Contaminant Fate and Transport : Film Permeability

Film Permeability

Film Index

Plastic tarps are used during soil fumigation to control fumigant emissions. They have been shown to be permeable to fumigant vapors, resulting in appreciable losses to the atmosphere. New low-permeability films are being developed to reduce fumigant emissions and increase chemical efficacy. A rapid, reliable and sensitive method is required to measure the permeability of various films.

An approach has been developed for estimating the mass transfer coefficient (h) of fumigant compounds diffusing across agricultural films. The h is a measure of the resistance to diffusion which, unlike other measures of permeability, is a property of the film-chemical combination and independent of the concentration gradient across the film. This method uses static sealed cells; fumigant vapor is spiked to one side of the film and the concentrations on both sides of the film are monitored until equilibrium. An analytical model is fitted to the data to obtain h. This model relies on a mass balance approach, and includes sorption to and diffusion across the film membrane. The method was tested using two polyethylene films and a very low-permeability film, and showed that the method produces a sensitive and reproducible measure of film permeability.

Film Table

Table 1. Mass transfer and sorption coefficients for fumigants diffusing through plastics at 20 °C, measured using triplicate static permeability cells ± standard error of the estimate. Values for α and kp in parenthesis are determined independently using Eq. 16 and 17; values of h in parenthesis are determined by nonlinear regression with these values of α and kp fixed.

	Methyl Bromide	Propargyl Bromide	Chloropicrin
1-mil HDPE
h (cm h^-1)	0.37±0.02	1.48±0.08	0.65±0.06
α (h^-1)	0.5±0.4	0.4±0.2	0.9±0.3
kp (cm)	0.3±0.1	1.2±0.02	1.8±0.2
4-mil black HDPE
h (cm h^-1)	0.14±0.01	0.48±0.02	0.25±0.03
α (h^-1)	0.21±0.09	0.13±0.04	0.32±0.06
kp (cm)	1.3±0.2	1.3±0.2	3.8±0.3
silver mylar†
h (cm h^-1)	<4.4×10-6	<1.3×10-5	<2.3×10-4
α (h^-1)	2.1±0.9	1.2±0.4	0.8±0.3
kp (cm)	1.09±0.07	5.2±0.4	11±2

†No permeation through the film detected; reported h is based on detection limit and Eq. 18. Regression to model to determine α and kp fixed h at the detection limit and C0 at 100%.

References

Yates, S.R., Gan, J., Ernst, F.F., Wang, D. and Yates, M.V. 1997. Emission of methyl bromide from agricultural fields. Rate estimates and methods of reduction. In: Fumigants: Environmental Fates, Exposure and Analysis, American Chemical Society Symposium Series No. 652, J.N. Seiber, J.A. Knuteson, J.E. Woodrow, N.L Wolfe, Yates, M.V. and S.R. Yates, editors, pp. 116-134.

Wang, D., Yates, S.R., Gan, J. and Jury, W.A. 1998. Temperature effect on MeBr volatilization: Permeability of plastic cover films. Journal Environmental Quality 26:821-827.

Papiernik, S.K., Gan, J., Knuteson, J.A. and Yates, S.R. 1999. Sorption of fumigants to agricultural films, Environmental Science and Technology 33:1213-1217.

Wang, D., Yates, S.R., Gan, J. and Knuteson, J.A. 1999. Atmospheric volatilization of methyl bromide, 1,3-dichloropropene, and propargyl bromide through two plastic films, transfer coefficient and temperature effect, Atmospheric Environment 33:401-407.

Papiernik, S.K., Yates, S.R. and Gan, J. 2001. An approach for estimating the permeability of agricultural films. Environmental Science and Technology 35:1240-1246.

Papiernik, S.K., Ernst, F.F. and Yates, S.R. 2002. An apparatus for measuring the gas permeability of films. Journal of Environmental Quality 31:358-361.

Papiernik, S.K. and Yates, S.R. 2002. Effect of environmental conditions on the permeability of high density polyethylene film to fumigant vapors. Environmental Science and Technology 36:1833-1838.

Last Modified: 11/04/2009