Effect of processing on the thermal stability of blends based on polyurethane. Part I

Polymer Degradation and Stability 69 (2000) 381±386

E€ect of processing on the thermal stability of blends based on polyurethane. Part I B.B. Khatua, C.K. Das * Materials Science Centre, Indian Institute of Technology, Kharagpur 721302, India Received 7 February 2000; received in revised form 7 March 2000; accepted 30 March 2000

Abstract The thermal stability of blends of polyurethane with three di€erent elastomers having reactive functional groups has been studied. The blends have been prepared by three di€erent blending techniques, keeping the blending ratio constant. It was found that the thermal stability of the blends was dictated by the blending technique. Blends prepared by the masterbatch technique degraded at a lower temperature. The degradation temperatures were increased when preblending and preheating/preblending techniques were adopted. This may be due to the formation of interchain crosslink bonds between the two elastomer phases, the extent of which was more in the preheated/preblended technique. # 2000 Published by Elsevier Science Ltd. Keywords: Polyurethane; Masterbatch; Preblending; Preheating preblending; Interchain crosslinking; Vulcanization

1. Introduction The blending of polymers is one of the ways of modifying the physical properties of a polymer in a desired way. Many studies have recently been devoted to polymer blends for obtaining new materials, which, in some cases, are better than the parent homopolymers. The mixing operation is an important step in blend preparation and it is well known that certain properties strongly depend on it [1±5]. When two polymers containing reactive functional groups are blended before addition of curatives, the blend is likely to have better performance properties, derived from the interchain crosslinking reaction via the polar functional groups. Das et al. have already studied a series of blend systems where interchain crosslinking occurs and showed how it a€ects the properties of the blends [6±15]. 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 [16±24]. This paper relates to our investigation of thermal stability of blends of polyurethane (AU) with other speciality elastomers, such as chlorosulfonated polyethylene (CSM), epichlorohydrin (EPH) and polychloroprene (CR) elastomers, as determined by Thermogravimetric Analysis (TGA). Polyurethane forms interchain crosslink bonds with CSM [11], EPH [12] and CR [13] elastomers at high

temperature without any curatives. Degradation is normally regarded as a kinetic process. It is however accompanied by the chemical bond dissociation [25] and there is a de®nite correlation between the chemical structure of a polymer and its degradation temperature [26,27]. Considering this, blends of polyurethane were prepared to study the e€ect of interchain crosslinks on the thermal stability of blends. 2. Experimental 2.1. Materials used

* Corresponding author. Tel.: +91-3222-55221; fax: +91-3222-55303. E-mail address: [email protected] (C.K. Das). 0141-3910/00/$ - see front matter # 2000 Published by Elsevier Science Ltd. PII: S0141-3910(00)00085-9

. Polyurethane: Vibrathane-5008 from Uniroyal Co., USA Chemical structure: . Chlorosulfonated polyethylene: CSM-350, M/S Tosoh, Japan Chemical structure: . Epichlorohydrin: Gechron-3100, M/S Nippon, Zeon, Japan Chemical structure: . Polychloroprene: Bayprene 210, Bayer Ltd., West Germany Chemical structure:

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B.B. Khatua, C.K. Das / Polymer Degradation and Stability 69 (2000) 381±386

Blends of polyurethane with the three elastomers were prepared at room temperature (25 C) in an internal mixer at a constant shear rate. To study the e€ect of interchain crosslinking on the thermal stability of the blends, the (50:50) blends of (AU:CSM), (AU:EPH) and (AU:CR) were prepared through three di€erent blending techniques, e.g. masterbatch technique, preblending technique and preheating preblending technique. Sulfur cure systems were used. The compounding formulations for the AU/CSM, AU/EPH and AU/CR systems are given in Tables 1±3, respectively. . Masterbatch technique: In this technique, the individual elastomers were ®rst mixed with the curatives and allowed to equilibrate for 24 h. The mixtures were then blended at (50:50) ratio. . Preblending technique: In the preblending technique, the two elastomers were ®rst blended at (50:50) ratio and the blend allowed to equilibrate for 24 h. The same amount of curatives as in the masterbatch technique was then incorporated into the blend.

. 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 in the blend after cooling. The amount of curative was 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 (2800 psi) up to their optimum cure time. Di€erential 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 to 600 C. To determine the glass transition temperature (Tg) of the blends, DSC was conducted in a Stanton Redcroft Thermal Analyzer, STA-625. 3. Results and discussion

Table 1 Compounding formulation for the AU/CSM blends

3.1. Thermal analysis of the AU/CSM blends

Elastomers

The low temperature DSC of the (50:50) AU:CSM blends was carried out to study the compatibility of the blends with reference to the e€ect of preheating. Two di€erent cases of the AU:CSM blends were considered having the same compounding formulations and blend ratio. In case of the (50:50) AU:CSM blend, without any heat treatment, prepared by the masterbatch techniques, the DSC plots showed two Tg values in the vicinity of ÿ17 and ÿ11.3 C. The low temperature DSC plot of a (50:50) AU: CSM preheated sample showed only one prominent Tg at ÿ9.2 C, indicating the better compatibility of AU and CSM in the blend. This study indicates that preheating the preblend followed by the curative addition can enhance the compatibility of the blend. The high temperature DSC/TGA of (50:50) AU: CSM blends, prepared by three di€erent blending techniques, was studied to ®nd the degradation pattern of the blends. From the TGA plots it was evident that the degradation occurred in two steps in each type of blend having the same blend ratio (Table 4). For (50:50) AU:CSM blend prepared by the masterbatch technique, the ®rst degradation (T1) started at a temperature of 183 C and continued up to 418 C with a faster rate where the second degradation (T2) occurred. 50% degradation (T50) and 90% degradation (T90) of the blend occurred at 351 and 550 C, respectively (Fig. 1). In the case of the (50:50) AU:CSM preblended sample, the ®rst degradation started at 214 C with a comparatively slower rate which lead to the second degradation at 421 C. T50 and T90 of the blend were found to be 355 and 548 C, respectively (Fig. 2). In the case of the sample

a

AU CSMb

Masterbatch

Preblend

Preheated preblend

50 50

50 50

50 50

a Curatives for AU (phr): MBT-2, MBTS-4, ZDC-1, ZnO-1, stearic acid-1.5, caytur-0.5, S-2. b Curatives for CSM (phr): MBT-2, MBTS-2, ZDC-1, ZnO-2, stearic acid-1.5, S-2.

Table 2 Compounding formulation for the AU/EPH blends Elastomers

Masterbatch

Preblend

Preheated preblend

AUa EPHb

50 50

50 50

50 50

a Curatives for AU (phr): MBT-2, MBTS-4, ZDC-1, ZnO-1, stearic acid-1.5, caytur-0.5, S-2. b Curatives for EPH (phr): MBT-1.5, TMTD-1.5, ZnO-1, stearic acid-1, S-1.5.

Table 3 Compounding formulation for the AU/CR blends Elastomers a

AU CRb a

Masterbatch

Preblend

Preheated preblend

50 50

50 50

50 50

Curatives for AU (phr): MBT-2, MBTS-4, ZDC-1, ZnO-1, stearic acid-1.5, caytur-0.5, S-2. b Curatives for CR (phr): MBT-2, ZDC-1, MgO-4, ZnO-5, stearic acid-1.5, S-2.

B.B. Khatua, C.K. Das / Polymer Degradation and Stability 69 (2000) 381±386

preheated followed by the curative addition, the degradation started at 216 C with much slower rate as compared to both the above blending techniques and lead to the second degradation at 428 C. T50 and T90 of the preheated preblend occurred at 372 and 534 C, respectively (Fig. 3). The DSC plots, in all the three types of blends are characterized by exothermic peaks. The heat of oxidative degradation was measured from the DSC curves (Table 4). It is observed that the heat evolved for the masterbatch sample was higher that those of the preblended and preheated preblended samples, suggesting the ease Table 4 Degradation temperatures, exothermic heat of oxidative degradation of the AU/CSM blends

383

of oxidative degradation of the masterbatch sample. Thus preblended and preheated preblended samples were more stable towards thermal degradation. Heat of vulcanization of the AU:CSM blends, without any curative, was studied. Table 5 represents the onset temperature and the heat of interchain crosslinking reaction of the blends. Three di€erent blend ratios were considered. The heat of reaction varied with the blend ratio, associated with the exothermic peak in each case. The heat of reaction was maximum for the (80:20) AU:CSM blend ratio, decreased with CSM addition and again increased to its 80% level while the onset temperature of reaction had maximum value at 50:50 level. The exothermic peaks revealed that some reaction occurred between the two elastomeric phases, the extent of which depends on the blend ratio.

Blends

T1 ( C)

T2 ( C)

T50 ( C)

T90 ( C)

Exothermic heat of degradation (J/g)

3.2. Thermal analysis of the AU/EPH blend

Masterbatch Preblend Preheated preblend

183 214 216

418 421 428

351 355 372

550 548 534

2230 2100 1780

The low temperature DSC of the (50/50) AU/EPH blends was conducted to study the compatibility of the

Fig. 3. DSC/TGA plot of the AU/CSM blend prepared by the preheating preblending technique. Fig. 1. DSC/TGA plot of the AU/CSM blend prepared by the masterbatch technique. Table 5 Onset temperatures and exothermic heats of vulcanization of the blends without any curative Blends

Fig. 2. DSC/TGA plot of the AU/CSM blend prepared by the preblending technique.

Onset temperature ( C)

Exothermic heat of vulcanization (J/g)

AU: CSM 80:20 50:50 20:80

100 107 105

97 70 86

AU: EPH 80:20 50:50 20:80

92 100 98

15 38 24

AU: CR 80:20 50:50 20:80

51 49 50

23 58 33

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B.B. Khatua, C.K. Das / Polymer Degradation and Stability 69 (2000) 381±386

blends with reference to the e€ect of preheating the preblend. In the present study, two di€erent cases have been considered, e.g. blends obtained by the masterbatch techniques and by preheating followed by curative addition, having the same compounding formulations with the same elastomer ratio. In both cases only one prominent Tg was observed, the position of which varied slightly depending on the blending techniques. The (50/50) AU/EPH blend (masterbatch technique) showed a single Tg at ÿ11 C indicating the better compatibility of the blend. In case of the blend of (50/50) AU/EPH preheating followed by curative addition, the Tg slightly shifted to the higher region which was in the temperature of ÿ6 C. High temperature DSC/TGA analysis of the (50:50) AU/EPH blends, prepared by three di€erent blending techniques with the same compounding formulations and elastomer ratio have been considered to correlate the e€ect of blending technique on the thermal stability. In each case single-step degradation occurred. The degradation temperatures and the exothermic heats of degradation of the blends are given in Table 6. In case of (50/50) AU/EPH blend obtained from the masterbatch techniques, the initial degradation (T1) started at 260 C and continued up to 375 C at a faster rate where the second degradation (T2) occurred. 50% degradation (T50) and 90% degradation (T90) of the blend were found to be 334 and 412 C, respectively (Fig. 4). For the preblended sample of (50/50) AU/EPH, without any heat treatment, the initial degradation (T1) occurred at 266 C and continued up to 380 C at a faster rate where the second degradation (T2) started. The temperatures T50 and T90 of the preblend also increased and started at 339 and 478 C, respectively (Fig. 5). In the case of the blend obtained by heating the preblend, followed by curative addition, the degradation started (T1) at relatively higher temperature, 275 C and continued up to 395 C at a faster rate where the second degradation (T2) started. The temperatures T50 and T90 of the preheated preblend again increased to at 345 and 501 C, respectively (Fig. 6). The exothermic peak characterizes the DSC plot of the three types of blends. The exothermic heat of oxidative degradation was highest for the masterbatch sample. This value was decreased from preblended sample to

preheated preblended sample, indicating the easier of oxidative degradation in the masterbatch sample than those of the preblended and preheated preblended sample. Heat of vulcanization was studied for the AU/EPH blends without any curative addition. Three di€erent cases have been considered having di€erent elastomer ratios. Table 5 gives the onset temperature, the heat of

Fig. 4. DSC/TGA plot of the AU/EPH blend prepared by the preblending technique.

Fig. 5. DSC/TGA plot of the AU/EPH blend prepared by the preblending technique.

Table 6 Degradation temperatures, exothermic heat of oxidative degradation of the AU/EPH blends Blends

T1 ( C)

T2 ( C)

T50 ( C)

T90 ( C)

Exothermic heat of degradation (J/g)

Masterbatch Preblend Preheated preblend

260 266 275

375 380 395

334 339 345

412 478 501

900 880 810

Fig. 6. DSC/TGA plot of the AU/EPH blend prepared by the preheating preblending technique.

B.B. Khatua, C.K. Das / Polymer Degradation and Stability 69 (2000) 381±386

interchain crosslinking reaction with the blending ratio. In all cases exothermic peaks were observed. Heat of vulcanization was highest in the case of 50/50 (AU/ EPH) blend suggesting interchain crosslinking between the elastomers without any curative, the extent of which depend on the blending ratio. 3.3. Thermal analysis of the AU/CR blend High temperature DSC/TGA of the (50/50) AU/CR blends prepared by three di€erent blending techniques have been considered to correlate the e€ect of blending technique on the thermal stability. In each case, the degradation occurred in three 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 7. For the (50/50) AU/CR blend prepared by the masterbatch technique, the ®rst degradation (T1) started at 210 C. The second degradation (T2) started at 312 C and let to the third degradation (T3) at 368 C. 50% degradation (T50) and 90% degradation (T90) of the blend started at 365 and 566 C, respectively (Fig. 7). In the case of the preblended sample of (50/50) AU/CR, without any heat treatment, the ®rst degradation occurred at 215 C and continued up to 320 C where second degradation started. The third degradation started at 372 C. T50 and T90 of the preblended sample were 367 and 556 C, respectively (Fig. 8). In the case of the

385

(50/50) AU/CR blend prepared by the preheating preblending technique, the degradation started at relatively higher temperature, 224 C and continued up to 324 C where second degradation occurred. The third degradation started at 377 C. T50 and T90 of the blend were 367 and 555 C, respectively (Fig. 9). The exothermic peak characterizes the DSC plot of the three types of blends. The heat evolved was highest for the masterbatch sample. The exothermic heat of oxidative degradation decreases in case of preblended and preheated preblended samples, the extent being more in the preheated preblended sample. This study also supports the higher stability of the preheated preblended sample towards thermal degradation. The heat of vulcanization of the blends without any curative has been studied. Three di€erent blend ratios have been considered. The onset temperature and heat of interchain crosslinking reaction for each blend ratio are shown in Table 5. In each case an exothermic peak was observed. The heat of interchain crosslinking reaction was highest in case of the (50/50) AU/CR blend. This suggested that the interchain crosslinking reaction occurred between the two elastomeric phases without any curative addition and the extent of the reaction depends on the elastomer ratio in the blend.

Table 7 Degradation temperatures, exothermic heat of oxidative degradation of the AU/CR blends Blends

T1 ( C)

T2 ( C)

T3 ( C)

T50 ( C)

T90 ( C)

Exothermic heat of degradation (J/g)

Masterbatch Preblend Preheated Preblend

210 215 224

312 320 324

368 372 377

365 367 367

566 556 555

3350 3270 3080

Fig. 7. DSC/TGA plot of the AU/CR blend prepared by the masterbatch technique.

Fig. 8. DSC/TGA plot of the AU/CR blend prepared by the preblending technique.

Fig. 9. DSC/TGA plot of the AU/CR blend prepared by the preheating preblending technique.

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4. Conclusions The blending technique has an important role in determining the thermal stability of the blends. Blends, prepared through the di€erent blending technique, having the same compounding formulation and same blending ratio di€er in thermal behaviour. Blends prepared by the masterbatch technique degrade at a lower temperature compared to the preblended and preheated preblended ones, with the same elastomer ratio. This may be due to the formation of interchain crosslinks in the blend of the two elastomers having reactive functional groups as a result of blending before addition of curatives. This extent of crosslinking is again increased when the preblend is subjected to heat treatment before curative addition. Hence, the thermal stability and the degradation temperature of the preblended and preheated preblended sample are increased because of the crosslinking between the elastomer phases, the extent of which was more in the preheated preblended sample. References [1] Paul DR, Newman S. Polymer blends. New York: Academic Press, 1978. [2] Paul DR, Barlow JW. J Macromol Sci, Macromol Rev 1980;C18:109. [3] Manson JA, Sperling LH. Polymer blends and composites. New York: Plenum, 1976.

[4] Olabisi O, Robeson LM, Shaw MT. Polymer±polymer miscibility. New York: Academic Press, 1979. [5] Utracki LA. Polymer alloys and blends. Munich: Hanser Publishers, 1989. [6] Tripathy AR, Ghosh MK, Das CK. Kautsch, Gummi, Kunststo€e 1992;45:626. [7] Tripathy AR, Das CK. Plast Rubb Comp Process Appl 1994;21:5. [8] Tripathy AR, Das CK. J Appl Polym Sci 1994;51:245. [9] Singha Roy SK, Das CK. Polymers and Polymer Composites 1995;3(6):403. [10] Singha Roy SK, Das CK. Polym Networks Blends 1996;6(1):9±13. [11] Khatua BB, Das CK. Int J Polym Mater, in press. [12] Khatua BB, Das CK. J Appl Polym Sci 2000;76:1367. [13] Khatua BB, Das CK. J Elastomers and Plastics, in press, 1999. [14] Maity M, Das CK. Int J Polym Mater 2000;45:123. [15] Maity M, Das CK. Int J Polym Mater, in press. [16] Alex R, De PP, De SK. J Polym Sci Polym Lett Ed 1989;27:361. [17] Alex R, De PP, De SK. Polym Commun 1990;31:367. [18] Alex R, De PP, De SK. Kautsch, Gummi Kunststo€e 1991; 44:333. [19] Mukhopadhyay S, De SK. J Appl Polym Sci 1992;45:181. [20] Mukhopadhyay S, De PP, De SK. J Appl Polym Sci 1991;43:347. [21] Bhattacharya T, De SK. Eur Polym J 1991;27:1065. [22] Mukhopadhyay S, Chaki TK, De SK. J Polym Sci Polym Lett Ed 1990;28:25. [23] Mukhopadhyay S, De SK. J Mater Sci 1990;25:4027. [24] Mukhopadhyay S, De, De SK. J Appl Polym Sci 1991;42:2773. [25] Matveyev YuI, Askadskii AA, Zhuravleva IV, Slonimskii GL, Korshak VV. Polymer Science 1981;23:9±122194. [26] Korshak VV. Thermostoikiye Polimery (Heat stable polymers). Nauka, Moscow, p. 411, 1969. [27] Korshak VV. Chemical structure and thermal stability of polymers, Nauka, Moscow, p. 419, 1970.