Process Control for organic coatings and thin layer production

Thin organic coatings and multilayer films

Measuring the thickness distribution of thin transparent coatings on transparent materials is a challenge, especially as an in-line process control technique. The challenge becomes greater the thinner the layers or the more similar the refractive indices are.
This challenge arises, for example, with

  • sub-micron organic coatings (lacquer, adhesive etc.) on polymer films
  • impregnations such as siliconization on polymer films or on paper
  • components of multilayer co-extrudes films
  • organic coatings on components, e.g. lubricants, release agents
  • ...

Using fluorescence dyes can help to face this challenge.  

Fluorescence

absorption, fluorescence, diffraction
© Fraunhofer IAP, Andreas Holländer

Dye solutions and a green laser demonstrate optical absorption (left), fluorescence (middle), and diffraction (right).

Silicone on paper
© Fraunhofer IAP

A silicone coating on paper in day light (left, improved contrast) and fluorescence image (right).

Some organic dyes (and also inorganic pigments) can absorb light of a particular wavelength (colour) and emit light with a longer wavelength (other colour). This effect is named fluorescence. Fluorescence can be measured with an extremely high sensitivity and, therefore, only a very small concentration of some ppm (parts per million) of a fluorescent dye is sufficient for its detection in a µm thick coating. Such a small concentration in a thin layer is not visible for the naked eye and it does not affect the properties of the coating material like its mechanical strength, modulus etc. Since the dye is dispersed in the organic material on a molecular level the fluorescence intensity is proportional to the thickness of the coating.

Adding a fluorescent dye to a lacquer will allow to monitor the thickness distribution by recording the fluorescence intensity distribution with a digital camera. Of course, the dye is not visible by the naked eye. As an example the picture shows a sheet of paper with a silicone coating where the silicone contains a fluorescence dye. While the coating is virtually invisible in day light the fluorescence can be recorded with a good sensitivity.

 

Inline process control of coating processes

Distribution of a lacquer coating thickness on a PET film
© Fraunhofer

The distribution of a lacquer coating thickness on a PET film was recorded at 3 m/min at a resolution in the range of mm.

The fluorescence intensity of a dye dissolved in a layer can be recorded on a running web. The fluorescence intensity represents the thickness at each point.

A simple equipment for the inline monitoring of fluorescence signals from polymer films was installed at Fraunhofer IVV. The picture shows the thickness distribution of a lacquer coating on a PET film as recorded with this machine.

While these test were run at 3 m/min we can estimate that the technique works at least up to web speeds of several 100 m/min with sub-mm lateral resolution.

The technology can be extended e.g. to multiplexing of several dyes in several components or layers, probing intrinsic material properties such as curing state with special dyes, and other intersting features.

 

 

Benefits

A 100 % monitoring of a coating homogeneity results in a new level of process stability and quality assurance. Problems in the coating process, defects etc. can be detected within a short period of time. A better control over the coating process can improve the quality of coatings and impregnations. This approach helps to avoid failure, saves costs, and improves customer satisfaction.

Beyond the monitoring, the data can be used to control the actual layer preparation process and to make it more precise. This opens up new opportunities in process optimization. For example, the coating thickness can be adjusted to the lowest possible level to assure the intended effect. This saves not only the costs for the coating but also reduces the energy demands for drying and curing.

 

Services

Our services include the design and installation of fluorescence-based in-line imaging systems including

  • the selection of suitable dyes
  • design of the optical system
  • feasibility studies
  • finding suppliers of the components
  • installation of the system in the clients equipment
  • troubleshooting