The present invention relates to a ball for use in playing games, especially to a pressureless ball, such as a pressureless tennis ball .

Conventional tennis balls as well as many other balls for playing games, have a wall or shell of elasto¬ meric material which is usually a composition of rubber or rubber-like materials. The wall forms a hollow sphere or core defining a cavity which is filled with gas, usu¬ ally air, at a required pressure. In case the cavity contains a gas at a pressure above that of the atmosphere, the ball is called a pres¬ surized ball. In case the cavity contains a gas having substantially the same pressure as that of the atmosphe¬ re, the ball is called a pressureless ball. Both the above types of tennis balls can be made to meet the requirements of the International Lawn Tennis Federation as regards diameter, weight, rebound and de¬ formation. In addition to meeting these requirements, a tennis ball should have a long life as well as good and consistent play behaviour.

Pressurized balls have a relatively short life as a result of the gas permeating through the wall of rubber or rubber-like materials. On the other hand, pressure¬ less balls have a substantially longer life but are dif- ficult to produce with the desired play behaviour.

There is therefore a need for an improved ball, especially a tennis ball, which will address these pro¬ blems.

According to the invention, there is provided a ball having a wall of elastomeric material defining a cavity that contains a filling which comprises a plura¬ lity of substantially closed cells.

By introducing such a filling in the cavity of the ball, the deformation behaviour, and especially the dy-

namic deformation behaviour of the ball may be control¬ led, such that the play behaviour of the ball may be ea¬ sily determined as desired.

It should be noted that the deformation behaviour of a pressurized ball to a large extent depends on the pressurized gas in the cavity, while the deformation be¬ haviour of a conventional pressureless ball is mainly due to the characteristics of the elastomeric material forming the wall of the ball. Thus, the force needed to compress the pressurized ball increases continuously, with increasing compres¬ sion, along a curve primarily determined by the elevated pressure in the cavity of the ball and increasing pro¬ gressively. In the conventional pressureless ball, the compression force follows an increasing curve which is more complex as a consequence of the major influence of the characteristics of the material of the elastomeric wall which also control the rebound of the ball.

By being able to modify both the characteristics of the elastomeric material forming the wall of the ball, and the characteristics of the filling comprising closed cells, the compression behaviour and, consequently, the play behaviour of the ball, especially the pressureless tennis ball, may easily be adapted to various require- ments.

Preferably, the cavity is substantially filled with the substantially closed cells in order that the comp¬ ression behaviour shall be affected. However, it is possible to let the substantially closed cells fill only a peripheral part of the cavity adjoining the inner side while some other material is used in the central part thereof, and still obtain a desired compression beha¬ viour.

In a preferred embodiment, the substantially closed cells are elastomeric in order to be able to adapt to the changing shape of the cavity during compression of

the ball. For the same reason, the substantially closed cells can be separate hollow bodies which may move rela¬ tive to each other during the change of shape of the ca¬ vity of the ball. Of course, neither do all the closed cells need to be separate hollow bodies nor do all the hollow bodies need to be separate.

Microspheres, especially polymeric microspheres are a preferred form of hollow bodies. They have the prefer- red elasticity and may be used in unexpanded as well as in expanded form. Further, microspheres have a low weight, such that they do not adversely affect the weight of the ball. In addition, such microspheres form an essentially leak-proof filling for the cavity of the ball.

Preferably, the expanded polymeric microspheres ha¬ ve a particle size in the range of from 15 μm to 150 μm, and more preferably from 30 μm to 100 μm. In a preferred arrangement, the filling has a weight of from 0.5 g to 4.0 g, preferably from 1.0 g to 3.0 g. Useful microsphe¬ res are available under the tradename EXPANCEL from Akzo Nobel, Sweden.

For ease of filling, preferably the microspheres are formed from an expandable material which may, for example, be provided in the form of a pellet or tablet of unexpanded material which is subsequently expanded in situ to substantially fill the cavity with expanded mic¬ rospheres.

In one embodiment of the invention, the unexpanded material is formed into a preform such as a pellet or tablet, inserted into the cavity and subsequently expan¬ ded to form the filling of microspheres.

The polymeric material may include a compatible binder material which may be selected from one or more of ethyl vinyl acetate and paraffin wax. The binder ma-

terial facilitates the forming of a preform of the unex¬ panded material.

The invention also provides a method for manufac¬ turing a ball having a cavity by forming a spherical shell of an elastomeric material and providing a filling in the cavity, said filling comprising a plurality of substantially closed cells, preferably microspheres.

The cavity may be filled with pre-expanded mic¬ rospheres. Alternatively, the filling is formed in situ in the cavity from unexpanded material. In this case, typically the unexpanded material is formed into a preform which is inserted into the cavity and subsequently converted in situ into polymeric microspheres. The preform may be a pellet or a tablet.

The microspheres may be polymeric microspheres. In one embodiment of the invention, the polymeric material includes a compatible binder such as ethyl vinyl acetate and/or paraffin wax. In one arrangement, the method comprises the steps of: inserting a preform of unexpanded material into at least one of a pair of hemispherical half shells; bonding the two half shells together to form the cavity; and expanding the material to form a filling of mic¬ rospheres which substantially fill the cavity.

Typically, the polymeric material is expanded by heat at a temperature from 80°C to 140°C. In an alternative arrangement, the method comprises the steps of: inserting a volume of expanded material between two hemispherical half shells of elastomeric material; and bonding the two half shells together to form the cavity.

The step of inserting a volume of expanded material in between the two hemispherical half shells of elasto¬ meric material may include first enclosing said volume of expanded material in a soft envelope. More precisely, unexpanded material may first be enclosed in a soft ma¬ terial and then expanded therein.

An embodiment of the invention, more specifically a tennis ball by way of example only, will be described in more detail such that the invention will be clearly un- derstood.

A pressurized tennis ball initially has good play¬ ing properties but the playability diminishes due to gas escape. This problem of loss of playing properties has been addressed by the present invention by filling the cavity with low weight microspheres, in this case poly¬ meric microspheres.

The polymeric microspheres used in the invention may be either pre-expanded or expanded in situ during the process of tennis ball manufacture. It has been found that the microspheres may be used in pure form or in combination with suitable polymeric binder materials.

The pre-expanded, unexpanded, polymer bound pre- expanded and polymer bound unexpanded materials are all referred to as the filling in this specification. The microspheres employed in the present invention are hollow thermoplastic particles having the following diameters: the pre-expanded microspheres have a particle size of 15-80 μm and unexpanded microspheres have a par¬ ticle size of 6-24 μm. However, the expanded particles may have a particle size from 10 to 150 μm, preferably in the range from 30 μm to 100 μm.

The microspheres contain a volatile liquid. The amount of this liquid is usually 5 to 50% by weight of the microsphere. The volatile liquid is conventionally a hydrocarbon, such as isobutan. It will be appreciated that the volatile liquid should be selected such that it

does not dissolve the thermoplastic material of the mic¬ rospheres.

When microspheres are used in the pure form, 0.5 g - 1.5 g is required to fill the core cavity. In the pre- sence of a binder, the total weight of the matrix is between 1.0 g and 3.0 g. A polymeric binder has been found to be useful during the manufacturing process of tennis balls. Suitable binder materials include ethyl vinyl alcohol (EVA) having a melting point of 95°C, and paraffin wax having a melting point of 50-52°C. The mic¬ rospheres are preferably mixed, in a separate process, with the polymeric binder material and formed into a preform such as pellets or tablets for simple dosing. A number of modifications may be made to the ball in order to accommodate the filling. For example, a lighter felt and/or different core weights may be used.

To manufacture tennis balls, various elastomeric materials are mixed with different chemical ingredients. The mixture is milled to a smooth consistency and fed into an extruder which forms the mixture into preforms. The elastomeric preforms are placed into a multicavity precision mould. Under pressure and heat, the preforms are formed into hemispheres each of which is one half of a tennis ball core. These halves are edge ground. The edges are then coated with adhesive in a double-carousel assembly unit.

Pellets containing pure or matrix bound unexpanded microspheres are added to half shells on one carousel. The half shells meet, one from each of the carousel units, one containing a pellet and the other being emp¬ ty.

The half shells adhesively tack together before en¬ tering a second cure process. This second cure perma¬ nently fuses the halves forming pressureless complete ball cores. The completed and resilient cores are con¬ veyed to grinding machines which abrade the surface of

the cores. A slightly rough surface permits a ball core to better retain adhesive and results in a good bond between ball and cover. Following grinding, the cores are coated with adhesive. The covers for the ball cores are cut from felt. The back of the rolls of felt are coated with adhesive in controlled quantities prior to cutting cover pieces therefrom. The cover pieces are edge coated with adhesi¬ ve which defines the seams of the tennis ball. From the cover operation, the tennis balls are moved to another press to undergo a third curing process. The application of heat and pressure in the third curing process assures a solid bond between cover and core. The time is also important both for the degree of cure and the degree of expansion of the microspheres. The microspheres should be fully expanded in this process and the cavity should be filled. The expansion temperatures range from 80°C to 140°C.

Removed from the curing press, the balls are then steam fluffed to raise the nap of the felt and dried to form finished tennis balls.

The tennis ball described above complies with the International Lawn Tennis Federation specification and has play properties at least as good as conventional tennis balls.

It will be appreciated that while the invention has been described with reference to tennis balls, it may also be possible to apply it to other play balls having a wall of elastomeric material defining a cavity, e.g. footballs, volleyballs, basketballs and squashballs.

Also, it is to be understood that modifications to the above described embodiment of the invention can be made by the person skilled in the art without departing from the spirit and scope of the invention. For example, other materials than microspheres may be used as long as

they possess substantially closed cells that -can affect the compression behaviour of the ball.