Golf ball

Disclosed is a golf ball having improved impact resilience, which comprises a core and a cover covering the core, wherein the cover is a lithium neutralized ionomer resion which is a copolymer of 10 to 20% by weight of an alpha, beta-unsaturated carboxylic acid and the rest of ethylene, of which 20 to 70 mol % of carboxyl groups are neutralized with lithium ion, and the ionomer resin has a melt index of 0.5 to 5.0 and a shore D hardness of at least 60. The lithium neutralized ionomer resin may be combined with a di- or tri- valent metal neutralized ionomer resin which is a copolymer of 10 to 20% by weight of an alpha, beta-unsaturated carboxylic acid and the rest of ethylene, of which 40 to 70 mol % of carboxyl groups are neutralized with di- or tri- valent metal ion, and which has a melt index of 0.5 to 5.0 and a shore D hardness of 60 to 80.

FIELD OF THE INVENTION 
The present invention relates to a golf ball which is covered by a specific 
cover material. 
BACKGROUND OF THE INVENTION 
A cover material of golf balls is an ionomer resin which is excellent in 
impact resistance, cut resistance and impact resilience. Two sorts of the 
ionomer resins, i.e. sodium neutralized ionomer resin and zinc neutralized 
ionomer resin, are commercially available under the trade names of 
"SURLYN", "HI-MILAN" and the like, and no other ionomer resins are 
available. 
The sodium neutralized ionomer resin has superior impact, resilience in 
comparison with the zinc neutralized one. In order to improve impact 
resilience of golf balls, U.S. Pat. No. 3,819,768 proposes that the sodium 
neutralized ionomer resin and zinc neutralized ionomer resin are mixed. 
However, since ionomer resin available are only two sorts, as mentioned 
above, the modification by the mixing method are lacking in variety. 
Japanese Kokai Publication (unexamined) 82768/1986 also proposes that the 
sodium neutralized ionomer resin, or a mixture of the sodium neutralized 
ionomer resin and the zinc neutralized ionomer resin, is additionally 
neutralized with other metal, such as lithium, potassium, cesium and 
rubidium to improve impact resilience. The ionomer resin is poor in 
reactivity in the additional neutralization. Accordingly, the ionomer 
resin used in the additional neutralization and an amount of it are so 
limited that it is difficult to obtain a high neutralizing degree. 
SUMMARY OF THE INVENTION 
It has been surprisingly found that an ionomer resin which is neutralized 
with lithium ion has higher impact resilience than the commercially 
available sodium or zinc neutralized ionomer resin. Thus, the present 
invention provides a golf ball which comprises a core and a cover covering 
the core, wherein the cover is a lithium neutralized ionomer resin which 
is a copolymer of 10 to 20% by weight of an alpha, beta-unsaturated 
carboxylic acid and the rest of ethylene, of which 20 to 70 mol % of 
carboxyl groups are neutralized with lithium ion, and the ionomer resin 
has a melt index of 0.5 to 5.0 and a shore D hardness of 60 to 80. 
The present invention also provides a golf ball which comprises a core and 
a cover covering the core, wherein the cover is a mixture of the above 
mentioned lithium neutralized ionomer resin with 10 to 90% by weight of a 
di- or tri- valent metal neutralized ionomer resin which is a copolymer of 
10 to 20% by weight of an alpha, beta-unsaturated carboxylic acid and the 
rest of ethylene, of which 40 to 70 mol % of carboxyl groups are 
neutralized with divalent metal ion, and the di- or tri- valent metal 
neutralized ionomer resin has a melt index of 0.5 to 5.0 and a shore D 
hardness of 60 to 80.

DETAILED DESCRIPTION OF THE INVENTION 
The lithium neutralized ionomer resin of the present invention is prepared 
by neutralizing with lithium ion an ionomer base polymer. The ionomer base 
polymer is a copolymer of 10 to 20% by weight of an alpha, 
beta-unsaturated carboxylic acid (e.g. acrylic acid, methacrylic acid 
etc.) and the rest of ethylene. If the alpha, beta-unsaturated carboxylic 
acid is less then 10% by weight, stiffness is poor. If it is more than 20% 
by weight, the obtained resin is brittle. The ionomer base polymer may be 
obtained by copolymerizing the alpha, beta-unsaturated carboxylic acid and 
ethylene. The copolymerization can be conducted by well known methods. The 
ionomer base polymer may also be prepared by treating the commercially 
available sodium neutralized ionomer resin in a nitric acid aqueous 
solution followed by rinsing with water to remove sodium ion. Treating 
conditions for removing sodium ion per se are known in the art. 
The neutralization may be carried out by mixing and melting the ionomer 
base polymer and a lithium ion source. Examples of the lithium ion sources 
are lithium hydroxide, lithium carbonate, lithium methoxide, lithium 
acetate and the like. A degree of neutralization is preferably 20 to 70 
mol %, more preferably 20 to 50 mol % of the free carboxyl group in the 
ionomer base polymer. If it is less than 20 mol %, the obtained resin is 
poor in stiffness and impact resilience. If it is more than 70%, stiffness 
is too high and feeling when one hit a golf ball is poor. The obtained 
resin also has poor moldability and is poor in impact resistance. 
The lithium neutralized ionomer resin of the present invention has a melt 
index of 0.5 to 5.0, preferably 0.8 to 3.0. The melt index is determined 
according to JIK-K 6760 (190.degree. C. and 2,160 g load). Melt indexes of 
less than 0.5 deteriorate flowability, moldability and impact resistance. 
Melt indexes of more than 5.0 deteriorate impact resilience and impact 
resistance. The ionomer resin also has a shore D hardness of 60 to 80, 
preferably 60 to 70. If it is less than 60, impact resilience is poor. If 
it is more than 80, the ball is too hard. 
Since the lithium neutralized ionomer resin has highly improved impact 
resilience in comparison with the commercially available sodium or zinc 
neutralized ionomer resin, it is suitable for a cover of the golf ball of 
the present invention. As the cover resin, the lithium neutralized ionomer 
resin is used solely or in combination with other ionomer resin. The other 
ionomer resin to be combined with the lithium neutralized ionomer resin 
may be any ionomer resins known in the art, but preferably be a di- or 
tri- valent metal neutralized ionomer resin which is a resin that 40 to 70 
mol %, preferably 40 to 60 mol %, of carboxyl groups of the above 
mentioned ionomer base polymer are neutralized with divalent metal ion. 
The di- or tri- valent metal neutralized ionomer resin can be commercially 
available zinc neutralized ionomer resins or those prepared by 
neutralizing the ionomer base polymer with di- or tri- valent metal ion, 
preferably divalent metal ion (e.g. magnesium ion, calcium ion, aluminum 
ion, barium ion, copper ion etc.). Neutralization of the ionomer base 
polymer may be carried out by melting and mixing the base polymer with a 
di- or tri- valent metal ion source. Examples of the di- or tri- valent 
metal ion sources are carbonates, acetates, hydroxides and oxides of the 
di- or tri- valent metal. If the degree of neutralization is less than 40 
mol %, stiffness and impact resilience are poor. If it is more than 70 mol 
%, the resin is very hard and feeling when one hit the ball is poor. 
Moldability and impact resistance are also poor. 
The di- or tri- valent metal neutralized ionomer resin of the present 
invention has a melt index of 0.5 to 5.0, preferably 0.7 to 2.0. Melt 
indexes of less than 0.5 deteriorate flowability, moldability and impact 
resistance. Melt indexes of more than 5.0 deteriorate impact resilience 
and impact resistance. The ionomer resin also has a shore D hardness of 60 
to 80, preferably 60 to 70. If it is less than 60, impact resilience is 
poor. 
In case where the cover is prepared from a mixture of the lithium 
neutralized ionomer resin and the di- or tri- valent metal neutralized 
ionomer resin, impact resilience are highly enhanced. A mixing ratio of 
lithium neutralized ionomer resin / di- or tri- valent metal neutralized 
resilience is 10 / 90 to 90 / 10, preferably 70 / 30 to 40 / 60. The 
ionomer resin mixture of the present invention also required to have a 
melt index of 0.5 to 3.0, preferably 1.0 to 2.5. If the melt index is less 
than 0.5, its flowability is too low and it is difficult to mold. Impact 
resistance is also deteriorated. If it is more than 3.0, both impact 
resilience and impact resistance are deteriorated. The ionomer resin 
mixture has a Shore D hardness of 60 to 80, preferably 60 to 70. The 
hardness of less than 60 deteriorates impact resilience. Mixing of the 
ionomer resins may be carried out using an extruder, such as a single 
screw extruder or a twin screw extruder which is preferred. 
The cover resin may further contain additives, if necessary. Examples of 
the additives are a pigment, a lubricant, a dispersant (calcium stearate 
etc.), an antioxidant, a stabilizer, a UV absorber, an antistat and the 
like. 
The golf ball of the present invention can be prepared by covering a golf 
ball core with the cover material. The covering method can be any methods 
used in this field, generally an injection molding. The golf ball core can 
be either a solid core which is solidly molded from rubber, or a 
thread-wound core which is prepare by winding rubber thread on a center 
material. 
The golf ball of the present invention has much improved impact resilience. 
EXAMPLES 
The present invention is illustrated with the following examples which, 
however, are not to be construed as limiting the present invention to 
their details. 
EXAMPLES 1 TO 5 AND COMATIVE EXAMPLES 1 TO 6 
A rubber composition was prepared by mixing 100 parts by weight of 
polybutadiene, 35 parts by weight of zinc acrylate, 16 parts by weight of 
zinc oxide and 1 part by weight of dicumyl peroxide. The composition was 
vulcanized at 150.degree. C. for 30 minutes to form a solid core having an 
average diameter of 38.4 mm. 
Preparation of Cover Resins 
Three ionomer resins (1), (2) and (3) were prepared by mixing the 
ingredients shown in Table 1 in a twin screw extruder. The resin was 
extruded to pellels at a temperature of 200 to 260 .degree. C. Extruding 
conditions were a screw diameter of 45 mm, a screw rotating rate of 200 
rpm and a screw L/D of 30. The cylinder temperature condition were as 
follow; 
______________________________________ 
Barrel 
1 2 3 4 5 6 7 8 
______________________________________ 
Temp. (.degree.C.) 
100 150 180 200 240 270 270 270 
______________________________________ 
Both an adapter and a die are set 270.degree. C. 
TABLE 1 
______________________________________ 
Resin (1) 
Resin (2) 
Resin (3) 
______________________________________ 
HIMILAN 1605 treated with 
100 100 98 
nitric acid* 
Lithium carbonate 
1.7 1.1 -- 
Magnesium hydroxide 
-- -- 3 
______________________________________ 
*Pellets of HIMILAN 1605 were ground to powder and added into a 3.5 N 
nitric acid solution. The solution was heated to reflux for 24 hours and 
then rinsed with water and dried under reduced pressure. In this 
treatment, metal ions were removed from the ionomer resin to reproduce 
ethylenemethacrylic acid copolymer. 
Preparation of a Golf Ball 
The core obtained above was covered with a cover resin shown in Table 2 by 
injection molding to form a golf ball. Mixing of the ionomer resins in 
Table 2 was conducted in the same conditions as the ionomer resins of 
Table 1. 
TABLE 2 
__________________________________________________________________________ 
Examples No. 
Neutralizing ion 
Resin name 
1 2 3 4 5 6 
__________________________________________________________________________ 
Lithium Resin (1).sup.1 
100 -- 50 70 50 40 
Resin (2).sup.2 
-- 100 -- -- -- -- 
Sodium HIMILAN 1605.sup.3 
-- -- -- -- -- -- 
HIMILAN 1707.sup.4 
-- -- -- -- -- 20 
Divalent metal 
HIMILAN 1706.sup.5 
-- -- 50 30 -- -- 
Resin (3).sup.6 
-- -- -- -- 50 40 
Melt index of cover resin 
1.4 3.0 1.8 1.9 1.2 1.4 
Hardness (Shore D) of cover resin 
68 66 72 71 70 72 
Ball weight (g) 45.2 45.1 45.1 45.0 45.0 45.1 
Ball compression (PGA) 
100.5 
99.2 100.3 
99.8 99.0 1001 
Ball coefficient of restitution.sup.7 
0.7803 
0.7790 
0.7844 
0.7856 
0.7821 
0.7816 
Ball durability.sup.8 
98 108 105 101 106 97 
__________________________________________________________________________ 
Comparative Examples No. 
Neutralizing ion 
Resin name 
1 2 3 4 5 6 
__________________________________________________________________________ 
Lithium Resin (1).sup.1 
-- -- -- -- -- -- 
Resin (2).sup.2 
-- -- -- -- -- -- 
Sodium HIMILAN 1605.sup.3 
100 -- 50 70 -- 50 
HIMILAN 1707.sup.4 
-- 100 -- -- 50 -- 
Divalent metal 
HIMILAN 1706.sup.5 
-- -- 50 30 50 -- 
Resin (3).sup.6 
-- -- -- -- -- 50 
Melt index of cover resin 
2.8 0.9 1.7 2.2 0.8 1.8 
Hardness (Shore D) of cover resin 
67 67 70 69 69 70 
Ball weight (g) 45.0 45.0 45.1 45.1 45.2 45.1 
Ball compression (PGA) 
99.4 100.3 
100.4 
100.0 
100.5 
98.0 
Ball coefficient of restitution.sup.7 
0.7711 
0.7715 
0.7759 
0.7750 
0.7748 
0.7730 
Ball durability.sup.8 
100 90 90 97 95 109 
__________________________________________________________________________ 
.sup.1 Ionomer resin (1); Methacrylic acid content is about 15%. The 
neutralizing ion is lithium and neutralizing amount is about 30 mol %. It 
has a melt index of 1.4. 
.sup.2 Ionomer resin (2); The neutralizing ion is lithium and neutralizin 
amount is about 20 mol %. It has a melt index of 3.0. 
.sup.3 HIMILAN 1605 available from Du PontMitsui Polychemicals Company, 
Ltd. which contains methacrylic acid in an amount of about 15% by weight. 
The neutralizing ion is sodium and neutralizing amount is about 30 mol %. 
It has a melt index of 2.8. 
.sup.4 HIMILAN 1707 available from Du PontMitsui Polychemicals Company, 
Ltd. which contains methacrylic acid in an amount of about 15% by weight. 
The neutralizing ion is sodium and neutralizing amount is about 60 mol %. 
It has a melt index of 0.9. 
.sup.5 HIMILAN 1706 available from Du PontMitsui Polychemicals Company, 
Ltd. which contains methacrylic acid in an amount of about 15% by weight. 
The neutralizing ion is zinc and neutralizing amount is about 60 mol %. I 
has a melt index of 0.9. 
.sup.6 Ionomer resin (3); The neutralizing ion is magnesium and 
neutralizing amount is about 45 mol %. It has a melt index of 1.9. 
.sup.7 Calculated from an initial velocity of a golf ball, when a 
cylindrical material of 198.4 g was struck to the ball at a speed of 45 
m/s 
.sup.8 A golf ball was struck to a board at a speed of 45 m/s at a 
temperature of 23.degree. C. and number of striking was determined until 
the ball was cracked. The number is expressed as an index number when the 
number of Comparative Example 1 is made 100. 
The lithium neutralized ionomer resin of the present invention provides 
higher impact resilience in comparison with a sodium neutralized ionomer 
resin commercially available, even if it is used solely as in Examples 1 
and 2. If the lithium neutralized ionomer resin is combined with the 
divalent metal neutralized ionomer resin, impact resilience is further 
improved as shown in Examples 3 to 6.