WATER SORPTION CHARACTERISTICS OF BANANA FIBRE/PHENOLFORMALDEHYDE COMPOSITES. Abstract

Transcription

1 Chapter 8 WATER SORPTION CHARACTERISTICS OF BANANA FIBRE/PHENOLFORMALDEHYDE COMPOSITES Abstract The theme of this chapter is water sorption characteristics of banana fibre reinforced phenol formaldehyde composites. In order to enhance the interfacial interactions between the PF matrix and the fibre, different types of chemical modifications were used. The influence of fibre loading, fibre surface modifications and fabrication methods of the composites on the kinetic parameters of water sorption was studied. The cured PF resin and the fibre reinforced composites were immersed in water at 25, 50 and 90 o C. Fabrication of composites by RTM considerably decreased the water uptake. Composites with 50 wt% banana fibre exhibited maximum water uptake. The moisture absorption was found to increase with increasing temperature and fibre loading. Among the chemically treated banana fibre composites, alkali treatment accompanied a decrease in water absorption due to the better adhesion between the fibre and the matrix. It was observed that the mechanism of water transport is of non-fickian type in all cases. Parameters like diffusion, sorption and permeability coefficients of the composites were determined. The experimental results were compared with theoretical modeling using Fickian model. The result of this chapter has been communicated to J. Adhesion Science and Technology.

2 298 Chapter Introduction Water absorption in polymer composites received considerable attention owing to its theoretical and practical significance, as they are important for many applications such as waste water treatment, packaging and building industry. Water sorption have effects on the physical properties of the composites and can affect the matrix structure and the fiber/matrix interface resulting in changes of bulk properties such as dimensional stability, mechanical and electrical properties. Water absorption behaviour of the fibre reinforced systems is important for many applications. Several investigators have evaluated the sorption properties of composites based on jute, pineapple, oil palm etc in different polymer matrices (1-3). It is now established that many polymer composite materials are sensitive to humidity. Natural fibers, being lignocellulosic, are hydrophilic in nature and are permeable to water (2). Incorporation of natural fibers into polymeric matrices generally increases the water sorption ability of the composite. The determining factors that affect water sorption of composites are the hydrophilicity of the individual components of the fibre, void content, diffusivity, the structural arrangement of the fibres within the matrix, permeability of the fibre, orientation of the fibres, the availability of free holes in the polymer, the number of polar sites, etc (4). The permeability of overall composite is however decided mainly by that of the fibres. Transport of water takes place through micro cracks that occur on the surface of the composite and also through micro channels, which occur inside the material due to defects. Bessadoc et al. (5) investigated the water sorption characteristics of alfa fibres. The results revealed that the treatments reduced the overall water uptake of alfa fibres, in particular, styrene treatment allows

3 Water sorption characteristics of 299 to increase significantly the moisture resistance of the fibres. Lin et al.(6) studied the moisture absorption and mechanical properties of wood flour filled polypropylene composites in hydrothermal environments. The degree of moisture absorption was found to be dependent on the modification of matrix, weight percentage, mesh size and surface treatment of wood flour. The moisture absorption and electrical conduction behaviour of composites with different fibre loadings were investigated by Wang et al. (7). According to them at high fibre loading when accessible fibre ratio is high, the diffusion process is the dominant mechanism; while at low fibre loading close to and below percolation threshold, percolation is the dominant mechanism. The over-estimate of accessible fibre ratio led to discrepancies between the observed and model estimates. Fraga et al. (8) conducted gravimetric and dielectric measurements to monitor water uptake in composites with glass, jute and washed jute fibres. The results obtained show that when the sizing is removed in fibres washed with hot water, the effective coefficient of diffusion increases. A non-linear behaviour has been found in the changes of the permittivity with water absorption in the frequency range 200 Hz to 1 MHz. In another interesting report concerning flax fibres, the influence of absorbed water on the tensile strength of fibres was investigated by Baley et al.(9) The authors observed that drying of flax fibres resulted in the modification of adhesion between cellulose microfibrils and matrix. This modification was believed to be due to the evolution of components ensuring the transfer of load between microfibrils and thus enhancing the strength of the cellular wall. This chapter is concerned with the moisture sorption characteristics of banana fibre reinforced phenol formaldehyde composites. The water sorption of these

4 300 Chapter 8 composites at different temperatures was analyzed. The diffusion coefficient, sorption coefficient, permeability coefficient, and the kinetic parameters of water diffusion of the composites were calculated. The effect of surface treatments on the water sorption kinetics of the composites was also studied. Experimental results were compared with theoretical predictions. 8.2 Results and discussion Effect of fibre loading Figures 8.1(a) and (b) delineate the molar uptake of banana fibre reinforced PF composites made by RTM and CM techniques respectively at different fibre loading as a function of immersion time at 25 o C. From the figure it can be seen that phenol formaldehyde resin shows very low water absorption. This three dimensionally cross linked thermoset matrix has practically no sorption. The slight absorption of water observed here could have occurred through the micro cracks present inside the material % 10% 20% 30% 40% 50% Q t (mole)% t 1/2 (min) 1/2 (a)

5 Water sorption characteristics of 301 Q t (mole)% % 10% 20% 30% 40% 50% t 1/2 (min) 1/2 (b) Figure 8.1 Mole percent water uptake of banana fibre/pf composites fabricated by (a) RTM and (b) CM techniques as a function of immersion time at 25 o C All the composite samples absorbed water very rapidly at the initial stage but resulted in a levelling off at a later stage. This indicates the attainment of equilibrium. The water penetration and diffusion are mainly through the fibre/ matrix interfacial region and cross sectional portion of the fibre by capillary mechanism. After the initial capillary uptake of water, as the banana fibre loading increased the water uptake also increased. This is due to the hydrophilic character of banana fibre. There are porous tubular structures present inside the banana fibre through which water can easily penetrate. At lower loadings, due to the low stability of the system, the interfacial failure as well as the deterioration of the composites is easier than that at higher loadings (10). The capillary region uptake is similar in all cases. For 50%

6 302 Chapter 8 fibre loading the equilibrium reaches after a long duration. A similar trend is observed for CM composites. SEM of the cross section of a banana fibre is given in Fig Figure 8.2 Scanning electron micrograph of the cross section of banana fibre The cross sections of the fibers also become the main source to the penetrating water. Unlike organic penetrants, water molecule is small and strongly associated through hydrogen bond formation. These water molecules form strong localised interactions with hydroxyl groups available on cellulose and lignin. The porous nature of the fiber results in water absorption by capillary action. Table 8.1 lists the effect of fibre loading on the Q values for composites.

7 Water sorption characteristics of 303 Table 8.1. Q values of banana fibre reinforced PF composites as a function of fibre loadings, temperatures and fabrication methods. Fibre content (wt%) Temperature ( o C) Q (mole %) RTM Q (mole %) CM

8 304 Chapter 8 Upon reinforcing PF resin with banana fibre a large increase in sorption takes place. On increasing the fibre loading, the extent of sorption and the Q values also increase. The presence of reinforcement modifies the response of the resin to humid environments. The diffused moisture may reduce the bond strength by the breaking of the bonds. The porous structure of fibre accelerates the penetration of water. Maximum sorption was observed for composite having 50% fibre loading. Composites fabricated by RTM and CM techniques exhibited similar trend. However, the RTM composites showed lower uptake than CM composites. Figure 8.3 shows water uptake by RTM and CM composites at 25 o C. The lower water uptake of composites prepared by RTM technique supports the increased fibre/matrix adhesion observed in the mechanical property measurements (chapter 5). Figures 8.4 and 8.5 represent the effect of fibre loading on water sorption at 50 o C and 90 o C respectively. It is manifested that as temperature increases, rate of diffusion increases for neat PF sample irrespective of the processing techniques (11). A similar trend is observed in the case of all composites. This is associated with the changes that occur at the interface at higher temperatures in composites. The activity of the water molecule also increased with temperature (12). As the activity of water molecule increases, the rate of diffusion also increases. As a result, the Q value increases with temperature. It is also clear from the sorption data that the water sorption proceeds very quickly in the first stage for a specific time. After the initial fast absorption, the rate of sorption decreases as equilibrium is achieved (11). Increase in temperature leads to thermal expansions, which pave way for water absorption through the micro cracks as well. Diffusion is related to the velocity of the diffusing molecules as

9 Water sorption characteristics of D = λc [8.1] 3 where c = mean velocity of molecules, λ = mean free path (distance travelled by molecules between two successive collisions). Since the mean velocity increases with temperature, diffusion follows the same trend RTM CM 2.0 Q t (mole)% t 1/2 (min) 1/2 Figure 8.3 Variation of molar water uptake with root time of 40wt% fiber composites fabricated by RTM &CM at 25 o C

10 306 Chapter 8 Q t (mole)% % 10% 20% 30% 40% 50% t 1/2 (min) 1/2 (a) % 10% 20% 30% 40% 50% Q t (mole)% t 1/2 (min) 1/2 (b) Figure 8.4 Mole percent water uptake of banana fibre/pf composites fabricated by (a) RTM and (b) CM techniques as a function of immersion time at 50 o C

11 Water sorption characteristics of 307 Q t (mole)% % 10% 20% 30% 40% 50% t 1/2 (min) 1/2 (a) % 10% 20% 30% 40% 50% Q t (mole)% t 1/2 (min) 1/2 (b) Figure 8.5 Mole percent water uptake of banana fibre/pf composites fabricated by (a) RTM and (b) CM techniques as a function of immersion time at 90 o C

12 308 Chapter 8 Scheme 8.1 Interaction of cellulose with water molecule The interaction between natural fiber and water molecule is represented in scheme 8.1. Two types of hydrogen bonds are present in the system. Inter molecular hydrogen bonds are formed between the hydroxyl groups in the cellulosic fiber and water, while intra molecular hydrogen bonds are formed with the hydroxyl groups of two cellulosic units in the form of chain.

13 Water sorption characteristics of RTM CM 2.0 Q (mole)% Fibre content (Wt%) Figure 8.6 Water uptake at equilibrium for composites, prepared by RTM and CM as a function of fibre content at 25 o C It is evident from the figure 8.6 that for composites fabricated by RTM the equilibrium water uptake is less than the CM composites Effect of treatment on water uptake Figures 8.7 (a-b), 8.8 (a-b) and 8.9 (a-b) illustrate the water absorption of treated banana fibre reinforced/pf composites at 25, 50 and 90 o C prepared by RTM and CM respectively.

14 310 Chapter 8 Q t (mole) % Untreated NaOH KMnO 4 Formic acid Silane Acetylated Benzoylated t 1/2 (min) 1/2 (a) Q t (mole)% Untreated NaOH KMnO 4 Formic acid Silane Acetylated Benzoylated t 1/2 (min) 1/2 Figure 8.7 (b) Mole percent water uptake of treated banana fibre/pf composites fabricated by (a) RTM and (b) CM techniques as a function of immersion time at 25 o C

15 Water sorption characteristics of 311 Q t (mole)% Untreated NaOH KMnO 4 Formic acid Silane Acetylated Benzoylated t 1/2 (min) 1/2 (a) Q t (mole)% Untreated NaOH KMnO 4 Formic acid Silane Acetylated Benzoylated t 1/2 (min) 1/2 (b) Figure 8.8 Mole percent water uptake of treated banana fibre/pf composites fabricated by (a) RTM and (b) CM techniques as a function of immersion time at 50 o C

16 312 Chapter 8 Q t (mole)% Untreated NaOH KMnO 4 Formic acid Silane Acetylated Benzoylated t 1/2 (min) 1/2 (a) Q t (mole) % Untreated NaOH KMnO 4 Formic acid Silane Acetylated Benzoylated t 1/2 (min) 1/2 (b) Figure 8.9 Mole percent water uptake of treated banana fibre/pf composites fabricated by (a) RTM and (b) CM techniques as a function of immersion time at 90 o C

17 Water sorption characteristics of 313 Table 8.2 lists the effect of chemical modification on the Q values for composites. Table 8.2 Q values of banana fibre reinforced PF composites as a function of chemical modifications, temperatures and fabrication methods. Fibre modification Untreated NaOH KMnO 4 Formic acid Silane Acetylation Benzoylation Temperature ( o C) Q (mole %) RTM Q (mole %) CM

18 314 Chapter 8 Temperature effect of sorption curves portrays that the treated fibre reinforced composites absorb water very rapidly at the initial stage as seen for the untreated one and later attain a saturation level with no further increase in water sorption. In chemically treated fibers, rearrangement of the cellulose fibrils leads to free spaces for the matrix resin to squeeze in and lesser space for the water molecules. The slope change after the initial fast absorption can be attributed to the overall change in the absorption mechanism. It has been observed in our studies that alkali treatment of the fiber surface increases the polarity of cellulose fibers and decreases the size of the fibers. Increased alkali concentration brings about more crystallinity to the fibers (13) probably due to the arrangement of the fibrils. Moreover, alkali treatment results in the leaching out of the amorphous waxy cuticle layer which holds the water molecule. This reduces the water sorption capacity of the fiber. In addition, chemical modification covers some of the surface pores in the fiber as well. Lignin, one of the components of the natural fibres has been reported to be the component responsible for restricting the hydrogen bond between the fibres and thereby their swelling capacity (14). Alkali treatment helps in the removal of this component and brings about a reduction in the swelling stresses. The cellulose fibrils are allowed to rearrange and there is an increase in the surface area of the fibres. This also lowers the equilibrium stresses involved. Fibrillation is observed in formic acid and KMnO 4 treated fibers due to the leaching out of waxes, gums and pectic substances which in turn make the fibre surface rough. When the fibre surface becomes rough and porous, the resin penetration becomes easy resulting in better fibre matrix interaction. This in turn decreases the water

19 Water sorption characteristics of 315 uptake. Shrinkage of the micropores and collapse of capillaries upon chemical treatment may also block the capillary absorption. The lower water uptake of these treated fibers composites are due to the improved fibre/matrix adhesion. Introduction of coupling agents, acetylation and benzoylation makes the fiber hydrophobic. The more hydrophobic is the fibre, the less is the fibre/matrix interaction. This leads to a decrease in the interaction between the fibre and matrix which facilitates the sorption process. The decreased fibre/matrix adhesion for the above three modifications is evident from the tensile and impact fracture properties discussed in chapter 6. The weak interface could lead to the formation of void structures within the composites, which facilitates water sorption (15). In the case of RTM composites, water absorption is found to decrease in the order acetylation > benzoylation > silane treated > untreated > formic acid treated > KMnO 4 treated > NaOH treated composites and for CM composites the water uptake is as follows: benzoylation > acetylation > silane treated > untreated > formic acid treated > KMnO 4 treated > NaOH treated composites. Here again, the water uptake is low for RTM composites compared to CM composites (16). This is clear from Fig. 8.10, which shows the variation of Q value for the treated composites fabricated by RTM and CM at 25 o C. As the temperature is increased from 25 to 90 o C the water uptake increased. In all cases maximum sorption is observed at 90 o C. This shows that temperature has a strong effect on the fibre/matrix interaction. At higher temperature the adhesive bonding is deteriorated. The temperature affects the penetration rate of water to the interface. Bellenger et al. (17) observed that in styrene crosslinked polyesters, the water equilibrium concentration is an

20 316 Chapter 8 increasing function of temperature. With increase of temperature, as a result of the expansion of volume fraction accessible to water molecules is increased and the water uptake got enhanced. The water absorption for the composites fabricated by RTM is less than CM composites (Figure 8.10) as observed in the case of untreated fibre composites. These results prove the better fibre/matrix adhesion of RTM composites. Lowest water uptake is seen for alkali treated fibre reinforced composites while benzoylated fibre composites exhibit the highest water uptake. Hence water sorption is dependent on the fibre and matrix structure and on the fibre/ matrix interface. 3.0 RTM CM 2.5 Q (Mole)% Fibre modification Figure 8.10 Variation of Q values for the composites fabricated by RTM and CM at 25 o C. (Fibre content 30Wt%). 1. untreated 2. alkali treated 3.KMnO 4 treated 4. formic acid treated 5. silane treated 6. acetic anhydride treated 7. benzoylated

21 Water sorption characteristics of 317 To study the mechanism of water sorption the kinetic parameters n and k diffusion coefficient and permeability coefficient of water sorption in different systems were analyzed using the following relationship. log (Q t /Q ) = log k + n log t 8.2 where Q t is the mole percent increase in uptake at time t, Q is the mole percent increase in uptake at equilibrium, t is the time, k is a constant characteristic for the polymer, which indicates the interaction between the polymer and water. The values of n and k are determined by linear regression analysis. The value of diffusion coefficient was calculated from the relationship D = π hθ 4Q where θ is the slope of the initial linear portion of the sorption curves and h is the thickness of the sample. The sorption coefficient has been calculated using the equation S = M 8.4 M P where M is the mass of water taken up at equilibrium and M P is the initial mass of the polymer. The permeability coefficient is the net effect of sorption and diffusion and is expressed as P = DS 8.5 Table 8.3 shows the values of the permeability coefficients of the composites at fibre loading fabricated by RTM and CM techniques. Table 8.4 gives the

22 318 Chapter 8 values of n and k values for banana fibre reinforced PF resin composites as a function of fibre treatments. The value of D increases with fibre loading and is found to be maximum for 50 wt% fibre loading. It is due to the inherent hydrophilic nature of the fibres. Similar trend is observed for CM composites. The D values of RTM composites are found to be lower than CM composites since the void content is lesser and fibre/matrix adhesion is higher in RTM composites. The value of k indicates the interaction between the polymer and water and n indicate the mode of diffusion. The value of n is much lower than 0.5. This is due to the penetration of water molecules through the interfacial regions and micro voids present in the composites, which are the other water absorption mechanisms in fibre reinforced composites. When the value of n = 0.5, diffusion obeys Fick s law and is said to be Fickian. This occurs when the segmental mobility of the polymer chains is faster than the rate of diffusion of permient molecules. When n > 1 the diffusion is said to be anomalous. When the value of n is between 0.5 and 1 the diffusion is non Fickian. The value of n clearly shows that the diffusion process deviates from the Fickian mechanism (Table 8.3 and 8.4). For chemically modified composites, the diffusion coefficient varies according to the treatment. Alkali treated fibre composites have lower diffusion coefficient. It is evident from Table 8.5 that diffusion coefficient increases with increase of temperature. The value is found to be maximum for composites at 90 o C and minimum for composites at 25 o C for a particular sample. The RTM composites have lower diffusion coefficient compared to CM composites and is attributed to lower void content and better fibre/matrix adhesion (Figure 8.11).

23 319 Water sorption characteristics of (a) (b) Figure 8.11 SEM of the fracture surfaces of the untreated banana fibre composite fabricated by (a) CM and (b) RTM techniques. Fibre content (30Wt%)

24 320 Chapter 8 Table 8.3 Fibre content (wt%) Effect of fiber loading on the n and k values of banana fibre/pf composites at different temperatures Temperature( o C) n RTM k (g/g/min 2 ) n CM k (g/g/min 2 )

25 Water sorption characteristics of 321 Table 8.4 Fibre modification Untreated NaOH KMnO 4 Formic acid Silane Acetylation Benzoylation Effect of surface modifications on the n and k values of banana fibre/pf composites at different temperatures. Fibre content (30Wt%) Temperature( o C) n RTM k (g/g/min 2 ) CM n k (g/g/min 2 )

26 322 Chapter 8 Table 8.5 Effect of fibre loading on sorption coefficient and permeability of banana fibre/pf composites at different temperatures prepared by RTM and CM. Fibre content (wt%) Temperature ( o C) Dx10 4 (cm 2 /s -1 ) CM S (g/g) Px10 4 (cm 2 / s -1 ) Dx10 4 (cm 2 /s 1 ) RTM S (g/g) Px10 4 (cm 2 / s -1 )

27 Water sorption characteristics of 323 Table 8.6 Fibre modification Untreated NaOH KMnO 4 Formic acid Silane Acetylation Benzoylation Effect of chemical modification on sorption coefficient and permeability of banana fibre/pf composites at different temperatures prepared by RTM and CM. Fibre content (30Wt%) Temperature ( o C) Dx10 4 (cm 2 /s -1 ) CM S (g/g) Px10 4 (cm 2 / s -1 ) Dx10 4 (cm 2 /s 1 ) RTM S (g/g) Px10 4 (cm 2 / s -1 )

28 324 Chapter 8 Permeability coefficient gives an idea about the amount of water permeated through uniform area of the sample per second. Permeability coefficient, which could be considered as the total effect of sorption and diffusion, can be estimated using equation 8.5. Tables 8.5 and 8.6 represent the effect of fibre loading and chemical modification on sorption coefficient and permeability of banana fibre /PF composites at different temperatures (40wt% fibre) prepared by RTM and CM respectively. The sorption and permeability coefficients vary with fibre loading, chemical modification and temperature. For untreated fibre reinforced composites, the permeability value increases as a function of fibre loading and is maximum for the composite having a fibre loading of 40wt%. In the case of treated fibre composites, P showed the least value of permeation for NaOH treated fibre composites fabricated by RTM, when compared to the untreated and other treated banana fibre composites. This is consistent with the other results obtained Modeling of water sorption The kinetics of water sorption for two phase materials consisting of one dense and one less dense phase are often fitted according to the following equations (18). Qt/Q = 4(τ/π) 1/2 for τ W /W(eq) = (1-8/π 2 1 ) exp [-(2n+1) 2 π 2 τ] for τ > (2n+1) 0 Where τ is a dimensionless parameter, τ = ( Dt / h 2 ) W is the initial weight of the composite sample and W(eq) is the final weight of the composite sample. The experimental and fitted diffusion curves for the composites fabricated by RTM having a fibre content of 30% at 25 o C are

29 Water sorption characteristics of 325 shown in Fig Here the absorption data have been plotted as Qt/Q versus the square root of τ. The experimental and fitted curve displayed strong deviation. This is due to the deviation of diffusion processes from Fickian behaviour which is clear from the n values given in Table Q t / Q Theoretical Experimental (RTM) Experimental (CM) τ 1/2 Figure 8.12 Experimental and theoretical diffusion curves for 30wt% banana fibre/pf composites 8.3 Conclusion The water absorption kinetics of banana fibre reinforced PF composites were analysed at different temperatures 25, 50 and 90 o C. The effects of fibre loading and chemical modification of fibres on the water absorption of the composites were evaluated. Neat PF resin shows minimum water absorption and as the amount of hydrophilic cellulose fibre increased the water uptake also increased. The fibre reinforced composites were found to absorb water very rapidly at the initial stage and later a saturation level was attained without

30 326 Chapter 8 any further increase in water sorption. Water sorption of RTM composites were comparatively less than that of CM composites due to the lower void content and high fibre/matrix interaction. The values of diffusion coefficient, sorption coefficient, permeability coefficient and mole percent uptake at equilibrium of alkali treated fibre reinforced composite were found to be lower than that of untreated and other treated composites irrespective of the fabrication techniques. This is due to the enhanced bonding between fibre and matrix through chemical treatment. As a result of chemical treatment the hydroxyl groups of cellulose binds with the matrix by hydrogen bonding and good fibre/matrix adhesion is established. Therefore the chances of hydroxyl groups coming in contact with water molecules are reduced considerably. The analysis also proved that water absorption was increased in the case of benzoylated, acetylated and silanised fibre composites. Here, after chemical treatment the fibre surface becomes hydrophobic, which decreases the fibre/matrix interaction, thus facilitating void formation and enhances watersorption. The sorption coefficient and permeation coefficient increased with increase in fibre loading. Diffusion studies revealed that the interfacial adhesion plays a vital role in the water transport process. Fickian model was used to study the diffusion process in banana fibre reinforced polyester composites and revealed deviation between experimental values and theoretical predictions. This may be due to the deviation of diffusion from Fickian behaviour. Scanning electron micrographs of fracture surfaces of RTM and CM composites indicated that fibre splitting and fibre pull out occurred more for CM composites. Thus fibre/matrix interaction substantially improved in RTM composites. Finally, it is highly relevant to add that water transport studies could be used as a probe to evaluate the strength of the interfacial adhesion in cellulose fibre reinforced polymer composites.

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