POLYMER CLADDINGS FOR OPTICAL FIBRE WAVEGUIDES

Abstract: An optical fibre comprising a core coated with a cladding composition having a lower refractive index than the core, said cladding composition comprising a fluori­ nated mono-acrylate, a polyfunctional cross-linking acry­ late being difunctional or higher, and a photoinitiator, said cladding composition comprising less than 0.3% by weight of a mono- or polyfunctional thiol and being cured or cross-linked.

Description: This invention relates to optical fibres and in particular to optical fibres comprising a glass core coated with a polymer cladding. The use of optical fibres as a means of transmitting data has received widespread interest. Information transfer using a modulated light beam guided by a glass or plastics fibre has been utilized in many applications including telecommunications and computer link-up and data base use. Advantages of the use of fibre optic linkages are very high information carrying capacity compared to metal wires carrying electrical signals and freedom from external interference. Optical fibres comprise a core, generally an inorganic glass such as fused silica or a synthetic resin, and a cladding of a material having a lower refractive index than the core, which cladding confines the light energy to propagate in the core by total internal reflection. The refractive index of fused silica is 1.458 at room temperature and there is a limited range of materials which have refractive indices below this value. The efficiency of propagation increases as the difference in refractive index between the core and the cladding increases. Generally, the refractive index of the cladding should be at least 0.03 units less than that of the core, preferably at least 0.05 units less. Prior art cladding materials include thermoplastic polymers which are coated on the optical fibre by melt extrusion through a die. This method of cladding suffers from the disadvantages that it is difficult to obtain thin coatings and the coatings tend to be loosely bonded to the silica core. Other polymers have been applied by solvent coating. However, solvent coating has the disadvantages that it may be necessary to coat a fibre several times until the desired thickness is obtained and the necessity of handling high solids solutions with attendant problems of bubbling of the coating. Furthermore, there is the additional problem of pollution of the environment during evaporation of the solvent. Cross-linkable polymeric coating compositions have been employed which are rapidly cured after coating by heating or exposure to ultra violet light. Examples of such compositions are disclosed in United States Patent Nos. 4099837, 4125644 and 4511209. British Patent No. 1262526 discloses an optical element e.g. a lens, view aperture etc.comprising a solid transparent base e.g. a thermoplastics material, on which is coated a fluorine-containing transparent, thermoset organic polymer having an index of refraction within 0.02 units of the base. The polymer is formed from a fluorine-free acrylic monomer and a fluorine-containing acrylic monomer, at least one of which monomers being polyfunctional. The specific coating compositions disclosed utilise at least 70% by weight of the fluorine free acrylic monomer. European Patent Publication No. 256765 discloses a cladding composition suitable for plastics optic fibres comprising: 1. a u.v. curable monofunctional acrylate or α-fluoroacrylate monomer; 2. a polyfunctional acrylate or α-fluoroacrylate compound having at least two acryloyl radicals, and 3. a photoinitiator. The refractive index of the cured composition is required to be at least 0.01 units less than the refractive index of the plastics core. The fluorinated acrylate or α-fluoroacrylate monomer preferably comprises at least 50%, more preferably from 50 to 98% by weight of the cladding composition with the polyfunctional acrylate or α-fluoroacrylate from 1 to 20% and the photoinitiator from 0.01 to 10% by weight of the composition respectively. Preferred monofunctional acrylate and α-fluoroacrylate monomers are represented by the following generale formulae: in which: Q and Y = H or F m is an integer from 1 to 12 n is an integer from 5 to 12, and p is an integer from 1 to 3 In the preferred practise of the invention, the monomer (1) comprises a mixture of from 1 to 40% of a compound represented by general formula (A) and from 99 to 60% of a compound represented by general formula (B). European Patent Publication No. 257863 discloses a cladding composition for a plastics clad silica or glass core optical fibre comprising: 1. a monofunctional fluorinated acrylate or α-fluoroacrylate composition comprising from 1 to 40% by weight of a highly fluorinated monomer represented by general formula (A) above and from 99 to 60% by weight of a lowly fluorinated monomer represented by general formula (B); 2. a polyfunctional acrylate or α-fluoroacrylate compound having at least two acryloyl radicals; 3. a photoinitiator, and where necessary 4. a stabilizer selected from thiodicarboxylic acid diesters and hindred phenols. United States Patent No. 4511209 discloses a cladding composition for plastic clad silica optical fibres comprising: (1) a highly fluorinated monofunctional acrylate with a refractive index below 1.38 and constituting more than 50% by weight of the composition, (2) a polyfunctional acrylate being trifunctional or higher serving as a cross-linking agent, (3) a mono- or polyfunctional thiol that functions as a synergist preferably a thiol containing silane eg. gamma-mercaptopropyl trimethoxy silane, and (4) a photoinitiator. The cladding compositions are dip or spray coated onto the fibre and exposed to ultra violet radiation to cure the coating. The hard clad optical fibres produced have attenuation often below 10 dB/km and exhibit superior temperature behaviour than silicone clad optical fibres. The present invention provides alternative cladding formulations for optical fibres. According to the present invention there is provided an optical fibre comprising a core coated with a cladding having a lower refractive index than the core characterised in that the cladding composition comprises: (a) one or more fluorinated mono-acrylates selected from the group consisting of (i) fluorinated mono-acrylates having a fluorinated cycloaliphatic radical having either a minimum (number) of three C-F bonds or in which at least 25% of the C-H bonds have been replaced by C-F bonds, whichever degree of fluorination is higher, and (ii) compounds of the general formula: in which: X represents H or an alkyl group of 1 to 5 carbon atoms, x is 1 or 2, R represents an alkyl group of 1 to 5 carbon atoms, and R f represents a fluoro-aliphatic radical or a fluorinated cycloaliphatic radical having either a minimum (number) of three C-F bonds or in which at least 25% of the C-H bonds have been replaced by C-F bonds, whichever degree of fluorination is higher, (b) a polyfunctional cross-linking acrylate being difunctional or higher, and (c) a photoinitiator, the composition comprising less than 0.3% by weight of a mono- or polyfunctional thiol and being cured or cross-linked. Also according to the invention there is provided a cladding composition for optical fibres comprising: from 50 to 95% by weight of a fluorinated mono-acrylate selected from the group consisting of (i) fluorinated mono-acrylates having a fluorinated cycloaliphatic radical having either a minimum (number) of three C-F bonds or in which at least 25% of the C-H bonds have been replaced by C-F bonds, whichever degree of fluorination is higher, and (ii) compounds of the general formula: in which: X represents H or an alkyl group of 1 to 5 carbon atoms, x is 1 or 2, R represents an alkyl group of 1 to 5 carbon atoms, and R f represents a fluoro-aliphatic radical or a fluorinated cycloaliphatic radical having either a minimum (number) of three C-F bonds in which at least 25% of the C-H bonds have been replaced by C-F bonds, whichever degree of fluorination is higher, from 2 to 35% by weight of a polyfunctional cross-linking acrylate being difunctional or higher, and from 0.5 to 20% by weight of a photoinitiator, the composition comprising less than 0.3% by weight of a mono- or polyfunctional thiol. The cladding composition of the invention may readily be applied by dip coating and can be immediately photopolymerised to cause curing or cross-linking, by exposure to ultra violet light to provide optical fibres having equivalent and often superior properties to those of the prior art. In particular the adhesion to glass of the cladding composition of the invention is superior to that of the composition of United States Patent No. 4511209. If the formulations of the invention additionally comprise other vinyl functionalized components e.g. (meth)acrylic silanes and (meth)acrylic acid the adhesion to glass is further increased. The cladding compositions of the invention differ from the compositions of United States Patent No. 4511209 in that they do not require the presence of a thiol synergist. The function of the synergist is not defined although suitable compounds are identified as being of the gamma-mercaptopropyl trimethoxy silane type. The most likely function of these compounds is to chain transfer the polymer chain to the mercapto function and then via the silane, bond the polymer to the core surface. This results in the polymer matrix having silane groups appended only at the termini of the polymer chains. In the composition of the present invention when adhesion enhancers, e.g., acrylic silanes, are employed it results in the incorporation of the adhesion enhancer through the matrix thereby significantly promoting adhesion. The fluorinated mono-acrylates used in the compositions of the invention possess one polymerisable vinyl group. The term "acrylate" is used in the generic sense and includes not only derivatives of acrylic acid, but also methacrylic and other modified acrylic acids. The fluorinated mono-acrylates may possess a fluorinated cycloaliphatic radical having either a minimum number of three C-F bonds or in which at least 25% of the C-H bonds have been replaced by C-F bonds, whichever degree of fluorination is higher. The fluorinated cycloaliphatic radical is generally a fluorinated, preferably saturated non-aromatic, cycloaliphatic radical. The chain may be interrupted by nitrogen or oxygen atoms bonded only to carbon atoms. A fully fluorinated group, such as perfluorocyclopentyl or perfluorocyclohexyl is preferred, but hydrogen or chlorine atoms may be present as substituents in the fluorinated aliphatic radical; generally not more than one atom of either is present in the radical for every two carbon atoms. The presence of a fluorinated cycloaliphatic radical provides the unobvious advantage of a tougher cured coating composition in which thermal oxidative degradation processes are retarded compared to acyclic compositions. It is then possible to extend the use of the cladded fibre to a broader temperature operating range without sacrificing the optical clarity and colourlessness of the coating. Specific examples of such compounds include: 1,1-dihydroperfluorocyclohexane carbinol acrylate, 1,1-dihydroperfluorocyclohexane carbinol methacrylate, 1,1-dihydroperfluorocyclopentane carbinol acrylate, and 1,1-dihydroperfluorocyclopentane carbinol methacrylate. Fluorinated monoacrylates of the following general formula may also be employed: in which: X represents H or an alkyl group, preferably CH₃, R f represent a fluoro aliphatic radical, preferably C y F 2y +1 in which y is an integer from 3 to 12, R represents an alkyl group, generally of 1 to 5 carbon atoms, and x is 1 or 2. The fluoroaliphatic radical R f is generally a fluorinated, preferably saturated, monovalent, non-aromatic, aliphatic radical of at least two carbon atoms. The chain may be straight, branched, or, if sufficiently large, cyclic, and may be interrupted by oxygen atoms or nitrogen atoms bonded only to carbon atoms. A fully fluorinated group is preferred, but hydrogen or chlorine atoms may be present as substituents in the fluorinated aliphatic radical; generally not more than one atom of either is present in the radical for every two carbon atoms. Preferably the radical contains a terminal perfluoromethyl group. Preferably, the fluorinated aliphatic radical contains not more than 20 carbon atoms. Specific examples of such compounds include: 2-(N-ethyl perfluoro octane sulphonamido)ethyl acrylate, 2-(N-ethyl perfluoro octane sulphonamido)ethyl methacrylate, and 2-(N-butyl perfluoro octane sulphonamido)ethyl acrylate. Mixtures of two or more fluorinated mono-acrylates may also be employed. The poly-functional cross-linking acrylates used in the invention are at least difunctional, preferably trifunctional or higher. The compounds generally have a molecular weight of less than 600. Typical tri- and tetrafunctional acrylates have the general formula: in which: X is as defined above, and R¹ represents an alkyl group, generally of 1 to 5 carbon atoms (e.g. methyl), hydroxy, or -OC(O)C(X)CH₂ in which X is as defined above. Suitable difunctional acrylates are of the general formula: in which: each X is as defined above and q is an integer from 3 to 8. Examples of cross-linking acrylates include: trimethylol propane tri(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,3-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, pentaerythritol tetra(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritol penta(meth)acrylate, and hydantoin hexaacrylate. Mixtures of cross-linking acrylates may be employed. The photoinitiator may comprise any of the photoinitiators known in the art e.g. d-hydroxyacetophenone type photoinitiators. Examples of photoinitiators are those commercially available from Ciba Geigy under the Trade Marks IRGACURE 651, IRGACURE 500, IRGACURE 184, and those commercially available from Merck under the trade names DAROCUR 1173 (2-hydroxy-2-methyl-1-phenyl-1-propanone) and DAROCUR 1116 (2-hydroxy-2-methyl-1-(4-isopropylphenyl)-1-propanone). In general suitable cladding compositions will have the following formulation: fluorinated mono-acrylate 50 to 95% by weight cross-linking acrylate 2 to 35% by weight photoinitiator 0.5 to 20% by weight. Preferably the components are selected within the following ranges: fluorinated mono-acrylate 75 to 95% by weight cross-linking acrylate 2 to 10% by weight photoinitiator 0.5 to 10% by weight. In addition to the three components the compositions preferably include an adhesion enhancer e.g. a compound having a vinyl functionality and being different from the fluorinated mono-acrylate and the cross-linking acrylate. The adhesion enhancers possess a single polymerisable vinyl group, and may be methacrylate, preferably acrylate silanes such as 3-tri(m)ethoxy-silylpropyl(meth)acrylate. Alternatively acrylic or methacrylic acid may be used as an adhesion enhancer. The adhesion enhancers generally increase the bonding strength of the cladding composition to the silica by at least 10%, generally at least 20% compared with cladding compositions in which the adhesion enhancers are absent. Generally, the adhesion improves as the concentration of adhesion enhancer is increased. However, the refractive index of the cladding material also rises as the proportion of non-fluorinated material is increased, which is undesirable. Thus a compromise must be made, and best results have been obtained with the adhesion enhancer present in the range 1 to 25%, preferably 2 to 15% by weight of the composition. The cladding compositions may also comprise a thermal stabilizer/antioxidant. Low loss optical fibres must pass a stringent cold/hot temperature cycling test. The optical fibres are kept at -65°C for four hours and monitored as a function of loss in dB/km. The fibres are then brought back to room temperature for two hours and thereafter held at +125°C for four hours. Again the temperature is brought to ambient for two hours and the loss of the optical fibre is recorded. It has been found that the presence of up to 5%, generally 0.001% to 5%, preferably 0.01 to 1% by weight of one or more thermal stabilizers/antioxidants compatible with the cladding composition provides improved stability to the cold/hot cycling test. Suitable stabilizers/antioxidants include low melting hindered phenols and thioesters. Specific examples include 2,6-di-tert-butyl-4-methyl phenol commercially available under the trade name ULTRANOX 226, octadecyl-3,5-di-tert-butyl-4-hydroxyhydrocinnamate commercially available under the trade name IRGANOX 1076, 2,6-di-tert-butyl-4-sec-butyl phenol commercially available under the trade names ISONOX 132 or VANOX 1320, and dilauryl thiodipropionate commercially available under the trade name CYANOX LTDP. A combination of thioester and hindered phenol has proved to be particularly effective. The core of the optical fibres of the invention is preferably formed of an inorganic glass, more preferably fused silica, but may optionally be formed of a synthetic resin. The cladding composition is selected to provide a refractive index lower than that of the core, preferably at least 0.03 units less, more preferably at least 0.05 units less than the refractive index of the core. The optical fibres of the present invention may be prepared by conventional techniques employing the cladding compositions of the invention. The optical fibres of the invention may also possess a protective layer such as those known in the art. For example a protective coating of a fluoropolymer e.g. poly (tetrafluoroethylene), may be coated as an extrusion by passing the clad fibre through a melt of the fluoropolymer. A suitable fluoropolymer is commercially available from Dupont under the trade name TEFZEL 210. The accompanying drawing illustrates suitable apparatus for preparing a coated optical fibre in accordance with the invention. A glass fibre 2 is drawn from a pure glass preform 4 held in a furnace 6 on a standard glass drawing tower. The tower is provided with a coating station comprising a coating cup 8 containing the cladding composition and the metering die 10. The coated fibre is passed immediately through an ultra violet curing station where it is photopolymerised and cooled on a take-up spool 14. A typical fibre will have an overall diameter of 200µm (microns) and a cladding thickness of 10µm (microns). The invention will now be illustrated by the following Examples. In the Examples the following components were employed: Mono-acrylate A: 2-(N-ethyl perfluoro-octane sulphonamido)ethyl acrylate commercially available from Minnesota Mining and Manufacturing Co., purified by extraction with ethanol. Mono-acrylate B: 1,1-dihydroperfluoro-octylacrylate (commercially available from Minnesota Mining & Manufacturing Co.) TMPTMA: trimethylolpropane trimethacrylate HHA: hydantoin hexa-acrylate (commercially available from Minnesota Mining & Manufacturing Co.) HDDA: hexanediol diacrylate acrylate silane: 3-trimethoxysilylpropyl methacrylate IG651: IRGACURE 651 commercially available from Ciba Geigy (benzildimethylketal) IG500: IRGACURE 500 commercially available from Ciba Geigy IG184: IRGACURE 184 commercially available from Ciba Geigy thiol synergist: gamma-mercaptopropyl trimethoxysilane EXAMPLE 1 A cladding composition was prepared by mixing the following components: Mono-acrylate A 92 parts by weight TMPTMA 4 parts by weight IG651 4 parts by weight Coated fibres were prepared by the technique described with reference to Figure 1 to produce coated fused silica optical fibres having an overall diameter of 200µm and a cladding thickness of 10µm. The coated fibre was tested using a Photon Kinetics spectrophotometer and an attenuation figure of 4.8 dB/km at 812 nm was recorded. Commercially available optical fibres in accordance with United States Patent No. 4511209 were tested under identical conditions and recorded an attenuation figure of 6.2 dB/km. EXAMPLES 2 TO 9 Comparison with fibres of US Patent No. 4511209 The formulations reported in Table 1 were prepared. (parts by weight) Example Mono-acrylate A Mono-acrylate B TMPTMA IG651 thiol silane acrylic acid acrylate silane 2 944 - 44 54 - - - 3(c) 890 - 44 54 60 - - 4 888 - 44 56 - 58 - 5 910 - 44 56 - - 56 6(c) - 935 58 41 - - - 7(c) - 874 56 41 - - 43 8(c) - 873 58 41 - 51 - 9(c) - 880 56 41 41 - - (c) = Comparative Example not in accordance with the invention. The refractive index of the uncured formulations were measured. The adhesion of the cured formulations to glass was measured by a drag test. The drag test consisted of coating the formulation to be tested on to a large glass slide. Lines were scored diagonally across the coating and a stylus to which a weight was attached was applied to the coating. The stylus was pulled across the coating. The weight applied to the stylus when a half of the lines are stripped away was recorded, in grams, as a measure of adhesion. The refractive indices of the uncured, uncross-linked formulation at sodium D line wavelength and adhesion values are reported in the following Table 2. Example Mono-acrylate Adhesion Refractive Index 2 A 240 1.393 3(c) A <160 1.397 4 A >800 1.397 5 A 720 1.396 6(c) B 240 7(c) B 400 8(c) B 800 9(c) B 160 (c) = Comparative Example not in accordance with the present invention. Examples 2, 4 and 5 show that the refractive index of the three component system was lower than that of the four component system of Comparative Example 3. Moreover, the glass adhesion of the 3 component system (Example 2) and the adhesion enhanced systems (Examples 4 and 5) was greater than that of the thiol silane containing formulation (Example 3). Coatings using mono-acrylate B (Examples 6 to 9) were subjectively much more brittle than those using mono-acrylate A. Examples 2 to 5 show the refractive index of the 3 component system was lower than the comparison formulation. Further that the glass adhesion of the 3 component system and adhesion enhanced systems was greater than that of the thiol silane containing formulation. Coating using mono-acrylate B (Examples 6 to 9) were subjectively much more brittle than those using mono-acrylate A. EXAMPLES 10 TO 13 Comparison of the effect of fluorinated acrylate on adhesion The formulations reported in Table 3 were prepared and refractive index and adhesion measurements were conducted as in Example 2. Example Mono-acrylate B Mono-acrylate A TMPTMA IG651 Adhesion Refractive Index 10(c) 928 - 57 40 240 1.356 11 - 927 54 44 640 1.392 12(c) 922 - 49 41 <160 not measured 13 - 946 51 44 720 not measured (c) = Comparative Example not in accordance with the invent These results indicate that the fluoro-octyl sulphonamido acrylate monomers exhibit advantageous adhesion properties. EXAMPLES 14 TO 16 Comparison of the effect of photoinitiator choice on adhesion The formulations reported in Table 4 were used to coat glass slides. The adhesion properties of the formulations were measured as in Example 2. Example Mono-acrylate A TMPTMA acrylic silane IG651 IG500 IG184 Adhesion 14 734 116 103 115 - - 1040 15 705 103 108 - 109 - 720 16 701 126 114 - - 100 800 All of these commercial photoinitiators effected acceptable levels of cure. EXAMPLES 18 TO 23 Comparison of the effect of cross-linking agent selection on adhesion The formulation reported in Table 5 were used to coat glass slides. The adhesion of the formulations was measured as in Example 2. Example Mono-acrylate A IG651 acrylate silane TMPTMA HHA HDDA Adhesion 18 704 60 101 101 - - 1040 19 696 61 103 50 - - 1200 20 715 60 105 - 114 - (a) 21 722 62 101 - 62 - 1200 22 696 63 108 - - 104 800 23 702 61 100 - - 51 720 (a) Very poor quality coatings led to inconsistent results EXAMPLE 24 Effect of acrylic silane concentration on adhesion To a formulation comprising Mono-acrylate A, IG651 and TMPTMA in the weight ratio 92:4:4 was added the following proportion of acrylate silane. The adhesion to glass was measured as in Example 2. The results are shown in Table 6 overleaf. weight % acrylate silane Adhesion 0 240 1 200 2.4 640 5 480 6 640 7.5 640 11 800 14 1040 19 800 22.5 1040 30 >>1040 It can be clearly seen that any amount of the acrylate silane over 1% was beneficial to core cladding adhesion. EXAMPLES 25 TO 28 Comparison with fibres of U.S. Patent No. 4511209 Optical fibres were prepared as in Example 1 using the following cladding compositions: Example Mono-acrylate A IG651 TMPTMA acrylic acid acrylate silane thiol synergist 25 88 4 4 - 4 - 26 88 4 4 4 - - 27(c) 88 4 4 - - 4 28(c) 72.1 0.9 23.3 - - 3.7 (c) = Comparative Example not in accordance with the presen invention. Example Attenuation at 820 nm (dB/Km) 25 22 26 11.9 27(c) 114 28(c) 48.4 (c) = Comparative Example not in accordance with the present invention. Comparison of adhesion by lap shear test 0.5 inch (12.8 mm) overlap. Example Immediately after cure After 24 hrs % Change 25 100.3 108.4 8.0 26 123.7 178.2 44.0 28(c) 66.6 71.1 6.7 (c) = Comparative Example not in accordance with the present invention. Adhesion of cladding to silica of Examples 25 and 26 containing acrylic acid or acrylate silane is considerably improved over the cladding of Comparative Example 28 containing thiol synergist. The improvement is particularly pronounced upon ageing. Example 29 Addition of thermal stabilizer Optical fibres were prepared as in Example 1 using a cladding composition of the following formulation: % by weight mono-acrylate A 88.5 TMPTA 5 acrylate silane 2 IG651 4 ULTRANOX 226 0.5 After thermal cycling at +125°C the fibre exhibited a permanent loss damage of 7 dB/km compared to 10 to 14 dB/km of fibres having the same cladding fromulation without ULTRANOX 226 thermal stabilizer. Example 30 Combination of two thermal stabilizers An optical fibre was prepared as in Example 1 using the following cladding formulation. % by weight mono-acrylate A 82.25 TMPTA 10 acrylate silane 5 DAROCUR 1116 2 CYANOX LTDP 0.5 IRGANOX 1076 0.25 After curing the clad fibre was extrusion coated with TEFZEL 210 in a conventional manner. The permanent loss damage of the resulting buffered optical fibre was 1.1 db/km after being annealed at +125°C for four hours. Example 31 A cladding composition was prepared by mixing the following components: 1,1-dihydroperfluorocyclohexane carbinol acrylate 88 parts by weight 2-ethyl-2-(hydroxymethyl)-1,3-propanediol triacrylate 5 parts by weight 3-(trimethoxysilyl)propyl methacrylate 5 parts by weight DAROCUR 1173 2 parts by weight Coated fibres were prepared and evaluated using the procedure according to Example 1. An attenuation figure of 5.73 dB/km at 820 nm was recorded. After curing the cladded fibre was extrusion coated with TEFZEL 210 in a conventional manner to provide a buffered optical fibre with an attenuation loss of 6.23 dB/km at 820 nm. The thermal stability of the TEFZEL buffered fibre was evaluated after maintaining it as 125°C for four hours and was recorded as 10.58 dB/km. The 1,1-dihydroperfluorocyclohexane carbinol acrylate used in this Example was prepared according to the procedure disclosed by D.W. Codding et al., "Journal of Polymer Science", 15, 518 (1955) except that the charge was 210 g trifluoroacetic anhydride, 79.8 g acrylic acid, 250 g perfluorocyclohexylmethylol, and 0.1 g phenathiazine as inhibitor; the reaction mixture was stirred for 2 hours after the reaction had subsided, and the product was purified by vacuum distillation. A comparison was made of the TEFZEL coated cladded fibre prepared in this Example with a commercially available hard clad silica fibre sold by Ensign-Bickford (Simsbury, CT). Both fibres were maintained at 125°C for four hours. The instant fibre was colorless and transparent and showed no increase in attenuation at 600 nm whereas the Ensign-Bickford fibre yellowed and showed an increase in attenuation of 120 dB/km at 600 nm. "Transparent" means that the cladded fibre when viewed under an optical microscope (e.g., at 100X), have the property of transmitting rays of visible light so that bodies beneath the fibre, for example, such as bodies having essentially the same nature as the fibre, can be clearly seen through the fibre. Improved properties of the cladded fibre can be realized by including antioxidant/thermal stabilizer in the formulation as has been shown in Example 30. "IRGACURE" (Ciba Geigy), "DAROCUR" (Merk), "ULTRANOX", "IRGANOX", "ISONOX", "VANOX", "CYANOX" and "TEFZEL" (Dupont) are all registered trade marks.

Claims: An optical fibre comprising a core coated with a cladding having a lower refractive index than the core characterised in that the cladding composition comprises: (a) one or more fluorinated mono-acrylates selected from the group consisting of (i) fluorinated mono-acrylates having a fluorinated cycloaliphatic radical having either a minimum (number) of three C-F bonds or in which at least 25% of the C-H bonds have been replaced by C-F bonds, whichever degree of fluorination is higher, and (ii) compounds of the general formula: in which: X represents H or an alkyl group of 1 to 5 carbon atoms, x is 1 or 2, R represents an alkyl group of 1 to 5 carbon atoms, and R f represents a fluoro-aliphatic radical or a fluorinated cycloaliphatic radical having either a minimum (number) of three C-F bonds or in which at least 25% of the C-H bonds have been replaced by C-F bonds, whichever degree of fluorination is higher, (b) a polyfunctional cross-linking acrylate being difunctional or higher, and (c) a photoinitiator, the composition comprising less than 0.3% by weight of a mono- or polyfunctional thiol and being cured or cross-linked. An optical fibre as claimed in Claim 1 in which the cladding composition is free from mono- or polyfunctional thiols. An optical fibre as claimed in Claim 1 or Claim 2 in which the fluorinated acrylate is selected from: 1,1-dihydroperfluorocyclohexane carbinol acrylate, 1,1-dihydroperfluorocyclohexane carbinol methacrylate, 1,1-dihydroperfluorocyclopentane carbinol acrylate, 1,1-dihydroperfluorocyclopentane carbinol methacrylate, 2-(N-ethyl perfluoro octane sulphonamido)ethyl acrylate, 2-(N-ethyl perfluoro octane sulphonamido)ethyl methacrylate, 2-(N-butyl perfluoro octane sulphonamido)ethyl acrylate, and, mixtures thereof. An optical fibre as claimed in any one of Claims 1 to 3 in which the polyfunctional cross-linking acrylate (b) is selected from compounds of the general formulae: in which: X represents H or an alkyl group of 1 to 5 carbon atoms, R¹ represents an alkyl group of 1 to 5 carbon atoms or in which X is as defined above, and p is an integer from 3 to 8. An optical fibre as claimed in any preceding Claim in which the polyfunctional cross-linking acrylate (b) is selected from trimethylol propane tri(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,3-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, pentaerythritol tetra(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritol penta(meth)acrylate, and hydantoin hexaacrylate and mixtures thereof. An optical fibre as claimed in any preceding Claim in which the cladding composition comprises: from 50 to 95% by weight of fluorinated mono-acrylate (a), from 2 to 35% by weight of polyfunctional cross-linking acrylate (b), and, from 0.5 to 20% by weight of photoinitiator (c). An optical fibre as claimed in Claim 6 in which the cladding composition comprises: from 75 to 95% by weight of fluorinated mono-acrylate (a), from 2 to 10% by weight of polyfunctional cross-linking acrylate (b), and from 0.5 to 10% by weight of photoinitiator (c). An optical fibre as claimed in any preceding Claim in which the cladding composition additionally comprises from 1 to 50% by weight of the total cladding composition of an adhesion enhancer having a vinyl functionality different from the fluorinated mono-acrylate (a) and the polyfunctional cross-linkable acrylate (b). An optical fibre as claimed in Claim 8 in which the adhesion enhancer is a (meth)acrylic silane. An optical fibre as claimed in any preceding Claim in which the cladding composition additionally comprises from 0.001 to 5% by weight of the total cladding composition of a thermal stabilizer/antioxidant. An optical fibre as claimed in Claim 10 in which the thermal stabilizer/antioxidant is selected from: 2,6-di-tert-butyl-4-methyl phenol, octadecyl-3,5-di-tert-butyl-4-hydroxyhydrocinnamate, 2,6-di-tert-butyl-4-sec-butyl phenol, dilauryl thiodipropionate and combinations thereof. An optical fibre as claimed in any preceding Claim in which the core is fused silica and the refractive index of the cladding is at least 0.03 units less than that of the core. A cladding composition for optical fibres comprising: from 50 to 95% by weight of a fluorinated mono-acrylate selected from the group consisting of (i) fluorinated mono-acrylates having a fluorinated cycloaliphatic radical having either a minimum (number) of three C-F bonds or in which at least 25% of the C-H bonds have been replaced by C-F bonds, whichever degree of fluorination is higher, and (ii) compounds of the general formula: in which: X represents H or an alkyl group of 1 to 5 carbon atoms, x is 1 or 2, R represents an alkyl group of 1 to 5 carbon atoms, and R f represents a fluoro-aliphatic radical or a fluorinated cycloaliphatic radical having either a minimum (number) of three C-F bonds in which at least 25% of the C-H bonds have been replaced by C-F bonds, whichever degree of fluorination is higher, from 2 to 35% by weight of a polyfunctional cross-linking acrylate being difunctional or higher, and from 0.5 to 20% by weight of a photoinitiator, the composition comprising less than 0.3% by weight of a mono- or polyfunctional thiol.

Inventors: BABIRAD STEFAN A, DODDS ALASTAIR STUART, HULME-LOWE ALAN GEORGE, SAVU PATRICIA M, BABIRAD, STEFAN A, DODDS, ALASTAIR STUART, HULME-LOWE, ALAN GEORGE, SAVU, PATRICIA M
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1989
Main classification: C03C25
IPC codes: B29C47, G02B6, B05C3, C03C25, C03C13