Nonoxidative crosslinking reactions in natural rubber. I. Determination of crosslinking groups

JOURNAL OF POLYMER SCIENCE: Polymer Chemistry Edition VOL. 13,2725-2734 (1975)

Nonoxidative Crosslinking Reactions in Natural Rubber. I. Determination of Crosslinking Groups

D. R. BURFIELD and S. N. GAN, Department of Chemistry, University of Malaya, Kuala Lumpur 22-11, Malaysia

Synopsis

The presence of both epoxide and amine groups as integral constituents of the natural rubber macromolecule have been demonstrated, by application of an HBr double titration technique. The number of such groups is dependent on the clonal source of the rubber and is found to decrease on storage in step with an increase in the plasticity of the rubber. The groups are believed to be involved in nonoxidative crosslinking reactions which occur in rubber on storage. Possible crosslinking mechanisms are proposed.

INTRODUCTION

In a recent publication' it was proposed that the observed hardening of rubber on storage was due to crosslinking reactions involving epoxide groups present in the rubber macromolecule. The proposal was shown to be in keeping with the findings of other worker^^-^ which had previously been in- terpreted in terms of aldehydic crosslinking reactions. The presence of epoxide groups was supported by Pb(OAc)4 degradation reactions, analogous to the method used by Flory and Leutner.6

In this paper further experiments are described which confirm the presence of epoxide groups in natural rubber (NR) and their role in crosslinking reactions.

EXPERIMENTAL

Isolation of Rubber

The natural rubber used in these experiments was isolated from freshly tapped rubber latex, of a variety of clones, and used immediately. Four dis- tinct methods of isolation were employed.

Method AC. The fresh latex was diluted to three times its volume with distilled water and the rubber coagulated by addition of 2% acetic acid. The soft coagulum was extensively washed with water, pressed into thin sheets and dried under vacuum at 5OOC.

Method BZ. The fresh field latex was diluted with an equal volume of water followed by the addition of one volume of benzene. The rubber, which forms a soft coagulum, was then washed, pressed into thin sheets and dried as above.

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0 1975 by John Wiley & Sons Inc.

2726 BURFIELD AND GAN

Method ME. The latex was diluted with one volume of water and added slowly, with stirring, to two volumes of methanol. The precipitated rubber was then washed and dried as described.

Method AUTO. The latex was stored loosely covered in the dark for 72 hr, during which time autocoagulation took place. The coagulum was then sheeted, washed, and dried.

Plasticity Determination

The Wallace plasticity and the plasticity retention index (PRI) of the rubber samples were determined according to the established test method^.^ The plasticity of a raw rubber sample is related to its molecular weight, although in a complex manner. Thus, the plasticity generally increases with increasing molecular weight.

The PRI is defined by'eq. (1):

where Paged is the plasticity of the rubber sample after heating at 140°C for 30 min in an air oven and PO is the plasticity of the untreated control. The PRI was developed as an objective test to measure the resistance of raw rubber to oxidative breakdown on heating. The actual value is, however, dependent upon the degree of chain scission due to oxidative degradation and upon the degree of crosslinking due mainly to nonoxidative processes. Values of PRI greater than 100 arise when the crosslinking reaction predominates.

Epoxidation of Synthetic Rubber

A synthetic polyisoprene sample (Goodyear Natsyn 200) of 97% cis-1,4 content was epoxidized by the slow addition of 40 ml of an 0.023M chloro- form solution of perbenzoic acid to 2.4 g of rubber dissolved in 200 ml benzene. The stirred reaction mixture was maintained at 5OC for 2 hr, by which time all the perbenzoic acid has been consumed. The reaction mixture was then concentrated to about 100 ml by flashing off solvent under vacuum, and the epoxidized rubber was isolated by precipitation with methanol. The modified rubber was subsequently purified by reprecipitation from benzene, followed by drying under vacuum at room temperature.

Determination of Epoxide Groups

Epoxide groups were determined by the direct titration method developed by Durbetaki.8 Thus about 1.2 g of rubber was dissolved in 80 ml of AR grade benzene and the resulting solution titrated with 0.02M HBr in glacial acetic acid, crystal violet indicator being used. Results quoted are the mean of duplicate titrations and are corrected for a blank titration with benzene alone. The visual endpoint produced by crystal violet indicator was fairly sharp and stable for more than 15 min, thus indicating that a negligible addition of HBr to the double bonds in the macromolecule occurred under titration conditions.

This method of analysis was found to be quantitative for low molecular

CROSSLINKING IN NATURAL RUBBER. I 2121

weight epoxides. Titration of epoxidized synthetic rubber yielded a value equivalent to 92% of the added perbenzoic acid. It is believed that this discrepancy is due to the nonquantitative nature of the epoxidation reaction rather than that of the titration technique.

RESULTS

Epoxide Determination

Specificity of Reaction. Since groupings other than epoxides could well be present in the rubber sample either as integral components of the macromolecule or as low molecular weight impurities, it was necessary to determine the specificity of the titration. The results, which are summarized in Table I, are similar to the findings of Durbetaki.8 It can be concluded that the titer value for a given rubber sample will be equivalent to the sum of the epoxy and/or amino groups in the sample. As it has been proposed’ that these two types of group are responsible for the crosslinking reaction, they will be re- ferred to collectively as “crosslinking groups.”

Removal of Low Molecular Weight Impurities. The effect of low molecular weight impurities on the rubber titration value was determined by

TABLE I Specificity of HBr Titration

Quantitative Functional group reaction Interference

I I -C-C-

‘d Yes

-CHO I

-C=CHCHO RNH, R,NH R3N

I 1 -c- c- I 1

NHI OH RCHCO H

I NH

-CO H -0OH

0 II

n I

--C-NH

-C=X-

Water

Yes

No

No Yes Yes Yes

Yes

Yes

No N o

No

Yes

No

None

None -

None ca. 0.1%

ca. 1%

-

None if less than 1%


TABLE I1 Effect of Purification on the Number of Crosslinking Groups

Number of

reprecipitations per chaina Number of crosslinking groups

43.2 41.5 42.1 43.1 41.1 41.6

Mean 42.1, standard deviation 0.8

a Calculated on basis of M,, = lo6.

successive reprecipitation of a rubber sample. Thus, rubber isolated by method AC was subject to purification by methanol reprecipitation from benzene. The results are summarized in Table 11.

Rather surprisingly, the titration value is unaffected by reprecipitation, and consequently only negligible amounts of titratable low molecular weight species are retained in rubber after isolation by this method. This can only mean that the crosslinking groups must be present as an integral part of the rubber chain.

Titration of an unpurified synthetic polyisoprene (Goodyear Natsyn 200) stabilized with an aromatic amine based antioxidant, yielded a titration figure of 11 f 1 groups per chain (calculated assuming Mn = lo6).* One methanol reprecipitation reduced the titration value to 5 f 1. This demonstrates that repeated reprecipitation is an adequate procedure for removal of low molecular weight impurities.

Clonal Variation of Crosslinking Groups. The number of crosslinking groups in rubber samples from four clonal latices were determined (Table 111). It is immediately apparent that there is considerable clonal variation. Similar clonal variation has been reported5 previously for the number of car- bony1 groups, the values of which (10-29 per chain) are however considerably lower than the number of crosslinking groups observed in this study. In addition, the number of crosslinking groups is seen to vary with the time of collection. Thus the collection of the two samples of PR107 was separated by a period of about 2 months. This may be related to the observation that the carbonyl group content is dependept on whether the tree has been continual- ly tapped or “rested” before collection of the c ample.^

Attempts to distinguish between the amino and epoxy group content by a potentiometric titration method have so far proved unsuccessful, because of the low concentration of these groups in rubber. The groups can however, be distinguished by a double titration technique, since the formation of the amine hydrobromide [eq. (2)]

Double Titration Technique.

+ RR”H + HBr + RR’NH‘Br- (2)

*The value of a,, = lo6 is arbitrarily assigned so as to provide a convenient system for expres- sion of the concentration of combined groups and does not necessarily correspond to the actual molecular weight of the rubber.

CROSSLINKING I N NATURAL RUBBER. I 2729

TABLE I11 Clonal Variation of Number of Crosslinking Groupsa

Clone

Number of crosslinking groups

per chain

Tjir 1 71 f 2 PB 48 58 f 1 RRIM 600 111 f 3 PR 107 50 f 1 PR 107 83 f 2

a Rubbers were isolated by using method AC except RRIM 600, which was isolated by method BZ. The two samples of PR 107 were collected at different times.

and the bromohydrin [eq. (3)] are both reversible, but

a t different pH. The pH dependence of the reverse reactions was demonstrated for reaction (2) by retitration of poly(ally1amine-co-methyl methacrylate) and for reaction (3) by retitration of epoxidized polyisoprene. Thus, the samples were initially titrated and precipitated into methanol, the pH of which was adjusted by addition of sodium hydroxide. The polymer was then washed repeatedly with excess methanol until the washings were neutral, after which the sample was dried and retitrated. The results are summarized in Table IV. It can be seen that, whereas the amine hydrobromide is quantitatively reversed at a pH 7.8, the bromohydrin is only slightly reversed under these conditions. Quantitative reversal of the bromohydrin does however occur a t pH 11. The results for the control sample show that the washing procedure is effective in removal of excess sodium hydroxide up t o pH 8 but that at pH 11.8 some contamination does occur.

Application of this method to a natural rubber sample (Table IV) shows quite clearly the presence of both pH-dependent reversible reactions. These results thus strongly support the presence of both amino and epoxy groups in natural rubber and enable the concentrations of such groups to be estimated.

TABLE IV pH-Dependent Reversibility of Bromohydrin and Hydrobromide Groups

Number of titratable groups per chain

Poly( allylamine- pH of Polyisoprene Epoxidized co-methyl Natural

precipitation (control) polyisoprene methacrylate) rubber

Initial sample 7 f 2 407 k 12 17 2 2 83 i 2 2.8 5 f 2 6 f 2a 7.8 4 t 2 35k 2 192 2 20 t 2

89 f 3 11.8 14 f 2 411 f 13

- -

-

a Reprecipitated a t pH 4.4.


TABLE V Application of Double Titration Technique to Clonal Rubbersa ___

Total number of Number of Number of

groups per chain per chain per chain crosslinking amino groups epoxide groups

Clone NTb NAC WE = N T -NA)

RRIM 600 91 t 1 1 6 ~ 1 75t 2 Tjir 1 71 f 2 14k 1 57 t 3 PB 48 58 k 1 1 2 f 1 46 f 2 PR 107 83t 2 2 0 t 2 63f 2

aRubbers were isolated by using method AC. Samples reprecipitated at pH 7.8. The rubbers are the same samples as those in Table 111, with the exception that the RRIM 600 sample had been stored for 2 months in a desiccator over silica gel.

b First titration. c Second titration.

Typical results for a variety of clonal rubbers are shown in Table V. The number of amino groups is found to be in the region of 12-20 groups per chain (0,017-0.028% N), which is similar to the nitrogen content of purified natural rubber?JO whereas the number of epoxy groups varies between 46 and 75 groups per chain (0.07-0.12% 0), which again is similar to reported oxygen contents of natural rubber.1° It should be emphasized that these fig- ures apply to a complete fraction of freshly isolated natural rubber, small amounts of gel present being dissolved by prolonged stirring; most literature analyses, on the contrary, refer to the sol rubber fraction alone.

Effect of Method of Isolation on Number of Crosslinking Groups and Plasticity.

The number of crosslinking groups, initial plasticity, and PRI value were determined on rubber samples from the same field latex but isolated by vari- ous methods. The results are summarized in Table VI. Both the number of crosslinking groups and the plasticity are affected by the method of isolation. In particular, the rubber isolated by autocoagulation shows a low value of crosslinking group together with a high initial plasticity. This rather suggests that the conditions of autocoagulation, which allow formation of amino acids through hydrolysis of proteins, favor ring opening of epoxide groups and crosslinking of the rubber macromolecule.

TABLE VI Effect of Method of Isolation on Number of Crosslinking Groups and Plasticitya

Number of Initial

isolation groups per chain Po PRI Method of crossIinking plasticity

~ ~~~

BZ ME AC AUTO

76 64 49 33

33.5 34.5 32.6 44.0

1 2 8 109 111 93

aClone RRIM 600 collected a t different time than that used in Tables I11 and V


Also interesting is the observation that the PRI value roughly parallels the number of crosslinking groups in the rubber. The PRI value is taken as a measure of the oxidizability of raw rubber. The actual value for a given rubber sample will depend on the extent of hardening during the test due to crosslinking reactions and the breakdown due to oxidative degradation. Since the rubber samples under comparison are obtained from the same source it is likely that the extent of oxidative breakdown will be similar in each case, and consequently the PRI value would be expected mainly to re- flect the variation in degree of crosslinking under test. These observations support the hypothesis that the epoxide groups are responsible for crosslinking reaction in rubber.

Effect of Storage on Plasticity and Number of Crosslinking Groups

The plasticity and number of crosslinking groups were determined over a period of 30 days for samples of rubber stored in an air oven at 6 O O C . The results are shown in Figure 1. It can be seen that the decrease in number of crosslinking groups again parallels the increase in plasticity except at long storage periods, when the plasticity probably decreases due to onset of oxidative degradation.

I0 2 0 30 STORAGE PERIOD (DAY)

Fig. 1. Variation of the number of crosslinking groups and Wallace plasticity with storage. Rubber from clone Tjir 1, stored a t 6OoC in an air oven.

The fairly small decrease compared to the total number of crosslinking groups, together with the rapid deacceleration in their rate of disappearance may well be rationalized, since the reaction will depend upon the number of amino groups in the sample which has been shown to be much lower than the epoxide group content.

DISCUSSION

The foregoing experiments have demonstrated the presence of both epoxy and amino groups chemically bound to the natural rubber macromolecule. The number of such groups is consistent with the typical oxygen and nitrogen analysis of purified natural rubber. The number of epoxy groups (30-75 per chain) is, however, considerably higher than the previously reported value of about 13 groups per chain derived from degradation experiments.l This is not altogether surprising, since the degradation value is dependent on two major


assumptions: (1) that the ring opening of epoxide groups by acid treatment of the latex is quantitative and (2) that the groups are randomly distributed over the whole molecular weight range. There is evidence that' the oxygen content of rubber is concentrated in the lower molecular weight range.11J2 If the distribution of epoxide groups is also weighted towards the smaller macromolecules, this would have the effect of reducing the number of epoxide groups determined by the degradation method, and would thus explain the apparent discrepancy.

The number of crosslinking groups is observed to decrease on storage in step with increasing plasticity of the rubber. This suggests that both epoxy and amino groups are involved in the crosslinking reaction, since involvement of amino groups alone, e.g., by interaction with carbonyl groups [eq. (4)], would leave the titratable number of crosslinking groups unchanged (Table I), as the resultant crosslink (I) can be titrated quantitatively.

H H

(4) I I

-C=O + H,N ----t -C=N- + H20 I

The actual mechanism of the crosslinking reaction is uncertain, however. It was proposed previously1 that amino groups could be incorporated into the rubber macromolecule by ring opening of epoxide groups with amino groups which abound in the latex [eq. (5)].

C9i CHJ I I

'0' I I -CH,-CH-C-CH,- - -CH,- CH-C-HZ- + RNH, (5)

/NH OH R

I1 I11

Reaction of these amine incorporated groups with a second epoxide group on an adjacent chain would give rise to crosslinks (IV).

-cH~--cH-C- CH,- I I

I CH3

Iv Such reactions would explain the observed crosslinking which occurs when

low concentrations of monofunctional amines, e.g., ethanolamine and cyclo- hexylamine, are added to rubber latex.13 Addition of excess amine would lead to ring opening of all epoxide groups and suppression of the crosslinking reaction, as is in fact 0b~erved.l~


There are, however, other possibilities. Thus amino acids V could partici- pate in crosslinking through reactions such as those leading to VI and VII, where -NHR represents an amine or amino acid group incorporated into rubber, and >N-R the resulting crosslink. The participation of amino acids in crosslinking reactions is consistent with the observation that they are able to increase the PRI value of raw rubber.14 Formation of crosslink species VII is accompanied by dehydration, which is clearly consistent with the accelera- tion of the crosslinking reaction under anhydrous conditions. In addition, amide formation could be an alternative explanation as to the decrease in number of “crosslinking groups” on storage, since the amide group is not titratable (Table I).

-CH~-CH- C- CH,- I I I I

NH OH

R-CH

C O P

V

I 1 NH OH I I

R-CH

C=O I I

or N-R I

-cH,-cH-C-CH~- I I I I

NH OH

R-CH + H20

c=o

VII 6 H

VI

The presence of ester linkages in factory crepe rubber has recently been demonstrated,15 and this lends further support to the possible presence of crosslinks such as VI and VII in storage hardened rubber. Hydroxylic groups such as required in structures 111, IV, and VI have also been indicated by analysis and characterization of the oxygen content of crepe rubber.1°J6

The postulation of reactions such as (5) is not completely without diffi- culty, since it has been shown that reactions of model compounds proceed very s10wly.’~ However, although reactions between amines and epoxides are very slow under completely anhydrous conditions,ls they proceed rapidly at


room temperature in aqueous media.lg Thus the ring-opening reaction would be expected to occur readily in rubber latex, but other reactions such as amide or ester formation would be expected to predominate in the rubber on drying.

The occurrence of more than one type of crosslinking reaction favored by different conditions is not unreasonable. Thus nonoxidative crosslinking reactions which occur readily in the latex phase have been demonstrated by the increase in viscosity of HA latex concentrate on storage.20 (The rate of viscosity increase in latex being faster than a dry rubber control.) On the other hand, it has been shown that the crosslinking reaction in dry rubber can be accelerated under rigorous drying condition^.^

CONCLUSION

The presence of epoxy and nitrogen-containing groups as integral constituents of the rubber macromolecules has been demonstrated. The number of epoxy groups is found to decrease on storage in step with increase in plasticity of the rubber. Consequently the epoxide groups are believed to be involved in crosslinking reactions, possibly together with amino acids either free or incorporated into the rubber chain.

The experimental findings do not rule out the possibility of other nonoxidative crosslinking reactions involving different functional groups in rubber. Nevertheless, the epoxy crosslinking reaction is believed to be the dominant process in the samples investigated.

References 1. D. R. Burfield, Nature, 249,29 (1974). 2. B. C. Sekhar, J . Polym. Sci., 48,133 (1960). 3. B. C. Sekhar, Proc. 4th Rubber Tech. Conf. (London), 1 (1962). 4. B. C. Sekhar, Malaysia Natural Rubber-New Presentation Processes, Rubber Research

5. B. C. Sekhar, Proc. Nat. Rubb. Res. Conf., Kuala Lumpur, 512 (1960). 6. P. J. Flory and F. S. Leutner, J. Polym. Sci., 3,880 (1948). 7. R.R.I.M. Test Methods for Standard Malaysian Rubbers, Rubber Research Institute of

8. A. J. Durbetaki, Anal. Chem., 28,2000 (1956). 9. R. F. Freeman, Proc. 3rd Rubber Tech. Conf. (London), 3 (1954).

Institute of Malaya, 1971.

Malaya, 1970.

10. T. Midgley, A. L. Henne, A. F. Shepherd, and M. W. Renoll, J. Amer. Chem. Soc., 57,2318

11. G. F. Bloomfield and E. H. Farmer, Z.R.Z. Trans., 16,69 (1940).

12. G. F. Bloomfield, Communication No. 273, Rubber Research Institute of Malaya, 1951. 13. A. D. T. Gorton, Rubber Ind., 142 (1974). 14. S. Kasinathan, M. Nadarajah, and A. S. L. Tirimanne, R.R.Z.C. Bull., 6 , Nos. 1 and 2

15. E. C. Gregg, Jr., and J. H. Macey, Rubber Chem. Technol., 46,47 (1973). 16. J. L. Bolland,Z.R.Z. Trans., 16,267 (1941). 17. T. Colclough, I.R.Z. Trans., 38, l l (1962). 18. W. H. Horne and R. L. Shriner, J. Amer. Chem. Soc., 56,2925 (1932). 19. D. R. Burfield, S. N. Gan, and R. H. Smithers, to be published. 20. A. D. T. Gorton, Rubber Znd., 111 (1974).

(1935).

(1971).

Received April 3, 1975 Revised May 12,1975

Nonoxidative crosslinking reactions in natural rubber. I. Determination of crosslinking groups

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