Thermoplastics, commodity

The commodity thermoplastics are the most-used plastics in this sector, accounting for 95% of all thermoplastic packaging. Polyolefins are the most important, accounting for more than 65% of the total plastic weight. Polyethylene terephthalate, polystyrene and PVC are also commonly used. 

Figure 2.17 shows that the cost per volume of the commodity thermoplastics is intermediate between steel and aluminium, which explains their development in packaging and the poor penetration of the much more expensive engineering thermoplastics. 

Consequently, commodity thermoplastics are the most used, with rigid and soft PVCs dominating. However, some engineering thermoplastics are used for specific applications that justify their cost. 

Table 2.37 points out some interesting properties for the medical applications of commodity thermoplastics. 

Thermoplastic Polymers. Most thermoplastic polymers are used in high-volume, widely recognized applications, so they are often referred to as commodity plastics. (We will elaborate upon the distinction between a polymer and a plastic in Chapter 7, but for now we simply note that a plastic is a polymer that contains other additives and is usually identified by a variety of commercial trade names. There are numerous databases, both in books [1] and on the Internet [2], that can be used to identify the primary polymer components of most plastics. With a few notable exceptions, we will refer to most polymers by their generic chemical name.) The most common commodity thermoplastics are polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC) and polystyrene (PS). These thermoplastics all have in common the general repeat unit -(CHX-CH2)-, where -X is -H for PE, -CH3 for PP, -Cl for PVC, and a benzene ring for PS. When we discuss polymerization reactions in Chapter 3, we will see that all of these thermoplastics can be produced by the same type of reaction. 

The chemical structures of thermosets are generally much more diverse than the commodity thermoplastics. The most common types of thermosets are the phenol-formaldehydes (PF), urea-formaldehydes (UF), melamine-formaldehydes (MF), epoxies (EP), polyurethanes (PU), and polyimides (PI). Appendix 2 shows the chemical structure of these important thermosetting polymers. 

Polypropylene (PP) is a semicrystalline commodity thermoplastic produced by coordination addition polymerization of propylene monomer [197]. Most frequently, stereospecific Ziegler-Natta catalysts are used in industrial processes to produce highly stereospecific crystalline isotactic (iPP) and syndiotactic (sPP) polymer with a small portion of amorphous atactic PP as a side product. Polymerization of non-symmetrical propylene monomer yields three possible sequences however, the steric effect related to the methyl side group highly favors the head-to-tail sequence. The occurence of head-to-head and tail-to-tail sequences produces defects along the PP chain [198]. Presence of such defects affects the overall degree of crystallinity of PP. 

The need to minimise production costs and hence the selling price of mineral fillers is a dominant theme running through the technology. Although compound cost reduction was one of the principal reasons for using fillers in the past, this is of less importance now, due to the fall in the cost of the commodity thermoplastics. Indeed, the cost in use of fillers (taking into account volume costs and the cost of incorporation) can now lead to a rise in overall compound costs and it is often the minimisation of this cost increase that has become important. 

Among the contrarians, Bayer has had for historical reasons a different approach to the problem of remaining a well-rounded producer of chemicals. It has always been a leader in such specialty chemicals as plastics and rubber additives and supplies master batches through its subsidiary Rhein Chemie. Its polymers are mainly centered on synthetic elastomers and specialty plastics like polyurethanes and polycarbonates that Bayer itself develops. These are not subjected to the cutthroat competition that commodity thermoplastics are. Bayer has remained involved in aroma chemicals with a well-known subsidiary, Haarmann Reimer. In pharmaceuticals the company occupies an honorable rank, although in the future its management may want to separate that branch from the rest of its activities in preparation for further alliances. Like BASF, Bayer considers its agrochemical line as a core business. However, unlike BASF, it is less interested in integrating upstream production to oil raw... 

Commercial plastic materials may conveniently be divided into five major classes. Commodity thermoplastics are families which are produced in volumes of a billion pounds or more per year. Engineering thermoplastics... 

TABLE 15.2 Commodity Thermoplastics in the United States and Canada... 

In summary, commodity thermoplastics are manufactured readily at low cost, and offer a combination of processability, mechanical, thermal, optical, and chemical properties that are useful in a wide range of mass markets and products. 

Maximum Use Temperature. The most frequent requirement for higher engineering performance is retention of properties at higher temperatures. Whereas most commodity thermoplastics soften and distort in boiling water, engineering thermoplastics are most often characterized by their ability to stand... 

PLA is a polymer that may not be well suited to injection moulding. Its rate of crystallisation is too slow to allow cycle times typical of those for commodity thermoplastics such as polystyrene. Stress induced crystallisation that can enhance PLA crystallisation is better suited to processes such as fibre spinning or biaxial orientation of film. 

R.G. Harvan (1997) Chemistry Industry, p. 212 - Polyethylene New directions for a commodity thermoplastic . 

Of the plastics that are currently found in the solid waste stream, commodities are by far the greatest component. Seventy eight % of this waste is comprised of commodity thermoplastics while 16% is thermosets and only 6% is engineering thermoplastics. Therefore the contribution of high performance blends and alloys to the waste stream is rather negligible. 

Polyolefins are the most widely used commodity thermoplastics. They are of immense interest to polymer community because of their simple chemical structures and fascinating hierarchical stmctural organizations possible. To date, the field of polyolefins remains one of the most vibrant areas in polymer research. 

Hopefully you are now familiar with the appearance and some typical properties of the commodity thermoplastics, and can recognise how some products have been made. You are now ready to study the microstructure and processing of polymers in more detail, and to find out how the properties can be related to the microstructure. 

Each commodity thermoplastics is available in many grades, either adapted to the requirements of a process, or to the demands of a product type. The major variants will be described and some of the jargon explained. A few of the important applications will be described, while others appear throughout the book. 

In recent times, commodity thermoplastics have become signiflcant contenders as matrices for glass fibre composites where environmental resistance is sought for example, polypropylene is now available in a pre-preg form for fusion bonding. Since it has low polarity and is also partially crystalline, moisture absorption is very low. 

Polyethylene, the most-used commodity thermoplastic, also shows wavelength-dependent behavior in photodegradation. Heacock [135] proposed UV radiation of about A = 257 nm to result in the formation of unsaturated and oxygenated structures in polyethylene, while the longer wavelength UV-A radiation was believed to cause mainly crosslinking reactions. Results from a monochromatic exposure study of low-density and high-density polyethylene... 

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