Application of polymer cement-based materials containing redistributable latex powder in architectural coatings
A mixture composed of adhesive redistributable latex powder polymer, cement, water, additives, etc., the material formed after condensation and hardening is called a polymer cement-based material.Such materials have the characteristics of good water resistance, durability, low price and environmental protection of cement-based materials, as well as the flexibility, water resistance, strong adhesion and high tensile strength of organic polymers.，
1.1 Physical and mechanical properties
The physical and mechanical properties of redistributable latex powder polymer cement-based materials mainly refer to their strength, elastic modulus, crack resistance, acid and alkali resistance, water resistance, permeability resistance, etc.Compared with cement-based materials and polymer materials, the tensile strength, flexural strength and bonding strength of redistributable latex powder polymer cement-based materials have been significantly improved, especially the bonding strength has been greatly improved. This is because of the good bonding effect of the polymer and the bonded matrix.The increase in tensile strength is mainly due to the reduction of microcracks and other structural defects.An interlocking network is formed between the polymer and the cement slurry. When cracks occur under stress, the redistributable latex powder polymer can “bridge” the microcracks and the cracks expand, so that the fracture toughness and deformation properties of the polymer cement-based material are improved.In addition, the improvement of the workability and dispersion of the mixture makes the structure of the hydrated cement slurry uniform and dense, and also promotes the improvement of tensile strength.
Due to the more dense structure, the permeability of the redistributable latex powder polymer cement-based material is greatly reduced, which also promotes the improvement of acid, alkali and water resistance.The ability of polymer cement-based materials to withstand temperature and humidity changes and circulation is better than that of cement-based materials. This is due to the presence of polymer films, the lower water-to-ash ratio and reasonable pore structure when using polymers.The elastic modulus of polymer cement-based materials is reduced. This is due to the fact that the elastic modulus of the polymer is much lower than that of the cement slurry. The elastic modulus is reduced. It is beneficial to improve the deformation of polymer cement-based materials, which is very conducive to the material's absorption of stress under variable temperature conditions.At the same time, due to the increase in the tensile strength of the polymer cement-based material, the elongation performance is improved, thereby reducing the formation of dry shrinkage cracks and temperature cracks.The reduction of dry shrinkage cracks is also related to the higher water retention of the polymer.Polymer cement-based materials decrease in strength as the temperature rises, and when the polymer content in the material is high, there may be slight flammability.Usually, the use temperature of polymer cement-based materials is limited to below 150℃.
1.2 Construction operation characteristics
Polymer cement-based materials often have better construction operation characteristics, but different polymers will bring different properties to polymer cement-based materials.For synthetic resin emulsion polymers, because they are rich in surfactants, the fluidity of the mixture can be increased; the dispersed polymer particles can also make cement and hydrated product particles easier to move relative to each other like “balls”, which can often significantly improve the construction properties of the mixture.Due to the dispersion of the polymer and the adsorption of surfactants on the cement particles, the hydration rate of the cement is slowed down, which has a retarding effect on the cement and prolongs the condensation time of the mixture.Under normal circumstances, these factors are very beneficial to the operability of construction.The hydrophobic and colloidal properties of the emulsion make the newly mixed polymer cement-based material have good water retention, thereby reducing the need for long-term wet maintenance.For polyvinyl alcohol water-soluble polymers, the aqueous solution is a colloid with great adhesion, which can significantly improve the adhesion of the mixture and effectively improve the construction operability.
1.3 Durability and corrosion resistance
The introduction of polymer has improved the physical and mechanical properties of the material, improved the microstructure and reduced the structural defects, significantly improved the permeability and freeze-thaw durability of the material, and made the material have good chemical resistance and good resistance to chlorine and salt penetration.
The cost of polymer cement-based materials is significantly higher than that of cement-based materials, but at present, in different application fields, most of them are in an acceptable range and are very competitive.These characteristics of polymer cement-based materials make them very suitable for the preparation of certain decorative architectural coatings and functional architectural coatings.
2 Modification of cement by polymer in polymer cement-based materials
In polymer cement-based materials, when the dominant material is cement, the material shows the characteristics of inorganic cement, but the performance is significantly improved; and vice versa.Both of these conditions exist in architectural coatings.
2.1 Cement is the leading material
When the dominant material is cement, the modification mechanism of polymer to cement can be roughly summarized as：
First, the polymer is evenly dispersed in the cement gel, which can prevent or reduce the evaporation of water and prevent the absorption of water by the dry matrix, so that the cement hydration effect is more perfect.
The second is that the addition of polymer increases the adhesion of the cement gelling material.In the cement-polymer system, both cement and polymer can act as adhesives; however, the bonding strength of the polymer film is much higher than the tensile strength of cement, which greatly improves the tensile strength of the system.The polymer film is evenly distributed in the material and forms a network that runs through each other with the cement gel. When subjected to external forces, this structure is conducive to the dispersion and transfer of stress, preventing or weakening the growth of cracks.
Third, there is a chemical effect between the polymer and the cement hydration product.In addition, during the condensation and hardening process of cement, a large number of pores or microcracks will inevitably occur and weak parts will be formed in the material.The added polymer is dispersed and gathered on the pore wall, forming a layer of polymer film on the pore wall or microcrack surface, which has an enhanced effect on these weak parts.Therefore, through the diffusion (self-adhesive) of polymer molecules, polymer particles form a continuous polymer film that is insoluble in water in the material, improving the adhesion to the interface and the modification of the material itself.The polymer film is distributed between the cement hydration products, which acts as a “bridge” when there is stress, effectively absorbing and transmitting energy.
2.2 Polymer as the leading material
The dominant material is polymer, which is more common in architectural coatings.At this time, while the material exhibits polymer properties, it obtains the water resistance, breathability and chemical modification effects conferred by the cement-based material, which significantly improves the performance of the composite material.
3application of polymer cement-based materials in architectural coatings
Polymer cement-based materials are mainly used as base materials in architectural coatings.Since the cement will condense and harden in a very short time after adding water, even if the retarding measures are taken, it can only be extended for a limited time, and it is of no practical significance as a product.Therefore, polymer cement-based materials are mainly used in the production of powdered and two-component architectural coatings.
3.1 Powdered architectural coatings
The polymer components of powdered polymer cement infrastructure construction coatings are divided into redistributable latex powder or polyvinyl alcohol fine powder.In the factory, the various components are mixed evenly to make coatings. When used, only water can be added and stirred evenly, such as coating coatings, porcelain-like coatings, etc.The advantage of this kind of coating is that it can be used by adding water and stirring, which is easy to transport and store, and avoids the preparation of a variety of raw materials on the construction site.
3.2 Two-component architectural coatings
Two-component architectural coatings are supplied in supporting packaging for powder and liquid materials.Compared with powdered architectural coatings, two-component architectural coatings also have their performance advantages, which are mainly reflected in their low cost.However, when the amount of preparation at a time is less than one package when used, weighing and matching are very troublesome.In addition, the packaging volume and transportation volume of two-component architectural coatings have also increased accordingly.Therefore, some architectural coatings that are demanding on cost and do not require long-distance transportation, such as polymer cement waterproof coatings, are still two-component architectural coatings; some architectural coatings, such as flexible putty for exterior walls, are powdered and two-component architectural coatings go hand in hand.
4 Performance characteristics of polymer cement infrastructure construction coatings
4.1 Performance advantages
In polymer cement infrastructure construction coatings, the polymer can form a film in a dry environment, while the cement is cured and hydrated in the presence of water. The combination of these two systems gives the coating film its physical and mechanical properties.The polymer gives the polymer cement-based coating good surface properties, which not only improves the stain resistance, but also retains the original breathability of the pure inorganic coating.Polymer cement-based construction coatings have many properties that are difficult to obtain for coatings prepared using only organic base materials, such as the adhesion strength, water resistance, and aging resistance to cement-based inorganic base materials, which far exceed the effects that can be achieved by using only organic or inorganic base materials.
4.2 Economic advantages
Polymer cement infrastructure construction coatings due to the use of low-cost cement, the amount of organic base material can be significantly reduced, thereby reducing the cost of coatings.On the other hand, liquid organic coatings need to solve storage properties, such as anti-precipitation, mildew, and maintaining stable viscosity of the coating. Many additives, such as thickeners, mildew inhibitors, antifreeze thawing agents, dispersants, etc., need to be used, and the performance requirements of these additives are high, so their cost accounts for a high proportion of the constituent materials of the coating (generally accounting for 15%~35% of the cost of coating raw materials), and in powdered or two-component polymer cement infrastructure construction coatings, these additives either do not need to be used at all, or in small quantities, or cheaper products (such as dispersants, Sodium hexametaphosphate, a product with dispersion properties and cheap, can be used to reduce the production cost of coatings.
4.3 Production and its technical advantages
According to the current supply of raw materials for architectural coatings, the vast majority of raw materials used in polymer cement infrastructure architectural coatings are powdered materials, and a small number of raw materials that may need to be further ground are also very small.Therefore, its production is only to mix various powdered materials evenly in a dry powder mixer. The production process is very simple, and the production technology is easy to master and control.Production equipment only needs dry powder mixers, powdered material packaging machines, quality control equipment, etc., with small investment.
4.4 Packaging and transportation
The packaging conditions required for liquid coatings are much stricter than for powdered architectural coatings. They not only require the packaging materials to be airtight, but also require high strength to prevent them from being damaged or leaking during transportation and storage, so the packaging cost is very high.On the other hand, general liquid coatings contain 40% to 55% water, and the amount of packaging materials required is also large, and transportation costs are also increased.It can be seen that powdered architectural coatings have great advantages in packaging and transportation.
4.5 Environmental advantages
It is generally believed that water-based architectural coatings are environmentally friendly products. In view of the needs of product promotion, many businesses refer to such coatings as green and environmentally friendly products.In fact, water-based coatings replace organic solvents with water as the dispersion medium of coatings, reducing the VOC (volatile organic compounds) emitted by coatings into the atmosphere.However, water-based coatings also bring new problems. For example, water-based coatings require the use of a large number of surfactants (dispersants, wetting agents, emulsifiers, etc.), mildew inhibitors, and freeze-thaw stabilizers.
In addition, because water-based coatings are easy to clean with water, a large amount of cleaning water is often used in production and construction. In many cases, waste coatings are even discharged directly into the sewers during the construction of coatings. The result of these practices is the discharge of a large number of surfactants, mildew inhibitors, freeze-thaw stabilizers, etc. into the river system, causing pollution.Compared with solvent-based coatings, the environmental protection of water-based coatings is unquestionable and is the inevitable direction of the development of coatings. However, water-based coatings are not called environmentally friendly products abroad, only products that can be used safely.Some countries also stipulate that coating companies are responsible for recycling packaging materials after use, and waste coatings must be treated uniformly to avoid flowing into rivers.In this sense, there are far fewer surfactants and other additives (such as fungicides, freeze-thaw stabilizers, and film-forming additives) in powdered architectural coatings that will have adverse effects on the environment, and the amount of cleaning water used in production and construction is much smaller. From the aspects of environmental protection and water saving, it undoubtedly has greater advantages.
4.6 Application restrictions
Polymer cement-based materials can currently only be used in a limited number of architectural coating varieties. One is that they can only be made into powdered products or two-component products packaged separately from liquid and powder; the other is that they can only be made into thick architectural coatings.For thin-quality, brushing, rolling, or coatings that need to be sprayed with a smooth surface, the current technical state is still difficult to use.
5The main varieties of polymer cement infrastructure construction coatings
Polymer cement-based architectural coatings can be divided into three categories: wall coatings, functional architectural coatings and wall putty. The main composition materials and performance characteristics are shown in Table 1.Due to the habits of the construction industry, building thermal insulation coatings are usually called thermal insulation pastes or thermal insulation mortars, but their composition and material substance belong to the category of coatings, and they were also used under the name of architectural coatings in the early stages of their development.
In short, redistributable latex powder polymer cement infrastructure architectural coatings, with their good technical, economic, environmental protection and other advantages, will be better developed in future practical applications, and will become a new class of important architectural coatings.