Plasma Surface Testing

There are a variety of methods used to measure how effective particular plasma processes are at acting on a surface. These methods can be applied to both untreated and treated surfaces, which is advantageous for comparing the state of a surface before and after it has been processed.

Molecules within the bulk of material are surrounded on all sides by other molecules. A new surface is created by breaking some of these bonds, which in turn requires energy. This means that at the surface there are some bonds which are no longer linked to others and are unsatisfied: this creates an excess of ‘stored’ energy on the surface.

This surplus surface energy is defined as the excess energy on the surface of a material compared with the bulk material itself.

But what happens when a liquid comes into contact with a surface? If the molecules that make up the liquid are more strongly attracted to each other than the surface, the liquid will bead up as the wetting is poor. The other side of this is that if the attraction between surface and liquid is stronger, then the liquid will spread more easily.

Therefore, if a surface has high surface energy it will be wet more easily, and as the wettability is simply described as the adhesion characteristics of the surface, the surface in question will be much easier to glue/print/paint upon or bond to.

Surfaces such as wax that are mostly made up of carbon-hydrogen (C-H) bonds have low surface energies and do not wet easily. Surfaces that are predominately oxygen-hydrogen (O-H) bonds have higher surface energies and are adhered to more easily by liquids. Materials with low surface energies are those such as polyethylene or polypropylene.

Plasma treatments can be used to attach oxygen containing species to a low energy surface in order to turn it into a higher energy surface. This technique can also be used to attach various functional groups to change the properties of the material at the surface.

Plasma bonding to a surface

Image credit: Henniker Scientific

Test Methods Test Fluids

Test fluids are easy to use with no specialized training and are cost effective, making them a good choice for measuring surface energy.

The fluids are available in increasing graduations (surface energy) and supplied as either bottled inks with a brush applicator, or marker pens. Both methods can be easily drawn over the surface in turn. The fluids with low energy will form beads on the surface, indicating the fluid has a lower energy than the surface. This process continues with a graduation of fluids until one no longer forms beads, but spreads evenly across the surface, indicating to the user that the energy levels of both surface and fluid are approximately equal.

Test fluids

Image credit: Henniker Scientific

The fluids are available in a blue, red or green colour with a range of surface energies that is given in units of dynes/cm or mN/m. Liquids in the blue colour are based on toxic formamide and are formulated to DIN ISO8296, so the results can be compared between any manufacturing site or laboratory that also tests with it. The red and green liquids are based on alcohols they are non-toxic; this means they aren’t strictly cross-site comparable but should be fine for most instances.

Individual Bottles & Sets Individual Test Fluid Bottles

Individual 12 ml Test Bottles with Brush Applicator Table 1  
Blue Green Red
Range Increment Range Increment Range Increment
18-28 mN/m 2 mN/m 24-72 mN/m 2 mN/m 34-46 mN/m 2 mN/m
30-46 mN/m 1 mN/m - - - -
48-72 mN/m 2 mN/m - - - -
105 mN/m n/a - - - -

Test Fluid Bottles Sets

Henniker Plasma offers all colours of liquid in sets of six (blue) or seven (green and red) separate bottles in a robust case. The table below shows their most popular sets.

Individual 12 ml Test Bottles with Brush Applicator in Sturdy Case Table 2
Blue Green Red
28, 38, 56, 64, 72 and 105 mN/m 24, 30, 36, 42, 48, 58 and 72 mN/m 34, 36, 38, 40, 42, 44 and 46 mN/m

Test Pens

Related Stories

Quick Test Pens

For a fast check to determine if a part has or has not been plasma treated, the Quick-Test pens are ideal. They can be refilled and come with a single ‘setting’ of 38 mN/m.

Test Pen Sets

This is a set of seven separate test marker pens in transparent carry case (red fluid energy levels seen in Table 1).

Easy Test Pens

The Easy-Test Pens may be filled with 100 ml of any fluid with the energy levels available shown in the green fluid range in Table 1.

Easy test pens

Image credit: Henniker Scientific

Test for Silicone Contamination

Silicones are polymers containing silicon, carbon and oxygen. Silicones can be present on many surfaces due to mould release agents or simply from leeching from ‘clean’ packaging, resulting in poor adhesion and bonding characteristics.

However, unlike carbon based polymers, only the organic functional groups may be removed via plasma treatment, which leaves a non-volatile silicate surface.

Time and effort can, therefore, be saved by testing for the presence of silicone. By using this simple test it can highlight issues that may be caused by manufacturing or post-treatment packaging and measures can be taken to rectify it rather than further up the processing line.

Silicone Contamination Test Kit

Henniker’s silicone contamination kit has all of the necessary equipment to conduct silicone contamination tests along with easy to follow instructions. These are protected by a small hard carry case, excellent for testing in the field.

Silicone contamination test kit

Image credit: Henniker Scientific

Contact Angle Measurement

Contact angle instruments use a rapid image capture camera along with analysis software to optically analyze the shape a drop of liquid makes when it is in contact with a surface.

This gives an insight into the surface energy as the shape of the droplet and contact angle can be used to determine energy levels. If the liquid is attracted to the surface (material with higher surface energy), it will form a low contact angle (theta < 90°). If the liquid is repelled by the surface, the contact angle will be higher (theta > 90°). This is, therefore, a quantitative measurement of how well the liquid wets the surface.

Contact angle measurement

Image credit: Henniker Scientific

Static Contact Angle Measurement Mode

Static Contact Angle is used to determine the highest possible contact angle at “equilibrium”. This measures the wetting characteristics of a substrate to check surface treatment, cleanliness and/or contamination effects.

Dynamic Contact Angle Measurement Mode

This test mode takes many sequential shots as a falling droplet lands on the surface to be measured. Each of the images is analyzed and the change in contact angle (wetting), volume (absorption) and spreading as a function of time is presented as a plot. It is most useful when troubleshooting problems related to dynamic processes such as printing, gluing and coating applications.

PGx Portable Contact Angle Instrument

The PGx is a fully automated Contact Angle Measurement Instrument that can determine surface wetting and absorption in-situ on nearly any surface regardless of size or shape. This is smallest contact angle measurement device available, with a footprint of only 90 mm x 55 mm and weighing just 400 g. The PGx contains features previously found only in larger laboratory instruments and enables direct surface testing of almost any 3D object without having to cut the sample to fit.

Specifications

  • 0.2 uL to 10 uL droplet size
  • 80 frames per second image capture
  • Touch down, impact and manual drop application modes
  • Built in, automatic calibration
  • Automatic sampling in both Static & Dynamic modes
  • adhesion (gluing, bonding)
  • absorption
  • surface contamination (cleaning)
  • wettability (printing, painting, coating)

 

Henniker Plasma logo

This information has been sourced, reviewed and adapted from materials provided by Henniker Plasma.

For more information on this source, please visit Henniker Plasma.

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