Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02342151 2001-02-22
PRODUCTION OF CONTINUOUS FIBRE FROM NANOFIBRES.
FIELD OF THE INVENTION.
The invention relates to a process for producing fibres or yarns from
nanofibres. More
particularly in the process of this invention the nanofibres are dispersed in
a solution of
high molecular weight polyethylene which is converted to a fibre in a gel
spinning
process. The fibres are stretched to align the nanofibres along the fibre. The
draw ratio is
15 or more. Subsequently, if desired. the polyethylene matrix can be cracked
and
evaporated by heating the fibre to about 500 C in an inert atmosphere or the
polyethylene
can be converted to carbon by heating the fibre in an atmosphere with a
controlled
oxygen content. Commercial gel spinning processes can be used with little or
no
modifications. The high molecular weight polyethylene solution with the
nanofibres
dispersed in it can be prepared as presently practiced or the high molecular
weight
polyethylene can be produced as a slurry in the spinning solvent using
polymerization
catalyst supported on the nanofibres, the spinning solution then can be
prepared by
heating this slurry.
BACKGROUND OF THE INVENTION.
Gel spinning processes are known and commercially practiced, for example those
described in US 4 137 394,
US 4 344 908 and US 5 342 567. In these processes a solution of high molecular
weight
of polyethylene ( a dope) in a suitable solvent is prepared. The molecular
weight of the
polyethylene is in the order of a million and more. The higher is the
molecular weight of
the polyethylene the stronger is the resultant fibre, however the higher is
the molecular
weight of the polyethylene the more difficult is to prepare the solution.
Polyethylene is
soluble only at temperatures above some 120 C, thus the boiling point of the
solvent must
be higher than this temperature, otherwise it would flash during spinning,
which is
undesirable. Suitable solvents are pure or mixed hydrocarbons boiling in the
range of
150-200 C or hydrocarbons having high boiling point such as mineral oils. Also
fluorinated or chlorinated hydrocarbons may be used although this is not
practiced in
CA 02342151 2001-02-22
producing gel spun fibres. The polyethylene solution is spun and cooled so
that the
polyethylene gels, thus yielding a gel fibre of polyethylene containing
solvent. To obtain
the final product the fibre must be stretched to many times its original
length and the
solvent removed. The stretching is done at an appropriate temperature in one
or multiple
steps. The solvent is removed by extraction with a light solvent or it is
evaporated or
stripped by steam in the case that a volatile solvent is used. The sequence of
stretching
and solvent removal may follow each other by say solvent removal - stretching -
stretching or stretching - solvent removal - stretching or by using a
different sequence.
None of the prior art teaches the option of adding an insoluble fibres such as
carbon
nanotubes to the dope.
High molecular weight polyethylene is known and is commercially available
under the
name Hostalen GUR, for example.
Nanofibres are known. For example carbon nanotubes are available in research
quantities
and may soon be available in commercial quantities. Presently only one method
for
organizing these nanofibres into an oriented fibre is described in Science,
290, 1331
(2000). A dispersion of carbon nanotubes is spun into a stream of poly (vinyl
alcohol)
solution in water. The process is slow and it is not commercial.
DETAILED DESCRIPTION OF THE INVENTION.
I have now found a process for producing a continuous fibre from nanofilaments
using a
gel spinning process.
In the process of the invention the nanofibres are added to the solution of
the high
molecular weight polyethylene in a suitable spinning solvent.
The polyethylene is a linear "high density" polyethylene prepared by any known
process.
Its molecular weight may be 100 000 to 10 000 000, preferably 500 000 to 4 000
000. A
polar monomer such as malefic anhydride may be grafted to the polyethylene
backbone to
provide enhanced bonding between the POLYETHYLENE and the nanofibres in the
final
product.
The nanofibres may be any fibres of a diameter in the order of 10 nm and of
the length to
diameter ration greater than 50, preferably greater than 100. Preferably
single wall carbon
nanotubes can be used.
The solvent may be a hydrocarbon or a similar solvent in which polyethylene is
soluble,
such as fluorinated or chlorinated hydrocarbon. A mixture of said solvents may
be used.
The normal boiling point of the solvent is above the temperature at which
polyethylene is
soluble in the solvent which is about 120 C. The solvent selection depends on
the
particular gel spinning process. Lower boiling solvent are advantageously used
when the
solvent is evaporated from the gel after spinning while high boiling solvents
such as
mineral oil have to be extracted from the gel. Either process has its
advantages and draw-
backs, as those skilled in the art would appreciate.
In preparing the solution the nanofibres and the high molecular weight
polyethylene can
be added to the solvent and in a stirred tank and while stirring the
temperature is
increased to the desired level. Care must be taken to exclude oxygen. The
vessel design
CA 02342151 2001-02-22
and the dissolution procedures are described in the prior art and are well
known to those
skilled in the art. The molecular weight and concentration of the polyethylene
and the
concentration of nanofibres in the spinning solution are adjusted so that
desired
processability and content of the nanofibres in the final product are
obtained.
Alternately, the fibre can be dispersed in a purified spinning solvent, and an
ethylene
polymerization catalyst is then deposited onto the nanofibres, the vessel is
pressurized
with ethylene which polymerizes and coats the nanofibres. The polymerization
catalyst
and the temperature are selected so that the resultant polyethylene is of the
desired
molecular weight. Upon heating to at least the solubilization temperature the
polyethylene dissolves and the solution can be used as the spinning solution
in the
subsequent gel spinning process. This method may provide a more uniform
dispersion of
the nanofibres.
The temperature of spinning solution must be above the minimum temperature at
which
the polyethylene is soluble in the solvent used, which is usually 120- 130 C.
However the
temperature should not be much higher than the normal boiling temperature of
the
solvent else it would flash at the spinneret exit which is undesirable.
The orientation of the nanofibres is the result of the shear gradient in the
spinneret
followed by the considerable reduction in the transverse dimension of the gel
fibre caused
by the solvent removal and drawing of the fibre. Thus for example a gel fibre
spun from a
dope which after cooling contains 5 volume % of the nanofibres and 5 volume %
of the
polyethylene and drawn 20 times receives orientation similar to a melt spun
fibre drawn
200 times.
Nanofibres may be added to the dope and converted to fibre using other
solution spinning
processes such as those used in the production of poly acrylo nitrile or
celulose acetate
fibres for example. Although the final orientation would be lower than that
achieved in
gel spun polyethylene fibres, the product may be quite suitable for some end
uses.
There are potentially many uses for such fibres utilizing their strength or
electrical
properties. However the technology is too new to anticipate all the uses.
