High temperature epoxy resins from hydrogenated quinoxalines

Details High temperature epoxy resins from hydrogenated quinoxalines High temperature epoxy resins from hydrogenated quinoxalines

1996-10-25

1998-01-20

Krass, Frederick

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

From heterocyclic reactant containing as ring atoms oxygen,...

528406, 540473, 540573, 544235, 544336, 544349, 544350, 544353, 544357, C07D40514, C08G 5932, C08G 5926

Patent

active

057102422

DESCRIPTION:

BRIEF SUMMARY
This is a 35 USC 371 of PCT/GB95/00326 filed Feb. 16, 1995.
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-glycidyl epoxy system, drawbacks of N,N-diglycidyl epoxy system, of which TGDDM is an example, is that they display reduced functionality in their reactions with amine hardeners because of intramolecular cyclisation reactions which compete with the desired cross-linking processes. This has the effect of reducing the glass transition temperature, T.sub.g, because it effectively preserves polymer mobility within the structure due to the reduction in the number of cross-links.
Whilst it is recognised that TGDDM resins have a dry T.sub.g value of around 260.degree. to 265.degree. C., in practice their use is limited to applications requiring a maximum service temperature of around 125.degree. C. This is because they have a tendency to pick up moisture from the atmosphere. Absorbed water has a plasticising effect on such resins, reducing the T.sub.g and hence limiting the maximum service temperature.
Attempts have been made to exploit commercially epoxy resins with a higher T.sub.g or reduced water affinity, thus providing a higher maximum service temperature, but none has shown an improvement over TGDDM in overall performance
It is therefore an object of the present invention to provide an epoxy resin system which overcomes some of the disadvantages displayed in prior art systems, with particular emphasis on eliminating intramolecular cyclisation and providing good resistance to oxidative aging.
It is a further object of the invention to provide a method of increasing the effective functionality of commercially-available materials and thereby increase their glass transition temperatures through enhanced cross-link density.
According to the invention there is provided a multi-functional epoxy resin derived from a precursor of formula (I): ##STR2## wherein R.sup.3 to R.sup.8 inclusive are independently selected from hydrogen, C.sub.1 to C.sub.3 alkyl or halo-alkyl, and wherein n=0, 1 or 2, and wherein R.sup.9 =hydrogen, alkyl or halogen. R.sup.9 position with hydrogen and self-coupled under acidic conditions in the presence of formaldehyde or benzaldehyde to give, after subsequent dehydrochlorination, a multi-functional compound of formula (II): ##STR3## and higher oligomers (III), wherein R.sup.3 to R.sup.8 inclusive are independently selected from hydrogen, C.sub.1 to C.sub.3 alkyl, halo-alkyl, or optionally-substituted aryl, and wherein n=0,1 or 2, and x=0 to 10 ##STR4##
In another aspect of the invention, the precursor (I) is substituted at the R.sup.9 position with alkyl or halogen and self-coupled in the presence of formaldehyde or benzaldehyde to give, after subsequent dehydrochlorination, a multi-functional compound of formula (IV): ##STR5## wherein R.sup.3 to R.sup.8 inclusive are independently selected from hydrogen, C.sub.1 to C.sub.3 alkyl, halo-alkyl, or optionally-substituted aryl, and wherein n=0, 1 or 2.
The multi-functional compound (IV) is a substituted version of compound (II) in which the formation of higher oligomers (III) is prevented by the substituents R.sup.9.
In the above-mentioned self-coupling reactions, when formaldehyde is used as the coupling reagent, R.sup.1 and R.sup.2 are both hydrogen. If benzaldehyde is used, R.sup.1 is C.sub.6 H.sub.5 and R.sup.2 is hydrogen.
Preferably, the heterocycles are six-membered rings and the substituents are all hydrogen, so that, in the above formulae, R.sup.1 to R.sup.8 inclusive are hydrogen and n=0.
In another preferred form of the invention, R.sup.1 is C.sub.6 H.sub.5, all other optional constituents being hydrogen.
Seven- and eight-membered nitrogen heterocycles may also be prepared,

REFERENCES:
patent: 2951822 (1960-02-01), Reinking
patent: 3429833 (1969-02-01), Porret
patent: 3828066 (1974-08-01), Porret

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