3.1 Characteristics of the bleached and unbleached eucalyptus fibers
Fig. 1 depicts the thermograms of the BEF and UEF. From the trends depicted, the initial degradation of the BEF (2870C) was slightly delayed relative to that of UEF (275.330C) (Fig. 1 (a)). Thermo-degradation rates of -0.018%/0C and -0.013%/0C were also recorded at 363.830C and 339.670C for the BEF and UEF respectively (Fig. 1(b)). The trend of results depicted by Fig. 1(a) and the values of degradation temperatures and rates extracted suggests that the BEF exhibited slightly higher thermal stability. This finding could be attributed to possible removal of hemicellulose, lignin and other volatile content from the fiber by the bleaching treatment [18].
Fig. 2 depicts the diffractometries of BEF and UEF with the intense and mild picks shown between 22-230C and 180C on the 2θ coordinate respectively. These intense and mild peaks depicted represents the crystalline and amorphous peaks of the fibers respectively. Little comparison of the two diffractometries suggests that the BEF exhibited slightly higher intensity which could be due to the resultant effects of bleaching treatment on the fiber, where volatile contents and impurities that could be present on the fibers might have been bleached-out. Perhaps, the aforementioned hypothesis has supported the earlier on explanation given in dissecting the cause of variabilities observed with the thermograms of the BEF and UEF (Fig. 1). More so, these slight differences in the diffractometries has also been reflected on the Crl of the BEF and UEF as shown in Table 2.
On the other hand, studies of the surface mophologies of both the BEF and UEF using the FTIR reveals that the bleaching treatment made on the fiber did not alter the surface morphology of the fiber as regards to the functional groups. It could be seen that the prominent peaks at around; 3300 cm-1 and 1000 cm-1 that have been reported to be for carboxyl and hydroxyl functional group [19].Overall, it could be concluded that the bleaching treatment made on the eucalyptus fibers has slightly modified the microstructure of the fiber but did not alter the functional groups on the surface morphology of the fiber.
Table 2 Crystallinity indexes of BEF and UEF
Eucalyptus Fiber Samples
|
I200 (2θ:220-230)
|
Inon-cry (180)
|
Crl (%)
|
BEF
|
6507.92
|
1965.15
|
69.80
|
UEF
|
5194.81
|
1820.38
|
64.95
|
3.2 Thermal properties of the brake pad composites
Table 2 presents the Td5 of 368,342 and 412°C for the BEFPC, UEFPC and PBC respectively. It could be observed that the BEFPC is having delayed 5% weight loss relative to the UEFPC, which could be attributed to the fact that bleached eucalyptus fibers is free from volatile contents, thus the samples exhibited higher thermal stability compared to the unbleached eucalyptus fibers. Similar finding has been reported elsewhere [20]. However, the Td5 of PBC exhibited higher thermal stability compared to the BEFPC and UEFPC. While on the other hand, residual values recorded at 1000 °Care; 68.1, 68.5 and 73.7 % for the BEFPC, UEFPC and PBC respectively. Overall, comparing the thermal stability of the composites and the braking temperature of automobile brake pad (350 °C) reveals that the thermal decomposition temperatures of the EFPBCs were within the operating temperature range.
3.3 Flexural properties of the brake pad composites
Fig. 3 depicts the higher magnitudes of flexural strength and flexural modulus of the EFPBCs compared to the PBC. Where the BEFPC achieves the highest flexural properties followed by the UEFPC, detail values of the flexural strength and modulus of the composites are presented in Table 3. It could be seen that the values achieved with the EFPBCs are higher than that the values of flexural strength achieved with Lapinus and Kevlar fiber reinforced phenolic resin composites (29.5 and 46.5 MPa respectively) [21]. This could be due to strong interfacial interaction between phenolic hydroxyl group of polybenzoxazine binder and highly polar lignocellulose hydroxyl and carboxylic groups of bleached eucalyptus fiber, which are abundantly available in eucalyptus fibers [22] [. The flexural modulus of the composites also improved by reinforcing with bleached eucalyptus fiber. Flexural modulus of 13.2 and 9.8 GPa were recorded for the BEFPC and UEFPC respectively. This shows that the flexural properties of the EFPBCs are higher than that of PBC. This desirable properties achieved could be attributed to the removal of lignin by bleaching treatment which enhances the cellulose contents in bleached fibers, owing to the fact that cellulose content of natural fibers possess high rigidity characteristics. The superior flexural property of the BEFPC could be attributed to the improved crystallinity of the BEF shown in Fig. 2(a) and Table 1. Similar findings where the impact of high rigidity characteristics of cellulose towards the improvement of mechanical properties of natural fiber-reinforced composites has been reported [23]. Interestingly, the values of the flexural properties of the EFPBCs achieved herein are within the recommended range of typical friction materials (flexural strength of 10.0-40.0 MPa and flexural modulus of 3.0-8.0 GPa) [24].
3.4 Dynamic mechanical properties of the brake pad composites
The storage modulus (E′) and loss tangent (tan δ) of the EFPC and PBC as a function of temperature are shown in Fig. 4. From the Figure, it could be observed that the storage modulus of EFPBC indicates either stability or stiffness. The storage modulus values of BEFPC and UEFPC at 30 °C were 5.25 and 5.10 GPa, respectively, while that of PBC was relatively lower (4.93 GPa) (Table 3). This finding suggests that the dimensional stability of the polybenzoxazine composite material is enhanced with incorporation of eucalyptus fibers. It was shown that the EFPBC materials have improved symmetrical shape. The higher storage modulus recorded with the BEFPC relative to UEFPC could be attributed to the slight micro-structural modification of the BEF explained under 3.1 sub-section.
It was found that the glass transition temperature (Tg ) values of BEFPC and UEFPC were at 219 and 237 °C, respectively, while that of PBC was lower (212 °C) (Table 3). This is due to the chemical composition of cellulose content in the eucalyptus fibers which could interact with a polybenzoxazine matrix and hinder the movement of the polymer chain (chain segment). That could lead to the requirement of temperature for the chain movement and correspond to the increased Tg value. These thermal transition behaviors of thermosetting matrix influence the wear performance of friction composite materials.
Table 3 Various properties of polybenzoxazine composites
Type of Composite
|
Various properties of the polybenzoxazine composites
|
|
Thermal properties
|
Flexural properties
|
Dynamic mechanical properties
|
|
Td5 (°C)
|
Residual weight at 1000 °C (%)
|
Flexural strength (MPa)
|
Flexural modulus (GPa)
|
Storage modulus at 30 oC (GPa)
|
Tg ( oC)
|
BEFPC
|
368
|
68.1
|
54.5
|
13.2
|
5.25
|
219
|
UEFPC
|
342
|
68.5
|
51.3
|
9.8
|
5.10
|
237
|
PBC
|
412
|
73.7
|
43.2
|
7.0
|
4.93
|
212
|
3.5 Tribological properties of the brake pad composites
3.5.1 Friction coefficient of the brake pad composites
Fig. 5 shows the friction coefficient of EFPBCs as a function of temperature. Friction coefficients of the BEFPC were found to be; 0.43, 0.45, 0.42, 0.39, 0.39 and 0.31, while that of UEFPC were found to be; 0.38, 0.39, 0.39, 0.37, 0.39 and 0.32, when the tests were carried out at 100, 150, 200, 250, 300, and 350 °C, temperatures respectively. It could be seen that when the temperature increased at the beginning, the polymer composite entered a rubber-like state and the contacting surface between the composites and the surface of test plate could be enhanced. The coefficient of friction therefore increased in the range of 100 to 150 °C. When the temperature exceeded 300 °C, the coefficient tended to decrease due to the decomposition of the eucalyptus fiber and binder. The higher values of the friction coefficients recorded with the BEFPC could be due its superior micro-structural stability exhibited presented in Fig. 2(a) and Table 2. In addition, BEFPC and UEFPC had coefficients of friction in the temperature range of 100 to 350 °C, ranging from 0.30 to 0.42, which falls within the range of values specified by TIS 97-2557 standards [25].
3.5.2 Specific wear rate of the brake pad composites
Fig. 6 shows specific wear rate of EFPBC and PBC as a function of temperature. The specific wear rates of the BEFPC of; 0.066×10-7, 0.091×10-7, 0.139×10-7, 0.250×10-7, 0.752×10-7, and 3.23×10-7 cm3/Nm were recorded, while those of UEFPC are; 0.065×10-7, 0.091×10-7, 0.183×10-7, 0.307×10-7, 0.756×10-7, 3.348×10-7 cm3/Nm at 100, 150, 200, 250, 300, and 350 °C temperatures respectively. It could be observed that, the specific wear rates of the EFPBCs at glassy state were slightly increased with an increasing of temperature from 100 to 250 °C and then drastically increased beyond 300 °C. This is attributed to the fact that glass to transition temperature of BA-35X at the rubbery state the composite could be easily deformed [26]. The results also showed that the wear rate of BEFPC tended to significantly increase over the tested temperature range of 250 to 300 °C to around 0.752×10-7cm3/Nm. Maximum values of 3.23×10-7 cm3/Nm was recorded at 300 to 350 °C, which could be due to the intense decomposition of eucalyptus fibers and other components with the BEF due to the bleaching treatment. The intense decomposition of the fibers may result in loosening of the bonding between the fibers and the matrix. However, the said specific wear rate is still within the range suggested by the brake pad industry standard for automotive products (TIS 97-2557).
3.6 Worn surfaces of of the brake pad composites
Fig. 7 presents the worn surfaces of EFPBCs after tribological tests at high temperature. Cracks and small grooves were observed on worn surfaces due to the abrasive wear mechanism similar to what was reported in the literature [27]. In addition, small cracks perpendicular to the sliding direction were also observed on surfaces of the composites reinforced with eucalyptus fibers. That means the fibers could lead to the increase of wear of the polybenzoxazine matrix. This phenomenon corresponds to the wear rate value generated in the wear test as shown in Fig. 4.