GB2042553A - Cross-linked cellular elastomers - Google Patents

Abstract

A cross-linked cellular elastomer is made from the reaction of one or more natural and/or synthetic rubbers, a cross-linking agent and a monomeric metallic salt of an ethylenically-unsaturated carboxylic acid and is expanded with a blowing agent. The metallic salt is preferably basic zinc methacrylate. The elastomer may be used to provide a core for a tennis-ball or a sponge for a table-tennis bat. The former use enables advantage to be taken of the properties hitherto only found in pressurised playballs, whilst the latter use leads to a marked increase in the speed of the ball when struck by the bat.

Description

SPECIFICATION Cellular elastomeric compositions This invention relates to cellular, elastomeric compositions and to the use of these compositions as components of items of sports equipment, in particular as components of playballs (especially tennisballs) and as components of sports strikingimplements (especially table-tennis bats).

More particularly, the present invention relates to a cross-linked, cellular, elastomeric composition which is the reaction product of a formulation including a natural rubber and/or one or more synthetic rubbers, a cross-linking agent and a monomeric metallic salt, the formulation being expanded by means of a blowing agent, and to the use of that composition in the production of a core for a tennis-ball and of a component of a table-tennis bat (hereinafter referred to as a "bat-sponge").

Currently-available tennis-balls are provided with hollow cores. These cores may either contain air at atmospheric pressure (in which case they are referred to as "pressureless cores") or they may contain air and/or an inert gas at a pressure of from 7 p.s.i. to 12 p.s.i. above atmospheric pressure (when they are referred to as "pressurised cores").

Authorities such as the International Tennis Federation (I.T.F.) lay down fairly stringent specifica tionsfortennis-ballswhich are to be used in "officially-recognised" play; these specifications define the weight, size, compressibility and rebound properties of the balls. "Pressurised" balls, as currently available, generally satisfy the l.T.F. specifications, but suffer from certain disadvantages in manufacture. First, arrangements must be made to ensure that such balls will retain their internal pressure both in storage and in use. Second, if such balls become puctured, the internal pressure will be lost.

Whilst the currently -available "pressureless" balls do not, of course, suffer from the disadvantages mentioned in the immediately-preceding paragraph, they have serious deficiencies of their own. "Pres- sureless" balls tend to give a poor "response" off the racket when played, tend also to "collapse" at high impact-speeds, and tend to show poor "rebound" characteristics when covered with the "traditional" Melton cloth.

We have found thattennis-balls having cores of a cross-linked, cellular elastomer according to the present invention approximate to the desirable playing properties of a "pressurised" ball, yet suffer from neither of the aforementioned disadvantages. For example, balls having cores according to the present invention can be punctured without affecting their playing properties.

We have also found that bat-sponges, made from cellular elastomers according to the present invention, provide novel playing characteristics, notably a substantial increase in the speed of a ball when played off the bat.

Accordingly, the present invention provides a cross-linked, cellular, elastomeric composition, the composition being the reaction product of a formulation which includes (i) a natural rubber and/or one or more synthetic rubbers; (ii) a cross-linking agent and (iii) a monomeric metallic salt of an ethylenicallyunsaturated carboxylic acid, the formulation being expanded by means of a blowing agent.

The present invention also provides components for items of sports equipment, the components being made from a cellular elastomeric composition as described in the immediately-preceding paragraph.

In one preferred embodiment, the present invention provides a core for a tennis-ball, the core comprising a cross-linked, cellular elastomeric composition, said composition being the reaction product of a formulation which includes (i) a natural rubber and/or one or more synthetic rubbers; (ii) a cross-linking agent and (iii) a monomeric metallic salt of an ethylenically-u nsaturated carboxylic acid, the formulation being expanded by means of a blowing agent.

In a second preferred embodiment, the present invention provides a bat-sponge (as hereinbefore defined) for a table-tennis bat, the bat-sponge being made from a cellular elastomeric composition as described in the immediately-preceding paragraph.

In the practice of the present invention, natural rubber, cis-polybutadiene rubber, styrene/butadiene rubber (SBR), nitrile rubber or a blend of any of these are preferred as the "fundamental" elastomer on which the formulation is built.

The cross-linking agent is preferably a peroxide, e.g. dicumyl peroxide.

The blowing agent may be any substance which will generate a non-toxic gas at the preferred reaction temperature. Thus 'physical' types of blowing agent (such as low-boiling-point solvents and fluorocarbons) or 'chemical' types of blowing agent (such as heat-sensitive materials in the form of fine powders or liquids which decompose at a particular temperature to yield a non-toxic gas) may be used.

The type and quality of blowing agent added must not, however, significantly alter the viscosity of the formulation. For this reason a decomposable solid such as benzene sulphonhydrazide, sold by Fisons Limited, as GENITRON BSH, is preferred. (GENIT RON is a Registered Trade Mark).

The monomeric metallic salt may be zinc dimethacrylate, zinc diacrylate, a mixture of these two or basic zinc methacrylate (the reaction product formed by reacting zinc oxide with methacrylic acid in a 1/1 molar ratio). Basic zinc methacrylate is preferred as it gives better reproducibility.

Other additives, well known to those skilled in the art, may be employed in making a composition according to the present invention. Such additives include the following: (a) Fillers Preferred fillers are carbon black (for tennis-ball cores) or titanium dioxide (for batsponges). In the production of bat sponges, a small amount of a pigment may be added to the formulation.

(b) cross-linking rate controlling agents. Many substances which are known per se as anti-oxidants or antiozonants can be used to control the cure-rate of the reaction. For example N - 1,3 - dimethyl - butyl - N - phenyl - p - phenylene diamine, available from Vulnax International Limited as PERMANAX 6PPD.

(PERMANAX is a Registered Trade Mark).

(c) Antioxidants A preferred antioxidant is 2,2 methylene - bis (4 - methyl - 6 - t - butyl phenol), available from Anchor Chemicals as Antioxidant 2246.

(d) ProcessingAids A preferred example of a processing aid is stearic acid. It is believed that stearic acid, in the formulation of the present invention, may improve the efficiency of the blowing agent.

Preferred embodiments ofthe present invention will be illustrated, merely by way of example, in the following description.

EXAMPLES I TO 111 Production of cellular tennis-ball cores according to the present invention was initially based on a black-filled cis-polybutadiene/basic zinc methacrylate compound. Atypical example of such a compound is formulation I shown in Table A (below). The core produced from this formulation had certain properties that met the specification of a conventional tennis ball but other factors, such as higher resilience, adversely affected playing properties.

Changes in the formulation and moulding conditions enabled a series of cores, with a range of hardness and resilience, to be produced and from these a softer core with considerably lower rebound was chosen for improved playing characteristics. An example of this formulation II, where resilience has been lowered by blending cis-polybutadiene with SBR.

Formulation Ill represents a still further developement and leads to the production of a tennis-ball that has all the static properties properties of a conventional ball.

Table A (Quantities are parts by weight) Ingredient Ex.l Ex.ll Ex.lll (a) Butadiene Rubber 100 45 (b) Styrene/butadiene Rubber - 55 100 (c) Metallic salt 20 12 15 (d) Cross-linking-rate 2 2 2 control agent (e) Filler (i) 30 - (e) Filler (ii) - 35 35 (f) Antioxidant 0.2 0.2 0.2 (g) Blowing agent 3 2.5 2.5 (h) Cross-linking agent 5 4 5 (j) Processing Aid 0.2 0.2 0.2 Notes to TABLEA (a) Available from Japan Synthetic Rubber Co. as JSR BRO 1 (b) Available from International Synthetic Rubber Co. Ltd. as INTOL 1502.

(c) Basic zinc methacrylate.

(d) Available from Vulnax International Limited as PERMANAX 6 PPD.

(e) (i) Carbon blacktype FT(FineThermal), available from Philblack as P33.

(ii) Carbon blacktype MT (Medium Thermal), available from Sevalco as SEVACARB MT.

(f) Available from Anchor Chemicals Limited as Antioxidant 2246.

(g) Available from Fisons Limited as GENITRON BSH.

(h) Available from Hercules Powder Company Limited as DICUP 40KE.

The words SEVACARB, INTOL, PERMANAX, GENIT RON and DICUP are Registered Trade Marks.

EXAMPLES lAND Il Referring to Formulations I and II in Table A (above), the compounding ingredients, except for the cross-linking agent and the blowing agent, were charged to a cold Banbury mixer and throughly mixed. The mixed compound was transferred to a water-cooled two-roll mill where the cross-linking and blowing agents were added and throughly dispersed. The compound was removed from the mill in strip form and the strips were fed to a cooled Barwell extruder. The extrudatewas a 1.5 inch diameter rod which was then cut into slugs measuring approximately 2 inches long and weighing 46 g. The slugs were dusted with French chalk to prevent sticking in the mould.

Slugs were loaded into a multicavity mould which had been pre-heated to 125"C between the platens of a press. The cavities were spherical in shape (with the mould closed) and were 1.73 inches in diameter.

The mould was closed ad pressure applied to keep the mould closed during the foaming stage. After 19 minutes, blowing agent de-composition and partial cross-linking had occurred. On opening the mould the pressure produced inside the cores by the evolution of nitrogen caused them to expand in a uniform manner until this pressure reduced to atmospheric pressure i.e. there was no residual internal pressure inside the cores. The diameter of the cores at this stage was 2.56 inches. The cores were quickly transferred to a second mould of cavity diameter 2.56 inches this having been pre-heated to 1600C, and left for 20 minutes under very low pressure to complete the cross-linking reaction.

The cores were removed from the mould and allowed to cool when a small amount of shrinkage occurred. The following properties were recorded: Example I Example Weight (g) 46 46 Diameter (inches 2.42 2.42 Forward compression (inches x 10-3) 340 480 Rebound from 100" (inches) 73 65 The cores were covered with Melton cloth in the conventional manner and play-tested. Playing characteristics were found to be very good and were an improvement overthose of conventional "pres- sureless" balls.

EXAMPLEIII Cores were produced from Formulation Ill in TABLE A (above) by the method given for Examples I and II. The following core properties were recorded: Weight (g) 46 Diameter (inches) 2.42 Forward compression (inches x 10-3) 260 Rebound from 100" (inches) 61 The cores were covered with Melton cloth in the conventional manner and play-tested. The playing characteristics were again good but these balls travelled faster through the air than those of Example II and were consequently more difficult to control.

In general the playing qualities of balls made according to Example II are preferred but the expert player might use balls prepared according to Example Ill if he required a 'fast' game.

For comparative purposes the properties of currently available "pressureless" and "pressurised" cores are as follows: Pressureless Pressurised Weight (g) 46 46 Diameter (inches) 2.42 2.42 Forward Compression (inches x 10-3) 240 250 Rebound from 100" (inches) 62 70-72 EXAMPLEIV This example describes the preparation of a tabletennis bat-sponge. The compound formulation was as shown in TABLE B (below).

TABLEB Ingredient Parts by weight (a) Butadiene Rubber 100 (b) Pigment 0.50 (c) Metallic salt 25 (d) Cross-linking-rate 1 control agent (e) Blowing agent 4 (f) Cross-linking agent 5 Notes to TABLEB (a) JSR BRO 1 (see notes to TABLE A) (b) PV fast yellow/titanium dioxide (1 :1 ratio) (c) Basic zinc methacrylate (d) Available from Uniroyal Inc. as NAUGARD445 (e) GENITRON BSH (see notes to TABLE A) (f) DICUP 40 KE (see notes to TABLE A) The compound was prepared as described in Examples I to Ill (above), the blowing agent and peroxide being added on a mill.

The compound was removed from the mill as a sheet about i inch thick. A portion of this sheet, weighing 225 g, was loaded into a mould cavity measuring 53/it S/e x inches, the mould having been pre-heated to 120"C. The volume of the material was such that it substantially filled the mould cavity in order to achieve the maximum rate of heattransfer.

The mould was closed and pressure applied to keep it closed during the foaming stage. After 9 minutes the mould was opened and the fully expanded, partially cross-linked product was removed and transferred to a second mould cavity measuring 83 x8lxi inches, this mould having been pre-heated to 1 500C. The mould was closed under minimal pressure and the cross-linking completed in 20 minutes.

The foamed slabs were removed from the mould and allowed to cool. The following properties were then recorder: Weight(g) 225 Specific gravity 0.3 Dimensions (inches) 8x8x Pendulum block resilience 96% The slabs of foam were sliced into thicknesses ranging from 1 - 2 mm. These were then cut to shape and applied to both faces of a table tennis bat. Pimpled rubber was then stuck to the foam layers and the finished bats play-tested.

The highly resilient foam produced playing characteristics not previously encountered in the game of table tennis i.e. the speed of the ball off the bat was dramatically increased. This allows wider scope in selecting the optimum pimpled rubber facing and will improve the prospects of producing a range of bats for a variety of styles of play.

Claims (14)

1. A cross-linked, cellular, elastomeric composition, the composition being the reaction product of a formulation which includes (i) a natural rubber and/or one or more synthetic rubbers; (ii) a cross-linking agent and (iii) a monomeric metallic salt of an ethylenicallyunsaturated carbovylic acid, the formulation being expanded by means of a blow- ing agent.

2. A composition according to Claim 1, in which the synthetic rubber is a cis-polybutadiene rubber, a styrene/butadiene rubber, a nitrile rubber or any combination thereof.

3. A composition according to Claim 1 or 2, in which the cross-linking agent is a peroxide.

4. A composition according to Claim 3, in which the cross-linking agent is dicumyl peroxide.

5. A composition according to any one of the preceding claims, in which the metallic salt is zinc dimethacrylate, zinc diacrylate, or a mixture thereof.

6. A composition according to any one of Claims 1 to 4, in which the metallic salt is basic zinc methacrylate (as hereinbefore defined).

7. A composition according to any of the preceding claims, in which the blowing agent comprises a solid material which decomposes during formation of the composition to yield a non-toxic gas.

8. A composition according to Claim 7, in which the blowing agent is benzene sulphonhydrazide.

9. A component of an item of sports equipment, the component comprising a cross-linked, cellular, elastomeric composition according to any one of Claims 1 to 8.

10. A component according to Claim 9 which is a core for a tennis-ball

11. A component according to Claim 9 which is a bat-sponge (as hereinbefore defined) for a tabletennis bat.

12. A core for a tennis-ball, substantially as hereinbefore described with reference to any one of Examples I to Ill.

13. A bat-sponge for a table tennis bat, substantially as hereinbefore described with reference to Example IV.

14. An item ofsportsequipmentwhich includes one or more components made from a composition according to any one of Claims 1 to 8.