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FLEXO Magazine : March 2008
TECHNOLOGIES & TECHNIQUES 32 FLEXO MARCH 2008 www.flexography.org liquefied and, as a result, exhibit a lower proportion of solid crystals. Harder waxes resist liquefying and have a relatively high- er proportion of crystals in the solid state to impart slip. ABRASION RESISTANCE. Abrasion is a phe- nomenon caused by the mechanical action of rubbing, scraping or erosion. Since it is intimately related to scratching and slip, it is not surprising that many slip aids also function as mar and abrasion resistance additives. Abrasion resistance is produced by a combination of basic characteris- tics such as elasticity, hardness, strength, toughness, lubricity, particle size and, in some cases, thickness. WATER REPELLENCY. Also known as water resistance, this is another important property obtained with waxes. As the name implies, this characteristic is the protection of a surface against water penetration. The mag- nitude of protection can be temporary or limited (water resistance or repellency) up to a nearly infinite period of time or unlimited based upon the kind of exposure (water- proof ). Water resistance generally implies resistance to water in the liquid state, where- as moisture resistance refers to protection against water in a gaseous or vapor state. Usually, paraffin waxes, including scale waxes (a lower refined paraffin grade containing up to 5 percent oil), perform very well, particu- larly on porous surfaces. The oil penetrates easily and quickly into the pores and fissures of the substrate, imparting a very hydropho- bic character to the treated surface. TEXTURIZING. Although coatings are usu- ally applied to provide optical effects (color, gloss or matting etc.), or to protect a sub- strate, some applications also require the surface to have tactile properties. By em- ploying a coating that incorporates coarse wax particles, a rough and uneven surface is created at the microscopic level that is very similar to that observed with matting agents. Because tactile properties are largely dependent on the coating formulation, it is important that the wax particles protrude through the coating layer and this requires a particle size larger than the film thickness. FORMULATING WITH WAX EMULSIONS Wax emulsions are now well established and extensively used in water-based for- mulations such as paints, coatings, inks and OPVs, textile and leather treatments, polishes, paper and corrugated coatings, etc. These ready-to-use wax emulsions can be easily incorporated into a formula by simple mixing. Their very fine particle size ensures thorough, homogeneous incorporation with other ingredients of the formulation, maxi- mizing the required effects. And because of their inherent smaller particle size are less likely to detract from gloss. Wax emulsions can be stabilized by either a steric mechanism (using non-ionic emulsifiers) or by an elec- trostatic mechanism (using ionic emulsifiers, most often anionics). Combining anionic and non-ionic emulsifiers provides the emulsion the optimum stability because wax particles are protected through both stabilization mechanisms. This is referred to as the electro-steric stabi- lization mechanism. In addition, each stabilization mechanism not only has its own advantages and limitations, but also significantly impacts the overall formula- tion giving added flexibility in formulating. The advantages and limitations of both surfactant families are presented in Table 2. The wax properties that have the great- est impact on formulation performance include the chemical composition, the molecular weight, the melting point, the hardness and, in the case of emulsions and dispersions, the particle size. The end ap- plication and the coating process (includ- ing the curing) also substantially influence selection of the most appropriate wax. When selecting a wax, it is important to consider several things: Melting point of the wax should be lower than the curing temperature when curing is required. This allows the wax to melt, migrate to the surface of the coating, re-crystallize as the coating cools and, eventually, form a continuous film that encourages blooming. Particle size and particle size distribu- tion should be chosen to allow par- ticles to migrate to the film surface. This is particularly important if a hard wax is selected as it will produce the ball bearing mechanism. Sometimes a wax emulsion with a smaller particle size performs equally well, provided that the concentration is correctly adapted. The particle size range can be controlled during the emulsifica- tion process in order to meet precise specifications. pH of the wax emulsion should be within approximately one unit of the system to which it is added. If neces- sary, the pH of the emulsion can usu- ally be adjusted using aqueous ammo- nia or acetic acid. Type of surfactant can also influ- ence compatibility with the other components, as well as the overall formula stability. Matching the emul- sion charge with the coating charge enhances stability. Order of component addition in water-based formulations can be a critical factor in maintaining stability. Agglomeration can be prevented and overall stability maximized by add- ing the wax emulsion last. A further dilution of the emulsion with soft or demineralized water before incorpo- ration can also reduce the shock. Regulatory aspects of waxes ,ifthe emulsion is intended for food contact use (in a coating or in a package), must be in compliance with applicable regu- lations (FDA, BfR, European Directives, etc.). Many waxes are available in vari- ous grades, may be chemically modi- fied, or may consist of any number of different chemical compounds; there- fore, a confirmation of the product's regulatory status should be provided by the manufacturer or supplier. ABOUT THE AUTHOR: This article was submitted by Michelman Inc. Michelman is a global manufacturer of performance-enhanc- ing barrier, functional, and decorative coatings for flexible film packaging, paperboard, and corrugated boxes; and additives and modi- fiers for many industries including coatings, inks, composites, and construction products. Michelman products related to this story include Michem® Lube, Michem® Emulsion, Michem® Glide and Michem® Guard. ILLUSTRATION A. ILLUSTRATION B.