GB2356868A

GB2356868A - Liquid crystal coating composition comprising liquid crystal material & curable precursor for an elastomeric continuous phase (matrix) polymer

Info

Publication number: GB2356868A
Application number: GB9928712A
Authority: GB
Inventors: William James Stephen Barker; Robert John Pugh
Original assignee: B&H Liquid Crystal Devices Ltd
Current assignee: B&H Liquid Crystal Devices Ltd
Priority date: 1999-12-03
Filing date: 1999-12-03
Publication date: 2001-06-06
Also published as: GB9928712D0

Abstract

A liquid crystal coating composition comprises a liquid crystal material and at least one curable polymer matrix precursor materials such as a material which forms a continuous phase elastomeric material upon curing (for example a polyurethane or polyester). The composition may be printed onto a substrate (for example of polymeric material, wood or metal) and subsequently cured to produce a polymer matrix enclosing the liquid crystal material 7. Such liquid crystal display devices may be used as labels, eg for indicating the temperature of packages of prepared foods. The matrix absorbs less light than conventional microcapsules, which means that the amount of liquid crystal material may be reduced.

Description

2356868 LIQUID CRYSTAL MATERIALS This invention relates to liquid crystal

materials, particularly (though not exclusively) those which are thermochromic (ie those whose colour is temperature dependent).

Many thermochromic liquid crystal materials are known, as are devices incorporating such materials (for example thermometers). Liquid crystal materials tend to exhibit long term instability when exposed to oxygen or moisture, and therefore need to be sealed from the atmosphere.

Typically, this is accomplished by micro-encapsulation within hollow polymeric spheres. However, this procedure suffers from the inherent drawback that a substantial proportion of light from the liquid crystal material is is absorbed, diffracted or diffused by the walls of the microcapsules. The problem may be mitigated to a certain extent by creating microcapsules with a large diameter and small wall thickness, but there is a practical limit beyond which an unacceptable number of microcapsules burst.

Normally the microcapsules are around 10 to 3Ogm in diameter, and have wall thicknesses of up to igm.

By employing microcapsules which are as large and thin- walled as possible, the amount of light lost may be minimised, but the degree of light loss is still such that a substantial volume of expensive liquid crystal material needs to be used in order to achieve an acceptable degree of visible colour change. The cost of producing such microencapsulated materials is accordingly relatively high, and liquid crystal materials are therefore precluded from use in various fields to which they would otherwise be ideally suited.

One such field is the food packaging industry. Food bacteria multiply at a faster rate as temperature increases, and food poisoning is becoming more prevalent due to the increase in availability of pre-cooked chilled food, the continuing lack of awareness of correct fridge temperatures and the trend towards less use of preservatives in foods generally. Different bacteria move from a dormant to an active state at different temperatures. E coli begins to become active at around 70C, and compila bacta at around 80C. Other bacteria react at different temperatures.

Storage temperatures should therefore be maintained below the point at which the more dangerous bacteria become active. Ideally, storage temperatures should be below SOC.

It is therefore a desirable objective to provide packages of prepared foods with means for indicating the temperature of the package. It would then be a straight forward matter for those involved in the transportation, storage, display and purchase of the goods to ascertain whether or not any individual item was being maintained at the correct temperature. Unfortunately, however, the above noted drawbacks associated with existing liquid crystal encapsulation techniques make the use of such materials impossibly uneconomic for mass incorporation into disposable packaging. There is therefore a need for an improved liquid crystal composition which exhibits the maximum degree colour change while utilising the minimum of liquid crystal material. There is also a need for an improved technique which permits the simple, quick and effective production of liquid crystal devices incorporating such materials.

The present invention addresses these problems, and provides a liquid crystal coating composition comprising a liquid crystal material and one or more curable polymer matrix precursor materials.

The composition (or "ink") may be printed onto any suitable substrate material (for example polymer materials, wood or metal surfaces). A curing step then results in the formation of a polymer matrix enclosing the liquid crystal material. The invention therefore also provides a liquid crystal display device comprising a substrate bearing a cured composition as defined above, in the form of a liquid crystal material retained within a polymer matrix.

The invention also extends to a method of manufacturing a liquid crystal display device, the method comprising the steps of: printing onto a suitable substrate a liquid mixture containing a liquid crystal material and one or more curable polymer matrix precursor materials; and curing said precursor material(s) to form a polymer matrix encasing said liquid crystal material.

By dispersing neat liquid crystal material in a polymer matrix, we have found that the intensity of colour exhibited by symbols printed, for example, on reel to reel equipment can be increased to acceptable levels, which are not achievable using conventional microcapsules, while also reducing the amount of liquid crystal material used by about 20-06. The increase in light transmittance is thought to be due to a number of factors. The absence of a multiplicity of microcapsule walls leads to a decrease in light absorption, but it is postulated by the applicants that further benefits accrue from a reduction in refraction and destructive interference. Within conventional microcapsules the liquid crystal droplets (of around 8 to 3OAm in diameter) adopt a spherulitic structure, which results in an optical axes running radially from the centre of the droplets, such axes being everywhere normal to the interior surface of the microcapsules. In contrast, within the polymer matrices of the present invention there is evidence of a micellar arrangement of liquid crystal aggregates, the micelles being less than 2 to 3Am in diameter and having optical axes which are aligned with the surface of the substrate. This results in high intensity colours due to constructive Bragg reflection.

Preferably, the polymer matrix is formed from a continuous phase elastomeric material, for example a polyurethane polymer. The elastomeric material may, for instance, comprise a polyester. Many different precursor materials will readily suggest themselves to those skilled in the art. Merely by way of example, polyols (such as diols and tetrols) may be used in conjunction with crosslinkers (such as di-isocyantes).

Liquid crystal materials according to the invention may be incorporated into labels for indicating the temperature of an item to which the label is attached. For such a purpose, the liquid crystal material is preferably chosen to possess a visible characteristic (such as colour or degree of opacity) which changes significantly at a predetermined temperature. The label may therefore readily indicate visually to an observer whether the item to which the label is attached is above or below the predetermined temperature.

The choice of liquid crystal material will depend upon is the precise use for which the device is intended. In principle, any suitable liquid crystal material may be used, for example chiral nematic crystals and cholesteryl compounds (such as benzoate, caprylate, p-chlorobenzoate, p chl orophenyl acetate, 3chloropropionate, cinnamate, 2,4 dichlorobenzoate, 3,4-dichlorobenzoate, 3,5 dinitrobenzoate, ethylcarbonate, formate, isobutyrate, isotridecylcarbonate, laurate (dodecanoate), linoleate, linoenate, methyl carbonate, myristate, p-phenyl benzoate, propionate, trichloroacetate, hemisuccinate, sulfate, undecanoate, stearate, sebacate, oleyl carbonate, caproate, chloride, chloroformate, isostearyl carbonate, nonanoate, p nonylphenyl carbonate and oleate).

Liquid crystal mixtures can be accurately formulated by those experienced in the art to indicate temperatures to within an accuracy of +0.250C.

The invention therefore provides a simple, cheap and accurate means of creating liquid crystal materials, for example for incorporation into temperature-indicating labels. Such labels may be used with advantage in the packaging of perishable foods, and also in many other fields where the temperature of an item is important. Liquid crystal materials according to the invention may for example be used to indicate the temperature of hot drinks, radiators and drugs requiring cool storage. They may also be used in containers for urine samples taken (for example) for drug testing, where it is important to ensure that the temperature of the sample corresponds to core body temperature in order to prevent tampering. Many further uses will readily suggest themselves to those skilled in the art.

The invention will hereinafter be described in more detail by way of example only, with reference to the accompanying drawings, in which:Figure 1 is a diagrammatic representation of an embodiment of food packaging label incorporating a liquid crystal material according to the invention, showing the display at a temperature less than SOC; and Figure 2 is a diagrammatic representation of the label of Figure 1, showing the display at a temperature above SOC.

The food packaging label (10) illustrated in Figures 1 and 2 comprises a polymeric film substrate having a black upper surface (liquid crystal materials are typically printed onto black backgrounds, in order to present their colour variations most clearly) Onto the substrate are printed two areas (12 and 14) of liquid crystal mixtures.

Both contain polymer matrix precursor material and one or more liquid crystal materials, the identity of the liquid crystal materials differing between the two areas. The liquid crystal material printed onto area 12 (Figure 1) is adapted to display a green colour at temperatures below SOC and to be transparent above this temperature, while the liquid crystal material printed onto area 14 (Figure 2) is adapted to display a red colour at temperatures above SOC and to be transparent below this temperature. Thus, an observer may readily determine whether an item to which the label is attached has a temperature of below SOC (display as shown in Figure 1) or above 50C (display as shown in Figure 2).

The compositions of the invention may be printed onto suitable substrates in conventional manner, for example onto a polymer film fed by reel to reel apparatus. Printing may conveniently by carried out by a gravure process, using a silk screen rotary head, but other methods may also be used.

The specific formulations of suitable liquid crystal materials to produce particular colours will be well known to those skilled in the art, and appropriate polymer matrix precursor materials will also be readily apparent. By way of example, there is set out below the formulation of a suitable composition adapted to produce a red colour at temperatures above SOC:

Cholesterol nonanoate 60g Cholesterol chloride 15g N(4-methoxybenzylidene) -4-butylanaline 8g 20g of the above mixture should be added to:

Capa 210, a 1000 molecular weight diol 30g Capa 316, a 1000 molecular weight tetrol 8g Butane diol 2g Isophorone di-isocyanate 25g

Claims

A liquid crystal coating composition comprising a liquid crystal material and one or more curable polymer matrix precursor material (s).
2. A composition according to claim 1, wherein the curable polymer matrix material(s) is/are chosen from those which form continuous phase elastomeric materials upon curing.
3. A composition according to claim 2, wherein the elastomeric material is a polyurethane polymer.
4. A composition according to claim 3, wherein the elastomeric material is a polyester.
5. A composition according to any preceding claim, wherein the precursor materials comprise at least one polyol and at least one crosslinker.
6. A composition according to claim 5, wherein the polyol is a diol or a tetrol or a mixture thereof and the crosslinker is a di-isocyanate.
7. A liquid crystal display device comprising a substrate bearing a cured composition according to any preceding claim, in the form of a liquid crystal material retained within a polymer matrix.

8. A label incorporating a device according to claim 7, the liquid crystal material being adapted to undergo a substantial change in a visible characteristic thereof at a predetermined temperature, whereby the label is adapted to indicate visually to an observer whether an item to which the label is attached is above or below said predetermined temperature.
8
9. A label according to claim 8, wherein said visible characteristic is the degree of opacity/transparency and/or the colour of the liquid crystal material.
10. An item of food packaging incorporating a label according to claim 7 or claim 8.
11. An item of food packaging according to claim 10, wherein said label is provided with a discrete area containing liquid crystal material adopted to exhibit a green colour at or below a predetermined temperature and/or a discrete area containing liquid crystal material adapted to exhibit a red colour at or above said predetermined temperature.
12. A method of manufacturing a liquid crystal display device, the method comprising the steps of: printing onto a suitable substrate a liquid mixture containing a liquid crystal material and one or more curable polymer matrix precursor materials; and curing said precursor material(s) to form a polymer matrix encasing at least a substantial proportion of said liquid crystal material.