Alkalization mechanism of cellulose in hydroxypropylcellulose preparation process

Abstract The role of alkali treatment of cellulose in hydroxypropylcelluloe (HPC) preparation has been studied from the viewpoint of the selective distribution of sodium hydroxide between HPC and cellulose phases. Generally it is considered to be important to prepare uniform alkali cellulose, whose calculated value of composition is C 6 H 10 O 5 NaOH. x H 2 O, prior to etherification of cellulose. Therefore, enough alkali to obtain alkali cellulose is used by major manufacturers of cellulose ethers. However, HPC having good solution qualities and performance properties can be prepared even from partially alkalized cellulose by using 0.2–0.4 molar sodium hydroxide per anhydroglucose unit. The results obtained from a series of experiments indicate the following mechanism for the formation of HPC. Hydroxypropylation is initiated in the alkalized portion of cellulose if such portion and propylene oxide are present together. As the hydroxypropylation proceeds, the liberated sodium hydroxide in HPC phase migrates into the cellulose phase due to the shift of distribution equilibrium of sodium hydroxide; the above tendency is enhanced by byproducts such as propyleneglycol. Some newly alkalized portions are present and hydroxypropylation follows. These processes are repeated, and then all portions of cellulose are alkalized and hydroxypropylated.