Cet article a été soumis comme:
Veilleux, J. and Rodrigue, D., Properties of Recycled PS/SBR Blends: Effect of SBR Pretreatment, Prog
Rubber Plast Recyc Tech, accepté, juillet (2015).
Résumé
Les propriétés du caoutchouc recyclé styrène-butadiène (SBR) mélangé avec du polystyrène vierge (PS) ont été étudiées sur une large gamme de concentrations en SBR (0-94% en poids). Les mélanges ont été préparés par extrusion bi-vis suivie par un moulage par injection. Un prétraitement en solution (toluène) a également été effectué pour améliorer la compatibilité entre les phases. Des essais d'extraction et de thermogravimétrie ont été utilisés pour déterminer la quantité de PS inséré dans les particules de SBR. L'analyse morphologique (SEM) couplée à des mesures physiques (densité et dureté) et mécaniques (traction, flexion, torsion et impact) a été effectuée pour évaluer le comportement final des mélanges. Les résultats montrent que le traitement proposé peut améliorer les propriétés, en particulier pour des concentrations supérieures à 50% en poids de SBR.
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4.1 Abstract
The properties of recycled styrene-butadiene rubber (SBR) blended with virgin polystyrene (PS) have been investigated over a wide range of concentrations (0-94% wt. SBR). The compounds were prepared by twin- screw extrusion followed by injection molding. A pre-treatment in solution (toluene) was also performed to improve the compatibility between the phases. Extraction tests and thermogravimetry were used to determine the amount of PS inserted in the SBR particles. Morphological analysis (SEM) coupled with physical (density and hardness) and mechanical (tension, flexion, torsion and impact) measurements were performed to assess the final behavior of the mixtures. The results show that the proposed treatment can improve the properties, especially at SBR concentrations greater than 50% wt.
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4.2 Introduction
Plastics are an integral part of everyday life. This is why the industry has developed considerably over the last 50 years. Thus, world production in 2013 approached the 300 million tonnes (67). Their good mechanical properties, versatility in terms of design, durability and lightness allowed to gradually replace materials such as wood, metal and glass in many applications (68). Their potential for different applications led to growing plastics demand (69). Unfortunately, the high consumption of plastics inevitably leads to large amounts of waste (68 2014, Catalytic activity of metal impregnated catalysts for degradation of waste polystyrene, 69). Thus, a way must be found to reduce the materials send to landfills each year which represents about 9.6 million tons of plastic in Europe (67).
The main problems related to plastics recycling are retrieval, sorting, transport and market fluctuations in terms of price and availability (1). This is particularly the case for polystyrene (PS). The latter is one of the most used because of its unique physic-chemical properties like low density, high stability and ease of processing.(69). Although PS represents only 7.1% of the global production of plastics (67), it alone constitutes 10% of the total weight of plastic waste in landfills (69), because PS can be found in many products commonly used and discarded such as food packaging, toys, as well as hygiene items and medical uses (53).
Another material which causes a number of environmental problems is rubber, especially used tires. With increasing industrialization and globalization, world production of rubber tires has increased dramatically (70). Overall, more than 280 million used tires are discarded in the United States each year (71). A solution to reuse these materials is needed as it is estimated that between 2 and 4 billion used tires are still stacked in landfills in the United States (71). The recovery of waste rubber is the most technical and economic approach to solve the problem of disposal. (38) Today, the most interesting process is downsizing by different methods (ambient or cryogenic grinding) to get a more or less fine powder called ground rubber tires (GTR), depending on the treatment used. This treatment also leads to a reduction of molecular weight and the rupture of the macromolecular structure of the rubber crumb allowing the material to be more easily used alone or within other matrices (38).
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Several studies have been made to give a new vocation to used tires. These studies have shown that mixing two materials is a low cost and effective way to improve the properties of each material (72-74). However, the performance of the mixtures are highly dependent on the nature and concentration of each component as they influence the interaction between both phases (75). For blends of immiscible polymers, the interfacial area and the compatibility between the phases must be optimized or the mechanical properties will be bad, especially for the elongation at break (74-76). This is why many researchers have studied ways to improve the compatibility between a thermoplastic matrix and rubber particles.
A technique to improve compatibility is to add a coupling agent to the mixture. The latter acts as an interphase between both materials.(77) For example, Kakroodi et al. (76) studied a mixture of GTR with high density polyethylene (HDPE) using maleated polyethylene (MAPE) as a coupling agent. This addition allowed to produce mixtures containing high rubber concentration (up to 90% wt.) and also doubled the tensile modulus. Furthermore, Higazy et al. have studied various mixtures of PS with styrene-butadiene rubber (SBR) and added glass fibers or talc.(63) The compatibility was analyzed by ultrasound and showed that glass fibers had a single peak on the curves in contrast to talc which that had two. Later, Phadke et al. found that the addition of natural rubber (NR) improved the adhesion between polypropylene (PP) and GTR (40%) by doubling the impact resistance (78). Finally, Ibrahim et al. studied the properties of PS and a triblock copolymer of styrene- butadiene-styrene (SBS). Young's modulus and the maximum stress decreased by a factor of two when SBS concentration increased from 10 to 20%, but impact resistance increased from 20 to 201 J/m between 0 and 60% SBS (72).
A second technique to improve compatibility is to create a link between both phases through a chemical reaction. For example, SBR can be grafted with PS molecules before mixing with a PS matrix. Grafting can increase the strength and the maximum stress of the mixture (64, 75). A very good review about the possibilities to mix GRT with various polymers such as low and high density polyethylene (LDPE, HDPE), ethylene vinyl acetate (EVA), PS and PP was written by Karger-Kocsis et al. (37). The authors presented different types of chemical agents according to the matrix used. Mechanical properties such as tensile modulus, maximum stress and hardness are briefly described. Moreover, a study showed that the addition of a silane coupling agent increased the mechanical properties of a polyester-based resin prepared from maleic anhydride, ethylene glycol and isophthalic acid in a ratio 1:2:1 with GTR (79). GTR surface treatment improved tensile modulus and strength from 1.07 to 3.52 GPa and 12.6 to 23.8 MPa, while the elongation at break decreased from 1.55 to 1.32% compared to the reference.
37 Another widely investigated technique is the partial devulcanization of the rubber which can partially regenerate the material to its original state. (80) Thus, by selectively breaking the crosslinking bonds, the polymer chains become more mobile which promotes mixing and entanglements. The main investigations on SBR chemical devulcanization has been made to introduce into matrices such as PS, EVA and polyvinyl chloride (PVC) (37). It was found that tensile modulus and strength decreased, while the elongation at break increased with GTR concentration. Blends of PP with GTR were also made by Luo et al. (75). The authors analyzed the thermal properties of mixtures and found that rubber incorporation leads to a progressive decrease of the melting temperature and an increase of the enthalpy of fusion. The trends for the mechanical properties were similar to the study of Karger-Kocsis et al. (37).
Finally, a solution treatment for SBR particles was proposed by Macsiniuc et al. (80). The idea is to swell the rubber particles with a solvent and to force the penetration of dissolved matrix molecules in the crosslinked structure of the rubber (80, 81). This work showed that a polystyrene-based mixture with recycled SBR with PS dissolved in tetrahydrofuran (THF) resulted in increased Young's modulus (468-652 MPa), tensile strength (5.14 to 9.39 MPa) and impact strength (35.2 to 50.1 J/m). However, this work was carried out in a batch mixer (discontinuous process) which limits its application to industrial scale (continuous process).
This article is a second step based on previous results obtained on mixtures of recycled PS with recycled styrene-butadiene rubber (SBR) (81). The compounds were produced by twin-screw extrusion followed by injection molding for a SBR range of concentrations between 0 and 62% wt. A pre-treatment in solution (toluene) was also performed to determine its effect on mechanical properties. The results showed that the proposed treatment leads to substantially improved quality of the blends (homogeneity and mechanical properties). The pre-treatment decreased Young's modulus, tensile strength and hardness when SBR concentration increased which allowed for a more elastic material because impact resistance and density increased. Following these results, two new series of mixtures were made and characterized, but this time with virgin PS to limit interference from a recycled matrix: variability and contamination. These new series have increased the range of SBR introduced into the PS (up to 94 wt% SBR) to compare the physical and mechanical properties between recycled and virgin matrices. In addition, the effect of the pre-treatment is analyzed and quantified here. Finally, the behavior at low SBR concentrations which had not been observed
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with the recycled PS due to limiting processing conditions of the mixtures will be analyzed. To do this, the new series will be produced under the same conditions as before (81).
4.3 Experimental
4.3.1. Materials
For the matrix, crystal polystyrene (Ineos Nova 3900) was provided by Nova Chemicals. The latter has an average molecular weight of 240 kg/mole, a density of 1.04 g/cm3 and a melt index of 38 g/10 min (200 °C and
5 kg) according to ASTM D1238. Recycled SBR rubber powder was supplied by Royal Mat (Canada) coming from shredded worn tires. It has a density of 1252±11 kg/m3 and has been previously sieved to collect only
particles less than 710 microns. For the pre-treatment, the same solvent as in the previous work was chosen (81): toluene (Anachemia, ACS grade).
4.3.2. Preparation
The polymer blends were produced in a wide range of SBR concentrations (0-94% wt.). For twin-screw extrusion a temperature profile between 165 and 175 °C from zone 1 to zone 10 was used. In addition, a speed of 100 rpm was used for the screws. For injection, the screw temperature profile was between 190 and 200 °C, while the mold was set at 30 °C. The mold cavities were selected to produce the geometries required for the different tests. More details are available in our earlier work (81).
Pre-treatment of SBR powder was made in the same conditions as described in our previous work (81), but a change was made to the protocol due to the use of pellets. The solution was heated to about 60 °C to promote dissolution of the polymer. Once the PS dissolved, the solution was cooled to room temperature. Thereafter, the following steps were performed: stirring for 24 h with a mechanical stirrer, vacuum filtration with a Buchner funnel and a paper coffee filter and finally drying at 60 °C.
39 To simplify the presentation, a code is used for the different mixtures. The number represents the SBR concentration, while the letter "s" is used to indicate that solution pre-treatment was made. For example, a mixture of 72% SBR with pre-treatment will be named "72s".
4.3.3. Characterization
First, the amount of PS absorbed into SBR particles by solution pre-treatment was estimated. To do this, a Soxhlet extraction with toluene was performed. For each extraction, 5 g of rubber powder was placed in a two- layer paper (Kimwipes) previously weighed. The extraction lasted overnight and the solvent was heated to its boiling point (about 111 °C (82)). Thereafter, the paper and the rubber powder were dried in an oven at 60 °C and weighed to determine mass loss. Three repetitions were made.
Thermogravimetric analysis (TGA) was performed using a TGA Q5000 IR apparatus (TA Instruments). About 2 mg of sample was placed in an aluminum pan. The measurements were performed by heating from 50 to 750 °C at a rate of 10 °C/min in a nitrogen atmosphere (25 ml/min). The mass loss is reported here.
The weight average molecular weight (Mw) and the polydispersity index (PI) were determined by size exclusion chromatography (SEC). The device used was a Viscotek HT-GPC with triple detection: high temperature refractive index, light scattering and viscometer. The device is also made up of 3 columns: Mixed PL gel B LS 10 m (300 x 7.5 mm). The flow rate was set at 1.0 ml/min and the eluent was 1,2,4- trichlorobenzene (TCB) of HPLC grade. In addition, the system temperature was set at 140 °C. All samples were prepared using a Vortex autosampler at a concentration of 4 mg/ml in TCB. The calibration method used to generate the data reported was based on polystyrene standards having a molar mass of 99 and 235 kg/mole (Malvern). These were also dissolved in TCB.
Differential scanning calorimetry (DSC) was performed on a DSC823e instrument (Mettler Toledo) with a cooling device using liquid nitrogen. In an aluminum pan, between 10 and 15 mg of sample was encapsulated. The measurements were carried out from -100 °C to 120 °C at a rate of 20 °C/min under a nitrogen atmosphere. The sample underwent an isotherm at this temperature for 2 minutes then cooled to -100 °C at
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the same speed. Finally, a second heating run was performed under the same conditions as the first one after a 5 minutes isotherm at -100 °C. The last heating curve was used to report the results.
Scanning electron microscopy (SEM) was used to investigate the particle size, as well as dispersion and contact between PS and SBR. To study the morphology of the compounds, a fracture in liquid nitrogen was made. The fractured surfaces were coated with an alloy of gold and palladium and the samples were examined under a voltage of 15 kV with a scanning electron microscope JEOL JSM-840A. The size of rubber particles before or after extrusion, with or without pre-treatment was also characterized using image analysis via the Image-Pro Plus software (Media Cybernetics). Dimensions (size) such as average diameter, area and perimeter were obtained. For particles after extrusion, the pellets were immersed in hot toluene (about 60 °C) to dissolve the matrix. To recover the SBR particles, vacuum filtration with a Buchner funnel and a coffee filter paper was performed. Finally, drying in a vacuum oven at a temperature of 60 °C produced the extracted SBR particles.
Density measurements were performed with a gas (nitrogen) pycnometer model Ultrapyc 1200e (Quantachrome Instruments, USA). For each sample, the test was repeated five times and the average is reported with standard deviations (less than 1%). For hardness, each sample was measured by the Shore D scale (model 307L durometer). The values reported are the average of twenty-five measurements.
Tensile tests were carried out using a model 5565 universal testing machine (Instron, USA) with a 5 kN load cell. Type V samples were produced directly from injection molding according to ASTM D638. The tests were performed at a speed of 1 mm/min at room temperature. For each concentration, at least five specimens were used and the average with standard deviation are reported. The data presented are the Young's modulus (E), the maximum stress (Y) and elongation at break (εb).
Flexural tests were performed on the same apparatus as tensile tests, but equipped with a 500 N load cell. Bending tests were performed at room temperature using a speed of 2 mm/min according to ASTM D790. The samples were produced by injection molding to obtain a rectangular samples with dimensions of 125x12.7x3 mm3. The distance between the supports was set at 60 mm and a minimum of three samples were tested to
41 The complex shear modulus (G*), storage (G') and loss (G") moduli, as well as their ratio (tan = G"/G') were measured on an ARES rheometer (Rheometric Scientific, USA). The samples were produced by injection molding in order to obtain rectangular bars with dimensions of 55x12x3 mm3. The tests were performed in the
linear viscoelastic range with frequency sweeps between 0.01 and 10 Hz with a deformation of 0.05% at room temperature. The average and standard deviation are reported from at least three measurements.
The Charpy impact strength was obtained with a pendulum weight of 242 g (1.22 J) on a Tinius Olsen (USA) apparatus model 104. The arm length of 279 mm results in a speed 3.3 m/s. Rectangular samples with dimensions of 60x10.15x3 mm3 were obtained by injection molding and were notched at least 24 hours before
the test through an automatic sample notcher model ASN (Dynisco, USA). The tests were performed according to ASTM D6110 at room temperature. The results are the average of ten measurements.
4.4 Results and Discussion
Soxhlet extractions were used to determine the amount of polystyrene inserted in the rubber particles through the solution pre-treatment. The first test was conducted on the SBR powder without pre-treatment. The results were used to determine a 6% mass loss for the neat SBR. This can be attributed to the loss of additives or small molecules present in the rubber crumbs. This information is important because it allows for a correction in the rubber mass as small molecules can be extracted during pre-treatment. Thereafter, the same test was performed on the treated rubber with PS in solution. This data was used to calculate the mass lost during extraction which is directly related to the amount of PS inserted. The experiment was repeated three times and allowed to obtain an average weight loss of 7.5%. This mass is related to the amount of PS inserted into SBR particles.
To validate the results obtained by Soxhlet extraction, TGA tests were also performed. Figure 1 presents the mass loss; i.e. the mass that was degraded when heated under nitrogen. The results show that more SBR in the mixture (less PS) led to higher residual mass (lower mass loss). For the blends, the analyses were performed on the pellets produced by extrusion (5-100% PS) for the series without pre-treatment. Subsequently, the same method was applied on pretreated SBR and a mass loss of 65.5% was observed. Based on the results of Figure 6, PS content can be approximated as 6.1% by interpolation which is in agreement with the result obtained by Soxhlet extraction (7.5%).
42 PS content (%) 0 20 40 60 80 100 Mas s l os s (% ) 50 60 70 80 90 100 110
Figure 6: Mass loss (residues) obtained by TGA for PS/SBR blends as a function of PS content
Soxhlet extraction was also applied on the samples (recycled PS) from our previous work, as this characterization was not performed originally (81). The value obtained is 10.5%. Thus, for the same type of treatment, a higher amount of recycled PS was inserted in the rubber particles. To explain this difference, the molecular mass was determine by SEC. The virgin PS has a weight average molar mass of 243 kg/mole (IP = 2.4), while the recycled PS has 217 kg/mole (IP = 2.2). Knowing that the diffusion of molecules in a medium decreases with increasing molecular weight, this can explains the difference in mass absorbed because treatment time and temperature were the same in both cases (83, 84).
Subsequently, DSC tests were made to determine the glass transition temperature (Tg) of both polystyrene. The recycled PS has a transition at 106.8 °C, while the virgin PS is at 88.2 °C. Generally, Tg increases with Mw (85), but the value can change due to the presence of impurities/additives which can explain why the recycled PS has a higher Tg despite its lower molecular weight (58). These impurities/additives are added during processing or the results of use. For example, flame retardants are generally added in expanded PS for better fire resistance (86, 87). The most used molecule is hexabromocyclododecane (HBCD) which is much smaller (642 g/mole) and soluble in the same solvents as PS. HBCD provides better fire resistance by raising the ignition temperature, as well as reducing the rate of combustion, flame spread and smoke creation (86) . It is especially used for injection molding applications subject to variable residence times at elevated temperatures and high shear stress(30). HBCD could also be inserted into the SBR particles and can explain the higher value obtained during extraction.
43 Figure 7 shows typical images of SBR particles and Table 3 summarizes the dimensions for the various treatments. The results indicate that the differences between the different types of particles are statistically insignificant because distributions are very wide (large standard deviations). This indicates that the various treatments had no effect on the distribution of particle sizes. It is possible to conclude that the extrusion and injection molding did not changed the size of the SBR particles by breakup or agglomeration, with or without the solution pre-treatment.
Figure 7: SEM micrographs of SBR powders: A) without treatment, B) after solution pre-treatment, C) after