Research Summary
Mechanism of Void Volatile Removal in Thermoplastic Prepreg during Oven Vacuum Bag (OVB) Processing
Authors: Danning Zhang (Ph.D MSEG), Dirk Heider, John W. Gillespie, Jr
Research Goal
• Investigation of nucleation of volatiles in composite domain
• Modeling of void volatile diffusion in resin and void domain
• Find out the temperature for diffusion to complete
Thermoplastic Composites and Oven Vacuum Bag (OVB) Processing
• High performance thermoplastic composites (TPCs) as primary structural materials
• OVB processing - potential cost effective alternative of autoclave
• Void dynamics and consolidation mechanisms are not fully understood!
Microstructure of Thermoplastic Prepreg
• Sealed and Statistically Distributed Voids
• Rough Surfaces and Permeable Interlayer Gaps
Proposed Mechanism for Thick Laminates
• Single layer void air diffusion followed by air flow through interlaye gaps
Nucleation of Volatiles
Why?
• Solubility decreases as temperature goes up during processing
• Supersaturation of volatiles: σ=C_b⁄C_i -1
Nucleation Types:
• Classic homogeneous nucleation
• Classic heterogeneous nucleation
--Require high super-saturation value (~>100)
• Growth of pre-existing voids
--Low super-saturation ratio, no new nuclei generated
Super-saturation of Volatiles
• Initial concentration is saturation at 298K under 1atm, 2atm during processing
• Assuming no gases exchange in composite domain
• Concentration follows Henry’s Law : C = S(T)*P

Nucleation Rate
• Nucleation for classic nucleation [unit: # of nuclei /m^3/s]


Simulation results of Non-isothemal Diffusion


Simulation results of Non-isothemal Diffusion


Summary
• No new air voids can be nucleated
• Air dissolved in composite may grow pre-existing void during processing
• At low heating rate, gas in the void of AS4/APC2 tape can be removed at relatively low temperature ~ before interlayer permeability reduce significantly
• Optimizing processing cycle with high heating rate and low temperature dwell
Acknowledgements
• Faculty Advisor: Prof. J. W. Gillespie, Jr.
• Committee Members: Prof. Suresh. G. Advani, Prof. Dirk. Heider, Prof. Michael Mackay
• CCM Researchers and Technician: Pavel Simacek, John Tierney, John Thiravong
• Funding: EADS