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Polyketone / Zyex Information Center

Ashaway's Zyex®- Based Tennis Strings: the New "Manmade Natural Gut"

By Steve Crandall
Vice President, Sales & Marketing
Ashaway Racket Strings


For some time now, we've been claiming that our new Zyex®-based Monogut® ZX and Dynamite® tennis strings are "manmade polymer equivalents to natural gut." But string manufacturers have been comparing their strings to natural gut for many years now: so much so, in fact, that the claim might be said to lack a certain degree of credibility! So, we think it's fair for people to demand that we explain ourselves: just how do MonoGut ZX and Dynamite strings qualify as "manmade natural gut?"

First, let's take a look at "real" natural gut, which has a number of properties that make it ideal for racquet strings. Made from the intestines of cows (not cats!), its performance has been unmatched since it was first introduced in the nineteenth century. What makes gut a superstar is its ability to stretch on impact and then rebound quickly without deformation or loss of tension. This quality is called "dynamic stiffness" and means several things: 1) the string absorbs the force of impact, not the racquet or the player's arm. In other words, gut plays "soft;" 2) it returns that force to the ball very quickly and efficiently, providing more power; 3) because it stretches, it allows the ball more dwell time on the strings. This lets you "pocket" the ball and enhances control; 4) gut maintains tension over the life of the string, enhancing its value. Gut is also quite durable when struck on the sweet spot, but not so much on off-hits, making it better for better players. On the downside, gut is very expensive and, without protective coatings, is very sensitive to moisture - it sags.

So the key to gut-like playability is dynamic stiffness, which can also be likened to elasticity. Zyex inventor Bruce McIntosh calls dynamic stiffness "the ability of a material to resist stiffening up as it is rapidly stretched and released - become 'boardy.'" It's measured in pounds per inch, and in laymen's terms, is a measure of how much a string stretches when it strikes the ball. The more stretch, the more power it returns to the ball and the less impact it has on your arm and elbow. The less it stretches, the more energy is lost in flattening the ball and the less total power is available for rebound.

As in golf, the lower the dynamic stiffness 'score', the better. For today's leading natural gut string, this number comes in around 102, according to the US Racquet Stringers Association (USRSA).

For years now manufacturers have been trying to find a synthetic material for string whose properties match the dynamic stiffness of natural gut. Some of the most popular today are polyester and co-polymer strings. Most polyester and co-polymer strings are solid monofilaments: a single filament extrusion with varied profiles and surface textures. These strings are not sensitive to moisture and do provide good abrasion resistance. But in terms of dynamic stiffness, polyester and co-polymer strings range between 226 and 242 lb./in. This is not bad compared with ultra-stiff aramid strings, but it's still quite stiff. In fact, high-end polyester and co-polymer monofilament strings have been called 'gut on steroids' and are designed for hard-hitting, high-end professional players. And their prices reflect that.

The chart below (Figure 1) shows comparative data of several strings, including the best selling Natural Gut. I have to qualify this chart by saying that, except for the Dynamic Stiffness numbers which were provided by USRSA, all the other tests were done in Ashaway's own lab, which is not an independent, certified lab. Nevertheless, the numbers should be close enough to give you a clearer picture of how dynamic stiffness works.


Comparative String Data Chart

The first column shows the tensile strength of the various strings, which is the force required to stretch the string close to its breaking point. Note that the leading polyester mono has better tensile strength than gut while all the others have less. So it's stronger, and one would think, provides more power. But not so. Look at the next column, "Elongation at 55 lb.," which is the amount a string stretches at racquet tension, expressed as a percentage of length. Polyester and co-polymer strings stretch less than half as much as gut, while both Zyex strings stretch more. This means that polyesters are much stiffer and return less power to the ball.

By comparison, the dynamic stiffness of Zyex is the closest to gut of any synthetic string material. That's why we call it a "manmade polymer equivalent to natural gut." This means it absorbs and returns impact force more like gut. It controls the ball more like gut, especially in thinner gauge strings, and holds tension as well, or perhaps even a bit better than gut. Zyex is impervious to moisture and offers excellent abrasion resistance to enhance durability. And, of course, it's cheaper than either gut or the high-end polyesters.

Better still, Zyex offers more variety. Unlike polyesters and co-polymers, which are primarily monofilament constructions, Zyex can be constructed as a multifilament (Ashaway Dynamite 17 Natural or Dynamite 18 Soft); as a multistranded monofilament (Ashaway Dynamite® 16 Tough); or, as a straight monofilament in our new MonoGut ZX and MonoGut ZX Red. This, along with a broader range of gauges than either polyesters or gut, allows players to more closely tailor their string choice to their style of play. We think that's a good story.


Dynamite 17 Natural Dynamite 18 Soft Dynamite 16 Tough MonoGut ZX
Unlike polyesters and co-polymers, which are primarily monofilament constructions, Zyex can be constructed as a multifilament (Ashaway Dynamite 17 Natural or Dynamite 18 Soft); as a multistranded monofilament (Ashaway Dynamite® 16 Tough); or, as a straight monofilament in our new MonoGut ZX and MonoGut ZX Red. This, along with a broader range of gauges than either polyesters or gut, allows players to more closely tailor their string choice to their style of play.

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