The Effect of Microwave Radiation on Tensile Properties of Silkworm ( B. mori ) Silk
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The Effect of Microwave Radiation on Tensile Properties of Silkworm (B. mori) Silk Emily J. Reed and Christopher Viney School of Engineering, University of California at Merced, 5200 North Lake Road Merced, CA 95343, U.S.A.
ABSTRACT We have developed a reproducible protocol for studying the effect of microwave radiation on the mechanical behavior of Bombyx mori cocoon silk. In the course of this work, we identified multiple improvements that can be made to ASTM F 1317-98, the standard according to which microwave oven power output is calibrated. Exposure to microwaves does not significantly affect mechanical properties of silkworm silk, if samples are kept in a desiccator after degumming (or after degumming and microwaving) and prior to testing in a dry environment. This finding contrasts with previous work in which samples were not kept in a desiccator, and were tested in a relatively humid environment. Because the effect of microwave radiation on the mechanical behavior of silk is sensitive to ambient moisture, meaningful comparison or pooling of test results acquired in different laboratories is contingent on standardization of both the sample storage environment and the environment in which samples are tested. Interpretation of the extensive existing literature on silk mechanical properties must take account of the reality that the sample storage and testing environments are not standardized and are usually not reported. INTRODUCTION A previous study [1] suggested that irradiation in a domestic microwave oven can markedly reduce the rate of stress relaxation (and, by inference, the creep rate) in both silkworm cocoon silk and spider major ampullate silk (drag line). It was also observed that many tensile properties of samples tested in constant strain rate experiments were enhanced by this treatment. We have accordingly set out to identify the dose of microwave energy that would optimize the improvements in properties. First, we had to ensure that the energy to which samples are exposed is dependably quantifiable, and we therefore systematically investigated the variables that can be significant during calibration of the microwave oven. The understanding gained is not limited to the specific context that prompted this study, but is relevant to the wider field of materials processing (and also to the processing of food). Second, we performed (tensile) stress-strain tests and stress relaxation studies on silkworm cocoon silk samples that were subjected to various histories of exposure to microwaves and environmental moisture. The results are directly applicable to decisions regarding the use of silk under conditions where microwave exposure is likely; for example as a reinforcing fiber in microwave-cured epoxy matrix composites, or as a component of the material from which the radome on an aircraft is constructed.
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EXPERIMENTAL DETAILS Microwave oven calibration A Panasonic microwave oven equipped with a rotating turntable (“The Genius Premier”, Model No. NN-S969BA, Danville, KY) was calibrated for use with
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