Do you know what Molten Colour is?

FAQ:”A client is asking me to measure something they call “Molten Colour’. Do you have any information on this? Do Hunterlab instrument have a function for this colour?”

“Molten Colour” is not a defined colour scale but refers to melting chemical crystalline or powder material such as Maleic or Phthalic Anhydride, into a liquid at an elevated temperature in the 140 – 250 C range.

Industrial Standard References for Molten Colour Applications

APHA/Pt-Co Colour measurement is a colour quality attribute of BPA Bisphenol A, Cresylic Acids, Maleic and Phthalic Anhydride; Cresylic Acids and aromatic hydrocarbons prepared by heating to a liquid form.

• ASTM D1686 Test Method for Colour of Solid Aromatic Hydrocarbons and Related Materials in the Molten State (Platinum-Cobalt Scale)
• ASTM D3366 Standard Test Method for Colour of Maleic Anhydride and Phthalic Anhydride in the Molten State and After Heating (Platinum-Cobalt Scale)
• ASTM D3627 Standard Test Method for Colour of Cresylic Acids (“C” Series Standards)
• ASTM D4789 Standard Test Method for Solution Colour of Bisphenol A (4,4´-Isopropylidenediphenol)

ASTM – American Society of Testing and Materials, West Conshohocken, PA USA www.astm.org

After preparation of the hot liquid sample as shown in HunterLab application note, AN 1030 Hot Liquids, liquid is measured in total transmission using CIE L*, a*, b* D65/10 colour values as a full colour descriptor, or typically using a specialized yellowness scale such as APHA/Pt-Co/Hazen Colour, Yellowness Index per ASTM E313, or Gardner Colour.

The higher the yellowness; the greater the presence of impurities. Typical APHA/Pt-Co/Hazen colour values are between 0 – 100 for these products.

FAQ: “Colour instruments measure colour in reflectance or transmission. Why not just measure the colour of the Maleic or Phthalic Anhydride in the crystalline or powder form?”

Impurities related to processing, storage or shipping result in absorbance in the blue region of the visible spectrum and visually show as trace amounts of yellowness. These small amounts of yellowness are visually lost in the light scattering from the crystalline or powder forms.

By melting the crystals or powder all this “optical noise” is removed. Standardization on cell or vial in transmission negates the effects of the cell and solvent leaving just the colour of the impurities to be measured by a specialized yellowness metric.