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MUG Resin: Formaldehyde-Free Wood Adhesive

Traditional wood adhesives, such as urea-formaldehyde (UF) resin, have long dominated the wood-based panel industry, but their formaldehyde emissions pose severe health and environmental risks. This low-toxic, formaldehyde-free co-condensed resin—synthesized from melamine (M), urea (U), and glyoxal (G)—combines excellent bonding strength, water resistance, and environmental compliance.

This article details MUG resin’s optimized synthesis process, curing performance, structural characterization, and industrial applications, serving as a comprehensive guide for wood processing professionals and chemical engineers.

Why MUG Resin? Core Advantages Over Traditional Adhesives

MUG resin addresses the critical pain points of conventional wood adhesives, offering unmatched value for modern sustainable manufacturing:

Formaldehyde-Free & Low-Toxic

Glyoxal replaces formaldehyde as the crosslinking agent, eliminating formaldehyde emission entirely. Glyoxal is low-volatile at room temperature and non-toxic, complying with global eco-friendly standards (e.g., CARB P2, EU REACH).

Balanced Performance

Bonding Strength: Meets Grade Ⅱ plywood requirements (GB/T 9846-2015) with dry shear strength of 2.11 MPa, cold water (24h) wet strength of 1.24 MPa, and hot water (3h) wet strength of 0.75 MPa.

Stability: Stable at room temperature as a golden yellow transparent liquid, with no crystallization or delamination during storage.

Processability: Moderate viscosity (178.6 mPa·s at optimal conditions) ensures uniform application and good wettability on wood surfaces.

Cost-Effective

Raw materials (melamine, urea, glyoxal) are readily available and affordable, with lower production costs than those of isocyanate-based or biomass adhesives (e.g., soy protein glue).

Optimized Synthesis Process of MUG Resin

The performance of MUG resin is highly dependent on synthesis conditions. Through systematic research, the optimal parameters are confirmed as follows:

Key Synthesis Parameters

Parameter	Optimal Range	Impact on Resin Performance
Molar Ratio (M:U:G)	0.13:0.30:1.0	Higher melamine content increases solid content and viscosity; excess melamine causes poor workability.
Reaction pH	5.0–6.0	Acidic conditions promote co-condensation; pH >8 leads to resin solidification; pH <4 reduces storage stability.
Reaction Temperature	70–80℃	Low temperature (＜60℃) results in incomplete reaction; high temperature (＞90℃) darkens resin color and increases viscosity.
Reaction Time	50 minutes	Shorter time (＜30min) causes insufficient crosslinking; longer time (＞70min) increases viscosity and production costs.

Step-by-Step Synthesis Workflow

1. Preparations: Add 145g of 40% glyoxal solution and 18g of urea to a three-necked flask equipped with a stirrer, condenser, and thermometer.
2. pH Adjustment: Use 30% NaOH solution to adjust the pH to 5.0–6.0.
3. Heating & Melamine Addition: Heat the mixture to 70–80℃, then add 0.13 molar equivalent of melamine (relative to glyoxal) and stir until dissolved.
4. Co-Condensation Reaction: Insulate at 70–80℃ for 50 minutes to complete the reaction.
5. Cooling & Discharging: Cool to below 40℃ and discharge. The final resin is a golden-yellow, transparent liquid with a 50.7% solid content and a viscosity of 178.6 mPa·s.

Curing Performance of MUG Resin

Dynamic Mechanical Analysis (DMA) reveals the curing behavior of MUG resin, which directly impacts wood panel hot-pressing processes:

Curing Characteristics

Onset Curing Temperature: 120.5–132.2℃ (lower than UF resin’s 140–150℃), reducing energy consumption during hot-pressing.
Peak Storage Modulus: 9764 MPa (at optimal conditions), indicating excellent mechanical strength after curing.
Thermal Stability: Decomposition temperature of 195–210℃, ensuring long-term stability of bonded wood products.

Influence of Synthesis Conditions on Curing

Molar Ratio: Increasing melamine content enhances crosslinking density, raising the storage modulus, but increasing the curing temperature.
pH Value: pH 5–6 yields the highest storage modulus, as acidic conditions facilitate the formation of stable C-N and C=N bonds.
Temperature: 70–80℃ promotes sufficient co-condensation, balancing curing speed and resin stability.

Application Performance: MUG Resin in Plywood

MUG resin is primarily used for bonding wood-based panels like plywood, particleboard, and MDF. Key application data:

Plywood Preparation

Spreading Rate: 180 g/m² (single side) for 1.5mm poplar veneers.
Hot-Pressing Parameters: 150℃, 300 seconds (5 minutes).
Post-Treatment: Balance the bonded plywood at room temperature for 24 hours before testing.

Mechanical Properties of Plywood

Performance Index	Value	GB/T 9846-2015 (Grade Ⅱ) Requirement
Dry Shear Strength	2.11 MPa	≥1.0 MPa
Cold Water (24h) Wet Strength	1.24 MPa	≥0.7 MPa
Hot Water (3h) Wet Strength	0.75 MPa	≥0.5 MPa

Application Advantages

Suitable for indoor and semi-outdoor wood products (e.g., furniture, interior decoration, packaging boxes).
Compatible with existing wood panel production lines, requiring no equipment modifications.
Low formaldehyde emission (undetectable), meeting E0/E1 standards for indoor air quality.

conclusion

Melamine-Urea-Glyoxal (MUG) resin revolutionizes the wood adhesive industry by addressing the formaldehyde emissions of traditional resins. Its optimized synthesis process (M:U:G = 0.13:0.30:1.0, pH 5–6, 70–80℃, 50min) yields a low-toxic, high-performance adhesive with excellent bonding strength and water resistance. Validated by ¹³C-NMR and XPS, the co-condensed structure ensures stable performance, while DMA analysis provides guidance for industrial hot-pressing processes.

As the wood processing industry shifts toward sustainability, MUG resin’s compliance with global environmental standards, cost-effectiveness, and process compatibility make it an ideal alternative to UF, MUF, and isocyanate-based adhesives. Its application in plywood, particleboard, and MDF will continue to grow, driving the development of green wood-based panel production.