Surface Treating: Promoting Adhesion

Published: March 01, 2011, By By Enercon Industries Corp.

If you're in the converting industry, you're likely amazed by the continually changing landscape of technically innovative products and processes. They elevate the profitability and productivity of the industry.

Most notably, flexible packaging relies heavily on these innovations to meet ever-increasing packaging cost and weight reduction requirements from sustainability-conscious consumer products companies. One key technology supporting the flexible packaging industry — surface treatment — lends evolving surface-functionalizing benefits with the latest generation of surface modification innovations.

Most packaging polymers, such as polyethylene (PE) and polypropylene (PP), are chemically inert and nonporous with relatively low surface tensions. This makes them non-receptive to adhesion with interfaces such as printing inks, coatings, and adhesives.

Surface treatment by corona or atmospheric plasma discharges are common choices for inducing an economical and effective modification to the surface characteristics of these types of substrates. Metallized films, nylon, vinyl, polyvinyl chloride (PVC), polyethylene terephthalate (PET), foils, coated papers, foams, and many other roll-to-roll substrates benefit from this categorical solution as well.

Corona treatment, as an “air plasma,” also is suitable for the treatment of three-dimensional objects such as injection-molded and blow-molded parts. But all that being said, what do each of these treatment processes really excel at doing in their latest state of the art, and what treatment process should be considered to provide the best solution for your application?

A corona treater in action.

Corona Treatment

With polymer-based packaging substrates, corona treatment excels at activating these surfaces by disassociating the oxygen molecules within the air ionizing discharge to free atomic oxygen (O) to bond to the ends of the substrate molecules following a free radical surface effect. Other initiating radicals include nitrogen (N), hydroxyl (-OH), and hydrogen (H), which influence the chemical functionalization of these surfaces.

Reactive groups, such as carbonyls, hydroxyls, hydroperoxides, aldehydes, ethers, or esters, ultimately are introduced to the surface. Also made present at the surface of these films are water-soluble, low-molecular-weight oxidized materials (LMWOM), formed by molecular scission during corona treatment.

Surface roughening on corona treated films is caused by the arc discharge impacts at the surface, and the interaction of LMWOM and water in a high humidity environment. Although LMWOM does not necessarily form a weak boundary layer, over-treatment can cause an excessive amount of oxidized materials that can hinder subsequent adhesion of inks, coatings, and adhesives to the corona treated film. Appropriately, corona treated surfaces ultimately will raise surface tension on these films to the preferred level.

So if the surface of polymer films and other non-polar substrates are made polar, how does corona treatment create the bond with solvent-borne, water-borne, and energy-curable inks, coatings, and adhesives? The free radicals will form carbonyl groups from the ozone created from the discharge, promoting adhesion. The more electrons that avalanche to the substrate, the shorter the molecular chains become and the more adhesion (polar) sites that are created (to a point). Adhesion occurs at the top atomic layers of the substrate.

Conventional solvent-borne inks will have better substrate wetting characteristics than water-borne liquids at these top layers because some of the solvents can “dissolve” the boundary layer (process additives, organic contaminations) and allow the ink to anchor to the substrate. Water-borne and ultraviolet/electron beam (UV/EB) liquids will not dissolve the boundary layer as readily and will require higher substrate treatment levels.

Corona discharge technology continues to evolve in performance to expand its versatility with a wider range of substrates and processes. Integration of certain ceramics that coat both the electrodes and the ground roll generate “high definition” discharges with ultra-high homogeneity and uniformity, leading to higher density and higher surface energy effects.

Reductions in the incidence of material wrinkling and backside treatment also diminish with these low heat discharges. Cast film extrusion, biaxial orientation systems, laminating, extrusion coating, and printing processes therefore all benefit from these developments.

Atmospheric Plasma

You've undoubtedly read or heard much about atmospheric plasma technology and the commercial models that have been introduced over the past ten years. Atmospheric pressure plasmas have been prominent over this period because of their technical significance in eliminating the need for batch-processing vacuum chambers to ensure the maintenance of a negative pressure, while in practice delivering very similar surface treatment results directly on the production line.

Although there are different kinds of atmospheric pressure discharges, the most commonly used on an industrial scale is the dielectric barrier discharge (DBD).

A DBD is a non-thermal radio frequency (RF) fourth-state-of-matter plasma with a gas discharge maintained between electrodes separated by at least one dielectric barrier. Atmospheric pressure DBDs usually consist of a multitude of transient micro-discharges of very short duration (several 10 ns), with diameters of about 0.1 mm and mean electron energies of typically 1-10 eV. Within these micro-discharges, the gas is excited, ionized, and dissociated, and highly reactive species are formed without a significant increase of the average gas temperature.

Documented advantages for atmospheric plasma discharge treatments of two dimensional web-based materials are rooted in observations that ion bombardment physically and chemically removes oxides and reducible compounds from surfaces, with many contaminations vaporized. In addition, gas molecules are accelerated to an excited state, releasing active chemical-free radicals and UV energy. Free radicals activate chemical reactions on surfaces, inducing intermolecular cross-linking.

When compared to corona discharges, atmospheric plasmas produce significantly more homogeneous and uniform surface activation across material surfaces and increase the micro-roughness of surfaces, with introductions of active species. Atmospheric plasma power densities are not high enough to damage polymeric materials, and voltage levels typically are too low to create back-side treatment.

The adhesion promotion mechanisms are similar to that of corona treatments, whereby adhesion occurs at the top angstroms of surface depth, but with the exception that the discharge is significantly more active and powerful with respect to ionic bombardment and without channeled arcing. Rather, the glow-like discharge at high frequency creates different mechanisms for enhancing wettability and adhesion.

With polymers, plasma surface bombardment causes hydrogen to be released or abstracted from the surface, which then can react with the functionalizing gases used in the treatment process and with small amounts of oxygen in the surrounding air. Also, surface oxidation is created with functional group formation. This surface reaction can be prescribed to create covalent bonding with the ink, coating, or adhesive formulation for enhanced surface adhesion characteristics.

Recently, fourth generation atmospheric plasma technology has been introduced that finally has broken through previous barriers relative to operating speed and treatment width. Moreover, new features and benefits, such as halving of gas consumption, tripling of electrode power output, automatic gap adjustment, and the elimination of ozone in corona modes, have made atmospheric plasma treatment systems even more practical and sustainable technologies for converting applications.

Expanded applications beyond difficult-to-treat flexible packaging substrates now include surface activation of photovoltaic materials, cleaning of foils and glass substrates, and improvements in the breathability and efficiency of textile materials. Table I summarizes key process operating specifications and strengths for both corona and atmospheric plasma treatments.

Knowing in advance the strengths and capabilities of specific types of surface modification technologies — and how these technologies can be tailored to specific converting application requirements — can generate new and perhaps unexpected revenue streams along with process cost savings.

Supplier Info

• Enercon Industries Corp. | www.enerconind.com

Rory A. Wolf is the VP-Business Development for Enercon Industries Corp., Menomonee Falls, WI. He holds a MBA from Marquette Univ. and has nearly 30 years of experience in the plastics and paper industries. He has authored more than 30 technical papers and articles. His most recent work includes the publishing of a new book titled Plastic Surface Modification: Surface Treatment and Adhesion.

Not To Be Overlooked

Innovations in flame plasma and ozone generation technologies also are playing pivotal roles in elevating productivity and profits. New flame plasma burner designs are providing better film treatment quality and a wider operating range for heat-sensitive films. And higher chill roll cooling efficiencies are improving surface temperature and treatment uniformity.

Ozone generation systems offer significant adhesion benefits within extrusion coating processes. Recent innovations are increasing ozone concentration and production levels with new air and oxygen ionization techniques. New ozone flow distribution technology also is allowing multiple extrusion systems to receive balanced ozone distribution from a single generator.

The views and opinions expressed in Technical Reports are those of the author(s), not those of the editors of PFFC. Please address comments to the author(s).