Why Are Water Tanks Black? The Job of the Outer Layer

In short: the outside of a water tank is black because black is the most effective protection against ultraviolet light. Carbon black absorbs UV before it can reach the polymer and turns it into a small amount of heat. Without that, sunlight slowly breaks the plastic apart — first a chalky surface, then brittleness, then cracks.

A water tank spends its whole life outdoors. It is the one appliance in the compound that never comes inside, never gets covered, and never gets a break from the sun. Everything about the outer layer follows from that.

What sunlight actually does to plastic

Polyethylene is a long chain of repeating molecules. Ultraviolet light carries enough energy to break the bonds in that chain — a process called photodegradation. In the presence of oxygen it accelerates.

You can watch it happen on any unprotected plastic left outside:

1. Colour fades. The first and most cosmetic stage.
2. The surface chalks. Rub it and a fine powder comes off on your hand. That powder is degraded polymer.
3. The plastic goes brittle. The broken chains are shorter, and short chains do not flex.
4. It cracks. Usually starting where the wall is thinnest or most stressed.

The tank does not spring a leak on day one and it does not fail dramatically. It quietly gets weaker for years and then splits somewhere it was already working hardest.

Why carbon black

Carbon black is the standard UV protection for outdoor polyethylene, and it has held that position for decades because nothing else does the job as cheaply or as well.

It works by absorption. UV that would otherwise be absorbed by the polymer chain is absorbed by the carbon particles instead and dissipated as a small amount of heat through the wall. The polymer behind it never receives the energy that would have broken it.

Two things follow that are worth knowing:

• It has to be through the material, not on it. Carbon black is compounded into the polyethylene before the tank is formed, so the protection is the wall rather than a coating on the wall. A painted-on finish scratches, and where it scratches the plastic underneath is unprotected.
• Darker blocks more. This is why the protective layer is black rather than a dark colour. There is no lighter pigment that performs as well against UV.

Why this matters more in Kenya

UV intensity depends on how high the sun gets, how much atmosphere the light passes through, and how much ozone is above you. Kenya scores badly on the first two.

Sitting on the equator means the sun passes close to directly overhead, so the light takes the shortest possible path through the atmosphere. Altitude compounds it — there is less air above Nairobi at 1,795 m than above a coastal city, and less air means less absorption on the way down.

The result is that Nairobi regularly reaches a UV index of 11 at midday, which is the top of the "extreme" band. A European summer typically peaks at 8 or 9.

A tank wall specified for a temperate climate is being asked to do noticeably more work here, all year round rather than for three months of it. The protection is not a nice-to-have.

The outer layer of a Vee Tank

The Vee Tank wall is moulded in three layers, and the outermost is black, for UV protection. It is the face that takes the sun, the rain and the knocks of standing outdoors, and it is the only one of the three that the weather ever touches.

Because the tank is blow moulded, the three layers are laid down together in one shot rather than added one after another. The black layer is not a skin applied to a finished tank — there is no join between it and the layer behind it, because they were never separate parts.

How to check a tank that is already standing

You do not need instruments. Go and look at one that has been outside for a few years:

• Rub the sunny side with a thumb. Chalk on your thumb means the surface is degrading.
• Compare the sunny side with the shaded side. A visible difference in colour or texture tells you UV is winning.
• Look at the top. The horizontal surfaces take the most direct sun and go first.
• Check around fittings and the lid rim. Degradation shows up first where the wall is stressed or thin.

A tank that chalks heavily after two or three seasons was not made with adequate UV protection, whatever the label said.

What the outer layer does not do

It is worth being precise, because the three layers of a tank wall get credited with each other's work.

The black outer layer protects the plastic from sunlight. It is not what keeps algae out of your water — that is a matter of stopping light passing all the way through the wall, which is the middle layer's job. And it is not the surface your water touches; that is the inner layer, and it is a different material for a different reason.

Three layers, three jobs.

The rest of the wall

• Premium outer layer, black, for UV protection
• Middle layer, grey, for added strength and opaqueness
• Inner layer, white virgin material, food grade and anti-algae

Six sizes from 1,000 to 10,000 litres, made in Kenya by Astral Industries Limited, with a 12-year warranty.

See the range, or ask us anything.

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Sources for the general material science in this post: Carbon black as a UV stabiliser for polyethylene and UV exposure and polyethylene storage tanks. UV index figures from the Nairobi UV index record and the German Federal Office for Radiation Protection's UV index scale.