Cross-linking tendency and photo-oxidation degradation in Silane-grafted LDPE insulation under accelerated cyclic weathering aging

Si-g-LDPE (Silane-grafted LDPE) is the based material of silane-crosslinked polyethylene (Si-XLPE) nominated to be used in renewable energy (photovoltaic) structures for the reason of its good behavior under weathering circumstances. The present study has an attempt to follow up the cross-linking process of Si-g-LDPE under cyclic accelerated weathering aging and to track its photo and thermo-oxidative degradation at both microscopic and macroscopic scales. The realized experiments consist to carry out cyclic accelerated weathering aging of 1152 hours, using QUV chamber, on films of Si-g-LDPE. The process of cross-linking is checked by using Hot-Set-Test measurements and Fourier Transform Infrared (FTIR) spectroscopy. Evolution of mechanical properties and carbonyl index are used as relevant precursors of photo and thermo-oxidation degradation. Additional information on the microstructural changes (crystallinity) and on the optical properties is gained by using X-ray difractometry and UV-visible measurements. Our findings lead to a result that Si-g-LDPE has a big ability to crosslink under cyclic weathering aging. The elastic behavior of the material was enhanced gradually with increasing aging time. The elastic behavior enhancement is a consequence of crosslink network density increase. FTIR results support perfectly the Hot-Set-Test results by the increase of single or multi-siloxane linkages absorption bands. These linkages modify the digital fingerprint of the material. Our results ascertained also that we can relate the elongation at break and tensile strength changes to the changes in the carbonyl index. Both characteristics are related to the photo and thermo-oxidation degradation. Photo-oxidation degradation leads to the decrease of mechanical properties and to the increase of carbonyl index. An abrupt and sharp increase at the beginning of aging in the crystallinity of the material followed by a level-off state was observed. Finally, the optical properties (band gaps, Urbach energy and dielectric constant) are greatly affected by the weathering aging process.