There are many processes which involve the absorbtion, and optional subsequent re-emission, of energy. For example, domestic storage heaters include blocks having a high thermal mass which are arranged to absorb heat from a heating element during the night time using cheaper electricity and to re-emit the heat during the day time. In addition, situations can be envisaged where it may be desirable to absorb electromagnetic radiation such as microwaves or radio waves in order to shield an article from the radiation.

It is an object of the present invention to address the problem of energy absorbtion and optional re-emission.

This invention is based on the discovery of a surprising property of optionally-substituted polyvinyl alcohol or compounds incorporating the same namely that energy can be absorbed under certain conditions and re- emitted at a later time.

According to a first aspect of the present invention, there is provided the use of a first material which includes optionally-substituted vinyl alcohol moieties as an energy absorbing means.

The term"optionally-substituted vinyl alcohol" moieties suitably refers to vinyl alcohol moieties wherein one or more hydrogen atoms have been optionally

substituted by other atoms or groups. Optional substituents include optionally-substituted alkyl groups.

Preferably, said vinyl alcohol moieties are unsubstituted.

Said optionally-substituted vinyl alcohol moieties are preferably components of a first polymeric moiety, for example, a polymeric chain. Said first polymeric moiety may be a moiety derived from a copolymer of optionally- substituted vinyl alcohol and at least one other monomer which preferably includes an alkenyl, more preferably a vinyl, group. Preferably, said at least one other monomer is hydrophobic in character when in water. Said copolymer of vinyl alcohol and said at least one other polymer preferably is highly random. It is found that the more random the copolymer, the greater the energy absorbtion.

Preferred optional substituents on said other monomer include carboxy, acid anhydride, acid halide and ester functionalities. Preferably, said optional substituents have the general formula-COOR wherein R represents a hydrogen or halogen atom or an optionally substituted, especially an un-substituted, alkyl group. More preferably, said optional substituents comprise alkanoate groups. Preferably, said at least one other monomer comprises a vinyl alkanoate compound, especially vinyl acetate.

Said first polymeric moiety may include at least 20 wt%, preferably at least 40 wt%, more preferably at least 60 wt%, especially at least 70 wt% of said optionally- substituted vinyl alcohol moiety. Said first polymeric moiety may include less than 95 wt%, preferably less than 90 wt%, more preferably less than 85 wt% of said optionally-substituted vinyl alcohol moiety.

Said first polymeric moiety may include at least 5 wt%, preferably at least 10 wt%, more preferably at least 15 wt% of said other monomeric moieties. Said first polymeric moiety may include less than 50 wt%, preferably less than 40 wt%, more preferably less than 30 wt%, especially less than 25 wt% of other monomeric moieties.

Preferably, said first polymeric moiety consists essentially of a moiety derived from a co-polymer of optionally-substituted vinyl alcohol and said at least one other monomer.

One or more electrolytes may be associated with said optionally-substituted vinyl alcohol moieties and/or with said first polymeric moiety. The use of electrolytes such as sulphates and/or thiosulphate is found to affect the energy absorbtion characteristics and more particularly, the temperature at which energy is absorbed and/or emitted by the energy absorbing means.

Said first material may include a second polymeric moiety with which said optionally-substituted vinyl alcohol moieties and/or said first polymeric moiety are associated.

Preferably, said first polymeric moiety is bonded to said second polymeric moiety.

A pre-cursor of said second polymeric moiety (ie. a compound with which said first polymeric moiety or a pre- cursor thereof may be reacted to bond said first polymeric moiety to said second polymeric moiety) may have a general formula

wherein A and B are the same or different and at least one comprises a polar atom or group, Rl and R2 independently represent non-polar atoms or groups and n represents an integer.

Preferably, Rl and R2 are independently selected from a hydrogen atom or an optionally substituted, preferably un-substituted, alkyl group. Preferably, R'and R2 represent the same atom or group. Preferably, R'and Rz represent a hydrogen atom.

Preferably, A and B are independently selected from optionally-substituted alkyl, cycloalkyl, cycloalkenyl, cycloalkynyl, aromatic and heteroaromatic groups. Where group A or B has a cyclic structure, five or, more preferably, six membered rings are preferred.

More preferably, A and B are independently selected from optionally substituted aromatic and heteroaromatic groups, with five or, more preferably, six-membered such groups being especially preferred. Preferred heteroatoms of said heteroaromatic groups include nitrogen, oxygen and sulphur atoms of which oxygen and especially nitrogen, are preferred. Preferred heteroaromatic groups include only one heteroatom. Preferably, a or said heteroatom is positioned furthest away from the position of attachment

of the heteroaromatic group to the group C=C. For example, where the heteroaromatic group comprises a six-membered ring, the heteroatom is preferably provided at the 4- position relative to the position of the bond of the ring with the group C=C.

Unless otherwise stated, optionally substituted groups described herein for groups A and B, may be substituted by halogen atoms, and optionally substituted alkyl, acyl, acetal, hemiacetal, acetalalkyloxy, hemiacetalalkyloxy, nitro, cyano, alkoxy, hydroxy, amino, alkylamino, sulphinyl, alkylsulphinyl, sulphonyl, alkylsulphonyl, sulphonate, amido, alkylamido, alkylcarbonyl, alkoxycarbonyl, halocarbonyl and haloalkyl groups. Preferably, up to 3, more preferably up to 1 optional substituents may be provided on an optionally substituted group.

Unless otherwise stated, an alkyl group may have up to 10, preferably up to 6, more preferably up to 4 carbon atoms, with methyl and ethyl groups being especially preferred.

Preferably, A and B each represent polar atoms or groups. Preferably, A and B each represent optionally- substituted aromatic or heteroaromatic groups wherein the "p"orbital of the aromatic groups are aligned with those of the group C=C. Preferably, A and B represent different atoms or groups.

Preferably, one of groups A and B includes an optional substituent which includes a carbonyl or acetal group with a formyl group being especially preferred. The other one of groups A and B may include an optional substituent which is an alkyl group, with an optionally

substituted, preferably un-substituted, Cl4 alkyl group, for example a methyl group, being especially preferred.

Preferably, group A represents a phenyl group substituted, preferably at the 4-position relative to the group C=C, by a formyl group or a group of general formula where x is an integer from 1 to 6 and each R3 is independently an alkyl or phenyl group or together form an alkalene group.

Preferably, group B represents a group of general formula wherein R4 represents a hydrogen atom or an alkyl or aralkyl group, R5 represents a hydrogen atom or an alkyl group and X-represents a strongly acidic ion.

Said first material may be used to absorb thermal energy, suitably from the surroundings. The thermal energy may be absorbed when the ambient temperature is in

a first range. When the ambient temperature falls below the minimum of the first range, then the thermal energy previously absorbed by the material may be re-emitted. Said first range may be adjusted by associating electrolytes with the first material. Advantageously, said first range may be 10°C to 20°C which makes said first material particularly suited to use in domestic heating situations.

Alternatively, said first material may be used to absorb electromagnetic waves, for example radio waves or microwaves. One example of such use is in masking a vehicle from detection by radar. In this case, said first material optionally in combination with a carrier and suitably in a fluid state, may be arranged around, for example by being sprayed around, said vehicle so that said first material absorbs electromagnetic energy which is, accordingly, not reflected back to a radar receiver so the vehicle is invisible to radar and is undetectable.

According to a second aspect of the invention, there is provided a method of absorbing energy at a locus, the method including the steps of: selecting a locus at which it is desired to absorb energy; and associating a first material which includes optionally-substituted vinyl alcohol moieties with the locus.

According to a third aspect of the invention, there is provided a combination comprising a first material as described according to said first aspect and apparatus for absorbing and/or emitting energy, for example heat.

Said apparatus may include means for controlling absorption and/or emission of energy. Said apparatus may be a heater.

According to a fourth aspect of the present invention, there is provided a method of preparing an energy absorbing material comprising associating optionally-substituted vinyl alcohol with a carrier.

The method is suitably used to prepare a material according to said first aspect.

The method preferably comprises preparing a first polymeric material, suitably by co-polymerizing said vinyl alcohol with one other monomer (which may, therefore, act as a carrier for such vinyl alcohol). A preferred monomer is a vinyl alkanoate material, especially vinyl acetate.

Thus, preferably, said first polymeric material comprises a poly (vinylalcohol-vinylacetate) co-polymer. It has been found that electrolytes can be used when making up aqueous liquids, which include said first polymeric material such that the electrolytes affect the energy absorbtion characteristics and more particularly the temperature at which energy is absorbed and/or emitted by the energy absorbing material. For example, sulphates are found to lower the minimum temperature at which energy is absorbed whereas the thiosulphates have an opposite effect.

Said method may include associating, for example reacting, said vinyl alcohol or a compound prepared by reaction of said vinyl alcohol with another compound, for example said first polymeric material, with a second polymeric material (which suitably provides said second polymeric moiety described above).

Said second polymeric material may be prepared by providing a compound of general formula

or a salt thereof in a solvent of a type in which ethene itself is generally insoluble and causing the groups C=C in said compound to react with one another to form a polymeric structure.

Preferably, said solvent is a polar solvent.

Preferably said solvent is an aqueous solvent. More preferably, said solvent consists essentially of water. Preferably, said compound of general formula I is provided in said solvent at a concentration at which molecules of said compound aggregate. An electrolyte may be present in the solvent to affect the aggregation of said molecules.

Aggregation of said compound of general formula I may be shown or inferred from the results of various analyses, for example from vapour pressure analyses, surface tension measurements, molar conductance measurements, molar volume measurements as described in Applicant's co-pending application number GB 96 19419.6 and any one or more of such analyses may be used. Preferably, said compound of general formula I is provided in said solvent at or above a concentration suggested by relevant vapour pressure measurements as being a point of aggregation of the compound.

It is believed that said molecules of compound I form aggregates or micelles in the solvent, with the C=C bonds aligned with one another so that the molecules effectively align substantially parallel to one another. Preferably,

the molecules align with groups A and B adjacent to one another.

Said compound of general formula I may be provided in said solvent at a concentration of at least 0.5 wt%, preferably at least 1.0 wt% and, more preferably, at least 1.5 wt%.

The groups C=C in said compound are preferably caused to react in a photochemical reaction. Preferably, the method comprises inducing a photochemical reaction, suitably using ultraviolet light. Preferably, in the method, light of up to 500 nm wavelength is used.

Preferred compounds of general formula I for use according to the present invention include those referred to on page 3 line 8 to line 39 of GB 2 030 575 B and said compounds are hereby incorporated into this specification.

Compounds of general formula I for use according to the present invention may be prepared as described in GB 2 030 575 B and such preparatory methods are also hereby incorporated into this specification.

Preferably, said vinyl alcohol or a moiety derived therefrom or incorporating the same, for example said first polymeric material, is reacted with said second polymeric moiety, suitably in an acid-catalysed Aldol condensation reaction.

According to a fifth aspect of the present invention, there is provided any novel material prepared or preparable in a method according to said second aspect.

Any feature of any aspect of any invention or embodiment described herein may be combined with any feature of any other aspect of any invention or embodiment described herein.

The invention will now be described, by way of example, with reference to the single accompanying figure which is a graph of heat capacity (aCp|kJ. kg-'K-') vs temperature (°C) for the gel prepared in Example 3.

Example 1 Preparation of polo (vinvlalcohol-vinvlacetate) co-polymer This was prepared using a standard method for example as described in"Experimental Plastics and Synthetic Resins", G. F. D'Alelio, published by Wiley.

Example 2 Assessment of heat absorbtion/release properties of copolymer (s) prepared in Example 1 A microcalorimetric experiment was set up in which the temperature was increased at a regular rate to a predetermined maximum and subsequently decreased at the same rate.

It was found that as the temperature was increased heat was absorbed from the surroundings and as the temperature was decreased, heat was given out in a similar manner to that illustrated in the figure hereto.

Example 3 Preparation of qel having heat absorbtion/release properties (i) Preparation of Poly 4-di (4-(N-methylpyridinyl))- 2,3-di (4- (1-formylphenyl) butylidene (CompoundIIshown below) An aqueous solution of greater than 1 wt% SbQ was exposed to ultraviolet light. This results in a photochemical reaction between the carbon-carbon double bonds of adjacent 4- (4-formylphenylethenyl)-1- methylpyridinium methosulphate molecules (I) in the aggregate, producing a polymer, poly (1,4-di (4- (N- methylpyridinyl))-2,3-di (4- (1-formylphenyl) butylidene (II), as shown in the reaction scheme below. It should be appreciated that the anions of compounds I and II have been omitted in the interests of clarity.

>1 °,'ow/w Aqueous solution UVirradiation

(ii) Preparation of gel reaction of co-polymer of Example 1 and the compound of Example 3 (i).

The copolymer of Example 1 (13g) and the compound of Example 3 (i) (87g of a 2% w/w solution) were reacted together in a standard Aldol condensation reaction to produce a gel.

Example 4 Assessment of heat absorbtion/release properties of gel prepared in Example 3 (ii).

The result of a microcalorimetric experiment on the gel in which the temperature was increased at a regular rate to a predetermined maximum and subsequently decreased at the same rate are provided in the figure hereto.

It will be noted that as the temperature was increased heat was absorbed from the surroundings and, as the temperature was decreased, heat was given out.

The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of the foregoing embodiment (s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.