Poly(vinyl chloride) (PVC)

Synonyms

Poly(1-chloroethylene); Polychloroethylene; PVC

Definition

Poly(vinyl chloride) (PVC) (CAS number 9002-86-2) is a synthetic resin made from vinyl chloride and is a member of a large family of polymers broadly referred to as “vinyls.” The chemical formula for vinyl chloride is H₂C=CHCl, and the formula for PVC is (H₂C–CHCl) _n , where n is the degree of polymerization. This polymer is one of the most widely used commercial thermoplastic materials. PVC is one of the earliest produced polymers and now one of the three most abundantly produced synthetic polymers [1, 2].

History

Vinyl chloride is a colorless, highly flammable gas with a faint sweet odor that was first reported by Justus von Liebig and his research student, Henri Victor Regnault, in 1835; Regnault also reported the polymerization of this compound in 1838 [3]. Regnault observed that vinyl chloride polymerized to form a white powder upon exposure to sunlight. Some later works revealed that the polymerized object was PVC, but, according to some other papers, the polymerized object was poly(vinylidene chloride). Thus, it is commonly believed that PVC was first prepared by a German chemist named Eugen Baumann in 1872, and Baumann is often credited as the inventor of PVC. The industrial development of PVC resins began approximately 50 years later. Full-scale commercial production began in Germany in 1931, while PVC was produced commercially in the USA in 1933. During World War II, PVC became the material of choice to protect electrical wires for military [4].

Process of Manufacture

Unlike other thermoplastics that are entirely derived from oil, PVC is manufactured from two starting materials: ethylene and sodium chloride. Ethylene is derived from a cracking process involving a feedstock based on oil. Chlorine is produced via the electrolysis of sodium chloride in salt water and is then combined with the ethylene obtained from oil. The resulting chemical is ethylene dichloride, which can be converted to vinyl chloride at high temperatures. Alternatively, vinyl chloride is obtained by reacting ethylene with oxygen and hydrogen chloride over a copper catalyst. Consequently, vinyl chloride and PVC consist of 57 % chlorine and 43 % hydrocarbon. Vinyl chloride boils at −13.4 °C and is normally stored and shipped as a liquid under pressure. Generally, PVC is produced via the free radical polymerization of vinyl chloride (Fig. 1) with initiators, such as organic peroxides, in thick-walled, high-pressure-rated steel vessels. Currently, PVC is produced using various processes to generate increasingly specialized PVC varieties that are tailored for specific end markets and new processing technologies. Commercially, PVC is mainly produced by solution, bulk, suspension, and emulsion polymerizations. Tacticity of PVC changes according to the reaction conditions. Most of the products are usually atactic [5].

Poly(vinyl chloride) (PVC), Fig. 1

Polymerization of vinyl chloride

Properties

Pure PVC is a white, brittle solid. The glass-transition temperature of PVC homopolymers is near 80 °C. The reported amount of crystallinity is in the range of between 5 % and 10 %. The presence of chlorine gives excellent fire resistance to PVC. Moreover, PVC exhibits excellent electrical insulation characteristics. These properties make PVC an ideal material for wire coating and architectural materials. Pure PVC is rigid, difficult to burn, and highly resistant to strong acids and bases, most other chemicals, and many organic solvents. The Flory-Huggins parameter (polymer-solvent interaction parameter) of PVC ranges from 19.2 to 22.1 (MPa)^1/2 [2]. In addition, PVC is not biodegradable. Due to its durability, PVC can be used in a variety of applications. Usually, thermoplastics are supplied as pellets. However, PVC resin is often supplied as a powder, and long periods of storage are possible because PVC resists oxidation and degradation. Because PVC is one of the least expensive plastics, easy to mold, and lightweight, it is predominant in the construction industry.

Applications

The major factors that favor the use of PVC for various applications are its range of flexibility, low toxicity, chemical stability, cost-effectiveness, ease of fabrication, biocompatibility, etc. More than 30 million tons of PVC were used worldwide in recent years. PVC is used in numerous applications that require a longevity ranging from short to extended periods of time. Approximately 60 % of the PVC that is used in various applications is expected to have a lifetime exceeding 40 years. The applications for PVC fall into two broad categories depending on its formulation.

Pure PVC is a glassy polymer at room temperature and is used for molding and profile extrusions with few additives. The typical products produced from glassy PVC include window frames, water pipes, credit cards, bottles, house siding, and containers. However, pure PVC is unstable when exposed to visible and UV light. To address these disadvantages and make PVC suitable for different applications, antioxidants and stabilizers are added.

To use PVC in flexible or elastomeric applications, plasticizers such as phthalates must be mixed with the PVC to lower the glass-transition temperature. This type of PVC typically contains 30–40 wt% plasticizers. To a large extent, the type and amount of plasticizer determines the properties of the plasticized composition. In this soft and flexible form, PVC is used in electrical cable insulation and many other applications where PVC replaces rubber. In the healthcare industry, flexible PVC, produced by the addition of phthalates, is used to make blood bags, feeding tubes, and many other items [6]. Many other applications include hoses, imitation leathers, cables, packaging films, vinyl flooring, and car interiors.

Environmental Impact, Sustainability, and Recycling

PVC degradation during processing and use has been one of the most essential elements of PVC science and technology. Moreover, although PVC use has grown tremendously over the last 70 years, health and environmental problems have been associated with its use, including the effects of the vinyl chloride, phthalate plasticizers, and other contaminants, such as dioxins and heavy metals. For example, vinyl chloride has been identified as a carcinogen. Recycling PVC materials is more difficult than other widely used plastics due to the thermal stability of PVC and the diverse additives used for various products. However, recently, the recycling efficiency of PVC has been improved. Recycled PVC is break to small chips, impurities removed, and the product refined to make pure white PVC. The Society of Plastics Industry recycling codes of PVC is 3 shown in Fig. 2.

Poly(vinyl chloride) (PVC), Fig. 2

The Society of Plastics Industry recycling codes of PVC