Title: Effects of Microwave Radiation on Selected Mechanical Properties of Silk
Name: Emily Jane Reed
Degree: Doctor of Philosophy
Institution: University of California, Merced, 2013
Committee Chair: Valerie Leppert
Impressive mechanical properties have served to peak interest in silk as an engineering material. In addition, the ease with which silk can be altered through processing has led to its use in various biomaterial applications. As the uses of silk branch into new territory, it is imperative (and inevitable) to discover the boundary conditions beyond which silk no longer performs as expected. These boundary conditions include factors as familiar as temperature and humidity, but may also include other less familiar contributions, such as exposure to different types of radiation.
The inherent variations in mechanical properties of silk, as well as its sensitivity to moisture, suggest that in an engineering context silk is best suited for use in composite materials; that way, silk can be shielded from ambient moisture fluctuations, and the surrounding matrix allows efficient load transfer from weaker fibers to stronger ones. One such application is to use silk as a reinforcing fiber in epoxy composites. When used in this way, there are several instances in which exposure to microwave radiation is likely (for example, as a means of speeding epoxy cure rates), the effects of which remain mostly unstudied.
It will be the purpose of this dissertation to determine whether selected mechanical properties of B. mori cocoon silk are affected by exposure to microwave radiation, under specified temperature and humidity conditions.
Results of our analyses are directly applicable wherever exposure of silk to microwave radiation is possible, including in fiber reinforced epoxy composites (the entire composite may be microwaved to speed epoxy cure time), or when silk is used as a component in the material used to construct the radome of an aircraft (RADAR units use frequencies in the microwave range of the electromagnetic spectrum), or when microwave energy is used to sterilize biomaterials (such as cell scaffolds) made of silk.
In general, we find that microwave exposure does not detract from the average mechanical properties of silk, but that it may increase the spread of data points around that average. Along the way, we come to a number of useful conclusions, summarized here:
Regarding silk in general
* Storage conditions can have a significant and enduring effect on tensile properties of degummed B. mori silk. Samples stored in a sealed container with desiccant (silica gel) have a lower yield stress and yield strain than samples stored without desiccant and they also relax more rapidly in stress relaxation tests. The ability of this silk to resist plastic deformation is optimized at intermediate hydration levels. Sensitivity to the humidity levels encountered by samples prior to testing complicates the interpretation of results, and makes inter-laboratory comparisons challenging. Silk storage conditions should therefore be reported--and, ideally, standardized--to enable useful comparison between studies.
* Differences in hand-reeling techniques can impose changes on the silk microstructure that significantly affect the results of tensile tests. Breaking strain and toughness were lower for the samples reeled by one person in our study, and the coefficient of variation was markedly higher for those samples in all tensile properties measured (yield strain, yield stress, stiffness, breaking strain, breaking stress, and toughness). Standardization of silk reeling technique is therefore necessary.
* Under our experimental conditions, tensile properties of B. mori cocoon silk annealed for 7 hr at 140<°>C do not significantly differ from those of silk taken from the same cocoon but not annealed. Tempered with knowledge about the sensitivity of silk to humidity and that degradation will occur at sufficiently high temperatures, this finding suggests that silk may be used in conditions significantly above room temperature without concern about changes in mechanical performance.
* Tensile properties of degummed silk from the inside surface of a B. mori cocoon do not differ significantly from those of silk taken from the outside surface, provided that samples used in the comparison have similar diameters. In combination with previous studies, this finding suggests that silk from any part of the cocoon may be used without concern over introducing a new source of variability into collected data, subject to the limitation that the sample diameters should be consistent.
* Silk color and cocoon size have a small to negligible effect on fiber tensile properties.
Regarding microwave oven calibrations
* The shape and aspect ratio of the calibration vessel can have significant effects on calibration results. Thus, these should both be specified in calibration standards.
* Calibration results depend on the position that the calibration vessel occupies in the microwave oven chamber. Thus the calibration vessel should consistently be placed at the same location that subsequent samples will occupy.
* Use of a large volume of water in calibrations gives a more accurate measure of the output power of the microwave oven; conversely, use of a smaller volume of water leads to a larger thermal gradient during the calibration, resulting in increased heat loss and ultimately an underestimate of the oven's output power.
* Calibrations performed with larger sample volumes avoid the complicating effects of standing waves of microwave energy, thus making the calibrations more reliable.
* Heat loss from the calibration vessel can occur during calibration of a microwave oven, such that the apparent power (as measured by the calibration) is less than the true output power of the oven. Microwave oven calibration standards should be refined to take account of this heat loss, in order to give a more accurate measure of the power that samples will be exposed to during a particular microwave treatment.
* Reproducible exposure of samples to microwave radiation requires measurement, not an assumption, of the magnetron start-up delay time.
Regarding microwave irradiated silk
* Under the experimental conditions reported here, silk is a poor absorber of microwave energy. Thus, silk can be used as a component in materials that are subjected to microwave processing, as well as materials that are subjected to in-service microwave radiation.
* While the mean values of mechanical properties were unaffected by the microwave treatments delivered in this study, the spread of breaking strength values as measured by the Weibull modulus increased with microwave exposure. The decrease in failure predictability of individual fibers suggests that silk can more appropriately be used in a composite material for situations where it will be exposed to microwave radiation, rather than relying on individual, isolated fibers for mechanical performance.
* In situations where microwave heating does affect the mechanical properties of silkworm (B. mori) silk (reported elsewhere), those effects are a result of changes that take place via a specific kinetic route that depends on rapid heating and cannot be accessed by a conventional thermal anneal.