Membrane-based VARTM: membrane and resin interactions

(Published on April 2006 – JEC Magazine #24)

SOLANGE C. AMOUROUX¹²,
D. HEIDER²³,
J. W. GILLESPIE JR¹²³

1 DEPARTMENT OF MATERIALS SCIENCE AND ENGINEERING,
2 CENTER FOR COMPOSITE MATERIALS,
3 DEPARTMENT OF CIVIL AND ENVIRONMENTAL ENGINEERING,
4 ELECTRICAL AND COMPUTER ENGINEERING DEPARTMENT,
UNIVERSITY OF DELAWARE

Numerous solutions have already been proposed to reduce the thickness gradient and potential for dry spots. In reference [2], the main existing solutions were presented and a multifunctional solution, introduced by EADS [3], was highlighted. The concept is the following: a membrane permeable to air and volatiles but impermeable to resin (fig.1) is added into the VARTM set-up (fig.2).

This supplemental layer provides a uniform compaction and an areal vacuum over the entire surface. Thanks to these properties, the thickness gradient can be decreased and the dry spots eliminated [4]. However, previous work [2] has shown that this innovative solution, although efficient in the case of epoxy resin systems, seems to be inadequate in other cases (such as vinyl-ester with high styrene content) . The choice of membrane is therefore driven by the choice of resin (and vice-versa) and this choice is critical since it affects not only the membrane behaviour during the infusion but also the final parts’ quality. In order to fully control the membrane-based process and extend its use to a wider range of resins, it is necessary to establish a fundamental understanding of the compatibility issues. The chosen approach to get more insight into the membrane-based process can be divided into three parts:

1. Characterisation of the membrane structure
2. Experimental observation of the compatibility phenomena
3. Simulation of the phenomena and development of adequate techniques to feed and validate the model

Membrane structure

The preferred membrane for this project is provided by W. L. Gore & Associates GmbH. It is made of PTFE nanofibres. A SEM picture of the membrane’s surface is shown in figure 3 whereas its crosssection is pictured in figure 4.

Exploratory experiments

Parts were infused with both resin systems of interest. In [2] and [5], these preliminary experiments were presented in detail, along with micrographs of the final parts (figs 5&6), which highlighted the important impact of compatibility on parts’ quality.

Simulation

Ultimately, the objective of the model is to predict the time it would take for the resin to go through the membrane. Recognizing that this time should be larger than the gel time to obtain a compatible system, it will be possible to tailor the membrane’s properties accordingly. The first step towards this most comprehensive model is the elaboration of an introductory model, which takes into account the obvious forces acting on the membrane. The assumptions and details of the derivation of this model can be found in reference [5]. It is interesting to indicate that this model is mainly based on classical transport through porous media (Darcy’s law) and surface science (Laplace equation).

The time of interest is given by:

where h is the resin viscosity (Pa.s), η the membrane thickness (m), ε the membrane porosity (%), r the average flow pore size (m), ΔP the vacuum applied during the VARTM process, γ the resin surface tension (N/m) and θ the contact angle of the {membrane, resin} system.

In order to validate the model, its results need to be compared to the experimental trials. The data required to feed the model are obtained using adequate methods.

Experimental techniques to determine relevant membrane properties

Among the chosen techniques, one is especially interesting, since it involves a unique set-up created at the Center for Composite Materials. Its purpose is to measure contact angle between substrates and fluids, statically and dynamically. The set-up is illustrated in figure 7. Its specificity is to use a high-speed camera which can easily capture the dynamic behaviour.

The above set-up provides the following pictures:

Utilizing the experimental data in the model shows that the theoretical prediction captures the main phenomena over a wide range of time scales. However, refinement is necessary to get more accuracy. The next steps are to evaluate the time dependence of the wetting parameters, such as the surface tension of the resin and the contact angle. The contact angle technique itself also requires some improvements to better capture the effect of the membrane roughness and the dynamic behaviour of the wetting parameters over longer (hours) periods of time. These specific issues are currently under investigation.

Future work

As part of our long-term future work, we will use the final model to extract the important membrane and resin parameters in order to create a design chart indicating the adequate membrane for a specific resin system. This chart will be validated with simple geometries and later applied to more complex geometries, such as stiffeners.

1 In this study, a compatible {membrane, resin} system refers to a system in which the membrane is able to remain impermeable to the resin until the part is cured.

Acknowledgements

The authors thank W. L. Gore and Associates GmbH for providing the membrane. Support from the Federal Aviation Administration and the Office of Naval Research is gratefully acknowledged.

References

1. D. Heider, J. W. Gillespie Jr, Journal of Advanced Materials, 36 (4) (2004), 11.
2. S. C. Amouroux, D. Heider, S. Lopatnikov and J. W. Gillespie Jr in Proc. of the American Society for Composites, Philadelphia PA, 2005.
3. J. Filsinger, T. Lorenz, F. Stadler and S. Utecht, U.S. Pat. 6,843,953 (Mar. 5, 2002), to EADS Deutschland GmbH (Munchen, DE).
4. W. Li, J. Krehl, J. W. Gillespie Jr, D. Heider and M. Endrulat, K. Hochrein, M. G. Dunham, C. J. Dubois. Journal of Composite Materials 38 (20), 2004.
5. S. C. Amouroux, D. Heider, S. Lopatnikov and J. W. Gillespie Jr in Proc. SAMPE International Technical Conference, 33, Seattle WA, 2005.