Evaluation of the Permeability of Agricultural Films to Various

Oct 5, 2011 - Yaorong Qian†*, Alaa Kamel†, Charles Stafford†, Thuy Nguyen†, William J. .... David M. Butler , Nancy Kokalis-Burelle , Joji Mur...
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Evaluation of the Permeability of Agricultural Films to Various Fumigants Yaorong Qian,†,* Alaa Kamel,† Charles Stafford,† Thuy Nguyen,† William J. Chism,† Jeffrey Dawson,‡ and Charles W. Smith‡ †

Biological and Economic Analysis Division and ‡Health Effects Division, Office of Pesticide Programs, U.S. Environmental Protection Agency

bS Supporting Information ABSTRACT: A variety of agricultural films are commercially available for managing emissions and enhancing pest control during soil fumigation. These films are manufactured using different materials and processes which can ultimately result in different permeability to fumigants. A systematic laboratory study of the permeability of the agricultural films to nine fumigants was conducted to evaluate the performance of commonly used film products, including polyethylene, metalized, and high-barrier films. The permeability, as expressed by mass transfer coefficient (cm/h), of 27 different films from 13 manufacturers ranged from below 1  10 4 cm/h to above 10 cm/h at 25 °C under ambient relative humidity test conditions. The wide range in permeability of commercially available films demonstrates the need to use films which are appropriate for the fumigation application. The effects of environmental factors, such as temperature and humidity, on the film permeability were also investigated. It was found that high relative humidity could drastically increase the permeability of the high-barrier films. The permeability of some high-barrier films was increased by 2 3 orders of magnitude when the films were tested at high relative humidity. Increasing the temperature from 25 to 40 °C increased the permeability for some high-barrier films up to 10 times more than the permeability at 25 °C, although the effect was minimal for several of these films. Analysis of the distribution of the permeability of the films under ambient humidity conditions to nine fumigants indicated that the 27 films largely followed the material type, although the permeability varied considerably among the films of similar material.

’ INTRODUCTION Preplant soil fumigation is often used for broad spectrum pest control in many high-value crops. Soil fumigants are applied to control weeds, plant pathogens, nematodes and insects. However, soil fumigants are susceptible to rapid emission after being applied to soil due to their high volatility. High levels of fumigant emissions may endanger the health of workers and bystanders and also contribute to volatile organic compounds in the air that adversely affect air quality.1 Use of agricultural plastic films (tarps) to cover the treated field after fumigation has been a common practice to reduce the fumigant emission and to minimize worker exposure to the fumigants. Recently, the U.S. Environmental Protection Agency (U.S. EPA) established a set of regulations requiring a suite of complementary mitigation measures to protect handlers, reentry workers, and bystanders from risks resulting from exposure to the soil fumigants.1 Among the requirements is the need for a buffer zone between treated and untreated sites to allow airborne fumigant residues to disperse before reaching bystanders. U.S. EPA is also granting “buffer zone credits”, which reduce buffer distances, to encourage users to employ practices that reduce emissions, such as the use of high-barrier tarps.1 This article not subject to U.S. Copyright. Published 2011 by the American Chemical Society

The benefits of using high-barrier films during fumigation have been documented in many studies.2 7 However, various agricultural films from many different manufacturers have been used by farmers and commercial organizations during the fumigation of soils to control the emission of fumigants and to retain fumigants in the soil for longer periods.5,8 11 Available films include low-density polyethylene (LDPE), high-density polyethylene (HDPE), metalized films, and multilayer films with imbedded barrier materials (e.g., polyamide and ethylene vinyl alcohol-EVOH), the latter often designated as virtually impermeable films (VIF) or totally impermeable films (TIF). The permeability of the films to fumigants varies widely with material composition, manufacturing technique, and manufacturer. In addition, the manufacturer assigned name designations (e.g., VIF, TIF) are somewhat arbitrary and permeability of these films may not directly correlate with their designations. Many factors including field conditions, temperature, fumigant type, film Received: May 26, 2011 Accepted: October 5, 2011 Revised: September 27, 2011 Published: October 05, 2011 9711

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Table 1. List of Agricultural Films Submitted by Various Manufacturers and Vendors Film Name

Description

Manufacturer

polyethylene films (PE): AEP Sun Film High Barrier

1.0 mil, clear polyethylene

AEP Inc.

Cadillac HDPE

1.25 mil, clear

Cadillac Products Packaging Co.

Canslit Embossed HDPE

0.6 mil, black

Canslit Inc. /Imaflex Inc.

Canslit Embossed LDPE

1.25 mil, black

Canslit Inc. /Imaflex Inc.

Pliant Embossed LDPE

1.25 mil, embossed LDPE

Pliant Corp

Canslit Metalized Canslit Metalized

1.25 mil black/silver 1.25 mil white/silver

Canslit Inc. /Imaflex Inc. Canslit Inc. /Imaflex Inc.

Pliant Metalized

1.25 mil, black/silver

Pliant Corp

Cadillac VIF

1.25 mil, black

Cadillac Products Packaging Co.

Can-Block VIF

0.8 mil, black

Canslit Inc./Imaflex Inc.

FilmTech VIF

1.25 mil, black

FilmTech Corp.

Ginegar Ozgard

1.25 mil, black

Ginegar Plastic Products, Ltd.

Ginegar VIF Guardian Olefinas VIF

1.25 mil, embossed black 1.2 mil, embossed black

Ginegar Plastic Products, Ltd. Guardian Agroplastics Olefinas USA

MidSouth VIF

1.25 mil, embossed black

Mid South Extrusion

Pliant Blockade Black

1.25 mil, black

Pliant Corp

Pliant Blockade White

1.25 mil, white/black

Pliant Corp

Metalized Films:

Virtually Impermeable Films (VIF):

Totally Impermeable Films (TIF): AEP-One

EVOH barrier, 1.0 mil, Clear

AEP Inc.

BayFilm

2 mil, black, contains halosulfuron-methyl

Bayer Innovation

Berry EVOH-High Barrier Berry High Barrier w/improved toughness

EVOH barrier, black EVOH barrier, black

Berry Plastics Berry Plastics

Berry EVOH-Supreme Barrier

EVOH barrier, black

Berry Plastics

Dow SARANEX A

black

Dow Chemical Co.

Dow SARANEX B

black

Dow Chemical Co.

Klerks/HyPlast TIF

clear

Klerk’s Plastic/HyPlast

Raven TIF VaporSafe

1.0 mil, EVOH barrier, clear

Raven Industries Inc.

Raven TIF VaporSafe

1.4 mil, EVOH barrier, black

Raven Industries Inc.

stretching, and gluing in the field, can potentially alter the permeability of the films as well.12,13 Previous studies have described analytical approaches for determining the permeability of agricultural films 14,15 and attempts have been made to characterize the films based on their permeability.16 18 However, because of the lack of a standard method for classifying the available films according to their permeability to fumigants and because of the limited available data on the permeability of these films to fumigants, it has been difficult for the regulatory agencies and fumigant applicators to systematically evaluate the permeability of the films on the market and to reliably develop buffer zone credits. In order to mitigate this uncertainty, a systematic laboratory test for the film permeability of commonly used agricultural films against several fumigants was conducted to establish a database of the film permeability for those fumigants. The effects of temperature and humidity on the film permeability to fumigants were also investigated. Permeability is expressed as a “mass transfer coefficient” (MTC) which is dependent only upon the properties of the film, the properties of the fumigant, and physical conditions of the environment (such as temperature and humidity) and are independent of compound concentrations within the test system.13 15 Results from this study could assist the management of fumigant emissions by growers and fumigant applicators

and help regulators in the evaluation of the benefits of agricultural film usage, assessment of fumigation risks, and reduction of the uncertainties in potential buffer zone credit calculations.

’ EXPERIMENTAL SECTION The test method used in this study was adapted from a previously published technique.14,15 The test apparatus consists of an airtight cylinder constructed from two stainless steel endcaps (chambers) separated by the test film. The basis for the test is that as fumigants penetrate the film over time, fumigant concentrations in the source chamber will decline while concentrations in the receiving chamber will increase until equilibrium is achieved. Test compounds were introduced into the chamber on one side of the film (source chamber). The concentrations of the test compounds from both sides of the film (source and receiving chambers) were monitored over time by analyzing the vapor in each chamber. The rate of change in the fumigant concentrations over time was used to calculate the mass transfer coefficient (MTC) of the film for each target compound as described previously.14,15 Agricultural Films and Fumigants. Twenty seven samples of agricultural films were obtained from 13 manufacturers or their authorized vendors in 2009 and 2010. The names of the tested 9712

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Table 2. Mass Transfer Coefficient (MTC, cm/h, Average of Triplicates) of 27 Films to Various Fumigants at 25°C and Ambient Humidity (Relative Humidity of 35%-45%) in the Laboratory MeBr

IOM

PPO

1,3 D, cis

1,3-D, trans

DMDS

MITC

PIC

SF

AEP Sun Film High Barrier Cadillac HDPE

0.8238 0.4800

1.4124 0.8157

0.5250 0.2896

4.2344 2.6985

5.6792 3.8916

3.8840 2.3719

8.5298 7.3816

1.5404 0.7104

0.0107 0.0060

Canslit LDPE

0.6269

1.0387

0.4189

3.2106

4.4027

2.9652

8.1979

1.2124

0.0089

Canslit HDPE

1.3611

2.1783

0.8553

6.3171

7.9845

5.6798

13.4456

2.2558

0.0200

Pliant Regular LDPE

1.1078

1.9215

0.7473

5.6506

7.3937

5.2480

11.7678

2.2710

0.0157

Canslit Metalized Black

0.0217

0.0360

0.0124

0.1286

0.1982

0.1107

0.3696

0.0362

0.0003

Canslit Metalized White

0.0185

0.0313

0.0111

0.1153

0.1796

0.1031

0.3030

0.0342

0.0002

Pliant Metalized

0.0570

0.0876

0.0359

0.3469

0.6251

0.2771

1.1435

0.0828

0.0006

PE Films

Metalized Films

VIFs Cadillac VIF

0.0085

0.0061

0.0053

0.0109

0.0232

0.0060

0.1093

0.0011

0.0020

Can-Block

0.0047

0.0018

0.0012

0.0012

0.0036

0.0008

0.0366

0.0001

0.0003

FilmTech VIF

0.0029

0.0015

0.0011

0.0019

0.0052

0.0009

0.0512

0.0001

0.0000

Ginegar VIF

0.0053

0.0030

0.0023

0.0033

0.0074

0.0017

0.0592

0.0005

0.0007

Ginegar Ozgard

0.0019

0.0007

0.0006

0.0005

0.0019

0.0003

0.0180

0.0001

0.0000

Guardian Olefinas VIF

0.0151

0.0109

0.0089

0.0235

0.0523

0.0120

0.2928

0.0016

0.0000

Mid South VIF Pliant Blockade, Black

0.0017 0.0045

0.0008 0.0020

0.0019 0.0013

0.0024 0.0022

0.0047 0.0050

0.0021 0.0013

0.0097 0.0527

0.0000 0.0010

0.0000 0.0001

Pliant Blockade, White

0.0057

0.0027

0.0015

0.0025

0.0067

0.0010

0.0823

0.0001

0.0001

AEP-One

0.0001

0.0000

0.00004

0.0002

0.0003

0.0002

0.0003

0.0002

0.0000

BayFilm

0.0001

0.0001

0.0001

0.0007

0.0009

0.0006

0.0016

0.0005

0.0001

Berry High Barrier

0.0000

0.0000

0.0000

0.0001

0.0001

0.0000

0.0002

0.0001

0.0000

Berry High Barrier w/improv toughness

0.0000

0.0001

0.0000

0.0001

0.0002

0.0002

0.0004

0.0000

0.0000

Berry Supreme Barrier Dow Saranex (A)

0.0009 0.0006

0.0001 0.0006

0.0002 0.0003

0.0002 0.0008

0.0003 0.0011

0.0002 0.0004

0.0002 0.0051

0.0002 0.0007

0.0000 0.0000

Dow Saranex (B)

0.0002

0.0002

0.0001

0.0003

0.0005

0.0002

0.0021

0.0002

0.0000

Klerks/HyPlast

0.0010

0.0001

0.0002

0.0000

0.0002

0.0000

0.0097

0.0000

0.0000

Raven VaporSafe 1.0 mil

0.0000

0.0000

0.0000

0.0000

0.0000

0.0000

0.0000

0.0000

0.0000

Raven VaporSafe 1.4 mil

0.0001

0.0001

0.0001

0.0002

0.0003

0.0002

0.0005

0.0002

0.0000

TIFs

films and their manufacturers are listed in Table 1. Fumigant standards were obtained from several sources. Methyl bromide (MeBr) was obtained from Chemtura Corp (Williamston, SC). Iodomethane (IOM), 1,3-dichloropropene (1,3-D, mixture of cis and trans isomers), dimethyl disulfide (DMDS), propylene oxide (PPO), and methyl isothiocyante (MITC, transformation product of metam sodium or dazomet during fumigation) were purchased from Sigma-Aldrich. Chloropicrin (PIC) was obtained from Arysta LifeScience (Cary, NC). Sulfuryl fluoride (SF) was obtained from Dow Chemical (Indianapolis, IN). Procedures. Films were tested initially under ambient laboratory humidity conditions, with the relative humidity ranging from 35 to 45%, and repeated under high relative humidity conditions to simulate the near saturated moisture conditions expected under a tarp in the field. Tests were carried out in a temperature controlled environmental chamber at 25 °C ((0.5 °C). Select VIFs and TIFs were tested again at 40 °C ((0.5 °C) under ambient humidity conditions. To obtain the high humidity conditions, 1 mL of distilled water was added to the source side of the permeability test cells and equilibrated overnight before adding fumigants in the cells. A visible amount of liquid water remained present in the cells at the end of each test, indicating that high humidity was maintained for the duration of the test.

The effective relative humidity in the source side of the permeability chamber was determined in a separate apparatus using a hand-held NIST (National Institute of Standards and Technology) traceable hygrometer and the relative humidity was found to be approximately 90% ((2%). Samples of agricultural films were placed between the two halfcells and the cells were joined together by epoxy glue to form a gastight seal. Aluminum tape was then applied to the outside of the cells to provide additional support and sealing of the apparatus. The cells were placed inside the temperaturecontrolled environmental chamber and equilibrated before fumigants were introduced. Generally, triplicate permeability cells were constructed for each film type and the calculated MTCs were the average of the triplicates. At the beginning of each test, about 30 40 mL of premixed fumigant vapor was placed into the source side of each permeability cell. Excessive pressure was released through a valve installed on the permeability cell. During the course of testing, gas samples (250 μL) from both receiving and source sides of the permeability cells were collected periodically and fumigant concentrations were measured. The fumigant concentrations in collected samples were measured with an Agilent 6890 gas chromatograph/5973 mass spectrometer system (GC/MSD) 9713

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Figure 1. Cross plot of the MTCs for MeBr and IOM of the 27 films at 25 °C and ambient humidity. Logarithmic scales are used to display the low MTC values. Films are grouped by manufacturer designation and reflect a range of MTC values within each group.

in the select ion monitoring (SIM) mode interfaced with an Agilent headspace autosampler. The temperature settings for the headspace autosampler were 80 °C (oven), 90 °C (loop), and 100 °C (transfer line). The GC column used was a DB-624 column (30 m  0.25 mm ID, 1.4 μm film thickness). The GC oven temperature was programmed to hold for 3 min at the initial condition of 40 °C. The temperature was then increased to 50 at 10 °C/min, held for 10 min, followed by raising the temperature to 110 °C at a rate of 20 °C/min. The MTCs were calculated based on the compound responses at each sampling time using the film permeability calculator (FilmPC v1.0.2), the permeability calculation software developed and provided by Scott Yates of the Agricultural Research Service, U.S. Department of Agriculture (ARS, USDA) at Riverside, CA. Most of the films tested were designated as VIF and TIF. The testing period for each of these films lasted up to 12 days. At the end of the testing period for some TIF films, some fumigants were still not detected in the receiving chamber of the permeability cells, or the detectable amount was so low that the calculated MTC was below 1  10 4. The calculated MTCs in these cases were treated as zero. The relative standard deviation for replicate analyses was, in general, within 30% for the MTCs that were above 0.001 cm/h, but usually higher for films with very low MTCs.

’ RESULTS AND DISCUSSION 1. Mass Transfer Coefficients of Films for Various Fumigants. The MTC results of the 27 films from thirteen manufacturers are listed in Table 2. The results clearly show that different films possess very different permeability for each compound. For example, the MTC for MeBr ranges from below 1  10 4 to 1.2 cm/h for the 27 different films tested, a range spanning 5 orders of magnitude (Table 2). The low density polyethylene (LDPE) films and high density polyethylene (HDPE) films, in general, have high permeability to all tested fumigants. Metalized PE films have lower permeability than the LDPE and HDPE films. The multilayered VIFs (usually with a polyamide barrier layer) generally have low permeability. The VIFs from different manufacturers, however, have vastly different MTC values. For example, the MTC for MeBr ranges from 0.0017 to 0.0151 cm/h for the nine VIFs, a 10-fold range, even though the MTC value is small (Table 2). The large

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Figure 2. Mass transfer coefficients (MTCs) of 27 tarps for different fumigants at 25 °C and ambient humidity. The MTCs of VIF and TIF films are low and lay nearly along the x-axis.

range in the MTCs among the VIFs for all the fumigants suggests that the permeability of the films with similar name designations, and with supposedly similar material and construction, can differ substantially in their permeability. The MTCs for TIFs (typically with an EVOH barrier layer) are generally lower than that of VIFs, with the MTC for MeBr as an example ranging from 1  10 4 to 0.0010 cm/h, a range of more than 2 orders of magnitude. The permeability variations among the different films are in agreement with previous reports.13,18 Because of the vastly different permeability of the films to various fumigants, it is difficult for users and regulators to estimate the emissions of fumigants in field applications without conducting detailed permeability determination of each film used in the field. In order to simplify the effort of estimating the fumigant emissions, managing the exposure risk, and granting relief from fumigant buffer zones (e.g., in the form of credits) if users opt to use less permeable films, the films may be broadly separated into different categories based on the distribution and clustering of the MTCs of various films to the fumigants. Figure 1 is a cross-plot of the MTC values of the 27 films for MeBr and IOM. The separation of films generally follows the manufacturer designations, for example, PE films are within one group with the highest permeability, metalized films are in another group with moderate permeability, and VIF films are in one group with low permeability. Although the separation is not distinct in the values of MTC for MeBr or IOM alone between VIF and TIF, all TIF films with the lowest MTC values can be grouped together using the combination of MeBr and IOM (Figure 1). The separations of these films using different fumigant combinations are generally similar. A more elaborated clustering analyses of the MTC values of individual fumigants and a composite value of all fumigants using a statistical approach produced similar groupings.19 Although the permeability of each film to different fumigants differed from each other, they generally track with each other among the different films (Figure 2). For example, sulfuryl fluoride was generally the slowest to permeate through a film compared to other fumigants. The calculated MTC of the films for SF ranged from below 1  10 4 to 0.0200 cm/h. MITC was the fastest fumigant to permeate through any film compared to other fumigants. The MTC for MITC in the LDPE and HDPE was as high as 13 cm/h. Calculated film permeability was generally similar for MeBr, IOM, PIC, and PPO. The MTCs for DMDS and both isomers of 1,3-D were generally higher than 9714

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Figure 3. Examples of time needed for a fumigant (25 °C and ambient humidity) to reach equilibrium across a film with MTCs of approximately (a) 10, (b) 1, (c) 0.1, and (d) 0.01 cm/h. The upper curve and data points indicate the amount of the fumigant in the source chamber (100% at time zero) at different sampling times after fumigant is placed in the source chamber. The lower curve and data points indicate the amount of fumigant in the receiving chamber (0% at time zero).

other fumigants for the same films (except MITC) though the trans-1,3-D generally moved faster through a film than the cisisomer. Similar patterns in the rate of permeation among different fumigants were also observed in another study.18 As a result, the groupings of films can be represented by select fumigants, such as MeBr-specific grouping or a global grouping using composite value for all fumigants.19 MTC and Emission Rate. To better understand the relationship between the MTC and the emission of fumigants through the agricultural films, the time needed for a fumigant to reach near equilibrium across the film in the source and receiving chambers in the laboratory is shown in Figure 3 for films with MTC values of 10, 1, 0.1, and 0.01 cm/h, a typical range for PE film, metalized film and VIF film. In theory, it will take an infinite amount of time to reach true equilibrium on both sides of the film.15,18 In practice, when the process reaches about 95% of the true equilibrium, this status can be considered near equilibrium.18 This practically defined end point is generally within the current analytical limitations and uncertainty of the techniques used to determine the target compounds and the MTC of the films. For a compound to reach near equilibrium across a film with a MTC of about 10 cm/h (e.g., the PE films for MITC) the time needed was less than 1 h (about 0.8 h, Figure 3a). It took about 8 h for a compound to reach the near equilibrium state at both sides of the film in a closed system if the MTC decreased to 1 cm/h (e.g., the PE films for MeBr, IOM, and PIC, Figure 3b). This observation is consistent with the general belief that the PE films are permeable to fumigants.13 When the MTC further decreased 10-fold to 0.1 cm/h (e.g., the metalized films to DMDS, PIC, IOM) it took about 3 days (about 80 h, a 10-fold increase) for the compound to reach such a state (Figure 3c). As the MTC

decreased another 10-fold to 0.01 cm/h (e.g., most of the VIF films for MeBr, IOM) it took about 800 h (a 10-fold increase) to reach near equilibrium (Figure 3d). The inversely proportional relationship between the increase in the time needed to reach the near equilibrium and the decrease in the MTC demonstrates the usefulness of this parameter as an indicator of film property. The time needed to reach near equilibrium in Figure 3 was for a closed system and for approximately 50% of the fumigants to permeate through the film. In actual field settings, the “receiving side” is open atmosphere. The expected time needed to reach the near equilibrium state in an open system (i.e., no fumigant remaining under the film) would be longer than that in a closed system because the entire amount of the fumigants would need to pass through the films to reach equilibrium, whereas only 50% of the fumigants needs to permeate through a film to reach equilibrium in a laboratory closed system. In one test with a PE film, the time needed for about 95% of the fumigants in the source chamber to pass through a film in an open system (simulated by using the test apparatus with no receiving chamber) was approximately 24 h for MeBr (Figure 4a and Table 1 in the Supporting Information). In comparison, it took approximately 7 h for MeBr to reach near equilibrium (about 53% in the source chamber) in the two chamber closed system described in this study (Figure 4b). In this case, it took approximately three to four times longer to reach near equilibrium in an open system. This could have implications when translating MTC values obtained from closed system tests in the laboratory to actual field conditions. Because other types of films have not been tested, it is unknown whether the time needed for about 95% of fumigants to permeate through in the simulated open system is longer than the closed system by similar extent. 9715

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Figure 4. Concentration change over time for MeBr in the source side in (a) an open system (MeBr permeating through the film into air. 0% is the equilibrium concentration in the source); and (b) a closed system (50% is the equilibrium concentration in the source) for one HDPE film. Symbols are the measured amount in triplicate tests. The measured MTC of this PE film for MeBr is 0.824 cm/h. The time needed to reach 95% of the equilibrium concentration (about 53% in the source side) is approximately 7 h in the closed system. The time needed to reach 95% of the equilibrium concentration (about 5% in the source side) in the open system is about 24 h. Each test was conducted in triplicate. Other compounds exhibit similar changes (Table 1 in the Supporting Information).

In actual field application of fumigants, the fumigants are injected into soil or drip applied into the soil. The amount of fumigant available for permeation through the film probably is much less than the amount applied. In addition to the absorption and retention of fumigants by soil, fumigants in soil are degraded over time.20,21 For example, PIC, 1,3-D, and MITC are subject to fast degradation, either by biological processes in soil, or chemical processes in soil and vapor. The amount of emission of fumigants and the time needed for fumigants to dissipate under tarps is therefore much more complex to estimate than the ideal conditions in the laboratory. Effects of Environmental Conditions on the Permeability. Previous studies reported that gas transport through plastic films could be affected by relative humidity 18,22,23 and temperature.12,13,24,25 Because agricultural films can be exposed to high humidity during soil fumigation, it is important to characterize the effects of relative humidity on film permeability to fumigant vapors. Relative Humidity. The permeability of agricultural films, particularly those with low MTC values, can be greatly affected by the moisture content in the air.18 In this study, when VIF films were exposed to high relative humidity conditions, the MTCs increased by up to 3 orders of magnitude (Table 3). Similar effects by humidity on the permeability of some VIF films were also reported previously.18 The MTCs

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of many VIF films under high relative humidity conditions were close to the MTCs seen for regular LDPE and HDPE films (Tables 2 and 3 and Figures 1 and 2 in the Supporting Information). High humidity only had a slight effect on the permeability of the PE films. The increase in the MTC of PE films due to high humidity was less than 20% for most fumigants (Tables 2 and 3) and was generally within the range of the analytical method uncertainties. These results of the minor humidity effects on the permeability of PE films are similar to the results of a previous report.13 In contrast, the VIF tarps nearly lost all their original impermeability for fumigants when tested under high humidity. The effects of humidity on the permeability of metalized films are smaller than that of the VIF tarps. The implication of these humidity effects on the permeability of films is that high humidity in the air under a tarp in the field could potentially compromise the fumigant retaining ability of the VIF films. The EVOH layer in the TIF films was an excellent barrier layer, reducing the MTCs to below 0.001 cm/h under ambient humidity conditions in laboratory tests. However, when the films were exposed to high relatively humidity, the ability of this barrier layer to block the passage of fumigants was greatly reduced (Table 3). The MTCs of the TIF films increased by 2 3 orders of magnitude under high relative humidity compared to that under ambient humidity conditions at the same temperature of 25 °C (Tables 2 and 3). However, the MTCs of the TIFs under high humidity remained low compared to the VIF under the same high humidity conditions. The MTC was below 0.1 cm/h for most of the TIFs for all the tested fumigants under high humidity conditions (Table 3), suggesting that the TIFs would perform better in reducing the emission of fumigants compared to other types of films. In addition to its affect on film permeability, elevated humidity may also influence the stability of fumigants in the vapor. MITC appeared to be significantly less stable under high humidity conditions during the tests, with over 80% of MITC degraded in two days (Figure 3 in the Supporting Information). In comparison, recoveries of SF, MeBr, IOM, and PPO from the cells remained largely between 70% and 120%. This effect was observed during the tests of various films. Any degradation of MITC in the source side of the high humidity test cell, relative to the receiving side, would have resulted in shorter apparent equilibrium time, and therefore the calculated MTC value for MITC was likely biased high. However, because MITC was the fastest compound to permeate any film and equilibrium reached in a relatively short time, the bias in the MTC of MITC caused by the degradation of MITC under high relative humidity conditions may not be substantial. It is possible that the presence of liquid water in the source side of the permeability cells under high humidity conditions may affect calculated MTC values if fumigants are partitioned into the liquid water. However, recoveries of most fumigants from the vapor phase suggest that the amount partitioned into the liquid phase is minimal. Recovery of MITC decreased rapidly to less than 20% of the initial amount within two days in all the tests (Figure 3 in the Supporting Information). If MITC had been sequestered by the small amount of liquid water (a few drops) in the source side, the decrease in the recovery would be sharp followed by a flattened pattern. Two TIFs that do not use EVOH barrier material showed no apparent effects from humidity. The MTCs of both Dow Saranex films were similar under high relative humidity and 9716

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Table 3. Mass Transfer Coefficient (MTC, cm/h, Average of Triplicates) of Agricultural Films to Various Fumigants at 25°C and High Humidity (Relative Humidity of 90%) films

MeBr

IOM

PPO

1,3-D, cis

1,3-D, trans

DMDS

MITC

PIC

SF

0.5000 1.2113

0.8686 2.1286

0.2913 0.7679

2.8621 8.8657

4.8446 12.9526

2.5951 7.8947

7.6397 23.9830

0.7754 2.6259

0.0072 0.0185

Canslit Metalized Black

0.0711

0.0828

0.0694

0.1340

0.1645

0.1257

0.3361

0.0919

0.0027

Pliant Metalized

0.1390

0.2030

0.0874

0.6072

0.9236

0.5360

2.1128

0.1713

0.0017

Cadillac VIF

0.1667

0.2066

0.2586

0.7273

1.3020

0.5438

6.6230

0.1563

0.0020

Can-Block

0.1430

0.1336

0.1395

0.3691

0.6684

0.2560

2.1650

0.0960

0.0010

FilmTech VIF Ginegar VIF

0.3542 0.2682

0.4533 0.3183

0.6339 0.3990

1.6355 1.1049

2.5607 2.3772

1.3169 0.8056

5.6756 10.7761

0.3887 0.2419

0.0026 0.0152

Ginegar Ozgard,

0.1631

0.2114

0.2460

0.8037

1.3461

0.6037

3.2284

0.1601

0.0010

Guardian Olefinas VIF

0.2662

0.3236

0.3430

1.0608

2.3161

0.7783

11.5732

0.2637

0.0023

Mid South VIF

0.0878

0.0892

0.1300

0.1851

0.2878

0.1473

0.7659

0.0774

0.0012

Pliant Blockade, Black

0.2826

0.3441

0.3632

1.0154

1.4204

0.7971

3.2613

0.2811

0.0061

Pliant Blockade White

0.7421

1.0520

0.9096

4.7243

7.9022

3.6111

15.9521

0.9499

0.0051

0.0133 0.6182

0.0131 0.7838

0.0333 0.2897

0.0250 3.0451

0.0382 4.6441

0.0199 2.8390

0.1512 7.4034

0.0029 0.9628

0.0001 0.0067

PE Films Cadillac, HDPE Pliant Regular Black LDPE Metalized Films

VIFs

TIFs AEP-One BayFilm Berry EVOH High Barrier

0.0355

0.0289

0.0780

0.0535

0.0929

0.0482

0.5031

0.0038

0.0009

Berry High Barrier w/Improv toughness

0.0337

0.0293

0.0641

0.0524

0.0803

0.0458

0.1755

0.0108

0.0002

Berry EVOH Supreme Barrier

0.0446

0.0446

0.0818

0.0626

0.0806

0.0587

0.1063

0.0157

0.0009

Dow SARANEX A

0.0009

0.0006

0.0007

0.0013

0.0021

0.0009

0.0397

0.0000

0.0000

Dow SARANEX B

0.0002

0.0001

0.0002

0.0015

0.0013

0.0003

0.0099

0.0004

0.0000

Klerks/HyPlast

0.0834

0.0929

0.1429

0.2745

0.5024

0.1963

2.0422

0.0758

0.0031

Raven TIF VaporSafe 1.4 mil Raven TIF VaporSafe 1.0 mil

0.0069 0.0115

0.0047 0.0118

0.0179 0.0288

0.0129 0.0168

0.0213 0.0275

0.0099 0.0186

0.0796 0.0743

0.0007 0.0005

0.0000 0.0001

ambient humidity conditions at 25 °C (Tables 2 and 3) for all the fumigant except for MITC, further evidence suggesting that the presence of liquid water did not impact the calculated MTC values significantly. The calculated MTC value for MITC was higher under high humidity conditions than ambient humidity conditions, likely the result of the bias caused by the degradation of MITC under high humidity conditions. Dow Saranex films use a different barrier layer, polyvinylidene chloride, which appeared to be not affected by the moisture in the air. Temperature. The effects of rising temperature on the permeability, in contrast, were much smaller than the effects of humidity, as shown by the change of the permeability of twelve VIF and TIF tarps at 40 °C. Several films, such as Berry Supreme Barrier, Raven 1.4 mil TIF, Ginegar VIF and Cadillac VIF films, showed almost no change between the MTCs measured at 25 °C and at 40 °C for MeBr, SF and IOM (Table 2 in the Supporting Information). The measured MTCs at both temperatures were within the range of expected experimental variability. The MTCs of several films, such as FilmTech VIF, Pliant Blockade (black), Ginegar Ozgard, MidSouth VIF, Guardian Olefinas VIF, and AEPOne, increased approximately 2 5 times at 40 °C compared with those at 25 °C for MeBr and IOM. The increase in the permeability at 40 °C compared to that at 25 °C was between f5 and 10 times for Dow Saranex and Klerks/Hyplast TIF. Similar increases in the permeability with temperature were

also observed previously,13,18 with a 2 5 times increase in the permeability generally expected for every 10 °C increase.18 The calculated MTC values for MITC, DMDS, 1,3-D, and PIC were more variable at 40 °C than that for MeBr, IOM, and SF. The laboratory determined MTCs for films were achieved under controlled, ideal conditions. During actual field fumigant applications and tarp placement, many factors could potentially alter the emission rate of fumigants through agricultural films, such as adsorption and retention of fumigants by soil, degradation of fumigants, variation of humidity under the tarp, air temperature, handling and installation techniques of the films, and deterioration of the films in the field. 13,25,26 Some compounds, such as MITC, 1,3-D, PIC, are unstable under high moisture conditions, particularly when exposed to sunlight (refs 26,27 and Figure 3 in the Supporting Information). Some fumigants may be quickly degraded, thus reducing their potential emission to atmosphere. Because fumigants are retained in the soil, emission through the tarp is much less compared to the scenario that all the fumigants are placed under the tarp and are available for permeation through the tarp. High relative humidity under the tarps can further increase emission, particularly when the temperature is high during day time. Prediction of actual fumigant emission rates is therefore complicated by many interrelated processes. 9717

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’ ASSOCIATED CONTENT

bS

Supporting Information. Additional tables and figures as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: (410)305-2636; fax (410-305)3091; e-mail: Qian.yaorong@ epa.gov.

’ ACKNOWLEDGMENT We are grateful that Elizabeth Kolbe reviewed the quality of all the data for this study. Chemtura, Dow Chemical, and Arysta LifeScience provided methyl bromide, sulfuryl fluoride, and chloropicrin standards, respectively. The MTC calculation application software was provided by Scott Yates. We thank the film manufacturers for sending sample films for testing. Detailed statistical analysis of the permeability and film ranking are documented in a U.S. EPA report.19 Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. EPA. The views expressed in this paper are those of the authors and do not necessarily represent those of the US EPA. No official Agency endorsement should be inferred. ’ REFERENCES (1) U.S. EPA pesticide registration website. http://www.epa.gov/ oppsrrd1/reregistration/soil_fumigants/implementing-new-safetymeasures.html (accessed October 17, 2011). (2) Austerweil, M.; Steiner, B.; Gamliel, A. Permeation of soil fumigants through agricultural plastic films. Phytoparasitica. 2006, 34 (5), 491–501. (3) Gao, S.; Trout, T. J. Surface seals reduce 1,3-dichloropropene and chloropicrin emissions in field tests. J. Environ. Qual. 2007, 36 (1), 110–119. (4) Gao, S.; Hanson, B. D.; Qin, R; Wang, D.; Yates, S. Developing agricultural practices to reduce emissions from soil fumigation using field plot tests. J. Environ. Qual. 2010, 0422, DOI:10.2134/jeq2009. (5) Wang, D.; Yates, S. R.; Ernst, F. F.; Gan, J.; Jury, W. A. Reducing methyl bromide emission with high-barrier film and reduced dosage. Environ. Sci. Technol. 1997, 31 (12), 3686–3691. (6) Wang, D.; Yates, S. R. Methyl bromide emission from field partially covered with a high-density polyethylene and a virtually impermeable film. Environ. Sci. Technol. 1998, 32 (17), 2515–2518. (7) Samtani, J. B.; Ajwa, H. A.; Goodhue, R. E.; Daugovish, O.; Kabir, Z.; Fennimore, S. A. Weed control efficacy and economics of 1,3dichloropropene and chloropicrin applied at reduced rates under impermeable film in strawberry beds. HortScience 2010, 45 (12), 1841–1847. (8) Chellemi, D. O.; Mirusso, J. Optimizing soil disinfestation procedures for fresh market tomato and pepper production. Plant Disease 2006, 90 (5), 668–674. (9) Gamliel, A.; Grinstein, A.; Klein, L; Cohen, Y; Katan, J. 1998. Permeability of plastic films to methyl bromide: Field study. Crop Prot. 1998, 17 (3), 241 248. (10) Gilreath, J. P.; Motis, T. N.; Santos, B. M. Cyperus spp. control with reduced methyl bromide plus chloropicrin doses under virtually impermeable films in pepper. Crop Prot. 2005, 24 (3), 285–287. (11) Papiernik, S. K.; Yates, S. R.; Dungan, R. S.; Lesch, S. M.; Zheng, W.; Guo, M. Effect of surface tarp on emissions and distribution of dripapplied fumigants. Environ. Sci. Technol. 2004, 38 (16), 4254–4262.

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(12) Wang, D.; Yates, S. R.; Gan, J.; Knuteson, J. A. Atmospheric volatilization of methyl bromide, 1,3-dichloropropene, and propargyl bromide through two plastic films: Transfer coefficient and temperature effect. Atmos. Environ. 1999, 33 (3), 401–407. (13) Papiernik, S. K.; Yates, S. R. Effect of environmental conditions on the permeability of high density polyethylene film to fumigant vapors. Environ. Sci. Technol. 2002, 36 (8), 1833–1838. (14) Papiernik, S. K.; Yates, S. R.; Gan, J. An approach for estimating the permeability of agricultural films. Environ. Sci. Technol. 2001, 35 (6), 1240–1246. (15) Papiernik, S. K.; Ernst, F. F.; Yates, S. R. An apparatus for measuring the gas permeability of films. J. Environ. Qual. 2002, 31 (1), 358–361. (16) Yates, S. R., Chellemi, D.; Browne, G.; Wang, D.; Gao, G.; Hanson, B.; Ajwa, H.; Kluepfel, D. Update of Film Permeability Measurements for the USDA-ARS Area-Wide Research Project. 2008 Annual International Research Conference on Methyl Bromide Alternatives and Emissions Reductions. http://mbao.org/2008/018Yates. pdf (accessed October 17, 2011). (17) Ajwa, H. Testing Film Permeability to Fumigants Under Laboratory and Field Conditions. 2008 Annual International Research Conference on Methyl Bromide Alternatives and Emissions Reductions. http://mbao.org/2008/035Ajwa.pdf (accessed October 17, 2011). (18) Papiernik, S. K.; Yates, S. R.; Chellemi, D. O. Standardized approach for estimating the permeability of plastic films to soil fumigants under various field and environmental conditions. J. Environ. Qual. 2010, DOI: 10.2134/jeq2010.0118. (19) Second Update To Health Effects Division Recommendations for Good Agricultural Practices and Associated Buffer Credits. 1/11/ 2011, Document ID: EPA-HQ-OPP-2005-0123-0748. http://www. regulations.gov/#!documentDetail;D=EPA-HQ-OPP-2005-0123-0748 (accessed October 17, 2011). (20) Ma, Q. L.; Gan, J.; Papiernik, S. K.; Becker, J. O.; Yates, S. R. Degradation of soil fumigants as affected by initial concentration and temperature. J. Environ. Qual. 2001, 30 (4), 1278–1286. (21) Dungan, R. S.; Yates, S. R. Degradation of fumigant pesticides: 1,3-dichloropropene, methyl isothiocyanate, chloropicrin, and methyl bromide. Vadose Zone J. 2003, 2 (3), 279–286. (22) Johansson, F.; Leufven, A. Food packaging polymer films as aroma vapor barriers at different relative humidities. J. Food Sci. 1994, 59 (6), 1328–1331. (23) Lagaron, J. M.; Gimenez, E.; Catala, R.; Gavara, R. Mechanisms of moisture sorption in barrier polymers used in food packaging: Amorphous polyamide vs. high-barrier ethylene-vinyl alcohol copolymer studied by vibrational spectroscopy. Macromol. Chem. Phys. 2003, 204 (4), 704–713. (24) Muramatsu, M.; Okura, M.; Kuboyama, K.; Ougizawa, T.; Yamamoto, T.; Nishihara, Y.; Saito, Y.; Ito, K.; Hirata, K.; Kobayashi, Y. Oxygen permeability and free volume hole size in ethylene-vinyl alcohol copolymer film: Temperature and humidity dependence. Radiat. Phys. Chem. 2003, 68 (3 4), 561–564. (25) Wang, D.; Yates, S. R.; Jury, W. A. Temperature effect on methyl bromide volatilization: Permeability of plastic cover films. J. Environ. Qual. 1998, 27 (4), 821–827. (26) Qin, R.; Gao, S.; McDonald, J. A.; Ajwa, H.; Shem-Tov, S.; Sullivan, D. A. Effect of plastic tarps over raised-beds and potassium thiosulfate in furrows on chloropicrin emissions from drip fumigated fields. Chemosphere 2008, 72 (4), 558–563. (27) Yates, S. R.; Gan, J.; Papiernik, S. K. Environmental fate of methyl bromide as a soil fumigant. Rev. Environ. Contam. Toxicol. 2003, 177, 45–122.

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