Propylene glycol is an organic diol (double alcohol) widely used in cosmetic preparations especially in perfumery. It is officially named as propane-1,2-diol in IUPAC nomenclature with a CAS number of 57-55-6. A syrupy substance, it is clear, odorless, and slightly sweet. There are different derivatives, or polymers, of propylene glycol and water, including PPG-9, PPG-12, PPG-15, PPG-17 PPG-20, PPG-26, PPG-30, and PPG-34, the number of which corresponds to the number of units of propylene glycol available in the compound. Propylene glycol can also be found in the muscles, especially after exercise, and can be metabolized into lactic acid.
Manufacturing Process
Propylene glycol can be produced from propylene oxide using either non-catalytic or catalytic methods. For the first one, a high temperature process of about 200˚C to 220˚C is used while for the latter, a 150˚C to 180˚C is utilized with ion exchange resin or small amount of sulfuric acid or alkali. Propylene glycol can also be sourced from glycerol, as a byproduct of bio diesel.
Properties
Propylene glycol has a low melting point of about -59 ˚C and boils at around 188.2 ˚C. The organic alcohol is soluble in water, acetone, and chloroform. Propane glycol serves as a humectant for its hydrophilic property and as an effective moisturizer. It also serves as a stabilizer for chemical preparations to prolong their shelf life. Propane glycol is biodegradable.
Physical hazard
Propylene Glycol is included by the US Food and Drug Administration (FDA) as one of the food additives considered “generally recognized as safe” (GRAS). As an indirect food additive, the organic alcohol is also permitted as a defoaming agent. A concentration limit of 50 percent for propylene glycol and its polymers is recommended by the CIR Expert Panel based on human irritation and sensitization tests. According to studies, diseased skin may develop irritation/sensitization reactions to propylene glycol. With normal skin tested under occlusive patches, it is suggested that it can be a potential skin irritant and is dependent on its strength. However, the National Toxicology Program’s (NTP) Center for the Evaluation of Risk to Human reproduction (CEHRER) Expert Panel in 2003 found out that propylene glycol poses no risk for reproductive or developmental toxicity to humans.
Uses
The pharmaceutical and food industry utilize propylene glycol USP, the food grade variety, for a wide range of applications. It can serve as an inert diluent for various medical formulas and as a coupling agent for cosmetic preparations like liquid soaps, lotions, and shaving foams. Propylene glycol is utilized as a solvent for perfumes and flavor concentrates. The FAO/WHO Expert Committee on Food Additives recommends a maximum daily intake of 25 mg. of propylene glycol/kg. body weight/day.
Industrially, propylene glycol serves as a raw material for unsaturated polyester resins, which can be later converted as gel coats, sheet molding compounds, offshore construction, and synthetic marble casts. It is also used as a solvent for inks and laundry soaps and as a stabilizer for hydraulic fluids. Propylene glycol is added to increase pliability of plastics and is a vital ingredient in making antifreeze and resins for paints and varnishes.
