[ACS Symposium Series] Polymeric Foams Volume 669 (Science and Technology) || Microcellular Foams

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<ul><li><p>Chapter 7 </p><p>Microcellular Foams </p><p>Vipin Kumar and John E . Weller </p><p>UW-Industry Cellular Composites Consortium, Department of Mechanical Engineering, Box 352600, University of Washington, Seattle, W A 98195 </p><p>Microcellular polymers refer to closed cell thermoplastic foams with an extremely large number of very small cells. Typically, one hundred million or more bubbles nucleate per cm3 of polymer, and each cell is of order 10 m diameter. These materials were first produced in the early 1980's with the objective of reducing the amount of polymer used in mass produced items, and have the potential to revolutionize the way thermoplastic polymers are used today. Microcellular plastics have been produced from a number of polymers and can be produced with densities ranging from 3 to 99 percent of the density of the solid material, offering engineers a new range of properties for design. Microcellular foams possess improved fatigue life and energy absorption characteristics as well as a high specific strength. It is expected that these materials will be utilized in a wide variety of applications in the next century. </p><p>Imagine solid polymers replaced by microcellular polymers with 20 to 40 percent reduction in material without compromising their function or required strength. Also look forward to composite microcellular materials with layers of solid and cellular materials designed to offer optimum mechanical and thermal properties and a minimal use of polymer. Created by processes without using any chemicals that are harmful to the environment, such materials may well become widely used by the turn of the century! </p><p>Microcellular polymers are closed cell plastic foams with a bubble densities in excess of 100 million bubbles per cm 3 and diameters of order 10 . The idea to introduce very small bubbles in plastics by gas nucleation was originally advanced by Professor Nam P. Suh ( i ) of the Massachusetts Institute of Technology around 1980 as a means to reduce the cost of many mass produced plastic items. The rationale was that if bubbles smaller than the critical flaws that already exist in plastics could be introduced in sufficient numbers, then the material density could be reduced while maintaining the essential mechanical properties. </p><p>The basic microcellular process is a two-step process shown in Figure 1. In the first step, the polymer sample is saturated by a pressurized, non-reacting gas in a pressure vessel. Over time, the polymer absorbs the gas, and eventually become saturated. When the polymer is saturated, it will not absorb any more gas, and the gas is distributed uniformly throughout the polymer. Typically, it takes several hours to several days for a thin polymer sheet to become saturated. Of course, the amount of time required to satu-</p><p> 1997 American Chemical Society 101 </p><p>Dow</p><p>nloa</p><p>ded </p><p>by N</p><p>ORT</p><p>H C</p><p>ARO</p><p>LIN</p><p>A S</p><p>TATE</p><p> UN</p><p>IV o</p><p>n A</p><p>ugus</p><p>t 20,</p><p> 201</p><p>2 | ht</p><p>tp://p</p><p>ubs.ac</p><p>s.org </p><p> Pu</p><p>blic</p><p>atio</p><p>n D</p><p>ate:</p><p> June</p><p> 1, 1</p><p>997 </p><p>| doi: 1</p><p>0.1021</p><p>/bk-19</p><p>97-066</p><p>9.ch00</p><p>7</p><p>In Polymeric Foams; Khemani, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997. </p></li><li><p>102 POLYMERIC FOAMS </p><p>Figure 1. Schematic of the microcellular process: (a) saturation of polymer with gas at room temperature; (b) bubble nucleation and growth at atmospheric pressure. </p><p>Dow</p><p>nloa</p><p>ded </p><p>by N</p><p>ORT</p><p>H C</p><p>ARO</p><p>LIN</p><p>A S</p><p>TATE</p><p> UN</p><p>IV o</p><p>n A</p><p>ugus</p><p>t 20,</p><p> 201</p><p>2 | ht</p><p>tp://p</p><p>ubs.ac</p><p>s.org </p><p> Pu</p><p>blic</p><p>atio</p><p>n D</p><p>ate:</p><p> June</p><p> 1, 1</p><p>997 </p><p>| doi: 1</p><p>0.1021</p><p>/bk-19</p><p>97-066</p><p>9.ch00</p><p>7</p><p>In Polymeric Foams; Khemani, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997. </p></li><li><p>7. K U M A R &amp; W E L L E R Microcellular Foams 103 </p><p>rate the polymer depends on the polymer itself, its diffusivity, its thickness, the saturation temperature, and sometimes the gas pressure. </p><p>This saturation step is usually carried out at room temperature since the solubility of most gases is higher at lower temperatures. Higher saturation temperatures may be employed however, to accelerate the diffusion of gas in the polymer. After the polymer has become saturated, it is removed from the pressure vessel and then foamed. In the second step of the process, the saturated sample is heated to the foaming temperature, which is close to the glass transition temperature of the virgin polymer. During this stage a large number of bubbles nucleate and grow. Bubble growth is controlled by holding the specimen at the foaming temperature for a certain length time. This time is commonly referred to as the foaming time. Since the growth rates of bubbles near the glass transition temperature are relatively low, it is possible to arrest the bubble growth in a controllable manner by quenching the foam in a low temperature bath. </p><p>Figure 2 shows scanning electron micrographs of a number of polymers foamed by this process. The microcellular structure for all of these foams is very homogenous, and the bubbles are typically between 5 to 25 in diameter. One exception is the PET foam, where the bubbles are an order of magnitude larger. The average bubble size in all microcellular foams is strongly influenced by the number of bubbles that nucleate, which in turn is a function of the concentration of gas in the sample after saturation. The density of the foams can be controlled by choosing an appropriate gas saturation pressure, and using a suitable foaming time and temperature. These parameters are specific to a given gas-polymer system, and for the most part must be determined experimentally. </p><p>Figure 3 shows scanning a scanning electron micrograph of a polystyrene sample showing the full specimen cross section. This sample was originally 0.40 mm thick and grew to a thickness of 0.65 mm after foaming. This example shows that thin sections of polymers normally made from solid polymer can indeed be foamed by the microcellular process (7 ). </p><p>Overview of Microcellular Polymers </p><p>The basic microcellular process outlined above has been successful with a number of amorphous polymers, and semicrystalline polymers with low levels of crystallinity. Some of these gas-polymers systems are discussed below. </p><p>Polyvinyl Chloride (PVC). Microcellular PVC foams have been produced by Kumar and Weller (2 ) using carbon dioxide as the nucleating gas. Bubble densities in the range 107-109 per cm 3 of polymer were achieved, and foams with relative density (density of the foam divided by the density of the polymer) in the range 0.15 to 0.94 have been produced. The sorption data for carbon dioxide in PVC (2 ) shows that for 2 mm thick specimens of a rigid PVC formulation saturated at 4.8 MPa and room temperature, the gas concentration at equilibrium is approximately 75 mg of carbon dioxide per gram of PVC, or about 7.5 percent by weight. Thus a very high concentration of gas molecules exists in the polymer matrix at saturation. As a result, there is significant plasticization of the polymer, and the glass transition temperature drops from 78.2 C for original PVC to an estimated 14 C upon saturation (2 ). Bubble nucleation and growth is therefore possible at very moderate temperatures, at or below the glass transition temperature of the original polymer. In the case of PVC, the lowest temperature at which microcellular foam was produced was 14 C </p><p>Figure 4 shows the range of relative densities for the family of microcellular PVC foams produced by heating the saturated PVC specimens to different temperatures. The lowest density foams (15 to 20 percent of the solid PVC density) are produced in the 100-115C temperature range. Note the nearly linear variation of foam relative density with foaming temperature in the range 56C to 105C, demonstrating that the foam density can be simply and accurately controlled by controlling the temperature to which the saturated plastic is heated. </p><p>Dow</p><p>nloa</p><p>ded </p><p>by N</p><p>ORT</p><p>H C</p><p>ARO</p><p>LIN</p><p>A S</p><p>TATE</p><p> UN</p><p>IV o</p><p>n A</p><p>ugus</p><p>t 20,</p><p> 201</p><p>2 | ht</p><p>tp://p</p><p>ubs.ac</p><p>s.org </p><p> Pu</p><p>blic</p><p>atio</p><p>n D</p><p>ate:</p><p> June</p><p> 1, 1</p><p>997 </p><p>| doi: 1</p><p>0.1021</p><p>/bk-19</p><p>97-066</p><p>9.ch00</p><p>7</p><p>In Polymeric Foams; Khemani, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997. </p></li><li><p>104 POLYMERIC FOAMS </p><p>Figure 2. Scanning electron micrographs showing examples of the structure created by the microcellular process: (a) PVC; (b) polycarbonate; (c) ABS; and (d) PET. </p><p>Figure 3. Overall view across a polystyrene sample originally 0.40 mm thick (7). The thickness after foaming was 0.65 mm. </p><p>Dow</p><p>nloa</p><p>ded </p><p>by N</p><p>ORT</p><p>H C</p><p>ARO</p><p>LIN</p><p>A S</p><p>TATE</p><p> UN</p><p>IV o</p><p>n A</p><p>ugus</p><p>t 20,</p><p> 201</p><p>2 | ht</p><p>tp://p</p><p>ubs.ac</p><p>s.org </p><p> Pu</p><p>blic</p><p>atio</p><p>n D</p><p>ate:</p><p> June</p><p> 1, 1</p><p>997 </p><p>| doi: 1</p><p>0.1021</p><p>/bk-19</p><p>97-066</p><p>9.ch00</p><p>7</p><p>In Polymeric Foams; Khemani, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997. </p></li><li><p>7. K U M A R &amp; W E L L E R Microcellular Foams 105 </p><p>Polycarbonate (PC). Creation of microcellular structure in polycarbonate was first reported in 1990 by Kumar, Weller and Hoffer (3 ). Carbon dioxide, due to its high solubility in polycarbonate, was used to achieve very high bubble nucleation densities in the range 1-10 109per cm 3 of polymer. Sorption data shows an equilibrium gas concentration of approximately nine percent by weight at a saturation pressure of 4.8 MPa. The number of bubbles nucleated was found to increase exponentially with increasing gas saturation pressure. A family of microcellular polycarbonate foams has been produced ranging in density from 10 to 99 percent of the density of solid polycarbonate. </p><p>Weller (24) has conducted a detailed characterization of the P C - C 0 2 system. The density of microcellular polycarbonate foams as a function of the foaming temperature for various saturation pressures is shown in Figure 5. Again, we see a linear relationship, suggesting that the foaming temperature is a critical parameter for controlling the density of the microcellular foams, and thus an effective means of in-process control of foam morphology. The data in Figure 5 is discussed further in the section on mechanical properties. </p><p>Polystyrene (PS). Microcellular polystyrene has been studied by a number of investigators (1,6,7,8,9,10,15 ). Martini et al. (1 ) produced microcellular polystyrene with cells in the 5-25 mm range and void fractions of 5 to 30 percent. Using higher nitrogen pressures for saturation, Kumar (10 ) extended the void fraction range to 80 percent. This was possible because the number of bubbles increased exponentially with saturation pressure, allowing for lower density foams to be produced while keeping the average bubble diameter in the microcellular range. </p><p>Polyethylene Terephthalate (PET). Kumar and Gebizlioglu (11 ), and Baldwin and Suh (12 ) have applied the microcellular process to foam PET using carbon dioxide. The results with PET have been most interesting. It has been found that with sufficiently long exposure to high pressure carbon dioxide, PET begins to crystallize. The resulting PET foams have a significantly higher crystalline content, and although mechanical property data is not yet available, these foams appear to be much stronger than microcellular foams from other polymers at comparable relative densities. </p><p>P E T G . The most recent addition to the family of polymers successfully processed by the microcellular process is PETG (22,23). Relative densities of three percent and higher have been achieved using carbon dioxide for bubble nucleation, see Figure 6. An interesting observation in C0 2 -PETG system is the relatively high foaming temperature to which the process appears to be stable. In Figure 6, the highest temperature at which foam was produced is 120 C, which is nearly 40 C above the of PETG. By comparison, in Polycarbonate-C02 system, the process becomes unstable at 160 C, about 10 C above the T g of polycarbonate. </p><p>Mechanical Properties of Microcellular Polymers </p><p>The available mechanical property data on microcellular foams is expected to increase as the number of researchers and our understanding of the processing characteristics of different gas-polymer systems grows. However, the data available so far is quite limited except for a few select systems. The results to date show that these novel materials offer many properties that were not previously available from either conventional foams or solid polymers. In many cases, the properties are improved over those of conventional foams, and in some cases a particular property may be better than that of the original, solid material. </p><p>The polycarbonate - C O ? system has received a great deal of attention since a wide range of processing conditions have been documented (24). Figure 5 shows the </p><p>Dow</p><p>nloa</p><p>ded </p><p>by N</p><p>ORT</p><p>H C</p><p>ARO</p><p>LIN</p><p>A S</p><p>TATE</p><p> UN</p><p>IV o</p><p>n A</p><p>ugus</p><p>t 20,</p><p> 201</p><p>2 | ht</p><p>tp://p</p><p>ubs.ac</p><p>s.org </p><p> Pu</p><p>blic</p><p>atio</p><p>n D</p><p>ate:</p><p> June</p><p> 1, 1</p><p>997 </p><p>| doi: 1</p><p>0.1021</p><p>/bk-19</p><p>97-066</p><p>9.ch00</p><p>7</p><p>In Polymeric Foams; Khemani, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997. </p></li><li><p>106 POLYMERIC FOAMS </p><p>50 60 70 80 90 100 110 120 130 </p><p>Foaming Temperature (C) </p><p>Figure 4. Plot of PVC foam relative density as a function of foaming temperature for specimens saturated with carbon dioxide at 4.8 MPa ( 2) </p><p>Dow</p><p>nloa</p><p>ded </p><p>by N</p><p>ORT</p><p>H C</p><p>ARO</p><p>LIN</p><p>A S</p><p>TATE</p><p> UN</p><p>IV o</p><p>n A</p><p>ugus</p><p>t 20,</p><p> 201</p><p>2 | ht</p><p>tp://p</p><p>ubs.ac</p><p>s.org </p><p> Pu</p><p>blic</p><p>atio</p><p>n D</p><p>ate:</p><p> June</p><p> 1, 1</p><p>997 </p><p>| doi: 1</p><p>0.1021</p><p>/bk-19</p><p>97-066</p><p>9.ch00</p><p>7</p><p>In Polymeric Foams; Khemani, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997. </p></li><li><p>7. K U M A R &amp; W E L L E R Microcellular Foams 107 </p><p>steady state process space for PC-CO system, where steady state refers to the state where all cell growth has completed. This figure illustrates the wide range of relative densities that can be produced in this system and the numerous processing conditions that can be employed. These different processing conditions can be used to produce foams that have the same relative density and different average cell size. For example, a sample could be saturated at 5 MPa and foamed at approximately 100 C or it could be saturated at 1 MPa and foamed at 140 C to produce a foam of the same density. The possibility of producing these foams is illustrated by the horizontal dashed line in Figure 5. For the two processing conditions discussed above, a relative density of approximately 0.5 will be produced. In addition, foams of different average cell size are produced as illustrated by the two indicated data points, which have the same relative density and cell sizes of approximately 5 and 22 . The ability to produce foams of the same density and different cell size has opened the possibility to explore the effect of cell size on the properties of microcellular materials. </p><p>The strength of microcellular foams is nearly proportional to the foam density. Figure 7 shows the relative tensile strength of several microcellular foams and some structural foams as a function of the relative density. For this plot, the relative tensile strength was computed by dividing the strength of the foam by the strength of the unprocessed material. The relative tensile strength of microcellular foams appears to be marginally b...</p></li></ul>

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