Rubber vulcanizates, recycling

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... 

Jacob C, De P.P., Bhowmick A.K., and De S.K. Recycling of EPDM waste. II. Replacement of virgin rubber by ground EPDM vulcanizates in EPDM/PP thermoplastic elastomeric composition, J. Appl. 

Klingensmith, B., Recycling, production and use of reprocessed rubbers, Rubber World, 203, 16, 1991. Drozdovskii, V.F., Production of comminuted vulcanizates, Prog. Rubber Plast. TechnoL, 14, 116, 1998. 

Ishiaku, U.S., Chong, C.S., and Ismail, H., Cure characteristics and vulcanizate properties of blends of a rubber compound and its recycled DE-VULC, Polym. Polym. Comp., 6, 399, 1998. 

The fourth trend is spurred by environmental sustainability concerns and the need for increased recyclability and reuse of polyolefin blends. In this regard, there is increasing replacement of PVC by polyolefin-polyolefin blends. There is also an increase in recyclability of EPDM mbber vulcanizates since EPDM is the fastest growing elastomer among synthetic rubber and the most used of nontire rubbers. Also, cryogenically ground rubber tires are being used as fillers for polyolefin blends such as LLDPE/HDPE. 

On the other hand, the mechanical properties of thermoplastic vulcanizates containing ground tire rubber have been investigated with the aim of increasing use of recycled rubber. The compositions tested included passenger car combined with EPDM, SBR rubber, isoprene rubber, and butadiene rubber. It was found that the particle size of the ground tire rubber had small effect on mechanical properties, but that the choice of the sulfur accelerator was significant [26]. 

Waste ground rubber tire powder was devulcanized and mixed with 30 per hundred rubber (phr) natural rubber to give satisfactory vulcanizate properties. An intermeshing counter-rotating twin-screw extruder with constant root and flight diameters of the screw was designed and installed for waste rubber recycling [31]. 

It is known that sulfur-vulcanized Natural Rubber (NR) can be completely recycled at 200 to 225°C by using diphenyldisulphide [41]. Recently, the effi-ciacy of various disulphides as recycling agents for NR and EPDM vulcan-izates were reported [42]. While complete de vulcanization was observed on sulfur-cured NR at 20b°C, a decrease in crosslink density of 90% was found when EPDM sulfur vulcanizates with diphenyldisulphide were heated to 275°C in a closed mold for 2 hours. At the same time, EPDM cured by peroxide showed a decrease in crosslink density of about 40% under the same conditions. 

Thermoplastic elastomer (TPE) blends have been broadly studied as a new class of materials. TPEs offer various advantages and require no state-of-the-art processing machinery, while scrap and rejects are recyclable. Blends can be homogeneous, phase separated or both. TPEs are multi-phase polymer systems consisting of hard and soft domains that can be copolymers or mechanical blends. This phase separation leads to materials having unique and viable commercial physical properties. TPEs exhibit the thermoplastic characteristics of the hard thermoplastic phase, and resilience as a result of the rubbery domains. TPEs based on natural rubber (NR) and thermoplastic blends are known as thermoplastic natural rubber (TPNR) blends. There are two types of TPNR, namely thermoplastic polyolefin (TPO) and thermoplastic vulcanizate (TPV).3... 

To date, with the exception of vehicle tires, TPEs have been replacing TS rubbers in many applications. Unlike natural TS rubbers, most TPEs can be reground and recycled, thereby reducing overall cost. The need to cure or vulcanize them is eliminated, reducing cycle times, and products can be molded to tighter tolerances. However there are TP vulcanizate (TPV) that provide property advantages. Most TPEs can be colored, whereas natural rubber is available mainly in black. TPEs also weigh 10 to 40 % less than rubber. 

PE/PP Recycled tire rubber Thermoplastic vulcanizate described (Syman-T NRI Industries) based on recycled tire rubber blend with PE/PP mixtures 169... 

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