New Approach to Surface Treatment

Flexible packaging comprised of polyolefins such as polypropylene has low levels of polar functional groups on the surface, and poor wettability and adhesion properties, making it difficult to apply other layers such as dyes, inks, adhesives and coatings. To enhance surface polarity, surface treatments such as flame, corona or plasma can be applied to improve wettability and adhesion. Plasma can also be used as a preparatory treatment for a photografting approach to achieve high stability in treatment and permanent surface changes.

Physical treatments such as corona are usually applied on the surface, introducing polar groups and enhancing surface energy. However, these treatments are not stable, and the resulting surface modification is uneven, showing variations during printing or coating processes.

Moreover, the improved adhesion of the functional layers is due to a purely physical interaction or to the formation of weak bonds. For these reasons, adhesion is weak and has low thermal and chemical resistance.

Atmospheric plasma glow-discharge technology has advanced within the converting industry to become an exceptionally effective discharge treatment process which homogenously modifies surface chemistry to increase the bonding of interfaces such as dyes, inks, adhesives and coatings to 3-D surfaces such as polypropylene parts.

Plasma treatment has significantly greater longevity than corona discharges, offers extraordinary longevity compared to corona, but is not permanent in nature. The introduction of an innovative photografting approach which delivers a proprietary molecule to a web surface prepared by atmospheric plasma treatment offers a method of improving adhesion of inks, coatings and adhesives on flexible-packaging substrates.

Plasma processing of materials has been a vital industrial technology in many areas including electronics, aerospace, automotive, and biomedical industries. This is because of the unparalleled capability of plasmas for production of chemically reactive species at a low gas temperature, while maintaining high uniform reaction rates over relatively large areas.

In the past, the majority of plasma processing has been done at low pressure in a vacuum chamber and viewed as a necessary processing requirement. In principle and practice, however, atmospheric pressure plasmas provide a critical advantage over widely used low-pressure plasmas, as they do not require expensive vacuum systems. The cost of materials processing is thus reduced substantially and materials issues related to vacuum compatibility are not of concern.

Over the past few years, roll-to-roll atmospheric pressure plasma-discharge systems have been developed for operation at commercial speeds. These systems produce glow discharges with a low gas temperature, typically below 300 degrees C., and provide efficient reaction rates for deposition over increasingly large surface areas.

Although there are different kinds of atmospheric pressure discharges, the most commonly used on industrial scale is the dielectric barrier discharge (DBD). A DBD is a nonthermal 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 microdischarges 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 microdischarges 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 2-D web-based materials are rooted in observations that ion bombardment physically and chemically removes oxides and reducible compounds from surfaces, vaporizing contaminations.

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, increase the micro-roughness of surfaces, with introductions of active species. Atmospheric plasma power densities are not high enough to damage polymeric materials.

The principle for using a unique photografting technique for 3-D surfaces initially requires the surface to be activated through physical pretreatment (suggested to be by atmospheric plasma in this paper). A proprietary liquid solution (solvent or water based) is applied after surface pre-treatment in amounts of 1g/m2. It is applied to surfaces using a coating unit, and then the carrier liquid is removed by evaporation. A reactive photoinitiator is then grafted onto the surface by UV light exposure.

Anchoring of the photoinitiator to the 3-D surface ensures practically unlimited storage stability, as the surface tension assumes permanence. It is the chemical/physical properties of the photoinitiator which support the enhancement of the surface tension of the treated polymer substrate, thereby improving wettability.

Also, grafting the photoinitiator onto the surface prevents it from migrating into the polymer mass. For this reason, the treatment has practically unlimited storage stability, provided exposure to UV light is avoided. The photoinitiator grafted from the carrier solution reacts during polymerization, forming a chemical bond between the interface (dye, ink, etc.) and 3-D part surface.

The proposed advantage of the plasma-photografting approach lies primarily in the matching of the homogeneous surface treatment of polymeric surfaces using atmospheric plasma glow-discharge technology with the uniformity of a photografting solution coated over structure surfaces. Moreover, the permanent change in surface tension would allow water-based, solvent-based and UV-based printing technologies to be used. Figures 1 and 2 diagram the surface preparation and photografting technique for a polymeric substrate.

Editor's note: For the complete text of this paper, including an illustrative peel-adhesion experiment, conclusions and references, see the online version of this article at www.convertingmagazine.com.