Morphological, Physical, and Mechanical Properties of Sugar-Palm (Arenga pinnata (Wurmb) Merr.)-Reinforced Silicone Rubber Biocomposites

The development of environmentally benign silicone composites from sugar palm fibre and silicone rubber was carried out in this study. The mechanical, physical, and morphological properties of the composites with sugar palm (SP) filler contents ranging from 0% to 16% by weight (wt%) were investigated. Based on the uniaxial tensile tests, it was found that the increment in filler content led to higher stiffness. Via dynamic mechanical analysis (DMA), the viscoelastic properties of the silicone biocomposite showed that the storage modulus and loss modulus increased with the increment in filler content. The physical properties also revealed that the density and moisture absorption rate increased as the filler content increased. Inversely, the swelling effect of the highest filler content (16 wt%) revealed that its swelling ratio possessed the lowest rate as compared to the lower filler addition and pure silicone rubber. The morphological analysis via scanning electron microscopy (SEM) showed that the sugar palm filler was evenly dispersed and no agglomeration could be seen. Thus, it can be concluded that the addition of sugar palm filler enhanced the stiffness property of silicone rubber. These new findings could contribute positively to the employment of natural fibres as reinforcements for greener biocomposite materials.


Introduction
The development of new materials is rapidly increasing to meet new demands in this era of advanced technologies. Research and fabrications on composite materials, especially the demand for this product is low as it only focuses in the rural areas. Abundant sugar palm fibre residues have not been utilised, which might cause a negative effect towards nature as it is burned in the field [50]. Thus, this encouraged this study to select sugar palm fibres as a filler for silicone rubber. Many studies have also carried out investigations on the properties of the sugar palm fibres, and promising results were obtained as the properties of sugar palm fibres are also comparable to other natural fibres [51].
Therefore, this study aims to report on the mechanical properties of the sugar-palmreinforced silicone rubber biocomposite via the uniaxial tensile test. Its viscoelastic behaviour was characterised under dynamic mechanical analysis (DMA) to determine its storage modulus, (E ), loss modulus, (E ), and tan δ, while the physical characteristics of the sugar-palm-reinforced silicone rubber biocomposite were investigated by analysing its density, moisture absorption content, and swelling behaviour. The morphology of the break specimens was also analysed using a scanning electron microscope (SEM).

Material
Silicone rubber (Ecoflex 00-30 Platinum Cure) supplied by Castmech Sdn Bhd, Ipoh, Malaysia was selected as the matrix for this study. The product came with two parts: Part A and Part B. The silicone mixture was mixed using a ratio of 1:1. The sugar palm fibres were supplied by the local people in Kuala Pilah, Negeri Sembilan, Malaysia.

Specimen Preparation
The fibres were harvested and washed using clean water to ensure contaminants such as dry leaves and dusts were removed from the fibres. The fibres were then let to dry at room temperature for 24 h. The cleaned sugar palm fibres were then crushed using the crushing machine followed by the planetary mono mill machine (Pulverisette Classic Line 6, Fritsch, Germany). To control the size of the sugar palm (SP) filler, the final step of the fibre preparation included the sieving process using a mesh with a frame size of 0.25 mm. The sugar-palm-reinforced silicone rubber biocomposite specimens were prepared with 0 wt% (Pure silicone rubber), 4 wt%, 8 wt%, 12 wt%, and 16 wt% filler compositions. The specimens were prepared by mixing both part A and part B of the silicone rubber. After constant stirring, the weighed fillers were inserted into the mixture and the stirring process was then continued. It is crucial to achieve homogeneity by stirring the mixture evenly to ensure that no accumulated fillers are found in the mixture. The mixture was then poured into the mould and was let to cure for four hours at room temperature.

Uniaxial Tensile Test
The mechanical tensile behaviour of the sugar-palm-reinforced silicone rubber biocomposite was investigated using a 3382 Universal Testing Machine (Instron, Norwood, MA, USA). The test was conducted based on the ASTM D412 standard with a speed rate of 500 mm/min. For each filler composition, nine samples were tested until a failure state was reached.

Dynamic Mechanical Analysis (DMA)
The viscoelastic behaviour of the silicone biocomposite specimens were studied via a dynamic mechanical analysis using DMA 8000 (Perkin Elmer, Waltham, MA, USA). Three specimens for each filler composition were prepared with dimensions of 30 mm × 5 mm × 3 mm. The viscoelastic behaviour of the samples was compared from the storage modulus, loss modulus, and tan delta parameters. All specimens were exposed to a tension mode with a constant frequency of 1 Hz. The test temperature was varied from room temperature up to 200 • C.

Density Test
Six specimens with dimensions of 20 mm × 20 mm × 3 mm for each composition were prepared and their density was determined using the Archimedes method. A Specific Gravity Measuring Kit (AD-1653, Cole-Parmer, Japan) was used to determine the weight of the specimens. Each specimen was weighed both in air and distilled water. Their density value was obtained from Equation (1).
where ρ is the density, A is the weight of the specimen in air, B is the weight of the specimen in water, ρ o is the density of distilled water (0.9973 g/cm 3 , temperature: 24 • C), and d is the density of air (0.001 g/cm 3 ).

Moisture Absorption Test
A similar procedure was carried out by [52] where six specimens were prepared for the moisture absorption test with dimensions of 50 mm × 50 mm × 2 mm. The specimens were then immersed in distilled water for 24 h at room temperature. The specimens were removed from distilled water, wiped, and dried thoroughly to ensure no excess of water was present on the surface. The specimen was then weighed to obtain the weight gain. The volume, % of moisture absorption was then calculated using Equation (2): where W f and W i are the final and initial weights of the specimens and ρ w and ρ c are densities of the water and composite specimen, respectively.

Swelling Test
Six specimens for each filler composition were prepared with dimensions of 50 mm × 25 mm × 2 mm. The test was conducted based on ASTM D471 where toluene liquid was chosen as the swelling agent. The test was carried out at room temperature. For the first 22 h of immersion, the weights of each specimen were taken for every two hours. The immersion was then continued for another 48 h with 24 h of interval. The swell rate of the specimen was then calculated using Equation (3): where ∆M is the change in mass (%), M 1 is the mass of the specimen before immersion (g), and M 2 is the mass of the specimen after immersion (g).

Scanning Electron Microscope (SEM)
The morphological surface of the specimens after the tensile test was examined using a TM3000 Scanning Electron Microscope (SEM) (Hitachi, Japan). The surfaces to be examined were coated with platinum using a sputter coater machine (SC7620, Polaron, Watford, UK) to avoid the specimen from charging in order to obtain better image quality.  showed the average tensile properties from nine specimens for each filler composition where the xand y-axes of the graph represent strain, ε, and engineering stress, σ E , respectively. It can be observed that the curves behaved highly nonlinearly where there was a slight concave downward pattern from 0 to 0.05 MPa for all specimens, as can be seen in the inset of Figure 1. After 0.05 MPa, the pure silicone rubber and 4 wt% curves continued to increase with a concave upward pattern until it reached a failure state, while the specimens with increased filler contents of 8 wt%, 12 wt%, and 16 wt% showed that the nonlinear elastic behaviour was reduced, making it almost linear-like. It could also be observed that a further increment in the sugar palm filler into the silicone rubber showed a gradual increase in the gradient of the graph. This indicates that the stiffness of the specimen increased with the filler content. These results are consistent with Ziraki et al.'s [53] findings where the further addition of polypropylene fibres into the silicone rubber increased the specimen's stiffness property. Gan et al. [54] reported similar outcomes where the stiffness property of the graphene nanoribbon-filled silicone rubber nanocomposite had improved significantly in comparison to pristine silicone rubber. Karthikeyan et al. also found that the addition of sisal fibre into the silicone rubber had enhanced the stiffness of the materials. This was attributed to the filler volume fraction as a higher filler content would increase the crosslink density of the silicone rubber to resist the deformation and thus increase the material's stiffness. In addition to that, the filler distribution plays an important role in ensuring the loads are uniformly transferred [55].

Results and Discussion
3.1.1. Uniaxial Tensile Test Figure 1 shows the stress-strain curve of pure silicone rubber and the sugar-palmreinforced silicone rubber biocomposite (4 wt%, 8 wt%, 12 wt%, and 16 wt%). The curves showed the average tensile properties from nine specimens for each filler composition where the x-and y-axes of the graph represent strain, ε, and engineering stress, σE, respectively. It can be observed that the curves behaved highly nonlinearly where there was a slight concave downward pattern from 0 to 0.05 MPa for all specimens, as can be seen in the inset of Figure 1. After 0.05 MPa, the pure silicone rubber and 4 wt% curves continued to increase with a concave upward pattern until it reached a failure state, while the specimens with increased filler contents of 8 wt%, 12 wt%, and 16 wt% showed that the nonlinear elastic behaviour was reduced, making it almost linear-like. It could also be observed that a further increment in the sugar palm filler into the silicone rubber showed a gradual increase in the gradient of the graph. This indicates that the stiffness of the specimen increased with the filler content. These results are consistent with Ziraki et al.'s [53] findings where the further addition of polypropylene fibres into the silicone rubber increased the specimen's stiffness property. Gan et al. [54] reported similar outcomes where the stiffness property of the graphene nanoribbon-filled silicone rubber nanocomposite had improved significantly in comparison to pristine silicone rubber. Karthikeyan et al. also found that the addition of sisal fibre into the silicone rubber had enhanced the stiffness of the materials. This was attributed to the filler volume fraction as a higher filler content would increase the crosslink density of the silicone rubber to resist the deformation and thus increase the material's stiffness. In addition to that, the filler distribution plays an important role in ensuring the loads are uniformly transferred [55].    15. This showed that pure silicone rubber exhibited the most flexible behaviour with the highest strain value but lacks the stiffness property as compared to the specimen with added sugar palm filler. With the increment in SP filler content, it gradually reduced the values of maximum tensile strength and strain. Unlike synthetic fibres, natural fibres exhibit a lower strength and durability [56] and, thus, could contribute to a lower mechanical tensile strength. The load transferred from the silicone rubber to the sugar palm filler was unable to effectively be absorbed by the filler due to the lack of durability of natural fibres in withstanding the load, and this resulted in an early breakage of the specimen with lower ultimate tensile strength and strain at break. These results are consistent with those of Ismail et al. [57] who reported that the increment in bamboo fibre into natural rubber decreased the tensile strength and elongation at break of the composite. Another similar finding was reported by Berahman et al. [58] where the increment in the filler content monotonically reduced the elongation at break of the samples.

Morphological Characterisation
To further support Table 1, Figure 2 shows the morphological images of the pure silicone rubber and sugar-palm-reinforced silicone rubber biocomposite after the tensile test. It could be observed that the SEM image of pure silicone rubber as shown in Figure 2a revealed a smooth surface with no formation of voids. The SEM images of specimens with added sugar palm filler (Figure 2b-e) displayed an uneven fracture surface as the surface became rougher with a higher filler loading. Despite having a low durability in SP fillers, the early breakage of the specimens could also be due to the irregular size of sugar palm fillers that caused an uneven load transfer in between the filler and the matrix. Furthermore, the addition of filler led to the formation of voids, as can be seen from the SEM images, due to the air trapped within the fillers and the matrix during the curing process. Namitha et al. [52] in their study also described a similar result. Furthermore, through the SEM images, the voids shown also proved why specimens with added sugar palm filler resulted in a significant drop in its tensile strength and strain values. However, these specimens also proved that the load was transferred effectively between the fillers and the matrix as most failure occurred at the fillers. The fillers were also well-dispersed and no SP filler agglomeration could be observed. The variation in the viscoelastic behaviour, storage modulus (E′), loss modulus (E″), and tan delta (tan δ) of pure silicone rubber, 4 wt%, 8 wt%, 12 wt%, and 16 wt% sugar- The variation in the viscoelastic behaviour, storage modulus (E ), loss modulus (E ), and tan delta (tan δ) of pure silicone rubber, 4 wt%, 8 wt%, 12 wt%, and 16 wt% sugar-palm-reinforced silicone rubber biocomposites are shown in Figure 3a-c in the temperature range from room temperature to 200 • C. It could be observed that without the sugar palm filler, pure silicone rubber exhibited the lowest storage modulus property and its stiffness property decreased (reaching negative values) with an increase in operating temperature. This indicates that the specimen's ability to retain the stiffness property by absorbing the energy received decreased across the temperature range. A similar trend could be observed where the storage modulus of specimens with added sugar palm filler increased steadily with the increase in temperature condition. It could also be seen that the stiffness property of 4 wt%, 8 wt%, 12 wt%, and 16 wt% specimens improved significantly by 70%, 100%, 190%, and 280%, respectively. These results revealed that the storage modulus was enhanced by the addition of sugar palm filler into the silicone rubber. The main cause of pure silicone rubber losing its ability to absorb the energy throughout the heating process may be due to the weak intermolecular forces among the polymer chain of -Si-O-Si- [59]. This would reduce the ability of the pure silicone rubber to store the energy upon receiving it, hence resulting in a low stiffness property. In contrast, the presence of sugar palm filler appeared to absorb most of the heat exposed by the silicone rubber and thus reduce the mobility of the silicone rubber polymer chain. This resulted in an improvement of the weak intermolecular forces of the polymer chain in the silicone rubber into stronger intermolecular forces. Thus, this improved the stiffness property of the specimen with added sugar palm filler as compared to pure silicone rubber. Furthermore, these results also showed that the interfacial adhesion between the sugar palm filler and the silicone rubber possessed good interfacial bonding as the load and heat-exposed area transferred effectively. As a result, the stiffness property was enhanced. A similar finding was also reported by Feuchtwanger et al. where the addition of NiMnGa increased the storage modulus of silicone rubber by 50% [60]. The results also showed an almost similar pattern where the storage modulus increased upon heating, especially for silicone rubber with the highest volume fraction of the bulk single crystal. palm-reinforced silicone rubber biocomposites are shown in Figure 3a-c in the temperature range from room temperature to 200 °C. It could be observed that without the sugar palm filler, pure silicone rubber exhibited the lowest storage modulus property and its stiffness property decreased (reaching negative values) with an increase in operating temperature. This indicates that the specimen's ability to retain the stiffness property by absorbing the energy received decreased across the temperature range. A similar trend could be observed where the storage modulus of specimens with added sugar palm filler increased steadily with the increase in temperature condition. It could also be seen that the stiffness property of 4 wt%, 8 wt%, 12 wt%, and 16 wt% specimens improved significantly by 70%, 100%, 190%, and 280%, respectively. These results revealed that the storage modulus was enhanced by the addition of sugar palm filler into the silicone rubber. The main cause of pure silicone rubber losing its ability to absorb the energy throughout the heating process may be due to the weak intermolecular forces among the polymer chain of -Si-O-Si- [59]. This would reduce the ability of the pure silicone rubber to store the energy upon receiving it, hence resulting in a low stiffness property. In contrast, the presence of sugar palm filler appeared to absorb most of the heat exposed by the silicone rubber and thus reduce the mobility of the silicone rubber polymer chain. This resulted in an improvement of the weak intermolecular forces of the polymer chain in the silicone rubber into stronger intermolecular forces. Thus, this improved the stiffness property of the specimen with added sugar palm filler as compared to pure silicone rubber. Furthermore, these results also showed that the interfacial adhesion between the sugar palm filler and the silicone rubber possessed good interfacial bonding as the load and heat-exposed area transferred effectively. As a result, the stiffness property was enhanced. A similar finding was also reported by Feuchtwanger et al. where the addition of NiMnGa increased the storage modulus of silicone rubber by 50% [60]. The results also showed an almost similar pattern where the storage modulus increased upon heating, especially for silicone rubber with the highest volume fraction of the bulk single crystal.  As for loss modulus, E , Figure 3b presents the viscous part of pure silicone rubber and the sugar-palm-reinforced silicone rubber biocomposite. The pattern of the graph of all sugar-palm-reinforced silicone rubber biocomposite (4 wt%, 8 wt%, 12 wt%, and 16 wt%) specimens showed a decrease in loss modulus as the temperature increased. Interestingly, the 16 wt% specimen possessed the highest viscous portion followed by 12 wt%, 8 wt%, and 4 wt% specimens. This was due to the presence of sugar palm filler, which absorbed the energy that was transferred from the silicone rubber. This resulted in a low elastic response of silicone rubber. Therefore, this indicates that the 16 wt% specimen exhibited the highest energy lost due to friction and molecular motions [61]. Dislocation of sugar palm filler might also occur during the test, and this would be a logical explanation for the energy loss. A higher filler content would result in more energy loss as more dislocation and internal friction took place. This result is in agreement with [62] where the presence of fillers had a significant effect on the loss modulus of the composites. It could also be observed that the 8 wt% specimen exhibited lower viscous properties in comparison to the 16 wt% and 12 wt% specimens, which was almost similar tos pure silicone rubber after 164 • C. Pure silicone rubber, however, displayed a different behaviour where the loss modulus increased up to 124 • C and then continued to decrease gradually with the heating temperature. The decrease trend of silicone rubber also showed that the performance of the silicone rubber in dissipating the energy was almost similar to those of 4 wt% and 8wt% specimens after 124 • C. This might due to the lower mobility of molecular chains of the silicone rubber as the internal friction in between the filler-filler and filler-matrix is lower as compared to the 12 wt% and 16 wt% specimens [63][64][65].
The tan delta or damping ratio is the effectiveness of the material to dissipate the energy via internal friction and molecular movement. From Figure 3c, pure silicone rubber demonstrated the highest damping performance where the trend increased up to 164 • C and declined rapidly as the heating continued as compared to specimens with sugar palm filler. The pure silicone rubber showed an ineffectiveness at absorbing and dissipating the energy as it may fail abruptly where the damping ratio dropped sharply after 164 • C until it reached negative tan delta values. The specimens with added sugar palm filler, on the other hand, possessed almost similar tan delta behaviour where it decreased with the increasing temperature test, as shown in the inset of Figure 3c. However, the data revealed that the 16 wt% specimen was better at dissipating energy as compared to the 4 wt%, while the 12 wt% and 8 wt% specimens showed an almost similar damping ratio. This indicates that higher-filler-addition specimens are good at energy absorption but have less potential in dissipating the energy. Although the sugar-palm-reinforced silicone rubber biocomposite was found to exhibit low damping capacity, it still had a better damping capacity in comparison to pure silicone rubber. The effect of the addition of sugar palm filler into the silicone rubber could be seen when the tan delta behaviour was different than the pure silicone rubber as it dropped abruptly to negative tan delta values. This similar trend of sugar-palm-reinforced silicone rubber biocomposites has also been reported in the previous literature [60,66,67] where the damping ratio decreases as the heating continues. The interactions of filler-matrix and filler-filler play an important role in storing the energy and dissipating it effectively. From the data obtained, it can be concluded that the addition of sugar palm filler decreases the mobility of the polymer chain of the silicone rubber and it shows that the sugar palm filler and silicone rubber have a strong adhesion as it tends to store the energy rather than dissipate it. This theory is in good agreement with Kamaruddin et al. [64] as the graphene content increases into the rubber band. Figure 4 illustrates the experimental density of the composites with various filler contents compared to the density of sugar palm filler. Compared to pure silicone rubber with an experimental density of 1.025 g/cm 3 , the sugar-palm-reinforced silicone rubber biocomposite's experimental density increased linearly with the filler content, as depicted in the inset of Figure 4. The increase in density was similar to the findings of Gao et al. [68] where the increment in filler content increased the density of the silicone rubber foam. Furthermore, Sanyang et al. [69] also found that the density of sugar palm starch/polylactic acid bilayer films increased significantly to 1.35 g/cm 3 with loadings up to 80 wt%. Another similar trend could also be observed in a study by Atikah et al. [70]. It was reported that when glass fibre was added into the sugar-palm-fibre-reinforced epoxy composite, the composite's density increased up to 1.4 g/cm 3 . In comparison, it revealed that the highest density value (16 wt%) in this study was actually much lower as compared to the saturated specimens of Sanyang's and Atikah's findings. an experimental density of 1.025 g/cm 3 , the sugar-palm-reinforced silicone rubber biocomposite's experimental density increased linearly with the filler content, as depicted in the inset of Figure 4. The increase in density was similar to the findings of Gao et al. [68] where the increment in filler content increased the density of the silicone rubber foam. Furthermore, Sanyang et al. [69] also found that the density of sugar palm starch/polylactic acid bilayer films increased significantly to 1.35 g/cm 3 with loadings up to 80 wt%. Another similar trend could also be observed in a study by Atikah et al. [70]. It was reported that when glass fibre was added into the sugar-palm-fibre-reinforced epoxy composite, the composite's density increased up to 1.4 g/cm 3 . In comparison, it revealed that the highest density value (16 wt%) in this study was actually much lower as compared to the saturated specimens of Sanyang's and Atikah's findings.  Figure 5 presents the average moisture absorption content for each specimen after 24 h of immersion in distilled water. Clearly, the results showed an increase in moisture absorption with the increment in filler content. The 16 wt% specimens exhibited the highest moisture absorption content of up to 3.245%, while pure silicone rubber possessed the lowest. Similar results were reported by Namitha et al. [71] where a higher volume of ceramic in silicone rubber led to a higher water intake. Obviously in this study, natural fibres are known for their hydrophilic properties as they are mainly composed of cellulose and hemicellulose layers. This natural hydrophilic behaviour that exists in natural fibres allows the specimens to absorb more moisture. Thus, a higher water absorption of the specimen was obtained as a higher filler content was further added. A similar increment  Figure 5 presents the average moisture absorption content for each specimen after 24 h of immersion in distilled water. Clearly, the results showed an increase in moisture absorption with the increment in filler content. The 16 wt% specimens exhibited the highest moisture absorption content of up to 3.245%, while pure silicone rubber possessed the lowest. Similar results were reported by Namitha et al. [71] where a higher volume of ceramic in silicone rubber led to a higher water intake. Obviously in this study, natural fibres are known for their hydrophilic properties as they are mainly composed of cellulose and hemicellulose layers. This natural hydrophilic behaviour that exists in natural fibres allows the specimens to absorb more moisture. Thus, a higher water absorption of the specimen was obtained as a higher filler content was further added. A similar increment in moisture absorption content on other matrices using sugar palm fibre as the reinforcement or filler was also reported. Atikah et al. [70] found that the incorporation of sugar palm fibre increased the moisture intake of the hybrid composite of up to 9%. In contrast, Sahari et al. [50] claimed that the increment in AP fibre (0-30 wt%) into plasticised sugar palm starch gradually reduced the moisture content from 25% to 14%. From this study, the highest filler addition resulted in a moisture content of 3.245%, which is very much lower than Sahari's findings. This is due to the hydrophobic property that is present in the silicone rubber that has the ability to repel water [71].

Moisture Absorption Behaviour
in moisture absorption content on other matrices using sugar palm fibre as the reinforcement or filler was also reported. Atikah et al. [70] found that the incorporation of sugar palm fibre increased the moisture intake of the hybrid composite of up to 9%. In contrast, Sahari et al. [50] claimed that the increment in AP fibre (0-30 wt%) into plasticised sugar palm starch gradually reduced the moisture content from 25% to 14%. From this study, the highest filler addition resulted in a moisture content of 3.245%, which is very much lower than Sahari's findings. This is due to the hydrophobic property that is present in the silicone rubber that has the ability to repel water [71].  Figure 6 displays the average swelling behaviour of both pure silicone rubber and the sugar-palm-reinforced silicone rubber biocomposite with 4 wt%, 8 wt%, 12 wt%, and 16 wt% of filler content. The results displayed the same pattern where for the first two hours, the swelling rate increased significantly. This indicates the specimens' ability to fully absorb the toluene liquid. Unlike the sugar-palm-reinforced silicone rubber biocomposite, pure silicone rubber continued to absorb more toluene for the next two hours, which made the curve rise with a new peak. Interestingly, it was found that the addition of 4 wt% of filler content significantly decreased the swelling ratio of the specimen as compared to the pure silicone rubber. This trend continued to decrease with the further increment in filler content where the highest filler content (16 wt%) showed the lowest swelling rate. The pattern of swell rate also showed that all specimens reached an equilibrium state after 22 h of immersion. 3.2.3. Swelling Figure 6 displays the average swelling behaviour of both pure silicone rubber and the sugar-palm-reinforced silicone rubber biocomposite with 4 wt%, 8 wt%, 12 wt%, and 16 wt% of filler content. The results displayed the same pattern where for the first two hours, the swelling rate increased significantly. This indicates the specimens' ability to fully absorb the toluene liquid. Unlike the sugar-palm-reinforced silicone rubber biocomposite, pure silicone rubber continued to absorb more toluene for the next two hours, which made the curve rise with a new peak. Interestingly, it was found that the addition of 4 wt% of filler content significantly decreased the swelling ratio of the specimen as compared to the pure silicone rubber. This trend continued to decrease with the further increment in filler content where the highest filler content (16 wt%) showed the lowest swelling rate. The pattern of swell rate also showed that all specimens reached an equilibrium state after 22 h of immersion.

Swelling
The highest swelling ratio from the pure silicone rubber is due to their weak intermolecular forces, and this increases the ability of the chain mobility in silicone rubber [72]. Moreover, single bonds in the Si-O-Si backbone chain of the silicone rubber will allow for more spaces to be available, thus resulting in the highest rate of swelling. In contrast, the addition of sugar palm filler certainly decreased the spaces for chain mobility to occur in silicone rubber as sugar palm dominated the spaces. This causes the movement of the chains to be restricted. As a result, this reduces the swelling rate of the sugar-palm-reinforced silicone rubber biocomposite specimens. This chain motion restriction is a result from the increase in crosslink density in silicone rubber [73,74]. This indicates that pure silicone rubber possessed the lowest crosslink density in comparison to the added sugar palm filler into the silicone rubber. Similar findings were also reported by Mostafa et al. [75] where the increment of carbon black content decreased the swelling rate induced by the higher crosslink density [75]. The highest swelling ratio from the pure silicone rubber is due to their weak intermolecular forces, and this increases the ability of the chain mobility in silicone rubber [72]. Moreover, single bonds in the Si-O-Si backbone chain of the silicone rubber will allow for more spaces to be available, thus resulting in the highest rate of swelling. In contrast, the addition of sugar palm filler certainly decreased the spaces for chain mobility to occur in silicone rubber as sugar palm dominated the spaces. This causes the movement of the chains to be restricted. As a result, this reduces the swelling rate of the sugar-palm-reinforced silicone rubber biocomposite specimens. This chain motion restriction is a result from the increase in crosslink density in silicone rubber [73,74]. This indicates that pure silicone rubber possessed the lowest crosslink density in comparison to the added sugar palm filler into the silicone rubber. Similar findings were also reported by Mostafa et al. [75] where the increment of carbon black content decreased the swelling rate induced by the higher crosslink density [75].

Conclusions
This study aimed to promote a bio-friendly composite material using sugar palm fibre as the filler and silicone rubber as the binder. From this study, the mechanical, physical, and morphological analysis of sugar-palm-reinforced silicone rubber biocomposite were analysed and it was found that the addition of sugar palm filler into silicone rubber increased the stiffness, density, as well as the moisture absorption content of the material significantly. The swelling test revealed that the increment in filler composition showed a lower swelling rate as compared to pure silicone rubber. The morphological images also displayed a good filler-matrix bonding where no agglomeration was observed. In conclusion, this study proves that the newly developed biomaterial, the sugar-palm-reinforced silicone rubber biocomposite, has great potential for many applications. Therefore, this study has enhanced significant knowledge pertaining to silicone biocomposite materials.

Conclusions
This study aimed to promote a bio-friendly composite material using sugar palm fibre as the filler and silicone rubber as the binder. From this study, the mechanical, physical, and morphological analysis of sugar-palm-reinforced silicone rubber biocomposite were analysed and it was found that the addition of sugar palm filler into silicone rubber increased the stiffness, density, as well as the moisture absorption content of the material significantly. The swelling test revealed that the increment in filler composition showed a lower swelling rate as compared to pure silicone rubber. The morphological images also displayed a good filler-matrix bonding where no agglomeration was observed. In conclusion, this study proves that the newly developed biomaterial, the sugar-palm-reinforced silicone rubber biocomposite, has great potential for many applications. Therefore, this study has enhanced significant knowledge pertaining to silicone biocomposite materials.