How to make the average particle size of superfine calcium carbonate primary particles reach 0.02～0.1μm is the basic index of superfine calcium carbonate, which is of course very important, otherwise it will not be called superfine calcium carbonate.
But strictly speaking, this only completes half of the task of producing ultrafine calcium carbonate. If the surface treatment technology and dispersion technology cannot keep up, the generated primary particles will agglomerate into large secondary particles with a particle size of several hundred nanometers.
Therefore, under the transmission electron microscope (TEM), the photos observed or taken can only reflect the size of the primary particles, and cannot reflect the actual size of the secondary particles after agglomeration (because certain techniques have been made during the sample preparation process). Treatment), the ultrafine calcium carbonate with serious agglomeration has a relatively small specific surface area measured by BET.
Therefore, only by using transmission electron microscope observation and specific surface area measurement to cooperate with each other, can we get a more scientific and comprehensive judgment on the fineness of ultrafine calcium carbonate, the crystal shape and dispersion of the particles.
The average particle size of superfine calcium carbonate is intrinsically related to its specific surface area. Some researchers use the sedimentation volume to determine the particle size. As we all know, in addition to the particle size (limestone quality, lime calcination quality, lime digestion, carbonization conditions, etc.), the factors that affect the sedimentation volume of calcium carbonate, there are also calcium carbonate crystal forms, and calcium carbonate crystal forms are diverse Because the same crystal shape is not very regular geometric morphology, it is impossible to be the same between each other, so the influence of crystal shape on the sedimentation volume is more complicated. Therefore, it is not scientific enough to determine the size of the particle size by the sedimentation volume method. Larger.
In order to effectively prevent the agglomeration of secondary particles, superfine calcium carbonate generally needs surface modification treatment, because the surface treatment process not only effectively improves the activity of superfine calcium carbonate, but also effectively prevents the agglomeration of secondary particles.
For a special kind of ultrafine calcium carbonate, in order to better meet the requirements of users, there are not only different crystal shapes and particle sizes, but also the choice of surface treatment agents, as well as experience and know-how in the treatment process.
Therefore, it is very difficult to use a unified standard to measure special varieties of superfine calcium carbonate for different purposes.
For ultrafine calcium carbonate, crystal form is also a very important technical indicator.
As we all know, ordinary light calcium carbonate is spindle-shaped. If it is used in PVC plastics, it will generate greater stress and whiten the plastic film. As superfine calcium carbonate, products with different crystal shapes should be produced according to different uses. E.g:
Ultra-fine calcium carbonate for plastics requires a simple crystal structure, a smaller pile volume, and a lower oil absorption value, so cubic or spherical is better. Ultrafine calcium carbonate with a particle size of 0.072μm has a certain strengthening effect on PVC plastics, which can make the surface of the product fine and clean, with good gloss and good electrical insulation performance. It is used in flexible cable materials. When the filling amount is increased by more than double, It can still maintain its performance in line with national standards. Used in plastic film, it can reduce whitening and increase low-temperature elongation; used in hard plastics, such as plastic doors and windows and various profiles, can improve impact strength, The notched impact strength can reach 49.1kJ/m2.
Superfine calcium carbonate for rubber has the best chain-like reinforcement. Chain-like ultra-fine calcium carbonate is formed by combining several or even dozens of calcium carbonate crystal grains in one direction. It has a three-dimensional structure in rubber and has good dispersibility. When mixing with rubber, calcium carbonate The chain breaks to form an active section, which is more firmly combined with the rubber chain, thereby greatly improving its reinforcing effect in the rubber. The order of reinforcement of different shapes of superfine calcium carbonate in rubber is from strong to weak: chain shape, needle shape, spherical shape and cubic shape.
The ink uses ultra-fine calcium carbonate, the cube is the best, which is also determined by the nature of the ink. In the resin ink, after filling the ultra-fine calcium carbonate, high gloss, good transparency and good fluidity are required. The cube is most beneficial to the gloss of the ink.
Precipitated calcium carbonate for paper coating generally does not belong to the category of ultrafine calcium carbonate. Most of its particle size is within the range of fine calcium carbonate (particle size 0.1-1μm), but one thing is the same, that is, there are certain requirements for crystal shape. Such as requirements for opacity (covering rate), high viscosity concentration, whiteness and ink absorption performance. Therefore, the flaky crystal form and the cubic crystal form are more ideal.
To sum up, it is not only necessary to ensure that the particle size of ultrafine calcium carbonate is 0.02-0.1μm, but also to determine the type of product crystal shape according to the application, in order to produce a marketable product.
The oil absorption value of ultrafine calcium carbonate is very important to its application performance, especially for plastics, coatings, and inks. If the oil absorption value is high, it will consume a lot of plasticizer when used in plastics; when used in coatings and inks, it will increase the viscosity. Therefore, the oil absorption value cannot be too high.
In addition to the physical properties of the substance itself, there are many other factors that affect the oil absorption value of a certain powder, and the particle size of the powder is an important factor.
Therefore, as ultra-fine calcium carbonate, the first consideration is the particle size of ultra-fine calcium carbonate, what is the dispersion performance, whether it has reached the specific surface area corresponding to its particle size, and on this basis, how to reduce its oil absorption value Only in this way can it have practical meaning. If its dispersibility is not good, the secondary particles will agglomerate very much and the specific surface area will be very small. In this case, even if the oil absorption value is low, it is of no practical significance in application.
Relatively speaking, the main content of ultrafine calcium carbonate (calcium oxide) should not be too strict within a certain range. Because, in most cases, ultra-fine calcium carbonate is used as a functional filler. It is the physical properties that affect the filler, but rarely the chemical properties. The main content of ultra-fine calcium carbonate is within a certain range. It can meet the requirements of its physical properties.
In the preparation process of ultra-fine calcium carbonate, in order to prepare ultra-fine calcium carbonate with various crystal shapes, good dispersion performance and excellent activation performance to meet the requirements of different products such as plastics, rubber, ink, etc., it is usually necessary to add certain control agents , Dispersant, surface treatment agent, etc., in general, these deliberately added "impurities" have no effect on the application performance of ultra-fine calcium carbonate, and are even beneficial. And because of the deliberate addition of these so-called impurities, the main content of ultrafine calcium carbonate (calcium oxide) must be affected.
Of course, certain harmful elements, such as iron, manganese, etc., should be strictly controlled whether they are brought into the raw materials or in the preparation process, because these harmful impurities not only affect the color of the product, but also accelerate the resin The degradation rate of itself accelerates the aging of the resin and should be strictly avoided. There are also some impurities such as silicon, aluminum, magnesium, etc., which not only adversely affect the whiteness of ultrafine calcium carbonate, but also adversely affect the operation in the production process. When choosing limestone raw materials, it must be controlled.
It can be seen that the main content of superfine calcium carbonate should not be too strict within an appropriate range, otherwise it will affect the development of some special varieties.
To sum up, the key development direction of ultrafine calcium carbonate is specialization, serialization, diversification and functionalization. It is believed that as long as the ultrafine calcium carbonate with a particle size of 0.02～0.1μm can be used in rubber, plastics, ink, coatings, papermaking, etc. regardless of crystal form, dispersibility and activation, it is unrealistic. According to different uses, specific and unique requirements should be put forward for particle size, crystal shape, specific surface area, oil absorption value, main content, etc., so that it can better meet the requirements of different industries and different users. Therefore, it is very difficult to use a unified standard to measure special varieties of superfine calcium carbonate for different purposes.