Cure kinetics and process modeling of a carbon-fiber thermoplastic-toughened epoxy resin prepreg
It is well known that the mechanical performance and fracture behavior of a thermosetting composite is inherently determined by the properties and characteristics of its constituents. However, the performance and behavior is also drastically influenced by the viscoelastic properties and status of the material during the cure process. This brings about a need to possess knowledge of the cure history of any composite product. Such knowledge is attainable by monitoring the material response to temperature and pressure cycles throughout the cure process. Nevertheless, changes to the cure and, equivalently, the manufacturing process influences the final cost of a composite product, thus, making it crucial to select an optimum cure profile conducive to both the desired thermo-mechanical properties as well as minimum cost. The present work investigates the cure kinetics and process behavior of a commercial carbon-fiber thermoplastic-toughened epoxy resin prepreg, IM7/977-2 UD. Experimental data and theoretical models are mostly demonstrated in the form of cure time and temperature functions, f(t,T). A comprehensive cure map is constructed based on this data in order to provide all the necessary information for design of an optimum cure profile. Material properties are measured over a broad range of isothermal cure profiles using advanced analytical techniques such as shear rheometry and Differential Scanning Calorimetry (DSC). Shear rheometry is utilized to quantify some important viscoelastic properties, such as complex viscosity (η*), shear storage modulus (G′), and shear loss modulus (G″), as well as to identify important cure transitions like gelation and vitrification. Thermal properties are obtained using DSC. These include heat flow (dH/dt), glass transition temperature (Tg), and degree of conversion (α). Before performing DSC experiments it is necessary to know a material’s decomposition temperature, and this is obtained through the use of Thermogravimetric Analysis (TGA). Since the material studied in this work is a thermoplastic-toughened epoxy prepreg, a variety of discrepancies in comparison to the kinetics of neat epoxies are observed. These inconsistencies invariably show up as variations in the values of Tg, ultimate heat of reaction (HU), and rate of reaction (dα/dt). For the 977-2 material, it is concluded that the addition of the thermoplastic agent to the epoxy significantly affects the progress of chemical reactions in addition to imparting a step transition in the progress of Tg at elevated isothermal cure temperatures (Tcure ≥ 180 °C). Furthermore, it is concluded that the existence of fibers among the polymer monomers alter the flow-ability of resin molecules throughout the cure process, resulting in early vitrification and lower HU over the entire range of Tcure. The variations observed in the values of HU at various Tcure may result in either under- or overestimation of α regardless of the relationship utilized to calculate it. In addition, a unique one-to-one relationship is established between Tg and α. Regarding the uncertainties present in the calculation of α, it is concluded that Tg is a better estimate of the state of the material at every desired stage of cure.
Thesis (M.S.)--Wichita State University, College of Engineering, Dept. of Mechanical Engineering.