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Unsaturated Polyester Composites
Journal of Colloid and Interface Science 242, 174–179 (2001) doi:10.1006/jcis.2001.7788, available online at http://www.idealibrary.com on
Effect of Silane Coupling Agent on Interphase and Performance of Glass Fibers/Unsaturated Polyester Composites Soo-Jin Park1 and Joong-Seong Jin Advanced Materials Division, Korea Research Institute of Chemical Technology, P.O. Box 107, Yusong, Taejon 305-600, South Korea Received December 1, 2000; accepted June 22, 2001; published online August 22, 2001
To improve the interfacial adhesion at interfaces between fibers and matrix, the γ -methacryloxypropyltrimethoxy silane (90 wt%, MPS) containing γ -aminopropyltriethoxy silane (10 wt%, APS) was applied to the surface treatment of glass fibers with different concentrations. The contact angle of silane-treated glass fibers and interlaminar shear strength and critical stress intensity factor (K IC ) for mechanical interfacial properties of glass fibers/unsaturated polyester were measured. According to the contact angle measurement, the silane-treated glass fibers did lead to an increase in surface free energy, mainly due to the increase of its specific (or polar) component. And mechanical interfacial properties of the composites were improved in the case of silane-treated composites compared with those as received and were shown in a maximum value in the presence of 0.2 wt% MPS/APS, due to the peptide bonding between C=O of MPS and –NH of APS. It revealed that the hydrogen bonding, which is one of the specific components of the surface free energy, between the glass fibers and the coupling agents plays an important role in improving the degree of adhesion at interfaces in a composite system. °C 2001 Academic Press Key Words: glass fiber; silane coupling agent; interfacial adhesion; surface free energy.
INTRODUCTION
Glass fibers are the most common of all reinforcing fibers for polymeric matrix composites. The principal advantages of glass fiber are low cost, high tensile strength, high chemical resistance, and excellent insulating properties. Fiber-reinforced polymer composites have seen a rapid rise in use in the past 30 years, due to their high strength and stiffness and light weight, compared with more traditional structural materials, such as steel and aluminum. The reason for this superior performance is the synergistic combination of the two, or more, constituent phases. This synergy is brought about by the interaction between the fibers and the polymeric matrix (1). It is well known that the interphase plays a key role in determining mechanical interfacial properties of composites. The efficiency of the stress transfer between fibers and matrix is determined not only by molecular interaction at interface, but 1 To whom correspondence should be addressed. E-mail: [email protected]. re.kr.
0021-9797/01 $35.00
C 2001 by Academic Press Copyright ° All rights of reproduction in any form reserved.
also by the properties of the formed interphase, in particular, its thickness and strength (2–4). One of the effective ways of creating interphase and, in this way, controlling composite properties is fiber surface treatment (5, 6) using sizings and film formers or coupling agents. To improve the interfacial adhesion at interface between the glass fibers and unsaturated polyester resin, silane coupling agents were used formerly on the fibers (7). At interface between the glass fibers and the silane coupling agent, the hydroxyl groups of the silanes and those of the glass fiber surface can react with each other through siloxane bonding or hydrogen bonding, as seen Fig. 1, which indicated the adhesion process of the silane coupling agents onto glass fibers (8). Early, Boerio et al. (9) found that the amino silane forms hydrolyzed polysiloxanes that are weakly bound and easily desorbed by water. It was suggested that such a nonpolar surface would require the amino silane to be adsorbed with the electron pair on the glass surface. And Yue and Quek (10) reported that the silane coupling agent containing amino silane coupling agent was improved in the mechanical interfacial properties of composites, compared with those of the single coupling agent used. Therefore, it can be suggested that the optimum condition of composites can be obtained by simultaneous introduction of silane coupling agent concentration onto the fiber surface. In recent years, the importance of acid–base interaction at interfaces has been demonstrated. In particular, the correlation between interfacial strength and such dynamic parameters of the surface as enthalpy of adsorption and surface free energy has been revealed (11–14). However, a strong interfacial interaction is a necessary but not sufficient condition for good stress transfer between the matrix and fibrous reinforcement. On the other hand, good adhesion between the fibers and the interphase can result in failure through the interphase/matrix boundary (2, 3). The degree of adhesion at interfaces between two, not identical, solids is greatly dominated by a weakly intermolecular or physical force, whereas the composite element formation is strongly primary or chemical bonding in nature (15, 16). The aim of this work is to propose the effect of silane coupling agent containing amino silane coupling agent based on degree of adhesion (or surface free energy) on resulting mechanical interfacial properties of the glass fiber-reinforced, unsaturated, polyester matrix composites.
174
SILANE COUPLING AGENT EFFECTS
175
FIG. 1. Surface structure of silane-treated glass fiber.
EXPERIMENTAL
volume fraction of bulk specimens was about 51 ± 3% for all composites.
Materials and Sample Preparation The glass fabrics (23 × 23 count/inch, 248 g/m2 ) used in this work were supplied from Hyun Dai Fiber Co. of Korea. And the unsaturated polyester (UP) matrix, R-235 (specific gravity: 1.11, viscosity: 2.8 poise), was ortho-phthalic acid type resin, which was supplied by Seiwon Chem. of Korea. Methylethylketone peroxide was selected as a hardener for cure. Silane coupling agents, γ -methacryloxypropyl trimethoxy silane (MPS) and γ -aminopropyltriethoxy silane (APS), supplied from Shinetsu Co. of Korea, were introduced to improve the interfacial adhesion of glass fibers/unsaturated polyester composites. The chemical structures of UP, MPS, and APS are given in Fig. 2. In order to treat the glass surface under different concentrations, the MPS (90 wt%) containing the APS (10 wt%) studied were prepared under a constant condition. In those solutions, cosolvent of methanol (95 wt% in total solvent) and deionized water (5 wt%) was used. And the MPS/APS concentration was varied from 0.1 to ∼0.5 wt% in total solvent. After the silane coupling agent was hydrolyzed at pH 4.0 for 1 h with acetic acid solution, the glass fibers were dipped in the hydrolyzed silane solution for 30 min. And it was dried at room temperature for 48 h. The composites for mechanical interfacial properties carried out by interlaminar shear strength (ILSS) and critical stress intensity factor (K IC ) were prepared in a hot press at 20 atm and 100◦ C for 1 h with a vacuum bagging method (17). The fiber
Fourier Transform Infrared (FT-IR) Spectroscopic and Contact Angle Measurements Infrared spectra of the casting samples were measured with a FT-IR spectrometer (Digital FTS-80, Bio-Rad). The scans were from 400 to 4500 cm−1 and required 40 s to complete. Contact angle measurements on glass fibers were performed using the Kruss Processor Tensiometer K12 equipped with an apparatus for fiber measuring (4). About 2 g of glass fibers were packed into an apparatus, and then mounted indirectly onto the measuring arm of the microbalance. The packing factor of fibers was measured for each silane-treated fiber by measuring the increase in weight per unit time at zero depth of immersion of a completely wetting liquid (in this study, n-hexane). The test wetting liquids used for contact angle measurements were n-hexane, deionized water, diiodomethane, and ethylene glycol. The surface free energy and their London dispersive and specific (or polar) components for the wetting liquids are listed in Table 1. Mechanical Interfacial Properties and Scanning Electron Microscopy (SEM) Observation The degree of adhesion at interfaces between fibers and matrix made with and without MPS/APS may be measured by
FIG. 2. Chemical structures of the UP, MPS, and APS used.
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TABLE 1 The Characteristics of Wetting Liquids Used in this Work Wetting liquid
γLL , mJ · m−2
γLSP , mJ · m−2
γL , mJ · m−2
η, mPa · S
ρ, g · cm−3
n-hexane Water Diiodomethane Ethylene glycol
18.4 21.8 50.42 31.0
0 51 0.38 16.7
18.4 72.8 50.8 47.7
0.33 1 2.76 17.3
0.661 0.998 3.325 1.100
γLL , London dispersive component of surface free energy. γLSP , specific component of surface free energy. γL , total surface free energy η, viscosity. ρ, density.
short-beam test for the interlaminar shear strength and critical stress intensity factor (K IC ) of the mechanical interfacial properties. For a rectangular cross section of the composites, the ILSS determined from three-point bending tests are calculated as (18) ILSS =
3F , 4bd
[5]
where F (N) is the rupture force, b (m) the width of the specimen, and d (m) the thickness of the specimen. It is well known that the fracture toughness is a critical property in resisting crack propagation loaded from matrix to fiber, which ought to be considered in the evaluation of a composite material for a real application. For a rectangular cross section of composites, the fracture toughness of the composites can be measured by a three-point bending test for K IC , as (19) K IC
PL = 3/2 Y, bd
[6]
where P (kN) is the rupture force, L (cm) the span between the supports, b (cm) and d (cm) the specimen thickness and width, and Y the geometric factor described in ASTM E399. The fracture surface of composites was examined using the scanning electron microscopy after measuring the short-beam flexural tests and documented in representative photomicrographs. RESULTS AND DISCUSSION
Fourier Transform Infrared Spectroscopy FT-IR spectroscopy was used to observe the silane coupling agent containing the amino silane adsorbed onto the glass fiber surface after hydrolysis. The FT-IR transmittance spectra of MPS/APS before and after hydrolysis are shown Fig. 3. The spectrum of Fig. 3a, before hydrolysis, shows two strong peaks at 2947 and 819 cm−1 due to the Si–CH3 group of MPS. The peaks at 3421 and 3371 cm−1 are assigned to the primary amine and the peaks at 943 and 820 cm−1 are observed due to the Si–OC2 H5 group of APS. After hydrolysis, intensity of C=O peak was reduced due to the peptide bonding between C=O of MPS and –N–H of APS. Two new peaks at 3367 and 1032 cm−1 due to the –OH and Si–OH group are observed in the spec-
FIG. 3. Transmission spectra of MPS/APS (a) before hydrolysis and (b) after hydrolysis.
trum of Fig. 3b, but the peaks or intensity of them at 813 and 2947 cm−1 decrease, resulting in efficient hydrolysis under this experimental condition. Contact Angle Measurements In the 1970s, Chwastiak (20) introduced the procedure for wicking rate measurements by enclosing the glass fiber bundle in a glass tube so that the porosity was fixed for a given strand of glass fibers. And he calculated the surface tension of fibers using the modified Washburn equation [21, 22]. c · ρ 2 · γL · cos θ m2 = , t 2η
[1]
where m is the weight of the penetrating liquid, t the flow time, c the packing factor, ρ the density of measuring liquid, γL the liquid surface tension, and η the viscosity of liquid. Table 2 shows the results of cos θ values on glass fibers treated with MPS/APS concentrations. As a result, the cos θ of deionized water on various silane-treated glass fibers largely increases up to 0.2 wt% concentration as the MPS/APS concentration increases (R 2 = 0.95). As mentioned above, it is clearly found that silane treatment leads to a change in fiber TABLE 2 The cos θ Values of Glass Fibers Treated with MPS/APS Concentration
Deionized water Diiodomethane Ethylene glycol
As received
0.1 wt%
0.2 wt%
0.3 wt%
0.5 wt%
0.33 0.66 0.64
0.48 0.62 0.56
0.60 0.57 0.59
0.57 0.62 0.59
0.52 0.62 0.56
SILANE COUPLING AGENT EFFECTS
surface nature, resulting in increasing hydrophilic properties. Above 0.2 wt% MPS/APS concentration, the silane treatment is formed in the thick layer on fibers, resulting in an increase in the contact angle of water. In other words, in case of excess silane concentration, the silane coupling agent is not always formed in hydrogen bonding with the glass fiber, due to the thick layer formed. Therefore, the fibers treated with excess silane concentration demonstrate the silane characteristics in nature, resulting from increasing the intermolecular equilibrium distance between fibers and silane or not exhibiting the specific component of surface free energy in a silane characteristic (11). It is well known that the knowledge of surface energetics at a given temperature of a solid has recently allowed significant progress in many academic and scientific fields involving two, not identical, molecular interactions at a certain intermolecular distance, such as adsorption (gas–solid), wettability (liquid– liquid), and adhesion (solid–solid) (11, 23). To quantitatively evaluate the surface free energy (γS ), and its dispersive force component (γSL ) and specific force component (γSSP ), for silane-treated glass fibers, we employed Kaelble’s (24) method as shown below. It is well known that Young’s Eq. [2] between the contact angle (θ), surface free energy (γS ), solid/liquid interfacial tension (γSL ), and surface free energy of standard liquid (γL ) is valid: (γS − γSL ) . cos θ = γL
[2]
Conversely, assuming the addition of intermolecular force, Fowkes (25) represented the surface free energy by sum of a contribution resulting from dispersive forces (γ L ) and a contribution resulting from specific forces (γ SP ): γ = γ SP + γ L .
[3]
177
FIG. 4. Plots of a linear fit method for increasing MPS/APS concentration on glass fiber-reinforced composites.
This equation the linear relationship between q shows us q that q γL (1 + cos θ)/ γLL and γLSP / γLL is valid. If the contact angle is measured for various liquids having different values of q SP L γ , we can plot γ (1 + cos θ )/ γ as a function γL ,qγLL , and L L q L
of γLSP / γLL to obtain a single straight line. From the gradiL ent of this line, γSSP can be derived, q and q γS can also be defined
from the extrapolated value of γLSP / γLL → 0. In Fig. 4, this method has been applied to the wetting data sets gained for the glass fibers with MPS/APS concentrations. Referring to the Owens and Wendt method, as shown in Fig. 4, it is obvious that the wetting data points for each concentration coincide quite well with a straight line and the least-squares linear fit of which is given by equation. And all the results
In addition, at the interface between a polar liquid (γ1 = γ1L + γ1SP ) and nonpolar liquid (γ2 = γ2L + γ2SP ) being in contact, Fowkes defined the interfacial tension, as follows: q [4] γ12 = γ1 + γ2 − 2 γ1L γ2L . For the interface between two liquids, Owens and Wendt (26) and Kaelble (24) expanded Fowkes formula [4] to obtain the following equation: q q γ12 = γ1 + γ2 − 2 γ1L γ2L − 2 γ1SP γ2SP .
[5]
Substituting Young’s Eq. [2] for Eq. [5] above and dividing q γLL , we get the following:
by
q q q γLSP γL (1 + cos θ) q = 2 γSL + 2 γSSP q . γLL γLL
[6]
FIG. 5. Surface free energies of glass fibers treated with MPS/APS concentration.
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PARK AND JIN
FIG. 6. Interlaminar shear strength (ILSS) and critical stress intensity factor (K IC ) of the composites with MPS/APS concentration.
in terms of surface free energies and their components of MPS/APS-treated fibers are shown in Fig. 5. As an experimental result, the total surface free energy, γS , of MPS/APS-treated glass fibers increases up to 0.2 wt% silane concentration. This surface free energy increase of silane-treated glass fibers can be attributed to the increase in hydroxyl groups, resulting in increased hydrogen bonding between the glass fibers and the silane coupling agent, as shown Fig. 1, which is one of the specific components of surface free energies (11). In particular, the surface free energy of composites was shown in a maximum value in the presence of 0.2 wt% MPS/APS on glass fibers. Mechanical Interfacial Properties Figure 6 shows the evolution of both ILSS and K IC in flexure of the composites with the MPS/APS concentration. From the
FIG. 7. Dependence of the interlaminar shear strength (ILSS) and critical stress intensity factor (K IC ) on γSSP .
FIG. 8. SEM micrographs of the composites: (a) as received, (b) 0.1 wt%, (c) 0.2 wt%, and (d) 0.5 wt%.
179
SILANE COUPLING AGENT EFFECTS
ILSS results, the presence of coupling agent does lead to an increase of ILSS of the composites, which can be related to the effect of increasing the degree of adhesion at interfaces among the three elements, i.e., fiber, matrix, and silane coupling agent. Also, the K IC value is the highest, about 3.98 MPa · m1/2 in the case of 0.2 wt% silane-treated composites due to the peptide bonding between MPS and APS. Meanwhile, the mechanical interfacial properties of composites decreased above 0.2 wt% silane concentration. This is attributed to the fact that the excess silane layer physisorbed onto the glass fiber surface forms the weak boundary layer at a high silane coupling agent concentration. This excess silane layer may act deficient in the composite interphase and cause a lubrication effect (27). Therefore, the mechanical interfacial properties of the composites decrease at a given higher silane coupling agent concentration. Good linearties between the specific component of surface free energy and both the resulting ILSS and K IC are shown in Fig. 7. As mentioned above, it is a consequence of the increased specific component of surface free energy of glass fibers treated with MPS/APS, resulting in enhancing the hydrogen bonding in an equilibrium distance. The fracture surfaces of representative specimens with MPS/ APS concentration were examined by SEM. The micrographs of these fracture surfaces are shown in Fig. 8. The fracture surfaces show remarkable differences, resulting from the change in fiber-silane coupling agent adhesion. The cohesive failure at unsaturated polyester matrix increases when the MPS/APS is presented, and the concentration is increased up to 0.2 wt%. But a marginal decrease in fracture surface is shown in above 0.2 wt% concentration. This again confirms that the excess silane layer makes the decreasing of the fracture toughness of the composites. CONCLUSION
The effects of silane coupling agent treatments on the fiber surface properties have been studied in terms of the surface energetics of the fibers and the mechanical interfacial properties, including ILSS and K IC , of the composites. From the experimental results, it is then noted that the silane coupling agent containing the amino silane does lead to an increasing of the mechanical interfacial properties of composites, due to the peptide bonding between C=O of MPS and –NH of APS. Also, the study of surface free energies and their components determined from the multiple testing liquids seems to correlate with the study of mechanical interfacial properties. This clearly results
in increasing the specific component of surface free energy for the intermolecular or physical bonding properties among three, not identical, elements of the composites. Consequently, both surface free energy and mechanical interfacial properties of the composites are shown in the maximum values in the presence of 0.2 wt.% MPS/APS. REFERENCES 1. Berg, J., and Jones, F. R., Compos. A 29A, 1261 (1998). 2. Tselios, C., Biliaris, D., Savidis, P., and Panayiotou, C., J. Mater. Sci. 34, 385 (1999). 3. Agrawal, R., Sazena, N. S., Sreekaka, M. S., and Thomas, S., J. Polym. Sci. Polym. Phys. 38, 916 (2000). 4. Mader, E., and Pisanova, E., Polym. Compos. 21, 361 (2000). 5. Park, S. J., Kim, M. H., Lee, J. R., and Choi. S., J. Colloid Interface Sci. 228, 287 (2000). 6. Frysz, C. A., and Chung, D. D. L., Carbon 35, 1111 (1997). 7. Elley-Briston, D. M., Bellinger, M. A., Sauer, J. A., and Hara, M., J. Polym. Sci. Polym. Phys. 37, 2705 (1999). 8. Thomas, B., Klaus-Jochen, E., Karina, G., and Hans-Jorg, J., Macromol. Chem. Phys. 200, 852 (1999). 9. Boerio, F. J., Schoenlein, L. H., and Grievenkamp, J. E., Appl. Spectrosc. 22, 203 (1978). 10. Yue, C. Y., and Quek, M. Y., J. Mater. Sci. 29, 2487 (1994). 11. Park, S. J., in “Interfacial Forces and Fields: Theory and Applications” (J. P. Hsu, Ed.). Dekker, New York, 1999. 12. Drzal, L. T., Sugiura, N., and Hook, D., Compos. Interfaces 4, 337 (1997). 13. Hanb¨ucken, M., Rottger, B., and Neddermeyer, H., Appl. Surf. Sci. 164, 91 (2000). 14. Cong, P., Nanao, H., Igari, T., and Mori, S., Appl. Surf. Sci. 167, 152 (2000). 15. Park, S. J., and Lee, J. R., J. Mater. Sci. 33, 647 (1998). 16. Tsai, Y. C., Chou, C. T., and Penn, L. S., in “In Contact Angle, Wettability and Adhesion” (K. L. Mittal, Ed.). VSP, Utrecht, 1993. 17. Park, S. J., Cho, M. S., and Lee, J. R., J. Colloid Interface Sci. 226, 60 (2000). 18. Choudhary, V., Fitzer, E., and Heine, M., Angew. Makcrom. Chem. 160, 17 (1988). 19. Chen, M. C., Hourston, D. J., and Sun, W. B., Eur. Polym. J. 31, 199 (1995). 20. Chwastiak, S., J. Colloid Interface Sci. 42, 298 (1973). 21. Park, S. J., Jin, J. S., Lee, J. R., and Kim, Y. K., J. Korean Ind. Eng. Chem. 11, 285 (2000). 22. Van Oss, C. J., “Interfacial Forced in Aqueous Media.” Dekker, New York, 1994. 23. Israelachvili, J., “Intermolecular and Surface Forces,” 2nd ed. Academic Press, London, 1992. 24. Kaelble, D. H., J. Adhesion 2, 66 (1970). 25. Fowkes, F. M., J. Phys. Chem. 66, 382 (1962). 26. Owens, D. K., and Wendt, R. C., J. Appl. Polym. Sci. 13, 1741 (1969). 27. Jang, J. S., Lee, J. Y., and Jeong, J. K., J. Appl. Polym. Sci. 59, 2069 (1997).
Effect of Silane Coupling Agent on Interphase and Performance of Glass Fibers/Unsaturated Polyester Composites Soo-Jin Park1 and Joong-Seong Jin Advanced Materials Division, Korea Research Institute of Chemical Technology, P.O. Box 107, Yusong, Taejon 305-600, South Korea Received December 1, 2000; accepted June 22, 2001; published online August 22, 2001
To improve the interfacial adhesion at interfaces between fibers and matrix, the γ -methacryloxypropyltrimethoxy silane (90 wt%, MPS) containing γ -aminopropyltriethoxy silane (10 wt%, APS) was applied to the surface treatment of glass fibers with different concentrations. The contact angle of silane-treated glass fibers and interlaminar shear strength and critical stress intensity factor (K IC ) for mechanical interfacial properties of glass fibers/unsaturated polyester were measured. According to the contact angle measurement, the silane-treated glass fibers did lead to an increase in surface free energy, mainly due to the increase of its specific (or polar) component. And mechanical interfacial properties of the composites were improved in the case of silane-treated composites compared with those as received and were shown in a maximum value in the presence of 0.2 wt% MPS/APS, due to the peptide bonding between C=O of MPS and –NH of APS. It revealed that the hydrogen bonding, which is one of the specific components of the surface free energy, between the glass fibers and the coupling agents plays an important role in improving the degree of adhesion at interfaces in a composite system. °C 2001 Academic Press Key Words: glass fiber; silane coupling agent; interfacial adhesion; surface free energy.
INTRODUCTION
Glass fibers are the most common of all reinforcing fibers for polymeric matrix composites. The principal advantages of glass fiber are low cost, high tensile strength, high chemical resistance, and excellent insulating properties. Fiber-reinforced polymer composites have seen a rapid rise in use in the past 30 years, due to their high strength and stiffness and light weight, compared with more traditional structural materials, such as steel and aluminum. The reason for this superior performance is the synergistic combination of the two, or more, constituent phases. This synergy is brought about by the interaction between the fibers and the polymeric matrix (1). It is well known that the interphase plays a key role in determining mechanical interfacial properties of composites. The efficiency of the stress transfer between fibers and matrix is determined not only by molecular interaction at interface, but 1 To whom correspondence should be addressed. E-mail: [email protected]. re.kr.
0021-9797/01 $35.00
C 2001 by Academic Press Copyright ° All rights of reproduction in any form reserved.
also by the properties of the formed interphase, in particular, its thickness and strength (2–4). One of the effective ways of creating interphase and, in this way, controlling composite properties is fiber surface treatment (5, 6) using sizings and film formers or coupling agents. To improve the interfacial adhesion at interface between the glass fibers and unsaturated polyester resin, silane coupling agents were used formerly on the fibers (7). At interface between the glass fibers and the silane coupling agent, the hydroxyl groups of the silanes and those of the glass fiber surface can react with each other through siloxane bonding or hydrogen bonding, as seen Fig. 1, which indicated the adhesion process of the silane coupling agents onto glass fibers (8). Early, Boerio et al. (9) found that the amino silane forms hydrolyzed polysiloxanes that are weakly bound and easily desorbed by water. It was suggested that such a nonpolar surface would require the amino silane to be adsorbed with the electron pair on the glass surface. And Yue and Quek (10) reported that the silane coupling agent containing amino silane coupling agent was improved in the mechanical interfacial properties of composites, compared with those of the single coupling agent used. Therefore, it can be suggested that the optimum condition of composites can be obtained by simultaneous introduction of silane coupling agent concentration onto the fiber surface. In recent years, the importance of acid–base interaction at interfaces has been demonstrated. In particular, the correlation between interfacial strength and such dynamic parameters of the surface as enthalpy of adsorption and surface free energy has been revealed (11–14). However, a strong interfacial interaction is a necessary but not sufficient condition for good stress transfer between the matrix and fibrous reinforcement. On the other hand, good adhesion between the fibers and the interphase can result in failure through the interphase/matrix boundary (2, 3). The degree of adhesion at interfaces between two, not identical, solids is greatly dominated by a weakly intermolecular or physical force, whereas the composite element formation is strongly primary or chemical bonding in nature (15, 16). The aim of this work is to propose the effect of silane coupling agent containing amino silane coupling agent based on degree of adhesion (or surface free energy) on resulting mechanical interfacial properties of the glass fiber-reinforced, unsaturated, polyester matrix composites.
174
SILANE COUPLING AGENT EFFECTS
175
FIG. 1. Surface structure of silane-treated glass fiber.
EXPERIMENTAL
volume fraction of bulk specimens was about 51 ± 3% for all composites.
Materials and Sample Preparation The glass fabrics (23 × 23 count/inch, 248 g/m2 ) used in this work were supplied from Hyun Dai Fiber Co. of Korea. And the unsaturated polyester (UP) matrix, R-235 (specific gravity: 1.11, viscosity: 2.8 poise), was ortho-phthalic acid type resin, which was supplied by Seiwon Chem. of Korea. Methylethylketone peroxide was selected as a hardener for cure. Silane coupling agents, γ -methacryloxypropyl trimethoxy silane (MPS) and γ -aminopropyltriethoxy silane (APS), supplied from Shinetsu Co. of Korea, were introduced to improve the interfacial adhesion of glass fibers/unsaturated polyester composites. The chemical structures of UP, MPS, and APS are given in Fig. 2. In order to treat the glass surface under different concentrations, the MPS (90 wt%) containing the APS (10 wt%) studied were prepared under a constant condition. In those solutions, cosolvent of methanol (95 wt% in total solvent) and deionized water (5 wt%) was used. And the MPS/APS concentration was varied from 0.1 to ∼0.5 wt% in total solvent. After the silane coupling agent was hydrolyzed at pH 4.0 for 1 h with acetic acid solution, the glass fibers were dipped in the hydrolyzed silane solution for 30 min. And it was dried at room temperature for 48 h. The composites for mechanical interfacial properties carried out by interlaminar shear strength (ILSS) and critical stress intensity factor (K IC ) were prepared in a hot press at 20 atm and 100◦ C for 1 h with a vacuum bagging method (17). The fiber
Fourier Transform Infrared (FT-IR) Spectroscopic and Contact Angle Measurements Infrared spectra of the casting samples were measured with a FT-IR spectrometer (Digital FTS-80, Bio-Rad). The scans were from 400 to 4500 cm−1 and required 40 s to complete. Contact angle measurements on glass fibers were performed using the Kruss Processor Tensiometer K12 equipped with an apparatus for fiber measuring (4). About 2 g of glass fibers were packed into an apparatus, and then mounted indirectly onto the measuring arm of the microbalance. The packing factor of fibers was measured for each silane-treated fiber by measuring the increase in weight per unit time at zero depth of immersion of a completely wetting liquid (in this study, n-hexane). The test wetting liquids used for contact angle measurements were n-hexane, deionized water, diiodomethane, and ethylene glycol. The surface free energy and their London dispersive and specific (or polar) components for the wetting liquids are listed in Table 1. Mechanical Interfacial Properties and Scanning Electron Microscopy (SEM) Observation The degree of adhesion at interfaces between fibers and matrix made with and without MPS/APS may be measured by
FIG. 2. Chemical structures of the UP, MPS, and APS used.
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PARK AND JIN
TABLE 1 The Characteristics of Wetting Liquids Used in this Work Wetting liquid
γLL , mJ · m−2
γLSP , mJ · m−2
γL , mJ · m−2
η, mPa · S
ρ, g · cm−3
n-hexane Water Diiodomethane Ethylene glycol
18.4 21.8 50.42 31.0
0 51 0.38 16.7
18.4 72.8 50.8 47.7
0.33 1 2.76 17.3
0.661 0.998 3.325 1.100
γLL , London dispersive component of surface free energy. γLSP , specific component of surface free energy. γL , total surface free energy η, viscosity. ρ, density.
short-beam test for the interlaminar shear strength and critical stress intensity factor (K IC ) of the mechanical interfacial properties. For a rectangular cross section of the composites, the ILSS determined from three-point bending tests are calculated as (18) ILSS =
3F , 4bd
[5]
where F (N) is the rupture force, b (m) the width of the specimen, and d (m) the thickness of the specimen. It is well known that the fracture toughness is a critical property in resisting crack propagation loaded from matrix to fiber, which ought to be considered in the evaluation of a composite material for a real application. For a rectangular cross section of composites, the fracture toughness of the composites can be measured by a three-point bending test for K IC , as (19) K IC
PL = 3/2 Y, bd
[6]
where P (kN) is the rupture force, L (cm) the span between the supports, b (cm) and d (cm) the specimen thickness and width, and Y the geometric factor described in ASTM E399. The fracture surface of composites was examined using the scanning electron microscopy after measuring the short-beam flexural tests and documented in representative photomicrographs. RESULTS AND DISCUSSION
Fourier Transform Infrared Spectroscopy FT-IR spectroscopy was used to observe the silane coupling agent containing the amino silane adsorbed onto the glass fiber surface after hydrolysis. The FT-IR transmittance spectra of MPS/APS before and after hydrolysis are shown Fig. 3. The spectrum of Fig. 3a, before hydrolysis, shows two strong peaks at 2947 and 819 cm−1 due to the Si–CH3 group of MPS. The peaks at 3421 and 3371 cm−1 are assigned to the primary amine and the peaks at 943 and 820 cm−1 are observed due to the Si–OC2 H5 group of APS. After hydrolysis, intensity of C=O peak was reduced due to the peptide bonding between C=O of MPS and –N–H of APS. Two new peaks at 3367 and 1032 cm−1 due to the –OH and Si–OH group are observed in the spec-
FIG. 3. Transmission spectra of MPS/APS (a) before hydrolysis and (b) after hydrolysis.
trum of Fig. 3b, but the peaks or intensity of them at 813 and 2947 cm−1 decrease, resulting in efficient hydrolysis under this experimental condition. Contact Angle Measurements In the 1970s, Chwastiak (20) introduced the procedure for wicking rate measurements by enclosing the glass fiber bundle in a glass tube so that the porosity was fixed for a given strand of glass fibers. And he calculated the surface tension of fibers using the modified Washburn equation [21, 22]. c · ρ 2 · γL · cos θ m2 = , t 2η
[1]
where m is the weight of the penetrating liquid, t the flow time, c the packing factor, ρ the density of measuring liquid, γL the liquid surface tension, and η the viscosity of liquid. Table 2 shows the results of cos θ values on glass fibers treated with MPS/APS concentrations. As a result, the cos θ of deionized water on various silane-treated glass fibers largely increases up to 0.2 wt% concentration as the MPS/APS concentration increases (R 2 = 0.95). As mentioned above, it is clearly found that silane treatment leads to a change in fiber TABLE 2 The cos θ Values of Glass Fibers Treated with MPS/APS Concentration
Deionized water Diiodomethane Ethylene glycol
As received
0.1 wt%
0.2 wt%
0.3 wt%
0.5 wt%
0.33 0.66 0.64
0.48 0.62 0.56
0.60 0.57 0.59
0.57 0.62 0.59
0.52 0.62 0.56
SILANE COUPLING AGENT EFFECTS
surface nature, resulting in increasing hydrophilic properties. Above 0.2 wt% MPS/APS concentration, the silane treatment is formed in the thick layer on fibers, resulting in an increase in the contact angle of water. In other words, in case of excess silane concentration, the silane coupling agent is not always formed in hydrogen bonding with the glass fiber, due to the thick layer formed. Therefore, the fibers treated with excess silane concentration demonstrate the silane characteristics in nature, resulting from increasing the intermolecular equilibrium distance between fibers and silane or not exhibiting the specific component of surface free energy in a silane characteristic (11). It is well known that the knowledge of surface energetics at a given temperature of a solid has recently allowed significant progress in many academic and scientific fields involving two, not identical, molecular interactions at a certain intermolecular distance, such as adsorption (gas–solid), wettability (liquid– liquid), and adhesion (solid–solid) (11, 23). To quantitatively evaluate the surface free energy (γS ), and its dispersive force component (γSL ) and specific force component (γSSP ), for silane-treated glass fibers, we employed Kaelble’s (24) method as shown below. It is well known that Young’s Eq. [2] between the contact angle (θ), surface free energy (γS ), solid/liquid interfacial tension (γSL ), and surface free energy of standard liquid (γL ) is valid: (γS − γSL ) . cos θ = γL
[2]
Conversely, assuming the addition of intermolecular force, Fowkes (25) represented the surface free energy by sum of a contribution resulting from dispersive forces (γ L ) and a contribution resulting from specific forces (γ SP ): γ = γ SP + γ L .
[3]
177
FIG. 4. Plots of a linear fit method for increasing MPS/APS concentration on glass fiber-reinforced composites.
This equation the linear relationship between q shows us q that q γL (1 + cos θ)/ γLL and γLSP / γLL is valid. If the contact angle is measured for various liquids having different values of q SP L γ , we can plot γ (1 + cos θ )/ γ as a function γL ,qγLL , and L L q L
of γLSP / γLL to obtain a single straight line. From the gradiL ent of this line, γSSP can be derived, q and q γS can also be defined
from the extrapolated value of γLSP / γLL → 0. In Fig. 4, this method has been applied to the wetting data sets gained for the glass fibers with MPS/APS concentrations. Referring to the Owens and Wendt method, as shown in Fig. 4, it is obvious that the wetting data points for each concentration coincide quite well with a straight line and the least-squares linear fit of which is given by equation. And all the results
In addition, at the interface between a polar liquid (γ1 = γ1L + γ1SP ) and nonpolar liquid (γ2 = γ2L + γ2SP ) being in contact, Fowkes defined the interfacial tension, as follows: q [4] γ12 = γ1 + γ2 − 2 γ1L γ2L . For the interface between two liquids, Owens and Wendt (26) and Kaelble (24) expanded Fowkes formula [4] to obtain the following equation: q q γ12 = γ1 + γ2 − 2 γ1L γ2L − 2 γ1SP γ2SP .
[5]
Substituting Young’s Eq. [2] for Eq. [5] above and dividing q γLL , we get the following:
by
q q q γLSP γL (1 + cos θ) q = 2 γSL + 2 γSSP q . γLL γLL
[6]
FIG. 5. Surface free energies of glass fibers treated with MPS/APS concentration.
178
PARK AND JIN
FIG. 6. Interlaminar shear strength (ILSS) and critical stress intensity factor (K IC ) of the composites with MPS/APS concentration.
in terms of surface free energies and their components of MPS/APS-treated fibers are shown in Fig. 5. As an experimental result, the total surface free energy, γS , of MPS/APS-treated glass fibers increases up to 0.2 wt% silane concentration. This surface free energy increase of silane-treated glass fibers can be attributed to the increase in hydroxyl groups, resulting in increased hydrogen bonding between the glass fibers and the silane coupling agent, as shown Fig. 1, which is one of the specific components of surface free energies (11). In particular, the surface free energy of composites was shown in a maximum value in the presence of 0.2 wt% MPS/APS on glass fibers. Mechanical Interfacial Properties Figure 6 shows the evolution of both ILSS and K IC in flexure of the composites with the MPS/APS concentration. From the
FIG. 7. Dependence of the interlaminar shear strength (ILSS) and critical stress intensity factor (K IC ) on γSSP .
FIG. 8. SEM micrographs of the composites: (a) as received, (b) 0.1 wt%, (c) 0.2 wt%, and (d) 0.5 wt%.
179
SILANE COUPLING AGENT EFFECTS
ILSS results, the presence of coupling agent does lead to an increase of ILSS of the composites, which can be related to the effect of increasing the degree of adhesion at interfaces among the three elements, i.e., fiber, matrix, and silane coupling agent. Also, the K IC value is the highest, about 3.98 MPa · m1/2 in the case of 0.2 wt% silane-treated composites due to the peptide bonding between MPS and APS. Meanwhile, the mechanical interfacial properties of composites decreased above 0.2 wt% silane concentration. This is attributed to the fact that the excess silane layer physisorbed onto the glass fiber surface forms the weak boundary layer at a high silane coupling agent concentration. This excess silane layer may act deficient in the composite interphase and cause a lubrication effect (27). Therefore, the mechanical interfacial properties of the composites decrease at a given higher silane coupling agent concentration. Good linearties between the specific component of surface free energy and both the resulting ILSS and K IC are shown in Fig. 7. As mentioned above, it is a consequence of the increased specific component of surface free energy of glass fibers treated with MPS/APS, resulting in enhancing the hydrogen bonding in an equilibrium distance. The fracture surfaces of representative specimens with MPS/ APS concentration were examined by SEM. The micrographs of these fracture surfaces are shown in Fig. 8. The fracture surfaces show remarkable differences, resulting from the change in fiber-silane coupling agent adhesion. The cohesive failure at unsaturated polyester matrix increases when the MPS/APS is presented, and the concentration is increased up to 0.2 wt%. But a marginal decrease in fracture surface is shown in above 0.2 wt% concentration. This again confirms that the excess silane layer makes the decreasing of the fracture toughness of the composites. CONCLUSION
The effects of silane coupling agent treatments on the fiber surface properties have been studied in terms of the surface energetics of the fibers and the mechanical interfacial properties, including ILSS and K IC , of the composites. From the experimental results, it is then noted that the silane coupling agent containing the amino silane does lead to an increasing of the mechanical interfacial properties of composites, due to the peptide bonding between C=O of MPS and –NH of APS. Also, the study of surface free energies and their components determined from the multiple testing liquids seems to correlate with the study of mechanical interfacial properties. This clearly results
in increasing the specific component of surface free energy for the intermolecular or physical bonding properties among three, not identical, elements of the composites. Consequently, both surface free energy and mechanical interfacial properties of the composites are shown in the maximum values in the presence of 0.2 wt.% MPS/APS. REFERENCES 1. Berg, J., and Jones, F. R., Compos. A 29A, 1261 (1998). 2. Tselios, C., Biliaris, D., Savidis, P., and Panayiotou, C., J. Mater. Sci. 34, 385 (1999). 3. Agrawal, R., Sazena, N. S., Sreekaka, M. S., and Thomas, S., J. Polym. Sci. Polym. Phys. 38, 916 (2000). 4. Mader, E., and Pisanova, E., Polym. Compos. 21, 361 (2000). 5. Park, S. J., Kim, M. H., Lee, J. R., and Choi. S., J. Colloid Interface Sci. 228, 287 (2000). 6. Frysz, C. A., and Chung, D. D. L., Carbon 35, 1111 (1997). 7. Elley-Briston, D. M., Bellinger, M. A., Sauer, J. A., and Hara, M., J. Polym. Sci. Polym. Phys. 37, 2705 (1999). 8. Thomas, B., Klaus-Jochen, E., Karina, G., and Hans-Jorg, J., Macromol. Chem. Phys. 200, 852 (1999). 9. Boerio, F. J., Schoenlein, L. H., and Grievenkamp, J. E., Appl. Spectrosc. 22, 203 (1978). 10. Yue, C. Y., and Quek, M. Y., J. Mater. Sci. 29, 2487 (1994). 11. Park, S. J., in “Interfacial Forces and Fields: Theory and Applications” (J. P. Hsu, Ed.). Dekker, New York, 1999. 12. Drzal, L. T., Sugiura, N., and Hook, D., Compos. Interfaces 4, 337 (1997). 13. Hanb¨ucken, M., Rottger, B., and Neddermeyer, H., Appl. Surf. Sci. 164, 91 (2000). 14. Cong, P., Nanao, H., Igari, T., and Mori, S., Appl. Surf. Sci. 167, 152 (2000). 15. Park, S. J., and Lee, J. R., J. Mater. Sci. 33, 647 (1998). 16. Tsai, Y. C., Chou, C. T., and Penn, L. S., in “In Contact Angle, Wettability and Adhesion” (K. L. Mittal, Ed.). VSP, Utrecht, 1993. 17. Park, S. J., Cho, M. S., and Lee, J. R., J. Colloid Interface Sci. 226, 60 (2000). 18. Choudhary, V., Fitzer, E., and Heine, M., Angew. Makcrom. Chem. 160, 17 (1988). 19. Chen, M. C., Hourston, D. J., and Sun, W. B., Eur. Polym. J. 31, 199 (1995). 20. Chwastiak, S., J. Colloid Interface Sci. 42, 298 (1973). 21. Park, S. J., Jin, J. S., Lee, J. R., and Kim, Y. K., J. Korean Ind. Eng. Chem. 11, 285 (2000). 22. Van Oss, C. J., “Interfacial Forced in Aqueous Media.” Dekker, New York, 1994. 23. Israelachvili, J., “Intermolecular and Surface Forces,” 2nd ed. Academic Press, London, 1992. 24. Kaelble, D. H., J. Adhesion 2, 66 (1970). 25. Fowkes, F. M., J. Phys. Chem. 66, 382 (1962). 26. Owens, D. K., and Wendt, R. C., J. Appl. Polym. Sci. 13, 1741 (1969). 27. Jang, J. S., Lee, J. Y., and Jeong, J. K., J. Appl. Polym. Sci. 59, 2069 (1997).