You've worked hard to fine-tune your dyeing and finishing process, then someone brings you a finished sample, and there is bad news: the shade is dull and yellow.
Yellowing can be one of the most frustrating problems in the textile manufacturing process. In order to understand how to counteract and control yellowing, let's first look at what causes it:
Often, mills will heat-set their fabrics to dimensionally stabilize them by increasing the crystallinity and, in turn, decreasing the number of amorphous regions of the fiber. This prevents shrinkage and distortion in subsequent processing by the mill or the consumer, but can sometimes lead to yellowing and discoloration.
Certain fabrics, especially nylon and nylon/spandex blends, are very sensitive to oxidative damage. When these fabrics are exposed to elevated temperatures, such as during heat-setting or dyeing, the fibers or other chemistry on their surfaces (e.g. softeners, knitting aids, etc.) can oxidize. This oxidation will lead to unwanted discoloration of the fabric, and can be especially detrimental to bright and/or light colored fabrics.
The first thing to examine if you find yourself with a discoloration issue is the heat exposure the fabric is seeing. Can you reduce the time and/or temperature of your heat-setting process and still achieve the desired results? If so, this is the easiest, and least expensive way to solve the issue. Heat-setting times can vary based on a number of factors, including fabric construction, yarn denier, and fiber type, but below is a table of general guidelines. If your process is well outside of these parameters, it might be worth considering a change.
Oxidation and yellowing can still occur during a properly controlled heat-setting process. In this case, there are certain chemistries that can be applied to the fabric to prevent discoloration. Antioxidants work by "sacrificing" themselves. These readily oxidizable compounds get oxidized instead of the fabric surface, preventing discoloration. Antioxidants have added benefits in addition to reducing yellowing. Since fewer dye sites are oxidized when heat-setting with an antioxidant, there are more free dye sites available for dyes. This leads to richer, deeper colors. The picture below illustrates the dramatic effect antioxidants can have on the final color of a fabric. In the first picture, the fabric, a nylon/spandex blend, was treated with Pomoco Antioxidant ADH and then heat-set. In the second picture, no antioxidant was used.
Discoloration and yellowing can be one of the most frustrating things for a dyer. However, with proper process controls combined with innovative chemistry, yellowing and discoloration can almost always be controlled. If you are experiencing yellowing in your process, we stand ready to help. Contact us today so we can get to work to resolve your issues.