What Does Film Thickness Mean?
Film thickness is the depth of the coating applied. Film thickness can be broadly classified as:
- Wet film thickness - Wet film thickness (WFT) is the thickness of wet paint or any coating film that is liquid-based. Wet thickness is at its peak right after the application of the coating. It decreases once volatile and solvents from the coating film undergo evaporation. While measuring wet film thickness, it is necessary to identify the amount of material that needs to be applied to achieve a particular dry film thickness that will give the best protection against damage, wear and corrosion.
- Dry film thickness - Dry film thickness (DFT) is the thickness of a coating as measured above the substrate after the coating has dried. This can consist of a single layer or multiple layers. DFT is measured for cured coatings (after the coating dries). The thickness of a coating depends on the application and type of process employed. Proper thickness should be determined by recommended coating system parameters.
Corrosionpedia Explains Film Thickness
The measurement of the wet film thickness of paints or coatings applied on materials can be very beneficial to determine the appropriate thickness of dry coating. Almost all top performing and protective coatings should meet specific requirements for dry film thickness. Therefore, a direct relation exists between wet film and dry film thickness.
The dry film and wet film ratio is identified by the volume of volatiles present in the applied coating as well as permitted thinning. In flat coatings, the wet film thickness is usually lower than the dry film thickness. Taking a wet film thickness measurement at the point of application is ideal, as it allows film adjustments and corrections at the point of application. Making a correction once the film has undergone chemical curing or drying takes substantial time and labor, and could result in film contamination. Moreover, it may result in problems like reduced integrity and adhesion of the coating.
In order to measure wet film thickness, various gauges are used. The most common are:
- Notched gauges - Suitable for non-even surfaces such as rough concrete blocks. They are also ideal for the field and shop, especially in identifying the estimated wet film thickness of commercial articles that have shape and size not suitable for other gauge types like thin edges, ellipses and others.
- Pfund and interchemical gauges - These gauges are appropriate for uniform surfaces and can provide sensitive and accurate measurements of wet film thickness compared to notched gauges.
DFT is a parameter that is considered during quality control or inspection processes. Durability of a coating is function of DFT, whereas the thickness of an individual coating layer in a coating system and the total system DFT reported specifies the coating's durability.
Types of DFT gauges include:
- Magnetic pull-off (Type I)
- Constant pressure probe (Type II)
- Electronic magnetic flux
- Eddy current gauge
Standards concerning DFT measurements include:
- ISO 2808
- BS 3900 Part C5
- BS 5494
- ASTM D1005
- SIS 184160