In situ formation of polymeric hydrogels has been studied to ascertain their ability to block regions with high permeability in reservoirs and enhance oil recovery. The difficulties of applying these gel systems are related, among other factors, to their fragility in the conditions of high temperature and salinity typically found in reservoirs. In this context, various options for modification of gelling systems have been studied, such as addition of reinforcement loads, for preparation of nanocomposite hydrogels with improved mechanical and thermal properties. Thus, the objective of this study was to assess the properties of hydrogels based on partially hydrolyzed polyacrylamide (PHPA), with different molar masses, crosslinked with aluminum citrate, in the presence or absence of bentonite clay. We call these systems composite hydrogels or conventional hydrogels, respectively, and prepared them in synthetic brine containing 29,940 ppm of total dissolved solids (TDS). The selected hydrogels were submitted to rheology tests and scanning electron microscopy for assignment of gel strength codes. The results showed it was possible to obtain composite hydrogels with enhanced thermal properties in comparison with conventional hydrogels, mainly at a temperature of 85°C. The rheological tests, by determining the viscous modulus (G”), elastic modulus (G’), and the ratio between the two (G”/G’), along with the tan delta factor, indicated competition in the interaction between the clay and crosslinker, due to the latter’s positive charges in relation to the negative charges present in the polymer chains. The SEM micrographs showed the presence of clay particles adhered in the polymer chain, improving the thermal resistance of the system.
Research Article
Evaluation of the Influence of Adding Clay in Polymeric Hydrogels Based on Phpa and Aluminum Citrate
https://doi.org/10.21203/rs.3.rs-3238156/v1
This work is licensed under a CC BY 4.0 License
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Conformance control
PHPA
hydrogels
clay
Despite increasing reliance on renewable energy, petroleum still accounts for 34% of global energy demand (GAO et al. 2018). Because it is a nonrenewable source and since an increasing portion of wells have reached maturity, interest has grown to improve the efficiency of oil production (BAGHERPOUR et al. 2019). According to Brazil’s National Agency for Petroleum, Natural Gas and Biofuels (ANP), mature fields are those that have been producing for 25 years or longer and/or that have produced 70% or more of the proven reserves (ANP, 2020).
Production from mature wells progressively declines as the reserves become exhausted (Tongwa & Baojun, 2015). Besides this, mature wells generate larger amounts of undesired water than new wells. Indeed, this produced water is the main byproduct of crude oil production.
There are three phases of oil recovery: primary, secondary and tertiary, or enhanced. Primary recovery relies on the natural energy of the reservoir, based on the pressure difference between the reservoir and borehole (Plankaert, 2005). Secondary recovery entails maintaining this pressure difference by injecting water or gas in the reservoir to increase its pressure. According to Nwidee and collaborators (2016), these two processes together can recover an average of 30% of the oil, with considerable variation depending on the reservoir conditions. Finally, enhanced oil recovery (EOR) employs various technologies, including injection of special fluids containing additives (chemical injection) that can alter the production conditions and displacement mechanism, seeking to improve the sweep efficiency and/or displacement of the oil to be recovered. The latter process is responsible for the largest portion of oil recovered, which can reach 60% together with the previous two phases (Giraldo et al., 2016; Lakatos & Lakatos-Szabo, 2008; Bai et al., 2015).
Among the methods to increase oil production is conformance control of the reservoir, which has the main objective of altering the profile of the reservoir rock formation to diminish its permeability, preventing the passage of injected fluids through regions with greater permeability. In this respect, the use of polymer hydrogels to block highly permeable regions is attracting increasing attention (Al-Muntasheri et al., 2009; Chung et al., 2011; El-Karsani et al., 2014; Abdulbaki et al., 2014; Goudarzi et al., 2015; Aldhaheri et al., 2016).
Polymer hydrogels are defined as hydrophilic semi-solids with a three-dimensional (3D) structure involving a liquid part. Among these hydrogels, the standouts are those based on polyacrylamide (PAM) and its derivatives, by forming crosslinks with some reticulation agents, or crosslinkers (Rathod & Mehta, 2015; Al-Muntasheri et al., 2007; Tessarolli et al., 2014; Sun et al., 2016, Aguiar, 2020).
With regard to crosslinkers, some are organic, where the crosslinking with the polymer chains is accomplished by forming covalent bonds with functional groups of polymers derived from PAM, such as polyethyleneimine (PEI) or chitosan (Al-Muntasheri et al., 2007; Jia et al., 2012; Sengupta et al., 2014; Ma et al., 2017; Jia & Chen, 2018). Other crosslinkers are metallic, based on Cr+ 3, Al+ 3 and/or Zr+ 4. These are recommended for interaction with polymers that have anionic groups, such as partially hydrolyzed polyacrylamides (PHPAs) (Moradhi-Araghi, 2000; Al-Muntasheri et al., 2007; He et al., 2017; Fraga et al., 2020).
Various difficulties can be encountered in applying these gelling systems in oil reservoirs, such as dilution of the system, loss of components due to adsorption on the rocks, and destabilization in the severe temperature and salinity conditions typically found in reservoirs, among others (Goudarzi et al., 2015; Tongwa & Baojun, 2015). These difficulties generate a constant need to adapt these materials to the application parameters. In this regard, composite and nanocomposite hydrogels have been widely studied for having better properties than conventional hydrogels, mainly involving thermal and mechanical stability of the systems obtained, which improves the blocking efficiency and oil recovery (Seright et al., 2003; Aldhaheri et al., 2016; Adewunmi et al., 2018; Lashari et al., 2018).
Among the reinforcement loads used to formulate these hydrogels are substances with a structure based on metal oxides or modified bentonite clay. Studies have shown that the respective properties are closely related to improving the performance of hydrogels (Dragan, E. S. & Dinu, M. V., 2020; Tessaroli et al., 2019; Tessaroli, et al., 2019b; Aguiar et al, 2020; Pereira et al., 2020; Pereira et al., 2020b; Pereira et al. 2022). Nevertheless, in this context we did not find any studies of the influence of natural bentonite clay (without structural modifications) on polymer hydrogels based on PHPA crosslinked with aluminum and evaluated in field conditions with regard to salinity and temperature of the fluid.
Therefore, we sought to develop and evaluate polymer hydrogel nanocomposites based on PHPA and aluminum citrate, reinforced with unmodified bentonite clay, to act as possible blocking agents in regions of high permeability and in temperature and salinity conditions typical of Brazilian oil reservoirs.
2.1. Materials
Partially hydrolyzed polyacrylamide (PHPA) samples, with average molecular masses of 3, 13 and 16x106 Da and hydrolysis degree between 28 and 30%, were provided by SNF Floerger (São Paulo, SP, Brazil) with respective trade names Flopaam 3130S, Flopaam 3330S and Flopaam 3530S. The inorganic crosslinker aluminum, in the form of citrate, was synthesized from citric acid (C6H8O7) and aluminum chloride hexahydrate (AlCl3.6H2O), both purchased from Sigma-Aldrich (São Paulo, SP, Brazil).
Sodium bentonite clay (Bent) was used on this work, with trade name Argel-40, provided by Bentonit União Nordeste Indústria e Comercio Ltda. The clays were milled and sieved to 200 mesh and used without purification. Table 1presents the main physical and chemical properties of the bentonite clay.
|
Physical and chemical properties (average values) |
|
|---|---|
|
Appearance |
Light yellow |
|
Water content |
9.0–13.0% |
|
Swelling capacity (2,0 g/100 ml) |
35 |
|
pH (2,0 g/100ml) |
7.5 |
|
Cation exchange capacity (CEC) |
110 meq/100 g |
|
Chemical analysis (average values) |
|
|
SiO2 |
62% |
|
Al2O3 |
19% |
|
Fe2O3 |
3% |
|
MgO |
2% |
|
CaO |
2% |
|
Na2O |
2.5% |
|
TiO2 |
< 1% |
|
K2O |
< 1% |
Table 2 shows the salts and their concentrations used to prepare the synthetic brine, which had a composition typical of the water injected in reservoirs, here called injection water. The following salts were used: NaCl, KCl, CaCl2.2H2O and MgCl2.6H2O, all supplied by Sigma-Aldrich (Rio de Janeiro, RJ, Brazil).
|
Composition |
Concentration (mgL− 1) |
|---|---|
|
Na+ / K+ |
11.309 |
|
Ca+ 2 |
117 |
|
Mg+ 2 |
109 |
|
SO4 − 2 |
5 |
|
HCO3− |
64 |
|
Cl− |
17.756 |
|
Total |
29.360 |
2.2- Preparation of the crosslinker solution
The aluminum citrate (AlCit) solution was prepared using a 15% by mass solution of aluminum chloride hexahydrate (AlCl3.6H2O) and a 50% by mass solution of citric acid (C6H8O7), according to the method developed by Fraga and collaborators (2020). For this purpose, the acid and chloride solutions were mixed under stirring for at least 40 min., and then the pH of the resulting solution (aluminum citrate) was adjusted to the range of 6.0–6.5 with a NaOH solution under constant stirring, after which the supernatant was filtered for use as crosslinker.
According to Fraga and collaborators (2020), the aluminum/citrate ratio of the crosslinking solution should be 2.3/1, because this was the only ratio at which the solution remained stable in their study. Starting from this ratio, we determined the volumes of aluminum chloride (AlCl3) and citric acid (C6H8O7) for synthesis of the crosslinker:
- aluminum chloride hexahydrate solution: 15% m/v, i.e., 27.15g of AlCl3 in 100 mL of solution;- citric acid solution: 50% m/v, i.e., 50 g of C6H8O7 in 100 mL of solution.
We chose an initial chloride volume of 30 mL, formed by 8.15g of AlCl3 and 0.91 g of aluminum (0.03 mols).
The aluminum/citrate ratio was 2.3/1, such that:
2.3 = 0.03 mol of aluminum/n mol of citrate
Therefore, n = 0.013 mol of citrate, corresponding to 2.46 g of citrate or 2.50 g of citric acid. Since the mass/volume ratio of the aluminum citrate solution was 50%, 2.50 g of citric acid corresponded to 5 mL of solution. Thus, we added 5 mL of the citric acid solution (50% m/v) to 30 mL of the aluminum chloride solution (15% m/v).
The ICP-OES analysis carried out by Fraga et al. (2020) indicated that in the pH range of 6.0–6.5, the free aluminum in the solution was 2.1% m/v. So, with the mass values of the reagents used to prepare the solutions, together with the concentrations of aluminum revealed by ICP-OES, it was possible to determine that 1% v/v of aluminum citrate contained 235 ppm of aluminum. Based on these calculations, the crosslinker aluminum citrate was synthesized and used to prepare the hydrogels. The results shown are related to the content of aluminum present in the solution.
2.3- Hydrogel preparation
The conventional (PHPA/AlCit) and composite (PHPA/AlCit/Bent) hydrogels were prepared according to the following steps. PHPA was initially dissolved in injection water (Table 2) during 72 h to obtain a homogenized solution. The clay was dispersed by mixing the powder in injection brine with an ultrasonic homogenizer (Cole-Parmer 500W) with frequency of 30% during 1 minute.
Then the dispersed clay was added to the polymer solution to prepare the composite hydrogel, where the mixture was stirred for 1 hour. The previously synthesized aluminum citrate was added to the polymer solution of polymer/clay mixture depending on the hydrogel being prepared, and the system was left under magnetic stirring for 40 minutes.
The gelling solutions were then transferred to sealed tubes and kept in an oven at 70°C or 85°C for 30 days. The detailed compositions of the three polymer hydrogels are listed in Table 3.
|
Polymer concentration (mg/L) |
Crosslinker concentration (mg/L) |
Clay (bentonite) concentration (mg/L) |
|---|---|---|
|
3500 |
325 |
0 100 200 300 |
|
400 |
||
|
550 |
||
|
4250 |
325 |
|
|
400 |
||
|
550 |
2.4- Hydrogel characterization
The hydrogels were evaluated after aging for 30 days by computing the gel strength code according to the bottle test, followed by rheological analysis and morphological examination by scanning electron microscopy (SEM).
2.4.1- Gel strength code
The bottle test involves visual analysis of the gelling time and final consistency attained by the hydrogel during the aging period (Sydansk, 1988). The gelling solutions are placed in sealed bottles so that the total volume of the system is equal to half the volume of the bottle. The gel consistency is determined by inverting the bottles and observing the characteristics of the system under the effect of gravity, ranked by an alphabetic code from A to J, where: (A) means no detectable gel formed; (B) denotes highly flowing gel; (C) means flowing gel; (D) is moderately flowing gel; (E) denotes barely flowing gel; (F) pertains to highly deformable non-flowing gel; (G) mean moderately deformable non-flowing gel; (H) denotes slightly deformable non-flowing gel; (I) means rigid gel; and (J) denotes highly rigid gel (SYDANSK, 1988; PEREIRA, et al; 2020).
2.4.2- Rheological analysis
We used a Discovery Hybrid Rheometer III from TA Instruments, with parallel plate geometry (P60) to measure the oscillatory dynamics at temperatures of 70 and 85° C. This was carried out by sweeping the oscillation frequency from 0.01 Hz to 10 Hz with 1% deformation (present in the linear viscoelastic region) to determine the storage (or elastic) modulus (G') and loss (or viscous) modulus (G"). While the elastic modulus was used to provide the strength estimation, the viscous modulus was used to determine the material’s stiffness through the tan delta (tan δ) metric, according to the following equation.
This is an important indication of the strength of a hydrogel, by representing the rate of energy dissipation by storage during a deformation cycle. Values of tan (δ) greater than 1 denote weak gels, while values less than or equal to 0.1 indicate strong gels and values in the interval 0.1 < tan(δ) ≤ 1 denote gels with moderate strength (OKAY, OPPERMANN, 2007).
2.4.3- Morphology
The morphology of the hydrogels was determined with a Hitachi TM3030 Plus scanning electron microscope. The micrographs were obtained with magnifications of 500x and 2000x. The samples after aging in an oven were frozen at -4 ºC and freeze-dried for 72 h and then kept in a desiccator until analysis.
3.1. Gel strength code
- Hydrogels prepared with polymer FP3130S
Table 4 reports the results of bottle tests of the hydrogels formed with polymer FP 3130S, at 70 and 85 ºC, without addition of bentonite (conventional hydrogels). We also used the polymer concentration of 2000 mg/L. However, since the results obtained were not promising, for the other systems this concentration was not used.
|
Polymer concentration (mg/L) |
Crosslinker concentration (mg/L) |
Time (days) T = 70 oC |
Time (days) T = 85 oC |
||||||||||
|
1 |
3 |
7 |
10 |
15 |
30 |
1 |
3 |
7 |
10 |
15 |
30 |
||
|
2000 |
325 |
A |
A |
A |
A |
A |
A |
A |
A |
A |
A |
A |
A |
|
400 |
A |
A |
A |
A |
A |
A |
A |
A |
** |
** |
** |
** |
|
|
550 |
A |
A |
A |
A |
A |
A |
A |
A |
** |
** |
** |
** |
|
|
3500 |
325 |
A |
D |
D |
E |
E |
E |
A |
D |
D |
E |
E |
E |
|
400 |
A |
D |
E |
E |
E |
E |
A |
D |
E |
E |
E |
E |
|
|
550 |
A |
E |
F |
F |
F |
F |
A |
E |
F |
F |
F |
F |
|
|
4250 |
325 |
A |
B |
E |
E |
E |
F |
A |
B |
E |
E |
E |
F |
|
400 |
B |
E |
E |
F |
F |
F |
B |
E |
E |
F |
F |
F |
|
|
550 |
B |
F |
F |
G |
* |
* |
B |
F |
F |
G |
* |
* |
|
| *destabilization (syneresis/gel breakdown) | |||||||||||||
| **precipitation | |||||||||||||
In the initial sweep of the conventional hydrogels utilizing polymer FP3130S, at both 70 and 85 ºC, gels were obtained with gel strength codes of D or better at the two polymer concentrations and the crosslinker concentration as of 325 ppm of Al+ 3 ions. This behavior was related to the minimum concentrations of the components for gel formation, as described by Smith (1995).
At the lowest concentration of PHPA FP 3130S (2000 mg/L) utilized, no gel was formed (code A), nor was precipitation of the polymer observed. This behavior might have been associated with the polymer chain closure, promoted both by the interaction of the anionic charges of the polymer chain with the divalent cations and by the combination between the action of the inorganic crosslinker and the interaction of the divalent cations (Ca+ 2 and Mg+ 2). This behavior was also observed by Zhang and collaborators (2016).
On the other hand, the increase in the concentrations of the polymer and crosslinker caused faster gel formation. This behavior might have been associated with the larger number of crosslinking sites in the polymer chain, even with the occurrence of interactions of the anionic groups of this chain with the divalent cations present in the water, promoting interactions with the crosslinker molecules, generating gels. However, this behavior was only observed at Al3+ concentrations greater than 325 ppm.
We stress that the increase of the temperature from 70 to 85 oC, in the case of polymer concentrations above 2750 ppm and at the highest crosslinker concentration caused an increase in the gel strength code, along with precipitation of the system at the highest concentrations of the gel components: 4250 ppm of PHPA and 550 ppm of Al3+. This behavior is in accordance with the findings of Oliveira and collaborators (2019).
The composite hydrogels prepared with polymer FP 3130S and aluminum citrate at all concentrations of bentonite clay used in this study (100, 200 and 300 ppm) precipitated within 1 day, irrespective of the storage temperature in the aging test (70 or 85 oC). According to Haraguchi and collaborators (2005), the surface of bentonite contains positive charges that establish different types of interactions with PHPA (hydrogen bonds and ionic interactions). In the presence of a crosslinker, this favored the closing of the polymer chain, mainly in the case of the polymer with the lowest molar mass.
- Hydrogels prepared with the polymer FP3330S
Table 5 reports the results of the bottle tests of the hydrogels formed by polymer FP 3330S at 70 ºC with and without the addition of bentonite.
The addition of bentonite caused a reduction of the stability time of the samples with the lowest concentrations of polymer and/or crosslinker. On the other hand, there was slight improvement of the gel strength of the samples with the highest concentrations of polymer and/or crosslinker.
Al-Munstasheri and collaborators (2007) suggested the existence of competition in the hydrolyzed groups of PHPA between bentonite and the crosslinker, and observed that the ionic interaction between the bentonite and the hydrolyzed group was less stable than the crosslinks, causing the stability of the gel to decrease. We found that the increase of the polymer concentration (more anionic sites) and crosslinker favored its interaction with the ionic groups of the polymer chains, causing greater stability of the systems.
We also verified that the increase of the bentonite concentration to 200 ppm produced more promising results, because the gels were more stable, mainly those with polymer concentration of 4250 ppm. These results showed the occurrence of slower gel formation, corroborating the suggestion of competition of the polymer/clay and polymer/crosslinker interactions, possibly favoring the formation of more stable gels. On the other hand, the increase of the bentonite concentration to 300 ppm favored polymer chain closure.
|
Polymer concentration (mg/L) |
Crosslinker concentration (mg/L) |
Clay (mg/L) |
Time (days) |
|||||
|
1 |
3 |
7 |
10 |
15 |
30 |
|||
|
3500 |
325 |
0 |
D |
E |
F |
F |
F |
F |
|
100 |
C |
D |
D |
* |
* |
* |
||
|
200 |
C |
C |
D |
D |
D |
D |
||
|
300 |
** |
** |
** |
** |
** |
** |
||
|
400 |
0 |
E |
F |
F |
F |
F |
F |
|
|
100 |
D |
D |
D |
D |
D |
* |
||
|
200 |
D |
E |
E |
E |
F |
F |
||
|
300 |
** |
** |
** |
** |
** |
** |
||
|
550 |
0 |
F |
F |
F |
F |
F |
G |
|
|
100 |
E |
F |
F |
F |
F |
H |
||
|
200 |
D |
E |
E |
E |
F |
F |
||
|
300 |
** |
** |
** |
** |
** |
** |
||
|
4250 |
325 |
0 |
E |
E |
F |
F |
F |
F |
|
100 |
D |
E |
E |
E |
* |
* |
||
|
200 |
C |
D |
D |
D |
D |
D |
||
|
300 |
** |
** |
** |
** |
** |
** |
||
|
400 |
0 |
E |
E |
F |
F |
F |
F |
|
|
100 |
D |
E |
E |
E |
E |
E |
||
|
200 |
C |
D |
D |
D |
D |
D |
||
|
300 |
** |
** |
** |
** |
** |
** |
||
|
550 |
0 |
F |
F |
F |
F |
F |
G |
|
|
100 |
E |
F |
F |
F |
G |
G |
||
|
200 |
E |
E |
E |
E |
E |
E |
||
|
300 |
D |
E |
F |
F |
F |
F |
||
| *destabilization (syneresis/gel breakdown) | ||||||||
| **precipitation | ||||||||
Table 6 reports the results obtained by the bottle tests of the hydrogels formed by the polymer FP 3330S, at 85 ºC with and without the addition of bentonite. As observed for the conventional hydrogels formed by the polymer FP3130S (Table 4), the systems’ stability decreased with increased temperature.
We also observed greater fluidity of the composite systems in comparison with the conventional hydrogels (Table 6), indicating the existence of less density of the bonds with the crosslinker in comparison with the conventional hydrogel systems.
|
Polymer concentration (mg/L) |
Crosslinker concentration (mg/L) |
Clay (mg/L) |
Time (days) |
|||||
|
1 |
3 |
7 |
10 |
15 |
30 |
|||
|
3500 |
325 |
0 |
F |
G |
G |
H |
* |
* |
|
100 |
D |
D |
D |
D |
D |
* |
||
|
200 |
C |
C |
D |
D |
E |
E |
||
|
300 |
** |
** |
** |
** |
** |
** |
||
|
400 |
0 |
F |
G |
H |
H |
I |
* |
|
|
100 |
D |
E |
E |
E |
F |
* |
||
|
200 |
C |
C |
D |
E |
F |
F |
||
|
300 |
** |
** |
** |
** |
** |
** |
||
|
550 |
0 |
F |
G |
H |
H |
I |
* |
|
|
100 |
D |
D |
E |
E |
F |
F |
||
|
200 |
D |
D |
E |
E |
F |
F |
||
|
300 |
** |
** |
** |
** |
** |
** |
||
|
4250 |
325 |
0 |
F |
G |
H |
H |
* |
* |
|
100 |
D |
E |
E |
E |
F |
F |
||
|
200 |
D |
E |
E |
E |
E |
F |
||
|
300 |
** |
** |
** |
** |
** |
** |
||
|
400 |
0 |
F |
G |
H |
H |
* |
* |
|
|
100 |
D |
E |
E |
E |
E |
E |
||
|
200 |
C |
D |
E |
E |
E |
E |
||
|
300 |
** |
** |
** |
** |
** |
** |
||
|
550 |
0 |
G |
I |
I |
* |
* |
* |
|
|
100 |
D |
E |
E |
E |
F |
F |
||
|
200 |
D |
E |
F |
F |
F |
F |
||
|
300 |
** |
** |
** |
** |
** |
** |
||
| *destabilization (syneresis/gel breakdown) | ||||||||
| **precipitation | ||||||||
Furthermore, the composite systems remained stable for a longer period in the majority of samples (when evaluated at temperature of 85 oC). According to Zhang and collaborators (2016), gelling systems based on PHPA at temperatures above 80°C can have a faster crosslinking process, possibly diminishing the stability time of the gels. We found that the competition with Al+ 3 ions by the hydrolyzed groups of the PHPA with the addition of bentonite delayed the crosslinking in the gel, contributing to greater stability of the systems.
As can also be noted in Table 6, the addition of 200 ppm of bentonite to the systems aged at 85°C increased the thermal stability of the hydrogels when concentrations of Al+ 3 greater than or equal to 400 ppm were added. In comparison with the conventional hydrogels (Table 5), there was slower increase of the gel strength, possibly generated by the competition between the clay and crosslinker for the active sites in the polymer chain. However, this delay was offset by the greater thermal stability caused by adding the clay.
Nevertheless, the hydrogels containing 300 ppm of bentonite became unstable in less than 24 hours. This behavior can also be related with the polymer chain closure, as observed in the tests at 70 oC
- Hydrogels prepared with the polymer FP3530S
The hydrogels prepared with polymer FP3530S, in comparison with those prepared with the other two polymers analyzed in this study, were more stable at both temperatures analyzed.
Table 7 shows the results obtained in the bottle tests of the hydrogels formed by polymer FP 3530S at 70 ºC, with and without addition of bentonite.
The systems based on polymer FP 3530S aged at 70°C (Table 7) had more promising results in terms of stability than those obtained with polymer FP3330S (Table 5) in the formation of conventional hydrogels. In general, there were reductions of the strength codes observed, related to the larger number of anionic sites in the chains of the polymer with higher molar mass, allowing greater interaction with the cationic species, causing slower formation of gels.
The increase of clay concentration was associated with less stable systems in the samples with lower polymer content. As the concentration of aluminum increased, the clay did not cause faster breakdown of the gel, but it did retard the evolution of the gel strength code in comparison with the conventional systems. A hypothesis to explain that observation is the greater interaction of the positive charges on the surface of the bentonite clay with the anionic sites of the PHPA chains, which are more sensitive to increased temperature, in detriment to the reduction of the number of ionic crosslinks between these sites and the ionic groups of the polyacrylamide molecules.
|
Polymer concentration (mg/L) |
Crosslinker concentration (mg/L) |
Clay (mg/L) |
Time (days) |
|||||
|
1 |
3 |
7 |
10 |
15 |
30 |
|||
|
3500 |
325 |
0 |
D |
D |
D |
E |
E |
E |
|
100 |
D |
D |
D |
E |
E |
E |
||
|
200 |
C |
D |
D |
D |
D |
E |
||
|
300 |
C |
C |
C |
D |
* |
* |
||
|
400 |
0 |
D |
D |
E |
E |
E |
E |
|
|
100 |
D |
E |
E |
E |
F |
F |
||
|
200 |
C |
C |
D |
E |
F |
F |
||
|
300 |
C |
C |
C |
D |
* |
* |
||
|
550 |
0 |
F |
F |
F |
F |
F |
F |
|
|
100 |
D |
D |
D |
D |
D |
D |
||
|
200 |
D |
D |
D |
D |
D |
E |
||
|
300 |
C |
D |
E |
E |
* |
* |
||
|
4250 |
325 |
0 |
D |
D |
E |
E |
E |
E |
|
100 |
D |
E |
E |
E |
F |
F |
||
|
200 |
D |
E |
E |
E |
E |
F |
||
|
300 |
C |
D |
D |
E |
E |
F |
||
|
400 |
0 |
F |
F |
F |
F |
F |
F |
|
|
100 |
D |
E |
E |
E |
E |
E |
||
|
200 |
C |
D |
E |
E |
E |
E |
||
|
300 |
C |
D |
D |
E |
E |
E |
||
|
550 |
0 |
F |
F |
F |
F |
F |
F |
|
|
100 |
D |
E |
E |
E |
F |
F |
||
|
200 |
D |
E |
E |
E |
E |
F |
||
|
300 |
D |
E |
E |
E |
E |
F |
||
| *destabilization (syneresis/gel breakdown) | ||||||||
Table 8 shows the results obtained in the bottle tests of the hydrogels formed with polymer FP 3530S at 85 ºC, with and without the addition of bentonite.
Once again, the increase in temperature caused faster gel formation (and increase of the strength codes), due to the greater availability of anionic sites in the polymer chains (Oliveira et al., 2019) to interact with the crosslinker to form the conventional hydrogels.
|
Polymer concentration (mg/L) |
Crosslinker concentration (mg/L) |
Clay (mg/L) |
Time (days) |
|||||
|
1 |
3 |
7 |
10 |
15 |
30 |
|||
|
3500 |
325 |
0 |
C |
E |
E |
E |
E |
E |
|
100 |
D |
D |
D |
E |
E |
* |
||
|
200 |
C |
D |
D |
D |
* |
* |
||
|
300 |
C |
C |
C |
D |
* |
* |
||
|
400 |
0 |
D |
F |
F |
F |
F |
F |
|
|
100 |
D |
D |
D |
D |
D |
D |
||
|
200 |
** |
** |
** |
** |
** |
** |
||
|
300 |
C |
C |
D |
D |
* |
* |
||
|
550 |
0 |
E |
F |
F |
F |
F |
F |
|
|
100 |
E |
E |
E |
E |
F |
* |
||
|
200 |
D |
E |
E |
F |
F |
* |
||
|
300 |
D |
E |
E |
E |
* |
* |
||
|
4250 |
325 |
0 |
C |
D |
E |
E |
E |
E |
|
100 |
D |
E |
E |
E |
E |
E |
||
|
200 |
D |
E |
E |
E |
E |
* |
||
|
300 |
C |
D |
D |
E |
* |
* |
||
|
400 |
0 |
E |
E |
E |
E |
E |
E |
|
|
100 |
D |
E |
E |
E |
E |
E |
||
|
200 |
C |
D |
E |
E |
E |
E |
||
|
300 |
C |
D |
D |
E |
E |
E |
||
|
550 |
0 |
F |
F |
G |
G |
G |
G |
|
|
100 |
F |
F |
F |
F |
F |
* |
||
|
200 |
E |
E |
F |
F |
G |
* |
||
|
300 |
E |
E |
F |
F |
F |
* |
||
| *destabilization (syneresis/gel breakdown) | ||||||||
| **precipitation | ||||||||
On the other hand, at 85 oC, the greater hydrolysis of the polymer chains promoted by the increase in temperature favored the interaction of these chains with the clay and crosslinker, decreasing the stability of the systems formed. Only for a few systems was there greater stability of the system in the presence of the clay, with slower evolution of the code observed for the gel, mainly when using the lowest clay concentration (100 ppm).
3.2. Rheological tests of the hydrogels prepared with the polymer FP 3330S
We did not conduct continuous shear and oscillatory rheological tests of the systems containing polymer FP3130S because the composite hydrogels were not stable for longer than 24 hours. In contrast, the hydrogels prepared with polymer FP3530S had the longest stability among the systems analyzed in this study.
Since the hydrogels prepared with polymer FP3330S presented the greatest variation in stability, in the presence or absence of the clay, at the two temperatures, we analyzed the rheological behavior of the systems based on the influence of the clay with this polymer. Thus, we carried out two experiments, varying both the crosslinker concentration and polymer concentration, at a constant clay concentration of 200 mg/L (the concentration that caused the best increase in the stability of the systems containing polymer 3330S – Tables 5 and 6).
-
Influence of crosslinker concentration on the viscoelastic properties of the conventional hydrogels and nanocomposites
To evaluate the variation of crosslinker concentration, keeping the polymer and clay concentrations fixed, we used the following compositions:
- polymer concentration: 3500 mg/L (represented by 3500P in the graph legends);
- crosslinker concentrations: 400 and 550 mg/L (represented by 400R and 550R, respectively, in the graph legends);
- clay concentration: 200 mg/L (represented by 200C in the graph legends).
Figure 1 shows the curves obtained for the elastic modulus (G’) and viscous modulus (G”) in function of the frequency for the polymer gels based on FP 3330S, with different crosslinker concentrations and 200 mg/L of clay, at 70 oC. It can be seen that an increase in the crosslinker concentration (Fig. 1a), while keeping the polymer concentration at 3500 mg/L, caused an increase in the values of G’.
According to Zolfagari and coworkers (2006), the increase of concentrations of inorganic crosslinkers causes faster formation of the three-dimensional network of the polymer gel with greater density of crosslinks, which increases the elastic component of the material.
We calculated the tan delta values, and the frequency selected in this and the other analyses was 1 Hz. The values are presented in Table 9, and it can be seen that increase of the concentration of Al+ 3 ions to 550 ppm caused a reduction of the tan delta values to the point of forming strong gels at aging time of 7 days (tan delta < 0.1). This demostrated an increase of the values of G’ in relation to G” of the polymer gel.
|
System Evaluated |
Tan Delta value at 1 Hz (70 oC) |
Tan Delta value at 1 Hz (85 oC) |
|---|---|---|
|
3500P 400R |
0.200 |
0.279 |
|
3500P 550R |
0.104 |
0.051 |
|
3500P 400R 200C |
0.074 |
0.139 |
|
3500P 550R 200C |
0.099 |
0.043 |
P – polymer concentration in ppm; R – crosslinker concentration in ppm; C – clay concentration in ppm.
Comparison of the results depicted in Fig. 1a with those of Fig. 1b shows that the addition of the bentonite clay at the concentration of 200 ppm had different influences on G’, according to the concentration of aluminum ions used. At the lowest Al+ 3 concentration (400 ppm), there was an increase of G’ in relation to the corresponding gel without addition of clay, contrary to the case of the system with higher Al+ 3 concentration (550 ppm), where the values of the G’ curve declined with the addition of the same clay quantity. That fact allows stating that the in the presence of a lower crosslinker concentration, with the polymer and clay concentrations fixed, the elastic component is represented by the sum of the ionic bonds formed between the polymer chain and the Al+ 3 ions and clay. With greater crosslinker concentration, competition occurred between the metal ions and the clay particles for access to the negative sites of the PHPA chain, which retarded the formation of crosslinks and caused a lower value of G’ for the same aging time. This rheological behavior explains the similarity of the gel strength codes shown in Table 5, where the difference in crosslinker concentration did not alter the gel strength code when using 200 ppm of clay.
Comparison of the rheological curves at 85 ºC (Fig. 2a) with the curves of the gels aged at 70 ºC (Fig. 1a) reveals a slight variation in the viscoelastic behavior of the systems without clay, as well as the values of tan delta (Table 9). There was an increase of the gel strength for higher crosslinker concentrations, observed by the lower tan delta values, indicating that a greater quantity of negative charges in the polymer chains (with the increase of the hydrolysis degree) promoted greater crosslinking, thus increasing the polymer network formed.
The presence of the clay load (Fig. 2b) reduced the values of G’, mainly observed at low frequencies, indicating the smaller formation of crosslinks in the polymer chains, lowering the elasticity of the gel. This behavior corroborates the findings reported in Table 6, of a small delay in the evolution of the gel strength code (the code varied by only one degree with the change of temperature). Furthermore, there was no significant change in the tan delta values (Table 9), indicating a change in the fluidity of the gels.
-
Influence of the concentration of the polymer on the visoelastic properties of the conventional hydrogels and nanocomposites
After investigating the variation in concentration of the crosslinker, we analyzed the influence of polymer concentration on the rheological behavior of the hydrogels while keeping the crosslinker and clay concentrations fixed. For that purpose, we used:
- polymer concentrations: 3500 and 4250 ppm (represented by 3500P and 4250P, respectively, in the graph legends);- crosslinker concentration: 550 ppm (represented by 550R in the graph legends);
- clay concentration: 200 ppm (represented by 200C in the graph legends).
The curves of G’ and G” presented in Fig. 3a, obtained at temperature of 70 oC, showed that the increase of the polymer concentration did not significantly alter the values of G’. In contrast, Karlum and collaborators (2018) reported that increased polymer concentration hastened the formation of crosslinks, making the gel more elastic in a shorter aging time frame. However, we believe the variation of the polymer concentration adopted in this study was not sufficient to cause that behavior, also considering the higher crosslinker concentration used to formulate the gels.
The results reported in Table 10 show that the increase of polymer concentration reduced the values of tan delta to the point of obtaining a strong gel with the polymer concentration of 4250 ppm. That fact can be related to the reduction of the G” values of the gels with greater polymer concentration (Fig. 3a), i.e., for the variables used in this work, the increase did not result in lower fluidity of the material.
The curves presented in Fig. 3b show that the addition of clay caused a reduction of the values of G’ of the gels at both polymer concentrations, with a smaller reduction for the gels with polymer concentration of 4250 ppm. This can be attributed to the mentioned competition for the hydrolized sites of the PHPA chain between the Al+ 3 ions (observada only at concentration of 550 ppm) and the bentonite particles, which winds up delaying the formation of crosslinks in the three-dimensional network of the polymeric gel. That effect is attenuated with greater availability of hydrolyzed sites in the polymer, due to the increase of its concentration.
|
System Evaluated |
Value of Tan Delta at 1 Hz (70oC) |
Value of Tan Delta at 1 Hz (85oC) |
|---|---|---|
|
3500P 550R |
0.104 |
0.049 |
|
4250P 550R |
0.067 |
0.049 |
|
3500P 550R 200C |
0.100 |
0.047 |
|
4250P 550R 200C |
0.071 |
0.034 |
| P – polymer concentration; R – crosslinker concentration; C – clay concentration | ||
Table 10 also shows a reduction in the values of tan delta of the composite gels with increased polymer concentration, but in comparison with the tan delta values without the addition of clay, the addition of the clay did not cause a significant variation of these values.
The variation of the polymer concentration at temperature of 85 ºC (Fig. 4) caused proportional changes in G’ and G” for both polymer concentrations (3500 and 4250 mg/L). In this regard, Table 10 indicates that the increase of polymer concentration did not drastically alter the tan delta values, since strong gels were obtained for all the compositions analyzed.
The curves shown in Fig. 4b indicate that the addition of clay caused a reduction of the values of G’ and G” of the gels prepared with both polymer concentrations, based on comparison with the curves in Fig. 4a. This can be attributed to the already described competition for active sites of the PHPA chain between the Al+ 3 ions and clay particles, delaying the formation of crosslinks in the three-dimensional network of the polymer gel. That effect was attenuated with greater availability of hydrolyzed sites of the polymer due to the increase of its concentration.
The results in Table 10 indicate little change in the tan delta values of the composite gels at both polymer concentrations in comparison with the tan delta values without clay addition. This demonstrates that the addition of bentonite clay did not significantly influence the strength of the gels formed, despite the same competition of clay particles with the metallic crosslinker for active sites.
3.3. Morphological analyses of the hydrogels prepared with polymers FP 3330S and FP 3530S
The SEM images of the composite hydrogels prepared with polymer FP3330S are shown in Fig. 5.
Micrographs 5 (a) and (b) show the formation of a structure with high porosity and low crosslink density, due to the low adhesion of the clay on the polymer matrix. In turn, micrographs 5 (c) and (d), of systems with greater polymer concentration, show increased interaction between the matrix and unmodified clay. This indicates a greater reinforcement capacity, with the formation of a structure that is less porous and with greater crosslink density. Besides this, the systems prepared with higher concentration of unmodified bentonite had denser crosslinks, with formation of a denser structure having fewer pores, indicating that the clay at higher concentration had greater interaction with the polymer (Figs. 5 (e) and (f)).
The SEM micrographs of the composite hydrogels prepared with polymer FP3530S are shown in Fig. 6.
For polymer FP3530, Figs. 6 (a) and (b) show the formation of a structure with high porosity and low crosslink density. This again can be attributed to the low adhesion of the clay particles on the polymer chains. Figures 6 (c) and (d), of systems with higher polymer concentration, do not show an increase of the interaction between the polymer matrix and the unmodified clay, and hence the formation of a porous structure with little interaction between the clay and polymer. Figures 6 (e) and (f) present hydrogels with higher concentration of unmodified clay. These have lower porosity and formation of a denser structure, indicating that a higher clay concentration favored the interaction with the polymer.
In general, we found that the morphological density of the systems was related to the clay concentration and molar mass of the polymer. In particular, lower molar mass enabled better interaction of the clay particles with the polymer chains.
We also noted from the morphology of the hydrogels that the gel strength was greater, due to the better interaction of the polymer with the clay load, for both polymers FP3330S and FP3530S.
Comparison of the systems of conventional hydrogels and nanocomposites prepared with different PHPA polymers evaluated in this study shows that the addition of different concentrations of clay negatively influenced the crosslinking process, causing a reduction of the gel strength code and stability period than of the similar systems of conventional hydrogels, aged at 70 ºC.
In general, the addition of 200 ppm of clay in the gelling systems promoted better hydrogels (higher strength codes and greater stability), mainly at 85 ºC and for fewer than 30 days, which did not occur with the clay concentrations of 100 and 300 ppm.
At 70 ºC, the concentration of aluminum citrate did not influence the rheological behavior of the gels, while at 85 ºC, greater quantities of crosslinkers intensified the assumed competition between the positively charged clay particles and the negatively charged aluminum citrate for the active sites of the PHPA. This diminished the elastic modulus (G’). An increase of polymer concentration caused an increase in the elastic modulus (G’) and a consequent reduction of the tan delta value.
The SEM micrographs revealed that low clay concentrations did not modify the polymer, with low adhesion of the particles on the polymer chain, generating porous surfaces.
FUNDING
The authors would like to acknowledge the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) and the Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ) for the financial support of this work.
AUTHOR INFORMATION
Authors and Affiliations
Universidade Federal do Rio de Janeiro, Rio de Janeiro - RJ, Brazil
Venâncio J. S. Neto; Priscila F. Oliveira and Claudia R. E. Mansur
Contributions
Conceptualization: [Priscila F. Oliveira] and [Claudia R. E. Mansur]; Methodology: [Priscila F. Oliveira] and [Claudia R. E. Mansur]; Formal analysis and investigation: [Venancio J. S. Neto]; Writing-original draft preparation: [Venancio J. S. Neto]; Writing-review and editing: [Claudia R. E. Mansur]; Funding acquisition: [Claudia R. E. Mansur]; Supervision: [Claudia R. E. Mansur].
Corresponding author
Correspondence to Claudia R. E. Mansur.
AVAILABILITY OF DATA AND MATERIAL
The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.
ETHICS DECLARATIONS
Ethical approval
Not applicable.
Competing interests
The authors declare no competing interests.
Consent for publication
Not applicable.
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No competing interests reported.