Note: Descriptions are shown in the official language in which they were submitted.
W O 96110478 PCT/GB95/02325
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' METHOD AND APPARATUS FOR FORMING ELONGATE PTFE MATERIAL
AND PTFE MATERIAL PARTICULARLY DENTAL FLO~~
' TECHNICAL FIELD
This invention relates to a method for forming
an elongate PTFE material, and also to a PTFE material.
The PTFE material of the invention has many possible
uses, for example in or as dental floss or other dental
care articles, and as a thread or yarn for sewing or
weaving, e.g. to make woven filters, and as a suture.
Application of the material as dental floss, or in
dental flossing devices, is described more below.
BACKGROUND ART
The improvement of the strength properties of
PTFE in the form of sheet, tape or rod by processes
involving heating and stretching is well known.
Particularly important has been the development of the
process of "expansion" of PTFE by W L Gore &
Associates, Inc. (Gore), which involves stretching an
unsintered PTFE article at a high rate of stretch at an
elevated temperature, to cause expansion (i.e. density
reduction, with increase of porosity) of the PTFE. The
isothermal expansion stage is followed by increase of
temperature, with the material retained in its expanded
state, to at least 327°C which effects the phenomenon
known as amorphous locking or node-locking, by the
formation of amorphous regions in the PTFE which appear
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to lock together the fibrils of PTFE. This process is
described for example in Gore's US Patents 3953566 and
3962153, where examples are given of biaxial and ,
uniaxial stretching of films, and uniaxial stretching
of rods. Sheets having two-dimensional strength, i.e.
substantially equal tensile strengths in two
perpendicular directions, are subjected only to biaxial
stretching.
Non-contact heating of an extruded PTFE article
while it is being stretched is also disclosed in GB-A-
2025835.
A different process, involving sintering of the
PTFE before stretching is described in EP-A-391887.
GB-A-1525980 describes a process of peeling a curtain
of threads from a sintered PTFE block, and stretching
the thread curtain while it passes over a curved heated
surface. The threads are then brought together to. form
a cable or yarn. The individual threads are very fine.
It is well known that unsintered PTFE articles,
such as extruded tape, differ much in their behaviour
in heat treatment from thermoplastic polyolefins such
as polyethylene. For completeness of prior art
disclosure in relation to the invention to be described
below, there is also mentioned GB-A-1287874 which ,
describes a process of producing fibres from a polymer
film in which the film is fed to a combined slitting
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and stretching zone in which apparently the cutting
device is heated. The cut fibres are stretched by
taking them up from the cutting tool at a rate faster
than the film feed rate_ After stretching, the fibres
pass over a curved guide surface, which may be heated.
Although PTFE is mentioned, the only detailed example
employs polypropylene film.
In GB-A-1541681, an orientable polymer strip is
passed over a heated fixed metal tube, to provide a
limited heating region in a stretching zone. PTFE is
mentioned, but the examples disclose use of
polyethylene, and the aim of the technique is to
minimize the width reduction occurring on stretchin;:,
while achieving high orientation of the polymer.
GB-A-1124109 discloses passing a polyolef~:.
body over a straight pressure edge, which is heater,
and stretching it so that a necking shoulde:- forms
the pressure edge. Biaxially oriented ribbons and
tapes are obtained. Polyethylene and polypropylene ar
employed in the examples.
The application of expanded PTFE material as
dental floss is described in US-A-4776358, in which
material obtainable from Gore in the form of tape is
~ used to form an elongate envelope containing cleaning
material. The exterior surface of this PTFE envelope
is not coated.
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A second dental floss made of PTFE is described in
EP-A-335466. Expanded PTFE of a particular strength
produced by the Gore expansion process is coated with
microcrystalline wax.
Another dental floss based on PTFE not made by the Gore
expansion process is described in WO 92/10978 (Westone).
The use of PTFE in dental floss is also mentioned
briefly in US-A-4836226.
It is desirable to employ, in a dental floss, a PTFE
material which in use is resistant to fibrillation, i.e. the
separation of fibrils from the main body of the material.
DISCLOSURE OF THE INVENTION
The present invention seeks to provide a method for the
formation of an elongate PTFE material, which can be simple
and employ inexpensive apparatus, while at the same time
allowing selection of properties of the PTFE product.
Particularly, it is desired to provide a PTFE product which
reduces or avoids fibrillation in use. The product can have
many uses, and can be particularly suitable for use as a
dental floss, and the process can produce an elongated PTFE
article of suitable dimensions for use as dental floss,
avoiding the need to slit the material longitudinally after
elongation. This avoids any defects which may be
WO 96110478 PCT/GS95/023Z5
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introduced by slitting after elongation.
According to the invention in one aspect there
is provided a method of forming an elongate PTFE
material, comprising passing an unsintered PTFE tape
across a heated surface in sliding contact therewith
while applying tension to the tape, wherein the
temperature of the heated surface, the passage speed of
the tape and the tension applied are such that the PTFE
tape, when its temperature is raised by contact with
the heated surface, is longitudinally stretched (i.e.
stretched in its direction of movement) with
simultaneous width and thickness reduction while in
contact with the surface. This longitudinal stretching
takes place at least partly when the tape is in contact
with the surface, but some stretching and width and
thickness reduction may continue after the contact
ceases. Since the tape is usually tensioned, there
also may be a small amount of stretching prior to
contact with the surface.
The PTFE tape may be any unsintered tape
susceptible to stretching under the conditions of the
method. Preferred'is an extruded tape as described
below. By the term unsintered tape, we include
~ unsintered tapes substantially unchanged in sintering
state after extrusion and also tapes having some degree
of sintering but which are capable of being
WO 96/10478 , PCT/GB95/02325
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longitudinally stretched in the method of the
invention.
Preferably the method includes the step of
effecting node-locking (amorphous locking) of the PTFE
of at least part of the tape after longitudinal
stretching. This node-locking step preferably takes
place while the PTFE tape is in contact with the heated
surface, but may take place szzbsequently. Heat may be
applied subsequently to effect node-locking and/or
sintering, e.g. by a further heater. Preferably the
contact surface is the sole source of heat for the
stretching step. When node-locking takes place in
contact with the heated surface, it follows immediately
after the longitudinal stretching, and it may be that
these two steps are to some extent combined. The node-
locking is typically effected by raising the
temperature of the PTFE to its melting or sintering
temperature range, i.e. to at least 327°C and
preferably to at least 346°C. After the node-locking,
the PTFE element generally undergoes substantially no
further longitudinal. stretching.
Preferably the contact path of the PTFE tape on
the heated surface is convexly curved but a flat
surface may in principle be used. Preferably; this
heated surface is part-cylindrical, the PTFE tape
preferably passing.circumferentially over the surface
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in a plane perpendicular to the axis.
The radius of curvature of .the contact path of
the PTFE tape on the heated surface is preferably in
the range 100 cm to 0.3 cm.
~'he temperature of the heated surface can be
selected within a wide range, preferably 35 to 550°C,
more preferably 200 to 500°C. To achieve node-locking
by the heating applied by the heated surface, its
temperature should be at least 320°C.
In the process of the invention, the PTFE tape
is heated mainly or entirely conductively by its one-
side contact with the heated surface, so that its
temperature increases as it passes across the surface.
At a certain point, the PTFE reaches a temperature such
that, at the tension applied at that point, it extends
longitudinally, with simultaneous width and thickness
reduction. This temperature is below the melting or
sintering temperature of PTFE. The thickness reduction
progressively brings the bulk of the material in the
cross-section, on average, closer to the heated
surface, so that the temperature distribution across
the cross-section may be reduced. On the other hand,
the material is now moving faster, due to its
longitudinal stretching. Overall, the effect is a
simultaneous width and thickness reduction with
longitudinal extension, i.e. a necking down of the
WO 96/10478 PCT/GB95102325
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material-, at least partly while in contact with the
heated surface.
It can be seen that this process is not ,
isothermal, in that the temperature of the tape is
progressively rising as it moves across the heated
surface and is in most or. all cases non-uniform across
the thickness of the material due to the one-side
contact, and furthermore the material is accelerating
as it passes across the surface, due to the
longitudinal stretching. Contact time with the heated
surface may vary widely depending on the particular
process and product, but for example is in the range of
0.5 to 10 seconds. The length extension (stretching
ratio) can therefore be selected, and can be very high.
Preferably in the invention, the extension (extended
length/original length) is at least 10, and may be in
the range 20 to 100, but it may be higher. In general,
the higher the length extension, the higher the tensile
strength of the extended material_
The width reduction of the tape occurring in
the stretching is preferably at least 50%, more
preferably at least 60o and most preferably at least
700.
As mentioned above, the node-locking or
amorphous locking preferably takes place during the
contact of the PTFE tape with the heated surface.
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Particularly in this case, the process of the present
inventions appears to be self-regulating. It is
surprising that the necking down of PTFE tape, as the
temperature rises, does not lead rapidly to its
rupture. Without wishing to be bound by theory, the
present applicants believe that uncontrolled extension
ofthe tape to rupture is prevented or controlled by
one or more of (i) node-locking, (ii) sintering, (iii)
cooling of the tape as it leaves the contact with the
heated surface and (iv) work hardening of the tape by
virtue of the stretching process.
It is, of course, possible to cause rupture of
the PTFE tape, for example if the tension is too high,
or the passage speed too slow so that the temperature
rises too high, or the temperture is too low to permit
the desired processes to occur. However, it has been
found in general relatively easy empirically to
establish conditions at which a particular desired
process of the invention operates, and to select those
conditions to achieve the desired extension (extended
length/original length). It has not been necessary to
monitor the tension applied. Indeed the exact value of
the applied tension may not easily be obtained, because
of friction losses at the contact between the PTFE tape
and the heated surface, and for example the friction
losses in the drive of a take-up roller. Rather, the
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process can be controlled by selection of appropriate
values, within wide ranges, of the temperature of the
heated surface, the contact length, the let-off speed .
and the take-up speed of the tape (i.e. input speed to
the stretching step and output speed from the
stretching step) and by selection of the strength of
the starting PTFE tape in one or both of the
longitudinal and transverse directions. The
longitudinal and transverse strengths of the starting
PTFE tape, and the ratio between them, also affect the
selection of the process parameters and the properties,
and dimensions of the stretched product.
The magnitude of the cross-sectional change,
i.e. necking ratio or change of cross-sectional area,
depends also on the ratio of the transverse to the
longitudinal strength of the starting tape.
The starting tape (precursor tape) may have any
suitable cross-section, and has a width greater than
its thickness. Preferably this tape has a flat face
which is the face which contacts the heated surface.
The width is preferably at least twice its thickness.
The thickness is preferably less than 5 mm, more
preferably less than I mm.
As already mentioned, control of product
dimensions (thickness and width) can be achieved by
selection of transverse/longitudinal tensile strength
WO 9611047 PCT/GB95/02325
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ratio (which will be referred to below as "strength
ratio"). Preferred for use in the invention are PTFE
tapes in which the longitudinal tensile strength is not
much greater than the transverse tensile strength.
Such a material can be produced by selection of a
suitable extrusion process, optionally with
calendering, in a known manner and may be a material in
which the fibrils produced by shear in the extrusion
process are, to a significant extent, transversely
aligned, i.e. transverse to the length direction. The
strength ratio is the ratio of tensile strength in the
transverse and to that in the longitudinal direction.
Preferably the material used has a ratio of transverse
tensile strength to longitudinal tensile strength of at
least 0.2, e.g. more than 0.5. This ratio may even be
greater than 1Ø There is in principle no upper limit
on this ratio, and it may be, for example, up to 3.
Such materials can be contrasted with~those subjected
to uniaxial extension in the Gore expansion process as
described, for example, in US Patent 3962153, which are
generally very much weaker in the transverse direction
than in the longitudinal direction. The material of
high strength ratio preferred for use in the invention
becomes more uniaxially oriented in the stretching step
of the invention, so that its fibrils become uniaxially
aligned to a greater extent. Thus there is a
WO 96110478 PCTlGB95/02325
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substantial gain in longitudinal strength, but there
may also be a loss of biaxiality i.e. a decrease of the
strength ratio. This is in contrast to some prior art ,
processes for polyolefin treatment in which it is
attempted to. retain biaxiality, i.e. to maintain
transverse orientation.
In the invention, the change of cross-sectional
area results from the longitudinal extension or
stretching of the PTFE tape. Depending on the starting
material and the process conditions, this longitudinal
extension may be accompanied by an increase of porosity
and reduction of density, or the porosity and density
may be maintained substantially constant.
In another aspect the invention can be defined
as a method of adapting the properties of an elongate
unsintered PTFE tape, preferably an extruded tape,
comprising uniaxially extending the tape longitudinally
with simultaneous width and thickness reduction, while
it is maintained in sliding contact with a heating
surface which effects heating of the tape.
Particularly preferred is a process where the tape has
a high strength ratio as discussed above. Also
preferred is node-locking (amorphous locking), while
the tape is in contact with the heating surface or
afterwards.
Processes for producing extruded unsintered
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PTFE tape for use in the invention are generally well
known. The material is preferably subjected to
calendering after extrusion, as is conventional, to
achieve a desired thickness and properties. The
extrusion die, and the calendering if employed, should
be selected so as to achieve the desired strength
ratio.
The term PTFE is here used, as is usual, to
describe a member of a range of polymers based on
polytetrafluoroethylene. For example, as is
conventional, small amounts of co-monomers may be
included, such as ethylene, chlorotrifluoroethylene or
hexafluoropropylene, provided that the properties of
the product are satisfactory.
Conventionally, PTFE is extruded with a
lubricant, which is subsequently removed, for example,
by heat cleaning or solvent extraction. The PTFE may,
as is conventional, include one or more fillers and/or
pigments. For dental floss at least, unfilled PTFE is
prefe-rred.
Preferably the contact surface used to
provide the heating of the tape-in the invention is
stationary but the invention .includes within its scope
the case where the contact surface is also moving, for
example at a slower speed than the lowest speed of the
PTFE tape_ The contact surface is preferably convexly
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curved with a preferred radius of curvature in the
range 100 cm to 0.3 cm, and preferably provides a
contact path length in the range 3 mm to 100 cm, ,
preferably 1 cm to 50 cm.
A plurality of PTFE tapes may be passed in
parallel over the heated surface, and taken up on a
common take-up device e.g. roller. These parallel
tapes may originate from a single wide tape which is
being slit prior to contact with the heated surface.
The apparatus used may therefore include slitting means
for slitting a PTFE tape longitudinally, upstream of
the contact surface.
The invention includes within its scope a
process of producing a tape, by the extension process
described above, which is subsequently sub-divided
longitudinally, e.g. by slitting, to provide material
of a suitable dimension for use as a dental floss.
It is found that the PTFE tape produced is
resistant to fibrillation, and is amenable to coating,
e.g. with wax.
The elongate stretched PTFE tape produced by
the process described above may be suitable for use as
it is in preparing a dental floss. Thus the product
can, as a result of the stretching process, have the
desired dimensions for a dental floss of tape shape
made of PTFE, e.g. a width in the range of 0.5 to 4 mm,
W O 96110478 - r' PCT/GB95/02325
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preferably 1-3 mm and a thickness in the range of 10 to
60 um. This dental floss may be coated with wax, in a
conventional manner, and the wax may include other
suitable adjuvants effective in dental health care.
5 Alternatively, the PTFE tape may be suitable for use
uncoated, as discussed below. The invention thus can
provide unwaxed dental floss.
The invention extends to PTFE tape produced by
the method of the invention described above.
10 By the method described above, it has been
found possible to prepare a PTFE tape having different
properties at its opposite faces. Such a tape may have
various uses, but in particular has been found to have
satisfactory properties for use as or in a dental
15 floss. -
Therefore, according to the invention in
another aspect, there is provided an integral PTFE tape
having opposite faces at which the respective physical
states of the PTFE material differ. Thus the
properties of the opposite faces of the PTFE tape are
different, and the physical state or structure of the
PTFE material varies through the thickness of the tape.
Thus for example, the opposite faces may differ in at
least one of the following properties:
(a) reflectance, i.e. reflectivity of light. Thus
one surface may be relatively matt and the
WO 96/10478 ~, ~ ~ ,,,, PCT/GB95/02325
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other have a higher glossiness.
(b) surface roughness. One surface may be
relatively smooth, compared with the other.
(c) hardness or conformability. As a result of
heat treatment, one surface may be harder or
less conformable than the other.
(d) coefficient of friction.
(e) peel strength after. pressing onto a surface.
This means that the adhesion of the two faces
of the tape, when pressed to the same test
surface, is different.
This difference of a property or properties
between the two faces typically results from different
mechanical and/or heat treatment (e.g. degrees of
sintering) of the PTFE material at the two faces. One
of the faces may be substantially unsintered, while at
the other face the PTFE material is at least partially
sintered, and may be fully sintered and may be node-
locked. The mechanical sliding of one face over the
heated surface may also affect the properties.
Preferably the tape is an extruded tape, and formed
from an extruded, unsintered element.
The thickness of the PTFE tape having different
properties or different physical states at the two
faces is preferably in the range Sum to 1mm, more
preferably l5.to 150um. The tape may have a tensile
W~ 9611047 . PCT/GB95/02325
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strength of at least 50MPa, preferably at least 75MPa.
In principle there is no width limitation on the tape.
The invention in another aspect therefore
provides a method of making a PTFE tape having opposite
faces at which the respective physical states of the
PTFE material are different, the method including the
step of subjecting an elongate PTFE tape to non-uniform
heating across its thickness simultaneously with
stretching it longitudinally to reduce its thickness
and width.
We have found that a PTFE tape having opposite
faces with different properties, as described above,
can be particularly useful as a dental floss, and can
be used as dental floss without a coating, such as a
conventional wax coating. Alternatively a coating to
adapt its surface properties may be applied.
Particularly where one face of the tape has a
relatively matt surface quality, with~a~coefficient of
friction higher than that of the other face, the
uncoated PTFE tape can be easily and conveniently
gripped by the fingers of the user. A conventional
PTFE tape which has a low coefficient of friction and
low conformability on both sides tends to slip through
the fingers of the user.
The other face of the PTFE tape having been
subjected to different heat treatment, for example to
WO 96/10478 _ PCT/GB95/02325
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node-lock or sinter it partially or wholly, the PTFE
tape as a whole has sufficient strength for use as a
dental floss. Preferably the uniaxial tensile strength
of the tape (on average) is at least 100 MPa.
The PTFE tape may be twisted along its length,
when used as a dental floss, so as to present regions
of higher coefficient of friction and regions of higher
conformability on each side of the floss.
The dental floss, consisting of or comprising
the PTFE tape described above, can be packaged in a
conventional manner, for example in a container such as
a plastics material box, with an outlet aperture
through which the tape can be pulled. The tape can be
packed loosely in the box, or wound on a spool in the
box. As is normal, the box may have a cutting means
for assisting the breaking of the floss, and this may
be a notch. Alternatively, the PTFE tape can be used
as a floss element in a flossing device, e.g. of the
type where the floss element extends between two
holding arms of a frame. The preferred dimensions of
the PTFE tape used as dental floss are given above.
Further advantages of the PTFE tape having
opposite faces of different properties have been found
to be that the tape can be wound more easily on a .
spool, than PTFE tape which has sintered faces at both
sides, such as is produced for example by the Gore
CA 02201499 2002-11-12
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expansion process described above. If the coefficient
of friction of the PTFE tape is low, it is difficult to
form a coherent spool, since the tape tends to slip off
the spool, e.g. when the spool is subjected to sideways
forces or impact or vibration. Hitherto it has been
customary to put a wax coating on the tape, before
winding it into a spool. However, the tape of the
present invention, without a surface coating, has been
found to be easily wound into a spool which retains
good coherence, and to be easily unwound from the spool
also. The higher "stickability" of the tape, i.e. the
adhesion between adjacent layers on the spool,
apparently gives coherence to the spool. These
advantages apply in use as a dental floss, where the
dental floss is wound on a spool in a packaging.
Furthermore, as a floss the PTFE tape can give a better
grip and feel to the user, for the reasons given above,
particularly when used uncoated, since there is no
coating which becomes applied to the user's fingers.
Accordingly, one aspect of the present invention
resides in an integral PTFE tape having opposite faces
at which the respective physical states of the PTFE
material differ, said opposite faces differing in at
least one of a) reflectance; b) surface roughness; c)
coefficient of friction; and d) peel strength after
pressing onto a surface.
In another aspect, the present invention resides
in a method of forming an elongate PTFE material,
comprising the step of passing an unsintered PTFE tape
CA 02201499 2002-11-12
19a
across a heated surface in sliding contact therewith
while applying tension to the tape, wherein the
temperature of the heated surface, the passage speed of
the tape and the tension applied are such that the PTFE
tape, when its temperature is raised by contact with the
heated surface, is longitudinally stretched with
simultaneous width and thickness reduction while in
contact with the surface, wherein the reduction in width
of said tape in said step is at least 50%.
In a further aspect, the present invention
resides in a method of altering the properties of an
elongate unsintered PTFE tape, comprising uniaxially
extending said tape longitudinally with simultaneous
width and thickness reduction while it is maintained in
sliding contact with a heating surface, wherein the
width reduction of said tape, as a result of its contact
with said heated surface, is at least 500.
In another aspect, the present invention resides
in a method of forming an elongate PTFE material,
comprising passing an unsintered PTFE tape across a
heated surface in sliding contact therewith while
applying tension to the tape, wherein the temperature of
the heated surface is at least 320° C., and the passage
speed of the tape and the tension applied are such that
the PTFE tape, when its temperature is raised by contact
with the heated surface, is longitudinally stretched
with simultaneous width and thickness reduction while in
contact with the surface.
CA 02201499 2002-11-12
19b
In a further aspect, the present invention
resides in a method of altering the properties of an
elongate unsintered PTFE tape, comprising uniaxially
extending said tape longitudinally with simultaneous
width and thickness reduction while it is maintained in
sliding contact with a heating surface, the temperature
of said heating surface being at least 320° C.
In another aspect, the present invention resides
in a method of forming an elongate PTFE material,
comprising passing an unsintered extruded PTFE tape
across a heated surface in sliding contact therewith
while applying tension to the tape, wherein the
temperature of the heated surface, the passage speed of
the tape and the tension applied are such that the PTFE
tape, when its temperature is raised by contact with the
heated surface, is longitudinally stretched with
simultaneous width and thickness reduction while in
contact with the surface.
In a further aspect, the present invention
resides in a method of adapting the properties of an
elongate unsintered extruded PTFE tape, comprising
uniaxially extending said tape longitudinally with
simultaneous width and thickness reduction while it is
maintained in sliding contact with a heating surface.
In another aspect, the present invention resides
in a method of making a PTFE tape having opposite faces
at which the respective physical states of the PTFE
material differ, comprising the step of subjecting an
unsintered extruded PTFE tape to non-uniform heating
CA 02201499 2002-11-12
19c
across its thickness simultaneously with stretching it
longitudinally to reduce its thickness and width, the
wide reduction of the tape in said step being at lest
500.
In a further aspect, the present invention
resides in a method of making PTFE dental floss,
comprising passing an unsintered PTFE tape across a
heated surface in sliding contact therewith while
applying tension to the tape, wherein the temperature of
the heated surface, the passage speed of the tape and
the tension applied are such that the PTFE tape, when
its temperature is raised by contact with the heated
surface, is longitudinally stretched with simultaneous
width reduction and thickness reduction while in contact
with the surface, said width reduction being at least
50%.
In another aspect, the present invention resides
in a method of making PTFE dental floss comprising
adapting the properties of an elongate unsintered PTFE
tape by uniaxially extending said tape longitudinally
with simultaneous width reduction and thickness
reduction while it is maintained in sliding contact with
a heating surface, said width reduction being at least
50%.
In a further aspect, the present invention
resides in a dental floss comprising an integral PTFE
tape having opposite faces at which the respective
physical states of the PTFE material differ.
CA 02201499 2002-11-12
19d
In another aspect, the present invention resides
in a dental floss comprising an integral PTFE tape
having opposite faces at which the respective
coefficients of friction are different.
In a further aspect, the present invention
resides in a dental floss comprising an integral PTFE
tape having opposite faces at one of which the PTFE
material is unsintered and the other of which the PTFE
material is at least partially sintered.
In another aspect, the present invention resides
in a method of making a dental floss comprising a PTFE
tape having opposite faces at which the respective
physical states of the PTFE material differ, the method
comprising the step of subjecting an unsintered PTFE
tape to non-uniform heating across its thickness
simultaneously with stretching it longitudinally to
reduce its thickness and width, the width reduction
being at least 50%.
BRIEF DESCRIPTION OF THE DRAWING
Embodiments of the invention will now be
described by way of non-limitative example, with
reference to the accompanying drawing which shows
diagrammatically a process and apparatus embodying the
invention. In the drawing, the single figure is a
diagrammatic side view of an apparatus embodying the
WO 96/10478 ~ PCT/GB95/02325
t t''~ ,a
~2U~
invention.
PREFERRED EMBODIMENTS OF THE INVENTION
In the figure, there is shown a coil 1 of
extruded, unsintered PTFE sheet S. The sheet S is
5 unwound from the roll 1 around a larger roller 2 which
is driven at a predetermined speed. The sheet S is
taken off the roller 2 around a small roller 3 and
passes across a slitter 4 which has an array of
parallel knives which subdivide the sheet S
10 longitudinally into a plurality of parallel tapes T.
Adjacent the slitter 4 is a floating roller 5 which
pushes the tapes downwardly to ensure the correct
position of the sheet S relative to the slitter 4. A
pair of rollers 6 are driven at a predetermined speed,
15 so as to grip the tapes T at their nip, so that the
rollers 2,6 provide tension across the slitter 4.
The parallel tapes T pass, via a guide 12 which
aligns the tapes and ensures their separation, to a
pairof rollers 7, which are driven at a predetermined
20 speed to provide the predetermined let-off speed for
the heating and stretching step which follows. The
tapes T pass in parallel over a predetermined length of
a stationary smooth convexly curved surface (radius of
curvature 48 mm) of a heater 8, which contacts one side
of each of the tapes. The tapes make sliding contact
with the heater 8. Two heaters (A and B) have been
W O 96110478 PCTlGB95/a2325
14 ~ '~
. , , ,~',~.~ ~.2~
21
used. Heater A is heated by a resistance heating
element embedded in a ceramic matrix material of the
heater, and its temperature can be set to a desired
value. This heater is available from Osram Sylvania
under the trade name Sylvatherm, and similar heaters
are available from Vulcan Refractories (UK). Such
heaters are sold for use as radiant heaters, but are
employed as contact heaters in the present process.
Heaters with surfaces of other materials than
ceramics may be employed. Heater B has a stainless
steel skin providing the contact surface. This surface
has a radius of curvature of 10 cm, a transverse lena~r,
of 35 cm and a length in the running direction of ~ cr,..
The surface is wire-brushed to give it a satin fi:m;:.
rather than a mirror finish. It is -clamped to ~ :~~.~.:- ~
aluminium block.
As they pass over the heater 8, the precurs::
tapes are subjected to heating and stretching, so ti~,:-
they extend longitudinally, with simultaneous width an::
thickness reduction. The extended tapes then pass over
the pair of rollers 9, wrzich are also driven at a
predetermined speed, higher than that of the rollers 7,
the speed ratio of the rollers 7,9 determining the
length extension of the tapes T. From the rollers 9,
the tapes pass to individual spooling devices, not
shown, where they are wound into packages.
WO 96/10478 .. PCT/GB95/02325
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C
f~l~~~
22
The heater 8 is mounted on a support 10, whose
vertical position can be adjusted by a height control
cylinder 11, operated pneumatically or hydraulically.
This allows removal of the heater 8 from the path of
the tapes entirely, and also allows adjustment of the
length of the contact path of the tapes on the heater
8. In the drawing the tapes T are shown tangential to
the heater surface, which is the position of minimum
contact length. In operation, the contact- length is
longer than this minimum, and the tapes T are deflected
downwardly from the horizontal position shown.
Examples 1 -6
The starting PTFE element is an approximate:-,
biaxial PTFE unsintered tape, made by extrusion any
calendering without filler, in which the fibriliar
structure is approximately equally dispose~ in the
lateral and longitudinal direction. The strength ra~:
is 0.82. Such a tape is fabricated from PTFE polymer
powder by extrusion with the aid of a lubricant through
a suitable die. The extruded tape is calendered to
give the desired thickness and strength ratio and then
treated ("degreased") by heating or solvent extraction
to remove the lubricant. Such techniques are well
known. A wide tape may be slit longitudinally, to
provide the starting element for the present process.
CA 02201499 2005-09-13
23
The starting element has dimensions of 230 um thickness
and 13.5 mm width, 5000 tex (being the mass in grams of a
fibre per 1000 metre length i.e. g/km), density 1.60 g/cm3,
longitudinal tensile strength of 6.7 MPa and transverse
tensile strength of about 5.6 MPa.
This tape was drawn by the apparatus shown in the
accompanying drawing at several different length extensions.
Heater A described above was employed. The let-off speed
(initial tape speed) was 12 cm/min., and the take-up speed
determined the length extension (e.g. a take-up speed of 900
cm/min gives an extension of 75). The heater surface
temperature was about 370°C. The contact length with the
curved surface of the heater was about 3 cm. Back tension
by the let-off roll is minimal, i.e. sufficient to achieve
the desired contact with the heater. This process can be
operated at a wide range of length extensions, and results
are given in Table 1 for a range which gave particular good
products for use as dental floss.
WO 96/10478 PCT/GB95102325
~ 1~~ .~
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24
TABLE 1
Example Length Dimensions Tex Density Longitudinal
extension strength
(ratio) ~m x mm g/cc MPa
- 1 230 x 13.5 5000 1.60 6.7
(starting
tape)
I 50 50 x 2.3 121 1.06 137
2 55 44 x 2.3 108 1.08 181
3 60 39 x 2.2 97 1.12 191
4 65 36 x 2.0 86 1.18 214
S 70 36 x 1.9 82 1.18 221
G 75 34 x 1.8 74 1.17 235
WO 96/10478 ~ ~ PCT/GB95/02325
' s - ~~; .~
Samples of these extended materials were used
as dental floss without further cutting to reduce
width, both uncoated and coated with wax in a
5 conventional manner, and were found satisfactory, being
very resistant to fibrillation in use and easily
gripped by the user's fingers.
All of the extended PTFE tapes of Table 1
exhibited different physical states at their two faces.
10 Particularly it was readily apparent on inspection in
each case that the reflectance, smoothness,
conformability and coefficient of friction was
different, at the two faces.
Thus the invention can provide PTFE elements
15 having high fibrillation resistance, particularly when
the starting material for the process has a strength
ratio in the range 0.2 to 3Ø As a process, the
invention can achieve a high stretch ratio (length
extension) with a short stretching path, by localised
20 high temperature contact heating.
Examples 7-11
Various precursor tapes of extruded and
- calendered unsintered unfilled PTFE were extended by
25 the method of the invention, using the apparatus of the
drawing, to make stretched tapes of various dimensions,
WO 96!10478 ' PCT/GB95/02325
~~~~~y~
26
densities and strengths. Details of the precursor
tapes, the process conditions and the product tapes are
given in Table 2. Heater A was employed for Examples 7 ,
to 11 and Heater B for Example 12.
WO 96/10478 ~ PCT/GB95/02325
~i~ ~ 27
N O
'iN
N
N
x
p. o
O O N O O O M
n0 ~Dr M tDt0U7 tn tnO tf] Q1O
X N O 1DN M u'7M O l0m N
W .-1r N O u7.-iO N N c l0v N w-1.-1.-iO c
ri
r-i
r
N 01
ri
p.X O
~7 N O .-1O O O
rtfN m N m lON O N tn O N O
N . . ~.. . . ~ ~pp m . O ,.a,.y
01,-iN O rl.-iO r M M V rlrlv M N riC
O
rlO
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E m r r o m o m
r0tnv~tDN m N .-i O O f1 O M N
X ~ r O vf7O O N ~ m m
W o1N M .--1~ .-1O .--1rlc W ~DO ~ON rl rlV
O
01O
M
N N
o.x o
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f[7c'r ~Dm l0tf7N . O r O . .-I~
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W 10CDU)O N rlO N O~c W M O M N .--1rle
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01 N tn(~01 O l0 O l0
(0 r ~ m m tt)N O cTO O 01M
X W io r r W ~n O m o vo
r-I W .-il0uf7O tnrlO .-1u7c c ~ rl.-r.-1.-IO M
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r M
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.-iX
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r N o m N a r o c
ro r ~ m o uoN o M r o
X N W .-iO N O tf7 M tnN
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N N Ql N O 11 O N
a~C O v fl..ic0.C W t~ -,v
G N -.-~ U G1.m a~S-m.~~ w U .-i
O r-ii.ui-~ U v7 t0N tTC_ riZ U .i
-.W ~0 W a~lafl.C ~6U \ v1
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W C N a o t~ ~ ~ N C ~- b~C
U -.1v7 ~ d C N .-1V7 .-1 47
N b S-1.C C1 ~ O i-~ N ~ 'O>, J..i
m ~ N a> >,C ~ ~ a-~~ ~- ~ .v.i>,
W > O~ a 'i O v1N U C J~rl a~X
m .-m C .a a a~rox .c .-1U -.~.-I
tntTG N tnN ~ Q7J-~~ U i~ p~t0 ~ f-a
o c ro~ x a a n,a m a .~'aX a c a ~
s~o s~~ a~v ~a rox v o .c-.~v o a~ a~ro
U ~-7F ~nE-~D E E W x U E 3 E .aF D
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WO 96/10478 . ~ . . PCT/GB95/02325
28
Table 2 shows that the product properties can
be selected over wide ranges, by variation of precursor
material and process. conditions. The products are.
suitable for dental floss and for uses other than
dental floss.
All the tape products of Table 2 exhibit
different physical structure and properties at their
two faces. tae classified the surface properties of the
tapes generally into two characteristic surface types:-
Type A . characterised by
(i) high hardness/low conformability
(ii) glossy surface
(iii) low adhesive property
(iv) low coefficient of friction
Type B . characterised by
(i) low hardness/high conformability
2p (ii) matt surface
(iii) significant adhesive property
(iv) higher coefficient of friction
than type A.
The product tape of Example 7 has two faces
both of type B, though differing slightly. This is
WO 96110478 ~ 1~ ~ ~ PCT/GB95/02325
!~;t, :~ r~
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29
thought to be because the temperature at which
stretching took place is below that associated with
either node locking or sintering.
The product tape of Example 11 has two faces
which are both of type A, though they differ slightly.
The temperature of stretching is relatively low
(compared with Examples 8,9 and 10) but, because the
precursor tape is relatively thin and the precursor
tape let-off speed (input speed) and the length
extension are both low, there is a longer contact time
with the heater surface resulting in more uniform
properties at the two faces of the product tape. Both
faces have the hard and glossy appearance, associated
with a sintered surface.
The product tapes of Examples 8,9 and 10
exhibited more sharply a difference of physical states
at the two faces. One face (that which contacted the
heater) is of type A, while the other face is of type
B.
Quantitative observations of these properties
of tapes of Examples 8,9 and 10 were as follows:
Gloss:
If two parallel wound packages of any of the
tapes of examples 8, 9 or 10 are compared (a) with the
heater contacting face wound outermost and (b) with the
heater contacting face wound innermost, an immediate
WO 96/10478 , ; F ' ~- ' , PCT/GB95/02325
2~(~ 'E ~9~
difference in light reflectance and surface gloss is
apparent. In particular when viewed in fluorescent
lighting the surface of package (a) appears to be ,
highly reflective with a high surface gloss. In
5 package (b) by comparison, reflectivity and surface
gloss are considerably reduced.
In addition, the package (a) has a slippery
nature with an apparent low coefficient of friction
whilst for package (b) the surface feels tacky with an
10 apparently higher coefficient of friction.
In addition with a cross-wound package of tape,
if the tape turns over during winding the difference in
surface gloss can immediately be detected.
Hardness/Conformability:
15 If these tapes are wound into cross-wound
packages and a length inspected with a binocular
microscope after removal from the package the following
features are observed. Signs of compression by contact
with the underlying tape geometry or the overlying tape
20 geometry can be seen in the form of characteristic
surface lines.
Under oblique illumination these surface lines
are much more pronounced on the matt surface (B-type
surface) than on the glossy surface (A-type surface).
25 Thus the softer texture of the B-type side of the tape
retains indentations caused by overlying tapes. This
WO 96114478 ~ PCT/GB95/02325
31
explains why such tapes do not fall or remove easily
from packages or bobbins even when unwaxed and why hand
grip can be maintained on the floss without the
necessity forwaxing.
Friction and adhesive properties:
If a given length of tape is wrapped partly
around and pressed gently into contact with a curved
glass surface, such as a surface of a Pyrex beaker, and
tension is applied on its free end to pull it
tangentially to the curved surface, the following is
observed. The matt surface (type B) strongly grips thF~
glass surface_ If the length in contact with the
surface is 5-10 cm the tape can be appreciably
stretched before it loses adhesion to the surface.
If the glossy surface (type A) is presses: _..
contact the tape can be more easily disloda~::.
PT FE tapes made by processes inve~:m:.a
sintering as described in US patents 3953566 and
3962153 and in EP-A-391887 similarly show no tendency
to adhere to the glass surface.
An extended tape made in accordance with
Example l2 above has been subjected to further
observation and test, as follows:
WO 96!10478 PCTIGB95I02325
=~... ' ' ''.''.
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Surface Roughness:
The technique employed is confocal microscopy,
which is available at the Materials Science Department
at the University of Surrey, England. The object under
test is scanned in several planes, one plane at a time,
using laser light and measuring absorption. A three-
dimensional optical image is produced, and profiles of
cross-sections can be generated. Gold coating of the
PTFE tape was necessary in order to reveal surface
details satisfactorily.
The results obtained showed that the Type A
surface (as identified above) of this sample-has higher
values of surface roughness than the Type B surface,
for each of six different roughness parameters. In
summary, the Type B surface is less rough by a facto:
of '~s to ~.
This result is initially surprising, sine tt:~
Type A surface is more glossy to the eye. Gloss
apparently depends on other factors as well as surface
profile.
Coefficient of friction:
Friction was measured using a reciprocating
tribology device at the Materials Department of Brunel
University, England. Four superimposed layers of the
tape were held on a flat steel surface by tension. A
WO 961104'78 PCT/GB95/02325
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33
glass countersurface is reciprocated in contact with
the sample, at a range of loads at the contact region.
The load applied was cycled from low values to high
ones and down again. The resistance to sliding is
measured. Coefficient of friction (COF) was found to
vary in dependence on load over parts of the load
range, and behaviour was not consistent as between the
load-increasing and load-decreasing parts of the cycle.
Therefore the results must be treated as tentative.
Nevertheless the Type A surface showed consistently
lower resistance to sliding than the Type B surface.
The~lowest COF value for the Type A surface is 0.01 at
low loads (up to 150 gf). At these loads, the COF of
the Type B surface was very much higher. The Type A
surface behaved more closely to what has been typically
observed in earlier work on sintered PTFE.
Peel strength:
Adhesion was measured between each surface of
the tape and four surfaces, i.e.-(i) the same surface
of the tape, (ii) the other surface of the tape, (iii)
a flat glass surface and (iv) a flat melamine surface,
by compressing two layers of the tape between a glass
microscope slide and a smooth melamine surface. A load
of 8 kgf was applied for about 10 hours,-at a
temperature of 14-18°C.
WO 96110478 I , PCT/GB95102325
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The peel strength was then measured. Three
arrangements A, B and C were tested, to give all
possible interfaces:-
Arrangement A
Time of load application . 9.5 h
Interface Peel strength (gf)
i Glass/surface type B 2.76/3.93/4.42*
ii Surface type A/surface type B 4.74
iii Surface type A/melamine 0
Arrangement B
Time of load application . 11.75 h
Interface Peel strength (gf)
i Glass/surface type A 0
ii Surface type B/surface type B 4.74
iii Surface type A/melamine 0
Arrangement C
Time of load application . 12.7 h
Interface Peel strength (gf)
i Glass/surface type B 4.42
ii Surface type A/surface type A 0
iii Surface type B/melamine 4.42/6.44* .
* indicates multiple tests.
WO 96!10478 PCT/GB95/02325
.;~,a~~ ~2o~4yy
In conclusion the type B surface of the tape
adhered to both surfaces of the tape and to glass and
melamine, whereas the type A surface adhered only to
the type B surface.
5 While the invention has been illustrated herein
by embodiments, it is not limited to those embodiments,
and modifications and variations may be made within the
scope and concept of the invention herein disclosed.
