temperature and esterification rate
The effect of water-carrying agent dosages on the equilibrium reaction temperature and esterification rate is shown in Table 2.
From Table 2, the esterification rate is the highest when the dosage of water-carrying agent is 70 g and the corresponding equilibrium reaction temperature is 119°C. In the range of water-carrying agent dosage from 50 to 70g, the esterification rate increases with the increase of water-carrying agent dosage, which results from the higher water-carrying efficiency (corresponding to higher dosage of water-carrying agent), leading to higher esterification rate. However, with continual increase of water-carrying agent dosage, the esterification rate decreased. This is attributed to the excessive amounts of water-carrying agent, leading to a low reactant concentration and a low esterification rate. Correspond- ingly, the equilibrium reaction temperature decreased with the increase of water-carrying agent dosage, which is also caused by the reflux of a great deal of cooled water-carrying agent.
Effects of catalysts and their dosages on the esterification rate
The concentrated sulfuric acid and p-toluene sulfonic acid were selected as catalysts. The ratio of acid to pentaerythritol is 1.25 and the dosage of water-carrying agent is 70 g in this experiment. The effects of catalysts and their dosages on the esterification rate are shown in Fig. 2.
From Fig. 2, the esterification rate is the highest when the molar ratio of p-toluene sulfonic acid to alcohol is 0.07 and the molar ratio of sulfuric acid to alcohol is 0.11. However, the real value of esterification rate is likely to be affected owing to the total amount of water containing a part of dehydrated water for the dehydrating property of concentrated sulfuric acid. Insufficient catalyst dosages can lead to incomplete esterifica- tion reactions; similarly, excessive catalyst dosages can also lead to low esterification rate, which is possibly because the excessive dosage of catalyst leads to the occurrence of side reactions. Overall, the p-toluene sulfonic acid is chosen as the esterification catalyst, and its molar ratio to alcohol is 0.07.
Effects of inhibitors and their dosages on the esterification rate
The hydroquinone and phenothiazine were selected as inhibitors in this experiment; besides, the ratio of acid to pentaerythritol, the water-carrying agent dosage, and the molar
Table 2–Equilibrium reaction temperature and esterification rate at different dosages of water-carrying agent
Water carrying agent, g Equilibrium reaction temperature, °C Esterification rate, %
50 128 58 60 122 57.9 70 119 66.5 75 118 62 80 117.8 63 85 117 62.4
Preparation and Characterization of Star-Shaped Polycarboxylate Superplasticizer 187
ratio of catalyst to alcohol are 1.25, 70 g and 0.07, respectively. The effects of inhibitors and their dosages on the esterification rate are shown in Table 3. The dosage is the molar percentage of inhibitor to AA.
From Table 3, phenothiazine plays an inhibiting effect at the lower dosages; whereas the hydroquinone can exhibit an effective inhibiting effect only at the relatively higher dosages. It also can be seen from Table 3 that the esterification rates are the highest when the molar ratios of hydroquinone and phenothiazine to AA are 4% and 1.1%, respectively. With the continual increase of inhibitor dosage, the esterification rate decreases. This is probably because the acrylic acid is apt to homopolymerize with each other rather than esterification reaction when the dosages of inhibitors are too low, leading to the decrease of esterification rate. With the continual increase of inhibitor dosage, the free radicals produced from reac- tion system will be excessively captured, which can interfere with or even eliminate the
Fig. 2–Esterification rates at catalysts’ different molar ratio to alcohol.
Table 3–Effects of inhibitors and their dosage on the esterification rate
Catalyst Dosage, % Esterification rate, %
Hydroquinone 2 94.9 3 95.0 4 95.3 4.5 93.3 5 93.0 Phenothiazine 0.2 88.7 0.5 93.1 0.8 92.4 1.1 95.6 1.4 91.7 1.7 94.2
role of catalyst and leads to the decrease of esterification rate. Therefore, the proper molar ratios of hydroquinone and phenothiazine to AA are 4% and 1.1%, respectively.
Effect of reaction times on the esterification rate
The esterification rate is also affected by the reaction time, and thus the investigated reaction times were selected from 4 hours to 8 hours. In this experiment, the ratio of acid to pentaerythritol, the water-carrying agent dosage, and the molar ratio of catalyst to alcohol are 1.25, 70 g and 0.07, respectively. The effect of reaction times on the esterification rate is shown in Fig. 3.
The results show that the esterification rate increases with the increase of reaction time in the range of 4-6 hours. The esterification rate directly affects the quality of esterification product, and further correlates to the performance of the final synthesized PCE. Besides, the esterification reaction time should be theoretically prolonged at a proper esterification temperature to ensure the esterification rate as high as possible. However, in Fig. 3, the esterification reaction rate gradually increases after reacting for 6 hours, and it changes slightly with the extension of reaction time. Based on economical considerations and energy conservation, the proper esterification time therefore is determined as 7 hours.
Characterization of esterification product
The esterification product in this study was characterized by 1H NMR (Fig. 4).
The esterification reaction occurred between the hydroxyl groups of pentaerythritol and the carboxyl groups of AA. During the process of the esterification reaction, its methy- lenes initially linked to the hydroxyl groups change to link to ester groups. As a result, the electron withdrawing effect of ester structure leads to its peak shifting to the lower field. From the peak positions in Fig. 4, the primary peaks of esterification product in 1H NMR
spectrum correspond to the three H atoms on the -CH=CH2 bonds (6.154ppm, 6.328ppm,
Fig. 3–Esterification rates for different reaction times.
Preparation and Characterization of Star-Shaped Polycarboxylate Superplasticizer 189
5.963ppm). The structure diagram displayed in Fig.4 clearly indicates the peaks and their corresponding H atoms in the molecular structure. All of the displayed characteristic peaks confirm the occurrence of esterification reaction and the indication of a relatively ideal molecular structure for the esterification product.
Effects of AA/IPEG ratios on fluidity performances of paste mixing with synthesized PCE
After the confirmation of structure of esterification product which is used for the subse- quent polymerization, the relevant polymerization conditions also should be determined. The effects of AA/IPEG ratios on the fluidity performances of paste mixing with synthe- sized PCE at a dosage of 0.15% are shown in Fig. 5.
The ratio of AA to IPEG affects PCE’s structure and performance in cement paste. From
Fig. 5, the fluidity of cement paste increased slightly with the increase of AA/IPEG ratio
in the range of 2.6-3.3. This is possibly because the effective adsorption sites increase caused by the increased proportion of carboxyl groups with the increase of AA/IPEG ratio. Thereafter, the fluidity of cement paste reached maximum at the AA/IPEG ratio of 3.3 and then decreased with a continual increase of AA/IPEG ratio. This is possibly because the continual increase of AA/IPEG ratio is equivalent to decrease the density of side chain, which is a key factor to determine the workability and efficiency of PCE. Too low density of side chain will lower the steric hindrance effect of side chain (polyoxyethylene), and thus leads to the decrease of cement paste fluidity.
Effects of initiator/IPEG ratios on fluidity performances of paste mixing with synthesized PCE
There were better fluidity performances of cement pastes at the initiator/IPEG ratio of around 0.3 by experiments, and thus the initiator/IPEG ratio was investigated in the range of 0.26-0.38. The effects of initiator (APS)/IPEG ratios on the fluidity performances of paste mixing with synthesized PCE at a dosage of 0.15% are shown in Fig. 6.
From Fig. 6, the fluidities of cement paste increased and then decreased with the increase of APS/IPEG ratio; besides, the cement paste exhibited the best fluidity at the APS/IPEG ratio of 0.28. The dosage of initiator affects the polymerization rate and molecular weight in a free radical polymerization. Too low dosage of initiator will produce fewer primary radicals, and thus cause incomplete polymerization. With the increase of initiator dosage, the polymerization rate is accelerated and the reaction becomes sufficient; furthermore, the suitable initiator dosage is helpful to obtain the product with suitable molecular
Fig. 5–Fluidities of cement paste mixing with PCE at different AA dosages.
Fig. 6–Fluidities of cement paste mixing with PCE at different initiator/IPEG ratios.
Preparation and Characterization of Star-Shaped Polycarboxylate Superplasticizer 191
weight, which optimizes its adsorption rate on the surfaces of cement particles and ensures good fluidity retention. Too high dosage of initiator will reduce the molecular weight of PCE, which can weaken the steric hindrance effect and further lead to the low fluidity performance.
Characterizations of polymerization product
The synthesized polymerization product was characterized by 1H NMR to confirm its
star-shaped structure. The 1H NMR spectrum of the synthesized polymerization product is
shown in Fig. 7.
From Fig. 7, the peaks at 4.5-5.0 ppm and 3.3-3.8 ppm were distinctly higher than other peaks, because they correspond to the H atoms of the polyethylene glycol connected with the arm structure. Also from Fig. 7, the peaks at 2.314 and 1.645 ppm correspond to the H atoms of methylenes, whose schematic structure diagram is also displayed in Fig. 7. It is well-known that the peaks corresponding to H atoms of methylenes will not be displayed at these positions if the esterification product does not polymerize with other monomers or forms other products. The total integral area for these two peaks also indicates many star- shaped structures have formed. All of these characteristic peaks confirm the polymeriza- tion reaction and the expected polymerization product with a star-shaped structure.
The FTIR spectra of the conventional comb-shaped PCE and synthesized star-shaped PCE are shown in Fig. 8, and these spectra were analyzed by means of other reported references.13,14
It can be seen from Fig. 8 that the two PCEs both exhibit characteristic peaks at around 1108cm-1 and 1554cm-1. The peak at 1108 cm-1 belongs to the vibration of polyethylene
glycol chain, and the characteristic peak at 2500-3300 cm-1 corresponds to its stretching
vibration. The peak at 1554 cm-1 belongs to the symmetric vibration of C=O bond in
carboxylic acid. By contrast, there is a large peak at around 1772cm-1 in the spectrum
of the star-shaped PCE but no peak at this position in the spectrum of the conventional
comb-shaped PCE. The peak in this position corresponds to the ester groups, and thus, this result proves the star-shaped structure of this PCE due to the esterification reaction. All of the analysis indicates that the star-shaped PCE has not only the conventional groups such as polyethylene glycol and carbonyl groups, but also the characteristic groups, i.e., ester groups, which confirm the completion of reactions and the achievement of the targeted structure.
Fluidity performances of the pastes mixing with PCEs
The star-shaped PCE was synthesized according to the above determined conditions; besides, the conventional PCE as reference was synthesized with the same conditions as the polymerization-step of star-shaped PCE. The fluidity performances of the cement pastes mixing with the two PCEs respectively at a dosage of 0.15% are shown in Fig. 9.
From Fig. 9, the saturation dosages of the synthesized star-shaped and conventional comb-shaped PCEs were about 0.3% and 0.4%, respectively. These results indicate that excellent fluidity performance can be achieved for lower dosages of star-shaped PCE, demonstrating its high workability efficiency. Furthermore, the fluidity values of the synthesized star-shaped PCE were higher than those of conventional comb-shaped PCE over the range of experimental dosages.
Adsorption mechanism of star-shaped PCE in cement paste
As is well-known, the carboxyl groups of PCE adsorbs on the surfaces of cement particles when added to the cement paste; besides, the hydrophilic part, i.e., polyethylene glycols of PCE can retain the fluidity of cement paste. Thus, the PCE’s adsorption behavior on the surfaces of cement particles is an important factor to investigate the mechanism of star-shaped PCE in cement paste. The adsorption behaviors and the relationship between
Fig. 8–FTIR spectra of the synthesized polymerization product.
Preparation and Characterization of Star-Shaped Polycarboxylate Superplasticizer 193
adsorption amount and fluidity for star-shaped and conventional PCEs in cement pastes are shown in Fig. 10 and Fig. 11 respectively.
It can be seen from Fig. 10 and Fig. 11, the adsorption amount and rate of star-shaped PCE are higher than those of conventional comb-shaped PCE, but also the similar results were displayed in terms of relationship between adsorption amount and fluidity. This is caused by their different molecular structures. For the star-shaped PCE compared with conven- tional PCE, its multi-arm structure leads to a stronger steric-hindrance effect; moreover, it
Fig. 9–Fluidities of cement pastes mixing with PCEs at different dosages (W/C=0.29).
Fig. 10–Adsorption behaviors of star-shaped and conven- tional PCEs in cement paste (PCE dosage=0.15:29).
has smaller hydrodynamic volume15 which leads to a higher contents of polar groups per
unit volume and has more adsorption sites which leads to a stronger affinity and a higher probability of surface adsorption. This is in accordance with the results reported by Kazuo Yamada.16 Also, with the hydration process, a part of conventional PCE molecules gradu-
ally lose the workability for the coverage of hydrate layer by its lower adsorption amount and more surfaces of cement particles. This well agrees with other researchers’ study17
investigating the effects of charge density of PCE on the dispersant behavior and showing that the dispersion effect of PCE is well correlated to the amount of adsorbed polymer and can be interpreted in terms of surface coverage. Then, we suppose that the star-shaped PCE still has other “arms” dispersed in the paste pore solution, and the hydrate layer is still covered by these free “arms”, leading to good dispersion retaining of cement particles in the cement paste. This speculative mechanism is diagramed in Fig. 12.
Based on the above results and analysis, the good fluidity performances of star-shaped PCE are attributed to its special multi-arm structure, which has higher energy efficiency to achieve a better workability in cement pastes. This star-shaped PCE is one member of a large PCE family which has very broad performances. It is meaningful for researchers to investigate the PCE with novel structure, which can diversify the PCE family.
CONCLUSIONS
Based on the results of this experimental investigation, the following conclusions are drawn:
1. A star-shaped PCE was successfully synthesized through two-step reactions: the first esterification between pentaerythritol and AA, and the second free radical polymerization among the esterification product, IPEG and AA.
Fig. 11–Relationship between adsorption amount and fluidity for star-shaped and conventional PCEs in cement paste.
Preparation and Characterization of Star-Shaped Polycarboxylate Superplasticizer 195
2. The esterification rate can reach above 95% at a water-carrying agent dosage of 70g, a catalyst/alcohol molar ratio of 0.07:1, an inhibitor (hydroquinone) /AA molar ratio of 0.04:1 (or phenothiazine/AA molar ratio of 0.011:1), and a reaction time of 7 hours; furthermore, in the second polymerization step, the best fluidity of cement paste was achieved at the initiator/AA/IPEG ratio of 0.28: 3.3: 1.
3. The structure of esterification product was characterized by its 1H NMR spectrum,
which confirms the occurrence of esterification; moreover, the structure of final synthe- sized star-shaped PCE was characterized by its 1H NMR and FTIR spectra, which confirm
the completion of subsequent polymerization and the achievement of an ideal star-shaped structure.
4. The saturation dosages of the synthesized star-shaped PCE and a conventional comb- shaped PCE are 0.3% and 0.4% respectively, indicating star-shaped PCE’s dosage improved efficiency; besides, the synthesized star-shaped PCE exhibits better fluidity performance and adsorption behaviors.
5. The improved performances of star-shaped PCE are attributed to its novel branched multi-arm structure with higher energy efficiency, and thus it can be considered as a new- type PCE to provide the theoretical basis and technological application in cement and concrete research.
AUTHOR BIOS
Xiao Liu is an associate professor at the College of Materials Science and Engineering
in Beijing University of Technology. He majored in Materials Science and Engineering and received his PhD from Beijing University of Chemical Technology. His research interests include functionalized polycarboxylate superplasticizer with high performance by molecular structure design, as well as the interface adsorption of polycarboxylate superplasticizer in cement paste system.
Fig. 12–Adsorption mechanism of star-shaped PCE in cement paste.
Ziming Wang is a professor at the College of Materials Science and Engineering in
Beijing University of Technology. He majored in Materials Science and Engineering and received his PhD from Beijing University of Technology. He is a vice executive secretary of Association of Concrete Admixture. His research interests include high-performance cement-based materials and rheology of cement paste.
Jie Zhu is a Researcher Engineer at Beijing BBMG Cement Energy Technology Co., Ltd. Ming Zhao is a master student in Beijing University of Technology.
Wei Liu is a master student in Beijing University of Technology. Dongjie Yin is a master student in Beijing University of Technology.
ACKNOWLEDGMENTS
The authors wish to express their gratitude and sincere appreciation to the National Natural Science Foundation of China (Grant number: 51208012), Research Fund of New Teachers for the Doctoral Program of Higher Education of China (Grant number: 3c009011201301), Project of Central Research Institute of Building and Construction Co., Ltd (Contract number: CBM2014Ky01-01) and Project of China Railway Engineering Materials Technology (Anhui) Ltd (Project number: 40009011201409) for financing this research work.
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