Hydroxypropyl methylcellulose (HPMC) is hygroscopic after drying but stable. Its solution is stable at PH 4-8 (in 1% solution 25℃). As the temperature increases, the viscosity of the solution decreases. Heating-cooling can reversibly convert HPMC from a solution state to a gel state. According to different grades and concentrations, the gelation temperature is 50 ～ 90 ℃. HPMC aqueous solution has anti-enzyme effect and maintains good viscosity stability during long-term storage. However, its aqueous solution is susceptible to attack by microorganisms, so preservatives should be added during storage. When HPMC is used as a thickener in ophthalmic eye drops, benzalkonium chloride is usually added as a preservative.
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