• Plasma treatment (Photo: Diener electronic)

    Plasma treatment (Photo: Diener electronic)

Polymer surface treatment

Contact angle measurements for determining wettability and adhesion

The use of water-based printing, adhesive and coating substances requires optimum pre-treatment of hydrophobic plastic surfaces. Different methods, such as corona processes, flame treatment or plasma treatment and the chemical action of ozone or fluorine increase the surface free energy and therefore ensure improved wetting, initial adhesion and long-term stability.

Our drop shape analysis measuring instruments determine the contact angle as a measure of the wettability. The effect of the treatment on wetting and adhesion can be accurately determined and optimized for the contacting substances based on a characterization of the surface free energy.

Applications of plastic surface treatment

  • Painting and other coatings
  • Printing of plastic parts and films
  • Bonding and sealing
  • Increasing the biocompatibility of implants

Characterizing the wettability based on the contact angle

The high surface tension of water in contact with usually low-energy polymer surfaces leads to poor wetting. Pre-treatment increases the surface free energy and therefore improves the wetting by water.

The difference in wetting before and after treatment can be measured by means of the contact angle using our drop shape analysis instruments. For quality assurance purposes, the contact angle can be non-destructively measured directly on site, and also for large samples, with the help of our mobile measuring equipment.

Determination of surface free energy

The surface free energy can be calculated from the contact angle with a number of liquids. Our measuring instruments with dosing units for up to eight liquids quantify the success of a pre-treatment process in this way. Repeat measurements at longer time intervals allow the long-term effect of treatment to be estimated.

The surface free energy is increased particularly by embedding polar groups in the polymer surface. Splitting the surface free energy into a polar and a dispersive fraction describes the exact degree of matching to polar, aqueous media.

Calculating the adhesion and the long-term stability

A complete interfacial chemical image of the contact between polymer and wetting substance is produced when the surface tension of the liquid is also characterized. The work of adhesion as a measure of the initial strength of a bond or coating and the interfacial tension as a destabilizing factor can then be calculated.

With the help of the pendant drop method, our instruments measure the surface tension without changing the test setup. The procedure for the overall characterization of coating substance and coated material has been defined in the norm DIN 55660.

KRÜSS Application Reports

TN318: Inline process control of wettability by means of contact angle measurement
on moving surfaces

With a feasibility study we show how contact angle measurements can be used for uninterrupted inline process controls by utilizing our fast Liquid Needle dosing unit and the ADVANCE API software interface for the integration in information systems.

AR280: Optimizing flame treatment of polymer surfaces

A supplier for the automotive industry was confronted with a very high reject rate when applying a decorative foil on flame-activated polypropylene. After unsuccessful efforts with test inks, non-
destructive on-site measurements of the surface free energy showed were the problem came from.

AR272: Why test inks cannot tell the full truth about surface free energy

The SFE is determined for 16 materials and plasma treated polymers. The differences – which are quite large in some cases – are explainable considering that ink tests do not take the polar part of SFE into account.

AR262: The Effect of an Oxygen-Helium Atmospheric Plasma on the Surface Energy of Medical Plastics

Surface modification of polymers by means of plasma treatment significantly improves the wettability by water. As a result of this discovery, higher biocompatibility of the plastics in medical applications can be expected.

AR256: How plastics lose their hydrophobia 

The increase in surface polarity by ozone treatment is proven with the help of contact angle measurement. The results also show the dependency of the degree of surface activation on the duration of the treatment.

AR253: Correlation of receding water contact angle data with moisture vapor transition rate on corona treated polypropylene packaging film

The increase in surface free energy of a PP film due to corona treatment is quantified by means of contact angle measurements. The undesirable increase in moisture permeability with excessive treatment duration correlates well with the measured receding angle.

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