CURED FIBROUS COMPOSITES PREPARED FROM THE RESINOUS REACTION PRODUCTS OF HYDROXY AROMATIC MONO-ALDEHYDES AND METHYLATED PYRIDINES OR METHYLATED PYRAZINES, AND PREPOLYMERS PREPARED FROM HYDROXY AROMATIC MONO-ALDEHYDES AND METHYLATED PYRAZINES

This invention relates to ther osetting polymers prepared from methylated pyridines or pyrazines and hydroxy aromatic mono-aldehydes.

In the aerospace and/or aircraft industry 5 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 10 below.

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

15 U.S. patent 4,163,740 discloses the preparation of solutions of polystyrlpyridines 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.

Bra sch, Chemische Berichte 42:1193-97 (1909) discloses the reaction, of methylated pyridines and salicylaldehyde to prepare monomeric hydroxy methyl stilbazoles.

Franke, Chemische Berichte 38:3724-28 (1905) discloses the reaction of methylated pyrazines with salicylaldehyde to prepare related monomeric compounds.

Related monomeric stilbazole compounds are also disclosed by Chiang et al., J. Org. Chem. 10:21-25 (1945). In each of these articles there is no disclosure of polymers.

A preprint paper by Yan et al., entitled "Styryl Pyridine Based Epoxy Resins", 183rd American Chemical Society National Meeting, Las Vegas, Nevada, March 25, 1982 (Volume 46, pages 482-488) teaches epoxy resins prepared from the reaction products of collidine, lutidine and picoline with hydroxybenzaldehyde.

This invention is directed to a cured fibrous laminate characterized as being prepared from the resinous reaction product of one or more hydroxy aromatic mono-aldehydes and one or more methylated pyridines or pyrazines having the structural formula

R R

1

- C

N 0 \C - - C

_

R R

where Z is N, C-CH ₃ , C-CH_-CH ₃ or C-H, R is hydrogen, methyl, or ethyl, whereby the total number of methyl groups on the ring is from 2-4.

This invention is also directed to a thermo¬ settable prepolymer characterized as the resinous reaction product of one or more hydroxy aromatic mono-aldehydes and one or more methylated pyrazines, the methylated pyrazines having the structural formula:

R R

I f

R R

wherein R is hydrogen, methyl or ethyl and the number ^• of methyl groups on the ring is from 2 to 4. '

Prepolymers are prepared by reacting alkylated pyridines or pyrazines such as methylated pyridines or methylated pyrazines with hydroxy aromatic mono-aldehydes in the presence of an acidic catalyst.

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

Useful catalysts include, for example, sulfuric acid, hydrochloric acid, ZnCl ₂ , acetic anhydride, A1C1 ₃ , toluene disulfonic acid, trichloro acetic acid, and acetic acid. The catalysts are used in amounts of from 0.5 to 20 weight percent based on ^" the total weight of the reactants, preferably 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-tetramethy pyridine, 2,3,5-trimethyl pyridine, 2,3,6-trimethyl pyridine, 2,4,5-trimethyl pyridine and 2,4,6-trimethyl pyridine.

Examples of useful hydroxy aromatic aldehydes are 2-hydroxy benzaldehyde, 3-hydroxy benzaldehyde and 4-hydroxy benzaldehyde.

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.

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 of the hydroxy aromatic aldehyde to the methylated pyridine or pyrazine ranges from 0.5:1 to 6:1, preferably from 1:1 to 3:1.

The thermosettable prepolymer (oligomer or resin) initially obtained is cured by press molding at a temperature from 180° to 300°C for a time of 1 to 8 hours. The resultant semicured polymer is further cured at a temperature from 250° to 300°C for a time of 2 to 10 hours to obtain the final thermoset fire resis- tant molding.

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

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

Example 1

In a 500-ml glass resin kettle, equipped with a stirrer, thermometer, nitrogen purge and reflux condenser, there was introduced 378.9 grams of

4-hydroxybenzaldehyde (3.11 moles), and 187.9 grams of 2,4,6-trimethylpyridine (1.55 moles) . The reaction mixture was heated and agitated until a homogenous mixture resulted. Then, 6.32 ml of concentrated sulfuric acid was added (equivalent to 2.0 weight % of total reactants). This mixture was reacted for four hours over a temperature range of 165°C to 195°C. The resulting product was a viscous, maroon-colored liquid. The reaction product is believed to be an oligomeric mixture based on bis(4-hydroxy styryl) methyl pyridine.

When allowed to cool to ambient temperature, a very hard and brittle solid formed. The properties of the product are

Melting Point Range = 115°C to 130°C. Elemental Weight % Analysis = 75.9% carbon,

4.0% nitrogen, 5.6% hydrogen.

IR Spectrum Analysis: The product totally lacked the aldehyde peak (1670 cm ^" ), thus indicating that.the hydroxybenzaldehyde was totally reacted. As expected, trans-unsaturation absorption bands were found to be present (970 cm~ ). Aromatic carbon-oxygen bonds were also determined to exist (1250 cm ^" ) due to the phenolic groups.

Approximately 30 grams of the prepolymer prepared as described above was dissolved in 30 cc of acetone in a shallow pan. Ten 3" x 3" (76 mm x 76 mm) sections of glass fiber mat were dipped in this prepolymer and acetone solution and then dried in an oven at 150°C and a high vacuum for 1.5 hours. After vacuum drying, the ten glass fiber prepregs were layed up on top of one another and then compression molded at 260°C and 400 psi (2758 kPa) for 3 hours.

Example 2

2,3,5,6-Tetramethylpyrazine (204 grams, 1.5 moles) and p-hyroxybenzaldehyde (733 grams, 6 moles) were added to a 2 liter resin kettle having a nitrogen purge. After complete dissolution, sulfuric acid (15.12 grams, 0.15 moles) was added to the reactor contents. The temperature was maintained between 176° and 198°C. After 3 hours, 4 minutes, the viscosity reached 740 centipoise (0.74 P"s). A dark hard glossy

solid was obtained after the reactor contents were cooled to room temperature. The solid was crushed to give a violet powder with a mortar and pestle. The violet prepolymer was dissolved in itiethanol. The methanolic solution of prepolymer was added to water which caused the prepolymer to precipitate. The precipitate was dried, crushed, redissolved in methanol, precipitated in water, and dried overnight at 120°C in a vacuum oven. The dried prepolymer was a light brown colored powder. The prepolymer melted between 177° and

192°C. It was compression molded at 255° to 290°C and 490 psi (3378 kPa) for 2 hours to give a cured black polymer with the following properties:

Glass transition temperature = 430° to 440°C (determined by DSC) weight loss in nitrogen @ 950°C = 38.5%

This polymer was post cured in an oven for 15 hours at 280°C which increased the glass transition temperature to 460°C and decreased the weight loss in nitrogen at 950°C to 31.8%.

Example 3

Using the same procedure of Example 2, 270.3 grams of 2,5-dimethyl pyrazine was reacted with 610.6 grams of 4-hydroxybenzaldehyde to prepare a dark brown prepolymer.