Studies on Porous and Morphological Structures of Expanded PTFE Membrane Through Biaxial Stretching Technique

ORIGINAL PAPER/PEER-REVIEWED Studies on Porous and Morphological Structures of Expanded PTFE Membrane Through Biaxial Stretching Technique By Xinmin Hao a,b, Jianchunn Zhang b, Yuhai Guo c,, Huapeng Zhang c Abstract A porous expanded polytetrafluoroethylene (eptfe) membrane was prepared from emulsion polymerized PTFE fine powders by a series of mechanical operations, which included extrusion, rolling, stretching and heat setting. Very small holes in the PTFE sheet were observed by SEM analyses after extrusion and rolling, and which were elongated and enlarged by longitudinal stretching. Fibrils between the slits are observed by SEM analyses. The second stretching operation, transverse stretching, provided a lattice-like porous structure. After heat setting, an island-like structure was formed, which is composed of billions of tiny inter-connected continuous fibrils and nodes. The porous structure was studied through SEM and the COULTER POROMETER tester. Results show that the mean pore size and porosity increase with an increase in longitudinal stretching ratio, transverse stretching ratio, and heat setting temperature. The mean pore size decreases and the porosity increases with an increase in transverse stretching rate. Remarkably, increasing transverse stretching rate increases porosity while the mean pore size decreases slightly, resulting in a membrane with a more uniform cell and denser cell structure. DSC, WAXD analysis and a. Donghua University, Shanghai, 200051, People s Republic of China) b. The Quartermaster Research Institute of the General Logistic Department of CPLA, Beijing, 100088, People s Republic of China) c. Shanghai New & Special Textile Material Research Center, Shanghai, 200082, People s Republic of China) mechanical testing show that mechanical processing and heat setting decrease both the crystallinity and crystallite size. Keywords Expanded PTFE membrane, biaxial stretching, porous structure, crystalline structure 1. Introduction Membrane technology has been significantly developed in the last three to four decades and has been extensively applied in human life, industrial fields and scientific research. Expanded polytetrafluorethylene (eptfe) membranes can be produced by a series of mechanical operations: extrusion, rolling and stretching [1,2,3]. The biaxially stretched eptfe membrane was produced by the first stretching (longitudinal stretching) operation parallel to the rolling direction and second stretching (transverse stretching) perpendicular to the first stretching. It is well known that PTFE has a high melting point, is chemically inert and strongly hydrophobic. The eptfe membrane has high strength and a smooth surface and each square centimeter contains billions of continuous superfine fibrils interconnected with each other. These properties make eptfe membranes ideally suited for a variety of applications and industrial processes, especially for harsh chemical environments and high temperatures. Furthermore, eptfe has low chemical extractability and has excellent biological compatibility. In China, during May and June, 2003, this eptfe laminated with fabric was worn by the medical staff in several hospitals without another single infection of medical personnel with the Severe Acute Respiratory Syndrome (SARS) virus[4,5]. 31 INJ Summer 2005

Researchers have developed biaxial stretching technology to produce expanded PTFE membranes [6,7,8,9,10,11,12]; however, few documents have been reported extensively on the influence of porous structure formation through biaxial stretching operations and the morphological structure changes during the processing. In this paper, the influence of the biaxial stretching process on the structure and morphology of expanded eptfe membrane is investigated. 2 Experimental 2.1 Sample Preparation The starting PTFE powder was produced by emulsion polymerization (CD123. Asahi Fluoropolymers Co.) with a number average molecular weight of 5 X 10 6 was mixed with naphtha as a lubricant (20%) and then extruded into a rod of 13mm in diameter at 180 C by use of an extruder. Thereafter, this rod was rolled between two metal rollers to form a sheet with a thickness of 120µm. The rolled sheet was longitudinally stretched between two pairs of rollers in an oven at 200 C, while the naphtha was being evaporated to produce the PTFE base sheet. Then the longitudinally stretched-ptfe base sheet was stretched along the transverse direction at 140 C and finally subjected to heat setting at about 280 C for several seconds. Heat setting of the stretched membrane is the last process necessary to stabilize the porous structure and ensure good dimensional and mechanical properties of the final eptfe membrane. The process machinery is shown in the foregoing paper[4]. 2.2 Measurements and Characterization Observation of surface morphological structure The eptfe membranes were sputtering coated with Au for the SEM observation using Amary-1845FE. Analysis of pore diameter and porosity Measurement of the pore diameter was performed with a COLULTER POROMETER after the sample eptfe membrane had been soaked in POROFIL liquid for several minutes. DSC Analysis A Perkin-Elmer DSC-7 calorimeter was used to analyze the thermal behavior of differently processed eptfe membranes, with the heating rate of 15 C/min up to 400 C. About 8mg PTFE membrane sample was used for each DSC measurement. The crystallinity of the membrane sample was calculated with equation (1): Xc = ( H/ H O ) X 100% (1) Where: H is the melting enthalpy of the samples, and H O is the melting enthalpy of eptfe with 100% crystallinity, which is assumed as 69J/g. WAXD Analysis The intensity profiles of the eptfe membrane samples were measured with Ni-filtered CuKa irradiation ( angstrom) using a Rigaku D/Max-B WAXD diffractometer with a tube operating voltage of 40kV. In order to eliminate the effect of the orientation induced by stretching on crystallinity measurement, all samples were cut into powder and then pressed into the sample holder during WAXD testing. The equatorial WAXD patterns of different eptfe membrane samples were obtained, and the crystallinity and crystallite size of different samples were calculated by equation (2) and Scherrer s equation (3) respectively after peak fitting, where the peak around 2q=18 is attributed to the crystalline diffraction of (100) planes and the weak peak around 2q=16 is attributed to the amorphous contribution. C = 100I c % Ic + KI a Where I C is the area of the resolved crystalline peak, and I a is the area of the resolved amorphous peak, K is 0.66. D (hkl) = kλ βcosθ Where D (hkl) is the crystallite size normal to the crystalline plane (hkl), here k is assumed to 0.89, λ is the CuKα radiation wavelength of 1.5418 angstrom, β is the height at half width of the resolved crystalline peak in radian, θ is the Bragg diffraction angle in degree. According to Bragg reflection equation (4), is in reverse proportion to, which represents the chain packing density of the crystalline domains and can be calculated from the diffraction patterns of the membrane samples. (2) 2dsinθ = nλ, (4) where d is the space between the diffracted crystalline planes, θ is the Bragg angle, and λ is the X-ray radiation wavelength 1.5418 Å, n is the diffraction order. Mechanical Testing The tensile tests of the membrane samples were conducted using an INSTRON 1122 with a sample size 20mm wide and 780mm long and conformed to Chinese standard GB1040-79. The loading rate was 100mm/min, and every sample was loaded to tensile break. 3. Results and Discussion 3.1 Effects of Processing Parameters on the Porous Structures of epfte membrane 3.1.1 Effect of the longitudinal stretching The results of the SEM observations after different longitudinal stretching are given in Fig. 1 and Fig. 2. The rolling and stretching direction in these two figures are the horizontal direction. From Figure 1, it can be seen that some holes on the PTFE base sheet are formed as a result of extrusion and rolling and they are elongated by the first longitudinal-stretching step. Samples in Figure 2 were all processed under the same longitudinal-stretching conditions with 5X, 6X and 8X longitudinal-stretching steps, at 4.8m/min stretching rate, and no heat (3) 32 INJ Summer 2005

hao layout 7/5/05 5:55 PM Page 3 (1) PTFE rolled sheet (2) 1X longitudinal-stretching PTFE sheet Figure 1 SEM MICROGRAPHS OF THE PTFE ROLLED SHEET AND THE LONGITUDINALLY-STRETCHED PTFE BASE SHEET (1) 5X longitudinal stretching (2) 6X longitudinal stretching (3) 8X longitudinal stretching Figure 2 EFFECT OF RATIO OF LONGITUDINAL STRETCHING ON THE SURFACE STRUCTURE OF THE MEMBRANE setting. Figure 2shows that the effect of ratio of longitudinal stretching on the surface structure of the membrane. The effects on porosity and mean pore size are shown in Table 1, showing that both properties increase with increasing longitudinal stretching. 33 INJ Summer 2005 3.1.2 Effect of the transverse stretching The results of the SEM observations after different transverse stretching degrees are shown in Figure 3. The longitudinal stretching ratio was maintained at 2X and the transverse

Table 1 EFFECT OF LONGITUDINAL STRETCHING RATIO ON POROSITY Stretching ratio 5 6 8 Porosity, % 76.4 87.6 89.1 Mean pore diameter, Ãm 0.35 0.47 0.51 stretching rate was also fixed and the ratio of transverse stretching was varied during the sample preparation from 2.1X to 8.5X. The transverse stretching rate was fixed at 4.8m/min and no heat treatment was employed. According to the data in Table 2, porosity and pore diameter are increased along with the increase of the ratio of transverse widening. Pore diameter and porosity are controlled in accordance with the final application of the product. Table 2 EFFECT OF TRANSVERSE STRETCHING RATIO ON POROSITY AND THE PORE DIAMETER Stretching ratio 2.1 2.9 4.7 8.5 Porosity, % 56.5 58.4 60.4 78.0 Mean pore diameter, µm 0.08 0.09 0.12 0.24 Table 3 EFFECT OF TRANSVERSE STRETCHING RATE ON POROSITY AND PORE DIAMETER Stretching rate, m/min 4.8 6 8 Porosity, % 60.4 64.2 70.8 Mean pore diameter, µm 0.12 0.11 0.09 3.1.3 Effect of the transverse stretching rate The effect of the transverse stretching rate on the pore diameter and porosity of the eptfe membrane is given in Figure 4 and Table 3, where the other processing parameters are all the same except the transverse stretching ratio. None of the membranes depicted in Figure 5 were heat-set and they were processed with 2X longitudinal stretching and 4.7X transverse stretching. Figure 3 EFFECT OF THE RATIO OF TRANSVERSE STRETCHING ON THE STRUCTURE OF THE MEMBRANE 1) 2.1X transverse stretching (2) 2.9X transverse stretching (3) 4.7X transverse stretching (4) 8.5X transverse stretching The data in Table 3 show that the higher the transverse stretching rate, the higher the porosity, further, the average pore diameter declines with the increase of the stretching rate. Literature data reveals that the temperature during transverse stretching has little effect on membrane stretchability. This is because the activation energy required for the steady growth of fibrils is 11.3 kj/mol [8,9], which makes it is easy to form pores in membrane by stretching membrane. 3.1.4 Effect of the heat setting temperature Heat setting is the last process in the production of microporous eptfe membrane, which stabilizes the structure and enhances the mechanical properties of the membrane. In order to counteract the thermal shrinkage and fix the microstructure of the membrane, the membrane was constrained during the heat setting. The temperature and time of heat setting have great influence on the porous characteristics, the dimensional stability and mechanical properties of the final membrane. Figure 5 displays the SEM photographs of the membrane samples and the effect of heat setting on the pore characteristics are shown in Table 4. Heat setting temperatures ranged from 245 C to 300 C and all the 34 INJ Summer 2005

(1) 4.8m/min (2) 6m/min (3) 8m/min Figure 4 EFFECT OF TRANSVERSE STRETCHING RATE ON THE SURFACE STRUCTURE OF THE MEMBRANE Table 4 EFFECT OF HEAT SETTING TEMPERATURE ON POROSITY Sample No. Heat setting Mean pore Minimum pore Maximum pore temperature diameter, µm diameter, µm diameter, µm 1 245 0.382 0.319 0.424 2 280 0.589 0.469 0.646 3 300 0.685 0.618 0.835 Table 5 CALCULATED RESULTS FROM DSC ANALYSIS Sample Enthalpy ( H),J/g Crystallinity X C,% T max, O C Sheet 59.636 86.4 342.7 Base sheet 57.549 83.4 342.7 Non-heat set 54.786 79.4 342.1 245 heat set 46.165 66.9 343.3(336.0) 280 heat set 32.852 47.6 335(343.7) 300 heat set 16.059 23.3 327 samples were subject to 2X longitudinal stretching, 8X transverse stretching, and the transverse stretching rate was set as 9m/min. From Table 4, we can see that the membrane pore diameter increases with the increase of heat setting temperature. The higher of the heat setting temperature, the larger of the node area, and the space between the nodes also increases. It may be that the fibrils connected by the nodes will break down at higher temperature. The pores are formed among fibrils which interconnect the nodes, and the pore diameter is dependent on the space among fibrils. 3.2 Effects of Processing Parameters on the Crystalline Structure and Mechanical Property of epfte membrane 3.2.1 DSC Analysis The DSC traces and corresponding calculated results of different membrane samples are given in Figure 6 and Table 5. From Figure 6 and Table 5, it can be seen that the crystallinity and the melting fusion of the membrane decreases with stretching and heat setting temperature. The higher the heat setting temperature, the lower the enthalpy and consequently the lower the crystallinity. From Figure 6, it can be seen that the melting of the eptfe membrane occurs at a range of temperatures, which suggests the non-perfection and/or non-uniformity of the crystalline structure of the membrane with less perfect and/or small crystalline domains melting first, and the melting range is widened by stretching and heat setting, and after stretching and heat setting the onset and maximum melting temperature of eptfe membrane is lowered. With 300 C heat treatment, the maximum melting temperature is lowered to about 327 C, which is the well documented melting temperature of PTFE recrystallized from the melt. The eptfe membrane is turned from opaque to transparent when the heat setting temperature is above 300 C, which suggests the effect of decrease of crystallinity with the increase of heat setting temperature. 35 INJ Summer 2005

Figure 5 EFFECT OF HEAT SETTING TEMPERATURE ON THE POROUS STRUC- TURE OF THE MEMBRANE Figure 6 DSC CURVES OF DIFFERENT PTFE MEMBRANE SAMPLES A discernable peak shoulder at lower temperature exists for all samples before heat setting, and it becomes more and more pronounced after heat setting and especially at 300 C heat setting. Only one lower temperature peak exists with peak temperature of about 327 C. Several documents have been published dealing with the melting and crystallization behavior of dispersion PTFE polymer. Suwa et al [13] attributed the higher temperature peak of DSC analysis to the linear part of the folded ribbon based on the microstructure model first put forward by Rahl et al [14] and confirmed by Chanzy et. al [15] through high-resolution electron microscopy. Further, the lower temperature peak was attributed to the folded region of the folded ribbon. As far as the chain extension in the crystalline lamellae or ribbon is concerned, some authors suggested the chains are folded with the chain axis perpendicular to the length of the crystalline bands like conventional polyethylene after recrystallization from the melt, and some authors like Suwa and Rahl at al concluded that chains are extended with the chain axis along the crystalline ribbons like polyethylene crystallized under high pressure for virgin dispersion polymer particles. Based on the above mentioned observation and discussion, it seems that with heat setting above 150 C the virgin PTFE polymer becomes more and more chain folded, but the chain extension and folded degree is still open to verification by other characterization means which is underway in our further study. 3.2.2 WAXD Analysis The WAXD profiles and corresponding calculated results are given in Figure 7 and Table 6. Table 6 shows that the crystallinity and crystallite size of the membrane is decreased with the stretching and heat setting processes, which suggests stretching and heat setting under constraint leads to the deterioration of crystalline domains of PTFE membrane, especially the crystalline domains perpendicular to the chain axis direction. After longitudinal stretching, the chain packing normal to the chain axis direction becomes smaller, and with biaxial stretching, the chain packing becomes a little larger, but after heat setting at 245 C and 280 C, the chain packing normal to the (100) planes restores and with 300 C heat setting the chain packing again becomes larger with recrystallization of small or defective crystallites under higher temperature. 36 INJ Summer 2005

Table 6 CALCULATED RESULTS OF THE WAXD ANALYSIS Sample Crystallite size, Å Crystallinity, % 2θ, O d,å Sheet 113.2 97.2 18.0 4.93 Base sheet 93.7 94.2 18.5 4.80 Non-heat set 84.9 94.9 17.9 4.96 245 heat set 89.3 88.6 18.5 4.80 280 heat set 79.9 86.0 18.5 4.80 300 heat set 65.3 80.7 18.0 4.93 3.2.3 Mechanical Property Heat setting serves as not only a dimensional stabilization and fixation means, but also a mechanical enhancement. With heat setting, the tensile breaking extension decreases, the tensile modulus increases and the tensile strength also increases a little, which is illustrated in Figure 8. Figure 7 WAXD ANALYSIS RESULTS OF DIFFERENT EPTFE MEMBRANE SAMPLES 4. Conclusions Emulsion polymerized PTFE particles with a number average molecular weight of 5X10 6 were used as the starting material to make eptfe membranes with the operations of extrusion, rolling, longitudinal stretching, biaxial stretching and heat setting. Small holes in the eptfe sheet are formed after extrusion and rolling, and are elongated into slits which become larger with increased longitudinal stretching. More fibrils are also observed by SEM as the fracture size increases. The second stretching operation, transverse stretching, provides a lattice-like porous structure. After heat setting, the island-like structure is formed; it is composed of billions of tiny inter-connected continuous fibrils and the nodes. The mean pore size and porosity increase with an increase in longitudinal stretching ratio, transverse stretching ratio, and setting temperature. The mean pore size decreases, and porosity increases with an increase in transverse stretching rate. The crystallinity and crystallite size of the membrane decreases with stretching and heat setting, and after heat setting, the crystallinity clearly decreases. Also after heat setting, the membrane can no longer be stretched, and the dimensional stability and mechanical properties are enhanced. Heat setting may change the extended chains in the lamellae into folded chains, which will be the subject of future work. Through controlling the stretching direction, stretching ratio, stretching rate and heat setting temperatures, eptfe membranes with different porous characteristics, morphology and mechanical properties can be engineered for different product end-uses. Acknowledgements: The financial support of this project from the Beijing Natural Science Foundation of China with the Program grant number of 2052016 is thankfully acknowledged. We are very grateful to Prof. Larry C. Wadsworth of the Materials Science & Engineering Department, University of Tennessee for helpful critical suggestions. Figure 8 TENSILE MECHANICAL PROPERTIES OF THE HEAT SETTED MEMBRANE SAMPLES Refernces 1. S.Oga. Japan Patent 42-13560, 1967. 2. R.W.Gore. U.S. Patent 3664906, 1972. 3. R.W.Gore. U.S. Patent 3953556, 1976. 4. Huang Jizhi, Zhang Jianchun, et al. European Polymer Journal 2004; 40: 667-671. 5. Hao Xinmin, Zhang Jianchun, et al. European Polymer Journal 2004; 40: 673-678. 6. Zhang Jianchun, Huang Jizhi et al. In: Processes technique of waterproof and permeable textile, China Textile Press; 2003. p.271. 7. Guo Yuhai. A Study on Waterproof & Moisture Permeable Textile Laminated with Microporous eptfe Membrane. Master Thesis, 1997. p32. 37 INJ Spring 2005

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