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Currently, the two major commercial methods to make gypsum wallboard water resistant (WR) are by incorporating hydrocarbon wax or polymethylhydrogensiloxane (PMHS or siloxane) into the gypsum slurry during board manufacture. In this paper, the chemistries and applications of wax, siloxane, gypsum and their interactions in the gypsum wallboard manufacturing process are discussed. A new laboratory method for comparisons of Volatile Organic Compound (VOC) emissions during a simulated wallboard drying schedule is also presented with data for a variety of siloxanes and wax emulsion. In addition, regulatory burdens associated with storage, handling and disposing unused siloxane is considered.
An emissions testing oven was designed and fabricated that replicated United States Environmental Protection Agency (EPA) Method 25 testing of plant stack emissions. Panel air emissions of VOCs over a typical plant drying schedule were determined and the types of air pollutants were identified at a third-party test site in Research Triangle Park, North Carolina. Second and third blocks of testing were done at the Henry Research & Development Center in Kimberton, Pennsylvania with samples sent to additional third parties for analysis of the air pollutants collected, in particular siloxane-related compounds.
Although panels made with wax or siloxane generated VOCs during the kiln drying process, the contribution of VOC emission levels from siloxane panels were substantially higher by as much as a factor ten times that of the wax-containing panels. The higher emissions are thought to be due to reactions and subsequent volatilization that occurs as the siloxane raw material is converted to the WR silicone form during the gypsum wallboard manufacturing process. Based on reviews of Title V EPA documents of wallboard manufacturers, emissions of particulate matter (PM) and PM under ten microns (PM 10) are also significantly increased when using siloxane (specifically particles at 2.5 microns and smaller, which are defined by the EPA as “fine particles”).
If government emissions permitting, and storage and disposal requirements are a concern for a manufacturing plant, wax emulsion use as a WR agent is recommended over siloxanes. Plants switching from wax emulsions to siloxane may need to decrease VOCs from other plant sources to stay in compliance with local EPA standards. In addition, mitigating any increased PM emissions to avoid air permit modifications and devoting resources to properly handle and dispose of unused siloxanes may be necessary.