Bleach-Active Compounds

In document 23550908 Laundry Detergents (Page 86-91)

3. Detergent Ingredients

3.3.1. Bleach-Active Compounds

The dominant bleaches in Europe and many other regions of the world are of the peroxide variety. Hydrogen peroxide is converted by alkaline medium to the active intermediate hydrogen peroxide anion according to the following equation:

H2O2+ OHÐ H2O + HO2

DetergentIngredients

The perhydroxyl anion oxidizes bleachable soils and stains. The usual sources of hydrogen peroxide are inorganic peroxides and peroxohydrates. The most frequently encountered source is sodium perborate (sodium peroxoborate tetrahydrate, NaBO3

· 4 H2O) [84] , which in crystalline form contains the peroxodiborate anion:

Peroxodiborate is hydrolyzed in water to form hydrogen peroxide. Sodium perborate monohydrate and sodium percarbonate, which dissolve more quickly than perborate tetrahydrate, have been increasingly used either as separate color-safe bleach or in bleach-containing detergents at the expense of sodium perborate tetrahydrate. The monohydrate version has an improved storage stability as compared with the tetrahy-drate version. Sodium perborate monohytetrahy-drate is the preferred ingredient for laundry products in countries having high ambient temperatures because, contrary to perborate tetrahydrate, the monhydrate does not cause caking of powder products upon storing at elevated temperatures for prolonged periods. In contrast to what its name would suggest, sodium perborate monohydrate contains no water of crystallization. Its mo-lecular structure is [85]:

On a per weight basis the monohydrate has a considerably higher content of active oxygen than the tetrahydrate. For this reason, the monohydrate has become the favorite form of perborate in compact detergents.

The use of sodium percarbonate has gained importance in those countries in which boron is either banned or restricted for environmental and regulatory reasons, or negatively discussed. In contrast to sodium perborate, sodium percarbonate is a true peroxohydrate (Na2CO3 · 1.5 H2O2). In order to achieve a good storage stability for the bleach, coated or stabilized percarbonate must be used.

The salts of peroxomono- and peroxodisulfuric acid and peroxomono- and perox-odiphosphoric acid are not significant as detergent bleaches. This results largely from their insufficient bleaching power in wash liquor, either because they are insufficiently hydrolyzed to hydrogen peroxide in an alkaline medium or because their oxidation potential is too low.

Comparison shows that sodium perborate and sodium percarbonate have the most preferred prerequisites for use as a detergent bleach additive. Oxygen bleaches have been reviewed [85] – [88].

The concentration of bleach-active hydrogen peroxide anion increases with pH value and temperature. Sodium perborate exhibits significantly less bleaching efficiency at temperatures below 60 C. Even at low temperatures (i.e., where the reaction equilib-rium is unfavorable), hydrogen peroxide anions are present in the wash water, but show

LaundryDetergents

only modest bleaching power (Fig. 51). The bleaching effect also increases markedly with increasing perborate concentration (Fig. 52) and time.

Many attempts were made in the 1980s to improve the performance of a detergent by using organic peroxy acids, for example, monoperoxyphthalic acid and diperoxy-dodecanedioic acid (DPDDA) salts, as bleach components:

Figure 51. Bleach performance of sodium perborate vs. temperature [89]

Initial perborate tetrahydrate concentration 1.5 g/L = 150 mg of active oxygen/L

DetergentIngredients

Monoperoxyphthalic acid monomagnesium salt

Diperoxydodecanedioic acid sodium salt

With these it is possible to obtain significant bleaching at a temperature as low as 30 C.

However, for several reasons (dye damage and cost effectiveness) a technical and commercial breakthrough with these peroxyacids has not been achieved so far.

Hypochlorite is used for bleaching in many global regions where laundry habits, such as cold water washing, cause sodium perborate to be less effective. In an alkaline medium, hypochlorite bleaches are converted to the hypochlorite anion:

HOCl + OHÐ ClO+ H2O

Hypochlorite bleach chemistry and applications have been reviewed [90] – [92].

Hypochlorite can be used in either the wash or the rinse cycle at concentrations between ca. 50 and 400 mg/L active chlorine (Fig. 53) and a wash liquor ratio (bath ratio) between 1 : 15 and 1 : 30.

Normally, an aqueous solution of sodium hypochlorite (NaOCl) is used as a source of active chlorine. Organic chlorine carriers (e.g., sodium dichloroisocyanurate), which are

Figure 52. Bleach performance of sodium per-borate vs. concentration (red wine soil) [89]

LaundryDetergents

hydrolyzed to hypochlorite in an alkaline medium, are only common in automatic dishwashing detergents. Figure 54 shows a comparison of the bleaching properties of sodium hypochlorite and sodium perborate as a function of temperature and pH value.

One of the major advantages of powdery sodium perborate over liquid sodium hypochlorite is the fact that whereas the latter must be added separately in either the wash or the rinse cycle, perborate can be incorporated directly into a powder laundry product. This results in a mild color-safe bleach. By contrast, successful addition of chlorine bleach solution, whether in a household or an institutional machine, is heavily dependent on experience of the user and on observing the manu-facturer's recommendations. Incorrect dosage of sodium hypochlorite may easily occur, and this may cause significant damage to laundry and colors. A further advantage of sodium perborate is its long shelf life, whereas sodium hypochlorite solutions have limited storage stability, which depends very much on the content of impurities, particularly traces of heavy metal ions. On the other hand, hypochlorite bleaches can be used in both wash and rinse cycles independent of temperature. They provide effective bleaching and disinfection (hygiene) even at very low temperature. However, because of its great reactivity and extraordinarily high oxidation potential, sodium hypochlorite, in contrast to sodium perborate, may cause problems with textile dyes and most fluorescent whitening agents, both of which often show poor stability in the presence of chlorine. Studies of washing and bleaching habits show that peroxide bleach use dominates in Europe, chlorine bleach being used predominantly in Med-iterranean countries. Hypochlorite bleach in either the wash or the rinse cycle is still the preferred bleaching agent in a large part of the world (cf. Table 17). The use of peroxide

Figure 53. Recommended concentrations of active chlorine as a function of wash temperature [93]

a) Maximum concentration of application; b) Minimum concentration of application

“Active or available chlorine” is calculated as twice the actual weight percent of chlorine in the hy-pochlorite molecule

Figure 54. Stain removal comparisons

& Hypochlorite (200 ppm active chlorine) + 0.15 % detergent; * Perborate bleach (28 ppm active oxygen) with protease enzymes + 0.15 % detergent; ~ 0.15 % Detergent only[94]

DetergentIngredients

(“oxygen”) bleach in laundry products has increased significantly in the USA, Japan, and other regions of the world during the 1980s and 1990s.

In document 23550908 Laundry Detergents (Page 86-91)

Related documents