Note: Descriptions are shown in the official language in which they were submitted.
- ~S7~5
A ~E~R--RESISTANr~ RUBBER PRODUCT AND A METHOD
OF MAKING S~ME
Wear-resistant rubber products are used to a large
extent in the mining industry, especially in chutes and,
primarily, in mills for grinding ore. In such mills,
use is made of a wear-resistant lining consisting of shell
plates and so-called lifters which extend into the clrum
mil~ and stir -the charge within the rotating mill drum
and which therefore are subjected to an exceptionally
high wear. In view hereof, the lifters must be replaced on
more than one occasion during the life of the mill lining
or, strictly speaking, the shell plates. ~ormally, the
worn lifters are replaced b~r new ones, and the worn lifters
fre~uently are scrapped. The worn-out lifters usually
still carry large quantities of rubber since the lifters
must be replaced before the grinding capacity which is
depenaent upon the lifter geometry, has decreased to an
unacceptably low level. In order to reduce waste of
liftex material, it has therefore been suggested/ for
instance in U.S. patent speci~ication 3,191r875 and
Swedish patent speci~ication 335,460, gradually to raise
the lifters in relation to the shell plates, thereby
to use up as much rubber as possible. It has also be
tried to recap rubber li~ters by using recapping methods
well known in connection with the recapping of motor
vehicle tires, the strength requirements necessitating
the technique of hot recapping.
However~ the recapping methods used in the tire
industry are not readily adapted to the recapping oflifters
because the worn rubber lifter must be thoroughly buffed
and washed and given a primer coating and because vulcan
i.zation must be carried out for a considerable per.iod o~
time under heat and pressure in expensive vulcanizing
moulds of steel in order to produce a bond of sufficient
strength. Furthermore, someo~ the known recapping methods
require access to special equipment. rrhe application of~/sb
q~
~ l s~
pre-vulcanized treads to used carcasses requires care-
ful shaping (abrading) of the rubber surface of the used
detail to make it conform to the shape of the new
vulcanized rubber detail which is vulcanized to the old
rubber detail.
~ It therefore is an object of the present invention
to provide a new wear-resistant rubber product which
has the properties of the original product and which,
besides, is readily manufactured. ~urthermore, the in-
vention comprises a novel method of making this product.
According to the invention, a wear-resistant rubber
product which is to be used as~ for example, a lifter
in grinding mills thus comprises at least two pre
fabricated parts interconnected by means of a binder
layer f in situ cast and vulcanized polyurethane having
a thickness of at least 2 mm (preferably at least 5 mm)~
The wear-resistant rubber product is made by placing the
prefabricated vulcanized rubber parts at a distance o~
at least 2 mm (preferably at least 5 mm) from one another
and filling the gap with a reacting urethane rubber
composition with excess isocyanate so that the pre~
fabricated parts on either side of the gap are contacted
substantially simultaneously by said urethane rubber
~omposition. The characteristic features of the invention
are stated in the claims.
The wear-resistant rubber product may be either
a new product or a recapped product in which parts of
a used corresponding wear-resistant rubber product have
been connected with at least one prefabricated vulcanized
wear-resistant rubber body by means of the in situ cast
and vulcanized urethane rubber layer, said prefabricated
and vulcanized rubber body having a profile adapted to
that of the future product. Since the conditions of pre-
f~bricating this rubber body are far more readily
controlled, a high-quality wear~resistant rubber body
can be obtained. Hot recapping by vu].canlzing an uncured
rubber compound to an old and worn product in a vulcanizing
mould always necessitates a compromise between on one hand
1 :1S 7~5
the vulcanization of the newly supplied rubber compound
and thus the properties and adhesive power of said
compound in the vulcanized state and, on the other hand,
the risk of overcuring or scorching of the rubber
material of the old and worn product to be recapped.
- When the invention is applied to the manufacture
or recapping of mill lif-ters, the product may be formed
of at leas-t two parts, one of which comprises a holder
and the other consis-ts of a prefabricated cured wear-
resistant rubber body of a profile suitable for the
future lifter. For the recapping, the part comprising
the holder consists of a worn lifter, the surface of
which is prepared by cleaning and, if necessary, also
by buffing. Such a worn lifter may comprise a metal
component with attachment means and a rubber part
vulcanized thereto. The prefabricated part or parts
used for the manufacture may consist of extruded or
moulded cured wear-resistant rubber bodies which,
however, must be relieved oftheirvlllcanization skin by
some simple surface treatment, such as sawing or cutting.
On the other hand, it is not always necessary to buff
the rubber bodies, and no primer need be applied. Thus,
the cured wear-resistant rubber bodies and the worn
lifter can be proc~ssed with equipment less expensive
than that used in, for example, the complete or partial
recapping of tires.
During manufacture, the prefabricated parts preferably
are placed on a casting substrate in such a manner that
the minimum spacing between their surfaces to be inter-
connected will be 2 mm, preferably 5 mm. In this manner,a gap is formed, the ends of which are closed to form
an upwardly open mould cavity into which the reacting
urethane rubber composition is then introduced, preferably
injected. ~he minirn~n spacing of 2 mm, preferably 5 Illtl, iS
conditioned by the requirement that it shall be possible
for the reacting urethane rubber composition safely to enter
and fill out the gap between the bodies and to contac-t the
bodies subs-tan-tially simultaneously.
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The rubber parts to be joined together may have
room temperature when contacted by the hot urethane
rubber composition. On the other hand, it is often
convenient to dry and preheat the prefabricated parts
before the urethane rubber composition is cast because
such drying and preheating reduces the risk of bubble
formation within the in situ cast urethane rubber
composition. Moreover, it was found that one can be
sure to obtain satisfactory strength values if the
prefabricated parts are preheated. The urethane rubber
composition preferably is introduced at a temperature of
75-150C. The upper limit is conditioned by the fact that
the time or pot life during which the compounded urethane rubber
composition can be used will be too short if the casting
temperature exceeds 150C.
The invention will be described in more detail in
the following, reference being had to the accompanying
drawing in which Figs. 1 and 2 dia~ramatically illustrate
two possible procedures in the manufacture of a wear-
resistant rubber product according to the present in-
vention.
Fig. 1 shows a casting mould 10 in which a worn
lifter 11 and a prefabricated wear resistant rubber body
12 have been placed lying on their sides, such that a
gap 13 having a minimum width of at least 2 mm is formed
therebetween. The gap is being filled with a reacting
urethane rubber composition from an injection nozzle 14
which is moved along the gap and in which the components
of the urethane rubber composition are mixed i~mediately
before the composition leaves the nozzle. The injection
nozzle preferably is inserted into the gap, thereby to
cause the liquid urethane rubber composition to rise
and gradually to expel the air within the gap and to
contact both rubber bodies substantially at the same time.
Fig. 2 shows the manufacture of a new wear-resistant
rubber body according to the present invention, the
components of which have not previously been used. In
this case, a sectional metal element 15 has been placed
.
~ 1 5 ~
in the center of the bottom of a casting mould 16. Two pre-
fabricated, cured wear-resistant rubber bodies 17, 1~ have
been rigged up in the casting mould by means of sma~l spacer
members (not shown) pro~ided at the ends thereof and, if
necessary, also at one or more points along the length of
the rubber bodies so that an inverted Y-shaped gap 19 is
formed bet~een the bodies 17, 18 and the sectional metal
element 15. A needle-shaped nozzle 20 is used for filling
the gap 19 with the liquid reacting urethane rubber composition.
The invention will now be illustrated in more detail,
reference being had to the following Examples.
Example 1
In this Example, use was made of a worn lifter,
the rubber part of which was formed of wear-resistant
rubber based upon styrene-butadiene rubber and having
a hardness of 60 Shore A. The worn lifter was cleaned
and slightly buffed on its surface. The lifter had a
leng-th of 1 m and a width of 2 dm~ The lifter and a
prefabricated sectional rubber element of the same rubber
type were placed overnight in a warming cupboard having
a temperature of 110C. The bonding surface of the pre-
fabricated sectional rubber element was conditioned by
removal of a surface layer~ The lifter was laid on its
side on a plastic fabric (polytetrafluoroethylene) Oll a
casting table. The sectional rubber element was placed
alongside of the lifter, such that the minimum distance
between this surface and the curved worn surface of the
lifter was at least 2 mm. The ends of the resulting gap
between the lifter and the sectional rubber element were
sealed by pressing mould side walls against the lifter
and the rubber sectional element. A reacting urethane
rubber composition was then injected into the gap by
means of the nozzle of a mixer/injector that was inserted
in the gap in order to fill said gap while e~pelling
the air. In this manner, the two rubber surfaces were
contacted with the reacting urethane rubber composition
substantially simultaneously, which is of essential
importance to the reliability of the bond.
L4S
For the urethane rubber composition, -the following
two-componen~ system was used:
Prepolymer
100 parts by weight polyester prepolymer having
about 6.5% of available iso-
cyana-te ~roups (MDI, i.e.
methylene-bisphenylene-
diisocyanate)
Curing agent
33.6 parts by weight hydroxyl-terminated polyester
10 3.7 parts by weight 1,4-butanediol
This recipe implies a curing agent content of 95%,
based upon the available isocyanate groups.
The two rubber details had a temperature of 90-100C,
and the urethane rubber composition injected into the gap
had a mixing temperature of 110C. After the gap had been
filled, the components were left on the casting table until
the urethane rubber composition had solidified and obtained
sufficient strength to allow handling of the lifter and
the sectional rubber element attached thereto. The product
was then stored at room temperature for 72 hours.
From the lifter and the sectional rubber element
vulcanized thereto-transverse test pieces having a thicknessof
25 mm were taken by sectioning. The ~ieces or samnles werethen
subjected to adhesion tests, so~called peeling tests. The
peel strength was found to be 26.5 kN/m, the rupture
occuring in the styrene bu-tadiene rubber material, not in
the joint between the urethane rubber and styrene butadiene
rubber materials.
Example 2
Example 1 was repeated, and both rubber components
were buffed. The peel strength was found to be somewhat
lower, 24.8 kN/m, and the rupture occured in the styrene
butadiene rubber material. This shows that it is not
necessary to buff the surfaces ln order to establish
adhesion between polyurethane and rubber, and that a
cut surface (Example 1) is sufficient. However, the
vulcanization skin of the prefabricated sectional rubber
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element must be removed.
E~ample 3
Example 1 was repeated, except that no drying and
preheating in a warminy cupboard occured, which means
that both rubber components had room temperature. The
peel strength was 14.6 kN~m, and the rupture occured
in ~he joint between the rubber materials. This test
shows that an acceptable strength is obtainable also
with cold rubber, but that higher strength values will
be obtained if the rubber components are preheated and
dried.
Example 4
In this Example, worn lifters and prefabricatecl
sectional rubber elements of the same material and having
the same dimensions as the lifter and the sectional rubber
element of Example 1 ~ere used As in Example 1/ the worn lifters
were cleaned and sliyhtly buffed, and a surface layer
of the sectional rubber elements was removed to expose
fresh rubber. The lifters and the sectional rubber ele-
ments were preheated overnight in a warming cupboardhaving a temperature of 100C. Each lifter was then
placed in a shallow casting mould which stood on a
casting table and had such dimensions that the lifter
in upright position, i.e. with the metallic holder
facing downwardly, fitted closely in the casting mould,
the edyes of which protruded beyon~ the rubber part of
the worn lifter. In a first test, the lifter was covered
with the urethane rubber composition stated in Example 1,
and the sectional rubber element was gradually pressed
down into the reactiny urethane rubber composition
within half a minute. In a second test, the sectional
rubber ele.ment was pre$sed down into the reacting urethane
rubber composition 2 minutes after the lifter placed
in -the casting mould had been covered with the urethane
rubber composition. 72 hours after casting, an adhesion
test was made, and the peel strength was found to be
18.6 kN/m in the first test and but 6.2 kN/m in the
second test. This Example shows that it is of essential
importance -that the rubber bodies to be joined together
will contact the urethane rubber composition substantially
simultaneously.
Example 2 was repeated with a ?olyether
prepolymer and a different curing agent composition. The
rubber material in the lifter and in the sectional rubber
element was the same as in Example 1. The prepolymer and
the curing agent composition were as follows:
1 0 ~y~
100% by weight pol.yether prepolymer with
about 7.7% of available
isocyanate groups (MDI)
Curing agent
4.6 parts by weisht 1,4-butanediol
15 3 2 parts by weight 1,3-butanediol
72 hours after casting, an adhesion test was carried
out, and the peel strength was found to be 25.4 kN/m.
The rupture occured in the styrene butadiene rubber
material, not in the joint between the rubber materials.
