HPMC (Hypromellose)

1: What is HPMC?

Hydroxypropyl methylcellulose ( (Propylene glycol ether of methylcellulose) is a methylcellulose modified with a small amount of propylene glycol ether groups attached to the anhydroglucose of the cellulose. The dry product contains 19 to 30 per cent of methoxyl (-OCH₃) groups and 3 to 12 per cent of hydroxypropyl (-OCH₂CHOHCH₃) groups. HPMC can be derived from tree fiber or cotton fiber.

2: How HPMC is made:
The cellulose ethers are manufactured by a reaction of purified cellulose with alkylating reagents (methyl chloride) in presence of a base, typically sodium hydroxide and an inert diluent. The addition of the base in combination with water activates the cellulose matrix by disrupting the crystalline structure and increasing the access for the alkylating agent and promotes the etherification reaction. This activated matrix is called alkali cellulose (Kirk-Othmer, 1993). During the manufacture of HPMC alkali cellulose reacts with methyl chloride to produce methyl cellulose and sodium chloride. Side reactions of the methyl chloride and sodium hydroxide produce methanol and dimethyl ether by-products. The methylcellulose is then further reacted with the staged addition of an alkylene oxide, which in the case of HPMC is propylene oxide (Kirk Othmer, 1993; Dow, 2002).

After this reaction, MC and HPMC are purified in hot water, where they are insoluble. Drying and grinding completes the process.

3: Chemicals agents and reactions:
RcellOH:NaOH + CH₃Cl + NaCl +RcellOCH₃ + x C H₃CH –O CH₂ + Rcel – OCH₃ +O (OCH₂ CHCH₃)xOH.

Methyl chloride (CH₃Cl) is colorless gas with a faint, sweet odor that is not noticeable at dangerous concentrations.Synthetic forms are a chlorinated hydrocarbon derived from petroleum, and a suspected carcinogen. (Lewis, 1992; NJ1998).

Propylene oxide is also a petroleum derivative.

4: Environmental concerns:
EPA – Clean Air Act Amendments – final rule for National emission standards for hazardous air pollution (NESHAP) includes cellulose ethers production source category as major sources of emissions of hazardous air pollutants.(FR67:112: 40044) June 11, 2002.