Thermal degradation of coir fiber reinforced low-density polyethylene composites

2.2.1 Alkali treatment

Firstly, chopped CF was washed with water to remove impurities. After that CF was soaked in 5% (w/v) NaOH solution, maintaining fiber/solution ratio of 1:10 (w/v). The fibers were kept in the solution for 72 h at ambient temperature [3], [19]. After that, fibers were continually washed with tap water followed by distilled water until the last trace of alkali was removed reaching approximately pH 7. Finally, they were air oven-dried at 70°C for 48 h to obtain alkali-treated fibers (A-CF).

2.2.2 Acrylic acid treatment

A-CF was soaked in a 1% acrylic acid solution maintaining fiber/solution ratio of 1:10 (w/v) for 20 min [14]. Then, the fibers were washed thoroughly and air oven-dried at 70°C for 48 h to obtain acrylic acid-treated fibers (AA-CF).

2.4.1 Fourier transform-infrared analysis

In order to investigate the effect of chemical treatment on CF, FTIR spectra was examined for untreated CF, A-CF, and AA-CF using an FTIR spectrometer (Nicolet 6700 series, USA). Potassium bromide (KBr) was used as reference substance. The samples were analyzed over the range of 4000–600 cm⁻¹ with a spectrum resolution of 4 cm⁻¹.

2.4.2 Scanning electron microscopy

Scanning electron microscope (Model LEO-435VP) was used for the morphological study of both reinforcing fibers and the interface between the fiber and the polymer matrix in the composite samples. Prior to their morphological observation, the samples were coated with gold to make them conductive.

2.4.3 Thermogravimetric analysis

Thermal analysis of untreated and treated fibers and composite containing these fibers were carried out using EXSTAR TG/DTA 6300 equipment. Samples of approximately 8–10 mg were heated steadily at a heating rate of 10°C/min from ambient temperature to 700°C under nitrogen atmosphere. The results were obtained in the graphical form: thermogravimetric (TG) and DTG curves. The test results were the average of three specimens for each composite sample.

2.4.4 X-ray diffraction

X-ray diffraction (XRD) was used to study the crystallinity of the composite samples. The XRD patterns were recorded using a Brucker AXS D8 diffractometer operating with Cu-Kα radiation (λ=1.5406 Å) at 40 kV and 30 mA in the range of 2θ=10–50° with a scan speed of 0.02°/s. The crystalline thickness (L) was calculated according to the Scherrer’s equation:

L=Kλβcosθ

where K is the Scherrer’s constant normally taken as 0.89, λ is the wavelength of the X-ray radiation (1.5406 Å for Cu), β is the full width at half maximum of diffraction peak in radian, and θ is the Bragg angle.

3.3.1 Thermal analysis of untreated and treated fibers

Before the analysis of the thermal decomposition of the natural fiber-reinforced polymer composites, it is important to understand the thermal behavior of the reinforcing natural fiber. TG and DTG curves of untreated and treated CF are shown in Figure 4A and B. TG curves of reinforcing fibers (CF, A-CF, and AA-CF) show distinct weight loss processes occurring at different temperatures (Figure 4A). The ligno-cellulosic fiber samples underwent three-stage degradation processes (Figure 4B).

Figure 4:

TG (A) and DTG (B) curves of coir fibers: untreated (CF), alkali-treated (A-CF), and acrylic acid-treated (AA-CF).

Thermal degradation of the ligno-cellulosic natural fiber mainly corresponds to the degradation of hemicellulose, cellulose, and lignin. Due to the presence of acetyl groups in hemicellulose, it shows lower thermal stability than the cellulose and lignin [22]. However, lignin is the most difficult one to decompose. Its degradation occurs in a wide temperature range due to its complex aromatic structure with various branches [23]. The first weight loss process for both untreated and treated CF observed between 30 and 150°C (~7.5% weight loss) indicates the vaporization of absorbed moisture and physically absorbed water [24]. The second weight loss process corresponds to the decomposition of the hemicellulose, pectin, the cleavage of the glycosidic linkage of cellulose, and part of lignin occurring under the temperature range of 200–300°C [25]. In this temperature range, the weight loss accounts for about 23% for untreated, 13% for A-CF, and 12% for AA-CF. It can be inferred from the results that the second stage decomposition rate is more prominent in the case of untreated fiber than in the case of treated fibers, which is also confirmed in the DTG curve, where no peak is seen for the treated fiber. However, a peak at 283°C is noticed for the untreated fiber, which indicates that the chemical treatment significantly removed the hemicellulose, pectin, and part of lignin from the CF. This result corroborates well with the result of FTIR analysis, which revealed the disappearance of the C=O band in the FTIR spectra of both A-CF and AA-CF. The third (and major) weight loss process (300–380°C) is mainly attributed to the decomposition of cellulose by breaking of glycosidic linkages of the glucose chain and decomposition of lignin but to a lesser extent [25]. From the DTG curves, the temperature at maximum weight loss rate is observed at around 325°C (39% weight loss) for untreated, 339°C (39.5% weight loss) for A-CF, and 340°C (43.7% weight loss) for AA-CF. The lower degradation temperature of the untreated fiber observed is because of the presence of hemicellulose and pectin, which exhibits low thermal stability, whereas the A-CF and AA-CF are more stable due to the removal of these components [26]. The last and fourth weight loss process above 380°C corresponds to the degradation of the rest of lignin and oxidative decomposition of the charred residue [24]. From the DTG curve, the degradation peak is observed to be reduced from 456°C (untreated) to 437°C and 423°C for A-CF and AA-CF, respectively. The higher degradation peak for the untreated fiber indicates the lower percentage of lignin in the treated fiber. This result corroborates with the result of FTIR analysis, which revealed that the C-O stretching acetyl group of lignin is weakened after the chemical treatment, especially after the acrylic acid treatment. Above 500°C residue mass can be observed. The untreated fiber has lower residue mass (1.2%) than A-CF (3.2%) and AA-CF (3.8%). The higher percentage of residue indicates better thermal stability of alkali and AA-CF.

3.3.2 Thermal analysis of composites

The TG and DTG curve of LDPE, as well as CF/LDPE composites at different fiber loading, are shown in Figure 5A and B. For the pure LDPE, the onset of thermal degradation (the temperature when the sample loses 5% of its weight) was observed at 376°C (Figure 5A), with the maximum weight loss rate at 462°C (77% weight loss) (Figure 5B), and complete decomposition was observed at around 529°C. DTG curve shows a single stage degradation process for pure LDPE matrix, while composites show more than one degradation stages. For the composite, the first degradation DTG peak mainly corresponds to the degradation of the constituents of CF, while the second main degradation DTG peak corresponds to the degradation of the LDPE matrix. Owing to the oxidative degradation of charred residue, the third small degradation is observed. From the TG and DTG curves, it is evident that the thermal stability of the LDPE matrix decreases with the addition of fiber. This is because of the thermal stability of the CF which is much lower than that of the pure LDPE matrix. For the composite containing 10 wt% CF, the degradation peak corresponding to CF is not well noticed, it may be because all the fibers were well protected by the LDPE matrix which is thermally more stable than the CF. For the 20 wt% CF composite, the first degradation peak corresponding to the degradation of CF is observed at about 346°C. The major weight loss (~72%) occurs at around 463°C corresponding to the degradation of LDPE matrix. However, the major weight loss DTG peak of the composite containing 30 wt% CF shifted to 33°C lower temperature than that of the composite containing 20 wt% CF. This result indicates that the composite containing 20 wt% CF has better thermal stability than composite containing 30 wt% CF and can be explained on the basis of better fiber-matrix interaction in 20 wt% CF composite. The low thermal stability at high fiber loading may be due to the more dominant fiber-fiber interaction than the fiber-matrix interaction which causes incomplete wetting of the fiber in the polymer matrix [12].

Figure 5:

TG (A) and DTG (B) curves of LDPE, and the CF/LDPE composites at different fiber content.

The TG and DTG curves of LDPE composites containing 20 wt% untreated, A-CF, and AA-CF are depicted in Figures 6A,B–8A,B, respectively. From the TG and DTG results (Figures 6–8), it is evident that the initial degradation peak for the A-CF and AA-CF/LDPE composites shifted to higher temperature compared to the untreated CF/LDPE composite. This may be because some of the constituents of the fiber such as wax, pectin, and hemicellulose, which have low thermal stability, were removed during the alkali and acrylic acid treatments. Between these two treated fiber composites, the composite containing AA-CF was recorded with higher initial degradation peak temperature (363°C) than that of the composite containing A-CF (354°C). This is due to the fact that acrylic acid treatment removed impurities, wax, pectin, and hemicellulose more effectively than that of the alkali treatment. However, the second broad main peak of the A-CF and AA-CF/LDPE composites shifted to lower temperature as compared to the untreated CF/LDPE composite. The TG and DTG results show maximum weight loss rate at around 463°C (~72% weight loss) for untreated, 451°C (~69% weight loss) for A-CF, and 453°C (~76% weight loss) for AA-CF composites. These results could be attributed to the poor interaction between the treated fiber and the LDPE matrix. It has been explained earlier that the waxy layer present on the CF surface provides a strong interfacial bonding between the fiber and the LDPE matrix [27]. Between these two treated fiber composites, the AA-CF exhibits more interfacial interaction with LDPE matrix than that of the A-CF. AA-CF surface is relatively more hydrophobic, resulting in better fiber wetting and bonding with the LDPE matrix [12].

Figure 6:

TG (A) and DTG (B) curves of CF/LDPE composites at 20 wt% fiber loading with or without compatibilizer.

Figure 7:

TG (A) and DTG (B) curves of A-CF/LDPE composites at 20 wt% fiber loading with or without compatibilizer.

Figure 8:

TG (A) and DTG (B) curves of AA-CF/LDPE composites at 20 wt% fiber loading with or without compatibilizer.

Figures 6–8 show the effect of MA-g-LDPE addition in the untreated, A-CF, and AA-CF-containing composites on the thermal degradation of these composites. From the TG and DTG results, it is evident that the addition of MA-g-LDPE to the untreated and treated fiber composites shifted both fiber and matrix degradation DTG peak to the higher temperature range compared to the same composite formulation without MA-g-LDPE. For the composite containing untreated fibers (Figure 6A and B), the addition of compatibilizer shifted the first and second degradation peaks from 346°C (~17% weight loss) and 463°C (~72% weight loss) to 360°C (~25% weight loss) and 466°C (~59% weight loss), respectively. For the composite containing A-CF (Figure 7A and B), the addition of compatibilizer shifted the first and second degradation peaks from 354°C (~18% weight loss) and 451°C (~69% weight loss) to 356°C (~21% weight loss) and 463°C (~75% weight loss), respectively. For the composite containing AA-CF (Figure 8A and B), the addition of compatibilizer shifted the first and second degradation peaks from 363°C (~18% weight loss) and 453°C (~76% weight loss) to 364°C (~19% weight loss) and 461°C (~75% weight loss), respectively. From these results, it is clear that the decomposition temperature and thermal stability of the composites are affected by the presence of the MA-g-LDPE in the composites. The composites containing untreated CF and A-CF with MA-g-LDPE show relatively higher percentage weight loss in the first degradation process (260–380°C), indicating lower thermal stability in this temperature range than the composites without MA-g-LDPE. However, the second major degradation process (380–500°C) for the composites with MA-g-LDPE was observed with lower percentage weight loss, indicating higher thermal stability in this temperature range than the composites without MA-g-LDPE. The addition of MA-g-LDPE generates strong covalent bond between the hydroxyl groups of the fiber and maleic anhydride groups of the compatibilizer, while LDPE of the compatibilizer becomes compatible with the LDPE matrix, resulting in better interfacial adhesion between the fiber and the matrix [12]. This improved interaction will promote more interaction between the decomposition processes of both fiber and polymer matrix; i.e. the degradation of fiber may accelerate the degradation of the polymer matrix. The composite containing untreated fiber with MA-g-LDPE is recorded with higher major degradation peak with minimum weight loss, although it has slightly lower thermal stability in the temperature range (260–380°C). Therefore, it can be inferred that the composite containing untreated fiber with MA-g-LDPE is thermally more stable than all the other composite samples. This is probably because the waxy layer present on the CF surface provides a strong interfacial bonding between the fiber and the LDPE matrix.