3.1. Cure Characteristics
The effect of the addition of different ratios of PBOs is first discussed in this section, because the curing process is crucial for the MRE samples to become crosslinked. In general, the results show a slight change that can affect the thermal properties.
Figure 1,
Figure 2 and
Figure 3 show the curing curves of NR-based MREs with three types of PBO: NO:AO, LMO:AO, and PO:AO, respectively. The experimental data were first filtered using the moving average method at every 30 data using Equation (3), where subscript
represents the variable at the
-th datum (
),
and
are the original data and the moving averaged data, respectively, and
is the size of the block. The moving standard deviation (
) is calculated based on the obtained moving average value as expressed in Equation (4) [
35], where
is the total number of data. Generally, three stages could be obtained from the curing characterization curves: induction, curing, and overcuring. The first stage, which usually occurred within 0–3 min, is referred to as the induction stage, representing the slow chemical reaction between rubber and additives. The second stage is attributed to the curing period, wherein the torque increases from approximately 3–8.5 min due to the occurrence of rubber molecular chains forming network structures. The final stage, which occurred beyond 15 min, corresponds to overcuring. A decreasing trend of the graph obtained at this stage is known as a reversion curve. The reversion occurred due to the breakage of the crosslink bonds caused by the temperature, and this process depends on the rubber type, vulcanization agent, temperature, and content of polysulfide crosslinks in the rubber network [
25,
27,
36]. Based on the literature, the reversion was measured using the calculation in Equation (2), whereby the lowest percentage of reversion is desirable for MRE materials [
36,
37,
38,
39].
From
Figure 1, we can see that the torque values slightly increased with the increase in the NO:AO ratio from 0:100 to 70:30 during the induction phase. Increasing the AO content provided no significant effect on the curing or reversion stages. However, to further evaluate the effects of PBO ratios, several crucial data were obtained from
Figure 1,
Figure 2 and
Figure 3, and are presented in
Table 4,
Table 5 and
Table 6. This is probably due to the breakdown of the chemical structure of oil between NR molecular chains. The addition of dispersion aids such as PBOs in the MRE samples weakens the intermolecular attraction forces between the polymer chains [
26]. NO 2 indicated a low reversion process when NO was mixed with low amounts of AO, compared to NO 1, NO 3, and NO 4. This may have been a result of the softening characteristics of AO, which improved the workability of the rubber composition to provide good processability [
4,
20]. Apparently, the NR-based MREs showed good results in terms of reversion behavior when NO was mixed with low amounts of AO.
The curing curves for NR-based MREs containing different ratios of LMO are shown in
Figure 2. The reversion process decreased when the ratio of LMO:AO increased. This could be due to the degradation of the polysulfide bonds in the rubber network when more AO was added to the sample. At 30 min, LMO 1 containing no AO showed the highest reversion. Meanwhile, the curves for other ratios—including LMO 2, LMO 3, and LMO 4—were overlapped, corresponding to insignificant effects on reversion. This may have been a result of the breakage of the crosslink bonds in the rubber chains during the mixing process. The results show that the crosslink bonds easily break without the presence of AO in the samples, and subsequently decrease the reversion curve. The sample with the presence of AO contributes to favorable reversion behavior, which corresponds to high resistance to the breakage of crosslink bonds.
Figure 3 depicts the curing curves for different ratios of PO:AO. Similar to NO:AO and LMO:AO ratios, all MRE samples using PO:AO exhibited reversion behavior after 15 min of the curing process. Increased reversion behavior was observed when the PO was mixed with high amounts of AO in the NR compounds. The reversion percentage of PO 2 was the lowest compared to PO 1, PO 3, and PO 4. This was due to the breakdown of paraffin side chains in the PO and AO at a ratio of 70:30, which resulted in decreased reversion in the NR network. In addition, AO has a good impact on the cure characteristics when it is mixed with PO. This may be due to the unsaturated ring compound in the oil, which is compatible with the NR network [
11]. PO 4 showed increased reversion behavior when it was mixed with the AO in the MRE samples. Aromatic content in the MRE samples might interact with PO, resulting in an increased reversion curve.
The values of scorch time (t
s1), cure time (t
90), minimum torque (M
L), maximum torque (M
H), and torque value difference (ΔM) that were obtained from curing curves of different ratios of PBOs are summarized in
Table 4,
Table 5 and
Table 6. The moving
for all NR-based MRE samples with different ratios of oils in
Table 4,
Table 5 and
Table 6 was found to be approximately <1 dNm for each sample. The value of
was obtained from the torque value for each sample.
According to the data in
Table 4, the addition of different ratios of PBOs affects the cure characteristics of the NR-based MREs. The value of scorch time (t
S1) represents the period of time required for the safe processing of the rubber compound. Based on
Table 4, NO 1, NO 2, and NO 3 showed very small effects on the scorch time, where the difference was only 0.01–0.02 min. This may have been due to the rising amount of additives during the induction stage to break down the intermolecular friction between rubber in the NR. Hence, the additives prevented the rubber chains from breaking down within the NR matrix, resulting in intermolecular friction and, consequently, causing the insignificant difference in the scorch time. Meanwhile, NO 4 samples indicated the highest scorch time of ~3.49 min due to the high AO content, which delayed the curing process. The long scorch time, corresponding to a slow chemical reaction between the rubber and additives, was favorable compared to the short scorch time in terms of providing enough processing safety. Meanwhile, the shortest scorch time had an influence on the crosslink, wherein the crosslinking started quickly in the compound [
7,
31,
37].
The optimal cure time (t
90) was equivalent to 8 min, which is the time required for the rubber compound to reach the state of curing 90% of the rubber, achieving optimal properties. Referring to
Table 3, NO 2 demonstrated the highest cure time, with a value of 8.79 min. Meanwhile, NO 4 provided the lowest value, which was 8.54 min. It can clearly be seen that the presence of low amounts of AO can prolong the cure time. According to Neau et al. [
40], three samples of NO were compared to AO, and all of the NO samples showed a longer cure time than AO due to the lower amount of sulfur compounds in the oil compared to AO. In addition, AO may accelerate the curing reaction in NR compounds, which contain a more reactive double bond. The presence of nitrogen and sulfur in the PBO is one of the factors that can accelerate the curing process and, consequently, affect the crosslink density [
40,
41].
The t
S1 values showed that LMO 1 had the lowest scorch time (t
s1) compared to other samples, at 3.32 min. This may be due to the poor compatibility of LMO with the rubber matrix during the mixing process. In this experiment, LMO 3—containing the same amounts of LMO and AO in phr—had the longest scorch time, while delaying the beginning of the vulcanization reactions [
37]. However, LMO 4, which contained 3 phr of LMO and 7 phr of AO exhibited almost the same ts
1 behavior as LMO 3. Unsaturated rings in the AO of the MRE samples might play important roles during mixing to delay the safety time.
Regarding the t
90 values, different ratios of LMO in the MRE samples exhibited distinct vulcanization times. LMO 3 and LMO 4 had similar behavior, whereby the LMO mixed with AO in those samples presented an increase in vulcanization time. This might be explained by the reduced amounts of AO, which does not help much when it is mixed with LMO. However, in comparison, at t
90, LMO 4 exhibited the longest scorch time because of the high amounts of AO. These findings reflect the influence of different ratios of oil on the compatibility of rubber compounds. In addition, AO can accelerate the curing reaction in NR compounds, which contain more reactive double bonds [
10,
26].
The results showed that each different NR-based MRE sample exhibited different cure characteristics due to different ratios of PO:AO. The value of tS1 for PO 1 showed the lowest time, at 3.19 min, and contained 10 phr of PO in the sample. This may be due to the incompatibility of the PO with the NR compound during the curing process, leading to a decreased value of tS1. Meanwhile, PO 4 exhibited the highest tS1 as compared to PO 1, PO 2, and PO 3. The highest amount of AO mixed with the PO might delay the vulcanization reactions at the beginning of the process. However, PO 3 also exhibited similar behavior to PO 4. The high aromatic content in the sample caused an increasing scorch time, which resulted in good and safe processing.
It can be seen that the t
90 of different ratios of PO increased when it was mixed with high amounts of AO. However, the PO 2 and PO 3 ratios did not significantly affect the vulcanization process, wherein both had similar values of vulcanization time. Referring to
Table 6, PO 4 demonstrated the highest cure time, with a value of 8.59 min. Meanwhile, PO 1 provided the lowest cure time, at 8.28 min, while PO 2 had a cure time of 8.43 min. It can clearly be seen that the presence of a high amount of AO in the sample can prolong the cure time. The mixture of PO with AO based on the aromatic content of the oil might increase the cure time. It can be concluded that the addition of the AO to the other PBOs influenced the curing characteristics in terms of scorch time and optimal cure time.
3.2. Microstructure Observation
Figure 4 illustrates the microstructure of isotropic MRE samples. All NR-based MRE samples exhibited a homogeneous distribution of CIPs. Clearly, the agglomeration of CIPs occurred in the MRE samples, as shown in
Figure 4a–d. In general, it is known that during the early stages of the mixing process, NR presented high viscosity and elasticity. To acquire a homogeneous distribution of the CIPs, high shear forces were required during early mixing to disperse the CIPs and mitigate the agglomeration during the fabrication process. As shown in
Figure 4a–d, increased porosity may occur due to the weak attraction between the NR, additives, and CIPs during the curing stage, consequently resulting in weak adhesion between the MRE sample components.
Based on
Figure 4b–d, the MRE samples formed large agglomerations and many pores in the presence of more AO. This may have been due to the existence of high aromatic content in the AO. The high aromatic content in the AO had weak interactions with the NO, NR phase, and CIPs within the MREs, decreasing the crosslink density, as shown in
Table 3. It can be concluded that the higher the aromatic content, the greater the possibility of agglomeration occurring. Meanwhile,
Figure 4a shows that a large agglomeration was observed in the MRE sample containing NO without AO, and the porosity still occurred in the sample. However, the greater number of CIPs embedded within the NR phase indicated a better interface between the NR and CIPs. Clearly, CIPs could blend well with the matrix and the dispersing aids. This could be due to the high content of NO, which has good compatibility with NR and is more stable compared to AO. Consequently, the higher the ratio of NO to AO, the better the compatibility between the NR phase and the CIPs.
Referring to
Figure 5a–d, the microstructures of different ratios of LMO:AO were randomly distributed within the MRE samples.
Figure 5a illustrates the large agglomeration of CIPs in the absence of AO. This may be due to the incompatibility of the NR phase with the LMO, which resulted in a decrease in crosslink density in the absence of AO. LMO has a low density, which means it is less prone to interact with the NR phase and CIPs within the MREs. Meanwhile,
Figure 5b–d show fewer agglomerations and decreased porosity in MRE samples containing the AO, as compared to
Figure 5a. It can be seen that the AO helps to reduce agglomeration within MRE samples when it is mixed with the LMO. The CIPs can blend well when the LMO is mixed with the AO. The unsaturated ring content in the AO has a high reactivity, which enables it to easily interact with the NR and CIPs. It can be concluded that the best ratio of LMO:AO with respect to the MRE microstructure is found in the samples with high AO content, since less agglomeration and porosity were observed, leading to an increase in the crosslink density, as shown in
Table 4.
The microstructure of the MREs’ cross-section for PO:AO was observed with different ratios, as shown in
Figure 6a–d. All of the samples exhibited similar random CIP distribution within the MREs. It can be seen that the agglomerations and porosity still formed within the MREs.
Figure 6a,b,d display large agglomeration within the MRE samples. Referring to
Figure 6a,b, a large agglomeration is formed in the MREs due to the high levels of paraffin side chains. The paraffin side chains have no double bond, which leads to less reactivity within the MREs, and the incompatibility of the NR phase with the PO. Meanwhile,
Figure 6c shows less agglomeration and porosity within the MREs when the ratio of PO:AO is 50:50, due to the existence of both aromatic and paraffinic content. At this ratio, the aromatic and paraffinic content contributed to well-blended CIPs in the NR phase. This ratio is considered when the PO and AO mixture is needed to reduce the agglomeration and porosity within the MREs. In conclusion, PO contributed to a reduction in agglomeration within MRE samples.
3.3. Magnetic Properties
Magnetic properties in MREs are important for evaluating the bonding between the particles and the matrix in MREs that produce a high MR effect and good mechanical properties [
15]. The magnetization curves exhibited narrow magnetic hysteresis loops for all MRE samples, corresponding to soft magnetic characteristics. The trend of the graph is similar for all samples, where the magnetization curves increase as the magnetic field strength increases from −8000 mT to 8000 mT. Then, the slope begins to decrease when the CIPs in the MRE samples reach their saturated condition, where the M
S value is obtained. The parameters that influenced the magnetic properties of the MRE samples were saturation magnetization (M
S), coercivity (H
C), and retentivity magnetization (M
R), the values of which are depicted in
Table 7,
Table 8 and
Table 9. As seen from the magnetization curves, the values of M
R and H
C in MRE samples can be obtained at the vertical and horizontal axes, respectively. Coercivity is defined as the magnetic field that is required to drive the reverse magnetization after being saturated. Meanwhile, the retentivity is a measure of magnetization that remains after the applied magnetic field is released.
Figure 7 shows the magnetization curves of NR-based MREs with different ratios of NO:AO. The magnetization curves of NO 3, where the ratio of PBO was 50:50, showed the lowest magnetization. Meanwhile, NO 1 had the highest magnetization curves compared to NO 2, NO 3 and NO 4, due to the high content of NO in the sample, which showed better magnetic properties.
Figure 8 shows the magnetic hysteresis loops for MRE samples with different ratios of LMO:AO. It can be observed that LMO 4 exhibited the highest value of magnetic saturation as compared to LMO 1, LMO 2, and LMO 3. This was probably due to the high content of AO mixed with the LMO. The content of AO might help LMO in terms of interaction and compatibility between CIPs and the NR matrix, resulting in better dispersion of CIPs within the MREs [
42].
The magnetization curves for MRE samples with different ratios of PO:AO are shown in
Figure 9. PO 1 shows the lowest magnetic saturation in the absence of the AO content. Meanwhile, PO 3 represents the highest value of magnetic saturation as compared to PO 1, PO 2, and PO 4. PO 3 might be the best ratio for magnetic properties due to the interaction of PO and AO when mixed one another at a ratio of 50:50.
Table 7 represents the M
S, M
R, and H
C for different ratios of PBO, where the values are obtained from
Figure 7. Apparently, NO 1 showed good magnetic properties, whereby the sample produced the highest magnetic saturation of 40.06 emu/g. The addition of AO at ratios of 70:30 and 50:50 resulted in a decrease in the value of magnetic saturation from 28.73 to 17.23 emu/g. However, magnetic saturation was slightly increased in the case of the NR-based MRE sample with a ratio of 70:30. It can be seen that, when not combining NO with AO, NR-based MRE samples exhibited better magnetic properties as compared to the samples containing AO. This could be due to the compatibility of the oil with the NR matrix, which offers a better dispersion of CIPs compared to NO 3. The compatibility and interaction between the matrix and the filler are the predominant factors in improving the performance of MREs. The presence of NO contributed to the enhancement of the magnetic properties of MREs, due to the compatibility between oil and NR. This is because NO is a stable compound due to its high levels of saturated rings, resulting in the CIPs becoming homogeneously dispersed in the morphological observation [
29].
Table 8 shows the magnetic properties of MREs for different ratios of LMO:AO. The addition of AO to LMO contributes to an improvement in magnetic properties, increasing the magnetic saturation from 45.90 to 64.87 emu/g. This improvement of magnetic properties could be due to the good compatibility between LMO and AO, which contain similar ring structures. Good compatibility helped in producing good distribution of CIPs, and subsequently contributed to high magnetic interaction among CIPs. Apparently, using LMO alone as a dispersing aid produced the lowest value of magnetic saturation, at ~32.53 emu/g. This was probably due to the poor dispersion of CIPs within NR-based MREs, as depicted in
Figure 5a. When LMO is mixed with a high content of AO, the unsaturated ring in AO may cooperate with the low density of LMO. The similar unsaturated rings of AO and LMO have a good interaction between NR and CIPs; as a result, incorporating a large amount of AO into LMO may improve compatibility within MREs, resulting in better CIP dispersion. This is confirmed by the microstructure shown in
Figure 5d, which represents a better dispersion of CIPs.
As stated in
Table 9, the MREs with different ratios of PO:AO showed different effects on magnetic properties. The decreasing content of PO from 100 to 50 wt% resulted in an increase in magnetic saturation from 31.00 to 55.71 emu/g. Meanwhile, the 30% content of PO at a ratio of 30:70 showed a decrease from 55.71 to 48.94 emu/g. The presence of unsaturated rings associated with double bonds in AO is compatible with the paraffinic side chains that exist in PO. Compatibility between PO and AO reached its maximum when the ratio was 50:50, as the magnetic saturation started to decrease at a ratio of 30:70. PO 3 showed the best results in the enhancement of magnetic properties, due to the CIPs being well dispersed. The highest magnetic properties among the different ratios of PBO above were those of LMO 4. This can be seen in the microstructure observation in
Figure 5d, where the CIPs were well dispersed within the NR matrix. Therefore, it can be said that using PBO as a dispersing aid might help in producing a better dispersion of CIPs, depending on the ratios.
3.4. Thermal Characteristics
The TGA curves of NR-based MREs with different ratios of PBOs measured in a nitrogen atmosphere are demonstrated in
Figure 10,
Figure 11 and
Figure 12. Meanwhile, differential thermal gravimetric (DTG) curves are shown in
Figure 13,
Figure 14 and
Figure 15. Two decompositions occur from the TGA curves. At stage I of all samples, it was found that the TGA curves decreased slightly, and no peaks were observed in the DTG curves, as shown in
Figure 13,
Figure 14 and
Figure 15. Stage I was defined as the degradation of volatile matter with certain additives that are added to the samples during the compounding process. Meanwhile, the degradation of polymers for NR in this temperature range is referred to as stage II; during this stage, the TGA curves were dramatically decreased, and the peaks were observed as illustrated in
Figure 10,
Figure 11 and
Figure 12. In general, the weight loss of NR-based MREs corresponded to the degradation of volatile matter and the NR matrix.
Based on
Figure 10, the temperature ranges for stage I and stage II of NR-based MRE samples using NO:AO are 260 to 343 °C and 343 to 465 °C, respectively. Stage II was associated with the endothermic peaks observed in the DTG curves at all PBO ratios.
Based on
Figure 10, NO 2 showed the lowest total weight loss of approximately 70 wt% due to the increasing of crosslink density within MRE samples, which could have improved the thermal resistance of the rubber compositions. The presence of a low amount of AO mixed with the NO caused an increase in crosslink density to occur during the curing process. Thus, the crosslink density increased, and the rubber chains were delayed, resulting in decrease in weight loss. In addition, this outcome was in parallel with the results of curing characterization, where NO 2 showed a low percentage of reversion during the curing process. This sample corresponded to the low amount of AO associated with the narrow peak that occurred in
Figure 10. The peaks occurred due to the heat absorbed and consequent breaking of the crosslink rubber chains. The presence of a high crosslink density caused the low weight loss. As such, NO 2 showed a better result in terms of thermal decomposition and a broader peak when NO was mixed with low amounts of AO as compared to other samples. NO 3 exhibits low weight loss at 75 wt% when mixed with the same amount of AO at a 50:50 ratio; this could be due to the increase in its crosslink density. This could be explained by the interaction between rubber and additives within MRE samples at a 50:50 ratio. The increase in the crosslink density resulted in decreased weight loss.
Meanwhile, NO 1 and NO 4 showed similar behavior, with high weight loss of approximately 80–90 wt% as compared to the other samples. This may have been a result of the decrease in crosslink density occurring in MRE samples during the curing process, resulting in reduced thermal stability. Hence, the amount of oil at the ratios of NO 1 and NO 4 might cause the degradation reactions to occur, as a result of which the crosslink bonds easily break. The material is easily decomposed due to the decrease in the formation of crosslink bonds in the rubber compound, making it break more easily. However, the effects of different ratios of PBO on the thermal degradation of MREs are different due to the presence of saturated rings in the NO [
39,
40,
43,
44]. Since the mass of CIPs was constant (60 wt%), and CIPs can only decompose at temperatures beyond 1000 °C, it can be deduced that the weight loss of NR-based MREs was predominantly influenced by different ratios of PBO.
Figure 13 shows the endothermic peaks in the DTG curves of NR-based MREs for different ratios of NO:AO. The peaks occurred at stage II from 343 to 465 °C due to the polymer degradation of the NR matrix. The broad peaks that were obtained in the DTG graph was showed similar trends. This could be due to the presence of unsaturated rings and saturated rings and consequently, the heat absorbed when the rubber chains were breaking.
Figure 11 shows TGA curves for different ratios of LMO:AO. The degradation in TGA curves can be observed at a temperature range of 200–500 °C. Stages I and II occurred at temperature ranges of 200 to 324 °C and 324 to 475 °C, respectively. All samples for different ratios of LMO:AO showed insignificant effects on the TGA curves, where the weight loss was shown to be approximately 60–65 wt%. This may have been due to the similar rings present in the AO and LMO, which are known as unsaturated rings. These rings showed compatibility when LMO was mixed with AO, which resulted in an insignificant effect on the degradation of the MREs.
Figure 14 shows the endothermic peaks in the DTG curves. The peaks occurred from 324 to 475 °C due to the polymer degradation of the NR matrix. The broad peaks that were obtained in the DTG graph for different ratios of LMO:AO showed similar trends. This could be due to the heat absorbed when the rubber chains were breaking. However, all samples exhibited similar trends.
Figure 12 shows the TGA curves for the different ratios of PO:AO. The significant weight loss for these samples occurred when the temperature range was at 200–500 °C. Referring to the TGA curves, stage I and stage II started at temperatures of 220 to 327 °C and 327 to 472 °C, respectively. The results of PO:AO samples showed an insignificant effect and similar behavior with respect to temperature. These results were identical to those of the LMO:AO samples, resulting in an insignificant effect. The addition of AO to PO exhibits good compatibility, as AO possesses an unsaturated ring and PO has paraffin side chains. The chemical structures of PO and AO caused high reactivity within MRE samples.
Figure 15 shows DTG curves for different ratios of PO:AO, and endothermic peaks were obtained at stage II. The DTG peaks were observed due to the breaking of crosslinked rubber chains. Then, the broad peaks showed similar trends in the DTG peaks for all samples. This could be due to the interaction of paraffin side chains with unsaturated rings, which caused the breaking of the crosslinks of rubber chains and, consequently, showed similar behavior in terms of weight loss.
The degradation temperature of MREs with different ratios of PBO is demonstrated in
Table 10,
Table 11 and
Table 12. T
onset represented the temperature when the NR matrix started to degrade, while T
end corresponds to the end of the degradation. T
max1 is the maximal temperature for the thermal degradation rate of PBO. Meanwhile, T
max2 is the maximal temperature for the thermal degradation rate of NR. Apparently, NO 1 showed the lowest value of T
onset at 260 °C without mixing with AO, increasing to 300 °C when NO was mixed with a low amount of AO at a ratio of 70:30 (NO 2). The addition of AO at ratios of 50:50 and 70:30 resulted in a decreasing value of T
onset from 290 to 280 °C. However, the T
onset was slightly increased in the case of NR-based MRE samples with ratios of 50:50. It could be said that mixing NO with AO at a ratio of 50:50 for NR-based MRE samples exhibited a better thermal degradation compared to the samples containing high amounts of AO. Hence, the high content of NO mixed with low amounts of AO delayed the occurrence of thermal degradation. The NO mixed with low amounts of AO showed the potential to improve the thermal properties, contradicting the results of the magnetic properties.
Table 11 shows the thermal degradation temperatures for different ratios of LMO:AO. The addition of AO to LMO with increasing ratios—from 0:100 (LMO 1) to 70:30 (LMO 4)—contributed to a better resistance to thermal degradation, where LMO 4 showed the highest value of T
onset, followed by LMO 3, LMO 2, and LMO 1, with values of 258, 242, 226 and 223 °C, respectively. The T
onset for all samples of LMO:AO increased with the increasing content of AO. The effects of different ratios of LMO:AO on the thermal degradation of MREs could be due to the presence of unsaturated rings, as explained in the discussion of TGA curves above. The heat transfer is expected to be faster with low amounts of AO, so the material will decompose at lower temperatures.
Thermal degradation for different ratios of PO:AO within MRE samples is shown in
Table 12. The sample of PO without AO showed the lowest value of T
onset, at 227 °C, where thermal degradation decreased dramatically. Meanwhile, the samples of PO 2, PO 3, and PO 4, where PO was mixed with increasing amounts of AO, showed an increasing thermal degradation temperature, where the values of T
onset were 235, 242, and 260 °C, respectively. These results contributed to delayed thermal degradation. It can be seen that AO delayed the degradation of the sample due to the unsaturated ring in AO, which contains a more reactive double bond within the NR matrix. Apparently, the best ratio of this sample to obtain thermal properties is PO 4, as compared to PO 1, PO 2, and PO 3. It should be noted that the thermal degradation increased with increasing amounts of AO content. Overall, NO 2 showed better thermal properties, since the value of T
onset was delayed at 300 °C as compared to other samples. Furthermore, this suggests that different ratios of PBO particularly NO to AO have a great influence on the thermal properties and weight loss of NR.