Tech Blog

Water resistance of melamine modified low toxicity urea formaldehyde resin

Urea formaldehyde resin is the most widely used adhesive in the artificial board industry, especially for plywood, particleboard, and medium-density fiberboard. They have low cost, fast curing, and good initial adhesion.

However, urea formaldehyde resin has two main drawbacks:
Poor water resistance-especially to boiling water, due to the presence of unstable C-N bonds and hydrophilic groups such as hydroxymethyl, carbonyl, amino, etc.
High formaldehyde emissions-both during the production process and throughout the lifespan of the panel.

This article focuses on the poor water resistance and high free formaldehyde content of urea formaldehyde resin. A certain amount of melamine is used for modification, and the modified urea-formaldehyde resin adhesive combines the advantages of melamine resin adhesives while compensating for their shortcomings.

Why can melamine improve urea formaldehyde resin?

Pure urea-formaldehyde resin contains many hydrophilic groups (hydroxymethyl, carbonyl, amino, imino), which are readily hydrolyzed in hot water, resulting in weak adhesive strength. Melamine has three main benefits:

Form a 3D cross-linked network to block hydrophilic sites.
Neutralizing acidic curing agents to slow down hydrolysis.
Reacts with free formaldehyde to reduce emissions.
By combining copolymerization (adding melamine during synthesis) and blending (mixing melamine resin with finished urea-formaldehyde resin), modified resins exhibit stronger water resistance and lower toxicity.

Experimental Design

1.Low‑Toxicity UF Resins Synthesized

Two series of low‑toxicity UF resins were prepared using an alkaline‑acid‑alkaline process with three urea additions:

JQ‑21 (A, B, C)	Unmodified (control)	1.05, 1.10, 1.15
JQ‑22 (A, B, C)	Copolymerized with 5% melamine (on urea weight)	1.05, 1.10, 1.15

Thus, six UF resins were tested.

2.Blending with Melamine‑Formaldehyde (MF) Resin

Each UF resin (JQ‑21 or JQ‑22) was blended with a pure melamine‑formaldehyde resin at different mass ratios:

MF:UF = 5:5, 4:6, 3:7, 2:8

3.Curing Systems

Two curing systems were evaluated:

Curing system 1: 5 mL curing agent (type not specified, but likely ammonium chloride‑based) per 100 g mixed adhesive.
Curing system 2: 0.5 g curing agent (different composition) per 100 g mixed adhesive.

4.Plywood Manufacturing and Testing

Substrate: Poplar veneer, 2 mm thick, moisture content 8–12%, three‑layer plywood.
Adhesive formulation: 100 parts mixed resin + 20 parts wheat flour + 5 parts water.
Spread rate: 280–320 g/m².
Hot pressing: 120 °C, 1.0 MPa, 3.5 min.
Testing: Bond strength after boiling water test (Type I plywood, GB/T 9846.3‑2004) and formaldehyde emission (desiccator method, GB/T 17657‑1999).

Results: Bond Strength (Water Resistance)

Effect of Blend Ratio and Copolymerization

The bond strength after the boiling-water test increased with increasing MF resin content in the blend. At MF:UF = 5:5, most formulations passed the Type I plywood standard (≥0.7 MPa required for Type I plywood in China). Results are shown below.

Table 1 – Bond strength with curing system 1 (MPa)

1.05	JQ‑21 (unmodified)	0.66	0.52	0.29	0.25
1.05	JQ‑22 (co‑polymerized)	0.76	0.54	0.42	0.25
1.10	JQ‑21	0.76	0.49	0.36	0.25
1.10	JQ‑22	0.93	0.60	0.47	0.34
1.15	JQ‑21	0.61	0.43	0.35	0.32
1.15	JQ‑22	0.65	0.59	0.47	0.30

Observations:

Copolymerized resins (JQ‑22) consistently gave higher bond strength than unmodified (JQ‑21) at the same blend ratio.
At a 5:5 blend, all JQ‑22 resins (F/U 1.05–1.15) met or exceeded the Type I standard (≥0.7 MPa). JQ‑22B (F/U=1.10) achieved 0.93 MPa – excellent.
Higher F/U molar ratios generally increased bond strength but also increased formaldehyde emissions.

Effect of Curing System

Curing system 1 (5 mL/100 g) yielded higher bond strengths for most formulations, whereas curing system 2 showed more stable performance across different blend ratios, especially for the copolymerized resins. For JQ‑22 resins with curing system 2, even the 4:6 blend often met the Type I standard (e.g., F/U=1.05 gave 0.76 MPa; F/U=1.10 gave 0.73 MPa).

Recommendation: For copolymerized melamine‑modified UF, curing system 2 is preferred for stability; for unmodified UF, curing system 1 gives better bond strength.

Results: Formaldehyde Emission

Effect of Modification

JQ‑21 (unmodified)	5:5	1.05	1.35
JQ‑22 (copolymerized)	5:5	1.05	0.90 (E₁ class)
JQ‑21	5:5	1.10	2.61
JQ‑22	5:5	1.10	1.57
JQ‑21	5:5	1.15	1.99
JQ‑22	5:5	1.15	1.39

Key findings:

Copolymerized resins (JQ‑22) consistently emitted less formaldehyde than their unmodified counterparts.
With curing system 1, all JQ‑22 blends at 5:5 achieved E₁ class (≤1.5 mg/L) per GB/T 9846.3‑2004. Some even reached ≤1.0 mg/L.
Curing system 2 gave higher formaldehyde emissions (mostly E₂ class), so curing system 1 is preferred for low emissions.

Effect of Blend Ratio

As the proportion of MF resin in the blend decreased, formaldehyde emissions initially dropped slightly, then leveled off. However, bond strength dropped significantly below 40% MF content. Therefore, a minimum of 40–50% MF in the blend is recommended to balance performance and emissions.

Practical Recommendations for Industry

UF resin F/U molar ratio	1.05–1.10 (lower is better for emission, but 1.10 gives stronger bond)
Melamine in copolymerization	5% on urea weight
Blend ratio (MF:UF)	5:5 (50% MF) for guaranteed Type I + E₁; 4:6 may work with optimized curing
Curing system	System 1 (5 mL/100 g) gives lower emission; System 2 gives more stable bond for copolymerized resins. Test both for your specific resin.
Hot press temperature	120 °C
Press time	3.5 min for 3‑layer plywood (2 mm veneer)

conclusion-Water resistance of melamine modified low toxicity urea formaldehyde resin

Melamine modification is an efficient method to improve the water resistance of low-toxicity urea formaldehyde resin while reducing formaldehyde release. The combination of copolymerization and blending performs better than individual blending. With appropriate proportions, molar ratios, and curing systems, melamine-modified urea-formaldehyde resin becomes an ideal ecological adhesive for outdoor and indoor wooden boards.

For plywood manufacturers seeking to improve durability, meet environmental standards, and control costs, melamine-modified urea-formaldehyde resin is the preferred choice.

Tech Blog

Water resistance of melamine modified low toxicity urea formaldehyde resin

Why can melamine improve urea formaldehyde resin?