Petrochemical-based plastic materials

In recent years, interest in plastics made from renewable materials (biological sources) has increased greatly. The drive towards increasing sustainability has enabled these plastics to become more competitive with petrochemical-based plastics. Many of these materials (though not all) are also biodegradable, which is also seen as a desirable attribute. Biodegradability will be discussed further in Chapter 16. 

Abstract Polyhydroxyalkanoate (PHA) initially received serious attention as a possible substitute for petrochemical-based plastics because of the anticipated shortage in the supply of petroleum. Since then, PHA has remained as an interesting material to both the academia and indusby. Now, we know more about this microbial storage polyester and have developed efficient fermentation systems for the large-scale production of PHA. Besides sugars, plant oils will become one of the important feedstock for the industrial-scale production of PHA. In addition, PHA will find new apphcations in various areas. This chapter summarizes the future prospects and the importance of developing a sustainable production system for PHA. 

A downside to the sustainable and extensive use of soy protein-based materials is their intrinsic reactivity and thus lower inertia when compared to most conventional petrochemical-based plastics. They are known to be sensitive to microbial spoilage and also to water due to hydrophilic nature of many amino acids constituting their primary structure and to the substantial amount of hydrophilic plasticizer required to impart thermo-processability and film flexibility. As a consequence, their mechanical properties and water vapor barrier properties in high moisture conditions are poor compared to synthetic films such as low-density polyethylene. 

Microbial PHA first received widespread attention during the petroleum crisis of the 1970s as a potential substitute for petrochemical-based plastics. Besides being a thermoplastic with properties comparable to that of PE, PHA are also completely biodegradable. The ability to produce PHA from renewable carbon sources also ensures a sustainable green chemistry process. A considerable amount of work has been focused on the production of various types of PHA for applications as commodity plastics. Initially, PHA were used to make everyday articles such as shampoo bottles and packaging materials. 

FIGURE 11.9 The business system for thermoplastics. With renewable-resource based products such as PLA all of the mass of the polymer originates from carbon dioxide. Conventional plastic materials use fossil resources. Both petrochemical-based plastics and PLA require process energy. The expectation is that the overall amount of fossil resources is less for PLA compared to petrochemical based products. 

Most of the plastics and synthetic polymers that are used worldwide are produced from petrochemicals. Replacing petroleum-based feedstocks with materials derived from renewable resources is an attractive prospect for manufacturers of polymers and plastics, since the production of such polymers does not depend on the limited supply of fossil fuels [16]. Furthermore, synthetic materials are very persistent in the environment long after their intended use, and as a result their total volume in landfills is giving rise to serious waste management problems. In 1992,20% of the volume and 8% of the weight of landfills in the US were plastic materials, while the annual disposal of plastics both in the US and EC has risen to over 10 million tons [17]. Because of the biodegradability of PHAs, they would be mostly composted and as such would be very valuable in reducing the amount of plastic waste. 

The base material producers are usually large chemical or material companies that manufacture products for broad markets such as petrochemicals or plastics. When demand warrants, they will produce materials specifically for the adhesive and sealant formulators. 

Different types of reactors are utilized for a wide variety of pyrolysis applications, including processing of waste plastics. The worldwide waste plastic pyrolysis systems utilize the fixed-bed designs of vertical shaft reactors and dual fluidized-bed, rotary kiln and multiple hearth reactor systems. The type of reactor used is chiefly based on material to be pyrolyzed and expected products from the pyrolysis. Stainless steel shaking type batch autoclave and stainless steel micro tubular reactors have also been used extensively [14]. Fluidized-bed reactors have been extensively used in producing raw petrochemicals from the pyrolysis of waste plastics [22, 24]. 

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