Waste polyamide

The hydrolysis of waste polyamides is widely used for monomer(s) recovery [5-12],... 

In 1994, DuPont [18] patented the ammonolysis process, in which PA6 and PA66 react with ammonia in the presence of a catalyst (ammonium phosphate) and as a result the corresponding monomers can be obtained. A high purity of the waste polyamides (98%) is required. The process conditions are 300-350°C and pressure of 68.9 MPa [18,40,41]. 

Lewis acids, such as the haUde salts of the alkaline-earth metals, Cu(I), Cu(II), 2inc, Fe(III), aluminum, etc, are effective catalysts for this reaction (63). The ammonolysis of polyamides obtained from post-consumer waste has been used to cleave the polymer chain as the first step in a recycle process in which mixtures of nylon-6,6 and nylon-6 can be reconverted to diamine (64). The advantage of this approach Hes in the fact that both the adipamide [628-94-4] and 6-aminohexanoamide can be converted to hexarnethylenediarnine via their respective nitriles in a conventional two-step process in the presence of the diamine formed in the original ammonolysis reaction, thus avoiding a difficult and cosdy separation process. In addition, the mixture of nylon-6,6 and nylon-6 appears to react faster than does either polyamide alone. 

The majority of polyamides used commercially are nylon-6,6 or nylon-6. The largest supply of waste for recycling of nylons is obtained from used carpets. Approximately 30-40% of die nylon produced in the world is used as carpet face fibers.1 Carpets consist of a multicomponent construction of face fibers and a primary and secondary backing. The face fibers are generally made of... 

This fall, the closed-loop Evergreen Nylon Recycling plant will start up in the US, a joint venture of DSM Chemicals North America and AlliedSignal. The facility will recover 45,000 m.t./year of caprolactam by depolymerising the fibres from 100,000 m.t./year of discarded nylon-6 carpets. Meanwhile in Germany, Lurgi is building the Polyamid 2000 AG facility. It will process 120,000 m.t./year of carpet waste and recover 10,000 m.t./ year of caprolactam from nylon-6 carpets and 13,000 m.t./ year of nylon-6-6 from nylon-6-6 carpets. 

CHEMICAL RECYCLING OF NON-WOVEN POLYAMIDE LINING WASTE... 

Polymers with hetero-atoms in the chain are suitable for chemical recycling of waste materials. In addition to depolymerisation (nylon 6) and solvolysis (nylon 6,6, PETP, PU) the degradation of aliphatic polyamides with dicarboxylic acids, diamines and cyclic anhydrides, especially trimellitic anhydride, becomes more and more important. The utilisation of the obtained fragments is described. 

Furan-2,5-dicarboxylic add also has tremendous industrial potential, because it could replace oil-derived diadds such as adipic or terephthalic acid as monomers for polyesters and polyamides [98, 99]. This diadd can be synthesized by Pt-catalyzed oxidation with 02 of 5-hydroxymethylfurfural the latter is obtained by acid-catalyzed dehydration of D-frudose or frudosans (inulin) the latter, however, are too expensive as starting materials, and yields from glucose-based waste raw materials are no higher than 40%. Therefore, the potential attractive option of furan-2,5-dicarboxylic acid will develop only after an effident generation of 5-hydroxymethylfurfural from forestry waste materials has been developed. The same compound is also the starting material for the synthesis of other interesting chemicals obtained by oxidative processes, such as 5-hydroxymethylfuroic add, 5-formylfuran-2-carboxylic add and the 1,6-dialdehyde. 

Figure 7.5 Classification result of shredded mixed electronics waste, superimposed onto a greyscale image of the waste fraction. Only objects that could be identified with a likelihood >95% have been classified. Classification colour code yellow metal, mostly aluminium red polymethyl metacry-late (PMMA) orange polyolefines (PE, PP) pink styrene polymers (PS, PS-E, etc.) violet polyamides (PA 6, PA 6.6, etc.) green acrylonitrile-butadiene-styrene (ABS) blue polyvinyl chloride (PVC).

The composition of feed polymers also has an important effect on the properties of products. In the experimental work of Miskolczi et al. commercial waste plastics from the packaging, electronic and automotive industry and the agriculture were used as raw materials. The samples contained high-density polyethylene (HDPE), low-density polyethylene (LDPE), polypropylene (PP), ethylene-propylene copolymer (EPC), polystyrene (PS), polyamide 6.6 (PA 6.6) and polyvinyl chloride (PVC). 

Automotive plastic waste components are, in addition to PP and PVC, styrene copolymers, rubber, polyamides and polyurethanes. 

Among the several kinds of polyamides composed of the large variety of acyclic and aromatic amino carboxylic acids or diamines and dicarboxylic acids, two Nylons are the most extensively applied in many fields. Nylon 6 and Nylon 6,6 are found in various waste streams, they may be present in pyrolysis recycling feeds as well. 

Finally, caprolactam could also be obtained by recycling polyamide-6 (PA-6) and polyamide-6,6 (PA-6,6) waste. Rhodia recycles about 30 000tons annually of PA-6 wastes in three different places in Europe. PA-6 production waste as well as used pure PA-6 wastes (fish nets and pure PA-6 fabrics) are used as starting products. PA-6,6 is also recycled. DSM and AlliedSignal opened a pilot plant in Richmond, Virginia, in 1997, where PA-6 carpets are depolymerized. The technology involves chemical processing of complete carpets without an expensive mechanical separation of fibers from the other carpet components. 

Utilization of a waste-product (2-methylglutaronitrile can be used as a co-monomer in the production of other polyamides, but the end-product niacin has an intrinsically much higher value). 

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