What is the future of water based inks and adhesives on raw BOPP?
Ronni Nielsen, Project Manager at Danish manufacturer Vetaphone, analyses the problem, and provides the solution.
Today, the use of water based inks and adhesives on raw BOPP is limited, without the added use of an expensive primer. The reason behind this is the limitation of the materials’ ability to react with oxygen, resulting in a maximum obtainable surface tension of 46 dyn/cm, after using Corona treatment. The only solution today is to buy BOPP that is already primed or has an inline primer for the purpose. The disadvantages of this are added cost, the fact that the primer is solvent based, and the total material thickness is greater. With the new EASI-Plasma technology, it is possible to obtain up to 60 dynes on BOPP, with by far the industry’s lowest gas consumption. With such reduced running costs, an ROI of as little as 12 months has been tried and proven in a commercial environment.
To explain how this is possible, we need to look at the chemistry of the BOPP surface. Corona is an electrical discharge, typically ranging between 30 and 40 kV. With this discharge, existing molecule chains are broken and new ones are created. The new molecule chains on the surface are mainly created from the oxygen in the ‘air gap’ from the uncontrolled atmosphere that surrounds us. The O2 molecules break into O-atoms, which then connect with the CH-based groups on the surface of the plastic film, creating the molecule chains that are shown on Figure 1.
The bi-product of this is that the Oxygen atoms combine into Ozone (O3), which then is removed from the area by the mandatory exhaust on the Corona treater.
EASI-Plasma shares some similarity with Corona, as the electrical discharge is achieved in the same way. But unlike Corona, which uses an uncontrolled atmosphere, EASI-Plasma works only with a highly controlled atmosphere, which for this application is Nitrogen based. By removing all the Oxygen, and replacing it by a controlled Nitrogen based atmosphere, EASI-Plasma is able to graft specific molecule chains on top of the surface. When aiming for high dyne levels on BOPP, the desired molecule chains created are predominantly Amine, Amide and Imide groups, as shown below in on Figure 2. But in addition, by removing Oxygen from the air-gap, there is no longer any Ozone created.
But just creating the required molecule groups is not enough. The knowledge of how to treat the surface evenly to the same dyne level, with the lowest possible amount of consumables, is the key to making the best and cheapest product. The secret lies in how to create the correct mix of molecule groups on each specific material in a highly controlled atmosphere.
The difference between Corona and the two types of EASI-Plasma treatment suited for high dyne levels on BOPP is illustrated on Figure 3.
The chart shows the limitation of oxidisation of the surface. After reaching 46 dyn/cm with Corona, merely increasing the power applied will not improve the surface tension further. However, with EASI-Plasma Standard Grafting, 56 dyn/cm were obtainable on this specific BOPP material, and by using Advanced Grafting, the EASI-Plasma treatment was able to obtain a surface tension of 60 dyn/cm. It’s worth noting that just as with Corona treatment, these two types of treatment do not benefit from increasing the power still further.
Looking at what EASI-Plasma does to the surface, is the reason that in the past people in the industry have concluded that it is a new and improved Corona treatment. But, to reach an accurate conclusion, you really need to look at all the variables involved. At the end of the day, the price per square metre of material is what matters. The consumers don’t really care how the plastic film was produced. For many materials, it is still possible to obtain a significant surface adhesion from Corona, so for these materials changing to EASI-Plasma, would give little benefit.
However, there is one great advantage of EASI-Plasma, which is that it does not necessarily need to be done inline. The reason behind this can be found in the condition known as ‘ageing’. After Corona treatment, the additives in the plastic film ‘try to get back to the surface’. With the molecular structure that Corona has created on the surface, this is quite easy. Depending on the amount of additives, this ageing effect can be measured from hours to weeks. The fact is, that it always decays until it reaches the ‘native’ level, which for BOPP in this example was 32 dyn/cm, as shown in the chart on Figure 3.
The effect of ageing of this BOPP material is shown in Figure 4.
The chart shows that the Corona treated material starts to decay, and within two months has aged back to its ‘native’ 32 dyn/cm. The EASI-Plasma Standard Grafting also decays at a similar pace, but the ageing stops at a higher level. Here, for example, it is 48 dyn/cm, before decaying slightly to 46 dyn/cm after six 6 months, where it stabilises. However, with the EASI-Plasma Advanced Grafting, there is no ageing at all. The achieved 60 dyn/cm on BOPP stays at that level even after 18 months.
As is the case with Corona, when using different materials, you need a different power per square area, also known as the Material Factor, which is measured in Watt·min/m2, to reach the desired dyne level. Not only is this factor different for each material, but even the same material from different suppliers can result in different Material Factor requirements, depending on the exact chemistry the producer is using to manufacture the product.
For EASI-Plasma it is not only the Material Factor that needs to be changed according to the material, but also the atmosphere, to create high and lasting dyne levels. By changing the gas-mix, which is all the Nitrogen based atmosphere really is, it is possible to tune the amount of the different molecule groups, as shown below in Figure 5.
The exact gas-mix is easily created in the Vetaphone laboratory, and all the gasses needed are available from any of the gas suppliers around the globe. The typical added running cost, compared with Corona, is between 0,30-0,50 ¢/m2. The gas consumption of an EASI-Plasma system is less than half that of any other system on the market today. And, not only does the system consume less gas, but the gas is also royalty free, meaning it can be bought from whichever supplier the customer chooses.
To date, the specific materials that Vetaphone has proven to obtain higher dyne levels with EASI-Plasma than Corona are: PP, OPP, BOPP, PVC, PET, and PVDC. The specific materials that Vetaphone has so far proven to obtain longer lasting dyne levels with EASI-Plasma are: BOPP, Fluorinated Polymers (FEP, ETFE, ECTFE), PE, PLA, COC, COP, and Textile. And that is just the beginning, because the recipe for many more materials will become available in the future as the Vetaphone chemists continue their R&D in the industry.
But, this technology is by no means totally new. It has been used for in laboratories since the 1990s. The difference now, is that it has moved to commercial production machines. In the past, the main limitation has been that the equipment available could not control the atmosphere. The solution from the suppliers of that technology was simply to increase the consumption of gas, which for small machines, low speeds, and short runs, proved in part to be successful. But by solving the problem this way, the producers in the market had created a situation where there was no return on their investment. Added to which, the equipment was sold only with a contract, where the producer had to buy the gas from the machine supplier. Clearly, this gave them no incentive to make the system more gas efficient. The other major problem was a lack of control of the atmosphere, even with high gas consumption, so the treatment was uneven. This is why this type of equipment never really made it into a production environment.
But that is all in the past. What EASI-Plasma offers today is a solution to these known problems. Not only is gas consumption lowered significantly, but the atmosphere is controlled throughout the entire production process, monitoring the influential factors and adjusting flow and gas mix accordingly. Today, as proof of quality, the customer can see logs of consumption, measurements of power, gas mix, and other deciding factors that ensure the atmosphere is controlled, and the process is within specification.
With no limitations on speed or width of the treatment, EASI-Plasma now gives users the ability to see it work on small-scale laboratory machines, and also in full size commercial production. If you would like to know more about EASI-Plasma, you can sign up for our free open house event where you will see the technology in action and learn more about how your production can benefit from our various applications. It will be held on the 25th March at the CPI (Coating Plasma Industrie) facility in Peynier, France.
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