petrochemical plasticizers such as phthalates provide only flexibility, but do not contribute to heat and light stability. As a result additional stabilizing additives must be added to supplement these plasticizers. Metallic stabilizers based on metals like cadmium, lead and barium are added to plastics to achieve stability to heat and light. But these stabilizers have high toxicity, and are unsuitable for many applications, particularly for food packaging and medical uses. Vegetable oil plasticizers achieve a double benefit by eliminating both the cost and the toxicity of metallic stabilizers. The final economics of use depend on the specific plastic formulation and the relative costs of the plasticizer versus the stabilizers which are displaced. In many cases the economic benefits of using vegetable oil derived plasticizers alone will justify their use 9.
The main obstacles to increasing the market for vegetable oil derived plasticizers is technological. In most applications plasticizers such as ESO are suitable for use only as secondary plasticizers. At higher levels, they may not mix properly into the plastics formulation, or may cause plastics formulations to become brittle. At current levels of technology, the markets for vegetable oil derived plasticizers are mature, and are likely to experience growth only with the growth of the plastics market. The American Soybean Association is investigating ways to improve the properties of vegetable oil derived plasticizers. This research could lead to their use as primary plasticizers, which are used at levels of up to forty percent by weight in some flexible plastics. If this can be achieved, the potential market for vegetable oil plasticizers, such as ESO, could be as high as 50 thousand tons 10.
There is a considerable incentive to motivate this type of research. Growing public concern about the environment and the potential health risks of industrial chemicals promises to increase the market for vegetable oil derived plasticizers. Concerns about air quality led to the Clean Air Act of 1990, which is restricting the use of volatile organic compounds like dioctyl phthalate. The potential toxicity of phthalates is an issue, particularly in the area of food packaging and medical applications. The disposal of plastics is a growing problem. Vegetable oil derived plasticizers, which do not evaporate or leach into soil or ground water after disposal, and have negligible toxicity, may provide an answer to these concerns.
1. Cavanaugh, Tim, "Plastics Additives '95," Chemical Marketing
Reporter, p. SR. 10, June 12 1995.
2. Information supplied by the American Soybean Association and Omnitech
International Inc.
3. "Phthalates Activate Estrogen Receptors,"Science News, 148:3, p. 47,
July 15, 1995.
4. Pryde, E., and Rothfus, J., "Industrial and Nonfood Uses of Vegetable
Oils," Oil Crops of the World, pp. 88- 111, Mcgraw Hill, New York, NY,
1989.
5 Product information supplied by Elf-Atochem North America.
6. Product information supplied by Witco.
7. Product information supplied by Elf-Atochem North America.
8. "Plasticizers; Manufacturing Handbook & Buyers Guide," Plastics
Technology, 40:8 p. 452, July 15, 1994.
9. Op cit Elf Atochem.
10. Information supplied by the American Soybean Association and Omnitech
International Inc.
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