THE EFFECT OF SIMPLE CARBOHYDRATES
ON THE VULCANISATION OF RUBBER.
BY
R. O. BISHOP AND E. RHODES.
INTRODUCTION.
In continuation of investigations on the vulcanisation effect:*
of Hevea Lipin (Quarterly 'Journal Vol. 2 No. 3) the question
arose as to the possible influence of the sugar with which I4pin is
known to be associated. The fact that, in spite of the presence
of sugars, the behaviour of raw rubber plus Lipin on the mixing
rolls is apparently similar to that of raw rubber plus lecithin,
suggests that the sugars might be without influence.
While it is recognised that, for the proper understanding of
their influence, it is essential to isolate and characterise the free
sugars present in latex, it was considered that the vulcanisation
effects which we had observed were sufficiently significant in their
possible bearing on plantation rubber variation to call for further
investigation, and although our subsequent observations cover only
a limited field somewhat divergent from the main line of work it
appears that the present results are worth recording.
In order to investigate this, mixings wrere made to determine
the effect of the simpler carbohydrates in rubber-sulphur vulcanisates, especially since there are records of the presence of carbohydrates occurring free in latex. (1), (2), (3), (4), (5), (6),
(7], (8).
At first the work was confined to mixing simple sugars with
plantation rubber on warm rolls.
The mixings were made by incorporating 10 grammes of
sulphur with 100 grammes of rubber on rolls at a temperature
of about 60°C. The crystalline sugar was added in the required
proportion to the plastic mass and the mixing was completed in
the ordinary manner. The amounts of sugar added as percentages on the raw rubber are shewn in Table I.
In all cases the mixing proceeded quite normally and there
were no signs of the peculiar effects observed when Hevea Lipiri
is incorporated with rubber. The subsequent testing of the
vulcanised mixes, however, gave stress-strain curves which did
not coincide with the controls. In almost every case there was
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151
evidence that the added sugar had affected the rate of vulcanisation of the rubber.
Since the results promised to have some significance, further
investigations were made. The subsequent observations, although
covering only a limited field, appear to be worth recording.
Table I gives the number of minutes of retardation or acceleration of vulcanisation produced by various sugars when mixed with
simple rubber-sulphur mixes.
The curves shewn in the accompanying diagrams were drawn
by a Scott machine adapted for ring test pieces. For purposes of
control, a rubber-sulphur mix without sugar was invariably made
at the same time as those containing sugars and test pieces were
cured in the same mould and tested at the same time. For this
reason the results recorded for acceleration or retardation produced by the sugar are strictly comparable with those of the
control sample. All the results recorded refer to this type of
control; for example, the retardation of 10 minutes recorded for
the mix containing 2 per cent, sucrose and slab rubber H.551/1,
was obtained against the control mix of this rubber, mixed, cured
and tested at the same time. Specimen curves are shewn m
figures I to VIII.
DISCUSSION.
From these results it is evident that the rate of cure of a fast
curing rubber (slab rubber) is retarded, while that of a normally
slow curing rubber is accelerated, by the addition of small quantities of sugars. These effects are neither uniform nor proportionate
to the amount of added sugar. The results do not appear to be
related to the constitution of the sugar.
Various authors have recorded the presence in Hevea latex
of true sugars as well as quebrachitol, and in view of the amounts
stated to be dialysable, it would appear that these sugars are likely
to exert some influence on the vulcanisation properties of plantation rubber. The means at our disposal have so far prevented us
from confirming the presence of free sugars, but it has been
possible to separate quebrachitol by a routine method. Daily
supplies of latex from trees of known history have been treated,
and the quebrachitol obtained has been bulked and purified. The
purity of the preparations has been determined by Melting Point
determinations. The material when pure is white and crystal
line, has a melting point of 191°C. and does not darken in colour
on storage or melting. In order to ascertain the influence of
quebrachitol on vulcanisation, mixes of normally fast and slow
curing rubbers containing 1, 2 and 3 per cent, of quebrachitol
were made in the manner described, together with control samples.
152
Figure VIII illustrates the results obtained. It will be seen
that the results are in complete agreement with those obtained
by Stevens H.P. (9) and that the effect of quebrachitol differs
essentially from that of the true carbohydrates in that retardation
of vulcanisation is produced in all types of rubber and that the
effect is approximately proportional to the amount of added quebrachitol.
Having examined the effect of sugars when mixed with dry
raw rubber, it was considered advisable to examine rubber prepared from latex to which sugars had been added.
For this
purpose, aqueous solutions of sugars were mixed with latex immediately before coagulating with acid in the ordinary way. From
this latex were prepared both crepe and slab rubber.
Control
samples of both varieties were prepared from another portion
of the same bulked latex to which no sugar had been added.
It should be remarked here that, in the earlier work on dry mixes,
the slow and fast curing rubbers were prepared from latices which T
although coming from the same field, were not necessarily identical in composition.
In the preparations from the treated and untreated latex of
the crepe rubber (samples H.584/2, H.SSl/2 and A.2/1. Table I)
the coagulum was machined into crepe within 24 hours after the
collection of the latex. In the case of the corresponding slab
rubber the coagulum was allowed to mature for seven days before
machining. The washing, machining and drying processes were
identical.
In every experiment in which sugars were incorporated with
latex, the above procedure was adopted and every sample of
rubber was compared with a control from a corresponding portion
of the same bulked latex. This method was adopted in order to
ascertain whether the accelerating or retarding effects of any
particular sugar on vulcanisation were influenced by the treatment
undergone by the coagulum previous to washing and machining.
Figs. VI and VII illustrate the effects produced by the addition
to latex of sucrose, dextrose, lactose and quebrachitol in amounts
calculated to give one per cent, on the dry rubber content of the
latex.
It has been recorded by Gorter (5) that the dialysable sugars
of latex may amount to 0.25 per cent, calculated on the latex.
The results obtained are in agreement with those obtained
when sugars are mixed with dry rubber, viz. that acceleration of
vulcanisation occurs with a slow curing (crepe) rubber and retardation of vulcanisation with a fast curing (slab) rubber when
both types are from the same bulked latex. On the other hand,
if a fast curing- rubber is prepared by dropping- latex into alcohol
153
and rapidly drying the coagulum in thin sheets, the effect of
added sugar when mixed with the dry rubber is a definite acceleration in rate of vulcanisation.
The fact that such comparatively inert, nitrogen-sulphur free
substances can exert such opposite effects appears' to us to be of
some importance. It has been shewn that sugars occur naturally
m latex and that their amount may vary with the tree and the
season of the year(3). Both these factors niay therefore affect
the quality of the resulting rubber in a manner depending on the
method of preparation. There is, at present, no evidence to shew
whether the effect of the sugars is achieved directly or through
the intermediate action of other substances.
REFERENCES.
1. K. Bamber.
2.
Circ. Royal Hot. Gardens, Ceylon.
(1899).
146.
C, Beadle and H. P. Stevens. Kolloicl Zeit. (1903). 13. 221.
3. C. Beadle and H. P. Stevens. Ind. Rubber Joimi. (1911). 41. 217.
4.
G. A. Vernet. Bull. Econon. de I'lndochme. (191.1). 14. 804.
5. K. Goiter. Archief voor de Rubber Cultunr (1917) 1, 375.
6. V. Grocnewegc. Mcd. Alg. Proefst. Landbouw. No. 11.
7.
G. S. Whitby.
"Plantation Rubber" 1920, p. 62,
'$. L. R. van Dillerr. Archief voor dc Rubber Cultuur (1922) 6. 263.
'9. H. P. Stevens. Joiini. Sac. Chem. Ind. (1917) 36. 368.