Polymer Recycling Research in the New Decade - American Chemical

Chapter 40

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Polymer Recycling Research in the New Decade G. Tesoro and Y. Wu Department of Chemistry, Polytechnic University, 6 Metrotech Center, Brooklyn, NY 11201

For about 10-15 years, research and development efforts have addressed the need for commercially viable technologies for the conversion of post use polymeric materials to new products of value. In industry, organizations, government laboratories, and universities around the world, approaches that would combine unique technical features with economic feasibility have been investigated with varying degrees of success. The major focus has been on thermo-plastics' remelting and reprocessing, and on regrinding of cured thermosets for blending with virgin resin in controlled amounts. The challenges of chemical processing or modification for recovered polymers and of experimental polymers designed for recycling have been defined more slowly. They are now active areas of research that will provide the basis for recycling technologies in the future. This paper will discuss some of the problems and progress associ-ated with the time scale of these creative new approaches, including, for example, pyrolysis, depolymerization of polyurethanes and novel experimental thermosets. The need for new solutions for problems of solid waste disposal has been apparent since the late nineteen seventies and early nineteen eighties, with the ever-increasing pace of plastics volumes produced, utilized for new applications and discarded. The issues of environmental contamination and of energy conservation are now driving forces for the development of new generations of recoverable polymers and of creative new technologies for utilization of polymeric materials post use. In 1980, the Department of Energy (DOE) established the Energy Conservation and Utilization Technologies (ECUT) program [1], with the mission to conduct generic, long-term high-risk applied research and exploratory development in areas pertaining to energy conservation. In the ECUT materials 0097-6156/95/0609-0502$12.00/0 © 1995 American Chemical Society In Plastics, Rubber, and Paper Recycling; Rader, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


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program, research on polymer recycling and recovery was an important element. For synthetic polymers, the initial focus was on technology for producing commercially viable commodities from automobile shredder residue [2]. Longer range efforts were contemplated on developing novel separation techniques, on recovery of value from specific polymer classes and on fundamental investigations of approaches to utilization and reuse of advanced polymeric materials and composites. The DOE/ECUT policy and program have been perhaps the most important factors in determining the progress of the nineteen eighties in polymer recycling research and development. The evaluation of current status and future needs can be attempted only in the context of accomplishments directly or indirectly related to this element of the DOE program. In this paper we will attempt to summarize the highlights of these accomplishments, concepts that have emerged as a basis for the research programs of the future, and problems that are now better understood regarding the time scale for technology transfer, and for commercial initiatives as well as the role of economics in every development. The subjects of natural polymers, biodegradable polymers, renewable resources and rubber require consideration of factors that do not necessarily apply to recycling of synthetic polymers and are beyond the scope of the discussion which follows. Highlights of the Eighties The conceptual approaches - or strategies ~ in the ECUT long-range research plan for utilization of polymer waste based on processes other than incineration or landfill were outlined in general terms in 1986 [3], as follows: 1. Direct Reuse: Processing by essentially known technologies for recovery of materials to be used either alone or in mixtures with virgin product. This approach has been valid primarily for thermoplastic uncontaminated materials. It has dominated engineering developments for recovery of PET and HDPE from beverage bottles and milk bottles. 2. Reuse after Separation and/or Modification: Processing of waste consisting of complex mixtures or contaminated materials that must be fractionated and/or modified for conversion to viable commercial products. Such industrial processes as reclaiming rubber from used automobile tires for blending with new rubber, and grinding of cured thermoset resins to be used as fillers, are examples of this approach. 3. Isolation of Well-Characterized Molecular Species from Recovered Polymers (Recovery of Value-Chemical Recycling): The complexity of this approach is evident from consideration of the numerous structures and properties in waste polymers, and of the changing character of products as newly developed materials reach the market place. This classification is approximately equivalent to the terminology of "secondary recycling" for approaches 1 and 2, and "tertiary recycling" for those defined as type 3 [4]. It can also serve to identify relatively short range

In Plastics, Rubber, and Paper Recycling; Rader, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


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engineering projects and long range research efforts aimed at creative new chemistries. The issues of energy conservation [5], of economic feasibility [6], of the challenge of markets [7] for recovered materials [7] are additional problems. These have been discussed in general terms, and also studied for numerous recycling processes and products that have been developed or proposed. However, the critical needs for continuing research and increased commitment to long range efforts, as well as for interdisciplinary activities in the industrial complex, have been only partially met as we begin the second decade of this effort. The research objectives identified about ten years ago [3] contemplated a new generation of recoverable polymers designed for recycling, the application of new, sophisticated techniques for reactions (e.g., fast pyrolysis) yielding well characterized chemical compounds, and optimization of chemical recycling processes providing recovery of value by classical reaction paths (e.g., hydrolysis, alcoholysis, glycolysis). Significant progress has been made towards each of these goals. Specific projects and objectives have been based initially on investigations of recovery of products from uncontaminated polymers available in significant volume in the waste stream. In addition, continuing research and development programs, with the active participation of industry, have provided a new basis for the (current and future) implementation of viable recycling processes for producing chemical compounds and materials of value. A summary of major research efforts that have provided new insight into the opportunities and problems of future research is shown in Table I. A brief review of status is discussed below. Polyester (PET). Recycling of polyethylene terephthalate (PET) from bottles has been a true success story. Over a period of 8-10 years, direct reuse of PET (and of HDPE base polymer) coupled with quality improvement technologies has evolved from early experimental programs on melting/reprocessing to the development and marketing of fiber grade PET in significant volume [8] and to the economically feasible recovery of monomers by methanolysis and glycolysis reactions [9]. The Chicago Board of Trades decision to launch electronic trading in recycled PET in 1994 reflects the growing role of market forces and the economic viability of those PET recycling processes that have reached commercial status. It is easy to understand the rapid growth of PET recovery from bottles, and of developments ranging from direct reuse to more demanding technologies for recovery of monomers or conversion to fibers. The availability of uncontaminated polymer and the relatively simple chemistry of polyethylene terephthalate have been important factors in this growth: the volume of reclaimed PET reportedly reached 400 million pounds in 1993. (N.Y. Times, Feb. 20,1994, p. 9). It is evident that there are favorable in the recovery of value from PET beverage bottles that have benefitted both technology and economics. Polyurethanes (Foams). In contrast to PET, the chemistry of polyurethanes is far more varied and complex. Progress on recycling has occurred on several fronts, including the use of regrind as filler in hot cure - molded foam - and recycling of glycolized RIM materials (12).


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Table I

Chronology of Research

Polymer Source



Research Objectives

Status and References

—

Auto shredder residue 1980-1985

Preliminary scanning of possibilities

PET (Beverage Bottles) 1985 to date

Ranging from direct reuse (melt/regrind) to chemical processing, monomer recovery, new products.

Commercial (8) - (9)

Polyurethanes (Foams) 1978 to date

Ranging from direct reuse to chemical approaches for monomer recovery.

Commercial (10) - (19)

Thermosets (Epoxy, Polyimides) 1988-1992

Experimental polymers designed for recovery. Novel materials.

Patents Issued (21) - (23)

Unsaturated Polyesters 1992-1994

Grinding of cured polymers as extenders for virgin resins. (Also recovery of monomers)

Extenders for virgin resin in SMC (24)-(26)

Polystyrene 1990 to date

Melt/reuse

Commercial (27)

Pyrolysis 1979 to date

Fluid bed pyrolysis of comingled waste polymers for monomer recovery.

Commercial (29) (a) (b)

Fast Pyrolysis 1990 to date

Recovery of monomers from comingled plastics.

Patents Issued (30) (a) (b) (c)

(2)

The history of research on polyurethanes (PU) recycling is long, but several approaches are reaching commercial maturity at this time. Documented technologies proposed range from regrinding and blending of waste material to the isolation of polyol and amine precursors of isocyanates by hydrolytic cleavage or by a process termed "hydroglycolysis" [10,11,12]. This "case history" provides an interesting view of the value of long range research investigations, of the time scale for the potential utilization of laboratory research results and of the constraints imposed by economic factors. In 1978, Grigat published an article [13] on clean hydrolytic cleavage of PU to recover starting polyol and the amine precursors of isocyanates. The documented research was part of a program financed by German government and industry, and designated as "recovery of raw materials from synthetic polymers". The author was the project leader for the


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development of continuous processes covered in detail by a final report issued in August, 1981 [14]. Commercial processes recently announced by Bayer, ICI, and Dow Chemical [15] have a scientific foundation in the research of the late seventies on hydrolysis in Germany [14] and on glycolysis in the U . S. [16]. Also, and in-depth investigation was reported by the scientific research staff of the Ford Motor Co. [17] on the reaction of polyether-based P U foam with dry atmospheric pressure steam (190-230°C) to yield high quality polyol, isomeric toluene diamines and C 0 as primary products. This was proposed as one approach to waste P U foam recovery at the time. In subsequent work on glycolysis of P U foam the Ford investigators [18, 19] examined the problems associated with "simple" glycolysis and the potential of a new Ford process termed "hydroglycolysis". Estimates of operating cost and process profitability (based on 1981 prices) showed at the time that economic incentive was "doubtful". The results of this extensive research on chemical processes for monomer recovery [10-19] are seemingly used commercially to a very limited extent, and development decisions on polyurethane recycling have apparently been governed by economic considerations. Thus, the history of research on PU recovery where new techniques can yield well characterized monomers and chemical compounds highlights a dilemma of this day, namely, the difficulty in the translation of successful, creative laboratory results to viable commercial processes with realistic economics.


2

Thermosets (Epoxy, Polyimide). The problem of recovery of thermosets has been an even greater challenge. The conventional wisdom of grinding cured resin and using it as filler has been recognized as an interim approach to utilization of cured resins. In spite of the relatively low volume of thermoset resins as compared to thermoplastics [20], recycling of thermosets was viewed as a major challenge when the long range plans of the ECUT materials program were formulated in the mid-eighties. Exploratory research on recovery of thermosets was initiated for epoxy resins [21], well characterized chemically and important technologically, particularly for advanced composites. The results of this research propose experimental polymers where the molecular structure is designed for recovery, and the properties are comparable to those of commercial resins currently employed. An overview of the chemistry and of a preliminary evaluation of economic feasibility [22] suggest that the approach is viable for epoxy resins. In subsequent research, it has been successfully applied to new, recoverable thermoset polyimide copolymers [23] where labile bonds are introduced for the purpose of solubilizing and reprocessing resins. To the best of our knowledge, the results published on these developments and covered by patents in 1989-1993 have not been explored in industry to date. There are significant opportunities for future development of the experimental polymers and for chemical modifications designed to impart special properties. The need for interactive research with industry for the optimization of these (and other) laboratory results is an objective for the next decade.



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Cured Unsaturated Polyesters. The recovery and isolation of well characterized chemical compounds of potential value from thermosets has also been explored for cured unsaturated polyesters, employing a different approach [24]. In this case, laboratory results of neutral hydrolysis have shown that current interest and possible regulations for recovery of SMC from automotive applications [25] may stimulate further work in industry in the future, perhaps aimed at studies of process optimization, and of development of unsaturated polyesters of improved properties from recovered oligomers. At this time, however, industrial work reported on recovery of cured unsaturated polyester has been focused primarily on defining the amounts of SMC regrind which can be incorporated in virgin material without impairing its properties [26]. Polystyrene. Recycling of polystyrene by chemical processing has not been a primary objective for research, partly because specific functional groups can be introduced by by employing comonomers in synthesis, and partly because the economics of chemical recovery and modification processes could not compete with current commercial production by regrind/reuse, for example of "Retain" resins (Dow Plastics-55% post consumer recyclate content) or "Dylite" (Arco Chemical-25% recyclate). The National Polystyrene Recycling Company (NPRC) is a multimillion dollars organization including several major polystyrene producers which was started in 1989 with the goal of facilitating industry efforts to recycle 25% of the PS produced in the U.S. for food service and packaging applications each year [27]. The growth of this industry endeavor is certain as an increasing number of PS recycling facilities is implemented by NPRC. Pyrolysis. For several years, the feasibility of recycling comingled post-use polymers by pyrolysis has been a controversial subject. However, several processes have been investigated in industry [28], and recycling technologies based on fluidized bed pyrolysis have been studied extensively at the University of Hamburg [29][a][b]. Research has focused on the suitability of plastic wastes, used tires, and oil residues as sources of well characterized purified olefins and other hydrocarbons. Pilot plants with capacity of 10-40 kg/hr of plastic and up to 120 kg/hr of used tires have been installed [29] and a semi-industrial plant using the Hamburg process has been built in Germany by the Asea Brown Boveri Company. According to a recent report, the American Plastics Council (a Washington, D . C , trade group) is promoting pyrolysis as a viable form of chemical recycling. Fast Pyrolysis. In a long range program at NREL, the objective of research on fluidized-bed high temperature pyrolysis has been to translate an understanding of pyrolytic events at the molecular level to the design of processes requiring minimum mechanical separation and purification of products. Mixed plastic waste streams including nylon 6 carpet, polyurethane, and PET-containing wastes have been selected for study [30(a)]. Recovery of pure monomer (caprolactam) from nylon 6 (caprolactam) has been attained. Molecular Beam Mass


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Spectrometry (MBMS) experiments have been employed in this research to evaluate the possibility of recovering specific monomers and compounds of interest from individual waste streams. Reportedly, polyurethane will be investigated in the future. Recently issued patents describe a process for fractionating organic and aqueous condensates made by fast pyrolysis of biomass material [30(b)] and "a method for controlling the pyrolysis of a complex waste stream of plastics and for converting the stream into useful high value monomers" [30(c)]. General Concepts. The technological advances in recycling of PET, Polyurethanes and Polystyrene are evident from this brief discussion of accomplishments. It is also clear that chemical research results for recovery of polyurethanes, thermosets, and for fast pyrolysis of comingled polymers have not been implemented commercially at this time, perhaps for reasons of economic feasibility, and partly because major developmental efforts requiring industry participation have not been available to date. It is with these considerations in mind that we can attempt to formulate research needs for the next decade. Future Research and Development Needs. Future needs for effective research can be defined on several levels, depending on the complexity of chemical problems, and on economic feasibility of potential solutions. It is desirable to realize the benefits of research results now available, by process optimization, by chemical modification of existing systems as recovered, and by continuing efforts, with appreciation of industrial interests. The time scale for implementing results of new laboratory research must be recognized. Changes in the composition and properties of materials require consideration, and interdisciplinary programs should be pursued. A prime example of this need is research for the recovery of polymer blends that are becoming increasingly important in the marketplace. Long range programs are essential for implementing new concepts, and for creating markets for new experimental materials designed with recycling in mind. An increasing commitment to long range projects on the part of the industrial complex is needed, devoting talent and funds to new initiatives as well as to the utilization of available knowledge, to scale-up experimentation, and to optimization for technology developed in the laboratory. This may necessitate cooperative programs of industrial groups with research in universities, whenever knowledge of industrial products and processes can be coupled with the results of preceding or concurrent scientific investigations, and where constraints imposed by policy and patent problems can be dealth with. In addition, a broad research program sponsored by government for the exploration of a new generation of polymers designed for recycling and earmarked for specific applications would be desirable. This would be a major undertaking, comparable in some aspects to the polymer element of the ECUT material program as proposed in the early 1980's [1], The underlying principles for such a program require fundamental knowledge of the relationships of properties with molecular structure, and also with effects of molecular assembly, intermolecular forces,



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morphology in the solid state, and interactions in multicomponent systems [31]. Thus, an inter-disciplinary effort is essential. New polymer systems that satisfy performance properties in established applications - and yet can be recovered and recycled economically, must be designed with these considerations in mind. This is a formidable challenge. On the basis of knowledge and experience gained during a decade of progress, it is evident that a long time is required for the development and scale up of creative new concepts potentially applicable to the design of recyclable polymeric materials. For example, the principle of reversible crosslinks explored for thermosets [21, 22] and the results of research on thermally controlled covalent bond formation [32] have demonstrated the time and effort required for implementing promising approaches in the search for systems that can be practical for industrial use. Briefly stated, chemical research needs for the next decade may be summarized as follows: • Optimization of approaches developed in the 1980's; • Research on recycling of newly available commercial materials (e.g., polymer blends); • Long range interactive programs of industry with universities; • Early assessment of economic feasibility; • Government sponsorship and leadership for projects designed to develop recyclable polymers for the future (1)

(2) (3) (4) (5) (6) (7) (8)

(9) (10)

(11)

References J. A. Carpenter, Jr. et al., Long Range Program Plan for F Y 1987-1994; ECUT Materials Program (ORNL), pp x-xx See also ORNL reports of the ECUT Materials program under Contract No. DE-AC05-840R21400 Plastics Institute of America (PIA), Secondary Reclamation of Plastic Waste - Phase I and Phase II, Technomic Publishing, Lancaster, PA, (1987) Reference 1 - Pages [2.2] - 2, 8, 9, 17 J. Leidner, Plastic Waste, M . Dekker, NY, pp 64-65 (1981) T. R. Curlee, Materials and Society, 12 (1), 1-45 (1988) T. R. Curlee, The Economic Feasibility of Plastics Recycling, Praeger N Y (1986) M . Alexander, Society of Women Engineers (SWE) The Challenge of Markets, pp 7-11, Nov/Dec (1993) a, V. Comello, R & D Magazine, October 25, 20-22, (1993) b, R. Leaversuch, Modern Plastics, October, 79-80, (1993) and July, 48-50 (1994) c, S. Diesenhouse, The New York Times, February 20, (1994) p 9 R. Leaversuch, Modern Plastics, July, 40-43 (1991) a, USP 4,196,148 (1980) to Ford Motor Co. b, USP 4,316,992 (1982) to Ford Motor Co. c, USP 4,328,368 (1992) to General Motors d, USP 5,089,571 (1992) to Dow Chemical Co. Materials World (6B), September, 485, (1993)


510 (12) (13) (14)


(15)

(16) (17) (18) (19) (20) (21) (22)

(23) (24) (25) (26) (27) (28) (29)

(30)

(31) (32)


Modern Plastics, November, 71-73, (1993) (From a paper presented at the Polyurethane World Congress, Vancouver, B.C., October 10-13, (1993) E . Grigat, Kunststoffe, 68 (5), 281-284, (1978) G. Niederlmann, G. Hetzel, H . Petersen, Report No. BMFT-FB-T-83-199 (1981), (NTIS, PC-A09/MFAOI) a, Modern Plastics, January, 47, (1991) b, Modern Plastics, February, 20, (1991) c, Plastics and Rubber International, June/July, 13, (1992) H . Ulrich et al. Polym. Eng. Sci. 18 (11), 844-848 (1978) J. L. Gerlock et al. J. Polym. Sci. (chem.), 18, 541-557 (1980) J. L. Gerlock, J. Braslaw, M . Zimbo, Industrial Engineering Chemistry Process Res. Dev., 23, (3), 545-552, July, (1984) J. Braslaw, J. Gerlock, Industrial Engineering Chemistry - Process Res. Dev., 23, (3), 552-557, July, (1984) T. R. Curlee - Preliminary Report to DOE by ORNL, Contract No. DEAC05-840R21400, April, (1989) G. C. Tesoro and V. Sastri, J. Appl. Polym. Sci. 39, 1425-1437; 1439-1457, (1990) G. C. Tesoro, H . Chum and A. Power, Proceedings of the 47th Annual Conference of the Composites Institute, Sc. 4C, 1-8 February, (1992) Y . Wu, G. C. Tesoro and I. Engelberg, in Emerging Technologies in Plastics Recycling, ACS Symposium #513, (G. Andrews and M . Subramanian, Editors), 186-196, (1992) G. C. Tesoro and Y. Wu, Advances in Polymer Technologies, 12, (2), 185196 (1993) Modern Plastics, April, 69-73 (1993) Proceedings, 49th Annual Conference Composites Institute, session 15, February, (1994) G. A Mackey, R. C. Westphal, R. Coughanouz, in Plastics Recycling (R. J. Ehrig, Editor) 109-129 (1992) Modern Plastics, January, 93, (1994) a, W. Kaminski, in Emerging Technologies in Polymer Recycling, ACS symposium #513 (G. Andrews and M . Subramanian, Editors), 6072 (1992) b, W. Kaminski and H . Rössler, Chemtech, February, 108-113, (1992) a, Report to DOE, Office of Industrial Technologies by NREL, Industrial Technologies Division, First Quarter, 14-16, (1994) b, H.L. Chum et al. USP 5,223,601 (1993) to Midwest Research Institute c, R.J. Evans et al. USP 5, 321, 174 (1994) to Midwest Research Institute H.R. Allcock, Science, 225, 1106-1112, (1992) L. P. Engle and K. B. Wagener, Rev. Macromol. Chem. Phys., C33 (3), 239-257, (1993)

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1995


Polymer Recycling Research in the New Decade - American Chemical

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