Method of producing string of polyamide and stringed rackets with such string

Abstract

string of polyamide is irradiated with dosages of 10⁴ to 10¹0 rads. It has been discovered by actual experience that in string which is treated in this way the cross linkage of the molecules is enhanced. Such string has higher elasticity and substantially lower damping when set into oscillation than untreated string and that a tennis racket produced with such string is superior to a racket with untreated polyamide string. The dosage is not so high as to cause the string to become brittle.

Description

REFERENCE TO RELATED DOCUMENTS

1. This application is a continuation-in-part of U.S. Pat. application Ser. No. 540,794, filed Jan. 14, 1975 to Harry Ferrari for METHOD OF PRODUCING STRING OR POLYAMIDE AND STRINGED RACKETS AND STRINGED MUSICAL INSTRUMENTS WITH SUCH STRINGS now abandoned.

2. The Use of Plastics and Elastometers in Nuclear Radiation--W. W. Parkinson and O. Sisman--Nuclear Engineering and Design 17(1971) pgs. 247-280 particularly pg. 269--Published by North Holland Publishing Co.

3. Strength of Irradiated Drawn and Undrawn Nylon--Hsiao, Y. C. Da, A. Haynes--British Journal of Applied Physics Vol. II July 1960 pgs. 277-279.

BACKGROUND OF THE INVENTION

This invention relates to FIGS. 2 and 3 showwould wound or twisted together and usually coated with anti-wear coating. The string 13 is fed from the feed reel 15 to the take-up reel 17 through a beam 19 of the particles. The energy of the beam 19 is so related to the speed of the string 13 that the energy of the particles impinging on the string 13 is adequate to provide the required rad dosage. For example, where the beam 19 is an electron beam the dosage is set by setting the beam current and the beam voltage.

Typically, a polyamide string of 0.055 inch diameter was irradiated with 10⁷ rads at about 70° F. and about 14.7 pounds per square inch pressure. The mechanical properties of the string before and after irradiation is shown in the following Table I:

TABLE 1
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Property       Unirradiated Irradiated
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Yield Strength 55,000 psi   54,200 psi
Tensile Strength
73,400 psi   73,000 psi
Modulus of Elasticity
0.32 × 106 psi
0.275 × 10 6 psi
Elongation     36%          33.1%
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The significant data is the modulus of elasticity. The effect of the
radiation is to decrease the modulus from 0.32×106 to
0.275×106. The modulus is defined as the change in stress
σ divided by the corresponding strain produced. Within the elastic
limit the stressstrain curve is nearly linear and the modulus, being the
slope, is constant. In FIG. 4 3 stress σ is
plotted vertically and strain horizontally. The curve 21 for the treated
string has a smaller slope than the curve 23 for the untreated string. It
is seen that for the same change, Δσ, in stress the string of
the lower modulus produces a higher strain than the string with the higher
modulus. The treated string with the lower modulus is more elastic, and
therefore in the case of tennis strings will follow the ball better than
untreated string which results in improved control over the ball.

However, the improved elasticity along alone is not adequate to produce a satisfactory string for a racket. The resiliency must also be increased which means that the internal damping or loss coefficient must be reduced. The loss coefficient is the decrement or the decrease in the vibration amplitude which occurs as a string vibrates after it is set into vibration. Essentially the loss coefficient determines the Q of the string and is high for low-loss coefficient and low for high-loss coefficient.

Table II below shows the improvement achieved in loss coefficient in polyamide string of 0.055 diameter by irradiation with a dosage of 10⁷ rads at about 70° F. and about 14.7 pounds per square inch pressure.

TABLE II
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Loss Coefficient
Preload-Pounds
Unirradiated
Irradiated
% Improvement
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40         9%         7%       22
60        14%        11%       21
80        16%        12%       25
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All data in Table II are an average of 4 measurements

To produce this table unirradiated and irradiated string of 0.055 inch diameter were subjected to tension by applying the forces shown in the left-hand column. The string was vibrated and the decrease in amplitude of the vibration per cycle was measured. The irradiated string (third column from left) manifested a substantially lower loss coefficient than the unirradiated string (second column). The net result is that the irradiated string has a harder rebound impact on the ball than unirradiated string.

FIG. 2 shows a tennis racket 25 with treated polyamide string 27 in accordance with this invention. Such a racket was found by actual use to be superior to a racket with untreated polyamide in its sharp and precise reflection of the ball. FIG. 3 shows a musical instrument 29 with string 31 according to this invention.

While preferred practice of this invention is disclosed herein, many modifications thereof are feasible. This invention is not to be restricted except insofar as is necessitated by the spirit of the prior art.

Claims

1. The method of substantially improving the elasticity and decreasing and damping of stringed rackets having string composed of polyamide for stringed apparatus which comprises improving the elasticity and decreasing the damping of said string by irradiating the string with radition radiation, the magnitude of the energy of said radiation being in the range between the energy for which cleavage is predominately produced and the energy for which linkage alone is produced, so that a decrease in the modulus of elasticity of the treated string is accompanied by a decrease in the damping loss coefficient of the treated string.

2. The method of claim 1 wherein the string is formed prior to its exposure to radiation and thereafter is irradiated progressively by being drawn through a beam of radiation.

3. The method of claim 1 wherein the string is irradiated with a dosage of 10⁴ to 10¹0 rads.

4. A string for stringed apparatus rackets having improved elasticity and reduced damping loss composed of polyamide irradiated with radiation, the magnitude of the energy of said radiation being in the range between the energy for which cleavage is predominately produced and the energy for which cross linkage alone is produced, so that a decrease in the modulus of elasticity of the treated string is accompanied by a decrease in the damping-loss coefficient of the treated string.

5. A stringed racket whose strings are composed of the strings of claim 4.

6. A stringed instrument at least one of whose strings is composed of the string of claim 4.

7. A string of polyamide for stringed apparatus the cross linkage of whose molecular chains is enhanced.

8. strings for a stringed racket, said racket having tensioned synthetic strings with resilience similar to natural gut, said strings being produced by providing a filament of a polyamide material and irradiating the filament with a radiation dosage of about 10⁴ to 10¹0 rads, thereby causing cross linking of molecules and increasing the resilience of said filament.

9. A stringed racket whose string is composed of the string of claim 8.