Improved, readily processable thermosetting polymer resins are being developed for use as matrix materials in composite-material structures in applications in which there are requirements for thermal stability and hot-wet performance. Heretofore, the development and utilization of polymer resins for such applications have been limited by adverse characteristics that include (1) toxicity of many of the chemicals used to make the resins; (2) poor performance during fires (excessive flame and smoke, dripping, and generation of hazardous substances); (3) significant absorption of water in humid environments, linked to delamination during rapid heating; and (4) in the cases of many of the polymers suitable for use at the highest temperatures, difficulties in the use of newer, less expensive fabrication techniques.
The present development effort, intended especially to mitigate the second and third of the above mentioned adverse characteristics, has thus far been focused on two promising classes of resins: cyanate esters and poly-p-phenylenes. Composite-material parts can be fabricated from these resins, by use of simple processing techniques, at least as easily as from the previously available resins that would otherwise be used. Also, in comparison with the polymers made from those previously available resins, the polymers made from these resins exhibit reduced uptake of water and greater thermo oxidative resistance.
The resins in question have the molecular structures shown in the figure. In experiments performed thus far, samples of resins and polymers were prepared as described below.
Toughened Cyanate Esters
These resins were derived from the di(cyanate ester) of bisphenol A (BADCy). Amorphous polycarbonate (APC) were used as a polymeric toughening agent. Samples of toughened cyanate esters were cast from mixtures that were formed by dissolving the toughening agent in methylene chloride and adding it to molten BADCy at temperatures between 80 and 100 °C. In some samples, the proportions of the ingredients by weight were 1 part APC and 10 parts BADCy; in other samples, the weight proportions were 1 part APC and 5 parts BADCy. Samples of neat BADCy were also cast. For each sample, the casting solution was poured into a mold, then baked at a temperature of 190°C for two hours), followed by 225°C for one hour, followed by 250°C for one hour, followed by 300°C for one hour.
Silane-Based Cyanate Esters
These resins were derived from the di(cyanate ester) of dimethyldi(p-phenol) silane [abbreviated “SiMCy”], which is a silicon-containing analog of BADCy. This resin melts at 59°C. Samples of this resin, without and with APC as a toughening agent, were cast in molds in the same manner as that of the toughened BADCy resins.
Phenyl ethynyl-terminated polyphenylene oligomers denoted generally as “P3-2300-PE” were synthesized and purified in powder form. Castings of P3-2300-PE were made by melting the powder in open molds at 180°C. The castings were subsequently cured at temperatures increasing gradually from 350 to 450°C. Samples of these polymers were subjected to a variety of thermal, thermomechanical, and water-absorption tests. The following conclusions were drawn from the test results:
- In comparison with other state-of-the-art polymers, these polymers exhibited greater thermo-oxidative stability and less uptake of moisture.
- During immersion in boiling water, significantly less water was absorbed by BADCy toughened with APC than by neat BADCy. The amount of water absorbed by SiMCy was about half that absorbed by neat BADCy.
- The char yield of SiMCy was greater than that of BADCy — most likely because of the formation of SiO2 at high temperature. The physical characteristics of SiMCy make it suitable as a “drop in” replacement for other cyanate esters in composite applications.
- The poly-p-phenylenes were successfully infused into carbon fiber cloths and mats.
- The thermal-stability and flame-resistance characteristics of the poly-pphenylenes were found to be of the same caliber as those of polyphenylene thermoplastics.
- The level of absorption of water by the poly-p-phenylenes during immersion was found to be about 0.5 weight percent — a level that is virtually identical to that of polyphenylene thermoplastics and significantly lower than the corresponding levels of many other state-ofthe- art high-temperature thermosetting polymer materials.
This work was done by Gregory R. Yandek and Darrell Marchant of the Air Force Research Laboratory, Andrew J. Guenthner and Michael E. Wright of the Naval Air Warfare Center, and Thomas K. Tsotsis of the Boeing Co.
This Brief includes a Technical Support Package (TSP).
High-Temperature Resins for Composite Materials
(reference AFRL-0049) is currently available for download from the TSP library.
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