Physics of tablet compression

The tablet compression process involves different steps of the rearrangement of particles within the die cavity and initial elimination of voids. It is very necessary for the academicians, students, production chemists, managers in the pharma background, to have the idea about the physics behind the tablet compression process.

1. PHYSICS OF TABLET
COMPRESSION
HEMANGA HAZARIKA
M. Pharm 1st Semester (2013 batch)
Roll no-MP/13/02 Dept. of Pharmaceutics
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2. Table of contents
a)Compression
b)Compression process
c)Properties of tablets influenced by compression
d)Factors in formulation development
e)Powder compression models
f)Compaction of powder
g)Role of moisture
h)Force-volume relationship
i)Conclusion
j)References
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3. Compression-the reduction in the bulk
volume of a material as a result of the removal of
the gaseous phase (air) by applied pressure.
In Pharmaceutical tablet manufacturing an
appropriate volume of granules in a die cavity is
compressed between an upper and lower punch to
consolidatethe material into a single solid matrix
which is subsequently ejected from the die cavity
as an intact tablet
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4. Tablet Compression
All tablets are made by a process of compression
Solid in the form of relatively small particles, is
contained in a die and a compression force of
several tonesis applied to it by means of punches
Two type of tablet press;
The extrinsic presshas one die and one pair of
punches
The rotary press has a larger number of dies
which are fitted, with their corresponding punches
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5. Process of tablet compression
It can be divided into three stages-
1)Filling
2)Compression
3)Ejection
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8. The process of compression
The subsequent events that occur in the
process of compression are…..
1)Transitional repacking
2)Deformation at the point of contact
3)Fragmentation and/or deformation
4)Bonding
5)Deformation of the solid body
6)Decompression, and
7)Ejection
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9. 1)Transitional repacking
The granules flow with respect to each other
with the finer particles entering the void between
the larger particles and the bulk density of the
granulation increased
Spherical particles undergo less particle
rearrangement then the irregular particles
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10. 2)Deformation at the point of contact
After closely packed of the granulation particles,
no further filling of the void can occur. A further
increase of compression force causes deformation
at the point of contact
Elastic deformation
Plastic deformation
Yield stress
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11. 3)Fragmentation and/or deformation
Fracture occurs when the stress within the
particle become great enough to propagate
Fragmentation cause furthers densification
with the infiltration of the smaller fragments
into the void space
With some materials fragmentation doesn’t
occur because the stress is released by
plastic deformation
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12. 4)Bonding
a.The mechanical theory-If only the mechanical
bond exists, the total energy of compression is
equal to the sum of the energy of deformation,
heat and energy absorbed for each constituent
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13. b.The inter molecular theory-The molecules(or
ions) at the surface of solid have unsatisfied
forces(surface free energy), which interact with
the other particles in true contact.
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14. c.The liquid surface film theory-Bonding to the
presence of a thin liquid film which may be
consequence of fusion or solution at the
surface of the particle induced by the energy of
compression. It may classified into two ways-
#Hot welding
#Cold welding
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15. Hot welding
 On macro scale, irregular particle shape ,there is
no. of points of contact.
 Application of load under appreciable force, results
in generation of frictional heat.
 If this heat is not dissipated, local rise in
temperature.
 This heat is sufficient to melt the contact surfaces.
 Melt solidifies gives rise to fusion bonding.
 Which results in increasing mechanical strength of
tablet.
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16. Cold welding
Particles approach each other very closely
(>50nm)
Their free surface energies result in a strong
attractive bond formation.
This bond depends on interior nature of the
particles.
This phenomenon is called cold welding
Cold welding results in increasing mechanical
strength of tablet.
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17. The influence of applied pressure on specific
surface area is shown in figure 1
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18. 5)Deformation of the solid body
Applied pressure further increased the bonded solid;
consolidatedtoward a limiting density by plastic and/or
elastic deformation of the tablet within the die
Strain : The relative amount of deformation produced on
a solid body due to applied force .
It is dimensionless quantity .
Compressive strain ,
Z=ΔH/Hο where,
H-Thickness
Stress(σ):
σ=F/A
here,FisforcerequiredtoproducestraininareaA
.
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19. As the applied pressure is further increased, the bonded
solid is consolidated toward a limiting density by plastic
and/or elastic deformation of the tablet within the die as
shown in Figure 2
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20. 6)Decompression
As the upper punch withdraw from the die cavity,
the tablet is confined in the die by a radial
pressure. Consequently any dimensional change
during decompression must occur in the axial
direction
Plastoelasticity (γ)
γ=[Hο/H–(H-H)/Hο-H]
where,
Hο,H,H=thicknessoftabletmassatonsetof
loading,atmax.appliedpressureandonejection
fromdie.
γ>9producetabletsthatarelaminatedorcapped.
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21. The process of compression has been described in terms
of the relative volume (ratio of volume of the compressed
mass to the volume of the mass at zero void) and applied
pressure as shown in Figure 4
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22. 7)Ejection
The lower punch rises and pushes the tablet
upward there is a continued residual die wall
friction.
As the tablet removed from the die the lateral
pressure is relievedand the tablet undergoes
elastic recovery with an increased (2-10%) of the
volume of that portion of the tablet removed from
the die
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23. The ratio of the pressure at time t to the maximum
pressure is plotted against the logarithm of time
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24. Properties of tablets influenced by
compression
1)Density and porisity
2)Hardness and tensile strength
3)Specific surface
4)Disintegration
5)Dissolution
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25. 1)Density and porosity
The apparent density of a tablet is exponentially
related to the compressional pressure
Porosity and apparent density are inversely
proportional
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27. 2)Hardness and tensile strength
There is a linear relationship between tablet
hardness and the logarithm of applied pressure
except at high pressure
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28. The radial tensile strength is proportional to the applied
pressure.
For an isotopic, Homogenous tablet, the radial and axial
tensile strength are equal
As applied pressure is increased, fragmentation results
in a stronger, radial tensile strength than axial tensile
strength
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29. The influence of concentration of providoneon the
tensile strengths of hydrous lactose is shown in figure 11
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30. 3)Surface area
Specific surface area is the surface area of 1g of
material
As the relationship between applied pressure
and apparent density is independent of the
material being compressed, the influence of
starch on the specific surface and porosityis not
significant
As the lactose granules, which were granulated
by adding 10%starch paste, are compressed,
the specific surface is increased to a maximal
value(four time that the initial value)
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31. The influence of applied pressure on the specific
surface area of a tablet is typified by Figure 15
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33. 4)Disintegration
Usually, as the applied pressure used to prepare
a tablet is increased, the disintegration time is
longer
There is an exponential relationship between the
disintegration time and the applied pressure, as
shown for aspirin and lactose in Figure 16
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35. 5)Dissolution
Four most common dissolution-pressure relations are-
Dissolution is more rapid as the applied pressureis
increased
Dissolution is slowedas the applied pressureis
increased
Dissolution is faster to a maximum, as the applied force
is increased, and then a further increasein applied
pressure slows dissolution
Dissolution is slowed to a minimum as the applied
pressure is increased, and then furtheran increasein
applied pressure speeds dissolution
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37. Factors in formulation
development
More than any other type of tablets, successful
formulations of direct compression tablets depend
on careful consideration of excipientproperties
and optimization of the compressibility, fluidity, and
lubricabilityof powder blends
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38. a)Compressibility
Formulation should be directedat optimizing
tablet hardness without applying excessive
compression force while at the same time
assuring rapid tablet disintegrationand drug
dissolution
A compression of the relative compressibility of
various direct-compression-fillersusing
magnesium stearate and stearic acid as
lubricants is presented in Figures 1 and 2
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41. b)Fluidity
The fluidity of tablet blends is important not
only from the direct effect on uniformity of
tablet weight, but also from the role it plays in
blendingand powder homogenecity
Fluidityof active ingredients become a factor
when the drug has been micronizedto
improve dissolution rate or provide more key
particles of drug per tablet
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43. c)Content uniformity
Particle size range of all components and the
more alike the particle densities, the less
chance for unbending or segregation
Smalland angularparticle shape of MCC
makes it difficult for higher density particles to
shift down through the spaces between the
blend of materials
Cellulose and starch products tend to have
lower true densities than sugars and inorganic
chemicals
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44. d)Lubrication
The overall mean particle size of direct-
compression blends is lessthan that for
granulations, higher concentrations of
lubricants are often needed
Length of blending becomes much more
critical in direct compression than in
lubricationof tablet granulations
The problem associated with the lubricating
direct compression blends can be divided into
two categories-a) Type and amount
needed to produce adequate lubrication
b)The softening effect of lubrication
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45. Powder compression models
#The Heckelequation
ln
1
/
E=kP+A
where E is the porosity of the powder bed and P the
applied compression pressure, A and k are parameters.
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47. # The Shapiro General Compression Equation
1
/
K= P
k= 3σ
0
Py, is commonly used as an indication of the
plasticity or hardness of a particle. This
assumption originated from an empirical
relationship between the parameter k and the yield
strength (σ
0)
# The Kawakitaequation
p
/c =
1
/
ab+
p
/
a
Where C is the degree of volume reduction,P is
the applied pressure, and a and b are parameters.
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48. Fig.Effectofincreasingcompressionalforcesonspecificsurface
areaofpowdermass
Increased surface area (from O to A), initial
particle fracture due to increased
compression point A. Particle rebonding
predominates and then surface area
decreases (from A to B).
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49. Compaction of powder
Thephysicsofcompactionissimplystatedas…
“Thecompressionandconsolidationoftwo
phasesduetoappliedforces”
COMPACTION CONSOLIDATION
•It is defined as formation
of solid geometry by
compression.
•The compaction takes
place in a die by action of
two punches, the lower
and upper by which
compression force is
applied.
It is in increasingin
mechanical strength of
material by particle-particle
interaction.
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50. *(Infig.dashlineisoriginalshapeandsolidlineisdeformed
shape.)
Diagramshowschangesingeometry
(strain)ofsolidbodyresultingfrom
varioustypesofappliedforces.Herethe
figure
a)Tensilestrain
b)Compressivestrain
c)Shearstrain
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51. Role of moisture
#As little as 0.02% moisture can affect the
proportion of applied forces transmitted to lower
punch.
#At 0.55% moisture the behavior is actually the
reverse of that for totally dry material.
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52. Force-volume relationship
#Compressionprocessendswhen,
bulkvolume=tappedvolume(porosity=0)
#Decreaseinporosityisduetotwoprocess.
1.FillinglargespacesbyInterparticulateslippage.
2.Fillingsmallvoidsbydeformationor
fragmentationathighload.
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53. Fig. Decreasing porosity with increasing compressional
forces
1.Initialrepacking
2.Elasticdeformation
3.Plasticdeformation
4.Compression
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54. Conclusion
Thephysicsoftabletcompressiongives
knowledgeofcompatibilityandflowabilityof
pharmaceuticalpowderwhichisessentialfor
formulationoftablets.
Thetendencyofmaterialforplasticdeformation,
fragmentationandelasticitycouldbeexpressed
andarecomparedwithdifferentmaterial.
Thebondingtheoriesintabletpreparationis
studiedtoincreasethestrengthoftablet.
Thedifferentparametersofpowderlikeflow
rates,effectofmoistureetc.arestudiedwith
thereeffectonthecompressionoftablet.
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55. References
1)Leon Lachman, Herbert A. Lieberman. Pharmaceutical
Dosage Forms: Tablets. Volume 1.Second edition; First
Indian reprint 2005,214-219
2)Leon Lachman, Herbert A. Lieberman. Pharmaceutical
Dosage Forms: Tablets. Volume 2. Second edition; First
Indian reprint 2005, 201-241.
3)Leon Lachman, Herbert A. Lieberman. The Theory and
Practice of Industrial Pharmacy; Special Indian Edition
2009, 66-99.
4)Jens ThuroCarstensen. Solid Pharmaceutics:Mechanical
Properties and Rate Phenomena; Tablettingand
Compression; University of Wisconsin, 173-214
5)Eugene L. Parrot. Compression; University of Iowa;221-241
6)Norman Anthony Armstrong; Tablet Manufacture; Welsh
School of Pharmacy, Cardiff University, U.K., 3653-3670
7)M. E. Aulton. Pharmaceutics: The Science of Dosage Form
Design; Second edition, 423-438.
8)Tilfamilien. Compression Analysis of Pharmaceutical
Powders: Assessment of Mechanical Properties and
Tablet Manufacturability Prediction.
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