1. Liquid polymer
The liquid polymers used for modification in cement mortar and cement concrete include epoxy resin and unsaturated polyester, and a curing agent is added when mixing with cement. Using liquid polymers to modify cement mortars and concrete must use systems that cure in the presence of water. After the polymer is mixed with cement mortar and concrete, the polymer’s curing reaction and the cement’s hydration should proceed simultaneously, thereby forming an interpenetrating network structure of the polymer and the cement gel. This structure can bond the aggregates more firmly and improve the performance of cement mortar and cement concrete.
2. Polymer emulsion
Polymer emulsions, dispersible polymer powders, and other polymers are the common polymers used as modifiers in commercial mortar. A stabilizer system formed by uniformly dispersing a substance in another liquid with fine particles is called an emulsion, which is used as a modifier in commercial mortar, usually by adding a polymerizable monomer in an emulsifier ( In the presence of surface active substances), it is obtained by dispersing fine particles in water. Generally, according to the type of charge on the polymer particles in the polymer emulsion, it is divided into three categories, namely, cationic emulsion (particles are positively charged), anionic emulsion (particles are negatively charged) and non-ionic emulsion (particles are not charged). ). The charge of the polymer particles is determined by the emulsifier used in the production of the emulsion. The emulsion used to modify concrete (mortar) mainly uses non-ionic emulsifiers during polymerization. Usually, the solid content of the polymer emulsion is 40% to 50%, including polymer, emulsifier, stabilizer, and so on.
Polyacrylate emulsion (PAE), vinyl acetate copolymer emulsion (EVA), styrene-acrylic emulsion (SAE), styrene-butadiene latex (SBR), neoprene latex (CR) wait are the most common polymer emulsions used in concrete (mortar) modification.
3. Performance and evaluation of emulsion
Common performance indicators of emulsions include solid content, residual monomer content, minimum film-forming substances, glass transition temperature, particle size and particle size distribution, compatibility, stability, pH value, viscosity, etc.
1. Solid content
The solid content is also called the non-volatile content, which involves calculating the amount of polymer and the water-cement ratio. The determination method of the non-volatile content is usually to take a certain quality of emulsion, dry it under a particular temperature environment, and then express it as a percentage of the dried emulsion mass relative to the original emulsion mass. However, the non-volatile content may also include other components other than the polymer; usually, the non-volatile content is calculated as the polymer content. However, the volatile content is used as the water in the emulsion, and the water in the emulsion must be considered when calculating cement’s water consumption.
2. Residual monomer content
The residual monomer content in the polymer emulsion that has not participated in the polymerization reaction is one of the essential properties of the polymer emulsion. If the residual monomer content is too high, it will not only increase the product cost and monomer consumption but also affect the stability of the emulsion. The hydrolysis of some monomers can change the emulsion system’s pH value, causing the emulsion’s storage stability to deteriorate. In addition, some monomers may have unbearable odors, which will affect the health of construction workers. , Therefore, it should control the residual monomer content in the polymer emulsion below 1%.
3. Glass transition temperature
Glass transition temperature refers to the temperature at which a polymer changes from an elastic state to a glass state, which is an important performance indicator of a polymer. Above this temperature, the polymer exhibits elasticity, and below this temperature, the polymer exhibits brittleness.
Since the polymer in the emulsion still maintains its nature, the emulsion also has a glass transition temperature, which is an index reflecting the hardness of the coating film formed by the polymer emulsion. The emulsion with high glass transition temperature has high hardness, gloss, good stain resistance, and is not easy to pollute. Other mechanical properties are also better. However, the glass transition temperature and the minimum film-forming temperature are also high, which brings some troubles to the use at low temperatures. This is a contradiction, and many of its properties will change when the polymer emulsion reaches a specific glass transition temperature. Significant changes occur, so it must control the proper glass transition temperature. As far as polymer-modified mortar is concerned, the higher the glass transition temperature, the higher the compressive strength of the modified mortar, and the lower the glass transition temperature, the better the low-temperature performance of the modified mortar.
4. Minimum film forming temperature
The minimum film-forming temperature refers to the minimum temperature at which the polymer particles in the emulsion are sufficiently mobile to coagulate with each other to form a continuous film. The polymer particles must form a close-packed configuration in the process of polymer emulsion forming a constant coating film. Therefore, in addition to the excellent dispersion of the emulsion, the conditions for creating a continuous film also include the deformation of the polymer particles, that is, the capillary pressure of water can pull the dispersed polymer particles together. When the water volatilizes, the spherical polymer particles are arranged more closely, which creates considerable pressure between the spherical particles. The closer the spherical particles, the more closely the spherical particles are arranged, and pressure will increase even more. When the particles contact each other, the pressure generated by the volatilization of water forces the particles to be squeezed and deformed to bond to form a coating film. More complex polymer particles are not easily deformed when subjected to external pressure, while softer polymer particles are easily deformed. If most of the polymer particles of the emulsion used as a modifier in construction mortar are thermoplastic resins, the lower the temperature, the greater the hardness and the more difficult it is to deform, so there is a minimum film-forming temperature problem. That is, under a particular temperature condition, after the water in the emulsion volatilizes, the polymer particles are still in a discrete state and cannot be integrated, so the emulsion cannot form a continuous uniform coating film due to the evaporation of water. When the temperature is higher than this specific temperature when the water evaporates, the molecules in each polymer particle will permeate, diffuse, deform, and gather to form a continuous transparent film. The lower limit of the film-forming temperature is called the minimum film-forming temperature.
The minimum film-forming temperature is an essential indicator of polymer emulsions, and it is necessary to use emulsions in low-temperature seasons. Taking appropriate measures can give the polymer emulsion a minimum film-forming temperature that meets the use requirements. For example, adding a plasticizer to the emulsion can soften the polymer so that the minimum film-forming temperature of the emulsion is significantly reduced, or use additives for polymer emulsions with a higher minimum film-forming temperature.
5. Particle size and distribution
The particle size of the polymer emulsion refers to the size of the particles in the polymer emulsion. According to the uniformity of the particle size of the polymer emulsion, it can be divided into monodisperse emulsion and polydisperse emulsion. The polymer emulsions produced in the actual industry are generally polydisperse. For polydisperse emulsions, in addition to the average particle size that can be used to represent its particle size, attention must also be paid to its particle size distribution, that is, the distribution of particles with different particle sizes. The particle size and distribution of the polymer emulsion greatly influence the emulsion’s viscosity, the film-forming properties, and the performance of the coating film. If the particle size of the polymer emulsion is small, its particles are easy to move, and it is easy to enter the gap between the cement and aggregate particles and form a close contact between the cement and aggregate particles. In addition, the emulsion with a smaller particle size has better permeability and is suitable for coating on porous substrates.
6. Stability
The stability of polymer emulsion refers to the ability of the emulsion not to break and coagulate when subjected to mechanical action, chemical medium, and temperature change. The stability of the polymer emulsion includes mechanical stability, chemical stability (compatibility), and the like.
- Mechanical stability
The mechanical stability of polymer emulsion mainly reflects its sensitivity to shear stress. It is prone to coalescence (demulsification) during mortar production if its performance is poor.
(2)Chemical stability
The chemical stability of polymer emulsion refers to its compatibility. When preparing commercial mortar, it must add various chemical substances such as pigments, fillers, and admixtures, and the emulsion should be compatible with these substances; otherwise, the mild one will affect the performance of the emulsion, and the severe one will cause demulsification. Therefore, when preparing mortar, it is necessary to select a polymer emulsion that is compatible with the mortar’s components.
7. pH value
The pH value of the polymer emulsion changes with the variety of the emulsion. For example, the pH value of the styrene-butadiene emulsion applied to cement modification is 10-11; the pH value of the acrylate emulsion is 7-9.
8. Viscosity
Viscosity is an index that characterizes the fluidity of a liquid. The viscosity of polymer emulsions can be measured by different methods, the most common of which is using a rotational viscometer. However, due to the viscosity’s of polymer dependence on the shear rate, the viscosity values measured by using different rotors and rotating speeds are different. In addition, the temperature also significantly impacts the viscosity, and the solid content also has a particular impact on the viscosity. For the same emulsion, the higher the solid content, the greater the viscosity.
