Significance Statement
Performance of composites in various applications such as aerospace and automobile is mainly dependent on properties of joints between the components. Adhesive bonding, a joining technology offers advantage in terms of low stress concentration at joint, great flexibility, manufacture ease and high adhesive strength to weight ratio.
From various researches, several theories put forward to account for adhesion between polymer adhesives and metal surfaces have been the mechanical interlocking, electrostatic attraction, diffusion and adsorption. Despite this, understanding of adhesion at the molecular level remains inadequate.
Researchers from Pusan National University (Republic of Korea) studied the interactions between iron Fe surface and diglycidyl ether of bisphenol A DGEBA epoxy adhesive by constructing adhesion models for three different adsorption configurations with the aid of density functional theory. Their study is published, Composite Science and Technology,
All density functional theory calculations were performed using Vienna ab initio simulation package with plane-wave basis set to solve the Kohn-Sham equations. Electron interactions were described with projector augmented wave method and spin-polarized generalized gradient approximation. The obtained lattice parameter was calculated to be 2.864Å which is in good accordance with experimental value of 2.867Å. Fe (100) surface was selected as adhesion surface because it’s the most thoroughly investigated and most stable surface after the (110) facet. For slab with five layers, the surface energy was well-converged at 2.378J/m2 which is good agreement with other theoretical results and experimental surface energy. One segment of diglycidyl ether of bisphenol A was used as an appropriate model for investigating adhesion mechanism of epoxy adhesives.
Three different configurations for the adsorption of epoxy adhesive on Fe (100) surface were; model A, in which an O atom of hydroxyl group in the epoxy adhesive is positioned at an on-top site of an iron atom on the surface, model B had epoxy adhesive rotated by 1800C from model A so that hydroxyl group in the epoxy adhesives oriented away from iron surface while model C had epoxy adhesive rotated by 900 from model A, so that an O atom of hydroxyl group in the epoxy adhesive is positioned at the on-top site of the adjacent iron atom.
Results of epoxy adhesive-Fe contributed on dispersion interaction adhesion energies of models A, B and C are 1.77, 1.74 and 0.01eV respectively. Dispersion corrections are significant in models A and B due to dispersion interactions between aromatic rings of the epoxy adhesive and the iron surface. After dispersion correction, Fe-O bond length shortened from 2.282Å to 2.160Å.
Lengths of bonds between C atoms of aromatic rings and surface iron atoms are in the range of 2.042-2.638Å and length of bond between O atom of hydroxyl group with iron atom is 2.160Å demonstrating strong interaction between epoxy adhesives and iron metal surface. It was also discovered that aromatic rings and hydroxyl groups have more significance in terms of adhesion energy when compared to ether groups.
Adhesion energies of model B were -3.47eV and -5.24eV without and with dispersion correction indicating slightly weaker adhesion compared to model A.
Distances between C atoms of aromatic rings and iron atoms of surface layer is in the range of 2.010-2.354Å which is longer than experimental value of Fe-C chemical bonds of 1.914-1.929Å. Distance between O atoms of ether groups and iron atoms of surface layer are 2.749Å and 2.784Å which are longer than experimental Fe-O chemical bonds (2.030-2.170Å). This result showed that aromatic rings have a stronger influence on adhesion performance. Model C also had the smallest adhesion energies of three models, -3.41 and -3.42eV without and with dispersion correction respectively. Distance between O atom of hydroxyl group and surface iron atom is 2.236Å which is longer than experimental observed Fe-C chemical bonds (2.030-2.170Å).
Bader charge analysis for model A, B and C showed |1.698e|, |1.764e| and |0.589e| were transferred from the iron surface to the epoxy adhesive. More charge was transferred from iron surface to the epoxy adhesives in models A and B than in model C due to -electrons of aromatic rings in models A and B interacting strongly with the valence d electrons.
Further results on density of states of epoxy adhesive-iron surface complexes showed that for models A and B, the density of states of the iron d orbital is moved slightly upward above Fermi level because of electron transfer from d orbital of the iron atom to the pZ orbital of the C atom and pZ orbital of the O atom. However for model C, upshift of iron d states is much smaller than models A and B.
This study provided important step in developing a basic understanding of the adhesion mechanism of epoxy resins on iron metal surfaces at the molecular scale.
Journal Reference
Ji Hye Lee1, Sung Gu Kang2, Youngson Choe3, Seung Geol Lee1. Mechanism of Adhesion of the Diglycidyl Ether of Bisphenol A (DGEBA) to the Fe(100) Surface. Composites Science and Technology, Volume 126, 2016, Pages 9–16.
[expand title=”Show Affiliations”]- Department of Organic Material Science and Engineering, Pusan National University, 2, Busandaehak-ro 63beon gil, Geumjeong-gu, Busan, 46241, Republic of Korea
- Office of Strategic Foresight, Korea Institute of S&T Evaluation and Planning (KISTEP), 68, Mabang-ro, Seocho-gu, Seoul, 06775, Republic of Korea
- Department of Chemical and Biomolecular Engineering, Pusan National University, 2, Busandaehak-ro 63beon gil, Geumjeong-gu, Busan, 46241, Republic of Korea
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