Characterization of Thermoplastic Prepreg for Process Modeling

Executive Summary

Process modeling of thermoplastic-matrix composite materials is ill-defined. The specifics of the characterization experiments and model fitting procedures have not been time-tested and developed, as is the case for thermosetting prepreg materials. Process modeling serves to provide many benefits to those manufacturing composite parts, including reducing the cost of manufacturing, and the risk associated with testing new processes. Thermoplastics are of growing interest to many industries including the aerospace industry, which also is one of the biggest customers of process simulation software today. Thermoplastics show benefits over thermosets such as shorter processing times, lower energy usage for storage, and good recyclability. For this project, carbon fiber-reinforced poly(ether ketone ketone) unidirectional tape prepreg (CF/PEKK) was characterized by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), rheology, and guarded heat flow metrology, to investigate the crystallization and melt kinetics and heat capacity, stress relaxation and modulus development, flow characteristics and linear thermal expansion, and steady-state thermal conductivity properties. Initial quasi-isothermal DSC experiments were used to determine the temperature where chain scission begins to occur, informing the upper processing temperature. Both neat and fiber-reinforced PEKK were subjected to modulated non-isothermal DSC experiments with 7 ramp rates and two or more replicates. Baselines were drawn for the cold crystallization, cooling crystallization, and melting thermal events, and crystalline conversion and conversion rates were analyzed. Specific heat capacity of neat PEKK was measured using modulated DSC, and a sapphire reference sample. CF/PEKK samples were subjected to temperature ramp and stress relaxation 3-pt bending DMA experiments using three replicates, and the results were analyzed. A rheometer was used to measure the flow characteristics of the neat resin, and the coefficient of thermal expansion for the fiber-reinforced material, with replicates for both experiments. A brass specimen was used to validate the CTE measurement methodology. Thermal conductivity calibrations were performed on three reference materials, followed by experiments performed on two CF/PEKK samples. The density of CF/PEKK was measured using a density determination kit. Uncertainty was quantified for all experiments, and the characterization data was packaged into a dataset for use by engineers at Convergent Manufacturing Technologies.

Technical Paper

Abstract:

A select high-performance thermoplastic prepreg composite material was characterized by differential scanning calorimetry, rheology, dynamic mechanical analysis, and guarded heat flow metrology, to investigate crystallization and melting kinetics, stress relaxation behavior, complex viscosity, heat capacity, flow characteristics, thermal conductivity, and thermal expansion properties. The dataset produced is used to build a full material model allowing for the simulation and prediction of processing outcomes, using Convergent Manufacturing Technologies' process simulation software.

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