U.S. patent application number 13/351867 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 | 20120277021 13/351867 |
Document ID | / |
Family ID | 47068309 |
Filed Date | 2012-11-01 |
United States Patent
Application |
20120277021 |
Kind Code |
A1 |
HIGUCHI; Hiroshi ; et
al. |
November 1, 2012 |
PRACTICE GOLF BALL
Abstract
A practice golf ball has a core made of a rubber composition
which includes a base rubber, a co-crosslinking agent, a
crosslinking initiator and a metal oxide, and has a cover which
encases the core and is made of a resin material. The
co-crosslinking agent is methacrylic acid. The resin material has a
breaking strength of 20 to 80 MPa and an elongation of 150 to 600%.
The ball is endowed with the properties required of practice balls
intended for long-term use, including better durability to cracking
and durability of appearance than ordinary game balls, and also
better durability to ball surface loss than the one-piece golf
balls which are commonly used as practice balls.
Inventors: |
HIGUCHI; Hiroshi;
(Chichibushi, JP) ; YAMAGISHI; Hisashi;
(Chichibushi, JP) ; ARAI; Daisuke; (Chichibushi,
JP) ; OZAWA; Yuichiro; (Chichibushi, JP) ;
OHIRA; Takashi; (Chichibushi, JP) ; SATO;
Katsunori; (Chichibushi, JP) |
Assignee: |
BRIDGESTONE SPORTS CO.,
LTD.
Tokyo
JP
|
Family ID: |
47068309 |
Appl. No.: |
13/351867 |
Filed: |
January 17, 2012 |
Current U.S.
Class: |
473/280 |
Current CPC
Class: |
A63B 37/0021 20130101;
A63B 37/0084 20130101; A63B 37/0019 20130101; A63B 37/0054
20130101; A63B 37/0074 20130101; A63B 37/0051 20130101; A63B
37/0006 20130101; A63B 37/0083 20130101; A63B 37/0017 20130101;
A63B 37/0087 20130101; A63B 37/0064 20130101; A63B 69/3655
20130101; A63B 37/0035 20130101; A63B 37/0066 20130101; A63B
37/0023 20130101; A63B 37/0033 20130101; A63B 37/0003 20130101;
A63B 37/0065 20130101; A63B 37/008 20130101; A63B 37/0063 20130101;
A63B 37/0031 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 |
2011-099853 |
Claims
1. A practice golf ball comprising: a core made of a rubber
composition comprising a base rubber and, as compounding
ingredients: a co-crosslinking agent, a crosslinking initiator and
a metal oxide; and a cover which encases the core and is made of a
resin material, wherein the co-crosslinking agent is methacrylic
acid, and the resin material has a breaking strength of from 20 MPa
to 80 MPa and an elongation of from 150% to 600%.
2. The practice golf ball of claim 1, wherein the ball has an
initial velocity of not more than 76 m/s.
3. The practice golf ball of claim 1, wherein the resin material
making up the cover is composed primarily of polyurethane, the
cover material has a Shore D hardness of from 30 to 57, and the
cover has a thickness of from 0.3 mm to 2.5 mm.
4. 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%.
5. 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%.
6. The practice golf ball of claim 5 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%.
7. The practice golf ball of claim 6 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 380; 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%.
8. The practice golf ball of claim 1, 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
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; the core
has a specific gravity of from 1.05 to 1.2; and, 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 BH, the core
deflection CH is from 2.0 mm to 7.0 mm, the ball deflection BH is
from 2.0 mm to 7.0 mm, and the ratio CH/BH is from 0.95 to 1.1.
9. The practice golf ball of claim 1 wherein the metal oxide is
zinc oxide, and the compounding ingredients are included in
respective amounts of from 10 to 40 parts by weight of methacrylic
acid, from 0.3 to 5.0 parts by weight of crosslinking initiator,
and from 15 to 30 parts by weight of zinc oxide, 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. 2011-099853 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 endowed
with the properties required of practice balls intended for
long-term use, including better durability to cracking and
durability of appearance than ordinary game balls, and also better
durability to ball surface loss than the one-piece golf balls which
are commonly used as practice balls.
[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 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 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.
[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] 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 at a golf driving range has
been difficult.
[0010] 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 rubber-forming rubber
composition, achieving good ball durability in long-term use at a
driving range has been difficult.
[0011] An example of the 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. Here and below, "surface loss" refers to the loss of material
from the surface of a golf ball when struck with a golf club.
[0012] 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 intended for long-term use,
including better durability to cracking and durability of
appearance than ordinary game balls, and also better durability to
ball surface loss than the one-piece golf balls which are commonly
used as practice balls.
SUMMARY OF THE INVENTION
[0013] We have discovered that, in the production of practice golf
balls having a core and a cover, by using methacrylic acid as a
co-crosslinking agent for the base rubber in the core-forming
rubber composition and by using a resin material having a breaking
strength of from 20 MPa to 80 MPa and an elongation of from 150% to
600% as the cover material, owing to the synergistic effects of
these materials, the resulting balls have better durability to
cracking and durability to abrasion than anticipated by golf ball
designers. This discovery has made it possible to obtain practice
golf balls which, even with long-term use, retain a good appearance
and a good flight performance, and moreover have better durability
to ball surface loss than the one-piece golf balls which are
commonly used as practice balls.
[0014] That is, in the present invention, by using methacrylic acid
as a co-crosslinking agent in the core-forming rubber composition
and by optimizing the amounts of methacrylic acid and crosslinking
initiator included in the composition, the ball can be made much
more durable to cracking than a game ball. In addition, by
selecting a resin material which has a high strength and good
elongation and using this resin material to form the cover, a
practice golf ball endowed with excellent durability to cracking,
durability to surface loss and durability to abrasion can be
obtained. Moreover, in the invention, by optimizing the
cross-sectional shape of the dimples formed on the ball surface, a
practice golf ball having excellent durability to markings (e.g., a
brand name or player number printed on the surface of the ball at
the time of manufacture) in long-term use can be obtained.
Furthermore, by optimizing the internal hardness profile of the
core, a golf ball having a good feel on impact can be obtained.
[0015] Accordingly, the invention provides a practice golf ball
having a core made of a rubber composition which includes a base
rubber and, as compounding ingredients: a co-crosslinking agent, a
crosslinking initiator and a metal oxide; and having a cover which
encases the core and is made of a resin material. The
co-crosslinking agent is methacrylic acid. The resin material has a
breaking strength of from 20 MPa to 80 MPa and an elongation of
from 150% to 600%.
[0016] The practice golf ball of the present invention preferably
has an initial velocity of not more than 76 m/s.
[0017] In the practice golf ball of the invention, the resin
material making up the cover may be composed primarily of
polyurethane, the cover material may have a Shore D hardness of
from 30 to 57, and the cover may have a thickness of from 0.3 mm to
2.5 mm.
[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 dimples 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] Preferably, in the practice golf ball of the invention, 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; the core
has a specific gravity of from 1.05 to 1.2; and, 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 BH, the core
deflection CH is from 2.0 mm to 7.0 mm, the ball deflection BH is
from 2.0 mm to 7.0 mm, and the ratio CH/BH is from 0.95 to 1.1
[0029] In the practice golf ball of the invention, the metal oxide
is preferably zinc oxide, and the compounding ingredients are
preferably included in respective amounts of from 10 to 40 parts by
weight of methacrylic acid, from 0.3 to 5.0 parts by weight of
crosslinking initiator, and from 15 to 30 parts by weight of zinc
oxide, per 100 parts by weight of the base rubber.
[0030] The practice golf ball of the invention is endowed with the
properties required of practice balls intended for long-term use,
including better durability to cracking and durability of
appearance than ordinary game balls, and also better durability to
ball surface loss than the one-piece golf balls which are commonly
used as practice balls.
BRIEF DESCRIPTION OF THE DIAGRAMS
[0031] FIG. 1 is a schematic cross-sectional diagram of a practice
golf ball according to one embodiment of the invention.
[0032] FIG. 2 is a schematic diagram of a core illustrating
positions A to F in the core hardness profile.
[0033] FIG. 3 is a schematic diagram showing an example of a dimple
cross-section.
[0034] FIG. 4A is a top view and FIG. 4B is a side view showing an
example of a dimple configuration.
[0035] 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
[0036] The objects, features and advantages of the invention will
become more apparent from the following detailed description, taken
in conjunction with the foregoing diagrams.
[0037] 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. Also, although it is not shown in Figs, a
paint layer such as a clear paint may be formed to the surface of
the ball.
[0038] 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, a metal oxide and, optionally, an
antioxidant. 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.
[0039] 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.
[0040] The polybutadiene has a Mooney viscosity (ML.sub.1+4,
(100.degree. C.)) which is preferably at least 30, more preferably
at least 35, even more preferably at least 40, and most preferably
at least 45, 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.
[0041] 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.
[0042] 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.
[0043] 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.
Preferred use can 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.
[0044] 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.
[0045] 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.
[0046] 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, although 98 wt % or less is preferred, and 95 wt % or less
is more preferred.
[0047] 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.
[0048] 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.
[0049] 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.
[0050] 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 can 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 %.
[0051] 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 is 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 %.
[0052] In the invention, methacrylic acid is an essential
ingredient which is employed 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 11 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, also giving the ball an unpleasant feel on impact.
[0053] 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.
[0054] 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 0.6 part by weight, and even
more preferably at least 0.9 part 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,
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.
[0055] 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.
[0056] 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.
[0057] 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.
[0058] 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 is 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.
[0059] 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.
[0060] 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.
[0061] 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.
[0062] 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.
[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, and even more preferably at least 65, 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, and even more preferably at least 59, 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, and even more preferably at least 61, 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, and even more preferably at least 60, 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, and even more preferably at least 57, but
is preferably not more than 75, more preferably not more than 73,
even more preferably not more than 71, and most 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, and even more preferably at least 54, 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 8, more preferably not more than 6, and even more
preferably not more than 4. The value of A-F is preferably at least
3, more preferably at least 6, and even more preferably at least 8,
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] No particular limitation is imposed on the cover resin
material in this invention, provided the material has a breaking
strength of from 20 MPa to 80 MPa and an elongation is of from 150%
to 600%. However, preferred use may be made of a thermoplastic
resin such as an ionomer resin or 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.
[0088] 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%, more preferably at least 250%, and most preferably at least
300%, 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 practice golf ball intended for long-term use can be
improved.
[0089] 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.
[0090] 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.
[0091] 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.
[0092] 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.).
[0093] 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 2.5 mm, more
preferably not more than 2.1 mm, even more preferably not more than
1.9 mm, still more preferably not more than 1.8 mm, and most
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.
[0094] 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.
[0095] 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,
still more preferably not more than 51, and most preferably not
more than 48. 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.
[0096] 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%.
[0097] 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.
[0098] 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.
[0099] 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%.
[0100] 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%.
[0101] 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 radius of curvature R representing
the roundness of the peripheral edge to be as large as possible
even in dimples having 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%.
[0102] 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.
[0103] 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.
[0104] 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.
[0105] 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 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 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%.
[0106] 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. As a result, a
white paint cracks and dirt is easy to adhere to the ball surface
for a log period of time. In the present invention, the durability
to cracking of the entire ball is enhanced by using polyurethane as
the cover material, and a good appearance in long-term use can be
maintained without causing the problems on cracking of a paint
because the material itself for forming a cover is whiten to finish
the ball surface with only a transparent clear paint as a paint
layer. Thus, in the inventive ball, 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 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 markedly in
long-term use.
[0107] 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.
[0108] 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.
[0109] The ball deflection (referred to here and below as "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. 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 ball obtained
as the finished ball will be very hard relative to the core
hardness (deflection).
[0110] 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.
[0111] Also, in this invention, the ball rebound (BV) has an upper
limit of preferably not more than 76.0 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 75.0 m/s. The lower limit 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. At a ball rebound (BV) in excess of 76.0 m/s, the ball may fly
so well that is passes over the netting at a driving range, thus
making the ball unsuitable for use as a practice golf ball having a
distance controlled for the driving range.
EXAMPLES
[0112] The following Examples and Comparative Examples are provided
to illustrate the invention, and are not intended to limit the
scope thereof.
Examples 1 to 4, Comparative Examples 1 to 4
[0113] 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 60 95 100 100 95 formulation IR2200 5 5 BR51 40 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 0.6 0.6 1.07 Zinc oxide 23 23
23 23 6 6 23 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
Titanium oxide 4 Vulcanization Temperature (.degree. C.) 170 170
170 170 160 160 170 conditions Time (minutes) 20 20 20 20 13 13
30
[0114] Details on the materials used in the core formulations in
the above table are provided below. [0115] (1) BR01: A butadiene
rubber synthesized with a nickel catalyst, available from JSR
Corporation; Mooney viscosity ML, 46 [0116] (2) IR2200: An isoprene
rubber, available from JSR Corporation; Mooney viscosity ML, 82
[0117] (3) BR51: A butadiene rubber synthesized with a neodymium
catalyst, available from JSR Corporation; Mooney viscosity ML, 36
[0118] (4) Perhexa C-40: An organic peroxide, available from NOF
Corporation [0119] (5) Percumyl D: An organic peroxide, available
from NOF Corporation [0120] (6) Zinc oxide: Available from Sakai
Chemical Co., Ltd. [0121] (7) Antioxidant: "Nocrac NS-6," available
from Ouchi Shinko Chemical Industry Co., Ltd. [0122] (8)
Methacrylic acid: Available from Kuraray Co., Ltd. [0123] (9) Zinc
methacrylate: Available from Asada Chemical Industry Co., Ltd.
[0124] (10) Zinc acrylate: Available from Nihon Jyoryu Kogyo Co.,
Ltd. [0125] (11) Titanium oxide: Available from Ishihara Sangyo
Kaisha, Ltd.
[0126] 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 Resin Pandex T8195 100 Additives Titanium
dioxide 3.5 Polyethylene wax 1.5 Shore D hardness 45
[0127] Details on the materials used in the cover composition in
the above table are provided below. [0128] "Pandex": A
thermoplastic polyurethane elastomer available under this trade
name from Dainippon Ink & Chemicals, Inc. [0129] Titanium
dioxide: Available under the trade name "Tipaque R550" from
Ishihara Sangyo Kaisha, Ltd. [0130] Polyethylene wax: Available
under the trade name "Sanwax 161P" from Sanyo Chemical Industries,
Ltd.
[0131] 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.
[0132] 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
[0133] The abbreviations and symbols relating to dimples which
appear in Table 3 are explained below. [0134] R: Radius of
curvature representing roundness provided at peripheral edge of a
dimple [0135] R/D ratio: Ratio of radius of curvature R to diameter
D [0136] N: Total number of dimples on surface of ball [0137] RA:
Collective number of dimples having an R/D ratio of at least 20%
[0138] ER: Ratio of RA to total number of dimples N [0139] 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 [0140] DER: Ratio of DE to total
number of dimples N [0141] 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. [0142] 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.
[0143] 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)
[0144] 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
[0145] 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
[0146] 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)
[0147] 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 balls 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
balls to traverse a distance of 6.28 ft (1.91 m) was measured and
used to compute the initial velocity.
Cover Material Hardness
[0148] 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)
[0149] 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 was
calculated from the measured values for five specimens.
Measurement of Coating Thickness
[0150] Lands (Y): The thickness of the clear coat at land areas at
intermediate positions between dimples was measured. [0151] Edges
(Z): The thickness of the clear coat at dimple edge areas was
measured.
[0152] 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
[0153] 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.
[0154] In addition, after the abrasion test described below had
been carried out, the total distance of the ball was again
measured.
Durability to Cracking
[0155] The ball was hit 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 (evaluation of
durability when ball is topped).
Abrasion Test
[0156] 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."
TABLE-US-00004 TABLE 4 Example Comparative Example 1 2 3 4 1 2 3 4
Core Type (1) (2) (3) (4) (5) (6) (6) (7) Diameter, mm 39.9 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 Deflection under 10-130 kg 2.6 2.5 3.8 3.1 2.75
2.75 2.75 compression (CH), mm JIS-C hardness 81 82 69 72 80 80 80
80 at core surface (A) JIS-C hardness 2 mm 76 77 63 64 75 75 75 75
inside core surface (B) JIS-C hardness 5 mm 79 79 67 69 77 77 77 77
inside core surface (C) JIS-C hardness 10 mm 74 74 66 69 71 71 71
71 inside core surface (D) JIS-C hardness 15 mm 69 69 64 67 67 67
67 67 inside core surface (E) JIS-C hardness 66 65 61 64 63 63 63
63 at core center (F) JIS-C hardness difference 2 3 2 3 3 3 3 3
between core surface and 5 mm inside core surface (A - C) JIS-C
hardness difference 15 17 8 8 17 17 17 17 between core surface and
center (A - F) Cover Type A A A A A A A Shore D hardness 45 45 45
45 45 45 45 Breaking strength, MPa 40 40 40 40 40 40 40 Elongation,
% 360 360 360 360 360 360 360 Specific gravity 1.15 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 1.4 Finished
Deflection under 10-130 kg 2.65 2.55 3.8 3 2.75 2.75 2.75 2.75 ball
loading (BH), mm Rebound (BV), m/s 74.2 74.3 74.1 73.5 76.2 76.6
76.6 74.6 Diameter, mm 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 Core
deflection/Ball deflection (CH/BH) 0.98 0.98 1.00 1.03 1.00 1.00
1.00 Dimples Type I I II II I I III I Clear Land areas (Y), .mu.m
16 16 15 15 16 16 17 16 coating Edge areas (Z), .mu.m 14 14 13 13
14 14 8 14 thickness Coating thickness ratio 88 88 88 88 88 88 47
88 (Z/Y .times. 100), % Distance HS 45, Spin rate, rpm 3370 3380
3090 3220 3360 3340 3340 3650 driver Total distance, m 223 223 215
213 233 235 235 221 HS 45, Total distance, m 220 220 212 211 230
232 228 215 driver (after abrasion test) Distance Total distance, m
-3 -3 -3 -2 -3 -3 -7 -6 difference Durability Durability At
incident 1103 1101 1050 1095 615 579 579 621 to cracking velocity
of 43 m/s Abrasion After 144 hours of good good good good good good
NG NG test abrasion with sand
[0157] The rubber material in Comparative Example 4 had a breaking
strength of 15 MPa and an elongation of 88%.
[0158] 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.
[0159] 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.
[0160] In the practice golf ball in Comparative Example 3, zinc
acrylate was included in the rubber composition and the cover was
composed primarily of an ionomer, as a result of which the
durability to cracking was very poor. In addition, because the
cover was composed primarily of an ionomer, the cover was too hard.
As a result, the durability of markings was very poor and the
flight performance following the abrasion test (following
evaluation of the durability of markings) decreased markedly.
[0161] The practice golf ball of Comparative Example 4 was a
one-piece golf ball composed of a rubber layer. Because rubber was
used, the breaking strength was poor and the elongation was low. As
a result, the durability to cracking, particularly the durability
to cutting, was poor, in addition to which surface loss readily
arose. Also, in the abrasion test, since a clear paint was formed
on the coating of white enamel paint, cracking occurs to the white
paint and dirty was easy to adhere to its surface so that the
appearance was poor when seen the appearance after the abrasion
test. Moreover, the decrease in flight performance following the
abrasion test (following evaluation of the durability of markings)
was somewhat large, and the spin rate increased.
[0162] Japanese Patent Application No. 2011-099853 is incorporated
herein by reference.
[0163] 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.
* * * * *