The new material, described in the journal Advances in Industrial and Engineering Chemistry, belongs to a family of polymers known as non-isocyanate polyurethanes, or NIPUs. The concept dates back to 1957, when chemists E. Dyer and H. Scott first demonstrated that cyclic carbonates and amines could be reacted to form urethane bonds without any phosgene chemistry. In the decades since, the approach has attracted renewed attention as a safer, more sustainable route to polyurethane-type materials. Instead of combining polyols with isocyanates, NIPU chemistry couples carbonates with amines, sidestepping the most dangerous reagents in the conventional process. But the technology has struggled to catch up in one crucial application: rigid foams for building insulation, where conventional polyurethane foams still dominate the market.

The problem, the researchers explain, is that non-isocyanate rigid polyurethane foams developed so far have suffered from two crippling weaknesses: low mechanical strength and poor flame retardancy. Building insulation must withstand compressive loads behind walls and cladding, and it must resist ignition in an age of increasingly strict fire codes. Earlier attempts to make green foams from glucose or starch produced materials that either crumbled under pressure or burned too readily for commercial use. Glucose-based foams made with maleic acid and glutaraldehyde, for instance, showed low thermal stability, while even tannin-based foams, though somewhat more fire-resistant, still fell short of the flame-retardant performance of commercial foams treated with conventional additives.

The Kyonggi team, led by corresponding author Sang-Bum Kim with first author Sung Yeol Lim, turned to an unlikely hero: tannic acid, the polyphenolic compound familiar to anyone who has tasted an over-steeped cup of tea. Tannic acid brings a rare combination of properties to polymer chemistry. Its multiple aromatic rings lend rigidity and strength to the polymer network, while its abundance of hydroxyl groups makes it highly reactive, readily forming bonds with carbonate reagents and crosslinking agents. Most intriguingly, tannic acid is naturally flame-retardant: it can suppress flames, absorb heat generated during combustion, and promote the formation of a stable char layer that shields the underlying material from heat and oxygen. In other words, the fire protection is built into the molecule itself rather than bolted on with halogenated additives.

The synthesis proceeded in two stages. First, the researchers dissolved tannic acid in water and reacted it with dimethyl carbonate at 65 degrees Celsius to produce carbonated tannin, a carbonate-bearing intermediate. Spectroscopic analysis confirmed the transformation: the infrared spectrum showed the disappearance of the carbonyl peak characteristic of dimethyl carbonate at 1740 wavenumbers and the emergence of a new carbonyl peak at 1711 wavenumbers, while proton nuclear magnetic resonance revealed a methoxy signal at 3.46 parts per million absent from the starting material. The carbonated tannin was then reacted with hexamethylenediamine at 90 degrees Celsius for two hours, yielding a tannic acid-based non-isocyanate polyurethane resin whose urethane bonds were confirmed by characteristic infrared peaks for the carbonyl, nitrogen-hydrogen, and carbon-oxygen groups.

The critical innovation lay in the formulation of the foam itself. The team blended the resin with three crosslinking agents, each playing a distinct role: hexamethylenetetramine and glutaraldehyde extend the polymer chains and build crosslink density, while citric acid reacts with the amine groups in the resin and acts as a foaming inducer, generating the gas bubbles that turn a liquid resin into a lightweight foam. By systematically varying the amounts of these three agents across six formulations, the researchers could dissect exactly how each one shaped the final material. The differences were dramatic. In the formulation lacking glutaraldehyde, the foam expanded rapidly and then collapsed entirely, demonstrating that sufficient crosslinking is essential to stabilize the expanding bubble structure. All formulations containing adequate crosslinker produced stable foams of similar density.

Compressive testing revealed a clear winner: the formulation with the highest crosslinking agent content, designated F-1, exhibited the highest compressive strength of all. Scanning electron microscopy explained why. The F-1 foam displayed the smallest and most uniform cells, a microstructure that distributes mechanical stress evenly across the material and prevents cracks from propagating. The microscopy also illuminated the role of each additive. Without hexamethylenetetramine, the crosslinking reaction slowed, reducing reaction heat and delaying curing so that growing cells merged into larger, weaker pores. Without citric acid, no cells formed at all, confirming its role as the foaming trigger. And with less glutaraldehyde, cell sizes grew larger for the same kinetic reasons. The message was consistent: faster, denser crosslinking yields finer, stronger foam.

The fire performance was where the material truly distinguished itself. In thermogravimetric analysis, the tannic acid foam left behind roughly 25 percent char residue after heating to 550 degrees Celsius in nitrogen, compared with only about 15 percent for a conventional polyurethane foam of similar density, reflecting the char-forming tendency of tannic acid’s aromatic structure. Adding citric acid shifted the maximum decomposition rate of the urethane bonds to higher temperatures, from around 285 to 303 degrees Celsius, confirming that the crosslinked network improved thermal stability. In limited oxygen index testing, the new foam required a higher oxygen concentration to sustain burning than the conventional foam, and derivative analysis attributed the protection to tannic acid’s aromatic rings, which promote a stable char barrier, and to phenoxy radicals that quench the oxygen free radicals released during polymer decomposition.

The ignition test provided the most vivid demonstration. When a butane torch flame was applied directly to the specimens for twenty seconds, the conventional polyurethane foam burned rapidly, retaining only 23.3 percent of its original mass. The tannic acid-based foam resisted ignition, and any flames that did appear extinguished themselves within roughly two seconds, leaving about 90 percent of the mass intact. Photographs taken after the test show the conventional foam reduced to charred wreckage while the tannic acid foam retained its shape, a striking visual testament to the power of molecularly embedded fire protection. Notably, flame retardancy varied little across the different crosslinking formulations, suggesting that the fire resistance stems primarily from the tannic acid backbone itself.

The study’s conclusions point toward a future where building insulation no longer requires either phosgene-derived isocyanates or halogenated flame retardants. By tuning the balance of hexamethylenetetramine, citric acid, and glutaraldehyde, the researchers showed that crosslinking agents can simultaneously enhance mechanical strength, thermal stability, and structural integrity, addressing the two key limitations that have kept non-isocyanate foams out of commercial buildings. Challenges remain before tannic acid foams line the walls of skyscrapers, including scaling the synthesis and meeting the full battery of building-code certifications. But the demonstration that a plant-derived polyphenol can deliver both strength and self-extinguishing fire behavior in an isocyanate-free foam marks a significant advance for sustainable materials chemistry, and a compelling argument that the safest flame retardant may be the one grown in a tree.

Subject of Research: Tannic acid-based non-isocyanate rigid polyurethane foam with tunable crosslinking for improved mechanical strength and flame retardancy

Article Title: Study on the mechanical properties and flame retardancy of tannic acid-based non-isocyanate rigid polyurethane foam according to the content of crosslinking agents

Article References: Lim, S. Y., Park, S. B., Kim, M. S., & Kim, S.-B. (2025). Study on the mechanical properties and flame retardancy of tannic acid-based non-isocyanate rigid polyurethane foam according to the content of crosslinking agents. Advances in Industrial and Engineering Chemistry, 1(1), Article 15. https://doi.org/10.1007/s44405-025-00015-y

Image Credits: AI Generated

DOI: 10.1007/s44405-025-00015-y

Keywords: non-isocyanate polyurethane, tannic acid, rigid foam, crosslinking agents, flame retardancy, mechanical properties, building insulation, sustainable polymers, thermal stability, char formation, green chemistry, phosgene-free synthesis

The appeal of basalt fiber begins with its origin. Unlike glass fiber, which requires carefully formulated raw materials, basalt fiber is produced by melting naturally occurring volcanic basalt rock at high temperatures and drawing it into continuous filaments. This single-source production route eliminates the need for complex batch formulations, and the resulting fiber offers a combination of properties that frequently matches or exceeds those of conventional glass fiber. The review highlights that basalt fibers deliver higher specific stiffness than glass fibers, meaning they provide greater rigidity per unit of weight, a critical advantage in any application where every kilogram matters, from aircraft interiors to logistics pallets and vehicle body panels.

What makes the thermoplastic angle particularly compelling is recyclability. Thermoset composites, which have historically dominated structural applications, cure permanently and are notoriously difficult to reprocess at end of life. Thermoplastic matrices, by contrast, soften when heated and can be remelted, reshaped, and in many cases welded, opening the door to genuine circular-economy manufacturing. The review emphasizes that combining basalt fiber with thermoplastics such as polypropylene, polyamide, polyetheretherketone, polylactic acid, and polyphenylene sulfide yields parts that are lighter, stiffer, more flame-resistant, and ultimately more sustainable than many incumbent solutions. The authors also point to cost reduction and improved interface durability as key performance advantages that could accelerate industrial adoption.

The chemistry of the fiber-matrix interface emerges as one of the central technical themes of the review. Basalt fibers, like all inorganic reinforcements, do not naturally bond strongly to polymer matrices, and weak interfaces translate directly into poor load transfer and disappointing mechanical performance. Researchers have therefore developed an extensive toolkit of surface treatments: silane coupling agents that chemically bridge fiber and matrix, polyurethane-based sizings tailored to elastomeric matrices, plasma treatments that activate the fiber surface, and even electrophoretic deposition of graphene onto individual filaments. Recent work reviewed by the authors shows that carefully engineered sizing agents, including hybrid flexible-rigid formulations and organic-inorganic networks built from carbon nanotubes, can dramatically improve interfacial shear strength and, with it, the tensile, flexural, and impact performance of the finished composite.

Processing technology forms another pillar of the review. Short-fiber composites, in which chopped basalt filaments are dispersed into the melt, can be processed on conventional extruders and injection molding machines, making them attractive for high-volume automotive parts. However, fiber breakage and orientation during processing strongly influence final properties, and the authors document how parameters such as melt temperature, screw speed, and mold design control fiber length distribution and therefore stiffness and strength. At the other end of the spectrum, continuous basalt fiber fabrics and tapes can be consolidated into high-performance laminates by compression molding, automated tape laying, or additive manufacturing. The review notes that 3D printing of continuous basalt fiber-reinforced polylactic acid has already demonstrated impressive mechanical properties, with raster orientation and fiber volume fraction emerging as decisive design variables.

The range of thermoplastic matrices explored with basalt fiber is striking. In commodity polypropylene, basalt fiber improves stiffness, thermal stability, and flame retardancy, with maleic-anhydride-grafted compatibilizers and elastomer toughening further refining the balance of properties. In engineering plastics such as polyamide 6 and 66, basalt reinforcement competes directly with glass fiber, and studies of in-situ polymerized polyamide 6 composites show that the fiber surface state strongly affects thermomechanical performance. At the high-performance end, basalt fiber has been combined with polyetheretherketone, polyetherimide, polyethersulfone, and polyphenylene sulfide to create laminates with exceptional wear resistance, flame retardancy, and even damage self-sensing capability, where the conductive network within the composite changes its electrical response as damage accumulates, effectively allowing the material to report its own condition.

Sustainability considerations run throughout the review, and the authors document a growing body of work on bio-based matrices. Basalt fiber has been successfully incorporated into polylactic acid, polybutylene succinate, polybutylene adipate-co-terephthalate, and bio-based polyamides, producing composites that pair a mineral reinforcement of volcanic origin with polymers derived from renewable resources. Studies of hybrid systems combining basalt with natural fibers such as flax, hemp, and wood fiber show synergistic effects, while research on recycled woven basalt fiber composites demonstrates that mechanical performance can be substantially retained through reprocessing cycles. The review also cites work on fire-retarded biobased polyamide 11 composites, which retained their structure, properties, and fire behavior after repeated reprocessing, an encouraging sign for closed-loop manufacturing.

Environmental durability, long a question mark for basalt composites, receives a balanced treatment. The authors summarize studies showing that basalt fibers and their composites can withstand corrosive environments, saltwater immersion, wet-dry cycling, ultraviolet weathering, and freeze-thaw exposure, with performance depending strongly on fiber chemistry and matrix selection. Marine applications feature prominently: basalt fiber-reinforced thermoplastic and epoxy composites have been evaluated for seawater ageing, marine decks, and structural components, and comparative studies suggest that thermoplastic matrices can offer advantages in long-term water resistance. In construction, basalt fiber-reinforced polymer bars and tendons have been investigated as concrete reinforcement, with research addressing creep, relaxation, and fatigue behavior under sustained and cyclic loads.

The application landscape described in the review is remarkably broad. In the automotive sector, basalt fiber-reinforced thermoplastics are positioned for interior and structural components where weight reduction drives fuel savings and emissions compliance. In aerospace, thermoplastic composites are gaining ground because they can be welded rather than adhesively bonded, speeding assembly, and basalt fiber offers a cost-effective alternative to carbon fiber for less critical structures. Hybrid armor systems combining basalt and Kevlar fabrics demonstrate ballistic performance benefits, while basalt composites are also being explored for wind turbine components, railway applications, sports equipment, thermal shielding, and even biomedical hard-tissue repair using polylactic acid matrices. The logistics sector, the authors note, stands to benefit from multifunctional, cost-effective parts that reduce vehicle weight across entire fleets.

Despite this momentum, the review is candid about the obstacles standing between laboratory promise and industrial standardization. Basalt fiber production still lacks the standardized grading and quality control enjoyed by the glass fiber industry, making it difficult for designers to specify materials with confidence. Variability in rock composition translates into variability in fiber properties, and the authors identify standardization as a key challenge that must be solved before widespread adoption can occur. Interface engineering remains part science and part craft, with sizing formulations often optimized empirically for specific fiber-matrix pairs. Yet the trajectory is clear: as the authors conclude, the combination of mechanical performance, thermal stability, flame resistance, recyclability, and raw material abundance positions basalt fiber-reinforced thermoplastic composites as one of the most promising material platforms of the coming decade, provided that industry and researchers can agree on the standards needed to trust rock spun from a volcano as a precision engineering material.

Subject of Research: Basalt fiber-reinforced thermoplastic composites: properties, processing, and applications

Article Title: Review: fundamentals, applications, and progress of basalt fiber-reinforced thermoplastic composites

Article References: Tayfun, Ü., Akar, A. Ö., & Yılmaz, V. M. (2026). Review: fundamentals, applications, and progress of basalt fiber-reinforced thermoplastic composites. Journal of Materials Science. https://doi.org/10.1007/s10853-026-13835-0

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13835-0

Keywords: basalt fiber, thermoplastic composites, fiber-matrix interface, sizing agents, polypropylene, polyamide, recyclability, flame retardancy, automotive applications, aerospace materials, additive manufacturing, sustainable materials

The material at the heart of the study is a hybrid nanocomposite that combines epoxy resin with two very different nanoscale fillers: multi-walled carbon nanotubes, or MWCNTs, and nanoclay. Each filler brings a distinct talent. Carbon nanotubes form percolating conductive networks that allow the coating to carry electrical current and respond to changes in temperature or chemical exposure. Nanoclay platelets, when properly dispersed into intercalated or exfoliated structures, act as physical barriers that slow heat transfer and the diffusion of combustible gases, while also stiffening the polymer matrix. The challenge has always been that these benefits do not scale independently. Adding more of one filler can disrupt the dispersion of the other, shift the curing behavior of the epoxy, or push one property past its optimum while another collapses.

Traditionally, researchers have navigated this compositional space through trial and error, or through statistical design-of-experiments methods such as response surface methodology. The Vietnamese team adopted a four-factor Box–Behnken design, an efficient experimental layout that samples the corners and center of a multi-dimensional formulation space without requiring every possible combination to be tested. That design was coupled with response surface modeling, but the authors went further by training a Random Forest model, an ensemble machine learning method that builds many decision trees on randomized subsets of the data and averages their predictions. The Random Forest approach substantially outperformed classical response surface methodology, achieving coefficients of determination, or R² values, of 0.986 for limiting oxygen index, 0.995 for tensile strength, and 0.998 for the logarithm of electrical conductivity.

Those numbers matter because they describe how confidently the model can predict real material behavior from formulation variables alone. An R² approaching 1.0 means nearly all of the variation in the measured property is captured by the model. In practical terms, the researchers could ask the algorithm what happens if the nanotube loading rises while the nanoclay fraction falls, and receive a reliable answer without mixing a single new batch of resin. To make the model interpretable rather than a black box, the team applied SHAP analysis, a technique borrowed from explainable artificial intelligence that quantifies how much each input factor contributes to each prediction, including the direction and nature of its influence across the formulation range.

The optimization itself relied on multi-objective desirability functions, a strategy that seeks a balanced formulation window rather than maximizing any single property in isolation. This distinction is central to the study’s philosophy. A coating with record-breaking flame retardancy but brittle mechanics would fail in service, just as a tough but flammable coating would fail certification. By defining desirability across fire performance, mechanical strength, conductivity, and sensing response simultaneously, the framework identified a composition where all properties land in an acceptable, mutually compatible zone. The authors report that this machine learning-assisted workflow reduced the experimental burden by approximately 50 to 65 percent compared with a full-factorial exploration of the same design space, a substantial saving in laboratory time, materials, and cost.

The optimized coating delivered impressive measured performance. Its limiting oxygen index, the minimum oxygen concentration in an atmosphere that sustains candle-like burning, reached 28.0 plus or minus 0.2 percent, a threshold associated with genuinely flame-retardant behavior. In cone calorimetry testing, the peak heat release rate was reduced to 720 plus or minus 9 kilowatts per square meter, indicating that the material releases heat far more slowly when exposed to fire. Mechanically, the coating achieved a tensile strength of 80.0 plus or minus 1.4 megapascals and a flexural modulus of 2.80 plus or minus 0.14 gigapascals, confirming that fire safety did not come at the expense of structural integrity. Electrical conductivity settled near 1.0 times ten to the minus four siemens per meter, a level low enough for insulation purposes yet sufficient for sensing.

That sensing capability is where the material becomes genuinely futuristic. The coating exhibited a thermal sensing response of 8.0 plus or minus 0.5 percent and an ammonia response of 12.0 plus or minus 0.9 percent, meaning its electrical resistance shifts measurably when temperature changes or when ammonia gas is present. This behavior arises from the percolating network of carbon nanotubes embedded in the epoxy. When the coating is heated, stretched, or exposed to certain vapor molecules, the tunneling gaps and contact geometry between nanotubes change, altering the conductive pathways. A coating that can report its own temperature, detect structural strain, or flag the presence of hazardous chemicals transforms a passive protective layer into an active element of a monitoring system, with obvious implications for fire early warning and structural health monitoring.

Crucially, the predictions held up in the laboratory. Prediction errors generally remained below 5 percent across the key properties, and the team reinforced confidence through repeated cross-validation and independent validation experiments on formulations the model had not seen during training. Structural analyses then explained why the optimized composition works so well. Microscopy and related characterization confirmed intercalated and exfoliated nanoclay domains dispersed through the epoxy, continuous conductive pathways formed by the carbon nanotubes, and strengthened interfaces between the epoxy matrix and both filler types. These three structural features jointly govern the multifunctional performance, and the machine learning model effectively learned to navigate the trade-offs among them without needing an explicit physical theory of each interaction.

The broader significance of the work lies in its reproducibility as a strategy rather than in any single formulation. Epoxy resins are ubiquitous in coatings, adhesives, electronics encapsulation, and composite matrices, and fire safety regulations continue to tighten across industries. Halogenated flame retardants, once standard, face increasing environmental scrutiny, pushing researchers toward nanofiller-based solutions such as clays, carbon nanotubes, graphene, and layered double hydroxides. At the same time, the literature on machine learning in polymer nanocomposites has grown rapidly, with studies applying neural networks and ensemble models to predict tribological, thermal, and dielectric properties. What distinguishes this study is the integration of the entire pipeline, from efficient experimental design through interpretable machine learning to multi-objective optimization and experimental confirmation, applied to a coating that must satisfy five demanding criteria at once.

The authors acknowledge support from Hanoi University of Industry through its Advanced Materials and Sustainable Technologies research group, and they report no competing interests. Their framework, they argue, offers a reproducible template for designing fire-safe, durable, and self-sensing epoxy nanocomposite coatings, and the approach extends naturally to other multifunctional material systems where competing property targets make exhaustive experimentation impractical. As laboratories worldwide confront similar multi-criteria design problems, the message of this research is clear: the fastest route through a vast formulation space may no longer be a systematic march through every combination, but a well-trained algorithm that knows where to look.

Subject of Research: Machine learning-assisted optimization of epoxy/MWCNT–nanoclay hybrid nanocomposite coatings for combined fire safety, mechanical performance, and self-sensing functionality

Article Title: (Machine learning-assisted optimization of multifunctional Epoxy/MWCNT–nanoclay nanocomposites for fire-safe, self-sensing coatings)

Article References: Nguyen, T. A., Dang, H. T., & Nguyen, V. H. (2026). (Machine learning-assisted optimization of multifunctional Epoxy/MWCNT–nanoclay nanocomposites for fire-safe, self-sensing coatings). Polymer Bulletin, 83(11), Article 633. https://doi.org/10.1007/s00289-026-06687-w

Image Credits: AI Generated

DOI: 10.1007/s00289-026-06687-w

Keywords: epoxy nanocomposites, multi-walled carbon nanotubes, nanoclay, flame retardancy, self-sensing coatings, machine learning, Random Forest, response surface methodology, Box–Behnken design, SHAP interpretability, structural health monitoring, polymer coatings

Glass fiber-reinforced unsaturated polyester composites are workhorse materials across the automotive, marine, construction and electrical industries. They combine high specific strength with low weight, corrosion resistance and cost-effective processing, which has made them a favored substitute for metals as manufacturers of electric and hybrid vehicles chase carbon emission targets and longer driving ranges. Yet the carbon-rich organic resin that binds these laminates together is inherently flammable. When polyester burns it can release dense smoke and toxic gases, a serious concern in passenger compartments, building panels and other enclosed spaces where fire safety regulations are strict.

Historically, the industry leaned on halogenated flame retardants such as brominated compounds, which are highly effective at quenching flames. Those chemicals have fallen out of favor, however, because combustion releases corrosive hydrogen chloride and related gases that threaten both human health and the environment, and restrictions under the European Union’s REACH framework have accelerated the shift toward halogen-free alternatives. Inorganic hydroxides and borates have emerged as leading candidates, and the Turkish team focused on two of the most widely available: aluminum hydroxide, supplied by Eti Maden with an average particle size of 12 to 18 micrometers, and zinc borate, supplied by Melos Kimya with particles averaging 3.2 micrometers.

Each additive fights fire through a distinct physical mechanism. ATH decomposes endothermically between roughly 220 and 300 degrees Celsius, absorbing heat from the flame zone while releasing water vapor and leaving behind an alumina residue that acts as a thermal barrier on the surface. Zinc borate operates over a higher temperature window, releasing crystal water between about 290 and 450 degrees Celsius and converting to boron oxide, which forms a glassy, dense layer that limits contact between the underlying polymer and oxygen. Because the two mechanisms complement one another, previous studies have reported synergistic effects when the minerals are used together, although the synergy typically becomes significant only when the combined filler content exceeds ten percent by weight.

To test these ideas in a structural laminate, the researchers produced composite panels by hand lay-up, embedding eight layers of 300 grams per square meter plain-weave E-glass fabric at a fixed 30 percent volume fraction in a commercial unsaturated polyester resin. The additives were dispersed into the resin using magnetic stirring at 650 revolutions per minute followed by 40 minutes of ultrasonication to prevent agglomeration, then cured with methyl ethyl ketone peroxide and cobalt naphthenate at room temperature before a two-hour post-cure at 50 degrees Celsius. Each mineral was tested alone at 5, 10 and 15 percent by weight, and hybrid formulations mixed the two additives in a one-to-one ratio at the same total loadings.

Mechanical testing revealed a familiar trade-off. The untreated laminate reached a flexural strength of 500.41 megapascals, and every flame-retardant formulation fell below that benchmark. The gentlest penalty came from 5 percent ATH, which retained about 96 percent of the neat flexural strength at 479.98 megapascals, the best value among all fire-retardant composites. Strength declined progressively as loading increased, dropping to 394.85 megapascals at 15 percent ATH, while zinc borate proved more damaging to bending performance, yielding values between 364.38 and 394.79 megapascals across its loading range. The 15 percent hybrid formulation managed only 302.62 megapascals, roughly 60 percent of the neat laminate, reflecting the stress concentrations that rigid inorganic particles introduce into the resin matrix and the resulting weakening of fiber-matrix load transfer under bending.

Impact resistance told a strikingly different story. In unnotched Charpy tests, the neat composite absorbed 135.41 kilojoules per square meter, but adding ATH raised that figure by roughly 43, 52 and 78 percent at 5, 10 and 15 percent loadings respectively, peaking at 241.03 kilojoules per square meter. The researchers attribute this toughening to crack deflection and tortuous fracture paths around the rigid inclusions, which dissipate energy even as the same particles create stress concentrations that erode bending strength. Zinc borate, whose glassy structure tends to facilitate crack propagation, delivered a more modest boost to 162.92 kilojoules per square meter at 15 percent, while the hybrid system reached 151.35 kilojoules per square meter, still comfortably above the untreated laminate.

The fire performance results were the centerpiece of the study. Under the UL-94 horizontal burning test, the neat laminate burned at 20.15 millimeters per minute. Fifteen percent ATH reduced that to 12.15 millimeters per minute and 15 percent zinc borate achieved 10.77, but the 15 percent hybrid formulation delivered the lowest rate of all at 10.23 millimeters per minute, a reduction of about 49 percent. To quantify the hybrid advantage, the team calculated a semi-empirical synergy coefficient comparing the measured burning rate with the average of the two single-additive systems at the same total loading. Values above one indicate performance beyond simple superposition, and the hybrids scored approximately 1.23 at 5 percent, 1.13 at 10 percent and 1.12 at 15 percent, supporting a genuine synergistic trend across the entire loading range.

Post-combustion examination of the charred residues helped explain why the combination works so well. ATH alone leaves a porous, cracked alumina layer that offers only partial protection, while zinc borate produces a glassy, dense and continuous borate phase that also promotes surface carbonization of the polymer. In the hybrid formulations, these two morphologies merge into a more compact, crack-resistant barrier that limits the transfer of heat and oxygen to the underlying material. The boron oxide layer effectively seals the surface while the endothermic water release from ATH cools the flame zone, a dual mechanism that prior studies had described and that this work confirms directly in a glass fiber-reinforced polyester system.

The authors position their findings within a deliberate low-loading strategy, keeping total inorganic content at or below 15 percent where conventional approaches often demand more than 30 percent of a single filler to achieve comparable fire performance. High filler loadings degrade resin fluidity, complicate processing and take a heavier toll on mechanical properties, so achieving a nearly 50 percent reduction in burning rate at modest loadings represents a practical formulation window for applications that require both fire safety and structural reliability. The team notes that UL-94 horizontal burning provides a screening-level indicator, and future work will incorporate limiting oxygen index and cone calorimetry to quantify heat release and smoke production. With relevance spanning automotive interior trim, structural panels, transportation components and electrical applications, the study offers a robust framework for comparing flame-retardant laminate formulations and demonstrates that ATH and zinc borate, used together in optimized proportions, can meaningfully improve fire behavior without sacrificing the mechanical integrity that makes glass fiber polyester composites so widely useful.

Subject of Research: Halogen-free flame retardancy and mechanical performance of aluminum hydroxide and zinc borate additives in glass fiber-reinforced unsaturated polyester composites

Article Title: Flame retardancy and mechanical effects of ZnB and ATH additives in glass fiber-reinforced polyester composites

Article References: Ari, B., Yilmaz, A. N. Y., & Bedeloglu, A. C. (2026). Flame retardancy and mechanical effects of ZnB and ATH additives in glass fiber-reinforced polyester composites. Journal of Materials Science: Polymers, 1(1), Article 9. https://doi.org/10.1007/s44493-026-00013-6

Image Credits: AI Generated

DOI: 10.1007/s44493-026-00013-6

Keywords: flame retardancy, glass fiber composites, unsaturated polyester, aluminum hydroxide, zinc borate, UL-94 test, flexural strength, Charpy impact test, hybrid additives, halogen-free flame retardants, automotive composites, synergistic effect

The research centers on microencapsulated phase change materials, or mPCMs, tiny core-shell particles in which a substance that melts and freezes at a useful temperature is locked inside a polymer wall. Phase change materials absorb large amounts of latent heat as they melt and release that heat as they solidify, acting as thermal batteries that buffer temperature swings. The catch is that the most common PCMs are solid-liquid paraffins that leak when they melt, which is why encapsulation is essential. Microencapsulation solves the leakage problem, dramatically increases the surface area available for heat transfer, and allows the particles to be blended directly into concrete, coatings, textiles, or packaging without any special handling.

What makes the Turkish team’s work novel is the shell material. Most commercial and laboratory mPCMs rely on formaldehyde-based resins such as melamine-formaldehyde or urea-formaldehyde, which can release harmful substances during synthesis and use, or on polystyrene and PMMA, which are flammable. The researchers instead dissolved waste PVC, sourced from a commercial leather company, together with 1-tetradecane, a paraffin that melts at around 5 degrees Celsius, in tetrahydrofuran and used a solvent evaporation technique to precipitate PVC shells around droplets of the paraffin. It is the first time waste PVC has been used this way for a low-temperature, cold-storage PCM, and the choice is doubly clever because PVC is inherently flame retardant, with a limiting oxygen index between 40 and 45, far above the threshold of 26 that separates flame-retardant polymers from ordinary combustible ones.

The synthesis itself is a piece of practical process chemistry. Solution A contained 26 grams of sodium chloride and 9 grams of gelatin dissolved in deionized water, with the salt raising the ionic strength to keep the water from dissolving the THF and the gelatin acting as a natural, biodegradable surfactant that stabilizes the emulsion. Solution B held the waste PVC powder and 1-tetradecane in THF. When B was dripped into the vigorously stirred A, droplets formed whose size depended on how they were added: a Pasteur pipette produced large particles, a micropipette produced small ones. The emulsion was then heated to 56 degrees Celsius so the THF evaporated, leaving behind hardened capsules that were filtered, washed with water and ethanol, and dried. Remarkably, the final particle diameter tracked the initial droplet size, giving the team a simple dial for tuning capsule dimensions, something conventional in-situ polymerization cannot easily achieve.

Two particle populations emerged. The small-particle capsules, designated mPCM/SP, averaged 145 micrometers, while the large-particle capsules, mPCM/LP, averaged 612 micrometers. Both were unimodally distributed, a sign of a well-controlled process. Fourier transform infrared spectroscopy confirmed that the characteristic C-Cl stretching peak of PVC at 1750 per centimeter persisted in the capsules and that the CH2 stretching bands of the paraffin core remained intact, indicating that core and shell coexist without strong chemical interaction, exactly what a good encapsulation should deliver. Differential scanning calorimetry showed that the capsules behave isothermally like the pure paraffin, with melting temperatures of 5.0 and 4.7 degrees Celsius and latent heats of 126.7 and 136.9 joules per gram for the large and small particles respectively. Encapsulation ratios reached 66.5 percent for the large particles and 71.9 percent for the small ones, squarely within the 60 to 90 percent range typical of the best polymer-shelled paraffin microcapsules in the literature.

One subtlety the authors confront head-on is supercooling. The encapsulated paraffin froze at slightly lower temperatures than the bulk material, a well-documented consequence of the shell adding thermal resistance and constraining nucleation within the confined core. Crucially, however, the capsules still solidified above 1 degree Celsius, comfortably inside the operating window for cold-chain and anti-icing service, and the shift is partly an artifact of the relatively fast 5 degrees per minute DSC scanning rate, which is known to exaggerate apparent supercooling. Repeated cycling showed deviations of less than 1 degree Celsius in transition temperatures and about 1.2 percent in latent heat, confirming that the phase change process remains fully reversible and that encapsulation has not degraded the PCM’s intrinsic properties.

Durability testing was equally convincing. The capsules endured 1,000 accelerated thermal cycles between -20 and 30 degrees Celsius, the equivalent of roughly 15 years of daily freeze-thaw service, with DSC signals reproduced almost perfectly after every 100 cycles and FT-IR spectra after 1,000 cycles showing no chemical change. Leak tests were brutal in their simplicity: samples were frozen at -18 degrees Celsius for 12 hours, then baked at 50 degrees Celsius for another 12 hours on filter paper. No visible PCM leakage appeared, and weight losses were a negligible 0.05 to 0.08 percent. Thermogravimetric analysis showed the capsules degrade at higher temperatures than the free paraffin, meaning the PVC shell genuinely protects the core.

The flame retardancy results may be the study’s most distinctive contribution. Because standard limiting oxygen index tests require rectangular samples that microparticles cannot form, the team built a calibration curve by burning reference plastics of known LOI, including expanded polystyrene, polyacrylonitrile, PET, nylon 66, and ABS, and correlating burning times with literature values. Using the resulting equation, they estimated LOI values of 27.6 percent for mPCM/SP and 28 percent for mPCM/LP, above the flame-retardancy threshold and a dramatic improvement over pure 1-tetradecane, whose LOI of 17 means it burns in ordinary air. The mechanism is intrinsic: when PVC decomposes it releases hydrogen chloride gas, which suppresses combustion and promotes a protective char layer. Unlike conventional flame-retardant PCM systems that rely on added ammonium polyphosphate or expandable graphite, this fire resistance comes free with the recycled shell material itself.

The application vision spans two very different worlds. In the sub-5-degree melting range, the capsules are natural candidates for cold-chain packaging, refrigerated transport containers, and insulated panels, where they would absorb heat whenever temperatures rise and release it back as things cool, smoothing out the fluctuations that spoil food and vaccines. In civil infrastructure, mixed into concrete pavements, bitumen binders, or surface coatings, the latent heat released as the capsules freeze could slow the drop of road surface temperature, delay ice nucleation, and reduce ice adhesion, potentially cutting dependence on corrosive chloride deicers. Earlier work by Farnam and colleagues demonstrated the pavement concept with other PCMs, and the Turkish team’s flame-retardant, waste-derived capsules would bring an added safety margin to exactly that use case.

Beyond the immediate applications, the study is a template for what materials scientists call waste valorization: converting an environmental liability into a high-value functional material. PVC is notoriously difficult to recycle through conventional plastic streams because its chlorine content and rheology make it incompatible with aliphatic polyesters and polyolefins, so it is usually landfilled or incinerated. Here, that same chlorine chemistry becomes an asset, delivering flame retardancy that other shell polymers must buy with additives. The process also avoids the nastier solvents common in encapsulation chemistry, using halogen-free THF whose high vapor pressure makes recovery straightforward. As the authors conclude, the work validates a route from a persistent waste problem to thermally reliable, fire-safe, size-tunable microcapsules, and it broadens the reach of phase change material technology into the low-temperature regime where cold chains, winter roads, and a warming world’s cold-storage demands increasingly intersect.

Subject of Research: Flame-retardant microencapsulated phase change materials synthesized from waste poly(vinyl chloride) for low-temperature thermal energy storage and anti-icing applications

Article Title: Novel flame retardant microencapsulated phase change materials from waste poly(vinyl chloride) for maintaining cold and anti-icing applications

Article References: Alkan, C., & Alakara, E. H. (2026). Novel flame retardant microencapsulated phase change materials from waste poly(vinyl chloride) for maintaining cold and anti-icing applications. Journal of Materials Science: Polymers, 1(1), Article 16. https://doi.org/10.1007/s44493-026-00017-2

Image Credits: AI Generated

DOI: 10.1007/s44493-026-00017-2

Keywords: phase change materials, waste PVC recycling, microencapsulation, thermal energy storage, flame retardancy, 1-tetradecane, anti-icing, cold chain, latent heat, solvent evaporation, supercooling, sustainable materials