Mechanical Behavior of Compacts Made of Binary Mixtures of Starch and Sodium Chloride: A Study of Factors
that Affect the Tensile Strength
Jovana Radojevic
Mentor: Antonios Zavaliangos Ph.D.
Drexel University, College of Engineering, Department of Materials Science & Engineering
Abstract
Tensile Strength of Mixtures
The purpose of this work was to try to understand the behavior of a
two component system in the case of two pharmaceutical excipients
– sodium chloride and starch, and examine the effects of different
factors on the mechanical behavior of compacts made of these
mixtures. Two different mixing methods were compared. The milling
time with zirconia balls affected the particle size of powders. The
presence of a soft phase during milling appears to have enhanced
the reduction of particle size in the brittle component of the mixture.
In both cases the tensile strengths of mixtures deviated from the rule
of mixtures between the individual components. The experiments
were performed at different times throughout the year and the results
suggest that the storage of powders over time can have an effect on
mechanical behavior of compacts. Moisture pick-up may have
affected properties of powders. The conclusion is that sodium
chloride and starch form interacting mixtures and that a mixture of
fixed composition can have a range of properties depending on the
particle size and the degree of powder evolution during storage.
Percolation theory and microcracking can offer some explanations of
the peculiar behavior of these mixtures.
Shown are results for compacts made at 100 MPa
The compacts that expanded the most were the weakest and vice versa, the compacts
that did not expand showed high strengths H0 – compact height immediately after compaction H – compact height 22 hours after compaction
Expansion of Mixtures Tensile Strength of Mixtures 1.07
9
Stored Ball Milled
1.06
Axial Expansion, H/H0
7
6
5
4
Fresh Ball Milled
2
Stored Mixed
Fresh Mixed
1.04
1.01
0.98
20
30
40
50
60
70
80
90
100
Stored Ball Milled
1
0.99
0
10
Fresh Ball Milled
1.02
1
0
Stored Mixed
1.03
0
10
20
30
% v/v starch
5
TS immediately
4
3
TS after 1 day
2
1
0
100
150
200
250
Compaction Pressure [MPa]
70
80
90
Microcomputed tomography analysis was done only on fresh
mixed compacts (due to the feature size); Arrow indicates
compaction direction
100
300
• Microcracking could cause expansion
• Extensive microcracks were observed
only in mixed compacts, which were the
ones that showed the most expansion
50% NaCl + 50% starch
stored ball milled
350
There is a strong correlation between the tensile
strength and expansion of mixture compacts
Tensile Strength [MPa]
RD
RD immediately
50
60
Diametral Compression Testing
6
RD after 1 day
50
% v/v starch
Comparison of strengths immediately
and 22 hours after compaction
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
40
Tensile strength, σT calculation
2
The stored ball milled
powders resulted in the finest
particle size distribution,
which led to superior
strengths of the compacts and
excessive plastic deformation
during tensile strength testing
F – breaking force in diametral compression
d – diameter of the compact t – height of the compact
Percolation Hypothesis
Fresh Ball Milled
NaCl
1.07
85/15 mix
50/50 mix
Stored Ball Milled
20/80 mix
10/90 mix
1.07
Starch
1.03
1.01
PB
A+B
0.99
0
300
1.05
1.05
1.03
Mixing Methods
With zirconia balls
Milling
Ball Milled
NaCl
+
starch
Without zirconia balls
12 hours
@ 60 rpm
100
200
Compaction Pressure [MPa]
300
0
100
200
Compaction Pressure [MPa]
• It seems that for both
groups of the mixed
compacts starch is
already percolated at
15% because those
compacts show
significant expansion
even though NaCl is
in majority (85%)
300 • For FRESH BALL
MILLED 20/80 mix
expanded more than
starch alone  NaCl
is possibly not
percolated anymore
• The percolation limit
of NaCl in STORED
BALL MILLED
mixtures is reached
at much lower
amounts ~5% of NaCl
300
Future Work
• Discrete element modeling (DEM) of the NaCl-starch mixture system
10 balls, each 12 mm in
diameter
+
0.99
0
Can we predict the properties of
the final compact?
1.03
1.01
0.99
Each of the ingredients has specific properties that
will affect the final product in a specific way
100
200
Compaction Pressure [MPa]
Stored Mixed
1.07
1.01
PAB
0
Fresh Mixed
1.07
The top surfaces of the broken compacts were examined using Zeiss Surpa HV50
Scanning Electron Microscope (SEM)
100
200
Compaction Pressure [MPa]
H/H0
PA
SEM Results
1.03
1.01
0.99
H/H0
All pharmaceutical compacts are made from powder mixtures
Active
Disintegrants
Based on the properties of the
pharmaceutical Excipients
ingredient (API)
Binders
Lubricants
individual components
5/95 mix
1.05
1.05
H/H0
3
Fresh Mixed
1.05
H/H0
Tensile Strength [MPa]
8
0
Motivation
SkyScan 1172 µCT Scans
NaCl
+
starch
• 20 g of mixture
• Volume fraction
of each phase
was considered
with respect to
the solid
volume of the
mixture
Mixed
Fresh – experiments performed within 3 weeks from receiving fresh powders
Stored – experiments performed 8 months after powders were received
• Simulate tensile strength testing and compare results with experimental
data
• Test percolation hypothesis with different particle sizes for mixtures
• Perform FTIR analysis on mixture samples to detect possible
chemical interactions between the constituent materials
Acknowledgements
Donation from the International Fine Particles Research
Institute is acknowledged.