Ind. Eng. Chem. Res. 2006, 45, 6489-6492
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Influence of Mercerization on the Dynamic Mechanical Properties of Bamboo, a Natural Lignocellulosic Composite Mahuya Das and Debabrata Chakraborty* Department of Polymer Science and Technology, UniVersity of Calcutta, 92, A.P.C. Road, Kolkata 700009, India
Bamboo fibers that have been treated in NaOH solutions of varying concentrations were subjected to differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA) studies, respectively. The moisture desorption peak (and the enthalpy values associated with it) was moved to higher values as the alkali concentration increased up to 15% and shifted to lower values beyond that temperature. A broad exotherm was observed in all of the DSC curves for alkali treatments up to 15%. Beyond that concentration, two comparatively smaller exothermic peaks appeared for the 20% and 50% alkali-treated samples. DMTA study of bamboo strip samples reveals that the room-temperature value for the storage modulus (E′) of the untreated bamboo strips is increased by ∼400% in the case of 15% alkali-treated samples, and the rate of decrease in the modulus over the temperature range of 140-180 °C is also maximum for those samples. The untreated bamboo samples show a primary loss modulus (E′′) peak at 111.8 °C, which is shifted to higher temperatures for alkali-treated samples. The damping parameter (tan δ) is also maximum for untreated samples. 1. Introduction Currently, industries are focusing more and more attention toward ligno-cellulosic based natural fibers as reinforcement for composites. They are relatively abundant and inexpensive. There are many reports on composites based on natural fibers.1 In the past, some attempts have been made to study the different characteristics of various natural fibers such as jute, sisal, pineapple leaf fiber, broom, and wood.2-6 Bamboo is another potential reinforcement, given its high strength. Literature work on bamboo and bamboo composites have been reported,7,8 but a thorough study about both the composite and bamboo itself still is needed. Bamboo belongs to the perennial grass family Bambusoideae, with weak stems or culms. It is a natural ligno-cellulosic composite, in which cellulose fibers are embedded in a lignin matrix. Bamboos occur mostly in natural vegetation of tropical, subtropical, and temperate regions and are abundant in tropical Asia. In these areas, bamboos have been used as a primary material for the construction of houses, scaffolding, ladders, and fencing. Bamboo culm itself resembles a unidirectional fiber-reinforced composite with many nodes along its length. The current availability of bamboo fibers is limited, and, hence, a hand-ful of studies that are related to the characterization of these fibers is available. Amada et al. have reported on the structure variation in bamboo, relative to cross section and height.9 Jain et al. have studied the mechanical properties of bamboo.7 The results of these studies revealed that the mechanical properties of bamboo vary along and across the cellulose fibers. Despande et al. have developed methods for the extraction of bamboo fibers, and they evaluated their mechanical properties.10 Das et al. have investigated the effect of mercerization on the mechanical properties of bamboo11 and bamboo strip-novolac resin composites.12 The fact that bamboo represents itself as a composite material draws attention to the evaluation of an important property for the composite: dynamic mechanical properties. In this present paper, the dynamic mechanical and thermal properties of * To whom correspondence should be addressed: Tel.: 91-033-23508386. Fax: 91-033-2350-9755. E-mail address: debchakrabarty@ yahoo.co.in.
bamboo strips were investigated to gain a better understanding about the temperature-dependent strength properties of bamboo. 1.1. Dynamic Mechanical and Thermal Analysis. Dynamic mechanical and thermal analyses at a selected fixed frequency over a range of temperature have grown as useful analytical techniques for the characterization of polymeric materials, such as homopolymers, copolymers, blends, and composites.13,14 The evaluation is based on the determination of the temperature dependence of the dynamic moduli, stress relaxation, mechanical loss, and damping phenomena. The results from dynamic mechanical thermal analysis (DMTA) studies give us information about the glass-transition temperature and associated features and can be used as a means of “fingerprinting” polymer types. Thus, the DMTA method is suitable for making sensitive conclusions about changes in the mobility of molecules and for investigating phase structure and as well as morphology. The DMTA properties of a composite material are dependent on the fiber content, the compatibilizer, additives, the orientation of the fiber, and the mode of testing. There are many studies related to the dynamic mechanical thermal (DMT) behavior of glass, carbon, and natural-fiber-reinforced polymer-matrix composites.15-17 Although syntheticfiber-reinforced composites have been extensively used for DMT analyses, similar studies that are relative to natural fiber composites are not abundant. Hassan et al. have reported a DMT study of esterified bagasse fiber.18 However, DMT analysis of any natural fiber itself (i.e., without any polymeric matrix) is not very common. Polymers are viscoelastic material. According to the DMTA principle, when internal molecular motion occurs, the material responses in a viscoelastic manner (i.e., mechanical stress is not instantaneous but develops before the relaxation time is reached). Thus, the strain response lags behind the stress. This phase lag results from the time necessary for molecular rearrangements and is associated with relaxation phenomena. The tangent of this phase lag is described as the damping factor and is denoted by “tan δ”. This mechanical damping or internal friction indicates the amount of energy that is dissipated as heat during the deformation of the material. The internal friction of material is important as a property index and for environmental
10.1021/ie0603971 CCC: $33.50 © 2006 American Chemical Society Published on Web 08/19/2006
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and industrial applications. The dynamic modulus or storage modulus (E′) and the loss modulus (E′′) are two other dynamic mechanical properties of materials that are observed when the materials are deformed under periodic forces. The storage modulus is an indicator of the stiffness of polymeric material under the dynamic stress and strain conditions and is dependent on the type of polymer matrix, interface, temperature, and frequency. The dynamic loss modulus or internal friction is sensitive to many types of molecular motion, transitions, relaxation processes, and structural heterogeneities and morphology of multiphase systems. Therefore, interpretations of the dynamic mechanical properties at the molecular level are of great scientific and practical importance in understanding the mechanical behavior of all polymeric materials. 2. Experimental Section 2.1. Materials. Bamboo that belonged to the variety Bamboosa balcua were supplied by the Forum of Scientists, Engineers and Technologists (FOSET) in West Bengal, India. It was supplied in the particulate form (30-36 mesh size) and also in strip form (with average dimensions of 100 mm × 15 mm × 1.5 mm). This specimen is used throughout the study. 2.2. Alkali Treatment. Bamboo fibers (both in strips and dust form) were soaked in caustic soda solution with varying concentration (10%, 15%, 20%, 25%, and 50%) at ambient temperature, maintaining a liquor ratio of 15:1. The fibers were kept immersed in the alkali solution for 1 h. The fibers then were copiously washed with distilled water, to remove any traces of alkali that may have been sticking to the fiber surface, and subsequently neutralized with 2% sulfuric acid solution. The neutrality was checked with litmus paper. The fibers then were dried in a hot air oven at 105 °C. 3. Testing 3.1. Differential Scanning Calorimetry (DSC) Analysis. DSC analysis of the bamboo dust samples was performed using a thermal analyzer (Mettler DSC 25 module). All of the measurements were made under a N2 flow (150 mL/min), keeping a constant heating rate of 10 °C/min and using an alumina crucible with a pinhole. 3.2. Dynamic Mechanical and Thermal Analysis (DMTA). DMTA measurements were performed on a TA Instruments model DMA 983 system in flexural bending mode with a peakto-peak amplitude displacement of 0.3 mm. The test samples were clamped between the ends of two parallel arms, mounted on low-force flexure pointers that allowed motion only in the horizontal plane. Typical clamped sample dimensions were 60.0 mm × 12.5 mm × 2 mm. The samples were tested in a nitrogen atmosphere in a fixed-frequency mode (1.0 Hz) and a heating rate of 5 °C/min. The temperature ranged from 30 °C to 220 °C, under a nitrogen atmosphere. All of the data used for this paper were from the second heat measurements in the dynamic mechanical analysis (DMA). This ensures that each sample has the same thermal history. A minimum of four samples was measured for each group of bamboo strip specimens. 4. Results and Discussion 4.1. DSC Analysis. The DSC curves of untreated and alkalitreated bamboo dust are shown in Figure 1, and the corresponding thermal characteristics are given in Table 1. Both the untreated and treated fibers exhibit an endothermic peak at
