SOAPS AND LAUNDRY PRODUCTS:LAUNDRY AIDS.

LAUNDRY AIDS

In general, laundry aids contribute to the effectiveness of laundry deter- gents and provide unique functions. Laundry aids include builders, bleaches, enzymes, fabric softeners, and optical whiteners.

Builders

Despite the considerable advances made in the production of the active detergent/surfactant chemicals starting during the early 1900s, progress in the use of detergents for heavy-duty (cotton) washing was still relatively slow. By the end of World War II, detergents had already displaced soaps to a considerable extent in the field of fine laundering and dishwashing. However, small amounts of dirt were being redeposited uniformly over the whole surface of clothing items washed in either the washbasin or the washing machine, thus giving the clothing a gray appearance. The breakthrough in the development of detergents for all- purpose laundering came in 1946, with the introduction of the first “built” detergent product in the United States. Originally created by Procter & Gamble in 1943, this detergent was a combination of syn- thetic detergents and “builders.” Soap for cotton washing had been built for many years with various alkaline materials, including carbonates, sili- cates, borax, and orthophosphates. A builder is any substance added to a surfactant to increase the efficiency of detergency. Builders enhance or maintain the cleaning action of the surfactant. The primary function of builders is to reduce water hardness, and this is accomplished either by sequestration or chelation (holding water hardness minerals in solution), by precipitation (forming insoluble precipitates), or by ion exchange (trad- ing electrically charged particles). Thus, because many consumer homes do not have water softeners installed on the premises, most modern laundry detergents soften the water themselves. In general, builders can also supply and maintain alkaline washing conditions, assist in preventing the redeposition of removed soils, and emulsify oil- and grease-based soils.

Essentially water softeners, builders continue to be included in mod- ern domestic laundry detergent formulations to assist the surfactant sys- tem in its action. The two major types of builders include inorganic builders and organic builders. Common inorganic builders, once widely used but now universally banned or legally restricted by many governments of the world, are phosphates, specifically sodium tripolyphosphate (STPP) (Na5P3O10). STPP buffers the washing water to a milder pH than would be obtained otherwise. The building process is accomplished via sequestration; specifically, STPP sequesters calcium and magnesium ions (ions that contribute to the formation of hard water) to form soluble complexes. This action softens the water and provides a mild alkaline environment, both of which result in more favorable water chemistry for optimum detergent action. STPP also allowed clay-type dirt to remain in suspension (a process called deflocculation). These phosphates vastly im- proved detergency performance, making synthetic detergents more suit- able for cleaning heavily soiled laundry (i.e., “heavy-duty” detergency). However, the large amount of phosphates released into the environment via STPP, specifically added to lakes and streams upon discharge, was found to be the leading cause of objectionable algal blooms. Phosphates are excellent nutrients for algae and other small aquatic plants inhabiting such places as lakes and streams. When a large amount of phosphates be- came available, these molecules acted as fertilizers and algal growth was uninhibited, leading to a process called eutrophication (meaning nutrition by chemical means). In short, the algal blooms covered the surface of the water bodies and synthesized large amounts of oxygen via photo- synthesis. However, as algal growth continued, the oxygen produced at the surface escaped into the atmosphere and prevented atmospheric oxy- gen from being obtained by other marine life (e.g., fish). Since algae carry out respiration and cause a net drain on the oxygen supply, the demand, called the biochemical oxygen demand, may exceed the supply and cause the destruction of much aquatic life. The problem may be accentuated as algal blooms block out sunlight required by other photo- synthesizing aquatic plants. In addition, the death of the overwhelmingly high populations of algae over time, with their associated settling to the bottom of the water bodies, allowed for extensive tissue decomposition by bacteria, the emission of obnoxious sulfurous odors, and thus further depletion of oxygen supplies. It was this death of fish and other aquatic animals, often occurring on a large scale in lakes and rivers covered by algae, that prompted environmental action through government legislation at the height of the phosphate controversy in the early 1970s. Ironically, the problem was actually found to be related to the total amount of detergent used by consumers, and the level of aquatic pollution, not the actual phosphate. Unpolluted lakes can readily absorb excess phosphate, as zooplankton will eat the algae that flourish and are themselves eaten by fish. However, the addition of phosphates to polluted waters leads to unchecked/unbalanced algal growth. In mid 1970, the major detergent producers announced plans to replace a large percentage of the phosphate as STPP (20 to 65 percent in detergent formulations) with sodium nitriloacetate (NTA), which also effectively binds water hardness ions and functions as a builder. However, by the end of the same year, NTA had already passed out of favor as a result of the finding that NTA also binds (sequesters) other potentially toxic metals (e.g., cadmium [Cd] and mercury [Hg]) and could have easily released these ions in a location where the results would be quite serious (e.g., across the placental barrier to a developing human fetus). In addition, NTA contains nitrogen, which is also a good fertilizer and nutrient source for algae.

The detergent industry has since offered a variety of phosphate re- placements, which prominently include other inorganic builders such as sodium carbonate, complex aluminosilicates called zeolites, and sodium silicates. As a precipitating builder, sodium carbonate (Na2CO3), or washing soda, acts as a builder by precipitating calcium ions, thus in- creasing the detergency of the surfactant system by softening the water.

However, the calcium carbonate precipitate (CaCO3) can elicit deleterious effects on automatic washing machines. In addition, an excess of carbonate ions within a body of water leads to a highly alkaline (basic; excess of OH- ions) condition, which can cause washed fabrics to be irritating to the skin and eyes. Zeolites are a complex of aluminum, silicon, and oxygen. When added to hard water as part of a synthetic detergent  formulation, the sodium salts of zeolites act like ion-exchange resins, ex- changing their sodium ions for ions within hard water (e.g., calcium). As ion-exchange builders, zeolites soften water and hold hard water ions in suspension (rather than being precipitated), and zeolite solutions are not nearly as alkaline as sodium carbonate solutions. Sodium silicates, also known as “water glass” or soluble silicates (e.g., Na2SiO3, Na2Si2O5, Na4SiO4), are produced from sand and sodium carbonate. As a precipitating builder, they precipitate both magnesium and calcium ions within hard water and also act as a corrosion inhibitor by protecting washing machine die-cast internal frames from rust formation during wash agitation. This builder also increases the effectiveness of the physical deter- gent capacity of powder detergent formulations.

An example of an organic builder included in synthetic detergent formulations is the sodium salt of carboxymethylcellulose (CMC). Al- though a French patent for the use of CMC as an additive to washing materials was thought to have been applied for in 1936, this patent was not developed extensively until World War II, when CMC was used in Germany. Initially used on a moderately large scale as an extender for soap, which was in short supply, it was later used as an additive to the synthetic detergents being produced as a substitute for soaps during war- time. After World War II, international intelligence reports on German industrial efforts were published, and the use of CMC as an additive to synthetic detergent powders, which eliminated redeposition of soil prob- lems, was noted. Treating pure cellulose with caustic soda and chlorace- tic acid produces CMC. Also considered a suspension agent, CMC increases the negative charge in fabrics, thus causing fabrics to repel dirt and prevent redeposition of dirt particles on clothing. CMC is best suited on fabrics derived from cellulose (e.g., cotton, rayon, etc.) and on fabric blends with cellulose components.

Bleaches

In general, all bleaches are oxidizing agents, which are used to whiten and brighten fabrics and assist in the removal of challenging stains. Some types of fabric stains are bound so tightly in place that they cannot be easily dissolved and must be destroyed instead. The colors of such difficult stains are often associated with weakly bound electrons, such as those involved in double bonds between atoms. Double bonds can give organic molecules their colors, and groups of atoms that give rise to color in molecules are called chromophores. Bleaches act by attacking the vulnerable electrons of stain molecules by using electron-removing atoms, including oxygen and chlorine, and destroying the chromophores in the stain molecules. Thus, bleaches tend to convert soils into colorless,

invisible soluble particles, which may then be removed by detergents and flushed out in the waste washing water. Alternately, the molecules may remain on the fabric but be no longer capable of absorbing visible light. Thus, while the oxidized form of stains is less highly colored compared with the reduced form, the oxidized form of the stain may remain on the garment in some cases.

The familiar domestic liquid laundry bleaches added separately to stained clothes are all generally aqueous (water-based) 5.25 percent sodium hypochlorite (NaOCl) solutions. This bleach is made by dissolving chlorine gas in a dilute solution of sodium hydroxide (12 to 16 percent) until the alkalinity is neutralized. The resulting solution is diluted to approximately 5 percent. This chemical acts as a bleach because the hypochlorite ion is a modest oxidizing agent that can oxidize many of the chemicals responsible for staining. Hypochlorite bleaches release chlorine rapidly, and these high concentrations of chlorine can also disinfect and deodorize fabrics. However, such high concentrations of chlorine can be quite damaging to fabrics, breaking apart fabric molecules and weakening the clothing item. These bleaches do not work well on poly- ester fabrics, often leading to a yellowing rather than a whitening effect. They sometimes destroy the chromophores in dye molecules, turning colored fabrics white. Other times, such bleaches modify the dye mole- cules and change the color of the fabric altogether. Other types of bleaches are available in solid forms, which are developed to release chlo- rine slowly into water to minimize the damaging effects to clothing. Symclosene (N3O3C3Cl3), a cyanurate-type bleach, is an example of a

bleach available in solid form.

In the 1950s, a prominent innovation and addition to synthetic deter- gents was detergent with oxygen bleach. Oxygen-containing (color-safe) bleaches assist in removing stains from almost all types of washable fabrics and work more gently than chlorine-based bleaches. Such bleaches frequently contain oxygen compounds such as sodium perborate (NaBO2- H2O2). As indicated by the formula, this bleach is a complex of NaBO2 and the powerful oxidizing agent hydrogen peroxide (H2O2). When added to hot water (above sixty-five degrees centigrade), this substance decomposes into hydrogen peroxide and sodium borate (Na2B4O7). The liberated hydrogen peroxide acts as a bleach, attacking double bonds of stain chromophores as it in turn decomposes to release oxygen (O2). This type of oxidation removes much of the stains on clothing while generally not affecting fast (permanent) fabric coloring. The tetraborate ion formed is also useful as a laundry aid builder, as it readily forms a com- plex with iron (Fe). Sodium tetraborate (Na2B4O7·10H2O) is also known as borax, and this product alone is marketed as a laundry aid. However, borates are somewhat toxic. While the potentially deleterious effects of borax exposure in humans are currently being investigated, boron is specifically toxic to citrus crops, and detergent runoff must be avoided during irrigation.

Oxygen-releasing bleaches are generally less active than chlorine bleaches and require higher temperatures, higher alkalinity, and higher concentrations for working efficiency. They are particularly effective when used during a clothing presoaking routine, as they require fairly high temperatures for effectiveness during the wash cycle. At lower water temperatures, an enzyme found in many biological stains (e.g., blood, grass, etc.) called catalase decomposes hydrogen peroxide quite rapidly and can destroy the perborate bleach within a few minutes. The higher water temperatures are required for oxygen-releasing bleaches to per- form effectively, as the catalase enzyme is easily deactivated at high tem- peratures. Used mainly for bleaching white-colored resin-treated polyester/ cotton fabrics, oxygen-releasing bleaches can protect these types of fab- rics longer and allow them to become whiter than can chlorine bleaching techniques. The rapid increase in the use of synthetic fibers for clothing manufacture, which are adversely affected by chlorine, has increased the use of sodium perborate.

Enzymes

Another class of laundry aid substances added to laundry detergents is the enzymes, or organic catalysts. The enzymes used are very stable and are readily isolated from microorganisms. In general, enzymes are proteins that catalyze specific reactions by binding to specific chemical products called substrates. These organic (carbon-based) biological catalysts tend to hasten chemical reactions without themselves becoming altered or destroyed. In general, laundry aid enzymes act by breaking down var- ious products (e.g., proteins and fats) that may bind stains to clothing. They act to cut up large stain molecules into smaller fragments that can then be washed away with water.

The use of enzymes for washing has a long history, starting with a patent in 1913 for soda plus a small amount of impure proteolytic (protein-digesting) trypsin enzyme marketed as a prewash. However, it had limited commercial success and was eventually replaced by a bacterial protease, which required neutral pH water conditions. Proteolytic enzymes had been tried as additives to washing powders in Germany in the 1920s and again in Switzerland in the 1930s with limited success. Eventually, better strains of enzymes were developed, with stability to a wider pH spectrum, stability against the addition of bleaches, and quicker ac- tion. In the late 1960s, a few presoaks and synthetic detergents appeared on the U.S. consumer market containing enzymes. Enzyme presoaks are used to soak items before washing to remove and decompose difficult protein-based stains and soils, including food, grass, and blood. When added to the wash cycle of automatic washing machines, they increase cleaning power. However, most enzymes are denatured at high temperatures, so detergents using enzymes usually perform best in warm (not necessarily hot) water. Technological advances are being developed (e.g., manipulation of enzymes isolated from bacteria inhabiting natural hot springs) that allow enzymes added to detergents to perform in high- temperature cleaning cycles. While enzymatic powders currently hold a large proportion of the household detergent market, the future of enzy- matic powder production remains obscure, because the effectiveness of these products has been questioned and concern over their safety with repeated use has been suggested.

The enzymes used are generally proteolytic (protein cleaving/digest- ing) and lipolytic (fat cleaving/digesting). Protease enzymes cause pro- teins to be hydrolyzed back into their constituent amino acid building blocks, and lipases cause ester linkages in fats to hydrolyze. The most common protein enzyme type is alkaline protease, which digests protein in alkaline conditions. Although enzymes can digest proteins in stains, they can also cause severe allergic reactions in humans. As such, they are often used in a granulated form, coated with chemicals including poly- ethylene glycol, which melts and releases the enzymes within the wash cycle. Lipases have been developed to increase the cleaning of fatty soils within clothing. Some genetically engineered lipases efficiently convert fats in food, cosmetics, and sweat into fatty acids and glycerol during the spin cycle of automatic laundering and during prespotting treatment reg- imens. Other enzymes include amylases, which are used in detergents to degrade starches to water-soluble sugars. They perform well in cold water temperatures because they hydrolyze the “starch glue” that binds the soil to the fabric. In addition, cellulases remove cellulose-based fine, fluffy microfibrils released from cotton after repeated washings. Such microfibrils cause characteristics such as stiffness and graying coloration in some fabrics, particularly noticeable in laundered towels.

Fabric Softeners

Another class of laundry aid substances added to laundry detergents is the fabric softeners. First developed in the 1950s as products added to the final rinse cycle, fabric softeners make fabrics softer and fluffed up and decrease static cling, wrinkling, and drying time. These products re- main on fabrics after laundering, allow for easier ironing of clothing, and provide a pleasant fragrance to fabrics. By the 1970s, fabric softeners were available as products added to the washing cycle, as part of multifunctional laundry products (detergents with added fabric softeners), and as separate sheets added to the dryer.

Liquid fabric softeners added to the rinse cycle are predominantly cationic surfactants. Cationic surfactants consist of two parts: a long water-insoluble hydrocarbon tail region and a small positively charged water-soluble head region. The most common cationic surfactants are called quaternary ammonium salts, because they possess four hydrocarbon groups attached to a nitrogen atom bearing a positive charge. These compounds are based on a positive ammonium ion, which itself is based on a positive nitrogen ion. In fabric softeners, a positive centrally located nitrogen ion forms covalent bonds with four hydrocarbon chains. A specific type of quaternary ammonium salt, which bears only two long carbon chains and two smaller groups of nitrogen and is used as a fabric softener, is called dioctadecy ldimethy lammonium chloride. The long hydrocarbon chains are hydrophobic and have a physical characteristic similar to most oily lubricants. Normally, when a fabric softener is applied to wet negatively charged fabric fibers, the softener will stick to the fabrics very strongly. Cationic surfactants are seldom used within the actual wash cycle, as washing deter- gent surfactants are predominantly anionic, possessing a negatively charged head region. The oppositely charged head regions would thus tend to clump together and precipitate from solution, destroying the detergent action of both.

As stated above, the cationic (positive) charge of fabric softeners has a strong affinity for wet negatively charged fabrics. As such, fabric soften- ers form a uniform layer one molecule thick on the surface of clothing fibers. The long hydrocarbon chains lubricate the fibers and reduce fric- tion and static. Previously treated clothing items are thus lubricated and experience weaker frictional forces when transferred to the automatic dryer. They also transfer fewer electrical charges as they tumble dry. As such, clothing flexibility and softness are both increased. However, while fabrics then possess a fluffy appearance and feel, the hydrophobic coating of the softener slightly reduces the ability of the fabric to absorb water. This is an issue for items such as towels and cotton baby diapers. In addition, cationic surfactants, although not good detergents, are mildly anti- septic, imparting a germicidal effect. They act as bactericidal agents because they coat, smother, and kill bacteria, and they may inactivate and cause imbalances in bacterial metabolic pathways and enzyme cascades.

Optical Whiteners (Fluorescers)

White fabrics, such as cotton, tend to absorb an increased amount of blue light and begin to appear yellow. Historically, “washing blue” was added when washing clothes so that cotton fabrics naturally aging to a yellowish hue would appear white. This blue dye absorbed red and green light, balancing the blue absorption of the fabric itself so that the fabric appeared colorless. Instead of using bluing, optical whiteners, or brighteners, are fluorescent dyes often added to modern synthetic detergents to increase the brightness of white fabrics. These brighteners are actually fluorescent dyes, organic compounds called blancophors (or colorless dyes; e.g., blancophor R), which contain four five-carbon rings and two centrally located SO3-Na+ groups that increase the solubility of the com- pound. They coat the fabrics during washing and convert the invisible ultraviolet (UV) light/radiation component of sunlight into an almost imperceptible blue tint. Instead of absorbing red and green light to balance the white appearance of fabrics, optical whiteners reintroduce the missing blue light. The absorption of this invisible light (mostly UV-A) is reemitted as visible light at the blue and violet part/end of the UV light spectrum. These brighteners restore the mixture of colors reflected to what a white fabric would naturally reflect. When exposed to sunlight, fabrics appear brighter, almost “whiter than white,” and the blue light camouflages any fabric yellowing. Many new clothing items already contain such dyes, but repeated washings remove them from the fabric.

Optical whiteners do not perform any cleaning action; thus, their only benefit is cosmetic, yielding more pleasing laundry. While clothing in fact may actually be dirty, optical whitening provides the deception of cleanliness. Because different types of fabrics carry different electrical charges (e.g., nylon has a positive charge, cotton a negative charge), oppositely charged fluorescers are often needed.