Dyne solutions have been the most common method of quality-checking material surface cleanliness for decades. Their ubiquity and low cost have led them to be heavily relied upon by manufacturers even though they are imprecise, destructive to surfaces, and harmful to the user. The science behind using dyne measurements is solid and trustworthy, but their use in a manufacturing context does little to help prevent or curb adhesion failure.
A dyne is a unit of measurement used to measure force. A dyne per centimeter is the unit used to measure the surface tension of liquids. Surface tension is a suitable way to estimate the surface energy of a material, which is an excellent indicator of adhesion failure or success. However, focusing on the kind of measurement dyne solutions provide is indirect and approximate.
The basic idea with dyne solutions is:
- The solutions typically come in a set with a number associated with each pen or ink
- When the ink is spread on a surface, it will either bead up or wet out depending on how the ink reacts with the "cleanliness" of the surface
- Manufacturers create a spec based on their adhesion goals and correlate that to a dyne pen number, then apply that pen number to a sample off the production line
- A technician who applies the ink with the agreed-upon number does a visual check; if they think the ink from that pen looks like it is wetting out enough, they give the thumbs up, and the process gets approved
In the video below, Dr. Giles Dillingham, Founder & Chief Scientist of Brighton Science (formerly BTG Labs), describes in greater detail how dyne solutions are used.
Within each of the points above lies an issue inherent to the use of dyne inks and pens. Looking closer at each one, it becomes very apparent that this method of surface cleanliness validation has some major drawbacks.
Not All Dyne Inks Are Created Equal
There is a lot about dyne inks that make them unreliable. Manufacturers have seen variations between brands of dyne pen makers so that one number is not equivalent among the different brands. This lack of consistency makes it impossible to know exactly what you are getting with the pens and if they are telling you what you need to know. Also, some brands provide numbers in a wider range than others, which may mean that the numbers in the range your surface is in may not even be available.
Dyne Ink Is Destructive
As was stated in the basic points about dyne inks above, the dyne inks can only be used on samples of parts and not the parts themselves. This is due to the caustic nature of the inks. They cannot be used on the production line due to their carcinogenic and teratogenic (harmfulness a fetus in utero) characteristics. They are also destructive to the parts they are being applied to, so the materials that are actually being used in the adhesion process are not the materials being tested. Additionally, there is a warning label on dyne ink packaging advising never to use them on metal because the ink cannot be cleaned off. However, this is a very common practice.
The dyne pens are also a threat because the tips of the pens can pick up contamination from one surface and transfer it to the surface being tested after. This leads to tests that are useless because it’s impossible to know what contamination the inks are indicating.
Bead Up or Wet Out?
The behavior of the dyne ink on a surface is similar to a gradient, and when a user is visually assessing what the ink is doing, it’s almost like a glass half full vs. glass half empty scenario. The characteristics of the ink behavior only have two possibilities; bead up or wet out. If users have varying opinions on what the thresholds for those qualities are, then they may provide a false positive or use a different pen when it is unnecessary to do so.
To properly read the activity of dyne ink on a surface and make an informed decision about how to proceed, a significant amount of training is required. Still, there is variation among trained users. This training also needs to include understanding about what the ink does to the surface of a material.
Adhesion occurs on the top 2-3 molecular layers of a surface. The dyne ink itself is a solvent and has a tendency to cut through these top few molecular layers – the most crucial element to adhesion. Therefore, the ink may be testing or reacting to layers that are below those layers that won’t actually be exposed in the adhesion application. This can lead to either false positives or just an inaccurate reading
Here's another video of Giles explaining more of the science behind the use of dyne solutions and some of their drawbacks.
Dyne Ink Isn’t Enough
In order to know what dyne ink number a surface should test, a lot of preliminary work needs to be done. A spec needs to be formulated based on the adhesion performance requirements of the finished product. This just means that the adhesion needs to stick to the degree prescribed by the development team. Surface quality inspections also need to be done throughout the production process to validate the effectiveness of treatments such as plasma or corona. Having a surface quality check before and after these procedures ensures a higher level of control over the surface quality. Being able to correlate those specs into a dyne measurement means either a lot of costly and imprecise trial and error or even more damaged sample parts. These options aren't typically feasible options for most manufacturers, and therefore using a dyne number as the standard can lead to reinforcing unsupported logic. The “when this number pen wets out, we are good to go” logic lacks any nuance and is a flimsy foundation for material surface quality control.
In this next video, Giles offers insight into why it is very hard to get reliable and trustworthy data from the use of dyne solutions.
Why Dyne? There is Another Way
Essentially, dyne ink tests are a go/no-go method that gives no indication of what precise changes need to be made if that part failed the test or any indication of why it may have succeeded. These methods are disguised as quantitative tests because they associate a number with the surface, but they offer little to no actionable data that can be relied on. Trends cannot be mapped, and the results are dubious at best.
A truly repeatable, quantitative surface energy measurement needs to be done directly on surfaces being bonded, coated, glued, sealed, printed on, or painted. This inspection test needs to be fast, easy, accurate, and (very importantly) non-destructive. The contact angle is a true quantitative process control method that has all of the implied benefits of a dyne solution without any of the drawbacks. Contact angle measurements are an accurate way to assess surface cleanliness and quality directly on real parts and components without sacrificing time, samples, or anyone's health.
To find out more about the contact angle alternative to costly dyne solution tests, download the "Manufacturer's Roadmap to Eliminating Adhesion Issues in Production" eBook. This guide gives clear insight into eliminating guesswork around adhesion so manufacturers can achieve consistent and successful results.