• Plastic bumper

    Plastic bumper

Coating of polymers

Methods for measuring wetting and adhesion

Polymers compete with traditional materials in almost all walks of life. Many coating applications for plastics, such as painting, printing and bonding, define requirements for the development of the materials and for the coatings. High quality is the result of specific interface measurements and the use of materials corresponding to these measurements.

Typical polymer-coating applications

  • Painting of plastics as a design element
  • Painting of packaging and films
  • Bonding with glass, metal, textiles or other synthetic materials
  • Superhydrophobic coatings for self-cleaning surfaces
  • Coating of contact lenses for comfort when wearing and good oxygen permeability

Energy balance between plastic and coating

A frequent problem with plastic coating is the incompatibility of the materials. The polymers used are difficult to wet, and water as the basis of many coatings provides poor wetting. To make both sides compatible, the surface free energy of the plastic is increased by pre-treatment and that of the coating reduced by the addition of surfactants. Our measuring equipment provides vital information for both tasks.

Surface free energy of the polymer

A range of methods is used to increase the surface free energy of plastics:

  • Corona treatment
  • Plasma treatment
  • Flame treatment
  • Chemical action

The polar fraction of the surface free energy, which is accessible by measuring the contact angle, is particularly informative, as this reflects the affinity with aqueous media. We have developed stationary laboratory units and non-destructive, mobile instruments for use on site for measuring the contact angle.

Surface tension of the coating and its adhesion and stability

Our tensiometers are used to analyse the surface of the coating phase, the liquid coating. The effectiveness and efficiency of the surfactants used are of interest here.

The polar and non-polar (dispersive) fractions of the surface tension can also be determined. The combined results for the solid and liquid phase give an informative overall picture:

  • The work of adhesion is a measure of the initial adhesion of a coating or a bond.
  • The interfacial tension goes hand-in-hand with the long-term stability; for a permanent bond, it must be as small as possible.

KRÜSS Application Reports

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 recending 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.

AR234: Determination of the surface free energy of electronic components

Comparative contact angle measurements on two electronic components show significant differences in their surface free energies. The differences in adhesion and wetting which are to be expected with one and the same potting compound mean a quality problem.