What is Dyne Testing?
Contents
Dyne testing is a method for measuring the wetting tension of a solid by spreading a liquid with a known surface tension across the solid’s surface. This approximates the amount of force required to spread a fluid across a surface, measured in dynes per centimeter (dynes/cm).
The higher the dyne value achieved, the higher the wetting tension of the solid surface. Higher wetting tensions, which approximate surface energies, are generally associated with higher rates of successful adhesion.
For years, dyne testing has been the prevalent approach to assessing the cleanliness and surface energy of material surfaces in terms of quality. Their widespread use and affordability have made them a popular choice among manufacturers, despite their lack of precision, potential damage to material surfaces, and risk to users. While the science supporting dyne measurements is reliable, their application in a manufacturing setting does little to avert or mitigate adhesion failure.
Dyne testing is based on the concept of surface energy or wettability, a process directly related to a surface's adhesive potential. When a substance interacts with a substrate or material, it can reveal information about the material's surface energy. The response of the dyne solution will vary depending on the surface energy.
During the application of dyne solution, it can either form beads or disperse. The user must pay attention to whether the solution forms beads or spreads. If beading occurs, this signifies lower surface energy in the material compared to the solution. Conversely, if the dyne solution forms a continuous layer, the material possesses a higher surface energy than the solution. This test, in theory, will tell the user the material’s dyne level.
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How is a Dyne Test Conducted?
Dyne testing gauges the surface energy of non-absorbent, polymer-based materials. Dyne test solutions typically come in a set with a number associated with each dyne test pen or dyne test ink.
- The initial step involves selecting a dyne test pen filled with a test liquid assumed to have lower surface energy than the test sample. The pen is then firmly applied to the substrate until it becomes saturated with the test liquid. This liquid is gently spread across the test sample's surface. When the ink is spread on a surface, it will either bead up or wet out, depending on how the ink reacts with the test sample’s surface.
The reaction of the test fluid determines the surface energy of the sample. The ‘precise’ surface energy is measured by finding the dyne solution or pen that spreads out and then just barely starts to bead up in the course of about 2 seconds, as defined by the ASTM standard (more details on the ASTM standard below).
- Manufacturers utilizing dyne testing will often create a specification based on their adhesion goals and correlate it to a dyne level. Then, they will apply that dyne test 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 appears to wet out enough, they will ‘pass’ the test and allow the part to move forward with the subsequent process step.
- It's recommended to perform the test in a spotless, flat area and to do so in at least three distinct places on the material sample.
- The test findings provide insights into the material's wetting properties and its ability to secure inks, coatings, or adhesives.
- Precautions should be followed throughout the procedure to prevent contamination to pen tips and dyne solution jars and ensure the validity of the results.
- Safety measures should be followed to limit worker exposure to the hazardous chemicals found in dyne pens and solutions.
- Common dyne test solutions include dyne test pens and dyne test ink. Dyne test ink can be applied using sterile cotton applicator swabs, brush applicators, and pens.
ASTM & ISO Standards for Dyne Testing
The primary benefit of this approach, grounded on the ASTM standard test method D2578-84 and ISO 8296, is the ability to conduct the test quickly and, in theory, ease of use on the production floor.
These standards outline the procedure for determining the wetting tension of polypropylene and polyethylene film and sheeting only. These specific guidelines should be followed to achieve the most consistent and reliable results on material surfaces. Even when following these guidelines carefully, variation between technicians and day-to-day activities has been found [R.J. Caimi, L.K. Derr, T.J. Dunn, Precision of the surface energy test, Converting Magazine, 10 (6) (1992) 62.]
Common Uses of Dyne Testing
According to the ASTM standard, dyne testing was specifically designed for polyethylene and polypropylene films. While employing this procedure and the specified chemicals for films made from different polymers is feasible, it may alter the surface energies at the gas-liquid and solid-liquid boundaries, impacting the contact angle and wetting tension. The user must determine the suitability and importance of using materials other than a polyolefin.
Dyne testing is commonly used in manufacturing environments where materials are tested before adhesion processes like bonding, sealing, coating, painting, and printing.
Another common use is validating material surfaces after surface treatment processes like corona treatment, flame treatment, and plasma treatment.
Limitations of Dyne Testing
Not all Dyne Tests Are Created Equal
Producers have observed discrepancies between the different brands of dyne pen manufacturers, making it impossible to equate one number across all brands. This inconsistency makes it challenging to understand precisely what the pens offer and whether they provide the necessary information. Additionally, some brands offer a broader range of numbers than others, implying that the required numbers for your surface might not even be accessible.
Human Error
One issue might be the inconsistency of lab staff when using dyne solutions with cotton swabs, causing a difference in the quantity collected by the swab, and consequently altering the test results. Furthermore, applying the liquid to the surface using a cotton tip varies from one individual to another. This application method often leads to inconsistent dyne-level results. If using a dyne pen, a similar issue can arise regarding the differences in amount of pressure used when applying the dyne pen between operators.
Contaminants could impact the test. Potential contaminants could be impurities present on the material’s surface, or in the case of dyne pens, on the pen tip from a former test. Surfactants mixed with certain polymers to modify their properties can likewise alter the outcomes. Hence, it's crucial that materials being tested are neither touched nor rubbed before the test to avoid contamination. Utilizing dyne pens, all impurities that are rolled over will be absorbed into the pen's felt, thereby contaminating the tip and permanently altering the way the ink interacts with the surface, similar to dragging a yellow marker through blue or purple marker ink.
The ink determines a pass/fail level, and these results are based on the user's interpretation of how the ink reacts to the surface, making the process very subjective (enormous room for error and debate). The nature of the ink's reactions only presents two outcomes; either it beads up or it wets out. If the users' perspectives on what constitutes the thresholds for these properties differ, they might give a misleading positive or opt for a different pen when there's no need. Interpretation can be made even more difficult depending on the color and texture of the material being tested. Additionally, the ASTM standard calls for the measurement to be made within two seconds of spreading the dyne ink. Inconsistent timing between technicians can change the interpretation of results.
Substantial training is crucial to accurately interpret the behavior of dyne ink on a surface and make a well-informed judgment on the next steps. However, even among those trained, variability still exists [see above reference to round-robin study]. This training should also incorporate knowledge about the impact of the ink on the material's surface.
These variabilities leave a lot up for human error.
Destructive to the Part & the User's Health
Dyne inks often contain a blend of at least two solvents, which have the potential to produce VOCs, also known as "volatile organic chemicals." These VOCs are recognized for their damaging effects on the environment and potential harm to those who handle them. Some dyne ink compositions are as simple as water and alcohol, but others carry either mild or significantly high toxicity levels toward humans.
A surface tension test called the ACCU dyne test, which utilizes fluids, was conducted on a material, and the findings were documented in a Safety Data Sheet (SDS) (updated at the close of 2022). The examination revealed that two associated chemicals, “2-ethoxyethanol and Methyl Violet 28,” are classified as hazardous under the 2012 OSHA Standard (29 CFR 1910.1200). According to the California Proposition 65 regulation, “2-ethoxyethanol is acknowledged by the State of California as a chemical that can induce birth anomalies or other reproductive damage." The European Chemicals Agency also identifies similar reproductive toxicity related to this specific chemical. Hence, certain dyne inks are environmentally harmful and deemed toxic to the individuals who use them.
Not only is dyne ink potentially damaging to the user, but it always damages the product. Once a dyne test has been conducted, the part is then rendered contaminated and unsuitable for further production use. This increases the rate of wasted product, and testing processes aren’t as reliable since it does not check each individual part surface.
Polymer Testing vs. Testing Other Materials
As mentioned, the ASTM standard for dyne testing was established based on examining non-absorbent polyethylene and polypropylene films. Consequently, using dyne tests on any other material should be studied comprehensively prior to use in order to ensure repeatability and reliability.
The Impact of Solvents
Many film manufacturers include a lubricant called slip in polymer formulations to reduce friction on the film as it passes through rollers and corona treatment during the cooling process. Slip is soluble in alcohol and many other solvents, including those found in dyne pens. In this case, the dyne pen ink could not indicate the presence of a slip because it dissolved the ink. However, while somewhat hydrophilic, slip is not very soluble in water, so water could react to its presence without dissolving it.
Other plasticizers, blooming agents, and processing additives in the polymer formulations can cause similar effects.
Read more about the effect of slip and dyne pens here.
Watch the Video: Water Contact Angle vs. Dyne Inks
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Best Alternatives to Dyne Testing
Contact angle measurements are an excellent validation check for cleaning processes and surface treatment processes like solvent cleaning, parts washing, ultrasonic cleaning, plasma treatment, corona treatment, and more. Contact angle measurements are applicable to almost any material—metals, composites, polymers, ceramics, glass, and more.
Contact angles provide quantitative, non-subjective data about the cleanliness and surface composition at a molecular level.
Tools that provide rapid contact angle measurements equip you with the capability to reliably evaluate the readiness of a surface for bonding, coating, or sealing right at the point of preparation. Unlike systems confined to laboratory settings, instruments like the Surface Analyst gather and share contact angle data across processes and functions. This provides unparalleled transparency and understanding of real-life adhesion processes, manufacturing, product development, and supply chain difficulties.
Fast
Through the push of a button, take quantitative contact angle measurements in less than 2 seconds.
Accurate
Intelligent software precisely measures variable material surfaces in unpredictable manufacturing environments.
Easy
Designed for use on the manufacturing floor, this portable device is not tethered to a tablet or computer. The handheld and automated Surface Analyst systems are safe alternatives to dyne ink or dyne pens that gauge surface condition and consistency by rapid, automated measurements of the water contact angle in a precise, controlled manner. Designed for use by operators at any level, this tool is easy to learn and can be used within minutes of training by anyone.
Non-Destructive
Surface Analyst technology utilizes highly purified water, effectively removing the opportunity for contamination that dyne ink might introduce to your manufacturing process. For those working on the front lines to produce the items we use daily, steering clear of hazardous substances is just one standard for ensuring a secure workplace - a factor every enterprise should place at the top of their list.
Download the full technical paper to learn the differences between dyne pens and contact angle measured using the Surface Analyst™. Download the paper here.
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Fill out the form to receive your personal copy of the eBook, "What is Dyne Testing?"
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How to Convert from Dyne Testing to Contact Angle
Transitioning to the new language of contact angle is challenging for some manufacturers. However, contact angle measurements, which are more precise and eliminate the chance of human error, can help overcome this.
With over two decades of expertise in material science, Brighton Science is well-equipped to assist manufacturers in transitioning from dyne inks to Surface Analyst solutions. Brighton Science can customize the Surface Analyst testing process to suit the specific needs of each manufacturer. The shift from dyne language to contact angle introduces a new protocol and enhances efficiency and precision.
To take a deep dive into the best practices of improving surface quality & process control, read this eBook, "Predictable Adhesion in Manufacturing Through Process Verification."
