Water Sorption Studies of Hybrid Biofiber-Reinforced Natural Rubber

Biomacromolecules 2005, 6, 2969-2979

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Water Sorption Studies of Hybrid Biofiber-Reinforced Natural Rubber Biocomposites Maya Jacob,† K. T. Varughese,‡ and Sabu Thomas*,† School of Chemical Sciences, Mahatma Gandhi University, Priyadarshini Hills P.O. Kottayam, Kerala, India-686 560, Central Power Research Institute, Polymer Laboratory, Bangalore, India-560 080 Received April 19, 2005; Revised Manuscript Received May 31, 2005

Hybrid biofibers (sisal and oil palm) were incorporated into natural rubber matrix. The water absorption characteristics of the composites were evaluated with reference to fiber loading. The influence of temperature on water sorption of the composites is also analyzed. Moisture uptake was found to be dependent on the properties of the biofibers. The mechanism of diffusion in the gum sample was found to be Fickian in nature, while in the loaded composites, it was non-Fickian. Sisal and oil palm fibers were subjected to different treatments such as mercerization and silanation. The effect of chemical modification on moisture uptake was also analyzed. Chemical modification was seen to decrease the water uptake in the composites. The thermodynamic parameters of the sorption process were also evaluated. Activation energy was found to be maximum for the gum sample. 1. Introduction Lignocellulosic fiber-reinforced composite materials are used more and more in all kinds of applications, and they are turning out to be the apple of the eye of the scientific community. The major interest in plant fiber-reinforced composites is due to their favorable properties such as low specific density, high strength, and good acoustic insulating properties. Among the different natural fibers, sisal and oil palm fibers appear to be promising materials because of the high tensile strength of sisal fiber and the toughness of oil palm fiber. Therefore, any composite comprising these two fibers will exhibit the above desirable properties of the individual constituents. The properties of a hybrid composite mainly depend on the fiber content, length of individual fibers, orientation, extent of intermingling of fibers, fiber-to-matrix bonding, and arrangement of both the fibers. The strength of the hybrid composite is also dependent on the failure strain of individual fibers. Maximum hybrid results are obtained when the fibers are highly strain-compatible. One interesting area of study in the case of composites is the moisture uptake of fiber-reinforced composites. A major drawback of vegetable fiber-reinforced composites is their affinity toward moisture, leading to low degrees of adhesion between fiber and matrix. This limitation can be remedied in the form of chemical modifications. A large number of studies have been conducted to analyze the water uptake in natural fiber-reinforced composites. * Corresponding author. E-mail: [email protected]. Tel: 91-4812730003, 91-481-2731036. Fax: 91-481-2561190. † Mahatma Gandhi University. ‡ Central Power Research Institute.

The water uptake behavior of sisal fiber-reinforced benzylated fir sawdust composites was investigated by Lu et al.1 The experimental results indicated that water absorption behavior of the composites was mainly controlled by the reinforcing fiber and the fiber/matrix interfacial characteristics. In a similar study, Alvarez et al.2 analyzed the water absorption behavior of composites made from a biodegradable matrix and alkaline-treated sisal fibers. The matrix in question was a commercial product called MaterBi-Y, which was based on a cellulose derivatives and starch system. The water absorption in natural fibers such as sisal, coir, luffa sponge, and cellulose (from pulp) reinforced polypropylene composites were investigated by Espert et al.3 The authors carried out the experiments at three different temperatures: 23, 50, and 70 °C. The process of absorption of water was found to follow the kinetics and mechanisms described by Fick’s theory. In addition, the diffusivity coefficient was found to be dependent on the temperature as estimated by means of Arrhenius law. A decrease in tensile properties of the composites was demonstrated, showing a great loss in mechanical properties of the water-saturated samples compared to the dry samples. An attempt to study the moisture uptake characteristics of hybrid systems was performed by Mishra et al.4 The composite systems chosen were sisal/glass and pineapple/ glass fiber-reinforced polyester composites. Composites were prepared by varying the concentration of glass fiber and by subjecting the biofibers to different chemical treatments. The authors observed that the water uptake of hybrid composites was less than that of unhybridized composites. In another interesting study, the water absorption behavior of short sisal fiber-reinforced polystyrene composites has been studied with special reference to fiber loading and

10.1021/bm050278p CCC: $30.25 © 2005 American Chemical Society Published on Web 09/08/2005

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fiber-matrix interface modification. The interface modifications performed were benzoylation, polystyrene maleic anhydride (PSMA) treatment, toluene diisocyanate treatment, and silane treatment The authors5 observed that upon chemical modification water uptake was found to decrease, while there was a definite amount of increase upon fiber loading. Bledzki6 looked into the influence of a coupling agent (maleic anhydride-polypropylene) on the water sorption properties of different types of wood fiber (hard wood fiber, soft wood fiber, long wood fiber, and wood chips) reinforced polypropylene composites. A reduction in water uptake was observed and was attributed to better compatibilization between fiber and matrix. The moisture absorption characteristics of flax and hemp fiber-reinforced unsaturated polyester resin was investigated by Burgueno et al.7 The authors found that that the biocomposite material systems, on average, absorbed 4 times as much moisture as the E-glass material system. The moisture uptake was also found to increase with fiber volume fraction. In an innovative study, the water uptake in selfreinforced composites of sisal fiber was analyzed by Lu et al.8 The authors prepared the composites by gradient plasticization along the radial direction of sisal fibers. Through slight benzylation treatment, skin layers of sisal fibers were converted into thermoplastic material, while the core of the fiber cells remained unchanged. Self-reinforced composites of sisal were then prepared using hot pressing, in which the plasticized parts of sisal serve as the matrix and the unplasticized cores of the fibers as the reinforcement. The water uptake of the prepared composites was found to be lower when compared to the sisal fiber. This was attributed to the presence of benzyl groups acting as barriers and blocking the gaps between the cells, thus preventing the entry of water. The water sorption characteristics of jute fiber-reinforced high-density polyethylene (HDPE) composites were investigated by Mohanty et al.9 In this work, jute fiber was subjected to chemical treatments such as mercerization, cyanoethylation, and addition of maleic anhydride grafted polyethylene (MAPE). They observed minimum water uptake in the treated composites and attributed it to better fiber/ matrix adhesion. The moisture uptake in MAPE-treated composites was found to be reduced by nearly 47%. Another interesting study involving biodegradable composites was analyzed by Tserki et al.10 The authors used cotton waste as a source of reinforcing fibers in biodegradable polyester (bionolle 3020). Maleic anhydride-grafted bionolle (bionolle-g-MA) was used as a compatibilizer. The compatibilizer was found to decrease the water sorption characteristics of the composites. In a recent study, the water and steam uptake of natural fiber-reinforced novolac resin was studied by Mishra et al.11 In this work, the natural and maleic anhydride esterified fibers of banana, hemp, and sisal were used. The absorption of water was found to increase with an increase in time from 2 to 30 h in all fiber composites tested. Among all the composites, the maximum absorption of water was found in hemp fiber composite and the minimum in maleic anhydride

Jacob et al. Table 1. Properties of Sisal Fiber Chemical Constituents (%) cellulose hemicellulose lignin wax ash Physical Properties diameter (mm) tensile strength (MPa) Young’s modulus (GPa) elongation at break (%) microfibrillar angle (°)

78 10 8 2 1

0.1212 530-630 17-22 3-7 20-25

Table 2. Properties of Oil Palm Fiber Chemical Constituents (%) cellulose hemicellulose lignin ash content Physical Properties diameter (µm) tensile strength (MPa) Young’s modulus (MPa) elongation at break (%) microfibrillar angle (°)

65 19 2

150-500 248 6700 14 46

treated sisal fiber composite. The maleic anhydride esterified fiber composite showed less absorption of water than the untreated fiber composites. Among all six composites, steam absorption was found to be maximum in untreated hemp fiber composite and minimum in maleic anhydride treated banana fiber composite. The above studies indicate that, though a large number of investigations have been carried out in the field of water sorption for biofiber-reinforced composites, the water transport properties of hybrid biofiber-reinforced natural rubber composites have not been looked into. This manuscript attempts to study the water uptake characteristics of sisaloil palm hybrid fiber-reinforced natural rubber composites. The influence of temperature on the diffusion process has also been monitored. The thermodynamics of the sorption process is also evaluated. 2. Experimental Section 2.1. Materials. Sisal fiber was obtained from Sheeba Fibers, Poovancode, Tamil Nadu. Oil palm fiber was obtained from Oil Palm India Limited. Natural rubber used for the study was procured from Rubber Research Institute of India, Kottayam. All other ingredients used were of commercial grade. The physical properties of sisal and oil palm fibers are given in Tables 1 and 2. 2.2.A. Alkali Treatment. Sisal and oil palm fibers of lengths 10 mm and 6 mm were treated for 1 h with NaOH solutions of concentrations 0.5%, 1%, 2%, 4%, respectively. The fibers were further washed with water containing acetic acid. Finally, the fibers were washed again with fresh water and dried in an oven.

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Water Sorption Studies of Rubber Biocomposites Table 3. Formulation of Mixes

(A) Formulation of Mixes A to E (Fiber Loading) ingredients

gum

B

C

A

D

E

NR ZnO stearic acid TDQa CBSb sulfur sisal fiber untreated fiber length (10 mm) oil palm fiber untreated fiber length (6 mm)

100 5 1.5 1 0.6 2.5

100 5 1.5 1 0.6 2.5 5

100 5 1.5 1 0.6 2.5 10

100 5 1.5 1 0.6 2.5 15

100 5 1.5 1 0.6 2.5 20

100 5 1.5 1 0.6 2.5 25

5

10

15

20

25

(B) Formulation of Mixes (phr) ingredients NR ZnO stearic acid resorcinol Hexac TDQ CBS sulfur sisal fiber treatment

fiber length, (10 mm) oil palm fiber treatment fiber length (6 mm) a

I

J

K

L

P

Q

R

100 5 1.5 7.5 4.8 1 0.6 2.5 15 0.5% NaOH 1 hr

100 5 1.5 7.5 4.8 1 0.6 2.5 15 1% NaOH 1 hr

100 5 1.5 7.5 4.8 1 0.6 2.5 15 2% NaOH 1 hr

100 5 1.5 7.5 4.8 1 0.6 2.5 15 4% NaOH 1 hr

100 5 1.5 7.5 4.8 1 0.6 2.5 15 0.4% silane F8261

100 5 1.5 7.5 4.8 1 0.6 2.5 15 0.4% silane A151

100 5 1.5 7.5 4.8 1 0.6 2.5 15 0.4% silane A1100

15 0.5% NaOH 1 hr

15 1% NaOH 1 hr

15 2% NaOH 1 hr

15 4% NaOH 1 hr

15 0.4% silane F8261

15 0.4% silane A151

15 0.4% silane A1100

2,2,4-trimethyl-1,2-dihydroquinoline. b N-cyclohexylbenzothiazyl sulfenamide. c Hexamethylene tetraamine.

2.2.B. Silane Treatment. The silanes used were 1,1,2,2perfluorooctyl triethoxy silane (F8261), 3-aminopropyl triethoxy silane (A1100), and vinyl triethoxy silane (A151). Solutions at 0.4% of the respective silanes were prepared by mixing with an ethanol/water mixture in the ratio 6/4 and allowed to stand for 1 h. The pH of the solution was maintained at 4 with the addition of acetic acid. Sisal and oil palm fibers were dipped in this solution and were allowed to stand for 1.5 h. The ethanol/water mixture was drained out, and the fibers were dried in air and then in an oven at 70°C until completely dry. 2.3. Preparation Of Composite. Formulation of mixes is shown in Table 3. Natural rubber was masticated on the mill for 2 min followed by addition of the ingredients. The composite materials were prepared in a laboratory two-roll mill (150 × 300 mm2). The nip gap, mill roll, speed ratio, and number of passes were kept the same in all the mixes. The samples were milled for sufficient time to disperse the fibers in the matrix at a mill opening of 1.25 mm. The bonding system consisting of resorcinol and hexamethylene tetraamine was incorporated for mixes containing treated fibers. The fibers were added at the end of the mixing process, taking care to maintain the direction of compound flow, so that the majority of fibers followed the direction of the flow.

3. Measurements Water sorption is evaluated in terms of weight increase for composite specimen immersed in distilled water at temperatures of 30, 50, and 70 °C. Freshly cut circular specimens of thickness of 2 mm were dried in a vacuum at room temperature for 2 days, and the weight of the dried specimen was measured using an electronic balance. The weighed specimens were then immersed in distilled water at different temperatures. The specimens were periodically removed from the water bath, and the surface moisture was wiped off. The weight gain of the specimen has been measured as a function of time until equilibrium or the saturated state of water uptake has been reached. Moisture absorption was determined by weighing the specimen on an electronic balance. The molar percentage uptake Qt for water by 100 g of the polymer was plotted against the square root of time. The Qt value is expressed as Qt )

Me(w)/Mr(w) Mi(s)

× 100

(1)

where Me(w) is the mass of water at equilibrium, Mr(w) is the relative molecular mass of water (18), and Mi(s) is the

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initial mass of the sample. When equilibrium was reached, Qt was taken as the molar percentage uptake at infinite time, that is, Q∞. 4. Results and Discussion 4.1. Moisture Uptake In Fiber-Reinforced Composites. All polymer materials absorb moisture in humid atmosphere and when immersed in water. Natural fibers absorb a high amount of moisture compared to synthetic fibers. Natural fibers being lignocellulosic are highly hydrophilic in nature and are permeable to water. Incorporation of natural fibers into polymeric composites thus generally increases the rates of water sorption ability. Moisture absorption takes place by three types of mechanisms, namely, diffusion, capillarity, and transport via micro cracks. Among the three, diffusion is considered to be the major mechanism. Water absorption largely depends on the water-soluble or hygroscopic components embedded in the matrix, which acts as a semipermeable membrane. The water penetration and diffusion are mainly through the fiber matrix interfacial region and the cross-sectional portions of the fibers by capillary mechanisms. The porous structure of the fiber facilitates the water absorption by capillary action. This mechanism involves the flow of water molecules along the fiber-matrix interface followed by diffusion from the interface into the matrix and the fiber. Microcracks can also pave the way for moisture transport involving the flow and storage of water within the cracks. Thus, fiber/matrix adhesion is an important factor in determining the sorption behavior of a composite. Moreover, fiber architecture has also been found to affect the moisture absorption. Moisture diffusion in polymeric composites has been shown to be Fickian as well as non-Fickian in character. The main factors which affect the diffusion process in a polymeric composite are as follows: (1) The polarity of the molecular structure, i.e., the presence of chemical groups capable of forming hydrogen bonds. (2) The degree of cross-linking. (3) The presence of residual monomers or other waterattacking groups. (4) Crystallinity. (5) Free hydroxyl groups. (6) Temperature. (7) Area of exposed surfaces. (8) Surface protection. In general, moisture diffusion in a composite depends on factors such as volume fraction of fibers, void volume, additives, humidity, temperature, orientation of reinforcement, nature of fiber (that is permeable or impermeable), area of exposed surfaces, diffusivity, reaction between water and matrix, and finally surface protection. The interaction between natural fiber and water molecules is represented in Figure 1. Two types of hydrogen bonds are present in the system. Inter-hydrogen bonds are formed between the hydroxyl groups in the cellulosic fiber and the water, while intra-hydrogen bonds are also formed between the two hydroxyl groups of two cellulosic units.

Figure 1. Schematic representation of the interaction of cellulose with water molecule.

In the case of natural fiber rubber composites, moisture uptake is mostly dependent on the fibers, as the rubber matrix is hydrophobic and therefore not apt for water penetration. Pure hydrocarbon rubber absorbs very little water, less than 1 ppt.12 This limited uptake takes place because of the small amount of protein that is present in the polymer. Water uptake in lignocellulosic fibers is greatly dependent on morphology and physical as well as chemical structures. Biofibers are composed of cellulose, hemicellulose, lignin, and waxy particles. Studies have indicated that penetration of water into the fibers occurs through the micropores present on the fiber surface.13 The absorption of water by polymers depends mainly on two factors, the availability of free nanosized holes in the polymer as well as the polar sites. There can be two states of water molecules present in the polymer: unbonded in the nanoholes of the polymer and bonded with hydrogen bonds to the polymer.14 The waxy materials present on the fiber will additionally help to retain the water molecules on the fiber. Pothen and Thomas15 investigated the effect of hybridization of a biofiber with a synthetic fiber on the water sorption properties of banana-glass fiber-reinforced polyester composites. They observed the water uptake to be quite low for the hybridized composites when compared to the unhybridized ones. In the present study, a natural rubber matrix is reinforced with two types of biofibers, viz., sisal and oil palm. The water sorption characteristics of the composites will therefore depend on the intrinsic properties of both sisal and oil palm fibers. 4.2. Fiber Loading. Figure 2 exhibits the variation of water uptake of sisal fiber-reinforced natural rubber composite (30 phr), oil palm fiber/natural rubber composite (30 phr), and sisal/oil palm hybrid fiber-reinforced natural rubber composites (30 phr), respectively. It can be observed that sisal-natural rubber (NR) composite exhibits the highest water uptake while the hybrid composite exhibits intermediate sorption characteristics. The water uptake is dependent on the chemical constituents present in sisal and oil palm fibers. Sisal fiber contains a higher content of hydrophilic holocellulose (23%) than oil palm fiber. This causes the composite to exhibit greater water uptake. Oil palm fiber has a higher content of lignin (57%) than sisal fiber (see Tables 1 and 2). Since lignin is a hydrophobic compound, it prevents the entry of water, and hence, oil palm fiberreinforced composites exhibit the lowest water uptake. Sisal fiber-reinforced natural rubber composite exhibits the highest value for diffusion coefficient as evident from Table 4. Figure 3 presents the variation of moisture sorption curves as a function of fiber loading at 30 °C. It can be seen that, as fiber content increases, the water uptake also increases.

Water Sorption Studies of Rubber Biocomposites


Figure 2. Variation of molar water uptake of sisal-NR composite and oil palm fiber-NR composite at 30 °C. Table 4 sample

diffusion coefficient at 30 °C (D) (cm2/min)

sisal/NR sisal-oil palm/NR oil palm/NR

1.4837 × 10-11 4.9326 × 10-11 1.9688 × 10-12

This indicates that the water sorption process is more dependent on the hydrophilic lignocellulosic fibers. All the samples showed a similar pattern of sorption, where the samples absorbed water very rapidly during the first stages, reaching a certain value, the saturation point, where no more water was absorbed and the content of water in the composites remained the same. The hydrophilic character of natural fibers is responsible for the water absorption in the composites, and therefore, a higher content of fibers leads to a higher amount of water absorbed. The gum sample shows Fickian diffusion, while the mechanism of diffusion in loaded samples is non-Fickian in nature. This is due to the presence of microcracks as well as the viscoelastic nature of the polymer. An interesting feature in this graph is the two-stage diffusion of the samples, which is prominent at 20 and 30 phr. In the first stage, absorption arises because of Fickian diffusion giving rise to an apparent saturation level. The prolonged exposure of the swollen samples to water results in fiber/matrix debonding. This leads to the collection of water in the voids and microcracks that are formed in the interfacial region. This results in the second-stage diffusion. The variation of water sorption curves as a function of fiber loading at 50 and 70 °C is depicted in Figures 4 and 5. Here, one can also see that maximum and minimum uptakes are exhibited by the composite containing 50 phr fiber and gum, respectively. The effect of temperature on the diffusion

Figure 3. Variation of molar water uptake with root time as a function of fiber loading at 30 °C.


process of composites A (30 phr) and E (50 phr) is presented in Figures 6 and 7. The mechanism of diffusion is found to be non-Fickian. It can also be seen that, as temperature increases, the rate of diffusion also increases. Diffusion is related to the velocity of the diffusing molecules by the equation given below 1 D ) λcj 3

(4)

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Figure 6. Variation of molar water uptake with root time of composite A at different temperatures.

where cj ) mean velocity of molecules and λ ) mean free path (distance traveled by molecules between two consecutive collisions). Since the mean velocity increases with temperature, diffusion also increases with temperature. 4.3. Chemical Modification. Mercerization is an economical and effective method used for improving the interfacial incompatibility between the matrix and the fiber and also to reduce water uptake of fibers. It improves the fiber surface adhesive characteristics by removing natural waxy materials, hemicellulose, and artificial impurities by producing a surface topography.16 In addition to this, alkali treatment can lead to fibrillation, or breaking down of fibers

Jacob et al.

Figure 7. Variation of molar water uptake with root time of composite E at different temperatures.

into smaller ones. All these factors provide a large surface area and give better mechanical interlocking between the fiber and the matrix and thus reduce water absorption. As the strength of NaOH increases, the amount of surface area created increases, providing better mechanical interlocking between the fiber and the matrix. Besides the removal of hemicellulose and waxes, the treatment with NaOH solution promotes the activation of hydroxyl groups of the cellulose unit by breaking the hydrogen bond. The authors in a previous study observed that mercerization of sisal and oil palm fibers in natural rubber composites resulted in enhanced tensile properties.17 Figure 8 presents the variation of moisture uptake at room temperature for the alkali-treated and untreated samples. It is obvious that alkali treatment has resulted in lowering of water uptake when compared to the treated samples. Also, among the chemically treated composites, composite containing fibers treated with 0.5% NaOH showed the highest water uptake while the composite containing fibers treated with 4% NaOH exhibited the minimum water uptake. As the concentration of NaOH increases, the adhesion between fiber and matrix increases, and hence, uptake of water decreases. Also, increased alkali concentration induces greater crystallinity to the fibers, thereby reducing the water sorption capacity of the fibers.17 It is also obvious from eq 3 that any factor that reduces the velocity and the mean free path of diffusing molecules lowers water sorption. The better adhesion between matrix and fibers decreases the velocity of the diffusing molecules, since there are fewer gaps in the interfacial region. Another factor is that stronger adhesion results in tighter packing within the rubber-fiber network, which means that the distance traveled by the diffusing water molecules between two consecutive collisions decreases (mean free path) and consequently results in lowering of

Water Sorption Studies of Rubber Biocomposites


debonding, leading to abnormal water uptake. Thus, the uptake of water will be greater because of the fiber debonding from the matrix. Moreover, there may also be a chance of degradation of the composite at 70 °C by the excessive water absorption through fibers and by the formation of microcracks or voids. Thus, after initial uptake, the contribution to water sorption is mainly from the hydrophilic fibers. The moisture resistance of composites comprising fibers treated with different types of silane coupling agents is presented in Figure 11. Before silane treatment, the fibers were pretreated with 1% NaOH. Out of the three types of silanes used, silane (F8261), vinyl silane, and amino silane, the maximum uptake of water is given by vinyl silane, while the lowest uptake is given by fluorosilane treated samples. Mercerization treatment prior to silane treatment is an effective means of advocating better property retention to composites exposed to moisture. The potential advantage of using silane coupling agents are their inherent natural attraction to both the natural fiber and the resin matrix. The reaction mechanisms are as follows. First, silane reacts with water to form silanol and an alcohol. Figure 8. Variation of molar water uptake with root time of composites untreated, I, J, K, and L, at 30 °C.

water uptake (see eq 4). It can be also seen from Figure 8 that the samples have a tendency to approach Fickian behavior as the concentration of NaOH increases. The better adhesion of composites containing alkali-treated fiber is also evident from scanning electron microscopy (SEM). Figure 9 a,b shows the tensile fracture surface of untreated and treated composite at 30 phr loading. The presence of holes is clearly visible in Figure 9a. This indicates that the level of adhesion between the fibers and the matrix is poor, and when stress is applied, it causes the fibers to be pulled out from the rubber matrix easily, leaving behind gaping holes. In Figure 9b, we can see the presence of a number of short, broken fibers projecting out of the rubber matrix. This indicates that the extent of adhesion between the fibers and the rubber matrix is greatly improved, and when stress is applied, the fibers break and do not wholly come out of the matrix. The rate of diffusion of water into the composite is timeand temperature-dependent. As temperature increases, activation of diffusion takes place, leading to weakening of fibermatrix adhesion and creation of microcracks or voids in the system. Because the matrix is hydrophobic in nature, variation of temperature largely affects the water absorption through the fiber. The composite containing fibers treated with 4% NaOH (L) is compared at various temperatures like 30, 50, and 70 °C in Figure 10. As the temperature is increased from 30 to 50 °C, the rate of diffusion increases and the water uptake increases. The samples show a Fickian mode of diffusion at 30 and 50 °C (i.e., a linearity is maintained between time and water absorption). When the temperature rises to 70 °C, it can be seen that the rate of diffusion increases. This is also clear from the calculation of the diffusion coefficient (Table 9). Another point to note is that, at high temperatures such as 70 °C, fiber-matrix adhesion decreases and there is a chance of fiber-matrix

NH2(CH2)3Si(OC2H5)3 + 3H2O f NH4(CH2)Si(OH)3 + 3C2H5-OH (5) In the presence of moisture, the silanol reacts with the hydroxyl group attached to the cellulose molecules of the fiber through an ether linkage with the removal of water. NH2(CH2)3Si(OH)3 + H2O + fiber-OH f NH2(CH2)3Si(OH)2-O-fiber + 2H2O (6) Similar reactions take place for fluorosilane and vinyl silane coupling agents. The rubber matrix gets attached to the organo functional group of the silane coupling agent either through a covalent bond or a hydrogen bond. Here, the possibility of hydrogen bonding is greater because of the presence of nitrogen in the amino group and fluorine in the fluorosilane group, while no such possibility exists in the vinyl group. The schematic sketch of the interaction between rubber, amino, and fluorosilane coupling agent and fiber is shown in Figure 12. This kind of bonding cannot be possible for the vinyl silane group. The hydrogen bond formed by the fluorine to the matrix will be stronger than the hydrogen bond formed by nitrogen to the matrix because of the high electronegative character of the fluorine atom. Thus, in fluorosilane-treated composites, fiber/matrix adhesion will be stronger compared to aminosilane-treated and vinylsilane-treated composites. Because of the stronger fiber/matrix adhesion, the uptake of water will be less in fluorosilane-treated composite as seen in the figure. The sorption curve is found to be anomalous, showing that diffusion takes place largely through the fiber. Thus, fluorosilane-treated fiber-containing composites show the lowest water uptake compared to other silane-treated samples. The sample Q, i.e., composite containing vinylsilane-treated fiber, exhibits the largest uptake of water; the sorption curve is found to be Fickian. Because the fibermatrix adhesion is very weak in vinylsilane-treated composite, maximum uptake of water takes place. Water uptake

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Figure 9. SEM tensile fracture surface of untreated (a) and 4% NaOH treated (b) sisal/oil palm reinforced natural rubber composites at 30 phr loading at magnifications of 100× and 100×, respectively. Table 5 sample gum

B

C

A

D

E

temperature (°C)

diffusion coefficient (D) (cm2/min)

30 50 70 30 50 70 30 50 70 30 50 70 30 50 70 30 50 70

1.19217 × 10-12 1.17898 × 10-11 1.20561 × 10-10 5.5255 × 10-11 2.35207 × 10-10 2.91285 × 10-10 4.9326 × 10-11 2.08886 × 10-10 1.33127 × 10-9 2.8815 × 10-10 3.3965 × 10-10 1.4990 × 10-9 5.7519 × 10-9 4.02964 × 10-8 9.1792 × 10-8 9.5092 × 10-8 4.0296 × 10-7 1.1767 × 10-6

Table 6

Figure 10. Variation of molar water uptake with root time of composite L at different temperatures.

temperature sorption coefficient sample (°C) (g/g) gum

is high mainly because of the fact that, as interfacial adhesion is weak, water can easily enter into the interfacial gaps that are present in the composite. Composites containing aminosilane-treated fibers show medium water uptake. In these composites, the adhesion to the matrix is not as strong as in fluorosilane-treated composite, but stronger than in vinylsilane-treated composite. Here, diffusion can occur both through the matrix as well as through treated cellulosic system. However, diffusion through the matrix is slightly more pronounced than through fibers, as the sorption curve is seen to approach a Fickian behavior. Another interesting observation is that composites containing fibers treated with 4% NaOH exhibit lower water uptake than silane-treated fiber-containing composites. Table 8 supports the above observations. The effect of temperatures on fluorosilane-treated fibercontaining composites is analyzed and is shown in Figure 13. As temperature increases, the activation of diffusion increases and water uptake increases. The sorption curve at

B

C

A

D

E

30 50 70 30 50 70 30 50 70 30 50 70 30 50 70 30 50 70

1.044 1.033 1.1064 1.0794 1.0792 1.1921 1.1377 1.2240 1.4008 1.2091 1.2534 1.5713 1.1768 1.2396 1.4693 1.2091 1.2134 1.5713

permeability coefficient ( P ) D × S) (cm2/min) 1.2446 × 10-12 1.2178 × 10-11 1.3338 × 10-10 5.9642 × 10-11 2.2542 × 10-10 3.4724 × 10-10 3.2782 × 10-10 4.1573 × 10-10 2.0997 × 10-9 5.5412 × 10-11 2.8260 × 10-10 1.8576 × 10-9 6.7688 × 10-9 9.3499 × 10-8 13.4869 × 10-8 11.4975 × 10-8 5.0507 × 10-7 1.8489 × 10-6

70 °C is a characteristic one showing deviation from 30 and 50 °C. It can be seen the uptake of water at 70 °C increases to a larger extent because of the fiber matrix debonding at


Water Sorption Studies of Rubber Biocomposites Table 7 sample

activation energy (kJ/mol)

gum B C A D E L P

76.588 65.768 43.764 27.353 15.768 10.443 23.4206 30.8889

Table 8

sample

diffusion coefficient ( D) (cm2/min)

sorption coefficient (g/g)

permeability coefficient ( P ) D × S) (cm2/min)

I J K L P Q R

1.8345 × 10-11 1.7012 × 10-11 1.5572 × 10-11 1.44 × 10-11 1.838 × 10-11 7.217 × 10-11 6.4 × 10-11

0.155116 0.13050 0.09910 0.1070 0.0896 0.1518 0.1247

2.846 × 10-12 2.22 × 10-12 1.5431 × 10-12 1.54 × 10-12 2.29 × 10-12 1.095 × 10-11 7.98 × 10-12

Table 9

sample L

P

temperature (°C)

diffusion coefficient ( D) (cm2/min)

sorption coefficient (g/g)

permeability coefficient ( P ) D × S) (cm2/min)

30 50 70 30 50 70

1.44 × 10-11 1.52 × 10-11 5.0622 × 10-12 1.838 × 10-11 2.016 × 10-11 4.68 × 10-12

0.1070 0.13440 1.00826 0.0896 0.1842 1.7731

1.54 × 10-12 2.042 × 10-12 5.104 × 10-12 2.29 × 10-12 3.7134 × 10-12 8.298 × 10-12

Figure 11. Variation of molar water uptake with root time of composites untreated, L, P, Q, and R, at 30 °C.

higher temperatures. This results in the formation of voids and microcracks into which water can easily seep. It is quite evident from the above observations that chemical modification has resulted in lowering of moisture uptake due to increased fiber/matrix adhesion. Interestingly, the sorption curves of the treated composites do not show any prominent two-stage diffusion. This is because in treated composites debonding does not take place because of better wetting between fiber and matrix. Hence, water cannot accumulate in the voids of the interfacial region. 4.4. Kinetic Parameters. The thermodynamic parameters of the sorption process can be calculated from diffusion data. The activation energy can be calculated from the equation log D ) log Do - ED/RT

(7)

where D ) diffusion coefficient, Do ) constant, and ED ) activation energy. A plot of log D against 1/T gives the value of the activation energy from the slope. 4.4.1. Diffusion Coefficient. The diffusion coefficient explains the rate at which a diffusion process takes place. It is the rate of transfer of the diffusing substance across the unit area of a section divided by the space gradient of concentration. The diffusion coefficient characterizes the ability of water molecules to diffuse into the fiber.

Figure 12. Schematic sketch of the interaction between rubber, silane (amino and fluoro) coupling agents, and fiber.

The diffusion coefficient can be calculated by the equation D)π

( ) hθ 4QR

2

(8)

where h is the initial thickness or initial average diameter of the sample and θ is the slope of the initial linear portion of the sorption curve. The slope is calculated from the graph of Qt versus root time.

2978


Jacob et al. Table 10

Figure 13. Variation of molar water uptake with root time of composite P at different temperatures.

4.4.2. Sorption Coefficient. The sorption coefficient (S) is calculated by the equation S)

MR Mo

(9)

where MR ) mass of the water taken up at equilibrium and Mo ) initial mass of the sample. It gives a measure of the extent of sorption. 4.4.3. Permeability Coefficient. The permeability coefficient gives an idea about the amount of water permeated through the uniform area of the sample per second. The permeability coefficient is given by equation P ) DS

(10)

The activation energy of the various samples is given in Table 7. It can be seen that activation energy is maximum for the gum sample and minimum for the sample containing 50 phr fiber loading. This is because the presence of fibers in the reinforced samples leads to a smooth influx of water into the composite. Therefore, the energy required for this process is relatively low. The gum sample, being hydrophobic, does not take up moisture. The chemically modified composites registered high activation energies indicating that entry of water is hindered by the presence of a strong interface. The diffusion coefficient was found to increase with fiber loading and temperature (Table 5). A higher diffusion coefficient leads to the conclusion that a large amount of moisture is absorbed by the sample. The permeability coefficient was also found to increase with loading and temperature (Table 6). Thermodynamic functions ∆S, ∆H, and ∆G were calculated by linear regression analysis using the Van’t Hoff equation ln Ks ) ∆S/R - ∆H/RT

(11)

∆G ) ∆H - T∆S

(12)

where Ks ) no. of moles of solvent sorbed at equilibrium/

sample

∆H (kJ/mol)

∆S (J/mol/K)

∆G (kJ/mol)

gum B C A D E L P

19.68902 18.26324 21.19826 17.70227 16.73088 21.54784 52.14362 77.69006

12.53153 15.46878 30.82041 18.28764 17.49016 34.77954 129.0251 209.1585

-3.77736 -4.66878 -9.31739 -5.52345 -5.28279 -10.5167 -39.0425 -63.2973

mass of the fiber, ∆S ) entropy of sorption, ∆H ) enthalpy of sorption, and ∆G ) free energy change. The thermodynamic parameters are presented in Table 10. The enthalpy and entropy of sorption are positive, indicating that the process is endothermic. It can also be seen that for the composites containing chemically treated fibers, ∆S and ∆H values are quite high. The free energy values are found to be negative for all the systems, indicating that the diffusion process is a spontaneous reaction due to the presence of hydrophilic biofibers. 5. Conclusion An investigation into the water sorption characteristics of sisal and oil palm hybrid fiber-reinforced natural rubber biocomposites was attempted. It was found that water uptake was mainly dependent on the properties of the lignocellulosic fibers. The mechanism of diffusion was found to be Fickian for the gum sample, while for the fiber-reinforced composites, non-Fickian behavior was found. The sorption curves of the composites showed a two-stage diffusion pattern, while this was absent in the sorption curves of composites containing treated fibers. Among the composites containing alkali treated fibers, moisture uptake was found to decrease with concentration of alkali. This was attributed to increased adhesion leading to decrease in velocity and mean free path of diffusing water molecules. Among the composites containing fibers treated with silane coupling agents, composites containing fibers treated with silane F8261 exhibited minimum water uptake. This was attributed to the strong hydrogen bond formed between the hydroxyl group of the fiber and the electronegative fluorine atom. The rate of diffusion was seen to increase with temperature because of weakening of fiber-matrix adhesion and formation of microcracks. The diffusion coefficient was found to be minimum for the gum sample and increased with the loaded composites. The activation energy was found to be maximum for the gum composite and minimum for the composite containing 50 phr fiber. The chemically modified composites registered high values of activation energy. The free energies of the composites were found to be negative. References and Notes (1) Lu, X.; Zhang, M. Q.; Rong, M. Z.; Shi, G. A.; Yang, G. C. Polym. Compos. 2003, 24 (3), 367-379. (2) Alvarez, V. A.; Ruscekaite, R. A.; Vazquez, A. J. Comput. Mater. 2003, 37 (17), 1575-1588. (3) Espert, A.; Vilaplana, F.; Karlsson, S. Composites, Part A 2004, 35 (11), 1267-1276.

Water Sorption Studies of Rubber Biocomposites (4) Mishra, S.; Mohanty, A. K.; Drzal, L. T.; Misra, M.; Parija, S.; Nayak, S. K.; Tripathy, S. S. Compos. Sci. Technol. 2003, 63 (10), 13771385. (5) Nair, K. C. M.; Thomas, S. J. Thermoplast. Compos. Mater. 2003, 16 (3), 249-271. (6) Bledzki, A. K. Appl. Compos. Mater. 2003, 10 (6), 365-379. (7) Burgueno, R.; Quagliata, M. J.; Mohanty, A. K.; Mehta, G.; T. Drzal, L. T.; Misra, M. Composites, Part A 2004, 35, 645-656. (8) Lu, X.; Zhang, M. Q.; Rong, M. Z.; Yue, D. L.; Yang, G. C. Compos. Sci. Technol. 2004, 64, 1301-1310. (9) Mohanty, S.; Verma, S. K.; Nayak, S. K.; Tripathy, S. S. Int. J. Plast. Technol. 2003, 7, 75-87. (10) Tserki, V.; Matzinos, P.; Panayiotou, C. J. Appl. Polym. Sci. 2003, 88 (7), 1825-1835.

Biomacromolecules, Vol. 6, No. 6, 2005 2979 (11) Mishra, S.; Naik, J. B.; Patil, Y. P. AdV. Polym. Technol. 2004, 23 (1), 46-50. (12) Mikheev, Y. A.; Zaikov, G. E. Polym. Polym. Compos. 2001, 9, 1. (13) Sreekala, M. S.; George, J.; Kumaran, M. G.; Thomas, S. J. Polym. Sci., Part B: Polym. Phys. 2001, 39, 1215-1223. (14) Luo, S.; Leisen, J.; Wong, C. P. Polym. Polym. Compos. 2001, 9, 1. (15) Pothan, L. A.; Thomas, S. J. Appl. Polym. Sci. 2003, 91 (6), 34214111. (16) Bledzki, A. K.; Gassan, J. Prog. Polym. Sci. 1999, 24, 221-274. (17) Jacob, M.; Thomas, S.; Varughese, K. T. Compos. Sci. Technol. 2004, 64, 955-965.

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Water Sorption Studies of Hybrid Biofiber-Reinforced Natural Rubber

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