Thermosets recycling technology

Plastic resins differ in terms of which recycling technologies are appropriate. Thermoplastics are more amenable to mechanical recycling than thermosets, which cannot be melted and reshaped. Typically, condensation polymers such as PET, nylon, and polyurethane are more amenable to feedstock recycling than addition polymers such as polyolefins, polystyrene, and PVC. Most addition polymers produce a complex product mixture that is difficult to use economically as a chemical feedstock, while condensation polymers usually produce relatively pure one- or two-component streams. 

Pickering S J (2006), Recycling technologies for thermoset composite materials - current status . Compos Appl Sci Manuf 37, 1206-1215. 

Utilisation of Thermoset Recyclate by Integrated Extrusion Technology... 

Pickering, S.J. (2006) Recycling technologies for thermoset composite materials-current status. Composites Part A, 37 (8), 1206-1215. 

Composites are inherently difficult to recycle because they are multiphase. The two or more phases/materials that constitute the composite are normally intermixed on a very fine scale, and trying to separate them complicates the recycling process. However, some techniques have been developed, with modest success, for reoyoUng polymer-matrix composites. Recycling technologies differ only slightly for thermoset-matrix and thermoplastic-matrix composite materials. 

Soft flexible rubbery behavior depends on long flexible polymer molecules in the form of random coils. Strength, heat and chemical resistance depend on attachment between the coils. Conventional rubber chemistry uses vulcanization, permanent thermoset primary covalent cross-links, usually by sulfiir plus metal oxides, to hold the coils together but this makes processing more difficult, and recycling very difficult. In the past 40 years, this technology has been supplemented by the... 

Research on the pyrolysis of thermoset plastics is less common than thermoplastic pyrolysis research. Thermosets are most often used in composite materials which contain many different components, mainly fibre reinforcement, fillers and the thermoset or polymer, which is the matrix or continuous phase. There has been interest in the application of the technology of pyrolysis to recycle composite plastics [25, 26]. Product yields of gas, oil/wax and char are complicated and misleading because of the wide variety of formulations used in the production of the composite. For example, a high amount of filler and fibre reinforcement results in a high solid residue and inevitably a reduced gas and oiFwax yield. Similarly, in many cases, the polymeric resin is a mixture of different thermosets and thermoplastics and for real-world samples, the formulation is proprietary information. Table 11.4 shows the product yield for the pyrolysis of polyurethane, polyester, polyamide and polycarbonate in a fluidized-bed pyrolysis reactor [9]. 

In 1990, the JRPS organized a tertiary committee, the RP Recycling and Treatment Council (RTC) for the solution of technological and social problems regarding recycling thermoset composites waste. 

Some new technology is being developed by which mixed plastics may be depolymerized at high temperatures into monomers that pan be easily separated and repolymerized. Pyrolysis is also effected on commingled plastics, as well as on thermosets, wherein the product consists of a fuel, that will be used for energy. In summary, the concept of tertiary recycling may be practical for some polymers while also effective for better recycling of mixed plastics or thermosets. 

Multilayer packages are more difficult to recycle, unless reworked and reused as an iimer layer in new multilayer films. The thermosets are in principle nonrecyclable, but in some cases technology exists for depolymerization or pyrolysis. In principle they may be reground into fillers (incorporating fiber-... 

Extensive studies on the ultrasonic devulcanization of rubbers and some preliminary studies on ultrasonic decrosslinking of crosslinked plastics were carried out [83-122]. It was shown that this continuous process allows one to recycle various types of rubbers and thermosets. As a most desirable consequence, ultrasonically devulcanized rubber becomes soft, therefore making it possible for this material to be reprocessed, shaped, and revulcanized in very much the same way as the virgin rubber. This new technology has been used successfully in the laboratory to devulcanize a ground tire rubber (GRT)... 

Adherent Technologies Inc. [8] has developed a process for the reclamation of carbon fibers from carbon/epoxy composites. It has studied the depolymerization of thermoset carbon fiber reinforced epoxy matrix composites using a low temperature (20 min at 325°Q catalytic tertiary recycling reclamation process and has been able to obtain a product with 99.8% carbon and 0.2% residual resin, with only a loss of about 8.6% in fiber tensile strength. The process can be economically viable, provided sufficient scrap feedstock is available. Possible applications for the recovered fiber include thermoplastic and thermoset molding compounds. 

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