Flammability behaviour of wood and a review of the methods for its reduction

Phosphorylated and carbamylated Kraft lignin for improving fire- and biological-resistance of Scots pine wood

In this study, Kraft lignin was modified by ammonium dihydrogen phosphate (ADP) and urea for achieving phosphorylation and carbamylation, aiming to protect wood against biological and fire attack. Scots pine (Pinus sylvestris L.) sapwood was impregnated with a water solution containing Kraft lignin, ADP, and urea, followed by heat treatment at 150 °C, resulting in changes in the properties of the Kraft lignin as well as the wood matrix. Infrared spectroscopy, 13C cross-polarisation magic-angle-spinning (MAS) nuclear magnetic resonance (NMR), and direct excitation single-pulse 31P MAS NMR analyses suggested the grafting reaction of phosphate and carbamylate groups onto the hydroxyl groups of Kraft lignin. Scanning electron microscopy with energy dispersive X-ray spectroscopy indicated that the condensed Kraft lignin filled the lumen as well as partially penetrating the wood cell wall. The modified Kraft lignin imparted fire-retardancy and increased char residue to the wood at elevated temperature, as confirmed by limiting oxygen index, microscale combustion calorimetry, and thermogravimetric analysis. The modified wood exhibited superior resistance against mold and decay fungi attack under laboratory conditions. The modified wood had a similar modulus of elasticity to the unmodified wood, while experiencing a reduction in the modulus of rupture.

Improving Mechanical Performance of Self-Binding Fiberboards from Untreated Perennial Low-Input Crops by Variation of Particle Size

Studies on self-binding hot-pressed fiberboards using agricultural byproducts aim to identify alternatives to scarce wood resources. Particle size and mixture significantly impact strength, although direct comparisons are difficult due to differences in study methods. We evaluated fiberboards made from the two perennial biomass crops Miscanthus and Paulownia and compared them to Picea (spruce), using five distinct particle size blends prepared from milled and sieved particles, respectively. The boards were evaluated for their modulus of elasticity, modulus of rupture, reaction to fire, water absorption, and thickness swelling. All specimens exhibited normal ignitability, as defined by Euroclass E according to EN13501-1. The results indicate that mechanical performance improves with increasing density, which correlates with higher proportions of finer particles. Notably, the finer Miscanthus blends and all Paulownia samples met the modulus of elasticity requirements of EN 622.

Impact of surface delignification on fire retardancy of wood treated with polyelectrolyte complexes

Wood is a natural composite widely employed as a residential building interior finishing. Although wood is readily available and offers benefits to the occupants, such as enhanced well-being, it is rarely employed in commercial construction due, amongst others, to the potential hazard of fire propagation. The application of flame retardant (FR) treatments leads to a reduction of wood flammability and supports wood as interior finishing. Polyelectrolyte complexes (PECs) deposition is an innovative surface treatment that has already proven its efficiency for fabrics. For wood, recent studies have highlighted that the weight gain impacted the fire-retardancy, and a minimum of 2 wt.-% was set to obtain fire protection. This study explored the potential of surface delignification to activate the wood surface and facilitate the PEC impregnation. Yellow birch (Betula alleghaniensis, Britt.) was surface delignified (0.3 mm) using sodium chlorite. The treatment impact on wood was evaluated by spectroscopy analysis (FTIR, Raman), and the increase in wood wettability was demonstrated (contact angle decreases from 50° to 35° after the surface delignification). Then, PECs consisting of polyethyleneimine and sodium phytate were surface impregnated in wood and delignified wood. The flame retardancy was evaluated using a cone calorimeter. Despite the increase in weight gain (1.5 wt.-% ± 0.3 wt.-% to 4.3 wt.-% ± 2.5 wt.-%), fire performance was not improved. This study demonstrates that lignin strongly affects char formation, even in the presence of PECs.

A novel modified nano-alumina composite sol for potential application in forest firefighting

Herein, modified ammonium polyphosphate wrapped nano-alumina (mAPP@Als) was first synthesized and then dispersed in traditional fire extinguishing solution (FES) to fabricate a FES-mAPP@Als composite sol. It was found that the phosphorus-silica containing units were attached onto the nano-alumina surface, and the mAPP@Als particles showed excellent dispersion level in FES with a single-domain particle size distribution range. Due to the synergistic effects of the phosphorus-nitrogen and silica-alumina flame retardant components, FES-mAPP@Als (5% concentration) coated wood exhibited improved limiting oxygen index (33.2%) and carbonization ability, and depressed heat release (41.9%) and smoke production (10.7%), as compared to the pristine wood. In addition, the FES-mAPP@Als composite sol showed enhanced fire-extinguishing and anti-reignition capacities compared to the FES. This research offers a novel composite sol fire extinguishing agent for fighting forest fires.

Eco-Friendly Multilayer Coating Harnessing the Functional Features of Curcuma-Based Pigment and Rice Bran Wax as a Hydrophobic Filler

This work aims to highlight the multiple features shown by curcuma-based pigment and rice bran wax, which can be selectively employed as bio-based additives for the realization of multilayer wood coatings with multiple functionalities, harnessing the capabilities of the two environmentally friendly fillers, in line with current environmental sustainability trends. The role of the two green materials on the morphology of the composite layers was examined through observations employing scanning electron and optical microscopy, revealing a strong alteration of the film's appearance, both its color and reflectivity. Additionally, their influence on the paint's resilience was assessed by exposing the samples to UV-B radiation and consecutive thermal shocks. The coating displayed a clear and uniform change in color because of substantial curcuma powder photo-degradation but it remained exceptionally stable when subjected to thermal stresses. Moreover, the protective properties of the coatings were evaluated by conducting liquid resistance tests and water uptake tests, while the hardness and the abrasion resistance of the coatings were assessed to evaluate the effect of the additives on the mechanical properties of the coatings. In conclusion, this study showcases the promising joint action of curcuma-based pigment and rice bran wax in multilayer coatings. This combination offers vibrant yellow tones and an appealing appearance to the paint, enhances the surface's water-repellent properties, and improves the mechanical resistance of the coatings.

High Leach-Resistant Fire-Retardant Modified Pine Wood (Pinus sylvestris L.) by In Situ Phosphorylation and Carbamylation

The exterior application of fire-retardant (FR) timber necessitates it to have high durability because of the possibility to be exposed to rainfall. In this study, water-leaching resistance of FR wood has been imparted by grafting phosphate and carbamate groups of the water-soluble FR additives ammonium dihydrogen phosphate (ADP)/urea onto the hydroxyl groups of wood polymers via vacuum-pressure impregnation, followed by drying/heating in hot air. A darker and more reddish wood surface was observed after the modification. Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, solid-state ¹³C cross-polarization magic-angle-spinning nuclear magnetic resonance (¹³C CP-MAS NMR), and direct-excitation ³¹P MAS NMR suggested the formation of C-O-P covalent bonds and urethane chemical bridges. Scanning electron microscopy/energy-dispersive X-ray spectrometry suggested the diffusion of ADP/urea into the cell wall. The gas evolution analyzed by thermogravimetric analysis coupled with quadrupole mass spectrometry revealed a potential grafting reaction mechanism starting with the thermal decomposition of urea. Thermal behavior showed that the FR-modified wood lowered the main decomposition temperature and promoted the formation of char residues at elevated temperatures. The FR activity was preserved even after an extensive water-leaching test, confirmed by the limiting oxygen index (LOI) and cone calorimetry. The reduction of fire hazards was achieved through the increase of the LOI to above 80%, reduction of 30% of the peak heat release rate (pHRR₂), reduction of smoke production, and a longer ignition time. The modulus of elasticity of FR-modified wood increased by 40% without significantly decreasing the modulus of rupture.

New Challenges in Wood and Wood-Based Materials II

Wood is a natural material that is available in large quantities and is easy to produce, making it the perfect material to consider for the circular economy [...]

Facile preparation of flame-retardant cellulose composite with biodegradable and water resistant properties for electronic device applications

The aim of the present study is to produce flexible, flame-retardant, water-resistant and biodegradable composite materials. The ultimate goal of this research is to develop simple processes for the production of bio-based materials capable of replacing non-degradable substrates in printed circuit board. Cellulose was chosen as a renewable resource, and dissolved in 1-ethyl-3-methylimidazolium acetate ionic liquid to prepare a cellulosic continuous film. Since flame retardancy is an important criterion for electronic device applications and cellulose is naturally flammable, we incorporated ammonium polyphosphate (APP) as a flame-retardant filler to increase the flame retardancy of the produced materials. The developed material achieved a UL-94 HB rating in the flammability test, while the cellulose sample without APP failed the test. Two hydrophobic agents, ethyl 2-cyanoacrylate and trichloro(octadecyl)silane were applied by a simple dip-coating technique to impart hydrophobicity to the cellulose-APP composites. Dynamic mechanical analysis indicated that the mechanical properties of the cellulosic materials were not significantly affected by the addition of APP or the hydrophobic agents. Moreover, the biodegradability of the cellulosic materials containing APP increased owing to the presence of the cellulase enzyme. The hydrophobic coating slightly decreased the biodegradability of cellulose-APP, but it was still higher than that of pure cellulose film.

Fabrication of Inorganic Coatings Incorporated with Functionalized Graphene Oxide Nanosheets for Improving Fire Retardancy of Wooden Substrates

Flame-retardant chemicals are frequently used within consumer products and can even be employed as a treatment on the surface of different types of materials (e.g., wood, steel, and textiles) to prevent fire or limit the rapid spread of flames. Functionalized graphene oxide (FGO) nanosheets are a promising construction coating nanomaterial that can be blended with sodium metasilicate and gypsum to reduce the flammability of construction buildings. In this work, we designed and fabricated novel and halogen-free FGO sheets using the modified Hummers method; and subsequently functionalized them by pentaerythritol through a chemical impregnation process before dispersing them within the construction coating. Scanning electron microscopic images confirm that the FGO-filled coating was uniformly dispersed on the surface of wooden substrates. We identified that the FGO content is a critical factor affecting the fire retardancy. Thermogravimetric analysis of the FGO coating revealed that higher char residue can be obtained at 700 °C. Based on the differential scanning calorimetry, the exothermic peak contained a temperature delay in the presence of FGO sheets, primarily due to the formation of a thermal barrier. Such a significant improvement in the flame retardancy confirms that the FGO nanosheets are superior nanomaterials to be employed as a flame-retardant construction coating nanomaterial for improving thermal management within buildings.

Highly Flame-Retardant and Low Heat/Smoke-Release Wood Materials: Fabrication and Properties

Wood is an important renewable material exhibiting excellent physical and mechanical properties, environmental friendliness, and sustainability, and has been widely applied in daily life. However, its inherent flammability and susceptibility to fungal attack greatly limit its application in many areas. Use of fire-retardant coatings and preservatives has endowed wood with improved safety performance; importantly, the cooperative effect of dual treatments on the burning behavior and flame retardancy of wood needs to be better understood. Here, a two-step treatment for wood is proposed, with a copper–boron preservative (CBP) and a fire-retardant coating. The thermal degradation and burning behavior of treated wood were investigated. The CBP formed a physical barrier on the wood surface, facilitating a charring process at high temperatures and thus suppressing the release of heat and smoke. Notably, the dual-treated wood exhibited lower heat release and reduced smoke emission compared with the mono-treated wood, indicating a cooperative effect between CBP and fire-retardant coatings, beneficial to the improvement of fire safety. This experimental work improved fire retardance and suppressed smoke release in flammable materials, and offers a new design for developing fire-retardant coatings.

Construction for Health; Reversing the Impacts

The health of humans and the planet are the most vital contemporary issues and essential components of the Sustainable Development Goals (SDGs). Scientists and professionals strive for integrated, evolving, healthy, and sustainable solutions encompassing biodiversity and industrial ecology, while offering viable economic attainments. The building industry, especially construction, is an extensive economic counterpart that largely influences health on various levels. On a practical scale, most direct or indirect impacts on health are related to conventional construction systems (CCSs), particularly their materialisations and implementation methods. Therefore, from a global perspective, emerging technologies or remodelled methods to accomplish sustainable use, reuse, and recycling, and improving the planet’s health to ensure the wellbeing of its inhabitants, are crucial. The current research is part of a broader study on “programmable construction systems” (PCSs), concentrating on “programmable construction materials” (PCMs) for health. Therefore, issues are reviewed, relevancies are addressed, and health-oriented concepts are discussed. Example concepts of formulation and the simplified toolkit creations follow the problems’ sources in a case study, providing insight into the resulting multiscale impacts on real-life practices. The results prove the method’s potential and validate its simplicity and applicability through an abstract examination of a newly built case study. Finally, the summarised outcomes of other extensive studies on societal preferences also confirm the feasibility of the hypothesis (i.e., the healthy materialisation) also from a social perspective.

Characterisation of the fire behaviour of wood: From pyrolysis to fire retardant mechanisms

Wood is undeniably the most useful and readily available natural raw material. However, the susceptibility of wood products to fire is one of the crucial challenges faced in the wood industry. The fire behaviour of wood is a very complex phenomenon due to the different constituents and their independent reactions to fire. This article presents a thorough overview of the flammability stages of wood. It covers pyrolysis, thermal oxidative decomposition, ignition, combustion and heat release as well as flame extinction mechanisms. In the area of flame retardancy, conventional wood fire retardants, nanocomposites fire retardants and wood modification processes are investigated. Factors such as wood species, moisture content, density, experimental conditions such as external heat flux, heat exposure time, wood permeability and porosity are some of the deterministic parameters characterising the fire behaviour. This paper is a one-stop-shop for researchers analysing wood flammability due to the inclusion of all aspects pertaining to the burning of wood.

Flame Retardant Coatings: Additives, Binders, and Fillers

This review provides an intensive overview of flame retardant coating systems. The occurrence of flame due to thermal degradation of the polymer substrate as a result of overheating is one of the major concerns. Hence, coating is the best solution to this problem as it prevents the substrate from igniting the flame. In this review, the descriptions of several classifications of coating and their relation to thermal degradation and flammability were discussed. The details of flame retardants and flame retardant coatings in terms of principles, types, mechanisms, and properties were explained as well. This overview imparted the importance of intumescent flame retardant coatings in preventing the spread of flame via the formation of a multicellular charred layer. Thus, the intended intumescence can reduce the risk of flame from inherently flammable materials used to maintain a high standard of living.

Fire Retardancy and Leaching Resistance of Furfurylated Pine Wood (Pinus sylvestris L.) Treated with Guanyl-Urea Phosphate

Guanyl-urea phosphate (GUP) was introduced into furfurylated wood in order to improve fire retardancy. Modified wood was produced via vacuum-pressure impregnation of the GUP–furfuryl alcohol (FA) aqueous solution, which was then polymerized at elevated temperature. The water leaching resistance of the treated wood was tested according to European standard EN 84, while the leached water was analyzed using ultra-performance liquid chromatography (UPLC) and inductively coupled plasma–sector field mass spectrometry (ICP-SFMS). This new type of furfurylated wood was further characterized in the laboratory by evaluating its morphology and elemental composition using optical microscopy and electron microscopy coupled with energy-dispersive X-ray spectrometry (SEM-EDX). The chemical functionality was detected using infrared spectroscopy (FTIR), and the fire resistance was tested using cone calorimetry. The dimensional stability was evaluated in wet–dry soaking cycle tests, along with the mechanical properties, such as the Brinell hardness and bending strength. The fire retardancy of the modified furfurylated wood indicated that the flammability of wood can be depressed to some extent by introducing GUP. This was reflected in an observed reduction in heat release rate (HRR2) from 454.8 to 264.9 kW/m2, without a reduction in the material properties. In addition, this leaching-resistant furfurylated wood exhibited higher fire retardancy compared to conventional furfurylated wood. A potential method for producing fire-retardant treated furfurylated wood stable to water exposure has been suggested.

Passive Fire Protection of Taeda pine Wood by Using Starch-Based Surface Coatings

The present paper reports the preliminary results relating to the development, subsequent application, and testing of environmentally benign starch-based formulations for passive fire protection of wood substrates. This study evaluated the effectiveness of starch colloid coatings applied onto the wood surface with a view to improving its performance when exposed to the external heat flux (35 kW/m²) during cone calorimetric tests. The formulations were prepared from aqueous colloid solutions of either starch alone, or in combination with inorganic salts, such as: sodium carbonate, Na₂CO₃, potassium carbonate, K₂CO₃, and diammonium hydrogen phosphate, (NH₄)₂HPO₄. The fire performance of Taeda pine wood samples, where their top surfaces were treated with these formulations, was compared with the control sample. The thermal and combustion characteristics of the tested samples were determined with the aid of thermo-gravimetric analysis (TGA), bomb and cone calorimetric techniques, and a steady state tube furnace coupled to an FT-IR spectrometer. A significant boost of fire protection was observed when starch formulations with added inorganic salts were applied onto the wood surfaces, compared with the control sample. For example, the presence of K₂CO₃ in starch colloid solutions resulted in a notable delay of the ignition and exhibited a reduction in the heat release parameters in comparison with the untreated wood substrate.

Durability and Fire Performance of Charred Wood Siding (Shou Sugi Ban)

Shou sugi ban, also known as yakisugi, or just sugi ban, is an aesthetic wood surface treatment that involves charring the surface of dimensional lumber, such as exterior cladding. The goal of this research is to examine the effect of shou sugi ban on the flammability and decay resistance of wood. Several species and variants of commercially available sugi ban were tested. The flammability was examined from the heat release rate curves using the oxygen consumption method and cone calorimeter. Durability was examined with a soil block assay for one white-rot fungus and one brown-rot fungus. The testing showed that the shou sugi ban process did not systematically improve the flammability or durability of the siding.

The Effect of Heat Flux to the Fire-Technical and Chemical Properties of Spruce Wood (Picea abies L.)

The paper assesses the influence of the heat flux on spruce wood (Picea abies L.) behavior. The heat flux was performed at 15, 20, 25, and 30 kW·m⁻². The fire-technical properties, such as the mass burning rate, charring thickness, charring rate, as well as the chemical composition (contents of the extractives, lignin, cellulose, holocellulose), of wood were determined. The highest burning rate of spruce wood of 0.32%·s⁻¹ was reached at the heat flux of 30 kW·m⁻². The charring rate ranged from 1.004 mm·min⁻¹ (15 kW·m⁻²) to 2.016 mm·min⁻¹ (30 kW·m⁻²). The proposed model of the charring process of spruce wood in time and appropriate thickness as a selected parameter is applicable in validation of the results of computer fire models in the design of fire protection of wooden buildings. The decrease in the holocellulose content mostly caused by the degradation of hemicelluloses was observed during thermal loading. The biggest decrease in hemicelluloses (24.94%) was recorded in samples loaded at 30 kW·m⁻². The contents of cellulose increased due to the structural changes (carbonization and crosslinking), the content of lignin increased as well due to its higher thermal stability compared to saccharides, as well as the resulting lignin condensation.

Innovative Polyelectrolyte Treatment to Flame-Retard Wood

Fire protection has been a major challenge in wood construction for many years, mainly due to the high flame spread risk associated with wood flooring. Wood fire-retardancy is framed by two main axes: coating and bulk impregnation. There is a growing need for economically and environmentally friendly alternatives. The study of polyelectrolyte complexes (PECs) for wood substrates is in its infancy, but PECs’ versatility and eco-friendly character are already recognized for fabric fire-retardancy fabrics. In this study, a new approach to PEC characterization is proposed. First, PECs, which consist of polyethyleneimine and sodium phytate, were chemically and thermally characterized to select the most promising systems. Then, yellow birch (Betula alleghaniensis Britt.) was surface-impregnated under reduced pressure with the two PECs identified as the best options. Overall, wood fire-retardancy was improved with a low weight gain of 2 wt.% without increasing water uptake.

Evaluation of Marblewood Dust’s (Marmaroxylon racemosum) Effect on Ignition Risk

The paper deals with the selected characteristics, such as moisture, average bulk density, and fraction size, of tropical marblewood dust (Marmaroxylon racemosum) that influence its ignition risk. Research was focused on sieve analysis, granulometric analysis, measurement of moisture level in the dust, and determination of the minimum ignition temperatures of airborne tropical dust and dust layers. Samples were prepared using a Makita 9556CR 1400W grinder and K36 sandpaper for the purpose of selecting the percentages of the various fractions (<63, 63, 71, 100, 200, 315, 500 μm). The samples were sized on an automatic vibratory sieve machine Retsch AS 200. More than 65% of the particles were determined to be under 100 μm. The focus was on microfractions of tropical wood dust (particles with a diameter of ≤100 µm) and on the impact assessment of particle size (particle size < 100 µm) on the minimum ignition temperatures of airborne tropical dust and dust layers. The minimum ignition temperature of airborne marblewood dust decreased with the particle size to the level of 400 °C (particle size 63 μm).

A thermally insulating vermiculite nanosheet-epoxy nanocomposite paint as a fire-resistant wood coating

Conventional fire-retardant composite coatings are typically made of organic-based materials that reduce flame spread rates. However, the associated chemical reactions and starting precursors produce toxic and hazardous gases, affecting the environment and contributing to climate change. Wood is one of the most common materials used in construction and households, and thin-film fire-retardant coatings are needed to protect it from fire. Here, we derive high-performance nanocomposite paint-based coatings from naturally occurring and highly insulating layered vermiculite. The coatings are made using different weight percentages of shear-exfoliated vermiculite nanosheets in an epoxy matrix and are brush-coated onto teak wood. A series of tests using coated wooden rods and standard fire retardancy tests confirm a reduction in flame spread and combustion velocity with minimal toxic smoke release. Samples coated with the vermiculite/epoxy nanocomposite paint resist fire propagation, and post-combustion analysis indicates their resistance to thermal degradation. Our results offer a novel and eco-efficient solution to minimize the flame propagation rate, enhancing char development, and expand the scope of applications of ultra-thin vermiculite in nanocomposite coatings as a fire retardant, exploiting its low thermal conductivity in thermal insulation systems.

Durability of Flame-Retarded, Co-Extruded Profiles Based on High-Density Polyethylene and Wheat Straw Residues

At present, little information is available in the scientific literature related to the durability (weathering resistance) of fire-retarded wood and natural fiber-reinforced thermoplastics. In this work, thermoplastic profiles for façade applications based on high-density polyethylene, wheat straw particles, and fire-retardants were extruded and their reaction-to-fire performance before and after artificial weathering evaluated. Profile geometries were either solid or hollow-core profiles, and fire-retardants (FR) were added either in the co-extruded layer or in the bulk. Various FR for inclusion in the co-extruded layer were screened based on UL-94 tests. For profile extrusion, two types of FR were chosen: a coated intumescent combination based on ammonium polyphosphate (APP) and an APP coated with melamine and without formaldehyde. Before weathering, the peak heat release rate (pHRR) and the total heat release (THR), which were determined using cone calorimeter measurements, were reduced by up to 64% and 67% due to the FR. However, even before weathering, pHRR of the profiles was relatively high, with best (lowest) values between 230 and 250 kW/m² under the test conditions. After 28 days of artificial weathering, changes in reaction-to-fire performance and color were evaluated. Use of the APP in the co-extruded layer worsened color change compared to the formulation without APP but the pHRR was not significantly changed. The influence of weathering on the fire behavior was small compared to the difference between fire-retarded and non-fire-retarded materials. Results from the cone calorimeter were analyzed with regard to ETAG 028, which provides requirements related to the durability of fire performance of building products. In many formulations, increase in THR was less than 20% compared to before weathering, which would place some of the profiles in class C or better (EN 13501-1). However, due to the high pHRR, at best, class D was obtained under the conditions of this study. In addition to cone calorimeter measurements, results from the single flame source test, limiting oxygen index determination and thermogravimetric analysis, are shown and discussed. Strength properties, water uptake and swelling of the profiles, thermal conductivity, and energy dispersive X-ray data are also presented.

Approaching Highly Leaching-Resistant Fire-Retardant Wood by In Situ Polymerization with Melamine Formaldehyde Resin

The objective of the work was to improve the leaching resistance of fire-retardant (FR) modified wood by the incorporation of a thermoset resin. Here, Scots pine (Pinus sylvestris L.) sapwood was impregnated with melamine formaldehyde (MF) resin and hydrophilic FRs guanyl-urea phosphate/boric acid by a vacuum-pressure treatment. Resistance to leaching of FR-modified wood was evaluated, after conducting an accelerated aging test according to European standard EN 84. Inductively coupled plasma analysis showed that the incorporation of MF resin significantly reduced the leachability of FRs. Scanning electron microscopy/energy-dispersive X-ray spectrometry revealed that the mechanism of water resistance was by doping the FRs into MF resin microspheres. Fourier transform infrared spectra showed the chemical functionality changes of FR-modified wood such as the formation of methylene bridges by drying the modified wood specimens. An increase in the thermal stability of FR-modified wood was confirmed by thermal gravimetric analysis. Excellent fire performance of FR-modified wood after leaching was affirmed by the limiting oxygen index and cone calorimeter tests.

Flame Retardant Properties of a Guanidine Phosphate-Zinc Borate Composite Flame Retardant on Wood

This work combines guanidine dihydrogen phosphate (GDP) and zinc borate (ZB) to modify wood via microwave-ultrasonic impregnation for realizing favorable flame retardancy and thermal stability, which were investigated by the limiting oxygen index (LOI), thermogravimetric analysis (TGA), and cone calorimetry tests (CONE). The treated samples show better performance in fire retardancy with the LOI value increasing to 47.8%, and the results of TGA indicate the outstanding thermal stability of wood. In addition, the decline of heat release rate, total heat release, smoke production rate, and total smoke production examined by CONE further demonstrates the achievement of excellent flame retardancy and smoke suppression properties of the GDP/ZB-modified wood.

Review of Functional Treatments for Modified Wood

Wood modification is now widely recognized as offering enhanced properties of wood and overcoming issues such as dimensional instability and biodegradability which affect natural wood. Typical wood modification systems use chemical modification, impregnation modification or thermal modification, and these vary in the properties achieved. As control and understanding of the wood modification systems has progressed, further opportunities have arisen to add extra functionalities to the modified wood. These include UV stabilisation, fire retardancy, or enhanced suitability for paints and coatings. Thus, wood may become a multi-functional material through a series of modifications, treatments or reactions, to create a high-performance material with previously impossible properties. In this paper we review systems that combine the well-established wood modification procedures with secondary techniques or modifications to deliver emerging technologies with multi-functionality. The new applications targeted using this additional functionality are diverse and range from increased electrical conductivity, creation of sensors or responsive materials, improvement of wellbeing in the built environment, and enhanced fire and flame protection. We identified two parallel and connected themes: (1) the functionalisation of modified timber and (2) the modification of timber to provide (multi)-functionality. A wide range of nanotechnology concepts have been harnessed by this new generation of wood modifications and wood treatments. As this field is rapidly expanding, we also include within the review trends from current research in order to gauge the state of the art, and likely direction of travel of the industry.

Experimental Study of Oriented Strand Board Ignition by Radiant Heat Fluxes

Wood and composite panel materials represent a substantial part of the fuel in many building fires. The ability of materials to ignite when heated at elevated temperatures depends on many factors, such as the thermal properties of materials, the ignition temperature, critical heat flux and the environment. Oriented strand board (OSB) without any surface treatment in thicknesses of 12, 15 and 18 mm were used as experimental samples. The samples were gradually exposed to a heat flux of 43 to 50 kW.m⁻², with an increase of 1 kW.m⁻². At heat fluxes of 49 kW.m⁻² and 50 kW.m⁻², the ignition times are similar in all OSB thicknesses, in contrast to the ignition times at lower heat fluxes. The influence of the selected factors (thickness and distance from the heat source) was analysed based on the experimentally obtained data of ignition time and weight loss. The experimentally determined value of the heat flux density was 43 kW.m⁻², which represented the critical heat flux. The results show a statistically significant effect of OSB thickness on ignition time.

Combustion Behavior and Thermal Degradation Properties of Wood Impregnated with Intumescent Biomass Flame Retardants: Phytic Acid, Hydrolyzed Collagen, and Glycerol

Wood is a natural renewable material with a porous structure widely used in construction, furniture, and interior decoration, yet its intrinsic flammability poses safety risks. Therefore, environmentally friendly flame retardants have received increasing attention. In this study, a water-soluble flame retardant, consisting of bio-resourced phytic acid (PA), hydrolyzed collagen (HC), and glycerol (GL), was used to improve the flame retardancy of wood ("PHG/wood") through full cell vacuum-pressure impregnation. Morphology and Fourier transform infrared analysis results show that the flame retardant impregnated the wood and adhered evenly to the wood vessels. A PA/HC/GL ratio of 3:1:1 (concentration of the flame retardant solution = 30%) maximized the limiting oxygen index (LOI, 41%) and weight gain (51.32%) for PHG-C30/wood. The flame retardant formed an expansive layer after heating, and the treated wood showed an improved combustion safety performance such that the fire performance index and residue of PHG-C30/wood were 75 and 126.8% higher compared with that of untreated wood, respectively. The peak and total heat release were also significantly reduced by 54.7 and 47.7%, respectively. The PHG/wood exhibited good carbon-forming performance and a high degree of graphitization after combustion. The dense carbon layer provides condensed phase protective action, and non-combustible volatile gases, such as H₂O, CO₂, and NH₃, are released simultaneously to dilute the fuel load in the gas phase. Thus, PHG is shown to be an effective flame retardant for wood.

Suitability and Modification of Different Renewable Materials as Feedstock for Sustainable Flame Retardants

Due to their chemical structure, conventional flame retardants are often toxic, barely biodegradable and consequently neither healthy nor environmentally friendly. Their use is therefore increasingly limited by regulations. For this reason, research on innovative flame retardants based on sustainable materials is the main focus of this work. Wheat starch, wheat protein, xylan and tannin were modified with phosphate salts in molten urea. The functionalization leads to the incorporation of phosphates (up to 48 wt.%) and nitrogen (up to 22 wt.%). The derivatives were applied on wood fibers and tested as flame retardants. The results indicate that these modified biopolymers can provide the same flame-retardant performances as commercial compounds currently used in the wood fiber industry. Besides, the flame retardancy smoldering effects may also be reduced compared to unmodified wood fibers depending on the used biopolymer. These results show that different biopolymers modified in phosphate/urea systems are a serious alternative to conventional flame retardants.

Investigation of proteinaceous paint layers, composed of egg yolk and lead white, exposed to fire-related effects

Fires can have a negative impact on the environment, human health, property and ultimately also on various objects of cultural heritage (CH). This paper deals with an investigation into the degradation of selected proteinaceous paint layers that were exposed to fire-related effects (i.e., fire effluents and/or high temperatures) in a modified cone-calorimeter system. Paint layers of egg yolk adhesive (E) and lead white tempera (E + LW) were exposed to fire-related impacts on top of a CH stack and in a specially designed CH test chamber. On the CH stack, the proteinaceous paint layers were exposed to fire effluents and high temperatures, while in the CH test chamber, the samples were exposed mainly to fire effluents. The molecular changes to the exposed paint layers were analysed by invasive and non-invasive spectroscopic analyses (i.e., FTIR and Raman spectroscopy) and complimented with pyrolysis-GC–MS, while the colour changes were evaluated using colourimetry. It was concluded that the proteinaceous binder degrades into aromatic amino acids and/or fatty acids after exposure to the overall impacts of the fire. Aromatic amino acids were detected by means of the FTIR and py-GC–MS analyses. In the case of the lead white tempera exposure, partial dissociation of the lead white pigment was confirmed by the detection of alteration products, such as lead oxide and lead carbonate. Moreover, the investigation of the E + LW samples exposed for longer times revealed the presence of lead carboxylates. On the other hand, no significant molecular changes were observed with the CH samples exposed to fire effluents in the CH test chamber. The research offered us an insight into the fire-induced effects on selected paints for the first time.

Hazards Resulting from the Burning Wood Impregnated with Selected Chemical Compounds

In the construction industry, a variety wooden products have been used for thousands of years, according to demand, accessibility/availability, and customers’ requirements. Wood is a preferred material due to its large range of properties, depending on the type of wood. It is an easily available and economically competitive material, and it is also extremely strong in relation to its weight. Therefore, it is used in the production of construction materials, building parts, and finishing components, as well as for furniture and decorative elements. Each of these products is commonly additionally chemically treated in order to improve its performance parameters. However, impregnated wooden products such as furniture and fence boards are often misused, including for house heating, waste incineration, bonfires, etc. For this reason, among the products of combustion, there is a whole range of different chemical compounds, frequently carcinogenic, and dangerous for health and the environment, for example, heavy metals. Knowledge in this field is important for professions, such as: firefighter, lifeguard, people dealing with environmental management, and units responsible for waste landfills. On the other hand, important recipients of this information are ordinary residents who, due to a lack of knowledge, use such materials as e.g., heating material.

Ignition of Deposited Wood Dust Layer by Selected Sources

The main waste of wood sanding technology is wood dust. The formation of wood dust affects its behaviour. Wood dust can be in a turbulent form and behaves explosively or in a settled form where it becomes flammable. Dust particles are barely detectable by the naked eye, wood dust still presents substantial health, safety, fire and explosion risks to employees. This article deals with the evaluation of ignition temperature and surface temperature of deposited wood dust samples by selected ignition sources. The influence of selected physical properties of wood dust, the size of the contact area between the ignition source and the combustible material, the spatial arrangement during the ignition and the application time of the ignition source are analysed. The paper describes the behaviour of a 15 mm deposited layer of wood dust of spruce (Picea abies L.), beech (Fagus silvatica L.). oak (Quercus petraea Liebl.) caused by three potential ignition sources—a hot surface, an electric coil and a smouldering cigarette. Prior to the experimental determination of the ignition temperature, dust moisture content which did not significantly affect the ignition phase of the samples, as well as sieve analysis of tested samples were determined. The lowest minimum ignition temperature on the hot plate, as an important property of any fuel, because the combustion reaction of the fuel becomes self-sustaining only above this temperature, was reached by the oak dust sample (280 °C), the highest by the spruce dust sample (300 °C). The ignition process of wood dust was comparable in all samples, differing in the ignition time and the area of the thermally degraded layer. The least effective ignition source was a smouldering cigarette.

Expandable Graphite as a Fire Retardant for Cellulosic Materials—A Review

A diversity of chemicals is used to produce fire retardants (FRs); some of the main group of chemicals are hazardous to the environment as well as to human life; however, expandable graphite (EG) can be a gateway to a more environmentally friendly FRs or intumescent fire retardants (IFRs). Researchers define intumescent as the swelling of a particular substance placed between a heat source and an underlying substrate when they are heated. EG is a material with extraordinary thermophysical and mechanical properties. The referred EG properties are unparalleled. EG is a low-density carbon material having a series of unique properties: developed specific surface, binder-free pressing capacity, stability to aggressive media, and low thermal conductivity. Therefore, EG is a promising material both for research work and for industrial applications. The primary goal of this literature review was to report current knowledge on the use of EG as a fire retardant for cellulose and cellulose-modified materials. EG is produced, among other methods, by thermal shock of graphite oxide under forming gas. When exposed to heat, EG will expand. The expansion mechanism was presented in this review. Equally important to this review is the knowledge related to cellulose thermal degradation and cellulose impact on the development of science and technology.

Thermal Characteristics of Expandable Graphite-Wood Particle Composites

According to the Fire Statistics Yearbook of the National Fire Agency of the Republic of Korea, the total number of fires in 2018 was 42,338, which resulted in 2500 victims and amounted to property damages of approximately 560 billion KRW. The number of fires in buildings where wood was used as a finishing material was 28,013 (66%) in that period. To minimize human and property damage, composite materials were prepared by mixing wood particles with expandable graphite. The physical and thermal properties of the composite materials were investigated. It was observed that the expansion rate increased by 341.7% according to the expandable graphite content. Additionally, the total heat released and the thermal conductivity decreased from 38.63 to 2.5 MJ/m² and from 24.62 to 7.8 W/m·K. The time to inactivity of white mouse in the smoke toxicity test was 14.9 min and exceeded the toxicity standard for flame retardant performance. The expandable graphite added to composite materials adopted worm-like shapes as a result of combustion, and it formed a fine lattice layer structure with 16–22 μm gaps that could reduce thermal conductivity. In addition, we can minimize the damage to people and property in the event of a fire.

Technical Performance Overview of Bio-Based Insulation Materials Compared to Expanded Polystyrene

The energy efficiency of buildings is well documented. However, to improve standards of energy efficiency, the embodied energy of materials included in the envelope is also increasing. Natural fibers like wood and hemp are used to make low environmental impact insulation products. Technical characterizations of five bio-based materials are described and compared to a common, traditional, synthetic-based insulation material, i.e., expanded polystyrene. The study tests the thermal conductivity and the vapor transmission performance, as well as the combustibility of the material. Achieving densities below 60 kg/m3, wood and hemp batt insulation products show thermal conductivity in the same range as expanded polystyrene (0.036 kW/mK). The vapor permeability depends on the geometry of the internal structure of the material. With long fibers are intertwined with interstices, vapor can diffuse and flow through the natural insulation up to three times more than with cellular synthetic (polymer) -based insulation. Having a short ignition times, natural insulation materials are highly combustible. On the other hand, they release a significantly lower amount of smoke and heat during combustion, making them safer than the expanded polystyrene. The behavior of a bio-based building envelopes needs to be assessed to understand the hygrothermal characteristics of these nontraditional materials which are currently being used in building systems.

Waterborne Intumescent Coatings Containing Industrial and Bio-Fillers for Fire Protection of Timber Materials

Flammability and combustion of softwood treated with intumescent coatings were studied in the present work. The formulations applied onto wood surfaces contained different ratios of industrial fillers, titanium dioxide TiO₂ and aluminium trihydroxide Al(OH)₃, and/or bio-fillers, eggshell and rice husk ash. Combustion behaviours of unprotected and protected wood samples have been examined with the aid of cone calorimetry performed under the varied levels of thermal flux ranging from 30 to 50 kW/m². The char residues obtained after the completion of cone calorimetry test at 40 kW/m² were analysed by the Raman spectroscopy. The fire protective properties of the studied coatings were strongly influenced by the nature of the fillers as well as by the intensity of thermal irradiance. The incorporation of bio-based fillers into the water-based intumescent formulations significantly improved fire resistance of wood substrates. For example, at 30 kW/m², the Effective Heat of Combustion was reduced by more than 40%, whilst the average Peak to Heat Release Rate had dropped from 193.2 to 150.3 kW/m² for the wood sample protected with the formulation incorporating two industrial and two bio-fillers. Moreover, an application of the studied coatings resulted in a notable reduction of the back surface temperature of the wood specimens.

Cookstove Smoke Impact on Ambient Air Quality and Probable Consequences for Human Health in Rural Locations of Southern Nepal

Residential emission from traditional biomass cookstoves is a major source of indoor and outdoor air pollution in developing countries. However, exact quantification of the contribution of biomass cookstove emissions to outdoor air is still lacking. In order to address this gap, we designed a field study to estimate the emission factors of PM2.5 (particulate matter of less than 2.5 µ diameter) and BC (black carbon) indoors, from cookstove smoke using biomass fuel and with smoke escaping outdoors from the roof of the house. The field study was conducted in four randomly selected households in two rural locations of southern Nepal during April 2017. In addition, real-time measurement of ambient PM2.5 was performed for 20 days during the campaign in those two rural sites and one background location to quantify the contribution of cooking-related emissions to the ambient PM2.5. Emission factor estimates indicate that 66% of PM2.5 and 80% of BC emissions from biomass cookstoves directly escape into ambient air. During the cooking period, ambient PM2.5 concentrations in the rural sites were observed to be 37% higher than in the nearby background location. Based on the World Health Organization (WHO)'s AirQ+ model simulation, this 37% rise in ambient PM2.5 during cooking hours can lead to approximately 82 cases of annual premature deaths among the rural population of Chitwan district.

Flame Retardancy of Wood Fiber Materials Using Phosphorus-Modified Wheat Starch

Biopolymer-based flame retardants (FR) are a promising approach to ensure adequate protection against fire while minimizing health and environmental risks. Only a few, however, are suitable for industrial purposes because of their poor flame retardancy, complex synthesis pathway, expensive cleaning procedures, and inappropriate application properties. In the present work, wheat starch was modified using a common phosphate/urea reaction system and tested as flame retardant additive for wood fibers. The results indicate that starch derivatives from phosphate/urea systems can reach fire protection efficiencies similar to those of commercial flame retardants currently used in the wood fiber industry. The functionalization leads to the incorporation of fire protective phosphates (up to 38 wt.%) and nitrogen groups (up to 8.3 wt.%). The lowest levels of burning in fire tests were measured with soluble additives at a phosphate content of 3.5 wt.%. Smoldering effects could be significantly reduced compared to unmodified wood fibers. The industrial processing of a starch-based flame retardant on wood insulating materials exhibits the fundamental applicability of flame retardants. These results demonstrate that starch modified from phosphate/urea-systems is a serious alternative to traditional flame retardants.

Bioinspired Struvite Mineralization for Fire-Resistant Wood

High-performance wood materials have attracted significant attention in recent years because of excellent property profiles achieved by relatively easy top-down processing of a renewable resource. A crucial flaw of the renewable wood scaffolds is the low flame retardancy, which we tackled by bioinspired mineralization in an eco-friendly processing step. The formation of the biomineral struvite, commonly found in urinary tract stones, was used for the infiltration of hierarchical wood structures with the necessary ions followed by an in situ synthesis of struvite by ammonium steam fumigation. Struvite decomposes prior to wood, which absorbs heat and releases nonflammable gas and amorphous MgHPO₄ resulting from the degradation, which promotes insulating char formation. As a result, the mineralized wood can hardly be ignited and the treatment strongly suppresses the heat release rate and smoke production.

Protecting Offspring Against Fire: Lessons From Banksia Seed Pods

Wildfires are a natural component in many terrestrial ecosystems and often play a crucial role in maintaining biodiversity, particularly in the fire-prone regions of Australia. A prime example of plants that are able to persist in these regions is the genus Banksia. Most Banksia species that occur in fire-prone regions produce woody seed pods (follicles), which open during or soon after fire to release seeds into the post-fire environment. For population persistence, many Banksia species depend on recruitment from these canopy-stored seeds. Therefore, it is critical that their seeds are protected from heat and rapid oxidation during fire. Here, we show how different species of Banksia protect their seeds inside follicles while simultaneously opening up when experiencing fire. The ability of the follicles to protect seeds from heat is demonstrated by intense 180 s experimental burns, in which the maximum temperatures near the seeds ranged from ∼75°C for B. serrata to ∼90°C for B. prionotes and ∼95°C for B. candolleana, contrasting with the mean surface temperature of ∼450°C. Many seeds of native Australian plants, including those of Banksia, are able to survive these temperatures. Structural analysis of individual follicles from these three Banksia species demonstrates that all of them rely on a multicomponent system, consisting of two valves, a porous separator and a thin layer of air surrounding the seeds. The particular geometric arrangement of these components determines the rate of heat transfer more than the tissue properties alone, revealing that a strong embedment into the central rachis can compensate for thin follicle valves. Furthermore, we highlight the role of the separator as an important thermal insulator. Our study suggests that the genus Banksia employs a variety of combinations in terms of follicle size, valve thickness, composition and geometric arrangement to effectively protect canopy-stored seeds during fire.

Preparation and characterisation of flame retardant encapsulated with functionalised silica-based shell

Intumescent fire retardant (IFR) coatings are nowadays considered as the most effective flame retardant (FR) treatment. Nevertheless, the principal compound in an IFR system, ammonium polyphosphate (APP), is highly sensitive to moisture and IFR coating effectiveness decreases quickly. The main objective of this study is to encapsulate APP in a hybrid silica-based membrane by sol-gel process using alkoxysilane tetraethoxysilane (TEOS) and methyltriethoxysilane (MTES) precursor. The morphology and structure of APP and microencapsulated ammonium polyphosphate (MAPP) were assessed by scanning electron microscopy and Fourier transforms infrared spectroscopy (FTIR). X-ray photoelectron spectroscopy (XPS) results revealed that APP was well encapsulated inside the polysiloxane shells. The thermal degradation of APP and MAPP was evaluated by thermogravimetric analysis. At 800 °C, the MAPP had higher char residue (70.49 wt%) than APP (3.06 wt%). The hydrophobicity of MAPP increased significantly with the water contact angles up to 98°, in comparison to 20° for APP.

Phytoremediation potential of poplar and willow species in small scale constructed wetland for boron removal

Boron (B) pollution is an expanding environmental problem throughout the world due to intensive mining practices and extensive usage of B in agricultural chemicals and industrial products in recent years. The purpose of this study was to investigate B removal performance of four poplar and four willow species in small scale Constructed Wetland (CW). Rooted cuttings of tested species were treated with simulated wastewater having five elevated B concentrations (0.5, 5, 10, 20 and 40 ppm). All the tested species could resist up to 20 ppm wastewater B supply and could regrow from their roots in the soil having maximum 15 mg/kg B content. The result of the study indicated that 65% ± 5.3 of B was removed from the wastewater in 5 ppm B treatment while the same efficiency decreased to 45% ± 4.6 at 40 ppm B supply. The average effect of sediment on B removal was found to be approximately 20% for all B treatments while the remaining part of the loaded B was removed from the CW within effluent (35-54%). Therefore, actual effects of plant species on B removal was ranged from 45% to 25% between 5 and 40 ppm B treatments. Mass B removal within plant body (phytextraction) comprised the 13-10% of total loaded B in CW while the remaining part of the loaded B (31-15%) was stabilized into the sediment with the effects of poplar and willow roots. These results presented clear understanding of effective B purification mechanisms in CWs. Boron phytextraction capacity of a plant species was less effective than its phytstabilization efficiency which increase filtering capacity of the sediment and stabilization of more B around the rhizosphere. In terms of their B removal ability, P.nigra and S.anatolica had the highest B removal capacities with phytextraction (20-11%) while S.alba, P.alba and S.babylonica had more phytstabilizaiton performance (40-15%) in CW. Disposal of B loaded plant material create another environmental costs for CW applications. Therefore, B loaded wood and leaf tissues were mixed and used for production of wooden panels in the study. Then a combustion test was applied on these panels to test their fire resistance. The results of the tests revealed much higher burning tolerance of the B loaded panels (5-20%) compared to controls. Annual harvesting, fast growing and deep rooting ability of the poplar and willow species with their high phytstabilization and phytextraction efficiencies make these species excellent tools to remove B from the polluted waters. Utilization of these species for B removal in large scale CWs is quite possible which should be also investigated in further studies.

Nanostructured Wood Hybrids for Fire-Retardancy Prepared by Clay Impregnation into the Cell Wall

Eco-friendly materials need "green" fire-retardancy treatments, which offer opportunity for new wood nanotechnologies. Balsa wood (Ochroma pyramidale) was delignified to form a hierarchically structured and nanoporous scaffold mainly composed of cellulose nanofibrils. This nanocellulosic wood scaffold was impregnated with colloidal montmorillonite clay to form a nanostructured wood hybrid with high flame-retardancy. The nanoporous scaffold was characterized by scanning electron microscopy and gas adsorption. Flame-retardancy was evaluated by cone calorimetry, whereas thermal and thermo-oxidative stabilities were assessed by thermogravimetry. The location of well-distributed clay nanoplatelets inside the cell walls was confirmed by energy-dispersive X-ray analysis. This unique nanostructure dramatically increased the thermal stability because of thermal insulation, oxygen depletion, and catalytic charring effects. A coherent organic/inorganic charred residue was formed during combustion, leading to a strongly reduced heat release rate peak and reduced smoke generation.

Graphene-Borate as an Efficient Fire Retardant for Cellulosic Materials with Multiple and Synergetic Modes of Action

To address high fire risks of flamable cellulosic materials, that can trigger easy combustion, flame propagation, and release of toxic gases, we report a new fire-retardant approach using synergetic actions combining unique properties of reduced graphene oxide (rGO) and hydrated-sodium metaborates (SMB). The single-step treatment of cellulosic materials by a composite suspension of rGO/SMB was developed to create a barrier layer on sawdust surface providing highly effective fire retardant protection with multiple modes of action. These performances are designed considering synergy between properties of hydrated-SMB crystals working as chemical heat-sink to slow down the thermal degradation of the cellulosic particles and gas impermeable rGO layers that prevents access of oxygen and the release of toxic volatiles. The rGO outer layer also creates a thermal and physical barrier by donating carbon between the flame and unburnt wood particles. The fire-retardant performance of developed graphene-borate composite and mechanism of fire protection are demonstrated by testing of different forms of cellulosic materials such as pine sawdust, particle-board, and fiber-based structures. Results revealed their outstanding self-extinguishing behavior with significant resistance to release of toxic and flammable volatiles suggesting rGO/SMB to be suitable alternative to the conventional toxic halogenated flame-retardant materials.