Rheolaser Coating: An Innovative Solution For Optimising Paints, Inks & Coatings Formulations

The use of paints, inks, and coatings across a wide range of industries has continued to grow over recent years, driven by high demand from the automotive, electronics, construction and aeronautics industries.

Both water and solvent borne formulations continue to be challenged to provide more competitive and environmentally friendly materials. Those working in the industry are often searching for new, original formulas that use more sustainable materials.

This means we need to look closely at controlling the characterisation of these innovative formulations. The main benefits of applying Formulaction's Rheolaser Coating to these processes are:

•  Formulating and reformulating biocompatible and solvent free solutions
•  Gaining precise knowledge and understanding of formulations, including drying kinetics, drying steps, etc.
• Evaluating the impact of the formulation ingredients, and film formulation parameters, i.e. Temperature, Humidity, Substrate, Film Thickness, etc.
•  Determining the optimal drying protocol time, such as time and temperature.

How was the study carried out?

The experiments were performed by Rheolaser Coating High Temperature on a bi-component epoxy adhesive at different temperatures.

The Rheolaser Coating high temperature measurement is based on an optical technology - Diffusing Wave Spectroscopy (DWS). This is where a laser illuminates the coating, and the photons travel through the coating thickness and interact with the coating's scatterers (Particles, Droplets, Polymers, etc).

The different optical paths of the photons create interfered backscattered waves forming an image on the camera composed of bright and dark spots = Speckle Image.

When the coating undergoes important microscopic dynamic activity (liquid-like), the speckle image will be fluctuating, some bright spots become dark and some dark spots become bright. The fluctuation speed is directly correlated to the scatterers motion and thus to the visco-elastic properties of the material.

Thanks to our dedicated image analysis, it is possible to determine a characteristic frequency, the microscopic dynamics (μD or mD), which directly correlates to the speckle image fluctuation.

For example, figure 2.a below shows a liquid sample drying / curing at a fixed temperature over time. A high value of μD indicates fast speckle image fluctuation, corresponding to a liquid sample (fast scatterers motion). On the other hand, a low μD value is a sign of a slower evolution of the speckle image and solid-like behaviour. Therefore, it is possible to precisely monitor curing kinetics of the coating.

For powder coatings, during curing at an elevated temperature, the coating changes from solid to liquid to solid form, thus the scatterers mobility change. A μD value decrease means that the  mobility decreases and vice-versa. Examples of these can be found further down in this post. It also possible to follow the microscopic mobility versus temperature during a temperature increase.

A low value of μD indicates low speckle image fluctuation, corresponding to a steady state of the sample. On the other hand, a peak of mobility (μD) is a sign of a faster evolution of the speckle image, generally corresponding to phase transition (figure 2.b).

What were the results of the study?

1.    Determination of the different drying steps and characteristic times
Figure 3 below shows the microscopic dynamics (mD) versus time for a 100 μm thick technical paint sample at 90 °C.

The graph shows a clear identification of the different drying steps (i.e. Evaporation, Packing, Drying) and allows us to determine the characteristic times (Open Time, Tack-Free, Dry-Hard, Dry-Through).

For this sample, at 90°C and for 100μm thickness, the evaporation stage takes 1 minute (the open time), the packing finishes after 6 minutes and the sample is completely dry after 9 minutes (dry through).

In conclusion, the instrument allows us to determine the different drying steps and characteristic times from Room Temperature up to 250°C.

2.    Determination of the influence of humidity and temperature

The instrument also allows the exploration of the differences in the drying kinetics between different formulations or at different drying conditions (e.g. Temperature, Humidity, Thickness, Substrate Porosity, etc).

Figure 4 (below) shows the drying kinetics of the same technical ink under different drying conditions. This includes the same temperature but at varying humidity (as shown by the red and blue curves) and the same humidity but at different temperatures (as shown by the blue and pink curves).

For all the experimental conditions, the instrument allows the identification of the different drying steps (i.e. Evaporation, Packing, Drying, etc.) and to determine the characteristic times, as displayed in the table inset (tc1 = Open time, tc2 = tack-free, tc3 = dry-through).

At the same temperature (30 °C), the increase of the humidity (from 30% to 60%) slows down the drying kinetics (blue and red curves) and at the same humidity (30%), the increase of the temperature (from 25°C to 40°C) accelerates the drying kinetics (blue and pink curves).

The instrument is sensitive to the temperature and humidity influence on the drying kinetics. In conclusion, this solution allows formulators to optimize the drying protocol and processes.

Due to its specific functionalities, the software also allows us to easily rank the drying kinetics of different formulations, temperatures, humidity, thicknesses, and substrates.

Therefore, the faster the microscopic dynamics evolution (mDE) increases, the faster the drying.

Figure 5 above shows the microscopic dynamics evolution (mDE) versus time for a technical ink at different humidity and different temperature. The pink curve, at 40°C and 30% humidity, shows the fastest mDE increase, so the drying is the fastest. The red curve, at 25°C and 60% humidity, shows the slowest mDE increase, so the drying is the slowest under these conditions.

Here, the variable parameters were temperature and humidity, but it is possible to vary the formulation ingredients, the thickness, and the substrate. The software also provides a quantitative information, the time ‘t90’ (seen in the red boxes, inset fig. 5).

The ‘t90’ corresponds to the time when the sample’s microscopic mobility is reduced by 90%, “90% dried”. The ‘t90’ is extremely useful for a wide range of applications where there is a need to optimize the process and determine when microscopic mobility reduces by 90%, so it is then possible to start the next processing step.

For some formulations, the ‘t90’ is associated with the ‘open time’. In conclusion, this solution allows the formulators to compare, rank and screen different formulations and drying conditions.

Conclusion

Formulaction’s Rheolaser Coating offers a new in-situ, non-invasive and efficient method to better understand different materials, allowing formulator’s to:

•    Monitor and know precisely the curing and drying kinetics
•    Determine the characteristic times of the film forming process
•    Analyse from room temperature up to 250 oC with humidity control
•    Evaluate the impact of the formulation, including the temperature, the thickness, the humidity, the substrate
•    Optimize the manufacturing protocol.

Other key benefits of this new technology include:

•    Objectivity and accuracy in monitoring the curing/drying
•    The option of performing analysis on any type of substrate
•    Enables both temperature and humidity control
•    Provides in-situ and contactless measurement
•    Sensitivity to mobility at the nanometer scale.

For more information about the Rheolaser Coating, please call us on 01442 876777 or email us at: sales@fullbrook.com.