Thermochromism: The Effects of Temperature on Pigments and Dyes
During manufacturing and production processes, things can get pretty heated - literally. Using heat or high temperature is common practice in plant operations to manufacture a product. Elevated temperature, however, can affect a product's physical appearance, particularly color. This color change from temperature, called thermochromism, is problematic for plastic, coating, and textile applications during quality control inspections.
A pigment or dye's reaction to high temperature depends on its chemical properties and material. Too hot of temperature or long exposure to high temperature may completely degrade the color of an object. In other instances, an object or sample drifts to another shade of color until it stabilizes back to room temperature. Certain colors are more susceptible to this drift, depending on the application.
In the textiles industry, for example, a textile material goes through a dye and dry process that often shifts its color temporarily. After being taken out of the dye bath, textiles are rinsed, finished, and then placed in a high temperature oven to dry. The sample is then removed from the oven after a standard drying time and set aside to condition, or cool down to ambient temperature. However, manufacturers may notice a change in the textile color during this time. A fabric dyed blue, for example, may drift to a reddish hue while it's still hot or warm from the oven and then slowly adjust to its correct blue color after stabilizing to room temperature. Because of this temporary drift in color, evaluating the sample against the standard during this "cool down" stage will deliver misleading results.
Quality control and process control procedures are required to ensure a product meets the defined color standard through an efficient process. An outline of these procedures should define how the product is to be evaluated, when and how often it should be inspected, and steps to be taken for troubleshooting color inconsistencies. When dealing with thermochromism, visual and instrumental evaluations should be performed.
After processing, the color of a sample may drift to another shade of color for a certain amount of time, as illustrated in the textile example above. It is recommended to assess the sample at scheduled times and record how long it takes to cool down and adjust to its correct color. Once the sample is stabilized to ambient temperature, visual and instrumental evaluations can be performed more accurately to identify if the sample color meets the standard.
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