There are alternatives to styrene and methyl methacrylate (MMA), however each of these replacement monomers are accompanied with a unique set of performance, regulatory and health issues that have inhibited wide-spread adoption across the industry. The result is that styrene and MMA remain the predominate constituents in open molding resins.
Optimizing Application Techniques – Addressing the emissions issues with spray application is a primary strategy in emissions reduction. This involves three elements:
- Controlling spray gun pressure to maximize transfer efficiency and minimize overspray;
- Training operators in optimized spraying techniques;
- Using containment flanges to capture overspray.
These optimization techniques are detailed in ACMA’s Controlled Spraying Handbook.
Spraying emissions are based on the evaporative surface area of the resin spray pattern. When atomizing a fluid stream the particle size distribution has a major effect on the evaporation of volatile components. As fluid tip pressures increase, the exiting resin stream is transformed into smaller droplet sizes. As the particle size decreases (more atomization) the spray pattern surface area increases in a non-linear fashion, resulting in a dramatic increase in volatile surface area. As described in the handbook, spray equipment should always be set up to operate at the lowest pressure that produces an acceptable spray pattern. Minimizing atomization is a primary factor in reducing spray application emissions.
Taking this concept one step further, the use of non-atomized application equipment develops the reduction in volatile surface area to the next level. A properly configured non-atomized spray gun produces a coherent flow stream or larger ligaments (extended droplets) that minimize resin surface area.
Operator training is another key component with the controlled spraying application technique. The position of the spray gun, bounding the mold perimeter and spray pattern angle relative the mold surface are contributing factors to optimizing the process.
The third element of controlled spraying is the use of containment flanges around the mold perimeter. Eliminating the spread of off-mold overspray reduces the volatile surface area of the applied resin, yielding substantial emissions reductions.
The advent of robotic application for spraying operations adds another element of optimization to the process. Once a robot is set up within the parameters of controlled spraying and “taught” the proper spray path sequence, the application consistency yields benefits. Over time, the accuracy of transfer efficiency can accumulate into measurable emissions reductions.
Resin Additives – The use of styrene suppressant additives is another significant strategic option for optimizing emissions reduction. While suppressants have little effect during the spraying or roll-out phases of the process, they are very effective during the quiescent (static) phase of curing. Suppressant additives must be carefully matched to specific resin formulations and testing conducted to arrive at a supportable emissions factor.