GB2258918A - Temperature indicators - Google Patents

Abstract

A thermal indicator for verifying whether a substance e.g. timber, rubber, cork or food has properly been heat-sterilised, comprises a pin, nail, plug or staple having a rigid member for insertion into a piece of timber. A head of the indicator carries or incorporates a thermally sensitive pigment which is arranged to change colour if the thermal sterilisation has been carried out properly. A pile of timber can be checked rapidly by looking at indicators which have been inserted into each individual piece.

Description

THERMAL INDICATORS The present invention relates to thermal indicators, and particularly to indicators for verifying when timber has been properly "pasteurised" (that is, heat-treated to ensure its sterility).

It is a requirement of the Authorities in a number of countries that timber should be heat-treated ("pasteurised") prior to import. The purpose is to ensure that imported timber is sterile, and does not contain any insects, worms or other undesirable parasites.

At present it is not easy for officers to check at the port of entry that the timber has been properly sterilised, and the present invention aims to alleviate this difficulty by providing a "tell-tale" which can be attached to individual pieces of timber prior to the sterilising heat-treatment process, and which will subsequently confirm that the timber has properly been treated.

Thermally sensitive pignments are of course already available as commercial products, and are typically incorporated in solvent or solvent/resin carrier systems. The need to apply these to smooth, uniform, chemically inert surfaces limits their usefulness.

Certainly, they are at present not useful in conjunction with timber, which is not only not smooth, uniform or chemically inert, but may also be wet or at least damp at the time of application.

According to a first aspect of the present invention there is provided a thermal indicator having a rigid shaft for insertion into a substrate, the indicator carrying or incorporating a thermally-sensitive pigment.

According to a second aspect of the present invention there is provided a method of verifying the thermal processing of a substrate, comprising inserting a rigid member of a thermal indicator into the substrate, and determining whether the processing has been correctly carried out according to the colour of a thermally-sensitive pigment carried on or incorporated into the thermal indicator.

The invention may be carried into practice in a number of ways, and one specific embodiment will now be described, by way of example.

The preferred embodiment takes the form of a tack, nail, staple or drawing pin comprising a head and, beneath that, one or more relatively narrow elongate shafts. The upper surface of the head is painted or impregnated with a suitable temperature sensitive material such as the paints supplied by Thermindex of Riverside Buildings, Unit 1, Dock Road, Connahs Quay, Deeside, Clwyd CH5 4DS. The head is preferably round.

The member for insertion into the substrate is preferably a shaft, but may be a pin, grip, clasp, blade or other rigid mechanical member.

Prior to a batch of timber being thermally sterilised, or otherwise subjected to elevated temperatures, a piece of timber has one of these thermal indicators driven into it. The indicators remain in the timber during the subsequent handling and shipping. In this way, a customs officer or other official who needs to check that the timber has properly been heat treated can easily do so simply by looking at the indicator colour. If the timber has not been heat treated, or if this has been done improperly (for example at too low a temperature or for too short a time) the indicator will be one colour; on the other hand, if heat treatment has been carried out properly the indicator will be another colour.

Typically, sterilising heat treatment (pasteurisation) requires that the temperature of the timber be raised to approximately 500 C for around four hours or to higher temperatures for shorter times. The ideal temperature sensitive material for this specific application will therefore be sensitive not only to temperatures, but also to the time at which a piece of timber has been held at a particular temperature.

Thermally indicating paints are ideal, but as these are typically moisture-sensitive it may prove necessary to encapsulate the paint within the head of the indicator so that it is not affected by atmospheric moisture or other chemicals. This could be achieved by providing a transparent layer of varnish or lacquer over the paint.

Alternatively, it might be possible to incorporate the thermally-sensitive treatment into the body of the indicator itself. One could, for example, make at least the head of the indicator out of a naturally transparent or translucent plastics material, with the thermally sensitive treatment incorporated therein.

Where the plastics material is moulded, it may be desirable to make use of cold moulding techniques to ensure that the thermally sensitive pigments do not activate during the moulding process itself. A suitable material for this purpose might be Methacrylate.

The exact form of the indicator is not critical. All that is needed, essentially, is some mechanical carrier for the thermally sensitive material which can be driven into timber. Options include drawing pins, pins, nails, staples or other articles of similarly driveable form. As mentioned above, the indicator preferably has a head and a shaft, rather like a drawing pin or a golf tee, but in an alternative form the indicator could simply be a pointed shaft without any separate head. In that case, either the end of the shaft could be coated with the thermally sensitive material, or suitable thermally sensitive pigments could be incorporated into the material from which the shaft is made.

The indicators could be driven into the timber by hand, but more conveniently they will be machine driven by an appropriate driver or gun. Ideally the indicators are driven into the timber sufficiently hard so that they are slightly recessed behind the surface of the timber.

Then, if it is desired subsequently to plane the surface in which the indicator has been driven, the indicator will not get in the way of the plane.

The indicator is desirably made from a suitable plastics material, and is of one piece. A plastics material indicator will not damage a plane, as a metal indicator might, and additionally does not present a hazard when high-frequency heating etc is employed.

Alternatively, if the indicator were to be made of metal or to have a metal or other heat-conducting shaft, the shaft could then act as a thermally-conducting pathway to modify the response of the thermally indicating material at or nearer the surface. One might for example have an indicator with an insulated head on a heat-conductive shaft.

To prevent the indicator from coming out of the timber, the member for insertion into the substrate may be barbed or corregated. It could also be hollow; and the indicator could take the form of a flat head with a flange around the edge, such as is found in a beer-bottle top.

In an alternative form, the indicator may comprise a plug for insertion into predrilled or preformed holes.

The plugs could be several centimetres in length in order to record or indicate temperature gradients within the timber.

It is also envisaged that the indicators may incorporate mechanical, chemical or other features which will indicate if tampering has occurred. The material of which the indicator is made may include specific "chemical fingerprints" to make the indicators difficult to counterfeit.

A similar sort of indicator could be used, with a suitable thermally-sensitive pigment, on substrates other than wood, for example rubber, cork, foodstuffs and so on.

Claims (25)

CLAIMS:

1. A thermal indicator having a rigid member for insertion into a substrate, the indicator carrying or incorporating a thermally-sensitive pigment.

2. A thermal indicator as claimed in claim 1 which takes the form of a pin or nail having a head at one end of the member.

3. A thermal indicator as claimed in claim 1 which takes the form of a headless pin or nail.

4. A thermal indicator as claimed in Claim 1 which takes the form of a staple.

5. A thermal indicator as claimed in Claim 1 which takes the form of a plug.

6. A thermal indicator as claimed in any one of the preceding claims in which the member is pointed at one end.

7. A thermal indicator as claimed in any one of the preceding claims which is made of a plastics material.

8. A thermal indicator as claimed in any one of Claims 1-6 which is made of metal.

9. A thermal indicator as claimed in any one of the preceding claims which is of one unitary piece.

10. A thermal indicator as claimed in any one of the preceding claims in which the member is barbed.

11. A thermal indicator as claimed in any one of the preceding claims in which the thermally-sensitive treatment comprises a coating which is applied to at least part of the indicator so as to to be visible when the indicator is inserted into a substrate.

12. A thermal indicator as claimed in any one of the preceding claims in which the thermally-sensitive pigment is incorporated into at least part of the material from which the indicator is made, so as to be visible when the indicator is inserted into a substrate.

13. A thermal indicator as claimed in any one of the preceding claims suitable for being driven into a substrate.

14. A thermal indicator as claimed in any one of the preceding claims in which the thermally-sensitive pigment is protected by a clear coating.

15. A thermal indicator as claimed in any one of the preceding claims in which the member is heat-conductive, whereby the indicator is sensitive to temperatures in the interior of a substrate.

16. A thermal indicator as claimed in any one of the preceding claims in which the thermally-sensitive pigment changes colour on being held at a particular temperature for a particular time.

17. A thermal indicator as claimed in any one of the preceding claims in which the thermally-sensitive pigment is arranged on activation to undergo an irreversible colour change.

18. A thermal indicator as claimed in any one of the preceding claims in which the member is a shaft.

19. A thermal indicator substantially as specifically described.

20. A method of verifying the thermal processing of a substrate, comprising inserting a rigid member of a thermal indicator into the substrate, and determining whether the processing has been correctly carried out according to the colour of a thermally-sensitive pigment carried on or incorporated into the thermal indicator.

21. A method as claimed in Claim 20 including driving the member into the substrate.

22. A method as claimed in Claim 21 including overdriving the member so that the entire indicator is recessed behind the surface of the substrate.

23. A method as claimed in Claim 22 including placing the member in a hole which is preformed or predrilled in the substrate.

24. A method of verifying the thermal sterilisation of timber, the method being as claimed in any one of Claims 20-23.

25. A method of verifying the thermal sterilisation of timber, substantially as specifically described.