U.S. patent application number 13/351946 was filed with the patent office on 2012-11-01 for practice golf ball.
This patent application is currently assigned to BRIDGESTONE SPORTS CO., LTD.. Invention is credited to Daisuke ARAI, Hiroshi HIGUCHI, Takashi OHIRA, Yuichiro OZAWA, Katsunori SATO, Hisashi YAMAGISHI.
Application Number | 20120277023 13/351946 |
Document ID | / |
Family ID | 47068311 |
Filed Date | 2012-11-01 |
United States Patent
Application |
20120277023 |
Kind Code |
A1 |
HIGUCHI; Hiroshi ; et
al. |
November 1, 2012 |
PRACTICE GOLF BALL
Abstract
In a practice golf ball having a core and a cover encasing the
core, the cover has a material hardness, expressed as the Shore D
hardness, of from 30 to 57. The core has an initial velocity CV
(m/s) and the ball has an initial velocity BV1 (m/s) such that the
value CV-BV1 satisfies the condition (CV-BV1)=0.7.times.(cover
thickness)-b, where 0.1.ltoreq.b.ltoreq.1.0. The ball has a soft
feel on impact like that of a one-piece golf ball and thus is able
to reduce strain on the player's body, in addition to which it has
much better durability to cracking than commonly used game balls,
good retention of appearance even with long-term use, and a
distance that is controlled so as to be appropriate for a driving
range.
Inventors: |
HIGUCHI; Hiroshi;
(Chichibushi, JP) ; YAMAGISHI; Hisashi;
(Chichibushi, JP) ; ARAI; Daisuke; (Chichibushi,
JP) ; OZAWA; Yuichiro; (Chichibushi, JP) ;
SATO; Katsunori; (Chichibushi, JP) ; OHIRA;
Takashi; (Chichibushi, JP) |
Assignee: |
BRIDGESTONE SPORTS CO.,
LTD.
Tokyo
JP
|
Family ID: |
47068311 |
Appl. No.: |
13/351946 |
Filed: |
January 17, 2012 |
Current U.S.
Class: |
473/280 |
Current CPC
Class: |
A63B 37/0021 20130101;
A63B 37/0084 20130101; A63B 37/0051 20130101; A63B 37/0003
20130101; A63B 37/0019 20130101; A63B 37/0006 20130101; A63B
37/0092 20130101; A63B 37/0017 20130101; A63B 37/0063 20130101;
A63B 37/0074 20130101; A63B 69/3655 20130101; A63B 37/0065
20130101; A63B 37/0023 20130101; A63B 37/0087 20130101; A63B
37/0033 20130101; A63B 37/005 20130101; A63B 37/0066 20130101; A63B
37/0031 20130101; A63B 37/0068 20130101 |
Class at
Publication: |
473/280 |
International
Class: |
A63B 69/36 20060101
A63B069/36; A63B 37/00 20060101 A63B037/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 27, 2011 |
JP |
2011-099803 |
Claims
1. A practice golf ball comprising a core and a cover encasing the
core, wherein the cover has a material hardness, expressed as the
Shore D hardness, of from 30 to 57; and the core has an initial
velocity CV (m/s) and the ball has an initial velocity BV1 (m/s)
such that the value CV-BV1 satisfies the condition
(CV-BV1)=0.7.times.(cover thickness)-b, where
0.1.ltoreq.b.ltoreq.1.0.
2. The practice golf ball of claim 1, wherein the ball initial
velocity BV1 is not more than 76 m/s.
3. The practice golf ball of claim 1, wherein the cover has a
thickness of from 0.3 mm to 1.9 mm and is formed primarily of
polyurethane.
4. The practice golf ball of claim 1, wherein the core is formed of
a rubber composition comprising a base rubber and, as compounding
ingredients: a co-crosslinking agent, a crosslinking initiator and
a metal oxide, said co-crosslinking agent being methacrylic
acid.
5. The practice golf ball of claim 1 wherein, when the core and the
ball are each compressed under a final load of 1,275 N (130 kgf)
from an initial load of 98 N (10 kgf), letting deflection by the
core be CH and deflection by the ball be BH1, the ball deflection
BH1 is from 2.0 mm to 7.0 mm, and the ratio CH/BH1 is from 0.95 to
1.1.
6. The practice golf ball of claim 1, wherein the core, when
compressed under a final load of 1,275 N (130 kgf) from an initial
load of 98 N (10 kgf), has a compression CH of from 2.0 to 7.0 mm;
the core has a hardness profile in which, letting A be the JIS-C
hardness at a surface of the core, B be the JIS-C hardness at a
position 2 mm inside the core surface, C be the JIS-C hardness at a
position 5 mm inside the core surface, D be the JIS-C hardness at a
position 10 mm inside the core surface, E be the JIS-C hardness at
a position 15 mm inside the core surface, and F be the JIS-C
hardness at a center of the core: A is from 60 to 88, B is from 54
to 83, C is from 56 to 85, D is from 54 to 80, E is from 51 to 75,
and F is from 48 to 72; the relative hardness conditions
A>B<C.gtoreq.D>E>Fare satisfied; the value A-F is not
more than 19; the core is formed in such a way that A has the
highest value among A to F; the value A-C is from 0 to 8; and the
core has a specific gravity of from 1.05 to 1.2.
7. The practice golf ball of claim 1, wherein the ball has formed
on a surface thereof a plurality of dimples, each dimple having a
spatial volume below a flat plane circumscribed by an edge of the
dimple, and the sum of the dimple spatial volumes, expressed as a
percentage (VR) of the volume of a hypothetical sphere representing
the ball were the ball to have no dimples on the surface thereof,
being from 0.8% to 1.7%.
8. The practice golf ball of claim 1, wherein the ball has formed
on a surface thereof a plurality of dimples which satisfy
conditions (1) and (2) below: (1) the dimples have a peripheral
edge provided with a roundness represented by a radius of curvature
R of from 0.5 mm to 2.5 mm; and (2) the ratio ER of a collective
number of dimples RA having a radius of curvature R to diameter D
ratio (R/D) of at least 20%, divided by a total number of dimples N
on the surface of the ball, is from 15% to 95%.
9. The practice golf ball of claim 8 which further satisfies
condition (3) below: (3) the ball has thereon a plurality of dimple
types of differing diameter, and the ratio DER of a combined number
of dimples DE obtained by adding together dimples having an own
diameter and an own radius of curvature larger than or equal to a
radius of curvature of dimples of larger diameter than said own
diameter plus dimples of a type having a largest diameter, divided
by the total number of dimples N on the surface of the ball, is at
least 80%.
10. The practice golf ball of claim 9 which further satisfies
conditions (4) to (6) below: (4) the number of dimple types of
differing diameter is 3 or more; (5) the total number of dimples N
is not more than 80; and (6) the surface coverage SR of the
dimples, which is the sum of individual dimple surface areas, each
defined by a flat plane circumscribed by an edge of the dimple,
expressed as a percentage of the surface area of a hypothetical
sphere representing the ball were the ball to have no dimples on
the surface thereof, is from 60% to 74%.
11. The practice golf ball of claim 4, wherein the compounding
ingredients are included in respective amounts of from 10 to 40
parts by weight of methacrylic acid as the co-crosslinking agent,
from 15 to 30 parts by weight of zinc oxide as the metal oxide, and
from 0.3 to 5.0 parts by weight of the crosslinking initiator, per
100 parts by weight of the base rubber.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This non-provisional application claims priority under 35
U.S.C. .sctn.119(a) on Patent Application No. 2011-099803 filed in
Japan on Apr. 27, 2011, the entire contents of which are hereby
incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a practice golf ball which
is suitable for use at such places as driving ranges. More
specifically, the invention relates to a practice golf ball which
has a soft feel on impact like that of a one-piece golf ball and
thus is able to reduce strain on the player's body, which has a
better durability to cracking and durability of appearance than
ordinary game balls, and which moreover has a distance that is
controlled so as to be appropriate for a driving range.
[0004] 2. Prior Art
[0005] In terms of the performance sought in a practice golf ball,
it is generally desirable to obtain ball characteristics which are
no different from those of the golf balls used to play an actual
round of golf. If, for example, a practice ball has a feel on
impact or a flight performance which differs from that of a game
ball, when the time comes to play an actual round, the golfer will
be unable to take full advantage of the skills acquired through
practice.
[0006] On the other hand, because of the size limitations of
driving ranges, there is a need today for low-distance practice
golf balls in order to keep the balls from flying out of the
driving range. Accordingly, there exists a desire for practice
balls which have a performance characterized by the same feel on
impact as a game ball, but a short flight distance.
[0007] Moreover, because practice golf balls are used over and over
again at a driving range, in order to reduce the costs of operating
the range, it is desirable for the balls to be capable of enduring
use for as long a period of time as possible even when repeatedly
hit. That is, because the golf balls used at a driving range are
repeatedly used over a long period of time by many golfers
practicing their skills, there is a desire for such practice balls
to have a durability, including a durability to cracking, which is
superior to that of ordinarily used game balls, and for the balls
to cause little strain to the body and thus not be tiring to
use.
[0008] As for the ball structure in a practice golf ball, for the
ball to be imparted with a flight performance and feel similar to
those experienced when playing an actual round of golf, it is more
desirable to use a two-piece solid golf ball than a one-piece
ball.
[0009] In addition, JP-A 2003-70936 describes, as a rubber
composition for the core of a two-piece solid golf ball, the
compounding of a given amount of zinc acrylate in
cis-1,4-polybutadiene rubber. However, here too, when zinc acrylate
is used in the rubber-forming rubber composition, achieving good
ball durability in long-term use at a driving range has been
difficult. Other prior art publications relating to this invention
include JP-A 59-49779, JP-A 2007-61614, JP-A 2007-301357, JP-A
2010-115485 and JP-A 2010-115486.
[0010] In view of the foregoing, it is an object of the present
invention to provide a practice golf ball endowed with the
properties required of practice balls for long-term use, including
a soft feel on impact like that of a one-piece golf ball which
enables strain on the player's body to be reduced, a much better
durability to cracking than ordinarily used game balls, good
retention of appearance even with long-term use, and a distance
that is controlled so as to be appropriate for a driving range.
SUMMARY OF THE INVENTION
[0011] We have discovered that, in a practice golf ball which has a
core and a cover, by arranging for the initial velocity difference
between the core and the ball to be small and to satisfy a specific
formula, the sense of structural unity at the time of impact
increases, giving the ball a soft feel on impact like that of a
one-piece golf ball, and thus enabling strain on the player's body
to be reduced. In addition, the ball has a better durability to
cracking than ordinary game balls, an excellent durability of
appearance, and a distance that is controlled so as to be
appropriate for a driving range.
[0012] Accordingly, the invention provides a practice golf ball
having a core and a cover encasing the core, wherein the cover has
a material hardness, expressed as the Shore D hardness, of from 30
to 57. The core has an initial velocity CV (m/s) and the ball has
an initial velocity BV1 (m/s) such that the value CV-BV1 satisfies
the condition (CV-BV1)=0.7.times.(cover thickness)-b, where
0.1.ltoreq.b.ltoreq.1.0.
[0013] The initial velocity BV1 of the practice golf ball of the
invention is preferably not more than 76 m/s.
[0014] In the golf ball of the invention, the cover may have a
thickness of from 0.3 mm to 1.9 mm and may be formed primarily of
polyurethane.
[0015] In the golf ball of the invention, the core may be formed of
a rubber composition composed of a base rubber and, as compounding
ingredients: a co-crosslinking agent, a crosslinking initiator and
a metal oxide, with the co-crosslinking agent being methacrylic
acid.
[0016] In the inventive golf ball, when the core and the ball are
each compressed under a final load of 1,275 N (130 kgf) from an
initial load of 98 N (10 kgf), letting deflection by the core be CH
and deflection by the ball be BH1, it is preferable for the ball
deflection BH1 to be from 2.0 mm to 7.0 mm, and for the ratio
CH/BH1 to be from 0.95 to 1.1.
[0017] Preferably, in the practice golf ball of the invention, the
core, when compressed under a final load of 1,275 N (130 kgf) from
an initial load of 98 N (10 kgf), has a compression CH of from 2.0
to 7.0 mm; the core has a hardness profile in which, letting A be
the JIS-C hardness at a surface of the core, B be the JIS-C
hardness at a position 2 mm inside the core surface, C be the JIS-C
hardness at a position 5 mm inside the core surface, D be the JIS-C
hardness at a position 10 mm inside the core surface, E be the
JIS-C hardness at a position 15 mm inside the core surface, and F
be the JIS-C hardness at a center of the core: A is from 60 to 88,
B is from 54 to 83, C is from 56 to 85, D is from 54 to 80, E is
from 51 to 75, and F is from 48 to 72; the relative hardness
conditions A>B<C.gtoreq.D>E>F are satisfied; the value
A-F is not more than 19; the core is formed in such a way that A
has the highest value among A to F; the value A-C is from 0 to 8;
and the core has a specific gravity of from 1.05 to 1.2.
[0018] The practice golf ball of the invention may have formed on a
surface thereof a plurality of dimples, each dimple having a
spatial volume below a flat plane circumscribed by an edge of the
dimple, and the sum of the dimple spatial volumes, expressed as a
percentage (VR) of the volume of a hypothetical sphere representing
the ball were the ball to have no dimples on the surface thereof,
being from 0.8% to 1.7%.
[0019] The practice golf ball of the invention may have formed on a
surface thereof a plurality of dimples which satisfy conditions (1)
and (2) below:
[0020] (1) the dimples have a peripheral edge provided with a
roundness represented by a radius of curvature R of from 0.5 mm to
2.5 mm; and
[0021] (2) the ratio ER of a collective number of dimples RA having
a radius of curvature R to diameter D ratio (R/D) of at least 20%,
divided by a total number of dimples N on the surface of the ball,
is from 15% to 95%.
[0022] The practice golf ball which satisfies above conditions (1)
and (2) preferably satisfies also condition (3) below:
[0023] (3) the ball has thereon a plurality of dimple types of
differing diameter, and the ratio DER of a combined number of
dimples DE obtained by adding together dimples having an own
diameter and an own radius of curvature larger than or equal to a
radius of curvature of dimples of larger diameter than the own
diameter plus dimples of a type having a largest diameter, divided
by the total number of dimples N on the surface of the ball, is at
least 80%.
[0024] The practice golf ball which satisfies above conditions (1)
to (3) preferably satisfies also conditions (4) to (6) below:
[0025] (4) the number of dimple types of differing diameter is 3 or
more;
[0026] (5) the total number of dimples N is not more than 380;
and
[0027] (6) the surface coverage SR of the dimples, which is the sum
of individual dimple surface areas, each defined by a flat plane
circumscribed by an edge of the dimple, expressed as a percentage
of the surface area of a hypothetical sphere representing the ball
were the ball to have no dimples on the surface thereof, is from
60% to 74%.
[0028] In the practice golf ball of the invention, it is preferable
for the compounding ingredients to be included in respective
amounts of from 10 to 40 parts by weight of methacrylic acid as the
co-crosslinking agent, from 15 to 30 parts by weight of zinc oxide
as the metal oxide, and from 0.3 to 5.0 parts by weight of the
crosslinking initiator per 100 parts by weight of the base
rubber.
[0029] The practice golf ball of the invention has a soft feel on
impact like that of a one-piece golf ball and thus is able to
reduce strain on the player's body, in addition to which it has a
much better durability to cracking than commonly used game balls,
good retention of appearance even with long-term use, and a
distance that is controlled so as to be appropriate for a driving
range.
BRIEF DESCRIPTION OF THE DIAGRAMS
[0030] FIG. 1 is a schematic cross-sectional diagram of a practice
golf ball according to one embodiment of the invention.
[0031] FIG. 2 is a schematic diagram of a core illustrating
positions A to F in the core hardness profile.
[0032] FIG. 3 is a schematic diagram showing an example of a dimple
cross-section.
[0033] FIG. 4A is a top view and FIG. 4B is a side view showing an
example of a dimple configuration.
[0034] FIG. 5 is a top view showing the markings that were placed
on the golf balls fabricated in the examples and the comparative
examples.
DETAILED DESCRIPTION OF THE INVENTION
[0035] The objects, features and advantages of the invention will
become more apparent from the following detailed description, taken
in conjunction with the foregoing diagrams.
[0036] The structure of the practice golf ball of the invention is
exemplified by, as shown in FIG. 1, a two-piece solid golf ball G
having a core 1 and a cover 2 which encases the core 1. The cover 2
typically has a plurality of dimples D formed on a surface thereof.
In the diagram, the core 1 and the cover 2 are shown as single
layers, although the core and the cover may each be composed of a
plurality of layers.
[0037] The core is obtained by vulcanizing a rubber composition
composed primarily of a rubber material. The rubber composition
used to form the core may be a rubber composition containing, for
example, a base rubber, a co-crosslinking agent, a crosslinking
initiator, and a metal oxide. If necessary, an antioxidant and
other compounding ingredients may also be included in the rubber
composition. The base rubber used in this rubber composition is
preferably polybutadiene. In the invention, as will be subsequently
described, it is preferable for the core cross-sectional hardness
to change in specific ways from the surface to the center of the
core, and for the core cross-sectional hardness distribution, also
referred to herein as the "core hardness profile," to be adjusted
within certain desired ranges. To this end, in formulating the
core, it is essential to suitably adjust, for example, the amounts
in which the various subsequently described compounding ingredients
are included, the vulcanization temperature and the vulcanization
time.
[0038] The polybutadiene used as the rubber component must have a
cis-1,4 bond content of at least 60 wt %, preferably at least 80 wt
%, more preferably at least 90 wt %, and most preferably at least
95 wt %. If the cis-1,4 bond content is too low, the rebound may
decrease. In addition, the polybutadiene has a 1,2-vinyl bond
content of preferably 2 wt % or less, more preferably 1.7 wt % or
less, and even more preferably 1.5 wt % or less.
[0039] The polybutadiene has a Mooney viscosity (ML.sub.1+4
(100.degree. C.)) of preferably at least 30, more preferably at
least 35, even more preferably at least 40, and most preferably at
least 45, but preferably not more than 100, more preferably not
more than 80, even more preferably not more than 70, and most
preferably not more than 60.
[0040] The term "Mooney viscosity" used herein refers to an
industrial indicator of viscosity (JIS K6300) as measured with a
Mooney viscometer, which is a type of rotary plastometer. This
value is represented by the unit symbol ML.sub.1+4 (100.degree.
C.), wherein "M" stands for Mooney viscosity, "L" stands for large
rotor (L-type), and "1+4" stands for a pre-heating time of 1 minute
and a rotor rotation time of 4 minutes. The "100.degree. C."
indicates that measurement was carried out at a temperature of
100.degree. C.
[0041] In order to obtain the rubber composition in a molded and
vulcanized form which has a good rebound, it is preferable for the
polybutadiene to have been synthesized using a rare-earth catalyst
or a Group VIII metal compound catalyst.
[0042] The rare-earth catalyst is not subject to any particular
limitation, although preferred use can be made of a catalyst which
employs a lanthanum series rare-earth compound. Also, where
necessary, an organoaluminum compound, an alumoxane, a
halogen-bearing compound and a Lewis base can be used in
combination with the lanthanum series rare-earth compound.
Preferred use may be made of, as the various above compounds, those
compounds mentioned in JP-A 11-35633, JP-A 11-164912 and JP-A
2002-293996.
[0043] Of the above rare-earth catalysts, the use of a neodymium
catalyst that employs a neodymium compound, which is a lanthanide
series rare-earth compound, is especially recommended. In such a
case, a polybutadiene rubber having a high cis-1,4 bond content and
a low 1,2-vinyl bond content can be obtained at an excellent
polymerization activity.
[0044] The polybutadiene has a polydispersity Mw/Mn (Mw being the
weight-average molecular weight, and Mn being the number-average
molecular weight) of preferably at least 2.0, more preferably at
least 2.2, even more preferably at least 2.4, and most preferably
at least 2.6. The upper limit is preferably 6.0 or less, more
preferably 5.0 or less, and even more preferably 4.5 or less. If
Mw/Mn is too low, the workability may decrease. On the other hand,
if Mw/Mn is too high, the rebound may decrease.
[0045] When the above polybutadiene is used as the base rubber, the
proportion of the overall rubber represented by the polybutadiene
is preferably at least 40 wt %, more preferably at least 60 wt %,
even more preferably at least 80 wt %, and most preferably at least
90 wt %. The above polybutadiene may represent fully 100 wt % of
the base rubber, although 98 wt % or less is preferred, and 95 wt %
or less is more preferred.
[0046] Illustrative examples of cis-1,4-polybutadiene rubbers which
may be used include the high-cis products BR01, BR11, BR02, BR02L,
BR02LL, BR730 and BR51, all of which are available from JSR
Corporation.
[0047] Rubber components other than the above polybutadiene may
also be used in the base rubber, insofar as the objects of the
invention are attainable. Illustrative examples of rubber
components other than the above polybutadiene include
polybutadienes other than the above polybutadiene, and other diene
rubbers such as styrene-butadiene rubbers, natural rubbers,
isoprene rubbers and ethylene-propylene-diene rubbers.
[0048] Isoprene rubbers which may be used include those having a
cis-1,4 bond content of at least 60 wt %, preferably at least 80 wt
%, and more preferably at least 90 wt %, and having a Mooney
viscosity (ML.sub.1+4 (100.degree. C.)) of at least 70, preferably
at least 75, and more preferably at least 80, with an upper limit
of 90 or less, and preferably 85 or less. For example, the product
IR2200 available from JSR Corporation may be used.
[0049] Styrene-butadiene rubbers which may be used include
solution-polymerized styrene-butadiene rubbers and
emulsion-polymerized styrene-butadiene rubbers. Compared with
emulsion-polymerized styrene-butadiene rubbers,
solution-polymerized styrene-butadiene rubbers do not contain
organic acids and low-molecular-weight components that arise from
the manufacturing process, and thus have a poor processability. In
addition, when a solution-polymerized butadiene-styrene rubber is
used in a core-forming rubber composition, the seasonal (winter
versus summer) difference in ball rebound is larger. On the other
hand, when an emulsion-polymerized styrene-butadiene rubber is used
in the core-forming rubber composition, compared with when a
solution-polymerized styrene-butadiene rubber is used, hardening of
the core due to temperature changes on account of seasonal
differences can be effectively prevented. Therefore, by using these
two types of styrene-butadiene rubber having different qualities in
a specific ratio, it is sometimes possible to suppress a decline in
resilience and a change in hardness during winter use while
maintaining a good processability. By way of illustration, use may
be made of the solution-polymerized products SBR-SL552, SBR-SL555
and SBR-SL563 (available from JSR Corporation) as the
solution-polymerized styrene-butadiene rubber, and use may be made
of the emulsion-polymerized products SBR 1500, SBR 1502, SBR 1507
and SBR 0202 (available from JSR Corporation) as the
emulsion-polymerized styrene-butadiene rubber. Ordinary,
commercially available, solution-polymerized styrene-butadiene
rubber has a styrene bond content of from 5 wt % to 50 wt %, and
emulsion-polymerized styrene-butadiene rubber has a styrene bond
content of from 15 wt % to 50 wt %.
[0050] The proportion of the overall rubber represented by rubber
components other than polybutadiene is preferably 0 wt % or more,
more preferably at least 2 wt %, and most preferably at least 5 wt
%, but preferably not more than 60 wt %, more preferably not more
than 40 wt %, even more preferably not more than 20 wt %, and most
preferably not more than 10 wt %.
[0051] In the invention, it is preferable to use methacrylic acid
as the co-crosslinking agent. Methacrylic acid is included in an
amount, per 100 parts by weight of the base rubber, of preferably
at least 10 parts by weight, more preferably at least 12 parts by
weight, even more preferably at least 14 parts by weight, and most
preferably at least 16 parts by weight. The upper limit in the
amount of methacrylic acid is preferably not more than 40 parts by
weight, more preferably not more than 35 parts by weight, even more
preferably not more than 30 parts by weight, and most preferably
not more than 25 parts by weight. Including too much methacrylic
acid may make the core too hard, giving the ball an unpleasant feel
on impact. On the other hand, including too little methacrylic acid
may make the core too soft, also giving the ball an unpleasant feel
on impact.
[0052] It is preferable to use an organic peroxide as the
crosslinking initiator in this invention. Examples of commercial
products that may be advantageously used include Percumyl D,
Perhexa 3M and Perhexa C40, all available from NOF Corporation.
These may be used singly or as combinations of two or more
thereof.
[0053] The crosslinking initiator is included in an amount, per 100
parts by weight of the base rubber, of preferably at least 0.3 part
by weight, more preferably at least 0.5 part by weight, and even
more preferably at least 0.7 part by weight. The upper limit in the
amount of crosslinking agent is preferably not more than 5.0 parts
by weight, more preferably not more than 4.0 parts by weight, even
more preferably not more than 3.0 parts by weight, and most
preferably not more than 2.0 parts by weight. Including too much
crosslinking initiator may make the core too hard, giving the ball
an unpleasant feel on impact and also substantially lowering the
durability to cracking. On the other hand, including too little
crosslinking initiator may make the core too soft, giving the ball
an unpleasant feel on impact and also substantially lowering
productivity.
[0054] It is preferable to use zinc oxide as the metal oxide in
this invention, although metal oxides other than zinc oxide may be
used insofar as the objects of the invention are attainable. The
metal oxide is included in an amount, per 100 parts by weight of
the base rubber, of preferably at least 15 parts by weight, more
preferably at least 17 parts by weight, even more preferably at
least 19 parts by weight, and most preferably at least 21 parts by
weight. The upper limit in the amount of metal oxide is preferably
not more than 30 parts by weight, more preferably not more than 28
parts by weight, even more preferably not more than 26 parts by
weight, and most preferably not more than 24 parts by weight.
Including too much or too little may make it impossible to obtain a
suitable weight and a suitable hardness and rebound.
[0055] In the practice of the invention, an antioxidant may also be
included in the rubber composition. For example, use may be made of
the commercial products Nocrac NS-6, Nocrac NS-30 and Nocrac 200,
all available from Ouchi Shinko Chemical Industry Co., Ltd. These
may be used singly or as combinations of two or more thereof.
[0056] The amount of antioxidant included per 100 parts by weight
of the base rubber, although not subject to any particular
limitation, is preferably at least 0.1 part by weight, and more
preferably at least 0.2 part by weight, but is preferably not more
than 1.0 part by weight, more preferably not more than 0.7 part by
weight, and even more preferably not more than 0.4 part by weight.
Including too much or too little antioxidant may make it impossible
to achieve a suitable core hardness gradient, as a result of which
a good rebound, good durability and good spin rate-lowering effect
on full shots may not be achieved.
[0057] In the practice of the invention, from a resource recycling
standpoint, a ground or abraded powder of vulcanized rubber may be
included in a small amount of 40 parts by weight or less per 100
parts by weight of the base rubber. In such a case, the ground or
abraded powder may be compounded in an amount, per 100 parts by
weight of the base rubber, which is more than 0 wt %, preferably at
least 2 wt %, and most preferably at least 5 wt %, but is
preferably not more than 40 wt %, more preferably not more than 35
wt %, even more preferably not more than 30 wt %, and most
preferably not more than 25 wt %. The ground or abraded powder of
vulcanized rubber is a vulcanizate which contains rubber and
unsaturated carboxylic acid or a metal salt thereof. It is
desirable for the ground or abraded powder of vulcanized rubber
used to have a particle size which is preferably at least 20 .mu.m,
more preferably at least 25 .mu.m, and most preferably at least 30
.mu.m, but is preferably not more than 1,000 .mu.m, more preferably
not more than 900 .mu.m, and most preferably not more than 800
.mu.m. Adding a crushed or abraded powder of vulcanized rubber has
such effects as improving the productivity of the vulcanizate and
increasing the durability to cracking. However, including too much
may markedly lower the workability of the rubber composition and
the productivity.
[0058] The core may be produced by using a known method to
vulcanize and cure the rubber composition containing the various
above ingredients. For example, production may be carried out by
using a mixing apparatus such as a Banbury mixer or a roll mill to
mix the rubber composition, compression molding or injection
molding the mixed composition in a core mold, then curing the
molded body by suitable heating at a temperature sufficient for the
organic peroxide and co-crosslinking agent to act, such as under
conditions of between about 100.degree. C. and about 200.degree. C.
for a period of from about 10 minutes to about 40 minutes. The core
hardness profile of the invention may be achieved by a combination
of the vulcanization conditions and adjustment of the rubber
formulation.
[0059] The core diameter, although not subject to any particular
limitation, is preferably at least 38.9 mm, and more preferably at
least 39.3 mm, but is preferably not more than 42.1 mm, and more
preferably not more than 41.1 mm. At a core diameter outside of
this range, the durability of the ball to cracking may worsen
dramatically and the initial velocity of the ball may decrease.
[0060] It is recommended that the specific gravity of the core be
at least 1.05, preferably at least 1.08, and more preferably at
least 1.1, but not more than 1.2, preferably not more than 1.15,
and more preferably not more than 1.13.
[0061] The core deflection under loading (referred to here and
below as "CH"), i.e., the deflection by the core when compressed
under a final load of 1,275 N (130 kgf) from an initial load of 98
N (10 kgf), is preferably at least 2.0 mm, more preferably at least
2.3 mm, and even more preferably at least 2.4 mm, but is preferably
not more than 7.0 mm, more preferably not more than 6.0 mm, even
more preferably not more than 5.0 mm, and most preferably not more
than 4.5 mm. If the core deflection CH is too small, the feel of
the practice golf ball on impact may be so hard as to make the ball
unpleasant to use. On the other hand, if the core deflection is too
large, the feel of the practice golf ball on impact may be so soft
as to make the ball unpleasant to use, in addition to which the
productivity may decline considerably.
[0062] The core rebound (also referred to as the core initial
velocity, CV), is preferably at least 65 m/s, more preferably at
least 68 m/s, even more preferably at least 71 m/s, and most
preferably at least 73 m/s, but is preferably not more than 76 m/s,
more preferably not more than 75.7 m/s, even more preferably not
more than 75.4 m/s, and most preferably not more than 75 m/s. A
core rebound outside of this range is undesirable because the
distance of the ball may dramatically decline or the ball may fly
so far as to pass over the netting at a driving range.
[0063] In the practice of the invention, as shown in the schematic
diagram of the core in FIG. 2, letting A be the JIS-C hardness at a
surface of the core, B be the JIS-C hardness at a position 2 mm
inside the core surface, C be the JIS-C hardness at a position 5 mm
inside the core surface, D be the JIS-C hardness at a position 10
mm inside the core surface, E be the JIS-C hardness at a position
15 mm inside the core surface, and F be the JIS-C hardness at the
center of the core, the respective values A to F, although not
subject to any particular limitations, preferably fall within the
specific ranges indicated below. By thus setting the hardness
profile at the core interior within specific ranges, both a
comfortable feel on impact similar to that of a game ball and also
a good durability to cracking can be obtained.
[0064] Letting A be the JIS-C hardness at the surface of the core,
the value of A is preferably at least 60, more preferably at least
63, even more preferably at least 65, and still more preferably at
least 67, but is preferably not more than 88, more preferably not
more than 86, and even more preferably not more than 84.
[0065] Letting B be the JIS-C hardness at a position 2 mm inside
the core surface, the value of B is preferably at least 54, more
preferably at least 57, even more preferably at least 59, and still
more preferably at least 61, but is preferably not more than 83,
more preferably not more than 81, and even more preferably not more
than 79.
[0066] Letting C be the JIS-C hardness at a position 5 mm inside
the core surface, the value of C is preferably at least 56, more
preferably at least 59, even more preferably at least 62, and still
more preferably at least 65, but is preferably not more than 85,
more preferably not more than 83, and even more preferably not more
than 81.
[0067] Letting D be the JIS-C hardness at a position 10 mm inside
the core surface, the value of D is preferably at least 54, more
preferably at least 57, even more preferably at least 60, and still
more preferably at least 63, but is preferably not more than 80,
more preferably not more than 78, and even more preferably not more
than 76.
[0068] Letting E be the JIS-C hardness at a position 15 mm inside
the core surface, the value of E is preferably at least 51, more
preferably at least 54, even more preferably at least 57, and still
more preferably at least 60, but is preferably not more than 75,
more preferably not more than 73, even more preferably not more
than 71, and still more preferably not more than 70.
[0069] Letting F be the JIS-C hardness at the center of the core,
the value of F is preferably at least 48, more preferably at least
51, even more preferably at least 54, and still more preferably at
least 57, but is preferably not more than 72, more preferably not
more than 70, and even more preferably not more than 68.
[0070] Moreover, in the above core hardness profile, it is
preferable for the relative hardness conditions
A>B<C.gtoreq.D>E>F to be satisfied, for the value A-F
to be not more than 19, for the core to be formed in such a way
that A has the highest value among A to F, and for the value A-C to
be in a range of from 0 to 8. If the above conditions are not
satisfied, the ball may have a diminished feel on impact and a
reduced durability to cracking.
[0071] The value of A-C is preferably within a range of from 0 to
8, with the lower limit being preferably at least 0, and more
preferably at least 1, and the upper limit being preferably not
more than 6, and more preferably not more than 4. The value of A-F
is preferably at least 3, more preferably at least 5, and even more
preferably at least 7, but is preferably not more than 19.
[0072] In the practice of the invention, the core may be
administered surface treatment with a solution containing a
haloisocyanuric acid and/or a metal salt thereof.
[0073] Prior to surface-treating the core with a solution
containing a haloisocyanuric acid and/or a metal salt thereof,
adhesion between the core surface and the adjoining cover material
can be further enhanced by abrading the surface of the core
(referred to below as "surface grinding").
[0074] Such surface grinding removes the skin layer from the
surface of the vulcanized core, and thus makes it possible to
enhance the ability of the solution of haloisocyanuric acid and/or
a metal salt thereof to penetrate the core surface and also to
increase the surface area of contact with the adjoining cover
material. Exemplary surface grinding methods include buffing,
barrel grinding and centerless grinding.
[0075] The above haloisocyanuric acid and metal salt thereof is the
compound shown in the following formula (I).
##STR00001##
In the formula, X is a hydrogen atom, a halogen atom or an alkali
metal atom. At least one occurrence of X is a halogen atom.
Preferred halogen atoms include fluorine, chlorine and bromine,
with chlorine being especially preferred. Preferred alkali metal
atoms include lithium, sodium and potassium.
[0076] Illustrative examples of the haloisocyanuric acid and/or a
metal salt thereof include chloroisocyanuric acid, sodium
chloroisocyanurate, potassium chloroisocyanurate,
dichloroisocyanuric acid, sodium dichloroisocyanurate, sodium
dichloroisocyanurate dihydrate, potassium dichloroisocyanurate,
trichloroisocyanuric acid, tribromoisocyanuric acid,
dibromoisocyanuric acid, bromoisocyanuric acid, sodium and other
salts of dibromisocyanuric acid, as well as hydrates thereof, and
difluoroisocyanuric acid. Of these, chloroisocyanuric acid, sodium
chloroisocyanurate, potassium chloroisocyanurate,
dichloroisocyanuric acid, sodium dichloroisocyanurate, potassium
dichloroisocyanurate and trichloroisocyanuric acid are preferred
because they are readily hydrolyzed by water to form acid and
chlorine, and thus play the role of initiating addition reactions
to the double bonds in the diene rubber molecules. The use of
trichloroisocyanuric acid provides an especially outstanding
adhesion-improving effect.
[0077] The haloisocyanuric acid and/or a metal salt thereof is
preferably dissolved in an organic solvent and used as a solution.
A known organic solvent may be used for this purpose, with the use
of an organic solvent which is soluble in water being especially
preferred. Examples include ethyl acetate, acetone and methyl ethyl
ketone. Of these, acetone is especially preferred on account of its
ability to penetrate the core surface. The use of a water-soluble
solvent is preferable because such solvents readily take up
moisture; either the moisture which has been taken up readily
undergoes a hydrolysis reaction with the haloisocyanuric acid
and/or a salt thereof deposited on the core surface or, when water
washing is used in a subsequent step, as the affinity to the core
surface increases, a hydrolysis reaction between the water and the
haloisocyanuric acid and/or a metal salt thereof more readily
arises.
[0078] When dissolved in an organic solvent, the content of the
haloisocyanuric acid and/or a metal salt thereof in the solution is
preferably at least 0.3 wt %, more preferably at least 1 wt %, and
more preferably at least 2.5 wt %. At less than 0.3 wt %, the
adhesion improving effect anticipated following core surface
treatment may not be obtained, possibly resulting in a poor
durability to impact. The upper limit in the content may be as high
as the saturated solution concentration. However, from the
standpoint of cost effectiveness, when prepared as an acetone
solution, for example, setting the upper limit in content to 10 wt
% is preferred. The core is immersed in the solution for a length
of time which is preferably at least 0.3 second, more preferably at
least 3 seconds, and even more preferably at least 10 seconds, but
is preferably not more than 5 minutes, more preferably not more
than 1 minute, and even more preferably not more than 30 seconds.
If the immersion time is too short, the desired effects of
treatment may not be obtained, whereas if the immersion time is too
long, a loss in ball productivity may occur.
[0079] The method of treating the core surface with a
haloisocyanuric acid and/or a metal salt thereof is exemplified by
methods which involve coating the surface of the core with a
solution of haloisocyanuric acid and/or a metal salt thereof by
brushing or spraying on the solution, and methods in which the core
is immersed in a solution of the haloisocyanuric acid and/or a
metal salt thereof. From the standpoint of productivity and high
penetrability of the core surface by the solution, the use of an
immersion method is especially preferred.
[0080] After the core has been surface treated with a solution
containing haloisocyanuric acid and/or a metal salt thereof, it is
preferable to wash the surface of the core with water. Water
washing of the core surface may be carried out by a method such as
running water, spraying, or soaking in a washing tank. However,
because the aim here is not merely to wash, but also to initiate
and promote the desired treatment reactions, too vigorous a washing
method will not be appropriate. Therefore, preferred use may be
made of washing by soaking in a washing tank. In such a case, it is
desirable to place the cores to be washed from about one to five
times in a washing tank that has been filled with fresh water.
[0081] Treating the core surface with a haloisocyanuric acid and/or
a metal salt thereof greatly improves adhesion between the core
surface and the cover. The reason for this is not well understood,
but is thought to be as follows.
[0082] The haloisocyanuric acid and/or a metal salt thereof,
together with the solvent, penetrates to the interior of the diene
rubber making up the core and approaches the vicinity of the double
bonds on the backbone. Water then enters the core surface,
whereupon the haloisocyanuric acid and/or a metal salt thereof is
hydrolyzed by the water, releasing the halogen. The halogen attacks
the double bonds on the diene rubber backbone located nearby, as a
result of which an addition reaction proceeds. In the course of
this addition reaction, the liberated isocyanuric acid is added,
together with the halogen, to the diene rubber backbone while
retaining the cyclic structure. The added isocyanuric acid has
three --NHCO-- structures on the molecule.
[0083] Because --NHCO-- structures are thereby conferred to the
core surface that has been treated with the haloisocyanuric acid
and/or a metal salt thereof, adhesion with the cover material
improves further. It is most likely because of this that the
durability of the golf ball to impact improves. Moreover, when a
polyurethane elastomer or polyamide elastomer having the same
--NHCO-- structures on the polymer molecule is used as the cover
material, the affinity increases even further, presumably
increasing the durability to impact.
[0084] When the addition of isocyanuric acid and chlorine (as the
halogen) to the surface of diene rubber has occurred, changes in
the bonding states before and after addition appear in an infrared
absorption spectrum as increases in the C.dbd.O bond (stretching)
absorption peak at 1725 to 1705 cm.sup.-1, the broad H--H bond
(stretching) absorption peak at 3450 to 3300 cm.sup.-1, and the
C--Cl bond absorption peak at 800 to 600 cm.sup.-1. Hence, by
measuring the IR absorption spectrum of a surface-treated core and
confirming increases in these absorption peaks, it is possible to
qualitatively confirm that isocyanuric acid and chlorine addition
to the diene rubber molecules at the core surface has indeed
occurred.
[0085] Following surface treatment, when the material at the
surface portion of the solid core is examined by differential
scanning calorimetry (DSC), no exothermic or endothermic peaks are
observed from room temperature to 300.degree. C. This means that
the functional groups which have been introduced maintain a stable
state within this temperature range. In other words, during molding
of the cover material, the functional groups which have been
introduced do not undergo degradation or the like due to heat, and
continue to be effective. Also, because melting in the manner of a
hot melt resin does not arise, deleterious effects on durability
and quality of appearance, such as resin bleed out to the parting
line, do not occur. In addition, the very fact that the material in
the surface portion of the solid core following the surface
treatment described above is stable may be regarded as evidence
that the isocyanuric acid having a melting point above 300.degree.
C. has been added with its molecular structure still intact.
[0086] Next, the material making up the cover which directly
encases the core is described.
[0087] In the practice of the invention, although not subject to
any particular limitation, the resin component in the cover is
preferably composed primarily of polyurethane. Use may be made of a
thermoplastic polyurethane elastomer or a thermoset polyurethane
resin, with the use of a thermoplastic polyurethane elastomer being
especially preferred.
[0088] The thermoplastic polyurethane elastomer has a structure
composed of soft segments formed from a polymeric polyol (polymeric
glycol) and hard segments formed from a chain extender and a
diisocyanate. Here, the polymeric polyol serving as a starting
material may be any which has hitherto been used in the art
relating to thermoplastic polyurethane materials, and is not
subject to any particular limitation. Exemplary polymeric polyols
include polyester polyols and polyether polyols. Polyether polyols
are more preferable than polyester polyols because thermoplastic
polyurethane materials having a high rebound resilience and
excellent low-temperature properties can be synthesized.
Illustrative examples of polyether polyols include
polytetramethylene glycol and polypropylene glycol.
Polytetramethylene glycol is especially preferred from the
standpoint of the rebound resilience and the low-temperature
properties. The polymeric polyol has an average molecular weight of
preferably from 1,000 to 5,000. To synthesize a thermoplastic
polyurethane material having a high rebound resilience, an average
molecular weight of from 2,000 to 4,000 is especially
preferred.
[0089] The chain extender employed is preferably one which has
hitherto been used in the art relating to thermoplastic
polyurethane materials. Illustrative examples include, but are not
limited to, 1,4-butylene glycol, 1,2-ethylene glycol,
1,3-butanediol, 1,6-hexanediol and 2,2-dimethyl-1,3-propanediol.
These chain extenders have an average molecular weight of
preferably from 20 to 15,000.
[0090] The diisocyanate employed is preferably one which has
hitherto been used in the art relating to thermoplastic
polyurethane materials. Illustrative examples include, but are not
limited to, aromatic diisocyanates such as 4,4'-diphenylmethane
diisocyanate, 2,4-toluene diisocyanate and 2,6-toluene
diisocyanate, and aliphatic diisocyanates such as hexamethylene
diisocyanate. Depending on the type of isocyanate, control of the
crosslinking reaction during injection molding may be difficult. In
this invention, the use of 4,4'-diphenylmethane diisocyanate, which
is an aromatic diisocyanate, is most preferred.
[0091] A commercial product may be advantageously used as the
thermoplastic polyurethane material composed of the above
materials. Illustrative examples include those available under the
trade names Pandex T8180, Pandex T8195, Pandex T8290, Pandex T8295
and Pandex T8260 (all available from DIC Bayer Polymer, Ltd.), and
those available under the trade names Resamine 2593 and Resamine
2597 (available from Dainichiseika Color & Chemicals Mfg. Co.,
Ltd.).
[0092] The cover has a thickness which is preferably at least 0.3
mm, more preferably at least 0.5 mm, and even more preferably at
least 0.7 mm, but is preferably not more than 1.9 mm, more
preferably not more than 1.8 mm, and even more preferably not more
than 1.7 mm. If the cover thickness is larger than the above range,
the ball rebound may decrease and the flight performance may
worsen. On the other hand, if the cover thickness is smaller than
the above range, the durability to cracking will decrease. In
particular, when the ball is hit thin, or "topped," the cover may
tear.
[0093] The cover has a specific gravity which is preferably at
least 1.13, more preferably at least 1.14, and even more preferably
at least 1.15, but preferably not more than 1.30, more preferably
not more than 1.20, and even more preferably not more than
1.17.
[0094] It is critical for the cover to have a material hardness,
expressed as the Shore D hardness, of at least 30, preferably at
least 35, and more preferably at least 38, but not more than 57,
preferably not more than 55, more preferably not more than 53, and
even more preferably not more than 51. If the Shore D hardness of
the cover material is higher than the above range, the appearance
performance in long-term use (referred to below as "durability of
markings," examples of markings being a brand name or player number
printed on the surface of the ball at the time of manufacture) will
decline, in addition to which the flight performance will markedly
decrease. On the other hand, if the Shore D hardness of the cover
material is lower than the above range, the durability to cracking
will markedly decrease and, particularly when the ball is topped,
the cover may tear. In addition, the spin rate becomes very high,
shortening the distance traveled by the ball.
[0095] The inventive golf ball, when initially measured, has a
deflection BH1 (mm) when compressed under a final load of 1,275 N
(130 kgf) from an initial load of 98 N (10 kgf) and an initial
velocity BV1 (m/s) and, when measured again after being left to
stand for 350 days following initial measurement, has a deflection
BH2 (mm) when compressed under a final load of 1,275 N (130 kgf)
from an initial load of 98 N (10 kgf) and an initial velocity BV2
(m/s), such that the difference BH2-BH1 is preferably not more than
0.2 mm, more preferably not more than 0.15 mm, and even more
preferably not more than 0.1 mm, and such that the difference
BV2-BV1 is preferably not more than 0.3 m/s, more preferably not
more than 0.2 m/s, and even more preferably not more than 0.1 m/s.
As a result, even in long-term use, the ball retains a good
appearance and a good flight performance.
[0096] Moreover, at the time of initial measurement, the deflection
BH1 of the practice golf ball of the invention when compressed
under a final load of 1,275 N (130 kgf) from an initial load of 98
N (10 kgf), although not subject to any particular limitation, is
preferably at least 2.0 mm, more preferably at least 2.3 mm, and
even more preferably at least 2.4 mm, but is preferably not more
than 7.0 mm, more preferably not more than 6.0 mm, even more
preferably not more than 5.0 mm, and most preferably not more than
4.5 mm.
[0097] Also, letting the respective deflections of the core and the
ball when each is compressed under a final load of 1,275 N (130
kgf) from an initial load of 98 N (10 kgf) be CH and BH1, the ratio
CH/BH1 is preferably at least 0.95, more preferably at least 0.96,
and even more preferably at least 0.97, but is preferably not more
than 1.1, more preferably not more than 1.08, and even more
preferably not more than 1.07. If the ratio CH/BH1 is too large,
the finished ball may have too small a deflection relative to the
core deflection. That is, the cover hardness may be too high, which
may lower the feel on impact and lead to a decline in appearance
with long-term use. On the other hand, if the ratio CH/BH1 is too
small, the cover may be too soft, greatly lowering the durability
to cracking, which may result in cracking of the cover when the
ball is topped. Also, the spin rate may become very high, possibly
leading to a decrease in distance.
[0098] The inventive ball also has, at the time of initial
measurement, a rebound BV1 which is preferably at least 65 m/s,
more preferably at least 68 m/s, even more preferably at least 71
m/s, and most preferably at least 73 m/s, but is preferably not
more than 76 m/s, more preferably not more than 75.7 m/s, even more
preferably not more than 75.4 m/s, and most preferably not more
than 75 m/s. A core rebound outside of this range is undesirable
because the distance of the ball may dramatically decline or the
ball may fly so far as to pass over the netting at a driving
range.
[0099] In the invention, it is critical for the core to have an
initial velocity CV (m/s) and the ball to have an initial velocity
BV1 (m/s), as defined above, such that the value CV-BV1 satisfies
the following condition.
(CV-BV1)=0.7.times.(cover thickness)-b
Here, 0.1.ltoreq.b.ltoreq.1.0, b being the value of the intercept
in the above formula.
[0100] At values of CV-BV1 outside of the above range, the
difference between the core rebound and the ball rebound becomes
large, deadening the feel of the ball, which may cause fatigue in
the player when striking the ball.
[0101] The practice golf ball of the invention typically has
numerous dimples formed on the surface thereof, each dimple having
a spatial volume below a flat plane circumscribed by an edge of the
dimple. Although not subject to any particular limitation, the sum
of the dimple spatial volumes, expressed as a percentage (VR) of
the volume of a hypothetical sphere representing the ball were the
ball to have no dimples on the surface thereof, is preferably in a
range of from 0.8% to 1.7%, the lower limit being more preferably
0.83%, even more preferably 0.85%, and most preferably 0.86%, and
the upper limit being more preferably 1.5%, even more preferably
1.3%, and most preferably 1.2%.
[0102] Also, although not subject to any particular limitation, the
dimples formed on the practice golf ball of the invention
preferably satisfy conditions (1) and (2) below. Although it is
preferable for both of the following conditions (1) and (2) to be
satisfied at the same time, it is acceptable for either one of
these conditions alone to be satisfied.
[0103] First, referring to FIG. 3, as condition (1), it is
preferable for the dimples to have a peripheral edge provided with
a roundness represented by a radius of curvature R in a range of
from 0.5 mm to 2.5 mm. The lower limit of the radius of curvature R
is more preferably 0.6 mm, and even more preferably 0.7 mm, and the
upper limit is more preferably 1.8 mm, and even more preferably 1.5
mm.
[0104] Next, as condition (2), it is preferable for the ratio ER of
a collective number of dimples RA having a radius of curvature R to
diameter D ratio (R/D) of at least 20%, divided by a total number
of dimples N on the surface of the ball, to be in a range of from
15% to 95%. Here, the ratio R/D is expressed as a percentage
(R/D.times.100%), a larger value indicating a dimple in which the
rounded part of the dimple accounts for a larger proportion of the
dimple size and which has a smoother cross-sectional shape. The
ratio ER indicates the number of such smooth dimples as a
proportion of the total number of dimples; by setting ER in a range
of from 15% to 95%, damage to the paint film at dimple edges can be
effectively suppressed. The upper limit in the ratio R/D, although
not subject to any particular limitation, is preferably not more
than 60%, and more preferably not more than 40%. The lower limit in
the ratio ER is more preferably 20%, and even more preferably 25%,
and the upper limit is more preferably 90%, even more preferably
85%, and most preferably 70%.
[0105] Also, although not subject to any particular limitation, it
is preferable for condition (3) to be satisfied. That is, as
condition (3), it is preferable for the ball to have thereon a
plurality of dimple types of differing diameter, and for the ratio
DER of a combined number of dimples DE obtained by adding together
dimples having an own diameter and an own radius of curvature
larger than or equal to a radius of curvature of dimples of larger
diameter than the own diameter plus dimples of a type having a
largest diameter, divided by the total number of dimples N on the
surface of the ball, to be at least 80%.
[0106] Generally, at a fixed dimple depth (see FIG. 3), the radius
of curvature R representing the roundness provided to the
peripheral edges of the dimples is smaller at smaller dimple
diameters D. However, above condition (3), by such means as
adjusting the depth, sets the roundness R represented by the
peripheral edge to be as large as possible even in dimples of a
small diameter D, thus forming dimples having a smooth
cross-sectional shape and, by setting the above ratio DER to at
least 80%, increases the proportion of such smooth dimples, more
effectively suppressing damage to the paint film. The ratio DER is
more preferably at least 85%, even more preferably at least 90%,
and most preferably at least 93%. The upper limit in the ratio DER
is 100%.
[0107] In addition, the dimples formed on the practice golf ball of
the invention, although not subject to any particular limitation,
preferably satisfy conditions (4) to (6) below. Although it is
preferable for all of the following conditions (4) to (6) to be
satisfied at the same time, it is acceptable for any one of these
conditions alone to be satisfied.
[0108] First, as condition (4), it is preferable for the number of
dimple types of differing diameter D on the ball to be 3 or more,
and more preferable for dimples of at least five types to be
formed. In this case, the diameters D of the dimples, although not
subject to any particular limitation, are preferably set in a range
of from 1.5 mm to 7 mm, the lower limit being more preferably 1.8
mm and the upper limit being more preferably 6.5 mm. The depths of
the dimples, although likewise not subject to any particular
limitation, are preferably set in a range of from 0.05 mm to 0.35
mm, the lower limit being more preferably ab 0.1 mm and the upper
limit being more preferably 0.3 mm, and even more preferably 0.25
mm.
[0109] As condition (5), the total number of dimples N on the
surface of the ball is preferably not more than 380, and more
preferably not more than 350. The total number of dimples N is even
more preferably in a range of from 220 to 340.
[0110] As condition (6), it is preferable for the dimples to be
formed in such a way that the surface coverage SR of the dimples,
which is the sum of individual dimple surface areas, each defined
by a flat plane circumscribed by an edge of the dimple (dash-dot
line in FIG. 3), expressed as a percentage of the surface area of a
hypothetical sphere representing the ball were the ball to have no
dimples on the surface thereof (broken line in FIG. 3), is in a
range of from 60% to 74%. At an SR greater than 74%, the intervals
between neighboring dimples become too narrow, which may make it
difficult to provide the dimple edges with a roundness having the
radius of curvature R specified in above condition (1). On the
other hand, at a surface coverage SR below 60%, the aerodynamic
performance decreases, as a result of which the distance traveled
by the ball may decrease. The surface coverage SR has a lower limit
of more preferably 65%, and even more preferably 68%, and an upper
limit of more preferably 73%.
[0111] To ensure a good appearance, it is preferable to apply a
clear paint to the surface of the inventive ball. The resulting
clear coat has a thickness at dimple lands (Y) which is preferably
at least 10 .mu.m, more preferably at least 12 .mu.m, and most
preferably at least 13 .mu.m, but is preferably not more than 30
.mu.m, more preferably not more than 25 .mu.m, and most preferably
not more than 20 .mu.m; and a thickness at dimple edges (Z) which
is preferably at least 8 .mu.m, more preferably at least 10 .mu.m,
and most preferably at least 11 .mu.m, but is preferably not more
than 28 .mu.m, more preferably not more than 23 .mu.m, and most
preferably not more than 18 .mu.m. Also, the ratio Z/Y of edge
areas (Z) to land areas (Y), expressed as a percentage, is
preferably at least 60%, more preferably at least 70%, and most
preferably at least 80%, but is preferably not more than 100%, and
more preferably not more than 95%. Outside of the above range, the
durability of markings at dimple edges decreases significantly in
long-term use.
[0112] The ball diameter is preferably at least 42 mm, more
preferably at least 42.3 mm, and even more preferably at least
42.67 mm, but is preferably not more than 44 mm, more preferably
not more than 43.8 mm, even more preferably not more than 43.5 mm,
and most preferably not more than 43 mm.
[0113] The ball weight is preferably at least 44.5 g, more
preferably at least 44.7 g, even more preferably at least 45.1 g,
and most preferably at least 45.2 g, but is preferably not more
than 47.0 g, more preferably not more than 46.5 g, and even more
preferably not more than 46.0 g.
EXAMPLES
[0114] The following Examples and Comparative Examples are provided
to illustrate the invention, and are not intended to limit the
scope thereof.
Examples 1 to 6, Comparative Examples 1 to 6
[0115] Rubber materials formulated as shown in Table 1 below were
furnished for the fabrication of practice golf balls in the
Examples and Comparative Examples. These rubber compositions were
suitably mixed using a kneader or roll mill, then vulcanized under
the temperature and time conditions in Table 1 to produce solid
cores in the respective Examples. Ingredient amounts in the table
below are shown in parts by weight.
TABLE-US-00001 TABLE 1 Type of Core (1) (2) (3) (4) (5) (6) (7)
Core BR01 100 60 95 95 100 100 formulation IR2200 5 5 5 BR730 95
BR51 40 Perhexa C-40 0.6 0.6 (40% dilution) Actual amount 0.24 0.24
of addition Percumyl D 1.07 1.07 0.8 0.8 1.07 0.6 0.6 Zinc oxide 23
23 23 23 23 6 6 Antioxidant 0.2 0.2 0.2 0.2 0.2 0.2 0.2 Methacrylic
acid 22.5 22.5 18 22.5 22.5 Zinc methacrylate 33 Zinc acrylate 33
Vulcanization Temperature (.degree. C.) 170 170 170 170 170 160 160
conditions Time (minutes) 20 20 20 20 20 13 13
[0116] Details on the materials used in the core formulations in
the above table are provided below. [0117] (1) BR01: A butadiene
rubber synthesized with a nickel catalyst, available from JSR
Corporation; Mooney viscosity ML, 46 [0118] (2) IR2200: An isoprene
rubber, available from JSR Corporation; Mooney viscosity ML, 82
[0119] (3) BR730: A butadiene rubber synthesized with a neodymium
catalyst, available from JSR Corporation; Mooney viscosity ML, 55
[0120] (4) BR51: A butadiene rubber synthesized with a neodymium
catalyst, available from JSR Corporation; Mooney viscosity ML, 36
[0121] (5) Perhexa C-40: An organic peroxide, available from NOF
Corporation [0122] (6) Percumyl D: An organic peroxide, available
from NOF Corporation [0123] (7) Zinc oxide: Available from Sakai
Chemical Co., Ltd. [0124] (8) Antioxidant: "Nocrac NS-6," available
from Ouchi Shinko Chemical Industry Co., Ltd. [0125] (9)
Methacrylic acid: Available from Kuraray Co., Ltd. [0126] (10) Zinc
methacrylate: Available from Asada Chemical Industry Co., Ltd.
[0127] (11) Zinc acrylate: Available from Nihon Jyoryu Kogyo Co.,
Ltd.
[0128] In each example, after the rubber composition formulated
from the ingredients shown in Table 1 was molded and vulcanized to
form a core, the surface of the core was abraded to a desired
diameter. Next, surface treatment of the core was carried out by
immersing the core for 30 seconds in an acetone solution of
trichloroisocyanuric acid (concentration, 3 wt %), then washing the
surface of the core with water. The core was then set in a mold for
injection molding the cover, and the cover composition shown in
Table 2 below was injection molded over the solid core. Ingredient
amounts in the table below are shown in parts by weight.
TABLE-US-00002 TABLE 2 A B C D E F Resin Himilan 7331 50 Himilan
1557 30 Himilan 1855 20 Pandex T8260 25 100 Pandex T8195 100 75 25
Pandex T8290 75 Pandex T8180 100 Additives Magnesium 1 stearate
Titanium dioxide 3.5 3.5 3.5 3.5 3.5 2.1 Polyethylene wax 1.5 1.5
1.5 1.5 1.5 Shore D hardness 45 50 40 58 25 50
[0129] Details on the materials used in the cover composition in
the above table are provided below. [0130] "Himilan": Ionomer
resins available under this trade name from DuPont-Mitsui
Polychemicals Co., Ltd. [0131] "Pandex": Thermoplastic polyurethane
elastomers available under this trade name from Dainippon Ink &
Chemicals, Inc. [0132] Magnesium stearate: Available from NOF
Corporation [0133] Titanium dioxide: Available under the trade name
"Tipaque R550" from Ishihara Sangyo Kaisha, Ltd. [0134]
Polyethylene wax: Available under the trade name "Sanwax 161P" from
Sanyo Chemical Industries, Ltd.
[0135] In order to form a predetermined dimple pattern on the
surface of the cover, a plurality of protrusions corresponding to
the dimple pattern were formed in the mold cavity, by means of
which dimples were impressed onto the surface of the cover at the
same time that the cover was injection molded. Details on the
dimples are given below in Table 3. The markings shown in FIG. 5
were printed on the ball surface.
[0136] In addition, the ball was clear-coated with a paint composed
of 100 parts by weight of polyester resin (acid value, 6; hydroxyl
value, 168) (solids)/butyl acetate/PMA (propylene glycol monomethyl
ether acetate) in a weight ratio of 70/15/15 as the base; 150 parts
by weight of a non-yellowing polyisocyanate, specifically a
hexamethylene diisocyanate adduct (available from Takeda
Pharmaceutical Co., Ltd. as Takenate D-160N; NCO content, 8.5 wt %;
solids content, 50 wt %) as the curing agent; and 150 parts by
weight of butyl acetate. In Comparative Example 4, a coating of
white enamel paint was applied as a base coat for clear
coating.
TABLE-US-00003 TABLE 3 Dimple Diameter D Depth R R/D N RA ER DE DER
SR VR No. Number (mm) (mm) (mm) ratio (number) (number) (%)
(number) (%) (%) (%) Configuration Dimple I 1 24 4.4 0.182 0.75 17
338 102 30 330 98 72 0.86 FIG. 4 2 204 4.2 0.175 0.8 19 3 66 3.6
0.165 0.8 22 4 12 2.7 0.135 0.9 33 5 24 2.5 0.105 0.9 36 6 8 3.4
0.145 0.6 18 Dimple II 1 24 4.4 0.207 0.7 16 338 102 30 330 98 72
0.99 FIG. 4 2 204 4.2 0.200 0.7 17 3 66 3.6 0.190 0.8 22 4 12 2.7
0.160 0.85 31 5 24 2.5 0.130 0.85 34 6 8 3.4 0.145 0.55 16 Dimple
III 1 24 4.4 0.216 0.5 11 338 36 11 306 91 72 0.99 FIG. 4 2 204 4.2
0.209 0.5 12 3 66 3.6 0.194 0.6 17 4 12 2.7 0.151 0.6 22 5 24 2.5
0.116 0.5 20 6 8 3.4 0.160 0.5 15
[0137] The abbreviations and symbols relating to dimples which
appear in Table 3 are explained below. [0138] R: Radius of
curvature representing roundness provided at peripheral edge of a
dimple [0139] R/D ratio: Ratio of radius of curvature R to diameter
D [0140] N: Total number of dimples on surface of ball [0141] RA:
Collective number of dimples having an R/D ratio of at least 20%
[0142] ER: Ratio of RA to total number of dimples N [0143] DE: Sum
of number of dimples having an own diameter and an own radius of
curvature larger than or equal to a radius of curvature of dimples
of larger diameter than the own diameter, plus number of dimples of
a type having a largest diameter [0144] DER: Ratio of DE to total
number of dimples N [0145] SR: Sum of individual dimple surface
areas, each defined by a flat plane circumscribed by an edge of the
dimple, expressed as a percentage of the surface area of a
hypothetical sphere representing the ball were the ball to have no
dimples on the surface thereof [0146] VR: Sum of individual dimple
spatial volumes, each formed below a flat plane circumscribed by an
edge of the dimple, expressed as a percentage of the volume of a
hypothetical sphere representing the ball were the ball to have no
dimples on the surface thereof
[0147] The physical properties of the cores and covers in the
respective examples of the invention and the comparative examples,
and the physical properties, distance, durability and feel of the
practice balls obtained in each example were measured or evaluated
as described below. The results are presented in Table 4.
Deflection of Core and Finished Ball (mm)
[0148] The deflection (mm) of the core or finished ball as the test
sphere when compressed at a rate of 10 mm/min under a final load of
1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) was
measured. The test was performed using a model 4204 test system
from Instron Corporation.
Cross-Sectional Hardness of Core
[0149] The core was cut with a fine cutter and the JIS-C hardnesses
at above positions B to F were measured in accordance with JIS
K6301-1975 after holding the core isothermally at 23.+-.1.degree.
C. (at two places in each of N=5 samples).
Surface Hardness of Core
[0150] JIS-C hardness measurements were carried out on the core
surface in accordance with JIS K6301-1975 after holding the core
isothermally at 23.+-.1.degree. C. (at two places in each of N=5
samples).
Rebound (Initial Velocity) of Core and Finished Ball
[0151] The initial velocity was measured using an initial velocity
measuring apparatus of the same type as the USGA drum rotation-type
initial velocity instrument approved by the R&A. The cores or
balls used as the samples were held isothermally at a temperature
of 23.+-.1.degree. C. for at least 3 hours, then tested in a room
temperature (23.+-.2.degree. C.) chamber. Ten balls were each hit
twice, and the time taken for the cores or balls to traverse a
distance of 6.28 ft (1.91 m) was measured and used to compute the
initial velocity.
Cover Material Hardness
[0152] A cover sheet was formed and, after holding the samples
isothermally at 23.+-.1.degree. C., the Shore D hardness was
measured in accordance with ASTM D-2240.
Durability to Cracking
[0153] The ball was hit thin ("topped") five times at the same
place with the leading edge of a number nine iron (X-BLADE GR,
manufactured by Bridgestone Sports Co., Ltd.) at a head speed (HS)
of 38 m/s, following which the ball was repeatedly struck against a
wall at an incident velocity of 43 m/s and the average number of
shots (N=3 balls) until cracking occurred was determined.
Abrasion Test
[0154] Ten golf balls and 3 liters of bunker sand were placed in a
magnetic ball mill having an 8 liter capacity and mixing was
carried out for 144 hours, following which the balls were visually
examined for any loss of the markings and to assess the degree of
surface scratching, the degree of loss of luster and the degree of
sand adhesion. The ball appearance was rated as "good," "fair" or
"NG."
Measurement of Coating Thickness
[0155] Lands (Y): The thickness of the clear coat at land areas at
intermediate positions between dimples was measured. [0156] Edges
(Z): The thickness of the clear coat at dimple edge areas was
measured.
[0157] The above measurements were carried out at three places on
each of two balls in the respective examples, and the average of
these measurements was determined.
Distance
[0158] A TourStage X-Drive 701 (loft angle, 9.degree.),
manufactured by Bridgestone Sports Co., Ltd., was mounted as the
driver (W#1) on a golf swing robot and struck at a head speed (HS)
of 45 m/s. Both the spin rate of the ball immediately after impact
and the total distance traveled by the ball were measured.
[0159] In addition, after the abrasion test described above had
been carried out, the total distance of the ball was again
measured.
Feel
[0160] Ten teaching professionals hit the test balls with a driver
(W#1) and rated the feel of the balls on impact as good, somewhat
hard (fair), or too hard (NG).
TABLE-US-00004 TABLE 4 Example 1 2 3 4 5 6 Core Type (1) (2) (3)
(4) (5) (5) Diameter, mm 39.9 39.9 39.9 39.9 39.9 39.9 Specific
gravity 1.118 1.118 1.118 1.118 1.118 1.118 Deflection under 10-130
kg 2.6 2.5 3.8 3.1 2.75 2.75 compression (CH), mm Rebound (CV), m/s
74.9 74.9 74.7 74.1 74.7 74.7 JIS-C hardness 81 83 69 72 80 80 at
core surface (A) JIS-C hardness 76 78 63 64 75 75 2 mm inside core
surface (B) JIS-C hardness 79 80 67 69 77 77 5 mm inside core
surface (C) JIS-C hardness 74 74 66 69 71 71 10 mm inside core
surface (D) JIS-C hardness 69 68 64 67 67 67 15 mm inside core
surface (E) JIS-C hardness 66 64 61 64 63 63 at core center (F)
JIS-C hardness difference between 2 3 2 3 3 3 core surface and 5 mm
inside core (A - C) JIS-C hardness difference between 15 19 8 8 17
17 core surface and center (A - F) Cover Type A A A A B C Shore D
hardness 45 45 45 45 50 40 Specific gravity 1.15 1.15 1.15 1.15
1.15 1.15 Thickness, mm 1.4 1.4 1.4 1.4 1.4 1.4 Finished ball
Deflection under 10-130 kg compression 2.65 2.55 3.8 3 2.6 2.8 30
days after production (BH1), mm Deflection under 10-130 kg
compression 2.55 2.5 3.7 2.9 2.5 2.7 350 days after measuring BH1
(BH2), mm Difference between BH1 and -0.1 -0.05 -0.1 -0.1 -0.1 -0.1
BH2, mm Rebound 30 days after 74.2 74.2 74.1 73.5 74 74.2
production (BV1), m/s Rebound 350 days after 74.2 74.3 74.2 73.6
74.1 74.2 measuring BV1 (BV2), m/s Difference between BV1 and 0 0.1
0.1 0.1 0.1 0 BV2, m/s Diameter, mm 42.7 42.7 42.7 42.7 42.7 42.7
Core initial velocity - 0.7 0.7 0.6 0.6 0.7 0.5 ball initial
velocity (CV - BV1) Core deflection/ball deflection 0.98 0.98 1.00
1.03 1.06 0.98 (CH/BH1) Dimples Type I I II II I I Clear coating
Land areas (Y), .mu.m 16 16 15 15 16 16 thickness Edge areas (Z),
.mu.m 14 14 13 13 14 14 Coating thickness ratio 88 87.5 88 88 87.5
87.5 (Z/Y .times. 100), % Distance HS 45, driver Spin rate, rpm
3370 3380 3090 3220 3270 3440 (30 days after Total distance, m 223
223 215 213 223 222 production) HS 45, driver Total distance, m 220
220 212 211 218 220 (after abrasion test) Distance Total distance,
m -3 -3 -3 -2 -5 -2 difference Durability Durability to At incident
1103 1095 1050 1095 1230 873 cracking velocity of 43 m/s Abrasion
test After 144 hours good good good good fair good (durability of
of sand abrasion markings) Feel Driver good good good good fair
good Comparative Example 1 2 3 4 5 6 Core Type (6) (7) (7) (5) (5)
(5) Diameter, mm 39.9 39.9 39.9 39.9 39.9 39.9 Specific gravity
1.118 1.118 1.118 1.118 1.118 1.118 Deflection under 10-130 kg 2.75
2.75 2.75 2.75 2.75 2.75 compression (CH), mm Rebound (CV), m/s
77.2 77.6 77.6 74.7 74.7 74.7 JIS-C hardness 80 80 80 80 80 80 at
core surface (A) JIS-C hardness 75 75 75 75 75 75 2 mm inside core
surface (B) JIS-C hardness 77 77 77 77 77 77 5 mm inside core
surface (C) JIS-C hardness 71 71 71 71 71 71 10 mm inside core
surface (D) JIS-C hardness 67 67 67 67 67 67 15 mm inside core
surface (E) JIS-C hardness 63 63 63 63 63 63 at core center (F)
JIS-C hardness difference between 3 3 3 3 3 3 core surface and 5 mm
inside core (A - C) JIS-C hardness difference between 17 17 17 17
17 17 core surface and center (A - F) Cover Type A A A D E F Shore
D hardness 45 45 45 58 25 50 Specific gravity 1.15 1.15 1.15 1.15
1.15 0.99 Thickness, mm 1.4 1.4 1.4 1.4 1.4 1.4 Finished ball
Deflection under 10-130 kg compression 2.75 2.75 2.75 2.40 2.8 2.9
30 days after production (BH1), mm Deflection under 10-130 kg
compression 2.5 2.5 2.5 2.4 2.7 2.75 350 days after measuring BH1
(BH2), mm Difference between BH1 and -0.25 -0.25 -0.25 0.0 -0.1
-0.15 BH2, mm Rebound 30 days after 76.2 76.6 76.6 73.9 74.2 73.5
production (BV1), m/s Rebound 350 days after 75.7 75.8 75.8 74.0
74.2 73.6 measuring BV1 (BV2), m/s Difference between BV1 and -0.5
-0.8 -0.8 0.1 0 0.1 BV2, m/s Diameter, mm 42.7 42.7 42.7 42.7 42.7
42.7 Core initial velocity - 1.0 1.0 1.0 0.8 0.5 1.2 ball initial
velocity (CV - BV1) Core deflection/ball deflection 1.00 1.00 1.00
1.15 0.98 0.95 (CH/BH1) Dimples Type I I III I I I Clear coating
Land areas (Y), .mu.m 16 16 17 16 16 16 thickness Edge areas (Z),
.mu.m 14 14 8 14 14 14 Coating thickness ratio 88 88 47 88 88 88
(Z/Y .times. 100), % Distance HS 45, driver Spin rate, rpm 3360
3340 3340 3170 3700 3300 (30 days after Total distance, m 233 235
235 223 218 220 production) HS 45, driver Total distance, m 230 232
228 215 216 208 (after abrasion test) Distance Total distance, m -3
-3 -7 -8 -2 -12 difference Durability Durability to At incident 615
579 579 1350 657 1020 cracking velocity of 43 m/s Abrasion test
After 144 hours good good NG NG good NG (durability of of sand
abrasion markings) Feel Driver good good good NG good good
[0161] The practice golf ball in Comparative Example 1 included
zinc methacrylate as the co-crosslinking agent in the core-forming
rubber formulation. As a result, the durability to cracking was
poor.
[0162] The practice golf ball in Comparative Example 2 included
zinc acrylate as the co-crosslinking agent in the core-forming
rubber formulation. As a result, the durability to cracking was
very poor.
[0163] The practice golf ball in Comparative Example 3 included
zinc acrylate as the co-crosslinking agent in the core-forming
rubber formulation. As a result, the durability to cracking was
very poor. In addition, the radius of curvature at the dimple edges
was small and the coating thickness ratio between the dimple edges
and the lands was small. As a result, the durability of markings
was very poor, in addition to which the flight performance
following the abrasion test (after evaluating durability of
markings) decreased.
[0164] In the practice golf ball in Comparative Example 4, the
cover was too hard. As a result, the durability markings was very
poor. Moreover, the fight performance following the abrasion test
(after evaluating durability of markings) greatly decreased.
[0165] In the practice golf ball in Comparative Example 5, the
cover was too soft. As a result, the durability to cracking was
poor. In particular, the durability to cutting decreased. Moreover,
the ball had a high spin rate on shots with a W#1, resulting in a
decreased distance.
[0166] In the practice golf ball in Comparative Example 6, the
cover was formed primarily of an ionomer. As a result, the
durability of markings was very poor, in addition to which the
flight performance following the abrasion test (after evaluating
durability of markings) greatly decreased.
[0167] Japanese Patent Application No. 2011-099803 is incorporated
herein by reference.
[0168] Although some preferred embodiments have been described,
many modifications and variations may be made thereto in light of
the above teachings. It is therefore to be understood that the
invention may be practiced otherwise than as specifically described
without departing from the scope of the appended claims.
* * * * *