Foamability of Thermoplastic Vulcanizates (TPVs) with Carbon Dioxide and Nitrogen

Transcription

1 Foamability of Thermoplastic Vulcanizates (TPVs) with Carbon Dioxide and Nitrogen S.G. Kim, C.B. Park *, B.S. Kang * and M. Sain Department of Chemical Engineering & Applied Chemistry, University of Toronto, Toronto, Ontario, M5S 3E5 * Department of Mechanical & Industrial Engineering, University of Toronto, Toronto, Ontario, M5S 3G8, Canada Received: 15 May 2005 Accepted: 27 January 2006 ABSTRACT The foamability of thermoplastic vulcanizate (TPV) has been investigated in a customized foaming system using carbon dioxide (CO 2 ) and nitrogen (N 2 ) as a physical blowing agent. TPV or dynamic vulcanizate is a special class of thermoplastic elastomer that is produced by technological blending of a rubber and a thermoplastic simultaneously. The rubbery part was dynamically cured in a thermoplastic matrix. The infl uence of blowing agent content and the processing conditions on the expansion behaviour, the cell-number density and the foam structure are discussed. The TPV foam with N 2 produced a uniform and fi ne cell structure with a smooth surface, indicating that N 2 could be a very good physical blowing agent for TPV material. INTRODUCTION Thermoplastic elastomers (TPE) exhibit the properties of conventional thermoset rubber yet can be processed in thermoplastic processing equipment. The great majority of TPEs have hetero-phase morphology, whether the TPE is derived from block copolymers, rubber-plastic compositions or ionomers. In general, the hard domains undergo molecular relaxation at elevated temperatures, thus allowing the material to flow. When cooled, the hard domains (i.e. the thermoplastic regions) again solidify and provide tensile strength at normal service temperatures (1,2). The soft domains give the material the elastomeric characteristics. The market of TPEs based on polyolefin rubber-plastic compositions has grown along two distinctly different product lines: one class consists of a simple blend and classically meets the definition Rapra Technology Limited

2 S.G. Kim, C.B. Park, B.S. Kang and M. Sain of a thermoplastic polyolefin (TPO), and in the other class the rubber phase is dynamically vulcanized giving rise to thermoplastic vulcanizate (TPV). Both the simple blend and the dynamically vulcanized TPE have found wide industrial applications (3). Thermoplastic vulcanizate (TPV) is prepared by melt mixing a thermoplastic with an elastomer in the presence of a small quantity of vulcanizing system which leads to the in-situ crosslinking of the rubber phase. TPV was first introduced by Gessler et al. (4) and since then has attracted great industrial attention (5). The vast majority of commercial TPV applications are made from EPDM in polypropylene, although other variants are emerging. TPVs are gaining popularity in a number of market segments including automotive, construction, appliances, medical and electronics. Automotive weatherstrip is a high potential market for TPV. Among various types of TPVs, those based on polyolefin thermoplastics such as PP and EPDM rubbers have gained great attention for various industrial applications especially in the automotive industries. This is due to the increased compatibility between these two polymer groups (6). Some examples of commercial TPV materials include those sold under the tradenames Santoprene by Advanced Elastomer System LP (AES), Sarlink by DSM, Uniprene by Teknor-Apex, Excelink by JSR, Forprene by Polyone, Nextrene by Thermoplastic Rubber Systems (TRS), Milastomer by Mitsui and Multiorene by Multibase, and so on (7). Static vulcanization, used commercially since the days of Charles Goodyear (8), involves the heating of a rubber stock (fully compounded and mixed with a cure system, usually sulfur) at a temperature of 130 to 180 C for a specified time, during which chemical crosslinks are formed between the macromolecules of the elastomer (e.g., natural rubber, butyl, EPDM etc.) (9,10). This process transforms the rubber into a tough, elastic, durable thermoset material (11). Dynamic vulcanization, on the other hand, embraces the curing of a rubber composition during its mixing or mastication, and one of the ingredients of this rubber composition must be a thermoplastic resin. This process results in a most TPV material with the properties of a conventional thermoset rubber, but which processes as a conventional thermoplastic. Dynamic vulcanization generates the same necessary crosslinks or three-dimensional polymer structures as static vulcanization. In dynamic vulcanization, however, these structures are generated in small rubber particles dispersed in the uncrosslinked thermoplastic polymer matrix as microgel. If the elastomer particles of such a blend are small enough and if they are sufficiently vulcanized, then the physical and chemical properties of the blend are generally improved (1,2). Coran, Das and Patel (12) demonstrated the effect of the size of the rubber particles and the degree of cure on the material properties. The foaming process starts when the high temperature and high pressure polymer/gas mixture is exposed to the atmospheric pressure upon exiting 20

3 the die. It consists of three fundamental steps: bubble initiation (nucleation), bubble growth and stabilization. Nucleation, or the formation of expandable bubbles, can begin within a polymer melt that has been supersaturated with the blowing agent. Once nucleated, a bubble continues to grow as the blowing agent diffuses into it. This growth will continue until the bubble stabilizes or ruptures (13-15). With careful tailoring of the processing conditions, microcellular foamed plastics with a cell density higher than 10 9 cells/cm 3 and a controlled volume expansion in the range of 1.5 to 43 times for the high density and low density applications are obtained (13-15). Foaming of TPV is no different than foaming of any other polymer. The first extensive investigation of foaming in TPV was carried out in 1992 by Dutta and Cakmak (16). They studied foaming of an olefinic thermoplastic elastomer; consisting of polypropylene (PP)/ethylenepropylene-diene-terpolymer (EPDM), commercially known as Santoprene thermoplastic elastomer, using a chemical blowing agent that releases N 2 upon decomposition. However, the foam densities they were able to achieve were limited to 0.5 g/cm 3. The foam structure was non-homogeneous and a wide cell size distribution was obtained. Later efforts have been taken by one of the major TPV suppliers, DSM of Netherlands, to develop a foaming technology which involves a water releasing chemical compound (WCC) as a chemical blowing agent (CBA) (17,18). DSM also introduces a foaming grade TPV, Sarlink. DSM claims that their foaming technology can be used to produce TPV foams with a density from 0.15 to 0.90 g/cm 3. The advantage of a CBA is that it doesn t require a special foaming extrusion system but CBA are usually expensive so an additional 10% to 15% cost will be added up to the final TPV foam product. In addition, due to unexpected reactions during decomposition of CBA and foaming, CBA-blown TPV foam usually has an open-celled structure which leads to more water absorption that is not favorable for the sealing purpose. Another commercialized TPV foaming technology is based on water as a physical blowing agent (PBA), which has been independently developed by Sahnoune from AES (19,20). Water is an attractive PBA from the standpoint of cost and environmental requirements. It also does not require special handling, is readily available, and the pressures involved in the foaming process are not as demanding as for other blowing agents. Sahnoune et al. (19,20) studied the effect of water concentration on foam density and cell nucleation under the various processing conditions for a TPV. His study found that, despite of their complex microscopic structure, TPV foam very much like conventional thermoplastics do. The water foaming method can achieve from density as low as 0.15 g/cm 3 but the cell size is around 200 to 300 µm. Spatiael et al. (21) studied the foaming behaviors of several TPV formulations containing various amounts of branched polypropylene resin with water as the blowing agent, while the extensional viscosity of the materials with different formulations was measured and considered. They indicated that the replacement of a small amount of linear 21

4 S.G. Kim, C.B. Park, B.S. Kang and M. Sain polypropylene with branched polypropylene improve the foam density and cellular structure. However, as the added content of branched polypropylene was increased, a worse foamability was observed. They concluded that there exists an optimal amount of branched content. More recently, Kropp et al. (22) carried out a comparative study of foaming three types of TPEs with carbon dioxide (CO 2 ) and using hydrocerol as a nucleating agent. These materials were a thermoplastic polyurethane (TPU), a styrene based TPE (SEBS) and again a PP/EPDM TPV. They found that foaming was most difficult with the PP/EPDM TPV. The foam density they could reach was 0.76 g/cm 3 and the foam structure was also not uniform. They found that a specific TPU-type showed the best foamability; a SEBS-type was also successfully foamed; and a PP/EPDM TPV especially developed for water foaming was most difficult to foam with CO 2 as the blowing agent. The extrusion of low-density TPV foams seems to be a very difficult process according to the previous. The challenge is partially due to the fact the only ingredient in the TPV composition that can be foamed is the PP matrix. For lower hardness grades, PP comprises only a small amount of the total composition. Therefore, in order to make TPV foam with a specific gravity of 0.3 g/cm 3, it may be necessary to foam the PP phase to a specific gravity of 0.05 g/cm 3. Additional complications are related to the rheological properties of TPVs. High loading of vulcanized rubber particles in the polymer melt may affect melt integrity and stability of the foaming process. The type of blowing agent used in plastic foam processing is critically important in determining the cell morphology of the produced foams. A long-chain physical blowing agent is typically used for low-density foam processing because of its low diffusivity and high solubility (23). However, because of the limitations of the long-chain blowing agents, significant efforts have been made to replace these blowing agents with environmentally safe and non-flammable blowing agents in low-density foam processing. Especially, inert gases such as CO 2 have been considered as a replacement blowing agent. Since the inert gas blowing agents have higher volatility because of their smaller molecular size, and therefore higher diffusivity than the long-chain blowing agents such as CFCs, HCFCs, pentane and butane, gaseous blowing agents can escape easily during expansion (23). Therefore, it is very difficult to obtain low density foam with a large expansion ratio with an inert gas. On the other hand, the long-chain blowing agents have low diffusivity because of their low volatility. This low diffusivity offers a tremendous advantage in controlling cell growth to achieve a very high expansion ratio since it is easier to control cell growth because of the slow growth rate and because gas escape is prevented. 22

5 In this study, we attempted to understand the foaming behaviors of TPV in general, with CO 2. The objective was to investigate the influence of the blowing agent concentration on the volume expansion ratio, the cell nuclei density and the foam structure under different processing conditions. EXPERIMENTAL Materials The base material used for creating the foamed TPV was a commercial TPV grade (Santoprene W228) with a shore A hardness of 68 that is specifically manufactured for foaming applications by Advanced Elastomers Systems, L.P. Its density is 0.97 g/cm 3 according to ASTM D92. Nitrogen (BOC Gas, 99.9% purity) and carbon dioxide gases (BOC Gas, 99.5% purity) were used in high-pressure experiments. Experimental Setup Figure 1 shows a schematic of the extrusion foaming system used in the experiment. A single screw extruder (Brabender, ) with a mixing screw (Brabender, ) of 30:1 L/D ratio and 3/4 diameter was used. The extruder had three zones of temperature control. A positive displacement pump was used to inject the physical blowing agent, and a diffusion enhancing device containing static mixer (omega, FMX S) was installed after the extruder to improve the dissolution of the physical blowing agent in the polymer melt. The other systems included a gear pump (Zenith, PEP-II 1.2 cc/rev) where the volumetric displacement was properly controlled by the motor, a heat exchanger for cooling the polymer melt that contains homogenizing static mixers (Labcore Model H ), and a cooling sleeve for the precise control of die temperature. A filamentary die with L/D /0.025 was used in this foaming experiment. Experimental Procedure The experimental procedure was as follows. The TPV pellets were first fed into the barrel through the hopper and were completely melted by the screw rotation and shear after zone 2 and before the gas injection port. Then a metered amount of physical blowing agent was injected into the extrusion barrel by the positive displacement pump and mixed intensively with the polymer melt stream. When the gas was injected into the extrusion barrel, the screw generated shear fields to completely dissolve gas in the polymer melt via mixing and 23

6 S.G. Kim, C.B. Park, B.S. Kang and M. Sain Figure 1. Schematic of the foaming extrusion system diffusion. Moreover, the static mixer was used to enhance the dissolution of the gas in the polymer melt. Ideally, a single-phase polymer/gas solution was generated, which went through the gear pump and fed into the heat exchanger where it was cooled to a designated temperature. The cooled polymer/gas solution entered the die and foaming occurred at the die exit through a process of thermodynamic instability induced via a rapid pressure drop. The melt and die temperatures were synchronized for simplicity in this experiment. While optimizing all the parameters, the melt and die temperatures were lowered gradually and samples were randomly collected at each designated temperature when no further change was observed in the pressure. The temperature profile was set to zone 1: 155 C, zone 2: 170 C, zone 3: 170 C, mixer: 170 C and gear pump: 170 C. The temperature of the heat exchanger and the die was lowered down from 170 C. A flow rate of the experiment was maintained at 10 g/min. Two kinds of physical blowing agents, i.e., N 2 (0.2, 0.5 and 1 wt%) and CO 2 (1 and 2 wt%) were used in the experiments. The results were compared and efforts were made to investigate which kind of blowing agent would give the best foamed structures for the TPV materials. 24

7 Characterization of Foams The foam samples were characterized by the volume expansion ratio and the cell-population density. The expansion ratio of the TPV foam was determined by measuring the weight and volume expansion of the sample. The volume of foam sample was determined by the water displacement method (ASTM D792). The volume expansion ratio (Φ) was calculated on the basis of the ratio of bulk density of pure TPV material (ρ p ) to the bulk density of foam sample (ρ f ) as follows (24) : The cell-population density was calculated from the microstructure obtained from the scanning electron microscope (SEM). The foam samples were fractured in liquid nitrogen and the fractured surface was coated with gold before SEM. The cell density (n) is defined as the number of cells per unit volume with respect to the unfoamed polymer. A certain area is chosen in the SEM photograph. First, the number of cells (n b ) in a defined area (L L) was determined and then the total number of cells per cubic centimeter was calculated as follows (24) : (1) where L is the side length in mm and Φ is the volume expansion ratio. It may be noted that we used the cell-population density defined with respect to the unfoamed polymer and the volume expansion ratio in this paper to better describe the processing-to-structure relationships since these parameters indicate how well cell nucleation and expansion were controlled during foam processing (24,25). (2) RESULTS AND DISCUSSION Effect of Die Pressure The foaming experiment was conducted with various concentrations of CO 2. The pressure profiles were shown at various temperatures in Figure 2. It noted that the die pressure decreased as the amount of injected CO 2 or N 2 was increased. The reduction in the die pressure was due to the plasticizing effect of the dissolved blowing agent in the polymer matrix (24). 25

8 S.G. Kim, C.B. Park, B.S. Kang and M. Sain Figure 2. Die pressure profiles of TPV sample for CO 2 The dissolution of the blowing agent in the polymer melt during extrusion strongly depends on the solubility of blowing agent. The viscosity of polymer melt drops as the free volume is increased. This plasticizing effect increases as the amount of PBA increases. We also observed that the die pressure with CO 2 was higher than with N 2 when the same amount of gas was injected ( for 1 wt% CO 2, Δ for 1 wt% N 2 ) as shown in Figure 2. This was due to the fact that N 2 has a large number of molecules than CO 2 for the same weight fraction. But as the N 2 content increased, the viscosity did not drop as much as in the case of CO 2. A possible explanation could be incomplete solubility of nitrogen in the polymer melt. Process Conditions and Volume Expansion The effects of the processing conditions and the amounts of blowing agent on the volume expansion ratio of extruded TPV material were investigated. The amount of blowing agent used was 0.2, 0.5 and 1 wt% for N 2 and 1 and 2 wt% for CO 2. Figure 3 depicts the volume expansion versus the die temperatures for TPV sample. One of the most critical factors affecting the foaming behavior of TPV is the amount of blowing agent injected. This shows that the largest expansion ratio achieved was a function of the amount of the blowing agent injected. The largest volume expansion ratio achieved for 0.2, 0.5 and 1 wt% N 2 was 2.0, 2.1 and 2.4. Their foam density was 0.49, 0.46 and 0.40 g/cm 3 26

9 Figure 3. Volume expansion ratio of TPV sample for CO 2 respectively. However, the largest volume expansion ratio achieved for 1 and 2 wt% CO 2 was 1.8 and 4.5. Their foam density was 0.54 and 0.21 g/cm 3, respectively. Figure 3 also shows that TPV has a slightly higher volume expansion ratio using N 2 compared to CO 2 in the entire range of processing temperature in the case of same wt% of gas content ( for 1 wt% CO 2, Δ for 1 wt% N 2 ). However, for the same number of molecules, CO 2 would produce a higher expansion ratio. For 2 wt% CO 2, there was an optimum temperature that produced the maximum expansion ratio. Above this temperature, the volume expansion ratio was governed by gas loss. Because of the high diffusivity of gas, the gas quickly escaped out to environment at elevated temperature. As the temperature decreased, the amount of gas lost decreased because of the decreased diffusivity, and as a consequence, a greater amount of retained gas in the foam resulted in an increased expansion ratio (24,25). By contrast, below the optimum temperature, the volume expansion ratio decreased as the temperature decreased, showing that the retained gas was not fully utilized to expand the foam because of the increased stiffness by crystallization of PP component in TPV, although the amount of gas lost could have been further decreased. So there was an optimum 27

10 S.G. Kim, C.B. Park, B.S. Kang and M. Sain temperature where the retained gas was fully utilized to maximize the expansion ratio of the extruded TPV foams. It is believed that the injected CO 2 dissolved in the polymer matrix completely and thereby, this typical mountain shape was obtained for the expansion ratio curves versus the temperatures. For N 2, there was little change of volume expansion as the temperature varied. It is thought that the injected N 2 did not dissolve in the PP matrix completely and the undissolved gas pockets caused a decrease in the expansion ratio (26). In fact, an initial hump was observed at the die exit for all temperatures when N 2 was processed. This supported the fact that the injected N 2 did not dissolve in the polymer matrix. At 1 wt% N 2, the pressure was too high and experiments could not be conducted at lower temperatures to observe the mountain shape. Foam Structure and Cell Density Typical cell structures of the TPV foams using CO 2 as a blowing agent are shown in Figure 4(a) and 4(b). The cells were relatively large, in the range of 80 to 140 microns for CO 2. However, the cells were much smaller, in the range of 20 to 80 microns for N 2. More importantly, the cell structures with both blowing agents were almost closed. Also, we observed that the cell size was smaller as the blowing agent content increased. The resulting foamed-tpv profiles with N 2 had uniform and fine cell structures with a smooth surface, indicating that N 2 could be a very good blowing agent for TPV materials. Because of the smaller expansion ratio due to the slow dissolution, some strategies need to be developed to overcome this weakness. Figure 5 depicts the cell density versus the melt temperature for TPV material. As CO 2 content was increased, the cell density was increased. This result was expected because the higher the concentration of blowing agent in the polymer matrix, the greater the driving forces to nucleate bubbles. When the polymer melt had more dissolved blowing agent, a greater thermodynamics instability was induces as the polymer melt exited from the nucleation nozzle because of the solubility drop. As a consequence, a higher cell population density was developed in the extruded foams (24,25). Figure 5 shows that the cell density increased as the blowing agent increased, as expected. However, it was observed that there was no significant change in cell density as the temperature varied. For the 1 wt% of CO 2, TPV foam with N 2 showed much higher cell density than with CO 2 in the entire range of processing temperature. This TPV material was very promising for N 2 foaming even with a very low concentration (0.2 wt%). But for CO 2, the results were not promising. The cell density was about ~10 6 cells/cm 3 for CO 2 (1 wt%) and 28

11 (a) 1 wt%, 150 C (b) 2 wt%, 150 C 100 µm (c) 1 wt%, 160 C (d) 2 wt%, 160 C (a) CO 2 Figure 4a. Cell structure of TPV sample foamed with (a) CO 2 ~10 7 cells/cm 3 for N 2 (1 wt%). In conclusion, good foam or uniform structure cannot be produced with CO 2. However, very fine TPV foam structure was observed with N 2 even though N 2 has lower solubility to PP component in TPV material than CO 2. These promising results need to be analyzed to be able to use this technology in industry. 29

12 S.G. Kim, C.B. Park, B.S. Kang and M. Sain (a) 0.2 wt%, 150 C (b) 0.5 wt%, 150 C (c) 1 wt%, 150 C 100 µm (d) 0.2 wt%, 160 C (e) 0.5 wt%, 160 C (f) 1 wt%, 160 C (b) N 2 Figure 4b. Cell structure of TPV sample foamed with (b) N 2 30

13 Figure 5. Cell density of TPV sample for CO 2 CONCLUSIONS Experimental studies were carried out to manufacture TPV foams in extrusion using CO 2. The volume expansion ratio and cell density increased as the amount of blowing agent increased. TPV foaming with N 2 showed lower expansion behavior than that of CO 2 for the same number of molecules. However, the TPV foam with N 2 produced a uniform and fine cell structure with a smooth surface, indicating that N 2 could be a very good physical blowing agent for a fine cell TPV foam. ACKNOWLEDGEMENT The authors are grateful to AUTO21 and CCMCP (Consortium for Cellular and Micro-Cellular Plastics) for the financial support of this project. We also acknowledge the kind donation of Santoprene materials from Advanced Elastomer Systems. 31

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