Prepreg curing process for preparing composites having superior surface finish and high fiber consolidation

1. A process for curing a prepreg, comprising the steps of

(A) Providing a prepreg comprising a thermosetting resin composition and fiber;

(B) Placing the prepreg under reduced pressure relative to atmospheric pressure;

(C) Exposing the prepreg under reduced pressure to a first elevated temperature relative to room temperature for a time sufficient to:

remove about 1% by weight to about 3% by weight volatile materials in the prepreg, based on the total weight of the prepreg and

increase viscosity of the prepreg to a range of about 1 to about 40,000 Poise;

(D) Exposing the prepreg under reduced pressure relative to atmospheric pressure to a first subsequent elevated temperature relative to room temperature and greater than the first elevated temperature for a time sufficient to cure the prepreg; and

(E) Exposing the cured prepreg to a second subsequent elevated temperature relative to room temperature that is less than any one or more of the first elevated temperature or the first subsequent or the second subsequent elevated temperatures with or without reduced pressure relative to atmospheric pressure.

2. The process of claim 1 , wherein the first elevated temperature is in the range of about 120° F. to about 350° F.

3. The process of claim 1 , wherein the any remaining volatile materials include water, residual solvents and/or monomeric volatile materials having a higher boiling point at atmospheric pressure relative to the materials having a low boiling point at atmospheric pressure.

4. The process of claim 1 , wherein the thermosetting resin composition comprises one or more epoxies, episulfides, oxetanes, thioxetanes, maleimides, nadimides, itaconimides, oxazines, cyanate esters, and oxazolines.

5. The process of claim 1 , wherein the fiber is a member selected from the group consisting of carbon, glass, aramid, boron, polyalkylene, quartz, polybenzimidazole, polyetheretherketone, polyphenylene sulfide, poly p-phenylene benzobisoxazole, poly p-phenylene benzobisthiazole, silicon carbide, phenolformaldehyde, phthalate, poly pyridobisimidazole and napthenoate.

6. The process of claim 5 , wherein the fiber is glass and is a member selected from the group consisting of S glass, S2 glass, E glass, R glass, A glass, AR glass, C glass, D glass, ECR glass, glass filament, staple glass, T glass and zirconium oxide glass.

7. The process of claim 5 , wherein the fiber is carbon and is made from polyacrylonitrile, pitch, rayon or acrylic.

8. The process of claim 1 , wherein the fiber is carbon having been sized with a benzoxazine-containing sizing agent.

9. The process of claim 4 , wherein the thermosetting resin composition is an oxazine and comprises one or more benzoxazines.

10. The process of claim 4 , wherein the oxazine is a benzoxazine comprising one or more of

wherein X is selected from the group consisting of a direct bond, CH 2 , C(CH 3 ) 2 , C═O, S, S═O and O═S═O, and R 1 , R 2 , R 3 and R 4 are the same or different and are selected from the group consisting of hydrogen, alkyl, alkenyl and aryl, and

wherein R is alkyl or aryl and R 4 is selected from hydrogen, halogen, alkyl and alkenyl.

11. The process of claim 4 , wherein the oxazine is a benzoxazine comprising one or more of

12. The process of claim 1 , wherein the reduced pressure is greater than 686 mm Hg (13.3 psi).

13. The process of claim 1 , wherein the first elevated temperature is about 200° F. and the time is about 2 hours.

14. The process of claim 1 , wherein the first subsequent elevated temperature of step (D) is about 360° F. and the time is about 2 hours.

15. The process of claim 1 , wherein the second subsequent elevated temperature of step (E) is about 90° F.

16. The process of claim 1 , wherein the cured prepreg is storage stable at room temperature.

17. The process of claim 1 , wherein the cured prepreg shows using c-scan substantially no detectable voids larger than approximately ⅛ inch.

18. The process of claim 1 , wherein the cured prepreg shows total void content of less than about 1 to 2% by volume determined by acid digestion using ASTM 3171-11.

19. The process of claim 1 , wherein after step (C) but before step (D) exposing the prepreg under reduced pressure to a second elevated temperature relative to room temperature, said second elevated temperature being greater than the first elevated temperature, for a time sufficient to remove remaining volatile materials in the prepreg.

20. The process of claim 19 , wherein the second elevated temperature is about 290° F. and the time is about 3 hours.