Note: Descriptions are shown in the official language in which they were submitted.
7~
HOECHST AKTIENGESELLSCHAFT HOE 90/F345 DCh.5~AP
Description
Heat-resistant optical moldings, and a process for ~heir
production
Optical fibers are widely used in ~he automotive and
illumination sectors and in medical diagnostics and in
particular in data transmission.
These fibers have usually been produced from glass, but
the range of applications is limited to a relatively
small number of certain applications due to their low
flexibilit~, in particular at xelatively large diameters,
and due to their low breaking strength under mechanical
load.
Thers has therefore been no lack of attempts to replace
glass by organic polymeric materials. Polymer fibers have
low specific gravity, but nonetheless high strength.
Their flexibility is retained even at relatively large
fiber diameters, and they are, furthermore, virtually
insensitive to mechanical load. A further advantage of
polymer fibers is that the ~tarting components ar~
inexpensive and easy to produce. However, the trans-
mission of optical fibers made from organic polymers
drops considerably as the temperature is increased, which
means that their use is limited to areas in which they
are not subjected to considerable heating. Thus, for
example, the maximum service temperature of polymethyl
methacrylate is 80~C.
In general, a polymeric optical fiber comprises a core of
relatively high refractive index and a cladding of lower
refractive index. This arrangement ensures that the light
does not leave the fiber due to reflection at the core/
cladding interface and thu~ remainR within the fiber. In
the case of optical fibers ~or light, it is therefore
~ 2 ~ 7~
necessary to apply a layer which entirely surrounds the
fiber. The application of layers of this type, for
example by dipping the fi~er into a solution containing
a polymer in dissolved form or by special spinning
processes r i5 expensive and complicated.
The ob~ect was therefore to provide optical moldings, in
particular having a core/cladding structure, which can be
obtained by a simple and economical proce~s, have high
flexibility, high brsaking s~rength and good transmission
and, in particular, can be used even at elevated
temperatures.
The present invention solves this ob~ect. It has been
found that fibers having a core/cladding s~ructure and
the abovementioned advantages can be obtained from
polymeric silazanes or hydridochlorosilazanes.
Such polysilazanes and polyhydridochlorosilazanes which
are suitable for the production of the molding~ according
to the invention were hitherto only used as precer~nic
materials which are pyrolized to give silicon nitride and
are described, for example, in DE-A-37 37 921
(US-A~4,935,481), DE-A-37 43 825 (US-A-4,939,225),
US-A-4,946,920 and US-A-4,931,513.
~he present invention accordingly provides a process for
the production of optical moldings from polysilazanes or
polyhydridochlorosilazanes, which are first pressed to
give moldings or dissolved in a solvent and extruded and
are sub~equently exposed to a ga~ atmosphere at a tem-
perature in the range from +10 to +200~C and a relative
humidity in the range from 20 to 100~ for from 0.1 to 24
hours.
These polysilazaneR can be prepared, ~or ex~nple, by
reacting aminochloroRilane~ (I) of the formula RSiCl2-NR-R
with 3.15 mol of ~nonia per mole of silane in THF,
fo~ning oligomers of the formula (II)
- 3 ~ 5~
R' R' R' R'
,~
~ N
Cl - Si - Cl + 3 n NH3 ~- Si - N~ 2 n NH4Cl
(I) (II~
where n is an integer.
Elimination of the dialkyl~mino group allows further
crosslinking of the oligomers with one another, giving
polysilazanes containing s~ructural units of the formula
S ~III)
(NH)1/2 1 ~ R' R'
~- Si - N ~ Si - N; - (III)
R and R' may in this case be identical or different
radicals. Possible radicals R and R' are, for example,
R = (Cl-C4)alkyl, vinyl or phenyl, and
R' = ~Cl-C4)alkyl.
In the abovementioned formula, x and y are the molar
fractions of the two structural unit~, where x + y = 1
and x can have a value in the range from 0.75 to 0.95.
Examples of other polysilazanes which can be used for the
production of the moldings according to the invention are
descxibed in the abovementioned publications.
To produce the moldings according to the invention, the
polysilazanes are either preæsed directly into the
desired shape or first dissolved in a solvent, for
example THF, hexane or toluene, and ~ubsequently spun to
give fibers in a piston spinning machine or extru~ion
spinning machine at an atmospheric humidity of less than
- 4 ~ 5~7~
1% (for example in dry nitxo~enl.
The moldings according to the invention having a core/
cladding structure are produced from poly3ilazane mold-
ings in a ~ubsequent step by introducing the poly-
sila2ane molding~ in~o a ga~ atmosphere having a defined
water content. Diffusion of the water through the surface
of the fibers cau~es breaking of the Si-NH-Si bonds.
Substitution of NH by O wi~h formation and elimination of
ammonia gives siloxanes, which are likewise cro~slinked
with one another. The atmosphere has a rPlative humidity
in the range from 20 to 100%, preferably from 30 to 90%.
The temperature is between 10 and 200C, pxeferably in
the range from 20 to 50C. ~he gas atmosphere is
generally purified air, but may preferably be nitrogen or
a noble gas, in particular helium, particularly prefer-
ably mixtures of these gases. The siloxanessilazane ratio
in the cladding layer which forms can be regulated via
the moisture content of the akmosphere as a function of
the temperature.
The minimum residence time of the moldings depend~ on the
respective temperature:relative humidity (RH) ratio. At
relatively low temperatures, a higher relative humidity
is preferred. The minLmum residence tLmes are determined
by measuring the optical transmis~ion of the moldings.
The time at which the transmission of the molding no
longer changes to a measurable extent is referred to as
the minimum residence time. The residence tIme~ are
furthermore dependent on the shape of the molding and on
the polysilazanes employed as starting component~. ~hey
are on average about 5 hours at 50 D C ~ about 2 hours at
80C and about 0.~ hour at 110C. Through a further
increase in temperature, it is possible to ~urther reduce
the minimum residence times. Xn khe process according to
the invention, residence times of from 0.1 to 24 hour~
resulted at temperatures in the range from 10 to 200C
and relative humidities in the range from 20 to 100%.
- 5 ~ ~ 7~
The optical moldings producQd by this process ha~e a
cladding comprising
from 1 to 3% by weight of O,
from 20 to 25% by weight of N and
from 30 to 40~ by weight of Si.
The core compri~es 100% of a polysilazane or polyhydr~do-
chlorosilazane. ~h~ thi~kne~6 o~ the cladding i~ in the
range from 0.1 to 5 ~m, preferably in the range from 0.2
to 2.0 ~m. The attenuation of the molding~ according to
the invention i~ from 0.1 to 10 dB/cm.
The silicon-containing core/cladding moldings produced by
the process according to the invention have high flexi-
bility, high stability under mechanical load and good
heat resi~tance up to 200C and are stable e~en under
extreme wea~hering conditions. Due to their high heat
resistance, they are particularly suitable for illumi-
nation and data-transmission applications (optical
fibers) and for optical ~ensors, in particular at
locations which experience considerable heating, such as,
for example, in automobiles.
Example 1:
A polymeric silazane fiber wa~ first produced under a dry
nitrogen atmosphere by means of a piston spinning
machine. The fiber was then stored for 48 hour~ at 25C
and a humidity of 80%, forming an optical cladding. The
diameter of the fiber was 1 mm.
The transmission of the fiber was measured using a light
source (HeNe laser) and a detector which was movable
along the fiber and a detectox at the end of the fiber.
The two detector~ are eguipped with Ulbricht globes in
order to prevent anisotropy effects. Thi~ constxuction
made it possible 'co measure the attenuation of the fiber
and its uniformity (center6 of scattering) (Figure 1~. A
- 6 ~ 77~
value of about 0.2 d~/cm and 633 nm was ohtained.
Example 2:
The ~easur~ment station wa~ u~ed to dekermine the
dependence of the attenuation on the residence ~ime in
the ga~ atmosphere. The minLmum xesidencs tLme necessary
was determined as the time at which the transmission no
longer changes to a measurable exten~. The re~ult for an
N2 atmosphere (95~ RH) was S hours ~or 50C, 2 hours for
80~Cr and 0.2 hours for 110C for a 1 mm fiber.
Example 3:
The fiber produced in Example 1 wa~ ~tored for 2 days at
150C undex purified air and at 80~ relative humidity,
Remeasurement of the at~enuakion gave no significant
change in the transmission.
Example 4:
The fibers produced as described in Example 1 were cured
at 100C and 2G0C under nitrogen and air. A change of
less than 1% in the transmission occurred.
Example 5:
A fiber with a diameter of 0.5 mm produced in accordance
with Example 1 was used as an optical sensor. To this
end, the fiber was placed in the vicinity of a liyht
source (halogen lamp~O The intensity of the light enter~
ing the fiber, which was proportisnal to the entire light
output of the lamp, could thus be employed to stabilize
the light output by means of a control circuit. The
temperature in the vicinity of the fiber was above 130C.
This arrangement made it pos~ible to keep the light
intensity entering the fib~r con~tant over several dayR
in an experiment for measuring vibrations by optical
means.
