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eye on and superfine (330 mesh) profiles. See Table 1. Overview HPC is chemically inert and non- toxic and has a white crystalline appearance, which makes it well suited for manufacturing pharmaceu- ticals. In addition to its superior binding ability, HPC has a low mois- ture-uptake upper limit—approxi- mately 15 percent after 2 days in a high humidity (40°C/80 percent RH) setting—compared to other popular binders, including polyvinylpyrroli- done (PVP) and hydroxypropyl methylcellulose (HPMC), both of which are more hygroscopic and continue to absorb moisture (more than 50 percent in the case of PVP) under the same conditions. HPC is soluble in water and a wide range of polar organic solvents, and its ther- moplasticity and cohesive properties make it a versatile ingredient with wide applications in IR solid dosage forms. The next sections review HPC's applications as a wet and dry binder and solubility enhancer in IR tablets and films. Wet and dry binding High-shear wet granulation. HPC polymers can be added as a binder solution or in dry powder form directly to the powder bed in a high-shear mixer [1]. As a dry mix binder, 3 percent concentration is often sufficient. The dry-mixing approach has several advantages: Less water is required, there is no need to prepare a binder solution, and the process is unaffected by the viscosity of a binder solution. In the subsequent kneading step, 15 percent water (by volume) is typically added to mixtures that include HPC regular and fine powders; for mixtures that include the SSL superfine powder (SSL-SFP) HPC, 12.5 percent water volume is optimal because that will produce the most uniform granule size distribution range. In general, the addition of less water volume produces granules that fall short of the target particle size distribution range, while higher amounts yield granules that exceed the target. Furthermore, the shorter the gran- ulation process, the harder and less friable the tablets will be and the Tablets & Capsules October 2014 37 Ryan Cheng Nisso America excipients Guest columnist Ryan Cheng describes the application and benefits of using spe- cial low-viscosity hydroxypropyl cellu- lose (HPC) to formulate immediate- release (IR) tablets and films and improve the solubility of actives. Oral solid dosage forms offer patients and consumers simplicity and convenience, and their formula- tion requires versatile excipients that imbue the product with good stabil- ity, remain effective even at low use levels, and meet all regulatory re - quirements. HPC is one such excipi- ent. It serves as a wet and dry binder, film former, and solubility enhancer, among other applications, in dietary supplements and pharmaceuticals. Nisso and several other companies manufacture pharmaceutical-grade HPC. The focus of this column is Nisso's special low-viscosity grades of HPC, which are the only ones of their type on the market. Each grade of our HPCs is assigned a specific viscosity range based on a 2 percent aqueous solution at 20°C. They include the Super Special Low (SSL) grade with a viscosity of 2.0 to 2.9 millipascal-seconds (mPa· s), the Special Low (SL) grade with a viscos- ity of 3.0 to 5.9 mPa·s, and the Low (L) grade with a viscosity of 6.0 to 10.0 mPa·s. Higher-viscosity grades are also available, and they are used in controlled-release formulations. The low-viscosity HPCs are offered as powders in two size profiles: regu- lar (40 mesh) and fine (100 mesh). The exception is the SSL grade, which is offered only in the regular Table 1 Summary of HPC grades SSL SL L M H Viscosity (mPa·s at 2% aqueous solution and 20°C) 2.0 – 2.9 3.0 – 5.9 6.0 – 10.0 150 – 400 1,000 – 4,000 Molecular weight (GPC method) 40,000 100,000 140,000 620,000 910,000 Regular (40 mesh) Yes Yes Yes Yes Yes Fine (100 mesh) - Yes Yes Yes Yes Superfine (330 mesh) Yes - - - - Availability