R esearch efforts for establishment of structure-property relationships. Flexural Creep Behavior of High Density Thermosetting Polyester Foams

Transcription

1 Flexural Creep Behavior of High Density Thermosetting Polyester Foams A. D AMORE AND L. NICOLAIS Department of Materials and Production Engineering University of Naples Naples, Italy M. NARKIS Department of Chemical Engineering Technion Haifa, Israel INTRODUCTION R esearch efforts for establishment of structure-property relationships have been focused largely on low-density foams in which the compressive behavior is of dominant importance [1]. A much wider spectrum of mechanical properties is of practical interest for highdensity foams, used as structural components. However, very little information has been published on rigid high-density foams [2]. Recently Masi, et al. [3] have shown that the tensile properties of high density rigid thermosetting polyester foams having cellular structure consisting of spherical voids could be predicted by simple rules. The tensile properties of tough foams based on polyethylene, polypropylene, and high impact polystyrene were studied by Pramuk [4] and analyzed empirically by plotting reduced properties against reduced densities on log-log scales (see Chapter 7, Reference 2 for additional studies on high density foams). Masi, et al. [3] suggested the following equation JOURNAL OF CELWLAR PLASTICS Volume 22-May X/86/ Technomic Publishing Co., Inc. 193

2 194 A. D AMORE, L. NICOLAIS, AND M NARKIS to replace the empirical log-log correlation. In this equation, Q f, orp, and er, pp are tensile strength and density of the foam and the solid polymer, respectively. The present paper is an extension of the behavior characterization of the high-density thermosetting polyester foams by flexural creep studies at different temperatures. EXPERIMENTAL Foamed sheets, 9 mm thick, were supplied by SNIAL Company, Italy. These foams were prepared by using standard unsaturated polyester/ styrene resins blended with blowing agents. Stable liquid foams were obtained by blending the resin with azo compounds or low-boiling compounds such as propane. The liquid foam was poured, or injected, into a mold at 80 C and the crosslinking reaction proceeded. This preparation method yielded foamed sheets having very thin skins (about 0.1 mm); the foam density can be considered as uniform [3]. The overall purpose of this work is to produce sheets of fiberglass reinforced polyester foams. The present work deals only with unreinforced polyester foams. Specimens 120 x 25 x 9 mm were tested in a flexural creep apparatus shown schematically in Figure 1. RESULTS AND DISCUSSION In recent years the technology of reaction injection molding (RIM) of high density thermosetting foams has become increasingly important. Figure 1. Schematic description of flexural creep instrument.

3 Flexural Creep Behavior of High Density Foams 195 RIM technology involves the use of highly reactive liquid components (polyurethane, polyester, or epoxy resins) injected into a mold in which they rapidly solidify by crosslinking and expand to form an integralskin high-density foam. The cellular structure of the core material of high-density foam is simple, consisting ideally of isolated spherical bubbles uniformly distributed in the continuous polymeric matrix. Such a cellular structure is simpler for purposes of analysis than complicated cell structures representing low-density foams unless the solid-skin effect is important. The skin effect in the present thermosetting polyester foams is negligible as stated in the experimental part [3]. Morphological studies of these foams have proved the sphericity of the voids whose size decrease upon increasing the foam s density [11]. Nevertheless, in the density range studied, the size of the spherical voids had no effect on the foam tensile properties, and only their content practically determines these properties [3]. Tensile or flexural creep curves usually consist of three characteristic regions: an initial region in which a high creep rate decreases exponentially, a second region of constant creep rate, and a third one in which the creep rate rapidly increases up to failure. Findley [5,6] suggested that the tensile creep of a large variety of polymers can be described by the following equation: where and E are the initial and time dependent strains, respectively, and m and n are empirical parameters with m being temperature and stress dependent. Nutting [7] suggested introducing stress (Q) dependency as follows: where a is the stress, e is the density, T is the absolute temperature, t is the time and A, K, and n are empirical constants. Alperstein, et al. [9] found that the compressive creep behavior of rigid PU foams obeyed the Findley equation. Creep studies as a function of temperature have shown an Arrhenius type dependence and a stress dependent activation energy. The present polyester foam respond, within the flexural stress range studied, in a linear viscoelastic manner as shown in Figure 2. Flexural creep results at different temperatures are given in Figures 3 and 4 for polyester foams having densities of 0.56 and 0.8 g/cm3, respectively. The creep data are represented by compliance-

4 196 A. D AMORE, L. NICOLAIS, AND M. NARKIS Figure 2. Flexural compliance-time curve at 80 C for three stresses. Foam density /cm3. time curves where J(t), the compliance, is equal to 1/E(t). The curves of Figures 3 and 4 were shifted horizontally to the 20 C creep curve. The validity of the superposition method for the systems under investigation is shown in Figures 5 and 6, where smooth master curves have been obtained. Compliance-time curves at different temperatures for the solid polyester matrix are shown in Figure 7. The experimental shift factors determined from Figures 3, 4, and 7 for the two foams and the solid polyester matrix are shown in Figure 8 in an Arrhenius plot. Figure 3. Flexural compliance-time curves at various temperatures. Foam density 0.56 glcm 3.

5 Flexural Creep Behavior of High Density Foams 197 Figure 4. Flexural compliance-time curves at various temperatures. Foam density 0.8 g/cm3. This figure shows that the shift factors for the polyester foams and the solid polyester matrix are practically indistinguishable. Thus, the void content in the range studied has no practical effect on the viscoelastic response of these materials. An activation energy of 71 Kcal/mole independent of the foam density and flexural stress in the experimental ranges studied was calculated from the Arrhenius plot given in Figure 8. This value is somewhat larger than other literature values [5,6] (for example, 48 Kcal/mole for PMMA in tensile creep). The com- Figure 5. Flexural compliance-time master curve obtained by shifting the data of Figure 3 to a reference temperature 20 C. Thermosetting polyester foam, density 0.56 g/cm3.

6 198 A. D AMORE, L. NICOLAIS, AND M. NARKIS Figure 6. Flexural compliance-time master curve obtained by shifting the data of Figure 4 to a reference temperature 20 C. Thermosetting polyester foam, density 0.8 g/cm3. pliance-time curves shown in Figure 3 are replotted in Figure 9 using the Findley s expression where J is the compliance at 60 sec. Typical values of the Findley s equation parameters are given in Table 1 for the 0.56 g/cm foam. Values of the exponent n are shown to slightly increase with temperature in the range studied. Foam modulus is dependent upon void content and can be predicted for the rigid high-density foams in question by the Kerner equation: Figure 7. Flexural compliance-time curves at various temperatures for the solid thermosetting polyester, density 1.16 g/cm3.

7 Flexural Creep Behavior of High Density Foams 199 Figure 8. Arrhenius plot of the experimental shift factors a(t) obtained from Figures 3, 4, and 7. where Ep is the modulus of composite, Ep is the modulus of polymer and A, B, and (c/>r)eff. are given by Figure 9. Correlation of the creep data with the Findley s equation J - Jo = mtn where Jo = J(60 sec).

8 200 A. D AMORE, L. NICOLAIS, AND M. NARKIS Table 1. Calculated m and n parameters of Findley s equation J - Jo = mtn for Jo = J (60 sec). Foam density 0.56 glcml. where vp is the Poisson ratio of the polymer, Ef is the elastic modulus of filler (practically equal to zero), 0 is the maximum packing fraction for the spherical voids and of is the actual volume fraction of the voids. Values of lip 0.3, (EflEP) 0, and c/>m = = = have been used. Given two polyester foams having two different densities and behaving in a linear manner, their moduli ratio or compliance ratio, at the same temperature and at the same time may be compared to the Kerner s predicted ratio. Thus, for the two foams demonstrated in Table 2, having densities 0.56 and 0.8 g/cm3 the predicted J1/J2 ratio, according to Kerner, is This value is compared in the Table with the experimental ratios showing reasonable agreement. In summary, this paper further verifies that the behavior of rigid high density thermosetting polyester foams is relatively simple. Known rules for solid materials can be used, either straightforwardly, or after their slight modification, to describe the behavior of the polyester foams. Table 2. Experimental compliance ratios (data taken from Figures 5 and 6) for two polyester foams having densities of 0.56 and 0.8 g/cm3. The predicted ratio is 2:18 according to Kerner.

9 Flexural Creep Behavior of High Density Foams 201 ACKNOWLEDGEMENTS Thanks are due to C.N.R., &dquo;progetto Finalizzato Chimica fine e Secondaria&dquo; for partial financial support and to Istituto G. Donegani for fellowship to one of us (A.D.). REFERENCES 1. Benning, J. C., Plastic Foams, Wiley-Interscience, N.Y. (1969). 2. Hilyard, N. C., Mechanics of Cellular Plastics, Appl. Sci. Pub., N.Y. (1982). 3. Masi, P., Nicolais, L., Mazzola, M., and Narkis, M., Polym. Eng. Sci., 24, 469 (1984). 4. Pramuk, D. F., Polym. Eng. Sci., 16, 559 (1976). 5. Findley, W N., SPE J., 16, 57 (1960). 6. Findley, W. N., Proc. Testing for Performance Trans., 30, 87 (1962). 7. Nutting, P., Proc. ASTM, 21, 1162 (1921). 8. Zaslawsky, M., SPE J., 24, 62 (1968). 9. Alperstein, D., Narkis, M., Kenig, S., and Siegmann, A., Polym. Composites, 5, 155 (1984). 10. Kerner, E. H., Proc. Phys. Soc. B., 69, 808 (1956). 11. Masi, P. and Nicolais, L., Technical Report, SNIAL Co. (1982).