The Effects of Binding Agents on Flow Properties

Flow is one of the important powder properties in tablet manufacture. A powder that does not flow well through the hopper results in an uneven distribution of the active ingredient in individual tablets. This poses content uniformity problems and may result in underdosing or overdosing when patients consume these tablets. Binding agents are added to a drug-excipients blend to improve its flow properties. They exert this action by altering particle size, shape and moisture content in the blend.

Particle Size

• Large particles flow better than smaller-sized particles. During the process of wet granulation, binding agents are added to fine powders. This causes a significant increase in particle size. As particle size increases, there is a lesser influence of adhesive forces on the powder's ability to flow. Granulated powders, therefore, show lesser adherence with the hopper walls and between particles, resulting in better flow properties.

Particle Shape

• The flow of powders is best when all particles have a similar shape. The process of granulating a tablet mix with a binding agent converts the powder into agglomerates, which have a uniformly spherical shape; therefore, such particles flow better than nongranulated blends.

Moisture

• The amount of moisture in a powder blend is another crucial factor determining the quality of powder flow. A moisture level that is too low does not allow sufficient agglomeration; too much moisture content in granules causes them to adhere to each other and the hopper surface. It is important to use appropriate concentrations of a binding agent to ensure moisture content for optimum flow of the granules.

Binding Agent Selection

• Several binding agents are available for use in the wet granulation of tablets. Corn and wheat starch, pregelatinized starch, microcrystalline cellulose and povidone are a few of the commonly used binding agents. Selecting the right one is crucial to ensure the binding agent promotes flow of powder and its compressibility. Binding agents that cause bonding that is too strong between particles tend to hamper tablet disintegration on ingestion by the patient, leading to slower drug delivery.