Research on the Synthesis and Performance of High-Performance Polycarboxylate Superplasticizers

Introduction

Polycarboxylate superplasticizers are an indispensable part of the field of new building materials. As an efficient admixture, they can form a good technical complementarity with new building materials, enhancing the overall performance of building materials. Their joint use can reduce energy consumption and environmental pollution, promoting the green development of the construction industry. Currently, it is of great practical significance to prepare high-performance polycarboxylate superplasticizers through scientifically effective process optimization experiments, optimize the synthesis process of PCEs, and reduce cement usage and achieve low-carbon concrete through effective molecular structure functional design based on simple technology and abundant raw materials.

Prepared the most effective polycarboxylate superplasticizer based on polyethylene glycol monomethyl ether (EPEG) by studying the effects of acid-to-ether ratio, dripping time, mercaptopropionic acid dosage, initiator dosage, reductant dosage, catalyst dosage, and polymerization temperature on its performance [1]. Used acrylic acid, trimethylammonium methacrylate chloride, and polyether as raw materials, and determined the optimal synthesis process through orthogonal experiments [2]. The amphoteric polycarboxylate superplasticizer synthesized at room temperature exhibited better dispersing performance and significant enhancement effects. Used the Hâ??Oâ??E51 system as an initiator-reductant to enable the polymerization of polyethylene glycol monomethyl ether (EPEG) with acrylic acid at room temperature, resulting in the preparation of high-performance polycarboxylate superplasticizer [3]. Compared with commercially available HPEG-type superplasticizers, PCE-E exhibited better water-reducing rate and slump retention performance.

Therefore, this paper uses ethylene glycol monovinyl polyethylene glycol ether (hereinafter referred to as EPEG) as the macromonomer, acrylic acid as the monomer, and introduces phosphate groups. It utilizes single-factor experiments to determine the impact of the acid-ether ratio, reaction temperature, and chain transfer agent dosage on the performance of the water-reducing agent. Scientific and effective optimization methods are employed to optimize its synthesis process. Through effective molecular structure functional design, it aims to reduce the use of cement and improve the dispersion and slump-retaining performance of concrete, based on simple technology and abundant raw materials.

Experiments

Raw Materials

Ethylene oxide-terminated polyethylene glycol (EPEG3000); acrylic acid (AA); polymeric functional monomer (small molecule unsaturated monomer with phosphate ester groups); hydrogen peroxide (H2O2); sodium hydrosulfite (SH); mercaptoethanol (ME); ferrous sulfate; caustic soda flakes; tap water (W). Cement (Qingpeng P�O42.5R); fly ash (Grade II); sand (machine-made sand, fineness modulus 2.4); continuously graded gravel; commercially available superplasticizers.

Synthesis

Add EPEG, AA, and deionized water to the four-necked flask and secure it. Set the temperature of the water bath and turn on the stirrer to dissolve them. Weigh AA and deionized water, dissolve them together to prepare solution A. Weigh ME, SH, and deionized water, dissolve them together to prepare solution B. Once the polyether macromonomer in the base solution is fully dissolved and reaches the set temperature, add the polymerization functional monomer, 1% ferrous sulfate solution, and Hâ??Oâ?? in sequence. Stir for 5 minutes, then turn on the automatic dropping device to add solution A and B drop by drop, initiating the reaction. After the dropping is complete, continue to maintain the temperature for 60 minutes. After the reaction is over, observe the temperature rise curve on the dropping instrument. If no abnormalities are found, a colorless transparent EPEGtype polycarboxylate high-performance superplasticizer is obtained.

Test Method

Cement slurry flow test: prepare the instruments cement slurry mixer, conical mold (upper diameter 36mm, lower diameter 60mm, height 60mm, metal product with smooth inner wall without seams), glass plate (400×400×5mm), stopwatch, steel ruler, scraper, etc. According to GB/T 8077-2012 "Test Method for Homogeneity of Concrete Admixtures", place the glass plate on a level position and wet the glass plate, conical mold, mixer, and mixing pot with a damp cloth to make their surfaces damp but not wet. Place the conical mold in the center of the glass plate and cover it with a damp cloth for use. At room temperature, weigh 300g of cement and pour it into the mixing pot. Add an appropriate amount of admixture and 87g of water, stir for a certain period of time. Quickly pour the mixed slurry into the conical mold and scrape it flat with a scraper. Then lift the conical mold vertically and start the stopwatch at the same time. Allow the cement slurry to flow on the glass plate for 30 seconds, then measure the diameter of the flowing part perpendicular to two directions with a ruler, and take the average value as the cement slurry flow rate.

Results and Discussion

Effect of Acid-Ether Ratio on the Performance of EPEG- type Polycarboxylate Superplastici-zer

Research has shown that the carboxyl groups in water reducers readily adsorb onto cement particles through electrostatic interactions, generating spatial repulsion that disperses the cement particles. If there are too few carboxyl groups, the electrostatic repulsion between cement particles is weak, and the spatial hindrance effect cannot be fully exerted [4]. As illustrated in Figure 1, the dispersion performance is optimal when the acid-to-ether ratio is 4.51 [5].

Figure 1: Effect of Acid-Ether Ratio on Dispersion Performance

Impact of the Amount of Polymeric Functional Monomer on the Dispersibility of EPEG-Type Polycarboxylate Super-plasticizer

The polymeric functional monomer is a small unsaturated monomer with phosphate ester groups, which carries two negative charges. The phosphate ester groups can rapidly adsorb and coat cement particles. When cement particles approach each other, electrostatic repulsion is formed, promoting their rapid dispersion. When the content of phosphate ester monomers is excessive, the excessive phosphate ester groups can cause chain transfer effects, making it difficult to control the structure and relative molecular mass of the water-reducing agent, and deteriorating the dispersion effect. As shown in Figure 2, when the amount of polymeric functional monomer is 2.22% of the mass of EPEG-3000, the dispersion effect is the best [6].

Figure 2: Effect of the dosage of polymeric functional monomers on dispersion

Effect of Temperature on the Dispersing Performance of EPEG-Type Polycarboxylate Superplasticizer

As shown in Figure 3, when the temperature is below 20°C, the fluidity tends to decrease. This is because the number of radicals generated in the reaction system is relatively low, resulting in a slower polymerization rate and incomplete reaction. As the temperature gradually increases, the number of radicals also gradually increases, accelerating the polymerization rate with the macromonomer and gradually improv-ing the dispersing performance of the water reducer. However, when the temperature exceeds 20°C, the radical polymerization rate becomes too fast, which can easily lead to explosion polymerization or side reactions, thereby causing a decrease in dispersing performance [7].

Figure 3: Effect of Temperature on the Dis-persing Performance of EPEG-Type Poly-carboxylate Superplasticizer

Effect of Chain Transfer Agent on the Dispersing Perfor-mance of EPEG-Type Polycarboxylate Superplasticizer

Free radical polymerization exhibits high reaction rates and readily forms high-molecular-weight polymers. A common method to regulate and control the molecu-lar weight of the synthesized polymers is through the use of chain transfer agents. The impact of the amount of chain transfer agent on the fluidity of the water reducer paste is illustrated in Figure 4. When the amount of chain transfer agent is low, it cannot effectively control the monomer conversion rate of the reaction, leading to larger molecular weights of the synthetic product. As the amount of chain transfer agent increases, the monomer conversion rate of the synthetic product decreases, resulting in a reduction in molecular weight and a gradual enhancement of disper-sion performance [8].

Figure 4: Effect of Chain Transfer Agents on the Dispersing Performance of EPEG-Type Polycarboxylate Superplasticizers

Effect of Acrylic Acid Dosage in the Base Solution on the Dispersion Performance of Epeg-Type Polycarboxylate Super-plasticizer

As the amount of AA in the base solution decreased (corresponding to an increase in the pH of the base solution), the experimental test values for the cement paste initially increased and then decreased. When the amount of AA in the base solution was 1.2% of the EPEG mass, the test value reached its maximum. The results indicate that as the pH increases, the degree of ionization of the carboxyl groups increases, leading to stronger electrostatic repulsion between polycarboxylate molecules. This causes the polycarboxylate molecular chains to become more extended, thereby improving the dispersion performance of C6-PCE. However, further reducing the amount of AA in the base solution, resulting in a further increase in pH, leads to a decrease in the number of charges on the polycarboxylate molecules, a reduction in electrostatic repulsion, and a curling of the molecular chains. Consequently, the dispersion performance of the polycarboxylate superplasticizer decreases. As show in figure 5.

Figure 5: Effect of acrylic acid dosage in the base solution on the dispersion performance of EPEG-type polycarboxylate superplasticizer

Infrared Spectroscopy Analysis

From Figure 6, it can be observed that a strong peak at 3431.88 cm^-¹ corresponds to the stretching vibration peak of intramolecularly associated hydroxyl groups; at 1281.16 cm^-¹ and 1240.58 cm^-¹, there are asymmetric stretching vibration peaks of ether bonds (C—O—C) or ester bonds (—COO), respectively, and a characteristic absorption peak of C=O appears at 1728.13 cm^-¹. These two peaks indicate the presence of ester groups. In summary, the molecular structure of this water-reducing agent contains hydroxyl, carboxyl, ester, and other groups, which is consistent with the expected synthetic structure.

Figure 6: Optimal process infrared spectrogram

GPC Analysis

Samples synthesized according to the optimal process were tested by Gel Permea-tion Chromatography (GPC),The test results are presented in Table 1.

                                                                        Table 1: Gel Chromatography Test Results

According to the table, the polydispersity index is 1.69, indicating a relatively concentrated molecular weight distribution. Additionally, the monomer conversion rate is 83.33%, which is high, suggesting that the reaction proceeds relatively completely and the monomer conversion rate is high.

Concrete Performance Verification

A comparative verification of concrete performance was conducted between the commercially available EPEGtype polycarboxylate superplasticizer and the synthesized superplasticizer. The concrete mix proportions and results are presented in Tables 2 and 3. The experimental data indicate that the initial spreading degree of the homemade superplasticizer is greater than that of the commercially available one, and it exhibits better workability. Additionally, the homemade superplasticizer experiences less loss in performance after 2 hours.

                                                                           Table 2: Mix Proportion of Concrete

                                                Table 3: Impact of Different Water-Reducing Agents on Concrete Performance

Conclusion

The optimal process parameters for synthesizing the EPEG polyether macromonomer are as follows: the concentrations of the polymeric functional monomer and chain transfer agent are 2.22% and 0.44% respectively, the acid-to-ether ratio is 4.51, the reaction temperature is 20°C and the dosage of acrylic acid in the base solution is 1.2%. Infrared spectroscopy analysis reveals that the majority of the monomers have participated in the radical copolymerization reaction, and the molecular structure is consistent with the expected design. Gel permeation chromatography analysis shows that the molecular weight distribution of the synthesized water reducer is relatively concentrated, and the monomer conversion rate is high, indicating that the reaction has proceeded relatively completely.

References

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