Field of the Invention
The present invention relates to multi-cellular polyurethane foam tyres for vehicles, particularly slow moving vehicles including industrial vehicles.
The invention relates more particularly to the tyre made with polyurethane foam to improve performance in the area of comfort, ride quality and energy required to drive by such tyres during running of vehicles.
Background of the Invention
Currently used vehicle tyres are generally pneumatic tyres, that is, the tyres must contain certain minimum air content under pressure to carry a specified load. The internal pressure is limited by the strength of the tyre structure and the travel comfort.
Pneumatic tyres are disadvantageous in the context that they require a very complex structural design to ensure an internal overpressure.
For control of an internal overpressure the tyre body must be reinforced with multi-layer, strength-carrying cord layers, and such layers have to be provided with rubber layers of different physical characteristics which can resist high-frequency stresses when the tyre is in use. The tyres must be also provided with a special insulating layer to preserve the internal overpressure during use.
Tyres may be formed by supplying a material in a flowable form into a mould and after the material hardens, removing the molded tyre from the mould. However, it may be desirable for moulded tyres to have a tread portion that exhibits different material characteristics than other portions of the tyre. For example, it may be desirable for the tread portion to have particular friction and wear-resistant characteristics that are different from other parts of the tyre, such as the portion of the tyre between the tread portion and the hub.

In tyres for machines it may be desirable to provide tyres having a relatively stiff material able to support high loads associated with the weight of the machine and any payload. In contrast, it may be desirable for the tread portion of such a tyre to be formed from a material that provides a desired level of traction and resistance to wear. However, it may be difficult to mold a tyre formed from a single material that results in this combination of desirable characteristics.
Attempts have been made to mold a polyurethane tread material onto a rubber tyre. However, the polyurethane tread material may tend to separate from the rubber tyre under certain operating conditions due, for example, to high speed, high loads, or high temperatures. As a result, it may be desirable to provide a molded tyre that provides a combination of desirable stiffness, traction, and wear resistance.
An example of a method of preparing a composite of polyurethane bonded to the surface of a cured rubber substrate and the resulting composite cushioned tyre is disclosed in U.S. Patent No. 4,669,517 to Krishnan ("the '517 patent"). In particular, the method of the "517 patent" requires that the rubber substrate contain a hydroxyl terminated polybutadiene polyol. The method includes first applying to a cleaned, cured rubber surface containing the polybutadiene polyol a coating of cyanuric acid followed by an additional coating of a phenol formaldehyde resin, and then applying a liquid polyurethane reaction mixture, which is cured to form the resultant composite. The method of the "517 patent" is particularly directed to the preparation of a tyre including a polyurethane carcass applied to a cured rubber tread, and to a tyre including a polyurethane tread applied to a cured rubber carcass.
Automotive vehicles, motor cars and trucks, can use flat free tyres on unpaved road or off road service Vehicles which do not carry spares, such as motorcycles, motor bikes, and bicycles, find flat free tyres invaluable.
So-called deflation proof tyres have been available for some years; these tyres all depend on a foamed elastomer filling The foamed filling has serious disadvantages, especially excessive heat build up within the tyre during service; filler breakdown during service. A major disadvantage is that the filling must be installed in the factory and thus this is expensive.

Though there are various manufacturing methods and materials for producing polyurethane foam tyres which are deployed across the world, it has always been a problem to manufacture multi-cellular polyurethane foam tyres with good performance.
Much work has been made since the early 1970's to overcome the inherent drawbacks in pneumatic tyres, primarily the risk and expenses of flat tyres, without sacrificing their performance characteristics.
One of the earlier methods was to fill an existing pneumatic tyre with polyurethane foam which involved a two component foam temperature system, where the components were metered and blended together, and then injected into the tyre or tube cavity. The chemicals reacted to form polyurethane foam which expanded and filled the volume normally occupied by compressed air.
The resulting tyre was virtually flat-proof (puncture-proof) and had found applicability in industrial equipments where durability is a primary consideration. The main drawbacks of this method are the costs involved, a urethane filled tyre assembly is more expensive than an air filled assembly and there is a noticeable reduction in the performance characteristics of the tyre for some applications compared to a pneumatic tyre, the weight is higher and the speed is limited as the urethane foam builds up and retains heat under the continual flexing resulting from rolling.
In another improvement, the tyre and tube was replaced with a hollow urethane elastomer shell. The urethane was stiff enough to support the load while the elastomer nature of the urethane combined with the hollow interior allowed the tyre to deflect to provide shock absorbing properties. The tyre was held on by tension member consisting of fiber or cord embedded into the tyre.
Tyres made of an elastomer require some method of preventing them from elongating and rolling off the rim in use, as elastomers were soft enough to provide a cushioning effect and have relatively low tensile modulii. Such methods normally involve embedding into the tyre a flexible tensile stiffness of the composite tyre such that the load required to stretch the tyre on and off the

rim is higher than the loads encountered in use. This allows the tyre to be mounted, yet prevents it from inadvertently rolling off in normal usage.
Materials used as tension members include natural and synthetic fiber and cord such as nylon, polyester and cellulose, and metal wires or coils.
A more recent innovation was to manufacture the tyre from self-skinning flexible microcellular polyurethane foam. In this concept the foam interior provides the flexibility for the shock absorption characteristics, while the unfoamed skin on the outer sur face provides the wear resistant riding surface. Various means can be used to hold the tyre on the rim including tension members and gluing of the tyre on the rim.
Reference may be made to US6343843 (B1), wherein a pneumatic tyre, a wheel rim, and an assembly of a pneumatic tyre and a wheel rim, provided with lawn-like materials for damping air resonance of the tyre cavity are disclosed. An assembly of a pneumatic tyre and a wheel rim where a foaming agent is injected into the tyre cavity is also disclosed.
Polyurethane foam products of this type are molded. In order to mold tyre shaped object the mold should be filled under some pressure to prevent surface defects caused by air pockets. This pressure can be obtained by injecting the preblended liquid urethane components into a sealed and pressurized mold, or else by rotating the mold while pouring in the preblended liquid urethane components. Both of these methods are used extensively in casting products from various materials.
Reference may be made to WO2013182477 (A1), wherein a tyre, the inner wall of which has a layer of polyurethane foam, characterized in that the polyurethane is 4,4'-diphenylmethane diisocyanate, and a polyol which has an ethylene oxide content greater than 50% (% by weight). This specific polyurethane formulation efficiently absorbs noise, is inexpensive and is easy to use by direct casting of the reagents in the tyre of the invention.

Reference may also be made to US201003 8005 (A1), wherein a tyre-rim assembly comprising a pneumatic tyre (1), a rim (2) and a sponge member (4) disposed in a tyre internal cavity (3) defined by the pneumatic tyre (1) and the rim (2), characterized in that the sponge member is described. US6374887 (B1) describes a vehicle tyre which includes a rubber or elastomer tyre body which has a circumferentially extending crown portion and opposite sidewalls joining the crown portion.
The preferred version of the prior art is to produce a polyurethane foam tyre with an embedded tension member. The preferred location of the tension member is near the inside diameter of the tyre. Current practice has two tension members located on each side of the tyre cross section near the inside diameter of the tyres. A particularly troublesome aspect is to ensure full coverage of the tension member by the urethane foam.
Preblended liquid urethane components expand as they react and form urethane foam. Thus to completely fill the mold cavity with unfoamed material is extremely wasteful as a volume of foam equal to the degree of expansion would be forced out of the mold. However, when only enough material to form the tyre is introduced into the mold, the liquid congregates at the outside diameter of the mold, especially if the mold is being rotated. As a result, tension members near the inside of the mold are not immersed in the liquid material. Instead the tension members must be engulfed by the expanding urethane foam which is already in a state best described as semi-solid, so molding flaws result. The difficulty of covering the tensipn member by this expanding semi-solid foam is increased if the tension member is located near a wall of the mold. Thus this method is used.
US2185398 (A) describes a Multicellular rubber tyre. United Kingdom Patent No. 1,502,808 discloses a structural formation and a manufacturing process of cordless injection-molded pneumatic tyres made of an elastomer having a high modulus of elasticity for the purpose of simplifying and rendering less expensive the material structure of pneumatic tyres as well as for simplifying the related manufacturing technology.

U.S. Pat. No. 4,230,169 discloses a pneumatic tyre which is made of an elastic material without carcass reinforcement and which is provided with a crown portion and sidewalls. Each sidewall terminates in a bead, and the parts between the sidewalls and the crown portion contain an annular thickening or bulge whose strength in the circumferential direction is higher than the strength of adjacent parts, given the fact that the annular thickening or bulge is made of a fibrous material or steel cable.
The multicellular polyurethane tyres produced till now solve the basic problem of punctures of pneumatic rubber tyres but the performance is not as good as a pneumatic rubber tyres. The current tyres have more rolling resistance, needs more energy to drive and less shock absorbing than a pneumatic tyres.
DETAILED DESCRIPTION OF THE INVENTION
The object of the invention is to provide a tyre which avoids the above drawbacks of the prior art and to improve the performance in the area of comfort, ride quality and energy required to drive compared to currently available multicellular polyurethane tyres.
It is a further object of this invention to provide a cast tyre which retains satisfactory performance characteristics during the service life of the tyre.
Multi-cellular polyurethane tyre is a novel invention in the polyurethane industry. Multi-cellular polyurethane tyre is a solid tyre, cast in centrifugal casting process using metallic moulds. The manufacturing process of the same is described below.
Polyol with multiple hydroxyl functional groups is agitated in a container. Subsequent to the initial agitation a catalyst is added which increases the rate of the reaction with Isocyanate. Foam stabilizer, hardener, water and colour are added to the polyol. All the ingredients are thoroughly mixed. The polyol mix is loaded in the polyol tank of a PU processing/mixing machine and on the other side isocyanate is loaded in the isocyanate tank of same machine. The chemicals (polyol and Isocyanate) are stored at designated temperature to maintain viscosity. Calibration is

done to set a perfect delivery output of the metering pump. Both the isocyanate and polyol pressure and temperature are set after the completion of delivery calibration to achieve a good quality of PU. Both isocyanate and polyol are injected inside the mixing chamber, are mixed and poured through the pouring nozzle of the mixing head of the machine into the moulds (The mixing ratio is set from 0.4 to 1.1 depending on product specification).
The chemical mixture is poured to the rotating mould which rotates at 200 to 600 RPM depending upon product specification. A heating coil present under the rotating mould maintains the requisite temperature of the mould. Mould temperature is kept from 55°C to 65°C depending on the product. Mould release is sprayed inside the mould before PU casting which helps to remove the tyre from the mould. A Tension member Polyester/ nylon cord is also put inside the mould, which is engulfed by the polyurethane foam while the tyre is formed, to give a high grip quality of the tyre with the rim (Optional). Post pouring after defined time (2 to 6 minutes depending on type of tyre), the tyre is de-moulded and thus a polyurethane tyre is produced. De-moulded tyre is finished to remove PU flushes generated in rotating moulding process. Finished tyres are cured at room temperatures for 24 to 72 hours before they are ready for usage.
This technology/technique has the advantage of being simple and inexpensive, it is also easy to implement in the tyre of the invention. This invention for tyres, is particularly intended to all slow moving vehicles of the passenger type as well as industrial vehicles. The invention and its advantages will be readily understood in the light of the description and examples which follow.
The tyre of the present invention has the essential feature that the inner wall is provided, covered for all or part of at least one layer of polyurethane polymer foam (abbreviated as "PU"), said PU is based on diphenylmethane diisocyanate (abbreviated as "MDI") and a polyol having an ethylene oxide content greater than 50% by weight; these compounds are described in detail below. Polyurethane, in general, is a polyisocyanate reaction product (compound carrying at least two isocyanate groups -NCO) and a polyol (compound containing at least two alcohol functions -ROH), polyol generally used with its fixed alcohol functions either polyethers or on polyesters.

The PU foam of the tyre of the invention has the uniqueness of being based on a specific diisocyanate compound, in this case an MDI, which is combined with a polyol itself is specific, namely a polyether having high levels of ethylene oxide.
As mentioned earlier Polyurethanes are closely related to polyureas which are the products of the reaction of Isocyanates (-N=C=O) with active hydrogen compounds (R-OH). That means Polyurethanes tyres are the products of Isocyanate and Polyol. Important catalyst such as Foam Stabilizer, Hardener, Water and Colour are added to help processing the polymer and to change the properties of the polymer to achieve high resilience and low energy retention. There are various types of Polyurethane foams available and are used according to requirements.
Production flow
Premixing of raw materials
• Polyol system preparation Loading to chemical processing and molding machine
• MDI and
• Polyol system
Selection of premixing of polyol as per requirement of particular tyre, for continuous core
density of foam and better resilience
Setting parameters of ratio of different raw material as per product requirement.
• Critical parameters Temperature, pressure and mixing ration of MDI and Polyol
• Ratio can be anything between 0.40 to 1.1 depending on product requirement and inputs
• Specific gravity of the Tyre decide the amount of raw material feeding
Mold preparation
• As per tyre size and design
• Pre-heating, cleaning
• Mold release systems applied on the mold
• Tension member for reinforcement

Molding technology
• Centrifugal casting: mold is rotated between 200 - 600 rpm (depending on the size, specification of the tyre)
• Raw material is fed to the mold while in rotation to ensure smooth foam core and top skin (0.3 to 1.5 mm) depending on the tyre requirement.
• Mold stops after feeding of material is complete
Semi-finished tyres are taken out from the mold after 120 - 360 sec, depending on the type of product
Post molding operation
• Initial finishing is done
• Post Initial finishing operation if any
• cured for another 24 hrs to 72hrs for making the tyre ready to use
• Post cure finishing operation if any
The proper mixture is made from high pressure jet of isocyanate & polyol inside mixing chamber of moulding machine which is poured in the rotating mould. This ensures that the material is distributed very evenly across the whole tyre circumference, which means that the tyres will run very smoothly. Due to centrifugal force in the rotational method for manufacturing tyres creates a special integral skin foam structure in which the foam density increases towards the outside wall of the tyre. The skin thickness varies from 0.3 to 1.5 mm based on RPM of mould as per product requirement. While the microcellular PU foam is nearly solid at the outer circumference, it's even surface enables brilliant rolling properties. Due to its good aging and weathering resistance, it retains its properties in long term. This resistance does not deter even when it remains in one position under heavy load for an extended period of time and is compressed on one side.

The close knit cellular structure is consisting of three parts, i.e., Thick outer skin, Inner core and Cord
Thick outer skin: Though the foaming structure is uniform inside the tyre but due to the centrifugal force that comes from the rotating mould, the maximum pressure goes to the outer skin of the tyre that is why the outer skin becomes a thicker and harder as compared with the rest portion. The density of the chemical at the outer skin is much more than that of the inner core. This thick outer skin gives the strength to handle the friction and the difference of hardness from the inner core gives a good bounce of the tyre.
Inner core: The inner core has a lesser hardness and lesser density than that of the outer skin but with uniform foam structure. This difference gives the required bounce. A good quality of the inner core is responsible for the longer life of a tyre.
Cord: Generally 2 to 4 layers of cord are used. But as per the requirement the number of layers can be reduced or increased. It gives the strength of the tyre to get stuck with the cycle rim and also gives the elastic property. No cord is present for the small size trolley tyres, the cord is an optional parameter. It is used as per requirement.
A major cause of tyre failure is tread separation, which has forced recalls by major tyre makers over recent years. Polyurethane tyres are moulded from a single piece and thus are far less likely to separate or undergo blowout. The centrifugal method for manufacturing of tyres creates a special integral skin foam structure (0.3 to 1.5 mm skin thickness depending oh tyre application) in which the foam density increases towards the outside of the tyre. While the microcellular PU foam is nearly solid at the outer circumference, its even surface enables brilliant rolling properties. Uniform foam structure is achieved in high pressure jet mixing of isocyanate and polyol in mixing chamber of machine and also in centrifugal mould under rotation and can be seen in tyre cross-sectional view. It is a solid tyre and that makes the tyre different from the other pneumatic tyres. Thus it protects the inner core and maintains the strength equally.

Longer life of a tyre depends upon the quality of the PU foam. Following quality of standard is maintained at the time of production.
• The nylon cord that is present inside the tyre should be correct as it creates proper grip with the metallic rim (otherwise tyre will get removed from metallic rim).
• The mixing ratio of Isocyanate & Polyol should be correct otherwise performance of tyre will get affected.
• The chemical temperature and the mold temperature should be as per standard to maintain to achieve good quality of tyre.
• Mold needs to be clean on regular basis to get a perfect pattern of a tyre as designed. Perfect pattern causes a good gripping quality with floor surface.
• The hardness on the top of the pattern and density of the tyre should be perfect to get a
good performance.
Thus longer life of tyres is achieved.
The invented tyre is highly resilent. The resilience is tested by various methods:-
1. PU sheet - Ball drop test: A foam sheet is made of same specification as used in a tyre. The equipment is made of glass tube and a steel ball, the sheet is kept under the glass tube and the steel ball is drop from a specified height and the rebound height of the steel ball is recorded and put as % of drop height.
2. Tyre is fitted on the rim and dropped on a flat surface from a specified height and the rebound height of the tyre is recorded and put as % of drop height. This is very much depended on the quality of the rim / wheel where as the test one is more of a laboratory test. The next test is more practical where dependence on rim / wheel is much lesser.
3. Tyre - Ball drop test: The equipment, made of glass tube and a steel ball, the tyre is kept under the glass tube and the steel ball is drop from a specified height and the rebound height of the steel ball is recorded and put as % of drop height.
Test and measuring Power to drive
The test rig to run a tyre is made with
■ One roller, which is the track
■ Drive, motor + gear + power unit, with power consumption recording equipment
■ Wheel fixing area,
■ Loading mechanism of the wheel
The whole set up is run without a tyre to measure the power / energy consumption at no load. The current and voltage is also recorded.

Assuming as P1
A tyre fitted on an assembled wheel is fitted on the rig and specifies load is applied on the tyre. The rig is started and ran for specified time and specified speed at predecided load. The power consumption meter will give energy consumption data. The current and voltage data is also recorded.
Assuming the value is P2
So the power / energy required P = P2 - P1
Different set of data can be recorded by
Varying speed keeping load constant,
Varying load keeping speed constant,
Varying speed and load,
The same test conducted on the New PU tyre and old PU tyre which can be compared and also this can be compared with Pneumatic tyres.
Resilience / rebound
Pneumatic: 50 - 60% depending on the air pressure
Prior art PU tyres: 20 - 25%
New invention: 40 - 52%
Foam structure
• Continuous homogenous foam structure with top skin, with more closed cells in
honeycomb foam structure
o Top skin having a very high abrasion resistance giving longer life. Even after top sking rubs off, tyre continues till the tread wears off.
o Continuous foam with closed cell in honeycomb structure allows to take contact pressure / sudden shocks to be distributed in all direction, where as prior art PU tyre the shock in more in direct direction on the pressure applied.
• Softer high resilience foam
o Better shock absorbing capabilities, more comfortable ride
o High resilience gives more reaction force than prior art PU tyres, resulting in lesser energy requirement to drive

o Lower hysteresis loss as more energy is transferred back, causing lesser heating of the tyre, allowing greater life and performance
The new tyre according to this invention has the following characteristics and advantages:
1. This new multicellular polyurethane tyre has an almost uniform foam structure in the tyre except a thick skin to protect the inner foam and a longer life of the tyre. This special polyurethane foam is with very high resilience and with very low energy retention compared to prior art multicellular polyurethane tyres, resulting in lower energy requirement to drive.
2. The tyre is made by special foam uniform structure with close knit cellular uniform structure giving it better performance like more of a single unit filled with air.
3. When a force / pressure is on the contact point, the close knit cell structure takes that force / pressure and distribute to all the cells in different directions.
4. This reduces the direct force / shock to the vehicle giving lesser shock and a more comfortable ride.
5. This distribution of pressure / energy to a wider area of the tyre (but not quite as an air tyre but a definite noticeable improvement from current products available), it retains much lesser energy than the current PU foam tyres.
6. The overall ride quality is comfortable.
7. The definite advantage over a pneumatic tyre is the user does not have to pump air in the tyre and there is no chance of punctures.
The manufacture of the PU foam is done in a manner known to those skilled in the art by reacting the mixture MDI / polyol (and other additives) in the presence of a blowing agent or foaming agent, or an agent precursor foaming.

CLAIMS
1. A high performance multicellular polyurethane tyre, wherein, the tyre is made of polyurethane foam which has a close knit uniform cellular structure consisting of three parts, viz, outer skin, inner core and cord.
2. A tyre as claimed in claim 1, which has a special integral foam structure in which the foam density increases towards the outside wall of the tyre.
3. A tyre as claimed in claim 1 and 2, wherein the polyurethane foam is microcellular and nearly solid at the outer circumference with a thick outer skin selectively from 0.3 to 1.5mm depending on the requirement, giving it enough strength to handle friction with its even surface, enabling brilliant rolling properties and difference in hardness from the inner core, giving a good bouncing effect to the tyre.
4. A tyre as claimed in claim 1 to 3, wherein the polyurethane foam is obtained from an isocyanate such as diphenylmethane diisocyanate (MDI) and a polyol.
5. A tyre as claimed in claim 1 to 4, wherein high performance means and includes very high resilence, very low energy retention, shock reducing, puncture proof, comfortable ride quality and long lasting.
6. A process for manufacturing high performance multicellular polyurethane tyre comprising casting the tyre in a centrifugal casting method from an isocyanate and a polyol in a metallic mould in presence of catalyst and other chemicals and under reaction conditions such as herein described creating a special integral foam structure of 0.3 to 1.5 mm skin thickness and in which the foam density increases towards the outside wall of the tyre.
7. A process as claimed in claim 6, wherein, the foam structure is microcellular and uniform which is achieved by high pressure jet mixing of isocyanate and polyol in a mixing chamber of machine and also in centrifugal mould under rotation.