In the construction process chain of coatings and inks, polishing is a specific technical term that refers to the degree of ease that the coating can be mechanically ground in the middle stage of the curing process. It does not evaluate the performance of the final coating, but quantifies the process cost and operability of the final coating to achieve the ideal surface condition for subsequent coating when it is used as an "intermediate layer". This property has clear practical value for systems that require multiple layers of coating, such as wood paints, industrial primers, and various types of putty.
1. Definition of polishing
Polishing specifically refers to the paint film or putty layer, which is polished with abrasives such as sandpaper after curing to the specified state, so as to produce a flat and rough matte surface.
Its engineering value is rooted in the inherent requirements of multi-layer coating systems. During the paint application process, almost every coat except the last coat needs to be properly sand. The core purpose of this process is twofold: the first is to completely eliminate defects, particles or poor leveling on the coating surface, and obtain a physically flat base surface; Second, and more critical, it is possible to produce a uniform, fine rough surface through sanding, which greatly increases the effective contact area between coatings and mechanical meshing, ensuring sufficient adhesion between layers (e.g. between primer and putty, putty and topcoat). Therefore, the polishing performance of the paint film is one of the important indicators to reflect the construction performance of a coating.
Ideal sanding often means a balance: a coating that is neither too hard to sand, time-consuming and abrasive, or not too soft or sticky to clog the sandpaper, sanding inefficiently and not get a clean surface.
2. Measurement methods and instruments
The evaluation of polishing needs to be based on a unified and comparable method. The Chinese national standard GB/T 1770-2008 "Determination of the polishing properties of coating and putty films" is currently a reliable test basis in China. This standard specifies an empirical test method to evaluate its grinding resistance by observing the change phenomenon of the coating surface and evaluating the difficulty of grinding after the specified number of polishing times of the test plate under the specified load.
1. Overview of the test methodology
The method described in the GB/T 1770 standard is a simulation test close to the actual construction. A dedicated grinding testing machine or equipment adapted to standard grinding heads is usually used. After the specimen is cured to the specified age, it is fixed on the testing machine so that the polishing surface faces the grinding head. During the test, a specified type of sandpaper is selected and a standardized load (weight) is applied to the grinding head to simulate the pressure of manual sanding. Subsequently, the drive grinding head performs a specified number of cycles (e.g. 50 revolutions) on the surface of the specimen. Evaluation methods include:
Qualitative evaluation: observe whether the surface after polishing is fluffed, whether there are any unpolished highlights remaining, whether the sandpaper is seriously blocked, etc.
Semi-quantitative evaluation: Some methods measure the geometric dimensions (e.g., chord length) that form wear marks after sanding and compare them with data.
2. Related instruments and equipment
Although the GB/T 1770 standard does not specify specific instrument models, abrasion/abrasion testing machines commonly used on the market are often used to perform standardized abrasive tests. For example, the Taber series abrasion testing machine is one of the widely adopted equipment. This type of equipment works by applying pressure to a standard grinding wheel (or grinding wheel) with a load weight, causing it to rotate and rub against the surface of the sample. By selecting sandpaper with different roughness (corresponding to grinding wheel models, such as CS-10, H-18, etc.) and different loads (such as 500g, 1000g), it is possible to simulate sanding conditions of different intensities. The rotational speed and load accuracy of the equipment have strict technical requirements to ensure the repeatability of the test results.
| Method/Device Type | Main features and application scenarios |
| GB/T 1770 standard method | Standardized empirical method, suitable for laboratory comparison and rating of coating film and putty film polishing performance -9. |
| Abrasion tester (e.g. Taber type) | It provides precise load and speed control, and the test results are highly repeatable, and are widely used in wear resistance and simulated grinding tests of coatings, plastics and other materials-2. |
3. Key factors affecting polishing
The polishing properties of coatings are not determined by a single factor, but are a combination of their internal chemical composition and microscopic physical structure. Research and production practice show that the following factors are the key to regulating polishing.
1. Chemical structure and cross-linking density of resin system
The nature of film-forming substances is crucial. For example, in water-based wood paints, the molecular structure design of acrylic latex has a significant impact on polishing. Studies have shown that the polishing performance can be improved by cross-linking carboxyl groups with zinc ions. However, the effect of ketonazine cross-linking on polishing was not obvious. In addition, the emulsion particles are designed with a "hard core-soft shell" structure, and by adjusting the core-shell mass ratio, the hardness and brittleness of the coating film can be effectively balanced, resulting in an easy-to-polish and dust-free surface.
2. The type and dosage of pigments, fillers and additives
The solid components in the formulation directly affect the resistance during sanding. High-hardness, large-sized fillers (e.g., quartz sand, alumina) can improve the wear resistance of coatings but may make sanding difficult. Conversely, certain functional additives, such as zinc stearate, are often used as "sanding aids" that migrate to the surface after the coating has cured, acting as a lubricating agent, making sanding smoother and sandpaper less likely to clog. In ink systems, the addition of specific polyamide powders can also improve surface smoothness and scratch resistance, which can also affect sanding behavior.
3. The curing state and physical properties of the coating
Fully cured coatings are usually hard and difficult to sand. Therefore, both the polishing test and the actual operation emphasize the "specified age", when the coating has dried and has a certain hardness, but the interior may not be fully cross-linked, and it is in the window period when brittleness is suitable for polishing. The basic physical properties of the coating, such as glass transition temperature (Tg), hardness, and toughness, together determine whether it will be crumbled into powder or plastically deformed when cut by abrasives.
4. Application points in the coating and ink industry
There are specific differences in the focus and testing methods of polishing in different segments.
Wood coatings – one of the most demanding areas for grinding. Whether water-based or solvent-based systems, primers and putty must have good sanding properties to provide the ideal base surface for the topcoat. There are many relevant standards, such as GB/T 23999 (water-based wood coatings), GB/T 23998 (solvent-based nitro wood coatings), etc., all of which list polishing as key indicators.
In the field of industrial and architectural coatings, including epoxy primers, automotive primers, etc., the abrasiveness is related to the adhesion of the interlayer and the durability of the overall coating system. The polishing of construction putty (indoor and exterior) is the core of construction, which is clearly stipulated in standards such as JG/T 298 and JG/T 157.
Inks & Specialty Coatings – For inks that need to be laminated or overprinted after printing, the surface properties of the cured film may involve similar "reprocessability". Some functional coatings, such as water-based UV-curable coatings for indoor wood products as specified in LY/T 3231, pose unique challenges to polishing due to their rapid curing properties.
Summary
All in all, polishing is a typical "process-oriented" performance indicator, which accurately acts on the intermediate link between coatings and inks from construction to film formation. Its evaluation relies on standardized empirical methods such as GB/T 1770 and precision instrument simulations. By analyzing the influencing factors such as resin cross-linking, filler addition, and curing kinetics, the formula can be designed in a targeted manner to make the coating exhibit appropriate polishing characteristics at a specific point in time. In-depth understanding and effective control of this parameter are indispensable technical links for efficient and high-quality multi-layer coating.