Effect of chemical treatment on thermal properties of bagasse fiber-reinforced epoxy composite

2.1.1 Bagasse fiber

Sugarcane residue was procured from a local sugar mill (Saharanpur, Uttar Pradesh, India. BF was sieved from the sugarcane residue for the removal of unwanted impurities. After separation, the moisture content of fiber was found to be 6–7%. The length of the BF varied from 5 to 10 mm with an average length of 8 mm. The fibers were washed with distilled water to remove any unwanted adhered impurities; this was followed by drying in a hot air oven at 70°C for 48 h to remove the moisture.

The epoxy resin (AW106) and the curing agent (HV953IN) are useful polymers and have good strength, toughness, and resilience. When both are mixed they have an excellent resistance to chemical and moisture attack, having outstanding electrical insulating properties and an absence of volatile matter on curing. It has a density 1.1 g/cm³. This was procured from M/S Petro Araldite Pvt. Limited (Manali, Chennai, India).

Sodium hydroxide (Min-assay 98%) was procured from M/S Himedia Laboratories Pvt. Limited (Mumbai, India).

Acrylic acid (Assay-min 99%) was procured from M/S Loba Chemie Pvt. Limited (Mumbai, India).

2.2.1 The treatment of the BF

BF was soaked with 1% sodium hydroxide (NaOH) for 30 min, followed by 1% acrylic acid for 1 h at ambient temperature [17]. The treatment cost is very small because sodium hydroxide and acrylic acid are available for USD 6.3/kg and USD 13/kg, respectively. The fibers were washed with distilled water to remove any unwanted adhered impurities. This was followed by drying in a hot air oven at 70°C for 48 h. During this treatment, the liquor ratio was maintained at 20:1 in a bath tub. This treatment causes disruption of hydrogen bonding in the network structure, thereby increasing the surface roughness. It also removes a certain amount of lignin, wax, and oils covering the external layer of the fiber surface [18]. It increases the amount of cellulose exposed on the fiber surface, which is attributed to raising the number of possible reaction sites. Further, acrylation is initiated by free radicals of the cellulose molecule. This led to a strong covalent bond formation that improved the thermal stability of chemically treated fibers.

Fiber-OH+NaOH→Fiber-O-Na+H2O

2.2.2 Preparation of composites

Epoxy resin (AW106) and curing agent (HV953IN) were mixed (as instructed by the manufacturer), followed by the addition (10, 20, 30, and 40 wt%) of untreated and treated BF, and stirred in a mixer for 10–15 min at a rotational speed of 2000 rpm to get a homogenous mixture. The mixture was poured into the mold, 300×300×10 mm³ and cured for 24 h at ambient temperature. A hand lay-up method was used to manufacture the composite material [19]. In order to avoid the problem of sticking of composite material, the mold was coated with a Teflon sheet; the formation of bubbles may create a problem. To overcome such problems, a rollover sheet with a heavy roller has been incorporated. Every composite cast was cured under a load of 25 kg for 24 h before it was removed from the mold.

2.2.3 Testing of thermal properties

The thermogravimetric analysis (TGA), differential thermal gravimetric (DTG), and differential scanning calorimetry (DSC) thermograms were taken from EXSTAR TG/DTA 6300 (RT Instruments Inc., Woodland, CA, USA) in a nitrogen atmosphere (flow rate 200 ml/min) at a constant heating rate of 10°C/min from ambient temperature to 800°C. A composite weighing between 8 and 12 mg was used. Here we observed the behavior of the respective sample due to thermal degradation.

2.2.4 FTIR spectroscopy

FTIR spectroscopy was used to analyze any changes in the functional group of the cellulose fibers after the chemical treatment of the fibers. Fiber samples and KBr mixed in the ratio of 1:10 were pressed into a disk for (FTIR NICOLET 6700, LabX, Midland, ON, Canada) spectroscopy measurement. The spectra were recorded with 32 scans in the frequency range of 4000–400 cm^-1 with a resolution of 40 cm^-1.

2.2.5 SEM morphology

The SEM studies were conducted by using the LEO 435 VP (LEO, Austin, TX, USA) with an acceleration voltage specification up to 30 kV, magnification range (10× to 300,000×), detection mode (secondary & backscattered electrons) and the images were recorded by printer and camera. The test sample was mounted on that being fused with silver gel and gold coated to avoid electrical charging during the test.

3.1.1 TGA and DTG analysis

TGA and DTG thermograms of neat epoxy, untreated BF, treated BF, untreated BF-epoxy composites, and treated BF-epoxy composites are shown in Figures 1–10. In TGA thermograms, there are three different zones of degradation. In the first zone (at temperature range 80–100°C), there is a small loss in weight due to the removal of moisture present in the sample. Because of the onset of the thermal decomposition process, there is a very high loss in weight in the mid zone of degradation and the last zone called the ultimate thermal degradation.

Figure 1:

TGA thermogram of epoxy, untreated BF, and treated BF.

Figure 2:

TGA thermogram of untreated and treated 10% BF-epoxy composites.

Figure 3:

TGA thermogram of untreated and treated 20% BF-epoxy composites.

Figure 4:

TGA thermogram of untreated and treated 30% BF-epoxy composites.

Figure 5:

TGA thermogram of untreated and treated 40% BF-epoxy composites.

Figure 6:

DTG thermogram of epoxy, untreated BF, and treated BF.

Figure 7:

DTG thermogram of untreated and treated 10% BF-epoxy composites.

Figure 8:

DTG thermogram of untreated and treated 20% BF-epoxy composites.

Figure 9:

DTG thermogram of untreated and treated 30% BF-epoxy composites.

Figure 10:

DTG thermogram of untreated and treated 40% BF-epoxy composites.

The TGA behavior of neat epoxy is shown in Figure 1 (curve A). A single-step decomposition process is observed for neat epoxy. It is observed that the degradation started at about 200°C. The range of the middle zone is 340–440°C of epoxy that have much rate of weight loss. From Figure 6 (curve A), the higher rate of weight reduction is (0.75 mg/min) observed at about 435°C due to degradation, and the degradation of the remaining part of the product for further degradation is observed at 650°C. The residue left after final degradation was 1.1%.

The TGA behavior of untreated and treated BFs is shown in Figure 1 (curves B and C, respectively). Untreated and treated BFs have 6–7% weight loss observed at 100°C, corresponding to the removal of moisture. The middle zone of the thermogram at around 260–450°C were almost the same, corresponding to the degradation of hemicelluloses. From Figure 6, (curves B and C), the higher rate of weight reduction is (1.93 mg/min) observed at about 329°C due to the degradation of hemicelluloses [20], and cellulose for untreated BF shifted to 333°C for treated BF; thereafter, a second decomposition step took place corresponding to the degradation of lignin and cellulose. The middle zone of the treated BF curve is more inclined as compared to the untreated BF curve; this is due to a lower content of lignin. The residue weight of untreated BF found 2.4%, and the treated BF is -2.7% due to presence of lignin in BF, which is responsible for the char.

The TGA behavior of untreated 10% BF-epoxy composite and treated 10% BF-epoxy composite is shown in Figure 2 (curves A and B, respectively). The starting weight of the reduced sample is observed at a temperature of 199°C; this is due to the removal of fiber present in the polymer matrix. The range of the middle zone is 300–440°C of the respective sample which has high rate of weight loss. From Figure 7 (curves A and B), the higher rate of weight reduction is (0.74 mg/min) observed at about 359°C for untreated 10% BF-epoxy composite and raised to 362°C for treated 10% BF-epoxy composite. The degradation of the remaining part of the product for the further degradation observed at 477°C for untreated composite raised to the 489°C for treated composite improved the thermal stability of the composite. The same type of observation was found in [21].

The TGA behavior of untreated 20% BF-epoxy composite and treated 20% BF-epoxy composite is shown in Figure 3 (curves A and B, respectively). The starting weight of the reduced sample observed at a temperature of 195°C corresponds to the removal of fiber present in the polymer matrix. The range of the middle zone is 320–430°C of the respective sample that has a large rate of weight loss. From Figure 8 (curves A and B), the higher rate of weight reduction is (0.85 mg/min) observed at about 359°C for the untreated 20% BF-epoxy composite and down to 356°C for the treated 20% BF-epoxy composite. The degradation of the remaining part of the product is observed at 500°C for the untreated composite and down to 483°C for the treated composite.

The TGA behavior of untreated 30% BF-epoxy composite and treated 30% BF-epoxy composite is shown in Figure 4 (curves A and B, respectively). The starting weight of the reduced sample observed at a temperature of 195°C corresponds to the removal of the fiber present in the polymer matrix. The range of the middle zone is 310–440°C of the respective sample that has a greater rate of weight loss. From Figure 9 (curves A and B), the higher rate of weight reduction (0.84 mg/min) is observed at about 354°C for the untreated 30% BF-epoxy composite and rose to 356°C for the treated 30% BF-epoxy composite; this improved the thermal stability of the composite. The degradation of the remaining part of the product for further degradation is observed at 500°C for the untreated composite and down to 484°C for the treated composite.

The TGA behavior of the untreated 40% BF-epoxy composite and the treated 40% BF-epoxy composite is shown in Figure 5 (curves A and B, respectively). The starting weight of the reduced sample observed at a temperature of 190°C corresponds to the removal of fiber present in the polymer matrix. The range of the middle zone is 310–440°C of the respective sample that has a greater rate of weight loss. From Figure 10 (curves A and B), the high rate of weight reduction is (0.89 mg/min) observed at about 354°C for the untreated 40% BF-epoxy composite and rose to 358°C for the treated 40% BF-epoxy composite; this improved the thermal stability of the composite. The degradation of the remaining part of the product for further degradation was observed at 500°C for the untreated composite and down to the 481°C for the treated composite.

Thus, from DTG analysis, it was observed that the thermal stability of the untreated BF composite was improved by the chemical treatment.

3.1.2 DSC analysis

The results of DSC thermogram of untreated BF and BF are shown in Figure 11. Both DSC thermograms have distinct exothermic peaks that can be seen in the respective figure. The corresponding ΔH value is given in Table 1.

Figure 11:

DSC thermogram of untreated BF and treated BF.

From Figure 11, in the DSC thermogram, untreated BF (curve I) has two exothermic peaks at 353°C and 455°C; this is due to the decomposition of hemicelluloses and α-cellulose, respectively. But in the case of treated BF (curve II), the hemicellulose peak was shifted to a higher temperature at 364°C; this is because of the removal of non-cellulosic matter such as hemicelluloses, pectin, etc. During the chemical treatment of fiber, the destruction of the bonding that is present in the fiber might have some influence and shifted the peak. The exothermic peak at 448°C is due to the degradation of cellulose in BF and formation of char [22].