Hydrogen peroxide in home-care formulations
By Elena Petrovicova
Detergent formulations for fabric washing must remove a wide variety of stains and soils from different fabric types under a broad range of wash conditions. Principal components of detergent formulations such as surfactants, builders, and enzymes remove primarily oily, greasy, particulate, and proteinacious soils and stains. However efficient, modern detergents still cannot remove some stains effectively without the aid of bleaching systems.
A bleaching system in the context of this article refers to materials that possess useful oxidative properties. Such materials can be included directly in a detergent formulation to remove bleachable stains such as those associated with tea, coffee, red wine, and various fruit and vegetable products. They can also be offered as laundry prewash and additives that have been formulated to boost the stain removal efficacy of detergents.
In recent years, manufacturers have offered a variety of products and regimens for more effective stain removal. Such regimentation of the laundering process, supported by innovation in laundry prewash and in-wash additives, has significantly boosted laundry detergent industry growth and fueled the need for the development of more effective but safe bleaching actives (Fig. 1).
Despite the large amount of research and data that has been generated over several decades and the numerous bleaching agents that have been identified and manufactured, only a few of these agents have made it into consumer household products or gained significant commercial importance.
These are chlorine bleaches, primarily sodium hypochlorite, and peroxygen (oxygen-based) bleaches, including sodium perborate, sodium percarbonate, liquid hydrogen peroxide, and peracid precursors.
Liquid chlorine bleach, or sodium hypochlorite (NaOCl), was the only household bleach available in the mid-1950s. It has remained virtually unchanged over the years, with only minor improvements in quality that have resulted from better manufacturing techniques. NaOCl is still the most commonly and widely used bleach in the United States.
Aqueous solutions of NaOCl are strong bleaching agents and powerful disinfectants. Their low cost and high availability make them affordable in most regions of the world. However, despite their historical dominance and low cost, they have some distinct disadvantages:
- They damage colored, silk, and wool fabrics and are therefore limited to use in white laundry.
- They have a strong odor.
- Accidental spillage or misuse can cause irreparable damage to fabrics.
Bleaching with NaOCl is also the subject of much debate due to the potential generation of chlorinated by-products, which are deemed harmful to the environment and have negative effects on septic tanks.
As a result, peroxygen bleaches are gaining traction not only in laundry but also in many other applications due to a combination of better environmental compatibility and better fabric or surface safety with fewer deleterious effects.
Hydrogen peroxide (HP), H2O2, is a common bleaching agent as well as the starting material for almost all other oxygen-based bleach systems. Its consumption in the United States is currently in the range of 2.2 million tons (2.0 million metric tons). H2O2 is a very weak acid; it is only slightly dissociated (pKa = 11.6), and it is relatively stable in undissociated form.
Sodium perborate (SPB) has been the predominant source of hydrogen peroxide in powder laundry formulations. SPB was first produced at the turn of the last century and has been used in European powder detergent formulations since then. In North America, SPB appeared only in 1980s, with the advent of fast-dissolving SPB monohydrate.
Dissolved in an aqueous medium, sodium perborate, NaBO3. H2O (also available as tri- and tetrahydrate), is hydrolyzed producing HP and sodium borate. It is considered, for the most part, stable and easy to handle and formulate into consumer products. However, it is not very effective below 160°F (71°C) at short US washing cycles lasting only 10–15 minutes. Despite many efforts to increase the activity of SPB to achieve its full performance potential at lower water temperatures, it remains only a fair stain remover under US washing conditions. It is more commonly used in Europe, where washing machines are manufactured with heating coils that can raise water temperature to the boiling point.
Until recently no aqueous liquid detergent formulation on the market contained HP. The situation changed in 1997, with the introduction of liquid bleach boosters containing liquid peroxide. Stabilization of peroxide in these aqueous products is achieved by an acidic pH, which is known to slow peroxide decomposition. In washing processes, the HP is activated by the alkalinity of the detergent. Some newer stabilizer packages make it possible to formulate weak alkaline HP solutions at concentrations up to 6%.
Sodium percarbonate (SPC) is another source for delivery of HP. Unlike SPB, it is not a true peroxygen compound, but rather a perhydrate (Na2CO3·1.5H2O2). The integrity of the adduct is due to bonding between carbonate anions and HP molecules within the crystal. Its rather confusing name has arisen because of historical uncertainties over its structure. In solution, SPC dissociates into sodium carbonate and HP, which further decomposes to molecular oxygen and water, making it an environmentally desirable material. The bleaching performance of SPC is considered, on an equal available oxygen basis, to be similar to SPB.
Peracids are considered to be excellent peroxy-based bleaches, the most important being peroxyacetic acid. However, they have a poor compatibility in formulations and are not very stable in anything other than acidic pHs. Consequently, they require special handling during transportation and storage; thus, their main use is limited to commercial and industrial laundering and bleaching processes.
One possible future source of HP in detergent formulations could be the use of oxidase enzymes, such as glucose oxidase, which uses glucose substrate to generate HP. Patents have already been published using this approach to generate peroxide. The advantage could be the replacement of solid peroxide sources with a substance with greater long-term stability and perhaps more activity than current, alkaline-stable equivalents.
Stain removal and peroxygen bleaching mechanisms
Stains are discolorations caused by intensely colored substances, which even in small amounts can affect the color and appearance of a fabric. Colored stains are usually either vegetable in origin (typically anthocyanin, carotenoid, or porphyrin in structure) or caused by artificial food colorants and cosmetic ingredients. Very often, stains are complex mixtures of food preparations and beverages. The main tasks of bleaches are to remove colored stains from fabric or aqueous solution by decolorizing them either in solution or on the fiber. This is achieved by chemical degradation of the chromophoric units present in colored soils. In the course of reaction, conjugated double bonds, for example, are disrupted so that the color disappears. Large molecules are broken down and polar groups are introduced, so that the stain becomes more hydrophilic and easier to remove or disperse.
Oxidation with peroxygen bleaches involves three reactions:
- Introduction and dissolution of the bleaching agent,
- Bleaching in solution of stains desorbed from the fibers, and
- Bleaching of stains located within the fibers.
Bleaching in solution has been shown to obey pseudo first-order kinetics. However, the kinetics and mechanisms for bleaching of stains residing in fibers are not sufficiently understood. Bleaching kinetics are complicated by the simultaneous existence of two processes: stain removal by nonoxidative detergency and oxidative destruction of stains.
HP-based bleaches have the advantage of being more fabric- and color-safe. They can also be used without seriously diminishing the effectiveness of enzyme products. The high oxidative potential of hypochlorites—the very property that makes them good bleaches—makes them nonselective and capable of oxidizing a broad spectrum of fabric dyes and fading them in the normal wash time.
In alkaline solution, HP dissociates:
H2O2 → H+ + HO2-
Although there is still debate, the generally accepted view today is that perhydroxyl anion, HO2-, a powerful nucleophile, is the most important active bleaching species. For most effective stain removal, pH values >10.5 and temperatures higher than 50°C are required. For use at lower temperatures, longer reaction times (soaking) or the addition of an activator can be employed to improve bleaching.
Activators are used to generate more active bleaching species by mixing acyl compounds with HP and generating peroxyacids. The acyl group, RCO-, is capable of being perhydrolyzed by the perhydroxyl anion from a suitable precursor to form a peroxyacid, RCO3H. Some of the few commercial activators are tetraacetyl ethylene diamine (TAED), nonanoyloxy benzenesulfonic acid sodium salt (NOBS), and lauroyloxy benzenesulfonic acid sodium salt (LOBS). The major barriers for activators into the commercial market are: (i) cost, (ii) the ability to control their bleaching potential due to the damage they might cause to the fabric, and (iii) toxicity, sensitization, and environmental impact.
Bleach catalysts, based on metal complexes, also show high potential as enhancers for peroxide bleaching. Their commercial future remains uncertain until the problem with fabric damage is solved. For further reading on some of these materials refer to the information provided on p. 622.
Factors influencing peroxygen bleaching
In addition to temperature and concentration, other important factors have the potential to improve efficacy, such as pH, surfactant, and builder presence. Depending on the formulation, some bleaches might be used in a well-buffered system; that is, in conjunction with carbonates or silicates, or with surfactants, such as in a heavy-duty powder detergent or powder laundry additives. Builders help retain sufficient alkalinity and pH values greater than 9 in wash water and enhance bleach performance.
In a case of liquid peroxide formulation, a slightly acidic pH provides long-term physical and chemical stability of an HP-containing formulation. However, such pH typically does not provide the most optimal bleaching conditions. Detergent, if used in combination with an HP-containing product, will provide mild alkaline conditions, usually between pH 7 and 9, thus further improving the bleaching performance of HP. The efficient removal of some stains might require pH > 10 and might be achieved by using systems that provide a pH “shift” to a highly alkaline pH range during the bleach application.
Fig. 2 illustrates the effect of washwater pH on removal of some dried-in stains with HP and SPC added along with nonenzymatic detergent. Generally, stain removal increases with increasing pH, and the extent of this effect is stain dependent. That is because pH affects the degree of deprotonation of the stain and with that the degree of susceptibility of the stain to the bleaching agent. Adhesion of the stain molecules to the fabric, the degree of which is fabric dependent (cotton, synthetic, or blend), may change its conformation and its accessibility to attack. Stain removal also depends on the physical state and the location of stain. The bleaching of stain residing within fibers is much more difficult than bleaching in the solution. The bleaching rates are much slower because the bleaching mechanism likely involves diffusion-dependent steps: (i) the stain diffusion from fibers into the bath where it is bleached or (ii) the bleaching agent diffusion into the fibers and bleaching the stain within the fiber.
Taking into consideration fundamental properties and behavior, HP can be formulated effectively within liquid-based cleaning products. For the use in powdered products, SPC or SPB can be used as very stable and useful delivery vehicles for generating the HP bleaching moiety. HP-based bleaches are gentler to colors and fibers, are odorless, and have very low environmental impact. HP is a safe and convenient material that is finding widespread use in household cleaning products.
Elena Petrovicova is research manager at Church & Dwight Co. Her work there involves research and development on new fabric-care products, including detergents, pretreaters, and in-wash boosters. Before coming to Church & Dwight, she worked in a variety of research capacities at Unilever Research, Edgewater, New Jersey, and TRI in Princeton, New Jersey, USA. She can be contacted at Elena.Petrovicova@churchdwight.com.
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