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.

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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.

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One of the keys to achieve and comply with lower consumption standards for automobiles thus higher mileage MPG, is to lower the weight of the vehicle. As increasingly plastic substrates are used for constructing a vehicle, including composites materials such as GFRP and CFRP, welding suddenly becomes less of an operational task as opposed to more adhesive bonding.

Adhesive suppliers develop better products to allow for structural bonding while also strategically link up with surface pre-treatment solution providers such as Plasmatreat, to allow for an effective but low cost bonding solution.

Advantages and drawbacks of adhesive bonding for composites*

Compared to mechanical fasteners, adhesive bonding provides many advantages:

• Mechanical fasteners require drilling holes in the parts, and this weakens the composites because it cuts through the reinforcing fibers and also creates weak points. Bonding improves tensile resistance.
• Bonded joints exhibit lower stresses concentrations than mechanical joints when holes are needed, and thus provides increased static strength,
• Risks of cracks propagation are reduced,
• Bonded joints provide always 10 to 25 % weight savings in primary and secondary structures,
• Bonded joints enable the design of smooth external structures,
• For large surfaces bonding costs less than mechanical assembly, because it needs less manpower
• Adhesive may join together all kinds of materials: metals, composites, plastics, wood etc…
• Adhesives can join very thin materials which could not be riveted or bolted,
• Adhesives can join dissimilar materials without the risk of galvanic corrosion,
• Adhesives may be flexible or rigid according to their formulation,
• Adhesives have an excellent resistance to fatigue.

However, adhesives bonding has also some drawbacks:

• Elevated temperature creep resistance is fair or even poor for some structural adhesives.
• Adhesives cannot be used in or near to the motors of automotive.
• In general adhesives do not resist to peel stresses, and this is a  drawback compared to welding for instance,
• Bonded parts cannot be dismantled easily,
• Bonding requires specific design so that the parts will be stressed only in shear mode,
• Bonding requires an excellent and specific surface preparation of the materials immediately before bonding,
• Bonded joints are difficult to inspect in a non destructive manner, although there are several NDT such as X rays, ultrasonic inspection, shearography, and others,
• Structural bonding requires an accurate mating of the parts because adhesives do not give high performances in thick joints,
• Water resistance of adhesives are often only fair,
• Durability of bonded joints must be assessed by difficult laboratory accelerated aging tests.

Pre-treatment with plasma (atmospheric in-line or vacuum low-pressure) can help overcome some of the drawbacks associated with bonding. Many car manufacturers are catching on.  Check out this new article in Automotive News:

http://www.autonews.com/apps/pbcs.dll/article?AID=/20130608/COPY01/306089999/adhesives-sought-for-making-cars-lighter-tougher#axzz2VqlIP6it

Till next time,

Andy

*Source: scribd.com/bonding-of-composite-materials

 

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Process Design Step 1:

Variables to consider when designing a surface modification program

 

I lied, partially.

I said that my next blog would be a trip report (sadly, Dyana said it was overcast) and power of plasma for modification of materials for the life sciences industry.   I’m not ready to jump into specific applications, rather want to start with some of the basics to a successful surface modification program.

I’ll start with Step 1.  Q&A.

The beginning of any lab development program typically involves a thorough Q&A session.   At the minimum, I want to know:

 

1.  Substrate

2.  Product environment

3.  Desired surface performance goal

 

The success, based on my experience, of a surface modification program relies on a thorough (if possible) understanding of these three items as 1 and 2 greatly impact 3.

For each question, there are 10s if not 100s of sub-questions that can shift outcome considerably.   I spoke about some of these recently at Hantel Technology  http://www.youtube.com/watch?v=gZemVc790oQ  and am summarizing a partial list of variables for each question below.

 

1. SUBSTRATE.  Tell me about (I’m polymer focused):
1. Resin selection/Metal properties
2. Composite properties
3. Manufacturing practice (molded, extruded, cast).  Are you starting with a machined part for R&D and then possibly considering molding for production.  We may talk about molecular weight distribution as well.
4. Cure mechanism
5. Cure temperatures
6. Hardness (durometer)/crystallinity
7. Topography
8. Tacticity
9. Additives (stabilizers, pigments, nucleating agents, plasticizers, etc)
10. Propensity for migration of additives
11. Propensity for molecular rotation
12. Finishes
13. Mold release materials
14. Machining debris
15. Moisture retain/absorption/adsorption
16. Cleanliness (and how is the substrate cleaned prior to plasma)
17. Manufacturing controls for said substrate
18. Throughput targets

 

2. ENVIRONMENT.  Once treated, please tell me about the next steps in processing and environment as these variables may impact surface performance and stability:
1. See Item #1.9 above.  Bloom, migration of Internal impurities
2. Adhesive technique (if bonding) and cure mechanism
3. Potential for oxidation
4. Chemical exposure
5. Sterilization technique
6. Subsequent assembly step (are you heat sealing?)
7. Subsequent cleaning steps and techniques (are you IPA wiping part 100X times during assembly?)
8. Handling (glove selection and practices)
9. Storage (Packaging materials, Temperatures)

 

3. SURFACE PROPERTIES.  What do you want as we have many variables to consider to provide the desired outcome.  Rather than listing the myriad of applications we practice, I’ll focus on variables that we consider in designing an experimental plan.

 

1. Type of equipment (Corona, Atmospheric, Low Pressure)
2. Steps and type of process (Cleaning, Etching, Activation, Functionalization, PECVD, Grafting, Crosslinking)
3. Chemistry (gas, liquid vapor, sublimated solids, combinations).
4.  Temperature of substrate, chamber, liquid/solid
5. Pressure (flow driven, throttled, pumping capacity)
6. Fixturing and fixture materials (does it contribute to dark space?)
7. Power (continuous, pulsed, duty cycle, frequency)
8. Time (3o seconds or 10 minutes)

BUT WAIT.  There is more!

VALIDATION TECHNIQUE/SURFACE ANALYSIS

Even the choice of how to validate the surface can impact the results.  Our chief technologist, Steve Kaplan, loves to say “don’t throw the baby out with the bathwater.”   It is not unusual for a customer to overlook the success of the process by improper selection of the validation method.    Test the product in the ultimate application.  Techniques used and considered at our laboratories include:

 

• Surface energy testing
• Dyne-cm, contact angle
• fluid choice
• Adhesion testing
• Wear and abrasion testing
• Friction testing
• Hardness testing
• Surface analysis
• X-ray Photoelectron Spectroscopy (XPS)
• Scanning Electron Microscopy (SEM)
• AFM Atomic Force Microscopy
• Fourier Transform Infrared Spectroscopy (FTIR)
• Chemical resistance
• Gas permeation / vapor barrier testing

 

This list isn’t to overwhelm.   I don’t expect answers to all of these questions nor do we screen every possible combination of variables.  We know where to start if you can provide us with the basics about  1 (Substrate), 2 (Environment of use) and 3 (Desired surface performance) so that we can design efficiently and effectively the best surface for your application.

Next blog…no promises.

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America and Canada are a truly great countries. Both regions are characterized by truly freedom loving people, both feature strong democracies despite the daily stalemates and political quabbles.  While their economies in general are powerful and have created some of the largest wealth per capita in the world, the recent 10-15 years have been marked by, in my view, myopic activities in industry: Manufacturing was given up on. With China achieving first “most favored nation status” and then later gaining access to the WTO, thus allowing for tax and duty favored imports, many manufacturing companies started to believe that they could not compete with China as well as other countries in the SE Asia region with their low wages and other low operating costs. Comprehensive new supply chain systems were set up, new operating and trading relationships were established, more and more company managers became ex-patriates. Some companies that wanted to continue producing product in North America were forced by large retailers such as WalMart to move their operations to a China location. The common crede became: Operating our production in China is the better way, there is no such future in North America.  

I disagreed from the Get-Go. I always believed that America needs manufacturing. One needs to build things to create value. Our countries cannot simply be service and consumption oriented societies. We saw what happened if when relied on the finance/banking sector alone. It created huge wealth only for a very few and when it all went wrong, we were all asked to pay the bill.

Manufacturing creates jobs at all levels, stimulates personal and professional creativity, helps shape products and processes and let us focus on the future by taking direct control. Plasmatreat works with manufacturers all over the world creating better and more productive operating environments. Here in Canada and the USA we have the potential to reclaim a top spot in the global arena of manufacturers. Designing and building product creates not only possibilities domestically but also sets the stage for successful exports. The USA in particular has been suffering from a negative trade deficit for several decades now. We need to think about reversing the flow of dollars into America not away from America. We need to support the Reindustrialization of America – we need to believe again in manufacturing. Plasmatreat together with our many industrial partners continuously are presenting ideas how to create competitive operating environments right here in North America. Our projects reach into various markets such as Solar, Medical, Packaging, Automotive and Electronics. We look foward to mastering the challenge to compete with low cost production countries, but we believe we can. Do you, too?

Till next time,

Andy

 

 

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