Preparation and swelling behaviors of a high temperature resistant superabsorbent using tetraallylammonium chloride as crosslinking agent

Accepted Manuscript Preparation and swelling behaviors of a high temperature resistant superabsorbent using tetraallylammonium chloride as crosslinking agent Xiaoyun Zhang, Xiangpeng Wang, Liang Li, Sisi Zhang, Ruonan Wu PII: DOI: Reference:

S1381-5148(14)00267-3 http://dx.doi.org/10.1016/j.reactfunctpolym.2014.12.006 REACT 3479

To appear in:

Reactive & Functional Polymers

Received Date: Revised Date: Accepted Date:

19 July 2014 8 December 2014 26 December 2014

Please cite this article as: X. Zhang, X. Wang, L. Li, S. Zhang, R. Wu, Preparation and swelling behaviors of a high temperature resistant superabsorbent using tetraallylammonium chloride as crosslinking agent, Reactive & Functional Polymers (2015), doi: http://dx.doi.org/10.1016/j.reactfunctpolym.2014.12.006

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Preparation and swelling behaviors of a high temperature resistant superabsorbent using tetraallylammonium chloride as crosslinking agent Xiaoyun Zhanga*, Xiangpeng Wanga, Liang Lib, Sisi Zhanga, Ruonan Wub a

College of Science, China University of Petroleum (East China), Qingdao 266580,

China b

Department of Petroleum Engineering, Texas Tech University, Lubbock, TX 79409,

USA

*Corresponding author. Tel.: +86-532-86981571; Fax: +86-532-86981791 E-mail: [email protected]

Abstract A new high temperature resistant superabsorbent was synthesized through solution polymerization of acrylamide (AM) and partially neutralized acrylic acid (AA), using tetraallylammonium chloride (TAAC) as crosslinker, ammonium persulfate (APS) as initiator. Parameters that influence water absorbency of the superabsorbent at 25°C and 200°C such as molar ratios of AM to AA, TAAC to AA, APS to AA and neutralization degree, were investigated. Swelling behaviors of superabsorbent prepared at the optimum conditions in different pH and saline solutions were studied. The swelling ratios of superabsorbent in distilled water and 1wt% NaCl solution at 250°C reach 287g/g and 69g/g, respectively.

Keywords: High temperature resistant superabsorbent; Tetraallylammonium chloride; Water absorption; Swelling; Crosslinking

1

1. INTRODUCTION Superabsorbent polymer (SAP) is a kind of important functional material which is crosslinked with a three-dimensional network structure. It exhibits strong capacity of swelling in water and retaining water even under certain pressure [1,2]. Traditionally, the SAPs were prepared using N, N-methylenebisacrylamide (NMBA) as crosslinker [3,4]. SAPs are primarily used as adsorbents for water and aqueous solution in room temperature environment, such as in the field of diapers, hygiene, agriculture and horticulture products [5-7]. However, most superabsorbents become water soluble when the temperature of external solutions is above 150°C, owing to the hydrolysis of amide bonds on the network crosslinking points [8]. This disadvantage restricts further application of SAP at higher temperature, such as being used as channel blocking agent for steam stimulated oil well [9]. During the past decade, efforts have been especially devoted to enhancing the high-temperature stabilities of the SAP [10,11]. Xie and Zhang used divinylbenzene (DVB) as crosslinker to prepare an anti-hydrolyzing superabsorbent [12], but the reported temperature was only 70 oC. Since DVB is volatile and insoluble in water, it cannot disperse well in water phase. This is detrimental to an effective copolymerization. Duran-Valencia et al reported a preformedn partical gel made from AM, VP and AMPS using NMBA as the crosslinker exhibited good thermal stability (130 oC) under reservoir conditions [13]. Bai et al extensively studied the preformed particle gel (PPG) for water conformance control [14-16]. The PPG can also be used as an economical gel blocking agent at temperature up to 150 °C. Zhang et al reported a similar SAP using the combination of NMBA and PT as crosslinking agents. Its temperature resistance reached 160°C [17]. Although this kind of polymer-based superabsorbent seems promising candidate, higher temperature resistant property is required before it can be used as steam channeling blocking agent, because the steam temperature is normally higher than 250 °C. The crosslinking agent may play a key role in preparation of a high temperature resistant superabsorbent. This provoked us to seek other polyfunctional chemicals as suitable crosslinkers. This article aims at preparing a novel high temperature resistant superabsorbent, using tetraallylammonium chloride (TAAC) as crosslinking agent. TAAC has good solubility in water but no chemical bonds that can be broken in high temperature aqueous media. During the copolymerization wither other vinyl 2

monomers, TAAC can cyclize to form two five membered rings. The cyclic structure centered with N atom will serve as crosslinking point of the network structure. This will certainly increase the structural stability and high temperature performance of the superabsorbent. The factors that affect the water absorbency such as the amount of crosslinker and initiator, the ratio of the monomers were tested. The high temperature resistant performance of the TAAC-crosslinked SAP was investigated.

2. EXPERIMENTAL 2.1. Materials Arcylic acid (AA, chemically pure, Beijing Eastern Chemical Works, China) was used directly. Acrylamide (AM), sodium hydroxide (NaOH), sodium chloride (NaCl), ammonium persulfate (APS), N,N-methylenebisacrylamide (NMBA), divinylbenzene and tetraallylammonium chloride were analytical grade and purchased from Sinopharm (Shanghai, China) and used as received. All other reagents used were of analytical grade and all solutions were prepared with distilled water. 2.2. Copolymerization of AM and partially neutralized AA The polymerization was carried out under nitrogen atmosphere in a four-necked flask equipped with a thermometer and gas inlet at 60 °C. Taken the sample with AA neutralization degree of 80%, AM to AA ratio of 40 mol %, TAAC to AA ratio of 0.12 mol %, APS to AA ratio of 0.12 mol % as a representative example, the detailed procedure is as follows: 7.2 g (0.1 mol) of AA and 2.84 g (0.04 mol) of AM were dissolved in 8 mL of distilled water. Then 16 g of 20% (0.08 mol) NaOH solution was added dropwise to neutralize the above solution. After being purged with nitrogen for 30 min to remove the dissolved oxygen, the mixed solution was heated in a water bath to 60oC, then 51.2 mg (0.12 mmol) of 50% TAAC solution and 27 mg (0.12 mmol) of APS (dissolved in 1 mL of water) were introduced into the monomer solution. The polymerization occurred normally within 5-10 min and was continued for 3 hrs. The resulting gel product was removed and cut into small pieces (0.1–0.5 cm in thickness), and then dried in a vacuum oven at 80°C to constant weight. In a similar manner, a series SAPs with varied monomer, crosslinker and initiator ratios were prepared. All the samples used were milled through 20-50 mesh screen. In addition, samples with NMBA and DVB as crosslinker were also prepared according to a similar procedure, except where the molar ratio of crosslinker to AA is twice what TAAC was used. 3

2.3. Swelling studies Water absorbency in distilled water at different temperature To determine water absorption capability, an accurately weighed sample (0.3 g) was immersed in 250 mL of distilled water for 24 h to reach swelling equilibrium at certain temperature. Measurement at high temperature was carried out in a JN-500B stainless steel pressure vessel (Qingdao Petroleum Instrument). Then the completely swollen gel was separated from unabsorbed water by filtering through a 100-mesh screen. The remaining gel was collected and weighed. The water absorbency of the sample (Qw) was calculated based on the following equation: Qw = (m2-m1) / m1

(1)

where m1 and m2 represent the weights of the dried sample and the swollen gel (g), respectively. Water absorbency at various pH solutions at 200°C The pH values of external solution were adjusted by addition of 10% NaOH or 0.1 mol/L HCl solutions, monitored with a PHS-3C pH meter (Shanghai INESA). The measurement of water absorbency of superabsorbent in different pH solutions at 200°C was similar to that in distilled water. Swelling behavior in saline solutions Saline solutions with different cations (Na+, Mg2+, Al3+) and same anion (Cl–) were prepared using NaCl, MgCl2∙2H2O and AlCl3∙ 6H2O, respectively. The determination of water absorbency of superabsorbent in different saline solutions was similar to that in distilled water. 2.4. Characterization FTIR

spectrum

was

obtained

on

a

Thermo

Nicolet

NEXUS

TM

-1

spectrophotometer in the range of 400-4000cm (KBr disc).Thermal stability of samples was investigated using a Perkin-Elmer-TGA 7 thermogravimetric analyzer under nitrogen atmosphere at a heating rate of 10°C/min. Morphology of the samples was examined by a HITACHI S-4800 scanning electron microscope (SEM) after coating with gold film.

3. RESULTS AND DISCUSSION 3.1. Preparation of the SAPs 4

In this work, solution polymerization protocol was applied and APS was used as initiator for the preparation the SAPs. As a traditional technology, the polymerization of acrylic monomer using NMBA as a crosslinker leading to SAPs has been well documented in the literature [18]. Although examples of using TAAC as a crosslinker have also been found, most of the literatures are patents and the related information about crosslinking mechanism is rare. But based on the fact that diallyammonium chloride forms a five-membered ring during free radical polymerization [19], it is reasonable to assume that TAAC builds two five-membered rings centered at the nitrogen atom (Scheme 1). In this way, the crosslinked networks are capable of keeping water and stable at elevated temperature, because no chemical bonds are liable to hydrolysis at elevated temperature. A representative IR spectrum of synthesized superabsorbent is shown in Fig.1. Characteristic peak at 3437cm-1 is due to NH2 and –OH groups (overlapped). The band at 2932cm-1 is ascribed to the -CH2- group on the polymeric chain. The absorption bands at 1558 and 1405 cm-1 are attributed to the appearance of –C=O from –COO– groups. And the peak at 1683cm-1 is attributed to the –C=O stretching of AM. Based on the above analysis, it can be concluded that superabsorbent polymer was successfully synthesized. 3.2. Effect of AM content on water absorbency at 25°C and 200°C As reported in previous study, the collaborative absorbent effect of –CONH2, –COONa and –COOH groups is superior to that of single –CONH2, –COONa or –COOH group [20]. When the AM was used as a comonomer for the synthesis of SAP, the ratio of AM to AA was critical to the absorbent performance. The effect of AM content on water absorbency of superabsorbent was studied by varying the AM/AA molar ratio from 0.25 to 0.55, whereas the other parameters were kept constant [Neutralization degree: 80 %; molar ratios (based on AA) of APS, TAAC were 0.12%, 0.12%, respectively]. As shown in Fig. 2, a similar profile can be observed at 25 °C and 200 °C. When the molar ratio of AM to AA is less than 0.4, swelling ratios increase with increasing amount of AM. As well known, the water absorbency is related to electrostatic repulsion of the gel and osmotic pressure between the gel and external solutions. Increasing the AM/AA ratio would decrease the sodium ions in the polymeric network, which increase the electrostatic repulsion by descreening the negative charges of –COO– groups, and thus water absorbency increases [21]. 5

However, when the molar ratio of AM to AA is over 0.4, swelling ratios decrease with further increasing the amount of AM. This is due to the fact that –COO– has a better hydrophilicity than –CONH2. When more AM is added into polymerization system, the swelling ratio will decrease [22-23]. Under our experimental conditions, the molar ratio of AM to AA in a feed ratio of 0.4 possesses the highest water absorbency, both at 25 °C and 200 °C. 3.3. Effect of TAAC content on water absorbency at 25°C and 200°C The influence of TAAC contents on superabsorbent water absorbencies was investigated by varying the TAAC to AA ratio with other parameters kept constant [Neutralization degree was 80%; molar ratios (based on AA) of APS, AM were 0.12%, 0.4, respectively]. AS shown in Fig.3, the SAP with TAAC/AA ratio of 0.12% exhibited maximum absorbencies of 287 g/g (at 25 °C) and 281g/g( at 200 °C). For TAAC to AA ratios less than 0.12%, the water absorbencies increase with the increase of TAAC. The water absorbency at 200 °C is obviously lower than that at 25 °C when the TAAC concentration is in the range of 0.06% to 0.10%. This indicates that the crosslinker content is a key factor on SAP absorbency, especially at high temperature. When the concentration of TAAC is very low [24-25], it is hard to form an effective three-dimensional network structure, the prepared superabsorbent performs certain water solubility. However, when an excessive amount of TAAC is added, the crosslinking density of the superabsorbent also increases. This would result in a decrease in the space between the copolymer chains and lead a decrease in water absorbency. Thus, superabsorbents with moderately crosslinked polymer networks can absorb and retain large quantities of aqueous fluids. The observation is similar to other SAP systems using NMBA as the crosslinker [26-27]. 3.4. Effect of AA neutralization on water absorbency at 25°C and 200 °C The effect of AA neutralization on water absorbency at 25 °C and 200 °C is shown in Fig.4. The water absorbency increased with the increase of the neutralization degree of AA till it reached a maximum at 80%. There are little differences between absorbencies at 25 °C and at 200 °C. This tendency can be explained by the cooperative effect between –COOH and –COO– groups [28]. With increasing the neutralization degree of AA, more –COOH groups are transformed into –COO–. The increased electrostatic repulsion of –COO– on the polymer chains leads to a higher 6

osmotic pressure between gel network and water. Besides, some hydrogen bonds between –COOH groups are destroyed during the process of transformation of –COOH groups to –COO– groups, resulting in a smaller crosslinking density. The water absorbency increases. However, a further increase of neutralization degree can introduce more sodium ions into the gel to react with –COO– groups, and the electrostatic repulsion reduces [29]. The water absorbency decreases accordingly. 3.5.Effect of initiator content on water absorbency at 25°C and 200°C Varying the amount of APS initiator affected absorbency (Fig.5), especially at high temperature. The water absorbency increases with increasing APS content and then decreases with further increasing content of APS. The maximum absorbencies were obtained at 0.12 mol% of APS. When APS content is low, the number of free radicals is small, leading to a low crosslinking degree and a low conversion rate [30]. However, when the molar ratio of APS to AA is higher than the optimum value, a great number of free radicals are generated, and this results in a short average chain length. The molecular weight of the polymer is low, thus some hydrogels exhibit water solubility [31], may have contributed to the observed decreased water absorbency. The optimum conditions thus obtained are: the molar ratios of AM to AA, TAAC to AA, APS to AA were 0.4, 0.12%, 0.12%, respectively. Thus, superabsorbent polymer prepared under this optimized conditions was used in the following text. 3.6.Swelling behaviors of superabsorbents prepared with different crosslinker in distilled water and 1wt% NaCl solution Swelling behaviors of superabsorbents prepared using different crosslinkers were comparatively investigated in distilled water and 1 wt% NaCl solution at different temperature (Fig.6). Three SAPs were prepared under same conditions except that the crosslinker contents were different. In cases of DVB and NMBA, molar ratio of crosslinker to AA is twice what TAAC was used, in consideration of that TAAC is a tetra-functional monomer. As shown in Fig.6, when NMBA was used as crosslinker, the water absorbency increased from 50 oC (249g/g) to 100 oC (284g/g). This is due to the decrease in crosslinking density caused by the thermal hydrolysis of the amide bonds. It is obvious that hydrogel becomes water soluble due to the collapse of the crosslinked 7

structure. Superabsorbent synthesized using DVB as crosslinker begins to dissolve at 100 °C and little water is retained at a higher temperature. However, superabsorbent synthesized from TAAC does not dissolve even at 250 °C and shows good water absorption capability. Its swelling ratios reach 287g/g and 69g/g in distilled water and 1 wt% NaCl solution, respectively. The results indicate that the structure of crosslinker plays a critical role in the thermal stability of hydrogel. As shown in Fig.7(A), when NMBA was used as crosslinker, the crosslinked amide bonds began to collapse due to hydrolysis at 100°C, which would certainly resulted in an increased water absorbency because of decreased crosslinking density. The hydrolysis process of amide bonds might be completed at 150°C, thus the hydrogel became water soluble. When DVB was used as crosslinker, it could not be uniformly dispersed in the monomer system because it is not soluble in water phase. This would lead to an ineffective crosslinking. The hydrogel thus formed would dissolve at high temperature. But, when TAAC was used as crosslinker, as depicted in Fig.7(B), there were no apparent structure changes for the hydrogel with the increase of temperature. During the polymerization process, per TAAC molecule forms two five-membered rings through cyclization reaction. The cyclic structural feature has great advantages over NMBA or DVB crosslinked SAPs. With increasing the temperature, the three-dimensional structure of the hydrogel could still maintain well. Even if one of the four C-N bonds is broken, the water absorbency would not be affected. 3.7. Water absorbency of superabsorbent in different pH solutions at 200°C The effect of pH value on swelling ratios of superabsorbents prepared under the optimum conditions (TAAC as crosslinker) at 200°C is shown in Fig.8. With the increase of pH value, the swelling ratios increase in the range of 1-5 and decrease quickly when the pH value is over 9. Based on previous studies, this can be explained by the cooperative relation between –COOH and –COO– groups. When pH of external solutions is less than 5, the –COO– groups can turn into –COOH groups, which decrease the negative charge repulsion and increase the association among –COOH groups because of the formation of hydrogen bond, leading to a larger crosslinking density, the water absorbency decreases with the increase of pH value [32]. On the contrary, when pH of external solutions is more than 9, the –COOH– groups can turn into COO– groups, the increased ionic strength causes a rapid decrease of osmotic pressure, the water absorbency decreases [33]. When pH of the 8

external solution is in the range of 5-9, little change in swelling ratios can be attributed to the buffer action of –COOH and –COO– groups with weak acid or base. 3.8. Water absorbency of superabsorbent in different saline solutions Water absorbencies in saline solutions are significant in many practical applications such as restraining the steam breakthrough and improving the effect of steam flood. Therefor the swelling behaviors of superabsorbents in different saline solutions were studied (Fig.9). It can be seen that the swelling ratios decrease with increasing the concentration of external saline solution. This can be explained by the osmotic pressure between gel networks and the saline solutions. With increasing the concentration of the salt solution, the osmotic pressure decreases [34], resulting in a decreased swelling ratio. Besides, multivalent cations decrease the water absorbency considerably. This can be attributed to the complexing ability of the carboxylate groups and formation of intramolecular and intermolecular complexes with multivalent cations (Ca2+, Al3+) [35], which lead to the increase of crosslinking density. Thus the water absorbency decreases consequently. 3.9.Thermal analysis The TGA curve of superabsorbent is shown in Fig.10. The P(AA-co-AM) high temperature resistant superabsorbent exhibits a two-stage weight loss intervals. Weight loss of ~17 wt% (50-350 oC) is due to the removal of absorbed water and bonded water. Then a sharp weight loss of ~33 wt% (350-490 oC) is observed, corresponding to the decomposition of superabsorbent molecules. The decomposition temperature of 350°C indicates a good thermal stability for the superabsorbent used at high temperature. 3.10.

Surface morphologies The surface structure of the hydrogel formed from the same SAP was observed

using SEM. Fig.11 shows the micrographs of gels at 25°C and 250°C, respectively. As can be seen from Fig.11, The surface morphologies of superabsorbent after adsorption were porous. Besides, the surface morphology of the gel at 250°C is similar with the gel at 25°C. The result proves that the superabsorbent has good high temperature resistance.

9

3. CONCLUSIONS A novel high temperature resistant superabsorbent was synthesized through solution polymerization based on AM and partially neutralized AA, using TAAC as crosslinker. The effects on swelling ratios of superabsorbent at 25°C and 200°C such as molar ratios of AM to AA, TAAC to AA, APS to AA and neutralization degree were investigated. The optimum conditions obtained were: the molar ratios of AM to AA, TAAC to AA, APS to AA were 0.4, 0.12%, 0.12%, respectively, the neutralization degree was 80%. The water absorbencies of the superabsorbent prepared using TAAC could reach 287g/g (in distilled water) and 69g/g (in 1wt% NaCl solution) at 250°C. Moreover, swelling behaviors of the product in various pH solutions at high temperature revealed that the superabsorbent is applicable in a wide pH range. Based on these properties, this kind of superabsorbent exhibited high application potentials, especially in the steam flooding of super heavy oil reservoirs.

Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Project No.21172264, No. 51374230), and the Key Technologies Research and Development Program of China (Grant 2011ZX05051-003).

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Figure Captions: Scheme 1. Synthesis of the superabsorbent polymer. Fig. 1. FTIR spectrum of superabsorbent. Fig. 2. Effect of AM content on water absorbency at 25°C and 200°C. Neutralization degree was 80%; molar ratios of APS, TAAC (based on AA) were 0.12%, 0.12%, respectively. Fig. 3. Effect of TAAC content on water absorbency in distilled water at 25°C and 200°C. Neutralization degree was 80%; molar ratios of APS, AM (based on AA) were 0.12%, 0.4, respectively. Fig. 4. Effect of AA neutralization on water absorbency at 25°C and 200 °C. Molar ratios of APS, TAAC, AM (based on AA) were 0.12%, 0.12%, 0.4, respectively. Fig. 5. Effect of APS content on water absorbency at 25°C and 200°C. Neutralization degree was 80%; molar ratios of TAAC, AM (based on AA) were 0.12%, 0.4, respectively. Fig. 6. Swelling ratios of superabsorbents with different crosslinker at different temperature. (A) in distilled water, (B) in 1wt% NaCl solution Fig. 7. Structure change of superabsorbent with increasing temperature. (A) NMBA as crosslinker, (B) TAAC as crosslinker Fig. 8. Swelling ratios in various pH solutions at 200°C. Fig. 9. Swelling ratios of superabsorbent in different saline solutions. Fig. 10. TGA curve of superabsorbent. Fig. 11. SEM micrographs of gels (a) at 25°C (b) at 250°C.

13

*

CH2

O

CH

OH(Na) +

N Cl-

CH2

g

CH2

CH2

m

CH

n *

CONH2

(NH4)2S2O8

COOH(Na) N Cl-

O NH2

*

CH2 CH

h

CONH2

CH2

CH2

p

CH2 CH

q *

COOH(Na)

Scheme 1

14

Transmittance (%)

80

70

60 2932

50 1683 1405 1558 3437

40 4000

3500

3000

2500

2000

1500

Wavenumber (cm-1)

Fig. 1.

15

1000

500

320 o

25 C

300

o

200 C QW (g/g)

280 260 240 220 200 180 0.25

0.30

0.35

0.40

0.45

Molar ratio AM/AA

Fig. 2.

16

0.50

0.55

320

25 ℃ 200 ℃

280

QW (g/g)

240 200 160 120 80 40 0 0.06

0.08

0.10

0.12

0.14

Molar ratio TAAC/AA (%)

Fig. 3.

17

0.16

0.18

300

o

25 C o 200 C

280

QW (g/g)

260 240 220 200 180 60

65

70

75

80

Neutralization degree (%)

Fig. 4.

18

85

90

320

o

25 C o 200 C

280 240

QW (g/g)

200 160 120 80 40 0 0.03

0.06

0.09

0.12

0.15

0.18

Molar ratio APS/AA (%)

Fig. 5.

19

0.21

350

NMBA as crosslinker DVB as crosslinker TAAC as crosslinker

(A)

300

QW (g/g)

250 200 150 100 50 0 50

100

150

200

250

Temperature (℃)

90

NMBA as crosslinker DVB as crosslinker TAAC as crosslinker

(B)

75

QS (g/g)

60 45 30 15 0 50

100

150

Temperature (℃ )

Fig. 6.

20

200

250

H H

H H

C N N C

C N N C

O

O

O

O

H H C N N C O

O

H H

H H C N N C

C N N C

O

O

O

H H C N N C O O

O

(A)

N

N

N

NH

N

N

NH

NH

N

N

N

NH

(B)

Fig. 7.

21

300 250

Qw (g/g)

200 150 100 50 0 0

2

4

6

8

pH

Fig. 8.

22

10

12

14

90 NaCl CaCl2

80 70

AlCl3

Qs (g/g)

60 50 40 30 20 10 0 0.1

0.2

0.3

0.4 Concentration( mol L-1)

Fig. 9.

23

0.5

100

Weight (%)

90 80 70 60 50 40 100

200

300

400

Temperature ( )

Fig. 10.

24

500

600

Fig. 11.

25