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FELLER FOR AN OPTICAL TRANSMISSION
ELEMENT HAVING AT LEAST ONE
BACKGROUND OF THE INVENTION
The invention relates to filler for an elongated optical transmission element which contains at least one optical waveguide and at least one protective covering enclosing the optical waveguide, the filler having a component in the form of a plasticizer.
U.S. Pat. No. 4342.500 discloses a cable which is high voltage-resistant and can therefore be used in the region of high voltage installations. If such a cable were filled only with crosslinked synthetic materials (polyurethane, epoxy resin, polyester resin, see column 1, lines 58 and 59), air gaps would then be unavoidable. Owing to the high field strength concentrations, these air gaps would lead to difficulties. For this reason, these existing plastics materials are plasticized by the addition of appropriate plasticizers, because it is possible in this way to avoid the formation of such undesirable cavities. Thus, the addition of plasticizers in this prior art reference has the sole purpose of plasticizing the existing plastics material sufficiently to achieve complete filling of all gaps with the plasticized plastics material and thus to overcome the difficulties with the high voltage fields. WO 92/00368 discloses a filler for optical waveguide cables, which filler may contain polypropyleneoxyglycol (PPG), as is evident from the examples in Table 1 on page 16 and from Examples 6-11 disclosed on page 14. However, these polypropyleneoxygiycols may be considered to be suitable for use only when having molecular weights of at least 3000. Molecular weights between 3000 and 8000 are preferably used, as disclosed in claim 1 of the reference. Page 14, lines 22 to 24, discloses that embodiments 6 and 7 are not suitable for use and that molecular weights of more than 3000 are therefore used. EP 029 198 91 discloses a filler for optical transmission elements which comprises a mixture of an oil and a thixotropic agent and additionally contains an organic thickener which completely or partially comprises halogenated and/or halogen-free hyarocarbon polymers. Other fillers likewise containing oils are described in DE 38 39 596 Al and U.S. Pat. No. 4,701,016. German Offenlegungsschrift 27 28 642 discloses a longitudinally water-tight optical waveguide cable in which swollen polystyrene in oil is used as a lead filling material or wire filler. GB-A 2 059 097 discloses an optical cable which contains a cable sheet comprising stabilized PVC material. Such cable sheets must be flexible and in particular the cable sheet cannot become rigid when low temperatures are encountered. In order to ensure sufficient flexibility at temperatures down to -50° C, the PVC compound of the cable sheet contains a di(2-ethylhexyi) sebacate (DOS) plasticizer. Specifically, the PVC compound may contain 45 to 70 parts by weight of the DOS plasticizer, 10 to 50 parts by weight of a filler and 3 to 8 parts by weight of stabilizers, relative to 100 parts by weight of the PVC. This compound is converted into granules, which can be applied to a cable core containing optical waveguides by means of an extruder. This prior art states nothing about the use of a filler, i.e., a material which can be arranged between a cable sheet and the optical waveguides.
The requirements which such wire fillers have to meet are that the optical waveguides are as far as possible not subjected to any impermissible tensile and/or compressive forces, i.e. the filler must not be too rigid. Since this should
also be the case at relatively low temperatures, for example -30°, the oils used have to meet particularly high requirements so that they do not harden excessively at the low temperatures. Furthemore, these fillers should be sufficiently
5 resistant to dripping out (drip test), and this is a critical requirement particularly at relatively high temperatures. By using thixotropic agents in the known wire fillers, it is possible to advantageously influence the behavior, particularly at relatively high temperatures, to which the use of
10 thickeners finally also contributes. However, the disadvantage of the known fillers is that their molecular composition is relatively non-uniform, owing to the different base components used. Furthermore, expensive base oils must be used if the wire filler is to meet high requirements. Moreover.
15 conventional fillers also interact with poly olefins .i.e. exhibit an increase in mass of >5% by weight. As a result, the properties of the polyolefin wire sheaths change.
SUMMARY OF THE INVENTION
20 It is the object of the invention to provide a filler which can be prepared in a particularly economical manner. This object is achieved, according to the invention, in the case of a filler of the type stated at the outset, if monomeric plasticizers are used; if the molecular weight of the mono
25 meric plasticizer is chosen between 200 and 2000 g/mol; if the plasticizer accounts for between 30% by weight and 99% by weight and preferably between 80% by weight and 95% by weight of the filler; and if the filler acquires its characteristic behavior through the proportion of plasticizer within
30 the total composition.
The invention is furthermore achieved if the plasticizers used comprise one or more PVC plasticizers but not including those which consist of polypropylene glycol; if the
35 plasticizer accounts for between 30% by weight and 99% by weight and preferably between 80% by weight and 95% by weight of the filler; and if the filler acquires its characteristic behavior through the proportion of plasticizer within the total composition.
4q Such plasticizers are relatively cheap as they are used in large amounts in chemistry. They also have the advantage that they are substantially inert, that is to say in particular that they do not attack or impair, for example, protective coverings or cable sheaths. Such plasticizers can be particu
45 lady readily processed, have a low vapor pressure and are mostly odorless, colorless and in general also light-stable, stable to low temperatures and heat-stable. A further advantage of the plasticizers is that they are not hygroscopic, which is important particularly in connection with optical
50 waveguides, owing to their H2 sensitivity. Moreover, such plasticizers are not hazardous to health, have low flammability and are not very volatile. When plasticizers are used, the increase in mass of polyolefin wire sheaths is <1% by weight.
55 Another major advantage of the plasticizers is that they have relatively high molecular uniformity of their compounds. In particular, so-called monomer plasticizers have complete molecular uniformity. However, polymer plasticizers also exhibit relatively high molecular uniformity of
60 their compounds. Hence, the properties, such as the temperature behavior, the flashpoint, the vapor pressure, the viscosity and the drip behavior, can be particularly readily adjusted and stabilized. The use of molecules having a relatively uniform composition has a particularly advanta
65 geous effect, especially on the drip behavior.
In the invention, plasticizers which are still liquid at 20° C. or have a viscosity of less than 15,000 mPa-s are
preferably used. Hence, the processing effected at room temperature (for example, introduction of the filler into a protective covering or into a cable sheath) can be carried out in a particularly simple and rapid manner.
Apart from the plasticizers, additional substances may be introduced into the filler, but the amount of the plasticizers should account for between 30% by weight and 99% by weight with the total composition, preferably between 80% by weight and 95% by weight. The characteristic behavior of the filler is thus imparted to it by the proportion of the plasticizer within the total composition.
Since the plasticizers are also readily compatible with one another and are miscible, it is also possible to introduce plasticizers having different chemical compositions into the filler. Hence, when the term plasticizer is used below, this means that it may also be a mixture of several different plasticizers.
A thickener, in particular in the form of an organic thickener, may also be added to the filler, and hydrocarbon polymer may advantageously be used. The amount of the thickeners in the filler should expediently be chosen to be between 1 and 5% by weight. Additives in the form of small spheres may also be introduced as thickeners into the filler, in particular the use of small hollow spheres is advantageous owing to their great compressibility and easy processability. The external diameters of such small spheres are expediently substantially smaller than the diameter of the optical waveguide, i.e. under 100 um.
The fillers may also expediently be mixed with further components and in particular an addition in the form of a thixotropic agent is expedient. Thixotropic agents have a particularly advantageous effect on the drip behavior. In particular, addition of finely divided Si02 (so-called colloidal silica—e.g. "Aerosil" from the firm Degussa) is expedient as a thixotropic agent. Furthermore, thixotropic agents my also comprise alumina and/or bentonites as well as mixtures of these substances. The addition of thixotropic agents should not exceed 30% by weight, and values between 7 and 12% by weight are particularly advantageous if the invention is used as a so-called wire or lead filler for optical waveguides. In such an optical transmission element, the filler is present between a protective covering and one or more optical waveguides.
For use as a thixotropic wire or lead filler, the following values are expedient:
Plasticizer: 80% by weight to 95% by weight
Thixotropic agent: 5% by weight to 15% by weight
Thickener: 1% by weight to 5% by weight
In the case of thixotropic core fillers, the following values are expedient:
Plasticizer: 30% by weight to 90% by weight
Thixotropic agent: 5% by weight to 15% by weight
Thickener: 1% by weight to 5% by weight
The proportion in the form of a plasticizer is expediently 55 chosen so that the filler as a whole has a viscosity of between 5000 mPa-s and 25,000 mPa-s and preferably between 10,000 mPa-s and 15,000 mPa-s, at 20° C. In the region of the lowest temperature of the transmission element (-30° C), the viscosity should expediently be less than 25,000 g0 mPa-s, while in the region of the highest temperature (+60° C.) the viscosity should expediently still be 5000 mPa-s.
As a result of the addition in the form of a thickener, the viscosity of the filler can be increased according to requirements.
It is advantageous if the molecular composition of the plasticizer in the filler is chosen to be as uniform as possible.
As a result of this, the filler then exhibits particularly uniform behavior and, for example, certain components do not begin to drip out at certain limiting temperatures (separation of low molecular weight components), while others still show no tendency to drip out. The difference in the chain length of the molecules of the plasticizer should expediently be chosen to be less than a factor of 10.
If monomeric plasticizers are used, the molecular weight is advantageously chosen to be between 200 and 2000 g/mol, and it is preferable to use values of between 200 and 800 g/mol. When polymeric plasticizers are used, it is expedient to choose the average molecular weight to be between 1000 and 10,000 g/mol, and preferably between 3000 and 6000 g/mol.
Plasticizers having particularly advantageous properties and a low price are PVC plasticizers, i.e. substances which are added to PVC-based plastic materials. PVC plasticizers also have the advantage that they exhibit particularly little migration. Particularly aromatic and/or aliphatic polycarboxylates when used as plasticizers, are inert and particularly cheap organic substances which have a low vapor pressure and are produced in large amounts.
Esters can be particularly advantageously used in this composition, in particular the following groups of esters are usable:
polyesters of adipic, sebacic and azelaic acid, phosphates,
hydroxycarboxylates and fatty esters.
Examples of plasticizers comprising phthalates in the above groups are:
Examples of plasticizers based on polyesters of adipic, sebacic and azelaic acid are:
Sebacic acid: 1,10-Decanedlcarboxylic acid with
1.2- ethanediol (ethylene glycol)
2.2- dimethyl- 1,3-propanediol
Azelaic acid: 1,9-Nonanedicarboxylic acid with
1.3- propanediol 1,2-propanediol