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.
We are developing computational models to elucidate the expansion and dynamic filling process of a polyurethane foam, PMDI. The polyurethane of interest is chemically blown, where carbon dioxide is produced via the reaction of water, the blowing agent, and isocyanate. The isocyanate also reacts with polyol in a competing reaction, which produces the polymer. Here we detail the experiments needed to populate a processing model and provide parameters for the model based on these experiments. The model entails solving the conservation equations, including the equations of motion, an energy balance, and two rate equations for the polymerization and foaming reactions, following a simplified mathematical formalism that decouples these two reactions. Parameters for the polymerization kinetics model are reported based on infrared spectrophotometry. Parameters describing the gas generating reaction are reported based on measurements of volume, temperature and pressure evolution with time. A foam rheology model is proposed and parameters determined through steady-shear and oscillatory tests. Heat of reaction and heat capacity are determined through differential scanning calorimetry. Thermal conductivity of the foam as a function of density is measured using a transient method based on the theory of the transient plane source technique. Finally, density variations of the resulting solid foam in several simple geometries are directly measured by sectioning and sampling mass, as well as through x-ray computed tomography. These density measurements will be useful for model validation once the complete model is implemented in an engineering code.
A series of experiments has been performed to allow observation of the foaming process and the collection of temperature, rise rate, and microstructural data. Microfocus video is used in conjunction with particle image velocimetry (PIV) to elucidate the boundary condition at the wall. Rheology, reaction kinetics and density measurements complement the flow visualization. X-ray computed tomography (CT) is used to examine the cured foams to determine density gradients. These data provide input to a continuum level finite element model of the blowing process.
Conformal coatings are used in space applications on printed circuit board (PCB) assemblies primarily as a protective barrier against environmental contaminants. Such coatings have been used at Sandia for decades in satellite applications including the GPS satellite program. Recently, the value of conformal coating has been questioned because it is time consuming (requiring a 5-6 week schedule allowance) and delays due to difficulty of repairs and rework performed afterward are troublesome. In an effort to find opportunities where assembly time can be reduced, a review of the literature as well as discussions with satellite engineers both within and external to Sandia regarding the value of conformal coating was performed. Several sources on the value of conformal coating, the functions it performs, and on whether coatings are necessary and should be used at all were found, though nearly all were based on anecdotal information. The first section of this report, titled 'Conformal Coating for Space Applications', summarizes the results of an initial risk-value assessment of the conformal coating process for Sandia satellite programs based on information gathered. In the process of collecting information to perform the assessment, it was necessary to obtain a comprehensive understanding of the entire satellite box assembly process. A production time-line was constructed and is presented in the second section of this report, titled 'Satellite Box Assembly', specifically to identify potential sources of time delays, manufacturing issues, and component failures related to the conformal coating process in relation to the box assembly. The time-line also allows for identification of production issues that were anecdotally attributed to the conformal coating but actually were associated with other production steps in the box assembly process. It was constructed largely in consultation with GPS program engineers with empirical knowledge of times required to complete the production steps, and who are familiar with associated risks from activities such as handling, assembly, transportation, testing, and integration into a space vehicle (SV) system. Finally, section three titled, 'Summary and Recommendations for Future Work', briefly summarizes what we have learned and describes proposed future work.
Foam encapsulants are used to encapsulate electromechanical assemblies for reasons such as shock mitigation, structural support, and voltage breakdown protection. Characterization of electrical properties of polymer encapsulants is important in situations where potting materials are in intimate contact with electrical components (e.g., printed wiring boards). REF308, REF320, RSF200, and EF-AR20 foams were developed for encapsulation in some potting applications at Sandia. Select electrical properties were measured for these Sandia encapsulants to characterize them for use in electromechanical potting applications. Dielectric constant with dissipation factors, volume resistivity, and dielectric strength were measured for REF308, REF320, RSF200, and EF-AR20 encapsulants. Fabrication of foam test specimens and the electrical test procedures will be discussed, and electrical testing results will be reported.