Four types of spray systems are tested in making crumb rubber based panels. The first one is a Binks gun system, a traditional spray device. Binks spray systems are designed for spraying paint or a liquid with very fine particles. The test shows that the gun is workable only with Mesh #40 crumb rubber. Spraying the mix with rubber particles larger than Mesh #40 will cause the "clogging" problem because of the Binks's nozzle is very small. Therefore, Binks guns can play a minor rule for the current application without modification to the nozzle.

The second one is a hopper type of spray gun (Wal Board). The feature of this spray device is that it has a large spray tip so coarse rubber particles can easily be "pushed" through the nozzle. The testing results show that this device can work well with the mix of crumb rubber up to 4.0 mm in a laboratory environment. Most of these spray devices have a small capacity, and the future task is to develop a device that can handle a large volume of the mix.

As aforementioned, increasing the rubber ratio in the mix is favorable to enhance the acoustic absorption capability. For this purpose the third spray device, a protocol of a spray system is designed and manufactured as part of the task implemented in the current study that can spray dry rubber particles. A wet-dry spray process is then planned that this spray gun disperses dry rubber particles along with another gun spraying the mix (wet), so a high rubber content can be achieved. The wet-dry process is in its infant stage, and more tests are needed to further characterize and quantify this process.

The fourth spray device tested is a new technology called Convergent Spray TechnologyTM. This technology was originally developed in National Aeronautics and Space Administration's (NASA) Marshall Space Flight Center, Alabama to apply heat-resistant coatings to the Space Shuttle's solid rocket boosters. The feature of this spray device is that crumb rubber and the bonding agent are mixed at the time they are sprayed. That is the mix is not "pre-mixed". Conventional spray devices require that the fluid or the mix are blended first, then flow within the spray system during spraying. As such, the parameter of viscosity for the fluid or the mix is introduced and is measured to make sure that the flowability is appropriate. A fluid/mix too thick or "high viscosity" will make the fluid/mix non-workable for the spray process. But for this Convergent Spray technology, this is no longer a problem, and the consequence is that now the crumb rubber ratio can be further increased. A number of specimens are made by this technology with a 1 to 1 ratio in weight between crumb rubber and the bonding agent, and they appear of good quality. This technology really shows a promising future.