I.1. Recycling of rubber

The problem of recycling rubber has existed since Charles Goodyear first discovered vulcanization in 1839. Disposal of used rubber into landfills has become increasingly prohibitive due to high costs, legislative pressures, public opinion, especially high environmental stress. Researchers pay much attention to recycling. Many rubber products are now recycled such as used tires.

Rubber recycling encompasses a range of rubber types, Scheirs [2] pointed out that approximately 94% of all the rubber consumed in the world was thermoset in nature and thermoplastic elastomers comprise the remaining 6% of rubber products. Besides automotive tires, other thermoset rubber articles that are entering the recycle stream include drive belts, automotive hoses and a variety of fabric reinforced components.

The first most important step is processing or reduction of whole tires into useful particle sizes. Sadhan [1] noted that the processes used for grinding of rubber are based on cutting, shearing, or impact, depending on the equipment and the grinding conditions. There were cutting, milling, extrusion, ambient and wet grinding, cryogenic grinding, etc.

Shredded tires are used as filler material in highway construction because they increase the abrasion resistance and enhance the resilience. Concentrations of tire rubber used for these purposes are 10% to 20%. [1]

Cross-linked networks rubber could be recycled via high-pressure, high-temperature sintering (HPHTS) process [1]. The rubber powder in HPHTS is believed to “sintering” due to rearrangement of chemical bonds across the rubber particle interfaces at elevated temperatures.

Since the development of a tire vacuum pyrolysis process from the laboratory to the industrial scale, the used tires could be converted to carbon black and oil by pyrolysis [1].

Ethylene propylene rubber EPM is a copolymer consisting of ethylene and propylene units as part of the main polymer chain. [29] It can be cross-linked with peroxides or radiation but not sulfur. EPM is used as an ethylene based plastic impact modifier and as a viscosity index improver for lubricating oils. When a non-conjugated diene is grafted on to the main polymer chain it becomes a terpolymer, ethylene propylene diene (EPDM, figure 1.1) and interchain sulfur cross-linking becomes possible. EPDM is largely unaffected by weather with very good resistance to ozone. Low temperature flexibility is very good and compares well with NR, and like NR and SBR, EPDM (with a lower polarity than NR) has very poor oil resistance. [29]

Chemical structure of EPDM rubber

Figure 1.1: Chemical structure of EPDM [29]

The image of the recycled EPDM rubber powder used in this project under microscopy was shown in Figure 1.2. The recycled EPDM rubber was produced by grinding EPDM rubber and the size is less than 0.1 mm. The focus on this work was to recycle the EPDM rubber by blending it with thermoplastic polymer to form the Thermoplastic/rubber-blends.

Image of recycled EPDM rubber under microscopy

Figure 1.2: Image of recycled EPDM rubber under microscopy

I.2. Rubber Recycling by Blending with Plastics

The best way to recycle rubber products would be to devulcanize and reuse them in the rubber industry [1]. Processes for devulcanization, including chemical, thermal, thermomechanical, and ultrasonic, have been worked out, but they are costly and not suitable for commercial application, particularly in manufacturing highly engineered produces like tire.

Sadhan [1] summed up that other alternative is to blend the crumb or ground rubber with a material having the ability to flow under heat and pressure, so that it can be shaped into useful articles at a reasonable cost. This can be accomplished by mixing finely ground rubber with plastics, along with necessary additives.

Thermoplastic elastomers (TPEs) combine the elastic and mechanical properties of thermoset crosslinked rubbers due to the melt processability of thermoplastics. Today, TPEs comprise the fastest growing rubber market. TPEs can be processed by a variety of techniques, such as extrusion blow moulding, injection moulding, vacuum forming and calendaring. So, production scrap and waste rubber after use can be recycled. Thermoplastic vulcanisates (TPVs) are a particular family of TPEs, which are produced via dynamic vulcanization of
non-miscible blends of a rubber and a thermoplastic, i.e. the selective crosslinking of the rubber while simultaneous melt mixing with the thermoplastic [3].

In recent years, TPEs and TPVs have replaced conventional rubbers in a variety of applications including appliance, automotive, medical, engineering, etc.[1] TPEs are made by copolymerization and by blending thermoplastics with a rubbery component. TPVs, on the other hand, are made by dynamically vulcanizing the rubber component in a rubber/thermoplastic blend during mixing. Sadhan [1] pointed out that both materials are processed like thermoplastics and are recyclable.

Thermoplastic elastomers (TPEs) exhibit the functional properties of conventional elastomeric material and can be processed with a thermoplastic processing machine [4].
Bhowmick and co-workers [6] considered that the plastic acts as a continuous phase allows for melt processing of the TPEs, whereas the dispersed rubber phase is responsible for rubber elasticity and other elastomeric properties of the blends. Various types of thermoplastics are used to prepare TPEs. These include polypropylene, low-density polyethylene, ultra-lowdensity polyethylene, linear low-density polyethylene, chlorinated polyethylene, polystyrene, polyamide, ethylene vinyl acetate (EVA) copolymer and poly (methyl methacrylate). [4]

Ethylene methylacrylate copolymer is one of the most thermostable olefin copolymers. It is used for the manufacture of films and sheets, for injection molding, extrusion coating and coextrusion. Some of the most lasting effects of the addition of EMA are the lowering of the Vicat temperature, the reduction of the flexural modulus of elasticity, a noticeable improvement in resistance to stress cracking. The thermal stability of the material is so high that temperatures of 315 to 330 0C can be used for extrusion coating. What is more, with a melting temperature of 150 0C EMA can be processed on normal LDPE plant to tubular film.[9] We choose EMA to blend with EPDM rubber due to its thermal stability and good mechanical properties.

 Thermoplastics such as PE, PP, and PVC are not only cheap, but also are available in a wide range of melt index and micro-structure, which can be used for blending with recycled rubber. In addition, a substantial quantity of commodity and engineering plastics is available as recyclate from municipal solid waste and manufacturing waste. This can be used to blend with scrap rubber, to further reduce product cost and alleviate the current environmental problems.[1] Considering the low cost, we choose LDPE to blend with EPDM rubber.

In short, there is a great scope for recycling both devulcanized rubber and ground rubber by blending with plastics. Whereas devulcanized rubber will act as a second polymeric component, vulcanized ground-rubber particles will act as a low-cost organic filler or extender. Polymeric in nature, ground-rubber particles provide additional advantage over inorganic fillers such as carbon black, talc, or silica for bonding better with matrix polymerafter suitable chemical or mechanical treatment or in the presence of a compatibilizer.[1]

I.3. Compatibilization

Datta and Lohse[5] has expressed that the earliest theories of the theories of the thermodynamics of polymer mixtures (blends and solutions) date back to 1941. The main equation is the Flory-Huggins-Staverman (FHS) expression for the free energy of mixing two


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