Optical fibre element comprising a polyacrylate coating layer reticulated by UV radiation and an H2-absorbing buffer

Abstract: The optical fibre element comprises at least one layer of coating constituted by a polyacrylate reticulated by UV radiation and at least one H₂-absorbing buffer, said polyacrylate comprising at least one photo-initiator of UV reticulation possibly together with an activator for it wherein said photo-initiator and its activator, if any, are selected from the group which comprises those photo-initiators and those activators which cause a reduction in the hydrogen absorption capacity of less than 30% in a H₂-absorbing buffer which has been in contact with said polyacrylate containing them for 30 days at 100°C in an air atmosphere saturated with water vapour at 150 mbar.

Description: The present invention relates to an optical fibre element which comprises a polyacrylate coating layer crosslinked by UV radiation and an H 2 -absorbing buffer. During the course of the present description and of the claims which follow the expression" optical fibre element" is used to indicate elements for the transmission and/or processing of optical signals such as, for example, optical fibre cables, joints for optical fibre cables, terminals for optical fibre cables, amplifiers with active core optical fibre, optoelectrical repeaters, optical fibre sensors and such like. It is known that, to enable them to be handled, optical fibres are coated with a polyacrylic protective sheath crosslinked by UV radiation. Said sheath of polyacrylates crosslinked by UV radiation consists preferably of a primary coating, of a secondary coating and, possibly, of an external coloured layer. Said sheath is usually obtained from mixtures of prepolymers (oligomers) having an acrylic base with reactive functional diluents, a photo-initiator and other secondary additives; according to the photo-initiator used, it is sometimes also necessary to add an activator. In the case of the primary and secondary coating, the respective mixtures are applied during the spinning step and crosslinked on line by UV radiation; in the case of the coloured layers, it is on the other hand preferred to apply the suitable mixture at a later stage to the fibre already protected by the primary and secondary layers. It is also known that other layers of polyacrylate crosslinked by UV radiation can be present in optical fibre elements having a more complex structure such as, for example, optical fibre tapes. It is lastly known that hydrogen exerts harmful effects on optical fibres and jeopardize their efficiency. It has therefore been proposed to incorporate, in the optical fibre elements, some compositions capable of capturing hydrogen before it exerts its harmful effects on the fibres. In the title, description and claims said compositions are, for short, indicated with the expression "H 2 -absorbing buffers". The Applicant has developed H 2 -absorbing buffers substantially consisting of compositions comprising at least one unsaturated organic compound of silicon and at least one hydrogenation catalyst selected from the group comprising the salts and the organic compounds of transition metals. Examples of these compositions are described in the US patent No. B-4,688,889 and in the Italian patent applications No.s 20,833 and 20,834, both dated 2 July 1990. GB-A-2 163 172 discloses an UV-curing composition suitable for use as a secondary cladding material for optical fibers. Said composition comprises an acrylic oligomer and a photopolymerization initiator. The composition may also comprise a photosensitization assistant (i.e. an activator). However, this document does not recognize that some initiators and/or activators poison the hydrogenation catalyst, based on a transition metal, of H 2 -absorbing buffers. It has now been unexpectedly observed that that some polyacrylates crosslinked by UV radiation poison the hydrogenation catalyst, based on transition metals, of the H 2 -absorbing buffers. Studies carried out with the object of identifying the cause of such poisoning have shown that the capacity to inactivate the H 2 -absorbing buffer is not an intrinsic characteristic of said polyacrylates but is to be looked for in some compounds added in order to foster UV radiation. More in particular, it has been found that said poisoning is due to some photo-initiators of UV crosslinking and/or to some activators, used together with with some photo-initiators, in spite of the fact that, on occasion, their quantity is only of the order of 10- 4 + 10- 5 parts by weight with respct to the quantity of H 2 -absorbing buffer in contact with the polyacrylate crosslinked by UV radiation. It has also been found that the poisoning of the hydrogenation catalyst also takes place when the poisoning photo-initiators and/or activators are incorporated in only the primary coating of the fibre and not in the secondary coating in direct contact with the H 2 -absorbing buffer. The photo-initiators and the activators which inactivate the H 2 -absorbing buffer are identified by means of a test, described in detail later, with which it is possible to measure the quantity of hydrogen absorbed by a H 2 -absorbing buffer before and after it has been placed in contact with the polyacrylate, crosslinked by UV radiation, for 30 days at 100°C in an air atmosphere saturated with water vapour at 150 mbar. On the basis of the experience acquired it has been established that the photo-initiators and/or their activators can be considered non-inactivators when the capacity of residual absorption of the H 2 -absorbing buffer, which has been in contact with the polyacrylate reticulated by UV radiation for 30 days at 100°C in an air atmosphere saturated with water vapour at 150 mbar, is greater than 70%. In one embodiment, the present invention relates to an optical fibre element according to claim 1. In a further embodiment, the present invention relates to an optical fibre cable according to claim 4. A typical example of H 2 -absorbing buffer suitable for determining whether a photo-initiator and its activator, if any, causes a reduction in the hydrogen absorption capacity after having been in contact, for 30 days at 100°C in an air atmosphere saturated with water vapour at 150 mbar, with a polyacrylate containing them is that described in example 1 of the US patent 4,688,889; said H 2 -absorbing buffer is substantially constituted by (a) 90 parts by weight of a vinyl- terminated dimethylsiloxane, with no chain unsaturations and having a content of unsaturated groups equal to 7.4 mmol per 100 g of product, b) 0.2 parts by weight of palladium powder having an average size 48 microns and (c) 10 parts by weight of colloidal silica. Examples of optical cables and of components thereof in the production of which it is useful to employ the present invention are those described in the following documents: US-B-4,688,889; UK-A-2,172,410; EP-A-280,275; FR-A-2,200,535; UK-A-1,598,540; UK-A-2,021,282;; UK-A-2,099,173; UK-A-2,164,471; UK-A-2,170,921; UK-A-2,174,822; US-B-4,143,942; US-B-4,153,332; US-B-4,199,224; US-B-4,491,386; US-B-4,491,387;; US-B-4,676,590 and US-B-4,690,498. Examples of suitable photo-initiators according to the present invention are those which do not contain any atoms of nitrogen, phosphorus or sulphur in their molecule and which do not require the addition of amino activators. Examples of suitable photo-initiators which do not contain any atoms of nitrogen, phosphorus or sulphur in their molecule and which do not require the addition of amino activators are benzoin and its alkyl ethers, acetophenone, dialkyl acetophenones, anthraquinone and its derivatives. Typical examples of photo-initiators according to the present invention are benzophenone, diethoxyacetophenone, dimethoxyphenylacetophenone, hydroxycyclohexylphenylketone and 2-hydroxy-2-methyl-1-phenylpropanone. The following tests and examples are used to illustrate the present invention without, however, limiting it in any way. TESTS The capacity of absorbing hydrogen on the part of H 2 -absorbing buffers was assessed by measuring the pressure drop in a hermetically-sealed container, containing the H 2 -absorbing buffer under examination in a hydrogen atmosphere. The apparatus used is an automatic commercial apparatus for the measurement of pressure in a range from 1000 to 1 mbar. The apparatus is accomplished by assembling a fixed-volume chamber provided with two valves (one of which is the pin-type to regulate the supply of hydrogen and the other of the usual type for connection to a vacuum pump) with a commercial pressure transducer type E 8510 connected to a commercial digital reader, both made by the firm Edwards Alto Vuoto S.p.A.. Inside the apparatus there is a glass container. The control unit providing a digital reading of the pressure has a resoluticn of 1 mbar, and the pressure indication is independent of the composition of the gas and of atmospheric pressure. The tests have been carried out a constant temperature of 23°C. After weighing the glass container to an accuracy of 0.01 g (weight A), the H 2 -absorbing buffer under examination was placed in the container; the glass container was then weighed a second time (weight B). The glass container holding the H 2 -absorbing buffer under examination was inserted into the apparatus and vacuum was applied for about 1 to 2 hours. After leaving the system in a static vacuum for at least 12 hours, the container was connected to a hydrogen cylinder until the digital pressure indicator indicated the desired pressure (generally some 500 or 1000 mbar). The hydrogen cylinder cock was closed, and the time as well as the hydrogen pressure were noted. After 24 hours the residual hydrogen pressure was read. The hydrogen absorption capacity expressed in norm cm 3 /g was calculated with the following formula: P - Pr x V x 273 1013 x 273 + C x B - A where P = initial hydrogen pressure, Pr = residual hydrogen pressure after 24 hours of test, C = temperature, in °C, during the test, V = free volume of the apparatus after insertion of the H 2 -absorbing buffer under examination, B = weight of the glass container with the H 2 -absorbing buffer, A = weight of the empty glass container. For each sample of H 2 -absorbing buffer the above test was carried out twice, and the average of the two values obtained was made. The above test was carried out on H 2 -absorbing buffers before and after having been in contact with optical fibres coated or with coating material films according to the methods described below. a) coated fibre A section of coated optical fibre was used having a length such as to respect the fibre/H 2 -absorbing buffer ratio existing in an optical fibre cable. Normally, such ratio is of 2 g of H 2 -absorbing buffer for each meter of optical cable. In the case of a single-fibre optical cable, the coated optical fibre was smeared with the H 2 -absorbing buffer whose hydrogen absorption capacity had been measured previously with the method described above. The coated optical fibre treated in this way was made to age at 100°C in a large sealed test tube in the presence of air (150mbar) and saturated water vapour. After one month, the H 2 -absorbing buffer was recovered and its residual hydrogen absorption capacity was measured with the method described above. b) coating material Three samples were prepared in the form of films having a thickness of 10-20mm by crosslinking in depth with UV radiation respectively: the material used for forming the primary coating, the secondary coating and the coloured layer of the optical fibre. The samples were prepared using the quantity of material with which about 5 meters of optical fibre are coated. Each sample was smeared with 10 g of H 2 -absorbing buffer for which the hydrogen absorption capacity had been determined previously with the method described above. Each sample was made to age as described under the previous point a); after one month, the H 2 -absorbing buffer was recovered and its residual hydrogen absorption capacity was measured with the method described above. EXAMPLE 1 Mixture for forming a layer of non-poisoning primary coating: silicon acrylate (EBECRYL ™ 350 by Radcure Specialties Inc., USA) 55 g aliphatic urethane acrylate (EBECRYL™ 230 by Radcure Specialties Inc., USA) 10 g hexandiole diacrylate (HDDA) 15 g isobornylacrylate 15 g diethoxyacetophenone 5 g hydrogen absorption capacity on the part of the H 2 -absorbing buffer: initial = 1.65 normal cm 3 /g final = 1.38 normal cm 3 /g EXAMPLE 2 Mixture for forming a layer of poisoning primary coating: silicon acrylate (EBECRYL™ 350 by Radcure Specialties Inc., USA) 55 g aliphatic urethane acrylate (EBECRYL™ 230 by Radcure Specialties Inc., USA) 10 g hexandiole dyacrylate (HDDA) 8 g isobornylacrylate 15 g benzophenone 5 g aminoacrylate (UVECRYL™ P115 by Radcure Specialties Inc., USA) 7 g hydrogen absorption capacity on the part of the H 2 -absorbing buffer: initial = 1.65 normal cm 3 /g final = 0.35 normal cm 3 /g The comparison between examples 1 and 2 shows that the poisoning is to be attributed to the aminoacrylate (activator). EXAMPLE 3 Mixture for forming a layer of non-poisoning secondary coating: epoxy acrylate (EBECRYL™ 600 by Radcure Specialties Inc., USA) 45 g aromatic urethane acrylate (EBECRYL™ 210 by Radcure Specialties Inc., USA) 5 g methacrylic acid 15 g trimethylolpropane triacrylate (TMPTA) 20 g tripropylenglycol diacrylate (TPGDA) 5 g dimethoxyphenylacetophenone 10 g hydrogen absorption capacity on the part of the H 2 -absorbing buffer: initial = 1. 70 normal cm 3 /g final = 1. 46 normal cm 3 /g EXAMPLE 4 Mixture for forming a layer of poisoning secondary coating: epoxy acrylate (EBECRYL™ 600 by Radcure Specialties Inc., USA) 45 g aromatic urethane acrylate (EBECRYL™ 210 by Radcure Specialties Inc., USA) 7 g methacrylic acid 15 g trimethylolpropane triacrylate (TMPTA) 20 g tripropylenglycol diacrylate (TPGDA) 5 g hydroxypropylphenyl ketone 5 g isopropylthioxanthone 3 g hydrogen absorption capacity on the part of the H 2 -absorbing buffer: initial = 1. 70 normal cm 3 /g final = 0.40 normal cm 3 /g The comparison between examples 3 and 4 shows that the poisoning is to be attributed to the isopropylthioxanthone (activator). EXAMPLE 5 Mixture for forming a non-poisoning coloured (ink) layer: polyester urethane acrylate (EBECRYL™ 810 by Radcure Specialties Inc., USA) 44 g tripropylenglycol diacrylate (TPGDA) 10 g trimethylolpropane triacrylate (TMPTA) 25 g white pigment (TiO 2 ) 5 g N-vinyl-2-pyrrolidone (diluent) 10 g 2-hydroxy-2-methyl-1-phenylpropane 6 g hydrogen absorption capacity on the part of the H 2 -absorbing buffer: initial = 1. 70 normal cm 3 /g final = 1.50 normal cm 3 /g EXAMPLE 6 Mixture for forming a poisoning coloured (ink) layer: polyester urethane acrylate (EBECRYL™ 810 by Radcure Specialties Inc., USA) 44 g tripropylenglycol diacrylate (TPGDA) 10 g trimethylolpropane triacrylate (TMPTA) 25 g white pigment (TiO 2 ) 5g N-vinyl-2-pyrrolidone (diluent) 9 g benzophenone 4 g 4,4'-bis(dimethylamino)benzophenone 3 g hydrogen absorption capacity on the part of the H 2 -absorbing buffer: initial = 1. 70 normal cm 3 /g final = 0. 48 normal cm 3 /g The comparison between examples 5 and 6 shows that the poisoning is to be attributed to the 4,4'-bis(dimethylamino)benzophenone (photo-initiator).

Claims: An optical fiber element comprising at least one coating layer and at least one H 2 -absorbing buffer, wherein said H 2 -absorbing buffer substantially is a composition comprising at least one unsatured organic compound of silicon and at least one hydrogenation catalyst selected from the group comprising salts and organic compounds of transition metals, characterized in that said coating comprises a polyacrylate crosslinked by UV radiation, in that said polyacrylate comprises at least one photo-initiator for UV crosslinking possibly together with an activator for said photo-initiator, and in that said photo-initiator does not contain any atoms of nitrogen, phosphorus or sulphur and does not require the presence of an amino activator, whereby said photo-initiator and its activator, if any, cause a reduction in the hydrogen absorption capacity of less than 30% in a H 2 -absorbing buffer which has been in contact with said polyacrylate containing them for 30 days at 100°C in an air atmosphere saturated with water vapour at 150 mbar. An optical fiber element according to claim 1, characterized in that the photo-initiator is selected from benzoin and alkyl ethers thereof, acetophenone, dialkyl acetophenones, anthraquinone and derivatives thereof. An optical fiber element according to claim 1 or 2, characterized in that the photo-initiator is selected from benzophenone, diethoxyacetophenone, dimethoxyphenylaceto phenone, hydroxycyclohexylphenylketone and 2-hydroxy-2-methyl-1-phenylpropanone. An optical fiber cable comprising an optical fiber element according to claim 1, 2 or 3.

Inventors: BOSISIO CLAUDIO, CAMPANA ANTONIO, BOSISIO, CLAUDIO, CAMPANA, ANTONIO
Applicants: PIRELLI CAVI SPA, PIRELLI CAVI S.P.A.
Application year: 1991
Main classification: G02B6
IPC codes: B01J20, C03C25, G02B6, C09D133