Cordura Fabric with Subtle Thin-Film Modifications Used for the Enhancement of Barrier Properties Against Toxic Gases (Part I)

Abstract

This study describes the modification of polyamide fabric with the commercial name Cordura to create a material that is more resistant to the permeation of toxic gases and compound vapors while remaining permeable to water vapor and air. The surface of Cordura was modified by applying a very thin film, deposited from a vapor organic phase at room temperature under the influence of γ-radiation. The permeability of the modified fabric to water vapor, organic vapors, organic dyes, and toxic substances (sulfur mustard gas) was determined and compared to the properties of the unmodified fabric.

1. Introduction

In the modern world, people are occasionally exposed to unusual and dangerous situations like car accidents, fires, mob clashes, and chemical spills. Rescuers, such as medical teams, firefighters, and police officers, are often subjected to hazards, including poisonous gases, for extended periods of time and therefore require special protective uniforms. An excellent material for these uniforms is available under the commercial name Saratoga, which consists of three layers: an outer layer made from a polymeric (e.g., polyurethane) coating that protects against dangerous substances; a strong, supportive middle layer made from extremely strong polyamide Cordura fabric; and an inner layer containing active carbon to absorb toxic substances and protect the rescuer. This three-layer laminate prevents toxic substances from the outside from permeating through the rescuer’s clothes while also allowing water vapor and air to permeate through to the outside. With these protective clothes, rescuers can safely work in dangerous conditions for several hours. The central Cordura polyamide 6,6 layer, which is extremely strong and scratch-resistant [1]—stronger than typical nylon and much stronger than polyester or cotton—plays a crucial role. This is the material of choice for special clothing, backpacks, camping and military equipment, and even footwear. To improve Cordura’s transmission properties, which entails reducing its permeability to toxic gases, the deposition of a very thin organic film under the influence of γ-radiation is proposed. Thin organized films that efficiently separate two phases are commonly found both in nature (for example, the Bi-Layer Phospholipid Membrane [2] of living cell walls) and in industry. A coupling layer made from a Self-Assembled Monolayer, which efficiently bonds the surface of electronic structures with a protective polymer [3,4,5] or a Poly(p-xylylene) Parylene [6,7] film, is proposed. This Parylene film effectively protects the Integrated Circuits from the environment and corrosion. This method was developed by M. Szwarc and improved by W. Gorham in the 1970s. In this method, a Parylene dimer (see Figure 1) is split into two radicals in a vacuum at temperatures exceeding 650°C. After the recombination of radicals, an extremely thin, highly oriented, well-organized Parylene film is formed. This Parylene film effectively protects the surface of Integrated Circuits, or any other electronic devices, from corrosion and harsh environments. In our research, we propose depositing a thin film onto Cordura fabric without using a Parylene dimer substrate, high-vacuum, or high-temperature conditions. Instead, we modify the surface of the soft polymeric fabric Cordura using products formed from the vapors of simple organic substances under RT conditions, normal pressure, and the influence of γ-radiation.

Samples of Cordura fabric, measuring 5 cm × 9 cm, were placed in a zip bag filled with an organic gas phase, generated via evaporation from filter paper soaked with an organic liquid (Figure 2). The zip bags were exposed to γ-radiation from Cs-137 with a 662 keV energy source (Figure 3).

The total proposed radiation dose is 10 kGy, with additional samples studied at 1.5 kGy and 3.0 kGy. For comparison, samples without irradiation were also prepared. The following organic substances were used to modify the Cordura fabric: Styrene, Tetraethoxysilane, Toluene, and Xylene. The obtained properties were compared to unmodified Cordura fabric and only γ-irradiated Cordura fabric. FTIR ATR spectroscopy was used to verify whether a thin, subtle, organic film was formed on the surface of Cordura fabric. The transmission properties of both modified and unmodified Cordura fabric were characterized by measuring the permeability of different organic dyes and water vapor (polar solvent), Hexane (non-polar solvent), and Toluene (partly polar solvent), and the results are presented. On the basis of these experiments, the most promising modified Cordura fabric was selected. Its protective properties against poisonous gas, such as sulfur mustard gas, were determined and compared to unmodified Cordura fabric. The proper modification route was selected. The complete research procedure is presented in Figure 4.

2. Materials and Methods

Cordura fabric (white 560-dtex-fc-weber-13508151747) was purchased from Spektrum (Spektrum Zoltán Szabó, Zabrze, Poland. Hexane, Styrene, Tetraethoxysilane, Toluene, and Xylene were purchased from Sigma–Aldrich. Filter paper 70 g/m² was purchased from Warchem Poland. The Cordura fabric was subjected to γ-irradiation modification. The tests were carried out using a radioactive source, ¹³⁷Cs. ¹³⁷Cs is an isotope with a gamma radiation energy of 662 keV and a low energy intensity in the irradiation process. The dose of the samples irradiated with Cs-137 was measured using a 30 cm³ ionization chamber at a set dose rate of 69 Gy/h. Zip bags with Cordura, organic-soaked filter paper, and a separating PE grid were placed around the uncollimated ¹³⁷Cs radioactive source. Irradiation was carried out under natural conditions, taking into account the environmental correction factor.

The list of dyes (Sun Chemical DIC) used for permeability tests is presented in

Table 1. Characteristics of used dye solutions.

No	Dye	Molecular Weight	Molar Concentration	Weight Concentration	Solvent
		g/mol	mmol/dm3	% w/w
1	Orasol Black X51				Toluene
2	Alkali Blue 6B	595.72	4.19	0.25	Water
3	Eriochrome Black T	461.40	4.34	0.2	Water
4	Orasol Black X55	deep reddish-black chromium compound		0.1	Toluene
5	Naphtol Blue Black	616.49	4.05	0.25	Water
6	1-(2-Pirydylo-azo)-2-Naftol	249.29	4.02	0.1	Ethanol
7	Xylenol Orange	700.61	4.28	0.3	Water
8	Beetle Juice (homemade)			5	Ethanol

. Approximately 0.1% w/w–0.3% w/w dye solutions, all with approximately the same molality of 4 mmol/dm³, were used. The used dyes and test solutions are presented in Table 1. Using sulfur mustard gas (98.1% purity, synthesized at the Military Institute of Chemistry and Radiometry), the γ-irradiation and permeability tests were conducted at the Military Institute of Chemistry and Radiometry, Poland. Note that few references are cited because we focused only on papers reporting on the permeability of toxic gases, such as sulfur mustard gas. FTIR ATR spectra were obtained using a Nicolet Avatar ( Thermo Nicolet Avatar device (USA), purchased from Spectrolab (Thermo representative) based in Łomianki near Warsaw, Poland) and a diamond-tip Gladiator, and Raman spectra were obtained using a Nicolet Almega (Thermo Nicolet Almega device (USA), purchased from Spectrolab (Thermo representative) based in Lomianki near Warsaw, Poland). The permeability of water, Hexane, and Toluene was obtained by measuring mass changes in 10 ml solvent-filled glass vials covered with the tested Cordura fabric. The permeability of sulfur mustard gas was tested using the method presented in [8]. All measurements were conducted in triplicate.

3. Results

3.1. FTIR ATR Analysis

The results of γ-irradiation of Cordura fabric at 10 kGy in the presence of different solvents, measured using FTIR ATR spectroscopy, are shown in Figure 5, Figure 6 and Figure 7 and Figures S1–S4 in the Supplementary Materials. In Figure 5, the FTIR ATR spectra of unmodified Cordura fabric are presented, and in Figure 6, the FTIR ATR spectra of Cordura fabric irradiated at 10 kGy are presented. The Raman spectra of unmodified Cordura fabric are presented in Figure 7. The FTIR ATR spectra of 10 kGy irradiated Cordura fabric in the presence of Styrene vapors (Figure S1); 10 kGy irradiated Cordura fabric in the presence of Tetraethoxysilane vapors (Figure S2); 10 kGy irradiated Cordura fabric in the presence of Toluene vapors (Figure S3); and 10 kGy irradiated Cordura fabric in the presence of Xylene vapors (Figure S4) are presented in the Supplementary Materials. All spectra are very similar, with strong signals from the benzene ring (~1600 and 1500–1430 cm⁻¹, strong to weak), with corresponding features in the Raman spectra (Figure 7). No differences were observed except for Cordura fabric irradiated in the presence of Tetraethoxysilane (TEOS) (compare Figure S2 and other spectra), where slightly different absorbencies resulting from the presence of hydroxyl groups were observed (region ~970 cm⁻¹; 1060 cm⁻¹; 3500 cm⁻¹).

3.2. Dye Permeability

To study surface properties, especially biological surfaces and their role in tissue growth and differentiation, special tools are required. For example, to characterize functionally graded surfaces for biological and biomedical applications, researchers from the Shanghai Technical University [9] used two very specific and sophisticated strategies: (1) The substrate was vertically placed in a container, and then the solution containing the functional component was added at a constant rate. The amount of the deposited component gradually changed along the vertical direction with increasing contact time. (2) A moving collector or mask was used to control the amount of the deposited component during jet printing or electrospray [9].

In this work, a similar procedure was applied, based on the interactions between the component (in this case, dye) and the substrate surface. A 0.2 ml drop of dye solution was deposited on the modified Cordura surface to observe the spreading of the dye (eight dyes listed in Table 1) and the permeation of the dye solution through the Cordura fabric to the supporting filter paper. Such “color” procedures are commonly used to test the barrier properties of different materials against poisonous gases and liquid drops [8,10,11,12].

The results of the experiments are illustrated in photographs presented in Figures S5–S14 in the Supplementary Materials. The images show Cordura fabric prior to dye deposition; results for eight dyes deposited on Cordura fabric irradiated at 0, 1.5, 3.0 and 10 kGy; results for eight dyes deposited on Cordura fabric irradiated in the presence of Toluene vapors 0, 1.5, 3.0 and 10 kGy; and the supporting filter paper after 48 hours under irradiated Cordura fabric, both with and without the presence of Toluene vapors. The same experiments were performed for Cordura fabric irradiated in the presence of Styrene, Tetraethoxysilane, and Xylene vapors. The results for all studied unmodified and modified Cordura fabrics, based on the deposited drops of dyes, were consistent. These observations are presented in

Table 2. Results of deposited dye drops on surface of unmodified/modified Cordura fabrics (with γ-radiation and organic vapors).

Dye	1	2	3	4	5	6	7	8
Paper	++	++	++	+++	++	+++	+++	+++
Cordura	++	+	+	+++	+	+++	+	+
Cordura and Radiation	++	+	+	+++	+	+++	+	+
Cordura + TEOS	++	+	+	+++	+	+++	+	+
Cordura + Toluene	++	+	+	+++	+	+++	+	+
Cordura + Styrene	++	+	+	+++	+	+++	+	+
Cordura + Xylene	++	+	+	+++	+	+++	+	+

Description of drop spreadability on tested Cordura fabrics: + only drop; ++ high drop spreadability; +++ very high drop spreadability.

.

Cordura fabric with/without modifications was impermeable to the majority of the eight dyes tested regardless of whether they were dissolved in Ethanol, Toluene, or water. Only dye 4 (Orasol Black X55 in Toluene) and dye 6 (1-(2-Pirydylo-azo)-2-Naftol in Ethanol) showed permeability through Cordura fabric. The filter paper placed under this fabric was stained. Very minor differences were observed in the spreading of dye 4; for example, on Cordura modified with Toluene vapors, it spread more easily than on Cordura that was only irradiated. The permeability results of dyes 4 and 6 through Cordura fabrics (after 48 hours) are presented in

Table 3. The permeability of dye 4 and dye 6 through γ-modified Cordura fabrics (description of staining a filter paper covered with Cordura fabric).

Cordura Fabric	Dye 4				Dye 6
Dose [kGy]	0	1.5	3.0	10.0	0	1.5	3.0	10.0
Cordura			+	++	+	++	++	+
Cordura/Styrene	++	++	+		++	++	++
Cordura/TEOS	+		++	++		+	++
Cordura/Toluene		+	++					+
Cordura/Xylene		+		+++	+			+++

Description of drop spreadability on the tested filter paper covered with Cordura fabrics: + only drop; ++ high drop spreadability; +++ very high drop spreadability.

.

Some permeability was also observed for dye 1 (Orasol Black X51 in Toluene) on irradiated Cordura (doses of 1.5 kGy and 10.0 kGy), Cordura/Styrene (doses of 0 kGy and 10.0 kGy), Cordura/TEOS (dose of 1.5 kGy), Cordura/Toluene (doses of 1.5 kGy and 3.0 kGy), and Cordura/Xylene (doses of 0 kGy and 3.0 and 10.0 kGy).

Based on the dye permeability results, it can be stated that the most impermeable Cordura fabrics were the samples irradiated in the presence of Toluene and Xylene vapors at doses lower than 3.0 kGy.

3.3. Water Vapor Permeability

The transmission properties of modified Cordura fabrics against different gas molecules with different physicochemical characteristics were determined. Table S1 in the Supplementary Materials presents the physicochemical characteristics of the studied water and organic solvents.

The water vapor permeabilities [g/m² h] were calculated on the basis of weight changes in glass vials filled with water and capped with modified Cordura fabric. The results are presented in Figure 8 for the Cordura that was irradiated alone. Figure 9 presents the results for Cordura irradiated in the presence of Styrene vapor, Figure 10 for Cordura irradiated in the presence of TEOS vapor, Figure 11 for Cordura irradiated in the presence of Toluene vapor, and Figure 12 for Cordura irradiated in the presence of Xylene vapor. The water vapor permeability was measured up to 166 hours under RT conditions; however, for the clarity of the figures, only the results for the first 68 h are presented.

It was evident that the highest water vapor permeability (WVP) was observed for uncapped glass vials. When they were capped with Cordura fabric, the WVP reduced to about 86% of the initial value (in relation to the permeability measured for uncapped vials). It was observed that increasing the radiation dose up to 10 kGy further reduced the WVP to about 73%. For lower radiation doses (0, 1.5 kGy, and 3.0 kGy), the differences between irradiated and non-irradiated Cordura fabric were negligible. When Cordura fabric was irradiated in the presence of organic vapors—Styrene, TEOS, Toluene, and Xylene—no decrease in the WVP was observed for the sample irradiated with a 10 kGy dose. The organic vapors showed a kind of “healing” effect, reducing the impact of high radiation doses. Here, “healing” effect refers to the reduction in the observed permeability changes. The WVP was reduced to about 84% compared to the uncapped vials. Additionally, a smaller reduction in the WVP was also observed for the Cordura irradiated in the presence of TEOS vapor: it was only reduced to about 86% of the initial value. The highest reduction in the WVP, indicating the highest blocking effect, was observed for Cordura fabric irradiated with a dose not exceeding 3.0 kGy in the presence of Toluene and Xylene vapors, with the WVP being reduced to about 77%.

3.4. Hexane Vapor Permeability

Different results were observed for the Hexane Vapor Permeability (HVP) when the glass vials filled with Hexane were capped with the modified Cordura fabrics. For all studied Cordura fabrics, the observed changes were negligible. All tested probes showed the same HVP values, whether non-irradiated or irradiated with doses up to 10 kGy and regardless of the presence of organic Styrene, TEOS, Toluene, or Xylene vapors. The differences did not exceed the calculation errors. Therefore, all the obtained results are collectively presented in Figure 13.

Note that all studied Cordura fabrics are highly permeable to the non-polar, volatile Hexane vapors, and the calculated HVP value is nearly 20 times greater than the WVP value. Only some very minor reductions in the HVP were observed for Cordura fabric irradiated in the presence of TEOS vapors, which can be attributed to the differences between the more polar Cordura fabric surface and non-polar Hexane molecules.

3.5. Toluene Vapor Permeability

The Toluene Vapor Permeability, when glass vials filled with Toluene were capped with modified Cordura fabrics, showed similar patterns to the Hexane Vapor Permeability. Because the observed changes were negligible, all studied Cordura fabrics, irrespective of whether they were non-irradiated or irradiated with doses up to 10 kGy, with or without the presence of organic Styrene, TEOS, Toluene, or Xylene vapors, showed nearly identical TVP values, with minor differences that did not exceed the calculation errors. It is noteworthy that all studied Cordura fabrics are highly permeable to the slightly polar, volatile Toluene vapor, and the TVP value is nearly 4 times greater than the WVP value. However, for the non-polar Hexane vapors, the HVP value is 20 times greater than the WVP value for polar water vapors.

3.6. Toxic Sulfur Mustard Gas Permeability

The permeability of sulfur mustard was measured in a chamber made of acid-resistant stainless steel 304. The chamber consisted of two parts separated by an acrylic ring, which enabled the assembly of the tested fabric samples in a repeatable manner, creating a barrier between the top and bottom of the chamber. The inside of the chamber had a 60 mm diameter, the top of the chamber was 20.4 mm in height, and the bottom was 28.7 mm. The upper part of the chamber had a glass window, allowing a view of the interior of the chamber (see Figure 14). The chamber was equipped with a set of two outlets plugged with rubber septa, ensuring the tightness of the system and enabling the collection of gas samples. The chamber parts were clamped using a special stand.

The following round 70 mm Cordura samples were prepared via die cutting:

• No irradiation, no chemical solvent vapor O-1; O-2; O-3;
• An amount of 3 kGy irradiation, no chemical solvent vapor R-1; R-2; R-3;
• An amount of 3 kGy irradiation, Toluene vapors T-1; T-2; T-3;
• An amount of 3 kGy irradiation, Xylene vapors X-1; X-2; X-3;
• An amount of 3 kGy irradiation, Tetraethoxysilane vapors D-1; D-2; D-3.

The gas samples were collected with a gas-tight 1000 μl syringe heated to 70 °C. The syringe was heated to prevent the adsorption of sulfur mustard on the glass surface of the syringe. The tests were performed by installing the fabric into the chamber with an acrylic ring, pipetting 5 μl of sulfur mustard on the fabric’s surface, and closing it by tightening the stand. Then, the gas sample was collected from the bottom part of the chamber after 45 seconds of sulfur mustard exposure to the fabric. The gas samples were tested with an Agilent Technologies 8890 Gas Chromatograph with a Flame Photometric Detector (FPD).

The results are shown in Figure 15, Figure 16 and Figure 17, which present chromatograms of sulfur mustard gas permeability for unmodified Cordura (0-1) (Figure 15), Cordura irradiated in the presence of Xylene (X-1) (Figure 16), and Cordura irradiated in the presence of TEOS (D-1) (Figure 17). The obtained results are presented in

Table 4. The results of the toxic sulfur mustard gas permeability.

Sample	Radiation Dose	Monomer	Retention Time	Sulfur Mustard Gas Peak Area	Sulfur Mustard Gas Peak Area Average
	kGy		min	pAsec	pAsec
0-1	0		1.766	114.38
0-1	0		1.763	82.19	79.8 ± 35.9
0-3	0		1.770	42.70
Rad-1	3.0		1.765	77.64
Rad-2	3.0		1.769	258.81	165.7 ± 90.7
Rad-3	3.0		1.765	160.75
D-1	3.0	TEOS	1.769	17 314
D-2	3.0	TEOS	1.761	15 529	23 106 ± 11 613
D-3	3.0	TEOS	1.764	36 476
T-1	3.0	Toluene	1.765	61.59
T-2	3.0	Toluene	1.766	105.22	93.9 ± 28.4
T-3	3.0	Toluene	1.768	114.91
X-1	3.0	Xylene	1.767	63.10
X-2	3.0	Xylene	1.763	146.19	94.5 ± 45.1
X-3	3.0	Xylene	1.764	74.14

. The first chromatogram peak results from the excess of carrier air, and the second peak results from the mustard gas.

The obtained sulfur mustard gas permeability results show that Cordura fabric is permeable to the sulfur mustard gas molecules, and after 3 kGy irradiation, it starts to become even more permeable. It is important to note that there are no statistically significant changes in the permeability in relation to the unmodified Cordura fabric when Cordura fabric is irradiated in the presence of Toluene or Xylene vapors. However, a small increase in sulfur mustard gas permeability after irradiation was observed (compare samples 0 and R; 79.8 pAsec and 165.7 pAsec). The small increase in permeability observed is somehow “healed” after the addition of aromatic Toluene or Xylene vapors. Therefore, the addition of aromatic organic vapors with even stronger “hydrophobic” properties is expected to further reduce the permeability of sulfur mustard gas. This idea is supported by the dramatic changes in sulfur mustard gas permeability observed for sample D, where Cordura fabric was irradiated in the presence of highly “hydrophilic” TEOS molecules. In comparison to the unmodified Cordura fabric, a nearly 300-fold increase in sulfur mustard gas permeability was observed. Sample D, irradiated in the presence of TEOS, is completely permeable to sulfur mustard gas molecules. It was demonstrated that the modification of Cordura fabric with hydrophilic precursors like TEOS is not effective for producing less permeable and more resistant Cordura fabric against sulfur mustard gas. The obtained results show a new direction for future research on Cordura fabric modification using highly hydrophobic, aromatic compounds while avoiding organic molecules with a tendency to form hydrophilic molecules.

4. Discussion

Cordura fabrics modified with γ-ray irradiation in the presence of organic vapors of Styrene, TEOS, Toluene, and Xylene revealed only small changes in their membrane structures. These differences were very subtle and were hardly detectable via FTIR ATR or Raman spectra analysis. Nevertheless, these differences were visible in the dye permeation experiments and the organic vapor permeabilities. The main aim of our work was to identify changes that reduced the permeability of organic species and preserved the permeability of water vapors. Therefore, for the final experiments measuring permeability to toxic vapors (sulfur mustard gas), Cordura fabric irradiated with doses up to 3.0 kGy in the presence of Toluene and Xylene vapors was selected. The properties of all samples were compared to Cordura irradiated at 3.0 kGy and unmodified Cordura fabric. After Cordura fabric irradiation with γ-rays in the presence of a hydrophilic precursor, TEOS, a strong increase in the sulfur mustard gas permeability was observed. Thus, in the following experiments, we plan to test organic vapors from compounds more hydrophobic than Toluene and Xylene.

5. Conclusions

The method for the synthesis of thin impermeable Parylene coatings was modified to create a similar coating, modifying the permeability of polyamide Cordura fabric. Cordura fabric was irradiated with γ-rays at doses up to 10 kGy in the presence of aromatic hydrophobic vapors (Toluene, Xylene, and Styrene) or hydrophilic precursor Tetraethoxysilane (TEOS) vapors under normal pressure and RT conditions. FTIR ATR analysis did not detect any changes in the Cordura fabric’s surface composition. Minor changes were observed in the experiments with the spreading of eight different dyes. The permeabilities of Hexane, Toluene, and water vapors were measured and compared for unmodified Cordura fabric, only γ-ray-irradiated Cordura fabric, and γ-ray-irradiated Cordura fabric in the presence of Toluene, Xylene, Styrene, and TEOS vapors. Only small changes in the permeabilities were observed. However, significant changes were observed in the experiments analyzing toxic sulfur mustard gas permeability. The unmodified Cordura fabric and only γ-ray-irradiated Cordura fabric revealed a similar permeability, which was slightly reduced when Cordura fabric was irradiated in the presence of aromatic-compound vapors. However, after irradiation in the presence of TEOS vapor, a nearly 300-fold increase in sulfur mustard gas permeability was observed.

This study highlights the importance of process optimization to guide future research on γ-ray irradiation in the presence of molecules of highly hydrophobic organic vapors while excluding molecules that act as precursors to hydrophilic compounds.

It is important to note that γ-radiation is a widely applicable method, allowing this method of modification to be performed on a larger, commercial scale with large rolls of irradiated Cordura fabric wound with filter paper presoaked in organic liquid. Therefore, this method has the potential to be commercialized on an industrial scale.