Question: We are having issues with bleeding/leeching of our dye/powder blend. We have run temperature tests and time tests to see if this would make a difference and only saw small changes to the bleeding. I am wondering if anyone has a solution?
Dr. Nick: Your first port of call really needs to be with your pigment supplier.
If this problem has surfaced after previously not having issues, you need to work together to identify what has changed.
If this problem has surfaced with a new color, you must consider whether the pigment mix can be re-formulated to reduce the issue.
Bear in mind that a typical custom color will need 4 or 5 individual pigment components, in order to achieve an accurate match. Any one of these components may have a greater tendency to leach out of the plastic than the others. The color of the bleed-out should give you an indication of which component is at fault.
Good performance in other molding processes (eg injection and blow molding) is unfortunately not a guarantee of adequate performance in rotomolding, which is a zero shear process.
Bleed-out of pigment in dry blends for rotomolding can be due to a number of different factors:
- Excessive pigment loading. The so-called “Old Rule” of pigment blending indicates a maximum addition rate of 100g pigment per 100 lb resin; this equates to a pigment addition rate of approx. 0.45% by weight. In many situations, this addition rate is excessive. Generally, any pigment loading in excess of 0.3% by weight is likely to become problematic, for a number of reasons.
- Pigment type. Different pigments will have different wetting characteristics and therefore different propensities to bleed out. Potentially problematic pigments include phthalocyanine blue and green, ultramarine blue, carbon black, along with many organic pigments (basically reds and yellows).
- Pigment format. The same basic pigment may be available in different particle sizes from different manufacturers and some pigments contain coatings, dispersion aids and other additives. Cheaper pigments tend to be contaminated with trace elements, which may also promote bleeding.
- Poor dispersion. Most pigments can be dispersed adequately by relatively low shear blending, but some exhibit a tendency to agglomerate. Increased shear mixing may help with this, but bear in mind that excessive temperature rise in the mix can create color shade changes. You may also get bleed-out on the blades of the blender.
Adding dispersants to the pigment mix can help; I have heard of zinc stearate being used for this purpose. However, be very cautious with this reagent; it can also promote serious warpage problems. I have also heard of polyethylene waxes being added as dispersion aids, but I have no direct experience of this myself.
It makes a lot of sense to test new pigment blends for their bleed-out characteristics, with a simple rub-off procedure on a rotomolded part. It’s best to leave the part for 24 hours after molding before testing.
Spray an approx. 5” x 5” area on the inside surface of the part with a household cleaner. Rub the sprayed area briskly with a white tissue. Even a non-bleeding pigment may show slight traces of color on the tissue, from the inside surface rub.
Now repeat the procedure on the outside surface of the part (side in contact with the mold). Any signs of color on the tissue after rubbing will indicate significant bleed-off issues.
Try to standardize every element of the procedure (eg type of cleaner, how many squirts per test, time between spraying and rubbing).
If you set up such a protocol internally and test all your current pigments with it (both problematic and non-problematic), you may start to observe trends that can help you avoid problems in the future.
Dr. Nick Henwood is the Technical Director for the Association of Rotational Molders. He has 30+ years of experience in rotomolding, specializing in materials development and process control. In 2022 he was inducted into the Rotational Molding Hall of Fame. He operates as a consultant, researcher and educator through his own company, Rotomotive Limited, based in UK.
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