Thermoplastic-recycled rubber elastomers

Ismail, H. and Suryadiansyah, S., Thermoplastic elastomers based on polypropylene/natural rubber and polypropylene/recycle rubber blends. Polymer Test., 21, 389, 2002. 

Another application of the injection system consists of recycling old and waste rubber. The reactive mixing of crumb rubber with a melted polymer represents an innovative way of recycling these waste rubbers [6]. Some thermoplastic elastomer may contain up to 60% recycled rubber derived from used vehicle tires, and thus through the reactive injection system the rubber scrap is revulcanized [7]. 

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... 

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. 

The market of PP/EPDM blends has grown dramatically because of its recycling abihty and processability by conventional thermoplastic processing equipment. The unique characteristics of thermoplastic elastomer made it an attractive alternative to conventional elastomers in a variety of markets. Liu et al. showed from the experimental blends (53) that materials cost reduction of between 30% to 50% is possible in comparison to commercial products if one applies the PP/EPDM blends to the construction of a basketball court, a tennis court, and a roller hockey rink, which were estimated around 7000, 14,000, and 40,000, respectively. The cost comparison took into account the percentage of rubber or PP used in experimental blend, the exponential factor for a scale-up process and the overall surface area of the specific applications. Among many possible application of this blend two readily feasible applications are roofing and flooring. 

Polypropylene (PP) in various forms (filled, unfilled, reinforced, rubber blended) finds the greatest usage of all automotive plastics. Its consumption is likely to accelerate as a result of new laws requiring recyclability, and because of the weight and cost reductions it offers. New fabrication techniques may also contribute to growth. For example, the monomaterial sandwich construction techniques developed by fabricators yield rugged and lightweight PP-based instrument and door panels. These parts consist of a sandwich of reinforced or neat PP substrate, a cross-linked PP foam, and a thermoplastic polyolefin elastomer (TPO) cover. 

PROPERTIES OF SPECIAL INTEREST In general, thermoplastic elastomers (TPE) provide the mechanical properties of rubber in combination with the processing characteristics of plastics recyclable Kraton D s are the lowest cost TPE (> 0.85 Ib ). 

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. 

Thermoplastic elastomers are defined by ASTM D 1566 as a family of rubberlike materials that, unlike conventional vulcanized rubber, can be processed and recycled like thermoplastic materials . A rubber is defined as a material that is capable of recovering from large deformations quickly and forcibly and retracts within 1 min to less than 1.5 times its original length after being stretched at room temperature (18 to 29° C) to twice its length and held for I min before release . 

Door seals have typically been made of rubber materials with metal inserts. The metal inserts allow the rnbber part to snap-fit onto the frame. There have been efforts to develop a thermoplastic elastomer seal that would be relatively easy to recycle and would meet the same performance requirements. These can also be molded in different densities or fitted over compatible plastics to eliminate the need the metal inserts. A good example is Ihe Mitsubishi Motors Mini-Cab (in Japan), which is a SUV-like commercial vehicle with a large rear cargo door. The rubber seal for the door was replaced with a TPE seal with the new part weighing 30% less [3]. 

The main advantage of such triblock copolymers is that they can be moulded and recycled simply by heating the material above the glass transition temperature of polystyrene, unlike classical vulcanised rubbers, which cannot be reused without degradation as they are chemically cross-linked. Such SBS-based materials are called thermoplastic elastomers. However, for various reasons, including cost, the commercial use of such polymers is rather limited compared with the use of natural and classical synthetic rubbers. 

These materials have the functional requirements of elastomers (extensibUity and rapid retraction) but processability similar to that of rigid thermoplastics. The principal advantages of the TPEs compared to vulcanized rubber are (1) reduction in compounding requirements, (2) easier and more efficient processing cycles, (3) scrap recycling, and (4) availability of thermoplastic processing methods. Generic classes of TPEs include... 

A thermoplastic elastomer (TPE) is a rubbery material with properties and functional performance very similar to those of a conventional thermoset rubber, yet it can be fabricated in the molten state as a thermoplastic. ASTM D 1566 defines TPEs as a diverse family of rubber-like materials that, unlike conventional vulcanized rubbers, can be processed and recycled like thermoplastic materials. Many TPEs meet the standard ASTM definition of a rubber, since they recover quickly and forcibly from large deformations, they can be elongated by more than 100 percent, their tension set is less than 50 percent, and they are sometimes insoluble in boiling organic solvents. Figure 4.35 indicates hardness ranges for various types of TPEs and conventional elastomers. 

Thermoplastic elastomers (TPEs) combine the physical properties of vulcanized rubber with the ease and economy of conventional thermoplastic processing. They are also well suited to reprocessing and recycling and minimize toxicity issues. Many types of thermoplastic elastomers are polymer blends cranprising a thermoplastic continuous phase in combination with a discontinuous vulcanized or unvulcanized elastomeric phase, which in the latter case could also be co-continuous. 

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