Flame-retardant recyclable epoxy networks: Strategies, mechanisms, and future directions

Songshan Lake Materials Laboratory

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Advanced flame-retardant recyclable epoxy resin system.

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Credit: Qingshan Yang, Pingan Song and Siqi Huo from University of Southern Queensland; Hao Wang from Swinburne University of Technology

Research teams from Swinburne University of Technology and University of Southern Queensland have provided a deep overview of the current state of the art of fire-retardant recyclable epoxy systems (FRREs) based on covalent adaptable networks. By integrating dynamic covalent bonds (DCBs) and flame-retardant groups into the epoxy crosslinking network can effectively improve fire safety and recyclability. However, how to balance the recyclability, flame retardancy, and network stability of FRREs remains a key challenge. This review provides valuable insights into the directional design of high-stability FRREs.

Epoxy resins (EPs) are fundament material in modern industries owing to their exceptional properties and thermal stability. These qualities make them indispensable in electronics, automotive, aerospace, and structural engineering applications. They are widely used in the fields of adhesives, coatings, electronics and electrical, construction, and carbon fibers-reinforced polymers (CFRPs) due to its excellent processability, mechanical properties, adhesive performance, corrosion resistance, and dimensional stability. However, there are two main drawbacks of EP materials in practical industrial applications: (i) high flammability due to their carbon-hydrogen-based structure; and (ii) non-recyclability/reprocessability caused by their permanently crosslinked network. These problems raise safety concerns and environmental issues. Currently, efforts to enhance fire safety often involve adding flame-retardant additives, which can compromise mechanical strength and complicate recycling processes.

Despite advancements in flame-retardant strategies, sustainability remains elusive. Conventional epoxy systems are predominantly thermosetting, rendering them difficult to recycle, and their fixed network structures tend to deteriorate over time, especially under environmental stress. The integration of reversible bonds through dynamic covalent chemistry (DCC) offers a promising avenue to imbue epoxy networks with recyclability and self-healing capabilities. Yet, achieving a delicate balance between flame retardancy, recyclability, and in-service performance has remained a significant hurdle. Depending on the difference of dynamic covalent reactions, FRREs can be categorized into carboxylic ester-based, phosphate ester-based, imine-based, disulfide-based, and Diels-Alder-mediated exchange. Nevertheless, the introduction of weak reversible linkages and labile flame-retardant structures inevitably compromise the structural integrity of the networks, resulting in reduced thermal stability, creep resistance, and long-term durability in harsh service environments. This contradiction between dynamic recyclability and network stability represents the central obstacle preventing FRREs from broader engineering applications. Following this, researchers proposed some feasible and potential solutions to construct high-stability FRREs, such as (i) controlling catalytic activity, (ii) incorporating high-energy DCBs, (iii) introducing rigid or conjugated groups, (iv) utilizing noncovalent interactions, (v) constructing hyperbranched and interpenetrating networks.

By rational structure design, future FRREs are expected to achieve the combined goals of excellent in-service performance, outstanding flame retardancy, and high-efficiency recyclability. Nevertheless, several challenges and technical barriers related to achieving an optimal balance between recyclability, flame retardancy, and long-term stability continue to hinder the large-scale application and commercialization of high-stability FRREs. To advance the practical implementation of these materials, future research should focus on optimizing molecular design strategies, conducting systematic mechanistic studies, employing theoretical modeling, and developing scalable manufacturing processes.

This review provides a comprehensive roadmap for the directional design and fabrication of FRREs, while also suggesting promising research pathways for developing high-stability FRREs.

This work has been recently published in the online edition of Materials Futures, a new international journal in the field of interdisciplinary materials science research.

Reference: Qingshan Yang, Yong Guo, Guofeng Ye, Cheng Wang, Asim Mushtaq, Min Hong, Pingan Song, Hao Wang, Siqi Huo. Fire-retardant recyclable epoxy systems based on covalent adaptable networks[J]. Materials Futures. DOI: 10.1088/2752-5724/ae17de

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Journal

Materials Futures

DOI

10.1088/2752-5724/ae17de 

Article Title

Fire-retardant recyclable epoxy systems based on covalent adaptable networks

Libcom.orgfiles.libcom.org/files/Bookchin M. Our Synthetic Environment.pdf

Our Synthetic Environment - Libcom.org

Modern agriculture may be distinguished from earlier forms of cultivation by its reliance on chemistry for soil nutrients and the control of insect infestations ...

 

Electrolysis can solve one of our biggest contamination problems

ETH Zurich

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Removing insecticides from contaminated soils – Patrick Domke (pictured) and other ETH researchers found the solution in electrolysis.

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Credit: Hannes Cullum / IVY FILMSTUDIO GmbH

They were once considered miracle workers – insecticides such as lindane or DDT were produced and used millions of times during the 20th century. But what was hailed as progress led to a global environmental catastrophe: persistent organic pollutants (POPs) are so chemically stable that they remain in soil, water and organisms for decades. They accumulate in the fatty tissue of animals and thus enter the human food chain. Many of these substances were banned long ago, but their traces can still be found today – even in human blood.

How to remediate such contaminated sites, be they soils, bodies of water or landfills, is one of the major unresolved questions of environmental protection. How can highly stable poisons be rendered harmless without creating new problems? Researchers at ETH Zurich, led by Bill Morandi, Professor of Synthetic Organic Chemistry, have now found a promising approach. Using an innovative electrochemical method, they are not only able to break down these long-lived pollutants but also to convert them into valuable raw materials for the chemical industry.

Converting pollutants into raw materials

A key distinction between this and previous work is that the carbon skeleton of the pollutants is recycled and made reusable, while the halide component is sequestered as a harmless inorganic salt. “The previous methods were also energetically inefficient,” says Patrick Domke, a doctoral student in Morandi’s group. He explains: “The processes were expensive and still led to outcomes that were harmful to the environment.” 

Together with electrochemistry specialist Alberto Garrido-Castro, a former postdoc in this group, Domke developed a process that renders the pollutants in question completely harmless. During this project, the two researchers were able to draw on the many years of experience of ETH professor Morandi, who has been working on the transformation of such compounds for years. “The key advance of this new technology is the use of alternating current to sequester the problematic halogen atoms as innocuous salts such as NaCl (table salt), while still generating valuable hydrocarbons,” says Morandi. 

Using electricity to break down toxins  

Electrolysis enables almost complete dehalogenation of pollutants under mild, environmentally friendly and cost-effective conditions. It cleaves the stable carbon-halogen bonds, leaving behind only harmless salts such as table salt and useful hydrocarbons such as benzene, diphenylethane or cyclododecatriene. These are actually sought-after intermediates in the chemical industry, for example, for plastics, varnishes, coatings and pharmaceutical applications. In this way, the technology not only contributes to the remediation of contaminated sites but also to the sustainable circular economy.

“What makes our process so special from a technical point of view is that we supply electricity using alternating current, similar to the electrical waveform delivered to households. It is one of the most cost-effective resources in chemistry,” explains Garrido-Castro. “Alternating current protects the electrodes from wear, which is why we can reuse them for many subsequent electrolysis cycles. In addition, the alternating current suppresses unwanted side reactions and the formation of poisonous chlorine gas, allowing the pollutant’s halogen atoms to be fully converted to inorganic salts.” The reactor used by the researchers consists of an undivided electrolysis cell in which dimethyl sulfoxide (DMSO) is used as a solvent – itself a by-product of the pulp process in paper production. 

A fully thought-out circular economy

The process can be applied not only to pure substances but also to mixtures from contaminated soils. Soil or sludge can therefore be treated without pre-treatment or further separation processes. A prototype of the reactor has already been successfully tested on classic environmental toxins such as lindane and DDT. “Our system is mobile and can be assembled on site. This eliminates the need to transport these hazardous substances,” explains Domke. 

Uniteddiversity.cooplibrary.uniteddiversity.coop/More_Books_and_Reports/Silent_Spring-Rachel_Carson-1962.pdf

Silent Spring - United Diversity Library

RACHEL CARSON. Page 8. Foreword. IN 1958, when Rachel Carson undertook to write the book that became Silent Spring, she was fifty years old. She had spent most ...

Nrdc.orgwww.nrdc.org/stories/story-silent-spring

The Story of Silent Spring - NRDC

Silent Spring took Carson four years to complete. It meticulously described how DDT entered the food chain and accumulated in the fatty tissues of animals, ...

Libcom.orgfiles.libcom.org/files/Bookchin M. Our Synthetic Environment.pdf

Our Synthetic Environment - Libcom.org

Modern agriculture may be distinguished from earlier forms of cultivation by its reliance on chemistry for soil nutrients and the control of insect infestations ...

 

Persistent environmental toxins accumulate in tissues already in the fetal stage

University of Oulu, Finland

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Persistent organic pollutants (POPs) begin to accumulate in the tissues of mammals already during the foetal stage, according to new research from the University of Oulu, Finland. The animal-model study found that environmental toxins had built up in the tissues of sheep raised in clean organic production, and that the same substances were transferred in notable amounts to the developing foetuses’ adipose tissue.

Persistent environmental toxins, such as PCBs and DDT, remain in nature for long periods without breaking down. They can accumulate in the fatty tissues of organisms and bioaccumulate through the food chain. These substances were previously used in industry and as insecticides, and although their use is now strictly regulated, they remain widespread in the environment.

The study analysed tissue samples from 15 organic ewes and their lambs shortly after birth, searching for the most common POPs. Almost all of the substances investigated were detected in both adult sheep and lamb tissues. All the compounds identified were able to cross the placenta, and the transfer was so effective that concentrations in the lambs’ tissues averaged 30–103 per cent of those measured in the mothers.

Previous research in humans has shown that environmental toxins present in maternal circulation can pass through the placenta. What this study newly demonstrates is that, in sheep, compounds accumulated in the mother’s adipose tissue are transferred to the developing foetus in almost the same proportion.

Because placental structure in sheep differs from that in humans, no direct conclusions can be drawn regarding human exposure. However, concentrations of POPs in adult human adipose tissue are on average higher than in sheep, underscoring the need for further research.

‘The results illustrate the widespread distribution of persistent environmental toxins and the ways in which they infiltrate every part of our surroundings. In epidemiological studies, POP concentrations measured from umbilical cord blood after birth have been linked to obesity, metabolic syndrome and lower IQ. What remains for future research to determine is the extent of the health effects that POPs accumulated in adipose tissue may have on the foetus and on the child’s later health,’ summarises doctoral researcher Ella Vuoti.

Original article: Ella Vuoti, Jerry Nguyen, Panu Rantakokko, Heikki Huhta, Panu Kiviranta, Juha Räsänen, Sanna Palosaari, Petri Lehenkari. Adipose tissue deposition and placental transfer of persistent organic pollutants in ewes. Environmental Research, Volume 287, 2025. 

Journal

Environmental Research

DOI

10.1016/j.envres.2025.123164 

Enantioselective maternal transfer of pesticide metabolite and its thyroid effects

KeAi Communications Co., Ltd.

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HOW THE TWO MIRROR-IMAGE FORMS OF O,P'-DDD DIFFER IN THEIR TRANSFER TO OFFSPRING AND THEIR EFFECTS ON THE THYROID SYSTEM IN ZEBRAFISH.

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Credit: Lili Niu, et al

A new study published in Environmental Chemistry and Ecotoxicology has revealed that two mirror-image forms of a persistent pesticide metabolite behave differently when they move from mother fish to their offspring — demonstrating their environmental risks. Chiralmolecules that are mirror images of each other, like left and right hands.

“We were motivated by a simple question,” says lead author Lili Niu. “Many pesticides exist in two mirror-image forms, but environmental assessments usually treat them as if they're the same. We wanted to know whether that assumption is actually safe, especially across generations.”

To answer this, the researchers fed adult zebrafish diets containing each form of o,p'-DDD for four weeks. They then measured how much of the chemical accumulated in the adults and how much was passed to their developing embryos, as well as tracked hatching success, deformities, survival, and changes in thyroid hormones, which are essential for healthy growth.

“We found that offspring consistently carried even higher chemical levels than their parents, showing that maternal transfer was highly efficient,” shares Niu. “The S-enantiomer accumulated 134-176% more in adults and over 100% more in their larvae than its mirror-image R-form. This led to more severe outcomes in the next generation, including increased mortality, malformations, and reduced hatching success in the S-DDD exposed groups.”

To understand why, the team used computer-based modeling to simulate how each form of the chemical interacts with key proteins involved in producing and regulating thyroid hormones. “These simulations showed that S-DDD binds more strongly to several of these proteins, helping explain its greater biological impact,” says Niu. “What surprised us most was how consistently the S-form caused stronger effects at every level we tested.”

Notably, a small structural difference in the molecule led to very large differences in how much accumulated in the fish, how it affected their hormone system, and how their offspring developed.

The team emphasised that understanding enantiomer-specific effects will help improve ecological risk predictions for long-lasting pollutants and support the development of more accurate environmental standards.

“If we ignore these differences, we risk underestimating long-term harm to wildlife,” adds Niu. “Our work shows that even very low exposure in parents can create meaningful risks for the next generation.”

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Contact the author: Lili Niu, Interdisciplinary Research Academy (IRA), College of Biological and Environmental Engineering, Zhejiang Shuren University, China, niulili@zjsru.edu.cn

The publisher KeAi was established by Elsevier and China Science Publishing & Media Ltd to unfold quality research globally. In 2013, our focus shifted to open access publishing. We now proudly publish more than 200 world-class, open access, English language journals, spanning all scientific disciplines. Many of these are titles we publish in partnership with prestigious societies and academic institutions, such as the National Natural Science Foundation of China (NSFC).

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Journal

Environmental Chemistry and Ecotoxicology

DOI

10.1016/j.enceco.2025.10.021 

Method of Research

Experimental study

Subject of Research

Animals

Article Title

Enantioselective maternal transfer of chiral pesticide metabolite o,p’-DDD in zebrafish (Danio rerio): Implications for intergenerational thyroid disruption.

COI Statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper

Uniteddiversity.cooplibrary.uniteddiversity.coop/More_Books_and_Reports/Silent_Spring-Rachel_Carson-1962.pdf

Silent Spring - United Diversity Library

RACHEL CARSON. Page 8. Foreword. IN 1958, when Rachel Carson undertook to write the book that became Silent Spring, she was fifty years old. She had spent most ...

Nrdc.orgwww.nrdc.org/stories/story-silent-spring

The Story of Silent Spring - NRDC

Silent Spring took Carson four years to complete. It meticulously described how DDT entered the food chain and accumulated in the fatty tissues of animals, ...

Libcom.orgfiles.libcom.org/files/Bookchin M. Our Synthetic Environment.pdf

Our Synthetic Environment - Libcom.org

Modern agriculture may be distinguished from earlier forms of cultivation by its reliance on chemistry for soil nutrients and the control of insect infestations ...