3.3 Effect of temperature on crystal structure and crystallinity
of different waxy materials
Figure 3 shows the crystal morphology of paraffin, beeswax, FHSO, HCO,
EGMD and Estercoat at 0, 25 and 50 °C. In agreement with what was
previously reported by Fei et al. [8] and Hwang et al. [20],
paraffin, beeswax, EGMD and Estercoat all showed needle-like and fibrous
crystal, however, their sizes and networks were different. It seemed
that paraffin, EGMD and Estercoat all had dendritic crystals which are
highly interconnected and formed junction points, while beeswax has much
finer crystals. FHSO crystallized into more ordered and larger crystals,
while HCO seems to have mixed crystal morphology (rosette, fibrous, and
irregular) at 25 °C which agree with what was reported by Yang et al.
[21]. As shown in Figure 3, when temperature was increased to 50 °C,
the crystal type of paraffin, beeswax, EGMD and Estercoat was not
significant altered, however, the crystal number and density seemed to
be decreased. When the temperature was lowered from 25 to 0 °C, crystal
morphology of paraffin was not significant changed, while beeswax had
denser crystals, and EGMD’s and Estercoat’s crystal density was reduced.
For FHSO, temperature did not have significant impact on their crystal
structures. For HCO, observation of the crystals became difficult when
temperature was lowered to 0 or increased to 50 °C. The changes in
crystal structure may correlate to the changes in coefficient of
friction of these materials as they significantly influence the physical
properties such as hardness of waxes. It was observed that the lower
crystal density would lead to higher surface coefficient of friction as
paraffin, beeswax, EGMD and Estercoat all have increased friction
coefficient when temperature was increased from 25 to 50 °C. The
increased temperature resulted in lower crystal density and led to a
softer material, which subsequently negative affected transfer film
forming and increased the real contact area at the interface, thus
increase the friction coefficient. For FHSO, no significant changes in
crystal structure was observed with temperature changes, thus, the
coefficient of friction of FHSO was quite consistent at low and high
temperatures. Although the crystal structure of HCO at 0 and 50 °C was
not clear for unknown reasons, it is reasonable to speculate that it was
not significantly changed as its friction coefficient was not
significantly affected.
Table 3 shows the enthalpy and relative crystallinity (RC) of the tested
waxes. In general, the RC of the materials decreased as the temperature
increased. However, RC of several materials including FHSO and HCO were
not sensitive to the variation of the equilibration temperature. This is
probably another reason for why their coefficient of friction was not
affect by the environmental temperature (Table 1c). Paraffin, beeswax,
EGMD and Estercoat all had the highest RC at 0 °C, and their RC
significantly decreased when the temperature was increased from 25 to 50
°C. Correspondingly, their coefficient of friction and wear loss all
significantly increased (Table 2c). Beeswax had significantly higher RC
at 0 than 25 °C, and accordingly, lower coefficient of friction and wear
loss was observed at 0 °C. However, EGMD and Estercoat had slightly
higher RC at 0 °C compared to 25 °C, but their coefficient of friction
and wear loss were actually higher at 0 °C. Factors other than the
crystallinity such as surface topography at different temperature may
also have played a role on the surface properties of the EGMD and
Estercoat. There is very limited study on the possible physical property
changes of waxes when they are conditioned under various temperature
after solidification. Studies on the surface characteristics under
various temperatures using laser scanning microscopy could be helpful
for a better understanding of surface property variations with
temperature.
Conclusions
Surface friction coefficient of waxes is shown to be significantly
influenced by normal load, sliding velocity and environmental
temperature. The wear loss of these waxes also varies significantly with
different sliding conditions. The transfer film forming ability and
hardness of these materials significantly affect their friction and wear
behaviors. Paraffin wax was susceptible to both normal load and sliding
velocity. Larger normal load resulted in lower friction coefficient
while higher sliding velocity led to higher friction coefficient of
paraffin. FHSO, HCO, beeswax, EGMD and Estercoat were less sensitive to
normal load and sliding velocity, and their friction coefficient were
almost consistent. Higher environmental temperature decreased crystal
density and crystallinity of paraffin, beeswax, EGMD and Estercoat, and
subsequently increased their friction coefficient. Increased normal
load, sliding velocity and environmental temperature, in general,
resulted more severe wear of all the tested materials. Overall, the
soybean oil-based Estercoat is confirmed to have friction and wear
properties comparable or better than paraffin under tested conditions
and can be a good ecofriendly substitute of paraffin. The information
provided in this study can also be used as a reference for correctly
designing systems for coating, conveying, packaging operations,
transporting and storing of papers and paperboards coated with these
materials.