Download Procedure for assessing the friction properties of vulcanisates in a

Kauchuk i Rezina, No. 4, 2011, pp. 26–27
Procedure for assessing the friction properties of
vulcanisates in a liquid medium
V.V. Vlasov and M.E. Solov’ev
Yaroslavl’ State Technical University
Selected from International Polymer Science and Technology, 39, No. 1, 2011, reference KR 11/04/26; transl. serial no. 16404
Translated by P. Curtis
Today there are many methods for determining the
abrasion of vulcanisates [1, 2], but in practically all
of them it is dry friction that is considered. In practice,
most mechanical rubber goods, including motor vehicle
tyres, undergo wear in a liquid medium, and therefore
it is not possible sufficiently accurately to predict the
wear resistance and friction properties of a particular
product under actual service conditions. We propose
a procedure for assessing the friction properties of
vulcanisates in air and in a liquid medium.
To conduct investigations, we developed a special
unit. The test method is similar to tests on a Grasselli
machine [3] and consists in determining the friction
properties of vulcanisates during slip of a specimen
pressed against the abrasive surface of a disc rotating
at constant speed under the action of a variable normal
force. An important difference from the Grasselli
machine is that design features (the friction unit, in a
vessel filled with liquid, is positioned at an angle of
20° to the horizontal) allow tests to be conducted in
a liquid medium. Because of this angle of the unit,
the formation of a wetted surface becomes possible.
For this, a certain volume of liquid is poured into the
unit, with which part of the rotating abrasive disc
is immersed in the liquid. The material used for the
abrasive disc was a material similar in properties to
the standard abrasive for conducting tests (GOST 34474). Specimens similar to those used on a Schopper
machine are used as the test specimen on the given
unit [3].
On the given unit, an assessment was made of
the abrasion and friction properties of a typical tread
vulcanisate based on a combination of natural rubber
and polybutadiene rubber and N-339 carbon black
© 2012 Smithers Rapra Technology
as a function of the contact pressure and the type of
medium in the contact zone. During abrasion of the
vulcanisate in air, the friction coefficient on a dry
abrasive surface is maximum. Here, strong heating
up and intense breakdown of the material occur. The
dependence of the friction coefficient on pressure
at the contact spot in the examined load range is
practically rectilinear, as shown in Figure 1. The nonabsolute linearity of the dependences may be due
to the fact that, under load, there is a change in the
stress–strain state of the specimen, which leads to a
certain change in the area of surface contact with the
abrasive surface.
As can be seen from Figure 1, the dependence of
the frictional force in water is S-shaped. This may be
due to the fact that, at low contact pressures, a layer
Figure 1. Dependence of the friction coefficient µ on the
contact pressure P for a specimen of standard vulcanisate:
1 – wetted surface; 2 – in water; 3 – dry surface
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of water appears between the abrasive surface and
the specimen, acting as a lubricant and lowering the
friction coefficient. With increase in the load and
consequently in the pressure at the contact spot, the
water is forced out and this layer breaks down, on
account of which there is an increase in the friction
coefficient, which has a value similar to that on a
dry surface.
The dependence of friction coefficient on contact
pressure on a wetted surface is most interesting (see
Figure 1). The given dependence is practically linear
and similar to the dependence obtained under dry
friction conditions, but, with identical values of the
pressure at the contact spot, the friction coefficient
is lower. This may be due to the fact that, when the
abrasive disc enters the layer of water, wetting and
aeration of the surface occur. Thus, the presence of
air bubbles in the air film at the contact spot is the
cause of reduction in the friction coefficient.
Figure 2. Dependence of the abrasion I on the contact
pressure P for the test vulcanisate: 1 – wetted surface; 2 – in
water; 3 – dry surface
As regards the abrasion of the rubber under the
given test conditions, under dry friction conditions it
is an order of magnitude higher than in water and on
a wetted surface, which is graphically demonstrated
in Figure 2. This may be due to the features of friction
described above.
Besides standard rubber, a test tread rubber
containing a combination of natural rubber and
polybutadiene rubber, 15 parts N-234 carbon
black, and 50 parts Zeosyl® was also subjected to
tests. Comparison of the friction properties of the
vulcanisates investigated under identical test conditions
(Figures 1 and 3) indicates that the standard rubber
has a slightly higher friction coefficient under dry
friction but a considerably lower friction coefficient
in water and on a wetted surface (see Figure 4) than
the test vulcanisate. This is due to the fact that the
test vulcanisate contains silicon dioxide filler Zeosyl®
which can be wetted by water [4, 5]. Therefore, on
account of the forces of surface tension of water,
appearing as a result of wetting of two surfaces (the
abrasive surface and the surface of the vulcanisate),
the friction coefficient increases.
Figure 3. Dependence of the friction coefficient µ on the
contact pressure for the test vulcanisate: 1 – wetted surface;
2 – in water; 3 – dry surface
Thus, the given procedure for assessing the cohesive
properties of vulcanisates under various contact
conditions has shown its effectiveness and may be used
to optimise the frictional properties of vulcanisates.
REFERENCES
1.
G.M. Bartenev and V.V. Lavrent’ev, Friction
and Wear of Polymers. Khimiya, Leningrad,
240 pp. (1972).
2.
G.M. Bartenev et al., Kauch. i Rezina, (3):20
(1969).
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Figure 4. Dependence of the friction coefficient µ on the
contact pressure P on the wetted surface for the standard (1)
and test (2) vulcanisates
International Polymer Science and Technology, Vol. 39, No. 4, 2012
3.
G.I. Brodskii et al., Abrasion of Vulcanisates.
Khimiya, Moscow, 240 pp. (1975).
4.
A.M. Pichugin, Material Science Aspects
of the Development of Tyre Vulcanisates.
Mashinostroenie, Moscow, 383 pp. (2008).
5.
I.M. Agayants, Rubber Worker’s Cookbook.
Gelioprint, Moscow, 120 pp. (2009).
Received 4.12.2010
© 2012 Smithers Rapra Technology
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