Preparation and properties of epoxidized natural rubber/poly (8-caprolactone) self-vulcanizable blends Yasuhisa Tsukahara,"' Tomio Yonemura,' Azanam Shah Hashim,b Shinzo Kohjiya' and Kyoji Kaeriyama" aDepartment of Materials Science, Kyoto Institute of Technology, Kyoto 606, Japan bPusat Pengajian Technology Industry, University Sains Malaysia, Penang, Malaysia 'Institutefor Chemical Research, Kyoto University, Uji 611)Japan Curing behaviour as well as mechanical properties of the binary blends of epoxidized natural rubbers (ENR) with end- carboxylated telechelic poly(ecapro1actone)s (XPCL) has been investigated for development of high-performance self- vulcanizable rubber blends. The binary blends of ENR with different degrees of epoxidation and XPCL of different molecular masses were prepared by using an open two-role mill and subsequently cured at 160-200 "C.It was found that the end-functional and crystallizable XPCLs cured ENR well in the absence of other additives and acted as an effective polymeric crosslinker for ENR to produce self-vulcanizable binary blends. The degree of chemical crosslinking depended on the degree of epoxidation of the ENR as well as the curing time and temperature. The relative contribution of the physical crosslinking via crystallization of poly(E-caprolactone) chains to that chemical crosslinking was controlled by the molecular mass of the XPCL to give elastomeric materials with various stress-strain curves. End-functionalized oligomers or polymers such as macromono- mers'.' and telechelic polymers334 have recently attracted much attention as useful building blocks for the development of high-performance polymeric materials. Utilization of telechelic polymers as one component in polymer blends might be interesting to achieve the reactive blending of two different polymers, in which desired properties associated with the telechelic polymers and the crosslinking points between the two components could be introduced simultaneously. Among many polymer pairs, a telechelic poly (E-capro- lactone)/epoxidized natural rubber pair might be interesting.Poly (E-caprolactone), a tough, crystalline polymer of moderate melting point, possesses the unique ability of being miscible with a variety of other polymers over a wide composition range.5,6 Epoxidized natural rubber (ENR) is a useful rubber material since the presence of the oxirane groups make it possible to perform various types of crosslinking reactions in addition to normal sulfur vulcanization through the double In addition, the presence of the oxirane group in natural rubber (NR) leads to its polar nature and the resulting ENR shows various useful properties depending on the degree of epoxidation, e.g.oil resistance, low gas permeability, and good adhe~ion.~*'~9'~ Since poly(s-caprolactone) is a biodegrad- able polyester and ENR is based on a natural polymer, their binary blends should also be biodegradable and thus useful as ecornaterial~.~~~'~ In this study, we report the experimental finding that blending of the telechelic dicarboxylated poly(ecapro1actone) (XPCL) into ENR and subsequent compression molding at elevated temperature cured ENR well in the absence of any other additives.This was clearly shown in the curing behaviour of the binary blends of the telechelic XPCL with ENR as well as the thermal and mechanical properties of the blends after the curing reaction.16 Generally, vulcanization of rubbers requires many additives in addition to sulfur as a crosslinking reagent, e.g. assisting reagent, accelerator, retarder, stabilizer, and so on. In contrast to this, our result indicates that simple blending of the telechelic XPCL and molding cures ENR well and the resulting blends show good elastomeric properties.This indicates that telechelic polymers are very useful for the preparation of self-vulcanizable blend materials. Experimental Samples of 50 and 25% epoxidized natural rubbers (ENR5O and ENR25), were supplied from Guthrie Co. Malaysia. Epoxidation of the isoprene units of the latex form of the natural rubber was performed by reaction with performic or peracetic acid.'-3 Dicarboxylated telechelic poly(8-caprolactone)s (XPCL)s of different molecular masses were prepared by reaction of the terminal hydroxy groups of poly(e- capro1actone)s (PCL)s (Daicel Co. Ltd., Japan) with phthalic anhydride in the presence of triethylamine and 4-dimethylamin- opyridine in tetrahydrofuran according to Scheme 1. After reaction, the XPCLs were purified by extraction to remove unreacted phthalic anhydride.The introduction of the carboxy- phenyl end-groups was confirmed by 'H NMR spectroscopy and gel permeation chromatography (GPC) using refractive index and UV detectors. 'H NMR spectra were measured on a Varian Gemini-200. The molecular masses of the XPCLs were determined by GPC using the standard polystyrene calibration curve. GPC was performed with chloroform as eluent at 35°C using a Tosoh HLC-802A instrument with G3000H and G5000H columns. XPCL Scheme 1 Preparation of XPCLs J. Muter. Chem., 1996, 6(12), 1865-1870 1865 Table 1 End-carboxylation of PCLs" sample PCLb/mmol -OH/mmol PTAc/mmol DMAPd/mmol TEA'/mmol THFf /ml XPCL 1000 25 0 50 0 250 0 41 7 291 7 500 XPCL2OOo 20 0 40 0 200 0 33 3 233 3 500 XPCL4OOo 12 5 25 0 1250 20 9 145 9 500 a Reaction tme 48 h, temperature 66"C a,w-Dihydroxypoly(&-caprolactone)Phthalic anhydnde 4-Dimethylaminopyridine Triethylamine Tetrahydrofuran The binary blend compounds of ENR and XPCL (or PCL) were prepared by using an open two-roll mill at room tempera- ture (20 "C) under circulation of cooling water These com- pounds were aged overnight, and then the curing behaviour of the compounds was monitored by time-torque curves at 160-200°C using a Curelastometer I1 (JSR Co Ltd) Using the cure curves as a guide, the compounds were cured at 180 "C for fixed times under a pressure of 100 kgf cm-2 (kgf= 980665N) using a hot-press machine to produce the ENR-XPCL (or PCL) blend sheet specimens Differential scanning calorimetric (DSC) curves, and dynamic and tensile mechanical properties of these blend specimens were measured :on a Seiko SSC-580/DSC-20, Rheology DVE-V4 rheospectro-65 60 55 50 45 meter and a Shinkotsushin Kogyo TCM-100 tensile testing elution count machine, respectively Electron microscopic photographs of ultrathin sections of the specimens were taken by using a Fig.1 GPC curve of XPCL4000 taken by RI and UV detectors, JEOL JEM1200EX-I1 instrument after staining with OsO, chloroform, 35 "C, one count is ca 1 ml Table 3 Blend ratios of ENR/XPCL and ENR/PCL binary blend Results and Discussion compounds' Dicarboxylated telechelic PCL (ENR/XPCL) 100 100 100 100 100 100The end-group transformation of the hydroxy PCL to XPCL ENR5O ----XPCLlOoo 30 -was carned out with an excess of phthalic anhydnde under XPCL2000 -30 ----the conditions shown in Table 1 'H NMR spectra of the XPCL4000 --20 30 40 50 XPCLs showed new aromatic proton signals of the carboxy- ENR-25 100 100 100phenyl end groups at 6 7 25-7 90 as well as a large decrease XPCLl000 30 --of the methylene proton signal intensity connected to the end XPCL2000 -30 ---30hydroxy group of PCLs at 6 3 32 The end-group functionality XPCL4000 of XPCLs estimated from the peak intensity of 'H NMR (ENR/PCL) spectra were 70-100% as shown in Table 2 together with the ENRSO 100 100 100 -molecular mass and the polydispersity index (M,/M,) deter-PCLlOOo 30 -PCL2Ooo -30 -mined by GPC Fig 1 shows the GPC curve of XPCL4000 as PCL4Ooo --30an example GPC curves of XPCLs detected by an RI detector showed almost equivalent behaviour to that obtained with a "Compounds were prepared using an open two-roll mill at roomUV detector, indicating the uniform introduction of the car- temperatureboxyphenyl end group XPCLs at 180 "C were homogeneous, but the ENR-PCL sheetsCuring behaviour prepared under the same conditions showed rather hetero- Blend ratios of the compounds prepared by an open two-roll geneous appearance and were not cured well This indicates mill are shown in Table 3 The blend compounds of ENR with that ENR can be well cured by telechelic dicarboxylated PCLs the original PCLs were also prepared for comparison The but not by dihydroxy PCLs time-torque curves of ENR-XPCL binary blend compounds Fig 2 shows the effect of the molecular mass of the XPCL without any other additives at 160-200°C showed a large component on the cure curves measured for ENRSO-XPCL torque increase after very short induction period ( 1-2 min) (XPCL 30 phr, phr =per hundred rubber) blend compounds On the other hand, the ENR-PCL blend compounds did not It is seen from Fig 2 that the period of the sharp torque rising show any torque increase in the time-torque curve and the region is almost unaffected by the molecular mass of the curve was almost the same as those of the green ENRs at the XPCL, however, the maximum torque (at 2 h) increases signifi- same curing temperature The hot-pressed sheets of ENR-cantly with decrease in molecular mass of the XPCL This corresponds to an increase in the crosslinking density due to Table 2 Characteristics of XPCLs the increase in the concentration of the end-functional groups with decreasing molecular mass Fig 3 shows the effect of GPC the cure temperature on the curing behaviour of (a) ENR50-XPCL4000 and (b) ENR25-XPCL4000 blend sample Mn Mw/M, fa compounds It is seen that the curing rate increases with increase in cure temperature The curing rates of XPCLlOoo 1300 2 04 140 ENR25-XPCLs [Fig 3(b)] are lower than those of the corre- XPCL2Ooo 2500 2 42 200 XPCL4Ooo 6400 175 152 sponding ENRSO-XPCLs [Fig 3(a)] Compounds containing ~~~ ~ ~ highly epoxidized ENR (ENRSO) exhibit higher torques at 'f =end group functionality estimated by 'H NMR spectroscopy comparable times relative to low epoxidized samples (ENR25), 1866 J Mater Chem, 1996,6(12), 1865-1870 lo+ 0 60 120 time/min Fig.2 Time-torque curves for ENRSO/XPCLs ( 100:30) blend com- pounds cured at 180°C: ---, XPCLlOOO ---, XPCL2000; -, XPCL4000 1.0 0.5 6 60 120 60 120 time/mi n Fig. 3 Effect of cure temperature on time-torque curves for (a) ENRSO/XPCLs ( 100:30) blend compounds and (b) ENR25/XPCLs (100:30) blend compounds. -, 160"C; ---,180 "C; ---, 200 "C indicating that the crosslinking formation is influenced by the degree of epoxidation in the ENR. It should be noted here that the end carboxylic groups provide an acidic environment which might cause cationic ring opening of the epoxy groups (furanization).This might depend on the molecular mass and the blend ratio of the XPCL. Further investigation on the crosslinking reaction will be discussed in a future paper. Thermal properties Poly(&-caprolactone) is a semicrystalline polymer and shows a melting point (T,) at ca. 50-60°C, and is slightly depen- dent on the molecular mass. Fig. 4 shows DSC curves for (a) ENRSO-XPCL ( 100:30) hot-pressed sheets and (b) ENR5O-PCL (100:30) sheets in the temperature range -100 to 100 "C (cure time is 14 min for all the samples). The glass transition (T,)of the ENR component is observed at the same temperature (ca. -20 "C) for all samples. This indicates that a phase-separated structure exists in these blends. However, the endothermic peak corresponding to T, of the XPCL component at ca.50-55°C differs considerably from those of the original dihydroxy PCL. T, peaks for ENR-PCL blends are sharp and the intensity is almost independent of the molecular mass of the PCL component. In contrast to this, 0Vc i A -100 -50 0 50 100 -100 -50 0 50 IOO temperature/"C Fig. 4 DSC thermograms for (a) ENRSO/XPCLs (---, XPCLlOOO;-.-, XPCL2000; -, XPCL4000) and (b) ENRSO/PCLs (---, PCL1000; -.-, PCL2000; -, PCL4000) binary blends. Heating rate: 10"C min-'. Cure time was 14 min and cure temperature was 180"C for all samples. the peak intensity is much reduced for ENR-XPCL blends and there is no T, peak for the blend XPCLlOOO of lowest molecular mass. The difference in the T, peak intensity between ENR-XPCL and ENR-PCL might be due to the existence of crosslinking points, which could restrict crystallization of the poly (&-caprolactone) chains.Mechanical properties Fig. 5 shows the temperature dependences of the dynamic mechanical modulus and the loss tangent for the ENRSO-XPCL (100: 30) blend sheets. It is seen that the large lowering of E' corresponding to Tgof the ENR component is observed at ca. -20 "C and there is a second reduction of E' due to the T, of XPCL component at ca. 50°C. The degree of the second reduction decreases significantly with decrease of molecular mass of the XPCL and for the XPCLl000 blend it is absent. The temperature dependences of loss tangent for XPCL4000 blend also show the corresponding maxima, but the second maximum is absent for the XPLClOOO blend.These results are consistent with the DSC measurements. It appears that the chain length of XPCLlOOO is too short to form crystalline domains under the restriction of the crosslinking points while the long XPCL4000 chains can form a substantial crystalline phase. Fig. 6 shows the effect of the molecular mass of XPCLs on -2L . ' ' . ' ' ' ' ' a * . ' ' * ' -100 -50 0 50 100 temperature/"(= Fig. 5 Temperature dependences of (a) dynamic storage modulus and (b) loss tangent for ENRSO/XPCLs (100:30) blend sheets. Cure time of the compounds was 14 min for all samples. ---, XPCLlOOO; -*-, XPCL2Ooo; -, XPCL4000. J.Muter. Chem., 1996, 6(12), 1865-1870 1867 0 100 200 300 400 strain (%) Fig. 6 Stress-strain curves for ENRSO/XPCL( 100 30) blend sheets measured (a) at room temperature (20 "C) and (b) in a hot water-bath (8OOC) Elongation rate lOOmmmin-' Cure time 14min for all samples ---, XPCLlOOO, --, XPCL2000, -, XPCL4000 the tensile stress-strain curves for the ENRSO-XPCL (100 30) blend sheets measured at (a) room temperature and (b) above T, of the poly(ecapro1actone) chain (8OOC) (cure time is 14 min for all the samples) It is seen that the modulus as well as the tensile strength (TB)and the elongation at break (EB) increases with increase in molecular mass of XPCLs Values of TB and EB as well as the stress at 50-600% elongation (M50-M600)are shown in Table 4 The increase in the modulus might be related to both the filler effect and the physical crosslinking of the crystalline phase of the XPCL component, since the effect of the crystalline phase becomes more prominent with increase in molecular mass of XPCLs as shown in Fig 4 and 5, while the extent of the chemical crosslinking through the end carboxy groups of XPCL and the epoxy groups decreases The stress-strain curves at 80°C [Fig 6(b)], show that the modulus increases in the order XPCL4000<2000 < 1000 above T,, indicating again that the Table 4 Tensile properties of ENRSO/XPCL (100/30) binary blends" sample XPCL- 1000 XPCL-2000 XPCL-4000 0 52 0 59 0 96 0 86 0 92 127 144 147 186 2 27 2 15 2 45 -3 17 3 35 --4 93 --8 77 2 57 11 48 325 400 645 "Cured at 180"C for 14 min 1868 J Muter Chew , 1996, 6(12), 1865-1870 degree of crosslinking formation increases with decrease of the XPCL chain length The marked increase in TB for ENR50-XPCL4000 [Fig 6(a)] might be related to strain-induced crystallization of the ENR component at large elongation The degree of the strain-induced crystallization would be expected to be reduced by an increase in the crosslinking density Large EBvalues are also due to the XPCL crystalline domains, which will be fragmented at large elongations, as will be discussed later, but still might be able to act as the physical crosslinking points to give large E, values Therefore, the good mechanical properties (increase in TB and EB)for ENR50-XPCL4000 [Fig 6(a)] might be ascnbed to the effect of the synergism of the physical crosslinking through the crystalline domains of poly(&-caprolactone) chains, the strain-induced crystallization of the rubbery component and the chemical crosslinking between the end carboxy groups and the epoxy groups The crystalline domains of XPCL4000 might reinforce the tensile properties of the blend Fig 7 compares the hysteresis and the permanent set in the stress-strain curves of ENRSO-XPCL (100 30) blend sheets during cyclic deformations up to 300% elongation It is seen that considerable hysteresis is observed in the blend with XPCL4000 The hysteresis as well as the permanent set decreases upon decreasing the molecular mass of the XPCL component and there is almost no hysteresis in the XPCLlOOO blend The observed hysteresis and permanent set might be ascribed to structural changes during elongation, especially the change of the crystalline domains of poly (s-caprolactone) 41 II --c 1st cycle 2nd .cH 3rd 12l0 XPCLlOOO +*2nd 1st cycle XPCL2000 4 +*2nd 1st cycle XPCL4000 I 0 100 200 300 400 strain (%) Fig.7 Effect of the molecular mass of XPCL on the elongation and recovery behaviours in the cyclic stress-strain curve at room temperature (20 "C) Elongation and recovery rate 100 mm min-' Table 5 Tensile properties of ENR/XPCL4000 (100/30) binary blends" l""'"'1 10 sample 0 500 strain (Yo) Fig.8 Influence of the degree of epoxidation and cure time on the stress-strain curves at room temperature (20 "C).Elongation rate: 100 mm min-'. ENRSO: -, 14 min; -.-, 60 min. ENR25: ..., 14 min; ___ ,60 min. chains, in which plastic deformation, destruction and fragmen- tation might occur by the large applied strain; fragmented small crystalline domains may, however, still be able to act as physical crosslinking points. Therefore, it is reasonable that the blend with XPCLlOOO did not show hysteresis because no crystalline phase was detected by DSC or by dynamic modulus measurements (Fig. 4 and 5). It is also seen from Fig. 7 that the hysteresis of XPCL4000 and 2000 blends in the second and third elongation cycles becomes very small and resembles that of the XPCLlOOO blend.This indicates that the crystalline domains are destroyed and fragmented by the first elongation resulting in the suppression of the effect of the crystalline domains and the blends show more elastic behaviour mostly through the chemical crosslinking points. Fig. 8 shows the effects of the degree of epoxidation of ENR as well as the cure time on the stress-strain curves for ENRSO and ENR25 blends with XPCL4000. These blends were cured for different times (14 and 60 min). The values of TB,EB and the stress at 50-800% elongation (A450-A4800)of these blend sheets are summarized in Table 5. The stress-strain curves of ENR25 blends show both lower modulus and lower TB but ENRSO ENR25 14 min 60 min 14 min 60 min 0.99 1.20 0.26 0.33 1.31 1.56 0.35 0.49 1.79 2.19 0.52 0.78 2.33 2.95 0.76 1.12 3.10 4.05 1.04 1.64 4.42 6.29 1.51 2.32 7.15 - 2.18 3.39 3.31 5.51 - - - 8.75 10.02 11.08 5.27 9.75 683 59 1 810 830 " Cured at 180"C.higher EB compared with those of ENR5O blends at the same cure time. Therefore, the use of less epoxidized ENR produces less rigid and more extensionable rubber materials. It is also seen that an increase in the cure time increases both the modulus and TBbut decreases EB. These are consequences of the change in the extent of the crosslinking reaction between the carboxy end-groups and the oxirane groups. Fig. 9 shows a transmission electron micrograph observed for an ultrathin section of the ENR50-XPCL4000 (100: 30) blend sheet, in which light islands of the poly(s-caprolactone) domain dispersed in the dark ENR matrix are seen.It is also observed that many long poly (s-caprolactone) crystalline lamellae grow into the rubbery matrix as light strips, the thickness of which is ca. 10 nm and uniform. Presumably, these crystalline lamellae play an important role in the reinforcement of the tensile mechanical properties of the ENR-XPCL4000 blend. Conclusion In conclusion, it was shown that the end-carboxylated telechelic XPCLs act as an effective polymeric crosslinker for ENR to produce the self-vulcanizable binary blends, which can be Fig. 9 Electron microphotographs of an ultrathin section of ENR50/XPCL4000 (100:30) blend cured at 180 "Cfor 14 min; stained with OsO,, magnification (a) x 5000, (b) x 30 000 J.Mater. Chem., 1996, 6(12), 1865-1870 1869 converted to elastomeric matenals by simple cunng at 160-200°C It was found that the degree of crosslinking between XPCL and ENR depended on the molecular mass of the XPCL and the degree of epoxidation in the ENR as well as the cure time and temperature It was also shown that the relative contnbution of the physical crosslinking to the chemi- cal crosslinking is controlled by the molecular mass of the XPCL to give various stress-strain curves This indicates that telechelic polymers are very useful for the preparation of a wide variety of self-vulcanizable blend materials Since poly(&- 3 4 5 6 7 8 9 ed M K Mishra, Polymer Frontier International, New York, 1994,p 161 Telechelic Polymers Synthesis and Applications, ed E J Goethals, CRC Press, Boca Raton, FL, 1989 Y Tezuka, Prog Polym Sci ,1992,17,471 J V Koleske and R D Lundberg, J Polym Sci A-2,1969,7,795 M Aubin and R E Prud’homme, Macromolecules, 1980,13,365 I R Geilling, Rubber Chem Technol, 1985,58,86 I R Geilling and N J Mornson, Rubber Chem Technol, 58,243 A S Hashim and S Kohjiya, Kautsch Gummi Kunstst, 46,203 985, 993, caprolactone) is a biodegradable polyester and ENR is based on a natural polymer, their binary blends could be also biodegradable and thus might be interesting as ecomaterials 10 11 A S Hashim and S Kohjiya, J Polym Sci, Polym Chem 1994,32,1149 S Jayawardena, D Reyx, D Durand and C P Pinazzi, Makr Chem .1984.185.19.2089 Ed , mol 12 C S L Baker, I R Geilling and P Newell, Rubber Chem Technol , The authors are indebted to Mr Y Fukui and Mr Y Ohtsuka 1985,58,67 of Daicel Chem Ind Co Ltd for supplying the hydroxy- terminated poly(&-capro1actone)s and for help in recording the electron microscopic photographs 13 14 15 R Alex, N M Mathew, P P De and S K De, Kautsch Gummi Kunstst ,1989,42,674 J E Potts, R A Clendinnmg, W B Ackart and W D Niegmh, ACS Polym Prepr ,1972,13,629 C G Pitt, F I Chasalow, Y M Klimas and A Schindler, J Appl References 16 Polym Sci ,1981,26,3779 Y Tsukahara, T Yonemura, A S Hashim, S Kohjiya, Polym Prepr Jpn , 1994, 43, 1298, International Rubber Conference 1 Y Yamashita and Y Tsukahara, in ModlJication of Polymers, eds (ZRC95Kobe),1995, p 74 C Carraher and J Moore, Plenum, New York, 1983, p 131 2 Y Tsukahara, in Macromolecular Design Concept and Practice, Paper 6/03150A, Received 7th May, 1996 1870 J Mater Chem, 1996, 6(12), 1865-1870