Size Reduction || pharmaceutics || Trekpharma

 

SIZE REDUCTION:

Definition: size reduction is a process of reducing the particle size of substance to a finer state of sub-division to small pieces, to coarse particles or to powder.

or

Size reduction or comminution is the process of reducing substances to smaller particles.

Objectives:

1.    Size reduction leads to increase of surface area.

Example 1: The rate of dissolution of solid drug particles increases many folds after size reduction. Griseofulvin, when administered in its micronized form shows around five times better absorption.

Example II: The absorptive power of charcoal and kaolin increases after size reduction due to increase in surface area.

2.    Size reduction produces particles in narrow size range. Mixing of powders with narrow size range is easier.

3.    Pharmaceutical suspensions require finer particle size. It reduces rate of sedimentation.

4. Pharmaceutical capsules, insufflations (i.e., powders inhaled directly into the lungs), suppositories and ointments require particles size to be below 60 μm size.

5.    Size reduction enhances the rate of drying.

6.    It facilitates easier and uniform mixing of several solid ingredients.

Factors affecting size reduction:

1.   Hard material: Hard materials like pumice and iodine are most difficult to comminute. During size reduction these types of materials will produce abrasive wear of milling surfaces, which will then contaminate the material.

2.    Fibrous materials: Crude drugs obtained from plants like glycyrrhiza, rauwolfia, ginger etc. are fibrous in nature and cannot be crushed by pressure. So, they may be size-reduced by cutter mill.

3.    Friable materials: Sucrose and dried filter cakes are friable (i.e. brittle) hence they are easy to comminute by hammer mill or fluid energy mill.

4   Plastic materials: Synthetic gums, waxes and resins become soft and plastic during milling. These low melting substances should be chilled (made cold) before milling. These types of materials are milled by using hammer mill and fluid energy mill.

5.   Hygroscopic materials: Hygroscopic materials absorbs moisture rapidly hence they must be comminuted inside a closed equipment like ball-mill.

6.    Thermolabile materials: Thermolabile materials like vitamins and antibiotics are milled inside chilled equipment.

7.   Inflammable materials: Fine dust, such as dextrin, starch and Sulphur, is a potential explosive mixture under certain conditions. All electrical switches should be explosive proof and the mill should be earthed properly.

8.    Particle size of the feed: For a mill to operate satisfactorily, the feed should be of proper size.

9.   Moisture content: Presence of more than 5% moisture hinders the milling process and produces a sticky mass.

Methods of size reduction:

Method

Common examples

Cutting

Scissors cutter mill

Compression

Roller mill crusher mill

Impact

Hammer mill

Attrition (pressure and friction)

Fluid energy mill

 

 

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