# Degradation of Low-Density Polyethylene Greenhouse Film Aged in Contact with Agrochemicals
## 4 Discussion
The main degradation mechanisms (photo-oxidation,thermo-oxidative degradation, photo-fragmentation, and cross-linking/chain scission) and the pivotal chemical reactions involved are detailed as follows:  
_(I)_ _Photo-Oxidation_  
Initiation step. The initial step involves the absorption of UV light by the polymer, leading to bond cleavage and the formation of free radicals. R-H + UVhv \(\rightarrow\) R+ H  
Propagation step. The free radicals react with molecular oxygen to form perox-radicals, which can then form hydroperoxides. R+ O2\(\rightarrow\)ROO-  
ROO-+ R-H\(\rightarrow\)ROOH + R-  
Decomposition step. Hydroperoxides are unstable and decompose to form alkoxy and hydroxyl radicals, leading to the formation of carbonyl compounds. ROOH\(\rightarrow\)RO-+ OH-  
RO-\(\rightarrow\)R\({}^{\prime}\).+ R-C(=O)-R\({}^{\prime\prime}\)  
Thus, in the photo-oxidation process, the general whole reaction can be resumed in  
Polymer + O2 + hv\(\rightarrow\)Carbonyl-containing compounds + Other degradation products  
_(II)_ _Thermo-Oxidative Degradation_  
Initiation. Thermal energy causes the homolytic cleavage of polymer chains, forming free radicals. R-H \(\xrightarrow{A}\) R + H-  
Propagation and Decomposition. As occurs for photo-oxidation, the free radicals react with oxygen to form peroxy radicals and hydroperoxides, which decompose to form carbonyl compounds and double bonds. R+ O2\(\rightarrow\)ROO-  
ROO-  
ROO-+ R-H\(\rightarrow\)ROOH+R-  
ROOH\(\rightarrow\)RO-+ OH-  
RO-\(\rightarrow\)R\({}^{\prime}\).+ R-C(=O)-R\({}^{\prime\prime}\)  
RO-\(\rightarrow\)R-CH=CH-R + R\({}^{\prime\prime}\)  
Thus, in the thermo-oxidation process, the general whole reaction can be resumed in  
Polymer + O2 + \(\Delta\)\(\rightarrow\)Carbonyl-containing compounds + Alkenes + Other degradation products  
_(III)_ _Direct Bond Scission_  
Moreover, UV radiation can directly cleave bonds in the polymer backbone, forming smaller fragments that can further react with oxygen and each other to form carbonyl compounds and double bonds. R-R\({}^{\prime}\) + hv \(\rightarrow\)R+ R\({}^{\prime}\). R+ O2\(\rightarrow\)R-C(=O)-R\({}^{\prime\prime}\)  
R+ R-\(\rightarrow\)R = R Thus, if direct bond scission occurs, the general whole reaction can be resumed in  
\[\text{Polymer + hv}\rightarrow\text{Carbonyl-containing compounds + Alkenes + Smaller fragments}\]  
_(IV) Cross-Linking and Chain Scission_  
During the first step of polymer decomposition, a cross-linking reaction could occur between free radicals formed during UV exposure and other polymer chains. In addition, the polymer chains can break, leading to the formation of smaller chains with terminal carbonyl groups and double bonds. \[\text{R-+ R}^{\prime}\rightarrow\text{R-R}^{\prime}\]  
\[\text{R-CH}_{2}\text{-CH}_{2}\text{-R}+hv\rightarrow\text{R-CH}\text{-+-CH}_{2} \text{-R}\]  
\[\text{R-CH}\text{+ O}_{2}\rightarrow\text{R-CHO}\]  
\[\text{R-CH}\text{+ R}\rightarrow\text{R-CH}\text{=CH-R}\]  
The general reactions involved in these processes can be resumed in  
\[\text{Polymer + }hv\rightarrow\text{Cross-linked polymer + Alkenes + Smaller chains with carbonyl groups}\]  
All the previous schematized degradation mechanisms lead to aldehydes, ketones, carboxylic acids, and alkenes as primary oxidation products. \[\text{Formation of Aldehydes: R-CH}_{2}\text{-R}^{\prime}+\text{O}_{2} \rightarrow\text{R-CHO}+\text{R}^{\prime}.\]  
\[\text{Formation of Ketones: R-CH}+\text{O}_{2}\rightarrow\text{R-C(=O)-R}^{\prime}\]  
\[\text{Formation of Carboxylic Acids: R-CHO}+\text{O}_{2}\rightarrow\text{R-COOH}\]  
\[\text{Formation of Alkenes: R-CH}+\text{R}\rightarrow\text{R-CH}\text{=CH-R}\]  
Degradation mechanisms were therefore magnified by the combined effects of photo-oxidation from solar radiation with the chemical actions of the agrochemicals, altering the polymeric matrix structure.