Often people discuss their concerns with Dyehouse SolutionsInternational; this month's question comes from a number of staff at a Reactivedyestuff manufacturer in India.


"We have received complaints from some ofour dealers about metamerism for some of our dye combinations. We understandwhat that is. But they also complain about phototropism. What is phototropism?And how can we prevent it?"


This article seeks to answer the queries of themanufacturers.


The Answer


There is some confusion within the textile industry concerninga number of terms which include:

  • phototropism
  • photochromism
  • dichroism
  • metamerism


For example, it is difficult to see how or why a dyestuffdealer should be complaining about, let alone seeing, phototropism. Furtherexamples of confusion follow. For example also, whilst metamerism with respectto changes in hue in different illuminants (light sources) is generally wellunderstood; there are other kinds of metamerism.


What is at issue here is not simply a question of "AcademicPrecision".


Before a problem can be solved, it must be correctlydiagnosed.

Our Epilogue has the final word on this.


Phototropism


We believe this is a phenomenon confined to reddish yellowdyes. Some reddish yellow dyes change shade on exposure to light. This appliesto dyes in aqueous solution, not on the fibre. It is believed to involve"Enol-Keto Tautomerism". The dye molecule can flip from one form tothe other by absorbing energy from light.


For this reason, when conducting optical studies on anyreddish yellow dye, the solution should always be stored in a dark cupboard forat least 4 hours after dissolution, and before spectrophotometric measurement.

We know of no such recommendation before spectrophotomericmeasurement of dyed substrates.


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About the Author


DavidParkes is a resourceful, innovative, result-oriented and self-motivatedTechnical & Marketing Consultant with a proven reputation as a worldleading authority on reactive dyestuffs. He has a career spanning over 40years, most of which were spent at ICI/Zeneca Colours, in various leadingcapacities. He has several pioneering efforts to his credit, including"Customer Improvement Audits." In his role as a consultant, he hasworked closely with many Asian industries, and has helped them improve theirprocesses and products immensely.