NZ207328A

NZ207328A - Extruding plastics coating onto optical fibres

Info

Publication number: NZ207328A
Application number: NZ207328A
Authority: NZ
Inventors: P G Hale
Original assignee: Int Standard Electric Corp
Priority date: 1983-03-15
Filing date: 1984-03-01
Publication date: 1986-07-11
Also published as: GB2138429B; AU2538884A; GB8307147D0; GB2138429A

Description

New Zealand Paient Spedficaiion for Paient Number £07328 20732 Priority Date(s): Complete Specification Filed: J. .*?• -W-Class: •••■•nrmw Publication Date: ,7)(?C~ P.O. Journal, No: | U ^TB A liS NEW ZEALAND THE PATENTS ACT, 1953 COMPLETE SPECIFICATION "METHOD OF MANUFACTURING OF PLASTICS * * " WAR 1984 COATED GLASS OPTICAL FIBRE" * 'WE, INTERNATIONAL STANDARD ELECTRIC CORPORATION , a Corporation of the State of Delaware, United States of America, of 320 Park Avenue, New York 22, New York, United States of America, hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: 207328 This invention relates to the manufacture of plastics coated glass optical fibres.

When glass optical fibre is made it is conventional to provide the freshly drawn fibre surface, typically but not necessarily a silica surface, with a primary plastics coating on-line with the fibre drawing operation in order to give a measure of protection for the glass surface against degradation by atmospheric attack. This coating is typically no more than about 50 microns thick, and usually a second thicker coating, a secondary coating, is applied at a later stage to give a more robust package. This secondary coating is typically provided by extrusion. Each of these coatings is a tight coating with substantially no voids between it and the material it envelopes, and hence, if it is subject to shrinkage in the axial direction over an appreciable length, then this shrinkage is transmitted to the fibre.

When a thermo-plastics sheath of crystallisable polymeric material is applied by extrusion to a metal wire to provide electrical insulation it is quite common to quench cool the extrudate by passing it through a water trough immediately after it leaves the extruder. The same practice has been employed for applying plastics coatings to optical fibres, but in this instance it has been found that quench cooling gives rise to subsequent problems as the plastics material later attempts to anneal into a state more closely 2073 2. S approximating to an 'equilibrium' state.

Quench cooling produces a relatively low density product in which the amount of crystallisation is well below the maximum value that is achievable for that material. Subsequent thermal excursions above the glass transition temperature are liable to produce a significant increase in the amount of crystallisation, and this is manifest by an increase in the axial (and radial) compression of the fibre which, if uncontrolled, can lead to a dramatic increase in attenuation at low temperatures due to fibre buckling. This occurs for instance when a quench cooled nylon secondary coating is subject to being temporarily heated to 70 to 80°, for example, in the course of extruding a cable sheath round an assembly of packaged fibres.

It has been found that, provided the line speed is not too fast, this problem can be greatly reduced by using still air at ambient temperature to cool the secondary coating extrudate as it leaves the extruder. This air cooling has to be over a long enough distance for the extrudate to pass right through the temperature band of peak crystallisation rate before it is subject to any fiercer cooling. By way of example it has been demonstrated that, with a line speed of 40 m/min., 0.85 mm diameter nylon coating can be satisfactorily annealed with cooling in static air at ambient temperature for a duration of seven and a half seconds. This 207328 corresponds to an air gap of 5 metres. However, as higher line speeds are attempted the requisite lengthening of the air gap to preserve the same duration of air cool becomes impractical, primarily because of the attendant requirement to increase the line tension to avoid excessive droop of the line catenary. In this context it is to be noted that the line tension dictates the axial strain induced in the fibre when the package is at room temperature, and it is desirable to be able to control this parameter to suit the particular requirements determined by the environmental conditions in which the package is going to be cabled, installed, and used.

According to the present invention there is provided a method of providing an extruded plastics tight coating of crystallisable polymeric material around a glass fibre, wherein on emerging from the extruder the extrudate is annealed in a forced flow of fluid maintained at a temperature which is in excess of the glass transition temperature of the polymeric material and is in the vicinity of the temperature at which the crystallisation growth rate is at its peak value, and in which flow the extrudate is maintained until it is crystallised to substantially the fullest extent achievable.

The forced flow annealing, provided for instance by passing the extrudate through a thermostatically controlled recirculating oil bath, or by directing hot air jets against its surface, is capable of providing more rapid cooling of 2073ZS the extrudate to the temperature band of maximum crystallisation rate and then holding the temperature within this band until crystallisation is effectively complete. In this way full crystallisation is capable of being achieved in s 5 shorter time, thus allowing a greater line speed for a given length of annealer. After sufficient crystallisation has been achieved, further cooling to below the glass transition temperature may be effected as quickly as is found convenient. This may be for instance by forced air cooling with 10 air at room temperature, or by quench cooling with cold water. It may also be noted that if the cooling fluid used to promote rapid crystallisation is a liquid, an effect of its greater density in comparison with air will be to provide an increased measure of buoyancy which is helpful in reducing 15 the minimum required tension in the line. If the fluid is a gas additional buoyancy can alternatively be achieved by directing gas flow upwardly around the extrudate.

Typically the temperature band of operation is about ± 10°C from the temperature of peak crystallisation rate, with 20 the limits of the band being at about 30% of the peak value or preferably at least 50% of that value. By the time the crystallisation has proceeded to 90% of the maximum achievable value for that material its mechanical properties are substantially those of the fully crystallised material to the 25 extent that subsequent short duration temperature excursions

Claims

207328 above the glass transition temperature do not cause any significant shrinkage. For a 0.85 mm diameter secondary coating of nylon 12 satisfactory annealing is preferably performed at a tempera-5 ture of about 15°C. A heated silicone oil may be used for this purpose, or alternatively heated air directed against the underside of, or around, the coated fibre for instance from jets disposed on either side. The crystallisation is substantially complete after an annealing period at this 10 temperature of about 5 seconds. 207328 What we claim is:

1. A method of providing an extruded plastics tight coating of crystallisable polymeric material around a glass fibre, wherein on emerging from the extruder the extrudate is annealed in a fluid maintained at a temperature which is in excess of the glass transition temperature of the polymeric material and is within a range of +/- 10% about the temperature at which the crystallisation growth rate is at peak value, and in which the extrudate is maintained until it is crystallised to substantially the fullest extent achievable with that material.

2. A method of providing an extruded plastics tight coating of crystallisable polymeric material around a glass fibre, wherein on emerging from the extruder the extrudate is annealed in a forced flow of fluid maintained at a temperature within the band over which the crystallisation growth rate is at least 30% of its peak value, and in which flow the extrudate is maintained until it is crystallised to at least 90% of the maximum crystallinity achievable with that material.

3. A method as claimed in claim 1 or 2, wherein the annealing fluid is a liquid.

4. A method as claimed in claim 1 or 2, wherein the annealing fluid is a gas.

5. - A glass optical fibre provided with an extruded plastics 16 MAY 1986 | RECEIVED 7 207323 coating of crystallisable polymeric material by the method claimed.in any preceding claim. INTERNATIONAL STANDARD ELECTRIC CORPORATION P.M. Conrick Authorized Agent 5/1/1223