Definition
An applicator is a laboratory tool used to prepare uniform, controllable thickness wet film coatings on the surface of a flat substrate. It is usually made of metal or plastic, with a straight rod or ring structure with precision machined notches, and the liquid sample (such as paint, ink, adhesive, etc.) is spread into a wet film of a specific thickness by mechanical scraping for subsequent drying, curing, and performance testing. The design and operation of the applicator must comply with a number of international and national standards to ensure the repeatability and comparability of coating film preparation.
Principle
The working principle of the applicator is based on mechanical scraping and fluid shearing. During operation, an appropriate amount of sample is placed at the front end of the substrate, and the applicator scrapes the sample at a constant speed and pressure, and the notched part (such as grooves or gaps) controls the thickness of the wet film remaining on the substrate. The thickness of the wet film mainly depends on the geometric depth of the notch of the applicator, and the relationship can be approximated as:
WFT = G × k
WFT is the thickness of the wet film, G is the notch depth of the applicator, and k is the correction coefficient related to the rheological properties of the material. During the coating process, the sample is sheared, and its rheological properties such as viscosity and thixotropy affect the uniformity of the final film layer.
Measurement method
The thickness of the wet film prepared by the applicator can be evaluated by combining theoretical calculations with instrumental measurements. The theoretical thickness is calculated based on the notch depth of the applicator and the volume of the material, while the actual thickness is affected by the operating conditions and material properties. Commonly used measurement methods include wet film comb gauge measurement, that is, using a toothed thickness gauge to detect the height of wet film immediately after coating; and dry film measurement, which is measured using a micrometer or film thickness gauge after the coating is fully cured. To ensure reliable results, it is often necessary to measure and average values at multiple locations, while recording parameters such as ambient temperature and humidity and coating speed.
Influencing factors
The uniformity and thickness accuracy of the coating film are affected by a variety of factors. The parameters of the applicator itself, such as notch processing accuracy, material hardness and edge flatness, directly affect the film thickness control. In terms of operating conditions, the coating speed, applied pressure and coating angle need to be stable, too fast may lead to uneven film layer, and uneven pressure will cause thickness fluctuations. Sample properties such as viscosity, leveling, thixotropy, and solids can also significantly affect film formation, and high viscosity materials may require higher scraping forces. Environmental factors such as temperature changes can cause changes in sample viscosity, and the surface flatness and cleanliness of the substrate cannot be ignored.
Applications
Coaters are widely used in industrial testing and R&D fields where the preparation of standard coatings is required. In the coatings industry, it is used to prepare samples for testing the adhesion, hardness, abrasion resistance and weather resistance of paint films. In the ink field, the applicator evaluates printability, color intensity and drying performance. The adhesive industry is used to prepare uniform adhesive layers to test bond strength. In addition, applicators are also a key sample preparation tool in laboratories in the fields of electronic materials, paper coating, textile processing, and optical films, supporting product quality control and formulation development.
Selection considerations
Choosing the right applicator requires a combination of technical parameters. First, the notch depth series of the applicator is determined based on the expected wet film thickness range, with common specifications ranging from microns to millimeters. In terms of material, the stainless steel coater has good wear resistance and is suitable for conventional samples; Plastic applicators are chemically resistant and can be used in specific chemical environments. In terms of structure, the straight rod applicator is suitable for most flat plate coatings, while the ring applicator is convenient for the preparation of circumferential coatings. When selecting a type, it is also necessary to refer to the size and accuracy requirements of relevant test standards, and consider the matching of substrate size and coating width. It is recommended to verify the suitability of the applicator through actual sample testing.