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Effect of cross-linking on the thermal stability and molar mass distribution of paraffin waxes
Polymer Degradation and Stability 73 (2001) 151–155 www.elsevier.nl/locate/polydegstab
Effect of cross-linking on the thermal stability and molar mass distribution of paraffin waxes F.M. Mhlongo, A.S. Luyt*, C.G.C.E. van Sittert School of Chemical Sciences, University of the North (Qwa-Qwa), Private Bag X13, Phuthaditjhaba 9866, South Africa Received 6 February 2001; accepted 17 March 2001
Abstract The influence of dicumyl peroxide (DCP) (5, 10 and 15% by mass) on the structure and thermal properties of two different paraffin waxes was investigated. For both waxes an increase of gel content was observed for increasing DCP concentration. Gel permeation chromatography, however, showed a decrease in intensity of the main peak, and the development of a lower molar mass peak. Differences were observed between the DSC curves for the untreated and treated samples, but these could not, with certainty, be linked to either cross-linking or degradation. TGA curves showed two well-developed decomposition steps for treated samples, which is in line with the development of a second peak in the GPC chromatogram. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Paraffin wax; Dicumyl peroxide; Degradation; Cross-linking; DSC; TGA; GPC
1. Introduction Cross-linking of waxes received little attention, but Brink and Dressler [1] found that the extent of crosslinking depends on the amount of cross-linking agent used. They found that when a hard paraffin wax was cross-linked in the presence of dicumyl peroxide (DCP), the congealing point of the cross-linked wax decreases with increasing peroxide/wax (m/m) ratio. The elasticity also increased and beyond 50/50 m/m peroxide/wax ratio, an insoluble, infusible, hard, brittle gel was obtained. Cross-linking initiated by thermal decomposition of peroxides results in a decrease in the crystalline content. This decrease is not extensive and corresponds to the gel content. For example, the crystalline portion in lowdensity polyethylene (LDPE) was found to drop from the original value of 41% to a value of 36.5% after cross-linking [2]. The number of cross-links formed in a polymer may be derived from the amount of decomposed peroxide and from the cross-linking efficiency. If the reaction conditions are chosen so that about five
* Corresponding author. Tel.: +27-58-713-0152; fax: +27-58-7130152. E-mail address: [email protected] (A.S. Luyt).
half-lives of peroxide decomposition are reached, the amount of undecomposed peroxide (about 3% of its original concentration) may be ignored [3]. In a structural investigation of chemically cross-linked low-density polyethylene, it was found that the gel fraction is high and shows no extremes, and that it increases slightly with considerable increases in the peroxide content. In addition, it was pointed out that the proportionality between gel content and concentration of peroxide does not exist at large concentrations [4]. Luyt et al. [5] investigated the extent of cross-linking of different paraffin waxes. They found that cross-linking increases with an increase in DCP/wax ratio. They also found that the solubility of the materials decreases with an increase in DCP/wax ratio. Luyt and Ishripersadh [6] investigated the cross-linking of three paraffin waxes in the presence of dicumyl peroxide, but here the samples were only mechanically mixed and then heated in the DSC and TGA. They therefore thermoanalytically followed the cross-linking process. In this paper we investigated the influence of peroxide cross-linking on the thermal properties and molar mass distribution of two different waxes in the presence of dicumyl peroxide.
0141-3910/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(01)00081-7
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F.M. Mhlongo et al. / Polymer Degradation and Stability 73 (2001) 151–155
Table 1 Gel content values Wax H1/DCP
% Gel
Samples Wax M3/DCP
% Gel
100/0 95/5 90/10 85/15
3.9 14.9 20.7 37.4
100/0 95/5 90/10 85/15
2.9 4.0 7.9 12.4
2. Experimental For cross-linking, hard (carbon distribution C28C120, molar mass 0.785 kg mol 1, density 940 kg m 3, melting point 104 C) and medium (carbon distribution C15-C78, molar mass 0.440 kg mol 1, density 900 kg m 3, melting point 72 C) waxes were mechanically mixed in 100/0, 95/5, 90/10, and 85/5 m/m ratios with dicumyl peroxide. These samples were pressed in a hot melt press at 160 C for 3 min. Thermogravimetric analysis (TGA) was carried out on a Perkin-Elmer TGA7 thermogravimetric analyzer in nitrogen atmosphere. Samples (5–10 mg) were heated at 10 C min 1 from 25 to 600 C. Differential scanning calorimetry (DSC) was carried out on a Perkin-Elmer DSC 7 thermal analyzer in nitrogen atmosphere. Samples (5–10 mg) were heated at a rate of 10 C min 1 from 25 to 140 C, cooled to 25 C at the same rate, and reheated. Gel permeation chromatography (GPC) was carried out on a Waters model 150-C ALC/GPC equipped with a refractive index detector and two Waters Styragel HR 4E (7.8300 mm) columns. A flow rate of 1.0 ml min 1 and injection volume of 200.00 ml was used. The analyses were performed at 100 C. Xylene was used as the mobile phase, and polystyrene standards, covering a broad molar mass range (1000 to 1,000,000), were used at concentrations of 0.05 g ml 1. A 12 h xylene extraction of the samples was used to determine the gel content.
3. Results and discussion Values for gel content, which are indicative of the extent of cross-linking, are summarized in Table 1. These data show that the cross-linking of wax increases with increasing peroxide content. There is a constant increase within the investigated concentration range, which confirms the conclusion that wax needs a higher concentration of DCP for cross-linking [7]. It is also clear from the tabulated values that the hard wax crosslinks more effectively than the medium wax, probably because of its longer chains. The GPC curves in Fig. 1 show a decrease in the intensity of the peak maximum with increasing DCP content up to 10%. This is accompanied by the development of a peak at lower molar mass values, which
Fig. 1. GPC curves for hard paraffin wax.
Fig. 2. GPC curves for medium paraffin wax.
may be indicative of degradation and chain scission. In the presence of 15% DCP the peak maximum increases to a value higher than that for untreated wax. It seems as if the wax is primarily degraded in the presence of up to 10% DCP, after which cross-linking becomes dominant. The GPC curves in Fig. 2 show a decrease in the intensity of the peak maximum with increasing DCP content. This decrease in peak maximum is also accompanied by the development of a peak at lower molar mass values, which may be indicative of degradation and chain scission. It seems as if the medium wax is primarily degraded in the presence of DCP, and any cross-linking, indicated by the increase in gel content values (Table 1), is not obvious from the GPC results. Fig. 3 shows the DSC (heating) curves for untreated and the treated hard paraffin wax samples in nitrogen atmosphere. The curve for untreated wax shows a multiple endothermic event with the first peak at 75 C, the second peak at 104 C and a peak shoulder at 90 C. This endothermic event indicates wax melting. There are slight differences in the shapes of this endotherm when
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Table 2 Onset temperatures, peak temperatures and melting enthalpies of untreated and treated hard paraffin wax Samples Wax/DCP
To,m/ C
Tp,c/ C
Hc/ J g
100/0 95/5 90/10 85/15
60.0 56.9 53.7 52.6
77.2 76.0 74.3 75.4
210.4 162.1 146.0 145.5
1
Table 3 Onset temperatures, peak temperatures and freezing enthalpies of untreated and treated hard paraffin wax
Fig. 3. DSC heating curves for hard paraffin wax samples.
Samples Wax/DCP
To,c/ C
Tp,c/ C
100/0 95/5 90/10 85/15
95.6 93.9 91.2 91.7
91.5 91.1 88.5 89.3
Hc/ J g
1
211.6 210.1 177.7 142.8
Table 4 Onset temperatures, peak temperatures and freezing enthalpies of untreated and treated medium paraffin wax Samples To,m/ C Tp,m/ C Hm/J g Wax/DCP 100/0 95/5 90/10 85/15
Fig. 4. DSC cooling curves for hard paraffin wax samples.
the wax is reacted with DCP. The onset temperatures of melting and the melting enthalpies for these samples are summarized in Table 2. These data show a decrease in onset temperature of melting, as well as melting enthalpy, with increasing dicumyl peroxide content. The peak temperature of melting initially decreases, followed by an increase in temperature. The decrease in peak temperature may indicate an increase in the extent of cross-linking with increasing peroxide content. It may also be the result of degradation of the wax, as seen in the discussion of the GPC results. Fig. 4 shows the DSC cooling curves for untreated and the treated hard paraffin wax samples in nitrogen atmosphere. The untreated wax shows two exothermic events. The first peak is at 92 C and the second peak at 70 C. The curves for wax samples mixed with increasing amounts of DCP show similar exothermic events. The results obtained from these curves are summarized in Table 3. These data show a decrease in onset temperature with increasing DCP content, with a slight increase
44.9 40.3 39.9 37.0
57.2 54.4 54.2 53.2
164.7 116.8 118.9 114.9
1
To,c/ C Tp,c/ C Hc/J g 54.1 51.7 52.1 51.2
49.6 47.5 48.5 46.0
1
141.8 123.9 99.6 90.4
when DCP is in excess of 10%. The enthalpy, however, continuously decreases with increasing DCP content. Cross-linking reduces crystallinity and, therefore, Tc and enthalpy decrease with an increase in DCP concentration. Cross-linking is, however, limited up to 10% DCP so that degradation may also have contributed to the observed trends. Figs. 5 and 6 show the DSC heating and cooling curves in nitrogen atmosphere for untreated and a treated medium paraffin wax. The results obtained from the heating and cooling curves are summarized in Table 4. These data show that there is a decrease in onset temperature of melting, as well as enthalpy of melting, with an increase in DCP content. This is probably the result of sample degradation, but it may also be the result of reduced crystallinity caused by cross-linking, or a combination of both. Figs. 7 and 8 show that untreated samples of both waxes decompose in a single step, while treated samples decompose in two clearly defined steps. For both waxes the onset temperatures for the first step are clearly lower than that for the decomposition of untreated waxes. Untreated hard paraffin wax starts decomposing at about 300 C, while the treated samples initially decompose at
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Fig. 5. DSC heating curves for medium paraffin wax samples.
Fig. 7. TGA curves for hard paraffin wax samples.
Fig. 6. DSC cooling curves for medium paraffin wax samples.
Fig. 8. TGA curves for medium paraffin wax samples.
Table 5 Temperatures of 5, 10, 20, 30 and 40% mass loss of hard paraffin wax samples in nitrogen atmosphere
Table 6 Temperatures of 5, 10, 20, 30 and 40% degradation of medium paraffin wax samples in nitrogen atmosphere
5%/
Samples Wax/DCP
T
100/0 95/5 90/10 85/15
211.4 190.1 175.2 161.0
C
T 10%/ C
T
242.6 239.2 196.2 185.1
266.7 339.4 322.2 306.1
20%/
C
T
30%/
278.8 264.6 355.8 341.5
C
T 40%/ C
Samples Wax/ DCP
T
287.8 384.2 381.0 362.9
100/0 95/5 90/10 85/14
225.0 195.5 185.9 164.9
about 150 C, followed by a second decomposition step starting at about 300 C. Medium paraffin wax shows similar behaviour, but the decomposition steps are not so well separated as in the case of hard paraffin wax. These observations are in line with the observation of the development of a lower molar mass peak during GPC analyses, which indicates the development of a lower molar mass wax fraction as a result of heating in the presence of DCP.
5%/
C
T
10%/
241.3 229.8 212.8 181.4
C
T
20%/
261.4 260.7 240.5 224.5
C
T 30%/ C
T
274.8 278.4 256.2 253.9
285.4 290.6 272.2 267.3
40%/
C
The thermal stabilities of cross-linked and uncrosslinked waxes were characterized in terms of the temperature of 5, 10, 20, 30 and 40% mass loss. The results are summarized in Tables 5 and 6. These data indicate that the thermal stability of the waxes decreases with an increase in DCP content, which will be the result of, inter alia, degradation. It is not yet clear what the influence of cross-linking will be on the thermal stability of waxes.
F.M. Mhlongo et al. / Polymer Degradation and Stability 73 (2001) 151–155
References [1] Brink A, Dressler F. Br Polym J 1969;1:37–40. [2] Chodak I. Prog Polym Sci 1995;20:1165–99. [3] Lazar M, Rado R, Rychly J. Adv Polym Sci 1990;95:149–97.
155
[4] Kunert KA, Soszynska H, Pislewski N. Polymer 1981;22:1355–60. [5] Nhlapo NS, Luyt AS, Vosloo HCM. J Appl Polym Sci 1998;70:1551–9. [6] Luyt AS, Ishripersadh K. Thermochimica Acta 1999;333:155–67. [7] Hlangothi SP, Luyt AS. Thermochimica Acta 2000;360:41–6.
Effect of cross-linking on the thermal stability and molar mass distribution of paraffin waxes F.M. Mhlongo, A.S. Luyt*, C.G.C.E. van Sittert School of Chemical Sciences, University of the North (Qwa-Qwa), Private Bag X13, Phuthaditjhaba 9866, South Africa Received 6 February 2001; accepted 17 March 2001
Abstract The influence of dicumyl peroxide (DCP) (5, 10 and 15% by mass) on the structure and thermal properties of two different paraffin waxes was investigated. For both waxes an increase of gel content was observed for increasing DCP concentration. Gel permeation chromatography, however, showed a decrease in intensity of the main peak, and the development of a lower molar mass peak. Differences were observed between the DSC curves for the untreated and treated samples, but these could not, with certainty, be linked to either cross-linking or degradation. TGA curves showed two well-developed decomposition steps for treated samples, which is in line with the development of a second peak in the GPC chromatogram. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Paraffin wax; Dicumyl peroxide; Degradation; Cross-linking; DSC; TGA; GPC
1. Introduction Cross-linking of waxes received little attention, but Brink and Dressler [1] found that the extent of crosslinking depends on the amount of cross-linking agent used. They found that when a hard paraffin wax was cross-linked in the presence of dicumyl peroxide (DCP), the congealing point of the cross-linked wax decreases with increasing peroxide/wax (m/m) ratio. The elasticity also increased and beyond 50/50 m/m peroxide/wax ratio, an insoluble, infusible, hard, brittle gel was obtained. Cross-linking initiated by thermal decomposition of peroxides results in a decrease in the crystalline content. This decrease is not extensive and corresponds to the gel content. For example, the crystalline portion in lowdensity polyethylene (LDPE) was found to drop from the original value of 41% to a value of 36.5% after cross-linking [2]. The number of cross-links formed in a polymer may be derived from the amount of decomposed peroxide and from the cross-linking efficiency. If the reaction conditions are chosen so that about five
* Corresponding author. Tel.: +27-58-713-0152; fax: +27-58-7130152. E-mail address: [email protected] (A.S. Luyt).
half-lives of peroxide decomposition are reached, the amount of undecomposed peroxide (about 3% of its original concentration) may be ignored [3]. In a structural investigation of chemically cross-linked low-density polyethylene, it was found that the gel fraction is high and shows no extremes, and that it increases slightly with considerable increases in the peroxide content. In addition, it was pointed out that the proportionality between gel content and concentration of peroxide does not exist at large concentrations [4]. Luyt et al. [5] investigated the extent of cross-linking of different paraffin waxes. They found that cross-linking increases with an increase in DCP/wax ratio. They also found that the solubility of the materials decreases with an increase in DCP/wax ratio. Luyt and Ishripersadh [6] investigated the cross-linking of three paraffin waxes in the presence of dicumyl peroxide, but here the samples were only mechanically mixed and then heated in the DSC and TGA. They therefore thermoanalytically followed the cross-linking process. In this paper we investigated the influence of peroxide cross-linking on the thermal properties and molar mass distribution of two different waxes in the presence of dicumyl peroxide.
0141-3910/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(01)00081-7
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F.M. Mhlongo et al. / Polymer Degradation and Stability 73 (2001) 151–155
Table 1 Gel content values Wax H1/DCP
% Gel
Samples Wax M3/DCP
% Gel
100/0 95/5 90/10 85/15
3.9 14.9 20.7 37.4
100/0 95/5 90/10 85/15
2.9 4.0 7.9 12.4
2. Experimental For cross-linking, hard (carbon distribution C28C120, molar mass 0.785 kg mol 1, density 940 kg m 3, melting point 104 C) and medium (carbon distribution C15-C78, molar mass 0.440 kg mol 1, density 900 kg m 3, melting point 72 C) waxes were mechanically mixed in 100/0, 95/5, 90/10, and 85/5 m/m ratios with dicumyl peroxide. These samples were pressed in a hot melt press at 160 C for 3 min. Thermogravimetric analysis (TGA) was carried out on a Perkin-Elmer TGA7 thermogravimetric analyzer in nitrogen atmosphere. Samples (5–10 mg) were heated at 10 C min 1 from 25 to 600 C. Differential scanning calorimetry (DSC) was carried out on a Perkin-Elmer DSC 7 thermal analyzer in nitrogen atmosphere. Samples (5–10 mg) were heated at a rate of 10 C min 1 from 25 to 140 C, cooled to 25 C at the same rate, and reheated. Gel permeation chromatography (GPC) was carried out on a Waters model 150-C ALC/GPC equipped with a refractive index detector and two Waters Styragel HR 4E (7.8300 mm) columns. A flow rate of 1.0 ml min 1 and injection volume of 200.00 ml was used. The analyses were performed at 100 C. Xylene was used as the mobile phase, and polystyrene standards, covering a broad molar mass range (1000 to 1,000,000), were used at concentrations of 0.05 g ml 1. A 12 h xylene extraction of the samples was used to determine the gel content.
3. Results and discussion Values for gel content, which are indicative of the extent of cross-linking, are summarized in Table 1. These data show that the cross-linking of wax increases with increasing peroxide content. There is a constant increase within the investigated concentration range, which confirms the conclusion that wax needs a higher concentration of DCP for cross-linking [7]. It is also clear from the tabulated values that the hard wax crosslinks more effectively than the medium wax, probably because of its longer chains. The GPC curves in Fig. 1 show a decrease in the intensity of the peak maximum with increasing DCP content up to 10%. This is accompanied by the development of a peak at lower molar mass values, which
Fig. 1. GPC curves for hard paraffin wax.
Fig. 2. GPC curves for medium paraffin wax.
may be indicative of degradation and chain scission. In the presence of 15% DCP the peak maximum increases to a value higher than that for untreated wax. It seems as if the wax is primarily degraded in the presence of up to 10% DCP, after which cross-linking becomes dominant. The GPC curves in Fig. 2 show a decrease in the intensity of the peak maximum with increasing DCP content. This decrease in peak maximum is also accompanied by the development of a peak at lower molar mass values, which may be indicative of degradation and chain scission. It seems as if the medium wax is primarily degraded in the presence of DCP, and any cross-linking, indicated by the increase in gel content values (Table 1), is not obvious from the GPC results. Fig. 3 shows the DSC (heating) curves for untreated and the treated hard paraffin wax samples in nitrogen atmosphere. The curve for untreated wax shows a multiple endothermic event with the first peak at 75 C, the second peak at 104 C and a peak shoulder at 90 C. This endothermic event indicates wax melting. There are slight differences in the shapes of this endotherm when
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F.M. Mhlongo et al. / Polymer Degradation and Stability 73 (2001) 151–155
Table 2 Onset temperatures, peak temperatures and melting enthalpies of untreated and treated hard paraffin wax Samples Wax/DCP
To,m/ C
Tp,c/ C
Hc/ J g
100/0 95/5 90/10 85/15
60.0 56.9 53.7 52.6
77.2 76.0 74.3 75.4
210.4 162.1 146.0 145.5
1
Table 3 Onset temperatures, peak temperatures and freezing enthalpies of untreated and treated hard paraffin wax
Fig. 3. DSC heating curves for hard paraffin wax samples.
Samples Wax/DCP
To,c/ C
Tp,c/ C
100/0 95/5 90/10 85/15
95.6 93.9 91.2 91.7
91.5 91.1 88.5 89.3
Hc/ J g
1
211.6 210.1 177.7 142.8
Table 4 Onset temperatures, peak temperatures and freezing enthalpies of untreated and treated medium paraffin wax Samples To,m/ C Tp,m/ C Hm/J g Wax/DCP 100/0 95/5 90/10 85/15
Fig. 4. DSC cooling curves for hard paraffin wax samples.
the wax is reacted with DCP. The onset temperatures of melting and the melting enthalpies for these samples are summarized in Table 2. These data show a decrease in onset temperature of melting, as well as melting enthalpy, with increasing dicumyl peroxide content. The peak temperature of melting initially decreases, followed by an increase in temperature. The decrease in peak temperature may indicate an increase in the extent of cross-linking with increasing peroxide content. It may also be the result of degradation of the wax, as seen in the discussion of the GPC results. Fig. 4 shows the DSC cooling curves for untreated and the treated hard paraffin wax samples in nitrogen atmosphere. The untreated wax shows two exothermic events. The first peak is at 92 C and the second peak at 70 C. The curves for wax samples mixed with increasing amounts of DCP show similar exothermic events. The results obtained from these curves are summarized in Table 3. These data show a decrease in onset temperature with increasing DCP content, with a slight increase
44.9 40.3 39.9 37.0
57.2 54.4 54.2 53.2
164.7 116.8 118.9 114.9
1
To,c/ C Tp,c/ C Hc/J g 54.1 51.7 52.1 51.2
49.6 47.5 48.5 46.0
1
141.8 123.9 99.6 90.4
when DCP is in excess of 10%. The enthalpy, however, continuously decreases with increasing DCP content. Cross-linking reduces crystallinity and, therefore, Tc and enthalpy decrease with an increase in DCP concentration. Cross-linking is, however, limited up to 10% DCP so that degradation may also have contributed to the observed trends. Figs. 5 and 6 show the DSC heating and cooling curves in nitrogen atmosphere for untreated and a treated medium paraffin wax. The results obtained from the heating and cooling curves are summarized in Table 4. These data show that there is a decrease in onset temperature of melting, as well as enthalpy of melting, with an increase in DCP content. This is probably the result of sample degradation, but it may also be the result of reduced crystallinity caused by cross-linking, or a combination of both. Figs. 7 and 8 show that untreated samples of both waxes decompose in a single step, while treated samples decompose in two clearly defined steps. For both waxes the onset temperatures for the first step are clearly lower than that for the decomposition of untreated waxes. Untreated hard paraffin wax starts decomposing at about 300 C, while the treated samples initially decompose at
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F.M. Mhlongo et al. / Polymer Degradation and Stability 73 (2001) 151–155
Fig. 5. DSC heating curves for medium paraffin wax samples.
Fig. 7. TGA curves for hard paraffin wax samples.
Fig. 6. DSC cooling curves for medium paraffin wax samples.
Fig. 8. TGA curves for medium paraffin wax samples.
Table 5 Temperatures of 5, 10, 20, 30 and 40% mass loss of hard paraffin wax samples in nitrogen atmosphere
Table 6 Temperatures of 5, 10, 20, 30 and 40% degradation of medium paraffin wax samples in nitrogen atmosphere
5%/
Samples Wax/DCP
T
100/0 95/5 90/10 85/15
211.4 190.1 175.2 161.0
C
T 10%/ C
T
242.6 239.2 196.2 185.1
266.7 339.4 322.2 306.1
20%/
C
T
30%/
278.8 264.6 355.8 341.5
C
T 40%/ C
Samples Wax/ DCP
T
287.8 384.2 381.0 362.9
100/0 95/5 90/10 85/14
225.0 195.5 185.9 164.9
about 150 C, followed by a second decomposition step starting at about 300 C. Medium paraffin wax shows similar behaviour, but the decomposition steps are not so well separated as in the case of hard paraffin wax. These observations are in line with the observation of the development of a lower molar mass peak during GPC analyses, which indicates the development of a lower molar mass wax fraction as a result of heating in the presence of DCP.
5%/
C
T
10%/
241.3 229.8 212.8 181.4
C
T
20%/
261.4 260.7 240.5 224.5
C
T 30%/ C
T
274.8 278.4 256.2 253.9
285.4 290.6 272.2 267.3
40%/
C
The thermal stabilities of cross-linked and uncrosslinked waxes were characterized in terms of the temperature of 5, 10, 20, 30 and 40% mass loss. The results are summarized in Tables 5 and 6. These data indicate that the thermal stability of the waxes decreases with an increase in DCP content, which will be the result of, inter alia, degradation. It is not yet clear what the influence of cross-linking will be on the thermal stability of waxes.
F.M. Mhlongo et al. / Polymer Degradation and Stability 73 (2001) 151–155
References [1] Brink A, Dressler F. Br Polym J 1969;1:37–40. [2] Chodak I. Prog Polym Sci 1995;20:1165–99. [3] Lazar M, Rado R, Rychly J. Adv Polym Sci 1990;95:149–97.
155
[4] Kunert KA, Soszynska H, Pislewski N. Polymer 1981;22:1355–60. [5] Nhlapo NS, Luyt AS, Vosloo HCM. J Appl Polym Sci 1998;70:1551–9. [6] Luyt AS, Ishripersadh K. Thermochimica Acta 1999;333:155–67. [7] Hlangothi SP, Luyt AS. Thermochimica Acta 2000;360:41–6.