Ionic Liquid Coating for

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Research Note pubs.acs.org/IECR

Curable Imidazolium Poly(ionic liquid)/Ionic Liquid Coating for Containment and Decontamination of Toxic Industrial ChemicalContacted Substrates Rhia M. Martin,† Dylan I. Mori,† Richard D. Noble,*,† and Douglas L. Gin*,†,‡ †

Department of Biological and Chemical Engineering, and ‡Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, United States S Supporting Information *

ABSTRACT: A curable, imidazolium-based room-temperature ionic liquid (RTIL) coating system has been developed for the containment and decontamination of toxic industrial chemicalcontacted surfaces. The curing times and mechanical properties of this poly(RTIL)/RTIL platform, which is based on alcohol− isocyanate step-growth polymerization chemistry, can be tuned by modifying the structures of the step-growth monomers, the stoichiometric ratio of linear (diol, A2) to cross-linking (triol, A3) RTIL monomers used, and the amount of free RTIL in the formulation. When applied to painted steel and rubber test substrates contacted with o-dichlorobenzene (o-DCB) (a polychlorinated biphenyl simulant), a 50:50 (A2:A3) step-growth alcohol-RTIL monomer mixture cross-linked with a stoichiometric amount of a commercial di-isocyanate monomer (B2) and containing 43 wt % free RTIL in the coating, reduced the o-DCB vapor amount by 96−99% compared to the uncoated, o-DCB-contacted control samples. This peelable, flexible, solid coating also removed by sorption up to >99% of the o-DCB liquid applied to the substrates after 24 h of contact.

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application of oxidizing solutions is often employed for the decontamination of TICs, but this approach often utilizes corrosive reagents.10,11 Furthermore, many TICs have a propensity to persist on and leach out of porous substrates, constituting a long-term hazard.12 Current oxidative decontamination strategies are insufficient at removing and degrading soaked-in contaminants.10 A recent approach to the containment and decontamination of toxic chemicals on contacted substrates is a spreadable, room-temperature ionic liquid (RTIL)-based gel coating material.13 In this multicomponent system, the RTIL (i.e., a molten organic salt at ambient conditions with negligible vapor pressure) acts as a stable fluid medium that can (1) envelop the contaminated area, (2) depress the vapor pressure of enveloped toxic compound, and (3) remove the absorbed contaminant via solubilization or reaction with an added reagent.14,15 However, the physically gelled RTIL material in this prior work is still a fluid. This feature makes it difficult to completely remove from the substrates after decontamination and to contain after removal. This initial RTIL gel coating has also only been demonstrated to work with a highly reactive chemical warfare agent (CWA) simulant and not with TICs, which include many less reactive compounds.

oxic industrial chemicals (TICs) are a group of chemicals produced or utilized in large quantities industrially that can lead to a variety of health problems or even death in humans.1 Because of the prevalence of TICs, there is significant concern of accidental exposure or unintentional release of TICs via leaks or spills during storage, transport, or use. This class of compounds includes carcinogens, reproductive hazards, and physical hazards. Exposure to many TICs can lead to irreversible damage to DNA, cellular proteins, and other macromolecules.1,2 Also, TIC vapor inhalation can lead to lung damage. Thus, the development of technologies or materials that effectively contain and/or decontaminate TICs-contacted surfaces and provide mitigation is an important area of research.2−5 Containment and decontamination procedures for TICs vary with the specific contaminant of interest; however, the principal method of limiting exposure of first-responders and technicians to these chemicals is the utilization of adsorbents. Specifically, activated carbon is frequently employed in protective garments that act as barrier materials but have limited lifespans due to the uptake capacity of the sorbent.6,7 Activated carbon exhibits an affinity for a range of organic compounds6 and is thermally stable and resistant to degradation in both acidic and basic environments.3 However, current adsorbent materials provide inadequate protection, as not all TICs are strongly adsorbed. Methods for modifying the surface of the carbon to alter its adsorptive abilities and the development of new adsorbent materials are active areas of interest.8,9 Similarly, nonspecific © XXXX American Chemical Society

Received: February 6, 2016 Revised: April 18, 2016 Accepted: May 18, 2016

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DOI: 10.1021/acs.iecr.6b00542 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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processes rapidly, (2) there are no volatile reaction byproducts, which aids vapor containment, (3) isocyanates are industrially used in resins, and (4) the structures and properties of the final resin can be tuned via modifications to the stoichiometry of the linear and cross-linking step-growth monomers, monomer structures, and free RTIL composition and loading.16 We examined the curing rate of our composite material by studying the reaction between the synthetically derived imidazolium-based alcohols and commercially available diisocyanates. The step-growth polymerization of these monomers forms the polymeric backbone of our coating system. To mimic conditions under which the coating system would eventually be applied, FT-IR spectroscopy was selected to monitor this reaction as thin films sandwiched between NaCl plates (Table 1).

Herein, we present a new imidazolium-based poly(RTIL)/ RTIL coating system based on alcohol−isocyanate step-growth polymerization chemistry (Scheme 1) that can be applied as a Scheme 1. Structures of the Step-Growth Monomers and Resulting RTIL-Based Polymer Network. This New Spreadable Coating Material Is a Two-Component System That Consists of an Imidazolium-Based Polymer Network and Free RTIL

Table 1. FT-IR Analysis of Thin Film Curing Timesa

a

FT-IR experiments were performed using 1 equiv di-isocyanate (B2), 1 equiv the total number of alcohol groups supplied by A2 or A3, 1.05 equiv [HMIM][Tf2N] and 0.1 equiv 1,4-diazabicyclo[2.2.2]octane at ambient temperature. Curing times were determined as a function of the reduction in the isocyanate IR stretching band at 2241 cm−1.

nonvolatile, viscous monomeric fluid onto TIC-contacted substrates and rapidly cured into a flexible, solid composite film. The applied coating effectively sequesters the TIC vapor and absorbs TIC liquid from the substrate. The curing time and mechanical properties of this new curable RTIL coating material can be tuned by modifying the structures of the step-growth monomers used, the stoichiometric ratio of linear (RTIL-diol, A2) to cross-linking (RTIL-triol, A3) monomers used, and the amount of free RTIL in the formulation. The remainder of the system is made up of a commercial organic diisocyanate monomer (B2) that reacts with the alcohol groups on the A2 and A3 monomers to form polymeric urethane linkages. The TIC sequestration and removal effectiveness of the coating system was initially demonstrated by applying a 50:50 (A2:A3) step-growth RTIL monomer system with a stoichiometric amount of toluene di-isocyanate (TDI) and 43 wt % free RTIL on o-dichlorobenzene (o-DCB)-contacted painted steel and rubber test substrates. This initial coating formulation was able to reduce the o-DCB vapor amount by 96−99% compared to uncoated o-DCB-contacted control samples and remove up to >99% of the o-DCB liquid applied to the substrates. Imidazolium-based RTIL coating materials have been demonstrated to encapsulate CWA simulant vapors and uptake soaked-in simulant from CWA-contacted surfaces.13 The ionic nature of both the polymer network and the liquid RTIL solvent provides good adhesion between the two phases. Additionally, alcohol-isocyanate step-growth chemistry was selected for the polymer component because (1) the reaction

First, we investigated the effect of modifications to the RTIL monomers on the curing rate. The first-generation vicinal diol A2 monomer 1 reacted with TDI quickly (entry 1). The symmetrical diol monomer 2a with primary alcohols had a similar curing time (entry 2); however, increasing the aliphatic chain length of the monomer increased the curing time significantly (entry 3).17 Also, employing a less electrophilic, aliphatic B2 monomer dramatically increased reaction time (entry 4). Additionally, substitution of the A3 monomer also resulted in very fast consumption of the B2 reaction partner (entry 5). Next, we prepared a series of free-standing cured films and evaluated their mechanical properties. These films were prepared by mixing a [HMIM][Tf2N] solution of the A2 and A3 monomers plus 1,4-diazabicyclo[2.2.2]octane (DABCO) polymerization catalyst with a [HMIM][Tf2N] solution of TDI (B2) (see the Supporting Information section IV for full details). An initial formulation consisting of a 50:50 (2a:3a) monomer ratio, a stoichiometric amount of TDI, and 1.05 equiv (43 wt %) [HMIM][Tf2N] afforded a transparent, flexible, cured film with a very low elastic modulus (5.3 ± 0.4 kPa) compared to other polyurethane elastomers and coatings.18−20 As expected, increasing the proportion of crosslinking monomer 3a to 2a resulted in less flexible films with higher elastic moduli (see Table S1 entries 2 and 3). However, further lowering the amount of A3 monomer by using a 75:25 B

DOI: 10.1021/acs.iecr.6b00542 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Research Note

Industrial & Engineering Chemistry Research (2a:3a) ratio did not result in a uniform film. Use of A2 monomers with longer alkyl chains (2b and 2c) at a 50:50 ratio with similarly chain-extended A3 monomers (3b and 3c, respectively) and the same amount of free RTIL led to flexible films with lower elastic moduli than the original 50:50 (2a:3a) film (see Table S1, entries 4 and 5). Additionally, increasing the amount of free RTIL resulted in the most flexible cured film (see Table S1, entry 6).21 Finally, we sought to demonstrate the ability of our curable RTIL composite coating system to contain TIC vapor and absorb liquid TIC from contacted substrates. The coating system is intended for single use, and the contaminated coatings should be disposed of as appropriate immediately following the decontamination procedure. For these proof-ofconcept trials, we selected an initial (i.e., representative) poly(RTIL)/RTIL coating prepared by reacting a 50:50 (2a:3a) monomer mixture with a stoichiometric amount of TDI that also contained 43 wt % free [HMIM][Tf2N] in the coating. This curable coating was applied as a mixture of the two sets of monomers (plus DABCO catalyst) as RTIL solutions to o-DCB-contacted painted steel and rubber as test substrates and allowed to cure in situ (see the Supporting Information for full details on methods and analyses used). Efforts to test wood and other very porous substrates are the subject of continued research, as current poly(RTIL)/RTIL formulations are not sufficiently viscous to prevent complete soak-through in their initial monomeric form into these substrates. o-DCB was selected as a simulant for polychlorinated biphenyls (PCBs).22 Once widely used but now regarded to be persistent pollutants, PCBs have been implicated in many health concerns, including hypertension, diabetes, cancer, and reproductive impairment.23−27 As a result, the development of methods for PCB removal and decontamination of PCBcontacted surfaces has gained significant attention.28−31 The applied poly(RTIL)/RTIL coating substantially reduced the concentration of o-DCB vapor in the headspace above both the o-DCB-treated painted steel and rubber, compared to their respective uncoated control samples. It was found that 96% of the o-DCB vapor was suppressed above the coated painted steel substrate relative to an uncoated painted steel sample, whereas effectively complete suppression of o-DCB vapor was achieved above the coated rubber substrate (i.e., o-DCB vapor could not be detected in the headspace by GC−MS) (see the Supporting Information). The applied coating also significantly reduced the amount of liquid o-DCB contained within the substrates after treatment, compared to the uncoated samples, as determined by extraction of soaked-in o-DCB from the samples (Figure 1). It was found that 65% of the o-DCB applied to the uncoated painted steel substrate could be removed by CHCl3 extraction. However, when the poly(RTIL)/RTIL coating described above was applied to a o-DCB-contacted painted steel substrate for 24 h, none of the o-DCB applied was extracted from the substrate after removal of the coating. This difference is consistent with successful uptake of the liquid o-DCB into the coating from the substrate. When o-DCB was applied to a blank rubber substrate (a swellable, hydrophobic, cross-linked polymer), 99% of the oDCB was extracted by CHCl3. When the poly(RTIL)/RTIL coating was applied and cured on the o-DCB-containing rubber sample, the amount of o-DCB that could be extracted from the rubber sample was reduced to 31% relative to the uncoated sample. This result shows substantial uptake of soaked-in o-

Figure 1. Total mole fraction of liquid o-DCB remaining in the uncoated control and coated substrates compared to the initial amount of o-DCB applied to each, after 24−25 h at ambient temperature. The values shown are the averages of three independent runs with standard deviation error bars.

DCB from the swellable rubber into the poly(RTIL)/RTIL coating. In summary, we have developed a new curable imidazoliumbased poly(RTIL)/RTIL coating system based on alcohol− isocyanate step-growth polymerization chemistry that effectively contains and decontaminates TIC-contacted surfaces. The curing time and mechanical properties of this new RTIL coating platform can be tuned by modifying the structures of the step-growth monomers, the stoichiometric ratio of linear A2 to cross-linking A3 monomers used, and the amount of free RTIL in the formulation. When applied to o-DCB-contaminated painted steel and rubber, the coating serves as a barrier to o-DCB vapor release from the surface and removes most of the liquid o-DCB from the substrates. We are currently developing more viscous hydroxylated imidazolium poly(RTIL) prepolymers for curing on very porous substrates such as wood.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b00542. Syntheses and characterization data of the imidazoliumalcohol monomers, procedure for preparing the curable coatings, elastic modulus data for a series of cured films, and procedures for the o-DCB vapor containment and liquid decontamination testing (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding from the Defense Threat Reduction Agency/Army Research Office through a Phase 2 SBIR grant with TDA Research, Inc. (Grant W911NF-14-P-0066) is gratefully acknowledged. We thank Dr. B. A. Voss for his work initiating this project. We also thank Drs. B. J. Elliott and W. A. Ellis at TDA Research for helpful discussions. C

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Novel Copolyether Macrodiols. J. Appl. Polym. Sci. 1996, 63, 1373− 1384. (20) Yokoyama, N. Properties of Polyurethane Coatings Containing Additives of Phenolic Compounds. J. Appl. Polym. Sci. 2006, 102, 2099−2106. (21) Using 1.5 equiv of [HMIM][Tf2N] did not produce a uniform film. (22) Lester, G. R. Catalytic destruction of hazardous halogenated organic chemicals. Catal. Today 1999, 53, 407−418. (23) Sharma, B.; Sharma, P.; Joshi, S. C. A Review on Reproductive Toxicity of Organochlorine Pesticides. World J. Pharm. Res. 2014, 3, 280−299. (24) Tang, M.; Chen, K.; Yang, F.; Liu, W. Exposure to Organochlorine Pollutants and Type 2 Diabetes: A Systematic Review and Meta-Analysis. PLoS One 2014, 9, e85556. (25) Everett, C. J.; Thompson, O. M. Dioxins, furans and dioxin-like PCBs in human blood: Causes or consequences of diabetic nephropathy? Environ. Res. 2014, 132, 126−131. (26) Quinete, N.; Schettgen, T.; Bertram, J.; Kraus, T. Occurrence and distribution of PCB metabolites in blood and their potential health effects in humans: a review. Environ. Sci. Pollut. Res. 2014, 21, 11951− 11972. (27) Everett, C. J.; Frithsen, I.; Player, M. Relationship of polychlorinated biphenyls with type 2 diabetes and hypertension. J. Environ. Monit. 2011, 13, 241−251. (28) Cheng, M.; Zeng, G.; Huang, D.; Lai, C.; Xu, P.; Zhang, C.; Liu, Y. Hydroxyl radicals based advanced oxidation processes (AOPs) for remediation of soils contaminated with organic compounds: A review. Chem. Eng. J. 2016, 284, 582−598. (29) Sharma, J.; Bhar, S.; Veerappapillai, S. Phytoremediation of Polychlorinated Biphenyls: A Brief Review. Res. J. Pharm., Bio. Chem. Sci. 2015, 6, 1466−1471. (30) Peng, W.; Fang, Z. D.; Qiao, H.; Hao, Q. L.; Zhang, K.; Yu, H. B. A Review on Remediation Technologies of PCBs from the Contaminated Soils or Sediments. Adv. Mater. Res. 2014, 955−959, 2238−2242. (31) Bonneau, J.-P.; Bonneau, C.; Bonneau, M.; Bonneau, V. Device and method decontaminating surfaces comprising one or a plurality of toxic products. PCT Int. Patent WO 2014135819, Sep. 12, 2014.

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DOI: 10.1021/acs.iecr.6b00542 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX