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Fracture toughness of thermoplastic co-poly (ether ester) elastomer—Acrylonitrile butadiene styrene terpolymer blends
ARTICLE IN PRESS
POLYMER TESTING Polymer Testing 25 (2006) 562–567 www.elsevier.com/locate/polytest
Material Properties
Fracture toughness of thermoplastic co-poly (ether ester) elastomer—Acrylonitrile butadiene styrene terpolymer blends P. Sivaramana, L. Chandrasekhara, V.S. Mishraa, B.C. Chakrabortya,, T.O. Vargheseb a
Naval Materials Research Laboratory, (DRDO), Anand Nagar (PO), Ambernath, Maharashtra 421 506, India b Central Institute of Plastic Engineering and Technology (CIPET), Cherlapally, Hyderabad 500 051, India Received 14 January 2006; accepted 24 March 2006
Abstract Acrylonitrile butadiene styrene terpolymer (ABS) was blended with thermoplastic co-poly (ether ester) elastomer (COPE) in different weight ratios in a twin screw extruder. Fracture toughness of the blends as a function of COPE weight percentage was studied. Fracture toughness was investigated using the J-integral by locus method developed by Kim et al. [B.H. Kim, C.R. Joe, D.M. Otterson, Polym. Test. 8 (1989) 119]. The fracture toughness of the blend with 5 wt% shows higher fracture toughness than pure ABS. The correlation of the topology of the fracture surface with the fracture toughness of the blends was carried out. The physical properties of the blends were studied. The addition of COPE improves the impact property, while flexural modulus shows a slight increase when ABS is blended with COPE at low concentration. The SEM micrographs show three-phase morphology. r 2006 Elsevier Ltd. All rights reserved. Keywords: Polymer blends; Fracture toughness; Mechanical properties; Morphology
1. Introduction Acrylonitrile butadiene styrene terpolymer (ABS) is one of the most important rubber-toughened thermoplastics, which is widely used due to its toughness and good surface gloss. ABS is a blend of styrene–acrylonitrile copolymer and polybutadiene and can be further blended with other polymers to broaden the scope of possible applications. For instance, ABS is blended with PC to improve low temperature impact strength and heat resistance [1,2]. ABS/PVC blends Corresponding author. Tel.: +91 251 2620 608; fax: +91 251 2620 604. E-mail address: [email protected] (P. Sivaraman).
0142-9418/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2006.03.007
have good flame retardency and impact strength [3,4]. ABS/PBT blends have good processability and improved chemical and heat resistance [5,6]. Reports on ABS blended with thermoplastic elastomers are scanty. The chemical and lowtemperature properties are improved when ABS is blended with TPU [7]. In the present work we report the fracture toughness and mechanical properties of ABS/thermoplastic polyether ester copolymer elastomer (COPE). It is expected that properties of ABS with respect to toughness, processability and thermal stability will be enhanced due to the blending with COPE. Linear fracture mechanics that is used for brittle polymers is inadequate for ductile polymers. In
ARTICLE IN PRESS P. Sivaraman et al. / Polymer Testing 25 (2006) 562–567
ductile polymers such as rubber-toughened plastics, there is a considerable amount of energy dissipation ahead of the crack tip due to plastic deformation during crack propagation. Hence, the J-integral concept was introduced by Rice to predict the fracture behaviour of ductile polymers [8]. The J-integral is an energy input which can be used as a design parameter. Many different methodologies are used to determine Jc-integral at crack initiation for ductile polymers. One successful method is the locus method developed by Kim and Joe [9–12]. The technique is based on the locus of crack initiation points on the load–displacement curve by visual observation. This method has been successfully utilized for highly deformable materials like Santoprone [9], Nylon/ABS [10], PP [11], PBT [12], PP/ EPDM blends [13,14], PVC/TPU blends [15] and PVC/NBR/TPU blends [16]. The objective of the present work is to analyze the fracture mechanics of ABS/COPE blends at different weight ratio of COPE using the J-integral by locus method. Fracture topology of the blends is also discussed and the effect of ABS/COPE blend composition on mechanical properties such as flexural and impact strength is analyzed. The correlation of the fracture toughness and mechanical properties with the morphologies of the blend is presented. 2. Experimental 2.1. Materials ABS (Absolac 200EP) used in this study was procured from Bayer ABS Limited, India. Thermoplastic copolyether ester, COPE (Hytrel 4069), was obtained from DuPont, India. ABS and COPE were predried at 80 1C for 4 h in an air-circulating oven before the blend preparation. 2.2. Blend preparation A Berstroff (Germany), twin-screw extruder was used for the melt blending of ABS and COPE. The predried granules of ABS and COPE were mixed mechanically in a bag and fed into the extruder. The processing temperature range was 150–230 1C and the screw speed was 150 rpm. The ABS/COPE blend compositions were 100/0, 97.5/2.5, 95/5, 90/10 and 85/15 and were designated HY0, HY2.5, HY5, HY10 and HY15, respectively. The extrudate was obtained as strands and passed through a waterbath
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and pelletized. Test specimens were injection moulded using a Windsor SP130 injection-moulding machine. The granules were predried at 80 1C before injection moulding to get specimens without defects. 2.3. Characterisation 2.3.1. Mechanical properties The flexural properties of the blends were evaluated using a Hounsfield universal testing machine (Model H50KS). The crosshead speed rate was 3 mm/min. The notched impact strength was tested using a Tinius Olsen 92T (USA) impact tester. Impact specimens were cut from the 6.35mm-thick sheet. Density measurement was carried out according to ISO 1183. 2.3.2. Determination of fracture energy Various methods and specimen configurations are available for the measurement of fracture toughness [17,18]. Here we have used three point bend specimens (width and thickness 6.4 and 12.7 mm, respectively) having varying crack sizes to evaluate the fracture energy in terms of the critical J integral value, Jc, utilizing the crack initiation locus line method [9–12]. The method determines Jc as 1 DU c , (1) B Da where B is the thickness of the specimen, a the initial crack length and Uc is the enclosed area between the loading line and the locus line. Three-point bend tests were performed on a Hounsfield Universal Testing Machine (model H50KS) at a speed of 1 mm/min. The span length of the fixture was kept 50 mm for all studies. The temperature of test was 27 1C and relative humidity was 50%. In preparing the test specimens, the initial crack was made by slotting and a sharp crack was then introduced by pushing a razor blade into the blunt notch. The a/W (initial crack/width) ratio was varied from 0.2 to 0.8. Load–displacement graphs were recorded and crack initiation points were marked on each loading line during the test. The crack tip area was illuminated from behind the three-point bend fixture so that the crack initiation points could be clearly seen. Due to slow crosshead speed, there was ample time to respond to the crack initiation process. Similar to the crack initiation point, maximum load point in the load versus deflection curves can be used to measure fracture toughness using the Jc ¼
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Eq. (1) (R-curves). Here Uc is replaced with UR which is the area enclosed by the locus line of maximum points of the load–displacement curves and the x-axis. For brittle materials, the maximum load point coincides with the crack initiation point, whereas for ductile materials this may not be the case. For ductile materials the maximum point can be easily identified as there is a sharp transition in the crack growth resistance curves (R-curves) [10]. 2.3.3. Scanning electron microscope (SEM) analysis Morphological studies were carried out using a Leo 1477 (UK) SEM. To study the fracture topology under SEM, the fractured surface was coated with gold without etching or staining. For morphological study, low temperature fractured samples were first etched with tetrahydrofuran (THF) and later stained with OsO4 and coated with gold. 3. Results and discussion 3.1. Fracture toughness of the blends Fig. 1. shows the typical load–displacement curves for ABS/COPE blends at different crack lengths. The unfilled circles denote the observed crack initiation points and line L1 denotes the locus of the crack initiation. The shape of the locus is dependant on the specimen length since the total deformation energy in the specimens prior to crack initiation is also dependant on the specimen length. Since the locus method determines Jc by partitioning the essential energy required for crack propaga-
tion, consistent Jc values are obtained regardless of the specimen length. The linear least-square-fitted slope was taken as Jc. The resistance to crack propagation at maximum load can be also used as a fracture-characterising parameter provided that the resistance curve shows a sharp transition between the crack initiation point and the point where plateau resistance begins. In Fig. 1, filled circles denote the observed maximum load for crack growth and line L2 denotes the locus of maximum load points. In the plot of UR/B versus a, linear least-square-fitted slope is taken as the Rmax value. Figs. 2 and 3 show the variation of essential energy needed for crack initiation per unit thickness (Uc/B), and crack growth resistance at maximum load per unit thickness (UR/B) with initial crack length (a), respectively. The result shows that for a given sample thickness the J value at the crack initiation point (Jc) is constant and R values at the maximum crack resistant point (Rmax) are constant. Fig. 4 shows the plot of Jc and Rmax values of the blends versus the concentration of COPE in the blends. It can be seen that the Rmax values are higher than the Jc values. For these blends, the maximum load point occurs well after the observed crack initiation point. Rmax characterises the fracture event associated with a certain amount of crack advancement at the maximum load point. For ductile materials, Rmax is always higher than Jc due to inherent increase in crack resistance associated with the crack growth. Fig. 4 also shows that HY5 blend shows the maximum Jc and Rmax values and, hence, better fracture toughness than the other
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Fig. 2. Crack initiation energy per unit thickness, Uc/B versus crack length, a of ABS/COPE blends.
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Fig. 3. Crack growth resistance at maximum load per unit thickness UR/B versus crack length (a) of ABS/COPE blends.
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0 0
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5
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COPE concentration (wt %) Fig. 4. Jc/Rmax and impact strength of the ABS/COPE blends as a function of COPE content.
blends studied. The addition of COPE increases the toughness of the blends except for blend HY15. The effect of blend composition on Jc and Rmax can be explained on the basis of interactions between the COPE particles. The fracture toughness depends upon the amount of rubber and the distance between the particles (ligament thickness). At low concentration of rubber, the interactions between the particles does not become significant until very large plastic strains have been reached. As the amount of rubber is increased, the rubber particles will begin to interact after a moderate amount of plastic strain. With further deformation, the diameter of the ligament between the rubber particles reduces even more by hole growth, which reduces
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the load carrying capacity. Consequently, the critical value for crack initiation Jc and critical value at maximum load for crack growth, Rmax, decreases by increasing the rubber concentration in the blends [19]. Fig. 5. shows the fracture surfaces for pure ABS and for the blends HY5 and HY15. The fracture surface topology indicates that the failure mode for the unmodified ABS as well as for the blends is by ductile tearing. The samples showed stress whitening which is characteristic of rubber-toughened plastics. When the samples with an initial crack were subjected to load, they initially resist the crack growth, but on increasing the applied load, they undergo spontaneous failure. When the material is tough enough to resist the crack propagation, the materials will stretch over a long distance until it reaches failure. From the SEM micrographs it can be seen clearly that the topology of fracture surface for the blend HY15 consist of holes which were formed during the crack propagation and, hence, it is expected that this blend should exhibit lower fracture toughness than HY5. This result collaborates with the observed values of Jc and Rmax. 3.2. Physical properties of the blends Impact strength of the blends as a function of COPE concentration is shown in Fig. 4. The variation of impact strength with the COPE content showed almost the same trend as that of Jc and Rmax values. Impact results show that all the blends exhibit higher impact strength than pure ABS, with HY5 showing the maximum value. The flexural modulus and strength as a function of COPE concentration is shown in Fig. 6. The strength increases from 0 wt% COPE and the curve shows a maximum around 2.5–5 wt% and then decreases. There is slight increase in modulus for HY2.5 and HY5 blends. The increase in modulus may be due to good interaction between the nitrile group of ABS and the ester group of COPE. Density measurement of the blends also shows that there is an increase in the density of the HY5 blend (Table 1). Increasing the rubber content further results in the modulus of the blends decreasing. At lower concentration of rubber, the interaction between ABS and COPE is more dominant than the effect of addition of low modulus rubber. However, at high concentration of COPE, the modulus factor is more dominant in the blends than the interaction between ABS and COPE.
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Table 1 Flexural modulus, flexural strength and density of ABS/COPE blends
Fig. 5. SEM micrographs of fracture surface of ABS/COPE blends: (a) HY0, (b) HY5, and (c) HY15.
3.3. Morphology The mechanical properties of the blends can be correlated with the morphology of the blends. SEM micrographs of the blends which were fractured at
Blend
Flexural modulus (MPa)
Flexural strength (MPa)
Density (g/cm3)
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2265 2274 2213 1972 1775
52.1 57.3 57.2 53.8 48.8
1.011 1.014 1.041 1.039 1.034
low temperature are shown in Fig. 7. ABS is a terpolymer containing styrene–acrylonitrile random copolymer (SAN) to which polybutadiene is grafted. Polybutadiene is dispersed in the SAN matrix as globular droplets. Hence, it can be seen from Fig. 7a that the pure ABS has a two phase morphology, polybutadiene droplets are dispersed in the SAN matrix. On the other hand, the blends show three-phase morphology (Fig. 7a and b). It can be seen that the COPE phase is also dispersed in the SAN matrix along with polybutadiene. The COPE particles can be seen from the micrographs as holes since they were etched out. The size of the COPE particles in HY5 blend was mostly in the range of 0.5–1 mm, whereas in HY15 blend it was 1–1.5 mm. Moreover, the number particles in HY15 was increased compared to HY5. It can be suggested that the amount of rubber content in ABS may not be sufficient to give maximum
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4. Conclusion Blends of ABS and COPE were made using a twin screw extruder. Fracture toughness of the blends was studied using the J-integral by locus method. The fracture toughness of the blend reached a maximum at 5 wt%. The fracture surface morphology also collaborated with the observed toughness results. The mechanical properties of the blend show a good synergic effect at around 2.5–5 wt% of COPE. SEM analysis shows that COPE droplets also dispersed in the SAN matrix along with PB.
Reference [1] M. Ishikawa, I. Chiba, Polymer 31 (1990) 1232. [2] M.P. Lee, H. Hiltner, B. Baer, Polymer 32 (1992) 675; M.P. Lee, H. Hiltner, B. Baer, Polymer 33 (1992) 685. [3] S.N. Maiti, U.K. Saroop, A. Mishra, Polym. Eng. Sci. 32 (1997) 27. [4] M.N. Sharma, J.S. Anand, A.K. Kulshreshtha, S.F. Xavier, S. Chakrapani, Int. J. Polym. Mater. 12 (1988) 165. [5] W. Hale, H. Keskkula, D.R. Paul, Polymer 40 (1999) 3353. [6] E. Hage, W. Hale, H. Keskkula, D.R. Paul, Polymer 38 (1997) 3237. [7] G. Damma, E. Mastuscelli, A. Zanetti, M. Zorzetto, J. Mater. Sci. 18 (1983) 89. [8] J.R. Rice, J. Appl. Mech. 35 (1968) 379. [9] B.H. Kim, C.R. Joe, Polym. Test. 7 (1987) 355. [10] D.M. Otterson, C.R. Joe, B.H. Kim, Polym. Test. 9 (1990) 219. [11] B.H. Kim, C.R. Joe, D.M. Otterson, Polym. Test. 8 (1989) 119. [12] B.H. Kim, H.S. Kim, J. Mater. Sci. 24 (1989) 921. [13] C.S. Ha, Y. Kim, W.J. Cho, J. Appl. Polym. Sci. 51 (1994) 1381. [14] C.S. Ha, Y. Kim, W.J. Cho, J. Mater. Sci. 31 (1996) 2917. [15] C.S. Ha, Y. Kim, L.K. Lee, W.J. Cho, Y. Kim, Polymer 39 (1998) 4765. [16] N. Manoj, P. Sivaraman, S. Praveen, R.D. Raut, L. Chandrasekhar, B.C. Chakraborty, J. Appl. Polym. Sci. 97 (2005) 1763. [17] ASTM Standard D 6068-96. [18] C.R. Bernal, P.M. Frontini, R. Herrera, Polym. Test. 11 (1992) 271. [19] D.S. Parker, H.J. Sue, J. Huang, A.F. Yee, Polymer 31 (1990) 2267. Fig. 7. SEM micrographs of ABS/COPE blends: (a) HY0, (b) HY5, and (c) HY15.
fracture toughness so that further addition of COPE (up to 5 wt%) in the SAN matrix increases the overall toughness of this particular grade of ABS.
POLYMER TESTING Polymer Testing 25 (2006) 562–567 www.elsevier.com/locate/polytest
Material Properties
Fracture toughness of thermoplastic co-poly (ether ester) elastomer—Acrylonitrile butadiene styrene terpolymer blends P. Sivaramana, L. Chandrasekhara, V.S. Mishraa, B.C. Chakrabortya,, T.O. Vargheseb a
Naval Materials Research Laboratory, (DRDO), Anand Nagar (PO), Ambernath, Maharashtra 421 506, India b Central Institute of Plastic Engineering and Technology (CIPET), Cherlapally, Hyderabad 500 051, India Received 14 January 2006; accepted 24 March 2006
Abstract Acrylonitrile butadiene styrene terpolymer (ABS) was blended with thermoplastic co-poly (ether ester) elastomer (COPE) in different weight ratios in a twin screw extruder. Fracture toughness of the blends as a function of COPE weight percentage was studied. Fracture toughness was investigated using the J-integral by locus method developed by Kim et al. [B.H. Kim, C.R. Joe, D.M. Otterson, Polym. Test. 8 (1989) 119]. The fracture toughness of the blend with 5 wt% shows higher fracture toughness than pure ABS. The correlation of the topology of the fracture surface with the fracture toughness of the blends was carried out. The physical properties of the blends were studied. The addition of COPE improves the impact property, while flexural modulus shows a slight increase when ABS is blended with COPE at low concentration. The SEM micrographs show three-phase morphology. r 2006 Elsevier Ltd. All rights reserved. Keywords: Polymer blends; Fracture toughness; Mechanical properties; Morphology
1. Introduction Acrylonitrile butadiene styrene terpolymer (ABS) is one of the most important rubber-toughened thermoplastics, which is widely used due to its toughness and good surface gloss. ABS is a blend of styrene–acrylonitrile copolymer and polybutadiene and can be further blended with other polymers to broaden the scope of possible applications. For instance, ABS is blended with PC to improve low temperature impact strength and heat resistance [1,2]. ABS/PVC blends Corresponding author. Tel.: +91 251 2620 608; fax: +91 251 2620 604. E-mail address: [email protected] (P. Sivaraman).
0142-9418/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2006.03.007
have good flame retardency and impact strength [3,4]. ABS/PBT blends have good processability and improved chemical and heat resistance [5,6]. Reports on ABS blended with thermoplastic elastomers are scanty. The chemical and lowtemperature properties are improved when ABS is blended with TPU [7]. In the present work we report the fracture toughness and mechanical properties of ABS/thermoplastic polyether ester copolymer elastomer (COPE). It is expected that properties of ABS with respect to toughness, processability and thermal stability will be enhanced due to the blending with COPE. Linear fracture mechanics that is used for brittle polymers is inadequate for ductile polymers. In
ARTICLE IN PRESS P. Sivaraman et al. / Polymer Testing 25 (2006) 562–567
ductile polymers such as rubber-toughened plastics, there is a considerable amount of energy dissipation ahead of the crack tip due to plastic deformation during crack propagation. Hence, the J-integral concept was introduced by Rice to predict the fracture behaviour of ductile polymers [8]. The J-integral is an energy input which can be used as a design parameter. Many different methodologies are used to determine Jc-integral at crack initiation for ductile polymers. One successful method is the locus method developed by Kim and Joe [9–12]. The technique is based on the locus of crack initiation points on the load–displacement curve by visual observation. This method has been successfully utilized for highly deformable materials like Santoprone [9], Nylon/ABS [10], PP [11], PBT [12], PP/ EPDM blends [13,14], PVC/TPU blends [15] and PVC/NBR/TPU blends [16]. The objective of the present work is to analyze the fracture mechanics of ABS/COPE blends at different weight ratio of COPE using the J-integral by locus method. Fracture topology of the blends is also discussed and the effect of ABS/COPE blend composition on mechanical properties such as flexural and impact strength is analyzed. The correlation of the fracture toughness and mechanical properties with the morphologies of the blend is presented. 2. Experimental 2.1. Materials ABS (Absolac 200EP) used in this study was procured from Bayer ABS Limited, India. Thermoplastic copolyether ester, COPE (Hytrel 4069), was obtained from DuPont, India. ABS and COPE were predried at 80 1C for 4 h in an air-circulating oven before the blend preparation. 2.2. Blend preparation A Berstroff (Germany), twin-screw extruder was used for the melt blending of ABS and COPE. The predried granules of ABS and COPE were mixed mechanically in a bag and fed into the extruder. The processing temperature range was 150–230 1C and the screw speed was 150 rpm. The ABS/COPE blend compositions were 100/0, 97.5/2.5, 95/5, 90/10 and 85/15 and were designated HY0, HY2.5, HY5, HY10 and HY15, respectively. The extrudate was obtained as strands and passed through a waterbath
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and pelletized. Test specimens were injection moulded using a Windsor SP130 injection-moulding machine. The granules were predried at 80 1C before injection moulding to get specimens without defects. 2.3. Characterisation 2.3.1. Mechanical properties The flexural properties of the blends were evaluated using a Hounsfield universal testing machine (Model H50KS). The crosshead speed rate was 3 mm/min. The notched impact strength was tested using a Tinius Olsen 92T (USA) impact tester. Impact specimens were cut from the 6.35mm-thick sheet. Density measurement was carried out according to ISO 1183. 2.3.2. Determination of fracture energy Various methods and specimen configurations are available for the measurement of fracture toughness [17,18]. Here we have used three point bend specimens (width and thickness 6.4 and 12.7 mm, respectively) having varying crack sizes to evaluate the fracture energy in terms of the critical J integral value, Jc, utilizing the crack initiation locus line method [9–12]. The method determines Jc as 1 DU c , (1) B Da where B is the thickness of the specimen, a the initial crack length and Uc is the enclosed area between the loading line and the locus line. Three-point bend tests were performed on a Hounsfield Universal Testing Machine (model H50KS) at a speed of 1 mm/min. The span length of the fixture was kept 50 mm for all studies. The temperature of test was 27 1C and relative humidity was 50%. In preparing the test specimens, the initial crack was made by slotting and a sharp crack was then introduced by pushing a razor blade into the blunt notch. The a/W (initial crack/width) ratio was varied from 0.2 to 0.8. Load–displacement graphs were recorded and crack initiation points were marked on each loading line during the test. The crack tip area was illuminated from behind the three-point bend fixture so that the crack initiation points could be clearly seen. Due to slow crosshead speed, there was ample time to respond to the crack initiation process. Similar to the crack initiation point, maximum load point in the load versus deflection curves can be used to measure fracture toughness using the Jc ¼
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Eq. (1) (R-curves). Here Uc is replaced with UR which is the area enclosed by the locus line of maximum points of the load–displacement curves and the x-axis. For brittle materials, the maximum load point coincides with the crack initiation point, whereas for ductile materials this may not be the case. For ductile materials the maximum point can be easily identified as there is a sharp transition in the crack growth resistance curves (R-curves) [10]. 2.3.3. Scanning electron microscope (SEM) analysis Morphological studies were carried out using a Leo 1477 (UK) SEM. To study the fracture topology under SEM, the fractured surface was coated with gold without etching or staining. For morphological study, low temperature fractured samples were first etched with tetrahydrofuran (THF) and later stained with OsO4 and coated with gold. 3. Results and discussion 3.1. Fracture toughness of the blends Fig. 1. shows the typical load–displacement curves for ABS/COPE blends at different crack lengths. The unfilled circles denote the observed crack initiation points and line L1 denotes the locus of the crack initiation. The shape of the locus is dependant on the specimen length since the total deformation energy in the specimens prior to crack initiation is also dependant on the specimen length. Since the locus method determines Jc by partitioning the essential energy required for crack propaga-
tion, consistent Jc values are obtained regardless of the specimen length. The linear least-square-fitted slope was taken as Jc. The resistance to crack propagation at maximum load can be also used as a fracture-characterising parameter provided that the resistance curve shows a sharp transition between the crack initiation point and the point where plateau resistance begins. In Fig. 1, filled circles denote the observed maximum load for crack growth and line L2 denotes the locus of maximum load points. In the plot of UR/B versus a, linear least-square-fitted slope is taken as the Rmax value. Figs. 2 and 3 show the variation of essential energy needed for crack initiation per unit thickness (Uc/B), and crack growth resistance at maximum load per unit thickness (UR/B) with initial crack length (a), respectively. The result shows that for a given sample thickness the J value at the crack initiation point (Jc) is constant and R values at the maximum crack resistant point (Rmax) are constant. Fig. 4 shows the plot of Jc and Rmax values of the blends versus the concentration of COPE in the blends. It can be seen that the Rmax values are higher than the Jc values. For these blends, the maximum load point occurs well after the observed crack initiation point. Rmax characterises the fracture event associated with a certain amount of crack advancement at the maximum load point. For ductile materials, Rmax is always higher than Jc due to inherent increase in crack resistance associated with the crack growth. Fig. 4 also shows that HY5 blend shows the maximum Jc and Rmax values and, hence, better fracture toughness than the other
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Fig. 2. Crack initiation energy per unit thickness, Uc/B versus crack length, a of ABS/COPE blends.
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blends studied. The addition of COPE increases the toughness of the blends except for blend HY15. The effect of blend composition on Jc and Rmax can be explained on the basis of interactions between the COPE particles. The fracture toughness depends upon the amount of rubber and the distance between the particles (ligament thickness). At low concentration of rubber, the interactions between the particles does not become significant until very large plastic strains have been reached. As the amount of rubber is increased, the rubber particles will begin to interact after a moderate amount of plastic strain. With further deformation, the diameter of the ligament between the rubber particles reduces even more by hole growth, which reduces
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the load carrying capacity. Consequently, the critical value for crack initiation Jc and critical value at maximum load for crack growth, Rmax, decreases by increasing the rubber concentration in the blends [19]. Fig. 5. shows the fracture surfaces for pure ABS and for the blends HY5 and HY15. The fracture surface topology indicates that the failure mode for the unmodified ABS as well as for the blends is by ductile tearing. The samples showed stress whitening which is characteristic of rubber-toughened plastics. When the samples with an initial crack were subjected to load, they initially resist the crack growth, but on increasing the applied load, they undergo spontaneous failure. When the material is tough enough to resist the crack propagation, the materials will stretch over a long distance until it reaches failure. From the SEM micrographs it can be seen clearly that the topology of fracture surface for the blend HY15 consist of holes which were formed during the crack propagation and, hence, it is expected that this blend should exhibit lower fracture toughness than HY5. This result collaborates with the observed values of Jc and Rmax. 3.2. Physical properties of the blends Impact strength of the blends as a function of COPE concentration is shown in Fig. 4. The variation of impact strength with the COPE content showed almost the same trend as that of Jc and Rmax values. Impact results show that all the blends exhibit higher impact strength than pure ABS, with HY5 showing the maximum value. The flexural modulus and strength as a function of COPE concentration is shown in Fig. 6. The strength increases from 0 wt% COPE and the curve shows a maximum around 2.5–5 wt% and then decreases. There is slight increase in modulus for HY2.5 and HY5 blends. The increase in modulus may be due to good interaction between the nitrile group of ABS and the ester group of COPE. Density measurement of the blends also shows that there is an increase in the density of the HY5 blend (Table 1). Increasing the rubber content further results in the modulus of the blends decreasing. At lower concentration of rubber, the interaction between ABS and COPE is more dominant than the effect of addition of low modulus rubber. However, at high concentration of COPE, the modulus factor is more dominant in the blends than the interaction between ABS and COPE.
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60
2500
55
2000
50
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45
Flexural Modulus
Flexural strength (MPa)
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Flexural modulus (MPa)
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Flexural strength 40
1000 0
2.5
5
7.5
10
12.5
15
COPE concentration (wt %) Fig. 6. Flexural modulus and strength of ABS/COPE blends.
Table 1 Flexural modulus, flexural strength and density of ABS/COPE blends
Fig. 5. SEM micrographs of fracture surface of ABS/COPE blends: (a) HY0, (b) HY5, and (c) HY15.
3.3. Morphology The mechanical properties of the blends can be correlated with the morphology of the blends. SEM micrographs of the blends which were fractured at
Blend
Flexural modulus (MPa)
Flexural strength (MPa)
Density (g/cm3)
HY0 HY2.5 HY5 HY10 Hy15
2265 2274 2213 1972 1775
52.1 57.3 57.2 53.8 48.8
1.011 1.014 1.041 1.039 1.034
low temperature are shown in Fig. 7. ABS is a terpolymer containing styrene–acrylonitrile random copolymer (SAN) to which polybutadiene is grafted. Polybutadiene is dispersed in the SAN matrix as globular droplets. Hence, it can be seen from Fig. 7a that the pure ABS has a two phase morphology, polybutadiene droplets are dispersed in the SAN matrix. On the other hand, the blends show three-phase morphology (Fig. 7a and b). It can be seen that the COPE phase is also dispersed in the SAN matrix along with polybutadiene. The COPE particles can be seen from the micrographs as holes since they were etched out. The size of the COPE particles in HY5 blend was mostly in the range of 0.5–1 mm, whereas in HY15 blend it was 1–1.5 mm. Moreover, the number particles in HY15 was increased compared to HY5. It can be suggested that the amount of rubber content in ABS may not be sufficient to give maximum
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4. Conclusion Blends of ABS and COPE were made using a twin screw extruder. Fracture toughness of the blends was studied using the J-integral by locus method. The fracture toughness of the blend reached a maximum at 5 wt%. The fracture surface morphology also collaborated with the observed toughness results. The mechanical properties of the blend show a good synergic effect at around 2.5–5 wt% of COPE. SEM analysis shows that COPE droplets also dispersed in the SAN matrix along with PB.
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fracture toughness so that further addition of COPE (up to 5 wt%) in the SAN matrix increases the overall toughness of this particular grade of ABS.