It is important to determine the properties of natural industrial minerals that are widely used for filling purposes in the paint industry. Particularly, properties such as particle size and distribution, color properties, oil absorption, density, surface area, and abrasiveness are important.
In this study, some technical information that we think will be useful for practitioners is presented regarding the test methods and devices used to determine these features.
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 the determination of particle size. A sieve assembly consists of a series of sieves of increasing aperture size and the sample is placed on the top sieve.
The sieve set is placed in the shaking device and the sieve set is vibrated for a certain time. As a result, the particles are dispersed (fractions) according to their size and mesh size of the sieves. Particle size analysis are usually performed using standard laboratory sieves with large sizes up to 38 microns.
The reason for this is that the method is very simple and inexpensive, and the material can be easily separated into its fractions. Sieve analysis is carried out (constant mass) until the sample mass no longer changes in the respective sieves.
Each sieve is weighed individually, the volume of each fraction is calculated in wt % and a distribution by mass is provided.
The steps of sieve analysis are:
• Start weighing,
• 5 -10 minutes elimination,
• Back weighing,
• Sieve cleaning.
Common errors encountered in sieve analysis are:
• Overloading sieves (blocking sieve holes),
• Old, worn, or damaged sieves or
• Data transfer errors.
It should also be taken into account that the aperture dimensions of the new sieves in accordance with the standards are subject to some tolerances. For example, a deviation of approximately ± 30 microns from the mean true aperture size of 1 mm sieves is allowed.
For a 100-micron sieve, this range is ± 5 microns (i.e. average actual aperture size is between 95-105 microns). On the other hand; the main problem in particle size analysis; the analysis of the dimensions that cannot be reached with standard laboratory sieves will be made with the existing methods.
Although the existing methods give reproducible results within themselves, there are some differences between them.
1.2. Dynamic Image Analysis
The Dynamic Image Analysis (DIA) technique involves a camera system in front of an illuminated background and a stream of particles passing in front of the camera. The system measures free-falling particles and suspensions and also demonstrates the air pressure dispersion of particles that tend to agglomerate.
Modern DIA systems analyse over three hundred images per second in real-time, detecting millions of individual particles in just a few minutes. This performance is based on fast cameras, bright light sources, short exposure times, and powerful software.
Unlike sieve analysis, DIA measures particles in a completely random fashion. Size and shape parameters are determined based on particle images.
The parameters defining the particle shape are:
• Sphericity,
• Symmetry,
• Convexity and
• Aspect ratio.
An important feature of DIA is its extremely high detection
sensitivity for coarse particles.
1.3. Laser Light Scattering
The laser diffraction technique, also known as Laser Light Scattering (SLS), is the most used method among fine particle measurement techniques in recent years. This method performs (indirectly) dimensional analysis on the estimation of the scattering behavior of the laser light incident on the particle.
The advantages of the method are:
• Wide dynamic measurement range,
• Flexible dispersion options,
• Measurement speed,
• Repeatability.
Speaking of diffraction, there are 2 types of terms:
• Diffraction angle,
• Diffraction pattern.
The former is dependent on particle size while the latter
is dependent on particle size distribution (PSD).
Particle size in the static laser light analysis method; is measured indirectly by detecting the intensity distributions of the laser light scattered by the particles. This method simply means that large (large) particles scatter light at small (low) angles, while small particles produce wide-angle scattering patterns.
Large particles produce a fairly sharp intensity distribution with distinct maximums and minimums at defined angles, while the light scattering pattern of small particles becomes more and more dispersed and the overall intensity decreases.
Multidimensional particles are particularly difficult to measure in particles with different dispersion because the individual luminescent signals of the particles are matched to each other.
Static Laser Light Scattering (SLS) is an indirect method that calculates particle size distributions on the basis of scattered light patterns created by particles.
Algorithms; the optical properties of particles such as sphericality, refractive index (RI), and absorption index (AI) are based on MIE theory. The biggest advantage of SLS is its wide measuring range.
The result obtained with SLS roughly corresponds to the X-area parameter (diameter of the equivalent circle). The various particle sizes measured are all associated with spherically shaped particles. Therefore, SLS always yields larger size distributions than image analysis.
Laser diffraction creates volumetric particle size distributions that allow the determination of a wide variety of parameters. For example, (Dv10) can be used to detect fine particles in the dispersion, while (Dv90) helps detect coarse particles in the dispersion.
There are two different optical theories (approaches), Fraunhofer and Mie theory, for calculating the particle size distribution by laser diffraction method:
• In the Fraunhofer approach, it is assumed that all particles are much larger than the wavelength of the rays (d > λ) and are in the form of an opaque two-dimensional circular ring.
• In Mie’s theory, it is accepted that all grains are transparent and spherical, and the difference between the deflection indices of the grains and their environment is small.
In a study carried out on micronized calcite products, it has been concluded that in the measurements made according to Mie and Fraunhofer’s theories, different results are obtained as the finer dimensions are approached and that the values of both theories approach each other as the sample to be measured gets larger.
For this reason, since almost all of the particle size distribution of micronized calcite products are made using devices based on the laser diffraction method, it seems that it would be healthier to make the measurements based on the “Mie theory”.
Especially after the publication of the ISO 13320 standard, an international standard for particle size analysis by laser diffraction was introduced, thus demonstrating that this technique is completely acceptable.
This standard states that accurate results can be obtained for particles larger than 50 microns with the Fraunhofer method and the Mie theory can be used for measurements with smaller particle distribution.
Mie theory, especially with the rapid development in computer (computation) technology; can reveal the difference in light diffraction resulting from the refractive index difference, the relative transparency of the particles, and the absorption coefficient differences.
2. Total Surface Area
It is also expressed as specific surface area. It is expressed as the surface amount in certain weight and volume (cm2 / g or m2 / g) of a product obtained as a result of size reduction processes such as crushing and grinding.
Today, different parameters such as particle size, particle shape, liberation size, and specific surface area are used to define the fragmented material. It is generally preferred to give the particle size distribution together with the specific surface area in scientific studies.
Specific surface area; particle size is a useful measure of characterization and roughness. For this purpose, the BET (Brunauer, Emmet, and Teller) device is widely used for surface area measurements, nano, and macropore (pore) size, and pore size distribution analyses in powder or bulk samples.
The low-temperature gas adsorbing technique provides the standard for measuring the total surface area of powders or porous materials. Measuring the surface area with the gas adsorbing method is basically based on measuring the amount of gas required for the gas molecules to form a monolayer on the sample surface to be measured.
Unbalanced forces between molecules at the boundary surface of a solid or liquid cause a concentration change.
3. Corrosiveness
The wear potential of filler minerals depends on the 3 properties of the mineral:
• Particle structure,
• Particle fineness-size,
• Hardness (Mohs).
As the fineness of the filler increases, the wear potential decreases. The most modern method for detecting the wear potential of fillers and pigments is the Einlehner AT 1000 / 2000 abrasion tester.
This device works using a cylindrical ceramic body with special slots and wire made of synthetic filament.
The material, which is put into the chamber as pulp at a certain solid ratio (such as 15%) is realized by measuring the weight loss under the sieve (mg loss / 100,000 revolutions) after a certain number of rotations of the device.
4. Whiteness
Today, modern color measurement, and color specification, are based on the CIE (International Commission on Illumination) system. This system was created in 1931, although new additions and corrections have been made since that date without any change in the basic structure and principles.
The CIE system is based on experimental observations rather than theories of color perception.
In color measurement, the light source, observer, and surface should always be considered [11]. Although X, Y, and Z tristimulus values can express the color numerically, they do not provide information about the color.
In order to make a more easily understandable definition of color, in 1976 CIE defined a system called the CIE Lab system, which has three coordinates as L*, a*, and b* calculated from X, Y, and Z tristimulus values. The “*” sign in these parameters is used to distinguish CIE formulas from similar formulas in different color systems that were developed before.
In 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 denoted by L: Here; L* is on the black-white (L*=0 for black, L*=100 for white) axis, a* is on the red-green (positive value is red, a negative value is green) axis, b* is yellow-blue (positive value is yellow, a negative value is green).value is located on the blue) axis.
Along with these, the brightness value (RY), which can also be called luminance or reflectance, can be obtained from the whiteness measurement results.
5. Density
Calculation of bulk quantity is an important issue for many technical or commercial reasons such as storage, packaging, and transportation of materials. The density of the part systems that make up a heap is called the bulk density.
It can also be called compressed density. In other words, it is the apparent density of the layer of mineral powder formed by vibration or compression. It is the mass per unit volume of the heap of mineral grains, including the grains and the spaces between them.
The bulk density is related to the particle type and void ratio of the particles forming the heap. Besides porosity, the shape and size of the grains are also important in calculating the bulk density.
The bulk densities of two different materials of the same weight and characteristics, coarse and fine, are different from each other. This is due to the change (increase or decrease) in the particle size, causing a change in the void volume between the parts.
The actual density measurement is carried out in devices also called helium pycnometers. This device finds volume and true density using Archimedes’ principle of fluid overflow and Boyle’s Law.
The gas used in the pycnometer is helium, which is preferred because helium exhibits ideal gas behavior and is an inert gas that can enter even the smallest pores of the sample to be analysed.
6. Dope Oil Absorption
Dope oil absorption standard TS 2583 EN ISO 787-5: General test methods for pigments and fillers Part 5-Determination of oil absorption value. The high oil absorption value of filler, for example, indicates that more resin must be used to form a composite, thereby increasing the cost of composite manufacturing.
In addition, this creates relatively high-viscosity compounds and dispersions that make it difficult to obtain high loadings.
Result
In paint applications, it is possible to determine the properties of filler minerals, as well as the selection of appropriate mineral fillers, by various analysis methods.
Performing or having these tests performed in a standard and reliable manner will also activate the function of the filler in the paint.
References
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Prof. Dr. Öner Yusuf Toraman
Niğde Ömer Halisdemir University
Mining Engineering Department
Ahmad Farid Ehsan
Niğde Ömer Halisdemir University
Graduate School of Natural and Applied Science
