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PY5010 Drug design and medicine development > Milling > Flashcards

Milling Flashcards

1
Q

What is comminution

A

Particle size reduction

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2
Q

What does size reduction affect?

A
  • Mean particle size

- Its distribution

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3
Q

Why do particles size need to be reduced?

A
  • Improves the efficiency of pharmaceutical processes e.g. mixing or suspension production
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4
Q

What is crack propagation?

A
  • High localised stress produces strain in the particles that cause bond rupture and propagation of crack through the regions which has discontinuities
  • Takes place during particle size reduction
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5
Q

What is Griffith’s theory?

A
  • Describes crack propagation as strain energy in specific region
  • mean stress around crack is called crack multiplier
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6
Q

Griffith’s theory equation

A 
Ok = 1+2(L/2r)
Ok = crack multiplier
L = length of crack
r = radius of curvature of the tip of the crack
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7
Q

What material is able to propagate a crack?

A

Brittle

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8
Q

How do plastic materials react to crack propagation?

A
  • Resist fracture at larger stress
  • Allow strain relaxation without crack propagation
  • Atoms or molecules slip over one another
  • Requires energy
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9
Q

Mohs’ scale

A

Table of minerals
Diamond has Mohs hardness > 7
Talc has Mohs hardness < 3

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10
Q

Brinell

A

Developed a quantitative measurement of surface hardness

  • Hard spherical indenter in contact with test surface and known constant load applied to the sphere
  • Permanent deformation is measured
  • Hardness calculated
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11
Q

How can you reduce the particle size of elastic materials?

A

Lowering the temperature to below glass transition (Tg)

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12
Q

How much of the energy affects particle size reduction?

A

Very small ( about 2% )

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13
Q

What happens to the rest of the energy in comminution?

A

Lost due to:

  • elastic/plastic deformation
  • interparticulate friction
  • particle-machine wall friction
  • heat, sound, vibration
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14
Q

What is Rittinger’s hypothesis?

A
  • Relates energy used in comminution to new surface area produced

E=Kr(Sn-Si)
Kr= Rittinger’s constant
Sn= New SA
Si = Initial SA

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15
Q

What is Kick’s theory?

A
  • Relates energy used in comminution to ratio of change in size
E = Kk log (di/dn)
Kk = Kick's constant
di = Initial particle diameter
dn = New particle diameter
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16
Q

Cutting methods

A
  • Cutter Mill
17
Q

How does cutter mill work?

A

Series of knives attached to horizontal rotor which acts against a series of knives attached to mill casing
Size reduction occurs by fracture of particle between the two knives

18
Q

Size reduction range of cutter mill

A

100-50000 mcm

- useful in producing coarse particle

19
Q

Impact methods

A

Hammer mill

Vibrational mill

20
Q

How does hammer mill work?

A

Consists of four or more hammers hinged onto central shaft enclosed in a rigid metal case
Hammers swing out radially from rotating central shaft at an angular velocity of 80 s-1
Strain rate is so high most particles undergo brittle fracture

21
Q

Size reduction range of hammer mill

A

50-5000 mcm

22
Q

How does vibrational mill work?

A

About 80% of mill filled with porcelain or steel balls and mill is vibrated

23
Q

Size reduction range of vibrational mill?

A

1-1000 mcm

24
Q

Compression methods

A

Runner mill

Roller mill

25
Q

How does runner mill work?

A

Using a mortar and pestle

26
Q

Size reduction range of runner mill?

A

20-10000 mcm

27
Q

How does roller mill work?

A

Two cylindrical rollers mounted horizontally and rotated along their long axes

  • one driven directly
  • second driven by friction
  • material drawn through gap between rollers
28
Q

Size reduction range of roller mill

A

500-100000 mcm

29
Q

Attrition method

A

Roller mill

30
Q

How does roller mill of attrition method work?

A
  • Two or three porcelain or metal rollers mounted horizontally with adjustable gap - upto 20 mcm small
  • Rollers rotate at different speeds and material is sheared as it passes through the gaps transferring from slower to faster roller
  • useful for suspensions, pastes and ointments
31
Q

Size reduction range of roller mill of attrition method?

A

1-200 mcm

32
Q

Combined impact and attrition method

A

Ball mill

Fluid energy mill

33
Q

How does ball mill work?

A

Hollow cylinder rotates on its horizontal longtitudinal axes

  • 30-50% of mill occupied with different diameter balls
  • smaller ones form fine product
34
Q

Size reduction range of ball mill

A

1-200 mcm

35
Q

How does fluid energy mill work

A

Hollow toroid has a diameter of 20-200mm and air is injected as high pressure jet through the nozzle forming zones of turbulence which ensures high levels of particle-particle collisions
Particle size classifier ensures material is retained

PY5010 Drug design and medicine development (2 decks)

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