Lecture 11- Particle Size Reduction Flashcards
Size reduction
-grinding, milling or particle size reduction= used for the process which break down particles into a smaller size
*milling equipment = used to reduce particle size to the desired degree
^degree= depends on the type of active ingredient it contains + its use
Important of size reduction
Surface area= particle size reduction increases the surface area of the substance. Increased surface= exposes the drug particle more to the body fluid
Effect of absorption= sparingly soluble drugs are absorbed more rapidly when administered in fine powder forms
Effect for viscosity= viscosity increases when the particle size is reduced
Extraction= if the drug is powdered + used for extraction; more surface area is exposed to the solvent for extraction
Important of size reduction
Mixing= fine particles give uniform mixing with a homogeneous mass.
Appearance= milling of the ointments; pastes/creams give them a smooth appearance. Also stabilises the preparation
Stability= velocity of the dispersed phase is dependent on the particle size
Small or large particles?
Good flowability= essential to ensuring uniform feeding of formulations to high-speed tablet machines
-determines uniformity of tablet + capsule weights = assuming uniform mixing of drug and excipients had been achieved
-smaller particles with high SA do not flow as well as larger particles
Surface hardness
-size reduction can be influenced by surface hardness of a material
-hardness of a material can be described by its position in a scale (MOHs scale)
Hard materials;
- difficult to comminute; may cause damage
Soft materials;
- difficult to mill, absorb large energy through plastic
Criteria for size reduction;
- large capacity
- require a smaller power input per unit of product
- yield a product of the single size distribution desired
Milling equipment & mechanisms of particle size reduction
*cutting methods; cutter mill
*compression methods; mortar + pestle, end-runner mill & edge-runner mill
*impact methods; hammer mill + vibration mill
*attrition methods; roller mills
*combined impact + attrition methods; ball mill and fluid energy mill
Cutting method
Size reduction takes place with the help of sharp blades
Occurs by fracturing particles between stationary and rotating knives
Screen is fitted to retain the material in the mill until a sufficient degree of size reduction is achieved
Compression method
Size reduction due to crushing by compression & shearing
Smaller scale- pestle and mortar
End-runner mill= modified from of pestle and mortar
Edge runner mill= pestle is mounted horizontally and rotates against the powder bed
^2 heavy wheels either made out of stone or metal connected by a shaft
-wheels rotate at its axis in a circular pan
-material is fed into the centre of the pan + is worked outwards by the action of wheels
-material is scraped from the bottom of the vessel and fed into the wheel-> gets crushed into powder
Impact methods; hammer mill
Hammers swing out radially from rotating shift
High velocity of hammers = brittle fracture to particles
Small particles= less prone to fracture than larger particles
Produce powder with narrow size distributions
Particles are retained within a mill by a screen which allows only adequately comminuted particles to pass through
Impact methods; vibration mill
Filled to approximately 80% total volume with porcelain/steel balls
During milling= whole body of the mill is vibrated= sized reduction occurs by repeated impaction
Milled particles fall through a screen at the base of the mill
Efficiency of vibratory milling > conventional ball milling
Attrition method; roller mill
Shearing of material between the rollers
2/3 mobile rollers are mounted horizontally with an adjustable gap
Rollers roll in opposite directions - may be smooth/corrugated
Rotates at diff speeds, material is sheared as it passes through the gap
Combined impact + attrition methods; ball mill
Milling Efficiency Depends On:
1. Feed Amount
Too much → cushioning effect, less impact, ↓ efficiency.
Too little → abrasive wear on mill, risk of contamination.
- Ball Size
Large balls → break coarse material.
Small balls → refine fine particles, fill spaces between large balls. - Speed of Rotation
*Cascading (Low speed)
Balls barely move → minimal size reduction
Attrition dominates (sliding/grinding).
*Centrifugation (High speed)
Balls stick to wall, no fall → no size reduction.
*Cataracting (Optimal speed ~2/3 critical)
Balls rise & fall on material → maximum impact + attrition
Most efficient for size reduction.
*check diagram
Combined impact and attrition methods;
Fluid energy mill
Slide 24
Combined impact and attrition methods; pin mill
Slide 25
