U.S. patent application number 13/351790 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 | 20120277020 13/351790 |
Document ID | / |
Family ID | 47068308 |
Filed Date | 2012-11-01 |
United States Patent
Application |
20120277020 |
Kind Code |
A1 |
HIGUCHI; Hiroshi ; et
al. |
November 1, 2012 |
PRACTICE GOLF BALL
Abstract
A practice golf ball has a core and a cover. The core is made of
a rubber composition which includes a base rubber, a
co-crosslinking agent, a crosslinking initiator and a metal oxide.
The base rubber is a mixture of polybutadiene with
styrene-butadiene rubber and/or isoprene rubber, and the
co-crosslinking agent is methacrylic acid. The core has a specific
deflection under a specific load, and the ball has a specific
initial velocity. The dimples on the surface of the ball have a
volume ratio VR of from 0.95 to 1.7.
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: |
47068308 |
Appl. No.: |
13/351790 |
Filed: |
January 17, 2012 |
Current U.S.
Class: |
473/280 |
Current CPC
Class: |
C08L 9/06 20130101; A63B
37/0051 20130101; A63B 37/0031 20130101; C08L 9/00 20130101; A63B
37/0064 20130101; A63B 37/0068 20130101; A63B 37/0087 20130101;
A63B 37/0066 20130101; A63B 37/0012 20130101; A63B 69/3655
20130101; C08L 9/00 20130101; A63B 37/0063 20130101; A63B 37/0084
20130101; C08K 5/09 20130101; C08L 9/06 20130101; C08K 5/09
20130101; A63B 37/0003 20130101; A63B 37/0033 20130101; A63B
37/0035 20130101; A63B 37/0074 20130101; A63B 37/0065 20130101;
A63B 37/0021 20130101; A63B 37/0017 20130101; C08K 5/09 20130101;
C08L 9/00 20130101; C08L 9/00 20130101 |
Class at
Publication: |
473/280 |
International
Class: |
A63B 69/36 20060101
A63B069/36 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 27, 2011 |
JP |
2011-099969 |
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 comprises a
resin component, wherein the base rubber is a mixture of
polybutadiene with styrene-butadiene rubber or isoprene rubber or
both; the co-crosslinking agent is methacrylic acid; the core, when
compressed under a final load of 1,275 N (130 kgf) from an initial
load of 98 N (10 kgf), has a deflection CH of from 2.5 mm to 7.0
mm; the ball has an initial velocity of not more than 70 m/s; and
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 0.95% to 1.7%.
2. 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; the resin component of
the cover is composed primarily of polyurethane; the cover has a
material hardness, expressed as the Shore D hardness, of from 30 to
57; the cover has a thickness of from 0.3 mm to 2.5 mm; 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 BH1, the
ratio CH/BH1 is from 0.95 to 1.1.
3. The practice golf ball of claim 1, wherein the ball has, upon
initial measurement, 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 also has, when measured
again after being left to stand for 350 days following initial
measurement, 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
not more than 0.2 mm and the difference BV2-BV1 is not more than
0.3 m/s.
4. 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%.
5. The practice golf ball of claim 4 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%.
6. The practice golf ball of claim 5 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%.
7. The practice golf ball of claim 1, wherein the polyurethane in
the resin component of the cover is a thermoplastic polyurethane
elastomer.
8. The practice golf ball of claim 7, wherein the thermoplastic
polyurethane elastomer comprises soft segments formed from a
polymeric polyether polyol and hard segments formed from an
aromatic diisocyanate.
9. The practice golf ball of claim 1, wherein the ball has, upon
initial measurement, a deflection BH1 when compressed under a final
load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) of
from 2.5 mm to 7.0 mm.
10. The practice golf ball of claim 1 wherein, in the rubber
composition making up the core, polybutadiene accounts for not more
than 80 wt % of the base rubber, styrene-butadiene rubber or
isoprene rubber or both account for from 20 wt % to 60 wt % of the
base rubber, and the compounding ingredients are included in
respective amounts of from 10 to 40 parts by weight of methacrylic
acid, from 15 to 30 parts by weight of metal oxide, from 0.3 to 5.0
parts by weight of crosslinking initiator, and from 0.1 to 1.0 part
by weight of an antioxidant, 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. 2011-099969 filed in
Japan on Apr. 27, 2011, the entire contents of which are hereby
incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a practice golf ball which
is suitable for use at such places as driving ranges. More
specifically, the invention relates to a practice golf ball which
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 the ability
to maintain a stable feel on impact and a stable flight performance
over a long period of time, and which, moreover, by further
limiting the flight of the ball, has a controlled distance for use
at very small driving ranges.
[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 describes, as a rubber
composition for the core of a two-piece solid golf ball, the
compounding of a given amount of zinc acrylate in
cis-1,4-polybutadiene rubber. However, here too, when zinc acrylate
is used in the rubber-forming rubber composition, achieving good
ball durability in long-term use at a driving range has been
difficult.
[0011] Also, JP-A 2004-180793 and JP-A 2008-149190 disclose golf
balls which use zinc acrylate in the core formulation and use a
thermoplastic polyurethane as the cover material. However,
drawbacks of such golf balls include a hard cover, a large decrease
in ball flight following abrasion of the ball surface, and poor
durability of markings ("markings" referring here to, for example,
a brand name or player number printed on the surface of the ball at
the time of manufacture).
[0012] In view of the foregoing, it is an object of the present
invention to provide a practice golf ball which 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 the ability to maintain a
stable feel on impact and a stable flight performance over a long
period of time, and which moreover, by further limiting the flight
of the ball, has a controlled distance for use at very small
driving ranges.
SUMMARY OF THE INVENTION
[0013] We have discovered that, in the production of practice golf
balls having a core made of a rubber composition and a cover
encasing the core, by using a mixture of polybutadiene with
styrene-butadiene rubber and/or isoprene rubber as the base rubber,
by using methacrylic acid as a co-crosslinking agent in the rubber
composition and also including a given amount of a crosslinking
initiator per 100 parts by weight of the base rubber, and moreover
by optimizing the core deflection under a specific load, the
initial velocity of the finished ball and the dimple volume ratio
VR, owing to the synergistic effects of these features, 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 a practice golf ball which, even
with long-term use, retains a good appearance and a good flight
performance, has a good feel on impact and, by further limiting the
flight of the ball, has a controlled distance.
[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 rubber composition, the ball can be made
much more durable to cracking than a game ball. In addition, by
having the cover material formed primarily of polyurethane, a
practice golf ball endowed with excellent durability to cracking
and durability to abrasion can be obtained. Moreover, 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 comprising 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 comprises a resin component. The base rubber
is a mixture of polybutadiene with styrene-butadiene rubber and/or
isoprene rubber, and the co-crosslinking agent is methacrylic acid.
The core, when compressed under a final load of 1,275 N (130 kgf)
from an initial load of 98 N (10 kgf), has a deflection CH of from
2.5 mm to 7.0 mm. The ball has an initial velocity of not more than
70 m/s. 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 0.95% to 1.7%.
[0016] 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; and the value A-C is from 0 to 8. The
core preferably has a specific gravity of from 1.05 to 1.2. The
resin component of the cover is preferably composed primarily of
polyurethane. The cover preferably has a material hardness,
expressed as the Shore D hardness, of from 30 to 57, and preferably
has a thickness of from 0.3 mm to 2.5 mm. When the core and the
ball are each compressed under a final load of 1,275 N (130 kgf)
from an initial load of 98 N (10 kgf), letting deflection by the
core be CH and deflection by the ball be BH1, the ratio CH/BH1 is
preferably from 0.95 to 1.1.
[0017] The practice golf ball of the invention preferably has, upon
initial measurement, 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, 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
not more than 0.2 mm and the difference BV2-BV1 is not more than
0.3 m/s.
[0018] The practice golf ball may have has formed on a surface
thereof a plurality of dimples which satisfy conditions (1) and (2)
below:
[0019] (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
[0020] (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%.
[0021] The practice golf ball which satisfies above conditions (1)
and (2) may satisfy also condition (3) below:
[0022] (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%.
[0023] The practice golf ball which satisfies above conditions (1)
to (3) may satisfy also conditions (4) to (6) below:
[0024] (4) the number of dimple types of differing diameter is 3 or
more;
[0025] (5) the total number of dimples N is not more than 380;
and
[0026] (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%.
[0027] The polyurethane in the resin component of the cover may be
a thermoplastic polyurethane elastomer, in which case the
thermoplastic polyurethane elastomer preferably includes soft
segments formed from a polymeric polyether polyol and hard segments
formed from an aromatic diisocyanate.
[0028] The practice golf ball of the invention preferably has, upon
initial measurement, a deflection BH1 when compressed under a final
load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) of
from 2.5 mm to 7.0 mm.
[0029] Preferably, in the rubber composition making up the core of
the practice golf ball of the invention, polybutadiene accounts for
not more than 80 wt % of the base rubber, styrene-butadiene rubber
and/or isoprene rubber account for from 20 wt % to 60 wt % of the
base rubber, and the compounding ingredients are included in
respective amounts of from 10 to 40 parts by weight of methacrylic
acid, from 15 to 30 parts by weight of metal oxide, from 0.3 to 5.0
parts by weight of crosslinking initiator, and from 0.1 to 1.0 part
by weight of an antioxidant, 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, the ability to maintain a
stable feel on impact and a stable flight performance over a long
period of time, and also, by further limiting the flight of the
ball, a controlled distance for use at very small driving
ranges.
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.
[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. A mixture of polybutadiene with a styrene-butadiene
rubber and/or an isoprene rubber is used as the base rubber in the
rubber composition. In the present 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 a 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 is 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] The above polybutadiene used in the base rubber accounts for
preferably not more than 80 wt %, more preferably not more than 70
wt %, even more preferably not more than 60 wt %, and most
preferably not more than 57 wt %, of the overall rubber. The lower
limit is preferably at least 40 wt %, more preferably at least 45
wt %, and even more preferably at least 48 wt %.
[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] In the present invention, a styrene-butadiene rubber (SBR)
and/or an isoprene rubber (IR) are used together with the above
polybutadiene rubber (BR). The styrene-butadiene rubber and the
isoprene rubber are described below.
[0049] Isoprene rubbers (IR) 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 (SBR) 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 styrene-butadiene and/or isoprene rubber account for a
proportion of the overall rubber which is preferably at least 20 wt
%, more preferably at least 30 wt %, even more preferably at least
40 wt %, and most preferably at least 43 wt %, but is preferably
not more than 60 wt %, more preferably not more than 55 wt %, and
even more preferably not more than 52 wt %.
[0052] The styrene-butadiene rubber accounts for a proportion of
the overall rubber which is preferably 0 wt % or more, more
preferably at least 10 wt %, and even more preferably at least 20
wt %, but is preferably not more than 60 wt %, more preferably not
more than 50 wt %, even more preferably not more than 40 wt %, and
most preferably not more than 35 wt %.
[0053] The isoprene rubber accounts for a proportion of the overall
rubber which is preferably 0 wt % or more, more preferably at least
9 wt %, and even more preferably at least 13 wt %, but is
preferably not more than 60 wt %, more preferably not more than 45
wt %, even more preferably not more than 35 wt %, and most
preferably not more than 25 wt %.
[0054] 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 12 parts by weight, even more preferably at
least 14 parts by weight, and still more preferably at least 16
parts by weight. The upper limit in the amount of methacrylic acid
is preferably not more than 40 parts by weight, more preferably not
more than 35 parts by weight, even more preferably not more than 30
parts by weight, and most preferably not more than 28 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.
[0055] 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.
[0056] 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.
[0057] 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.
[0058] 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.
[0059] 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.
[0060] In the practice of the invention, from a resource recycling
standpoint, a ground or abraded powder of s vulcanized rubber may
be included in a small amount of 40 parts by weight or less per 100
parts by weight of the base rubber. In such a case, the ground or
abraded powder may be compounded in an amount, per 100 parts by
weight of the base rubber, which is more than 0 wt %, preferably at
least 2 wt %, and most preferably at least 5 wt %, but is
preferably not more than 40 wt %, more preferably not more than 35
wt %, even more preferably not more than 30 wt %, and most
preferably not more than 25 wt %. The ground or abraded powder of
vulcanized rubber is a vulcanizate which contains rubber and
unsaturated carboxylic acid or a metal salt thereof. It is
desirable for the ground or abraded powder of vulcanized rubber
used to have a particle size which is preferably at least 20 .mu.m,
more preferably at least 25 .mu.m, and most preferably at least 30
.mu.m, but is preferably not more than 1,000 .mu.m, more preferably
not more than 900 .mu.m, and most preferably not more than 800
.mu.m. Adding a crushed or abraded powder of vulcanized rubber has
such effects as improving the productivity of the vulcanizate and
increasing the durability to cracking. However, including too much
may markedly lower the workability of the rubber composition and
the productivity.
[0061] 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.
[0062] 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.
[0063] The core has a specific gravity of preferably at least 1.05,
more preferably at least 1.08, and even more preferably at least
1.1, but preferably not more than 1.2, more preferably not more
than 1.15, and even more preferably not more than 1.13.
[0064] 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 at least 2.5 mm, preferably at least 2.6 mm, and
more preferably at least 2.7 mm, but is not more than 7.0 mm,
preferably not more than 6.0 mm, more preferably not more than 5.5
mm, and most preferably not more than 5.2 mm. If the core
deflection CH is too small, the feel of the practice golf ball on
impact will 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 will be so soft as to make the
ball unpleasant to use, in addition to which the productivity may
decline considerably.
[0065] The core rebound (also referred to as the core initial
velocity, CV), is preferably at least 60 m/s, more preferably at
least 63 m/s, even more preferably at least 66 m/s, and most
preferably at least 67 m/s, but is preferably not more than 71 m/s,
more preferably not more than 70.5 m/s, and even more preferably
not more than 70 m/s. A core rebound outside of this range is
undesirable because the distance of the ball may dramatically
decline, or the ball may fly so far as to pass over the netting at
a very small driving range.
[0066] In the present 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, it is preferable for the respective values A to F to
fall within the specific ranges indicated below. By thus setting
the hardness profile at the core interior within specific ranges, a
comfortable feel on impact similar to that of a game ball and also
a good durability to cracking can be obtained.
[0067] 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
62, and even more preferably at least 63, but is preferably not
more than 88, more preferably not more than 85, and even more
preferably not more than 81.
[0068] 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 56, and even more preferably at least 58, but
is preferably not more than 83, more preferably not more than 80,
and even more preferably not more than 76.
[0069] 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 58, and even more preferably at least 60, but
is preferably not more than 85, more preferably not more than 82,
and even more preferably not more than 78.
[0070] 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.
[0071] Letting E be the JIS-C hardness at a position 15 mm inside
the core surface, the value of E is preferably at s 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,
and even more preferably not more than 71.
[0072] 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.
[0073] 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.
[0074] The value of A-C is preferably within a range of from 0 to
8, with the lower limit being preferably more than 0, more
preferably at least 1, and even more preferably at least 2, 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 2, more preferably at least 4, and even
more preferably at least 6, but preferably not more than 19, more
preferably not more than 18, even more preferably not more than 17,
and most preferably not more than 16.
[0075] 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.
[0076] 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").
[0077] 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.
[0078] 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.
[0079] 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.
[0080] 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.
[0081] 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.
[0082] 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.
[0083] 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.
[0084] 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.
[0085] 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.
[0086] 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.
[0087] 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.
[0088] 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.
[0089] Next, the material making up the cover which directly
encases the core is described.
[0090] In this invention, a thermoplastic resin such as an ionomer
resin or polyurethane may be used as the resin component of the
cover. In particular, the use of a resin material composed
primarily of polyurethane is preferred. Specifically, 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.
[0091] The thermoplastic polyurethane elastomer preferably 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.
[0092] 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.
[0093] 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 is 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.
[0094] 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.).
[0095] 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, and still more preferably not more than 1.7 mm. If the
cover thickness is larger than the above range, the ball rebound
will decrease, worsening the flight performance. On the other hand,
if the cover thickness is smaller than the above range, the
durability to cracking will decrease. In particular, when the ball
is hit thin, or "topped," the cover is likely to tear.
[0096] 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.
[0097] 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 54, even more preferably not more than 51,
and still more preferably not more than 50. If the Shore D hardness
of the cover is higher than the above range, the appearance
performance in long-term use (durability of markings) will decline,
in addition to which the flight performance will markedly decrease.
On the other hand, if the Shore D hardness of the cover is lower
than the above range, the durability to cracking will greatly
decrease and, particularly when the ball is topped, the cover is
likely to tear. In addition, the spin rate becomes very high,
shortening the distance traveled by the ball.
[0098] The practice golf ball of the invention 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. It is
critical for 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,
to be in a range of from 0.95% to 1.7%, the lower limit being
preferably 0.97%, and more preferably 0.98%, and the upper limit
being preferably 1.5%, more preferably 1.3%, and most preferably
1.2%
[0099] 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.
[0100] 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.
[0101] 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%.
[0102] 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%.
[0103] 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%.
[0104] 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.
[0105] 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 at least 0.1 mm, and even
more preferably at least 0.13 mm, and the upper limit being more
preferably not more than 0.3 mm, and even more preferably not more
than 0.25 mm.
[0106] 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.
[0107] 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%.
[0108] 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 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.
[0109] 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.
[0110] 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.
[0111] The ball has, upon initial measurement, a deflection BH1,
when compressed under a final load of 1,275 N (130 kgf) from an
initial load of 98 N (10 kgf), of preferably at least 2.5 mm, more
preferably at least 2.6 mm, and even more preferably at least 2.65
mm, but preferably not more than 7.0 mm, more preferably not more
than 6.0 mm, even more preferably not more than 5.5 mm, and most
preferably not more than 5.0 mm. 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 the deflections of the core
and the ball be respectively CH and BH1, the ratio CH/BH1 is
preferably at least 0.95, more preferably at least 0.96, and even
more preferably at least 0.97, but is preferably not more than 1.1,
more preferably not more than 1.09, and even more preferably not
more than 1.08. If the ratio CH/BH1 is too large, the deflection of
the finished ball may be too small relative to the core deflection.
That is, because the cover hardness increases, 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/BH1 is too small,
the cover becomes very soft, which may significantly lower the
durability to cracking and lead in particular to cracking of the
cover when the ball is topped. In addition, the spin rate may
become too high, resulting in a shorter distance of travel by the
ball.
[0112] In addition, the ball has, upon initial measurement, a
rebound BV1 which is preferably at least 60 m/s, more preferably at
least 63 m/s, even more preferably at least 65 m/s, and most
preferably at least 67 m/s, but must not be more than 70 m/s and is
preferably not more than 69.8 m/s, more preferably not more than
69.6 m/s, and most preferably not more than 69.4 m/s. Outside of
the foregoing range, the distance traveled by the ball may greatly
decrease, or the ball may fly so far as to pass over the netting,
making it unsuitable for use as a controlled-distance practice golf
ball at very small driving ranges.
[0113] Moreover, in order to achieve a good durability over a long
period of time, the practice 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.
EXAMPLES
[0114] The following Examples and Comparative Examples are provided
to illustrate the invention, and are not intended to limit the
scope thereof.
Examples 1 to 8, Comparative Examples 1 to 11
[0115] Rubber materials formulated as shown in Table 1 below were
furnished for the fabrication of practice golf balls in the
Examples and Comparative Examples. These rubber compositions were
suitably mixed using a kneader or roll mill, then vulcanized under
the temperature and time conditions in Table 1 to produce solid
cores in the respective Examples. Ingredient amounts in the table
below are shown in parts by weight.
TABLE-US-00001 TABLE 1 Type of Core (1) (2) (3) (4) (5) (6) (7) (8)
Core BR01 55 55 55 100 60 100 95 formulation IR2200 20 15 15 5
BR730 100 BR51 40 SBR0202 25 30 30 Perhexa C-40 0.6 0.6 (40%
dilution) Actual amount 0.24 0.24 of addition Percumyl D 0.9 1.1
0.9 1.2 0.8 0.6 0.6 0.8 Zinc oxide 23 23 22.5 23.5 23 9.5 9.5 23
Antioxidant 0.2 0.2 0.2 0.2 0.2 0.1 0.1 0.2 Methacrylic acid 25.5
27 17 29 18 22.5 Zinc 26 methacrylate Zinc acrylate 26 Titanium
oxide 4 Vulcanization Temperature (.degree. C.) 170 170 170 170 170
160 160 170 conditions Time (minutes) 20 20 20 20 20 13 13 30
[0116] Details on the materials used in the core formulations in
the above table are provided below. [0117] (1) BR01: A butadiene
rubber synthesized with a nickel catalyst, available from JSR
Corporation; Mooney viscosity ML, 46 [0118] (2) IR2200: An isoprene
rubber, available from JSR Corporation; Mooney viscosity ML, 82
[0119] (3) BR730: A butadiene rubber synthesized with a neodymium
catalyst, available from JSR Corporation; Mooney viscosity ML, 55
[0120] (4) BR51: A butadiene rubber synthesized with a neodymium
catalyst, available from JSR Corporation; Mooney viscosity ML, 36
[0121] (5) SBR0202: An emulsion-polymerized styrene-butadiene
rubber, available from JSR Corporation [0122] (6) Perhexa C-40: An
organic peroxide, available from NOF Corporation [0123] (7)
Percumyl D: An organic peroxide, available from NOF Corporation
[0124] (8) Zinc oxide: Available from Sakai Chemical Co., Ltd.
[0125] (9) Antioxidant: "Nocrac NS-6," available from Ouchi Shinko
Chemical Industry Co., Ltd. [0126] (10) Methacrylic acid: Available
from Kuraray Co., Ltd. [0127] (11) Zinc methacrylate: Available
from Asada Chemical Industry Co., Ltd. [0128] (12) Zinc acrylate:
Available from Nihon Jyoryu Kogyo Co., Ltd. [0129] (13) Titanium
oxide: Available from Ishihara Sangyo Kaisha, Ltd.
[0130] In each example, after the rubber composition formulated
from the ingredients shown in Table 1 was molded and vulcanized to
form a core, the surface of the core was abraded to a desired
diameter. Next, surface treatment of the core was carried out by
immersing the core for 30 seconds in an acetone solution of
trichloroisocyanuric acid (concentration, 3 wt %), then washing the
surface of the core with water. The core was then set in a mold for
injection molding the cover, and the cover composition shown in
Table 2 below was injection molded over the solid core. Ingredient
amounts in the table below are shown in parts by weight.
TABLE-US-00002 TABLE 2 A B C D E F G Resin Himilan 50 7331 Himilan
30 50 1557 Himilan 20 1855 Himilan 50 1601 Pandex 25 100 T8260
Pandex 100 75 25 T8195 Pandex 75 T8290 Pandex 100 T8180 Additives
Mag- 1 1 nesium stearate Titanium 3.5 3.5 3.5 3.5 3.5 2.1 2.1
dioxide Poly- 1.5 1.5 1.5 1.5 1.5 ethylene wax Shore D hardness 45
50 40 58 25 50 60
[0131] Details on the materials used in the cover composition in
the above table are provided below. [0132] "Himilan": Ionomer
resins available under this trade name from DuPont-Mitsui
Polychemicals Co., Ltd. [0133] "Pandex": Thermoplastic polyurethane
elastomers available under this trade name from Dainippon Ink &
Chemicals, Inc. [0134] Magnesium stearate: Available from NOF
Corporation [0135] Titanium dioxide: Available under the trade name
"Tipaque R550" from Ishihara Sangyo Kaisha, Ltd. [0136]
Polyethylene wax: Available under the trade name "Sanwax 161P" from
Sanyo Chemical Industries, Ltd.
[0137] 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
was printed on the ball surface.
[0138] 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 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
SR VR No. Number (mm) (mm) (mm) ratio (number) (number) (%)
(number) (%) (%) (%) Configuration Dimple I 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 II 1 24 4.4 0.237 0.65 15 338 102 30 330 98 72
1.14 FIG. 4 2 204 4.2 0.230 0.65 15 3 66 3.6 0.220 0.75 21 4 12 2.7
0.190 0.8 30 5 24 2.5 0.160 0.8 32 6 8 3.4 0.145 0.45 13 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
[0139] The abbreviations and symbols relating to dimples which
appear in Table 3 are explained below. [0140] R: Radius of
curvature representing roundness provided at peripheral edge of a
dimple [0141] R/D ratio: Ratio of radius of curvature R to diameter
D [0142] N: Total number of dimples on surface of ball [0143] RA:
Collective number of dimples having an R/D ratio of at least 20%
[0144] ER: Ratio of RA to total number of dimples N [0145] 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 [0146] DER: Ratio of DE to total
number of dimples N [0147] 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 [0148] 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.
[0149] 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 Tables 4 and
5.
Deflection of Core and Finished Ball (mm)
[0150] 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
[0151] 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
[0152] JIS-C hardness measurements were carried out on the core
surface in accordance with JIS K6301-1975 after holding the core
isothermally at 23.+-.1.degree. C. (at two places in each of N=5
samples).
Rebound (Initial Velocity) of Core and Finished Ball
[0153] The initial velocity was measured using an initial velocity
measuring apparatus of the same type as the USGA drum rotation-type
initial velocity instrument approved by the R&A. The cores or
balls used as the samples were held isothermally at a temperature
of 23.+-.1.degree. C. for at least 3 hours, then tested in a room
temperature (23.+-.2.degree. C.) chamber. Ten balls were each hit
twice, and the time taken for the cores or balls to traverse a
distance of 6.28 ft (1.91 m) was measured and used to compute the
initial velocity.
Cover Material Hardness
[0154] A cover sheet was formed and, after holding the samples
isothermally at 23.+-.1.degree. C., the Shore D hardness was
measured in accordance with ASTM D-2240.
Durability to Cracking
[0155] The ball was hit thin ("topped") five times at the same
place with the leading edge of a number nine iron (X-BLADE GR,
manufactured by Bridgestone Sports Co., Ltd.) at a head speed (HS)
of 38 m/s, following which the ball was repeatedly struck against a
wall at an incident velocity of 43 m/s and the average number of
shots (N=3 balls) until cracking occurred was determined.
Abrasion Test
[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."
Measurement of Coating Thickness
[0157] Lands (Y): The thickness of the clear coat at land areas at
intermediate positions between dimples was measured. [0158] Edges
(Z): The thickness of the clear coat at dimple edge areas was
measured.
[0159] 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
[0160] 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.
[0161] In addition, after the abrasion test described above had
been carried out, the total distance of the ball was again
measured.
Feel
[0162] 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 Core Type (1) (2)
(3) (3) (2) (2) (3) (3) Diameter, mm 39.9 39.9 39.9 39.3 40.5 41.1
39.9 39.9 Specific gravity 1.118 1.118 1.118 1.118 1.118 1.118
1.118 1.118 Deflection under 10-130 kg 4 2.8 5 5 2.8 2.8 5 5
compression (CH), mm Rebound (CV), m/s 67.7 68.4 69.7 69.7 68.4
68.4 69.7 69.7 JIS-C hardness 71 79 64 64 79 79 64 64 at core
surface (A) JIS-C hardness 66 74 59 59 74 74 59 59 2 mm inside core
surface (B) JIS-C hardness 68 76 61 61 76 76 61 61 5 mm inside core
surface (C) JIS-C hardness 66 71 61 61 71 71 61 61 10 mm inside
core surface (D) JIS-C hardness 63 68 59 59 68 68 59 59 15 mm
inside core surface (E) JIS-C hardness 59 64 55 55 64 64 55 55 at
core center (F) JIS-C hardness difference between core 3 3 3 3 3 3
3 3 surface and 5 mm inside core (A - C) JIS-C hardness difference
between core 12 15 9 9 15 15 9 9 surface and center (A - F) Cover
Type A A A A A A B C Shore D hardness 45 45 45 45 45 45 50 40
Specific gravity 1.15 1.15 1.15 1.15 1.15 1.15 1.15 1.15 Thickness,
mm 1.4 1.4 1.4 1.7 1.1 0.8 1.4 1.4 Finished ball Deflection under
10-130 kg 3.8 2.7 4.75 4.7 2.75 2.75 4.65 4.85 compression 30 days
after production (BH1), mm Deflection under 10-130 kg 3.7 2.6 4.65
4.6 2.7 2.7 4.6 4.8 compression 350 days after measuring BH1 (BH2),
mm Difference between BH1 and BH2, mm -0.1 -0.1 -0.1 -0.1 -0.05
-0.05 -0.05 -0.05 Rebound 30 days after production 67.3 67.8 69.1
68.9 68 68.2 69 69.3 (BV1), m/s Rebound 350 days after measuring
67.4 67.8 69.1 69 68 68.2 69.1 69.3 BV1 (BV2), m/s Difference
between BV1 and BV2, m/s 0.1 0 0 0.1 0 0 0.1 0 Diameter, mm 42.7
42.7 42.7 42.7 42.7 42.7 42.7 42.7 Core initial velocity - 0.4 0.6
0.6 0.8 0.4 0.2 0.7 0.4 ball initial velocity (CV - BV1) Core
deflection/ball deflection (CH/BH1) 1.05 1.04 1.05 1.06 1.02 1.02
1.08 1.03 Dimples Type I II I I II II I I Clear coating Land areas
(Y), .mu.m 15 17 15 15 17 17 15 15 thickness Edge areas (Z), .mu.m
13 15 13 13 15 15 13 13 Coating thickness ratio 88 88 88 88 88 88
88 88 (Z/Y .times. 100), % Distance HS 45, driver Spin rate, rpm
3180 3360 2960 2980 3380 3480 2880 3050 (30 days after Total
distance, m 194 188 192 191 189 190 192 193 production) HS 45,
driver Total distance, m 191 184 189 188 185 186 187 191 (after
abrasion test) Distance Total distance, m -3 -4 -3 -3 -4 -4 -5 -2
difference Durability Durability to At incident 1035 1118 930 1170
930 773 1050 788 cracking velocity of 43 m/s Abrasion test After
144 good good good good good good fair good (durability of hours of
sand markings) abrasion Feel Driver good good good good good good
fair good
TABLE-US-00005 TABLE 5 Comparative Example 1 2 3 4 5 6 Core Type
(4) (5) (5) (6) (7) (7) Diameter, mm 39.9 37.3 42.3 39.9 39.9 39.9
Specific gravity 1.128 1.119 1.118 1.118 1.118 1.118 Deflection
under 10-130 kg 1.8 3.8 3.8 3.8 3.8 3.8 compression (CH), mm
Rebound (CV), m/s 73.7 74.7 74.7 77 77.4 77.4 JIS-C hardness 90 59
69 70 70 70 at core surface (A) JIS-C hardness 88 63 63 65 65 65 2
mm inside core surface (B) JIS-C hardness 87 67 67 68 68 68 5 mm
inside core surface (C) JIS-C hardness 81 66 66 66 66 66 10 mm
inside core surface (D) JIS-C hardness 74 64 64 62 62 62 15 mm
inside core surface (E) JIS-C hardness 70 61 61 58 58 58 at core
center (F) JIS-C hardness difference between core 3 2 2 2 2 2
surface and 5 mm inside core (A - C) JIS-C hardness difference
between core 20 8 8 12 12 12 surface and center (A - F) Cover Type
A A A A A G Shore D hardness 45 45 45 45 45 60 Specific gravity
1.15 1.15 1.15 1.15 1.15 0.99 Thickness, mm 1.4 2.7 0.2 1.4 1.4 1.4
Finished ball Deflection under 10-130 kg 1.9 3.4 3.9 3.8 3.8 3.3
compression 30 days after production (BH1), mm Deflection under
10-130 kg 1.9 3.4 3.9 3.5 3.5 3 compression 350 days after
measuring BH1 (BH2), mm Difference between BH1 and BH2, mm 0 0 0
-0.3 -0.3 -0.3 Rebound 30 days after production 74.4 73.4 74.7 76
76.4 77.0 (BV1), m/s Rebound 350 days after measuring 74.4 73.4
74.8 75.1 75.5 76.6 BV1 (BV2), m/s Difference between BV1 and BV2,
m/s 0 0 0.1 -0.9 -0.9 -0.4 Diameter, mm 42.7 42.7 42.7 42.7 42.7
42.7 Core initial velocity - -0.7 1.3 0.0 1.0 1.0 0.4 ball initial
velocity (CV - BV1) Core deflection/ball deflection (CH/BH1) 0.95
1.12 0.97 1.00 1.00 1.15 Dimples Type I III I I I I Clear coating
Land areas (Y), .mu.m 15 17 15 15 15 15 thickness Edge areas (Z),
.mu.m 13 8 13 13 13 13 Coating thickness ratio 88 47 88 88 88 88
(Z/Y .times. 100), % Distance HS 45, driver Spin rate, rpm 3480
2830 3400 3100 3070 3040 (30 days after Total distance, m 223 210
217 224 226 228 production) HS 45, driver Total distance, m 219 203
214 221 223 213 (after abrasion test) Distance Total distance, m -4
-7 -3 -3 -3 -15 difference Durability Durability to At incident
1425 1680 585 555 518 300 cracking velocity of 43 m/s Abrasion test
After 144 good NG good good good NG (durability of markings) hours
of sand abrasion Feel Driver NG NG good good good good Comparative
Example 7 8 9 10 11 Core Type (7) (5) (5) (5) (8) Diameter, mm 39.9
39.9 39.9 39.9 42.7 Specific gravity 1.118 1.118 1.118 1.118 1.121
Deflection under 10-130 kg 3.8 3.8 3.8 3.8 compression (CH), mm
Rebound (CV), m/s 77.4 74.7 74.7 74.7 JIS-C hardness 70 69 69 69 80
at core surface (A) JIS-C hardness 65 63 63 63 75 2 mm inside core
surface (B) JIS-C hardness 68 67 67 67 77 5 mm inside core surface
(C) JIS-C hardness 66 66 66 66 71 10 mm inside core surface (D)
JIS-C hardness 62 64 64 64 67 15 mm inside core surface (E) JIS-C
hardness 58 61 61 61 63 at core center (F) JIS-C hardness
difference between core 2 2 2 2 3 surface and 5 mm inside core (A -
C) JIS-C hardness difference between core 12 8 8 8 17 surface and
center (A - F) Cover Type A D E F Shore D hardness 45 58 25 50
Specific gravity 1.15 1.15 1.15 0.99 Thickness, mm 1.4 1.4 1.4 1.4
Finished ball Deflection under 10-130 kg 3.8 3.40 3.9 3.6 3.1
compression 30 days after production (BH1), mm Deflection under
10-130 kg 3.5 3.4 3.9 3.5 3.1 compression 350 days after measuring
BH1 (BH2), mm Difference between BH1 and BH2, mm -0.3 0.0 0 -0.1 0
Rebound 30 days after production 76.4 73.9 74.2 73.5 74.6 (BV1),
m/s Rebound 350 days after measuring 75.5 74.0 74.2 73.6 74.7 BV1
(BV2), m/s Difference between BV1 and BV2, m/s -0.9 0.1 0 0.1 0.1
Diameter, mm 42.7 42.7 42.7 42.7 42.7 Core initial velocity - 1.0
0.8 0.5 1.2 ball initial velocity (CV - BV1) Core deflection/ball
deflection (CH/BH1) 1.00 1.12 0.97 1.06 Dimples Type III I I I I
Clear coating Land areas (Y), .mu.m 17 15 15 15 15 thickness Edge
areas (Z), .mu.m 8 13 13 13 13 Coating thickness ratio 47 88 88 88
88 (Z/Y .times. 100), % Distance HS 45, driver Spin rate, rpm 3070
2900 3520 3110 3650 (30 days after Total distance, m 226 214 209
211 221 production) HS 45, driver Total distance, m 219 206 207 199
215 (after abrasion test) Distance Total distance, m -7 -8 -2 -12
-6 difference Durability Durability to At incident 518 1305 612 960
621 cracking velocity of 43 m/s Abrasion test After 144 NG NG good
NG NG (durability of markings) hours of sand abrasion Feel Driver
good NG good good good
[0163] In the practice golf ball of Comparative Example 1, the
deflection of the finished ball was too small. As a result, the
ball had too hard a feel on impact, making it uncomfortable to use
as a practice golf ball.
[0164] In the practice golf ball of Comparative Example 2, the
cover was too thick, as a result of which the ball had a dull feel
on impact. In addition, the radius of curvature at the dimples
edges was small and the edge-to-land coating thickness ratio was
small, as a result of which the durability of markings was very
poor. Moreover, the flight performance following the abrasion test
(following evaluation of the durability of markings) decreased.
[0165] In the practice golf ball of Comparative Example 3, the
cover was too thin. As a result, the durability to cracking was
poor, with the durability to topping being particularly poor.
[0166] In the practice golf ball of Comparative Example 4, zinc
methacrylate was included as the co-crosslinking agent in the
core-forming rubber formulation. As a result, the durability to
cracking was poor.
[0167] In the practice golf ball of Comparative Example 5, zinc
acrylate was included as the co-crosslinking agent in the
core-forming rubber formulation. As a result, the durability to
cracking was very poor.
[0168] In the practice golf ball of Comparative Example 6, zinc
acrylate was included in the rubber composition and the cover was
composed primarily of an ionomer. As a result, the durability to
cracking was very poor. In addition, because the cover was composed
primarily of an ionomer, it was too hard, as a result of which the
durability of markings was very poor and the flight performance
following the abrasion test (following evaluation of the durability
of markings) greatly decreased.
[0169] In the practice golf ball of Comparative Example 7, zinc
acrylate was included, as a result of which the durability to
cracking was very poor. In addition, the radius of curvature at the
dimple edges was small and the edge-to-land coating thickness ratio
was small, as a result of which the durability of markings was very
poor. Moreover, the flight performance following the abrasion test
(following evaluation of the durability of abrasions)
decreased.
[0170] In the practice golf ball of Comparative Example 8, the
cover was too hard, as a result of which the durability of markings
was very poor. In addition, the flight performance following the
abrasion test (following evaluation of the durability of markings)
greatly decreased.
[0171] In the practice golf ball of Comparative Example 9, the
cover was too soft, as a result of which the durability to cracking
was poor. When the ball was topped in particular, the cover ended
up cracking. In addition, the spin rate was too high, reducing the
distance traveled by the ball.
[0172] In the practice golf ball of Comparative Example 10, the
cover was formed by using an ionomer as the resin component. As a
result, the durability of markings was very poor. In addition, the
flight performance following the abrasion test (following
evaluation of the durability of markings) greatly decreased.
[0173] The practice golf ball of Comparative Example 11 was a
one-piece golf ball composed of a single layer. As a result, the
durability to cracking was poor and the spin rate was high. In
addition, the decrease in the flight performance following the
abrasion test (following evaluation of the durability of markings)
was somewhat large.
[0174] Japanese Patent Application No. 2011-099969 is incorporated
herein by reference.
[0175] 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.
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