Hydroxypropyl methyl cellulose (HPMC) is a polymer used for multiple applications in fields such as construction and ophthalmology. It is also known as hypromellose, which is the international, non-proprietary name (INN) for this substance. The name is not patented by any drug company, and can be used without fear of trademark or copyright infringement. HPMC is a solid substance at room temperature and is typically beige in color.
One of the most important properties of hydroxypropyl methyl cellulose is that, in a water-based solution, it does not have a gelation property when heated. Instead of precipitating out as methylcellulose does, it causes the entire solution to congeal into a semi-flexible mass that can no longer flow. The more HPMC that is present, the lower the critical temperature needed to achieve this effect.
The ability of hydroxypropyl methyl cellulose to absorb water is what has led to one of its most common uses — as a substitute for human eye tear-film. When this substance is added to eye drops or other ophthalmologic aids, it increases the amount of time the drops spend on the cornea by absorbing the water that is present and preventing early evaporation. Theoretically, the longer a lubricant substance stays on an eye, the less irritation suffered. Solutions containing 2% hydroxypropyl methyl cellulose are typically used during orbital surgery to protect the corneas.
In addition to being used as a ocular protector, hypromellose is also used in the construction industry to make adhesives, and to mitigate the presence of water. The leading manufacturer of HMPC for industrial applications is Dow Wolff Cellulosics GmbH, and one of their most popular products is Walocel™ which is a thickening agent. When used in a construction environment, the function of HPMC is very similar to hydroxy ethyl methyl cellulose (HEMC).
This compound is also used as a food additive for thickening and emulsifying liquids. It is recognized in the Codex Alimentarius, which is Latin for food book — an internationally recognized set of guidelines which pertain to food safety and use. Hydroxypropyl methyl cellulose also carries code E464 in the European Union, making it a known and listed food additive.
HPMC is a semi-synthetic compound that is non-toxic to humans. It must be treated carefully, however, as it does react strongly if exposed to oxidizing agents. It is also combustible. This compound is listed by the Chemical Abstracts Service (CAS) as number 9004-65-3. These numbers are assigned to chemicals in order to help chemists locate them in searchable databases.