Rotomoulding breaks the assumption every other plastics process is built on. There is no screw, no die, and essentially no shear. The polymer powder simply melts, flows, coalesces and sinters against a hot mould wall as the mould rotates.

That one fact changes everything about how you colour the part.

Why ordinary masterbatch does not work in rotomoulding

A masterbatch is designed to be broken apart by mechanical energy. In an extruder or an injection screw, the pigment concentrate is subjected to intense shear that pulls the carrier apart and distributes the pigment through the host polymer. The masterbatch is doing half the work; the machine is doing the other half.

Rotomoulding provides none of that half. There is nothing to disperse the pellet except heat and time.

Feed standard pelletised masterbatch into a rotomoulding charge and you get exactly what you would expect: streaks, swirls, uneven shade, and unmelted or partly melted colour concentrate visible in the wall. The tank is not defective structurally, but it is unsellable.

This is why rotomoulders do not colour the way film and moulding shops do.

The three ways rotomoulded parts are actually coloured

1. Dry blending pigment powder with the resin powder. The cheapest route. Pigment powder is tumbled with the ground polyethylene powder, sometimes with a small amount of an adhesion aid so the pigment sticks to the resin particles rather than segregating.

Cheap, fast, and workable for a forgiving shade. But it is the weakest option on consistency: pigment can segregate in the charge box, it dusts, and it coats the mould over time. Colour matching between batches is difficult, and mould release and surface finish can suffer.

2. Compounded colour – pigment extruded into the resin, then pulverised. The polymer is fully compounded with pigment on an extruder – proper dispersion, under proper shear – and the coloured pellets are then ground to rotomoulding powder, typically around 35 mesh.

Every particle of powder now already contains its pigment. Nothing has to disperse in the mould. This gives the best colour consistency, the best surface finish and the best mechanical retention, and it is what serious tank makers use for critical colours.

It is also the most expensive and the least flexible – you commit to a colour at the compounding stage, not at the machine.

3. A masterbatch designed for rotomoulding, pulverised with the resin. A middle path. A high-let-down concentrate on a carrier closely matched to the tank resin, ground together with the natural powder so it is already intimately mixed at the particle scale.

For this to work the concentrate has to be formulated for it – very well dispersed at source, on a low-MFI carrier compatible with rotomoulding grade LLDPE, and pulverisable without smearing. A general-purpose masterbatch pelletised for injection moulding is not that.

What this means when you buy: ask specifically for a rotomoulding grade and ask how it is intended to be introduced. If the answer is “just add it at the machine”, the supplier has not formulated for your process.

Why the long thermal cycle punishes weak formulations

Rotomoulding is slow and hot. A tank spends a long period – commonly in the range of ten to twenty minutes – in an oven at high temperature, with the polymer molten for a substantial part of it.

Compare that with an injection cycle where the polymer is molten for seconds.

That extended heat history means:

  • Pigments must have genuine long-term heat stability, not just a peak-temperature rating. A pigment that survives 30 seconds at 250 °C may not survive 15 minutes at 220 °C.
  • The antioxidant package is doing real work. Rotomoulding grades are stabilised for it; your colour system must not undermine that.
  • Organic pigments with marginal heat resistance shift shade or degrade outright over the cycle. Inorganic pigments are generally the safer choice for tanks.

If your shade drifts between a short cycle and a long one, the pigment selection is wrong.

UV protection: tanks live outdoors for a decade

A water tank sits on a roof in full sun for its entire service life. UV stabilisation is not optional and it is the main thing separating a tank that lasts from one that chalks and cracks in a few summers.

Two things carry it:

The stabiliser package – HALS-based, dosed for the required life. Rotomoulding grades from the resin producer usually arrive pre-stabilised at a stated UV rating, and the colour system must add to that rather than dilute it.

The pigment itself. Carbon black in the outer layer is an extremely effective UV screen, which is why the classic Indian three-layer tank runs black on the outside. The white or light inner layer is there for the water – light exclusion inside the tank inhibits algal growth, and a light interior makes the tank easier to inspect and clean.

That three-layer construction is a colour specification with three different jobs, which is why it should be specified layer by layer rather than as “black and white tank”.

And the same rutile rule applies as everywhere else: any white or light layer with sun exposure must use rutile titanium dioxide, never anatase. Anatase is photocatalytic and will accelerate exactly the degradation the UV package is there to prevent.

Potable water: the part that is not negotiable

A water storage tank is a food-contact article in everything but name.

Rotationally moulded polyethylene water storage tanks in India are covered by an Indian Standard, and the colour system has to be compatible with it. In practice that means:

  • Pigments free of heavy metals – no lead chromates, no cadmium
  • Compliance with the migration and extraction limits that apply to potable water contact
  • No odour or taste transfer, which is a real risk with some organic pigments and with any carrier that is not fully stabilised
  • Documentation you can hand to a customer or an inspector

Confirm the current standard and amendment with your supplier and get the compliance documentation in writing before you commit to a shade. “Food grade” on an email is not documentation.

What to put on your enquiry

  • Resin: rotomoulding grade LLDPE, MFI and existing UV rating
  • Wall construction – single layer, or multi-layer with the colour of each
  • How you intend to introduce colour: dry blend, pulverised together, or fully compounded
  • Powder mesh size
  • Oven cycle time and peak internal air temperature
  • Required outdoor life
  • Potable water compliance requirement
  • Target shade with a physical reference, plus wall thickness

FAQs

Can I use normal masterbatch in rotomoulding? Generally no. Rotomoulding has almost no shear, so a standard pelletised masterbatch never disperses and shows as streaks and swirls. You need either a rotomoulding-specific concentrate pulverised with the resin, a dry-blended pigment, or fully compounded coloured powder.

What is the best way to colour a rotomoulded water tank? Compounded colour – pigment extruded into the resin under proper shear, then pulverised – gives the best consistency, surface finish and mechanical performance. Dry blending is cheaper but less consistent.

Why do rotomoulded parts need more heat-stable pigments? Because the polymer is held molten for many minutes rather than seconds. A pigment rated for a short peak temperature can still shift shade or degrade over a long rotomoulding cycle.

Why are Indian water tanks black on the outside and white inside? Carbon black on the outer layer is a highly effective UV screen for a tank that lives in full sun. The light inner layer excludes light from the stored water, which discourages algal growth, and makes the interior easier to inspect.

What pigment restrictions apply to potable water tanks? No heavy metal pigments, compliance with the applicable migration and extraction limits, and no taste or odour transfer. Ask your supplier for written compliance documentation against the current Indian Standard for rotationally moulded polyethylene water storage tanks.

What powder mesh size is used for rotomoulding? Around 35 mesh is the common working figure. Particle size and shape affect flow, sintering and surface finish, so any coloured powder needs to match the natural resin powder it is blended with.