Aramid fibers are made from high molecular weight polymers that are highly crystalline and have either a high or no glass transition temperature. They are highly resistant to heat decomposition, have inherent flame resistant properties and are frequently used in apparel and in protective clothing for special environments where their flame resistant properties are required. Fabrics made of aramid fibers for the protection of military personnel must meet minimum strength requirements as defined in MIL-C-83429A.
Processing and/or dyeing certain aramid fabrics, particularly with highly polar organic solvents, degrades the mechanical properties of these fabrics. Aramid fibers are available from the fiber producers as solution-dyed aramid yarn which is prepared by solution dyeing, in which a quantity of dye or pigment is mixed with the molten resin prior to extrusion of the resin into fine fibers. As an alternative to solution-dyed, manufacturer-supplied aramid yarns, fiber suppliers recommend a complicated exhaust dyeing procedure with a high carrier (acetophenone) content. A more recent aramid dyeing procedure is described in U.S. Pat. No. 4,525,168 in which acid or anionic dyes are introduced into aramid fibers by coupling the dye to a dye site receptor which, in turn, is attached to the fiber. The fiber is first swollen in a strong polar solvent and, while in the swollen condition, a substance capable of forming a strong chemical bond with the anionic dye is introduced.
Another procedure for dyeing specific aramids, namely poly(m-phenyleneisophthalamide) fibers, is described in commonly-assigned Cates et al U.S. Pat. No. 4,710,200. In this procedure, fibers of the meta isomer are swollen in an aqueous dye bath containing with the dye a significant quantity of a polar organic solvent swelling agent such as N-methylpyrrolidone, dimethylsulfoxide (DMSO) or dimethylacetamide.
While care is taken to avoid significant and objectionable loss of fiber strength, the polar organic solvents used in the dyeing procedures described above are notorious for damaging the fiber itself. Our invention restores fiber tearing strength and increases fiber breaking strength for dyed aramids up to 45%. Similarly, it is useful to increase the strength of undyed or greige aramid fibers to further enhance their already substantial tearing strength and breaking strength. Our invention increases tearing strength of greige aramid fabrics as well.