In the wet massing of powders and powder blends, the rheological behavior of the wet powder masses not only plays a critical role in the unit process but also influences the attributes of the product. The physical properties of the powder excipients, such as particle size and size distribution, shape, surface area, bulk and tapped density and surface morphology, are a major source of variability in the rheological behavior of wet powder masses and the quality attributes of the final product. The objective of the present investigations was to study the rheological behavior of wet masses containing hydroxypropyl methylcellulose (HPMC) obtained from two sources (Methocel from Dow and Pharmacoat from Shin-Etsu) using a mixer torque rheometer. In order to simulate a real formulation, diltiazem HCl (DTZ) (40% loading) was used as part of the substrate powder mass. Hydroxypropyl cellulose (HPC) was used as the binder. Since HPMC is water-soluble, isopropyl alcohol (IPA) was used as the wet massing liquid. The rheological behavior of the wet powder masses was studied as a function of mixing time and amount of wet massing liquid (IPA). The rheological profiles obtained for DTZ-Methocel and DTZ-Pharmacoat exhibited same magnitude for mean torque, however, for DTZ-Pharmacoat the peak was more extended than that for DTZ-Methocel. The extended peak for DTZ-Pharmacoat indicated that the wet mass will stay suitable during the process for larger quantities of the wet massing liquid before turning into paste and becoming unsuitable for the process as compared with the DTZ-Methocel system. The mixing kinetics of the two powder systems appeared to be quite different. These differences in the rheological behavior of the wet masses may be attributed to the difference in the particulate and surface properties of the two HPMCs. Some of the properties of the two HPMCs, such as particle size and size distribution, surface area, surface morphology and DSC thermograms, explain the difference observed in their rheological behavior. The difference in the rheological profiles of the two DTZ-HPMC systems indicated superiority of Pharmacoat over Methocel considering their wet massing behavior.
Properties of HPMC (Hydroxypropyl MethylCellulose)
Hydroxypropyl MethylCellulose is a non-ionic cellulose ether, in the appearance of white powder, odorless and tasteless. It is soluble in water, most polar organic solvents, and the appropriate proportion of ethanol/water, propanol /water and dichloroethane, but insoluble in diethyl ether, acetone, and anhydrous alcohol. In cold water, it will swell into a clear or slightly turbid colloidal solution.
Its aqueous solution has surface activity, high transparency and stable properties. Hydroxypropyl MethylCellulose has thermal gelation properties, so after heating the aqueous solution will precipitate and form gels, and dissolve again after cooling. Products of different specifications have different gelling temperatures. Its solubility varies with the viscosity, and the lower the viscosity is, the greater the solubility will be. There are some differences in the properties of Hydroxypropyl MethylCellulose of different specifications. The dissolution of Hydroxypropyl MethylCellulose in water is not affected by the pH value.
1. Stability
Hydroxypropyl MethylCellulose has many features such as thickening power, salt discharge performance, pH stability, water retention, excellent film-forming properties, extensive resistance to enzymes, dispersion and adhesion.
In latex paints and water-soluble resin coatings, Hydroxypropyl MethylCellulose acts as film-forming agents, thickeners, emulsifiers and stabilizers, making the coating film with good wear resistance, leveling property, and adhesion, and improving the surface tension, pH stability, and compatibility with metal color materials. In the water-based paints, Hydroxypropyl MethylCellulose has excellent storage stability, maintains uniform viscosity, makes pigments uniformly dispersed, and basically won’t be affected by the pH value.
2. Water-holding Property
In putty, the main roles of celluloses are water retention, adhesion, lubrication and so on. Hydroxypropyl MethylCellulose has high water-holding performance, surface opticity and improves uniformity. This helps to avoid crusting and off-powder phenomena, improve resistance to shrinkage, cracking and vertical flow, and to improve the work efficiency since the painting job is comfortable and saves effort.
3. Dispersibility
In the production of synthetic resin, Hydroxypropyl MethylCellulose plays the role of protective colloidal agents and can effectively prevent polymeric particles from agglomerating. In the floating polymerization of vinyl chloride (VC), the disperse system has a direct impact on the product, PVC resin, and the quality of processing and products. It helps to improve the thermal stability of the resin and control the particle size distribution(that is, adjust the density of PVC). PVC resins made from high-quality cellulose ethers not only can ensure that the performance meets international standards, but also have apparent physical properties, fine particle characteristics and excellent melting rheological behavior.
4. Adhesion
In such materials as the adhesive mortar and surface mortar in the thermal mortar system, celluloses mainly plays a role of bonding, water retention and increasing strength. It also helps to improve construction, water-retaining property and resistance to hanging. Furthermore, it improves the resistance to vertical flow, shrinkage and cracking, and thus improves the bonding strength and the work efficiency.
5. Thickening Property
In interface agents, Hydroxypropyl MethylCellulose mainly functions as the thickening agent and can increase the the tensile strength and flexural strength. It also helps to improve surface coatings, enhance adhesion and improve the bonding strength of mortar. Furthermore, good permeability can improve the uniformity of the interface, enhance lubricity and fluidity of mortar, make coating easier and thus improve work efficiency.
6. Constructability
In the bonding of tiles, tile adhesives can function as water-retaining agent and adhesives, and can greatly improve construction performance of adhesives, allow longer opening hours and greater cohesion, and prevent falling of tiles due to excessive water loss. Hydroxypropyl MethylCellulose has the properties of improving the workability, good water-retaining property, enhancing adhesion and high resistance to hanging. Hydroxypropyl MethylCellulose can improve the sticking and tiling efficiency, and also enhance bonding strength and shear strength.
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