Note: Descriptions are shown in the official language in which they were submitted.
2158805
HOECHST AKLl~SELLSCHAFT HOE 94/F 923 Dr.RL-nu
Werk Gendorf
Composites of polytetrafluoroethylene, intermediate
products for these and processes for their production
Description
Polytetrafluoroethylene, PTFE in the following, is a
valuable material not least because of its powerful
antia & esive properties and its high melt viscosity.
However, these properties make hon~; ng with other
materials exceptionally difficult. Thus, the low surface
tension of PTFE does not allow wetting by a & esives, and
welding as a rule fails because of the high melt viscos-
ity, which necessitates welding temperatures in the
region of 370C, which is detrimental to many of the
materials to be hon~e~ with it, in particular plastics.
The invention was thus based on the object of producing
composite bodies or shaped articles in which PTFE is
firmly h~n~eA to a lower-melting plastic, where this
plastic can also be very much less heat-stable than PTFE.
This object is achieved according to the invention by
compression-molding at a temperature of 350 to about
400C a layer of PTFE with a copolymer which is arranged
in a flat form, has predominantly tetrafluoroethylene
(TFE) units and acts as a hot-melt a & esive with respect
to PTFE, and a fiber material which is arranged in a flat
form, and is stable up to at least 400C. The copolymer
is expediently employed for this purpose in the form of
a film. It can also be applied to the PTFE in the form of
granules or a powder, but it must then be ensured that
the copolymer does not substantially melt around the
fiber material on the opposite side of the sheet-like
structure formed.
The intermediate product thus obt~; n~ is storage-stable
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and can be marketed in the form of rolls or sheets,
depending on the thickness.
However, the intermediate product can also be further
processed to give the end product, the composite body or
shaped article, directly, if necessary after cooling to
a suitable temperature, which depends on the properties
of the plastic to be bonded to it. For this, the plastic
which can be processed as a melt, arranged in a flat
form, is compression molded with the intermediate pro-
duct. "Arranged in a flat form" in the following andabove is understood as meaning a film or a layer of a
powder or granules, or of a paste, and also a sheet. The
processing temperature and the compression molding
pressure depend on the nature and amount of the plastic
which can be processed as a melt and are to be chosen by
the expert according to his technical knowledge or to be
determined with the aid of simple preliminary experi-
ments. During this procedure, the plastic melts around
the side of the fiber material arranged in a flat form
which has remained exposed, a sheet-like structure
(laminate) which is bonded inseparably and has outstand-
ing properties being formed. The sheet-like structures
thus obtained can in turn be further laminated - on the
side of the lower-melting plastic - for example by
compression molding with a film of the same plastic or a
plastic which is compatible with this.
Preferred embodiments of the invention are explained in
more detail below.
The PTFE is employed as a film or sheet of greater or
lesser thickness, depending on the intended use of the
composite body. If necessary, to eliminate unevenness in
the surfaces of the PTFE material and/or of the compres-
sion mold, a suitable intermediate layer can be intro-
duced between the mold and the composite, for example a
sheet-like structure - which is of course sufficiently
heat-stable - of a silicone material, for example of a
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silicone rubber/glass fiber fabric laminate.
Possible copolymers which act as hot-melt adhesives with
respect to PTFE are primarily the copolymers which have
already been employed to date for composites of PTFE with
itself or other materials, that is to say copolymers
having perfluoroalkyl perfluorovinyl ether units (US-A
3 946 136) or a perfluoroalkene which differs from TFE,
such as hexafluoropropene (US-A 2 833 686). Copolymers of
90 to 99.5% by weight of TFE and 0.5 to 10% by weight of
perfluoroalkyl perfluorovinyl ether having alkyl radicals
of 1 to 10 carbon atoms, in particular 1 to 3 carbon
atoms, are particularly preferred. Copolymers of 95 to
99% by weight of TFE and 5 to 1~ by weight of perfluoro-
(alkyl vinyl) ether are particularly suitable.
Suitable fiber materials are woven fabrics or nonwovens
of sufficiently heat-stable fibers of glass, ceramic,
carbon, metal, such as stainless steel, or plastics which
have a high heat stability, such as condensation products
of terephthalic acid and p-phenylenediamine.
It is decisive that the fiber material arranged in a flat
form allows mechanical anchoring for the melt and can
thus act as the bonding member or intermediate layer
between the PTFE and the plastic which can be processed
as a melt. It is thus also possible, for example, first
to bring more or less loose fibers into a flat form,
with, for example pulverulent, copolymer which acts as a
hot-melt adhesive, and then to use this as the basis for
the process according to the invention.
Plastics which can be processed as a melt and which can
be employed are all the customary industrial thermo-
plastics, for example the relatively low-melting polymers
poly(vinyl chloride) (PVC), polyethylene or
polypropylene, processing depending on the properties of
this plastic. Cooling of the intermediate product to the
suitable further processing temperature will therefore in
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general be necesæary.
One embodiment of the invention relates to lamination of
the interme~;ate product with a paste of a plastic which
can be processed as a melt, in particular a PVC paste.
The layer thickness of the paæte applied can vary: if the
layer is to serve merely as a binder for a subsequently
applied sheet-like structure - film or sheet - of the
same plastic or of a plastic compatible with this, a
layer thickness of up to about 100 ~m, preferably 50 to
80 ~m, is sufficient. The product thus obtained can then
be further processed directly or, after cooling, at a
later point in time. However, the layer thickness of the
lamination can also be a few millimeters, for example up
to 3 mm, whereupon correspo~; ng sheets are obtained
after cooling. Suitable pastes of plastic are known to
the expert. PVC pastes are described, for example, in DE-
A 42 26 289 and the literature mentioned therein.
In a particular embodiment of the process, however,
further processing to give the composite body can also be
carried out with sufficiently heat-stable plastics in one
step, for example with liquid-crystal polymers, such as
completely aromatic copolyesters.
The most favorable processing conditions depend on the
properties of the starting materials employed. The
pressure is thus expediently also maintained during
cooling. It may be advantageous to carry out the compres-
sion molding under inert gas protection or under reduced
atmospheric pressure. The formation of bubbles or hollow
cavities can thus be suppressed by lamination in vacuo.
The process can be carried out industrially on the known
laminating machines. Vacuum multilayer presses or twin-
belt presses, if continuous laminates are to be produced
from roll to roll, have proved suitable.
The composite bodies thus obtained are distinguished by
advantageous properties, which often go beyond those of
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the components. Thus, for example, a thermal expansion
similar to that of steel or ceramic can be achieved in
composite bodies having a thin PTFE layer. The load
capacity under pressure is very high. The heat distortion
point and the welding properties can be adjusted within
wide limits.
The thickness of the composite bodies according to the
invention can likewise vary within wide limits and can be
less than 100 ~m up to several millimeters.
Since all the composite bodies are based on the known
good properties of PTFE, they are particularly suitable
as a sliding bearing material.
The invention is explained in more detail in the follow-
ing Examples.
Example 1 (inter~e~;ate product)
- A glass silk fabric, 192 g/m2 (type 91121 from Inter-
glas),
- a film of a polymer of 96% by weight of TFE and 4~ by
weight of perfluoro(n-propyl vinyl) ether, melt flow
index (MFI) 10 g/minute at 372C under a load of 5 kg,
thickness 25 ~m and
- a PTFE sheet, thickness about 2 mm,
dimensions of all the specimens 300 x 180 mm, were
introduced into a vacuum multilayer press - from the top
downwards.
As pressure compensation against slight unevenness in the
press platen plates, a glass fiber fabric/silicone rubber
laminate about 3 mm thick was placed on the PTFE sheet.
Heating up from room temperature to 370C (which was
maintained for 12 minutes) took place in the course of
about 35 minutes. Thereafter, the press was cooled to
230C by air cooling in the course of about 85 minutes,
21588 0S
and then cooled to about 30C in the course of a further
60 minutes. The mean specific compression molding pres-
sure was 0.6 bar (0.5 to 0.8 bar).
Strips 20 mm wide were cut off from the intermediate
product thus obtained and the peel resistance was tested
at a pull-off rate of 100 mm/minute in an angled peel
test analogous to DIN 53 282. The peel resistance was 25
to 40 N/10 mm strip width.
Example 2 (further processing of the intermediate
product)
An 80 ~m thick layer of a PVC paste i8 knife-coated onto
the glass fiber side of the intermediate product accord-
ing to Example 1 and is gelled at 180C. The sheet-like
structure thus obtained is suitable for the production of
coatings of good surface slip.
However, the sheet-like structures can also be bonded -
immediately after gelling or else if appropriate after
cooling and reheating, in a separate process step - to a
PVC film or sheet at 180C under a pressure of 1 bar in
a press.
Example 3 (one-stage process)
A sheet-like structure is produced from the PTFE sheet,
the film and the glass silk fabric in accordance with
Example 1, but is compression molded without cooling at
about 350C under a pressure of 115 bar with a 100 ~m
thick film of a completely aromatic copolyester (~VECTRA
C 950, melting temperature 320 to 325C, commercial
product of Hoechst AG).
