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Effect of processability on the thermal stability of the blends based on polyurethane (Part III)
Polymer Degradation and Stability 70 (2000) 263±267
www.elsevier.nl/locate/polydegstab
Eect of processability on the thermal stability of the blends based on polyurethane (Part III) M. Maity, B.B. Khatua, C.K. Das * Materials Science Centre, Indian Institute of Technology, Kharagpur, 721302, India Received 7 March 2000; received in revised form 10 April 2000; accepted 18 May 2000
Abstract The thermal stability of blends of polyurethane with two dierent elastomers having reactive functional groups has been studied. The blends have been prepared by two dierent blending techniques, keeping the blending ratio constant. It was found that the thermal stability of the blends was dictated by the blending technique. The preblended sample degraded at an earlier temperature. The degradation temperatures were increased when a preheating preblending technique was adopted. This may be due to the formation of interchain crosslink bonds between the two-elastomer phases on heat treatment. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Polyurethane; Preblending; Preheating preblending; Interchain crosslinking; Polyblend; Dicumyl peroxide
1. Introduction Polymer blending has become an inseparable part of polymer processing. It is widely accepted as the most economic and fastest way to produce new polymeric materials with a balanced combination of properties. [1±3]. The mixing operation is an important step in blend preparation and it is well known that certain properties depend on it [4±6]. Two dierent reactive polymers can chemically interact at high temperature to form interchain crosslink polyblends. Das et al. have developed such interchain crosslinkable elastomer±elastomer [7±10] and elastomer±plastic blend [11]. Recent studies showed that polyurethane is able to crosslink with EVA [12] and CSM [13] at high temperature without any curatives. De and co-workers have reported that rubbers having appropriate functional groups interact with each other when blended and thus crosslink at high temperature in the absence of any curatives [14±22]. In the previous paper [23] we have reported the eect of blending technique towards the thermal stability of the blends of polyurethane with chlorosulfonated polyethylene (CSM), epichlorohydrin (EPH) and chloroprene (CR) elastomers. The present study was undertaken with * Corresponding author. Tel.: +91 3222 55221; fax: +91-322255303. E-mail address: [email protected] (C.K. Das).
a view to investigate the thermal stability of the blends of polyurethane with two dierent speciality elastomers, such as chlorinated polyethylene (CPE) and ethylene acrylic elastomers (Vamac). We have already reported that polyurethane forms interchain crosslink bonds with CPE [24] and Vamac [25] elastomers at high temperatures without any curatives. Degradation is a kinetic process and accompanied by the chemical bond dissociation [26]. Thus there is de®nite correlation between the chemical structure of a polymer and its degradation temperature [27,28]. Considering this blends were prepared to study the eect of interchain crosslinking reaction on the thermal stability of blends. 2. Experimental 2.1. Materials used Polyurethane (AU) : Vibrathane- 5004 from Uniroyal Co., USA.
Chlorinated polyethylene (CPE) : CM 013 from Bayer Germany.
0141-3910/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(00)00125-7
264
M. Maity et al. / Polymer Degradation and Stability 70 (2000) 263±267 Table 2 Compounding formulation for the AU/Vamac blends Elastomers
Preblend
Preheated preblend
Ethylene acrylic elastomer (Vamac): Vamac±GR from Du Pont, USA.
AU Vamac DCP
50 50 4
50 50 4
Dicumyl peroxide (DCP) : Varox 40 KE from Vanderbilt Comp, USA.
Dierential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) of the blends were carried out with a Shimadzu thermal analyzer (DT-40) in air at a heating rate of 10 C/min, in the temperature range 25±600 C. To determine the glass transition temperature (Tg) of the blends [29,30], Dierential scanning calorimetry (DSC) was conducted in a Stanton Redcroft thermal analyzer, STA-625.
Blends of polyurethane with the two elastomers were prepared at room temperature (25 C) in an internal mixer at a constant shear rate. To study the eect of interchain crosslinking to the thermal stability of the blends, the (50:50) blends of (AU: CPE), and (AU: Vamac) were prepared through two dierent blending techniques, e.g. preblending technique and preheating preblending technique. DCP was used as the curative. The compounding formulations for the AU/CPE and AU/ Vamac systems are given in Tables 1 and 2, respectively. 1. Preblending technique: in this technique, the two elastomers were ®rst blended at (50:50) ratio and the blend allowed to equilibrate for 24 h. Then DCP was incorporated into the blend as curative. 2. Preheating preblending technique: for the preheating preblending technique, the preblend was subjected to heat treatment at 150 C for 15 min and then the curatives were incorporated into the blend at room temperature (25 C). The amount of curative was the same as with the preblending technique. The continuous cure characteristics and the processability of the blends were studied in a Monsanto rheometer (R-100) at 150 C. Blends were then allowed to cure in a hot press at 150 C under constant pressure (20 MPa) up to their optimum cure time. Table 1 Compounding formulation for the AU/CPE blends Elastomers
Preblend
Preheated preblend
AU CPE DCP
50 50 2
50 50 2
3. Results and discussion 3.1. Thermal analysis of the AU/CPE blends To study the compatibility of the blend with reference to the eect of preheating of the preblend, the low temperature DSC of the (50:50) AU/CPE has been studied. For (50:50) AU/CPE preblended sample a single Tg was observed at ÿ44 C, whereas in case of the (50:50) AU/ CPE preheated preblended sample, the Tg value shifted to the higher temperature side (ÿ40 C). The Tg value in the blend may have been shifted to the higher temperature side due to crosslinking by the addition of curatives. But in the case of the preheated preblended technique, a higher shifting of the Tg was observed. This is probably because of the interchain crosslinking reaction occurring between the two elastomer phases during the heat treatment. That again, enhances the phase adhesion, aecting the segmental motion. High temperature DSC/TGA of the (50/50) AU/CPE blends prepared by two dierent blending techniques were considered to correlate the eect of blending technique on the thermal stability. In each case, the degradation occurred in two steps, associated with exothermic peaks in the DSC curves. The degradation temperatures and the exothermic heats of oxidative degradation of the blends are given in Table 3. For the preblended sample the initial degradation (T1) started at 254 C and continued up to 376 C where the second degradation (T2) started. 50% degradation (T50) and 90% degradation (T90) of the sample started at 417 and 510 C, respectively (Fig. 1). For the preheated sample of (50:50) AU/CPE, the degradation started at 303 C and lead to the second degradation at 411 C. T50 and T90 of the sample occurred at 442 and 514 C, respectively (Fig. 2). The delayed degradation for preheated preblended sample suggested that the preheated
M. Maity et al. / Polymer Degradation and Stability 70 (2000) 263±267
265
Table 3 Degradation temperatures, exothermic heat of oxidative degradation of the AU/CPE blends Blends(AU/CPE)
T1( C)
T2( C)
T50( C)
T90( C)
Exothermic heat of degradation (J/gm)
Preblend Preheated preblend
254 303
376 411
417 442
510 514
1430 1340
Table 4 Onset temperatures and exothermic heats of vulcanization of the blends without any curative Blends
Fig. 1. DSC/TGA plot of the AU/CPE blend prepared by the preblending technique.
Onset temperature (0C)
Exothermic heat of vulcanization (J/gm)
(AU: CPE) 80: 20 50: 50 20: 80
130 116 122
11 22.7 16
(AU: Vamac) 80: 20 50: 50 20: 80
164 110 122
15.8 21.9 17.2
clearly corroborates the earlier ®nding that the interchain crosslinking between the two elastomeric phases which probably occurred to a maximum extent at the (50:50) AU/CPE ratio. 3.2. Thermal analysis of the AU/Vamac blends
Fig. 2. DSC/TGA plot of the AU/CPE blend prepared by the preheating preblending technique.
samples were more stable towards thermal degradation because of extra crosslinking introduced between the two phases. The exothermic peak characterizes the DSC plots of the two types of blends. The heat of oxidative degradation was measured from the DSC curves (Table 3). It is observed that the heat evolved for the preblended sample was higher than the preheated preblended samples, suggesting the ease of oxidative degradation of the preblended sample. Heat of vulcanization was also studied for the three blend systems reported in Table 4. It was observed that the heat of vulcanization was more at (50:50) AU/CPE blend ratio accompanied by low onset temperature. This
Low temperature DSC of the (50:50) AU/Vamac preblended and preheated preblended samples was conducted to study the compatibility of the blend. In both the cases two Tg values were observed indicating the blend is incompatible. The preblended sample showed the Tg values at ÿ32 and at ÿ10 C, whereas the preheated preblended sample showed the Tg values at ÿ27 and at ÿ8.2 C. The shifting of Tg values to the higher temperature side for the preheated preblended samples may be due to the certain amount of extra crosslinking between two elastomers on heat treatment. High temperature DSC/TGA plots of (50:50) AU/ Vamac preblended and preheated preblended sample are shown in Figs. 3 and 4, respectively. In each case, the degradation occurred in two steps, associated with exothermic peaks in the DSC curves. The degradation temperatures and the exothermic heats of oxidative degradation of the blends are given in Table 5. For (50:50) AU/Vamac preblended samples the initial degradation (T1) occurred at 246 C and continued up to 366 C where the second degradation (T2) started. 50% degradation (T50) and 90% degradation (T90) of the samples occurred at 425 C and 540 C, respectively (Fig. 3). In the case of the blend obtained by heating the
266
M. Maity et al. / Polymer Degradation and Stability 70 (2000) 263±267 Table 5 Degradation temperatures, exothermic heat of oxidative degradation of the AU/Vamac blends
Fig. 3. DSC/TGA plot of the AU/Vamac blend prepared by the preblending technique.
Blends (AU/Vamac)
T1 ( C)
T2 ( C)
T50 ( C)
T90 ( C)
Exothermic heat of degradation (J/g)
Preblend Preheated preblend
246 252
366 377
425 432
540 524
1680 1430
without any curatives. Three dierent cases were taken to study the eect of blend ratio on the onset temperature of crosslinking reaction and the heat of reaction. The onset temperature and the heat of crosslinking reaction are given in Table 4. In each case an exothermic peak was observed. The heat of reaction varied with blend ratio. The heat of reaction was highest for the (50:50) AU/Vamac blend ratio and the temperature of onset of the reaction also dropped more than for the other two blends. This suggests that two polymers react at high temperature in the absence of any curative and the extent of reaction depends on the blend ratio. 4. Conclusion
Fig. 4. DSC/TGA plot of the AU/Vamac blend prepared by the preheating preblending technique.
preblend, followed by the curative addition, the degradation occurred at relatively higher temperature. The initial degradation (T1) started at 252 C and continued up to 377 C where second degradation (T2) occurred. 50% degradation (T50) and 90% degradation (T90) of the sample occurred at 432 and 524 C, respectively (Fig. 4). From the above trend of change, it is evident that the preheated preblended sample is thermally more stable than the preblended sample. Again it was also supported from the DSC plot. In both cases the degradation was associated with exothermic peaks. the amount of heat of oxidative degradation was measured from DSC curves. It was found that the heat evolved for the preblended sample during oxidative degradation was more than the preheated preblended sample (Table 5). Evolution of higher amount of heat suggested the easier of oxidative degradation of the preblended sample. However, the amount of heat decreased for the preheated preblended sample, suggesting the higher stability of the preheated preblend towards oxidation. This may be due to the formation of a new chemical bond on heat treatment. The heat of vulcanization has been studied for AU/ Vamac blends. The AU and Vamac have been blended
There is a de®nite correlation between the chemical structure of a polymer and its degradation temperatures. Blends, prepared through the dierent blending techniques, having the same compounding formulation and same blending ratio dier in thermal behaviour. The early degradation of the preblended sample compared to the preheated preblended sample with the same elastomer ratio indicates the presence of some extra chemical linkages between the two phases. Interchain crosslinking reaction occurs in the blend of the two elastomers having reactive functional groups as a result of blending before addition of curatives. This extent of crosslinking increases when the preblend is subjected to heat treatment before curative addition. Thus, the thermal stability and the degradation temperature of the preheated preblended sample increases signi®cantly. References [1] Olabisi O, Robeson LM, Shaw MT. Polymer±polymer miscibility. New York: Academic Press, 1979. [2] Utracki LA. Polymer alloys and blends. Munich: Hanser Publishers, 1989. [3] Paul DR, Newman S. Polymer blends. New York: Academic Press, 1978. [4] Paul DR, Barlow JW. J Macromol Sci, Macromol Rev 1980;C18:109. [5] Manson JA, Sperling LH. Polymer blends and composites. New York: Plenum Press, 1976. [6] La Mantia FP, Valenza A, Acierno D. Polymer Degrad Stab 1985;13(1±4):1±9. [7] Singha Roy SK, Das CK. Polym Compos 1995;3:5. [8] Tripathy AR, Das CK. J Appl Polym Sci 1994;51:245.
M. Maity et al. / Polymer Degradation and Stability 70 (2000) 263±267 [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
Tripathy AR, Das CK. Plast Rubb Comp Process Appl 1994;21:5. Singha Roy SK, Das CK. J Polym Engg 1995;14:175. Mishra JK, Das CK. Int, J Polym Mater, in press. Maity M, Das CK. Int J Polym Mater 1998;45:1±2, 123. Khatua BB, Das CK. Int J Polym Mater, (in press). Alex R, De PP, De SK. J Polym Sci, Polym Lett Ed 1989;27:361. Alex R, De PP, De SK. Polym Commun 1990;31:67. Alex R, De PP, De SK. Kautsch, Gummi Kunstst 1991;44:333. Mukhopadhyay S, De SK. J Appl Polym Sci 1992;45:181. Mukhopadhyay S, De PP, De SK. J Appl Polym Sci 1991;43:347. Bhattacharya T, De SK. Eur Polym J 1991;27:1065. Mukhopadhyay S, Chaki TK, De SK. J Polym Sci Polym Lett Ed 1990;28:25.
[21] [22] [23] [24] [25] [26] [27] [28] [29] [30]
267
Mukhopadhyay S, De SK. J Mater Sci 1990;25:4027. Mukhopadhyay S, De SK De. J Appl Polym Sci 1991;42:2773. Khatua BB, Das CK. Polym Degrad Stab 2000;69:381. Maity M, Das CK. Polym Int, in press. Maity M, Das CK. Polym Plast Tech Engg, in press. Matveyev Yu I, Askadskii AA, Zhuravleva IV, Slonimskii GL, Korshak VV. Polym Sci 1981;23:9±12±2194. Korshak VV. Thermostoikiye polimery (Heat stable polymers). Moscow,: Nauka, 1969. p. 411 Korshak VV. Chemical structure and thermal stability of polymers. Moscow: Nauka, 1970. p. 419. Thomas S, George A. Eur Polym J 1992;28:145. Thomas S, Gupta BR, De SK. J Vinyl Technol 1987;9:71.
www.elsevier.nl/locate/polydegstab
Eect of processability on the thermal stability of the blends based on polyurethane (Part III) M. Maity, B.B. Khatua, C.K. Das * Materials Science Centre, Indian Institute of Technology, Kharagpur, 721302, India Received 7 March 2000; received in revised form 10 April 2000; accepted 18 May 2000
Abstract The thermal stability of blends of polyurethane with two dierent elastomers having reactive functional groups has been studied. The blends have been prepared by two dierent blending techniques, keeping the blending ratio constant. It was found that the thermal stability of the blends was dictated by the blending technique. The preblended sample degraded at an earlier temperature. The degradation temperatures were increased when a preheating preblending technique was adopted. This may be due to the formation of interchain crosslink bonds between the two-elastomer phases on heat treatment. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Polyurethane; Preblending; Preheating preblending; Interchain crosslinking; Polyblend; Dicumyl peroxide
1. Introduction Polymer blending has become an inseparable part of polymer processing. It is widely accepted as the most economic and fastest way to produce new polymeric materials with a balanced combination of properties. [1±3]. The mixing operation is an important step in blend preparation and it is well known that certain properties depend on it [4±6]. Two dierent reactive polymers can chemically interact at high temperature to form interchain crosslink polyblends. Das et al. have developed such interchain crosslinkable elastomer±elastomer [7±10] and elastomer±plastic blend [11]. Recent studies showed that polyurethane is able to crosslink with EVA [12] and CSM [13] at high temperature without any curatives. De and co-workers have reported that rubbers having appropriate functional groups interact with each other when blended and thus crosslink at high temperature in the absence of any curatives [14±22]. In the previous paper [23] we have reported the eect of blending technique towards the thermal stability of the blends of polyurethane with chlorosulfonated polyethylene (CSM), epichlorohydrin (EPH) and chloroprene (CR) elastomers. The present study was undertaken with * Corresponding author. Tel.: +91 3222 55221; fax: +91-322255303. E-mail address: [email protected] (C.K. Das).
a view to investigate the thermal stability of the blends of polyurethane with two dierent speciality elastomers, such as chlorinated polyethylene (CPE) and ethylene acrylic elastomers (Vamac). We have already reported that polyurethane forms interchain crosslink bonds with CPE [24] and Vamac [25] elastomers at high temperatures without any curatives. Degradation is a kinetic process and accompanied by the chemical bond dissociation [26]. Thus there is de®nite correlation between the chemical structure of a polymer and its degradation temperature [27,28]. Considering this blends were prepared to study the eect of interchain crosslinking reaction on the thermal stability of blends. 2. Experimental 2.1. Materials used Polyurethane (AU) : Vibrathane- 5004 from Uniroyal Co., USA.
Chlorinated polyethylene (CPE) : CM 013 from Bayer Germany.
0141-3910/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(00)00125-7
264
M. Maity et al. / Polymer Degradation and Stability 70 (2000) 263±267 Table 2 Compounding formulation for the AU/Vamac blends Elastomers
Preblend
Preheated preblend
Ethylene acrylic elastomer (Vamac): Vamac±GR from Du Pont, USA.
AU Vamac DCP
50 50 4
50 50 4
Dicumyl peroxide (DCP) : Varox 40 KE from Vanderbilt Comp, USA.
Dierential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) of the blends were carried out with a Shimadzu thermal analyzer (DT-40) in air at a heating rate of 10 C/min, in the temperature range 25±600 C. To determine the glass transition temperature (Tg) of the blends [29,30], Dierential scanning calorimetry (DSC) was conducted in a Stanton Redcroft thermal analyzer, STA-625.
Blends of polyurethane with the two elastomers were prepared at room temperature (25 C) in an internal mixer at a constant shear rate. To study the eect of interchain crosslinking to the thermal stability of the blends, the (50:50) blends of (AU: CPE), and (AU: Vamac) were prepared through two dierent blending techniques, e.g. preblending technique and preheating preblending technique. DCP was used as the curative. The compounding formulations for the AU/CPE and AU/ Vamac systems are given in Tables 1 and 2, respectively. 1. Preblending technique: in this technique, the two elastomers were ®rst blended at (50:50) ratio and the blend allowed to equilibrate for 24 h. Then DCP was incorporated into the blend as curative. 2. Preheating preblending technique: for the preheating preblending technique, the preblend was subjected to heat treatment at 150 C for 15 min and then the curatives were incorporated into the blend at room temperature (25 C). The amount of curative was the same as with the preblending technique. The continuous cure characteristics and the processability of the blends were studied in a Monsanto rheometer (R-100) at 150 C. Blends were then allowed to cure in a hot press at 150 C under constant pressure (20 MPa) up to their optimum cure time. Table 1 Compounding formulation for the AU/CPE blends Elastomers
Preblend
Preheated preblend
AU CPE DCP
50 50 2
50 50 2
3. Results and discussion 3.1. Thermal analysis of the AU/CPE blends To study the compatibility of the blend with reference to the eect of preheating of the preblend, the low temperature DSC of the (50:50) AU/CPE has been studied. For (50:50) AU/CPE preblended sample a single Tg was observed at ÿ44 C, whereas in case of the (50:50) AU/ CPE preheated preblended sample, the Tg value shifted to the higher temperature side (ÿ40 C). The Tg value in the blend may have been shifted to the higher temperature side due to crosslinking by the addition of curatives. But in the case of the preheated preblended technique, a higher shifting of the Tg was observed. This is probably because of the interchain crosslinking reaction occurring between the two elastomer phases during the heat treatment. That again, enhances the phase adhesion, aecting the segmental motion. High temperature DSC/TGA of the (50/50) AU/CPE blends prepared by two dierent blending techniques were considered to correlate the eect of blending technique on the thermal stability. In each case, the degradation occurred in two steps, associated with exothermic peaks in the DSC curves. The degradation temperatures and the exothermic heats of oxidative degradation of the blends are given in Table 3. For the preblended sample the initial degradation (T1) started at 254 C and continued up to 376 C where the second degradation (T2) started. 50% degradation (T50) and 90% degradation (T90) of the sample started at 417 and 510 C, respectively (Fig. 1). For the preheated sample of (50:50) AU/CPE, the degradation started at 303 C and lead to the second degradation at 411 C. T50 and T90 of the sample occurred at 442 and 514 C, respectively (Fig. 2). The delayed degradation for preheated preblended sample suggested that the preheated
M. Maity et al. / Polymer Degradation and Stability 70 (2000) 263±267
265
Table 3 Degradation temperatures, exothermic heat of oxidative degradation of the AU/CPE blends Blends(AU/CPE)
T1( C)
T2( C)
T50( C)
T90( C)
Exothermic heat of degradation (J/gm)
Preblend Preheated preblend
254 303
376 411
417 442
510 514
1430 1340
Table 4 Onset temperatures and exothermic heats of vulcanization of the blends without any curative Blends
Fig. 1. DSC/TGA plot of the AU/CPE blend prepared by the preblending technique.
Onset temperature (0C)
Exothermic heat of vulcanization (J/gm)
(AU: CPE) 80: 20 50: 50 20: 80
130 116 122
11 22.7 16
(AU: Vamac) 80: 20 50: 50 20: 80
164 110 122
15.8 21.9 17.2
clearly corroborates the earlier ®nding that the interchain crosslinking between the two elastomeric phases which probably occurred to a maximum extent at the (50:50) AU/CPE ratio. 3.2. Thermal analysis of the AU/Vamac blends
Fig. 2. DSC/TGA plot of the AU/CPE blend prepared by the preheating preblending technique.
samples were more stable towards thermal degradation because of extra crosslinking introduced between the two phases. The exothermic peak characterizes the DSC plots of the two types of blends. The heat of oxidative degradation was measured from the DSC curves (Table 3). It is observed that the heat evolved for the preblended sample was higher than the preheated preblended samples, suggesting the ease of oxidative degradation of the preblended sample. Heat of vulcanization was also studied for the three blend systems reported in Table 4. It was observed that the heat of vulcanization was more at (50:50) AU/CPE blend ratio accompanied by low onset temperature. This
Low temperature DSC of the (50:50) AU/Vamac preblended and preheated preblended samples was conducted to study the compatibility of the blend. In both the cases two Tg values were observed indicating the blend is incompatible. The preblended sample showed the Tg values at ÿ32 and at ÿ10 C, whereas the preheated preblended sample showed the Tg values at ÿ27 and at ÿ8.2 C. The shifting of Tg values to the higher temperature side for the preheated preblended samples may be due to the certain amount of extra crosslinking between two elastomers on heat treatment. High temperature DSC/TGA plots of (50:50) AU/ Vamac preblended and preheated preblended sample are shown in Figs. 3 and 4, respectively. In each case, the degradation occurred in two steps, associated with exothermic peaks in the DSC curves. The degradation temperatures and the exothermic heats of oxidative degradation of the blends are given in Table 5. For (50:50) AU/Vamac preblended samples the initial degradation (T1) occurred at 246 C and continued up to 366 C where the second degradation (T2) started. 50% degradation (T50) and 90% degradation (T90) of the samples occurred at 425 C and 540 C, respectively (Fig. 3). In the case of the blend obtained by heating the
266
M. Maity et al. / Polymer Degradation and Stability 70 (2000) 263±267 Table 5 Degradation temperatures, exothermic heat of oxidative degradation of the AU/Vamac blends
Fig. 3. DSC/TGA plot of the AU/Vamac blend prepared by the preblending technique.
Blends (AU/Vamac)
T1 ( C)
T2 ( C)
T50 ( C)
T90 ( C)
Exothermic heat of degradation (J/g)
Preblend Preheated preblend
246 252
366 377
425 432
540 524
1680 1430
without any curatives. Three dierent cases were taken to study the eect of blend ratio on the onset temperature of crosslinking reaction and the heat of reaction. The onset temperature and the heat of crosslinking reaction are given in Table 4. In each case an exothermic peak was observed. The heat of reaction varied with blend ratio. The heat of reaction was highest for the (50:50) AU/Vamac blend ratio and the temperature of onset of the reaction also dropped more than for the other two blends. This suggests that two polymers react at high temperature in the absence of any curative and the extent of reaction depends on the blend ratio. 4. Conclusion
Fig. 4. DSC/TGA plot of the AU/Vamac blend prepared by the preheating preblending technique.
preblend, followed by the curative addition, the degradation occurred at relatively higher temperature. The initial degradation (T1) started at 252 C and continued up to 377 C where second degradation (T2) occurred. 50% degradation (T50) and 90% degradation (T90) of the sample occurred at 432 and 524 C, respectively (Fig. 4). From the above trend of change, it is evident that the preheated preblended sample is thermally more stable than the preblended sample. Again it was also supported from the DSC plot. In both cases the degradation was associated with exothermic peaks. the amount of heat of oxidative degradation was measured from DSC curves. It was found that the heat evolved for the preblended sample during oxidative degradation was more than the preheated preblended sample (Table 5). Evolution of higher amount of heat suggested the easier of oxidative degradation of the preblended sample. However, the amount of heat decreased for the preheated preblended sample, suggesting the higher stability of the preheated preblend towards oxidation. This may be due to the formation of a new chemical bond on heat treatment. The heat of vulcanization has been studied for AU/ Vamac blends. The AU and Vamac have been blended
There is a de®nite correlation between the chemical structure of a polymer and its degradation temperatures. Blends, prepared through the dierent blending techniques, having the same compounding formulation and same blending ratio dier in thermal behaviour. The early degradation of the preblended sample compared to the preheated preblended sample with the same elastomer ratio indicates the presence of some extra chemical linkages between the two phases. Interchain crosslinking reaction occurs in the blend of the two elastomers having reactive functional groups as a result of blending before addition of curatives. This extent of crosslinking increases when the preblend is subjected to heat treatment before curative addition. Thus, the thermal stability and the degradation temperature of the preheated preblended sample increases signi®cantly. References [1] Olabisi O, Robeson LM, Shaw MT. Polymer±polymer miscibility. New York: Academic Press, 1979. [2] Utracki LA. Polymer alloys and blends. Munich: Hanser Publishers, 1989. [3] Paul DR, Newman S. Polymer blends. New York: Academic Press, 1978. [4] Paul DR, Barlow JW. J Macromol Sci, Macromol Rev 1980;C18:109. [5] Manson JA, Sperling LH. Polymer blends and composites. New York: Plenum Press, 1976. [6] La Mantia FP, Valenza A, Acierno D. Polymer Degrad Stab 1985;13(1±4):1±9. [7] Singha Roy SK, Das CK. Polym Compos 1995;3:5. [8] Tripathy AR, Das CK. J Appl Polym Sci 1994;51:245.
M. Maity et al. / Polymer Degradation and Stability 70 (2000) 263±267 [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
Tripathy AR, Das CK. Plast Rubb Comp Process Appl 1994;21:5. Singha Roy SK, Das CK. J Polym Engg 1995;14:175. Mishra JK, Das CK. Int, J Polym Mater, in press. Maity M, Das CK. Int J Polym Mater 1998;45:1±2, 123. Khatua BB, Das CK. Int J Polym Mater, (in press). Alex R, De PP, De SK. J Polym Sci, Polym Lett Ed 1989;27:361. Alex R, De PP, De SK. Polym Commun 1990;31:67. Alex R, De PP, De SK. Kautsch, Gummi Kunstst 1991;44:333. Mukhopadhyay S, De SK. J Appl Polym Sci 1992;45:181. Mukhopadhyay S, De PP, De SK. J Appl Polym Sci 1991;43:347. Bhattacharya T, De SK. Eur Polym J 1991;27:1065. Mukhopadhyay S, Chaki TK, De SK. J Polym Sci Polym Lett Ed 1990;28:25.
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267
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