Soaps, detergents and agrochemicals: Synthetic detergents: surfactants, formulation and manufacture

3.1 Surfactants come in four families, and the synthetics do not form scum in hard water

A surfactant, short for surface-active agent, has a water-hating tail and a water-loving head; Kent calls this an amphipathic structure. In water the tails cluster into micelles, heads outward, and oily soil dissolves in their cores. Kent classifies surfactants by the charge on the head.

Anionic surfactants have a negative head. They include soap, linear alkylbenzene sulphonate (LAS), alcohol sulphates (AS), alcohol ether sulphates (AES), alpha olefin sulphonates and methyl ester sulphonates. Nonionic surfactants carry no charge; the head is a chain of ethylene oxide units, as in alcohol ethoxylates (AE), or a sugar, as in alkyl polyglycosides. Shreve notes they clean better at low temperatures and foam little, which suits drum washers. Cationic surfactants have a positive head, usually a quaternary ammonium group. They clean poorly but are antistatic and germicidal, so they go into fabric softeners and disinfectants. Amphoterics such as betaines carry a pH-dependent charge and are mild but costly, so they go into shampoos.

Why did detergents displace soap? Shreve gives the reason: soap forms insoluble calcium and magnesium salts with hard water, which precipitate as scum and cut foaming and cleaning, while the hardness salts of detergents stay dissolved or dispersed. Kent adds that synthetics were first developed after the First World War, when edible fats were scarce. Kent's US figures show the speed: 2,306 million lb of soap and 30 million lb of synthetics in 1940; by 1960, 1,230 million lb of soap and 3,940 million lb of synthetics, 76% of the total.

Soap is not dead. In Kent's 2003 world table it is still 19.8 of 40.1 billion lb of surfactants, almost half, because in many countries it remains the main product for washing both people and clothes. LAS is the largest synthetic, at 6.4 billion lb.

• Kent (2012), Ch 36 Soap, Fatty Acids, and Synthetic Detergents · pdf pp. 1451-1458
• Shreve (1984), Ch 29 Soap and Detergents · pdf pp. 547-552
• Dryden (1997), IIIB Soaps and Detergents; Glycerin · pdf pp. 306-307

3.2 A laundry detergent is a recipe in which the surfactant is a minority

Surfactants are rarely more than about a fifth to a third of a laundry powder. Kent's typical spray-dried recipes, in the table, have 0-20% LAS and 0-20% alcohol ethoxylate; the rest is builders, fillers and small doses of performance additives.

Builders soften the water. Calcium and magnesium in hard water weaken anionic surfactants, so builders lock the ions away. Sodium tripolyphosphate (STPP) does this by sequestration, forming a soluble complex with calcium. Shreve calls proper phosphate building the key that made the detergent industry possible: it also keeps soil in suspension and works with the surfactant, so less surfactant is needed. STPP was 20-35% of a phosphate powder (Kent) and 30-50% in Shreve's and Dryden's formulations.

Then came the phosphate problem. In the 1960s phosphate from detergents was linked to algal growth, or eutrophication, in lakes, and US states banned it. The first substitute, NTA, was dropped in 1970 over toxicity fears later judged unfounded for laundry use. The lasting replacements are zeolite A, a crystalline aluminosilicate that swaps its sodium for calcium inside its pores, and soda ash, which precipitates calcium as carbonate. Zeolite removes little magnesium, so the two are used together. In 1980 phosphates were still half the US builder market and zeolites 12% (Shreve).

The minor ingredients do the visible work. Enzymes digest particular stains: proteases cut protein, lipases fat, amylases starch, and cellulases trim fibre fuzz to keep colours bright (see Industrial enzymes). Peroxygen bleaches oxidise coloured stains. Optical brighteners, at only 0.05-0.2%, are fluorescent dyes that absorb ultraviolet light and re-emit blue-white light, masking yellowing. Sodium carboxymethylcellulose and polyacrylates stop soil settling back on the cloth. Sodium sulphate, 10-30% of many powders, does little in the wash: it is filler that controls density and flow.

• Kent (2012), Ch 36 Soap, Fatty Acids, and Synthetic Detergents · pdf pp. 1464-1466
• Shreve (1984), Ch 29 Soap and Detergents · pdf pp. 549-550, 558-560
• Dryden (1997), IIIB Soaps and Detergents; Glycerin · pdf p. 307

3.3 Powders are spray dried or agglomerated; liquids are simply mixed

A detergent factory has two halves: making the active and making the product. Shreve's Procter & Gamble flowsheet shows both. Alkylbenzene is sulphonated with oleum at about 55 C in a "dominant bath", a loop that recirculates at least 20 times the throughput through a cooler, because the reaction is fast (about 96% complete in under a minute) and strongly exothermic, and overheating darkens the product. Fatty alcohol is sulphated in a second loop and the acids neutralised with caustic soda, releasing six to eight times as much heat again. Kent notes that modern plants prefer sulphur trioxide diluted to 4-8% in dry air, in falling-film reactors: the capital is higher, but the product is 95-98% active against about 90% from oleum, with far less spent acid. The SO₃ is often made on site from sulphur, as in a Sulphuric acid plant.

Spray drying turns a wet slurry into hollow beads. Surfactant paste, builders and most additives are mixed in a crutcher at 64-72% solids, pumped at 300-1,200 psig and sprayed from nozzles near the top of a tower 40-200 ft high and 10-35 ft wide. Hot gas at 250-270 C enters near the bottom and rises against the falling droplets, leaving at 110-140 C. Spray drying gives density control (0.20-0.80 g/cm3), solubility and high output, but Kent names its drawbacks: high capital and energy use. Heat-sensitive enzymes, bleach and perfume are added afterwards, as 2-15% of the formula.

Agglomeration builds granules by spraying liquid surfactant or silicate onto dry powders in a mixer, then conditioning and screening. It needs less capital and energy, and its denser product cuts packaging cost, but it cannot carry as much surfactant.

Liquids need only mixing vessels or in-line mixers; they are 50-60% water. Kent reports liquids at 60-70% of the North American laundry market despite a higher cost per wash.

• Shreve (1984), Ch 29 Soap and Detergents · pdf pp. 559-562
• Kent (2012), Ch 36 Soap, Fatty Acids, and Synthetic Detergents · pdf pp. 1458-1461, 1466-1470

3.4 The money is in raw materials and brands, not in the plant

Washing products show a sharp split between cheap ingredients and valuable packs. Shreve notes that energy in soap making matters little next to raw materials, packaging and distribution. Dryden states that oils and fats account for a large share of the cost of toilet and laundry soap. For detergents, Kent observes that surfactant raw material prices rise with the number of processing steps, so a petrochemical LAS and a coconut-based alcohol sulphate compete on cost per unit of cleaning, not on origin.

Supply is a mix of make and buy. Shreve reports that the big soap and detergent companies make their own LAS, fatty alcohol sulphates and fatty acids, in hundreds of millions of pounds, but buy oleum, Caustic soda, phosphates and most additives.

Scale works differently in the two halves. Sulphonation and spray drying reward large plants: a tower making up to 80,000 lb an hour is a big fixed investment that must run full. Agglomeration and liquids need little capital, so small producers can enter. Indian experience shows both. Dryden lists a market in the mid-1990s led by Hindustan Lever, Tata Oil Mills, Procter & Gamble, Godrej and Colgate-Palmolive, alongside local players such as Nirma. Detergents made up Rs 2,000 crore, 67%, of turnover above Rs 3,000 crore. Detergents reached India only in 1956, yet India had become the second largest detergent market after the United States, about 1.71 million tonnes worth Rs 30 billion and growing 10% a year, with a large unorganised sector selling low-cost washing products.

The pattern for an investor follows from these facts. The cost floor is set by raw materials whose prices are made elsewhere, in palm plantations and petrochemical complexes. The premium is earned by brands and distribution. The plant in between rarely confers advantage unless scale or a better process cuts the cost of the active.

• Shreve (1984), Ch 29 Soap and Detergents · pdf pp. 547, 550, 565
• Kent (2012), Ch 36 Soap, Fatty Acids, and Synthetic Detergents · pdf pp. 1453, 1467-1470
• Dryden (1997), IIIB Soaps and Detergents; Glycerin · pdf pp. 307-308, 314-316

See it in the table

Check your understanding

Why does soap lose cleaning power in hard water while LAS largely does not?

A powder maker must remove STPP. Which pair of builders most directly replaces it?

What is the main economic drawback of spray drying compared with agglomeration?