Note: Descriptions are shown in the official language in which they were submitted.
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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. ~ ;
~.
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
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"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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* trade mark
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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
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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;
~t~
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
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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
