Rheological evaluation of sealant
materials
Introduction
Sealants are used to join components
together while acting as a gap filler
and forming a barrier. The barrier may
be necessary to exclude air, water or
weather, oils, liquids or sound. The
sealant must have the following
properties:
•
Adhere strongly to the
components to be joined
•
Be easily applicable by sealant
gun, manual knife or pressing
•
Not slump or creep after
application
•
Be temperature cycle resistant
•
Be cost effective and
environmentally friendly
A sealant is made up of a number of
components including: polymer,
plasticizer, filler, pigment, adhesion
promoter, curing/cross-linking system
and others (e.g. desiccant, stabiliser).
The filler may be as much as 40
weight per cent of the formulation and
therefore has a large effect on the
rheological properties of the material.
1
The polymers used in sealant type
materials include acrylics,
polyurethanes and polysulfides.
Figure 1 – Preshear creep experiment
10
8
Rheological Analysis
The experiments reported in this
note were performed on a Bohlin
rheometer at 20ºC. A cone and
plate measuring system (4º/40mm)
was used. The sealant samples
used in the study contained stearic
acid modified magnesium
carbonate, which had different
levels of coating on its surface. The
mineral was present in the
formulation at ~12 weight per cent.
Strain
Fillers are used in sealant products to
reduce the cost of the material and/or
to act as both a rheology control and
reinforcement agent. The materials
range from fumed silica, which is
relatively expensive and very
effective, to natural products such as
calcium carbonate and silica, which
are relatively cheap and not as
effective. Recently, coated
precipitated calcium carbonates
(CPCCs) have been used in sealants
as they are effective and intermediate
in cost. The coating material is usually
stearic acid which anchors to the PCC
as the calcium salt during processing.
Sample C
6
4
Sample B
2
Sample A
0
0
Figure 1 shows a plot of compliance
(deformation per unit stress) against
time. The assumption is that the
larger the compliance, the greater is
the amount of slump experienced by
the sealant. The data in Figure 1
shows that sample A would be
expected to slump less than sample
B, which would slump less than
sample C. The amount of coating
increases from sample A to B to C.
This correlates with the assumption
that as the level of coating is
Bohlin application note
MRK617-01
20
40
60
80
Time (s)
Figure 2 – Preshear creep experiment
Pre-shear creep
In this experiment, the material is
subjected to a high shear stress
(2500 Pa) for a period of time long
enough to break down the structure
(15 sec.). This simulates the
application of the sealant by, for
example, a sealant gun. The
shearing is stopped and
immediately a constant stress is
applied to the material. This test
was designed to mimic the slump
test which is used to characterise
these materials.
Increasing
coating
level
Sample F
10
8
6
Sample E
Increasing
coating
level
4
Sample D
2
0
0
20
40
60
80
Time (s)
increased the filler particles would be
better dispersed, thus increasing the
'flowability' of the material.
Figure 2 shows a similar plot but
includes samples D to F which have
progressively higher levels of coating
than sample C. It is interesting to note
that on increasing the level of coating
from that on C to that on D, the
amount of apparent 'slump'
decreases. This suggests that the
material has become less well
dispersed and/or that the dispersing
mechanism has changed. On further
increasing the level of coating (from E
to F) the dispersability (and thus
'flowability') appears to increase
again. The experiments show obvious
differences between these samples –
significantly the conventional slump
experiment only indicated that sample
F 'slumped' more than the others, but
was not discerning between samples
A to E.
the conventional test, it was not
possible to assess if the samples A
and D had been rheologically
matched. The pre-shear creep
experiment shows that a reasonable
rheological match had been produced
in each case, particularly with
samples
D and H.
Stress Viscometry
Note that the time axis on the graph
has been expanded to show the short
viscosity = shear stress/shear rate.
Figure 4 – stress viscometry curves
Figure 3 – Preshear creep experiment
2.0
1
Sample A
Sample H
1.5
0.1
Viscosity (MPas)
Strain
Sample D
1.0
Sample F
0.001
Sample A
0.0001
0.0
0
2
4
6
8
100
10
500
200
1000
Shear stress (Pa)
Time (s)
Figure 3 shows a plot of the two
samples A and D – also plotted on the
same curve are two samples G and
H, which are matches for samples A
and D respectively.
The stress is then increased stepwise and the process is repeated. The
curves show that there is a
discernable difference between the
viscosities of the samples at low
shear stresses, but the curves
become asymptotic at high shears.
The small differences in the samples,
even at low stresses, indicates the
value of using rheological
experiments to simulate the
deformation processes to which the
materials are subjected, as in the preshear creep experiments.
0.01
Sample G
0.5
Figure 4 shows the viscometry
profiles of the two samples with the
most extreme coating levels In this
experiment a stress is applied to the
sample and the resulting shear-rate is
measured. The viscosity is calculated
from the simple relationship:
time-scale behaviour of the samples.
This time-scale more closely
simulates that of interest after the
material has been applied.
Furthermore, if only high shear
measurements were used to
characterise the samples then
differences in the materials would not
be obvious.
The mineral used in samples G and H
has been prepared by a different
process to previously prepared
mineral samples, but gave the same
level of coating as A and D. Since
differences in the 'slump' of the
materials were not discernable using
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2
Bohlin application note
MRK617-01