In coatings, inks, slurries, and related industries, fineness is a core technical parameter for evaluating the degree of dispersion of solid particles in liquid media. It does not refer to the size of a single particle, but is an indicator of the size of the largest pigment or filler particle mass in the system after grinding and dispersing. The quality of fineness is directly related to the appearance, stability and all key properties of the final product.
1. Definition of fineness
Fineness, here specifically refers to grinding fineness. It describes the size of solid particle aggregates such as pigments, fillers and other solid particles contained in the liquid dispersion system such as color paint, varnish, and printing ink after a specific grinding process.
This index has a clear engineering orientation - it mainly reflects the crushing and dispersion effect of the dispersion process on the original pigment aggregates (commonly known as "pigment aggregates"). An ideal fineness value means that there are no excessive particles in the product that affect surface quality and function. Therefore, fineness is a key quality control point in the production process to check whether the grinding and dispersion process meets the standards.
It needs to be distinguished from the concept of "particle size" or "particle size distribution" in the basic chemical industry, which describes the overall thickness of powder feedstocks. The latter is usually measured using equipment such as laser particle size meters, which reflect the statistical distribution of all particles; The former focuses more on the upper limit of the scale of the largest particles remaining in the system after processing.
2. Core test method - scraper fineness meter method
The standard test method commonly used in the industry is the scraper fineness meter method, which is ISO 1524 and corresponds to the Chinese national standard GB/T 1724-2019 (replacing the earlier GB/T 6753.1-2007).
Method principle
This method uses a single- or double-groove precision metal plate (scraper fine meter) with a wedge-shaped groove on the surface of the surface. During the test, an appropriate amount of sample is dropped on the deepest end of the trench, and a special scraper is used to scrape the surface at a constant speed and pressure, so that the sample is flat in the trench. After a few seconds, observe where the particles appear in the trench under the specified lighting conditions. The scale value (usually in microns, μm) corresponding to this position is the grinding fineness of the sample.
Interpretation of the results
This reading represents the approximate size of the "largest particle" in the sample. It is not an average, but an upper indication of the particle size of the product. The smaller the reading, the more thoroughly the grinding dispersion and the finer the texture of the product.
Key points and specification selection
The standard specifies that a 100 μm fineness plate is recommended for routine testing. For products with higher fineness requirements, 50 μm or 25 μm fineness plates can be used, but the standard clearly states that the latter two need to be operated by skilled laboratory personnel for reliable results.
For readings less than 10 μm, the standard recommends that special caution should be exercised.
The scraper fineness meter itself is a common laboratory instrument, which is also reflected in official procurement, and its parameters such as range, index value, material, etc. are clearly specified.
3. The fine-grained control logic and influence of different industries
Although the test principles are similar, the control logic for fineness in different industries is based on the performance requirements of their end products.
Coatings industry
The fineness directly affects the appearance and protective function of the paint film. Excessive particles can lead to rough and reduced gloss on the paint film's surface, and may become weak points in the paint film, affecting its corrosion resistance and durability. For topcoats, especially high gloss or clear varnishes, the fineness requirements are extremely strict, usually below 20 μm or even lower.
Ink industry
In printing inks, fineness is the lifeline that determines print quality and efficiency. Insufficient ink fineness will directly lead to unclear, unfull, and even scratched prints. For high-speed printing, fine particles can ensure good ink fluidity and transferability; At the same time, the fineness also affects the coloring, gloss and drying performance of the ink.
Electronic paste and special industries
In electronic pastes (such as conductive pastes, resistance pastes), fineness control rises to the height that determines the electrical performance and reliability of products. For example, improving the fineness of tungsten slurry can significantly reduce its resistivity and improve its conductivity. At the same time, fineness affects the sintering density of the slurry, the resolution of printing lines, and the sharpness of edges, which is the key to ensuring the miniaturization and high performance of electronic products.
Other related industries
cement industry- Its "fineness" is often expressed as the specific surface area (m²/kg) or sieve allowance, and the control goal is to optimize the hydration reaction speed and final strength, which is different from the "grinding fineness" purpose of controlling particle agglomeration.
Textile fiber industry——Its "fineness" refers to the diameter or linear density of the fiber itself, and the test method mostly uses optical projection method or airflow meter method, which belongs to a different category from the fineness of the dispersion system described in this paper.
4. Key factors and control ideas affecting fineness
Unqualified fineness, the essence is insufficient dispersion, and its root cause usually comes from the following aspects:
material factors
The original particle size and aggregate hardness of the pigment/filler itself.
Compatibility of pigments with resins (binders).
The type and dosage of dispersion additives in the formula.
Process and equipment
Types of grinding equipment: such as three-roller machines, sanding machines, ball mills, etc., different types of equipment produce different shear forces. For example, the three-roller machine generates high shear forces through the difference in speed and pressure between the rollers, which can effectively depolymerize hard particles.
Grinding process parameters: including grinding time, media filling rate, equipment speed, operating temperature and the number of grinding passes.
Pre-dispersion effect: whether the pre-mixing process is sufficient before entering the high-efficiency grinding equipment.
Process control
Fineness should be used as an online quality control point to be sampled and tested regularly during the grinding process. For example, the initial fineness can be recorded and retested after each grinding time or pass until the fineness reaches a predetermined standard and stabilizes, thereby scientifically determining the grinding process parameters.
5. Summary
In summary, fineness is a core quality yardstick throughout the manufacturing process of high value-added dispersed products such as coatings and inks. It is quantitatively monitored by a standardized scraper method, and its values are closely related to the performance and reliability of the final product. A deep understanding of the technical connotation of fineness, strict adherence to standards for testing, and systematic collaborative control from the perspective of formula, process and equipment are the solid foundation for achieving product quality and performance consistency.