Effect of anisotropy on photo-mechanical oxidation of polyethylene

✍ Scribed by C Jin; P.A Christensen; T.A Egerton; J.R White

• Publisher: Elsevier Science
• Year: 2003
• Tongue: English
• Weight: 484 KB
• Volume: 44
• Category: Article
• ISSN: 0032-3861
• DOI: 10.1016/s0032-3861(03)00550-0

✦ Synopsis

Blown polyethylene (PE) film was exposed to UV irradiation while under tensile stress. Tests were conducted on unpigmented PE and on a series of five PEs containing TiO 2 pigments with different photo-activities. The yield strengths measured in tensile tests on unexposed films were typically 10% greater in the longitudinal (machine) direction than in the transverse direction. UV exposures were conducted with stress applied both parallel and transverse to the machine direction. The chemical degradation was followed using the FTIR carbonyl index. For all six materials, tensile stress accelerated carbonyl group development. Some samples cracked during UV exposure in tension and did so in a shorter time if the stress was applied in the machine direction than if it was transverse to it. The carbonyl index at the onset of cracking was lower for longitudinal samples than for transverse samples.

Unpigmented PE was slightly more sensitive to transverse strain and the anatase-pigmented PE slightly more sensitive to longitudinal strain. The carbonyl production rate for two of the rutile pigmented grades showed very little dependence on the straining direction whereas for a third the increase in rate was significantly greater when the deformation was applied in the machine direction than for transverse stretching. With the fourth rutile-pigmented grade, transverse stretching produced the greater effect. These differences cannot be correlated with the photoactivity of the different pigments. The highest ratios of longitudinal/transverse enhancement were found with the least photoactive rutile pigment and with anatase-the most photoactive of all those tested.