Lipophilic Super-Absorbent Swelling Gels As Cleaners For .
Lipophilic Super-Absorbent Swelling Gels as Cleaners forUse on Weapons Systems and PlatformsWP- 1761Final ReportVeera M. Boddu1, Sophie Minori Uchimiya2, Masahiko Ohta3, Kazuki Sada3,Christopher Myers1, Wayne Ziegler4, Thomas Torres512U. S. Army Engineer Research and Development Center, Champaign, IL 61822, USAAgricultural Research Service, U.S. Department of Agriculture, New Orleans, LA 70124, USA34Department of Chemistry, Hokkaido University, Sapporo, Hokkaido 060-0810, JapanU. S. Army Research Laboratory, Aberdeen Proving Grounds, MD 21005-5069, USA5U.S. Naval Facilities Engineering Service Center, Port Hueneme, CA 93043, USA
This report was prepared under contract to the Department of Defense StrategicEnvironmental Research and Development Program (SERDP). The publication of thisreport does not indicate endorsement by the Department of Defense, nor should thecontents be construed as reflecting the official policy or position of the Department ofDefense. Reference herein to any specific commercial product, process, or service bytrade name, trademark, manufacturer, or otherwise, does not necessarily constitute orimply its endorsement, recommendation, or favoring by the Department of Defense.
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TABLE OF CONTENTSList of Figures . vList of Tables . viiList of Acronyms . viiiAcknowledgements . ixAbstract . 1Objective . 1Background . 2Technical Approach . 8Materials and Methods . 8Synthesis . 9Characterization . 9Swelling Studies . 9Critical Temperature Studies . 10Cyclic Temperature Change Test . 10Compression Strength. 10Oil Absorption . 11Grease Cleaning. 11Results and Discussion . 11Characterization of NG-18 Gels . 11Swelling Behavior . 13Critical Solution Temperature . 17Thermal Cycling of NG-18-1% gels . 18iii
Compression Studies . 20Oil Absorption . 20Grease Cleaning. 23Recyclability of NG-18 Gels . 25Cost Assessment. 27Cost Reporting . 27Cost Analysis . 28Cost Comparison . 30Toxicity . 31Conclusions . 31References Cited . 31Appendices 34Appendix A: Military Performance Specification for Degreasing Solvent MIL-PRF-680B . 35Appendix B: Material Safety Data Sheets (MSDS). 48Appendix C: NG-18 Gel Study- Additional Pictures and Data Tables . 63iv
List of FiguresFigure 1.Figure 2.Figure 3.(a) Dry lipophilic polyelectrolyte gel (EG-18), (b) EG-18 gelswollen in tetrahydrofuran (THF) (ε 7.6), (c) Dry neutralanalogue (NG-18), (d) NG-18 gel swollen in THF. Figures are inscale with one another. Figure adapted from Reference [3].4Preparation of candidate lipophilic tetraalkylammoniumtetraphenylborate polyelectrolyte gel (EG-18) and its neutralanalogue (NG-18). Figure adapted from Reference [3]5Swelling degree (Q) of lipophilic polyelectrolyte gels (EGn wheren represents alkyl chain length of polyacrylate polymer backbone,as shown in Figure 2) and neutral analogues (NGn) in organicsolvents (in increasing order of polarity from left to right). Figureadapted from Reference [4].7Figure 4.FTIR spectra of stearylacrylate12Figure 5.Compression strength of swollen NG-18-1% , THF has a breakingpoint around 0.371 MPa.13Swelling degree of NG-18-x% at 25ºC in various solvents after (a)24 h, (b) 48 h and (c) 72 h14Temperature dependence of swelling degree (Q) of NG-18-1% invarious solvents.15Figure 8.Thermal response of NG-18 gels16Figure 9.Swelling degree changes of NG-18 with time in THF17Figure 10.(a) Temperature dependence of transmittance at 700 nm of NG-181% gel swollen in THF. (b) Photograph of transparent state at 25 C. (c) Photograph of opaque state at 0 C.18(a) Results of thermal cycling (25 C 0 C) test for NG-18 gels:changes of (circle) transmittance at 700 nm, (triangle) swellingdegree in THF. (b) Photograph of transparent state at 9th step. (c)Photograph of opaque state at 10th step.19Figure 6.Figure 7.Figure 11.v
Results of thermal cycling of (25ºC 0ºC) NG-18-1% gel swollenin cyclohexane19Compression strength of swollen (a) NG-18-1% and (b) NG-180.5% in toluene20Figure 14.Oil absorption properties of NG-18 gels.21Figure 15.Metal surface cleaning properties of NG-18 gels.21Figure 16.Photographs of metal coupons (a)-(c) soaked in SAE-30 oil, (d)-(f)soaked in the mixture of SAE-30 oil and alumina powder, (g)-(i)immersed in NG-18-1% gel, and (j) immersed in NG-18-0.5% gel(circles indicate the alumina remaining area).22Photographs of soiled metal parts cleaning tests (a) before and (b)after cleaningwith NG-18-1% gels swollen in THF23Figure 18.Grease absorption properties of NG-18 gels24Figure 19.Grease Cleaning power of NG-18 gels24Figure 20.Cyclic surface clening process with swollen NG-18-1% gel in THF25Figure 21.Photographs of metal coupons (a) before and (b) after immersingthe NG-18 gel: from left side, cycle1-5. (c) Absorbed amount ineach cycle.26Photographs of (a) collected solution, (b) filtrated residualparticulates, and (c) residual oil after evaporation. (d) Ratio ofcollected solution amount for the weight of gel during cycliccleaning process26Photographs of metal coupons immersed in gel: (left) NG-18-1%,(center) NG-18-0.5%, and (right) toluene63Metal coupon before (b), and after (a) being contaminated withMIL-PRF-10924 grease. Metal coupons cleaned with NG-18-0.5%(c)-(d) for 13 minutes and 32 seconds, TCE for 5 minutes 12seconds (e) and NG-18-1% for 12 minutes 58 seconds (f)63Figure 12.Figure 13.Figure 17.Figure 22.Figure 23.Figure 24.vi
List of TablesTable 1.Lagergren first and second order rate constants (k1 and k2) for swelling ofthe NG-18 gels . 16Table 2.Chemical cost and amounts needed to make one kilogram of dry NG-18 gels . 28Table 3.Chemical amounts of NG-18 and trichloroethylene needed to fill a 3400 Lcleaning tank to various levels . 29Table 4.Cost per batch of NG-18 gels compared to trichloroethylene when cleaningtank is filled to various levels . 30Table 5.Cost over time of NG-18 gels compared to trichloroethylene when cleaningtank is filled to various levels assuming no depreciation and one use perday. . 30Table 6.Experimental grease absorption data of NG-18 gels . 64Table 7.Cleaning power data of NG-18 gels. 64Table 8.Swelling data of NG-18-x% gel in various solvents for 24 h, 48 h, and 72 h. . 65vii
List of AcronymsAIBNazobisisobutylonitrileASTMAmerican Society for Testing and MaterialsDMSODimethylsulfoxideDoDDepartment of DefenseEG-18Octadecylacrylate-co-ethylene glycol dimethacrylate tetraalkylammoniumtetraphenylborate polyelectrolyte gelEGDMAEthylene glycol dimethacrylateFTIRFourier transform infraredHAPsHazardous air pollutantsMIBKMethylisobutylketoneMSDSMaterial Data Safety SheetNG-18Stearylacrylate-co-ethylene glycol dimethacryale neutral AStearylacrylateSPOTASustainable Painting Operation for the Total etrahydrofuranVOCsVolatile organic compoundsviii
AcknowledgementsWe wish to acknowledge the financial support provided by SERDP under the project no. WP1761. We would also like to acknowledge the Program Manager (Mr. Bruce D. Sartwell) andthe SERDP Science Advisory Board for advice given to us during the project.ix
AbstractIncreasingly stringent environmental regulations on volatile organic compounds (VOCs)and hazardous air pollutants (HAPs) demand the development of disruptive technologies forcleaning weapons systems and platforms. Currently employed techniques such as vapordegreasing, solvent, aqueous, or blast cleaning processes suffer from shortcomings inenvironmental friendliness, personnel health and safety, cleaning efficiency, cost-effectiveness,management of contaminated cleaning media, or in maintaining the integrity of equipmentmaterial surfaces. We propose to use novel lipophilic super-absorbent swelling gels as adisruptive solid state cleaning technology that will facilitate the Department of Defense (DoD) inovercoming limitations of currently employed cleaning techniques. Lipophilic super-absorbentswelling gels have been developed that will not only absorb the oil and grease from thesemachine parts, but will also act as an automated sweeper due to the self-generating mechanicalforce of the gel. An octadecylacrylate-co-ethylene glycol dimethacrylate (ODA-co-EGDMA)tetraalkylammonium tetraphenylborate lipophilic polyelectrolyte gel (EG-18) andpoly(stearylacrylate-co-ethyleneglycol dimethacrylate) (SA-co-EGDMA) neutral gel (NG 18)were evaluated for swelling and oil sorption capacity. The results were compared with acommercially available alkylstyrene copolymer (imbiber beads). For each gel, the swellingdegree and oil absorption capacity were quantitatively investigated at 0-60 C using a variety ofpolar and nonpolar solvents. The cross-linking of the polymers was studied using infraredspectroscopy, and the compression strength was determined. The cleaning tests were performedon metal coupons using ASTM G122-96(2008) methods. Cleaning tests were also performed onfield samples obtained from a Naval cleaning facility. NG-18 and EG-18 gels removedparticulate contaminants and absorbed oils and grease on metal and non-metal surfaces withoutcausing abrasion. The gels are also recyclable. The cleaning ability of the gels was comparedwith the standard solvent cleaner trichloroethylene (TCE) following ASTM G122-96(2008) testmethods and MIL-PRF-680B procedure with MIL-PRF-10924 test grease. Polymer gel cleanersexhibited analogues extent and rate of cleaning as the TCE. In conclusion, the recyclablesuperabsorbent polymer cleaners developed in this research will allow drastic reduction in theuse of VOC containing solvents and HAP release.ObjectiveThe overall objective of the proposed research was to develop an environmentally benigndisruptive technology for cleaning metal and non-metal surfaces. The ability of two superabsorbent polymer gel systems for removing oil, grease and particulates from metal and plasticsurfaces was initially evaluated. Upon successful proof of the surface cleaning ability of thesegel systems, further research will focus on improving the gel performance by design andsynthesis of additional polymer gel systems. Further research will address the post-cleaning gelremoval method, the use of non-fluorinated compounds in gel synthesis, and an evaluation oftoxicity and environmental fate-and-effects of the gels. The proposed cleaner is in solid form andis VOC-exempt, HAP-free, non-toxic, non-corrosive, non-ozone depleting, recyclable, and self-1
generates the energy necessary for the cleaning function, thereby affording a new cost-effective,environmentally friendly cleaning technology. We hypothesized that lipophilic super-absorbentswelling gels would, upon contacting oil and grease on the metal and non-metal surfaces, exertenough mechanical forces by swelling to remove particulate matters, oil, and grease on thematerial surfaces simultaneously. Also, that the super-absorbent gels would exhibit low frictionbehaviors and therefore not stick to, or cause damage on the surface of metal and nonmetalmaterials. Existing solvent and blast cleaning technologies pose environmental concerns bothduring (VOC production from organic solvents and HAP production from forced-air blastcleaning processes) and after (disposal of waste streams for solvent cleaning; cleanup of blastedcontaminants for forced air cleaning) the cleaning operation. In addition, these techniquesrequire on-site equipment such as the soaking bath and air compressor, and often necessitateoperation in a confined, well-ventilated space. Current limitations stated above call for aportable cleaning technology that will not pose environmental or health threats during or after thecleaning operations. The overall study aimed to utilize intricate designing of lipophilic superabsorbent swelling gels through careful selection of polymer backbone and ionic components,and the cross linking density for improved cleaning ability of the lipophilic swelling gels. Aftersuccessful proof-of-concept, a follow-on project will be proposed to address other issuesincluding the method for removing the gels after swelling, the use of non-fluorinated compoundsin gel synthesis, and an evaluation of toxicity and environmental fate-and-effects of the gels.BackgroundEnvironmentally benign VOC-exempt, and HAP-free surface cleaning technology, will supportongoing DoD programs such as the Sustainable Painting Operations for the Total Army(SPOTA). Technology developed in this research will result in dramatic overall reductions ofVOCs and HAPs emissions from DoD surface cleaning operations. Polyelectrolyte gels are ionicpolymer networks composed of charged polymer chains and freely mobile counter-ions.Polyelectrolyte super-absorbent wet-swelling hydrogels are known to undergo a dramatic butreversible volume change by absorbing large quantities of water. The polyelectrolyte hydrogelsswell in water because of (1) osmotic pressure induced by freely mobile counter-ions within thepolyelectrolyte, (2) increased entropy arising from the solvation of polymer ions and counterions, (3) electrostatic repulsion between the oppositely charged ions within the polyelectrolytegel, and (4) stretching of polymer chains between crosslinks caused by the increase in entropyassociated with mixing polymer with solvent [1, 2]. Polyelectrolyte hydrogels have found a widerange of applications in diapers, inks and display devices, separation media, and clean up ofaqueous spills. Polyelectrolyte hydrogels are particularly useful for a wide range ofenvironmental applications, because expansion and contraction of the gels can be engineered tobe triggered by small changes in environmental parameters such as temperature, pH, and ionicstrength. However, until recently, reports on gels that will swell by absorbing large quantities ofnonpolar organic solvents were nearly nonexistent. In nonpolar solvents, most polyelectrolytegels collapse, because the oppositely charged ions within the gel form ion pairs that thenaggregate, rather than becoming solvated.In 2007, Sada (a research collaborator on this project) and colleagues [3] reported, for the firsttime, a novel class of lipophilic polyelectrolyte gels bearing positively charged repeating units2
(substituted tetraalkylammonium with long alkyl chains) and negatively charged counter-ions(substituted tetraphenylborate; TFPB-) that swell dramatically but reversibly by absorbingorganic solvents having various polarities (ε 1.9-46; the lower the dielectric constant (ε), theless polar the solvent). Superior swelling ability in nonpolar solvents (illustrated in Figure 1, (a)(b) is enabled by making both the polymer chains and the counter-ions lipophilic, preventingcounter-ions from forming ion pairs, thereby enabling the solvation of ionic gel components insolvents. Lipophilic polyelectrolyte gel presented in Figure 1 (a)-(b) is hereby termed EG-18and will serve as a candidate cleaner in this proposal. Figure 1 (c)-(d) illustrates swellingbehavior of NG-18, a neutral analogue of EG-18 that does not contain the ionictetraalkylammonium tetraphenylborate unit. As shown in Figure 1, neutral gel NG-18 swells to amuch lesser extent than the ionic EG-18 gel. Neutral polymer gels swell in organic solventsbecause of the stretching of polymer chains between crosslinks caused by the increase in entropyassociated with mixing polymer with solvent.Additional swelling mechanisms ofpolyelectrolyte gels such as the solvation of ionic groups do not exist in neutral gels. Therefore,neutral gels may be of limited use as cleaners compared to
environmentally friendly cleaning technology. We hypothesized that lipophilic super-absorbent swelling gels would, upon contacting oil and grease on the metal and nonmetal surfaces, exert - enough mechanical forces by swelling to remove particulate matters, oil, and grease on the material surfaces simultaneously.
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