Natural gut strings are made from the intestines of cows. Natural gut has a low dynamic stiffness, and plays soft even when strung at high tension. This allows players to string their rackets tight to improve ball control, without losing much power and without greatly increasing impact shock. Natural gut also has excellent tension retention (it maintains tension over the life of the string). However, natural gut is expensive and, without protective coatings, is very sensitive to moisture (it sags).
Synthetic String Construction
Since the first nylon strings were developed in the 1970s, manufacturers have sought an alternative, more economical synthetic material whose properties match those of natural gut.
Synthetic strings comprise three main elements:
|Core||The core is the central load-bearing member and determines the string's characteristics, such as its stiffness and resiliency. There are two varieties of core construction multifilament and monofilament.|
|Monofilament cores comprise a single solid extrusion of material. It may also be constructed of a few smaller-gauge fibres that have been chemically bonded together so they behave as a single thick filament. Originally, monofilament strings were made from nylon, but most are now made from polyester, or a co-polymer with properties very close to polyester.|
|Multifilament cores are made from thousands of thin fibres twisted together. The fibres are not chemically bonded and are free to stretch somewhat independently from one another. Multifilament cores are therefore less stiff than monofilament cores of the same material, but are also less durable. Multifilament strings are most commonly made from nylon, but can incorporate other materials such as polyurethane, Zyex, Vectran, Kevlar, and others.|
|Jacket||The jacket protects the core from abrasion. Jackets may either be twisted or braided over the core. Twisting creates a somewhat smoother surface, making it easier to string in the racket. Braiding produces a more textured surface, for more bite on the ball.|
|Outer coating||Strings are usually coated with a resin which bonds the other layers together, creating a smooth finished surface.|
Synthetic String Materials
|Nylon||Nylon is the most popular string material for amateur players due to its low cost. However, nylon loses tension more rapidly than other string materials. The term synthetic gut is often used specifically to describe nylon monofilament strings.|
|Polyester (Poly)||A stiff and durable string material. Holds tension better than nylon.|
|Co-poly||Polyester blended with other softer polymers.|
|Polyetheretherketone||PEEK for short. Has superior tension retention over nylon and a dynamic stiffness closest to gut of any synthetic string material.|
|Composite||Manufacturers often combine yarns made from different materials (eg. different nylon polymers) into a single string, to achieve a balance of properties.|
|Proprietary||Some manufacturers also employ proprietary material technologies such as Vectran, Kevlar and Zyex: