Center for Composite Materials - University of Delaware
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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

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