Table of Contents
Aim Of This Article
This article aims to give you information on what treatment life means and importantly how it’s effected so that you can control and measure it’s affects to ensure that you have a stable process for using treated materials.
In the article we’ll speak about surface energy, if you want to know more about that you can read What Is Surface Energy? which gives an overview of surface energy and the methods for measuring it.
We’ll also consider surface treatment in general rather than look at specific machines and equipment, if you want to find out more about the technology then you can read about Atmospheric Plasma, Vacuum Plasma and Corona Treatment.
Let’s get in to treatment life of Plasma and Corona!
Beware Second Hand Information and Theories!
The first and key point to the question of how long does plasma treatment or corona treatment last and what is the treatment life is that it varies depending on the material at hand, the treatment level required, the age of the material and several other factors which make it impossible to answer the question with a single sentence.
I will go through the different ways treatment is lost, what’s actually happening and how you can prolong treatment life during this article, but so many of our customers are sure they know the answer to the question because a colleague told them, or they read something online once, maybe even an adhesive supplier or someone ‘in the know’ has told them the truth about surface energy and treatment life! But with many things in life, the real answer is much more complicated than it might first seem.
So as we start to answer the question, make sure to keep an open mind and forget that your old colleague Steve said that treatment lasts 24 hours only and that’s the answer. You’ll find there’s quite a bit more to it than that!
What Does Plasma And Corona Do To The Surface?
The fundamental process behind both plasma and corona treatment is that high voltage breaks up molecules in the air, mostly the oxygen molecules which creates free radicals, ions and excited species.
If there are light oils and greases on the surface, this energy is transferred to them and they react to form simple gases like carbon dioxide and water vapour which are blown away. For materials like many metals, ceramics and glasses, treatment mostly stops here.
For polymers, including epoxies and polyurethanes, some of these excited molecules implant themselves in to your material, leaving new chemistry on the surface which is good for wetting and adhesion.
This new chemistry is usually less than ten molecular layers deep, it is purely at the interface between material and the process you’re going to do next, printing, coating etc. This doesn’t change the material to make it brittle, or discolour the material or change it in any other way.
Your new surface has permanent, stuck in place molecules if it’s a polymer or simply has a perfectly pristine surface if it’s too inert to react with the plasma.
So Does Surface Treatment Life Last Forever?
The new molecules on the surface are there and they are there for good. But there are four key ways that treatment could decay with time:
External Contamination
External contamination from the air or more likely you! Whether it’s the common example of finger prints as you handle the part or airborne sprays, greases and debris landing on the treated area; after treatment you have made a fantastic surface to bond to.
Hopefully your ink or other process can get there first, but it is a race. Handling a part with gloves, keeping it away from dirty environments and avoiding wiping or leaving the part exposed will always help prolong treatment life.
Migrating Contamination
Internal oils blooming or migrating to the surface can be a huge problem, materials with high additive content can be killers for treatment. Highly plasticised PVC is one of the most well known problem materials as the plasticiser can migrate quickly, leaving a layer on the top of the material and completely cover up the treatment.
Rubbers, TPE and other soft materials are often likely culprits for this risk.
Molecular Movement
Molecular movement takes place on most materials, even a solid has some level of movement and the treatment we’ve added is entirely on the surface, which for material stability at the surface isn’t especially good.
A high concentration of this new chemistry at the surface might mean that molecules rotate or move to dissipate treatment and leave you with half of your treatment hidden under the surface where it isn’t useful. Uncrosslinked and softer materials have this risk the most.
Physical Removal
Physically removing treatment can be done because the treatment is a layer on the very top of the material. Abrading the surface will be enough to take away hundreds of molecular layers, significantly deeper than the treatment itself.
Allowing parts to be packaged poorly so they rub and scratch against each other in transit isn’t a good option either. Keeping parts away from other surfaces is important for materials with high quality finishes such as screens or painted materials and it’s also important for surface treated materials too.
The first three can be quicker in heat, when exposed to strong UV light or other aggressive environments, and when subjected to extra cold temperatures, deposits from ice can be left behind too.
At one end of the spectrum, a soft, oily, greasy silicone that’s travelled by boat from China is likely to give us no more than 30s worth of treatment life. At the other end, a highly cross linked, low additive and freshly moulded medical grade polymer could last more than five years.
Protecting Treatment and Prolonging Lifetime
Treatment life of plasma and corona can be protected to give prolonged treatment life, but there are things to consider.
The key point to understand is which of the methods is causing you to lose treatment.
Treatment life being lost due to external contamination is the easiest to avoid, we always use aluminium foil to wrap parts for trials and tests because you only have minimal point contact, unlike a cling film, and the way foil is manufactured means it has a perfectly clean surface – just make sure to get the standard foil and not the lubricated or nonstick version.
If treatment life is being lost due to migration of additives or molecular movement then this can be tricky to stop, but keeping parts at cooler temperatures is likely to slow this process down.
Finally, if abrasion and physical removal of treatment is causing problems, it’s worth knowing that treatment is only a few molecular layers deep. This is why treatment is invisible and why it doesn’t cause problems such as discolouring or making the material brittle. Treat the important surface as a non touch surface and protect it to avoid issues.
What Surface Energy and Treatment Level Do I Need?
This is another killer question that gets overlooked because people often think that treatment is all or nothing. A rough guide is to measure surface energy of the material as received and wherever the number starts, add 10 dynes/cm by plasma or corona treatment and expect to start seeing improved performance.
Add more than this and you’ve got room for treatment to drop over time. Most materials will drop a little with time but a common example is the classic 38 dyne requirement for polyethylene.
PE starts at around 30 dynes when it’s blown as a film. To get good adhesion we want the 38 dyne ink to pass which means that it is more than that number, so it’s at least around 40 dynes which is 10 dynes more than where it started. This gives good performance for printers when it’s converted into printed packaging.
Many manufacturers of PE will treat higher to give a longer treatment life and depending on the level of additives and age of material, the amount of treatment that can be accepted will vary and the higher the additive content, the faster the treatment will drop off.
In this expected graph, we see that treating as high as we can for the different additive content versions of PE means that the highest content version will fall below 38 dynes within 24 weeks, the standard additive content will last around 38 weeks and the ultra low additive content version will last well over a year.
These figures are expected and the assumption is that all are stored in the same way, with nobody contaminating or ruining treatment in any way other than additive blooming and molecular movement over time.
In reality the trends may change if the material is stored poorly, or its shipped multiple times and thrown around without care. But the trend should always be similar.
A Few More Examples of Treatment Life
As with a lot of questions in life, the answer is nearly always ‘how long is a piece of string?’ As even with similar materials, each supplier will have a slightly different set of ingredients which have more or less additives, or include varying amounts of regrind or recycled content.
So here are a few examples which are fairly common. However, add a UV stabiliser, or keep the materials in a hot and dusty environment and expect different results:
Polypropylene: PP being the common wonder material luckily has a good shelf life, normally weeks, but often months if stored correctly. Years isn’t unheard of!
Polycarbonate: PC is generally a good material to work with, it can often have a very long treatment life, well into years. Although variants with higher additive contents can be a problem.
TPE: being a range of materials and not an exact variation, the range can be wide. Harder TPEs are likely to last longer, with softer ones potentially having a shorter life. It would be sensible to think hours to days, with the possibility of weeks.
Nylon: with a wide variety of materials falling under the PA nylon family, we can see some interesting results. Nylon, often coupled with additives like glass fibres, is hygroscopic and can absorb water from the air. A freshly moulded or extruded nylon with hard fillers and low oil content is easy to treat and should last weeks and months. An older nylon left to absorb moisture is completely different and might not treat well at all, even when it does treat well it is unlikely to have the best lifetime.
PEEK: an excellent engineering polymer, poly ether ether ketone treats well, has a low additive content and has a rigid molecular structure. PEEK can often last years, especially when protected from airborne contamination.
All polymer materials react a little differently so if you want to find out your specific material properties, you can send them to us to check out or buy dyne pens online at Ebble.Shop.
Treatment Life Of Metals
Metals will have a treatment life the same as any other material, here are two of the common situations.
Metals such as stainless steel and gold: these materials are very resistant to plasma and so receive a strong cleaning effect only. The surface left behind is fantastic for bonding, but there’s a twist. They are so good for bonding, potentially having a surface energy (we’ll discuss in a moment) more than twenty times higher than a polymer which means they attract more than other materials. This can lead to an exposed metal having a very short lifetime, sometimes less than an hour, although days and weeks aren’t uncommon if the surface is protected.
Metals such as Aluminium: rarely found as the bare metal, aluminium always has an oxide layer which forms immediately after the metal is formed or exposed. When subjected to plasma, it is likely that oils and greases are removed, but the highly energetic nature of the plasma will help form additional oxides. These oxides form fast, and densely and can actually give a strongly bound layer which is good for bonding. Treatment life here again varies, but weeks to months is typical.
A Warning From Lessons Learnt
While material treatment life can last significant amounts of time, you never know when someone will open the bag, rub the part and say the phrase ‘this doesn’t look treated to me’ before then placing the part back in the bag and on the shelf. The part now has a greasy layer where it’s been touched and this could lead to production problems.
The reality is to minimise the risk by treating and processing as soon as possible. Educate your team about contamination and make sure you know when a part was treated, and when it’s due to drop off. Keep parts stored sensibly and use a stock rotation system.
If you’re treating and storing, or using subcontract plasma treatment for your components like that done by our Ebble Manufacturing side of the business, being aware of the risks and limitations is key to ensuring your parts are still in good condition when you use them.
How To Be Sure Of Treatment Life Of Surface Treatment
The only way to be sure of the treatment life of your material in your manufacturing environment with your handling procedures is to measure the surface energy over time.
While you’re measuring surface energy you should also measure performance of the part so you can be sure what the minimum real world treatment level is that you need.
In this example, the material starts at 30 dynes and with treatment goes up to 72 dynes.
Performance goes up with surface energy, passing the minimum performance level for the process at around 48 dynes.
This is only an example and we could see a variety of performance curves including drop off due to over treatment and steep or shallow spikes. So plotting Surface Energy vs Performance is a key way of understanding your process and knowing at what point you should be rejecting parts for this reason.
This graph adds two more scenarios; over treatment and material damage.
In both of these cases the treatment level and surface energy will measure high and that might look good. However performance has a peak before dropping off and potentially going lower than the minimum threshold of your process.
It is often advisable to know a treatment level window that is checked immediately so you can understand where you need to aim for and then look to understand treatment life.
This concept goes a little beyond simple ‘how long does surface treatment last’ and moves in to the full range of testing that we look to understand when working with companies to determine their project parameters.
Couple this information with the different parameters that the plasma and corona treatment systems work with and there’s quite a lot to think about in the early days to get everything right. In reality the overtreatment and material damage curves are usually easy to avoid because they need a lot of treatment or significant amount of power to damage the material in such a way. But it is certainly worth thinking about!
Conclusion and Considerations
Treatment life is smart to measure and understand. Knowing whether you have a short or a long treatment life will be useful for periods of shut down such as holidays or line stoppage.
Understanding treatment, measuring it and knowing how to handle parts is key to having good quality parts, and this information should be given to all involved in processing, and not just the quality department.
Always be aware of things changing in your environment because a new polymer grade, an intense heatwave, a supplier changing the water content of their adhesive or parts being left outside could all play a part in the performance of your process.
