Self-healing adhesion on polymer coatings

SELF-HEALING ADHESION ON POLYMER COATINGS

1 _{Laboratory of Materials and Interface Chemistry, Department of Chemical Engineering and}

Chemistry, Eindhoven University of Technology, Den Dolech 2, 5612AZ Eindhoven, The Netherlands – e-mail: y.s.deshmukh@tue.nl

Keywords: Coatings, Self-healing Adhesion, Dopamine, Hydrogen Bonds ABSTRACT

Supramolecular self assembling hydrogen bonded polymeric materials are of great importance due to its practical relevance. Well designed molecules based on these concepts   can   produce   polymeric   materials   with   “responsive”   properties   like   self   healing even in coating application. The reversible, non-covalent hydrogen bonding interactions are a recurring design principle for these materials.

Here, we report a concept for repairing mechanical damage at the coating/substrate interface. A dopamine based hydrogen bonded self-healing system was investigated and its use for self-healing adhesion purposes will be discussed. The presence of catechol bonds in the dopamine molecules promotes strong adhesion with the metal surface whereas flexible hydrogen bonding motifs ensure inter/intra molecular hydrogen bonding at the interface with polymer coating. The reversible character of the hydrogen bonds will ensure the repair of the adhesive bonds between the coatings and the substrate upon re-contact at the interface.

1. INTRODUCTION

Polymer coatings are an essential part of our daily life since they can either decorate or protect surfaces from influential aspects of the environment, e.g. from moisture, UV-radiation, chemical attack or mechanical damage. The performance and lifetime of coatings is strongly dependent on the coating materials properties but also on its adhesion to the substrate. Routine handling and usage of coatings typically leads to permanent damage which results in the loss of its protective or decorative functions. Very often, this damage occurs at the primer coating/substrate interface by the rupture of the adhesive chemical bonds, leading to the exposure of the underlying material which becomes vulnerable to further degradation and damage, e.g. corrosion, in the case of metal substrates.

To overcome the occurrence of such damage, extensive research has been performed to introduce self-healing strategies on coatings, which will have the ability to repair the damage, either in an intrinsic or extrinsic way[1,2][1]. Many strategies have  now  been  reported  to  recover  “bulk”  damage  and  a  few  more  are  now  focusing   on repairing surface properties[3][2] and other functionalities. However, successful healing processes which occur at the coating/substrate interface and repair the adhesion between dissimilar materials have not been reported so far.

In this paper, we report a new strategy for self-healing adhesion, based on dopamine molecules[4] and reversible hydrogen bonds. In our approach we have hydrogen

bonding moieties covalently connected to other molecules strongly attached bonded to a metal substrate, which should provide self healing characteristics at the coating/substrate interface.

2. RESULTS AND DISCUSSION

New molecules having hydrogen bonding moieties and strong metal binding characteristics were designed. The schematic of the molecule having the desired functionalities is presented in the Figure 1.

Figure 1: Scheme of a coating having the desired functionalities at interface to provide the self-healing adhesion behavior

The reversible character of the hydrogen bonds will ensure the repair of the adhesive bonds between the coatings and the substrate (bulk polymer coating) upon re-contact at the interface.The scheme of a self healing coating molecule connected via flexible reversible supra-molecular moieties (hydrogen bonding) is presented in Figure 1. The presence of amide moities at the interface will ensure re-establishment of hydrogen bonding interaction after the mechanical damage.

Figure 2 represents the mechanism of the self-healing adhesion. Figure 2a shows schematically the adhesion before damage and Figure 2b represents the coating after damage, i.e. upon loss of adhesion, represented as a microscopic void. If the flexible reversible supra-molecular moieties from the coating and the substrate re-connect, the adhesion is restored (reversible process from Figure 2b to 2a).

Figure 2: Schematic representation of (a) a coating containing supramolecular moieties connected via supramolecular interactions to a substrate; When stresses in

the film are too high, the coating/substrate adhesion can be lost (a  →  b). After the stress is relieved, the adhesion may be restored (b →  a).

To verify this concept and the recovering from the loss of adhesion, we will make use of a technique known as Johnson-Kendall-Roberts setup (JKR), which is named after its inventors. Two surfaces can be brought in contact and separated in a controlled fashion. Analysis of the contact force and contact area allows quantifying the adhesion between both surfaces. This setup can be used to bring the same surfaces in contact again (re-adhesion). This technique combined with AFM measurements will be used to demonstrate the self healing adhesion concept of the newly designed hydrogen bonded molecules.

3. CONCLUSIONS

We will report a new molecules based on dopamine and supramolecular hydrogen bonding which can be used for preparation of coatings with self0-healing adhesion. To rectify the adhesion/re-adhesion of the molecule at the interface, a Johnson-Kendall-Roberts setup will be used. The results will further complimented with AFM measurements.

ACKNOWLEDGMENT

The authors would like to thank SenterNovem / Agentschap NL IOP Self healing materials for providing research funding.

REFERENCES

[1] A. Dimopoulos, J. L. Wietor, M. Wubbenhorst, S. Napolitano, R. A. T. M. van Benthem, G. de With, R. P. Sijbesma, Macromolecules, 2010, 43, 8664.

[2] S. van der Zwaag, Self-healing materials: an alternative approach to 20 centuries of materials science, Vol. 100, Dordrecht, The Netherlands, 2007.

[3] T. Dikic, W. Ming, R. A. T. M. van Benthem, A. C. C. Esteves, G. de With, Adv. Mater. 2012, 24, 3701.

[4] H. Lee, S. M. Dellatore, W. M. Miller, P. B. Messersmith, Science 2007, 318, 426.