Color Change to Outer ShellOuter 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 high temperature blends are flame and heat resistant and have the ability to self extinguish when the source of ignition is removed. However, these Outer Shell fabrics are 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 by causing the material to fade, or to exhibit color change.
When an Outer Shell shows signs of discoloration, generally what has occurred is a condition known as dye sublimation. It has been our experience, confirmed by TenCate, the producers of the majority of Shell fabrics, that the color change to the Outer Shell material is generally indicative of a thermal exposure. Laboratory tests have shown that the dyestuff will normally begin to exhibit change at temperatures in the vicinity of 450°F, with or without direct flame impingement.
As a general rule of thumb, the discoloration by itself is not 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 (1) laundering the shell in a washing machine that has an agitator, where the mechanical action of the machine will cause any extraordinarily weak fibers to tear, and (2) 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. Since the Moisture Barrier layer is located closer to the Outer Shell, it is imperative that it be thoroughly inspected. Many of the films used in the production of Moisture Barrier fabrics will degrade at lower temperatures than the Outer Shells and we recommend that if the Shell has become discolored, the Moisture Barrier should be evaluated using the cup test to ensure that there has been no loss of water, chemical and/or bloodborne pathogen resistance. The cup test can be found in our Basic Inspection Guidelines under Moisture Barriers.