Polyurethane (aka Urethane) has been widely used as a cover material in premium golf balls due to its unique structural characteristic and performance properties. The most noticeable performance benefit achieved from polyurethane is excellent greenside spin with soft feel. Polyurethane exhibits strong elastomeric behavior on the golf course while also having much better cover shear durability than more common ionomer (aka Surlyn) covered balls.
Polyurethane generally comes in two forms: Thermoset and Thermoplastic. They are differentiated by their polymer crosslink structure and their ability to reverse their crosslinked polymer chains under high heat.
Thermoplastic Polyurethanes (TPUs) have a reversible material structure with lightly crosslinked bonds that can be melted at high temperatures (~ 400° F) in order for it to be more easily processed. TPUs still possess elastomeric behavior but with the benefit of being easier to process.
Thermoset Polyurethanes, on the other hand, have a non-reversible material structure, meaning its cross-linked polymer chains can’t be broken with high temperature. The main advantage of thermoset polyurethane lies in its strong crosslinked bonds that provide excellent mechanical properties compared to thermoplastic polyurethane. Increasing crosslinking density of polyurethane creates strong chemical bonds between molecules and enhances material hardness, compression set, flex modulus, tensile, and thermal properties by decreasing mobility of the crosslinked polymer chains. When used as a cover material in a golf ball this means a more durable cover while still maintaining great elastomeric (aka. spin) properties. Compared to thermoplastic urethane, a thermoset urethane is very difficult to process and mold into a cover material. It requires casting or compression molding to make a thin cover layer and achieve good ball performance, which are often costly and create challenges with insert centering and consistent cover thickness.
Table 1 below shows the changes in mechanical properties when chemical crosslinking is introduced into a thermoplastic urethane (TPU). As expected, all of the mechanical properties such as Shore D hardness, flexural modulus, and elastic modulus significantly increase after crosslinking. Additionally, the abrasion resistance of crosslinked polyurethane increases more than 3 times than that of thermoplastic polyurethane, ultimately resulting in better cover shear durability.
Table 1. Comparison of mechanical properties between thermoplastic and thermoset polyurethanes.
A similar result is shown below when these materials are used in golf ball covers. Figure 1 compares the shear cut resistance of golf balls having thermoset and thermoplastic polyurethane covers. The test was performed with a robot hitting golf ball with a wedge having new grooves at a controlled speed. The higher index number represents better cover shear durability that golfers would notice on the golf course. The results confirm that the cover shear property of polyurethane is greatly improved after chemical crosslinking.
Figure 1. Shear test results on golf balls covered with two different types of polyurethane (A) thermoplastic and (B) thermoset.
So how did we apply this knowledge to Callaway’s urethane covered golf balls?
Our objective was to take all the processing benefits of a thermoplastic polyurethane, combine them with the material properties a thermoset polyurethane, and apply them to a golf ball cover.
Callaway Golf Ball R&D decided to think outside the box and combine the best features of a thermoset material (strong cross-linking for great durability) with the best features of a thermoplastic material (thin injection molding) to develop a unique cover material unlike anything on the market. The result is a Thermoplastic Polyurethane (TPU) that is chemically crosslinked after the molding process. This unique TPU shows high melt flow during injection molding in order to make thin, consistent cover layers. And yet the final product is fully crosslinked to provide the same benefit as a traditional thermoset polyurethane cover.
The key benefit of this technology that is unachievable with traditional thermoset covers is the unique polymer chain structure that occurs during the TPU injection process. During injection, the TPU material flows around the insert in a way that aligns the polymer molecules parallel to the direction of flow, causing better chain alignment (as shown in Figure 2). The chain alignment occurs prior to the crosslinking reaction and once the crosslinking reaction is complete, the chain alignment is locked-in to create additional strength. For golfers playing Chrome Soft or Chrome Soft X, this increase in mechanical strength and elasticity means better cover durability and more greenside spin.
Figure 2. Comparison of chain structure in cover layer prepared by two different processing methods;
(A) cast/compression molding does not achieve chain alignment, (B) injection molding creates polymer chains aligned along flow direction.
The red lines in this Figure represent mechanical crosslinks.
The result has been our most successful product line to date with Chrome Soft. We created a highly durable, extremely elastic, high performance polyurethane cover material that provides great spin around the green and excellent durability in play. It has an extremely thin, consistent cover layer that provides consistent performance, while also allowing us to pass along cost savings to our customers and offer American-made premium golf balls for less than our competitors.
Not all golf ball cover materials are equal, and not all urethanes are created equal. As a result of our uniquely crosslinked Polyurethane, Chrome Soft and Chrome Soft X outperform other premium golf balls in short game performance and durability.
About the Author:
Hong is a Materials Manager at Callaway Golf Company joined in 2016. He worked at TaylorMade Golf Company as a Senior Materials Manager for 13 years. He received PhD degree from Case Western Reserve University in Macromolecular Science. After receiving his PhD degree, he worked at Wright Patterson Air Force Laboratory and US Naval Research Laboratory for 4 years.