HALS stabilization of LDPE films used in agricultural applications

The mechanism of stabilization of low density polyethylene (LDPE) films used in agricultural applications with hindered amine light stabilizer (HALS) has been studied under the influence of natural weather for 750 d. The structural modifications in the exposed films were monitored by Fourier transform infrared spectroscopy (FTIR), while the examination of the surface behavior was performed by scanning electron microscopy (SEM). Finally, the stability of the material was evaluated by measuring the percent elongation at break. The performance of HALS in the films was analyzed through a comparative study carried out on unstabilized LDPE samples exposed to the same weathering conditions for 170 d. The FTIR results have shown that HALS reduces significantly the rate of formation of ketone groups in the exposed films due to deactivation of the excited ketonic carbonyls. This indicates the role of HALS to prevent the occurrence of Norrish I and II reactions. On the other hand, the development of vinyl groups at 909 cm–1 during the exposure period of 750 d is attributed to the photolysis of the ester group of the HALS localized at 1 734 cm–1 in the film and the photooxidation of the polymer. The decrease observed in the ester band suggests the loss of the stabilizer. For the unstabilized LDPE film, the results have revealed a fast increase in both ketonic and vinyl groups attributed mainly to Norrish II reactions. This resulted in chain scission responsible for both the rapid drop in elongation at break and the considerable deterioration of the surface of the exposed film. On the contrary, the presence of HALS in the film resulted in a large retention of the elongation at break. Moreover, SEM analysis reveals the role of HALS to prevent the surface damage by reducing the number and size of microcracks.