Microwave Initiated Gum Arabic Grafted Poly L-Lactic Acid Bionanocomposite: Adhesive Application
5.2. Results and Discussion
5.2.2. Single lap shear test
5.2.2.3. Adhesive performance for glass substrates
Different parameters such as compression load, compression time and adhesive concentration are responsible for adhesive strength. All these three parameters were optimized as per the table 5.3 in which 2000 N, 10 min. and GA-g-PLA(15%) were selected as optimized compression load, compression time and adhesive concentration respectively. The study was investigated to check the adhesive properties of GA-g-PLA by single lap shear test. The substrates were bind together using synthesized adhesive by applying the adhesive on both the surfaces of the laminates and then applying load to the joint for binding the substrates together.
The effect of this compression load was also checked by varying the load and compression time. The effect of 10 and 15 wt.% of GA in bionanocomposite was analyzed for investigating adhesive strength. The experimental run for optimizing the parameters to get the best adhesive strength are systematically shown in table 5.3.
Table 5.3. Experimental run protocol for optimizing adhesive parameters for single lap shear test.
Experiment No.
Adhesive concentration (%)
Compression load (N)
Compression time (min)
1 GA-g-PLA(10%)
200
10 1000
2000
2 GA-g-PLA(10%) Best compression load
2 5 10 3
GA-g-PLA(10%)
Best compression load Best compression time GA-g-PLA(15%)
Optimization of parameters
Compression Load
One of the relevant parameter for determining the adhesive strength is compressive pressure applied during the cure of the adhesive. The compressive pressure was increased from 200 to 2000 N for conducting the test. Figure 5.1 shows the results of tensile lap-shear tests for GA-g-PLA(10%) with varying compression load. The average adhesive strength of 93, 111.5 and 230.5 MPa was calculated for compression load at 200, 1000 and 2000 N respectively.
On analyzing the bond stress-slip relations in figure 5.1 (i), (ii) and (iii); the behaviour of the adhesive describes that after an initial linear branch, the trend become nonlinear which confirmed the nonlinear behaviour of the GA-g-PLA(10%) adhesive (Khshain et al., 2015).
From the trend in figure 5.1 (i)-(iii), it was observed that the adhesive GA-g-PLA(10%) has elastic properties. After reaching the maximum load, the initiation of damage in the bond line starts. No cracking in the bond line was observed from the plots was confirmed as after reaching the maximum load, sudden decrease in shear stress was observed with displacement (Chataigner et al., 2011). The adhesive shear stress reaches close to zero within 3 to 10 min which depends on the test conditions. From the results, it was concluded that the best adhesive strength was achieved at 2000 N compression load.
Figure 5.1. Optimization of compression load at (i) 200 N, (ii) 1000 N, (iii) 2000 N, (iv) comparison of shear strength at different compression loads.
Compression time
The single lap shear tests at various compression time are presented in this section. For GA-g-PLA(10%), the debonding mechanism at interface was observed by the load versus length graphs. The range of the amount of adhesive retained in the adhesive line is between 50
the compression time beyond 10 min, no increase in shear strength was achieved as the curing of the adhesive was completed within 10 min. From the results it was determined that the 10 min of compression time for the adhesive system gives the highest shear strength.
Figure 5.2. Optimization of compression time for (i) 2 min, (ii) 5 min, (iii) 10 min, (iv) comparison of shear strength at different compression time.
Adhesive concentration
Other important parameter optimized was the concentration of GA in composite. The 10 and 15 % was compared and the adhesive strength was determined. The debonding between the GA-g-PLA(10%) adhesive-glass substrate occurred resulted in cohesive failure. The best shear strength was obtained for GA-g-PLA(15%) on applying 2000 N compression load for 10 min.
This proved to be the best combination of concentration and compression load which has ability to increase the joint rigidity drastically. Hence, for this combination adhesive has the capability to develop its full resistance capacity. For GA-g-PLA(15%), it was observed that, when the shear stress increases, the shear cracks develop on the substrate resulting in substrate failure. However, the adhesive bond line was intact and no deformation was observed in it.
The substrate failure was observed for GA-g-PLA(15%), as the shear strength of the adhesive was larger than the tensile strength of the substrate. The adhesive strength was not obtained for 20% composites, because the high viscosity decreased the surface wettability.
Figure 5.3. Optimization of GA concentration for (i) 10%, (ii) 15%, (iv) comparison of shear strength at different GA concentration (
♦
represents cohesive failure,▲ represents substrate failure).
The adhesion property of the prepared adhesive is due to the chain entanglement of GA-g-PLA molecules. The significant improvement in the shear strength of GA-g-PLA(10%) adhesive was observed with increase in the compression load and compression time as shown in figure 5.1 and 6.2. Whereas, the adhesive strength was improved drastically only by increasing the adhesive concentration from GA-g-PLA(10%) to GA-g-PLA(15%) as shown in figure 5.3. The GA-g-PLA(15%) showed the maximum adhesive strength which resulted in substrate failure due to more entanglement of GA-g-PLA molecular chains which leads to more adhesive power.
From the results, we have concluded that the adhering capacity is basically dependent on the adhesive concentration. Figure 5.4 shows the substrate failure during analysis for GA-g-PLA(15%) adhesive concentration, when the laminates were prepared by applying 2000N compression load for 10 min. It was also concluded that the active sites created on the GA backbone increased with increase in GA concentration during reaction which leads to more points of linkages between adhesive and adhering surface. Similarly, the single lap shear test was conducted for granite substrates at optimized conditions as shown in figure 5.4 (b). Hence, the prepared bionanocomposite proved to be excellent adhesive for glass as well as granite substrates.
Figure 5.4. Substrate failure occurred in (a) glass laminates, (b) granite laminates (c) comparison of adhesive shear strength between glass and granite substrates.