GB2104225A

GB2104225A - Testing plastics coatings on optical fibres

Info

Publication number: GB2104225A
Application number: GB08124129A
Authority: GB
Inventors: Michael Richard Barker; Mahesh Kumar Ramniklal Vyas
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1981-08-06
Filing date: 1981-08-06
Publication date: 1983-03-02
Also published as: GB2104225B

Abstract

The state of cure and/or adhesion of a plastics coating on an elongate member, such as an optical fibre, is determined by measuring the load (peel load) on the fibre (28), by means of a load cell (12) mounted on a cross member (10) driven by an electric motor (1), whilst holding (at 14) the fibre in a tensioned monofilament loop (30 - Figure 3). The loop serves to fracture the coating at a particular peel load (A - Figure 5) which is indicative of the state of cure. After such fracturing, and whilst the load continues to be increased, the monofilament loop peels the coating back, the extent of peel back being indicative of the coating adhesion. <IMAGE>

Description

SPECIFICATION Testing plastics coatings on optical fibres This invention relates to testing plastics coatings on elongate members, in particular optical fibres, and specifically to testing the state of cure and adhesion of a primary plastics coating applied to optical fibres.

Silica glass optical fibres generally have a soft plastics primary coating, which is relatively thin, applied thereto whilst the fibre is still warm from the drawing process. This coating is also referred to as an on-line coating, since it is applied in a process carried out in tandem with the drawing process. The purpose of the primary coating is to protect the underlying fibre during storage on a take-up reel and provide it with increased tensile strength. A thicker harder secondary plastics coating may be provided later.

A typical optical fibre of this type has a diameter of 125 microns. The primary coating is typically a silicone resin, such as that marketed by Dow Corning under the designation Sylgard (Registered Trade Mark) 182, which is about 40 microns thick. The secondary coating may be about 400 microns thick and be a polyamide resin.

Several process variables can effect the uniformity, adhesion and curing of the primary coating. For example, the wetting of the silica glass fibre depends on the temperature of the drawn fibre at the entry point to a coating cone, and on the quality and homogeneity of the masterbatch resin. Furthermore, the coating concentricity depends to some extent on the alignment of coating slides located at the base of the coating cone. It is ths apparent that without good process optimisation and control, variability in the manufactured coated fibre can rise.

According to one aspect of the present invention there is provided a method of testing the adhesion of a plastics coating applied to the circumference of an elongate member, comprising the steps of arranging the coated elongate member in a monofilament loop, tensioning the loop to a predetermined value, applying a gradually increasing load to the elongate member in a direction substantially normal to the applied loop tension whereby to fracture the coating without adversely affecting the elongate member, and monitoring the load, the plastics coating being fractured at a first load value and peeled from the elongate member with subsequently increasing load.

According to another aspect of the present invention there is provided apparatus for use in determining the state of cure and/or adhesion of a plastics coating applied to an elongate member, comprising means to apply a gradually increasing load to the coated elongate member along its longitudinal axis, means to circumferentially fracture the coating and peel the coating therefrom, whilst the load is applied, and means to record the load value at least when the coating is fractured.

An embodiment of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 shows, somewhat schematically, a view of a peel test machine; Figure 2 shows, schematically, a plan view of one monofilament loop forming arrangement for the machine of Figure 1; Figure 3 shows on an enlarged scale the looped monofilaments on a coated fibre prior to fracture of the on-line coating; Figure 4 shows a coated optic fibre after fracturing of the coating and peeling back thereof to form a collar; Figure 5shows a graph of peel load versus peeled length, and Figure 6shows a graph of mean peak load versus curing temperature.

The peel test machine shown in Figure 1 comprises a two-way constant horse power motor 1 arranged within a base 2 for driving, via a belt 3, two lead screws 4 and 5, arranged within respective C-section support members 6 and 7 mounted to the base 2 and interconnected by a cross-bar 8. The lead screws 4 and 5 can rotate relative to phosphor bronze bushes 9 which are mounted to the support members 6 and 7.

A crosshead 10 is arranged between the lead screws 4 and 5 and interconnected thereto via threaded phosphor bronze bushes 11, so that movement of the crosshead vertically with respect to the base 2 is determined by the motor 1, which is suitably geared to provide a predetermined speed of movement of the crosshead 10, typically 50 mm per minute although other speeds may be employed.

A load cell 12 is mounted centrally on the crosshead 10 and coupled to a suitable parallel-faced grip 13, or alternatively to a collet-type grip. The output of the load cell 12 can be monitored either on a remote digital millivoltmeter (not shown) or alternatively it can be displayed on a suitable chart recorder (not shown).

A nylon monofilament loop tensioning assembly 14 is located at the base of the machine. The assembly (Figures 1 and 2) comprises a pair of base plates 15 secured on opposite sides of a slot in the top face of the base 2. The base 2 includes a front face 16 which is also slotted (at 17), the slots in the top and front faces being in communication with one another to permit access to a monofilament loop, as will be apparent from the following. Two monofilament guide blocks 18 are secured to the base plates 15 on opposite sides of the slot. A respective micromanipulator slide assembly 19 is mounted to each base plate 15. Movable table 26 of the left hand micromanipulator slide assembly 19 carries a first bobbin 20 wound with nylon monofilament and a Correx (Registered Trade Mark) gram dial gauge 21.

Movable table 27 of the left hand micromanipulator slide assembly 19 carries a second bobbin 22 also wound with nylon monofilament and clamp 23.

The upper face of each guide block 18 is provided with a pair of parallel monofilament guide slots 24 (Figure 2). A centrally apertured plate 25 (omitted from Figure 2 for clarity) may be arranged to connect the top of the guide blocks 18 and effectively to close the top of the slots 24. The plate 25 serves to restrict the vertical displacement of the loop and in addition defines a constant loop size. In alternative arrange menus the guide blocks 18 are carried by the respective movable tables and the plate 25 is omitted, or mounted to only one guide block.

The bobbins 20 and 23 are each wound with high quality nylon monofilament, typically 0.18 mm diameter nylon filament fishing line. The filament from bobbin 20 is fed through a monofilament clamp 28 carried by the dial gauge arm, through one slot 24 of the guide block 18 and back through the other slot 24 of the same guide block 18 to the clamp 28, thus leaving a loop between the guide blocks 18. The filament from bobbin 22 is fed through clamp 23, through one slot 24 of the associated guide block, through the loop of monofilamentfrom bobbin 20, and back through the other slot of the associated guide block to the clamp 23. In a test a length of coated optical fibre 28 is passed through the interlinked loop 30 of monofilament thus formed (Figure 3) and one end of the length is clamped in grip 13.

The clamps 23 and 28 are tightened.

By moving the micromanipulatortables 26 and 27 the tension of the interlinked monofilament loop 30 is adjusted to, for example, 100g as indicated on the Correx gram dial gauge 21. The loop tension is then released by pushing, for example manually, the gauge clamp 28 to the right whilst simultaneously setting the crosshead 10 in motion. Then the gauge clamp 28 is released, thus subjecting the optical fibre to the loop tension while it is pulled through the loop. The load on the fibre is measured by the cell 12. When the maximum stress the coating will withstand is reached, the coating is fractured. With subsequent increasing load the coating is peeled off the fibre by the loop and a collar 29 built up. Each length of coated fibre is subjected to several tests along its length. The nylon monofilament loop is renewed after a small number of tests.

Atypical peel load versus peeled length curve obtained from tests conducted on a Sylgard coated optical fibre is shown in Figure 5. Initially when the loop tension is applied to the fibre an increase in peel load is obtained, due to the resistance offered by the Sylgard coating together with the net drag force resulting from pulling the fibre against the loop. The load reaches a maximum value (A) when the Sylgard coating fractures and peeling ensues, with a consequent initial drop in the load (B). As peeling continues there is a build-up of the Sylgard coating in the form of a collar, with a gradual increase in peel load (C). The value of the peel load at the peak A, the point of fracture of the coating, is used as a criteria for ascertaining the state of cure of the Sylgard coating, whereas the extent of peel back is indicative of the adhesion of the coating.

The results of such peel tests may be employed to optimise the operation of fibre drawing and coating production apparatus in order to ensure proper curing of the Sylgard coating. In such apparatus, since the curing oven length is fixed and the fibre draw speed is kept constant this means that the dwell time for the on-line coated fibre in the curing oven is essentially constant. Thus by carrying out peel tests on fibre whose on-line coating is cured at different temperatures, the optimum curing oven temperature, corresponding to the maximum peel load, can be determined. Figure 6 shows a graph of mean peak load versus curing oven temperature for one particular fibre drawing and coating apparatus, and the optimum temperature for the curing oven deduced therefrom.

Whereas the invention has been described with respect to the testing of plastics coatings applied to optical fibres, it may alternatively be employed to test plastics coating applied to other elongate members which are "hard" relative to the coating and thus not affected by the monofilament loop. Whilst a two loop-portion loop (interlinked) has been described the method and apparatus of the present invention may alternatively employ a single filament wound to form a loop around the coated elongate member, means being provided, if necessary, to ensure that the loop lies substantially in a single plane normal to the axis of the elongate member. Whilst the load cell and clamp is described as being arranged vertically over the loop, alternatively the arrangement may be horizontal.

Claims

1. A method of testing the adhesion of a plastics coating applied to the circumference of an elongate member, comprising the steps of arranging the coated elongate member in a monofilament loop, tensioning the loop to a predetermined value, applying a gradually increasing load to the elongate member in a direction substantially normal to the applied loop tension whereby to fracture the coating without adversely affecting the elongate member, and monitoring the load, the plastics coating being fractured at a first load value and peeled from the elongate member with subsequently increasing load.

2. A method as claimed in claim 1 wherein the monofilament loop comprises two separate monofilament loop portions which are interlinked.

3. A method as claimed in claim 1 or claim 2 wherein the elongate member is an optical fibre and the plastics coating comprises a cured primary coating thereon, and wherein the first load value is indicative of the state of cure of the primary coating.

4. A method of determining the optimum operating temperature for an oven employed for curing a plastics primary coating applied to an optical fibre, comprising curing a number of lengths of coated optical fibre at different temperatures, testing each of the cured coated fibres by a method as claimed in claim 3 and identifying the curing temperature providing the maximum first load value.

5. Apparatus for use in determining the state of cure andior adhesion of a plastics coating applied to an elongate member, comprising means to apply a gradually increasing load to the coated elongate member along its longitudinal axis, means to circumferentially fracture the coating and peel the coating therefrom, whilst the load is applied, and means to record the load value at least when the coating is fractured.

6. Apparatus as claimed in claim 5, wherein the means to circumferentially fracture the coating com prises a n a rra an arrangement for forming and clamping a first length of a monofilament in a first loop, an arrangement for forming and clamping a second length of monofilament in a second loop, which first and second loops are interlinked to form a loop in which a coated elongate member is arranged in use of the apparatus, and means for maintaining a predetermined loop tension.

7. Apparatus as claimed in claim 6, wherein the load applying means includes a grip means, for the elongate member, adapted to be driven in both directions along the longitudinal axis of an elongate member to be tested by a reversible electric motor, and a load cell for determining the load applied to an elongate member when tensioned between the loop and the grip means in use of the apparatus.

8. A method of testing the state of cure and/or adhesion of a primary plastics coating applied to an optical fibre substantially as herein described with reference to the accompanying drawings.

9. Apparatus for use in determining the state of cure and/or adhesion of a primary plastics coating applied to an optical fibre substantially as herein described with reference to and as illustrated in Figures 1 to 3 of the accompanying drawings.