One of our many mantras at BTG Labs is that all manufacturers who bond, coat, seal, paint and print stand a better chance of fully understanding and controlling their manufacturing process when it’s defined as an adhesion process. We believe this because adhesion is quite literally and figuratively, the glue that holds the structure of the product and the process together.
Any way the adhesion goes, so goes the manufacturing process. If adhesion fails, then the whole thing falls apart. For that glue to hold strong and true, the surfaces of the materials of the parts being bonded, coated, sealed, painted and printed have to go through rigorous preparation steps prior to the assembly stage of the process. Every step of the manufacturing process is designed to further prepare those parts for the final assembly and finishing steps.
As manufacturing becomes more complex with innovative structural geometries, novel materials and automated processes, the surface treatments have had to keep up. Traditional cleaning methods such as aqueous baths, solvent wiping by hand or industrial washers may not be sufficient to consistently prepare the entirety of every surface, with all the weird nooks and crannies, and create a bondable surface.
Surface treatment means altering the molecular constitution of a surface to make it interact with an adhesive, ink, coating, paint or another surface to create a strong bond between the two. Adhesion is fundamentally a chemical reaction that occurs between the top 1-3 molecular layers of surfaces coming together. The reason glue is sticky is because it forms chemical bonds with the surface it’s on. This is the same principle for what makes all adhesion successful.
Controlling the chemical composition of the top few molecular layers of a surface can guarantee good adhesion and be the cornerstone of a predictable adhesion process. Therefore, all the surface treatments and preparation before the product is assembled need to be calibrated and monitored with this in mind.
Surface Treatments to Guarantee Good Adhesion
There are several surface treatment methods that have become commonplace in adhesion processes and choosing the most appropriate one is vital. Some variables to consider when designating the correct equipment and technique include the fact that different materials have surfaces with different inherent levels of chemical reactivity so they may require more general or intense surface activation. Some materials can be treated in batches and some require a more precise treatment to target very specific areas.
Here’s an overview of some surface treatment options with some insight into how to ensure they are being used with the utmost efficiency and optimization.
Plasma, Flame and Corona Treatments –
All three of these methods use equipment for the expressed purpose of chemically altering the surface of a material to make it more amenable to adhesion. Each method differs in execution and is used for different applications but they all bombard the targeted surface with chemicals to break existing molecular bonds and create a chemically reactive surface, eager to adhere.
There are two types of plasma treatment: atmospheric and vacuum. Both types alter the surface by oxidating them. Vacuum plasma treatment allows control over more variables such as gas composition and ratios and is done in a chamber where a vacuum has been pulled. Because of this, it is the preferred method for coatings. Eye glass coatings, for example, require highly specific gas compositions in order to create protective or non-reflective coatings, but the process contains more steps and takes up more time. On the other hand, atmospheric plasma treatment uses a nozzle pulling it’s gas directly from the atmosphere and contains fewer controls, so it typically applies to small treatment areas. But, it better applies to continuous processes for large scale manufacturing as it can fit in right on the assembly line.
Corona treatment works by discharging high-voltage, high-frequency electricity from an electrode in a ceramic tube that runs the length of the roll of material needed to be activated. The electricity is sent through the material to an electrically-grounded, metal roll called the treater roll, that the material is wrapped around. This interaction between the electrode and the metal roll creates a visible flash on the surface of the material roll as it moves between the two components.
Flame treatment uses a carefully controlled blend of natural gas and air to create a hot, oxygen rich plasma. First, the heat removes contaminants. Then, after contaminant removal, the oxygen rich plasma activates the surface by partial oxidation. This process is typically used on low energy surfaces that can be difficult to adhere to, such as plastics and composites. Flame is the hottest of these processes, so polymers treated using flame could be prone to melt or create what's referred to as LMWOM or low-molecular-weight oxidized material (such as aliphatic hydrocarbons) that could interfere with bonding. This is one reason why film companies use corona, which is generally considered the coldest of the three types.
These are all extremely effective in preparing a surface to the proper state of cleanliness, but there are certain concerns that need to be understood and monitored:
- Parts can be under-treated or over-treated and either state can adversely impact adhesion.
- Parts often enter the cleaning process with varying degrees of cleanliness. These treatment tools need to be calibrated at the correct surface treatment level, and a surface measurement inspection device is essential in this calibration. The inspection device must also be used to monitor any changes occurring as the treatments are used because their parameters may vary over time during use.
- Excessively soiled parts may not be adequately cleaned because these methods are not designed to remove oils or large particulate.
- Overtreatment can volatilize and damage the surface, adversely impacting adhesion.
Chemical Etching -
Chemical etching is a highly precise cleaning and surface activation process that uses baths of temperature-regulated chemicals to selectively remove material to clean and produce metal parts. This a useful technique, but if the process is not tightly controlled, it can result in chemical contamination and variations in its effectiveness. This is a common method in microelectronics and medical devices where precision and high-reliability are extremely important.
Vapor Degreasing –
This is a version of a solvent bath that functions on the principle that solvent vapor will breakup and remove contaminants from a material surface. In this method the temperature of a solvent bath is raised by a heating coil and the vapor emanating from the solvent rises into a chamber that is holding the part being treated. The solvent vapor removes the contaminant by condensing on and then dripping off the surface of the part. Often the solvent can be recovered and reused making this an economical option, however, it needs to be closely monitored to ensure that the solvent is properly distilled and contaminant free when reused. The solvent also needs to be suitable for the material and entirely rinsed before adhering to the part.
Laser Ablation -
Laser ablation is an extremely precise cleaning technique that works through the removal (ablation) of tiny fractions of a material surface by a focused, often pulsed, laser beam. The laser irradiates the surface to remove atoms and can be used for drilling extremely small, deep holes through very hard materials, producing thin films or nanoparticles on a surface, preparing surfaces in a micro- and nano-controlled fashion, and so on. Some issues with laser ablation when it is not closely monitored is the micro-debris it can create that fuses to the surface in a manner that is very difficult to clean off. Also, overtreating can cause the material to be converted to a plasma.
No matter what cleaning methods are used, it is important to realize unexpected contamination can happen upstream or downstream from any operation. Because parts are usually crossing different departmental boundaries, it is essential that all personnel have an understanding of the sensitive nature of surfaces, the consequences of eschewing proper procedures and the risk of inadvertently introducing contaminants.
How to be Prepared for Adhesion
Most manufacturing companies have experienced, and likely are currently experiencing, some kind of adhesion failure. Sometimes these can be minor yet pervasive (like ink not sticking properly to a polymer label) and sometimes these can be catastrophic safety issues (like a lens on a car sensor malfunctioning because a coating wasn’t keeping out moisture). And when adhesion failure occurs, the root cause analysis and remediation can be extremely difficult, especially if you don’t have help from adhesion process experts or surface quality monitoring equipment.
In order to truly be sure your surfaces are prepared for adhesion, your process needs to be prepared for adhesion. Total preparation is key. Designing adhesion processes with the chemical nuances of adhesion in mind, and testing the quality of surfaces based on the science of adhesion makes sure you are creating the most bondable surfaces possible and measuring the variables that most affect adhesion.
To learn more about how to set up an adhesion process that consistently produces the highest quality products, download our eBook: Predictable Adhesion in Manufacturing Through Process Verification.