COMPRESSION AND COMPACTION, introduction, principle

1. COMPRESSION AND
COMPACTION
PRESENTED BY
NIVEDITHA G
M PHARM(1st sem)
Dept of pharmaceutics

2. Introduction
• Tablets constitutes of almost 70-80% of the all dosage forms.
• Tablets are generally manufactured by 3 main processes.
• Compaction represent one of the most important unit
operations in the pharmaceutical industry.
• Compaction is the situation in which materials are subjected
to some level of mechanical force.
• The physics behind the compaction is stated as the
compression and consolidation of the two phase system due to
applied force.
• While considering the compaction and compression of tablets
we had to taken the properties of powder into the consideration
as they are involve in the progress of compression and
compaction.

3. PHYSICS OF TABLET
COMPRESSION,COMPACTION AND
CONSOLIDATION
o Compression:
Compression of powder means reduction
of bulk volumes of material as a result of
displacement of a gaseous phase under
pressure.
Consolidation :
Consolidation is an increase in the
mechanical strength of a material resulting
from particle –particle interactions.(Increasing
in mechanical strength of mass).

4. Compaction of powder
• Compaction of powder is a general term used to describe
the situation in which these materials are subjected to
some level of mechanical force.
• In pharmaceutical industry the effect such process are
particularly important in various ways. such as:
A. In the manufacturing of tablet and granules,
B. In the filling of hard gelatin capsule shell,
C . In powder handling in general.
The physics of compaction may be simply started as “the
compaction and consolidation of two phase (particulate solid
and gas )system due to applied force” Therefore
Compaction =compression=consolidation

5. Tablet compression machine makes the
tablet by passing the granules in die with lower
and upper punch.
Tablets are being formed by compressing
the granules using the compression machine.
A tablet formation takes place by
combined pressing action of two punches and
die.

7. Solid air interface:
• Atoms or Ions are located at the surface of any solid particle
are exposed to a different distribution of intramolecular and
intermolecular bonding forces than those within the particle.
• This is indicated as unsatisfied attractive molecule force ,
extending out some molecular distance of the solid surface .
• This condition gives rise to free surface energy of the solid
which plays a major role between a particle and the
environment .
• Many important phenomena e.g. adsorption , attrition , rate of
dissolution , crystallization are the fundamental properties.
• Because of these unsatisfied bonding forces identically
particle attract together .

8. a. Flow from the hopper,
b. Relative motion of the mixture,
c. Production of granules,
d. Compression to produce granule or tablets etc.
• The overall resistance to relative movement of the
particle may be markedly affected by two other
factors :
• Firstly , many powder of the pharmaceutical intersect
readily develop intrastatic force specially when
subjected to internal friction.

9. • Although these particle contact and separation
are pre-requisite .
• The charge develop depend on the particular
material involved and the type of motion
produced in it .
• Usually electrostatic force is relatively small
but may be significant because they act over a
greater distance of the molecular force .

10. • The second factor namely the presence of an
adsorbed near the moisture on the particle.
• When particle approaches one another closely
enough , this field of moisture can formed
liquid bridge which holds the particle together
surface tension effect and a negative capillary
pressure.

11. Angles of repose :
It is the maximum angel that can be obtained
between the free standing surface of a powder heap
and horizontal plane. This is expressed by ϕ and
defined by the following equation :
Tanϕ=2h/D
Value of ϕ rarely less than 200 and value of up to 400
indicates reasonable flow potential .Above 500
however the powder flows only with great
difficulty.

13. Angle of repose for various particles

14. Flow rate:
Resistance to movement of particle especially for
granular powder with little cohesiveness may be assist by
determining their flow rate.
Through a circular orifice for instance tablet dies.
Flow experiment with the mixture of different size
fraction of the same material can be particularly valuable
because in many instances they exists optimum proportion
that leads to a maximum flow rate .
For this system when the proportion of the fine particle
exceeds 40% there is a dramatic fall in the flow rate.

15. A simple indication of the ease with which a
material can be induced to flow is given by the
application of a compressibility index(I).
I =(1-V/VO) -100%
Where ,V= Volume occupied by a sample of the
powder after being injected to a standardized
tapping procedure .
Vo=volume before tapping .
Values of below 15% usually gives rise to good flow
characteristics but reading above 25%indicates poor
flow ability.

17. Air space:
The mass of the powder sample can be determined with a great
accuracy but the measurement of volume is more complicated.
The main problem arises in defining the volume of bulk powder.
Three types of air space can be differentiated:
a. Open intraparticulate voids:
Those within a single particle but open to the external
environment.
b.Closed intraparticulate voids:
Those are within a single particle but open to the external
environment.
c. Interparticulate voids:
The air spaces between individual particles.

18. Therefore atleast 3 interpretation of “powder volume” may be
proposed
1.True volume:
True volume is the total volume of the solid particle, which
excludes all spaces greater than the molecular dimension. It is
denoted by Vt.
2.The granular volume or particle volume:
It is the cumulative volume occupied by the particle including all
intraparticulate voids. It is denoted by Vg.
3.The bulk volume :
It is the cumulative volume occupied by the entire powder mass
under the particular packing achieved during the measurement .

20. When studying this phenomena resulting in a
change in volume.
It may be convenient to consider volume V,of the
sample under experiment condition relative to
the true volume Vt.
Relative volume ,vR=V/Vt
VR decreases and tends towards to unity as all
the air is eliminated from the mass .This
phenomenon occurs in tabletting.

22. Porosity:
The voids present in the powder mass may be more
significant than the solid components .For this reason, a
second dimensionless quantity , the ratio of the total
volume of void space(VV ) to the bulk volume of the
material is often selective to monitor the progress of
compression . This ratio is referred to as porosity of the
material.
Porosity, E =Vv/Vb
VV =Vb-Vt
E=(Vb –Vt)/Vb
E=(1-Vt/Vb).100%

23. Density:
The ratio of mass (weight)to volume is known as the density of
the material .Three different densities for powdered solids ,based
on the following ratios,
M/Vt =pt the true density
M/Vg =pg the granular density
M/Vb =pb the bulk density
M is the mass of the sample .
Comparing the density p of a sample under specific test
conditions with the true density(sometimes called theoretical
density) of the material leads to the dimensionless quantity pr, the
relative density ,

24. Pr = p/pt
During compressional processes ,relative density to a
maximum of unity when all air spaces have been
eliminated.
Effect of Applied Forces:
Deformation:
When any solid body is subjected to opposing forces,
there is a slight change in its geometry.
Depending upon the nature on the applied load, the
relative amount of change (deformation) produced by
such forces is a dimensionless quantity called strain.

25. For example, if a solid rod is compressed by
forces acting by each ends to cause a reduction
in length of H from an uploaded length of Ho.
Then the compressive strength Z is expressed by
following equation,
Z= H/HO
The ratio of the force (F)necessary to produce
this strength to the area (A)over which it is
called the stress.

26. Stress,
ᵟ =F/A
Because most powder mass contain air space
true analogue behavior to the solid body should
not be expected.
Under low porosity condition compression do
provide a useful way of interpreting
experimental observation.

28. Process of Tablet Compression
Compression is the process of applying
pressure to the material. There are six different
stages involved in the tablet formulation. These
are:
1. Transitional repacking or particle rearrangement
2.Deformation
3.Fragmentation
4.Bonding
5.Deformation of the solid body
6.Ejection.

29. Process of tablet compression :
• 1.Transitional repacking /particle rearrangement.
 Generally it occurs at low pressure.
 It depends on particle size of distribution and shape.
 Reduction in the relative volume of powder bed into
closure packing structures.
 The granules flow with respect to each other with the
final particles entering the void between the larger
particles and the bulk density of the granulation is
increased.

30. • Spherical particle undergo less rearrangement
than the irregular particles.
• As spherical particles tends to assume close
packaging.
• As pressure increases, relative particle
movement become impossible, inducing
deformation.

33. 2.Deformation
• When the particles of the granulation are so closely packed
that no further fillings of voids can occur ,a further increase in
the compression force causes deformation at that point of
contact.
• Change in shape of material occurs.
• At a certain point, the packing characteristics of the particles,
reduced space or porosity of inter-particulate friction will
prevent any further rearrangement of particles.
• At this point further reduction in the compact volume results in
elastic or plastic deformation of particles.

34. Both plastic and elastic deformation may occur
although one type predominates for a given
material.
Elastic deformation : If the deformation disappears
entirely on the removal of the load or pressure, it is
called elastic deformation. In another word ,When
deformation returns to the initial stage, it is called
elastic deformation.
Plastic deformation :If the deformation does not
disappear entirely on the removal of the load or
pressure ,it is called plastic deformation. In another
word ,When deformation does not return to the
initial stage ,it is called plastic deformation.

35. • yield stress: The force required to initiate a plastic
deformation is known as yield stress.
• Plastic deformation is believed to create the
greatest number of clean surfaces . Because it is a
time dependent process.
• Higher rate of force application should lead to the
formation of less new clean surfaces and thus
resulting in weaker tablets.
• Furthermore, since tablet formation is dependent
on the formation of new clean surfaces , high
concentrations or over mixing of materials that
form weak bonds result in weak tablets.

37. 3.Fragmentation
• As compression force increase deformed particles
start fragmentation due to high load, particles breaks
into smaller fragments leading to formation of new
bonding areas.
• Fragmentation leads to further densification with the
infiltration of the smaller particles in the voids.
• The mechanism of fragmentation and plastic
deformation are not independent because both the
phenomena modify particle size distribution.
• With some materials fragmentation doesn’t occurs
because the stresses are released by plastic
deformation.

38. • Some particles undergo structural breakdown
called as brittle fracture.

39. • Fragmentation do not occur when applied
stress is balanced by a plastic deformation.
• Change in shape.
• Sliding of groups of particle (viscoelastic
flow).

40. 4.Bonding of particles
• After the fragmentation of the particles, as the
pressure increases, formation of new bonds between
the particles at that contact area occurs.
• The hypothesis favoring for increase in mechanical
strength of bed of powder when subjected rising
compressive forces can be explained by theories.
a) The Mechanical Theories
b) The Intermolecular Theories.
c) Liquid –Surface Film Theory

41. • THE MECHANICAL THEORIES:
• It occurs between irregularly shaped particles.
• Also Increases the number of contact points Between the
particles.
• The mechanical theory proposes that under pressure the
individual particles undergo Elastic/Plastic Deformation
and that the edges of the particle intermesh deforming a
mechanical Bond.
• Mechanical interlocking is not a major mechanism of
bonding in pharmaceutical tableting.
• Total energy of compression=Energy of deformation+
Heat+ Energy absorbed for each constituent .

42. THE INTERMOLECULAR THEORIES:
• The Molecules at surface of solids have unsatisfied forces
which interact with the other particle in true contact.
• Absolutely clean surface will bond with the strength of the
crystalline material ,whereas adsorbed materials restrict
bonding.
• Under Pressure Molecule in true contact between new clean
surface of the granule are close enough so that vanderwal
forces interact to consolidate the particles.
• Material containing plenty OH group may also create
hydrogen bond between molecule. e .g., MCC is believed to
undergo significant hydrogen bonding during tablet
compression.

43. • The intermolecular forces theory and the liquid-
surface film theory are believed to be the major
bonding mechanisms in tablet compression.
• LIQUID –SURFACE FILM THEORY:
• Due to the applied pressure ,the particles may
melt (due to lowering of melting point)or dissolve
(due to increased solubility).
• Many pharmaceutical formulations require a
certain level of residual moisture to produce high
quality tablets.

44. • The role of moisture in the tableting process is supported by
the liquid –surface film theory. Thin liquid films forms ,Which
bond the particles together at the particle surface.
• The energy of compression produces melting or liquefaction of
the particles at the contact areas. As the pressure is withdrawn
the melted ingredients solidifies causing fusing of the
particles.
• In addition the solubility of the solution at the particle
interface under pressure is increased and as the pressure is
released it gets super saturated and followed by subsequent
solidification or crystallization thus resulting in the formation
of bonded surfaces.

45. 5.DEFORMATION OF SOLID BODY:
• On further increases of the pressure ,the non- bonded
solid is consolidated towards a limiting density by
plastic/elastic deformation.
• When upper punch is withdrawn, tablet is confined to
the die by a radial pressure therefore any dimensional
change during decompression should occur in axial
direction.
• Capping can occur if decompression does not occur
simultaneously in all direction.

47. • 3 Stages of force necessary to eject a finished
table,
1. Peak force required to initiate ejection.
2. Small force required to push tablet up to die-
wall.
3. Decline force as tablet emerge from die.

48. • When lubrication is inadequate and /or “slip-
stick” conditions occur between the tablet and
the die wall, owing to continuing formation
and breakage of tablet /die wall adhesions.
Worn dies, which cause the bore to become
barrel-shaped ,give rise to a similar abnormal
ejection force trace and may lead to failure of
the tablet structure.

49. Various forces involved in compression:
1. Frictional forces
2. Distribution forces
3. Radial Forces
4. Ejection Forces

50. • Various Forces involved in compression:
1. Frictional forces
Inter-particulate friction:
Reduced Glidants( E.g:Colloidal silica )
Die wall friction
Reduced Lubricants (E.g.Mag.stearate)
2.Distribution forces:

51. Consolidation:
An increase in the mechanical strength of the
material resulting from particle or particle
interactions.(Increasing in mechanical strength of
the mass)
Consolidation process:
 Cold welding
 Fusion bonding

52. CONSOLIDATION
• Consolidation process:
• Cold welding :
when the surfaces of two particles approach each other closely
enough,( e.g .,at separation of less than 50 nm),their free
surface energies result in a strong attractive force , this process
is known as cold welding .
• Fusion bonding:
• contact of particles at multiple points upon application of
load, produces heat which fusion or melting .If this heat is not
dissipated ,the local rise in temperature could be sufficient to
cause melting of the contact area of the particles.
• Upon removal of load it gets solidified giving rise to fusion
bonding and Increase the mechanical strength of mass.

53. Factors affecting consolidation
Both “cold” and “fusion” welding ,the process is
influenced by several factors, including
1. The chemical nature of the materials
2. The extent of the available sources
3. The presence of surface contaminants
4. The intersurface distances
Friction:
The action of one surface or object rubbing against
another.

55. EFFECT OF FRICTION
In this two major components of frictional forces are;
1.Interparticulate friction
2.Die wall friction
1.Interparticulate friction :
 This arises at particle/particle contacts and can be
expressed in terms of a coefficient of inter particulate
friction.
 It is expressed in term of 𝜇𝑖
 It is more significant at low applied loads.
 Materials that reduce this effect are referred as glidants.
 eg ; colloidal silica.

56. • 2.Die wall friction :
 This results from material being pressed against the die
wall and moved down it;
 It is expressed in terms 𝜇𝑊.
 This effect becomes dominant at high applied forces
when particle rearrangement has ceased and is
particularly important in tabletting operation.
 Most tablets contain a small amount of an additive
designed to reduce die wall friction; such additives are
called lubricants.
 Eg :Magnesium stearate.

57. Distribution Forces:
• Most investigations of the fundamentals of tabletting
have been carried out on single-station presses(sometimes
called eccentric presses),or even on isolated punch and
die sets in conjunction with a hydraulic press.
• This simple compaction system provides a convenient
way to examine the process in greater detail.
• More specifically ,the following basic relationships
apply. Since there must be an axial (vertical)balance of
forces:

58. 𝐹𝐴 = 𝐹𝐿 + 𝐹𝐷
𝐹𝐴 =Force applied to the upper punch
𝐹𝐿 =proportion of it transmitted to the lower punch
𝐹𝐷 =reaction at the die wall due to friction at this
surface.

60. • Because of this inherent difference between
the force applied at the upper punch and that
affecting material close to the lower punch ,a
mean compaction force 𝐹𝑀, ℎ𝑎𝑠 been
proposed, where
FM = FA +FL2
• FM is the friction –independent measure of
compaction load.

61. • In single –station presses, where the applied force
transmission decays exponentially .
• Then the more appropriate geometric mean force
will be.
FG =(FA .FL)0.5
• Use of these force parameters are probably more
appropriate than use of FA.
• While determining the relationship between
compressional force and such tablet properties
such as tablet strength.

62. Development of radial force:
The compressional force is increased and any repacking of
the tabletting mass is completed, the material may be
regarded to some extent as a single solid body.
Compression force applied in one direction
(e.g.,Vertical)results in a decrease H in the height.
In the case of an unconfined solid body, this would be
accompanied by an expansion in the horizontal direction of
D
The ratio of these two dimensional changes is known as the
poisson ratio of the material, defined as:

63. The material is not free to expand in the horizontal
plane because it is confined in the die.
consequently ,a die radial die wall force FR
develops perpendicular to the die wall surface,
materials with larger poisson ratios giving rise to
higher values of FR .
Classic friction theory can then be applied to deduce
that the axial frictional force FD is related to FR by
the expression.

64. Die wall lubrication:
Best lubricant has low shear strength and strong
cohesive tendencies.
Lubricant forms a film of low shear strength at the
interface between tabletting mass and die wall.

66. FORCE VOLUME RELATIONSHIP:
End of compressional process is when bulk
volume=tapped volume.
Porosity(E)=0
Decrease in porosity is due to two process
1.Filling of large spaces by interparticulate slippage.
2.Filling of small voids by deformation or fragmentation
at higher loads.
A more complex sequence of events during compression
process involves four stages as shown in fig.

68. Decreasing porosity with increasing
compressional force
i. Single ended pressing
ii. Initial repacking
iii. Elastic deformation
iv. Plastic deformation
v. compression

69. Heckel plots:
The Heckel equation is based upon analogues
behavior to a first order reaction ,Where the
pores in the mass are the reactant, that is:
Log1/E=KyP=Kr
Ky is a material dependent constant inversely
proportional to its yield strength S.
Kr is a related to the initial repacking stage.

70. For cylindrical tablet ,
P=4F/Π.D2
P=applied pressure
D=tablet diameter
F=applied compressional force
E=100.(1-4W/pt .ᴫ.D2.H)
Here, W=Weight of tabletting mass .
Pt=true density
H=thickness of tablet.
Type a:Soft material (e.g, Nacl)
Type b:Hard material (e.g., lactose)

72. Crushing strength of tablet is directly
proportional to KY.
Application of Heckel plot :
Used to check lubricant efficacy.
For interpretation of consolidation mechanisms
.
Duberg and nystom distinguish between
plastic and elastic deformation characteristics
of a material.

73. LIMITATINS:
The plot is linear only at pressure,
The plot can be influenced by the time of
compression and the degree of lubrication.

75. Reference :
Share Lachman &Liebermann Industrial
pharmacy.
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