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\begin{document}

\title{The friction and wear behaviors of vegetable oil-based waxes, natural
beeswax, and petroleum paraffin wax}



\author[1]{Tao Fei}%
\author[2]{Kangzi Ren}%
\author[1]{Tong Wang}%
\affil[1]{University of Tennessee Knoxville}%
\affil[2]{Eurofins Suppplement Analysis Center}%


\vspace{-1em}



  
  \date{May 07, 2024}


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\selectlanguage{english}
\begin{abstract}
Quantitative analyses on the coefficient of friction of common coating
waxes are necessary and essential for designing systems for coating,
conveying, packaging operations, transporting and storing of papers and
paperboards, while analyses on wear behavior can be helpful for
predicting performance durability of the coating surface. In this study,
we investigated the friction and wear behaviors of six waxes including
four commercial waxes and two soybean oil-based wax developed in our lab
for bulk coating on cardboard. The effect of normal load, sliding
velocity, and environmental temperature was evaluated. The friction
coefficient of different waxes varies with sliding conditions. Higher
normal load, sliding velocity, and environmental temperature resulted in
significantly greater wear loss. Crystalline morphology and
crystallinity were affected by environmental temperature, and they
correlate to the variations in friction coefficient and wear loss of
these materials. Overall, the Estercoat developed in our lab had
comparable frictional properties and much less wear than paraffin wax
under tested conditions and can be a good substitute for paraffin wax.%
\end{abstract}%



\sloppy


\textbf{Keywords:}

Coating waxes; friction and wear behavior; effect of sliding conditions;
crystalline morphology

and crystallinity

Highlights:

\begin{itemize}
\tightlist
\item
  Friction coefficient and wear loss of different waxes varies with
  sliding conditions.
\item
  Crystalline morphology and crystallinity were affected by
  environmental temperature.
\item
  The changes in crystalline morphology and crystallinity correlate with
  the variations in friction coefficient and wear loss.
\item
  The soybean oil-based Estercoat developed in our lab has friction and
  wear properties comparable or better than paraffin wax under tested
  conditions.
\end{itemize}

\textbf{Introduction}

Waxes are commonly used for paper and corrugated coating. As a coating,
surface properties including friction and wear behavior are critical
factors as they are related to many issues regarding printing
runnability, print quality, sliding, durability and storage. Friction is
defined as the resistance to motion that occurs whenever a solid body
contacts with another, and wear represents the damage to or removal of
material from one or both side of solid surfaces that are in contact
during motion {[}1{]}. Friction and wear are all consequences of
materials' interaction at the contact point, and a better understanding
of how different waxes response to such interaction leads to rational
design of methods for applying coatings or new applications in which
they can be utilized.

Currently, the major market for waxes is still packaging which
represents 30\% of the total 3 billion pounds annual North American wax
consumption according to American Fuel \& Petrochemical Manufacturers
{[}2{]}. Regarding papers used for printing, high friction is typically
desired since it helps maintain good printing register {[}3{]}. While
for paperboard or corrugated coatings, the friction of coating material
must be carefully optimized since too high or too low surface friction
can lead to many problems. During the manufacture of packages, too
little friction can cause packages to slip off the inclined conveyor
belts and cause problem to the downstream processing. Lack of friction
may also cause storage problems as packages can slide off the stacks or
pallets. While too much friction can significantly slow down the
packaging progress at the delivery chutes {[}4{]}. To correctly design
systems for coating, conveying, packaging operations, transporting and
storing of papers and paperboards, quantitative analyses on coefficient
of friction of commonly used coating waxes are essential. Whereas,
studies on wear behavior of the waxes can be helpful to predict
performance durability of the coating surface.

Commonly, the simplest friction ``law'' (Amontons's law) is used to
describe friction, and it is stated that force of friction F, is
proportional to the normal force FN, meaning ideally coefficient of
friction (\selectlanguage{greek}μ\selectlanguage{english}=F/FN) only depends on the nature of the surface. However,
Coulomb in 1821 has found that the coefficient of sliding friction
depends on sliding speed and normal force, while coefficient of static
friction depends approximately logarithmically on time (Persson, 2000).
Temperature is another important factor that could significantly affect
\selectlanguage{greek}μ \selectlanguage{english}of materials. When the temperature rises, the thermal movement ability
of moving units could be improved and the space between molecules is
increased due to thermal expansion. Consequently, physical properties
such as \selectlanguage{greek}μ \selectlanguage{english}can be significantly affected. The sliding velocity, normal
load and environmental temperature all closely relate to the practical
situations that the wax coating can experience. Thus, a study on how
these factors affect the \selectlanguage{greek}μ \selectlanguage{english}of different wax coating materials would be
of a significance to the coating industries. However, a literature
search indicated that most of the studies were performed on materials
such as metals, and very few information was available on wax coatings.
Therefore, a systematic study on wax coating materials is needed. In
this study, we investigated the frictional and wear behaviors of waxes
and tried to better understand the relationship between frictional and
wear behavior and physical and thermal properties of materials. We
hypothesized that the friction coefficient of these coating materials
will be affected by their hardness positively related to normal load and
sliding speed. We also hypothesized that the coefficient of friction is
related to the waxes' melting profile and crystal structure and will
increase as the surface temperature of the materials increases.

To test our hypotheses, we compared friction coefficient and wear loss
of 6 waxes at different normal load, sliding velocity and environmental
temperature, and characterized their physical and thermal properties.
These waxes are paraffin wax, beeswax, fully hydrogenated soybean oil
(FHSO), hydrogenated castor oil (HCO), ethylene glycol mono/diester
(EGMD) and Estercoat as described later. Although approaches for
characterization of the friction and wear behavior are dependent on the
scale and complexity of the system under investigation {[}5-7{]}, and
field tests are necessary, such model study can serve as a guide for
further improving material surface properties and for industries to
better design their systems if similar materials and tribological
situations are encountered.

\textbf{Materials and Methods}

FHSO was provided by Stratas Foods (Memphis, TN). Paraffin wax and
beeswax were provided by Michelman, Inc. (Cincinnati, OH). HCO was
purchased from Acme-Hardesty Co. (Blue Bell, PA). EGMD and Estercoat was
synthesized using the method described as in the following section.

\emph{2.1 Synthesis of EGMD and Estercoat}

EGMD and Estercoat are soybean oil-based wax like materials developed in
our lab for corrugated coating {[}8{]}. It was synthesized following the
method reported in our previous study {[}8{]} with minor modifications.
Stearic acid and ethylene glycol were mixed in a round bottom flask at a
molar ratio of 1:0.6 with 1 wt\% of amberlyst-15 as catalyst. The
mixture was heated at 95 \selectlanguage{ngerman}°C in an oil bath for 8 hours to synthesize a
mixture of ethylene glycol mono and diester (EGMD). Hot filtration was
used to remove the catalyst after the reaction and the collected liquid
was cooled and saved. Mono and diacylglycerol (50/50) mixture of 5 wt\%
was then mixed into the previously collected EGMD at 75 °C to form the
Estercoat. Both the EGMD and Estercoat were used for tribological tests.

\emph{2.2 Measurement of coefficient of friction}

The selected waxy materials were melted at 5 °C higher than their
melting point and pool into a cylindrical mold (40 mm in diameter × 25
mm in height). The molten waxes were allowed to solidify under room
temperature (23 °C) for 2 hours before removed from the mold. The wax
disc was then taped on the platform of a Discovery Hybrid Rheometer (TA
Instruments, New Castle, DE) equipped with tribo-rheometry accessories
for measurement of coefficient of friction. The test was conducted
following the procedures reported by Krasmik and Schlattmann (2015)
{[}9{]} except that a three-ball on plate setup (Figure 1) was used.
Normal load (F\textsubscript{Load}), sliding velocity (\selectlanguage{greek}υ\selectlanguage{english}) and
temperature were varied to determine their influence on friction and
wear behaviors of different waxes. The bottom surface of samples in
contact with glass were tested. When testing the effect of normal load,
normal load of 5, 10 and 15 N were used at a fixed sliding velocity of 5
rad/s under room temperature (25 \selectlanguage{ngerman}°C). For evaluating the effect of
sliding velocity, 1, 5 and 10 rad/s were used with fixed normal load of
5 N under room temperature. For evaluating the effect of temperature,
environment temperature of 0, 25 and 50 °C were tested at fixed normal
load (5 N) and sliding velocity (5 rad/s). Three samples were prepared
for each test, and each sample was measured once.

\emph{2.3 Measurement of wear loss}

The depth of the wear track was measured as the indicator of wear loss,
by using a laser microscope (S-neox, Sensofar Metrology, Scottsdale, AZ)
following the method reported by Yamamoto et al. (2013) {[}10{]} with
minor modifications. The laser wavelength used was 530 nm and the
resolution was 75 nm. The depth of the wear track and the
cross-sectional area profile were precisely determined. Figure 2 shows
typical data of the wear track profile obtained from the laser
microscope.

\emph{Observation of wax crystallization using Polarized Light
Microscopy}

To determine how the environmental temperature affects the crystal
structures and subsequently physical properties such as coefficient of
friction and wear, the microstructure of the coating materials was
observed using PLM. Samples were prepared following the method reported
by Fei et al. {[}8{]} with minor modifications. A small amount of wax
was loaded on the glass microscopy slides and heated in an oven at 5 °C
above the melting temperature of waxes for 30 minutes. A preheated cover
slide was then slipped over the molten wax to produce a thin film. The
prepared slides were cooled at room temperature for one hour and then
analyzed with a Differential Interference Contrast (DIC) microscope
(Olympus BX53, Olympus Corporation, MA, USA) at 0, 25 and 50 °C using
the CellSens Dimension software (Olympus Corporation, MA, USA). The
crystal images were taken at 100× magnification.

\emph{Determination of relative crystallinity using DSC}

The melting profile of these coating materials was determined by using a
differential scanning calorimeter (Discovery DSC, TA Instruments, New
Castle, DE, USA). The temperature program started with a 1-min
equilibration at 25 °C, following by 8 min conditioning at 0, 25 or 50
°C, and then heating to 110 °C at 10 °C/min and a 2-min hold at 110 °C.
The heat of fusion measured by DSC is used to characterize the
crystallinity as reported by Yao and Wang {[}11{]}. The degree of
crystallinity (\%) of various waxes relative to paraffin wax at 25 °C
was estimated by the following equation:

RC (relative crystallinity, \%) = 100 × \selectlanguage{greek}Δ\selectlanguage{english}HS / \selectlanguage{greek}Δ\selectlanguage{english}HB

where \selectlanguage{greek}Δ\selectlanguage{english}HS is the heat of fusion of the tested wax and \selectlanguage{greek}Δ\selectlanguage{english}HB is the heat of
fusion of paraffin wax conditioned at 25 \selectlanguage{ngerman}°C.

\textbf{Results and Discussion}

\emph{3.1 Effect of normal force, sliding velocity and environment
temperature on coefficient of friction}

The measurements were conducted directly on wax samples instead of
cardboard samples coated with the waxes. This is because that the tested
waxes are designed for bulk coating in which the wax forms a thick layer
on the cardboard surface, and testing on the wax samples and coated
cardboard samples would make negligible difference. Moreover, testing
directly on the wax samples help us understand the effect of these
boundary conditions on the material itself. Table 1a shows the
coefficient of friction of different waxes under various normal loads.
It can be seen that only paraffin's \selectlanguage{greek}μ \selectlanguage{english}was significantly affected by
normal load. The \selectlanguage{greek}μ \selectlanguage{english}of paraffin decreased when the normal load was
increased from 5 to 10 N, and further increase of normal load to 15 N
did not lead to further decrease in \selectlanguage{greek}μ \selectlanguage{english}(Table 1a). The decrease in \selectlanguage{greek}μ \selectlanguage{english}of
paraffin is probably due to the formation of a paraffin film at the
interface which can serve as a solid lubricant with the increased normal
load. A paraffin layer was also observed on the stainless-steel balls
which may have changed the surfaces in contact and resulted in a reduced
\selectlanguage{greek}μ. \selectlanguage{english}As 10 N already resulted a transfer film and a coating on the
stainless-steel balls, no further significant decrease in \selectlanguage{greek}μ \selectlanguage{english}was observed
by further increasing normal load to 15 N. Zhou et al. {[}12{]} also
observed similar phenomenon when evaluating tribological properties of
polyurethane modified polyamide-based composites. As the applied load
increases, the friction heat increased quickly and a stable transfer
film which would act as a solid lubricant can be produced by the melting
material, thus reduced the friction coefficient {[}12{]}. FHSO and HCO
which are harder than paraffin generated powder during sliding and did
not form the transfer film, thus no significant variation in \selectlanguage{greek}μ \selectlanguage{english}was
observed. EGMD and Estercoat are harder than paraffin but softer than
FHSO and HCO, and the wear-off material of EGMD and Estercoat generated
during sliding was not powdery. However, a smooth transfer film was
probably also not formed. Therefore, no significant changes in \selectlanguage{greek}μ \selectlanguage{english}(Table
1a) for these two materials. Beeswax's \selectlanguage{greek}μ \selectlanguage{english}was not significantly affect by
the increased normal load either. It is the softest material compared to
the other five, and the stainless-steel balls were already fully coated
with the wax at a low normal load (5 N). With no significant change on
the surfaces in contact, no further formation of the transfer film, the
\selectlanguage{greek}μ \selectlanguage{english}was not significantly affected with increased normal load. Comparing
the different materials, beeswax had the highest \selectlanguage{greek}μ; \selectlanguage{english}EGMD, Estercoat and
paraffin behaved similarly; and FHSO and HCO had lower \selectlanguage{greek}μ \selectlanguage{english}than the other
four materials at all three normal load levels (Table 1a). In addition
to the transfer film formation ability, the different \selectlanguage{greek}μ \selectlanguage{english}of different
materials may also be attributed by their physical properties such as
hardness. By Bowden's friction theory {[}13{]}, the friction coefficient
may be expressed in terms of \selectlanguage{greek}μ \selectlanguage{english}= Ars/N (where Ar is the real contact
area, which is directly proportional to the friction force, s is the
shear strength, and N is the applied load). Higher hardness may lead to
lower degree of deformation of the material at the contact point, and
thus smaller real contact area between the material and the
stainless-steel balls. Consequently, at the same applied load, materials
with higher hardness would have lower friction coefficient.

Table 1b shows the changes in \selectlanguage{greek}μ \selectlanguage{english}of selected waxes with different sliding
velocities. For paraffin wax, its \selectlanguage{greek}μ \selectlanguage{english}first decreased when sliding
velocity was increased from 1 to 5 rad/s, and then increased when
sliding velocity was further increased to 10 rad/s. The decrease in \selectlanguage{greek}μ
\selectlanguage{english}can be explained by the same reason stated in the previous section, that
a transfer film was formed to serve as a solid lubricant {[}12{]}.
However, at higher sliding velocity (10 rad/s), the paraffin film may
have been melted by the friction heat and transferred away. HCO had a
decreasing trend in \selectlanguage{greek}μ \selectlanguage{english}when sliding velocity was increased, and a
significant reduction in \selectlanguage{greek}μ \selectlanguage{english}was observed when sliding velocity was
increased to 10 rad/s (Table 1b). HCO powders which may have lubricant
effect were generated during the sliding. The HCO powder was tested in
pharmaceuticals as a lubricant to prevent ingredients from clumping
together and from sticking to the tablet punches or capsule filling
machine {[}14{]}. With the increase in sliding speed, the friction heat
may also accumulate quickly at the interface, leading to the softening
and spreading of the material, which can promote the formation of a
stable transfer film. The thin and stable transfer film then can act as
the solid lubricant and reduces the friction coefficient of HCO at 10
rad/s {[}12{]}. For beeswax, the increased sliding velocity resulted in
a significant increase in \selectlanguage{greek}μ \selectlanguage{english}(Table 1b), this is likely due to the severe
deformation and an increase in the real contact area with the increased
sliding speed {[}15{]}. While for FHSO, EGMD and Estercoat, sliding
velocity had no significant effect on their \selectlanguage{greek}μ \selectlanguage{english}(Table 1b) indicating that
they are less susceptible to shear. Overall, beeswax had the highest \selectlanguage{greek}μ
\selectlanguage{english}at all sliding velocities tested. EGMD, Estercoat, and paraffin wax
again had similar frictional behavior, while FHSO and HCO had lower \selectlanguage{greek}μ
\selectlanguage{english}compared to the other four materials (Table 1). Physical properties of
materials such as hardness again may have attributed to the friction
coefficient's different responses to different sliding velocities
{[}13{]}.

Table 1c shows the effect of temperature on coefficient of friction of
different waxes. The \selectlanguage{greek}μ \selectlanguage{english}of paraffin and beeswax increased significantly
when the environmental temperature was increased to 50 \selectlanguage{ngerman}°C. The elevated
temperature led to softening of paraffin and beeswax (melting point both
is about 62-66°C), which can subsequently cause severe deformation and
larger real contact area at the contact point. Severe deformation and
larger real contact area all can result in higher \selectlanguage{greek}μ\selectlanguage{english} {[}13{]}. Increase
temperature may also lead to faster vibration of molecules and cause a
local uplifting of the surface, which could result in higher surface
roughness and subsequently increase friction coefficient {[}16{]}. Low
temperature did not have significant impact on paraffin, however, it
reduced the coefficient of friction of beeswax (Table 1c). Beeswax was
significantly hardened at freezing temperature, and the increased
hardness could result in a decreased \selectlanguage{greek}μ \selectlanguage{english}and less wear {[}13{]}. For FHSO
and HCO, temperature (neither low nor high) had no significant effect on
\selectlanguage{greek}μ \selectlanguage{english}(Table 1c). HCO has a much higher melting point (86-88 \selectlanguage{ngerman}°C) compared to
the other five selected waxes, and it is expected that no significant
changes in \selectlanguage{greek}μ \selectlanguage{english}would be seen at 50 \selectlanguage{ngerman}°C. Although FHSO has a lower melting
point (about 66 °C), with a sharp melting profile, severe softening of
FHSO may not occur when the materials is heated at a temperature lower
than its melting. Thus, less significant impact of temperature on
coefficient of friction was observed when heated at 50 °C. The high
hardness of FHSO and HCO may also have attributed to their friction
coefficients' low susceptibility to environmental temperature {[}17{]}.
For EGMD and Estercoat, it was surprising to observe that both decrease
and increase in temperature resulted higher \selectlanguage{greek}μ \selectlanguage{english}(Table 1c). The reasons
for this observation were not fully understood, and a study on the
changes of the crystal structure of the EGMD and Estercoat at low
temperature and high temperature was conducted and discussed in a later
section to help explain this.

\emph{3.2 Effect of normal force, sliding velocity and environment
temperature on wear loss}

The sliding caused substrate worn and wear tracks with different depth
was observed. However, the weight difference of the sample before and
after sliding was little and cannot accurately represent the
significance of the boundary conditions on wear loss. Therefore, wear
loss was characterized by precisely measuring the depth of the wear
track, and Table 2a shows the surface wear loss of different coating
materials under different normal loads at fixed sliding velocity of 5
rad/s. It can be seen that all the materials had increased wear loss
when the normal load was increased. Others have reported similar trend
although different materials were used for the evaluation {[}12, 18{]}.
As the normal load increases, the friction heat produced at the contact
surface is generated faster leading to a higher transient temperature,
and the waxes would melt and adhere to the surface of the counterface,
thus resulted in a more severe wear. Among the different materials,
beeswax at all normal load levels had the most severe wear, while HCO
had the least wear (Table 2a). The difference in wear of different
materials under same sliding conditions is also very likely determined
by their hardness. Among these materials, HCO has the highest hardness,
followed by FHSO, Estercoat, EGMD, paraffin and lastly beeswax. The wear
loss of these materials (from shallow to deep) seems to follow the same
order of their hardness, thus the higher hardness of the material, the
less wear. Others also have reported that materials with lower hardness
and softening temperature can be transferred away from the friction
surface more easily, resulting in more wear loss {[}12{]}. Overall, our
Estercoat which has higher hardness than paraffin had less wear
indicating that under similar frictional conditions, Estercoat may have
better performance durability than paraffin.

Table 2b shows the wear loss of different coating materials under
different sliding velocities with fixed normal load of 5N at room
temperature (25 \selectlanguage{ngerman}°C). It was observed that the wear loss of all the
materials except that of EGMD increased as sliding velocity was
increased. When sliding velocity was increased, the sliding distance was
also increased as the sliding time was kept constant, and longer sliding
distance theoretically leads to more wear loss. However, the wear loss
of EGMD surprisingly decreased with the increased sliding velocity. The
wear behavior of coatings is strongly dependent on their ability to form
the transfer film at the interface {[}18, 19{]}. It is possible that
EGMD has formed a transfer film which prevented further contact of the
steel ball with the surface by the high shearing force under the high
sliding velocity. Such film can be useful for preventing fatigue
deformation and reducing plough effect for coatings {[}12{]}. Similar to
the data shown in table 1, harder material tends to be more wear
resistant. Beeswax having high coefficient of friction and being the
softest material had the most wear, while HCO which has high hardness
and low coefficient of friction had the least wear at different sliding
velocities. FHSO and Estercoat were similar at low sliding velocity,
while Estercoat has better performance at high sliding velocity. This is
probably due to Estercoat having better transfer film forming ability
than FHSO under high shearing force. The Estercoat also outperformed
paraffin at different sliding velocity, indicating that the Estercoat as
a surface coating probably has better performance durability.

The effect of environmental temperature on wear was also evaluated.
Samples went through sliding at fixed normal load and sliding velocity
(5 N and 5 rad/s, respectively) under different temperatures, and the
wear loss was quantified. Table 2c shows that the wear loss of paraffin,
FHSO, and beeswax all increased when the temperature was increased. The
wear loss of HCO, EGMD and Estercoat was at the lowest when at room
temperature, however, increased when the temperature was either lowered
or elevated. The low temperature may have negatively affected the
formation of transfer film, while the high temperature could have
increased the transfer away of the coating on the counterface, thus all
resulted in increased wear loss {[}12, 18, 19{]}. The wear loss of
paraffin, beeswax, EGMD and Estercoat all corresponded to their changes
in surface coefficient of friction with temperature. Low temperature did
not have significant impact on paraffin, however, it reduced the
coefficient of friction of beeswax. While the elevated temperature
resulted in higher surface friction coefficient of paraffin and beeswax
and may have led to softening and transferring away of paraffin and
beeswax at the interface {[}12, 18{]}, thus more severe wear at high
temperature. For EGMD and Estercoat, their surface coefficient of
friction was higher at lowered and elevated temperatures than when at
room temperature, which matched their wear behavior. The coefficient of
friction of HCO and FHSO was not significantly affected by temperature,
however, their wear loss was significantly affected. It is suspected
that the different response of wear loss to temperature may relate to
the crystal structure and melting profile of the different materials.
Overall, the Estercoat would outperform paraffin according to their
observed friction and wear behaviors.

\emph{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.

\textbf{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.

\textbf{ACKNOWLEDGEMENTS:}

This work is funded by the United Soybean Board.

\textbf{Reference}

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\textbf{Figures and Tables}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

Figure 1. The three-ball on plate setup for tribological property
measurements (TA instrument).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

Figure 2. Typical wear track (top) and the profile (bottom) measured by
laser microscope\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image7/image7}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image8/image8}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image9/image9}

\end{center}
\end{figure}

Figure 3. Crystal structures of paraffin, beeswax, fully hydrogenated
soybean oil (FHSO), hydrogenated castor oil (HCO), ethylene glycol mono-
and diester (EGMD) and Estercoat under 0, 25 and 50 \selectlanguage{ngerman}°C

Table 1. Summary of coefficient of friction of different waxy materials
under (a) different normal load, (b) different sliding velocity and (c)
different environmental temperature.\selectlanguage{english}
\begin{longtable}[]{@{}llll@{}}
\toprule
c. & Environmental Temperature (\selectlanguage{ngerman}°C) & Environmental Temperature (\selectlanguage{ngerman}°C) &
Environmental Temperature (\selectlanguage{ngerman}°C)\tabularnewline
\midrule
\endhead
& Normal Load (N) & Normal Load (N) & Normal Load (N)\tabularnewline
& 5 & 10 & 15\tabularnewline
Paraffin & 144.77\selectlanguage{ngerman}±3.25\textsuperscript{cB} &
196.09\selectlanguage{ngerman}±7.45\textsuperscript{bB} &
254.73\selectlanguage{ngerman}±28.10\textsuperscript{aB}\tabularnewline
FHSO & 76.71\selectlanguage{ngerman}±4.92\textsuperscript{bC} & 122.54\selectlanguage{ngerman}±11.95\textsuperscript{aB}
& 138.60\selectlanguage{ngerman}±10.89\textsuperscript{aD}\tabularnewline
HCO & 6.76\selectlanguage{ngerman}±0.27\textsuperscript{bD} & 11.72\selectlanguage{ngerman}±3.23\textsuperscript{aC} &
12.13\selectlanguage{ngerman}±2.77\textsuperscript{aE}\tabularnewline
Beeswax & 314.81\selectlanguage{ngerman}±30.96\textsuperscript{cA} &
454.31\selectlanguage{ngerman}±77.26\textsuperscript{bA} &
837.55\selectlanguage{ngerman}±4.46\textsuperscript{aA}\tabularnewline
EGMD & 156.75\selectlanguage{ngerman}±7.23\textsuperscript{bB} &
161.90\selectlanguage{ngerman}±17.06\textsuperscript{bB} &
196.99\selectlanguage{ngerman}±4.60\textsuperscript{aC}\tabularnewline
Estercoat & 90.18\selectlanguage{ngerman}±3.11\textsuperscript{bC} &
125.34\selectlanguage{ngerman}±6.36\textsuperscript{aB} &
134.36\selectlanguage{ngerman}±3.68\textsuperscript{aD}\tabularnewline
b. & Sliding Velocity (rad/s) & Sliding Velocity (rad/s) & Sliding
Velocity (rad/s)\tabularnewline
& 1 & 5 & 10\tabularnewline
Paraffin & 87.28\selectlanguage{ngerman}±2.19\textsuperscript{cC} &
144.77\selectlanguage{ngerman}±3.25\textsuperscript{bB} &
291.62\selectlanguage{ngerman}±3.90\textsuperscript{aB}\tabularnewline
FHSO & 37.74\selectlanguage{ngerman}±2.03\textsuperscript{cE} & 76.71\selectlanguage{ngerman}±4.92\textsuperscript{bC} &
167.60\selectlanguage{ngerman}±23.76\textsuperscript{aC}\tabularnewline
HCO & N/A & 6.76\selectlanguage{ngerman}±0.27\textsuperscript{bD} &
21.00\selectlanguage{ngerman}±1.61\textsuperscript{aE}\tabularnewline
Beeswax & 123.87\selectlanguage{ngerman}±9.87\textsuperscript{cB} &
314.81\selectlanguage{ngerman}±30.96\textsuperscript{bA} &
385.55\selectlanguage{ngerman}±18.80\textsuperscript{aA}\tabularnewline
EGMD & 162.71\selectlanguage{ngerman}±12.39\textsuperscript{aA} &
156.75\selectlanguage{ngerman}±7.23\textsuperscript{aB} &
110.83\selectlanguage{ngerman}±2.34\textsuperscript{bD}\tabularnewline
Estercoat & 52.74\selectlanguage{ngerman}±2.56\textsuperscript{cD} &
90.18\selectlanguage{ngerman}±3.11\textsuperscript{bC} &
109.98\selectlanguage{ngerman}±5.00\textsuperscript{aD}\tabularnewline
c. Environmental Temperature (°C) Environmental Temperature (°C)
Environmental Temperature (°C) 0 25 50 Paraffin
134.18±9.95\textsuperscript{bB} 144.77±3.25\textsuperscript{bB}
532.67±11.21\textsuperscript{aB} FHSO 46.89±2.33\textsuperscript{cDE}
76.71±4.92\textsuperscript{bC} 150.81±3.44\textsuperscript{aD} HCO
21.10±1.87\textsuperscript{bE} 6.76±0.27\textsuperscript{cD}
64.02±3.62\textsuperscript{aE} Beeswax 80.09±6.61\textsuperscript{cCD}
314.81±30.96\textsuperscript{bA} 1,100.68±59.18\textsuperscript{aA} EGMD
559.84±57.02\textsuperscript{aA} 156.75±7.23\textsuperscript{cB}
447.28±27.22\textsuperscript{bC} Estercoat
132.27±4.86\textsuperscript{bBC} 90.18±3.11\textsuperscript{cC}
426.14±6.14\textsuperscript{aC} & & &\tabularnewline
\bottomrule
\end{longtable}

Table 2. Summary of wear loss of different waxy materials under (a)
different normal load, (b) different sliding velocity and (c) different
environmental temperature.\selectlanguage{english}
\begin{longtable}[]{@{}llll@{}}
\toprule
c. & Environmental Temperature (\selectlanguage{ngerman}°C) & Environmental Temperature (\selectlanguage{ngerman}°C) &
Environmental Temperature (\selectlanguage{ngerman}°C)\tabularnewline
\midrule
\endhead
& Normal Load (N) & Normal Load (N) & Normal Load (N)\tabularnewline
& 5 & 10 & 15\tabularnewline
Paraffin & 144.77\selectlanguage{ngerman}±3.25\textsuperscript{cB} &
196.09\selectlanguage{ngerman}±7.45\textsuperscript{bB} &
254.73\selectlanguage{ngerman}±28.10\textsuperscript{aB}\tabularnewline
FHSO & 76.71\selectlanguage{ngerman}±4.92\textsuperscript{bC} & 122.54\selectlanguage{ngerman}±11.95\textsuperscript{aB}
& 138.60\selectlanguage{ngerman}±10.89\textsuperscript{aD}\tabularnewline
HCO & 6.76\selectlanguage{ngerman}±0.27\textsuperscript{bD} & 11.72\selectlanguage{ngerman}±3.23\textsuperscript{aC} &
12.13\selectlanguage{ngerman}±2.77\textsuperscript{aE}\tabularnewline
Beeswax & 314.81\selectlanguage{ngerman}±30.96\textsuperscript{cA} &
454.31\selectlanguage{ngerman}±77.26\textsuperscript{bA} &
837.55\selectlanguage{ngerman}±4.46\textsuperscript{aA}\tabularnewline
EGMD & 156.75\selectlanguage{ngerman}±7.23\textsuperscript{bB} &
161.90\selectlanguage{ngerman}±17.06\textsuperscript{bB} &
196.99\selectlanguage{ngerman}±4.60\textsuperscript{aC}\tabularnewline
Estercoat & 90.18\selectlanguage{ngerman}±3.11\textsuperscript{bC} &
125.34\selectlanguage{ngerman}±6.36\textsuperscript{aB} &
134.36\selectlanguage{ngerman}±3.68\textsuperscript{aD}\tabularnewline
b. & Sliding Velocity (rad/s) & Sliding Velocity (rad/s) & Sliding
Velocity (rad/s)\tabularnewline
& 1 & 5 & 10\tabularnewline
Paraffin & 87.28\selectlanguage{ngerman}±2.19\textsuperscript{cC} &
144.77\selectlanguage{ngerman}±3.25\textsuperscript{bB} &
291.62\selectlanguage{ngerman}±3.90\textsuperscript{aB}\tabularnewline
FHSO & 37.74\selectlanguage{ngerman}±2.03\textsuperscript{cE} & 76.71\selectlanguage{ngerman}±4.92\textsuperscript{bC} &
167.60\selectlanguage{ngerman}±23.76\textsuperscript{aC}\tabularnewline
HCO & N/A & 6.76\selectlanguage{ngerman}±0.27\textsuperscript{bD} &
21.00\selectlanguage{ngerman}±1.61\textsuperscript{aE}\tabularnewline
Beeswax & 123.87\selectlanguage{ngerman}±9.87\textsuperscript{cB} &
314.81\selectlanguage{ngerman}±30.96\textsuperscript{bA} &
385.55\selectlanguage{ngerman}±18.80\textsuperscript{aA}\tabularnewline
EGMD & 162.71\selectlanguage{ngerman}±12.39\textsuperscript{aA} &
156.75\selectlanguage{ngerman}±7.23\textsuperscript{aB} &
110.83\selectlanguage{ngerman}±2.34\textsuperscript{bD}\tabularnewline
Estercoat & 52.74\selectlanguage{ngerman}±2.56\textsuperscript{cD} &
90.18\selectlanguage{ngerman}±3.11\textsuperscript{bC} &
109.98\selectlanguage{ngerman}±5.00\textsuperscript{aD}\tabularnewline
c. Environmental Temperature (°C) Environmental Temperature (°C)
Environmental Temperature (°C) 0 25 50 Paraffin
134.18±9.95\textsuperscript{bB} 144.77±3.25\textsuperscript{bB}
532.67±11.21\textsuperscript{aB} FHSO 46.89±2.33\textsuperscript{cDE}
76.71±4.92\textsuperscript{bC} 150.81±3.44\textsuperscript{aD} HCO
21.10±1.87\textsuperscript{bE} 6.76±0.27\textsuperscript{cD}
64.02±3.62\textsuperscript{aE} Beeswax 80.09±6.61\textsuperscript{cCD}
314.81±30.96\textsuperscript{bA} 1,100.68±59.18\textsuperscript{aA} EGMD
559.84±57.02\textsuperscript{aA} 156.75±7.23\textsuperscript{cB}
447.28±27.22\textsuperscript{bC} Estercoat
132.27±4.86\textsuperscript{bBC} 90.18±3.11\textsuperscript{cC}
426.14±6.14\textsuperscript{aC} & & &\tabularnewline
\bottomrule
\end{longtable}

\&\selectlanguage{english}
\begin{longtable}[]{@{}llll@{}}
\toprule
c. & Environmental Temperature (\selectlanguage{ngerman}°C) & Environmental Temperature (\selectlanguage{ngerman}°C) &
Environmental Temperature (\selectlanguage{ngerman}°C)\tabularnewline
\midrule
\endhead
& 0 & 25 & 50\tabularnewline
Paraffin & 134.18\selectlanguage{ngerman}±9.95\textsuperscript{bB} &
144.77\selectlanguage{ngerman}±3.25\textsuperscript{bB} &
532.67\selectlanguage{ngerman}±11.21\textsuperscript{aB}\tabularnewline
FHSO & 46.89\selectlanguage{ngerman}±2.33\textsuperscript{cDE} & 76.71\selectlanguage{ngerman}±4.92\textsuperscript{bC}
& 150.81\selectlanguage{ngerman}±3.44\textsuperscript{aD}\tabularnewline
HCO & 21.10\selectlanguage{ngerman}±1.87\textsuperscript{bE} & 6.76\selectlanguage{ngerman}±0.27\textsuperscript{cD} &
64.02\selectlanguage{ngerman}±3.62\textsuperscript{aE}\tabularnewline
Beeswax & 80.09\selectlanguage{ngerman}±6.61\textsuperscript{cCD} &
314.81\selectlanguage{ngerman}±30.96\textsuperscript{bA} &
1,100.68\selectlanguage{ngerman}±59.18\textsuperscript{aA}\tabularnewline
EGMD & 559.84\selectlanguage{ngerman}±57.02\textsuperscript{aA} &
156.75\selectlanguage{ngerman}±7.23\textsuperscript{cB} &
447.28\selectlanguage{ngerman}±27.22\textsuperscript{bC}\tabularnewline
Estercoat & 132.27\selectlanguage{ngerman}±4.86\textsuperscript{bBC} &
90.18\selectlanguage{ngerman}±3.11\textsuperscript{cC} &
426.14\selectlanguage{ngerman}±6.14\textsuperscript{aC}\tabularnewline
\bottomrule
\end{longtable}\selectlanguage{ngerman}

\&\selectlanguage{english}
\begin{longtable}[]{@{}llll@{}}
\toprule
c. & Environmental Temperature (\selectlanguage{ngerman}°C) & Environmental Temperature (\selectlanguage{ngerman}°C) &
Environmental Temperature (\selectlanguage{ngerman}°C)\tabularnewline
\midrule
\endhead
& 0 & 25 & 50\tabularnewline
Paraffin & 134.18\selectlanguage{ngerman}±9.95\textsuperscript{bB} &
144.77\selectlanguage{ngerman}±3.25\textsuperscript{bB} &
532.67\selectlanguage{ngerman}±11.21\textsuperscript{aB}\tabularnewline
FHSO & 46.89\selectlanguage{ngerman}±2.33\textsuperscript{cDE} & 76.71\selectlanguage{ngerman}±4.92\textsuperscript{bC}
& 150.81\selectlanguage{ngerman}±3.44\textsuperscript{aD}\tabularnewline
HCO & 21.10\selectlanguage{ngerman}±1.87\textsuperscript{bE} & 6.76\selectlanguage{ngerman}±0.27\textsuperscript{cD} &
64.02\selectlanguage{ngerman}±3.62\textsuperscript{aE}\tabularnewline
Beeswax & 80.09\selectlanguage{ngerman}±6.61\textsuperscript{cCD} &
314.81\selectlanguage{ngerman}±30.96\textsuperscript{bA} &
1,100.68\selectlanguage{ngerman}±59.18\textsuperscript{aA}\tabularnewline
EGMD & 559.84\selectlanguage{ngerman}±57.02\textsuperscript{aA} &
156.75\selectlanguage{ngerman}±7.23\textsuperscript{cB} &
447.28\selectlanguage{ngerman}±27.22\textsuperscript{bC}\tabularnewline
Estercoat & 132.27\selectlanguage{ngerman}±4.86\textsuperscript{bBC} &
90.18\selectlanguage{ngerman}±3.11\textsuperscript{cC} &
426.14\selectlanguage{ngerman}±6.14\textsuperscript{aC}\tabularnewline
\bottomrule
\end{longtable}\selectlanguage{ngerman}

\&\selectlanguage{english}
\begin{longtable}[]{@{}llll@{}}
\toprule
c. & Environmental Temperature (\selectlanguage{ngerman}°C) & Environmental Temperature (\selectlanguage{ngerman}°C) &
Environmental Temperature (\selectlanguage{ngerman}°C)\tabularnewline
\midrule
\endhead
& 0 & 25 & 50\tabularnewline
Paraffin & 134.18\selectlanguage{ngerman}±9.95\textsuperscript{bB} &
144.77\selectlanguage{ngerman}±3.25\textsuperscript{bB} &
532.67\selectlanguage{ngerman}±11.21\textsuperscript{aB}\tabularnewline
FHSO & 46.89\selectlanguage{ngerman}±2.33\textsuperscript{cDE} & 76.71\selectlanguage{ngerman}±4.92\textsuperscript{bC}
& 150.81\selectlanguage{ngerman}±3.44\textsuperscript{aD}\tabularnewline
HCO & 21.10\selectlanguage{ngerman}±1.87\textsuperscript{bE} & 6.76\selectlanguage{ngerman}±0.27\textsuperscript{cD} &
64.02\selectlanguage{ngerman}±3.62\textsuperscript{aE}\tabularnewline
Beeswax & 80.09\selectlanguage{ngerman}±6.61\textsuperscript{cCD} &
314.81\selectlanguage{ngerman}±30.96\textsuperscript{bA} &
1,100.68\selectlanguage{ngerman}±59.18\textsuperscript{aA}\tabularnewline
EGMD & 559.84\selectlanguage{ngerman}±57.02\textsuperscript{aA} &
156.75\selectlanguage{ngerman}±7.23\textsuperscript{cB} &
447.28\selectlanguage{ngerman}±27.22\textsuperscript{bC}\tabularnewline
Estercoat & 132.27\selectlanguage{ngerman}±4.86\textsuperscript{bBC} &
90.18\selectlanguage{ngerman}±3.11\textsuperscript{cC} &
426.14\selectlanguage{ngerman}±6.14\textsuperscript{aC}\tabularnewline
\bottomrule
\end{longtable}\selectlanguage{ngerman}\selectlanguage{english}
\begin{longtable}[]{@{}lllllll@{}}
\toprule
Materials & Enthalpy (J/g) & Enthalpy (J/g) & Enthalpy (J/g) & Relative
Crystallinity (\%) & Relative Crystallinity (\%) & Relative
Crystallinity (\%)\tabularnewline
\midrule
\endhead
& 0 \selectlanguage{ngerman}°C & 25 \selectlanguage{ngerman}°C & 50 \selectlanguage{ngerman}°C & 0 \selectlanguage{ngerman}°C & 25 \selectlanguage{ngerman}°C & 50 \selectlanguage{ngerman}°C\tabularnewline
Paraffin & 188.5\selectlanguage{ngerman}±0.8 & 184.9\selectlanguage{ngerman}±2.1 & 150.5\selectlanguage{ngerman}±0.3 & 102.0 & 100.0 &
81.4\tabularnewline
FHSO & 179.9\selectlanguage{ngerman}±3.6 & 181.9\selectlanguage{ngerman}±9.6 & 167.3\selectlanguage{ngerman}±17.3 & 97.3 & 98.4 &
90.5\tabularnewline
HCO & 130.0\selectlanguage{ngerman}±6.1 & 128.1\selectlanguage{ngerman}±8.6 & 119.0\selectlanguage{ngerman}±2.9 & 70.3 & 69.3 &
64.4\tabularnewline
Beeswax & 168.5\selectlanguage{ngerman}±1.7 & 164.7\selectlanguage{ngerman}±5.3 & 93.6\selectlanguage{ngerman}±1.4 & 91.1 & 89.1 &
50.6\tabularnewline
EGMD & 183.0\selectlanguage{ngerman}±30.9 & 169.1\selectlanguage{ngerman}±44.8 & 82.5\selectlanguage{ngerman}±15.3 & 99.0 & 91.5 &
44.6\tabularnewline
Estercoat & 177.0\selectlanguage{ngerman}±34.7 & 164.9\selectlanguage{ngerman}±0.8 & 101.7\selectlanguage{ngerman}±20.8 & 95.7 & 89.2 &
55.0\tabularnewline
For abbreviations, see footnote of Figure 3. & For abbreviations, see
footnote of Figure 3. & For abbreviations, see footnote of Figure 3. &
For abbreviations, see footnote of Figure 3. & For abbreviations, see
footnote of Figure 3. & For abbreviations, see footnote of Figure 3. &
For abbreviations, see footnote of Figure 3.\tabularnewline
\bottomrule
\end{longtable}

\selectlanguage{english}
\FloatBarrier
\end{document}

