Laundry detergent powder is a complex mixture of chemical compounds designed to effectively clean clothing and textiles. The development of modern laundry detergents involves a sophisticated understanding of chemistry, materials science, and consumer needs. This article delves into the raw materials used in laundry detergents, their functions, and the principles behind formulating effective cleaning products.
I. Raw Materials for Laundry Detergent Powder
The efficacy of laundry detergent depends on a carefully balanced combination of various ingredients, each serving a specific purpose in the cleaning process. These components can be broadly categorized into several groups:
A. Surfactants B. Washing Aids C. Builders D. Bleaching Agents E. Enzymes F. Additives for Special Functions
Let’s explore each of these categories in detail.
(A) Surfactants
Surfactants are the main active ingredients in laundry detergent. Previously, the quality of detergents was judged by the amount of surfactants in their formulas. Now, detergent manufacturers generally use a combination of two or more surfactants. Sometimes, even with a lower surfactant content, the synergistic effects between multiple surfactants can give the detergent good cleaning and dirt-removal capabilities. Many surfactants have excellent dirt removal, wetting, foaming, dispersing, emulsifying, and solubilizing abilities. The main types of surfactants commonly used in laundry detergent are as follows:
(1) Anionic surfactants: Such as sodium alkylbenzene sulfonate (LAS), sodium fatty alcohol sulfate, sodium fatty alcohol polyoxyethylene ether sulfate (AES), sodium alpha-olefin sulfonate (AOS), methyl ester sulfonate (MES), etc. (2) Non-ionic surfactants: Such as alkylphenol polyoxyethylene ether, fatty alcohol polyoxyethylene ether (AEO), etc.
Currently, most laundry detergents sold in the market are compound detergents. These typically use sodium alkylbenzene sulfonate as the main active agent, combined with AOS, AES, MES, AEO, and other surfactants. Additionally, due to the poor biodegradability of sodium alkylbenzene sulfonate and increasing environmental concerns, there’s a trend towards replacing it with AS and MES. Furthermore, alkylphenol polyoxyethylene ether is rarely used in laundry detergent production due to its environmental toxicity and unfriendliness.
(B) Washing Aids
1.Phosphates
There are many types of phosphates, but the main one used in laundry detergent is sodium tripolyphosphate (STPP).
(1) Sodium Tripolyphosphate: Commonly known as “pentasodium,” its molecular formula is [formula omitted]. It appears as a white powder, soluble in water, and its aqueous solution is alkaline. It has a good ability to coordinate with metal ions, not only softening hard water but also coordinating with metal components in dirt, helping to break down dirt during washing and thus improving cleaning effectiveness.
Sodium tripolyphosphate also has a “surface-active” effect during washing, swelling and solubilizing proteins in dirt; promoting emulsification of fatty substances; dispersing solid particles and preventing re-deposition of dirt; and maintaining a certain alkalinity in the washing solution, acting as a buffer. These effects make sodium tripolyphosphate an excellent washing aid.
When incorporated into laundry detergent, sodium tripolyphosphate also prevents product caking, keeping the product in a dry, granular state, which is important for product shaping.
Due to these various effects, sodium tripolyphosphate is a very important washing aid, widely used in laundry detergents with addition amounts ranging from 10% to 40%.
(2) Substitutes for Sodium Tripolyphosphate: Although sodium tripolyphosphate is an excellent washing aid, its discharge leads to eutrophication (also known as over-fertilization) of water bodies, polluting water resources. Therefore, over the past 20 years, researchers developing detergents have done extensive work on substitutes for tripolyphosphates. These substitutes are mainly organic chelating agents, high-molecular-weight electrolytes, and molecular sieves.
Organic chelating agents are organic compounds that can chelate metal ions such as calcium and magnesium, softening water by enclosing metal ions within the chelating agent molecules. Commonly used organic chelating agents include ethylenediaminetetraacetic acid tetrasodium salt (EDTA-4Na) and nitrilotriacetic acid (NTA). While organic chelating agents can soften hard water, they don’t have the emulsifying and dispersing effects on dirt that tripolyphosphates do.
Among high-molecular-weight electrolytes, sodium polyacrylate has been developed as a main additive. It also has a chelating effect on multivalent metal ions and can improve the dirt removal ability of detergents in hard water. Sodium polyacrylate can also adsorb onto the surface of washed items and dirt, increasing the electrostatic repulsion between them, which is beneficial for dirt removal. It can also increase the dispersibility of dirt, preventing re-deposition. Using sodium polyacrylate in combination with STPP provides good washing aid effects.
Molecular sieves are also a promising washing aid that can partially replace STPP. Also known as synthetic zeolites, they are crystalline aluminosilicates. Molecular sieves can be classified by pore size, with “4A” zeolite being used as a washing aid. Its molecular formula is [formula omitted]. Molecular sieves can soften water by exchanging ions in their crystal lattice with those in water. Besides softening hard water, they can also adsorb washed-off dirt, aiding in dirt removal. Using molecular sieves together with STPP shows significant washing aid effects, but if molecular sieves completely replace STPP, the dirt removal and anti-redeposition effects are not ideal.
2.Sodium Silicate
Sodium silicate, commonly known as water glass or liquid glass, has the molecular formula [formula omitted]. Commercial sodium silicate is available as granular solids or viscous aqueous solutions. The properties of water glass change with the ratio of Na2O to SiO2 in the molecule. If [formula omitted], this ratio n is called the modulus. The lower the modulus, the higher the alkalinity and water solubility; conversely, the higher the modulus, the lower the alkalinity and water solubility. The water glass used in detergents has a modulus of 1.6-2.4, which hydrolyzes in water to form silicic acid sol.
Adding water glass to laundry detergent has significant washing aid effects. Firstly, sodium silicate has a buffering effect on the pH of the solution, maintaining it at a weak alkaline level, which is beneficial for dirt removal. Secondly, the colloidal solution produced by its hydrolysis has a dispersing effect on solid dirt particles and an emulsifying effect on oil stains. Adding water glass to laundry detergent can also increase the mechanical strength, flowability, and uniformity of the powder granules.
The disadvantage of water glass is that the silicic acid sol formed by hydrolysis can be adsorbed by fibers and is difficult to wash away, making fabrics feel rough after drying. Therefore, the amount of water glass added to laundry detergent should not be excessive.
3. Sodium Sulfate
Anhydrous sodium sulfate is a white crystal or powder, commonly known as salt cake. Sodium sulfate with 10 molecules of crystallization water is commonly known as Glauber’s salt. Sodium sulfate is often added to laundry detergent as a filler to reduce costs. When used in combination with anionic surfactants, the increase of Na+ in the solution increases the surface adsorption of anionic surfactants and promotes the formation of micelles in the solution, thereby reducing the surface tension of the washing liquid, which is beneficial for wetting and dirt removal. The addition of sodium sulfate can also reduce the viscosity of the material, facilitating the formation of laundry powder. Considering these properties, sodium sulfate is a good filler, generally added to laundry detergent at 20% to 45%.
4. Sodium Carbonate
Industrial-grade sodium carbonate is commonly known as soda ash or washing soda. In laundry detergent, it acts as an alkaline agent and filler, saponifying dirt and maintaining a certain pH value of the laundry detergent solution, which aids in dirt removal. Sodium carbonate has a water-softening effect, reacting with Ca2+ and Mg2+ in hard water to form insoluble carbonates. However, sodium carbonate is strongly alkaline and can only be used in low-grade laundry detergents. It should not be added to high-grade detergents for washing silk and wool textiles.
5. Anti-redeposition Agents
Anionic surfactants commonly used in laundry detergents, such as sodium dodecylbenzene sulfonate, can remove dirt adhered to fibers, but compared to soap, their ability to emulsify and suspend dirt is inferior. This leads to the drawback of removed dirt re-depositing on fibers, resulting in poor anti-redeposition ability and causing washed clothes to appear gray or yellow. To overcome this, anti-redeposition agents must be added to synthetic detergents.
(1) Sodium Carboxymethyl Cellulose (CMC): CMC has excellent anti-redeposition ability. Its mechanism mainly involves CMC adsorbing onto fiber surfaces, weakening the re-adsorption of dirt by fibers. Additionally, CMC can surround dirt particles, keeping them stably dispersed in the washing solution. CMC’s adsorption is most significant on cotton fibers, making its anti-redeposition effect best for cotton fabrics, while its effect on wool and synthetic fiber fabrics is less ideal.
(2) Polyvinylpyrrolidone (PVP): PVP is a synthetic polymer compound. PVP used as an anti-redeposition agent has an average molecular weight of 10,000 to 40,000. It has good dispersing ability for dirt and provides excellent anti-redeposition effects for cotton and various synthetic fiber fabrics. K-15 and K-30 represent PVP with molecular weights of 10,000 and 40,000, respectively. PVP not only has strong anti-redeposition ability but also good solubility in water, doesn’t coagulate with inorganic salts, and has good compatibility with surfactants. Therefore, PVP is an excellent anti-redeposition agent, though its drawback is its high price.
6. Bleaching Agents
Bleaching agents added to laundry detergents are generally oxidants. During washing, they can oxidize and destroy colored dirt molecules, not only removing heavy stains but also making clothes white and colors vibrant. The main bleaching agents used in detergents are sodium hypochlorite and peroxy salts. Common bleaching agents are as follows:
(1) Sodium Perborate: Its molecular formula is NaBO3·4H2O. It’s not easily soluble in cold water but soluble in hot water. When heated in aqueous solution, it decomposes to release H2O2 and O2 with bleaching effects. NaBO2 also has some washing aid properties. Therefore, adding sodium perborate to laundry detergent will improve stain removal ability and increase whiteness.
The bleaching effect of sodium perborate is highly temperature-dependent, requiring temperatures above 80°C for effective bleaching. To make sodium perborate effective at lower temperatures, activators need to be added, such as sodium isononanoylbenzene sulfonate, which can lower the activation temperature to 40°C.
(2) Sodium Percarbonate: Its molecular formula is 2Na2CO3·3H2O2. In aqueous solution, it decomposes into Na2CO3 and H2O2, thus having both bleaching and alkaline effects. Sodium percarbonate has a bleaching effect at 50°C without the need for activators, and it’s cheaper than sodium perborate. Percarbonates decompose at lower temperatures and more easily when absorbing moisture. To prevent catalytic decomposition by heavy metals, metal chelating agents like EDTA should be added to the formula to improve storage stability.
7. Fluorescent Whitening Agents
To increase the whiteness of clothes after washing, the traditional method was to add a small amount of blue dye to the laundry detergent. This added a tiny amount of blue to the fabric, which complemented the original slight yellow color, visually increasing the apparent whiteness. However, this method reduced the brightness of light reflected by the fabric. This whitening method is called blue tinting.
Modern fluorescent whitening agents are fluorescent substances that can absorb invisible ultraviolet light and emit blue-violet fluorescence with a wavelength of 400-500nm. This blue-violet fluorescence complements the original slight yellow color of the fabric, increasing whiteness. Unlike blue tinting, it not only increases whiteness but also brightness, allowing the fabric to reflect more light. The amount of fluorescent whitening agents added to laundry products is very small, generally about 1% of the active substances in the detergent.
8. Enzymes
Enzymes themselves are proteins that can catalyze certain chemical reactions. For example, proteases can convert proteins into water-soluble amino acids. Adding enzymes to detergents can effectively promote the breakdown and removal of dirt. There are many types of enzymes, and the main ones used in detergents include proteases, lipases, cellulases, and amylases.
9. Foam Stabilizers and Defoamers
For hand-washing laundry detergents, the foaming ability and foam stability are very important. Appropriate foam can carry dirt, and the foam also serves as an indicator of the rinsing degree of clothes. However, when using washing machines, too much foam can interfere with the effective operation of the machine. When formulating laundry detergents, the amount of foam needs to be controlled according to different application purposes. The foam of detergents can be adjusted by selecting different types of surfactants and changing their proportions, or by adding foam stabilizers or defoamers.
Betaine-type amphoteric surfactants and alkanolamides are commonly used foam stabilizers, especially effective when combined with sulfonate and sulfate anionic surfactants, and they also have cleaning functions themselves. Tertiary amine oxides also have good foam stabilizing effects, with common ones including lauryl dimethyl amine oxide and myristyl dimethyl amine oxide.
For washing machine detergents, lower foaming power is needed, requiring the addition of small amounts of foam inhibitors in the formula. Adding fatty acid soaps to detergents made with sulfonates, sulfates, and non-ionic surfactants can have a defoaming effect. The degree of foam suppression increases with the saturation and carbon number of fatty acids, with C22 fatty acid soap being an excellent foam inhibitor. Polyethers and silicone oils are also commonly used foam inhibitors. Incorporating these substances into laundry detergent creates low-foam detergents. Among them, polyethers have been commonly used in recent years to produce low-foam detergents, typically copolymers of ethylene oxide and propylene oxide, which provide good defoaming effects when added to low-foam laundry detergents.
10. Solvents and Cosolvents
Laundry detergents contain many inorganic salts, which can reduce the solubility of surfactants. To keep all components in a dissolved state, cosolvents need to be added. Common cosolvents include sodium p-toluenesulfonate, sodium xylenesulfonate, and urea.
II. Formula Design for Laundry Detergent Powder
Formula design is a crucial step in laundry detergent production, as the quality of the formula affects the entire production process and product quality. Currently, there is no complete theoretical basis to guide formula development, so it relies on experiments and practical experience. The following principles can be used as a reference for formula design:
(1) Selection of Active Ingredients: For spray drying formation, due to the high temperatures involved, it’s preferable to choose thermally stable active ingredients for laundry detergent, such as LAS, AOS, and alkyl sulfonates. Non-ionic surfactants are not heat-resistant and should be added during post-mixing. Currently, compound laundry detergents generally use sodium alkylbenzene sulfonate as the main active agent, combined with sodium fatty alcohol sulfate, AES, and others.
(2) Foam Issues: Hand-washing detergents typically require more foam, so foam boosters and stabilizers should be considered in the formula. Machine-washing detergents prefer less foam, so low-foaming ingredients like sodium stearyl sulfate, non-ionic surfactants, fatty acid soaps, or other defoamers can be added.
(3) pH Value: The pH value of heavy-duty laundry detergent solutions is generally 9.5-10.5, which is quite alkaline and not suitable for washing protein fiber textiles like silk and wool. For washing silk and wool textiles, it’s best to use a mild, neutral laundry detergent. The key to formulating neutral laundry detergent is to achieve good washing effects without adding sodium tripolyphosphate and other alkaline agents.
(4) Other Considerations: Add appropriate amounts of anti-redeposition agents (such as CMC), anti-caking agents (such as sodium p-toluenesulfonate), and fluorescent whitening agents as needed.