Melamine–Glyoxal–Glutaraldehyde Wood Panel Adhesives without Formaldehyde

(MGG’) resin adhesives for bonding wood panels were prepared by a single step procedure, namely reacting melamine with glyoxal and simultaneously with a much smaller proportion of glutaraldehyde. No formaldehyde was used. The inherent slow hardening of this resin was overcome by the addition of N-methyl-2-pyrrolidone hydrogen sulphate ionic liquid as the adhesive hardener in the glue mix. The plywood strength results obtained were comparable with those obtained with melamine–formaldehyde resins pressed under the same conditions. Matrix assisted laser desorption ionisation time of flight (MALDI ToF) and Fourier transform Infrared (FTIR) analysis allowed the identification of the main oligomer species obtained and of the different types of linkages formed, as well as to indicate the multifaceted role of the ionic liquid. These resins are proposed as a suitable substitute for equivalent formaldehyde-based resins.


Introduction
Melamine-formaldehyde and melamine-urea-formaldehyde resins and adhesives are extensively used in particular for impregnated paper surface overlays and for plywood and particleboard panel binders [1]. The problem with these resins is now the presence of formaldehyde and its emission, as this chemical has been reclassified to carcinogenic category 1B and mutagen category 2 according to the "Classification, Labelling and Packaging of substances and mixtures" (CLP) of the EU Regulations.
Glyoxal has been chosen to substitute formaldehyde in some of these resins due to its capacity for giving clear adhesive resins and thus presenting the same appearance as UF-, MF-and MUF-bonded wood panels.
Glyoxal is nontoxic (LD50 rat ≥ 2960 mg/kg; LD50 mouse ≥ 1280 mg/kg when compared to formaldehyde's LD50 of 75 mg/kg) and non-volatile in the conditions in which formaldehyde is volatile when used in panels bonded with formaldehyde-based adhesives [2,3]. Because of advantages such as its mature production technology, low cost and easy biodegradation, glyoxal (G) has been widely used as a green environmental additive in the papermaking and textile industries [4]. In wood adhesive, it is mainly used to substitute formaldehyde (F) partially or totally, in applications as a crosslinking agent or curing agent in natural wood adhesives such as tannin-based adhesives [5,6], lignin-based adhesives [7] and protein-based adhesives [8]. While glyoxal is nontoxic and non-volatile it is also much less reactive than formaldehyde and the presence of two vicinal aldehyde groups partially, but only partially, offsets this drawback. Moreover, the literature about glyoxal-based resins application for wood adhesives and resins is rather scarce.
Recently, much work has been conducted to prepare and study the reaction mechanism and structure of environmentally friendly urea-based aminoresins by choosing glyoxal (G) to substitute

Fourier Transform Infrared Spectrometry (FTIR)
To confirm the presence of relevant structures, a Fourier Transform Infra Red (FTIR) analysis was carried out using a Shimadzu IRAffinity-1 spectrophotometer (Kyoto, Japan). A blank sample tablet of potassium bromide, ACS reagent from ACROS Organics (Acros Otganics, Geel, Belgium), was prepared for the reference spectrum. A similar tablet was prepared by mixing potassium bromide with 5% w/w of the sample powders to be analyzed. The spectrum was obtained in transmission measurement by combining 32 scans with a resolution of 2.0 (Perkin-Elmer, Villebon, France).

Plywood Preparation and Testing
The performance of the MGG' resins was tested by preparing laboratory plywood panels and evaluating their shear strength dry, after 24 h cold water soaking, and after 2 h in boiling water, tested wet. Triplicate three-ply laboratory plywood panels of 450 mm × 300 mm × 5 mm were prepared for each MGG' adhesive resin and 2 mm poplar (Populus tremuloides) veneers. To the glue mixes was added 10% by weight [HNMP] [HSO4 − ] ionic liquid as hardener on total MGG' resin solids. The glue spread used was of 260 g/m 2 double glue line, and hot pressing time was of 6 min at 150 °C and 1.5 MPa pressure. After hot pressing, the plywood was stored under ambient conditions (20 °C and 12% moisture content) for 48 h before testing according to China National Standard GB/T 14074 (2006) [19] and China National Standard GB/T17657 (1999) [20] which require a minimum average shear strength of 0.7 MPa on five specimens tested, and European Norm EN 636:2012 (2012) [21].

Results and Discussion
The same principle of using an ionic liquid as a hardener that has been used for urea-glyoxal resins [12,17] was applied to melamine-glyoxal (MG) and melamine-glyoxal-glutaraldehyde (MGG') resins for which, N-methyl-2-pyrrolidone hydrogen sulphate ([HNMP] [HSO4 − ]) was used as hardener. As this material is relatively expensive it was prepared in the laboratory according to the method of Wang et al. [15] and used with a minimum of purification [12,17]  The MG resin evolved into the MGG' resins given the better results that were obtained with such a combination. The double reaction, in two successive steps of melamine with glyoxal and glutaraldehyde has led to clear resin with no formaldehyde, each presenting distinct characteristics. These are shown in Table 1 which also reports the strength results of laboratory plywood panels prepared by using the different melamine-glyoxal-glutaraldehyde resins to which had been added 10% of [HNMP] HSO4 − ionic liquid. These satisfy the relevant requirements of international standards for both dry strength and strength after 24 h cold water soaking. In the two best cases they also satisfy the requirements for exterior grade bonds as their strength is still acceptable after the boiling water test.

Adhesives Bonding Performance
The first characteristic that can be noticed from Table 2 is that the proportion of glutaraldehyde increases the viscosity, and more interestingly the dry and wet tensile strengths increase up to a proportion of 20% molar of glutaraldehyde. A higher proportion nonetheless causes the viscosity to increase to such an extent that the resin solidifies in the reactor. Equally remarkable is the finding ( Table 2) that the tensile strength increases after 24 h cold water soak. The effect is very marked and becomes more evident as the proportion of glutaraldehyde increases. At first it was thought that the water repellent -(-CH2-)3-chain of glutaraldehyde caused the effect, but while this contributes to maintaining the dry strength once in water it does not justify the marked increase in strength noticed. It does however, contribute also to the resistance of the bond to boiling water as the bond performance improves as the relative proportion of glutaraldehyde increases.
Thermomechanical analysis experiments has shown that the hardening of the resin occurs in two phases (indicated with two arrows in Figure 1) a second increase in modulus occurring as the temperature, hence the curing time increases. This appeared to indicate that while the two aldehydes have been added simultaneously in the reactor, one is less reactive (glutaraldehyde) and had both The MG resin evolved into the MGG' resins given the better results that were obtained with such a combination. The double reaction, in two successive steps of melamine with glyoxal and glutaraldehyde has led to clear resin with no formaldehyde, each presenting distinct characteristics. These are shown in Table 1 which also reports the strength results of laboratory plywood panels prepared by using the different melamine-glyoxal-glutaraldehyde resins to which had been added 10% of [HNMP] [HSO 4 − ] ionic liquid. These satisfy the relevant requirements of international standards for both dry strength and strength after 24 h cold water soaking. In the two best cases they also satisfy the requirements for exterior grade bonds as their strength is still acceptable after the boiling water test.

Adhesives Bonding Performance
The first characteristic that can be noticed from Table 2 is that the proportion of glutaraldehyde increases the viscosity, and more interestingly the dry and wet tensile strengths increase up to a proportion of 20% molar of glutaraldehyde. A higher proportion nonetheless causes the viscosity to increase to such an extent that the resin solidifies in the reactor. Equally remarkable is the finding ( Table 2) that the tensile strength increases after 24 h cold water soak. The effect is very marked and becomes more evident as the proportion of glutaraldehyde increases. At first it was thought that the water repellent -(-CH 2 -) 3 -chain of glutaraldehyde caused the effect, but while this contributes to maintaining the dry strength once in water it does not justify the marked increase in strength noticed. It does however, contribute also to the resistance of the bond to boiling water as the bond performance improves as the relative proportion of glutaraldehyde increases.
Thermomechanical analysis experiments has shown that the hardening of the resin occurs in two phases (indicated with two arrows in Figure 1) a second increase in modulus occurring as the temperature, hence the curing time increases. This appeared to indicate that while the two aldehydes have been added simultaneously in the reactor, one is less reactive (glutaraldehyde) and had both reacted with the melamine in the reactor, one of them was contributing to cross-linking earlier than the other aldehyde. Considering the difference in reactivity of the two aldehydes when the MGG' resins was prepared, this should indicate that a MG resin is formed on to which some glutaraldehyde is also eventually linked. Cross-linking then, will depend first from the more numerous, still reactive hydroxyethyl groups formed by the first addition of glyoxal onto melamine to form bridges. Second from the less reactive hydroxypentyl groups yielded by the grafting reaction of glutaraldehyde onto the MG resin. This would explain the two phases of hardening of the MGG' resin seen in Figure 1. To show that this was the case, matrix assisted laser desorption ionisation time of fight (MALDI-ToF) mass spectrometry analysis of different resins and intermediates was carried out. This presented several points of interest: (i) first of all the composition and oligomers distribution in the MG and in particular the MGG' resin; (ii) second, and of equal interest, was to find out what was the interaction of the ionic liquid with both the two aldehydes, with melamine, with the MGG' resin so as to finally deduce its mechanism of action.
Polymers 2018, 10, 22 5 of 19 reacted with the melamine in the reactor, one of them was contributing to cross-linking earlier than the other aldehyde. Considering the difference in reactivity of the two aldehydes when the MGG' resins was prepared, this should indicate that a MG resin is formed on to which some glutaraldehyde is also eventually linked. Cross-linking then, will depend first from the more numerous, still reactive hydroxyethyl groups formed by the first addition of glyoxal onto melamine to form bridges. Second from the less reactive hydroxypentyl groups yielded by the grafting reaction of glutaraldehyde onto the MG resin. This would explain the two phases of hardening of the MGG' resin seen in Figure 1. To show that this was the case, matrix assisted laser desorption ionisation time of fight (MALDI-ToF) mass spectrometry analysis of different resins and intermediates was carried out. This presented several points of interest: (i) first of all the composition and oligomers distribution in the MG and in particular the MGG' resin; (ii) second, and of equal interest, was to find out what was the interaction of the ionic liquid with both the two aldehydes, with melamine, with the MGG' resin so as to finally deduce its mechanism of action.  hydroxyethyl groups formed by the first addition of glyoxal onto melamine to form bridges. Second from the less reactive hydroxypentyl groups yielded by the grafting reaction of glutaraldehyde onto the MG resin. This would explain the two phases of hardening of the MGG' resin seen in Figure 1. To show that this was the case, matrix assisted laser desorption ionisation time of fight (MALDI-ToF) mass spectrometry analysis of different resins and intermediates was carried out. This presented several points of interest: (i) first of all the composition and oligomers distribution in the MG and in particular the MGG' resin; (ii) second, and of equal interest, was to find out what was the interaction of the ionic liquid with both the two aldehydes, with melamine, with the MGG' resin so as to finally deduce its mechanism of action.

Analysis of Chemical Species Formed in the Reactions Analysed by MALDI-ToF Spectrometry
The MALDI-ToF of the two aldehydes first shown is for just the MALDI spectra of glyoxal and glutaraldehyde after reaction with the ionic liquid (Figures 3a,b and 4a-c). The species identified are shown in Tables 3 and 4. Two main trends are noticeable:

Analysis of Chemical Species Formed in the Reactions Analysed by MALDI-ToF Spectrometry
The MALDI-ToF of the two aldehydes first shown is for just the MALDI spectra of glyoxal and glutaraldehyde after reaction with the ionic liquid (Figure 3a,b and 4a-c). The species identified are shown in Tables 3 and 4. Two main trends are noticeable: Polymers 2018, 10, 22 6 of 19

Analysis of Chemical Species Formed in the Reactions Analysed by MALDI-ToF Spectrometry
The MALDI-ToF of the two aldehydes first shown is for just the MALDI spectra of glyoxal and glutaraldehyde after reaction with the ionic liquid (Figures 3a,b and 4a-c). The species identified are shown in Tables 3 and 4. Two main trends are noticeable:

Analysis of Chemical Species Formed in the Reactions Analysed by MALDI-ToF Spectrometry
The MALDI-ToF of the two aldehydes first shown is for just the MALDI spectra of glyoxal and glutaraldehyde after reaction with the ionic liquid (Figures 3a,b and 4a-c). The species identified are shown in Tables 3 and 4. Two main trends are noticeable:                       Two main trends are noticeable, as shown below.

1.
Linear oligomerization of the aldehyde by aldol condensation, which indicates that IL has some catalytic effect on the autocondensation of the aldehyde (Scheme 2).
peak 724 Da = glutaraldehyde heptamer by aldolcondensa tion, with Na + but also the alternatives as for the 424 Da peak ---Two main trends are noticeable, as shown below.
1. Linear oligomerization of the aldehyde by aldol condensation, which indicates that IL has some catalytic effect on the autocondensation of the aldehyde (Scheme 2). Scheme 3. Examples of complexes between aldol condensates and ionic liquid Figure 5 and Table 5 show the MALDI-ToF of the interaction between melamine and the ionic liquid. Only three peaks could be interpreted, namely the melamine peak, the ionic liquid peak and that of a coordination compound between two ionic liquid molecules and one single melamine at 536-539 Da ( Table 4). The rest are fragments either coming from impurities or fragments generated in the MALDI. It is clear that the melamine itself is not affected by the IL, although species like that of 536-539 Da could facilitate reaction with the aldehydes.  Table 5. Species formed by reaction of IL with melamine alone. Only two peaks could be interpreted. The rest are fragments either coming from impurities or fragments generated in the MALDI. It is clear that the melamine itself is not affected by the IL, although species like the 538 Da could facilitate reaction with the aldehydes.

Peak Chemical species
Scheme 3. Examples of complexes between aldol condensates and ionic liquid. Figure 5 and Table 5 show the MALDI-ToF of the interaction between melamine and the ionic liquid. Only three peaks could be interpreted, namely the melamine peak, the ionic liquid peak and that of a coordination compound between two ionic liquid molecules and one single melamine at 536-539 Da ( Table 4). The rest are fragments either coming from impurities or fragments generated in the MALDI. It is clear that the melamine itself is not affected by the IL, although species like that of 536-539 Da could facilitate reaction with the aldehydes. Scheme 3. Examples of complexes between aldol condensates and ionic liquid Figure 5 and Table 5 show the MALDI-ToF of the interaction between melamine and the ionic liquid. Only three peaks could be interpreted, namely the melamine peak, the ionic liquid peak and that of a coordination compound between two ionic liquid molecules and one single melamine at 536-539 Da ( Table 4). The rest are fragments either coming from impurities or fragments generated in the MALDI. It is clear that the melamine itself is not affected by the IL, although species like that of 536-539 Da could facilitate reaction with the aldehydes.  Table 5. Species formed by reaction of IL with melamine alone. Only two peaks could be interpreted. The rest are fragments either coming from impurities or fragments generated in the MALDI. It is clear that the melamine itself is not affected by the IL, although species like the 538 Da could facilitate reaction with the aldehydes.

Peak
Chemical species    In the case of MG resins several types of glyoxal bridges have been shown to form and link melamine molecules [13], such as the following (Scheme 4).  In the case of MG resins several types of glyoxal bridges have been shown to form and link melamine molecules [13], such as the following (Scheme 4).      In the case of the IL being present aldol condensation of the aldehyde occurs and species in which melamine is linked to aldol condensed aldehydes occur. For both the resin MGG' with and without IL condensation oligomers of melamine-glyoxal and of melamine-glyoxal-glutaraldehyde are present. Thus species such as the following are the result (Scheme 6).  In the case of the IL being present aldol condensation of the aldehyde occurs and species in which melamine is linked to aldol condensed aldehydes occur. For both the resin MGG' with and without IL condensation oligomers of melamine-glyoxal and of melamine-glyoxal-glutaraldehyde are present. Thus species such as the following are the result (Scheme 6).  In the case of the IL being present aldol condensation of the aldehyde occurs and species in which melamine is linked to aldol condensed aldehydes occur. For both the resin MGG' with and without IL condensation oligomers of melamine-glyoxal and of melamine-glyoxal-glutaraldehyde are present. Thus species such as the following are the result (Scheme 6).  In the case of the IL being present aldol condensation of the aldehyde occurs and species in which melamine is linked to aldol condensed aldehydes occur. For both the resin MGG' with and without IL condensation oligomers of melamine-glyoxal and of melamine-glyoxal-glutaraldehyde are present. Thus species such as the following are the result (Scheme 6).  In the case of the IL being present aldol condensation of the aldehyde occurs and species in which melamine is linked to aldol condensed aldehydes occur. For both the resin MGG' with and without IL condensation oligomers of melamine-glyoxal and of melamine-glyoxal-glutaraldehyde are present. Thus species such as the following are the result (Scheme 6). In the case of MG resins several types of glyoxal bridges have been shown to form and link melamine molecules [13], such as the following (Scheme 4).   In the case of the IL being present aldol condensation of the aldehyde occurs and species in which melamine is linked to aldol condensed aldehydes occur. For both the resin MGG' with and without IL condensation oligomers of melamine-glyoxal and of melamine-glyoxal-glutaraldehyde are present. Thus species such as the following are the result (Scheme 6). Where a glutaraldehyde is linked to a melamine of a MG resin skeleton. Also species in which an aldehyde that has undergone aldol condensation has reacted and linked with the melamine are present in the MGG'+IL resins. For example the peak at 356-357 Da (333 + 23 Da of Na + ) in Figure 6 represent just one of these species issued by the aldol condensation of three glyoxals linked to a melamine molecule. Aldehydes react readily with melamine, but when a dialdehyde reacts after one of the two aldehyde groups has reacted and before the second one reacts aldol condensation can occur involving this second, still free aldehyde group. Thus either (i) the glyoxal units have condensed by aldol condensation for subsequently the residual aldehyde group to react with melamine; or (ii) one glyoxal molecule has reacted with melamine and the other glyoxals have then reacted on the residual aldehyde function by aldol condensation (Scheme 8). Where a glutaraldehyde is linked to a melamine of a MG resin skeleton. Also species in which an aldehyde that has undergone aldol condensation has reacted and linked with the melamine are present in the MGG' + IL resins. For example the peak at 356-357 Da (333 + 23 Da of Na + ) in Figure 6 represent just one of these species issued by the aldol condensation of three glyoxals linked to a melamine molecule. Aldehydes react readily with melamine, but when a dialdehyde reacts after one of the two aldehyde groups has reacted and before the second one reacts aldol condensation can occur involving this second, still free aldehyde group. Thus either (i) the glyoxal units have condensed by aldol condensation for subsequently the residual aldehyde group to react with melamine; or (ii) one glyoxal molecule has reacted with melamine and the other glyoxals have then reacted on the residual aldehyde function by aldol condensation (Scheme 8). This is not the only case occurring in the MGG' + IL resin case, as for example the 452-453 Da peak in Figure 6, this being a species formed by aldol condensation of three glutaraldehydes linked to a melamine (Scheme 9). This is not the only case occurring in the MGG' + IL resin case, as for example the 452-453 Da peak in Figure 6, this being a species formed by aldol condensation of three glutaraldehydes linked to a melamine (Scheme 9). Scheme 8 . Reaction of a glyoxal aldol condensate on melamine. This is not the only case occurring in the MGG' + IL resin case, as for example the 452-453 Da peak in Figure 6, this being a species formed by aldol condensation of three glutaraldehydes linked to a melamine (Scheme 9

Analysis of Reactions by Fourier Transform Infrared (FTIR) Spectrometry
FTIR analysis of the hardened MG and MGG' resins ( Figure 7 and Table 7) shows all the main groups of the melamine and reacted glyoxal. These are listed in Table 6. The more interesting feature is the existence of the peak at 1234 cm −1 of C-O-C stretching. Its presence indicates that substituted methylene ether-like bridges (-CHR-O-CHR-) are formed in the case of MG resins just as they form in melamine-formaldehyde resins (MF, -CH2-O-CH2-). These substituted methylene ether-like bridges (-CHR-O-CHR-) clearly rearrange to substituted ethylene bridges (-CHR-CHR-) by the elimination of water as already shown in a previous study [13].

Analysis of Reactions by Fourier Transform Infrared (FTIR) Spectrometry
FTIR analysis of the hardened MG and MGG' resins ( Figure 7 and Table 7) shows all the main groups of the melamine and reacted glyoxal. These are listed in Table 6. The more interesting feature is the existence of the peak at 1234 cm −1 of C-O-C stretching. Its presence indicates that substituted methylene ether-like bridges (-CHR-O-CHR-) are formed in the case of MG resins just as they form in melamine-formaldehyde resins (MF, -CH2-O-CH2-). These substituted methylene ether-like bridges (-CHR-O-CHR-) clearly rearrange to substituted ethylene bridges (-CHR-CHR-) by the elimination of water as already shown in a previous study [13]. In Figure 8 there is a comparison of the FTIR spectra of IL, MG+IL and MGG'+IL. Other than small peaks corresponding to the addition of the small amounts of IL the two resin spectra show the same peaks as shown in Figure 7. The probable presence of ethers in the structure is supported also by the existence of MALDI-ToF peaks such as those at 395 Da (371 + 23 Da Na + ) and at 355 Da (without Na + ) and 379 Da (with Na + ) in Figure 6 which correspond to the two structures below (Scheme 10).  In Figure 8 there is a comparison of the FTIR spectra of IL, MG + IL and MGG'+IL. Other than small peaks corresponding to the addition of the small amounts of IL the two resin spectra show the same peaks as shown in Figure 7. The probable presence of ethers in the structure is supported also by the existence of MALDI-ToF peaks such as those at 395 Da (371 + 23 Da Na + ) and at 355 Da (without Na + ) and 379 Da (with Na + ) in Figure 6 which correspond to the two structures below (Scheme 10). In Figure 8 there is a comparison of the FTIR spectra of IL, MG+IL and MGG'+IL. Other than small peaks corresponding to the addition of the small amounts of IL the two resin spectra show the same peaks as shown in Figure 7. The probable presence of ethers in the structure is supported also by the existence of MALDI-ToF peaks such as those at 395 Da (371 + 23 Da Na + ) and at 355 Da (without Na + ) and 379 Da (with Na + ) in Figure 6 which correspond to the two structures below (Scheme 10).

Summary of Effects
Having identified some of the more relevant of the oligomers that were found and shown that melamine and glyoxal do react and form resins which also cross-link it is of interest to summarize what is then the role of ionic liquids.
1. IL appears to catalyse the reaction of aldehydes, and aldehydes pre-reacted with urea or melamine, to yield aldol condensation. It allows aldol condensation even of aldehydes that have been pre-reacted with melamine or even with wood lignin. An example (Scheme 11):  In Figure 8 there is a comparison of the FTIR spectra of IL, MG+IL and MGG'+IL. Other than small peaks corresponding to the addition of the small amounts of IL the two resin spectra show the same peaks as shown in Figure 7. The probable presence of ethers in the structure is supported also by the existence of MALDI-ToF peaks such as those at 395 Da (371 + 23 Da Na + ) and at 355 Da (without Na + ) and 379 Da (with Na + ) in Figure 6 which correspond to the two structures below (Scheme 10).

Summary of Effects
Having identified some of the more relevant of the oligomers that were found and shown that melamine and glyoxal do react and form resins which also cross-link it is of interest to summarize what is then the role of ionic liquids.
1. IL appears to catalyse the reaction of aldehydes, and aldehydes pre-reacted with urea or melamine, to yield aldol condensation. It allows aldol condensation even of aldehydes that have been pre-reacted with melamine or even with wood lignin. An example (Scheme 11): Scheme 10. Glyoxal ether bridges formed by water elimination between two melamine molecules.

Summary of Effects
Having identified some of the more relevant of the oligomers that were found and shown that melamine and glyoxal do react and form resins which also cross-link it is of interest to summarize what is then the role of ionic liquids. 1.
IL appears to catalyse the reaction of aldehydes, and aldehydes pre-reacted with urea or melamine, to yield aldol condensation. It allows aldol condensation even of aldehydes that have been pre-reacted with melamine or even with wood lignin. An example (Scheme 11): Or even with wood lignin [22,23] ( Scheme 12). This latter opens up the possibility of reaction with the lignin in the wood substrate, and thus of some covalent bonding between an aldehyde based adhesive and the substrate. While to advance such a hypothesis is possibly premature, it might also contribute to explain the good bonds obtained with aldehyde adhesives catalysed by ionic liquids.
2. IL catalyses the hardening of melamine-aldehyde resins to decrease hardening temperature, energy of activation of the hardening/condensation reaction and thus equally to improve their performance at equal temperature. 3. Moreover, IL affects the wood substrate by demethylation of lignin [22][23][24] rendering the substrate even more receptive to any type of adhesion.
However, it cannot be excluded that also other effects are induced by the presence of ILs.

Conclusions
Melamine-glyoxal-glutaraldehyde (MGG') resins which use no formaldehyde have been shown to be capable of bonding interior and exterior grade plywood under industrially significant press conditions when an ionic liquid is used as the hardener. Small proportions of glutaraldehyde improved the water resistance of the panel bonds obtained. These resins showed hardening in two steps due to the different reactivity of the two aldehydes used. The role of the ionic liquid was fundamental in the performance obtained, its role being not just limited to the function of hardener but also: (i) to decrease markedly the energy of activation of hardening of the MGG' resins; (ii) to favour some limited aldol condensation of the aldehydes in the hardening of the MGG' resins, even on aldehyde sites pre-reacted with melamine; (iii) to cause demethylation of lignin and thus rendering the wood substrate more prone to adhesion [23,24]; and (iv) to favour possibly the reaction of aldehyde groups in the resin with the lignin of the substrate.