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

Abstract

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.

1. Introduction

Wood bonding facilitates the effective application of wood in furniture, wooden structures, and the building sector [1], where wood-based products are frequently encountered in the form of glued laminated timber (glulam), particleboard, fiberboard, plywood, oriented strand-board, and other products that are bonded with different resin types [2].

Structural glulam is one of the oldest and a high-performance engineered wood products. It is a product consisting of two or more layers of wood, glued together, and is defined as a material created from suitably selected and prepared pieces of wood, in a straight or curved shape, with the fibers of all pieces practically parallel to the longitudinal axis [3]. Glulam is most frequently applied in furniture, sports facilities and equipment, in load-bearing elements, such as beams, arched columns, parts of bridges, shipbuilding and aeronautical structures, among others [4], mainly due to its versatility and sustainability. Nevertheless, issues including long-term durability, moisture susceptibility, and formaldehyde emissions restrict their effectiveness in demanding applications [5].

Almost any wood species, both hardwoods and softwoods, can be utilized in the production of glulam, provided that the wood’s physical and chemical properties, as well as bonding ability, are acceptable. Douglas spruce, larch, southern pine, hem fir, and spruce are mainly used in the USA, while in Europe, and especially in the Scandinavian countries, there is a particular preference for fir, spruce, and pine species [2]. The wood species, wood structure and properties, potential defects presence, the dimensions/cross section of wooden elements of glulam, their moisture content, density, adhesive type and adhesion conditions applied, as well as any potential treatment applied during the production process, are all significant factors that affect the properties of the glulam final product [6]. Norway spruce wood, which is also used in the current work, demonstrates a natural gloss, without axial resin ducts, though often with axial parenchyma. The heartwood and sapwood do not differ; the wood is light, soft, and white to light brownish yellow, and the boundaries between early and latewood are usually pronounced [7].

Thermosetting cold-bonding adhesives like epoxy, phenolic (phenol–resorcinol) are typically used for bonding of glulam and other wood-based products. Other natural or synthetic adhesives may also be utilized, depending on the application of glulam. Melamine–urea–formaldehyde (MUF) is initially produced in powder form and then combined with a catalyst, incorporating urea to create a colorless, liquid resin. Although it can tolerate hot pressure (120–150 °C), it can also function well at lower pressing temperatures. It is a resin characterized by excellent resistance to water, high resistance to both dry and humid environments, and the ability to cure quickly in high-frequency presses. Even though the particleboards and fiberboards with MUF exhibit low adhesive strength, this adhesive provides long-lasting glue bonds [8]. MUF adhesive is considered appropriate for use in plywood products made of either softwood or hardwood species, for the connection of lamellas, dowels, mortise and tenon elements, and other wood-to-wood joint bonding [9,10].

Liu et al. [11] reported that MUF exhibited good bonding performance at room temperature, but showed poor thermal resistance, with the structures being significantly degraded when the temperature increased above 200 °C. In contrast, phenol-resorcinol formaldehyde (PRF), exposed to elevated temperatures (150 °C), demonstrated similar bonding strength to that of solid wood. Jiang et al. [12] investigated the delamination strength of five-layer glulam products made of four different European species (Fraxinus excelsior, Fagus sylvatica, Larix decidua, and Picea abies) and adhesives of PRF and Isocyanate Polymer Emulsion (EPI), MUF, and Polyurethane (PUR). Delamination strength tests (EN 391, method A) showed that only Norway spruce (Picea abies) glulam provided satisfying delamination strength, independently of the adhesive type. Knorz et al. [13], based on EN 302-2, investigated the delamination strength and shear performance of Fraxinus excelsior L. glulam, glued with PRF, MUF—1 (100/20), MUF—2 (100/50), PUR, and EPI, with varying closed bonding times as a parameter to be studied. Significant differences were found between the delamination strength of different adhesives and bonding times, with the best performance to be recorded by PRF with the long bonding time, and it was the only resin to fulfil the delamination strength standard requirements. The penetration of the adhesive, as well as the adhesion line thickness, was found to be strongly related to bonding time.

Martins et al. [14] examined the bonding efficiency of Pinus pinaster glulam made of eight different resins (PRF, MUF, EPI, and 1C-PUR), focusing on different amounts of adhesive and pressures applied during bonding. The delamination (EN 14080 [15], Method A) and shear strength test demonstrated excellent performance in all PRF cases. Although MUF, EPI, and 1C-PUR resins presented satisfying bonding, the use of a hardener was necessary, especially for 1C-PUR. The shear strength was not significantly affected by the adhesive type, though differences were observed in wood failure rates between EPI and PUR. Although some research has been implemented so far, a gap has been detected in the existing literature towards the optimization approach of the MUF system in glulam and other wood-based products manufacturing.

The structural reliability of glulam in construction can be enhanced by optimizing the performance of MUF resins, which are extensively utilized in such engineered wood products [16]. Towards the attempts of optimization of MUF adhesive formulation and contribution to its current and real-world applications, the current work investigates the impact of varying chemical composition of MUF adhesive formulation on the bonding performance (in terms of delamination resistance and shear strength) of three-layer glulam samples that are manufactured using Norway spruce (Picea abies L.) wood pieces bonded with five different MUF resin formulations. Identifying the most effective MUF formulation for structural applications and improving MUF’s resilience can prolong the lifespan of glulam, minimize waste, and encourage environmentally friendly building practices. Thoroughly assessing and comparing the mechanical characteristics, durability, and bonding quality of spruce glulam beams constructed using five versions of commercial MUF resins would also provide manufacturers vital information to assist them choose the optimum adhesives for long-lasting, high-performance glulam and other engineered wood composites production.

2. Materials and Methods

The experiments were conducted both at the AUTH laboratory (School of Forestry and Natural Environment) and the glulam production company of Euroco S.A. (Thessaloniki, Northern Greece).

2.1. Preparation of Raw Materials and Glulam Production

Three (3)—layer glulam products were manufactured using defect-free sawn pieces of Norway spruce (Picea abies L.) timber originating from Sweden, at the dimensions of 6 × 12 × 100 cm (in thickness, width, and length, respectively). The specific species of spruce is included among those proposed by the standard as appropriate for testing of glulam (EN 14080). For the transformation of the material, from roundwood to sawn-wood pieces, a vertical band saw was used to avoid losing much wood material during the cutting process and then, after the conditioning of those (20 °C, 65% relative humidity) till constant weight, a planner was also used in order all the surfaces to be smooth enough to be glued and pressed.

In this work, 60 pieces of spruce wood were used, on which five different versions of the melamine–urea–formaldehyde (MUF) adhesive type were applied (

Table 1. Composition and characteristics of the five versions of MUF resin used in the experiments.

Property	Glue Version 1: MUF	Glue Version 2: MUF + % Glycerin	Glue Version 3: MUF	Glue Version 4: MUF	Glue Version 5: MUF
Viscosity at 25 °C (cps)	3461	2900	4740	9000	12,000
pH at 25 °C	9.6	9.65	9.64	9.7	9.6
Solid content (3 h, 120 °C) (%)	67.15	68.80	70	68	67.7
Water resistance at 25 °C	1:0.5	1:0.7	1:2.2	1:1.5	1:1

). More specifically, the five different versions of MUF adhesive had a ratio of 25% melamine and 75% urea-formaldehyde, with differences in the amounts of their components, as presented in Table 1. Two different hardener ratios and two different application quantities were applied in the current study, towards the optimization attempts of the MUF adhesive system.

A total of 20 glulam products (Figure 1) were manufactured, all consisting of three layers, and of dimensions 18 × 12 × 100 cm (in thickness, width, and length, respectively).

Specifically, in this work, 10 glulam products were manufactured applying the typical (correct) amount of glue and hardener mixture, as defined by ISO 12578 [17] and applying the whole procedure of glulam testing described in EN 14080. Specifically, 350 g of glue and hardener mixture was applied per m². From these 10 glulam products, five glulams were prepared (Experiment 1) applying a 4:1 glue–hardener ratio in the mixture, to 0.12 m² of the sawn timber surface. A total of 100 g of 4:1 glue–hardener mixture material was applied, which corresponded to 37.5 g of glue and 12.5 g of hardener, in each layer (adhesion line) of the glulam. In the other five glulam products (Experiment 2), a total of 100 g of a 5:1 ratio of glue–hardener mixture material was applied, which corresponded to 40 g of glue and 10 g of hardener, in each layer of the glulam (adhesion line).

As concerns the production of the rest 10 glulam products, they were prepared applying four times the amount of adhesive that is usually applied and specified by the standards, i.e., 400 g of the adhesive–hardener mixture were used on 0.12 m² of wood surface, in contrast to the previous 10 glulam products (as described above), where only 100 g of the adhesive–hardener mixture were used. Five glulam products were produced (Experiment 3), applying a 4:1 glue–hardener ratio material to 0.12 m² of the wood surface (totally 400 g of mixture material); one glulam product was produced for each adhesive system version, while in the remaining five glulam products (Experiment 4) a mixture of glue–hardener was applied in a ratio of 4:1, respectively. Specifically, for each version of the studied adhesive, a glue–hardener mixture of a 4:1 ratio was applied to 0.12 m² of wood surface (total 400 g of mixture material).

For the glulam products manufacturing, appropriate pieces of wood were selected, to be of low moisture content, which according to EN 336 [18], EN 338 [19] and EN14080 [15] should be 12% ± 2%. An electronic hygrometer (GANN, Germany) was used for the moisture content assessment (Figure 2A). Specifically, the mean value of moisture content of the boards ranged between 10.5 and 14.2%.

The sawn timber pieces were passed through the planer to ensure flat and smooth surfaces for the best application of the adhesive and suitable for bonding [20]. The adhesive (mixtures with hardener in the appropriate ratios) was spread evenly on both wooden surfaces, intended to be bonded, using a suitable comb, to achieve a uniform adhesive line thickness (Figure 2B), recording in parallel the exact amount of the adhesive mixture applied to each surface via weight measurements before and after the application.

The glued wooden elements were then placed, each one on the top of the other, for the glulam specimens assembly, which were placed in a hydraulic press for the application of pressure (0.4–1.2 MPa), for approximately 18 h (the norm requires at least 6 h). Totally, 20 three-layer glulam products were prepared, which were allowed to be conditioned under stable conditions (20 °C, 65% relative humidity) till constant weight, and their surfaces were planed again. In line with EN 14080 [15], the dimensions of the prepared glulam products were 12 cm in width and 1 m in length (parallel to grain) (Figure 3A). Of these 20 glulam products, a total of 231 glulam specimens of smaller dimensions were manufactured (Figure 1 and Figure 3B). Based on EN 14080 [15] (delamination test standard), all samples were sanded at their edges, using emery cloth (100 mm). The initial weight and dimensions of the samples were measured with a scale of high accuracy (3 decimals), as well as the dimensions of adhesion surface areas (Figure 3C).

Afterwards, the samples were placed in the apparatus of Delamination Test, which included two different elements, the tank where air pressure (Vacuum pump) was applied to the samples for 30 min (0 to 6 bar pressure) (Figure 4A,B), and then, a second machine in which a 2-h water pressure (Water pump) was applied (8.6 bar). Totally, the samples remained in this apparatus for 2.5 h. The tank was filled with water, and air was then decompressed inside the tank. Air pressure was applied using a vacuum pump for 30 min. After this duration, water supply was allowed in the tank, and a two-hour cycle of applying water pressure to the samples followed. Then, the water pressure was switched off, the tank was emptied of water, the pressure gauge dropped to zero, the tank lid was removed, all water was removed, and the wet samples were also removed from the tank and transferred to the drying chamber for the delamination test [15] (Figure 4C).

According to [15], three methods could be applied: method A (65 °C, 15% rh and air velocity 2–3 m/s), method B (70 °C, 9% rh and air velocity 2–3 m/s), and method C (30 °C, 30% rh and air velocity 2–3 m/s). In the current work, method B was selected to be applied. Specifically, after 13 h of the chamber’s operation, the samples were removed from the chamber and conditioned under stable conditions (20 °C, 65% rh) for approximately 7 days till constant weight, and remained there till the time of physical and mechanical properties investigation/specimens’ characterization. The weight and dimensions of each specimen were recorded again after the conditioning process to detect any changes, potential loss of extractives, or increase in mass.

2.2. Characterization of Glulam Specimens

According to the delamination test standard [15], the samples were exposed to cyclic water immersion and drying processes, and then, they were examined visually to determine the stability of the connections. The plastic ruler measurement was applied again to measure the total area of the glue line, taking into consideration both sides of the sample. A specific lamp was used to identify any detachment on the sample, which was marked with a pen when a separation or cracking was detected. Delamination length was also recorded. Finally, the total delamination length for each sample was calculated. According to EN 14080 [15], as well as [21,22,23], the allowed delamination percentage per sample is up to 40% (max), and the allowed mean delamination percentage is up to 8% (max). The Delamination Coefficient (A) was calculated based on [17,21,22,23], applying the formula for the Delamination Coefficient (1):

A = (L₂/L₁) ∗ 100

(1)

where A: Delamination Coefficient (%), L₁: Total delamination length before test (cm), L₂: Total delamination length of the two plates after the test.

A universal testing machine (UTM) was used to conduct block shear tests at a crosshead speed of 1.0 mm/min. Shear testing was used to assess the percentage of wood failure (concluding the adhesive failure). To identify notable variations (p < 0.05) among adhesive formulations, ANOVA (one-way) and Tukey’s HSD test were employed. The program of SPSS Version 28 (provided through AUTH University) was used in the results of the current work.

3. Results

The results from the first experiment (Experiment 1) are presented in Figure 5. It is observed that the samples individually and all of them fulfil the requirements of the respective standard of [15] (Figure 5). In the delamination test, in which method B was applied, Coefficient A (%) of the samples does not exceed 40%. However, the mean value of coefficient A of the samples was found to be 14.05%, which exceeds the permitted limit of 8%, as defined by the standard, and therefore, Glue 1 with a ratio of 1:4 (hardener–glue) and also having applied the correct amount of hardener–glue mixture, does not meet the requirements, therefore it should be rejected. Glues 1, 2, and 4 provided similar results of mean A% delamination coefficient values, while Glues 3 and 5 marked statistically significant differences from all the other glue categories of the current study.

The results of the first experiment show that the samples individually do not meet all the requirements of the norm [15]. Delamination test of glue lines applying method B, that is, per sample, the A% coefficient should not exceed 40%, as sample 4 has an A (%) coefficient of 44.44%. Therefore, the average value is a value that exceeds the acceptable limit of 8%, as defined by the standard, and is equal to 10.69%, so Glue 1, with a ratio of 1:4 and with the correct (typical) amount of hardener–glue mixture, does not meet the requirements and is also rejected.

In the first experiment carried out, the results of the samples, which were glued with Glue 3, showed that the samples individually meet all the requirements of the standard. According to the delamination test of Glue Lines (method B), Coefficient A (%) should not exceed 40%. In this study, the mean value was measured to be 5.42%; therefore, it does not exceed the permitted limit of 8%, as defined by the standard. Therefore, Glue 3 with a ratio of 1:4 and with the “correct”/typical amount of hardener–glue mixture meets the requirements and is considered “accepted”.

Additionally, in Experiment 1, the results of the samples, which were glued with Glue 4, showed that the samples individually meet all the requirements of the Glulam production norm. In the delamination test (method B), the Coefficient A (%) should not exceed 40%, as mentioned previously. However, the mean value was 12.49%, exceeding the permissible limit of 8%, as defined by the standard; therefore, Glue 4 with a ratio of 1:4 and with the “correct” amount of hardener–glue mixture does not meet the requirements and can be considered rejected.

In the first experiment carried out, the results of the samples, which were glued with Glue 5, showed that the samples individually do not meet all the requirements. In the delamination test (method B), coefficient A% should not exceed 40%, as samples 1 and 5 have a coefficient A (%) equal to 51.55% and 46.67%, respectively. Therefore, it follows that the mean value also exceeds the permitted limit of 8%, as defined by the standard, and is equal to 21.40%, so a Glue 5 ratio of 1:4, with the correct amount of hardener–glue mixture, does not meet the requirements and is considered rejected.

In the second experiment (Experiment 2), the results of the samples, which were glued with Glue 1, showed that the samples individually all meet the requirements (Figure 6). In the delamination test (method B), the coefficient A% should not exceed 40%. However, the mean value was found to be 10.71%, exceeding the permissible limit of 8%, as defined by the standard; therefore, Glue 1 with a ratio of 1:5 and with the correct amount of hardener–glue mixture does not meet the requirements and is considered rejected. According to statistical analysis results, the mean value of the A% delamination coefficient of Glue 1 differed statistically significantly from all the other glue categories of the current experiment. Statistically significant differences were also detected between Glue 3 and all glue categories, except for Glue 5.

Furthermore, in Experiment 2, the results of the samples, which were glued with Glue 2, demonstrated that the samples individually meet all the requirements. The mean value of the A (%) coefficient was measured to be 5.36%, which does not exceed the permissible limit of 8%, as defined by the standard. Therefore, Glue 2 with a ratio of 1:5 and with the “correct” amount of hardener–glue mixture meets the requirements and can be accepted. In the second experiment, the results of the samples glued with Glue 3 showed that all samples, both individually and collectively, met the requirements. In the delamination test (method B), the Coefficient A% should not exceed 40%, as mentioned above. Furthermore, the mean value was 1.46%, and it did not exceed the permissible limit of 8%, as defined by the standard; therefore, Glue 3 with a ratio of 1:5 and with the “correct” amount of hardener–glue mixture meets the requirements and is considered “accepted”.

In the second experiment, the samples glued with Glue 4 showed that they individually and collectively meet the requirements. In the delamination test (method B), the Coefficient A% should not exceed 40%. Furthermore, the mean value was 3.79%, which does not exceed the permissible limit of 8%, as defined by the standard. Therefore, Glue 4 with a ratio of 1:5 and with the “correct” amount of hardener–glue mixture meets the requirements and can be considered “accepted”. The results of the samples that were glued with Glue 5 showed that the samples individually and all meet all the requirements. In the delamination test (method B), the Coefficient A% should not exceed 40%, while the mean value was measured to be 2.42%, which does not exceed the permissible limit of 8%. Therefore, Glue 5 with a ratio of 1:5 and with the “correct” amount of hardener–glue mixture meets the requirements and can be considered “acceptable”.

In the third experiment (Experiment 3), the results of the samples, which were glued with Glue 1, revealed that the samples individually meet all the requirements of the standard (Figure 7). In the delamination test of Glue Lines (method B), the Coefficient A% should not exceed 40%, while the mean value was found to exceed the permissible limit of 8%, as defined by the standard, and is equal to 11.32%. Therefore, Glue 1 with a ratio of 1:4 and with the application of four times the amount of glue in the hardener–glue mixture does not meet the requirements and is rejected. The statistical analysis revealed that the mean value of the A% delamination coefficient of Glue 4 differed significantly from all the other glues’ respective values. Glues 1 and 5 marked similar levels of A% coefficient, while both of them differed statistically from the rest of the glue types (2, 3, and 4).

In the third experiment carried out, the results of the samples that were glued with Glue 2 showed that the samples individually meet all the requirements. In the delamination test, as mentioned previously (method B), the Coefficient A% should not exceed 40%. The mean value indeed did not exceed the permitted limit of 8%, as defined by the standard, and is equal to 5.19%. Therefore, Glue 2, with a ratio of 1:4 and with the application of four times the typical (correct) amount of glue in the hardener–glue mixture, meets the requirements and is considered acceptable. All the above results and conclusions are also depicted in the illustration of Figure 7.

In the third experiment carried out, the results of the samples, which were glued with Glue 3, showed that the samples individually meet all the requirements. Delamination Test of Glue Lines (method B) refers to the fact that the Coefficient A% should not exceed 40%. In the current case, the mean value did not exceed the permitted limit of 8%, as defined by the standard, and is equal to 3.58%. Therefore, Glue 3 with a ratio of 1:4 and with the application of four times the amount of glue in the hardener–glue mixture, meets the requirements and can be considered as acceptable. The samples, which were glued with Glue 4, demonstrated that they individually meet all the requirements of the respective standard. The mean value here did not exceed the permitted limit of 8%, as defined by the standard, and is equal to 0.48%. Therefore, Glue 4, with a ratio of 1:4 and with the application of four times the amount of glue in the hardener–glue mixture, meets the requirements and is considered accepted.

In the third experiment carried out, the results of the samples, which were glued with Glue 5, showed that the samples individually meet all the requirements. Delamination Test of Glue Lines (method B) requires the Coefficient A% not to exceed 40%. However, the mean value here exceeded the permissible limit of 8%, as defined by the standard, and is equal to 11.97%. Therefore, Glue 5, with a ratio of 1:4 and with the application of four times the amount of glue in the hardener–glue mixture, did not meet the requirements and is rejected.

In the fourth experiment carried out (Experiment 4), the results of the samples, which were glued with Glue 1, showed that the samples individually meet all the requirements(Figure 8). Delamination Test of Glue Lines (method B) indicates that the Coefficient A% should not exceed 40%. However, the mean value exceeds the permissible limit of 8%, as defined by the standard, and is equal to 17.42%. Therefore, Glue 1, with a ratio of 1:1 and with the application of four times the amount of glue in the hardener–glue mixture, does not meet the requirements and is rejected. According to the statistical analysis of the results, the A% delamination coefficient values of Glues 3 and 4 were found to be similar, while both of them differed significantly (from a statistical point of view) from the rest of the glue types (1, 2, and 5). Furthermore, Glue 2 marked a significantly lower (the lowest) value of delamination level than all the other glue types of the specific experiment.

In the fourth experiment carried out, the samples that were glued with Glue 2 showed that they individually meet all the requirements. The mean value exceeds the permissible limit of 8%, as defined by the standard, and is equal to 8.43%. Therefore, Glue 2, with a ratio of 1:1 and with the application of four times the amount of glue in the hardener–glue mixture, does not meet the requirements and is rejected. The results of the samples, which were glued with Glue 3, revealed that the samples individually did not meet all the requirements of the standard. Samples 1, 2, 3, 5, and 6 recorded a delamination coefficient of 69.13%, 46.19%, 46.41%, 66.74% and 53.83%, respectively. The mean value exceeds the permissible limit of 8%, as defined by the standard, and is equal to 40.37%. Therefore, Glue 3 with a 1:1 ratio and with the application of a fourfold amount of glue in the hardener–glue mixture does not meet the requirements and is rejected.

Furthermore, in the fourth experiment carried out, the results of the samples, which were glued with Glue 4, showed that the samples individually do not meet all the requirements of the norm. Delamination test of glue lines (method B) refers to the fact that the Coefficient A% should not exceed 40%. Samples of 1, 4, 5, 6, 8, 9, and 11 presented a delamination coefficient of 74.27%, 57.81%, 45.86%, 45.54%, 44.84%, 43.82% and 51.12% respectively. The mean value exceeded the permissible limit of 8%, as defined by the standard, and is equal to 43.28%. Therefore, Glue 4, with a ratio of 1 to 1 and with the application of four times the amount of glue in the hardener–glue mixture, does not meet the requirements and is rejected. The results of the samples, which were glued with Glue 5, showed that the samples individually did not meet all the requirements. The Coefficient A% should not exceed 40%, though sample 9 presented a delamination factor of 64.06%. The mean value exceeded the permissible limit of 8%, as defined by the standard, and was found to be equal to 20.16%. Therefore, Glue 5, with a ratio of 1:1 and with the application of four times the amount of glue in the hardener–glue mixture, did not meet the requirements and can be considered as rejected.

4. Discussion

The experimental findings indicate that, when put through delamination testing in accordance with EN 14080 [15] (Method B), the five adhesive formulations’ performances varied significantly. The hardener-to-glue ratio and the quantity of adhesive used were crucial factors in determining compliance with the standard, highlighting the need for accurate formulation and application in structural glulam bonding.

Except for one experimental scenario (Experiment 4, 1:1 ratio), Glue 3 demonstrated the most consistent compliance among the tested adhesives, fulfilling the standards. In Experiments 1–3, the mean delamination coefficient A (%) remained considerably below the 8% limit, indicating dependable bonding performance in normal application circumstances.

Additionally, Glue 2 performed satisfactorily, especially at a hardener-to-glue ratio of 1:5, where its mean A% (5.36%) showed a high level of resistance to delamination. These results are consistent with earlier research highlighting how well-formulated hardeners can improve moisture resistance and decrease adhesive brittleness [21,22,23].

However, with mean A (%) values ranging from 10.69% to 17.42%, Glue 1 failed to meet the criterion in every setup that was evaluated (Figure 9). This persistent underperformance indicates potential chemical formulation shortcomings such as inadequate cross-linking or inadequate moisture resistance. In similar tendencies, Glues 4 and 5 showed inconsistent performance; although they functioned well at a ratio of 1:5, their A (%) values were higher than allowed limits under excessive adhesive loading (4× application) or higher hardener concentrations (1:4). The necessity of adhesive-specific optimization in industrial settings is highlighted by this nonlinear behavior.

In comparison to the 1:4 ratio (Experiment 1), the 1:5 hardener-to-glue ratio (Experiment 2) generally increased delamination resistance. Glue 5, for example, worked satisfactorily at 1:5 (A% = 2.42%) after failing at 1:4 (A% = 21.40%), indicating that a lower hardener concentration would improve flexibility and bond durability. This conclusion is consistent with research conducted by Boursier et al. [24], which found that using too much hardener can result in adhesive layers that are brittle and prone to breaking under pressure.

It is interesting to note that performance was not always improved by applying four times as much glue as was typically used (Experiments 3–4). When the same excess glue was applied at a 1:1 ratio, Glue 3 displayed severe delamination (A% = 40.37%); however, Glue 4 demonstrated remarkable results at this level (A% = 0.48% at a 1:4 ratio). This implies that excessive use could interfere with the kinetics of curing, especially in adhesives that are susceptible to stoichiometric imbalances. Interestingly, despite generally acceptable mean values in the current experimental work findings, certain individual samples (Glue 5, Sample 9 in Experiment 4: A% = 64.06%) showed significant delamination. Localized adhesive flaws, uneven spread, or substrate irregularities could all result in these outliers, which call for more stringent manufacturing quality control. With careful calibration of hardener ratios, Glues 2 and 3 are the most practical options for industrial applications.

The short-term delamination resistance under controlled conditions was the main emphasis of this study; long-term durability tests that include mechanical fatigue, humidity exposure, and thermal cycling would further evaluate adhesive performance, while the mechanisms underlying adhesive failure would also be more clearly evidenced by microscopic examinations of failed bond lines, which could direct formulation advancements, even though it was not possible to be provided in the current work. Future research should investigate the chemical alterations of poor-performing adhesives and assess how they respond to environmental stressors in practice.

5. Conclusions

The findings highlighted how important the adhesive composition, hardener ratio, and application amount are in determining glulam products’ resistance to delamination. Although Glues 2 and 3 showed strong adherence to the norm of EN 14080, other formulations’ failures underscore the necessity of adhesive-specific modification. The study revealed that Glue 1 was repeatedly rejected because its delamination coefficient A (%) exceeded the 8% limit in all evaluated mixing ratios and glue volumes. On the other hand, under certain conditions, Glues 2 and 4 only achieved the allowed delamination limits, suggesting only partial effectiveness. These glues worked well at a 50% success rate, especially when using a 1:5 hardener-to-glue ratio with the right/typical amount of glue or a 1:4 ratio with four times the usual glue amount. Significantly, Glue 2, a blend of glycerin, urea–formaldehyde, and melamine (MUF), showed remarkable promise, suggesting that further research could make it more feasible.

With a low success rate of 25%, Glue 5 was unreliable and only achieved compliance in one scenario: a 1:5 hardener-to-glue ratio with the appropriate glue quantity. As a result, it was considered inappropriate for use in industrial settings. With a 75% success rate, Glue 3 was the best performer; it only failed in the 1:1 hardener-to-glue ratio when the amount of glue was quadrupled. Given its impressive performance, further study is advised to improve its formulation and guarantee consistent success under all tested circumstances.

In summary, even though Glue 3 is now the best-performing adhesive, Glue 2 may still be a good substitute if more study and development are conducted. According to the results, adhesive reliability for industrial applications might be greatly increased with only minimal adjustments, highlighting the need for exact formulation control. More research should be focused on improving adhesive formulation in order to eliminate the single failure scenario, either by changing the curing conditions or the hardener ratio. These findings have a practical application in that they provide glulam manufacturers with a clear, performance-based selection criterion that allows them to select an MUF resin that guarantees product reliability, satisfies structural requirements, and prolongs the service life of buildings in demanding environments.