Brownfield Vapor Barriers: Chemical Compatibility, Testing, and Advances in Materials Science

A new composite membrane system, Geo-SealTM, has been developed that offers exceptional chemical resistance for use as a vapor barrier at brownfield sites. Data generated in controlled laboratory conditions indicate the composite membrane to have < 0.2X the volatile organic compound partitioning when compared to spray applied latex/asphalt vapor barriers. More importantly, data generated under both liquid and gas permeability tests indicate that the new composite membrane system limits the transmission of volatile organic vapors. Data indicated the Geo-Seal membrane to resist contaminant permeation breakthrough for a period 18X longer than that of simple asphalt/latex membranes and to allow for < 0.16X the rate of VOC permeance of the asphalt/latex membranes. INTRODUCTION Brownfield site development often requires the use of a contaminant vapor barrier to inhibit volatile organic contaminants remaining on-site from migrating into the newly constructed buildings, potentially impacting indoor air quality. Historically plastic sheet materials such as high density polyethylene, known for chemical resistance, have been applied as contaminant vapor barriers. The use of these materials, however, requires labor-intensive cutting and seaming to ensure a continuous and cohesive barrier to vapor migration. This installation process can be intensive, difficult, and costly when applied to construction foundations with multiple penetrations (e.g. piping, electrical conduits). In recent years “spray applied” latex/asphalt membrane-type waterproofing materials have been widely promoted for brownfield vapor barrier use. While easy to apply and proven to retard water migration through concrete, the use of these latex/asphalt materials for repelling volatile organic constituents (VOCs) such as benzene and chlorinated solvents may be complicated by the affinity of latex/asphalt for VOCs. It is widely recognized that asphalt/latex-based products are, in fact, highly susceptible to partitioning by VOCs, particularly chlorinated dry cleaningtype solvents. BACKGROUND Spray Applied Asphalt/Latex Membranes. Asphalt/latex membranes are chemically described as bitumen/polystyrene emulsions that are spray-applied in the presence of calcium chloride salt solutions. Simply put, the salt solution “breaks the emulsion” upon mixing when applied forming a continuous layer of bitumen-styrene as the material dries upon a surface. Depending on the exact formulation, the emulsion material may also have clay or calcium carbonate added as a “filler” or “builder” which allows for varying of key characteristics such as viscosity, flexibility, etc. Geo-SealTM Composition. Geo-SealTM (Land Science Technologies, San Clemente, CA, USA) is a unique composite membrane (patent pending) that incorporates the ease of application associated with spray applied asphalt/latex membranes with the chemical resistance, low chemical permeability, and mechanical strength of high density polyethylene (HDPE). The Geo-Seal membrane incorporates all the positive aspects of 60 mil asphalt/latex membranes plus the two outer layers of proprietary HDPE. Hydrophobic vs Lipophilic. All asphalt/latex membrane materials are hydrophobic (water repelling) due to the petroleum (bitumen) content. This is why these materials tend to have both low adsorptivity toward water (water does not partition into the membrane itself) and low permeance with regard to water vapors (very little water vapor moves through the membrane). Asphalt/latex membranes make for excellent water-proofing and damp-proofing materials. Conversely, asphalt/latex membrane materials are lipophilic (oil attracting, or nonpolar). When contacted with oils they absorb the oil. In the same fashion, non-polar VOCs like benzene or perchloroethene (PCE) tend to partition into the membrane itself. This is very well documented. In fact, this is why the “dry cleaning” industry has adopted the use of PCE to remove bitumen from clothes...the PCE partitions into the bitumen and extracts it from the fabric. Likewise gasoline is commonly used as a cleaner to remove tar. SOLVENT EXPOSURE TESTING Any solvent exposure testing relevant to the use of materials for under-slab VOC contaminant vapor barriers should test or model the true long term exposure of the barrier material to the specific contaminant of concern. In the case of testing latex/asphalt contaminant vapor barrier material for exposure to volatile organic contaminants (e.g. benzene, PCE, trichloroethene (TCE), etc.) the most important factor to consider is the long term adsorption of the contaminant into the membrane itself. Over time the lipophilic membrane material will continue to absorb contaminant until some point in the future when it reaches equilibrium and/or becomes “saturated”. The period of time required to reach saturation is dependent upon the contaminant type, its concentration in the soil pore gas, temperature, pressure, and its specific partitioning coefficient toward the specific asphalt/latex membrane under testing. The standard analytical method for solvent exposure testing is generally considered to be ASTM D-543 (ASTM D-543-06). In this test the specific membrane material (latex/asphalt) is exposed to the specific contaminant of concern (e.g. PCE) within the specific medium of concern (air) for a period of 7 days. The amount of weight gained by the membrane is a direct measure of the absorption of the contaminant by the membrane material. When little absorption occurs it can be said that there is little reactivity or change of the membrane with exposure. This test however, will only indicate the absorption (partitioning) which occurs within the 7 day period when the membrane is subjected to the contaminant at the specific concentration tested. It does not indicate the total potential absorption (partitioning) that may occur over the lifespan of the membrane in an actual field application. In order to understand the long term effects of a membrane’s exposure to solvents one has to either 1) test the membrane under low volatile organic vapor (VOC) concentrations for an extended period of timeuntil the partitioning equilibrates (this could be many years depending on how low the vapor concentration is) or 2) run the test at very high concentrations to ensure saturation within the test period. At the point of saturation with VOCs, asphalt/latex membranes show very different characteristics, particularly with regard to VOC permeation, weight, dimensions, and tensile strength. It is widely known that unprotected asphalt/latex membranes absorb significant contaminant vapors as the VOC partitions into the bitumen fraction of the membrane itself. Eventually this leads to saturation of the membrane, membrane swelling, softness, etc. General Asphalt/Latex Solvent Exposure Testing. In work conducted by an independent laboratory experienced in asphalt/latex membrane formulation, ASTM D543 was conducted on varying formulations in the presence of hexane vapors. Specific formulations and test results are presented below in Table 1. It is appropriate to note that in all of the varying formulations a weight gain of greater than 10% was observed indicating that asphalt/latex membranes by their very chemical makeup absorb (partition) VOC vapors when properly exposed to the VOC. TABLE 1. VOC Solvent Exposure Testing of Various Asphalt/Latex Membranes By ASTM D-543 Employing Hexane Vapors*