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.