About Propylene Glycol. Propylene Glycol (a.k.a. Propane-1,2-diol) is a clear, colorless, non-corrosive, but strongly hygroscopic difunctional alcohol in such a way that it must be protected from exposure to the atmosphere. Unique among the glycols, Propylene Glycol has low toxicity to humans. The largest application of Propylene Glycol is in the production of polyester resins, although it also has a wide application in the food, pharmaceutical, and cosmetic industries. Propylene Glycol also has great solvent properties for several polar organic materials, such as phenols, alcohols, dyes, natural products, and even some resins. Moreover, due to its property of reducing the water freezing point, PG has been used as an option for aircraft deicing and anti-icing fluids. This commodity was first manufactured in 1859 by the hydrolysis of propylene glycol diacetate However, it was only in 1931 that it was first commercialized and used as a substitute for glycerol in pharmaceuticals.

The main industrial process for manufacturing PG involves the direct hydrolysis of propylene oxide with water. This results in the simultaneous production of Propylene Glycol, dipropylene glycol, and tripropylene glycol, which are then separated through distillation. This separation consists of the water-glycol mixture being dehydrated in successive columns, followed by vacuum distillations to obtain these three glycols in high-purity form. The remaining mixture of higher glycols is a process residue and has limited commercial use. The process generates minimal waste and the resulting glycols are environmentally friendly, as they are readily biodegradable and have low toxicity.

Another remarkable chemical route to produce Propylene Glycol is the hydrogenolysis of glycerol. This process can be applied to produce both industrial and USP PG grades. In resume, it consists of glycerol transforming into acetol and then the conversion of acetol into Propylene Glycol. These two reactions are carried out in the presence of a metallic catalyst.

Propylene Glycol Handling. Propylene Glycol possesses a noncorrosive nature, enabling its transportation and storage in containers crafted from materials such as stainless steel, aluminum, and lined steel. While carbon steel containers are also permissible, it is important to note that extended storage periods may result in minimal iron contamination. For the storage of pharmaceutical (USP) grade, stainless steel containers are commonly employed. Furthermore, it is crucial to acknowledge that this material exhibits hygroscopic properties, meaning it has a tendency to absorb moisture from the surrounding atmosphere. Therefore, it is essential to safeguard it from unnecessary exposure to the atmosphere.

Propylene Glycol Production. Propylene Glycol was first prepared in the 19th century, from the hydrolysis of propylene glycol diacetate. Nowadays, commercial production of PG is based on the hydrolysis of propylene oxide, which can be obtained by the dehydrochlorination of propylene chlorohydrin, and by the oxidation of propylene by an organic hydroperoxide. Raw materials and the respective production processes employed in the manufacturing of Propylene Glycol are listed below.

Propylene Glycol Uses. The uses and applications of Propylene Glycol may vary according to its specification. The main forms of Propylene Glycol are industrial grade (99.5 wt% minimum purity) and USP grade (also 99.5 wt% minimum purity, but higher quality standards in terms of specific impurities, such as max. 0.5 ppm iron).

Unsaturated polyester resins account for the majority of the commercial use of Propylene Glycol. Such resins, in turn, are used in thermoset composites.