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233Cellular Polymers, Vol. 33, No. 5, 2014

Thermoplastic Elastomer Foams Based on Recycled Rubber


Paridokht Mahallati and Denis Rodrigue*

Department of Chemical Engineering, Université Laval, Quebec City, Quebec, G1V 0A6, Canada

Summary

In this work, different concentrations (0, 35, 50 and 65%wt.) of recycled rubber (ethylene-propylene diene, EPDM) were blended with virgin polypropylene (PP) to produce thermoplastic elastomer resins via twin-screw extrusion. Then, the samples were pelletized and foamed by injection molding using a chemical blowing agent (azodicarbonamide, ADC). In particular, the molding process was optimized to determine the effect of processing parameters like blowing agent content, mold temperature, and injection conditions (pressure, velocity, etc.). From the samples obtained, a complete morphological (skin thickness, cell size and cell density) and mechanical characterization was performed including density and hardness. The results obtained showed that it is more difficult to produce a good foam structure with increasing recycled rubber content.

INTrODuCTION

In recent years, thermoplastic elastomer (TPE) foams have received a lot of attention in both scientific and industrial areas due to their specific properties such as low density, chemical resistance, thermal and acoustic insulation and energy-absorbing performance. The main applications of thermoplastic elastomer foams are in textile, packaging and automotive industries such as automotive doors and seals, as well as building and construction profiles (Rodrigue and Khodabakhsh, 2005; Ding et al., 2013).

On the contrary, polyolefins like polypropylene (PP) are widely used in many engineering applications because of their high mechanical and physical properties while having relatively low cost. The main advantage of PP over polyethylene (PE) and polystyrene (PS) is higher strength than polyethylene

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234 Cellular Polymers, Vol. 33, No. 5, 2014

Paridokht Mahallati and Denis Rodrigue

with better impact strength than polystyrene. Polypropylene also provides a higher service temperature range and good temperature stability. It has been shown to be a versatile thermoplastic resin that can be modified to meet a wide range of specific requirements (Park and Cheung, 1997; Gómez-Gómez et al., 2013). Polypropylene foams have been considered as a good replacement for other thermoplastic foams in packaging heavy goods and industrial applications because of excellent physical characteristics and low material cost (Weber et al., 2000). However, PP foams do not offer a high enough state of physical properties to fully compete in the foaming market. To improve its impact toughness and extend its application range to lower temperatures, a number of studies on PP toughening were performed by using economical elastomers which is a successful way of reducing cost while maintaining or even improving properties (Zhang et al., 2008; Tian et al., 2013).

At the same time, large quantities of rubber products are consumed in the world every year. As they consist of crosslinked elastomers (vulcanized) they cannot be reprocessed directly. Therefore, a way of recycling these products is to grind the elastomers by several well-known milling procedures to obtain small elastomer particles which can be used as filler in polymeric matrices. Then, blending theses recycled elastomers with thermoplastic resins has attracted a lot of attention, not only to make use of the waste material and solve a major environmental problem, but also to provide plastic/rubber compounds at a much lower cost and to obtain impact-modified thermoplastic elastomers (Nevatia et al., 2002; Tantayanon and Juikham, 2004; Grigoryeva et al., 2005; Lee et al., 2007; Xin et al., 2009a; 2009b; El-Nemr and Khalil, 2011). Thermoplastic elastomers based on recycled elastomers have shown that their mechanical properties are comparable to that of commercially available TPE (Ausias et al., 2007). As a result, it would be interesting to study the processability of PP/recycled elastomer blends for foaming applications.

Several processes such as extrusion, injection molding, compression molding, and micro-foaming have been developed to produce polymer foams. But injection foam molding is one of the most commercially important processes to mold a broad range of complex parts, either using a physical (PBA) or a chemical (CBA) blowing agent. PBA are compounds releasing gases as a result of physical processes such as evaporation and desorption at higher temperatures or reduced pressures. Because of environmental benefits, carbon dioxide and nitrogen are nowadays more and more in demand for use as physical foaming agents. On the other hand, CBA are solids decomposing in a specific temperature range releasing gases like carbon dioxide or nitrogen. The most frequently used CBA are azodicarbonamide and sodium bicarbonate (Huang et al., 1998; Weber et al., 2000; Sahnoune et al., 2001; Faruk et al., 2007; Zhang et al., 2011). Despite the type of blowing agent selected, the



basic principles of foam formation are gas dissolution and homogenization, bubble nucleation, bubble growth, and bubble stabilization. Nucleation, or the formation of expandable bubbles, can begin inside a polymer melt that has been supersaturated by the blowing agent. Once nucleated, a bubble keeps on growing as the blowing agent diffuses into it. This growth will continue until the bubble stabilizes or ruptures (Naguib et al., 2004).

In the present experimental work, chemical foaming of thermoplastic elastomers based on PP and recycled ethylene-propylene diene (r-EPDM) is performed by injection molding. In this project, the effect of r-EPDM and chemical blowing agent contents are studied, as well as injection shot size and mold temperatures. From the samples produced, a complete characterization is made including density, morphology and mechanical properties.

EXPErImENTaL

The materials used in this study were polypropylene (PP), recycled ethylene-propylene diene rubber (r-EPDM) and a chemical blowing agent. The polypropylene (density = 0.90 g/cc and melt flow index = 35 g/10 min) was purchased from LyondellBasell Canada Inc. under the trade name of SV955. The r-EPDM (density = 1.29 g/cc and average particle size = 0.48 mm) was supplied by Royal Mat Inc. The chemical blowing agent (CBA) was an activated azodicarbonamide with the commercial name of CELOGEN 754A produced by ChemPoint USA. CELOGEN 754A has a decomposition temperature between 165-180°C with a gas yield of 200 cc/g.

For the purpose of this work, the first step is to produce the TPE matrix based on PP and r-EPDM (with 0, 35, 50 and 65%wt.) via melt blending. This step was done in a co-rotating twin-screw extruder (Leistritz ZSE-27, Germany) with a L/D ratio of 40 (total of 10 zones). The r-EPDM was fed in the main hopper of the extruder (zone 1) and a secondary feeder (side-stuffer) was used at zone 4 to feed PP. The extruder was operated with a flat temperature profile of 180°C (for all the zones) at 130 rpm with a total flow rate of 5 kg/h and a 3 mm die. The extrudate was directly quenched in water at the die exit and pelletized. The pellets were finally dried at 80°C for 6 h to remove any residual water. More details on processing and conditions have been reported by Mahallati and Rodrigue (2014).

Foaming was performed by dry blending of the chemical blowing agent (0, 0.75 and 1.5%wt.) with the TPE pellets and then injection molded in a Nissei PS60E9ASE (Japan). Injection molding was done using a rectangular mold of 115×25.4×3 mm3 with a screw temperature profile between 210 and 220°C from the rear to the injection nozzle. Two different mold temperatures were



used: 30 and 70°C. Different injection shot size values (SM) were optimized to have different density reduction for each foamed sample.

Finally, samples were punched out from the molded plates and testing was done directly on the different geometries.

Foam morphology was analyzed with a scanning electron microscope (JEOL model JSM-840A) at 15 kV. The foamed samples were fractured in liquid nitrogen, sputtered with a gold-palladium blend in vacuum and surface characteristics were studied under different magnifications.

To evaluate the average cell size and cell density as well as skin thicknesses, an image analysis software (Image Pro Plus) was used. The measurements were performed in the center of the molded part in the transverse direction (perpendicular to the flow direction).

Density measurements were carried out according to ASTM D2856 using a Quantachrome model Ultrapyc 1200e nitrogen gas pycnometer. The values reported were the average of five runs.

Tensile tests were performed by an Instron model 5565 testing machine at room temperature according to ASTM D638 using at least 5 dumbbell shaped specimens (type V). Each sample was strained using a crosshead speed of 10 mm/min with a 500 N load cell.

Notched Charpy impact was done according to ASTM D256 at room temperature. For each composition, 10 samples were tested on a Tinius Olsen model 104. The samples were first notched with an automatic sample notcher Dynisco model ASN 120m.

Flexural modulus was performed according to ASTM D790 with five repetitions for each sample. An Instron model 5565 with a 50 N load cell and a crosshead speed of 10 mm/min at room temperature was used to perform the flexural tests (three-point bending). The specimens had dimensions of 75×12.9×3 mm3 and the span was 60 mm.

Shore D type durometer (PTC Instruments Model 307L) was used to measure hardness according to ASTM D2240. Ten repetitions were done to get the average.

rESuLTS aND DISCuSSION

It is known that the properties of foams depend not only on the polymer matrix, but also on the cellular structures of the foam. Accordingly, studying the fractured surface of the foams specimen by SEM is important to get a full picture of the morphology inside a molded part. Typical structure of



PP/r-EPDM (65/35) blends, foamed PP and foamed PP/r-EPDM (65/35) are shown in Figure 1. The foams have a relatively uniform structure. Figure 1a shows good interfacial compatibility and dispersion of the recycled elastomer (r-EPDM) in the PP matrix. Comparing Figure 1b and 1c, it is obvious that the presence of r-EPDM particles produced a greater number of nucleating sites (heterogeneous nucleation) leading to smaller cell sizes. It also increases the matrix viscosity hindering even more cell growth with increasing concentration.

Figure 1. Typical morphology of selected samples at different magnifications: (a) PP/r-EPDM (65/35), (b) and (d) PP foamed with 0.75%wt. CBA, (c) and (e) PP/r-EPDM (65/35) foamed with 0.75%wt. CBA

(a)

(b) (c)

(d) (e)



Table 1 represents the sample compositions tested with their respective processing conditions such as r-EPDM loading and chemical blowing agent concentration (CBA%wt), different mold temperatures as well as lowest (L) and highest (H) injection shot sizes (SM) to produce foams with different densities. For the morphological characterization, as reported in Table 1, the average cell size was calculated as the number average of at least 100 cells, and average cell density (Nf) defined as the number of cells per cubic centimeter of foam (cells/cm3), as well as foams total skin thickness ratios (total skin thickness divided by total sample thickness) are reported. Cell density was calculated by Equation (1), where n is the number of bubbles in the micrograph of area A in cm2 (Gosselin and Rodrigue, 2005):

N

F=

nA

3/2

(1)

According to Table 1, foaming neat PP and PP/r-EPDM blends resulted in different density decrease. In general, increasing the blowing agent concentration from 0.75 to 1.5%wt decreased even further density. The same conclusion is obtained by decreasing shot size. Also, adding more r-EPDM increases density as its density (1.29 g/cc) is higher than neat PP (0.900 g/cc). For cell density and cell size, both parameters have opposite trends as the amount of gas is fixed in each sample; i.e. creating more bubbles consumes more rapidly the gas and less gas molecules are available for cell growth. The results of Table 1 show that neat PP has larger cells (68-196 microns) and lower cell density (0.39-2.44x105 cells/cc) than samples containing r-EPDM (34-152 microns with 0.31-14.79x105 cells/cc). Finally, the matrix composition, shot size, blowing agent content and mold temperature all have a direct effect on total skin thickness since values between 23 and 67% were obtained. Nevertheless, the complex interactions between all the parameters studied does not enable a clear trend to be determined for each individual effect and more work would be needed to completely understand these relationships. But in general, skin thickness has a direct effect on core morphology. Since no cells are nucleated in the skins, more blowing agent is available for cell growth in the core of the samples leading to larger cell sizes. Besides, thicker skins reduce heat transfer, leading to higher temperatures in the core region; consequently lower cell density is the result of larger cell size consuming the blowing gas (Rodrigue and Leduc, 2003; Tovar-Cisneros et al., 2008). Nevertheless, since foam morphology is known to play an important role in controlling mechanical properties, these properties are reported next.



Table 1. Formulation, processing conditions and morphological properties of PP/r-EPDM blendsr-EPDm loading (%wt)

CBa (%wt)

mold temp. (°C)

Injection shot size

(mm)

average density (g/cc)

average cell size

(µm)

average cell density

(105 cells/cc)

Total skin thickness

(%)

0 0 30 41 0.872 - - -

0 0.75 30 31.7 0.803 81 ± 56 2.20 ± 0.87 33.3 ± 0.1

0 0.75 30 33 0.811 150 ± 78 0.39 ± 0.27 39.9 ± 0.1

0 1.5 30 31.2 0.799 68 ± 42 2.44 ± 0.69 42.8 ± 0.1

0 1.5 30 32.2 0.791 98 ± 81 2.02 ± 1.29 49.6 ± 0.1

0 0 70 41 0.899 - - -

0 0.75 70 31.7 0.794 116 ± 87 1.81 ± 1.90 35.7 ± 0.1

0 0.75 70 33 0.821 196 ± 40 0.51 ± 0.11 55.4 ± 0.2

0 1.5 70 31.2 0.784 155 ± 45 0.88 ± 0.10 46.0 ± 0.2

0 1.5 70 33 0.810 141 ± 50 1.15 ± 0.37 46.9 ± 0.1

35 0 30 41 0.989 - - -

35 0.75 30 30.2 0.890 52 ± 32 6.14 ± 1.67 28.0 ± 0.1

35 0.75 30 31.7 0.896 100 ± 29 2.36 ± 1.09 53.3 ± 0.2

35 1.5 30 27.8 0.924 62 ± 27 5.09 ± 1.51 44.8 ± 0.1

35 1.5 30 28.9 0.937 44 ± 29 5.54 ± 2.88 29.0 ± 0.1

35 0 70 41 1.009 - - -

35 0.75 70 30 0.893 63 ± 34 3.18 ± 1.15 29.0 ± 0.1

35 0.75 70 31.2 0.901 94 ± 34 1.97 ± 0.58 34.5 ± 0.1

35 1.5 70 29 0.928 57 ± 37 3.68 ± 1.11 30.3 ± 0.1

35 1.5 70 31.2 0.971 196 ± 58 0.09 ± 0.04 67.0 ± 0.3

50 0 30 41 1.032 - - -

50 0.75 30 30.2 0.959 77 ± 46 1.55 ± 0.81 34.0 ± 0.1

50 0.75 30 31.2 0.980 89 ± 39 2.72 ± 1.39 44.3 ± 0.1

50 1.5 30 28 0.999 34 ± 17 14.79 ± 4.73 28.3 ± 0.1

50 1.5 30 29 1.029 44 ± 21 2.37 ± 0.83 45.0 ± 0.1

50 0 70 41 1.048 - - -

50 0.75 70 30.2 0.964 45 ± 25 3.57 ± 0.35 26.9 ± 0.1

50 0.75 70 31 0.985 152 ± 63 0.31 ± 0.12 66.0 ± 0.2

50 1.5 70 29 0.986 104 ± 41 1.70 ± 0.59 29.9 ± 0.1

65 0 30 41 1.085 - - -

65 0.75 30 30.2 1.037 50 ± 26 3.30 ± 2.76 23.0 ± 0.1

65 0.75 30 31 1.056 71 ± 32 0.88 ± 0.41 39.6 ± 0.2

65 1.5 30 29 1.065 40 ± 19 5.04 ± 1.56 31.2 ± 0.1

65 1.5 30 30 1.065 89 ± 26 1.93 ± 0.72 47.2 ± 0.2

65 0 70 41 1.114 - - -

65 0.75 70 30.2 1.053 49 ± 27 2.05 ± 0.43 30.9 ± 0.1

65 1.5 70 29 1.043 41 ± 21 4.62 ± 2.00 36.6 ± 0.1



In order to characterize the influence of r-EPDM content and blowing agent on PP/r-EPDM foam blends, tensile properties are presented in Figures 2-4. In general, mechanical properties are mostly dependent on density, but also on cellular structure: cell size, cell density and cell size distribution (Saiz-Arroyo et al., 2013). Tensile properties are mostly related to density because the amount of material in a part is controlling how much stresses it can sustain. By foaming the blends, tensile strength (Figure 2) and modulus (Figure 3) decreased with increasing CBA content, but also by adding an elastomer phase (r-EPDM).

Figure 2. Tensile strength of PP and PP/r-EPDM blends as a function of CBA content for different mold temperatures (30 and 70°C). L and H represent the lowest and highest injection shot sizes (see Table 1 for details)

As seen in Figure 2, foaming neat PP with 1.5%wt CBA leads to a tensile strength drop of 39% (from 29.9 to 18.3 MPa at a mold temperature of 30°C (8% density reduction)). This property shows the same reduction while adding 1.5%wt blowing agent to PP/r-EPDM blends of 35/65 (tensile strength reduction from 9.8 to 6.0 MPa at a mold temperature of 30°C). Due to the incorporation of flexible r-EPDM in the PP matrix, there is a decreasing trend



in tensile strength by adding more r-EPDM. Blends of PP/r-EPDM 35/65 showed a 69% tensile strength loss in comparison with neat PP (from 29.8 to 9.2 MPa at a mold temperature of 70°C).

As presented in Figure 3, unfoamed samples have a higher tensile modulus compared with foamed ones. Especially for PP samples in which tensile modulus decreased from 406 to 281 MPa (31% drop) by adding 1.5%wt CBA at a mold temperature of 30°C. In the case of PP/r-EPDM blends, the same behavior can be seen. For example, in the case of PP/r-EPDM 50/50 blends, a 35% drop in tensile modulus is produced with the addition of 1.5%wt. blowing agent (from 134.0 to 86.6 MPa at a mold temperature of 30°C). Furthermore, adding r-EPDM to neat PP showed a general loss of tensile modulus for all PP/r-EPDM blends. The highest value of tensile modulus was for neat PP which decreased substantially (84% difference) for PP/r-EPDM samples of 35/65 (from 409 to 66.1 MPa at a mold temperature of 70°C).

Overall, the results of Figures 2-3 indicated that a wide range of tensile properties can be obtained by a simple change of r-EPDM and CBA content.

Figure 3. Tensile modulus of PP and PP/r-EPDM blends as a function of CBA content for different mold temperatures (30 and 70°C). L and H represent the lowest and highest injection shot sizes (see Table 1 for details)



According to Figure 4, elongation at break for foamed samples in comparison with unfoamed samples is lower and this property increased by increasing r-EPDM loading. The highest amounts of elongation at break (higher than 100%) can be observed for the foams and blends with 65%wt r-EPDM. By considering the gas cells as particles with zero modulus and the fact that gas cells act as sites for stress concentration, subsequently this leads to a decrease in all performance characteristics such as tensile strength, tensile modulus and elongation at break (Zhang et al., 2008). Nevertheless, the samples having r-EPDM showed elongation at break similar or even higher than neat PP (unfoamed) which is a major achievement here.

Figure 4. Elongation at break of PP and PP/r-EPDM blends as a function of CBA content for different mold temperatures (30 and 70°C). L and H represent the lowest and highest injection shot sizes (see Table 1 for details)

Figure 5 presents the Charpy impact strength results. Here, impact strength is decreased when the blends are foamed. Charpy impact testing represents the amount of energy absorbed by the samples during fracture and Charpy impact strength can be used as a measure of blend toughness. It can be seen that the addition of 0.75%wt blowing agent to neat PP leads to a significant decrease



of impact resistance of around 57%. As CBA content increases, the blends became more brittle but still the impact strength was lower in comparison with unfoamed samples. As expected, by increasing the elastomer content, the impact strength was increased for foamed and non-foamed samples. For example, adding 65%wt r-EPDM produced an impact strength 459% higher than neat PP (from 33.4 to 186.9 J/m at a mold temperature of 70°C). An even more important increase (716%) of impact strength was obtained by adding 65%wt r-EPDM to PP foamed with 1.5%wt CBA. This behavior is due to the fact that r-EPDM is a rubber which is softer and can better absorb the energy through elastic deformation. Good impact strength improvement shows the possibility to adjust this property by r-EPDM content.

Figure 5. Charpy impact strength of PP and PP/r-EPDM blends as a function of CBA content for different mold temperatures (30 and 70°C). L and H represent the lowest and highest injection shot sizes (see Table 1 for details)

Figure 6 shows that the flexural modulus of foamed samples mostly increases with increasing mold temperatures from 30 to 70°C which is related to the production of thicker skins (see Table 1). For PP foams with 0.75%wt CBA and constant injection shot size, increasing mold temperature from 30 to 70°C



improved flexural modulus by 4% (from 1300 to 1356 MPa), while an increase of 39% was obtained for total skin thickness (from 39.9 to 55.4%). The same trend is observed for PP/r-EPDM foams. For example, a 35/65 PP/r-EPDM foam has a 34% increase in total skin thickness (23.0 to 30.9%) which led to a 6% increase in flexural modulus (299.0 to 281.4 MPa). On the other hand, by foaming the blends there is a general decrease of flexural modulus. For example, foaming PP with 1.5%wt CBA (at a mold temperature of 30°C) produced a 27% flexural modulus reduction. As expected, adding r-EPDM to the foams and the blends also decreased the flexural moduli due to the elastomeric nature of the material (lower stiffness). As presented in Figure 6, flexural modulus decreased by 82% by adding 65%wt r-EPDM to PP (1796.0 to 318.5 MPa at a mold temperature of 70°C).

Figure 6. Flexural modulus of PP and PP/r-EPDM blends as a function of CBA content for different mold temperatures (30 and 70°C). L and H represent the lowest and highest injection shot sizes (see Table 1 for details)

For thermoplastic elastomers and their foams, hardness is an important parameter depending on the final application for the materials. Figure 7 presents the Shore-D values comparison of foamed and unfoamed samples



with increasing r-EPDM content. As r-EPDM loading increases from 0 to 65%wt, an expected decrease of hardness is observed which is related to increased elasticity of the samples. For example, adding 65%wt r-EPDM to PP decreased hardness by 27% (from 74.8 to 54.8 at a mold temperature of 30°C). In addition, by foaming the blends, hardness decreases as expected. For example, a 6% reduction of Shore-D value was produced by adding 1.5%wt CBA to neat PP, while a 12% decrease was obtained by adding 1.5%wt CBA to a 50/50 PP/r-EPDM blend.

Figure 7. Shore-D hardness of PP/r-EPDM foams and blends as a function of r-EPDM content at a mold temperature of 30°C and the lowest (L) injection shot size (see Table 1 for details)

CONCLuSIONS

In this work, a blend of polypropylene and recycled EPDM (PP/r-EPDM) was used to produce a thermoplastic elastomer resin with different rubber content (0-65%wt.). The blends were then injection molded with different chemical blowing agent concentrations (0-1.5%wt) to produce a cellular structure. Furthermore, processing parameters like shot size (low and high) and mold temperatures (30 and 70°C) were studied. From the samples produced, a complete characterization in terms of density, morphology, and mechanical properties (tension, flexion, impact and hardness) was performed.

First, a complex interaction between composition and processing conditions was observed where a wide range of morphological parameters were obtained in terms of average cell size (34-196 microns), cell density (0.31-14.79x105 cells/cc) in the core and total skin thickness (23-67%).



In general, mechanical properties were found to decrease with increasing CBA (less material available to sustain the stresses) and r-EPDM (introduction of an elastomer phase) contents. Nevertheless, adding 65%wt of r-EPDM to PP foams with 1.5%wt CBA improved impact strength by 716%. From the wide range of properties obtained, these results opened up the way to produce impact modified polymer foams by the simple addition of low costs recycled rubber particles. The final properties of the parts can be adjusted by a careful selection of composition (CBA and r-EPDM contents) and processing conditions (mostly shot size and mold temperature).

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