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Surface Treatment Discussion
by Russell E. Smith

While most of this website focuses on how to measure and characterize the surface properties — most notably wettability — of polymers, foil, coated paper, and other non-absorptive materials, it seems appropriate to delve a bit into the reasons these surfaces must be altered, as well as the various common methods of surface treatment, and how treatment affects surface chemistry.

Because plastics are composed of non-polar, long-chain molecules, their surfaces have little free energy, and are essentially inert. This is most notably true of fluorocarbons, silicones, polyolefins, and vinyls. Unlike high energy materials (such as metals and ceramics), plastics lack the available bond sites offered by charged ions distributed over the surface. Without this molecular attraction, liquids fail to wet the surface, resulting in poor adhesion and coverage.

This problem, while universal, is especially troublesome for processes such as printing, coating, and laminating, which work with fast-moving webs. For these purposes, the free energy (ability to attract and anchor a liquid) of the surface must significantly exceed the surface tension (resistance to spreading) of the liquid, or dewetting occurs readily, producing waste. Worst of all, some problems, like on-the-shelf delamination or ink liftoff, cannot be seen until a job is finished and shipped to the customer.

During the dawn of plastics technology, chemical priming was the only solution. Soon after, the first rudimentary machines were developed to treat polymer surfaces, increasing their polarity and surface energy, and making them acceptable for the various converting operations. These flame and corona treaters gradually evolved through the 1960s and 1970s. With solvent-based systems predominant, major breakthroughs came slowly. The advent of waterborne — and then energy-cured — systems changed this. Suddenly, marginal treat levels (34 to 38 dynes/cm) caused serious quality problems. Films that had been readily printable exhibited pinholes, fisheyes, ink liftoff, and other discrepancies. To remedy this, treatment levels of 40 dynes/cm and higher were needed at the press (or laminator, etc.).

In the 1980s and 1990s, surface treating equipment improved consistently and dramatically. Since then, the technology has continued to evolve, with the most significant breakthrough being the widespread use of atmospheric pressure plasma treatment. Converters now enjoy equipment which not too long ago would have been exclusively in the domain of the research lab. Custom blending of process gas (nitrogen, ammonia, carbon dioxide, oxygen-enhanced, and fluorine-based treating are all readily attainable), lane treating, synchronized pulse treating, and other options are widely employed.

Corona treaters are available for sheet or web applications in conventional, bare-roll, or convertible configurations; three dimensional and robotized treatment is readily achievable. Feedback loops make real-time output adjustments based on web speed or dwell time, temperature, humidity, or other process variables virtually immediate, with all process data permanently-retainable for subsequent data analysis as needed. The materials used in today's dielectrics are of superior quality: they last longer, run cooler, and transfer more power to the web; impedance matching is usually automatic to maximize energy output and reduce thermal stress. Finally, the electronic components and robustness of design employed in modern treaters make them far superior to earlier designs.

Meanwhile, the safety and control systems integral to flame treaters have also improved greatly, increasing their efficiency and making them competitive for some operations which were once handled strictly with corona. And, cold plasma treaters are now in extensive commercial use — a trend that will likely continue as their capital and operating costs diminish. Atmospheric plasma, a blend of corona and plasma technologies, already enjoys an integral role in web applications, and we can expect to see a continuing interest in both batch- and continuous- low pressure cold plasma use as well.

These machine advances translate to major quality and productivity enhancements for printers and converters. Specific advantages include improved coverage and faster drying; faster machine speed capabilities; better bond strength; longer treatment life; and the ability to use solvent-free coating and ink systems even on critical quality, six- or eight- color process jobs.

Waterborne coatings have obvious environmental advantages, are less toxic, and generally are less prone to creating off-flavors in food packaging. Unfortunately, they are also harder to handle — their surface tension is higher than that of solvent-based coatings, and, moreover, they are less able to incorporate contaminants, additives, and the short chain partials common in polyolefins and other low energy polymers. UV formulations generally share these traits. Additionally, their high viscosity inhibits rapid leveling (needed for uniform cure), and they tend to shrink during curing. These problems can be effectively overcome with appropriate surface treatment.

We will briefly discuss corona, flame, and plasma treatment. Other methods include ozone, ultra-high frequency electrical discharge, UV or laser bombardment, and chemical priming.

Corona treatment is the commonest choice for converters. A corona treating system can be thought of as a capacitor. High voltage is applied to the electrode. Between the electrode and the ground roll is a dielectric, comprised of the web, air, and an insulator such as silicone, epoxy, glass, or ceramic. The voltage buildup on the electrode ionizes the gas in the electrode/web gap, creating the highly energized corona. This excites the gas molecules, re-forming them into a variety of free radicals, which then bombard the surface, increasing its polarity by distributing free bond sites across it, and creating changes in the surface morphology which also serve to improve adhesion.

There are two basic treater designs; conventional (dielectric-covered roll) and bare-roll (dielectric-covered electrode). Only bare-roll systems can be used on conductive webs; conventional systems short out. But, conventional systems are more efficient, and have fewer problems associated with heat build-up on the electrodes. Therefore, they are preferred by film extruders and extrusion coaters. Bare-roll systems are ideal for converters who process various materials, especially foils and plastics which were pre-treated initially. They are well suited to “bump” treating — subjecting the web to treatment immediately prior to printing, coating, laminating, or metallizing. This not only re-energizes the surface, it also removes contaminants or bloomed additives which may have invaded it. Several manufacturers also offer convertible systems, which can be operated in either configuration.

Corona treaters are easy to install and use, can usually be adjusted for varying web widths, produce uniform treatment when operated properly, and are quite cost-effective. But there is a downside: Backtreatment can cause blocking and poor heat-sealing; corona treatment decays rapidly with handling and age, especially in heat and humidity; static buildup can require in-line deionization; attempting to increase surface energy by more than 10-12 dynes/cm is often inadvisable — pinholing, surface degradation, and accelerated treatment decay rate can result from overtreatment. Finally, the process produces ozone, which must be neutralized before release to the atmosphere.

Flame treatment is commonest for molded pieces such as bottles, tubing, and automotive parts. However, it is also widely used to treat films, foils, coated board, and other substrates. Like corona, it induces an ionized airstream, which alters the surface as it impinges upon it. This is accomplished by burning an ultra-lean gas mixture, whose excess oxygen is rendered reactive by the high temperature.

Advantages of flame treatment include freedom from ozone, pinholing, and backtreatment. Also, flame treating can achieve treatment levels above 70 dynes/cm even on polyolefins. Flame treatment is also considerably more stable than is corona — dyne level decay is much slower. A slight hazing may preclude use on optical grade films and some packaging materials. Capital costs tend to be higher than for corona treatment, but, depending on the cost of fuel, operating costs can be significantly lower.

Cold plasma treatment is typically run in batch mode, but recent improvements are making it more attractive to producers of high-end specialty webs. Traditional cold plasma treatment requires a partial vacuum; a selected gas is introduced into an evacuated chamber and ionized by a radio frequency (rf) field. The rf field excites the gas molecules, creating a blend of neutral atoms and reactive radicals formed from free electrons. Three processes occur when these free radicals bombard the surface: ablation (“cleaning” it by removing its outer molecular layer); crosslinking (interconnection of long-chain molecules); and activation (impartation of reactive molecules, which, in an oxygen-rich atmosphere, increases surface energy).

Plasma treatment is powerful, with the ability to provide specific and predictable surface functionalization to meet the most demanding and specialized requirements. It will likely remain at the cutting edge of polymer surface treatment technology for some time.

All three of these techniques alter the polymer's surface chemistry. The presence of carbonyl and hydroxyl groups — absent on untreated surfaces and introduced by any of these treatment methods in the presence of oxygen — enhances wettability, allowing inks, coatings, and adhesives to flow out and coat uniformly. At the same time, these free radical sites provide the molecular bond that adheres the coating to the surface and keeps it firmly anchored after it dries or cures. Therefore, the test methods discussed elsewhere throughout this website are appropriate for predicting both wetting and adhesion.