U.S. patent application number 09/138074 was filed with the patent office on 2001-12-13 for process for producing golf ball.
Invention is credited to IHARA, KEISUKE, INOUE, MICHIO, KASASHIMA, ATSUKI, MASUTANI, YUTAKA, SHIMOSAKA, HIROTAKA.
Application Number | 20010050447 09/138074 |
Document ID | / |
Family ID | 18359598 |
Filed Date | 2001-12-13 |
United States Patent
Application |
20010050447 |
Kind Code |
A1 |
INOUE, MICHIO ; et
al. |
December 13, 2001 |
PROCESS FOR PRODUCING GOLF BALL
Abstract
A method of manufacturing a golf ball is disclosed. A cover is
molded through use of either a thermoplastic resin having a group
capable of reacting with an isocyanate group or a thermoplastic
resin containing a compound having two or more groups capable of
reacting with an isocyanate group. Subsequently, a polyisocyanate
compound is caused to permeate into the surface layer of the cover
to thereby cause the reaction between the polyisocyanate compound
and the group(s) capable of reacting with the isocyanate group. As
a result, a modified layer having excellent properties is formed at
the surface of the cover formed from thermoplastic resin. In this
case, a non-yellowing polyurethane resin is preferably used as the
thermoplastic resin having a group capable of reacting with an
isocyanate group, and 4,4-diphenylmethane diisocyanate (MDI) is
preferably used as the polyisocyanate compound.
Inventors: |
INOUE, MICHIO; (SAITAMA,
JP) ; IHARA, KEISUKE; (SAITAMA, JP) ;
SHIMOSAKA, HIROTAKA; (SAITAMA, JP) ; MASUTANI,
YUTAKA; (SAITAMA, JP) ; KASASHIMA, ATSUKI;
(SAITAMA, JP) |
Correspondence
Address: |
SUGHRUE MION ZINN MACPEAK & SEAS
2100 PENNSYLVANIA AVENUE NW
WASHINGTON
DC
20037
|
Family ID: |
18359598 |
Appl. No.: |
09/138074 |
Filed: |
January 22, 1999 |
PCT Filed: |
December 24, 1997 |
PCT NO: |
PCT/JP97/04801 |
Current U.S.
Class: |
264/232 ;
264/129; 264/279.1; 264/340 |
Current CPC
Class: |
A63B 37/0003 20130101;
B29C 37/0078 20130101; A63B 37/0075 20130101; A63B 37/12 20130101;
C08L 75/04 20130101; C08G 18/7664 20130101; A63B 45/00 20130101;
A63B 37/0074 20130101; B29L 2031/545 20130101; B29K 2075/00
20130101; C08J 7/12 20130101; C08G 18/7671 20130101; C08L 75/04
20130101; A63B 37/0024 20130101; B29L 2031/54 20130101; C08L 75/04
20130101 |
Class at
Publication: |
264/232 ;
264/279.1; 264/129; 264/340 |
International
Class: |
B29C 035/02; B29C
071/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 24, 1996 |
JP |
8-343155 |
Claims
1. A method of manufacturing golf balls, comprising the steps of:
molding a cover through use of either a thermoplastic resin having
a group capable of reacting with an isocyanate group or a
thermoplastic resin containing a compound having two or more groups
capable of reacting with an isocyanate group; and causing a
polyisocyanate compound to permeate into the surface layer of the
cover to thereby cause the reaction between the polyisocyanate
compound and the group(s) capable of reacting with the isocyanate
group, whereby a modified layer is formed at the surface of the
cover.
2. A method of manufacturing golf balls according to claim 1,
wherein a non-yellowing polyurethane resin is used as the
thermoplastic resin.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method of manufacturing a
golf ball having a cover formed from a thermoplastic resin, and
particularly to a method of manufacturing a golf ball in which the
surface layer of a cover formed from a thermoplastic resin is
modified so as to impart excellent characteristics to the
cover.
BACKGROUND ART
[0002] Solid golf balls such as two-piece golf balls and
thread-wound golf balls are usually manufactured by the steps of
enclosing a solid core or thread-wound core with a cover by
compression molding or injection molding, forming dimples in the
cover, and stamping markings on and applying a coating onto the
surface of the cover. In this manufacturing process, thermoplastic
resins such as ionomer resins, polyolefin resins, and polyurethane
resins are often used as materials for covers.
[0003] When thermoplastic resins are used as materials for covers
of golf balls, covers and dimples in covers can be readily formed
by compression molding or injection molding. However, covers formed
from thermoplastic resins may be unsatisfactory in heat resistance,
wear resistance, compression resistance, solvent resistance,
chemical resistance, or like physical properties. Also, imparting a
certain performance to a cover is impossible unless a thermoplastic
resin used can impart the property to the cover.
[0004] By contrast, use of thermosetting resins as materials for
covers of golf balls enables an improvement of physical properties
of covers and can impart to covers performance which thermoplastic
resins cannot. However, use of thermosetting resins as cover
materials limits the method of molding to compression molding and
involves difficulty in setting molding conditions. As a result, the
formation of covers and dimples becomes difficult to carry out.
[0005] In recent years, in order to meet demand for improved
resilience, spin properties, and feel on impact, covers have
employed a multilayered structure consisting of a plurality of
layers having different physical properties, i.e. multi-piece
covers have been employed. However, even in the case of using
either thermoplastic resins or thermosetting resins as materials
for covers, constituent layers of a multilayered cover are
difficult to be uniformly formed to a thickness of not greater than
1 mm. Consequently, the thickness of each layer or the overall
thickness of a cover becomes relatively thick, resulting in failure
to obtain desired performance.
[0006] In view of the foregoing, an object of the present invention
is to provide a method of manufacturing golf balls capable of
improving physical properties of the cover of a golf ball,
imparting to the cover performance which cannot be imparted by
thermoplastic resins alone, and reducing the thickness of each
layer of a multilayered cover or the overall thickness of the
cover, by modifying the surface layer of a cover formed from an
easy-to-mold thermoplastic resin.
DISCLOSURE OF THE INVENTION
[0007] To achieve the above object, the present invention provides
a method of manufacturing golf balls, comprising the steps of:
molding a cover through use of either a thermoplastic resin having
a group capable of reacting with an isocyanate group or a
thermoplastic resin containing a compound having two or more groups
capable of reacting with an isocyanate group; and causing a
polyisocyanate compound to permeate into the surface layer of the
cover to thereby induce the reaction between the polyisocyanate
compound and the group(s) capable of reacting with the isocyanate
group, whereby a modified layer is formed at the surface of the
cover.
[0008] According to the method of the present invention, a cover is
molded from either a thermoplastic resin having a group (a reactive
group) capable of reacting with an isocyanate group or a
thermoplastic resin containing a compound (a reactive compound)
having two or more reactive groups. Subsequently, a polyisocyanate
compound is caused to permeate into the surface layer of the cover
to thereby cause the reaction between the polyisocyanate compound
and the reactive group(s) contained in the thermoplastic resin,
causing a number of urethane linkages and urea linkages, i.e. a
bridge structure. Thus is modified the surface layer. Through
appropriate selection of the above thermoplastic resin, reactive
groups, reactive compound, and polyisocyanate compound, desired
physical properties can be imparted to the surface layer of a
cover.
[0009] In the present invention, a golf ball is manufactured
according to, for example, the procedure shown in FIGS. 1A and 1B.
First, as shown in FIG. 1A, a solid core 2 formed from
polybutadiene rubber or the like is enclosed with a single-layer
cover 4 through injection molding or a like molding of a
thermoplastic resin. Next, as shown in FIG. 1B, a polyisocyanate
compound is caused to permeate into the surface layer of the cover
4 to thereby cause the reaction between the penetrating
polyisocyanate compound and a reactive group contained in the
thermoplastic resin of the cover 4, whereby the surface layer is
modified to obtain a modified layer 6.
BRIEF DESCRIPTION OF DRAWINGS
[0010] Sectional views showing an example of the steps of a method
of manufacturing a golf ball according to the present
invention.
BEST MODE FOR CARRYING OUT THE INVENTION
[0011] Examples of thermoplastic resins usable as materials for the
cover of a golf ball in the present invention include the following
three types.
[0012] (1) A thermoplastic resin having a group capable of reacting
with an isocyanate group (a reactive thermoplastic resin).
[0013] (2) A thermoplastic resin having no group capable of
reacting with an isocyanate group (a nonreactive thermoplastic
resin) mixed with a reactive compound.
[0014] (3) A reactive thermoplastic resin mixed with a reactive
compound.
[0015] A reactive thermoplastic resin may be any thermoplastic
resin which contains in its structure a group capable of reacting
with an isocyanate group, for example, a group having active
hydrogen such as hydroxyl group, carboxyl group, amino group,
urethane linkage, and urea linkage. Examples of a thermoplastic
resin having such a group include polyurethane resins, polyamide
resins, polyester resins, acrylic resins, and cellulose resins. A
number of reactive groups may be introduced, as needed, into these
thermoplastic resins in manufacture thereof through
copolymerization with comonomers having a group capable of reacting
with an isocyanate group.
[0016] Also, thermoplastic resins having no reactive group can be
converted to reactive thermoplastic resins in manufacture thereof
through copolymerization with monomers having a reactive group.
Examples of such nonreactive thermoplastic resins include
polyolefin resins such as polyethylene and polypropylene,
polystyrene resins, polyvinyl chloride resins, AS and ABS resins,
vinyl acetate resins, polycarbonate resins, and acetylcellulose
resins.
[0017] For the aforementioned thermoplastic resins having no
reactivity (nonreactive thermoplastic resins), the method of the
present invention can be carried out by adding thereto a compound
having two or more groups capable of reacting with an isocyanate
group, i.e. two or more groups having active hydrogen. Examples of
such a reactive compound include: the aforementioned reactive
thermoplastic resins; low molecular weight compounds such as
polyol, polyamine, and polycarboxylic acid; and oligomers such as
polyester-polyol, polyether-polyol, and polyester-polycarboxylic
acid.
[0018] Examples of preferred low molecular weight reactive
compounds include: low molecular weight polyol compounds such as
ethylene glycol, 1,4-butanediol, 1,6-hexanediol,
1,5-naphthylene-di-.beta.-dihydroxyethyl ether,
hydroquinone-.beta.-dihydroxyethyl ether, trimethylolpropane,
glycerin, and hexanetriol; low molecular weight polyamines such as
ethylenediamine, propylenediamine, butylenediamine,
hexamethylenediamine, 3,3'-dichlorobenzidine,
3,3'-dichloro-4,4'-diaminodiphenylmethane, and
2,5-dichlorophenyl-1,4-diamine; low molecular weight amino-alcohols
such as amino-ethyl alcohol, 3-amino-chlorohexanol, and
p-aminophenyl-ethyl alcohol; low molecular weight aliphatic
polycarboxylic acids such as oxalic acid, malonic acid, succinic
acid, glutaric acid, adipic acid, pimelic acid, fumaric acid,
maleic acid, methylmaleic acid, methylfumaric acid, itaconic acid,
citraconic acid, mesaconic acid, acetylenic acid, malic acid,
methylmalic acid, citric acid, isocitric acid, and tartaric acid;
and aromatic polycarboxylic acids such as phthalic acid,
terephthalic acid, isophthalic acid, trimellitic acid,
1,2,3-benzenetricarboxylic acid, 1,3,5-benzenetricarboxylic acid,
pyromellitic acid, benzenehexacarboxylic acid,
naphthalenedicarboxylic acid, naphthalenetricarboxylic acid,
naphthalenetetracarboxylic acid, diphenyltetracarboxylic acid,
diphenyl ether tetracarboxylic acid, and azobenzenetetracarboxylic
acid.
[0019] Examples of reactive compounds include: the aforementioned
polyester polyol and polyester polycarboxylic acid, each formed
from polyol and polycarboxylic acid; polyamide amine and polyamide
carboxylic acid, each formed from polyamine and polycarboxylic
acid; and the aforementioned polyether polyol which is formed by
polymerizing alkylene oxide such as ethylene oxide or propylene
oxide with polyol or polyamine used as a polymerization
initiator.
[0020] Reactive compounds used in the present invention are not
limited to the aforementioned low molecular weight compounds and
oligomers, but may be polymers such as polyurethane resins,
polyamide resins, polyester resins, acrylic resins, melamine
resins, urea resins, phenolic resins, and alkyd resins.
[0021] A reactive compound as mentioned above may be mixed with a
thermoplastic resin before the resin is molded into a cover, or may
be caused to permeate into a molded cover. In view of time required
for penetration and the fact that some reactive compounds are
difficult to permeate, a reactive compound is preferably mixed with
a thermoplastic resin before the resin is molded into a cover.
[0022] When a reactive compound is mixed with a nonreactive
thermoplastic resin, the reactive compound is mixed in a
concentration of 0.0001 mol to 1 mol based on a reactive group for
100 g of the thermoplastic resin. When a reactive compound is
contained in a concentration of less than 0.0001 mol, the cross
linking density attained by a polyisocyanate compound becomes
relatively low. As a result, an object of the present invention is
not sufficiently achieved. By contrast, a concentration in excess
of 1 mol is uneconomic and may impair physical properties which a
cover must have.
[0023] A polyisocyanate compound used in the present invention is a
compound having two or more isocyanate groups. Any polyisocyanate
compounds which have conventionally been used in the art related to
polyurethane resins may be used in the present invention. Examples
of such polyisocyanate compounds include: aromatic diisocyanates
such as 2,4-tolylene diisocyanate and 2,6-tolylene diisocyanate and
the mixture thereof, 4.4-diphenylmethane diisocyanate (MDI),
m-phenylene diisocyanate, and 4,4'-biphenyl diisocyanate; aliphatic
diisocyanates such as tetramethylene diisocyanate, hexamethylene
diisocyanate (HDI), and octamethylene diisocyanate; aromatic
aliphatic diisocyanates such as xylene diisocyanate; and
triisocyanates such as 4,4', 4"-triphenylmethane triisocyanate,
2,4,4'-biphenyl triisocyanate, and 2,4,4'-diphenylmethane
triisocyanate.
[0024] Other polyisocyanate compounds usable in the present
invention include adducts or isocyanate polymers, each having two
or more isocyanate groups and obtained by the reaction between any
of the aforementioned diisocyanates or triisocyanates and a
compound having two or more hydroxyl groups such as diol or triol.
These polyisocyanate compounds may be used singly or in
combination.
[0025] Preferred examples of the polyisocyanate compound usable in
the present invention include aromatic diisocyanates, among which
MDI is particularly preferred. Aromatic diisocyanates, particularly
MDI, have high reactivity with a reactive group in a thermoplastic
resin. Therefore, if aromatic diisocyanate, particularly MDI, is
used as the polyisocyanate compound, the effect of the present
invention is satisfactorily obtained. By contrast, aliphatic
polyisocyanate such as HDI has low reactivity with a reactive group
contained in a thermoplastic resin, and slowly reacts with the
reactive group, although it is usable in the present invention.
Therefore, if aliphatic polyisocyanate is used as the
polyisocyanate compound, the effect of the present invention may
not be satisfactorily obtained.
[0026] In the method of manufacturing golf balls according to the
present invention, a cover is molded from a thermoplastic resin as
mentioned previously. Subsequently, a polyisocyanate compound is
caused to permeate into the surface layer of the cover to thereby
cause the reaction between the polyisocyanate compound and a
reactive group(s) contained in the thermoplastic resin, thereby
modifying the surface layer. Thereafter, the surface of the cover
is stamped with markings and coated appropriately, as needed, to
thereby obtain a product golf ball.
[0027] In this case, preferred thermoplastic resins are ionomer
resins, polyolefin resins, polyurethane resins, polyamide resins,
and mixtures thereof, among which polyurethane resins are
particularly preferred. Among polyurethane resins, non-yellowing
polyurethane resins generally have low glass transition
temperatures, resulting in excellent resilience. Further,
non-yellowing polyurethane resins prevent the discoloration of a
cover with time and maintain the white color of the golf ball for a
longer period. Non-yellowing polyurethane resins mean polyurethane
resins containing polyisocyanate other than aromatic
polyisocyanate. Particularly preferred non-yellowing polyurethane
resins usable in the present invention are polyurethane resins
containing aliphatic polyisocyanate such as tetramethylene
diisocyanate, HDI, 2,4,4-(2,4,4)-trimethylhexamethylene
diisocyanate, octamethylene diisocyanate, lysine diisocyanate, or
like polyisocyanate, since they impart excellent resilience and
increased travel distance. The term "polyurethane resins"
encompasses polyurethane elastomers. In the present invention,
non-yellowing polyurethane elastomers, particularly polyurethane
elastomers containing aliphatic polyisocyanate, are most
advantageously used, since these elastomers have excellent
resilience and therefore increase travel distance, while
effectively preventing discoloration of a cover with time. When
these thermoplastic resins are used as materials for covers,
preferred polyisocyanate compounds usable therewith are
2,4-tolylene diisocyanate and MDI, especially MDI, in light of high
reactivity with a reactive group contained in a thermoplastic
resin. Therefore, in the present invention, the most preferable
result is attained through use of a non-yellowing polyurethane
elastomer as the thermoplastic resin and MDI as the polyisocyanate
compound.
[0028] A cover is molded by the injection molding method wherein a
thermoplastic resin is directly injection-molded onto a solid core
or thread-wound core or by the compression molding method wherein
semispheric cover materials (half cups) of a thermoplastic resin
are put on a solid core or thread-wound core and then
compression-molded. However, the molding method is not limited
thereto. Dimples may be formed at the same time that or after a
cover is molded.
[0029] Examples of a method of causing a polyisocyanate compound to
permeate into the surface layer of a cover include the following
methods: (1) a polyisocyanate compound is heated to obtain a molten
liquid thereof; (2) a polyisocyanate compound is dissolved in an
appropriate organic solvent to obtain a solution thereof; and (3) a
polyisocyanate compound is gasified through application of heat to
obtain a gas thereof. These methods particularly prefer relatively
low molecular weight polyisocyanate compounds. In the methods (1)
and (2), a golf ball having a molded cover is immersed in a
polyisocyanate compound liquid or solution, or the liquid or
solution is applied onto the cover surface. In the method (3), a
golf ball having a molded cover is placed in a polyisocyanate
compound gas. In order to accelerate penetration of a
polyisocyanate compound, a polyisocyanate compound in the form of
either liquid or gas may be caused to permeate under pressure, or a
liquid or gas of a polyisocyanate compound may be heated to such a
temperature as not to impair a cover, for example, a temperature of
50.degree. C. to 200.degree. C.
[0030] A polyisocyanate compound is caused to permeate into a cover
formed from a thermoplastic resin preferably in a density of 0.1 to
70 parts by weight in 100 parts by weight of the portion of the
cover permeated with the polyisocyanate compound. If the density is
less than 0.1 parts by weight, a required cross linking density
will not be achieved. By contrast, if the density is in excess of
70 parts by weight, the cross linking density will become
excessively high, and an unreacted polyisocyanate compound will
remain in a cover. The amount of penetration of a polyisocyanate
compound can be altered as needed in accordance with a required
cross linking density or a reactive group existing in or a reactive
compound contained in a thermoplastic resin.
[0031] The reaction between a polyisocyanate compound, which has
permeated into the surface layer of a cover as described above, and
a reactive group contained in a thermoplastic resin proceeds over
long hours even at room temperature. Thus, heating is not a
mandatory condition for the reaction. However, in general, a golf
ball whose cover has been permeated with a polyisocyanate compound
is preferably heated at an appropriate temperature of 50.degree. C.
to 200.degree. C. for several minutes through several hours so as
to accelerate a reaction of an isocyanate group, i.e. a cross
linking reaction.
[0032] In the present invention, the thickness of a cover before
modification is preferably 1.0 mm to 3.0 mm, particularly
preferably 1.5 mm to 2.5 mm; and the thickness of a modified layer
is preferably 0.1 mm to 1.0 mm, particularly preferably 0.2 mm to
0.8 mm. As mentioned previously, conventionally, constituent layers
of a multilayered cover are difficult to be uniformly formed to a
thickness of not greater than 1 mm. Consequently, the thickness of
each layer or the overall thickness of a cover becomes relatively
thick, resulting in failure to obtain desired performance. By
contrast, by forming a modified layer having a thickness of the
above range in a cover having a thickness of the above range, the
following advantages (a) to (d) are obtained. The thickness of a
modified layer is substantially identical to the depth of
penetration of a polyisocyanate compound into the surface layer of
a cover (the thickness of the portion of the cover permeated with
the polyisocianate compound).
[0033] (a) A cover can assume a multilayered structure consisting
of a plurality of layers which are different in a physical
property, while the entire thickness of the cover is maintained at
1.0 mm to 3.0 mm. Accordingly, a multilayered cover is formed such
that the thickness of each layer and the entire thickness thereof
are thin, and thus desired performance can be obtained.
[0034] (b) According to the aforementioned penetration methods (1)
to (3), a polyisocyanate compound permeates into the surface layer
of a cover to a uniform thickness. Thus is obtained a golf ball
whose cover is composed of layers having uniform thicknesses and
which has excellent symmetricalness.
[0035] (c) Constituent layers of a conventional multilayered cover
are molded separately from each other, and thus the layers are not
integrated together. This multilayered structure involves a
potential separation of layers from each other or a potential
failure in proper transmission of a force applied to a ball from an
outer layer to an inner layer when the ball is hit, resulting in a
potential deterioration in the durability of a cover and ball
performance. By contrast, the aforementioned methods (1) to (3)
provide a multilayered cover composed of integrated layers. Thus,
when a ball is hit, no separation of one layer from the other
occurs, and a force applied to the ball is properly transmitted
from an outer layer to an inner layer, resulting in an improvement
in the durability of a cover and ball performance.
[0036] (d) The performance of a cover can be altered by changing
the hardness of the surface layer of the cover through the
aforementioned modification. For example, when modification is
carried out such that the hardness of a surface layer becomes
higher than that of an inner layer, the obtained multilayer cover
provides improved durability and reduced spin motion as compared
with a cover whose surface layer is not modified. Thus, even a
material which is said to be unsuitable for a cover of a golf ball
due to poor durability can be used as a material for a cover. In
this case, the hardness of a cover before modification is
preferably 40 to 60 on the Shore D scale, particularly preferably
45 to 55; and the hardness of a modified layer is preferably 60 to
70 con the Shore D scale, particularly preferably 65 to 70. By
contrast, when modification is carried out such that the hardness
of a surface layer becomes lower than that of an inner layer, the
obtained multilayer cover provides higher spinability as compared
with a cover whose surface layer is not modified, while a reduction
in resilience is minimized. In this case, the hardness of a cover
before modification is preferably 60 to 70 on the Shore D scale,
particularly preferably 65 to 70; and the hardness of a modified
layer is preferably 40 to 60 on the Shore D scale, particularly
preferably 45 to 55. In the latter case, the difference in hardness
(Shore D) between the modified layer and the unmodified inner layer
is preferably 5 to 25, particularly preferably 10 to 25.
EXAMPLES
[0037] Two-piece golf balls of Examples 1 and 2 and Comparative
Examples 1-3 shown in Table 2 were manufactured by selectively
using cores having composition A or B shown in Table 1 and by
covering the cores with covers having the following compositions-
In Examples 1 and 2, each golf ball was immersed in molten liquid
of a polyisocyanate compound for a predetermined time, and then
heated at a predetermined temperature (cure temperature) for a
predetermined time (cure time), whereby the cover of the golf ball
was modified. The conditions of the modification are shown in Table
2. The covers of Comparative Examples 1-3 were not modified.
[0038] In Table 1, BR01 (product of Japan Synthetic Rubber Co.,
Ltd.) was used as polybutadiene rubber, and Percumyl D (product of
NOF Corp.) was used as dicumyl peroxide. The specific gravity,
hardness, and initial speed of the cores of Compositions A and B
are shown in Table 1. The hardness shown in Table 1 represents a
deformation of a core under a load of 100 kg. The initial speed
shown in Table 1 represents the initial speed of a core as measured
when hit by a No.1 Wood at a head speed of 45 m/s through use of a
hit testing machine.
[0039] (Cover Composition: Polyurethane)
[0040] A cover was formed exclusively of a non-yellowing
thermoplastic polyurethane elastomer containing aliphatic
diisocyanate. The surface hardness of the cover of this composition
was 48 on the Shore D scale.
[0041] (Cover Composition: Ionomer a)
[0042] A cover was formed of a mixture of Surlyn 8120 product of Du
Pont), Hi-milan 1706 (product of Du Pont-Mitsui Polychemicals Co.,
Ltd.), and Hi-milan AM7316 (product of Du Pont-Mitsui Polychemicals
Co., Ltd.) (mixing ratio of 50:25:25 (by weight)). The surface
hardness of the cover of this composition was 48 on the Shore D
scale.
[0043] (Cover Composition: Ionomer b) A cover was formed of a
mixture of Hi-milan 1557 (product of Du Pont-Mitsui Polychemicals
Co., Ltd.) and Hi-milan 1601 (product of Du Pont-Mitsui
Polychemicals Co., Ltd.) (mixing ratio of 50:50 (by weight). The
surface hardness of the cover of this composition was 60 on the
Shore D scale.
1 TABLE 1 Composition (wt. %) Core Composition A B Polybutadiene
rubber 100.0 100.0 Zinc acrylate 21.5 21.5 Zinc oxide 12 26.3
Dicumyl peroxide 1 1 Specific gravity 1.07 1.16 Hardness 3.41 3.41
Initial speed (m/s) 78.12 77.28 W1:HS45
[0044]
2 TABLE 2 Example Comparative Example 1 2 1 2 3 Core A A B A A
Composition Cover Poly- Poly- Ionomer Poly- Ionomer Composition
urethane urethane b urethane a Modification Conditions
Polyisocyanate MDI MDI . . . Compound Temp. of Molten 70.degree. C.
70.degree. C. . . . Liquid Immersion Time 10 min 20 min . . . Cure
Temperature 80.degree. C. 80.degree. C. . . . Cure Time 1 hr 1 hr .
. . Product Golf Ball Outer Diameter 42.7 42.7 42.7 42.7 42.7 (mm)
Weight (g) 45.5 45.5 45.5 45.5 45.5 Hardness (mm) 2.9 2.8 2.5 3.1
3.1 Cover Hardness (Shore D) Outer Surface 60 60 60 48 48 Hardness
Inner Surface 48 52 60 48 48 Hardness Travel Distance Test W1:HS50
Initial Speed 77.0 77.0 77.4 77.0 77.0 (m/s) W1:HS45 Spin speed
(rpm) 2700 2600 2500 2800 2800 Carry Travel 215 216 216 214 214
Distance (m) Total Travel 226 227 227 224 223 Distance (m) Feel on
Impact Good Good Bad Good Good Degree of 3.5 3.5 3.5 3.0 3.5
Discoloration (.DELTA.Y1) Cut Resistance Good Good Good Bad Bad
Abrasion Good Good Mod- Bad Bad Resistance Index erate
[0045] The golf balls of Examples and Comparative Examples were
measured for their cover hardnesses and abrasion resistance, and
were subjected to a travel distance test, a feel-on-impact test, a
discoloration test, and a cut resistance test, according to the
following procedures. The results are shown in Table 2. The
hardness of a product golf ball shown in Table 2 represents a
deformation of the ball under a load of 100 kg.
[0046] (Measurement of Cover Hardness)
[0047] A Cover was peeled off each product golf ball, and its outer
surface hardness and inner surface hardness were measured.
[0048] (Travel Distance Test)
[0049] Through use of a hit testing machine, the golf balls were
hit by a No. 1 wood at a head speed of 45 m/s. The initial speed,
carry travel distance, and total travel distance were measured.
[0050] (Feel-on-Impact Test)
[0051] The golf balls were subjected to sensory evaluation test for
feel on impact in which three professional golfers hit the golf
balls with a No. 1 wood and evaluated the feel on impact. The
evaluation criteria are as follows:
[0052] Good: Feel on impact is good
[0053] Bad: Feel on impact is bad
[0054] (Discoloration)
[0055] Each golf ball was subjected to an accelerated discoloration
test, and the degree of discoloration of a cover was measured. In
this test, through use of an accelerated discoloration testing
machine (Type FM-1, product of Suga Testing Machine K.K.), each
golf ball was continuously irradiated by means of a mercury lamp
for discoloration test (H400-F, product of Toshiba Corp.) for 24
hours, and the yellowness index was measured. The yellowness
indexes (YI) before and after irradiation were measured according
to the reflection method (JIS-K7103) through use of a multi-light
source spectrocolorimeter (Type MSC-1S-2DH, product of Suga Testing
Machine K.K.). Based on these measurements, the yellowing degree
was calculated in accordance with the following equation. The
thus-obtained .DELTA.Yl is shown as the degree of discoloration. If
.DELTA.Yl obtained through the equation is greater than zero, the
yellowness index has increased; the greater the value of .DELTA.Yl,
the greater the degree of discoloration.
[0056] .DELTA.Yl=YI-YI.sub.0
[0057] .DELTA.Yl:Yellowing degree
[0058] YI: Yellowness index after irradiation
[0059] YI.sub.0: Initial yellowness index of a cover
[0060] (Cut Resistance)
[0061] A no. 9 iron was attached to a hitting robot, and the
hitting robot was caused to perform a top-hit shot (shot for
hitting the top of a ball) at a head speed of 40 m/s- Subsequently,
visual check was performed to evaluate the state of the hit portion
of each golf ball. The evaluation criteria are as follows:
[0062] Good: A slight dent was observed but no other damage was
found on the cover.
[0063] Bad: The cover was cut and damage was conspicuous.
[0064] (Abrasion Resistance Index)
[0065] A pitching wedge was attached to the hitting robot, and the
hitting robot was caused to hit each of the golf balls in an
ordinary manner at a head speed of 30 m/s. Subsequently, visual
check was performed to evaluate damage on the surface of the hit
golf ball. The evaluation criteria are as follows:
[0066] Good: No abnormality was found on the hit surface.
[0067] Moderate: Slight fine splits were found on the hit
surface.
[0068] Bad: Conspicuous fine splits were found on the hit
surface.
[0069] As is apparent from Table 2, in the case where a
polyisocyanate compound is caused to permeate into the surface
layer of a cover formed from a non-yellowing thermoplastic
polyurethane elastomer as in Examples 1 and 2, whereby the surface
layer is modified so that the hardness thereof becomes greater than
that of the inner surface of the cover, when a golf ball is hit
with a No 1 wood, the golf ball undergoes a reduced amount of spin
and therefore travels a longer distance. In addition, the cut
resistance and abrasion resistance of the golf ball are
enhanced.
[0070] Industrial Applicability
[0071] As mentioned previously, according to the method of
manufacturing golf balls of the present invention, the surface
layer of a cover formed from an easy-to-mold thermoplastic resin is
modified, thereby improving physical properties of the cover,
imparting to the cover performance which cannot be imparted by the
thermoplastic resin alone, and reducing the thickness of each layer
of a multilayered cover or the overall thickness of the cover.
* * * * *