Advancing gelation front cure technique

#The present invention relates generally to fabrication and curing methods for composite materials, and more particularly to a method for controlling resin cure in the fabrication of composites.

#In existing fabrication processes for composite materials, resin cure often requires removal of volatile constituents through partially cured resin as a result of the curing process proceeding generally inwardly of the material. The resultant cured composite structure is therefore frequently characterized by an unacceptably high void volume and degree of microcracking.

#The invention has substantially reduced in critical importance previously existing problems with curing processes for composite materials, and finds basis in part in the recognition that the mechanism and rate of removal of volatile constituents from the curing process changes upon gelation of the resin. By maintaining and controlling a preselected temperature differential across the thickness of a composite laminate during the curing process, and allowing one side of the laminate to breathe, a gel front is generated in the laminate which advances with cure of the resin from a heated nonbreathing side of the laminate to the cooled breathing side. Volatile constituents from the curing process are transported toward the breathing side of the laminate ahead of the front through a less viscous, substantially uncured portion of the laminate which facilitates removal of the volatile constituents, avoids stresses internal of the laminate structure which would otherwise result in cracking, and substantially eliminates void formation resulting from trapped residual volatiles. The invention has particular application in fabricating thick or complex composite structures utilizing a condensation curing polymer.

#The invention will be clearly understood from the following detailed description of representative embodiments thereof read in conjunction with the accompanying drawings wherein:

#FIG. 1 is a schematic of a hydraulic press setup useful in practicing the advancing gel front method of the invention;

#FIG. 2 is a graph of advancing gel front temperature differential with time for a sample of FM 5064, a phenolic resin impregnated graphite laminate prepreg;

#Referring now to FIG. 1 of the drawings, shown therein is a schematic of a hydraulic press setup useful in practicing the advancing gel front method of the invention. Hydraulic press 11 includes top platen 11t and bottom platen 11b having connected thereto means for controlled heatinq or cooling of the platens by heater 12 and coolant source 13 connected to platens 11b,11t. Heater 12 may be connected in conventional fashion to both platens 11b,11t for selective heating of one or both of them; top platen 11t may be selectively cooled as by cooled water circulation. Press 11 may otherwise be substantially conventional with a capacity of up to about 12 tons, cooling capacity of one platen down to about 20.degree. C., and heating capacity up to about 200.degree. C.

#Laminate sample 15 to be cured according to the method of the invention is placed between the platens 11b,11t of press 11 within a flexible vacuum enclosure 17 substantially as shown in FIG. 1. In the practice of the invention, sample 13 may comprise thermally cured resin formulations of substantially any type, the method being particularly applicable to monitoring cure of condensation polymers in composite sheet material of from about 0.635 to 2.54 mm in thickness and cured at temperatures of up to about 163.degree. C. Enclosure 17 is supported on a steel plate 18 in thermal contact with platen 11b for conducting heat from platen 11b into enclosure 17 to sample 15. Vacuum source 19 is operatively connected to enclosure 17 through vacuu port 20 and valve 21 for selective evacuation of enclosure 17. Flexible peripheral seal 23 between enclosure 17 and plate 18 may be included to seal enclosure 17 under the influence of vacuum source 19.

#Within enclosure 17, sample 15 is sandwiched between a pair of peel plies 25 of porous Teflon or similar material providing means to readily separate sample 15 for examination following cure as described below. Sample 15 and peel plies 25 ar sandwiched between bleeder cloths 27 one of which is in laminar contact with breather mat 29 for facilitating removal of volatile constituents and otherwise for evacuating enclosure 17. Sheet 30 of Teflon or the like may be included between the assembly of 15,25,27 and plate 18 in order to absorb resin flow and prevent bonding of bleeding materials.