## 4 Conclusions  
Rubber crumbs have been treated with 1%, 4%, 7%, and 10% NaOH concentrations to determine the optimum NaOH concentration that can provide strong adhesion between the polyester matrix and the rubber filler in the rubber polyester composites. The treatment is to modify the rubber surface by mechanically etching the surface and to provide the rubber with a rougher surface, induced more  
Figure 16: Effect of rubber contact angle on tensile strengths. functional groups, and removed a passive hydrophilic layer, which prevents a good polyester adhesion to the rubber. Tensile testing, FTIR analysis, contact angle measurement, and a microscopic surface texture study were conducted to evaluate the effects of the treatments. The following conclusions are drawn from the analysis of this work. 1. NaOH treatment provides the rubber with a good roughness surface, induced more functional groups that increase the polarity of the rubber, removed the passive hydrophilic layer and insoluble ions from the rubber surface, and enhanced the hydrophilicity and wettability of the rubber. 2. However, NaOH treatment causes some degree of damage to the rubber structure, which decreased the mechanical properties of the composites. 3. The optimum concentration of NaOH for the rubber crumbs treatment is 7%, which has sufficiently etched the rubber surface, producing numerous micro cracks which give rubber a rougher surface, thus providing a larger surface area for polyester to adhere to the rubber. The 10% NaOH treatment showed comparable surface properties as seen in 7% of treatment. 4. Rubber that has been treated with 7% and 10% NaOH has a lower contact angle measurement which indicates a better wettability of the rubber surface. 5. Rubber treatment with a higher NaOH concentration further modified the rubber surface, intensively oxidized the rubber, generated more polar groups and provided better rubber surface morphology, which improved the rubber wettability and hydrophilicity. 6. Higher concentration of NaOH treatment has altered the rubber to some degree which increases the rubber surface energy that makes the rubber surface has a lower contact angle. 7. Increased rubber surface energy is the result of higher polarity of the rubber surface, rougher rubber surface, as well as the removal of the passive hydrophobic zinc stearate layer from the rubber surface and the present of soluble sodium ions which has been converted from the insoluble zinc ions. 8. Therefore, as the rubber has better wettability and hydrophilicity, the polyester is capable of adhering well to the rubber surface, ensuring maximum bond and resulting in stronger interfacial adhesion between the two surfaces. 9. On the other hand, rubber that has been treated with a lower concentration of NaOH; 1% and 4% treatments have a higher surface contact angle due to insufficient surface modification. Weaker alkaline treatment is not capable of sufficiently etching the rubber surface and cannot effectively disrupt the rubber chains, and therefore generates fewer polar groups. As a consequence, the rubber surface has poor wettability and hydrophilicity. Conceptualization, M.N. and S.M.S. ; methodology, M.N. ; investigation, M.N. ; resources, R.N. ; data curation, M.J.; writing--original draft preparation, M.N. and R.N. ; writing--review and editing, S.M.S., and M.J.; visualization, M.N. ; supervision, S.M.S., R.N., and M.J.; project administration, M.N. ; funding acquisition, S.M.S. All authors have read and agreed to the published version of the manuscript. This research was funded by UNIVERSITI PUTRA MALAYSIA through grant number Putra Grant IPS (9607000).