Method of forming a cured elastomer and golf balls

1. A method of forming a golf ball or a part of a golf ball, comprising

(a) forming a first thermoplastic hemispherical shell and a second thermoplastic hemispherical shell from a mixture comprising an elastomer, a first initiator, a second initiator, and a member selected from the group consisting of unsaturated carboxylic acids, metal salts of unsaturated carboxylic acids, and combinations thereof, wherein each of the first and second hemispherical shells has a cavity for receiving a spherical component and an edge that mates to the other of the hemispherical shells;

(b) positioning a spherical component between diametrically opposed first and second hemispherical shells;

(c) compression molding the first and second hemispherical shells around the spherical component to form a unitary layer of cured elastomer surrounding the spherical component,

wherein the compression molding step (c) comprises heating the first and second hemispherical shells at a first crosslinking temperature to partially crosslink the elastomer to a first crosslink density, then heating the first and second hemispherical shells at a second crosslinking temperature that is higher than the first crosslinking temperature to further crosslink the elastomer to a final crosslink density;

wherein, prior to heating the first and second hemispherical shells at the first crosslinking temperature to partially crosslink the elastomer to a first crosslinking density, the mixture further comprises a third free radical initiator and is heated at a third crosslinking temperature lower than the first crosslinking temperature to partially crosslink the elastomer to a third crosslinking density less than the first crosslinking density, wherein the elastomer is thermoplastic at the third crosslink density; and

further wherein either

(i) the first free radical initiator has a half-life of from about 0.2 minutes to about 5 minutes at the first crosslinking temperature, the second free radical initiator has a half-life of from about 0.2 minutes to about 5 minutes at the second crosslinking temperature, the third free radical initiator has a half-life of from about 0.2 minutes to about 5 minutes at the third crosslinking temperature, the second crosslinking temperature is higher than the first crosslinking temperature by at least about 30° C., and the first crosslinking temperature is higher than the third crosslinking temperature by at least about 20° C.;

or

(ii) the first free radical initiator has a first one-minute half-life temperature, the second free radical initiator has a second one-minute half-life temperature that is higher than the first one-minute half-life temperature by at least about 30° C., the third free radical initiator has a third one-minute half-life temperature that is lower than the first one-minute half-life temperature by at least about 20° C., the first crosslinking temperature is from about 20° C. lower to about 20° C. higher than the first one-minute half-life temperature, the second crosslinking temperature from about 20° C. lower to about 20° C. higher than the second one-minute half-life temperature, and the third crosslinking temperature from about 20° C. lower to about 20° C. higher than the third one-minute half-life temperature.

2. A method according to claim 1 , wherein the partial crosslinking at the third crosslinking temperature is carried out in the compression molding step (c).

3. A method according to claim 1 , wherein the partial crosslinking at the third crosslinking temperature is carried out in the forming step (a).

4. A method according to claim 1 , wherein the partial crosslinking at the third crosslinking temperature is carried out before the forming step (a).

5. A method according to claim 4 , wherein the partial crosslinking at the third crosslinking temperature is carried out in an extruder.

6. A method of forming a golf ball or a part of a golf ball, comprising

(a) forming a first thermoplastic hemispherical shell and a second thermoplastic hemispherical shell from a mixture comprising an elastomer, a first initiator, a second initiator, and a member selected from the group consisting of unsaturated carboxylic acids, metal salts of unsaturated carboxylic acids, and combinations thereof, wherein each of the first and second hemispherical shells has a cavity for receiving a spherical component and an edge that mates to the other of the hemispherical shells and wherein the forming comprises heating the first and second hemispherical shells at a first crosslinking temperature to partially crosslinking the elastomer to a first crosslink density at which the elastomer is thermoplastic;

(b) positioning a spherical component between diametrically opposed first and second hemispherical shells;

(c) compression molding the first and second hemispherical shells around the spherical component to form a unitary layer of cured elastomer surrounding the spherical component wherein the compression molding comprises heating the first and second hemispherical shells at a second crosslinking temperature that is higher than the first crosslinking temperature to further crosslink the elastomer to a final crosslink density;

wherein, prior to heating the first and second hemispherical shells at the first crosslinking temperature to partially crosslinking the elastomer to a first crosslink density, the mixture further comprises a third free radical initiator and is heated at a third crosslinking temperature lower than the first crosslinking temperature to partially crosslink the elastomer to a third crosslinking density less than the first crosslinking density, wherein the elastomer is thermoplastic at the third crosslinking density; and

further wherein either

(i) the first free radical initiator has a half-life of from about 0.2 minutes to about 5 minutes at the first crosslinking temperature, the second free radical initiator has a half-life of from about 0.2 minutes to about 5 minutes at the second crosslinking temperature, the third free radical initiator has a half-life of from about 0.2 minutes to about 5 minutes at the third crosslinking temperature, the second crosslinking temperature is higher than the first crosslinking temperature by at least about 30° C., and the first crosslinking temperature is higher than the third crosslinking temperature by at least about 20° C.; or

(ii) the first free radical initiator has a first one-minute half-life temperature, the second free radical initiator has a second one-minute half-life temperature that is higher than the first one-minute half-life temperature by at least about 30° C., the third free radical initiator has a third one-minute half-life temperature that is lower than the first one-minute half-life temperature by at least about 20° C., the first crosslinking temperature is from about 20° C. lower to about 20° C. higher than the first one-minute half-life temperature, the second crosslinking temperature from about 20° C. lower to about 20° C. higher than the second one-minute half-life temperature, and the third crosslinking temperature from about 20° C. lower to about 20° C. higher than the third one-minute half-life temperature.

7. A method according to claim 6 , wherein the partial crosslinking at the third crosslinking temperature is carried out in the forming step (a).

8. A method according to claim 6 , wherein the partial crosslinking at the third crosslinking temperature is carried out before the forming step (a).

9. A method according to claim 8 , wherein the partial crosslinking at the third crosslinking temperature is carried out in an extruder.

10. A method of forming a golf ball or a part of a golf ball, comprising

(a) heating a mixture comprising an elastomer, a first initiator, a second initiator, and a member selected from the group consisting of unsaturated carboxylic acids, metal salts of unsaturated carboxylic acids, and combinations thereof at a first crosslinking temperature to partially crosslinking the elastomer to a first crosslink density at which the elastomer is thermoplastic;

(b) forming the mixture into a first thermoplastic hemispherical shell and a second thermoplastic hemispherical shell, wherein each of the first and second hemispherical shells has a cavity for receiving a spherical component and an edge that mates to the other of the hemispherical shells;

(c) positioning a spherical component between diametrically opposed first and second hemispherical shells;

(d) compression molding the first and second hemispherical shells around the spherical component to form a unitary layer of cured elastomer surrounding the spherical component wherein the compression molding comprises heating the first and second hemispherical shells at a second crosslinking temperature that is higher than the first crosslinking temperature to further crosslink the elastomer to a final crosslink density;

wherein, prior to heating the mixture at the first crosslinking temperature to partially crosslinking the elastomer to a first crosslink density, the mixture further comprises a third free radical initiator and is heated at a third crosslinking temperature lower than the first crosslinking temperature to partially crosslink the elastomer to a third crosslink density less than the first crosslinking density, wherein the elastomer is thermoplastic at the third crosslink density; and

further wherein either

(i) the first free radical initiator has a half-life of from about 0.2 minutes to about 5 minutes at the first crosslinking temperature, the second free radical initiator has a half-life of from about 0.2 minutes to about 5 minutes at the second crosslinking temperature, the third free radical initiator has a half-life of from about 0.2 minutes to about 5 minutes at the third crosslinking temperature, the second crosslinking temperature is higher than the first crosslinking temperature by at least about 30° C., and the first crosslinking temperature is higher than the third crosslinking temperature by at least about 20° C.; or

(ii) the first free radical initiator has a first one-minute half-life temperature, the second free radical initiator has a second one-minute half-life temperature that is higher than the first one-minute half-life temperature by at least about 30° C., the third free radical initiator has a third one-minute half-life temperature that is lower than the first one-minute half-life temperature by at least about 20° C., the first crosslinking temperature is from about 20° C. lower to about 20° C. higher than the first one-minute half-life temperature, the second crosslinking temperature from about 20° C. lower to about 20° C. higher than the second one-minute half-life temperature, and the third crosslinking temperature from about 20° C. lower to about 20° C. higher than the third one-minute half-life temperature.

11. A method according to claim 10 , wherein the partial crosslinking at the third crosslinking temperature and the partial crosslinking at the first crosslinking temperature are carried out in an extruder.

12. A method according to claim 1 , wherein the elastomer comprises polybutadiene.

13. A method according to claim 1 , wherein the second crosslinking temperature is higher than the first crosslinking temperature by at least about 50° C. or the first free radical initiator's one-minute half-life temperature is at least about 50° C. lower than the second free radical initiator's one-minute half-life temperature.

14. A method according to claim 1 , wherein the first crosslinking temperature is from about 100° C. and at most about 190° C., the second crosslinking temperature is from about 165° C. to about 280° C., and the third crosslinking temperature is from about 50° C. to about 130° C.

15. A method according to claim 1 , wherein the first free radical initiator is used in an amount of at most about 40 parts by weight based on 100 parts by weight of a total combined weights of the first free radical initiator and the second free radical initiator; and further wherein the third free radical initiator is used in an amount of at most about 25 parts by weight based on 100 parts by weight of a total combined weights of the first free radical initiator, the second free radical initiator, and the third free radical initiator.

16. A method according to claim 1 , wherein the heating at the first crosslinking temperature is carried out for from about 0.5 minute to about 15 minutes; wherein the heating at the second crosslinking temperature is carried out for from about 1 minute to about 30 minutes; and wherein the heating at the third crosslinking temperature is carried out for from about 0.5 minute to about 15 minutes.

17. A golf ball formed by a method according to claim 1 , the golf ball having a center, a cover, and at least one intermediate layer between the center and the cover, wherein the unitary layer formed is an intermediate layer.

18. A golf ball formed by a method according to claim 1 , the golf ball having a center, a cover, and at least one intermediate layer between the center and the cover, wherein the unitary layer formed is the cover.

19. A golf ball formed by a method according to claim 6 , the golf ball having a center, a cover, and at least one intermediate layer between the center and the cover, wherein the unitary layer formed is an intermediate layer.

20. A golf ball formed by a method according to claim 10 , the golf ball having a center, a cover, and at least one intermediate layer between the center and the cover, wherein the unitary layer formed is an intermediate layer.