Pastes
Pastes differ from ointments and creams in showing essentially dilatants flow due to the high concentration of insoluble medicament present in them. They are characterized by definite yield value and increase in the resistance to flow with increased force of application.
As compared to ointments they are usually stiffer, less greasy and more absorptive. As they absorb serum secretions, pastes are used for acute lesions having a tendency to ooze.
Historically the pastes were introduced in about 1900; Unna’s and Lassar’s pastes are still used in practice.
The usual concentration of insoluble powders in pastes may be 20% or more and may consist of starch, zinc oxide, calcium carbonate etc. Zinc Oxide Paste USP contains as high as 50% of powdered medicament.
Pastes are chiefly used as vehicles for atringent and antiseptic agents. They are not suitable for application to hairy parts like scalp because they form a densely melted mass.
Example 01: To prepare Compound Aluminium Paste
Method of Preparation: Mix the aluminium powder and zinc oxide with liquid paraffin until smooth.
Uses:
Mild astringent, protective and antiseptic Example 2 : To prepare Resorcinol and Sulphur Paste
Method of Preparation:
Prepare emulsifying ointment and triturate the resorcinol, precipitated sulphur and zinc oxide with a portion of the emulsifying ointment until smooth. Gradually incorporate the remaining quantity of the emulsifying ointment.
Example 02: To prepare Zinc and Salicylic Acid Paste (Lassar’s Paste)
Method of Preparation:
Melt white soft paraffin in a glass container. Incorporate the zinc oxide salicylic acid, and starch stir until cold.
Example 03: To prepare Zinc and Coal Tar Paste
Method of Preparation:
Melt the emulsifying wax at 70°C and add coal tar and 22.5 g of yellow soft paraffin and stir at 70°C until completely melted. Add the remainder of the yellow soft paraffin and cool at 30°C. Add zinc oxide and the starch with constant stirring and stir until cold.
