Introduction

Plastic foams are extensively employed in various sectors such as construction, transportation, and packaging due to their lightweight properties, ease of processing, and capabilities in impact resistance, sound absorption, and thermal insulation1,2,3,4,5,6. Notably, the trade of expanded polystyrene (EPS) foam alone in the United States, the European Union, and China surpassed $270 million7 in 2022 (Fig. 1a, b). However, a substantial concern arises from the fact that over 90% of these products are derived from non-renewable petrochemical sources8, including widely used foams like polypropylene (PP), EPS, and polyurethane (PU)9,10,11 (Fig. 1a). The production processes of these materials not only generate heavy carbon emissions, thereby exacerbating climate change12, but also produce non-biodegradable wastes that break down into microplastics, endangering ecosystems and sustainable development due to their environmental persistence and toxicity13,14,15,16.

Fig. 1: Overview and merits of All-Cel foam.
Fig. 1: Overview and merits of All-Cel foam.
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a Global plastic production categorized by polymer types in 2023. b Total import and export statistics for EPS in the USA, EU, and China. c Fabrication of All-Cel foam through solvent-induced cellulose molecular assembly. The schematic diagram was conceived by the authors and created using Adobe Photoshop CC 2019 and Adobe Illustrator 2023 software. d Radar plots comparing the key attributes of All-Cel foam with those of conventional plastic foams.

In response to the growing crisis of plastic pollution, the Fifth United Nations Environment Assembly has advocated for robust measures to eliminate such pollution17. Replacing these materials with renewable biomass, such as cellulose—the most abundant polysaccharide on Earth—presents a sustainable alternative18,19,20,21,22,23,24,25. Unfortunately, transforming cellulose into durable, shapeable foam often involves complex pre-treatments (such as chemical oxidation and mechanical fibrillation) or the use of harmful crosslinking and foaming agents, which pose additional environmental and health risks26,27,28,29,30,31. Additionally, to prevent pore collapse during drying process, energy-intensive methods such as freeze-drying and supercritical drying are commonly employed, which limits the scalable applications of cellulose-based foams32. Conversely, employing solvent exchange or induction to regulate hydrogen bonding interactions between cellulose molecules facilitates the design and arrangement of cellulose at the molecular level23,33,34,35,36,37, resulting in ordered or gradient network structures. This molecular-level induced recombination offers the potential to unlock promising avenues for the development of high-performance cellulosic foams. Nevertheless, cellulosic foams still encounter challenges in integrating several desirable properties, including lightweight design, high strength, moldability, and thermal insensitivity.

Here, we introduce a gentle and scalable molecular-scale approach for creating cellulosic foams through ethanol-induced cellulose molecular programmed assembly and the construction of a gradient porous structure. This technique not only facilitates the preservation of network entanglement among cellulose molecules but also enables direct foaming under ambient conditions, culminating in a product we have termed “All-Cel foam”. Created from prevalent cellulose and employing scalable, recyclable molecular techniques, All-Cel foam offers an environmentally friendly solution for energy-efficient applications, notably in building construction (Fig. 1c).

Due to the entangled cellulose molecular network and the evolved honeycomb-like gradient porous structure, All-Cel foam distinguishes itself by exhibiting a high compressive modulus of 11.8 MPa, a low density of 0.12 ± 0.01 g/cm3, excellent thermal stability up to 264.1 °C, a low thermal conductivity ranging in 0.047–0.062 W m−1 K−1, and along with various forming and processing characteristics. Moreover, its biodegradable and recyclable molecular nature promotes a sustainable material cycle, significantly reducing environmental impacts. Compared to conventional plastic foams such as PP, EPS, and PU, All-Cel foam excels in both performance and eco-sustainability1,27 (Fig. 1d). This breakthrough presents vast potential for revolutionizing the applications of foams in construction and transportation, aligning with global efforts towards more sustainable materials.

Results

Fabrication and characterization of All-Cel foam

The conceptual diagram presented in Fig. 2a depicts the fabrication of All-Cel foam from biomass cellulose via ethanol-induced a molecular assembly process. An aqueous solution of zinc chloride/formic acid (ZnCl2/FA) is used to disrupt both intermolecular and intramolecular hydrogen bonds within cellulose at room temperature, creating a homogeneous cellulose molecular system (noted as cellulose/ZnCl2/FA). During this treatment, the cellulose molecules undergo esterification with formic acid. This chemical modification is confirmed by Fourier transform infrared spectroscopy, which shows a distinct absorption peak at 1712 cm−1, and by solid-state ¹³C nuclear magnetic resonance spectroscopy, which reveals a new peak at 164 ppm. These spectral features are indicative of the conversion of cellulose’s hydroxyl groups (–OH) into formate ester groups (–OCHO), as detailed in Supplementary Fig. 1. This conversion facilitates the subsequent programmed self-assembly of cellulose molecules and the construction of honeycomb-like gradient morphology in All-Cel foam.

Fig. 2: Design and structural characterizations of All-Cel foam.
Fig. 2: Design and structural characterizations of All-Cel foam.
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a Schematic illustration depicting the fabrication of All-Cel foam through the solvent-induced molecular assembly and the construction of gradient structure. These images were conceived by the authors and created using Adobe Illustrator 2023 software. b SEM image showcasing the dense surface structure of All-Cel foam. c Digital photograph displaying the cross-section of All-Cel foam. d SEM images highlighting the gradient cross-sectional structure of All-Cel foam. e Graph showing the density values corresponding to foams with varying thicknesses. f Two-dimensional (2D) wide angle X-ray scattering (WAXS) pattern of All-Cel foam, illustrating its crystalline structure. g Digital images demonstrating the functionality of All-Cel foam; on the left, a 1.3 g sample of the foam supports a 500 g weight, and on the right, it remains atop a dandelion, emphasizing its high mechanical strength and lightweight characteristics.

Following the creation of homogeneous cellulose molecular system, ethanol molecules are introduced into this molecular system to induce the assembly of cellulose molecules (left of Fig. 2a). Ethanol molecules competitively disrupt the coordination and hydrogen bonding between cellulose molecules and ZnCl2/FA solvent, while forming new hydrogen bonding with –OH and –OCHO groups of cellulose molecules. This shifts the hydrogen bonding interaction from cellulose-solvent to cellulose-cellulose interactions, inducing chain conformational contraction and facilitating supramolecular entanglement. At the cellulose/ZnCl2/FA-ethanol interface, these effects occur most rapidly, leading to a dense cellulose hydrogen bonding network at the surface (Fig. 2a, middle part and Fig. 2b).

The formation of a dense molecular network constrains cellulose chain mobility and acts as a barrier to hinder the diffusion and penetration of ethanol molecules into the interior of cellulose molecular system. As a result, ethanol exchange and cellulose molecular rearrangement in deeper regions proceed more gradually. The extended kinetic window allows greater chain relaxation and coarsening of the pore network38,39. This spatial variation in network formation kinetics exhibits gradient assembly characteristics and forms an entangled molecular network structure–near the surface regions develop tightly packed architecture with smaller pores, and the interior shows larger pores (right of Fig. 2a). Ultimately, this process results in All-Cel foam with the honeycomb-like gradient porous structure presented in Fig. 2c, d. Then, Supplementary Fig. 2a further demonstrates that pore size increase with increasing distance from the foam surface. Furthermore, the microscopic morphology of All-Cel foam’s pore walls also exhibits a porous structure (Fig. 2di–iii), with pore sizes increasing from approximately 120 nm near the surface to 600 nm at the base (Supplementary Fig. 2b). The density of All-Cel foam (with 7 wt% cellulose content) gradually decreases with increasing thickness, ultimately stabilizing at a low density of 0.12 ± 0.01 g/cm3 (Fig. 2e). This behavior suggests that the formylated cellulose molecules in the ZnCl2/FA system gradually reassemble, resulting in a densely packed surface and an internal gradient porous structure. In contrast, cellulose dissolved in a ZnCl2/H2O solvent without formylation experiences pronounced volumetric shrinkage and forms a dense bulk material following ethanol-induced assembly (Supplementary Fig. 3) due to the rapid reformation of hydrogen bonds between the abundant hydroxyl groups on the unmodified cellulose chains. As shown in Supplementary Figs. 4–6, time-resolved in situ imaging combined with statistical analysis confirms the pore evolution process, demonstrating All-Cel foam develops gradient porous structures across cellulose molecular systems of different concentrations. At higher cellulose concentrations, less ethanol influx is required to trigger the cellulose-cellulose assembly to form a connected network, leaving cellulose chains less time to move and causing the structure to lock in earlier than in dilute systems40. Consequently, the gradient pore structure of All-Cel foam evolves more slowly as the cellulose concentration increases.

The structural configuration imbues All-Cel foam with both lightweight and robust properties. Rheological data from Supplementary Fig. 7 demonstrates that the storage modulus (G’) exceeds the loss modulus (G”), signifying a transition from liquid-like to stable solid-like behavior, affirming the strength of the ethanol-induced porous cellulose structure. The one-dimensional (1D) and 2D WAXS patterns exhibit typical diffraction features that suggest an isotropic yet ordered aggregation structure within All-Cel foam (Fig. 2f and Supplementary Fig. 8a). This structural arrangement correlates with the Type II crystalline structure observed in the X-ray diffraction patterns in Supplementary Fig. 8b, indicating that ordered molecular linkages play a crucial role in the structural integrity of All-Cel foam.

Furthermore, the inductively coupled plasma optical emission spectrometry (ICP-OES) analysis confirms trace amounts of Zn2+ (0.02 wt%) in All-Cel foam, showing predominantly cellulose-driven assembly through van der Waals forces and hydrogen bonding. The low surface tension of ethanol facilitates the formation of a stable gradient porous network under ambient conditions. As a result of this attractive molecular assembly strategy, All-Cel foam can support approximately 400 times its weight, demonstrating substantial load-bearing capacity while maintaining the ability to hold a dandelion without significant deformation (Fig. 2g).

Mechanical properties of All-Cel foam

We conducted further analyses on the mechanical properties of All-Cel foam, comparing its performance against conventional petrochemical foams such as PP, EPS, and PU. As depicted in Fig. 3a, All-Cel foam displays typical behavior for cellular materials under uniaxial compression, characterized by three distinct stages in the stress-strain curves: initial linear viscoelasticity at low strains, followed by a plateau of plastic yielding at intermediate strains, and finally densification at high strains. The compressive modulus of All-Cel foam is measured at 11.8 ± 1.3 MPa, substantially higher than those of PP, EPS, and PU foams (Fig. 3b). At a compression strain of 60%, the compressive strength of All-Cel foam is 5.3 ± 0.5 MPa, which is 22 to 29 times greater than those of the aforementioned petrochemical foams (Fig. 3c). The superior strength and modulus of All-Cel foam are attributed to the molecular entanglements of cellulose chains, the reinforcement from hydrogen bonds and van der Waals forces, and the cohesive honeycomb-like gradient architecture, enhancing the toughness of the material.

Fig. 3: Superior mechanical properties of All-Cel foam.
Fig. 3: Superior mechanical properties of All-Cel foam.
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a–c Compressive properties represented by stress-strain curves (a), compressive modulus (b), and compressive strength at 60% strain (c) of All-Cel foam compared to commercial plastic foams. d–f Bending properties illustrated by flexural stress-strain curves (d), flexural strength (e), and flexural modulus (f) of All-Cel foam versus commercial plastic foams. g Digital photographs showing the All-Cel foam’s mechanical robustness. h Comparison of the impact resistance between All-Cel foam and commercial plastic foams. Values in (b, c, e, f, and h) represent their means ± SDs from n = 5 independent samples.

Additionally, All-Cel foam demonstrates high flexural resistance. In three-point bending tests, All-Cel foam exhibited a multi-stage stress-strain curve (Fig. 3d), starting with increasing stress with strain, followed by a sequence of stress drops during further deformation. This behavior reflects the progressive failure of the internal gradient porous structure of All-Cel foam. When subjected to bending, the gradient porous architecture enables gradual and distributed stress transfer, leading to improved toughness and energy absorption capacity of the All-Cel foam41. This structure results in All-Cel foam achieving high flexural strength and modulus, quantified at 2.67 ± 0.12 MPa and 37.02 ± 4.11 MPa, respectively—values significantly exceeding those of common petrochemical foams (Fig. 3e, f). In terms of impact resistance, All-Cel foam also outperforms its counterparts. During a free-fall impact test, a 256 g iron ball dropped from 30 cm caused significant damage to PP, EPS, and PU foams, while All-Cel foam maintained structural integrity, absorbing the impact with minimal damage (Fig. 3g). The impact resistance of All-Cel foam is notably high at 152 ± 9 J/m, surpassing that of traditional foams (Fig. 3h).

These mechanical properties are likely the result of the programmed assembly of cellulose induced by ethanol, which promotes robust hydrogen bonding and molecular entanglement at the micro level, coupled with a gradient porous structure at the macro level. Collectively, these factors create a durable and resilient network within the All-Cel foam.

Thermal properties and fire-retardant treatment

The operational effectiveness of foam materials in various applications heavily relies on their stability, particularly under fluctuating and elevated temperature conditions. Dynamic mechanical analysis results reveal that All-Cel foam demonstrates superior thermo-mechanical stability, enduring temperatures up to 264.1 °C, which exceeds the performance of PP, EPS, and PU foams (Fig. 4a and Supplementary Fig. 9). The storage modulus of All-Cel foam remains consistent at approximately 36 MPa up to 200 °C, a threshold where commercial plastic foams lose functionality. When subjected to a 200 °C hot plate for 10 min, EPS and PP foams softened completely, and PU foam exhibited structural failure upon direct contact. In contrast, All-Cel foam maintained its size and structural integrity, showcasing excellent thermal stability (Fig. 4b and Supplementary Fig. 10).

Fig. 4: Thermal properties and fire-retardant treatment of All-Cel foam.
Fig. 4: Thermal properties and fire-retardant treatment of All-Cel foam.
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a The storage modulus of All-Cel foam compared to commercial plastic foams. b Digital images illustrate the appearance of All-Cel foam alongside commercial plastic foams at ambient temperature (25 °C) and elevated temperature (200 °C), visually demonstrating its superior thermal stability. c–e Cone calorimeter tests of All-Cel/PA foam compared to commercial PP, EPS, and PU foams: heat release rate (HRR) in (c), total heat release (THR) in (d), and smoke production rate (SPR) in (e). f The limiting oxygen index (LOI) for All-Cel/PA foam, underlining its enhanced resistance to ignition and combustion. g Digital images from the vertical burning test of All-Cel/PA foam showcase its ability to resist and extinguish fire, confirming its practical application in fire-sensitive environments.

Fire safety is a crucial attribute for foam materials, particularly in construction applications where they play a key role in safeguarding lives and property. By treating All-Cel foam with a phytic acid alcohol solution and subsequent drying, we produced All-Cel/PA foam, a composite wherein cellulose is combined with phytic acid to boost its flame-retardant capabilities (Supplementary Fig. 11). Cone calorimetry tests quantified the combustion characteristics of All-Cel/PA foam (Fig. 4c–e and Supplementary Table 1). Notably, All-Cel/PA foam exhibits a peak heat release rate of 109.41 kW/m², reflecting reductions of 64.56%, 66.76%, and 69.59% compared to PP, EPS, and PU foams, respectively. Further, it displays a total heat release rate of 5.10 MJ/m², significantly lower than those of PP (37.26 MJ/m²), EPS (24.00 MJ/m²), and PU (17.53 MJ/m²).

The limiting oxygen index (LOI) values demonstrate that PP, EPS, and PU all register below 20%. In stark contrast, All-Cel/PA foam achieves an impressive LOI of 53.6%, underscoring its enhanced flame-retardant properties (Fig. 4f). During vertical burning tests according to the UL-94 standard, both All-Cel/PA foam and commercial plastic foams were exposed to two consecutive 10-s flame intervals (Fig. 4g and Supplementary Fig. 12). The PP, EPS, and PU foams ignited quickly and burned vigorously. Conversely, All-Cel/PA foam showcased desirable flame retardancy, as flames were promptly extinguished upon removal of the burner. This enhanced fire resistance is attributed to the ability of phytic acid to promote char formation in All-Cel foam. Specifically, during combustion, phytic acid decomposes to phosphoric acid and polyphosphoric acid, which catalyze the dehydration of glycosidic bonds and hydroxyl groups in the cellulose chains. This facilitates the carbonization of cellulose, leading to the formation of a continuous and thermally stable char layer. The char layer acts as a physical barrier that effectively insulates the underlying All-Cel foam from heat and oxygen, thereby suppressing flame propagation. In addition, phytic acid releases phosphorus-containing radicals (such as PO· and HPO·) that scavenge reactive hydrogen (H·) and hydroxyl (OH·) radicals in the flame zone, interrupting the chain reactions in the combustion process42,43.

Thermal insulation performance and building energy simulation

Thermal conductivity is a critical parameter for evaluating the efficacy of insulation materials. Increasing the concentration of the cellulose molecular system in All-Cel foam from 5 wt% to 9 wt% results in a slightly higher thermal conductivity (Fig. 5a). The thermal conductivity values for All-Cel foams range from 0.047 to 0.062 W m−1 K−1 at 30 °C, which are comparable to those of commercial plastic foams and other reported cellulose-based foams dried at ambient conditions1,20,21,24,27,44,45,46,47,48,49, thus affirming its insulation performance (Fig. 5b).

Fig. 5: Thermal insulation performance and building energy evaluation of All-Cel foam.
Fig. 5: Thermal insulation performance and building energy evaluation of All-Cel foam.
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a Thermal conductivity variation of All-Cel foam with differing concentrations of the cellulose molecular system. Data are presented as the mean value ± SDs, for n = 3 replicates. b Comparison of the thermal conductivity between All-Cel foam, petrochemical-based plastic foams, and other cellulose-based foams. c, d Infrared thermal images demonstrating the temperature of an iron box after 60 min of infrared heating without and with All-Cel foam insulation. e Temperature progression of the iron box over 60 min of infrared irradiation. f Annual energy consumption for building models located in various cities: with EPS foam or All-Cel foam as middle-wall insulation layer. g Calculated annual energy savings achieved in different global cities by implementing All-Cel foam. The authors designed this figure with the global national administrative boundary dataset, which was provided by the Resource and Environment Science Data Platform with permission (https://www.resdc.cn/data.aspx?DATAID=205).

To evaluate the thermal insulation performance of All-Cel foam, it was placed between an iron box and a high-power incandescent lamp, serving as a thermal barrier. Infrared thermographic imaging showed a rapid temperature increase in the bare iron box, reaching 112 °C after 60 min of exposure (Fig. 5c). In contrast, employing All-Cel foam as an insulating layer significantly slowed the temperature rise, with only an 8.2 °C increase after 60 min, despite the foam’s surface temperature reaching approximately 180 °C within the first 10 min (Fig. 5d). This translates to an 87.3% reduction in the temperature increase of the iron box, strongly validating the superior thermal insulation capability of All-Cel foam (Fig. 5e).

The mechanical strength and thermal insulation properties of All-Cel foam make it highly suitable for building envelopes, providing dependable structural support while enhancing energy efficiency. To assess the energy-saving efficacy of All-Cel foam, building energy simulations were conducted using EnergyPlus. These simulations compare the annual total energy consumption of a mid-rise apartment building model with and without All-Cel foam as the wall insulation layer, across various climatic zones of representative cities (Supplementary Fig. 13). The results show that All-Cel foam achieves energy consumption comparable to EPS foam, an established insulation material that is widely used in the middle-wall insulation layer (Fig. 5f). Moreover, Fig. 5g demonstrates that the energy-saving effects of All-Cel foam are more significant in temperate climates than in tropical regions, notably reducing annual energy consumption in Reykjavik by an average of 33.7%. These experimental and simulation findings collectively affirm the thermal insulation properties of All-Cel foam, underscoring its potential in energy-efficient and sustainable building applications.

Formability, recyclability, biodegradability, and environmental impacts of All-Cel foam

Conventional plastic foams are typically produced through physical, chemical, or mechanical foaming processes, combined with molding techniques such as compression, extrusion, or injection to achieve the desired shape. In contrast, All-Cel foam is produced via a facile method at ambient temperature, where solvent exchange facilitates the assembly of cellulose molecules to form a porous structure. This method eliminates the use of blowing agents and toxic cross-linkers, significantly reducing the emissions of volatile organic compounds and greenhouse gases. The fabrication of All-Cel foam is streamlined into a three-step casting process: initially, the cellulose molecular system is poured into a specific mold. The process then progresses to ethanol exchange to ensure cellulose reconstruction and foam structure formation. The final step involves ambient pressure drying, which results in All-Cel foam in various shapes, such as flower-shaped and drop-shaped structures (Fig. 6a).

Fig. 6: Characteristics of All-Cel foam: formability, recyclability, biodegradability, and environmental impact.
Fig. 6: Characteristics of All-Cel foam: formability, recyclability, biodegradability, and environmental impact.
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a Digital images showcasing All-Cel foams shaped into various forms. The flower-shaped (i) and raindrop-shaped (ii) foams were prepared by pouring cellulose molecular solution into corresponding molds, followed by ethanol exchange. For the wave-shaped (iii) and bridge (iv) shaped foams, preformed All-Cel foam panels were first softened in water, then placed into custom molds, and finally reshaped via ethanol exchange. b Demonstration of the capability to process large-sized All-Cel foam panels (47 × 28 × 3 cm), highlighting the scalability and potential for practical applications such as thermal insulation and structural panels. c Illustration of the recyclability process for All-Cel foam. The used All-Cel foam can be shredded into chips and re-dissolved to form a cellulose system. The regenerated All-Cel foam is fabricated via pouring the cellulose system into a mold to create a new foam embryo, and followed by ethanol-induced self-assembly. This process demonstrates All-Cel foam’s sustainable lifecycle. d Biodegradability tests of All-Cel foam and traditional plastic foams (PP, EPS, and PU) over time under natural soil environment, with intervals ranging from 0 to 160 days. As degradation time progressed, the All-Cel foam gradually lost structural integrity and visibly decomposed, whereas the traditional plastic foams remained nearly unchanged throughout the entire period. e Assessment of the environmental impacts of All-Cel foam relative to commonly used plastics (PP, EPS, and PU), with each impact category normalized by the material with the highest impact.

Additionally, All-Cel foams can be processed into desired forms through a method similar to compression molding. For example, a 3D wavy All-Cel foam can be softened in water and transformed into a soft foam panel. This panel is then placed into a bridge-shaped mold and treated with ethanol to reinforce its structure. After drying, the foam retains the 3D bridge-like shape (Fig. 6a). This shaping capability arises from the molecularly assembled network of All-Cel foam, which differs from conventional nanocellulose foams (CNF foams). As shown in Supplementary Fig. 14, the CNF foam tends to lose structural integrity when processed in water, making it difficult to shape into desired shapes. In contrast, All-Cel foam not only maintains well-defined 3D shapes but also preserves its mechanical robustness. Moreover, a simple silanization treatment endows the All-Cel foam with hydrophobicity, enabling it to retain a compressive modulus of 11.9 ± 0.8 MPa after immersion in water for 24 h (Supplementary Fig. 16). This robust mechanical stability further broadens the potential application range of All-Cel foams.

The robust intermolecular interactions between cellulose molecules ensure that the All-Cel foam maintains its structural integrity in water, enabling it to be reprocessed into different shapes multiple times (Supplementary Fig. 15). Further extending these processing routes, All-Cel foam can be processed into large-size products, as evidenced by the successful fabrication of a foam piece measuring 47 cm in length, 28 cm in width, and 3 cm in thickness (Fig. 6b), demonstrating significant potential for large-scale applications.

Since All-Cel foam is composed of cellulose molecules, it can be efficiently recycled at the end of its life and reprocessed into recycled foam products (Fig. 6c). Used All-Cel foams are broken down into chips, redispersed in the solvent to form a uniform cellulose system, and then reformed using molds after an ethanol-induced molecular assembly and ambient drying process. This simple recycling process facilitates closed-loop material recycling with strong economic feasibility. The recycled foam demonstrates comparable density and thermal conductivity; however, the observed reduction in compressive modulus may be attributed to a decrease in the degree of polymerization of the cellulose chains during the re-dissolution process (Supplementary Fig. 17). The shortening of cellulose chains inherently limits their ability to form effective supramolecular entanglements, thereby weakening physical crosslinking network of recycled foam. Moreover, the reduced chain length and entanglement density may weaken inter-chain interactions within the pore walls of recycled foam, thereby compromising the mechanical strength. To mitigate the mechanical deterioration of recycled All-Cel foams, incorporating reinforcing nanofibers such as cellulose nanofibers or mineral platelets could enhance structural integrity. In addition, mild chemical crosslinking using biodegradable agents may improve network stability while maintaining sustainability. Moreover, the ZnCl2/FA aqueous solution generated during the fabrication of All-Cel foam can be collected and reused through a simple distillation process (Supplementary Fig. 18). The recovery rate of the solution is 85.5 ± 5.1% after five cycles. Furthermore, the All-Cel foam processed using the recycled cellulose/ZnCl2/FA system maintains its mechanical integrity, demonstrating compressive moduli in the range of 5.1–7.8 MPa.

All-Cel foam also demonstrates impressive biodegradability in natural soil environments (Fig. 6d). When buried in soil at a depth of 6 cm, the All-Cel foam began to swell and showed slight surface erosion after 30 days while maintaining its structural integrity. By 90 days, microbial metabolism had compromised the structure, and it completely degraded after 160 days. In contrast, commercial plastic foams retained their original morphology over the same period. A life cycle assessment (LCA) was conducted to evaluate the environmental sustainability of All-Cel foam compared to mainstream commercial plastic foams such as PP, EPS, and PU (Fig. 6e, Supplementary Figs. 19 and 20, and Supplementary Tables 2–5). All-Cel foam outperforms PU foams in most LCA indicators and is comparable to PP and EPS foams in terms of carbon footprint, which is 24%, 41%, and 56% lower than PP, EPS, and PU, respectively, indicating reduced greenhouse gas emissions. Given its scalability, recyclability, and biodegradability, All-Cel foam represents a promising environmentally sustainable alternative to traditional plastic foams.

Discussion

In this study, we report a solvent-induced molecular assembly approach to create sustainable cellulose foams. The structural design of All-Cel foam primarily depends on the incorporation of ethanol to facilitate programmed molecular reorganization. Specifically, ethanol molecules disrupt the hydrogen bonding interactions within cellulose/ZnCl2/FA system, liberating the cellulose molecules and promoting their regional assembly. Upon rapid ethanol exchange at the foam surface, cellulose molecules rapidly recombine, resulting in a dense surface layer. This dense layer serves as a key structural feature that produces the gradient porosity of All-Cel foam. It slows the diffusion of ethanol into the cellulose system, resulting in varying rates of cellulose assembly at different depths. Near the surface, the higher concentration of ethanol leads to rapid solidification, creating a compact region with fine pores. In contrast, deeper regions experience a longer relaxation time, allowing for more extensive aggregation and the formation of honeycomb-like, gradient pore architectures within the All-Cel foam. This work provides attractive insights into structural control through solvent exchange dynamics and opens avenues for the rational design of porous cellulose materials.

All-Cel foam is characterized by its lightweight and strong properties. For instance, All-Cel foam exhibits a low density of only 0.12 ± 0.01 g/cm³, while demonstrating a high compressive modulus exceeding 11.8 MPa and thermal stability above 264 °C, surpassing conventional plastic foams such as PP, EPS, and PU. Additionally, All-Cel foam provides excellent thermal insulation, and when combined with phytic acid, it offers superior flame resistance, enhancing safety in building applications. The foam’s moldability enables it to be easily shaped into various forms, facilitating scalable production. Compared with other cellulose-based foams and aerogels (Supplementary Table 6)20,24,44,50,51, All-Cel foam features a gradient porous structure formed via ethanol-induced molecular assembly under ambient and scalable processing conditions, delivering high performance and versatile moldability. Its recyclability and biodegradability further support it as a sustainable alternative to conventional plastic foams for structural and thermal insulation applications.

Altogether, the development and applications of All-Cel foam underscore a significant shift towards more sustainable materials technology in industries traditionally dominated by petrochemical products. All-Cel foam is entirely derived from biomass, and its high compressive modulus and thermal properties, combined with the capacity to form complex shapes, give it a competitive advantage over conventional materials. Notably, the integration of phytic acid not only elevates its flame resistance but also illustrates the potential of combining cellulose with other naturally derived compounds to boost performance. Such properties make All-Cel foam a viable option beyond petrochemical paradigms.

While the current results are promising, understanding the potential impacts of upscaling—such as the preservation of foam integrity and properties, the consistency of cellulose source quality, and the economic viability—will be crucial. Looking forward, the scalability of All-Cel foam production poses a critical area for further research. Addressing the life cycle impacts and end-of-life scenarios, such as recyclability and biodegradability in more detail will enhance our understanding of the environmental benefits and limitations of All-Cel foam. The commitment to a sustainable approach invariably involves continuous improvement and iteration of material processing and properties to meet diverse application needs and regulatory standards. In conclusion, All-Cel foam not only presents a sustainable alternative but also pushes forward the frontiers of material science for environmental innovation.

Methods

Fabrication of All-Cel foam

A ZnCl2/FA mixture solution was prepared by stirring at room temperature, using a molar ratio of ZnCl2:H2O:FA at 1:2:2. Subsequently, a 7 wt% cellulose solution was obtained by dispersing cellulose in the ZnCl2/FA mixture and continuously stirring for 60 min at ambient conditions, until a transparent solution was formed. This solution was centrifuged to remove air bubbles and subsequently poured into cube molds. It was then immersed in ethanol to create the porous structure. Afterward, the material was air-dried to evaporate the ethanol, producing the All-Cel foam. To fabricate the All-Cel/PA foam with enhanced fire resistance, the porous structure was immersed in a 3 wt% PA ethanol solution and subsequently air-dried.