PFAS-FREE NANOFIBER BARRIER LAYER WITH ENHANCED AIR PERMEABILITY, DURABILITY, AND WATER REPELLENCY

Abstract

The present disclosure describes non-PFAS nanofiber barrier layers for protective clothing, including chemical and biological protective suits (CBPS). The barrier layers offer both excellent protection against various hazards and reduced physiological, psychological, and thermal stress.

Claims

1. An electrospun nanofiber barrier membrane comprising: a. polyurethane; b. polydimethylsiloxane; and c. SiO2; wherein a surface of the membrane has a structure that mimics lotus leaves.

Description

DETAILED DESCRIPTION

[0008] The present disclosure describes non-PFAS nanofiber barrier layers for protective clothing, including chemical and biological protective suits (CBPS). The barrier layers offer both excellent protection against various hazards and reduced physiological, psychological, and thermal stress.

[0009] A non-PFAS polyurethane (PU)-polydimethylsiloxane (PDMS)/SiO.sub.2 electrospun nanofiber barrier layer is disclosed herein. In some implementations, the surface structure of the barrier layer mimics lotus leaves to improve protection and mechanical performance in desired applications while eliminating reliance on PFAS. PU has excellent elastic properties, making it an excellent material for textiles. The combination of hydrophobic PU-PDMS polymers with adjusted hierarchical nanofiber surface structures (nanowires) generates a barrier layer with excellent hydrophobicity. Meanwhile, incorporating hydrophobic SiO.sub.2 nanoparticles on the surfaces of nanofibers has the potential to further enhance hydrophobicity and oleophobicity.

[0010] The disclosed non-PFAS electrospun PU-PDMS/SiO.sub.2 nanofiber barrier layers with nanowire surface structures enable production of a new generation of CBPS with exceptional comfortability and protection. Protective clothing made of the disclosed nanofiber barrier layer exhibits superior particle filtration, anti-pathogen properties, and chemical adsorption and degradation capabilities, as well as reduced physiological, psychological, and thermal stress. Moreover, use of the disclosed membranes will reduce the use of PFAS, thereby decreasing detrimental impacts of PFAS in the environment and within the human body. The nanofiber barrier membrane is well suited to replace conventional membrane technologies, thus alleviating the disadvantages associated with currently used membranes such as bulkiness, weight, moisture retention, and environmental impacts. The disclosed membranes will allow significant reduction in overall costs by improving safety, task completion, and the life cycle of CBPS.

[0011] The PDMS/PU-SiO.sub.2 nanofiber membranes deliver superior filtration efficiencies of particles and aerosols while maintaining excellent breathability. Additionally, the inclusion of PDMS and SiO.sub.2 improves the liquid repellency of the membranes.

[0012] The nanofiber barrier membranes meet targeted particle filtration, antipathogenicity, and chemical adsorption and degradation requirements for CBPS. The disclosed membranes also meet targeted weight, air permeability, mechanical, and fluid repellency requirements.

[0013] In some implementations, the polydimethylsiloxane/PU-SiO.sub.2 (P-SiO.sub.2-PU/PDMS) electrospun nanofiber barrier membrane is treated using plasma enhanced chemical vapor deposition (PECVD) to further enhance chemical resistance. The membrane effectively blocks hazardous contaminants, including particles (>99%), biological threats (>99%), and chemical hazards (exceeding the Class 3 requirements set forth in NFPA 1994 standard). It is lightweight, exhibits fire retardancy (Class 1), and has high breathability, having both low thermal (>220 W/m and vapor resistance (<25 Pa m2/W). As a result, use of the disclosed membrane in CBPS holds great potential for enhancing user safety, task performance, and the overall sustainability and life cycle of protective ensembles.

[0014] In some implementations, the PECVD membrane resists chemical warfare agents (CWA). Chemical permeation resistance may be achieved by using a dual protection mechanism involving both the nanofiber and a plasma coating. During the initial contact of the CWA (e.g., DMMP, TCE or DMS) with the composite protective structure, the repellent properties of the coating will help reduce absorption of majority of the CWA. The CWA to bead up and fall off the surface of the coated membrane. If a small amount of CWA passes through the initial repellent layer, it will be captured by the nanofiber layer. This two-stage protection mechanism reduces the absorption of CWAs below targeted limits.

[0015] In some implementations, the membranes may be assembled with Nomex and Supplex Nylon fabrics to optimize the mechanical properties of the protective ensembles. To optimize the mechanical properties of the protective ensembles, PECVD-treated nanofibers may be coated onto fabric. This approach allows seamless integration into the currently used protective ensembles manufacturing processes, improving performance without significant cost or production challenges.

Fabrication of PECVD Treated PU Nanofiber Barrier Membranes

[0016] PU solution composition and electrospinning parameters were adjusted and fine-tuned in a non-systematic way to optimize the parameters which produce stable and consistent nanofiber membranes. To prepare the PU/PDMS solution, DMF is added to a container and heated to 80 C., after which LiCl is dissolved therein. TPU is slowly added to the DMF/LiCl solution and the resulting solution is heated to 110 C. and stirred for 1 h. In parallel to preparation of the TPU/DMF/LiCl solution, THF is added to a separate container and heated to 90 C., after which PDMS is dissolved therein. The solution is cooled to ambient temperature and PDMS binder is added, followed by sonication for 30 min. The TPU/DMF/LiCl and THF/PDMS solutions are then combined and cooled to ambient temperature. The solutions are then stirred or sonicated for 1 h. Ethyl methacrylate (EMA) may be used as compatibilizer for better mixing of PU and PDMS. Hydrophobic SiO.sub.2 nanoparticles may be added to increase the water repellency and fire retardancy for some applications.

[0017] A combined TPU/DMF/LiCl and THF/PDMS polymer solution was electrospun to form nanofiber membranes. Electrospinning parameters, namely voltage, spinning distance, injection rate, and winding speed, were adjusted to optimize the stability of the electrospinning jet and facilitate fiber formation. Modulating these parameters resulted in various membrane microstructure properties with different fiber diameters, pore sizes, and surface structures. Standardized spinning parameters for the most stable electrospinning process and finest fiber structure were identified, thereby producing consistent PU membranes with optimal protection and breathability. A stable electrospinning process ensures no dripping of the polymer solution along the spinneret or early drying/webbing formations along the spinning rod, ensuring strong Taylor Cones and well-visible spinning. It was demonstrated that a stable electrospinning can be accomplished with 40-65 kV voltage, 100-120 mm distance, 1.5-2.3 mL/min injection rate, and 0.8-2.2 m/min winding speed. In addition, a systematic approach was applied to understand the effect of individual parameters on the resultant morphology and orientation of the nanofibers produced.

[0018] A PECVD coating was applied to improve liquid repellency properties and barrier performance against liquid penetration. Two types of plasma polymer films were evaluated on the surface of nanofiber membranes: (a) plasma polymerized hexamethyldisiloxane (HMDSO) and (b) plasma polymerized ethylene and butadiene (PCEB).

[0019] For HMDSO, liquid HMDSO was used as the precursor monomer to form a topcoat. A container filled with HMDSO was heated to a temperature of about 45-50 C. Upon reaching the set temperature, a needle valve between the HMDSO container and a vapor source mass flow controller was opened. HMDSO was introduced at a flow rate of 16-32 mL/min into a reaction chamber containing nanofiber barrier samples. The pressure of the reaction chamber was maintained at 10 Pa and was then gradually increased at a rate of 5 Pa/min until a final pressure of 40 Pa was achieved. External RF power was continuously supplied to the reaction chamber over the course of the reaction. This system resulted in a homogenous topcoat of silane-based plasma polymers on nano-roughed nanofiber membranes.

[0020] For PCEB, gaseous ethylene and butadiene were used in various flow ratios. Flow of precursors from gas containers through stainless steel tubes was regulated by Brooks mass flow controllers. Flow rates of 5-20 sccm for ethylene and 3-10 sccm for butadiene were used. The flow was introduced into a stainless-steel cylindrical chamber which had samples positioned on the live electrode. The working pressure of the reactor was varied from 10-80 Pa in different conditions. External RF power was supplied to the reaction chamber via a connection to the live electrode with a matching network, and the supplied power ranged between 5-20 W. According to the gas flow conditions and the combinations of working pressure and applied power, various types of films were formed on surfaces of textile substrates.

Characterization of PECVD Treated PU Nanofiber Barrier Membranes

[0021] Scanning electron microscopy (SEM) analysis was conducted to examine the microstructure of the PECVD treated PU nanofiber membranes. A bilayered membrane was confirmed, as the electrospun fibers were smooth with no morphological defects. Moreover, an extremely thin layer of nanofiber (42.5 m) coated on a fabric were visible in transversal (cross-section) SEM images, indicating that two morphologically distinct layers were created. Top-view SEM images revealed the fine fiber diameters, as further confirmed via fiber diameter measurements. The minimum fiber diameter (60-75 nm) of the PU membrane was achieved at electrospinning conditions of 65 kV voltage, 107 mm distance, and 37% humidity. Additionally, fiber diameter measurements showed a narrow size distribution for both layers via SEM, thus confirming the stability of the spinning jet that led to producing uniform fiber size with significant consistency. Porosity analysis of individual layers using gravimetric measurements showed that both layers feature over 90% porosity. SEM validation confirmed the ability to generate a PU membrane with fine fiber diameter and thus high porosity. This level of control over the membrane's microstructure properties is essential for delivering barrier membrane with optimal protection and breathability properties.

[0022] It is important for barrier membranes to have excellent tensile and stretchability properties in protective ensembles, because it is inevitable for the fibrous membranes to get stretched when the protective ensemble is in use. Thus, cyclic loading-unloading tensile performance of the PU barrier membrane was investigated using a TA-850 dynamic mechanical analyzer with the tensile clamp. The membranes were tailored into 3 mm wide strips and were stretched at a speed of 40 mm/min. Energy to fracture (toughness) of nanofiber membranes were obtained by calculating the area under the tensile stress-strain curve. Tensile mechanical properties during loading and unloading cycles of membranes were tested at a rate of 100%/min. Furthermore, linear strain recovery ratios (elasticity) of membranes were determined in a cyclic tensile test. The results showed the PU nanofiber membrane have over 32% elasticity at different strains of 100, 150, 200, 250, and 300%, which is higher than conventional PTFE membranes (30%). The membranes were also repeatedly stretched at a large deformation of 300% without breakage.

Permeability and Performance of PECVD Treated PU Nanofiber Barrier Membranes

[0023] Three methods were used to evaluate the breathability of the PECVD treated nanofiber membranes.

[0024] First, air permeability studies using the ASTM D737 test method were carried out. In this testing method, air is passed perpendicularly through a fabric until a prescribed differential air pressure is achieved. When this pressure is achieved, the airflow is measured and used to determine the air permeability of the fabric. The equipment used in this test was a Frazier low pressure air permeability machine with a conditioning atmosphere of 21 C. and 65% relative humidity. It was observed that different types of composite constructions give rise to differing levels of air permeabilities. Varying thicknesses lead to differences in air permeabilities.

[0025] Second, ASTM F1868-17 was used to measure the evaporative resistance of a fabric/PECVD-nanofiber/fabric assembly toward the flow of moisture from the skin to the environment. To note, evaporative interchange between people and their environments is dependent on many other factors other than the steady-state resistance values of clothing materials. Airflow velocity of 1.00.1 m/s was used in the testing, with a sample thickness of about 0.8 mm. The back side (or the inner lining of the ensemble) was used for contact with the hot plate. To remove wrinkles from the fabric, smoothing was done without compressing the fabrics. The result showed that the fabricated PECVD-PU nanofiber sandwiched by two layers of Nomex exhibit a low evaporative resistance of 9.67 Pa m2/W.

[0026] Third, the rate of transmission of water vapor through the composite structure was also studied by the ASTM E96 test method. Water vapor permeability is the time rate of water vapor transmission through a unit area of a flat material of unit thickness, induced by the vapor pressure difference between two specific surfaces under specified temperature and humidity conditions. A brief experimental method is described below. First, 30 mL of distilled water was poured into a test dish. Next, sealant was applied to the rim of the test dish. Afterwards, a sample of fabric was affixed onto the rim of the test dish ensuring good adhesion to the rim and no wrinkles. Then the humidity and temperature sensor and the test dish with the sample on it were placed inside a test chamber set to 32.2 C. The mass of the test dish, temperature, and humidity were recorded over a period of 22-24 hours. Calculations for water vapor transmission rates were performed according to guidelines in the ASTM E96 test document.

[0027] A water contact angle analysis to assess the hydrophobicity of the PECVD treated PU nanofiber membranes was also performed. Wettability testing based on dynamic water contact angle (DCA) revealed the PU nanofiber membrane to be highly hydrophobic with a CA of 140 without treatment. The PECVD treatment significantly improved the hydrophobicity of the membranes resulting in a CA of 152, indicating the membrane's ability to repel water and other liquids without requiring the use of fluoropolymers. This property is essential for applications requiring liquid resistance and makes the membrane suitable for a wide range of protective applications

[0028] The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the invention disclosed herein. Although the various inventive aspects are disclosed in the context of certain illustrated embodiments, implementations, and examples, it should be understood by those skilled in the art that the invention extends beyond the specifically disclosed embodiments to other alternative embodiments and/or uses of the invention and obvious modifications and equivalents thereof. In addition, while a number of variations of various inventive aspects have been shown and described in detail, other modifications that are within their scope will be readily apparent to those skilled in the art based upon reviewing this disclosure. It should be also understood that the scope of this disclosure includes the various combinations or sub-combinations of the specific features and aspects of the embodiments disclosed herein, such that the various features, modes of implementation, and aspects of the disclosed subject matter may be combined with or substituted for one another. The generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the disclosure. Thus, the present disclosure is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

[0029] Each of the foregoing and various aspects, together with those summarized above or otherwise disclosed herein, including the figures, may be combined without limitation to form claims for a device, apparatus, system, method of manufacture, and/or method of use.

[0030] All references cited herein are hereby expressly incorporated by reference.