Summary of the third issue:
Effect of polymer emulsion on properties of inorganic silicate coatings
Evaluation method and interaction of compatibility between emulsion and potassium silicate
Performance requirements of inorganic silicate coatings for polymer emulsions
Columnist Xu Kaibin
Shanghai aorun Chemical Co., Ltd
Emulsion for inorganic silicate coating
Introduction
Polymer emulsion has been widely used in traditional inorganic silicate coatings since the polymer emulsion was commercialized in the late 50s of last century. Polymer emulsion assisted potassium silicate, used together as a binder, has produced the second generation of silicate emulsion Silicate (Emulsion Paint). Polymer emulsion has improved the stability of inorganic coatings, making the first generation of two component pure inorganic silicate coatings become more convenient single component inorganic coatings, and at the same time, improve the performance of inorganic coatings. The third generation of inorganic coatings (Sol-silcate Paint) that appeared in the early part of this century combined silica sol with potassium silicate as inorganic binder, still need polymer emulsion as a supplementary binder to assist the inorganic binder to achieve the ideal physical and chemical properties.
The environment of inorganic silicate coating is quite different from that of ordinary latex paint. The function of polymer emulsion in inorganic coatings is also different from that of ordinary latex paint. Therefore, inorganic silicate coating has special requirements for the properties of polymer emulsion. Choosing the right polymer emulsion and correct coating formulation is as important as having a major impact on the properties of the final coating product.
In order to help select the appropriate polymer emulsion in inorganic silicate coatings, the emulsion of inorganic silicate coating is discussed from the following aspects: the influence of polymer emulsion on the performance of inorganic silicate coating, the evaluation method and compatibility of emulsion and potassium silicate, and the performance requirements of inorganic silicate coating on polymer emulsion.
Effect of polymer emulsion on properties of inorganic coatings
The first generation of two component pure inorganic silicate coatings were mixed in the construction site, and all the raw materials had been mixed together in the production of one component silicate latex inorganic coatings. Polymer emulsion is added to inorganic coatings according to DIN18363 standard or revised JG/T26-XXXX building interior and exterior wall liquid inorganic coating standard requirements, according to the amount of non-volatile polymer organic content up to 5%. That is to say, there are two kinds of binder in a coating system: one is inorganic binder sodium silicate or alkali silicate, the other is organic binder polymer emulsion.
The properties of these two binders are completely different, whether in liquid paint or in dry paint film. After construction, chemical reaction of water glass forms inorganic silicate matrix, and pigments and fillers in coating can also participate in the reaction. The polymer emulsion is coated by physical process only after construction.
In the inorganic silicate coating system, polymer emulsion can not only play a role in stabilizing the system and improving the rheological properties, but also improving many properties. The most important thing is to improve the washing resistance and water resistance, especially the early dry and wet wiping powder properties and water resistance, reduce the phenomenon of salting out and powdering, increase the flexibility of the film, improve adhesion and reduce the skin opening. Risk of cracking and falling off, etc.
Improve washability
Washing resistance is one of the most important and basic performance requirements of interior wall coatings. The single component pure inorganic coating without polymer emulsion is poor in washing resistance, whether it is silica sol or potassium silicate alone, or silica sol complex acid potassium silicate as binder. It is difficult to achieve the scrubbing resistance of the revised JG/T26-XXXX building interior and exterior wall liquid inorganic coating standard 5000. Second. In practical application, it is also very important for the anti dry wipe or wet wipe powder performance of wall coating. Due to the high permeability of inorganic binder and the lack of coating effect on pigments and fillers, it often causes the phenomenon that inorganic coating surface pigments and fillers are easy to drop powder. Therefore, it puts forward higher requirements for the anti washability of inorganic coating, which is close to GB / T 9756-2018 standard for superior products (6000 times) is also appropriate.
If the pure inorganic binder is kept and active filler is added into the formula to participate in the curing reaction, the scrub resistance can be improved to a certain extent, but to achieve 5000 times of scrub resistance, it is necessary to carefully screen various raw materials in the formula and scientifically design the formula; or in the form of two-component, depending on the addition of inorganic or organic acid, acid salt (sodium bicarbonate, phosphorus Curing agents such as sodium dihydrogen acid, metal salt (calcium chloride, ammonium chloride, sodium fluosilicate, aluminum phosphate), metal oxide (zinc oxide, calcium oxide, magnesium oxide) and metal powder (zinc powder, aluminum powder) can significantly improve the washability.
The addition of a small amount of polymer emulsion (< 5%, W/W) in inorganic coatings can easily increase the scrubbing resistance of coatings, meeting the requirements of standards and practical applications. As shown in Table 5, the washing resistance of the pure inorganic coating formulation (formula one or two, three) is very difficult to meet the 5000 requirements, but after the 5-10% polymer emulsion is added to the coatings (formula four or five, seven), whether it is by GB/T 9756-2018 requires seven days of curing or 21 days of curing according to JG / t26-xxxx standard. The second generation of inorganic coating (formula IV and V) with potassium silicate as inorganic binder and the third generation of inorganic coating (formula VII) with potassium silicate and silica sol as inorganic binder can achieve more than 5000 times of washability, or even 10000 times of washability; only silica sol can be used The formula of inorganic binder six can not reach 5000 of scrubbing resistance, but the scrub resistance is also improved obviously after adding emulsion. Taking into account the limitation of coating performance requirements and standards on polymer organic content, the amount of polymer emulsion (solid content of about 47 + 2%) in inorganic silicate coating is generally suitable for 5-8%, which can meet the standard requirements and achieve better performance.
Table 5 Comparison of washability of inorganic coatings
Improve water resistance
Water is the enemy of buildings, especially for the exterior wall, which should have better waterproof and air permeability. When the water resistance is poor, the water infiltrated into the outer wall will freeze in winter, and the volume expansion will produce great expansion stress, which will damage the building; corrosive gas can also damage the building by changing water into acid. When the air permeability is poor, the diffusion of water vapor is blocked. On the one hand, the film may blister and fall off. On the other hand, the enrichment of water will bring adverse effects on wall insulation. Therefore, the paint must be water repellent and air permeable to achieve the ideal protection of the wall.
Inorganic coating has the best air permeability among all architectural coatings due to its porosity. Generally, it is easy to meet the requirements of wall or relevant standards for air permeability. The detection of air permeability is shown in Figure 20. The water vapor permeability or equivalent static air layer thickness SD (or water vapor diffusion resistance) is used to express the air permeability. JG / T 512-2017 standard and ISO standard divide the water vapor transmittance of exterior wall coating into three levels, as shown in Table 6. Inorganic coatings can generally pass the requirements of the highest level III (≥ 150g / m2 · d).
Figure 20 schematic diagram of air permeability test method
The lower the water absorption is, the better. Inorganic coating has strong water absorption because of its porosity and high salt content, which is difficult to meet the requirements of exterior wall coating. The water absorption test is carried out as shown in Figure 21. JG / T 512-2017 and ISO standards also divide the water absorption of exterior wall coating into three levels, as shown in Table 6. The pure inorganic coating without emulsion is generally high in water absorption, which is grade I, > 0.50kg/m2. H0.5.
Fig. 21 schematic diagram of water absorption test method of architectural coating
Table 6 classification requirements for water vapor transmittance and water absorption of building coatings
From the perspective of coating protection, the smaller the water absorption is, the better the air permeability is. The ideal exterior wall system should have neither water absorption nor water vapor diffusion resistance, but in fact, it is impossible. The better the air permeability of general coating, the higher the water absorption, such as pure inorganic coating; or the lower the water absorption, the worse the air permeability, such as elastic coating. A good wall coating should keep the balance of air permeability and water absorption: W ≤ 0.5 kg / m2 · h0.5, SD ≤ 2m, that is, the shadow part of Figure 22; and the closer it is to the origin, the better the water resistance, air permeability and protection function. The best exterior wall coating should be w ≤ 0.1 kg / m2 · h0.5, SD ≤ 0.14m.
Figure 22 requirements for air permeability and water absorption of exterior wall coating based on exterior wall protection theory
The air permeability of inorganic coatings is easy to meet the requirements, but the water absorption is difficult. One of the most effective ways to reduce the water absorption of inorganic coatings is to add polymer emulsion. As shown in Fig. 23, with the increase of emulsion dosage, the water absorption of inorganic coatings decreases.
Fig. 23 influence of the amount of emulsion on water absorption in inorganic coatings
It is found that the water-proof and air permeability of inorganic coatings for exterior walls in Europe meet the most ideal requirements, i.e. w ≤ 0.1 kg / m2 · h0.5, SD ≤ 0.14m, while the permeability of interior wall inorganic coating is up to SD ≤ 0.14m, and the water absorption is different according to the different application places. The products with grade I, II and III requirements are available in the market.
The revised JG / T 26-xxxx standard for liquid inorganic coatings for interior and exterior walls of buildings requires the water vapor transmittance of inorganic coatings for interior and exterior walls to be ≥ 400 and 150g / m2 · D respectively, which is similar to that in Europe. However, due to the lack of familiarity with the detection method of this index in China, there is no requirement for other coating standards to adopt this index. We need to remind you that although there is no requirement for water absorption in the standard, it does not mean that this index is not important. In fact, we must strictly control this index in the development and production of inorganic coating for exterior wall, otherwise, there will be major quality problems in actual application.
Reduce salting out and pulverization
In the future, water glass in inorganic coating will precipitate on the surface of paint film in the form of alkali metal carbonate, which will lead to the phenomenon of salting out and pulverization, resulting in the problems of dry wipe off powder and color blooming. There are many ways to reduce salting out, such as high modulus potassium silicate instead of low modulus potassium silicate, replace the second generation inorganic coating formula with the third generation inorganic coating formula, and add polymer emulsion as much as possible under the standard permissible conditions.
The addition of polymer emulsion in inorganic coatings can significantly reduce salting out. The type and dosage of polymer emulsion have great influence on salting out. As shown in Figure 24, in the second generation inorganic coatings (potassium silicate as inorganic binder), the amount of styrene acrylic emulsion A and B with the same vitrification temperature and the same solid content were all added to 7.5%. When the paint film was stored in the same indoor environment for 8 months, the inorganic coatings containing emulsion B were salting out more clearly than the inorganic coatings of emulsion A. Compared with that of A and B, B was also much better than A. The same emulsion B has better water resistance and lower water absorption than A. B has better water resistance and lower water absorption. The film has less tendency to absorb and expand and soften, and better scrubbing resistance. At the same time, because of the strong hydrophobicity of paint film, salt has less chance to migrate from paint film to coating surface through water as the medium.
Fig. 24 comparison of salting out resistance of the same amount of emulsion A and B in the second generation inorganic coatings
Fig. 25 shows that the two second generation inorganic coatings with the same styrene acrylic emulsion dosage of 5% and 7.5% are stored in indoor environment for 8 months after salting out. The results show that the polymer emulsion is 7.5% when the emulsion is 5%.