A range of surface treatment technologies are available to improve a substrate’s surface characteristics and make it more receptive to inks, adhesives and coatings
Corona treatment has been used for many years in blown film extrusion and converting lines, for increasing the surface tension of plastics films, to prepare them for subsequent printing, coating or laminating processes. According to Italian specialist Ferrarini & Benelli, the foremost technological driver is the increase in power rates. The development of innovative materials and additives capable of complying with certain specifications required by the packaging market (such as biodegradable materials, metallocene, lubricants, anti-blocking agents, anti-static agents, anti-fogging additives) has resulted in changes in the composition of polymers, and, consequently, in their reaction to the corona treatment.
Therefore, optimum results can only be achieved by increasing the power rate. Higher power is also necessary to ensure the same quality results at higher production speeds. Another major driver for innovation has been the integration of automatic power control and delivery systems. The possibility of automatically adapting the power to the actual surface of the electrode during the discharge stage allows considerable energy saving and repeatability of results.
In addition, the advent of digital systems has made it possible to introduce important control functions, essential for guaranteeing treatment uniformity during the production process regardless of the speed and width of the treated film.
Digital systems have optimum yield rates and therefore require less electrical energy while ensuring the same treatment efficiency. They also allow all parameters and functions to be remotely controlled.
An advanced function now incorporated in Ferrarini & Benelli’s corona generators is the Wet Start feature, which makes it possible to start the corona discharge even with wet electrodes, roller and insulators; and to keep the discharge live in case of short circuits caused by ambient humidity. Another feature of the company’s corona stations is the quick extraction/mounting system of the electrodes. The unit can be easily removed for maintenance and quickly re-inserted in its original position without requiring further manual adjustments. An external air-gap adjustment (the distance between the electrode and the roller) also helps to minimise downtime.
Treatment of thick materials
A new technology from Germany’s Plasmawerk Hamburg utilises the advantages of corona for the treatment of various materials of any thickness, including non-wovens, foams and fibre-based products. While the advantages of corona treatment for thin and flexible polymer film have been established for decades, it has not been possible to transfer them to the treatment of polymer foams or non-wovens, due to the use of ecologically and economically problematic fluorination.
Now, Plasmawerk claims its EDGE system has made the efficiency of corona treatment available to polymer foams without the harmful effects of fluorination. EDGE (Enhanced Discharge GEometry) is a new type of corona discharge that can be easily integrated into existing extrusion or converting lines, to treat material widths up to 3,000mm.
Substrates such as metal foils and polymer foams, and fibre-based materials such as textiles and non-wovens, contain some properties to allow treatment similarly to web-like films, but in other ways differ significantly and therefore require suitable pretreatment solutions.
Alternatively to normal corona treatment, plasma jets are often used, especially when treating three-dimensional parts. Although this method is generally adequate for the pretreatment of materials of any thickness, its implementation is quite problematic because of the scaling complexities of the jets over the full width of the web, the high temperature and the very high energy level needed.
According to Plasmawerk, excessive voltages are never needed with EDGE system, regardless of the material being treated, because the ignition of the main discharge is stabilised, equalised and simplified through added pilot discharge within the EDGE electrode. In conventional systems, the discharge is operated only between the high voltage electrode and the roller used as a counter electrode. EDGE’s stabilisation and equalising effects are claimed to avoid disruptive discharges, perforation of the material, discharges at the edges and consequent damage of the web.
Plasmawerk says its new technology "excels at treating foam and non-wovens properly". Open-cell foams will not be damaged by local streamers, and closed-cell foams do not run the risk of being burned or perforated. The high efficiency of the plasma source (the very low power needed) is also said to create treatment possibilities for thermally delicate materials.
In addition, the company’s CAPS (Controlled Atmosphere Plasma System) technology, allows application of a controlled gas atmosphere inside the EDGE electrode. This controls the chemical processes in plasma and on the surface to be treated, resulting in functional coatings on the outer polymer monolayer said to allow adhesion of adhesives and coatings that "far exceeds" the results of common corona treatment.
Atmospheric plasma surface treaters offer converters special capabilities for materials that are difficult to treat or have stringent treatment requirements. These systems are designed to clean, etch and functionalise polymers, textiles, non-wovens, and metallised surfaces.
Imparting high surface energy and long lasting surface activation with specific gas chemistries is said to make them ideal for applications that other surface treatment technologies are unable to achieve.
US company Enercon offers plasma surface treatment in customised system configurations, to suit the speed and type of material to be treated. The company says its Plasma3 and Plasma4 technologies offer the lowest operating costs by optimising gas flow rates with treatment effectiveness.
Ferrarini & Benelli has also developed atmospheric plasma treatment, designed to provide optimal wettability of polymers and metals, ensuring the adhesion of inks, glues and coatings. This FB Plasma 3D system is said to be particularly suited to treatment of high thickness surfaces.
Another new development, to be introduced at this month’s ICE Europe event, is Kalwar’s Calvasol system – claimed to allow the functionalisation of most flexible materials with anti-fog, anti-block, anti-static, release, or long term adhesion properties. Said to "meet the challenge between saving resources and quality enhancement in an environmentally friendly and extremely efficient way", the patent pending system is a combination of the German company’s Calvatron atmospheric corona technology with a water-based aerosol, which is sprayed directly into the corona discharge. Film is transferred through the system similar to a normal corona treater, while the aerosol is applied to its surface with the help of the corona discharge.
There is no need for low pressure chambers and, unlike the current procedure – first pre-treatment then coating – the combination of two steps into one saves time and resources. The coatings applied range from 3-150nm, so there is generally no need for additional drying processes, which again saves "a tremendous amount of energy", states Kalwar. As the reactive chemistry will be fixed directly on top of the surface of the film, the effect is immediate active and generally independent of environmental influences such as humidity or temperature.
Calvasol can be used for anti-fog functionalisation of the surfaces of packaging films. Although anti-fog is not new for the market, the challenge is to combine it with, for example, a peel effect, sealing layer or materials for a better oxygen barrier. There are currently two traditional ways to process anti-fog on films: batching and coating. However, batching with anti-fog additives is not easy and the results can be unfavourable or, in the case of co-extruded films, very expensive. Coating requires the film to be processed either in a special coating line or in a print unit of a printing press.
Although the anti-fog results of coating are acceptable, significant disadvantages of this process include: high consumption of anti-fog solution; solvent-based chemistry (up to 90%); high energy consumption for the necessary cross-linking process; and the additional step in a press or coater.
The integration of Calvasol in laminating lines is said to have resulted in very good anti-fog results on polyolefin and thermoplastic films (for both cold and hot fog). No additional processing of the film in a coating line or press is necessary; no additional energy consumption due to cross-linking is necessary; immediate supply of the films is possible due to the use of water-based and FDA-approved chemistry; and chemistry consumption is low (only a nanoscaled functional "layer" is applied).
Also said to offer the possibility of providing an anti-static layer, even at 20% humidity, Calvasol can be easily integrated into existing lines (higher line speeds up to 500m/min and working widths up to 6,000mm are said to be possible). A new integrated winder in the lab line at Kalwar’s plant in Halle/ Westfalen, Germany, is equipped with a Calvasol system and can process films and foams up to 1,500mm working width and 300m/min line speed.
Flame on board
Established as an effective method of surface treatment on extrusion coating lines throughout the world, flame treatment is considered most important for the pre-treatment of paperboard prior to lamination of extruded polyethylene in the manufacture of aseptic packaging such as juice cartons.
According to UK-based specialist Aerogen, the widespread acceptance of flame treatment within the extrusion coating industry is due to the flame proving to be a more effective method of surface preparation for paperboard than other form of treatment. This is because it has greater ability in applying surface oxidisation, plus the added benefits of removing small surface fibres and pre-warming the surface. Ultimately, flame treatment contributes to significantly highly durable adhesion between the substrate and the coating. This ensures that the laminated layers will remain intact together for the lifetime of the carton product.
While flame treatment has been established in the liquid packaging industry for almost 40 years, Aerogen says recent innovations make it especially effective on the latest production lines. New features within the flame control system help address current issues such as environmental impact and evolving production demands, including thinner substrates and faster line speeds.
The process applies a linear flame across the web that activates the surface of the paperboard as it passes at high speed. To provide the burner with a fuel mixture, a combustion generator system controls the flame for process optimisation and safety in an industrial environment.
Aerogen’s latest system is controlled by an oxygen analyser which assesses the products of combustion of the flame. This allows automatic control of the flame even if there are subtle changes to the energy of the gas supply’s calorific content – as there often are – and adjusting the flame accordingly to compensate for those changes. Control by the oxygen analyser optimises the flame for the best adhesion results and the most efficient gas consumption.
In regard to safety, the flame has a very short dwell time on the surface of the material to apply the treatment, so there is no risk of burning or igniting the paperboard surface. Also, the treatment system is stringently interlocked to immediately shut the flame off in the event of a web break anywhere on the production line, or in the event of an emergency line stop.
In operation, the objective of the flame treatment control mechanisms is to maintain the relevant process parameters as, says Aerogen, this will result in consistent and repeatable treatment and, ultimately, consistent adhesion for the entire substrate surface processed.
Those parameters are: flame profile (the shape of the flame); flame energy (the size of the flame); flame consistency (the air:gas fuel ratio by means of the oxygen analyser); automatic burner positioning, to place the flame precisely against the paperboard web; and automatic deckling (the width of the flame relative to the web width).
Aerogen says its equipment is effective at up to 650m/min line speed and 3,000mm web width, and using gases in most international regions. New features such as automatic deckling and automatic burner positioning are aimed at developing flame treatment functionality to even higher line speeds.
A corona treater is a key component of many lamination production lines, as the adhesion promoter to improve bond strength between the lamination layers. So treater reliability is an essential consideration. "When purchasing look at the long costs rather than the short term purchase price," stresses UK manufacturer Corona Supplies. "Machine downtime and production waste will always cost you more. The correct size of treater and power supply, correct number of discharge electrodes, base roll sized to suit production line widths, speeds and film tensions, will with today’s technology generally only need periodic cleaning to ensure continuous operation.
"So it’s really outside influences that determine the reliability of your corona unit," the company continues. "Dirt and contamination from the factory atmosphere can enter the discharge electrodes; if a treater has been off for some time, moisture/dampness can be present within the station – not good when high voltage is applied. Debris/particles from the substrate being treated can cover the discharge electrodes, leading to high voltage sparking, potential pin-holing of the substrate and corona machine breakdown."
Corona Supplies offers "a proven solution" to avoid contamination build-up within the discharge treater. The Reverse Air-Flow (RAF) corona system, in use on many lamination and coating lines in the UK and Europe, is said to increase treater performance and reliability. It provides filtered and conditioned air directly into the corona discharge face, resulting in a more uniform corona, improved treat levels, and self-cleaning for the discharge electrodes, whether ceramic or metal. The corona-produced ozone is then extracted from the hood compartment surrounding the electrode assemblies. This prevents contamination build-up directly on the electrodes, avoiding frequent electrode failure and system shutdowns. If high atmospheric moisture is present, an in-line air duct heater can be fitted to ensure all dampness is removed from the discharge electrodes before the corona is switched on.
For treatment of electrically conductive substrates such as aluminium foil and metallised films, the corona station incorporates the Triple-Treat ceramic electrodes, each set mounted in a quick release removable cartridge – "a simple operator action, with no adjustment needed on re-fitting".
"How do you evaluate the performance of your corona treater?" asks Tom Gilbertson, VP application engineering at Enercon. "Most converters measure dyne level, but dyne level only measures wetting out," he stresses. "The real test is the treater’s ability to promote adhesion to your satisfaction. It’s not unheard of for two different corona treaters to produce the same dyne level, but yield different adhesion results. Obviously, all corona is not created equal.
"Clearly, a smoother, higher density and more uniform corona provides superior treatment and ultimately better adhesion results," continues Gilbertson. "High definition corona increases the likelihood of adhesion success, but your corona system must also handle the power requirements of your application. Three elements determine the effectiveness of the applied watt density: the film, the electrode and the base (ground) roll. Power generator size is dependent on the application and the effectiveness of the electrode/roll configuration.
"Ceramic electrodes are less sensitive to high operating temperatures than their metal counterparts. But, like corona, not all ceramic electrodes are created equal. For example, Enercon’s H system features special ceramic electrodes offering an extremely high dielectric constant and superior capability for high power densities. The electrode’s capabilities are maximised with a cooling system that acts as both a temperature stabiliser and an ozone removal system. Applying 50% more power per square inch to each electrode results in a higher treatment level. This can mean fewer electrodes are required to generate the same treatment level.
"The roll coating’s ability to dissipate heat generated during the treating process is critical. Heat buildup on covered rolls is very destructive and can only be mitigated by increasing the roll diameter to allow a period of cooling between cyclic positioning under the electrode," says Gilbertson.
An alternate solution is to use a ceramic coating. Unlike conductive ceramic roll systems, Enercon’s H system features a proprietary ceramic blend formulation said to be highly effective at withstanding the heat generated by treating – allowing high treatment levels from a small footprint. This high definition corona system "virtually eliminates" film wrinkling, puckering, pin-holing and backside treatment. It can also be used to treat both conductive and non-conductive films.