The role of phosphites in PVC products

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This post was last edited by Lai Kunhui on 2017-12-25 at 13:15. The role of phosphites in PVC products. I. Research on the aging mechanism of PVC. Polyvinyl chloride (PVC) is the world’s second-most widely used general-purpose resin; it is a material with a wide range of applications. It can be used to manufacture rigid products with high strength and hardness, such as pipes and fittings, doors and windows, as well as packaging materials. It can also be combined with plasticizers to produce very soft products, such as films, sheets, wires and cables, floors, synthetic leather, coatings, and other consumer goods. PVC plastic has excellent resistance to acids and alkalis, wear, combustion, and good insulating properties. However, it has poor stability to light and heat. Without a heat-stabilizer, polyvinyl chloride begins to decompose at 100°C, and the decomposition accelerates above 130°C. Upon heating, hydrogen chloride gas is released, causing it to change color. Ultraviolet rays and oxygen in sunlight cause photo-oxidative degradation of polyvinyl chloride, which reduces its flexibility and eventually leads to brittleness. At the same time, the aforementioned good mechanical and chemical properties decline rapidly. Therefore, stabilizers and other additives must be added to PVC resin in order to produce the various products required. The thermo-oxidative aging of PVC is complex; it is generally believed that the mechanism behind the dehydrochlorination during the thermal degradation of PVC is caused by a free-radical chain reaction. Some literature reports that the thermal degradation process of PVC is divided into two steps. (1) Dehydrochlorination: The double bonds, branching points, residual initiator end groups, and oxygen-containing structures present in PVC generate free radicals upon thermal or photoactivation. Under the initiation of these free radicals, the reactive chlorine atoms are removed to produce hydrogen chloride, while conjugated polyolefins are formed simultaneously ； (II) Formation of longer-chain polyolefins and aromatic rings: As degradation progresses further, the chlorine atoms on the allyl groups become highly unstable and are easily removed, resulting in the formation of longer-chain conjugated polyolefins; this is what is known as \"zipper-like\" dehydrogenation. At the same time, a small number of C-C bonds break and cyclize, yielding small amounts of aromatic compounds. Among them, decomposition and dehydrochlorination are the main causes of PVC aging. II. Study on the mechanism of action of phosphite antioxidants. Phosphites are classified as auxiliary antioxidants and play an important role in antioxidant systems. In addition to their excellent ability to decompose hydrogen peroxides, they also possess good capabilities for protecting color. Furthermore, it can also increase the processing temperature of polymers, and exhibits good synergistic effects with hindered phenol antioxidants and light stabilizers. In PVC formulations, phosphites are generally considered to act primarily as chelating agents. When used alone, they do not provide significant stabilizing effects; however, when combined with metal soaps, they can chelate metal chlorides, thereby improving heat resistance and weather resistance while maintaining transparency. It can also be used in combination with organotin compounds and epoxides to exhibit a synergistic effect. Phosphites affect the stability of PVC in various ways: (1) they capture hydrogen chloride released during PVC degradation, thereby suppressing its autocatalytic effect. (2) Replace the unstable allyl chloride atoms in PVC molecules to suppress dehydrochlorination. (3) Passivate harmful metal ions. (4) Antioxidant effect. (5) Inhibit free radicals. Furthermore, phosphites also have a good effect on improving the transparency of PVC plastics containing lead salts, barium salts, and calcium salts. III. Introduction to the Development of Phosphite Antioxidants. There are many types of phosphites, including aryl phosphites, alkyl phosphites, (alkylated aryl) phosphites, and alkylaryl mixed phosphites. The commonly used phosphites in PVC formulations include the following: triphenyl phosphite (TPP), phenyldiisooctyl phosphite (PDOP), diphenyliododecyl phosphite (DPDP), trinonylphenyl phosphite (TNPP), bisphenol A phosphite (1500), and others. However, as the industry develops, environmental protection requirements are becoming increasingly stringent, and restrictions on the use of nonylphenol, bisphenol A, and free phenols are also getting more strict. Traditionally, most phosphites are synthesized using triphenylphosphite as a substrate, which inevitably results in residual free phenol. To meet market demands, Evergreen Technology has introduced two new series of phosphites: (1) odorless, low-VOC phosphites in the EGPHOS ML series ； (II) Phenol-free, low-flavor phosphite EGPHOS PL series. Jiangsu Evergreen New Materials Technology Co., Ltd. specializes in the field of phosphite antioxidants. Since its establishment over 20 years ago, the company has adhered to the corporate philosophy of \"quality first, continuous innovation, win-win cooperation, and sustainable development.\" It strives for steady progress and ongoing improvement, aiming to become a professional manufacturer of phosphite antioxidants that serves polymer material producers around the world. IV. Conclusion At present, phosphites are used in small amounts in PVC formulations, and sometimes they are even not necessary at all; however, due to the unique properties arising from their structure, phosphites can improve certain characteristics of PVC. Furthermore, phosphite antioxidants can reduce the amount of primary stabilizers required, especially those expensive organic tin stabilizers. Therefore, the scientific and proper use of phosphites will give PVC better processing and performance properties.