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MATERIALS AND INTERFACES Static and Dynamic Mechanical Properties of Vinylester Resin Matrix Composites Reinforced with Shellac-Treated Jute Yarns Dipa Ray and S. P. Sengupta* Department of Materials Science, Indian Association for the CultiVation of Science, 2 A & B Raja S. C. Mallick Road, JadaVpur, Kolkata - 700032, India
A. K. Rana Indian Jute Industries’ Research Association, 17 Taratola Road, Kolkata - 700088, India
N. R. Bose Central Glass and Ceramic Research Institute, 196 Raja S. C. Mallick Road, JadaVpur, Kolkata - 700032, India
Natural fiber-reinforced composites are currently used for various types of applications. However, to improve the fiber/resin bonding at the interface, some suitable chemical modification of the fibers is required. In the present study, an attempt has been made to use an inexpensive, easily available, and water-soluble (in alkaline medium) natural resin shellac for the surface treatment of jute yarns. Jute yarns were treated with 1%, 2%, and 5% shellac solution, and these treated jute yarns were used as reinforcing material in vinylester resin matrix composites. The composites were subjected to flexural tests, and the flexural properties were found to be highest in the case of 1% treated composites. A dynamic mechanical study also showed the same trend, and the storage modulus was found to be highest for the 1% treated composites at room temperature. The fractured surfaces of the composites, as observed by SEM, were correlated with the mechanical properties. Introduction Natural fibers are inexpensive, easily available, renewable, and biodegradable cellulosic materials and have a significantly higher specific strength property than synthetic fibers. They have therefore attracted the attention of the scientific community and become increasingly useful as a raw material for manufacturing cost-effective and environment-friendly composite materials. The biodegradability of the natural fibers contributes effectively to a healthy environment, making these materials highly desirable, and at the same time, their low cost and easy availability significantly increase their commercial viability. However, their high level of moisture absorption and poor wetting characteristics with organic resins lead to a weak bond at the fiber/matrix interface and ultimately result in the production of composites of low mechanical properties. To overcome this disadvantage, a suitable surface treatment of the fibers is required prior to composite fabrication. Many experiments on the surface modification of natural fibers have been reported by several workers;1-8 however, a technologically and commercially successful alternative is yet to emerge. In the present study, an attempt is made to use an inexpensive, easily available, and water-soluble (in alkaline medium) natural resin shellac for the surface treatment of jute yarns. * To whom correspondence should be addressed. Tel.: +91-0332473 4971. Fax: +91-033-2473 2805. E-mail:
[email protected].
Figure 1. Aleuritic acid, present as a mixed anhydride with a compound containing two fused six-membered rings in shellac.
Experimental Section Materials. Jute yarns (8 lb) (white jute, Corchorus capsularis) were wrapped in black paper, kept in sealed polythene bags, and stored at 65% relative humidity (RH) and 25 °C. Vinylester resin used was of grade FB-701, a Ruia Chemicals product. Methyl ethyl ketone peroxide (MEKP), cobalt naphthenate, and N,N-dimethylaniline were used as the catalyst, the accelerator, and the promoter, respectively. Shellac is a natural resin of animal origin and consists of an excretion from a coccid insect that lives in the twigs of certain trees found in India, Thailand, and other countries in the East Indies. Shellac is a complex mixture of esters, anhydrides, and lactones of aliphatic and aromatic polyhydroxycarboxylic acids. Among these is found aleuritic acid, which is 9,10,16-trihydroxy palmitic acid. In one component of the resin, aleuritic acid is believed to be present as a mixed anhydride with a compound containing two fused six-membered rings, shown in Figure 1. Shellac Treatment. Shellac resin is acidic in character, and consequently, it is soluble in alkaline water solutions. Diethanolamine, an alkaline ammonia derivative, is a water-soluble
10.1021/ie0512800 CCC: $33.50 © 2006 American Chemical Society Published on Web 03/16/2006
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Figure 2. Chemical structure of vinylester resin.
liquid. Its combination with shellac has excellent emulsifying properties. Shellac solutions were prepared by adding dry shellac lumps slowly to water in the presence of 0.5% diethanolamine, and the shellac dissolved in the water only upon boiling. The concentration of base (diethanolamine) was chosen as 0.5% because that was found to be the threshold concentration below which the shellac could not be dissolved in the solution. Three shellac solutions of concentrations 1%, 2%, and 5% (by weight) were prepared in three separate beakers. Jute yarns were wrapped in a GoodBrand WrapReel machine, and the wrapped jute reels were soaked in 1%, 2%, and 5% shellac solutions in three separate containers at 30°C maintaining a liquor ratio of 15:1. The yarns were kept immersed in the shellac solutions for 30 min. They were allowed to dry at room temperature for 48 h and then oven dried at 100 °C for 6 h. The shellac absorption was 3.5%, 4.8%, and 7.6%, respectively, for the 1%, 2%, and 5% shellac-treated jute yarns. Sample Preparation. Jute/vinylester composites containing untreated and shellac-treated jute yarns were fabricated in the form of cylindrical rods of 6-mm diameter. Hollow cylindrical glass tubes of 6-mm internal diameter were used as molds. The resin was mixed with accelerator, promoter, and catalyst (2% each). The jute yarns were dried in an oven at 100 °C for 4 h prior to use and then soaked in the mixed resin, and the wetted yarns were pulled through the glass tube by hand. The samples within the glass tubes were cured at room temperature for 24 h and then cured in an oven at 80 °C for 4 h. The glass tubes were then broken cleanly to release the composite rods. Composites reinforced with 36 wt % jute were prepared for the investigation. Vinylester resin rods were cast using glass tubes as the molds by the same method as used for preparing the composites. Test Methods. Composites reinforced with the treated and untreated jute yarns were tested for their flexural strength under three-point bending in an Instron 4303 machine in accordance with standard method ASTM D790M-81. Test specimens were 120-mm-long cylindrical rods having a diameter of 6 mm. A span of 100 mm was employed with a crosshead speed of 2 mm/min. The flexural strength and flexural modulus were determined using the following equations
flexural strength )
8FL πd3
flexural modulus )
4mL3 3πd4
and
where F is the load, L is the span, d is the diameter of the specimen, and m is the slope of the initial straight-line portion of the load-displacement curve. Surface characteristics of the untreated and treated jute yarns and the fractured surfaces of the composites were investigated by SEM in a LEO S 440 instrument, using a voltage of 15 kV. Dynamic mechanical analyses (DMA) of the composite samples were carried out on a TA Instruments DMA 983 apparatus. The test specimen was clamped between the ends of two parallel arms, mounted on low-force flexure pivots allowing
Figure 3. SEM micrographs of (a) untreated, and (b) 1%, (c) 2%, and (d) 5% shellac-treated jute yarns.
motion only in the horizontal plane. The testing was done in a nitrogen atmosphere at a fixed frequency of 1.0 Hz (oscillation
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Table 1. Flexural Properties of 36 wt % Jute-Reinforced Vinylester Composites Reinforced with Untreated and Shellac-Treated Jute Yarns modulus (GPa)
flexural strength (MPa)
breaking energy (J)
toughness (MPa)
breaking strain (mm/mm)
sample
value
std dev
value
std dev
value
std dev
value
std dev
value
std dev
untreated 1% 2% 5%
10.670 11.650 10.520 9.742
0.54 0.62 0.65 0.42
136.1 139.4 128.7 121.2
0.56 0.83 0.44 0.91
0.5021 0.5009 0.4389 0.4371
0.07 0.04 0.03 0.02
0.0105 0.0101 0.0088 0.0087
0.001 0.0008 0.0006 0.0018
0.026 0.025 0.023 0.024
0.002 0.001 0.001 0.003
amplitude of 0.3 mm) and a heating rate of 5 °C/min. The samples were evaluated in the temperature range of 40-210 °C. Results and Discussion Shellac, having plenty of -OH and -COOH groups in its structure (Figure 1), can take part in hydrogen bonding with the fibers, as well as with the vinylester resin, which also bears -OH groups in the repeating units of its backbone chain (Figure 2). In our experiments, we used three concentrations (1%, 2%, 5%) of shellac in alkaline solution for fiber treatment. The surface morphologies of the untreated and shellac-treated jute yarns are shown in Figure 3a-d. The increased deposition of shellac on the surface of the treated jute yarns is clearly evident in the images.
The flexural properties of 36 wt % jute-yarn-reinforced vinylester resin composites before and after shellac treatment are reported in Table 1. For each data point, a minimum of five samples were tested, and the value given is the mean of those results. An improvement was observed in the case of 1% shellactreated composites, where the modulus was improved by 9%. However, no improvement was observed for 2% and 5% shellactreated composites. The flexural strength also showed a similar trend, although the improvement was not very significant. The breaking energy and the breaking strain values did not show any significant change in the treated composites. The variations of the flexural strength and modulus of the composites with shellac concentration are shown in Figure 4a and b, respectively. As shellac is expected to play the role of a coupling agent between the resin and the jute fiber, we can say that the interfacial strength of these composites will depend on the juteshellac, shellac-resin, and jute-resin bond strengths
Itotal interfacial strength ) I1 + I2 + I3 where I1 ) Ijute-shellac bond strength, I2 ) Ishellac-resin bond strength, and I3 ) Ijute-resin bond strength. In the case of the untreated composites, only I3 is active. For shellac-treated jute-reinforced composites, all of the factors play active roles. Interactive dominance of one factor over the other controls the composite strength properties. In the case of 1% treated composites, I1 + I2 + I3 all contribute significantly. As the shellac content was increased, the contributions of I1 and I2 became more predominant, and that of I3 was lowered. It can be observed from the test results that higher amounts of shellac coating (2% and 5%) did not help to improve the interfacial strength, but the presence of shellac in small amounts might be creating an optimum balance of all three factors, enhancing the composite strength properties.
Figure 4. Variations of the (a) flexural strength and (b) modulus of untreated and 1%, 2%, and 5% shellac-treated jute-yarn-reinforced composites.
Figure 5. Variation of the storage modulus values of untreated and 1%, 2%, and 5% shellac-treated jute-yarn-reinforced composites as a function of temperature.
Ind. Eng. Chem. Res., Vol. 45, No. 8, 2006 2725 Table 2. Results of DMT Analysis of 36 wt % Raw and Shellac-Treated Jute-Yarn-Reinforced Vinylester Composites
sample vinylester resin untreated jute/ vinylester composite 1% shellac-treated jute/vinylester composite 2% shellac-treated jute/vinylester composite 5% shellac-treated jute/vinylester composite
storage modulus at 35 °C 1.862 7.549 8.106 6.303 5.821
peak 1 peak 1 peak 2 peak 1 peak 2 peak 1 peak 2 peak 1 peak 2
temperature (°C)
loss modulus (MPa) Emax
damping parameter tan dµRξ
Emax
tan dµRξ
298.0 655.7 331.0 835.8 271.6 684.9 174.3 612.7 189.8
5.225 0.1661 0.1774 0.2153 0.1576 0.2379 0.1816 0.2191 0.1778
100.2 104.4 131.4 105.5 132.5 104.3 137.9 106.1 137.3
158.4 111.0 134.4 109.7 133.6 109.1 137.3 109.7 137.9
In the dynamic mechanical analysis also, the highest storage modulus at room temperature was observed in the case of 1% treated composites, which is in agreement with the flexural modulus values. Figure 5 illustrates the variation of the storage moduli (E′) of the untreated/shellac-treated jute-yarn-reinforced composites as a function of temperature. As the temperature was raised, the storage modulus values showed a significant fall in the temperature range of 90-140 °C. In Figure 5, the initial part of the curve is the glassy zone, where there is no molecular mobility in the composites. This is followed by a transition zone where molecular mobility is induced in the material, resulting in a significant fall in the storage modulus values. Finally, there is a modulus plateau region at higher temperature, where the matrix resin behaves like a rubber. The incorporation of reinforcing yarns into the matrix raises its storage modulus compared to that of the neat resin and also improves the rubbery plateau region, which is imperative for the reinforcement to improve the thermal-mechanical properties of the composites at higher temperature. The effect of the reinforcement is more significant in the rubbery zone than in the glassy zone, as the abrupt drop in the modulus of the matrix resin was restricted by the fiber stiffness.9 The storage modulus values of the composites obtained at 40 °C showed a trend similar to that observed for the flexural modulus values of the composites at room temperature. The 1% shellac-treated jute-reinforced composites exhibited the highest modulus values under both static and dynamic conditions (shown in Figures 4b and 5, respectively). Table 2 reports the loss modulus (Emax′′) values and the Emax′′ peak temperatures of the untreated/shellac-treated jute-yarnreinforced vinylester composites. The vinylester resin showed
a loss modulus peak at 100.2 °C that was attributed to the mobility of the resin molecules. This peak is also considered to be the glass transition temperature (Tg) of the resin.9-11 Figure 6 shows the temperature dependencies of the loss modulus values of the composites. A second rise is also evident in the loss modulus curve of each of the composites, which is referred to as peak 2 in Table 2. This can be explained by the fact that two distinct regions of restricted mobility exist in a composite. The polymer chains nearest to the reinforcing material are tightly bound, and their mobility is highly restricted. Beyond the tightly bound chains remain the loosely bound chains, which are more restricted in mobility than the bulk polymer but not as restricted as the tightly bound chains at the interface. The first transition peaks in the composites could be due to the mobility of the loosely bound chains away from the interface, and the slight increase in temperature (from 100.2 °C for the neat resin to approximately 104-106 °C for the composites) could be due to the restricted mobility of the chains compared to the neat resin. The second transition peak appeared at 130132 °C, which could be attributed to the mobility of the tightly bound chains at the interface. Similar two-peak regions in the composite samples have also been reported by others.9-11 A similar set of double peaks was observed in the E′′ vs T curves of the wood-polymer composites,12 where the higher transition peak was assigned to the initiation of micro-Brownian motion of the immobilized polymer molecules in the vicinity of the solid surface and the other peak was due to the polymer molecules in the bulk phase. The E′′ value corresponding to Tg was higher in the composites than in the neat resin (298 MPa) (Table 2). The
Figure 6. Variation of the loss values of untreated and 1%, 2%, and 5% shellac-treated jute-yarn-reinforced composites as a function of temperature.
Figure 7. Variation of the damping parameter of untreated and 1%, 2%, and 5% shellac-treated jute-yarn-reinforced composites as a function of temperature.
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interface, which could easily undergo a larger viscous dissipation, resulting in a higher loss modulus value. Damping Parameter. Similarly to the loss modulus curves, the damping curves (Figure 7) also exhibit two peaks, one around 109-111 °C and the other around 133-137 °C. The ratio of the loss modulus to the storage modulus (E′′/E′), i.e., tan δ, was very high for the unreinforced resin. Incorporation of the reinforcing yarns in the matrix resin restricted the mobility of the resin molecules, increased the storage modulus values, and lowered the viscoelastic lag between the stress and the strain; hence, the tan δ values were decreased in the composites.9 The lowering of the tan δ values in the composites compared to the neat resin was also due to the fact that there was less matrix by volume to dissipate the vibrational energy.9 The fracture surfaces of the untreated and 1%, 2%, and 5% treated composites are shown in Figure 8a-d, respectively. In the untreated composites (Figure 8a), the lumens of the fractured fiber bundles are clearly visible. In the treated composites, deposited shellac is evident in the micrographs. As the shellac concentration was increased, the deposited shellac layers on the jute fiber surface were evident more distinctly at the fractured surface. In the case of 5% treated composites, it is apparent from the micrograph that not much resin could penetrate the thick shellac layer onto the fiber surface, consequently lowering the strength of the composites. Conclusion
Figure 8. Fracture surfaces of (a) untreated and (b) 1%, (c) 2%, and (d) 5% shellac-treated jute-yarn-reinforced composites.
loss modulus value was highest (837 MPa) for the first transition peak in the 1% treated composites. Loss modulus values are known to give a measure of the viscous response of that material and to indicate that portion of the material that will flow under stress. Hence, the highest loss modulus value in the 1% treated composites for the first transition peak could be due to the reduced volume of the free resin molecules away from the
The effect of shellac treatment of reinforcing jute yarns on the mechanical properties of composites is reflected in the results of flexural and dynamic mechanical tests on the composites. (i) The storage modulus values of the composites obtained at 40 °C showed a trend similar to that obtained in the case of the flexural modulus values of the composites at room temperature. The 1% shellac-treated jute-yarn-reinforced composites exhibited the highest modulus value in their flexural test, and this was also reflected in the dynamic mechanical analysis study. (ii) This suggests that, at a 1% concentration of shellac solution, the H-bonding between the jute fiber, resin, and deposited shellac reached an optimum balance, where their interactions imparted the highest modulus to the composite. (iii) All of the composites showed a significant fall in the storage modulus values in the temperature range 90-140 °C. (iv) Two transition peaks were evident in the loss modulus curves of all of the composites. The first transition peak could be due to the mobility of the loosely bound chains beyond the interface, and the second transition (at a higher temperature), which appeared as a small rise, could be due to the initiation of the micro-Brownian motion of the tightly bound chains at the interface. (v) The highest loss modulus value of the first transition peak in the 1% treated composites indicates the highest viscous dissipation of the loosely bound chains away from the interface in these composites, which could be due to lowering of the volume of the free resin molecules. This suggests a larger interphase region, resulting in higher mechanical properties of the composites, which is in good agreement with the obtained results. (vi) The incorporation of reinforcing yarns increased the storage modulus values of the composites compared to that of the unreinforced resin by restricting the mobility of the resin molecules and lowered the viscoelastic lag between the stress and the strain, thus lowering the damping in the composites. As a concluding remark, it can be said that, abundantly available, low-cost natural resins such as shellac can be utilized
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effectively, in their virgin form or in some suitably modified form, for the surface treatment of natural fibers. However, the experimental results indicate that only small quantities of shellac can be effective in enhancing the composite strength. More research will be carried out in the future to explore the natural fiber/natural resin/synthetic resin combinations that are also highly favorable from an environmental point of view. Acknowledgment The authors are indebted to the Council of Scientific & Industrial Research, Government of India, for providing financial assistance during the course of the investigation. Directors, Indian Association for the Cultivation of Science (IACS), Central Glass & Ceramic Research Institute (CG&CRI), and Indian Jute Industries’ Research Association (IJIRA) are deeply appreciated for their interest and facility support. Thanks are due to Mr. K. Banerjee of CG&CRI for his help in sample preparation. Mrs. M. Sarkar of IJIRA is gratefully acknowledged for her support in carrying out the Instron and DMA tests. Mr. S. Shome and U. S. Kundu of GSI are also gratefully acknowledged for recording the SEM images. Literature Cited (1) Bledzki, A. K.; Gassan, J. Composites reinforced with cellulose based fibres. Prog. Polym. Sci. 1999, 24, 221. (2) Lindstrom, T., Wagberg, L. An overview of some possibilities to modify fibre surfaces for tailoring composite interfaces. In Proceedings of the 23rd Risφ International Symposium on Materials Science; Denmark, 2002; p 35.
(3) Rana, A. K.; Mandal, A.; Mitra, B. C.; Jacobson, R.; Rowell, R.; Banerjee, A. N. Short jute fibre-reinforced polypropylene composites: Effect of compatibiliser. J. Appl. Polym. Sci. 1998, 69, 329. (4) Bisanda, E. T. N.; Ansell, M. P. The effect of silane treatment on the mechanical and physical properties of sisal-epoxy composites. Compos. Sci. Technol. 1991, 41, 165. (5) Ray, D.; Sarkar, B. K. Characterization of Alkali Treated Jute Fibers for Physical and Mechanical Properties. J. Appl. Polym. Sci. 2001, 80, 1013. (6) Prasad, S. V.; Pavithran, C.; RohatgI, P. K. Alkali treatment of coir fibres for coir-polyester composites. J. Mater Sci. 1983, 18, 1443. (7) Gassan, J., BledzkI, A. K. Alkali treatment of jute fibres: Relationship between structure and mechanical properties. J. Appl. Polym. Sci. 1999, 71, 623. (8) Ray, D.; Sarkar, B. K.; Rana, A. K.; Bose, N. R. The Mechanical Properties of Vinylester Resin Matrix Composites Reinforced with AlkaliTreated Jute Fibres. Composites A 2001, 32, 119. (9) Ray, D.; Sarkar, B. K.; Das, S.; Rana, A. K. Dynamic mechanical and thermal analysis of vinylester resin matrix composites reinforced with untreated and alkali-treated jute fibres. Compos. Sci. Technol. 2002, 62, 911. (10) Pothan, L. A.; Potschke, P.; Thomas, S. The static and dynamic mechanical properties of banana and glass fibre woven fabric reinforced polyester composites. In Proceedings of the ACUN-3 (Technology ConVergence in Composite Application) 5-9 Feb 2001; University of New South Wales, Sydney, Australia, 2001; pp 452-460. (11) Pothan, L. A.; Thomas, S. Polarity parameters and dynamic mechanical behaviour of chemically modified banana fiber reinforced polyester composites. Compos. Sci. Technol. 2003, 63, 1231. (12) Hon, D. N.-S., Shiraishi, N., Eds. Wood and Cellulose Chemistry; Marcel Dekker Inc.: New York, 1991.
ReceiVed for reView November 18, 2005 ReVised manuscript receiVed January 15, 2006 Accepted February 6, 2006 IE0512800
