Publications

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The use of self-assembling, pre-polymer materials in 3D printing is rare, due to difficulties of facilitating printing with low molecular weight species and preserving their reactivity and/or functions on the macroscale. Akin to 3D printing of small molecules, examples of extrusion-based printing of pre-polymer thermosets are uncommon, arising from their limited rheological tuneability and slow reactions kinetics. The direct ink write (DIW) 3D printing of a two-part resin, Epon 828 and Jeffamine D230, using a self-assembly approach is reported. Through the addition of self-assembling, ureidopyrimidinone-modified Jeffamine D230 and nanoclay filler, suitable viscoelastic properties are obtained, enabling 3D printing of the epoxy–amine pre-polymer resin. A significant increase in viscosity is observed, with an infinite shear rate viscosity of approximately two orders of magnitude higher than control resins, in addition to, an increase in yield strength and thixotropic behavior. Printing of simple geometries is demonstrated with parts showing excellent interlayer adhesion, unachievable using control resins.

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Physical stress relaxation in rubbery, thermoset polymers is limited by cross-links, which impede segmental motion and restrict relaxation to network defects, such as chain ends. In parallel, the cure shrinkage associated with thermoset polymerizations leads to the development of internal residual stress that cannot be effectively relaxed. Recent strategies have reduced or eliminated such cure stress in thermoset polymers largely by exploiting chemical relaxation processes, wherein temporary cross-links or otherwise transient bonds are incorporated into the polymer network. Here, we explore an alternative approach, wherein physical relaxation is enhanced by the incorporation of organometallic sandwich moieties into the backbone of the polymer network. A standard epoxy resin is cured with a diamine derivative of ferrocene and compared to conventional diamine curing agents. The ferrocene-based thermoset is clearly distinguished from the conventional materials by reduced cure stress with increasing cure temperature as well as unique stress relaxation behavior above its glass transition in the fully cured state. The relaxation experiments exhibit features characteristic of a physical relaxation process. Furthermore, the cure stress is observed to vanish precipitously upon deliberate introduction of network defects through an increasing imbalance of epoxy and amine functional groups. We postulate that these beneficial properties arise from fluxional motion of the cyclopentadienyl ligands on the polymer backbone.

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When diethanolamine (DEA) is used as a curative for a DGEBA epoxy, a rapid “adduct-forming” reaction of epoxide with the secondary amine of DEA is followed by a slow “gelation” reaction of epoxide with hydroxyl and with other epoxide. Through an extensive review of previous investigations of simpler, but chemically similar, reactions, it is deduced that at low temperature the DGEBA/DEA gelation reaction is “activated” (shows a pronounced induction time, similar to autocatalytic behavior) by the tertiary amine in the adduct. At high temperature, the activated nature of the reaction disappears. The impact of this mechanism change on the kinetics of the gelation reaction, as resolved with differential scanning calorimetry, infrared spectroscopy, and isothermal microcalorimetry, is presented. It is shown that the kinetic characteristics of the gelation-reaction of the DGEBA/DEA system are similar to other tertiary-amine activated epoxy reactions and consistent with the anionic polymerization model previously proposed for this class of materials. Principle results are the time-temperature-transformation diagram, the effective activation energy, and the upper stability temperature of the zwitterion initiator of the activated gelation reaction. It is established that the rate of epoxide consumption cannot be generically represented as a function only of temperature and degree of epoxy conversion. The complex chemistry active in the material requires specific consideration of the dilute intermediates in the reaction sequence in order to define a model of the reaction kinetics applicable to all time-temperature cure histories.

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The accelerated aging behavior and aging state of a 30 year old field retrieved polysulfide elastomer was examined. The material is used as an environmental thread sealant for a stainless steel bolt in a steel threaded insert in an aluminum assembly. It is a two component curable polysulfide elastomer that is commercially available in a similar formulation as was applied 30 years ago. The primary goal of this study was to establish if aging over 30 years under moderate aging conditions (mostly ambient temperature and humidity) resulted in significant property changes, or if accelerated aging could identify developing aging pathways which would prevent the extended use of this material. The aging behavior of this material was examined in three ways: A traditional accelerated thermo-oxidative aging study between 95 to 140°C which focused on physical and chemical properties changes, an evaluation of the underlying oxidation rates between RT and 125°C, and an assessment of the aging state of a small 30 year old sample. All three data sets were used to establish aging characteristics, their time evolution, and to extrapolate the observed behavior to predict performance limits at RT. The accelerated aging study revealed a relatively high average activation energy of ~130 kJ/mol which gives overconfident performance predictions. Oxidation rates showed a decreasing behavior with aging time and a lower E a of ~84 kJ/mol from time - temperature superposition , but also predicted sufficient additional performance at RT. Consistent with these projections for extended RT performance, only small changes were observed for the 30 year old material. Extrapolations using this partially aged material also predict ongoing use as a viable option. Unexpected RT degradation could only develop into a concern should the oxidation rate not trend lower over time as was observed at elevated temperature. Considering all data acquired in this limited aging study , there are no immediately apparent concerns with this material for ongoing use. ACKNOWLEDGEMENTS We thank Lisa Deibler for providing us with a small sample of field aged and new commercial material.

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We are developing computational models to help understand manufacturing processes, final properties and aging of structural foam, polyurethane PMDI. Th e resulting model predictions of density and cure gradients from the manufacturing process will be used as input to foam heat transfer and mechanical models. BKC 44306 PMDI-10 and BKC 44307 PMDI-18 are the most prevalent foams used in structural parts. Experiments needed to parameterize models of the reaction kinetics and the equations of motion during the foam blowing stages were described for BKC 44306 PMDI-10 in the first of this report series (Mondy et al. 2014). BKC 44307 PMDI-18 is a new foam that will be used to make relatively dense structural supports via over packing. It uses a different catalyst than those in the BKC 44306 family of foams; hence, we expect that the reaction kineti cs models must be modified. Here we detail the experiments needed to characteriz e the reaction kinetics of BKC 44307 PMDI-18 and suggest parameters for the model based on these experiments. In additi on, the second part of this report describes data taken to provide input to the preliminary nonlinear visco elastic structural response model developed for BKC 44306 PMDI-10 foam. We show that the standard cu re schedule used by KCP does not fully cure the material, and, upon temperature elevation above 150°C, oxidation or decomposition reactions occur that alter the composition of the foam. These findings suggest that achieving a fully cured foam part with this formulation may be not be possible through therma l curing. As such, visco elastic characterization procedures developed for curing thermosets can provide only approximate material properties, since the state of the material continuously evolves during tests.

As part of the Light Water Reactor Sustainability Program, science - based engineering approaches were employed to address cable degradation behavior under a range of exposure environments. Experiments were conducted with the goal to provide best guidance for aged material states, remaining life and expected performance under specific conditions for a range of cable materials. Generic engineering tests , which focus on rapid accelerated aging and tensile elongation , were combined with complementar y methods from polymer degradation science. Sandia's approach, building on previous years' efforts, enabled the generation of some of the necessary data supporting the development of improved lifetime predictions models, which incorporate known material b ehaviors and feedback from field - returned 'aged' cable materials. Oxidation rate measurements have provided access to material behavior under low dose rate thermal conditions, where slow degradation is not apparent in mechanical property changes. Such da ta have shown aging kinetics consistent with established radiati on - thermal degradation models. ACKNOWLEDGEMENTS We gratefully acknowledge ongoing technical support at the LICA facility and extensive sample handling provided by Maryla Wasiolek and Don Hans on. Sam Durbin and Patrick Mattie are recognized for valuable guidance throughout the year and assistance in the preparation of the final report. Doug Brunson is appreciated for sample analysis, compilation and plotting of experimental data.

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