Technical article
May 2014
57
Two slip additives (SA) are used for the
study. These slip agents are designed to
be used as additives in a resin compatible
system to modify surface characteristics
including COF reduction. To evaluate the
synergistic effect of the slip additives,
three samples were made as shown in
Table 1
, with each formulation having a
total additive content of 1.25 per cent
by weight. Sample A and B are made
with 1.25 per cent single SA content and
sample C has both the slip agents, with
a total SA content of 1.25 per cent. The
neat resin DGDA-6318 BK is utilised as the
control.
2.2 Brabender Batch Mixing
Blending of the formulations was con-
ducted in a Brabender model Prep-Mixer.
This is a three-piece mixer with a 420ml
mixer chamber volume equipped with
Cam mixing blades. The total volume used
was approximately 294ml, in line with
the guideline for best mixing with the
Bradender mixer, which is 70 per cent.
There are three thermocouples which
measure three separate temperature
zones of the mixer. The first thermocouple
measures front plate, the second thermo-
couple measures centre of the chamber
and the third thermocouple extends into
the centre of the mixer bowl chamber,
which measures the actual temperature of
the sample.
The mixing bowl was pre-heated at 180°C,
and then the resin and SA1 (if present)
were added while the blades were rotating
at 20rpm. Note that since slip additive 1
(SA1) is a master batch with 50 per cent
slip agent, 2.5 per cent of the master batch
needs to be added to the formulation
to get 1.25 per cent SA1 content. SA2 (if
present) was then added last at 10rpm
after all materials were in the flux stage.
The rotating blades were then increased
to 20rpm after the SA additives were fully
incorporated into the polymer melt.
Mixing was continued for ten minutes, and
the sample was taken out by reversing
the blades at 10rpm. The rest of the
sample was removed by disassembling
the front plate and removing by hand
using a Brabender knife. Compounded
materials were then placed between two
mylar sheets and pressed flat in a press for
further processing.
2.3 Plaque Preparation
Compounded material was first pre-
weighed to the desired amount and placed
in between two mylar sheets. Outside the
mylar sheets were two aluminium sheets
and stainless steel mould plates. The mylar
was in contact with the material to prevent
sticking to the metal plates. The filled
mould was then placed into the press at
180°C (+5°C or – 5°C). The press was closed
and pressed at 500psi for five minutes
followed by 2,500psi for five minutes.
The cooling system was set to cool the
moulded plaques at the rate of 10°C per
minute. The plaque was taken out when the
temperature reached 35°C.
2.4 COF Measurement
The coefficient of friction (COF) is
measured according to the ASTM
procedure D1894 using a tribometer.
HDPE balls made by Precision Plastic Ball
Co were used for the measurements.
For each sample, the friction force was
measured at two points for each normal
force of 100N, 200N and 300N. The slope
of normal force vs friction force was
used to calculate the COF. Each point
of measurement was done with a new
HDPE ball and repeated for 40 cycles to
demonstrate the effect of surface wear on
COF. The data reported here are the COF
values obtained on the 40
th
cycle.
3 Results and
Discussion
The coefficient of friction measured on
plaques is shown in
Figure 1
. Sample A
which has 1.25 per cent of SA2 shows ~30
per cent reduction in COF and sample
B which has 1.25 per cent of SA1 shows
~40% reduction of COF over the control.
If the COF reduction of the mixture of SA1
and SA2 went as a weighted average, then
the COF of sample C should be a weighted
average of the sample A and sample B.
However, when both the additives are
added so that the total additive content
is 1.25 per cent, a synergy in the slip
behaviour is seen, resulting in a ~50%
reduction of COF over the control.
To understand the origin of the synergy
between the slip additives, Atomic Force
Microscopy (AFM) was used to image
the surface of the plaques used for COF
measurements. The reason for this is
because AFM is a surface technique and is
least affected by the depth of the plaque
and would give the best understanding of
COF, which is a surface phenomenon.
AFM images of plaques made from samples
A, B and C are shown in
Figure 2
. The figure
shows
distinct
differences
between
the surface morphologies of the three
samples. In
Figure 2a
, containing just SA2,
the HDPE banded spherulites of the base
resin are still visible in the topology. The
corresponding phase image (
Figure 2d
)
shows no phase domains which indicate
a surface with homogeneous viscoelastic
behaviour. This suggests that the surface
is covered by a thin superficial layer of
migrated SA2 on top of the HDPE resin.
Tapping mode AFM will typically probe
sample to a depth of ~20nm. This tapping
depth allows the bottom layer spherulite
structure of the HDPE to be captured
in the image, but is slightly blurry due
to the surface layer of SA2. The surface
roughness of sample A plaque is 4.2nm,
which is ~50 per cent less than the surface
roughness of the neat HDPE plaque.
Jacket Material Formulation
Samples Description Resin
SA1
SA2
Total SA%
A
Resin + SA2 98.75%
1.25%
1.25
B
Resin + SA1 97.50% 1.25%
1.25
C
Resin + SA1
+ SA2
97.75%
1%
0.25
1.25
Control
Resin
100%
▲
▲
Table 1
:
Sample description
Coefficient of Friction
Control
▲
▲
Figure 1
:
Coefficient of friction measured on plaques with equal additive content showing synergy between the two
additives
