Patent Application
20160053122
The present invention provides a suspension and method for improving the corrosion protection of substrates which comprise or consist of steel. It has been discovered that if an organic derivative of a bentonite clay and a modified urea are added to a dispersion of flakes of Zn, Al and/or Mg, or alloys thereof, the resulting suspension demonstrates a rheological profile which is ideally suited for coating steel substrates. In fact, the inventive suspension enables coating with a high consistency of performance. The performance of the suspension is already reached after one coating process i.e. even after a dip/spin coating process is conducted only one once. In conclusion, steel substrates can be provided with a high-quality corrosion resistant coating in a more economical way compared to the prior art.
According to the state of the art, fasteners (e.g. iron made screws or nuts, or clips and non-threaded fasteners) are coated with solvent- or water-based coatings comprising zinc/aluminium flakes by a dip/spin process to protect them from corrosion.
The first stage of the dip/spin process is to put the coating in a container at the right dry solid content. In the dip stage, fasteners are put in a basket, which is plunged in the container. In the spin stage, the wet coated fasteners are pulled out of the coating container and the basket is spun at high speed to eject the excess coating.
Subsequently, coated screws are downloaded on a belt and driven to a drying zone, and finally go to an oven for curing of the dried coating. This way the first layer of coating is deposited on each fastener.
Usually, a second coat is dip/spun the same way after cooling the fasteners.
With this two-coat process, regularly a minimum of 24 g/m2 of coating weight is reached. To test the performance of the coating, a salt spray test is the most common test to account for corrosion resistance performance. By using the two-coat process, good salt spray performance is obtained, unless the edges of the fasteners are not covered.
However, the dip/spin process generally suffers the disadvantage of providing a small coating weight per coat and a lack of homogeneity in thickness of the cured coating on the fasteners. The main reason for this deficiency is found in the coating compositions demonstrating bad flow behavior which is linked to the huge density difference between the pigments and the continuous phase. Thus, resulting coatings show poor settling resistance—on storage or during application—and poor homogeneity in thickness on the surface of the fasteners.
In conclusion, the consistency in overall performance which end users, car manufacturers and fasteners manufacturers want is not reached.
Another drawback in the state of the art is that consistency in corrosion resistance is only reached with the two-coat process and off a coating application assembly line. On the assembly line, fasteners are transported and often mechanically damaged. Because of mechanical damage to the fasteners on assembly lines, the consistency in performance of existing systems is poor.
The object of the present invention was thus the provision of coating compositions for improving the corrosion inhibition of steel substrates which overcome the deficiencies of the prior art. Furthermore, the object was to provide a method for protecting steel substrates from corrosion.
The object is solved by the suspension for improving the corrosion inhibition of substrates comprising or consisting of steel according to claim 1, the method for protecting substrates comprising or consisting of steel from corrosion and the use of the inventive suspension according to claim 15. The dependent claims highlight preferred embodiments of the invention.
According to the invention, a suspension for improving the corrosion inhibition of substrates comprising or consisting of steel is provided, wherein the suspension comprises or consists of
The inventive suspension has the advantage that a high consistency of performance is reached only after a one-coat process of coating i.e. even after the dip/spin process is employed only one time. Thus, corrosion protection is achieved in a faster i.e. more economical way.
In summary, the inventive suspension has the following advantages compared to products of the prior art:
In terms of Rheology, the properties of the inventive suspension compared to the prior art solutions can be summarized as follows:
The above rheological behavior is achieved by the inventive combination of the first and second rheological modifier. In this regard, it has been discovered that the organic derivative of a bentonite clay boosts the flow curve in the medium to high shear viscosity values whereas, at the same time, the modified urea increases very much the viscosity values in the low shear rate viscosity domain. This turned out to be the ideal rheological profile for a coating suspension for steel substrates.
According to a preferred embodiment of the invention, the liquid comprises or consists of a non-polar, medium-polar and/or polar solvent, preferably an aromatic solvent and/or hydroxylated solvent, more preferably a binder solution.
The inventive suspension may comprise the flakes in a concentration range of 30 to 60 wt.-%.
Preferably, the flakes have a size of 5 to 50 μm in length and/or a parallepipedic (lamellae) shape. The advantage of flakes and, in particular, flakes of parallepipedic (lamellae) shape is that the particles have a higher surface to volume ratio compared to other particle shapes like e.g. spheric shape.
In a further preferred embodiment of the invention, the first rheological additive comprises or consists of particles, preferably particles having a size of less than 1 μm.
The organic derivative of the bentonite clay may have
The first rheological additive is preferably a phyllosilicate clay which is preferably modified for an increased solubility in organic solvents.
It is particularly preferred if the first rheological additive comprises or consists of at least one member of the Bentone SD® family (of Elementis Specialities), or an equivalent thereof, preferably Bentone® SD-1 or SD-3.
The second rheological additive may comprise fibres. Preferably, the second rheological additive has thixotropy enhancing properties.
In a preferred embodiment, the second rheological additive comprises or consists of BYK®-410 (trademark of BYK Additives & Instruments).
In a particular preferred embodiment, the inventive suspension comprises
Further, a method for protecting substrates comprising or consisting of steel from corrosion is provided, wherein the method comprises or consists of the following steps:
Preferably, in step a), the substrate is coated by contacting the substrate at least partially with the inventive suspension and subsequently removing excess suspension, preferably by spinning at a force ranging from 20 g to 40 g.
The coating may be effected by spraying the substrate with the suspension and/or dipping the substrate into the suspension, preferably with subsequently spinning and/or draining the substrate.
In step c), the curing of the coating may be effected at a temperature of 220 to 280° C., preferably 230 to 250° C., more preferably 230 to 240° C. for a time of 10 to 40 minutes, preferably 20 to 30 minutes.
Finally, it is suggested to use the inventive suspension for coating substrates, preferably for coating fasteners, more preferably for coating screws, nuts, clips and/or non-threaded fasteners.
With reference to the following Examples and Figures, the subject according to the invention is intended to be explained in more detail without wishing to restrict said subject to the special embodiments shown here.
Flow curves were recorded of the inventive coating suspension or the reference product, respectively (see
In conclusion, the viscosity of the inventive suspension is significantly higher than the viscosity of the reference product at all tested shear rates.
Viscosity recovery curves were recorded of the inventive coating suspension or the reference product, respectively (see
Since a sag happens within 30 seconds from the start of recovery (at the time point of 30 seconds in
From this experiment, it can be seen that the starting viscosity of approx. 0.02 Pa·s rises steeply both for the inventive suspension and the reference product. However, at a recovery time of approx. 30 seconds (i.e. at time point 60 seconds), the reference product only recovers to a viscosity of approx. 20 Pa·s whereas the inventive suspension recovers to a viscosity of approx. 400 Pa·s (see
In conclusion, this experiment strongly indicates that reference product displays strong heterogeneity within the storage container and over time.
In conclusion, this experiment strongly indicates that the inventive suspension displays high homogeneity within the storage container and over time.
Fasteners were coated with a one-coat process employing the inventive suspension or the reference product, respectively. In said process, fasteners were dipped in the inventive suspension or the reference product, respectively, for the same period of time (approx. 10 to 15 seconds or slightly longer if they had a recessed head).
During the spin procedure, fasteners were spun at 220 rpm (centrifugal force is approx. 20-25 g) for 8 seconds. It turned out that the coating weight per treated surface is much higher (approx. 10 g/m2 more) if the fasteners are coated with the inventive suspension compared to the reference product (see Table 1).
One group of fasteners was coated with the inventive suspension in a one-coat process to have a coating weight per surface of approx. 24.6 g/m2.
For comparison of roughly equal coating thicknesses, another group of fasteners was treated in a two-coat process with the reference product and finally had a coating weight per surface of approx. 20-21 g/m2.
Both groups were subjected to a 720 hours Salt Spray Test. After the test, fasteners which were coated with the inventive suspension in a one-coat process demonstrated a roughly equal performance in corrosion resistance like fasteners coated with the reference product in a two-coat process.
| Number | Date | Country | Kind |
|---|---|---|---|
| 13164011.0 | Apr 2013 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2014/057795 | 4/16/2014 | WO | 00 |
