BUFFER LAYER FOR STRINGS

This application claims priority to U.S. Provisional Application Serial No. 60/866,199, which is hereby incorporated by reference hereby.

BACKGROUND

The strings for sports equipment (e.g., tennis raquets) or musical instruments are usually coated with a thin layer at their outmost surface to improve their durability, spin, feeling, etc.

Polyamide (nylon), polyester, and other polymers have been used to coat on strings. Nanocomposites, such as clay and carbon nanotube reinforced nylon 6 nanocomposites, having better physical properties than neat nylon 6, are of a potential to be highly durable string coating materials with other functionalities. The reinforcing polymeric composites using nano-sized clay particles with high aspect ratio have been investigated since the 1980's (see U.S. Patent No.

4,739,007). Strings are usually polymer materials with a multi-layer structure - core filament, wrapping filaments on the core filament, and coating. For the strings with multi-layer structures, coating materials are required to match the base materials and have good melt-flow properties (acceptable viscosity) at certain temperature to allow them to be penetrated into the gaps between the wrapping filaments. Viscosity of a nanocomposite is usually higher than neat nylon 6 at the same temperature. Thus, the nanocomposite may not easily penetrate into the gaps between the wrapping filaments. Figure 1 shows an SEM image of a cross-section view of a nylon 6/clay nanocomposite coated on a wrapping filament. It can be seen that the nanocomposite material did not successfully fill out the gaps. Many defects were left in the string which will result in unacceptable durability of the strings. The gaps will result in chipping-off or unacceptable durability of coatings during high impact hitting of balls. More over, due to creation of the gaps, coatings also fail to a fix the filaments on the core materials of the string. Figure 2 shows the chipped materials from filaments and coatings after high impact tests on the strings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows an SEM image of a cross-section view of a nylon 6/clay nanocomposite coated on a wrapping filament; Figure 2 shows an SEM image of chipped materials from filaments and coatings after high impact tests on a string;

Figure 3 A illustrates a cross-section of a core filament with wrapping filaments surrounding it;

Figure 3B illustrates a buffer layer applied onto the wrapping filament; Figure 3 C illustrates a coating applied onto the buffer layer; and

Figure 4 illustrates another embodiment of the present invention.

DETAILED DESCRIPTION

Although polymer nanocomposites have higher physical/mechanical properties than neat polymer materials, they normally have higher viscosity or melt- flow during an extrusion or coating process. To solve this problem, a thin buffer layer is used to coat on the multi-filament wrapped string to fill the gaps. The polymers of the buffer-layer coating have a high melt-flow (low viscosity) during coating process to fill all the gaps between the filaments, and the filaments are fixed by the coatings onto base core materials.

Example 1 : A coating system with a nylon 6 buffer layer

Figure 3A illustrates a cross-section of a string for coating comprised of one monofilament core 301 wrapped with smaller-diameter multi-filaments 302. Neat nylon 6 pellets as may be obtained from UBE Industries Inc. (product name: UBE SF 1018 A) are melted. The buffer layer coating 303 is applied by an extrusion process at temperatures ranging from 22O ⁰ C to 27O ⁰ C. The thickness of the buffer layer 303 may be from 10 to 100 micrometers. The gaps between the multi- filaments 302 are fully filled by the neat nylon 6 coating.

A wear-resistant coating 304 is then coated (Figure 3C) by an extrusion process at temperatures ranging from 24O ⁰ C to 28O ⁰ C. A nylon 6/clay or nylon 6/carbon nanotube nanocomposite may be employed as the wear-resistant coating material 304. The nylon 6 nanocomposite produced by in-situ polymerization may contain 4% nano-clay filler. Other nylon 6 nanocomposites produced by melt-compounded process may also be used for the wear-

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resistant coatings 304. Except for the clay, carbon nanotubes, ceramic particles such as SiO ₂ and AI2O3, or glass particles may be used to make nylonβ nanocomposites. The Nylon 6 nanocomposites may also be modified by rubber modifiers to improve the ductility and toughness. The thickness of the wear-resistant coating may be from 1 to 100 micrometers.

Example 2: A coating system with a nylon 11 buffer layer

Again referring to Figure 3 A, the string for coating is one monofilament core 301 wrapped with smaller-diameter multi-filaments 302. Neat nylon 11 may be obtained from ARKEMA Inc. Nylon 11 has a very good melt flow at temperatures over 22O ⁰ C. Good impact strength and shear strength also make nylon 11 a good buffer layer material. In Figure 3B, the buffer layer coating 303 is applied by an extrusion process at temperatures ranging from 19O ⁰ C to 27O ⁰ C.

The thickness of the buffer layer 303 may be from 10 to 100 micrometers. The gaps between the multi-filaments 302 are fully filled by the neat nylon 11 coating.

Referring to Figure 3C, a wear-resistant coating 304 is then coated by an extrusion process at temperatures ranging from 24O ⁰ C to 28O ⁰ C. Nylon 6/clay or a nylon 6/carbon nanotube nanocomposite may be employed as the wear-resistant coating material 304. The nylon 6 nanocomposite produced by in-situ polymerization may contain 4% nano-clay filler. Other nylon 6 nanocomposites produced by melt-compounded process may also be used for the wear-resistant coating 304. The nylon 6 nanocomposites may also be modified by rubber modifiers to improve the ductility and toughness. The thickness of the wear-resistant coating 304 may be from 1 to 100 micrometers.

Except for the extrusion process to deposit a coating on the string, other methods such as spraying, dipping, spin coating, brushing, painting, and immersing processes can be used to deposit a coating on the surface of strings. Nylon 6 nanocomposites may be melted at higher than 19O ⁰ C and extruded to deposit a coating on the strings. Nylon 6 nanocomposites may be dissolved in a solvent such as formic acid and sprayed, dipped, spin coated, brushed, painted, or immersed to deposit a coating on the string at room temperature or elevated temperatures. The solvent may be then removed by a follow-up process such as an evaporation method.

Figure 4 illustrates another embodiment of the present invention. Essentially, the coated string structure of Figure 3 C is then coated again with smaller-diameter multi-filaments 401. A buffer layer coating 402, similar to layer 303, is applied by an extrusion process at temperatures ranging from 19O ⁰ C to 27O ⁰ C. The thickness of the buffer layer 402 may be from 10 to 100 micrometers. The gaps between the multi-filaments 401 are fully filled by the neat nylon 11 coating. A wear-resistant coating 403 is then coated by an extrusion process at temperatures ranging from 24O ⁰ C to 28O ⁰ C. Nylon 6/clay or a nylon 6/carbon nanotube nanocomposite may be employed as the wear-resistant coating material 403. The nylon 6 nanocomposite produced by in-situ polymerization may contain 4% nano-clay filler. Other nylon 6 nanocomposites produced by melt-compounded process may also be used for the wear-resistant coating 403.

The nylon 6 nanocomposites may also be modified by rubber modifiers to improve the ductility and toughness. The thickness of the wear-resistant coating 403 may be from 1 to 100 micrometers.