Q A What is a Plasma?
Solid, liquid and gas are the three states of matter we are all familiar with. We can move between the states by adding or removing energy (e.g. heating/cooling). If we continue to add enough energy (for a plasma, we add electrical energy), gas molecules will become ionised (lose one or more electrons) and so carry a net positive charge. If enough molecules are ionised to affect the overall electrical characteristics of the gas the result is called a plasma. Plasmas are, therefore, quite rightly, often referred to as the fourth state of matter.
A plasma contains positive ions, electrons, neutral gas atoms or molecules, UV light and also excited gas atoms and molecules, which can carry a large amount of internal energy (plasmas glow because the light is emitted as these excited neutral particles relax to a lower energy state). All of these species can and do interact with any surface placed in contact with the plasma. By choosing the gas mixture, power and other conditions we can quite precisely tune, or specify, the effects of the plasma upon the surface.
Q A What does a Plasma Treatment do?
Plasma treatment modifies the surface of an object. It doesn’t affect the object in any other way and doesn’t change the bulk of the material at all. We can ‘tune’ the plasma to change the adhesion characteristics of the surface. We can make the surface easier to bond, print onto, glue etc. We can also introduce a range of other surface properties such as liquid repellency or low friction.
Q A How does the Plasma do that?
The plasma creates reactive molecules, electrons, ions and UV light. These interact in various ways with a surface. Organic contamination, in the form of long-chain hydrocarbons, can be broken down by the plasma and removed from the surface, leaving an ultra-clean surface. On polymers, the plasma also breaks surface bonds, and then terminates these bonds with oxygen-containing polar molecules, greatly improving the adhesion characteristics of the material by improving its wettability.
The 4 major effects of plasma are;
- Surface cleaning: removal of hydrocarbons which impede adhesion
- Micro-sandblasting: which increases the surface area and therefore wettability
- Cross-linking or branching of surface molecules: which impedes the migration of waxy additives from the bulk material to the surface
- Modification of surface chemical structure: to increase the surface energy and render a surface wettable and also to impart other specific functionality such as liquid repellency
Q A Which materials can benefit from plasma treatment?
Nearly all materials can benefit from plasma treatment. Metals, glass and ceramics can all be rendered ultra-clean. Polymers and rubbers can all be made easier to glue, print, paint and bond and the final mechanical bond strength is usually greatly enhanced. We can also treat powders, textiles, fabrics, and composite materials.
- engineering polymers
- optics & lenses
- medical devices
Q A Can I treat heat sensitive parts with Plasma?
Yes. Our type of plasma is often referred to as ‘cold plasma’ and should not be compared to e.g plasma cutting torches, which are a very different form of plasma. There is very little heat transfer to an object during processing.
Q A How long does plasma treatment take?
This really depends on the part but in general, can be anywhere from 20 seconds to just a few minutes for the activation to improve bonding. Removal of organic contamination can take a little longer depending on the degree of contamination.
Q A How long does plasma treatment last?
This depends on the material and subsequent handling and storage. It can be from minutes for glass, and metals to many months for e.g. polypropylene. It is always better to print, paint, glue etc. as soon as possible following plasma treatment but the ‘relaxation’ of the surface is asymptotic and even after days the surface is often much more active compared with an untreated surface.
The reason for material dependency is that most commercial polymers contain additives such as anti-oxidants, mould release agents and anti-block agents, which are oily or wax-like. These are incorporated to improve manufacturability and are designed to migrate to the surface.
For more information read our knowledge article where we discuss the main factors that influence the lifetime of plasma surface treatments and explain how these factors can affect different materials.
Q A How good is a bond after plasma treated parts have been glued, printed, painted?
Plasma modified surfaces form a covalent bond with many adhesives. The resulting bonding enhancement can range from a 2-fold to a 10-fold improvement in lap-shear strength and peel-strength. Some materials can even be bonded in the absence of adhesive, such as PDMS to glass, of particular importance in the production of microfluidic devices.
Q A How large can the parts be?
We supply plasma treatment equipment ranging in size from small bench-top machines no larger than a domestic microwave oven to thousands of litres in volume for large industrial applications.
Q A Can 3D objects be plasma treated?
Absolutely. The plasma is a gas phase process and the plasma will usually get everywhere that gas can get. There is a restriction for treating the inner diameter of long thin tubes where typically the plasma effects are less when the tube length > x10 diameter.
Q A Can I treat only specific regions of an object?
Parts can be masked if treatment is undesirable in some areas, however, plasma treatment is not usually detrimental and so most parts are wholly treated.
Q A Can I plasma treat material in-line?
We supply both batch-plasma treaters (small and large scale) and also in-line plasma treaters (atmospheric plasma ‘pen’). In-line plasma treaters have a relatively small treatment area and so usually require multiples to cover larger areas and/or robot handling for 3D objects.
Q A Does plasma generate any harmful waste?
The main by-products of batch-plasma treaters are CO, CO2 and water vapour, none of which are present in toxic quantities. In-line plasma treaters produce quantities of NOx which are removed with standard extraction hoods.
Q A What is surface energy?
Surface energy is defined as the excess energy at the surface of a material compared with the
bulk material itself.
When a liquid comes into contact with a surface, if the molecules of the liquid are attracted to each other more strongly than to the surface, then the liquid won’t wet the surface very well, instead of forming beads. Conversely, if there is a larger attraction to the surface then the liquid will spread out more.
It follows that if a particular surface has higher surface energy it will wet more easily and, since
the ability to wet a surface is, in turn, a simple definition of the adhesion characteristics of the
surface, it will be easier to glue/print/paint or bond to that surface.
Q A How does plasma treatment increase surface energy?
Plasma treatments aim to convert low energy surfaces to higher energy surfaces by attaching oxygen
containing species to the surface.
Q A How do I measure the effectiveness of plasma treatment?
From the definitions above, a measurement of surface wetting or surface energy can indicate the effectiveness of plasma treatment. We provide a range of measurement techniques including surface energy test inks and pens as well as contact angle measurement instruments, which measure the shape and angle of a drop of liquid in contact with the surface.
Q A Which vacuum pump should I choose?
Henniker Plasma’s range of vacuum plasma treatment systems (HPT and Nebula models) include a suitable vacuum pump which is required for the low pressures needed during plasma operation.