Research articleEvaluation of the influence of temperature and relative humidity on the permeability of four films to the fumigant dimethyl disulfide
Graphical abstract
Introduction
Pre-plant soil fumigation is widely used to control soil-borne pests in many high-value crops throughout the world (Minuto et al., 2006). Dimethyl disulfide (DMDS) is an alternative fumigant to methyl bromide (MB) that was phased out globally due to its stratospheric ozone-depleting properties. DMDS does not damage the ozone layer. It is emitted from many natural sources such as bacteria, fungi, plants and animals. Because it of its ability to control pests and diseases in soil, DMDS has been registered in the United States as Paladin® (Mcavoy and Freeman, 2013).
DMDS has a high volatility which results in rapid emissions from the soil shortly after application. These emissions reduce its concentration in the soil and its potential to control pests and diseases (Conkle et al., 2016). Previous research reported that barrier film on the surface of the soil during fumigation reduced the exposure risk to non-target organisms and the environment, increased the concentration of fumigants in soil, and reduced the fumigant dosage (Wang et al., 1997a, Gao and Trout, 2007, Gao et al., 2011, Qin et al., 2011). Covering the soil surface with a barrier film is commonly used to reduce the risk of DMDS contact with non-target organisms (Qian et al., 2011). The United States established regulations to encourage users to reduce the risk of non-target organism exposure to DMDS by using measures such as high-barrier films to reduce fumigant emissions (EPA, 2011).
Most films are made of polymer materials that have an ability to restrict the movement of a small molecular gas. Generally, the permeation of a fumigant through a film appears to be primarily diffusion-controlled (Waack et al., 1955). The gas molecule moves through the film from a high to a low concentration side. However, the permeability of a film to a gas appears may be more complex and involve several stages: (1) The gas molecule is adsorbed onto the film surface; (2) Gaseous molecules dissolve in the film; (3) The gas achieves a dissolution equilibrium on the high concentration side; (4) A gas molecule diffuses to the low concentration side of the film driven by the concentration gradient; And (5) Desorption of gas molecules from the low concentration side of the film and release to the atmosphere (Rogers, 1985).
The characteristics of a film are determined by the capacity of its polymer matrix to sorb gas molecules and by its ability to allow gases to diffuse through them (Zhang et al., 2001). The permeability of a film is affected by extrinsic and intrinsic factors. The intrinsic factors such as molecular polarity, crystallinity, chain stiffness and cohesive energy density are determined by the polymer material itself (Rogers, 1985, Waack et al., 1955). Different films are formed by different polymerization processes. This creates films that have different polymer structures and surface characteristics, as well as different interactions with fumigants. Therefore, the polymer material is the essential factor that determines the permeability of a film to a gas.
Polyethylene (PE) film can be manufactured as low-density (LDPE) film or high-density (HDPE) film, according to the molecular weight and chain structure. LDPE and HDPE are commonly used in soil fumigation. They were reported to be permeable to DMDS and other soil fumigants (Austerweil et al., 2006, Fang et al., 2017, Qian et al., 2011). Over time they have been replaced by Totally Impermeable Film (TIF) but these are more expensive than standard PE film and unaffordable for use by many farmers. TIF is produced by co-extruding multiple layers of polyethylene with ethylene vinyl alcohol copolymer (EVOH) (Stevens et al., 2016). Other films that have been used with fumigants include polyvinyl chloride (PVC), polyvinylidene chloride (PVDC) and EVOH. Their structures are shown in Table 1.
PE is the most common film consisting of a long-chain, linear structure or branch structure, with typical crystalline polymers. Polymerization of PE is symmetrical but its cohesive energy as a material is low, which allows gases to transverse through it thereby increasing its permeability (Waack et al., 1955).
PVC film is one of the most important commercial polymers due to its low production cost. It is typically used in the construction and engineering industries (Reinecke and Mijangos, 1997).
PVDC film and EVOH film are considered high barrier films due to their copolymer characteristic (Lingmei, 2007, Zhang et al., 2001). PVDC film is widely used as packaging material for food and medicine. PVDC film contains polymers that have two chlorine atoms with a strong negative charge as well as a molecular symmetry with a high cohesive energy. As a result, PVDC has low permeability constant and is a good barrier to gases. Importantly, the hydrophobic base chlorine and symmetrical molecular structure results in PVDC remaining relatively unaffected by relative humidity (RH).
EVOH film is produced by the complete hydrolysis of ethylene vinyl acetate random copolymer (Zhang et al., 2001). The barrier properties of EVOH depend on the copolymerization ratio of ethylene and vinyl alcohol, which is negatively correlated with the ethylene ratio and positively correlated with the ratio of ethylene alcohol. However, because of the existence of ethylene alcohol hydroxyl group, the hydroxyl group and water molecules form hydrogen bonds under high RH causing a reduction in its permeability (Koros, 1990, Sacharow, 1986, Zhang et al., 2001).
Previous research reported that the permeability of films was affected by extrinsic environmental factors such as temperature (Qian et al., 2011, Zhang et al., 2001) RH (Johansson and Leufven, 1994, Papiernik et al., 2011) and the number of layers in the film. Many studies reported that the permeability of a film to a fumigant increases with increasing temperature (Gamliel et al., 1998, Papiernik et al., 2002). High RH had a slight effect on the permeability of PE film but a significant effect on the permeability of TIF to most fumigants (Papiernik et al., 2011, Qian et al., 2011). PE films had a high permeability to DMDS, but metalized films and multilayer films imbedded with polyamide (e.g., Virtually impermeable films, VIF) or ethylene vinyl alcohol (e.g., TIF) had a low permeability to DMDS (Papiernik et al., 2011, Qian et al., 2011). The permeability of multilayer films to DMDS was also reported to vary widely with temperature and RH (Qian et al., 2011).
In order to find a film more suitable than PE for soil fumigation, we undertook a systematic evaluation of PE, PVC, PVDC and EVOH films to determine their permeability to DMDS under field conditions that we could simulate in the laboratory. We used the “Mass Transfer Coefficient” (MTC) is determine film permeability, as this method is independent of the concentration of fumigant either side of the film and dependent only on the material of film, the physicochemical properties of the fumigant and the conditions in the environment such as temperature and RH (Papiernik et al., 2001, Papiernik et al., 2002).
The objective of our research was to determine the suitability of these films for use with DMDS. To determine the level of DMDS emissions from these films, we simulated in the laboratory the RH and temperature conditions as well as the fumigant injection and film management typically used in the field. The results of this work should provide practical guidance on the most suitable barrier film to use with DMDS.
Section snippets
Sources of chemicals, soil and plastic film
Analytical grade DMDS (99% purity) was purchased from Chengdu Best New Materials Co., Ltd. (Chengdu, China). Ethyl acetate (GC-MS/HPLC grade) was purchased from Thermo Fisher Scientific Shanghai Instruments Co., Ltd. (Shanghai, China). Anhydrous sodium sulfate was obtained from Xilong Scientific Chemicals Co., Ltd. (Shantou, China).
Soil was collected from the top 20 cm of the soil in the Yanqing District of Beijing, China (115.97° E, 40.47° N). The soil texture is a sandy loam (60.02% sand,
Permeability of four films to DMDS
The results of the permeability tests of the four films (Table 2) showed that they were divided into two groups: PE and PVC (high permeability) and PVDC and EVOH (low permeability). PE and PVC are similar and have high h values indicating a high permeability to DMDS, whereas PVDC and EVOH are similar and have low h values indicating a low permeability to DMDS. According to the differences between the h values for the two groups of film, PE and PVC are about 20 times more permeable to DMDS than
Conclusions
This research confirmed that PVDC and EVOH films are relatively impermeable to the fumigant DMDS. DMDS has a high volatility which results in rapid emissions from the soil shortly after application. Our research showed that at a temperature of 26–30 °C, PVDC and EVOH barrier films retained about 98% of the DMDS applied to the soil for more than 60 h.
The cumulative emissions of DMDS from PVDC and EVOH films also reduced significantly and were at least 15-times lower than the other films tested.
Funding and acknowledgments
This research was supported by the National Natural Science Foundation project of China (No. 31572035) and the National Key Research and Development Program of China (No. 2017YFD0201600). We thank Dr Tom Batchelor for editorial comments on the manuscript and Zhaolu Zhou for her technical assistance on graphics.
Notes
The authors declare no competing financial interest.
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