Performance of Five Melamine–Urea–Formaldehyde (MUF) Resins in Spruce Three-Layer Glulam Bonds: Adhesion, Durability, and Mechanical Properties

Towards the creation of a long-lasting and high-performing glulam-product, the optimization of melamine–urea–formaldehyde (MUF) adhesive solutions in order to be in line with worldwide trends of building and cutting-edge material science is a matter of first-priority. However, glulam performance is still highly determined by the efficiency of adhesive bonds, which highlights the necessity of thorough resin and bonding examination. To identify the most effective MUF formulation for structural applications, this study examines the delamination resistance, of spruce three-layer glulam, applying five resins based all on MUF adhesive (EN 14080), differentiating in terms of hardener–resin ratios (1:4 and 1:5) and the applied adhesive amount (1:4 and 1:1) according to ISO 12578. The results revealed that some of the adhesives (A and E) were not suitable for use, the adhesives B and D require further processing, since both achieved a wood failure of 50% in the four applied experiments, while only adhesive C provided almost excellent results in all cases. When the hardener to glue ratio was 1:5 or 1:1, and the application was four times the typical amount of glue, the delamination test requirements were fulfilled, while none of the experiments with a ratio of 1:4 exhibited satisfying adhesion and strength, something that raises concern since this is the ratio recommended by the glulam-production standard. A thorough understanding of MUF adhesive formulations and adhesion mechanisms were approached, which is crucial towards the optimization of wood-based products especially of high-strength requirements as glulam. The hardener-to-glue ratio and the quantity of adhesive were highlighted as crucial factors, underlying the need for accurate formulation and application in structural glulam bonding, while more stringent manufacturing quality control seems to be a necessity.