Sabyasachi Gaan

Hybrid wood materials with improved fire retardance by bio-inspired mineralisation on the nano- and submicron level

Inspired by natural matrix-mediated biomineralisation, we present an artificial calcification approach for wood, which predominately targets the hardly accessible nanoporous cell wall structure rather than the micron-sized void system of the cell lumina. CaCO3 can be deposited with this method deep inside the wood structure. Mineralisation of the wood cell wall architecture with CaCO3 offers a green alternative to conventional fire-retardant systems.

Recent Advances for Flame Retardancy of Textiles Based on Phosphorus Chemistry

This paper aims at updating the progress on the phosphorus-based flame retardants specifically designed and developed for fibers and fabrics (particularly referring to cotton, polyester and their blends) over the last five years. Indeed, as clearly depicted by Horrocks in a recent review, the world of flame retardants for textiles is still experiencing some changes that are focused on topics like the improvement of its effectiveness and the replacement of toxic chemical products with counterparts that have low environmental impact and, hence, are more sustainable. In this context, phosphorus-based compounds play a key role and may lead, possibly in combination with silicon- or nitrogen-containing structures, to the design of new, efficient flame retardants for fibers and fabrics. Therefore, this review thoroughly describes the advances and the potentialities offered by the phosphorus-based products recently developed at a lab-scale, highlighting the current limitations, open challenges and some perspectives toward their possible exploitation at a larger scale.

Recent Developments in Organophosphorus Flame Retardants Containing P-C Bond and Their Applications

Organophosphorus compounds containing P-C bonds are increasingly developed as flame retardant additives due to their excellent thermal and hydrolytic stability and ease of synthesis. The latest development (since 2010) in organophosphorus flame retardants containing P-C bonds summarized in this review. In this review, we have broadly classified such phosphorus compounds based on the carbon unit linked to the phosphorus atom i.e., could be a part of either an aliphatic or an aromatic unit. We have only considered those published literature where a P-C bond was created as a part of synthetic strategy to make either an intermediate or a final organophosphorus compound with an aim to use it as a flame retardant. General synthetic strategies to create P-C bonds are briefly discussed. Most popular synthetic strategies used for developing P-C containing phosphorus based flame retardants include Michael addition, Michaelis–Arbuzov, Friedels–Crafts and Grignard reactions. In general, most flame retardant derivatives discussed in this review have been prepared via a one- to two-step synthetic strategy with relatively high yields greater than 80%. Specific examples of P-C containing flame retardants synthesized via suitable synthetic strategy and their applications on various polymer systems are described in detail. Aliphatic phosphorus compounds being liquids or low melting solids are generally applied in polymers via coatings (cellulose) or are incorporated in the bulk of the polymers (epoxy, polyurethanes) during their polymerization as reactive or non-reactive additives. Substituents on the P atoms and the chemistry of the polymer matrix greatly influence the flame retardant behavior of these compounds (condensed phase vs. the gas phase). Recently, aromatic DOPO based phosphinate flame retardants have been developed with relatively higher thermal stabilities (>250 °C). Such compounds have potential as flame retardants for high temperature processable polymers such as polyesters and polyamides. A vast variety of P-C bond containing efficient flame retardants are being developed; however, further work in terms of their economical synthetic methods, detailed impact on mechanical properties and processability, long term durability and their toxicity and environmental impact is much needed for their potential commercial exploitations.

Mineralization of wood by calcium carbonate insertion for improved flame retardancy

Abstract Wood can be considered as a highly porous, three-dimensional organic scaffold. It can be mineralized to create hierarchically structured organic-inorganic hybrid materials with novel properties. In the present paper, the precipitation of CaCO3 mineral in Norway spruce and European beech wood has been studied by alternating impregnation with aqueous and alcoholic electrolyte solutions. Microstructural imaging by SEM and confocal Raman microscopy shows the distribution of calcite and vaterite as two CaCO3 polymorphs, which are deposited deep inside the cellular structure of the wood. The confined microenvironment of the wood cell wall seems to favor a formation of vaterite, as visible by XRD and Raman spectroscopy. In view of a practical application, the mineralization of wood opens up ways for sustainable wood-based hybrid materials with a significantly improved fire resistance, as proven via pyrolysis combustion flow calorimetry and cone calorimetry tests. Beyond that, this versatile solute-exchange approach provides an opportunity for the incorporation of a broad range of different mineral phases into wood for novel material property combinations.

Recent studies on the decomposition and strategies of smoke and toxicity suppression for polyurethane based materials

The widespread application of polyurethane-based materials promotes its development and increases the requirement of flame retardancy and smoke toxicity properties. This review provides insight into recent studies related to thermal degradation, smoke and toxicity production for polyurethane-based materials. Factors influencing smoke production, smoke and toxicity suppression, mechanisms of polyurethane decomposition and its evaluation methods are summarized. Recent polyurethane smoke suppression strategies include the use of metal-based compounds (metal oxides, metal salts, metal–organic hybrids and metal hydroxides), melamine, carbon-based additives as well as other miscellaneous additives. The mechanism of action of these additives are also summarized in this review.

Phosphorus-containing polyimide fibers and their thermal properties

Phosphorus-containing polyimide was synthesized by solution polycondensation reaction of bis(3-aminophenyl) methyl phosphine oxide with 4,4′-(4,4′-isopropylidenediphenoxy)bis(phthalic anhydride). The chemical characterization of polyimide was done using FTIR and NMR spectroscopy. Also, the influence of different parameters on the rheological and thermal behavior of the polyimide was investigated. We were able to prepare uniform submicron- or nano-sized fibers by electrospinning highly viscous polyimide solutions. The diameters of the electrospun fibers increased from 58 nm to 347 nm as the concentration of the polyimide solution was raised from 10 to 24 wt%. Isoconversional kinetic analysis of the thermal decomposition of the polyimide was performed using Friedman and Ozawa–Flynn–Wall methods. The thermal decomposition mechanism of polyimide was evaluated and proposed using thermogravimetric analyzer-Fourier transform infrared spectroscopy (TGA-FTIR) and pyrolysis gas chromatography mass spectroscopy (Py-GC-MS). The results showed a complex degradation process of the main chain, which started with the scission of weaker bonds such as ether, alkyl, imide and some aromatic groups. The char residue of the polyimide fibers from TGA experiments were analyzed using scanning electron microscopy (SEM). It exhibited a dense structure, with a homogeneous dispersion of phosphorus atoms.

Physical and thermal properties of poly(ethylene terephthalate) fabric coated with electrospun polyimide fibers

Two analogous polyimides (PIs) containing flexible isopropylidene units were prepared. One was based on 4,4′-oxydiphthalic anhydride and 4,4′-(4,4′-isopropylidenediphenyl-1,1′-diyldioxy) dianiline and the other was based on 4,4′-(4,4′-isopropylidenediphenoxy) bis(phthalic anhydride) and bis(3-aminophenyl) methyl phosphine oxide. The ability of these two PIs to form uniform nanoscaled fibers was investigated by electrospinning technique. At optimal spinning conditions, PI fibers were electrospun onto the surface of woven poly(ethylene terephthalate) (PET) support to form a bilayer composite structure. These new fabric systems were analyzed regarding morphology, air permeability, wetting properties, and thermal stability. It was expected that the new PET/PI mats would possess enhanced properties compared with the initial woven PET fibers due to the excellent properties of PIs. Experimental results showed that PET woven substrate coated with electrospun PI fibers had improved values of air permeability, water contact angle and thermal stability when compared with the initial woven PET fibers.

Flammability of Cellulose-Based Fibers and the Effect of Structure of Phosphorus Compounds on Their Flame Retardancy

Cellulose fibers are promoted for use in various textile applications due their sustainable nature. Cellulose-based fibers vary considerably in their mechanical and flammability properties depending on their chemical composition. The chemical composition of a cellulose-based fiber is further dependent on their source (i.e., seed, leaf, cane, fruit, wood, bast, and grass). Being organic in nature, cellulose fibers, and their products thereof, pose considerable fire risk. In this work we have compared the flammability properties of cellulose fibers obtained from two different sources (i.e., cotton and peat). Compared to cotton cellulose textiles, peat-based cellulose textiles burn longer with a prominent afterglow which can be attributed to the presence of lignin in its structure. A series of phosphoramidates were synthesized and applied on both cellulose textiles. From thermogravimetric and pyrolysis combustion flow analysis of the treated cellulose, we were able to relate the flame retardant efficacy of the synthesized phosphorus compounds to their chemical structure. The phosphoramidates with methyl phosphoester groups exhibited higher condensed phase flame retardant effects on both types of cellulose textiles investigated in this study. In addition, the bis-phosphoramidates exhibited higher flame retardant efficacy compared to the mono-phosphoramidates.

One-Pot Synthesis of P(O)-N Containing Compounds Using N-Chlorosuccinimide and Their Influence in Thermal Decomposition of PU Foams

Synthesis of intermediate containing P(O)-Cl bonds is the key to converting P(O)-H bonds to P(O)-N. In this work we have performed chlorination reactions of different H-phosphinates and H-phosphonates using N-chlorosuccinimide as an environmentally-benign chlorinating agent. The chlorination reaction showed high yield and high selectivity for transformation of P(O)-H bonds into P(O)-Cl analogues, resulting in an easily separable succinimide as the by-product. Using a one-pot synthesis methodology, we have synthesized a series of P(O)-N containing derivatives whose synthesis was found to be dependent on the reaction solvents and the starting materials. The synthesized P(O)-N compounds were incorporated in flexible polyurethane foam (FPUF) and screened for their influence in thermal decomposition of FPUFs using thermogravimetric analysis (TGA) and a microscale combustion calorimeter (MCC). All solid P(O)-N compounds influenced the first-stage decomposition of FPUFs, which resulted in an accelerated decomposition or temporary stabilization of this stage. However, the liquid P(O)-N derivatives volatilize at an earlier stage and could be active in the gas phase. In addition, they also work in condensed phase via acid catalyzed decomposition for FPUFs.

Enhanced flame-retardancy and controlled physical properties of flexible polyurethane foams based on a shear-responsive internal network

Solid additives are commonly used in manufacturing flexible polyurethane foams (FPUFs) to incorporate novel functionality for various application purposes. However, the viscosity change as a result of solid addition in the FPUF formulation can adversely alter the physical and mechanical properties of foam materials. Here, we report a shear-responsive internal network based on –P–N–H⋯O– interfacial hydrogen bonds between the polyether–polyol chain and a solid flame-retardant (FR) of 6,6′-(ethane-1,2-diylbis-(azanediyl))-bis-9,10-dihydro-9-oxa-phosphaphenanthrene-10-oxide (EDAB-DOPO) and its utilization in FPUFs. This interfacial interaction can stabilize the FR-polyol suspensions against sedimentation and it can be destroyed by shear force before foaming to control the fluid viscosity and thus control the physical–mechanical properties of FPUFs. The excellent dispersibility of FR particles from suspensions is well preserved in FPUFs by the optimized processes, which significantly enhances the flame retardancy of FPUFs with low FR content to achieve a HF1 rating in UL 94 test. The design of a shear-responsive internal network between solid additives and a polymer matrix provides a simple and practical method for producing functional foam composites.