Shampoo is a basic hair care product representing the largest segment of hair care cosmetics. Shampoo is typically in the form of a viscous liquid with some exception of waterless solid form such as a bar. Shampoo was developed to replace soap for cleansing scalp and hair by removing unwanted sebum, dandruff, environmental dust, and residues of hair care products. Most of the dirt including sebum are water insoluble and cannot be effectively removed by water alone. Therefore, a shampoo containing a combination of surfactants is necessary. The content of surfactants in a shampoo is typically between 10% and 20%.
Two widely used preservatives, DMDM hydantoin and imidazolidinyl urea are found in many shampoos, to prevent fungal and bacterial spoilage. They release formaldehyde to kill germs.
Another broad-spectrum biocide is isothiazolinone and the related methylisothiazolinone and methylchloroisothiazolinone.
Sodium benzoate is another preservative used in shampoos. It kills bacteria, fungi, and yeasts, and works well in acidic mixtures.
Another bactericide used is 2-bromo-2-nitropropane-1,3-diol， also known as Bronopol.
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