Thermoset resin-based composite materials are widely used in the aerospace industry, mainly because of their high stiffness-to-weight and strength-to-weight ratios. A major issue with the use of thermoset resins in fiber composites is processinduced residual stresses that are formed from resin chemical shrinkage during the curing process. These residual stresses within the composite material ultimately result in reduced durability and residual deformations of the final product. Polybenzoxazine (PBZ) polymer resins have demonstrated near-zero volumetric shrinkage during the curing process. Although the low shrinkage of PBZ is promising in terms of reduced process-induced residual stresses, little is known about the physical causes. In this work, molecular dynamics (MD) simulations are performed with a reactive force field to predict the physical properties (gelation point, evolution of network, mass density, and volumetric shrinkage) and mechanical properties (bulk modulus, shear modulus, Young's modulus, Poisson's ratio, and yield strength) as a function of the cross-linking density and thermal properties (glass transition temperature). The MD modeling procedure is validated herein using experimental measurements of the modeled PBZ resin. The results of this study are used to provide a physical understanding of the zero-shrinkage phenomenon of PBZ. This information is also a critical input to future process modeling efforts for PBZ composites.

Molecular dynamics simulation of a thermoset network and the glass transition by heating and cooling.

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Adding conductive fillers to an insulating polymer matrix produces composites with unique properties. Varying amounts of carbon black (0.33, 0.67, and 1 wt%) and graphene nanoplatelets (5, 10, 15, and 20 wt%) were added to epoxy. In addition, a few carbon black/graphene nanoplatelet/epoxy formulations were also fabricated. The conductivity and tensile properties were determined and analyzed. The single filler composites containing 5 and 10 wt% graphene nanoplatelet and 0.33 wt% carbon black could be used for electrically insulating applications. Composites containing 15 and 20 wt% graphene nanoplatelet could be used for static dissipative applications. The following composites could be used for semi-conductive applications: 0.67 wt% carbon black/epoxy, 1 wt% carbon black/epoxy, 0.33 wt% carbon black/5 wt% graphene nanoplatelet/epoxy, and 0.33 wt% carbon black/10 wt% graphene nanoplatelet/epoxy. At the 95% confidence level, the combination of 0.33 wt% carbon black with 5 wt% graphene nanoplatelet caused the composite electrical resistivity (1/electrical conductivity) to significantly decrease from ∼1015 ohm-cm to ∼104 ohm-cm. It is likely that the highly branched, high surface area carbon black is forming an electrically conductive network with graphene nanoplatelets. Concerning single filler composites, adding ≤1 wt% carbon black did not significantly lower the composite tensile strain; however, adding graphene nanoplatelet did decrease tensile strain and increase modulus. One possible application for the 10 wt% graphene nanoplatelet/epoxy composite is in Polymer Core Composite Conductors for power transmission lines, which need to be electrically insulating, have improved thermal conductivity (increased from 0.2 to 0.3 W/m-K), increased tensile modulus (increased from 2.7 to 3.3 GPa), and good tensile strength (70 MPa) and strain (3.3%).

Huntsman–Merrimack MIRALON® carbon nanotubes (CNTs) are a novel, highly entangled, commercially available, and scalable format of nanotubes. As-received and acid-treated CNTs were added to aerospace grade epoxy (CYCOM® 977-3), and the composites were characterized. The epoxy resin is expected to infiltrate the network of the CNTs and could improve mechanical properties. Epoxy composites were tested for flexural and viscoelastic properties and the as-received and acid treated CNTs were characterized using Field-Emission Scanning and Transmission Electron Microscopy, X-Ray Photoelectron Spectroscopy, and Thermogravimetric Analysis. Composites containing 0.4 wt% as-received CNTs showed an increase in flexural strength, from 136.9 MPa for neat epoxy to 147.5 MPa. In addition, the flexural modulus increased from 3.88 GPa for the neat epoxy to 4.24 GPa and 4.49 GPa for the 2.0 wt% and 3.0 wt% as-received CNT/epoxy composites, respectively. FE-SEM micrographs indicated good dispersion of the CNTs in the as-received CNT/epoxy composites and the 10 M nitric acid 6 h treatment at 120 °C CNT/epoxy composites. CNTs treated with 10 M nitric acid for 6 h at 120 °C added oxygen containing functional groups (C–O, C=O, and O=C–O) and removed iron catalyst present on the as-received CNTs, but the flexural properties were not improved compared to the as-received CNT/epoxy composites.