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Exposure months

Figure 5.3. Percent elongation at break versus exposure time for greenhouse film exposed on model greenhouse and aluminum racks.

films exposed on both the greenhouse and racks. Figure 5.3 presents change in percent elongation at break versus exposure time for films exposed in racks and greenhouse films. Both the films have shown a downward trend, but a faster drop in measured property is observed for film exposed on operating greenhouse.

The higher rate of degradation on film exposed on greenhouses is attributed to the higher temperatures, humidity, and wind stresses, conditions that prevailed in an enclosed greenhouse tunnel [38, 40]. Furthermore, the higher degradation of greenhouse-exposed films may also be due to sulfur and halogen-based chemicals, which have adverse effects on HALS-stabilized LDPE films.

Effects of Chemical Treatment on Greenhouse Films The crops grown inside a greenhouse tunnel need specialized treatment depending on the type of insects/pests that would attack the plants. Pesticides used for spraying under greenhouse are mainly sulfur- or halogen-based compounds. It has been experienced by researchers that sulfur-containing additives, such as distearyl thiodipro-pionate (DSTDP) may have an antagonistic effect on the lifetime of HALS-based films [29, 34, 41, 42]. HALS also suffer when used with halogenated flame-retardants [29, 42]. The lifetime of the PE films will depend on the nature of pesticides/insecticides used, frequency of spraying, method of application, and design of the greenhouse.

It has been shown that single spraying of metam sodium during a one-year period is sufficient to degrade it. However, if the soil is covered with a film sheet during application of nematicide and kept down for 4 weeks, the film was protected [43].

A study was conducted by Epacher and Pukawzky [44] on three types of films having three different stabilizer packages exposed to 24 commercial pesticide formulations. These films were soaked for 1 h, 1 day, and 1 week duration. Artificially accelerated weathering on the same films were carried out under dry conditions for 300 h and 600 h inside a Xenotester.

Table 5.7 Effect of Pesticides and Irradiation on the Ultimate Elongation of PE Films

Percent Elongation after Treatment for 1 day and Xenotest Time (h)

Pesticide

Package A'

Package Bb

Actellic 351.8 394.4

Chinetrin 314.4 318.3

Neoron 324.6 356.6

Sulfur 339.6 309.3

222.3 292.0

330.0 282.7

52.9 317.3

230.5 323.8

50.3 287.7

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