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Nuclear magnetic resonance profiling of chemical warfare agent simulant mass-transport through a multilayered polymeric coating.

Abstract Temporary peel able coatings (TPCs) protect military equipment surfaces against chemical warfare agents (CWAs) by absorbing liquid contamination, thereby reducing the contact and vapor hazards to personnel. An understanding of the liquid CWA mass-transport mechanisms governing sorption is critical toward optimizing coating properties for reducing these hazards. We applied Nuclear Magnetic Resonance profiling to resolve mass-transport mechanisms of the CWA bis(2-chloroethyl) sulfide (sulfur mustard) simulant, methyl salicylate (MS), through a two-layered polyurethane TPC, where each layer absorbs liquids using different mechanisms (passive diffusion through pores and solubilization with the liquid). Depth profiles obtained at increasing time-points post-contamination demonstrated (a) dynamics of MS volume spread through the coating, (b) polymer swelling by a significant increase in the thickness of one of the layers and its relationship with overall coating thickness and contamination mass, and (c) preferential sites within the bulk for MS localization. Information of the type obtained from this exemplar system can be correlated to the physiochemical properties of the liquid contaminant as well as contact hazard and vapor hazard measurements to facilitate next generation coating development. Moreover, time-resolved determination of multiple liquid mass-transport mechanisms in an optically opaque multilayered coating were demonstrated noninvasively.

Keywords Multilayered coating, Nuclear magnetic resonance, Mass-transport, Desorption, Polymer swelling


Military equipment coated with chemical agent-resistant coating (CARC) improves the former's ability to withstand the damaging effects of chemical warfare agents (CWAs) and decontamination formulations. (1) However, the resistant properties of this coating mean that low volatility chemicals can remain as free liquids on these surfaces and pose prolonged contact and vapor hazards to personnel. Temporary peelable coating (TPC) technology helps to minimize contact and transfer hazards by absorbing the liquid contamination on the surfaces. (1)

Two types of polyurethane TPCs were developed, which are referred to as Coatings A and B in the present article. Uptakes of CWAs or their simulants into these absorbent-protective coatings occur through different physiochemical mechanisms. The absorption of CWA into Coating A is driven by the polymer solubility parameter that results in the physical swelling of the coating. However, the performance of Coating A to reduce the contact/vapor hazards is limited to smaller CWA drop sizes, since absorption by solvation results in a localized build-up of agent that may saturate the coating. This issue was addressed by improving the surface-wetting properties through the application of a super-absorbent-topcoat (SAT), resulting in a multilayered TPC (Coating B). The topcoat has a morphologically "cracked" surface that facilities CWA spreading by capillary filling, while macropores and mesopores increase the surface area to facilitate agent transport and absorption into the coating.

As part of a wider research program to accurately assess the contact and vapor hazards from the CWA-contaminated materials, there is a need to develop analytical methods to determine mass-transport of CWA in the material bulk (diffusion, solubility within the material matrix, and capillary flow), which together with the surface contamination determines the hazards. (2) Moreover, an improved understanding of CWA mass-transport will facilitate the development of predictive models of post-decontamination hazards and enable extrapolation to a wider range of agent--material pairings. The mass-transport of a liquid through a material depends on the physiochemical properties of the material, such as solubility toward the liquid, porosity, capillarity and polymer swelling, as well as the physiochemical properties of the liquid (such as molecular weight, p[K.sub.a], viscosity, surface tension, vapor pressure. and solubility toward the material) and other environmental factors (temperature and humidity).

Development of TPC technology and optimization of its performance to reduce CWA vapor and contact hazards can be achieved by determining the CWA mass-transport characteristics during absorption and desorption in relation to TPC composition and other parameters such as polymer packing or coating thickness. In particular information concerning the bulk spread of CWA provides a necessary weighting factor in vapor emission calculations for the accurate assessment of the vapor hazard elicited by contaminated materials such as TPCs. The bulk spread of CWA also determines the absorptive capacity per unit volume of the TPC, which influences its surface contamination.

The Nuclear Magnetic Resonance-MObile Universal Surface Explorer (NMR-MOUSE) is an emerging noninvasive spectroscopic technique that has been applied to the measurement of liquid ingress into a variety of porous and opaque materials: for example, water into concrete, (3) quantitative monitoring of oil ingress into heterogeneous layered fabrics, (4) tracking skin care ingredients into human skin and its hydra-tion. (5), (6) In the field of coatings, the ingress of alkyd coatings into different porous substrates and their drying processes were monitored by NMR imaging with GARField, which provides a strong gradient at right angle to the magnetic field for depth profiling. (7) The study demonstrated that proton depth profiles could be used to monitor the evaporation rate of the solvent component of the coating as well as chemical crosslinking during curing within the bulk of the porous substrates. Ulrich et al (8) used the NMR-MOUSE to monitor penetration of water at different salt concentrations into an acrylic paint film by obtaining relaxation decay profiles from a static position within the film bulk. Swelling of the coating upon water ingress was estimated by measurement of film length and by an increase in NMR signal intensity as water molecules occupied more free volume.(8)

The present study reports an NMR-MOUSE depth-profiling study of multilayered Coating B and its isolated Coating A component upon contamination with methyl salicylate (MS). MS was selected as a simulant for the CWA bis(2-ehloroethyl) sulfide (sulfur mustard) due to its similar boiling point, viscosity, surface tension, and molecular weight. We applied one-dimensional (1D) depth profiling to (i) resolve preferential sites for localization of absorbed liquid within the polymer coating, (ii) determine the relationship between simulant-induced polymer swelling and sorption of the TPC component within the multilayered coating, and (iii) calculate changes in the volume spread of absorbed liquid within the TPCs with time.


TPC contamination

TPCs A and B were supplied by AkzoNobel Aerospace Coatings (The Netherlands). NMR measurements were performed on contaminated TPCs by placing them onto a glass slide (0.8-1.0-mm thickness. VWR International, Germany) and depositing 5 or 10 [micro]L of MS using a Gilson pipette. After absorption into the coating, a second glass slide was placed on the surface of the TPC before measurement.

NMR-MOUSE measurements

High-resolution depth profiles were obtained using the PM2 Profile NMR-MOUSE (Act Mobile NMR Solutions, Aachen, Germany) (Fig. 1). The PM2 consists of two permanent magnet blocks with opposite polarization, which are placed on an iron yoke.(9) This provides a strong magnetic gradient (35 T [m.sup.-1]) that is parallel to and extends above the magnet surface in the depth direction (z-axis). When this is coupled to a Carr-Purcell-Meiboom-Gill (CPMG) echo radiofrequency (RF) pulse sequence (emitted by an RF coil positioned between the magnets), a sensitive volume (8 mm (l) x 8 mm (w) x 10 [micro]m (d)) is established at a fixed distance above the RF coil. The vertical movement of the RF coil--magnet assembly is controlled using a high-precision lift that steps the sensitive volume through the sample in micron-sized increments in the z-direction with a maximum working depth of 2400 pm. Spin echoes are acquired at each step, which can be processed to obtain spatially resolved 1D profiles of both the NMR relaxation rate and the absolute volume of liquid present at a given position across the sensitive volume.


The NMR data were collected using 64 spin echoes (echo time, 88 [micro]s). The [T.sub.1] (spin-lattice relaxation time) of MS was used to inform the time required between repetitions of the basic sequence in the CPMG pulse sequence and to achieve the maximum sensitivity per unit time; a repetition time ([R.sub,T]) of 1.5 * [T.sub.1] (1350 ms) was used in experiments. The Fourier transform of 256 CPMG scans were averaged and used to obtain the signal amplitude at each depth. Thus, the time taken to obtain a single CPMG echo was 5.8 min ([R.sub.T] x number of scans). The step size determines the number of data points in the depth profile and in turn contributes to the total acquisition time of a depth profile: [R.sub.T] X number of scans x number of data points in the depth profile. Depth profiles of TPCs were obtained at 10-[micro]m step sizes and a 10-[micro]m sensitive volume resolution, which typically required between 1.6- and 2-h collection times.

Data processing

NMR depth profiles

NMR data were acquired using Prospa v2.2.24 (Magritek, New Zealand). The proton signal axis in the depth profile was calculated by integration of the initial part of the CPMG echo envelope and normalized so that the proton signal between the uncontaminated and contaminated TPCs could be compared. The following procedure was used to determine thc absolute amount of contaminant at a given depth within the absorbent test coupon. The calculation below uses MS as an example:

(a) The sensitive volume is positioned into a pure solution of MS (in a glass vial). The mass of pure MS in the sensitive volume is equal to the density of MS multiplied by the sensitive volume [1.174 g c[m.sup.-3] X 0.00064 c[m.sup.3] = 7.5136 x [10.sup.-4] g.

(b) The raw probe output amplitude of the pure MS sample is normalized to a value of 1 (which is also equivalent to MS mass of 7.5136 x [10.sup.-4] g).

(c) Depth profiling of an MS-contaminated material using this normalization value provides a volume fraction of MS at each depth (when subtracted from the background signal of the TPC at each depth); these values can be multiplied by 7.5136 x [10.sup.-4] g to provide the absolute amount of MS at each depth.

Surface contamination area measurenients

Image processing to determine surface area contamination in photographs was carried out using Image J v.1.40 g (National Institute of Health, USA) by scale setting and then freehand selection of the contaminated region.

Results and discussion

Drop spread volume determination from 1D depth profiles

A simple model system was used to develop a method for determining the spread volume of a liquid in an absorbent material from 1D depth-profile data; this informed spread volume calculations of CWA simulant in bulk TPC in the experiments reported in the proceeding sections.

Filter paper was placed on a glass slide, and 6.5 mg (5.7 [micro]L) of poly(ethylene) glycol (PEG; molecular weight 200 Da) was deposited on its surface. The PEG drop was allowed to absorb and spread into the filter paper before a second glass slide was placed on its surface. The sample was placed onto the sample stage (above the probe head), and a photograph (Fig. 2a) was taken before initiation of a 1D depth profile. Since the measured [T.sub.1] value of pure PEG is short (180 ms), acceptable signal-to-noise ratio (SNR) depth profiles were obtained in a relatively short period (9.2-min collection time) at 10-[micro]m-depth resolution. The signal amplitude scale of the depth profile was converted into milligrams of PEG based on the data processing method detailed in the Methods section (Fig. 2b). Note that the background proton signal from the filter paper was negligible.


The spread of the PEG drop on the filter paper was easily visualized as shown in the photograph in Fig. 2a (blue circle). Assuming that the spread of PEG on the filter paper could be approximated to a cylinder ([PI][[gamma].sup.2]h), the volume spread of PEG on the filter paper (Figure 2) was calculated as 37 [mm.sup.3] (where [gamma] is the radius of the blue circle, and h is the depth of the filter paper, indirectly determined from the depth profile of the PEG-contaminated filter paper shown in Fig. 2b). This total spread volume (37 [mm.sup.3]) was subtracted from the yellow outlined area in Fig. 2a to determine the proportion of the total contamination analyzed by the sensitive volume, which was calculated as 28%. The sensitive volume (red box in Fig. 2a) is centrally positioned in respect of the probe head, i.e., at the center of the green cross in Fig. 2a.

The PEG mass at the probed volume (depicted by the red box in Fig. 2b) was calculated as 28% of the initial contamination dose (1.8 mg) since the initial PEG loading dose was known (6.5 mg). By summing the masses of PEG measured at each depth in Fig. 2b, we obtained the expected total PEG mass of 1.8 mg. This result indirectly confirmed the x-y location of the sensitive volume which insured that contaminated coatings were appropriately positioned on the probe head for depth profiling and validated the quantitation method.

The total contamination spread volume could be determined from the 1D depth profiling data (Fig. 2b) using the following calculation. As calculated from the depth-profile data in Fig. 2b, 1.8 mg of PEG was present within 9.6 [mm.sup.3] (the lateral dimensions of the sensitive volume x by the thickness of the filter paper). This was 3.6 times less than the initial contamination dose (6.5 mg). Assuming that the mass of contaminant per unit volume outside the probed region was the same as within the probed region (1.8 mg PEG/9.6 [mm.sup.3]), the total contamination volume of PEG in the filter paper was 35 [mm.sup.3] (3,6 x 9.6 [mm.sup.3]). This value (35 [mm.sup.3]) was similar to the total spread volume calculated earlier (37 [mm.sup.3]), where we approximated the total volume spread of PEG on the filter paper to a cylinder.

Depth profiles of uncontaminated TPCs

Typical 1D depth profiles of the Coatings A and B are shown in Figs. 3a and 3b, respectively. The top surface of the TPC is labeled for each depth profile; this is the surface that initially comes into contact with the CW simulant.


The measured proton density across each depth profile for each TPC suggests differences in polymer chain packing and/or chemical composition. A large proportion of the mass of Coating A is a polyurethane resin binder, which is expected to he present throughout this TPC. However, a distinct step profile in the bulk proton signal of Coating A was measured, where a higher proton density was observed toward its bottom surface. This feature is likely due to (a) sedimentation during the drying process (perhaps influencing polymer chain packing density and distribution of inorganic proton-deficient pigments); and/or (b) self-assembly of proton-containing coating components used to reduce the adhesion of the TPC film to the coated substrate without impacting on the cohesion of the film.

For the multilayered Coating B, a sharp reduction in proton signal is observed in its bulk (Fig. 3h), which is indicative of the interfacial region associated with the boundary between its components (SAT and Coating A). The latter is supported by the depth profile of Coating A alone (Fig. 3a), which demonstrates lower proton signal toward its surface and is expected to be at the interface in the Coating B system.

Depth profiling of Coating A after contamination with MS

Coating A was expected to swell upon absorption of MS by solvation. This was supported by the depth measurement of Coating A contaminated with 10 [micro]L MS (loading dose 11.8 mg) that showed an overall increase in its thickness by 70 [micro]m at 0.9 h post-contamination (Fig. 4). The upper glass slide was removed between 0.9 and 24 h time points to facilitate desorption of MS from the coating. Comparison of the depth profiles obtained at these time points demonstrates a reduction in the swelling (Fig. 4) together with a concomitant fall in signal intensity across its entire depth.


In Fig. 4, the boundary (b) differentiates the higher and lower proton signals observed in this coatings bulk (i.e., significant differences in the chemical composition and/or polymer packing density). The greatest loss in MS between 0.9 and 24 h is evident within the bottom side of the coating ([A.sub,2]) that gives rise to the higher proton signal (gray shaded area [A.sub.2] (3 arbitrary units (a.u.)) > A1 (1 a.u.) in Fig. 4).

NMR depth profiling also revealed locations in the uncontaminated Coating A bulk that demonstrated low proton signals (indicated by arrows on the black trace in Fig. 4). Upon contamination of this coating (green trace), these are the same locations that demonstrate high proton signals (red and blue color-matched arrows in Fig. 4). Possible explanations for this phenomenon may be that (i) MS diffuses and concentrates into regions within the bulk, which have low polyurethane chain packing; and/or, (ii) proton-deficient additives (pigments) within this TPC congregate and have an affinity for MS.

Depth profiling of multilayered Coating B after contamination with MS

MS (5 [mciro]L; 5.9 mg) was dosed onto Coating B and left to absorb, after which a glass slide was placed on top to prevent desorption. Figure 5a shows the depth profile of Coating B before and after contamination. The contaminated coating profile was obtained 2-h post-contamination; the time required to collect the depth profile of the uncontaminated and contaminated coatings was 2 h. An increase in proton signal is observed across the entire depth of the MS-contaminated Coating B. The residual proton intensities between the contaminated and uncontaminated coatings were used to calculate the bulk MS mass profile for the Coating B sample (Fig. 5a).


Figure 5b shows a diagram illustrating the central position of the MS deposition site on Coating B with respect to the probe head. By summing the MS mass measured at each depth, a total MS amount or 6.2 mg was obtained, which was near to mass balance (106%). Thus, these data suggest that all of the MS dosed onto Coating B was initially located within a volume of 8 mm (l) x 8 mm (w) x 210 [micro]m (d) (i.e., within the lateral dimensions of the sensitive volume and thickness of the coating).

The highest proton signal in the depth profile of the contaminated Coating B co-localized with the Coating A and SAT interface of the uncontaminated Coating B ([I.sub.1]; solid drop-down line in Fig. 5a). In fact, the position of the Coating A--SAT interface was still apparent in the contaminated coating ([I.sub.2]), however shifted in position relative to the uncontaminated coating. The reason for this shift was attributed to the expected polymer swelling of the Coating A layer of Coating B, which resulted in the total thickness of Coating B to increase by 60 pm upon contamination. Table 1 shows the differences in the thickness of the two layers of Coating B, before and after MS contamination: these were determined by measuring the distance to the right (Coating A component) or left (SAT component) of [I.sub.1] (uncontaminated coating) or [I.sub.2] (contaminated coating) in Fig. 5. Although the SAT component exhibits an increase in thickness upon contamination, the increase to the thickness of the Coating A component is significantly greater.
Table 1 Thickness measurements of the two layers in
the multilayered Coating B before and after MS contamination

                 Component thickness ([mu]m)

                     SAT          Coating A

Uncontaminated        72                 63
Coating B

Contaminated          79                 90
Coating B

Desorption experiments were performed to further monitor the relationship between total Coating B thickness and the bulk swelling features over time, Figure 6a shows depth profiles obtained from the same Coating B sampled at consecutive time-points after MS contamination. Note the depth profiles of uncontaminated Coating B (black trace) and at 2-h post-contamination (green trace) (Fig. 5a) are reproduced in Fig. 6a for comparison. A depth profile was collected at 18-h post-contamination without removal of the upper glass slide (red trace; Fig. 6a). At this time point, the amount of MS at the probed location was calculated as 72% of the initial loading dose (4.2 mg). This may suggest lateral spreading of the MS within the bulk of Coating B, thereby reducing the intensity at the position of the probed volume. Moreover, the interface shifted to a position in the coating bulk that was closer to the uncontaminated Coating B suggesting a reduction in swelling of the Coating A component.


Subsequent depth profiles were obtained at 44- and 70-h post-contamination, again without removal of the upper-glass slide (blue and pink traces; Fig. 6a). These depth profiles were not significantly different than the 18-h-depth profile, indicating an equilibrium state within the probed region.

As detailed above, the total contamination spread volume of PEG on filter paper was calculated using 1D depth-profiling data and a known value for the initial contamination density. In a similar manner, the volume spread of the MS contaminant in the Coating B sample at 18-h post-contamination was also determined. From the ID depth-profile data (Fig. 6a, red trace), 4.2 mg of MS was measured within 9.6 [mm.sup.3] of the coating (the lateral dimensions of the sensitive volume x by the thickness of the coating). This is 1.4 times less than the initial contamination dose (5.9 mg). Assuming that the mass of contaminant per unit volume outside the probed region is the same as within the probed region (4.2 mg MS/9.6 [mm.sup.3] [equivalent to] 0.44 mg MS per [mm.sup.3]), the total contamination volume of MS in Coating B is 13 mm3 (1.4 x 9.6 [mm.sup.3]). Thus, the MOUSE can be used to determine the wetting characteristics within the TPC, which is an important feature in the development of the coating's ability to reduce the contact hazard on its surface.

The upper glass slide was removed at 75-h post-contamination and then replaced before acquisition of the 77-h depth profile (Fig. 6b). Removal of the glass slide accelerated the desorption process and is obvious by the significant reduction in the proton signal amplitude across the entire depth of the coating system. However, the Coating A component appears to desorb at a faster rate compared to the SAT component. This is realized by comparison of the 18-h/ 44-h/70-h depth profile with the 77-h profile. In fact, the Coating A signal at this time point (77 h) is of very similar intensity to its matched region within the uncontaminated coating (centering at 30 [micro]m in Fig. 6 b). In contrast, the signal intensity of the SAT layer at 77 h is higher than that of its matched region within the uncontaminated coating system (centering at 130 [micro]m in Fig. 6b). This could be indicative of relative differences in desorption rates from the SAT and Coating A layers. Alternatively, the higher proton signal in the SAT layer at 77 h could be due to mass-transport of MS from the Coating A component into the SAT component during desorption.

The effect of multilayered Coating B thickness on MS bulk distribution

Figure 7 shows the depth profile of a sample of Coating B that is 2.6 times thicker (410-[micro]m thick) than the sample used in the earlier analysis (160-[micro]m thick). The depth profile of this thicker Coating B sample after contamination with 5 [micro]L MS (5.9 mg) is also shown in Fig. 7. As in the case of the previous sample, a glass slide was placed on its surface to prevent desorption after post-contamination.


An increase in proton signal is observed across the entire depth of Coating B after MS contamination (Fig. 6). The mass of MS was calculated as 105% of the loading dose and is initially located (upon absorption) within a volume of 8 mm (I) x 8 mm (w) x 210 [micro]m (d); this supports the earlier results reported for the thinner Coating B sample, where 106% of the loading dose was measured in the Coating B bulk shortly after contamination.

However, unlike the previous (thinner) Coating B sample (Fig. 5a), contamination of the thicker Coating B sample does not present with an intensity maximum in its bulk that co-localizes with the lowest signal in the bulk of the untreated coating (Fig. 7). This indicates an absence of swelling of the Coating A component. Since an identical loading density was applied to both Coating B samples, the absence of this feature in the thicker sample is attributed to the spreading of the simulant within the thickness of the bulk such that the amount of MS per unit area is less than that measured in the thinner sample. In turn, this may result in less agent partitioning into the Coating A component to induce swelling. Interestingly, the overall depth of the thicker Coating B sample did not change upon contamination, further supporting the association of the swelling feature with the overall thickness of the TPC.


Using a simple model system of PEG absorbed into filter paper, it was shown that the total spread volume of a liquid in the material bulk could be calculated from ID depth-profile data. This method was subsequently employed to determine the volume spread dynamics of MS upon absorption into a multilayered coating system. Thus, NMR-MOUSE can be used to determine CWA--simulant wetting in the TPC bulk, which is an important factor in coating development that informs its ability to reduce the surface contact hazard.

The experimental results indicate regions within the bulk of Coating A, which were sites for preferential MS localization. Further targeted molecular characterization with complementary analytical techniques can be used to examine the relationship of these sites to the physiochemical properties of the absorbed liquid.

The proton depth profile of the optically opaque multilayered Coating B revealed bulk features that were used to differentiate its individual components (a super-absorbent topcoat and Coating A). Time-resolved determination of multiple liquid mass-transport mechanisms in this coating were observed after contamination with MS: Liquid absorption and subsequent polymer swelling of the Coating A component were realized by an increase in the thickness of this layer, which was validated upon MS exposure of the isolated Coating A material. The release of the glass slide from the surface of the coating promoted vapor desorption, as realized by the reduction in proton intensity across the depth of the multilayered coating and in the reduction of the swelling feature. Further experiments indicated a relationship between coating thickness, contamination dose, and polymer swelling within the multilayered coating. Although these findings require validation with coatings that are bonded to a chemically hard surface and with a CWA rather than a simulant, information of this type can be correlated to contact hazard and vapor hazard measurements to facilitate next generation coating development.

Acknowledgments The authors gratefully thank the Dstl Science and Technology Centre for funding, and Dr. Jurgen Kolz (Act-Aachen, Germany) for helpful discussions during experiments.


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J. Coat.Technol. Res., 9(6) 735-743, 2012

[c] UK Crown 2012

E. Gazi *, S. J. Mitchell

Detection Department, Defence Science and Technology Laboratory, Porton Down. Salisbury SP4 OJQ, UK e-mail:

DOI 10.1007/s11998-012-9416-8
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Author:Gazi, Ehsan; Mitchell, Steven J.
Publication:JCT Research
Date:Nov 1, 2012
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