The thermal decomposition of oxidized polyethylene wax

Polymer Degradation and Stability 15 (1986) 183-187

Short Communication The Thermal Decomposition of Oxidized Polyethylene Wax

ABSTRACT Differential scann&g calorimetric and thermogravimetric analyses of oxidized polyethylene wax were carried out in oxygen and nitrogen atmospheres. Information was obtained about its thermal stability and pattern of degradation. It exhibited high thermal stability comparable with that of a low molecular weight polyethylene.

INTRODUCTION Controlled oxidation of low molecular weight polyethylenes leads to the formation of an interesting class of products, 1 referred to as oxidized polyethylene waxes, which have recently found wide application as additives in the petroleum industry. 2 They usually impart good anticorrosion properties to lubricating oils and greases and have also been incorporated in petroleum-derived and naturally occurring waxes in order to improve their thermal properties. 3 Since these products are subjected to high temperatures, especially during blending or in service, it is appropriate to investigate their thermal behaviour and thermal degradation in different atmospheres using thermoanalytical techniques. 4-6 183 Polymer Degradation and Stability 0141-3910/86/$03-50 © Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Great Britain

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Dhoaib AI-Sammerrai, VVejdanSelim

EXPERIMENTAL

Materials The oxidized low molecular weight polyethylene wax (Airy= 950; acid number, 60 mg K O H g - 1; mp ~ 105 °C) and the low molecular weight polyethylene (_~r = 1000) were prepared at PRC, Baghdad.

Apparatus and experimental procedure Differential scanning calorimetric (DSC) and thermogravimetric (TG) measurements were carried out in a Heraeus TA 500 thermal analyser. In DSC, 5-10 mg samples were heated at 5 °C min-1 in an aluminium crucible in nitrogen or oxygen. Pure, dry aluminium oxide powder was used in the reference cell. TG and DTG curves were recorded simultaneously using 5-10mg samples in a platinum crucible and at a heating rate of 10°C min-1 in flowing nitrogen or oxygen (15mlmin-~). All measurements were performed in duplicate.

RESULTS AND DISCUSSION DSC and TG curves for the low molecular weight polyethylene are shown in Fig. 1. Significant weight loss only begins at about 350°C under nitrogen, whereas, in oxygen, an exothermic reaction starts at about 190 °C and weight loss commences at approximately 180 °C, corresponding to the first exothermic peak in the DSC trace. These facts suggest that, in oxygen, degradation due to surface oxidation is occurring, while, in nitrogen, the major transition corresponds mainly to volatilization. DSC measurements for oxidized polyethylene wax under oxygen and nitrogen are shown in Fig. 2. A small broad endothermic peak, attributed to softening and melting, is recorded between 75 and 120°C with a maximum at 110 °C in both atmospheres. This is followed under oxygen by a complex exothermic system starting at about 190 °C and terminating at ,-~400 °C,iwhich is attributed to oxidative degradation. The higher onset temperature for the oxidized polyethylene wax compared with that (180°C) of the unoxidized polyethylene may be related to the higher thermal stability of the former compound which has already undergone

Thermal decomposition of oxidized polyethylene wax

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DSC and TG curves for low molecular weight polyethylene obtained under nitrogen and oxygen.

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DSC curves for oxidized polyethylene wax obtained under nitrogen and oxygen.

Dhoaib AI-Sammerrai, Wejdan Selim

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Fig. 3. TG and D T G traces for oxidized polyethylene wax in nitrogen,

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Fig. 4.

TG and D T G traces for oxidized polyethylene wax in oxygen.

oxidation. On the other hand, under nitrogen, an endothermic effect was recorded at approximately 325 °C, and is attributed mainly to the start of volatilization of the oxidized wax. The TG and DTG curves for oxidized polyethylene wax under nitrogen and oxygen are recorded in Figs 3 and 4, respectively. The traces indicate that the weight loss process is more complex under oxygen than under nitrogen. The single peak recorded under nitrogen which lies close to the endothermic offset (325 °C) in the corresponding DSC trace, with an initial temperature of transition (Ti) of 322 °C, a maximum temperature of transition (Tmax) of 490 °C and a final temperature of transition (Tf) of 550°C, is attributed mainly to the total volatilization of the compound with no residual material remaining. On the other hand, the weight loss process in oxidized polyethylene wax under oxygen gas indicates the presence of multiple transitions commencing at 203°C and terminating at 545°C, again with no residue. These transitions are associated with a complex degradation process involving extensive oxidation with the eventual formation of oxides of carbon and water. An effect corresponding to this weight loss was not detected in the corresponding DSC trace as an endothermic offset from the baseline, since the exothermic oxidation effect is so massive and intense that it must overwhelm the effect of the weight loss process. Table 1 presents the partial weight loss recorded at 100°C intervals. Under nitrogen, weight loss proceeds slowly up to 400°C and then increasingly rapidly between 400 and 500 °C. Under oxygen, a marked increase in weight loss was recorded over 300°C, reaching a maximum

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Thermal decomposition of oxidized polyethylene wax

TABLE 1 Thermogravimetric Data a on Polyethylene(PE)and Oxidized Polyethylene Wax (OPE) Temperature range (°C) 0-100 100-200 200-300 300-400 400-500 500-600

Per cent weight lost in nitrogen

Per cent weight lost in oxygen

PE

OPE

PE

OPE

0 0 1.0 12.5 55.5 31.0

0 0 0-7 13.5 52.5 33.3

0 0.5 14.0 24.2 34.3 27.0

0 0 9.6 25.4 40.8 24-2

a From TG curves. between 400 and 500 °C. This behaviour is attributed to the enhancement of breakdown of the polymeric compounds under the oxidizing conditions of oxygen gas with the formation of lower molecular weight c o m p o u n d s which tend to volatilize at lower temperatures. REFERENCES 1. G. D. Hobson, Modern petroleum technology, Elsevier Applied Science Publishers Ltd, London (1975). 2. M. W. Ranney, Lubricant additives, Noyes Data Corporation, New Jersey (1973). 3. D. AI-Sammerrai, Improvement of thermal stabilities of paraffin and microcrystalline waxes. Paper presented at the Scientific Conference on Chemical Engineering, Baghdad (1985). 4. J. Chiu, Polym. Prep. Amer. Chem. Sot., Div. Polym. Chem., 14, 846 (1973). 5. M. I. Pope and M. D. Judd, Differential thermal analysis, Heyden and Son Ltd, London (1977). 6. I. G6m6ry, J. Thermal Anal., 11,327 (1977).

Dhoaib AI-Sammerrai & Wejdan Selim Petroleum Research Centre, PO Box 10039, Jadiriyah, Baghdad, Iraq (Received: 18 February, 1986)