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FLEXO Magazine : March 2008
30 FLEXO MARCH 2008 www.flexography.org TECHNOLOGIES & TECHNIQUES Wax On Understanding The Properties Waxes Impart to an Ink or Coating Among the numerous additives available today for coatings and inks, waxes have a significant im- pact on many formulations or processes. Even if used in relatively small quantities--- typically below 3 percent solids content of the total composition---waxes impart or improve attributes such as slip and lubri- cation, abrasion resistance, anti-blocking, matting and water repellency, all of which are critical in the coating and ink arenas. Waxes are often classified as surface condi- tioner additives WAX DEFINITION AND TYPES The term "wax" encompasses a broad range of naturally occurring and syn- thetic compounds constituted from high fatty acid esters (typically C36 - C50) or from polymers (700 < molecular weight < 10,000). They differ from fats in that they are generally harder and less greasy. It is important to realize that chemical compo- sition alone does not define a wax. Rather, the term "wax" should be used as a generic term for materials that are or have the fol- lowing properties: Solid at 20°C, varying in consistency from soft and plastic to brittle and hard. Melting point of at least 40°C without decomposing, (this characteristic uniquely distinguishes waxes from oils and natural resins). Relatively low viscosity at temperatures slightly above the melting point and non-stringing (i.e., producing drop- lets---a characteristic that excludes most resins and plastics). Today, there are a large variety of waxes available, often classified according to their origin. Table 1 provides a summary of the waxes used most often in industrial applications. MECHANISMS OF WAX ACTION To have a significant impact on the coat- ing or ink properties, the wax must migrate to the surface and be present in sufficient quantity to impart the desired character- istic. Several migration mechanisms are generally proposed. THE BLOOMING MECHANISM. Molten wax particles float (or bloom) to the surface. The coating cools and re-crystallization of wax particles takes place, forming a thin but continuous wax-enriched surface layer. Generally, the softer the wax or lower the melting point, the more predominant is the blooming mechanism. The compat- ibility between the wax emulsion and other formulation components determines the wax migration rate (see Illustration A). THE BALL BEARING MECHANISM. In this case, solid wax particles migrate individu- ally or protrude through to the surface. By protruding slightly above the coating sur- face like ping-pong balls floating on a pool of water, they act as a physical spacer and prevent another surface from coming into close contact. Hard and high melting point waxes like HDPE, as well as PTFE, which exhibits wax-like characteristics, operate using this mechanism under certain con- ditions. Both the particle density and the extent of protrusion influence the magni- tude of the effect (see Illustration B). Once at the surface, the layer of wax particles has the ability to modify the Coefficient of Friction (CoF) of the sub- strate, imparting the desired characteristics. This explains why waxes are often classified as "surface conditioner additives." FUNCTIONAL ATTRIBUTES ANTI--BLOCKING. This term defines a non- stick condition between two surfaces or the resistance to adhesion between two sur- faces under the influence of temperature, relative humidity or even pressure. A very well known example of a blocking condi- tion is when a freshly painted window frame is closed too soon. Sometimes, it can be very difficult to open the window again. Factors affecting blocking include the coat- ing surface free energy, topography of the coatings, hardness, and glass transition temperature (Tg) of the polymer. HDPE, paraffin and carnauba waxes are typically used to counteract blocking. Anti-blocking agents are used extensively for items that are coated, dried and immediately stacked or rolled up for storage or shipment. SLIP AID. Slip represents the ability of two surfaces to glide over each other without causing any mechanical damage. Good slip properties require that the slip addi- tive concentrate to the surface during and immediately after application and curing. The harder the wax, the better the slip properties. This is explained by the fact that wax crystals in the solid state are the main factor responsible for the characteristic of slip. Softer waxes tend to be more easily TABLE 1. TABLE 2.