4.8 Case studies and examples
4.8.2 Host/guest systems: controlled release of active agents
an alkyd-melamine enamel. The coatings revealed promising active corrosion inhibition properties when applied on aluminium alloy and steel substrates (Leite Cavalcanti et al., 2012 ).
4.8.2 Host/guest systems: controlled release
encapsulation of low molecular weight inhibitors, zeolites and mesoporous oxides were considered as potential host candidates. Benzotriazole (BTA) was chosen as guest and the effect of releasing this molecular corrosion inhibitor from a coating was investigated for copper containing aluminium alloys. Water penetrating into scratches in the coating was considered to trigger the BTA release following competitive adsorption on the host surface.
Computer-based simulation aided several considerations. Prior to the laboratory work, MD and GCMC simulation methods were applied to identify promising zeolite (host) structures which were capable of BTA uptake and permit technologically relevant BTA loadings. The simulations allowed acquisition of atom-level understanding of the loading and release mechanisms and enabled selection of optimum host/guest systems prior to synthesis. Once the host materials were synthesised, the release properties were evaluated experimentally.
As shown in Fig. 4.12 , the GCMC simulation predictions initially predicted that 20 wt.% loading of corrosion inhibitors can be achieved within the nano-zeolite ‘cages’ (Fig. 4.2 ). MD simulations of the diffusion within the functionalised particles showed that displacement of inhibitor molecules by water is effi cient and can be controlled by adapting the zeolite structure and chemistry. In particular, the silica/alumina ratio of the zeolites turned out to govern the release rate.
4.12 BTA loading resulted from the GCMC simulation for different types of Zeolite structures. The FAU (faujasite) zeolite structure and BEA (beta polymorph A) zeolite structures were considered, the numbering refers to the Al/Si ratio (Fraunhofer IFAM).
25.0 22.5 20.0 17.5 15.0
BTA loading (%)
FAU-382 FAU-23.6 FAU-62 FAU-10 FAU-190 BEA-49.2 BEA
External pressure (hPa) 12.5
10.0 7.5
0 20 40 60 80 100 120 140 160
In the laboratory, BTA-loadings as high as the ones predicted by computer-based simulation could be achieved as confi rmed by thermogravi- metric analysis (TGA). Moreover, these investigations showed that immobilised BTA is only released at temperatures above 150 °C which may be considered as a maximum curing temperature for typical coatings. The thermally triggered as well as the water - triggered BTA release turned out to be primarily governed by the silica/alumina ratio of the zeolites, confi rming the trends obtained from simulation.
The host/guest nanomaterials as well as the paint formulations were continuously developed during the project. The project partners proved that the functionalised porous materials could not only be loaded with effective corrosion inhibitors using various nano-zeolite ‘cages’ but also that these nano-‘cages’ retain the desired effect even when dispersed in the fi nal coating. In addition to developing the novel corrosion protection fi llers, it was found that the combination of these particles with carbon nanotubes can increase the versatility of the coatings. Simulations indicated that the nano-zeolite and carbon nanotube particles remain well-dispersed throughout the polymer matrix. Concerning this matrix, the challenge was to fi rst create an accurate model of the crosslinked system – this was achieved by developing a novel procedure to simulate the formation of bonds between reactive sites. Experiments showed that by scratching the coating, one end of multiwall carbon nanotubes is ‘pulled out’ of the polymer matrix, leaving spaces in this fracture surface. When the resulting capillary voids are fi lled by a water fi lm, water molecules may reach nearby nano- zeolite cages and displace adsorbed corrosion inhibitor molecules which then can contribute to the protection of bare metal exposed by the scratch.
Moreover, carbon nanotube particles embedded and loaded in such way might contribute to increasing the fracture toughness of the composite coatings during the mechanical impact and, thus, make a contribution to protecting the substrate by impeding the formation or the growth of a scratch. Investigations concerning the effectiveness of such contribution to passive corrosion protection are ongoing.
From an industrial perspective, it was important that appropriate quantities, i.e. up to 5 wt.% of (loaded) nano-zeolites and carbon nanotube particles could be added to the paint formulation without impairing essential properties such as paint adhesion, hardness or the initial viscosity of the fl uid paint. Moreover, additional functionalities could be implemented in the coatings. For instance, an electrical conductivity due to the carbon nanotubes permitted the detection of scratches by measuring changes of the resistance between several contact points attached to the surface of the coating. Concerning the structure of the coatings, electron micrographs revealed that the functional particles remain uniformly dispersed throughout the polymer matrix as desired, as shown in Fig. 4.13 for the case of a coating
fi lled with zeolite hosts. Finally, the host/guest systems were investigated within several industrially relevant paint formulations. Industry standard 1000-hour salt spray tests, run at elevated temperatures after scratching/
damaging the modifi ed nanoSISAM-coating, applied to aircraft aluminium alloy AA2024-T3 showed no blistering in the affected area.
In summary, the investigations showed that functional fi llers such as nanomodules or multi-functional coatings such as nanoSISAM systems offer novel concepts for ambitious material developers in research and industry.