New Approaches to Investigate and Prevent Oxidation of Polypropylene

Sammanfattning: When employing chemiluminescence to evaluate stabiliserperformance in a polymer, the induction time or the time toreach maximum chemiluminescence intensity is often used. Insuch an experiment the longest part of the experiment isnormally the time before the oxidation starts. In the presentwork oxidation was initiated in stabilised polypropylene (PP)by means of UV light or unstabilised PP, and the spreading ofoxidation was then followed using imaging chemiluminescence(ICL). As a result of this initiation, oxidation started at thebeginning of the experiment, which eliminated the inductiontime. The rate of the subsequent spreading of oxidation to therest of the sample was used to determine the performance ofdifferent stabilisers in PP. It was shown that such experimentsare reproducible and can be used to compare differentantioxidants with respect to their efficiency. However, therate of spreading accelerated during the experiments.The gaseous products formed during the experiment were foundto influence the results by increasing the rate of spreading.In order to remove the influence of the gas phase, a device wasconstructed that directed an air flow over the sample surface.Even without the gas phase the spreading of oxidation occurred,but was now ascribed to bulk phase spreading. This spreadingshowed a linear spreading rate and did not accelerate. Thecause of the acceleration is probably a build-up of reactivegaseous species in the sample chamber. In order to clarifypossible volatile oxidation products capable of spreadingoxidation via the gas phase, formaldehyde, acetic acid,ethylene, water and iso-butylene were added in small amounts tothe inlet air in the ICL sample chamber. Only formaldehyde andacetic acid were found to increase the spreading of oxidation.However, acetic acid did this by interacting with thestabiliser, whereas formaldehyde increased the oxidation alsoin unstabilised PP.To further study the oxidation of PP, a combined instrumentwas constructed where a microcalorimeter (MC) was connected toa photomultiplier via an optic fibre so that a simultaneous CL(Counts/s) and MC (Joule/s) signal could be collected. Thisinstrument provides complementary data that gives moreinformation than if each technique had been used separately. Sofar, it has been shown that the MC response occurs earlier thanthe CL response at low extents of oxidation. However, at higherextents of oxidation the reverse has been observed.In addition to studying commercial antioxidants for PP a newantioxidant concept based on a synergistic mixture of organotellurium compounds and hindered phenols was developed andfound to be superior to standard antioxidant formulations.Moreover, hyperbranched hindered phenols of differentgenerations were developed and tested in PP. The intention wasto discover low migrating antioxidants. These were found to besuperior to the commercial hindered phenol Irganox 1010 whenused in liquid, low viscous systems, but did not performequally well in PP. This is believed to be due to restrictedmobility. However when combining them with the organo telluriumantioxidants, mixtures that performed well also in PP wereachieved.