Scale inhibitors with a hyper-branched structure: preparation, characterization and scale inhibition mechanism

Abstract

In order to improve the scale inhibition efficiency of existing scale inhibitors for industrial water and to reduce the phosphorus pollution of water bodies, a new type of scale inhibitor with a hyper-branched structure has been developed in this study. First, an AB′ type of functional monomer (AMA) was synthesized from maleic anhydride (MAH) and propylene glycol, then copolymerized with monomer B (MAH) through radical polymerization, resulting in a hyper-branched polycarboxylic acid. The synthesis conditions, such as reaction temperature and time, monomer ratio and initiator dosage, have been investigated for obtaining the expected hyper-branched polymer with good scale inhibition performance. The scale inhibition efficiency of the obtained products was determined according to their resistance to the crystallization of calcium sulfate and calcium carbonate under the optimal application conditions. The experimental results show that the hyper-branched polycarboxylic acid provides a scale inhibiting efficiency for CaCO₃ and CaSO₄ as high as 95.2% and 92.3%, respectively. In addition, XRD analysis showed that the good scale inhibition of the hyper-branched polycarboxylic acid is attributed to its ability to inhibit and destroy the formation of crystals, changing the crystal forms of the calcium scale. This conclusion indicates that the prepared hyper-branched polycarboxylic acid has great application potential in the treatment of industrial water.

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1 Introduction

Industrial water contains large amounts of calcium, magnesium, barium and other metal ions, and these metal ions are able to form scale in the form of carbonate and sulfate due to external condition changes.¹ Scale in equipment or pipes decreases the heat conduction severely. What’s worse, it may even lead to explosion, causing huge economic losses.² Therefore, adding scale inhibitors into the circulating water is one of the most effective methods of preventing the formation of scale.³

Present commercial scale inhibitor products are mainly natural materials, phosphoric acids, sulfonic acids and carboxylic acids, etc. Natural materials used for inhibition are usually neglected due to their unstable composition and lower scale inhibiting efficiency,⁴ though they can be easily extracted and are environmentally friendly.⁵ Phosphoric acid, which has a stable chemical structure, high resistant temperature and high scale inhibition efficiency, has been widely used in the past ten years.⁶ However, due to it being a water pollutant, phosphoric acid was replaced by novel sulfonic acids and carboxylic acids. The synthesis of sulfonic acid is tedious and expensive.⁷ In comparison, carboxylic acid can be prepared relatively easily, exhibits good scale efficiency and can be used in harsh environments, making it widely used in industry nowadays.⁸

Research has demonstrated that chemical inhibitors mainly prevent the formation of calcium sulfate or calcium carbonate crystals. Thus scale inhibitors are generally waterborne and their molecules usually contain a large amount of groups that have strong complexation abilities, but do not contain nitrogen, phosphorus and other nutrient elements to guard against water contamination.⁹

Hyper-branched polymers are characterized by their high solubility, low viscosity, good fluidity and large number of terminated functional groups.¹⁰ Once immersed in water, they can be easily adsorbed onto the calcium carbonate crystal surface, whilst the large number of cavities inside the hyper-branched molecules will wrap around the small crystals. In addition, the three-dimensional structure of hyper-branched polymers can notably disrupt the crystal formation and growth of calcium carbonate. Therefore, these characteristics give the hyper-branched polymers a special advantage for use in scale inhibition.¹¹

In this paper, a novel type of carboxyl-terminated waterborne hyper-branched polyester (HBP) has been designed, synthesized and characterized. First, the carboxyl and ester groups were introduced into the hyper-branched polyesters to increase the solubility of calcium carbonate, calcium sulfate and calcium phosphate in water but also to enhance the coordination ability of the HBP with calcium ions. Then, the scale inhibition mechanism of the developed hyper-branched polymers was investigated and proposed.

2 Experimental

2.1 Materials

Monomers maleic anhydride (MAH, 99%), which was used as received, and allyl alcohol (99%), which was dried by anhydrous Na₂SO₄, were both purchased from Aladdin Chemical Reagent Corp (Shanghai, China). Benzoyl peroxide (BPO) was purchased from Kelong Chemicals Corp. Ltd. and recrystallized with tetrahydrofuran (THF). All solvents, including chloroform, dichloromethane, benzene and THF were analytical grade, and they were used as received without further purification.

2.2 Synthesis of AB′ functional monomer and HBP

A mixture of allyl alcohol and MAH was added into a 250 mL four-necked flask, equipped with a mechanical stirrer and thermometer in a thermostatic oil bath, and the reaction was left for 12 hours within the temperature range of 40–55 °C. Vacuum evaporation was then conducted for 2 hours to remove any unreacted MAH and allyl alcohol. Finally, a clear, light yellow AB′ functional monomer (AMA) was obtained with a yield as high as 90%.

The HBP was synthesized by copolymerization of AMA with MAH. In a 250 mL flask, the initiator BPO was added into dry THF​ to dissolve, then added the mixture to the reaction vessel, the reaction was heated to 50 °C and left for 24 hours. The product was precipitated and washed with methylbenzene, then extracted in chloroform or dichloromethane for 24 hours to remove residual MAH and dried under vacuum. After that, a series of HBPs were synthesized, and saturated NaHCO₃ solution was used to dissolve and neutralize the obtained HBPs in order to increase their water solubility and stability within certain pH regions.

2.3 ¹H-NMR and FTIR characterization of HBP

A ¹H-NMR (AM 300 MHz Nuclear Magnetic Resonance, Bruker USA) spectrum of the HBP was recorded with D₂O and DMSO-d₆ as a mixed solvent. FT-IR spectra of the HBP were recorded using KBr pellets in a Nicolet MX-1E FTIR spectrometer (Nicolet, Japan) between 4000 and 400 cm⁻¹.

2.4 Molecular weight and polydispersity of HBP

Gel permeation chromatography (GPC) was used to determine the molecular weight and polydispersity of the HBP. GPC was carried out with a Waters 515 pump and a Waters 2410 refractive-index detector equipped with an OH pak KB-803 HQ column at 20 °C. The polyethylene glycol (M_w = 5900) was used as the standard of weight-average molecular weight and 0.2 mol L⁻¹ NaNO₃ was the eluent.

2.5 Dispersed properties of HBP

The dispersion properties of the HBP were investigated in an Fe₂O₃ suspending solution. The Fe₂O₃ suspending solution was prepared by dissolving FeSO₄ in distilled water, and then a little Fe₂O₃ was added. The composition of the solution was: Ca²⁺ (CaCO₃) = 150 mg L⁻¹, Fe²⁺ = 10 mg L⁻¹, and the pH of the solution was 9.0. The absorbance and transmittance of the Fe₂O₃ suspension were determined with a spectrophotometer (721, Shanghai XIPU instrument co., LTD) at 420 nm. Before measurement, the suspending solution was stirred for 15 min and left for 5 hours at 50 °C.¹²

2.6 Scale inhibiting efficiency of HBP

The scale inhibiting efficiency was measured according to Chinese GB16632-2008T.¹³ The solution containing Ca²⁺ (9.20 g L⁻¹ CaCl₂), HCO₃⁻ (7.36 g L⁻¹) or SO₄²⁻ (10.66 g L⁻¹) was prepared first, then varied amounts of scale inhibitor were added, and the mixtures were kept in a circulating water system of 70 °C for 24 hours. The Ca²⁺ concentration was recorded by titration with EDTA and the scale inhibiting efficiency (E) of the HBP was calculated according to eqn (1). Here, M₀ indicates the initial Ca²⁺ concentration of the mixed solution, M₁ represents the Ca²⁺ concentration of the solution without scale inhibitor, and M₂ represents the Ca²⁺ concentration of the solution with scale inhibitor.

2.7 Morphology of calcium scale crystals

The preparation method of CaCO₃ crystals is as follows: the precipitation of calcium carbonate was carried out at room temperature in a glass vessel with a magnetic stirrer. Equal volumes of CaCl₂, NaHCO₃ and scale inhibitor solutions were mixed simultaneously under vigorous stirring. The well-mixed solutions were left to stand under static conditions for 24 hours. The crystalline CaCO₃ was harvested by centrifugation and washed with distilled water twice to remove the residual HBP that was not incorporated into the crystals.¹⁴

XRD measurement was accomplished with Rigaku D/max-RB apparatus (Tokyo, Japan) and a powder diffractometer, and with image-plate photography using graphite monochromatized Cu Kα radiation. The data was collected from 10–70° with a scanning rate of 5° per min and analyzed with JCPDS files.

Scanning electron microscopy (SEM) images were recorded using a field emission scanning electron microscope (Sirion 200 SEM, Philips, 5 kV or 7401F SEM, JEOL, 1 kV). Prior to imaging by SEM, the samples were sputtered with a thin layer of gold.

3 Results and discussion

3.1 Characterization of HBP

The HBP was synthesized from the functional monomer (AMA) and the monomer MA based on radical copolymerization. It is well known that MAH is a powerful electron-accepting monomer and can easily copolymerize with electron-donating monomers, preferring alternating copolymerization. Due to its activity, the vinyl pendent groups of MAH in the linear molecule can be the branching point.¹⁵ The reaction process is described in Fig. 1.

Fig. 1 Synthesis of the hyper-branched polymer by radical copolymerization of AMA and MAH.

3.1.1 FTIR and ¹H-NMR. Fig. 2 shows the FTIR spectra of the AMA and the HBP. Compared with AMA, the peak at 1640 cm⁻¹, which is the stretching vibration of –C=C–, is weakened in the HBP. Peaks at 1850 cm⁻¹, 891 cm⁻¹ and 831 cm⁻¹, which are due to bending vibrations of =C–H from the AMA, were produced. The peak at 1060 cm⁻¹ has disappeared, which is due to the bending vibrations of =C–H from the MA. The FTIR analysis indicated that the AMA and MAH were involved in the copolymerization and the expected HBP was obtained.

Fig. 2 FTIR spectra of the hyper-branched polymer and AMA.

Fig. 3 illustrates the chemical shifts of hydrogen in the monomer AMA and the HBP.

Fig. 3 ¹H NMR spectrum of the AMA (a) and the hyper-branched polymer (b).

The ¹H NMR of the AMA is shown in Fig. 3(a). The chemical shifts were assigned at 5.3 ppm (COOHCH=CHCOOCH₂CH=CH), 6.3 ppm (COOHCH=CHCOOCH₂CH=CH) and 4.6 ppm (COOHCH=HCOOCH₂CH=CH).

Compared with the AMA, the chemical shifts of the prepared HBP show a number of differences. The peak at 6.3 ppm, assigned to –CH=CH–, has obviously decreased and a peak has appeared at 1.9 ppm, assigned to –CH–CH₂, implying that the –CH=CH₂ group of MAH has been partially incorporated during copolymerization with the AMA. Moreover, the peak at 5.3 ppm, assigned to –CH=CH₂, has disappeared completely. This evidence agrees with a total reaction of the AMA with the MAH during the copolymerization process.

The ¹H NMR and the FTIR spectrum show that the carboxylic-terminated hyper-branched polymer has been successfully synthesized.

3.1.2 Degree of branching (DB). Degree of branching (DB) is one of the main parameters during characterization of the structure of the hyper-branched polymer. Usually, the DB is obtained through NMR spectra. Based on the ¹H NMR spectra, the DB can be calculated with the proposed formula by Frechet.¹⁶where D, T, and L are the proportions of dendritic, end and linear structural units, respectively. The characteristic chemical shifts of the dendritic, end and linear structural units of hydrogen are around 6.35 ppm, 2.52 or 1.9 ppm, and 3.72 ppm. Substitution of the corresponding peak areas of the dendritic, end and linear structural units into eqn (2), allows the DB of the HBP to be calculated.

Table 1 shows the relationship between the DB of the obtained HBP and initiator concentration. With the increase of initiator concentration, the conversion rate of the monomer is increased, which provides more branching points, leading to a higher DB of the HBP. As shown in Table 1, when the concentration of the initiator was less than 3%, the DB of the HBP was below 0.3. However, if the concentration of the initiator was more than 3%, the DB would abruptly increase to more than 0.4. This may be because only a small number of double bonds from the maleic acid groups in AMA take part in the copolymerization. When there are higher concentrations of BPO, the maleic acid groups in AMA link with other AMAs, resulting in products with a higher DB.

Table 1 The DB of the HBP prepared with different amounts of initiator

Sample	Initiator concentration^a (%)	Degree of branching
a Initiator concentration refers to the amount of initiator accounting for monomer mass. Monomer ratio of AMA and MA is 1 to 1.2.
HBP-1	1	0.182
HBP-2	2	0.208
HBP-3	3	0.244
HBP-4	4	0.402
HBP-5	5	0.439

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3.1.3 The molecular weight and molecular weight distribution. Table 2 and Fig. 4 show the influence of the monomer ratio of MAH with AMA on the molecular weight and weight distribution of the obtained HBP. With an increase of monomer ratio, the number-average molecular weight of the HBP gradually decreases correspondingly. Compared with MAH, AMA has higher activity due to the double bond, so the higher MAH/AMA ratio resulted in lower molecular weight.

Table 2 Molecular weight and molecular weight distribution of HBP

Sample	Molar ratio of MAH and AMA^a	Number-average molecular weight	Weight-average molecular weight	Polydispersity
a Ratio of monomers: the initiator concentration is (4%) wt of AMA and MA.
HBP-6	0.8	5468	6258	1.144
HBP-7	1.0	4986	14 869	3.028
HBP-8	1.2	4854	5796	1.194
HBP-9	1.5	3012	5583	1.854
HBP-10	1.8	1789	3303	1.846

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Fig. 4 GPC analysis of the HBP when prepared with different ratios of MAH/AMA.

3.2 The scale inhibition properties of the HBP

3.2.1 The influence of DB on scale inhibition efficiency. Compared to linear polymers, the HBP has a larger surface area, terminated functional groups and a typical three-dimensional structure. Immersed in water, the terminal carboxylic acid group of the HBP may be adsorbed onto the surface to prevent the formation of calcium scale crystals and to destroy any crystals that have already formed by changing the crystal forms of the calcium scale.¹⁷

Fig. 5 shows the relationship between scale inhibition efficiency and the DB of the HBP. When the DB is lower than 0.2, the scale inhibition efficiency of the HBP on calcium carbonate and calcium sulfate is close to that of linear polymers,¹⁸ such as polyacrylic acid (E = 0.73–0.75, 20 ppm). However, when the DB is higher than 0.3, the scale inhibition efficiency suddenly goes beyond 93%.

Fig. 5 The relationship between scale inhibition efficiency and the DB of the HBP (the concentration of HBP is 20 ppm).

3.2.2 The influence of molecular weight of the HBP on scale inhibition efficiency. Fig. 6 illustrates the relationship between the molecular weight of the HBP and its scale inhibition efficiency. The HBP, with an M_n lower than 2000, cannot form a three-dimensional structure, so it will be hard for it to distort the calcium scale crystals and to prevent the formation of crystals. HBPs with an M_n of 3000–5000 have good scale inhibition efficiency. Once the molecular weight is more than 5000, the scale inhibition efficiency will be reduced. If the molecular weight of the HBP is too large, part of the carboxyl will be closed in within the molecule and loses the function of scale inhibitor.¹⁹

Fig. 6 The relationship between the inhibition efficiency and M_n (the concentration of HBP is 20 ppm).

3.2.3 The influence of scale inhibitor concentration on scale inhibition efficiency. Fig. 7 shows that the influence of the HBP concentration on its scale inhibition efficiency. It shows that, as the scale inhibition efficiency becomes higher with an increase of the HBP concentration, especially when the adopted concentration is from 5 ppm to 20 ppm, the scale inhibition efficiency of calcium carbonate and calcium sulphate were basically stable. Therefore, with the same concentration, the developed hyper-branched polycarboxylic acid performs much better than the existing scale inhibitors, such as phosphate (sodium tripolyphosphate, E = 0.87, 20 ppm) and polymaleic acid (E = 0.81–0.83, 20 ppm). The hyper-branched polycarboxylic acid, developed in this research, contains a large number of terminal carboxyl groups, which have a strong chelating effect with Ca²⁺, and the chelating effect can stop the crystal formation of calcium sulfate and calcium carbonate. Moreover, the number of carboxyl groups of the HBP surpasses that of the linear polyacrylic acid at the same molecular weight, therefore excellent scale inhibition performance can be achieved with lower concentrations of the HBP.

Fig. 7 The relationship between scale inhibition efficiency and the HBP concentration.

3.2.4 The influence of pH on scale inhibition efficiency. The influence of pH on the scale inhibition efficiency is shown in Fig. 8. We can see that, with an increase of the pH value, the scale inhibition efficiency of the HBP on calcium sulfate and calcium carbonate increases. However, when the pH value is more than 9, the scale performance of the HBP declines. The probable reason is that, as the OH⁻ has a stronger chelation ability with Ca²⁺ than the carboxyl group of the HBP, higher OH⁻ concentrations will hinder the dispersion and chelation effect of the HBP on Ca²⁺, leading to the lower scale inhibition efficiency. Fig. 8 illustrates that pH 6–9 turned out to be the optimum condition for the scale inhibition reaction of the HBP. A supersaturated solution of calcium carbonate, typically, was at pH = 8–8.5, so the HBP still meets the requirement of preventing the scaling of calcium carbonate and calcium sulfate.

Fig. 8 The relationship between the pH value of the solution and scale inhibition efficiency.

3.3 Morphology of calcium scale crystals

3.3.1 SEM of calcium carbonate crystals. Fig. 9 illustrates that the calcium carbonate crystals are regular with a smooth surface. After adding 20 ppm HBP-3 into a calcium carbonate solution, the surface of the calcium carbonate crystal appears to have a large number of small cracks,²⁰ while HBP-5 makes the surface become full of honeycomb defects, and the cracks become deeper. It indicates that, as the DB of the HBP increases, the growth of calcium carbonate becomes messy, and large and unstable crystals are formed.²¹ As a result, an increase in the DB of the HBP plays a significant role in preventing the growth of calcium carbonate crystals.

Fig. 9 SEM images of calcium carbonate scale (HBP conc. is 20 ppm); (a) control, (b) HBP-3 (DB = 0.244), (c) HBP-5 (DB = 0.439).

3.3.2 XRD analysis of calcium carbonate. The XRD was performed to look at the crystalline species of calcium carbonate when in the presence of different scale inhibitors, including the MAH and the HBP with varied DB. Fig. 10(a)–(d) give their XRD spectra. The major diffraction peaks at 29.36° and 43.28° are attributed to stable calcite crystals, and the peaks at 32.16° and 35.64° are attributed to the metastable aragonite crystals of calcium carbonate.²² Fig. 10(c) shows major peaks at 26.25°, 29.42° and 47.76°, indicating that the crystal of calcium scale consists of calcite, metastable state aragonite, and unstable vaterite, respectively.²³ After adding the HBP-4 (with a high DB), the peak at 29.42° disappeared, and the peaks at 24.8°, 26.8° and 44.1° were still present, which implied that the HBP with high DB would result in the unstable vaterite instead of the stable calcite and metastable aragonite. These phenomena are consistent with the study of S. Yu et al.²⁴

Fig. 10 The XRD pattern of calcium carbonate crystals with different scale inhibitors (the concentration of the HBP is 20 ppm). (a) Control, (b) MAH, (c) HBP-1 (DB = 0.18), (d) HBP-4 (DB = 0.40).

The crystallization degrees of calcium carbonate in the presence of different scale inhibitors were calculated and listed in Table 3. It shows that the crystallization degree of calcium carbonate decreases with HBP addition.²⁵ Moreover, the HBP with a higher DB would make the calcium carbonate have a lower crystallization degree.

Table 3 The calcium carbonate crystallization degree and crystalline form obtained with various scale inhibitors

Sample	Scale inhibitor	Crystallization degree (%)	Crystalline form
1	0	95.3	Calcite
2	MA	92.3	Calcite, aragonite
3	HBP-1	68.5	Vaterite, calcite
4	HBP-4	60.6	Vaterite, calcite

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3.3.3 Dispersion performance analysis and scale inhibition mechanism. As depicted in the experimental sections, calcium carbonate crystals can be formed in oversaturated solutions. On one hand, carboxyl-terminated hyper-branched polymers have a larger surface area, so can chelate with more Ca²⁺. Thus, the HBP could prevent the formation of calcium carbonate crystallite. Fig. 11 shows that as the DB of the HBP increases, the transmittance of Fe₂O₃ solution (that was used for testing the dispersion property of the scale inhibitor) is reduced and the absorbance increases, indicating that the HBP has an obvious impact on the dispersion performance because of its three-dimensional structure.

Fig. 11 The relationship between dispersion performance and the DB of the HBP.

This shows that the existing calcium carbonate can be dispersed and can be prevented from forming big crystals by use of the developed HBP.

4 Conclusion

A series of waterborne hyper-branched polycarboxyl acids, with different DB values and molecular weight, have been successfully synthesized through radical copolymerization. The optimum reaction conditions are as follows: a polymerization temperature of 60 °C, a reaction time of 20 hours, a suitable monomer mole ratio of MA : AMA = 1.2 : 1 and an initiator conc. that is 4%(wt) of the total monomers. The ideal conditions for scale inhibition performance were at pH = 6–8, with 5 ppm of the HBP for calcium carbonate and 20 ppm for calcium sulfate. Both the molecular weight and the DB of the HBP can impact its scale inhibition efficiency. More specifically speaking, the HBP with an M_n between 3000–5000 and a higher DB would improve the scale inhibition efficiency. Investigation of the scale inhibition mechanism of the HBP indicated that, because of its excellent complexation and dispersion ability, the hyper-branched polyoxylate acid could prevent the formation of calcium scale crystals by absorption, dispersion and distortion of the crystal lattice. Therefore, this study has provided theoretical support for applying HBPs to the scale inhibition field, as well as to its structure design for more useful HBPs.

Acknowledgements

The authors acknowledge the financial support from the Natural Science Foundation of China (21474114).

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