Soil redeposition

The term redeposition has already been used several times. Because most cleaning processes are batch processes, there will always exist the possibility that soils removed from the substrate will be redeposited onto the surface as a result of a lack of colloidal stability in the dispersed soils. For oily soils removed by solubilization, the process is thermodynamically driven so that it is essentially a one-way street and redeposition will be minimal. Solid soils, on the other hand, cannot be solubilized and redeposition must be retarded by other kinetically controlled means. Emulsified oily soils, where they occur, must be handled similarly. 

As pointed out earlier, one major role of surfactants at solid interfaces is to impart a degree of colloidal stability to finely divided particles in aqueous solutions. The adsorption of ionic surfactants at the soil-water and substrate-water interfaces produces an electrical double layer that retards the mutual approach of 

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Synthetic fabrics can also be finished to achieve a number of specific characteristics (199). For example, increased electrical conductivity can improve the antistatic character of polyester. Similarly, finishes that improve hydrophilic character also improve properties related to soil release and soil redeposition (199,200). 

Various hydrophilic finishes have been used in connection with soil release and soil redeposition during laundering. These finishes can impart a degree of antistatic protection to the treated materials however, the main functions of these finishes are to release soil and to prevent the redeposition during laundering. 

Excellent stability and suspendabi1ity are provided to the highly-alkaline formulations by a combination of Acusol 810 and Acusol 820 stabilizers. Both formulations can be pumped with a peristalic pump. Acusol 445 low-molecular-weight poly-acylic acid improves overall cleaning, prevents soil redeposition and, in the slurry formulations, aid in the precipitation and dispersion of the hydrated STPP. 

Tt is often desirable to increase the surface free energy of a synthetic polymer to alleviate such processing or end-use problems as static buildup, poor wettability, dyability, printability, or adherability, and poor stain-release or soil redeposition properties in textile applications. One method for accomplishing this is to graft copolymerize hydrophilic monomers onto the polymer molecules in the surface using ionizing radia-... 

Antiredeposition Polymers. The surfactants employed in synthetic detergents are effective in removing soil and stains from the surface of the washed fabric. Under heavy soil loading, there is a tendency for these soils to redeposit on the fabric. Various charged polymers have been found to prevent soil redeposition. Sodium carboxymethylcellulose (NaCMC) is effective in reducing soil redeposition at a formulation level of 0.1-0.5 percent. 

In addition to the desired repellency effects, other undesirable fabric properties are often found with repellent finishes. These include problems with static electricity, poor soil removal in aqueous laundering, stiffer fabric hand, greying (soil redeposition) during aqueous laundering and increased flammability. Some fabric properties that are often improved by repellent finishes include better durable press properties, more rapid drying and ironing, and increased resistance to acids, bases and other chemicals. Table 6.1 shows typical textile applications for repellent fabrics and their requirements. 

Other fabric properties that are enhanced by soil-release finishes include protection from soil redeposition during laundering and absorbency or transport of liquid water. Fabrics with increased absorbency provide garments that feel more comfortable under hot, humid conditions, thus leading the name comfort finish that has often been applied to fabrics treated with soil-release agents. 

AATCC Test Method 151 Soil redeposition, resistance to launder-omeler method, AATCC Technical Manual, American Association of Textile Chemists and Colorists, Research Triangle Park, NC, 1999, 267-268. 

CMC is used primarily in surfactants and PVP is used as bath additive. Further, CMC also forms a protective colloid with the soil and thus prevents soil redeposition on the cloth. The emulsion of oil and dirt should be stable, otherwise there will be redeposition. Ordinary soap and detergent is a good emulsifying agent. 

FIG. 8.24 Clay/oily soil redeposition of typical HDLDs. A, B, C, and D represent commercial liquid detergents, ranging from low-cost to premium brands, with and without the addition of a low-molecular-weight polyacrylate (pAA) homopolymer. 

PVP is a nonionic water-soluble polymer that interacts with water-soluble dyes to form water-soluble complexes with less fabric substantivity than the free dye. Additionally, PVP inhibits soil redeposition and is particularly effective with synthetic fibers and synthetic cotton blends. The polymer comprises hydrophilic, dipolar imido groups in conjunction with hydrophobic, apolar methylene and methine groups. The combination of dipolar and amphiphilic character make PVP soluble in water and organic solvents such as alcohols and partially halogenated alkanes, and will complex a variety of polarizable and acidic compounds. PVP is particularly effective with blue dyes and not as effective with acid red dyes. 

Cationic surfactants cause deposition of detergent residues loaded with soil, which are present in the liquor because of incomplete rinsing [1], Some alteration of the fabric appearance may occur even in the absence of softener. Some soil redeposition on clothes may indeed take place in the wash, resulting in lightly soiled whites turning gray or yellowish and colors to become dull [35],... 

Soils present in the washing liquor decrease the amount of DHTDMAC that deposits onto fibers, since DHTDMAC also adsorbs onto the soil particles, which are negatively charged. As a result, the softening efficacy is impaired. Furthermore, because of quaternary adsorption, the negative charges on the soil particles and on the fabrics disappear, favoring soil redeposition. 

Since the adsorption of nonionics onto soil or substrate does not significantly increase its electrical potential at the Stem layer, this mechanism of soil removal is probably not a major one for nonionics, and nonionics are generally not as effective as anionics for the removal of particulate soil (Albin, 1973). On the other hand, they appear to be very effective for producing steric barriers (see below) for the prevention of soil redeposition. 

Oily Soil Nonpolar soil has been found to be removed from hydrophobic substrates (e.g., polyester) more effectively by POE nonionics than by anionics (Fort, 1968 McGuire, 1975), and investigations of this type of soil removal have concentrated on the use of POE nonionics. POE nonionics have also been found (Rutkowski, 1971) to remove oily soils and prevent their redeposition at lower bath concentrations than anionics (i.e., nonionic surfactants are more efficient for these purposes than anionics). The greater efficiency of nonionics in soil removal is presumably due to their lower CMCs in the prevention of soil redeposition it is probably due to their greater surface coverage per molecule when adsorbed on substrate and soil. 

Nonionics have been shown also to be more effective than ionics in the removal of oily soil from relatively nonpolar substrates (polyester, nylon). On cotton, however, a relatively hydrophilic fiber, anionics can outperform nonionics in detergency, and both of these are superior to cationics (Fort, 1968). The effects here may be due to differences in the orientation of adsorption of the different types of surfactants on the different substrates. On nonpolar substrates and soils, POE nonionics are adsorbed (Chapter 2) from aqueous solution via dispersion forces or hydrophobic bonding with their hydrophobic POE groups oriented toward the adsorbent and their hydrophilic POE groups toward the bath. Adsorption of the surfactant in this fashion on the substrate lowers the substrate-bath interfacial tension jSB and facilitates soil removal (equation 10.3) adsorption in this fashion on both substrate and soil produces a steric barrier that inhibits soil redeposition. 

Synonyms Vinylpyrrolidone/acrylic acid copolymer Uses Surface-act. anti-soil redeposition agent in laundry detergents stabilizer for colloidal disps. 

In general, nonpolar (oily) soil is removed from hydrophobic substrates by polyethylene oxide nonionics better than by anionics. Also, nonionics are found to remove oily soils better than anionics at lower temperatures, due to lower CMC, and are generally better at preventing soil redeposition because of the greater surface area covered per molecule when adsorbed on substrate or soil. ... 

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