SAMPE Journal - January/February 2016 - 44

PHD STUDENT DIVISION AWARD ARTICLE
Development of a Process Window for Minimizing Volatile-Induced
Surface Porosity in the Resin Transfer Molding of
a Benzoxazine/Epoxy Blend
M. Anders*, J. Lo, T. Centea, S. Nutt
M.C. Gill Composites Center, Viterbi School of Engineering, University of Southern California,
Los Angeles, CA
*Corresponding author's e-mail: anders@usc.edu

Abstract
We consider the design of cure temperature cycles for the resin transfer molding (RTM) of carbon-fiber laminates
using a prototype blended resin that requires a two-stage cure cycle to prevent volatile-induced surface porosity.
While a two-stage cure cycle improves surface quality, it also adds significant cycle time to the processing,
therefore a methodology to identify the fastest cure cycle that avoids surface porosity is desired. We propose the
development of a process map, which enables the design of cure cycles with optimal intermediate dwells. First,
the resin cure kinetics are characterized using a "model-free" isoconversional method. Next, thermogravimetric
analysis (TGA) and rheological dynamic analysis (RDA) are used to determine a threshold mechanical state, above
which the release of volatile species can no longer occur. Finally, molded composite samples are fabricated in a
highly-instrumented lab-scale RTM tool, which features temperature and pressure sensors, as well as a transparent
mold wall that enables in situ observation of surface porosity formation. The results are combined into a process
map that shows, for any intermediate dwell temperature, the allowed dwell time window to produce porosityfree parts. Furthermore, by considering an RTM tool with a given tolerance for temperature control, the process
window can be used to design the fastest cure cycle that properly accommodates for the magnitude of thermal
gradients present. Finally, to validate the effectiveness of this approach, a molded sample is fabricated using
an "optimized" cure cycle designed with the process map, and the surface quality is compared to the baseline
cases. Altogether, the work clarifies the complex mechanisms that can lead to surface porosity formation during
the RTM processing of a novel resin, and provides a practical, science-based methodology for identifying process
modifications that can reduce defect levels.
Introduction
Resin transfer molding (RTM) is a popular composite
manufacturing technique for producing small to
medium sized parts with low microstructural defect
levels, excellent surface finishes, and potentially
complex geometries1. Voids are the most common
type of defect encountered in RTM, and are often
a result of incomplete preform saturation during
injection. Air can become trapped within the mold
cavity due to improper injection pressure2 or gate/
vent placement3, and considerable efforts have been
made to develop models and protocols that ensure
successful injection for arbitrary mold geometries.
However, voids can also arise from another source,
namely volatiles released by the resin after injection,
during the curing phase of the RTM process.
In the present study, we investigate volatileinduced porosity in context of a prototype resin,
currently under development for RTM of primary
aerospace structures. his blended formulation
contains both benzoxazine and epoxy constituents,
as well as a proprietary catalyst. The benzoxazine
44

component imparts excellent resistance to moisture
uptake and chemically aggressive environments, as
well as favorable flammability, smoke, and toxicity
(FST) properties. The resin's epoxy component acts
to toughen the otherwise brittle benzoxazine matrix,
and also increases the Tg by raising the maximum
theoretical cross-link density4. While this resin
has many potential benefits compared to standard
aerospace epoxies, it also presents some additional
challenges. One concern is an increased resin volatility,
which must be suppressed by the positive hydrostatic
pressure applied during RTM processing. A previous
study5 has shown that the minimum "critical pressure"
required to suppress volatile release for this resin is
~200 kPa (absolute).
While the minimum required pressure is easily
within the limits of most standard RTM systems,
the challenge lies in maintaining mold cavity
pressure during chemical cure shrinkage caused by
polymerization. Despite the total cure shrinkage of
this resin being relatively low (around 1%6) compared
to other thermoset resins, the density increase is
SAMPE Journal, Volume 52, No. 1, January/February 2016

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