It is no surprise that the 2012 TAPPI Extrusion Coating Course tackled each of its namesake topics as it provided 50 industry professionals and eight students with information on this critical converting process. North America and three other countries from around the world were represented in the classes.
The course covered equipment technology (from extruders to dies to auxiliary equipment) materials (from basic resins to specialty resins to additives), process controls (purging, winding and maintenance), and safety for extrusion coating & laminating process. A panel discussion concluded each half-day session, giving learners their opportunity to ask the faculty questions with subject matter still fresh.
The principal success factor for a successful extrusion coated or laminated process is adhesion of the molten extrudate to one or more substrates. Porous substrates (e.g. paper or polymeric nonwovens) often allow the extrudate to penetrate and mechanically entangle with fibers on the substrate’s surface. These materials, polymer rheology, extrudate temperature, and chill roll cooling produce entanglement and adhesive strength. Denser, nonporous substrates (e.g. most polymeric films) depend on chemical bonding to the extrudate, but many physical characteristics of the extrudate affect the nature, durability and dynamics of this chemistry.
Attraction to OxygenWhile chemistry (and chemical reactions) involves a complex and diverse science, the major influence on adhesion of extrudates to substrates requires the attraction of electrons from various other atoms to oxygen and other “electrophilic” (electron-loving) chemicals. “Oxidizing” a hot extrudate for a certain time in the “air gap” of course addresses this need directly. Most of the effectiveness of adhesion-promoting resins (carboxylic acids, acetates, esters) results from their oxygen content. Functionality of “electrophilic” amines provides the adhesive effects of extrusion primers. Similarly, corona treating freshly-extruded films generates compounds with oxygen or nitrogen functionality.
Chemical bonding to the substrates is of course dependent on chemical reactivity of the substrate’s surface. Polyethylene and polypropylene without treatment have unreactive chemistry on their surface. Polyester and nylon films provide more reactive surfaces. For all of these films, contaminates on the surface can cover up this chemical activity so that an extrudate is unable to interact chemically. Various kinds of in-line “surface treatment” (e.g. corona treatment, flame treatment) can remove such contaminates to expose inherent chemical activity.
Covalent bonds, the strongest type, usually do not form during extrusion coating/ laminating. Rather hydrogen bonds, in which an electrophilic atom draws a nearby hydrogen atom to it, predominate. Other chemical forces such as ionic interactions (for ionomers) and the van der Waals force also help increase adhesion.
While these chemical interactions are real, they need energy and molecular proximity. The heat of the extrudate is the usual source of the former (cooling the extrudate too much in the air gap or too quickly on the chill drum weakens adhesion). Managing the latter involves intimate “wet out” of the substrate(s) by the extrudate. The molten resin must literally contact with substrates sufficiently closely and uniformly to allow chemical interaction and bonding to occur.