EP1131215A1

EP1131215A1 - Tyres

Info

Publication number: EP1131215A1
Application number: EP99957377A
Authority: EP
Inventors: John Alexander Kennedy; William Arthur Brady
Original assignee: Individual
Current assignee: Kairfree Ltd
Priority date: 1998-11-21
Filing date: 1999-11-22
Publication date: 2001-09-12
Also published as: WO2000030873A1; HK1040071A1; CA2351826A1; AU1510100A; CN1367741A

Abstract

A tyre core (10; 20; 30) comprises a copolymer of ethene with a vinyl acetate. The core is for location between a wheel rim and a tyre casing. The core may replace a pneumatic tube, or may be provided in conjunction with a pneumatic tube.

Description

TYRES

The present invention relates to tyres, and in particular but not exclusively to tyres for vehicles such as cycles, wheelchairs and the like. One aspect of the invention relates to a tyre core intended to replace the inner tube of conventional pneumatic tyres, and to the composition and method of manufacture thereof. Other aspects of the invention relate to other features and elements of tyre construction, composition and fitting.

Pneumatic tyres provide excellent load support and shock absorbing properties in many applications. For example, many vehicles, such as automobiles, aircraft, military vehicles, motorcycles, bicycles, wheelchairs and the like are provided with pneumatic tyres. Pneumatic tyres generally comprise an outer tyre casing and an inner tube which is located between the wheel rim and the tyre casing and which is inflated with gas, typically air, to fill the rim-tyre casing space; the inflated inner tube provides the means for pneumatic support of the vehicle and retains the tyre casing on the rim. Other tyres, known as "tubeless" tyres, comprise a gas impermeable tyre casing the bead of which is sealed to the wheel rim. Although such tubeless tyres are now used in the great majority of automobile applications, such tyres are unsuitable for application in which the wheel rim is not readily sealable, for example, in spoked wheels as commonly used on bicycles. Although pneumatic tyres provide excellent support and cushioning they have some drawbacks . For example, the tyre material must have a low permeability to the gas contained therein but remain flexible to maintain the cushioning effect of the tyre. The number of materials which fulfil these requirements tends to be rather limited. Such materials include butyl rubbers and halogenated butyl rubbers. These materials are expensive compared to conventional rubbers, which do not possess the required degree of gas impermeability. The valves of such inner tyres are also required to seal sufficiently well so that leakage is kept to a minimum.

Although the abovementioned drawbacks may be overcome by use of particular rubber materials and well engineered valves, a more fundamental problem exists with respect to failure of such gas inflated tyres. For example, sudden tyre failure, commonly known as a "blow-out", may pose a serious danger to the occupants of a vehicle. Tyre failure may be a result of a weak spot in the tyre material, or puncturing by, for example discarded nails or screws, shards of glass or metal, or thorns and the like, as often found on the ground over which vehicle tyres travel. A further drawback of gas-filled tyres occurs when a slow leak exists, either from a small area puncture or through a poorly sealed valve. Such slow punctures, or more particularly the subsequent loss of pressure, result in an increased contact area of the tyre with the ground or contact surface, uneven and increased tyre wear, and increased rolling resistance, and may adversely affect vehicle handling.

Previously, the abovementioned disadvantages were addressed by use of various forms of puncture resistant liners which, in the case of inner tube containing tyres, seek to prevent objects passing through the tyre casing from penetrating the inner tube, or by use of solid or semi-solid foam tyres or tyre cores.

A major drawback, however, of certain foam filled tyres or cores is that the foam is readily flexed or deformed. In certain materials, this may result in excessive heat build-up in the tyre in use, which may lead to a breakdown of the foam material resulting in reduced support, increased rolling contact and subsequent tyre damage. A certain degree of flexibility however is necessary to provide the desired cushioning properties as provided by the conventional gas- filled pneumatic tyre. Harder, or less flexible tyre filling materials, such as the non- foam material of Gomberg (US Reissue 29,890) are claimed to have reduced heat build-up properties. However, a reduction in cushioning may occur, and such tyres may be relatively expensive as more material is required than those employing foam cores. Moreover harder materials have the disadvantage of causing excessive vibration within the wheel which can loosen screws, spokes and the like, and radically reduce wheel bearing life, thus significantly increasing the wear and tear on such wheels and the vehicles housing them. A further major drawback of many "puncture-proof " tyres is that many such tyres are constructed integrally with the wheel, and thus specially produced, or adapted, wheel rims are required. Specialist equipment is also necessary for constructing and fixing such puncture-proof tyres on the wheel rims. Furthermore, integral tyre-wheel constructions have the disadvantage that, should damage to the tyre occur, complete removal and replacement of the wheel would be required. US 4,416,844, US 4, 683,929 and US Re 29,890 describe foam- filled deflation-proof pneumatic tyres which are produced by injecting the filling material into the outer tyre casing and wheel rim cavity, to create an integral arrangement as described above . US 3,348,597 describes a polyurethane foamed rubber tyre which is formed directly on the axle. US 3,987,832 describes a solid wheel which comprises a hard radially extending core enclosing a hub sleeve. US 4,033,395 describes a tyre wherein the outer tyre casing and core are integral.

It is further noted that many known foam cores have a skin covering the surface of the core which is intended to reduce abrasion and damage to the core. In EP 0057917 Al the skin also protects the open-cell core from filling with water in wet conditions. This document describes the core as being 10% larger in cross section than the tube cavity in which it is housed, it is believed this would make the fitting of such a tyre difficult. Furthermore, since such a core is not circumferentially symmetrical - having a portion for extending into the rim and engaging the spoke ends - orientation of such a core on fitting to a wheel would be necessary, which may increase the fitting difficulty.

A number of polyurethane "puncture-proof " tyres are currently available. This material has the considerable disadvantage of producing toxic emissions of isocyanates and dense black smoke when ignited, thus posing environmental problems on disposal.

Other problems associated with polyurethane and rubber materials include, for a non-pneumatic tyre construction:

marked extreme vibration of the wheel; lack of grip of the tyre on the contact surface; - occurrence of flat spots on the tyre due to abrasion during skidding of the tyre on the contact surface; considerable drag and high rolling resistance; static build-up which results in sparking, thus use in flame- free areas or where sparking is undesirable would be prohibited; slippage from wheel rim when side pressure is applied to the tyre, for example when a vehicle is cornering.

It is anticipated that a puncture-resistant inner core of similar polyurethane or rubber materials would encounter similar problems.

The above problems contribute to provide, in the case of vehicular application of such tyres, a poor ride quality. Safety may also be compromised. It is an object of embodiments of certain aspects of the present invention to obviate or mitigate the abovementioned disadvantages of gas-filled pneumatic tyres and existing so-called solid puncture or deflation-proof tyres described in the art. It is a particular object of one embodiment of the present invention to provide a tyre core or insert which may be used in place of or in combination with conventional inflatable inner tubes. It is intended that the advantages provided by conventional pneumatic inner tubes are retained, such as superior cushioning support, without risk of deflation to provide a reliable alternative or complement to pneumatic tyres.

According to a first aspect of the present invention, there is provided a composition for use in the manufacture of a tyre core, wherein said composition comprises a copolymer of ethene with a vinyl acetate. The copolyτner thus formed is known as an ethylene vinyl acetate copolymer

(hereinafter described as EVA) .

Preferably, the vinyl acetate is un-substituted.

It should be noted that a number of physical properties have been established for a material (an EVA) found to be suitable for use as a tyre core in accordance with an embodiment of the present invention. Some properties of this material which may be advantageous are represented as follows and the properties of other materials employed in tyre cores according to aspects of the present invention should preferably be within the ranges or values shown.

PROPERTY UNIT TEST METHOD VALUE. S)

HARDNESS (SHORE A) DEGREE ASTM D2240 30 to 75

TENSILE STRENGTH Kg/cm² ASTM D1623 25 to 30

ELONGATION % ASTM D1623 200 to 300

TEAR STRENGTH Kg/cm ASTM 1623 10 to 15

FOAMING RATE TIMES U.S.I. 2 to 5

ADHESION STRENGTH Kg/cm U.S.I 2.2 to 3.2

SHRINKAGE % ASTM D955 2 to 5

Most preferably, the properties are:

PROPERTY UNTT TEST METHOD VALUE (S.

HARDNESS (SHORE A) DEGREE ASTM D22 0 35 to 55

TENSILE STRENGTH Kg/cm² ASTM D1623 27 ELONGATION % ASTM D1623 250

TEAR STRENGTH Kg/cm ASTM 1623 11 FOAMING RATE TIMES U.S. I. 3 ADHESION STRENGTH Kg/cm U.S.I 2.8 SHRINKAGE % ASTM D955 3

It is believed that perhaps the most important of these properties is the hardness, due to the significant bearing on rider comfort, for cycle applications, vehicle handling and rolling resistance.

Certain EVA compositions have been found to possess such properties . EVA compositions are preferred because they are relatively light weight, that is of a lower density compared to existing polyurethane compositions. EVA tyre cores, tyre inserts and tyres have also been found to provide relatively low rolling resistance which, without

wishing to be bound by theory, is believed to be due to lower absorption of energy than polyurethane, as hysteresis losses are lower. This may also contribute to the superior low heating properties of EVA when flexed and deformed, unlike polyurethane foams which rapidly heat up in use.

EVA also provides the required hardness, while retaining the desirable flexural properties of tyre cores, for example memory properties, which ensure good shape recovery following removal of a deforming force. This ensures good replication of the desirable properties of conventional pneumatic tyres.

EVA also desirably does not, in general, contain ozone depleting substances nor heavy metals, thus conferring additional environmental benefits in EVA product manufacture or disposal .

In other aspects of the invention, materials other than EVA may be utilised to form tyre cores having such desirable properties. For example, it is anticipated that formulations of ethylene-propylene rubbers (EPR and EPDM) containing low polypropylene (in the region of 15%) , and also Hytel rubber may be utilised.

The amount of vinyl acetate monomer incorporated into the preferred EVA copolymer in relation to the ethene content by weight (w/w) may be between 30% and 60%.

Preferably the amount of vinyl acetate is between 30% and

50%, more preferably between 30% and 40%. A most preferred amount of vinyl acetate is 33%.

Advantageously, said EVA may be cross-linked EVA, wherein the cross -linking is achieved by any suitable method. The composition may be in the form of a foam. Foaming may be achieved by any suitable method in the art, for example mechanical agitation or chemical means . The foam is preferably of closed cell construct, the closed cell construct offering the advantage that liquid, typically water, will not be taken up by the tyre, and an outer "skin" is not required. In other embodiments a "skin" may be desirable, for example to protect the core from abrasion or to facilitate fitting, and may be formed as a consequence of the manufacturing or forming method. The composition may contain one or more agents independently selected from the following classes: lubricants, cross -linking agents, foaming agents and fillers. It is to be understood that more than one agent may be selected from each of the above classes. Any suitable lubricant may be used. Typical lubricants include long chain carboxylic acids, for example stearic acid.

Any suitable cross-linking agent commonly known in the art may be used, such as dicumyl peroxide (DCP) . Suitable foaming agents include those commonly known in the art such as azodicarbonamide (ADCA) .

Suitable fillers include any suitable calcium salt and those fillers commonly known in the art such as mica, silica, calcium carbonate, carbon black and clay.

A preferred formulation has the following constituents :

EVA 0 - 120 parts by weight (w/w) of the total formulation

Stearic Acid 1 - 5 parts (w/w

DCP 1 - 5 parts (w/w

ADCA 2 - 6 parts (w/w

Calcium carbonate 30 - 60 parts (w/w

Preferably the amounts of each component are

EVA 100 part ^■.s (w/w

Stearic Acid 1 part (w/w

DCP 1 part (w/w

ADCA 2 parts (w/w

Calcium carbonate 40 - 50 parts (w/w

According to a second aspect of the present invention there is provided a tyre core or insert having a Shore (A) hardness of 30° - 70°, preferably 35° - 60°. The most preferred Shore hardness will depend on the intended use of the core, for example most preferably 38° to 40° for a leisure and commuting cycle, and 48° to 50° for a wheelchair.

The tyre core may be formed for fitting within a tyre casing, or may be formed within a tyre casing, or the tyre casing formed around the core, or the casing and tyre core formed simultaneously.

According to a third aspect of the present invention there is provided a tyre produced using the composition according to the first aspect.

It is to be understood that the tyre may be of the type which is moulded as a unit piece containing a core made using the composition of the first aspect and an outer, abrasion resistant skin or casing. Alternatively and preferably, the composition may be used to make a tyre core which may be inserted into a conventional pneumatic tyre casing in place of or in combination with a pneumatic inner tube. Such a core may be any suitable shape, including a torus shaped core. Such a torus shaped core has a circular cross-section, however other cross-sectional shapes may be desirable in certain circumstances, such as half -moon or crescent shapes, where a pneumatic tube is to be retained. The core may have a smooth, irregular or discontinuous surface or profile; a smooth surface may facilitate fitting while a rougher surface may facilitate grip between the tyre elements and prevent or limit relative movement therebetween. The characteristics of different areas or sections of the core may differ, for example the core may include a relatively stiff or hard central element, to minimise rolling resistance, and softer side portions, to provide greater grip when cornering, or a similar effect may be possible by providing a triangular core profile. The core may also be configured such that by reversing the core orientation within the tyre casing it is possible to provide the tyre with different characteristics; each half of the core section may be formed from a material having different properties, for example a relatively soft material and a relatively hard material. The core may also be configured to facilitate contact with the spoke nuts within the rim, to support the nuts and also to assist in preventing the nuts vibrating loose from the spokes. This may be achieved by providing the core with an appropriate profile, or by forming the inner surface of the core in a softer material which will readily deform to extend into the rim. The core may contain one or more cavities of gas such as air, or such cavities may be filled with gels, foams or other materials possessing particular desirable properties. For example a torus shaped core having a central cavity may be provided, resulting in a tubular configuration.

The core may not necessarily fill the entire wheel rim - tyre casing cavity, for example a pneumatic tube or element may be provided, or separate elements may be provided to engage and support the spoke nuts . Where a core is used in conjunction with a pneumatic inner tube it may be preferred that the core is located directly beneath the tyre casing, with the inner tube sandwiched between the wheel rim and the core. Tyres of this arrangement have been found to retain the desirable properties of solely pneumatic tyres while being effectively puncture proof. This mode of construction may be particularly applicable to half-moon or crescent shaped cores.

According to a fourth aspect of the present invention there is provided a method of producing a tyre core, the method comprising the steps of : providing an EVA composition; mixing said composition; moulding said mixed composition under pressure; and shaping said moulded composition to form a tyre core. It is to be understood that any suitable mixing method may be used and the components of the EVA composition may be mixed in any suitable order. A preferred mixing apparatus is a two roll mill which is commonly known in the art. Preferably the EVA composition is introduced into the mill in the order of: EVA, lubricant, cross-linking agent, foaming agent, filler.

The roll mixing may be conducted at any suitable temperature conveniently provided by controlling the temperature of the rolls. It has been conveniently found that good mixing is achieved with one roll set at a temperature in the region of 100°C and the other roll at the

lower temperature of about 60°C.

Moulding may be achieved using any suitable method. Conveniently this is achieved by injection moulding. The moulded composition, once removed from the mould may then be shaped using any suitable method to form a tyre core. For example pieces of the moulded composition may be cut to form rods of the composition. The offcuts may be used for other purposes, for example for location in a tyre rim adjacent to the spoke nuts. The ends of the rods may then be joined to form the tyre core. Joining may be accomplished using adhesive applied to each end of the rod. The rod ends are preferably flat cut, however other profiles suitable for bonding may be used. In an alternative embodiment, the ends of the rods remain free, to facilitate fitting; this may allow a rod to be fitted to, for example, a bicycle wheel without having to remove the wheel from the bicycle.

It should be understood that one or more of the above process steps may be carried out by utilizing an extrusion technique . For example a screw extruder may be used to mix the composition prior to injection moulding thereof. According to a fifth aspect of the present invention there is provided an apparatus for forming a puncture resistant tyre core comprising means for shaping an EVA composition into a rod configuration.

This may be achieved by using a suitably arranged cutting head. Conveniently the surface of such a tyre core is relatively rough and therefore grips the wheel rim and inside tyre surface, thus reducing slippage and disengagement of the tyre from the wheel.

According to a further aspect of the present invention there is provided a tyre core for location in the volume defined between a tyre casing and a wheel rim, the tyre core comprising a resilient rod having free ends, the rod length being selected to conform to the length of the volume between the casing and rim.

The invention also relates to a method of fitting such a core to a tyre.

As noted above, fitting of the core may be facilitated by providing the core in rod form. Also, stock keeping, and production of tyre cores may be simplified, as a retailer may be supplied with long lengths of rod which may be cut to size in accordance with individual customers' requirements. Such tyre cores also facilitate accommodation of variations in rim and tyre casing dimensions; rims and tyre casings tend to be provided in a range of "standard" diameters and widths, however different manufacturers tend to produce different rim and tyre configurations, and the ends of tyre cores in accordance with this aspect of the invention may be trimmed or shortened accordingly to provide a "perfect" fit.

The various aspects of the present invention will now be described by way of example, with reference to the accompanying drawings, in which: Figure 1 is a sectional view of a tyre core in accordance with an embodiment of an aspect of the present invention;

Figure 2 is a sectional view of a tyre core in accordance with a further embodiment of the present invention; and

Figure 3 is a sectional view of a tyre core in accordance with a still further embodiment of the present invention . Reference is first made to Figure 1 of the drawings, which illustrates a section of a tyre core 10 in accordance with an embodiment of the present invention. The core 10 is circular in cross section and is intended to replace a conventional pneumatic inner tube, that is the core 10 is located between a tyre rim and the tyre casing. The core is of an EVA foam composition, as will be described in due course .

Figure 2 of the drawings illustrates a section of an EVA tyre core or insert 20 in accordance with a further embodiment of the present invention. The core 20 is part- circular in section and is intended to be inserted in the crown of a tyre, between an existing pneumatic tube and tyre casing. It has been found that, when the tube is inflated to its normal recommended pressure, as used in the absence of the core 20, the tyre retains the desirable properties of a solely pneumatic tyre, but is effectively puncture proof. It has also been found that the core 20 is easier to fit than the core 10 described above. Figure 3 of the drawings illustrates a section of an

EVA tyre core 30 in accordance with a still further embodiment of the present invention. The core 30 is rectangular in section and is intended to be inserted in the crown of a relatively small diameter "balloon" tyre, such as utilised in electrically-powered wheelchairs, between an existing pneumatic tube and tyre casing. As with the core 20 described above, it has been found that, when the tube is inflated to its normal recommended pressure, as used in the absence of the core 30, the tyre retains the desirable properties of a solely pneumatic tyre, but is effectively puncture proof. Surprisingly, it has been found that the rectangular form of the core 30 does not have an adverse effect on the tyre performance; it appears that the properties of the preferred EVA composition, as described below, are such that the core 30 may deform to fill the tyre casing, and in doing so does not lose the advantageous properties of the composition. The manufacture and testing of the core 10 will now be described in some detail, and it will be apparent to those of skill in the art that the manufacturing process may be adapted as required in order to produce the other cores 20, 30 described above.

Mixing of formulation

100 parts (w/w) of ethylene vinyl acetate copolymer (available under the tradename EVATHENE VE 630) having a vinyl acetate monomer unit content of 33% w/w compared to

the ethylene monomer content was added to a two roll mill. Stearic acid 1 part (w/w) , DCP 1 part (w/w) , ADCA 2 parts (w/w) and calcium carbonate 40 - 50 parts (w/w) were then added to the mill in that order.

The rolls of the mill were set at temperature of 100°C

and 60 °C. Once mixed, the formulation was then removed from the mill and then introduced into a 6 ounce - 150 ton injection moulding machine.

Moulding of formulation

Injection moulding was conducted using a mould at 170°C

and an injection pressure of 700 kg cm^"2. Following injection, the material contained within the mould was cooled by cooling the mould with water. The cooling water had a temperature of 20°C on input to the mould. The

cylinder temperatures of the injection moulding machine were 140 °C and 180 °C. The moulding timing was as follows:

Injection: 10 to 20 seconds

Cooling: 40 to 90 seconds

Mould open and close: 20 seconds

Moulding Cycle therefor: 70 to 130 seconds.

The resultant moulded EVA composition was removed from the mould. The dimensions of the sheet depended on the mould used, but typically were 40 - 41" x 64 - 65" x 40 mm and 42 - 43" x 65 - 66" x 50 mm.

The moulded EVA composition, following removal from the mould, was then cut into strips using a band saw. The strips were then fed into a cutting machine to produce cylindrical rods.

Various diameters of rod could be made by using different cutting heads in the cutting machine. Typical sizes of rod have the following diameters: 1 . 75 " - 1 . 95 " ; 2 . 0 " - 2 . 10 " ; 1 . 25 " - 1 . 375 " .

These three size ranges are suitable for insertion into a multitude of outer tyre casing sizes.

A rod was then cut to the required length for the particular wheel to which it was to be fitted, and the ends glued together using impact glue containing Tolvene and Polene R.C. to form a torus shape. The torus could then be inserted directly into a conventional wheel, for example a bicycle wheel in place of the inner tube therein.

The tyre core had a shore hardness of 50° (ASTM D2240) .

Details of testing of such a tyre core are set out below.

Example 1

A company based on a large site uses a fleet of 24 bicycles to facilitate movement of personnel around the site. Half of the fleet of bicycles were fitted with tyre cores 10 as described above and used over a 12 month trial period. Users found that the 12 test cycles performed in a similar manner to the bicycles fitted with conventional pneumatic tubes, providing comparable rider comfort, handling capabilities and tyre casing wear, and of course did not experience any punctures or other tyre failures. At the end of the trial period the company selected to abandon pneumatic tubes and fitted the tyre cores 10 to the remainder of their fleet of bicycles.

Example 2 A wheel builder, who builds wheels by hand for cycles and wheelchairs, was supplied with a number of 26" tyre cores 10. Over a six month period cycles fitted with the tyre core were ridden over a variety of terrain, giving a comfortable ride, and exhibiting no problems. In particular, it was noted that no tyres were forced off the rim, even under extreme pressure and stress, in contrast to the behaviour of existing "solid" polyurethane tyres. Further, the wheels to which the cores were fitted experienced no adverse effects, notably no hub damage, no loosening of spokes, and no effect on the trueness of the wheels .

Example 3

A number of tyre cores 10 were supplied to a cycle road racing team, who carried out a number of tests and comparisons with existing products, as described below.

A rolling resistance test was carried out at Quarry Hill, Stanton by Dale, Derbyshire, England.

From a standing start, at a fixed point, the exercise was carried out to find and compare the distance travelled, the maximum speed gained and the average speed, using a normal leisure "mountain bike" cycle and measured by Cateye (trade mark) Computer.

The reasoning for using Quarry Hill was because of its long "straight" when it flattened out. The weather and road conditions were good.

Comparisons were made between:

1. A normal Continental (trade mark) "Cross Country" tyre with pneumatic inner tube @ 50 lbs psi.

2. A normal Continental "Cross Country" tyre with a tyre insert or core, as the core 10, in accordance with an embodiment of the invention.

3. A Green (trade mark) tyre (a "solid" polyurethane tyre) .

The same cycle and rider, whose weight was 12 st 2 lbs, was used throughout the test to maintain continuity and therefore giving no advantage to any one product .

RESULTS

Normal Continental Cross Country Tyre And Pneumatic Tube Size - 26 x 1.90

Average speed 15.3 mph Maximum speed 28.5 mph

Distance travelled 0.68 miles Tyre Insert Inside A Normal Continental Cross Country Tyre - Size 26 x 1.90

Average speed 14.0 mph

Maximum speed 23.0 mph Distance travelled 0.58 miles

Green Tyre - Size 26 x 1.90

Average speed 12.4 mph

Maximum speed 19.0 mph

Distance travelled 0.46 miles

A rolling road test was conducted, in conjunction with Sturmey Archer, Nottingham, England using their test equipment .

The tyre insert was fitted to a 26" ATB wheel, with a payload of 72.5 kg and run continuously for 100 hours, at a continuous speed of 15 mph therefore covering 1,500 miles non-stop .

Throughout the test the wheel was monitored for friction or excessive heating, which was not over-apparent throughout.

On completion, the wheel was taken apart and the tyre and tyre insert were examined for damage.

There was visible wear to the tyre but no more than would normally be expected after 1,500 miles. The tyre insert showed signs of wear as follows :

Dust particles were present which indicated the insert had settled into the wheel rim. Again, this was to be expected and would not be so prominent under normal conditions. It will be apparent to those of skill in the art that the tyre cores demonstrated excellent roll, resilience and cushioning properties, until now only associated with conventional pneumatic tyres. In particular there was a lack of vibration with minimum friction and hence road drag. The wheels remained light-weight and the tyres demonstrated full memory capabilities. Furthermore, the tyres did not absorb water in wet conditions and exhibited good road holding capabilities. The tyres also did not generate static electricity thus enabling use in flammable vapour areas or where sparks\electric discharges are prohibited. These road tests demonstrate that the required degree of hardness and resilience in a foam tyre may be achieved while maintaining the desirable flexibility and hence cushioning effects, until now only seen in pneumatic tyres, thus demonstrating superiority over existing polyurethane foam tyres .

It should be understood that the above examples are not limiting on the scope of the present invention and variations within the scope are allowed. For example the hardness of the final composition may be chosen for particular applications such that simulation of differing conventional tyre pressures is achieved. For example a shore hardness of 48 to 50° has been found to be particularly suitable for wheelchair use, whereas a shore hardness of 38 to 40° has been found to be particularly suitable for leisure and commuting bicycles.

Alternative methods of moulding the EVA composition may be chosen without departing from the scope of the present invention. For example tube-shaped moulds which give a rod shape of composition, or torus shapes, may be used. Moulding may also be accomplished by extruding the composition through a suitably sized die orifice.

Claims

1. A tyre core comprising a copolymer of ethene with a vinyl acetate, the copolymer thus formed being an ethylene vinyl acetate copolymer (EVA) .

2. The tyre core of claim 1, wherein the vinyl acetate is unsubstituted.

3. The tyre core of claim 1 or 2 , wherein the amount of vinyl acetate monomer incorporated into the copolymer in relation to the ethene content by weight (w/w) is between 30% and 60%.

4. The tyre core of claim 3, wherein the amount of vinyl acetate monomer incorporated into the copolymer in relation to the ethene content by weight (w/w) is between 30% and 50%.

5. The tyre core of claim 4, wherein the amount of vinyl acetate monomer incorporated into the copolymer in relation to the ethene content by weight (w/w) is between 30% and 40%.

6. The tyre core of claim 5, wherein the amount of vinyl acetate monomer incorporated into the copolymer in relation to the ethene content by weight (w/w) is 33%.

7. The tyre core of any of the preceding claims, wherein the copolymer is cross-linked EVA.

8. The tyre core of any of the preceding claims, wherein the composition is in the form of a foam.

9. The tyre core of claim 8, wherein the foam is preferably of closed cell construct.

10. The tyre core of any of the preceding claims, wherein the core composition contains one or more agents selected from the following classes: lubricants, cross-linking agents, foaming agents and fillers.

11. The tyre core of claim 10, wherein the composition contains a lubricant including long chain carboxylic acids.

12. The tyre core of claim 11, wherein the composition includes a steric acid lubricant.

13. The tyre core of any of claims 10 to 12, wherein the composition contains a dicumyl peroxide (DCP) cross- linking agent .

14. The tyre core of any of claims 10 to 13, wherein the composition contains an azodicarbonamide (ADCA) foaming agent .

15. The tyre core of any of claims 10 to 14, wherein the composition contains a calcium salt filler.

16. The tyre core of any of the preceding claims having the following formulation:

EVA 80 - 120 parts by weight

(w/w) of the total formulation

Stearic Acid 1 - 5 parts (w/w) DCP 1 - 5 parts (w/w)

ADCA 2 - 6 parts (w/w)

Calcium carbonate 30 - 60 parts (w/w) .

17. The tyre core of claim 16, having the following formulation:

EVA 100 parts (w/w)

Stearic Acid 1 part (w/w)

DCP 1 part (w/w)

ADCA 2 parts (w/w)

Calcium carbonate 40 - 50 parts (w/w) .

18. The tyre core of any of the preceding claims, wherein the hardness (Shore A) of the core is between 30° and 75°.

19. The tyre core of claim 18, wherein the Shore (A) hardness of the core is between 35° and 55°.

20. The tyre core of any of the preceding claims, for use in a cycle, having a Shore (A) hardness of 38° to 40°.

21. The tyre core of any of claims 1 to 19, for use in a wheelchair, having a Shore (A) hardness of 48° to 50°.

22. The tyre core of any of the preceding claims, wherein the core is torus shaped.

23. The tyre core of any of claims 1 to 21, wherein the core is part circular in section.

24. The tyre core of any of claims 1 to 21, wherein the core is rectangular in section.

25. The tyre core of any of the preceding claims, wherein the tyre core comprises a resilient rod having free ends.

26. A method of forming a tyre, the method comprising: providing a wheel rim; providing a tyre casing; locating the casing on the rim; and locating a tyre core in accordance with any one of the preceding claims between the rim and the casing.

27. The method of claim 26, further comprising: locating a pneumatic tube between the tyre core and the wheel rim.

28. A method of producing a tyre core, the method comprising the steps of : providing an EVA composition; mixing said composition; moulding said mixed composition under pressure; and shaping said moulded composition to form a tyre core.

29. A tyre core having a Shore (A) hardness of 30° - 70°.

30. The tyre core of claim 29, having a Shore (A) hardness of 35° - 60°.

31. The tyre core of claim 30, for use with a cycle, having a Shore (A) hardness of 38° to 40°.

32. The tyre core of claim 31, for use with a wheelchair, having a Shore (A) hardness of 48° to 50°.

33. A tyre core for location in the volume defined between a tyre casing and a wheel rim, the tyre core comprising a resilient rod having free ends, the rod length being selected to conform to the length of the volume between the casing and rim.