GB2048726A - Coating Optical Fibres - Google Patents

Abstract

The speed at which optical fibres can be coated with plastics is increased by generating a velocity field in the liquid polymer, 37, contained in a coating applicator, parallel to the direction 31 in which the optical fibre is drawn through the applicator. In one form of applicator the liquid polymer is forced under pressure from a first chamber 34 to a second chamber 37 through a small bore tube 35, through which the optical fibre passes. In a second embodiment (Fig. 2) the velocity field is generated by rotating rollers 15. <IMAGE>

Description

SPECIFICATION Improvements in or Relating to Coating Glass Fibres with Plastics The present invention relates to a plastics applicator for coating glass fibres with plastics.

Optical fibres made from glass and intended for use in telecommunications are-frequently coated with plastics to protect the fibre from mechanical damage and attack by water vapour etc. In a known method of coating optical fibres with plastics, the fibre is drawn through a crucible filled with-liquid polymer, e.g. silicon resin, having a small tapered nozzle in its base (bottom wall). The fibre emerging from the base of the crucible is coated with plastics, and is then passed through a curing oven. If the speed at which the fibre is drawn through the crucible is increased the depth of the meniscus surrounding the fibre at its point of entry into the liquid polymer increases.If the speed of the fibre motion relative to the liquid polymer is increased above about 0.35 m/sec the meniscus passes through the entire length of the coating rucible so that the fibre is'sheathed with a gas layer and is therefore not coated with plastics.

Thus an effective limit is placed on the maximum operating speed of a fibre production unit using this coating technique.

The present invention is concerned with a modification to this technique whereby, fibre can be coated at high speed.

It is believed that the prime reason for the prior art techique failing at high fibre speeds is the high relative velocity between the fibre and the polymer. The present invention seeks to reduce this high relative velocity by creating a flow pattern in the polymer whereby liquid contiguous with the fibre has a velocity imparted to it parallel with the direction of fibre motion by an agency other than the motion of the fibre itself. In two different embodiments of the invention this motion is created by:- (a) locating two rotating drums immersed in the polymer, close to, but not in contact with the fibre, (b) by pumping the liquid polymer through a narrow tube through which the fibre passes.

It should be emphasised that any contact between the fibre and a solid surface or element such as a roller or brush would cause surface damage to the fibre resulting in dramatic loss of fibre strength. Thus in the embodiment in which rollers are employed it is essential that they do not contact the fibre.

Details of known techniques for coating optical fibres can be found in the following references: U.S. Patent specification 4,1 15,087; "Continuous Coating of Glass Fibres" S. Torza J. App. Phys 47 No. 9 1 976 P401 7 et seq; "Optical Fibre Protection by Solution Plastic Coating" P. W.

France and P. L. Dunn, Proc. 2nd European Conference on Optical Fibre Communications, Paris 1976; "On Line Fibre Coating Using Tapered Nozzles" P. W. France, P. L. Dunn and M. H. Reeve Proc. 3rd European Conference on Optical Fibre Communication Munich 1977; "Plastic Coated Glass Fibres for Optical Communications" P. W.

France Proc. "Plastics in Telecommunications II" Conference Plastics and Rubber Institute at IEE London 1978.

According to the present invention there is provided a plastics applicator for coating a glass fibre with plastics comprising a vessel through which a glass fibre can be drawn along an axis hereinafter called the fibre axis, without touching any solid surface, a nozzle located in a wall of said vessel, on said fibre axis, and means for inducing a pattern of liquid flow in said vessel, such that when said vessel is filled with a liquid polymer and a fibre is drawn through said vessel and said nozzle, said means for inducing imparts to liquid polymer adjacent said fibre along at least part of said fibre length, a velocity substantially parallel to the direction of motion of said fibre.

Said means for inducting a pattern of flow may comprise a pair of powered rollers rotatable in opposite senses and mutually separated by a small gap located on said fibre axis. Said rollers may be connected to a source of power by a mechanical linkage.

Said vessel may comprise a 1 st compartment connected to a second compartment by a narrow tube located on said fibre axis, said means for inducing a pattern of liquid flow comprising said narrow tube and pump means for pumping polymer from said 1 st compartment into said second compartment.

Said nozzle may be a tapered nozzle located in a bottom wall of said 1 st compartment, through which a fibre can be drawn from said vessel.

An entrance port may be provided, on said fibre axis, in a top wall of said 2nd compartment of said vessel, through which a fibre can be drawn into said vessel.

Said vessel may be water cooled.

A partially evacuable compartment, through which said fibre passes may be mounted externally of, and on said top wall of said 2nd compartment.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 is a schematic representation of an optical fibre production and coating unit.

Fig. 2 is a section through a plastics applicator comprising a 1 st embodiment of the invention.

Fig. 3 is a section along line X-X of Fig. 2.

Fig. 4 is a schematic representation of a plastics applicator comprising a second embodiment of the invention.

Fig. 5 is a section through a known plastics applicator.

Referring now to the drawings and Fig. 1 in particular, a glass optical fibre 43 is drawn from soda-boro-silicate glass contained in a double crucible 44 housed in a furnace 1. The fibre passes through a diameter monitor 2, through a plastics applicator 4, a curing oven 5, and around pulley 45. The fibre is drawn from the system at constant speed by a capstan 6 and, finally, is spooled onto a take up drum 7 at constant torque.

The plastics applicator 4 is fed with liquid polymer from reservoir 3. The polymer employed is a heat curing silicon resin marketed under the name Sylgard by Dow Corning. In the production and coating unit of Fig. 1 either a known type of plastics applicator or a plastics applicator according to the present invention may be employed. The known type of applicator of which the present invention is an improved development, is shown in section in Fig. 5. It consists of a double walled vessel or crucible having an inner wall 19 and an outer wall 18, water can be pumped through the space 8 between the inner and outer walls. The fibre is drawn along the central axis 31, (fibre axis) of the vessel.

The bottom wall of the crucible contains a nozzle 16, and the entire assembly 9 is detachable enabling different nozzle sizes to be employed. A fibre guide 11 having a tapered passage located on the fibre axis is mounted in the crucible, which is covered by top wall 12 passage 10 communicates with the reservoir 3 of Fig. 1. The nozzle 1 6 is tapered, as is the passage in fibre guide 11. Typical nozzle dimensions are:- minimum diameter 300 microns, length 2mm. half angle of taper 30. The taper serves to generate centralizing forces on the fibre when it is drawn through the nozzle, the crucible of course being filled with polymer.

Using this type of plastics applicator drawing speeds for the fibre must not substantially exceed 0.35 m per second. If this is exceeded, the fibre will not be coated with plastics, because the velocity of the fibre drags a gaseous sheath right through the applicator.

More information on this phenomena can be found in the following references: "Dynamic wetting of glass and polymer fibre"-G. Inverarity Br. Polym.J. 1969 1 P245-25 1; and "UV cured epoxy-acrylate coatings on optical fibres Il- Dynamic coating and short term strength" by S.

Schonhom et Al-Coatings and Plastics Preprints Am. Chem. Soc. 37 Prt 2P256-258.

Using the modified plastics applicator of Figs. 2 and 3 how ever, higher fibre drawings speeds may be achieved while still obtaining adequate plastics coating. The plastics applicator shown in Figs. 2 and 3 is similar to that of Fig. 5, but with the addition of rollers 1 5. Like features are ndicated by like reference numerals in Fig. 5 and Figs. 2 & 3, no additional explanation of features already described will be given.

The rollers 15 have a pair of intermeshing pinion gears 14 disposed at one end, so that they rotate in opposite senses. The rollers and gears are mounted on shafts 22 and 23, which are held by a support cage 45 attached to support rod. The roller mounted on shaft 23 is driven by a pair of bevel gears 24, one of which is mounted on shaft 23, and the other of which is mounted on shaft 13. Shaft 1 3 is driven by a motor (not shown).

The shaft 13 is held by bearing sleeves in the crucible top wall 12. The gap between rollers 1 5 is located on the fibre axis 16, and is such that the rollers just fail to touch a fibre passing through the crucible on the fibre axis.

In use, the crucible is filled with liquid polymer, so that both the rollers 1 5 and the fibre guide 11 are immersed in liquid. The liquid level 46 is shown in Fig. 2. The rollers are then driven so that the periphery of the rollers adjacent the-gap between the rollers moves in the direction (downward) in which the fibre being coated is drawn through the crucible. Liquid is swept round with the rollers and propelled through the gap between the rollers in the direction of fibre motion, thus tending to reduce the relative velocity between the fibre and the liquid. This enables adequate coating of the fibre to be achieved even at high fibre drawing speeds.

Finally a feature of the plastics applicator common to both the applicator of Fig. 5, and the applicator of Figs. 2 and 3 should be mentioned.

The nozzle assembly 9, the fibre guide 11, and the applicator lid 12 are each formed in two halves. The fibre axis 1 6 intersects the division line between the two halves of these components. This construction is used to facilitate setting up the apparatus at the start of a run.

Initially a fibre is pulled from the double crucible 41 (Fig. 1) with the nozzle assembly 9, the fibre guide 11 and the applicator lid 1 2 removed. The fibre is pulled down, through furnace 5 over pulley 45, round capstan 6 and is attached to take up drum 7. The split elements 9, 11 and 12 are then fastened in position, the coating applicator is filled with polymer, and power is applied to the rollers so that coating commnces.

It should be emphasised that during the coating operation, the fibre does not touch any solid surface in the coating applicator, especiaily not the walls of nozzle 1 6 or the rollers 15.

The embodiment, about to be described with reference to Fig. 4, is the preferred embodiment, since it avoids the problems associated with rollers operating in the liquid polymer. This time the flow of liquid polymer is generated by pumping it through a narrow tube. The plastics applicator has an upper chamber 34, and a lower chamber 37, linked by a narrow tube 35 located on the fibre axis. As with the plastics applicators already described, the lower chamber 37 is equipped with a fibre guide 38 and a tapered nozzle 39. In use liquid polymer is pumped from chamber 37 via pipe 40, by a pump (not shown) to pipe 33 and thence into chamber 34 and back into chamber 37 through tube 35. The inner wall 42 of the chamber 34 is shaped to assist in producing a downwardly directed liquid flow, and to ensure that the fibre is not subjected to a fluid flow which would tend to displace it from the vertical. The fibre enters chamber 34 through a suitably tapered nozzle 36.

In order to minimize the amount of gas drawn into the liquid polymer, a partially evacuable chamber 47 is mounted on the top wall of chamber 35. The chamber 47 can be connected to a vacuum pump by pipe 32. The fibre enters the chamber 47 through an adjustable iris diaphragm 30.

In use liquid polymer is forced through tube 35 with a velocity approaching that of the moving fibre. The fluid flow pattern together with the fibre velocity tends to minimise the upward leakage of liquid through nozzle 36. The degree of evacuation in chamber 47 is determined empirically. The various hole sizes, and tube diameters, are sufficiently large to permit easy insertion of the fibre during the start up procedure. As with the previous plastics applicators, it should be emphasised, that in use the fibre does not touch any solid surfaces as it passes through the applicator.

Claims (1)

1. Claims

   1. A plastics applicator for coating a glass fibre with plastics comprising a vessel through which a glass fibre can be drawn along an axis, herein after called the fibre axis, without touching any solid surface, a nozzle located in a wall of said vessel, on said fibre axis, and means for inducing a pattern of liquid flow in said vessel such that when said vessel is filled with a liquid polymer and a fibre is drawn through said vessel and said nozzle, said means for inducing imparts to liquid polymer adjacent said fibre, and along at least part of said fibre length, a velocity substantially parallel to the direction of motion of said fibre.

   2. A plastics applicator as claimed in claim 1 wherein said means for inducing a pattern of flow comprises a pair of powered rollers rotatable in opposite senses and mutually separated by a small gap located on said fibre axis.

   3. A plastics applicator as claimed in claim 2 wherein said rollers are connected to a source of power by a mechanical linkage.

   4. A plastics applicator as claimed in claim 1 wherein said vessel comprises a first compartment connected to a second compartment by a narrow tube located on said fibre axis, said means for inducing a pattern of liquid flow comprising said narrow tube and pump means for pumping liquid polymer from said 1 st compartment into said second compartment.

   5. A plastics applicator as claimed in claim 4 wherein an entrance port is located on said fibre axis, in a top wall of said 2nd compartment of said vessel, through which a fibre can be drawn into said vessel.

   6. A plastics applicator as claimed in either claim 4 or 5 wherein there is provided a partially evacuable compartment, through which said fibre axis passes, mounted externally of, and on said top wall of said 2nd compartment, said entrance port communicating between said 2nd compartment and said partially evacuable compartment.

   8. A plastics applicator as claimed in any previous claim wherein said vessel is water cooled.

   9. A plastics applicator substantially as hereinbefore defined within reference to Figs. 2 and 3 of the accompanying drawings.

   1 0. A plastics applicator substantially as hereinbefore defined with reference to Fig. 4 of the accompanying drawings.

   11. An optical fibre production unlit substantially as hereinbefore described with reference to Fig. 1 and containing a plastics applicator as claimed in any previous claim.