Let's know in depth the behavior of functional loads for epoxy anticorrosive coatings of medium solid content.
by Hubert Oggermüller and Bodo Essen*
Anticorrosive paints contribute significantly to the conservation of metal constructions and structures. These coatings often contain zinc phosphate as an anticorrosive active pigment but gradually legislation places limits on its use.
The question arises whether high-performance functional loads based on Neuburg Siliceous Earth (TSN) can compensate for the loss of efficiency caused by a lower phosphate concentration.
Therefore, formulations with different zinc phosphate and mineral filler contents were evaluated in saline and humid atmospheres. In addition, the additional use of amino silane as a promoter of adhesion was evaluated. This publication includes only the first part of the study, the full version can be found in www.hoffmann-mineral.com
Experimental
- Base formulation: The starting formulation of this work was a two-component bottom or commercial first based on a standard epoxy resin and a polyamide resin as a hardener. The 68% solids and volatiles (VOC) content of 430 g/l indicate the "solid media" character of the formulation. Fig. 1 shows the control formula, which in addition to the active anticorrosive pigment contains a classic combination composed of talc and barium sulfate.
- Loads used: Talc has an average oil absorption and laminar structure. As a competitive load for anticorrosive applications, the mixture of quartz, mica and chlorite is shown, which has a mixed corpuscular and laminar structure. The particle size is somewhat larger than that of talc. The oil absorption and the specific surface are similar.
We compared these loads with the performance of NRT, which is a natural combination of corpuscular silica, cryptocrystalline and amorphous, with laminar kaolin: a loose mixture impossible to separate by mechanical procedures. As a result of its natural formation the silica fraction contains cryptocrystalline silica particles with a primary particle size of about 200 nm in diameter, which are covered by amorphous oval-shaped silica.
The program of experiments was performed with two modified types whose interaction with the polymer matrix has been adjusted by silanization of the surface of the charge. Aktisil PF 777 is a version of TSN that has been hydrophobically modified with an alkyl silane, while Aktisil AM has been distinguished with an amino silane.
Variations in formulation: As shown in Fig. 2, the zinc phosphate fraction was partially or totally replaced by talc at equal volume. In a second step the mineral load was varied. To maintain the same PVC and due to differences in density, the load quantities had to be varied appropriately. The amount of precipitated barite remained constant.
- Criteria for evaluating corrosion protection:
For the evaluation of the efficiency in the corrosion protection, the tests were carried out according to different standards. Humidity according to DIN EN ISO 6270-2 CH and saline atmosphere according to DIN EN ISO 9227.
Results
Rheology and leveling: Rheological properties were determined in an MCR 300 rheometer (Paar). The rheological activity of Aktisil PF 777 is shown with thixotropic properties with a high viscosity at low stress. The largest structural increase is shown in the low-effort region. Parallel to this is creep. As a result, drip resistance is reduced in vertical applications, even in large thicknesses. For thin thicknesses a more marked surface structure can be observed than with talc.
In addition, this rheological effect of Aktisil PF 777 offers the possibility of not including in the formulation the bentone commonly used as a rheological additive. For better leveling the preferred load would be the Aktisil AM.
1.1.Hardness: the evolution of the hardness of the coatings was evaluated with a König pendulum for 14 days. The hardness of the control formulation and the reduced content of zinc phosphate with talc are at a low level. Batches with Aktisil AM and mineral blending show a similar trend but with an advantage for Aktisil AM, although the final hardness is significantly higher than in the case of talc.
Of interest seems the unusually rapid increase in hardness when using Aktisil PF777, which reaches a hardness 27s higher after two days. The final hardness with Aktisil PF777 is the highest. In short, freshly painted surfaces should have greater resistance to mechanical loads.
Corrosion protection in sandblasted steel
- Reduced zinc phosphate content: the adhesion to the substrate of all formulations before the exposure tests was outstanding, as a cross-weft adhesion test resulted in GT0. No deterioration was observed after the tests in 1000h in a humid atmosphere and with surface scratching to substrate.
In highly corrosive environments, such as the one simulated with the saline atmosphere test, the differences between the different formulations become more noticeable.
Fig. 5 shows representative results for formulations with 2.5 p/w zinc phosphate after an exposure of 1000h. The control formulation for comparison was also included. The reduction in the amount of zinc phosphate in the talc formulation results in unfavorable results in terms of surface barrier. Although only localized blisters appeared, after decapping it becomes evident that there are areas of weakness that would lead to a loss of adhesion. Optically darker areas with no coating residue indicate a deterioration in adhesion strength.
The mixture of quartz, mica and chlorite increases the intensity of the blistering. Despite containing the same zinc phosphate as the other formulations, marked perforations of the coating are observed. Corrosive attack on the substrate is an indication of the complete loss of protective function.
On the contrary, with the TSN excellent results are obtained in terms of surface. Aktisiles not only prevent the advance of corrosion, but also the appearance of blisters compared to the similar formulation with talc. In particular the Aktisil AM impresses with a blister-free surface and offers good protection, similar to the control formulation.
In surface scratching, the formulation with talc and reduced zinc phosphate content leads to an increase in corrosion formation. With regard to delamination (Fig. 6, bottom) and corrosion, the formulation offers protection similar to that of control. In any case, the first rust drilling, away from the scratching zone, indicates the beginning of the loss of efficiency in surface protection.
The natural blend of quartz, mica and chlorite is unsatisfactory.
Aktisil PF777 and Aktisil AM have results almost at the same level as the control formulation and comparable with talc in terms of delamination width. As an advantage appears in both cases a reduced intensity in corrosion in the scratching in the form of oxide. (Fig. 6, top).
Obviously the increased resistance to anodic solution contains corrosion in scratching. This is evident after decapping the surface and observing the marked reduction in corrosion undercoating and depth of corrosion. In particular, the Aktisil PF777 offers a very high resistance, so that an almost total absence of corrosion near scratching is obtained. In combination with the reduction in zinc phosphate dosage, Aktisil PF777 acts synergistically and the protective function in the striped area even exceeds the control formulation with high zinc phosphate content.
The results in the cross-scratch test confirm the very good adhesion 24 hours after exposure.
Conclusion
The decrease in corrosion protection due to the reduction of anticorrosive pigment cannot be compensated by loads such as talc or the quartz/mica/chlorite mixture, as heavy scratch corrosion and an early loss of surface protection occur in the saline atmosphere test, indicating a loss of performance.
On the other hand, surfaces that have loads such as Aktisil PF777 and Aktisil AM offer high anticorrosive efficiency when the pigment is reduced and even when it is completely removed, since excellent adhesion, excellent resistance to water condensation and exceptional resistance to the saline atmosphere are evident.
In short, with the Aktisiles we obtain a better performance than with other loads such as talc and the quartz/mica/chlorite mixture and using a small dosage of zinc phosphate we can have a comparable performance with a paint manufactured with conventional loads and high dose of phostate.
* Representatives of the company Hoffmann Mineral. Learn more at www.hoffmann-mineral.com
* Adaptation: Félix Vicente Mondéjar and Diego Rincón Gil.
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