TPP stands for Thermal Protective Performance. The TPP rating of a fabric or composite refers to its thermal insulation characteristics when protecting the wearer from fire. TPP is measured using a combination of flame and radiant heat sources with a heat flux of 2 cal/cm2-sec. The flame is applied to the outer surface of a four-inch-square area of the fabric or composite. The time required to reach the equivalent of a second-degree burn at the copper calorimeter on the other side of the sample is recorded. This time (in seconds), multiplied by the heat flux of the exposure, gives the TPP rating. The higher the TPP rating, the more thermal protection a fabric or composite provides the wearer.
Thermal Damage Tolerance is a qualitative measure of the stability of a fabric after flame exposure. After a fabric is exposed to flame, does the material break open, form a hard char, or remain flexible? DuPont™ Nomex® fabrics will form a flexible char when exposed to flame; upon cooling, the char will harden. Fabrics with high levels of DuPont™ Kevlar® remain flexible after flame exposure.
Nomex® IIIA is a blend of 93% Nomex® with 5% Kevlar® added for flame break-open resistance and 2% P140 added for improved anti-static performance.
Nomex® with Kevlar® represents the family of outer shells made by manufacturers that contain blends of high levels of Kevlar® (50-60%). The higher Kevlar® content helps increase the cut resistance and thermal damage tolerance of these fabrics. Some manufacturers are now using Kevlar® filament yarn, the same form used in body armor, to help further strengthen fabric performance.
Filament yarns are continuous strands of fiber (as opposed to spun yarns, which are made up of short pieces of crimped fiber that are twisted together). Filament fibers tend to be stronger and have lower friction properties than spun yarns. Filament yarns are used in high-strength outer shells and low-friction inner liner fabrics.
Producer-colored fibers are colored during the fiber-making process, while piece-dyed fabrics are colored after the fabric is woven together. Producer-colored fibers generally have better color stability to light and heat exposures. Piece-dyed products tend to come in a wider variety of shades.