Plasma treatment: the better bond.

Plasma treating of plastic surfaces prior to coating or bonding is no longer a closely held secret, nor is it still limited to small parts. Now plasma is scaling up for larger automotive and medical applications.

Plasma--a kind of spooky state of matter in which gas molecules come apart and become intensely reactive--is coming into its own as a surface preparation prior to printing, bonding and coating plastic parts. Such treatment makes part surfaces harder, rougher, more wettable, less wettable, or easier to print on or stick to. Plasma is also used to treat film and even powder additives like pigments and fillers to make them more dispersible.

Plasmas were first used commercially 30 years ago to deposit metal coatings on turbine blades for aircraft engines. Plasmas for surface treating plastics go back 15-20 years, but they were used mainly in a handful of high-value products like contact lenses, electronics, and aerospace components to make them more wettable. Makers of these products kept plasma treatment secret for competitive reasons.

Many of the entrepreneurial R&D firms that originally developed plasma-treating applications were bought out in the late 1980s by larger companies that were able to bring more financial and engineering clout to developing treating equipment. Now, with military and semiconductor markets drying up, these companies are broadening their search to adapt plasma surface technologies to automotive and medical applications for painting, bonding, printing and cleaning. "The exciting thing about plasma is that the world doesn't know about it," says Richard DeLarge, owner of Plasma Etch Corp., an equipment producer. "Every week you stumble on something new."

Plasma-treating equipment is relatively expensive, so it's not for low-value-added commodity products. In some instances, however, it may allow an inexpensive material to behave like an expensive one (e.g., by making paint adhere to a PP or TPO car bumper as well as it would to polyurethane). For cases where plasma treatment makes economic sense, equipment makers insist there's no better bond. Depending on the resin, it can make bonds 10-400 times stronger than can other surface-activation techniques.

Plasma treatment of plastics has many other advantages over the wet-chemical or abrasive processes it replaces, not the least of which is environmental: No wet chemicals, fumes, flames, or sand blasting are involved. Instead, plasma treating is dry and, depending on residence time, relatively cool (from ambient to about 390 F) so it doesn't harm materials that shouldn't get wet or hot.

WHAT'S A PLASMA?

A plasma is a gas that is "excited' by passing electric energy through it so that the gas molecules break down into ions, free electrons, and highly reactive "free radicals." All these particles move about at high speeds, bumping into other gas molecules, splitting them apart and coming together again. This activity generates a glow of visible as well as ultraviolet light. Everyday examples of plasmas are lightning during storms, the blue spark of an electrical short circuit, and the light from neon or fluorescent bulbs. All the time in a plasma, some particles are returning to a stable gas again, shedding energy. The unstable ions and free radicals and the intense uv radiation of dissipating plasma energy make the plasma reactive. All this stops the moment the electricity is off, so plasma activity is highly controllable.

In commercial plasma treating, the activating energy can come from long or short radio waves or microwaves (40-400 kHz at the low end, 13.56 MHz in the middle and 2.45 GHz at the high end). Special industrial frequency bands are designated so as not to disturb air-traffic control or radio transmissions. The frequency most widely used among plasma-equipment makers is 13.56 MHz.

The shorter the wavelength, the more active the gas excitation. Microwaves make the most active plasma and the most efficient, its proponents say. "As you go up in frequency, you're able to create more of a plasma with more active species," says Wayne Landman, sales director of GaSonics/IPC, whose equipment uses either radio-frequency (RF) or microwaves. Stephen Kaplan, general manager of Himont/Plasma Science, whose equipment uses RF, says microwave treatment can be less uniform than RF: "That's why you get hot and frozen places in a microwave dinner. Such uneven treatment from microwave plasma on a bumper could mean some painted spots would peel later."

That view is hotly disputed by firms using microwave-generated plasmas. Gerhard Winter, general manager of Technics Plasma (which uses only microwaves for surface modification, not RF), says microwave plasma treatment has no uniformity problems and isn't comparable to a microwave oven, since the oven is not under low pressure vacuum and contains no electrically charged gas particles.

The degree of ionization--and reactivity--in a plasma varies greatly. Winter of Technics Plasma says RF-generated plasmas contain 0.01-0.1% active species or electrically charged particles, whereas microwave-generated plasmas have 1-10% active species.

In either case, gas molecules and ions in a plasma remain at room temperature even when excited. Only the electrons get very hot (20,000-90,000 F). Such so-called "cold" plasmas for surface treating plastics are made in a vacuum chamber, typically at 0.5-10 torr.

Most plasma-treatment systems aren't adjustable in terms of energy introduced into the gas. Some systems have a setpoint and can turn the RF energy on and off to control temperature of the part being treated. Others circulate cooling oil through shelves in the chamber to control part temperature.

TABULAR DATA OMITTED

TABLE 2--PEEL STRENGTH(1) IMPROVEMENT AFTER
PLASMA TREATING
                                Untreated      Plasma Treated
Resin                           (lb/in.)       (lb/in.)
ETFE Fluoropolymer(2)           0.1            15.8
FEP Fluoropolymer(2, 3)         0.1            10.4
Polymide(2)                     4              16
PTFE Fluoropolymer(2)           0.1            2.2
PFA Fluoropolymer(2)            0.1            8.3
PFA Fluoropolymer(3)            Very low       8.3
RTV Silicone(2)                 Very low       23
Silicone (2)                    0.4            19
1 Film sample epoxy bonded to itself. 2 From a technical paper
by Edward Liston of GaSonics/IPC. 3 From a technical paper by
S. Kaplan, W.P. Hansen of Himont/Plasma Science.

Plasmas created at atmospheric pressure by any of several high-voltage discharge processes are generically known as corona treatment, though companies involved say the technology in this field is broader than what is commonly referred to as corona treating. High-voltage discharge treatment makes a localized plasma around an electric arc, like the plasma created around a bolt of lightning, rather than filling a vacuum chamber with plasma. Applications and equipment for high-voltage discharge treatment are continuous, in-line and somewhat shorter-lived in treatment effect than vacuum plasmas and won't be dealt with in this article.

HOW A VACUUM PLASMA WORKS

Some scientists say free radicals in a cold plasma attack the resin surface by breaking polymer chains and insinuating themselves into the resin's structure through chemical linkages with carbon atoms. Others say uv radiation causes free radicals in the plasma like carboxyl (COOH) or hydroxyl (OH) groups to grab carbons from the resin surface, turn into C|O.sub.2~ and dissipate. In fact, plasma surface treatment involves multiple effects. "It has four major effects on polymeric substrates: surface cleaning, micro-etching, crosslinking and surface activation," says GaSonics senior staff scientist Edward Liston. "These four effects occur concurrently, and depending on processing conditions and reactor design, one or more may predominate."

Oxygen is the primary gas used for surface treating plastics, though some plasma processes use nitrogen, nitrous oxide, helium, argon, ammonia, tetrafluoromethane, water vapor, or air. Depending on process conditions, oxygen can perform all four surface functions, though it is said to be most effective in surface activation. With RF plasma, adding nitrogen to oxygen enhances plasma activity. With microwave plasma, Technics Plasma uses only oxygen and says research on reinforced PP shows that nitrogen weakens bond strength. Helium or argon are the preferred gases for crosslinking if used "in the total absence of oxygen or other free-radical scavengers," says GaSonics' Liston. Fluorine-containing gas is used for water repellency. And hydrogen-containing plasma is often used for degreasing industrial parts. Argon, which is inert, is also used for plasma cleaning--it may be compared to "molecular sandblasting." Other gases like oxygen or carbon tetrafluoride clean chemically as well as physically. Unlike solvent cleaning, plasma cleaning leaves nearly no residue to inhibit adhesion. It also uses no ozone-depleting CFCs.

Surface activation with plasma involves adding polar groups to the first molecular layer of a resin so as to raise surface energy--i.e., wettability. For example, the surface energy of untreated PP is 30 dynes. To print on PP, its surface energy must be raised to 40 dynes; for bonding and coating, to 50 dynes.

Treatment time for plastic surface activation can vary from a few seconds to over an hour, depending on reactor design. Typical treatment times are 2-15 min. GaSonics says PP syringes are treated instantaneously by the thousands in a batch process prior to printing, while Advanced Surface Technology Inc. president Dr. In-Houng Loh says up to an hour is required for a complete plasma polymerization process for specialized biomedical applications. There are also continuous plasma systems for high-speed treatment of films and fibers.

A process can be designed to use plasmas for several interacting objectives. Increased wettability creates microcracks or micropitting in the surface that allow adhesive to be drawn into the polymer surface. At the same time crosslinking occurs at the surface, which further anchors the two bonded surfaces. Designing for plasma treatment can be tricky because "different treatments that produce the same surface energies can yield very different bond strengths," says GaSonics' Liston. Plasma treating also makes plastics bondable below their melting points--a plus when low-temperature material is bonded to a high-temperature plastic.

Plasma surface treatment can also be applied to the surface of microbeads or powders to increase dispersibility in paint or in polymers. In Osaka, Japan, Nippon Paint Co. is said to plasma treat powders for colors and additives for better dispersion. Plasma processes are widely used to etch or remove polymer surfaces in circuits of microchips--far more complex applications than surface treatment because they may involve timed sequential processes using up to eight different gases. And if left long enough, plasmas can remove coatings or even vaporize a whole part in a condition known as ashing.

HOW IT'S BEING USED

Plasmas can be used to treat a range of resins, including some with notoriously inert surfaces such as polyolefins, engineering thermoplastics, fluoropolymers, TPOs and elastomers.

* Coating: Applications in commercial production include treating golf balls so paint won't chip when they're repeatedly struck with hardened-steel clubs. Windshields for helicopters are also treated to enhance wettability before coating. And TPO bumper fascia are treated so paint will stick.

* Bonding: A GaSonics system is used for surface activation of balloon catheters to help attach the balloon to the catheter. A system delivered earlier this year by Yield Engineering is being used in production for adhesion of stainless steel to polyimide in an anti-lock brake mechanism. And a system from Himont/Plasma Science is used to bond PU foam "flesh" to an acetal spring in a prosthetic foot.

A special case is potting of electrical components. One commercial application treats a PBT car ignition housing to promote adhesion of the potting medium to the inside.

* Cleaning: A GaSonics unit is used for surface activation of PP syringes for cleaning and printing. For bigger cleaning jobs, plasma may be used as the final cleaning step in a two-step process, following solvent cleaning.

* Enhancing wettability: Applications include small tubes for drawing blood and ink tubes for Parker pens. Increased wettability on the inside of the tube draws blood or ink up into it faster. A Technics Plasma system is used at a Mercedes sub-contractor in Germany to make sideview mirrors hydrophilic prior to painting.

OPERATING CONCERNS

At its simplest, plasma treatment equipment consists of a vacuum chamber and pump and a couple of electrodes generating radio frequencies. Because it can be this uncomplicated, there are processors in the U.S., Europe and Japan that have built their own plasma-treatment systems. Toyota Motor Corp. in Japan is said to use a microwave-generated plasma system built by a subsidiary to treat bumpers before painting.

Larger plasma equipment builders say not all of their small competitors build to OSHA electrical standards, especially those for shielding against non-ionizing or RF radiation. Also, generating RF energy can involve high voltages, which has caused some injuries, pointing up the need for proper safety controls and shielding for operators. But vacuum plasma systems are generally better shielded and use lower voltages than some equipment for corona treating continuous webs. However, vacuum chambers have their own associated safety concerns. On at least one occasion, some 10 years ago, a large vacuum chamber at an aerospace company imploded.

Another concern is that systems using oxygen (as most do) cannot use petroleum-based oils to lubricate the vacuum pump because it could explode. (That happened last year to a West Coast toll plasma treater, which used the wrong oil and blew the roof off its building.) Instead, pumps must be lubricated with synthetic oils that cost $1000-1500/gal.

Nonetheless, plasma advocates say their process still compares favorably with the environmental costs and operating hazards of some wet-chemical surface-treating alternatives.

WHO'S WHO IN EQUIPMENT

For many years there were only a few sources of plasma equipment. Today, the market is broad enough for equipment makers to specialize:

* Advanced Plasma Systems Inc. makes treating equipment using either high (13.56 MHz) or low (20-100 kHz) RF. Its patented gas-flow technology, using two gas inlets and two exhaust valves, can reverse the direction of gas flow for more uniform distribution, the company says.

Advanced Plasma builds standard vacuum chambers from 12 to 36 in. square. Batch systems can tumble small parts in a plasma environment like a washing machine. The company also builds custom reactors, like two for General Electric Co. that are each big enough to hold a jet engine. (They're used to bond cowlings onto composite materials.) Advanced Plasma also offers continuous plasma systems for treating fabric, cable or film, indexing material forward through a series of three vacuum chambers with vacuum locks in between. The company has some joint developments with Air Products & Chemicals Inc. of Allentown, Pa.

* Advanced Surface Technologies Inc. is a three-year-old R&D firm that does surface-modification research in the biomedical field. It also custom develops plasma reactors like one for a medical implant manufacturer, which uses a cylindrical reactor chamber 18 in. diam X 2 ft long. "Some of the chemistry this can offer is very, very novel," says company president Loh. Also, in a 2 X 3 ft reactor, the firm is plasma treating powders for greater dispersibility.

* GaSonics/IPC (formerly Branson/IPC and before that International Plasma Corp., which was founded in 1968) is one of the oldest in the field and says it has the largest installed base and field-service organization. It offers batch processing systems as well as continuous systems that can treat fiber, web or film at up to 500 ft/min. The continuous system has "loadlocks" and a series of three to five chambers with progressively increasing levels of vacuum. (The company has also built two continuous air-to-air systems.) Its equipment uses both microwave and RF energy.

GaSonics says it has commercial systems bonding silicone gaskets to phenolic connectors; surface-activating medical angioplasty catheters for cleaning and bonding; treating engineering plastic parts for paint adhesion; cleaning acrylic lenses before protective coating; and improving bond adhesion of brake shoes and auto headlamp sockets.

* Himont/Plasma Science Inc. (started in 1984 as Plasma Science and bought by Himont Inc. three years ago) is regarded even by some competitors as the most technically advanced because of its push into larger reactors. Plasma Science recently built five large (8 X 4 X 6 ft) plasma reactors for auto bumpers, including one for Europe and two for U.S. car makers. It also operates its own commercial installation for continuous plasma surface treating of fibers on a toll basis for Allied-Signal Inc., Morristown, N.J. Plasma Science can toll treat fabrics, nonwovens and films up to 60 in. wide. Its equipment uses 13.56-MHz RF energy.

* Leybold AG (formerly Leybold-Heraeus GmbH) makes equipment primarily for vacuum deposition of metals on metal or plastics (e.g., compact discs). It is developing plasma surface treating applications in Hannau, Germany (the company has U.S. offices in Connecticut). It has standard chamber sizes ranging from 40-in. square to huge chambers 6.5 ft in diam. X 10 ft long. Leybold uses microwave as well as RF, AC and DC energy.

* LFE Corp., in its former Tracer Lab Div., made what were probably the first commercial plasma systems in the U.S. in 1964. Some of the earliest applications were printing on PE film and treating petri dishes for enhanced wettability.

LFE makes standard small reactors for surface treating and cleaning: 4 in. diam X 6 in. and 10 in. diam X 18 in., using 13.56 MHz RF energy.

* March Instruments Inc. (which bought the product line of the former Tegal Corp. 10 years ago) makes small and large systems, primarily for R&D in industries like cable, medical devices and optics. Models range from small benchtop R&D systems (starting at around $14,000) to large custom production units costing upwards of $100,000. March's batch systems all use RF energy (13.56 MHz). The company has designed but not yet built a continuous system.

* Plasma Etch Corp. started 12 years ago building large plasma etching systems (up to 18 X 24 in.) for printed circuit boards. Three years ago the firm expanded into surface-treating equipment for plastic parts and says it has 20 such systems in the field, almost all in medical applications to make surfaces hydrophilic or hydrophobic. Standard and custom systems range from a benchtop lab model (6 in. diam. X 7 in. deep) for $13,000 up to the largest standard system (30 in. wide X 48 in. high X 24 in. deep) and custom systems as big as 4 ft square. Plasma Etch uses a patented temperature-control system adapted to surface treatment from the more precise plasma etching process.

* Plasma Electronics GmbH uses RF energy but treats parts outside of the RF field by flowing plasma gas over them (this is called a secondary plasma).

* Plasma-Therm Industrial Products Inc. announced at NPE last year that it was entering the surface activation plasma market and then withdrew shortly after. Plasma-Therm continues to supply plasma etching systems for semiconductors.

* Polar Materials Inc. was founded in 1984 as an R&D firm and is now commercializing systems for surface modifying plastics. Polar's specialty is large, custom reactors for continuous treating of film, small parts and powder. For example, Polar encloses a roll of film in a single large vacuum chamber (much like metalizing films). Prices range from $50,000 for small systems to $1 million for a complex in-line set-up. Polar also plasma treats powders itself on a toll basis "by the 100,000 lb" using a patent-applied-for process. Its in-house system can also treat film continuously in 40-in. wide x 20-in. diam. rolls.

* Technics Plasma GmbH (bought three years ago by the Krauss-Maffei Group) uses microwave plasma (2.45 GHz) and oxygen. Technics Plasma says it introduced microwave plasma in the early '80s. It uses only microwave activation for surface treatment, though it uses RF and lower-frequency plasmas for micro-electronic applications. Technics Plasma displayed what it calls the largest commercial plasma treatment system in the world at NPE last year, a two-chamber model 4002. One chamber loaded while the other treated multiple parts (treating a bumper prior to painting, for example, took 4.5 min). The machine, which costs about $1 million, went to American Fibrit Inc. in Battle Creek, Mich., after the show for treating PP instrument panels for the Volkswagen Golf and Jetta. Polyurethane foam padding sticks directly to the treated surface without glue, primer or adhesion promoter.

* Yield Engineering Systems (YES) Inc. is known primarily for making plasma equipment to clean silicon wafers in microelectronics manufacturing, but six years ago it introduced a product line for surface modification. Vacuum chamber sizes range from 7 X 16 X 16 in. for a lab model up to 30 X 30 X 9 in. high. YES uses either microwave (2.45 GHz) or RF power.