Computational Studies of Crosslinked Epoxy Resins

Scientists at BHP Steel Research Laboratories and RMIT University have used Accelrys' Amorphous Cell and PCFF force-field (now called COMPASS) to model and predict crosslink density and sites in epoxy resins, and also estimate barrier and adhesion properties.

This simulation methodology should prove invaluable in the research and design of new epoxy-resin coatings, useful in e.g. as primer coatings for steel, with improved barrier and adhesion properties.

The search for new and improved coating systems requires a detailed knowledge and understanding of structure/property relationships of the potential candidates. Computational techniques, which can predict the bulk and interfacial properties of organic coatings,¹ can be used to complement, refine, and guide experimentation, and thus significantly aid, enhance, and accelerate the search for and design of new coating systems.

Irene Yarovsky of RMIT University, Melbourne, Australia, and Evan Evans of BHP Steel Research Laboratories, Port Kembla, Australia, report in the scientific journal Polymer² an in silico methodology for the construction of molecular models of crosslinked polymeric networks, in particular low molecular weight water-soluble epoxy resins cured using different crosslinking agents. These systems are currently being scrutinised as potential candidates as primer coatings for steel.

The researchers studied three water-based primer model systems, each consisting of a phosphated epoxy resin and crosslinkers (CYMEL 1158: tributoxymethylmelamine and/or CYMEL 1172: tetramethylol glycoluril), with water as solvent.

Amorphous Cell was used to generate ten atomistic models of each resin/crosslinker/water systems at appropriate densities and concentrations. The resulting 3D periodic systems were then equilibrated using the PCFF force-field in the NVT ensemble. To simulate the curing process, the systems were then heated to 600 K for 100 ps MD simulations and then equilibrated at 300 K for a further 200 ps MD simulations. A script was developed which allowed elimination reactions to occur when appropriate reactive sites came within a cutoff distance of one another. The reaction radius was found to be optimal at 6 Angstroms. Upon reaction the byproduct (butanol in the case of CYMEL 1158 and water in the case of CYMEL 1172) was removed from the cell.

The networked models created in this fashion were studied in the bulk for properties such as shrinkage and transport of penetrants, and also adsorbed onto surfaces to study interfacial adhesion phenomena.

The simulations enabled the scientists to obtain the thermodynamic and transport properties of the three model coating systems. In more detail, the simulated results, which proved to be in good agreement with experimentation, revealed:

• The crosslink density and degree of unreacted crosslinking sites. Important in performance design.
• Curing shrinkage. Highly relevant to the synthesis and environmental performance of potential coatings
• Barrier properties - the mobility of oxygen and water in the crosslinked systems. Vital to anti-corrosion considerations
• Adhesion properties. The interface between the cured epoxy systems and alumina was modeled, revealing important properites important in coating design.

The ability to generate realistic crosslinked systems has long been desired in polymer simulation. The development of such networks is now possible using Amorphous Cell and Accelrys force-fields.

Irene Yarovsky comments, "Once established, the approach has been applied more widely to a large group of candidate polymers and allowed BHP to narrow their synthesis and testing effort to the selected best performers predicted by the simulation".

References

1. I. Yarovsky, Aust. J. Phys., 407, 50, 1997.
2. I. Yarovsky and E. Evans, Polymer, 963-969, 43, 2002.