Thermoplastic dual core having positive hardness gradient inner and outer core layers
A golf ball includes an inner core layer including a thermoplastic highly-neutralized ionomer formed from a copolymer of ethylene and an α,β-unsaturated carboxylic acid, an organic acid or salt thereof, and sufficient cation source to neutralize the acid groups of the copolymer by 80% or greater. The inner core has a geometric center hardness and a surface hardness to define a first positive hardness gradient having a first slope, S 1 . An outer core layer is disposed about the inner core and is formed from a second thermoplastic highly-neutralized ionomer. The outer core layer has an interior hardness and an outer surface hardness greater than the interior hardness to define a second positive hardness gradient having a second slope, S 2 . The golf ball also includes a cover layer.
1. A golf ball comprising:
an inner core layer comprising a first thermoplastic highly-neutralized ionomer comprising a copolymer of ethylene and an α,β-unsaturated carboxylic acid, an organic acid or salt thereof, and sufficient cation source to neutralize the acid groups of the copolymer by 80% or greater, and having a geometric center hardness and a surface hardness greater than the geometric hardness to define a first hardness positive gradient having a first slope, S 1 ;
an outer core layer disposed about the inner core, the outer core comprising a second thermoplastic highly-neutralized ionomer comprising a copolymer of ethylene and an α,β-unsaturated carboxylic acid, an organic acid or salt thereof, and sufficient cation source to neutralize the acid groups of the copolymer by 80% or greater, and having an interior hardness and an outer surface hardness greater than the interior hardness to define a second positive hardness gradient having a second slope, S 2 ; and
an outer cover layer;
wherein the ratio of S 2 to S 1 is about 3.0 or greater.
2. The golf ball of claim 1 , wherein the inner core layer has an outer diameter of about 0.5 inches to about 1.13 inches.
3. The golf ball of claim 1 , wherein the acid groups of the copolymer of the first or second thermoplastic highly-neutralized ionomer are neutralized by 90% or greater.
4. The golf ball of claim 3 , wherein the acid groups of the copolymer of the first or second thermoplastic highly-neutralized ionomer are neutralized by about 100%.
5. The golf ball of claim 1 , wherein the organic acid or salt thereof of the first or second thermoplastic highly-neutralized ionomer comprises barium, lithium, sodium, zinc, bismuth, chromium, cobalt, copper, potassium, strontium, titanium, tungsten, magnesium, cesium, iron, nickel, silver, aluminum, tin, or calcium salts, or salts of fatty acids.
6. The golf ball of claim 5 , wherein the fatty acid salt comprises stearic acid, behenic acid, erucic acid, oleic acid, linoelic acid or dimerized derivatives thereof.
7. The golf ball of claim 6 , wherein the organic acid or salt thereof comprises a magnesium salt of oleic acid.
8. The golf ball of claim 1 , wherein the inner core layer or outer core layer comprise a second polymer selected from the group consisting of a partially-neutralized ionomer and an acid copolymer.
9. The golf ball of claim 1 , wherein an angle, θ, is about 90° or greater, wherein θ is defined by the equation:
θ
=
180
-
(
180
π
(
tan
-
1
(
S
2
)
-
tan
-
1
(
S
1
)
)
)
.
10. The golf ball of claim 9 , wherein θ is about 100° or greater.
11. The golf ball of claim 10 , wherein θ is about 110° or greater.
12. The golf ball of claim 11 , wherein θ is about 120° or greater.
13. The golf ball of claim 1 , further comprising an inner cover layer disposed between the outer cover layer and the outer core layer.
14. A golf ball comprising:
an inner core layer comprising a first thermoplastic highly-neutralized ionomer comprising a copolymer of ethylene and an α,β-unsaturated carboxylic acid, an organic acid or salt thereof, and sufficient cation source to neutralize the acid groups of the copolymer by 90% or greater, and having a geometric center hardness and a surface hardness greater than the geometric hardness to define a first hardness positive gradient having a first slope, S 1 ;
an outer core layer disposed about the inner core, the outer core comprising a second thermoplastic highly-neutralized ionomer comprising a copolymer of ethylene and an α,β-unsaturated carboxylic acid, an organic acid or salt thereof, and sufficient cation source to neutralize the acid groups of the copolymer by 90% or greater, and having an interior hardness and an outer surface hardness greater than the interior hardness to define a second positive hardness gradient having a second slope, S 2 ; and
an outer cover layer;
wherein an angle, θ, is about 90° or greater, wherein θ is defined by the equation:
θ
=
180
-
(
180
π
(
tan
-
1
(
S
2
)
-
tan
-
1
(
S
1
)
)
)
.
15. The golf ball of claim 14 , wherein θ is about 100° or greater.
16. The golf ball of claim 15 , wherein θ is about 110° or greater.
17. The golf ball of claim 16 , wherein θ is about 120° or greater.
18. The golf ball of claim 14 , wherein a ratio of S 2 to S 1 is about 1.0 or greater.