CA1098925A - Pressurized game ball (e.g. tennis ball) having long lasting pressure retention - Google Patents

Abstract

INFLATED GAME BALL HAVING
LONG LASTING PRESSURE RETENTION

ABSTRACT OF THE DISCLOSURE

A pressurized game ball including an air-permeable elastomeric wall defining a cavity containing a compressible inflation gas with the improvement being that the inflation gas includes predetermined mixed amounts of air and sulfur hexafluoride (SFe) gas which effectively enables the ball to retain its pressurized state within a desired range of pressures for a period of time significantly longer than the ball would remain pressurized if the inflation gas were air alone.

Description

s INFLATED GAME BALL HAVING
LONG I~STING PRESSURE RETENTION

~A~'bh~;lD ~r '13 -NVENTION

The present invention generally relates to pressurized game balls and more pa.rticularly to an improved tennis ball having an a.ir-permeable elastomeric wall de~ining a fluid pressurized cavity9 or the like. The invention has been found especially useful and success~ul when embodied as an improved tennis ball and will herein be described as such. ~ ;
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DESCRIPTION OF THE PRIOR ART

Conventionally, the cavities of rubber articles such as tennis balls have been inflated with air, such as from a standard factory air line, although it is also known to :~:
use other inflating substances such as nitrogen, ammoni.a, and the like. However, air has been by far the most commonly used substance because of its ease of use for inflation~
its negligible cost and its availability. .Although a tennis ball inflated with air initially has satlsfactory playabil-ity, it is unable to retain its rebound and playability unless it is kept in a pressurized atmosphere when not in use, since the air permeates the rubber wall or core of the ball and gradually escapes.
The invariable loss of the internal air pressure in game balls having no valve ~or re-inflation~ such as tennis ballsg presently makes it necessary9 or at least very desir-ableg to package air-inflated tennis balls in a pressurized container as soon as they are manufactured~ since their

- 2 -"shelf-life" outside of a pressurized pa~kage is relatively short, i.e., the ball's internal pressure falls below the minimum pressure required for satisfa.ctory performance. The use of pressurized containers is an additional expense incidental to manufacturing and marketing the balls.
Pertinent prior puhlications where inflatable articles are pressurized with gases of large molecular slze includes the following:
U.S. No. 330~7,040 discloses the use of severa.l gases for inflating tires and the like to impart a smoother ride to the vehicle which are described as gases having a '~low gamma" of less than about 1.25 which relates exclusively to compressibility and not to permeabllity. The gases listed include SF~ among several other gases as being a "low gamma" gas. Union of South Africa No. 73/8777, pub-lished Janua~y 18, 1973, discloses the use of perfluoro-propane gas (C3F~) and DuPont Freon F-114 (Cl2CFCF3) to inflate game balls for prolonged pressure retention. In experimental work with the present invention, F-114 was found to be unsuitable and C3Fa, though being somewhat more suitable, is a relatlvely rare and expensive gas without sufficient commercial demand to bring its price and avail-abillty into commercial consideration as an inflating medium.
As will later become ap~arent~ SF~ was found to be sub-stantially more suitable in terms of extended pressureretention, material cost and ready availability. U.S.
Mo. 29~97,291 dlscloses an auto type hydraulic shock absorber using a hydraulic volume compensator of special low permeability film such as nylon inflated with DuPont Freons as a foam eliminator. United States No. 2,779,066 discloses thermal insulation members such as fiberglass enclosed with a laminated gas imperme-able film such as "Mylarl'*and "Saran'r*and filled with a ther~ally low con-ductive gas such as DuPont F-12. F-12 was found to be unsuitable for re-taining pressure in game balls.
It is an object of the present invention to provide an inflating medium as a component for pressurized ar~icles such as tennis balls which will not permeate the elastomeric cavity wall as readily as air.
The invention pro~ides a pressurized tennis ball having improved pressure retention properties including an elastomeric gas-permeable wall defining a hollow cavity containing an inflation gas under a pressure above atmospheric pressure, the improvement wherein said inflation gas comprises air and sulfur hexaf]uoride. The tennis ball will maintain the internal gas pressure required for good service for a substantially extended period of time. The inflating medium makes it possible to package and store the balls in other than pressurized containers for extended periods prior to use.

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If air is included in the ball with approximately atmospheric partial pressure and the SF6 is the remaining partial pressure, the longer lasting pressure lifetime is solely the result of 510w loss of the SF6 gas to atmospheric pressure.
However, there is an additional useful and advantageous character-istic of a ball inflated with sulfur hexafluoride or other low permeability gas. When air is included in the ball, its partial pressure as part of the total pressure can be provided at less than atmospheric pressure.
Under these conditions, the total pressure inside the ball tends to slowly increase from the permeation of air into the ball concurrently with a slow decrease of the total pressuTe due to the loss of the SF6 ou~ of the ball.
The initial pressure for the air component is that which would maintain the internal pressure of the air/SF6 system within the acceptable range for the longest period of time. This dual mechanism of the present invention is capable of extending the useful playing lifetime of an inflated ball even further. FurthermoreJ the extent of the pressure increase from the permeation of air into articles inflated according to the present disclosure ~and hence ~.

the useful lifetime) can be controlled within limits by the relative concentrations of air and low permeability gas used to pressurize the articles.
The SF~, as used with air in the present invention, is inexpensive, readily available and superior as a low permeabilit~ gas for maintaining sufficient playing pressure in a game ball over a substantially extended playing period as compared to air pressurized balls.

DESCRIPTION OF A PREFERRED EMBODIME~T

The subject invention is applicable to a game ball having a resilient elastomeric wall defining a hollow cavity which is pressurized and maintained in a pressurized condition with a compressible fluid or gas. The present invention is especially useful in tennis balls wherein the elastomeric wall or core of the ball is made ~rom natural rubber or equivalent elastomeric compounds known in the tennis ball art.
A tennis ball consists essentially of a hollow rubber core covered with a cloth~ usually a felt, composed o~ wool and nylon. The International Lawn Tennis Federation re~uires that the ~ollowing specification be met at a temperature of 20C and a relative humidity of 60~:
1. Diameter ('go-no-go' gauges), 2.575-2.700 i~.
(65.4-68.6mm) 2. Weight~ 2-2~ oz. (56.70-58.47g).

3. Rebound from 100 in. (2.54m) on to concrete, 53-58 in. (1.35-1.47m).

4~ (a) Deformation under 18 lbf (8.2 kg~) load, 0~230-0,290 in. (5.85-7.35mm).
(b) Deformation under 18 lbf (8.2 kgf) load on recovery after ball has been compressed through 1 in. (25.4mm), 0,355~0.425 in.
(9-10.8mm).
The test in 4(a) measures the 'compression' or 'hardness' property of the ball, and that in 4(b) measures hysteresis after the ball has been compressed through 1 in.
(25.4mm). The tests are carrled out on a special 'Stevens' machine. (British Patent No. 230,250).
The core halves of a conventional pressure type - tennis ball, which is fabricated together and inflated du~
ing fabrication to an internal air pressure of about 17 psl (117 kPa) gauge or about 32 psi absolute (220 kPa absolute)~
will generally have satisfactory rebound as long as a minimum pressure of about 13 to 15 psi (89,7-103 kPa) gauge or 28 to 30 psia (192-203 kPa absolute) is maintained.
It has been discovered that sulfur hexafluoride (SF~) as a low permeability gas mlxed with proper proportions o~
air achieve the ob~ective of providing an in~lation medium which is retained by the elastomeric walls within the cavit,y at acceptable pressures over a substantially greater extended period. Such a gas needs characteristics as follows:
1. The molecules of the gas are sufficiently large and chemically appropriate to deter their permeation through such elastomeric walls either through solubility or dif~usion;

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2. The vapor pressure of the gas with a proper amount of air is adequate to maintain desired operatirlg pressures within the ball through the ordinary range of temperatures in which the article is used; and 3. The flammability and toxicity properties of the gas are sufficiently low so that no hazards exist either in manu~acturing or in consumer use.
The following is an example of tests of tennis balls utilizing sul~ur hexafluoride (SF6) gasO
Two pairs of tennis ball cores were prepared without the felt coveringj the first pair was inflated with normal laboratory alr and the second pair was pressurized from ambient to final pressure w,~th air at ambient pressure plus sulfur hexafluoride gas. All of the cores had 103 ~ kPa (15 psi) gauge or (30 psia) pressure when they were ; initially inflated. From the partial pressures of air and sulfur hexa~luoride gas in the balls containing sulfur hexafluoride, the concentration of sulfur hexa-:
fluoride was 5005 volume percent.
The two responses used to monitor pressure changes inside the cores were deflection under 80 N (18 lbs.) load and percent rebound from a granite surface. As gas is lost from the balls and pressure decreases, their deflections increase and their rebound values decrease.
To aid in comparison, all values of deflection were divlded by the measurements made on the zeroth day, i.e. normalized.
The left hand column for each gas indicates the number of days of aging at room temperature and pressure which had elapsed between production and the listed rebound and de-flection readings. The deflection and rebound results are the averages from duplicate cores.

TABLE I
AIR S~a Days Rebound Def1ectiDn Days Rebound Deflection O 1.000 1.000 0 1.000 1.000 lo o . g~4 1.003 8 0.9~5 o .988 17 o .963 1.012 15 0.981~ o .964 lo 24 o . g60 1.006 22 o . g84 o . ~55 31 o . gl~g 1.061 29 o .978 o . gg4 52 0.959 1.052 50 0. 982 0 ~ 973 z36 o .894 1.231 234 0.980 1.067 The data clearly show that the cores containing the sulfur hexafluoride retained their rebound and deflection properties much longer which is a dlrect result of longer gas retention. The data are for comparison and are only proportional to tennis association standards for completed tennis balls, Since the pressure differential (~P) o~ the low permeability gas and of the air influences the permeation of the respective gases through the elastomeric wall of a game ball, said pressure differential affects the continuing pressure and pressure variation within the ball.
If the partial pressure of the air within the ball is below atmospheric and the remaining partial pressure is that of a low permeability gas such as SFB, the total pressure in the ball will initially increase until the air :- " . . , .. ~ . .

partia.l pressure in the ball is equal to that o~ the air outside the ball. Such pressure i.ncrea.se occurs because air permeates into the ball at a somewhat grea.ter rate than the rate at which the low permeability gas permeates ouk

5 OI the ball. As the air dif.~erential pressure across the wall o:f the ball approaches zero, then any further change in pressure of the ball will be due to the slow permeation o:f the S~ out through the wall of the ball.
For example, a tennis ball can be inflated with air and SF~, to a pressure of 30 psia (203 kPa absolute) with the partial pressure of the air being about 12 psia (82 kPa absolute) and the partial pressure of the SFB being about 18 psia (124 kPa absolute). Additional air will permeate into the ball and the SFe, will gradually and more 15 slowly permeate out of the ball. The total pressure within the ball will gradually increase to between about 32 (220 kPa absolute) and 33 psia (227 kPa absolute) (17-18 psia) until the air pressure equalizes while~the SF6 slowly begins to decline. Thereafter the total pressure will con-20 tinue to slowly decline due solely to the very slow per-meation of the SF~ through the wall of the ball. The ball will remain playable (unless the felt is first worn off`) until the total pressure declines to about 28 psia (13 psig) 25 (192 kPa absolute), corresponding to the proper rebound range for conventlonal tennis balls pressurized with air alone.
As seen from Table I above, the balls inflated wit the air/S:F6 system had negligible pressure decline aiter 30 234 days (about 9 ~ months). Conceivably, and pro~ecting - ~.0 s Table I, the balls with the air/SF~ system of Table I can have a shelf life of many months without a pressurized package and thereafter last ln playable condition for many more months before the total pressure in and, hence the rebound of, the ball declines below a playable level.
Further3 a ball manufactured with partial pressure of the air below atmospheric, as in the above example, would retain a playable pressure for yet additional months.
As a second example, a tennis ball could be inflated to 15 psig (103kPa) with pure SF8. Thereafter5 air would permeate into the ball until the air partial pressure in the ball equals atmospheric pressure, resulting in an un-desirable playing pressure initially approaching 30 psig (203 kPa) Experiments have not as yet been made to substitute the principal respectiqe components of air, oxygen (2 ) and nitrogen (Na)~ to establish whether or not either a or Na could be used respectively in lieu of air to provide a partial pressure in a game ball along with an additional appropriate partial pressure of SF~. However, it is con-sidered that these gases and other gases having permeability rates no greater than air can be used.
The foregoing description will suggest other embodiments and variations to those skilled in the art, all of which are intended to be included in the spirit of the invention as herein set forth

Claims (8)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A pressurized tennis ball having improved pressure retention properties including an elastomeric gas-permeable wall defining a hollow cavity containing an inflation gas under a pressure above atmospheric pressure, the improvement wherein said inflation gas comprises air and sulfur hexafluoride.

2. The ball of claim 1 wherein said air is in amount sufficient to provide a partial pressure in a range above atmospheric pressure.

3. The ball of claim 1 wherein said air is provided in an amount sufficient to provide a partial pressure below atmospheric pressure.

4. A tennis ball having improved pressure retention properties com-prising an elastomeric gas-permeable wall defining a hollow cavity containing a compressible inflation gas under pressure, said inflation gas including:
a) air having a partial pressure equal to ambient pressure; and b) sulfur hexafluoride in an amount sufficient to significantly improve the pressure retention properties of said ball as compared to the pressure retention properties of said ball when filled with air.

5. In a tennis ball including an elastomeric gas-permeable wall defining a hollow cavity containing a compressible inflation gas system under pressure above atmospheric pressure, the improvement wherein said inflation gas system comprises: (a) air in an amount sufficient to provide a partial pressure in a range up to but not including about atmospheric pressure; (b) sulphur hexafluoride in an amount sufficient to provide an additional partial pressure above atmospheric pressure to a desired total pressure within said cavity; and (c) said inflation gas being effective to enable said cavity to retain its pressurized state within a selected range of pressures for a period of time significantly longer than would said cavity remain so pressurized if said inflation gas were air alone.

6. The ball of claim 5 wherein said air is in amount sufficient to provide a partial pressure below atmospheric pressure.

7. The ball of claim 6 wherein said air is provided in an amount sufficient to provide a partial pressure of between about 12 psia and atmospheric.

8. The ball of claim 5 wherein said air is in amount sufficient to provide a partial pressure above atmospheric pressure.