This thesis summarises and compares the influences ofdifferent additives (photosensitisers, pro-oxidants, abiodegradable filler) on blown low-density polyethylene (LDPE)films during photo- and thermal degradation with respect tochemical changes in the polymer matrices and identifieddegradation products. Additives studied were irondimethyldithiocarbamate (FeDMC), nickel dibutyldithocarbamate(NiDBC), carbon black, starch, a pro-oxidant that consisted ofmanganese stearate together with styrene butadiene rubber (SBR)or natural rubber (NR) and a master batch containing linearLDPE, starch and pro-oxidant. A variety of different analysistechniques, such as size exclusion chromatography (SEC),infrared (IR) spectroscopy, differential scanning calorimetry(DSC), gas chromatography-mass spectrometry (GC-MS),chemiluminescence (CL) and thermogravimetric analysis (TGA),were applied.
The degradation environment and the type of additives hadsignificant effects on the rate of degradation and on theamount of degradation products. As expected, the degradationwas faster and more effective at longer exposure times andhigher temperatures.
LDPE containing FeDMC showed the highest molar massreduction, the highest carbonyl index and the highest amount ofdegradation products after photo-oxidation due to thephoto-sensitising role of the iron. Combination of FeDMC andNiDBC showed the lowest photo-degradability during the testperiod since NiDBC acted as photo-antioxidant and increased theinduction period. LDPE containing starch and pro-oxidant showedthe next highest rate of photo-oxidation due to the presence ofchromophoric groups, but LDPE with either starch or pro-oxidantalone showed a lower sensitivity to photo-oxidation than whenbothwere present.
Samples containing pro-oxidant were the only samples thatshowed an increased susceptibility to thermo-oxidation due tothe presence of unsaturation in the rubber phase of thepro-oxidant and the catalytic effect of manganese stearate onthe decomposition of hydroperoxides. Starch acted as anantioxidant, whereas the other additives had no analysableeffect on the thermal degradation rate of LDPE. LDPE withpro-oxidant containing NR showed faster degradation ratesduring thermo-oxidation than LDPE with pro-oxidant containingSBR, due to higher degree of oxidisable sites (unsaturations)in the NR chains.
A wide variety of degradation products were identified.Generally addition of the used additives were not found tochange the composition of the degradation products so as tomake them less environmentally acceptable compared to those ofLDPE without additives. Low molecular weight carboxylic acids,ketones, hydrocarbons, and γ -lactones were the mainvolatile degradation products identified from the gas phaseabove the photo- and thermo-oxidised solid samples. Carboxylicacids, dicarboxylic acids, ketoacids and ketones were thepredominant degradation products, extracted from the solidpolymer matrix after thermo-oxidation at 100 °C. Mainlysimilar degradation products were identified in both photo- andthermo-oxidised samples with some exceptions. 2,5-hexanedioneand 1-alkenes were identified only in photo-oxidised samples.Only in the samples containing pro-oxidant with styrenebutadiene rubber (SBR) were the following products found:benzaldehyde, acetophenone, benzothiazole, benzoic acid, benzylbenzoate, and two unidentified aromatic compounds.
Keywords:LDPE, polyethylene, oxidative degradation,photo-oxidation, thermo-oxidation, degradation products,photosensitisers, pro-oxidant, starch, master-batch, gaschromatography mass-spectrometry, GC-MS, solid phasemicroextraction, SPME.
Institutionen för polymerteknologi , 2001. , 86 p.
LDPE, polyethylene, oxidative degradation, photo-oxidation, thermo-oxidation, degradation products, photosensitisers, pro-oxidant, starch, master-batch, gas chromatography mass-spectrometry, GC-MS, solid phase microextraction, SPME