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Effect of relative humidity during tabletting on matrix formation of hydrocolloids: densification behavior of cellulose ethers.

The aim of this research was to investigate the elastic-plastic deformation behavior of the cellulose ethers hydroxypropylmethylcellulose (HPMC), hydroxyethylmethylcellulose (HEMC), and sodium carboxymethylcellulose (NaCMC) at relative humidities (RH) of 38, 57, and 75% and assess how the release of drugs embedded in such matrices is affected by the inner structure of the tablets formed during tabletting. Sorption and desorption isotherms and glass transition temperature were determined between 32 and 75% RH. The materials were equilibrated at 38, 57, and 75% RH and tabletted to a range of graded maximum relative densities. Pressure-time and displacement-time curves were analyzed by use of the Heckel function and a modified Weibull function (pressure-time only). After equilibration at the different RHs, all materials were in the glassy state. The respective degrees of polymerization had negligible effect on the absolute content of water, the sorption isotherms, and finally the densification behavior. At 38% RH, NaCMC contains the same amount of water as HPMC and HEMC, but deforms less plastically than the latter. This is attributed to tight binding of the water of hydration in the former. With increasing RH, NaCMC becomes only a little more plastic than both HPMC and HEMC, although it contains more than twice the amount of water. The binding strength of water and its molecular mobility, not the amount, seems to determine the readiness for volume reduction under load.