U.S. patent application number 10/969809 was filed with the patent office on 2006-04-20 for strings for racquets.
Invention is credited to Chaokang Chu, Dean J. Gambale.
Application Number | 20060084532 10/969809 |
Document ID | / |
Family ID | 36181491 |
Filed Date | 2006-04-20 |
United States Patent
Application |
20060084532 |
Kind Code |
A1 |
Chu; Chaokang ; et
al. |
April 20, 2006 |
Strings for racquets
Abstract
Novel racquet strings and methods for making the same. A polymer
cover combined with a low temperature adhesive is provided to the
strings. The string of the present invention may employ a
conventional string, such as a string having a center core
comprising gut or synthetic material such as nylon, and a polymer
cover impregnated with low temperature adhesive. The polymer cover
covers the string along at least a portion of the length of the
string.
Inventors: |
Chu; Chaokang; (Hockessin,
DE) ; Gambale; Dean J.; (Wilmington, DE) |
Correspondence
Address: |
W. L. Gore & Associates, Inc.
551 Paper Mill Road
P.O. Box 9206
Newark
DE
19714-9206
US
|
Family ID: |
36181491 |
Appl. No.: |
10/969809 |
Filed: |
October 20, 2004 |
Current U.S.
Class: |
473/524 ;
473/537 |
Current CPC
Class: |
A61M 2205/3372 20130101;
A63B 51/02 20130101; A61M 2205/8206 20130101; A61M 2205/3653
20130101; A61B 17/3474 20130101; A63B 51/10 20130101; A63B 60/42
20151001; A61M 13/003 20130101 |
Class at
Publication: |
473/524 ;
473/537 |
International
Class: |
A63B 49/00 20060101
A63B049/00; A63B 51/02 20060101 A63B051/02 |
Claims
1. A racquet string comprising: a) a string; and b) a composite
comprising a polymeric membrane having at least some porosity and
an adhesive disposed within said at least some porosity, the
composite covering at least a portion of said string.
2. The racquet string of claim 1 wherein said at least some
porosity is filled with the adhesive.
3. The racquet string of claim 2 further comprising an adhesive
layer disposed upon at least one surface of said polymeric
membrane.
4. The racquet string of claim 3 wherein the adhesive layer is
discontinuous.
5. The racquet string of claim 3 wherein the adhesive layer is
continuous.
6. The racquet string of claim 1 in which the adhesive comprises a
low temperature adhesive.
7. The racquet string of claim 6 in which the low temperature
adhesive is UV cured.
8. The racquet string of claim 6 wherein the low temperature
adhesive further comprises at least one filler material.
9. The racquet string of claim 8 wherein the at least one filler
material comprises at least a material selected from the group
consisting of: ceramics, metals, metal coated fillers, metallized
fillers, inorganic oxides, carbon, pigments, lubricants and
polymers.
10. The racquet string of claim 6 wherein the low temperature
adhesive comprises at least a material selected from the group
consisting of urethane acrylates and cationic epoxies.
11. The racquet string of claim 1 in which the composite is
helically wrapped around at least a portion of the string.
12. The racquet string of claim 1 in which the composite is
longitudinally wrapped around at least a portion of the string.
13. The racquet string of claim 1 wherein the polymeric membrane
comprises fluoropolymer.
14. The racquet string of claim 13 wherein the fluoropolymer is
expanded polytetrafluoroethylene.
15. The racquet string of claim 1 in which the composite has a
thickness of less than about 5 percent of the string diameter.
16. The racquet string of claim 1 in which the composite has a
thickness of less than about 3 percent of the string diameter.
17. The racquet string of claim 1 in which the composite has a
thickness of less than about 1 percent of the string diameter.
18. The racquet string of claim 1, wherein the string comprises
natural gut.
19. The racquet string of claim 1, wherein the string is a
mono-filament.
20. The racquet string of claim 1, wherein the string comprises a
plurality of filaments.
21. The racquet string of claim 20, wherein said filaments are of
substantially the same diameter.
22. A racquet string having a diameter of less than about 1.34 mm
and having dynamic modulus of less than about 30 kN/m and a
durability of at least about 2200.
23. A racquet string having a diameter of less than about 1.25 mm
and having dynamic modulus of less than about 30 kN/m and a
durability of at least about 500.
24. A racquet string having a diameter of less than about 1.25 mm
and having dynamic modulus of less than about 30 kN/m and a
durability of at least about 1000.
25. A racquet string having a diameter of less than about 1.20 mm
and having dynamic modulus of less than about 30 kN/m and a
durability of at least about 500.
26. A racquet string having a diameter of less than about 1.20 mm
and having dynamic modulus of less than about 30 kN/m and a
durability of at least about 800.
27. The racquet string of claim 1, wherein the string comprises
synthetic fibers.
28. The racquet string of claim 27, wherein the synthetic fibers
comprise polyamide.
29. The racquet string of claim 28, wherein the synthetic fibers
comprise nylon.
30. The racquet string of claim 27, wherein the synthetic fibers
comprise polyester.
31. The racquet string of claim 27, wherein the synthetic fibers
comprise PEEK.
32. A racquet comprising: a) a frame; b) a string disposed in the
frame, and c) a composite comprising a polymeric membrane having at
least some porosity and an adhesive disposed within said at least
some porosity, the composite covering at least a portion of the
string.
33. The racquet of claim 32 wherein the at least some porosity is
filled with the adhesive.
34. The racquet of claim 33 further comprising an adhesive layer
disposed upon at least one surface of the polymeric membrane.
35. The racquet of claim 34 wherein the adhesive layer is
discontinuous.
36. The racquet of claim 34 wherein the adhesive layer is
continuous.
37. The racquet of claim 32 in which the adhesive comprises a low
temperature curable adhesive.
38. The racquet of claim 38 in which the low temperature adhesive
is UV cured.
39. The racquet of claim 37 wherein the low temperature adhesive
further comprises at least one filler material.
40. The racquet of claim 39 wherein the at least one filler
material comprises at least a material selected from the group
consisting of: ceramics, metals, metal coated fillers, metallized
fillers, inorganic oxides, carbon, pigments, lubricants and
polymers.
41. The racquet of claim 37 wherein the low temperature adhesive
comprises at least a material selected from the group consisting of
urethane acrylates and cationic epoxies.
42. The racquet of claim 32 in which the composite is helically
wrapped around at least a portion of the string.
43. The racquet of claim 32 in which the composite is
longitudinally wrapped around at least a portion of the string.
44. The racquet of claim 34 wherein the polymeric membrane
comprises a fluoropolymer.
45. The racquet of claim 44 wherein the fluoropolymer is expanded
polytetrafluoroethylene.
46. The racquet of claim 32 in which the composite has a thickness
of less than about 5 percent of the string diameter.
47. The racquet of claim 32 in which the composite has a thickness
of less than about 3 percent of the string diameter.
48. The racquet of claim 32 in which the composite has a thickness
of less than about 1 percent of the string diameter.
49. The racquet of claim 32, wherein the string comprises natural
gut.
50. The racquet of claim 32 wherein the string is a
mono-filament.
51. The racquet of claim 32, wherein the string comprises a
plurality of filaments.
52. The racquet of claim 49, wherein said filaments are of
substantially the same diameter.
53. The racquet of claim 32, wherein the string comprises synthetic
fibers.
54. The racquet of claim 53, wherein the synthetic fibers comprise
polyamide.
55. The racquet of claim 54, wherein the synthetic fibers comprise
nylon.
56. The racquet of claim 53, wherein the synthetic fibers comprise
polyester.
57. The racquet string of claim 53, wherein the synthetic fibers
comprise PEEK.
58. A racquet comprising a frame and a string disposed within the
frame, the string having a diameter of less than about 1.34 mm and
having dynamic modulus of less than about 30 kN/m and a durability
of at least about 2200.
59. A racquet comprising a frame and a string disposed within the
frame, the string having a diameter of less than about 1.25 mm and
having dynamic modulus of less than about 30 kN/m and a durability
of at least about 500.
60. A racquet comprising a frame and a string disposed within the
frame, the string having a diameter of less than about 1.25 mm and
having dynamic modulus of less than about 30 kN/m and a durability
of at least about 1000.
61. A racquet comprising a frame and a string disposed within the
frame, the string having a diameter of less than about 1.20 mm and
having dynamic modulus of less than about 30 kN/m and a durability
of at least about 500.
62. A racquet comprising a frame and a string disposed within the
frame, the string having a diameter of less than about 1.20 mm and
having dynamic modulus of less than about 30 kN/m and a durability
of at least about 800.
63. A racquet string comprising: a) a core; and b) a composite
comprising a polymeric membrane having at least some porosity and
an adhesive disposed within at least some of said at least some
porosity, the composite covering at least a portion of the
core.
64. A racquet comprising: a) a frame; b) a string comprising a
core, the string disposed within the frame; and c) a composite
comprising a polymeric membrane having at least some porosity and
an adhesive disposed within said at least some porosity, the
composite covering at least a portion of the core.
65. The method of covering a racquet string comprising the steps
of: a) providing a racquet string; b) providing a polymeric
membrane having at least some porosity c) filling at least some of
said at least some porosity with an adhesive to form a composite;
and d) wrapping at least a portion of the string with the
composite.
66. A racquet string comprising: a) a string; and b) a composite
comprising: an expanded polytetrafluoroethylene membrane having at
least some porosity, an adhesive substantially filling said at
least some porosity, and a continuous adhesive layer disposed upon
at least one surface of said expanded polytetrafluoroethylene
membrane, said composite covering at least a portion of said
string.
67. A racquet string comprising: a) a string; and b) a composite
comprising an expanded polytetrafluoroethylene membrane having at
least some porosity, an adhesive substantially filling said at
least some porosity, and a discontinuous adhesive layer disposed
upon at least one surface of said expanded polytetrafluoroethylene
membrane, said composite covering at least a portion of said
string.
Description
BACKGROUND
[0001] The present invention relates to strings for sporting
applications, and particularly to strings for racquets such as
tennis racquets, badminton racquets, squash racquets, racquetball
racquets and the like.
[0002] Racquet strings must satisfy competing requirements. In a
tennis racquet, for example, the principal requirements are
playability and durability and it is difficult to satisfy both
requirements in a single racquet string type. String construction
and material selection has heretofore required a compromise between
acceptable playability and durability.
[0003] During play, particularly in tennis, the ball is usually hit
with some degree of spin. To generate spin, the strings are brushed
against the ball to impart a tangential force to it. This brushing
action causes the individual strings to slide over one another and
wear against each other. The rubbing action of one string against
another as well as the impact of the ball creates notches in the
strings at the inter-string contact point. These notches are the
primary reason for string breakage; as the notching becomes more
severe, the tensile strength of the string weakens and eventually
it breaks. The friction between the ball and the string during
contact with the string surface also causes some string wear.
[0004] Several materials have been used in racquet strings in order
to achieve a balance of durability and playability. One material,
natural gut, enjoys a reputation for unmatched playability.
Unfortunately, gut strings have a short life due to notching and
wear. Few recreational players use gut strings because they are
expensive and wear quickly. Many synthetic string materials, sizes
and constructions have been proposed as alternatives to gut. Such
synthetics generally are more durable than gut, but are not as
playable.
[0005] The most common synthetic material is nylon. Although more
modern fibers, such as PEEK and aramid fibers, such as Kevlar.RTM.
are used in racquet strings, nylon multi-filament strings are
generally accepted as among the most playable synthetic materials.
Nylon strings demonstrate improved durability over gut, but even
nylon strings are subject to frequent breakage by certain players,
particularly power hitters and those who hit the ball with a lot of
spin.
[0006] Nylon strings have been proposed in many mono-filament and
multi-filament constructions, the more durable strings being the
mono-filaments and the more playable strings being the
multi-filaments. Within the range of mono-filaments and
multi-filaments there are a variety of constructions that have been
used to either tailor the durability or the playability of the
string.
[0007] Coatings have been proposed to improve the abrasion
resistance of strings. For example, strings have been dipped or
coated with polytetrafluoroethylene in an attempt to reduce the
friction between strings, which causes notching. Other attempts
have included adding hard, abrasion resistant coatings to the
exterior of a string. Such coatings have generally failed because
they are inelastic and do not adhere well to the nylon surface as
the string stretches in use.
[0008] To improve the durability of nylon strings, the addition of
high strength fibers such as aramids to multi-filament
constructions has been proposed. However, the addition of stiff
aramid fibers to a string matrix dramatically reduces the
playability of the strings. Nomex, which has better elastic
properties than other aramids, has been added to the core of nylon
strings with some success with regards to durability, but with a
significant tradeoff with regard to playability.
[0009] An accepted measure of playabiltiy is dynamic modulus, which
is the ratio between the increase of tension and the elongation of
a string caused by dynamic impact. This is a measure of how stiff a
string is under dynamic conditions similar to that of being struck
with a tennis ball during play. To be playable, a racquet string
must show elastic properties under dynamic conditions and deform
under a given impact. Strings with low dynamic modulus are less
stiff and therefore have better playability than strings with a
high dynamic modulus, which do not stretch as much and therefore
feel stiff. Gut strings may have a dynamic modulus of as low as
17-26 kN/m. In contrast, high strength fibers such as Kevlar may
have a dynamic modulus of 88 kN/m to 140 kN/m or more. Nylon
strings have a dynamic modulus in a range of about 25 kN/m to about
45 kN/m.
[0010] Another factor affecting both durability and playabiltiy is
string size, or gauge. For example, a 16 gauge string generally has
a larger diameter than a 17 gauge string. Accordingly, the 16 gauge
string may last longer. But string size is critical to playability,
and thinner strings play better.
[0011] Higher gauge strings or thinner diameter strings play better
in part because they are more effective at imparting spin to a
ball, such as a tennis ball, because thin strings cut deeply into
the felt cover of a tennis ball, gripping it to impart the spin
necessary for player control. Thicker strings do not penetrate the
ball cover as deeply. Thinner strings also deflect more for a given
impact. This increase in deflection reduces the shock that the
player feels and returns more energy to the ball giving the player
more power. Furthermore, thick strings increase wind resistance to
racquet swing to a surprising degree.
[0012] While the dynamic modulus of an individual string is
indicative of its playability, racquets are actually strung with a
crossed pattern of strings called a string bed. The strings
extending from top to bottom of the racquet head are called the
main strings, while those crossing the racquet head are called the
cross strings. When the strings move within the string bed, the
main strings slide and rub against the cross strings. The resultant
friction between strings causes energy loss. This energy loss may
also affect playability.
SUMMARY
[0013] The present invention includes improved strings for racquets
and methods for making the same.
[0014] The string of the present invention may employ a
conventional string, such as a string having a center core
comprising gut or synthetic material such as nylon, and a polymer
cover impregnated with adhesive. The adhesive may be low
temperature adhesive. The polymer cover covers the string along at
least a portion of the length of the string. As the term "adhesive"
is used herein it is intended to mean a material that will form a
bond between the polymer cover and the base string. As the term
"low temperature adhesive" is used herein it is intended to
designate any adhesive that will either form a bond when processed
at a temperature less than about 300.degree. C. More preferably,
the low temperature adhesive comprises any adhesive that will
either cure or form a durable bond at less than about 275, 250,
225, 200, 175, 150, 125, 100, 75, 50, or 25.degree. C.
[0015] In one aspect, the invention provides a string, and a
composite comprising a polymeric membrane having at least some
porosity and an adhesive disposed within the at least some
porosity, the composite covering at least a portion of the
string.
[0016] In another aspect, the polymer cover has at least some
porosity. In another aspect of the invention, at least some of the
porosity is filled with an adhesive by applying the adhesive to one
or more surfaces of the polymer cover. In an alternative embodiment
of the invention, at least some of the porosity is filled, for
example, by imbibing or impregnating the porous polymer cover, with
adhesive.
[0017] In a still further aspect, the adhesive is applied to at
least one surface of the polymer cover and at least some of the
porosity is filled with an adhesive.
[0018] In an alternative embodiment of the invention, the adhesive
is a low temperature adhesive.
[0019] In yet another aspect of the invention, a suitable low
temperature adhesive can be applied to at least one surface of the
polymer cover and the low temperature adhesive may form a durable
bond between the string and cover material. In this aspect, the
adhesive may be continuous or discontinuous.
[0020] In order to provide the highest compatibility with a wide
variety of underlying string materials, it may be desirable to
provide an adhesive material that can be applied, and if necessary
cured, at or near room temperature, such as through use of pressure
sensitive adhesives, radiation curable adhesives, or the like.
Thus, in another aspect of the invention an adhesive is provided
that is cured through exposure to ultraviolet light (hereinafter
"UV" light) or an electron beam (hereinafter "EB").
[0021] In yet another aspect of the invention, the polymer cover
comprises ePTFE.
[0022] In another aspect, the composite has a thickness of less
than about 5% of the racquet string diameter. Preferably, the
composite has a thickness of less than about 3% of the racquet
string diameter. Most preferably, the composite has a thickness of
less than about 1% of the racquet string diameter.
[0023] In still another aspect, the string is of monofiliment
construction. Preferably, the string is of multifilament
construction. In this aspect, the filaments of substantially the
same diameter are preferred.
[0024] In another aspect, the racquet string of the present
invention includes a base string constructed of Nylon, PEEK or
gut.
[0025] In a still further aspect, the invention provides for an
adhesive comprising at least one filler material. In this aspect,
the filler material may be selected from the group consisting of
ceramics, metals, metal coated fillers, metallized fillers,
inorganic oxides, carbon, pigments, lubricants and polymers.
[0026] In another aspect, the adhesive comprises a urethane
acrylate or a cationic epoxy.
[0027] In yet another aspect of the invention, the cover is
helically wrapped around the base string.
[0028] In another aspect the invention is a racquet string having a
diameter of less than about 1.34 mm and having dynamic modulus of
less than about 30 kN/m and a durability of at least about
2200.
[0029] In a still further aspect, the invention is a racquet string
having a diameter of less than about 1.25 mm and having dynamic
modulus of less than about 30 kN/m and a durability of at least
about 500 and at least about 1000.
[0030] In a yet another aspect, the invention is a racquet string
having a diameter of less than about 1.20 mm and having dynamic
modulus of less than about 30 kN/m and a durability of at least
about 500 and at least about 800.
DESCRIPTION OF THE DRAWINGS
[0031] The operation of the present invention should become
apparent from the following description when considered in
conjunction with the accompanying drawings, in which:
[0032] FIG. 1 is a three-quarter perspective view of a racquet;
[0033] FIG. 2 is a schematic drawing of a porous film of the
invention wherein at least some of the porosity of the film is
filled with adhesive;
[0034] FIG. 3 is a schematic drawing of a porous film of the
invention wherein substantially all of the porosity of the film is
filled with adhesive;
[0035] FIG. 4 is a schematic drawing of a porous film of the
invention wherein at least some of the porosity of the film is
filled with adhesive and one surface of the film is provided with a
relatively thin layer of adhesive;
[0036] FIG. 5 is a schematic drawing of a porous film of the
invention wherein substantially all of the porosity of the film is
filled with adhesive and one surface of the film is provided with a
relatively thin layer of adhesive;
[0037] FIG. 6 is a schematic drawing of a porous film of the
invention wherein substantially all of the porosity of the film is
filled with adhesive and both surfaces of the film are provided
with a relatively thin layer of adhesive;
[0038] FIG. 7 is a schematic drawing of a porous film of the
invention wherein at least some of the porosity of the film is
filled with adhesive, but the adhesive is not coincident with the
surfaces of the film;
[0039] FIGS. 8a through 9b show string constructions according to
the invention.
[0040] FIG. 10 is a perspective view of the apparatus used to
determine string durability.
[0041] FIG. 11 is a schematic diagram of the apparatus used to
determine elastic modulus of strings.
[0042] FIG. 12 is a perspective view of the apparatus used to
determine the elastic modulus of strings.
DETAILED DESCRIPTION OF THE INVENTION
[0043] The present invention relates generally to improved racquet
strings.
[0044] The invention solves the problem of string durability
without diminishing the playability of the string. This is
accomplished by wrapping (or otherwise covering) at least a portion
of the string with a polymer cover The cover should be sufficiently
durable to withstand abrasion occasioned by ball impact and string
movement.
[0045] The polymer cover has at least some porosity. As used
herein, "porosity" refers to the property or state of a material
having voids or interstices. The cover may be impregnated with an
adhesive by applying adhesive to one or more surfaces of the
polymer cover. By utilizing a polymer cover comprising at least
some porosity, at least some of the porosity is filled with the
adhesive.
[0046] This novel construction uniquely combines the hardness of a
durable adhesive with the lubricity of a polymer having a low
coefficient of friction. A polymer string cover alone may not
provide adequate abrasion resistance; however, the inventors have
found that by filling at least some porosity of the polymer cover
with adhesive, abrasion resistance may be significantly improved,
while playability is retained. In this way, the porous polymer
provides a lubricious matrix that supports the highly abrasion
resistant adhesive. The abrasive resistant adhesive is bound within
this matrix and, despite its hardness, does not flake from the
flexible string. Moreover, the lubricious polymeric membrane
reduces friction between strings. Reducing friction at the
intersection of strings may further improve playability by
improving energy return, and further reduces string breakage by
inhibiting notches.
[0047] In an aspect of the invention, a suitable low temperature
adhesive can be applied to at least one surface of the polymer
cover and the low temperature adhesive may form a durable bond
between the string and cover material.
[0048] The porous polymer cover of the present invention improves
durability by providing a wear-resistant surface, but avoids the
problem of restricting elongation or movement of the string.
Moreover, by filling at least some, or substantially all, of the
porosity of the cover with adhesive, durability of the string is
further improved.
[0049] It has been discovered that the porous polymer can be
altered to withstand substantial wear and abrasion during use. Wear
and abrasion resistance can be improved by, for example, careful
selection of the adhesive used, the addition of certain filler
materials, as well as the amount of porosity filled with the
adhesive. Thus, by careful selection of adhesive type, amount of
adhesive used, and filler materials (if used), an extremely durable
and abrasion resistant cover can be fabricated to inhibit
notching.
[0050] The present invention also solves the problem of string
contamination. In applications such as tennis, grass and clay
courts in particular may expose the string to contaminants. The
polymer cover protects the core from abrasive contaminants that
contribute to premature wear, such as clay and silica which are
transferred from the court surface to the string by ball
impact.
[0051] Materials suitable for use in the porous polymer cover of
the present invention include, but are not limited to, the
following fluoropolymers: polytetrafluoroethylene (PTFE),
particularly porous expanded PTFE (ePTFE); fluorinated ethylene
propylene (FEP); polyethylene, including ultrahigh molecular weight
polyethylene; perfluoro alkoxy resin (PFA); polyurethane;
polypropylene; polyester; polyimide; and polyamide.
[0052] Although the invention includes use of any porous polymer
cover materials, particularly preferred are porous fluoropolymer
films, with PTFE and ePTFE being even more preferred. The porosity
of the porous polymer cover can be either partially or
substantially filled with adhesive. For example, a relatively small
amount of adhesive can be supplied to a select portion of the film
porosity, while leaving most of the porosity of the film unfilled.
In an aspect of the invention, adhesive can be evenly distributed
throughout the porosity of the cover from one side of the cover to
the other side, while still leaving at least some porosity
unfilled. Moreover, in a further aspect of the invention,
substantially all of the porosity of the film can be filled with
adhesive to perhaps result in better abrasion resistance and better
adhesion.
[0053] Turning to the figures, FIG. 1 shows a tennis racquet with
main strings (20) and cross strings (22). FIG. 2 illustrates a
porous cover material 1, where at least some of the porosity 2 is
filled with adhesive 3. FIG. 3 illustrates a porous cover where
substantially all of the porosity 2 is filled with adhesive 3. FIG.
4 illustrates an aspect of the invention wherein at least some of
the porosity 2 is filled with adhesive 3 and an additional surface
layer of adhesive 4 is supplied to one surface of the film. FIG. 5
illustrates an aspect of the invention where substantially all of
the porosity 2 has been filled with adhesive 3 and an additional
surface layer of adhesive 4 is supplied to one surface of the film.
FIG. 6 illustrates an aspect of the invention where substantially
all of the porosity 2 has been filled with adhesive 3 and both
surfaces of the cover are supplied with a surface layer of adhesive
4 and 5. FIG. 7 illustrates an embodiment in which some of the
porosity 2 is filled with adhesive 2 and both sides of the film are
supplied with a surface layer of adhesive. Although covers with any
amount of porosity may be used, preferably the cover has a bulk
density about 0.7 g/cc, before filling with adhesive.
[0054] A preferred cover material is a porous fluoropolymer
material such as uniaxially expanded PTFE. This material has
demonstrated exceptional durability without affecting the
playability of the base string. Porous expanded PTFE, such as that
made in accordance with U.S. Pat. Nos. 3,953,566; 3,962,153;
4,096,227; and 4,187,390, comprises a porous network of polymeric
nodes and interconnecting fibrils. These kinds of material are
commercially available in a variety of forms from W. L. Gore &
Associates, Inc., Newark, Del.
[0055] Expanded PTFE is formed when PTFE is heated and rapidly
expanded by stretching in at least one direction in the manner
described in the above listed patents. The resulting expanded PTFE
material achieves a number of exceptional properties, including
exceptional strength in the direction of expansion, and
exceptionally high flexibility, and conformability. The strength
properties in both the longitudinal and transverse directions of
the ePTFE may be altered in the expansion process, or by other
means known in the art to achieve the desired effect or
property.
[0056] As the term "expanded PTFE" is used herein, it is intended
to include any PTFE material having a node and fibril structure,
including in the range from a slightly expanded structure having
fibrils extending from relatively large nodes of polymeric
material, to an extremely expanded structure having very long
fibrils interconnected by small nodes. The fibrillar character of
the structure is identified by microscopy. While the nodes may
easily be identified for some structures, many extremely expanded
structures consist almost exclusively of fibrils with very small
nodes.
[0057] When a porous polymer cover material is used, at least some,
or substantially all, of the porosity of the porous polymer cover
can be filled with adhesive. Additionally, adhesive can also be
provided as a continuous or discontinuous coating on one or both
sides of the cover. As used herein, "discontinuous" means that the
adhesive does not fully cover the surface of the underlying cover.
"Continuous" means without holes or gaps extending through the
adhesive coating (i.e. fully covering the surface of the underlying
cover). The exact amount of adhesive used depends upon a number of
variables. For example, adding more adhesive may further improve
durability and abrasion resistance, but may also increase string
mass, which may affect playability. Providing less adhesive may
result in less durability and reduced abrasion resistance. However,
less adhesive may tend to preserve playability. Once the cover has
been impregnated or otherwise filled with adhesive, the preferred
percent mass of adhesive to ePTFE is 45%.
[0058] In order to provide the highest compatibility with a wide
variety of underlying string materials, it may be desirable to
provide a low temperature adhesive that can be applied, and if
necessary cured, at or near room temperature, such as through use
of pressure sensitive adhesives or radiation curable adhesives, or
the like.
[0059] Low temperature adhesives include any adhesive that will
either cure or form a durable bond when processed at a temperature
of less than about 300.degree. C. Suitable low temperature
adhesives include any suitable thermoset resin. For example,
suitable thermoset resins include epoxies (including acrylated
epoxies), polyurethanes, phenolics, and other thermosets. Suitable
thermoplastic resins include, for example, polyethylene,
polypropylene, polystyrene, polyvinyl chloride, polyurethanes, and
fluoropolymers such as THV (tetrafluoroethylene,
hexafluoropropylene, and vinylide fluoride), HTE
(hexafluoropropylene, tetrafluoroethylene, and ethylene), EFEP
(ethylene tetra fluoro ethylene based copolymer), ETFE (ethylene
tetrafluoroethylene), and PVDF (polyvinylidine fluoride), and
blends thereof. Other thermoplastic resins are also useful,
provided that they are processable at temperatures of less than
about 300 C.
[0060] Thermally activated adhesives which can cure or form a
durable bond when the adhesive is heated, such as THV 220
(tetrafluoroethylene, hexafluoropropylene, and vinylide fluoride,
available from Dyneon, LLC) and adhesives which can be caused to
cure through chemical reaction, such as known moisture cure
adhesives (e.g., polyurethane prepolymers, etc.) or other
chemically activated adhesives, can also be used.
[0061] In a preferred embodiment, the low temperature adhesive
comprises UV-curable adhesive. As used herein, UV-curable is
defined as a material that will react under UV light to cure or
form a durable bond. The UV light is provided by a lamp with
suitable spectral intensity, spectral dosage and wavelength. Those
of skill in the art will appreciate that curing with UV light may
be carried out at various rates, and that the distance between the
sample being cured and the UV lamp can be varied, provided the
appropriate spectral dosage is applied. In an aspect of the
invention, the UV curable material can also be sensitive to visible
light. However, preferred conditions are present only under the UV
spectrum (100-400 nm). In this range, the underlying core material
will not be damaged during the processing of the string. Suitable
UV-curable adhesives include, but are not limited to, epoxies,
acrylated epoxies, acrylated urethanes, acrylated silicones,
acrylated polyethers, acrylated polyester, acrylated polybutadiene,
and acrylated fluoropolymers. Specific examples of these adhesives
include acrylated aliphatic oligomers, acrylated aromatic
oligomers, acrylated epoxy monomers, acrylated epoxy oligomers,
aliphatic epoxy acrylates, aliphatic urethane acrylates, aliphatic
urethane methacrylates, allyl methacrylate, amine-modified
oligoether acrylates, amine-modified polyether acrylates, aromatic
acid acrylate, aromatic epoxy acrylates, aromatic urethane
methacrylates, butylene glycol acrylate, stearyl acrylate,
cycloaliphatic epoxides, cylcohexyl methacrylate, ethylene glycol
dimethacrylate, epoxy methacrylates, epoxy soy bean acrylates,
glycidyl methacrylate, hexanediol dimethacrylate, isodecyl
acrylate, isooctyl acrylate, oligoether acrylates, polybutadiene
diacrylate, polyester acrylate monomers, polyester acrylate
oligomers, polyethylene glycol dimethacrylate, stearyl
methacrylate, triethylene glycol diacetate, and vinyl ethers.
Preferred UV-curable adhesives include, for example, urethane
acrylates and cationic epoxies.
[0062] It may be desirable to utilize a solvent to aid in providing
adhesive to the porosity of the porous polymer cover. The ratio of
solvent material to adhesive can vary and will also be readily
determinable by the skilled artisan. Preferable solvent materials
will also be apparent to one skilled in the art and include, for
example, alcohols, ketones, etc. A preferred solvent is isopropyl
alcohol (IPA). When a solvent material is utilized, the solvent
material can be easily removed or driven off once the adhesive is
provided to at least some of the porosity of the porous polymer
cover as desired.
[0063] In a further aspect of the invention, the low temperature
adhesive can be combined (e.g., mixed, blended, etc.) with a
suitable filler material. Suitable filler materials may include,
but are not limited to, ceramics, metals, inorganic oxides, metal
coated materials, metallized materials, carbon, pigments and
polymers, which can be provided in any suitable form (e.g.,
particulates, fibers, etc.) Preferably, fillers are in nanoparticle
size. Filler materials may be desirable to alter certain properties
of the covered string (e.g., to improve abrasion resistance, or to
provide color, etc.). Use of solvent may be particularly useful
when at least partially filling the porosity of a porous cover with
an adhesive/filler material combination.
[0064] The adhesive may be applied to the cover by a variety of
methods known in the art. With regard to porous polymer covers,
suitable adhesive application means include, for example, coating
techniques (e.g., dip coating or spray coating), solvent imbibing,
vacuum assisted coating, pressure assisted coating, nip coating,
and other suitable means which would result in the adhesive filling
at least some of the porosity of the porous polymer cover.
[0065] As stated above, a preferred porous polymer cover is
expanded PTFE. At least a portion of the porosity of the expanded
PTFE is filled with low temperature adhesive. In an aspect of the
invention, substantially all of the porosity of the expanded PTFE
film is filled with low temperature adhesive. Furthermore, one or
more surfaces of the expanded PTFE may be provided with a
relatively thin surface layer of low temperature adhesive for
bonding the cover to the base string. Such surface layer(s) of
adhesive can be either continuous or discontinuous. In a preferred
embodiment the surface layer(s) of adhesive is a continuous layer.
Preferably, the film is impregnated with an adhesive/solvent
solution, thus allowing good penetration of the adhesive into the
porosity of the film. Impregnating is accomplished by first
preparing an adhesive/solvent solution, and second, combining this
solution with a porous film like expanded PTFE. Solvents such as
alcohols and ketones are capable of dissolving adhesives so that
the adhesive can penetrate and occupy the porosity of the porous
film. There are many suitable adhesives (e.g., urethanes, epoxies,
etc.) that can be dissolved in suitable solvents. In an aspect of
the invention, the adhesive is UV-curable urethane-acrylate. This
adhesive will also cure by other mechanisms such as through heating
and chemical reaction.
[0066] The mass of adhesive delivered to the expanded PTFE film (or
other polymer cover material) can be regulated by the solvent to
adhesive ratio in the solvent/adhesive solution and by the rate at
which it is applied. A spreading mechanism can be used to
distribute the adhesive/solvent solution after it contacts the film
surface. Once the film has accepted the adhesive/solvent solution,
or becomes impregnated, the mechanical characteristics of the film
can change and it may have the tendency to shrink. In order to
stabilize the film, a suitable liner can be provided to the film
following this step. An example of a suitable liner material is
polyester release film. Another suitable liner material may be a
silicone-coated paper. In any event, both the liner and the film
can be contacted together and placed into a forced air oven. The
heated air can be blown across the flat side of the film oriented
with the non-liner side toward the air stream. This drives off the
solvent and leaves the adhesive within the porosity of the film.
The film can be removed from the liner before applying the film to
the string.
[0067] Once the low temperature adhesive has been provided to at
least one surface of the polymer cover, and the low temperature
adhesive has at least partially filled, or is otherwise provided
to, the porosity of the cover (and the solvent driven off, if a
solvent is used), the cover can then be placed in contact with the
string and the low temperature adhesive can then be cured.
[0068] The cover of the present invention may be applied in a
variety of manners while maintaining the benefits of the present
invention. The cover may be wrapped longitudinally (in a "cigarette
wrap" manner), or as a continuous and seamless tube surrounding the
string. Preferably, the string is helically wrapped with a cover
material. In this embodiment, the string may be provided with a
cover in the form of a wrapped polymer layer having overlapping
edges to form a continuous cover or with non-overlapping edges. The
polymer layer may optionally be heated to thermally bond the
overlapped edges together. The cover may or may not include an
adhesive coating on its outwardly facing surface. The adhesive
coating serves to adhere the wraps to the base string and may also
provide an additional protective layer to shield the cover from
wear and contamination.
[0069] Although particularly preferred base string materials
include gut or nylon materials, cores comprising other synthetic
materials or aramid fibers may also benefit from the use of covers
made and applied in accordance with the present invention. However,
the covers are particularly attractive when used in combination
with highly playable strings, such as nylon or gut strings.
Although gut and nylon are typical materials for strings, another
preferred material for the string of the invention is PEEK. PEEK
strings may provide better durability than nylon and demonstrate
acceptable playability.
[0070] Regardless of the type of base string, once the string is
provided with the cover, the adhesive can be cured to result in the
covered string of the invention.
[0071] The particular curing mechanism used, such as heat, UV/EB
radiation, and chemical reaction, will depend on the type of
adhesive used. One preferred adhesive is urethane-acrylate, which
is capable of curing via heating and/or UV radiation. The preferred
mechanism for curing this adhesive on a gut or synthetic string is
UV radiation because of its relatively low temperature
application.
[0072] As discussed above, prolonged high temperature processes can
degrade the performance of strings with gut or synthetic components
by compromising the properties of the materials therein. Degraded
performance may be observed as a reduction of durability or an
increase in dynamic modulus. It is therefore desirable to process
strings at temperatures that do not change string performance.
Thus, in an aspect of the invention preferred low temperature
adhesives include adhesives that bond or can be cured at a
temperature of about 150.degree. C. or less and, in a further
aspect of the invention, at a temperature of about 120.degree. C.
or less. More preferably, the low temperature adhesive comprises
any adhesive that will either cure or form a durable bond at less
than about 100, 75, 50, or 25.degree. C.
[0073] To cure the adhesive by UV/EB radiation, the covered string
can be placed in tension to keep the covered string straight.
Important parameters for the UV curing process are spectral
intensity of UV light, measured by Wafts/cm.sup.2, and spectral
dosage of UV light, measured by Joules/cm.sup.2. The preferred
light intensity, wavelength and dosage depend upon the selection of
photoinitiators and the formulation of the adhesive blend, and are
readily determined by one of skill in the art. Upon exiting the UV
oven, the string should have a tack free surface, indicating that
the adhesive has cured.
[0074] In an aspect of the invention, a single layer of expanded
PTFE, having been stretched in the longitudinal and transverse
directions and impregnated with adhesive, is provided to the base
string. This is accomplished by helically wrapping the string at a
pitch angle measured from the end of the string. This construction
is believed to provide excellent strength and durability while
maintaining the playability of the base string.
[0075] Without intending to limit the scope of the present
invention, the following examples illustrate how the present
invention may be made and used:
EXAMPLES
Example 1
[0076] An example of a string according to the present invention
was prepared by helically wrapping a 1.19 mm diameter
multi-filament nylon string that was obtained from Prince Mfg. Co.
with a polymer film impregnated with UV-curable adhesive. The
string was made in the following manner:
[0077] Expanded PTFE film with a thickness of about 0.015 mm was
obtained from WL Gore and Associates, Inc., Newark, Del. The
expanded PTFE film had a bulk density of 0.7 g/cc, and was further
characterized by a matrix tensile strength of about 41,000 psi in
the longitudinal direction and a Bubble Point of 68 psi.
[0078] A 30 wt. % adhesive solution was prepared in isopropyl
alcohol for impregnating the expanded PTFE film. The adhesive
composition is 60 wt. % aliphatic polyester based urethane
diacrylate oligomer blended with ethoxylated trimethylol propane
triacrylate (available from Sartomer Company, Exton, Pa. as
CN963E75), 32 wt. % triacrylate acid ester (available from Sartomer
Company as CD9052), and 8 wt. % Genocure DMHA, available from Rahn
USA Corp., Aurora, Ind. This solvent-adhesive solution was
dispensed and spread evenly across the expanded PTFE film. A
polyester release film grade UV5010 was used as a liner and
combined with the film as the solvent-adhesive solution penetrated
the expanded PTFE film. Both the liner and impregnated film were
sent through an oven (set at about 120.degree. C.) to drive off the
solvent. The film was removed from the oven and a substantially
fully impregnated structure with adhesive coincident with both
surfaces of the film and a thin surface coat of adhesive present on
the liner side was recovered. The thin surface coat substantially
covered the expanded PTFE surface.
[0079] The 3.56 mm wide impregnated film was wrapped in a
non-overlapping helical fashion around the base string at a pitch
angle of 32 degrees while leaving little or no gap between the film
layers contacted. The resultant construction was a string with a
single layer of impregnated film covering the entire length of the
string.
[0080] The covered string was fed through a 300 Watt F300S
Electrode-less UV Lamp System provided by Fusion UV Systems, Inc.,
Gaithersburg, Md. The UV lamp was equipped with an H-bulb and F6
light shield for wire/cable applications with 360.degree.
reflection. UV dosage to cure the adhesive was controlled by the
line speed, which was set to 20 ft/min. Prior to inserting the
string, the UV oven was purged with nitrogen to remove oxygen from
the oven.
[0081] Once each string exited the UV lamp system, it was observed
to have a tack-free surface, indicating that the impregnated
adhesive had cured. It was further noted that the cover conformed
to the string. The covered string diameter was 1.24 mm.
[0082] The string was installed in a racquet and was found to have
excellent playability (that is, the playability was at least equal
to that of comparable diameter multifilament nylon strings as
measured by the dynamic modulus). During play, the strings felt
smoother and did not require repositioning as frequently as an
uncovered string.
[0083] Moreover, the durability was significantly improved. During
play tests, the strings exhibited noticeably less notching at
string contact points. The covered string was also tested on a
durability tester, and the durability was reported in Table 1
below. The inventive strings show significant durability
improvement over a comparable diameter nylon string, without
increasing the dynamic modulus.
Example 2
[0084] A second, slightly smaller example string was made according
to the present invention by helically wrapping a 1.13 mm diameter
multi-filament nylon string that was obtained from Prince Mfg. Co.
with a polymer film impregnated with UV-curable adhesive. The
string was made in the manner described above in Example 1 by
wrapping the base string with the same ePTFE film impregnated with
a low temperature adhesive used in Example 1.
[0085] The final string diameter was 1.18 mm. As reflected in Table
1, the inventive string shows much better durability than a
comparable diameter nylon string, and a similar dynamic
modulus.
Test Methods
Durability
[0086] The durability test apparatus is depicted in FIG. 10. Tennis
balls were alternatively fired at 60 MPH from two ball machines 15,
15' such that the balls contacted a simulated racquet frame 17 at a
rate of one every 4 seconds. Ball speed was measured at each ball
machine using laser speed recording equipment. The simulated
racquet string bed 30 was constructed of an 83/4''.times.111/2''
rectangular aluminum frame. The string bed was strung with a
pattern of 16 main strings 32 and 18 cross strings 34. Delrin.RTM.
grommets (not shown) were used to reduce string wear at the frame.
The frame was strung at a tension of 58 pounds. The frame was
placed perpendicular to the ground such that the discharges 36 of
the ball machines were both 25'' from the center of the sting bed.
The balls traveled along flight path 38 and contacted the string
bed at a horizontal angle of 60.degree. and an upward vertical
angle of 15.degree.. This was intended to simulate top-spin
action.
[0087] The test was conducted at an ambient temperature of
20.degree. C. Ten new Tretorn Micro X 90 balls were loaded into the
ball machines. As each ball machine alternately fired the balls at
the string bed, the main strings moved back and forth against the
cross strings. After impact, the balls were continuously fed back
into the ball machines. The balls were continuously fired from both
machines until a string broke. Durability was measured and recorded
as the number of impacts at which the string broke.
Dynamic Modulus
[0088] An apparatus for testing the dynamic modulus of a string is
depicted in FIG. 12 and schematically illustrated in FIG. 11. A
string sample 40 was held horizontally in metal clamps 42, 42'
spaced 340 mm (12.6 in) apart. Two metal bars 44, 44' were
positioned between the clamps just contacting the string to support
the string. The bar centerline spacing was 300 mm. The string was
tensioned at 28 kg. The test was conducted at an ambient
temperature of 20.degree. C.
[0089] A pendulum 46 was swung into the string to contact the
string at the centerpoint of the span between the support bars. The
pendulum includes a 0.8'' (20.3 mm) flat head hammer face 48, which
makes contact with the string. The pendulum weighs 720 g, and its
barycenter is 450 mm from the rotation point. At impact, the
angular speed is 5.35 rad/s, resulting in a hammer speed of 3.18
m/s.
[0090] When the hammer face hits the string, the maximum deflection
of the center point of the string was measured using laser
measurement equipment. The maximum tension increase in the string
was also monitored using a load cell attached to one end of the
string. From the maximum deflection (D.sub.max) and span
L.sub.Orig, the total lengthwise stretch (.DELTA.L) was calculated
according to the formula: .DELTA.L=L.sub.Max-L.sub.Orig where
L.sub.orig is the original string length and Lmax is the maximum
string length. Maximum string length is determined by the equation:
L Max = 2 ( L Orig 2 ) 2 + D Max 2 ##EQU1## Where D.sub.max is the
maximum deflection The dynamic modulus, k, may be calculated by
dividing the maximum change in string tension (.DELTA.T) as
measured by the load cell by the total lengthwise stretch
(.DELTA.L) at impact. Dynamic modulus has units of kN/m. Bubble
Point
[0091] The Bubble Point test provides an estimation of maximum pore
size. Liquids with surface free energies less than that of
stretched porous PTFE can be forced out of the structure with the
application of a differential pressure. This clearing will occur
from the largest passageways first. A passageway is then created
through which bulk air flow can take place. The air flow appears as
a steady stream of small bubbles through the liquid layer on top of
the sample. The pressure at which the first bulk air flow takes
place is called the bubble point and is dependent on the surface
tension of the test fluid and the size of the largest opening.
[0092] The Bubble Point is measured using the procedures of ASTM
F316-86 as guideline. Isopropanol was used as the wetting fluid to
fill the pores of the test specimen. The test sample is placed in a
filter holder (available from Millipore Corporation, Billerica,
Mass.), covered with a support screen and the locking ring of the
holder attached. The top of the holder is then filled with
isopropanol, and the holder is attached to an air supply with a
regulated control valve. The holder is placed under a magnifying
lens with a light and the air pressure is increased until a
continuous stream of bubbles is seen coming through the support
screen covered with isopropanol.
[0093] The Bubble Point is the pressure of air required to displace
the isopropanol from the largest pores of the test specimen and
create the first continuous stream of bubbles detectable by their
rise through a layer of isopropanol covering the porous media.
Matrix Tensile Strength
[0094] Tensile strength of ePTFE materials including ePTFE films is
measured using an INSTRON tensile testing machine with pneumatic
cord and yarn grip jaws. The machine tested 0.25 inch wide samples
using a 1 inch jaw separation distance and a crosshead speed of 10
inches/minute. Matrix tensile strength of porous PTFE samples is
determined by the formula: (2.2 g/cc.times.tensile
strength)/density of tested material, where 2.2 g/cc is taken
to
[0095] be the density of non-porous PTFE. TABLE-US-00001 TABLE 1
Durability Dynamic Modulus String Diameter (Impacts) (kN/m)
Comparative 1.24 mm 398 29.66 Example 1 Inventive 1.24 mm 1163
25.65 Example 1 Comparative 1.19 mm 344 25.76 Example 2 Inventive
1.18 mm 910 24.99 Example 2
* * * * *