There are numerous obstacles to maintaining a material surface that is clean and fully prepared for adhesion in a manufacturing process. But there is one element that carries through the entire adhesion process and has a massive effect on the adhesion performance of the final product: TIME.
Time takes its toll on surfaces. Time can be a sinister interference to preserving a surface that is the most amenable to adhesion.
Whether you are coating, printing, bonding, sealing or painting the actual adhesion operation takes place after several steps have been employed. Time factors in to each and every one of these points along the way.
How can you fight against a relentless force like time? Well, just like every variable that can affect the success of an adhesion process, the impact of time needs to be understood, measured and controlled.
Here are four crucial questions to ask when understanding how time comes into play in an adhesion process:
- What material are you manipulating?
- How are you manipulating it?
- What is the environment you are manipulating it in?
- How is it being handled, stored and shipped?
Getting clear, well-defined answers to these questions puts control in the hands of manufacturers to prevent adhesion failure or to eradicate it at the root cause.
Aging does not manifest itself in all materials equally. When thinking about how time affects different types of materials, you can lump them into three distinct categories:
Crystalline materials - this group includes metals, glass and ceramics with highly rigid molecular structure. These materials have high energy, which means they have very reactive surfaces. They age rapidly and are easily contaminated.
Thermosets - carbon fiber reinforced polymers, polyurethane and other materials that are very moldable upon creation, but become extremely rigid after curing (usually with heat) make up this category. These have a lower energy than the crystalline materials and require manipulation like fracturing through abrasion or activation through plasma treatment.
Thermoplastics - this name is used to describe any polymer that is rigid when cooled but elastic and soft when heated. This includes plastics like polyethylene, polypropylene, nylon and others. These are prepared for adhesion processes like printing on a plastic bottle or label by using corona, flame or plasma treatment. One issue that is typically unique to thermoplastics is the tendency of molecules that are found in the bulk of the material to “bloom” to the surface over time and create a surface that is not chemically suitable for the desired adhesion.
As stated above, what you do to prepare a surface for adhesion depends greatly on what the material is. These various treatment, cleaning and preparation steps have different shelf lives as well.
These charts display how different polymers (PTFE and HDPE) receiving the same treatment reacted differently to the plasma treatment and remained active and ready for adhesion for different lengths of time. In these trials, water contact angle measurements were used to gauge the quality of the surface over time. Each colored line represents various distances the plasma treatment was administered at. The dotted line represents the polymer before treatment. So you can see how, over time and depending on the material, the effect of treatment can wear off and lead to adhesion failure.
Another way that time comes into play with treatment and preparation is figuring out how long to treat or clean. It is possible to overtreat a surface and actually damage the material with overexposure to plasma or over-abrading and wearing away the surface.
This is an important point because if all of these processes were being performed in a sterile vacuum then time would scarcely be a factor. But the environment is typically one that exposes the material surfaces to many things that affect how long treatments and preparations are effective.
There are two ways that environments affect adhesion, through absorption and physical contact.
Absorption can look like a hot environment (due to ventilation and machinery creating heat) making thermoplastics deteriorate quickly and allowing those contaminants under the surface to bloom to the top. Also, if cleaning and treatment is being done near machinery performing tasks such as molding, forming or abrading, then chemical debris can contaminant the atmosphere of the area and ultimately contaminate the critical bond surface that needs to be controlled to ensure adhesion.
Physical contact not only refers to how material is handled by employees but also the chambers in which treatment and cleaning is happening. Soiled washing fluid that hasn’t been adequately changed will inevitably produce parts that will not adhere. Similarly, if there is unforeseen contaminants within treatment chambers where plasma treatments are being done then these can be easily transferred to material surfaces.
Taking the time to understand how these areas and environments can, over time, become detrimental to maintaining clean and active surfaces is critical to adhesion success.
The time it takes a material or part component to reach a manufacturing facility, the time it spends in storage and the time it spends in whatever container it is held in all have tremendous effects on every operation thereafter.
It may seem obvious that contamination and surface degradation can take place on a truck or before the part is under the manufacturer’s supervision. There are some unexpected ways that time in storage can truly alter the surface of a material.
Watch this video to hear from Dr. Giles Dillingham, BTG Labs' CEO and Chief Scientist, explain the details of how time in shipping and storage has a significant impact on the surface properties of a material:
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This chart shows how various molecules changed on the surface after a part was stored in plastic wrapping. These changes occur directly as a result of the transferring of elements from the plastic. Both carbon and fluorine have an impact on adhesion and the amount found of the surface increased significantly after exposure to the plastic wrapping. In order to ensure perfect adhesion there must be a precise chemical make up of the surface. As you can see, the contact angle changed dramatically (from a 62 degree angle to a 105 degree angle) after storage.
The impact of time is multifaceted and needs to be controlled on many fronts. Not giving adequate thought to how time is a key player in the state of material surfaces means not controlling and managing every aspect of an adhesion process. It is necessary to call upon Materials Science experts who can complete a holistic ageing study to show exactly how certain aspects of time’s role in manufacturing affects material surfaces during your process. This will allow manufacturers to precisely interrogate where they need to implement surface quality specifications and inspections to control and monitor the process.
Download the “Predictable Adhesion in Manufacturing Through Process Verification” ebook to learn about all the variables to consider when creating a reliable adhesion process.