Crack resistance test of epoxy resins under thermal shock

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1 Polymer Testing 21 (2002) Test Method Crack resistance test of epoxy resins under thermal shock Shengguo Wang a,*, Qingxiu Li 1, Wei Zhang a, Hongwei Zhou a a Department of Macromolecular Science, Fudan University, Handan Road, Shanghai , People s Republic of China Received 12 April 2001; accepted 20 June 2001 Abstract In this study, a new test method for evaluating the crack resistance of epoxy resins under thermal shock is proposed. A series of epoxy resins of varying toughness are tested by this method. We also study influence of geometry and dimensions of molds on the experimental results. It is shown that crack resistance of epoxy resins under thermal shock can be evaluated by this method. Our study also suggests that this method can be performed conveniently in laboratories and factories so that it can be widely applied in many fields to evaluate the crack resistance of epoxy resins Elsevier Science Ltd. All rights reserved. Keywords: Crack resistance; Epoxy resins; Thermal shock 1. Introduction Epoxy resins are widely used in coating, adhesive, insulating, flooring, laminating and casting applications because of their excellent mechanical and thermal properties such as high strength, elastic modulus and glass transition temperature [1]. However, they show low resistance to crack initiation and propagation under thermal shock. Therefore, it is necessary to perform crack resistance test under thermal shock of epoxy resins before they are used. Although many thermal shock tests have been employed to evaluate the crack resistance of metallic and ceramic materials [2 4], few investigations have been focused on thermosetting resins. In some methods [5,6], an embedded steel ring bonded by a strain gauge was used to evaluate the internal stress of epoxy resins, but it is not a direct way to evaluate the crack resistance of materials under thermal shock by measuring their internal stresses. Crack resistance testing of epoxy resins * Corresponding author. Tel.: ; fax: address: wangshengguo@etang.com (S. Wang). 1 Current address: School of Forestry and Wood Products, Michigan Technological University, Houghton, MI, USA. is undertaken for assessment of them with regard to their suitability for a given application, not for obtaining typical value data. Two kinds of inserts suggested by IEC have been embedded into epoxy resins to perform thermal shock tests. This test method is not sensitive enough to distinguish crack resistance of materials with varying toughness, due to low stress concentration induced by the inserts. Kojima et al. made a thermal shock test machine to estimate thermal shock resistance of epoxy resins [7]. This method has not been widely used due to the special equipment and restrictive experimental conditions. Therefore, it is important to propose a kind of method that can be performed conveniently and reflect the crack resistance of epoxy resins under thermal shock directly in laboratories and factories. In this study, we propose a new test method to evaluate crack resistance of epoxy resins under thermal shock. First, repeated thermal shock tests by the new method were performed on epoxy resins of varying toughness to demonstrate its feasibility. Then, we studied influence of geometry and dimensions of molds on crack resistance test results. 2. The crack resistance test method In this study, we devise a series of molds including inner cores, outer frames and underpans to evaluate the /02/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S (01)

196 S. Wang et al. / Polymer Testing 21 (2002) 195 199 Fig. 1. Molds for crack resistance test under thermal shock. (a) Hexagon inner core. (b) Hexagram inner core. (c) Outer frame.

1, D of hexagram inner cores are 7, 5, 4 cm, respectively. D of hexagram inner core is 7 cm. D of outer frame is 8 cm. Epoxy resins are cast into the space between the inner core and the outer frame.

2 196 S. Wang et al. / Polymer Testing 21 (2002) Fig. 1. Molds for crack resistance test under thermal shock. (a) Hexagon inner core. (b) Hexagram inner core. (c) Outer frame. (d) Photo of assembly of hexagram inner core and outer frame. crack resistance of epoxy resins under thermal shock. The geometry of the molds is shown in Fig. 1, D of hexagram inner cores are 7, 5, 4 cm, respectively. D of hexagram inner core is 7 cm. D of outer frame is 8 cm. Epoxy resins are cast into the space between the inner core and the outer frame. Thickness of the specimen is 5 cm. If the specimen is too thin, it will easily crack, and if the specimen is too thick the changing temperature conditions in the specimen will be different. The groove shown in Fig. 1 is used for keeping the same thickness of specimens. Thermal stresses will be set up due to the differences in the thermal expansion (or contraction) of epoxy resins and steel molds. The stresses loaded at the tip of the epoxy specimen constrained by hexagon inner core and outer frame are shown in Fig. 2. These three stresses cause the specimen crack at their tip. This kind of loading mode belongs to the cleavage or tensile-opening mode [8]. Thermal stresses can be expressed as follows: s a E T (1) Where a E T is difference of thermal expansion coefficient between epoxy resin and steel is modulus of epoxy resin is the change of temperature Fig. 2. Stresses loaded at the tip of epoxy specimen tested by hexagram inner core. From Eq. (1), we can see that a is determined by. Due to crack initiation and propagation relating to a, changing temperature of epoxy resins could control the cracking. This is the principle of the method. In this study, the thermal shock temperatures are 50 C and 18 C, which can be attained easily in many laboratories and factories.

3 S. Wang et al. / Polymer Testing 21 (2002) Materials and experiment procedures 3.1. Materials Molding epoxy compounds used for this study are composed of epoxy resin, curing agent, accelerator and toughening agents, The epoxy resin is diglycidylether of bisphenol A with a trade mark name DER331. The curing agent is methyl hexahydrophalic (MeHHPA) and the accelerator is benzyl dimethylamine (BDMA).The toughening agents are dibutyl phthalate (DBP), C glycidyl ether (Epodil748), poly ethylene glycol (PEG), carboxyl-terminated poly (butadiene-acrylonitrile) copolymers (CTBN), ZR rubber (a commercial rubber purchased from Tsinghua university), respectively. The neat epoxy resin compounds without modification are tested as control systems. The neat epoxy compounds are fabricated with 88 parts of curing agent, 0.14 parts of accelerator per 100 parts of resin by weight, while the modified epoxy compounds have added toughening agents Sample preparation The molds including outer frames and inner cores are cleaned and mopped with acetone, then put into an oven at 80 C for 45 min. The molds are taken out, coated with mold release agent, constrained with bolts and put into the oven at 140 C for 2 h to make the release agent form a membrane on the molds. After that the molds are taken out of the oven and cooled to a lower temperature about 90 C. Epoxy resin compounds are cast into the space between the inner core and outer frame and the epoxy specimens in molds put into the oven for curing. The curing procedure is as follows: 3 h at 90 C, 2 h at 110 C, 1 h at 130 C and then 12 h at 140 C Crack resistance test under thermal shock The epoxy resin samples in molds are taken out of the oven at 140 C and immediately flushed with water at 0 C for 3 min. They are then put into a refrigerator with a temperature of 18 C. After 3 h, we take the molds out and observe whether the samples crack or not. If a specimen cracks it is not subjected to the thermal shock cycle. The rest of the specimens are put into the oven at 50 C for 2 h. Then they are taken out and examined for whether or not they have cracked. A thermal shock test cycle includes 18 C for 3 h and 50 C for 2 h. The number of specimens for each epoxy compound is 5, if a specimen cracks the percent of crack is 20%, and so on. Thermal shock testing continues until all the specimens crack. 4. Results and discussion 4.1. The feasibility of the test method The crack resistance of basic epoxy compounds without modification and epoxy compounds containing 10 parts CTBN, PEG, Epodil 748, DBP by weight per 100 parts epoxy resin under thermal shock is shown in Fig. 3. We can see that the more to the right the crack line lies, the better is its crack resistance. The effect of toughing agents on crack resistance is as follows: CTBN PEG Epodil748 DBP 0, which can be explained by their toughening mechanisms. The increase in crack resistance of CTBN modified epoxy resins is the best because of the capability of the added elastomer to separate out as a discrete phase while curing. This can arrest crack propagation to a great extent when the materials undergoes fracture by disturbing the applied stress over the whole bonded area [9]. Hydroxyl groups on both ends of PEG react with epoxy resin so that PEG incorporates into networks by forming a block structure and increases the flexibility and mobility of networks, which makes it easier to dissipate stresses loaded on the specimens. Epodil 748 is a kind of reactive diluent or toughening agent, containing only one epoxide group capable of reacting with the curing agent. It reduces the functionality and crosslink density of the system and, therefore, improves the crack resistance of epoxy resins. DBP is a kind of non-reactive diluent or toughening agent, incapable of reacting with epoxy or curing agent. It only reduce viscosity of systems and has little effect on improving crack resistance. In this study, we also investigated the influence of CTBN amounts on crack resistance of epoxy resins under thermal shock. The amounts of CTBN were 1, 5, 10, 15, 20, 30 parts by weight per 100 parts epoxy resin. The results are shown in Fig. 4, where it is apparent that the higher the amount of CTBN the better the crack resistance. This can be explained by the definition of crack resistance. A material s crack resistance is defined by its ratio of toughness to yield stress, K IC /s y. The higher the CTBN amount, the higher the toughness and the lower the yield stress [10]; hence, the higher crack Fig. 3. Crack resistance of neat and toughened epoxy resins.

4 198 S. Wang et al. / Polymer Testing 21 (2002) Fig. 4. Crack resistance of epoxy resins toughened by varying amounts of CTBN. Fig. 5. Crack resistance of epoxy resin tested by varying dimensions of hexagram inner cores. resistance of epoxy resins. However, crack resistance of epoxy resins toughened by 5 and 10 parts CTBN is the same. This may be due to insufficient number of test specimens which could be solved by changing from 5 to 10 test specimens. From the results obtained, we can see that this kind of method can be used conveniently to evaluate crack resistance of epoxy resins with varying toughness under thermal shock Influence of geometry and dimensions of molds on experimental results There are two kinds of inner cores used in this study: hexagon and hexagram, the angles at the tips of the hexagon are twice that of the hexagram. Thermal shock test results for epoxy compounds which are formulated by 90 parts of MeHHPA, 10 parts of ZR rubber, 6 parts of PEG400 and 0.14 parts of BDMA per 100 parts of resin, tested with hexagon and hexagram inner cores of the same size are shown in Table 1. The epoxy specimens with the same formulation all fracture within 4 cycles when using hexagram inner core, while none fractures within 4 cycles when using the hexagram inner core. This suggests that the angle of the tip of the inner core greatly influences the results. The influence of the dimensions of the inner cores on experimental results has also been investigated. Crack resistance results of the same epoxy compounds tested Table 1 Crack resistance of epoxy resin with the same formulation tested by hexagram and hexagon inner cores Failure Percent Cycle Hexagon Hexagram 0 20% 0% 1 60% 0% 2 60% 0% 3 80% 0% 4 100% 0% by the same outer frame and different dimensions of inner cores 7, 5 and 4 cm are shown in Fig. 5. We can see that epoxy specimens become difficult to crack with decrease of the inner core s dimension. In the next study, we will investigate the relationship between the results tested by small and large size inner cores. 5. Conclusions As demonstrated by the experimental results, the thermal shock test method proposed by us is suitable for evaluating crack resistance of epoxy resins and other thermosetting resins. This kind of method can be manipulated conveniently and needs no special equipment. Although there are some problems in this method, for example, the number of thermal shock cycles of CTBN toughened epoxy resins is 24 (equal to 5 days) so that the testing process is too long, this method are still practicable in many fields and these problems could be overcome by selecting suitable size and geometry of molds. In the next study, we will continue improving this test method for further and wider applications. References [1] J.C. Salamone, et al., Concise Polymeric Materials Encyclopedia, CRC Press, Boca Raton, FL, [2] A. Weronski, T. Hejwowski, Thermal Fatigue of Metals, Marcel Dekker, New York, [3] L.V. Kravchuk, G.A. Schneider, G.E. Petzow, Thermal Shock and Thermal Fatigue Behavior of Advanced Ceramics, Kluwer, Dordrecht, [4] S. Ishihara, T. Goshima, K. Nomura, T. Yoshimoto, Crack propagation behavior of cermets and cemented carbides under repeated thermal shocks by the improved quench test, J. Mater. Sci. 34 (1999) [5] M. Shimbo, M. Ochi, T. Inamura, M. Inoue, Internal stress of epoxide resin modified with spiro ortho-ester type resin, J. Mater. Sci. 20 (1985) [6] M. Shimbo, M. Ochi, Y. Shigeta, Shrinkage and internal stress during curing of epoxide resins, J. Appl. Polym. Sci. 26 (1981)

5 S. Wang et al. / Polymer Testing 21 (2002) [7] Y. Kojima, T. Ohta, M. Matsushita, M. Takahara, T. Kurauchi, Thermal shock resistance of plastic IC package, J. Appl. Polym. Sci. 41 (1990) [8] A.J. Kinloch, R.J. Young, Fracture Behavior of Polymers, Applied Science Publishers Ltd, Barking, [9] C. Gouri, R. Ramaswamy, K.N. Ninan, Studies on the adhesive properties of solid elastomer-modified novolac epoxy resin, Int. J. Adhesion 20 (2000) [10] Y.Z. Zhang, C. Shen, Study of toughened epoxy resin by using CTBN, Chin. Mater. Engng 5 (1995)

5.1 Introduction. Contents

5.1 Introduction. Contents Contents Chapter 5 GLASS REINFORCED EPOXY 5.1. Introduction 5.2. Experimental 5.3. Results and discussion 5.4. Conclusion 5.1 Introduction The effect of matrix toughening on the mechanical properties of