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2 min read

Wettability Defined and Correlation to Bonding

By Frank Hild on August 28, 2018

Wettability Defined and Correlation to Bonding

There is usually a good correlation between bonding and wetting. Wettability can be defined as “the ability of a solid surface to reduce the surface tension of a liquid in contact with it such that it spreads over the surface and wets it.”

So, it seems intuitive that good wetting would automatically result in strong bonds. However, this is not always the case. Two different cases where this correlation can break down are:

  • Case #1 - Where the surface is wettable but the structure beneath (the "bulk property" of the material) is too weak to have good bond strength.
  • Case #2 - Where the surface is not wettable by water but there is still excellent bonding.

In this post, we will explore two examples of the first case: the bonding of PTFE (Teflon®) and the bonding of a waxy or oily surface.

Example #1 – Poor Bonding due to Inherent PTFE Surface Weakness

Polytetrafluoroethylene (PTFE) can be plasma treated to promote good wetting by water or adhesives; however, when the surface is bonded, the measured bond-strength is about half to three-quarters of that obtained by using a commercially acquired sodium etchant. The reason is that the surface structure of PTFE is very weak due to almost no cross-linking within the material composition. The top layer of the polymer will shear off with the adhesive, even if the surface is treated with plasma to give good and uniform wetting.

To get good bond-strengths between PTFE and an adhesive, it is necessary to use a surface treatment that cross-links to a significant depth within the polymer (usually 1 micron or more), such as the aforementioned sodium etching method. Plasma treatments generally only affect the top 0.01 microns of the material, so the resulting surface treatment is just not thick enough to give a strong bond even though it is wettable and bondable by the adhesive.

Example #2 – Poor Bonding Due to Oily/Waxy Contamination Layer

As a second example of a weak surface layer; it is easy to plasma-treat a waxy or oily surface and make it completely wettable and bondable by adhesives. However, these bonds will show almost no strength because the adhesive is not bonded to the substrate - only the surface contamination layer. This is the ultimate example of a weak boundary layer. It is also the primary fault of using a "wetting test" as the sole quality control test for plasma treatments. The apparent surface of the part may be completely wettable but still give very poor bonding because that surface is really a layer of cross-linked contaminate.

The Bottom Line

These are examples of scenarios where plasma treatment is not the best approach for improving bondability; there are many other situations where it is appropriate. The key is to partner with engineers with the expertise to use the best method for your specific situation. Let us know what you are dealing with, and we’ll let you know how best to proceed!

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Topics: Plasma Treatment
2 min read

Why Choose Plasma Surface Treatment of Plastics to Enhance Adhesion?

By Dave Biering on January 2, 2018

Why Choose Plasma Surface Treatment of Plastics to Enhance Adhesion?

Good adhesion of manufactured parts. Whether you want to adhere one part to another, or secure paint to a component, good adhesion can be difficult to achieve in the regular manufacturing process. Plasma surface treatment can help you enhance adhesion at a good value, and with excellent results - but the process remains a mystery to many designers. Let’s take a look behind the curtain to learn why plasma cleaning is often recommended: 

Adhesion. It sounds like a simple concept, but can be quite complex to achieve. To start, your parts must be ultra clean before you can even attempt to adhere parts, and virtually any contamination can reduce the bond strength between surfaces. Even parts that have been “cleaned” can still have a monolayer of contamination (or 0.1 microgram/cm2) remaining after rinsing with a liquid solvent. Once these cleaners evaporate, up to 0.2 drops/cm2 of liquid containing 10ppm of non-volatile organic material can be left behind, which will also interfere with bonding. Plasma treatment can completely remove all trace amounts of organic contamination to promote better adhesion. 

This is where low-pressure plasma (or vacuum plasma) is your friend. Plasma surface treatment eliminates even the thinnest layer of contamination to promote the best bonding results. Unlike atmospheric treatments, plasma treatments are placed in a vacuum chamber where complete, 3-D treatment of the entire component can be achieved. No area is left untreated with plasma (unless they are intentionally masked).

To demonstrate this power of plasma, I wanted to share this research article I recently read, which cites the high-adhesion properties of PTFE after plasma treatment. It notes the impact of “Heat and power on the adhesion of PTFE was very clear… the adhesion was so strong that the (material) tore apart instead of separating from the PTFE.” The article goes on to note that the bond strength lasted even a year after treatment.

Imagine the implications of your part or paint adhesion lasting for a year – or even for the lifetime of your product, which is often achieved.

Beyond cleaning, plasma surface treatments offer other advantages to enhance bonding:

Parts will remain thermally stable
The pressure inside the plasma chamber is maintained well below the 500mT, deformation point to eliminate the risk of thermal deformation to your components.
Surface ablation is minimized
Plasma surface treatment will not alter the transmittance clarity of treated parts. Your bond will remain true and not have an impact on secondary painting processes.
Repeatable with each manufacturing batch
All phases of the plasma process are computer controlled so your process can be repeated every time.

Does enhanced adhesion sound like a good match for your manufacturing process? You may want to start by reading our Surface Modification technical paper. Our plasma surface expertise spans hundreds of industries and applications.

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Topics: Plasma Treatment
2 min read

Ask the Expert: Dry Plasma Cleaning vs. Wet Cleaning Processes: Which method is best?

By Dave Biering on September 6, 2016

Dry Plasma Cleaning vs. Wet Cleaning Processes: Which method is best?

How can I remove trace contaminants from inorganic substrates?

Which cleaning method will achieve the best bonding results?

These questions were recently posted to our Ask The Experts portal. Proper cleaning of a substrate is a critical first step in many manufacturing processes, particularly for the medical, aerospace, and electronics industries. So which method will render substrates atomic-level clean and ready for bonding? We recommend plasma cleaning for a number of reasons:

Contamination is perhaps the greatest barrier to achieving a smooth bonding of materials. But in order to achieve just the right base surface for a successful bond, most manufacturers use conventional (wet) cleaning methods. This approach can include chemical rinsing, wiping or scrubbing, vapor degreasing, or ultrasonic baths. But here’s the challenge with wet cleaning methods; despite appearing clean upon initial inspections, there’s usually a fine film of gross contamination left behind. These can include organic residuals (machining oils, wax, grease, and polishing compounds), dust and other substances. So your “clean” substrates are anything but, and these materials will impede any attempt at surface bonding.

For best bonding results, we recommend a dry cleaning process; plasma cleaning. Plasma primes any surface for secondary manufacturing processes by removing all traces of contamination. It even removes the materials that wet cleaning methods leave behind. Read about the importance of surface preparation.

Look to plasma cleaning to provide:

  • A truly clean base surface for enhanced bonding
  • Uniform 3-D coverage of the entire substrate (even complex geometries
  • Earth-friendly processing method to eliminate harmful solvents
  • No impact on the dimensional tolerance of components
  • Increased manufacturing yields
  • Fewer in-field failures
  • Lifetime treatment duration compared to wet treatment methods

Plasma cleaning by industry

In the medical industry where sanitation is critical, the plasma process does not adversely affect the biocompatibility of medical products. Instead, it promotes fine cleaning of glasses such as microscope slides, lenses, optical devices and pump components. Electronic devices such as hearing aid components and circuit boards have also been successfully plasma cleaned via potting and encapsulation. In aerospace, plasma has given the wings of aircraft better wind resistance. These are just a few examples of the power of plasma!

While plasma may not be the ideal choice for every application, in most cases there is no better way of achieving the right cleaning level to bond contrasting materials. Why not fill out a Surface Modification Design Worksheet to discuss your specs with our team?

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Topics: Plasma Treatment
2 min read

What is Plasma Treatment? Here’s How it Works

By Dave Biering on June 28, 2016

What is Plasma Treatment? Here’s How it Works.

In honor of this recent commencement season, today we’re sharing a lesson right from our bearing resource center, TriStar University. Here’s a back-to-basics review of how plasma treatment works. Essentially, plasma treatment is a low-pressure gas process that removes all traces of organic contamination to improve the outcomes of secondary applications. 

Here’s how it works:

Have you ever experienced the power of plasma? You only need to look at fluorescent lights or neon signs to see the plasma process in action. Both of these light applications use the visible light that is released by plasma discharge.

Plasma treatment occurs when gas is exposed to an energy source such as electricity or microwave, and becomes a mixture of ions, radicals, free electrons and other types of molecular fragments. The resultant plasma treatment is the means by which all traces of organic contamination are removed. 

How is plasma treatment used?

Plasma treatment is used to treat the surface of virtually any material, including metal, polymer, glass, elastomer, ceramics and others. It increases bonding, which includes the adhesion of one part to another, and the bonding of the thin coatings (or thick layers) of adhesives, overmolding or potting resins to the substrate. 

Plasma can treat materials that are too hydrophobic (non wettable) or too hydrophilic (wettable) for the application they are intended for. The process can be classified into two categories; atmospheric and low-pressure or vacuum, as both use energy to ionize gas. Corona atmospheric treatments are generally used to treat larger substrates, and can easily make commodity-grade polymers wettable to improve coating adhesion. Explore the advantages of corona treatments. Low-pressure plasma treatments incorporate a vacuum chamber instead of direct contact with an open electrical charge. 

Who’s using plasma treatment?

  • Medical and biotech manufacturers incorporate plasma treatment to combat contamination for micro cleaning.
  • Aerospace companies use it to enhance bond strength.
  • Electronic manufacturers incorporate plasma to protect sensitive components in potting.
  • Printing companies use plasma to better adhere water-based inks and screen prints on devices.

Can plasma surface modification improve your manufacturing results? Just fill in a Surface Modification Design Worksheet to discover the potential, or connect with the Experts for advice!

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Topics: Plasma Treatment
1 min read

Plasma Treatment of Medical Filters

By Dave Biering on January 8, 2015

Plasma Treatment of Medical FiltersIt is no secret that medical manufacturers are turning to plasma treatment to help them improve the performance of their products. Whether used to alter PEEK and PVC to accept time-released medications or even to improve a plastic-to-metal bond in medical tools; plasma treatment has virtually unlimited potential for the medical and pharmaceutical fields. 

Today I wanted to share how we treated blood filters that are used in orthopedic surgery. Our team is proud to help ensure patient safety by treating the filters that inhibit platelet clotting and assist with gas exchange during surgery.

Our client cited they were having difficulty ensuring that blood would adhere to the polymeric filter material during surgery. Without a good bond, there was a real chance that air bubbles could form during surgery, which could lead to the formation of potentially deadly blood clots. 

Through our plasma treatment, we were able to modify the filter so blood would “wet” and create an intimate contact with the tubing. Plasma treatment has resulted in the client’s filters now boasting a greatly-improved molecular adhesion. 

Learn more about changing medical plastics with plasma treatment (below).

Topics: Plasma Treatment
1 min read

3 Key Benefits of Surface Modification

By Dave Biering on December 9, 2014

TriStar Plastics Corp. - 3 Key Benefits of Surface ModificationReading through some of our older posts, I noticed that we received some great feedback on our coverage of Plasma Surface Modification Improves Drone Aerodynamics. A topic that is still timely today. In this instance, our client needed to improve the bond strength on the wing edge of their drones. The end result after plasma treatment was a superior bond adhesion that excelled at resisting wind forces.

Today I wanted to review 3 key benefits of surface modification for all industries.

Plasma surface modification is often described as a low-pressure gas with electricity running through it. It contains ions, free radicals, excited molecules and UV light. When exposed to an energy source such as electricity or microwave, it becomes a mix of ions, free electrons and other types of molecular fragments. The resultant plasma treatment removes all traces of organic contamination. 

When properly applied, plasma treatment can:

  • Microscopically modify the surface of a polymer substrate to improve mechanical bond strength without altering the haze, transmittance or clarity of the material 
  • Clean surfaces to improve the surface wetting and adhesion of paints, coatings and markings
  • Functionalize groups (carboxyl (HOOC-), carbonyl (-C=O-), hydroxyl (HO- and others) to the polymer substrate to significantly increase the surface energy for bonding; particularly in applications where aqueous-based adhesives require a bond strength that can’t be obtained with conventional cleaning techniques.

Surface modification can benefit industries ranging from medical and biotech, to electronics and even consumer goods. For additional benefits, get your free copy of our Surface Modification technical paper! Or just Ask an Expert for advice!

Topics: Surface Modification Plasma Treatment
1 min read

Tips to Bond Silicone Rubber to Aluminum

By Dave Biering on September 23, 2014

Plasma cleaning of metal parts dramatically increases the success of overmolding.We’ve had a few questions lately about the best method of bonding rubber to aluminum before overmolding. It seems this bonding combination poses a tough challenge. And in many cases, we’re finding that companies want to achieve better bond strength without using harsh solvents. So how can you achieve one without the use of the other? Plasma cleaning is the answer.

Plasma vacuum cleaning prepares metal substrates by removing all contaminants without the use of solvents, and with no damage to the components. Plasma is a one-step and solvent-free process to improve bonding silicone rubber to aluminum ― or virtually any other material combination. Because when you begin with a spotless component you can dramatically increase the success of overmolding. 

You can check out our video for additional information or submit a Surface Modification Design Sheet to submit your specs for a quote. Learn how surface modification can also help prepare polymer substrates for bonding.

Topics: Surface Modification Bonding Plasma Treatment Rubber
1 min read

Benefits of Plasma Cleaning

By Kevin Smith on October 4, 2011

  • Remove organic contaminants by chemical reaction (O2) or physical ablation (Argon plasma)
  • Eliminate the use of chemical solvents as well as storage and disposal of solvent waste
  • Clean surfaces with microscale porosity or microchannels not suitable for solvent cleaning due to surface tension limitations
  • Render most surfaces hydrophilic; decrease water droplet contact angle and increase surface wettability
  • Promote adhesion and enhance bonding to other surfaces
  • Prepare surface for subsequent processing (e.g. printing, painting, coating, potting)
  • Clean surface without affecting the bulk properties of the material
  • Can treat a wide variety of materials as well as complex surface geometries; examples include:
    • Semiconductor wafers and substrates (Si, Ge)
    • Glass slides and substrates
    • Optics and optical fibers
    • Oxides (quartz, indium tin oxide (ITO), TiO2, Al2O3); mica
    • Gold and metal surfaces
    • Atomic force microscopy (AFM) cantilever tips

Tell us about your experience with plasma cleaning.

 

Topics: Surface Modification Plasma Treatment