In the coatings, inks and printing coatings industry, wet film thickness refers to the physical thickness of the liquid coating in the initial stage after application and before curing. Unlike dry film thickness, which characterizes final performance, wet film thickness is a dynamic critical process parameter for real-time monitoring of the construction process. Its core value is that through the real-time measurement and control of wet film thickness, it can directly predict and ensure whether the final dry film thickness meets the design specifications, so as to achieve upfront and accurate management of coating performance, material consumption and production costs.
1. Definition of wet film thickness
Wet film thickness refers to the geometric thickness of the coating material after it is applied to the substrate in liquid form before the curing process begins. It is directly determined by the construction process and material rheological properties, and is a process variable.
In engineering practice, wet film thickness measurement is mainly used for immediate guidance and deviation correction during construction. By quickly measuring the newly formed wet film on site, operators can immediately determine whether the current construction parameters (such as spray pressure, scraper clearance, roll speed, etc.) are appropriate and adjust them in time to avoid problems such as poor masking, sagging, incomplete curing, or cost overruns caused by a single coat that is too thin or too thick.
The conversion relationship between wet film thickness and dry film thickness is central to understanding its importance. The relationship between the two is determined by the volumetric solids of the coating, which can be obtained from the technical data of the paint manufacturer. Its basic calculation formula is:
Wet film thickness = dry film thickness / percentage of solids by coating volume
For example, if the thickness of the dry film is required to reach 150 microns, and the volume solids of the paint used is 80%, the theoretical value of the wet film thickness that needs to be controlled during construction should be 187.5 microns. If diluent is added to the construction, it needs to be corrected in the calculation. This relationship clearly shows that wet film thickness is the direct input variable to achieve the target dry film thickness.
2. Measurement method of wet film thickness
The international standard ISO 2808 and its equivalent national standard GB/T 13452.2 specify in detail a variety of film thickness measurement methods, including wet film thickness. For the measurement of wet film thickness, it is mainly divided into contact mechanical measurement method and non-contact measurement method.
1. Contact mechanical measurement method
This method is currently the most widely used and low-cost technology in the field, and its principle is that one part of the measuring instrument is in contact with the substrate under the wet film, and the other part touches the surface of the wet film, and the height difference between the two is the thickness of the wet film. Commonly used instruments include wet film wheel gauges (roller type) and wet film tooth gauges (comb tooth type). Among them, wet film tooth gauges are the most common on construction sites because they are very easy to operate. According to national metrology specifications, JJF1484 the measurement range of such tools usually covers (5~100) μm, and their calibration is clearly specified. It should be noted that the mechanical contact method may cause systematic errors due to pigment particle retention, and the surface of the substrate is required to be relatively flat.
2. Non-contact measurement method
In response to the need for high-precision, non-destructive, or special coatings such as uncured photoresist coatings, non-contact technologies have been developed, such as photothermal (ATO). These methods do not require contact with the wet film surface and enable continuous, real-time online monitoring, especially suitable for precision coating industries, such as slit coating where wet film thickness is less than 15 microns.
| Measurement method type | Main features and typical application scenarios |
| Contact mechanical measurement method (wet film tooth gauge/wheel gauge) | Easy to operate and low cost, suitable for rapid inspection and process adjustment on the construction site. It is the most common method at present-1-5. |
| Non-contact measurement methods (e.g. photothermal methods) | Non-destructive, online real-time monitoring for high-precision coatings (e.g., electronic, optical films) or wet film surfaces where contact is not allowed-1-3. |
3. Key factors affecting the thickness of the wet film
The formation and stability of wet film thickness do not exist in isolation, but are affected by the coupling of material properties, construction conditions and substrate conditions.
1. Material properties – the rheological properties of the coating/ink
The viscosity of a coating is the primary intrinsic factor that determines the thickness of the wet film it can form and maintain under different construction conditions. Studies have shown that in order to obtain a coating with a specified thickness and good workability, the range of matching combinations of the viscosity of the coating with the chosen application tool (e.g., roller, scraper type) can be very limited.
2. Construction process parameters
Construction tools and process settings are external means to directly control the thickness of the wet film. For example, in scraping, a small adjustment of the scraper gap can directly change the thickness of the wet film; In spraying, the gun aperture, movement speed, and pressure together determine the deposition rate. A study on polyurethane waterproof coatings showed that the choice of coating tool had a significant impact on the accuracy and consistency of film thickness.
3. Substrate status
The surface roughness, flatness, porosity, and whether it is in a vertical or horizontal position of the substrate can affect the leveling, penetration, and final wet film uniformity of the liquid coating. A rough or porous substrate can absorb more of the paint liquid component, resulting in a thinner local wet film thickness.
4. Key points of application of wet film thickness in different industrial fields
Different industries focus on wet film thickness control differently based on the requirements of their final product performance.
Protective coating(e.g. steel structure, ship)
This field has strict anti-corrosion contract specifications for dry film thickness. Therefore, wet film thickness measurement is the core means of "process control" during the construction process to ensure that each coating can achieve the minimum wet film thickness required for calculation, so as to achieve the target dry film thickness and anti-corrosion life at one time.
Precision coating and printing industry(e.g., functional films, printed electronics)
This area often pursues extremely thin and uniform coatings. For example, in microgravure coating, precise control is required to obtain wet film thicknesses ranging from sub-microns to tens of microns. Small fluctuations in wet film thickness can directly affect the conductive, optical or barrier properties of products, so high-precision online non-contact monitoring systems are often used.
Inks and textile coatings
For functional coatings, the initial wet film thickness (gluing thickness) is the fundamental variable that determines the final physical properties of the finished product, such as water pressure resistance, moisture permeability, and mechanical strength. Studies have confirmed that the adjustment of wet film thickness systematically changes the thickness and functionality of the finished film.
In summary, wet film thickness is a pivotal parameter that connects the coating formulation, construction process and final coating performance. From simple on-site gauge measurements to complex online optical monitoring, the precise control of them reflects the modern industrial quality control concept from "result inspection" to "process prevention". An in-depth understanding of the connotation, measurement method and influencing factors of wet film thickness is of great significance for achieving designable, predictable and reproducible coating properties.