What is plasma?
Here on Earth we are familiar with three states of matter: solid, liquid and gas, but the most common state of matter in the universe is plasma. Plasma is a higher energy form of matter that makes up the sun and every object we can see outside the solar system. On Earth plasma is present in neon lights, flames and electric discharges.
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Plasma carries high energy and makes up over 90% of the entire universe. |
Why plasma is uniquely useful
Plasma is a form of matter that has higher energy than the corresponding solid, liquid and gas. However the energy that is found in solids, liquids and gases is largely carried in the form of kinetic energy – or heat. In plasma the energy is also due to electrons being split from the atomic nucleus creating ions and free electrons. This means that plasma can carry a lot of energy and is highly reactive, but it is not necessarily hot. Openair® plasma systems employ the beam of ions and free electrons to clean and activate surfaces without burning them.
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| For solids, liquids and gases, as the energy increases so does the temperature. However plasma can carry high energy yet not be very hot. |
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How plasma cleans surfaces
In Openair® plasma the beam is created from the air by a pulsed current that excites the molecules. The resulting ions and free electrons then remove static electricity and dust, as well as vaporize contaminants, including waxes and silicone residues.
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The ions and free electrons in the plasma beam break down contaminants. Organic compounds are oxidized and turned mostly to water and carbon dioxide. |
How plasma functionalizes surfaces
Often the surfaces of plastics are chemically unreactive because their long polymer chains are low-energy and have no functional groups, therefore they are difficult to glue. The ions and free electrons in the beam of plasma blast low-energy atoms off the polymer and replace them with functional groups that are more reactive like –OH and –NH. The surface of the material can be visualized as Velcro™ tape with closed hooks. The ions and free electrons create hooks that make the surface more attractive to liquids, and consequently easier to wet.
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The ions and free electrons in the plasma create Velcro-like hooks on the surface of the material. |
How plasma makes materials wettable
A key step required for effective coating, bonding and printing is for the paint/glue/ink to wet the material. If it beads, like raindrops on a waxed car, the result will be patchy. Plasma promotes wettability by first cleaning the surface to remove contaminants, then it increases surface energy by changing its structure.
Many liquids are self-loving, so they do not spread out on a surface unless their energy is similarly high. The liquid can be visualized as having only Velcro-like loops, so in order to spread out, the surface needs to have a lot of hooks. Then the loops can attach to the hooks.
Water is particularly self-loving so paints, glues and inks that are water-based are incompatible with low-energy materials, including polymers commonly used in manufacturing.
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| A plastic surface will normally repel water, but when the surface is activated water will wet the surface evenly. |
Plasma turns liquid-hating materials into liquid-lovers
How much a material attracts a liquid can be expressed by its surface energy (measured as dynes per cm). Water has a high surface energy of 72 dynes/cm so when it meets a low-energy material like polypropylene (PP) that has a surface energy of below 28 dynes/cm the water will not be attracted and it will bead. When the polypropylene is treated with Openair® plasma its energy can be increased to over 72 dynes/cm, at which point the water will wet it completely.
It can be extremely useful to make materials wettable because then they can be bonded, printed or painted a lot more easily and without using organic solvents that are damaging to the environment.
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The surface energy of materials can be expressed as dynes per cm.
Most polymers cannot be wetted until their surface energy is increased to
about
70 dynes/cm. |
Openair® plasma makes the unbondable bondable
Plastics are useful because they are stable and non-reactive. However these same characteristics make them a challenge to bond. By treating the surfaces with plasma the non-reactive surfaces become activated. This can be visualized as the material developing atom-sized Velcro hooks that form cross-links. These hooks can then bond firmly to paint, glue and ink. Or in the case of injection molding processes, materials that would otherwise be incompatible can be molded together without the need for adhesives.
In some cases Openair® plasma can improve the strength of bonding by a hundredfold.
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| Plasma can activate the surface of incompatible materials so they can be bonded together. |
The benefits of surface functionalization for manufacturers
Manufacturing is becoming increasingly competitive so to remain profitable it is necessary to continually innovate and improve performance. Improving performance includes: improving the strength and quality of bonded assemblies, increasing throughput, lessening environmental impact and reducing material costs. Often improvements can be achieved by switching to new lower cost materials and employing new fabrication techniques. Plasma can play a vital role because it makes materials easier to process.
The list of potential benefits for adopting plasma technology include the following:
• More efficient painting and coating
• Assemblies bonded more strongly
• Use lower cost materials
• Adopt more efficient production processes (like multi-shot injection molding)
• Eliminate environmentally problematic solvents
In particular, old methods of preparing and priming surfaces can be eliminated and solvent-based adhesives, paints and inks can be replaced by less costly water-based equivalents.
Discover more about the
science of plasma and its use in industry
The use of atmospheric plasma in industrial applications is relatively new so the amount of documented information in the public domain is still limited. Some of the most complete applied research has been conducted at the University of Cincinnati and in Germany by the Fraunhofer Institute for Production Engineering and Applied Materials Science in Bremen. We have a library of papers. If you have a specific application in mind we would be pleased to supply the relevant material.
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