Melamine–Glyoxal–Glutaraldehyde Wood Panel Adhesives Without Formaldehyde

polymers

Article Melamine–Glyoxal–Glutaraldehyde Wood Panel Adhesives without Formaldehyde

Xuedong Xi ID , Antonio Pizzi * ID and Siham Amirou

LERMAB, University of Lorraine, 27 rue Philippe Seguin, 88000 Epinal, France; [email protected] (X.X.); [email protected] (S.A.) * Correspondence: [email protected]; Tel.: +33-6-2312-6940

Received: 10 November 2017; Accepted: 21 December 2017; Published: 24 December 2017

Abstract: (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.

Keywords: ionic liquids; melamine resins; wood adhesives; plywood; ionic liquids role; MALDI-ToF; FTIR

1. 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

Polymers 2018, 10, 22; doi:10.3390/polym10010022 www.mdpi.com/journal/polymers Polymers 2018, 10, 22 2 of 18 formaldehyde (F) partially or totally so as to eliminate or markedly decrease formaldehyde (F) emission [9–12]. Urea–glyoxal (UG) and urea–glyoxal–formaldehyde (UGF) resins have been successfully synthesized and used as wood adhesive for plywood, interior decoration and furniture material [10,11] as well as for particleboard [12]. Equally, melamine–glyoxal resins have been formulated and while they were capable of yielding reasonable bond strength to satisfy relevant plywood standards, they did not yield internal bond (IB) strength results for particleboard to the level needed to satisfy relevant standards [13]. This was shown to be due to the energy of activation of cross-linking of the melamine-glyoxal (MG) resins being higher than formelamine-formaldehyde ( MF) resins [13]. Acceptable gel times are obtainable only at temperatures higher than 150 ◦C, thus too high to be able to properly cure the resin in a particleboard core where the max temperature reached is of 110–120 ◦C. This rendered possible some applications such as MG resins impregnated paper surface overlays were the resin is in direct contact with the hot platen of the press at 180 ◦C but showed such resins limitations as binders for wood particleboard [13]. Ionic liquids are salts in the liquid state. In general, the term is restricted to salts whose melting point is below some arbitrary temperature, such as 100 ◦C. They are salts in which the ions are poorly coordinated, this causing these solvents to be liquid below 100 ◦C. Ionic liquids are largely made of ions and short-lived ion pairs. Ionic liquids are becoming of considerable importance in many applied ﬁelds of technology [14–16]. The potential market of ionic liquids is large. Currently, ionic liquids have an estimated market value of 1.3 billion US dollars per year, mainly as replacement for traditional organic solvents in petrochemical and pharmaceutical industries [14]. Their market is expected to increase rapidly, this likely upsurge being attributed to the greener characteristics of these materials and good solubility in water and organic solvents [14]. Recently, the energy of activation of cross-linking of urea–glyoxal resins was effectively and markedly decreased by the use of ionic liquids (IL) as resin hardeners and additives [12,17]. This made it possible to obtain the same performance of UF resins by using UG + IL resins to even bond particleboard. The same approach has then been used in this article for melamine–glyoxal resins with the added variation of using a small proportion of glutaraldehyde (G’) further condensed with the MG resin to yield a melamine–glyoxal–glutaraldehyde (MGG’) adhesive presenting even better water resistance of the wood panels bonded with it.

2. Materials and Methods

− 2.1. Preparation of N-Methyl-2-Pyrrolidone Hydrogen Sulphate ([HNMP] [HSO4 ]) Ionic Liquid − [HNMP] [HSO4 ] ionic liquid was synthesized according to the method of Wang et al. [18]. 1.0 mol N-methyl pyrrolidone (LERMAB, Epinal, France was weighed and placed into a 250 mL three-neck ﬂask. Then 1.0 mol sulphuric acid (Carlo Erba, Milan, Italy) was added dropwise into the 250 mL three-neck ﬂask with a dropping funnel under continuous mechanical stirring at room temperature. A considerable amount of white smoke appeared during the dropwise addition of acid. The mixture became a sticky, light yellow transparent liquid after stirring at 80 ◦C for 4 h. The resultant − liquid N-methyl-2-pyrrolydone hydrogen sulphate ([HNMP] [HSO4 ]) was washed three times with ether and ethyl acetate, respectively, then it was dried under vacuum to remove the ether and ethyl acetate.

2.2. Preparation of MG and MGG’ Resins The MG resins were prepared at M/G molar ratio: 1:6, according to the method of Deng et al. [13]. Glyoxal (40% water solution, Sigma-Aldrich, St.Louis, MO, USA) was placed in the three-neck ﬂask and the pH was adjusted to 4.5 using 33% NaOH. Subsequently, melamine was added, and the mixture was heated to 60 ◦C for 1 h. The reaction mixture was cooled to room temperature ready for use. Polymers 2018, 10, 22 3 of 18

Under the same reaction conditions, different parts of glyoxal were substituted with glutaradehyde (50% water solution, Sigma Aldrich, St. Louis, MO, USA) to prepare MGG’ resins. The relative molar proportions of glyoxal to glutaraldehyde are shown in Tables1 and2.

Table 1. Proportions of melamine (M), glyoxal (G) and glutaraldehyde (G’) used in the preparation of the MGG’ resins.

Relative Mixing Molar Ratio Glyoxal (40% Glutaraldehyde Resin Type Melamine (g) Glyoxal:Glutaraldehyde Solution) (g) (50% Solution) (g) MG 100:0 25.2 174 0 MGG’1 95:5 25.2 163.5 12 MGG’2 90:10 25.2 156.6 24 MGG’3 85:15 25.2 147.9 36 MGG’4 80:20 25.2 139.2 48 MGG’4 75:25 25.2 130.5 60

Table 2. Plywood results for melamine-glyoxal-glutaradehyde (MGG’) resins. MG resin of mole ratio M/G = 1:6, pH = 4.5, reacted at 60 ◦C for 1 h. Three layers plywood were prepared and tested for − tensile shear strength. Ten per cent by weight [HNMP] [HSO4 ] ionic liquid was used as hardener.

Relative Mixing Mole 24 h Cold 2 h Boiling Name of Solid Dry Shear Ratio of Viscosity/mPa·s Water Shear Water Shear Resin Content/% Strength/MPa Glyoxal:Glutaraldehyde Strength/MPa Strength/MPa MG 100:0 49 390 0.6 0.63 – MGG’1 95:5 48.9 410 0.6 0.9 – MGG’2 90:10 48.8 540 0.59 1.02 – MGG’3 85:15 48.7 1720 0.76 1.39 0.45 MGG’4 80:20 48.5 2890 0.73 1.48 0.7 MGG’4 75:25 solidiﬁed in the reactor “–” means the samples broke.

2.3. 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).

2.4. Matrix Assisted Laser Desorption Ionisation Time of Flight (MALDI-ToF) Spectrometry Samples for Matrix assisted laser desorption ionization time-of-flight (MALDI-ToF, AXIMA Performance, Shimadzu, Manchester, UK) analysis were prepared first dissolving 5 mg of sample powder in 1 mL of a 50:50 v/v acetone/water solution. Then 10 mg of this solution is added to 10 µL of a 2,5-dihydroxy benzoic acid (DHB) matrix. The locations dedicated to the samples on the analysis plaque were first covered with 2 µL of a NaCl solution 0.1 M in 2:1 v/v methanol/water, and predried. Then 1 µL of the sample solution was placed on its dedicated location and the plaque is dried again. MALDI-ToF spectra were obtained using an Axima-Performance mass spectrometer from Shimadzu Biotech (Kratos Analytical Shimadzu Europe Ltd., Manchester, UK) using a linear polarity-positive tuning mode. The measurements were carried out making 1000 profiles per sample with two shots accumulated per profile. The spectrum precision is of +1 Da.

2.5. 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, Polymers 2018, 10, 22 4 of 19 Polymers 2018, 10, 22 4 of 18 2.5. Plywood Preparation and Testing The performance of the MGG’ resins was tested by preparing laboratory plywood panels and testedevaluating wet. Triplicate their shear three-ply strength dry, laboratory after 24 h plywood cold water panels soaking, of and 450 after mm 2 h× in300 boiling mm water,× 5 tested mm were preparedwet. Triplicate for each MGG’three-ply adhesive laboratory resin plywood and 2pane mmls poplarof 450 mm (Populus × 300 mm tremuloides × 5 mm )were veneers. prepared To thefor glue − mixeseach was MGG’ added adhesive 10% by resin weight and 2 [HNMP] mm poplar [HSO (Populus4 ] ionic tremuloides liquid) veneers. as hardener To the on glue total mixes MGG’ was resin added solids. 2 ◦ The glue10% spreadby weight used [HNMP] was of [HSO 2604 g/m−] ionicdouble liquid as glue hardener line, andon total hot MGG’ pressing resin time solids. was The of glue 6 min spread at 150 C and 1.5used MPa was pressure. of 260 g/m After2 double hot glue pressing, line, and the hot plywood pressing was time stored was of under 6 min ambientat 150 °C conditions and 1.5 MPa (20 ◦C and 12%pressure. moisture After content)hot pressing, for 48 the h beforeplywood testing was st accordingored under to ambient China National conditions Standard (20 °C and GB/T 12% 14074 (2006)moisture [19] and content) China for National 48 h before Standard testing according GB/T17657 to China (1999) National [20] which Standard require GB/T a 14074 minimum (2006) [19] average and China National Standard GB/T17657 (1999) [20] which require a minimum average shear shear strength of 0.7 MPa on ﬁve specimens tested, and European Norm EN 636:2012 (2012) [21]. strength of 0.7 MPa on five specimens tested, and European Norm EN 636:2012 (2012) [21]. 3. Results and Discussion 3. Results and Discussion The sameThe same principle principle of of using using an an ionic ionic liquidliquid as a hardener that that has has been been used used for forurea–glyoxal urea–glyoxal resinsresins [12,17 [12,17]] was appliedwas applied to melamine–glyoxal to melamine–glyoxal (MG) (MG) and melamine–glyoxal–glutaraldehydeand melamine–glyoxal–glutaraldehyde (MGG’) − resins(MGG’) for which, resinsN for-methyl-2-pyrrolidone which, N-methyl-2-pyrrolidone hydrogen hydrogen sulphate sulphate ([HNMP] ([HNMP] [HSO4 [HSO]) was4−]) used was used as hardener. as As thishardener. material As is this relatively material expensiveis relatively it expensive was prepared it was inprepared the laboratory in the laboratory according according to the methodto the of Wangmethod et al. [ 15of ]Wang and usedet al. [15] with and a minimum used with a of minimum puriﬁcation of purification [12,17] (Scheme [12,17]1 (Scheme). 1).

H 80° C + - NCH3 + H2SO4 NCH3 HSO4

O O

− − SchemeScheme 1. Preparation 1. Preparation of Nof-methyl-2-pyrrolidone N-methyl-2-pyrrolidone hydrogenhydrogen sulphate sulphate ([HNMP] ([HNMP] [HSO [HSO4 ]) 4 ]).

The MG resin evolved into the MGG’ resins given the better results that were obtained with such The MG resin evolved into the MGG’ resins given the better results that were obtained with a combination. The double reaction, in two successive steps of melamine with glyoxal and suchglutaraldehyde a combination. has Theled to double clear resin reaction, with no in formal two successivedehyde, each steps presenting of melamine distinct characteristics. with glyoxal and glutaraldehydeThese are shown has led in toTable clear 1 which resin withalso reports no formaldehyde, the strength eachresults presenting of laboratory distinct plywood characteristics. panels Theseprepared are shown by using in Tablethe different1 which melamine–glyoxal–g also reports thelutaraldehyde strength results resins of to which laboratory had been plywood added 10% panels preparedof [HNMP] by using HSO the4− differentionic liquid. melamine–glyoxal–glutaraldehyde These satisfy the relevant requirements resins of tointernational which had standards been added for 10% − of [HNMP]both dry [HSO strength4 ] ionicand strength liquid. after These 24 satisfyh cold water the relevant soaking. requirements In the two best of cases international they also satisfy standards the for both dryrequirements strength for and exterior strength grade after bonds 24h as cold their water strength soaking. is still acceptable In the two after best the cases boiling they water also test. satisfy the requirements for exterior grade bonds as their strength is still acceptable after the boiling water test. 3.1. Adhesives Bonding Performance 3.1. AdhesivesThe first Bonding characteristic Performance 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 The first characteristic that can be noticed from Table2 is that the proportion of glutaraldehyde proportion of 20% molar of glutaraldehyde. A higher proportion nonetheless causes the viscosity to increases the viscosity, and more interestingly the dry and wet tensile strengths increase up to a increase to such an extent that the resin solidifies in the reactor. Equally remarkable is the finding proportion(Table 2) of that 20% the molar tensile of strength glutaraldehyde. increases after A higher 24 h cold proportion water soak. nonetheless The effect is causes very marked the viscosity and to increasebecomes to such more an evident extent as that the theproportion resin solidifies of glutaralde in thehyde reactor. increases. Equally At first remarkableit was thought is that the the finding (Tablewater2) that repellent the tensile –(–CH strength2–)3– chain increases of glutaraldehyde after 24 h coldcaused water the effect, soak. but The while effect this is verycontributes marked to and becomesmaintaining more evident the dry asstrength the proportion once in water of glutaraldehydeit does not justify the increases. marked Atincrease first itin was strength thought noticed. that the waterIt repellentdoes however, –(–CH contribute2–)3– chain also of glutaraldehydeto the resistance causedof the thebond effect, to boiling but while water this as contributesthe bond to maintainingperformance the dry improves strength as the once relative in water proportion it does of not glutaral justifydehyde the marked increases. increase in strength noticed. It does however,Thermomechanical contribute alsoanalysis to the experiments resistance has of theshow bondn that to the boiling hardening water of as the the resin bond occurs performance in two phases (indicated with two arrows in Figure 1) a second increase in modulus occurring as the improves as the relative proportion of glutaraldehyde increases. temperature, hence the curing time increases. This appeared to indicate that while the two aldehydes Thermomechanicalhave been added simultaneously analysis experiments in the reactor, has on showne is less thatreactive the (glutaraldehyde) hardening of the and resin had occursboth in two phases (indicated with two arrows in Figure1) 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 Polymers 2018, 10, 22 5 of 19 reacted with the melamine in the reactor, one of them was contributing to cross-linking earlier than Polymers 2018, 10, 22 5 of 18 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 alsois also eventually eventually linked. linked. Cross-linking Cross-linking then, then, will will depend depend first first from from the the more more numerous, numerous, still still reactivereactive hydroxyethylPolymershydroxyethyl 2018, 10, groups22groups formedformed byby thethe firstfirst additionaddition ofof glyoxalglyoxal ontoonto melaminemelamine toto formform bridges.bridges. SecondSecond5 of 19 fromfrom thethe lessless reactivereactive hydroxypentylhydroxypentyl groupsgroups yieldedyielded byby thethe graftinggrafting reactionreaction ofof glutaraldehydeglutaraldehyde ontoonto hydroxyethyl groups formed by the first addition of glyoxal onto melamine to form bridges. Second thethe MGMG resin.resin. This would would explain explain the the two two phases phases of of hardening hardening of of the the MGG’ MGG’ resin resin seen seen in Figure in Figure 1. To1. from the less reactive hydroxypentyl groups yielded by the grafting reaction of glutaraldehyde onto Toshow show thatthat this this was was the the case, case, matrix matrix assisted assisted laser laser desorption desorption ionisation ionisation time time of fight fight (MALDI-ToF) the MG resin. This would explain the two phases of hardening of the MGG’ resin seen in Figure 1. To massmass spectrometryspectrometry analysisanalysis ofof differentdifferent resinsresins andandintermediates intermediates waswas carriedcarried out.out. ThisThis presentedpresented show that this was the case, matrix assisted laser desorption ionisation time of fight (MALDI-ToF) severalseveral pointspoints ofof interest:interest: (i)(i) firstfirst ofof allall thethe compositioncomposition andand oligomersoligomers distributiondistribution inin thethe MGMG andand inin mass spectrometry analysis of different resins and intermediates was carried out. This presented particularparticular thethe MGG’MGG’ resin;resin; (ii)(ii) second,second, andand ofof equalequal interest,interest, waswas toto findfind outout whatwhat waswas thethe interactioninteraction several points of interest: (i) first of all the composition and oligomers distribution in the MG and in ofof thethe ionicionic liquidliquid withwith bothboth thethe twotwo aldehydes,aldehydes, withwith melamine,melamine, withwith thethe MGG’MGG’ resinresin soso asas toto finallyfinally particular the MGG’ resin; (ii) second, and of equal interest, was to find out what was the interaction deducededuce itsits mechanismmechanism ofof action.action. of the ionic liquid with both the two aldehydes, with melamine, with the MGG’ resin so as to finally deduce its mechanism of action.

FigureFigure 1.1.Thermomechanical Thermomechanical analysisanalysis (TMA)(TMA) curvescurves showingshowing thethe ModulusModulus ofof ElasticityElasticity (MOE)(MOE) asas aa Figure 1. Thermomechanical analysis (TMA) curves showing the Modulus of Elasticity (MOE) as a functionfunction ofof temperature temperature for for the the hardening hardening of of the the MGG’ MGG’ adhesives adhesives from from Tables Tables1 and1 and2. 2. function of temperature for the hardening of the MGG’ adhesives from Tables 1 and 2. Thermomechanical analysis curves of the MG resin hardening in respect of the hardening of the Thermomechanical analysis curves of the MG resin hardening in respect of the hardening of the MG Thermomechanicalresin + 3% IL and MG analysis resin + curves 5%IL (Figureof the MG 2) shows resin hardening clearly the inrise respect at a much of the lower hardening temperature of the MG resin + 3% IL and MG resin + 5%IL (Figure2) shows clearly the rise at a much lower temperature MGof the resin MOE + 3% of ILthe and IL catalysedMG resin resins+ 5%IL indicating (Figure 2) thei showsr much clearly faster the curing. rise at aThis much confirms lower temperaturethe role of IL of the MOE of the IL catalysed resins indicating their much faster curing. This conﬁrms the role of IL ofin the lowering MOE of of the the IL energy catalysed of activation resins indicating of condensation their much and faster hardening curing. of This MG confirmsresins. the role of IL in lowering of the energy ofof activationactivation ofof condensationcondensation andand hardeninghardening ofof MGMG resins.resins.

MG+5%IL

MG+3%IL

Figure 2. Comparative thermomechanical analysis curves of the rate of hardening of MG resins with Figure 2. Comparative thermomechanical analysis curves of the rate of hardening of MG resins with Figureand without 2. Comparative addition of thermomechanical IL. analysis curves of the rate of hardening of MG resins with and without addition of IL. and without addition of IL. 3.2. 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:

Polymers 2018, 10, 22 6 of 19

Polymers3.2. Analysis2018, 10 of, 22Chemical Species Formed in the Reactions Analysed by MALDI-ToF Spectrometry 6 of 18

ThePolymers MALDI-ToF 2018, 10, 22 of the two aldehydes first shown is for just the MALDI spectra of glyoxal6 of 19 and 3.2.glutaraldehyde Analysis of Chemical after reaction Species with Formed the inionic the liquid Reactions (Figures Analysed 3a,b by and MALDI-ToF 4a–c). The Spectrometry species identified are 3.2. Analysis of Chemical Species Formed in the Reactions Analysed by MALDI-ToF Spectrometry shown in Tables 3 and 4. Two main trends are noticeable: The MALDI-ToFThe MALDI-ToF of theof the two two aldehydes aldehydes ﬁrstfirst shownshown is is for for just just the the MALDI MALDI spectra spectra of glyoxal of glyoxal and and glutaraldehydeglutaraldehyde after after reaction reaction with with the the ionic ionic liquidliquid (Figures (Figure 33a,ba,b and and 4a–c).4a–c). The The species species identified identiﬁed are are shownshown in Tables in Tables3 and 3 4and. Two 4. Two main main trends trends are are noticeable: noticeable:

(a) (a)

(b) (b) Figure 3. MALDI-ToF spectrum of the interaction between glyoxal and N-methyl-2-pyrrolydone Figure 3. N Figure 3. MALDI-ToF spectrum of − the interactioninteraction betweenbetween glyoxalglyoxal andand N-methyl-2-pyrrolydone-methyl-2-pyrrolydone hydrogen sulphate ([HNMP] HSO4−) ionic liquid: (a) 20–200 Da range; (b) 200–500 Da range. hydrogen sulphate ([HNMP] [HSO − ]) ionic liquid: (a) 20–200 Da range; (b) 200–500 Da range. hydrogen sulphate ([HNMP] HSO44) ionic liquid: (a) 20–200 Da range; (b) 200–500 Da range.

(a)

Figure 4. Cont.

Polymers 2018, 10Polymers, 22 2018, 10, 22 7 of 187 of 19 Polymers 2018, 10, 22 7 of 19

Polymers 2018, 10, 22 7 of 19 Polymers 2018, 10, 22 7 of 19 Polymers 2018, 10, 22 7 of 19

(b)

(b) (b) (b) (b)

(c)

Figure 4. MALDI-ToF spectrum of the interaction between glutaraldehyde and N-methyl-2- (c) pyrrolydone hydrogen sulphate ([HNMP] HSO4−) ionic liquid: (a) 20–200 Da range. (b) 200–500 Da (c) range.Figure (c) 500–900 4. MALDI-ToF Da range. spectrum of( cthe) interaction(c) between glutaraldehyde and N-methyl-2- Figurepyrrolydone 4. MALDI-ToF hydrogen sulphatespectrum ([HNMP] of the HSOinteraction4−) ionic betweenliquid: ( a)glutaraldehyde 20–200 Da range. and (b )N 200–500-methyl-2- Da Figure 4. MALDI-ToFFigurepyrrolydonerange. (c4. ) spectrum500–900MALDI-ToFTable hydrogen Da3. Species range.of sulphate spectrumthe formed interaction ([HNMP] of bythe the HSOinteraction reactionbetween4−) ionic of between liquid:glyoxalglutaraldehyde ( a with)glutaraldehyde 20–200 IL, byDa andMALDI. range. andN (-methyl-2-b )N 200–500-methyl-2- Da Figure 4. MALDI-ToF spectrum of the interaction− between glutaraldehyde and pyrrolydonerange. (c) 500–900 hydrogen Da range. sulphate ([HNMP] HSO4 ) ionic liquid: (a) 20–200 Da range. (b) 200–500 Da pyrrolydone hydrogen sulphate ([HNMP] HSO4−) ionic liquid: −(a) 20–200 Da range. (b) 200–500 Da N-methyl-2-pyrrolydonerange. (c) 500–900 hydrogenTable Da 3.range. sulphate Species formed ([HNMP] by the [HSOreaction4 of]) glyoxal ionic liquid:with IL, by(a )MALDI. 20–200 Da range. range. (c) 500–900 Da range. (b) 200–500 Da range. (c) 500–900PeakTable Da 3. range.Species formed by the reaction of glyoxalChemical with IL,Species by MALDI. Table 3. Species formed by the reaction of glyoxal with IL, by MALDI. Table 3. SpeciesPeak formed by the reaction of glyoxal withChemical IL, by MALDI. Species Table 3. SpeciesPeak formed by the reaction of glyoxal withChemical IL, by MALDI. Species 199–200 Da = calculatedPeak 197 ChemicalNCH Species3 HSO - H 4 PeakDa, proton Peak = 198 Da ChemicalChemical Species Species 199–200 Da = calculated 197 ONCH3 HSO - H 4 199–200Da, proton Da == 198calculated Da 197 NCH3 HSO - H 4 199–200 Da = calculated 197 ONCH3 - Da, proton = 198 Da HSO4 199–200 Da = calculated 197 Da, proton = 198 Da O H Da, proton = 198 Da O OH O 199–200 Da = calculated 197 NCH3 - 138 Da = 140 Da, glyoxal dimer HSO4 HH O OHCHO Da, proton = 198by aldol 138Da Da condensation = 140 Da, glyoxal dimer O OH O H OH CHO 138by aldol Da = condensation140 Da, glyoxal dimer O OH 138 Da = 140 Da, glyoxal dimer by aldol condensation H CHO 138by aldol Da = condensation140 Da, glyoxal dimer OH H OH CHO by aldol condensation O OH O OH 2 x O OH O O OH 138 Da = 140 Da,By aldol glyoxal condensation dimer as: HCHO H CHO 2 x O O OH By aldol condensationBy as:aldol condensation as: 2 xH HCHOCHO H OH CHO by aldol condensation O O OH By aldol condensation as: H CHO 2 x HCHOOH OH By aldol condensation as: CHO HCHO H OH O OH OH O 234 Da = 230 Da calculated, OH 234 Da = 230 Da calculated, plus protonated, by aldol O OH OH O Polymers 2018, 10234, 22 Da = 230 Da calculated, H O OHH 8 of 19 plus protonated,+ by aldol O O OH OH O condensation, with Na234plus Da protonated, = 230 Da calculated, by aldol H H condensation, with Na+ 2 x O O OHOHOH OHO By aldol condensation234plus Da protonated, as:= 230 Da calculated, by aldol+ H O HOH OH HCHO condensation, with Na HCHO pluscondensation, protonated, with by Naaldol+ H O OH OH H O OH OH OH O OOH OH OH OH O condensation, with Na+ O OH OH H H H H 366–367 Da = 366 Da + O OH OH OH O OH OH OH 366–367 Da = 366 Da calculated, without Na NCH3 + O OH OH O 234 Da = 230calculated, Da calculated, without Na NCH3 H O plus protonated, by aldol HO H + O OH OH condensation, with Na O O H N CH3 H3C N 387 Da = 366 Da + 23 Da = as O OH OH O O O OH OH O O above but with Na+ H H H H O OH OH OH O OH OH OH + 487–488 Da = with Na NCH3 NCH3 H O O

Table 4. Species formed by the reaction of glutaraldehyde with IL, by MALDI.

Peak Chemical species

199–200 Da = calculated 197 Da, NCH3 - HSO4 proton = 198 Da without Na+ H O

222 Da = as 199–200 but with Na+, however it is also possible O OH 224 Da = 225 Da calculated with H H Na+ obtained by aldol OH O condensation

328.6 Da = 304 Da + 23 = 327 Da O OH OH O calculated, glutaraldehyde trimer by aldol condensation H H with Na+ OH OH

OO O OH OH O by aldol condensation, 3 x H H H H thus as OH OH

O OH O OH H H but also at 325 Da (observed 324 H H OO OO N Da) thus more probable: CH3 NCH3 H O O

as well as coupling with HSO4− O OH OH O (see example of the 424 Da peak). H H OH O 424 Da = 426 calculated,

glutaraldehyde tetramer by N CH3 H aldol condensation with Na+ O but also

Polymers 2018, 10, 22 8 of 19

O OH OH OH O O OH OH OH O

Polymers 2018, 10, 22 H H H 8 of 19 Polymers 2018, 10, 22 H H H H 8 of 19 366–367 Da = 366 Da O OH OH OH O OH OH OH NCH3 Polymers 2018, 10, 22 + 8 of 19 calculated, without Na NCH3 Polymers 2018, 10, 22 H O 8 of 19 Polymers 2018, 10, 22 O HOH OH OH O O OH OH OH O 8 of 19 O Polymers 2018, 10, 22 O H OH OH OH O H HO OH OH OH O H 8 of 18 O OH OH OH O O OH OH OH O 366–367 Da = 366 Da O OH OH OH O OH OH OH H H H H H O OH OH OH OH H ONCHOH3 OH OH OH + O OH OHO OH O O OH OH OH O 366–367 Dacalculated, = 366366–367 Da without Da = 366 Na Da O NCHOHO 3OH OHOH OH OO OHOHOH OHOH OH H H H OH H H NCHO 3 3 366–367 Da+ = 366 Da + H O OH OH OH H H O OH OH OH H calculated, without Na ONCH3 N calculated, without Na NCH3 O OH OH CHOH3 NCHO 3 OH OH OH 366–367 Da = 366 Da Table 3. Cont. H O H 3C N + NCH 3 calculated, without Na H NCH3 O 3 + O OHO OH O O O OH OH O O 387 Dacalculated, = 366 Da +without 23 Da Na= as O NCHH 3 O OH O + H H H H above but with Na O O Peak O OH ChemicalOH OH SpeciesO OH OH OOH O OH OHO HOH O OH OH OH + NCH 487–488 Da = with Na N NCH3 O NCH HOCH3 NCH3 O H3C N N CH O H O H3 OO OH H COHN OO O O OH OH H O O O 3 O 387 Da = 366 Da + 23 Da = as NH OO OH OH O CHO 3 O OH OH O O 387 Da = 366 Da + 23 Da = as N N H3C N H CH3 CH H H H + 3 O OH OHH CH OCN NO above but with Na O OH OH O OH H 3 3 H 387 Da = 366 Da ++ 23 Da = as H O OH OH OH above but with Na O OHO OHOHOH OOHOH OOOH O OO OHOH OHOH O O O O 387 Da = 366 Da ++ 23 Da = as O OH OH OH ONCHOH OH OH H 387 Da = 487–488366 Da +Da 23 = Dawith = Naas + + H H H 3 487–488above butDa =with with Na Na NCH3 387 Da = 366 Da + 23 Da = as above but with+ H NCHO 3 OH OH OH H H O OH OH OH H above+ but with Na + H NCH3 H H H + above but with+ 487–488Na Da = with Na HO OH OH OH NCHOO 3 OH OH OH Na 487–488 Da = with Na Table 4. Species +formed by the reaction ofH glutaraldehyde withO IL, by MALDI. O OHNCHOH3 OH NCHO 3 OH OH OH 487–488 Da = with Na OO + NCHH 487–488 Da = with Na 3 NCHO 3 OH O NCH3 O H O Peak O Chemical species TableTable 4. Species 4. Species formed formed by by the the reaction reaction of of glutaraldehydeglutaraldehyde with with IL, IL, by by MALDI. MALDI. Table 4. Species formed by the reaction of glutaraldehyde with IL, by MALDI. Table 4. Species formed by the reaction of glutaraldehyde with IL, by MALDI. Table 4. Species formedPeak by the reaction of glutaraldehydeChemical with IL, species by MALDI. Table 4. SpeciesPeak formed by the reaction of glutaraldehydeChemical with species IL, by MALDI. 199–200 Da = calculatedPeak 197 Da, ChemicalNCH species3 - Peak Chemical speciesHSO 4 proton = 198 Da without Na+ H Peak Chemical Species O Peak199–200 Da = calculated 197 Da, ChemicalNCH species3 - HSO4 199–200 Da = calculated 197+ Da, 199–200proton Da == calculated198 Da without 197 NaDa, NCHNCHH 3 HSO-- 199–200 Da = calculated 197 Da, NCH3 HSO-44 199–200 Da = calculated 197222 Da,Da proton proton= as 199–200 = =198 198 Da but Da without with+ Na+ HH HSO4 proton = 198 Da without Na + O without Na+ proton = 198 Da without Na O OHH Na+, however it is also possible O O OH OO 224 Da = 225 Da calculated with H 224 Da222 = 225Da = Da as 199–200calculated but with H 199–200 Da = calculated 197 Da, H NCH3 - Na+ obtainedNa222+, however Da =by as aldol 199–200 it is also but possible with O OH HSO4 222 Da = as 199–200 but+ with 222 +Da = as 199–200 but with O H OHOH O proton = 198 Da224+Na without Da, however = 225 DaNa it calculated is also possible with H 222 Da = as 199–200 but withNacondensation+ Na, howeverNa, however+, however it is italso it is is also possiblealso possible possible H OO OH Na224+ obtained Da = 225 by Da aldol calculated with H 224 Da = 225 Da calculated with H O OH O H H 328.6224 Da Da Na= =225+ +obtained304 Da Da calculated + by 23 aldol = 327 with Da H 224 Da = 225 Da calculated withcondensation Na+ obtained by aldol O H OH OH O + Na obtained by aldol OH O condensation calculated,Na obtainedcondensation glutaraldehyde by aldol OH O 328.6condensation Da = 304 Da + 23 = 327 Da OH O 222 Da = condensationtrimeras 199–200 by aldol but condensation with H O OH OH O H trimer by328.6 aldol Da =condensation 304 Da + 23 = 327 Da + calculated, glutaraldehyde OO OH OH OH O Na , howeverwith Nait328.6 is+ also Da = possible 304 Da + 23 = 327 Da OH OH 328.6with NatrimerDacalculated, = 304 by aldolDa glutaraldehyde + condensation23 = 327 Da H O OH OH O H 328.6 Da = 304 Da + 23 = 327 Dacalculated, calculated, glutaraldehyde glutaraldehyde O OH OH H O 224 Da = 225 Datrimer calculated+ +by aldol with condensation H OH OH H trimer by aldol condensationcalculated, withwith Na Na glutaraldehyde HH H + trimer by+ aldol condensation OH OH Na obtained bywith aldol Na HOO O OH OH H O trimer withby aldol Na+ condensation OHOH OH O by aldol condensation, 3 x condensationwith Na+ H OOH OHHO OHOH OH O H thus asby aldol condensation, 3 x thus as OO O OH OH OHOH O by aldol condensation, thus as H OOH H O OH OH OH thusby aldolas condensation, 3 x 328.6 Da = 304 Da + 23 = 327 Da OH OH by aldol condensation, 3 Ox H OHH H OH O H thus as OOH H H O OH OH H O thus as OH OH calculated,by glutaraldehydealdol condensation, 3 x O OH O OHOH OH H H H H O OH H O OH H thus as H H H trimer by aldol condensation H OH OHH but also at 325 Da (observed 324 O OH H O OH H OO H OO + but also at 325 Da (observed 324 O OH OH OHO OH H but also at 325 Da (observed 324 Da) thus more probable: OOH OON with Na H H CH Da) thus more probable: H N CH3 H but also at 325 Da (observed 324 NCH3 Da) thus more probable: H NCHOO3 H CHOO3 NCH3 O but also at 325 Da (observed 324 O OH H O NOOHOO OOH O CH Da) thus more probable: N 3 O NCH3 H H CH3 Da) thus more probable: O H O OOH NCHH 3 O OH OH O but also at 325 Da (observed 324 OH O OO by aldol condensation, 3− x OO as wellas aswell coupling as coupling with with HSO HSO4 4− O N Da) thus more probable: H H H CH3 H OO NCH OHOH OHOH O O thus as −as well as coupling with HSO4− 3 as well as coupling with [HSO(see example(see] (seeexample exampleof the of 424the of 424Da the Da − H OHO OH 4 as well as coupling with HSO4 O OH OH O peak).(see example of the 424 Da HH OO OH OH O H H 424 Da peak). peak). (see example of the 424 Da peak). H OHOH O O H 424 Da424 = Da426 = calculated, 426 calculated, H H 424 Dapeak). = 426 calculated, 4− OH O 424 Da = 426 calculated, glutaraldehydeas well as coupling tetramer with byHSO aldol O OH O OH Polymersglutaraldehyde424 2018 Da, 10 =, 22426 calculated, tetramer by OHN O CH3 9 of 19 Polymersglutaraldehyde 2018, 10, 22 tetramer by O OH N OHCH3 O 9 of 19 + glutaraldehyde424 Da = 426 tetramer calculated, by H H H condensation with Na but(see also exampleglutaraldehyde of the 424 tetramer Da by+ N CH3 aldol condensation with +Na H H H aldol condensationglutaraldehyde with tetramer Na+ by O N HCH3 but also ataldol 325 condensation Da (observed with 324 Na + H OO H peak).but aldol also condensation with Na OO O H + OHO O N but alsoaldol condensation with Na CH Da) thus 424more Da probable: but= 426 also calculated, O OH O OH O 3 but also NCHO 3 OH OH O H O glutaraldehyde tetramer by H N CH3 H and also HO H H aldol condensation with Na+ OH O and also and also OHNCHO O 3 NCH as well asbut coupling also with HSO4− 3 O O OH OH O (see example of the 424 Da O H H peak). OH O O OH OH O 424 Da = 426 calculated,but also protonated + HSO4− 2 − HSO4 but also protonated + [HSO4 ] such as as − O OH N OHCH O glutaraldehydebut also tetramer protonated by + HSO4 H 23 H H HSO + OH OH 4 aldol condensationsuch as with Na H H O 524 Da = glutaraldehydebut also pentamer by524 aldolcondensation, Da = glutaraldehyde OH OH + — with Na but also the alternatives as for the 424pentamer Da peak by 524 Da = glutaraldehyde 624 Da = glutaraldehyde hexamer by aldolcondensation,aldolcondensa — with Na+ but also the alternatives as for thepentamer 424tion, Da with peak by Na+ aldolcondensa --- 724 Da= glutaraldehyde heptamer by aldolcondensation,but also the + — with Na+ but also the alternatives as for thetion, 424alternatives with Da peak Na as but alsofor the the 424 Da --- alternativespeak as 624 Dafor = theglutaraldehyde 424 Da peakhexamer by aldolcondensa 624 Da = glutaraldehyde tion, with Na+ hexamerbut also by the --- aldolcondensaalternatives as tion,for with the Na424+ Da but alsopeak the --- 724 Daalternatives = glutaraldehyde as for theheptamer 424 Da by peakaldolcondensa tion, with Na+ 724 Da = glutaraldehyde but also the --- heptamer by alternatives as aldolcondensafor the 424 Da + tion,peak with Na but also the --- Two main trendsalternatives are noticeable, as as shown below. 1. Linear oligomerizationfor the 424 Daof the aldehyde by aldol condensation, which indicates that IL has some catalytic effectpeak on the autocondensation of the aldehyde (Scheme 2).

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 2. Example of glutaraldehyde aldol condensation

2. Formation of complexes between the aldol-derived oligomers formed with either N-methyl pyrrolydone or HSO4−, or both (Scheme 3).

Scheme 2. Example of glutaraldehyde aldol condensation

2. Formation of complexes between the aldol-derived oligomers formed with either N-methyl pyrrolydone or HSO4−, or both (Scheme 3).

Polymers 2018, 10, 22 9 of 19

O OH OH O

H H and also OH O NCH3

O OH OH O but also protonated + HSO4− 2 HSO4 such as H H OH OH

524 Da = glutaraldehyde pentamer by aldolcondensa tion, with Na+ but also the --- alternatives as for the 424 Da peak 624 Da = glutaraldehyde hexamer by aldolcondensa tion, with Na+ but also the --- alternatives as for the 424 Da peak 724 Da = glutaraldehyde heptamer by aldolcondensa Polymers 2018, 10, 22 tion, with Na+ 9 of 18 but also the --- alternatives as Two main trendsfor are the noticeable, 424 Da as shown below. peak 1. Linear oligomerization of the aldehyde by aldol condensation, which indicates that IL has some catalytic effect on the autocondensation of the aldehyde (Scheme2). Two main trends are noticeable, as shown below. 2. Formation of complexes between the aldol-derived oligomers formed with either N-methyl − 1. Linearpyrrolydone oligomerization or [HSO4 of], orthe both aldehyde (Scheme by 3aldol). condensation, which indicates that IL has some catalytic effect on the autocondensation of the aldehyde (Scheme 2).

Scheme 2. Example of glutaraldehyde aldol condensation Polymers 2018, 10, 22 Scheme 2. Example of glutaraldehyde aldol condensation. 10 of 19 Polymers 2018, 10, 22 10 of 19 2. Formation of complexes between the aldol-derived oligomers formed with either N-methyl pyrrolydone or HSO4−, or both (Scheme 3).

SchemeScheme 3.3. ExamplesExamples ofof complexescomplexes betweenbetween aldolaldol condensatescondensates andand ionicionic liquid.liquid 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 Figure5 and Table5 show the MALDI-ToF of the interaction between melamine and the ionic liquid.Figure Only 5 three and Tablepeaks 5 could show be the interpreted, MALDI-ToF namely of the the interaction melamine between peak, the melamine ionic liquid and peakthe ionic and liquid. Only three peaks could be interpreted, namely the melamine peak, the ionic liquid peak and liquid.that of Onlya coordination three peaks compound could be interpreted,between two namely ionic liquidthe melamine molecules peak, and the one ionic single liquid melamine peak and at that of a coordination compound between two ionic liquid molecules and one single melamine at that536–539 of a Dacoordination (Table 4). Thecompound rest are betweenfragments two either ioni comingc liquid frommolecules impurities and oneor fragments single melamine generated at 536–539 Da (Table4). The rest are fragments either coming from impurities or fragments generated in 536–539in the MALDI. Da (Table It is 4). clear The that rest the are melamine fragments itself either is comingnot affected from by impurities the IL, although or fragments species generated like that the MALDI. It is clear that the melamine itself is not affected by the IL, although species like that of inof the536–539 MALDI. Da could It is clear facilitate that thereaction melamine with theitself aldehydes. is not affected by the IL, although species like that 536–539of 536–539 Da Da could could facilitate facilitate reaction reaction with with the the aldehydes. aldehydes.

Figure 5. MALDI-ToF spectrum of the interaction between melamine and N-methyl-2-pyrrolydone Figure 5. MALDI-ToF spectrum of −the interaction between melamine and N-methyl-2-pyrrolydone Figurehydrogen 5. MALDI-ToF sulphate ([HNMP] spectrum HSO of4 the) ionic interaction liquid. 500–700 between Da melamine range. and N-methyl-2-pyrrolydone − hydrogen sulphate ([HNMP] HSO4 )−ionic liquid. 500–700 Da range. hydrogen sulphate ([HNMP] [HSO4 ]) ionic liquid. 500–700 Da range. Table 5. Species formed by reaction of IL with melamine alone. Only two peaks could be interpreted. TableThe rest 5. areSpecies fragments formed either by reaction coming of from IL with impurities melamine or fragments alone. Only generated two peaks in thecould MALDI. be interpreted. It is clear Thethat restthe aremelamine fragments itself either is not coming affected from by impurities the IL, although or fragments species generated like the in 538 the Da MALDI. could Itfacilitate is clear thatreaction the melaminewith the aldehydes. itself is not affected by the IL, although species like the 538 Da could facilitate reaction with the aldehydes.

Peak Chemical species Peak Chemical species

O HSO- N -4 − O HSO4 200 Da = [HNMP] HSO4 , calculated 197 H3C N − 200Da, Da+ protonation. = [HNMP] HSO4 , calculated 197 H3C NH Da, + protonation. NH 536–539 Da = 539 Da calculated (516 + Na+ N N + N N 536–539= 539) Da = 539 Da calculated (516 + Na = 539) H2NNHN H2NNHN N - N HSO4 H3C - HSO4 H3C O O

Polymers 2018, 10, 22 10 of 19

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.

Polymers 2018, 10, 22Figure 5. MALDI-ToF spectrum of the interaction between melamine and N-methyl-2-pyrrolydone10 of 18 hydrogen sulphate ([HNMP] HSO4−) ionic liquid. 500–700 Da range.

Table 5. SpeciesTable formed 5. Species by formed reaction by of reaction IL with ofmelamine IL with melamine alone. alone. Only twoOnly peaks two peaks could could be interpreted.be interpreted. The rest areThe fragments rest are fragments either coming either coming from impurities from impurities or fragments or fragments generated generated in in thethe MALDI. MALDI. It Itis clear is clear that thethat melamine the melamine itself isitself not is affected not affected by the by IL, the although IL, although species species like like the 538the 538 Da couldDa could facilitate facilitate reaction with the aldehydes. reaction with the aldehydes.

Peak Peak ChemicalChemical Species species

O HSO- N 4 200 Da = [HNMP] HSO4−, calculated 197 − H3C 200 Da = [HNMP]Da, [HSO+ protonation.4 ], calculated 197 Da, NH + protonation.

+ N N 536–539 Da = 539536–539 Da calculated Da = 539 Da (516 calculated + Na+ =(516 539) + Na = 539) H2NNHN N - HSO4 H3C Polymers 2018, 10, 22 O 11 of 19 Polymers 2018, 10, 22 11 of 19

In the case of MG resins several types of glyoxal bridges have been shown to form and link InIn the the case case of of MG MG resins resins several several types types of of glyoxal glyoxal bridges bridges have have been been shown shown to to form form and and link link melamine molecules [13], such as the following (Scheme 4). melaminemelamine molecules molecules [13], [13], such such as as the the following following (Scheme (Scheme 44).).

(a) (b) (a) (b)

SchemeScheme 4.4.Type Type of of bridges bridges formed formed in in MG MG resins. resins Scheme 4. Type of bridges formed in MG resins

They also form and link very branched resins of structure of the type below (Deng et al. 2017) TheyThey also also form form and and link link very very branched branched resins resins of of structure structure of of the the type type below below (Deng (Deng et et al. al. 2017) 2017) (Scheme 5). (Scheme(Scheme 5).5).

SchemeScheme 5.5.Type Type ofof branched branched oligomers oligomers formed formed in in MG MG resins. resins Scheme 5. Type of branched oligomers formed in MG resins

TableTable6 6and and Figure Figure6a–c, 6a–c, show show what what occurs occurs when when the the resin resin MGG’ MGG’ resin resin is is formed formed when when IL IL is is added. added. Table 6 and Figure 6a–c, show what occurs when the resin MGG’ resin is formed when IL is added.

(a) (a)

Polymers 2018, 10, 22 12 of 19 Polymers 2018, 10, 22 12 of 19

(b)

(c)

Figure 6. MALDI-ToF spectrum of the formation of the MGG’ resin in the presence of N-methyl-2- pyrrolydone hydrogen sulphate ([HNMP] HSO(4c−)) ionic liquid: (a) 20–200 Da range. (b) 200–500 Da range. (c) 500–800 Da range. Figure 6. MALDI-ToF spectrum of the formation of the MGG’ resin in the presence of N-methyl-2- − pyrrolydone hydrogenTable 6. sulphate Oligomers ([HNMP] formed byHSO the4 reaction) ionic liquid: of MGG’ (a )with 20–200 and Da without range. IL. ( b) 200–500 Da range. (c) 500–800 Da range.

Peak Chemical species Table 6. Oligomers formed by the reaction of MGG’ with and without IL.

Polymers 2018, 10, 22 149–151 Da = Melamine + 23 11 of 18 Da (Na+) with and without IL Peak Chemical species 159 Da = G + G’ by aldol 149–151 Da = Melamine + 23 Tablecondensation 6. Oligomers with formed and by the reaction of MGG’ with and without IL. Da (Na+) with and without IL without IL. Small with IL 159 DaPeak = G + G’ by aldol Chemical Species 184–185 Da = M(Glyox)1 + 149–151 Da = Melaminecondensationwithout + 23 DaNa (Na+with in MGG’) and with with and IL without IL without IL. Small with IL 159 Da = G + G’ by aldol197.8 condensation Da = [HNMP] with HSO and4−

without IL. Small184–185 with(only IL presentDa = M(Glyox) in MGG’1 with + 184–185 Da = M(Glyox)withoutIL)1 without Na+ inNa MGG’in MGG’ with IL with IL − 197.8 Da = [HNMP]197.8 [HSO234 Da Da = ]= (only[HNMP] aldol present condensation HSO in4 MGG’− of 4 with IL) + Polymers(onlyglyoxal 2018 present, 10 ,+ 22 Na in existsMGG’ only with in 13 of 19 234 Da = aldol condensation of glyoxal + Na+ exists PolymersIL) 2018 MGG’+IL, 10, 22 13 of 19 only in MGG’ + ILPolymers 2018, 10, 22 13 of 19 Polymers 2018, 10, 22 13 of 19 234331.5 Da = Da aldol = 328.6 condensation Da of Polymers 2018, 10, 22 O OH OH OH O 13 of 19 331.5 Da = 328.6 Da glutaraldehyde+ILglutaraldehyde+IL+ by by aldol aldol Polymersglyoxal 2018 ,+ 10 Na, 22 exists only in+ H NNHN OH 13 of 19 condensation occursBut but isalso relatively 331.5 = small310 + inNa (333 2 OH H condensation occurs but is H N NNH2 MGG’+IL + H2NNHN OH MGG’ + IL Butcalculated also 331.5 deprotonated = 310 + Na (333 = +332) H2NNHN But also 331.5 = 310 + Na (333 NNOHOH N H OHNNH2 relatively small in MGG’+IL OHN NNH2 calculatedBut also 331.5deprotonated = 310 + Na = 332)+ (333 H2NNHN NN 331.5in MGG’+ILDacalculated = 328.6 deprotonated Da = 332) NNOH H N NNH2 + O H NNHNNNOH OHOHH OH O in calculatedButMGG’+IL also 331.5 deprotonated = 310 + Na = (333 332) 2 NH2 NNNN glutaraldehyde+ILin MGG’+IL by aldol NNOH HN NNHNH 2 But also 331.5 = 310 + Na+ (333 H2NNHN NN2 incalculated MGG’+IL deprotonated = 332) NH2NH2 + H NNOH HN NHNNH2 H But also 331.5 = 310condensation + Na (333 calculated occurs but is NH2 2 incalculated MGG’+IL deprotonated = 332) NH2 NN NNOHOH H OH NH deprotonated = 332)relatively in MGG’ small + IL in MGG’+IL NN2 in MGG’+IL NH2 OH NH2 H NH2 OHOH H 2NNHN NH N NNH2 HH 395 Da = 392 Da (369 + 23) H NNHH2NNHN N OH 2 N NNHH O 395 Da395 = 392Da = Da 392 (369 Da (369+ 23) + 23) + NNOHOHN H NNH calculated. MGG’+IL with Na H2NNHN OO + NNOH NH NNNNHOHH calculated.395calculated. Da = 392MGG’+IL Da MGG’+IL (369 with + 23) with Na + Na H NNHNNN OH OHH 2 NNNNOHOH H O + NNNH2 OH HN NNH 395 Da = 392 Da (369calculated.395 + 23) Da calculated. = 392 MGG’+IL Da (369 MGG’ +with 23) + IL Na H2NNHN NH O NHNH2 N NNNNH2 OH + calculated.395 Da = 392 MGG’+IL Da (369 +with 23) Na+ NN2 OH H NH2 with Na NNNH2 OH O calculated. MGG’+IL with Na+ NNNH2 OH H NNNH2 OH NH2 O O O NHO 2 NH2HO O HO O HOHO HO NH2OH HOHO O HO OOH HO N N N OH 439 Da = MG5 + Na+, present in HO N O NHO N O 439 Da = MG5 + Na+, present in O O + OHO N N N OH O 439only Da MGG’= MG5 + Na , present in HO ON HON O only MGG’ O N N O + HO N N N OH only439 MGG’ Da = MG5 + Na , present in HO N NHO + O HO N N N OH O 439 Da = MG5 + Na only439, present Da MGG’ = inMG5 only + MGG’Na+, present in NH O N N O + N NHN N only439 Da MGG’ = MG5 + Na , present in HON N O O O HONH O only MGG’ HON N O NH HO O NH OHOH HO O HH + + OO HNHNN NH OH 496496 Da Da = 491 = 491 Da Da (with (with Na Na), ), HO O NN NNHNNHH O HNN NH OH 496probably Daprobably = 491 multiprotoated, Da multiprotoated, (with Na+), in in HO O + H NNOHOHOHNHH NNH 496 Da = 491 Da (with Na ), probably + OH OHOHHNN NH NNOH O probably496MGG’+ILMGG’+IL Da =multiprotoated, 491 (= GMGMG’)(=Da GMGMG’) (with Na in ), N NNNNHOHH HO HNNNN NHOH OHH multiprotoated, in MGG’probably496 Da + IL= 491(=multiprotoated, GMGMG’) Da (with Na +in), OH NH NNOH H O MGG’+IL (= GMGMG’) NH2 N NNH OH HNNNN NHOH H NH2 probably496 Da = 491multiprotoated, Da (with Na +in), OH NNNH2 OH O MGG’+IL (= GMGMG’) NNNH2 OH HN NNH H OH NH O MGG’+ILprobably multiprotoated,(= GMGMG’) in NH NN2 OH H OH NN2 OH H O O NNNH2 OH MGG’+IL (= GMGMG’) NHO 2 O NH HO NH HO 2 HOHO O 2 HO O OH 521 Da = 516/518 calculated = HO OH NH2 521 Da = 516/518 calculated = HO N O HON N O 5 + 5 MG G’ without Na or MG+ G’ HO N N N OH O 521 Da = 516/518 calculated521MG Da5 G’= 516/518 without = MG5G’ calculated Na without+ or MG Na= 5G’ O HO O HO O + without Na+ of MGG’+IL O N N O or MG G’ without Na of MGG’+IL + HO N N N OH 5 MG521without5G’ Da without = Na516/518+ of Na MGG’+IL calculated or MG5 G’ = HO ON HON O + O O 536–539 Da = MG G’ + Na + + HO N N N OH 5withoutMG521536–539 5DaG’ Na without= 516/518+ of Da MGG’+IL = NaMG calculated 5orG’ MG+ Na5 G’= HO N NNHO O + O 536–539 Da = MG5G’+ + Na O HO N N OHN OH O withoutMG521 5DaG’ =without Na 516/518+ of MGG’+IL Na calculated or MG 5 G’= N N + O HOOH O O 536–539 Da = MG5G’ + Na N N N N O withoutMG5G’ without Na+ of MGG’+IL Na or MG 5G’ N N + O HOOH O O 536–539 Da += MG5G’ + Na N O without Na of MGG’+IL HON N O 557.4 Da = 558 Da5 calculated+ = OH 536–539 Da = MG G’ + Na H2N N N NH2 O + HOOH O 536–539557.4MG Da4G Da 2=’ of558 = MGG’+ILMG Da5 G’calculated + Nawithout = H2N N N NH2 O N N 557.4 Da = 558 Da calculated+ = MG G ’ of HOOH O 557.4MG NaDa4G 2 =’ of558 MGG’+IL Da4 calculated2 without = H2N N NH2 + HON N O MGG’ + IL without Na + NH MG557.4Na4G 2’ Daof MGG’+IL= 558 Da calculatedwithout = H2NH N NH2 + 566 Da without Na+, in MGG’ N N 566 Da without Na , in+ MGG’ H NaMG557.4 4G Da2’ of = MGG’+IL558 Da calculated without = H N N H2NN NNH NH2 + + 2 H N N 581.7 Da = 581 calculated566 Da = MG without4G2’ with Na , Na in MGG’in NH N NH2 NaMG557.4+581.7 4G Da2’ ofDa = MGG’+IL558 = 581 Da calculated calculated without = = H2N NHN NH2 + H2N N N HN N N N MGG’+IL 566 Da without Na , in MGG’ NH H N N NH + NaMG+MG 4G24’G of2’ MGG’+ILwith Na+ in without MGG’+IL NH N N 2 588 Da with Na . 588581.7 Da in Da both = 581 MGG’ calculated and+ = H2N N N H N N 566 Da without Na , in MGG’ N N N H N NH + NH2 NHNH 2 581.7Na Da 4 2= 581 calculated+ = H N N NHN N N N MGG’ + IL MG588G Da’ with with Na Na in+.+ 588 MGG’+IL Da in 2 NH2 566 Da without Na , in MGG’ N N NH NH N NH2 + H N NNH NHNH N N MG581.74G2’ Dawith = 581Na calculated in MGG’+IL+ = 2 H 2N N N N NH 566both Da without MGG’ and +Na MGG’+IL, in MGG’ N 2N NH2 NNH2 NH2 588 Da with Na +. 588 Da in H N NHN N NH MG581.74G Da2’ with = 581 Na calculated in MGG’+IL = 2 2 N N N N + N N N NHN2 NH2 588both Da withMGG’ Na and. 588 +MGG’+IL Da in NHH2N NHN NH2 MG581.74G Da2’ with = 581 Na calculated in MGG’+IL = 2 N N N N + N N NH2 both588 MGG’ Da with and Na MGG’+IL.+ 588 Da in H2NHN N NHNH2 MG4G2’ with Na in MGG’+IL 2 N N N N + NH2 both588 Da MGG’ with and Na MGG’+IL. 588 Da in NHH2N N NH2 2 N N + NH2 both588 Da MGG’ with and Na MGG’+IL. 588 Da in H2N N NH2

both MGG’ and MGG’+IL H2N N NH2

Polymers 2018, 10, 22 11 of 19

Polymers 2018, 10, 22 14 of 19 In the case of MG resins several types of glyoxal bridges have been shown to form and link Polymers 2018, 10, 22 12 of 18 melamine moleculesPolymers [13], 2018 such, 10, 22 as the following (Scheme 4). 14 of 19 OH OH H HH Polymers 2018, 10, 22 H 14 of 19 H2N N N N N N Polymers736–738 2018 ,Da 10, 22= 738 Table 6. Cont. N N NH142 of 19 OH OH H OH Polymers 2018, 10, 22 + HHOHH 14 of 19 deprotonated, without Na In N N N N N N H2N N N OHN N N OH H N N NH2 both736–738 MGG’Peak Daand = 738MGG”+IL ChemicalHH Species H NH OHOH OH + H NH OH N deprotonated, without Na In H2N N N HHN N N H 736–738 Da = 738 OH OH N HON NH2 H H2N N N OH NHHHON N OH H both736–738 MGG’ Da and= 738 MGG”+IL + HONNH N O O N N NH2 deprotonated, without Na In NHN N N N OH H2N N N OH N N N OH deprotonated,736–738 Da = 738 without Na+ In N N N HON NH2 both MGG’ and MGG”+IL+ OH N N N N + HONH O HO NHO OH 736–738 Da = 738 deprotonated,deprotonated, without without Na NaIn In N N N N OHN both MGG’ and MGG”+IL NH N HON both MGG’ and MGG”+IL OH NH OH N both MGG’ and MGG”+IL HOH O HO O HO NH HHNH H OH H (a) HO O HO (b) N OH H N N N OH O OH HOOH 2 H N N N N N N HO O HHHO O H H 795–796 Da = without Na+ in OH OH OH H2N N N OH OHOH O N N H N N N N N N + HHN N H N NH both795–796 MGG’ Daand = withoutMGG’+IL Na in OHOH OH OH H OH O H2N N N HHN N N H H NH OH OH N N N OH both MGG’ and MGG’+IL + H NH N 795–796 Da = without Na in H2N N N OH NHHN N OH H H O NHN N N N HON OH + NNH N N N 795–796both+ MGG’ Da = without and MGG’+IL Na in H2NHON N O OH N HON N OH O 795–796 Da = without Na in both MGG’ and N N O N HON N + N N N N OH 795–796 Da = without Na in HONH O OH HO OH O MGG’ + IL both MGG’ and MGG’+IL N N NHO OHN N N N N both MGG’ and MGG’+IL NH HO HONH N HO O O HO NH NH OH HO O 125 HO 125 N 125 125 O HOOH HO O HO M MO OH 993 Da = 992 Calculated + 125 M 124 M 26 993 Da = 992 Calculated + 125 125124 12526 + M 125 + protonatedprotonated with with Na Na, only+, only in in M 125GGMMGMMGM125 M 125 + ++Na 993 Da = 992 Calculated + 1252626 124 26 Na + 125M 125 993 Da = 992 CalculatedMGG’MGG’ + protonated with Na+ , 125 M 993protonated Da = 992 Calculated with Na , only+ in M G124 MGM26 125 + + (c) 26M M(d) Na only in MGG’ + 125 GG M M protonated993 Da = 992 with Calculated Na , only + in M GMM124MGM26 125 + + MGG’ 26125125 5959 125125 Na protonated with Na+, only in M G MGM125 + + MGG’ 26 M G M Na 125 59 125 MGG’ M G M 59 Scheme 4. Type of bridges formed in125 MG resins125 M G M125 125 59 125 124 60 124 M 124 124 60 12426 125M 124 1010 Da = 1008 Da calculated + M 26 + + 124 G 60 M124125GMM Na + They also form 1010and Da link = 1008 very Da branched calculated +resins of structureM G of the124 type124 below+ (Deng et al. 2017) 1010 Da = 1008 Da calculatedprotonated, + protonated, only in MGG’ M12526GM Na 124 60 124 124M G'124 + + 1010 Da = 1008 Da calculated + GGM G M26 GM101 Na (Schemeonly 5). in MGG’ protonated, only in MGG’ 124 M 1010 Da = 1008 Da calculated + 59M 60 124 59 124M124G'+ + protonated, only in MGG’ GGG M 26GGMM 101G' Na 1010 Da = 1008 Da calculated + M59GGG M 59124GM101 + Na+ protonated, only in MGG’ 59 59 M59 GG' GG124 G101 protonated, only in MGG’ 59 59 M 59G' GGG 59101 59 59 H N N NH 59 2 2 H NG N NH 2 59 2 H N N NH 2H2N N NH22 H2N NN NHNH2 2 H2N N NH2 NNHN NN H N N NH H 2 NH 2 H2N N H NH2 H NN NN N N H2N N NH2 1169 Da = 1168 Da calculated, H2N N N H H NHNHN N N N N + H NH N NHNH 1169 Da = 1168 Da calculated,Without WithoutNa+ Na NH HH NH NN N N 1169 Da = 1168 Da calculated, H2N N H N NH H H O 1169 Da = 1168 Da calculated, H2N N N NH N N H H NH H NN NN H HNN + H2N N NH NH H N N H OHN 1169Without Da =+ 1168 Na Da calculated, NH Without Na N 2 N NNH NH N NH N N N O Scheme1169 Da = 5. 1168 +Type Da calculated,of branched Holigomer2N N N N Ns formed NHNin MGN resinsN NH N NH O Without Na NH N N ON HN N N NH N N N NHN 2 N OH O + NH2 N N HN N NH N N Without Na H2N N NHN 2 HN 2 N N OH N NHN2 NH N N N N OH O NH N NH N HON N O 2 N N NHH2N N N NH2 NH HN N N NHN OH NH H2N N NH2 O N 2 NH2 Table 6 and Figure 6a–c, show what occurs when the2 resinN N MGG’ resinO N isN formedNH2 when IL is added. H N N NH HONH N NH NH2 H22N N NH2 2 H2N 2 HO O N N N NH2 HO NH H2N N NH2 H2NH2N 2 In the case of the IL being present aldol condensationHO of the aldehydeN occurs and species in H2N which melamineIn the case is of linked the IL to being aldol presentcondensed aldol aldehydes condensation occur. of For the both aldehyde the resin occurs MGG’ and with species and in withoutInwhich theIn the case ILmelamine condensationcase of theof theis IL linked ILbeing oligomers being to present aldolpresent of condensed melamine–glyoxal aldolaldol condenconden aldehydessationsation and occur.of ofof the melamine–glyoxal–glutaraldehyde the Foraldehyde aldehyde both the occurs resin occurs andMGG’ andspecies with species inand in are present.In the caseThus of species the IL such being as presentthe following aldol condenare the sationresult (Schemeof the aldehyde 6). occurs and species in whichwhich withoutmelamine melamine IL condensationis linkedis linked to to oligomersaldol aldol condensed condensed of melamine–glyoxal aldehydes occur. occur. and ofFor Formelamine–glyoxal–glutaraldehyde both both the theresin resin MGG’ MGG’ with withand and which melamine is linked to aldol condensed aldehydes occur. For both the resin MGG’ with and withoutwithoutare IL present. condensation IL condensation Thus species oligomers oligomers such as of theof melamine–glyoxal melamine–glyoxalfollowing are the result and and of (Scheme of melamine–glyoxal–glutaraldehyde melamine–glyoxal–glutaraldehyde 6). arewithout present. IL condensation Thus species sucholigomers as the of following melamine–glyoxal are the result and (Scheme of melamine–glyoxal–glutaraldehyde 6). are present.are present. Thus Thus species species such such as as the the following following are thethe result result (Scheme (Scheme 6). 6).

(a) Figure 6. Cont.

Polymers 2018, 10, 22 13 of 18 Polymers 2018, 10, 22 12 of 19

(b)

(c)

FigureFigure 6. 6. MALDI-ToFMALDI-ToF spectrum spectrum of the of formation the formation of the MGG’ of the resin MGG’ in the resin presence in the of N presence-methyl-2- of − pyrrolydoneN-methyl-2-pyrrolydone hydrogen sulphate hydrogen ([HNMP] sulphate HSO ([HNMP]4−) ionic liquid: [HSO4 (a])) ionic20–200 liquid: Da range. (a) 20–200 (b) 200–500 Da range. Da range.(b) 200–500 (c) 500–800 Da range. Da range. (c) 500–800 Da range.

In the case ofTable the IL 6. beingOligomers present formed aldol by condensation the reaction of of MGG’ the aldehyde with and without occurs andIL. species in which melamine is linked to aldol condensed aldehydes occur. For both the resin MGG’ with and without IL condensation oligomersPeak of melamine–glyoxal and of melamine–glyoxal–glutaraldehydeChemical species are present. Thus species such as the following are the result (Scheme6). Polymers149–151 2018 Da, 10 ,= 22 Melamine + 23 15 of 19 Da (Na+) Owith andO without IL HO HO 159HO Da = G + G’ byOH aldol OH OH N N N H NNHN H O 2 H2NNHN condensationO with and N NNH2 N NNH N N O without IL. Small with IL NNOH H NNOH H NN NH NNOH 184–185 Da = M(Glyox)1 NH2 NH2 HO O NH2 NH2 without Na+ in MGG’ with IL

H2N N NH2 197.8 Da = [HNMP] HSO4−

(only present in MGG’ with N N NH IL) H H H2N N N 234 Da = aldol condensation of N N NH2 + N N glyoxal + Na exists only in NH N N

MGG’+IL NH2 N N NH2 331.5 Da = 328.6 Da H2N ON NH2 OH OH O glutaraldehyde+IL by aldol Scheme 6. Oligomers types formed in MG resins condensation occurs butScheme is 6. OligomersH types formed in MG resins. H OH OH relativelyDue exclusively small in toMGG’+IL the condensation of melamine and glyoxal these species are present together with species on which glutaraldehyde is definitely grafted on. Some of these are (Scheme 7):

OH H OH HHH H2N N N N N N N N NH2 OH N N OH N N N N NH NH N HO HO O HO O OH

H2N N NH2 H2N N NH2 N N N N NH H NH H H H2N N N H H N N N N N N N N O NH N N HN N N OH NH2 N N NH N O N NH2 NH H2N N NH2 2 HO N H2N Scheme 7. Oligomers formed in MGG’ resins showing reaction of glutaraldehyde and glyoxal

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).

Polymers 2018, 10, 22 15 of 19

O O HO HO HO OH OH OH N N N H NNHN H O 2 H2NNHN O N NNH2 N NNH N N O NNOH H NNOH H NN NH NNOH

NH2 NH2 HO O NH2 NH2

H2N N NH2

N N

NH H H H2N N N N N NH2 N N NH N N

NH2 N N NH2

H2N N NH2 Polymers 2018, 10, 22 14 of 18 Scheme 6. Oligomers types formed in MG resins

Due Dueexclusively exclusively to the to condensation the condensation of melamine of melamine and glyoxal and glyoxal these these species species are present are present together together withwith species species on which on which glutaraldehyde glutaraldehyde is defini is deﬁnitelytely grafted grafted on. Some on. Some of these of these are (Scheme are (Scheme 7): 7):

OH H OH HHH H2N N N N N N N N NH2 OH N N OH N N N N NH NH N HO HO O HO O OH

H2N N NH2 H2N N NH2 N N N N NH H NH H H H2N N N H H N N N N N N N N O NH N N HN N N OH NH2 N N NH N O N NH2 NH H2N N NH2 2 HO N H2N

SchemeScheme 7. Oligomers 7. Oligomers formed formed in MGG’ in MGG’ resins resins showing showing reaction reaction of glutaraldehyde of glutaraldehyde and andglyoxal glyoxal.

Where a glutaraldehyde is linked to a melamine of a MG resin skeleton. 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 Also species in which an aldehyde that has undergone aldol condensation has reacted and with the melamine are present in the MGG’+IL resins. For example the peak at 356–357 Da (333 + 23 linked with the melamine are present in the MGG’ + IL resins. For example the peak at 356–357 Da Da of Na+) in Figure 6 represent just one of these species issued by the aldol condensation of three (333 + 23 Da of Na+) in Figure6 represent just one of these species issued by the aldol condensation of glyoxals linked to a melamine molecule. Aldehydes react readily with melamine, but when a 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 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 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 glyoxal units have condensed by aldol condensation for subsequently the residual aldehyde group to to react with melamine; or (ii) one glyoxal molecule has reacted with melamine and the other glyoxals 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). havePolymers then 2018 reacted, 10, 22 on the residual aldehyde function by aldol condensation (Scheme8). 16 of 19

OH OH OH O

H2N N NH H OH OH OH NN

NH2

SchemeScheme 8 8. . Reaction of a glyoxal aldol condensatecondensate onon melamine.melamine.

This is not the only case occurring in the MGG’ + IL resin case, as for example the 452–453 Da 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 peak in Figure6, this being a species formed by aldol condensation of three glutaraldehydes linked to to a melamine (Scheme 9). a melamine (Scheme9). OH OH OH O

H2N N NH H OH OH NN

NH2 Scheme 9. Reaction of a glutaraldehyde aldol condensate on melamine.

3.3. 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].

Table 7. FTIR spectra of hardened MG and MGG’ resins.

Wavelength/cm−1 Group 812 triazine bending vibration 1074.3 C–O stretching vibration 1235 C–O–C (ether group) stretching vibration 1460.1 Methylene C–H bending vibration 1543 Triazine C=N bending vibration 1550.8 Triazine C=N bending vibration 1678 C=O(carbonyl) stretching vibration 1735.9 C=O(ketone) stretching vibration 2739 C–H stretching vibration 2962.7 C–H(aldehyde) stretching vibration 3100–3200 N–H, O–H stretching vibration

Polymers 2018, 10, 22 16 of 19

OH OH OH O

H2N N NH H OH OH OH NN

NH2 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

Polymerspeak in2018 Figure, 10, 226, this being a species formed by aldol condensation of three glutaraldehydes linked15 of 18 to a melamine (Scheme 9).

OH OH OH O

H2N N NH H OH OH NN

NH2 Scheme 9. Reaction of a glutaraldehyde aldol condensate on melamine.

3.3. Analysis of Reactions by Fourier Transform InfraredInfrared (FTIR)(FTIR) SpectrometrySpectrometry FTIR analysis of the hardened MG and MGG’ re resinssins (Figure 77 andand TableTable7 )7) shows shows all all the the main main groups of the melamine and reacted glyoxal. These are listed in Table 6 6.. TheThe moremore interestinginteresting featurefeature is the existenceexistence ofof thethe peakpeak atat 12341234 cmcm−−1 ofof C–O–C C–O–C stretching. Its Its presence indicates that substituted methylene ether-likeether-like bridgesbridges (–CHR–O–CHR–)(–CHR–O–CHR–) are are formed formed in in the the case case of of MG MG resins resins just just as as they they form form in melamine–formaldehydein melamine–formaldehyde resins resins (MF, (MF, –CH2–O–CH2–). –CH2–O–CH2–). These These substituted substituted methylene methylene ether-like ether-like bridges (–CHR–O–CHR–)bridges (–CHR–O–CHR–) clearly rearrange clearly rearrange to substituted to subs ethylenetituted bridges ethylene (–CHR–CHR–) bridges (–CHR–CHR–) by the elimination by the elimination of water as already shown in a previous study [13]. ofPolymers water 2018 as,already 10, 22 shown in a previous study [13]. 17 of 19

Table 7. FTIR spectra of hardened MG and MGG’ resins.

Figure 7. FT-IR spectra of hardened MGMG andand MGG’MGG’ resins.resins.

In Figure 8 there is Tablea comparison 7. FTIR spectra of the of FTIR hardened spectra MG andof IL, MGG’ MG+IL resins. 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 shownWavelength/cm in Figure 7. The−1 probable presence ofGroup 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 812 triazine bending vibration Na+) and 379 Da (with Na+)1074.3 in Figure 6 which correspond C–O stretching to the vibrationtwo structures below (Scheme 10). 1235 C–O–C (ether group) stretching vibration 1460.1 Methylene C–H bending vibration 1543 Triazine C=N bending vibration 1550.8 Triazine C=N bending vibration 1678 C=O(carbonyl) stretching vibration 1735.9 C=O(ketone) stretching vibration 2739 C–H stretching vibration 2962.7 C–H(aldehyde) stretching vibration 3100–3200 N–H, O–H stretching vibration

In Figure8 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 Figure7. The probable presence of ethers in the structure is supported

Figure 8. FTIR spectra of IL, MG+IL resin and MGG’+IL resin.

OH H H

H2N N NH N NNH2 H2N N NH N NNH2 O O NNOH OH NN NNOH OH NN

NH NH NH NH 2 2 2 2 Scheme 10. Glyoxal ether bridges formed by water elimination between two melamine molecules.

3.4. 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):

Polymers 2018, 10, 22 17 of 19 Polymers 2018, 10, 22 17 of 19

Figure 7. FT-IR spectra of hardened MG and MGG’ resins. Figure 7. FT-IR spectra of hardened MG and MGG’ resins. Polymers 2018, 10, 22 16 of 18 In Figure 8 there is a comparison of the FTIR spectra of IL, MG+IL and MGG’+IL. Other than smallIn peaks Figure corresponding 8 there is a comparison to the addition of the of theFTIR sm spectraall amounts of IL, of MG+IL IL the twoand resinMGG’+IL. spectra Other show than the + alsosamesmall by peaks the existence ascorresponding shown of in MALDI-ToF Figure to the 7. additionThe peaks probable of such the presence assm thoseall amounts atof 395ethers Daof inIL (371 the + structuretwo 23 Daresin Na isspectra )supported and atshow 355 also the Da + + (withoutbysame the peaks existence Na as) shown and of MALDI-ToF 379 in DaFigure (with peaks7. NaThe such )probable in as Figure those presence6 atwhich 395 Daof correspond ethers(371 + 23in Dathe to Nastructure the+) twoand at structuresis 355 supported Da (without below also + (SchemebyNa the+) and existence 10 379). Da of (with MALDI-ToF Na+) in Figure peaks 6such which as thosecorrespond at 395 Dato the(371 two + 23 structures Da Na ) and below at 355 (Scheme Da (without 10). Na+) and 379 Da (with Na+) in Figure 6 which correspond to the two structures below (Scheme 10).

Figure 8. FTIR spectra of IL, MG+IL resin and MGG’+IL resin. FigureFigure 8. 8.FTIR FTIRspectra spectraof ofIL, IL,MG MG+IL + IL resin resin and and MGG’+IL MGG’+IL resin. resin. OH H H OH H H H2N N NH N NNH2 H2N N NH N NNH2 O O H2N N NH N NNH2 H2N N NH N NNH2 O O NNOH OH NN NNOH OH NN NNOH OH NN NNOH OH NN NH NH NH NH 2 2 2 2 NH NH NH NH 2 2 2 2 Scheme 10. Glyoxal ether bridges formed by water elimination between two melamine molecules. Scheme 10. Glyoxal ether bridges formed by water elimination between two melamine molecules. 3.4. Summary of Effects 3.4. Summary of Effects Having identified some of the more relevant of the oligomers that were found and shown that melamineHaving and identiﬁedidentified glyoxal dosome react of theand more form relevantresins which of the also oligomers cross-link that it were is of foundinterest and to shownsummarize that melaminewhat is then and the glyoxal role of do ionic react liquids. and form resins which also cross-link it is of interest to summarizesummarize what is then thethe rolerole ofof ionicionic liquids.liquids. 1. IL appears to catalyse the reaction of aldehydes, and aldehydes pre-reacted with urea or 1. IL appears to catalyse the reaction of aldehydes, and aldehydes pre-reacted with urea or 1. melamine,IL appears to catalyseyield aldol the condensation. reaction of aldehydes, It allows and aldol aldehydes condensation pre-reacted even of with aldehydes urea or melamine, that have beenmelamine, pre-reacted to yield with aldol melamine condensation. or even It wiallowsth wood aldol lignin. condensation An example even (Scheme of aldehydes 11): that have beento yield pre-reacted aldol condensation. with melamine It allowsor even aldol with condensationwood lignin. An even example of aldehydes (Scheme that 11): have been pre-reacted with melamine or even with wood lignin. An example (Scheme 11): Polymers 2018, 10, 22 18 of 19

O CH CHO

CHO H2NNCN CHO H2NNCHN CHO + CHO OH OH NN NN

NH2 NH2 Scheme 11. Example of aldol condensation on an aldehyde prereacted withwith melamine.melamine

Or even with wood lignin [[22–23]22,23] (Scheme(Scheme 12 12).).

Scheme 12. Example of aldol condensation on an aldehyde prereacted with lignin

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 [22–24] 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.

4. 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.

Acknowledgments: The LERMAB of the University of Lorraine is supported by a grant overseen by the French National Research Agency (ANR) as part of the Laboratory of Excellence (Labex) ARBRE. The first author thanks the China Scholarship Council for the study bursary granted to him.

Author Contributions: Xuedong Xi and Siham Amirou contributed equally to the preparation of the MGG’ resin and the ionic liquid and the performance of the MALDI-ToF analysis. Antonio Pizzi analyzed and interpreted all the MALDI-ToF spectra, and deduced from the data all the reactions occurring and wrote the paper.

Conflicts of Interest: The authors declare no conflict of interest.

Polymers 2018, 10, 22 18 of 19

O CH CHO

CHO H2NNCN CHO H2NNCHN CHO + CHO OH OH NN NN

NH2 NH2 Scheme 11. Example of aldol condensation on an aldehyde prereacted with melamine Polymers 2018, 10, 22 17 of 18 Or even with wood lignin [22–23] (Scheme 12).

SchemeScheme 12.12. Example ofof aldolaldol condensationcondensation onon anan aldehydealdehyde prereactedprereacted withwith lignin.lignin

This latter opens up the possibility of reaction with the lignin in the wood substrate, and thus of This latter opens up the possibility of reaction with the lignin in the wood substrate, and thus some covalent bonding between an aldehyde based adhesive and the substrate. While to advance 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 such a hypothesis is possibly premature, it might also contribute to explain the good bonds obtained with aldehyde adhesives catalysed by ionic liquids. with aldehyde adhesives catalysed by ionic liquids. 2. IL catalyses the hardening of melamine–aldehyde resins to decrease hardening temperature, 2. energyIL catalyses of activation the hardening of the hardening/condensati of melamine–aldehydeon resinsreaction to and decrease thus equally hardening to improve temperature, their performanceenergy of activation at equal of temperature. the hardening/condensation reaction and thus equally to improve their 3. Moreover,performance IL ataffects equal temperature.the wood substrate by demethylation of lignin [22–24] rendering the 3. substrateMoreover, even IL affects more the receptive wood substrate to any type by demethylationof adhesion. of lignin [22–24] 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. However, it cannot be excluded that also other effects are induced by the presence of ILs. 4. Conclusions 4. Conclusions Melamine–glyoxal–glutaraldehyde (MGG’) resins which use no formaldehyde have been shown to beMelamine–glyoxal–glutaraldehyde capable of bonding interior and exterior (MGG’) grade resins whichplywood use under no formaldehyde industrially havesignificant been shown press toconditions be capable when of bonding an ionic interior liquid andis used exterior as th gradee hardener. plywood Small under proportions industrially of signiﬁcant glutaraldehyde press conditionsimproved the when water an ionic resistance liquid isofused the panel as the bonds hardener. obtained. Small proportionsThese resins of showed glutaraldehyde hardening improved in two thesteps water due resistanceto the different of the panelreactivity bonds of obtained. the two Thesealdehydes resins used. showed The hardening role of the in ionic two steps liquid due was to thefundamental different reactivity in the performance of the two aldehydesobtained, its used. role The being role not of just the ioniclimited liquid to the was function fundamental of hardener in the performancebut also: (i) to obtained, decrease its markedly role being the not energy just limited of acti tovation the function of hardening of hardener of the but MGG’ also: (i)resins; to decrease (ii) to markedlyfavour some the limited energy aldol of activation condensation of hardening of the alde of thehydes MGG’ in the resins; hardening (ii) to favour of the someMGG’ limited resins, aldoleven condensationon aldehyde ofsites the aldehydespre-reacted in with the hardening melamine; of the(iii) MGG’ to cause resins, demethylation even on aldehyde of lignin sites pre-reacted and thus withrendering melamine; the wood (iii) substrate to cause demethylationmore prone to adhesion of lignin [23,24]; and thus and rendering (iv) to favour the wood possibly substrate the reaction more proneof aldehyde to adhesion groups [23 in,24 the]; andresin (iv) with to favourthe lignin possibly of the the substrate. reaction of aldehyde groups in the resin with the lignin of the substrate. Acknowledgments: The LERMAB of the University of Lorraine is supported by a grant overseen by the French Acknowledgments:National Research AgencyThe LERMAB (ANR) as of part the of University the Laborato of Lorrainery of Excellence is supported (Labex) by ARBR a grantE. overseen The first author by the Frenchthanks National Research Agency (ANR) as part of the Laboratory of Excellence (Labex) ARBRE. The ﬁrst author thanks the China Scholarship Council for the study bursary granted to him. the China Scholarship Council for the study bursary granted to him. AuthorAuthor Contributions: Xuedong Xi and Siham Amirou contributed equally to the preparation of the MGG’ resin and thethe ionicionic liquidliquid andand the the performance performance of of the the MALDI-ToF MALDI-ToF analysis. analysis. Antonio Antonio Pizzi Pizzi analyzed analyzed and and interpreted interpreted all theall the MALDI-ToF MALDI-ToF spectra, spectra, and and deduced deduced from from the the data data all theall the reactions reactions occurring occurring and and wrote wrote the paper.the paper.

ConﬂictsConflicts ofof Interest:Interest: TheThe authorsauthors declaredeclare nono conﬂictconflict ofof interest.interest.

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