By Zhao Tong, CAS Shanxi Institute of Coal Chemistry
Synthetic polymer materials are extensively used in industrial production and in daily life. With huge quantities of wastes generated, however, environmental safety is seriously threatened. Conventional methods of disposal, such as landfill and burning, not only generate large amounts of greenhouse gases but also may discharge aromatic and nitrogen-containing toxic small molecules to pollute the soil, atmosphere and water sources and endanger the environment overall. Pyrolysis and traditional chemical degradation use high temperature, enforced oxidation, or strong acids to produce nonselective openings in chemical bonds of polymers. Products generated by such treatment methods are complicated, and their treatment processes will inevitably discharge some nitrogen-containing, halogen, or aromatic toxic small molecules that affect human health and the living environment.
Atoms such as carbon, oxygen, and nitrogen in polymer materials are bonded together in a highly orderly form. In case they are converted during the treatment process into chemicals with high added value, while partially or totally retaining their orderly features, not only can the discharge of toxic small molecules be avoided or reduced, important economic value can also be obtained. CAS Shanxi Institute of Coal Chemistry has used the aqueous-phase coordinated unsaturated metal efficient degradation system, developed through the conversion of catalytic cellulose at the early stage, and selectively opened C-N bonds, aromatic ester bonds, acylamide bonds, ether bonds, urea bonds and carbamate bonds in various polymer materials. In this way synthetic polymers such as epoxy resin composites, unsaturated polyester resin composites, nylon, polyester, polyether, and polyurethane, can have efficient degradation and recycling.
Controllable degradation and all-ingredient recycling of epoxy resins
Epoxy resins have excellent cohesion, corrosion resistance, shaping, and thermal stability. They have gained extensive applications in electrics, electronics, machinery, buildings and cultural/sports appliances as coatings, adhesives and shaping materials. They can be compounded with carbon fiber, in particular, to get composites with high strength and low density. Such composites are indispensable materials for aviation, spaceflight, and environment-friendly vehicles. Due to the great stability of their molecular structures, however, the recycling and use of carbon fiber with high added value is difficult.
Researchers have achieved selective breaking of C-N bonds in metal ion catalytic epoxy resins coordinated by an aqueous system, while maintaining the stability of C-C bonds and C-O bonds. When epoxy resins degrade, they generate small molecular compounds with definite structures. Degradation products float on the surface of the reaction fluid and can very easily be separated from the reaction system. Degradation products contain amino groups and can be used to cross-link agents in the synthesis of epoxy resins. Research on the degradation of carbon fiber-reinforced epoxy resin composites has also been conducted. It has discovered that the degradation rate of carbon fiber-reinforced epoxy resins is greater. After degradation, carbon fiber settles at the bottom of the reactor, whereas degraded resins float on the surface of the reaction fluid. The reaction system can be reused 8 times with no evident reduction of its reactivity.
Efficient degradation of unsaturated polyester resins
Unsaturated polyester resins are insoluble and infusible materials with a three-dimensional network structure generated through the cross-linking of polyester polymers with unsaturated bonds with cross-linking monomers (such as styrene). Composites that are produced from unsaturated polyester resins and fiber glass are called fiber glass reinforced plastics. The production scale is 6.00 million t/a in China. The treatment of its wastes is becoming a social problem.
The technology developed by researchers can selectively open ester bonds in their molecular structures and degrade unsaturated polyester resins to generate phthalic anhydride, binary alcohol derivatives, and styrene/maleic acid copolymer SMA. The corresponding separation and purification process has also been developed. Phthalic anhydride and binary alcohol derivatives can be recycled for use in the synthesis of resins. SMA can be used in dispersants and plastics modifiers.
Multi-level degradation and recycling of polyurethane
Great quantities of polyurethane materials are used in packaging, clothes, vehicles, and sealing. The production scale is over 10.00 million t/a in China. Researchers have in an aqueous-phase system selectively opened carbamate bonds and urea bonds in polyurethane to generate small molecular products with high added value, such as polyether (p-polyether urethane), toluene diamine (TDI precursor), and MOCA. These degradation products have different phases and different levels with the reaction system and are therefore convenient for recycling and separation. Polyether compounds generated, such as polytetrahydrofuran, undergo degradation and recycling in a solvent-free system to get tetrahydrofuran monomer. The content is over 99% and the yield reaches 95%. The degradation reaction temperature of the technology is 100-400℃. The technology has mild reaction conditions and is convenient for commercialization.
Oriented degradation of polyester fibers (resins)
Polyester is produced from ethylene glycol and terephthalic acid through ester bond condensation. The production scale is 40.00-50.00 million t/a in China. Polyester is mainly used in mineral water bottles and polyester fibers. The aqueous-phase coordinated unsaturated metal ion catalytic system developed by researchers can efficiently degrade polyester into ethylene glycol and terephthalic acid. In the temperature reduction of the reaction system, terephthalic acid separates out in crystal form. The purity can reach 99.8% and the yield is 90%.
Extraction of polyethylene from composite films
Polyethylene/polyester and polyethylene/nylon composite films are packaging materials with extensive applications in China today. Polyethylene/epoxy resin composite wastes are inevitable byproducts in the production of large-size composites such as windmill blades. The technology developed by researchers can easily remove polyester, nylon, and epoxy resin in these films through degradation to get polyethylene with a content of as high as 99%. The technology used in this process is convenient for commercialization. The reaction temperature is 120-180℃ and the reaction system can be reused several times.
