Synergistic Fire Resistance of Nanobrick Wall Coated 3D Printed Photopolymer Lattices.

Photopolymer additive manufacturing has become the subject of widespread interest in recent years due to its capacity to enable fabrication of difficult geometries that are impossible to build with traditional manufacturing methods. The flammability of photopolymer resin materials and the lattice structures enabled by 3D printing is a barrier to widespread adoption that has not yet been adequately addressed. Here, a water-based nanobrick wall coating is deposited on 3D printed parts with simple (i.e., dense solid) or complex (i.e., lattice) geometries. When subject to flammability testing, the printed parts exhibit no melt dripping and a propensity toward failure at the print layer interfaces. Moving from a simple solid geometry to a latticed geometry leads to reduced time to failure during flammability testing. For nonlatticed parts, the coating provides negligible improvement in fire resistance, but coating of the latticed structures significantly increases time to failure by up to ≈340% compared to the uncoated lattice. The synergistic effect of coating and latticing is attributed to the lattice structures' increased surface area to volume ratio, allowing for an increased coating:photopolymer ratio and the ability of the lattice to better accommodate thermal expansion strains. Overall, nanobrick wall coated lattices can serve as metamaterials to increase applications of polymer additive manufacturing in extreme environments.

Nanobrick Wall Multilayer Thin Films with High Dielectric Breakdown Strength.

Current thermally conductive and electrically insulating insulation systems are struggling to meet the needs of modern electronics due to increasing heat generation and power densities. Little research has focused on creating insulation systems that excel at both dissipating heat and withstanding high voltages (i.e., have both high thermal conductivity and a high breakdown strength). Herein, a polyelectrolyte-based multilayer nanocomposite is demonstrated to be a thermally conductive high-voltage insulation. Through inclusion of both boehmite and vermiculite clay, the breakdown strength of the nanocomposite was increased by ≈115%. It was also found that this unique nanocomposite has an increase in its breakdown strength, modulus, and hydrophobicity when exposed to elevated temperatures. This readily scalable insulation exhibits a remarkable combination of breakdown strength (250 kV/mm) and thermal conductivity (0.16 W m-1 K-1) for a polyelectrolyte-based nanocomposite. This dual clay insulation is a step toward meeting the needs of the next generation of high-performance insulation systems.

Non-Isocyanate Polyurethane Bio-Foam with Inherent Heat and Fire Resistance.

Polyurethanes (PUs) are versatile and widespread, particularly as flexible and rigid foams. To avoid isocyanates and other toxic reagents required for synthesis, such as phosgene, alternative synthetic routes have been utilized to produce non-isocyanate polyurethanes (NIPUs). A thermally and flame-resistant rigid NIPU was produced from environmentally benign and bio-sourced ingredients, requiring no catalyst or solvents. A foamed structure was obtained by the addition of glutaraldehyde and four different carboxylic acids: malic acid, maleic acid, citric acid, and aconitic acid. The resulting morphology, thermal degradation, and flame resistance of each foam were compared. The properties vary with each carboxylic acid used, but in each case, peak thermal degradation and peak heat release are postponed by >100 °C compared to commercial rigid PU foam. Furthermore, in a butane torch test, NIPU foams exhibit an 80% higher remaining mass and a 75% reduction in afterburn time, compared to commercial polyurethane. This bio-based polyurethane eliminates the hazards of traditional PUs, while imparting inherent thermal stability and flame resistance uncharacteristic of conventional foams.

Polyelectrolyte Complex with Controllable Viscosity by Doping Cu2+ Protects Nylon-Cotton Fabric against Fire.

Nylon-cotton (NC) blend fabrics are widely used in military and industrial applications, but their high flammability still remains a serious problem. In an effort to effectively and quickly impart flame retardancy to the NC fabric, it was treated by simply blade coating with a Cu2+-doped polyelectrolyte complex (CPEC) that consists of ammonium polyphosphate (APP), polyethylenimine (PEI), and copper sulfate. The viscosity of the CPEC can be adjusted by altering the content of CuSO4, which controls the amount of extrinsic and intrinsic ion pairs. By adjusting the proportion and content of PEI, APP, and CuSO4, CPEC suitable for treating the NC fabric was obtained. Only 0.067 wt % Cu2+ was needed to adjust the viscosity and impart self-extinguishing behavior in a vertical burning test. This simple two-step treatment provides a promising technology to protect flammable polymeric substrates with ultralow metal-doped polyelectrolyte complexes.

Biorenewable Polyelectrolyte Nanocoating for Flame-Retardant Cotton-Based Paper.

Cotton-based raw paper, made of 100% cellulose, is used to make humidity-sensing, cottonid for bio-architecture applications. Despite its renewability and excellent mechanical properties, it is inherently flammable. In an effort to reduce its flammability, thin films of fully renewable and environmentally benign polyelectrolytes, chitosan (CH) and phytic acid (PA), were deposited on raw paper via layer-by-layer (LbL) assembly. Only four bilayers (BL) of the CH/PA coating are required to achieve self-extinguishing behavior, with a 69% reduction in peak heat release rate measured by microscale combustion calorimetry. These results demonstrate that this renewable intumescent LbL-assembled film provides an effective flame-retardant treatment for these environmentally friendly, climate-adaptive construction materials and could potentially be used to protect many cellulosic materials.

Layer-by-Layer Deposition: A Promising Environmentally Benign Flame-Retardant Treatment for Cotton, Polyester, Polyamide and Blended Textiles.

A detailed review of recent developments of layer-by-layer (LbL) deposition as a promising approach to reduce flammability of the most widely used fibers (cotton, polyester, polyamide and their blends) is presented. LbL deposition is an emerging green technology, showing numerous advantages over current commercially available finishing processes due to the use of water as a solvent for a variety of active substances. For flame-retardant (FR) purposes, different ingredients are able to build oppositely charged layers at very low concentrations in water (e.g., small organic molecules and macromolecules from renewable sources, inorganic compounds, metallic or oxide colloids, etc.). Since the layers on a textile substrate are bonded with pH and ion-sensitive electrostatic forces, the greatest technological drawback of LbL deposition for FR finishing is its non-resistance to washing cycles. Several possibilities of laundering durability improvements by different pre-treatments, as well as post-treatments to form covalent bonds between the layers, are presented in this review.

Clay-Filled Polyelectrolyte Complex Nanocoating for Flame-Retardant Polyurethane Foam.

Polyurethane foam (PUF) is a highly flammable material typically used for cushioning in furniture and automobiles. A polyelectrolyte complex coating containing polyethylenimine, ammonium polyphosphate, and halloysite clay was applied to PUF using a two-step deposition process in an attempt to reduce its flammability. Electron microscopy confirms that this conformal thin film preserves the porous morphology of the foam and adds 20% to the foam's weight. Directly exposing coated foam to a butane torch flame yields a 73% residue after burning while keeping the internal structure of the foam intact. Cone calorimetry reveals a 52.5% reduction in the peak heat release rate (pkHRR) of the clay-based coating compared to that of the uncoated foam. This significant reduction in pkHRR and preservation of the porous structure of the foam highlights the utility of this easy-to-deposit, environmentally benign treatment to reduce the foam's flammability.

Efficient Heat Shielding of Steel with Multilayer Nanocomposite Thin Film.

In an effort to protect metal substrates from extreme heat, polymer-clay multilayer thin films are studied as expendable thermal barrier coatings. Nanocomposite films with a thickness ranging from 2 to 35 µm were deposited on steel plates and exposed to the flame from a butane torch. The 35 µm coating, composed of 14 deposited bilayers of tris(hydroxymethyl)aminomethane (THAM)-buffered polyethylenimine (PEI) and vermiculite clay (VMT), decreased the maximum temperature observed on the back side of a 0.32 cm thick steel plate by over 100 °C when heated with a butane torch. Upon exposure to high temperature, the polymer and amine salt undergo pyrolysis and intumesce, subsequently forming a char and blowing gas. The char encases the nanoclay platelets, and a ceramic bubble is formed. The macro-scale bubble, in tandem with the nanocomposite coating properties, increases resistance to heat transfer into the underlying metal substrate. This heat shielding behavior occurs through radiative effects and low aggregate through-plane conductivity resulting from multilayer nanodomains and intumesced porosity (i.e., conduction through the gas as the film expands to form a ceramic bubble). These relatively thin and lightweight films could be used to protect important metal parts (in automobiles, aircraft, etc.) from fire-related damage or other types of transient high-temperature situations.

Super Gas Barrier of a Polyelectrolyte/Clay Coacervate Thin Film.

Transparent polymeric thin films with high oxygen barrier are important for extending the shelf life of food and protecting flexible organic electronic devices. Polyelectrolyte/clay multilayer nanocoatings are shown to exhibit super gas barrier performance, but the layer-by-layer assembly process requires numerous deposition steps. In an effort to more quickly fabricate this type of barrier, a polyelectrolyte/clay coacervate composed of branched polyethyleneimine (PEI), poly(acrylic acid) (PAA), and kaolinite (KAO) clay is prepared and deposited in a single step, followed by humidity and thermal post-treatments. When deposited onto a 179 µm poly(ethylene terephthalate) (PET) film, a 4 µm coacervate coating reduces the oxygen transmission rate (OTR) by more than three orders of magnitude, while maintaining high transparency. This single-step deposition process uses only low-cost, water-based components and ambient conditions, which can be used to for sensitive food and electronics packaging.

Environmentally-Benign Phytic Acid-Based Multilayer Coating for Flame Retardant Cotton.

Chemically bleached cotton fabric was treated with phytic acid (PA), chitosan (CH) and urea by means of layer-by-layer (LbL) deposition to impart flame retardant (FR) behavior using only benign and renewable molecules. Samples were treated with 8, 10, 12 and 15 bilayers (BL) of anionic PA and cationic CH, with urea mixed into the aqueous CH solution. Flammability was evaluated by measuring limiting oxygen index (LOI) and through vertical flame testing. LOI values are comparable to those obtained with commercial flame-retardant finishes, and applying 10 or more bilayers renders cotton self-extinguishing and able to pass the vertical flame test. Microscale combustion calorimeter (MCC) measurements show the average reduction of peak heat release rate (pHRR) of all treated fabrics of ~61% and the reduction of total heat release (THR) of ~74%, in comparison to untreated cotton. Decomposition temperatures peaks (T1max) measured by thermogravimetric analyzer (TG) decreased by approximately 62 °C, while an average residue at 650 °C is ~21% for 10 and more bilayers. Images of post-burn char indicate that PA/CH-urea treatment is intumescent. The ability to deposit such a safe and effective FR treatment, with relatively few layers, makes LbL an alternative to current commercial treatments.

Environmentally Benign and Self-Extinguishing Multilayer Nanocoating for Protection of Flammable Foam.

Most current flame-retardant nanocoatings for flexible polyurethane foam (PUF) consist of passive barriers, such as clay, graphene oxide, or metal hydroxide. In an effort to develop a polymeric and environmentally benign nanocoating for PUF, positively charged chitosan (CH) and anionic sodium hexametaphosphate (PSP) were deposited using layer-by-layer (LbL) assembly. Only six bilayers of CH/PSP film can withstand flame penetration during exposure to a butane torch (∼1400 °C) for 10 s and stop flame spread on the foam. Additionally, cone calorimetry reveals that the fire growth rate, peak heat release rate, and maximum average rate of heat emission are reduced by 55, 43, and 38%, respectively, compared with uncoated foam. This multilayer thin film quickly dehydrates to form an intumescent charred exoskeleton on the surface of the open-celled structure of polyurethane, inhibiting heat transfer and completely eliminating melt dripping. This entirely polymeric nanocoating provides a safe and effective alternative for reducing the fire hazard of polyurethane foam that is widely used for cushioning and insulation.

Contact electrification between identical polymers as the basis for triboelectric/flexoelectric materials.

Polymer contact electrification offers the possibility to harvest mechanical energy using lightweight, flexible and low-cost materials, but the mechanism itself is still unresolved. Several recent studies confirm heterolytic covalent bond breaking as the mechanism for surface charge formation. Here it is shown that the reason for the formation of surface charge by contacting two identical polymers results from the fluctuation in the surface irregularities, and that contacted materials with a greater porosity or surface roughness differential result in a greater generation of surface charge. Porosity and surface roughness create uneven surface length percentage changes in the lateral direction during deformation, which changes the charge density across the surface during relaxation. Multilayered membranes exhibit flexoelectric properties upon pressing and releasing by generating charge without separating individual membrane layers. This new insight deepens the understanding of polymer contact electrification and highlights better ways to prepare triboelectric or flexoelectric nanogenerator devices.

High Modulus, Thermally Stable, and Self-Extinguishing Aramid Nanofiber Separators.

Mechanically and thermally robust separators offer an alternative approach for preventing battery failure under extreme conditions such as high loads and temperatures. However, the trade-off between electrochemical performance and mechanical and thermal stability remains an ongoing challenge. Here, we investigate aramid nanofiber (ANF) separators that possess high moduli and self-extinguishing characteristics. The ANF separators are formed from the dissolution of bulk Kevlar fibers and their subsequent vacuum-assisted self-assembly. Thermogravimetric analysis shows a high 5 wt % decomposition temperature of 447 °C, which is over ∼175 °C higher than commercial Celgard separators. The ANF separator also possesses a high Young's modulus of 8.8 GPa, which is ∼1000% higher than commercial separators. Even when dry or when soaked in battery electrolyte, the ANF separators self-extinguish upon exposure to flame, whereas commercial separators melt or drip. We show that these features, although adventitious, present a trade-off with electrochemical performance in which a lithium nickel manganse cobalt (NMC) oxide-based battery possessed a reduced capacity of 123.4 mA h g-1. Considering the separator holistically, we propose that the ANF separator shows an excellent balance of the combined properties of high modulus, flame-resistance, thermal stability, and electrochemical stability and might be suitable for extreme environment applications with further testing.

Mica-Based Multilayer Nanocoating as a Highly Effective Flame Retardant and Smoke Suppressant.

Highly flammable polyurethane foam (PUF) remains a key risk factor associated with bedding and upholstered furniture, contributing to the yearly destruction of property and loss of lives. In an attempt to tackle this issue and develop a more benign flame retardant for PUF, a mica-based nanocomposite was deposited using layer-by-layer assembly. Chitosan (CH) and poly(acrylic acid) (PAA) were used to stabilize high-aspect-ratio mica. Foam treated with eight bilayers of CH- and PAA-stabilized mica preserves the porous foam structure, prevents melt dripping, and self-extinguishes during a 10 s torch test, while uncoated foam is completely consumed. When exposed to 35 kW/m2 heat flux during cone calorimetry, the peak heat release rate is reduced by 54% and less-volatile molecules are released during combustion, resulting in a 76% reduction in the total smoke release. This multilayer coating serves as an environmentally benign template for flame-retarding PUF and various other three-dimensional substrates.

Salt doping to improve thermoelectric power factor of organic nanocomposite thin films.

Thermoelectric materials with a large Seebeck coefficient (S) and electrical conductivity (σ) are required to efficiently convert waste heat into electricity, but their interdependence makes simultaneously improving these variables immensely challenging. To address this problem, bilayers (BL) of poly(diallyldimethylammonium chloride) (PDDA) and double-walled carbon nanotubes (DWNT), stabilized by KBr-doped poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) were deposited using layer-by-layer (LbL) assembly. Doping PEDOT:PSS with KBr, prior to DWNT dispersion and LbL assembly, results in a six-fold improvement in electrical conductivity with little change in the Seebeck coefficient. A maximum power factor (PF = S 2 σ) of 626 ± 39 µW m-1 K-2 is obtained from a 20 BL PDDA/PEDOT:PSS-DWNT film (∼46 nm thick), where PEDOT:PSS was doped with 3 mmol KBr. This large PF is due to the formation of a denser film containing a greater proportion of DWNT, which was influenced by the charge-screening effects imparted by the salt dopant that separates PSS from PEDOT. This study demonstrates a relatively simple strategy to significantly increase the thermoelectric performance of fully organic nanocomposites that are useful for low temperature thermoelectric devices.

UV-protection from chitosan derivatized lignin multilayer thin film.

Lignin is one of the most abundant renewable materials on the earth. Despite possessing useful antioxidant and UV absorbing properties, its effective utilization in technology has been hampered by its relative insolubility and difficulty to process. In this work, a simple chemical derivatization process is utilized which yields water-soluble lignin possessing anionic carboxylate groups. These carboxylate groups give lignin polyanionic behavior and enable its utilization in the growth of a functional film via layer-by-layer (LbL) assembly with biologically sourced chitosan. The growth mechanism of this film is hypothesized to be a result of both hydrogen bonding and ionic interactions. The film demonstrates excellent UV-absorptive capability. A 100 nm thick chitosan/lignin coating was applied to a poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) film and shown to reduce its degradation sixfold over the course of a 1 hour exposure to harsh UV light. This is the first demonstration of lignin being utilized in a fully biologically derived LbL film. Utilization of lignin in LbL assembly is an important step in the development of renewable nanotechnology.

Enzymatic Modification of Polyamide for Improving the Conductivity of Water-Based Multilayer Nanocoatings.

Enzymatic modification, using a protease from Bacillus licheniformis (Subtilisin A), was carried out on polyamide 6.6 (PA6.6) fabric to make it more amenable to water-based nanocoatings used to impart electrical conductivity. The modified PA6.6 fibers exhibit a smoother surface, increased hydrophilicity due to more carboxyl and amino groups, and larger ζ-potential relative to unmodified polyamide. With its improved hydrophilicity and surface functionality, the modified textile is better able to accept a water-based nanocoating, composed of multiwalled carbon nanotubes (MWCNT) stabilized by sodium deoxycholate (DOC) and poly(diallyldimethylammonium chloride) (PDDA), deposited via layer-by-layer assembly. Relative to unmodified fabric, the enzymatically modified fibers exhibit lower sheet resistance as a function of PDDA/MWCNT-DOC bilayers deposited. This relatively green technique could be used to impart a variety of useful functionalities to otherwise difficult-to-treat synthetic fibers like polyamide.

Crosslinkable-Chitosan-Enabled Moisture-Resistant Multilayer Gas Barrier Thin Film.

Chitosan-based films exhibit good oxygen barrier that degrades when exposed to high humidity. In an effort to overcome this drawback, a multilayer nanocoating consisting of crosslinkable chitosan (CHQ) and poly(acrylic acid) [PAA] is deposited on polyethylene terephthalate (PET) using layer-by-layer assembly. Chitosan is functionalized with glycidyl methacrylate to introduce acrylic functionalities within the film. The deposited films are crosslinked using a free radical initiator and this crosslinking is confirmed by FTIR and reduced film thickness. A 10-bilayer (BL) crosslinked CHQ/PAA film, which is only 165 nm thick, results in a 36× reduction of the oxygen transmission rate of PET at 90% relative humidity. To achieve these same results without crosslinking, a 15-BL unmodified chitosan (CH)/PAA film, which is almost 5× thicker, must be deposited on PET. This environmentally friendly, transparent nanocoating is promising for food packaging or protection of flexible electronics, especially in high-humidity environments.

Unusually fast and large actuation from multilayer polyelectrolyte thin films.

Polymers responsive to external stimuli (e.g., electric field, chemical vapor, light) are of great interest for smart materials such as sensors and soft robotics. A vapor-driven multilayer polymer actuator, capable of fast and large-scale actuation, is described here. This Janus-like actuator is prepared with two polyelectrolyte multilayer systems (polyethylenimine (PEI)/poly(acrylic acid) (PAA) and polyurethane (PU)/poly(acrylic acid) (PAA)) using layer-by-layer assembly (LbL). The differing hydrophilicity of these two nanocoatings results in different swelling behavior in water and organic solvents, which leads to vapor-responsive mechanical motion. The bending/curling degree of this polymeric actuator can be precisely controlled by changing the thickness ratio of the two layers. A vapor sensor was constructed to demonstrate the environmental detection ability of this unique actuator.

Extreme Heat Shielding of Clay/Chitosan Nanobrick Wall on Flexible Foam.

Flexible polyurethane foam (PUF) is widely used in bedding, transportation, and furniture, despite being highly flammable. In an effort to decrease the flammability of the polymer, an environmentally friendly flame retardant coating was deposited on polyurethane foam (PUF) via layer-by-layer assembly. Treated foam was subjected to three different fire scenarios, 10 s torch test, cone calorimetry, and a 900 s burn-through test, to evaluate the thermal shielding behavior of an eight bilayer chitosan/vermiculite clay nanocoating. In each fire scenario, the nanocoating acts as a thermal shield from the flames by successfully protecting the backside of the PUF, whereas the side directly exposed to the flame results in a hollowed nanocoating that maintains the complex three-dimensional porous structure of the foam. Cone calorimetry reveals that the coating reduces the peak heat release rate and total smoke release by 53 and 63%, respectively, whereas a temperature gradient greater than 200 °C is observed across a 2.5 cm thick coated foam sample during the rigorous burn-through fire test. The thermal shielding behavior of this polymer/clay nanocoating makes this system very attractive in improving the fire safety of polyurethane foam used for insulating applications.