Outer shell materials used in the production of today's protective clothing are blends of very high temperature-resistant fibers, such as KEVLAR® and PBI®. These inherently flame- and heat-resistant blends have the ability to self-extinguish when the source of ignition is removed. However, these highly engineered outer shell fabrics are extremely difficult to dye and the dyestuff used to color the material is simply not able to withstand the same high temperatures as the fibers. Dyestuff will also react to exposure to ultraviolet rays, found in both natural and artificial light, causing the material to fade or to exhibit color change.
When an outer shell shows signs of discoloration, generally what has occurred is dye sublimation. It has been our experience, confirmed by TenCate, that the color change to the outer shell material is generally indicative of either exposure to ultraviolet light or as the result of a thermal exposure. Laboratory tests have shown the dyestuff will normally begin to exhibit change in temperatures somewhere in the vicinity of 450°F, with or without direct flame impingement.
Discoloration by itself is not necessarily cause for alarm, since the heat and flame characteristics are inherent to the material and cannot be washed off or worn out. However, any material should be checked for continued tensile strength whenever discoloration is noted. This can be accomplished by using manual pressure to pull and poke at the discolored areas. If the fabric can be torn, it should be considered too weak for service and immediately removed from the field.
If any outer shell material has been exposed to enough heat to cause discoloration, then the liner system must also be carefully checked, especially the moisture barrier layer which is located closer to the outer shell. We recommend that if the outer shell has become discolored, the moisture barrier should be evaluated using the hydrostatic water test method.