## 3 Results and Discussion
### Contact Angle Measurement  
The strength of the composites depends so much on the capacity of the filler to distribute the load evenly throughout the composites. It is therefore highly dependent on the well-dispersed fillers in the composites and the good interfacial interaction between the fillers and the matrix [51; 52]. Good interfacial adhesion of the filler-matrix is strongly influenced by the surface properties of the filler, such as surface energy, polarity, surface morphology, and surface hydrophilicity. The surface of the fillers must be cleared from any obstructions and impurities that prevent it from being well coated by the matrix. Stronger adhesion requires the fillers to have lower surface energy so that the well matrix coverage on the rubber can be achieved [24]. The surface of the rubber gloves is generally hydrophobic due to the vulcanization process and the surface coating. These properties help to minimize cross-contamination during use. Therefore, surface modification is essential to alter the surface properties to improve the wettability of the rubber surface to facilitate strong interfacial adhesion between the rubber fillers and the polyester matrix. The effect of surface modification towards surface properties can be assessed by measuring the contact angle; it measured the angle between the tangent to the water droplet at the touch point and the rubber surface. Smaller contact angle means the surface is hydrophilic because the water droplet can simply wet or easily occupy the surface of the rubber [37]. This mechanism provides better matrix wetting on the rubber surface, enhances adhesiveness between rubber and matrix and strengthens the composites. Different NaOH concentration treatment for rubber crumbs altered the rubber surface differently. Figure 14 shows the degree of contact angle decreased as the NaOH concentration increased. The 1% and 4% NaOH treated rubber have a high contact angle of at 62.27\({}^{\circ}\) and 59.39\({}^{\circ}\), respectively, and a  
Figure 13: Modulus of elasticity of the rubber polyester composites. noticeable decrease in contact angle from the 4% NaOH to 7% NaOH treatment can be observed. An approximately 27% decrease in contact angle degree for the 7% NaOH treated rubber compared to the 4% treated rubber was observed. It is worth to note that contact angle of 7% and 10% NaOH treated rubber provides almost similar degree of contact angle. This result showed that 7% is the optimal concentration of NaOH which adequately altered the rubber surface. The rubber surface treated with 7% NaOH obtained the best surface properties that provide better interfacial adhesion between rubber and polyester, resulting in maximum improvement for the composites of rubber-polyester. The decrease in the contact angle due to the increased concentration of NaOH indicated the wettability of rubber surface was increased [17], which clearly demonstrated in Figure 15. Water droplets on the surface of 7% and 10% of NaOH treated rubber tend to occupy a larger surface, which showed an improvement in surface wettability. However, the water droplet on the 1% and 4% NaOH treated rubber are more spherical, which covers less of the rubber surface [11; 27; 28; 37; 46; 47; 53; 54]. Figure 14: Contact angles of treated rubber crumbs. Figure 15: Water droplet profiles on NaOH treated rubber crumbs surface: (**a**) 1%, (**b**) 4%, (**c**) 7%, and (**d**) 10%. The wettability of the rubber surface increases due to many factors, such as the removal of hydrophobic elements such as zinc stearate, which diffused onto the rubber surface and formed a passive hydrophobic layer. Sodium hydroxide converted zinc stearate into sodium stearate, which is water-soluble and easy to rinse-off. This removal of sodium stearate leads to significant changes in the surface properties of the treated rubber. It increased the area for contact surface between the rubber and the matrix and strengthening the bond between them [20; 21; 23; 39; 42; 53; 55].