CHAPTER 4 WATER ABSORPTION BEHAVIOR AND ACCELERATED AGING EFFECTS

Transcription

1 1 CHAPTER WATER ABSORPTION BEHAVIOR AND ACCELERATED AGING EFFECTS.1 INTRODUCTION The mechanical properties of the polymer matrix composites depend on the properties of their constituents and their interaction with the environmental conditions such as moisture and temperature. All polymers and composites absorb moisture in humid atmosphere and when immersed in water. Natural fibers absorb more moisture when compared to synthetic fibers. The main drawback of natural fibers is their hydrophilic nature that lowers their compatibility with hydrophobic polymer matrices. This leads not only to weak interfacial adhesion between the fiber and the matrix but also results composites having high water absorption characteristics that reduce their utility in many applications. Fiber treatment by chemical modification and effective hybridization of natural fibers with stronger and more corrosionresistant synthetic fibers like glass or carbon are some of the techniques adopted to hopefully overcome these drawbacks. Further, the dimensional changes in the material when the structure is exposed to humid environment or elevated temperature have detrimental effects on its performance in terms of degradation of properties such as tensile strength, flexural strength, impact strength etc. Hence for effective utilization of the material, it is essential to know the moisture absorption behavior of the material under humid atmospheric conditions.

2 11 Accelerated aging tests are carried out in order to shed light on the long-term performance of the material without actually carrying out the long-term test. Such tests are carried out by exposing the samples to accelerated aging environments like ultraviolet radiation, thermal aging, elevated temperature, soil burial, boiling water immersion etc. A brief review of the literature which discuss the environmental aspects and durability issues of composites materials with special emphasis to natural fiber-based composites is presented in chapter. In this chapter, water absorption behavior and effects of accelerated aging (boiling water immersion and thermal aging) on mechanical properties and dimensional stability of woven jute and jute-glass fiber hybrid composites are described. The materials were characterized for water absorption as per ASTM D57. The effect of hybridization of glass fiber on diffusivity of water and residual properties of jute composites for different periods of immersion in water was investigated. The effect of glass fiber addition on tension properties and dimensional stability of jute composites subjected to accelerated aging for different durations was examined. Fractographic study was also carried out to investigate the degradation mechanism in composites upon aging.. WATER ABSORPTION BEHAVIOR..1 Theoretical Aspects of Diffusion-a Briefing Based on the Fick s law of diffusion (Bunsell and Renald 5), which assumes that the water penetration into a material depends on the moisture concentration gradient c / z and time, the moisture weight gain (G m ) as a function of time is given by equations (.1) and (.) (Mallick 1997).

3 1 G m = and G m = M % 8 D. =1- % exp z t D. for z t h. >.5 (.1) h M m 1/ M % D = z t D for z t % h. <.5 (.) h M m where t = time in seconds, z = distance inside composite in the direction of diffusion (mm), D z = diffusion coefficient in z direction (mm /s), h = total thickness of the laminate (mm), M = Moisture uptake at any time t, M m = Moisture uptake at fully saturated condition, The term D z is a measure of the speed at which moisture is diffused (or penetrates) in the z direction of the composite material. The z direction is usually taken as the thickness direction of the composite. G m can be measured experimentally by weighing the specimen at various times during exposure to a moist environment. Equation (.) indicates that the moisture absorption depends on the square root of time. Moisture intake in composite materials is therefore plotted as a function of the square root of time. Writing the equation (.) for two different instances of exposure period t 1 and t as, M M 1 m D t h z 1 1/ (.3) M M m Dzt h 1/ (.) Subtracting (.3) from (.), and rearranging, we get,

4 13 h M M 1 D z = 1 (.5) M m t t Equation (.5) shows that if the quantities M m and the linear slope of the moisture absorption curves, i.e. (M M 1 ) / ( t - t 1 ) are known, the composite diffusion coefficient can be easily evaluated. Since most moisture diffusion experiments include moisture diffusion through six surfaces, the value of D z in equation (.5) is in error (Collings and Copley 1983). For true one dimensional diffusion coefficient D z, a correction factor for edge effect given by Shen and Springer (1976) can be used giving, h h D=D z 1 w l (.6) where, w and l are the laminate width (mm) and length (mm) respectively... Experimental Procedure The test specimens were cut from the laminated panels AJ, H1, H and H3 (Details are given in Table 3.1 in Chapter 3) to the size recommended in ASTM D 57 (76. 5.) mm. Edges of the samples were sealed with polyester resin. Samples in groups of three were dried in an oven for hours at 7 C. The weights of the dry samples were recorded. The samples were then immersed in water at room temperature. Samples were periodically taken out of the water, surface water was wiped off with soft cloth or tissue paper and weighed to the nearest.1 g and the samples were re-immersed in water immediately. Weight readings of the samples were taken till the samples reached the equilibrium value. The rate of moisture uptake is fairly rapid in the early stages of conditioning with the rate of moisture uptake

5 1 decreasing with time. Hence the weight is recorded for shorter interval period in the early stages (once in a day) followed by a gradual decrease in the frequency of measurement as the rate of weight gain reduces. The percentage uptake of water was determined using the equation (.7). W w W (.7) W M (%) = d x1 d where, W w and W d are the weights of the wet and dry sample respectively...3 Strength Degradation Strength degradation studies are essential to know the long term behavior of the composites, after being exposed to water or humid atmosphere. Based on the experimental results that were presented in chapter 3, the hybrid laminates H with 16.5% glass fiber weight fraction in a total fiber weight fraction of % was taken as an optimum combination with a good balance between the properties and cost (which increases with the increase in glass fiber content). Hence this laminate and all jute laminate (for comparison purpose) were selected for strength degradation study. Test samples for tension, flexural and ILSS test were prepared from these laminates and then immersed in water. Tests were conducted on these specimens for two different periods of immersion, viz., 3 days and 65 days in order to determine the degradation of strength due to absorption of water. All the tests were conducted at room temperature on closed loop servohydraulic MTS 81 Material Test System. Geometry of the test samples and test conditions were same as described in chapter 3 except that emery end tabs were used for holding the samples in hydraulic grips instead of aluminium end tabs. Three identical samples were tested in each case and average results were taken.

6 15.. Results of Water Absorption Behavior...1 Diffusion Co-efficient and Applicability of Fick s Law Figure.1 shows the percentage of water absorbed by all jute and hybrid laminates for various periods of immersion. Moisture absorption (%) Hours 1 13 Hours Hours 1 36 Hours 16 Hours 8 Hours Glass fiber content (wt %) Figure.1 Moisture absorption vs. glass fiber content It can be observed from the figure that, for all periods of immersion, the extent of water absorption decreases considerably with the addition of glass fiber weight fraction in the form of extreme glass plies. This reduction is due to the fact that impermeable glass fibers act as barriers and prevent direct contact between jute and water. Figure. depicts the percentage of moisture absorption as a function of square root of the time t for all jute as well as jute glass hybrid combinations. Each point in the graph is the average of three data points. The initial portions of the curves are linear and all materials have attained the saturation level after 15 h of soaking period. The maximum moisture absorption for AJ, H1, H, and H3 are found to be 13.%, 8.%, 5.5%, and.5% of the specimen weight respectively.

7 16 Moisture absorption (%) AJ H1 H H3 1 3 Square root of time, hr 1/ 5 Figure. Moisture absorption vs. square root of the time The diffusion co-efficient is calculated using the expressions (.5) and (.6) from slope of the moisture absorption vs. t 1/ plots (Figure.). The results of these calculations are presented in the Table.1. Table.1 Saturation moisture content and diffusion coefficient Laminate Saturation Moisture level % Slope of the M Vs t 1/ curve Diffusion co-efficient without edge effect ( 1-7 ) mm /sec Diffusion co-efficient with edge effect ( 1-7 ) mm /sec AJ H H H It was reported (Rao and Balasubramanian 1981) that the diffusivity reduces with increasing fiber volume fraction in an impermeable fiber composite and decreasing fiber volume fraction in a permeable fiber composite. Same trend is also observed in the hybrid laminate under consideration, where glass (impermeable) fiber volume fraction is increased

8 17 while the jute (permeable) fiber volume fraction is reduced. The variation of diffusion co-efficient with glass fiber loading is shown in Figure.3. Diffusion coefficient (x 1e-6) mm /sec without edge effect with edge effect Glass fiber weight fraction, % Figure.3 Diffusion co-efficient vs. glass fiber content The reduction in diffusion coefficient is significant with 8.% addition of glass (one extreme layer of glass ply on either side) to jute composite. Further addition of glass results only in partial reduction in diffusion co-efficient. In order to evaluate the applicability of Fickian diffusion model for the materials under consideration, experimental values of non-dimensional parameters (M/M m ) and (D z.t / h ) are plotted along with the analytical curve from equations (.1) and (.), in Figure.. M/Mm Fick's law H1 H H3 AJ D z. t / h.1 1. Figure. Correlation between analytical model and experimental data

9 18 The figure reveals that the experimental data is in good agreement with Fickian curve for hybrid H3, and fairly good for hybrids H1 and H. The deviation of experimental data with the analytical curve in case of all jute composites, indicate that the diffusion in these composites is Fickian up to D z.t/h =.5 (t in hours), after which it behaves as non-fickian.... Degradation Mechanism and Residual Properties The mechanical properties of jute and jute-based composites after immersion in water are found to degrade which may be due to degradation of its constituents like fiber and matrix or may be due to weakening of fiber-matrix interface. Degradation in reinforcements plays an important role in strength and stiffness reduction of fiber reinforced composites as they are the major load-carrying constituents. The high percentage of water absorption in jute composites (13%) is due to hydrophilic nature of jute fiber that causes its swelling and shrinkage resulting in weakening of the fiber strength and fiber-matrix adhesion (Moe Moe Thwe and Kin Liao 3) with the diffusion of water. Water molecules would be attracted by the hydrophilic groups of polyester causing swelling of the matrix. The dimensional instability of the matrix under the influence of water may result in debonding of the fiber-matrix interface. The interfacial adhesion characteristics have a significant effect on the load carrying capacity of a fiber reinforced composite. When the composites have been immersed in water, the capillarity would conduct the water molecules to the material and fills in the voids and cracks in the composites. Such water filled voids at the interface results in interfacial debonding. Differences in moisture expansion coefficients of fiber and matrix also contribute to progressive debonding and therefore weakening of the material. The appearance of jute (AJ) and hybrid laminate (H) before and after immersion in water for 3 days is shown in Figures.5 and.6 respectively. The figures reveal clear darkening of jute laminates and whitening of hybrid laminates. The darkening of jute laminates is due to loss of surface matrix upon immersion in water and formation of fungus due to

10 19 hydrophilic nature of jute and whitening of hybrid laminate is due to clear visibility of glass fabric on loss of matrix. Before immersion After immersion Figure.5 Jute laminate (AJ) before and after immersion in water for 3 days Before immersion After immersion Figure.6 Hybrid laminate (H) before and after immersion in water for 3 days The variation in tensile properties with the period of immersion is shown in Figures.7(a) and.7 (b). Tensile strength, MPa All jute Hybrid Period of immersion, Days Tensile Modulus, GPa All jute Hybrid Period of immersion, Days a) Tensile strength b) Tensile modulus Figure.7 Influence of water absorption on tensile properties

11 11 It appears from the figure.7(a) that the tensile strength is considerably decreased with the increase in the period of immersion. When the hybrid laminate sample is subjected to tensile loading, both jute and glass fibers plies are uniformly strained. However, for two reasons, jute fiber plies fails first and the entire load has to be transferred to glass fiber plies. One reason being low extensibility of jute fiber ( %) (Jochen Gassan and Bledzki 1999) than the glass fiber extensibility (.5-3.%) (Paul Wambua et al 3). The second reason is low tensile strength of jute fiber which is further reduced upon immersion in water (due to greater affinity of jute fiber to water absorption). Thus, different extensibilities of two fibers, different coefficient of moisture expansions of two fibers and, water at the jute and glass fiber surface, results in weakening of jute-glass ply interface bonding. Hence when jute fiber plies fails, load will not be transferred to glass fiber plies effectively. Therefore the rate of tensile strength reduction in hybrid composite is more (7.%) when compared to jute composite (1.%) for the first immersion period of 3 days. However, during 3 to 65 days of immersion in water, the rate of degradation of tensile strength in jute composite is found to be more (11.%) than jute-glass hybrid composite (9.57%) as can be seen from the slope of the curve in Figure.7(a). This could be because, greater damage to jute fiber due to longer period of immersion may impart more detrimental effect on tensile strength than weakening of jute-glass ply interface bonding. At the end of 65 days of immersion period, the tensile strength of all jute and hybrid composite is reduced by 1.5% and 16.1% of their respective strengths before immersion. The tensile strength of hybrid composite, after 65 days (9 MPa) is still higher than the strength of the unexposed jute sample (8 MPa). The influence of water absorption on the tensile modulus of jute and hybrid composites is shown in figure.7(b). For jute composite, the decrease

12 111 in the tensile modulus is 7.3% and 3% and for hybrid composite, the decrease is.6% and 3.%, after 3 and 65 days of immersion in water, respectively. The degradation in the flexural properties after 3 and 65 days of immersion in water is shown in figure.8(a) and.8(b). The figure reveals that the degradation is significant only in the first period (3 days) with a reduction in the flexural strength of % in jute composites and 31% in hybrid composites. No further loss of strength is noticed beyond 3 days of immersion in water. Flexural strength, MPa All jute Hybrid Period of immersion, Days Flexural Modulus, GPa All jute Hybrid Period of immersion, Days a) Flexural strength b) Flexural modulus Figure.8 Influence of water absorption on flexural properties The reduction in flexural modulus is % and 56% for two different periods in jute composite as indicated in figure.8(b). In hybrid composite, the flexural modulus is reduced by 3% over an immersion period of 3 days and no further reduction in the modulus was noticed beyond this period. Figures.9(a) and.9(b) shows a comparison between flexural fractured surface of the sample before immersion in water and after 65 days of immersion in water. The damage caused to the fibers as well as loss of matrix resulting in weak fiber-matrix interfacial bonding can be

13 11 observed from SEM image of fractured surfaces after 65 days of immersion in water. Similar observations can be made for other properties also. a b Figure.9 SEM image (Mag 1) of flexural fractured tension surface (a) Before immersion (b)after immersion The effect of water absorption on interlaminar shear stress is shown in Figure.1. It appears from the figure that the reduction in interlaminar shear stress is continuous. A decrease in ILSS of 16.3% and 3% in jute composites and 18% and 8% in hybrid composites is observed respectively, for two the periods of immersion namely, 3 days and 65 days. ILSS, MPa All jute Hybrid Period of immersion, Days Figure.1 Influence of water absorption on ILSS

14 113.3 ACCELERATED AGING TESTS In the present section, the effect of accelerated aging on tensile properties and dimensional stability of jute fabric reinforced isothalic polyester composite (AJ) and jute-glass fabric reinforced isothalic polyester hybrid composite (H) has been experimentally investigated and compared. Accelerated aging tests were conducted by boiling water immersion and thermal aging for different durations. In order to avoid weakening of the bond between the tab and specimen upon exposure to accelerated aging conditions, all the specimens were fabricated in the dog bone shape with the geometry shown in Figure All dimensions are in mm Figure.11 Geometry of the accelerated aging tension test specimen.3.1 Accelerated Aging (Boiling Water Immersion) Test samples in four groups for each type of laminate, with three samples in each group were immersed in boiling water for 3, 5 and 7 hours and subsequently tested for tensile properties. One group of samples was tested without immersion for comparison purpose. Young s modulus was evaluated by the slope of the initial portion of load-deflection plot..3. Accelerated Aging (Thermal Aging) For thermal aging, samples in three groups with three samples in each group were kept in an air circulating oven (Figure.1) at 8 o C for 3, 5 and 7 days and subsequently tested for tensile properties.

15 11 Figure.1 Air circulating oven used for thermal aging test.3.3 Dimensional Stability of Composites Dimensional stability test was conducted by monitoring the dimensional and weight changes in the samples, before and after immersion in boiling water and heating in oven for different durations. The samples after aging were cooled down and conditioned at room temperature. The dimensions and weights of the samples were measured before conducting tension tests..3. Results of Accelerated Aging Tests.3..1 Effect of Accelerated Aging on Tensile Strength Figure.13 compares the effect of boiling water immersion on tensile strength of jute and hybrid laminates. Reduction in tensile strength, % AJ H Boiling water exposure time, Hours Figure.13 Tensile strength reduction vs. boiling water immersion time

16 115 The figure demonstrate that the degradation in tensile strength is more for all jute laminates when compared to hybrid laminates for all durations of immersion in boiling water. The decrease in the tensile strength is sharper with the increase in the exposure time in boiling water. This is due to the fact that the rate of diffusion of water into the composite is time dependent. Swelling of the fiber increases with exposure time in boiling water, as a result of which crack may be formed at the surrounding matrix for the flapping of the swollen fiber (Kuruvilla Joseph and Sabu Thomas 1995). This may attribute to penetration of more water into the composite, resulting in the reduction in fiber-matrix adhesion leading to decrease in the tensile strength. The maximum reduction in the tensile strength after 7 hours of immersion for all jute and hybrid laminate is 9% and % respectively. The significant reduction in tensile strength even in hybrid laminate may be attributed to the penetration of water through the edges of the samples. Figure.1 compares the effect of thermal aging on tensile strength of jute and hybrid laminates for different periods. Reduction in tensile strength, % AJ H Exposure period in oven, Days Figure.1 Tensile strength reductions vs. thermal aging period The figure reveals that, for both all jute and hybrid laminates, the reduction in tensile strength is drastic from 3 to 5 days of exposure, beyond which the reduction is negligible. Unlike for boiling water immersion, the reduction in tensile strength was more significant for hybrid laminates at 5

17 116 and 7 days of exposure to 8 o C. One of the reason for this could be weakening of jute and glass fiber ply interfacial bonding due to different co-efficient of thermal expansion of the two fibers. Further, glass fiber plies offer poor resistance to tensile strength at higher temperature. The maximum reduction in tensile strength for all jute and hybrid laminate after 7 days of exposure to 8 o C is found to be 1.69% and 17.36% respectively. Since these figures are less than that for boiling water immersion, it can be concluded that boiling water immersion has more detrimental effects on tensile strength than thermal aging for the durations considered in this study. The damage caused to the jute fibers (which results in loss of tensile strength) after immersion in boiling water for 7 hours and exposure to 8 o C for 7 days are clearly visible in SEM images of the tensile fractured surfaces shown in Figures.15(b) and.15(c) in comparison to the SEM image of the fractured surface before aging in Figure.15(a). Damaged fibers a b c Damaged fibers Figure.15 SEM images (Mag 1) of jute laminates (a) Before aging (b) After aging in boiling water for 7 h (c) After exposed to 8 o C for 7 days

18 117 Figure.15 also reveal that the damage caused to the jute fibers is more severe for boiling water immersion than exposure to thermal aging..3.. Effect of Accelerated Aging on Tensile Modulus Figure.16 compares the effect of boiling water immersion on tensile modulus of jute and hybrid laminates. Reduction in tensile modulus, % AJ H Boiling water exposure time, Hours Figure.16 Tensile modulus reduction vs. boiling water immersion time The decrease in tensile modulus in jute laminates with the immersion time is associated with water absorption. Jute fiber being hydrophilic in nature absorbs more water than inorganic fibers like glass. Therefore, due to the high water content after immersion in boiling water, the stiffness of the cellulose fibrils drops considerably which results in significant reduction in tensile modulus. The maximum reduction in the tensile modulus of all jute and hybrid laminate after 7 hours of boiling water immersion is found to be % and 33% respectively. The significant reduction in tensile modulus even for hybrid laminate is again due to penetration of water through the sample edges. Sealing of edges by resin may not allow the seal to remain intact when the samples are exposed to accelerated aging conditions.

19 118 Figure.17 compares the effect of thermal aging for different periods on tensile modulus of jute and hybrid laminates. Reduction in tensile modulus, % AJ H Exposure period in oven, Days Figure.17 Tensile modulus reduction vs. thermal aging period Similar trend is obtained as for tensile strength i.e. reduction in tensile modulus was more significant for hybrid laminate than jute laminate for 7 days of exposure to 8 o C. The maximum reduction in tensile modulus for all jute and hybrid laminate after 7 days of exposure to 8 o C is % and 6.67% respectively. The results reveal that boiling water immersion has more detrimental effects on tensile modulus than thermal aging Effect of Accelerated Aging on Dimensional Stability Figure.18 shows the percentage increase in the thickness of the laminates for different durations of immersion in boiling water, as a result of swelling of jute fibers. In case of all jute laminate, there is constant increase in the thickness of the laminate whereas in hybrid laminates; no significant increase in the thickness is noticed beyond 3 hours of immersion. After 7 hours of immersion in boiling water, the thickness swelling in all jute laminates is found to be 8.11%, whereas in hybrid laminate it is just.%.

20 119 Increase in thickness, % AJ H Boiling water exposure time, Hours Figure.18 Increase in thickness (%) vs. boiling water exposure time The percentage increase in area as a function of boiling water immersion is shown in Figure.19. For both all jute and hybrid laminates, a constant increase in the area is noticed upon increase in the immersion time. After 7 hours of immersion in boiling water, the area in all jute laminates is increased by 8.66%, whereas in hybrid laminates it is 6%. 1 Increase in area, % 8 6 AJ H Boiling water exposure time, Hours Figure.19 Increase in area (%) vs. boiling water exposure time The increase in weight of the samples as a function of boiling water immersion time is shown in Figure.. Unlike thickness and area variation, it is observed that the percentage increase in weights of both jute and hybrid laminates is less for 7 hours when compared to that for 3 hours. The loss of

21 1 weight between 5 hours to 7 hours of immersion time may be due to deterioration of matrix particles with increased immersion time. Increase in weight, % AJ H Boiling water exposure time, Hours Figure. Increase in weight (%) vs. boiling water exposure time From the results of this study, it can be concluded that hybrid laminates have better dimensional stability than all jute laminates due to protective glass layers that prevent direct contact of jute with boiling water. The effect of thermal aging on dimensional stability of jute and hybrid laminates is compared in Figures Reduction in thickness, % AJ H Exposure period in oven, Days Figure.1 Reduction in area (%) vs. exposure period in oven

22 11 Observation of figures.1 and. reveals marginal increase in the reduction in thickness and area of all jute laminates with the increase in exposure period. However, for hybrid laminate, reduction in thickness (%) and area (3%) remains almost constant for all durations of exposure. The maximum reduction in thickness and area of all jute laminates after 7 days of exposure to 8 o C is 3.98% and.6% respectively. 5 Reduction in area, % 3 AJ H Exposure period in oven, Days Figure. Reduction in thickness (%) vs. exposure period in oven Reduction in weight, % AJ H Exposure period, Days Figure.3 Reduction in weight (%) vs. exposure period in oven For both all jute and hybrid laminates, weight reduction is almost constant irrespective of exposure period (Figure.3). For all jute laminates, the maximum reduction in weight after 7 days is found to be 5.7% and for hybrid laminates, it is 3.8%.

23 1. CONCLUDING REMARKS One of the major drawbacks of jute fiber composites that limit their use in numerous structural applications is their affinity towards moisture absorption. The effects of using protective glass layers on water absorption behavior, residual mechanical properties and dimensional stability of woven jute fabric reinforced isothalic polyester composites were investigated by exposing both the jute and hybrid samples to water under normal and accelerated aging (boiling water and thermal aging) conditions. Based on this investigation, the following major conclusions are derived. Maximum water absorption level of untreated jute fabric reinforced isothalic polyester resin is found to be 13%, which can be significantly reduced by the inclusion of glass fiber. The diffusion co-efficient of jute composites can be substantially reduced with 8. wt% addition of glass fiber as one extreme ply on either side. Further addition of glass does not lead to significant reduction in diffusion co-efficient. The tensile, flexural and interlaminar shear properties degrade with the increase in water immersion period, with the rate of degradation being more in jute composite (except for tensile strength) than jute-glass hybrid composite. Boiling water immersion causes considerable damage to the jute fibers and has more detrimental effects on tensile properties than thermal aging for both types of laminates. Thermal aging has marginal effect on dimensional stability of all jute laminates and negligible effect on dimensional stability of hybrid laminates.