Stress Relaxation Behaviour of PALFnDPE Composites

Chapter 7 Stress Relaxation Behaviour of PALFnDPE Composites The results presented in this chapter have been communicated for publication to Journal of Reinforced Plastics and Composites. 7.1 Introduction S tress relaxation is the most fundamental experiment used for characterising the viscoelastic properties of materials. It relates to stress decay occurring with time when a solid is subjected to constant strain. The measurement of stress relaxation is considered to be a very useful experiment for predicting the long term characteristics of polymeric materials as it represents the basic time dependent response from which other time dependent responses such as creep can be obtained.' The characteristic feature of a uniaxial stress relaxation experiment is that the strain remains constant while the corresponding stress and stress rate decreases with time. The stress relaxation modulus is increased by rigid fillers and decreased by elastomeric ones up to the point where dewetting or crazing becomes pronounced. The rate of stress relaxation greatly increases after the onset of dewetting for both rigid and elastomeric

Effect of irradiation on stress relaxation of polyethylene has been studied by ~hateja.~ Tieghi er 01.' have established that the relaxation curves of a series of crosslinked polyester resins can be superimposed by a shift along the logarithmic time axis, with shift factors. Kumar er behaviour have investigated the stress relaxation of polyacetal-thermoplastic polyurethane elastomer blends and demonstrated that the stress relaxation modulus values measured at different strains can be superimposed by a shift along the logarithmic time axis to yield master curves of modulus over an extended time period. Stress relaxation behaviour of short fibre filled elastomers with respect to strain level, strain rate, fibre loading, fibre orientation and temperature have been studied.728 Thomas and co-workers have investigated the stress-relaxation behaviour of short sisal fibre reinforced natural rubber composites.9 In this chapter, the results of studies on the stress relaxation behaviour of short pineapple fibre reinforced polyethylene composites in tension are reported. The effects of strain level, pre-strain, fibre content, fibre length, chemical treatment, orientation and ageing on the relaxation behaviour have been investigated. It is shown that it is possible to superimpose stress relaxation curves at different strain level leading to a master curve. 7.2 Results and discussion 7.2.1 Effect of fibre length There are two important mechanisms that can lead to stress re~axation.~"~ (i) Physical stress relaxation due to molecular rearrangement requiring little primary bond formation or breakage and (ii) chemical stress relaxation due to chain scission or crosslink formation. Under normal conditions, both physical and chemical stress relaxation will occur simultaneously. Figure 7.1 shows the stress relaxation behaviour of PALFLDPE composites with different fibre length. In the case of composites based on 2 mm

and 6 mm long PALF fibres, the experimental data appear to lie on two straight lines. The first line is of greater slope and applies for shorter times (< 300 s) and the second line is valid for longer times. The initial relaxation may arise from reamangementfreorientation at the fibre-ldpe interfa~e.'.~ However, when 10 mm long fibres are used, linear relationship is observed between relative stress and log t.., 1 1.5 2 2.5 3 3.5 4 Figure 7.1. Stress relaxation curve of PALFILDPE composites with different fibre length (Successive graphs are displaced upward by '0.05 for clarity; Strain level 4%. fibre loading 20%). The effect of fibre length on the rate of relaxation of PALFLDPE composite is given in Table 7.1. It is evident from the table that for a given time, the relative stress decreases with fibre length. From Figure 7.2, it is observed that the relaxation modulus is higher in the case of composites containing 6 and 10 mm long fibres compared to 2 mm due to the greater reinforcing effect in these

composites. It was shown earlier (Chapter 3) that 6 mm was the optimum length for good reinforcement. The low relaxation modulus of composite with 2 mm long fibres may be due to the insufficient stress transfer and puflout of fibre from the matrix on the application of strain. Table 7.1. Rate of relaxation (%) of PALFLDPE composites with different fibre length (Fibre loading 20%, strain level 47'0) F~bre length (mm) Range (s) lo1- lo2 lo2-10' 10'- 104 2 9.1 10.2 10.7 6 9.5 9.2 8.8 10 8.2 11.3 11.9 200 1 1 I 1 2 3 4 Figure 7.2. Tensile stress relaxation modulus against time for PALFILDPE composites with different fibre length (Strain level 4%. fibre loading 20%).

7.2.2 Effect of fibre loading The stress relaxation behaviour of LDPE and PALF filled LDPE at different fibre loading is shown in Figure 7.3. In all cases, a two step relaxation is observed, It can be seen that even in the case of LDPE two linear regions are observed. The relative extent of linear region is dependent on the fibre content 0.6' 1 1 1.5 2 2.5 3 3.5 4 Figure 7.3. Stress relaxation curve of PALFiLDPE composites at different fibre loading (Successive graphs are displaced upward by 0.05 for clarity; strain level 4%. fibre length 6 mm). Stress relaxation is greatly reduced by the addition of fibres to the matrix particularly in the initial period (Figure 7.3). The rates of relaxation for the composites are provided in Table 7.2. It is seen that in all cases, the earlier slope is greater than the latter; however, at longer times the relaxatiot~ rate is nearly same for all the filled systems. Rigid fillers tend to decrease both the elastic and viscous

components of stress relaxation so long as there is no dewetting of the particles. In the fibre filled system, it is clear from the figure that as the fibre loading increases stress relaxation decreases. The effect of fibre loading on retained stress can be attributed to the elastic nature of fibre. Also the fibre-matrix interface bonding has a greater effect. The composite with 30% fibre content is found to exhibit single relaxation process. It is interesting to note that the cross over time is shifted to lower values on addition of PALF; however, increasing the fibre loading has marginal effect on the crossover time. LDPE has some amount of crystallinity. The initial portion may be attributed to relaxation of crystallites and the latter portion to viscous flow of PE chains. Table 7.2. Rate of relaxation (?h) of PALFkDPE composites at different fibre loading (Fibre length 6 mm, strain level 4%). Fibre loading (wt %) 0 10 20 30 lo1- lo2 12.3 13.9 9.5 7.2 Range ( 4 lo2-103 12.3 10.7 9.2 7.2 lo3- lo4 8.3 8.1 8.8 7.2 In order to further understand the stress relaxation behaviour of PALFILDPE composites, the relaxation modulus [E,(t)] is plotted against log time and shown in Figure 7.4. It is evident from the figure that the relaxation modulus increases with fibre loading. The trend is as expected since the stress relaxat~on modulus, wt), is increased by rigid fillers up to the point where dewetting or crazing becomes pronounced.z~3 The percentage decrease of wt) of LDPE after a time span of 10,000 second is 43% whereas in the case of composites containing 30% fibre loading, the decrease is only 18%.

I 0' I 1 2 3 4 Figure 7.4. Tensile stress relaxation modulus against time for PALFLDPE composites at different fibre loading (Strain level 4%). 7.2.3 Effect of fibre orientation The effect of fibre orientation on the stress relaxation behaviour of PALFILDPE composite containing 20% fibre loading is shown in Figure 7.5. The relaxation rate of randomly oriented composite is much higher than longitudinally and transversely oriented composites (Table 7.3). It was found that longitudinally oriented composite exhibits lower relaxation than transversely oriented composites. In transverse fibre orientation, the fibres are aligned perpendicular to the direction of force application and the major relaxation is due to the polymer. However, the time of transition of the two relaxation processes is almost constant except in randomly oriented composite.

1.1 * LONGITUDINAL TRANSVERSE x RANDOM Figure 7.5. Dependence of stress relaxation behaviour of PALFLDPE composites on fibre orientation (Successive graphs are displaced upward by 0.05 for clarity; strain level 4%, fibre loading 20%). Table 73. Rate of relaxation (%) of PALFLDPE composites with different fibre orientation (Fibre loading 20%, strain level 4%). Fibre orientation Range (s) 10'- lo2 lo2- lo3 lo3-- lo4 Longitudinal 9.5 9.2 8.8 Transverse 7.0 7.6 7.6 Random 15 15 12.8 7.2.4 Effect of chemical treatments The influence of different types of fibre treatment on stress relaxation behaviour of PALFLDPE composites is shown in Figure 7.6. As in the case of

untreated composite, DCP and BPO treated composites exh~bit two relaxation processes with nearly same crossover time. Interestingly, the data for silane (Silane A1 72, isocyanate (PMPPIC) and alkali treated composites appear to lie on a single straight line. As discussed in Chapter 4, these treatments considerably improve the mechanical properties. 0.6 1 1.5 2 2.5 3 3.5 4 Figure 7.6. Stress relaxation curve of PALFILDPE composites at different chemical treatments (Successive graphs are displaced upward by 0.05 for clarity; strain level 4%. fibre loading 20%). Due to enhanced fibre-matrix adhesion, it is possible that rearrangementslreorientation at the fibre-matrix interface do not contribute considerably to the stress relaxation process. Similar results were obtained in the case of short jute fibre reinforced NBR composites7 The relaxation rates of treated PALFILDPE composites given in Table 7.4 show that there is only marginal differences.

Table 7.4. Rate of relaxation (%) of PALFLDPE composites at different chemical treatment (Fibre loading 20%, strain level 4%). The variation of relaxation modulus [E,(t)] with time for untreated and treated PALFLDPE composites is shown in Figure 7.7. E,(t) values are considerably enhanced by treatment due to increased fibre-ldpe interface adhesion. The extent of increase of E,(t) in treated composites (at any time) depends on the nature of treatment. The maximum increase is exhibited by BPO treated composites. This trend in wt) data is similar to that observed earlier in statlc modulus data (Figure 4.16). The increase in E,(t) of treated composites is due to the grafting of polyethylene on to cellulose fibres by combination of cellulose and polyethylene radicals as discussed earlier (Chapter 4).

500 450 - A UNTREATED PMPPIC x SILANE BPO A DCP 0 NaOH 150 1 2 3 4 Figure 7.7. Tensile stress relaxation modulus against time for PALFLDPE composites at different chemical treatments (Strain level 4%, fibre loading 20%). 7.2.5 Effed of strain level Figure 7.8 shows the stress relaxation behaviour of PALFLDPE composite at different strain levels. It can be seen that the relative stress values recorded as a function of time for different strains exhibit different trends. At 2% strain, the relaxation rate due to the initial process is less than that due to the latter mechanism. At 4% strain the reverse behaviour is observed. Interestingly, the linear curve obtained at 6% indicates operation of a single relaxation mechanism (also see data in Table 7.5). Since the strain level is near the failure elongation (8-lo%), it is possible that rearrangements at the fibre-matrix interface has already taken place and hence do not contribute to the relaxation. As strain increases, the probability of a chain segment possessing enough energy to flow increases and more viscous flow takes place.'3 It is also concluded that the relaxation mechanism is a physical one involving the rearrangement of molecular chains or aggregates.

I 1 1.5 2 2.5 3 3.5 4 I Figure 7.8. Effect of strain level on stress,relaxation curves of PALFILDPE composites at 20% fibre loading (Successive graphs are displaced upward by 0.05 for clarity). Table 7.5. Rate of relaxation (%) of PALFLDPE composites at different strain level. Fibre loading 20%. Strain level (%) Range (4 10'- lo2 1 lo2-103 10)- lo4 The stress relaxation modulus at a given time may be independent of strain at small strain. However, at higher initial fixed strain, the relaxation modulus decreases faster (Figure 7.9).

1001 1 15 2 2.5 3 3.5 4 Figure 7.9. Tensile stress relaxation modulus against time for PALF/I,DPE composite at various strain levels (Fibre loading 20%). In this study an empirical horizontal shift of wt) versus time curve is made and the horizontal shift factor is computed as per procedure reported by Kumar el The master curve is constructed with reference to lowest strain value and is shown in Figure 7.10. The stress relaxation curves of composites at different strain level are superimposed and a smooth master curve is obtained. The construction of master curve enables one to predict the long term mechanical behaviour of the composite. Tieghi er stated that in classical viscoelastic terms the effect of strain is to shift the whole relaxation spectrum by a constant amount a, which depends on the value of applied strain.

Figure 7.10. Master relaxation modulus curve versus log time for PALFkDPE composites (Fibre loading 20%). 7.2.6 Effect of pre-strain Stress relaxation measurements were made on samples which had previously been stretched to 1% elongation, strain was released and then allowed to recover. Figure 7.1 1 shows the effect of pre-strain on the relaxation modulus of composite. it is evident from Table 7.6 that the rate of relaxation is faster in the composite, which has been prestrained. When the system is subjected to strain initially, orientation of molecules take place. By the application of strain again, (i.e. prestraining), orientation of the molecules will be fast, and relaxation of stress takes place to a greater extent

0 6 1 1.5 2 2.5 3 3.5 4 Figure 7.11. Stress relaxation curve of PALFLDPE composites-effect of prestrain (Fibre loading 20%). Table 7.6. Rate of relaxation (%) of PALFLDPE composites--effect of prestrain (Fibre loading 200/a, strain level 4%). Composite Range (s) 10'- lo2 lo2.- 10' 10'- lo4 Control 9.5 9.2 8.8 Prestrain 8.2 12 11.4 7.2.7 Effect of thermal treatment Stress relaxation measurements have also been made on PALFLDPE composites after exposing to 55OC and 75OC for 24 h. The thermal treatment produces interesting effect on the relaxation behaviour as shown in Figure 7.12. The data points for thermally treated samples lie on a single straight line. This is true for PMPPIC treated sample also, for both temperatures. The relaxation rates (Table 7.7) are found to increase on heat treatment.

0.6 J 1 1.5 2 2.5 3 3.5 4 Figure 7.12. Stress relaxation curve of PALFILDPE composites-effect of thermal treatment (Successive graphs are displaced upward by 0.05 for clarity; strain level 4%, fibre loading 20%). Table 7.7. Rate of relaxation (%) of PALFLDPE composites--effect of thermal treatment (Fibre loading 20%, strain level 4%). Composite- Range (4 Temperature (OC) 10'- lo2 lo2- lo3 10"- lo4. Untreated 28 9.5 9.2 8.8 75 8.8 12.8 11.6 PMPPIC 28 6.4 6.3 6.3 55 10.7 10.5 10.6 75 10.7 10.7 10.7

The effect of thermal treatment on relaxation modulus of untreated and PMPPIC treated composites is shown in Figure 7.13. It is observed that Wt) is marginally increased in the case of untreated composites. However, in the case of PMPPIC treated composite, the increase in E,(t) is considerable (about 18%). Thus, the results indicate that heating a thermoplastic composite near its softening temperature improves the modulus due to better interfacial adhesion. Improvement in mechanical properties of thermoplastic composites due to short time ageing at elevated temperature was reported earlier by Joseph et al." and Maldas el al." This was attributed to annealing leading to development of transcrystalline region at the fibre-matrix interface. It is believed that the increase on Wt) of PALF/LDPE composites on heat ageing is also due to similar cause. UNTREATED 2e0c PMPPIC 2e0c X UNTREATED 75 C PMPPIC 55% A PMPPIC 75 C I 1 150 1 1.5 2 2.5 3 3.5 4 Figure 7.13. Tensile stress relaxation modulus against time for PALF/L.DPE composites-effect of thermal treatment (Strain level 4%).

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