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Polypropylene glycol (PPG) is a colorless, odorless, viscous liquid that is a polymer of propylene oxide. It is also known as polypropylene oxide. There are a number of different forms that vary in the number of molecular units in the polymer. It is an ingredient in many household products and in cosmetics, and is widely used in industry for the manufacture of other products. As of 2011, the main method of production of this chemical requires raw materials that come from the petrochemical industry; however, alternative methods that are not dependent on fossil fuels are being investigated.
The chemical composition of polypropylene glycol can be summarized as H(C3H6O)nOH, where C3H6O is propylene oxide and n is the number of occurrences of this molecular unit — commonly between 300 and 4,000. It thus consists of repeating units of propylene oxide with a hydrogen atom at one end and a hydroxyl (OH) group at the other. A number of different forms of PPG are commercially available. They are generally named according to molar mass — more or less equivalent to the number of propylene oxide units in the polymer, for example PPG 425, PPG 1200 and PPG 4000.
In the normal manufacturing process, propylene — a by-product of gasoline manufacture — is converted to propylene oxide. This compound is then polymerized, using a strong base, such as potassium hydroxide, as a catalyst. In the quest to reduce reliance on fossil fuels, alternative means of production are being sought. One proposed method involves converting glucose from corn to lactic acid, which can then be used with a copper catalyst to synthesize PPG.
In many respects, polypropylene glycol resembles polyethylene glycol (PEG) in its properties. The viscosity of the liquid increases, and its solubility in water decreases, with increasing molar mass, or molecular weight. The molar mass is dependent on the number of molecular units in the polymer.
Many of the uses of polypropylene glycol relate to its physical, rather than chemical, properties, although its low toxicity is an important factor in many of its applications. PPG polymers are used in many cosmetic and personal care products to absorb and retain moisture and as thickeners. Confusingly, the Cosmetic, Toiletry & Fragrance Association (CTFA) has its own system of acronyms for these compounds — for example, PPG-9 in this system is PPG 425 in terms of molar mass. PPGs are used as defoaming agents in many industrial processes — including food production, as lubricants and in the study of rheology. They are also used in the manufacture of some forms of polyurethane, flexible epoxy resins and radiation-curable coatings.