This invention relates to thermosetting prepolymers prepared from methylated pyridines or pyrazines, aromatic dialdehydes, isopropenyl phenols, and with or without bismaleimides.

In the aerospace and/or aircraft industry there is a need for light weight fire resistant polymeric composites for interior and exterior use. A recent development in this critical area was the discovery that composites based on polystyrylpyridines are useful in this field of endeavor. The key patents are outlined below.

It is known from U.S. patent 3,994,862 that polystyrylpyridine thermosetting prepolymers and cured polymers can be obtained by reacting methylated pyridines and aromatic dialdehydes.

U.S. patent 4,163,740 discloses the preparation of solutions of polystyrylpyridines in various organic solvents such as ethyl acetate, propanol, and methyl- ethylketone.

U.S. patent 4,362,860 discloses related polystyrylpyridines terminated with vinyl pyridine.

This invention is directed to thermosetting prepolymers prepared by reacting (A) one or more aromatic dialdehydes, (B) one or more azine compounds having the formula

R R

I !

where Z is N, C-CH-, C-CH ₂ -CH_ or C-H R is hydrogen, methyl, or ethyl, whereby the total number of methyl groups substituted on the ring is in the range from 2-4, and (C) a member of the group consisting of (1) monomeric para or meta_ isopxopenyl phenols having the formula

wherein R-, is hydrogen or a group inert to the polymerization reaction such as hydroxy, halogen, nitro, or ainino groups, (2) dimers of said phenols, and

(3) mixtures thereof.

These thermosetting polymers are useful to make high temperature resistant composites with, for example, fiber glass or carbon fibers. The advantage of this invention is that the prepolymers made herein cure at lower temperatures and faster than the known uncapped polystyrylpyridine prepolymers. Additionally,

SUB8T_TUTL_ S.

the cured polymers obtained in accordance with the methods described in this invention posses superior ablative properties as seen by their very high char yields.

A preferred embodiment of the invention is to further react the prepolymer prepared above with a bismalei ide compound having the formula

where Z is a divalent organic radical having at least two carbon atoms and selected from the group consisting of aliphatic groups, alicyclic groups, aromatic groups, and groups having at least two aryl residues or cyclohexyl residues bonded together by methylene, sulfonyl, or oxygen linkages or combinations thereof and X is independently hydrogen, halogen, an alkyl group, of.1-4 carbons, or an aryl group of.6-8 carbons.

The prepolymers of this invention are prepared by reacting alkylated azine compounds such as methyl pyridines and methyl pyrazines with aromatic dialdehydes in the presence of an acidic catalyst. After formation of oligomers, monomeric or dimeric isopropenyl phenols are added to cap the oligomers.

The azine compounds and the aldehydes are heated to a temperature in the range from 130 to 230°C, preferably in the range from 170 to 190°C for a period of time from 0.5 to 6 hours and preferrably 1 to 2 hours. The reaction is conducted in the absence of oxygen and suitably with a nitrogen purge.

Useful catalysts that can be used include, for example, sulfuric acid, hydrochloric acid, zinc chloride, acetic anhydride, aluminum trichloride, toluene disulfonic acid, trichloro acetic acid, and acetic acid. The catalysts are used in amounts from 0.5 to 20 weight percent based on the total weight of the reactants and preferrably in amounts from 2 to 5 weight percent. It is to be understood that the reaction can proceed in the absence of catalyst but the reaction time is much longer.

Examples of useful pyridines are 2,3-dimethyl pyridine, 2,4-dimethyl pyridine, 2,5-dimethyl pyridine, 2,6-dimethyl pyridine, 3,4-dimethyl pyridine, 3,5-dimethyl pyridine, 3,5-dimethyl-2-ethyl pyridine, 2,3,4,6-te.tramethyl pyridine, 2,3,5-trimethyl pyridine, 2,3,6-trimethyl pyridine, 2,4,5-trimethyl pyridine and 2'4,6-triιiιe hyl pyridine.

Examples of useful mononuclear aromatic dialdehydes have the formula:

wherein X is independently hydrogen, chlorine, bromine, fluorine, or a monovalent hydrocarbon radical such as an alkyl group of 1-4 carbons, an aryl group of 6-8 carbons such as phenyl, tolyl, xylyl, an oxyalkyl group of 1-4 carbons or an oxyaryl group of 6-8 carbons.

Preferred compounds within the scope of the above formula are terephthalic aldehyde, phthalic aldehyde, and isophthalic aldehyde.

Examples of useful pyrazines are 2,5-dimethyl pyrazine, 2,3-dimethyl pyrazine, 2,5-dimethyl pyrazine, 2,3,5-trimethyl pyrazine and 2,3,5,6,-tetramethyl pyrazines.

Examples of useful bismaleimides are

N,N'-ethylenebismaleimide, N,N'-ethylenebis- (2-methylmaleimide), N,N'-trimethylene bismaleimide, N,N'-tetramethylene bismaleimide, N,N*-hexamethylene bismaleimide, N,N'-l,4-cyclohexylene bismaleimide, N,N'-m-phenylene bismaleimide, N,N'-p-phenylene bismaleimide, N,N'-2,4-tolylene bismaleimide, N,N'-2,6-tolylene bismaleimide, N,N'-(oxydi-p-phenylene)* bismaleimide, N,N'-(oxydi-p-phenylene)bis

(2-methylmaleimide) , N,N'-(methylenedi-p-phenylene)- bismaleimide, N,N'-(methylenedi-p-phenylene)- -bis(2-methylmaleimide), N,N'-(methylenedi- -p-phenylene)bis(2-phenylmaleimid ^* e), N,N'-(sulfonyldi-p-phenylene)bismaleimide, N,N'-(thiodi-p-phenylene)bismaleimide, N,N'-(dithiodi-p- phenylene)-bismaleimide, N,N'-(sulfonyldi-m-phenylene)- bismaleimide, N,N'-(o,p-isopropylidenediphenylene)- -bismaleimide, N,N'-(isopropylidenedi-p-phenylene)- bismaleimide, N,N'-(o,p-cyclohexylidenediphenylene)- bismaleimide, N,N'-(m-xylene)bismaleimide, N,N'-(p-xylylene)bismaleimide, N,N'-(4,4-p-triphenylen )- bismaleimide, N,N'-(p-phenylenedioxydi-p-phenylene)- bismaleimide, N,N'-(methylenedi-p-phenylene)bis- (2,3-dichloromaleimide), and N,N'-(oxydi-p-phenylene)- bis(2-chloromaleimide) .

The bismaleimides can be used in a weight ratio range of from one weight of maleimide to 4 of prepolymer and 4 weight units of maleimide to one of prepolymer.

The use of the bismaleimides in this invention gives prepolymers that cure at lower temperatures and at a faster rate.

It is to be understood that the foregoing reactants can be used alone or in combination as in an initial mixture of each or by the sequential addition during the reaction to achieve beneficial results.

The molar ratio range of the aldehyde to the azine compounds can be from 0.2:1 to 4:1 and preferrably in the range of 0.66:1 to 1.5:1.

The molar ratio range of the isopropenyl phenol or dimers thereof to the azine compounds can be from 0.05:1 to 4:1 and preferably in the range of 0.2:1 to 1:1.

The prepolymer (oligomer or resin) initially obtained is cured by press molding at a temperature range of 150 to 300°C for a time of 1 to 8 hours. The resultant semicured polymer is further cured at a temperature range of 250 to 300°C for a time of 2 to 10 hours to obtain the final fire resistant molding.

Composites are made by adding heat resistant fibers to the prepolymer before the press molding. Examples of useful heat resistant fibers to be used herein are graphite fibers, fiber glass, aramid fibers, and asbestos fibers.

The following examples are presented to further illustrate but not limit the invention.

Example 1 :

174.6 grams (1.3 moles) of terephthalaldehyde and 163.5 (1.35 moles) of 2,4,6-trimethylpyridine were placed in a 500-cc glass resin kettle equipped with a mechanical stirrer, modified Dean-Start apparatus for collection of water, infrared heating lamps, thermometer, and nitrogen purge system. The resin kettle was purged with nitrogen for 5 minutes and the infrared lamps turned on. At a 90°C pot temperature the terephthal- aldehyde began to melt. The mechanical stirrer was started and a 5 to 15-cc/min K , purge maintained. At a 110°C pot temperature 5 cc of concentrated sulfuric acid was added. The reaction mixture was heated to 160°C for V _* _ hours, collecting H ₂ 0 overhead. 43.0 grams (0.35 moles) of p-isopropenyl phenol was added and the reaction mixture maintained between 150 and 160°C for H_ hours. The reaction mixture was then cooled to room temperature, washed* with acetone and methanol, filtered, and dried in a vacuum oven at 90°C and 30 inches of Hgr vacuum (about 1600 Pa) for 45 minutes.

Prepolymer properties: Melting Point - 130-170°C

IR-hydroxy function - 3100-3600 cm ^"

Isopropenyl function - 885-900 cm ^" GPC - molecular weight - 900 Mn„

12.5 grams of the prepolymer was compression molded at 204-220°C and 500 psi (3447 MPa) for 1 hour. The cured block sample obtained after pressing was post cured in an atmospheric oven at 240°C for four hours. Properties of the cured material are as follows:

3

Glass Transition Temperature (T ) = 320°C by DMA Thermal Stability by Thermal Gravimetric Analysis (TGA)

5% wt loss in nitrogen and air at 500°C 75% char yield in nitrogen @ 950°C

Example II

7.0 grams of _sr,N*-methylenedi-p-phenylenebis- -maleimide was mixed with 7.0 grams of the prepolymer prepared as described in Example I. The prepolymer mixture was compression molded in a 2" x 2" (51x51 mml) plate at 180°C and 500 psi (3447 MPa) for 1 hour. The cured copolymer block sample was post cured in an atmospheric oven at 240°C for 4 hours. Properties of the cured material are as follows:

Tg by DMA = 315°C Thermal stability (TGA) = 5% wt loss in N = 460°C

5% wt loss in air = 450°C 63% char yield in ₂ @ 950°C.

Example III

Isopropenyl capped polystyrylpyridine powder from Example I and N,N ^T -methylenedi-p-phenylenebis maleimide powder were blended together in a ratio of 1/1 by weight. 4.4 grams of this material was alternated with 3 7/8" squares of glass cloth in a mold which had a cavity of 4"x4"xl/8 ^u (102x102x3.2 mm). The glass cloth weighed between 2.92 and 2.96 grams per piece and seven pieces were used before the top plate of the mold was positioned in place. The mold was placed in a heated press and the resin was cured for 2 hours at 900 psi (6205 MPa) and 180°C. A rigid, dark brown composite was obtained which did not support combustion when exposed to an open flame.

Example IV

153.5 grams (1.15 mole) of terephthalaldehyde and 103 grams (0.75 mole) of 2,3,5,6-tetramethylpyrazine were placed in a 500 cc glass resin kettle equipped with a mechanical stirrer, modified Dean-Stark apparatus for collection of water, infrared heating lamps, thermometer, and nitrogen purge system. The infrared lamps were turned on and a 5-10 cc/min nitrogen purge was initiated. At a 105°C pot temperature the reactants had melted and the mechanical stirrer was started.

Approximately 5 cc of concentrated sulfuric acid was added and the reaction mixture was heated to 160-170°C for 1 hour, collecting water overhead.* The reaction mixture was then cooled to between 150° and 158°C and - 48.2 grams of para-isopropenyl phenol was added dropwise as a 50/50 by weight solution in acetone. The reaction mixture was maintained between 150° and 158°C for 45 minutes before cooling to ^* room temperature. The prepolymer was purified by dissolving in a 50/50 THF/acetone mixture and precipitating- in water. The precipitated prepolymer was then filtered and dried in a vacuum oven.

Prepolymer Properties: melting point = 175-185°C

IR absorbtions = 3100-3000 cm Hydroxy 885-900 cm Isopropenyl

GPC wt. average ol. wt. = 4800

51.5 grams of the prepolymer was compression molded at a temperature between 210° and 230°C and 500 psi (3447 MPa) for 1 hour in a 4"x4"xl/8" (101x101x3.1"mm) plate. The sample was then post cured at 230°C for 12 hours in an atmospheric oven. Properties of the cured material are as follows:

Glass transition temp. (Tg) = >380°C by DSC Thermal stability (TGA) = 67% char yield in nitrogen 0°C.