Fracture Behaviour of Epoxy Resins Modified With Liquid Rubber and Crosslinked Rubber Particles under Mode I Loading

Transcription

1 Fracture Behaviour of Epoxy Resins Modified With Liquid Rubber and Crosslinked Rubber Particles Under Mode I Loading Fracture Behaviour of Epoxy Resins Modified With Liquid Rubber and Crosslinked Rubber Particles under Mode I Loading Makoto Imanaka 1*, Keisuke Ikeda 1, Yoshinobu Nakamura 2, and Masaki Kimoto 3 1 Department of technology education, Osaka University of Education, Kashiwara, Osaka , Japan 2 Department of Applied Chemistry, Osaka Institute of Technology, Asahi, Osaka , Japan 3 Division of Material Technology, Technical Research Institute of Osaka Prefecture, Izumi, Osaka , Japan Received: 7 May 2014, Accepted: 7 August 2014 SUMMARY Three-point bending tests were conducted with two kinds of rubber-modified epoxy resins: a liquid rubber-modified epoxy resin and a nano-elastomer particle-modified one. In most studies on rubber-toughened epoxy resins, fracture toughness has been evaluated by critical stress intensity factor or critical energy release rate, where the crack is assumed to propagate rapidly, and stable crack propagation has not been considered. However, stable crack propagation was observed for the present rubber-modified epoxy resins. To take into account crack propagation, fracture toughness was characterized using the crack-growth resistance curves (R-curve). In the present study, the evolution behaviour of the damage zone near the crack tip was observed using a video-microscope for the two kinds of rubber-modified resins. Furthermore, to investigate microscopic fracture mechanisms, the fracture surfaces were observed by scanning electron microscopy, and side views near the crack tip were also observed by a confocal laser scanning microscopy. Keywords: Three-point bending test, Rubber-toughened epoxy resin, R-curve 1. INTRODUCTION Rubber-modified epoxy resins have been used as structural adhesives among other functions because they exhibit satisfactory static strength and fracture toughness simultaneously. Numerous studies have been conducted on the fracture toughness of rubbermodified epoxy resins 1-3. There are two types of rubber-modified epoxy resin. In liquid rubber-modified epoxy resin the rubber particles are precipitated by phase separation from a homogeneous phase, and rubber particles are well dispersed in the resin. The other type is cross-linked or core shell rubber particles in epoxy resin. In this type the rubber particles are mixed with epoxy resin, aggregation of the particles occurs, and the dispersion state is inferior to that of * To whom all correspondence should be addressed. imanaka@cc.osaka-kyoiku.ac.jp Smithers Information Ltd., 2015 the precipitation type. However, rubber particle-modified resins are regarded as useful because of their excellent characteristics in heatproofing and so on. Recently, it was reported that the toughing mechanisms for liquid rubber-modified epoxy resins differ from those for core-shell rubber particle-modified epoxy resins 4. However, there are few reports on the fracture mechanisms of recently developed nano-elastomer particlemodified resins 5. In this study, three-point bending tests were conducted with two kinds of rubber-modified epoxy resins: one was a liquid rubber-modified epoxy resin and the other was a nano-elastomer particle-modified epoxy resin. In most studies on rubber-toughened epoxy resin fracture toughness has been evaluated by critical stress intensity factor or critical energy release rate, where the crack is assumed to propagate rapidly and stable crack propagation has not been considered. However, stable crack propagation was observed for the present rubber-modified epoxy resins. To take into account crack propagation, fracture toughness was characterized using the crack-growth resistance curve (R-curve). Here, the evolution behaviour of the damage zone near the crack tip was observed using a video-microscope for the two kinds of rubber-modified resins. Furthermore, to investigate microscopic fracture mechanisms, the fracture surfaces were observed using a scanning electron microscope (SEM), and side views of near crack tip were also observed by confocal laser scanning microscopy. 2. Experimental procedures The compositions of the two types of resins used in this study are given in Table 1. One was a carboxy-terminated Polymers & Polymer Composites, Vol. 23, No. 6,

2 Makoto Imanaka, Keisuke Ikeda, Yoshinobu Nakamura, and Masaki Kimoto Table 1. Formation of epoxy resins Composition CTBN-modified resin Epoxy resin Rubber particles (CTBN1300x8) Piperidine XER-modified resin Epoxy resin Rubber particles (XER-91) Piperidine Epoxy resin:epikote 828 (Japan Epoxy Resin Co. Ltd) CTBN (Liquid rubber) (Ube Co. Ltd) XER91 (Crosslinked rubber particles (JSR) Particle size is about 70 nm butadiene-acrylonitrile (CTBN)- modified system, and the other was a cross-linked rubber particle (XER)- dispersed system. The curing agent was piperidine for both rubber-modified systems, and the curing conditions were 20 hours at 393K. Fracture toughness of the resins was measured for single-edge notched bend (SENB) specimens using a three-point bending test as illustrated in Figure 1 whose shape and sizes were determined by ASTM-E A. To introduce the pre-crack, an acute incision was made on the base of the slot of the SENB specimen maintained at 373 K by using a new razor blade (Microtome Knives, T-40, Nippon Microtome Laboratory Co. Ltd. Japan). Damage evolution near the crack tip was observed using a videomicroscope simultaneously with the load-displacement curve, which enabled comparison of crack propagation and damage evolution with the corresponding load-displacement curve. The value of the J-integral for the SENB specimen was calculated by Eq. (1) from the integral intensity of the load-displacement curve on the basis of the method in ASTM-E-99A. (1) (2) where K is the stress intensity factor given by Eq. (2), W and B the width and thickness of the specimen, respectively, a 0 the pre-crack length, P M the maximum load, v the Poisson s ratio, U p the plastic work and α=a 0 /W. In Eq. (1), the first and second terms represent the elastic and plastic components, respectively; the former Figure 1. Shape and sizes of the single-edge notch bending specimens 9.0 g 1.0 g 0.5 g 9.0 g 1.0 g 0.5 g can be obtained for the K value from Eq. (2). To evaluate the latter term, it is necessary to calculate the plastic work, U p which can be obtained by δmax νp = pdν 0 pl where n pl is the plastic part of the load-line displacement curve, and n plmax is the maximum value of n pl. A more detailed calculation method is available elsewhere 6. Furthermore, the fracture surfaces were observed using a SEM, and internal defects from side views of the near the crack tip were also observed by a confocal laser scanning microscopy. 3. Experimental results and discussion Figure 2 shows stress-stain curves under tensile loading for unmodified and rubber-modified resins. As shown in this figure, the unmodified resin exhibited little yielding behaviour. On the other hand, the rubbermodified resins showed a clear yielding behaviour; yield stress for the XERmodified resin was about 20% greater than that for the CTBN-modified resin. Figures 3a and b show the load and displacement curves and the crack growth behaviour for the CTBN- and XER-modified specimens, respectively. In addition, photos of damage evolution near the crack tip corresponding to points A, B and C are also shown. The crack initiated at near the maximum load, and the crack propagated according to the decrease in the load for the both rubber-modified resins, wherein after the peak load the slope of the load-displacement curve for XER-modified resin was lower than that for CTBN-modified resin. Accordingly the crack propagation rate for the XER-modified resin was lower than that for the CTBN-modified resin. Furthermore, the peak load for the XER-modified resin was higher than that for the CTBN-modified resin. As shown in the stress whitening zone at points A, B and C, for the rubbermodified resins the stress whitening 400 Polymers & Polymer Composites, Vol. 23, No. 6, 2015

3 Fracture Behaviour of Epoxy Resins Modified With Liquid Rubber and Crosslinked Rubber Particles Under Mode I Loading zone increased with the crack length. The degree of the whiteness for the XER-modified resin was stronger than that for the CTBN-modified resin. Furthermore, the extent of the stress whitening zone for the XER-modified resin was larger than that for CTBNmodified resin at each point. Figure 2. Stress-strain curves for the dumbbell specimen Figure 4 shows the relationship between the width of stress the whitening zone and crack length. As shown, the width for the CTBNmodified resin reached a constant value at about 1 mm crack length, whereas the width for the XER-modified resin increased with crack length until about 2 mm. This indicated that the increase in the stress whitening zone with crack length for the XER-modified resin was greater than that for the CTBNmodified resin. The relationship between J-value and crack length is shown in Figure 5. In the initial stage of crack propagation the difference in J-value between the XERmodified resin and the CTBN-modified was small. However, the gradient of the R-curve for the XER-modified resin was greater than that for the CTBNmodified resin. Hence, the difference in the R-curve increased with the increase in crack length. This indicated that the crack growth resistance for the XERmodified resin was greater than that for the CTBN-modified resin. This trend corresponded to the crack growth behaviour as shown in Figure 3, and the evolution behaviour of the damage zone as shown in Figure 4. Figure 3. Load and crack length vs. displacement curves Polymers & Polymer Composites, Vol. 23, No. 6,

4 Makoto Imanaka, Keisuke Ikeda, Yoshinobu Nakamura, and Masaki Kimoto Figure 4. Width of the process zone vs. crack growth Figure 5. Plots of J-integral against crack growth whereas a wide range of 凹凸 patterns was observed for XER-modified resin. Scanning electron micrographs of uncracked stress whitening area were compared with those of the cracked area to explore the fracture mechanism, the locations of uncracked and cracked stress whitening areas are shown in Figure 7. Similar to Figure 6, the unloaded specimen was cooled in liquid nitrogen and then broken immediately. Scanning electron micrographs of the fracture surfaces are shown in Figures Figure 6. Macroscopic view of fracture surfaces Figure 7. Photo of area near the crack tip Figure 6 shows an example of macroscopic view of the fracture surfaces with XER- and CTBNmodified resins. In addition, the fracture surface was obtained as follows: the SENB specimen was loaded to generate crack growth then unloaded rapidly. To prevent a further increase in the stress whitening zone, the unloaded specimen was cooled in liquid nitrogen and then broken immediately. As shown in this figure, a thin linear fracture pattern was observed for CTBN-modified resin, 402 Polymers & Polymer Composites, Vol. 23, No. 6, 2015

5 Fracture Behaviour of Epoxy Resins Modified With Liquid Rubber and Crosslinked Rubber Particles Under Mode I Loading Figure 8 shows scanning electron micrographs of uncracked and cracked stress whitening areas of CTBNmodified specimens. Figure 8a shows both cracked and uncracked areas. Ridges propagated in a direction inclined by about 45 from the direction of crack propagation in the uncracked area, whereas ridges propagated parallel to the direction of crack propagation in the cracked area. Then as indicated in Figure 8c-f, voids dispersed uniformly for both the uncracked and cracked areas, wherein the appearance ratio and size of the voids for the cracked areas were greater than those for the uncracked areas. These observations agreed with the general trend of fracture mechanisms for rubber-toughened epoxy resins: shear bands formed to connect the voids, which caused plastic deformation of the matrix resin, and increased fracture toughness. Figure 9 also shows scanning electron micrographs of uncracked and cracked areas of XER-modified specimens. Figure 9a shows both cracked and uncracked areas. In Figures 9c and e which are uncracked areas, a block type fracture pattern was observed whose shape and size was distributed uniformly. On the other hand, in the cracked area as shown in Figures 9b, d, and f, a fracture pattern tearing in the direction of Figure 8. Scanning electron micrographs of CTBN-modified resin Figure 9. Scanning electron micrographs of XER-modified resin Polymers & Polymer Composites, Vol. 23, No. 6,

6 Makoto Imanaka, Keisuke Ikeda, Yoshinobu Nakamura, and Masaki Kimoto crack propagation was observed. In addition, the particle size of XERmodified resin was about 70 nm, hence high magnification SEM observations were necessary to investigate the dispersion of the voids. Scanning electron micrographs with 20,000 X magnification in Figure 9e and f show the dispersion of the voids in uncracked and cracked areas, respectively. For CTBN-modified resins the voids were dispersed uniformly, whereas voids were localized in XER-modified resins in both uncracked and cracked areas. The reason why a block-type fracture appeared in uncracked areas may be microcrack propagation to connect the localized voids, which may also be a reason of stress whitening. Then, such a microcrack may be a core of plastic deformation, which may bring about the tearing pattern as shown in Figures 9b, d, and f in the cracked area. This may be one reason for the improvement in fracture toughness in the XER-modified resin. Figure 10 shows scanning electron micrographs with 100,000 X magnification near the localized voids. Compared with uncracked areas, cracked areas were found to have small shallow voids in uncracked areas, whereas in cracked area the voids were large and deep. This may indicate promotion of void growth due to plastic deformation in the cracked areas. to be voids. On the other hand, for XER-modified resin block type defects dispersion around the crack as shown in Figure 11b, such a block type fracture pattern was also observed in the uncracked area in scanning electron micrographs. Hence, the defect pattern shown in Figure 11b are reflected the block type fracture pattern observed by scanning electron micrographs. These observations indicated that for XERmodified resin, block type microcracks were formed from localized voids, where these microcracks may bring about plastic deformation of the matrix phase, which may be one cause of high crack-growth resistance in XERmodified resin. 4. Conclusions Three-point bending tests were conducted with two kinds of rubbermodified epoxy resins: one was a liquid rubber-modified epoxy resin and the other was a nano-elastomer particlemodified epoxy resin. Crack-growth resistance curves (R-curve) and Figure 10. Scanning electron micrographs of XER-modified resin at high magnification Figure 11. Confocal laser scanning micrographs Figure 11 shows the damage state in the internal area from the side of the specimen near the crack tip observed using a confocal laser scanning microscope. Gaps generated by voids or cracks appeared in images produced by this method. Internal defects in these areas are shown, wherein the surface view is also shown. As shown in Figure 11a, for CTBN-modified resin, uniformly sized defects were dispersed around the crack: this dispersion behaviour of the defects resembled those of the voids that were observed using the SEM. Hence, these defects observed by confocal laser scanning microscopy were expected 404 Polymers & Polymer Composites, Vol. 23, No. 6,

7 Fracture Behaviour of Epoxy Resins Modified With Liquid Rubber and Crosslinked Rubber Particles Under Mode I Loading evolution of the damage zone near the crack tip were compared for the two kinds of rubber-modified epoxy resins. Furthermore, to investigate microscopic fracture mechanisms, the fracture surfaces were observed using a SEM, and the damage state in the internal area from side views of the near crack tip were also observed by confocal laser scanning microscopy. 1. The increase in stress-whitening zones with crack length for XERmodified resin was greater than that for the CTBN-modified resin. 2. In the initial stage of crack propagation the difference in J-value between XER-modified resin and CTBN-modified resins was small, however, the gradient in R-curve values for XER-modified resin was greater than that for CTBN-modified resin, hence, the difference in R-curve increased with the crack length. 3. For CTBN-modified resins voids were dispersed uniformly, whereas voids were localized for XERmodified resins. 4. For XER-modified resin, a blocktype fracture pattern was observed in the uncracked area, whereas in the cracked area a fracture pattern tearing in the direction of crack propagation was observed. The reason why the block-type fracture appeared in the un-cracked area may be microcrack propagation to connect the localized voids. Then such a microcrack may be a core factor for plastic deformation. This may be one reason for the improved fracture toughness of XER-modified resins. References 1. Yee A.F., Pearson R.A., J. Mat. Sci. 21, (1986) Pearson R.A., Yee A.F., J. Mat. Sci. 21, (1986) Pearson R.A., Yee A.F., J. Mat. Sci. 24, (1989) Sue H.-J., Meitin E.I.G., Picklman D.M., Bott C.J., Colloid Polym.Sci., 274, (1996) Sue H.-J., Gam K.T., Bestaoui N., Clearfield A., Miyamoto M., Miyake N., Acta Materialia, 52, (2004) ASTM Designation: E a, Standard test method for measurement of fracture toughness Annual Book of ASTM Standards, Section 3, Vol. 03,01,2000. Polymers & Polymer Composites, Vol. 23, No. 6,

8 Makoto Imanaka, Keisuke Ikeda, Yoshinobu Nakamura, and Masaki Kimoto 406 Polymers & Polymer Composites, Vol. 23, No. 6, 2015