Stress relaxation study of fillers for directly compressed tablets.
It is possible to assess viscoelastic properties of materials by means of the stress relaxation test. This method records the decrease in pressing power in a tablet at its constant height. The cited method was used to evaluate the time-dependent deformation for six various materials: microcrystalline cellulose, cellulose powder, hydroxypropyl methylcellulose, mannitol, lactose monohydrate, and hydrogen phosphate monohydrate. The decrease in pressing powering of a tablet during a 180 s period was described mathematically by the parameters of three exponential equations, where the whole course of the stress relaxation is divided into three individual processes (instant elastic deformation, retarded elastic deformation and permanent plastic deformation). Three values of the moduli of plasticity and elasticity were calculated for each compound. The values of elastic parameters ATihave a strong relationship with bulk density. The plastic parameters PTirepresent particle tendency to form bonds. The values of plasticity in the third process PT3ranged from 400 to 600 MPas. Mannitol had higher plasticity and lactose monohydrate on the contrary reduced plasticity. A linear relation exists between AT3and PT3for the third process. No similar interpretation of moduli calculated on the basis of three exponential equations has been realized yet.
Department of Pharmaceutical Technology, Faculty of Pharmacy in Hradec Králové, Charles University in Prague, Czech Republic.
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M
Rehula
Department of Pharmaceutical Technology, Faculty of Pharmacy in Hradec Králové, Charles University in Prague, Czech Republic.
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R
Adamek
Synpo, Pardubice, Czech Republic.
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V
Spacek
Fillers; Hydrogen bonds; Stress relaxation; Viscoelasticity; van der Waals bonds