Polyurea-based Hybrid Composites for CBRN Protection

Either considering chemical munitions or chemical improvised explosive devices, the armies should be able to counter their effects from two different perspectives: the agent used and the way to disperse it. While the agent to be delivered can be more or less lethal, the real factor that gives efficiency to the attack is the manner the agent is delivered. The state-of-the-art tested garment indicate that current knowledge regarding protective equipment for chemical warfare agents remains very limited. In this context, the present study aimed at the possibility of using the MWCNT-OH-based polyurea-composite material, obtained by our group in a previous study, in the field of protection against hazardous chemicals.

polyesters, polyether amides or polyurethanes between two layers of textile that permits the passage of water vapour, but do not let the toxic chemical penetrate, allow that water vapour pass to the exterior making the clothes more comfortable and helping to maintain the physiological stability [5,11].
The present study considered the available data from literature, but also aimed at the possibility of using the novel polyurea-based composite material obtained by our group for ballistic PE [12] also in hazardous chemicals protection.

Materials
The same materials and the same fabrication conditions were employed as in [12]. In brief, polyurea (PU) and four MWCNT-OH-polyurea derivatives (PUCs) have been obtained from respective components (Table 1), starting from a 10% MWCNT-OH masterbatch. The polymer obtained has been applied by direct continuous-layer spraying through a pressurized container at 150 bar and 65 ° on various layers, in different thicknesses: 0.55, 1.20, 1.75 and 2.30 mm.
Three types of gloves have been used as support layer: nylon-cotton safety gloves (Glove1) and cotton safety gloves (Glove2) from DALGECO®, and cotton safety gloves from DRÄGER®. In figure 1 are illustrated the steps pursued for the support materials (chemical protection materials) obtainment, by simple operations and equipment. Thermogravimetric analysis Thermogravimetric curves have been recorded on a Q500TA instrument, under nitrogen atmosphere, at a 10 °C/min. heating rate from room temperature to 600 °C.
Scanning electron microscopy SEM and EDX have been carried out on a VEGA II LMU equipment, at a 3.5 nm resolution and 30 keV, and an EDX analyzer Bruker AXS Microanalysis AG, Germany.

Protection evaluation against CWAs
The PUCs have been tested against CWAs and the protection time has been determined, in agreement with [13]. The protection time (the protection capacity) is the time from material contamination until penetration occurrence on the face opposite to the contaminated one [14].
In terms of protection evaluation, the most difficult testing environment has been employed: HD (blister agent), GD (soman) and Vx (nerve agents) have been employed in 10 g/m 2 and 50 g/m 2 contamination densities for an envisaged protection time of 24 hours, as per the maximum values given in the NATO standard [13], specific to equipment most frequently to be in contact with CWAs.
Protection time determination has been made in a glove-box Jacomex workstation using no. 6 DRS system (Device for the materials verification against CWA droplets) (figure 2).

Fig. 2. Sample contamination process
CWAs diffuse through the protection equipment as the concentration gradient decreases, until they penetrate the entire thickness of the material and pass from the contaminated to the opposite part.
The emphasis of the CWA penetration through materials has been made by colorimetric analysis. The CWAs that penetrate the testing material get into contact with the indicator tissue impregnated with pH indicator (Congo Red and chloramine), chlorinates and forms hydrogen chloride that determines the colour change from red to blue, which emphasizes the material penetration by the CWA.
Representative samples from five different spots have been tested in triplicate, as following: the sample is inserted in no. 6 DRS device above a layer of indicator tissue and a layer of cotton tissue and kept at 36.51 C for 15 minutes. Further, the sample is contaminated with distilled 10 or 50 g/m 2 HD/GD/Vx and again kept at 36.51 C. In figure 2 is shown the process after this conditioning. The contamination moment marks the beginning of the test. Every 30 minutes the sample is verified for HD penetration. The result of the test is the lowest value obtained for the protection time determined.
Determinations on polyurea (PU) and on four-layer thicknesses of PUCs have been performed (Table 2).

Results and discussions
Various types of chemical hazards are encountered daily both at home and in industry. These substances include household chemicals, toxic industrial chemicals and CWs, and their approach depends on the tactical situation [15]: if the priority is life-saving, first-responders contamination risk must be assessed and implemented. The presence or even the potential presence of a CWA in a major incident conducts to a very challenging management of the incident due to the fact that, in the first place, the unrecognized hazards may cause first-responders to become casualties themselves. Secondly, responders capability should not be diminished due to their PE, which also may cause physical and psychological stress.
In this context, new materials for protection garment are under research worldwide against CWAs resistance. One of the most researched materials in our group has been a bromobutyl/butyl rubber, which confers a renowned 24-hour protection against CWAs (10 g/m2 HD) [14]. Furtheron, the very good results obtained for MWCNT-OH-polyurea material in terms of ballistics protection/dynamic impact [12] made the present study become of interest.

Influence of the MWCNT concentration on thermal properties of MWCNT-OH-reinforced polyurea composites
Individual PE is a key piece of clothing not only in terms of protection from hostile environments, but also as regarding heat and cold extremes. Whilst protective clothing may be designed primarily for non-thermal hazards (e.g., chemical or biological hazards), another important challenge in all protective clothing remains users comfort, i.e. thermal stress management.
Thermograms and their derivatives are given in figure 3, and the main data obtained in Table 3. From figure 3 it has been observed that all the composites present a similar behaviour, being very stable up to ~230 ºC, with a <9% weight loss, which is very important when taking into account the polymer-textile sandwich structure for protection clothing, representing an important basis for the design of modern protective technologies and for the evaluation of individual PE materials at full-ensemble scale.  Figure 4 gives representative microphotographs taken from the MWCNT-OH masterbatch, from the PUC layer and an EDX evaluation of the OH groups dispersed through the layer. From the analysis of the SEM micrographs obtained, one may notice a homogenous dispersion of the carbon nanotubes in the polymer matrix. The qualitative dispersion is also satisfactory, with a narrow distribution of the chemical species, this being due to the filler introduced as masterbatch in the amine used for the PUC fabrication. a b c Fig. 4

Evaluation of the materials CBRN protection level
Evaluation of the CWAs penetration through PUC is presented in Table 4. The same results have been obtained for all the three types of support layers, which conducts, from the beginning, to a positive conclusion regarding various types of synthetic or natural polymers to be used as support materials as protective clothing against CBRN agents. From Table 4, one may notice the fact that the standard polyurea (PU) does not achieve the minimum requirements necessary for being used as CBRN protection material. Further, all the PUCs offer a good protection time, in agreement with [13], above 24 hours. Since, next to the protection evaluation, the study aimed at achieving also a good economical ratio, and the exposure time to the three CWAs has been prolonged for all the materials and for all the layer thicknesses. In case of the 0.55 mm-thickness, the first three materials offer at least a 72-hour protection time (against the most persistent chemical agent), while PUC4 has only a 60-hour protection time. This means that MWCNT-OH ratio in the material's mixture is very important, lowering its ratio conducting to protection capability diminishment.
As regarding the prominence of the layer thickness, as expected, the thicker layers offer a better protection. Economically speaking, this translates in the fact that PUC3 contains the lowest MWCNT-OH ratio where the protection time is still over one week (168 hours) in case of a 2.30 mm-layer, which reduces dramatically the cost of the product. Thus, PUC3 may be used both in CBRN individual and collective protection. PUC4 could also be of choice standing on the utility of the finite material to be employed as protection materials.

Conclusions
After having achieved very good results in terms of ballistic protection in a previous study for a MWCNT-OHbased polyurea composite, the results on determinations regarding PUC protection capacity conclude that the new solution also offers much more than the maximum protection time foreseen by the international standards (24 hours) for a contamination density of 50 g/m 2 of a persistent CWA.
Thus, next to excellent physico-chemical properties, the materials may be successfully used for the fabrication of individual and collective PE against CWAs.