Polyamide sheathed optical waveguide fibers

A method for manufacturing sheathed high tensile strength optical waveguides having drawn glass fiber optical cores in which the cores are passed through a molten thermoplastic polyamide to form an adhering coating which is then cooled to form a uniform sheath. The sheath is flexible over a temperature range of -40.degree. C. to +80.degree. C. and the polyamide contains dimeric fatty acid as one of its monomer components and has a glass transition temperature below -10.degree. C.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method for manufacturing a high tensile 
strength optical waveguide fiber in which a primary layer of a 
thermoplastic polyamide is applied to optical fibers after they are drawn, 
and while the polyamide is molten. 
2. Statement of the Related Art 
Optical waveguides consist essentially of glass fibers which have a 
refractive index profile such that incident light is guided in them around 
curves. Accordingly, they may be used as a light and/or signal 
transmission medium. The successful use of glass optical waveguides 
requires maintaining the high tensile strength of the glass fibers and 
avoiding increases in attenuation. To maintain the high tensile strength 
of the glass fibers, the optical fibers are sheathed immediately after 
drawing in at least one protective layer of plastic. The optimal layer 
thickness of the plastic film layer is determined by its critical 
mechanical properties, such as E-modulus and hardness, and is generally in 
the range from 10 to 100 microns (.mu.). 
To avoid attenuation losses, the glass fibers have to be surrounded by a 
flexible material which does not show any phase transitions, particularly 
over the required in-use temperature range of -40.degree. C. to 
+80.degree. C. Thus, the glass transition temperature of the material must 
be &lt;-40.degree. C. while the change in modulus over the above-mentioned 
temperature range should amount to less than 2 powers of ten. Neither 
should there be any change in length at temperatures in the range from 
-40.degree. to +80.degree. C. This type of layer is called a primary 
coating. If required, another protective layer (i.e. a secondary coating) 
may be applied to the primary coating. 
In the coating of optical fibers by lacquering, the bare fiber passes 
immediately after drawing through one or generally several coating units 
each followed by a drying zone. The coating units may be charged with 
non-reactive coating materials, i.e. polymers soluble in organic solvents, 
such as cellulose acetate, polyvinylidene fluoride or polyester imide. 
Hitherto, the use of commercial polyamides containing dimerized fatty acid 
has been curtailed by their inadequate low-temperature flexibility, 
because their glass transition temperatures are higher than -10.degree. C. 
One of the disadvantages of the above-mentioned lacquering technique is 
that, in general, only very thin layers (approx. 5.mu.) can be uniformly 
applied in each coating cycle. Although a layer thickness of approx. 
30.mu. per coating cycle can be achieved where thermally crosslinkable 
polysiloxanes are used, the protective film formed in this way is soft and 
critically lacking in mechanical strength. 
In addition to the thermosetting films applied by lacquering, coating 
materials based on acrylates of epoxide, polyurethane and silicon 
prepolymers which can be crosslinked by shortwave light have recently been 
introduced. Although film thickness of from 20 to 50.mu. per coating cycle 
can be achieved with systems of this type, the aging behavior of these 
coating materials and their effect on the static fatigue of the optical 
fibers have not yet been resolved. The problems presented by the residual 
monomer content also have not been satisfactorily solved. 
DESCRIPTION OF THE INVENTION 
Other than in the operating examples, or where otherwise indicated, all 
numbers expressing quantities of ingredients, reaction conditions, or 
defining ingredient parameters used herein are to be understood as 
modified in all instances by the term "about". 
This invention affords a manufacturing method which does not have the 
disadvantages of lacquering and which enables a high tensile strength 
optical waveguide to be produced in one pass while still providing 
adequate mechanical protection of the waveguide core. Advantages of this 
invention are that the polyamides utilized: show high chemical stability 
with respect to the surface of the glass core and to any corrosive 
environments; have no adverse effect on the optical properties of the 
waveguide; critically permit utilization of the waveguide at temperatures 
of -40.degree. C. up to +80.degree. C.; and avoid the problems arising 
from residual monomers or encapsulated low molecular weight volatiles 
emanating from polycondensation found in many other thermoplastic 
polymers. Furthermore, the method of this invention critically permits 
protective layers to be applied to the optical fibers uniformly and at 
high speeds, and with almost immediate non-tacky solidification. 
This invention more specifically affords a method for manufacturing a high 
tensile strength optical waveguide in which, after the glass core fibers 
are drawn, they are passed through a melt of certain thermoplastic 
polyamides as described below, which melt is at a temperature of 
170.degree. to 260.degree. C., preferably 200.degree. to 215.degree. C. 
The fibers with a molten polyamide layer are then immediately and very 
quickly cooled, preferably by gentle application of a drying gas which is 
at a temperature of 20.degree. to 30.degree. C. The preferred gas is 
ambient air, although any inert gas such as nitrogen or argon is suitable. 
The quick cooling is important to ensure a uniform coating and to prevent 
the coated fibers from sticking to objects in an undesirable manner. 
Coating in this manner using the particular polyamides described below is 
previously unknown. 
Polyamides useful in this invention must have a glass transition point 
below -10.degree. C., preferably below -40.degree. C. and must retain 
flexibility over the temperature range of -40.degree. C. to +80.degree. C. 
showing a change in tension modulus over this temperature range of less 
than 2 powers of ten while maintaining a stable length. Particularly 
suitable polyamides have a torsion modulus of 10.sup.9 N/m.sup.2 at 
-40.degree. C. and of 10.sup.8 N/m.sup.2 at +50.degree. C. The softening 
point of suitable polyamides should be above 100.degree. C., preferably 
above 130.degree. C., most preferably above 150.degree. C. and at 
210.degree. C. their viscosity should be approximately 1,000 to 10,000 
mPa.s. 
Polyamides which meet all of the above criteria, and which also display the 
earlier mentioned advantages, are difficult to find. Particularly useful 
polyamides are synthesized from a combination of dimeric fatty acid, 
monomeric fatty or aliphatic acid, polyether diamine, and a lower 
molecular weight diamine. In all instances more than one species of each 
component may be used. 
More specifically, useful polyamides include the condensation products of: 
(a) at least one dimeric fatty acid present in 20 to 60, preferably 35 to 
49.5, most preferably 40 to 48 mol %; 
(b) at least one C.sub.6-22 monomeric fatty acid or C.sub.6-22 aliphatic 
dicarboxylic acid present in 0.5 to 20, preferably 1 to 15, most 
preferably 2 to 10 mol %; 
(c) at least one polyether diamine of the formula 
EQU H.sub.2 N--R.sub.1 --O--(RO).sub.x --R.sub.2 --NH.sub.2, (I) 
wherein 
x is an integer from 1 to 80, preferably 8 to 80, most preferably 8 to 40, 
R is a C.sub.1-6, preferably C.sub.2-6, aliphatic hydrocarbon which may be 
branched or linear, 
R and R.sub.1 are each a C.sub.2-6 aliphatic or cycloaliphatic, preferably 
aliphatic, hydrocarbon, which may be the same or different, present in 1 
to 50, preferably 2 to 35, most preferably 4 to 25 mol percent; and 
(d) at least one C.sub.2-40 lower molecular weight diamine present in 15 to 
50, preferably 15 to 48, most preferably 25 to 46 mol %. 
Dimeric fatty acids that may be used as component (a) are generally 
polymeric fatty acid mixtures resulting from the polymerization of 
unsaturated fatty acids, containing at least 50% (by weight) dimers with 
monomers and trimers to 100%. Mixtures containing at least 70% dimers are 
preferred, at least 90% dimers is more preferred, and most preferred is a 
mixture containing 90 to 98% dimers, 1 to 7% monomers, and trimers to 
100%. The dimers should be formed from C.sub.12-22, preferably 
C.sub.14-20, most preferably C.sub.16-18 fatty acid monomers. The monomers 
are usually unreacted monomer starting material which may be unsaturated 
and/or branched and the trimers are usually over-reacted starting 
material. These dimeric fatty acid mixtures are commercially available, 
and a preferred mixture has 96% dimer, 3% trimer, and 1% monomer, in which 
the starting (monomeric) fatty acid was a C.sub.16-18 mixture containing 
70% by weight of a C.sub.18 fatty acid. 
Monomeric fatty acids that may be used as component (b) should have 6 to 
22, preferably 12 to 22 carbon atoms, examples of which are stearic, 
oleic, palmitic, a mixture of at least 50% palmitic with the balance to 
100% myristic, or any combination of the foregoing acids. Aliphatic 
dicarboxylic acids that may be used as component (b) should have 6 to 22 
carbon atoms and preferably are mixtures having chain length ranges of 
C.sub.6-12 or C.sub.12-22 such as tall oil acids. Useful acids also 
include adipic, azelaic, sebacic, decanedicarboxylic, and mixtures 
thereof. Mixtures of the above monomeric fatty acids and aliphatic 
dicarboxylic acids may also be used. 
Polyether diamines of formula I used as component (c) are known compounds, 
many of which are commercially available. Polyethers with two terminal 
amino moieties built up from branched or unbranched butane diols, pentane 
diols and/or hexane diols are useful as well as mixed ethers with two 
terminal amino moieties. Examples of preferred, commercially available, 
polyether diamines are: bis-(2-aminopropyl)-polyoxypropylenes and 
bis-(3-aminopropyl)-polytetrahydrofurans, having molecular weights of 500 
to 5,000, especially 700 to 2,500. 
Lower molecular weight diamines used as component (d) should have 2 to 40, 
preferably 2 to 20, most preferably 2 to 6 carbon atoms, and are known 
compounds. These include aliphatic diamines which may be branched or 
linear, for example, ethylenediamine, 1,3-diaminopropane 
1,4-diaminobutane, neopentyldiamine, hexamethylenediamine, 
trimethylhexamethylenediamine, and their mixtures. Dimer diamines (i.e. 
diamines obtained from dimeric fatty acids in which the carboxyl moieties 
are substituted by amino moieties) are also useful. Useful cycloaliphatic 
diamines include: diaminodicyclohexylmethane; 
3-aminomethyl-3,5,5-trimethylcyclohexylamine; and their mixture. Aromatic 
diamines such as diaminodiphenylmethane, arylaliphatic diamines such as 
xylylenediamine, and heterocyclic diamines such as piperazine, 
dimethylaminopiperazine, and dipiperidylpropane are also suitable. 
Aliphatic and dimer diamines are preferred. 
The above polyamides may be prepared in any known manner by melt 
condensation. Typically, the acid components react with the amine 
components at 150.degree. to 250.degree. C., and the water of reaction is 
removed by distillation, under vacuum, and/or the use of an azeotrope. The 
amine or acid number can be influenced in a known manner by the proper 
control of the reaction and the mol ratio of the acid or amino moieties 
present. 
The above polyamides are described in published German patent application 
No. 31 11 226 and corresponding U.S. patent application No. 06/678,230, 
both of which are entirely incorporated herein by reference. 
Other suitable polyamides are those disclosed in U.S. Pat. No. 4,062,828, 
which retain their flexibility at temperatures as low as -20.degree. C. 
preferably -40.degree. C., and which may be identified by simple 
experimentation. U.S. Pat. No. 4,062,828 is also incorporated herein by 
reference. 
Optical fibers having a single layer of a polyamide according to this 
invention are generally adequate for use as optical waveguides. However, 
if desired, a secondary layer of a different polymer may be applied using 
any known methods, such as lacquering, or melt dipping (under conditions 
that do not disturb the primary polyamide layer). The secondary layer does 
not have to meet the flexibility requirements imposed on the primary 
layer. Accordingly, commonly used polymers such as a nylon-type polyamide, 
polyethylene, copolymer of vinyl acetate and ethylene/propylene, 
polyester, thermoplastic elastomer, polyfluoroethylene, or the like, may 
be used for the secondary layer.

EXAMPLE 1 
A thermoplastic polyamide useful in this invention was produced from the 
following constituents: 
(a) 653.1 g of dimerized fatty acids* 
(b) 12.6 g of tall oil fatty acid 
(b) 59.1 g of sebacic acid 
(c) 167.1 g of bis-(3-aminopropyl)-polytetrahydrofuran (M.W. 1,100) and 
(d) 81.9 g of ethylene diamine 
using the following procedure: 
The carboxylic acids initially introduced were initially heated under 
nitrogen to around 60.degree. C. and the other reaction components 
subsequently added. The reaction mixture was then heated for 1 hour to 
230.degree. C. and left at that temperature for 1 hour, followed by 
evacuation to 15 mbar over the next 60 minutes at a constant temperature. 
After cooling to 210.degree. C., the reaction product was discharged and 
its determined characteristics were: 
Amine number: 5.2 
Acid number: 1.4 
Softening point: +168.degree. C. 
Glass transition temperature: -45.degree. C. 
Torsion modulus: -40.degree. C. 10.sup.9 N/m.sup.2 ; +50.degree. C. 
10.sup.8 N/m.sup.2. 
FNT * A mixture derived from C.sub.16-18 fatty acids containing 70% C.sub.18 
and having 96% dimer, 3% trimer, and 1% monomer, all percentages by 
weight. 
EXAMPLES 2-5 
The condensation reaction was performed in a glass flask properly equipped 
with tubes, initially in a nitrogen atmosphere and with agitation. The 
transferred carboxylic acids were first heated to approx. 60.degree. C. 
and the other reaction components were then added. The mixture was heated 
to 230.degree. C. within 1 hour, and this temperature was maintained for 1 
hour. During the next hour, a vacuum of 15 mbar was established at 
constant temperature. After cooling to 120.degree. C. the reaction product 
was drained off and isolated for determination of its properties. 
The following reaction components were used: 
(a) dimerized fatty acid (I) with 72% dimer content, 
(a) dimerized fatty acid (II) with 96% dimer content, 
(b) tall oil fatty acid, 
(c) bis-(3-aminopropyl)-polytetrahydrofuran, MW 750 (polyether diamine A). 
(c) bis-(3-aminopropyl)-polytetrahydrofuran, MW 1,100 (polyether diamine 
B). 
(c) bis-(2-aminopropyl)-polyoxypropylene, MW 2,000 (polyether diamine C). 
(d) ethylenediamine. 
(d) diamine prepared via the nitrile of a 96% dimerized fatty acid and 
followed by hydrogenation (dimer diamine). 
The amounts used as well as the amine and acid number of the reaction 
product are recorded in the table below, under the consecutive example 
numbers. Also listed are the softening point (R+B, ASTM E-28) and the 
flexibility values, found at low temperatures. The latter was determined 
by wrapping a test piece with the dimensions 20 mm.times.170 mm and a 
thickness of 1 mm by 360 deg around a brass cylinder with a diameter of 
25.6 mm. The tests were carried out with decreasing temperatures (test 
piece and cylinder in temperature equilibrium) to determine the lowest 
temperature at which three of five test pieces survived the wrapping test 
without breaking. 
TABLE 1 
______________________________________ 
Content (g) 
Component Ex. 2 Ex. 3 Ex. 4 
Ex. 5 
______________________________________ 
Dimeric fatty 759.2 795.2 
795.2 
acid I 
Dimeric fatty 820.8 -- -- 
acid II 
Tall oil fatty 
59.8 34.2 59.8 
59.8 
acid 
Ethylenediamine 
81.0 63.5 83.2 
82.8 
Dimer diamine -- 173.4 -- -- 
Polyether 112.4 168.7 -- -- 
diamine A 
Polyether -- -- 123.6 
-- 
diamine B 
Polyether -- -- ' 380.0 
diamine C 
______________________________________ 
Analytic data for Examples 2-5 is as follows: 
TABLE 2 
______________________________________ 
Softening 
Flexibility 
Example Amine No. Acid No. point (.degree.C.) 
to (.degree.C.) 
______________________________________ 
2 0.7 9.8 +105 -50 
3 5.5 1.2 +96 -60 
4 7.6 1.0 +106 -50 
5 2.9 1.4 +105 -60 
______________________________________ 
COMATIVE EXAMPLES 6 and 7 
As described in preceding Examples 2-5, polyamide condensation products 
were prepared from the following batches: 
(Example 6) 
802.5 g polymeric fatty acid I 
55.5 g tall oil fatty acid 
65.1 g ethylenediamine 
91.5 g 4,7,10-trioxatridecan-1,13-diamine 
(Example 7) 
810.0 g polymeric fatty acid I 
45.0 g tall oil fatty acid 
63.0 g ethylenediamine 
91.8 g 4,9-dioxadodecan-1,12-diamine 
Analytic data obtained in the same manner as for Examples 2-5 is shown in 
the following table. 
TABLE 3 
______________________________________ 
Comparative Softening 
Flexibility 
Example Amine No. Acid No. point (.degree.C.) 
to (.degree.C.) 
______________________________________ 
6 3.2 5.2 +93 -30 
7 4.2 1.6 +92 -25 
______________________________________ 
The polyamides of Comparative Examples 6 and 7 are markedly inferior to 
those of Examples 1-5 because their flexibility does not extend to 
temperatures of at least -40.degree. C., as is critical in this invention. 
EXAMPLE 8 
A melt of the polyamide of Example 1 was used at 205.degree. to 210.degree. 
C. for coating an optical fiber by melt dipping and cooling in accordance 
with the teachings of this invention. The optical fiber obtained in this 
way was covered by a uniform polyamide layer 60.mu. thick. The optical 
properties of the optical fiber were not adversely affected, i.e., no 
optical attenuation was observed.