Biobased Foams: A Critical Review of Their Synthesis, Performance and Prospective Applications

Abstract

Foams, as a type of porous materials, have found broad functional and structural application in heat and sound insulation, the mitigation of mechanical vibrations and impacts, packaging, etc. This paper aims to comprehensively review recently developed biobased foams (BBFs) with a comparison with their counterparts—namely, synthetic polymer foams—in terms of their foaming methods, physical and mechanical properties, and broad applications. A brief introduction to general foams, polymeric foams, and BBFs is provided, followed by a comparison of the related foaming methods; physical, mechanical, and chemical properties; and current and prospective applications. Several main polymer foaming methods (e.g., physical, chemical, and mechanical foaming) and their unique features are further examined in detail. The structure-related properties of polymeric foams (e.g., mass density, thermal conductivity, and rate effects in mechanical responses) are discussed, and the fundamental linearly viscoelastic models are summarized to account for the simple rate effect in the mechanical moduli of polymeric foams under varying loading rates. Furthermore, specific focus is placed on the foaming processes and material properties of sustainable BBFs (e.g., soybean-based, corn-based, and starch-based foams) and their potential to substitute conventional synthetic polymer foams. The technical challenges in processing BBFs are discussed, and the most promising applications of BBFs are then considered.

1. Introduction

1.1. Foams—A Type of Lightweight Cellular Material with Versatile Applications

A foam is a material dispersion that is formed by trapping gases in the shape of bubbles distributed in a liquid, solid, or gel, resulting in cellular microstructures [1]. A foam is a colloidal system where the gaseous phase is the dispersed phase and the liquid or solid is the continuum phase. Foams are characterized by their cellular structure, which gives them a high surface-area-to-volume ratio and a low mass density. This unique structure can be either temporary, e.g., the bubbles in a carbonated drink or beer, or permanent, like in solid polymeric foams. The properties of a solid-state foam–e.g., its mechanical strength and stiffness, thermal insulation, mitigation of mechanical vibrations and impact, and sound absorption—are highly dependent on its material type, the size and distribution of its voids (cells), and whether these cells are interconnected (open-cell) or sealed off from one another (closed-cell). In particular, engineering foams, as a subcategory of foams, are a class of lightweight, cellular materials that are designed and manufactured for specific, high-performance applications. Unlike simple foams used for packaging or cushioning, engineering foams are produced with precise control over their cell structure, mass density, and chemical composition to achieve targeted mechanical, thermal, and chemical properties. Engineering foams are often used as the core materials in sandwich-structured composites, where they provide exceptional specific strength and stiffness (ratios of mechanical strength and stiffness over mass density). The unique cellular architecture of these foams—either open-cell, closed-cell, or a hybrid—allows them to excel in broad engineering fields, e.g., thermal and acoustic insulation, energy absorption, and structural support, among others [2,3,4,5]. Foams can be generally categorized into three large groups according to their material constitutions, i.e., polymeric, metallic, and ceramic foams [6,7]. A brief comparison of the materials, foaming methods, properties, and typical applications of polymeric, metallic, and ceramic foams is tabulated in

Table 1. A brief comparison of polymeric, metallic, and ceramic foams [2,5,6,8,9,10,11].

	Polymeric Foams	Metallic Foams	Ceramic Foams
Materials	Synthetic and biobased polymers: PU, PS, PVC, PI, PO, PLA, etc.	Metals and alloys: Most commonly aluminum, but also steel, titanium, copper, nickel, etc.	Metal oxides (Al2O3, ZrO2), carbides (SiC), nitrides (e.g., Si3N4), and other ceramic materials.
Foaming Methods	Physical: Blowing agents (e.g., CO2, N2) dissolved into polymer melts to expand upon a pressure drop. Chemical: Foaming agents added to decompose during heating, to release gas to create the cellular structure. Mechanical: Gas mechanically whipped into polymer melts. 3D printing: Directly produces voids and cellular structures via computerized printing.	Powder metallurgy: Metal powder mixed with a foaming agent, compacted, and then heated to expand. Direct foaming: An inert gas (e.g., N2) injected directly into a molten metal to create bubbles. Investment casting: A polymeric foam template impregnated with a liquid metal, and the polymer is then removed. 3D printing.	Polymeric sponge replica: A polymeric foam immersed in a ceramic slurry to form a ceramic replica after burning away the polymer at high temperatures. Direct foaming: A gas introduced into a ceramic suspension to form cells after consolidation and sintering. Gel-casting: A ceramic slurry is gelled around a gas phase. 3D printing.
Properties	Thermal: Excellent thermal insulation due to the low thermal conductivity. Mechanical: Low mass density, high energy absorption, good cushioning capabilities, and high compressibility. Generally low mechanical strength compared to solid polymer counterparts. Chemical: Good corrosion resistance, generally with low thermal stability and susceptible to degradation at high temperatures.	Thermal: High thermal and electrical conductivity, suitable for heat exchangers. Mechanical: Lightweight, high specific strength and stiffness, and excellent energy/crash absorption. High compressive strength, especially for closed-cell foams. Chemical: Tailorable for high-temperature stability and corrosion resistance, depending on the base metal and the anticorrosion surface coating if applied.	Thermal: Extremely high thermal stability and very low thermal conductivity, suitable for high-temperature insulators. Mechanical: High specific compressive strength and hardness, typically brittle. Good resistance to wear. Chemical: High chemical stability and corrosion resistance, even in harsh environments.
Applications	Insulation: Building insulation, refrigerators, and refrigerated containers. Cushioning and packaging: Mattresses, seat cushions, and impact protection. Filtration: Air and liquid filtration systems. Biomedical: Tissue engineering scaffolds and drug delivery systems.	Aerospace and automotives: Lightweight structural components, crash absorbers, and passive damping materials. Thermal management: Heat exchangers, heat sinks, and thermal control systems. Biomedical: Bone scaffolds and orthopedic implants.	Molten metal filtration: To remove impurities from liquid metals like aluminum and steels. Catalyst supports: High specific surface area and thermal stability to carry catalysts in chemical reactors and exhaust systems. High-temperature insulation: Furnace linings and heat shields. Pollution control: Filters for diesel particulate matter and other high-temperature environments.

[2,3,4,5,6,7,8,9,10,11]. In addition, based on the Web of Science database, the number of annual journal articles published with the keyword “foam” in the years between 1990 and 2025 is shown in Figure 1a. From this, it can be seen that the research on foams has increased rapidly since the early 1990s and became very active after 2010, reaching a plateau at present. In contrast, the number of annual journal articles published with the keywords “polymer foam” or “polymeric foams” in the same time period is shown in Figure 1b, indicating that the research on polymeric foams represents only a small portion (with the number of annual publications below 1/20) of the research on foams. It largely reached its plateau after 2015, indicating the relatively small number of research groups around the world focusing on polymeric foam research. Furthermore, the Web of Science database indicates that a much smaller number of journal articles have been published in the literature with the keywords of “biobased foam” or “bio-based foam”, among which the first research article was published in 2015. Thus, research on BBFs is a growing field to be further explored. Herein, BBFs are defined as polymeric foams that are composed of base polymers that are completely or partially derived from biomass and its derivatives (e.g., soybeans and soy straws, corn and corn stalks, starches, vegetable oils, agricultural residuals, wood debris, etc.). With the fast development of modern green chemistry, more and more polymers have been derived from biomass to substitute petroleum-based synthetic polymers, and there is no clear boundary between biobased and synthetic polymers, as polymers are defined according to their atomic elements and molecular/chain structures. For instance, PLA and many types of PU can be both biobased and synthetic.

1.2. Polymeric Foams and Biobased Foams (BBFs)

So far, polymeric foams have occupied a large share of foam materials and played important roles in various industrial applications and human life, which has motivated extensive and in-depth research activity on their materials, foaming methods, characterization, performance modeling, and exploration of broader new applications. A polymeric foam can be defined as a two-phase system consisting of a gaseous phase of bubbles randomly distributed within a polymeric resin [12]. Polymeric foams can be further categorized into synthetic polymer foams and biobased foams (BBFs), and the latter are produced using biomass, e.g., soybeans, corn starches, celluloses, vegetable oils, etc. Commonly used synthetic polymeric foams available in the market include those composed of petroleum-derived PS, PU, PVC, phenolics, PE, PP [13], and epoxy [14]. BBFs typically include those composed of biomass-derived PLA, BPUs, PHAs, starch- and cellulose-based materials [13], PBAT, PPC, PCL, BEFs [15], PVA [14], and BPFs [16].

Synthetic polymer foams have been extensively integrated into various industrial and household products, as illustrated in Figure 2, due to their favorable features, e.g., low-cost and well-established manufacturing methods, low mass density, high compressibility and flexibility, good impact resistance, high thermal insulation, excellent sound absorption, etc. [12,17]. In addition, synthetic polymer foams also bear intrinsic technical deficiencies, e.g., low mechanical strength, fast environmental degradation, and limited recycling options, among others.

Table 2. Brief comparison of synthetic polymer foams and BBFs [12,13,14,15,16,17,18,19,20,21,22,23,24,25,26].

	Synthetic Polymer Foams	BBFs
Advantages	Excellent thermal and acoustic insulation: Superior insulation properties with wide application in construction, refrigeration, and automotive domains. High specific mechanical stiffness/strength (ratio of stiffness/strength over mass density) and compressibility: Cellular structures allowable for lightweight materials with high strength and rigidity. Versatility and durability: High tailorability in a wide range of properties, from flexible cushioning to rigid structural supports. High resistance to moisture and a long service lifetime. Cost-effectiveness and high availability: Fabrication from petroleum-based sources, low-cost commodity materials with well-established manufacturing processes. Good processability: Well-explored and established manufacturing techniques (e.g., extrusion, injection molding, bead foaming, etc.) for feasible fabrication of complex shapes and uniform cell structures of various polymers.	High sustainability and renewable resources: Sourced from biomass (e.g., corn starch, soy protein, and agricultural waste) to reduce dependence on fossil fuels and to benefit a more circular economy. Biodegradability: Many BBFs are biodegradable or compostable to offer a sustainable end-of-life option and to mitigate plastic waste in landfills and oceans. Reduced carbon footprints: Produced from renewable feedstocks and ecofriendly BAs to lower greenhouse gas emissions compared to their synthetic counterparts. Tailorable mechanical properties: Formulated to achieve a range of mechanical properties, from flexible to rigid, suitable for applications in packaging, insulation, etc. Enhanced functionality: Prospective BBFs with unique properties, e.g., antimicrobial activity and improved fire resistance using natural additives.
Disadvantages	Environmental impact: Derived from fossil fuels, contributing to depletion of nonrenewable resources and significant carbon footprints. Production often involves toxic chemicals and releases VOCs. Nonbiodegradability: Majority of synthetic polymer foams are nonbiodegradable and persist in landfills for centuries, leading to widespread plastic pollution and formation of microplastics. Recycling challenges: Difficult to recycle due to complex compositions, contamination issues, and high costs of separation and reprocessing. Flammability: Some synthetic foams (e.g., PU) are highly flammable and release toxic smoke during combustion and thus require addition of fire retardants. Potential health risks: Production processes generate harmful chemicals. Some foams release gases or VOCs and cause respiratory issues or allergic reactions in sensitive individuals.	Higher production costs: Higher costs of raw materials and novel manufacturing processes for BBFs than for traditional synthetic polymer foams. Performance limitations: Some BBFs unable to match the performance of synthetic polymer foams in some specific applications, e.g., thermal stability, moisture resistance, or long-term durability. Processability challenges: Poor “foamability”, leading to difficulties in achieving a fine, uniform cellular structure and requiring specific additives or process modifications. Food vs. fuel/land use concerns: Use of agricultural crops as a feedstock for biopolymers and related BBFs results in competition with food production and raises concerns about land use. Lack of standardization: Lack of industrial standardized regulations for disposal and recycling of BBFs results in confusion and difficulties in proper end-of-life management.

shows a comparison of the technical features of synthetic polymer foams and BBFs [17,18,19,20,21]. To date, a number of successful polymer foaming methods have been formulated, e.g., emulsion freeze-drying, mechanical stirring, particulate leaching/solvent casting, and 3D printing [22,23,24], which are reviewed in detail in Section 2. In a polymer foaming process, typical source foaming materials include the base polymeric resins, blowing agents (BAs), and additives used in foam extrusion and injection [24]. Furthermore, in the case of the synthesis of PU or phenolic foams, additional materials are needed when applying chemical or mechanical methods, e.g., polyol- or resol-type phenolic resins, isocyanates, BAs, surfactants, and curing agents.

Figure 3 illustrates the foaming mechanism, which plays a crucial role in controlling the structure and performance of the resulting polymeric foam, including the physical, thermal, and mechanical properties [12,26]. Polymer foaming is a process of bubble nucleation and growth that results from the addition of a BA to the polymeric matrix, followed by cell growth and the subsequent formation of the final cellular structure [14,25,26,27]. Among others, Kabir et al. [28] conducted an experimental study to investigate the dependencies of the performance of structural polymeric foams upon the foam mass density and cell morphology (e.g., the cell size, shape, and type). Similarly to other types of engineering foams, synthetic polymeric foams and BBFs can be further grouped into open- and closed-cell foams according to their cell morphologies [28].

Table 3. Comparison between open- and closed-cell polymeric foams [26,27,28].

	Open-Cell Polymeric Foams	Closed-Cell Polymeric Foams
Structure	Interconnected cells to result in a porous, sponge-like structure. Foams are more compliant.	Completely enclosed and isolated cells. Foams are more dense and rigid than open-cell foams.
Mass Density	Generally, lower mass density, typically ranging from 6.5 to 20 kg/m3, compared to closed-cell polymer foams.	Generally, high mass density, typically ranging from 27 to 50 kg/m3.
Heat Resistance R-Value	Lower R-value, with typical values of ~8.5 to 9.5 per cm.	Higher R-value, with typical values of ~16.5 to 17.5 per cm.
Water Resistance	Porous, nonwaterproofing cellular structure capable of absorbing water and moisture due to the large surface area of the open cells.	High water resistance and strong moisture barrier. Sealed cells prevent water and other liquids from passing through the foam.
Soundproofing	Excellent sound absorption. Open cells capable of trapping sound waves, ideal for noise reduction.	Useful in sound blocking. More dense and rigid cellular structures, helpful for sound transmission reduction.
Flexibility	Soft and flexible, easily compressed, suitable to be used for cushioning.	Stiffer and more rigid. Able to maintain high structural integrity and not easily compressed.
Air Permeability	Air- and vapor-permeable.	Air- and vapor-impermeable, suitable for use as air and vapor barriers.
Applications	Furniture cushions, mattresses (e.g., memory foams), soundproofing panels, and interior insulation in above-grade walls.	Lightweight protective packaging, automotive parts, structural reinforcement, building and roofing insulation, flotation in marine applications.
Cost	Low cost due to the lower mass density and use of less material.	More expensive due to the higher mass density and use of more material.
Structural Support	Unable to provide sufficient structural stiffness and strength for structural supports.	Able to provide substantial structural strength and stiffness for structural supports.

shows a comparison of the structures, properties, and applications of open and closed cellular polymeric foams. In addition, synthetic polymeric foams can be categorized into thermoplastic and thermosetting foams according to their response to heat. In principle, the base polymers of thermoplastic foams have linear or branched structures that can soften and become fluid when heated, allowing them to be repeatedly melted, reshaped, and reprocessed. In contrast, the base polymers of thermosetting foams have a crosslinked network structure that becomes rigid after foaming, and they cannot be remelted, reshaped, or reprocessed when heated [29]. Furthermore, synthetic polymeric foams and BBFs can be categorized into biodegradable and nonbiodegradable according to their ability to decompose naturally. Biodegradable polymer foams can decompose into simpler substances like CO₂ and water under the action of microorganisms and other living organisms, while nonbiodegradable polymer foams can resist decomposition by natural processes and persist in the environment for extended periods [15,19,29,30,31,32].

Table 4. Comparison between biodegradable and nonbiodegradable polymeric foams [17,19,28,29,30,31,32].

	Biodegradable Polymer Foams	Nonbiodegradable Polymer Foams
Source Materials	Derived from renewable, natural resources. Examples: PLA (from corn starch), starch-based polymers, PHAs (from microbial fermentation), PBS, PBAT, etc.	Derived from nonrenewable, petroleum-based resources. Examples: PA, PE, PI, PS, PU, etc.
Chemical Structure	Functional groups, e.g., ester or amide linkages, to be scissored by microorganisms and hydrolysis to form polymer chains susceptible to natural degradation.	Stable carbon–carbon backbones to resist microbial enzymes and natural degradation processes.
Cellular Structure	Produced as both open-cell and closed-cell foams. Challenging to achieve stable, cell-controllable structures due to material limitations such as poor melt strength, rapid crystallization, etc.	Easily manufactured into both open-cell and closed-cell structures with high stability. Closed-cell foams capable of trapping gas molecules and vapors for use as superior insulation.
Thermal Properties	Generally, lower thermal stability, sensitive to high processing temperatures, with a lower service temperature. Narrower processing window between melting and thermal degradation, posing challenges in foaming without additives and modified processes. Good thermal insulation with R-values typically lower than those of high-performance closed-cell nonbiodegradable foams. Thermal performance affected by moisture absorption.	Generally, higher thermal stability, withstanding a wider range of temperatures for high-heat applications. Wider processing window, allowing easier foaming and more consistent quality using conventional equipment. Excellent thermal insulation, particularly for closed-cell foams, with high R-values due to gas-filled, sealed cellular structures.
Mechanical Properties	Lower mechanical strength, thermal stability, and long-term durability. Typically brittle with poor impact resistance and lower fracture toughness. Compressive strength and resilience can be improved through additives and blending.	Excellent mechanical strength, durability, and resistance to environmental factors. Suitable for structural applications with requirements of high structural integrity and longevity.
Environmental Effects	Designed to biodegrade into natural byproducts (e.g., CO2, water, etc.) in landfills. Capable of reducing long-term environmental impacts and emissions of greenhouse gases and harmful chemicals.	Nonbiodegradable with long-term stability, able to exist in environment for a long time and potential to break down into microplastics, which leads to soil and water pollution.
Foaming Challenges	More complex foaming process due to material limitations, e.g., low melt strength, easy thermal degradation, and poor processability, making it difficult to achieve desired foam structure.	Well-established and mature foaming processes for efficient production of foams with consistent, predictable, and controllable properties.
Cost	Commonly more expensive due to use of renewable resources and less mature manufacturing methods. Costs decreasing with massive production.	More cost-effective due to mature production methods and use of abundant, petroleum-derived raw materials.
Foam Examples	PLA foam for packaging inserts, starch-based foam for food packing and disposable foodware, PHA foam for tissue scaffolds.	PS foam (Styrofoam®) for coffee cups and packaging, PU foam for building insulation and mattresses, PE foam for packaging and cushioning.
Common Applications	Short-term uses, with disposal and natural degradation as a concern (e.g., protective packaging, disposable food containers, etc.) Consumer goods and textiles (e.g., cushions in footwear, automotive seats, furniture upholstery, and bedding materials in pillows and mattresses). Thermal and acoustic insulation in construction. Bioengineering and biomedical applications (e.g., wound dressings, tissue scaffolding for cell growth and tissue regeneration, and biomedical implants with self-dissolution over time). Agricultural applications (e.g., mulch films, seed coatings, controlled fertilizer release, agrochemical delivery, water storage, etc.). Gas and liquid filtration.	Long-term, high-performance applications with concerns about durability, insulation, moisture resistance, and air sealing capabilities (e.g., sound and heat insulation, vibration and impact absorption, building and roof insulation, automotive parts, furniture cushions, etc.). Lightweight fill and insulation materials in construction (e.g., foam insulation in building structures and filling materials in road and bridge construction). Medical devices with long-term stability and performance (e.g., orthopedic implants, surgical meshes, diagnostic equipment, etc.). Lightweight cushioning and structural reinforcing materials in automobile parts and interior decor. Marine floatation devices (e.g., life jackets, buoys, and floating decks).

shows a comparison of the materials, structures, properties, and applications of biodegradable and nonbiodegradable polymeric foams.

It was reported that, in 2024, the global market for polymeric foams had an estimated value of USD 162.90 billion, and it was projected that the market would grow to USD 588.99 billion by 2030 [33]. In particular, the PU foam market amounted to USD 55.70 billion and was projected to reach USD 94.77 billion by 2032 [34]. Despite the huge global market for synthetic polymer foams, they also lead to environmental concerns, such as creating plastic waste in landfills and contributing to severe environmental pollution. In the last two decades, a number of government policies and environmental protection laws have been enacted to reduce the production and use of petroleum-based polymeric products, since they are the main sources of plastic waste and cannot degrade easily in the natural environment. Therefore, biodegradable polymeric materials have attracted significant attention, since their use can contribute to a better circular economy via developing sustainable biobased products; one example of this is BBFs. BBFs are composed of biomass, and they are environmentally friendly, sustainable, and typically easier to degrade in the environment under the compound effects of biological, chemical, and physical processes, while synthetic foams cannot degrade with such effects [13,32] (see also Table 4).

To date, numerous BBFs have been manufactured from sustainable biobased materials, including vegetable oils, soybeans, corn, cellulose, lignin, and other agricultural residues. These biomass materials have also been used as reinforcing fillers, crosslinkers, and plasticizers. Naturally, BBFs possess low mechanical strength and poor water resistance, and the latter is due to their high ratio of hydrophilic groups, e.g., hydroxyl (-OH), carboxyl (-COOH), and amino (-NH₂) groups, existing commonly in biomass-derived chemicals. In addition, various inorganic and biobased fillers have also been used to improve the physical and mechanical performance of BBFs. These inorganic fillers include nanosized clays (nanoclays), carbon-based fillers, magnesium hydroxide (Mg(OH)₂), glass and carbon fibers, etc. In contrast, biobased fillers include silk cocoons, walnuts, hazelnut shells, sugarcane bagasse, coconuts, bamboo fibers, nanocrystalline celluloses, soy proteins, wood flour, etc. [35,36,37,38,39,40]. However, more in-depth research is still needed to explore their manufacturing, mechanical and thermal characterization, performance modeling, and broad new applications. Furthermore, similarly to synthetic polymer foams, BBFs can also be categorized into thermoplastic and thermosetting BBFs according to their mechanical behavior when heated. A brief comparison of thermoplastic and thermosetting BBFs in terms of their materials, structures, foaming methods, properties, and applications is tabulated in

Table 5. Brief comparison of thermoplastic and thermosetting BBFs [13,18,19,36,37,38,39,40].

	Thermoplastic BBFs	Thermosetting BBFs
Examples	PLA, starch-based foams, PHAs, cellulose foams.	Biobased PU, EVO foams, bio-based phenolic foams.
Polymer Structure	Composed of long, linear or branched polymer chains that are not chemically bonded to each other. Chains are held together by weaker intermolecular forces. Upon heating, intermolecular forces between neighboring chains are overcome to allow chains to slide into each other. The structure can be amorphous or semicrystalline.	Characterized by 3D, rigid networks of permanently crosslinked polymer chains. Strong covalent bonds formed during curing to prevent chains from moving and to result in a solid that cannot be remelted. Typically formed with a more rigid molecular structure.
Cellular Structure	Able to be tailored with open- or closed-cell structures. Closed-cell foams are more rigid and better for insulation, while open-cell foams are more flexible. Cell morphology highly depends on processing parameters, e.g., temperature and pressure drop rate.	Cellular structure (open- or closed-cell) controlled by reaction kinetics and foaming process.
Properties	Recyclability: Easy to melt and be reprocessed under heating. Heat resistance: Lower thermal stability and softening or melting upon heating. Viscosity: Lower viscosity than thermoset BBFs. Flexibility: Commonly more flexible than thermosets. Biodegradability: Many thermoplastic BBFs are biodegradable, but not all.	Heat resistance: High thermal and dimensional stability due to crosslinked networks; not meltable. Strength: Typically more rigid with higher compressive strength. Durability: Excellent chemical and creep resistance. Recyclability: Difficult to recycle due to irreversible chemical bonds.
Manufacturing Process	Typical melt-based processes, e.g., extrusion or injection molding. Heated to a molten state, mixed with a blowing agent (physical or chemical), and then cooled to solidify and trap gas bubbles. Examples: Extrusion foaming, bead foaming, and batch foaming.	Typical one-step processes with liquid precursors (e.g., polyol and isocyanate for PU) mixed with a catalyst and blowing agent. Chemical reactions happen to create a highly crosslinked solid foam. Irreversible process with material unable to be reprocessed by melting.
Blowing Agents	Physical blowing agents: Supercritical CO2 or N2 to create a porous structure. Chemical blowing agents: Chemicals to generate gases upon heating.	Chemical blowing agents: Water reacting with a precursor to generate gases (e.g., CO2 in PU synthesis). Physical agents: Evaporation due to exothermic curing reaction.
Applications	Packaging materials, disposable foodservice items, protective foams, heat insulation.	Thermal insulation (e.g., construction, refrigeration), structural components (e.g., automotive, aerospace), cushioning, sound absorption.

[13,18,19,36,37,38,39,40].

The material properties and applications of BBFs are fundamentally determined by a complex interplay between the base polymer material, its chain structure, and the chosen foaming method. This relationship dictates the mass density, cellular structure, and physical and mechanical properties of the resulting BBFs and their potential for industrial and household applications. Among these, the inherent characteristics (e.g., the melt strength, viscosity, chain structure, and fundamental groups) of the base polymer are the factors that determine the specific forming method and related parameter choices. For example, many biobased polymers, particularly linear ones like PLA, have low melt strength and viscosity, which poses a critical challenge for foaming, since the polymer melt needs to be strong enough to trap and stabilize gas bubbles. If the melt is too weak, the gas will escape, leading to cell collapse and a shrunken, nonuniform foam. In addition, polymers with a linear chain structure (e.g., some types of PLA) are often difficult to foam due to their low melt strength, but introducing branching or crosslinking to the polymer chains can significantly improve foamability. A branched or crosslinked structure can increase the mechanical strength and viscosity of the polymer melt, which enables it to better retain the gas and stabilize the foam cells. This is a common strategy to overcome the limitations of linear biobased polymers. In addition, the case of BPUs, the types of polyols (the soft segments) and isocyanates (the hard segments) and their respective functional groups play a crucial role. The ratio and nature of these components determine the final properties and uses of the foams, from rigid and strong to soft and flexible [13,15,16,17].

Due to the technical importance of polymeric foams in industry and human life as well as the limited technical reviews on this topic available in the literature, the present study aimed to conduct a comprehensive review of the typical foaming methods of polymeric foams and their physical and mechanical properties, as illustrated in Figure 4 [41], with an emphasis on BBFs, i.e., those based on soy proteins, corn, and starches. The present review also sought to expose their technical deficiencies and limitations as well as to explore their prospective applications.

2. Common Polymer Foaming Methods

To date, physical, chemical, and mechanical foaming methods have been well established as mature methods for the production of a variety of polymeric foams with well-formed cellular structures for diverse applications [25,28,41,42]. Several types of BAs and polymeric materials have been utilized for polymer foaming, as listed in

Table 6. Common polymeric foams fabricated by typical foaming methods [13,14,15,16,22,23,24,25,26,27,43,44,45,46,47,48,49,50,51,52,53,54,55,56].

Foaming Technique	Typical Synthesized Polymeric Foams	Use of BAs
Extrusion	HDPE, LDPE, PP, PS, PVC, PU, PTFE, starch-based, PLA	Water, CO2, N2, HCs, HFCs
Injection	PP, PS, PE, PI6, PET, PU, PLA, PHA, biobased	Water, ADC, CO2, N2, HCs, HFCs, Sc-CO2
Chemical	PU, PE, PVC, PP, PI6	Water, ADC, sodium bicarbonate, citric acid
Mechanical	Epoxy resin, PP, phenolic, PU, biobased	Water, air

. Below, we briefly introduce the main mature foaming methods and their features.

2.1. Physical Foaming

Physical foaming aims to mix or diffuse the BA into a polymer melt at a specific temperature and pressure to promote the polymeric foam’s expansion into a cellular structure. Four steps are involved in this foaming process: (1) the development of a homogenous mixture of the polymeric matrix and BA, (2) nucleation formation, (3) the growth of bubbles, and (4) the formation of a stable cellular structure.

2.1.1. Extrusion Foaming

Extrusion foaming is a simple physical foaming method; as illustrated in Figure 5a, it is a continuous foaming process that converts a polymer melt into a porous polymer structure with expanded dimensions in terms of either a continuous structure (into the die) or discrete foam pellets with controllable dimensions [44,45,46,47]. During this process, a BA is first uniformly mixed with a polymer melt under controlled process parameters (e.g., temperature, pressure, etc.). The resulting mixture undergoes severe shearing and heating to ensure its homogeneity. When the homogenous mixture reaches the metal die, its pressure drops rapidly, resulting in the dissolved gas being supersaturated, and bubbles are nucleated and produced to create a cellular structure. The design of an extruded polymeric foam depends not only on the gas diffusivity and solubility, polymer melt viscosity, depressurization, zero-shear viscosity, and relaxation time but also on the effective control of the process parameters, e.g., the temperature profile, pressure gradient, screw speed, and die geometries [22,25,45,55]. In this process, single-screw extruders, co-rotating twin extruders, or tandem extruders are typically utilized for foam fabrication [56]. Extrusion foaming is an economically favorable foaming process with a high throughput, as well as offering versatile shapes and final foam properties [57], because this foaming technique enables one to produce foams with ideal cellular structures. For example, clay-reinforced PLA nanocomposite foams can be prepared to exhibit enhanced cell densities and expansion ratios at high nanoclay content, as shown in Figure 5b [58].

2.1.2. Injection Molding Foaming (IMF)

Injection molding is a commonly used foaming technique for the production of polymeric foams, with its foaming process illustrated in Figure 6a. The SEM micrograph of the cellular structure of an open-cell PP foam prepared by means of IMF is shown in Figure 6b. The foaming process of the IMF technique is based on mixing and dissolving the BA in the base polymer to create a polymer melt at a specific temperature and pressure, which is then injected into the mold to produce the polymeric foam [51,59]. During this process, the pressure of the polymer melt drops substantially and bubbles nucleate, grow, and release from the polymer melt to form the porous cellular structure of the foam [51,60]. Tailored cellular structures can be developed via adjusting the process parameters, e.g., the polymer melt and mold temperature, injection volume and speed, holding and cooling time, time delay [59], and backpressure [51].

The foam injection molding technique offers several technical merits, e.g., high productivity, continuous production, improved foam quality, reduced material and energy consumption, enhanced mechanical stiffness and impact resistance, etc. [51,62]. However, a few technical challenges are still present, including the need for specific equipment with modifications, i.e., specialized injection molding systems [26], weight reduction limitations, and the need for careful monitoring of the process parameters to avoid premature foaming behavior, nonuniform cellular structures, and surface defects (e.g., swirl marks) [63,64].

2.2. Chemical Foaming

In the chemical foaming process, polymeric foams are chemically synthesized through one or more chemical reactions between the polymeric matrix and the BAs, in which the mixture reacts under heating to produce gases and bubbles and the creation of cellular structures is facilitated. Alternatively, polymeric foams in this category can be produced when two polymers are mixed and react to produce inert gases to ultimately form the cellular structure, as illustrated in Figure 7 [22,26]. For instance, the reaction of isocyanates with water produces CO₂ and amine for foam formation [65]. This foaming process is exothermic, which brings technical challenges in terms of maintaining process control. In addition, foams produced in this foaming process may carry harmful chemical residues that create allergic effects, which restricts the application of some types of foams in sensitive environments. As an example, Cui et al. [66] produced electrically conductive CB/CPPC foams with a uniform cellular structure via a two-step foaming process involving melt blending and ADC, which resulted in a reduced threshold for the electrical percolation of the foams and offered a technical route to producing large-scale porous materials with negative temperature coefficients (NTCs).

2.3. Mechanical Foaming

Mechanical foaming is a simple polymer processing technique for the production of polymeric foams with vigorous mechanical agitation (high shear mixing) [3,4]. The mechanical agitation functions to diffuse air into the resin mixture, serving to generate bubbles and create the cellular structure in the resulting foam. Stirring is carried out at a controlled speed, as shown in Figure 8 [67]. For instance, a polyol or resol is used to produce PU [68] or phenolic [69] foams via mechanical foaming, with the addition of the necessary chemical components, e.g., isocyanates, BAs, catalysts, surfactants, and additives. Jin et al. [22] reported that mechanical foaming is a convenient process compared to other foaming methods. This foaming process is simple to control and relatively green and safe, without the release of toxic gases into the working environment, and it has a low cost and high efficiency for massive foam production.

3. Foam Properties

In polymeric foams, sufficient physical and mechanical properties are crucial for their structural stability and durability in short-/long-term applications. In addition, their mechanical properties are fundamentally important for shock absorption, packaging, and other industrial applications of polymeric foams.

3.1. Typical Physical Properties of Polymeric Foams

3.1.1. Mass Density

Mass density is one of the fundamental physical parameters of foams and is crucial for foam stability, structural integrity, and material costs. Low-density foams exhibit relatively poor mechanical performance owing to their larger cells and thinner wall sizes. In contrast, high-density foams generally possess enhanced mechanical stiffness and strength due to their smaller cell sizes, thicker wall sizes, and more uniform structures. The mass density has a direct impact on the overall foam performance, including the weight [70], mechanical properties [71], heat transfer [72], and material cost. The factors that influence the mass density of a foam include not only the foaming materials [73,74], but also the used foaming method [75], as previously shown in Table 6.

For instance, in their experimental study of polymer foaming, Thirumal et al. [72] observed that, when the mass density of a tested PUF decreased from 116 to 42 kgm⁻³, the corresponding compressive strength of the foam dropped from 85.3 to 19.6 Ncm⁻², and a power-law relationship existed between the compressive strength of the PUF and its mass density. It has been evidenced that high-density foams carry improved mechanical strength, hardness, toughness, and resistance to fracture, owing to their small, uniform, and dense cell structures. Nevertheless, low-density PUFs show poor mechanical properties owing to their larger cell sizes and thinner walls, which make them more prone to failure under external loading.

In addition, Kabir et al. [28] investigated the mechanical behavior of a crosslinked PVC and a rigid PUF and revealed that the mass density of the foams directly correlates with their mechanical strength. High-density foams demonstrate higher tensile strength and stiffness, as shown in Figure 9. Moreover, the fracture toughness and impact resistance of a polymeric foam depend upon its mass density.

3.1.2. Thermal Conductivity (TC)

TC is essential for foams used as isolating materials. In principle, lower TC is desirable for temperature-relevant thermal insulation usage, e.g., the packaging of refrigerated or frozen foods, vaccines, and other temperature-sensitive materials [20,76,77]. TC (k) is a quantitative measure of a material’s ability to conduct heat [76], with the SI unit Wm⁻¹K⁻¹, and is defined as

$$k=\left({\displaystyle \frac{q}{t}}\right){\displaystyle \frac{L}{A∆T}},$$

(1)

where q is the heat flux (Wm⁻²), L is the heat transfer distance (i.e., the thickness) (m), A is the area of the body through which heat is transferred (m²), and $∆T$ is the temperature difference (K).

Three heat transfer modes exist in nature, i.e., thermal conduction, convection, and radiation. Thermal convection is negligible in polymeric foams, especially in closed-cell foams with sealed and restricted air flow. In addition, thermal conduction occurs in a solid material, and gas conduction also happens within pores. Thermal radiation is also possible in foams. Thus, the overall TC (k_t) of a foam can be expressed as [20]

$$k_t=k_s+k_g+k_{r.}$$

(2)

where k_s is the thermal conductivity of the foam material, k_g denotes the thermal conductivity due to gases, and $k_r$ represents the thermal radiation coefficient.

Figure 10 illustrates the heat transfer through a layer consisting of an insulation foam, which is employed to reduce the effective thermal conductivity of the layer via potentially controlling the foam density and cell structure (open or closed), optimizing the chemical composition, selecting a proper foaming method, and tuning the process parameters. To tailor the thermal properties of a polymeric foam, an optimized ratio between the base polymer and the BA is crucial in order to design a closed-cell morphology to trap gases and to reduce the mass density. Any unevenness may lead to an irregular or enlarged cellular structure, which can negatively influence the heat transfer in a polymeric foam.

Table 7. Comparison of thermal conductivity data and applications of open- and closed-cell PUFs [77].

PUF	$\mathsf{\rho }$ (kgm−3)	$\mathsf{\lambda }$ (Wm−1K−1)	VP	Insulation Applications
Open cell	35–60	0.035–0.042	Higher	Interior uses, e.g., interior walls, ceilings, and attic areas
Closed cell	8–15	0.026–0.028	Lower	Exterior uses, e.g., exterior walls in wet conditions

$\mathsf{\rho }:$ apparent mass density;$\mathsf{\lambda }:$ thermal conductivity; VP: vapor permeability.

shows the values of thermal conductivity and the thermal insulation applications of typical open- and closed-cell PUFs [77].

The TC of polymeric foams depends not only upon their physical, chemical, and structural factors—e.g., the thermal conductivity of the polymer and gas phases, the closed-cell structure that traps gases within the cells, the thermal radiation between foam cells, the cell morphology and foam density, etc. [78,79]—but also on the ratio of isocyanate and hydroxyl groups, catalyst concentration, surfactants, and BAs [74].

3.2. Mechanical Properties of Polymeric Foams

3.2.1. Experimental Observations

Mechanical properties are fundamental for the practical application of polymeric foams. Sufficient mechanical strength and modulus are essential for foams to exhibit a load-carrying capacity. Three main factors govern the mechanical properties of a foam, i.e., the foaming material, the processing method, and the foam morphology [80]. The major parameters influencing the foam’s mechanical properties include the ratio of open cells, the cell size, the shape of the cells or their geometrical anisotropy, the thickness of the cell walls, and the distribution of the solid material between the struts and faces [80,81]. The tensile strength, modulus, and fracture toughness of a foam are strongly governed by the foam’s mass density, the cell orientation, and the degree of base polymer crosslinking [82]. Typically, the mechanical properties of polymeric foams are poorer; therefore, additives are commonly employed in foaming to enhance the mechanical properties of the resulting polymeric foams. For instance, reinforcing fibers were used to enhance cell nucleation and cell morphology [73]. Such a reinforcing strategy can improve the mechanical performance of polymeric foams. When 1.0 wt.% carbon nanofibers (CNFs) were added to a PUF, the resulting CNF-reinforced PUF showed enhancements of 86% in the tensile modulus, 35% in tensile strength, 57% in compressive strength, 40% in the compressive modulus, 40% in flexural strength, and 45% in the flexural modulus [83].

To evaluate the mechanical behavior of a solid material, a stress–strain diagram under uniaxial compression is essential. Malai et al. [84] conducted a uniaxial compression test on an EPS foam (mass density: 20 kgm⁻³) and obtained the compressive stress–strain diagram, which consisted of three characteristic compressive zones, as shown in Figure 11. Zone 1 represents the linearly elastic compression region at compressive strain below 1–2%, where the buckling and bending of the cell walls of the foam can occur. Zone 2 reflects the compression transition from the elastic to plastic regions. In this zone, the maximum amount of strain energy is stored in the foam due to the release of air from the closed cells, as well as the buckling and plastic deformation of the cell walls. Zone 3 (densification) indicates the transition of the internal microstructure of the foam from a cellular structure to the stacking of the cell walls; this is due to the collapse of the cellular structure upon the release of trapped air from the closed cells under large compressive strain.

3.2.2. Rate Effects in Mechanical Properties of Polymeric Foams

The mechanical properties of polymers and particle-/fiber-reinforced polymer matrix composites (PMCs) exhibit obvious rate effects (i.e., temperature and strain rate effects) in terms of their stress responses, strength and moduli, ultimate tensile strain, and fracture toughness, as well as in their viscoelastic and viscoplastic behavior [29,85,86,87,88,89,90]. Polymeric foams also demonstrate a certain extent of strain rate sensitivity, as evidenced by their enhanced elastic moduli, as well as plateau stresses along with decreasing densification strain [91]. A one-dimensional (1D) power-law constitutive law involving the strain rate was reported in the literature [91]:

$$\sigma \left(\epsilon \right)={\sigma }_0\left(\epsilon \right) {\left({\displaystyle \frac{\dot{\epsilon }}{{\dot{\epsilon }}_0}}\right)}^{n\left(\epsilon \right)},$$

(3)

$$n\left(\mathsf{\epsilon }\right)=a+b\mathsf{\epsilon } \mathrm{f}\mathrm{o}\mathrm{r} {10}^{-3}\le \dot{\mathsf{\epsilon }}\le {10}^2 \left(s^{-1}\right),$$

(4)

where${\sigma }_0\left(\epsilon \right)$ and ${\dot{\epsilon }}_0$ are, respectively, the quasi-static stress–strain relationship and a strain rate constant, and a and b are two material constants determined via fitting the experimental data.

In addition, when only considering the linearly viscoelastic properties of polymers and polymeric foams, their stress responses exhibit simple strain rate effects under uniaxial tension and compression tests. Below, we examine the strain rate effect in the stress responses of three fundamental linearly viscoelastic solids based on the Maxwell, Kelvin–Voigt, and three-parameter solid models, which are the simplest and most rational approaches to understanding the viscoelastic behavior of polymeric foams [29]. To simplify the discussion, we assume a linearly viscoelastic polymeric foam sample subjected to a displacement–control test with a constant strain rate. For a uniaxial tension specimen of length L₀ and uniform cross-sectional area A₀ subjected to a constant displacement rate (speed) V₀, the engineering tensile strain ε and strain rate $\dot{\epsilon }$ can be expressed as

$$\epsilon ={\displaystyle \frac{\Delta L}{L_0}},$$

(5)

$$\dot{\epsilon }={\displaystyle \frac{d\epsilon }{dt}}={\displaystyle \frac{d}{dt}}{\displaystyle \frac{\Delta L}{L_0}}={\displaystyle \frac{1}{L_0}}{\displaystyle \frac{d\Delta L}{dt}}={\displaystyle \frac{V}{L_0}}.$$

(6)

For a liquid-like Maxwell linearly viscoelastic foam with the material model shown in Figure 12, the constitutive law is

$$\dot{\epsilon }={\displaystyle \frac{\dot{\sigma }}{E}}+{\displaystyle \frac{\sigma }{\eta }},$$

(7)

where E is the stiffness of the linearly elastic spring and η is the kinetic viscosity of the linearly viscous damper (Newtonian) in the Maxwell model.

Under a constant strain rate V/L₀ and the stress response for time t > 0, Equation (7) can be expressed as

$${\displaystyle \frac{\dot{\sigma }}{E}}+{\displaystyle \frac{\sigma }{\eta }}={\displaystyle \frac{V}{L_0}},$$

(8)

which leads to the stress response

$$\sigma (t)=C{e}^{-{\displaystyle \frac{E}{\eta }}t}+\eta {\displaystyle \frac{V}{L_0}},$$

(9)

and C is the integration constant to be determined by the initial constant σ(0) = 0, i.e., C = −ηV/L₀. Thus, the rate effect of the Maxwell model can be expressed as

$$\sigma (t)=\eta {\displaystyle \frac{V}{L_0}}(1-e^{-{\displaystyle \frac{E}{\eta }}t}).$$

(10)

With the tensile strain of ε(t) = (V/L₀)t, the stress–strain relationship can be extracted from Equation (10) after eliminating t by t = ε/(V/L₀) as

$$\sigma =\eta {\displaystyle \frac{V}{L_0}}(1-e^{-{\displaystyle \frac{E}{\eta }}{\displaystyle \frac{L_0}{V}}\epsilon }),$$

(11)

and the corresponding effective modulus E_e is

$$E_e={\displaystyle \frac{d\sigma }{d\epsilon }}=E{e}^{-{\displaystyle \frac{E}{\eta }}{\displaystyle \frac{L_0}{V}}\epsilon },$$

(12)

i.e., the effective modulus E_e decays exponentially with strain ε for a Maxwell viscoelastic polymeric foam under a constant strain rate $\dot{\epsilon }=L_0/V$.

Similarly, for a solid-like linearly viscoelastic polymeric foam with the material model (Kelvin–Voigt model) shown in Figure 13, the constitutive law is

$$\sigma =E\epsilon +\eta \dot{\epsilon }.$$

(13)

Under a constant tensile strain rate V/L₀ and the stress response for time t > 0, Equation (13) can be recast as

$$\sigma =E\epsilon +\eta {\displaystyle \frac{V}{L_0}},$$

(14)

which results in a linear relationship between the tensile stress σ and tensile strain ε with a constant shift $\eta \dot{\epsilon }$. In this case, the effective modulus is constant as E and, therefore, there is no rate effect in this case.

Moreover, for a three-parameter linearly viscoelastic polymeric foam with the material model shown in Figure 14, the constitutive law is

$$E_1{E}_2\epsilon +E_2{\eta }_1\dot{\epsilon }=(E_1+E_2)\sigma +{\eta }_1\dot{\sigma },$$

(15)

where E₁ and E₂ are, respectively, the stiffnesses of the two linearly elastic springs, and η₁ is the kinetic viscosity of the linearly viscous damper in the model.

Under a constant strain rate V/L₀ and the stress response for time t > 0, Equation (15) can be expressed as

$$\dot{\sigma }+{\displaystyle \frac{E_1+E_2}{{\eta }_1}}\sigma ={\displaystyle \frac{E_1{E}_2}{{\eta }_1}}{\displaystyle \frac{V}{L_0}}t+E_2{\displaystyle \frac{V}{L_0}}.$$

(16)

The stress response for t > 0 can be solved from Equation (16) as

$$\sigma (t)=C{e}^{-{\displaystyle \frac{E_1+E_2}{{\eta }_1}}t}+{\displaystyle \frac{E_1{E}_2}{E_1+E_2}}{\displaystyle \frac{V}{L_0}}t+{\displaystyle \frac{E_1}{E_1+E_2}}{\displaystyle \frac{V}{L_0}}\eta ,$$

(17)

where C is the integration constant to be determined by the initial constant σ(0) = 0, i.e., C = −[E₁/(E₁ + E₂)](V/L₀)η. Thus, the rate effect of the three-parameter linearly viscoelastic model can be expressed as

$$\sigma (t)={\displaystyle \frac{E_1}{E_1+E_2}}{\displaystyle \frac{V}{L_0}}\eta (1-e^{-{\displaystyle \frac{E_1+E_2}{{\eta }_1}}t})+{\displaystyle \frac{E_1{E}_2}{E_1+E_2}}{\displaystyle \frac{V}{L_0}}t.$$

(18)

With the constant tensile strain of ε(t) = (V/L₀)t, the stress–strain relationship can be obtained from (18) after eliminating t by using t = ε/(V/L₀) as

$$\sigma (t)={\displaystyle \frac{E_1}{E_1+E_2}}{\displaystyle \frac{V}{L_0}}\eta (1-e^{-{\displaystyle \frac{E_1+E_2}{{\eta }_1}}{\displaystyle \frac{L_0}{V}}\epsilon })+{\displaystyle \frac{E_1{E}_2}{E_1+E_2}}\epsilon ,$$

(19)

and the corresponding effective modulus E_e is

$$E_e={\displaystyle \frac{d\sigma }{d\epsilon }}=E_1{e}^{-{\displaystyle \frac{E_1+E_2}{{\eta }_1}}{\displaystyle \frac{L_0}{V}}\epsilon }+{\displaystyle \frac{E_1{E}_2}{E_1+E_2}},$$

(20)

i.e., the effective modulus E_e is the effective modulus of the two linearly elastic springs superimposed with an exponentially decaying term with respect to strain ε.

4. Sustainable Biobased Foams (BBFs): A Promising Future

Sustainable BBFs represent a rapidly developing field with a promising future, driven by growing environmental concerns and the shift towards a circular economy [16,17]. These foams are produced from renewable resources, e.g., soybeans, corn, sugarcane, vegetable oils, potatoes, and agricultural residues, and offer a viable alternative to traditional petroleum-based foams. As a subgroup of polymeric foams, BBFs can also be categorized into thermoplastic and thermosetting, as well as biodegradable (e.g., PHAs, PLA, starch-and cellulose-based, PBAT, PPC, PCL, PVA) and nonbiodegradable (e.g., BPUs, BEFs, BPFs, etc.). Among others, some BEF, BPF, and BPU foams are produced from renewable raw materials, e.g., PU (100% soybean oil-based isocyanate) and bio-polyphenolic foams (100% wood tannin). These materials are described in Table 1 in the publication by X. Wang et al. [13], cited by the authors of the reported publication. The same publication also lists bio-polyphenolic foams (25% birch bark oil) and biobased polyurethane (PU) foams produced from liquefied biomass-based polyols and other additives (Figure 3), containing a smaller share of renewable raw materials [13].

To date, intensive research efforts have been dedicated to exploring and producing low-cost, lightweight, property-controllable BBFs from broad biomass materials. In addition, many types of biomass materials can be used as reinforcing fillers, crosslinkers, or polyol sources for the production of foams [92,93]. BBFs, as sustainable alternative materials to petroleum-based foams, can reduce greenhouse emissions and contribute to decreasing the overall carbon footprint of manufacturing. In addition, biobased materials can be used as additives or to produce bio-polyols and bio-phenols after chemical modification [40,94]. For example, biobased PU foams (BPUFs) are prepared from plant oil-derived polyols, which are reacted with isocyanates to produce cellular structures. Similarly, biomass components, e.g., corn stalks, dried distiller’s grains, and wheat straw—can be chemically modified following the same process to yield biobased polyols [95]. In contrast, castor oil is an exceptional source material for the production of BBFs because it naturally contains hydroxyl groups, which are able to react with isocyanates without additional modification [96].

4.1. Soybean-Based Foams (SBFs-1)

SBFs-1 are sustainable and ecofriendly BBFs with multiple technical merits, e.g., a light weight, superior thermoinsulating capability, high specific strength, and excellent energy-absorption capacity for shock, vibration, and sound. Such BBFs show rapidly growing application in thermal insulation [74], cushioning [12,70], antiflaming [94,97], and packaging [93]. SBFs-1 have been synthesized by means of physical, chemical, and mechanical foaming methods. In fact, soybeans cover ~60% of the global plant oil production, and nearly 15% of these plant oils are used for various industrial purposes. The largest shares of soybean oil are produced in the United States, Brazil, and Argentina [98], and 68% of this soybean oil is consumed for food applications in the United States, while the rest is used for biodiesel and polyol production [99].

For the synthesis of PUFs, soybean oil is chemically modified via epoxidation, followed by the nucleophilic opening of the oxirane rings using mono-alcohols [100] to produce polyols. Soybean oil-derived polyols (SBPs), with lower hydroxyl (OH) values, lower OH functionality (i.e., two as the threshold limit), and an enhanced molecular weight (Mw), are preferable for the production of flexible PUFs. Functionality of more than two values can lead to a rigid structure, thereby increasing the stiffness. However, crosslinked networks at these values are difficult to achieve [96,100,101]. SBPs have been optimized to enhance their internal microcellular structures, to maintain the fluid flowability before gelation, and to confirm their structural stability to avoid foam shrinkage. In contrast, some lab-created bio-polyols may face difficulties in stabilizing cell openings and controlling foam shrinkage [100]. A series of soy polyols have been developed with varying OH values ranging from 28 to 224 mg KOHg⁻¹, along with hydroxyl functionality values from 0.48 to 4.9 [100]. SBPs are reacted with isocyanates, and the selection of isocyanates can significantly influence the structures and properties of the resulting soybean-based polyurethane foams (SBPUFs). Campanella et al. [102] reported that toluene diisocyanate (TDI) can lead to closed-cell structures and rigid foams, while methylene diphenyl diisocyanate (MDI) and modified MDI can result in open-cell structures with enhanced flexibility. Moreover, water is commonly used as a BA for the production of SBPUFs. Experimental investigations of the influence of water as a BA in SBPUFs indicate that, with increasing water content, the foam mass density decreases, the morphology varies, and the cell size increases along with the thinner cell walls. However, at lower water content, uniform cellular structures can be achieved [102,103].

In addition, soy protein products can be used for the production of BBFs, e.g., defatted soy flour (DFS), soy protein concentrate (SPC), and soy protein isolate (SPI) [24,70]. These biomaterials have been used to improve the foam’s performance, e.g., using fillers with polyols as crosslinkers [36,104] or blends with a thermoplastic polymer like PLA using extrusion foaming [24,105]. For instance, Bote et al. [99] prepared biobased rigid polyurethane foams (BRPUFs) from soybean meal-derived polyols with other components, as shown in Figure 15, for a thermal insulation application; this resulted in a closed-cell structure with improved compressive strength of ~250 kPa. Moreover, SPI was used as a reinforcing filler in castor oil to prepare a PUF, which showed a higher mass density of 91 kgm⁻³ and enhanced compressive strength of up to 0.24 MPa, as well as a closed-cell ratio of 76%, i.e., increasing the mass density and compressive strength by 41.2% and 230.4%, respectively, compared to the neat PUF [93]. In addition, using the physical foaming method, SPC was used as a blending component with PLA to produce a biocomposite foam via extrusion using Sc-CO₂ foaming, which improved the interfacial compatibility and provided heterogeneous nucleation sites, resulting in higher cell densities and smaller cell sizes. Due to the hydrophilic nature of SPC, it significantly enhanced water absorption and resulted in an accelerated degradation rate [24].

Furthermore, Liu et al. [105] prepared PLA/SPC blended foams using the extrusion foaming method with chemical BAs (CBAs). The resulting foams demonstrated the significant effect of the foaming temperature and CBAs on the cell and foam mass densities. Dhaliwal et al. [98] fabricated SBPUFs for use in thermal insulation. The resulting foams exhibited significantly enhanced properties, with an 8% improvement in thermal resistivity, a ~512% enhancement in compressive strength, and 287% higher tensile strength compared to the control samples. These results indicate that soy-based polyols can effectively substitute petroleum-based polyols to produce BBFs, with the main physical and mechanical properties being comparable and even superior to those of their petroleum-based counterparts available in the market [98]. In addition, researchers have prepared RPUFs with phosphorus additives to enhance their flame retardancy and smoke suppression capabilities. The obtained foams showed significantly enhanced fire resistance, decreasing the peak heat release rate by ~40%, overall heat release by ~35%, and total smoke generation by ~49% compared to unmodified RPUFs [106]. The typical properties of soybean-based foams are summarized in

Table 8. Properties of soy-based foams.

Foam	Density (kgm−3)	${\mathit{\sigma }}_{\mathit{c}}$ (kPa)	${\mathit{E}}_{\mathbf{c}}$ (MPa)	${\mathit{\sigma }}_{\mathit{t}}$ (kPa)	${\mathit{E}}_{\mathbf{t}}$ (MPa)	Eb%	$\mathit{\lambda }$ (Wm−1K−1)	Ref.
SBPUF	54.9–98.8	61–137	-	47–115	-	-	0.025–0.027	[74]
SBRPUF	28.9–32.4	148–229	-	-	-	-	0.022	[92]
SBRPUF	41.0–41.7	203–257	-	-	-	-	-	[99]
SBFPUF	15–34	-	-	1700–2500	0.18–0.29	52–76	-	[102]
SBRPUF	-	390–425	-	-	-	-	0.028–0.030	[95]
SBPUF	36–39	210–250	-	-	-	-	-	[40]
SBRPUF-FR	30.4–39.6	-	-	-	-	-	-	[94]
SBRPUF	37–55	128–148	1.90–2.90	-	-	-	-	[107]

SBFPUF: soy-based flexible polyurethane foam; SBRPUF: soy-based rigid polyurethane foam; FR: flame retardant; ${\mathsf{\sigma }}_{\mathrm{c}}:$compressive strength; ${\mathrm{E}}_{\mathrm{c}}:$ compressive modulus; ${\mathsf{\sigma }}_t:$tensile strength; $E_t$: tensile modulus; Eb: elongation at break (%);$\lambda :$thermal conductivity.

.

4.2. Corn-Based Foams (CBFs)

Corn-based materials offer another low-cost, sustainable, and renewable biomaterial source for the production of BBFs with various applications, e.g., fire resistance, thermal insulation, and packaging. CBFs can be prepared by extrusion [108] and chemical [109] foaming methods. Corn polyols, corn starch, corn/corn husk fibers, and corn straws have been used for BBF fabrication. Corn polyols have been produced via chemical epoxidation [110,111,112] or thiol–ene reactions [113]. Furthermore, biocomposite foams have been fabricated via corn-derived additives, e.g., plasticizers (corn starch) [114], blends (sorbitol and corn polyols) [112], reinforcements (corn straw) [115], corncob fibers [116], and corn stover lignin [117].

Lui et al. [108] prepared corn starch/PVA foams by extrusion and investigated their thermal and structural properties. The resulting BBFs demonstrated reduced thermal stability and improved uniform cellular structure, as evidenced by SEM. Furthermore, Polat et al. [118] examined the effects of corn husk fibers, kaolin clay, and beeswax on crosslinked corn starch foams. The addition of beeswax and kaolin clay enhanced the water resistance, while adding the fiber/kaolin clay/beeswax combination yielded the lowest water absorption (9%). The mechanical properties were improved by the addition of fibers and kaolin clay but reduced when adding beeswax alone. The addition of beeswax and kaolin clay enhanced the cellular size and modified the thermal properties via decreasing the degree of crosslinking. These insights suggest that optimized material formulation can enhance the structural integrity and water resistance of biodegradable corn-based foams.

Additionally, PUFs can be produced from corn-derived polyols by means of chemical methods for different applications, e.g., cushioning and flame retardancy. Ugarte et al. [112] prepared a rigid PUF by mixing sorbitol and corn polyol for use in thermal insulation, which led to closed-cell structures with reduced cell sizes and higher mass densities. Enhanced mechanical properties in the resulting PUFs were achieved, e.g., the specific elastic modulus and compressive strength, densification strain, etc., as shown in

Table 9. Properties of corn-based foams.

Foam	Density (kgm−3)	${\mathsf{\sigma }}_{\mathbf{c}}$ (kPa)	${\mathbf{E}}_{\mathbf{c}}$ (MPa)	Sm (MPa)	$\mathsf{\lambda }$ (Wm−1K−1)	Ref.
CBFPUFs	80.6–83.6	16.5–20.3	-	28.8–48.9	-	[109]
SCBRPUFs	66–103	1015–396	0.984–0.213	-	35.4–36.4	[112]
CBPUF-FR	35	81–120	-	-	-	[120]

CBFPUFs: corn-based flexible polyurethane foams; SCBRPUFs: sorbitol and corn-based rigid polyurethane foams; CBPUF-FR: corn-based polyurethane foam—flame retardant; ${\mathsf{\sigma }}_{\mathrm{c}}$: compressive strength; ${\mathrm{E}}_{\mathrm{c}}:$ compressive modulus; S_m: storage modulus; $\mathsf{\lambda }$: thermal conductivity.

[108,111,119]. Moreover, the performance of corn-based flexible PUFs (CBFPUFs) can be optimized via varying the ratios of the gelling catalyst, blowing catalyst, and surfactant, as shown in Figure 16. Such a strategy resulted in varying surfactant content and improved foam stability, and varying the ratio of the blowing and gelling catalysts may result in a uniform cell morphology. The optimized foam demonstrated improved compressive strength, increased rebound resilience, and superior thermal stability, comparable to the conventional PUFs [108].

Furthermore, flame-retardant corn-based polyurethane foams (CBPUFs) were fabricated using corn oil-derived polyols, which showed a uniform cell structure, closed cells constituting more than 95%, and moderate compressive strength, and the addition of dimethyl methyl phosphonate (DMMP) led to the enhanced flame-retardant properties of the resulting CBPUFs [120]. Similarly, with the addition of inorganic clays, a flexible CBPUF was synthesized with lower compressive strength [110]. It was shown that clay did not significantly influence the foam’s performance. The typical properties of corn-based foams are summarized in Table 9.

4.3. Starch-Based Foams (SBFs-2)

SBFs-2 are another type of low-cost, biodegradable, water-soluble, and nontoxic BBF that could represent perfect alternatives to conventional PS foams—a type of nondegradable foam associated with increasing concern regarding environmental pollution [121]. These BBFs have been extensively used for trays, containers [122], cushions [119], food packaging [123], and thermal and vibration insulation [124]. Starch is a polysaccharide composed of amylose and amylopectin, in which the former is a linear polymer consisting of glucose units, while the latter is composed of glucose and also contains a branched structure, as well as a higher molar mass [125]. SBFs-2 are typically derived from natural sources, e.g., potato, corn, cassava, and oca starch [122,126]. Starch is able to absorb a large amount of water due to its hydrophilic nature. Different modification methods, e.g., acetylation, esterification, and silylation, have been commonly used to increase its water resistance for packaging applications. With proper chemical modification, hydrophobicity can be incorporated into starch to reduce its water absorption. In addition, in order to improve the moisture resistance and mechanical properties of SBFs-2, different approaches have been investigated, e.g., crosslinking with glyoxal or citric acid and the incorporation of natural fillers and agricultural residues (e.g., corn husk fibers, cotton fibers, malt bagasse, sesame cake, and sugarcane bagasse) [126]. SBFs-2 are typically prepared by gelatinization via mixing starch with other components, e.g., water, BAs, plasticizers, and additives. The gelatinized paste is then processed using extrusion, heat pressing, baking, or solvent exchange/vacuum freeze-drying, in which the controlled temperature and pressure cause the material to expand and form a cellular structure [125,127,128]. Furthermore, Duan et al. [128] applied a two-step extrusion process to produce SBFs-2, as shown in Figure 17, in which starch-based pellets were compounded with a small quantity of polysaccharide-based cellulose and starch crystals and then fed into a single-screw extruder for foaming. The starch crystals acted as both nucleating and reinforcing agents to generate a uniform cellular structure, achieving a mass density comparable to that of EPS, and significantly enhanced the compressive strength, elastic resilience, thermal stability, and water resistance [128].

In addition, Shogren et al. [127] prepared starch foam trays by first dry-mixing starch with guar gum to increase the mixture viscosity and magnesium stearate (Mg(C₁₈H₃₅O₂)₂) to facilitate mold release. Then, distilled water was added to form a batter with specific solid content. This batter was baked in a heated, closed steel mold using a lab-made baking machine, where the starch gelatinized, expanded, and dried to form a foam tray. The process parameters—e.g., the batter volume, mold temperature, and baking time—were controlled to ensure uniform foam formation in trays [127]. Furthermore, Reis et al. [129] used starch, glycerol, and PLA, which were mixed for pellet formation using a single-screw extruder. Again, the pellets were processed in an extruder to process sheets, which were subsequently thermoformed in a mold. During the expansion phase, a hydraulic press was utilized to press the mold, and the sheets were prepared by molding at 100 °C under 100 bar for 2 min to create expanded trays. In addition, Tibalia et al. [122] prepared starch-based food containers by thermopressing; the containers were reinforced with orange peel (up to 10% w/w) and exhibited both improved water resistance and mechanical properties. In contrast, sugarcane bagasse containers showed a water absorption value of ~10.95% and tensile strength of 6.59 MPa, outperforming rice straw containers. Degradation tests showed that the prepared containers were fully biodegradable, resulting in promising alternatives to conventional Styrofoam^® for packaging [101]. Kaisangsri et al. [130] prepared biodegradable foam trays by mixing cassava starch with plant fibers and chitosan and using a hot-mold baking process in an oven at a controlled temperature of 250 °C for 5 min. These BBFs showed tensile strength of 944 kPa and elongation of 2.43%, and both the water absorption index (WAI) and water solubility index (WSI) were superior to those of PS foams.

Guan et al. [116] prepared ecofriendly composite foams by blending starch acetate with either corncob fibers or cellulose for packaging applications. Their experimental results indicated that corncob fibers were able to enhance the compressive strength and mass density, while cellulose led to a more uniform cellular structure and increased moisture resistance. Lopez-Gil et al. [131] prepared SBFs-2 reinforced with grape waste, thistle waste, barley, and straw fibers by means of a microwave foaming process. The resulting foam had sufficient mechanical stiffness and strength suitable for structural applications. For thermal insulation applications, Han et al. [124] investigated SBFs-2 derived from waxy corn starch and high-amylose corn starch at different mixing ratios. The SBFs-2 were prepared with high amylose content, exhibiting increased viscosity, irregular cell structures, higher mass densities, enhanced flexural strength, improved water resistance, and downgraded thermal insulation performance. However, SBFs-2 containing high fractions of amylopectin showed more regular cell structures, lower densities, higher water absorption, and enhanced thermal insulation performance. The typical properties of SBFs-2 are summarized in

Table 10. Densities and mechanical properties of starch-based foams.

Foam	Density (kgm−3)	${\mathsf{\sigma }}_{\mathbf{c}}$ (kPa)	E (MPa)	${\mathsf{\sigma }}_{\mathbf{t}}$ (MPa)	Ref.
Starch-based	98.68	2.77–9.14	0.0235–0.146	-	[119]
Starch	14–34	-	142–450	-	[127]
Starch	19.94–32.53	75–125.10	-	-	[128]
Starch/PLA	119–129	-	185–294	6.1–11.5	[129]
Starch composites	21.1–45.7	40–114	-	-	[132]

${\mathsf{\sigma }}_{\mathrm{c}}$: compressive strength; E: elastic modulus;${\mathsf{\sigma }}_{\mathrm{t}}$: tensile strength.

.

5. Challenges in BBF Manufacturing

BBFs are promising substitutes for conventional foams. However, their large-scale production and commercialization still face various technical challenges in terms of process consistency, scalability, cost-effectiveness, reproducibility, and feasible adoption in industry. Specific technical challenges are as follows.

• High manufacturing costs of BBFs compared to those of conventional synthetic foams.
• Pretreatment of biomass materials and extraction of bio-polyols and bio-phenols for processing of biobased PU and bio-phenolic foams [77].
• Difficulties in maintaining uniform cell morphology, including cell size, cell density, cell wall thickness, and struts of BBFs.
• High viscosities and lower reactivities of soy-based polyols, which complicate processing and foam formulation.
• Technical challenges in uniformly dispersing additives to achieve optimal mechanical performance. The incorporation of soybean husk-derived ashes as a filler into rigid PUFs may disrupt the cellular structure and hinder uniform cell formation and quality. Limited chemical interaction between the fillers and the polymeric resins results in weak interfacial adhesion and poor mechanical properties [107].
• Relatively lower mechanical and thermal properties of BBFs compared to conventional synthetic counterparts. Feasible additives (e.g., plant fibers and nanomaterials) and chemical modification are desired to further enhance the physical and mechanical properties of BBFs [13].
• Poor moisture resistance of BBFs due to hydrophilic nature of constituents, e.g., starch, cellulose, and protein foams. Chemical modification is sought to improve the water resistance and hydrophobicity of BBFs for high performance and durability in applications.
• Inadequate standardized life-cycle assessment (LCA) data for biobased materials, e.g., lignin and plant-derived polyols, which complicates the assessment of their impacts on the environment [133].
• Replacement of bio-polyols with lignin to overcome negative environmental impacts of bio-polyols with compromised mass density and thermal conductivity, since bio-polyols are not always environmentally friendly [133].
• Inherent brittleness and water sensitivity of starch-based materials negatively influence physical and mechanical properties of resulting BBFs. Additional chemical modifications, plasticizers, coatings, and additives need to be rationally explored to address the weaknesses and enhance the desired properties [124].

6. Promising Applications of BBFs

• Automobile and aerospace industries: Applications in vehicle interiors, door panels, and aerospace insulation due to their light weight and thermal resistance properties.
• Thermal and acoustic insulation: Installation in buildings, fire-resistant structures, refrigerators, sound-absorbing walls, vibration dampers, and other thermal and acoustic barriers.
• Medical field: Potential uses in tissue engineering, wound healing, medicine and vaccine storage, etc. [20,134,135].
• Agriculture: Potential applications in controlled agricultural practices, e.g., greenhouse thermal insulation and hydroponic substrates. Sound and thermal insulation and moisture resistance of BBFs can provide improved plant growth conditions. Dsouza et al. [133] reported that biobased PU and phenolic foams, especially those improved with lignocellulosic materials, showed huge potential for hydrophilic usage, including environmental remediation, floral arrangements, and hydroponic germination.
• Sensors: Use of BBFs in wearable electronics and low-temperature sensors in medical, food, and environmental monitoring [66], pressure sensing, and electromagnetic interference applications [136].
• Packaging solutions: Broad applications of low-cost BBFs for food trays, containers, and loose filling materials for the packaging of commercial products.
• Other uses: Superhydrophobic foam membranes for waste oil sorption, active oil and water separation [137], wastewater treatment [138], and antibacterial protection [40].

7. Concluding Remarks

The growing demand for BBFs has drawn significant attention toward the development of sustainable biobased substitutes for conventional synthetic polymer foams. BBFs offer a myriad advantages in terms of sustainability, renewability, biodegradability, nontoxicity, cost-effectiveness, sufficiency, and positive impacts on the circular economy. BBFs also exhibit sufficient mechanical, thermal, and flame resistance properties, comparable to those of their traditional synthetic counterparts available in the market. For various applications, BBFs can be produced using physical, mechanical, chemical, backing, and compression molding foaming methods. Moreover, the structural features of BBFs—e.g., open- versus closed-cell configurations, cell anisotropy, wall thickness, and morphology—strongly influence their mechanical strength, elastic moduli, toughness, thermal conductivity, etc.

However, the physical and mechanical characterization of BBFs is needed to elucidate their structure–property relationships and to achieve or even surpass the performance of their conventional synthetic counterparts in terms of mechanical strength, thermal stability, water resistance, and commercial viability. Additionally, innovative strategies—e.g., chemical modification and the incorporation of various fillers (e.g., natural and synthetic fibers and nanoparticles)—can be further formulated to enhance these properties. Moreover, it is essential and technologically desirable to explore novel physical and chemical treatments to optimize the structural stability, mechanical strength and stiffness, resilience, flame retardancy, and biodegradation efficiency of BBFs.

Therefore, to develop ecotechnologically successful BBFs, research must be aligned with industrial requirements and commercial viability regarding the foam’s performance, production, and scalability. In addition, LCAs for BBFs should take into account the diverse end-of-life scenarios, e.g., environmental composting, recycling, and landfill disposal. It is necessary to consider that BBFs, which exhibit significant biodegradability during their service lives, may experience compromised long-term mechanical and thermal insulation properties when integrated into structural parts. Thus, an in-depth understanding and optimal design regarding the long-term biodegradation of BBFs is critical. Consequently, the manufacturing, deployment, and recycling of BBFs are expected to lead to broad green, sustainable, and recyclable applications of foams in various industrial sectors and human life.