Studies on the Water Absorption Properties of Short Hemp–Glass Fiber Hybrid Polypropylene Composites SUHARA PANTHAPULAKKAL AND MOHINI SAIN* Center for Biocomposites and Biomaterials Processing Faculty of Forestry University of Toronto, Toronto, Canada M5S 3B3
ABSTRACT: Hemp fiber is one of the inexpensive and readily available bast natural fibers and hemp-fiber reinforced polymer composite products have gained considerable attraction for automotive interior products. Though extensive research has been made on the performance evaluation of these composite materials, not much data is available on the moisture absorption of the composites, which restricts their use in exterior applications. This study aims to investigate the moisture absorption of short hemp fiber and hemp-glass hybrid reinforced thermoplastic composites to study their suitability in outdoor applications. The water absorption properties and its effect on the tensile properties of hemp and hemp/glass fiber hybrid polypropylene (PP) composites prepared by an injection molding process were investigated. Effect of hybridization on the water uptake and the kinetics of moisture absorption of the hemp fiber composites were evaluated by immersing the hybrid composite samples in distilled water at different temperatures, of 40, 60 and 80 C. The composites showed a Fickian mode of diffusion; however, a deviation was observed at higher temperature and may be attributed to the microcraks developed at the interface and dissolution of the lower molecular weight substances from the natural fibers. Equilibrium moisture content (Mm) showed that water up take of 40 wt% hemp fiber reinforced PP composites was highest and incorporation of glass fiber decreased the water uptake significantly (40%). Equilibrium moisture content was found to be independent of temperature, while diffusion coefficient (D) was increased with temperature. Effect of water absorption on the tensile properties of the composites showed that there is a significant reduction in strength and stiffness. It was observed that hybridization with glass fibers did not have any significant effect on the strength properties of the aged samples. The tensile properties of the re-dried aged samples showed an increased retention of strength properties after drying; however a complete recovery of the properties has not been achieved. This indicated that water absorption is not a physical process and permanent damage occurred to the composite after aging. KEY WORDS: natural fiber composites, hemp, fiber, water absorption, tensile properties.
*Author to whom correspondence should be addressed. E-mail:
[email protected]
Journal of COMPOSITE MATERIALS, Vol. 41, No. 15/2007 0021-9983/07/15 1871–13 $10.00/0 DOI: 10.1177/0021998307069900 ß 2007 SAGE Publications
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INTRODUCTION NATURAL FIBER reinforced polymer composites have experienced a tremendous growth in the composite industry because of their eco-friendliness and cost effectiveness [1–3]. They have already found use in building, construction, automotive, and packaging applications. Despite the attractiveness and environmental advantages of the natural fiber composites, their poor hygrothermal resistance compared to synthetic fiber reinforced plastics or engineering thermoplastics restricts their use in many structural as well as outdoor applications. The absorbed moisture results in to the deterioration of mechanical properties since the water not only affects the unfilled polymer matrices physically and/or chemically but also attacks the hydrophilic natural fiber as well as the fiber-matrix interface [4–14]. In order to expand the realm of natural fiber composites, it is required to increase the moisture diffusion resistance and thereby increase the retention of the properties after aging. Several research works have been reported on the hygrothermal aging of natural fiber, such as wood fiber, sisal, flax, pineapple leaf fiber, jute, oil palm, and bamboo fiber, reinforced thermoset resins and thermoplastics, which shows the relevancy of the subject [4–14]. Most of the studies in this area are mainly focused to improve the inherent weakness of the natural fiber composites and has been accomplished by increasing the hydrophobicity of the natural fibers through various physical and chemical pretreatments of the fibers [5,8–10,12,14]. Incorporation of compatibilizers also enhances the resistance to moisture through improved interaction between the fibers and the polymer matrix [7,8,11,13,14]. Hybridization with a small amount of moisture resistant and corrosion resistant synthetic fibers is another technique by which the strength and the stiffness as well as the moisture resistance of the natural fiber composites can be improved. Hybrid fiber composite materials can be comprised of a combination of more than one type of fibers in the same matrix. Though in principle several fibers can be incorporated into the hybrid system, a combination of only two types of fiber would be the most beneficial. By hybridization, it is possible to achieve a balance between performance properties and cost of the composites, which would not be obtained with a single kind of reinforcement. In other words, by careful selection of reinforcements it is possible to engineer the material to better suit various practical requirements with economic benefits. Researchers have reported that incorporation of glass fibers with natural fibers like sisal, bamboo, and oil palm empty fruit bunch (OPEB) fibers, in thermoset as well as thermoplastic matrix resulted in improved performance [15–18]. Hemp fiber is one of the inexpensive and readily available bast natural fibers and has attracted considerable attention of researchers and auto-parts manufacturers in Europe and North America. Several researchers exploited the reinforcing potential of hemp fibers in developing thermoplastic and thermoset composites using different forms of hemp fibers and various processing techniques [19–26]. However, most of the studies concentrated on the performance properties and not many data are available on the durability of hemp fiber composites [27–29]. Misra and Naik studied water absorption of various natural fiber filled novolac resin and reported that hemp fiber–filled composites showed highest water absorption [27]. Recently, Rouison et al. from our group have studied the durability of resin transfer molded hemp fiber–polyester resin with the help of magnetic resonance imaging technique [29]. We have already reported that short hemp/glass fiber hybrid composites have enough potential to be used for structural applications where high stiffness is required [30]. In this study the effect of hybridization
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of hemp fibers with glass fibers on the water absorption properties of the hemp fiber reinforced polypropylene composites were studied in order to study suitability of these composites in outdoor applications. Special emphasis was given to the study of the mechanism of water absorption from the kinetic characteristics and the effect of aging on the tensile properties of the composites.
EXPERIMENTAL Materials Polypropylene PP 6331 obtained from Himont was used as the polymer matrix for the composites. Hemp fibers with a fiber length of 12 mm used in the study was native hemp grown in Ontario and was obtained from Hempline Inc., Ontario. E-glass fibers with 6 mm length were used together with hemp fibers for making hybrid composites and were obtained from Plastic World, Canada. The compatibilizer used in the preparation of composites was Orevac-CA 100, which is a maleated polypropylene supplied by Arkema, Canada.
Processing Formulation of the composites prepared for this study is given in Table 1. Total weight percentage of the fibers in the composites was fixed at 40. The ingredients were melt blended using a Brabender Plasticorder at 170 C and at 60 r.p.m. for 5 min. The meltblended materials were allowed to cool to room temperature and then granulated using a C.W. granulator. Granulates prepared were then injection molded into standard ASTM test specimens for tensile, flexural and impact strength determination. Injection molding conditions used were: injection, temperature, 200 C; injection time, 8 s; cooling time, 25 s; and mold opening time, 2 s.
Water Absorption Studies Water absorption studies were performed following the ASTM D570-98 method. In order to study the kinetics of water absorption, six injection molded impact specimens of every sample were submerged in distilled water at different temperatures, 40, 60, and 80 C. The samples were taken out periodically and weighed immediately, after wiping out the water on the surface of the sample, using a precise four-digit balance to find out the
Table 1. Formulation of composites. Designation of samples A B C D
PP (wt%)
Glass fiber (wt%)
Hemp fiber (wt%)
Compatibilizer (wt%)
55 55 55 55
0 5 10 15
40 35 30 25
5 5 5 5
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content of water absorbed. All the samples were dried until constant weight with the four-digit balance, previously to be immersed in water. The percentage of water absorption at any time, t, Mt, was calculated by the equation: WðtÞ Wð0Þ 100 ¼ water absorption, Mt ð%Þ Wð0Þ where W(t) is the weight of the sample at time t, and W(0) is the initial weight of the sample (at t ¼ 0). The percentage equilibrium moisture absorption, Mm, was calculated as an average value of several constitutive measurements that showed no appreciable additional absorption. To study the effect of water absorption on the mechanical properties, six tensile test specimens of every sample were immersed in distilled water at room temperature and determined the water content periodically as explained before until the content of water remained invariable or remained more or less the same.
Tensile Test Tensile properties were measured with a standard computerized testing machine (Sintech Model 20) in accordance with the ASTM D-638 procedure at a crosshead rate of 12.5 mm/min. Tensile tests were performed on the dry samples before aging, wet samples after the samples reached the saturation limit and on re-dried aged samples in order to investigate the mechanism and extent of deterioration of properties after aging. Drying of the wet samples was carried out at 60oC for 5 days in an air oven.
RESULTS AND DISCUSSION Water Uptake Water absorption curves of injection molded PP, hemp/glass fiber hybrid composites (A–D) at 40 C is shown in Figure 1, where percentage of water absorbed is plotted against the square root of the soaking time. Each data point represents the average of six samples. In all the samples, except PP, percentage moisture absorption, Mt, increases steadily with t1/2 in the initial stage and then tends to level off following the saturation point, indicating a Fickian mode of diffusion. The equilibrium moisture content at the saturation level (Mm) of PP and the composites are summarized in Table 2. Water absorption of polypropylene is 0.40% and upon reinforcing with 40 wt% of hemp fiber a large increase in water uptake (Mm ¼ 8.5) was observed. This is attributed to the hydrophilic character of natural fibers since the matrix had little effect on the amount of water absorbed. A high amount of natural fiber in the composite showed a high uptake of water. The variation in the water uptake of the hemp-glass hybrid PP composites as a function of glass fiber content at 40 C is also evident in Figure 1. Incorporation of glass fiber in the hemp fiber PP composites decreased the equilibrium moisture content significantly and is attributed to the removal of hydrophilic natural fiber with the glass fiber in the composite. Incorporation of 5 wt% of glass fiber decreased the maximum moisture content in the composite by 10% and
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Water absorption (%)
8
6
4
2
0 0
10 Polymer
20 Time1/2 (h)
0% glass
5% glass
30 10% glass
40 15% glass
Figure 1. Water absorption curves of hemp fiber composites with different glass fiber content at 40 C.
Table 2. Equilibrium moisture content of composites. Equilibrium moisture content, %
Content of glass fiber, wt%
40 C
60 C
80 C
PP 0 5 10 15
0.49 8.55 7.67 6.25 5.07
0.49 8.57 7.66 6.38 5.20
0.50 8.39 7.41 6.20 5.06
shows a steady decrease with further addition of glass fiber. Equilibrium moisture content of the composites is decreased by 40% with the addition of 15 wt% glass fiber content. Effect of temperature on the water absorption behavior can be understood from the water absorption curves of PP and composites at different temperatures, 40, 60, and 80 C (Figures 1–3). At all temperatures PP shows the lowest water absorption, whereas hemp fiber composites exhibited the maximum water uptake. Though there is an increase in the initial uptake of water with increase in temperature, it is evident that in all the cases, equilibrium moisture content remains more or less the same indicating that Mm is independent of temperature (Table 3). The higher uptake of water may be due to the micro cracks developed on the surface and inside the material and/or natural fiber swelling due to moisture and the resulting fiber debonding from the matrix due to the moist and high temperature environment. As a result of the enhanced initial uptake of water, increase in the immersion temperature has considerably shortened the time required to reach equilibrium. For example, by increasing the temperature from 40 C to 60 C the time required to reach equilibrium is decreased by about 170 h (Figures 1 and 2) and further decrease was observed when the temperature increased
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Water absorption (%)
8
6
4
2
0 0
5 Polymer
10
15
0% glass
20 Time1/2 (h) 5% glass
25
30
10% glass
35
40
15% glass
Figure 2. Water absorption curves of hemp fiber composites with different glass fiber content at 60 C.
10
Water absorption (%)
8
6
4
2
0 0
10
20
30
40
Time1/2 (h) Polymer
0% glass
5% glass
10% glass
15% glass
Figure 3. Water absorption curves of hemp fiber composites with different glass fiber content at 80 C.
from 60 C to 80 C (Figures 2 and 3). Another important observation noticed is that at 80 C, moisture uptake reaches a maximum and then decreases thereafter. This may be due to the enhanced leaching of low molecular weight soluble materials from natural fiber at high temperature that leads to material loss after prolonged period of water immersion [31].
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Kinetics of Water Absorption Moisture absorption into the composite materials is considered by three major mechanisms and they include. (i) diffusion of water molecules inside the microgaps between polymer chains; (ii) capillary transport of water molecules into the gaps and flaws at the interface between fibers and the polymer due to the incomplete wettability and impregnation; and (iii) transport of water molecules by micro cracks in the matrix, formed during the compounding process [13,16,32]. Though all three mechanisms are active, the overall effect can be modeled conveniently considering the diffusion mechanism. There are three different kinds of diffusion behavior and they include Case I or Fickian diffusion, Case II and non-Fickian or anomalous diffusion [13,33,34]. The three cases of diffusion can be distinguished theoretically by the shape of the sorption curve, which is represented by the empirical equation: Mt ¼ ktn Mm where Mt is the moisture content at time t, Mm is the moisture content at the equilibrium and k and n are constants. The value of coefficient n shows different behavior between the three cases. For Fickian diffusion n ¼ 1/2, while for Case II n ¼ 1 and for anomalous diffusion n shows an intermediate value (1/2
