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Figure 1. Sample E-coated carbon fiber plate (6 in. x 6 in. x 0.25 in.) after pressing at 180°C. American Ceramic Society Bulletin, Vol. 95, No. 3 | www.ceramics.org 29 Credit: WJS Concepts LLC Trial 3: Two or three resins based on the results determined in Trials 1 and 2. Each trial required about one workweek of in-house time and about three days of off-site analysis and planning. In-house qualification testing—some done by the contractor, some done by the client's technicians—included: • Polymer viscosity at 65°C; • Polymer cure time at 180°C; • Room-temperature hardness of polymer on fabric (although subjective, the material could not be brittle); • Room-temperature tackiness of polymer on fabric (although subjective, the material could not stick strongly to nitrile gloves); • Ceramic yield from neat prepreg polymer, with a target of >70% from the original tacky resin; • Cracking after pyrolysis (some cracking was expected, but the client did not want "just powder clinging to the fiber tows"); and • Long-term weight loss at 1,100°C. Phase 1 results Table 1 provides test data and observations for each formulation in the three trials. The contractor fabricated Trial 1 samples during the first week on-site, including about six hours during the first day for equipment set-up and checks. Trial 1 polymers were all low- to medium- viscosity liquids. Sample A had low molecular weight with too many reactive functional groups, resulting in a too-highly crosslinked cured polymer (very brittle) but a high ceramic yield. Sample B had a higher molecular weight because of bulky nonreactive side groups added to reduce reactivity of the polymer. However, the cured material was too brittle and pyrolyzed to larger chunks because of less shrinkage and cracking. Sample C was softer because of insufficient cross-linking during curing. It had too low a molecular weight, which resulted in significant volatiles during pyrolysis and low yield of porous ceramic. The contractor fabricated Trial 2 formulations on-site in the sixth week of the program. The work resulted in polymers that were solid at room temperature but melted below 95°C. These polymers were designed with higher molecular weights and longer chains than Trial 1 polymers. Sample D had few available reactive groups, resulting in low cross-link density and a softer, low-yield ceramic material than Sample C, despite the higher molecular weight. This sample showed evidence of remelting as the cured chunks molded together during pyrolysis. Sample E had slightly higher molecular weight with twice the reactive side groups of Sample D. This formulation resulted in coated fabric that was slightly brittle—the fabric would craze if bent over the edge of a table. However, the material performed well in most other tests. The contractor fabricated Trial 3 polymers on-site during the 11th week of the program in an attempt to meld the best of the first two trials and further optimize the material. Sample F was formulated as a slightly higher molecular weight (i.e., longer average chain length) version of Sample E, with about 5% less reactive functional groups. Yield was low relative to Sample E, but acceptable. Sample G was formulated with the same proportion of reactive side groups as Sample E, but with bulky side groups instead of longer polymer chains. This polymer was too soft after curing, likely because of the side groups, and had a lower yield because of loss of side groups during pyrolysis. The client's R&D manager decided that formulations of Samples E and F produced materials with adequate properties to continue to Phase II. The manager also decided that projected costs would be tolerable, even if ~$2/lb higher than the target in small volumes. The client expected to meet the cost target at a volume of 1 metric ton/month. Final results and follow-up In three months—for less than onehalf the price of a Phase I Small Business Innovation Research grant—the client Table 1. Qualification test results for Trials 1–3 Sample 65°C viscosity (cP) 180°C cure Hardness of fabric at Ceramic yield (%) Pyrolysis cracking Mass loss at 1,100°C Comments time (min) room temperature for 500 h (%) Trial 1 A < 20 0.05 Brittle 88 Massive 0.2 Chunky powder B 94 0.53 Less brittle 80 Less cracking 1.1 Large chunks C 190 12.5 Softer, but brittle 60 Pores, friable 4.5 Weak porous chunks Trial 2 D 221 30 Somewhat soft 62 Solid piece, large pores 1.2 Remelted during pyrolysis E 320 20 Hard, slightly brittle 81 Solid piece, few pores 0.8 No evidence of remelting Trial 3 F 265 22 Stiff, but flexible 79 Some cracks, few pores 0.6 Shiny black after pyrolysis G 558 40 Soft, sticky 70 Some cracks, few pores 2.4 Dull black after pyrolysis


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