Theories for Stain Resistance

Discoloration of a textile by a stain involves the same physical-chemical interactions which govern the sorption of a textile dye by fibers. Hence, staining can be described as an unwanted dyeing process. Consequently, the objective of a stain-resist agent is to prevent dyeing of fibers by any colored species which may con- 

Dyeing of fibers occurs in two steps (1) adsorption of the dye on fibers and (2) diffusion of the dye into fibers. Hence, the staining of carpet can be prevented by (1) hindering adsorption of the staining colorant on fibers and (2) retarding its diffusion in fibers. Obviously, if adsorption on fibers can be prevented, a diffusion into the interior of fibers cannot occur. 

The first defense line against staining is the fluoiinated soil retardant (see Chapter 13) which reduces soiling with particulate soil and hinders wetting of fibers. When a liquid is spilled on the carpet, the fluorinated repellent provides time for the removal of the liquid by blotting before the liquid wicks into the carpet. However, if the liquid is allowed to wick into the carpet, a stain-resist agent in the fiber surface or inside the fibers must prevent staining. 

Because the synthetic dyes used in foods and beverages are bright colors, leaving a visible discoloration on a textile, the stain-resist drive focused first on synthetic food colors. A red dye, FD C Red 40 found in beverages, became the standard test substance. 

Most synthetic food dyes contain sulfonic acid groups (see Section 14.3), like the acid (anionic) dyes used to dye nylon. It is well known that acid dyes are sorbed onto nylon in an acid medium. The sorption of the dye decreases with increasing pH and drops abruptly at a neutral pH [28]. In an acid medium, the amine groups in nylon are protonated and form a cationic site for the acid dye [29]. The 

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