There are various recognized methods for dyeing and/or incorporating additives into fibers and other fibrous materials, including methods which utilize organic solvents such as dimethylsulfoxide ("DMSO"), dimethylacetamide ("DMAc"), dimethylformamide ("DMF"), N-methylpyrrolidone ("NMP"), N-cyclohexyl-2-pyrrolidone ("CHP"), acetophenone, acetanilide and methyl benzoate. Exemplary systems utilizing these organic solvents have been proposed in U.S. Pat. Nos. 3,558,267 to Langenfeld, 3,741,719 to Ramanathan et al, 4,525,168 to Kelly, and U.S. Pat. No. 4,898,596 to Riggins et al and U.K. Patent Nos. 1,275,459 to Gruen and 1,282,113 to Burtonshaw et al. In addition to these organic solvents it has been proposed in U.S. Pat. No. 4,780,105 to White et al to dye aromatic polyamides with a cationic dye and a dye assistant including N-substituted phthalimides (e.g., a mixture of N-butylphthalimide and isopropylphthalimide).
Despite the general availability of dyeing methods using various carriers, there continues to be a desire for process improvements which enhance the dyeability of fibers and fabrics formed therefrom. For example, improvements in dye exhaustion and more uniform dyeing tend to improve the aesthetic characteristics of the fibers. Another example is that certain dyeing techniques result in the undesirable side effect of retention of solvent by the fibers. Retention of solvent can adversely affect both the appearance and the flame resistance of the fibers. This is particularly frustrating when dyeing fibers such as the aromatic polyamides, i.e., poly(m-phenyleneisophthalamide) ("Nomex.RTM.") and poly(p-phenyleneterephthalamide) ("Kevlar.RTM."), polybenzimidazoles, aromatic polyimides, and the like which have excellent flame resistant properties but also are difficult to dye. In order to dye these fibers, some of the various organic solvents and carriers, e.g., DMAc, DMSO, DMF, acetophenone, etc., and the N-substituted phthalimides have to be used, but these compounds may present health problems for some employees, and if non-water soluble, also tend to be retained by the fibers after dyeing. Thus, prior art techniques for dyeing these fibers can be counter-productive in that one of the problems caused by retention of solvent or carrier, e.g., decreased flame resistance, requires addition of a significantly large portion of a flame retardant to compensate for the reduction in the flame resistant properties of the fibers.
It is therefore an object of the present invention to provide an improved method for dyeing fibrous materials, and particularly to provide a method which provides one or more particularly advantageous results, including increased efficiency in dye exhaustion, more uniform dyeing and reduction of the retention of solvent. The improved method is particularly advantageous when used with difficult to dye fibers such as aromatic polyamide fibers including Nomex.RTM. and Kevlar.RTM. fibers, polybenzimidazole fibers, aromatic polyimide fibers, fibers of copolymers of the monomers thereof and blends thereof.