Recycling of Plastic Materials Part 2 potx

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 al- ready given in Figure 4. Methanolysis of PET-waste The waste is treated with methanol (in a ratio 1/2 to 1/10), usually under pres- sure at high temperatures (160-310 o C) in the presence of transesterificationand (or) depolymerization catalysts. 17 Once the reaction is completed, DMT is recrystallised from the EG-methanol mother liquor, and distilled to obtain poly- merization-grade DMT. Also EGandmethanolare purified by distillation.East- man Kodak has been using such a process for recycling of X-ray films for 25 years, and itis still improving theprocess, 18 e.g., by usingsuperheated 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-waste 19 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 prac- tice. Figure 5 gives a flow chart of both processes. While both systems are com- pletely 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 profitability questionable. In addition, the simultaneous (with TPA) regenera- tion of ethylene glycol is difficult, ecologically undesirable (requiring the use of organic halogenated solvents), and not economical. Concerning alkaline hydro- lysis, the profitabilityis strongly determined bythe necessity of expensivefiltra- 8 PET Film Recycling W. De Winter 9 Figure 5. Flow chart of acid- and base-catalyzed PET degradation. tion and precipitation steps. To our knowledge, recycling of PET-waste by hydrolysis is not practiced on a production scale at present. This situation even persists in spite of the fact that the majority of newer industrial PET-synthesis plants are based on the TPA-process rather than on the DMT-process. 20 INCINERATION Another approach which can be used to recycle plastics, particularly when they contain alarge amount of impuritiesand other combustible solids(if such is a case, it is important to keep them away from landfills), is more recently called “quaternary recycling”, and consists of the energy recovery from the wastes by burning. 21 Research along this line has been performed, particularly in Europe and Japan, since the early 1960s. Strong emphasis has been laid on an optimiza- tion of incinerators with regard to higher temperature of their operation and re- duction of the level of air pollution. PET has a calorific value of ca. 30.2 MJ/kg, which is about equivalent to that of coal. It is thus ideally suited for the incineration process. The combustion of plastics, however, requires 3 to 5 times more oxygen than for conventional incin- eration, produces more soot, develops more excessive heat, and incineration equipment had to be adapted in order to cope with these problems. Several processes have been worked out to overcome these technological draw- backs. 22-27 Examples include Leidner’s continuous rotary-kiln process, Baliko’s process for glass-reinforcedPET, Crown Zellerbach Corporation’scombined sys- tem for wood fibre and PET to provide steam to power equipment, and ETH-Zu- rich’s fluidized bed system for pyrolysis, especially of photographic film, i.e., in combination with silver recovery. The latter system raises the additional prob- lem of the formation of toxic halogenated compounds, stemming from the pres- ence of silver halides. Typical operation conditions take place at temperatures around 700 o C. At lower temperatures, waxy side-products are formed, leading to clogging. At higher temperatures, in turn, the amount of the desirable fraction of mononuclear aromatics decreases. A representative sample, pyrolysed under optimized conditions, yields, in addition to water and carbon, aromatics like benzene and toluene, and a variety of carbon-hydrogen and carbon-oxygen gases. Studies have been performed 1 to avoid formation of dioxines and disposal of residual ashes containing heavy metals and other stabilizers. 10 PET Film Recycling be resolved; however, quite a few residual hurdles will have to be taken 25 before an economically feasible and ecologically accepted industrial technical process will be available. BIO- AND PHOTO-DEGRADATION Although there certainly has never been a great incentive for making unstable polymers, the idea of making photo- or bio-degradable polymers has long ex- isted, 28,29 and quite a bit of effort has gone into research along these lines. For such a process, of course, limitations with regard to the percentage of allowable impurities do not exist. Photodegradation Special photodegradable polymers 30 were synthesized for the purpose of hav- ing them destroyed after use(e.g., in a landfill). Another approach was theincor- poration of suitable groups (e.g., carbonyl) in the polymer backbone in order to make polymer photodegradable by sunlight or UV (see Figure 6). A problem arises due to the fact that light exposure conditions on a landfill cannot be regu- lated. The maindifficulty, however, seems tobe practically insurmountable: it is W. De Winter 11 Figure 6. Photodegradable monomers and polymers. 31 At present it seems that most problems arising during incineration of PET can hardly possible to combine rapid degradation upon exposure to light in a landfill after use with a good light-stability of the film during service. This contradictio in terminis is probably the reason why this method never really caught on. 29 An- other problem is a combination ofdesired properties with favorableeconomics. Biodegradation The main difference between biodegradation and photodegradation lies in the possibility to create in a landfill an environment completely different from that encountered under normal storage conditions; e.g., microorganisms which can destroy plastic films may be added to a landfill. In spite of the fact that substantial research time was spent on studies in this field, it is claimed 32 that surprisingly little is understood about the molecu- lar-level interaction between polymers and microorganisms. This can be ex- plained by a poorly defined environment (in a landfill), and by a large number of complex parameters involved in the process: methods of evaluation based solely on changes in physical properties are thus unsuitable for forming conclusions, similar to the evaluations based only on biogas production. Specifically for poly- esters, however, a number ofinteresting data are available. Esterases (ester-hy- drolyzing enzymes) and also some microorganisms are known to biodegrade polyesters at a reaction rate depending upon the polyester structure. 29,33 While many aliphatic polyesters, specifically poly(hydroxy fatty acids) - e.g., the BIOPOL 34-36 packaging material commercialized by ICI - are suited for biodegradation, the aromatic polyesters (e.g., PET) do not possess this prop- erty. 32,37-39 Another approach consists of mixing small amounts of biodegradable poly- mers, e.g., polysaccharides,with a regular polymer(e.g., a polyolefin), inorder to make the end-product destroyable as well. Examples of polysaccharides/poly- ethylene have been commercialized. 38 Mixtures of starch with other polymers, 40 12 PET Film Recycling including PET, have been studied, 34 but no commercialization of the latter mix- ture is known so far. The fact, however, that the starch additive is only needed in small amounts, which hardly alters the properties of an original polymer, might show some promise for future applications. One has to realize, however, that the thermal stability of starch derivatives above 230 o C is limited, whereas the PET-film extrusion temperature is in the range of 280 o C. There also remain some controversies about the completeness of the degradation of polymer/starch mixtures. Although the development of biodegradable plastics is still in progress, it is be- coming evident that theenormous market potential, forecast some yearsago, re- quires a real breakthrough in order to be attained. 41,42 The main reason for this setback is probably the fact that organic polymers do not biodegrade fast enough. 43,44 CONCLUSIVE REMARKS • From the data presented in this overview, it seems obvious that there ex- ists a clear hierarchy in PET-film recycling technologies. The most impor- tant criteria of classification are, first of all, the degree of “purity” of PET-scrap to be handled, and secondly, the economics of the process. • For the cleanest PET grade, the most economical process, i.e., direct re-use in extrusion, is self-explanatory. • For less clean PETsamples, it is still possible tore-use them after the modi- fication step (partial degradation, e.g., by glycolysis) at a reasonably low price. • More contaminated PET-film waste must be degraded into the starting monomers, which can be separated and re-polymerized afterwards, of course, at a higher cost. At present, only the methanolysis process is ex- ploited industrially, as opposed to hydrolysis processes, which are kept in reserve. • Finally, the most heavily contaminated PET-shreds have to beincinerated. Here, however, economics may not be favorable enough for industrial de- velopment. As an alternative, those PET-shreds are brought to a landfill. Perhaps in future more attention will be given to modification of PET-films in such a way that they may become biodegradable, if the process can be ac- celerated or if a real breakthrough becomes available. W. De Winter 13 REFERENCES 1 F. P. Boettcher, ACS Polymer Preprints, 32 (2), 114 (1991). 2. W. De Winter, Die Makromol. Chem., Macromolecular Symposia No. 57, 253 (1992). 3. Anon., Plastics Bulletin, 174, 6 (Jan-1992). 4. N. Basta et al., Chem. Eng., 97, 37 (Nov-1990). 5. Brit. Pat. 1.476.539 (1977) to Barber-Colman Co. 6. Anon., Manufacturing Chemist, 66, (Mar-1987). 7. L. Hellemans, R. De Saedeleer, and J. Verheijen, US Pat. 4,008,048 (1977) to Agfa-Gevaert. L. Jeurissen and F. De Smedt, Brit. Pat. 1,486,409 (1977) to Agfa-Gevaert. J. Tempels, Brit. Pat. 1,432,776 (1976) to Agfa-Gevaert. 8. W. Fisher, US Pat. 2,933,476 (1960) to du Pont. 9. J. Burke, in Plastics Recycling as a Future Business Opportunity, Technomic Publishing Co, Pennsylvania, USA, (1986). 10. K. Datye, H. Raje, and N. Sharma, Resources and Conservation, 11, 117 (1984). 11. D. Gintis, Die Makromol.Chem., Macromolecular Symposia, 57, 185 (1992). 12. R. Richard et al, ACS Polymer Preprints, 32 (2), 144 (1991). 13. A. Petrov and E. Aizenshtein, Khim. Volokna, 21 (4), 16 (1979). 14. US Pat. 3,884,850 to Fiber Ind. 15. Anon., Mod. Plast. Int., 20, 6 (1990). 16. A. M. Thayer, Chem. Eng. News, (Jan. 13, 1989). 17. Brit. Pat. 784,248 (1957) to du Pont. 18. Anon., Eur. Chem. News, 30 (Oct. 28, 1991). 19. H. Ludewig, Polyester Fibers, Chemistry and Technology, Wiley Int. Publ., 1971. 20. H. Schumann, Chemiefasern Textil, 11, 1058 (1990). U. Thiele, Kunststoffe, 79 (11), 1192 (1989). 21. T. Randall Curlee, The Economic Feasibility of Recycling, Praeger Publishers, New York, 1986. 22. Leidner, Polymer Plastics Techn. & Eng., 10 (2), 199 (1978). 23. S. Baliko, Energiagazdalkodos, 28 (11), 496 (1987). 24. D. Vaughan, M. Anastos, and H. Krause, Rpt. Battelle Columbus Lab., EPA-670/2-74-083, (Dec-1974). 25. R. Hagenbucher et al, Kunststoffe, 80 (4), 535 (1990). 26. K. Niemann and U. Braun, Plastverarbeiter, 43 (1), 92 (1992). 27. W. Kaminsky et al., Chem. Ing. Techn., 57 (9), 778 (1985). 28. Guillet, Chem. Eng. News, 48, 61 (May 11, 1970). 29. F. Rodriguez, Chem. Techn., 409, (Jul-1971). 30. G. Smets, Chem. Magazine, 481, (Sep-1989). 31. Brit. Pat. 1,128,793 (1968) to E. Kodak. 32. G. Loomis et al., ACS-Polymer Preprints, 32 (2), 127 (1991). 33. R. Klausmeier, Soc. Chem. Ind., London, Monogr., 23, 232 (1966). 34. Anon., Neue Verpackung, 1, 50 (1991). 35. J. Emsley, New Scientist, 50, 1 (Oct. 19, 1991). 14 PET Film Recycling 36. A. Steinbuchel, Nachr. Chem. Techn. Lab., 39 (10), 1112 (1991). 37. P. Klemchuk, Mod. Plastics Int., 82, (Sep-1989). 38. J. Evans and S. Sikdar, Chemtech, 38, (Jan-1990). 39. K. Joris and E. Vandamme, Technivisie, 179, 5, (1992). 40. R. Narayan, Kunststoffe, 79, 1022 (1989). 41. N. Holy, Chemtech, 26, (Jan-1991). 42. A. Calders, Technivisie, 156, 8 (Nov-1990). 43. Anon., Mod. Plast., 20 (1), 72 (1990). 44. H. Pearce, Scient. European, 14, (Dec-1990). W. De Winter 15 The Importance and Practicability of Co-Injected, Recycled Poly(ethylene terephthalate)/Virgin Poly(ethylene terephthalate) Containers Eberhard H. Neumann Nissei ASB GmbH, Mündelheimer Weg 58, D-4000 Düsseldorf 30, Germany INTRODUCTION In several European countries, packaging items, whether they are made of plastic, paper, metal, etc., are under governmental and public pressure. Well-known are: • actions to ban all plastic bags in one southern European country • the boycott of plastic packages in certain alpine villages • Denmark’s ban of metal cans for beverages • Switzerland’s removal of all PVC-packages • Germany’s set-up of a mandatory deposit on beverage bottles made of plas- tic and limiting sales on non-refillables. The list of restrictions on packages and their markets in Europe could be end- less. Increasing environmental concerns, overloaded landfills and inadequately equipped or even not existing garbage incineration units are calling for solu- tions. Out of many proposals two solutions are always highlighted in public discus- sions: • refillable and returnable packaging articles to reduce the amount of house- hold refuse • recycling of post-consumer packages. Regarding recycling systems for post-consumer plastic bottles, companies have already installed plants in North America which are profitable operations. However, these systems are leading to a converting technology which trans- forms discarded plastics into a range of second-use commodities, as well as low E. H. Neumann 17 cost base specialties. This paper is intended to show a technology, whereby discarded plastic bottles for beverages, food and household items - being post-consumer ware - can be used to manufacture the same range of packaging articles (bottles) for which they were originally made. Additional goal is to assure the highest level of safety offered by the original products made from virgin plastic. BASIC TECHNOLOGY Injection molding technologyinvolves the injection ofmolten plastic into one or several cavities via a hot-runner system (melt-channel distribution system) and rapid cooling of a preform to a low temperature. At this point the freshly manu- factured article can be ejected from the cavity. In multilayer technology, more than one plastic resin is injected into the cavity. The different resins are molten in separate injection units, conveyed in sepa- rate hot-runner channels, under pressure and high temperature, to an injection nozzle, which is the gate area for the molten plastic, into the cavity. This injection nozzle consists of an outside and an inside tube. Both plastic streams are brought together after beingreleased from the nozzle and they bond together because of a high pressure (up to 300 bar) and a high melting tempera- ture (see Figure 1). MANUFACTURING PROCESS OF MULTILAYER BOTTLES CONTAINING REGRIND For the process, a machine used had one injection unit for virgin PET and an- other injection unit for reground PET flakes. The individual steps of the process are described below. Drying of PET resin and PET flakes (Figure 2) The drying of a virgin PET resin and reground PET flakes at temperature lev- els of 160-180 o C to below 0.005% moisture content is essential for the production of amorphous multilayer polyester bottles. Polyester is an effective desiccant. The water absorption depends on a relative humidity, residence time, tempera- ture, and dimension of the flakes. When flakes containing moisture are heated up to the melting temperature, hydrolytic degradation occurs lowering a viscosity of the melt that results in en- hanced ability of preforms/bottles to crystallize (milky appearance). 18 PET Co-injection Molding [...]... and would add to the cost of the reground polymer material The intrinsic viscosity was 0.03 to 0.05 dl/g lower compared to that of virgin PET Such difference is negligible Only extreme variation in viscosities of melt-layers can lead to pro2 cessing problems The size of flakes was in general between 9 and 49 mm E H Neumann Figure 3 Bottle shape and the thickness of layers 23 24 PET Co-injection Molding... equipment) 3.75 3.75 5 Rent for space 8 DM/m2 per month: 500 h 0 .28 0.35 6 Labour (1/3 operator) 33 DM/h 11.00 11.00 7 Maintenance 20 % of machine price 9.71 12. 43 8 Entire cost per hour 88.75 106.97 106.97 9 Cost and output per hour (production cost) = DM/PC 0.0647 0.0780 0.0780 10 Total resin cost including scrap = DM/PC 0.1 12 0.0 721 0.0655 100% virgin PET 62. 17 60% 4.97 PET-regrind 70% PET-regrind... are fed by gravity into the screw of the injection unit Co-injection molding of virgin and reground PET flakes The virgin PET as well as the reground PET flakes are melted inside the separate injection units by means of external electrical heating of the injection barrel and the applied shear forces of the screw, driven by a hydraulic motor (see Figure 1) E H Neumann 21 The PET melt, either coming from... stretch-blow-molding or by applying thermal energy from the 22 PET Co-injection Molding outside by moving the heater-pots around the preform and/or by allowing a heated core rod to plunge into the center of the preform, thus influencing the thermal profile of a preform wall from the inside After thermal conditioning is accomplished, the preform is transferred into the blow mold of the stretch-blow-molding station Here... alternative method By this method the first preform is made of a thin layer of virgin PET In the next step, melt from reground PET flakes is injected into the exterior of the preform layer made of virgin PET Such technology requires two injection preform molds and subsequently a five station machine, compared with a standard machine which has four TRIALS OF CO-INJECTING VIRGIN PET AND REGROUND PET FLAKES The... preform (made out of one melt stream) Conditioning and stretch-blow-molding Thermal conditioning is the next step of the multilayer preform production The purpose of thermal conditioning of a given preform is to provide the necessary temperature distribution in a preform After leaving the injection mold and undergoing cooling process, the preform has a cross-sectional temperature distribution of an upside-down... the hopper The flakes coming 20 PET Co-injection Molding Figure 2 Close-loop drying system for PET regrind from less stretched bottle regions, i.e the neck and bottom recrystallize during the drying process Crystallized flakes will not stick together The dried reground PET flakes are fed via a feeding-extruder into the throat of the satellite injection (2nd and 3rd) unit of the multilayer single stage... circumferentially inflated by air pressure, to match the shape of the blow mold The final bottle is cooled down due to the contact heat losses on the metal surface of the blow mold The stretch-blow-molding process leads to a biaxial orientation of the macromolecules resulting in better mechanical properties and lowering the gas permeation of bottles Double-layer preforms The injection molding technique... ASB -25 0 T-Series machine (multilayer type) was used The processing parameters, as chosen, were within standard set-ups The bottle shape for these trials and its layer distributionare shown in Figure 3 Trial results The manufactured bottles were of a high quality When supplied with a clean regrind, the transparency was similar to that of comparable monolayer bottles Physical and mechanical properties of. .. majority of micro-organisms found on PET bottle surfaces is washed away during the cleaning process of the post-consumer reground PET flakes The remaining minor amounts do not survive the drying and processing temperature o in the injection molding unit, which reaches 180 and 27 0 C, respectively Contamination by foreign substances There is a possibility that PET bottles are used for storage of substances . 179, 5, (19 92) . 40. R. Narayan, Kunststoffe, 79, 1 022 (1989). 41. N. Holy, Chemtech, 26 , (Jan-1991). 42. A. Calders, Technivisie, 156, 8 (Nov-1990). 43. Anon., Mod. Plast., 20 (1), 72 (1990). 44 Praeger Publishers, New York, 1986. 22 . Leidner, Polymer Plastics Techn. & Eng., 10 (2) , 199 (1978). 23 . S. Baliko, Energiagazdalkodos, 28 (11), 496 (1987). 24 . D. Vaughan, M. Anastos, and H Columbus Lab., EPA-670 /2- 74-083, (Dec-1974). 25 . R. Hagenbucher et al, Kunststoffe, 80 (4), 535 (1990). 26 . K. Niemann and U. Braun, Plastverarbeiter, 43 (1), 92 (19 92) . 27 . W. Kaminsky et al.,