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Effect of dimethylpolysiloxane liquid on the cryogenic tensile strength and thermal contraction behavior of epoxy resins
Cryogenics 61 (2014) 63–69
Contents lists available at ScienceDirect
Cryogenics journal homepage: www.elsevier.com/locate/cryogenics
Effect of dimethylpolysiloxane liquid on the cryogenic tensile strength and thermal contraction behavior of epoxy resins Jin Woo Yi, Yu Jin Lee, Sang Bok Lee, Wonoh Lee, Moon Kwang Um ⇑ Composites Research Center, Korea Institute of Materials Science, 797 Changwondaero, Changwon, Gyeongnam, Republic of Korea
a r t i c l e
i n f o
Article history: Received 12 November 2013 Received in revised form 24 January 2014 Accepted 29 January 2014 Available online 8 February 2014 Keywords: Epoxy reins Tensile strength Cryogenic Thermal properties Composites
a b s t r a c t Dimethylpolysiloxane liquid was blended with diglycidyl ether of bisphenol-A epoxy resin including anhydride curing agent to improve the tensile strength of the epoxy resin at 77 K without any increase in its coefficient of thermal expansion (CTE). A bifunctional polymer, silicone-modified epoxy resin (SME), was also added to the mixture as a compatibilizer. The results of UV transmittance for the blend resin showed that the incorporation of the SME could stabilize effectively spherical domains of the siloxane liquid which was immiscible with the epoxy matrix. The tensile strengths of the blend resins at both room temperature and 77 K were measured and SEM analysis for the fractured cross sections was carried out to verify the toughening behavior of the liquid droplets. The results indicated that even small amount of addition of the siloxane liquid (0.05 phr) coupled with SME (20 phr) could enhance the tensile strength at 77 K by 77.6% compared to that of the neat epoxy resin. This improvement is attributed to the fact that the solid and s droplets can disperse the localized stress and interrupt the crack propagation by cavitation mechanism followed by multiple generation of numerous micro-deformation. From the CTE measurement, the siloxane liquid has no influence on the thermal contraction behavior of the blend resin. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Fiber-reinforced polymer composites (FRPC) have been widely utilized in the various applications such as aerospace, leisure sports, automotive industry, and military parts. The FRPC also have been taken into consideration as a promising material in cryogenic applications due to their exceptional mechanical properties even under extreme low-temperature conditions [1–3]. One of critical factors to be considered for the usage in the severe environment is the discrepancy of the thermal contraction between reinforcing fibers and matrix resins. The difference of the contractions which directly correspond to the coefficient of thermal expansion (CTE) can lead to the interfacial failure resulting in deterioration of mechanical properties of the composites, in particular, reinforced with continuous fibers [4]. Another important point is the improvement of low temperature strength or toughness of the matrix resins itself. In general, highly cross-linked thermoset resins are mainly used as a matrix for FRPC. Since the resin has poor resistance to crack formation and propagation due to its brittle behavior at low temperature, it is necessary to increase the strength or toughness at low temperature conditions. Otherwise, the formed
⇑ Corresponding author. Tel.: +82 55 280 3315; fax: +82 55 280 3498. E-mail address: [email protected] (M.K. Um). http://dx.doi.org/10.1016/j.cryogenics.2014.01.014 0011-2275/Ó 2014 Elsevier Ltd. All rights reserved.
cracks throughout the resin may decrease the barrier characteristics as well as the mechanical properties of the composites when they would be used as a composite outer shell of a hydrogen storage tank or structural materials for a liquefied nitrogen gas vessel [5]. Many researchers have reported the results about the improvement of mechanical properties of the resins using organic/inorganic additives and fillers. Inorganic particles of micro or nano size such as SiO2 [6–8], Al2O3 [9,10], ZrW2O8 [11], nanoclay [12– 14], AlN [15] and carbon nanotubes [16] can increase the mechanical properties and simultaneously decrease the CTE of the resins. However, the resins have some problems as matrix for continuous fiber-reinforced composites. At least more than 10 wt% of the fillers should be added into matrix resin to obtain practically positive effect on the CTE and mechanical properties. This high amount increases viscosity of the resin, which can make it difficult to infiltrate the resin into the reinforcement. Since most of the aggregated or undispersed filler are filtered by the reinforcement, the distribution state of the filler throughout the thickness is not uniform. Moreover, some surface treatments to enhance the interfacial adhesion between the resin and the fillers are additionally necessary. Toughened epoxy resins contain, in general, the rubber-type additives such as carboxyl-terminated butadiene acrylonitrile
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(CTBN) [17–19], amine-terminated butadiene acrylonitrile (ATBN) [20], hydroxyl-terminated butadiene acrylonitrile (HTBN) [21], silicone-modified resin [22,23], urethane-modified resin [24], polydimethylsiloxane particle [25], and hyperbranched polymer [26–31]. These materials show better toughness and mechanical properties than neat epoxy resin at the low temperature as well as room temperature condition. Therefore, they have been applied as a potential material and have realized in the cryogenic applications. However, there are still some drawbacks when they are used as matrix resin of fiber-reinforced composites or as additive for the resin. For instance, since the thermal contraction of the resin becomes very high due to the rubber additives [17], the mismatch of the thermal contraction with the reinforced fibers can give rise to delamination of the fiber–matrix interface. Therefore, it is of great practical importance to find out a proper filler or additive for a matrix which is capable of improving the toughness at low temperature without significantly impairing the desirable engineering performance such as the thermal contraction and the composite fabrication process. In this work, in order to improve the properties of matrix resin, we have focused on a dimethyl siloxane based liquid that has a very low Tg (Glass transition temperature) and small viscosity changes with variation in temperature, but incompatible with epoxy resin. Sometimes, the liquid has been compounded with some thermoplastic resins to improve the ambient and low temperature impact strength. The main purpose of this paper is to investigate the effect of the siloxane liquid on the mechanical and thermal behaviors at low temperatures of general epoxy resins. In addition, the toughening mechanism of the organic additive was also investigated.
bles could have any effects on the property of samples. After the degassing, the mixtures were poured into an open rectangular mold and then cured at 120 °C for 120 min. The compositions of the blend epoxy resins were shown in Table 1. Dog bone specimens of the blend resins were machined according to the ASTM standard D638. The tensile properties were evaluated on a universal tensile tester with a cross head speed of 5 mm/ min. The temperature conditions were room temperature (RT) and 77 K respectively. In particular, the low temperature was achieved by spraying liquid nitrogen in a cryostat designed and manufactured in our laboratory. Data were taken from an average of at least five specimens. Coefficient of thermal expansion (CTE) of the blend resins was measure by the reported method [32]. The equipment for the cryogenic CTE consists of a temperature controller, a temperature monitor, and a temperature sensor (DT-670, LakeShore). First, the specimen to which a strain gauge was attached was put in a chamber and the ambient temperature was lowered to 173 K. And then the apparent strain was measured while slowly rising the temperature. The CTE was calculated through calibration with a standard material, titanium silicate. The strain gauge (FCA-5-11-1L) was purchased from Tokyo Sokki Kenkyuio. Analysis of the fracture morphologies of the blend resins was conducted by scanning electron microscopy (SEM). In order to estimate the dispersion state of the siloxane liquid in the epoxy resin, the percent transmittances of the blend resins were measured. The blend resins for the measurement were prepared by stirring mechanically at 500 rpm followed by being degassed for 60 min at 60 °C. These uncured blends were transferred to quartz cells and the transmittance was measured at the wavelength of 500 nm by UV–Vis spectroscopy.
2. Experiments 2.1. Materials A diglycidyl ether of bisphenol-A type epoxy resin (YD-128, Kukdo Chemical Co., South Korea) with an epoxy weight equivalence (Weight in grams of resin containing 1 mol equivalent of epoxide, g/eq) in the range of 184–190 g/eq and a viscosity of 11,500–13,500 mPa s at 25 °C was used as a base resin. The anhydride type curing agent (KBH-1089, Kukdo Chemical Co., South Korea) was mixed with the base resin. Dimethylpolysiloxane based liquid (KF-96) with a viscosity of 1000 mPa s at 25 °C produced by Shin-Etsu Co. was used as an additive for the epoxy resin to enhance the properties. Silicone-modified epoxy resin (KSR-1000, Kukdo Chemical Co., South Korea) with an epoxy weight equivalence of 1100–1300 g/eq was used to increase the compatibility between the epoxy resin and the siloxane liquid. The chemical structures of the resin and the additives are shown in Fig. 1. 2.2. Preparation and characterization of the blend resin In order to well disperse the siloxane liquid into the epoxy resin and control the droplet size, the epoxy blends were prepared by two different dispersing procedures: a mechanical stirrer and a homogenizer. At first, the base resin (YD-128) and the siliconemodified epoxy resin (SME) was pre-mixed and the mixture was agitated vigorously for 20 min at 60 °C by the mechanical stirrer until the mixture became clear. Next, 0.05 parts per hundred resin (phr) of siloxane liquid and 90 phr of curing agent (KBH-1089) were added to the mixture and subsequently it was dispersed by the mechanical stirrer or the homogenizer. The rotational speeds of each method were 500 rpm and 5000 rpm, respectively. The dispersed mixtures were degassed for 60 min at 60 °C under vacuum to remove trapped air during the dispersion because the air bub-
3. Results and discussion 3.1. Effect of SME on the dispersion stability of siloxane liquid It is known that general epoxy resin and siloxane liquid are completely immiscible each other due to the relatively non-polar and inactive characteristic of the liquid with siloxane groups compared to the epoxy resin including hydroxyl and epoxide groups. Therefore, even though they seem to be well dispersed in early stage of the mixing, a dispersed phase, for example the siloxane liquid, eventually coalesces and consequently a phase separation occurs. On the other hand, in order to stabilize the dispersed phase, a bifunctional polymer can reduce the interfacial tension between the continuous and the dispersed phase. With regards to this point, a silicone-modified epoxy resin (SME) has both alkyl siloxane and epoxide groups on the backbone as shown in Fig. 1. Each group on the SME can assist the siloxane liquid to form its spherical domain by placing SME at the interface. A schematic diagram in Fig. 2 depicts a concept of the stabilization. In order to confirm this effect, we compared the percent transmittances of the blend resins containing the siloxane liquid with or without the SME. Fig. 3a shows a picture of the neat epoxy resin and the resin with only 0.05 phr of the siloxane liquid. It is seen that these two resins are almost clear. When transmittance of the neat epoxy resin (Ref 1) was set as a baseline, the percent transmittance of the other was measured at 94.3% indicating that there was no noticeable difference in the transmittance. This result can be explained by the phase separation. Most of the siloxane liquid coalesce and float on the top due to its lower density (0.97 g/cm3) resulting in no change of the transmittance. However, the blend resin containing SME and the siloxane liquid was relatively opaque shown in Fig. 3b. The percent transmittance significantly decreased to 70.4% compared to the clear reference resin only with the silicone-modified epoxy
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Fig. 1. Chemical structures of base resin and additives: (a) Diglycidyl ether of bisphenol-A type epoxy resin, (b) silicone-modified epoxy resin and (c) siloxane liquid.
Table 1 Weight ratios of the materials in the blend epoxy resins. Sample ID
Hundred resin
Neat epoxy SL005 SME20 SME20SL005 M SME20SL005H
Base resin
SME
100 100 80 80 80
– – 20 20 20
Curing agent (phr)
Siloxane liquid (phr)
Dispersion
90 90 90 90 90
– 0.05 – 0.05 0.05
Mechanical stirrer Homogenizer Mechanical stirrer Mechanical stirrer Homogenizer
epoxy resin possesses both siloxane and epoxy groups, it can stabilize effectively the liquid droplets in the epoxy resin.
3.2. Effect of the siloxane liquid on the RT and cryogenic mechanical properties
Fig. 2. Schematic diagram for the stabilization.
resin (Ref 2). This drop is attributed to well-dispersed siloxane liquid droplets which has a different refractive index from that of the epoxy resin. Even after some periods of time, the blend kept the opaque state. This means that since the silicone-modified
The tensile strengths of the blend resins at RT and 77 K were shown in Fig. 4. It is probable that the tensile strengths of the blend resins at RT are almost independent of the addition of SME and siloxane liquid. This is because the additives rarely affect the ductility of the blend resin due to their small content. In general, unmodified epoxy resin exhibits relatively ductile behavior at RT and this can be also seen from the stress–displacement curves of the blend resins (Fig. 5a). The stiffness of the blend resins was same resulting in the insensitivity of their room temperature strengths. Moreover, it is difficult to expect typical plastic energy dissipation of multiphase materials by localized deformations or particle cavitation because the dispersed phase is liquid at RT. On the contrary, the strengths at 77 K dramatically changed. According to the Fig. 5b, the significant difference of the stiffness trend at 77 K as with at RT among the blend resins was not observed indicating that they all showed obviously typical brittle behaviors, but the resultant strengths were not similar. The tensile strength of the neat epoxy resin at 77 K decreased by 28.5% and the strengths of the other resins at 77 K was increased by maximum 23.9% or
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Fig. 3. Comparison of the transmittance of the blend resins: (a) phase separation and (b) stable dispersion.
Fig. 4. Tensile strengths of the blend resins.
maintained compared to those at RT. The drop in the cryogenic tensile strength of the neat resin may be due to formation of invisible defects during the fabrication. Ideally, the tensile strength at 77 K should be higher than at RT owing to the increased modulus. However, from a practical point of view, the resin under the cryogenic temperature becomes more sensitive to unexpected defects at which a macro crack would initiate and finally the resin would be fractured. Therefore, it is inferred that the strength at 77 K
strongly depends on how effectively the resin can resist the crack propagation. In order to verify the reason of the toughening of the blend resins, the surface morphologies of the samples after being tested at 77 K were investigated. Fig. 6 shows the fracture surfaces of the cross sections. As shown in Fig. 6a, a crack initiation point was observed at the sample and then the cracks propagated through the smooth fracture surface. In particular, the fractured surfaces spread to the edge of the specimen with a radial form. This indicates the rapid progress of the fracture and consequently poor resistance to the crack growth. In case of the resin with 20 phr of SME (Fig. 6b), the radial pattern was also observed. However, it is seen that since the crack propagation was obstructed resulting in relatively spherical type ridges, the radial tip did not reach the edge. This morphology which probably originates from the higher molecular weight of the SME is expected to provide slight increase in the strength. Fig. 7 shows the comparison on the fracture morphologies of the blend resin with different droplet sizes which were controlled by a dispersing method. A homogenizer can crush the secondary phase into smaller size than a mechanical stirrer due to the high speed rpm and the narrow gap between the stationary part and the moving part. Therefore, more population and smaller size (5 lm) of the droplets were observed throughout the fracture surface of the resin mixed by the homogenizer. This resin showed the highest tensile strength at 77 K. From the SEM images shown in
Fig. 5. Representative stress–displacement curves of the blend resins at (a) RT and (b) 77 K.
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Fig. 6. SEM images of the fracture surfaces of the samples after being tested at 77 K: (a) neat epoxy and (b) SME20.
Fig. 7. SEM images of the fracture surfaces of SME20SL005H after being tested at 77 K: (a) entire sample, (b–c) magnified by 300 times and (d–e) magnified by 2000times.
Fig. 7a, any long radial fracture pattern was not observed and many cavitied droplets were uniformly distributed (Fig. 7b and c). Furthermore, there were several short furrows running out of the cavitied droplet (Fig. 7d and e). Possibly, those morphologies may be
the proof of crack energy absorption followed by the interruption of the propagation. This can accelerate multiple generations of numerous micro-deformations. As shown in Fig. 8b and c, relatively large (10–20 lm) and less populated droplets were
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Fig. 8. SEM images of the fracture surfaces of SME20SL005 M after being tested at 77 K: (a) entire sample and (b–c) magnified by 300 times.
distributed on the resin mixed by the mechanical stirrer. The cryogenic tensile strength decreased and the fracture surface was relatively smooth compared to that of the resin including smaller droplets (Fig. 8a). From the images, it is inferred that such large droplets more than 10–20 lm in diameter act as a pore rather than a toughening domain. 3.3. Thermal contraction behaviors of the blend resins When FRPC is exposed to cryogenic temperatures, internal stresses are generated because of the mismatch of the CTEs between fibers and matrix, which can result in the interfacial detach. Some research papers reported that interfacial adhesion between fillers and matrix resin at a low temperature would rather become stronger than at RT due to the thermal shrinkage induced by the big differences in the CTEs [10,12,16]. When a composite is reinforced with the filler that is discontinuous and dispersed as isolated phase in the matrix and consequently they can shrink isotropically, the interfacial failure could not be observed. However, a different behavior for the shrinkage is expected when the filler is a continuous fiber, for instance, a textile preform fabricated by the continuous fibers or the filler is a continuous carbon fiber that has different CTEs in the fiber direction and the thickness direction. Since the thermal contraction of such reinforcements must be restrained in certain direction while the matrix contracts isotropically, the interfacial failure cannot be avoided. From this perspective, toughened epoxy resins which have been considered as effective method have a problem because their CTE is no doubt increased with the addition of rubber additives. Therefore, effect of additives on the CTE is so critical and they should be estimated. Fig. 9 exhibits the CTEs of the blend resins from RT to 173 K. The results indicated that there was no significant difference of the CTE between the neat epoxy resin and the blend resins with SME and siloxane liquid unlike conventional toughened epoxy resins with rubber additives due to the small adding amount.
Fig. 9. CTE of the blend resins.
bifunctional polymer, SME (20 phr). The cryogenic tensile strength of the blend resins have been drastically improved by up to 77.6% compared to that of neat epoxy resin. It is concluded that the liquid droplets in a diameter of 5 lm in the matrix enhanced more effectively the strength than the droplets in 10–20 lm by dispersing the localized stress and interrupting the propagation of cracks through the promoted deformation mechanism such as cavitation of the droplets and multiple generation of numerous micro-deformation. From the CTE results, the siloxane liquid has no detrimental effect on the thermal contraction of the blend resins. Consequently, the blend epoxy resin is promising matrix to be employed in cryogenic applications. Acknowledgement This work was supported by a grant from the basic research program supported by Korea Institute of Materials Science (KIMS).
4. Conclusion A small amount (0.05 phr) of siloxane liquid which was completely immiscible with general epoxy resins was favorably stabilized in the form of a spherical domain in the matrix by adding a
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Contents lists available at ScienceDirect
Cryogenics journal homepage: www.elsevier.com/locate/cryogenics
Effect of dimethylpolysiloxane liquid on the cryogenic tensile strength and thermal contraction behavior of epoxy resins Jin Woo Yi, Yu Jin Lee, Sang Bok Lee, Wonoh Lee, Moon Kwang Um ⇑ Composites Research Center, Korea Institute of Materials Science, 797 Changwondaero, Changwon, Gyeongnam, Republic of Korea
a r t i c l e
i n f o
Article history: Received 12 November 2013 Received in revised form 24 January 2014 Accepted 29 January 2014 Available online 8 February 2014 Keywords: Epoxy reins Tensile strength Cryogenic Thermal properties Composites
a b s t r a c t Dimethylpolysiloxane liquid was blended with diglycidyl ether of bisphenol-A epoxy resin including anhydride curing agent to improve the tensile strength of the epoxy resin at 77 K without any increase in its coefficient of thermal expansion (CTE). A bifunctional polymer, silicone-modified epoxy resin (SME), was also added to the mixture as a compatibilizer. The results of UV transmittance for the blend resin showed that the incorporation of the SME could stabilize effectively spherical domains of the siloxane liquid which was immiscible with the epoxy matrix. The tensile strengths of the blend resins at both room temperature and 77 K were measured and SEM analysis for the fractured cross sections was carried out to verify the toughening behavior of the liquid droplets. The results indicated that even small amount of addition of the siloxane liquid (0.05 phr) coupled with SME (20 phr) could enhance the tensile strength at 77 K by 77.6% compared to that of the neat epoxy resin. This improvement is attributed to the fact that the solid and s droplets can disperse the localized stress and interrupt the crack propagation by cavitation mechanism followed by multiple generation of numerous micro-deformation. From the CTE measurement, the siloxane liquid has no influence on the thermal contraction behavior of the blend resin. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Fiber-reinforced polymer composites (FRPC) have been widely utilized in the various applications such as aerospace, leisure sports, automotive industry, and military parts. The FRPC also have been taken into consideration as a promising material in cryogenic applications due to their exceptional mechanical properties even under extreme low-temperature conditions [1–3]. One of critical factors to be considered for the usage in the severe environment is the discrepancy of the thermal contraction between reinforcing fibers and matrix resins. The difference of the contractions which directly correspond to the coefficient of thermal expansion (CTE) can lead to the interfacial failure resulting in deterioration of mechanical properties of the composites, in particular, reinforced with continuous fibers [4]. Another important point is the improvement of low temperature strength or toughness of the matrix resins itself. In general, highly cross-linked thermoset resins are mainly used as a matrix for FRPC. Since the resin has poor resistance to crack formation and propagation due to its brittle behavior at low temperature, it is necessary to increase the strength or toughness at low temperature conditions. Otherwise, the formed
⇑ Corresponding author. Tel.: +82 55 280 3315; fax: +82 55 280 3498. E-mail address: [email protected] (M.K. Um). http://dx.doi.org/10.1016/j.cryogenics.2014.01.014 0011-2275/Ó 2014 Elsevier Ltd. All rights reserved.
cracks throughout the resin may decrease the barrier characteristics as well as the mechanical properties of the composites when they would be used as a composite outer shell of a hydrogen storage tank or structural materials for a liquefied nitrogen gas vessel [5]. Many researchers have reported the results about the improvement of mechanical properties of the resins using organic/inorganic additives and fillers. Inorganic particles of micro or nano size such as SiO2 [6–8], Al2O3 [9,10], ZrW2O8 [11], nanoclay [12– 14], AlN [15] and carbon nanotubes [16] can increase the mechanical properties and simultaneously decrease the CTE of the resins. However, the resins have some problems as matrix for continuous fiber-reinforced composites. At least more than 10 wt% of the fillers should be added into matrix resin to obtain practically positive effect on the CTE and mechanical properties. This high amount increases viscosity of the resin, which can make it difficult to infiltrate the resin into the reinforcement. Since most of the aggregated or undispersed filler are filtered by the reinforcement, the distribution state of the filler throughout the thickness is not uniform. Moreover, some surface treatments to enhance the interfacial adhesion between the resin and the fillers are additionally necessary. Toughened epoxy resins contain, in general, the rubber-type additives such as carboxyl-terminated butadiene acrylonitrile
64
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(CTBN) [17–19], amine-terminated butadiene acrylonitrile (ATBN) [20], hydroxyl-terminated butadiene acrylonitrile (HTBN) [21], silicone-modified resin [22,23], urethane-modified resin [24], polydimethylsiloxane particle [25], and hyperbranched polymer [26–31]. These materials show better toughness and mechanical properties than neat epoxy resin at the low temperature as well as room temperature condition. Therefore, they have been applied as a potential material and have realized in the cryogenic applications. However, there are still some drawbacks when they are used as matrix resin of fiber-reinforced composites or as additive for the resin. For instance, since the thermal contraction of the resin becomes very high due to the rubber additives [17], the mismatch of the thermal contraction with the reinforced fibers can give rise to delamination of the fiber–matrix interface. Therefore, it is of great practical importance to find out a proper filler or additive for a matrix which is capable of improving the toughness at low temperature without significantly impairing the desirable engineering performance such as the thermal contraction and the composite fabrication process. In this work, in order to improve the properties of matrix resin, we have focused on a dimethyl siloxane based liquid that has a very low Tg (Glass transition temperature) and small viscosity changes with variation in temperature, but incompatible with epoxy resin. Sometimes, the liquid has been compounded with some thermoplastic resins to improve the ambient and low temperature impact strength. The main purpose of this paper is to investigate the effect of the siloxane liquid on the mechanical and thermal behaviors at low temperatures of general epoxy resins. In addition, the toughening mechanism of the organic additive was also investigated.
bles could have any effects on the property of samples. After the degassing, the mixtures were poured into an open rectangular mold and then cured at 120 °C for 120 min. The compositions of the blend epoxy resins were shown in Table 1. Dog bone specimens of the blend resins were machined according to the ASTM standard D638. The tensile properties were evaluated on a universal tensile tester with a cross head speed of 5 mm/ min. The temperature conditions were room temperature (RT) and 77 K respectively. In particular, the low temperature was achieved by spraying liquid nitrogen in a cryostat designed and manufactured in our laboratory. Data were taken from an average of at least five specimens. Coefficient of thermal expansion (CTE) of the blend resins was measure by the reported method [32]. The equipment for the cryogenic CTE consists of a temperature controller, a temperature monitor, and a temperature sensor (DT-670, LakeShore). First, the specimen to which a strain gauge was attached was put in a chamber and the ambient temperature was lowered to 173 K. And then the apparent strain was measured while slowly rising the temperature. The CTE was calculated through calibration with a standard material, titanium silicate. The strain gauge (FCA-5-11-1L) was purchased from Tokyo Sokki Kenkyuio. Analysis of the fracture morphologies of the blend resins was conducted by scanning electron microscopy (SEM). In order to estimate the dispersion state of the siloxane liquid in the epoxy resin, the percent transmittances of the blend resins were measured. The blend resins for the measurement were prepared by stirring mechanically at 500 rpm followed by being degassed for 60 min at 60 °C. These uncured blends were transferred to quartz cells and the transmittance was measured at the wavelength of 500 nm by UV–Vis spectroscopy.
2. Experiments 2.1. Materials A diglycidyl ether of bisphenol-A type epoxy resin (YD-128, Kukdo Chemical Co., South Korea) with an epoxy weight equivalence (Weight in grams of resin containing 1 mol equivalent of epoxide, g/eq) in the range of 184–190 g/eq and a viscosity of 11,500–13,500 mPa s at 25 °C was used as a base resin. The anhydride type curing agent (KBH-1089, Kukdo Chemical Co., South Korea) was mixed with the base resin. Dimethylpolysiloxane based liquid (KF-96) with a viscosity of 1000 mPa s at 25 °C produced by Shin-Etsu Co. was used as an additive for the epoxy resin to enhance the properties. Silicone-modified epoxy resin (KSR-1000, Kukdo Chemical Co., South Korea) with an epoxy weight equivalence of 1100–1300 g/eq was used to increase the compatibility between the epoxy resin and the siloxane liquid. The chemical structures of the resin and the additives are shown in Fig. 1. 2.2. Preparation and characterization of the blend resin In order to well disperse the siloxane liquid into the epoxy resin and control the droplet size, the epoxy blends were prepared by two different dispersing procedures: a mechanical stirrer and a homogenizer. At first, the base resin (YD-128) and the siliconemodified epoxy resin (SME) was pre-mixed and the mixture was agitated vigorously for 20 min at 60 °C by the mechanical stirrer until the mixture became clear. Next, 0.05 parts per hundred resin (phr) of siloxane liquid and 90 phr of curing agent (KBH-1089) were added to the mixture and subsequently it was dispersed by the mechanical stirrer or the homogenizer. The rotational speeds of each method were 500 rpm and 5000 rpm, respectively. The dispersed mixtures were degassed for 60 min at 60 °C under vacuum to remove trapped air during the dispersion because the air bub-
3. Results and discussion 3.1. Effect of SME on the dispersion stability of siloxane liquid It is known that general epoxy resin and siloxane liquid are completely immiscible each other due to the relatively non-polar and inactive characteristic of the liquid with siloxane groups compared to the epoxy resin including hydroxyl and epoxide groups. Therefore, even though they seem to be well dispersed in early stage of the mixing, a dispersed phase, for example the siloxane liquid, eventually coalesces and consequently a phase separation occurs. On the other hand, in order to stabilize the dispersed phase, a bifunctional polymer can reduce the interfacial tension between the continuous and the dispersed phase. With regards to this point, a silicone-modified epoxy resin (SME) has both alkyl siloxane and epoxide groups on the backbone as shown in Fig. 1. Each group on the SME can assist the siloxane liquid to form its spherical domain by placing SME at the interface. A schematic diagram in Fig. 2 depicts a concept of the stabilization. In order to confirm this effect, we compared the percent transmittances of the blend resins containing the siloxane liquid with or without the SME. Fig. 3a shows a picture of the neat epoxy resin and the resin with only 0.05 phr of the siloxane liquid. It is seen that these two resins are almost clear. When transmittance of the neat epoxy resin (Ref 1) was set as a baseline, the percent transmittance of the other was measured at 94.3% indicating that there was no noticeable difference in the transmittance. This result can be explained by the phase separation. Most of the siloxane liquid coalesce and float on the top due to its lower density (0.97 g/cm3) resulting in no change of the transmittance. However, the blend resin containing SME and the siloxane liquid was relatively opaque shown in Fig. 3b. The percent transmittance significantly decreased to 70.4% compared to the clear reference resin only with the silicone-modified epoxy
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Fig. 1. Chemical structures of base resin and additives: (a) Diglycidyl ether of bisphenol-A type epoxy resin, (b) silicone-modified epoxy resin and (c) siloxane liquid.
Table 1 Weight ratios of the materials in the blend epoxy resins. Sample ID
Hundred resin
Neat epoxy SL005 SME20 SME20SL005 M SME20SL005H
Base resin
SME
100 100 80 80 80
– – 20 20 20
Curing agent (phr)
Siloxane liquid (phr)
Dispersion
90 90 90 90 90
– 0.05 – 0.05 0.05
Mechanical stirrer Homogenizer Mechanical stirrer Mechanical stirrer Homogenizer
epoxy resin possesses both siloxane and epoxy groups, it can stabilize effectively the liquid droplets in the epoxy resin.
3.2. Effect of the siloxane liquid on the RT and cryogenic mechanical properties
Fig. 2. Schematic diagram for the stabilization.
resin (Ref 2). This drop is attributed to well-dispersed siloxane liquid droplets which has a different refractive index from that of the epoxy resin. Even after some periods of time, the blend kept the opaque state. This means that since the silicone-modified
The tensile strengths of the blend resins at RT and 77 K were shown in Fig. 4. It is probable that the tensile strengths of the blend resins at RT are almost independent of the addition of SME and siloxane liquid. This is because the additives rarely affect the ductility of the blend resin due to their small content. In general, unmodified epoxy resin exhibits relatively ductile behavior at RT and this can be also seen from the stress–displacement curves of the blend resins (Fig. 5a). The stiffness of the blend resins was same resulting in the insensitivity of their room temperature strengths. Moreover, it is difficult to expect typical plastic energy dissipation of multiphase materials by localized deformations or particle cavitation because the dispersed phase is liquid at RT. On the contrary, the strengths at 77 K dramatically changed. According to the Fig. 5b, the significant difference of the stiffness trend at 77 K as with at RT among the blend resins was not observed indicating that they all showed obviously typical brittle behaviors, but the resultant strengths were not similar. The tensile strength of the neat epoxy resin at 77 K decreased by 28.5% and the strengths of the other resins at 77 K was increased by maximum 23.9% or
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Fig. 3. Comparison of the transmittance of the blend resins: (a) phase separation and (b) stable dispersion.
Fig. 4. Tensile strengths of the blend resins.
maintained compared to those at RT. The drop in the cryogenic tensile strength of the neat resin may be due to formation of invisible defects during the fabrication. Ideally, the tensile strength at 77 K should be higher than at RT owing to the increased modulus. However, from a practical point of view, the resin under the cryogenic temperature becomes more sensitive to unexpected defects at which a macro crack would initiate and finally the resin would be fractured. Therefore, it is inferred that the strength at 77 K
strongly depends on how effectively the resin can resist the crack propagation. In order to verify the reason of the toughening of the blend resins, the surface morphologies of the samples after being tested at 77 K were investigated. Fig. 6 shows the fracture surfaces of the cross sections. As shown in Fig. 6a, a crack initiation point was observed at the sample and then the cracks propagated through the smooth fracture surface. In particular, the fractured surfaces spread to the edge of the specimen with a radial form. This indicates the rapid progress of the fracture and consequently poor resistance to the crack growth. In case of the resin with 20 phr of SME (Fig. 6b), the radial pattern was also observed. However, it is seen that since the crack propagation was obstructed resulting in relatively spherical type ridges, the radial tip did not reach the edge. This morphology which probably originates from the higher molecular weight of the SME is expected to provide slight increase in the strength. Fig. 7 shows the comparison on the fracture morphologies of the blend resin with different droplet sizes which were controlled by a dispersing method. A homogenizer can crush the secondary phase into smaller size than a mechanical stirrer due to the high speed rpm and the narrow gap between the stationary part and the moving part. Therefore, more population and smaller size (5 lm) of the droplets were observed throughout the fracture surface of the resin mixed by the homogenizer. This resin showed the highest tensile strength at 77 K. From the SEM images shown in
Fig. 5. Representative stress–displacement curves of the blend resins at (a) RT and (b) 77 K.
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Fig. 6. SEM images of the fracture surfaces of the samples after being tested at 77 K: (a) neat epoxy and (b) SME20.
Fig. 7. SEM images of the fracture surfaces of SME20SL005H after being tested at 77 K: (a) entire sample, (b–c) magnified by 300 times and (d–e) magnified by 2000times.
Fig. 7a, any long radial fracture pattern was not observed and many cavitied droplets were uniformly distributed (Fig. 7b and c). Furthermore, there were several short furrows running out of the cavitied droplet (Fig. 7d and e). Possibly, those morphologies may be
the proof of crack energy absorption followed by the interruption of the propagation. This can accelerate multiple generations of numerous micro-deformations. As shown in Fig. 8b and c, relatively large (10–20 lm) and less populated droplets were
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Fig. 8. SEM images of the fracture surfaces of SME20SL005 M after being tested at 77 K: (a) entire sample and (b–c) magnified by 300 times.
distributed on the resin mixed by the mechanical stirrer. The cryogenic tensile strength decreased and the fracture surface was relatively smooth compared to that of the resin including smaller droplets (Fig. 8a). From the images, it is inferred that such large droplets more than 10–20 lm in diameter act as a pore rather than a toughening domain. 3.3. Thermal contraction behaviors of the blend resins When FRPC is exposed to cryogenic temperatures, internal stresses are generated because of the mismatch of the CTEs between fibers and matrix, which can result in the interfacial detach. Some research papers reported that interfacial adhesion between fillers and matrix resin at a low temperature would rather become stronger than at RT due to the thermal shrinkage induced by the big differences in the CTEs [10,12,16]. When a composite is reinforced with the filler that is discontinuous and dispersed as isolated phase in the matrix and consequently they can shrink isotropically, the interfacial failure could not be observed. However, a different behavior for the shrinkage is expected when the filler is a continuous fiber, for instance, a textile preform fabricated by the continuous fibers or the filler is a continuous carbon fiber that has different CTEs in the fiber direction and the thickness direction. Since the thermal contraction of such reinforcements must be restrained in certain direction while the matrix contracts isotropically, the interfacial failure cannot be avoided. From this perspective, toughened epoxy resins which have been considered as effective method have a problem because their CTE is no doubt increased with the addition of rubber additives. Therefore, effect of additives on the CTE is so critical and they should be estimated. Fig. 9 exhibits the CTEs of the blend resins from RT to 173 K. The results indicated that there was no significant difference of the CTE between the neat epoxy resin and the blend resins with SME and siloxane liquid unlike conventional toughened epoxy resins with rubber additives due to the small adding amount.
Fig. 9. CTE of the blend resins.
bifunctional polymer, SME (20 phr). The cryogenic tensile strength of the blend resins have been drastically improved by up to 77.6% compared to that of neat epoxy resin. It is concluded that the liquid droplets in a diameter of 5 lm in the matrix enhanced more effectively the strength than the droplets in 10–20 lm by dispersing the localized stress and interrupting the propagation of cracks through the promoted deformation mechanism such as cavitation of the droplets and multiple generation of numerous micro-deformation. From the CTE results, the siloxane liquid has no detrimental effect on the thermal contraction of the blend resins. Consequently, the blend epoxy resin is promising matrix to be employed in cryogenic applications. Acknowledgement This work was supported by a grant from the basic research program supported by Korea Institute of Materials Science (KIMS).
4. Conclusion A small amount (0.05 phr) of siloxane liquid which was completely immiscible with general epoxy resins was favorably stabilized in the form of a spherical domain in the matrix by adding a
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