Thermal ageing of acrylonitrile-butadiene copolymer

Polymer Degradation and Stability 70 (2000) 1±4

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Thermal ageing of acrylonitrile-butadiene copolymer F. Delor-Jestin a,*, N. Barrois-Oudin b, C. Cardinet c, J. Lacoste a, J. Lemaire a a

Laboratoire de Photochimie MoleÂculaire et MacromoleÂculaire, Universite B. Pascal, Ecole Nat. Sup. de Chimie de Clermont-Ferrand, F-63174 AubieÁre, Cedex, France b RENAULT, Direction de la Recherche, F-92508 Rueil Malmaison, France c HUTCHINSON, Centre de Recherche, F-45120 Chalette/Loing, France Received 7 December 1999; accepted 28 December 1999

Abstract The thermal ageing at 100 C of vulcanized acrylonitrile-butadiene copolymer (NBR) is described. Analysis was based both on infrared spectroscopy (surface analysis) and on physical properties analysis (tensile and microhardness tests). Both methods appear to be complementary to describe the two competitive phenomena (oxidation and crosslinking) involved in the elastomer ageing. It is emphasised that the mechanical behaviour of aged NBR is di€erent in comparison with chloroprene rubber. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Acrylonitrile-butadiene copolymer; NBR; Thermo-oxidation; Re¯exion-infrared spectrometry; Mechanical properties

1. Introduction Acrylonitrile-butadiene copolymers (or nitrile butadiene rubbers, NBR) have generally good resistance towards oils and low gas permeability. Their use in automotive applications is interesting but their ageing resistance is limited because of the unsaturated backbone of the butadiene part. Past studies concerned especially the thermal oxidation at high temperature of NBR [1,2]. The photo-oxidation at long wavelengths (above 300 nm) and the thermal oxidation at 60 C of unvulcanized copolymer have also been studied [3,4] and degradation mechanisms were proposed. The observed changes in IR spectra of NBR photooxidized ®lms were almost the same as those described for butadiene rubber (BR, [5]) and styrene butadiene rubber (SBR, [6]) exposed in identical experimental conditions. A typical band in the hydroxyl region is characteristic from hydroperoxides and alcohols groups. In the carbonyl region of IR spectra acids and ketones (saturated and unsaturated) were detected. In the deformation vibration region reported results have shown a decrease of unsaturation bands. This phenomenon * Corresponding author. E-mail address: ¯[email protected] (F. Delor-Jestin).

was associated with crosslinking reactions [7,8]. Acrylonitrile units are not acting as preponderant reactive sites during photo-oxidation. The nitrile absorption band at 2240 cmÿ1 has shown no signi®cant change. Low temperature thermo-oxidation of NBR [3,5] gave a lot of similarities with photo-oxidation. The hydroperoxides observed during photo-oxidation (35 C) are unstable at 60 C. Oxidation products like ketones are photochemically unstable. However, they accumulate during thermo-oxidation. The attribution of photoproducts has been detailed in recent work [9]. Another study of vulcanized cis-1,4 PB concerned the changes of microstructures on the surface of samples [10]. The heterogeneity of the thermo-oxidized material (140 C) was characterized by optical microscopy. XPS (X-ray photoelectron spectroscopy) analyses showed the decrease of carbon at% and the increase of oxygen at% on the samples' surfaces. This phenomenon was attributed to the formation of new groups CˆO or CÿO. In the case of vulcanized NBR, thermal analysis (ATD, ATG) were given to determine the temperature interval of thermal stability [11]. The study also gave the evolution of residual deformation under constant de¯ection. In the present paper we report new results on degradation of vulcanized and formulated NBR. The ®rst part of our study allows us to understand the chemical evolution of NBR during thermal ageing. We will try later to determine the in¯uence of ®llers and di€erent

0141-3910/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(00)00035-5

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F. Delor-Jestin et al. / Polymer Degradation and Stability 70 (2000) 1±4

NBR (Krynac 2980 from POLYSAR) was kindly provided by HUTCHINSON (France). The nitrile content is around 26% (weight). Two formulations N1 and N2 were studied. Their compositions are given in the following list:

The choice of the analytical technique and the use of a horizontal attenuated total re¯ectance (HATR) device ±±equipped with a germanium crystal Ð is reported in another paper [12], typically the analysis concerns the ®rst micrometers. Mechanical properties as elongation at break were measured on an Adamel Lhomargy apparatus (100 daN, speed 200 mm/min). Thermal ageing was performed at 100 C in a ventilated air oven with natural convection. Microhardness was determined with a Karl Frank apparatus on sheets of NBR.

Composition (weight)

N1

N2

3. Results

Gum NBR Carbon black Zinc oxide Sulfur vulcanization

79.2 0 4.2 10.7

35.7 49.6 1.6 7.8

additives on oxidation. The second part of our study will concern the evolution of static mechanical properties. 2. Experimental

Some antioxidants and additives are also added to formulations N1 and N2. Sheets of NBR (2 mm thick) were used directly for thermal ageing and re¯ection FTIR analysis (NICOLET Impact) or tensile tests (ADAMEL LHOMARGY DY-35).

3.1. Part A: chemical evolution during thermo-oxidation of formulated NBR The N1 formulation (without carbon black) was analysed by ATR-IR spectroscopy at di€erent times of thermal ageing (100 C). The analysis shows the formation of a wide absorption band (3000±3600 cmÿ1) in the hydroxyl region. Two maxima are observed in the carbonyl region (1660 and 1580 cmÿ1, Fig. 1A). The 1660 cmÿ1 band is associated with the presence of new unsaturated carbonyl groups. The spectrum subtraction

Fig. 1. ATR-FTIR spectra of NBR N1 after thermo-oxidation at 100 C. A ±± Carbonyl region; B ±± substract spectrum (385 h ±± initial time); C ±± region 2300±2100 cmÿ1.

F. Delor-Jestin et al. / Polymer Degradation and Stability 70 (2000) 1±4

(see Fig. 1B) clearly underlines these changes. We also see in the very beginning of oxidation the disappearing on the surface, of bands attributed to formulation additives (1515 and 1495 cmÿ1). In the deformation vibrations region a shoulder at 1410 cmÿ1 is detected. The unsaturation loss is rather important; a strong decrease of the band intensity (at 970 cmÿ1) is observed, this band is characteristic of the trans-1,4 structure of the butadiene part. The nitrile band at 2240 cmÿ1 shows no signi®cant change during thermo-oxidation (see Fig. 1C). From past studies on unvulcanized copolymer we know that thermal ageing is closely associated with the butadiene part of the copolymer [3,4]. The thermooxidation was characterized with IR spectroscopy by the appearance of hydroxyl bands (hydroperoxides and alcohols) and carbonyl bands (saturated, unsaturated acids and ketones). The disappearance of bands typical of 1,4- and 1,2- unsaturation was also reported. In the case of vulcanized formulations the carbonyl region is specially modi®ed during thermal ageing. Carboxylates are observed at 1580 and 1410 cmÿ1. This is due to the presence of zinc oxide as is found for other elastomers (chloroprene rubber ±± CR [13], ethylene propylene diene monomer ±± EPDM [14]). The nitrile band shows no signi®cant change and the decrease of the double bonds during oxidation is also detected (970 cmÿ1); these results con®rm the preferential attack on butadiene units. The N2 formulation (with carbon black) was analyzed in the same conditions as N1. During thermal ageing at 100 C the formation of a wide band from 3000 to 3600 cmÿ1 of weak intensity was also observed. A short induction period of 30 h is observed. In the carbonyl region a wide band centered at 1580 cmÿ1 is detected (Fig. 2). A new band at 1740 cmÿ1 is characteristic of lubricant, the band intensity is decreasing during thermal ageing. The progressive loss of di€erent additives is also detected (shoulders at 1515 and 1495 cmÿ1). In the deformation vibrations region, a shoulder at 1410 cmÿ1 is observed as in the case of N1. The evolution of the

Fig. 2. ATR-FTIR spectra of NBR N2 after thermo-oxidation at 100 C; carbonyl region.

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970 cmÿ1 band shows a decrease of trans-1,4 unsaturation. The nitrile band does not change (2240 cmÿ1). An SF4 treatment of thermo-oxidized NBR was performed to follow the evolution of the band at 1580 cmÿ1. We then observed the conversion of the 1580 cmÿ1 band into a band at 1840 cmÿ1 typical of acid ¯uorides (1840 cmÿ1) [15,13]. That con®rmed the attribution of the 1580 cmÿ1 band to zinc carboxylates. With IR analysis the in¯uence of ®llers and additives on thermal ageing can be determined. The addition of carbon black brings a short induction period. With other elastomers (CR, EPDM) and di€erent kinds of carbon black the induction period can be more important (around 250 h, [13,14]). 3.2. Part B: mechanical properties evolution during thermo-oxidation of formulated NBR First we determined the evolution of modulus, elongation at break and tensile strength in static conditions for the formulation N2. The decrease of elongation at

Fig. 3. A ±± Evolution of the elongation at break percentage for a NBR N2 sample. B ±± Evolution of the tensile strength for a NBR N2 sample. C ± Evolution of microhardness for a NBR N2 sample.

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F. Delor-Jestin et al. / Polymer Degradation and Stability 70 (2000) 1±4

Fig. 4. Correlation between the chemical evolution and the elongation at break of thermo-oxidized NBR N2.

break after thermo-oxidation at 100 C is clearly shown in Fig. 3A. The induction period is around 30 h and the evolution is then rather important. After 300 h in the oven a loss of 43% of elongation at break can be noticed. The tensile strength increased at the beginning of thermal ageing then kept stable (see Fig. 3B). The rigidity of thermo-oxidized material is more important; this evolution can be associated with both crosslinking reactions and lubricant loss. Microhardness measurements were made on the thermo-oxidized samples surface. This evolution is going quite fast. An increase of microhardness is observed (Fig. 3C). The NBR mechanical properties have a signi®cant evolution after 30 h at 100 C. This material kept a good resistance at break (14 MPa after 300 h at 100 C). In the case of polychloroprene [13] the tensile strength decreased during thermo-oxidation. The chain scissions content could partially explain this di€erence. 4. Conclusion NBR formulation showed a limited resistance to thermal oxidation. The di€erences of mechanical evolution between NBR, CR and EPDM have been underlined. As a conclusion, it is possible to determine a correlation of carboxylate index (chemical oxidation) with

mechanical property evolution. Fig. 4 clearly shows that the decrease of mechanical properties such as elongation at break is well correlated with the thermal oxidation of the polymer matrix even if a small part of the sample is concerned by an important oxidation [12]. Oxidation pro®les (based on successive abrasions of the surface) across rubber samples demonstrated that oxidation is essentially located in the ®rst hundred microns (of a 2 mm thick sample). However, small but signi®cant oxidation was also detected in the core of thermo-oxidized rubbers. When a strong evolution of carbonyl products is observed, the loss of mechanical properties is also important. Acknowledgements The authors are grateful to HUTCHINSON (Chalette/ Loing, France) which supplied experimental materials and RENAULT (Rueil Malmaison, France) for ®nancial support.

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