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acrylate rubber reactive blends
PII: S0141-3910(98)00044-5
Polymer Degradation and Stability 62 (1998) 575±586 # 1998 Elsevier Science Limited. All rights reserved Printed in Great Britain 0141-3910/98/$Ðsee front matter
Thermal degradation and ageing behaviour of novel thermoplastic elastomeric nylon-6/acrylate rubber reactive blends Abhijit Jha & A. K. Bhowmick* Rubber Technology Centre, Indian Institute of Technology, Kharagpur 721302, India (Received 25 November 1997; accepted 12 January 1998) The thermal degradation behaviour of thermoplastic elastomeric blends of nylon6 and an acrylate rubber (ACM) has been studied by non-isothermal thermogravimetry. The thermal stability of the blends is low compared to that of nylon6, which is explained on the basis of `ester-amide interchange' reaction at high temperature during degradation. The exchange reaction eectively degrades the nylon-6 phase in the blends. The melt reaction between nylon-6 and ACM which occurs during the blending operation, slightly improves the stability of the blends. Ageing of dynamically cured 40/60 (w/w) nylon-6/ACM blend, which is thermoplastic elastomeric (TPE) in nature, has been carried out in air at various temperatures and times. The results show excellent heat resistance of the TPE at elevated service temperature. # 1998 Elsevier Science Limited. All rights reserved
1 INTRODUCTION
produce a graft or block copolymer at the interface, which, in turn, compatibilizes the two phases. The preparation and properties of novel heat and oil resistant thermoplastic elastomers from nylon-6 and acrylate rubber blend have been reported recently.4 The blends are reactive in nature which is manifested in the rise of mixing torque during melt blending in a Brabender Plasticorder at 220±235 C. Based on infra-red analysis, a plausible mechanism of reaction between the two polymers was proposed, as shown in Scheme 1. The in¯uence of interaction on mechanical, dynamic mechanical, rheological and swelling behaviour of the blends has been reported.4±6 The purpose of development of such blends is to improve the heat and oil resistance properties further as compared to the conventional commercial blends. In this paper the thermal stability of the blends is examined in two ways, (a) dynamic thermogravimetry in the temperature range of 300±600 C in nitrogenatmosphere to study the thermal degradation characteristics of the blends and the pure polymers and (b) ageing in the temperature range of 150± 200 C in air for prolonged time to check its applicability as a heat resistant material at elevated
Thermoplastic elastomers prepared from rubber± plastic blends have gained considerable interest in recent years due to their dual characteristics of rubbers and plastics.1,2 The ideal morphology of such blends comprises ®nely divided rubber particles dispersed in a minimum volume of plastic.3 The in-situ or dynamic vulcanization of the rubber phase improves the elastic recovery and other mechanical properties of the blends needed for commercial applications. Apart from morphology, the most important factor which controls the overall performance of thermoplastic elastomeric blends is the compatibility (i.e. technological compatibility) or the adhesion between the phases. Reactive blending is known to be one of the most important methods to induce compatibility between two immiscible polymers. Two polymers having proper reactive groups on their backbone can react with each other during melt blending to
*To whom correspondence should be addressed. Fax: 913222 55303; e-mail: [email protected] 575
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Scheme 1. Plausible mechanism of melt reaction between nylon-6 and ACM at 220 C.
service temperatures. Only 40/60 (w/w) nylon-6/ ACM dynamically vulcanized blend, which has been proven to be thermoplastic elastomeric in nature according to strength and elongational properties, has been examined in the later case to check its applicability as heat resistant thermoplastic elastomer. This can give us an idea of the service temperature range where it can be used in practical purpose without any appreciable loss of its physical properties. The results are also substantiated by dynamic mechanical thermal analysis (DMTA) and infrared (IR) analysis to ®nd out the nature and the cause of the degradation. Thermogravimetric analysis (TGA) is a useful technique for studying the thermal decomposition of polymers and its application can be extended to the determination of the kinetic features of degradation.7 TGA coupled with other analytical tools (e.g. GC±MS, pyrolysis±MS etc.) were used extensively to study the degradation of various pure polymers and copolymers over the last 40 years. On the other hand, very little information is available on the degradation behaviour of one polymer in the presence of another, as in the case of various blends, composites etc. As most polymer blends are heterogeneous, morphological characteristics have an important bearing on the types of reaction which can occur in degrading blends. The degradation
properties of polymethylmethacrylate (PMMA), polyvinylchloride (PVC) and polystyrene (PS) in the presence of various second polymers were reviewed by McNeill et al.8 It is observed that in most of the cases the products of degradation of one polymer greatly in¯uence the degradation behaviour of the other polymer. The interactions between the two polymers result in some cases destabilization; in other cases the thermal stability of the blend is improved. Lizymol and Thomas.9 and Goh10 reported the eect of miscibility on the thermal stability of various blends. Kole et al.11 studied the degradation behaviour of Siliconeethylenepropylene diene rubber blends and discussed the eect of compatibilization of the blends on its thermal degradation characteristics. Choudhury et al.12 observed that the thermal stability of thermoplastic elastomeric natural rubber±polypropylene blend is enhanced by the presence of a minor amount of ethylene propylene diene rubber or chlorinated polyethylene, which acts as interfacial agent between the two phases. 2 EXPERIMENTAL 2.1 Materials Nylon-6 (Ultramid B3) in pellet form [viscosity number (solution 0.005 g/ml. sulphuric acid)= 150 ml/g] was supplied by BASF, Germany. Acrylate rubber (ACM) NIPOL AR51 (sp.gr.=1.1 at 25 C) was obtained from Nippon Zeon Co. Ltd., Tokyo, Japan. It is reported to have an epoxy cure site and made from ethyl acrylate monomer. Hexamethylene diamine carbarnate, DIAK#1 (Du Pont), was supplied by NICCO, India. 2.2 Preparation of thermoplastic elastomeric compositions Nylon-6 and ACM were dried at 100 C for 24 h in vacuum, prior to blending. The blends were prepared in batches of 50 gm polymer in a Brabender Plasticorder (PL2000-3) mixer with roller type rotor at a mixer set point temperature of 220 C. The rotor speed was kept at 40 rpm. Nylon-6 was ®rst charged and melted for 2 min. After the nylon had melted, ACM was added and mixed for dierent times under the same conditions. The change of mixing torque with mixing time along with the stock temperature was recorded for each blend. As the mixing continued, the stock temperature gradually
Thermal degradation and ageing behaviour
rose to 235 C and the torque value showed an upward trend. After a speci®ed time of mixing, the resulting blend was quickly removed from the mixer and passed through the close nip-gap of a water cooled two-roll mill to stop the reaction. A similar procedure was followed for all the blends. The blend samples which showed thermoplastic elastomeric behaviour were also subjected to dynamic vulcanization. In the above processing stage, hexamethylene diamine carbarnate (HMDC) was added after a speci®ed time of mixing. The mixing was continued for another 5 min after HMDC addition and the blends were treated as before. 2.3 Molding of thermoplastic elastomer Test specimens (about 1.2 mm thick) were prepared by compression molding at 230 C in a frame-andplate mold between well released aluminium foils for 2 min for all the samples and immediately cooled by passing water under pressure. The samples were removed from the mold when the temperature came down to 100 C. Appropriate test specimens were die cut from the moulded sheets and used thereafter. 2.4 Thermogravimetric analysis (TGA) Thermogravimetric analysis was carried out in a Stanton Redcroft STA-625 thermal analyser, from ambient temperature to 600 C at a programmed heating rate of 20 C/min in nitrogen atmosphere. The onset of degradation (To ) and the temperature at which the rate of mass loss is at a maximum (Tmax ) were evaluated for each sample. The activation energy of degradation and the pre-exponential factor were determined using standard software. 2.5 Infrared (IR) spectra analysis The FTIR-ATR spectra were obtained with a Shimadzu FTIR spectrophotometer, model 8101 using 45 KRS5 prism at room temperature. The samples were scanned from 4000 cmÿ1 to 400 cmÿ1 with a resolution of 4 cmÿ1 and 40 scans were averaged for each spectrum. 2.6 Ageing study Ageing tests were performed in a Multi Cell Ageing Oven of Toyoseiki, Tokyo, Japan in the temperature range of 150±200 C for dierent times up to 7 days.
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2.7 Mechanical and dynamic mechanical analysis Tensile tests were performed according to ASTM D 412-80 test method by using dumb-bell shaped test pieces, which were punched out from the moulded sheets with BS-E-type die. The tests were carried out in a Zwick Universal Testing machine (UTM), model 1445 at 252 C and a cross-head speed of 500 mm/min. The results reported here are averages of three samples. The error in this measurement was 10%. Dynamic mechanical analysis of the blends and the individual polymers were performed on a Rheometric Scienti®c MK-II DMTA in the bending dual cantilever mode. The experiments were carried out at a frequency of 10 Hz, at a heating rate of 2 C/min and a double strain amplitude of 64 m over a temperature range of ÿ100 C± 15 C. The storage modulus, E0 , loss modulus, E00 and loss tangent, tand were measured for each sample in this temperature range. 3 RESULTS AND DISCUSSION 3.1 High temperature thermal degradation behaviour of nylon-6/ACM blends Figures 1 and 2 show the thermogravimetric curves of pure nylon-6, ACM and 40/60 (w/w) nylon-6/ACM blend and their corresponding derivative curves (DTG) respectively. Pure ACM shows an initiation temperature, To , of about 300 C and the temperature corresponding to its maximum rate of decomposition, Tmax , at 411 C. Pure nylon-6 shows a higher initiation temperature of 363 C with a Tmax , value at 455 C. Also, a minor degradation (only about 4%) step is observed for nylon-6 at around 250 C. In the case of the 40/60 (w/w) blend, it is observed that (i) the initiation of degradation shifts to higher temperature compared to that of ACM (i.e. 350 C), (ii) the degradation at 250 C is absent or diminished, and (iii) the Tmax value shifts towards lower temperature (i.e. 401 C) compared to that of the nylon-6. Also, the blend shows two stage decomposition with the other Tmax value at 420 C. A drastic lowering of the thermal stability of the blend sample compared to that of nylon-6 indicates that some chemical interaction between the two polymers occurs in the blends at high temperature during degradation and the products thus formed destabilize the nylon-6 phase in the blend.
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Fig. 1. TGA plot of pure nylon-6 (- - - -), pure ACM (ÐÐ) and 40/60 (w/w) blend of nylon-6/ACM (±.±.±).
Fig. 2. Derivative thermogravimetry (DTG) plot of pure nylon-6 (- - - -), pure ACM (ÐÐ) and 40/60 (w/w) blend of nylon-6/ACM (±.±.±).
As a result, TGA of the blend is close to that of ACM. Figure 3 shows the derivative thermogravimetry of nylon-6/ACM blends at three dierent plastic to rubber weight ratios. For 40/60 (w/w) blend, the
degradation is a two-step process with corresponding Tmax values at 401 C and 420 C, but in the case of 50/50 (w/w) blend, the second decomposition step is very slow. For 60/40 (w/w) blend the decomposition is distinctly a one-step process. Also, there is a minor point to note that Tmax corresponding to the ®rst decomposition step is slightly shifted by about 7 degrees to the higher temperature side with increase of the plastic content. To examine the eect of dynamic vulcanization on the thermal stability of the nylon-6/ACM blends, TGA was carried out for 40/60 nylon-6/ ACM (W/W) blend dynamically cured with 0.5 phr of HMDC. It is found that the above dynamically cured blend also shows two-step degradation (not shown in the ®gure). The Tmax value of the ®rst step remains unchanged but that of the second step (i.e. Tmax2 ) shifts slightly towards the higher temperature side (about 7 C). This can be explained on the basis of the fact, that during curing, a three dimensional network is formed in the rubber matrix which delays the thermal degradation of the same by trapping the end chain radicals generated in the depropagation step. We have seen that nylon-6 and ACM react with each other during melt-blending in Brabender Plasticorder at the temperature range of 220± 235 C.4 The reaction for dierent durations produces dierent amounts of graft copolymer of nylon-6 and ACM at their phase boundaries. To
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examine the eect of the extent of interfacial reaction (Scheme 1) between nylon-6 and ACM on the high temperature stability of a particular blend, mixing was carried out for dierent times for 40/60 (w/w) blend, and their degradation behaviour was examined through TGA. Figure 4 shows such DTG curves. It is clear that with increasing extent of the reaction, the fraction of the blend decomposed in the second stage is increased. Also the Tmax values corresponding to each step are unchanged, implying that their positions are independent of the interaction between the two phases. From the above results, it is clear that meltmixed nylon-6/ACM blends degrade at a lower temperature compared with pure nylon-6 indicating a strong interaction between the polymers at high temperature. The eect of the melt-reaction between the two components i.e. the formation of nylon-6/ACM graft copolymer only delays the degradation further to a second step process and the rate of second step decomposition increases with increasing the extent of melt reaction between the two polymers. Also it is interesting to note that the blends with higher amount of nylon-6 degrade by a one-step process again. The activation energy
and pre-exponential factor for nylon-6, ACM and 60/40 (w/w) nylon-6/ACM blend which show single-step degradation have also been determined using Freernan±Caroll's13 method. Nylon-6 shows much higher activation energy of degradation (e.g. 64 kcal/mole) and pre-exponential factor (i.e. InZ=44) than that of ACM (eg. Ea=50 kcal/mole; InZ=36). However, the 60/40 (w/w) blend shows an intermediate value (eg. Ea=53 kcal/mole, InZ=38). To explain the above results i.e. the interaction between nylon-6 and ACM at higher temperature, the possibility of `ester-amide exchange' reaction is postulated as discussed later. It is known that polyacrylates depolymerise through an unzipping mechanism to yield a quantitative amount of monomers in the temperature range 300±400 C.16 The primary thermal decomposition processes of nylon-6 were investigated by several authors.14,15 The mass spectra of the thermal degradation products evolved from nylon-6 shows that an intramolecular exchange process is the preferred thermal decomposition mechanism (Scheme 2). Decomposition through a -hydrogen transfer reaction with loss of water appears to occur as a secondary thermal process (Scheme 2(b)), evolving products with ole®n and nitrile
Fig. 3. DTG plot of nylon-6/ACM blends with dierent plastic to rubber weight ratio. 40/60 (w/w) (±.±.±), 50/50 (w/w) (- - - -) and 60/40 (w/w) (ÐÐ).
Fig. 4. DTG plot of 40/60 (w/w) nylon-6/ACM blend melt mixed for dierent times at 220 C. (1) 5 min (2) 7 min (3) 9 min (4) 11 min and (5) 13 min.
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Scheme 2. Thermal degradation pathways of nylon-6.
groups. There is also another pathway of depropagation of nylon-6 reported at 250 C through the formation of monomer (i.e. caprolactam) by backbiting reaction of the terminal amino groups (Scheme 2(c)), the presence of which is also re¯ected in the small amount of weight loss in the TGA curve of nylon-6 near 250 C. The absence of this step of degradation in the blend with ACM suggests that the melt reaction between nylon-6 and ACM blocks the terminal amino groups (as shown in Scheme 1(a)) and substantially decreases the rate of monomer formation. This hypothesis can also be supported by the fact that acetylation of the terminal amino groups of nylon-6 decreases the rate of monomer formation when the polymer is subjected to a temperature of about 250 C.16 The above melt-reaction can also explain the higher initiation temperature of the blends compared to that of ACM because, the labile epoxy groups of ACM are stabilized after its reaction with nylon-6 during the melt blending operation. It is well known that suitable polyesters and polyamides undergo exchange reactions to produce block and graft copolymers at the interphase during heating the mixture at a temperature range of 250±300 C.17±19 For example, in the case of a nylon-6 and polycarbonate (PC) system, the exchange reaction occurs during melt blending of the two polymers in an internal mixture at 250 C.20 It is also interesting to note that the products of the exchange reaction between nylon-6 and PC are responsible for a marked reduction in the thermal stability of the blends compared to that of the pure nylon-6 and PC. In line with the above facts, a similar type of ester-amide exchange reaction can reasonably be postulated for nylon-6/ACM system also in the temperature range of 250±300 C during degradation. The probable reaction pathways can be envisaged as shown in Schemes 3(a) and (b). The 3(b) pathway is more probable because the majority of the amino end groups of nylon-6 are
Scheme 3. Mechanism of ester-amide interchange reactions between nylon-6 and ACM.
blocked by its reaction with epoxy groups of ACM as discussed earlier. Thus, reaction 3(b) eectively degrades the nylon-6 molecules into lower molecular weight fragments in the blend and hence reduces its stability to a great extent. Also the nylon-6/ACM graft copolymers formed at the interphase by the exchange reaction at high temperature (>250 C) are joined by weak amide linkages due to steric eects of the adjacent ethyl ester groups and these amide linkages are susceptible to decomposition at lower temperature, which, in turn, can initiate the depolymerisation of both the phases and thus increase the rate of decomposition of the blends as compared to that of ACM. 3.2 Ageing behaviour of nylon-6/ACM based thermoplastic elastomer The ageing properties of the dynamically vulcanized 40/60 (w/w) nylon-6/ACM blend were studied at 150, 175 and 200 C over various periods of time. Figure 5 shows the changes in tensile strength and elongation at break during ageing at 150 C with time. The tensile strength shows a marginal increment in the early stage of ageing and decreases again with time after 3 days. The elongation at break also shows the same trend. This can be attributed to the post curing reaction of the ACM phases at 150 C during the early stage of ageing which increases both the tensile strength and elongation-at-break values. However, after 3 days, the degradation of both the phases may be predominant leading to a decrease in mechanical properties. In the case of 40/60 nylon-6/ACM (w/w) blend (without dynamic vulcanization), it is
Thermal degradation and ageing behaviour
found that after ageing at 150 C for 3 days, the elongation-at-break retains about 90% of its original value compared to that of the unaged sample with no increment in its tensile strength. Figure 6 shows the tensile strength and elongation-at-break of the blend aged at 150, 175 and 200 C for 3 days alongwith those of the unaged sample. The enhanced properties are due to the same reason discussed above. It is interesting to note that the mechanical properties of the above blend do not deteriorate to a signi®cant extent in the temperature range of 150±200 C, implying excellent heat resistant properties. To examine the nature and relative amount of degradation of both the polymeric phases at high temperature ageing, DMTA was performed on the same aged samples. Figure 7 shows the DMTA results of pure acrylate rubber, nylon-6 and 50/50 (w/w) blend of nylon-6 and ACM (unaged) in terms of temperature dependence of tand in the range between ÿ100 to 150 C. Nylon-6 shows two loss peaks in the above temperature range which have been labelled and occurring at 98 and ÿ50 C respectively. The a peak is assigned to Tg , which involves the motion within the amorphous phase and strongly depends on the crystallinity of the material, while the damping peak is attributed to the carboxyl group of the polyamide forming hydrogen bonds. In the case of ACM, three transitions are observed at 0, ÿ35 C and ÿ73 C, out of which the main transition at 0 C is ascribed to the glass±rubber transition or Tg . The 50/50 (w/w) blend shows two damping peaks, one at ÿ2.5 C corresponding to the Tg of ACM and
Fig. 5. Mechanical properties versus number of days ageing at 150 C for thermoplastic elastomeric 40/60 (w/w) nylon-6/ ACM, dynamically vulcanized blend.
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Fig. 6. Mechanical properties versus temperature of ageing (for 3 days) for thermoplastic elastomeric 40/60 (w/w) nylon6/ACM, dynamically vulcanized blend.
another broad peak at 85 C due to Tg of nylon-6, suggesting the microheterogeneity of the blend (i.e. two phase morphological structure). However, the most interesting feature observed in the blend sample is the appearance of a secondary tand peak of the rubber phase at 22.5 C which signi®es the existence of partially immobilized ACM chains grafted to the nylon-6 matrix.4 The mobility of the rubber chains when grafted to the plastic matrix is expected to be greatly reduced relative to that of a chain in the bulk polymer, with the mobility increasing gradually with increasing distance from the boundary. Thus, a layer of restricted chain mobility is formed in the outer shell of the rubber particles near the phase boundaries, the formation of which is re¯ected in the appearance of a new secondary tan peak at higher temperature region. Figure 8 shows the DMTA results of the samples aged at 150 C for dierent durations. No signi®cant change is observed in the tand peak heights and the glass transition temperatures of the ACM phase after ageing. However, it is found that with increasing time of ageing, both the peak position and the peak height of the secondary damping peak of the rubber phase have changed progressively. The above trend is very much prominent when the tan curves are plotted for the samples aged at dierent temperatures of ageing for a ®xed period of time (i.e. 3 days) as shown in Fig. 9(a). With increasing temperature of ageing, the secondary tand peak of the rubber phase is diminished and a broad transition occurs in the sample temperature region. Also, the peak height of the Tg of
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Fig. 7. Temperature dependence of tan of pure nylon-6 (- - - -), pure ACM (ÐÐ) and 50/50 (w/w) nylon-6/ACM blend (±.±.±).
Fig. 8. Temperature dependence of tan of thermoplastic elastomeric nylon-6/ACM blend aged at 150 C for dierent times. 0 days (ÐÐ), 3 days (- - - -) and 5 days (±.±.±).
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Fig. 9. (a) Temperature dependence of tan of thermoplastic elastomeric nylon-6/ACM blend aged for three days at dierent temperature. Control sample (ÐÐ), 150 C (-x-x-x), 175 C (±.±.±) and 200 C (- - - -). (b) Temperature dependence of storage modulus (E0 ) of thermoplastic elastomeric nylon-6/ACM blend aged for 3 days at dierent temperatures. Control sample (ÐÐ), 175 C (- - - -) and 200 C (±.±.±).
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the rubber phase increases with extent of ageing. It is also found that the peak height of the Tg of nylon-6 phase aged at 200 C shows a signi®cant increment compared to that of the unaged sample. As the secondary transition of the rubber phase signi®es the rubber chains grafted to the nylon-6 matrix, the above results suggest that during ageing, the chemical bonds between nylon-6 and ACM break down predominantly. This decreases the adhesion between the two phases leading to a decrease in its tensile strength and elongation values. Also, the increase in the height of the Tg of nylon-6 phase indicates that during ageing, the crystalline phase of the nylon-6 matrix undergoes thermo-oxidative degradation leading to an increase in its amorphous content which is also clear from log E0 versus temperature curves of the same aged samples (Fig. 9(b)). It is observed that during ageing, the colour of the blend sample changes from brown to blackish brown progressively, which indicates the occurrence of some chemical reactions between the polymers and the oxygen present in the air. To
study the nature of such a reaction, the aged samples were analysed by IR spectroscopy. Figure 10 represents the FTIR spectra of the samples aged at 150, 175 and 200 C for 3 days along with that of the unaged control sample. The assignments of various peaks found in nylon-6 and ACM are also given in Table 1 for clari®cation. It is clear from the ®gure that during ageing, signi®cant changes are observed in the spectral behaviour of the blends, particularly in the ranges 1700±1750 cmÿ1, 1610±1650 cmÿ1 and 1490±1560 cmÿ1. The peak at 1728 cmÿ1 which is due the >C=0 stretching frequencies of ethyl ester units of ACM and that of the free carboxyl groups of nylon-6 splits into three major peaks e.g. 1740, 1728 and 1713 cmÿ1. This indicates the thermooxidative scission of the amide linkages of nylon-6 to generate free carboxyl groups which absorb strongly in this region. Also, there is a possibility of thermal oxidation of the ACM chains and methylene units of nylon-6 which may generate dierent kinds of carbonyl groups at higher temperature (e.g. above 175 C). The most interesting changes occur in the region of Amide I
Fig. 10. FTIR spectra of thermoplastic elastomeric nylon-6/ACM vulcanizates, aged for 3 days at dierent temperatures. (1) unaged sample, (2) 150 C, (3) 175 C and (4) 200 C.
Thermal degradation and ageing behaviour Table 1. Assignment of infrared bands of (a) Nylon-6 and (b) Acrylate rubber Wave number, cmÿ1 (a) Nylon-6 3316 2936, 2862 1728
Assignment
1456 800
N-H stretching absorption of amide C-H bending >C=0 stretching of ÿCO2H end group >C=0 stretching of amide (Amide I band) N-H bending absorption (Amide II band) C-H bending -C-C- stretching
(b) Acrylate rubber 1728 1446 1375 1240 1159
>C=0 stretching of the ethyl ester unit CH3 assym. deformation Sym. CH3 deformation C-O-C assym. stretching C-O-C sym. stretching
1634 1541
Scheme 4. Thermal oxidative reaction pathway of nylon-6.
and Amide II bands of nylon-6. The peak intensity at 1647 cmÿ1 decreases relative to that of the >C=0 stretching frequency at 1728 cmÿ1 and the peak at 1541 cmÿ1 clearly splits into two peaks, one at 1544 cmÿ1 and the other at 1518 cmÿ1. The above results can be explained on the basis of formation of `imide' linkages on nylon-6 chains during thermal oxidation through the formation of unstable amide-hydroperoxide intermediate.21 The probable mechanism of formation of imides is shown in Scheme 4. The imide linkages are unstable and initiate further degradation of the nylon-6 chains. The Amide I band of the imide groups occur at 1670 ÿ1740 cmÿ1 (due to >C=0 stretching frequency) and Amide II band (due to N±H bending frequency) appears at 1510 ÿ1520 cmÿ1.22 Thus the formation of the imide groups explains the generation of new peaks both at 1740 cmÿ1 and 1518 cmÿ1. 4 CONCLUSIONS The thermal degradation behaviour of thermoplastic elastomeric blends of nylon-6 and acrylate rubber (ACM) has been studied by non-isothermal thermogravimetry. Also the ageing of dynamically
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vulcanized 40/60 (w/w) nylon6/ACM blend, which is thermoplastic elastomeric in nature, has been carried out at various temperatures and times. The following conclusions can be drawn from the above studies. (a) The melt-mixed nylon-6/ACM blends degrade at a lower temperature compared to that of nylon-6 indicating a strong interaction between the polymers at high temperature. To explain the above interaction, the possibility of ester±amide exchange reaction is postulated. (b) The eect of the melt-reaction between the two components ie, the formation of nylon6/ACM graft copolymer only delays the degradation further to a second step process and the rate of second step decomposition increases with increasing the extent of melt reaction between the two polymers. (c) The mechanical properties of the thermoplastic elastomeric 40/60 (w/w) blend (dynamically cured) do not deteriorate to a signi®cant extent in the temperature range of 150±200 C, implying its excellent heat resistant properties. (d) The DMTA results of the aged sample suggest that during ageing, the bonds between nylon-6 and ACM break down predominantly. (e) The FTIR studies of the aged sample indicate the formation of imide linkages on nylon-6 chains through thermal oxidation process. ACKNOWLEDGEMENTS The authors are thankful to the Department of Science and Technology, New Delhi for funding the project. The authors also wish to thank Mr Shyamal Mukherjee of Central Research Facility for his kind help in recording the TGA thermograms. REFERENCES 1. Legge, N. R., Holden, G. and Schroeder, H. E. (ed.), Thermoplastic Elastomers: A Comprehensive Review. Hanser, Munich, 1987. 2. Walker, B. M. (ed.), Handbook of Thermoplastic Elastomers. Van Nostrand Reinhold, New York, 1979. 3. Coran, A. Y., in Handbook of ElastomersÐNew Development and Technology, ed. A. K. Bhowmick and H.L. Stephens. Marcel Dekker, New York, 1988, p. 249. 4. Jha, A. and Bhowmick, A. K., Rubber Chem. Technol., 1997, 70, 798.
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5. Jha, A., Bhattacharyya, A. K. and Bhowmick, A. K., Polymer Networks and Blends, 1977, 7, 177. 6. Jha, A. and Bhowmick, A. K., J. Appl. Polym. Sci., in press. 7. Budrugeac, P. and Segal, E., Polym. Degrad. Stab., 1994, 46, 203. 8. McNeill, I. C., in Developments in Polymer DegradationÐ I, ed. N. Grassie. Applied Science, London, Chapter 6. 9. Lizymol, P. P. and Thomas, S., Polym. Degrad. Stab., 1993, 41, 59. 10. Goh, S. H., Thermochim. Acta, 1994, 215, 291. 11. Kole, S., Chaki, T. K., Bhowmick, A. K. and Tripathy, D. K., Polym. Degrad. Stab., 1993, 41, 109. 12. Choudhury, N. R., Chaki, T. K. and Bhowmick, A. K., Thermochim. Acta, 1991, 176, 149. 13. Freeman, E. S. and Caroll, B., J. Phys. Chem., 1958, 62, 394. 14. Luderwald, I. and Kricherdorf, H. R., Angew. Makromol. Chem., 1976, 56, 173.
15. Ballistreri, A., Garozzo, D., Giurida, M., Impallomeni, G. and Montaudo, G., Polym. Degrad. Stab., 1988, 23, 25. 16. Kelen, T. Polymer Degradation. Van Nostrand Reinhold, New York, 1982, Chapter 3, p. 48. 17. Porter, R. S. and Wang, L. H., Polymer, 1992, 33, 2019. 18. Kotliar, A. M., J. Polym. Sci., Macromol. Rev., 1981, 16, 367. 19. Gattiglia, E., Turtorro, A. and Pedemonte, E., J. Appl. Polym. Sci., 1989, 38, 1807. 20. Montaudo, G., Puglisi, C. and Samperi, F., J. Polym. Sci., Polym. Chem. Edn, 1994, 32, 15. 21. Mark, H. F., Bikales, N. M., Overberger, C. G. and Menges, G. (ed.), Encyclopedia of Polymer Science and Engineering. John Wiley and Sons, New York, 1985, Vol 4, p. 668. 22. Socrates, G., Infrared Characteristic Group Frequencies. Wiley Interscience, New York, 1980, Chapter 10, p. 77.
Polymer Degradation and Stability 62 (1998) 575±586 # 1998 Elsevier Science Limited. All rights reserved Printed in Great Britain 0141-3910/98/$Ðsee front matter
Thermal degradation and ageing behaviour of novel thermoplastic elastomeric nylon-6/acrylate rubber reactive blends Abhijit Jha & A. K. Bhowmick* Rubber Technology Centre, Indian Institute of Technology, Kharagpur 721302, India (Received 25 November 1997; accepted 12 January 1998) The thermal degradation behaviour of thermoplastic elastomeric blends of nylon6 and an acrylate rubber (ACM) has been studied by non-isothermal thermogravimetry. The thermal stability of the blends is low compared to that of nylon6, which is explained on the basis of `ester-amide interchange' reaction at high temperature during degradation. The exchange reaction eectively degrades the nylon-6 phase in the blends. The melt reaction between nylon-6 and ACM which occurs during the blending operation, slightly improves the stability of the blends. Ageing of dynamically cured 40/60 (w/w) nylon-6/ACM blend, which is thermoplastic elastomeric (TPE) in nature, has been carried out in air at various temperatures and times. The results show excellent heat resistance of the TPE at elevated service temperature. # 1998 Elsevier Science Limited. All rights reserved
1 INTRODUCTION
produce a graft or block copolymer at the interface, which, in turn, compatibilizes the two phases. The preparation and properties of novel heat and oil resistant thermoplastic elastomers from nylon-6 and acrylate rubber blend have been reported recently.4 The blends are reactive in nature which is manifested in the rise of mixing torque during melt blending in a Brabender Plasticorder at 220±235 C. Based on infra-red analysis, a plausible mechanism of reaction between the two polymers was proposed, as shown in Scheme 1. The in¯uence of interaction on mechanical, dynamic mechanical, rheological and swelling behaviour of the blends has been reported.4±6 The purpose of development of such blends is to improve the heat and oil resistance properties further as compared to the conventional commercial blends. In this paper the thermal stability of the blends is examined in two ways, (a) dynamic thermogravimetry in the temperature range of 300±600 C in nitrogenatmosphere to study the thermal degradation characteristics of the blends and the pure polymers and (b) ageing in the temperature range of 150± 200 C in air for prolonged time to check its applicability as a heat resistant material at elevated
Thermoplastic elastomers prepared from rubber± plastic blends have gained considerable interest in recent years due to their dual characteristics of rubbers and plastics.1,2 The ideal morphology of such blends comprises ®nely divided rubber particles dispersed in a minimum volume of plastic.3 The in-situ or dynamic vulcanization of the rubber phase improves the elastic recovery and other mechanical properties of the blends needed for commercial applications. Apart from morphology, the most important factor which controls the overall performance of thermoplastic elastomeric blends is the compatibility (i.e. technological compatibility) or the adhesion between the phases. Reactive blending is known to be one of the most important methods to induce compatibility between two immiscible polymers. Two polymers having proper reactive groups on their backbone can react with each other during melt blending to
*To whom correspondence should be addressed. Fax: 913222 55303; e-mail: [email protected] 575
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Scheme 1. Plausible mechanism of melt reaction between nylon-6 and ACM at 220 C.
service temperatures. Only 40/60 (w/w) nylon-6/ ACM dynamically vulcanized blend, which has been proven to be thermoplastic elastomeric in nature according to strength and elongational properties, has been examined in the later case to check its applicability as heat resistant thermoplastic elastomer. This can give us an idea of the service temperature range where it can be used in practical purpose without any appreciable loss of its physical properties. The results are also substantiated by dynamic mechanical thermal analysis (DMTA) and infrared (IR) analysis to ®nd out the nature and the cause of the degradation. Thermogravimetric analysis (TGA) is a useful technique for studying the thermal decomposition of polymers and its application can be extended to the determination of the kinetic features of degradation.7 TGA coupled with other analytical tools (e.g. GC±MS, pyrolysis±MS etc.) were used extensively to study the degradation of various pure polymers and copolymers over the last 40 years. On the other hand, very little information is available on the degradation behaviour of one polymer in the presence of another, as in the case of various blends, composites etc. As most polymer blends are heterogeneous, morphological characteristics have an important bearing on the types of reaction which can occur in degrading blends. The degradation
properties of polymethylmethacrylate (PMMA), polyvinylchloride (PVC) and polystyrene (PS) in the presence of various second polymers were reviewed by McNeill et al.8 It is observed that in most of the cases the products of degradation of one polymer greatly in¯uence the degradation behaviour of the other polymer. The interactions between the two polymers result in some cases destabilization; in other cases the thermal stability of the blend is improved. Lizymol and Thomas.9 and Goh10 reported the eect of miscibility on the thermal stability of various blends. Kole et al.11 studied the degradation behaviour of Siliconeethylenepropylene diene rubber blends and discussed the eect of compatibilization of the blends on its thermal degradation characteristics. Choudhury et al.12 observed that the thermal stability of thermoplastic elastomeric natural rubber±polypropylene blend is enhanced by the presence of a minor amount of ethylene propylene diene rubber or chlorinated polyethylene, which acts as interfacial agent between the two phases. 2 EXPERIMENTAL 2.1 Materials Nylon-6 (Ultramid B3) in pellet form [viscosity number (solution 0.005 g/ml. sulphuric acid)= 150 ml/g] was supplied by BASF, Germany. Acrylate rubber (ACM) NIPOL AR51 (sp.gr.=1.1 at 25 C) was obtained from Nippon Zeon Co. Ltd., Tokyo, Japan. It is reported to have an epoxy cure site and made from ethyl acrylate monomer. Hexamethylene diamine carbarnate, DIAK#1 (Du Pont), was supplied by NICCO, India. 2.2 Preparation of thermoplastic elastomeric compositions Nylon-6 and ACM were dried at 100 C for 24 h in vacuum, prior to blending. The blends were prepared in batches of 50 gm polymer in a Brabender Plasticorder (PL2000-3) mixer with roller type rotor at a mixer set point temperature of 220 C. The rotor speed was kept at 40 rpm. Nylon-6 was ®rst charged and melted for 2 min. After the nylon had melted, ACM was added and mixed for dierent times under the same conditions. The change of mixing torque with mixing time along with the stock temperature was recorded for each blend. As the mixing continued, the stock temperature gradually
Thermal degradation and ageing behaviour
rose to 235 C and the torque value showed an upward trend. After a speci®ed time of mixing, the resulting blend was quickly removed from the mixer and passed through the close nip-gap of a water cooled two-roll mill to stop the reaction. A similar procedure was followed for all the blends. The blend samples which showed thermoplastic elastomeric behaviour were also subjected to dynamic vulcanization. In the above processing stage, hexamethylene diamine carbarnate (HMDC) was added after a speci®ed time of mixing. The mixing was continued for another 5 min after HMDC addition and the blends were treated as before. 2.3 Molding of thermoplastic elastomer Test specimens (about 1.2 mm thick) were prepared by compression molding at 230 C in a frame-andplate mold between well released aluminium foils for 2 min for all the samples and immediately cooled by passing water under pressure. The samples were removed from the mold when the temperature came down to 100 C. Appropriate test specimens were die cut from the moulded sheets and used thereafter. 2.4 Thermogravimetric analysis (TGA) Thermogravimetric analysis was carried out in a Stanton Redcroft STA-625 thermal analyser, from ambient temperature to 600 C at a programmed heating rate of 20 C/min in nitrogen atmosphere. The onset of degradation (To ) and the temperature at which the rate of mass loss is at a maximum (Tmax ) were evaluated for each sample. The activation energy of degradation and the pre-exponential factor were determined using standard software. 2.5 Infrared (IR) spectra analysis The FTIR-ATR spectra were obtained with a Shimadzu FTIR spectrophotometer, model 8101 using 45 KRS5 prism at room temperature. The samples were scanned from 4000 cmÿ1 to 400 cmÿ1 with a resolution of 4 cmÿ1 and 40 scans were averaged for each spectrum. 2.6 Ageing study Ageing tests were performed in a Multi Cell Ageing Oven of Toyoseiki, Tokyo, Japan in the temperature range of 150±200 C for dierent times up to 7 days.
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2.7 Mechanical and dynamic mechanical analysis Tensile tests were performed according to ASTM D 412-80 test method by using dumb-bell shaped test pieces, which were punched out from the moulded sheets with BS-E-type die. The tests were carried out in a Zwick Universal Testing machine (UTM), model 1445 at 252 C and a cross-head speed of 500 mm/min. The results reported here are averages of three samples. The error in this measurement was 10%. Dynamic mechanical analysis of the blends and the individual polymers were performed on a Rheometric Scienti®c MK-II DMTA in the bending dual cantilever mode. The experiments were carried out at a frequency of 10 Hz, at a heating rate of 2 C/min and a double strain amplitude of 64 m over a temperature range of ÿ100 C± 15 C. The storage modulus, E0 , loss modulus, E00 and loss tangent, tand were measured for each sample in this temperature range. 3 RESULTS AND DISCUSSION 3.1 High temperature thermal degradation behaviour of nylon-6/ACM blends Figures 1 and 2 show the thermogravimetric curves of pure nylon-6, ACM and 40/60 (w/w) nylon-6/ACM blend and their corresponding derivative curves (DTG) respectively. Pure ACM shows an initiation temperature, To , of about 300 C and the temperature corresponding to its maximum rate of decomposition, Tmax , at 411 C. Pure nylon-6 shows a higher initiation temperature of 363 C with a Tmax , value at 455 C. Also, a minor degradation (only about 4%) step is observed for nylon-6 at around 250 C. In the case of the 40/60 (w/w) blend, it is observed that (i) the initiation of degradation shifts to higher temperature compared to that of ACM (i.e. 350 C), (ii) the degradation at 250 C is absent or diminished, and (iii) the Tmax value shifts towards lower temperature (i.e. 401 C) compared to that of the nylon-6. Also, the blend shows two stage decomposition with the other Tmax value at 420 C. A drastic lowering of the thermal stability of the blend sample compared to that of nylon-6 indicates that some chemical interaction between the two polymers occurs in the blends at high temperature during degradation and the products thus formed destabilize the nylon-6 phase in the blend.
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Fig. 1. TGA plot of pure nylon-6 (- - - -), pure ACM (ÐÐ) and 40/60 (w/w) blend of nylon-6/ACM (±.±.±).
Fig. 2. Derivative thermogravimetry (DTG) plot of pure nylon-6 (- - - -), pure ACM (ÐÐ) and 40/60 (w/w) blend of nylon-6/ACM (±.±.±).
As a result, TGA of the blend is close to that of ACM. Figure 3 shows the derivative thermogravimetry of nylon-6/ACM blends at three dierent plastic to rubber weight ratios. For 40/60 (w/w) blend, the
degradation is a two-step process with corresponding Tmax values at 401 C and 420 C, but in the case of 50/50 (w/w) blend, the second decomposition step is very slow. For 60/40 (w/w) blend the decomposition is distinctly a one-step process. Also, there is a minor point to note that Tmax corresponding to the ®rst decomposition step is slightly shifted by about 7 degrees to the higher temperature side with increase of the plastic content. To examine the eect of dynamic vulcanization on the thermal stability of the nylon-6/ACM blends, TGA was carried out for 40/60 nylon-6/ ACM (W/W) blend dynamically cured with 0.5 phr of HMDC. It is found that the above dynamically cured blend also shows two-step degradation (not shown in the ®gure). The Tmax value of the ®rst step remains unchanged but that of the second step (i.e. Tmax2 ) shifts slightly towards the higher temperature side (about 7 C). This can be explained on the basis of the fact, that during curing, a three dimensional network is formed in the rubber matrix which delays the thermal degradation of the same by trapping the end chain radicals generated in the depropagation step. We have seen that nylon-6 and ACM react with each other during melt-blending in Brabender Plasticorder at the temperature range of 220± 235 C.4 The reaction for dierent durations produces dierent amounts of graft copolymer of nylon-6 and ACM at their phase boundaries. To
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examine the eect of the extent of interfacial reaction (Scheme 1) between nylon-6 and ACM on the high temperature stability of a particular blend, mixing was carried out for dierent times for 40/60 (w/w) blend, and their degradation behaviour was examined through TGA. Figure 4 shows such DTG curves. It is clear that with increasing extent of the reaction, the fraction of the blend decomposed in the second stage is increased. Also the Tmax values corresponding to each step are unchanged, implying that their positions are independent of the interaction between the two phases. From the above results, it is clear that meltmixed nylon-6/ACM blends degrade at a lower temperature compared with pure nylon-6 indicating a strong interaction between the polymers at high temperature. The eect of the melt-reaction between the two components i.e. the formation of nylon-6/ACM graft copolymer only delays the degradation further to a second step process and the rate of second step decomposition increases with increasing the extent of melt reaction between the two polymers. Also it is interesting to note that the blends with higher amount of nylon-6 degrade by a one-step process again. The activation energy
and pre-exponential factor for nylon-6, ACM and 60/40 (w/w) nylon-6/ACM blend which show single-step degradation have also been determined using Freernan±Caroll's13 method. Nylon-6 shows much higher activation energy of degradation (e.g. 64 kcal/mole) and pre-exponential factor (i.e. InZ=44) than that of ACM (eg. Ea=50 kcal/mole; InZ=36). However, the 60/40 (w/w) blend shows an intermediate value (eg. Ea=53 kcal/mole, InZ=38). To explain the above results i.e. the interaction between nylon-6 and ACM at higher temperature, the possibility of `ester-amide exchange' reaction is postulated as discussed later. It is known that polyacrylates depolymerise through an unzipping mechanism to yield a quantitative amount of monomers in the temperature range 300±400 C.16 The primary thermal decomposition processes of nylon-6 were investigated by several authors.14,15 The mass spectra of the thermal degradation products evolved from nylon-6 shows that an intramolecular exchange process is the preferred thermal decomposition mechanism (Scheme 2). Decomposition through a -hydrogen transfer reaction with loss of water appears to occur as a secondary thermal process (Scheme 2(b)), evolving products with ole®n and nitrile
Fig. 3. DTG plot of nylon-6/ACM blends with dierent plastic to rubber weight ratio. 40/60 (w/w) (±.±.±), 50/50 (w/w) (- - - -) and 60/40 (w/w) (ÐÐ).
Fig. 4. DTG plot of 40/60 (w/w) nylon-6/ACM blend melt mixed for dierent times at 220 C. (1) 5 min (2) 7 min (3) 9 min (4) 11 min and (5) 13 min.
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Scheme 2. Thermal degradation pathways of nylon-6.
groups. There is also another pathway of depropagation of nylon-6 reported at 250 C through the formation of monomer (i.e. caprolactam) by backbiting reaction of the terminal amino groups (Scheme 2(c)), the presence of which is also re¯ected in the small amount of weight loss in the TGA curve of nylon-6 near 250 C. The absence of this step of degradation in the blend with ACM suggests that the melt reaction between nylon-6 and ACM blocks the terminal amino groups (as shown in Scheme 1(a)) and substantially decreases the rate of monomer formation. This hypothesis can also be supported by the fact that acetylation of the terminal amino groups of nylon-6 decreases the rate of monomer formation when the polymer is subjected to a temperature of about 250 C.16 The above melt-reaction can also explain the higher initiation temperature of the blends compared to that of ACM because, the labile epoxy groups of ACM are stabilized after its reaction with nylon-6 during the melt blending operation. It is well known that suitable polyesters and polyamides undergo exchange reactions to produce block and graft copolymers at the interphase during heating the mixture at a temperature range of 250±300 C.17±19 For example, in the case of a nylon-6 and polycarbonate (PC) system, the exchange reaction occurs during melt blending of the two polymers in an internal mixture at 250 C.20 It is also interesting to note that the products of the exchange reaction between nylon-6 and PC are responsible for a marked reduction in the thermal stability of the blends compared to that of the pure nylon-6 and PC. In line with the above facts, a similar type of ester-amide exchange reaction can reasonably be postulated for nylon-6/ACM system also in the temperature range of 250±300 C during degradation. The probable reaction pathways can be envisaged as shown in Schemes 3(a) and (b). The 3(b) pathway is more probable because the majority of the amino end groups of nylon-6 are
Scheme 3. Mechanism of ester-amide interchange reactions between nylon-6 and ACM.
blocked by its reaction with epoxy groups of ACM as discussed earlier. Thus, reaction 3(b) eectively degrades the nylon-6 molecules into lower molecular weight fragments in the blend and hence reduces its stability to a great extent. Also the nylon-6/ACM graft copolymers formed at the interphase by the exchange reaction at high temperature (>250 C) are joined by weak amide linkages due to steric eects of the adjacent ethyl ester groups and these amide linkages are susceptible to decomposition at lower temperature, which, in turn, can initiate the depolymerisation of both the phases and thus increase the rate of decomposition of the blends as compared to that of ACM. 3.2 Ageing behaviour of nylon-6/ACM based thermoplastic elastomer The ageing properties of the dynamically vulcanized 40/60 (w/w) nylon-6/ACM blend were studied at 150, 175 and 200 C over various periods of time. Figure 5 shows the changes in tensile strength and elongation at break during ageing at 150 C with time. The tensile strength shows a marginal increment in the early stage of ageing and decreases again with time after 3 days. The elongation at break also shows the same trend. This can be attributed to the post curing reaction of the ACM phases at 150 C during the early stage of ageing which increases both the tensile strength and elongation-at-break values. However, after 3 days, the degradation of both the phases may be predominant leading to a decrease in mechanical properties. In the case of 40/60 nylon-6/ACM (w/w) blend (without dynamic vulcanization), it is
Thermal degradation and ageing behaviour
found that after ageing at 150 C for 3 days, the elongation-at-break retains about 90% of its original value compared to that of the unaged sample with no increment in its tensile strength. Figure 6 shows the tensile strength and elongation-at-break of the blend aged at 150, 175 and 200 C for 3 days alongwith those of the unaged sample. The enhanced properties are due to the same reason discussed above. It is interesting to note that the mechanical properties of the above blend do not deteriorate to a signi®cant extent in the temperature range of 150±200 C, implying excellent heat resistant properties. To examine the nature and relative amount of degradation of both the polymeric phases at high temperature ageing, DMTA was performed on the same aged samples. Figure 7 shows the DMTA results of pure acrylate rubber, nylon-6 and 50/50 (w/w) blend of nylon-6 and ACM (unaged) in terms of temperature dependence of tand in the range between ÿ100 to 150 C. Nylon-6 shows two loss peaks in the above temperature range which have been labelled and occurring at 98 and ÿ50 C respectively. The a peak is assigned to Tg , which involves the motion within the amorphous phase and strongly depends on the crystallinity of the material, while the damping peak is attributed to the carboxyl group of the polyamide forming hydrogen bonds. In the case of ACM, three transitions are observed at 0, ÿ35 C and ÿ73 C, out of which the main transition at 0 C is ascribed to the glass±rubber transition or Tg . The 50/50 (w/w) blend shows two damping peaks, one at ÿ2.5 C corresponding to the Tg of ACM and
Fig. 5. Mechanical properties versus number of days ageing at 150 C for thermoplastic elastomeric 40/60 (w/w) nylon-6/ ACM, dynamically vulcanized blend.
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Fig. 6. Mechanical properties versus temperature of ageing (for 3 days) for thermoplastic elastomeric 40/60 (w/w) nylon6/ACM, dynamically vulcanized blend.
another broad peak at 85 C due to Tg of nylon-6, suggesting the microheterogeneity of the blend (i.e. two phase morphological structure). However, the most interesting feature observed in the blend sample is the appearance of a secondary tand peak of the rubber phase at 22.5 C which signi®es the existence of partially immobilized ACM chains grafted to the nylon-6 matrix.4 The mobility of the rubber chains when grafted to the plastic matrix is expected to be greatly reduced relative to that of a chain in the bulk polymer, with the mobility increasing gradually with increasing distance from the boundary. Thus, a layer of restricted chain mobility is formed in the outer shell of the rubber particles near the phase boundaries, the formation of which is re¯ected in the appearance of a new secondary tan peak at higher temperature region. Figure 8 shows the DMTA results of the samples aged at 150 C for dierent durations. No signi®cant change is observed in the tand peak heights and the glass transition temperatures of the ACM phase after ageing. However, it is found that with increasing time of ageing, both the peak position and the peak height of the secondary damping peak of the rubber phase have changed progressively. The above trend is very much prominent when the tan curves are plotted for the samples aged at dierent temperatures of ageing for a ®xed period of time (i.e. 3 days) as shown in Fig. 9(a). With increasing temperature of ageing, the secondary tand peak of the rubber phase is diminished and a broad transition occurs in the sample temperature region. Also, the peak height of the Tg of
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Fig. 7. Temperature dependence of tan of pure nylon-6 (- - - -), pure ACM (ÐÐ) and 50/50 (w/w) nylon-6/ACM blend (±.±.±).
Fig. 8. Temperature dependence of tan of thermoplastic elastomeric nylon-6/ACM blend aged at 150 C for dierent times. 0 days (ÐÐ), 3 days (- - - -) and 5 days (±.±.±).
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Fig. 9. (a) Temperature dependence of tan of thermoplastic elastomeric nylon-6/ACM blend aged for three days at dierent temperature. Control sample (ÐÐ), 150 C (-x-x-x), 175 C (±.±.±) and 200 C (- - - -). (b) Temperature dependence of storage modulus (E0 ) of thermoplastic elastomeric nylon-6/ACM blend aged for 3 days at dierent temperatures. Control sample (ÐÐ), 175 C (- - - -) and 200 C (±.±.±).
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the rubber phase increases with extent of ageing. It is also found that the peak height of the Tg of nylon-6 phase aged at 200 C shows a signi®cant increment compared to that of the unaged sample. As the secondary transition of the rubber phase signi®es the rubber chains grafted to the nylon-6 matrix, the above results suggest that during ageing, the chemical bonds between nylon-6 and ACM break down predominantly. This decreases the adhesion between the two phases leading to a decrease in its tensile strength and elongation values. Also, the increase in the height of the Tg of nylon-6 phase indicates that during ageing, the crystalline phase of the nylon-6 matrix undergoes thermo-oxidative degradation leading to an increase in its amorphous content which is also clear from log E0 versus temperature curves of the same aged samples (Fig. 9(b)). It is observed that during ageing, the colour of the blend sample changes from brown to blackish brown progressively, which indicates the occurrence of some chemical reactions between the polymers and the oxygen present in the air. To
study the nature of such a reaction, the aged samples were analysed by IR spectroscopy. Figure 10 represents the FTIR spectra of the samples aged at 150, 175 and 200 C for 3 days along with that of the unaged control sample. The assignments of various peaks found in nylon-6 and ACM are also given in Table 1 for clari®cation. It is clear from the ®gure that during ageing, signi®cant changes are observed in the spectral behaviour of the blends, particularly in the ranges 1700±1750 cmÿ1, 1610±1650 cmÿ1 and 1490±1560 cmÿ1. The peak at 1728 cmÿ1 which is due the >C=0 stretching frequencies of ethyl ester units of ACM and that of the free carboxyl groups of nylon-6 splits into three major peaks e.g. 1740, 1728 and 1713 cmÿ1. This indicates the thermooxidative scission of the amide linkages of nylon-6 to generate free carboxyl groups which absorb strongly in this region. Also, there is a possibility of thermal oxidation of the ACM chains and methylene units of nylon-6 which may generate dierent kinds of carbonyl groups at higher temperature (e.g. above 175 C). The most interesting changes occur in the region of Amide I
Fig. 10. FTIR spectra of thermoplastic elastomeric nylon-6/ACM vulcanizates, aged for 3 days at dierent temperatures. (1) unaged sample, (2) 150 C, (3) 175 C and (4) 200 C.
Thermal degradation and ageing behaviour Table 1. Assignment of infrared bands of (a) Nylon-6 and (b) Acrylate rubber Wave number, cmÿ1 (a) Nylon-6 3316 2936, 2862 1728
Assignment
1456 800
N-H stretching absorption of amide C-H bending >C=0 stretching of ÿCO2H end group >C=0 stretching of amide (Amide I band) N-H bending absorption (Amide II band) C-H bending -C-C- stretching
(b) Acrylate rubber 1728 1446 1375 1240 1159
>C=0 stretching of the ethyl ester unit CH3 assym. deformation Sym. CH3 deformation C-O-C assym. stretching C-O-C sym. stretching
1634 1541
Scheme 4. Thermal oxidative reaction pathway of nylon-6.
and Amide II bands of nylon-6. The peak intensity at 1647 cmÿ1 decreases relative to that of the >C=0 stretching frequency at 1728 cmÿ1 and the peak at 1541 cmÿ1 clearly splits into two peaks, one at 1544 cmÿ1 and the other at 1518 cmÿ1. The above results can be explained on the basis of formation of `imide' linkages on nylon-6 chains during thermal oxidation through the formation of unstable amide-hydroperoxide intermediate.21 The probable mechanism of formation of imides is shown in Scheme 4. The imide linkages are unstable and initiate further degradation of the nylon-6 chains. The Amide I band of the imide groups occur at 1670 ÿ1740 cmÿ1 (due to >C=0 stretching frequency) and Amide II band (due to N±H bending frequency) appears at 1510 ÿ1520 cmÿ1.22 Thus the formation of the imide groups explains the generation of new peaks both at 1740 cmÿ1 and 1518 cmÿ1. 4 CONCLUSIONS The thermal degradation behaviour of thermoplastic elastomeric blends of nylon-6 and acrylate rubber (ACM) has been studied by non-isothermal thermogravimetry. Also the ageing of dynamically
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vulcanized 40/60 (w/w) nylon6/ACM blend, which is thermoplastic elastomeric in nature, has been carried out at various temperatures and times. The following conclusions can be drawn from the above studies. (a) The melt-mixed nylon-6/ACM blends degrade at a lower temperature compared to that of nylon-6 indicating a strong interaction between the polymers at high temperature. To explain the above interaction, the possibility of ester±amide exchange reaction is postulated. (b) The eect of the melt-reaction between the two components ie, the formation of nylon6/ACM graft copolymer only delays the degradation further to a second step process and the rate of second step decomposition increases with increasing the extent of melt reaction between the two polymers. (c) The mechanical properties of the thermoplastic elastomeric 40/60 (w/w) blend (dynamically cured) do not deteriorate to a signi®cant extent in the temperature range of 150±200 C, implying its excellent heat resistant properties. (d) The DMTA results of the aged sample suggest that during ageing, the bonds between nylon-6 and ACM break down predominantly. (e) The FTIR studies of the aged sample indicate the formation of imide linkages on nylon-6 chains through thermal oxidation process. ACKNOWLEDGEMENTS The authors are thankful to the Department of Science and Technology, New Delhi for funding the project. The authors also wish to thank Mr Shyamal Mukherjee of Central Research Facility for his kind help in recording the TGA thermograms. REFERENCES 1. Legge, N. R., Holden, G. and Schroeder, H. E. (ed.), Thermoplastic Elastomers: A Comprehensive Review. Hanser, Munich, 1987. 2. Walker, B. M. (ed.), Handbook of Thermoplastic Elastomers. Van Nostrand Reinhold, New York, 1979. 3. Coran, A. Y., in Handbook of ElastomersÐNew Development and Technology, ed. A. K. Bhowmick and H.L. Stephens. Marcel Dekker, New York, 1988, p. 249. 4. Jha, A. and Bhowmick, A. K., Rubber Chem. Technol., 1997, 70, 798.
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5. Jha, A., Bhattacharyya, A. K. and Bhowmick, A. K., Polymer Networks and Blends, 1977, 7, 177. 6. Jha, A. and Bhowmick, A. K., J. Appl. Polym. Sci., in press. 7. Budrugeac, P. and Segal, E., Polym. Degrad. Stab., 1994, 46, 203. 8. McNeill, I. C., in Developments in Polymer DegradationÐ I, ed. N. Grassie. Applied Science, London, Chapter 6. 9. Lizymol, P. P. and Thomas, S., Polym. Degrad. Stab., 1993, 41, 59. 10. Goh, S. H., Thermochim. Acta, 1994, 215, 291. 11. Kole, S., Chaki, T. K., Bhowmick, A. K. and Tripathy, D. K., Polym. Degrad. Stab., 1993, 41, 109. 12. Choudhury, N. R., Chaki, T. K. and Bhowmick, A. K., Thermochim. Acta, 1991, 176, 149. 13. Freeman, E. S. and Caroll, B., J. Phys. Chem., 1958, 62, 394. 14. Luderwald, I. and Kricherdorf, H. R., Angew. Makromol. Chem., 1976, 56, 173.
15. Ballistreri, A., Garozzo, D., Giurida, M., Impallomeni, G. and Montaudo, G., Polym. Degrad. Stab., 1988, 23, 25. 16. Kelen, T. Polymer Degradation. Van Nostrand Reinhold, New York, 1982, Chapter 3, p. 48. 17. Porter, R. S. and Wang, L. H., Polymer, 1992, 33, 2019. 18. Kotliar, A. M., J. Polym. Sci., Macromol. Rev., 1981, 16, 367. 19. Gattiglia, E., Turtorro, A. and Pedemonte, E., J. Appl. Polym. Sci., 1989, 38, 1807. 20. Montaudo, G., Puglisi, C. and Samperi, F., J. Polym. Sci., Polym. Chem. Edn, 1994, 32, 15. 21. Mark, H. F., Bikales, N. M., Overberger, C. G. and Menges, G. (ed.), Encyclopedia of Polymer Science and Engineering. John Wiley and Sons, New York, 1985, Vol 4, p. 668. 22. Socrates, G., Infrared Characteristic Group Frequencies. Wiley Interscience, New York, 1980, Chapter 10, p. 77.