Inorganic Silicate Coatings Technology VOL.004-Pigments and Fillers for Inorganic Silicate Coatings
Like other architectural coatings, pigment and filler are one of the most used and important raw materials in inorganic silicate coatings, except varnish, the total amount of pigment and filler in white paint and color paint is generally 30-50%, so pigment and filler play an important role in the performance of inorganic silicate coatings, and even play a decisive role in some properties.
Filler is the largest amount of raw materials in inorganic coatings, although it accounts for the total cost is not high, but it has an important effect on many properties, and it is almost decisive for some properties such as stability; except for a small number of especially bright colors, white pigment will be used in the majority of the colors; as in the case of other architectural coatings, it is seldom to use the color pigment directly, but to use the color paste to adjust the color. Fillers for inorganic coatings, white pigments and color pastes for color mixing are discussed below.
Fillers for inorganic paints
Fillers can reduce the cost of paints, improve the covering efficiency of titanium dioxide by separating, and improve the strength, anti-cracking, gloss, hardness, scrub resistance and many other properties of paints. Fillers commonly used in latex paint heavy calcium, talcum powder, mica powder, wollastonite powder, kaolin, diatomaceous earth, quartz powder, etc. can be used in inorganic silicate coatings, but again, all have a greater risk.
Inorganic silicate coatings in the choice of filler than in latex paint is much more demanding, inorganic silicate coatings on the filler in the polyvalent metal ions requirements are very strict, a small amount of Ca2 +, Mg2 +, Zn2 +, Cu2 +, Fe3 + and other ions can cause the paint seriously thickened or even gelatinized and scrapped. And the higher the valence of metal ions, the stronger the inorganic binder potassium silicate and silica sol coagulation ability. Different valence metal ions on the negatively charged colloidal system of the coagulation capacity of the ion valence of about 6 times the ratio, that is, Me + : Me2 + : Me3 + = 16: 26 : 36 = 1 : 64 : 729. Visible bivalent metal ions of the coagulation capacity of the metal ions for a valence of the metal ions of the 64 times, the trivalent metal ions of the coagulation capacity of the metal ions of the divalent is more than ten times. It is because of the trivalent ions strong coagulation ability, the stability of inorganic coatings destructive, laboratory disperser shaft a small amount of rust can make the original stable inorganic coatings gel agglomeration and deterioration.
Fillers for inorganic coatings can be divided into two main categories, one is carbonate filler, the other is silicate filler (containing the chemical composition of silica filler, such as diatomaceous earth, silica micropowder, etc.), the following two types of fillers are discussed separately.
Carbonate filler
Carbonate fillers are mainly calcium carbonate and magnesium carbonate. Calcium carbonate and magnesium carbonate are carbonates of alkaline earth metals, which are similar in nature but also slightly different. For inorganic coatings should be most concerned about their solubility is different: magnesium carbonate solubility product (Ksp = 6.82 × 10-6) than the solubility product of calcium carbonate (Ksp = 3.36 × 10-9) is much larger, so magnesium carbonate slurry in the magnesium ion concentration of calcium carbonate slurry than the concentration of calcium ions in the slurry to be much larger, magnesium carbonate can easily lead to inorganic coatings after the thickening, and even the gel agglomeration and deterioration, so the Magnesium carbonate should not be used as filler for inorganic coatings. Calcium carbonate can be used in inorganic coatings due to the relatively low solubility in water, calcium ion concentration is small, the stability of inorganic coatings have less impact, can be used in inorganic coatings. Also based on the solubility of magnesium carbonate, hard water containing Ca2+, Mg2+ hydrolyzed during boiling to form scale is mainly composed of CaCO3 and Mg(OH)2, Ca2+ hydrolyzed to generate insoluble CaCO3, Mg2+ hydrolyzed end product is not MgCO3, but more insoluble Mg(OH)2.
Calcium carbonate is further subdivided into heavy and light calcium depending on the production process. Natural calcium carbonate ore is physically crushed to a certain fineness as heavy calcium, while light calcium is produced from calcium carbonate ore by a series of chemical processes such as high-temperature calcination and then carbonization and reduction. Although the chemical composition of light calcium is the same as heavy calcium, but because of the light calcium in the production process is difficult to ensure complete carbonization, the alkalinity of the final product will be higher than the heavy calcium, but also shows that the light calcium contains a certain concentration of highly reactive calcium ions, which can be reacted with potassium silicate or silica sol, which seriously affects the stability of the inorganic coatings, and therefore can not be used in inorganic coatings. For the same reason, light calcium should be used with caution in emulsion paint, if the quality of light calcium is a little poor or the amount used is too large, it may cause serious thickening of emulsion paint. It is for this reason that large paint companies do not use light calcium in building emulsion paints in order to ensure product stability.
Calcium bicarbonate is the most important, largest and most economical filler for architectural paints. It can also be used in inorganic architectural paints, and is the most commonly used filler in inorganic paints in Europe. However, as long as the ore is natural, its chemical composition will inevitably fluctuate, and may contain other impurities that are more difficult to control; and calcium carbonate and magnesium carbonate, which are both carbonates in nature, are often associated, which results in a high risk when using heavy calcium as a filler for inorganic coatings. In the southern part of China, the calcium carbonate content of the heavy calcium ore is higher, while the heavy calcium ore in the northern part of the country generally has low calcium carbonate content and higher magnesium carbonate content, which makes the use of it more risky.
Different origins of more than thirty kinds of calcium bicarbonate samples analysis found that these are “heavy calcium” in the domestic sales to the construction of the paint factory to use the chemical composition of the filler is very different, there is magnesium oxide content <1% of calcium bicarbonate, more magnesium oxide content of 1 ~ 20% of the samples between the varying; and even magnesium oxide content of more than 20% of samples, in fact, this is the dolomite powder. In fact, this is dolomite powder, the chemical composition is not calcium carbonate but magnesium carbonate. The magnesium oxide content of these huge differences in the heavy calcium in the packaging and no difference, are in the name of heavy calcium carbonate on the market, because in the ordinary latex paint they can be used, there is really no big difference; but inorganic coatings are completely different, only magnesium oxide is strictly controlled, the content of <1% of the true “heavy calcium” can be used in inorganic coatings, otherwise it is impossible to produce stable inorganic coatings. In short, inorganic coatings for the selection of heavy calcium filler is the most important is to strictly control the content of magnesium oxide in the heavy calcium, only magnesium oxide content of less than 1% of the heavy calcium can be used in inorganic coatings.
Silicate filler
Silicate filler types more than carbonate, architectural coatings commonly used silicate (silica) fillers are kaolin, talcum powder, mica powder, wollastonite powder, feldspar, diatomaceous earth, silica micropowder (quartz powder) and so on. Silicate fillers are more stable in inorganic coatings than heavy calcium, so these silicate fillers can be used in inorganic coatings if they are of good purity. As in the case of heavy calcium, as long as it is a natural mineral, it may fluctuate in chemical composition due to various factors such as different origins or different stratigraphic structures of the same origin, and its stability cannot be guaranteed if it is mixed with magnesium carbonate or other metal ions that contain soluble polyvalent ions.
Calcined kaolin, like heavy calcium, is one of the most commonly used fillers in architectural interior and exterior wall coatings, with large oil absorption and fine particle size, it can separate titanium dioxide, prevent the over-polymerization phenomenon of titanium dioxide, improve the covering efficiency of titanium dioxide, and at the same time, provide better dry covering power, which is used in large quantities for both interior and exterior wall latex coatings. Calcined kaolin is basically free of reactive polyvalent metal ions and has good stability in inorganic coatings, so it can be used as filler in inorganic coatings. It is because calcined kaolin is stable and controllable in inorganic coatings, while the chemical composition of other natural fillers fluctuates greatly, the quality is more difficult to control, so some domestic suppliers of raw materials for inorganic coatings recommended formulas filler only calcined kaolin, and no other fillers. This kind of simplified formula can indeed ensure that inorganic coatings have better stability, but it will also bring some negative effects. Carefully study the European inorganic coatings recommended formulations, you will find that very few of the calcined kaolin such as ultra-fine fillers, and the main use of bicarbonate of calcium, talcum powder, mica powder and other coarse particle size fillers. This is due to the curing of inorganic binder can only form a three-dimensional network skeleton, there are a large number of pores in the film, there is a certain dry hiding power, not like organic coatings as good as the powder coating effect, if too much use of these ultra-fine fillers such as calcined kaolin will result in the film void rate further increase, densification is poorer, the contraction of stress is greater, and the film strength of the consequences of the poorer, which leads to a reduction in the resistance to scrubbing, This leads to a series of problems such as reduced scrubbing resistance, powdering of the coating film, poor constructability when drying too fast, color blossoming, peeling and cracking of the coating film, and so on.
Washed kaolin is also one of the fillers commonly used in architectural coatings, but due to its high impurity content, the stability of inorganic coatings have a greater impact on the use of a greater risk, generally not used in inorganic coatings.
Talcum powder is mainly composed of water-containing magnesium silicate, the molecular formula is Mg3[Si4O10](OH)2, for the flaky or fibrous structure, which can reduce the drying shrinkage and deformation of the paint film, and it has a high degree of chemical inertia, softness, hydrophobicity, and other unique properties, which can be used in inorganic coatings to improve the film’s resistance to cracking, water resistance, seepage resistance, and color stability. Due to the high oil absorption of talcum powder, so do not add too much, generally not more than 10% is good. It should be reminded that in the domestic high-quality talc is mostly used in cosmetics and other high value-added products, talc used in the coating industry due to lower prices, most of the content is low, used in inorganic coatings with higher risk. Aorun chemical laboratory on the market to collect several kinds of talcum powder in inorganic coatings in the test, found that the stability of all poor, the reason for this result is not talcum powder can not be used in inorganic coatings, but due to the lack of purity, impurities brought into too many metal ions.
Mica powder is a kind of silicate with layer structure, the sheet thickness can be up to 1μm or less, the diameter to thickness ratio is large, it has good chemical inertness, toughness, acid and alkali resistance, corrosion resistance and anti-cracking properties, it is a kind of excellent inorganic coatings filler, it has good stability in inorganic coatings, and at the same time, it can improve anti-cracking property of inorganic coatings. However, it should be noted that its dispersion is slightly poor, the dosage is generally controlled at 5% or less is better, while the cost is higher.
The chemical composition of wollastonite is CaSiO3, with needle-like structure, white with slightly gray or slightly red, chemically inert, can be used in inorganic coatings. Such as mica powder, talcum powder has good anti-cracking, and its oil absorption is small, so you can use a larger amount without causing dispersion difficulties, reduced scrubbing and paint film powder and other issues, the disadvantage is that wollastonite general whiteness is poor, too much use will affect the whiteness of the paint film. With other silicate fillers, wollastonite powder purity of inorganic coatings is very important, due to the nature of carbonates often associated with wollastonite, wollastonite of higher purity is still a scarce resource.
Diatomaceous earth is a kind of siliceous sedimentary rock of biological origin, mainly composed of the remains of ancient diatoms, the chemical composition of which is dominated by SiO2, and its chemical properties are stable enough to be used in inorganic coatings, but due to its porous and high-surface-area properties that can result in too high a porosity in the coating film, it is generally not often used in inorganic coatings.
Quartz powder (also called silica micropowder) a hard, wear-resistant, chemically stable minerals, its main component is SiO2, Mohs hardness of 7, good stability in inorganic coatings, can be used as filler in inorganic coatings, can improve the film strength and wear resistance. Due to the high hardness of quartz, difficult to crush and grind, dry quartz powder may contain more due to the wear and tear of production equipment produced by the iron and affect the stability of inorganic coatings, water method of production of quartz powder containing iron and other impurities are less than the dry method, stability is better, but the processing cost of labor and materials, so the price is higher.
Inorganic coatings filler use skills
Fillers in inorganic coatings can generally be based on heavy calcium, supplemented by silicate fillers. You can choose two kinds of different fineness of calcium bicarbonate for reasonable grading, in order to improve the denseness of the coating film; silicate filler can improve the anti-cracking, water resistance and anti-permeability and so on.
The use of different fineness of filler for reasonable grading, used to improve the hiding power, film density and mechanical strength and other comprehensive performance, but not too much use of ultra-fine filler, as far as possible without affecting the appearance of the use of a slightly thicker filler, you can reduce the phenomenon of paint film powder, but also to improve the stability of the paint. Because the finer the filler, the larger the specific surface area, the higher the solubility in water, the more dissolved into the polyvalent metal ions will be; at the same time, the finer the filler, the longer the processing of crushing and grinding, the longer the impurity ions such as iron from the grinding media with the more, the worse the stability.
Almost all raw materials in inorganic coatings will affect the stability of the coating, including coating water. The most important of these many factors affecting stability are three raw materials, namely fillers, inorganic binders (silica sol and potassium silicate) and stabilizers. Among them, filler is the first element that affects the stability of inorganic coatings, and the second and third important ones are inorganic binder (potassium silicate and silica sol) and stabilizer respectively. There are many reasons why filler is the first element to determine the stability of inorganic coatings. One is the filler in the inorganic coatings in a large amount, even if only a small amount of active metal ions on the stability of the impact is also very large; if the filler in the high content of magnesium carbonate, magnesium carbonate in the coatings in the existence of the dissolution of the equilibrium of a certain concentration of magnesium ions will be dissolved to free out of the potassium silicate or silica sol in the reaction of the reactive silanol-based, magnesium ion concentration is reduced, the original dissolution equilibrium is destroyed, magnesium carbonate will continue to ionize, generate a new magnesium ion to continue to work with the silanol-based reaction. New magnesium ions continue to react with the silanol group until the coating gel curing. This is also hot storage can be accelerated inorganic coatings viscosity, wash resistance is reduced because the high temperature at the same time to accelerate the filler dissolution and curing reaction speed. The second is that since potassium silicate, silica sol and stabilizers are synthetic chemicals, they are relatively more stable and controllable in quality; whereas most fillers are natural minerals with greater fluctuations in chemical composition and are more difficult to control. If the polyvalent metal ions in the filler cannot be controlled strictly and effectively, even if the most stable potassium silicate and silica sol are used, it is difficult to produce stable inorganic coatings by adding sufficient amount of stabilizers. On the contrary, if the filler is of high quality, stable and reliable, no polyvalent metal ions, as long as the coating formula is reasonable, even if the use of ordinary potassium silicate can produce viscosity control, will not be gel curing inorganic coatings, but only after the thickening of more stabilized potassium silicate will be slightly more.
From the above analysis, it can be seen that the majority of quality problems that lead to thickening and even gelation of inorganic coatings are caused by the fluctuation of filler quality. However, a few domestic raw material suppliers in order to pursue commercial interests, deliberately exaggerate the facts, publicize his inorganic coating system formula is simple, do not select the filler, which is very dangerous and incorrect. Because the stabilization of potassium silicate or silica sol must retain a certain degree of chemical activity, that is, to retain a certain number of polyvalent metal ions can be reacted or self-condensation crosslinking curing silanol group, when the filler or other materials into the polyvalent metal ions will certainly react to the stability of the impact. To do not really pick the filler must be organic passivation of all the silanol base, but this is not only high cost, more critical is this “do not pick the filler” of potassium silicate and silica sol must be no active silanol groups, completely inert, lost as a binder role.
I came into contact with a number of coatings customers, including mega-coating enterprises feedback such a problem: the laboratory has done a lot of testing, in a longer period of time during the gap period of the formula for many times to reproduce the self-perception has been able to effectively control the stability of inorganic coatings, but once the pilot production out of the stability of the inorganic coatings on the failure. In most cases, the reason for this problem is that the filler used in the pilot test is different from the filler used in the experiment. Laboratory samples happen to be very good quality, so the experimental formula has been more stable, while the pilot test is usually carried out after a long time of research and development, at this time the production of fillers with the experimental filler has been very different, and coatings engineers do not realize this problem, resulting in the occurrence of the problem.
