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

"We have received complaints from some of our dealers about metamerism for some of our dye combinations. We understand what 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 the manufacturers.

The Answer

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

• phototropism
• photochromism
• dichroism
• metamerism

For example, it is difficult to see how or why a dyestuff dealer should be complaining about, let alone seeing, phototropism. Further examples of confusion follow. For example also, whilst metamerism with respect to changes in hue in different illuminants (light sources) is generally well understood; there are other kinds of metamerism.

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

Before a problem can be solved, it must be correctly diagnosed.

Our Epilogue has the final word on this.

Phototropism

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

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

We know of no such recommendation before spectrophotomeric measurement of dyed substrates.

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About the Author

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