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Unusual thermal degradation of maleic anhydride grafted polyethylene.

INTRODUCTION

Polymers, in general, exhibit lower thermal stability in air than in nitrogen because of oxidative degradation. However, some polymers have better thermal stability in air, such as poly(methyl methacrylate) (PMMA) and ethylene-vinyl acetate copolymer (EVA). The stabilizing effect of oxygen in thermal degradation of PMMA under air is explained by the formation of thermally stable radical species that suppress unzipping of the polymer (1). EVA is thought to be the result of the formation of a protective layer to delay decomposition (2).

Polyethylene (PE) is a thermoplastic polymer consisting of long chains of the monomer ethylene and widely used as an insulator for electric cables, which are usually operated at a temperature higher than 70[degrees]C (3). Thermal stability of PE is critical for reliable power supply and has been studied extensively under air (4) and under nitrogen (5-9). PE is known to undergo oxidative decomposition by random chain scission forming a wide range of volatile products (4-9). In our previous article (10), poly(ethylene-co-glycidyl methacrylate) (PEGMA) shows better thermal stability in air at lower heating rates. It is proposed that the epoxy groups in PEGMA react with the hydroxyl groups from thermal degradation and form a stable crosslinking structure.

In this article, maleic anhydride grafted polyethylene (manPE) is studied. manPE is a common compatibilizer for PE with inorganic (11) or organic materials (12). Maleic anhydride group is chemically similar to epoxy group of PEGMA and can also react with other functional groups as hydroxyl and amide groups. It is thus interesting to know whether manPE thermally degrades in a similar way to that of PEGMA.

EXPERIMENTAL

Materials

manPE (trade name: Fusabond [R] E MB265D) from Du Pont, USA is a commercially available grade with a melt flow index (MFI) of 12.3 g/10 min. (190[degrees]C X 2.16 kgf, ASTM D1238). The material was dried at 50[degrees]C in a vacuum oven for 120 h before testing.

Characterizations

Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were conducted with a Perkin Elmer TGA-6 thermogravimetric analyzer and PYRIS-1 DSC under a nitrogen or air flow of 50 ml/min. In each scan 10-12 mg of the sample was used.

RESULTS AND DISCUSSION

Dynamic TGA Study

Figure 1 shows TGA scans of manPE in nitrogen at various heating rates. The curves shifted to a higher temperature with increasing heating rate in consistent with previous studies on the thermal degradation of PE (5), (13). Weight loss temperatures ([T.sub.d]) determined from the TGA curves by the bitangent method also increased with heating rate (Fig. 2). Figure 1 also shows the derivative thermogravimetry (DTG) data of manPE in nitrogen. Under nitrogen, manPE degraded in a single step and the temperature of the maximum degradation rate of manPE was shifted toward higher temperatures with increasing heating rate.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

In general, TGA curves for most polymers would shift to a higher temperature with increasing heating rate and several explanations had been proposed to account for this shifting. Some consider that a change of mechanism of thermal degradation has occurred when there is a change in heating rate; others consider inefficient heat transfer from furnace to samples with increasing heating rate (4-9).

In contrast to a simple shifting in nitrogen, TGA curves of manPE in air (Fig. 3) showed a considerably different pattern when heating rate was raised. The weight loss temperatures ([T.sub.d]) increased with heating rate initially up to 7[degrees]C/min, decreased between 7 and 10[degrees]C/min, and increased again beyond 10[degrees]C/min. Heating rate dependence of [T.sub.d] was summarized in Fig. 2. To increase heating rate in a TGA, dynamic experiment will generally delay the evaporation of degraded products and resulted in a higher [T.sub.d] (4-9). Such a delaying behavior was observed for manPE in nitrogen (Fig. 2) and also in our previous study (10), where the [T.sub.d] value for neat PE increases with heating rate (heating rate = 5, 10, 20, 30, 40, 50, and 60[degrees]C/min.). The unusual decomposition behavior of manPE in air seems not to come from the mechanistic effect; a similar delaying phenomenon was observed for PEGMA decomposed in air (10), which is attributed to the formation of a stabilizing crosslinking structure.

[FIGURE 3 OMITTED]

The DTG curves for manPE degraded in air contains irregularities depending on heating rate and three typical curves are shown in Fig. 4. At a low heating rate of 3[degrees]C/min, there is no obvious weight loss until a sharp peak occurs at 417[degrees]C, and a second peak appears at 432[degrees]C. At a medium heating rate of 8[degrees]C/min, a shoulder was observed at 337[degrees]C and two obvious peaks appear at 395 and 450[degrees]C. At a higher heating rate of 15[degrees]C/min, a first peak appears as a small shoulder at 338[degrees]C, a second peak appears at 434[degrees]C, and some irregular peaks appears at higher temperature. The small peaks (337-338[degrees]C) appearing at medium and high heating rate were thought to the loss of maleic anhydride groups, similar to the deacetylation of EVA copolymer decomposed in air (2). At a lower heating rate, however, the small peak was not observed and the main decomposition temperature shifted to a higher value (417[degrees]C). There maybe some reactions taking place with maleic anhydride groups to slow down the rate of decomposition.

[FIGURE 4 OMITTED]

Mechanisms

Almost all thermal oxidations of vinyl polymers at moderate temperatures are postulated to involve the hydroperoxide radical in the propagation step of the degradation. A mechanism of degradation in oxygen atmosphere is as following (1), (14):

The initiation step of degradation in oxygen produces the radical precursors.

Initiation RH [right arrow] R* + *H (1)

When the radical precursors react with oxygen, a peroxy radical intermediate is produced during the propagation step and results in a small increase in the sample mass at 220-260[degrees]C (Fig. 3).

Propagation R* + [O.sub.2] [right arrow] ROO* (2)

The ROO* species can suppress unzipping of the polymer and degradation follows only random scission that produces only high molecular weight species, which do not vaporize. A noticeable delay in the mass loss is observed in TGA measurements (1).

Reactive ROO* would extract hydrogen from other polymer chains and produce hydroperoxide species and another polymer radical.

ROO* + RH [right arrow] ROOH + R* (3)

Hydroperoxides have two ways to decompose, reactions (4) and (5), and the latter has a lower activation energy (105-125 kJ/mole) (15).

ROOH [right arrow] RO* + *OH (4)

ROOH + RH [right arrow] RO* + R* + [H.sub.2]O (5)

The alkoxyl radical can extract hydrogen from other polymer chains and generates radical species and hydroxyl groups.

RO* + RH [right arrow] ROH + R* (6)

The radical species (R*) thus generated accelerate the oxidative degradation. On the other hand, the hydroxyl groups (ROH) could react with maleic anhydride group to form a stable crosslinking structure in manPE and retard further degradation, as shown in Scheme 1. Sufficient time is allowed in slow heating for the hydroxyl groups and maleic anhydride groups to go through the stabilizing crosslinking reaction. At a higher heating rate, however, the oxidizing radical species R* generated in (6) dominate over ROH and accelerate the degradation.

DSC Analysis

Figure 5 shows the DSC scans of manPE in nitrogen and in air at a heating rate of 3[degrees]C/min. Both curves show a melting endothermic temperature at 133[degrees]C and a decomposition temperature at about 413[degrees]C, which are similar to the results obtained from TGA measurements. A broad exothermic peak (170-240[degrees]C) in air, not observed in nitrogen followed the endothermic melting peak and could be ascribed to a thermal crosslinking reation of the maleic anhydride groups of manPE with hydroxyl groups, which are formed during degradation (16), (17).

[FIGURE 5 OMITTED]

Thermal stability of manPE should be enhanced if the crosslinking reaction between ROH and the maleic anhydride groups is allowed to proceed to a higher degree. Figure 6 shows the dynamic TGA scans at a rate of 10[degrees]C/min for samples held at 220[degrees]C for various periods of time. As could be seen, the [T.sub.d] value shifted from 363 to 421[degrees]C with the holding time increased from 0 to 9 min.

[ILLUSTRATION OMITTED]

[FIGURE 6 OMITTED]

Isothermal TGA Study

Figure 7 shows the isothermal TGA curves of manPE at 350[degrees]C for 60 min. The specimens was heated at a rate of 10[degrees]C/min to 220[degrees]C, h at that temperature for different periods of time, and then heated again at a rate of 10[degrees]C/min to 350[degrees]C. At 350[degrees]C, manPE degraded and the residual weight were, respectively, 73.7, 86.9, 95.8, and 96.8 wt% for the holding periods of 0, 3, 6, and 9 min at 220[degrees]C. More residues were obtained for specimen held longer at 220[degrees]C. The results support the hypothesis that the maleic anhydride groups of manPE, at 220[degrees]C, could participate in the stabilizing crosslinking reactions, as described above.

[FIGURE 7 OMITTED]

Figure 8 shows the isothermal TGA curves for manPE heated to 350[degrees]C at different heating rate and held at that temperature for 60 min. As can be seen in Fig. 8, the residual weight is 97.2, 96.9, 73.6, and 69.8 wt% for heating rate is 5, 7, 10, and 20[degrees]C/min, respectively. A slower heating rate gave rise to higher residual weight indicating that a slower heating rate allows the maleic anhydride groups to have adequate time to undergo thermal crosslinking reactions.

[FIGURE 8 OMITTED]

Results from both of the above two heating profiles suggested that a stabilizing crosslinking structure would form when the specimen were subject to lower heating rates or kept at 220[degrees]C for sufficient time. Enhanced thermal stability was observed for lower heating rates and prolonged heating at 220[degrees]C.

CONCLUSIONS

In contrast to the general behavior of a polymer, manPE showed delayed thermal degradation in air when heated at a lower rate. Degradation of manPE is controlled by two counteractive factors of chain scission and crosslinking reaction. The radical species R* generated in thermal decomposition accelerate the chain scission and reduce the thermal stability; on the other hand, the hydroxyl groups (ROH) formed during the degradation react with the maleic anhydride groups to form a stabilizing crosslinking structure and enhance the thermal stability. Both slow heating rate and prolonged heating at 220[degrees]C encourage the formation of the crosslinking structure by allowing adequate time for the stabilizing reaction to occur between ROH and the maleic anhydride groups and thus led to higher thermal stability of manPE.

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Correspondence to: Jiann-WEn Huang; e-mail: jw.huang@msa.hinet.net DOI 10.1002/pen.21129

Published online in Wiley InterScience (www.interscience.wiley.com). [C] 2008 Society of Plastics Engineers

Jiann-Wen Huang, (1) Wei-Chieh Lu, (2), Mou-Yu Yeh, (2) Chih-Hsiang Lin, (3) I-Shou Tsai (3)

(1) Department of Styling and Cosmetology, Tainan University of Technology, Yung Kang City 710, Taiwan, Republic of China

(2) Department of Chemistry, National Cheng Kung University, Tainan City 701, Taiwan, Republic of China

(3) Department of Fiber and Composite Materials, Feng Chia University, Seatwen, Taichung, Taiwan 40724, Republic of China
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Author:Huang, Jiann-Wen; Lu, Wei-Chieh; Yeh, Mou-Yu; Lin, Chih-Hsiang; Tsai, I-Shou
Publication:Polymer Engineering and Science
Article Type:Technical report
Geographic Code:1USA
Date:Aug 1, 2008
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