## 1 Introduction  
Rapid growth in the manufacture of rubber products has led to a large number of different types of rubber waste worldwide. Rubber products are non-biodegradable and will have a serious environmental impact if not properly handled [1]. Several works have been published that have used rubber products as part of the effort to ensure that rubber does not end up in landfill. Waste rubber products have been widely used in composite materials that offer better toughness, higher strength, abrasion resistance, and good insulation properties. They are used in a variety of applications such as marine, construction, sports, and recreation [2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15]. However, the use of rubber has an adverse effect on the composites as it weakens the adhesion between the rubber and the matrix and reduces the strength of the composite. Rubber is a hydrophobic material with low surface energy, which prevents the matrix from making a good adhesion to the rubber. Rubber products also contain many additives that over time are diffused to the surface and form a passive layer, which inhibits the matrix adhering well with the rubber [16; 17; 18; 19; 20; 21; 22; 23]. For composite materials, adhesion between the filler and matrix is highly reliant on the surface properties that include surface polarity, surface energy, and surface morphology. Therefore, the surface properties of the fillers require modification in order to improve bonding with the matrix [24]. Much research has been conducted to modify the surface properties of rubber to strengthen the adhesion of the rubber with the matrix. Potential methods include alkaline treatment using sodium hydroxide (NaOH); acidic treatments using sulfuric acid, potassium permanganate, or silane; and many more [16; 21; 25; 26; 27; 28; 29; 30; 31]. Surface treatments will oxidize the rubber surface and form more polar groups, which will attach to the polar groups in the matrix. This phenomenon will increase the rubber surface energy thus improving the rubber hydrophilicity and wettability. The surface treatments also etched the rubber surface and it becomes rougher which allows better coverage of the matrix onto the rubber [22; 23; 26; 32]. Treatment with NaOH is common for rubber because it was generally easy, inexpensive, and provides excellent results in improving adhesion between the rubber filler with matrix. NaOH modifies the rubber surface by mechanically etching the surface, resulting in more surface roughness allowing better matrix coverage on the rubber. The rougher the surface of the rubber, the better the rubber wettability properties and adhesion between the matrix and rubber. This indicates that NaOH treatment enhanced the hydrophilic properties of rubber [16; 20; 33; 34; 35]. NaOH also triggered degradation to some of the rubber chains, which creating more functional groups. The more functional groups appeared on the rubber surface the higher rubber surface polarity. The treatment also contributes to the presence of hydrophilic elements such as carboxyl group and increases the functional O-H groups therefore improving the wettability of the rubber surface [27; 35; 36; 37]. Moreover, NaOH is known as a heavy duty cleaner that helps remove impurities on the rubber surface, such as dirt, oil, or passive layer, which prevents the matrix adhering well to the rubber. For example, insoluble zinc stearate (one of the ingredient in rubber compounds) diffused to the top and formed a passive layer on the rubber surface. NaOH also chemically converted the insoluble element in rubber formulations, such as zinc ion, into water-soluble sodium ions that could easily be washed away. Several works have been published showing that treatment with NaOH provides good rubber surface properties to improve adhesion between rubber and matrix [16; 23; 38; 39; 40; 41]. The aim of this paper is to study the effects of different sodium hydroxide concentration treatments on the rubber surface.