How to: Make a color disperson

Discussion in 'Silver Gelatin Based Emulsion Making & Coating' started by Photo Engineer, Sep 11, 2010.

  1. Photo Engineer

    Photo Engineer Subscriber

    Apr 19, 2005
    Rochester, NY
    Multi Format
    I have been asked numerous times how one can make a dispersion of couplers for making coatings. So, here goes. We will be making a cyan coupler dispersion.

    You will need 1 gram of 2,5 di-octyl, 4 chloro-phenol. This is a 2 equivalent cyan coupler with ballast. It can be t-octyl or even decyl, the ballast is not strictly important at this point.

    You will need 1 gram of either tri cresyl phosphate or N,N-diethyl Lauramide. This is the primary coupler solvent.

    You will need some ethyl acetate.

    In a beaker, mix 1 gram of the coupler with 1 gram of the primary coupler solvent and heat on a water bath to just below the boiling point of ethyl acetate ( less than 77 C) and add ethyl acetate until the coupler has dissolved.

    In a separate beaker prepare and melt 10 g of 250 BI photograde gelatin in 90 g of distilled water at 40 C. Add to this about 2 - 3 drops of 10% Alkanol XC surfactant.

    Using a homogenizer ( - we used Polytron units -, begin stirring the gelatin mixture while adding the coupler / solvent mixture and while ramping up the speed of the homogenizer. Make sure that the homogenizer is under the surface of the gelatin and is not whipping in any air. The result should look like cream and be of that consistency. When there are no oil drops left, rinse the beaker of coupler/solvent with the gelatin dispersion and continue mixing for a few more moments so as to not waste any coupler.

    Now, chill set an noodle the dispersion, then wash it until all odor of ethyl acetate is left. Wring out the washed dispersion and weigh. This is the weight to base the coupler percentage on. If it were just 100 grams, this would be a 1% coupler dispersion in 10% gelatin. If coated as is, you would be coating about 100 mg/ft square of coupler and 1000 mg/ft square of gelatin if you used a 5 mil undercut blade.

    The idea is to mix this 1:1 with an emulsion, and then test it. Coated with a 5 mil blade this will be about 50 mg/ft square of coupler and should give a good dmax on paper. At 7 mil, this should give a good dmax on film.

    You have made and tested a 1:1 cyan coupler dispersion. The primary coupler solvent may be either one of the ones mentioned above. There are many others, but these are most common.

    The key to this is the use of a homogenizer. Next up will be making non solvent gelatin mixtures for coating using "Fischer" couplers or Azo dyes.

    NOTE: Ethyl acetate is very flammable. Take precautions when using it to prevent fires. This includes the wash step in which the water solution of ethyl acetate is being washed down the sink.

    Last edited by a moderator: Sep 11, 2010