GB2127561A - Method and apparatus for investigating fibre surfaces - Google Patents

Abstract

Apparatus for analysing the surface of a fibre 32 comprises a chamber 10 adapted to receive the fibre, means 12, 13 for introducing a fluid such as a gas and vapour mixture eg helium and water vapour, into the chamber and means 11 for measuring a change in a property of the fluid as a result of exposure to the fibre. Means for measuring a change in a property of the fluid may be a katherometer (thermal conductivity gauge) or a hygrometer. <IMAGE>

Description

SPECIFICATION Method and apparatus for investigating fibre surfaces The present invention relates to the quality control of artificial fibres, particularly those, eg carbon fibres, used as feinforcing media in a composite.

The mechanical properties of fibre reinforced composites, for example carbon fibre reinforced plastics (CFRP) are affected by the strength of the adhesive bond between the surface of the fibre and the matrix. If this bond is too weak then loads applied to the composite are not transferred fully to the fibre via the fibre-matrix interface and poor mechanical properties result. On the other hand, if the bond is too strong the composite is brittle and its strength is markedly dependent on the presence of notches and other defects. In order to achieve optimum composite properties the strength of the interfacial bond has to be controlled.

During the final stages of production, carbon fibres are usually given a surface treatment (normally oxidative) which strengthens the fibreresin bond in the final CRFP composite and therefore it is the level or amount of this surface treatment which it is necessary to control in order to achieve consistently satisfactory composite properties.

The effects of this surface treatment are twofold: (1) removal of carbon from the fibre surface may lead to changes in surface topography, eg roughening of the surface might increase the strength of the fibre-resin bond. (2) changes in the chemical nature of the surface-oxygen and oxyhydrogen complexes remain chemisorbed on the fibre surface after surface treatment. One or more or these surface groups might react chemically with the resin or its associated curing agents and thus affect the fibre-resin bond strength.

If the adhesive bond is chemically based then assessment of the level of surface treatment would invoive measuring the surface density of the group or groups involved in the chemical bonding process.

The present invention provides apparatus and a method for making such a measurement.

According to a first aspect of the present invention therefore, apparatus for analysing the surface of a fibre comprises a chamber adapted to receive a quantity of the fibre, means for introducing a fluid into the chamber, and means for measuring a change in a property of the fluid as a result of exposure to the fibre.

Preferably the means for introducing a fluid is adapted for introducing a gas or vapour.

A suitable basis upon which to anaiyse the surface of a reinforcing fibre such as a carbon fibre may be sorption, that is to say that the quantity of a fluid which is sorbed, physically, chemically or mechanically, at the surface of the fibre in given conditions may, according to the discovery underlying the present invention, be indicative of the extent to which the surface of the fibre is optimum for the purposes of the fibre-matrix bond In such circumstances the means for measuring a change in a property of the fluid may include means, for example a thermal conductivity gauge or katharometer, for measuring the change in relative quantities of the constituents thereof.

Alternatively, if water sorption indicates the nature of the surface, the means for measuring a change in the property of the fluid may include a hygrometer, perhaps one of the electrolytic type.

According to an important feature of the first aspect of the invention the apparatus may be arranged for continuous or intermittent on-line operation, that is to say analysis of the surface of fibre as it emerges from manufacture. To this end the chamber may have an inlet and an outlet for the fibre, both the inlet and the outlet incorporating a seal, for example a mercury seal, isolating the atmosphere within the chamber. The apparatus may have means such as an inlet heater upstream of the inlet for preparing the incoming fibre for analysis, eg ensuring it is dry, and means, such as an outlet heater downstream of the outlet, for removing sorbate from the fibre.

According to a second aspect of the present invention, a method of analysing the surface of a fibre comprises exposing the fibre, in a controlled environment, to a fluid, and measuring a change in the fluid as a result of the exposure.

Preferably the fluid is a gas or vapour and measuring the change in the fluid is effected by comparing the quantity of the constituents before and after contact with the fibres. It may be a gas and vapour mixture, eg helium and water vapour, the helium acting as a carrier and the water as the sensing medium.

According to a feature of the second aspect of the invention the analysis of the surface of the-fibres may take place at a stage in the production of the fibre just prior to sizing with matrix material. The process may incorporate heating the fibre both prior to analysis to clean it and after analysis to remove sorbate.

Apparatus and a method for analysing the surface of a reinforcing fibre will now be described by way of example with reference to the accompanying drawings, of which: figure lisa schematic diagram of the apparatus, figure 2 is a schematic diagram of a katharometer therefor, figure 3 shows a katharometer electrical circuit, figure 4 illustrates a hygrometer for use with the apparatus.

The apparatus illustrated in figure 1 comprises a sampling tube 10, a katharometer 1 a gas reservoir 12, a vapour reservoir 13, and an expansion chamber 14. The sampling tube 10 incorporates inlet and outlet V-tube mercury seals 15, and 16, respectively, and inlet and outlet ovens 1 7 and 18 respectively.

The katharometer 11, see figure 2, consists of a stainless steel block 20 containing two reference filaments, R1, Raw and two measurement filaments M1,M2 arranged for connection in a wheatstone bridge and arrayed with the reference filaments in a reference chamber 21 and the measurement filaments in a measurement chamber 22, each chamber 21, 22 having an inlet and an outlet.

The wheatstone bridge is more fully illustrated in figure 3, which shows a recorder 30 connected across terminals 1 and 2 and a supply and potential divider 31 between terminal 5 and terminals 3 and 4.

To operate the apparatus illustrated in figures 1 and 3 the sampler tube inlet is arranged to receive carbon fibre 32 immediately upon emerging from the surface treatment stage of manufacture and prior to sizing with matrix material.

In operation of the apparatus, carbon fibre 32 enters the tube 10 and is heated by the oven 1 7 to about 1 450C. It then passes through the mercury seal 1 5 and is exposed to an atmosphere consisting of water vapour in helium. This atmosphere is derived from the helium reservoir 12 and the water reservoir 13, part of the helium passing through a water saturated filter in the reservoir 13. En route to the tube 10 it passes through the chamber 21 in the katharometer 11 where its conductivity affects the resistance of the reference filaments R, and R2. After the tube 10 the atmosphere is passed over the measurement filaments M1 and M2 in the measurement chamber 22 of the katharometer 11. The carbon fibre leaves the tube 10 via the mercury seal 1 6 and the desorption oven 18.

The wheatstone bridge circuit of figure 3 associated with the katharometer 11 will have been balanced using the potential divider 31 witout fibre in the tube 10, so that readings on the recorder 30 provide an indication of vapour sorption by, and hence surface state of, the carbon fibre.

In a particular example of the apparatus and method described with reference to figures 1 to 3 the interior of the sample tube 10 is coated with an hydrophobic material, viz the solution of dimethyl-dichloro-silane in chloroform sold by Hopkins and Williams under the trade name 'Repelcote'. The helium was arranged to carry of the order of 6000 ppm water or less. The absorption chamber in the sample tube 10 was maintained at OOC and the ovens at 1450C.

In an alternative embodiment, an electrolytic hydrometer cell, see figure 4, replaces the katharometer 11. The hydrometer comprises a PTfE rod 40 surrounded by a glass tube 41. There is an annular measuring chamber 42 therebetween with an inlet 43 and an outlet 44 for the fluid under scrutiny. The exterior of the rod 40 carried two adjacent platinum wires 45 in spiral grooves, while the surface of the rod is coated with a solution 46 containing phosphorous pentoxide.

The mode of operation of the hygrometer is that the atmosphere under scrutiny is introduced into the chamber 42 whilst a potential difference is applied between the platinum wires 43. Water sorbed by the phosphorus pentoxide is electrolysed due to the potential difference, and the electrolysis current, which is detectable, is proportional to the amount of the water in that atmosphere.

Claims (15)

1. Apparatus for analysing the surface of a fibre comprising a chamber adapted to receive a quantity of the fibre, means for introducing a fluid into the chamber, and means for measuring a change in a property of the fluid as a result of exposure to the fibre.

2. Apparatus as claimed in claim 1 and wherein the means for introducing a fluid is adapted for introducing a gas or vapour.

3. Apparatus as claimed in claim 1 or claim 2 and wherein the means for measuring a change in a property of the fluid is a thermal conductivity gauge or katherometer.

4. Apparatus as claimed in claim 1 or claim 2 and wherein the means for measuring a change in a property of the fluid is a hygrometer.

5. Apparatus as claimed in any one preceding claim and wherein the apparatus is arranged for continuous or intermittent on-line operation.

6. Apparatus as claimed in claim 5 and wherein the chamber is provided with an inlet and outlet for the fibre, both the inlet and the outlet incorporating a seal to isolate the atmosphere within the chamber.

7. Apparatus as claimed in claim 5 or claim 6 and wherein the apparatus has means for preparing the incoming fibre and means for removing sorbate from the fibre downstream of the outlet.

8. Apparatus as claimed in claim 7 and wherein the means for preparing the incoming fibres and the means for removing sorbate downstream of the outlet are heaters.

9. A method of analysing the surface of a fibre using apparatus as claimed in any one preceding claim comprising exposing the fibre, in a controlled environment to a fluid, and measuring a change in the fluid as a result of the exposure.

10. A method as claimed in claim 9 and wherein the fluid is a gas or vapour or a gas and vapour mixture.

11. A method as claimed in claim 9 or claim 10 and wherein the change in the fluid is measured by comparing the quantity of the constituents of the fluid before and after contact with the fibres.

12. A method as claimed in any one of claims 9, 10 or 11 and wherein the fluid is a mixture of helium and water vapour.

13. A method as claimed in any one of claims 9, 10, 11 or 12 and wherein analysis of the surface of the fibres takes place at a stage in the production of the fibre just prior to sizing with matrix material.

14. A method as claimed in any one of claims 9, 10, 11., 12 or 13 and wherein the-fibre is heated prior to analysis and after analysis.

15. Apparatus as claimed in any one of claims 1 to 8 inclusive substantially as hereinbefore described and with reference to the drawings.

1 6. A method as claimed in any one of claims 9 to 14 inclusive substantially as hereinbefore described with reference to the example.