Page 64 - Handbook of Plastics Technologies

THERMOPLASTICS

2.4 CHAPTER 2

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both film and solid product applications. The content of starch in these blends can range
up to 50 percent by weight, and the materials can be processed on conventional processing
equipment. A product developed by Warner-Lambert call Novon is also a blend of poly-
mer and starch, but the starch contents in Novon are higher than in the material by Fertec.
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In some cases, the content can be over 80 percent starch.
Polylactides (PLAs) and copolymers are also of interest in biodegradable applications.
This material is a thermoplastic polyester synthesized from ring opening of lactides. Lac-
29
tides are cyclic diesters of lactic acid. A similar material to polylactide is polyglycolide
(PGA). PGA is also thermoplastic polyester but formed from glycolic acids. Both PLA
and PGA are highly crystalline materials. These materials find application in surgical su-
tures and resorbable plates and screws for fractures, and new applications in food packag-
ing are also being investigated.
Polycaprolactones are also considered in biodegradable applications such as films and
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slow-release matrices for pharmaceuticals and fertilizers. Polycaprolactone is produced
through ring opening polymerization of lactone rings with a typical molecular weight in
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the range of 15,000 to 40,000. It is a linear, semicrystalline polymer with a melting point
near 62°C and a glass transition temperature about –60°C. 32
A more recent biodegradable polymer is polyhydroxybutyrate-valerate copolymer
(PHBV). These copolymers differ from many of the typical plastic materials in that they
are produced through biochemical means. It is produced commercially by ICI using the
bacteria Alcaligenes eutrophus, which is fed a carbohydrate. The bacteria produce polyes-
33
ters, which are harvested at the end of the process. When the bacteria are fed glucose,
the pure poly hydroxybutyrate polymer is formed, while a mixed feed of glucose and pro-
34
pionic acid will produce the copolymers. Different grades are commercially available
that vary in the amount of hydroxyvalerate units and the presence of plasticizers. The pure
35
hydroxybutyrate polymer has a melting point between 173 and 180°C and a T near 5°C.
g
Copolymers with hydroxyvalerate have reduced melting points, greater flexibility, and im-
pact strength, but lower modulus and tensile strength. The level of hydroxyvalerate is 5 to
12 percent. These copolymers are fully degradable in many microbial environments. Pro-
cessing of PHBV copolymers requires careful control of the process temperatures. The
material will degrade above 195°C, so processing temperatures should be kept below
180°C and the processing time kept to a minimum. It is more difficult to process unplasti-
cized copolymers with lower hydroxyvalerate content because of the higher processing
temperatures required. Applications for PHBV copolymers include shampoo bottles, cos-
36
metic packaging, and as a laminating coating for paper products.
Other biodegradable polymers include Konjac, a water-soluble natural polysaccharide
produced by FMC; Chitin, another polysaccharide that is insoluble in water; and Chitosan,
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which is soluble in water. Chitin is found in insects and in shellfish. Chitosan can be
38
formed from chitin and is also found in fungal cell walls. Chitin is used in many bio-
medical applications, including dialysis membranes, bacteriostatic agents, and wound
dressings. Other applications include cosmetics, water treatment, adhesives, and fungi-
cides. 39
2.2.3 Cellulose
Cellulosic polymers are the most abundant organic polymers in the world, making up the
principal polysaccharide in the walls of almost all of the cells of green plants and many
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fungi species. Plants produce cellulose through photosynthesis. Pure cellulose decom-
poses before it melts and must be chemically modified to yield a thermoplastic. The chem-
ical structure of cellulose is a heterochain linkage of different anhydrogluclose units into
high-molecular-weight polymer, regardless of plant source. The plant source however

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