Oxidation of natural rubber and its derivatives prepared by epoxidation and subsequent hydrobromination

Eur. Polym. J. Vol. 26, No. 10, pp. 1055-1060, 1990 Printed in Great Britain. All rights reserved

0014-3057/90 $3.00 + 0~00 Copyright © 1990 Pergamon P ~ pie

OXIDATION OF N A T U R A L RUBBER A N D ITS DERIVATIVES PREPARED BY EPOXIDATION A N D SUBSEQUENT HYDROBROMINATION NGUYEN VIET BAC, LEVON TERLEMEZYAN* and MARIN MIHAILOV Central Laboratory for Polymers, Bulgarian Academy of Sciences, 1040, Sofia, Bulgaria (Received 22 December 1989; received for publication 9 March 1990)

Abstract--The oxidations in air of epoxidized natural rubber (ENR) and its hydrobrominated product were studied and compared with the case of natural rubber (NR). By following the i.r. characteristic bands at 580, 840, 870, 1720, 1760 and 3200-3600cm -t during the oxidation, it was demonstrated that the hydrobrominated product of ENR is the most unstable both at room temperature and at 150°. NR oxidizes more quickly and to a greater extent than ENR. The oxidation proceeds through consumption of double bonds, leading to the formation of new oxygen-containing functional groups (hydroperoxide, hydroxyl, carbonyl, carboxyl... ), changing the structure and properties of the materials. The absence of associated hydroxyl groups in the OH spectral region of hydrobrominated ENR products of low epoxide contents suggests the presence of randomly distributed epoxide groups in the polymer and indicates the random character of the epoxidation process of NR in latex.

Methods and analysis Oxidation at room temperature was followed using films cast directly from solutions on KBr windows. The oxidation at high temperature was performed with thin films east from solution on KBr pellets using a heating cell supplied by Carl-Zeiss, Jena. The spectra were recorded on Specord M-80 (Carl-Zeiss) during the exposure and the absorbances of characteristic bands were analysed using the methyl deformation band at 1380cm -t as internal standard.

INTRODUCTION

In our previous paper, using infrared spectroscopy (i.r.S) we reported on the structural modification of natural rubber ( N R ) during epoxidation in latex and subsequent hydrobromination of the epoxidized natural rubber ( E N R ) [1]. Except for a paper dealing with the ozonolysis of E N R [2] up to now the oxidation of non-vulcanized E N R and its polymer-analogues resulting from hydrobromination has not been studied. There is also no published account of the use of i.r.S to study the oxidation of N R and the above mentioned polymeranalogues. Continuing our study of N R derivatives by i.r.S, we now present the results of an investigation of the oxidation of N R and its polymer-analogues in thin films under various treatments. The paper also presents and discusses in detail the spectral characteristics of the hydroxyl region for hydrobrominated E N R (HBE).

RESULTS AND DISCUSSION

Oxidation o f N R and its polymer-analogues at room temperature

EXPERIMENTAL PROCEDURES

Materials NR latex centrifuged full ammonia latex of total solid content (TSC) of 61.2-61.3wt% (Qualitex) was used. Samples were taken after TSC determination for preparing solutions in toluene and subsequent casting of thin film for spectral studies. ENR samples with epoxide contents from 3 to 57 mol% were prepared by in situ epoxidation of NR latex using 30% aqueous H202 and formic acid [3], HBE was obtained by direct titration of ENR with HBr in glacial acetic acid. All samples were carefully purified and dried at 4if' under vacuum to constant weight before analysis.

The oxidation of N R could be performed in various ways, leading to different degrees of oxidation, even to degraded products. R e a c t i o n under mild, carefully controlled conditions l e a d s to E N R samples of different epoxide contents without ring-opening side products [1, 3, 4]. The modification of the structure and spectra of E N R and H B E products were studied in detail previously [1]. These reactions are represented schematically as follows: O H:O2/ HCOOH

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*To whom all correspondence should be addressed.

Scheme 1. In situ epoxidation of NR and subsequent hydrobromination of ENR product. 1055

NGUYENVIET BAC et

1056

In the present work, the structural modifications of NR and its derivatives were studied during the oxidation under various conditions. The stabilities of NR, ENR and HBE in air were compared by exposure at room temperature for various periods. The experiments were made for ENR samples having from 7 to 58 mol% epoxide groups and for HBE obtained from ENR having from 7 to 25 tool% epoxide groups. Figure 1 shows the most characteristic regions in representative i.r. spectra of NR, ENR and HBE samples exposed to air at room temperature and at

also for ENR-7 and ENR-57 (the number following ENR indicates the epoxide content in mol%). The spectra of HBE samples differ substantially from that of the original ENR in the region of cis-ethylene and hydroxyl groups. During the exposure, the most profound changes in HBE structure appeared in the carbonyl and hydroxyl regions. The initial weak ester carbonyl band at 1730 cm -1, due to natural impurities accompanying NR [8] increased in intensity and shifted toward 1720 cm- ~(ketone band [9]) because of the formation of new ketone and aldehyde groups during the oxidation of rubber chains. After 8-10 days, this region also contains a band for carboxylic vibration at 1760 cm -1 [9] which increases with the exposure time (Fig. 3, curve 3). It is seen that HBE is an unstable polymer-analogue of NR. The oxidation proceeds easily, consuming double bonds and leading to the formation of new oxygen-containing functional groups. The final product of HBE is a film of light brown to black colour depending on the original epoxide content (the higher the epoxide content, the darker is the colour) as compared to the colourless to slightly yellow elastomeric film of ENR and NR after the same treatment.

150 °.

Variations of absorbances of characteristic bands in spectra of thin film NR during the exposure are shown in Fig. 2. It is seen that NR oxidizes slowly and gradually in air. The characteristic bands for double bonds at 580, 840 and 1665cm -~ [1,5,6] continuously decreased; simultaneously the carbonyl band at 1730 cm -t slowly increased (see Fig. 2, curves 1-3 and 4, respectively). For ENR products, the most characteristic bands are at 580, 840 (residual double bonds) and 870 cm -] (asym. epoxide ring stretching vibration [7]). Variation of these bands for a typical sample having 25 mol% epoxide groups (ENR-25) during the exposure is also presented in Fig. 2 (dotted lines). It is seen that all the characteristic bands remain practically unchanged during the exposure. The epoxide and carbonyl bands slightly increased while the bands for double bonds slightly decreased (see Fig. 2, curves 6, 8 and 5, 7, respectively). This conclusion applied

9

The accelerated oxidation was studied at 150° using thin films cast from solution on KBr. Figure 1 includes also the most important regions in the i.r. spectra for some representative samples exposed to

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Oxidation of NR and its derivatives

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high temperature oxidation (curves 3, 6 and 9). Under these conditions, the reaction proceeds very rapidly, the most important period being the first 10-15 min. Figures 4, 5 and 6 show the variations of absorbances of characteristic spectral bands during the high temperature oxidation of NR, ENR and HBE, respectively. The changes of specific bands of cis-polyisoprene at 580 and 840 cm- J as well as of the oxygen-containing groups (carbonyl, carboxyl and hydroxyl at 1720, 1760 and 3460 cm-~, respectively) during the oxidation of NR are shown in Fig. 4. The oxidative

reaction of NR has a short induction period of 2-3 min, caused by natural antioxidant present in NR [8]. During this induction period, a continuous accumulation of active centres (hydroperoxide groups [10]) occurs and the reaction is characterized by a very low rate. As evident from Fig. 4, all the characteristic spectral bands vary extremely slowly during this period (Fig. 4, first 2-3 min). Thereafter an autoaccelerated thermo-oxidative reaction proceeds rapidly, then levelling up to ca 18 min (Fig. 4, curves 1-3). This levelling is probably due both to the decrease of the concentration of double ~ + ~

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NGUYEN VIET BAC et al.

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oxidation at 150°. Assignments of the bands as in Fig. 3.

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Fig. 6. Variation of characteristic bands of HBE-7 during oxidation at 150 . Assignments of the bands as in Fig. 3. bonds and to the limitation of the diffusion of active centres from the surface to the interior of the solid film. The oxidation of NR at high temperature leads to more profound changes in the structure and proceeds much more quickly than at room temperature. As already mentioned for HBE at low temperature, the initial weak ester carbonyl band at

1730 cm-1 increased in intensity and shifted toward 1720 cm-1 (ketone band [9]). After only 5-7 rain, a stretching vibration for carboxylic groups also appears and grows gradually (Fig. 4, curves 1 and 2). The difference between the high and low temperature oxidation is evident also in the OH region. In contrast to the room temperature oxidation of NR where very small changes are visible in the spectrum (Fig. 1, curves 1 and 2), it oxidizes readily at high temperature and a broad band for associated OH groups appears (Fig. 1, curve 3). In the oxidation process, both hydroperoxide and hydroxyl groups could be formed but the characteristic hydroperoxide band at 3530-3560cm-' [11] was not separately observable in the presence of the much stronger band of associated OH groups at 3200-3600 cm -~ which increases during the heat treatment. Macroscopically, the dry, transparent, colourless initial film of N R became opaque after oxidation. It was light yellow to yellow in colour, showing tackiness typical of low molecular NR. The intrinsic viscosity in tetrahydrofuran decreased from 4.12 for initial NR to 0.98 dl/g for oxidized NR. Thus the high temperature oxidation in air leads to the formation of oxygen-containing functional groups at the expense of double bonds. The reaction at later stages leads to chain-scission, decreasing the molecular weight (destructive oxidation). The ENR samples show the same characteristic behaviour of NR. During the oxidation, double bonds were consumed and new oxygen-containing groups were formed but ENR oxidized more slowly and levelled at lower extent of reaction (see Fig. 5). The relative absorbances of characteristic bands of NR, ENR and HBE before and after the heat treatment are presented in Fig. 7a,b, respectively. For ENR products, it is seen that the decrease of double bond content and the increase of carbonyl and carboxyl contents are smaller than for NR (see Fig. 7b and compare curves 4 and 5 in Figs. 4 and 5). As in the case of room temperature oxidation, the oxidation at 150° is fastest for HBE. It proceeds without induction period and levels up later (at c a 20 min). The consumption of double bonds and the formation of new oxygen-containing groups are very rapid. Experiments showed that, at lower initial epoxide contents, HBE products readily oxidize, accumulating OH group; however for the HBE products obtained from ENR having initial OH content higher than 13-15mo1%, the OH content decreases during the treatment at 150° after reaching a maximum. The C------Obands reach a maximum and then decrease also (see Fig. 6, curves 1 and 2). The decrease of intensity of the bands for oxygen-containing groups is probably due to the evolution of low molecular volatile products (H20, CHzO , CO2... [12]) The final products after oxidation of NR and ENR are similar (yellow, sticky films) but for HBE, dark brown to black, solid films are formed. High temperature oxidation demonstrates also that ' HBE is the most unstable member of this series. Compared with NR, ENR is more stable but it suffers also profound structural changes under this severe treatment.

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Characteristic of OH band in HBE product before and during the oxidation process The study of spectral characteristics of ENR and HBE revealed an interesting feature in the OH region of the HBE series. In this region, the absorption of pure ENR (product without ring-opening side reactions) is similar to that for NR. The reaction of HBr with ENR leads to the presence of Br and OH groups on the backbone. As shown previously [1], the C--Br vibration is observable in the 600-650 cm -j region as a weak band. The OH band in HBE samples is more interesting and was carefully investigated. Figure 8 presents a series of characteristic OH bands (vOH) of HBE-7 during oxidation at 150 °. The initial spectrum of HBE-7 shows a sharp band at 3570 cm-' and a shoulder at 3620cm-L This shoulder at 3620cm -' is the rest of a sharp peak for isolated terminal monohydroxyl groups. This band is clearly observable only in very dilute solution of low molecular alcohols in non-polar solvent where the association is excluded [13, 14]. In the solid film, it was observed as a shoulder in spectra of HBE obtained from ENR having low epoxide contents ( < 15 mol%). The band at 3570 cm-~ also appears to be due to hydroxyl groups. Normally, the hydrogen bonded OH groups in the dimeric state show a characteristic sharp band at ca 3515cm ~ [13, 14]. However, in our case, the OH group is bonded to the polymer backbone; the mobility is much more limited than for low molecular alcohols (even in dilute solution). The steric hindrance due to conformational and configurational reasons makes the contacts between OH groups attached to the main chain more difficult, weakening hydrogen bonds. In that case, the OH specific absorption is shifted toward higher wave numbers as demonstrated for other similar cases [15]. Thus, the band at 3570 cm -~ is assigned to the stretching vibration of

monohydroxyl groups in dimeric state (in dimer association). If OH groups are numerous and close together, their interaction must lead to association and the single sharp band in 3570cm -l should not be observable. High concentration of OH groups attached to the polymer chain could be obtained either by high temperature oxidation or by hydrobromination (via ring-opening reaction) of ENR of 3460 3040

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1060

NGUYENVIETBACet al.

higher epoxide contents. Our experimental data show that the oxidation really leads to associated OH groups. Figure 8 shows that, after only a few minutes at 150°, the initial sharp band at 3570cm -~ disappears and a broad, strong band of OH associated groups of increasing intensity is observable. As already mentioned, the formation of OH (and C------O) groups occurs at the expense of double bonds. Actually, the band for the double bond (v = CH) at 3040cm-' in the stretching vibration region decreased from a medium intensity to merely a shoulder after heat treatment at 150° (see Fig. 8 from curve 1 to 7). On the other hand, we have obtained the same associated OH band immediately after hydrobromination of ENR samples of higher epoxide contents. The intensity of this OH band is proportional to the initial epoxide content [16]. All the results show that, at lower epoxide contents, the epoxide groups must be randomly distributed along the polymer backbone. Only at higher epoxide contents, the presence of vicinal epoxide groups becomes more frequent; after hydrobromination, the probability for association of OH groups (both intermolecular and intramolecular) becomes appreciable. For ENR subjected to hydrobromination, this is the case for products having >25 mol% epoxide groups. In this way, i.r. demonstrates the random distribution of epoxide groups along the ENR chain. This result confirms the mechanism of random epoxidation of NR in latex as proposed by other authors [4, 17]. CONCLUSIONS The oxidations in air of NR, ENR and its hydrobrominated product HBE, were studied and compared. It was demonstrated that HBE is the most unstable, both at room and at high temperature. NR oxidizes more quickly and to a greater extent than ENR. The oxidation proceeds through a consumption of double bonds, leading to formation of new oxygencontaining groups (hydroperoxide, hydroxyl,

carbonyl, carboxyl...), changing the structure and properties of the materials. The absence of associated groups in the OH spectral region for HBE products of lower epoxide contents suggests the presence of randomly distributed epoxide groups in ENR and indicates the random character of the epoxidation of NR in latex. Acknowledgement--We thank the Committee for Science and Higher Education for financial support (Contract No. 338).

REFERENCES 1. Ng. V. Bac, M. Mihailov and L. Terlemezyan.3. Polym. Mater. 7, 55 (1990). 2. M. C. S. Perera, J. Elix and J. H. Bradbury. J. appl. Polym. Sci. 36, 105 (1988). 3. I. R. Gelling. Br. Pat. 2.113.692 (1983). 4. D. R. Burfield,K. L. Lim and K. S. Law. J. appl. Polym. Sci. 29, 1661 (1984). 5. K. V. Nelson, L. S. Skrypova and N. Kozlov. Vibrational Spectra and Molecular Processes in Rubber (edited by B. V. Stolyarov), p. 87. Khimia, Moscow (1965). 6. J. L. Binder. J. Polym. Sci. A 1, 37 (1963). 7. C. Roux, R. Pautrat, R. Cheritat, F. I_.¢dranand J. C. Danjard. J. Polym. Sci. C 16, 4687 (1969). 8. A. S. Kuzminskii, K. N. Leznev and Y. S. Zuev. Oxidation of Rubbers and Vulcanizates, p. 36. GKI, Moscow (1957). 9. K. Nakanishi and Ph. H. Solomon. Infrared Absorption Spectroscopy, p. 38. Holdenday, San Francisco (1977). 10. N. Grassi and G. Scott. Destruction and Stabilization of Polymers, p. 26. Mir, Moscow (1988). 11. H. H. Zeiss. J. Am. chem. Soc. 75, 997 (1953). 12. E. M. Bevilacqua. Autoxidation and Antioxidant (edited by W. O. Lundberg), p. 871. Wiley, New York (1962). 13. U. Liddel and E. D. Becker. Spectrochim. Acta 10, 70 (1957). 14. W. C. Coburn Jr and E. Grundwald. J. Am. chem. Soc. 80, 1318 (1958). 15. L. Nakanishi and P. H. Solomon. Infrared Absorption Spectroscopy, p. 25. Holdenday, San Francisco (1977). 16. Ng. V. Bac, L. Terlemezyan and M. Mihailov. Paper presented at X X V I Colloqium Spectroscopic International, Sofia, July 1989, Abstract Vol. 2, p. 53. 17. J. E. Davey and M. J. R. Loadman. Br. Polym. J. 16, 134 (1984).