Cynthia A Gosselin, Ph.D., Director, The ChemQuest Group

The film thickness of a paint or coating plays a critical role in the performance of the final product. While formulations govern inherent characteristics, the resulting in-service functionality, aesthetics and long-term performance are highly dependent upon application film thickness. For that reason, technical data sheets specify film thickness in coordination with the resulting physical testing performance for a wide variety of products.

However, paint cost is one variable that is tightly controlled, especially in high volume coil coating; and paint is the highest line-item consumable cost when analyzing coil coating applications. As a result, there has been a decades long initiative to more tightly control film thickness throughout the entire coil to minimize costs while still guaranteeing ultimate performance.

ASTM has many specifications that govern the measurement of film thickness (Table 1). Most of those measurements are generated from samples at the end of a coil, with the expectation that if all production variables remain constant, that end line measurement represents the entire coil.  While not a bad approximation (it has, after all, worked relatively well enough for decades), this one data point at the end of the coil does not reflect any variation that may have been induced by surface texture or gauge tolerance. Rather, constant monitoring of film thickness throughout the process is the only way to ensure a completely accurate representation of film thickness. Until recently, this has been easier said than done in a dynamic high speed industrial operation.

But technology eventually comes to the rescue. In 2020, a new specification: D8331/D8331M Standard Test Method for Measurement of Film Thickness of Thin-Film Coatings by Non-Destructive Means Using Ruggedized Optical Interference was introduced in ASTM Subcommittee D01.53. This represented a new non-destructive measurement system based on ruggedized optical interference (ROI) that transforms signal outputs into coating film thicknesses using digital formulas (recipes). This technology can be used in real time during coating applications or in a laboratory on sample panels.

The term “ruggedized” is purposely used to distinguish this device as one suitable for an industrial environment, as opposed to other optical instruments that must be used in a controlled laboratory atmosphere. This is important because less robust equipment could generate inaccuracies and constant maintenance issues – and perhaps still not provide the detail and reproducibility needed for real-time measurements.

This new advancement in instant, non-contact multi-layer film thickness measurement technology (NMM) was first commercialized in 2008 by Sensory Analytics (now part of Industrial Physics) under their SpecMetrix® brand.1 Ruggedized optical interference is a patented, non-destructive thin-film measurement technology used for real-time monitoring of coatings, paint, oil, wax and other fluidized systems. Film thickness is determined by analyzing the reflection of light waves through the coating layer or layers. Measurements can be obtained on opaque, transparent, dry or wet coatings.

The newest algorithms can measure coating thickness as low as 0.25 µ on a moving strip or web. Some specialized roll-to-roll lab models can measure thickness starting at 0.3 µ. The measurement technology can be used on coils, films, foils, printed packaging, webs, flat sheets, small samples or finished parts.

Since this technique can measure wet and dry films, coaters can position one device near the coating roll, measure wet film thickness and correlate it with another device at the end of the line on the cured/dry paint. This allows for almost instantaneous correlation of wet to dry film thickness – and thereby more consistent coating weight control throughout the coil without line stops.

Ruggedized Optical Interference Method

In practice, a light beam is projected onto a coated surface. As light travels through the coating and reflects off of the substrate, the overlapping light waves form an interference pattern. The system spectrometer reads the resulting interference pattern and calculates the exact thickness of each coating layer.

In ruggedized optical interference, a measurement “recipe” is a standardized digital algorithm that translates spectral reflection data into physical film thickness. These recipes are created by pairing material-specific data with mathematical models of light interference.

Baseline optical constants such as refractive index (n) and the extinction coefficient (k) of both the coating layer and substrate are catalogued across a wide range of wavelengths. Specially developed software utilizes Fresnel equations and the Transfer Materials Method (TMM). TMM is a mathematical technique used to analyze wave propagation to simulate how light will behave. In optics, TMM evaluates the path of light using two matrices. The interface matrix calculates reflection and transmission coefficients at the boundary of two layers (Fresnel’s equations). The propagation matrix accounts for the phase shift and attenuation a wave accumulates while traveling through a single, uniform layer of thickness. Cascading these 2×2 matrices computes the total system matrix, giving rise to total reflectance and transmittance for varying wavelengths and incident angles. The model calculates constructive and destructive interference patterns based on the formula:

2nt = mλ

where t = film thickness, n = refractive index, m = interference order and λ = wavelength.

The final verified model is saved as a digital formula (the recipe). During real time measurement, the spectrometer reads the reflected light and software curve fitting or Fast Fourier Transform algorithms match live data against a recipe to calculate the thickness.

Since digital recipes are used to determine film thickness, there was initial concern from  paint companies that they would have to divulge their formulas in order to allow coil coaters to use the recipes. Paint manufacturers spend significant research and resources to formulate with specialized pigments, resins and additives to generate their specific systems. Divulging this type of information would be disadvantageous, because rivals could conceivably reverse engineer paint chemistries to gain a “less expensive” advantage.

ASTM 8331 has taken this concern into account. To ensure the protection of proprietary information and specialized chemical formulas, standard operating practices used within the specification eliminated the need for divulging formulas. Rather, the ability of the gauge to output reproducible, standardized digital recipes is aligned with physical reference standards, as opposed to specific formulary combinations. This way, there is no need to disclose any proprietary chemical information. Technology based on optical reflection and wave phase shifts masks the trade secret details of any specific formulation. The recipes can also be fine-tuned to consider a particular customer substrate/paint combination that is unique to the coater.

In theory, to date, pretreatments and conversion coatings can also be measured. However, in practice, measuring these coating weights is quite difficult. ROI technologies are designed for thicker coatings as low as 0.25µ. Pretreatments are often applied under 0.5µ (and are usually reported as coating weights as opposed to thicknesses) and may not create a distinct optical interference pattern different enough from the substrate. In addition, if the surface roughness profile is heavy or rough, the optical signal can scatter, making it hard for sensors to calculate a distinct layer measurement. It may be necessary to develop an in-line wet film measurement configuration to accommodate many pretreatments and conversion coatings on particular substrates and place another device directly after the pretreatment section of a paint line.

The standard has now undergone its first five-year review. Coaters that have experience using and fine-tuning the technique weighed in and made the standard even more useful. Libraries of recipes have been generated for a wide range of coating/substrate combinations. The final frontier will be the ability to measure, in real time, pretreatments and conversion coatings.

Table 1: Comparison of Methods for Measuring Organic Film Thickness

Feature / Capability Ruggedized Optical Interference XRF DJH Bore Gauge Eddy Current/ Magnetic Tooke Gauge Ultrasonic Gravimetric (Weigh-Strip-Weigh)
ASTM Specification D8331 B568 D5796 D7091 D4138 D6132 None
In line/ real time Yes (up to 150 measurements/sec) Yes No No No No No
Non-contact Yes Yes No No No No No
Wet Film Yes No No No No No No
Dry Film Yes Yes Yes Yes Yes Yes Yes
Multi-layer Yes No Yes No Yes No No
Substrate Independence High Medium Medium Low Medium Medium High

References

1SpecMetrix technical presentation