4.1 Fourier Transform Infrared Spectroscopy (FT-IR) of Rosin acid and its derivative
FTIR spectra of the raw material (Rosin acid), intermediate product (MA), and final comonomer (TM) are presented in Fig. 3. The band at 2927 cm− 1 of the rosin acid spectrum (Fig. 3a) indicated the presence of –CH stretching. The stretching vibration of the carbonyl group of carboxylic acid appeared at 1695 cm− 1. The absorption peak at 1284 cm− 1 was due to the -CO stretching of carboxylic acid. The peaks in the spectrum of maleopimaric anhydride (MA) at 1845 and 1700 cm− 1 are due to -CO stretching of the anhydride group (Fig. 3a). In the spectrum of TM (Fig. 3b), the peaks at 1650, 985 and 916 cm− 1 indicated the presence of terminal C = C bonds. The peak that appeared in 1725 cm− 1 was assigned to the carbonyl group and peaks observed at 2924 and 2849 cm− 1 were due to the presence of methyl and methylene groups.
4.2 Fourier Transform Infrared Spectroscopy (FT-IR) of Chicken feather and biocomposites
FTIR spectra of the chicken feather and the composites loaded with varying percentages of rosin acid derivative are presented in Fig. 4. Chicken feather fiber (curve a) showed characteristic absorption bands at 3290 cm− 1 corresponding to N-H symmetric amide stretching. The peaks at 2962 and 2930 cm− 1 observed were due to asymmetric and symmetric stretching vibrations of methyl (CH3) in keratin protein. The peaks at 1652 and 1534 cm− 1 were due to –C = O symmetrical stretching of amide (I) and N-H bending vibration of amide (II). The absorption band that appeared at 710 cm− 1 was due to alkyl thiols (C-S), which originated from the amino acid cysteine. In the FT-IR spectra of composites (curve b-e), loaded with different percentages of TM, the intensity of the N-H peak that appeared for chicken feather at 3290cm− 1 was found to be decreased and ultimately almost diminished in all the composites.
In all the composites, the C = C stretching peak of TM was shifted from 1650 cm− 1 to 1638 cm− 1 (curve c), and 1633 cm− 1 (curve d, e), and the intensity of the peaks were found to reduce. Also the C = O stretching peak appeared at 1725 cm− 1 for MAESO/CF/TM0 (curve b) was found to shift to 1720 cm− 1, 1716 cm− 1 and 1715 cm− 1 for the samples MAESO/CF/TM20, MAESO/CF/TM30 and MAESO/CF/TM40 respectively. The peaks corresponding to asymmetric and symmetric stretching of CH3 were shifted to lower wave numbers. It was observed that the C = O peak intensity got reduced after the incorporation of TM in the formulations. The reduction in peak intensities and shifting of the peaks indicated a better interaction and formation of crosslinked structure among all the components of the composites.
4.3 Density Functional Theory (DFT) Study
DFT study was carried out for all the components involved in the synthesis of the cross-linker product MAESO/TM/CF as shown in Fig. 2. Structures of the reactants were optimized following the information gathered from available literature based on the crystal structure. A probable cross-linked structure has been reported for the resultant product also supported by the DFT calculations. Optimized structures of Rosin acid, MAESO, TM, CF, and a unit of the cross-linked product (MAESO/TM/CF) are shown in Fig. 5. and Fig. 6. Hessian calculations exhibited absence of negative imaginary frequencies, which indicated the optimized structures to be true minima on the potential energy surface. The cross-linked product MAESO/TM/CF composite consists of hydroxyl, epoxy, and carbonyl functional groups. The calculated vibrational frequencies of the important functional groups are listed in Table 2 and were in accordance with the experimental values obtained from FTIR spectroscopy (Figs. 3 and 4).
Table 2
Experimental and theoretical infrared spectral data of the optimized complexes
Name Code
|
Functional Groups
|
|
Frequencies (cm− 1)
|
Rosin acid
|
-CH stretching
|
Exp.
Calc
|
2953
2927
|
-C = O stretching
|
Exp.
Calc
|
1789
1695
|
-C-O stretching
|
Exp.
Calc
|
1291
1284
|
TM
|
-CH stretching
|
Exp.
Calc
|
2980
2924
|
-C = O stretching
|
Exp.
Calc
|
1735
1725
|
-C = C (terminal)
|
Exp.
Calc
|
1706
1650
|
-C-O stretching
|
Exp.
Calc
|
1223
1231
|
MAESO/TM/CF
|
-CH stretching
|
Exp.
Calc
|
2991
2962
|
-C = O stretching
|
Exp.
Calc
|
1801 − 1734
1725 − 1715
|
-C = C (terminal)
|
Exp.
Calc
|
1700 − 1683
1650 − 1633
|
4.4 Thermogravimetric analysis (TGA)
The thermal property of the composites was investigated by TGA. Table 3 presents the initial decomposition temperature (Ti), maximum pyrolysis temperature (Tm), and residual weight (%) of the specimen. The values presented (Table 3) were the average of three different samples. After incorporation of rosin derivative into the composites the Ti values increases. Rosin derivative played a remarkable role in improving the thermal properties as it could enhance the interaction between the resin and chicken feather fibers and thereby restricted the mobility of volatile degraded products out of the crosslinked network structure. Samples with 30 wt% of TM (MAESO/CF/TM30) showed maximum thermal resistance. The lower thermal resistance shown by the composite prepared with 40 wt% of TM (MAESO/CF/TM40) was due to the presence of higher amount of TM which might have lowered the interaction among the constituents of the composites. The residual weight percentage of the composite loaded with 30 wt% of TM was found to be maximum due to char formation on the surface while exposed to heat and blocking the air to penetrate into the composites. The formation of highest amount of char was as a consequence of the development of an improved crosslinked structure produced by the interaction between CF, resin, and TM.
Table 3
Thermal analysis of the composites
Samples
|
Initial decomposition temperature (°C)
(Ti)
|
Maximum pyrolysis temperature
(°C)
(Tm)
|
Temperature of decomposition (°C) (Td) at different weight loss (%)
|
Residual weight (%) at 600°C
|
1st step
|
2nd step
|
20%
|
40%
|
60%
|
80%
|
CF
|
225
|
288
|
308
|
262
|
311
|
407
|
572
|
13
|
MAESO/CF/TM0
|
211
|
370
|
411
|
296
|
355
|
405
|
585
|
18
|
MAESO/CF/TM20
|
231
|
371
|
414
|
294
|
357
|
406
|
582
|
18
|
MAESO/CF/TM30
|
234
|
367
|
414
|
297
|
357
|
408
|
-
|
20
|
MAESO/CF/TM40
|
221
|
297
|
393
|
279
|
328
|
410
|
527
|
16
|
4.5 Limiting Oxygen Index (LOI) study
The LOI values of the composites loaded with varying amounts of TM are presented in Table 4. The values were found to increase after the inclusion of TM up to 30wt% and after that, the values decreased beyond that percentage. The increase in values might be due to the participation of the phenanthrene ring of TM to form a crosslinked structure between resin, CF, and TM. Also, it provided a thermal barrier promoting char formation and thus enhanced the thermal stability of the prepared composites. Char shielded the surface of the samples from heat and lowered the loss of mass rate in the course of thermal decomposition, thus giving an enhanced flame resistivity. At a higher percentage of TM, the LOI values decreased which might be due to weak interaction among the constituents of the composites.
Table 4
Limiting Oxygen Index (LOI) test of the composites
Samples
|
LOI (%)
|
Flame Description
|
Smokes and Fumes
|
Char
|
MAESO/CF/TM0
|
38
|
Small localized flame
|
Dark Fumes
|
Medium
|
MAESO/CF/TM20
|
44
|
Small localized flame
|
Dark Fumes
|
Medium
|
MAESO/CF/TM30
|
55
|
Small localized flame
|
Dark Fumes
|
Medium
|
MAESO/CF/TM40
|
50
|
Small localized flame
|
Dark Fumes
|
Medium
|
4.6 Ultraviolet resistance test
Figure 7. represents the weight loss of the samples prepared with varying percentages of TM which were subjected to a UV environment for several days. In all the samples, with or without TM, weight loss % was found to be increased initially due to moisture absorption. The loss in the material due to degradation was minimal than the initial decrease in loss in weight of the samples. The sample without TM showed highest weight loss (%) followed by the TM loaded samples after 70 days of irradiation. After the incorporation of TM, the rate of weight loss decreased. Composite loaded with 30 wt% of TM showed minimum weight loss. This might be due to interaction among chicken feather fiber, resin, TM leading to the formation of a crosslinked structure which slowed down the photodegradation method and enhanced the UV resistance.
Figure 8. shows the FTIR spectra of UV exposed samples which were investigated by the shifting in the intensities of carbonyl peak. In all the composites, the –OH stretching peak appeared in the 3455 − 3450 cm− 1 range. The peaks between 2932 − 2927 cm− 1 were due to –CH stretching. It was observed that the intensity of the carbonyl peak at 1726 cm− 1 was more for the composite without rosin acid derivative. However, after the incorporation of rosin acid derivative, the intensity of the carbonyl peak decreased and shifted to 1725 cm− 1 and 1720 cm− 1 respectively indicating the formation of a better crosslinked structure and thus imposing restriction on the composites from photodegradation.
The change in carbonyl index values of the composites with/without rosin derivative after 70 days of UV exposure are shown in Fig. 9. Composite with rosin derivative (MAESO/CF/TM0) showed more carbonyl index values compared to the composites loaded with rosin acid derivative. The increase in carbonyl index values was could be because of chain scission between the matrix and the fiber. The carbonyl value index decreased up to the 30 wt% addition of rosin derivative after that it increased. The rosin derivative might have stabilized the composites by protecting them from UV radiation and slowed down the process of photodegradation. Similar UV stability of wood polymer composite was reported by Mandal et al. [29]. However, at 40 wt% of rosin derivative loading (MAESO/CF/TM40), the carbonyl index values increased. The reason might be due to excess amount of TM leading to weak interaction between the constituents of the composites.
4.7 Mechanical property
The mechanical and hardness values of the composites with various percentages of TM before and after UV exposure are presented in Table 5. The addition of TM into the composites contributed immensely to the tensile, flexural, and hardness strength. Composites loaded with 30wt% of TM showed maximum values and beyond that, the values decreased. The enhancement in the mechanical properties could be due to enhanced interaction among the CF, TM, and resin. Also, the existence of a large phenanthrene ring in the TM provided stiffness to the composites. The decrease in mechanical properties might be due to the presence of the unreacted components in the composites which resulted in poor adhesion among the chicken feather fiber based polymer composites.
Table 5 also shows the mechanical properties of the UV degraded composites after 70 days of irradiation. The results shows reduced tensile, flexural, and hardness values of the composites. Highest loss in all the properties was observed for the composites without TM. However, the loss was less significant in the composites having various percentages of TM. The minimal loss in the properties of the specimens was could be the consequence of enhanced crosslinked structure between the TM, CF, and resin. Further, the presence of TM might have delayed the photodegradation process offering resistance to the composites.
Table 5
Mechanical properties of the composites before and after UV degradation study
Samples
|
Tensile Strength
(MPa)
|
Flexural strength
(MPa)
|
Hardness
(Shore D)
|
Before
|
After
|
Before
|
After
|
Before
|
After
|
MAESO/CF/TM0
|
10.05 (± 0.46)
|
6.82 (± 0.98)
|
29.68 (± 1.67)
|
23.4 (± 0.79)
|
71.6 (± 1.52)
|
67.33 (± 1.52)
|
MAESO/CF/TM20
|
10.26 (± 1.09)
|
7.31 (± 0.99)
|
30.03 (± 1.12)
|
26.42 (± 0.57)
|
74.66 (± 0.5)
|
71.6 (± 1.60)
|
MAESO/CF/TM30
|
13.97 (± 0.93)
|
10.35 (± 1.01)
|
37.80 (± 0.79)
|
33.25 (± 0.99)
|
80.33 (± 1.52)
|
77.8 (± 0.76)
|
MAESO/CF/TM40
|
12.71 (± 0.65)
|
9.67 (± 1.27)
|
32.41 (± 0.78)
|
28.33 (± 0.66)
|
77 (± 1.32)
|
73.33 (± 1.60)
|
4.8 Biodegration study
Figure 10. shows the bacterial growth studied for four weeks on the composites loaded with various percentages of TM, Bacillus cereus and Pseudomonas aeruginosa were used for biodegradation of the samples. The bacterial growth and degradation rate in the broth culture medium containing mineral salts and samples as a sole carbon source were evident after one week of incubation. The bacterial growth was found to be enhanced steadily with rise in the time of the bacterial exposure. In the sample loaded with 30 wt% of TM, initially, the bacterial growth was exponential and then it became stationary (no further increase in growth over time), and for the composites loaded with 20 and 40 wt% of TM, the bacterial growth was found to be higher but decreased after two weeks of incubation, possibly due to accumulation of inhibitory by-products produced by the microorganism. The degradation pattern in the case of the composite without TM was found to increase till four weeks of incubation. The high rate of degradation of the samples could be attributed to the hydrolytic enzyme production (including keratinases and proteases) capability of the bacterium Bacillus cereus and Pseudomonas aeruginosa that could degrade the proteins present in the chicken feather. Several studies reported the production of keratinases and peptidases by Bacillus sp. (including Bacillus cereus and B. licheniformis) using chicken feather as the sole carbon and nitrogen source [31–33]. Samples loaded with 20 and 40 wt% of TM showed higher degradation than 30 wt% TM loaded samples probably due to lower crosslinking among the components of the composites [34].
4.9 Morphological analysis
The SEM micrographs of the composites with/without rosin derivative are shown in Fig. 11. Voids and surface roughness appeared in the composites without TM (MAESO/CF/TM0). From the SEM images, it is evident that the surface roughness of the samples decreased after the incorporation of TM. With the increase in the TM percentage up to 30 wt%, the void spaces and surface roughness were found to decrease implying an enhanced interaction and compatibility among the constituents of the composites. However, in the composite loaded with 40 wt% of TM (MAESO/CF/TM40), the surface was found to be uneven due to aggregation of excess amount of TM. The composites were exposed to UV rays for 70 days. The SEM micrographs of the samples exposed to UV are shown in Fig. 12. It could be observed that due to photodegradation cracks appeared on the surface of the composites and surface roughness increased. The surface of the composite without TM (MAESO/CF/TM0) appeared rougher than the composites with TM. The addition of TM into the composites retarded the development of cracks onto the bulk of the composites. The rosin derivative promoted the interaction among the resin, CF, and TM which in turn minimized the degradation of the composites against UV rays.
Figure 13. shows the SEM micrographs of the degraded samples caused by bacterial growth. Physical breakdown of the composites without TM compared to the composites loaded with TM after the microbial attack was evident from the images. It was also observed that the areas where fibers were exposed were partially damaged whereas those deeply embedded in the matrix are unaffected. The decrease in degradation of composites with TM was due to the formation of a strongly interconnected network which lowered the accessibility and reactivity of the microorganisms. The SEM image of the composite loaded with 30 wt% of TM (MAESO/CF/TM30) showed minimum cracks compared to the other composites which revealed the influence of TM on the rate of degradation of the composites.
4.10 Water absorption test
The water uptake of the composites is shown in Fig. 14. The water uptake capacity was very high in the beginning and later it reduced for all the samples. Water uptake was found to be less for the composites loaded with TM compared to unloaded composites. The existence of the phenanthrene ring structure of TM and the crosslinked network in the composites might be the reason which inhibited the inflow of water thoroughly into the samples and hence significantly reduced the water absorption value. Composite loaded with 30 wt% of TM (MAESO/CF/TM30) showed the least swelling. The swelling again increased for the composite loaded with 40 wt% of TM (MAESO/CF/TM40), which might be due to the presence of an excessive quantity of TM in the polymer matrix leading to weak interaction among the constituents of the composites followed by collapsing of the network structure.
4.11 Chemical Resistance Test
Figure 15. shows the chemical resistance test for the composites which were carried out in 4% acetic acid and 4% NaOH solution. In all the samples, swelling increased with increase in time. Highest swelling was shown by the composite prepared without rosin derivative. The swelling was found to decrease after the incorporation of rosin derivative which helps to form the crosslinked structure and thus decreases the accessibility of chemicals into the samples. It was also observed that composites were more resistive in acid solution than in alkali solution. The higher swelling in alkali solution might be due to enhanced interaction between NaOH, chicken feather, resin, and TM. Both the acid and alkali solution composite with 30 wt% of TM (MAESO/CF/TM30) showed least swelling. The composite containing 40 wt% of TM (MAESO/CF/TM40) showed less resistance to chemicals due to the dilution effect leading to weak interaction between the constituents of the composites.