The two major commercial methods for making gypsum board water resistant are through incorporation of hydrocarbon wax emulsions or polymethylhydrogensiloxane (PMHS) into the gypsum slurry during board manufacture. While the chemistry and water resistance (WR) mechanisms of these two additives are quite different, they accomplish the same basic task in gypsum board. Any effective WR additive must reduce the level of water absorbance from over 40% for non-WR board to the ASTM specifications of less than 10% for Gypsum Sheathing and less than 5% for Water Resistant Gypsum Backing Board. Currently the US market share by plant of wax emulsions and PMHS is about 55% and 45% respectively1. While both additives enjoy significant utilization in the industry, their overall environmental impact and worker safety aspects are dramatically different. In order to better understand these differences, Henry has recently published a technical white paper covering the EH&S considerations of both additives2. In this paper the chemistries and applications of wax emulsion, siloxane, gypsum and their interactions in the gypsum wallboard manufacturing process are discussed. In order to better understand quantitatively the differences in volatile organic content (VOC) contribution, a new laboratory method for comparison of VOC emissions during a simulated wallboard drying schedule was created. This method requires a special oven apparatus that was designed and fabricated to simulate EPA's Method 25 for testing plant stack emissions. Panel air emissions of VOCs over a typical plant drying schedule were determined and the types and quantities of air pollutants were identified. Although panels made with both wax emulsions and siloxane generated VOCs during the kiln drying process, the contribution of VOC emissions levels from siloxane panels were substantially higher, by as much as a factor 100 times during peak emissions of the wax emulsion containing panels. In addition, the VOCs created by siloxane contained a number of Hazardous Air Pollutants (HAPS) that may be a concern. It was also observed from various literature sources that siloxane contributed significantly to emissions of particulate matter (PM) and more importantly PM fewer than 10 microns (PM10), while the use of wax emulsions does not contribute to PM emissions in the wallboard process. According to the EPA, PM10 poses a health concern because it can be inhaled into and accumulate in the respiratory system. Particles less than 2.5 micrometers in diameter (PM2.5) are referred to as "fine" particles and are believed to pose the greatest health risks. Because of their small size, fine particles can lodge deeply into the lungs where they more vulnerable to injury. Roughly one out of every three people in the United States is at a higher risk of experiencing PM2.5 related health effects. This paper concluded that in situations where government emissions permitting is a concern for a specific manufacturing plant, wax emulsion use as a WR agent is recommended over siloxanes. Plants attempting to switch from wax emulsions to siloxanes need to address the specific HAPS created during the drying process, total VOCs emitted in order to stay in compliance with local EPA standards, and make efforts to mitigate the increased PM emissions (especially PM10 and less) otherwise air permit modifications may be required.
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