Thermosets, processing recycling

Styrene-butadiene block copolymers belong to a new class of polymers called thermoplastic elastomers (TPE). Products made from these polymers have properties similar to those of vulcanized mbbers, but they are made from equipment used for fabricating thermoplastic polymers. Vulcanization is a slow and energy-intensive thermosetting process. In contrast, the processing of thermoplastic elastomers is rapid and involves cooling the melt into a rubberlike solid. In addition, like true thermoplastics, scrap from TPE can be recycled. 

Rear [35] estimated that the typical conventional thermoset process takes around 2-6 times longer and the total non-recydable scrap is 10-50%. In contrast, the TPE scrap is usually 2-5% and is recyclable... 

In the case of cellulose, the reinforcement provided by cellulose fibers in the properties of thermosetting composites is recently discussed in the work of Thakur and Thakur (2014), including polyester, epoxy, phenolic and vinyl ester matrixes. The main properties that justify the utilization of cellulose fibers and their composites were considerable are toughness, flexibUify, easy processing, recyclability, and eco-fiiendliness. 

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. 

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. 

Plastics processing, 19 536-563 extrusion, 19 539-549 molding, 19 549-556 thermoplastic resins, 19 536, 537-539 thermosetting resins, 19 556-559 Plastics recycling, 21 372, 446-461 economics of, 21 457-459 impurity separation in, 21 448 materials separation in, 21 447-448 recent developments in, 21 456-457 types of plastics in, 21 449-457 Plastics recycling industry, U.S., 21 458-459... 

Results are presented of experiments undertaken by Gaiker in the manufacture of sandwich panels containing foam cores based on PETP recycled by a solid state polyaddition process developed by M G Ricerche. Panels were produced with glass fibre-reinforced unsaturated polyester and epoxy resin skins, and allthermoplastic panels with PE, PP, PS and glass fibre-reinforced PETP skins were also produced. EVA hot melt adhesives and thermoset adhesives were evaluated in bonding glass fibre-reinforced PETP skins to the foam cores. Data are presented for the mechanical properties of the structures studied. 

This change is signihcant in that most components of MSW are either biodegradable or recyclable with relatively modest effort. For example, glass, paper, and metals have been recycled for decades. By contrast, the recycling of plastics is more difficult partly because there are so many different kinds of plastic and partly because some plastics (thermosetting plastics) cannot he remelted, an important hrst step in any recycling process. 

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... 

Pyrolysis is a tertiary or feedstock recycling technique capable of converting plastic waste into fuels, monomers, or other valuable materials by thermal and catalytic cracking processes. This method can be applied to transform both thermoplastics and thermosets in high-quality fuels and chemicals. Moreover it allows the treatment of mixed, unwashed plastic wastes. 

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