Chemical and morphological changes of environmentally degradable polyethylene films exposed to thermo-oxidation

Thermo-oxidation of blown low density polyethylene (LDPE) films modified with different combination of biodegradable filler, prooxidant and photosensitizers was conducted in oven at 60 and 100 degrees C for a period of 14 days. Volatile and semivolatile degradation products were extracted by solid phase micro extraction (SPME) technique and identified utilizing gas chromatography-mass spectrometry (GC-MS). Chemical and morphological changes were monitored and these are given as carbonyl index, crystallinity and melting behavior, molecular weight and molecular weight distribution. The samples containing solely prooxidant showed the highest susceptibility to thermal degradation during the test period. The second most degradable samples were LDPE modified with 20% masterbatch (containing starch and a prooxidant). LDPE containing only starch did not show any degradation during the test period. The major degradation products were homologous series of carboxylic acids, ketones, hydrocarbons and lactones. 4-Oxopentanoic acid, 5-oxohexanoic acid and benzoic acid were identified only in LDPE containing prooxidant (LDPE-PO) and LDPE modified with 20% masterbatch (LDPE-MB). A small number of aldehydes (3-methyl pentanal, benzaldehyd and 2-propyl 5-oxohexanal) were identified solely in LDPE-MB. Esters could be identified only from LDPE-Starch and pure-LDPE samples. The crystallinity of all the samples increased after aging at 60 degrees C except for LDPE-Starch which showed no significant change in crystallinity. The melting thermograms of LDPE-PO and LDPE-MB (first heating) exhibited low temperature shoulders around 75 degrees C (after treatment at 60 degrees C) and appears to move downward with increasing exposure temperature (treatment at 100 degrees C). The shoulders near 115 degrees C (second heating) increase with increasing exposure temperature which is due to a preferential scission at the tertiary carbon atom as observed by increased crystalline melting point. ATR and transmission FTIR show that the absorbance of carbonyl containing groups is almost the same on the surface as in the bulk for virgin samples and samples aged at 60 degrees C. Opposite to the other materials, LDPE-MB samples aged at 100 degrees C show a much faster increase in the absorbance of carbonyl containing groups in the bulk of the film than on the surface layer. This indicates that the bulk of the latter films are more labile than the surface which could be a consequence of higher starch concentration on the surface than in the bulk. (C) 1998 Elsevier Science Ltd. All rights reserved.