The present invention relates to epoxy resins and in particular to epoxy resins for use in fibre-reinforced composite materials suitable for aerospace applications where high temperatures may be encountered, for example due to aerodynamic heating and proximity to engines and exhaust gases.

Currently, the most widely-used high performance carbon fibre composite materials are based on the tetra-functional N-diglycidyl epoxy system, όw[N,N-όw(2,3- epoxypropyl)-4-aminophenyl]rnethane (TGDDM). One of the drawbacks of N,N- diglycidyl epoxy systems, of which TGDDM is an example, is that they display reduced functionality in their reactions with diamine hardeners because of intramolecular cychsation reactions which compete with the desired cross-linking processes. This has the effect of reducing the glass transition temperature, T _g , because it effectively preserves polymer mobility within the structure due to the reduction in the number of crosslinks.

Whilst it is recognised that TGDDM resins have a dry T _g value of around 260 to 265°C, in practice their use is limited to applications requiring a maximum service temperature of around 125°C. This is because they have a tendency to absorb moisture from the atmosphere. Absorbed water has a plasticising effect on such resins, reducing the T _g , and hence limiting the maximum service temperature.

Attempts have been made to exploit commercially epoxy resins with a higher T _g or reduced water affinity, thus providing a higher maximum service temperature, but none has shown an improvement over N,N-diglycidyl epoxy systems in overall performance.

There is thus a general desire to provide a means to increase the effective functionality of commercially-available materials without detriment to other properties in order to

increase T _g through enhanced cross-link density and thereby produce epoxy resin systems with enhanced maximum service temperatures.

The problem has been tackled in United Kingdom Patent Application 9403563 by modifying the chemistry of tetrafunctional N,N-diglycidyl epoxy systems to produce an epoxy in which the glycidyl groups exhibit a reduced tendency to undergo cyclisation reactions during cure.

It is an object of the present invention to provide an epoxy resin system which overcomes some of the disadvantages displayed in prior art systems, and a particular object to provide an epoxy resin system which exhibits increased functionality so as to increase T _g through enhanced cross-link density and thereby increase service temperatures.

According to the invention there is provided a multi-functional epoxy resin precursor having the formula (I):

CH ₂ ClCH(OH)CH ₂ (I)

wherein R] to R ₃ inclusive are independently selected from hydrogen, C, to C ₃ alkyl or haloalkyl, or halogen. In preferred embodiments, the precursor (I) may be substituted in at least one of the R,, R ₂ , and R ₃ positions with a halogen, or alternatively substituted in each ofthe R _b R ₂ , and R ₃ positions with hydrogen.

In a further aspect the invention also provides a multi-functional epoxy resin derived from a precursor having the formula (I).

The precursor is derived from 3,4'-diaminodiphenyl ether, DDE, which has been reacted with epichlorohydrin, ECH, to produce the tetrachlorohydrin ofthe precursor of formula (I). This may be dehydrochlorinated to form a tetrafunctional epoxy resin having general formula (II):

wherein R, to R ₃ inclusive are independently selected from hydrogen, Ci to C ₃ alkyl or haloalkyl, or halogen. The three positions labelled R,, R ₂ , R ₃ can be used to incorporate other functional groups such as halogens or methyl groups that can provide beneficial properties. For example, it is documented that incoφoration of halogens into an epoxy can reduce water absoφtion. Unsubstituted (with R,=R ₂ -=R ₃ =H) and dehydrochlorinated the tetrafunctional compound is similar to TGDDM, differing by the ether linkage instead of a -CH ₂ - linkage and the meta N- diglycidyl group instead ofthe para.

However, the precursor (I) contains an active site para to the meta nitrogen which enables it to be reacted further before dehydrochlorination. In a preferred aspect ofthe invention this site is utilised to produce an epoxy resin derived from the precursor (I), which resin comprises the dehydrochlorination product of a coupled product

comprising a precursor (I) as claimed in any one of claims 1 to 3 coupled with a second N-diglycidyl epoxy precursor through its active site para to the meta nitrogen to produce a molecule having functionality greater than four. The invention thus enables production of epoxy resins with an increased number of N-diglycidyl groups compared with TGDDM and thus, although cychsation reactions are not inhibited, resins with overall higher functionality than TGDDM can be obtained with the attendant potential advantages of raised glass transition temperature and hence operating temperature through enhanced cross-link density. Substitutents in positions ortho to the nitrogens would sterically hinder the achievement of the planar conformation of the C — N bond which is necessary to activate the coupling reactions. Substitution at these positions is therefore ruled out.

The precursor (I) may be coupled, typically under acidic conditions in the presence of formaldehyde, with a second precursor of similar general formula (I) through the active site. This enables production of an epoxy resin comprising an 8-functional compound of formula (III), which is the dehydrochlorination product of a coupled product comprising a first precursor of general formula (I) coupled with a second precursor of general formula (I) through the active site:

in which R| to R^ inclusive are independently selected from hydrogen, C* to C ₃ alkyl or haloalkyl, or halogen.

The second compound of general formula (I) may be identically substituted to the first, so that in the resultant self-coupled product (III) Rι=R,, R ₂ =R ₅ and R^R*. In the alternative, the second precursor of general formula (I) can be differently substituted to the first. In such cases it should be appreciated that the products of the coupling reaction will include, as well as the cross-coupled product of the two precursors, products ofthe self-coupling of each precursor.

Further coupling reactions at other sites may also produce higher oligomers having functionality in excess of eight.

In a preferred example of compound (III), the epoxy resin is substituted at one or both of the R ₂ and R ₅ positions with a halogen which is preferably chlorine. The remaining positions are preferably substituted with hydrogen.

Altematively, a precursor of general formula (I) may be coupled with an N-diglycidyl epoxy precursor of different structure. This second precursor may conveniently be selected to have functionality of two so as to produce a 6-functional compound as a coupled product, and may for example be selected to be an aniline derivative which so as to produce, after coupling to a precursor of general formula (I), typically under acidic conditions in the presence of formaldehyde, to produce, and subsequent dehydrochlorination of the resultant coupled product, a 6-functional product of general formula (IV) below, in which R, to R ₅ inclusive are independently selected from hydrogen, C, to C ₃ alkyl or haloalkyl, or halogen. In a preferred example of compound (IV), the epoxy resin is substituted the R ₅ position with a halogen with

chlorine being particularly preferred. The remaining positions are preferably substituted with hydrogen.

Embodiments ofthe invention will now be described by way of example only.

Example l . DDEUC (II, Rs = H)

87.30g (0.8mol) of 3-aminophenol, 82.92g (O.όmol) of potassium carbonate, 750ml of N,N-dimethylacetamide and 400ml of toluene were reacted under nitrogen at 140°C, removing the water by-product as an azeotrope with the toluene. When complete (after approximately 5 hours), 112.88g (0.8mol) of 4-fluoronitrobenzene was added, the temperature increased to 150°C and the reaction monitored by reverse phase HPLC until complete (approx. 4 hours). After cooling, the solution was filtered and the solvents removed on a rotary evaporator. The product was then dissolved in 400ml of methanol and precipitated by the dropwise addition of water. The fine yellow precipitate was then filtered and dried in a vacuum oven at 40°C. The product, 3-amino-4'-nitrodiphenyl ether, was then hydrogenated with 60psi H ₂ at

60°C using a palladium oxide catalyst with ethanol as the solvent to produce 3,4'- diaminodiphenyl ether, DDE.

64.08g (0.32mol) of DDE, 150ml of benzene and 19.2g (0.32mol) of acetic acid were mixed and heated to 60°C under nitrogen. 236.88g (2.56mol) of epichlorohydrin was added dropwise over 6 hours monitoring the exothermic reaction by HPLC until complete (after approximately 14 hours). The excess solvents and reactants were removed by rotary evaporation, isolating the viscous amber tetrachlorohydrin of DDE.

This was dissolved in 300ml of butanone and reacted with 61.44g (1.54 mol) of NaOH for 3 hours at 60°C under nitrogen. After cooling the solution was filtered and the solvent removed from the filtrate leaving a pale amber coloured resinous product.

Example 2: DDESC (III, Rs = H)

3,4'-diaminophenyl ether, DDE, was first produced in like manner to example 1 by reacting 3-aminophenol with 4-fluoronitrobenzene to form 3-amino-4'-nitrodiphenyl ether which was then hydrogenated. The DDE was reacted with epichlorohydrin as in the previous example and the viscous amber tetrachlorohydrin of DDE isolated from the resultant solution.

23g of HCl, 92mls of H ₂ O and 80ml of 1,4-dioxan were added and heated to 45°C under nitrogen. When dissolved, 15.58g (0.19mol) of a 37% solution of CH ₂ O was added and the reaction monitored by reverse phase HPLC until complete (approx. 1 hour). After cooling, the solution was neutralised with 10% aqueous NaOH, upon which it separated into an organic and an aqueous layer. The aqueous layer was decanted off and the organic layer dried thoroughly using a rotary evaporator.

Finally, this solid organic product was dissolved in 300ml of butanone and reacted with 61.44g (1.54mol) of NaOH for 3 hours at 60°C under nitrogen. After cooling the solution was filtered and the solvent removed from the filtrate leaving an amber coloured resinous product.

Examplg ; Cl ₂ -J3DES£ (III, R ₂ , ₅ = Cl, other Rs=H)

87.30g (0.8mol) of 3-aminophenol, 82.92g (O.όmol) of potassium carbonate, 750ml of N,N-dimethylacetamide and 400ml of toluene were reacted under nitrogen at 140°C, removing the water by-product as an azeotrope with the toluene. When complete (after approximately 5 hours), 140.44g (0.8mol) of 3-chloro-4- fluoronitrobenzene was added, the temperature increased to 150°C and the reaction monitored by reverse phase HPLC until complete (approx. 4 hours). After cooling, the solution was filtered and the solvents removed on a rotary evaporator. The product was then dissolved in 400ml of methanol and precipitated by the dropwise addition of water. The fine yellow precipitate was then filtered and dried in a vacuum oven at 40°C. The product, 3-amino-2'-chloro-4'-nitrodiphenyl ether, was then hydrogenated with 60psi H ₂ at 60°C using a palladium oxide catalyst with ethanol as the solvent to produce 2'-chloro-3,4'-diaminodiphenyl ether, DDE.

The synthesis then proceeded in the same manner as for DDESC (example II) above, except that 0.32 mol of the resultant 2'-chloro-3,4'-diaminodiphenyl ether is used instead of DDE. This produced a chlorinated equivalent product to the DDE self coupled resin.

Example 4: DDEXC (IV, R ₅ = Cl, other Rs=H))

63.79g (0.5mol) of 3-chloroaniline and 15.01g (0.25mol) of acetic acid were mixed and heated to 50°C under nitrogen. 185.06g (2.0mol) of epichlorohydrin was added dropwise over 6 hours monitoring the exothermic reaction by HPLC until complete

(approx. 14 hours). The excess solvents and reactants were removed by rotary evaporation, isolating the pale amber dichlorohydrin of 3-chloroaniline.

O.l lmol of the dichlorohydrin of 3-chloroaniline and O.l lmol of the tetrachlorohydrin of DDE, prepared as indicated previously, were mixed with 24g of HCl, 63ml of H ₂ O and 40ml of 1,4-dioxan and heated to 45°C under nitrogen. When dissolved, 13.39g (0.165mol) of a 37% solution of CH ₂ O was added and the reaction monitored by reverse phase HPLC until complete (approx. 4 hours). After cooling, the solution was neutralised with 10% aqueous NaOH, upon which it separated into an organic and an aqueous layer. The aqueous layer was decanted off and the organic layer dried thoroughly using a rotary evaporator.

Finally, this solid organic product was dissolved in 250ml of butanone and reacted with 26.4g (0.66mol) of NaOH for 3 hours at 60°C under nitrogen. After cooling the solution was filtered and the solvent removed from the filtrate leaving an amber coloured resinous product.

In the coupling stage of the above synthesis of DDEXC the dichlorohydrin of 3- chloroaniline is coupled with the tetrachlorohydrin of DDE. Due to the differing reactivities the tetrachlorohydrin of DDE has a tendency to self couple at the reactive site forming DDESC (as example 2), and then the dichlorohydrin of 3-chloroaniline couples at a site meta to the para-nitrogen (R ₂ , R ₃ , R ₅ , Rs on formula III) giving a cross-coupled product of greater than 8 functionality. The final product is thus a mixture comprising some of the above in addition to the respective self-coupled products ofthe two components and the desired cross coupled product of example 4.

In table 1 below properties are given for the exemplified resins, cured with 65% of the stoichiometric amount of the diamine hardener 4,4'-diaminodiphenylsulphone, DDS. For the puφoses of comparison the table also lists MY9512, a standard epoxy

resin formulation for materials having aerospace applications whose major component is TGDDM.

Sample τ _g (°o Water Abs Toughness Thermal Aging at 70°C Klc T _g (°C) after 83% rh (%) MN/m ^{(3 2)} 3 mths at 150°C

DDEUC 257 4.3

DDESC 317 5.7 0.41 313

C1 ₂ -DDESC 308 5.0 0.46 303

DDEXC 289 4.7 0.47 288

MY9512 268 4.5 0.46 266

Table 1 : A comparison ofthe properties ofthe example materials and MY9512