Rotomoulding is a fantastic process that is versatile, flexible, and open to design. By using colours, you can differntiate your products and make them unique.
by Angélique Brocatus, Business Developing Powders, A. Schulman

Colours are not only used as visual appreciation but can be used for safety reasons as well, think about road bollards.


To produce a colour, different methods in the industry can be used. You can use dryblending, by adding a pigment to your natural polymer, via a low speed or high speed blender. Or you can make a fully compounded colour via an extrusion process.

Colouring via dryblending

The advantage of using dryblending is that it’s cheaper than a fully compounded colour. One of the disadvantage is that the colour consistency is difficult to achieve and the final part can look ”washed out”. Pigments are very fine, which gives a higher pollution risk in the production environment, on the mould and can cause health issues for the operator. The mechanical properties, such as impact and tensile elongation have a negative effect when using dryblended colours.


Using a high speed blender gives you a better intensity compared to low speed blender, but it’s still 25 to 50% less efficient than compounding. In general, the % of pigment used for dryblending colours is 0.15-0.20%. Even by adding such a low % via high speed blending, there will be a detrimental effect on the impact properties.

Below graphs give the effect of the pigment on impact for certain colours between high speed blender and a compounded colour @ -20°C.




Impact Blue @ -20°C Impact Red @ -20°C Impact Green @ -20°C

Besides the mechanical properties, the visual effect is important. As your product makes you unique, you want your coloured part remain bright and opaque for many years, and not fading away with cracks as the dryblended part in the right picture is showing.

Colouring via compounding

With compounding, the raw material supplier is using a two step process. First they make a homogenous mix of the polymer, MB, and eventually additives via extrusion. Afterwards they are grinding the compounded granules in the correct particle size.


With a compounded colour, you will have a constant colour, very good opacity, no pollution, good mechanical properties, and it’s the only method for use in Food contact applications.

But, even with compounding, colours may fade in a very short time. Therefore your raw material supplier should use the best masterbatch for your application. Not only a good UV stabilized polymer should be used, also the masterbatch itself is very important. If your supplier is using a high UV stabilized polymer with an unstable masterbatch, it will have an influence on your end product.


The example of the green outdoor storage grit salt bin is made with a UV stable blue pigment and an unstable yellow pigment in the formulation. It results in that all the yellow dissappeared within months and the outside bin turns blue.

Therefore a masterbatch needs to have a high lightfastness, high weatherability, and good heat stability for outdoor applications:



  • The colorants ability to withstand exposure to UV-radiation with change of appearance and is measured in a scale from 1 to 8 and is evaluated against a blue wool scale.
  • The scale measures the different in colour comparison with time of exposure.
  • Normally tested with artificial light in accelerated tests ex. WOM test.
  • The standard of this test is the weather conditions in Florida.


  • The colorants ability to withstand colour change, depending on environmental stress such as: visible light, UV light, oxygen, temperature, moisture, atmospherical pollutions (such as sulfur-dioxide or acid rain), and ozone.
  • Tests are normally made in an outdoor test area, but can also be tested in a WOM.

Heat stability

  • The temperature that a pigment colour can withstand for five minutes without a big colour change.
  • This is tested in an injection moulding machine with a residence time of five minutes. Then a colour plate is moulded and the colour deviation is measured.

Rotomoulding parts, made with compounded colours gives you a product with good mechanical properties with stable colours.

Schulman is a reliable raw material supplier, providing a consistent quality compounded colour, only using high UV polymers with fully stabilized masterbatch, specially developed for your application.

With in-house knowledge from our masterbatch division, we can offer you the exclusivity you’re looking for.

For more information, please contact your Account Manager or Angélique Brocatus-Kooijmans, Business Development Manager Powders, or visit our website

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