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Recycling of Plastic Materials Part 2

Tham khảo tài liệu 'recycling of plastic materials part 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 8 PET Film Recycling MONOMER RECOVERY Although monomer recovery is the oldest recycling method and can be used to recover PET-waste having a high degree of impurity it is regrettable that it is not the most economical method. The earliest methods of PET synthesizing were based preferentially on the use of dimethyl terephthalate DMT which could be better purified than terephthalic acid TPA therefore methanolysis is discussed before hydrolysis. The chemical principles of both processes are already given in Figure 4. Methanolysis of PET-waste The waste is treated with methanol in a ratio 1 2 to 1 10 usually under pressure at high temperatures 160-310oC in the presence of transesterification and or depolymerization catalysts.17 Once the reaction is completed DMT is recrystallised from the EG-methanol mother liquor and distilled to obtain polymerization-grade DMT. Also EG and methanol are purified by distillation. Eastman Kodak has been using such a process for recycling of X-ray films for 25 years and it is still improving the process 18 e.g. by using superheated methanol vapor to allow the use of ever more impure PET-waste. Important factors which have to be dealt with in this process are avoiding coloration and keeping down the formation of ether-glycols. Hydrolysis of PET-waste19 Although aromatic polyesters are rather resistant to water under atmospheric conditions compared with other polymers they can be completely hydrolyzed by water at higher temperatures and under pressure. For practical purposes however particularly to speed up the process use has to be made of catalysts. Acidic as well as alkaline catalysts have been studied and worked out in practice. Figure 5 gives a flow chart of both processes. While both systems are completely realistic their usefulness under practical production conditions remains controversial. As far as acid hydrolysis is concerned the large acid consumption and the rigorous requirements of corrosion resistance of the equipment make .