Size Reduction | PHARMACEUTICS-I

Chapter - 5

Size Reduction

Contents

1. Definition

2. Importance of Particle Size Reduction

3. Factors Affecting Size Reduction

4. Methods of Size Reduction 

Size reduction or comminution is the process of reducing drugs (vegetable and chemical substances) into smaller pieces, coarse particles or fine powder.

Importance of Particle Size Reduction

1. To increase the rate of solution in case of chemical substances, because reduction of the particle size increases the surface area for the action of solvent.

2. To allow the rapid penetration of the solvent (menstruum), in case of crude drugs for the extraction of active constituents from vegetable and animal drugs.

3. To get a uniform powder because particle size reduction helps in uniform mixing of drugs, required for preparing different formulations for administration.

4. To increase the rate of absorption of a drug. The smaller the particle size, the greater is the rate of absorption.

5. To improve the stability of certain pharmaceutical dosage form, such as suspensions. The rate of sedimentation decreases to a large extent by reducing the particle size of the drug.

6. To help in the process of separation of solids from liquids by filtration or by sedimentation. The rate of filtration or sedimen tation depends upon the particle size.

Factors Affecting Size Reduction 

1. Hardness: The hardness of the material affects the process of size reduction. It is easier to break soft material to a small size than hard material.

2. Toughness: The crude drugs of fibrous nature or those having higher moisture content, are generally tough in nature. A soft but tough material may present more problem in size reduction, than a hard but brittle substance.

3. Stickiness: Stickiness causes a lot of difficulty in size reduction. This is due to the fact that material adheres to the grinding surfaces or sieve surface of the mill. It is difficult to powder a drugs of having gummy or resinous nature, if the method used for size reduction generates heat. Complete dryness of material may help to overcome this difficulty.

4. Material structure: Materials which show some special struc ture may cause problem during size reduction e.g. vegetable drugs which have cellular structure, generally produce long fibrous particles on its size reduction. Similarly a mineral substance having lines of weakness. produces flake like particles on its size reduction.

5. Moisture content: The presence of moisture in the material influences a number of its properties such as hardness, toughness or stickiness which in its turn affects the particle size reduction. The material should be either dry or wet. It should not be damp. The material having 5% moisture in case of dry grinding and 50% moisture in wet grinding does not create any problem.

6. Softening temperature: Waxy substances such as stearic acid or drugs containing oils or fats, become softened during the size reduction processes, if heat is generated. This can be avoided by cooling the mill.

7. Purity required: Various mills used for size reduction often cause the grinding surfaces to wear off and thus impurities come in the powder. If a high degree of purity is required, such mills must be avoided. Moreover, the mills should be thoroughly cleansed between batches of different materials in order to maintain purity.

8. Physiological effect: Some drugs are very potent. During their particle size reduction in a mill, dust is produced which may have an effect on the operator. In such cases, the enclosed mills may be used to avoid dust.

9. Ratio of feed size to product size: To get a fine powder in a mill, it is required that a fairly small feed size should be used. Hence it is necessary to carry out the size reduction process in several stages, using different equipment e.g. preliminary crushing followed by coarse powder and then fine grinding.

10. Bulk density: The output of the size reduction of material in a machine, depends upon the bulk density of the substance.

METHODS OF SIZE REDUCTION

1. Cutting ( cutter mill)

2. Compression (roller mill)

3. Impact (hammer mill, disintegrater mill)

5. Attrition (ball mill)

6. Combined Impact and Attrition ( fluid energy mill)

SELECTION OF DEGREE OF SIZE REDUCTION

The basic principles governing selection of degree of size reduction are as foliows:

1. Drugs required to be administered in powders and suspension in mixtures are reduced to fine powder.

2. Drugs to be injected in suspension form are reduced to very fine powder.

3. Drugs which are to be extracted, need size reduction, which depends upon the following factors:

(a) Solvent or menstruum: Water has more penetration power than other organic solvents such as alcohol (60% or more). Therefore, a higher degree of size reduction is required with alcoholic as menstruum than with water.

(b) Drug/menstruum ratio: The rate of solution is greater in case of a larger than a small volume of solvent. Moreover, the size reduction of a solute also hastens the solution formation. Therefore, in preparing a weak preparation, the entire drug or coarse grade of drug can be used but for a stronger preparation, fine degree of size reduction is required.

(c) Physical character of the drug and localisation of the con stituents The menstruum is slowly penetrated into hard woody drugs than into soft drugs. Therefore in the case of hard drugs it becomes necessary to reduce them to a fine powder than in the case of soft drugs for the same process of extraction and menstruum. The localisation of active principles is an important factor in deciding the degree of size reduction of drugs e.g. active constituents of digitalis leaves are localised chiefly in the harder tissues of leaves and hence require a fine degree of size reduction.

(d) Process: In infusion and decoction processes the menstruum used is water and the drug/menstruum ratio is large. As such there is no need of a higher degree of size reduction. In the case of maceration and percolation processes, the drugs/men struum ratio is comparatively smaller than for infusion and decoction. Therefore, a higher degree of size reduction is required. The same is also very essential in case the men struum is alcohol and extraction process is percolation.

LEVIGATION AND ELUTRIATION

Levigation

• Levigation is the process of wet grinding.

• The material is converted into paste with water by using colloidal mill or a mill resembling an edge-runner mill.

• The process is used for the preparation of light kaolin, chalk, red and yellow mercuric oxide, calamine and camphor etc.

Elutriation

• Elutriation is the process of separation of fine particles and coarse particles from a paste obtained after levigation.

• The paste is mixed with a large volume of water and the mixture is allowed to stand for a short period during which the heavy coarse particles with high density settle down on the bottom of the vessel.

• In small scale grinding, conical flask can be used but on a large scale, elutriation tanks are used (For detail see chapter on size separation).