BOND AGENT SELECTION AND MIX DESIGN

The first task in investigating the spray technology is the selection of the bonding agent. The guideline in the selection is that the bonding agent should be a member of the rubber family with respect to its chemical compositions[8]. Following this guideline, two polymer compounds, one is acrylic and the other is alkyd, are selected because both acrylic and alkyd belong to the rubber family chemically and are commercially available. The acrylic-based compound is water-soluble and the alkyd-based compound is oil-based. A number of mixes of an acrylic compound with crumb rubber are made. It is shown that a strong bond is developed between the acrylic compound and crumb rubber. A number of mixes of an alkyd based compound with crumb rubber are also made, and, in comparison to the bonding strength between the acrylic compound and crumb rubber, an stronger bond between the alkyd compound and rubber particles is observed. From the bonding strength point of view, the alkyd-based compound is more preferable because it can better serve the purpose of having a strong bonding strength.

But, the drawback is that the spray device is filled with the residual of rubber particles "glued" with the oil based compound like alkyd after application and is problematic with respect to the cleaning which will require the use of a large quantity of chemical thinners, and take a quite amount of time. As a consequence, it is quite costly both in time and money to maintain and clean the spray devices dispersing alkyd compounds. On the other hand, only water is needed to wash the spray device after it is used in dispersing the acrylic based soluble compound. In addition, the cost for alkyd based compounds is more expensive than that of acrylic based compounds.

Therefore, the choice is made to select the acrylic-based compounds as the bonding agent. But, a thin layer of alkyd polymer will be sprayed to the panels made by the mix of the acrylic-based agent with rubber particles as a finishing coat. This thin layer will sever the purpose of coloring panels and at the same time provide a protective coating for the panel to increase the panel durability (it is typical that an acrylic based paint can last 10 years to15 years, but an alkyd based paint can last 15 years to 20 years or longer). Therefore, such a selection to have the acrylic compound as the bonding agent, and the alkyd compound used as the protective coating reflects an optimum approach in utilizing the two polymers for their contribution to serve the spraying technology.

The next task is to determine the mix ratio of crumb rubber to the bonding agent. For better sound absorption, a higher percent of crumb rubber in the mix is desirable. However, too much rubber particles in the mix will make the mix very "sticky" or "dry". As such, the flowability of the mix will be reduced so that the spraying process can become non-operable. So a balanced ratio is needed in the mix design. Based on a number of tried mixes with different ratios, it is shown that the percent of crumb rubber may vary from 50% to 80% in weight for every 100% of the bonding agent also in weight. Such a mixing ratio can generally make the spraying process workable. It is also noticed that the mixing ratio selection depends on the particle size of crumb rubber. Because the bonding agent "coats" rubber particles, when larger sized rubber particles are used, lesser amount of the bonding agent is needed to coat those rubber particles. Thus the ratio of higher percent of crumb rubber can be reached by using larger rubber particles. The size of crumb rubber used in this study varies between 0.25 mm to 4.0 mm.