U.S. patent application number 13/606325 was filed with the patent office on 2014-03-13 for golf ball.
This patent application is currently assigned to BRIDGESTONE SPORTS CO., LTD.. The applicant listed for this patent is Daisuke ARAI, Hiroshi HIGUCHI, Takashi OHIRA, Yuichiro OZAWA, Katsunori SATO. Invention is credited to Daisuke ARAI, Hiroshi HIGUCHI, Takashi OHIRA, Yuichiro OZAWA, Katsunori SATO.
Application Number | 20140073455 13/606325 |
Document ID | / |
Family ID | 50233818 |
Filed Date | 2014-03-13 |
United States Patent
Application |
20140073455 |
Kind Code |
A1 |
OZAWA; Yuichiro ; et
al. |
March 13, 2014 |
GOLF BALL
Abstract
The invention provides a golf ball endowed with an improved
distance and durability. The ball has a core and a cover, the core
being formed of a rubber composition containing a base rubber, a
co-crosslinking agent, a crosslinking initiator and a metal oxide.
The co-crosslinking agent is methacrylic acid. The core has a
hardness profile in which, letting (A) be the JIS-C hardness at the
core surface, (B) be the JIS-C hardness at a position 2 mm in from
the surface toward the center of the core, (C) be the JIS-C
hardness at a position 5 mm in from the surface toward the center
of the core, (D) be the JIS-C hardness at a position 10 mm in from
the surface toward the center of the core, (E) be the JIS-C
hardness at a position 15 mm in from the surface toward the center
of the core, and (F) be the JIS-C hardness at the center of the
core, (A) to (F) have specific hardnesses.
Inventors: |
OZAWA; Yuichiro;
(Chichibu-shi, JP) ; ARAI; Daisuke; (Chichibu-shi,
JP) ; HIGUCHI; Hiroshi; (Chichibu-shi, JP) ;
OHIRA; Takashi; (Chichibu-shi, JP) ; SATO;
Katsunori; (Chichibu-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
OZAWA; Yuichiro
ARAI; Daisuke
HIGUCHI; Hiroshi
OHIRA; Takashi
SATO; Katsunori |
Chichibu-shi
Chichibu-shi
Chichibu-shi
Chichibu-shi
Chichibu-shi |
|
JP
JP
JP
JP
JP |
|
|
Assignee: |
BRIDGESTONE SPORTS CO.,
LTD.
Tokyo
JP
|
Family ID: |
50233818 |
Appl. No.: |
13/606325 |
Filed: |
September 7, 2012 |
Current U.S.
Class: |
473/372 |
Current CPC
Class: |
A63B 37/0018 20130101;
A63B 37/0063 20130101; A63B 37/002 20130101; A63B 37/0083 20130101;
A63B 37/0012 20130101; A63B 37/0065 20130101; A63B 37/0074
20130101; A63B 37/008 20130101; A63B 37/0087 20130101; A63B 37/0084
20130101 |
Class at
Publication: |
473/372 |
International
Class: |
A63B 37/06 20060101
A63B037/06 |
Claims
1. A golf ball comprising a core and a cover, wherein the core is
formed of a rubber composition comprising a base rubber, a
co-crosslinking agent, a crosslinking initiator and a metal oxide,
the co-crosslinking agent being methacrylic acid; wherein 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 in from the surface toward a center of the core, (C) be the
JIS-C hardness at a position 5 mm in from the surface toward the
center of the core, (D) be the JIS-C hardness at a position 10 mm
in from the surface toward the center of the core, (E) be the JIS-C
hardness at a position 15 mm in from the surface toward the center
of the core, and (F) be the JIS-C hardness at the center of the
core: (A) is from 75 to 95, (B) is from 70 to 90, (C) is from 68 to
88, (D) is from 65 to 80, (E) is from 58 to 73, (F) is from 50 to
65, the hardness relationship
(A)>(B).gtoreq.(C)>(D)>(E)>(F) is satisfied, the value
(A)-(F) is at least 20, the core is formed in such a way that (A)
has the highest hardness value among (A) to (F), and the value
(A)-(C) is from 3 to 15; and wherein the core has a specific
gravity of from 1.05 to 1.2.
2. The golf ball of claim 1, wherein the crosslinking initiator is
included in an amount of from 1.2 to 5 parts by weight per 100
parts by weight of the base rubber.
3. The golf ball of claim 1, wherein the rubber composition further
includes a fatty acid metal salt.
4. The golf ball of claim 3, wherein the fatty acid metal salt is
zinc stearate.
5. The golf ball of claim 3, wherein the fatty acid metal salt is
included in an amount of from 0.1 to 5 parts by weight per 100
parts by weight of the base rubber.
6. The golf ball of claim 1, wherein the cover is formed of a resin
material which has a breaking strength of from 20 to 80 MPa and an
elongation of from 150 to 600%.
7. The golf ball of claim 1, wherein the metal oxide in the rubber
composition is zinc oxide.
8. The golf ball of claim 1, wherein the core has a deflection
(CH), when compressed under a final load of 1,275 N (130 kgf) from
an initial load of 98 N (10 kgf), of from 2.0 to 7.0 mm.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a golf ball composed of a
core and a cover. More specifically, the invention relates to a
golf ball which has a reduced spin rate and thus can achieve an
increased distance, and which moreover has a good durability even
in long-term use.
[0002] In contemplating the ball construction of golf balls
intended for use in competition, to impart a flight performance and
feel of the sort typically experienced when playing a round of
golf, it is more desirable to use a two-piece solid golf ball than
a one-piece ball. Particularly in golf balls composed of a core and
a cover, conferring the ball with an increased distance owing to a
reduced spin rate is one of the most desirable characteristics
because it enables the game to be played with a competitive
advantage.
[0003] As is widely known, two-piece solid golf balls are composed
of a core and a cover, with the core being a rubber crosslinked
material of certain desirable properties obtained by using a base
rubber composed primarily of cis-1,4-polybutadiene rubber to which
compounding ingredients such as a co-crosslinking agent, a metal
oxide and an organic peroxide have been added. For example, JP-A
59-49779 teaches, as the rubber composition for the core of a
two-piece solid golf ball, the compounding of a given amount of
zinc methacrylate as a co-crosslinking agent in
cis-1,4-polybutadiene rubber. However, when zinc methacrylate is
used in this way in a core-forming rubber composition, achieving
good ball durability in long-term use has been difficult.
[0004] In addition, JP-A 2003-70936, JP-A 2007-61614, JP-A
2007-301357, JP-A 2010-115485, JP-A 2010-115486, JP-A 2004-180793,
JP-A 2008-149190, JP-A 2009-195761, JP-A 2005-27814, and JP-A
2010-269147 all describe, as rubber compositions for the cores of
two-piece solid golf balls, the compounding of given amounts of
zinc acrylate in cis-1,4-polybutadiene rubber. However, here too,
when zinc acrylate is used in the core-forming rubber composition,
achieving good ball durability in long-term use has been
difficult.
[0005] Prior art relating to this invention is described in JP-A
2002-126128, which is directed at a one-piece golf ball having an
optimized internal hardness profile from the surface toward the
center of the ball. However, this prior-art golf ball lacks a
satisfactory durability to "surface loss" when hit with the sharp
portion of a clubhead, such as the leading edge of an iron.
SUMMARY OF THE INVENTION
[0006] Accordingly, the object of the present invention is to
provide a golf ball which has a reduced spin rate and thus can
achieve an increased distance, and which moreover has a good
durability even in long-term use.
[0007] The inventors have conducted extensive investigations in
order to attain the above objects and, as a result of repeated
improvements in core-forming rubber compositions which contain
cis-1,4-polybutadiene rubber and use methacrylic acid as the
co-crosslinking agent, have found that by specifying, in the core
hardness profile, hardness differences and the hardness gradient
between the surface and the core, the spin rate of the golf ball
can be reduced, enabling an increased distance to be achieved, in
addition to which the ball can be conferred with has a good
durability even in long-term use.
[0008] Accordingly, the invention provides the following golf
balls.
[1] A golf ball comprising a core and a cover, wherein the core is
formed of a rubber composition comprising a base rubber, a
co-crosslinking agent, a crosslinking initiator and a metal oxide,
the co-crosslinking agent being methacrylic acid; wherein 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 in from the surface toward a center of the core, (C) be the
JIS-C hardness at a position 5 mm in from the surface toward the
center of the core, (D) be the JIS-C hardness at a position 10 mm
in from the surface toward the center of the core, (E) be the JIS-C
hardness at a position 15 mm in from the surface toward the center
of the core, and (F) be the JIS-C hardness at the center of the
core: (A) is from 75 to 95, (B) is from 70 to 90, (C) is from 68 to
88, (D) is from 65 to 80, (E) is from 58 to 73, (F) is from 50 to
65, the hardness relationship
(A)>(B).gtoreq.(C)>(D)>(E)>(F) is satisfied, the value
(A)-(F) is at least 20, the core is formed in such a way that (A)
has the highest hardness value among (A) to (F), and the value
(A)-(C) is from 3 to 15; and wherein the core has a specific
gravity of from 1.05 to 1.2. [2] The golf ball of [1], wherein the
crosslinking initiator is included in an amount of from 1.2 to 5
parts by weight per 100 parts by weight of the base rubber. [3] The
golf ball of [1], wherein the rubber composition further includes a
fatty acid metal salt. [4] The golf ball of [3], wherein the fatty
acid metal salt is zinc stearate. [5] The golf ball of [3], wherein
the fatty acid metal salt is included in an amount of from 0.1 to 5
parts by weight per 100 parts by weight of the base rubber. [6] The
golf ball of [1], wherein the cover is formed of a resin material
which has a breaking strength of from 20 to 80 MPa and an
elongation of from 150 to 600%. [7] The golf ball of [1], wherein
the metal oxide in the rubber composition is zinc oxide. [8] The
golf ball of [1], wherein the core has a deflection (CH), when
compressed under a final load of 1,275 N (130 kgf) from an initial
load of 98 N (10 kgf), of from 2.0 to 7.0 mm.
BRIEF DESCRIPTION OF THE DIAGRAMS
[0009] FIG. 1 is a schematic cross-sectional diagram of a golf ball
according to one embodiment of the invention.
[0010] FIG. 2 is a schematic diagram of a core illustrating
positions A to F in the core hardness profile.
[0011] FIG. 3 is a schematic diagram showing an example of a dimple
cross-section.
[0012] FIG. 4A is a top view and FIG. 4B is a side view showing an
example of a dimple configuration.
[0013] 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
[0014] The invention is described more fully below.
[0015] The golf ball of the invention has a structure which,
although not subject to any particular limitation, is exemplified
by, as shown in FIG. 1, a two-piece solid golf ball G having a core
1 and a cover 2 that encases the core. The cover 2 has a surface on
which, typically, a plurality of dimples D are formed. In the
diagram, the core 1 and the cover 2 are each shown as single
layers, although either or both may be composed of a plurality of
layers.
[0016] The core is obtained by vulcanizing a rubber composition
composed primarily of a rubber material. The rubber composition
used to form the core includes a base rubber, a co-crosslinking
agent, a crosslinking initiator and a metal oxide, and may
optionally include also an antioxidant, an organosulfur compound, a
fatty acid metal salt and an inert filler. The base rubber used in
this rubber composition is preferably polybutadiene. In the
invention, as will be subsequently described, it is critical for
the core cross-sectional hardness to change in a specific way from
the surface to the center of the core, and for the core
cross-sectional 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.
[0017] The polybutadiene used as the rubber component must have a
cis-1,4 bond content of at least 60% (here and below, "%" refers to
percent by weight), preferably at least 80%, more preferably at
least 90%, and most preferably at least 95%. 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% or
less, more preferably 1.7% or less, and even more preferably 1.5%
or less.
[0018] The polybutadiene has a Mooney viscosity (ML.sub.1+4
(100.degree. C.)) which is preferably at least 30, more preferably
at least 35, and even more preferably at least 40, but is
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.
[0019] 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.
[0020] In order to obtain the rubber composition in a molded and
vulcanized form having a good rebound, it is preferable for the
polybutadiene to be one that has been synthesized using a
rare-earth catalyst or a Group VIII metal compound catalyst.
[0021] 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 may be used in
combination with the lanthanum series rare-earth compound. The
compounds mentioned in JP-A 11-35633, JP-A 11-164912 and JP-A
2002-293996 can be advantageously used as the various above
compounds.
[0022] 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.
[0023] The polybutadiene has a molecular weight distribution Mw/Mn
(Mw being the weight-average molecular weight, and Mn being the
number-average molecular weight) of at least 1.0, 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 resilience
may decrease.
[0024] When the above polybutadiene is used as the base rubber, the
proportion of the overall rubber represented by the polybutadiene,
although not subject to any particular limitation, 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.
[0025] 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.
[0026] 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.
[0027] A solution-polymerized styrene-butadiene rubber or an
emulsion-polymerized styrene-butadiene rubber may be used as the
styrene-butadiene rubber (SBR). For example, use may be made of the
solution-polymerized products SBR-SL552, SBR-SL555 or SBR-SL563
(available from JSR Corporation) as the solution-polymerized
styrene-butadiene rubber, and use can be made of the
emulsion-polymerized products SBR 1500, SBR 1502, SBR 1507 or SBR
0202 (available from JSR Corporation) as the emulsion-polymerized
styrene-butadiene rubber.
[0028] The styrene bond content in the above styrene-butadiene
rubber is preferably at least 5 wt %, more preferably at least 10
wt %, even more preferably at least 15 wt %, and most preferably at
least 18 wt %. The upper limit is preferably not more than 35 wt %,
more preferably not more than 30 wt %, even more preferably not
more than 25 wt %, and most preferably not more than 22 wt %. If
the styrene bond content is too high, due to temperature changes on
account of seasonal differences, the core will become harder and
large changes will occur in the rebound. On the other hand, if the
styrene bond content is too low, the ease of operation during
surface grinding of the core will dramatically decrease.
[0029] The isoprene rubber (IR) may be one having a cis-1,4-bond
content of at least 60%, preferably at least 80%, and more
preferably at least 90%, and having a Mooney viscosity
(ML.sub.1+4(100.degree. C.)) of at least 60, preferably at least
70, and more preferably at least 80, but not more than 90, and
preferably not more than 85. By way of illustration, use may be
made of, for example, the commercial product IR2200 (available from
JSR Corporation).
[0030] The styrene-butadiene rubber and/or isoprene rubber accounts
for a proportion of the overall base rubber which is preferably at
least 1 wt %, more preferably at least 5 wt %, and more preferably
at least 10 wt %. The upper limit is preferably not more than 40 wt
%, more preferably not more than 37 wt %, and even more preferably
not more than 35 wt %. Outside of this mixing ratio, temperature
changes on account of seasonal differences may cause the core to
become harder and give rise to large changes in the rebound.
[0031] Methacrylic acid is used as the co-crosslinking agent in the
present invention. Here, methacrylic acid is included in an amount,
per 100 parts by weight of the base rubber, of preferably at least
5 parts by weight, more preferably at least 7 parts by weight, even
more preferably at least 12 parts by weight, and most preferably at
least 13 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, likewise giving the ball an unpleasant
feel on impact.
[0032] It is preferable to use an organic peroxide as the
crosslinking initiator. Examples of commercial products that may be
advantageously used for this purpose include Percumyl D and Perhexa
C40 (both from NOF Corporation), and Trigonox 29-40b (from Akzo
Nobel N.V.). These may be used singly or as combinations of two or
more thereof.
[0033] The crosslinking initiator is included in an amount, per 100
parts by weight of the base rubber, of preferably at least 1.2
parts by weight, more preferably at least 1.25 parts by weight, and
even more preferably at least 1.3 parts by weight. The upper limit
in the amount of crosslinking initiator is preferably not more than
5.0 parts by weight, more preferably not more than 4.0 parts by
weight, and even more preferably not more than 3.5 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.
[0034] Zinc oxide is preferably used 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 5 parts by weight, more preferably
at least 7 parts by weight, even more preferably at least 12 parts
by weight, and most preferably at least 13 parts by weight. The
upper limit in the amount of metal oxide 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 or too little may make it impossible to obtain a suitable
weight and a suitable hardness and rebound.
[0035] An inert filler may be included in the rubber composition.
Preferred use may be made of, for example, barium sulfate, calcium
carbonate or silica as the inert filler. Any one of these may be
used alone or two or more may be used in combination. The amount of
inert filler included per 100 parts by weight of the base rubber
may be set to preferably at least 1 part by weight, and more
preferably at least 5 parts by weight. The upper limit in the
amount of inert filler included may be set to preferably not more
than 50 parts by weight, more preferably not more than 40 parts by
weight, and even more preferably not more than 30 parts by weight.
If the amount of inert filler included is too large or too small, a
suitable weight and a good hardness and rebound may not be
achieved.
[0036] In the invention, an organosulfur compound is included in
the rubber composition. Illustrative examples include thiophenols,
sulfides, alkylmercaptans, and preferably pentachlorothiophenol and
metal salts of pentachlorothiophenol. The amount of the
organosulfur compound included per 100 parts by weight of the base
rubber is preferably at least 0.01 part by weight, more preferably
at least 0.03 part by weight, and even more preferably at least
0.05 part by weight. The upper limit is preferably not more than 3
parts by weight, more preferably not more than 2.5 parts by weight,
and even more preferably not more than 2 parts by weight.
[0037] 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.
[0038] 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 0.15 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.
[0039] A fatty acid metal salt may be included in the rubber
composition. By doing so, compared with conventional rubber
compositions in which methacrylic acid is used as the crosslinking
agent, the vulcanization time for the core-forming rubber
composition is shortened, making it possible to increase
productivity. Illustrative examples of the fatty acid metal salt
include the zinc salts and magnesium salts of higher fatty acids
such as stearic acid, palmitic acid, 12-hydroxystearic acid,
behenic acid, oleic acid, linoleic acid, linolenic acid, arachidic
acid, lignoceric acid, lauric acid and myristic acid. These may be
used singly or two or more may be used in combination.
[0040] The amount of fatty acid metal salt included per 100 parts
by weight of the base rubber is preferably at least 0.1 part by
weight, more preferably at least 0.15 part by weight, and even more
preferably at least 0.2 part by weight. The upper limit is
preferably not more than 5 parts by weight, and more preferably not
more than 3 parts by weight.
[0041] 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 this 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 3,000 .mu.m, more preferably
not more than 2,000 .mu.m, and most preferably not more than 1,500
.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.
[0042] 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 200.degree. C. for a
period of from 10 to 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.
[0043] The core diameter, although not subject to any particular
limitation, is preferably at least 38.0 mm, more preferably at
least 38.9 mm, and even 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.
[0044] 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.
[0045] The core deflection under loading, 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) (which deflection is referred
to here and below as "CH"), 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 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 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.
[0046] 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
in from the surface toward the center of the core, (C) be the JIS-C
hardness at a position 5 mm in from the surface toward the center
of the core, (D) be the JIS-C hardness at a position 10 mm in from
the surface toward the center of the core, (E) be the JIS-C
hardness at a position 15 mm in from the surface toward the center
of the core, and (F) be the JIS-C hardness at the center of the
core, the respective values (A) to (F) 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 and also a good durability to cracking can be obtained.
[0047] Letting (A) be the JIS-C hardness at the surface of the
core, the value of (A) is preferably at least 75, more preferably
at least 76, and even more preferably at least 77, but is
preferably not more than 95, more preferably not more than 93, and
even more preferably not more than 91.
[0048] Letting (B) be the JIS-C hardness at a position 2 mm in from
the surface toward the center of the core, the value of (B) is
preferably at least 70, more preferably at least 73, and even more
preferably at least 75, but is preferably not more than 90, more
preferably not more than 87, and even more preferably not more than
85.
[0049] Letting (C) be the JIS-C hardness at a position 5 mm in from
the surface toward the center of the core, the value of (C) is
preferably at least 68, more preferably at least 70, and even more
preferably at least 72, but is preferably not more than 88, more
preferably not more than 86, and even more preferably not more than
84.
[0050] Letting (D) be the JIS-C hardness at a position 10 mm in
from the surface toward the center of the core, the value of (D) is
preferably at least 65, more preferably at least 66, and even more
preferably at least 67, but is preferably not more than 80, more
preferably not more than 78, and even more preferably not more than
76.
[0051] Letting (E) be the JIS-C hardness at a position 15 mm in
from the surface toward the center of the core, the value of (E) is
preferably at least 58, more preferably at least 59, and even more
preferably at least 60, but is preferably not more than 73, more
preferably not more than 71, and even more preferably not more than
69.
[0052] Letting (F) be the JIS-C hardness at the center of the core,
the value of (F) is preferably at least 50, more preferably at
least 51, and even more preferably at least 52, but is preferably
not more than 65, more preferably not more than 63, and even more
preferably not more than 61.
[0053] Moreover, in the above core hardness profile, it is
preferable for the hardness relationship
(A)>(B).gtoreq.(C)>(D)>(E)>(F) to be satisfied, for the
value (A)-(F) to be at least 20, 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 3 to 15. If the above
conditions are not satisfied, the ball may have a diminished feel
on impact and a reduced durability to cracking.
[0054] The value of (A)-(C) has a lower limit of preferably at
least 3, and more preferably at least 4, and an upper limit of
preferably not more than 15, more preferably not more than 13, and
even more preferably not more than 11. The value of (A)-(F) has a
lower limit of preferably at least 20, and more preferably at least
21, and an upper limit of preferably not more than 50, more
preferably not more than 40, and even more preferably not more than
35.
[0055] In the practice of the invention, the core may be subjected
to surface treatment with a solution containing a haloisocyanuric
acid and/or a metal salt thereof.
[0056] 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").
[0057] 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.
[0058] The haloisocyanuric acid and metal salts thereof are
compounds of 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.
[0059] 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 on the diene rubber molecules. The use of
trichloroisocyanuric acid provides an especially outstanding
adhesion-improving effect.
[0060] The haloisocyanuric acid and/or a metal salt thereof is
preferably used as a solution obtained by dissolution in water or
an organic solvent.
[0061] When water is used as the solvent, the content of the
haloisocyanuric acid and/or a metal salt thereof in the treatment
solution, although not subject to any particular limitation, may be
set to preferably at least 0.5 part by weight, more preferably at
least 1 part by weight, and even more preferably at least 3 parts
by weight, per 100 parts by weight of water. If the content of
haloisocyanuric acid and/or a metal salt thereof is too low, the
adhesion improving effect expected after core surface treatment may
not be obtained and the durability to impact may be poor. The upper
limit is the saturated solution concentration. However, from the
standpoint of cost effectiveness, it is preferable to set the upper
limit to about 10 parts by weight per 100 parts by weight of water.
The core is immersed in the treatment solution for a length of time
which, although not subject to any particular limitation, may be
set to preferably at least 0.3 second, more preferably at least 3
seconds, and even more preferably at least 10 seconds. The upper
limit is preferably not more than 5 minutes, and more preferably
not more than 4 minutes. If the immersion time is too short, the
anticipated treatment effects may not be obtained, whereas if the
immersion time is too long, a loss in ball productivity may
occur.
[0062] In cases where an organic solvent is used, a known organic
solvent may be employed, 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, the affinity of water to the core surface increases, along
with which a hydrolysis reaction between the water and the
haloisocyanuric acid and/or a metal salt thereof more readily
arises.
[0063] 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
even 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 about
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. The upper limit is preferably not more than 5 minutes, and
more preferably not more than 4 minutes. 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.
[0064] 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.
[0065] 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, the washing method
should be one that is not too vigorous. Accordingly, 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.
[0066] 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.
[0067] First, 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
a double bond 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 its cyclic structure. The added isocyanuric acid has
three --NHCO-- structures on the molecule.
[0068] 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 molecules is used as the cover
material, the affinity increases even further, presumably
increasing the durability to impact.
[0069] 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
thus 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.
[0070] In cases where, using an organic solvent, the addition of
isocyanuric acid and chlorine 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 N--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 diene rubber molecules at the core
surface has indeed occurred.
[0071] Next, the material making up the cover which directly
encases the core is described.
[0072] No particular limitation is imposed on the cover resin
material in this invention, provided the material has a breaking
strength of from 20 to 80 MPa and an elongation of from 150 to
600%. However, preferred use may be made of an ionomer resin or a
thermoplastic resin such as polyurethane. The use of a resin
material composed primarily of polyurethane is especially
preferred. For example, 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.
[0073] The breaking strength of the cover resin material is at
least 20 MPa, preferably at least 25 MPa, more preferably at least
30 MPa, and most preferably at least 35 MPa, but not more than 80
MPa, preferably not more than 75 MPa, more preferably not more than
70 MPa, and most preferably not more than 65 MPa. The elongation of
the cover resin material is at least 150%, preferably at least
200%, and more preferably at least 250%, but not more than 600%,
preferably not more than 550%, more preferably not more than 520%,
and most preferably not more than 490%. The breaking strength and
elongation (tensile tests) are measured by methods in general
accordance with JIS K 7311-1995. By using such a cover resin
material having a breaking strength and an elongation in the
above-indicated ranges, the durability to cracking, durability to
surface loss and durability to abrasion desired of a golf ball
intended for long-term use can be improved.
[0074] 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 they enable
thermoplastic polyurethane materials having a high rebound
resilience and excellent low-temperature properties to 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.
[0075] 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.
[0076] 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.
[0077] 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.).
[0078] 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 may decrease. In
particular, when the ball is hit thin, or "topped," the cover may
tear.
[0079] 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 is preferably not more than 1.30, more
preferably not more than 1.20, and even more preferably not more
than 1.17.
[0080] The cover material has a Shore D hardness which is
preferably at least 30, more preferably at least 35, and even more
preferably at least 38, but is preferably not more than 57, more
preferably not more than 55, even more preferably not more than 53,
and most preferably not more than 51. If the Shore D hardness of
the cover is higher than the above range, the appearance
performance in long-term use (durability of markings) may decline,
in addition to which the flight performance may markedly decrease.
On the other hand, if the Shore D hardness of the cover is lower
than the above range, the durability to cracking may markedly
decrease and, particularly when the ball is topped, the cover may
tear. In addition, the spin rate may become very high, possibly
shortening the distance traveled by the ball.
[0081] The 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 ratio (VR) with respect to
the volume of a hypothetical sphere representing the ball were it
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.
[0082] Also, although not subject to any particular limitation, the
dimples formed on the golf ball of the invention preferably satisfy
conditions (1) and (2) below. Although satisfying both of the
following conditions (1) and (2) at the same time is preferred, it
is acceptable for either one of these conditions alone to be
satisfied.
[0083] 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 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.
[0084] 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 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%.
[0085] 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 having 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 N of dimples
on the surface of the ball, to be at least about 80%.
[0086] 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. However, above condition (3), by such means as adjusting
the depth, sets the radius of curvature R representing the
roundness of the peripheral edge to be as large as possible even in
dimples having a small diameter, thus forming dimples having a
smooth cross-sectional shape, and also increases the proportion of
such smooth dimples by setting the above ratio DER to at least 80%,
in this way 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%.
[0087] In addition, the dimples on the 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.
[0088] 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 0.1 mm and the upper
limit being more preferably 0.3 mm, and even more preferably 0.25
mm.
[0089] As condition (5), the total number N of dimples on the
surface of the ball is preferably not more than 380, and more
preferably not more than 350. The total number N of dimples is even
more preferably in a range of from 220 to 340.
[0090] 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 it to have no
dimples on the surface thereof (broken line in FIG. 3), is from 60
to 74%. At a surface coverage 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 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%.
[0091] In one-piece golf balls, because rubber has a somewhat
yellow color, a white enamel paint is generally applied as a first
coat, following which a clear paint is applied. In the inventive
ball, in order to ensure a good appearance, it is preferable to
apply a clear paint to the surface of the ball. The resulting clear
coat has a thickness at dimple lands (Y) which is at least 10
.mu.m, preferably at least 12 .mu.m, and most preferably at least
13 .mu.m, but is not more than 30 .mu.m, 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 at least 8 .mu.m, preferably
at least 10 .mu.m, and most preferably at least 11 .mu.m, but is
not more than 28 .mu.m, 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
(Z/Y.times.100), is at least 60%, preferably at least 70%, and most
preferably at least 80%, but is not more than 100%, and preferably
not more than 95%. Outside the above range, the durability of
markings at dimple edges decreases dramatically in long-term
use.
[0092] The ball diameter is at least 42 mm, preferably at least
42.3 mm, and more preferably at least 42.67 mm, but is not more
than 44 mm, preferably not more than 43.8 mm, more preferably not
more than 43.5 mm, and even more preferably not more than 43
mm.
[0093] 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.
[0094] The ball deflection (BH) 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. Moreover, when the core and ball are each compressed under
a final load of 1,275 N (130 kgf) from an initial load of 98 N (10
kgf), the ratio CH/BH of the respective deflections CH and BH 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/BH is too large, the hardness
(deflection) of the finished ball will be very hard relative to the
core hardness (deflection). That is, because the cover becomes
harder, the feel on impact may decrease and the quality of the
appearance may decline with long-term use. On the other hand, if
the ratio CH/BH is too small, the cover will be very soft, which
may significantly lower the durability to cracking and lead to
cracking of the cover, particularly when the ball is topped. In
addition, the spin rate may undergo a large increase, which may
result in a shorter distance of travel by the ball.
[0095] Moreover, in the invention, the finished ball has an initial
velocity (BV) with a lower limit of at least 70 m/s, preferably at
least 70.5 m/s, and more preferably at least 71 m/s, and with an
upper limit of preferably not more than 77.62 m/s. If the initial
velocity (BV1) of the ball falls below the above lower limit, the
distance traveled by the ball may decrease. On the other hand, if
the initial velocity (BV) of the finished ball exceeds the above
upper limit, the ball will be in violation of the R&A Rules of
Golf.
[0096] Moreover, from the standpoint of ensuring a good durability
over a long period of time, the golf ball of the invention, upon
initial measurement, 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.
[0097] As described above, the golf ball of the invention has a
reduced spin rate and can thus achieve an increased distance, and
moreover has a good durability even in long-term use.
EXAMPLES
[0098] The following Examples and Comparative Examples are provided
to illustrate the invention, and are not intended to limit the
scope thereof.
Examples 1 to 9, Comparative Examples 1 to 10
[0099] Rubber materials formulated as shown in Table 1 below were
furnished for the fabrication of 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 parts by weight No. 1 No. 2 No. 3 No. 4 No.
5 No. 6 No. 7 No. 8 No. 9 No. 10 No. 11 No. 12 No. 13 No. 14 No. 15
Core BR01 100 100 100 80 80 80 80 100 100 100 100 95 formu- IR2200
20 12.5 5 5 lation SBR SL563 20 7.5 15 BR730 100 100 100 Perhexa
C-40 0.6 0.6 0.6 (40% dilution) Actual amount 0.24 0.24 0.24 of
addition Percumyl D 1.3 1.5 1.5 3 3 1.3 1.3 1.3 1.3 1.07 0.8 0.6
0.6 0.6 1.07 Zinc oxide 23 23.5 23 23 23 22.5 22.5 22.5 22 23 23 6
6 9.5 23 Antioxidant 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2
0.2 0.2 0.1 0.2 Methacrylic 24 22.5 20 22.5 20 23.5 23.5 23.5 22
22.5 22.5 22.5 acid Zinc 33 methacrylate Zinc acrylate 33 26 Zinc
stearate 1.7 0.3 0.3 0.3 0.3 1.7 1.7 1.7 1.7 Zinc salt of penta-
0.1 0.1 1 1 chlorothiophenol Titanium oxide 4 Vul- Temperature 170
170 170 170 170 170 170 170 170 170 170 160 160 160 170 cani-
(.degree. C.) zation con- Time (minutes) 18 18 18 18 18 18 18 18 18
20 20 13 13 13 30 ditions Details on the materials used in the core
formulations in the above table are provided below. (1) BR01: A
butadiene rubber synthesized with a nickel catalyst, available from
JSR Corporation; Mooney viscosity ML, 46 (2) IR2200: An isoprene
rubber, available from JSR Corporation; Mooney viscosity ML, 82 (3)
SBR SL563: A solution-polymerized styrene-butadiene rubber,
available from JSR Corporation; styrene bond content, 200 (4)
BR730: A butadiene rubber synthesized with a neodymium catalyst,
available from JSR Corporation; Mooney viscosity ML, 55 (5) Perhexa
C-40: An organic peroxide, available from NOF Corporation (6)
Percumyl D: An organic peroxide, available from NOF Corporation (7)
Zinc oxide: Available from Sakai Chemical Co., Ltd. (8)
Antioxidant: "Nocrac NS-6," available from Ouchi Shinko Chemical
Industry Co., Ltd. (9) Methacrylic acid: Available from Kuraray
Co., Ltd. (10) Zinc methacrylate: Available from Asada Chemical
Industry Co., Ltd. (11) Zinc acrylate: Available from Nihon Jyoryu
Kogyo Co., Ltd. (12) Titanium oxide: Available from Ishihara Sangyo
Kaisha, Ltd.
[0100] 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 subsequently 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.
TABLE-US-00002 TABLE 2 A B C D Formulation Himilan 1557 50 (pbw)
Himilan 1601 50 Himilan AM7327 50 Surlyn 6320 50 Pandex T8260 25
Pandex T8195 100 75 Magnesium stearate 1 1 Titanium dioxide 3.5 3.5
2.1 2.1 Polyethylene wax 1.5 1.5 Details on the materials used in
the cover composition in the above table are provided below.
"Himilan": Ionomer resins available under this trade name from
DuPont-Mitsui Polychemicals Co., Ltd. "Pandex": Thermoplastic
polyurethane elastomers available under this trade name from DIC
Bayer Polymer, Ltd. "Surlyn": An ionomer resin available under this
trade name from E.I. DuPont de Nemours & Co. Magnesium
stearate: Available from NOF Corporation Titanium dioxide:
Available under the trade name "Tipaque R550" from Ishihara Sangyo
Kaisha, Ltd. Polyethylene wax: Available under the trade name
"Sanwax 161P" from Sanyo Chemical Industries, Ltd.
[0101] 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. 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/propylene glycol monomethyl ether acetate (PMA) in a weight
ratio of 70/15/15 as the base; 150 parts by weight of a
non-yellowing polyisocyanate, specifically an adduct of
hexamethylene diisocyanate (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 11, 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
No. Number (mm) (mm) (mm) ratio (number) (number) (%) (number) (%)
SR VR 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 0.216 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 The abbreviations and symbols
relating to dimples which appear in Table 3 are explained below. R:
Radius of curvature representing roundness provided at the
peripheral edge of a dimple R/D ratio: Ratio of radius of curvature
R to diameter D N: Total number of dimples RA: Collective number of
dimples having an R/D ratio of at least 20% ER: Ratio of RA to
total number of dimples N DE: Sum of the number of dimples having
an own diameter and having an own radius of curvature larger than
or equal to a radius of curvature of dimples of larger diameter
than the own diameter, plus the number of dimples of a type having
a largest diameter DER: Ratio of DE to the total number of dimples
N 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 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
[0102] 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
golf 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)
[0103] The deflections (mm) of, as the test spheres, the cores and
finished balls when compressed at a rate of 10 mm/min under a final
load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf)
were measured. The tests were performed using a model 4204 test
system from Instron Corporation.
Cross-Sectional Hardness of Core
[0104] 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
[0105] 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 of Core and Finished Ball (Initial Velocity)
[0106] 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 and
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 samples were each hit
twice, and the time taken for the samples to traverse a distance of
6.28 ft (1.91 m) was measured and used to compute the initial
velocity.
Cover Material Hardness
[0107] 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.
Breaking Strength and Elongation (Tensile Tests)
[0108] The resin materials and rubber materials were formed into 2
mm thick sheets, and held in a 23.+-.1.degree. C. atmosphere for
two weeks. These samples were shaped into dumbbell-shaped test
specimens in accordance with JIS K 7311-1995, and the specimens
were subjected to measurement in a 23.+-.2.degree. C. atmosphere at
a test rate of 5 mm/s, also in accordance with JIS K 7311-1995. The
average breaking strength and elongation of each material were
calculated from the measured values for five specimens.
Measurement of Coat Thickness
[0109] Lands (Y): The thickness of the clear coat at land areas at
intermediate positions between dimples was measured. [0110] Edges
(Z): The thickness of the clear coat at dimple edge areas was
measured.
[0111] 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
[0112] 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 the ball was 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.
[0113] In addition, the total distance of the ball was measured
again following the marking durability test above.
Durability to Cracking
[0114] The durability of the golf ball to cracking was evaluated
using an ADC Ball COR Durability Tester produced by Automated
Design Corporation (U.S.). This tester functions so as to fire a
golf ball pneumatically and cause it to repeatedly strike two metal
plates arranged in parallel. The incident velocity against the
metal plates was set at 43 m/s. The number of shots required until
the golf ball cracked was measured, and the average value for five
golf balls (N=5) was determined.
Abrasion Test
[0115] 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 markings and to assess the degree of
surface scratching and the degree of loss of luster due to abrasion
by the sand, as well as the degree of sand adhesion. The ball
appearance was rated as "Good," "Fair" or "NG."
Feel
[0116] 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 7 8 9 Core Type No. 1
No. 2 No. 3 No. 4 No. 5 No. 6 No. 7 No. 8 No. 9 Diameter, mm 39.9
40.7 39.9 40.7 39.9 39.9 39.9 39.9 39.9 Specific gravity 1.12 1.12
1.12 1.13 1.12 1.12 1.13 1.13 1.12 Deflection under 10-130 kg 2.85
2.75 3.0 2.9 3.2 3.2 2.85 2.9 2.85 compression (CH), mm Rebound
(CV) 75.1 76 76.7 77 77.7 74.3 74.7 74.6 74.5 JIS-C hardness at
core surface (A) 82 85 82 83 81 81 86 83 85 JIS-C hardness 2 mm
inside 77 81 79 80 79 77 81 77 81 core surface (B) JIS-C hardness 5
mm inside 77 79 76 77 74 76 79 75 78 core surface (C) JIS-C
hardness 10 mm inside 69 70 69 73 69 67 74 69 73 core surface (D)
JIS-C hardness 15 mm inside 65 63 60 66 62 62 65 64 66 core surface
(E) JIS-C hardness at core center (F) 59 57 55 60 57 56 58 57 58
JIS-C hardness difference between core 5 6 6 6 7 5 7 8 7 surface
and 5 mm inside core surface (A - C) JIS-C hardness difference
between 23 28 27 23 24 25 28 26 27 core surface and center (A - F)
Cover Type A B A A A A A A A Shore D hardness 45 50 45 45 45 45 45
45 45 Breaking strength, MPa 40 37 40 40 40 40 40 40 40 Elongation,
% 360 260 360 360 360 360 360 360 360 Specific gravity 1.15 1.15
1.15 1.15 1.15 1.15 1.15 1.15 1.15 Thickness, mm 1.4 1.0 1.4 1.0
1.4 1.4 1.4 1.4 1.4 Finished Deflection under 10-130 kg loading 2.8
2.65 2.9 2.9 3.1 3.1 2.8 2.85 2.8 ball 30 days after production
(BH1), mm Deflection under 10-130 kg loading 2.7 2.55 2.8 2.8 3 3
2.7 2.75 2.7 350 days after measuring BH1 (BH2), mm Difference
between BH1 and BH2, mm -0.1 -0.1 -0.1 -0.1 -0.1 -0.1 -0.1 -0.1
-0.1 Rebound 30 days after production 74.5 75.7 76.1 76.7 77.1 73.8
74.1 74 73.9 (BV1), m/s Rebound 350 days after measuring 74.5 75.6
76.1 76.6 77.1 73.8 74.1 74 73.9 BV1 (BV2), m/s Difference between
BV1 and BV2, m/s 0 -0.1 0 -0.1 0 0 0 0 0 Diameter, mm 42.7 42.7
42.7 42.7 42.7 42.7 42.7 42.7 42.7 Core initial velocity - Ball
initial velocity (CV - BV1) 0.6 0.3 0.6 0.3 0.6 0.5 0.6 0.6 0.6
Core deflection/Ball deflection (CH/BH1) 1.02 1.04 1.03 1.00 1.03
1.03 1.02 1.02 1.02 Dimples Type I I I I I I I I I Clear Land areas
(Y), .mu.m 16 16 16 16 16 16 16 16 16 coating Edge areas (Z), .mu.m
14 14 14 14 14 14 14 14 14 thickness Coating thickness ratio (Z/Y
.times. 100), % 88 88 88 88 88 88 88 88 88 Distance HS 45, driver
Spin rate, rpm 2900 2850 2900 2950 2890 2850 2950 2950 2950 Total
distance, m 227 233 234 237 239 224 226 225 224 HS 45, driver Total
distance, m 224 230 231 234 236 221 223 222 221 (after abrasion
test) Distance Total distance, m -3 -3 -3 -3 -3 -3 -3 -3 -3
difference Durability Durability to At incident 1100 900 1100 850
1050 1000 1100 1100 1100 cracking velocity of 43 m/s Abrasion test
After 144 hours of Good Good Good Good Good Good Good Good Good
abrasion with sand Feel Driver Good Good Good Good Good Good Good
Good Good
TABLE-US-00005 TABLE 5 Comparative Example 1 2 3 4 5 6 7 8 9 10
Core Type No. 10 No. 11 No. 10 No. 12 No. 13 No. 14 No. 13 No. 10
No. 10 No. 15 Diameter, mm 39.9 39.9 38.5 39.9 39.9 39.9 39.9 39.9
39.9 42.7 Specific gravity 1.12 1.12 1.12 1.12 1.12 1.12 1.12 1.12
1.12 1.12 Deflection under 10-130 kg 2.6 3 2.6 2.75 2.75 3.8 2.75
2.6 2.6 compression (CH), mm Rebound (CV) 74.9 74.3 74.9 77.2 77.6
77.4 77.6 74.9 74.9 JIS-C hardness at core surface (A) 81 73 81 80
80 70 80 80 80 80 JIS-C hardness 2 mm inside 76 65 76 75 75 65 75
75 75 75 core surface (B) JIS-C hardness 5 mm inside 79 70 79 77 77
68 77 77 77 77 core surface (C) JIS-C hardness 10 mm inside 74 70
74 71 71 66 71 71 71 71 core surface (D) JIS-C hardness 15 mm
inside 69 68 69 67 67 62 67 67 67 67 core surface (E) JIS-C
hardness at core center (F) 66 65 66 63 63 58 63 63 63 63 JIS-C
hardness difference between core 2 3 2 3 3 2 3 3 3 3 surface and 5
mm inside core surface (A - C) JIS-C hardness difference between 15
8 15 17 17 12 17 17 17 17 core surface and center (A - F) Cover
Type A A A A A C A C D Shore D hardness 45 45 45 45 45 60 45 60 45
Breaking strength, MPa 40 40 40 40 40 17 40 17 12 Elongation, % 360
360 360 360 360 100 360 100 120 Specific gravity 1.15 1.15 1.15
1.15 1.15 0.99 1.15 0.99 0.99 Thickness, mm 1.4 1.4 2.1 1.4 1.4 1.4
1.4 1.4 1.4 Finished Deflection under 10-130 kg loading 2.65 2.9
2.5 2.75 2.75 3.3 2.75 2.25 2.75 2.75 ball 30 days after production
(BH1), mm Deflection under 10-130 kg loading 2.55 2.8 2.4 2.5 2.5 3
2.5 2.3 2.6 2.75 350 days after measuring BH1 (BH2), mm Difference
between BH1 and BH2, mm -0.1 -0.1 -0.1 -0.25 -0.25 -0.3 -0.25 0.05
-0.15 0 Rebound 30 days after production 74.2 73.8 73.7 76.2 76.6
77.0 76.6 74.1 73.7 74.6 (BV1), m/s Rebound 350 days after
measuring 74.2 73.8 73.7 75.7 75.8 76.6 75.8 74.2 73.8 74.7 BV1
(BV2), m/s Difference between BV1 and BV2, m/s 0 0 0 -0.5 -0.8 -0.4
-0.8 0.1 0.1 0.1 Diameter, mm 42.7 42.7 42.7 42.7 42.7 42.7 42.7
42.7 42.7 42.7 Core initial velocity - Ball initial velocity (CV -
BV1) 0.7 0.5 1.2 1.0 1.0 0.4 1.0 0.8 1.2 Core deflection/Ball
deflection (CH/BH1) 0.98 1.03 1.04 1.00 1.00 1.15 1.00 1.16 0.95
Dimples Type I I II I I I II I I I Clear Land areas (Y), .mu.m 16
15 17 16 16 16 17 16 16 16 coating Edge areas (Z), .mu.m 14 13 8 14
14 14 8 14 14 14 thickness Coating thickness ratio (Z/Y .times.
100), % 88 88 47 88 88 88 47 88 88 88 Distance HS 45, driver Spin
rate, rpm 3370 3250 3090 3360 3340 3040 3340 3190 3320 3650 Total
distance, m 223 222 219 233 235 237 235 224 221 221 HS 45, driver
Total distance, m 220 219 212 230 232 222 228 216 209 215 (after
abrasion test) Distance Total distance, m -3 -3 -7 -3 -3 -15 -7 -8
-12 -6 difference Durability Durability to At incident 1103 1103
1575 615 579 300 579 1350 1020 621 cracking velocity of 43 m/s
Abrasion test After 144 hours of Good Good NG Good Good NG NG NG NG
NG abrasion with sand Feel Driver Good Good Good Good Good NG Good
NG Good NG
[0117] The rubber material in Comparative Example 10 had a breaking
strength of 15 MPa and an elongation of 88%.
[0118] In the golf ball in Comparative Example 1, the hardness
profile was not optimized and the spin rate was high. As a result,
the distance traveled by the ball decreased.
[0119] In the golf ball in Comparative Example 2, the hardness
profile was not optimized and the spin rate was high. As a result,
the distance traveled by the ball decreased.
[0120] In the golf ball in Comparative Example 3, the hardness
profile was not optimized, the cover was too thick and the spin
rate was high. As a result, the distance traveled by the ball
decreased. In addition, the dimple edge radius of curvature R was
small, as a result of which the durability to abrasion was poor and
the flight performance underwent a large decrease.
[0121] In the golf ball in Comparative Example 4, zinc methacrylate
was used as the co-crosslinking agent in the core formulation. As a
result, the finished ball underwent large changes over time in the
deflection and rebound (initial velocity), and the durability to
cracking was poor.
[0122] In the golf ball in Comparative Example 5, zinc acrylate was
used as the co-crosslinking agent in the core formulation. As a
result, the finished ball underwent large changes over time in the
deflection and rebound, and the durability to cracking was
poor.
[0123] In the golf ball in Comparative Example 6, zinc acrylate was
used as the co-crosslinking agent in the core formulation, in
addition to which the cover had a small breaking strength and a
small elongation. As a result, the finished ball underwent large
changes over time in the deflection and rebound, and the durability
to cracking was poor. Moreover, the cover was hard, giving the ball
a poor feel on shots with a driver.
[0124] In the golf ball in Comparative Example 7, zinc acrylate was
used as the co-crosslinking agent in the core formulation. As a
result, the finished ball underwent large changes over time in the
deflection and rebound, and the durability to cracking was poor.
Moreover, the dimpled edges had a small radius of curvature R, as a
result of which the finished ball had a poor durability to abrasion
and exhibited a large decrease in flight performance.
[0125] In the golf ball in Comparative Example 8, the hardness
profile was not optimized and the spin rate was high. As a result,
the distance traveled by the ball decreased. Moreover, the cover
had a small breaking strength and a small elongation, as a result
of which the finished ball had a poor durability to abrasion and
exhibited a large decrease in flight performance. In addition, the
cover was hard, giving the ball a poor feel on shots with a
driver.
[0126] In the golf ball in Comparative Example 9, the hardness
profile was not optimized and the spin rate was high. As a result,
the distance traveled by the ball decreased. Moreover, the cover
had a small breaking strength and a small elongation, as a result
of which the finished ball had a poor durability to abrasion and
exhibited a large decrease in flight performance.
[0127] In the golf ball in Comparative Example 10, the surface
rubber material in the one-piece construction had a small breaking
strength and a small elongation, as a result of which the finished
ball had a poor durability to cracking and a poor durability to
abrasion, and exhibited a large decrease in flight performance.
* * * * *