Depolymerization commercial application

Depending on the chemical agent used to break down the polymer, different depolymerization routes can be envisaged glycolysis, methanolysis, hydrolysis, ammonolysis, etc. In the following sections of this chapter, these alternatives are reviewed for those condensation polymers having the most significant commercial applications. It must be pointed out that a majority of the studies on chemical depolymerization of plastic wastes is reported in patents works published in the scientific literature are relatively scarce. 

Polylactic acid (PLA) has been produced for many years as a high-value material for use in medical applications such as dissolvable stitches and controlled release devices, because of the high production costs. The very low toxicity and biodegradability within the body made PLA the polymer of choice for such applications. In theory PLA should be relatively simple to produce by simple condensation polymerization of lactic acid. Unfortunately, in practice, a competing depolymerization process takes place to produce the cyclic lactide (Scheme 6.10). As the degree of polymerization increases the rate slows down until the rates of depolymerization and polymerization are the same. This equilibrium is achieved before commercially useful molecular weights of PLA have been formed. 

Microcrystalline Cellulose. Microcrystalline cellulose is a purified, partially depolymerized cellulose that occurs as a white, odorless, tasteless, crystalline powder composed of porous particles. It is widely used in pharmaceutical dosage forms, primarily as a filler-binder in oral tablets and capsules with both wet granulation and direct compression processes. Microcrystalline cellulose was marketed first in 1964 by the FMC Corporation under name Avicel PH in four different particle size grades, each with different properties.37 Addition of Avicel into a spray-dried lactose-based formulation overcame compressibility problems. At the same time, the lactose enhanced the flowability of the Avicel products available at that time. The direct compression tableting process became a reality, rather than a concept, partially because of the availability of Avicel. As of 2007, Avicel PH is commercially available in 10 types with different particle size, density, and moisture grades that have different properties and applications (Table 7.6).38 Other brands of microcrystalline cellulose are also available on the pharmaceutical market, including Pharmacel 101 and 102 from DMV International and Emcocel 50 M and 90 M from JRS Pharma. 

Converted starches, also called thin-boiling starches, are produced by degradation of the starch chains into small segments. They can be cooked in water at higher concentrations than native starches. Low-viscosity starches are needed in applications where a high solid starch paste with a pumpable and workable viscosity is required. There are four classes of commercial converted starches dextrins (hydrolysis in solid-state) acid-modified starches (hydrolysis in a slurry) oxidized starches and enzymically depolymerized starches. 

Microcrystalline cellulose is a purified, partially depolymerized cellulose that occurs as a white, odorless, tasteless, crystalline powder composed of porous particles. It is commercially available in different particle sizes and moisture grades that have different properties and applications. 

In 1977, Tokiwa and Suzuki reported that some lipases, which are extracellular enzymes that usually cleave esters in oils and fats, are also able to attack ester bonds in some aliphatic polyesters and can depolymerize such materials [38]. Aliphatic polyesters, however, exhibit only limited useful properties for many applications. Aromatic polyesters, such as PET and PBT, which are widely applied because of their excellent properties, are not attacked by hydrolytic enzymes. This led to the development of aliphatic-aromatic polyesters as biodegradable plastics that present a compromise between biodegradability and material properties [39]. Recently, however, Muller et al. [40] have isolated a hydrolase (TfH) from Thermofibida fusca which is able to depolymerize the aromatic polyester PET at a high rate in contrast to other hydrolases such as lipases. They have demonstrated for the first time that commercial PET can be effectively hydrolyzed by an enzyme at a rate that does not exclude a biological recycling of PET. The effective depolymerization of PET with the enzyme TfH will result in water... 

The depolymerization can be prevented by incorporating monomeric units with higher thermodynamic ceiling temperatures into the polymer. Thus, a-methyl styrene/methyl methacrylate copolymers have achieved a certain commercial importance as heat-stable, transparent polymers for special applications. 

The most important commercial exploitation of this phenomenon has been the development of positive radiation resists for use in the semiconductor industry. A number of methacrylates and butene sulfones have been developed for these applications (42-44). Another important class of materials which undergo large-scale degradation of the main chain are the aliphatic polysulfones. For example, in 1981 Bowmer and O Donnell (45) examined the 5delds of a number of aliphatic polysulfones as a fimction of temperature and discussed these results in terms of the change in the equilibrium between polymerization and depolymerization as the ceiling temperature is approached. This aspect of radiation chemistry has had far-reaching consequences for our modem society. 

Copolymers of glycolide L(-)lactide have been commercialized for biomedical applications and are high strength biodegradable thermoplastic materials. Poly(glycolic acid) and copolymers with D,L-lactides are presumed to be biodegradable, although the role of chemical hydrolysis vs. enzymatic depolymerization in this process remains open to debate. 

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