Utilization of waste expanded polystyrene: Blends with silica-filled natural rubber
Highlights
► Tensile strength of the silica filled blend is comparable with silica filled NR. ► Modulus and compression set were the best for compatibilized NR/EPS blends. ► Tear strength has increased by 25% for compatibilized blends. ► A 5% waste EPS can be incorporated into NR compounds as a waste management measure.
Introduction
The significance of polymer recycling has greatly increased over the years. Expanded polystyrene is a major constituent of plastic wastes. Recycling of EPS involves various technological challenges stemming mainly from its low bulk density (15–50 kg/m3) [1]. Owing to its light weight, buoyancy, thermal insulation, dimensional stability, chemical resistance, electrical properties, hygienic appearance, low cost etc., EPS is used for a large number of applications. Today, global consumption of EPS exceeds 3 million tons with an increase of approximately 6% a year [2], [3]. After use, EPS usually ends up in landfills or is incinerated. Its nuisance value in the environment is high because of its large volume. Added to this are the high transportation costs associated with shipping low bulk density waste EPS. [4]. In response to the high cost of disposal and growing public opposition to land filling, a number of recycling strategies for EPS has been devised. If the waste EPS can be used in other polymers by cost effective methods then the economics of recycling can be favourable.
Any reduction in the amount of waste EPS will have a salutary impact on the environment in terms of reduction in volume of waste. Moreover, in view of the rising price of NR, replacement of NR even by a small percentage of a waste material can have considerable impact on the economic viability of NR processing. Hence, blending EPS with NR has been done in this study to gainfully utilize waste EPS. Although there is hardly any information in the literature about NR–EPS blends, there are some detailed references about studies on thermoplastic elastomers based on NR–PS blends [5], [6].
The properties of a blend are dependent on phase behaviour, blend ratio and cross linking levels [7], [8]. Immiscible blends are preferable over miscible blends because in miscible blends only an average of the individual properties is obtained [9]. However, very often many of the immiscible blends exhibit poor mechanical properties as well as unstable morphology. Even though both NR and EPS are nonpolar materials, they exhibit poor adhesion between the interfaces. Therefore, compatibilization of the blend is necessary to improve its properties to usable levels.
Addition of compatibilizers or interfacial agents to polymer blends affects the flow behaviour because of the interactions between the blend components via the blend compatibilization. This results in improved interfacial adhesion and physical properties [10], [11], [12]. Grafting reactions could induce significant changes in the polymer chain relating to chemical composition, polymer chain structure and molecular mass. These parameters are responsible for the interactions between polymers [10]. MA grafting of polymers has been increasingly used as a means of compatibilization. The effect of the addition of different compatibilizers such as bromobutyl rubber (BIIR) and maleic anhydride on the elastic behaviour of natural rubber/butyl rubber (NR/IIR) blend has been studied by Hamza et al. [13]. Grafting of natural rubber and nitrile rubber on low density polyethylene has been performed successfully using acrylic acid and maleic anhydride [14]. The grafted compounds were found to be superior compared to nongrafted compounds in mechanical properties, chemical resistance, aging, and so on [15]. The incorporation of MA-functionalized polypropylene (PP-g-MA) as a compatibilizer agent in PP/rubber blends has been studied with the aim of improving the dispersion and interaction of rubber within the PP matrix [16], [17]. With different rubber types, it has been observed that maleic anhydride grafted styrene–ethylene–butadiene–styrene (MA-g-SEBS) rubber is most effective for toughening of PET [18], [19]. The grafting performance by MA on SBS was reported by Lasalle et al. [20].
The present study uses MA compatibilized NR/EPS blends for subsequent blending with more of NR followed by vulcanisation by conventional methods. It has been shown during earlier studies that cardanol separated from cashew nut shell liquid (CNSL) is a good substitute for petroleum based NR plasticisers [21]. Hence this study uses cardanol as plasticiser as an additional step to conserve fossil fuel based substances and utilizes a cheap agrobyproduct.
Section snippets
Materials
EPS used in this study was purchased from S-tech Thermocool Industry, Cherthala, Kerala with a density value of 15 Kg/m3. Waste EPS was not selected for the study because of the need to get a clean material for the initial studies. Natural rubber (ISNR-5) was obtained from Rubber Research Institute of India; Kottayam, Kerala. Other rubber chemicals were of commercial grade. Cardanol was separated from commercial grade CNSL (purchased from Vijayalaksmi Cashew Exports, Kollam, India) by
Fourier Transform Infrared Spectroscopy
The blends obtained from Brabender Plasticorder were Soxhlet extracted for 48hours. After extraction of the nonreacted MA by water, the nongrafted sample was solution cast using toluene as solvent. The solvent was evaporated in a hot air oven at 60 °C for 30 min before recording the IR spectra. The grafted sample was found to only swell in toluene. It was directly used for IR study after drying. The FTIR spectra were recorded using a Thermo Nicolet FTIR Spectrometer Model Avtar 370 instrument.
Fourier Transform Infrared Spectroscopy
A representative infrared spectrum of the non-grafted and grafted NR/EPS blends with 1% MA is shown in Fig. 1. An intense characteristic band at 1778 cm−1 and a weak absorption band at 1854 cm−1 are observed. These bands can be assigned to grafted anhydride rings. They are due to symmetric (strong) and assymetric (weak) carbonyl (CO) stretching vibrations of succinic anhydride rings grafted on PS (polystyrene) and NR [28], [29], [30]. This proves the presence of grafted anhydride groups on the NR
Conclusion
In this work, non-degradable and not easily recyclable waste EPS has been substituted for 5% NR by a compounding process. Most properties of the blend show remarkable improvement on compatibilisation with MA. For an optimum MA concentration of 1%, the tensile strength is comparable with that of silica filled NR. Properties like modulus at 300% elongation and compression set were the best for compatibilized blends. Tear strength shows an impressive 25% improvement in the case of compatibilized
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