Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a process for retread-
ing a worn tyre by the direct application of a premoulded tread
to the carcass of t~e tyre.
Conventional processes for retreading worn tyres may
be divided into two groups: namely a) Elot processes b) ~ow
temperature processes.
In the hot process, the not ye-t vulcan:ized t~ead is
applied to a p~eviously rasped carcass. ~lhe assembly of the
carcass and the tread is placed inside a special mould and then,
with heat provided by steaml hot gas or hot air, the vulcanization
reaction is effected at high temperatures. Whilst vulcanlzation
is taking place, the design of the tread is also formed and this
naturally depends on the shape of the mould.
Such hot processes have various disadvantages. In
particular such disadvantages include fast ageing of the finished
product because of the high temperatures involved and high plant
costs due;to the large number of moulds needed to have a
sufficiently flexible production.
While in other hot processes, theaforesaid technology
continues to be used, the use of a prevulcanized tread in
retreading tyres has been proposed.
This makes it possible to in part, avoid the problem of
moulds. However it introduces another substantial difficulty,
namely double vulcanization which substantially reduces the
mechanical characteristics of the tyre.
In the conventional low temperature processes the tread
is applied to the carcass of the tyre to be retreaded, using an
intermediate strip of adhesive material. The adhesive strip,
having a natural rubber base and containing, inter alia, an accel-
erator and a vulcanizing agent, is maintained at a low temperature,protected by a special casing in order to prevent undesired
prevulcanization and only when it is to be used may the said
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strip be applied to the ~read.
Other low temperature processesUsed, instead of a
single strip of adhesive material, two separate strips, again
wi-th a rubber base, one of which contains the accelerator and
the other, the vulcanizing agent. These particular strips also
have to be kept carefully isolated until the time they arè to be
used so as to prevent undesired reactions. Durin~ the vulcaniza-
tion phase the complete assembly is enclosed in a protective
casing which is suitably taut so that it is tightly up ayainst
the surface of the tread and the sidewalls of the tyre. The
casing is provided with a valve to externally relieve the air
trapped between it and the tyre during the fitting of the casing.
This prevents oxidation during the vulcanization or alternations
from taking place in the chemical-physical characteristics of
the material which is subsequently vulcanized.
The application of t~e strips of adhesive material and
of the tread to the carcass of the tyre is carried out through
rolling operations performed which favour lateral expulsion of
the air trapped between the carcass, the strips o~ adhesive mat-
erial and the tread during the application. In particular, theconventional procedure is to effect the rolling operation with
a reciprocating motion in a direction diagonal to the surface
of the tread at the same time that the tyre is being rotated.
The cold processesl which are widely used, however have
various disadvantages. In particular the strips of adhesive
material are likely to slide outside the area in which the ;-
tread has to be applied and it is di~ficult to get the final
thickness of the strips of adhesive material to be uni~orm because, ~-
due to the aforesaid tendency to slide the material accumulates
in some places, whilst in others it is in short supply and can
even be insuf~icient. Due to the a~orementioned disadvantages,
the bond between the tread and the carcass is irregular and areas - -
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are formed which become unstuck and these considerably shortenthe life of the retreaded tyre.
A further disad~antage, of significant importance, is
the air which, during the vulcanization is encapsulated between
the protective casing and the tread since, despite the care with
which the protective casing is put into position and the techni-
cal expedients used, it is not possible to eliminate fully the
presence of air inside the said casing and thus a cerkain quantity
of air, though oE minimal volume, is always trapped in the
interstices of the surface of the tread. r~his notably reduces
the heat transmission coefficient, thereby inhibiting proper
vulcanizing reticulation. Furthermore, in all the aforesaid
low temperature processes, -the strips of adhesive material between
the tread and the carcass, whether consisting of one or more layers,
have to be kept away from the tread and separate ~rom each
another under special ambient conditions, duly protected by a
special casing in order to prevent prevulcanization which greatly
reduces their adhesi~e properties. Only at the time they are to
be used can the strips be applied to the tread or to the carcass
of the tyre and this renders the task of the operator extremely
long and arduous.
- The present invention pro~ides a process for retreading
a worn tyre which is not subject to the aforesaid disadvantages
and which comprises the direct application to the carcass of
the tyre of a single assemhly comprising a premoulded tread and
a strip or sheet of adhesive material, with which assembly the
sheet of adhesive material may be prepared and applied to the ;
tread when the tread is being formed, without any danger of
pre w lcani2ation, the application of the sheet to the tread is
effected without any undesiredsliding occurring and with the
optimum certainty of an absence o~ air between the tread and
the sheet, and there being essentially no air between the
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protective c~sin~ a~d the tyre during the vulcanization.
According to the present invention in the low tempera-
ture process, the sheet of adhesive material with which to anchor
the tread to the carcass of the tyre is prepared from an admixture
comprising at least one non-vulcanized bonding material, at least
one vulcanization accelerator at least one vulcanizing agent, a
relatively high percentage in weight of co-vulcanizing resins,
that is about 7.6 to 8.2~ by weight oE the total composition, and
an agent for the prevention of scorching, capable o~ vulcanizing
the non-vulcanized bondin~ mater.ial at a temperature around 100C
and to stabilize ~he reagents and thus prevent commencement of
the vulcanization, up to temperatures of bet.ween 35C and 40C
inclusive, and the sheet of adhesive material is bonded to the
inner surface of the tread by rolling and simultaneous perfora-
tion of the said sheet so as to expel air trapped between the
tread and the sheet of adhesive material, via the perforations
in the said sheet of adhesive material.
According to the present invention there~ore there is
provided a process for retreading a worn tyre comprising a carcass
having side walls and a rolling surface which process comprises
providing a pre-moulded tread, preparing a sheet of adhesive
material from an admixture comprising at least one vulcanizable
; bonding material, at least one vulcanization accelerator, at least
one vulcanizing agent, a high weight percentage of co-vulcanizing
resins, and an anti-scorching agent capable of preventing vulca-
nization of said sheet at a temperature up to 40C, said sheet
being vulcanizable at temperatures of about 100C, forming an
assembly by bonding said sheet to an inner surface of said tread
by means of rolling during which said sheet is perforated to cause
expulsion of air trapped between the tread and the sheet through
said perforations, applying the assembly to the carcass with:the
adhesive sheet forming an intermediate layer between the tread
and the rolling surface of
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the carcass enclosing the assembly and carcass with a protective
casing which extends around the tread and the side walls in an
air-tight manner, and inserting the enclosed assembly and carcass
in a vulcanization oven to effect vulcanization.
The present invention will be further illustrated by
way of the accompanying drawings :in which:
Fig. 1 is a block flow diagram of the stages in the
process for retreading a ~yre accordin~ to one embodiment of the
present invention;
Fig. 2 is a schematic side view showing how the assembly
of the premoulded tread and the sheet of adhesive material is
achieved in the process o~ Fig. l;
Fig. 3 shows a cross sectional view of an assembly of
the tread, the sheet of adhesive material, the carcass of the tyre
and the protective casing mounted on a supporting rim in the
process of Fig. l; and
Fig. 4 is a schematic side elevation of the assembly
of Fig. 3 in a vulcanizing oven.
The tread 1 is produced in accordance with conventional
techniques, preferably giving it a particular section as shown
in Fig. 3. In particular the tread 1 is provided with an inner
surface la which as will be seen hereinafter, match the rolling
surface of the carcass of the tyre 2. The inner surface la is
suitably arcuate and has a radius to allow the single assembly
constituted by the tread and the sheet of adhesive material to
bond perfectly to the pre-treated rolling surface of the carcass
2 of the tyre.
The preparation of the rolling surface of the carcass
2 of the tyre involves rasping operations with which all residual
parts of the tread to be reformed are removed from the carcass of
the tyre 2. The rasping operation is par~icularly intense
adjacent the sidewalls of the carcass 2 of the tyre an~ at the -
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conclusion of these rasping operations, the rolling surface 2a
is arcuate, substantially as shown in Fig. 3, with a radius of
curvature R.
The radius R is considered when forming the tread 1 so
that the inner surface la of -the tread is arcuate with a radius
of curvature Rl identical to the radius I~. The surfaces la o~
the tread 1 and 2a of the carcass 2 of the tyre are thus on the
same planes and match each other.
This operating stage avoids the substantial problem
which is experienced when preparing the tread.and the carcass of : :
the tyre on the basis of conventional c~rtain techniques, whereby
the tread is provided with a perfectly flat inner surface and the
adhesive material is built up in the areas close to the sidewalls .
of the carcass of the tyre, so as to bring the rolling surface
into the right condition to receive the tread.
The process according to the invention requires the
preparation of a sheet of adhesive material 3 to be applied to -`
the tread at the time it is being formed, so that the single
assembly of the tread and the sheet of adhesive material can ~:
subsequently be stored without the risk of any kind of reaction
occurring. These characteristics are obtained by dosing reagents : . :
which allow the sheet of adhesive material to be produced with /.. ~:.
a high percentage of co-vulcanizing resins and also by a suitable
percentage of an anti-scorching agent which prevents the previously . ~.:
mentioned undesired reactions from taking place. The reagents :
used in the production of the sheet of adhesive materialmay varyboth
qualitatively and quantitatively depending upon the composition ~
of the tread. By way of an example, the composition by weight is . -
- given below of the reagents for producing a standard sheet of :. -
adhesiye material suitable for most of the tyres currently on . .::
the market: .
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COMPONE:NTS P RTS IN WEIGHT
Natural rubber or polyisoprene 100
zinc oxide 5-10
Stearic acid 0.5-1.2
Anti-oxidant (Poly 2-3-4 trimethyl
dihydroquinaldine)
Anti-ozonant
(N-phenyl - N - cyclohexyl para 1-2
phenylendiamine)
Pine tar 5-10
Co-vulcanizing resins (Polyhalethyl phenol) 15
Adhesive agents (liquid coumarone/indene resin) 10
suna weld (Polymer 780) 10
Insoluble.. Sulphur 3~5
Accelerators (diphenyl guanadine and 1-3
mercaptobenzothiazole)
High structure carbon black 30
Anti-scorching agent (supplied under the trademark 0,05
Santocard PVI)
The high percentage of co-vulcanizing resins produces
a stabiliæed product which retains indefinitely its adhesive
properties. If the lower figure of the ranges is taken, the
percentage by weight of the co-vulcanizing resins is 8.2~ and7 if
the higher figure is taken the percenta~e by weight of co-
vulcanizing resins is 7.6%.
The addition of a suitable percentage of an agent for
the prevention of scorching prevents undesired prevulcanization
and makes it possible for the sheet of adhesive material 3 to be -
applied to the tread 1 and the properties of the assembly of the
; tread and the sheet of adhesive material to remain unchanged. ~ .
The sheet 3 produced in accordance with the composi-
tion given above and subjected to "Monsanto" rheometric:tests,
using a sample vulcaniæed at 100C, gave the following physical
characteristics as a function of time: :
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2h 3h 4h
Modulus at 300~ 63 64 85
Tensile stress 238 237 235
Ultimate elongation 640 610 560
Shore Test A 46 49 53
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The stages for the produc~ion o~ ~he sheet of adhesive
material comprise precomminuting/mixing the mixture for the rubber
with the combined action of mechanical processing/temperature
and the use of peptization means until a degree of viscosity is
obtained such that in the subsequent mixing with reinforcing,
charging and vulcanizing substances, these may be suitably broken
up in the mixture; a first un-operative stage for a period of
approximately 12 hours; dosing and mixing of the components that
form the mixture (excluding the vulcanizing agents and the
accelerators) in open mixers at low temperature; a second un-oper- ~-
ative stage for approximately 12 hours to allow perfect diffusion
of the reagents and the inert bodies forming the mixture; mixing -
in the mixture the accelerators and the vulcanizing agents in an
open mixer at low temperature suitably 30 - 40C; calendering -
in sheets of a small thickness; and cutting of the sheets to a
size corresponding to the dimensions of the tread on which they
will be applied.
Once the sheet 3 has been prepared in accordance with
the aforesaid method, it is applied to the smoothed surface la
of the premoulded tread 1, over the contact surface of which a
special rubber base bonding solution is spread.
In the process according to the invention, as shown in
Fig. 2, the application of the sheet of adhesive material 3 to the -
; tread 1 requires the tread 1 and the sheet 3 to be passed ~etween
normal drive and pressure rollers 17 and between two rollers 4
and 5, the latter having over its full rolling sur~ace a series
of sharp tips 6 dimensioned to perforate the sheet 3 until the
- surface la of the tread 1 is reached.Subsequently, inside a
vacuum chamber 7, expulsion of the air trapped between the tread
1 and the sheet of adhesive material 3 is expelled via the perfor-
ations in the sheet 3 and the air is purged from the perforations.
Prior to the assembly of the tread 1 and the sheet of
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adhesive material 3 leaving the chamber 7, the free surface o~
the sheet 3 has a protective film 3a of polythene or some other
synthetic material applied to it to prevent air ~rom returning
to the perforations in the sheet 3 and to conserve the chemical
physical characteristics of the sheet 3 against dust and extra- ~;
neous bodies~
In the retreadin~ stage o~ a tyre, the assembly of the
tread 1 and the sheet of adhesive material 3 is thus pre-formed
ready to be applied to the rolling surface of the previously
rasped, smoothed and solution treated carcass 2 of the tyre.
The application is effected in accordance with conventional tech-
niques and without there being any risk of the assembly slipping
over the carcass 2 of the tyre~ this mainly being due to the
fact that the sheet 3 already forms a unitary body with the
tread l.
Upon completion of this stage, the tyre is mounted
also according to conventional techniques, on a special rim lO
subsequent to the placement of a protective casing 8 around the
tyre so as to enshroud the tread l and the sidewalls 9 of the tyre,
~0 with its edges maintained tightly pressed against the sidewalls
by the rim 10. The protective casing 8 is provided with a valve
ll which allows the air trapped inside the casiny at the time -
of fitting on to the tyre to be discharged externally. When -
this stage has been completed, the tyre is then placed inside
a ~ulcanizing oven 12.
In the process o~ the present invention~ prior to
commencement of the actual vulcanization, the protective casing
8 is ~onnected to a vacuum apparatus 14 via a pipe 13 so as
to fully purge ~ir from the inslde of the casing 8, even air that
normally trapped in the interstices of the tread l. Then by
means of a pipe 15, air is passed inside the carcass 2 of the -
tyre or into an inner tube 16 in the said tyre, until a pressure
is buil~. up which roughly corresponds to the normal workiny
pressure of the tyre.
The oven 12 is set in operation to cause ~ulcanization
suitably at an optimum temperature of 100C with a suitable
heating fluid preferably hot air or steam. The oven 12 is
pressurized at a pressure slightly below that of the tyre until
the necessary conditions are reached which give rise to the
vulcanization reaction, providing a per~ect union between the
tread 1 and the carcass 2 of the tyre, with the physical-chemi~al
characteristics of the vulcanized material being ~ully guaranteed.
The presence of the anti-scorching agent in the sheet .
of adhesive material 3 advantageously provides an initial retard-
ation in the vulcanization reaction, so as to allow the operator
to intervene, even after the operation of the oven 12, particular-
ly in cases where some handling error is detected subsequently
to the placing inside the oven 12 of the tyre prepared for :
vulcanization.
"Monsanto" rheometric tests carried out at 100C show
that unlike the sheets of conventional adhesive mate~.ial, the
sheet produced in accordance with the process of the present
.; invention remains perfectly stabilized at temperatures up to .
35C ~ 40C, delaying the commencement of the vulcanization by
approximately 20 - 30 minutes.
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