Thermosetting polymer recycling

Details are given of the development of energy and material recycling processes for thermosetting polymer composites. Applications in the cement industry and in coal fired fluidised bed combustion plants are discussed. 3 refs. 

Thermoplastics are more suitable for recycling than elastomers or thermosetting polymers. Thermoplastics can be heated above their melting temperatures and then recast into new shapes. Elastomers and thermosets, on the other hand, have extensive cross-linking networks that must be destroyed and then reformed in the process of recycling. Processes that destroy cross-linking, however, generally break down the polymer beyond the point at which it can be easily reconstituted. 

Thermosetting polymers are more difficult to recycle than thermoplastic polymers because —... 

When a thermoplastic polyurethane elastomer is heated above the melting point of its hard blocks, the chains can flow and the polymer can be molded to a new shape. When the polymer cools, new hard blocks form, recreating the physical crosslinks. We take advantage of these properties to mold elastomeric items that do not need to be cured like conventional rubbers. Scrap moldings, sprues, etc. can be recycled directly back to the extruder, which increases the efficiency of this process. In contrast, chemically crosslinked elastomers, which are thermosetting polymers, cannot be reprocessed after they have been cured. 

Thermosensitive hydrogels, 13 743 THERMOSET Thermoset recycling pyramid, 13 780-781 Thermoset elastomers, 20 71 Thermoset epoxy resins, curing of, 10 421 Thermoset flexible polyurethane foams properties of, 25 461 Thermoset matrix composites, 21 456 Thermo set molding properties of diallyl isophthalate, 2 262t Thermoset polymers, 25 455 cured, 10 425... 

Chemical recycling of polymers is an important target of modem society that consumes a great deal of petroleum-based polymeric materials. Although the chemical recycling of thermosetting polymers is also required, it is difficult because of their inherent insoluble and infusible properties. 

Recycling polymers The starting materials for the synthesis of most polymers are derived from fossil fuels. As the supply of fossil fuels is depleted, recycling plastics will become more important. Thermosetting polymers are more difficult to recycle than thermoplastic polymers because only thermoplastic materials can be melted and remolded repeatedly. 

Provide one example each of a thermoplastic polymer and a thermosetting polymer. Which type of polymer is easier to recycle Which type is more durable Explain. 

Some synthetic polymers can be recycled and some cannot. So-called thermoplastic polymers, usually composed of hnear or only shghtly branched molecules, can be heated and formed and then reheated and reformed. Therefore, they can be recycled. On the other hand, thermosetting polymers, which consist of molecules with extensive three-dimensional cross-hnking, decompose when heated, so they cannot be reheated and reformed. This makes them more difficult to recycle. 

The same technique can be used to dye a material that is otherwise difficult to dye. An ethylene-propylene copolymer rubber was reacted first with maleic anhydride, then with an aromatic amine dye in an extruder to produce a dyed rubber.81 Dye sites can also be inserted into polyolefins by grafting them with dimethylaminoethyl methacrylate, using azo or peroxide catalysts in an extruder.82 jV-Vinylimidazole has been grafted to polyethylene in an extruder with the help of dicumylperoxide.83 The product was mixed with an acrylic acid-modified polypropylene and used to compatibilize polyethylene and polypropylene. This could be helpful in the recycling of mixed polyolefins from municipal solid waste. Recycling of cross-linked (thermoset) polymers is more of a problem because they cannot be remelted in an extruder. However, they can be if... 

Plastics are materials that can be formed into various shapes, usually by the application of heat and pressure. Thermoplastic materials can be reshaped. For example, plastic milk containers are made from the polymer polyethylene. These containers can be melted down and the polymer recycled for some other use. In contrast, a thermosetting plastic is shaped through irreversible chemical processes and, therefore, cannot be reshaped readily. An elastomer is a material that exhibits rubbery or elastic behavior. When subjected to stretching or bending, an elastomer regains its original shape upon removal of the distorting force, if it has not been distorted beyond some elastic limit. Rubber is the most familiar example of an elastomer. 

Some plastics are biodegradable while others must be recycled. Plastics can be either thermoplastics or thermosetting polymers. The thermoplastics are plastics, like polyethylene, that can be heated and still be molded over again to take on different shapes. On the other hand, the thermosetting plastics undergo a chemical reaction that is not reversible. Therefore, they solidify into one final shape. 

Thomas, R., Vijayan, P., Thomas, S. Recycling of thermosetting polymers. In Fainleib, A., Grigoryeva, O. (eds.) Recent developments in polymer recycling, pp. 122-129. Transworld Research Network, Kerala (2011)... 

The ability of polymer composites, especially thermoset polymers and their composites, to be repaired, recycled and bonded depends on the physical and chemical nature of the polymer surface combined with the manufacturing process. The interfaces can be numerous. 

Competitive penetration of polypropylene into other applications has primarily taken place in polyethylene, polystyrene, polyvinyl chloride (PVC), thermoplastic polyester, nylon-6 or -6/6, and sometimes directly from metals or thermoset polymers like phenolic or reinforced reaction injection molded (RIM) urethane. The reasons for market penetration by PP replacement vary widely with an assortment of material design options chemical resistance, heat resistance, recycleability, processability, economics, and aesthetics. 

Polyolefins in fact lend themselves to this straightforward recycling procedure. Other thermoplastics like polyamides and polyesters need more careful handling because of their sensitivity to moisture absorption and molecular degradation. Thermosets are, by definition, not recyclable in the true sense however by comminuting and recompounding with fresh monomers, most thermoset polymers can in effect be recycled without significant loss of performance. 

Polymers are used in many medical applications, such as flexible wound dressings, surgical implants, and prosthetics. Some of these polymers are thermoplastic, which means that they can be melted and reshaped, or heated and bent. Others are thermosetting, which means that their shape is determined as part of the chemical process that formed the polymer. Thermosetting polymers cannot be reshaped easily and are not easily recycled, whereas thermoplastic polymers can be melted down and cast into new shapes in different products. 

Bakelite. the first commercially produced polymer, contains monomer units of phenol and formaldehyde. If an item made of Bakelite were broken, it could not be melted down and re-formed. Is Bakelite a thermoplastic or thermosetting polymer Can Bakelite be recycled ... 

In fact, RIM was the first plastic to be approved for bumper fascia in North America to meet the low-temperature crashworthiness demanded by some OEMs. Unfortunately, due to process complexity, RIM could not be fabricated at a high enough rate to meet the demands of large car platforms. Faster cycle time for part production is delimited by the speed of curing chemistry and the ability of the resin to flow into molds. Furthermore, the automotive industry wanted to move away from the use of thermoset polymer systems that cannot be remelted. This attribute of thermosets was labeled by the industry as non-recyclable, and the industry made room for other thermoplastics such as TPO that can be remelted. Reaction-injection-molded parts are made from urethane epoxies, polyesters, and polyamides. A study of cost versus performance reported that RIM gives the best cost-performance characteristic for composite materials and is competitive with steel. ... 

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