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Comparing PEEK, Nylon, and PU-Based Strings

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

Generally known for its toughness and abrasion resistance, when used in string PU permeates the nylon sheath, chemically bonding the whole into a single, unitized matrix. Unlike UltraKill 17, which has a woven nylon wear layer over a PEEK multifilament core, MultiKill 17 has no separate wear layer, just the smooth PU surface. Because of the material's natural adhesive properties, this surface helps grip the ball.

Ever since we introduced our first polyurethane (PU) based string at the end of last season - MultiKill^® 17 - people have been asking me to explain the difference between it and our Zyex^® (PEEK) based strings like our PowerKill^® and UltraKill^® strings. I've gone over some of this before, but as traditional nylon multifilament, PU-based nylon multifilament, and PEEK multifilament strings comprise the largest proportion of strings in use today, the comparisons bear repeating. Besides, my mother always said she had to repeat things to me at least three times, and I think that's good advice.

The first thing that needs to be said is that how a string plays and feels for you is much more important than how it technically compares to another string. This is not just to say that everybody is different - which is rather a cliché - but that the same string will really play and feel differently for different players. If this were not true, there would be only one string!

PEEK stands for polyether ether ketone, the chemical name for the Zyex brand polymer filaments we use in our string. PEEK is a very robust high-temperature engineering grade polymer, noted for its toughness and durability. PEEK monofilaments and fibers have excellent abrasion resistance (over five times better than Kevlar). They also have better dynamic stiffness ratios than any other synthetic string material, giving them the ability to stretch and snap back quickly, returning energy and power to the ball (we call it "soft power"). Finally, PEEK has exceptionally low creep, which is the ability of a string to retain tension when strung in a frame, extending the life of the string.

As for nylon, I'd wager that nearly everyone who has ever played racquetball has used nylon strings at some point. Nylon has been the dominant material for racquetball string since the game first began and still remains very popular. Nylon strings are relatively inexpensive, they are quite responsive, and players love their "feel" and "playability."

Nylon strings don't spring back quite as quickly as Zyex but depending on the design and gauge of the string, can be either quite stretchy or quite stiff, with dynamic stiffness ratings averaging around 190 lb./in, compared to Zyex at 145. Nylon's main drawback is that it loses tension and wears more quickly than other string materials leading to a shorter life in the racquet. It is also not as durable for power players.

Enter PU, or polyurethane, which is one of - if not the - most versatile polymers known, with some $80 billion worth produced annually. Generally known for its toughness and abrasion resistance, when used in string it permeates the nylon sheath, chemically bonding the whole into a single, unitized matrix. Unlike UltraKill 17, which has a woven nylon wear layer over a PEEK multifilament core, MultiKill 17 has no separate wear layer, just the smooth PU surface. Because of the material's natural adhesive properties, this surface helps grip the ball.

In adding PU to nylon, our intention was to add durability to the nylon for heavy hitters, and improve abrasion resistance, which would reduce notching and improve the life of the string. MultiKill 17 does these things very well. The added strength improves durability, especially on off-center hits near the edge of the frame. But what surprised us was that the PU also improved the tension holding ability of the string while retaining nylon's soft feel and comfortable playability.

Needless to say, we were very eager to get comments from our play testers when MultiKill 17 was ready, especially from our best-known player, Cliff Swain. After playing with nylon-based string for years, Cliff has become a strong advocate for our Zyex-based PowerKill 17, and even sells his Cliff Swain brand racquets factory-strung with it. So, when he tried PU-based MultiKill 17, the first thing he noticed was that it did not string as tight as his PowerKill. In fact, he said he had to string about 4 lbs. tighter - about 10% -- to get the feel he was used to. But once strung he found the MultiKill also held tension better than a traditional multifilament nylon string. It did not "relax" as much after stringing and remained quite stable after that.

Cliff also noticed the PU seemed to add power to the string. "I love the playability, the dynamic stiffness, and the sharpness that I get with the Zyex strings," he said, "but there is something about the polyurethane, nylon filament combination that also gives me excellent power. But it's power of a different sort."

We found that comment quite intriguing and will explore it at greater depth in next spring's issue. We'll also review the comments from several of our other well-known play testers.

ZYEX^® is a registered trademark of Victrex Ltd.