Prepregs for use in building lay-ups of composite materials and processes for their preparation
The invention concerns a process for making a scrim-reinforced prepreg for use in building low-porosity lay-ups, characterized in that a conventional prepreg is formed of a fibrous reinforcement and a heat-curable resin, the prepreg having suitable viscosity and sufficient tackiness to hold a scrim which is adhered to the prepreg by applying light pressure, so that the scrim is impressed onto the prepreg to such a degree, that less than half of the circumference of the scrim strands becomes coated by the prepreg-resin.
1. A method for making a prepreg assembly having a grid of channels through which air can escape comprising the steps of:
providing a first prepreg ply comprising a fibrous reinforcement located in a heat-curable resin matrix, said first prepreg ply having a surface;
providing a second prepreg ply comprising a fibrous reinforcement located in a heat-curable resin matrix, said second prepreg ply having a surface;
locating a scrim between said first prepreg ply and said second prepreg ply, said scrim comprising a plurality of strands that form a grid; and
pressing the surfaces of said first and second prepreg plies against said scrim to form said prepreg assembly, said surfaces being pressed together so as to form the air escape channels that are located between the surfaces of said first and second prepreg plies and along said plurality of strands, said air escape channels forming a grid of channels through which air can escape from said prepreg assembly.
2. A method for making prepreg assembly according to claim 1 wherein said fibrous reinforcement in said first and second prepreg plies comprises fibers selected from the group consisting of glass fibers, carbon fibers, graphite fibers, boron fibers, ceramic fibers and aramid fibers.
3. A method for making a prepreg assembly according to claim 1 wherein said fibrous reinforcement in said first prepreg ply comprises fibers that are unidirectionally aligned.
4. A method for making a prepreg assembly according to claim 1 wherein said fibrous reinforcement in said second prepreg ply comprises fibers that are unidirectionally aligned.
5. A method for making a prepreg assembly according to claim 3 wherein said fibrous reinforcement in said second prepreg ply comprises fibers that are unidirectionally aligned.
6. A method for making a prepreg assembly according to claim 1 wherein said heat-curable resin in said first and second prepreg plies comprises a thermoset resin.
7. A method for making a prepreg assembly according to claim 1 wherein said strands are thermoplastic.
8. A method for making a prepreg assembly according to claim 1 wherein the strands have a substantially round cross-section.
9. A method for making a prepreg assembly according to claim 1 wherein said first and second prepreg plies comprise an edge and wherein said scrim extends outwardly beyond said edge.
10. A method for making a prepreg assembly according to claim 1 wherein said scrim comprises strands selected from the group consisting of polyester strands, polyamide strands, polyethersulphone strands, polypropylene strands, viscose staple yarn, meta-aramid strands, para-aramid strands, jute strands, flax strands and cotton strands.
11. A method for making a prepreg assembly according to claim 1 wherein the amount of heat-curable resin matrix in said prepreg assembly is from 25 to 45 weight percent of the prepreg assembly.
12. A method for making a prepreg assembly according to claim 8 wherein the diameter of said strands is between 200 and 600 microns.
13. A method for making a prepreg assembly according to claim 1 wherein said warp direction strands are from 3 to 60 millimeters from each other.
14. A method for making a prepreg assembly according to claim 1 wherein said strands that are crosswise to said warp direction strands are from 3 to 60 millimeters from each other.
15. A method for making a prepreg assembly according to claim 13 wherein said strands that are crosswise to said warp direction strands are from 3 to 60 millimeters from each other.
16. A method for making a prepreg assembly according to claim 6 wherein said thermoset resin is selected from the group consisting of epoxy resin, cyanate ester resin, polyester resin, vinyl ester resin and bismaleimide resin.
17. A method for making a prepreg assembly according to claim 16 wherein said thermoset resin is epoxy resin.
18. A method for making prepreg assembly according to claim 17 wherein said thermoset resin comprises difunctional epoxy resin and trifunctional epoxy resin.
19. A method according to claim 1 which includes the additional step of curing said prepreg assembly to form a cured product.