About Polyether Macromonomers and Polycarboxylate Superplasticizer（PCE）
1. Development history
Polycarboxylate superplasticizer（PCE） is the latest generation of concrete admixture, known as the third-generation high-performance water-reducing agent. Compared with the previous generation of naphthalene- based water-reducing agents, Polycarboxylate superplasticizer（PCE） have the characteristics of higher water-reducing rate and better cement adaptability. At the same time, there is no process wasted water and waste gas generated during the production process of the Polycarboxylate superplasticizer（PCE）, which is a green environmental protection material.
Polycarboxylate superplasticizer（PCE） is a high molecular polymer with amphiphilic properties, usually a polyoxyethylene ether macromonomer with a terminal double bond and an unsaturated carboxylic acid small molecule monomer, copolymerized under the action of an initiator Reactive synthesis, in which the terminal alkenyl groups of the large and small monomers form the molecular backbone through copolymerization, and the polyethylene glycol segments of the polyether macromonomers form the structural side chains . Compared with traditional ordinary water-reducing agent products, the remarkable feature of Polycarboxylate superplasticizer（PCE） is the designability of molecular structure. In the water reducer molecule, the unsaturated double bond of the macromonomer end group is polymerized to form the polycarboxylic acid backbone, and the -COO-Na, -SO3-Na and other groups directly connected to the molecular backbone form " “Multi-point anchoring” is adsorbed on the surface of cement particles; while the polyethylene glycol branch of the macromonomer interacts with water molecules through hydrogen bonding to form a solvated polymer chain layer on the surface of the cement particles, which uses steric hindrance to make the cement particles Disperse to achieve water reducing effect. Therefore, the improvement of the water-reducing agent is inseparable from the upgrading of the macromonomer, and the improvement of the molecular structure of the macromonomer can greatly improve the production process and product performance of the Polycarboxylate superplasticizer（PCE）.
Early polycarboxylate superplasticizer products are esters, which macromer is a methyl group polyglycol ether (the MPEG). Ether -based macromonomer polycarboxylate superplasticizer is now mainstream products macromer, macromer mainly by such small molecules of different structures unsaturated alcohol starter ethoxylation reaction synthesis with an end Polyethylene glycol ether with double bond. According to the different molecular structure of the initiator, the synthesized macromonomers can be divided into three types: vinyl alcohol 3- carbon macromonomer (allyl polyethylene glycol ether-APEG), vinyl alcohol 4-carbon and 5- carbon macromonomer macromonomer ( isobutenyl polyglycol ether -HPEG, isopentenyl polyglycol ether -TPEG), and vinyl ethers with 2 + 2 2 + 4 type macromonomer (EPEG, VPEG).
With the rapid development of the Polycarboxylate superplasticizer（PCE） industry, China's polyether macromonomer industry has also made considerable progress, with both output and production capacity continuing to increase, reaching the highest level in history in 2017. The total sales volume and structure of polyether monomers from 2013 to 2017 are shown in Table 1.
2. Advantages of EPEG monomer
The main difference between vinyl ether EPEG monomers and the existing vinyl alcohol 4-carbon and 5- carbon macromonomers lies in the molecular structure of the monomer initiator, ethylene glycol monovinyl ether. In EPEG macromonomer by converting points structural features child, unsaturated double bonds in the molecular structure an oxygen atom directly attached, form a group of the molecular structure of CO bonds. This change in molecular structure shifts the electron cloud distribution of the double bond, thereby improving the charge environment of the unsaturated double bond in the macromonomer, making the reactivity of the double bond in the macromonomer greater than that of the general macromonomer. More, easier to carry out the polymerization reaction.
On the other hand, because the double bond in the molecule is a substituted structure, the steric resistance of the polyether side chain swing is further reduced, so that the polyether side chain swings more freely and the range of motion is larger; the degree of freedom of the polyether side chain swing The increase improves the encapsulation and entanglement of the polyether side chain, so that the synthesized Polycarboxylate superplasticizer（PCE） has higher adaptability, especially for the situation of poor sand and gravel quality and high mud content.
In the reaction of 4-carbon 5-carbon monomers with acrylic acid, the constant k11 is relatively large, resulting in a mismatch between the reactivity ratios of large and small monomers. For example, when a 4-carbon monomer (HPEG) and acrylic acid (AA) undergo a copolymerization reaction, the reactivity ratios of the two are: r1(AA)=1.70, r2(HPEG)=0.017, the active monomer acrylic acid is more prone to self-polymerization . Therefore, in order to reduce the R11 reaction and promote the R12 reaction, the concentration of acrylic acid in the reaction system must be reduced. Therefore, it is necessary to adopt the method of dripping acrylic acid and control the dripping time to 2~3h, which is relatively long. The double bond activity of EPEG-type monomers is high, the constants k12 and k21 are relatively large, and the copolymerization chain extension reaction is easier to proceed. The increase in the concentration of acrylic acid will not cause the rapid occurrence of the R11 reaction. Therefore, the dripping time of acrylic acid can be greatly shortened. It takes 30 minutes to add dropwise to complete the reaction. This monomer can be reacted in a one-pot method. The prepared dripping liquid is added to the reaction system at one time, and the reaction can be completed after stirring for 30-40 minutes. The performance of the synthesized water reducing agent is also excellent, which can greatly simplify the reduction of polycarboxylic acid. The production process of liquid.
The reactivity ratio of EPEG-type monomers and small acrylic monomers determines that their reaction is closer to the ideal constant ratio copolymerization, and the reaction process is easier to control. Therefore, the reaction process can be adjusted more appropriately. In terms of reaction temperature, the reaction temperature of 3-carbon monomer is 60°C~70°C, which requires a higher reaction temperature; 4-carbon 5-carbon monomer usually reacts at 30°C~40°C; while EPEG monomer has a lower reaction temperature The requirement is very low. During the polymerization reaction, the reaction can be completed at a room temperature higher than 5°C without heating. In addition, the advantages of EPEG macromonomers are also reflected in the starter ethylene glycol monovinyl ether. The initiator is produced by the direct reaction of acetylene gas and ethylene glycol, and no unnecessary by-products are produced. The detailed process is shown in Figure 5. In the reaction process, acetylene is an excess raw material, and the unreacted acetylene can be recycled and reused through the recycling system, without causing environmental pollution, and has significant advantages of sustainable development.
3. Future development direction
In recent years, China's domestic polyether monomer market has continued to develop, and the product structure has undergone significant changes. There are endless varieties of new functionalized macromonomers, which fully reflects the overall industrial chain of domestic Polycarboxylate superplasticizer（PCE） macromonomers. The rapid development and continuous improvement of the level also reflect the continuous improvement of the quality requirements of the large monomers by many water-reducing agent‘s manufacturers.
With the commissioning of new and expanded ethoxylation projects in China and the improvement of production technology, the domestic production capacity of polyether monomers will be further expanded, and competition among enterprises will become more intense. Therefore, in addition to continuously improving the quality of their own products and optimizing process technology, domestic polyether monomer manufacturers should also focus on the development and application of new functional polyether monomers to meet the needs of different functional water-reducing agents. Meanwhile, with Zhong Guo environmental remediation efforts continue to increase, both the initiator of the manufacturer, or water reducer manufacturers are constantly improving their production technology, provide more environmentally friendly, green products and processes. This also means that the development of macromonomers in the future, from the production of initiators to the ethoxylation process, needs to pay more attention to meeting environmental protection requirements.
Under this development trend, the emergence of vinyl ether macromonomers has provided a new direction for domestic macromonomer manufacturers. At present, many macromonomer manufacturers have launched the production of vinyl ether macromonomers, It has also been widely used in water manufacturers. The outstanding advantages of vinyl ether macromonomers in terms of production technology and product performance will make it the main trend of macromonomer development in the future. At the same time, the production of vinyl ether macromonomer starters has now entered the stage of large-scale production, and the price of the starter will continue to decline, making vinyl ether macromonomers more competitive in the market.
The new polyether macromonomer represented by EPEG macromonomer has no pollution in the production process of the starting agent, can adapt to low-temperature synthesis of polycarboxylic acid water reducer, has high double bond activity, simple synthesis process, and water reducer performance With excellent characteristics, and the entire production process meets the requirements of green, low-carbon and environmental protection, it will surely become the mainstream variety in the polycarboxylate water-reducing agent market, with good economic efficiency and promotion value. At the same time, the development of EPEG macromonomers can also further promote Polycarboxylate superplasticizer（PCE） products, which will continue to develop in the direction of functionalization, specialization and ecologicalization in the future.
