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 (μ=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
μ 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 μ 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 μ 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.
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.