Corona sparks new interest for treating large 3-D parts.
Coronas, also known as "atmospheric" or "nonvacuum" plasmas, are ionized gases created by electricity passing through air. They can react with a plastic surface to improve adhesion for printing, painting or laminating. Coronas were first used to help ink stick to thin film, known as web treating, then expanded to three-dimensional surface treating of small round items like bottles or syringes. Larger 3-D parts were typically treated by priming, flame treating or abrasives. Now corona treating is replacing some of these approaches because it's less likely to mar, melt or otherwise damage part surfaces, especially important for parts requiring Class A finishes. Corona treating is of special interest in allowing PP to be substituted for ABS since PP isn't paintable without treatment, while ABS is easily painted.
"There's a lot going on in the European automotive market with corona treating of large parts for painting and gluing," says Ole Jensen, general manager of Tantec AS in Denmark (sister company to Tantec Inc. in the U.S.). "ABS plastics are going out and being replaced with PP." Jensen cites the example of PP interior door panels for the '92 Volkswagen Golf, which are treated to bond carpet trim. PP instrument panels for VW are also corona treated for PU foam adhesion.
In the U.S., TPO air bag covers are being corona treated for scuff-coat adhesion at United Technologies' Automotive Engineered Systems Div. in Niles, Mich. TPO airbag covers are treated for paint adhesion at Davidson Interior Trim/Textron in Dover, N.H. And corona is also used on EPDM car-door weather stripping at several of Schlegel Corp.'s plants around the world (Schlegel is headquartered in Madison Heights, Mich.). These are just a few examples.
Another advantage of corona treating is environmental. To provide paint adhesion to TPO and PP parts, corona treating replaces solvent-based primers that contain as much as 90% VOCs. Replacing primers also makes finishing critical parts more precise. Corona treating air-bag covers allows them to be painted with one coat, for a more precise fit into the breakaway joint in the steering wheel. Corona treating also allows C|O.sub.2~/water-blown urethane foams to adhere to plastic parts, replacing CFC-blown foams, which adhere easily.
CORONA VS. PLASMA
A closely related technology for increasing wettability of plastic parts is vacuum-chamber plasma treating. Like corona, plasma treating uses an electromagnetic energy to ionize gas molecules, which in turn react with the plastic surface. Key differences are that plasma treating takes place in a vacuum, unlike corona processes, and plasma can use gases other than air to achieve a wider variety of surface-modification effects. Nonetheless, there is a somewhat fuzzy line between the two processes, at least in the opinions of some treatment hardware suppliers.
Large parts like car bumpers are being treated for adhesion with vacuum chamber plasmas. Vacuum plasmas have the advantage of being able to treat nooks and crannies in large parts more easily than most coronas can. An intriguing exception is that hollow parts can be filled with gases other than air and then corona treated. Lectro Engineering Co.'s Poly Foam Treater draws a partial vacuum in hollow parts like ice chests, salad bars or office furniture, fills the parts with what the company describes as "a controlled amount of a conductive gas mixture," then passes the parts through its treating tunnel to improve adhesion to foam filling.
Advantages of corona over vacuum plasma are that coronas readily lend themselves to continuous production, whereas vacuum plasma is primarily a batch process. Also, most corona equipment is much less expensive than vacuum plasma equipment.
FOUR TYPES OF 3-D TREATERS
Tantec general manager Mark Blitshteyn says commercial electric-discharge systems can be classified by frequency and voltage into four types: low-frequency and high-frequency arc, spark, and glow. Low-frequency arc and spark discharges operate at about 60 Hz (the frequency of standard a-c current). High-frequency arcs operate at 20-30 KHz and glow types run at up to 2 MHz.
Low-frequency arc systems (mostly ungrounded) are made by Arcotec, Corotec, Enercon Industries, and Tantec. Arcs form between positive and negative electrodes close together in the head like the gap in a spark plug. They generate plasma and blow it in a plume about 50-70 mm wide and 25 mm long onto the part surface. Treatment area is narrow (2-7 in.) even when two or three heads are lined up, so these systems aren't used to treat big parts for painting but rather for cables, pipes or areas on car battery cases. Electrodes are mounted close (3-4 mm) to the parts (or parts conveyor), and dwell time may be 0.5-1 sec.
Suppressed-spark discharge is a 30-yr-old system available only from Lectro Engineering. One of the few continuous systems for large parts, it's an ungrounded tunnel with insulated electrodes built into the walls, creating a region of ionized air through which parts pass on a conveyor. The electrical field in the tunnel reaches 240,000 volts at 60 Hz. Parts being treated can be feet away from the electrodes and can even overlap. Dwell time is controlled by conveyor speed.
Glow-discharge systems, offered only by Electro-Technic Products (ETP), is by far the highest frequency commercial system, transmitting over 100,000 volts at 2 MHz. This ungrounded discharge can treat cylindrical profiles like cable, reaching 270|degrees~ around the product circumference with one electrode. ETP consultant John Logeman says the company has developed even higher frequency systems on an experimental basis, up to 6 MHz. Other firms say they're developing higher frequency systems, too.
High-frequency corona discharge systems are made by Arcotec, Kalwar, Kasuga-Denki, Tantec and 3DT. They use up to 60,000 volts at frequencies of 20-30 kHz to treat small oriented parts with a grounding electrode under the conveyor. Systems in development have hundreds of high-frequency electrodes in the form of wires, chains or filaments to treat large, irregular parts, but the cost approaches that of vacuum plasma systems.
Yet another high-frequency system that is almost in commercial application may be easier to use. Nippon Paint Co. calls it "a noncontact type plasma system" that creates plasma strong enough to treat bumper surfaces for virtually any coating "with an open distance of 12 in. or more between electrodes, allowing bumpers to pass continuously through the electrodes."
WHAT CORONA CAN AND CAN'T DO
Small parts and extruded profiles are usually treated inline while still warm from the mold or die because warm parts accept treatment more readily, says Enercon. Coronas and plasmas can treat virtually all thermoplastics, thermosets and elastomers, but the surface effect wears off. Treatment lasts longest on polyolefins like HDPE, shortest on polyester; so treated parts must be used within hours or days. Resins with slip additives must be used right after treating because slip agents migrate to the surface, undoing the corona effect. Resins with conductive metal or carbon fillers can't be treated with grounded systems, but reportedly can with ungrounded systems like those from ETP and Lectro-Treat.
Not only do corona treatments have a shelf life, but because the surface treatment is only a few hundred angstroms thick, it's easily scratched. So pretreated parts that are stored should have dust blown off with an air jet, not wiped.
Not every part shape is corona treatable. Enercon and ETP systems can treat concave shapes only 2 in. deep without special electrodes. And systems where electrodes must be as close as 3-4 mm from the part surface pose problems for big parts, which may warp in cooling. If some electrodes get too close, they can scorch the parts.
Production rates for corona surface treaters vary with the type of plastic substrate, additives in the substrate, and the material being bonded. Lectro Engineering says an 8-in. tunnel treats HDPE bottles without additives at conveyor speeds of 8 ft/min. At that rate, 10,000 4-oz bottles/hr can be treated to a water-wettable surface-energy level of 70 dynes/|cm.sup.2~. Tantec says its controlled high-frequency arc system treats large irregular parts like car bumpers in a 30-sec cycle. Bonding decorative carpet to car doors takes 45 sec and raises surface energy from 29 dynes/|cm.sup.2~ to 52.
Corona treating can build up static. The bigger the mass of the part, the bigger the charge. Treating a car bumper would build up a long-lasting static charge that could cause an explosion in a paint booth later on if the static weren't removed. Corona treating filled rather than empty bottles for printing can also bring nasty surprises. Faberge last year created a shampoo bottle requiring 40% ink coverage. In a trial run, 50,000 bottles were filled before corona treating and printing. Then every time anyone opened a bottle, they got a static shock.
The extremely high voltages (10,000-30,000v) of corona discharge is always a safety concern. Most equipment has multiple shut-off provisions. For instance, if cooling air to the electrodes (in systems with insulated electrodes) stops, the system shuts off before it damages the electrode or the part. Incoming voltage also varies, and can hurt treatment quality. So United Technologies Automotive Engineered Systems Div. in Niles, Mich., added voltage alarms to its corona system. "We studied the minimum voltage levels for paint adhesion and set alarms to sound if the voltage drops too low," says a company engineer.
COMMERCIAL 3-D CORONA SYSTEMS
The following companies make commercial 3-D corona and atmospheric plasma treaters, though not all market them in North America:
* Ahlbrandt-System GmbH, Lauterbach, Germany, makes a small 3-D corona treater for small bottles and sells it in Europe. (In the U.S. Ahlbrandt's web corona systems are built and sold by Enercon.)
* Arcotec Oberflaechentechnik GmbH of Wimsheim, Germany (represented in the U.S. by 3DT) offers air-blown arc-discharge corona equipment, called Arcojet, in a range of frequencies.
* Corotec Corp. is a 17-yr-old company that makes ungrounded, air-blown plasma-discharge equipment. Its Plasma-Jet treaters have been used for about five years on medical parts. They also treat cable and pipe for printing, exterior auto trim for metalizing, and foam weatherstrip for flocking. Units have 2-in.-wide single heads or 3- to 4-in.-wide dual heads.
* ETP has very high-frequency systems (up to 2 MHz) used to treat big parts like car bumpers and interior trim. ETP's system, developed in 1984, creates electric fields that are said to treat irregular surfaces up to 12 in. wide and 10 in. from the electrodes. (The higher the frequency, the wider the surface area covered by the plasma.) ETP's other applications include treating doll faces for pad printing and PP battery cases for printing. ETP's smallest system, model BD-85, treats plastic contact lenses for coating.
* Enercon uses an air-blown electric arc to create an ungrounded atmospheric plasma. Its Dyne-a-Mite line of treaters comes with single, dual and triple electrode heads (for 2.5-, 5- and 7-in. treatment widths, respectively). Heads must be mounted 0.25 in. from part surface. Enercon says its system is used to treat PE, PP, PET, PVC, PS, nylon, polycarbonate, and fluoropolymers. It's also being used to treat pultruded thermoset polyester profiles to enhance adhesion of a thermoplastic extruded surface coating.
* Kasuga-Denki Inc. in Japan has been working in corona treatment for at least 15 years, making high-frequency, high-voltage, atmospheric plasma systems and low-frequency corona systems for surface treatment. Kasuga-Denki also builds MHz-range equipment for sterilizing medical equipment, but not for surface modification.
* Klaus Kalwar GmbH of Germany has made surface treating equipment for 30 years. Kalwar, together with Freudenberg in Germany, developed a patented system for 3-D treating large parts using chain electrodes and robotic part handling. Kalwar also has a pass/fail quality-control system that identifies whether parts received adequate treatment. Kalwar has family ties to Societe de Transfert Technologique (STT) of France.
* Lectro Engineering Co. has built Lectro-Treat tunnels for over 30 years and claims to have over 1000 units in use worldwide. The largest is 20 ft long and has a 45 x 22 in. treating tunnel for a large rotomolded part. Another application is an HDPE salad bar for lunch counters, measuring 4.5 ft long x 3.5 ft high x 4 ft wide. It's treated for PU foam filling and requires a 7-min cycle. Lectro uses 240,000-v discharge at 60 Hz. Tunnels range from $18,000 for 4 x 4 in. diam. to $40,000 for 30 x 30 in. A patented new Polyfoam Treater, which uses gas vacuum like a true plasma system, is treating instrument panels in Japan. Lectro represents STT in the U.S.
* Nippon Paint Co. Ltd. of Osaka, Japan (which has a U.S. branch), is believed to be developing an atmospheric plasma system for bumpers in cooperation with a Japanese car maker. According to patent literature, the system may use hundreds of fine, multi-ply wires as electrodes, which trail over the part surface. Another system described in the patents resembles a "plasma car wash" in which a bumper, mounted on a grounding electrode on a conveyor, passes twirling, brush-like electrode rollers, which treat the sides, ends and top surfaces.
* Plasma Elektronics in Germany makes small, ungrounded 3-D treaters using both low and high frequency, as well as vacuum chamber plasma treatment systems.
* Societe de Transfert Technologique (STT) of La Ferte, France, related through family ties to Kalwar in Germany (their respective founders are brothers-in-law), uses ungrounded electrodes for surface treatment of cables, wires and syringes. Its systems can treat parts up to 12.9 ft wide x 4 in. deep. STT is represented by Lectro Engineering.
* Sister firms Tantec AS and Tantec Inc. have systems treating big 3-D auto parts such as bumpers. Tantec Inc. makes both low- and high-frequency 3-D corona systems, ranging from $15,000 for the smallest to $l50,000.
* 3DT is a two-year-old company (started by former Tantec people) with high-frequency (20-30 kHz), grounded and ungrounded treatment systems in medical and automotive applications. 3DT also says it has the only system for maintaining output voltage despite drops or surges in incoming voltage.
Corona or Plasma--What's in a Name?
"There are no classification or standards for this industry, none whatever," notes Mark Blitshteyn, general manager of Tantec Inc. "Every high-voltage discharge system is called a corona, when they're really all atmospheric plasmas."
It's definitely in fashion to call 3-D corona systems "plasmas" in order to distinguish them from traditional web treaters. Certainly the electric discharge equipment in 3-D systems is nothing like web systems. Three-D coronas may use output frequencies a thousand times higher or lower than web treaters (60 Hz-2 mHz for 3-D corona treaters vs 3-50 kHz for web treaters).
A convenient visible distinction used by corona machinery makers is to call grounded discharges coronas and ungrounded ones atmospheric plasmas. "They look different," says Corotec president Bruce Stobbe. "Coronas are typically purple, plasma is purple at the far end of the discharge and yellowish at the electrode."
Both grounded and ungrounded systems exhibit a wide range of voltage and frequency. Lower voltage is needed at high frequency, and conversely, higher voltage compensates for a drop in frequency. Coronas tend to use higher voltage (and lower frequency) to reach the grounding electrode. Atmospheric plasmas with only emitting electrodes tend to have lower voltage and high frequency. Corona's higher voltage gives off more ozone, or extra free radicals. S corona systems need venting or catalytic reactors to remove ozone, plasmas don't.
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|Author:||Schut, Jan H.|
|Date:||Feb 1, 1993|
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