Method of producing string of polyamide and stringed rackets and stringed musical instruments with such strings

String of polyamide is irradiated with dosages of 10.sup.4 to 10.sup.10 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.

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. Das, A. 
Haynes -- British Journal of Applied Physics Vol. II July 1960 pgs. 
277-279. 
BACKGROUND OF THE INVENTION 
This invention relates to stringed apparatus such as stringed rackets and 
stringed instruments and has particular relationship to the strings 
forming a part of such apparatus. The expression stringed apparatus as 
used in this application means stringed rackets and stringed instruments 
and other devices to which this invention is applicable. To a large extent 
this application is confined to stringed rackets both in the interest of 
concreteness and also because the remarkable results which gave rise to 
this invention have been experienced with rackets. Since the string 
according to this invention has properties which are advantageous in 
stringed musical instruments as well as stringed rackets, musical 
instruments as well as rackets are within the scope of this invention. 
In accordance with the teachings of the prior art, string for stringed 
rackets are made of animal gut, polymer materials such as polyamide which 
is sold under the name NYLON, or metals such as steel. Gut is considered 
to be superior for stringed rackets from the standpoint of the advantage 
which its use gives to the player. The principal advantage of gut is that 
it operates with less internal damping than other materials, and the ball 
rebounds harder and with less loss of energy on impact. Essentially 
damping is a measure of resilience; when the damping is low, the 
resilience is high and when the damping is high, the resilience is low. 
Gut has high resilience. However, gut is high in cost, variable in 
quality, has only modest strength, has low durability, and is unable to 
withstand for any length of time high moisture or humid environments. The 
above disadvantages have limited the use of gut to players who are willing 
to sacrifice high cost and relatively high probabiltiy of failure for 
better playing characteristics. Polyamides have good durability and good 
resistance to water or dampness. However, they lack the low damping of 
gut. 
Steel has found limited use because it transfers the numerous impacts of 
the ball to the frame substantially undamped and is damaging to the frame 
particularly if it is made of materials other than steel. Steel is also 
damaging to tennis balls resulting in excessive wear and low life. 
A wide variety of materials have been used for strings of musical 
instruments such as gut, steel, polymers such as polyamides, aluminum, 
composites such as steel--or aluminum-wound gut or polyamide. The exact 
material selected depends on the instrument and tonal qualities desired 
for any particular string. Normally, to achieve lower notes, the density 
of the strings is increased, for example, steel-wound gut or polyamide is 
used; and the tension on the string is reduced. 
It is an object of this invention to overcome the above disadvantages of 
the prior art and to provide string of polyamide having low internal 
damping for stringed apparatus. It is another object of this invention to 
provide improved stringed apparatus of which polyamide string having low 
internal damping shall form a part. 
SUMMARY OF THE INVENTION 
This invention arises from the discovery that by irradiating polyamide with 
elementary particles which inject energy into its molecular structure its 
internal damping is reduced. In addition, it has been discovered that the 
elasticity of polyamides is increased. The irradiation may be produced by 
electrons, neutrons, gamma rays, protons, alpha particles, X-rays or other 
particles or combinations of these particles. The radiation dosage should 
be between 10.sup.4 and 10.sup.10 rads. A rad is the quality of radiation 
which leads to the absorption of 100 ergs of energy per gram of irradiated 
material. 
In accordance with this invention the polyamide string is drawn and reeled 
and progressively irradiated as it passes from the feed reel to a take-up 
reel. The string as it is irradiated moves at a speed of about 240 feet 
per second. The tension impressed on the string is small but sufficient to 
enable the take-up reel to wind the string at the desired speed. Typically 
the string has diameter of about 0.055 inch and the tensional force is 5 
to 10 pounds. The tension in pounds-per-square inch is the force divided 
by the cross-sectional area of the string, that is, typically 
5/.pi.(0.0275).sup.2 to 10/.pi.(0.0275).sup.2. The irradiation at 10.sup.4 
to 10.sup.10 rads takes place at room temperature and pressure; that is, 
at about 70.degree. F and about 14.7 pounds per square inch. The dosage of 
the radiation and not the dose rate is important in producing the desired 
improvement. The dosage is measured in rads. 
The string so treated is then used in stringed apparatus such as stringed 
rackets or stringed musical instruments. Actual experience with a tennis 
racket strung with polyamide string irradiated with dosages of between 
10.sup.4 and 10.sup.10 rads has revealed that the racket in play functions 
like a racket strung with gut string as far as the damping factor is 
concerned. In addition, because the elasticity of the string is increased 
the control of the ball is improved. It is to be noted that because the 
damping is low, the high elasticity does not militate against effective 
elastic rebound of the ball from the racket. 
In polymers such as polyamide, radiation produces competing reactions, 
scission or cleavage and cross linking depending on the dosage of 
radiation, temperature environment and other conditions. The cleavage 
reaction, which predominates at low irradiation doses, results in the 
breakdown of the long polymer chains into smaller chains. The 
cross-linking reaction, which predominates at high irradiation doses, 
results in the binding together of molecules of the polymer into a network 
structure. At intermediate doses there is a mixture of the two reactions. 
Cleavage normally results in a decrease in tensile strength and modulus of 
elasticity, whereas cross-linking increases both the tensile strength and 
the modulus of elasticity. If the dosage is excessive, complete 
cross-linkage may take place but the string becomes brittle. It is 
desirable that the dosage be such as to enhance the cross-linkage without 
producing brittle string. 
The optimum irradiation dose to be used depends on many factors such as the 
specific polyamide used, string thickness, irradiation environment 
(atmosphere and temperature), etc., and the intended end use of the 
product; however, it should normally vary between 10.sup.4 and 10.sup.10 
rads at the room temperature and pressure. The irradiation can be achieved 
by any of the particles listed above. 
Strings produced in the practice of this invention are applicable to any 
stringed rackets including tennis, badminton, squash and paddle-ball 
rackets and to other sports equipment such as backboards for baseball, 
tennis, etc. The invention is also useful for improving the resilience of 
fishing lines as desired. 
Another application for irradiated strings is for stringed musical 
instruments as mentioned. The irradiation studies conducted on polyamide 
strings for stringed rackets reveal that irradiation under the proper 
conditions improve the mechanical properties of strings for stringed 
musical instruments, thereby greatly increasing their value or 
alternatively broadening the flexibility by which they may be used.

The apparatus shown in FIG. 1 includes a generator 11 of elementary 
particles, for example, an electron-beam generator. The generator 11 is of 
the type which emits its particles into the atmosphere. The polyamide to 
be irradiated is formed into string 13 and wound on a feed reel 15. 
Usually the string is formed of a plurality of threads would 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 
10.sup.7 rads at about 70.degree. 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 I 
______________________________________ 
Property Unirradiated Irradiated 
______________________________________ 
Yield Strength 55,000 psi 54,200 psi 
Tensile Strength 
73,400 psi 73,000 psi 
Modulus of Elasticity 
0.32 .times. 10.sup.6 psi 
0.275 .times. 10.sup.6 psi 
Elongation 36% 33.1% 
______________________________________ 
The significant data is the modulus of elasticity. The effect of the 
radiation is to decrease the modulus from 0.32 .times. 10.sup.6 to 0.275 
.times. 10.sup.6. The modulus is defined as the change in stress .sigma. 
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 stress .sigma. 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, .DELTA..sigma., 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 is not adequate to produce a 
satisfactory string. 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 
10.sup.7 rads at about 70.degree. F and about 14.7 pounds per square inch 
pressure. 
TABLE II 
______________________________________ 
Loss Coefficient 
Preload-Pounds 
Unirradiated 
Irradiated 
% Improvement 
______________________________________ 
40 9% 7% 22 
60 14% 11% 21 
80 16% 12% 25 
______________________________________ 
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.