Polyamidoamine-epichlorohydrin Research Articles

Effects of dehydration time on performances of polyamidoamine-epichlorohydrin resin and its modified soybean-based adhesive

Soybean-based adhesive has been commercialized in the wood industry owing to its several advantages, such as no formaldehyde, good bonding properties, and renewability. However, the large-scale application of soybean-based adhesive is greatly limited by its significantly higher cost than urea-formaldehyde resin. Thus, in this study, a cost-effective polyamidoamine-epichlorohydrin (PAE) resin with improved crosslinking efficiency to soybean meal was prepared from branched polyamidoamine (PAA) with reduced dehydration time at high temperatures. Test results from GPC, FTIR and NMR analyses showed that traditional PAA synthesis involving 3-h dehydration time at 180 °C resulted in over-branching of PAA resin and poor crosslinking efficiency of PAE resin and decreased water resistance of obtained soybean-based adhesive. PAA resin synthesized at appropriate dehydration time (1 h) remained sufficient secondary amine groups for being grafted by epichlorohydrin to form effective PAE resin with more azetidinium groups. As a result, the water resistance of soybean-based adhesive crosslinked by the optimal PAE-1 resin prepared from 1 h-dehydrated PAA-1 significantly improved by 42.9 % compared with that of soybean-based adhesive crosslinked by traditional PAE resin, attributing to forming denser and stronger crosslinking networks after thermally cured. Consequently, this optimal PAE-1 could reduce the cost of soybean-based adhesive due to decreasing 16.7 wt% of PAE dosage without compromising bonding property and the energy consumption of PAA/PAE synthesis. Therefore, an appropriate decrease in the dehydration time of PAA resin at a high temperature provides an effective, economical, and energy-saving strategy to improve the bonding property of soybean-based adhesives.

Understanding polyamidoamine epichlorohydrin (PAAE) retention in paper

Polyamidoamine epichlorohydrin (PAAE) is a permanent wet strength resin used in papermaking. When applied to paperboard, some amount of resin is retained in the sheet, and some is lost to the white water. The papermaker usually knows the amount of PAAE charged to the pulp but has no idea how much chemical is retained in the sheet. In addition, the influence of PAAE dosage, freeness, zeta potential, and pulp kappa number variability on PAAE retention is not well understood. Factorial design experiments using unbleached and bleached softwood (loblolly pine) kraft pulps were conducted to understand the factors that affect PAAE retention. The results revealed that PAAE retention, wet tear index, and tensile index not only depended on the PAAE charged of the pulp but also depended significantly on the pulp freeness. In lieu of freeness, zeta potential data can be used to predict PAAE retention. In addition, at similar freeness, bleached pulp has the highest retention of PAAE compared to low and high kappa unbleached kraft pulps. The results also suggest that lignin may have potential as a wet strength agent.

A novel approach utilising phosphorylated casein for fire-retardant and formaldehyde-free medium density wood fibreboards

Casein has been employed in composite materials as a flame retardant (FR). However, in the current research, casein has not been applied to a medium density fibreboard (MDF) material system, especially in a formaldehyde-free MDF material system, to investigate its impact on mechanical and fire-resistant performance. Polyamidoamine-epichlorohydrin (PAE) resin has been utilised as a binder to manufacture formaldehyde-free MDF samples in this research. Phosphorylation was conducted to enhance the flame-retardant ability of casein. This chemical treatment resulted in the significant improvement on the char-forming performance of the casein, leading to the dramatic reduction of the peak heat release rate (PHRR) by ⁓45% compared with non-treated casein. The char formation of the phosphorylated casein in the MDF sample also decreased its PHRR more effectively by ∼18% with the drop of the THR value (∼40 MJ/m2) than that of the casein. Besides, the addition of the phosphorylated casein to the MDF based on the PAE binder did not diminish the binder`s bonding performance, resulting in the improvement of the flexural modulus by ∼23% and flexural stress by ∼16%, while the IB strength retained compared with that of the control MDF sample. The current study clearly demonstrates that the jointly use of phosphorylated casein and PAE is a promising avenue to overcome one of the ongoing challenges in the MDF material system with regard to mitigating the reduction in mechanical performance associated with the introduction of flame-retardant materials.

A Room‐Temperature Curing Plant Protein‐Based Adhesive with High Strength and Flame Retardancy for Heat‐Free Adhesion

AbstractPlant protein‐based adhesives are gaining traction owing to their low cost and eco‐friendliness. However, achieving flame retardancy and long‐term water resistance in them under room‐temperature curing conditions remains a challenge due to the hydrophilicity and low reactivity of plant proteins. Herein, a novel adhesive synthesized from soybean meal (SM), activated sodium alginate (aSA), nano‐hydroxyapatite (nHA), and polyamidoamine‐epichlorohydrin (PAE) resin addresses this challenge. aSA as a reactive bio‐based cross‐linker formed covalent cross‐linking structures with SM matrix at room temperature, while the nHA‐induced biomineralization and PAE‐constructed supramolecular cross‐linking promote water drainage from the adhesive, preventing water erosion of the adhesive structure. The developed adhesive shows versatility across multiple substrates, with its wet shear strength on wood reaching 0.76 MPa, surpassing that of the commercial aldehyde‐based adhesive. The adhesive is effective over a wide temperature range from room temperature to 150 °C due to the reactivity of the PAE resin above 60 °C. Moreover, it exhibits excellent flame retardancy (limiting oxygen index of 31.2%) owing to its dense structure and the abundance of N‐containing and P‐containing components. This work is expected to break the monopoly of petroleum‐based adhesives in the realm of room‐temperature adhesives.

Fabrication of an Ultrastrong Formaldehyde-Free Wood Adhesive with an Organic-Inorganic Cross-Linking Network.

The wood industry faces challenges in producing eco-friendly, high-performance, and formaldehyde-free adhesives. In this study, carboxylated styrene-butadiene rubber (XSBR) was blended with polyamidoamine-epichlorohydrin (PAE) resin, and a controlled amount of CaCO3 powder was incorporated to create an adhesive with exceptional strength. The resulting three-layer plywood demonstrated remarkable dry and wet shear strengths of 3.09 and 2.36 MPa, respectively, and of 2.27 MPa after boiling water tests, comparable to that of phenolic resins. Additionally, the adhesive exhibited strong adhesion across various materials including glass, metal, etc. This exceptional performance was due to two primary factors: (1) the high-density chemical cross-linking reaction and the physical entanglement between XSBR and PAE; (2) the organic-inorganic hybrid involving metal ion complexation developed by CaCO3, which fostered molecular chain connections and enhanced the adhesive-material interface. These findings offer valuable references for further research in the field of wood adhesives.

Polyamidoamine epichlorohydrin (PAAE) wet-strength agent: Generations, application, performance, and recyclability in paperboard and linerboard

Polyamidoamine epichlorohydrin (PAAE) is the preeminent permanent wet strength additive used in papermaking. Wet strength additives are used to improve paper resistance to a rupture force in wet environments. The invention of PAAE in 1957 was an innovation, as it improved paper properties by giving superior wet strength in humid or wet conditions. It was rapidly adopted by the industry. Despite PAAE’s long history, the mechanism of PAAE interaction with fiber has not been fully understood. Therefore, fundamental understanding of PAAE mechanism needs to be investigated to improve its utilization in making sustainable paper products. These areas include an understanding of repulping methods and optimal PAAE dosages for better cost and performance. This paper investigates different generations of PAAE, the application method, and its impact on paper recyclability. Three generations of PAAE are currently on the market with at least two newer iterations under development. Critical application parameters that need to be understood include determination of the bonding mechanism, optimal dosage, and retention parameters. The main drawback of PAAE application is it makes paper recycling difficult. Several repulping methods are proposed for better recyclability and sustainability in the papermaking process.

An innovative biomass‐based wood adhesive generated from peony seed meal: Its preparation and properties

AbstractPlant protein‐based adhesives, particularly those derived from soybean meal (SM), have been available for various product applications. However, the high cost of SM has limited large‐scale manufacturing, despite sufficient production capacity. Peony seed meal (PSM) is abundant in proteins, polysaccharides, and polyphenols, the protein in PSM contains highly hydrophobic amino acids, resulting in low water solubility and water holding capacity. It is worth investigating whether the unique composition of hydrophobic amino acids in PSM protein can enhance the water resistance of PSM‐based adhesives. Additionally, the lower price of PSM, which is approximately two‐thirds of the cost of SM, makes it a more valuable raw material for adhesives. Herein, we report a simple and practical approach for preparing PSM‐based wood adhesives by utilizing the highly active cross‐linking agent of polyamidoamine‐epichlorohydrin (PAE). The results showed PSM‐based adhesives possessed excellent adhesion, especially the adhesive of “PSM‐36%”, with the wet bond strength and the aged bond strength reaching 1.60 and 0.83 MPa, respectively, meeting the standards outlined for indoor‐use plywood (≥0.98 MPa). Mechanistic studies indicated that the adhesion mechanism of such PSM‐based adhesives might involve high solid content, low viscosity, high thermal stability, and the formulation of covalent bonds and hydrogen bond networks.

Effect of Protein Surface Hydrophobicity and Surface Amines on Soy Adhesive Strength.

Soy is considered one of the most promising natural materials for manufacturing wood adhesives due to its low cost, high protein content, and ready availability. However, more cost-effective ways of improving its wet shear strength are needed to achieve wider market acceptance. Protein adhesive wet strength depends on the use of (typically expensive) crosslinking additives as well as the processing/denaturation of the protein. It has been commonly stated in the literature that protein denaturation leads to higher bond strength by activating the surface and exposing the reactive groups. Therefore, we investigated how differences in surface reactive groups (surface hydrophobicity and reactive amine groups) brought on with different denaturation treatments relate to bonding performance. Fourteen soy protein isolates (SPIs) with different denaturation histories were investigated. Characterization of the SPIs included surface hydrophobicity, surface amine content, extent of protein hydrolysis, and bond strength (wet and dry, with and without polyamidoamine epichlorohydrin (PAE) crosslinking agent) by ASTM D7998. The molecular weight patterns showed that proteins denatured by extensive hydrolysis had very low bond strengths. Adding the crosslinker, PAE, improved all the shear strength values. We found that the number of water-accessible reactive amine groups on protein surfaces had no impact on the adhesive strength, even with the amine-reactive crosslinker, PAE. Conversely, increased surface hydrophobicity was beneficial to adhesive strength in all cases, though this correlation was only statistically significant for wet strength without PAE. While, in general, denatured proteins are typically thought to form better bonds than native state proteins, this work suggests that it matters how proteins are denatured, and what surfaces become exposed. Denaturation by hydrolysis did not improve bond strength, and extensive hydrolysis seemed highly detrimental. Moreover, exposing hydrophobic surface groups was beneficial, but exposing covalent bond-forming reactive amine groups was not.

Multifunctional protocatechuic acid-polyacrylic acid hydrogel adhesives for wound dressings.

Multifunctional hydrogel adhesives are highly desirable in wound healing applications, yet their preparation often requires complex material system design to achieve. Herein, a straightforward one-pot two-step polymerization method is developed to prepare adhesive hydrogels for wound dressing based on protocatechuic acid (PCA), polyacrylic acid (PAA), and polyamidoamine-epichlorohydrin (PAE), where PCA provides the catechol groups for strong adhesion, PAA serves as the primary polymer matrix, and PAE acts as a bridge connecting PCA and PAA. This design results in a PAA-PAE-PCA hydrogel having a remarkable instant 90-degree peeling interfacial toughness of 431 J m-2 on porcine skin, which is further amplified to 615 J m-2 after 30 minutes. The hydrogel also possesses the desired features for wound dressing, such as self-healing, antioxidant, anti-UV and antibacterial properties, good cytocompatibility, strong adhesion in use and weak adhesion on removal, as well as reversible and wet adhesion. Finally, in vivo data reveal that the PAA-PAE-PCA hydrogels can significantly accelerate wound healing, as evidenced by a noticeable reduction in the wound area and a diminished inflammatory response. Collectively, these results endorse the obtained multifunctional hydrogel as a promising candidate for wound healing and related fields.

Novel bridge assistance strategy for tailoring crosslinking networks within soybean-meal-based biocomposites to balance mechanical and biodegradation properties

The cost-effective fabrication of environmentally friendly soybean meal (SM)-based biocomposites with tailorable biodegradability, mechanical properties, and water resistance remains a challenge. Herein, a novel bridge assistance strategy to effectively balance the performance of SM-based biocomposites was designed by constructing multiple tailorable networks using diethylenetriamine (DETA), melamine-urea-glyoxal (MUG), and polyamidoamine-epichlorohydrin (PAE) resins. The Schiff’s base reaction between DETA and MUG yielded a more reactive, branched DETA@MUG crosslinker, which could efficiently form co-crosslinks with long-chained PAE. Owing to the stronger bridge interactions of DETA, the obtained PAE/DETA@MUG co-crosslinks were more stable than those formed using PAE and MUG alone. Consequently, the PAE/DETA@MUG co-crosslinker system effectively reinforced the multiple networks and the performance of the SM-based biocomposites via the formation of more DETA@MUG–protein, PAE–protein, PAE–carbohydrate, and PAE–wood crosslinks, resulting in improvements in the dry tensile strength (158.2%) and water resistance (461.9%). Moreover, the multiple networks formed by branched DETA@MUG were beneficial for balancing the interior forces of the obtained SM-based resin, thereby improving its toughness by 64.3%. The DETA bridge also decreased the numbers of stable PAE–wood and PAE–SM bonds in the SM-based biocomposites while ensuring the necessary PAE/DETA@MUG co-crosslinks, which improved the biodegradation rate by 16.2% without compromising the mechanical properties and water resistance. Therefore, this bridge assistance strategy is highly suited for the commercial production of SM-based polymers and biocomposites with balanced mechanical properties, water resistance, and biodegradability.

Effects of wood dyestuff on the bonding properties of soybean-based adhesive for commercial plywood production

The two main components of commercial soybean-based adhesives (SA), polyamidoamine-epichlorohydrin (PAE) resin and soybean protein, are extremely ion-sensitive. When SA is used to bond dyed wood substance and/or mix with anion dyes for labelling wood composites during commercial manufacturing, the regularly used anion dyes in the wood industry may produce significant differences in the bonding capabilities of SA. In the current study, three common tricolor anion dyes in the wood industry were chosen to evaluate their impacts on the structures and properties of soybean protein, PAE resin, and PAE-modified SA, respectively. Analytical data from FTIR, XRD, TGA, NMR, sol-gel test, and plywood evaluation indicated that dye ions had varying effects on the bonding properties of SAs. The addition of chosen red and yellow dyes to SAs enhanced plywood cycling wet strengths by 41.7–42.8%, whereas the blue dye had little impact. The promoter interactions of red- and yellow-dye ions on the unfolding of globular structures of soybean protein, the self-crosslinking of PAE resin, and the sufficient co-crosslinking of soybean protein by PAE resin all contributed to the formation of compact three-dimensional networks of cured SAs. As a result, this work illuminates a novel method for conveniently improving the bonding capabilities of SAs by adding appropriate dyes.

Effective and environmentally safe self-antimildew strategy to simultaneously improve the mildew and water resistances of soybean flour-based adhesives

Wide applications of soybean-based adhesives in the production of wood composites are significantly restricted by mildew attacks. Increasing mildew resistance without increasing costs and causing environmental risks and/or loss of bonding properties remains challenging. This study proposes a self-antimildew strategy that involves the easy conversion of mildewing primary amines of soybean proteins of soybean flour (SF) into mildew-tolerant secondary amine linkages based on the structural design and synthesis of polyamidoamine–epichlorohydrin (PAE) resins. This effectively and environmentally safely improves the mildew resistance of SF-based adhesives without using additional mildew preventives and simultaneously enhances the bonding properties. NMR and FTIR results confirmed that the azetidine content of the PAE resin increased by 123.1% as the epichlorohydrin ratio increased from 0.75 to 1.2. The SF-based adhesive fabricated from the optimal PAE resin improved the mildew resistance by ≥ 367% and water resistance by 61.3% owing to the effective conversion of primary amines into secondary amine linkages via sufficient protein-PAE cross-linking. Furthermore, the addition of 0.45 wt% of environmentally safe and low-cost sodium formate into self-antimildew SF-based adhesives could maintain the desired mildew resistance and reduce the dosage of expensive PAE resin by 33% without compromising the bonding properties. This novel antimildew strategy allows cost-effective and environmentally safe improvements in both antimildew and mechanical performances, thereby allowing cleaner production of protein-based adhesives and composites.

Improving Bond Performance and Reducing Cross-Linker Dosage of Soy Protein Adhesive via Hyper-Branched and Organic-Inorganic Hybrid Structures.

Eco-friendly soybean protein adhesives could be an ideal substitute for replacing traditional formaldehyde-based adhesives in wood industry. However, a large number of cross-linking agents are required in soy protein adhesive formulations to obtain sufficiently performing properties. Inspired by the high performance of nacre and branched structures, a hyper-branched amine (HBPA) was synthesized and grafted to graphene oxide (GO), generating a hyper-branched amine-functionalized GO (FGO). A novel soy protein-based adhesive was developed by mixing FGO with soy protein (SPI) and a low dose polyamidoamine-epichlorohydrin (PAE). Results showed that the addition of only 0.4 wt% FGO and 0.75 wt% PAE to the SPI adhesive formulation enhanced the wet shear strength of plywood to 1.18 MPa, which was 181% higher than that of the adhesive without enhancement. The enhanced performance is attributed to the denser cross-linking structure and improved toughness of the adhesive layer. Using FGO in the adhesive formulation also greatly reduced the concentration of the additive cross-linker by up to 78.6% when compared with values reported in the literature. Thus, using a hyper-branched functionalized nano-material to form an organic-inorganic hybrid structure is an effective and efficient strategy to reinforce the composites and polymers. It significantly reduces the chemical additive levels, and is a practical way to develop a sustainable product.

Ovalbumin has unusually good wood adhesive strength and water resistance

AbstractThe interest in non‐formaldehyde adhesives has reinvigorated studies using proteins from plant and animal sources, with an emphasis on plant‐based sources, especially the oilseed plants such as soybean. Although animal‐based sources have received much less attention, and ovalbumin from egg whites has not been studied at all, it provides a surprising low viscosity and high wet strength as a wood adhesive. Of the animal and plant proteins used in the past, egg protein has been used for centuries as a binder in tempera, for book illumination, and for gilding, but not as a water‐resistant wood adhesive. The isolated ovalbumin is the best, but dried egg whites and whole egg have good adhesive strength. The parameters for ovalbumin wood bonding are examined with thin veneers using ASTM D 7998 and plywood tests using D 906. Sodium metabisulfite, sodium periodate, urea, and polyamidoamine‐epichlorohydrin (PAE) improve the wet bond strength, but the surfactants CTAB and SDS at low concentrations do not. Although the high protein content of the ovalbumin is important for good strength, the low viscosity of the protein adhesive provides further insight into protein–protein interactions.

A soybean adhesive with excellent hygrothermal resistance and enhanced mildew resistance via optimal synthesis of polyamidoamine–epichlorohydrin resin

Despite the fact that soybean-based adhesives offer huge advantages such as sustainability and being eco-friendly, their wider application in wood composites has been severely restricted by the poor resistance towards wet environments and mildew. In this study, a soybean-based adhesive with excellent hygrothermal and enhanced mildew resistance was developed by optimizing the synthesis of a polyamidoamine–epichlorohydrin (PAE) resin with varying epichlorohydrin (ECH) content and applying formic acid as a terminating agent. FTIR, NMR analyses and plywood evaluation indicated that the ECH content had a significant effect on the hygrothermal resistance of the PAE-modified soybean adhesive. This is attributed to the greater number of azetidine groups resulting from higher ECH content, which effectively crosslinked the soybean meal to form compact networks. However, owing to the more powerful mildew inhibition of formic acid than sulfuric acid and ECH-derived organochlorine, higher ECH content also led to a higher amount of organochlorine in the PAE resin but required less formic acid to terminate PAE polymerization, causing reduced mildew resistance of the soybean adhesive. The PAE resin synthesized with 50% ECH was optimum for formulating a soybean adhesive for wood composites, which showed superior hygrothermal and mildew resistance compared to commercial adhesives.

Double-network strategy for a cost-effective soybean meal-based adhesive with required and stable water resistance for structural use

Although soybean meal (SM)-based adhesives, modified by polyamidoamine-epichlorohydrin (PAE) resin exhibit satisfactory characteristics to replace formaldehyde-based adhesives, their broad commercialization is still limited due to poor water resistance and high cost. This study employed a novel double-network strategy to effectively crosslink SM and fabricated an SM-based adhesive with required and stable water resistance using a co-crosslinker (PM) composed of PAE and melamine-urea-glyoxal (MUG) resins. Analyses including 1 H NMR, 13 C NMR, FTIR, TGA, and XPS revealed that the robust double-network structures were constructed from MUG-SM and PAE-SM networks, which were further reinforced by the co-crosslinking between PAE and MUG resins. Thus, up to 40% of the expensive PAE could be substituted by low-cost MUG resin, resulting in a novel SM/PM adhesive with the required water resistance for structural use at a 16% reduced cost. Furthermore, PM exhibited much better storage stability than PAE and MUG resins. The presented double-network strategy sufficiently promotes the cost-effective application of SM-based adhesives in wood composites and extends it from indoor to outdoor products. • PAE resin blended with MUG resin to produce a PM resin via co-crosslinking. • Double-network structure constructed from PM-soybean meal crosslinking. • Enhanced adhesion properties produced by a double-network structure revealed. • Storage stability of PM resin in 60 days traced and characterized. • SM/PM adhesive reduced cost by 16% with comparable water resistance.

Conversion of soybean oil extraction wastes into high-performance wood adhesives based on mussel-inspired cation-π interactions

As a soybean oil extractive byproduct, high temperature defatted soy meal (HSM) presents great potential as a raw material for vegetable protein adhesives to replace aldehyde-based adhesives in the wood-based panel production. However, the application has been hindered by its poor cold-pressing adhesive performance. Herein, a novel HSM-based adhesive with excellent cold-pressing adhesion performance was developed based on mussel-inspired cation−π interactions. Highly reactive polyamidoamine-epichlorohydrin (PAE) and folic acid (FA) were added into an HSM-based adhesive to construct a dual-network system stabilized by strong cation−π interactions. The coacervate formed by PAE and FA served as an “internal adhesive” to bond HSM particles together, yielding high initial viscosity but easy sizing. As expected, the prepared adhesive exhibited an excellent cold-pressing bonding strength of 423 kPa, showing a 295% improvement compared to the soy protein (SP) adhesive. To improve the hot-pressing bonding strength of the adhesives, inorganic calcium carbonate (CaCO3) particles were introduced into the adhesive system to build an organic–inorganic hybrid adhesive system. The wet shear strength of the SPAE-FA-CaCO3 adhesive significantly improved from 0.63 MPa to 0.96 MPa, meeting the requirements for the practical application. This method provides a novel strategy to exploit high-performance vegetable protein-based wood adhesives.

Improved Wood-Bond Strengths Using Soy and Canola Flours with pMDI and PAE.

The surprising lack of literature on using the very common wood adhesive polymeric methylenediphenyl diisocyanate (pMDI) with protein adhesives may be because of perceived poor improvement of protein wet strength. Reacting pMDI with the flour (soy or canola) before adding water unexpectedly improves wood bonding compared to adding the pMDI to an aqueous protein slurry. Mixing the liquid pMDI with the oilseed flour produces a free-flowing powder with up to 50% of pMDI to flour by weight. The mixture slowly reacts since the isocyanate band in the infrared spectra remains for several days but diminishes with time. Adding pMDI increases the dry and wet strength of wood bonds using Automated Bonding Evaluation System (ABES) testing and levels off at about 50%. Similarly, adding the polyamidoamine-epichlorohydrin (PAE) cross-linker to the oilseed flour increases dry and wet bond strength, but the effect levels off at about 20% of PAE. However, the combination of these two cross-linkers added to the flours results in greater dry and wet shear strength than either one alone. In addition to tests using ABES (ASTM D 7998), the increase in strengths is also observed—but with a diminished effect—in bonding plywood using the interior plywood strength test ASTM D 906.

An aqueous polyisocyanate adhesive with excellent bond durability for engineered wood composites enhanced by polyamidoamine-epichlorohydrin co-crosslinking and montmorillonite hybridization

Recent trends toward mass-timber buildings and tall timber structures necessitate more stringent requirements for the bond durability of adhesives for engineered wood composites. In this work, a cost-effective strategy for manufacturing a novel aqueous polyisocyanate adhesive with excellent hygrothermal resistance for engineered wood composites was produced via co-crosslinking with polyamidoamine-epichlorohydrin (PAE) resin followed by organic–inorganic hybridization with montmorillonite clay (MMC). Test results confirmed that PAE co-crosslinking effectively increased the crosslinking density of aqueous polyisocyanate adhesive meanwhile prolonged working life. Hybridization of PAE-crosslinked polyisocyanate with MMC resulted in compact crosslinked networks and notably increased hygrothermal resistance. Consequently, this novel aqueous polyisocyanate adhesive exhibited excellent bond durability of plywood in terms of a wet aged strength of 1.73 MPa and a strength reduction ratio of only 18.4% after 7 boiling-dry hygrothermal aging cycles, which satisfies the requirements for manufacturing engineered wood composites as good construction and building materials.

Preparation and characterization of a melamine-urea-glyoxal resin and its modified soybean adhesive

Although polyamidoamine-epichlorohydrin (PAE)-modified soybean meal (SM) adhesives exhibit desired bond properties and technological applicability, their commercial application is still limited by the high cost of PAE resin. In this study, a formaldehyde-free crosslinking agent, melamine-urea-glyoxal (MUG) resin for SM adhesive, was developed based on investigations of the effects of melamine content and storage time on the structure and properties of MUG resin and MUG-modified SM adhesive. The test results confirmed that melamine could effectively copolymerize with glyoxal and urea to form an aldehyde-terminated MUG resin that was crosslinkable to soybean protein. The melamine content in MUG had important effects on the bond properties of the SM adhesive, attributed to the incorporation of aromatic triazine rings into the aliphatic urea-glyoxal chains and the greater reactivity and functionality of melamine than urea. However, the addition of melamine at more than 10% led to a shorter storage time of MUG resin and worse bond properties of MUG-modified SM adhesive. Owing to the further reactions of residual aldehyde and amino groups of MUG resin, the water resistance of the MUG-modified SM adhesive decreased with the storage time of the MUG resin. The allowable storage of the optimal MUG-10 resin was 3 weeks to ensure an acceptable bonding quality. Due to the much lower cost of MUG resin than PAE resin, it is beneficial for the wide commercial applications of formaldehyde-free SM adhesives in wood composites.