Crosslinked polymer materials, recycling

A growing volume of waste materials, especially vulcanized rubbers and crosslinked polymers are proving difficult to recycle. As an alternative to their disposal in landfills, there have been many attempts to grind these materials and use the products as a substitute for fillers in composite materials. Other non-plastic materials such as glass, paper, natural fibrous materials, and fly ash are also used for filler replacement. There is extensive literature on the use of ground tires as filler replacements. This is a specialized topic with only a minor relationship to fillers. 

Polyurethane is a very common crosslinked polymer and many materials produced from it end up as waste. Given the quantity of material, recycling is a major problem. Cryogenic pulverization systems have been developed which can process PU foam to particles smaller than 1 mm (preferably <100 pm)/ " These particles are homogenized with polyol and then reacted with isocyanates to produce foam. This foam with 5% pulverized PU foam has a density equivalent to a similar foam produced without the recycled material. A further increase in filler content causes a density increase. Pulverized PU foam particles were also tried as a filler in natural rubber vulcanizates with good results. Figure 12.10 shows the effect of PU... 

The degradative radiation-recycling of PTFE led to a successful pilot-scale plant producing 12 tons/year recycled powder at Sumitomo, Japan [9], For similar polymerdegrading industrial developments several other candidates are very promising. Among other synthetic polymer products, discarded automobile tires represent a major environmental concern, in an amount close to 10 Mtons/a. A promising method is mentioned in the literature [9] in which the vulcanized rubber product is crushed at low temperature, irradiated at a dose rate of 100 kGy, and milled repeatedly, if necessary. The reclaimed de-crosslinked material can be added to an extent 10 - 15% to various new rubber blends. 

This paper describes novel approaches to the exploitation of both furan monomers and a specific facet of furan reactivity in order to synthesize either conjugated oligomers incorporating the heterocycle in their backbone, or polymeric structures which can be crosslinked and returned to linear structures through the reversible chemistry of the Diels-Alder reaction. The first family of compounds showed interesting features in terms of conductivity, luminescence, mesogenic character and photoactivity. The second class of materials owes its interest to the possibility of recycling otherwise intractable polymers, e.g. tires, thanks to a simple thermal process. 

---