Biofoam

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Biofoams are biological or biologically derived foams, making up lightweight and porous cellular solids. A relatively new term, its use in academia began in the 1980s in relation to the scum that formed on activated sludge plants. [1] [2]

Contents

Biofoams is a broad umbrella term that covers a large variety of topics including naturally occurring foams, as well as foams produced from biological materials such as soy oil and cellulose. Biofoams have been a topic of continuous research because synthesized biofoams are being considered as alternatives to traditional petroleum-based foams. [3] [4] [5] Due to the variable nature of synthesized foams, they can have a variety of characteristics and material properties that make them suitable for packaging, insulation, and other applications.

Naturally occurring foams

Foams can form naturally within a variety of living organisms. For example, wood, cork, and plant matter all can have foam components or structures. [6] Fungi are generally composed of mycelium, which is made up of hollow filaments of chitin nanofibers bound to other components. [7] Animal parts like cancellous bone, horseshoe crab shells, toucan beaks, sponge, coral, feathers, and antlers all contain foam-like structures which decrease overall weight at the expense of other material properties. [6] [8] [9]

Structures like bone, antlers, and shells have strong materials housing weaker but lighter materials within. Bones tend to have compact, dense external regions, which protect the internal foam-like cancelous bone. [6] [8] The same principle applies to horseshoe crab shells, toucan beaks, and antlers. [6] [9] [10] The barbs and shafts of feathers similarly contain closed-cell foam. [6] [11]

Protective foams can be formed externally by parent organisms or by eggs interacting with the environment: tunicate egg mix with sea water to create a liquid-based foam; tree frog eggs grow in protein foams above and on water (see Figure 1); certain freshwater fish lay eggs in surface foam from their mucus; deep sea fish produce eggs in swimbladders of dual layered foams; and some insects keep their larvae in foam. [12] [13]

Biomimetic synthetic foams

Honeycomb

Figure 2: Depiction of various honeycomb patterns 10338 2021 262 Fig1 HTML.webp
Figure 2: Depiction of various honeycomb patterns

Honeycomb refers to bioinspired patterns that provide a lightweight design for energy absorbing structures. Honeycomb design can be found in different structural biological components such as spongy bone and plant vasculature. Biologically inspired honeycomb structures include Kelvin, Weaire and Floret honeycomb (see Figure 2); each with a slightly different structure in comparison to the natural hexagonal honeycomb. These variations on the biological design have yielded significantly improved energy absorption results in comparison to traditional hexagonal honeycomb biofoam. [14]

Due to these increased energy absorption performances, honeycomb inspired structures are being researched for use inside vehicle crumple zones. By using honeycomb structures as the inner core and surrounding the structure with a more rigid structural shell, these components can absorb impact energy during a crash and reduce the amount of energy the driver experiences. [15]

Aerogel

Figure 3: Bioinspired Macroscopic Carbon Tube Aerogel (CTA) compared with polar bear fur at high magnification 1-s2.0-S2451929419302025-gr1.jpg
Figure 3: Bioinspired Macroscopic Carbon Tube Aerogel (CTA) compared with polar bear fur at high magnification

Aerogels are able to fill large volumes with minimal material yielding special properties such as low density and low thermal conductivity. These aerogels tend to have internal structures categorized as open or closed cell structures, [17] the same cell structure that is used to define many 3-dimensional honeycomb biofoams. Aerogels are also being engineered to mirror the internal foam structures of animal hairs (see Figure 3). These biomimetic aerogels are being actively researched for their promising elastic and insulative properties. [16]

Material properties

Foam cell structures

A foam is considered open-celled if at least two of its facets are holes rather than walls. [18] In this case the entirety of the load on the foam is on the cross-beams that make up the edges of the cell. [6] If no more than one of the walls of the cell are holes, the foam is considered closed-celled in nature. [18] For most synthetic foams, a mixture of closed cell and open cell character is observed due to cells rupturing during the foaming process and then the matrix solidifying. [18] [5]

The mechanical properties of the foam then depend on the closed cell character of the foam as derived by Gibson and Ashby: [19]

Where E is the elastic modulus, ρ is the density of the material, φ is the ratio of the volume of the face to the volume of the edge of the material, and the subscript s denotes the bulk property of the material rather than that of the foam sample.

Liquid and solid foams

For many polymeric foams, a solidified foam is formed by polymerizing and foaming a liquid polymer mixture and then allowing that foam to solidify. [5] [12] [18] [3] Thus, liquid foam aging effects do occur before solidification. In the liquid foam, gravitational forces and internal pressures cause a flow of the liquid toward the bottom of the foam. [12] [20] This causes some of the foam cells to form into irregular polyhedra as liquid drains, which are less stable structures than the spherical structures of a traditional foam. [12] These structures can however be stabilized by the presence of a surfactant. [20]

The foam structure before solidification is an inherently unstable one, as the voids present greatly increase the surface free energy of the structure. [12] [20] In some synthetic biofoams, a surfactant can be used in order to lower the surface free energy of the foam and therefore stabilize the foam. In some natural biofoams, proteins can act as the surfactants for the foams to form and stabilize. [12]

Fiber reinforcement

During the solidification of synthetic biofoams, fibers may be added as a reinforcement agent for the matrix. [18] [4] This additionally will create a heterogeneous nucleation site for the air pockets of the foam itself during the foaming process. [18] However, as fiber content increases, it can begin to inhibit formation of the cellular structure of the matrix. [4]

Applications

Packaging

Figure 4. PLA being produced through ring-opening polymerization of lactide. PLA Ring-Opening Polymerization of Lactide.jpg
Figure 4. PLA being produced through ring-opening polymerization of lactide.

In relation to packaging, starches and biopolyesters make up these biofoams as they are adequate replacements to expanded polystyrene. [22] Polylactic acids (PLAs) are a common form of the basis of these biofoams since they offer a substitute for polyolefin-based foams that are commonly used in automotive parts, pharmaceutical products, and short life-time disposable packaging industries due to their bio-based and biodegradable properties. [23] PLA comes from the formation of lactide produced from lactic acid due to bacterial fermentation through ring-opening polymerization, in which the process is shown through Figure 4. [21]

PLA does not have the most desirable traits for biodegradability in the packaging industry as it contains a low heat distortion temperature and has unfavorable water barrier characteristics. [22] On the other hand, PLA has been shown to have desirable packaging properties including high ultraviolet light barrier properties, and low melting and glass transition temperatures. [24] As of recently, PGA has been introduced in the packaging industry as it is a good solvent and comparable to PLA. Table 1 shows the characteristics of both biofoams and how they compare. [25] As shown, PGA contains a strong stereochemistry structure which in turn causes it to have high barrier and mechanical properties making it desirable for the packaging industry. [25] The study of mixing both PGA and PLA has been explored by using copolymerization in order for PGA to help enhance the barrier properties of PLA when used in packaging. [25]

Tg (°C)Tm (°C)Tensile strength (MPa)Young's modulus (GPa)Elongation at break (%)Flexural strength (MPa)Flexural modulus (GPa)
PLA57-58140-180532.45923.4
PGA35-40220-230115716.42227.8

Table 1: The properties of PLA in comparison to PGA. [25]

Biomedical

Figure 5: The FFF-based 3D printed PLA undergoing compression testing and its shape after heating the sample greater than its glass transition temperature (Tg). Compression of FFF-based 3D printed PLA.jpg
Figure 5: The FFF-based 3D printed PLA undergoing compression testing and its shape after heating the sample greater than its glass transition temperature (Tg).

The most popular biofoam in the use of biomedical devices is PLA as well. PLA's properties are also desirable in biomedical applications, especially in combination with other polymers. [26] Specifically, its biocompatibility and biodegradability make it favorable in tissue engineering through the use of FDM-3D printing. [26] PLA does well in these printing environments as its glass transition temperature as well as shape memory is small. [21] In recent studies, PLA has been specifically combined with hydroxyapatite (HA) in order to make the modulus of the sample more favorable for its application in repairing bone failure. [26] Specifically in tissue engineering, HA has also been shown to generate osteogenesis by triggering osteoblasts and pre-osteoblastic cells. [27] HA is a strong material, which makes it ideal to add to PLA, due to the fact that PLA has weak toughness with a 10% elongation before failure. [26] FFF-based 3D printing was used as well as compression tests demonstrated in Figure 5. [21] The results showed that there was a self-healing capability of the sample, which could be used in certain biomedical practices. [21]

Environmental impact

Figure 7: Biodegradation of PU cubes over 12 weeks. Degradation was analyzed through A) change in appearance, B) cube mass, and C) maximum force at 50% compression force deflection (CFD). Error bars indicate sample standard deviations of the triplicate measurements. For compost and soil mass loss, p < 0.01, and for compost and soil CFD, p < 0.01 (Table S2). AlgaeFoam Degredation.jpg
Figure 7: Biodegradation of PU cubes over 12 weeks. Degradation was analyzed through A) change in appearance, B) cube mass, and C) maximum force at 50% compression force deflection (CFD). Error bars indicate sample standard deviations of the triplicate measurements. For compost and soil mass loss, p < 0.01, and for compost and soil CFD, p < 0.01 (Table S2).

With recent attention toward climate change, global warming, and sustainability, there has been a new wave of research regarding the creation and sustainability of biodegradable products. This research has evolved to include the creation of biodegradable biofoams, with the intention to replace other foams that may be environmentally harmful or whose production may be unsustainable. Following this vein, Gunawan et al. [28] conducted research to developed “commercially-relevant polyurethane products that can biodegrade in the natural environment”. [28] One such product includes flip-flops so as part of the research a flip-flop made from algae derived polyurethane was prototyped (see Figure 7). [28] [29] [30] [31] This research ultimately resulted in the conclusion that in both a compost and soil environment (different microorganisms present in each environment) significant degradation occurs in polyurethane foam formulated from algae oil. [28]

Figure 6: A) General polyester polyol and polyurethane (PU) syntheses and structure. B) Algenesis algae-based PU flip-flop prototype Molecule AlgaePUSandal.jpg
Figure 6: A) General polyester polyol and polyurethane (PU) syntheses and structure. B) Algenesis algae-based PU flip-flop prototype

Similarly, research has been done where algae oil (AO) and residual palm oil (RPO) have been formulated into foam polyurethane at different ratios to determine what ratio has the optimum biodegradability. RPO is recovered from the waste of palm oil mill and is a byproduct of that manufacturing process. After undergoing a tests to determine biodegradability as well as a thermogravimetric analysis, the team determined that the material could be utilized in applications such as insulation or fire retardants depending on the AO/RPO ratio. [5]

Another focus of biofoam research is the development of biofoams that are not only biodegradable, but are also cost-effective and require less energy to produce. Luo et al. have conducted research in this area of biofoams and have ultimately developed a biofoam that is produced from a “higher content of nature bioresource materials” and using a “minimal [number of] processing steps”. [32] The processing steps include the one-pot method of foam preparation published by F. Zhang and X. Luo in their paper about developing polyurethane biofoams as an alternative to petroleum based foams for specific applications. [33]

Ongoing research

Research efforts have been put into using natural components in the creation of potentially biodegradable foam products. Mycelium (Figure 8), chitosan (Figure 9), wheat gluten (Figure 10), and cellulose (Figure 11) have all been used to create biofoams for different purposes. [7] [34] [35] [18] The wheat gluten example was used in combination with graphene to attempt to make a conductive biofoam. [35] The mycelium-based, chitosan-based, and cellulose-based biofoam examples are intended to become cost effective and low density material options. [7] [34] [18]

Related Research Articles

<span class="mw-page-title-main">Biopolymer</span> Polymer produced by a living organism

Biopolymers are natural polymers produced by the cells of living organisms. Like other polymers, biopolymers consist of monomeric units that are covalently bonded in chains to form larger molecules. There are three main classes of biopolymers, classified according to the monomers used and the structure of the biopolymer formed: polynucleotides, polypeptides, and polysaccharides. The Polynucleotides, RNA and DNA, are long polymers of nucleotides. Polypeptides include proteins and shorter polymers of amino acids; some major examples include collagen, actin, and fibrin. Polysaccharides are linear or branched chains of sugar carbohydrates; examples include starch, cellulose, and alginate. Other examples of biopolymers include natural rubbers, suberin and lignin, cutin and cutan, melanin, and polyhydroxyalkanoates (PHAs).

<span class="mw-page-title-main">Polyurethane</span> Polymer composed of a chain of organic units joined by carbamate (urethane) links

Polyurethane refers to a class of polymers composed of organic units joined by carbamate (urethane) links. In contrast to other common polymers such as polyethylene and polystyrene, polyurethane is produced from a wide range of starting materials. This chemical variety produces polyurethanes with different chemical structures leading to many different applications. These include rigid and flexible foams, and coatings, adhesives, electrical potting compounds, and fibers such as spandex and polyurethane laminate (PUL). Foams are the largest application accounting for 67% of all polyurethane produced in 2016.

<span class="mw-page-title-main">Mycelium</span> Vegetative part of a fungus

Mycelium is a root-like structure of a fungus consisting of a mass of branching, thread-like hyphae. Fungal colonies composed of mycelium are found in and on soil and many other substrates. A typical single spore germinates into a monokaryotic mycelium, which cannot reproduce sexually; when two compatible monokaryotic mycelia join and form a dikaryotic mycelium, that mycelium may form fruiting bodies such as mushrooms. A mycelium may be minute, forming a colony that is too small to see, or may grow to span thousands of acres as in Armillaria. The network of mycelium acts similar to human brains, in the way that mycelium is used to send electrical signals to the fruiting bodies of mushrooms. These electrical signals can be used to convey information or warn about incoming danger.

<span class="mw-page-title-main">Surfactant</span> Substance that lowers the surface tension between a liquid and another material

Surfactants are chemical compounds that decrease the surface tension or interfacial tension between two liquids, a liquid and a gas, or a liquid and a solid. Surfactants may function as emulsifiers, wetting agents, detergents, foaming agents, or dispersants. The word "surfactant" is a blend of surface-active agent, coined c. 1950.

<span class="mw-page-title-main">Foam</span> Form of matter

Foams are materials formed by trapping pockets of gas in a liquid or solid.

In organic chemistry, a polyol is an organic compound containing multiple hydroxyl groups. The term "polyol" can have slightly different meanings depending on whether it is used in food science or polymer chemistry. Polyols containing two, three and four hydroxyl groups are diols, triols, and tetrols, respectively.

<span class="mw-page-title-main">Polylactic acid</span> Biodegradable polymer

Polylactic acid, also known as poly(lactic acid) or polylactide (PLA), is a thermoplastic polyester with backbone formula (C
3H
4O
2)
n or [–C(CH
3)HC(=O)O–]
n, formally obtained by condensation of lactic acid C(CH
3)(OH)HCOOH with loss of water. It can also be prepared by ring-opening polymerization of lactide [–C(CH
3)HC(=O)O–]
2, the cyclic dimer of the basic repeating unit.

<span class="mw-page-title-main">Polypropylene glycol</span> Chemical compound

Polypropylene glycol or polypropylene oxide is the polymer of propylene glycol. Chemically it is a polyether, and, more generally speaking, it's a polyalkylene glycol (PAG) H S Code 3907.2000. The term polypropylene glycol or PPG is reserved for polymer of low- to medium-range molar mass when the nature of the end-group, which is usually a hydroxyl group, still matters. The term "oxide" is used for high-molar-mass polymer when end-groups no longer affect polymer properties. Between 60 and 70% of propylene oxide is converted to polyether polyols by the process called alkoxylation.

<span class="mw-page-title-main">Bioplastic</span> Plastics derived from renewable biomass sources

Bioplastics are plastic materials produced from renewable biomass sources, such as vegetable fats and oils, corn starch, straw, woodchips, sawdust, recycled food waste, etc. Some bioplastics are obtained by processing directly from natural biopolymers including polysaccharides and proteins, while others are chemically synthesised from sugar derivatives and lipids from either plants or animals, or biologically generated by fermentation of sugars or lipids. In contrast, common plastics, such as fossil-fuel plastics are derived from petroleum or natural gas.

<span class="mw-page-title-main">Polyisocyanurate</span> Type of plastic typically used for thermal insulation

Polyisocyanurate, also referred to as PIR, polyiso, or ISO, is a thermoset plastic typically produced as a foam and used as rigid thermal insulation. The starting materials are similar to those used in polyurethane (PUR) except that the proportion of methylene diphenyl diisocyanate (MDI) is higher and a polyester-derived polyol is used in the reaction instead of a polyether polyol. The resulting chemical structure is significantly different, with the isocyanate groups on the MDI trimerising to form isocyanurate groups which the polyols link together, giving a complex polymeric structure.

<span class="mw-page-title-main">Foam latex</span>

Foam latex or latex foam rubber is a lightweight form of latex containing bubbles known as cells, created from liquid latex. The foam is generally created though the Dunlop or Talalay process in which a liquid latex is foamed and then cured in a mold to extract the foam.

<span class="mw-page-title-main">Biodegradable plastic</span> Plastics that can be decomposed by the action of living organisms

Biodegradable plastics are plastics that can be decomposed by the action of living organisms, usually microbes, into water, carbon dioxide, and biomass. Biodegradable plastics are commonly produced with renewable raw materials, micro-organisms, petrochemicals, or combinations of all three.

Biodegradable polymers are a special class of polymer that breaks down after its intended purpose by bacterial decomposition process to result in natural byproducts such as gases (CO2, N2), water, biomass, and inorganic salts. These polymers are found both naturally and synthetically made, and largely consist of ester, amide, and ether functional groups. Their properties and breakdown mechanism are determined by their exact structure. These polymers are often synthesized by condensation reactions, ring opening polymerization, and metal catalysts. There are vast examples and applications of biodegradable polymers.

A thermoset polymer matrix is a synthetic polymer reinforcement where polymers act as binder or matrix to secure in place incorporated particulates, fibres or other reinforcements. They were first developed for structural applications, such as glass-reinforced plastic radar domes on aircraft and graphite-epoxy payload bay doors on the Space Shuttle.

In polymer chemistry, the term prepolymer or pre-polymer, refers to a monomer or system of monomers that have been reacted to an intermediate-molecular mass state. This material is capable of further polymerization by reactive groups to a fully cured, high-molecular-mass state. As such, mixtures of reactive polymers with un-reacted monomers may also be referred to as pre-polymers. The term "pre-polymer" and "polymer precursor" may be interchanged.

<span class="mw-page-title-main">Polyurethane foam</span> Material used for thermal insulation

Polyurethane foam is a specialist material used for thermal insulation and other applications. It is a solid polymeric foam based on polyurethane chemistry.

<span class="mw-page-title-main">Nanocellulose</span> Material composed of nanosized cellulose fibrils

Nanocellulose is a term referring to nano-structured cellulose. This may be either cellulose nanocrystal, cellulose nanofibers (CNF) also called nanofibrillated cellulose (NFC), or bacterial nanocellulose, which refers to nano-structured cellulose produced by bacteria.

Polyurethane dispersion, or PUD, is understood to be a polyurethane polymer resin dispersed in water, rather than a solvent, although some cosolvent maybe used. Its manufacture involves the synthesis of polyurethanes having carboxylic acid functionality or nonionic hydrophiles like PEG incorporated into, or pendant from, the polymer backbone. Two component polyurethane dispersions are also available.

Waterborne resins are sometimes called water-based resins. They are resins or polymeric resins that use water as the carrying medium as opposed to solvent or solvent-less. Resins are used in the production of coatings, adhesives, sealants, elastomers and composite materials. When the phrase waterborne resin is used, it usually describes all resins which have water as the main carrying solvent. The resin could be water-soluble, water reducible or water dispersed.

<span class="mw-page-title-main">Mycelium-based materials</span>

Mycelium, the fungal equivalent of roots in plants, has been identified as an ecologically friendly substitute to a litany of materials throughout different industries, including but not limited to packaging, fashion and building materials. Such substitutes present a biodegradable alternative to conventional materials.

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