Some Tests Applied to Filler Minerals

Paints and coatings sector – determining the properties of natural industrial minerals commonly used as fillers is important. Particularly particle size and distribution, color properties, oil absorption, density, surface area and abrasiveness stand out as key characteristics. This study presents technical information that practitioners will find useful regarding the test methods and equipment used to determine these properties.

1. Particle Size Analysis

The most commonly used methods for determining particle size are: • Sieve Shaking System, • Dynamic Image Analysis (DIA), • Laser Light Scattering (Laser Diffraction).

1.1. Sieve Analysis

Sieve analysis is the most widely used method for determining particle size. A sieve set consists of a series of sieves with increasing aperture sizes, and the sample is placed on the topmost sieve. The sieve set is placed in a shaking device and vibrated for a specific period. As a result, particles are distributed according to their size and the mesh apertures of the sieves (fractions). Particle size analyses are generally performed on coarse sizes down to 38 microns using standard laboratory sieves. This is because the method is simple and inexpensive, and the material can be easily separated into fractions. Sieve analysis is performed until the residual sample mass on the relevant sieves no longer changes (constant mass). Each sieve is weighed individually, the volume of each fraction is calculated as a percentage by weight, and a mass-related distribution is obtained [38].

The steps of sieve analysis are as follows:

• Initial weighing, • Sieving for 5-10 minutes, • Re-weighing, • Sieve cleaning.

Common errors encountered in sieve analysis are:

• Overloading of sieves (blocking of sieve apertures), • Worn, damaged or defective sieves, or • Data entry errors. It should also be noted that the aperture sizes of new sieves conforming to standards are subject to certain tolerances. For example, 1 mm sieves are permitted an average actual aperture size deviation of approximately ± 30 microns. For a 100 micron sieve, this range is ± 5 microns (meaning the average actual aperture size is between 95-105 microns). On the other hand, the main problem in particle size analysis is determining which of the existing methods should be used to analyze sizes that cannot be measured with standard laboratory sieves. Although the existing methods produce repeatable results within themselves, there are some differences between them.

1.2. Dynamic Image Analysis

Dynamic Image Analysis (DIA) technique consists of a camera system in front of an illuminated background and a flow of particles passing in front of the camera. The system measures freely falling particles and suspensions, and simultaneously demonstrates the dispersion of particles tending to agglomerate by air pressure. Modern DIA systems analyze more than three hundred images per second in real time, detecting millions of individual particles within just a few minutes. This performance is based on high-speed cameras, bright light sources, short exposure times, and powerful software. Unlike sieve analysis, DIA measures particles in a completely random manner. Size and shape parameters are determined based on particle images.

The parameters that define particle shape are:

• Sphericity, • Symmetry, • Convexity, and • Aspect ratio. An important feature of DIA is its extremely high detection sensitivity for coarse particles [46].

1.3. Laser Light Scattering

Among fine particle measurement techniques, the method most frequently used in recent years is laser diffraction, also referred to as Laser Light Scattering (SLS). This method performs size analysis (indirectly) based on estimating the behavior of the laser light incident on the particle in scattering.

The advantages of the method are as follows:

• Wide dynamic measurement range, • Flexible dispersion options, • Measurement speed, • Repeatability.

When discussing diffraction, 2 types of terms stand out:

• Diffraction angle, • Diffraction pattern. The first depends on particle size, while the second depends on particle size distribution (PSD). In static laser light analysis methodology, particle size is measured indirectly by detecting the intensity distributions of laser light scattered by particles. This method is simply based on the principle that large (coarse) particles scatter light at small angles, while small particles produce wide-angle scattering patterns. Large particles produce a fairly sharp intensity distribution with marked maxima and minima at defined angles, while the light scattering pattern of small particles gradually becomes dispersed and the overall intensity decreases. Due to the matching of individual light-scattering signals from particles, measurement of polydisperse particles with different size distributions is particularly difficult. Static Laser Light Scattering (SLS) is an indirect method that calculates particle size distributions based on the scattered light patterns produced by particles. The algorithms are based on the assumption that particles are spherical, with optical properties such as refractive index (RI) and absorption index (AI) based on MIE theory. The greatest advantage of SLS is its wide measurement range. The result obtained with SLS roughly corresponds to the X-area parameter (equivalent circle diameter). All measured particle sizes are related to spherically shaped particles. For this reason, SLS always gives wider size distributions than image analysis. Laser diffraction creates volumetric particle size distributions that allow for the determination of a wide variety of parameters. For example, (Dv10) can be used to detect fine particles in the distribution, while (Dv90) helps to detect coarse particles in the distribution. For calculation of particle size distribution using the laser diffraction method, there are two different optical theories (approaches): Fraunhofer and Mie theory: • In the Fraunhofer approach, all particles are assumed to be much larger than the wavelength of the rays (d > λ) and to be opaque two-dimensional circular ring-shaped. • In Mie theory, all particles are assumed to be transparent and spherical in shape, and the difference between the refractive indices of particles and the medium in which they are located is assumed to be small [5]. In a study conducted on micronized calcite products, it was found that different results were obtained as smaller sizes were approached when measurements were made according to Mie and Fraunhofer theories, and that the values of both theories approached each other as the sample size increased. For this reason, since micronized calcite product particle size distributions are nowadays carried out almost entirely using equipment based on the laser diffraction method, it appears that performing measurements based on "Mie theory" would be more appropriate. Particularly after the publication of ISO 13320 standard [37], an international standard has been established for particle size analysis by laser diffraction, thereby demonstrating that this technique is completely acceptable. This standard states that accurate results can be obtained with the Fraunhofer method for particles larger than 50 microns, and Mie theory can be used for measurements with smaller particle distributions. Mie theory, particularly with rapid advances in computer (computational) technology, can demonstrate the difference between light diffraction resulting from differences in refractive index, the relative transparency of particles, and differences in absorption coefficients.

2. Total Surface Area

Also referred to as specific surface area. It is expressed as the amount of surface in a given weight and volume of a product obtained as a result of size reduction operations such as crushing and grinding (cm² / g or m² / g). Today, various parameters such as particle size, particle shape, liberation size and specific surface area are used in characterizing particulate materials. In scientific studies, it is generally preferred to present particle size distribution together with specific surface area. Specific surface area is a useful measure of particle size, characterization and roughness. For this purpose, BET (Brunauer, Emmet and Teller) equipment is widely used in surface area measurements on powder or bulk samples, and in nano and macropore (pore) size and pore size distribution analyses. The standard for total surface area measurements of powders or porous materials is provided by low-temperature gas adsorption technique. Surface area measurement by gas adsorption method is based fundamentally on measuring the amount of gas required to form a monolayer of gas molecules on the surface of the sample to be measured. At the interface between a solid or liquid and its surroundings, the imbalance of intermolecular forces results in concentration changes.

3. Abrasiveness

The abrasion potential of filler minerals depends on three properties possessed by the mineral: • Particle structure, • Particle fineness – size, • Hardness (Mohs). As the fineness of the filler increases, the abrasion potential decreases. The most modern method for determining the abrasion potential of fillers and pigments is the Einlehner AT 1000 / 2000 abrasion test apparatus. This equipment operates using special sockets made of synthetic flame and a cylindrical ceramic body with wire. The material, placed as a pulp at a specified solids percentage (such as 15%), in the reservoir is measured as weight loss below the sieve (mg loss / 100,000 revolutions) after a specified number of rotations of the equipment.

4. Whiteness

Modern color measurement today is based on the CIE (International Commission on Illumination) system. This system was created in 1931, yet despite this, new additions and corrections have been made since that date without changes to the basic structure and principles. The CIE system is based on experimental observations rather than color perception theories. In color measurement, the light source, observer, and surface must always be considered. X, Y and Z tristimulus values, while able to express color numerically, do not provide information about color. To provide a more easily understood definition of color, in 1976 the CIE defined a system called the CIE Lab system with three coordinates calculated from X, Y and Z tristimulus values in the form of L*, a* and b*. The "*" symbol in these parameters is used to distinguish CIE formulas from similar formulas in other previously developed color systems. In the CIE L*a*b* color system, differences in colors and their locations are determined according to L*, a*, b* color coordinates. The "whiteness value," which can also be expressed as Lightness, is represented by L: Here, L* is on the black-white axis (L*=0 for black, L*=100 for white), a* is on the red-green axis (positive value for red, negative value for green), and b* is on the yellow-blue axis (positive value for yellow, negative value for blue). In addition, the brightness value (also called luminance or reflectance) (Ry) can also be obtained from whiteness measurement results.

5. Density

For many technical or commercial reasons such as storage, packaging and transportation of materials, the calculation of bulk quantity is an important matter. The density of the particle systems that make up a bulk is called bulk density. It can also be referred to as compacted density. That is, the apparent density of the layer of mineral powder formed by vibration or compression. The mass per unit volume of the bulk made up of mineral particles, including the particles themselves and the voids between them. Bulk density is related to the particle type and void fraction of the bulk. In calculating bulk density, in addition to porosity, the shape and size of the particles are also important. Two separate materials of coarse and fine size with the same weight and properties have different bulk densities. This is because changes in particle size (increase or decrease) cause changes in the volume of voids between particles. True density measurement is performed in equipment also called helium pycnometry. This equipment uses Archimedes' fluid displacement principle and Boyle's Law to find volume and true density. The use of helium as the gas in pycnometry is preferred because helium exhibits ideal gas behavior and is an inert gas that can penetrate even the smallest pores of the sample being analyzed.

6. DBP Oil Absorption

The DBP oil absorption standard is TS 2583 EN ISO 787-5: General test methods for pigments and fillers Part 5 – Determination of oil absorption value. A high oil absorption value of a filler indicates that, for example, more resin is required to create a composite, and therefore composite production costs will increase. Furthermore, this situation creates dispersions with relatively high viscosity and high loadings that are difficult to achieve.

Conclusion

In paint applications, in addition to the selection of appropriate mineral fillers, the determination of the properties of these minerals is possible through various analysis methods. Performing or having these tests conducted in a standard and reliable manner will also enhance the function of the filler within the paint. Prof. Dr. Öner Yusuf Toraman Niğde Ömer Halisdemir University Department of Mining Engineering Ahmad Farid Ehsan Niğde Ömer Halisdemir University Graduate School of Natural and Applied Sciences

References

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