Waterborne paints, both for architectural applications, for wood and for metal, in some countries are commonly packaged in metal packaging, more precisely, resin-lined cans. The surface of such cans is covered with epoxy phenolic resin from the inside, just like the lid. The resin-lined surface protects the metal can against contact with waterborne paint, but it is not the only form of protection against contact with the paint.
Introduction
These types of resin-lined cans are common for latex paints in the US and Canada (Figure 1), especially 1 US pint (1 US fl.oz), 1 US quart (32 US fl.oz) or 1 gallon (128 US fl.oz). Resin-lined cans containing latex paints can also be found in some European countries, e.g. Poland, UK, or Finland. In the case of small packages, such as 1 US quart (32 US fl.oz), you can often find cans that combines plastic with a metal rim and resin-lined lid. An example of such a can is shown in Figure 2, along with a fully metal resin-lined can of the same volume.
It would seem that since the can is resin-lined inside, there is no problem with contact of liquid waterborne paint with the metal can and there is also no problem of corrosion. However, we cannot forget about the resin-lined lid and rim, which in this case is crucial from the point of view of contact with the paint. After filling on the production line, the can has a press-on lid that rubs against the rim. When opening a can, a metal opener or screwdriver is usually used, which may further damage the resin coating. When painting, the excess paint from the brush is wiped off and remains on the rim of the can.
After painting, e.g. the first layer, the can is closed, then opened and then closed again many times, which damages the resin coating, and contact with liquid paint may cause rust. When opening the can again, rust fragments fall into the paint, contaminating it (Figure 3).
POS-tinting cannot be omitted. In some dispensers, pigment concentrates are dispensed without the need to open the can by piercing the lid, which is then closed with a cork and shaken. All of this can lead to an increased risk of paint contacting the metal can and causing it to rust. All this means that latex paints packed in metal resin-lined cans require additional protection, as it is impossible to avoid the epoxy resin coating being damaged and the liquid paint causing the can to rust. Such additional protection are flash rust inhibitors, whose role is not only to protect cans.
Flash rust inhibitors
Flash rust inhibitors are additives to waterborne paints designed to protect metal elements against rust when in contact with waterborne paints. First of all, they are dedicated to DTM (direct to metal) paints based on waterborne binders, in which the standard corrosion inhibitor for long-term protection is effective only after the coating dries.
When applying waterborne paint to a metal substrate, water and all other paint components come into contact with the metal surface, which may cause the appearance of the so-called flash rust immediately after applying the paint, because the standard corrosion inhibitor does not work yet. This is where flash rust inhibitor plays a role as an additional ingredient providing protection against corrosion while the paint is still wet. I described this in more detail in my article in PPCJ – click here. This is the case with metal paints, where protection of the substrate is important.
In the case of latex paints applied to mineral substrates, there is no need to protect the substrate against flash rust. There is a need to protect cans against rust and here flash rust inhibitors are also used, usually at modified dosage.
Flash rust inhibitors are usually sodium nitrite based solutions or newer generation ones are nitrite-free. Their effectiveness depends on compatibility with the binder in the paint, as well as the level of dosage in the formulation.
Experimental
Like any additive, flash rust inhibitors require application studies regarding the effectiveness of specific dose levels, compatibility with binders and determining the impact on individual parameters of paints and coatings for which they may be responsible. One such test is to determine the effectiveness of the cans against rust. Such case studies are carried out in the Spektrochem development laboratory at the stage of developing flash rust inhibitors and for those already on the market at the request of their manufacturers.
The results of long-term stability tests for a control sample (containing no flash rust inhibitor) and two inhibitors in the same doses in the entire formulation are discussed below. Tests were carried out for the formulation of gloss latex wall paint (PVC 19%) based on pure acrylic polymer dispersion. The tests were carried out according to an in-house procedure. The paints were placed in new, resin-lined metal cans and tightly closed. The cans were mixed to coat the inside walls and lid with the tested paints, then placed under accelerated storage stability conditions, and then stored in the laboratory at ambient temperature. The total duration of the test is 7 months. After this time, the samples were opened to assess the degree of rust.
Figure 4 shows the appearance of paints in cans after opening them for 7 months. The control sample shows very severe rusting of the lid, rim and rust fragments inside the can. Paint with Inhibitor A shows local rust spots on the lid and small rust fragments that have fallen into the can. Paint with inhibitor B shows no rust on the lid, rim and can.Paint with inhibitor B shows no rust on the lid, rim or inside of the can.Figure 4 shows the appearance of paints in cans after opening them for 7 months. The control sample shows very severe rusting of the lid, rim and rust fragments inside the can. Paint with Inhibitor A shows local rust spots on the lid and small rust fragments that have fallen into the can. Paint with inhibitor B shows no rust on the lid, rim or inside of the can (only slight synerersis).
After opening the cans, an additional thing was noticed. The control paint had a putrid odor. After measuring the pH, it turned out that it had dropped so drastically that the in-can preservative probably stopped microbiologically protecting the paint. The presence of microbiological contamination is evidenced by a brown discoloration on the paint surface that is not caused by rust. A similar observation was made on the sample with Inhibitor A, which also showed a putrid odor and a brown discoloration of syneresis.
Control paint, after re-closing the lid, showed gassing and bubbles leaking through the leak between the lid and the rim (Figure 5).
The paint with inhibitor B, the effect of which was confirmed in tests for good protection against rust, did not show a pH shift after 7 months of testing. This proves that corrosion inhibitors protect the contents of the can on many levels. However, their selection must be confirmed by application studies, as shown in the example presented, in which two inhibitors used in the same doses showed different effects. Although inhibitor A showed significant anti-rust properties, the corrosion process and the pH disturbance of the paint caused the in-can preservative to stop working. This also shows that it would not be sufficient to filter the paint to remove rust fragments, as it would not be suitable for painting due to microbial deterioration.
Summary
The presented results of application studies show how important it is to provide appropriate tools for the formulator in the form of case studies describing the effective operation of additives such as flash rust inhibitors. The determined dose of the inhibitor was only effective for one of them, while the other one, despite relatively good effectiveness in protecting the can against rust, was not able to buffer the paint in terms of disturbed stability and microbiological protection. Such comparisons allow us to show not only the effectiveness in the relevant area of application, which was flsh rust and protection of the can against rust, but also secondary parameters.
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