Water is everywhere…just not always where we want it. This past winter, the Eastern U.S. was inundated with snow and ice, while California and the Western U.S. are suffering from extreme drought. Availability of water is a growing concern around the world as it is a vital resource for the seven billion (and counting) people on Earth.

While representing only a small fraction of water usage as compared to that used in agriculture and for energy production, industrial use of water is on the rise. A staggering 18.2 billion gallons of fresh water are used every day for industrial use in the U.S. – about 4% of the total water used for all purposes.

Manufacturing one ton of automotive steel requires about 75,000 gallons of water. Actual manufacture of the vehicle itself requires an additional 39,000 gallons. Two and a half gallons of water are needed to produce a gallon of gasoline – and 20 gallons are needed to produce a pint of beer!

Critical part cleaning, for adhesion-related applications, represents a significant portion of industrial water use. Since the replacement of Freon and other solvent cleaning processes starting in the 1980s, U.S. industrial use of aqueous cleaning processes has become the norm.

But aqueous cleaning is a cost-intensive process, whether you’re looking at it from an environmental standpoint, a dollars-and-cents standpoint, or a labor standpoint.

Water needs to be delivered; detergents, surfactants and other chemicals are added and need to be kept in balance to control the washing process; additional processing is required to treat the waste water for recycling or disposal. Rinse water must also be clean and controlled. There is an old saying that “You are only as clean as your last rinse.”

Even after the part has been washed, it is still not ready for bonding, coating, painting, or printing. It needs to be dried, requiring additional energy costs, equipment footprint, time, and labor.

Furthermore, as mentioned above, water tends to not always go where we want it. Even with the best washing and drying processes, water’s influence remains at the molecular surface where adhesion occurs, interfering with a strong bond.

Molecular water resides in the oxides of aluminum and other metals. Water is absorbed and bonded within many polymers such as ABS and nylon. However, water molecules are not usually well-bonded and do not provide a robust bonding surface.

What if there was a better way for critical cleaning of organic contaminants? What if a process did not use water or other liquids but instead actually removed water from the molecular surface while vaporizing organic contaminants? It would be even better if this nano cleaning could be accomplished without touching the part and a chemical activation could occur to chemically bond to the adhesive, coating paint, or printing ink.

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Plasma Clean

My colleague, Wally Hansen, and I have just released a new review paper that may be of interest to those of you in the aerospace industry.

As you know, aerospace has had decades of experience with metal bonding, sealing, and corrosion protection. However, many of these methods have not lent themselves well to automation, and they also generate environmental waste.

But new applications are emerging for atmospherically deposited nano coatings that can address these shortcomings. These nano coatings are formed by direct injection of select vapor chemistries into the plasma jet. Such molecular surface modifications yield long-term, environmentally stable interfaces for bonding.

Cleaning

Plasma surface treatment begins with a fine cleaning of the substrate to remove loosely bound organic contaminants (on metal oxide surfaces, the oxide layer is either removed or pre-conditioned for adhesion).

Atmospheric plasma jets provide a multi-faceted cleaning action. First, the charged species within the plasma neutralizes electrostatically bound particles. Next, a vortex of pressurized gas blows away unbound solids and oils. Finally, the substrate is bombarded by reactive gas plasma species.

Low molecule weight contaminants on the substrate surface are efficiently reduced into nascent compounds and removed from the material system. This final phase of cleaning takes place on the molecular scale.

Coating

After cleaning, a different plasma jet technology deposits a functional coating. The coating thickness is controlled by parameters such as distance to substrate and speed. Coatings have been developed that simultaneously provide adhesion promotion and corrosion protection of the bond line. Furthermore, some of these coatings act as reliable tie layers, effectively bridging contact between inorganic and organic systems. There is opportunity to replace liquid primers or otherwise laborious surface pre-treatments used in composite bonding.

So far, atmospherically deposited plasma coatings have demonstrated utility as a release layer, an adhesion promoter, or a corrosion barrier. In practice, the plasma-deposited nano coating combines with conventional protective top coats to amplify corrosion resistance and suppress ingress at damage sites.

The atmospherically deposited plasma coatings show substantial improvement in corrosion performance even at a thickness of less than 500 nm.

Conclusion

Atmospheric plasma cleaning and coating can offer significant advantages in surface preparation of metals and composite systems for bonding and sealing in the aerospace industry. The technology is scalable and amenable to automation as it enables high-throughput material processing.

Furthermore, some plasma implementations are able to combine multiple process steps into a single step while reducing waste and reducing operating costs – without compromising performance.

Plasma treatments are a permanent and covalent substrate modification.  However many references note diminishing effects of plasma treatments with time.  One generalized conclusion is that the plasma modification is a temporary effect.  This conclusion is not inherently accurate or applicable to all plasma and material systems.  In truth there are many factors that govern the success and longevity of a plasma modification.  Research in plasma lacks harmonization in equipment, setup/configuration, and material selection.  These are key variables in a plasma modification.  Results from one method may not necessarily translate well to another experimental setup or class of material.  For this reason some engineering reviews of gas plasma do more to confound than to elucidate the scientific dialogue within industry.

Equipment design is of particular relevance in plasma industry.  This includes but is not limited to the electrode configuration, matching, RF frequency, and equipment geometry.  Many apparatus used in academia boast custom fabricated equipment or custom modification to existing tools.  Their equipment exemplifies engineering capabilities.  In my opinion the effectiveness of the equipment to a material system is specific and rarely generalizable to all materials or apparatus.

Plasma chemistry and substrate material should be matched correctly.  Some polymer systems may be either resistant or sensitive to specific plasma chemistry.  It is not enough to report gas, pressure, and power.   A complete characterization should understand the plasma stoichiometry and a hypothesis of the surface interaction.  Furthermore it must be accepted that many polymer systems are mobile, may swell with gas or moisture, or may undergo relaxation mechanisms.  Therefore be careful to consider pairing a material system with appropriate plasma source and plasma chemistry.

I sell gas plasma technology.

This can be confusing, as there are several types of plasmas, both naturally occurring (such as the Northern Lights, lightning, and stars) and human-made (such as those used in neon signs, fluorescent lights, and plasma televisions).

From the examples above, it’s clear that plasma generates both light and energy. Plasma can also be used to modify – or, more specifically, molecularly re-engineer – other materials.

My company sells plasma technologies and processes for modifying a myriad of materials. Typically, the application is a surface cleaning and activation – either to prepare plastic or metal for a subsequent coating or bonding step — or thin film coatings that may be used to change the barrier or coefficient properties of a surface.

There are many different ways to manufacture human-made plasma.
We use primarily atmospheric and low-pressure plasma technologies in our work. There are a number of benefits to the low-pressure approach:

1.   For starters, the working environment is a primary plasma. In a primary plasma, there is a greater mean free path of the particles before a collision.

This sustained energy is ideal for modifying the interstices of porous media (such as a non-woven or sintered polymer), or for use inside complex nano-scale vias or channels. With atmospheric processes, on the other hand, the mean free path is very short, so the treatment area is limited.

2.   Low-pressure plasma offers chemistry versatility. Many different gases and vapors can be used, safely and economically. Low-pressure plasma is often used as a replacement technology for wet chemistry processes, providing greater control, lower costs, and lower risk of workplace exposures that could lead to accident or injury.

Additionally, unlike many wet chemistry processes, rinsing and curing is not required with a low-pressure process. This means a much shorter processing time, minutes versus hours in some cases.

In atmospheric processes, use of these chemistries may be dangerous and quantities required to generate the plasma may not be economical. This is one of the reasons that our Openair® technology uses just air. It’s incredibly cheap, readily available, and great for many industrial high-speed surface preparation processes.

3.   When using a low-pressure technique, multiple steps may be run in a single process. A part may be exposed to a cleaning gas chemistry (to remove contaminants from a surface) as well as an activation or coating process in a single run. It is not unusual for a single plasma process to replace two to three manual steps, eliminating overhead costs associated with transporting product, labor and materials.

4.   Another advantage is that the low-pressure process provides an extremely controlled environment. The process is conducted in a vacuum chamber with exacting control of gas flow, time, and power. Variations in the day-to-day environment are removed, and the precise process is readily reproducible. Additionally, cleanliness is assured, whether the process is practiced in a clean room or on an industrial manufacturing floor.

5.   Low-pressure technology allows for permanent, stable results. This means that a large batch of parts may be treated and stored prior to use. Or, alternately, parts may be shipped to other manufacturing sites for final assembly.

6.   Our low-temperature process enables treatment of thermally sensitive materials, and the process is free from electrical potential. Therefore, conductive materials may be safely modified.

7.   The total cost of consumables, including energy, gases/liquids, and maintenance parts, is typically less than $5 per hour. Furthermore, there are no additional costs for hazardous waste disposal, as none is created.

8.   The technology offers high-batch throughput:

•   Line speeds, in our standard R2R equipment, are up to 100 fpm with again, no time require for curing or drying steps.
•   Cycle times, during batch processing, range from 60 seconds to 20 minutes. Because a single batch may include hundreds or even thousands of parts, this means that each individual part is treated in mere fractions of a second.

Our job is to accurately evaluate your application and select the most appropriate technology solution for your production goals, be it low-pressure, corona, flame, or Openair. In rare cases, a simple IPA wipe may be all that’s needed to solve your adhesion problems!

Thanks for reading. If you have any questions, I welcome your calls and emails.