Tech Briefs

Study attempts to determine if polypropylene nanofiber materials can be used in air filtration systems to remove toxic vapors.

The Toxic Industrial Chemical/Toxic Industrial Material (TIC/TIM) Task Force MFR#1 published in February 2009 focuses on inhalation hazards in an operational environment and provides a list of compounds prioritized based on toxic hazard and the likelihood of an encounter. With these types of vapor threats, cartridge-based air purifying respirators are used to protect the warfighter against chemical exposure. Traditional air purification materials often rely on porous carbons such as activated carbon or activated charcoal. Ongoing efforts seek to improve the performance of carbon materials in air purification applications as well as provide alternative materials.

For this study, polypropylene nanofiber materials provided by Apollo NanoTech Inc were evaluated for their potential use in removing vapor phase targets. A Thermolyne incubator (Compact Series 5000) was modified to conduct water vapor transport studies based on guidance provided by the ASTM E 96 protocol. Water vapor transport through a material is determined by measuring the rate of water loss through the material over a period of time.

Images of as received nanofiber materials: hydrophilic (A), hydrophobic (B), and sheet type (C) variations.

A scintillation vial (20 mL) was filled with 16.9 mL deionized water (± 0.1 mL) over which the sample material was sealed with parafilm (water 1.27 cm from sample surface). The sample was weighed and placed in the incubator. Drierite was used to lower the humidity in the incubator and a dry nitrogen stream was flowed across the surface of the sample (250 sccm). Weight measurements were collected at 30 to 45 min intervals using an analytical balance. The temperature of the incubator was 25°C (±1°C). This instrument was used to evaluate permeation of water through the various functionalized fabrics.

The temperature in the custom environmental chamber was controlled using a probe inside the chamber that adjusts an Air-Therm ATX heater. Mass flow controllers, regulated by an inline Vaisala humidity probe, governed the ratio of humid to dry air entering the chamber. An Aerosol Vapor Liquid Assessment Group (AVLAG) test cell was used for these evaluations.

The AVLAG cell was set up for single flow diffusive penetration testing using a single air or nitrogen stream. The “headspace” above the swatch was stagnant, and the differential pressure above and below the swatch was zero. A sample (2.54 × 2.54 cm) was sandwiched between two supports with 0.64 cm2 circular openings. The sample assembly was placed in the AVLAG cell and equilibrated to the desired humidity for 2h. Target was introduced by placing liquid drops on top of glass wool using a repeating dispenser. Challenge was applied to the surface of the sample in the static region of the AVLAG cell; therefore, evaporation was not a significant consideration. A direct line from the permeation cell to a dedicated FID allowed for continuous monitoring of target concentrations. The FID used Peak Simple, six-channel data acquisition software (SRI) for signal capture and peak integration. Excess flow from the direct line (above 50 mL/min) was filtered through a carbon scrubber.

Microwave modification of fabrics was used for modification of the polypropylene sheets. The initiation solution was prepared by mixing 5 mL ammonium hydroxide (28 – 30%) with 92 mL of isopropanol. To this solution, 3 mL tetraethyl orthosilicate (TEOS) was added to the ammonium hydroxide solution. The fabric substrate was fully submerged in the TEOS mixture and removed to a glass, microwave safe dish. The sample was microwaved using 1,200W for 30s. This process was repeated for a total of three cycles. Treated fabric was dried at 100°C for 30 min.

To prepare the sol, 1.9g Pluronic P123, 0.5g mesitylene, and 2.12g 1,2-bis (trimethyoxysilyl)ethane (BTE) were mixed with 16g ethanol at room temperature with a magnetic stir bar in a sealed container. At this point, 6.07g 0.1 M HNO3 was added dropwise, and stirring continued for 6h. The TEOS treated fabric was dipped into the prepared sol at a rate of 270 mm/min. The sample was hung to dry in a 60°C oven for 24h followed by drying in a vacuum oven at 60°C for an additional 24h. The fabric sample was then immersed in ethanol at 60°C for 48h to extract surfactant.

The sample was rinsed with additional ethanol and dried overnight at 60 – 65°C. To functionalize the sorbent material with primary amine groups, the fabric was submerged in a solution of 3-aminopropyltriethoxy silane (APS) in toluene at 0.5% volume/volume for 1h. Samples were then rinsed thoroughly with toluene and dried at 100°C. The porphyrin was added to this sample by submerging in a solution of 0.6 mg/mL porphyrin in 0.1 M 2-(N-morpholino) ethansulfonic acid (MES) buffer pH 5.5 with 5 mg 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC). Samples were incubated overnight before rinsing thoroughly with water and drying at 100°C overnight.

Samples were evaluated using the permeation system with 2-chloroethyl ethyl sulfide (half mustard; CEES), dimethyl methylphosphonate (DMMP), and methyl salicylate (MES) as the targets. Evaluations used 1 μL of the targets. The total exposed area in the AVLAG system was 0.64 cm2 providing surface exposure concentrations of 16.7 g/m2 for CEES; 18.0 g/m2 for DMMP; and 18.3 g/m2 for MES.

This work was done by Brandy J. White, Martin H. Moore, and Brian J. Melde for the Naval Research Laboratory. NRL-0074

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Permeation Tests on Polypropylene Fiber Materials (reference NRL-0074) is currently available for download from the TSP library.

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