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Effect of water absorption on the thermal–mechanical properties of HTPB modified DGEBA-based epoxy systems
ARTICLE IN PRESS
POLYMER TESTING Polymer Testing 26 (2007) 262–267 www.elsevier.com/locate/polytest
Material Properties
Effect of water absorption on the thermal–mechanical properties of HTPB modified DGEBA-based epoxy systems N.G. Berrya,b, J.R.M. d’Almeidab,, F.L. Barciac, B.G. Soaresc a
Centro de Pesquisa e Desenvolvimento Leopoldo Ame´rico M. de Mello (CENPES), Petrobras, Cidade Universita´ria, Q.7, Ilha do Funda˜o, 21941-598, Rio de Janeiro, RJ, Brazil b Departamento de Cieˆncia dos Materiais e Metalurgia, Pontifı´cia Universidade Cato´lica do Rio de Janeiro, Rua Marqueˆs de Sa˜o Vicente 225, Ga´vea, 22453-900, Rio de Janeiro, RJ, Brazil c Instituto de Macromole´culas, Universidade Federal do Rio de Janeiro, Centro de Tecnologia, Bl. J, Ilha do Funda˜o, 21945-970, Rio de Janeiro, RJ, Brazil Received 14 September 2006; accepted 23 October 2006
Abstract Three DGEBA-based epoxy systems modified by the incorporation of hydroxyl-terminated polybutadiene (HTPB) were aged by water immersion. These systems have different macromolecular structures, and comprise a monophasic system (EPI) and two biphasic systems (EPH and EPA) differing in the size of the elastomeric domains. The diffusion coefficient and the water saturation value were successfully determined using the Fickian diffusion model. Both parameters were larger for the monophasic system. The effect of water absorption on the glass transition temperature was evaluated by DMA. The results show that at the beginning of the immersion process the absorbed water forms strong dipole–dipole interactions, increasing the values of Tg. Plasticization follows for longer immersion times. From the DMA results, it was also observed that the monophasic system has a more rigid structure. The biphasic system with small elastomeric domains (EPA) has a flexible network, indicating that the HTPB domains are chemically linked to the epoxy matrix. The small value of tan-d for the biphasic EPH formulation was attributed to poor elastomeric–matrix interaction. r 2006 Elsevier Ltd. All rights reserved. Keywords: Epoxy resins; HTPB; Toughness; Water absorption; DMA
1. Introduction Nowadays, polymers are widely used in many applications that range from everyday household goods to matrices for advanced composites. Whatever the intended use, the effects posed by the Corresponding author. Tel.: +55 21 3114 1842;
fax: +55 21 3114 1236. E-mail address: [email protected] (J.R.M. d’Almeida). 0142-9418/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2006.10.009
operational conditions on the physical properties of polymeric materials have to be carefully analyzed to establish the proper maintenance procedures and/or to determine the life of the component. One of the many examples in which aging of a polymer is of prime importance is in the gas and oil industries, where polymers and polymer matrix composites are being used both as structural elements or as repairs to oil pipes [1–3]. When used as a repair, adhesion of the composite to the metallic substrate is of primary importance,
ARTICLE IN PRESS N.G. Berry et al. / Polymer Testing 26 (2007) 262–267
and polymers with enhanced toughness are being developed to fulfill this requirement. To increase the toughness of brittle thermoset polymers, like epoxies, different fillers, such as fibers, elastomers and even thermoplastic polymers can be used [4–6]. Another way to improve resin toughness is to introduce flexible segments on the macromolecular network [7,8]. In a recent work, Barcia et al.[9,10] showed that the macromolecular structure of the common diglycidyl ether of bisphenol-A, DGEBA, epoxy resin can be conveniently changed by the incorporation of hydroxyl-terminated polybutadiene, HTPB. The systems analyzed by these authors were a blend of DGEBA and HTPB, and block copolymers developed from the reaction between the epoxy monomer and functionalyzed HTPB. In this last case, two different epoxy systems were developed: one with HTPB functionalyzed with isocyanate groups and the other with HTPB functionalyzed with carboxyl groups. The DGEBA/HTPB blend (EPH) is a biphasic system, with HTPB domains dispersed in the rigid thermoset epoxy matrix [9]. The main advantage of this system is the ease of its preparation. It is, however, possible that HTPB domains could migrate with time, reducing its applicability. The DGEBA system formulated with HTPB functionally modified with carboxyl groups (EPA) is also a biphasic system, but the elastomeric domains obtained have a smaller size than those of the EPH formulation [10]. The epoxy system formulated with HTPB functionally modified with NCO groups (EPI) is a monophasic, translucent material. These epoxy formulations provided better impact properties, and good adhesion behavior to aluminum substrates, than the unmodified epoxy monomer. The stability of these properties with time, particularly, in hot and humid environments is yet to be determined. Therefore, in this work a study was undertaken to determine the water absorption behavior, and the effect of water on the dynamical–mechanical properties of these modified epoxy formulations.
263
agent was a mixture of diethylenetriamine and triethylenetetramine (Shell Chemical Co., EPICURE 3140) with a number of amine groups corresponding to 378 g/equiv. Hydroxyl-terminated liquid polybutadiene (HTPB) (trade name: Liquiflex H, Petroflex Ind. Com. S.A., Brazil) presents a ¯ n Þ of 3000 and number-average molecular weight ðM a hydroxyl number of 0.8 g/mequiv. Carboxyl-terminated polybutadiene (CTPB) was an in-house product prepared by reacting HTPB with maleic anhydride, in a stoichiometric epoxy/ anhydride molar ratio, as reported elsewhere [10]. Isocyanate-terminated polybutadiene (NCOTPB) was another in-house product prepared by reacting HTPB with a small excess (around 10%) of toluene diisocyanate (TDI) related to the amount of OH groups in the HTPB, in the presence of dibutyl tin dilaurate as a catalyst, as described elsewhere [9]. 2.2. Modification and curing procedure of epoxy resin All polymers were prepared from stoichiometric mixtures of the epoxy resin and the hardener. Epoxy modified with HTPB (EPH) was prepared by mixing both components previously degassed for 60 min in a vacuum oven at 80 1C. Then, the hardener was added and gently stirred for about 5 min to ensure proper dispersion of the hardener. The resulting mixture was degassed for 10 min, cast into molds and cured at 100 1C for 120 min. Epoxy networks modified with CTPB (EPA) were prepared by pre-reacting the epoxy resin with 10 wt% CTPB using triphenyl phosphine (0.2 wt%) as a catalyst. The reaction was carried out at 80 1C under nitrogen atmosphere for 24 h. After the pre-reaction, the hardener was added and the mixture was cast into molds and cured at 100 1C for 120 min. Epoxy networks modified with NCOTPB (EPI) were prepared by pre-reacting the epoxy resin with 10 wt% of NCOTPB using dibutyl tin dilaurate as a catalyst, at 80 1C for 120 min. After the pre-reaction, the hardener was added and the curing process was performed as above.
2. Experimental 2.3. Experiments 2.1. Materials The epoxy resin (RE) was a diglycidyl ether of bisphenol A type (Shell Chemical Co., EPON 828) with an epoxide equivalent of 192 g/equiv. The cure
Test specimens 76 mm long, 25 mm wide and 3.2 mm thick, were used for the water absorption tests, following the recommendations of the ASTM D-570 standard of test specimens for sheet materials
ARTICLE IN PRESS N.G. Berry et al. / Polymer Testing 26 (2007) 262–267
264
(item 4.2 of the above cited standard). The methodology used to remove the specimens from the soaking medium and determine the weight gain also followed the procedures recommended by ASTM D-570 standard. The weight of the specimens, before and after the beginning of water absorption, was measured within 70.1 mg, at room temperature (2473 1C). Three specimens were used per epoxy formulation. The water absorption and diffusion process was modeled using the solution of the Fick equation for an infinitely large plate of thickness h, namely [11]: 1 M% 8X 1 ¼1 2 exp½Dð2n þ 1Þ2 p2 t=h2 p n¼0 ð2n þ 1Þ2 M1
(1) where M% is the mass of water absorbed at a time t, MN is the mass of water absorbed at saturation and D is the diffusion coefficient. This equation converges rapidly as t increases. Therefore, one can use the first term of the series as a good approximation, and for values of M%/MNo0.5, i.e. for short times, Eq. (1) can be written as [11] rffiffiffiffiffiffi M % 4 Dt ¼ (2) M1 h p Dynamic mechanical analysis (DMA) was performed to determine the glass transition temperature, Tg, and the loss and storage modulus of the
epoxy systems. Specimens 20 mm long, 10 mm large and 2 mm thick were tested on a Rheometric Scientific MK3 equipment using the three-point bending fixture. The experimental setup of the equipment was: heating rate of 5 1C/min; frequency of 3 Hz; N2 atmosphere. 3. Experimental results and discussion Fig. 1 shows the experimental data for the weight gain vs. time curves. The values reported are the average results from three specimens for each of the modified epoxy systems, as well as for the unmodified RE. As one can see, all materials show an almost linear relationship between the weight gain and the square root of the immersion time at the beginning of the absorption process. This behavior is well described by Eq. (1), showing that the initial stage of water absorption behavior is governed by the Fickian diffusion model. Therefore, the water concentration gradient is the driving force that leads to water absorption in these epoxy systems. The solid lines in Fig. 1 represent the adjusted curves. The values obtained for MN and D are listed in Table 1. From the data obtained, it can be seen that the diffusion coefficient changed when the epoxy monomer was modified with HTPB. The monophasic EPI system presented the smaller value, and the biphasic EPA system has a diffusion coefficient higher than that of the bare epoxy. It can be
5.0 4.5
EPI EPH EPA RE
4.0
Weight gain (%)
3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0
20
40
60
80
100
Time (days1/2) Fig. 1. Weight gain vs. square root of time curve showing the water absorption behavior of the epoxy systems analyzed.
ARTICLE IN PRESS N.G. Berry et al. / Polymer Testing 26 (2007) 262–267 Table 1 Diffusion coefficient (D, mm2/s) and saturation value (MN, %) obtained from the experimental weight gain vs. time curve
Table 2 Variation of Tg with the time of immersion Epoxy system
Epoxy system RE MN (%) D (mm2/s)
EPH
EPA
265
Time of immersion, t (days)
EPI
2.9570.13 3.1270.20 2.4670.06 4.6170.37 1.17 107 9.55 108 1.72 107 5.22 108
highlighted here that the values listed in Table 1 are consistent with the values cited for other epoxy systems [12,13]. As can also be seen in Fig. 1, saturation was not reached during the immersion time used in this work. The values of MN were thus obtained by extrapolation using Eq. (1). The extrapolated values, Table 1, show that the monophasic EPI system will absorb more water than the biphasic systems (i.e., EPH and EPA) and the bare epoxy (RE), which can be detrimental for its use in longterm applications. This result, however, is in accordance to the differences found between the diffusion process of monophasic and biphasic polymers, where one phase absorbs more water than the other. For these latter structures, where phases with different water absorption behavior coexist, MN is expected to be lower since the volume fraction of the absorbing phase is reduced [14]. The values of the glass transition temperature, Tg, obtained from the DMA are shown in Table 2. These values correspond to the peak of the tan d curves. Fig. 2 shows, as an example, the experimental tan d curves for the specimens as fabricated (time of immersion ¼ 0). It can be seen that for the initial stages of immersion the exposure of the epoxy systems to water produced a steady increase of Tg values for all the systems. These results point to the direction that the immersion process is increasing the rigidity of the macromolecular network at the first water absorption stages. Although the plasticizing effect of water, with the consequent decrease in Tg values, is well described in the literature [12,15], some results show that, given the right conditions, the absorbed water can promote strong dipole-dipole interactions, increasing the rigidity of the resin [16,17]. This kind of bond, known as Type II, has a higher activation energy for the desorption process, and is considered as an irreversible bond. The amount of water forming Type II bonds increases with temperature and with the time of immersion
RE EPH EPI EPA
0
45
150oto200
83 75 89 85
88 83 92.7 87
88 83 91.4 92.6
Fig. 2. Tan d curve of the as fabricated epoxy systems.
[16], corroborating the experimental results listed in Table 2. As one can see from the values listed on Table 2, except for EPA, the values of Tg remain almost constant for times of immersion greater than 45 days. This behavior could be indicating that for these epoxy formulations a steady macromolecular structure, fully saturated with dipole–dipole interactions, was obtained and, therefore, after that time plasticization will begin to occur. Table 3 lists the values of tan d as a function of the immersion time. From the values reported, it is apparent that the modifications performed affected the macromolecular structure in different ways. For the as fabricated materials, the monophasic EPI formulation shows a smaller value of tan d than the unmodified epoxy, RE, indicating the development of a more rigid structure. This result corroborates the higher Tg value measured for this system, Table 2. The tan d values of the biphasic EPH and EPA formulations showed different trends. The EPA formulation has the higher tan d value, which can be associated with a more flexible network. This can be an indirect indication that the HTPB domains are chemically linked to the epoxy matrix, and the system had its toughness increased by the
ARTICLE IN PRESS N.G. Berry et al. / Polymer Testing 26 (2007) 262–267
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Table 3 Variation of tan d and storage modulus (MPa) with the immersion time System
RE
EPH
EPA
EPI
Immersion time, t (days)
tan d E0 , 25 1C E0 , 150 1C a
0 0.80 1113 13.7
45 0.62 482 4.9
150a 0.56 420 5.2
0 0.68 897 8.4
45 0.66 226 3.2
150a 0.55 271 3.6
0 0.86 1008 14.6
45 0.64 350 3.8
150a 0.52 310 3.9
0 0.69 1320 18.1
45 0.57 556 8.6
150a 0.47 361 4.8
150oto200.
the variation of E0 with temperature for the four formulations analyzed before immersion, Fig. 3(a), and after 45 days of immersion, Fig. 3(b). In agreement with the variation observed for tan d, the EPI formulation has the higher value of E0 and EPH the lowest. The data in Table 3 also show that all formulations were strongly affected by immersion in distilled water, with the storage modulus both at 25 1C and at 150 1C showing a very sharp decrease with immersion time. 4. Conclusions
Fig. 3. Storage modulus vs. temperature curves: (a) as-fabricated samples, (b) after 45 days of immersion.
introduction of these elastomeric domains. For the physically blended EPH formulation, the small value of tan d could reflect the lack of interaction between the elastomeric HTPB domains and the epoxy matrix [18]. After immersion, the values of tan d were consistently lowered for all formulations, pointing to a decrease of matrix mobility with immersion time, which agrees with the results of Tg and the correlated increase of the rigidity due to the formation of strong dipole–dipole interactions [16]. The values of the storage modulus, E0 , at 25 1C and 150 1C are also listed in Table 3. Fig. 3 shows
The water uptake behavior of the DGEBA-based epoxy resins was affected by the microstructure developed due to the incorporation of HTPB. The monophasic EPI system, obtained with HTPB functionalyzed with isocyanate groups, showed a higher value for MN, and a lower diffusion coefficient than the biphasic EPH and EPA systems. The water absorption process for all systems could be, however, described by the Fickian diffusion model. At the initial stages of the absorption process the values of the glass transition temperature were increased, which was attributed to the formation of strong dipole–dipole interactions and the consequent increase of the rigidity of the macromolecular networks. For longer periods of time, plasticization began to occur. For the period of time analyzed, however, it was not possible to predict which of the systems was the most affected by water absorption. The dynamic mechanical analysis showed that the monophasic EPI formulation has a more rigid structure than the neat epoxy, indicating a strong interaction of the NCO functionalyzed HTPB with the epoxy matrix. The behavior of the biphasic systems was dependent on the interaction of the HTPB domains with the DGEBA resin. The EPA
ARTICLE IN PRESS N.G. Berry et al. / Polymer Testing 26 (2007) 262–267
formulation, obtained with HTPB functionally modified with carboxyl groups, developed a flexible structure, indicating good interaction between the rigid epoxy matrix and the compliant polybutadiene domains. The result for the blended EPH formulation shows that a weak interaction was developed between the epoxy resin and the elastomeric domains. Acknowledgments The authors acknowledge the Brazilian Agency CNPq and CENPES-Petrobras for the financial and technical support.
[7]
[8]
[9]
[10]
[11]
[12]
References [1] M.J. Cowling, E.M. Smith, S.A. Hashin, I.E. Winkle, Performance of adhesively bonded steel connections for marine structures, in: Proceedings of the EVALMAT 89, Kobe, Japan, Nov 20-23, 1989, p. 827–834. [2] M. Davis, D. Bond, Principles and practices of adhesive bonded structural joints and repairs, Int J Adhesion & Adhesives 19 (1999) 91–105. [3] R.F. Bayer, A.C. Bifulco, New Practices with Composite Material for Pipeline Rehabilitation, in: Proceedings of the Rio Pipeline 2003 Conference, Rio de Janeiro, Brazil, Oct 21-23, 2003, CD-Rom. (in Portuguese). [4] C.B. Bucknall, A.H. Gilbert, Toughening tetrafunctional epoxy resins using polyetherimide, Polymer 30 (1989) 213–217. [5] R.A. Pearson, A.F. Yee, Influence of particle size and particle size distribution on toughening mechanisms in rubber modified epoxies, J.Mater.Sci. 26 (1991) 3828–3844. [6] V.D. Ramos, H.M. Costa, V.L.P. Soares, R.S.V. Nascimento, Modification of epoxy resin: a comparison of
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different types of elastomer, Polym.Testing 24 (2005) 387–394. J.R.M. d’Almeida, S.N. Monteiro, The Influence of the Hardener/Resin Ratio on the impact behavior of an epoxy system, J. Polym. Eng. 18 (1998) 235–248. D. Egan, C.D. Weber, Tough enough—the development and use of reactive liquid polymers as tougheners for epoxy resin, Adhesives Age 43 (1) (2000) 24–29. F.L. Barcia, M.A. Abraha˜o, B.G. Soares, Modification of epoxy resin by isocyanate-terminated polybutadiene, J. Appl. Polym. Sci. 83 (2002) 838–849. F.L. Barcia, T.P. Amaral, B.G. Soares, Synthesis and properties of epoxy resin modified with epoxy-terminated liquid polybutadiene, Polymer 44 (2003) 5811–5819. C.H. Shen, G.S. Springer, Moisture absorption and desorption of composite Materials, J. Comp. Mater. 10 (1976) 2–20. P. Moy, F.E. Karasz, Epoxy-water interactions, Polym.Eng. Sci, 20 (1980) 315–319. C. Maggana, P.J. Pissis, Water sorption and diffusion studies in an epoxy resin system, J. Polym. Sci: Part B – Polym Phys 37 (1999) 1165–1182. P.M. Jacobs, F.R. Jones, Diffusion of moisture into twophase polymers. Part 1—The development of an analytical model and its application to styrene–ethylene/ butylenes-styrene block copolymer, J. Mater. Sci. 24 (1989) 2331–2336. P. Peyser, W.D. Bascom, The anomalous lowering of the glass transition of an epoxy resin by plasticization with water, J. Mater. Sci. 16 (1981) 75–83. J. Zhou, J.P. Lucas, Hygrothermal effects of epoxy resin. Part I: the nature of water in epoxy, Polymer 40 (1999) 5505–5511. J. Zhou, J.P. Lucas, Hygrothermal effects of epoxy resin. Part II: variations of glass transition temperature, Polymer 40 (1999) 5513–5522. P. Bussi, H. Ishida, Composition of the continuous phase in partially miscible blends of epoxy resin and epoxidized rubber by dynamic mechanical analysis, Polymer 35 (1994) 956–966.
POLYMER TESTING Polymer Testing 26 (2007) 262–267 www.elsevier.com/locate/polytest
Material Properties
Effect of water absorption on the thermal–mechanical properties of HTPB modified DGEBA-based epoxy systems N.G. Berrya,b, J.R.M. d’Almeidab,, F.L. Barciac, B.G. Soaresc a
Centro de Pesquisa e Desenvolvimento Leopoldo Ame´rico M. de Mello (CENPES), Petrobras, Cidade Universita´ria, Q.7, Ilha do Funda˜o, 21941-598, Rio de Janeiro, RJ, Brazil b Departamento de Cieˆncia dos Materiais e Metalurgia, Pontifı´cia Universidade Cato´lica do Rio de Janeiro, Rua Marqueˆs de Sa˜o Vicente 225, Ga´vea, 22453-900, Rio de Janeiro, RJ, Brazil c Instituto de Macromole´culas, Universidade Federal do Rio de Janeiro, Centro de Tecnologia, Bl. J, Ilha do Funda˜o, 21945-970, Rio de Janeiro, RJ, Brazil Received 14 September 2006; accepted 23 October 2006
Abstract Three DGEBA-based epoxy systems modified by the incorporation of hydroxyl-terminated polybutadiene (HTPB) were aged by water immersion. These systems have different macromolecular structures, and comprise a monophasic system (EPI) and two biphasic systems (EPH and EPA) differing in the size of the elastomeric domains. The diffusion coefficient and the water saturation value were successfully determined using the Fickian diffusion model. Both parameters were larger for the monophasic system. The effect of water absorption on the glass transition temperature was evaluated by DMA. The results show that at the beginning of the immersion process the absorbed water forms strong dipole–dipole interactions, increasing the values of Tg. Plasticization follows for longer immersion times. From the DMA results, it was also observed that the monophasic system has a more rigid structure. The biphasic system with small elastomeric domains (EPA) has a flexible network, indicating that the HTPB domains are chemically linked to the epoxy matrix. The small value of tan-d for the biphasic EPH formulation was attributed to poor elastomeric–matrix interaction. r 2006 Elsevier Ltd. All rights reserved. Keywords: Epoxy resins; HTPB; Toughness; Water absorption; DMA
1. Introduction Nowadays, polymers are widely used in many applications that range from everyday household goods to matrices for advanced composites. Whatever the intended use, the effects posed by the Corresponding author. Tel.: +55 21 3114 1842;
fax: +55 21 3114 1236. E-mail address: [email protected] (J.R.M. d’Almeida). 0142-9418/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2006.10.009
operational conditions on the physical properties of polymeric materials have to be carefully analyzed to establish the proper maintenance procedures and/or to determine the life of the component. One of the many examples in which aging of a polymer is of prime importance is in the gas and oil industries, where polymers and polymer matrix composites are being used both as structural elements or as repairs to oil pipes [1–3]. When used as a repair, adhesion of the composite to the metallic substrate is of primary importance,
ARTICLE IN PRESS N.G. Berry et al. / Polymer Testing 26 (2007) 262–267
and polymers with enhanced toughness are being developed to fulfill this requirement. To increase the toughness of brittle thermoset polymers, like epoxies, different fillers, such as fibers, elastomers and even thermoplastic polymers can be used [4–6]. Another way to improve resin toughness is to introduce flexible segments on the macromolecular network [7,8]. In a recent work, Barcia et al.[9,10] showed that the macromolecular structure of the common diglycidyl ether of bisphenol-A, DGEBA, epoxy resin can be conveniently changed by the incorporation of hydroxyl-terminated polybutadiene, HTPB. The systems analyzed by these authors were a blend of DGEBA and HTPB, and block copolymers developed from the reaction between the epoxy monomer and functionalyzed HTPB. In this last case, two different epoxy systems were developed: one with HTPB functionalyzed with isocyanate groups and the other with HTPB functionalyzed with carboxyl groups. The DGEBA/HTPB blend (EPH) is a biphasic system, with HTPB domains dispersed in the rigid thermoset epoxy matrix [9]. The main advantage of this system is the ease of its preparation. It is, however, possible that HTPB domains could migrate with time, reducing its applicability. The DGEBA system formulated with HTPB functionally modified with carboxyl groups (EPA) is also a biphasic system, but the elastomeric domains obtained have a smaller size than those of the EPH formulation [10]. The epoxy system formulated with HTPB functionally modified with NCO groups (EPI) is a monophasic, translucent material. These epoxy formulations provided better impact properties, and good adhesion behavior to aluminum substrates, than the unmodified epoxy monomer. The stability of these properties with time, particularly, in hot and humid environments is yet to be determined. Therefore, in this work a study was undertaken to determine the water absorption behavior, and the effect of water on the dynamical–mechanical properties of these modified epoxy formulations.
263
agent was a mixture of diethylenetriamine and triethylenetetramine (Shell Chemical Co., EPICURE 3140) with a number of amine groups corresponding to 378 g/equiv. Hydroxyl-terminated liquid polybutadiene (HTPB) (trade name: Liquiflex H, Petroflex Ind. Com. S.A., Brazil) presents a ¯ n Þ of 3000 and number-average molecular weight ðM a hydroxyl number of 0.8 g/mequiv. Carboxyl-terminated polybutadiene (CTPB) was an in-house product prepared by reacting HTPB with maleic anhydride, in a stoichiometric epoxy/ anhydride molar ratio, as reported elsewhere [10]. Isocyanate-terminated polybutadiene (NCOTPB) was another in-house product prepared by reacting HTPB with a small excess (around 10%) of toluene diisocyanate (TDI) related to the amount of OH groups in the HTPB, in the presence of dibutyl tin dilaurate as a catalyst, as described elsewhere [9]. 2.2. Modification and curing procedure of epoxy resin All polymers were prepared from stoichiometric mixtures of the epoxy resin and the hardener. Epoxy modified with HTPB (EPH) was prepared by mixing both components previously degassed for 60 min in a vacuum oven at 80 1C. Then, the hardener was added and gently stirred for about 5 min to ensure proper dispersion of the hardener. The resulting mixture was degassed for 10 min, cast into molds and cured at 100 1C for 120 min. Epoxy networks modified with CTPB (EPA) were prepared by pre-reacting the epoxy resin with 10 wt% CTPB using triphenyl phosphine (0.2 wt%) as a catalyst. The reaction was carried out at 80 1C under nitrogen atmosphere for 24 h. After the pre-reaction, the hardener was added and the mixture was cast into molds and cured at 100 1C for 120 min. Epoxy networks modified with NCOTPB (EPI) were prepared by pre-reacting the epoxy resin with 10 wt% of NCOTPB using dibutyl tin dilaurate as a catalyst, at 80 1C for 120 min. After the pre-reaction, the hardener was added and the curing process was performed as above.
2. Experimental 2.3. Experiments 2.1. Materials The epoxy resin (RE) was a diglycidyl ether of bisphenol A type (Shell Chemical Co., EPON 828) with an epoxide equivalent of 192 g/equiv. The cure
Test specimens 76 mm long, 25 mm wide and 3.2 mm thick, were used for the water absorption tests, following the recommendations of the ASTM D-570 standard of test specimens for sheet materials
ARTICLE IN PRESS N.G. Berry et al. / Polymer Testing 26 (2007) 262–267
264
(item 4.2 of the above cited standard). The methodology used to remove the specimens from the soaking medium and determine the weight gain also followed the procedures recommended by ASTM D-570 standard. The weight of the specimens, before and after the beginning of water absorption, was measured within 70.1 mg, at room temperature (2473 1C). Three specimens were used per epoxy formulation. The water absorption and diffusion process was modeled using the solution of the Fick equation for an infinitely large plate of thickness h, namely [11]: 1 M% 8X 1 ¼1 2 exp½Dð2n þ 1Þ2 p2 t=h2 p n¼0 ð2n þ 1Þ2 M1
(1) where M% is the mass of water absorbed at a time t, MN is the mass of water absorbed at saturation and D is the diffusion coefficient. This equation converges rapidly as t increases. Therefore, one can use the first term of the series as a good approximation, and for values of M%/MNo0.5, i.e. for short times, Eq. (1) can be written as [11] rffiffiffiffiffiffi M % 4 Dt ¼ (2) M1 h p Dynamic mechanical analysis (DMA) was performed to determine the glass transition temperature, Tg, and the loss and storage modulus of the
epoxy systems. Specimens 20 mm long, 10 mm large and 2 mm thick were tested on a Rheometric Scientific MK3 equipment using the three-point bending fixture. The experimental setup of the equipment was: heating rate of 5 1C/min; frequency of 3 Hz; N2 atmosphere. 3. Experimental results and discussion Fig. 1 shows the experimental data for the weight gain vs. time curves. The values reported are the average results from three specimens for each of the modified epoxy systems, as well as for the unmodified RE. As one can see, all materials show an almost linear relationship between the weight gain and the square root of the immersion time at the beginning of the absorption process. This behavior is well described by Eq. (1), showing that the initial stage of water absorption behavior is governed by the Fickian diffusion model. Therefore, the water concentration gradient is the driving force that leads to water absorption in these epoxy systems. The solid lines in Fig. 1 represent the adjusted curves. The values obtained for MN and D are listed in Table 1. From the data obtained, it can be seen that the diffusion coefficient changed when the epoxy monomer was modified with HTPB. The monophasic EPI system presented the smaller value, and the biphasic EPA system has a diffusion coefficient higher than that of the bare epoxy. It can be
5.0 4.5
EPI EPH EPA RE
4.0
Weight gain (%)
3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0
20
40
60
80
100
Time (days1/2) Fig. 1. Weight gain vs. square root of time curve showing the water absorption behavior of the epoxy systems analyzed.
ARTICLE IN PRESS N.G. Berry et al. / Polymer Testing 26 (2007) 262–267 Table 1 Diffusion coefficient (D, mm2/s) and saturation value (MN, %) obtained from the experimental weight gain vs. time curve
Table 2 Variation of Tg with the time of immersion Epoxy system
Epoxy system RE MN (%) D (mm2/s)
EPH
EPA
265
Time of immersion, t (days)
EPI
2.9570.13 3.1270.20 2.4670.06 4.6170.37 1.17 107 9.55 108 1.72 107 5.22 108
highlighted here that the values listed in Table 1 are consistent with the values cited for other epoxy systems [12,13]. As can also be seen in Fig. 1, saturation was not reached during the immersion time used in this work. The values of MN were thus obtained by extrapolation using Eq. (1). The extrapolated values, Table 1, show that the monophasic EPI system will absorb more water than the biphasic systems (i.e., EPH and EPA) and the bare epoxy (RE), which can be detrimental for its use in longterm applications. This result, however, is in accordance to the differences found between the diffusion process of monophasic and biphasic polymers, where one phase absorbs more water than the other. For these latter structures, where phases with different water absorption behavior coexist, MN is expected to be lower since the volume fraction of the absorbing phase is reduced [14]. The values of the glass transition temperature, Tg, obtained from the DMA are shown in Table 2. These values correspond to the peak of the tan d curves. Fig. 2 shows, as an example, the experimental tan d curves for the specimens as fabricated (time of immersion ¼ 0). It can be seen that for the initial stages of immersion the exposure of the epoxy systems to water produced a steady increase of Tg values for all the systems. These results point to the direction that the immersion process is increasing the rigidity of the macromolecular network at the first water absorption stages. Although the plasticizing effect of water, with the consequent decrease in Tg values, is well described in the literature [12,15], some results show that, given the right conditions, the absorbed water can promote strong dipole-dipole interactions, increasing the rigidity of the resin [16,17]. This kind of bond, known as Type II, has a higher activation energy for the desorption process, and is considered as an irreversible bond. The amount of water forming Type II bonds increases with temperature and with the time of immersion
RE EPH EPI EPA
0
45
150oto200
83 75 89 85
88 83 92.7 87
88 83 91.4 92.6
Fig. 2. Tan d curve of the as fabricated epoxy systems.
[16], corroborating the experimental results listed in Table 2. As one can see from the values listed on Table 2, except for EPA, the values of Tg remain almost constant for times of immersion greater than 45 days. This behavior could be indicating that for these epoxy formulations a steady macromolecular structure, fully saturated with dipole–dipole interactions, was obtained and, therefore, after that time plasticization will begin to occur. Table 3 lists the values of tan d as a function of the immersion time. From the values reported, it is apparent that the modifications performed affected the macromolecular structure in different ways. For the as fabricated materials, the monophasic EPI formulation shows a smaller value of tan d than the unmodified epoxy, RE, indicating the development of a more rigid structure. This result corroborates the higher Tg value measured for this system, Table 2. The tan d values of the biphasic EPH and EPA formulations showed different trends. The EPA formulation has the higher tan d value, which can be associated with a more flexible network. This can be an indirect indication that the HTPB domains are chemically linked to the epoxy matrix, and the system had its toughness increased by the
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266
Table 3 Variation of tan d and storage modulus (MPa) with the immersion time System
RE
EPH
EPA
EPI
Immersion time, t (days)
tan d E0 , 25 1C E0 , 150 1C a
0 0.80 1113 13.7
45 0.62 482 4.9
150a 0.56 420 5.2
0 0.68 897 8.4
45 0.66 226 3.2
150a 0.55 271 3.6
0 0.86 1008 14.6
45 0.64 350 3.8
150a 0.52 310 3.9
0 0.69 1320 18.1
45 0.57 556 8.6
150a 0.47 361 4.8
150oto200.
the variation of E0 with temperature for the four formulations analyzed before immersion, Fig. 3(a), and after 45 days of immersion, Fig. 3(b). In agreement with the variation observed for tan d, the EPI formulation has the higher value of E0 and EPH the lowest. The data in Table 3 also show that all formulations were strongly affected by immersion in distilled water, with the storage modulus both at 25 1C and at 150 1C showing a very sharp decrease with immersion time. 4. Conclusions
Fig. 3. Storage modulus vs. temperature curves: (a) as-fabricated samples, (b) after 45 days of immersion.
introduction of these elastomeric domains. For the physically blended EPH formulation, the small value of tan d could reflect the lack of interaction between the elastomeric HTPB domains and the epoxy matrix [18]. After immersion, the values of tan d were consistently lowered for all formulations, pointing to a decrease of matrix mobility with immersion time, which agrees with the results of Tg and the correlated increase of the rigidity due to the formation of strong dipole–dipole interactions [16]. The values of the storage modulus, E0 , at 25 1C and 150 1C are also listed in Table 3. Fig. 3 shows
The water uptake behavior of the DGEBA-based epoxy resins was affected by the microstructure developed due to the incorporation of HTPB. The monophasic EPI system, obtained with HTPB functionalyzed with isocyanate groups, showed a higher value for MN, and a lower diffusion coefficient than the biphasic EPH and EPA systems. The water absorption process for all systems could be, however, described by the Fickian diffusion model. At the initial stages of the absorption process the values of the glass transition temperature were increased, which was attributed to the formation of strong dipole–dipole interactions and the consequent increase of the rigidity of the macromolecular networks. For longer periods of time, plasticization began to occur. For the period of time analyzed, however, it was not possible to predict which of the systems was the most affected by water absorption. The dynamic mechanical analysis showed that the monophasic EPI formulation has a more rigid structure than the neat epoxy, indicating a strong interaction of the NCO functionalyzed HTPB with the epoxy matrix. The behavior of the biphasic systems was dependent on the interaction of the HTPB domains with the DGEBA resin. The EPA
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formulation, obtained with HTPB functionally modified with carboxyl groups, developed a flexible structure, indicating good interaction between the rigid epoxy matrix and the compliant polybutadiene domains. The result for the blended EPH formulation shows that a weak interaction was developed between the epoxy resin and the elastomeric domains. Acknowledgments The authors acknowledge the Brazilian Agency CNPq and CENPES-Petrobras for the financial and technical support.
[7]
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[11]
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