## 3 Results and Discussion
### Tensile Strength  
The mechanical performance of the composite is highly dependent on the ability of the matrix and the filler to adhere well to each other. In general, the incorporation of waste rubber crumbs into composites resulted in a decrease in tensile strength caused by poor interfacial adhesion between rubber and matrix. Rubber is hydrophobic in nature and has low surface energy, which prevents the matrix from coating well on the rubber surface, therefore adhesion between the matrix and rubber is not good [23; 48]. However, the rubber surface treatment is expected to improve the interfacial bonding of rubber crumbs to the polyester matrix. Figure 6 shows the rubber-polyester composite's tensile strength and elongation at break of composite with untreated rubber crumbs and composite with rubber treated with different NaOH treatment concentrations. From the graph it can observed that treatment of NaOH to the rubber caused a decrease in tensile strength towards the composite; however, the tensile strength improved when the concentration of the NaOH increased. Treatment with NaOH provides excellent roughness to the surface, induces more functional groups, and removes the hydrophilic layer, which collectively enhance adhesion between rubber and the polyester. However, NaOH also caused damaged to the rubber structure and thus degrades the mechanical properties [23]. The increasing trend of tensile strength with the increase of NaOH concentration is because the higher concentration mechanically etched the rubber surface to a greater extent, producing numerous micro-cracks and forming much smaller cracks, making the rubber surface rougher. A rougher rubber surface provides a larger surface area for polyester to adhere on the rubber. This ensures maximum adhesion between rubber and polyester, enhanced the interfacial bonding, and improved the tensile strength of the composites [23; 35; 36; 48; 49]. From the tensile strength result, it can be noticed that composite tensile strength with 10% NaOH treated rubber is relatively comparable with composites of 7% NaOH. This suggests that the 7% NaOH treatment offers the ultimate modification to the rubber surface in order to achieve the desired surface properties. Rubber naturally has very high elasticity; however, treatment with NaOH may cause a slight decrease in elasticity. This can be observed as the elongation at break of the composite with 1% NaOH treated rubber declined when compared to the composite with untreated rubber. However, elongation at break of the composite with treated rubber shows a slight increase as the NaOH concentration increased, which is a similar trend to the composite tensile strength as discussed earlier. The higher NaOH concentration modified the rubber surface for better adhesion with polyester thus reflecting an increase of elongation at break for the composite. Figures 7-10 displayed the SEM rubber surface morphology, in which a significant effect of the different treatment concentration on the roughness of the rubber surface can be observed. Figures 7 and 8 showed that the rubber surface with 7% and 10% NaOH treatment were rougher with many microcracks and the cracks are much smaller, these attributes lead to a larger rubber surface area. While Figures 9 and 10 revealed that the rubber crumbs treated with lower NaOH concentration, 1% and 4% NaOH, respectively, which had a smoother surface with less micro-cracks and cracks are larger. A smooth surface inhibits the good coating of polyester on the rubber, and consequently the bonding is weaker, leading to lower tensile strength. A smoother surface typically has a higher contact angle with poor wettability properties; therefore, the polyester could not be well wetted on the rubber and did not have a sufficient physical bonding system to strengthen the adhesion. As a consequence, rubber and polyester will experience poor interfacial adhesion resulting in lower tensile strength. Figure 6: Tensile strength and elongation at break of the composites with different NaOH concentration.