Common Methods for Recycling EPS

Figure 7. Global EPS consumption by regions in 2013, adapted from [14].

In Australia, it is estimated that more than 40,335 ton of expanded polystyrene from packaging were thrown away by year. According to the Plastics and Chemical Industry Association (PACIA) only 2,775 ton of EPS was collected and recycled in Australia during 2011-2012, which represents a recycling rate of 6.9% [18]. In Brazil the recycling rate grew from 6.7% in 2007 to 9.3% in 2009 [19] mainly due to mechanical recycling process.

argumentations such as toxic emissions have received much attention and resistance from concerned parties against incineration process (British Plastic Federation). In the latter, mechanical recycling is a popular recovery path for manufacturers and is carried out on single-polymer waste streams. Anyway, the market for recycled materials only worth if the quality of the final product is close to that of the original. Despite these drawbacks, mechanical plastic recycling is thought to be the most used, when ones takes into account the waste‘s energy, natural resources and environmental pollution [22, 24-25].

Expanded polystyrene (EPS) is commonly used for insulation and packing materials.

However, the EPS used for packing materials is often discarded after their first use, which results in a large amount of polymer waste. Continuous disposal of EPS in landfills leads to serious environmental problems. Due to the increase in landfill costs and the decrease of the landfill space, alternatives for the disposal of polystyrene materials must be developed [25-26]. Recycling of this waste has recently received significant attention all over the world due to the changes in both regulatory and environmental issues [26].

Generally the EPS recycling has three types of methods. The first one is the material recycling, where there is a reduction in the volume of EPS by heating, solvent or friction.

Then, PS can be recovered and re-used as raw material in daily products, construction materials and so on. The second one is chemical recycling, which is aimed at recovering the styrene monomer to re-use it as chemical resource. The third one is thermal recycling, an effective method for the contaminated EPS waste, which it can be used for energy production through combustion. Because the bulk volume of the waste EPS (due contains so much air), it becomes unfeasible to transport to recycling facilities [27]. So, many researchers have been developing potential solvents for this problem. Noguchi et al. [28-30] developed a new recycling system for waste EPS using a natural solvent. In their study, the authors proposed a new system for EPS recycling, which uses orange oil (d-limonene) as the EPS shrinking agent. This recycling process can reproduce polystyrene (PS) with the same mechanical properties of the original. Additionally, the limonene can be re-used at least 10 times. In a study performed by Amianti and Botaro [31], a new recycling method of waste EPS was demonstrated. Concrete was impregnated with polystyrene (CIP) by dissolving EPS in a mixture of acetone and ciclohexane. As a consequence, it resulted in an economic, efficient and easily applied material, which reduces the permeability of pre-cast concrete surfaces.

Thereby, reducing the rate of degradation and increasing overall durability. Shin et al. [21]

proposed a method for recycling EPS using N,N-dimethylacetamide, tetrahydrofuran and dimethylformamide as solvents. The authors developed submicron and nanofibers of recycled EPS from electrospinning process by mixing these fibers with the conventional glass filter media. The results showed that the addition of the nanofibers improved the efficiency of the filter media from 68 to 88%. However, the dissolution of waste EPS in solvents has some drawbacks. Unfortunately, these chemical techniques usually involve the use of hazardous solvents that can cause health diseases. On the other hand, after use, these solvents can be recovery, recycled or properly disposed without causing environmental damage.

On the other hand, chemical recycling is used to obtain styrene monomers and others chemical products. In contrast to condensation polymers as poly(ethylene terephthalate) [33], PS cannot be easily recycled to its monomer by simple chemical methods. So, thermochemical recycling techniques such as pyrolysis are generally applied. PS can be thermally depolymerized at relatively low temperatures in order to obtain the monomer styrene with high selectivity [22]. Kaminsky [34-35] reached a 65 wt% yield of styrene using

a fluidized bed reactor at 580°C. Ward et al. [36] in a later publication using the same experimental reactor at 520°C reported the generation of an oil composed of 82.8% w/w of styrene. This was achieved by further distillation of the liquid fraction after pyrolysis.

Furthermore, Liu et al. [37] studied the pyrolysis of PS in a laboratory fluidized bed reactor in the temperature range of 450–700°C. The yield of styrene reached a maximum of 78.7 wt% at 600°C. The same yield of styrene monomer from the pyrolysis of polystyrene has been reported by Bouster et al. [38]. The uncatalyzed pyrolysis of polystyrene has been reported to produce 83 wt% conversion of low viscosity oil consisting mainly of styrene, with a gas and char yield each with less than 5 wt% [39]. Fast heating rates such as those produced in a fluidized bed reactor and higher temperatures result in the thermal cracking of the pyrolysis vapors and a higher yield of gas and lower yield of oil [40]. Miskolczi et al. [41] studied thermal degradation of waste EPS into fuel like diesel oil. The temperature range of 410–

450°C was used in the process. They found out that an increase in the degradation temperature improved yield of both gas and liquid. Nearly complete cracking could be attained at 450°C. The addition of organic compounds such as naphthalene in the pyrolysis of polystyrene waste has been shown to enhance styrene yield [42]. In contrast, the addition of other polymers as polyethylene, led to higher gas release rates and lower liquid release rates [43]. The use of different solid-acid catalysts such as zeolites, alumina and silica-alumina at a temperature of around 350°C has been reported to significantly modify the selectivity of the styrene, since the main products are benzene, ethylbenzene and cumene [44]. Zhang et al.

[45] used several basic catalysts and noted an increase of the yield of the monomer when compared to thermal or acid-catalyzed degradation. Barium oxide was proposed as the most effective catalyst for the pyrolysis of waste polystyrene. The monomer yield was of 76 wt% at 350.8°C in a batch reactor with a continuous nitrogen flow. Finally, it has been shown that the pyrolysis products can be a strong function of the raw material, reactor configuration, polymer-catalyst contacting pattern and the average molecular weight of the polymer [22, 38, 44, 46]. The pyrolysis of EPS and others plastic materials to fuel is gaining momentum and being adopted due to its efficiency over the others recycling processes [47].

According to the literature review, waste EPS can be recycled in many ways when it comes to the end of its life. The recycling method takes into account the technical, economic and environmental considerations [48]. Alternative systems using recycled materials contribute to environmental waste reduction and the development of sustainable products.

EPS thermo-mechanical recycling process is a cheap alternative to recycle EPS without using hazardous solvent or generate toxic emissions. In addition, it can promote the integration of pickers and industries for the development of recycling-based materials. Generally, the addition of wood flour waste to recycled EPS results in a composite feasible from both mechanical and environmental point of view [49-50].

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Experimental Methods

Materials

The EPS waste was obtained from Associação de Recicladores Serrano, a sorting unit from Caxias do Sul, Brazil, with melt flow index (MFI) of 20g/10min (200ºC/5 kg). Wood flour (WF) of Pinus elliottii was obtained from Madarco Co., Caxias do Sul, Brazil, with a particle size ranging between 53-105 μm. The wood flour used in this study is a waste from the lumber industry without any kind of preliminary chemical treatment. The poly(styrene-co-maleic anhydride) oligomer supplied by Sartomer Co., Exton/USA (SMA2000) used as the coupling agent, contains 30 wt% of maleic anhydride groups and a weight average molecular weight of 7500 g/mol. The amount of coupling agent incorporated was 2 wt%.

EPS thermo-mechanical recycling and composite preparation

The waste EPS samples were acquired from the packaging of electronic goods and home appliances. The adhesives and papers were removed before EPS recycling. EPS was molded by compression in a hot press at 130ºC during 5 min in order to reduce the apparent density.

Then, the resulting EPS plates were grinded in a rotary knife mill to obtain EPS flakes. The methodology used for recycling EPS waste is shown in Figure 8. The wood flour was dried in an oven with air circulation at 105°C for 24 h. Samples with 10, 20, 30 and 40 wt% of wood flour with and without 2 wt% of SMA2000 and EPS flakes were processed in a co-rotating twin-screw extruder at 200 rpm. The nine barrel temperature zones were controlled at between 160ºC and 190ºC. Specimens for mechanical tests were injection molded at a barrel temperature of 180ºC and mold temperature of 40 ± 2ºC.

Mechanical testing

The tensile tests were conducted according to ASTM D638 at a crosshead speed of 5 mm.min^-1, with a 50-mm extensometer, using an EMIC DL 3000 analyzer. The flexural tests were performed on the same equipment according to ASTM D790 at a crosshead speed of 1.5 mm.min^-1. Izod impact strength was measured with a CEAST Resil 25 pendulum using unnotched specimens according to ASTM D256. Each test value was calculated as the average of at least five independent measurements.

Density and morphological study

The density values for five waste EPS samples, before and after the compression molding, were determined according to NBR 11949-07 and calculated as the mass volume ratio. The EPS samples had a volume superior of 30 cm³. For the EPS-r (recycled EPS), obtained after extrusion and injection molding, and the composite material, the density was determined according to ASTM D792-00. Void content was determined according to ASTM D2734.

Studies on the morphology of the composites were carried out using a SHIMADZU Superscan SS-550, scanning electron microscope (SEM). The cryo-fracture surface specimens were sputter-coated with gold.

Figure 8. Thermo-mechanical process used for EPS recycling.