Polyurethane foams functionalized with sulfonic acid groups are used in this study to exchange lead (Pb2+) ions from aqueous solutions. Toluene-2, 4-diisocyanate, 2,6-diisocyanate (TDI) was reacted with Polypropylene glycol 1200 (PPG) in 2 : 1 molar ratio to form a linear prepolymer. The linear prepolymer was further polymerized using N,N-bis(2-hydroxyethyl)-2-aminoethanesulfonic acid (BES), which acts both as a chain extender and an ion-exchanger for Pb2+ ions. The functionalized polyurethane foam was characterized by Fourier transform infrared spectroscopy (FTIR), gel permeation chromatography (GPC), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX). The Pb2+ ion exchange capacity was determined using an Inductively Coupled Plasma Mass Spectrometer (ICP-MS). The maximum Pb2+ ion exchange capacity of the foam was found to be 51 ppb/g from a 100 ppb Pb2+ solution over a period of two hours. In addition, pH analysis was carried out on the foam composition with the best Pb2+ ion removal capacity. The pH results based on two-hour exposures showed that the functionalized polyurethane foam performed better at lower pH levels.
Polyurethane foams are considered to be one of the best commercially available insulation materials. They possess good thermal insulating properties, low moisture-vapor permeability, high resistance to water absorption, and a relatively high specific strength. Another advantage of polyurethane foam systems is that the synthesis can be tailored to various specific applications. The major components of polyurethane foams are an isocyanate and a polyol (or a mixture of polyols). A blowing agent and a catalyst are used to accelerate the foam formation. The foams can be synthesized as open-cell foams or as closed-cell foams based on the initial raw materials concentration. Recently, the cellular form of this polymer has intrigued researchers to explore other novel applications for this material.
Open cell polyurethane foams have been shown to exhibit a reasonable amount of ion exchange capacity and hence are being considered as suitable ion exchange media for heavy metal ions removal [
Functionalization of polyurethane foams by surface modification has led to systems capable of adsorbing heavy metal and trace metal ions. Surface modification of the foam usually involves the coupling of a sorbent to the polyurethane foam through an AZO group [
In a study published by Jang et al. [
However, the research summary described above does not address bulk functionalization of polyurethane foams for selective elimination of heavy metal ions. To address this we focused our research work on the development of a bulk functionalized polyurethane foam system using a chain extender. Chain extenders are low-molecular multifunctional species. They can be used to balance the backbone structure of polymers [
Polypropylene glycol 1200 (PPG; Sigma Aldrich Co. LLC) was dried in a vacuum oven at 70°C for 24 hours. Toluene diisocyanate (TDI; 2.4–80% and 2.6–20%, Alfa Aesar), Dimethyl Sulfoxide (DMSO; Alfa Aesar, 99.9% pure), N,N-bis(2-hydroxyethyl)-2-aminoethanesulfonic acid (BES; Alfa Aesar, 99% pure), Dibutyltin dilaurate (DBTL, Sigma Aldrich, 95% pure), Polysiloxane surfactant (Sigma Aldrich), and Nitrogen gas (Airgas, O2 free UHP) were used.
The experimental set up consists of a 3-neck round bottom reaction flask placed in an oil bath fitted with a mechanical stirrer and a condenser at the center neck, nitrogen gas inlet and outlet at the right neck, and a drop funnel at the left neck. The reaction was carried out at 65–70°C in an inert atmosphere. The 3-neck flask was initially charged with TDI and allowed to stabilize at 70°C in a saturated nitrogen atmosphere. A drop funnel was filled with a preweighted amount of PPG which was added drop wise and allowed to react with TDI for 3-4 hours until an initial isocyanate content of 11-12% was reached as described in ASTM D5155 procedure.
A preweighted amount of BES dissolved in DMSO (amount of DMSO used was based on the solubility limit of BES in DMSO) at 70°C was added drop wise into the reaction flask. The reaction was allowed to proceed until a final isocyanate content of 7-8% was reached. The tin catalyst was added at the end and the prepolymer was decanted into a glass mold. Based on the amount of PPG used, a preweighted amount of distilled water was added as a blowing agent along with the surfactant. The mixture was then mixed using a mechanical stirrer at 2500–3000 rpm for 10–15 seconds. This initiates the reaction of water with the remaining isocyanate groups forming an intermediate compound and eventually releasing CO2 gas to form the cellular structure [
In order to determine the optimum (1) chain extender (BES) content, (2) molar ratio of PPG and TDI, and (3) chain extender reaction time (CERT), a set of experiments were designed as discussed below. The objective of these experiments was to characterize the effect of these parameters on the physical properties and performance (i.e., ion exchange capacity) of the foam. Table
Foam composition based on various processing variables.
Variable | Sample | PPG (g) | TDI (g) | BES (g) | DMSO (g) | DBTL (g) | Surfactant (g) | Water (g) | CERT (min) |
---|---|---|---|---|---|---|---|---|---|
BES/DMSO | A1 | 50 | 18.3 | 0 | 0 | 2 | 0.25 | 6 | 40 |
A2 | 3.4 | 7.8 | |||||||
A3 | 6.7 | 15.7 | |||||||
A4 | 10.1 | 23.5 | |||||||
|
|||||||||
PPG/TDI | B5 | 50 | 9.1 | 6.7 | 15.7 | 2 | 0.25 | 6 | 40 |
B6 | 18.3 | ||||||||
B7 | 27.4 | ||||||||
|
|||||||||
CERT | C9 | 50 | 18.3 | 6.7 | 15.7 | 2 | 0.25 | 6 | 60 |
C8 | 40 | ||||||||
C10 | 90 |
The functionalized polyurethane foam samples were characterized by a BRUKER vector 22 FTIR with a DTGS (deuterated triglycine sulfate) detector and a PIKE 3–12 multibounce Zn-Se variable angle ATR. The incidence angle was set to 80 during characterization. The foam samples were cut into strips of
The polyurethane foam samples were analyzed using a Varian ProStar HPLC system using a PLgel 5mm mixed C column. HPLC grade dimethyl formamide (DMF) was used as the eluent with a flow rate of 1 mL/min. The UV detector was set to detect sample at a wavelength of 569 nm. Foam samples were dissolved in DMF to get a 0.5% w/v concentration and 25
An Inductively Coupled Plasma Mass Spectrometer (ICP-MS) was used for trace metal analysis at ppb levels. The demolded functionalized foam was cut into one gram cubes, maintaining a constant volume. Polyurethane foam requires homogeneous rearrangement of its polymeric bonds for effective ion exchange, as the foam does not experience stress-free conditions after synthesis [
Standard Pb2+ solutions of 100 ppb were prepared from a stock solution of 1 part per million (ppm). The conditioned foams were soaked in 25 mL of Pb2+ solution in a beaker with a magnet. The beaker was then covered to avoid contamination and placed on a stirrer. Foam cubes were submersed in the Pb2+ solution for various time periods. 10 mL of each sample was filtered into 15 mL centrifuge tubes. Nitric acid was added to stabilize the Pb2+ ions in the sample during storage and for ICP-MS analysis.
The study samples were prepared according to Table
The FTIR spectrum of the foam samples A1 to A4 with varying BES/DMSO content is shown in Figure
FTIR spectrum of samples A1–A4.
Sung and Schneider [
FTIR spectrum of samples B7–C10.
BES is a chain extender which links the isocyanate groups in the linear polymer chains through its hydroxyl groups, thereby increasing the molecular weight of polyurethane. DMSO is a strong organic solvent that dissolves polyurethane as well as BES. Since BES is insoluble in other organic solvents, the use of DMSO cannot be eliminated from the foam synthesis process. Hence, both BES and DMSO have an opposite effect on the molecular weight of the resulting polyurethane system.
GPC analysis shows that the retention time needed to elute higher molecular weight or larger molecules decreases as the DMSO content and CERT increase during synthesis. This can be seen in Figure
Retention times of foam samples.
Sample | Peak retention time (mins) | ||||
---|---|---|---|---|---|
A1 | 11.4 | ||||
A2 | 9.5 | 11.3 | |||
A3 | 8.6 | 11.1 | 11.6 | ||
A4 | 7.7 | 11.0 | 11.7 | 13.7 | 14.6 |
B7 | 9.2 | 11.4 | |||
C9 | 9.4 | 11.0 | 11.4 | 13.9 | |
C10 | 9.4 | 9.7 | 10.6 | 11.7 | 15.3 |
High and low molecular weight compounds of functionalized polyurethane foam samples based on BES/DMSO content.
Samples A2 and B7 seem to show a similar elution trend. A2 has a lower amount of DMSO and B7 has a higher TDI content in its composition. This provides additional isocyanate groups for functionalization, therefore lowering the ability of DMSO to break the polymer chains during synthesis. Hence, increasing the TDI content in the foam composition without affecting the structural integrity of the foam may be a way to reduce the cleaving effect of DMSO.
Similarly, Figure
High and low molecular weight compounds of functionalized polyurethane foam.
Foam samples were analyzed using a Topcon SM-300 SEM/EDX instrument. The samples were coated using a sputter coater before analysis. Figure
SEM image of polyurethane foam at 50x magnification.
EDX elemental analysis.
ICP-MS is a powerful analytical technique that allows detection of trace elements at parts per billion and parts per trillion levels. Lead ion exchange capacity of the synthesized foam was measured using ICP-MS.
The synthesized foam samples were cut into symmetrical cubes, weighing about 1 gram, and were conditioned in 2N NaCl solution for 4 hours before soaking in a 100 ppb lead solution for a period of 2 hours. The lead solution was filtered and analyzed by ICP-MS to determine the final lead concentration in the solution. The experiments were repeated with two grams of foam samples. The results of this analysis are shown in Figure
Lead removal capacity based on foam weight.
Foam samples A1 (0 moles BES) showed some Pb2+ removal in the absence of sulfonic functional groups, which confirms that the mechanism of lead ion removal is not only by ion exchange but may also be due to surface adhesion or absorption [
An increase in lead removal capacity for all the foam samples was observed when 2 grams of the foam was soaked in Pb2+ solution. This is clearly due to the availability of additional functional groups for ion exchange which increased the Pb2+ ion removal capacity from 48 ppb/g to 53 ppb/2g of A3 foam.
Based on the above results, A3 foam samples were analyzed at 6.5 and 8.5 pH levels over a period of 1 to 48 hours, as shown in Figure
pH analysis of functionalized polyurethane foam.
The lead removal capacity of the foam seemed to taper down and saturate after 2 hrs. Lead removal capacity of the foam also seemed to decrease as the pH of the solution increases to 8.5. This may be due to the neutralization of the sulfonic acid groups at higher pH levels which in turn diminishes the availability of functional groups for ion exchange [
Polyurethane foam made of PPG and TDI segments with sulfonic functional groups was synthesized using BES chain extender dissolved in DMSO for Pb2+ ion removal from aqueous solutions. Various parameters, such as PPG/TDI ratio, BES/DMSO content, and CERT, were studied by designing a set of ten experiments. The outcome of each experiment (i.e., combination) was recorded to analyze the physical characteristics of the functionalized foam. The foams were conditioned in 2N NaCl solution to improve the ion exchange capacity and to realign the polymeric bonds.
FTIR characterization of the foam samples confirmed the presence of sulfonic groups in the polyurethane backbone. GPC characterization confirmed polymer chain disintegration due to higher DMSO content and increased CERT. This was further confirmed by ICP-MS analysis showing the lead removal capacity of the foams. Foam samples containing higher DMSO exhibit lower Pb2+ removal rate since the functional groups were not available uniformly throughout the bulk of the foam. EDX analysis confirmed the presence of Pb2+ in the foam samples soaked in a standard lead solution. Foam samples synthesized with 1 : 2 mole ratio of PPG : TDI and 0.6 : 4 moles of BES : DMSO (based on moles of TDI) exhibited the best performance of lead removal. The measured lead removal capacity of the foam was 48 ppb/g and 51 ppb/2g. pH analysis of the foam showed reduction in its lead removal capacity at higher pH levels due to neutralization of sulfonic acid groups which lowered the number of functional sites available for ion exchange.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors would like to thank the members of Water Equipment Policies (WEP) and Industry-University cooperative research program (I/UCRC) for their support through the course of this research work. The authors would also like to extend their gratitude to Susan Krezoski from the Department of Chemistry and Biochemistry, UW-Milwaukee, for scheduling and coordinating the use of ICP-MS, and Dr. Xiangyang Liu and Dr. Marianna Orlova of the UW-Milwaukee, biotech facility, for training and coordinating the use of GPC.