3.1 Optimizing cross-linking time
The response surface morphology based on the design of central composite using Design expert 7.0.0 software was used to maximize the bio-degradability and minimizing the water absorption and permeability of the films. To choose the best cross-linking time, 0% w/w, 10% w/w and 20% w/w of SFR/PVA film was cross-linked for 0, 5, 10, 15, 20 ,25,30 and 35 minutes. The best time for making cross-link was chosen based on the degree of degradability and minimum swelling ratio.
The best time selected was used to cross-link PVA films with varying cross-linker concentration and the properties were characterized to narrow down the final cross-linking parameters [21].
The presence of hydroxyl group in PVA attracts water restricting the mobility of polymer chains thereby forming hydrogen bond with the PVA chains. Furthermore, the incoming water molecules displace the PVA chains causing swelling. Cross-linking reduces the hydroxyl groups on PVA as a result, the hydrophilicity of the polymer is reduced (figure 6 a). In addition cross-linking restricts the movement of PVA chains, which upon the amount of water entering between the polymers chains gets limited
3.2 Water swelling study
Water swelling evaluation was carried out after the samples were kept in oven at 50 °C for 2 hrs.
The water absorption phenomena were rapid at the beginning and slowly decreased reaching at 35 minutes of immersion. The lowest swelling ratio at 5.68% BA /PVA cross-linked sample was attained at 35 minutes (Figure 7). Swelling ratio of pure PVA (170%), PVA with 5.68 % BA without cross-linking showed a lower swelling ratio (87.52%). The reduction of swelling ratio after crosslinking demonstrate the synergetic interaction between PVA, Sisal fiber and cross-liker. This related to the reduction of the free hydroxyl group of the polymer chains due to the formation of ester bond between the sisal fiber and the polymer (Figur3 6a) [23] [22].
To obtain optimal concentration of boric acid (BA) the swelling ratio was evaluated by fixing the time at 35 minute, PVA cross-linked with various concentrations of boric acid (BA). As displayed in (Figure 8) the result demonstrate a minimum swelling ratio of 48% was achieved for 5.688% BA/PVA cross-linked samples with significant (p < 0.05) reduction in swelling ratio was due to the formation of cross-linked networks that makes hydroxyl group reduced and restricted the movement of the polymer chains. This indicates the synergetic effect between the PVA and crosslinking [23].
3.3 Selection of boric acid concentration
Once the cross-linking time was fixed to be 35 minutes, PVA with various concentrations of boric acid (BA) was cross-linked for 35 minutes and was characterized by a swelling ratio. The optimal concentration of boric acid was chosen based on the lowest swelling ratio.
The swelling ratio of PVA (Figure 8) was measured to be 170%. After cross-linking with boric acid for 35 minutes, the swelling ratio was found to decrease as the concentration of the cross-linker increased. A minimum swelling ratio of 48% was achieved for 5.688% BA/PVA cross-linked samples. This significant (p < 0.05) reduction in swelling ratio was due to the formation of cross-linked networks, which not only reduced the hydroxyl groups on PVA but also restricted the movement of the polymer chains. Since the minimum swelling ratio was observed in the 5.68% BA-CL-PVA sample, 5.68% boric acid (BA) was chosen for the synergistic study of cross-linking and reinforcement.
3.4 Sisal fiber concentration and Swelling ratio
To evaluate the maximum concentration of Sisal fiber and the swelling ratio of SFR/PVA (Figure 9) at 20% SFR ratio the minimum value reached 48% after which, it becomes nearly constant. The main reason of swelling ratio reduction in a fiber –reinforced composite was due to the reduction of hydrophilic OH group of PVA because of the formation of hydrogen bond between the OH of the sisal fiber and the PVA. The presence of a large number of hydroxyl groups can improve the interfacial binding between the matrix and the filler via hydrogen bonding. To improve the interfacial binding between sisal fiber and PVA, chemical cross-linking was done using boric acid. To see the effect of cross-linking on PVA the SFR kept constant (20%) and 5.68% BA was added. The result shows lower water absorption of PVA due to the synergetic effect of Sisal fiber and boric acid with PVA [24].
3.5 Bio-degradability test
To check whether the films were biodegradable or not, biodegradability test was done on soil burial experiment. The biodegradability study (Table 1 and Figure 10) showed that all the films were biodegradable.
Table 1: Degradation of different samples in the soil with time (day)
Volume fraction
|
Weight loss or percentage of degradability with time (days)
|
Number of days
|
7
|
14
|
21
|
28
|
35
|
42
|
49
|
56
|
63
|
0/100
|
15
|
19
|
29
|
33
|
39
|
43
|
46
|
51
|
59
|
100/0
|
20
|
24
|
27
|
34
|
43
|
46
|
55
|
62
|
70
|
10/90 ST
|
20
|
28
|
37
|
40
|
44
|
50
|
53
|
60
|
67
|
10/90 SNT
|
10
|
15
|
29
|
36
|
47
|
51
|
56
|
62
|
68
|
20/80 ST
|
16
|
25
|
29
|
32
|
38
|
42
|
50
|
56
|
62
|
20/80 SNT
|
21
|
26
|
40
|
49
|
56
|
60
|
66
|
70
|
73
|
Conventional Plastic
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
3.6 Water permeability test
The water permeability tested by keeping the film in water at the interval of 5 minutes for 24 hours. The result shows increase in water permeability as pore or gap on the surface increase. The water permeability significantly decrease when PVA reinforced with sisal fiber and cross linked with BA. A minimum permeability of 8.4% and 8.5% was achieved for 5 mm and 10 mm fiber length for 24 hour. This shows that more fineness of the size of the material results in less percentage of permeability (Figure 11).
Synergistic effect of reinforcement and cross-linking
The synergistic effect of reinforcement and cross-linking (Figure 12) resulted in a significant change in the swelling ratio of the composite. The swelling ratio of 10% SFR-CL-PVA was 123%, while that of 15% and 4% boric acid-cross-linked PVA was 89%. After the integration of 20% sisal fiber reinforced (SFR) and cross-linked with 5.68% BA, the swelling ratio was decreased up to 32% (lower swelling value), which was higher than the reduction due to reinforcement or cross-linking alone. The formation of a strong cross-linked network worked together with the reinforcement to reduce the swelling ratio. The main cause of this significant reduction in swelling ratio was the enhancement of interfacial binding between PVA and SFR, the formation of cross-linked networks, and the effective interaction between SFR, PVA, and the boric acid (BA) cross-linker.
3.7 Statistical Analysis of Different Process Variable Optimization on water absorption the bio-plastic films
The experimental design software 7.0.0 was employed to optimize the cross linking parameters for maximum degradability of the film and minimum water absorption. The effect of interaction parameters is shown in Figure 13
Equation in relation to coded factors
1/Swelling ratio (R) = +100.33+6.54 * A-12.02 * B-10.21* C-3.13*A*B-29.38 *A*C-9.58* B*C+4.1 *A2+9.95* B2+1.93 *C2--------------------------------------------------------------- (3.1)
Equation in relation to factors
1/swelling ratio
(R)=+319.38759+88.72907concentration11.34524temprature+5.93809time0.25070concentrationtem
perature0.97938concentrationtime0.025543temperaturetime+4.11367+0.063651+2.13908E-003----------------(3.2)
The importance and accuracy of ANOVA was tested. The result shows as displayed in table a liner interaction with coded factor A which is related to cross-linking concentration. The cross-linking concentration factors A^2, B^2 and C^2 quadratic related to percentage with significance variable less than 0.05 and mean value greater than 0.05 as shown in table 2
Table 2: Regression coefficients and significance of response surface quadratic model for water absorption
Factor
|
Coefficient
Estimate
|
df
|
Standard
Error
|
95% CI
Low
|
95% CI
High
|
VIF
|
Intercept
|
100.33
|
1
|
0.067
|
100.18
|
100.48
|
|
A-Concentration
|
6.54
|
1
|
0.045
|
6.44
|
6.64
|
1.00
|
B-Temperature
|
-12.02
|
1
|
0.045
|
-12.12
|
-11.92
|
1.00
|
C-Time
|
-10.21
|
1
|
0.045
|
-10.31
|
-10.11
|
1.00
|
AB
|
-3.13
|
1
|
0.058
|
-3.26
|
-3.00
|
1.00
|
AC
|
-29.38
|
1
|
0.058
|
-29.51
|
-29.25
|
1.00
|
BC
|
-9.58
|
1
|
0.058
|
-9.71
|
-9.45
|
1.00
|
A^2
|
4.11
|
1
|
0.044
|
4.02
|
4.21
|
1.02
|
B^2
|
9.95
|
1
|
0.044
|
9.85
|
10.04
|
1.02
|
C^2
|
1.93
|
1
|
0.044
|
1.83
|
2.02
|
1.02
|