Physics of tablet compression

Physics of tablet
compression
Nitin
M.pharmacy
IPS KUK

Tablet
 Tablets are solid dosage forms consisting of
active ingredient(s) and suitable pharmaceutical
excipients. They may vary in size, shape, weight,
hardness, thickness, disintegration and
dissolution characteristics, and in other aspects.
They may be classified, according to the method
of manufacture, as compressed tablets or molded
tablets.

Types of tablet
1)compressed tablets
2)sugar coated tablets
3)film coated tablets
4)enteric coated tablets
5)effervescent tablets
6)chewable tablets
7)dispersible tablets
8)sustained release tablets

9)multilayer tablets
10)sublingual tablets
11)buccal tablets
12)implant tablets
13)hypodermic tablets
14)solution tablets
15)vaginal tablets

Compressed tablet manufacture
•The classification of manufacturing methods
wet granulation: suitable for drugs that are stable to moisture and
heat
dry granulation: suitable for drugs that are sensitive to moisture
and heat
powder compression : suitable for drugs that are sensitive to
moisture and heat, fill material possessing, good flowability and
compressibility
granulation
direct
compression
crystal compression：suitable for drugs with proper crystal
form and good flowability

The classification of tablet presses
 Tablet presses:
a. single-punch presses
b. multi-station rotary presses

The main components of single-punch
tablet press
Core components:
die
lower punch
upper punch

COMPACTION : it is defined as ‘compression, consolidation̕
of a two- phase (particulate solid-gas) system due to the
applied force .
COMPRESSION : A reduction in the bulk volume of the
material as a result of displacement of the gaseous phase.
CONSOLIDATION: Increase in the mechanical strength of
the material resulting from particle-particle interactions.
DEFINITIONS:

When some external forces applied on the powder reduction in the bulk volume of
the powder. Stages involved in the bulk reduction of powdered solid
COMPRESSION :

Rearrangement / Repacking also occur On applying more force Particles undergo certain
type of deformation.
Two types of deformations :
1.Elastic deformation : Removal of upload act like rubber comes to original place,
usually all solids undergo elastic deformation.
Ex:- Acetyl salicylic acid, MCC.
2. Plastic deformation : They won’t come back to its original volume, completely
reduction in the bulk volume. [ When shear strength of particles
less than the tensile (breaking) strength of the particles ]
Brittle fracture : Shear strength more than the tensile strength
EX:- sucrose

Consolidation
 When surface of two particles approach each other (<50 nm), their free surface
energies result in a strong attractive forces a process known as cold welding.
 Because of the roughness of particles actual area involved in bonding may be small.
On the macro scale, most particles are irregular in shape with many points of contact in a
bed of powder.
 Applied load was transmitted through this contacts, result in generation of
considerable frictional heat if this heat was not dissipated the local rise in temperature
occurs and causes melting of contact area of particles. This melt solidifies giving rise to
fusion bonding.

Cold and fusion welding processes are influenced by
 Chemical nature of materials
 Extent of available surface
 Presence of surface contaminants
 Intersurface distances
Consolidation also depends on the degree and type of crystallinity.

Interparticulate friction:
• friction between particle/particle and expressed as coefficient
of “interparticulate friction”
addition of glidants
Die wall friction:
• arises as material being pressed to die wall and
moved down it, can be expressed as
“coefficient of die wall friction”
 addition of lubricants
EFFECTS OF FRICTION

Force distribution
 Most of the investigations of the fundamentals of tabletting have
been carried out on single station presses or even on isolated punch
and die sets in conjunction with a hydraulic press.
 This compaction system provides a convenient way to examine the
process in greater detail.
More specifically the following basic relationships apply.

The axial balance of forces:
FA=FL+FD
FA=force applied to the upper punch
FL= force transmitted to the lower punch
FD= reaction at the die wall to the friction at this surface.
The mean compaction force (FM):
FM = (FA+F)L/2
A recent report confirms that FM offers a practical friction- independent measure of
compaction load, which is generally more relevant than FA.
The geometric mean force (FG):
FG = (FA . FL)0.5

Die wall lubrication:
Die wall lubricants act by forming a film of low shear strength between tabletting mass and
die wall. Chemical bonding occurs between boundary lubricant and the surface of the die wall
as well as at the edge of the tablet.
 Best lubricants have low shear strength but strong cohesive tendencies at right angles to the
plane of shear. Shear strength of lubricants can be measure by “punch penetration”.
Material Shear strength
Stearic acid 1.32
Hard paraffin 1.86
Potassium stearate 3.07
Sodium stearate 3.32
Boric acid 7.16

Ejection force:
 Radial die wall forces and die wall friction affect the ejection of tablet. Force required
to eject a tablet follows 3 stages.
1. A peak required to ejection by breaking of tablet and die wall adhesions.
2. A smaller force required to push the tablet up the die wall.
3. Declining force of ejection as the tablet emerges from the die.
 Variations in this pattern occurs especially when lubrication is inadequate, slip-stick
conditions. Worn dies causes the die bore to become barrel shaped and may leads to
structural failure of tablets .Ex: well lubricated system has low FE.

 After the completion of compression air spaces are removed i.e Vb=Vt and E=0.
 Residual porosity is required, a relation exists between applied force FA and remaining
porosity ‘E’. Decreased porosity was due to. Filling of air spaces by interparticulate slippage
Filling of small voids by deformation (or) fragmentation at higher loads. This process was
expressed as
E0= initial porosity
E = porosity at pressure P
K1 K2 K3 K4 = constants
But this data applies to only few materials such as alumina and magnesia and establishes that
degree of compression depends upon E0. To eliminate this, experiment are carried out on
tablet masses of same Vt and variable initial values of Vb
Force volume relationships:

More complex events in compression involves 4 stages
• Initial repacking materials followed by elastic deformation.
• Elastic limit is reached, plastic deformation/brittle fracture dominates.
• All voids are eliminated.
• Compression of solid crystal lattice.
In tabletting process after applying compressional force the relation b/w applied pressure (P)
and porosity (E) become linear over the range of pressure.
Shapiro equation:
LogE=LogE˳ - K.P E0= Porosity when the pressure is ‘o’
K = Constant
Walker equation:

Heckel equation is based upon analogous behavior to a first order reaction, where the pores
in the mass are reactant.
Ky= material dependent constant, but inversely proportional to material yield
strength(S)
Ky= 1/3s
Kr= related packing stage (E0)
These relations may be established by measuring applied force (F) and movements of
Punches during compression cycle and translating this data into applied pressure (P), for a
cylindrical tablet.
D= diameter of tablet
,
Heckel plots:

Similarly,
W= weight of tablet
= true density
H= thickness of tablet at that point.
 Heckel plots identifies the predominant form of deformation for a given
sample.
 Soft materials undergo plastic deformation readily, retains different
degrees of porosity based on initial packing of die, which was influenced by
the size distribution, shape etc of original particles.
 Harder materials have high yield strength undergo compression by
fragmentation i.e breakdown of larger particles to form denser packing.

 Type (a) plots have higher slope than type (b)
from this, we can expect that these materials have lower yield strength(S).
 Hard, brittle materials are more difficult to compress than softer ones, fragmentation occurs
in hard materials & plastic deformation in soft materials.

Two regions of Heckel plots represents
1. Initial repacking stage
2.Subsequent deformation process

 Crushing strength can be correlated with value of Ky. i.e Ky larger –
harder tablets. This information was utilized for binder selection to
particular material.
Heckel plots are influenced by
• Degree of lubrication
• Size of the die.
 Residual porosity in particular formulations provide good mechanical
strength, rapid water intake and hence good disintegration characteristics.

Physics of tablet compression

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Physics of tablet compression