# Degradation of Low-Density Polyethylene Greenhouse Film Aged in Contact with Agrochemicals
## 4 Discussion  
From these results, it is worth highlighting a significant effect of the deterioration of the mechanical properties of the film, due to the likely synergistic effect induced by both the agrochemicals and the artificial aging. In particular, the samples treated with the fungicide are much more affected by the worsening of mechanical performances. This result could be correlated to specific interactions and reactions occurring between Metalaxil-m and polyethylene during UV-accelerated weathering. Although a detailed description of the chemical phenomena is outside of the current paper's aim, it is possible to hypothesize that the active fungicide, whose structure is made of [N-(2,6-dimethylphenyl)-N-(methoxy-acetyl)-DL-alanine ester], shows chemical functional groups sensitive to redox-photocatalytic processes, able to generate a series of active radicals, strongly contributing to the photodegradation Norrish reactions of polyethylene, leading to the scission of the main chain [41,42]. Differently from the fungicide, the acetamiprid--whose chemical structure is typical of a chloronicotinyl compound including monochloropyridine, a nitrile, and a carboxamidine--follows different photodegradation mechanisms, strongly correlated to the specific environment. It is particularly enhanced in a water environment, due to the presence of pyridine and double and triple carbon nitrogen groups [43]. It is also worth highlighting that the mechanical properties of all the samples treated with double fungicide and insecticide spraying follow slower degradative kinetics, as shown in Figures 4 and 5. It is likely that the higher concentration of both active compounds provides a thicker coating on the PE surface, thus preserving the polymer from the faster degradative phenomena. This outcome is significant since the double treatment supports an improved action of both compounds, which protects the PE-LD film from the fast photodegradation phenomenon. This occurrence could be taken into consideration both to enhance the action of the functional compounds and to preserve the mechanical performances of the polymer matrix for longer times. Nevertheless, after four weeks of film permanence in the aging chamber (equivalent to about six months of real aging conditions on a greenhouse located in Southern Italy, at least in terms of global energy received), the PE-LD plastic film should be replaced, since, after coming into contact with a pesticide or fungicide, it has lost 50% of its \"as new\" elongation at break, thus its useful life may be considered as finished. The impact of the contamination of the film material by these agrochemicals in the use phase is considerable. The effect on the material in the use phase reflects a consequent hindrance of the material, from a recycling perspective. Indeed, as the material enters the recycling stage, its chances of becoming a closed-loop recycled material are rather low. When PE-LD film comes into touch with the fungicide, this behavior is particularly evident. As shown in Figure 5b, the material has almost entirely lost its mechanical properties, after four weeks of artificial aging. Further reprocessing through mechanical recycling would be therefore impossible, since the material becomes brittle and fragmented. This is especially important when taking into account the extra breakdown mechanisms that thermo-mechanical processes cause in polymers during mechanical recycling [44,45]. These findings appear to be consistent with the literature outcomes presented in the earlier sections, corroborating the existence of a correlation between the absorption intensity showed by a variation in the CI value, with the degradation of the plastic film. Since the Carbonyl Index acts as an indicator of the extent of degradation in the plastic films used in greenhouses, the main chemical reactions occurring during plastic UV aging responsible for its variation must be mentioned.