![]() We should distinguish PPA, as a so-called semi aromatic polyamide, from fully aromatic polyamides, such as Kevlar, as their properties, uses and chemistry are distinct enough that they can be distinguished and treated as separate polymers. This underlines that unlike the other high-performance engineering plastics, its properties can be somewhat tuned, and thus can be used in more general applications, compared to the other high-performance engineering plastics. 4 This means that two manufacturers can offer a product called PPA, with similar mechanical and thermal performances, but the underlying monomers can be significantly different. ASTM indicates that a manufacturer must maintain a minimum of 55 molar % of either or both of these acids, in order to call their resin PPA. It is the result of mixing a combination of terephthalic and/or isophthalic acid with various diamines. PPA is different from most existing polymers, as it does not have a singular chemical structure. Among these niche polymers, we find polyphthalamide (PPA), which melts at range of 296–325☌, depending in its composition. Polyether ether kethone, with a melting point of 340☌, can be found in fuel cell membranes and electronic data storage devices. Polyphenyelene sulphide is used a solder for electrical parts, with a melting point of 280☌, 3 and has the particular characteristic of being able to be “cured” (that is, to be thermally treated in order to increase the degree of cross-linking between polymers), despite being considered a thermoplastic material. Poly p-xylene or parylene, has a melting point of 420☌, 3 and is used as a coating in medical devices or for microlectrodes. Thus, they are employed in very niche applications. These polymers have melting temperatures well above the range of both engineering plastics and commodity plastics and can keep mechanical properties in temperatures in which other polymers would just melt or undergo thermal degradation. Some examples include polyphenylene sulfide, polyetherimide, polyetherketone (PPS, PEI, PKK, respectively) among others. Within engineering plastics, some niche products exist, making up less than 10% of all engineering plastics currently used. 2 The other 30% is mostly made up of engineering plastics, with the corresponding increase in mechanical and thermal properties. Together, commodity plastics make up more than 70% of all plastics currently produced, according to Plastics Europe. The main focus of the plastic industry is on so-called commodity plastics (plastic used in applications in which their mechanical properties do not have to exceed materials such as metals, e.g., plastic bags, food packaging, containers for household goods, etc.) These include polypropylene, polyethylene, polyvinyl chloride, and polyethylene terephthalate (PP, PE, PVC, PET, respectively). Millions of tons of plastic are produced every year, 1 in an ever increasing number of different polymer grades, blends and composites. Plastic materials have come a long way since the inception of the first synthetic polymer, Bakelite.
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