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Description  |
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The invention relates to a string suitable for tennis rackets and like
applications comprising at least one synthetic resin monofil.
Strings consisting at least partially of a synthetic resin are
conventional, see, for example, DOS (German Unexamined Laid-Open
Application) No. 2,728,339 and DOS No. 1,703,132. The strings are utilized
for various purposes, especially as strings for musical instruments, as
well as for stringing rackets, especially tennis, squash, badminton
rackets, etc., and also as strings for bows and crossbows, and here the
above enumeration merely cites examples. For all of these applications,
the strings, bow strings, or the like must exhibit specific properties
with respect to the tensile strength and the elongation upon short-term
and repeated stress. After such a stress, the strings or the like must
return rapidly and completely to their initial length. Finally, the
strings or the like are to possess also a high strength under the various
conditions occurring during use, especially a high abrasion resistance, a
good flexibility, an extensive independence of the properties from tht
environmental conditions, as well as in total a high stability with
respect to the various loads to which the strings are exposed during
mounting to the various supports for which the strings are intended. The
profile of the requirements for tennis racket strings is shown, for
example, in the periodical Test, No. 6, (1978), pp. 512-517.
Gut strings have been used for a long time for musical instruments and for
stringing high-quality tennis rackets. The recuperative capacity of these
gut strings, i.e., their capability of rapidly and completely reassuming
the original length after a short-term or repeated stress, is excellent.
Furthermore, in case of gut strings, the increase in length or the
elongation, in dependence on the tensile force exerted, is linear and
remains practically unchanged from one load cycle to the next, which is
evidence of sluggish flow. However, all tensile elongation force curves
exhibit steps or jumps produced by the onset of tearing of certain
individual fibers or also the unraveling or untwining of turns of the
strings provided with a twist. The above-described phenomena
correspondingly reduce the lifetime of gut strings. In gut strings, the
lifetime is clearly proportional to the diameter thereof; however, on the
other hand, it is not readily possible to simply increase this diameter
since this leads to various disadvantages, namely, in particular, with
respect to the elasticity of the string under tensile stress.
Furthermore, gut strings do not exhibit a uniform quality, either, since
this quality depends on the gut utilized (sheep, cattle, pig gut), as well
as on the storage conditions for the strings and on the moisture
conditions ambient at the moment the strings are used. Since natural gut
strings show a high moisture absorption, a consequence of which is the
occurrence of dimensional changes, i.e., elongations of the string, th
elastic characteristic changes very considerably and to the disadvantage
of the players. Moreover, gut strings are pronouncedly expensive to
manufacture.
In recent times, a number of different strings has been developed,
consisting at least partially of a synthetic resin, especially
thermoplastic synthetic resins. These strings, as hereafter described,
have a structure which frequently is more or less complicated, for
example:
1. Monofil strings, heretofore, have been conventionally extruded from
polyamide, such as nylon, from modified polyvinyl chloride, from
polyurethane, or from a polyester, such as polyethylene terephthalate, or
also from a polyolefin such as polyethylene or polypropylene. The
manufacture of these strings is economical and thus desirable, and the
strings exhibit high strength during use. (It will be understood that the
term "monofils" refers to relatively-thick, extruded fibers or threads
having a diameter of at least 0.1 millimeter, preferably up to 1.5
millimeters which are extruded as individual fibers.) However, these
monofil strings have the disadvantage that--even after being stressed by a
relatively weak tensile force--the strings return only gradually into
their initial condition, due to their internal friction, and the
disadvantage that the strings experience irreversible lengthening under a
relatively-high tensile stress. Besides, extruded monofil strings become
brittle, inter alia, at low temperatures; this holds true expecially for
polyamide strings.
2. Other strings consisting of a bundle of parallel multifilaments, which
are not impregnated into the core and, which are merely surrounded,
namely, entirely on the outside, by an envelope or casing of an extruded
synthetic resin material. The thus-constructed strings exhibit the
disadvantage that the strings show little resistance to bending stresses
and in practical usage, the thin envelope has only a poor abrasion
resistance.
3. Another group of strings are in the strings of a flat bundle of parallel
multifilaments impregnated by extrusion with a thermoplastic material, for
example, with a polyamide, wherein the flat bundle (or the strap, or the
ribbon) obtained in this way is subsequently twisted at an elevated
temperature. The strings produced in this way exhibit the disadvantage
that the turns unravel when the string is subjected to tensile stress.
4. Yet another group of strings represents a combination of the
aforementioned types of strings with respect to their structure. For
example, these strings include strings with a monofil, an extruded core of
a thermoplastic material which is surrounded, for reinforcement purposes,
with windings of a thread, a strap, or a ribbon, or which is surrounded by
a casing or a braided envelope, wherein the casing is impregnated. The
provision of a reinforcing thread or the like increases the tear strength
of the string only if its ultimate elongation is higher than that of the
other string component, e.g., a thread or filament to be reinforced. In
general, the reinforcing threads, for example, metal, carbon, or boron
filaments, have a higher ultimate tensile strength and a higher modulus of
elasticity, but a lower ultimate elongation than the string component,
e.g., the monofil to be reinforced. If the ultimate elongation of the
reinforcing thread is exceeded, then the original thread or filament which
is reduced in cross section, is the sole bearer of the stress. Besides,
such multifilament strings are considerably more expensive in their
manufacture than the monofil strings.
The predominant part of the strings presently manufactured, which consist
at least partially of a synthetic resin, have hysteresis curves disclosing
an initial flow and indicating that, after a number of successive
stresses, a permanent elongation remains. For all these reasons, the
presently available strings produced from synthetic resin materials are
not fully satisfactory, especially if the strings are to be used as
stringing for tennis rackets. Although the conventional strings wherein at
least one process step takes place for the extrusion of thermoplastic
materials, particularly, for the production of a monofil core or an
impregnation of a band of multifilaments, can be manufactured very
economically as well as continuously and rapidly, the
artificially-produced strings are, on the other hand, not competitive in
quality with the natural gut strings, namely, primarily due to the
specific properties, especially the inadequate recovery power of the
thermoplastic materials employed as well as the insufficient elasticity.
The multifilament synthetic resin strings are still inferior as a tennis
racket stringing to the high-quality natural gut strings with regard to
playing characteristics, and are at most comparable to the lower quality
natural gut strings. Furthermore, the purely synthetic resin monofls
exhibit the poorest playing properties, due primarily to an inadequate
elasticity. In the production of conventional monofil and multifil or
multifilament synthetic resin strings, polyamide 6 and 6.6, as well as, to
a minor extent, polyethylene terephthalate are preferably utilized.
Furthermore, another disadvantage to natural gut strings as well as
synthetic resin strings, especially of polyamide, is the moisture
absorption and emission, respectively. Depending on the respective
atmospheric humidity and the ensuing moisture content of the strings, the
strings, in their strung condition, can contrast or expand in length. Even
in case of a synthetic resin of such high quality as polyamide 6 and 6.6,
dimensional variations of about 2% are still observed upon a change in
relative atmospheric humidity of 25-80%; in case of natural gut strings,
these variations are about 4%. On account of these dimensional changes, a
reduction in the pretensioning force of the tightened string occurs with
an increase in the moisture content of the strings; thus, for example, the
stringing of a tennis racket becomes relaxed and the ball is no longer
accelerated. Upon a reduction in the moisture content of the strings, on
the other hand, a shortening of the strings occurs and the pretensioning
forces are increased; the tightened string becomes harder. A particular
disadvantage of strings made of polyamide is due to the fact that, with a
moisture content of the polyamide of about 3% occurring at atmospheric
humidities of 50% relative humidity, the glass transition zone of
polyamide 6 and 6.6 is already at about 20.degree. C. For this reason,
these strings have a high attenuation and the strings exhibit a poorer
restoring power due to internal friction. Besides, in these polyamide
strings, strong variations in elasticity are experienced in case of
temperature changes.
In German patent application No P 29 14 606.3, a string has been suggested
made of a synthetic resin material, which is better adapted to the gut
strings and simultaneously exhibits the advantages of the conventional
strings of synthetic resin materials. This string consists of at least one
monofil of polyvinylidene fluoride (PVDF).
It has now been found that these synthetic resin strings of polyvinylidene
fluoride do not exhibit a high attenuation at room temperature and thus
rebound faster; that these strings are, at the same time, substantially
more elastic than all heretofore known synthetic resin strings, and that
these strings do not age as quickly. Strings of polyvinylidene fluoride
approximate with respect to their elasticity and their elastic
characteristics, the properties of high-quality natural gut strings.
Moreover, as compared with strings of natural gut and especially
polyamide, the disadvantage inherent in the polyamide due to dimensional
changes by moisture absorption is avoided, since moisture absorption of
polyvinylidene fluoride in the saturated condition is below 0.2%. This
means that polyvinylidene fluoride is substantially more
moisture-resistant than the nowadays customary synthetic resin strings of
polyamide. This holds true, in particular, for the temperature range from
+15.degree. to 50.degree. C., which is important for practical use.
Besides, strings of polyvinylidene fluoride are among those of all
synthetic resin monofils which have maximum weatherability.
The elasticity of the polyvinylidene fluoride monofil, with a pretensioning
strength of the string in the range between 170 and 320 N, is adapted to
that of a natural gut string. This elasticity of the polyvinylidene
fluoride monofil can be set for the thicknesses under consideration so
that identical playing properties are attained independently of the
thickness. Another essential property resides in that the relaxation of
the polyvinylidene fluoride monofil is equal to or lower than in natural
gut strings, with a comparatively applied pretensioning strength of 200 N.
Accordingly, the strings made from polyvinylidene fluoride monofils are
distinguished by high elasticity with a low relaxation. The elasticity for
the string of polyvinylidene fluoride monofils, with a pretensioning
strength of 200 N, can reach between 2.0 to 5.0.multidot.10.sup.-4
N.sup.-1, preferably approximately 3.3.multidot.10.sup.-4 N.sup.-1.
The ratio between high elasticity of the monofils and low relaxation
thereof, required for a string, especially also for stringing ball game
rackets, is attained in polyvinylidene fluoride monofils by axially
stretching the polyvinylidene fluoride monofil at a ratio between 1:3 to
1:10, preferably 1:4 to 1:5. By the choice of the stretching temperature,
the stretching conditions, and the residence time, the elasticity can be
greatly varied above an elongation of 7-8%. Thus, it is possible to
produce strings having the desired elasticity merely by a stretching step
under heated conditions. However, if these strings have been strung a
relatively long period of time, it is found that, on the one hand, tension
is reduced while, on the other hand, the elasticity drops as well.
Therefore, to set the elasticity desired at room temperature and to make
the elasticity uniform over long periods of time, a preliminary stretching
step is executed at elevated temperatures so that the strings have an
elasticity higher by about 40-70% than desired, and the elasticity is
brought to the desired value by at least one subsequent stretching step
under cold conditions. By the level of the respectively applied tension,
the length of the linear dynamic elongation range can be determined.
It is thus especially advantageous to subject the polyvinylidene fluoride
monofils to at least one more cold stretching step. By this subsequent
stretching operation, the relaxation of the monofil is decreased, in
particular, but the elasticity is likewise reduced, along with the
ultimate elongation. Therefore, this subsequent stretching step has its
limits.
A preferred field of application of monofils of polyvinylidene fluoride is
the use as stringing of ball rackets, especially tennis rackets. In this
utilization, the advantages of high weatherability and unresponsiveness to
moisture are fully exploited. Additionally, monofil strings of
polyvinylidene fluoride combine playing characteristics approaching those
of high-quality natural gut strings with a corresponding elasticity and
relaxation behavior. However, it has been found that monofils of
polyvinylidene fluoride have, in the temparature range from -20.degree. to
30.degree. C., a low attenuation, measured with a measuring frequency of
about 1 Hz. Due to this low attenuation and the concomitant low energy
dissipation, stringing systems with monofils of polyvinylidene fluoride
has a very good ball acceleration. However, the vibrations imparted to the
stringing of polyvinylidene fluoride monofils by the impact of a ball fade
rather gradually, so that arm joints are placed under mechanical stress
for a relatively long time period. It is possible, of course, to utilize
ball rackets having a higher inherent attenuation by constructing the
frame of the ball racket correspondingly, so that even monofils of
polyvinylidene fluoride exhibit, with such rackets, only a very brief
reverberation. Such ball rackets with stringing of pure polyvinylidene
fluoride monofils are gentler on the arm and yet show a satisfactory ball
acceleration.
Yet, the problem exists of eliminating also in monofils on the basis of
polyvinylidene fluoride the aforedescribed disadvantage of low attenuation
especially in the temperature range from -20.degree. to +30.degree. C., to
shorten the reverberating of the strings, and simultaneously to retain the
excellent properties when using polyvinylidene fluoride for strings.
This problem has been solved according to this invention in a string of at
least one monofil of a synthetic resin by using a synthetic resin mixture
consisting of 99-86% by weight of polyvinylidene fluoride and 1-14% by
weight of at least one polyacrylate. By the use of a polyacrylate in
addition to the polyvinylidene fluoride, attenuation can be increased,
i.e., it has been made possible thereby to shorten the reverberation of
the respective monofils under a corresponding load.
Preferred polyacrylates are homopolymers such as polymethyl methacrylate,
polymethyl acrylate, polyethyl acrylate and polypropyl acrylate, or
mixtures of these polyacrylates. An advantageous further development of
the synthetic resin mixture for the monofils according to this invention
provides that the polyacrylates used are copolymers of the vinylidene
fluoride monomer with at least one acrylate monomer, preferably selected
from the group of the alkyl acrylates or alkyl methacrylates, wherein the
alkyl groups are preferably lower alkyl groups of 1-4 carbon atoms and
wherein the acrylate proportion of the mixture is calculated from the
acrylate proportion of the copolymers. Preferably employed are mixtures of
polyvinylidene fluoride, pure acrylates, and vinylidene fluoride-acrylate
polymers. Monofils produced from such blends can be adjusted with respect
to their damping behavior, and the damping especially in the desired
temperature range of -20.degree. to +30.degree. C. can be increased to a
degree as it is especially desirable in the use as stringing for ball
rackets, and as it is known from natural gut strings.
The acrylate copolymers of vinylidene fluoride monomers with at least one
acrylate monomer are copolymers prepared from a monomeric mixture
containing 0-30% by weight of the above-described acrylate monomer and
90-70% by weight of vinylidene fluoride. The copolymer having such a
composition can be of a type as manufactured by suspension polymerization,
emulsion polymerization, graft polymerization, or by some other suitable
polymerizing method.
A preferred blend for producing the monofils contains, besides
polyvinylidene fluoride, up to 8% by weight of at least one polyacrylate
homopolymer and up to 25% by weight of at least one polyvinylidene
fluoride-acrylate copolymer. A weight ratio is preferred wherein
polyvinylidene fluoride amounts to 90-95% by weight of the mixture and the
acrylate components contained therein amount to 10-5% by weight. By
changing the proportions of polymethyl methacrylate or the methyl, ethyl,
and propyl acrylate proportions in the polyvinylidene fluoride copolymers,
the attenuation curves of the total mixture for the monofils can be
adjusted so that the reverberation of strings made from the monofils can
be minimized.
The invention has an especially good effect if the string is made of at
least one monofil, and fully proves itself in case of a monofil string,
which is of maximum interest from an economic viewpoint, combining the
advantages of natural gut strings with the heretofore known advantages of
synthetic resin strings. However, the string can also consist of several
monofils of a mixture of polyvinylidene fluoride and at least one
polyacrylate, which are twisted, braided, plied, or similarly joined
together. This is also to cover strings of a complete structure,
exhibiting still other additional components besides synthetic resin
monofils.
With the great stress to which strings are exposed in the tensioned
operating condition, wearing through can occur even in case of the
monofils of this invention at the points of intersection of two strings.
To avoid this, the invention proposes to coat the surface of the monofil
so that frictional resistance is lowered and abrasion resistance is
raised, for example, with polytetrafluoroethylene or silicone oils. The
increased abrasion resistance prolongs service life. This is also
advantageous for these strings when used as stringing for ball rackets.
Such coating also makes these strings more advantageous than strings of
natural gut.
The string of this invention is essentially characterized by its
elasticity, which is essentially dependent on the properties of the
polyvinylidene fluoride in the monofil.
As in the case of other macromolecular compounds, some of the properties of
polyvinylidene fluoride, especially the degree of crystallinity, depend on
the thermal prehistory of the material as well. While an
extensively-amorphous material of good flexibility is produced by rapid
cooling after processing, a gradual cooling or tempering at about
135.degree. C. leads to highly crystalline materials which, with a higher
density, exhibit a higher modulus of tension and flexion and have an
improved long-term stability. The process for producing a string in
accordance with the invention provides that a rod of polyvinylidene
fluoride and polyacrylate is extruded at a melt temperature of the
polyvinylidene fluoride of between 250.degree. and 350.degree. C.,
preferably between 260.degree. and 280.degree. C., and cooled to a
temperature of between 60.degree. and 150.degree. C., preferably between
130.degree. and 145.degree. C., and is axially stretched at this
temperature, whereupon the thus-obtained monofil is cooled to room
temperature (about 20.degree. C.) and then stretched under these cold
conditions. By the combination according to this invention of the process
steps of warm-stretching and a cold, but relatively minor subsequent
stretching of the monofils, the excellent properties required for a string
are attained, namely, an elastic behavior approaching that of natural gut
strings and remaining uniform over long periods of time, and a reduction
in the relaxation of the polyvinylidene fluoride to a value acceptable for
playing characteristics. Preferably, the cold stretching of the monofil is
conducted to such an extent that an elongation of the monofil takes place
by 1-3%. This extent of cold stretching is sufficient to attain the
desired reduction in relaxation. During the subsequent cold-stretching
step, the knot tear strength and the ultimate elongation are hardly
altered practically, whereas the elasticity rises somewhat. The attainable
elasticity, knot tear strength, and ultimate elongation of the monofil
also depend on the temperature at which the warm-stretching step is
carried out. The temperature during the warm stretching and also the
stretching ratio, preferably chosen to be between 1:3 and 1:10,
preferably, 1:4 to 1:5, likewise depend on the required final thickness or
diameter of the monofil string. To obtain, for example, a final thickness
of 1.2 to 1.5 mm for a monofil string, the thickness of the rod to be
stretched must be chosen to be between 2.7 and 3.4 mm at a stretching
ratio of 1:5 and between 3.4 and 4.2 mm at a stretching ratio of, for
example, 1:8.
It is also advantageous to subject the monofils, warm-stretched at a
temperature of between 130.degree. and 145.degree. C., prior to cooling to
room temperature, furthermore to a temperature lying somewhat above the
stretching temperature, to diminish stresses.
The desired cold-stretching step is achieved according to this invention,
for example, by winding up the monofil with a uniform tensile force of at
least 200 N, preferably 230-280 N, (N=Newton=kg.multidot.m/sec.sup.2) and
allowing it to remain wound up under tension for at least five minutes,
preferably for up to one hour or optionally longer, until it is passed
onto its use after having been relieved of its tension.
For the preferred use of the string of this invention as stringing for ball
rackets, especially tennis rackets, monofils are employed having an
elasticity of 2.7-3.6.times.10.sup.-4 N.sup.-1 with a pretensioning force
of 200 N, an ultimate elongation of 16-30%, a tear strength of between 300
and 500 N/mm.sup.2, with a diameter of 1.2-1.5 mm.
The invention will be explained in greater detail below using a tennis
racket string as the example. The properties of significance for practical
use of the string of this invention made of polyvinylidene fluoride and
polyacrylate for stringing tennis rackets will be briefly described below.
It is furthermore to be noted that strings for tennis rackets, when being
strung, are tightened with a pretensioning force, depending on the type of
game of the player, of between 150 N and 300 N, preferably about 200 N.
The following requirements result.
(a) Tensile Strength:
On the basis of the stress-strain diagram of a tennis ball, an estimate can
be made that about 50-250 N of force is absorbed by the stringing of the
tennis racket. These forces are distributed among the individual strings
of a tennis racket in various ways. Since, in general, the longitudinal
strings have a higher pretensioning than the transverse strings, these
forces are absorbed to a greater degree by the longitudinal strings and to
a lesser degree by the transverse strings. The force exerted by a ball on
a string should, per estimate, not be more than 50 N in case of average
players. This force is added to the pretensioning force of a string of
160-300 N, by which a string is pretensioned during mounting.
(b) Tension Relaxation:
The tennis racket strings are pretensioned, depending on the player and the
type of game, with 160-300 N, predominantly with 200 N during stringing.
With an increasing tension, the deformation distance is reduced and the
contact time between the ball and the stringing is shortened so that
generally the guidance of the ball is poorer and a high speed of the
tennis racket is required for accelerating the ball. The tension of the
strings should vary with the time to a minimum extent, i.e., the tension
relaxation is to be low. Furthermore, the tension is to change only to a
minimum extent due to the effects of temperature and moisture.
(c) Knot Tear Strength:
The strings must have an adequate knot tear strength, but it is possible to
reduce the force acting on the knot by repeatedly turning the string
during stringing.
(d) Elasticity Behavior:
One of the most important properties of the tennis racket strings is the
elasticity behavior in case of tensile forces of about 200 N. This can be
varied within a certain range, due to the nonlinearity of the
stress-strain diagram, by the choice of the tension and/or by the diameter
of the strings. This property has an effect on the ball acceleration, the
ball control, and the stress on the elbow joint. Too long a deformation
path causes too low a ball acceleration, and a deformation path which is
too short causes a poor ball control. the natural gut strings, according
to all experience gained heretofore, exhibit an elasticity behavior
ensuring a satisfactory ball control and acceleration as well.
(e) Restoring Power:
The string, after short-term stress, is to return rapidly to its initial
condition. This means that the internal friction of the material utilized
is to be low. However, if the internal friction is too low, an undesired
reverberation occurs, for example, in ball rackets. A measure for inner
friction is the attenuation.
(f) Abrasion Resistance:
The wear characteristic is determined, on the one hand, by the rubbing
together of two strings at the points of intersection of a stringing, but,
on the other hand, also by dust and dirt.
The following description relates to the manufacture of a monofil string
from a polyvinylidene fluoride modified according to this invention, which
is to be used as a tennis racket string, and this string is investigated
with respect to its properties and compared with a multifil high-quality
synthetic resin string of polyamide (PA) of the type "Hy-O-Sheep" by
Rucanor GmbH, Cologne, and a natural gut string of the type "Victor
Imperial" by Hoffman von Cramm KG, Unteraching, and a monofil string of
pure polyvinylidene fluoride. The monofil of this invention to be
evaluated consists of 79% by weight of polyvinylidene fluoride and 3.5% by
weight of PMMA (polymethyl methacrylate) and 17.5% by weight of a
vinylidne fluoride-ethyl acrylate graft polymer with 20% by weight of
ethyl acrylate. Table 1 shows the compilation of a comparison of the
mechanical properties in tabular form for the monofil strings described
herein.
The string of this invention as well as the properties of the string made
of the modified polyvinylidene fluoride (i.e., containing polyacrylates)
as compared with the above-specified natural gut string, the synthetic
resin string of polyamide, and a monofil of polyvinylidene fluoride are
illustrated in the accompanying figures, wherein:
FIG. 1 is a perspective view of a monofil made from the modified
polyvinylidene fluoride of this invention;
FIG. 1a is a perspective view of a twisted bundle of three modified
polyvinylidene fluoride monofils;
FIG. 2 shows a stress-strain diagram for the different strings;
FIG. 3 shows the elasticity as a function of the pretensioning force
applied to each string; and
FIG. 4 shows the dependency of the shear modulus and attenuation on the
temperature.
The elasticity .alpha. indicated in the following table and in FIG. 3 is
defined as the ratio of the change in elongation .DELTA..epsilon. at a
change in force .DELTA..kappa.
.alpha.=.DELTA..epsilon./.DELTA..kappa.
.DELTA..epsilon.=change in elongation
.DELTA..kappa.=change in force
for a reversible deformation.
The tear strength was determined according to DIN 53 455. Concerning the
knot tear strength, a knot was made in the thread to be tested, and then
the tear strength was measured on the thread in the tensile strength test
according to DIN 53 455. The characteristic values are compiled in Table
1.
Abrasion resistance was determined by means of a special test instrument.
Two strings are clamped in place in intersecting relationship, each with a
pretensioning force of 200 N. The tensioning clamps for one string are
fixed, the tensioning clamps for the other string are movable. The movable
string is pulled from about under the fixed string and extended upwardly.
With a speed of 100 cycles per minute, this string is moved to and fro.
The point of intersection of both strings can shift to and fro during
testing by about 10 mm. The abrasion resistance determined with this
device is most advantageous in case of the tennis racket strings made of
polyamide. The natural gut strings differ very greatly with respect to
abrasion resistance. There are strings which rupture already after 600
cycles, whereas other rupture only after 2000 cycles. PVDF monofils
exhibit an abrasion resistance slightly poorer than that of natural gut
strings. In contrast, PVDF monofils and/or modified PVDF monofils
according to the invention, the surface of which has been coated, for
example, with "Teflon," exhibit abrasion resistance values substantially
higher than those of natural gut strings. These properties are shown in
the following Table 1.
TABLE 1
__________________________________________________________________________
Comparison of Mechanical Properties
Tear Ultimate
Knot Tear
Density
Thickness
Strength
Elongation
Strength
Abrasion Resistance
String g/cm.sup.3
(mm) (N) % (N) Number of Cycles
__________________________________________________________________________
Natural gut string (1)
"Victor Imperial"
1.33 1.3 440 15 200 2000-15000
Synthetic resin string (2)
PA
"Hy-O-Sheep" 1.11 1.4 750 25 400 50 000
PVDF Monofil string (3)
1.78 1.3 480 20 350 300-2000
Acrylate-modified
PVDF monofil string (4)
1.68 1.3 570 28 350 2000
Acrylate-modified
PVDF string with
"TEFLON" coated surface
1.68 1.3 570 28 350 20000-50000
__________________________________________________________________________
A comparison of the characteristic values of tennis racket strings made of
monofils from acrylate-modified PVDF according to this invention with
high-quality natural gut and "synthetic resin strings" shows that the
modified PVDF monofil string does not exhibit a large number of the
disadvantages displayed by the natural gut strings as well as the
conventional synthetic resin strings, and is substantially improved in its
attenuation properties as compared with strings made purely of PVDF.
The stress-strain diagram of FIG. 2 shows the characteristic differences in
natural gut strings and synthetic resin strings. A striking feature here
is, in particular, that the polyamide strings exhibit a great force
increase at higher elongations. The elasticity can be determined from the
stress-strain diagram of FIG. 2 in accordance with the above-described
formula, and is illustrated as a function of the pretensioning in FIG. 3
for the various strings. It can be seen that monofils on the basis of
polyvinylidene fluoride exhibit, at pretensioning forces of about 150 N to
350 N, approximately the same elasticity as high-quality natural gut
strings. In contrast thereto, the synthetic resin strings of polyamide
show a substantially-lower elasticity at a pretensioning force of 200 N.
FIG. 4 shows the temperature curves of the linear modulus and the
attenuation, measured in a torsion pendulum test according to DIN 53 445.
The strings of natural gut and on the basis of polyvinylidene fluoride
change their shear modulus in a temperature range up to +20.degree. C.
only to an insignificant extent. In contrast thereto, synthetic resin
strings on the basis of polyamide undergo extensive changes in this
temperature range, which is due to the fact that the glass transition
temperature of the polyamide is lowered by the relative humidity content,
wherein moisture saturation at 23.degree. C. and 50% relative humidity
signifies a glass transition temperature of 20.degree. C.
A comparison of the attenuation curves of natural gut string, polyamide
string, pure PVDF monofil, and acrylate-modified PVDF monofil according to
this invention as illustrated in FIG. 4 shows that the highest attenuation
is exhibited by the saturated polyamide string in the temperature range to
be considered under practical conditions, at 23.degree. C. and 50%
relative humidity. In contrast, a completely dry polyamide string has a
very low attenuation. A markedly lower attenuation is exhibited by the
natural gut string, saturated with moisture at 23.degree. C. and 50%
relative humidity. The lowest attenuation is displayed by the pure PVDF
string. Such a low attenuation means that such strings will reverberate
longer, causing a longer-term vibration of a ball racket. The
polyacrylate-modified PVDF string of this invention displays a desirable
attenuation characteristic approaching that of natural gut string in the
temperature range under consideration.
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