Note: Descriptions are shown in the official language in which they were submitted.
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METHOD OF BONDING URETHANE FORMULATIONS
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TO RUBBER
The present invention relates to a method of bonding
uncured urethane formulations to substrates and particu-
larly to rubber or synthetic rubber substrates.
The desirability of achieving maximum bonding strength
between urethane formulations and cured rubber or
synthetic rubber substrates has long been known in the
art. The bonding of urethane formulations to cured
rubber or synthetic rubber substrates has been one of
the more difficult problems experienced in the rubber
10 industry and also with users of rubber products. The
conventional use of adhesives, primers, bonding agents,
etc., available commercially has been unable to provide
a urethane-to-rubber bond that is equal in pull strength
to a rubber-to-rubber bond.
15 Laboratory adhesion tests using conventional methods of
applying primers or adhesives agents, such as cyanoacry-
lates, allowing them to dry and then applying a urethane
formulation as a liquid or a paste and allowing it to
cure, normally produces a bond strength ranging from 1
20 to 30 pli (pounds per linear inch). A desirable bond
strength should be at least 80 pli.
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Summar~ of the Invention
We have found in the present inventian that if the
uncured urethane is immediately applied to a rubber or
synthetic rubber surface wetted with adhesive before the
adhesive dried, the bond between the urethane, after
curing, and the rubber or synthetic rubber surface is
increased three to four times over the maximum bond
strength achieved in prior methods where the adhesive
was permitted to dry prior to application of the urethane
onto the surface.
The invention of the present application has many uses
such as repairihg tears or worn spots in rubber-based
articles subject to considerable wear. For example, the
method of the present invention could be used effectively
in repairing conveyor belts of a rubber or synthetic
rubber material, patching tires which have been torn or
ripped and even recapping of tires.
As will be appreciated by a person s~illed in the art,
there is a great economic need fQr a method which will
permit the user to take advantage of the higher abrasion
2Q xesistance of cured urethane and the cold-cure charac-
teristics of many urethane formulations will result in
sub5tantial energy and cost savings. The ability of
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the user to build on a rubber or synthetic rubber
substrate with a cold-curing system that has good
adhesion offers a wide range of cost~saving industrial
applications.
The present invention provides a method of bonding uncured
urethane formulations to a cured rubber substrate compri-
sing the steps of:
(a) cleaning the substrate surface;
(b) coating the cleaned substrate surface with a
liquid cyanoacrylate adhesive;
(c) coating the substrate surface, wetted with the
cyanoacrylate adhesive, with a liquid or paste
urethane formulation while the adhesive is still
wet; and
(d) permitting the urethane formulation to cure.
The uncured urethane is applied in liquid or paste form
until the surface is built up to the desired form or
thickness. The urethane is then allowed to cure and
after curing has been completed, the repaired surface
2Q can be buffed or shaped as desired and can then be used
with the repaired area containin~ the urethane prcviding
greater abrasion resistance than the surrounding rubber
base material.
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Following the procedure outlined above, it is possible
to obtain bond strengths in excess of 100 pli.
A particular use of the invention believed to have great
commercial significance, is the use of the invention in
recapping tires. In conventional practice in recapping
tires, a tire is buffed to remove the worn tread and an
adhesive is applied to the buffed area. When the
adhesive is no longer wet, an uncured rubber is applied
over the adhesive. The tire with the rubber thereon is
then placed in a steel mold where, with the application
of heat, the rubber is cured and in the process the
rubber is bonded to the tire. The conventional recapping
process requires the consumption of considerable energy.
In the practice of the present invention in recapping
of tires, the tire is buffed and all buffed material
removed from the tire. The clean tire is then wetted
with a cyanoacrylate adhesive, an;d before the adhesive
has dried, a coating of uncured urethane liquid or paste
is immediately applied to the wetted surface. Before
2a the urethane has an opportunity to cure, the tire with
the urethane liquid or paste thereon is placed into a
mold and additional uncured urethane is pumped into the
mold cavity so as to complete the process of building
up the desired thickness of tread material on the outer
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surface of the tire. The urethane i~ then allowed to
cure and with fast curing urethane formulations, the
tire can be removed from the mold in about 15 minutes.
This time of mold removal can be varied depending upon
the cure rate of the particular urethane formulation
used, It is preferred that the exothermic reaction of
the curing urethane be mild, thus enabling a greater
variety of materials to be used in making the mold.
A preferred technique would be to mount the buffed and
cleaned tire on a mandrel so that it can rotate in a
vertical plane. While rotating, the adhesive is
sprayed upon the tire and as the tire continues to rotate,
the uncured liquid or paste-like urethane material is
applied to the adhesive-wetted surface of the tire. The
lS time element is critical so as to insure the application
of the urethane to the surface while the adhesive is
still wet. Preferably, the time element would be about
five seconds with most urethane formulations considered
for practical use. The time element can be controlled
2Q either by moving a urethane discharging nozzle closer
to the point where an adhesive nozzle sprays the adhe-
sive on the tire or just rotating the tire faster,
These are all techniques which can be practiced by a
person skilled in t~e art so long as they utilize the
inventive concept of applying the initial uncured
urethane to the tire while the adhesive is still wet.
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Because of the mild exothermic reaction produced by the
curiny of various ;Eormulations of urethane, the tire with
the initial coating of urethane can be placed in a mold
which is formed of plastic material rather than metal
molds as conventionally used.
Preferably, the plastic mold would hold the tire casing
in a vertical position and one or more inlets to the
interior of the mold would enable additional uncured
urethane material to be pumped into the mold cavity
until it is completely filled with uncured urethane. An
air vent would be located at the top of the mold so
that as the uncured urethane enters the interior of the
mold, entrapped air is forced out of the air-release
vent at the top of the mold. The air vent could also
be used as an indicator to~tell when the mold is filled
because the first indication of urethane material in
the air-vent opening would be an indication t,hat the
mold is filled.
In conventional recapping, mold time varies with the
size of the tire being capped, As tire size and tread
thickness increase~ demold time increases. The reverse
is true for urethane~capped tires, The larger the
urethane mass, the faster it will react. Thus, small
and large tires will take approximately the same demold
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time.
Because urethane has significantly better abrasion
resistance than rubber, it would be possible to recap
a tire casing with a thinner layer of urethane and
still obtain the same length of service as would be
obtained from a recapped tire having a much thicker
layer of rubber material. It is estimated that with
the practice of the invention as described and applied
to automotive tires recapped with urethane material,
lQ it would be possible to obtain from 80,000 to 100,000
miles of service compared with from 25,000 to 40,000
miles for rubber tires.
Formulations of the urethane materials which could be
used in the practice of this invention can be readily
produced by persons skilled in the art from known
chemistry techniques in the production of urethanes.
A specific reference to the compositions and availahle
sources of commercial supply of urethanes which can be
used in the practice of this invention will be found
2Q in Chapter Seventeen of the publication RUBBER
TECHN~LQGY~ edited by Maurice Morton and published
b~ Von Nostrand Reinhold Company. To the urethane -
mater~al, anhydrous- fillers can be added, as is
known in the art, to introduce properties of hardness,
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abrasion resistance, tear strength and pigmentation.
~uch anhydrous fillers include~ for example~ a variety
of talcs, hydrated aluminum oxides, calcium carbonate,
diatomaceous earth, silica smoke, carbon black, fiber-
glass, titanium oxide, zinc oxide, bentonite and suchother fillers which are commonly used in urethane
technology, The fillers are used in proportions to
produce the characteristics of the properties desired
in the urethane material.
10 The preferred cyanoacrylate adhesive, in the practice
of the present invention, would be a composition contain-
ing alkyl esters of cyanoacrylic acid as disclosed in
U.S. Patent No. 2,794,788. Such adhesives can be of
different viscosities. We have found that a medium
15 viscosity of from 100 to 120 CP Brookfield at 78 F is
preferable to prevent excessive penetration of the
adhesive into the rubber substrate and to provide
adequate "working time" to apply the polyurethane
coating while the adhesive is still wet.
20 For additiunal information regarding cyanoacrylate
adhesives, attention is directed to the article entitled
"CYANOACRYLATE ADHESIVES" pages 569 to 580 from the
second edition of "HANDBOOK OF ADHESIVES" published by
yon Nostrand Reinhold ,Company.
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In the foregoir.g description and in the claims ! the use
o~ the terms "rubber" and "synthetic rubber" are not
intended to co~er silicone rubber.
