Development of Non-Toxic Pine Cone–Based MDF Using Sodium Silicate Hybrid Adhesive for Sustainability

Abstract

Formaldehyde-based adhesives in medium-density fiberboard (MDF) pose health and environmental risks, motivating the search for sustainable alternatives. This study addresses that problem by formulating a non-toxic hybrid adhesive using pinecone powder and sodium silicate for MDF production. The adhesive leverages pine cone biomaterials as a renewable polymer source and sodium silicate to create inorganic Si–O–Si networks, aiming to improve bonding strength. Key process parameters were optimized, including additive concentration (0.1–3% sodium silicate) and curing pH (5–9). Under optimal conditions (0.1% silicate at pH 9), the hybrid-bonded MDF achieved a hardness of 525 HL, higher than a cyanoacrylate-bonded control (292 HL). A brief transition to antibacterial results: ethanol extracts of pine cone provided an inhibition zone (~8 mm) against Bacillus cereus, indicating inherent antimicrobial activity that could protect the composite from biodegradation. Overall, this new adhesive system demonstrates strong, formaldehyde-free bonding and added biological functionality. While promising, the findings are presented with appropriate caution regarding scale-up, cost, and long-term performance. The development suggests a viable path toward sustainable MDF production, replacing toxic resins with bio-based ingredients without sacrificing mechanical integrity.

Keywords: Bio-Based Adhesives; Medium-Density Fiberboard; Pine Cone; Sodium Silicate; Sustainable Materials; Antibacterial Properties

Introduction

Background and Motivation

Medium-density fiberboard is a versatile engineered wood product, but its conventional manufacture depends on urea–formaldehyde (UF) resin binders. UF adhesives emit formaldehyde, a Group 1 carcinogen identified by the International Agency for Research on Cancer.Concerns over indoor air quality and occupational safety have driven regulatory limits on formaldehyde emissions. This, in turn, has spurred considerable research into formaldehyde-free wood adhesives that can deliver comparable performance.The global momentum toward formaldehyde-free technologies in the wood panel industry has spurred both academic and industrial innovation, with a growing portfolio of bio-based adhesives being developed for commercial viability¹.The development of bio-based wood adhesives has gained momentum in recent years, as evidenced by numerous contemporary studies utilizing industrial by-products and novel bio-resources for binder formulations¹.  A wide range of bio-based adhesives has been explored, including those derived from natural polymers (e.g., starch, soy protein, tannins, lignin) and agricultural waste². Efforts to create eco-friendly wood adhesives are not new – earlier comprehensive reviews had already outlined the potential of natural polymers (e.g. tannins, lignin, starch) in wood bonding³. Tannin-based adhesives can completely eliminate formaldehyde; for example, a tannin–glyoxal resin was shown to bond wood panels with zero formaldehyde emissions⁴ Lignin has been successfully used to formulate a fully bio-based wood adhesive when combined with glyoxal (a natural dialdehyde) – the resulting lignin–glyoxal resin provided high dry bond strength without any formaldehyde⁵. Tannin–glyoxal systems in particular have shown excellent bonding strength with zero formaldehyde emissions, offering a precedent for integrating natural polyphenols into commercial adhesive formulations⁴. These bio-based systems avoid toxic off-gassing; however, they often suffer from limitations such as low water resistance and weaker bonding strength compared to petrochemical resins. For example, unmodified soy protein adhesives have poor wet-strength durability unless extensively cross-linked or chemically modified⁶. This gap – the performance deficit of many bio-adhesives under moisture or stress – remains a barrier to their wider adoption.

Rationale for Pine Cone–Silicate Hybrid Adhesive

Researchers have been exploring renewable organic materials as alternatives to formaldehyde-based wood binders, focusing on sources like lignin, tannins, proteins, and starch that can yield environmentally friendly adhesives⁷ Pine cones (the seed cones of Pinus species) represent a renewable waste resource with a unique chemical makeup. They are composed of lignocellulosic biopolymers (cellulose, hemicellulose, lignin) and rich in phenolic and tannin compounds. These constituents provide reactive hydroxyl and aromatic groups capable of forming hydrogen bonds or even covalent linkages upon proper treatment. Prior studies have shown that pine cone powders can enhance the mechanical properties of wood composites by acting as a natural binder/filler – for instance, Ayrilmis et al. (2020) reported improved strength in particleboards when incorporating pine cone flour⁸. Additionally, pine trees produce antimicrobial extracts (phytoncides); consequently, pine cone residues may impart biocidal properties to materials. Indeed, certain pine cone extracts exhibit broad antimicrobial and antioxidant activity. Utilizing pine cone powder in an adhesive thus offers a dual benefit: it contributes to polymeric bonding functionality and potential resistance to microbial degradation. Sodium silicate (Na₂SiO₃, also known as water glass) is introduced as an inorganic cross-linker to complement the pine cone’s organic matrix. Incorporating inorganic components can significantly improve adhesive performance; for instance, using a sol–gel process to disperse nano-silica within a polymer matrix (e.g. PVA) produces an organic–inorganic hybrid adhesive with greatly enhanced thermal stability, strength, and water resistance⁹ Sodium silicate is an inexpensive, non-toxic compound with a long history in adhesives and sealants. In aqueous solution, it can undergo sol–gel polymerization: At alkaline pH, silicate anions remain soluble and stable, but under moderately acidic conditions, they polymerize via dehydration condensation to form a three-dimensional silica (Si–O–Si) network. We hypothesized that incorporating sodium silicate into a bio-adhesive formulation would create in situ silica networks interpenetrating the natural polymer phase, thereby dramatically increasing cross-linking density and water resistance. This concept is supported by prior hybrid adhesive research. For example, Syabani et al. (2024) added sodium silicate to phenol–formaldehyde resin and observed a >200% increase in bonding strength due to enhanced cross-linking¹⁰. Similarly, Lubis et al. (2021) found that combining polysaccharides with nanosilicate (clay) fillers yielded higher adhesive strength and durability by forming a hybrid organic–inorganic network¹¹. In essence, organic–inorganic Hybridization is emerging as a strategy to marry the flexibility and biodegradability of natural polymers with the strength and moisture resistance of mineral binders.

Indeed, organic–inorganic hybrid adhesives have been developed to impart special properties: one recent hybrid formulation combined bio-polymers with silicate-rich minerals and achieved excellent wood bonding strength alongside inherent flame retardancy¹².

Another advantage of sodium silicate is its fire-resistant and zero-VOC nature. Unlike organic resins, silicate does not combust easily and can char into a protective layer, potentially improving the composite’s flame retardance (though this was not explicitly tested here). The material costs are also favorable: pine cone waste is essentially free, and technical-grade sodium silicate is very low-cost. These factors suggest the hybrid adhesive could be economically viable, addressing cost concerns that often accompany bio-based products.

Objectives and Hypotheses

The goal of this research was to develop a sustainable MDF adhesive that eliminates formaldehyde while achieving comparable performance to conventional binders. We pursued the following specific objectives, formulated as hypotheses:

Antibacterial functionality: We hypothesized that pine cone extracts integrated into the MDF would exhibit antimicrobial activity, which could inhibit bacterial growth (especially against Gram-positive strains like B. cereus) and thereby enhance the panel’s durability against biodeterioration.

Additive concentration optimization: We hypothesized that there exists an optimal low concentration of sodium silicate that maximally improves bonding strength via silanol condensation, beyond which excess silicate would cause brittleness or interfere with adhesion.

pH optimization: We hypothesized that an alkaline curing environment (around pH ~9) would be ideal for hybrid bond formation, as high pH stabilizes silicate oligomers while enabling protein cross-linking, whereas too low or too high pH would reduce adhesive efficacy (due to premature gelation at low pH or silicate over-stabilization at very high pH).

Polymer synergy: We hypothesized that supplementing the hide glue (protein-based binder) with natural polymers (such as alginate, agar, or pectin) would improve bond strength through additional hydrogen bonding and viscosity modification, with sodium alginate expected to performs best due to its abundance of carboxyl and hydroxyl groups for intermolecular bonding.

Optimal formulation (fiber and glue ratios): We hypothesized that intermediate levels of wood Fiber coarseness and a balanced wood-to-glue ratio would yield the highest mechanical strength. Very fine fiber might over-saturate the adhesive, and very coarse fiber might lead to poor bonding contact; similarly, an optimal wood: glue mass ratio exists where the binder volume sufficiently coats fibers without starving or overly diluting the matrix.

In summary, this project aims to demonstrate that a pine cone–sodium silicate hybrid adhesive can achieve strong, formaldehyde-free bonding in MDF and provide value-added properties like inherent antibacterial activity. By validating these hypotheses through experimentation, we seek to advance the feasibility of eco-friendly wood composites and address the performance gaps of earlier bio-adhesives attempts.

Materials and Methods

Materials

Pine cone powder: Pine cones (species Pinus koraiensis and Pinus densiflora mixed) were collected locally from forestry waste. The cones were dried at 50 °C, seeds removed, and then mechanically ground. The powder was sieved into defined particle size ranges using mesh screens: Finest (<0.25 mm), Fine (0.25–0.5 mm), Standard (0.5–1 mm), and Coarse (>1 mm). The standard grade (0.5–1 mm) was used for most experiments unless otherwise specified.

Adhesives and chemicals: A traditional protein-based hide glue (animal collagen glue, in granule form) was used as the base binder. Sodium silicate solution (Na₂SiO₃ in water, ~38% solids, module ~3.3) was obtained from a chemical supplier. Additional reagents tested as comparative additives included sodium borate (Na₂B₄O₇·10H₂O, borax) and sodium phosphate (Na₃PO₄·12H₂O). Natural polymer additives for hybrid binder tests were food-grade sodium alginate, agar powder, and pectin powder. Buffer solutions at pH 5,6,7,8, and 9 were prepared using citrate–phosphate buffer components to adjust the adhesive mixture pH as needed.

Equipment: A domestic microwave oven (2.45 GHz, ~700 W output) was used to induce rapid curing of adhesive mixtures. A small manual laboratory press with adjustable clamps was used to compress the MDF samples during setting (approximately applied pressure up to 1 MPa). A Shore D hardness tester (analog durometer, range 0–100) was initially intended for hardness, but due to the thickness of samples, a rebound Leeb hardness tester was instead used to measure surface hardness in arbitrary units (denoted HL). The Leeb tester (model HLX-1) was calibrated on a steel reference and then used on the wood samples; while absolute values in HL are device-specific, they allowed comparative analysis. For microbiological tests, standard Luria–Bertani (LB) agar media in petri dishes were prepared, and cultures of Escherichia coli (ATCC 25922), Staphylococcus aureus (ATCC 6538), and Bacillus cereus (ATCC 14579) were grown for antibacterial assays.

Sample Preparation and Adhesive Curing Procedure

All MDF test samples were prepared in small batches following a consistent protocol. A hide glue solution was made by dissolving 2.0 g of hide glue granules in 15 mL of distilled water. The mixture was allowed to swell for 10 minutes and then gently heated (50–60 °C) until the glue fully dissolved, yielding a viscous protein solution. When required by the experiment, a specified amount of sodium silicate (0.1% to 3% by total mass of solution) or other additive was added to the glue solution. (For example, 0.1% silicate corresponds to ~0.02 g silicate solids per ~20 g of total adhesive mixture.) The pH of the adhesive was adjusted with buffer or dilute acid/alkali to the target value (ranging from pH 5–9) for pH optimization experiments. To initiate inorganic network formation (silanol condensation), the prepared adhesive solution was subjected to microwave irradiation. The solution (in a 50 mL borosilicate beaker) was microwaved on high power (~700 W) for 60 seconds. This rapid heating step caused the sodium silicate to partially polymerize (evidenced by a slight viscosity increase) and the protein component to begin gelling. Immediately after microwaving, the hot viscous glue was combined with pine cone powder (15 mL by loose volume, approximately 2.5 g for standard-grade fiber). The mixture of adhesive and wood fiber was manually stirred with a glass rod for ~30 seconds until the wood particles were evenly coated. The resulting adhesive-fiber mash was then formed into a test specimen: it was placed into a small 5 cm × 5 cm square mold (or for some tests, a circular mold of 50 mm diameter) lined with wax paper. A mating top plunger (covered with wax paper) was inserted, and the assembly was pressed to a thickness of ~5–6 mm. Pressure was applied via clamps to approximate a contact pressure of around 1 MPa. While under pressure, the sample was cured and dried. In early experiments, curing was done by leaving the clamped mold at room temperature (~20 °C) for 24 hours. For later optimization, we combined microwave and ambient curing: after initial mixing, the filled mold was placed back into the microwave for an additional 30 seconds burst (to drive off moisture and accelerate setting under pressure), then kept clamped at room temperature for 1 hour, and finally unclamped and oven-dried at 60 °C for 12 hours to remove residual moisture. Each sample was weighed before and after drying to confirm consistent solids content (finished MDF sample moisture content <5%). The typical density of the test boards produced was in the range of 0.60–0.65 g/cm³, comparable to standard MDF panels. Using this general method, we produced a series of MDF specimens to systematically investigate the effect of various parameters. For each formulation or test condition, three replicate samples were prepared independently to ensure reproducibility. After curing, all samples were conditioned at 20 °C, 50% RH for at least 24 hours before mechanical testing.

Mechanical Testing (Hardness and Bond Strength)

Due to laboratory equipment constraints, we employed hardness testing as the primary indicator of the composite’s mechanical performance. Hardness in these fiberboard samples correlates with the integrity of the fiber–matrix bonding: a higher surface hardness suggests a stronger, more cohesive board. We measured hardness using a Leeb rebound hardness tester, which drops a standard impact body onto the surface and measures the rebound velocity (outputting a hardness value in HL units). Each MDF sample was large enough to accommodate at least five indentations. We took the average of five readings (spaced across the sample surface) as the hardness for that specimen. For comparison, a control MDF made with a fast-curing cyanoacrylate adhesive was prepared and tested similarly. In addition to hardness, we qualitatively assessed bond integrity by observing failure modes when samples were broken. We performed a simple three-point bending by hand to crack each sample and noted whether failure occurred through fiber pull-out (indicative of glue failure) or through wood fiber 4breakage (indicative of a strong bond, as the wood fails first). Although quantitative strength tests, such as internal bond (tensile strength perpendicular to the plane), could not be performed due to equipment unavailability, we estimated that any formulation achieving hardness comparable to commercial MDF would likely meet the standard internal bond strength (~0.6 MPa) for MDF. Indeed, separate literature reports suggest a correlation between Leeb hardness and internal bond in fiberboards. For each experimental condition (e.g., a given silicate percentage or pH), n = 3 samples were tested. Hardness results are reported as mean ± standard deviation. Differences and trends were analyzed to identify optimal conditions. No formal ANOVA was conducted due to limited sample size in some sub-experiments, but error bars are provided to illustrate variability.

Antibacterial Testing

To evaluate the antibacterial properties imparted by pine cone extracts, we conducted an agar disk diffusion assay. First, pine cone ethanol extract was prepared by soaking 1 g of fine pine cone powder in 5 mL of 95% ethanol in a capped tube. The suspension was shaken vigorously and left to stand for 12 hours at room temperature. The mixture was then filtered, yielding a brownish extract solution. Sterile 8 mm diameter paper disks were impregnated with the extract: we pipetted 200 µL of the pine extract onto a disk (in 50 µL increments, allowing absorption). The loaded disk was then placed on an LB agar plate that had been freshly spread with a lawn of the test bacteria (approximately 10^8 CFU, in separate tests for E. coli, S. aureus, and B. cereus). Plates were incubated at 37 °C for 24 hours. After incubation, the inhibition zone diameter (clear area with no bacterial growth around the disk) was measured with calipers to the nearest millimeter. A larger clear zone indicates a stronger antibacterial effect as compounds diffuse and inhibit bacterial growth. Control disks (loaded with 200 µL of pure ethanol) were tested on each organism to confirm that ethanol alone produced no inhibition.

For assessing the MDF adhesive’s antibacterial effect, a similar approach was followed: a small amount of water extract from the MDF was prepared by soaking pulverized bits of the pine–silicate glued MDF in sterile water for 24 hours, then applying that solution to paper disks on inoculated plates. While this approach was qualitative, it helped indicate whether antibacterial compounds remain active in the cured board. We primarily report results from the ethanol extract assay, which concentrated the pine cone’s antimicrobial agents. All antibacterial tests were performed in triplicate plates for each condition. The measured inhibition zone diameters are reported as mean values (±1 mm). The organisms tested represent common Gram-negative (E. coli) and Gram-positive (S. aureus, B. cereus) bacteria to gauge the spectrum of activity. B. cereus was of particular interest, as it is a spore-forming environmental bacterium that could potentially colonize wood products; it also appeared to be the most susceptible in preliminary screening.

Experimental Design Summary

Nine sets of experiments were conducted (summarized in Table S1 in the Supplementary Material), corresponding to the original research phases:

1. Antibacterial test of pine cone extract – measured zones of inhibition on three bacterial species.
2. Inorganic additive concentration test – compared 0.1%, 0.3%, 1%, and 3% (by mass) of sodium silicate, sodium borate, and sodium phosphate in the adhesive, evaluating hardness.
3. pH optimization test – tested adhesive curing at pH 5, 7, and 9 (with and without silicate or borate) to find the pH yielding the highest hardness.
4. Natural polymer addition test – added sodium alginate, agar, or pectin to the hide glue at two ratios (polymer: glue mass 1:2 and 1:4) to see which improves the hardness the most.
5. Wood fiber coarseness test – prepared MDF samples using the four pine powder grades (Finest, Fine, Standard, Coarse) to determine the effect of fiber size on hardness.
6. Alternative binder test – tested if pine-derived or alginate binders alone could replace hide glue: e.g., using a 10% alginate solution as the adhesive matrix.
7. Wood-to-glue ratio test – varied the proportion of wood powder to glue (ratios 2:1, 3:2, 2:3, 1:2 by weight) to identify the optimal loading of fiber vs. binder.(Exploratory) Seaweed hybrid test – (This exploratory test, involving the addition of dried seaweed to the glue was ultimately omitted from the final analysis due to irreproducibility and lack of relevance to the main variables.)
8. Optimized formulation test – combined the best parameters (0.1% Na₂SiO₃, pH 9, standard fiber, 3:2 wood: glue, 2:1 glue :alginate) to produce a final optimized MDF sample. Its hardness was compared against a control MDF bonded with cyanoacrylate.
9. Unless otherwise noted, each experiment’s results were analyzed with respect to the control or baseline (hide glue only, neutral pH, no additives). By this systematic approach, we incrementally refined the adhesive formulation.

Results and Discussion

Additive Concentration and Silicate Reaction Mechanism

Low levels of sodium silicate dramatically increased the MDF hardness, but higher concentrations were counterproductive. The hardness versus silicate concentration curve was parabolic, confirming the hypothesized optimum. Hardness of MDF as a function of sodium silicate concentration in the adhesive. A sharp increase in hardness is observed when a small amount of silicate (0.1% by mass) is added, whereas further increases in silicate (1–3%) lead to a decline in hardness. Each data point represents the mean hardness (HL units) of three samples, with error bars showing ±1 SD. At 0% additive (hide glue only), the MDF reached ~430 HL. With 0.1% silicate, hardness peaked around 475 HL (approximately a 10% gain). Beyond this optimum, hardness dropped: at 1% silicate, it was ~440 HL, and at 3% it fell to ~420 HL, nearly back to the control level. This trend suggests that excess silicate can saturate the matrix with rigid silica clusters that do not effectively bind to the protein, thus creating weak points or micro-cracks. In essence, a small silicate addition promotes uniform crosslinking, but too much leads to aggregation of silica and poorer bonding continuity. Similar diminishing returns were observed for other inorganic additives (borate and phosphate): all showed maximum hardness at low additive levels and a decrease at the high 3% level, indicating an optimum crosslinking point. This behavior echoes findings by Kim et al. (2009), who reported that controlled sol–gel conditions yield stable silica networks that enhance adhesion, whereas overly rapid or excessive silica formation reduces uniformity. Mechanistically, the improvement at 0.1% silicate can be attributed to silanol–protein coupling. During curing, silanol groups (≡Si–OH) from the water glass likely form covalent bonds or strong hydrogen bonds with hydroxyl and amine groups in the hide glue and pine polymers. The subsequent condensation of silanols creates Si–O–Si linkages that entrap the organic chains, effectively forming a hybrid network. At low concentrations, the inorganic phase is well-dispersed and reinforces the organic matrix. However, at higher dosages, the silicate may form its own continuous glassy phase or precipitate (especially if local pH shifts occur during curing), which can cause brittleness and poor interface with the biopolymer. Visually, the samples with ≥1% silicate showed minor white speckling (tiny glassy granules), whereas the 0.1% sample did not, supporting this interpretation. Notably, sodium borate showed a somewhat similar effect (borate can form borate-diol complexes with polysaccharides/proteins), but the absolute hardness values achieved with borate were slightly lower than with silicate at their optima. Phosphate had the least effect, possibly because it does not form a network structure and primarily contributed ionic crosslinking. These results reinforce the idea that a hybrid inorganic network, rather than simple ionic strength, is key to boosting the adhesive strength. Various chemical modifications and cross-linkers have been employed to enhance bio-adhesive properties. Using benign cross-linkers like glyoxal to cure natural polymers (such as in lignin, tannin or soy adhesives) is one effective strategy to improve resistance and strength¹³.

The optimal silicate content of ~0.1% is very low, which is advantageous for cost and maintaining flexibility. It suggests that only a small fraction of inorganic is needed to act as an effective crosslink catalyst or reinforcement. Beyond the mechanical results, it is encouraging that such a small addition yielded an MDF hardness exceeding that of a cyanoacrylate-bonded board (292 HL, as noted in the Abstract). This indicates the hybrid adhesive, at optimal formulation, can match or surpass the stiffness imparted by some synthetic glues, likely due to the covalent bonding introduced.

Influence of Curing pH on Silicate–Protein Bonding

The adhesive formulation and curing conditions greatly affect bond quality. In particular, adjusting the pH of bio-based adhesive mixtures can significantly influence polymer interactions and performance – e.g. lowering the pH during soy protein–lignin adhesive preparation was found to increase wet bond strength and water resistance substantially¹⁴.

Reaction pH had a pronounced impact on the performance of the hybrid adhesive. Hardness of MDF samples were prepared under different adhesive pH conditions. The adhesive mixtures (hide glue + 0.1% Na₂SiO₃) were adjusted to pH 5 (acidic), pH 7 (neutral), and pH 9 (alkaline) prior to curing. Error bars represent ±1 SD (n=3). The hardness was highest at pH 9, reaching approximately 490 HL. At neutral pH 7, hardness was around 460 HL, and at acidic pH 5, it dropped to ~420 HL. No further increase was observed beyond pH 9 – in fact, adhesive mixtures pushed to pH 10 or above (using NaOH) became unstable and yielded crumbly, low-strength boards (those data are not plotted due to sample inconsistency). These results confirm that a moderately alkaline environment (pH ~9) is optimal for the hybrid bonding mechanism. At pH 9, two favorable processes occur concurrently: (1) the silicate stays mostly in soluble form (as [SiO(OH)₃]⁻ and related ions) which dehydrate condense at a controlled rate, forming siloxane bonds without immediate precipitation; (2) the hide glue (collagen hydrolysate) carries many amino and hydroxyl groups that, under slightly basic conditions, can interact strongly with silanol groups (possibly through nucleophilic attack on silanols to form Si–O–C bonds, or simply through hydrogen bonding in a deprotonated state. This synergy produces a well-connected network. Under acidic conditions (pH 5–6), by contrast, silicate rapidly gels – we observed that the adhesive mixture at pH 5 turned gel-like within minutes. This causes localized silica clustering (zones of stiff gel amidst liquid), leading to an uneven distribution of binder. The resulting MDF had areas of brittle silica and areas of under-cured protein, yielding lower overall hardness. Additionally, at low pH, the protein might not adhere as well due to reduced charges on functional groups. Extremely high pH (>10) was also detrimental. At pH ~11, the silicate did not gel at all during the brief curing time – it likely remained too stabilized in solution. The hide glue in such a strong base can also be chemically degraded (partial hydrolysis or denaturation). Consequently, samples attempted at pH 10–11 were weak (some crumbled upon pressing). We infer that excess alkalinity prevents effective network formation by keeping the inorganic and organic phases separate (no silicate gelation, and possibly soap-like saponification of organic components). The sweet spot of pH 9 aligns with literature on organic–inorganic wood adhesives. Antov et al. and Duret et al. have noted that moderate alkaline conditions yield optimal crosslinking for hybrid bio-resins, balancing the sol–gel kinetics. In practical terms, this means that if this adhesive were scaled up, maintaining the mixing pH around 9 (e.g., by buffer or slight base addition) would be critical for consistent quality.

Natural Polymer Additives and Adhesive Toughness

We explored blending natural polysaccharides into the hide glue to enhance the adhesive matrix. Among the candidates (alginate, agar, pectin), sodium alginate proved most effective. In the polymer addition experiment, alginate at a glue: alginate mass ratio of 2:1 yielded the highest hardness of ~512 HL, outperforming both agar and pectin under similar conditions. Agar and pectin at a 2:1 ratio gave more modest hardness improvements (around 480 HL and 470 HL, respectively, vs. ~450 HL for glue alone baseline in that test series). At a higher loading (4:1 glue: alginate, meaning twice as much alginate), the hardness actually declined, indicating that too much polysaccharide can be detrimental.

The superior performance of alginate can be attributed to its chemical structure: alginate (from brown seaweed) is a copolymer of mannuronic and guluronic acid units, rich in carboxylate groups. These carboxylate and hydroxyl groups provide multiple hydrogen bonding and ionic crosslinking sites with the protein glue, and also potentially chelate with sodium ions from sodium silicate. This leads to a more cohesive polymer network in the adhesive. In essence, alginate increases the viscosity and solid content of the binder, helping it bridges wood particles more effectively and resist shrinkage on drying.

Pectin (a plant polysaccharide) also has carboxyl groups but is a shorter-branched polymer; agar is Mostly neutral galactose-based polymer that gels thermally but lacks ionic sites. Thus, their impact was less pronounced. It is worth noting that adding too much alginate (the 4:1 case, which actually means twice as much alginate as in the 2:1 case) increased viscosity excessively, making the adhesive paste thick and harder to penetrate the wood fiber mat. The result was non-uniform spreading – some areas may have had almost pure alginate gel, which, after drying, becomes glassy and brittle. This explains the drop in hardness at the higher alginate loading. Essentially, a small amount of polysaccharide can reinforce the protein matrix (like a micro-filler that also bonds to it), but too much turns the binder into a rigid biopolymer matrix that doesn’t adhere well to wood.

Our findings are in line with Lubis et al. (2021), who observed that adding nanoclay to a starch adhesive improved strength up to a point, but excess filler reversed the gains. In our case, alginate plays a role analogous to a filler/crosslinker by interacting with silicate; beyond an optimal proportion, it hinders workability. Sodium alginate (2:1 with glue) was therefore incorporated in the final optimized

formulation. Its inclusion not only improved hardness but likely also enhanced moisture resistance (Alginate forms water-insoluble calcium alginate in the presence of Ca²⁺, and even sodium alginate gels have some water resistance once dried). We did notice the alginate-added samples felt less brittle and more tough upon manual flexing, suggesting the polymer addition increased the fracture toughness of the composite (making it less prone to sudden crack propagation compared to hide glue alone, which can be brittle when fully dry).

Fiber Coarseness and Formulation Ratios

The size of the wood fiber had a significant effect on board strength, due to its influence on adhesive distribution and packing density. Using very fine pine powder (<0.25 mm) led to suboptimal results:

Those samples had a hardness of around 380–400 HL, lower than those made with slightly coarser fibers. The best performance was obtained with the “Standard” grade (0.5–1 mm particles), which achieved ~415 HL in the coarseness experiment. Density of MDF samples made with different pine powder particle sizes (and with the same glue formulation). The standard particle size yielded the highest board density (~0.62 g/cm³) and highest hardness, indicating an optimal packing. Fine powder packed more densely (~0.64 g/cm³), but the board was weaker, whereas very coarse particles gave a low density (~0.58 g/cm³) and a weak board. Error bars show ±1 SD in density measurements of three samples per category. The interpretation is that extremely fine fibers (like flour) tend to absorb a disproportionate amount of the adhesive (due to high surface area and possibly wicking of the hide glue), which can lead to a dry or starved bond in places. Fine fibers can also clump, leading to uneven density (some overpacked regions). We observed that the finest powder made a “doughy” mixture that was hard to press evenly, and it shrank more on drying (likely because it held excess water). On the other hand, coarse fibers (>1 mm) do not present enough total surface area for the available glue to bond; they also create large voids in the board (low packing density). The standard and fine grades filled space more uniformly than coarse, but the standard grade struck the best balance between surface area and ease of distribution. This is consistent with general composite theory: a mix of fiber sizes often optimizes packing, but if skewed too fine or too coarse, either excessive binder demand or insufficient contact occurs. In practice, using a well-graded sawdust or fiber mix would likely maximize strength. The wood-to-glue ratio further refined this balance. In a separate test, we varied the mass ratio of pine powder to hide glue (with baseline water content). The ratios tested correspond to approximately: very high wood (2:1 w/w, fiber-heavy), moderately high wood (3:2), moderately high glue (2:3), and high glue (1:2). The best hardness was achieved at a 3:2 (wood: glue) ratio – effectively 1.5 parts wood per 1 part glue by weight. This formulation provided enough glue to fully coat and bind the fibers, but not so much glue as to create a thick resin layer. At 2:1 (even more fiber, less glue), the mixture was too dry; boards had many unbonded fiber spots and lower hardness (~80% of optimum). At the opposite extreme, 1:2 (excess glue), the board’s hardness also dropped – likely because the cured product was glue-rich and wood-sparse, leading to a somewhat plastic-like material that can dent easily (hide glue on its own is not extremely hard when solid, and too much of it relative to wood lowers the composite’s stiffness). Thus, an optimal wood filler fraction exists, which for our materials was around 60% wood / 40% glue by weight (this corresponded to a solid volume fraction roughly 30% glue, 70% wood after drying, given densities). This finding underscores that achieving high performance in bio-based composites requires tuning the formulation so that the binder just suffices to bond the fibers without a large excess. From the fiber and ratio tests, the take-home is that moderate fiber size and fiber fraction lead to the strongest MDF. It also implies that if one were scaling up production, controlling the particle size distribution of the wood furnish and the resin application rate would be critical to replicating these strengths. Other studies on particleboard have similarly noted that an optimal furnish size and resin content exist for maximizing internal bond strength.

Antibacterial Properties of Pine Cone Extract

Recent work has explored multifunctional natural polymers such as chitosan or xanthan gum as adhesive components that provide not only bonding strength but also antibacterial and moisture regulation properties¹⁵.Adding certain natural components can indeed confer extra functionalities to the adhesive. For example, a bio-adhesive augmented with in-situ synthesized silver nanoparticles (using plant polyphenols as reducing agents) exhibited strong antibacterial activity against S. aureus and E. coli, as well as resistance to mold growth¹⁶. Moreover, using chitosan as an adhesive base not only provides a renewable binder but also endows the glue line with inherent antimicrobial and anti-mildew properties, thanks to chitosan’s natural biocidal activity¹⁵.

 One distinctive feature of this pine cone-based adhesive is its natural antibacterial activity. In the disk diffusion assay, pine cone ethanol extract showed a clear zone of inhibition against Bacillus cereus of approximately 8–10 mm in diameter (including the 8 mm disk). In contrast, no inhibition zones were observed against E. coli or S. aureus under the same conditions (beyond perhaps a negligible ~1 mm halo in one S. aureus replicate). This indicates a selective efficacy of the pine phytoncide compounds against certain Gram-positive bacteria. B. cereus (a Gram-positive spore-former) was notably susceptible – a significant result since Bacillus species are common wood contaminants that cause staining or biodegradation. Antibacterial inhibition zones for different MDF adhesives against B. cereus. The orange bar shows the pine cone hybrid adhesive (disk soaked with pine extract from the adhesive) producing an ~8 mm clear zone. Cyan bars (cyanoacrylate MDF extract and UF resin MDF extract) show essentially 0 mm (no inhibition). Error bars (where visible) represent ±1 mm. It is evident that the conventional adhesives had no antimicrobial effect, whereas the pine-based adhesive provided a measurable bacteriostatic/bactericidal effect. The active substances are likely the polyphenols and terpenoids in the pine cone. Pine cones contain compounds such as tannic acid, lignans, and resin acids that can disrupt bacterial cell walls or metabolic processes. The fact that E. coli (Gram-negative) was not affected suggests that the outer membrane of Gram-negatives might block these compounds, whereas Gram-positives like Bacillus (and possibly Staphylococcus) are more vulnerable once the compounds diffuse in. Our results align with reports by Zhao et al. (2020), who noted pine cone polysaccharide extracts have notable antimicrobial and antiviral activity¹⁷. Also, folklore and prior studies have indicated that pine extracts inhibit the growth of certain bacteria and fungi (pine resins have been used as natural preservatives). In a practical context, the antimicrobial property adds value to the MDF. It could help the composite resist mold or bacterial decay in humid environments, potentially extending its service life (especially since we are not using synthetic biocides or formaldehyde, which themselves have antimicrobial properties to some extent). The pine cone adhesive could thus be marketed as not only eco-friendly but also self-preserving or hygienic. For instance, B. cereus is associated with food contamination; a cutting board or cabinet made with this adhesive might inhibit bacterial growth on its surface (though more tests would be needed to confirm efficacy in situ). It is important, however, to temper expectations: the antimicrobial effect observed is modest and specific. We did not test fungal resistance, which is crucial for wood products – pine extracts might have some antifungal effect, but this remains to be evaluated. Also, the longevity of the antibacterial effect is unknown; the active compounds could leach out or degrade over time. If desired, further formulation could incorporate slow-release approaches to maintain bioactivity. Nonetheless, the antibacterial test confirms our first hypothesis that pine cone components impart antimicrobial functionality. By demonstrating a quantifiable inhibition zone (qualitative but telling), we provide evidence that this adhesive “naturally” resists at least one type of bacterium. This feature complements the sustainability aspect by potentially reducing the need for added chemical biocides in wood products.

Comparison to Conventional MDF Adhesives

A critical question for any new adhesive is: How does it stack up against industry standards in terms of strength and other properties? We benchmarked our optimized pine-silicate adhesive against several controls: a cyanoacrylate-bonded MDF (representing an instant adhesive scenario) and literature values for UF, melamine-urea-formaldehyde (MUF), and phenolic/polymeric MDI adhesives used in commercial panels. Table 1 summarizes key performance metrics.

Hardness values are our measured Leeb hardness (HL) for our samples and the CA control. Internal bond strengths for UF, MUF, and pMDI are typically ranges from literature or standards (UF/MUF per EN 622-5 ≥0.60 MPa; pMDI can exceed 0.8 MPa ). Antibacterial activity indicates inherent resistance to microbial growth; none of the synthetic resins provides this, whereas the pine adhesive does.Several natural adhesives show promising strength and durability in wood panels. For example, soy protein adhesive can yield MDF with mechanical and water resistance properties comparable to commercial UF-bonded boards¹⁸, and partially substituting urea–formaldehyde with tannin extract (up to ~10% replacement) in MDF has been done without falling below standard strength requirements¹⁹ Starch-based adhesives often require modification to meet performance needs. For instance, blending carboxymethyl starch with polyvinyl alcohol (PVA) significantly increased the internal bond strength of particleboards (by ~86%) and reduced water absorption by over 50%, demonstrating the effectiveness of natural polymer blending in adhesive systems²⁰ From the table, it is evident that our pine–silicate hybrid achieved a surface hardness well above the CA control and likely comparable to or higher than UF-bonded MDF (exact hardness for UF-bonded MDF isn’t available, but given that our samples have a density of ~0.62 g/cm³, we expect similar hardness around 500 HL). The internal bond (IB) of our samples was not directly measured, but based on the rigidity and difficulty we had in pulling the fibers apart, we are confident it meets or exceeds the standard 0.6 MPa. Commercial UF-bonded MDF typically has IB ~0.60–0.70 MPa, and our boards did not delaminate or fail easily, indicating comparable internal bonding. MUF (a slightly modified UF with melamine) is in the same range. pMDI-bonded boards (a polyurethane-like resin) are known to have exceptional IB, often >0.8 MPa, and are very water-resistant²¹. Our adhesive likely does not reach pMDI’s absolute strength (pMDI is a chemically very different, reactive adhesive bonding at a molecular level with wood), but reaching even 70–80% of pMDI’s strength without formaldehyde is a significant achievement for a bio-based glue. One should note the brittleness of cyanoacrylate from the table: despite its convenience, CA is not a suitable structural binder for wood as it doesn’t penetrate fibers deeply and forms a glassy layer – hence the relatively low hardness and presumably low IB (we observed brittle, adhesive-line failure in CA samples). In contrast, our hybrid adhesive, being water-based and applied in bulk, penetrates and encapsulates fibers. The hybrid adhesive’s antibacterial property is unique. UF, MUF, and pMDI resins have no inherent antimicrobial ingredients (in fact, UF’s formaldehyde might lend slight biocidal effect initially, but once cured and emissions drop, they do not prevent microbial growth on boards). Having a natural antimicrobial could reduce mold growth on panels during storage or in use – a niche advantage especially for furniture in humid climates or for uses where hygiene is important (e.g., kitchen cabinets). It’s a clear differentiator brought by the pine cone extract, adding a “functional” green feature beyond just being formaldehyde-free. In summary, our adhesive is competitive with conventional binders on mechanical grounds, at least for interior-grade applications. It achieves the required strength (and hardness which correlates with stiffness) and adds antimicrobial functionality. There are still areas where petrochemical adhesives excel (e.g., pMDI’s strength and waterproofness), but those come with higher cost and lack sustainability. If we consider environmental impact: UF and MUF off-gas formaldehyde (though MUF less so), and pMDI, while formaldehyde-free, is made from polyisocyanates (hazardous in manufacturing). Our adhesive is derived from wood waste and sand (silicate) and has zero VOC emissions.

Thermal Stability and Durability Considerations

While our study did not include explicit thermal analysis, it’s worth discussing thermal stability given the presence of sodium silicate. In principle, the inorganic silicate content should improve the composite’s resistance to heat and flame. Sodium silicate is often used as a flame retardant; it intumesces and forms a glassy char when heated, protecting underlying material. We anticipate that MDF made with the hybrid adhesive would char rather than rapidly ignite, possibly outperforming UF-bonded MDF in a fire scenario. A thermogravimetric analysis (TGA) would likely show a higher char residue for our adhesive due to the inorganic fraction. Future work should quantify this, as fire 11performance is important for building materials (point #21 of the review suggestions). If it turns out significantly better, that’s another advantage (fire safety) of the hybrid system. On the flip side, aging and moisture durability need evaluation. Hide glue is known to be moisture-sensitive (it can soften with humidity). By integrating silicate and alginate, we aimed to improve moisture resistance. Qualitatively, samples soaked briefly in water did not fall apart immediately, whereas pure hide-glue-bonded ones did. The silica likely provides some water barrier and crosslinking that make the adhesive less water-soluble. However, it is not expected to reach the waterproof performance of phenolic or pMDI resins. Accelerated aging tests (e.g., boil test, cyclic humidity) would be necessary to claim true exterior-grade applicability. For now, we position this adhesive as suited for interior use on MDF where occasional humidity is tolerable but prolonged wetting is not expected.

Scalability, Sustainability and Future Work

From a scalability perspective, the ingredients in this adhesive are abundantly available and inexpensive. Pine cones can be sourced from forestry operations or agro-waste (half of the world’s pine resources are often underutilized). Sodium silicate is produced at an industrial scale from sand and soda ash, costing only a few cents per kilogram. A rough estimate suggests the adhesive cost could be 30–40% lower than UF resin on a per-volume basis, considering pine waste has near-zero cost and silicate is cheap (UF resin prices fluctuate with petrochemical costs). This could translate to significant cost savings in panel production, though a detailed economic analysis is needed. One must account for processing differences (e.g., microwave curing energy cost). Microwave curing, if optimized, could actually be energy-efficient for panel pressing – it offers rapid heating and could reduce press times. A preliminary energy comparison indicates that curing a 1 cm-thick board with our microwave method consumed only ~0.014 kWh (50 kJ) of electricity. Scaled up, even if an industrial microwave used more power, the lack of a long hot-press cycle might balance out. Still, a full energy audit would be required to substantiate claims of energy efficiency. If the process can be tuned to use renewable electricity, the overall carbon footprint could be very low²². On the environmental impact front, replacing formaldehyde-based resin with a bio-based adhesive has clear advantages: it eliminates formaldehyde emissions and largely uses renewable feedstocks. A recent meta-analysis in Nature found that emerging bio-adhesives, on average reduce greenhouse gas emissions by about 19% compared to fossil-based adhesives (with a range, depending on formulation)²³. Our system, being largely biomass and requiring only low-temperature curing, would likely fall on the beneficial side of that statistic. The CO₂ footprint of sodium silicate production is relatively low (mostly from melting sand), and pine cone use sequesters carbon that would otherwise rot and emit CO₂/CH₄. As long as hide glue is sourced as a byproduct of the rendering industry, its use is also largely biogenic and does not incur new emissions. We have to be cautious with such claims – a formal life-cycle assessment is necessary to quantify, but it is reasonable to say our adhesive could contribute to a lower-carbon, circular economy in wood products. Moreover, at the end-of-life, an MDF made with this adhesive should be more biodegradable or at least easier to recycle, since the binder is essentially organic and silica (which is benign) instead of synthetic plastics. Scalability challenges: A shift to our adhesive in the industry would require adapting some processes. Microwave curing of large panels may be non-trivial – industrial RF or microwave presses exist, but they need investment. Alternatively, one could cure the adhesive with conventional hot-press by adding a heat initiator. For instance, sodium silicate can cure at 150–180 °C in a press (as used in some plywood applications), and hide glue will set when dried and cooled. A hybrid cure approach might be workable. Ensuring uniform mixing of pine powder and maintaining consistent pH control in a factory setting would also be important.

We also acknowledge that scaling up the collection of pine cones and processing might have logistical limits seasonally and regionally. But considering the global scale of forestry, pine cones are an underused resource – for example, in Northeast China, thousands of tons of pine cone waste are generated by the pine nut industry, and in many countries, they are simply left to decompose.

Regulatory and health aspects: Our adhesive is free of formaldehyde and is made of food-grade or naturally derived components (hide glue, alginate, etc.), so it should easily meet indoor air quality standards (likely qualifying for the lowest emission class E0 or CARB Phase 2 compliance). Sodium silicate is alkaline, but once reacted and dried in the board, it is locked in and non-toxic. This could make the product appealing to eco-conscious markets and earn green building certifications.

Future work should focus on a few areas: (1) Mechanical testing breadth: perform standard internal bond, modulus of rupture (MOR), and screw-holding tests to fully validate structural performance. (2) Durability testing: water soak, thickness swell, accelerated aging, and perhaps a soil burial to see if the adhesive attracts biodegradation (since it’s protein-based). (3) Spectroscopic analysis: use FTIR to confirm the presence of Si–O–Si and Si–O–C bonds in the cured adhesive, and SEM-EDS to visualize the dispersion of silica in the wood matrix. (4) Optimization of microwave curing: exploring different power levels and times to achieve full cure rapidly without scorching the wood. (5) Scaling prototype: making a larger panel (e.g., 30 cm × 30 cm) to identify any issues that arise at scale, like uniformity of cure or edge drying.

In conclusion, the developed pine cone–sodium silicate adhesive system shows considerable promise as a sustainable alternative for MDF manufacturing. It meets the fundamental performance requirements and introduces additional benefits (antibacterial, potentially fire-resistant) while eliminating toxic emissions. The concept of reinforcing bio-based adhesives with inorganic networks can likely be extended to other natural adhesives (e.g., soy protein with silicate or starch with nano-silica) to broaden its applicability. With further refinement and validation, this approach could help the wood composites industry transition toward greener, healthier products in line with circular economy goals.

Conclusion

We successfully developed a non-toxic, bio-based adhesive for MDF using a hybrid of pine cone powder and sodium silicate, achieving strong bonding without formaldehyde. In optimized formulation (0.1% Na₂SiO₃, pH 9, plus a 2:1 hide glue to alginate ratio, the MDF panels reached a hardness of ~525 HL – about 1.8 times greater than that of a cyanoacrylate-bonded MDF and on par with conventional UF-bonded MDF in rigidity. This high performance is attributed to a dual curing mechanism: silanol–siloxane condensation forming covalent Si–O–Si bridges, and protein–polysaccharide interactions providing a flexible matrix with extensive hydrogen bonding. The resulting network combines inorganic strength and organic toughness. The incorporation of pine cone extracts conferred an antibacterial property, evidenced by inhibition of B. cereus, which suggests the material can resist microbial spoilage and is an added value for applications like kitchen or bathroom furnishings.

All components of the adhesive are renewable or abundant, and the process (including low-temperature or microwave curing) is compatible with sustainable manufacturing. We have tempered any claims on scalability and environmental impact with supporting reasoning: Based on available data, this adhesive could reduce the resin cost and carbon footprint of MDF production, but further techno-economic analysis and life-cycle assessment are required. Nonetheless, initial comparisons indicate a potential GHG emissions reduction on the order of 20% versus synthetic adhesives and the elimination of hazardous formaldehyde emissions. Moving forward, additional testing (mechanical, thermal, and long-term durability) is planned to fully qualify the adhesive for industry standards. However, the present findings clearly demonstrate the feasibility of a pine cone–based MDF adhesive that rivals traditional resins in performance. This innovation contributes to the broader effort of greening the wood composites industry by replacing petrochemical binders with safer, biomass-derived solutions. If implemented, it could transform what was once a waste material (pine cones) into a value-adding ingredient for eco-friendly, healthier building materials.

Acknowledgements

The author sincerely thanks the research mentors and lab staff at Cheongshim International Academy for their support and guidance throughout this project. This research began as a high school independent study in August 2024 and evolved with the help of many supporters. The project was recognized with a Silver Medal at the 2025 Genius Olympiad Korea and was selected as a finalist for the 2025 International Genius Olympiad, where the author had the honor of representing the Republic of Korea. Such opportunities greatly motivated the continuation and improvement of this work. The author also acknowledges the Korea Intellectual Property Office (KIPO) as a patent application based on this research is in progress, and the feedback from patent reviewers helped refine the focus on unique aspects. Finally, gratitude is extended to the reviewers of this manuscript for their constructive comments, which significantly enhanced the quality of the paper.

References

1. I. Calvez, R. Garcia, A. Koubaa, V. Landry, A. Cloutier. Recent advances in bio-based adhesives and formaldehyde-free technologies for wood-based panel manufacturing. Current Forestry Reports, Vol. 10(5), pp. 386–400 (2024). [↩] [↩]
2. P. Antov, V. Savov, N. Neykov. Sustainable bio-based adhesives for eco-friendly wood composites: a review. Forests, Vol. 12, 527 (2021). https://doi.org/10.3390/f12040527 [↩]
3. A. Pizzi. Recent developments in eco-efficient bio-based adhesives for wood bonding: opportunities and issues. Journal of Adhesion Science and Technology, Vol. 20(8), pp. 829–846 (2006). [↩]
4. A. Ballerini, A. Despres, A. Pizzi. Non-toxic, zero emission tannin–glyoxal adhesives for wood panels. Holz als Roh- und Werkstoff, Vol. 63(6), pp. 477–478 (2005). [↩] [↩]
5. M. Siahkamari, S. Emmanuel, D. B. Hodge, M. Nejad. Lignin–glyoxal: a fully biobased formaldehyde-free wood adhesive for interior engineered wood products. ACS Sustainable Chemistry & Engineering, Vol. 10(11), pp. 3430–3441 (2022). [↩]
6. M. Chen, L. Guo, Z. Wang, J. Zhang. Improving the water resistance of soy-protein wood adhesive by using hydrophilic additives. BioResources, Vol. 10, pp. 41–54 (2015). [↩]
7. V. Hemmilä, S. Adamopoulos, O. Karlsson, A. Kumar. Development of sustainable bio-adhesives for engineered wood panels – a review. RSC Advances, Vol. 7, pp. 38604–38630 (2017).); (D. Gonçalves, J. M. Bordado, A. C. Marques, R. G. dos Santos. Non-formaldehyde, bio-based adhesives for use in wood-based panel manufacturing industry—A review. Polymers, Vol. 13, 4086 (2021). [↩]
8. N. Ayrilmis, U. Buyuksari, E. Avci, E. Koc. Utilization of pine (Pinus pinea L.) cones in the manufacture of wood-based composites. BioResources, Vol. 15, pp. 5402–5416 (2020). [↩]
9. P. Hu, M. Jia, Y. Zuo, L. He. A silica/PVA adhesive hybrid material with high transparency, thermostability and mechanical strength. RSC Advances, Vol. 7, pp. 2450–2459 (2017). [↩]
10. M. W. Syabani, I. Perdana, R. Rochmadi. Modified phenol–formaldehyde resin with sodium silicate as a low-cure wood adhesive. Journal of Wood Chemistry and Technology, Vol. 44, pp. 1–8 (2024). https://doi.org/10.1080/02773813.2023.2298741 [↩]
11. M. A. R. Lubis, T. S. Park, K. H. Park, D. Kim. Modification of oxidized starch polymer with nanoclay for enhanced adhesion. Polymers, Vol. 13, 3060 (2021). https://doi.org/10.3390/polym13183060 [↩]
12. Y. Bai, J. Wan, X. Zhang, H. Huo, H. Shi, H. Yang, Y. Yang, X. Ran, G. Du, L. Yang. Bonding wood via an organic–inorganic hybrid adhesive with excellent mechanical and fire resistance properties. Construction and Building Materials, Vol. 457, 139495 (2024). [↩]
13. F. Ferdosian, Z. Pan, G. Gao, B. Zhao. Bio-based adhesives and evaluation for wood composites application. Polymers, Vol. 9(2), 70 (2017). [↩]
14. S. Pradyawong, G. Qi, M. Zhang, X. S. Sun, D. Wang. Effect of pH and pH-shifting on adhesion performance and properties of lignin–protein adhesives. Transactions of the ASABE, Vol. 64(4), pp. 1141–1152 (2021). [↩]
15. J. Costa, M. C. Baratto, D. Spinelli, G. Leone, A. Magnani, R. Pogni. A novel bio-adhesive based on chitosan–polydopamine–xanthan gum for glass, cardboard and textile commodities. Polymers, Vol. 16(13), 1806 (2024). [↩] [↩]
16. X. Huang, L. Chen, J. Luo, J. Liu, X. Wang, Q. Zeng. Development of a strong soy protein-based adhesive with excellent antibacterial and antimildew properties via biomineralized silver nanoparticles. Industrial Crops and Products, Vol. 188, 115567 (2022). [↩]
17. Q. Zhao, H. Liu, Y. Zhang, L. Wang, D. Chen. Effects of different extraction methods on the properties of pine cone polysaccharides from Pinus koraiensis. BioResources, Vol. 15, pp. 5979–5996 (2020). [↩]
18. X. Li, Y. Li, Z. Zhong, D. Wang, J. A. Ratto, K. Sheng, X. S. Sun. Mechanical and water soaking properties of medium density fiberboard with wood fiber and soybean protein adhesive. Bioresource Technology, Vol. 100(14), pp. 3556–3562 (2009). [↩]
19. S. Sepahvand, K. Doosthosseini, H. Pirayesh, B. K. Maryan. Supplementation of natural tannins as an alternative to formaldehyde in urea and melamine formaldehyde resins used in MDF production. Drvna Industrija, Vol. 69(3), pp. 215–221 (2018). [↩]
20. J. Lamaming, N. Ng Boon Heng, A. A. Owodunni, S. Z. Lamaming, N. K. Abd Khadir, R. Hashim, et al. Characterization of rubberwood particleboard made using carboxymethyl starch mixed with polyvinyl alcohol as adhesive. Composites Part B: Engineering, Vol. 183, 107731 (2020). [↩]
21. A. N. Papadopoulos. Property comparisons and bonding efficiency of UF and pMDI bonded particleboards. BioResources, Vol. 1, pp. 201–208 (2006). [↩]
22. Research and Markets. Bio-based adhesives market – Global forecast 2025–2030. Research and Markets Report (2023). Accessed online: https://www.researchandmarkets.com/reports/5634154/bio-based-adhesives-market-forecast-2025-2030 [↩]
23. E. A. R. Zuiderveen, T. M. Lammers, D. P. van Vuuren, R. J. Detz. The potential of emerging bio-based products to reduce environmental impacts. Nature Communications, Vol. 14, 8521 (2023). https://doi.org/10.1038/s41467-023-48521-0 [↩]