Microwave Assisted Selective Hydrolysis of Polyamides from Multicomponent Carpet Waste

Abstract

against landfills, and incineration does not retain the intrinsic material value of the polymeric materials which is a waste of resources. [6] Synthetic carpets usually consist of a multicomponent system, where several different polymeric materials constitute the fibers and backing, respectively. The advantage of the multicomponent system is the specific properties and performances of the different polymers. This is, however, a disadvantage when it comes to recycling, as sorting and separation become challenging and polymers are seldom miscible. They also have different characteristic behaviors such as solubility and processability. [7,8] This calls for new techniques to recycle carpet waste to respond to environmental and economic concerns and to generate valuable products from the waste. [6,[9][10][11][12] Many synthetic multicomponent carpets comprise of polyamide (PA) fibers (polyamide-6 (PA-6) and/or polyamide-66 (PA-66)) with a polypropylene (PP) backing, as well as an adhesive to ensure that the fibers are well connected to the backing. Additives such as dyes, repellents, and inorganic fillers (e.g., calcium carbonate, CaCO 3 ) are also commonly added. [8,13] PA fibers are often chosen, because of their high melting points and high abrasion resistance. [6] Typically, the PAs make up around 50% of the carpet mass, making recycling of the PA section a viable option. [14] The recycling can take place through, for example, selective hydrolysis of the PA component present in the synthetic carpet. [15] Chemical recycling converts waste into new building blocks such as monomers, oligomers or functional chemicals, that in turn can be utilized to synthesize new polymers or plasticizers. [16,17] In this way, the value of the material can be retained or in some cases, it can even increase. [18] This is a potent option for PAs, since they can be depolymerized by hydrolysis of the amide bond, resulting in the cyclic monomer ε-caprolactam [19] or the linear monomer aminocaproic acid from polyamide-6 and adipic acid and hexamethylenediamine from PA-66. These studies show that high temperatures and pressures are effective tools for depolymerization. [3,5,6,20] Successful depolymerization of PA-6 was also achieved by microwave assisted recycling under high temperatures and preasures. Concentrated phosphoric acid (C = 0.50 g mL −1 ) was for example used as a catalyst, leading to a product mixture containing 90% aminocaproic acid, the linear form of ε-caprolactam during 12 minutes at 240 °C. This catalyst was chosen due the the high dipole Selective hydrolysis of polyamide-6 (PA-6) and polyamide-66 (PA-66) from commercial multicomponent PA-6/PA-66/polypropylene (PP) carpet is demonstrated by a microwave-assisted acid catalyzed hydrothermal process, yielding monomeric products and solid polypropylene residue. First, an effective method is established to chemically recycle neat PA-6 and PA-66 granules using microwave irradiation. The optimized, hydrochloric acid (HCl) catalyzed process leads to selective production of monomers...

Electrocatalytic conversion of waste polyamide‐66 hydrolysates into high‐added‐value adiponitrile and hydrogen fuel

“…Unlike PET, which can be readily hydrolyzed by base or hydrolase, 14 PA‐66 is resistant to light, abrasion, and strongly basic conditions. However, the amide bond of PA‐66 can be readily degraded into monomers under mild conditions with hydrolyzed acid 15–17 . In view of this, as shown in Figure 1A, the integrated system for PA‐66 upcycling contained several reaction steps: (i) H 2 SO 4 ‐catalyzed PA‐66 (Equation 1), (ii) hydrolysis product separation, (iii) KOH‐assisted filtered hydrolysate (Equation 2), and (iv) electroreforming of KOH‐assisted PA‐66 hydrolysate (Equation 3), including hexamethylenediamine (HMD) oxidation (Equation 4) and paired hydrogen evolution reaction (HER) (Equation 5).…”

Synthesis, Characterization, and the Potential for Closed Loop Recycling of Plant Oil-Based PA X.19 Polyamides

“…Hydrolysis reactions are commonly used for the chemical recycling of polyesters, such as poly­(ethylene terephthalate) (PET), and has also been applied recently for the closed loop recycling of long-chain polyesters . The hydrolysis of polyamides, however, is more complicated and often requires long reaction times and harsh conditions. , Microwave-assisted hydrolysis is a promising tool for the chemical recycling of polyamides with reduced reaction times and energy consumption. − …”

Recycling and Degradation of Polyamides

“…Urška Češarek et al [128] found in their study of the chemical recycling of aliphatic polyamides that when HCl was used as an acid catalyst with external microwave radiation at 200 • C and a 1.25 HCl/amide molar ratio, PA-66 could be completely converted into constituent monomers in 10 min. Subsequently, Eva Bäckström et al [129] found that polyamide-6 (PA-6) and polyamide-66 (PA-66) would be selectively hydrolyzed by microwave-assisted hydrolysis of industrial multi-component polyamide-6 (PA-6)/ polyamide-66 (PA-66)/polypropylene (PP) carpets, which may provide new ideas for the separation and degradation of composite materials.…”