Poly monomer recycling process

Figure 13.7 Block flow diagram of current polyfmethyl methacrylate) (PMMA) monomer recycling process. MIB, Methyl isobutyrate MA, methyl acrylate MMA dimer, 1,4-cyclohexane dicarboxylic acid dimethyl ester [87]. Modified from Kikuchi Y, Hirao M, Sugiyama H, Papadokonstantakis S, Hungerbuehler K, Ookubo T, et al. Design of recycling system for poly(methyl methacrylate) (PMMA). Part 2 process hazards and material flow analysis. Int J Life Cycle Assess 20i4 i9(2) 307—19.

Other materials in waste that is thermally processed also were studied by pyrolytic techniques, typically with the purpose of regenerating the monomers or of obtaining other useful small molecules. For example, pyrolytic studies were performed for the evaluation of the possibilities for re-utilization of nylon carpet waste [7], the recycling of thermoset polymeric composites [8], the recovery of methyl methacrylate from poly(methyl methacrylate) waste [9], as well as for other raw material recovery from pyrolysis of plastic waste [10]. The results of incineration of various other types of waste also were studied at model scale [11, 12). These studies were applied to specific waste materials associated with the manufacturing process or to municipal solid waste [13-15)... 

Poly(methyl methacrylate) can be converted to monomer for recycling by passage through a continuous twin screw extruder at a little over 400°C.65 The 95% yield of 95-99% purity compares favorably with the 60-80% yield of 60-80% purity of the batch process, which is ten times slower. 

Selected chemolysis processes for PET are illustrated in Figure 9.8 and tabulated in Table 9.6. These reactions yield either the original monomers or products that can be converted to other monomers. Hydrolysis can be effectively used with PET and polyurethane waste plastics in feedstock recovery (Zia et al., 2007). Reaction conditions employed are varied and these selected references do not cover them exhaustively. Aromatic polyesters, PET and poly(butylene terephthalate), have been studied intensively for feedstock recovery. PET is extensively used in soda bottles and less than 30% of the product is mechanically recycled. 

There are forms of recycling that are less familiar —in that they do not directly recycle the materials into new artifacts. Tertiary recycling breaks down the plastic to its component monomers, which can be repolymerized. This requires pyrolysis of the materials, sometimes called thermal cracking, and is most easily accomplished with step growth polymers such as polyamides and polyesters but is also possible for poly(methyl methacrylate). The processes require specialized plant, so are capital intensive, and a large-scale operation is necessary for economic viability. 

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