FORM 2
THE PATENTS ACT, 1970
(Act 39 of 1970)
AND
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION (See section 10; rule 13)
A HYBRID BELT COMPOSITION FOR HEAVY DUTY VEHICLE TIRE
APPLICANT
CEAT LIMITED
An Indian Company
of RPG HOUSE, 463, Dr. Annie Besant Road,
Worli, Mumbai 400 030,
Maharashtra, India
INVENTORS
Sujith Nair, Biswaranjan Dash, Vivek Purohit and
Rajasekar Velusamy all are Indian Nationals
of Ceat Limited,
Getmuvala, Chandrapura, Halol
Panchmahal, Halol - 389 350, Gujarat, India
PREAMBLE TO THE DESCRIPTION
The following specification describes the invention and the manner in which it is to be performed.

FIELD OF THE INVENTION
The present invention relates to a rubber composition for steel belt coating of a radial tire. Particularly the present invention relates to a hybrid belt skim composition for heavy duty tires. The present invention also relates to process for preparation of skim composition and use thereof.
BACKGROUND OF THE INVENTION
Tires have a steel belt that imparts stiffness to the tire. Steel-belted tires, commonly called radial tires. A tire is required to have excellent physical characteristics such as grip performance, abrasion resistance etc. As the radial tires for use in the conventional trucks and buses, there have been used a radial tire body-reinforced by a belt composed of one or more and a carcass. The heavy duty tires for use in trucks and buses employing steel cords as reinforcing elements for the belt and the carcass are used not only on fully-paved and well-conditioned good roads for instance a route for the exclusive use of motor vehicles such as expressway or the like but also on running roads partly including bad roads with an inferior road surface such as a construction road or the like.
Steel cords are extensively used as a reinforcement material in radial tires, high pressure hydraulic hoses, rubber tracks and heavy duty conveyor belts. The use of steel cords leads to improved strength, stiffness, stability and uniformity in tires. Good adhesion between the rubber compound and the steel cord in the tire is critical to the tire‟s performance. Not only will good adhesion ensure safe tire service under a variety of severe road and environmental conditions (e.g., moisture, salt water, etc.), it will also increase the tire‟s lifetime. In order to guarantee a sufficient level of adhesion between the steel cord and rubber, the properties of both the metal surface and the rubber must be optimized.
Various rubber compositions are known which are developed to improve adhesion requirements.

The present invention addresses the above problems and provides an improvement on properties of the heavy duty radial tires comprising the steel belt of hybrid skim composition. The present invention thus relates to a skim composition for steel belt that enhances a steel cord to rubber coating adhesion without affecting is processability.
SUMMARY OF THE INVENTION
In an aspect the present invention provides hybrid belt skim composition for radial
tires.
In another aspect the present invention provides process for preparation of said
skim composition and use thereof.
According to the present invention, a radial tire with improved adhesion property
comprising a combined reinforcement which comprises hybrid belt composed of
layers with steel cords bonded to skim composition of the present invention.
BRIEF DESCRIPTION OF DRAWINGS
The foregoing summary, as well as the following detailed description of the present invention, will be better understood when read in conjunction with the appended drawings. For the purpose of assisting in the explanation of the invention, there are shown in the drawings embodiments which are presently preferred and considered illustrative. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown therein.
Fig. 1 is a perspective view of a reinforcing layers used to form the belt; Fig. 2 is a perspective view of the hybrid belt of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
In describing and claiming the invention, the following terminology will be used in accordance with the definitions set forth below. Unless defined otherwise, all

technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any sensing device, methods, system and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred sensing device, methods, system and materials are described herein. As used herein, each of the following terms has the meaning associated with it in this section. Specific and preferred values listed below for individual components, substituents, and ranges are for illustration only; they do not exclude other defined values or other values within defined ranges for the components and substituents.
The terms “preferred” and “preferably” refer to embodiments of the invention that may afford certain benefits, under certain circumstances. However, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful, and is not intended to exclude other embodiments from the scope of the invention.
As used herein, the terms “comprising,” “including,” “having,” “containing,” “involving,” and the like are to be understood to be open-ended, i.e. to mean including but not limited to.
The term "phr" means parts per hundred parts of rubber by weight, and is a measure common in the art wherein components of a composition are measured relative to the total of all of the elastomer (rubber) components.
Thus, before describing the present invention in detail, it is to be understood that this invention is not limited to particularly exemplified systems or process parameters that may of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments

of the invention only, and is not intended to limit the scope of the invention in any manner.
In one aspect the present invention provides a hybrid belt comprising the skim
composition for radial tires.
The skim composition comprises natural rubber, poly butadiene rubber, silica,
cobalt salt, carbon black and silane coupling agent.
According to the present invention the heavy duty truck radial tire comprises total
4 layers of belts. The first belt which is just above the carcass ply is called
transition belt, 2nd and 3rd belt layers are called „the working belts”, this is
because of the fact that they are intended to bear load. The 4th layer is called
protector belt.
In an aspect the present invention provides a skim composition for working belt where adhesion is very critical. Whereas in first and 4th belt layer tackiness/stickiness is crucial for tire building process. Generally, silica compounds have less tack. The present invention provides hybrid concept in which the working layers are made with high adhesion compound and transition & protector layers are made with high tack and moderate adhesion.
The objective of the present invention is to enhance a steel cord to rubber coating adhesion without affecting processability by using hybrid belt layers in a tire wherein the belt comprises unique combination of adhesion promoter resins, silica, silane coupling agent and rubber blends.
According to the present invention a radial tire comprises plurality of belt layers for example a transition belt, working belt and protector belt wherein the working belt layers comprise the present skim composition for working belts and conventional carbon black composition for protector belt and transition belt making it a hybrid belt for a tire.

The term "hybrid belt" as used herein refers to a belt comprising different belt layers of one or more compositions of the present invention for steel cord coating of the belt. The term “Skim compound” or Skim composition” are synonyms and refers to coating compound for coating steel cord of heavy duty tires.
Conventionally, single compound is used for coating all four belts. The present invention provides a hybrid belt where two different compositions for belt layers are used (1) novel skim compound for working belt layers and (2) conventional carbon black compound for protector belt and transition belt as shown in fig. 2.
In another embodiment the present provides a radial tire with superior adhesion properties comprising hybrid belt layers.
The hybrid belt comprises one or more layers comprising skim composition when applied for steel cord coating has superior adhesion properties.
In another embodiment the present composition is used in working belt of a radial tire where adhesion is very critical. In an embodiment the skim composition of the present invention is used in first and fourth belt layer where tackiness/stickiness is crucial for tire building process.
In an embodiment the skim composition of the present invention comprises
composition of adhesion promoter resins, silica, silane coupling agent and rubber
blends.
The silica compounds have less tack. Hence, the hybrid concept is adopted in
which working layers made with high adhesion compound and transition &
protector layers are with high tack and moderate adhesion.
Preferably blend of natural rubber and polybutadiene rubber is used which improves mixing and calendaring process.

Use of silica deteriorates adhesion and processability of the tire. The present invention also solves this problem by providing the skim composition wherein silica and carbon black are used as fillers in a specific amount which achieves superior adhesion property as compared to conventional carbon black compositions.
In an embodiment skim composition comprises blend of cobalt salt, resorcinol
formaldehyde resin and hexamethoxymethlylmelamine(HMMM) to get the
desired results.
In another aspect the present invention provides skim composition for hybrid belt
of heavy duty tires.
In an embodiment the skim composition of the present invention is as represented
in below table.
Ingredient Range (phr)
Rubber 80-90
Poly Butadiene Rubber 10-20
Filler 45-55
Cobalt Salt 0.3-1.0
Carbon Black 5-15
Silane Coupling Agent 4.2-4.7
Tackifying Resin 1.5-5
Adhesion Promoter Resins 1.0-6.0
Antioxidant 6PPD 0.5-2.5
Ingredient Range (phr)
Rubber 80-90
Poly Butadiene Rubber 10-20
Filler 45-55
Cobalt Salt 0.3-1.0
Carbon Black 5-15
Silane Coupling Agent 4.2-4.7
Tackifying Resin 1.5-5
Adhesion Promoter Resins 1.0-6.0
Antioxidant 6PPD 0.5-2.5
The rubber used is natural rubber and/or poly butadiene rubber. The natural rubber is one the major components of the rubber blend used in this invention.
A reinforcing agent/filler known to those skilled in the art may be used in the rubber composition either by themselves or in combination with other reinforcing fillers. In particular embodiments of the composition disclosed herein, the filler is

silica. The silica used include Ultrasil VN3G, MFIL200G which is reinforcing non-petroleum based filler used to improve the physical, dynamic and adhesion performance of the compound.
The cobalt salt used is selected from the group comprising cobalt naphthanate, cobalt boroacylate, and cobalt neodecanoate or cobalt stearate.
The silane coupling agent used include, but not limited to, bis[3-
(triethoxysilyl)propyl] disulfide, bis[3-(triethoxysilyl)propyl] tetrasulfide,
trialkoxymercaptoalkyl-silane, S-[3-(triethoxysilyl)propyl]ester or octanethionic
acid which acts as a coupling agent between silica and rubber. It reacts with the
silanol groups of silica and forms stable covalent chemical bonds during mixing.
It also makes stable covalent chemical bonds with the rubber during the process of
vulcanization and thus resulting excellent filler-polymer interaction in the
compound.
The tackifying resin used is selected from the group comprising hydrocarbon resin, Phenolic resin, p-t-butyl phenol Resin.
The adhesion promoter resins used is selected from the group comprising Resorcinol formaldehyde resin and hexamethoxymethlylmelamine(HMMM).
The antioxidant used is selected from the group comprising N'-phenyl-p-phenylenediamine (6PPD) or 2,2,4-Trimethyl-1,2-Dihydroquinoline(TMQ).
The skim composition of the present invention can further comprise various other additives known in the art, which are selected from zinc oxides, vulcanization accelerators, aging inhibitors and pre vulcanization inhibitors and the like.
In an embodiment the present invention comprises a hybrid belt composition comprising 80-90 phr of natural rubber, 10-20 phr of poly butadiene rubber, 45-55

phr of silica, 0.3-1.0 phr of cobalt salt, 5-15 phr of carbon black, 4.2-4.7 phr of silane coupling agent optionally one more additives for hybrid belt.
In an embodiment the hybrid belt composition is used for making hybrid belt of radial tire of a vehicle.
In another aspect the present invention provides a process for preparation of skim composition and use thereof.
The process is performed in Banbury & Sheeted out two-roll mill which is machinery traditionally used for mixing rubber compounds.
According to the present invention the process for preparation of skim compound comprises the following steps
a) Master batch mixing of ingredients as per the formulation except Sulphur
& Accelerators.
b) Master batch from previous step mixed with sulphur and accelerators.
The skim compound obtained is sheeted out in 6mm at the end of second step in
twin screw sheeter.
The skim compound is then coated or bonded to steel cord to enhance the adhesion property.
In an embodiment the process for coating the steel cord with skim compound of the present invention comprises coating steel cord through calendaring process where each belt is calendared separately and assembled together in tire building machine.
According to the present invention, a radial tire with improved adhesion property comprising a combined reinforcement consisting of hybrid belt composed of layers with steel cords bonded to skim compound of the present invention.

The skim compound of the present invention is particularly for use in the manufacture of heavy duty tires comprising steel-belted structure.
The present invention has following advantages.
1) The present skim composition poses superior rubber to steel belt cord bonding and easy processing. In heavy duty radial tires, adhesion between 2nd & 3rd belt (i.e working belt) is very critical in order to prevent belt separation failures under severe loading conditions. The composition is based on general purpose rubber with predominantly silica filler and with unique combination of additives. The Silica filler is a key for excellent adhesion and low heat buildup.
2) Silica filler enhances the bonding between steel belts, even with minor dosage of bonding agent and sulphur.
3) Natural rubber provides strength and tack.
4) Presence of polybutadiene rubber helps in processing (especially calendaring) and controlling viscosity.
5) Adhesion promoters improve rubber to steel bonding.
The present invention will now be more particularly described with reference to the following examples. It is to be understood that these are intended to illustrate the invention and in no manner to limit its scope.
Examples: Example 1
Compositions
Compos ition 1 Compos ition 2 Compos ition 3 Compos ition 4 Compos ition 5
Natural Rubber 100 100 85 100 80
Poly Butadiene Rubber - - 15 - 20
Compos Compos Compos Compos Compos
ition 1 ition 2 ition 3 ition 4 ition 5
Natural Rubber 100 100 85 100 80
Poly Butadiene
- - 15 - 20
Rubber

Carbon Black
55 60 20 45 5
Silica - - 41 30 55
Silane Coupling Agent - - 3.5 2.5 4
Cobalt Salt 0.8 0.8 0.8 0.8 0.5
Adhesion Promoter Resins - 5.5 1.5 1.5 6
Zinc oxide 8 10 10 10 8
Tackifying Resin - 6 4 2.5 9
Antioxidant 6PPD 3 1.5 3 1.5 3.5
Sulphur 6.5 5 6 6 4.5
DCBS 1 1 2 1.5 2

Result
Pullout adhesion 100.0 106.1 121.0 121.8 129.9
Retention after heat ageing (80deg X 7 days) 100.0 108.9 122.2 127.4 137.0
Natural polyisoprene rubber, Nickel Catalyzed Poly butadiene rubber, Carbon black – N326, Silica- MFIL200G,Silane coupling agent – bis[3-(triethoxysilyl)propyl] disulfide, Cobalt salt – Boroacylate. Above mentioned compositions are mixed in laboratory banbury mixer (refer example 10 for detailed steps). Pullout adhesion is done as per ASTM D2229
Example 2
Tack value after calendaring:

Composition
All 4 belt with Hybrid belt ( 1&4 with
composition 1 composition 1, 2&3 with composition 5)
Rubber 100% NR Blend of Natural rubber and polybutadiene
Filler Carbon black Predominantly Silica
Cobalt salt Cobalt Boroacylate Cobalt Boroacylate
Adhesion promoters No Resorcinol formaldehyde
resin/Hexamethylene
melamine

Compound Property
Tackiness in 8.9 7.6
Newton[Original] – 2&3
belt
Tackiness in 6.2 1.7
Newton[After 4 days
storage] – 2&3 belt
*Index higher the better
Result and conclusion:
Tackiness retention is good for belt skim compound 1&4 compared to 2&3.This is
essential for assembling coated belt in tyre building process.
Example 3
Tire Adhesion and Belt Endurance test:
Belt endurance test is done on pulley wheel drum by increasing load step by step
at constant speed till the failure.
Tyre Adhesion Test procedure:

A Sample of 25mm width was cut from the tyre and slitting of40mm is done between 2nd and 3rd belt layer and the edges are fixed in UTM and pulled apart. The force per unit is noted as Kg/inch. The testing done for both fresh as well as pulley wheel tested tire samples.
Result:

Composition
All 4 belt with Hybrid belt ( 1&4 with
composition 1 composition 1, 2&3 with composition 5)
Rubber 100% NR Blend of Natural rubber and polybutadiene
Filler Carbon black Predominantly Silica
Cobalt salt Cobalt Boroacylate Cobalt Boroacylate
Adhesion promoters No Resorcinol formaldehyde
resin/Hexamethylene
melamine

Compound Property
Tyre belt endurance 100 112
Tyre adhesion (Fresh) 100 132
Tyre adhesion after 100 159
endurance
Tyre belt endurance is direct measure of belt performance in the field. Endurance depends on belt to belt adhesion. Higher the tyre adhesion betters the endurance performance and the lower belt failures in service. Comparative study of conventional and hybrid belt concept is given in example 8. As mentioned in example 8, tyre adhesion (both fresh & after endurance test) is significantly high when hybrid belt is used. That is reflected in the belt endurance as well.

Example 4
Rubber compound Preparation:
a) Prepared in Banbury & Sheeted out two-roll mill which is machinery traditionally used for mixing rubber compounds.
b) Two step mixing in Banbury - (1) Master batch mixing of ingredients as per the formulation except Sulphur & Accelerators. (2)Master batch from previous step mixed with sulphur & Accelerators.
c) Rubber compound sheeted out in 6mm at the end of second step in twin screw sheeter.
d) Critical parameters: (1) Mixing time, temperature & energy (2) Viscosity.
Coating Process: Steel cord coating is done through calendaring process. Individual belts calendared separately and assembled together in tire building machine.
a) Critical parameters: (1) Gauge consistency of calendared belt (2) Tackiness
14

We Claim
1. A hybrid belt composition comprising
80-90 phr of natural rubber 10-20 phr of poly butadiene rubber 45-55 phr of silica 0.3-1.0 phr of cobalt Salt 5-15 phr of carbon black and 4.2-4.7 phr of silane coupling agent
2. The composition as claimed in claim 1 further comprises tackifying resin, Adhesion Promoter Resins, Antioxidant 6PPD.
3. The composition as claimed in claim 1 wherein said cobalt salt is selected from the group comprising cobalt naphthanate, cobalt boroacylate, cobalt neodecanate or cobalt stearate.
4. The composition as claimed in claim 1 wherein said silane coupling agent is selected from bis[3-(triethoxysilyl)propyl] disulfide, bis[3-(triethoxysilyl)propyl] tetrasulfide, trialkoxymercaptoalkyl-silane, S-[3-(triethoxysilyl)propyl]ester or octanethionic acid.
5. The composition as claimed in claim 1 wherein said tackifying resin is selected from the group comprising hydrocarbon resin, Phenolic resin, p-t-butyl phenol Resin.
6. The composition as claimed in claim 1 wherein said adhesion promoter resins is selected from the group comprising Resorcinol formaldehyde resin and hexamethoxymethlylmelamine(HMMM).

7. The composition as claimed in claim 1 wherein said antioxidant is selected from the group comprising N'-phenyl-p-phenylenediamine (6PPD) or 2,2,4-Trimethyl-1,2-Dihydroquinoline(TMQ).
8. A process for preparation of hybrid belt composition comprising steps of

a) Master batch mixing of ingredients of the composition as claimed in claim 1 and
b) Master batch from previous step mixed with sulphur and accelerators to obtain hybrid belt composition with enhanced adhesion property.
9. A radial tire comprising a hybrid belt as claimed in claim 1.