Effect of Carbon Black on Microcellular Behavior of Ethylene- Octene Copolymer Vulcanizates

Transcription

1 Effect of Carbon Black on Microcellular Behavior of Ethylene-Octene Copolymer Vulcanizates Effect of Carbon Black on Microcellular Behavior of Ethylene- Octene Copolymer Vulcanizates Nimai C. Nayak, * P. Ganga Raju Achary, Suchilipsa Das, and Sanija Begum Micro-Nano Materials Laboratory, Department of Chemistry, Institute of Technical Education and Research, Siksha O Anusandhan University, Bhubaneswar, Odisha-75, India Received: 24 October 2, Accepted: 9 December 2 SUMMARY The morphology of the microcellular ethylene-octene copolymer (EOC) of both unfilled and conducting carbon black (Ensaco 25G) filled compounds with different blowing agent and carbon black loading was studied using SEM photomicrographs. The systematic variation in the average, minimum and maximum cell size and cell density with different blowing agent and filler loading was observed. Some physical properties such as relative density, hardness, tensile strength, elongation at break, modulus and tear strength decreases with increase in blowing agent and filler loading. However, the stress relaxation behaviour is found to be independent of blowing agent loading. Keywords: Polymer composites, Micro cellular ethylene-octene copolymer, Carbon black, Blowing agent, Physico-mechanical properties INTRODUCTION The natural or synthetic cellular and microcellular materials (i.e., wood, cancellous bone, sponge, cork, corals etc.) have widely been used []. The high strength to weight ratio of wood, insulating properties of cork and balsa and cushioning properties of cork and straw have helped to develop a broad range of synthetic cellular polymers that are increasingly being used structurally for insulation, in cushioning, and in systems for absorbing energy from impacts. Cellular polymers are a class of polymer whose apparent density decreases substantially by the presence of numerous cells disposed throughout the * Correspondence to: Nimai C.Nayak. nimainayak@soauniversity.ac.in Smithers Information Ltd. 24 Cellular Polymers, Vol., No. 2, 24 7

2 Nimai C. Nayak, P. Ganga Raju Achary, Suchilipsa Das, and Sanija Begum masses [2]. The cellular polymers constitute a two-phase (solid and gas) system in which the solid phase is continuous and composed of either natural or synthetic polymer. The gas phase is distributed in voids or pockets called cells. If these cells are interconnected, the material is termed as open cell, and if the cells are discrete and the gas phase of each one is independent of that of the other cells, the material is termed as closed cells [, -5]. Polymeric foams are gaining interest for various applications because they have a large strength to weight ratio, as well as being cost effective as compared to other light-weighted structural materials [6]. Elastomeric foam or cellular rubber can be produced either with open or closed-cell structure. Commercially available rubber based polymeric foams are produced from synthetic polymers or rubbers such as ethylene vinyl acetate (EVA) copolymer, natural rubber( NR), ethylene propylene diene (EPDM) terpolymer, acrylonitrile butadiene rubber (NBR), polychloroprene rubber (CR) and acrylonitrile butadiene-polyvinyl chloride blend (NBR/PVC) [7]. Foams can be reinforced by adding rigid filler or short glass fiber to the polymer before it is foamed. The effect of the reinforcing fillers such as carbon black, silica and silicate on the polymeric foams is complicated, and hence they can affect the cell growth process, change the cell geometry and physical properties. Carbon black is a major component of many rubber products and used for reinforcement and modification of physical properties. The effect of carbon black on the morphology and physical-mechanical properties of microcellular EPDM rubber vulcanizates have been studied [8, 9]. Kim and co-workers [] and Le and Choi [] studied the foaming characteristics and physical properties of natural rubber foams filled with carbon black. Xin et al. [2] investigated the effect of carbon black on microcellular structure and physical properties of chlorinated polyethylene rubber foams. Thermoplastic elastomers (TPE) are a new class of materials that exhibit the properties of conventional thermoset rubbers that can be processed with thermoplastic processing equipments. DuPont Dow elastomer has introduced POEs under the brand name ENGAGE. They are ethylene-octene copolymers produced via advanced INSITE TM catalyst and process technology designed to be processed like thermoplastic but can be compounded like-elastomers []. The exceptional performance of ENGAGE is attributed to extraordinary control over polymer structure, molecular weight distribution, uniform comonomer composition and rheology. They are being considered for use in diverse applications such as cushioning agents, gaskets, and particularly good alternative for sealing application due to their structural regularity and non-toxic composition. Cellular elastomers made from this POE have superior tear strength, elongation and resiliency in comparison with other cellular elastomers (i.e., EPDM, Neoprene, PVC and PVC/ Nitrile) [4]. The effect of 72 Cellular Polymers, Vol., No. 2, 24

3 Effect of Carbon Black on Microcellular Behavior of Ethylene-Octene Copolymer Vulcanizates CaCO, silica and aluminium silicate on microcellular behaviour of ethyleneoctene copolymer has been studied [5-7]. In the present study we have investigated the effect of carbon black (Ensaco 25G) loading on morphology and physico-mechanical properties of microcellular ethylene octane copolymer vulcanizates with reference to the variation of blowing agents and carbon black loading. EXPERIMENTAL Materials The ethylene octene copolymer (Engage 85) contains 25% wt octene monomer with a melt flow index of.5 g/ min (9 C /2.6 kg), density.868 gm/cc, and Mooney viscosity,ml +4 (2 C) 5 (M/s Dow Elastomer Co, USA). The conducting carbon black ENSACO 25 G was supplied by M/S TIMCAL, Singapore. These carbon nanoparticles (CNP) are round in shape with mean diameter of about 4 nm, BET nitrogen surface area of 65 m 2 g, Oil absorption number (OAN) is 9 cm g, volume resistivity Ωcm and apparent density.8. They are associated to aggregates of several hundreds of nanometer long having a branched structure. These CNP belong to the conductive carbon black family and the volume conductivity of the powder is Ωcm. They are synthesized by partial oil oxidation of carbo-chemical and petrochemical origin. Zinc oxide and stearic acid used was of chemically pure grade having specific gravity 5.55 and.82 respectively of Loba Chemicals, India. The dicumyl peroxide (DCP) used was of 98% purity (M/s Aldrich Chemical Company, USA). The blowing agent used was mikrofine ADC-2 manufactured by High Polymer Lab, New Delhi, India. It is a chemical blowing agent based on azodicarbonamide having decomposition temperature is 58±2 C. The gas yield is 85 cc/g at STP. Compounding and Sample Preparation The compositions of both unfilled and carbon black filled vulcanizates re given in Table. Three concentrations of the blowing agent (ADC) typically used in industrial formulations, 2 and 4 parts per parts of the resin (phr) were studied [8]. Cellular Polymers, Vol., No. 2, 24 7

4 Nimai C. Nayak, P. Ganga Raju Achary, Suchilipsa Das, and Sanija Begum Table. Formulations of carbon black (Ensaco 25G) reinforced vulcanizates in phr Mix No. Engage CB Azodicarbonamide ZnO Stearic acid DCP (Mikrofine ADC-2) G G G 2 G 4 EB 2 EB 2 EB 22 EB 24 EB 4 EB 4 EB EB 44 The mixing was carried out in a Brabender Plasticoder (model, PLE ) having cam type rotors. The co-polymer was first melted at 8 C with rotor speed of 6 rpm for two minutes followed by addition of other ingredients. With subsequent incorporation of filler-mixing was continued for another five minutes to ensure homogeneous distribution of ingredients. Finally curative was added and mixing was continued for another two minutes. The hot mix was taken out and passed through a tight nip two-roll mill to form sheet. The curing characteristics of the vulcanizates were determined using a Monsanto Rheometer (model R ) according to ASTM D 284 and ASTM D 5289 procedures at 6 C with an arc of oscillation of. The vulcanizates were moulded at 6 C to 8% of their respective optimum cure times in an electrically heated hydraulic press at a pressure of 5 MPa. All sides of the mould were tapered to to facilitate the expansion of the microcellular product and for better mould release. Expanded microcellular sheets were post cured at C for one hour in an electrically heated air oven. Characterization Mooney viscosity (ML +4 at C) of the samples were determined (according to ASTM D ) using a Negretti automatic shearing disc viscometer (model MK-, UK). The density of the samples was determined from the sample weight in air and water according to ASTM D 792 method. 74 Cellular Polymers, Vol., No. 2, 24

5 Effect of Carbon Black on Microcellular Behavior of Ethylene-Octene Copolymer Vulcanizates The hardness of the samples was measured according to ASTM D224 using durometer and Shore A scale at room temperature. The stress-strain properties such as tensile strength, elongation at break, modulus and tear strength were determined on a Hounsfield 45 universal testing machine at room temperature ( C) (according to ASTM D ). The crescent tear strength and trouser tear strength were also measured in the Hounsfield (model UTM) as per ASTM D The stress relaxation measurements were carried out as per ASTM D-574. SEM Studies The SEM micrographs of the razor-cut surfaces of microcellular were recorded at 2kV using ZEISS EVO 6 scanning electron microscope with Oxford EDS detector, and were analyzed for better understanding of the cell structures. The surfaces of the samples were gold-coated for five minutes under an argon atmosphere (in vacuum) using a sputtering technique before performing the experiment. RESULTS AND DISCUSSION Curing Characteristics The curing characteristics of blowing agent and carbon black containing vulcanizates at 6 C, obtained from the Monsanto rheographs, are summarized in Table 2. For both unfilled as well as carbon black filled vulcanizates, the maximum rheometric torque decreases significantly on increasing blowing agent concentration whereas minimum rheometric torque decreases slightly. The scorch time (t s2 ) or time to incipient cure is a measure of the time at which premature vulcanization of the materials occurs. It is the time taken for the minimum torque value to increase by two units. It is seen that the scorch time increases marginally with increase in blowing agent concentrations for unfilled vulcanizates. For Ensaco25G carbon black filled vulcanizates scorch time decreases marginally with increase in filler loading as well as increase in blowing agent loading. The cure time decreases with increase in carbon black loading. This may be due to increase in the viscosity of the vulcanizates with increase in carbon black loading for which shorter time is required for the beginning of the vulcanization process. However, the cure time increases marginally with increase in blowing agent loading for both unfilled as well as carbon black filled vulcanizates. During Cellular Polymers, Vol., No. 2, 24 75

6 Nimai C. Nayak, P. Ganga Raju Achary, Suchilipsa Das, and Sanija Begum curing, the blowing agent also starts decomposition. Thus, curing is affected by the exothermic decomposition of the blowing agent. Therefore, the actual rheometric characteristics of the vulcanizates cannot be obtained but the resultant effect of curing and blowing is obtained [8, 5-7]. Table 2. Rheometric characteristics of unfilled and carbon black (Ensaco 25G) filled vulcanizate Mix No. G G G 2 G 4 EB 2 EB 2 EB 22 EB 24 EB 4 EB 4 EB 42 EB 44 Minimum Torque (ML) (dn m) Maximum Torque (MH) (dn m) Scorch Time at 6 C (t s2 ) (min) Curing Time at 6 C (t 9 ) (min) Mooney Viscosity ML ( C; +4) Morphology of the Razor-cut Surfaces of Microcellular EOC- Vulcanizates The SEM photomicrographs of various unfilled and carbon black (Ensaco 25 G) filled vulcanizates of different compositions are compared in Figure. All vulcanizates exhibit closed-cell structure. The quantitative image analysis of the photomicrographs was carried out to determine the average cell size (or diameter, D) [9]. The density of both unfoamed and foamed samples was determined by using the method of buoyancy. Then the density of the foamed sample was divided by the density of the unfoamed samples to obtain the relative density (ρ r ) of the vulcanizates: ρ r = ρ f ρ s () 76 Cellular Polymers, Vol., No. 2, 24

7 Effect of Carbon Black on Microcellular Behavior of Ethylene-Octene Copolymer Vulcanizates The cell density (N) (number of cells per unit volume (cm ) of the microcellular vulcanizates at maximum expansion were calculated by using the relation [2]: N = nm2 A /2 ϕ (2) where N is the cell density, n is the number of cells in the SEM micrograph, M is the magnification factor, A is the area of the micrograph (cm 2 ), and f is the volume expansion ratio of the polymer foam, which was calculated by the Equation (). ϕ = ρ s ρ f () where ρ s and ρ f are densities of the samples before and after foaming respectively. The results are summarized in Table. The number of cells per unit volume (cell density) increases on increasing blowing agent concentrations for both (the unfilled as well as carbon black filled) type of vulcanizates. The average cell size decreases with increase in blowing agent concentrations for the unfilled vulcanizate. In case of phr blowing agent loaded vulcanizates, the average cell sizes are less than that of 2 and 4 phr blowing agent loaded vulcanizates. It is due to the fact that phr blowing agent is not enough to form microbubbles inside the carbon black filled dense vulcanizates. It is evident from the figures that with increase in carbon black loading to 4 phr, the cell density decreases for phr blowing agent loading but on increasing of blowing agent loading beyond phr, the cell density increases. With increase in blowing agent loading from phr to 2 phr, the average cell size increases for carbon black loaded vulcanizates. However with further increase in blowing agent concentration to 4 phr, the average cell size and maximum cell sizes decreases. It is evident that addition of Ensaco 25G filler increases the number of cells, and also decreases the average cell size. Increase in number of cells may be considered due to nucleation effect by carbon black surfaces. The decrease in cell size is due to increase in melt viscosity and secondly due to increase in cure rate by incorporation of Ensaco 25G carbon black both of which retard the growth of the cells. Cellular Polymers, Vol., No. 2, 24 77

8 Nimai C. Nayak, P. Ganga Raju Achary, Suchilipsa Das, and Sanija Begum Figure. SEM photomicrographs of microcellular Ethylene-octene copolymer vulcanizates: (a) G, (b) G 2, (c) G 4, (d) EB 2, (e) EB 22, (f) EB 24, (g)eb 4, (h) EB 42, (i) EB 44 Physical Properties It is clear from the Table that the relative density is more pronounced in case of carbon black filled vulcanizates. With increase in carbon black loading, the gas permeability decreases, and thus there is less possibility of the loss of gas by diffusion. As a result, more gas remains in the rubber matrix. The hardness of the vulcanizates decreases with increase in blowing agent loadings but increases with increase in carbon black loading. The enclosed gas inside the rubber matrix has little elastic property, and hence hardness decreases with increase in blowing agent loading. It is observed that the expansion ratio increases with increase in blowing agent loading for unfilled as well as carbon black loaded vulcanizates. However, the expansion ratio decreases with increase in carbon black loading for phr blowing agent loading. It is a well known fact that the foam expansion process 78 Cellular Polymers, Vol., No. 2, 24

9 Effect of Carbon Black on Microcellular Behavior of Ethylene-Octene Copolymer Vulcanizates Table. Physical properties of unfilled and carbon black filled EOC vulcanizates Mix No. G G 2 G 4 EC 2 EC 22 EC 24 EC 4 EC 42 EC 44 Relative Density (ρ f /ρ s ) Hardness (Shore A) Average cell size (µm) 6±.2 94±. 78±. 75±.2 4±. 95±.2 65±. 76±. 72±.2 Volume expansion ratio(f) Cell density (N ) (cells/cm ).8 x 7 6. x 7. x 8.4 x 7 2. x x x 6.4 x 8 5. x 8 Table 4. Physical properties of unfilled and Ensaco 25G filled microcellular vulcanizates Mix No. G G G 2 G 4 EB 2 EB 2 EB 22 EB 24 EB 4 EB 4 EB 42 EB 44 Tensile Strength (MPa) Elongation at break(%) Modulus (%) (MPa) Modulus (2%) (MPa) Modulus (%) (MPa) Tear Strength (N/mm) is primarily controlled by the polymer matrix modulus, melt viscosity properties and gas diffusion rate [2-22]. The density increases with increase in carbon black loading. Because of its higher density, the modulus as well as melt viscosity increases which decreases the expansion ratio. Tensile strength, elongation at break, modulus and tear strength values of the vulcanizates are summarized in Table 4. Tensile strength, elongation at break, modulus Cellular Polymers, Vol., No. 2, 24 79

10 Nimai C. Nayak, P. Ganga Raju Achary, Suchilipsa Das, and Sanija Begum values decreases with increase in blowing agent loading for the unfilled and carbon black filled vulcanizates. Both the tensile strength and modulus increases for the 2 phr carbon black loaded vulcanizates. However, with increase in carbon black loading to 4 phr, the tensile strength decreases but modulus goes on increasing. Though the tear strength increases with increase in carbon black loading, it decreases with increase in blowing agent concentration. It is also observed that the decrease in relative tensile strength is sharper than the, 2 and % relative modulus. This is due to the fact that the maximum of the cell size (i.e., flaws) affects the tensile strength but not the modulus [8]. Moreover, % modulus is more than and 2% modulus. Similar results were also reported in case of silica filled microcellular vulcanizates [6]. The increase in relative modulus may be due to an increase in gas pressure inside the cells [6]. Stress Relaxation Behaviour The stress relaxation behaviour of the unfilled as well as the Ensaco25G carbon black filled solid as well as microcellular vulcanizates are obtained by stretching the samples at a constant strain level of %. Figure 2 and Figure show the decay of stress with time for the unfilled and 2 phr carbon black filled microcellular vulcanizates. The rate of decay is almost similar to the unfilled and carbon black filled microcellular vulcanizates. It shows that the relaxation behaviour is independent of the blowing agent loading, i.e., the density of the closed cell microcellular vulcanizates. The simulation study of the stress relaxation behaviour of microcellular vulcanizates is done using the classical Maxwell model [2]: σ (t) = σ e (t/τ r ) [4] ln σ σ t = τ r t [5] where s (t) is the stress at time t, σ is the stress at t= and τ r is the relaxation time. Therefore a plot of ln σ versus time (t) gives a straight line with the σ t slope reciprocal of the relaxation time τ r. Figures 4 and 5 are the Maxwell 8 Cellular Polymers, Vol., No. 2, 24

11 Effect of Carbon Black on Microcellular Behavior of Ethylene-Octene Copolymer Vulcanizates Figure 2. Stress relaxation behaviour of unfilled ethylene-octene copolymer vulcanizates Figure. Stress relaxation behaviour of 2 phr Ensaco 25G carbon black filled ethylene-octene copolymer vulcanizates Cellular Polymers, Vol., No. 2, 24 8

12 Nimai C. Nayak, P. Ganga Raju Achary, Suchilipsa Das, and Sanija Begum Figure 4. Maxwell plot for stress relaxation behavior of unfilled vulcanizates Figure 5. Maxwell plot for stress relaxation behavior of 2 phr Ensaco 25G carbon black filled vulcanizates plots for the unfilled and 2 phr carbon black filled vulcanizates respectively. Similar results were observed for 4 phr carbon black filled vulcanizates. These figures illustrates that the Maxwell model fits with the experimental stress relaxation for the unfilled (G, G, G 2,, G 4 ) as well as 2 and 4 phr carbon 82 Cellular Polymers, Vol., No. 2, 24

13 Effect of Carbon Black on Microcellular Behavior of Ethylene-Octene Copolymer Vulcanizates black filled solid (EB 2, EB 4 ) vulcanizates. However, the model does not fit with experimental data for the carbon black filled microcellular vulcanizates (EB 2, EB 22, EB 24, EB 4, EB 42, and EB 44 ). The relaxation time (τ r ) for the vulcanizates obtained from the Maxwell model are represented in Figure 6. According to the Maxwell model the relaxation time found to be independent of carbon black as well as blowing agent loading. Figure 6. The relaxation time (t r ) for the vulcanizates obtained from the Maxwell model Fracture Nuclei in Tensile Failure When the material breaks in simple extension by characteristic tearing from a small nick in one edge, the energy stored in the specimen at break, (i.e., the work required to break) is simply related to the catastrophic tearing energy. The tearing energy of the foam [24] (T f,) can be expressed as: T f = 2KE f l [6] Where K is numerical constant having a value of about 2, [] l is the depth of the flaw and E f is the strain energy density at failure in the bulk of the test piece for the foam. Cellular Polymers, Vol., No. 2, 24 8

14 Nimai C. Nayak, P. Ganga Raju Achary, Suchilipsa Das, and Sanija Begum Table 5. Tear energy and calculated flaw size of unfilled and Ensaco 25G filled microcellular ethylene octane copolymer vulcanizates Mix No. G G 2 G 4 EB 2 EB 22 EB 24 EB 4 EB 42 EB 44 Trouser Tear Resistance (N /mm) Tear Energy (T f ) (KJ/m 2 ) Strain Energy Density(E f )(KJ/m ) Calculated Cell size (l) (µm) It can be assumed that tensile rupture occurs by catastrophic tearing of the flaw, and is estimated by using the above expression. It can be calculated by using the measured tear energy and strain energy density of the microcellular vulcanizates [25]. The tear energy of the various microcellular samples were calculated from the values of tear strength of the trouser specimen (ASTM D-574). The equation used for the calculation of tear strength is: T f = 2 F t [7] T f = 2 x T f where T f is the trouser tear strength and T f is the tear energy. Strain energy density can be calculated from the tensile strength and the elongation break of the dumbbell-shaped specimen. The results are summarized in Table 5. It is observed that theoretical values of the flaws depth (l) are larger than the corresponding maximum cell size. The mean value of the ratio of the theoretical depth of the flaws and the maximum cell size is.. For a perfectly regular foam structure, the tear tip diameter would be expected to be twice the pore diameter because of the random arrangement of the pores in the space [22]. Imperfections in the foam will lead to local deviations of tear from a linear front and hence give rise to corresponding larger effective diameter at the tip. 84 Cellular Polymers, Vol., No. 2, 24

15 Effect of Carbon Black on Microcellular Behavior of Ethylene-Octene Copolymer Vulcanizates CONCLUSIONS Maximum rheometric torque decreases with increase in blowing agent concentrations. The cure time increases with increase in blowing agent but decreases with carbon black loading. The average cell size decreases with increase in blowing agent concentrations for the unfilled vulcanizates. One phr blowing agent (Mikrofine ADC-2) loading is not sufficient to form Ensaco 25G loaded microcellular product. With increase in blowing agent loading in carbon black loaded vulcanizates, the average cell sizes decrease. The cell density increases with increase in blowing agent loading. The expansion ratio increases with increase in blowing agent loading but decreases with increase in carbon black loading. The relative density of the vulcanizates decreases with increase in blowing agent loading. Tensile strength, elongation at break, tear strength and modulus decreases with increase in blowing agent loading for both unfilled as well as Ensaco 25G carbon black filled vulcanizates. The stress relaxation behaviour is independent of blowing agent loading (i.e., density) of closed cell microcellular vulcanizates. Maxwell model fits with unfilled, unfilled microcellular and carbon black filled solid vulcanizates but do not fit with carbon black filled microcellular vulcanizates. In tensile rupture specimens, the theoretically calculated flaw size is. times larger than that of the maximum cell size measured from SEM photomicrographs. ACKNOWLEDGEMENTS The authors are thankful to Mr Frederic Lye of TIMCAL, Singapore for providing Ensaco 25G. REFERENCES. Gibson L.J. and Ashby M.F., (998), Cellular Solids, Structure and Properties, Pergamon Press, Oxford. 2. ASTM D8-8c, (982), Definition of terms relating to plastics. American Society for Testing and Materials, Philadelphia.. Blow C.M. and Hepburn C., (982), Rubber Technology and Manufacture. 2 nd edition, Butterworth, London. 4. Babbit R.O., (978), The Vanderbilt Rubber Handbook. Vanderbilt Company Inc., Norwalk, CT. Cellular Polymers, Vol., No. 2, 24 85

16 Nimai C. Nayak, P. Ganga Raju Achary, Suchilipsa Das, and Sanija Begum 5. Klempner D. and Frisch K.C., (99), Handbook of Polymeric Foams and Foam Technology, Hanser Publishers, Munich. 6. Altsta dt V., Diedrichs F., Lenz T., Bardenhagen H. and Jarnot D., Polym. Polym. Compos., 6 (998) Laakso R., Guffey V. and Vara R., Rubber World, 28 (28) Guriya K.C. and Tripathy D.K., J. Appl. Polym. Sci., 62 (996) Mahapatra S.P. and Tripathy D.K., Cell. Polym., 2 (24) Kim J.H., Choi K.C. and Yoon J.M., J. Ind. Eng. Chem., 2 (26) Lee E.K. and Choi S.Y., Korean J. Chem. Eng., 24 (27) Zhang B.S., Lv X.F., Zhang Z.X., Liu Y., Kim J.K. and Xin Z.X., Mater. Des., (2) 6-.. ENGAGE A polyolefin Elastomer, Manual, Du-Pont NEN, Wilmington, DE. 4. Sherman L.M., Plastic Technology, (996) Nayak N.C. and Tripathy D.K., Cell. Polym., 9 (2) Nayak N.C. and Tripathy D.K., J. Appl. Polym. Sci., 8 (22) Nayak N.C. and Tripathy D.K., J. Mater. Sci., 7 (22) Reyes-Labarta J.A., Olaya M.M., and Marcilla A., J. Appl. Polym. Sci., 2 (26) Sims G.L.A. and Khumiteekoh C., Cell. Polym., (994) Zhai W., Leung S.N., Wang l., Naugib H.E. and Park C.B., J. Appl. Polym. Sci., 6 (2) Park C.B., Baldwin D.F. and Suh N.P., Polym. Eng. Sci., 5 (995) Wang J., Cheng X.G., Yuan M.J., He J.S., Polymer, 42 (2) Sperling L.H., Introduction to physical polymer science, John Wiley, New York, USA, Ch.8, p.75 (986). 24. Gent A.N., Thomas A.G. J. Appl. Polym. Sci., 2 (959) Rivilin R.S. and Thomas A.G., J. Polym. Sci., (95) Cellular Polymers, Vol., No. 2, 24