WO1997019141A1

WO1997019141A1 - Adhesives

Info

Publication number: WO1997019141A1
Application number: PCT/GB1996/002832
Authority: WO
Inventors: Gordon Nelson
Original assignee: Bioinvicta Limited
Priority date: 1995-11-17
Filing date: 1996-11-18
Publication date: 1997-05-29
Also published as: GB9523581D0; AU7583396A

Abstract

The present invention concerns methods of manufacturing of adhesives comprising cross-linking first and second molecules each of which has at least one group selected from the group of hydroxy aromatic, dihydroxy aromatic, hydroxy phenone and amino, the first molecule being linked to the second molecule via at least one quinone, together with adhesives produced according to the method. Also provided is the use of same molecules in a method of manufacture of an adhesive. The invention further provides a method of manufacturing thermostably bound solids, together with the use of gelatin, soya and casein in a method of manufacture of thermostably bound solids.

Description

Adhesives

The present invention concerns a method of cross-linking molecules each of which has at least one chemical group selected from the group of hydroxy aromatic, dihydroxy aromatic, hydroxy phenone and amino. The present invention also concerns molecules made by such a method, and in particular concerns adhesives made by the method.

Adhesives are widely used both domestically and industrially. They are typically applied to dry first and second surfaces in order to bond the surfaces together, e.g. as bonding agents to bond together particulate matter, or to adhere other solid materials such as woods and metals. Most adhesives bind dry surfaces more strongly than the same wet surfaces and the addition of water to bound surfaces typically causes an irreversible, or substantially irreversible, weakening of the bond. The bonding of an adhesive can be significantly impaired by the competition of water for surface area to bind by the dispersive effect of water on the adhesive. Adhesives also display a significant plasticity as the temperature is raised, particularly as the temperature reaches approximately 100°C.

Due to this fact, bioadhesives such as the polyphenolic proteins produced by the phenol glands ofthe mussel genus Mytilus have become of particular interest (see, for example Waite and Tanzer, 1981, Science, 212: 1038) and methods for the preparation and isolation of bioadhesive peptides have been developed (e.g. US Patent No. 4,687,740). However, the preparation and isolation of bioadhesive peptides can be both costly and inconvenient.

It is now found that such methods of preparation of bioadhesives are unnecessary and that in fact bioadhesives may be readily prepared from a wide range of existing, widely available molecules. The bioadhesives of the present invention may present enhanced properties when compared to those of the prior art, particularly with regard to their mechanical strength, heat resistance and water-resistance.

According to the present invention there is provided a simple, convenient, inexpensive method of manufacture of an adhesive comprising cross-linking first and second molecules each of which has at least one chemical group selected from the group of hydroxy aromatic, dihydroxy aromatic, hydroxy phenone and amino, the first molecule being linked to the second molecule via at least one quinone.

At least one of the molecules may comprise at least one amino acid or a derivative thereof having either an amino, hydroxy aromatic, dihydroxy aromatic or hydroxy phenone group. The amino acid or derivative thereof may be selected from any one ofthe group of tyrosine, 3,4-dihydroxy phenylalanine, lysine, asparagine, proline, hydroxyproline, glutamine and arginine.

At least one of the molecules may comprise either gelatin or chitosan. A molecule may for example comprise a peptide or a protein, for example a gelatin such as fish gelatin. Alternative molecules which may be used include myosin, actin, casein and soya The first and second molecules may be soluble in water. Either one or both of the first and second molecules may be solubilised in water. The molecule may be solubilised in water by the addition of an organic acid. The organic acid may have either a hydroxy aromatic, dihydroxy aromatic, hydroxy phenone or an amino group. The organic acid may, for example, be acetic acid, 4-hydroxybenzoic acid, 1,4-hydroxy phenylpyruvic acid or 1 ,4-amino benzoic acid.

It is found that, surprisingly, molecules, in particular molecules containing amino acids, may be readily linked together to form adhesives (see 'Experimental' below). The significance of this invention is further increased by the fact that the molecules to be linked may be water-soluble (although, of course, other solvents could be used) and that the linking reaction may occur in aqueous solution. Existing molecules such as gelatin and chitosan, or any other selected molecule, but particularly molecules having a linear structure, may be linked. This use of readily available, inexpensive, molecules means that the method of the present invention may be used to produce inexpensive products. If the molecules are insoluble in water then they may be solubilised, for example by reaction with an organic acid. The linking reaction or reactions may then cause the molecules to return to their original hydrophobic state, producing a water-resistant adhesive. Indeed, water may even be observed to form as a film on the surface ofthe mixture of reacting molecules as they are linked. This use of aqueous solution also allows the linking to act to adhere surfaces which are water- absorbent (see below) and allows the creation of strongly-bound materials.

When soluble molecules are used, for example chitosan, it is preferable to obtain a solution with as high a concentration of chitosan as possible with the appropriate modification (resulting from solubilisation). Solutions may have a concentration of 5-15 % chitosan, although up to 20% may be readily generated. However, the high concentrations (e.g. 20%) lead to high viscosity. Chitosan solutions of 1-5% w/v also display some adhesive properties.

Gelatin based adhesives may be prepared as a powdered product. Hot water may be added to a measured aliquot and stirred immediately prior to making a bond. A two part system involving (a) a gelatin solution and a benzoquinone solution or (b) a solution of gelatin with catechol and a tyrosinase solution may be prepared and mixed manually or mechanically prior to bond formation.

Gelatins may be hydrolysed, for example chemically or enzymatically, in order to solubilise them. Non-hydrolysed gelatins may of course be used, although they require heating in order to increase their solubility, and so any substrate to which they will be applied may also be heated in order to prevent the unwanted solidification ofthe gelatin at ambient temperatures

Gelatin solutions for use in the present invention may additionally comprise biocides (e.g. sporicides or bactericides) in order to facilitate their storage.

Due to the fact that the molecules to be bound may have one or more of a range of groups, various reactions may occur in order to form the linkage or linkages between the molecules. Additionally, the molecules ofthe present invention have been found to be useful as binding agents for e.g. metals and particulate materials such as sand, clay and MgO (see Experimental below). For example cores for use in the foundry industry may be made by binding together particulate matter such as green sand (ordinary untreated sand), black sand and recycled core sand.

The first and second molecules may both have an amino group, the linkage being made by reacting a quinone with the amino groups of the first and second molecules.

The quinone may be benzoquinone or a molecule having a benzoquinone group. The benzoquinone may be selected from any one of the group of 1,2- benzoquinone, 1,3-benzoquinone and 1,4-benzoquinone. The benzoquinone may be synthesised by the oxidation of dihydroxybenzene, hydroxybenzene, hydroxy phenone or a molecule containing a group thereof.

Alternatively or additionally, the first molecule may have a first group comprising a hydroxy aromatic, dihydroxy aromatic or hydroxy phenone group and the second molecule may have an amino group, the linkage being formed between the first group and the amino group by the oxidation of the first group and the reaction of the resultant group with the amino group. The first group may be oxidised into a quinone, the resultant quinone being reacted with the amino group.

Alternatively or additionally, the first molecule may have a first group comprising a hydroxy aromatic, dihydroxy aromatic or hydroxy phenone group and the second molecule may have a second group comprising a hydroxy aromatic, dihydroxy aromatic or hydroxy phenone group, the linkage being formed between the first and second groups by the oxidation of the first and second groups and the reaction of the resultant groups. The first and second groups may be oxidised into quinones.

Hence various reactions and combinations of reactions may occur between the molecules to form the linkage or linkages, all ofthe linkages being via a quinone.

The aforementioned oxidation may be catalysed by an enzyme. The enzyme may for example be selected from any one ofthe group of tyrosinase, phenolase, potato phenol oxidase and laccase.

Naturally occurring carbohydrate polymers which are substituted with hydroxy aromatic residues may also be cross-linked. For example, ferulic acid (3- hydroxy-4-methoxy benzoic acid) substituted carbohydrates can be extracted from crops such as corn husks and maize. Alternatively, synthetic ferulic acid substituted polymers may be synthesised, for example by esterification of organic molecules or by other processes which retain the phenolic structure.

The enzyme tyrosinase hydroxylates monophenols such as hydroxybenzoic acid and oxidises the resultant diphenols. Diphenols such as catechol can also be transformed into the quinone derivative. The quinone binds with available amino groups on polymers such as proteins such as gelatin or an amino polysaccharide such as chitosan or an amino derivative of chitosan. Alternatively, chitosan may be derivatised in order to provide substrate groups suitable for oxidation by tyrosinase catalytic activity. The oxidation of molecules having a group comprising a hydroxy aromatic, dihydroxy aromatic or hydroxy phenone group may be the oxidation by tyrosinase of 1,2- dihydroxybenzene, 2-hydroxy phenone or hydroxybenzene or a molecule containing a group thereof.

Hence the rate of oxidation of groups may be conveniently controlled by an enzyme, allowing the rate of linkage to be readily controlled.

A water proofing agent may be added to the adhesive. Water proofing agents include vegetable pitch, Mystolene (Ciba Geigy, MK9) and Persistol (BASF). Other water proofing agents may of course be used, particularly those which are water soluble.

Also provided according to the present invention is adhesive comprising cross-linked molecules according to the present invention. Such a molecule may be an adhesive.

The adhesives of the present invention may adhere to water-absorbent substrates. They may adhere to a substrate having a hydroxy aromatic, dihydroxy aromatic, hydroxy phenone or amino group.

For example, the substrate may be wood, leather, cotton, paper, carpet or a textile. Also provided according to the present invention is the use of first and second molecules each of which has at least one group selected from the group of hydroxy aromatic, dihydroxy aromatic, hydroxy phenone and amino in a method of manufacture of an adhesive, the first molecule being linked to the second molecule via at least one quinone.

Also provided according to the present invention is a method of manufacturing thermostably bound solids comprising mixing a solution of one of the group of gelatin, casein and soya with the solid to be bound and heating the mixture such that substantially all ofthe solvent is removed from the mixture.

Also provided according to the present invention is the use of gelatin, soya and casein in a method of manufacture of thermostably bound solids, the solids being mixed with a solution of either gelatin, soy or casein and the mixture being heated such that substantially all ofthe solvent is removed.

Gelatin, soya and casein according to the present invention are particularly useful in the foundry industry (see Experimental below) for the production of thermostable cores. Cores may be made for example using particulate matter such as sand, clay and MgO, gelatin and inorganic silicates, borates, phosphates, fluorides which can be cured by carbon dioxide gas or air.

The presence of gelatin and subsequent baking has been found to overcome the problem of cores becoming friable (i.e. crumbly), which is encountered when high ratios of silicate are used in order to speed up the reaction. Decoring (the removal from the core of the moulded article) and demoulding (the removal of pattern pieces from the set core) from casting in the presence of gelatin is simpler and easier than when using conventional resin binding agents and produces less finning characteristics in the castings.

The sand ofthe core can be recycled by abrasion alone, washing not being required as with high silicate systems and decoring is substantially improved.

Where sand blocks have been heated to above 800 °C, the sand block may be easily crumbled and on cooling the sand re-used to make another core.

Water proofing agents such as ZnO, phosphates and perborates may be added to the mixture of gelatin/casein/soya and particulate matter.

Other uses of the adhesives and methods ofthe present invention include continuous casting, for example of steel; the manufacturing of insulating sleeves; the adhesion of woods to e.g. concrete; the adhesion of glass fibres to form insulating material; and the adhesion of bricks.

Wood may be adhered to concrete for application as a gripper rod to concrete flooring adhesive. Water proofing agents can be added.

Sand and gelatin (or soya or casein) formulations (see Example 9 below - core bonding) can be used to adhere materials derived from glass fibre called 'Shotts' as insulation material. Solid blocks of significant strength can be prepared in 3 minutes at 230 °C. Existing technology involves baking for 2 hours at 200 °C, and so a significant saving in energy can be made.

Adhesives and compounds (gelatin, soya and casein) (see bond shell sand - Example 11 below) according to the present invention can be used to adhere bricks, for example "Specials" - bricks with unusual shapes etc. Pigments may be added to the adhesives and compositions ofthe present invention in order to colour them to match the brick colour.

The invention will be further apparent from the following description, with reference to the several figures ofthe accompanying drawings, which show, by way of example only, forms of linkage of molecules.

Ofthe figures:

Figure 1 shows the solubilisation of chitosan by acetic acid and 4- hydroxybenzoic acid, preparing it for subsequent linkage reactions.

Figure 2 shows the solubilisation of chitosan by acetic acid and 4- hydroxy phenylpyruvic acid, preparing it for subsequent linkage reactions. Experimental

Example 1

Preparation of chitosan solution

The drawings illustrate steps in the method of preparing bioadhesives with good strength characteristics using quinones, enzyme generated quinones and amino group-containing natural polymers. In the illustrated reactions, the acetate and the aromatic acid are used to solubilise the chitosan, which is not per se soluble in water. The concentrations ofthe acids are chosen to ensure that both acids react with the amino groups ofthe chitosan.

Preparation of a 1% w/v chitosan solution

A 1% w/v solution of chitosan is made by dissolving 1 g of chitosan in 1% v/v acetic acid. Any chitosan which is not modified will not be solubilised. A 1% w/v chitosan solution was also prepared with a mixture of 0.5% w/v amino benzoic acid and 0.5% v/v acetic acid. A 1% chitosan solution was also prepared with 1% w/v amino benzoic acid. However, approximately 10% ofthe chitosan did not go into solution, requiring the acid concentration to be increased slightly in order to solubilise the chitosan.

Example 2

Tyrosine glucan is mixed with the chitosan solution of Example 1 to give a solution of

1% (w/v) chitosan and 10% (w/v) tyrosine glucan .

Example 3

Hydroxy benzoic acid is mixed with the chitosan solution ofExample 1 to give a solution of 1% (w/v) chitosan and 10% (w/v) hydroxy benzoic acid. Example 4

A mixture of amino benzoic chitosan and hydroxy benzoic chitosan is mixed with the chitosan solution ofExample 1 to give a solution of 1% (w/v) chitosan, 5% (w/v) amino benzoic chitosan and hydroxy benzoic chitosan.

Example 5

Varying quantities of between 200 and 5000 mg of benzoquinone are added to 100 g of the solutions of Examples 2, 3 and 4 and the adhesive allowed to form.

Example 6

Varying quantities of between 200 and 5000 mg of catechol (1,2-dihydroxybenzene) together with between 3 and 5 mg of mushroom tyrosinase are added to 100 g of the solutions of Examples 2, 3 and 4 and the adhesive allowed to form.

Example 7

3-5 mg of mushroom tyrosinase is added to 100 g ofthe solutions of Examples 2 and 4 and the adhesive allowed to form.

Example 8

A 25% w/v - 45% w/v gelatin solution is prepared at 60 °C and either 200-5000 mg benzoquinone added to 100 g of the gelatin solution, or 200-5000 mg catechol with tyrosinase added to 100 g ofthe gelatin solution. In experiments, both acidic and alkaline porcine and bovine gelatin are used. When using powdered gelatin, it is dissolved in water by heating to above 40°C. When using preparations such as 45% w/v Fresh Water fish gelatin (Sigma Chemicals), they are liquid at room temperature and suitable for cross-linking. All adhesives produced exhibited good adhesive properties beyond that associated with gelatin alone.

The bonding ability ofthe adhesives (of Examples 5, 6, 7 and 8) is tested by using it to adhere wood strips and determining the shear strength ofthe bound strips under varying loads, at varying temperatures and after immersion in water for various periods of time and after various drying periods. Bonding ability was also tested with other materials such as metals.

Best shear strength results for Example 5 with beech wood substrates have been obtained with a benzoquinone level of 1000-2000 mg.

Adhesive materials ofthe above Examples adhere beech wood strips with shear strengths up to 3620 N and in general the wood breaks first in about 50% ofthe bonds. Many other absorbent substrates such as card, paper, cotton and leather are also bonded strongly, as are metals.

The adhesive preparations exhibit water resistance, and are resistant to organic solvents and high temperatures. Films prepared from gelatin based bioadhesive preparations are also resistant to permeation by organic solvents.

Table 1 shows the shear strength of gelatin/quinone bonds used to adhere beech wood strips (1.25 cm overlap) at indicated weeks of storage (+ denotes wood failure). Table 2 shows the shear strengths of gelatin/quinone bonds on wood strips ( 1.25 cm overlap) at different temperatures (+ denotes wood failure).

By comparison (to the data in Tables 1 and 2), a typical resin based wood adhesive, although having a greater initial strength, lost over 40 % of its strength at 40°C due to adhesive failure (rather than wood failure), and 95% ofthe adhesive shear strength was lost at 100°C.

On total immersion in water, there were clear reductions in uptake in many cases, though no gelatin/quinone bioadhesive sample failed to absorb at least some water. Best results to date have been obtained with a gelatin/benzoquinone preparation where the adhesive sample absorbed only 26% of its weight of water after 1 hour immersion, and 123% after 24 hours. The proteinaceous material alone took up 2'/₂ times its own weight of water in a similar time scale.

After four days immersion a typical protein/quinone bond between beech wood strips remains intact although there is measurable loss of strength. However, when dried out, the bond strength returns almost to its original value. By comparison many resin based commercial adhesives fall apart after four days immersion with no opportunity for recovery on drying.

Table 3 shows amino polysaccharide chitosan and its derivatives both alone and mixed with protein used as adhesives, and having improved strength on cross-linking with quinones or enzyme generated quinones. Foundry Industry

In the foundry industry, liquid phase substances such as metals and alloys are cast into cores, the core determining the shape the substances will take when they solidify to form moulded articles. Cores comprise particulate materials such as sand and MgO moulded into a shape. This is achieved by the addition to the particulate material of a binding agent, the mixture of the particulate material and binding agent (and any other chosen additive) then being placed in a "box" (for example a hot box or cold box - see below), pattern pieces being placed in the mixture to determine the shape it will take when set, and the mixture then being set. The pattern pieces are then removed (the core is demoulded) to give a core which is used for moulding, moulded articles being replicas ofthe pattern pieces.

Example 9

Core Bonding-'Hot Box' Process

Solid cores were prepared comprising 97.5% (w/v) green sand (untreated ordinary sand) and 2.5% gelatin (from a 50% w/v gelatin solution). Sample mixtures were prepared using 200 g of sand and 5 ml of either 50% w/v liquid (at RTP) fish gelatin or hydrolysed (chemically or enzymatically hydrolysed) gelatin. The core was prepared by heating the sample mixture at 230 ^CC for 3 minutes. The heating time could be reduced by about 1 minute by adding benzoquinone (100-500 mg per 200 g of sand) before addition of the gelatin. The core could be formed more quickly or more slowly by varying the baking temperature, with the proviso that the temperature had to be at least about 100 °C. The addition of a water proofing agent (2.5% of mixture weight) e.g. vegetable pitch, Mystolene (Ciba Geigy, MK9) or Persistol (BASF - HP) allowed the set cores to be immersed in water without disruption. Non hydrolysed gelatin e.g. 300 bloom bovine gelatin was also used, the solution being heated to above its melting point before mixing with the sand. The sand was also preheated before addition ofthe gelatin in order to prevent its premature solidification.

Example 10

Hot Box/Cold Box

Cores were made using sodium silicate and curing (setting) with either CO₂, air or hot air at room temperature. Cores were made by adding to sand 0.5-3 % (w/v) of a 50% gelatin solution, the sand mixture already comprising 1-5 % (w/v) sodium silicate solution. At this level after gassing with 10 psi (about 29.3 kg.cm²) CO₂ for 30 seconds to 1 minute a core was formed which could be handled. Further strength was obtained by subsequent baking of the set core above 100 °C, for example for 2-3 minutes at

230 °C, or at 120 °C for 30 minutes. The cores were post-coated with refractory coatings and further baked, the refractory coatings aiding decoring and producing improved pattern pieces.

Example IQ

Quick cold set systems

Formulations of gelatin with quinone were admixed into sand using a continual mixer and the resulting mixture dumped onto or into a pattern wherein cross-linking produced bonding strength and consolidated the moulding.

Example 1 1 Core Adhesive Conventionally, sand is pre-coated in resin to become "Shell Sand". Cores form spontaneously on heating. The resulting sand blocks (moulds) are adhered using an oil based adhesive. Gelatin water based materials can also be used. Using 50% gelatin (liquid at room temperature) and clay as filler, a material is prepared containing as little as 20% water which is gunned or trowelled onto the mould surfaces, and on application to hot moulds (for example about 130 °C) the mould components stick together. The addition of benzoquinone speeds up the curing of the bond. However, the addition of benzoquinone reduces the shelf-life of the water based adhesive as the material can thicken in a relatively short time (perhaps 1 hour after being mixed) and therefore cannot always be applied successfully to the mould surface.

Example 12

Continuous Casting of Steel

Silicate and gelatin (with or without benzoquinone) are mixed with substantially inorganic materials such as refractory materials, and gunned onto a hot concrete surface, set to a one inch (2.5 cm) thick refractory interphase. Molten steel is cast against the interphase, which acts as an insulator. Fibrous cellulosic or glass materials are be used as additives. The surface ofthe film becomes glassy/vitreous and when the steel hardens, the mould is easily removed, for example spontaneously or mechanically.

Example 13

Manufacturing of insulating sleeves

When preparing moulds for casting, excess molten metal is added which is taken up into sleeve which insulates and keeps metal molten until all solidification and moulding is complete. The methods ofthe present invention can be used to produce both core formers and adhesives.

Table 1 :

Sample (weeks of storage) Strength (N) Wood Breakage (+/-) powdered formulation 21 °C

Gelatin 2 1463.16 -

Gelatin + Benzoquinone 16 2860.62 +

Gelatin + Hydroquinone 16 2687.7 +

Gelatin + Catechol + tyrosinase 16 2856.7 +

Gelatin - 25% w/v

Benzoquinone, hydroquinone and catechol - 1 % w/v

Tyrosinase - 0.005% w/v

Table 2:

Shear Strength (N)

Sample RT°C 40°C 80°C 100°C gelatin+benzoquinone 3000 + 2480 + 1713 + 1742 + gelatin+hydroquinone 3033 + 2340 + 2555 + 1436 + gelatin+catechol+tyrosinase 3067 + 2376 + 2126 + 1773 +

Gelatin - 25% w/v

Benzoquinone, hydroquinone and catechol - 1% w/v

Tyrosinase - 0.005% w/v Table 3

shear strength (N) chitosan/gelatin (4: 1) 5093 chitosan/ge latin (4: l)/catechol/tyrosinase 5459 chitosan/gelatin (1 :4) 2596 chitosan/gelatin (1 :4)/benzoquinone 4168 chitosan gelatin (1 :4)/catechol/tyrosinase 3835 chitosan 3015 chitosan/catechol/tyrosinase 3684

Gelatin - 25% w/v

Benzoquinone and catechol - 1% w/v

Tyrosinase - 0.005% w/v

Where ratios are given, they refer to proportion of 20% chitosan acetate to dry weight of gelatin. Where no ratio is given, chitosan concentration = 12% w/v

Tests were performed with a beech wood substrate.

Claims

1. A method of manufacture of an adhesive comprising cross-linking first and second molecules each of which has at least one group selected from the group of hydroxy aromatic, dihydroxy aromatic, hydroxy phenone and amino, the first molecule being linked to the second molecule via at least one quinone.

2. A method according to claim 1, at least one ofthe molecules comprising at least one amino acid or a derivative thereof having either an amino or hydroxy group.

3. A method according to claim 2, the amino acid or derivative thereof being selected from any one ofthe group of tyrosine, 3,4-dihydroxy phenylalanine, lysine, asparagine, proline, hydroxyproline, glutamine and arginine.

4. A method according to any one of the preceding claims, the first and second molecules being soluble in water.

5. A method according to claim 4, at least one ofthe first and second molecules being solubilised in water.

6. A method according to claim 5, the molecule being solubilised in water by the addition of an organic acid.

7. A method according to claim 6, the organic acid having either a hydroxy aromatic, dihydroxy aromatic, hydroxy phenone or an amino group.

8. A method according to claim 7, the organic acid being selected from any one ofthe group of acetic acid, 4-hydroxybenzoic acid, 1,4-hydroxy phenylpyruvic acid and 1 ,4-amino benzoic acid.

9. A method according to any one ofthe preceding claims, the first and second molecules both having an amino group, the linkage being made by reacting a quinone with the amino groups ofthe first and second molecules.

10. A method according to claim 9, the quinone being benzoquinone or a molecule having a benzoquinone group.

1 1. A method according to claim 10, the benzoquinone being selected from any one of the group of 1.2-benzoquinone, 1.3-benzoquinone and 1 ,4-benzoquinone.

12. A method according to either one of claims 10 or 1 1, the benzoquinone being synthesised by the oxidation of dihydroxybenzene, hydroxybenzene. hydroxy phenone or a molecule containing a group thereof.

13. A method according to any one of the preceding claims, the first molecule having a first group comprising a hydroxy aromatic, dihydroxy aromatic or hydroxy phenone group and the second molecule having an amino group, the linkage being formed between the first group and the amino group by the oxidation of the first group and the reaction ofthe resultant group with the amino group.

14. A method according to claim 13, the first group being oxidised into a quinone, the resultant quinone being reacted with the amino group.

15. A method according to any one ofthe preceding claims, the first molecule having a first group comprising a hydroxy aromatic, dihydroxy aromatic or hydroxy phenone group and the second molecule having a second group comprising a hydroxy aromatic, dihydroxy aromatic or hydroxy phenone group, the linkage being formed between the first and second groups by the oxidation ofthe first and second groups and the reaction ofthe resultant groups.

16. A method according to claim 11, the first and second groups being oxidised into quinones.

17. A method according to any one of claims 12, 14 and 16, the oxidation being catalysed by an enzyme.

18. A method according to claim 17, the enzyme being selected from any one of the group of tyrosinase, phenolase, potato phenol oxidase and laccase.

19. A method according to either claim 14 or 16, the oxidation being by tyrosinase and being of 1.2-dihydroxybenzene, 2-hydroxy phenone or hydroxy¬ benzene or a molecule containing a group thereof.

20. A method according to any one ofthe preceding claims, also comprising the addition of a water proofing agent.

21. An adhesive comprising cross-linked molecules according to any one of the preceding claims.

22. An adhesive according to claim 21 , wherein it adheres to a substrate having a hydroxy aromatic, dihydroxy aromatic, hydroxy phenone or amino group.

23. The use of first and second molecules each of which has at least one group selected from the group of hydroxy aromatic, dihydroxy aromatic, hydroxy phenone and amino in a method of manufacture of an adhesive, the first molecule being linked to the second molecule via at least one quinone.

24. A method of manufacturing thermostably bound solids comprising mixing a solution of one ofthe group of gelatin, casein and soya with the solid to be bound and heating the mixture such that substantially all ofthe solvent is removed from the mixture.

25. The use of gelatin, soya and casein in a method of manufacture of thermostably bound solids, the solids being mixed with a solution of either gelatin, soy or casein and the mixture being heated such that substantially all of the solvent is removed.