Fabrication of Fine-celled PP/Ground Tire Rubber Powder Composites Using Supercritical Carbon Dioxide

Transcription

1 Fabrication of Fine-celled PP/Ground Tire Rubber Powder Composites Using Supercritical Carbon Dioxide Fabrication of Fine-celled PP/Ground Tire Rubber Powder Composites Using Supercritical Carbon Dioxide Zhen-Xiu Zhang 1,2, Lin Li 2, Zhen Xiang Xin 1, and Jin Kuk Kim 2 * 1 Key Laboratory of Rubber-Plastics, Ministry of Education, Shandong Provincial Key Laboratory of Rubber-Plastics, Qingdao University of Science and Technology, Qingdao, , China 2 School of Nano and Advanced Materials Engineering, Gyeongsang National University, Gyeongnam, Jinju, , South Korea Received: 23 February 2011, Accepted: 14 March 2011 Summary PP/Ground Rubber Tire (GRT) powder microcellular blend foams were prepared by supercritical carbon dioxide. The effects of blend composition like the content of GRT and maleic anhydride-grafted polypropylene (PP-g-MA) on crystallinity, solubility, diffusivity, morphology and mechanical properties of PP/ GRT microcellular composites were studied. The results showed that the PP/ GRT composite foams with a unique bimodal (large and small) cellular structure, in which the large-cells embrace a GRT powder. Depending on the composition, generally, the higher content of GRT results in the smaller cell sizes, higher cell densities and relative densities, whereas the 20 wt% GRT composite shows the lowest relative density. The mechanical properties of the microcellular PP/ GRT composite foams are directly related to the blend composition and the processing conditions. The PP-g-MA/GRT (50/50) produced microcellular foams with a very fine and uniform cell structure, lower relative densities and improved mechanical properties. Keywords: Polypropylene, ground rubber tire, foam, composite *Corresponding author: Prof. Jin Kuk Kim, School of Nano and Advanced Materials Engineering, Gyeongsang National University, Gyeongnam, Jinju, , South Korea. Tel: (+82) (0) ; Fax: ; rubber@gnu.ac.kr Smithers Rapra Technology, 2011 Cellular Polymers, Vol. 30, No. 3,

2 Zhen-Xiu Zhang, Lin Li, Zhen Xiang Xin, and Jin Kuk Kim Introduction Due to its outstanding characteristics and low cost, polypropylene (PP) has been considered as a substitute for other thermoplastic foam materials [1]. But it does not provide a high enough state of physical properties at a given flexibility to fully compete in the cellular elastomer market [2]. To improve the impact toughness and extend its application range, a number of studies on toughening PP with rubber have been made in the last 20 years [3], Recycling of ground rubber tire powder is our preference from the ecological and economical point of view. The usage of GRT powder as dispersed phase in PP matrix offers an interesting opportunity for recycling of scarp rubber, so the PP/GRT composites foaming technology is to fulfill the requirement of lower cost, lighter weight and better fuel economy, therefore presents an important milestone in many applications. Polymers are often blended to create new functions, which each polymer alone cannot express, or to compensate for the weakness in the mechanical properties of each polymer. In polymeric foaming also, polymer blends are often used to create fine cell structures [4]. Han et al. [5] have reported the well-mixed and poorly mixed PS/9 wt% PMMA blends were foamed with CO 2, and the results showed that the well-mixed blends were rather homogeneous, whereas the poorly mixed blends clearly showed a dominant small cell phase and larger cells spread as stripes through the foamed sample. Doroudinai et al. [6] studied the foaming of high-density polyethylene (HDPE)/isotactic polypropylene (PP) blends. They found that a fine cellular structure could be created in the blend polymer by choosing a suitable temperature, whereas little or no foaming took place in each neat polymer. Siripurapu et al. [7] investigated PS/poly(vinylidene fluoride) (PVDF) and poly(methyl methacrylate) (PMMA)/ PVDF blends for foaming to expand the operating windows and concluded that blending PS with PVDF was not suitable for foaming because of their immiscibility, whereas PMMA/PVDF could be foamed at various operating conditions. Related to the situation of poorly mixed blends is the case of blending non-miscible polymers, foaming of these blends can result in remarkable foam morphologies. Taki et al. [4] have produced foamed poly(ethylene glycol) (PEG)/polystyrene (PS) blends showed a unique bimodal (large and small) cell structure, in which the large-size cells embraced a PEG particle. The mechanical properties of microcellular foams processed in batch method have rarely been reported. Kumar et al. [8] studied the tensile behaviors of microcellular foamed polymer polycarbonate foams, Matuana et al. [9] studied the mechanical properties of microcellular polyvinyl chloride (PVC) foams, Sun et al. [10-11] examined the mechanical properties of polysulfone, polyethersulfone and polyphenylsulfone microcellular foams. Recently, Fu et al. [12] investigated the effect of nanoclay on the mechanical properties of 112 Cellular Polymers, Vol. 30, No. 3, 2011

3 Fabrication of Fine-celled PP/Ground Tire Rubber Powder Composites Using Supercritical Carbon Dioxide PMMA/clay nanocomposite foams. The tensile behaviors of TPO were also investigated by Wong et al. [13], they reported the effects of foaming conditions and relative density on the mechanical properties of microcellular TPO foams. The purpose of this study was to investigate the relationships of processing, structure and properties of microcellular PP/GRT composites, and at last generation of fine-celled PP/GRT composite. When polymer blends are foamed, the cellular structure is determined not only by the morphology, crystallinity and viscosity of the blend polymers but also by the solubility and diffusivity of the physical foaming agent in the polymers. Therefore, there is a possibility of creating various cell structures by the blending of polymers. In this study, PP/ GRT composites were foamed by pressure-quench method described by Goel and Beckman [14] using CO 2 as foaming agent, the effect of blend composition on the desorption behavior of CO 2 and crystallinity of PP/GRT composite, and its relationship with cell structure as well as mechanical properties of foamed PP/GRT composites were investigated, the mechanism of a unique cellular structure of foamed PP/GRT composite was also investigated. Experimental Materials Polypropylene (R520Y) supplied by SK Corporation, which has a melt flow index (MFI) of 1.8 g/10 min (ASTM D1238), a density of 0.9 g/cm 3, and melting point of 153 C was used as a matrix in this experiment. Maleic anhydride grafted polypropylene (PP-g-MA) was prepared by our research group [30], which has MAH grafting degree of wt%, melting point of 151 C. The GRT powder was ground by wet grinding method and was supplied by Hongbok Industry, Korea. The composition of scrap rubber is: polymer content of 48.5% with natural rubber (NR) and styrene-co-butadiene rubber (SBR) in 25% and 75% ratio respectively. The other composition of waste rubber was organic additives, carbon block and ash content 13.4%, 27.7% and 10.4%, respectively. Its particle size was characterized to be µm as shown in Figure 1. Commercial grade CO 2, an environmentally friendly PBA, with a purity of 99.95% was supplied by Hyundai Gas Inc. Preparation PP/ GRT Composites by Twin Screw Extruder PP/GRT composite samples were prepared at different ratios, as shown in Table 1. All experiments were performed by using a modular intermeshing Cellular Polymers, Vol. 30, No. 3,

4 Zhen-Xiu Zhang, Lin Li, Zhen Xiang Xin, and Jin Kuk Kim Figure 1. SEM microphotograph of μm GRT powder (scale bar is 50 µm) Table 1. Formulation and blend conditions of PP/GRT composites GRT PP PP-g-MA Barrel Temperature Screw Speed C 100 rpm twin screw extruder (D=19 mm, L/D=40/19, BauTech). The screw speeds were kept at 100 rpm, the barrel temperature was maintained at 200, 210, 220, and 230 C from the hopper to the die. The extrudate was pelletized and dried under vacuum at 80 C for 24 h to remove any residual water. The plate samples of the blends with 2.0 mm thickness were compression-molding at 180 C for 6 min. Preparation and Analysis of PP/GRT Composites Foams Microcellular foaming experiments were performed in a batch process. A schematic of the batch-foaming process is shown in Figure 2. Plate samples that were 2.0 mm thick, 60.0 mm long, and 4.0 mm wide were enclosed high-pressure vessel. The vessel was flushed with low-pressure CO 2 for about 3 min and pressurized to the saturated vapor pressure CO 2 at room temperature and preheated to desired temperature. Afterward, the pressure was increased to the desired pressure by a syringe pump (ISCO260D) and maintained at this pressure for 2 h to ensure equilibrium absorption of CO 2 by the samples. After saturation, the pressure was quenched atmospheric 114 Cellular Polymers, Vol. 30, No. 3, 2011

5 Fabrication of Fine-celled PP/Ground Tire Rubber Powder Composites Using Supercritical Carbon Dioxide Figure 2. The schematics of batch-foaming process pressure within 3 s and the samples were taken out. Then foam structure was allowed to full growth during rapid depressurization. The foam morphology was characterized by utilizing a scanning electron microscope (SEM, Philips XL 30S). The foamed samples were cooled in liquid nitrogen and fractured to produce a clean and intact surface with minimum plastic deformation. They were then gold coated by using a sputter coater for enhanced conductivity. The average cell size and cell density were analyzed by utilizing the ImageJ software. The cell sizes, cell densities and relative densities were characterized. The cell diameter (D) is the average of all the cells on the SEM photo, usually more than 100 cells were measured. d D = i n i n i (1) Where n i was the number of cells with a perimeter-equivalent diameter of d i. The density of foam and unfoamed samples was determined from the sample weight in air and water respectively, according to ASTM D 792 method A. Then the density of the foamed sample is divided by the density of the unfoamed sample to obtain the relative density (ρ r ). The volume fraction occupied by the microvoids (V f ) was calculated as: V f = 1 ρ f ρ m (2) Where r m and r f are the density of the unfoamed polymer and foamed polymer respectively. Cellular Polymers, Vol. 30, No. 3,

6 Zhen-Xiu Zhang, Lin Li, Zhen Xiang Xin, and Jin Kuk Kim The cell density (N 0 ) based on the unfoamed sample was calculated as: N f = V f π 6 D3 (3) N f N 0 = 1 V f (4) Where V f is the volume fraction occupied by the microvoids, N f is the cell density based on the foamed sample. To study the effect of GRT on CO 2 solubility and diffusivity, experiments of CO 2 sorption and desorption were performed. At high temperatures, temperature control and the accurate reading of pressure and volume are very difficult, so the absorption measurement was carried out at 45 C and desorption was characterized at room temperature. Although the testing temperatures are not the actual foaming temperature, the results can provide some general trend. The experimental procedure was similar to the employed by Berens et al. [15] flat plate samples 60 mm in length, 20 mm in width, and 2 mm in thickness of PP or PP/GRT composites were placed in a high-pressure vessel that was connected to a syringe pump. The samples were saturated with CO 2 at 15 MPa and 45 C for 24 hours. They were then quickly taken out of the high-pressure vessel and placed on a high-resolution balance. The CO 2 desorption curve (weight loss with time) was recorded. The crystallization temperature and crystallinity of the PP/GRT composites were investigated using a differential scanning calorimeter (DSC, Q20, TA Instruments). For each sample, 6 10 mg was sliced from the compressionmolded specimens and placed in a hermetic aluminum pan under 50 ml/min nitrogen flow. The samples were heated from room temperature to 200 C at 10 C/min. The crystallinity was calculated from the specific heat required for melting (ΔH m ) by integrating the area under the corresponding peak and dividing this value by the heat of fusion for the pure crystalline phase of polypropylene (ΔH mo ), J/g. The tensile mechanical properties of microcellular PP/GRT composite foams were tested, following the ASTM D638 procedure on a Tensometer 2000 (Bong Shin) mechanical testing machine at room temperature. Foamed samples were allowed to desorb the gas for at least 2 weeks before property characterization. PP/GRT composites foam samples used for tensile testing had a thickness ranging from 2 mm to 4 mm, depending on the expansion 116 Cellular Polymers, Vol. 30, No. 3, 2011

7 Fabrication of Fine-celled PP/Ground Tire Rubber Powder Composites Using Supercritical Carbon Dioxide ration of the sample under different foaming conditions. The displacement rate of the crosshead was 10 mm per min. Tensile strength was calculated as the load force divided by the initial cross-sectional area of the specimen. The modulus of elasticity was obtained by calculating the slope of the stressstrain curves in the elastic region. The elongation at break of the sample was calculated in terms of percent elongation. A minimum of five specimens was tested for each set of foaming conditions. Results and Discussion Effect of Blending on Crystallinity The results of the effect of GRT content and compatibilizers on crystallinity are summarized in Table 2. Random PP is a semi-crystalline polymer, and its mechanical properties are greatly affected by its overall crystallinity, the foaming behavior of a semi-crystalline polymer largely depends on the crystallization temperature [16]. Therefore, it is important to investigate the effects of GRT content and PP-g-MA on the crystallization behavior of blends. The crystallization temperature (T c ) and crystallinity of all PP/GRT composites decreased when the GRT content was increased. This effect could be explained by the lowered mobility of polymer chains in the PP/GRT matrix, which resulted from the presence of dispersed GRT particles. The presence of the rubber powder restricts the growth of the crystalline phases and thereby leading to the amorphous regions. The PP-g-MA and PP-g-MA/ GRT composite showed reduced crystallinity compared with PP and PP/ GRT composite. This was ascribed to the reduced perfection of the crystals, deriving from the anhydride groups, and the presence of possible branching. Table 2. Crystallization temperature, crystallinity of PP and PP/GRT composites Sample T c, C Crystallinity of Blends (%) Crystallinity of Polymer (%) PP PP-g-MA PP/GRT (80/20) PP/GRT (60/40) PP/GRT (50/50) PP-g-MA/GRT (50/50) Cellular Polymers, Vol. 30, No. 3,

8 Zhen-Xiu Zhang, Lin Li, Zhen Xiang Xin, and Jin Kuk Kim Solubility and Diffusivity of PP/GRT Composites It is known that the foamability of polymers is affected by the sorption of gas in the polymer and that the mechanisms of cell nucleation and cell growth are influenced by the amount of gas dissolved in the polymer and the rate of gas diffusion. Desorption isotherm curves for the PP/GRT composites are illustrated in Figure 3. After releasing the pressure in the high-pressure vessel, the samples still remained unfoamed due to the low temperature applied, and thus the dimensions of the samples keep unchanged. The initial stage of the desorption curve of CO 2 from the polymer is linear with respect to time [16], and the y-intercept back to time zero yields the solubility of the samples in the experiment. By extrapolating from desorption curves, the solubility of CO 2 in PP and PP/GRT composites was obtained (see Table 3). The composites Figure 3. Desorption curves of PP/GRT composites Table 3. Solubility and diffusivity of foamed PP and PP/GRT composites Sample Solubility (wt%) Diffusivity (cm 2 /s) PP PP/GRT(80/20) PP/GRT(60/40) PP/GRT(50/50) PP-g-MA/GRT(50/50) Cellular Polymers, Vol. 30, No. 3, 2011

9 Fabrication of Fine-celled PP/Ground Tire Rubber Powder Composites Using Supercritical Carbon Dioxide exhibited a slightly higher solubility than pure PP. This can be explained that the poor adhesive between PP and GRT, there exist some microvoids in the blends and it can hold amount of gas, this is one reason, another reason is because of the lower crystallinity of the blends. Diffusivities of CO 2 in the samples were determined using the following equation for Fickian diffusion [16] through a flat plate: M t = 4 D M π 0.5 t 0.5 L (5) where M t is the mass gain by the sample at time t, M the maximum mass gain, D the diffusivity of CO 2 in the polymer, and L the thickness of the sample. M is the equilibrium solubility of CO 2 at the conditions of the experiment. The diffusion coefficient of CO 2 in the sample was obtained from the slope of M t /M plotted against t 0.5 /L. As our expectation that blends exhibit high diffusivities because of the addition of GRT to the polymer leads to poor adhesion between the GRT and the polymer matrix. The poor surface adhesion of the polar GRT to the nonpolar polymer provides a channel through which gas can quickly escape from the blends. Effect of GRT Content The effect of GRT powder on the cell structure of PP/GRT composites were identified by comparing the results from experiments with various GRT contents of 0 wt%, 20 wt%, 40 wt% and 50 wt%, the foaming temperature and saturation pressure were fixed at 155 C and 12 MPa, respectively. Figure 4 are the scanning electron micrographs of foamed PP/GRT composites with different concentration of the scrap rubber powder. The comparison shows that existence of GRT powder caused deterioration of microcellular structure, larger cells, and the cells become non-spherical and non-uniform. The figures suggest that foaming PP/GRT composites, promotes further detachment of GRT from the matrix, caused by bubbles formation at the interface. This is because the poor adhesion between PP and GRT easily led to the coalescing and a two-phase separation at the boundary, and higher solubility and higher diffusivity make the bubble growth rate faster. As illustrated in Figure 3, the solubility and diffusivity of CO 2 in PP/GRT composites were larger than those in pure PP. So during the manufacturing process at foaming PP/GRT composites, big cells occurred, the foaming effect caused by CO 2 is so extreme that it causes the cells to explode during depressurization. Cellular Polymers, Vol. 30, No. 3,

10 Zhen-Xiu Zhang, Lin Li, Zhen Xiang Xin, and Jin Kuk Kim (a) (b) (c) (d) Figure 4. Effect of GRT content on microcellular structure of PP/GRT composite foams at 12 MPa, 155 C, (a) 0 wt%, (b) 20 wt%, (c) 40 wt%, (d) 50 wt% It is well known that PP and GRT are immiscible with each other, and islandsea morphology was established at these weight fractions: GRT became islands, and PP made the matrix. Some GRT was lost when the samples were prepared for SEM observations, and this resulted in voids in the matrix. As it can be seen in the picture, an increase in the GRT weight fraction made the number of the island increase. Figure 5 shows an enlarged SEM micrograph of the PP/GRT (50/50). It clearly shows a bimodal cell size structure, in which the average diameter of the smaller cell was less than 55 µm and the cell size of the larger cell was about 420 µm, a GRT was located at a large cell in the foams. Counting the cells whose diameters were larger than 55 µm at the six different positions in each micrograph and averaging the counted numbers gave the average number of large cells. The number density of large cells (N f ) was calculated by the Equation (3). Figure 6 shows the relationship between the number density of large cells and the weight fraction of GRT in the blend. As the fraction of the GRT increased, the number of large cells increased. This indicated that the large cells originated at the boundary between the PP and GRT, the small cells originated in the PP phase of the blend. 120 Cellular Polymers, Vol. 30, No. 3, 2011

11 Fabrication of Fine-celled PP/Ground Tire Rubber Powder Composites Using Supercritical Carbon Dioxide Figure 5. Enlarged SEM micrograph of the PP/GRT composite foam Figure 6. Relationship between the GRT content, average size of large cells and number density of large cells of PP/GRT composite foams at 12 MPa and 155 C As it can be seen in the micrographs of the PP/GRT composite foams that existence of many smaller cells around larger cells. The diameter of the smaller cells was calculated from the cross-sectional area of the cells under the assumption that a cell took a spherical shape. Figure 7 represents how the cell diameter and density of the cells in the blend foam were changed by Cellular Polymers, Vol. 30, No. 3,

12 Zhen-Xiu Zhang, Lin Li, Zhen Xiang Xin, and Jin Kuk Kim Figure 7. Effect of the GRT content on the average cell size and cell density of small cells the concentration of GRT. As illustrated in Figure 4 and 7, the size of cells in the foamed PP/GRT (80/20) composite became bigger than that of pure PP foamed under the same conditions. Blending GRT with PP created numerous large cells at the boundary between the matrix polymer and the dispersed GRT powder, and at the same time, it enlarges the size of the cells nucleating at the PP matrix. This is because the foaming behavior of semicrystalline polymers is affected significantly by their crystallinity [16-17]. The aforementioned DSC results demonstrated that the total crystallinity of the PP/GRT composites decreases with increasing of content of GRT. Moreover, GRT powder plays the role of a nucleating agent. Therefore the PP/GRT (80/20) composite shows the big cell size, however further increase in GRT content, leads to a smaller cell size in the foamed PP/GRT, this may be due to that the higher viscosity of blends. The overall cell density is much lower than that of pure PP; this could be due to the heterogeneous nucleation induced in the presence of GRT. In heterogeneous nucleation, the number of nucleated cells depends strongly on the number of nucleating site (or the distribution of nucleating agent) [18]. The filler distribution and size, together with mixed nucleating regimes (heterogeneous and homogeneous) are deemed to affect the final cell distribution and size. A formation mechanism of the cellular structure observed in the PP/GRT composite foaming is summarized in Figure 8. Before the foaming, the 122 Cellular Polymers, Vol. 30, No. 3, 2011

13 Fabrication of Fine-celled PP/Ground Tire Rubber Powder Composites Using Supercritical Carbon Dioxide (a) (b) (c) (d) Figure 8. Schematic diagram of the formation mechanism of the bimodal cellular structure: (a) initial state, (b) bubble nucleation and growth, (c) bubble explode and (d) Formation of the bi-modal structure island (GRT powder)-sea (PP) morphology was established because of the immiscibility of both the polymers. The dispersibility of GRT in the PP matrix was changed by the weight fraction and could be controlled by blending. When the PP/GRT composite polymer was foamed, because of the poor adhesion of PP and GRT, bubble nucleation and growth occurred at the interface of PP and GRT at the initial stage of foaming. Because of the higher diffusivity of CO 2 in PP/GRT composite, the gas accumulates at the boundary of PP and GRT, and bubbles become larger than that nucleated in PP matrix. At last, the bubble to explode during depressurization. This phenomenon created a bimodal cell structure, in which the larger cell embraced a GRT. Analyzing the graphs in Figure 9, it is clearly seen that the relative density decrease initially and then increased with increasing GRT weight fraction at all processing conditions. Addition of 20 wt% GRT powder promote heterogeneous nucleation, however the higher initial CO 2 concentration may lead to bubble coalescence at the weak surface between PP and GRT during bubble growth and eventually makes bubbles explode causing lower relative Cellular Polymers, Vol. 30, No. 3,

14 Zhen-Xiu Zhang, Lin Li, Zhen Xiang Xin, and Jin Kuk Kim Figure 9. Effect of GRT weight fraction on the relative density of foamed PP/GRT blends at different processing conditions (a) 12 MPa, (b) 16 MPa, (c) 20 MPa 124 Cellular Polymers, Vol. 30, No. 3, 2011

15 Fabrication of Fine-celled PP/Ground Tire Rubber Powder Composites Using Supercritical Carbon Dioxide density. Further increase of GRT content will increase the melt viscosity and diffused rate of CO 2 from PP matrix, it also causes more resistance to cell nucleation and growth. In addition, since GRT powder cannot be foamed, the more the amount of GRT in the sample is, the larger the relative density. The effect of GRT on the elastic modulus and tensile strength of foamed PP/ GRT blends at different foaming conditions can be observed from Figures 10 and 11. The mechanical properties such as elastic modulus and tensile strength decrease, as the GRT weight fraction in the PP/GRT blends increased except for the 20 wt% GRT blend. These results were expected since the PP act as a plastic segment which contributes most of the tensile mechanical strength of the PP/GRT blends. However, for 20 wt% GRT blend, the results are not favorable. A proposed reason is that the cell nucleation and growth may occur at the interface region of the GRT and PP. Consequently, it could detach the GRT from the PP matrix causing deterioration of PP and GRT adhesion, the bubble explode and caused some larger bubbles than high weight fraction of GRT blend. PP-g-MA/GRT Composite Foam Thermoplastic vulcanizate (TPV) is a special class of thermoplastic elastomers (TPEs) made of a rubber/plastic polymer mixture in which the rubber phase is highly vulcanized. One of major criteria for a thermoplastic vulcanizate is that elongation at break is more than 100%. For PP/GRT (50/50) composite, tensile strength is 11.5 MPa and elongation is only 39.4%. In order to obtain TPV based on GRT, PP is replaced by PP-g-MA. The reactivity of MA group in PP-g-MA and phenolic OH group in GRT can enhance the compatibility between PP and GRT. 23 As shown in Figure 12, it can be observed that tensile strength is 11.9 MPa and elongation is 210.4% for PP-g-MA/GRT (50/50) composite, namely we have successfully prepared TPV based on GRT. The cell morphologies of microcellular PP/GRT (50/50) and PP-g-MA/GRT (50/50) composite are presented in Figure 13, the foaming temperature and saturation temperature were fixed at 155 C and 20MPa respectively. As shown in Figure 13, significant improvement on the cell morphology can be seen in the PP-g-MA/GRT composite. As the aforementioned, the foaming PP/GRT (50/50) composite, promotes further detachment of GRT from the matrix, which resulted from the fact that there was almost no interfacial adhesive between GRT powder and PP matrix, and the majority of GRT powder were pulled out and the big bubbles remained. However, PP-g-MA made the morphology undergo a considerable change in the interfacial behavior. The cell size becomes smaller and uniform, it clearly shows that the connection Cellular Polymers, Vol. 30, No. 3,

16 Zhen-Xiu Zhang, Lin Li, Zhen Xiang Xin, and Jin Kuk Kim Figure 10. Effect of GRT weight fraction on the tensile strength of foamed PP/GRT blends at different processing conditions (a) 12 MPa, (b) 16 MPa, (c) 20 MPa 126 Cellular Polymers, Vol. 30, No. 3, 2011

17 Fabrication of Fine-celled PP/Ground Tire Rubber Powder Composites Using Supercritical Carbon Dioxide Figure 11. Effect of GRT weight fraction on the tensile modulus of foamed PP/GRT blends at different processing conditions (a) 12 MPa, (b) 16 MPa, (c) 20 MPa Cellular Polymers, Vol. 30, No. 3,

18 Zhen-Xiu Zhang, Lin Li, Zhen Xiang Xin, and Jin Kuk Kim Figure 12. Comparison of mechanical properties between PP/GRT (50/50) composite and PP-g-MA/GRT (50/50) composite (a) (b) Figure 13. Microcellular structure of PP/GRT composite foams at 20 MPa, 155 C: (a) PP/GRT=50/50, (b) PP-g-MA/GRT=50/50 of GRT and PP matrix, because chemical interactions between the phenolic hydroxyl groups in GRT and maleic anhydride groups in PP-g-MA in which an interphase between the dispersed phase (GRT) and the polymer matrix (PP) was formed, the changes should be attributed to the compatibilization imparted by PP-g-MA/GRT binary blend. From Figures 14-15, it can be observed that the relative density and cell size of PP/GRT composites are higher than, whereas the average cell density is lower than the PP-g-MA/GRT composite. It is known that the mechanism of 128 Cellular Polymers, Vol. 30, No. 3, 2011

19 Fabrication of Fine-celled PP/Ground Tire Rubber Powder Composites Using Supercritical Carbon Dioxide cell growth is governed by the stiffness of the gas/polymer matrix, the rate of gas diffusion, and the amount of gas loss, the poor surface adhesion of PP-GRT provides a channel through which CO 2 can quickly escape from the composites [25]. However, the presence of PP-g-MA/GRT decreases the possibility so as to facilitate cell nucleation and growth. So the PP-g-MA/ GRT composite has lower relative density, smaller average cell size, higher cell density and uniform cell structure. In order to characterize the difference of foamed PP/GRT and PP-g-MA/GRT composites, the tensile properties were also measured. Figure 16 presents the comparison of tensile strength of the PP/GRT and PP-g-MA/GRT microcellular composites. It can be seen that foamed PP-g-MA/GRT composite has lower tensile strength; this is caused by the lower relative density PP-g-MA/GRT composite as aforementioned. The tensile moduli of the foamed composites were similar to the tensile strength trends, which are presented in Figure 17. The elongation at break for PP/GRT and PP-g-MA/GRT (50/50) microcellular composites are summarized in Figure 18. It is can be seen that the foamed PP-g-MA/GRT composite with higher elongation, it was increased almost 80% compared with PP/GRT composite. Because in the presence of PP-g- MA which leads to the formation of chemical interactions between the carbon black of GRT and maleic anhydride, thereby leading to formation of interphase between the GRT and the PP matrix. Figure 14. Comparison of the average cell size and cell density of foamed PP/GRT and PP-g-MA/GRT composites Cellular Polymers, Vol. 30, No. 3,

20 Zhen-Xiu Zhang, Lin Li, Zhen Xiang Xin, and Jin Kuk Kim Figure 15. Comparison of the relative density of foamed PP/GRT and PP-g-MA/GRT composites at different processing conditions; (a) 12 MPa, (b) 16 MPa, (c) 20 MPa 130 Cellular Polymers, Vol. 30, No. 3, 2011

21 Fabrication of Fine-celled PP/Ground Tire Rubber Powder Composites Using Supercritical Carbon Dioxide Figure 16. Comparison of The tensile strength of foamed PP/GRT with PP-g-MA/GRT composites at different processing conditions; (a) 12 MPa, (b) 16 MPa, (c) 20 MPa Cellular Polymers, Vol. 30, No. 3,

22 Zhen-Xiu Zhang, Lin Li, Zhen Xiang Xin, and Jin Kuk Kim Figure 17. Comparison of the tensile modulus of foamed PP/GRT with PP-g-MA/GRT composites at different processing conditions; (a) 12 MPa, (b) 16 MPa, (c) 20 MPa 132 Cellular Polymers, Vol. 30, No. 3, 2011

23 Fabrication of Fine-celled PP/Ground Tire Rubber Powder Composites Using Supercritical Carbon Dioxide Figure 18. Comparison of the Elongation at break of foamed PP/GRT with PP-g-MA/ GRT composites at different processing conditions; (a) 12 MPa, (b) 20 MPa Conclusions The microcellular foams of PP/GRT composites were produced by pressure quench method using supercritical carbon dioxide as blowing agent, the effects of blend composition on the crystallinity, sorption behavior of CO 2, and corresponding cellular morphology and mechanical properties of foamed PP/ Cellular Polymers, Vol. 30, No. 3,

24 Zhen-Xiu Zhang, Lin Li, Zhen Xiang Xin, and Jin Kuk Kim GRT composites were investigated. The experimental results showed that with the addition of scrap rubber powder, the crystallinity of PP/GRT composites was decreased, whereas the solubility and the diffusivity of CO 2 increased in the PP/ GRT composites. The PP/GRT composite showed a unique cellular structure in which large cells embraced a GRT powder and small cells existed around large cells. The mechanism of creating such cell structure could be explained by the morphology of the blends, the solubility and diffusivity of CO 2 in PP/ GRT composites, this might lead to the development of an efficient nucleating agent of polymer foaming. The mechanical properties of the microcellular PP/GRT composite foams are directly related to the processing conditions and composition. Under the same processing conditions, the mechanical properties of foamed PP/GRT composites vary with the blend composition. The PP/GRT (80/20) showed the lowest relative density and mechanical properties. The PP-g-MA/GRT (50/50) produced microcellular foams with a very fine and uniform cell structure, and enhanced the mechanical properties. Acknowledgement We are grateful for financial support from the Shangdong Natural Science Foundation (Nos. ZR2010EM044) and Doctoral Found of QUST. References 1. H.E. Naguib, C.B. Park, U. Panzer, and N. Reichelt, Strategies for Achieving Ultra Low-density Polypropylene Foams, Polym. Eng. Sci., 42(7), (2002). 2. N.C. Nayak and D.K. Tirpathy, Effect of aluminum silicate filler on morphology and physical properties of closed cell microcellular ethylene octene copolymer, J. Mat. Sci., 37, (2002). 3. J.Z. Liang and R.K.Y. Li, Rubber Toughening in Polypropylene: A Review, J. Appl. Polym. Sci., 77(2), (2000). 4. K. Taki, K. Nitta, S. Kirhara, and M. Ohshima, CO 2 foaming of Poly(ethylene glycol)/polystyrene Blends: Relationship of the Blend Morphology, CO 2 Mass Transfer, and Cellular Structure, J. App. Polym. Sci., 97, (2005). 5. X. Han, J. Shen, H. Huang, D.l. Tomasko, and L.J. Lee, CO 2 foaming based on polystyrene/poly(methyl methacrylate) blend and nanoclay, Polym. Eng. Sci., 47, 103 (2007). 6. S. Doroudiani, C.B. Park, and M.T. Kortschot, Processing and characterization of microcellular foamed high-density polyethylene/isotactic polypropylene blends, Polym. Eng. Sci., 38, , (1998). 134 Cellular Polymers, Vol. 30, No. 3, 2011

25 Fabrication of Fine-celled PP/Ground Tire Rubber Powder Composites Using Supercritical Carbon Dioxide 7. S. Siripurapu, Y.J. Gay, J.R. Royer, J.M. Desimone, R.J. Spontak, and S.A. Khan, Generation of Microcellular Foams of PVDF and Its Blends Using Supercritical Carbon Dioxide in a Continuous Process, Polymer, 43, (2002). 8. V. Kumar, M. VanderWel, J. Weller, and K.A. Seeler, Experimental Characterization of the Tensile Behavior of Microcellular Polycarbonate Foams, Journal of Engineering Material Technology, 116(4), (1994). 9. L.M. Matuana, C.B. Park, and J.J. Balatinecz, Structures and Mechanical Properties of Microcellular Foamed Polyvinyl Chloride, Cellular Polymers, 17(1), 1 16, (1998). 10. H. Sun, G.S. Sur, and E.J. Mark, Microcellular Foams from Polyethersulfone and Polyphenylsulfone: Preparation and Mechanical Properties, European Polymer Journal, 38(12), (2002). 11. H. Sun and J.E. Mark, Preparation, Characterization, and Mechanical Properties of Some Microcellular Polysulfone Foams, J. Appl. Polym. Sci., 86(7), (2002). 12. J. Fu and H.E. Naguib, Effect of nanoclay on the mechanical properties PMMA clay nanocomposite foams, Journal of Cellular Plastics, 42(4), (2006). 13. S. Wong, H.E. Naguib, and C.B. Park, Effect of Processing Parameters on the Cellular Morphology and Mechanical Properties of Thermoplastic Polyolefin (TPO) Microcellular Foams, Advances in Polymer Technology, 26(4), (2007). 14. S.K. Goel and E.J. Beckman, Generation of microcellular polymeric foams using supercritical carbon-dioxide.1. Effect of pressure and temperature on nucleation, Polym. Eng. Sci., 34(14), (1994). 15. A.R. Berens, G.S. Huvard, R.W. Korsmeyer, and F.W. Kunig, Application of Compressed Carbon Dioxide in the Incorporation of Additives into Polymers, J. Appl. Polym. Sci., 46, 231 (1992). 16. S. Doroudiani, C.B. Park, and M.T. Kortschot, Effect of the Crystallinity and Morphology on the Microcellular Foam Structure of Semicrystalline Polymers, Polym. Eng. Sci., 36(21), (1996). 17. D.F. Baldwin, C.B. Park, and N.P. Suh, Microcellular Processing Study of Poly (Ethylene Terephthalate) in the Amorphous and Semicrystalline States, Polym. Eng. Sci., 36(11), (1996). 18. S.H. Lee, B. Maridass, and J.K. Kim, Dynamic Reaction Inside Co-rotating Twin Screw Extruder. II. Waste Ground Rubber Tire Powder/Polypropylene Blends, J. Appl. Polym. Sci., 106(5), (2007). Cellular Polymers, Vol. 30, No. 3,

26 Zhen-Xiu Zhang, Lin Li, Zhen Xiang Xin, and Jin Kuk Kim 136 Cellular Polymers, Vol. 30, No. 3, 2011