Dimensional, Crosslinkable Oligomers

The present invention relates to multidimensional oligomers that include a hub and a plurality of radiating arms, each arm terminating at the periphery in a crosslinking end cap moiety. Such compounds have relatively low molecular weight, but cure to high performance composites useful at high temperatures.

Epoxies dominate the composite industry today primarily because they are relatively low-cost and are easy to use. Epoxies, however, have low thermal stabilitites and tend to be brittle. There is a need for high performance, temperature-resistant composites made curing inexpensive, "commodity" starting materials that will be useful in conditions where epoxies cannot be used. The present invention describes oligomers that fulfill these requirements and present great promise for engineering composites, particularly for aerospace applications.

Composites possessing glass transition temperatures greatly in excess of their curing temperatures can be prepared from multidimensional oligomers formed by the condensation of "commodity" starting materials. The oligomers have the general formula: ##STR1## wherein w=an integer greater than 2 and not greater than the available number of substitutable hydrogens on the Ar group;

Ar=an aromatic moiety; ##STR2## R=an organic radical having a valence of four; R.sub.1 =any of lower alkyl, lower alkoxy, aryl, phenyl, or substituted aryl (including hydroxyl or halo-substituents);

j=0, 1, or 2;

E=allyl or methallyl;

G=--CH.sub.2 --, --S--, --O--, or --SO.sub.2 --;

Q=an organic radical of valence two, and preferably a compound selected from the group consisting of: ##STR3## As will be explained, these oligomers are prepared by the condensation of an aromatic hub and a suitable end cap moeity with or without a chain-extending group (Q) to provide short-armed, multidimensional oligomers of high thermal stability.

Multidensional morphologies in crosslinking oligomers produce composites having solvent resistance, high glass transition temperatures, and toughness upon curing. The resins and prepregs are readily processed prior to curing. The cured composites have glass transition temperatures (melt temperatures) in excess of their curing temperatures. Such compounds can be readily made from "commodity" starting materials that are readily available at relatively low cost. The composites are cost competitive with epoxies, but possess better physical properties for aerospace applications (especially higher use temperatures).

Particularly preferred oligomers of the present invention have the general formula: ##STR4## wherein Ar=an aromatic radial; Y=a crosslinking end cap;

w=an integer greater than 2 and not greater than the available number of substitutable hydrogens on the Ar group;

P=--CONH--, --NHCO--, --O--, ##STR5## R=an organic radical having a valency of four, and, preferably, a residue of pyromellitic dianhydride, benzophenonetetracarboxylic dianhydride, or 5-(2,4-diketotetrahydrofuryl)-3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride

The crosslinking end cap (Y) is preferably a phenylimide having a formula: ##STR6## R.sub.1 =any of lower alkyl, lower alkoxy, aryl, or substituted aryl (including hydroxyl or halo- on any replaceable hydrogen);

j=0, 1, or 2; and

G=--CH.sub.2 --, --S--, --O--, or --SO.sub.2 --.

The most preferred end caps include: ##STR7## wherein n=1 of 2 (preferably 2);

j=0, 1, or 2 (preferably 1); G and R.sub.1 are as previously defined (with R.sub.1 preferably being ##STR8##

These multidimensional oligomers are made by the condensation of aromatic hub monomers with the end cap reactants in an inert atmosphere. For example, the hub might be ##STR9## and the end cap, a radical as illustrated above terminated with an acid halide to form an amide linkage (NHCO) between the hub and the end cap. Alternatively, the hub might include the acid halide and the end cap the amine so that the condensation will yield an amide of opposite orientation (CONH). Ester or ether multidimensional oligomers of this general type are made in accordance with Examples I through VII of our copending application U.S. Ser. No. 810,817 by reacting an acid halide and a phenol. Diimide linkages are formed by reacting an amine-terminated hub with a dianhydride and an amine-terminated end cap.

The hub (Ar) precursor preferably is selected from the group consisting of phenyl, naphthyl, biphenyl, azalinyl (including melamine radicals) amines or acid halides, or triazine drivatives described in U.S. Pat. No. 4,574,154 (incorporated by reference) to Okamoto of the general formula: ##STR10## wherein R.sub.2 is a divalent hydrocarbon residue containing 1-12 carbon atoms (and, preferably, ethylene).

Substantially stoechiometric amounts of the reactants are usually mixed together in a suitable solvent under an inert atmosphere to achieve the condensation. The reaction mixture may be heated, as necessary, to complete the reaction. Any of the oligomers can be used to form prepregs by application of the oligomers in a suitable solvent to suitable prepregging materials, and the prepregs can be cured in conventional vacuum bagging techniques at elevated temperatures to produce composites that have use temperatures in excess of their cure temperatures. The crosslinking end caps apparently bind the composites into a complex, 3-dimensional network upon curing by chemical induction or heating to yield a product having high thermal stability than the core temperature.

Compounds of the formulae: ##STR11## can also be synthesized with an Ullmann ether synthesis followed by a Friedel Craft reaction, as will be further explained. ##STR12## wherein q=--SO.sub.2 --, --CO--, --S--, or --(CF.sub.3).sub.2 C--, and preferably --SO.sub.2 -- or --CO--.

To form the ##STR13## compounds, preferably a halo- substituted hub is reacted with phenol in DMAC with a base (NaOH) over a Cu Ullmann catalyst to produce an ether "star" with active hydrogens para- to the either linkages. End caps terminated with acid halide functionalities can react with these active aryl groups in a Friedel Craft reaction to yield the desired product. For example, 1 mole of trichlorobenzene can be reacted with about 3 moles of phenol in the Ullmann ether reaction to yield an intermediate of the general formula: ##STR14## This intermediate can, then, be reacted with about 3 moles of (Y) -COCl to produce the final, crosslinkable, ether/carbonyl oligomer.

Similarly, to form the ##STR15## compounds, the hub is extended preferably by reacting a halo- substituted hub with phenol in the Ullmann ether synthesis to yield the ether intermediate of the ##STR16## compounds. This intermediate is mixed with the appropriate stoichiometric amounts of a diacid halide of the formula XOC--Q--COX and an end cap of the formula ##STR17## in the Friedel Craft reaction to yield the desired, chain-extended ether/carbonyl star and star-burst oligomers.

The end caps (Z) crosslink at different temperatures (i.e., their unsaturation is activated at different curing temperatures), so the cap should be selected to provide cured composites of the desired thermal stability. That is the backbone of the oligomer should be stable to at least the cure temperature of the caps. The multidimensional morphology allows the oligomers to be cured at a temperature far below the use temperature of the resulting composite, so completely aromatic backbones connected by heteroatoms are preferred to enhance the thermal stability.

U.S. Pat. No. 4,604,437 is incorporated by reference. That patent describes a polymer made from substituted, unsaturated, bicyclic imides having end caps of the formula: ##STR18## wherein E=allyl or methallyl, and n=1 or 2.

These bicyclic imide end caps are prepared from the analogous anhydride by condensation with an amine, and provide oligomers that cure in a temperature range between DONA (dimethyloxynadic) and nadic caps.

While essentially any dianhydride (aliphatic or aromatic can be used to form the diimide oligomers of the present invention, aromatic dianhydrides, such as pyromellitic dianhydride or benzophenonetetracarboxylic dianhydride, are preferred for cost, convenience, and thermal stability in the cured composite. If an aliphatic dianhydride is used, preferably the dianhydride is 5-(2,4-diketotetrahydrofuryl)-3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride (MCTC).

End caps of the formula ##STR19## are prepared by reacting an amine-substituted benzene, such as aniline, with an anhydride in the manner outlined in U.S. Pat. No. 4,604,437. One process for making the precursor anhydrides is described in U.S. Pat. No. 3,105,839.

While preferred embodiments have been shown and described, those of oridinary skill in the art will recognize variation, modifications, or alterations that might be made to the embodiments that are described without departing from the inventive concept. Accordingly, the description should be interpreted liberally, and the claims should not be limited to the described embodiments, unless such limitation is necessary to avoid the pertinent prior art.