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Introduction

In this Head Scratcher we are going to show you several similar scenarios/problems from around some developing markets mostly in Africa that have similar/slight different causes, which all involve solvent, alkyd-based products and Colour changes.

Scenario 1: Eggshell enamel changing in a Cape Town block of flats

A customer has taken a premium eggshell enamel white and painted the interior of a large block of flats. After nine to 12 months, 90 percent of the block looks white, however one section on the bottom floor (as seen in the picture below) sees very little natural sunlight, and is distinctly cream in Colour (more intense where there is less sunlight). The customer now wants a detailed scientific explanation of what happened and a correction specification. The customer does not want a solvent-based top coat.

Cause:

As you know, alkyds continue to carry on cross-linking all through their service life. But the cross-linking takes different routes in the presence of sunlight, as opposed to in no or low sunlight environments. In this case, when the eggshell paint is more in the shade, the yellower it will become.  Most emulsion polymer-based water-based systems do not yellow under these circumstances so using an emulsion polymer-based high sheen or semi-gloss coating will solve the issue. But it must be based on an adhesion-promoted polymer or it will not adhere well to the very hard/smooth cross-linked alkyd system especially under wet humid conditions.

Scenario 2: The Christmas break Colour change in Zimbabwe

Like in most southern African countries, painting contractors in Zimbabwe are in a rush to finish their painting jobs by mid-December so that they can go off on their summer holidays, which last three to four weeks. So, they normally take shortcuts and have two teams of painters working, one painting the walls with water-based paints and one team painting white gloss or eggshell. Then on about 15 December, they close up the building and go off on leave. In this instance, when they came back in mid-January all the white gloss or eggshell enamel had turned very cream. They of course blamed the product but what really happened?

Hint: Think what happens when water-based paint dries. What comes off from the film quite quickly and can attack the still-wet solvent/alkyd-based film?

Cause:

In this case the yellowing/Colour change is due to a combination of two things:

1. The ammonia or amine coming out of the wet water-based paint and into the still-wet solvent-based paint caused the alkyd cross-linking mechanism to accelerate the yellowing, probably by a factor of 10 -20 times as fast, compared to yellowing in the dark. This is why a lot of paint companies use yellowing in ammonia; it offers an accelerated way to judge how badly the dry film will yellow with time.
2. Of course, with the rooms being closed up dark for a month, the process was even more effective.

Scenario 3: The hospital mortuary in Namibia

In Windhoek, a customer painted a hospital mortuary freezer area with a long oil/urethane/Hydro-Carbon-based white primer top coat. The freezer was switched off and at normal room temperature at the time of painting. The area has now been in use for a year and the hospital is complaining that the whole room has turned distinctly cream (see the picture below).

An investigation revealed the whole area was cleaned three to four times a week with a highly alkaline disinfectant cleaner and the room is left in the dark for long periods. Also the standard temperature in the freezer area is -5 to -10°C. The hospital now wants an explanation and two alternative specifications: one solvent and one water-based that must not yellow and must take constant scrubbing.

Cause:

In this case it’s again due to a combination of three causes:

1. The chemicals in the cleaner accelerate the yellowing process
2. Dead bodies give off a slight amount of amine-like gases as they decay
3. Mortuaries are mostly kept in the dark for long periods

The cold temperature is actually a bit of a red herring as in fact the lower the temperature, the less likely the cross-linking will take place. So, you might find that the yellowing in a mortuary with a broken-down refrigeration unit may be much worse than the one where it is working.

For repainting, use either:

1. A self-cross-linking, adhesion-promoted sheen or semi-gloss or
2. A chlorinated rubber or vinyl enamel

As long as either system is allowed to dry for circa seven days before the temperature is dropped to -5 to -10°C, both will work, and give the desired cleaning properties, without yellowing.

Scenario 4 The UK school rooms

For the last six months an RSA contractor has been painting a new school. The specification was as follows:

1. Paint the walls and ceilings with water-based white paint
2. Paint the wooden doors/door frames and metal central heating radiators with white gloss

After the December holidays, the contractor came back in January, the door and window frames were slightly cream and the radiators distinctly cream, as was the skirting board behind the very old fridge in the staff room. The school authorities want a detailed explanation for the Colour change on the radiators and skirting board. The white on the radiators is especially getting more yellow by the day, as they are in frequent use during the cold January weather. The school also wants a repainting specification guaranteed not to turn yellow.

Cause:

Of course, heat accelerates the alkyd-curing mechanism and the hotter it is, the faster it cures, especially if the radiator is kept hot and in the dark. Also, the presence of amine from the water-based paint would have made it worse but in this case, the heat is the main culprit.

Old fridges use ammonia as part of their cooling systems so that accelerated the yellowing of the skirting board behind the fridge.

Solution: Self-cross-linking adhesion-promoted emulsion-based water-based paints or 2K Pu is best way to go for the radiators, with the water-based being the easiest/most cost-effective way to go.

Scenario 5 The gloss enamel Colour change in Tanzania

A Tanzanian manufacturer has been making a second quality enamel with circa 40 percent water, plus a combination of a polysaccharide-type water-in-oil emulsifier and a surfactant. He has been doing this for 10-15 years without problem. He recently increased the water content to 50 percent, and started to get complaints that the paint turns pink in the container and turns cream with a pinky cream undertone on the substrate. He wants an explanation of what is occurring and how he can stop it.

Cause:

This the rarest form of yellowing/Colour change I have seen only two to three times in my 36-year career. One theory is that somehow the water was not completely emulsified when it was added and the cobalt drier is somehow leaching out into the water phase. This causes not only the pink Colour but also causes the yellowing mechanism to go in a different direction.

How do you stop it? That’s a difficult one as we are not 100 percent certain of the mechanism. Getting the water emulsification correct and reducing the amount of water will help a lot. Also, some people say that pre-emulsifying the water into an alkyd containing the polysaccharide not only gives more reproducible results but less chance of the pink Colour occurring.

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