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  • Color difference

    Posted by crisbaysauli on February 20, 2015 at 12:37 am

    Hello colleagues. I am developing a balm type whitening cream that uses iron oxides as colorant. After multiple trials of the same formula, I sometimes obtain a little variation in color. What can be the cause of this?

    Chemist77 replied 9 years, 4 months ago 4 Members · 7 Replies
  • 7 Replies
  • belassi

    February 20, 2015 at 2:15 am

    Iron oxide pigments are so strong in colouring power that I suspect it’s simply a measuring problem.

  • Bobzchemist

    February 20, 2015 at 7:48 pm

    Unlike dyes, which produce the same color every time the same amount of dye from the same lot is added to a batch, iron oxides are pigments, which are not soluble, so the color comes from suspended oxide particles, rather than from solutions of individual dye molecules.

    The intensity of the color of an iron oxide suspension depends largely on the number of iron oxide particles suspended in the product. For any given weight of iron oxide pigment, the color will intensify as the iron oxide particles are ground finer and made more numerous.
    The issue is complicated by the fact that iron oxide particles agglomerate (stick to each other) readily, so not only do you have to grind/de-agglomerate the particles, you also have to make sure that they do not re-agglomerate in your product. 
    Pigment wetting can also be an issue - a pigment particle that is completely wetted will appear darker/more intense than one that still has air bubbles trapped on its surface.
    In order to get consistent batch-to-batch color using iron oxide pigments, you have to do four things:
    1) weigh your pigments very precisely
    2) grind/mill/homogenize them into your batch exactly the same way every time
    3) Use the right amount of wetting agents, suspending agents and anti-agglomeration ingredients
    4) adjust your pigment amounts every time you get a new lot of pigments
  • crisbaysauli

    February 24, 2015 at 4:55 am

    @Belassi maybe this is one factor, since our weighing scales have a precision of 0.005 g. I’ll have to check on that.

    @Bobzchemist is it acceptable to establish a Light-Standard-Dark shade? 
  • Bobzchemist

    February 24, 2015 at 6:55 pm

    I’m sorry, but I don’t quite understand. Are you talking about QC standards?

  • crisbaysauli

    February 27, 2015 at 1:57 am

    Yes @Bobzchemist. Like in labels and packaging, they establish a LSD standard. Can this also be applied to bulk products?

  • Bobzchemist

    February 27, 2015 at 3:15 pm

    Yes, absolutely. This is typical in situations where products are not being matched with a colorimeter, and also where the QC department does not really have trained color evaluators.

    In this kind of situation, you have to allow for the fact that the untrained eye can’t tell when a match is close enough. The procedure I’ve used in the past uses microscope slides. You put 1 drop of each standard on its own slide. Then, you put a drop of the batch on both sides of each of the standards (this eliminates the confusion of “is the standard on the right or the left” - the standard is always in the middle) Then, you cover each of the slides with another slide, and evaluate the color - if the batch is darker than the dark standard, or lighter than the light standard, it fails.
    With enough experience, color evaluators won’t need the dark and light standards anymore - but in some cases, acquiring this level of experience takes years.
  • Chemist77

    February 28, 2015 at 5:25 am

    @Bobzchemist Absolutely agree with you as I have seen in our lab where it takes years to develop that special ‘color eye’. 

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