Acid Ionic Liquids As a New Hardener in Urea-Glyoxal Adhesive Resins

polymers

Article Acid Ionic Liquids as a New Hardener in Urea-Glyoxal Adhesive Resins

Hamed Younesi-Kordkheili 1,* and Antonio Pizzi 2

1 Department of Wood and Paper Sciences and Technology, Faculty of Natural Resources, Semnan University, Semnan 35131-19111, Iran 2 LERMAB-ENSTIB, University of Lorraine, Epinal 88000, France; [email protected] * Corresponding: [email protected]; Tel.: +98-911-355-4324; Fax: +98-233-362-6299

Academic Editor: Frank Wiesbrock Received: 23 January 2016; Accepted: 17 February 2016; Published: 24 February 2016

Abstract: The effect of acidic ionic liquid (IL) as a new catalyst on the properties of wood-based panels bonded with urea-glyoxal (UG) resins was investigated. Different levels of N-methyl-2-pyrrolidone hydrogen sulfate ([HNMP] HSO4 (0, 1, 2, 3 wt %)) were added to prepared UG resin. The resin was then used for preparing laboratory particleboard panels. Then, the properties of the prepared panels were evaluated. The structure of the prepared UG resin was studied by 13C NMR, and thermal curing behavior of the resin before and after the addition of IL was measured by DSC. Additionally, the main oligomers formed in the UG reaction were identified by matrix-assisted laser desorption/ionization time-of-flight (MALDI TOF) mass spectroscopy. The results indicated that IL can be used as an efficient catalyst for UG resin. The physicochemical tests indicated that the addition of [HNMP] HSO4 from 0 to 3 wt % decreased the pH value of the glue-mix, and the pH decreased on curing to the same level as urea-formaldehyde resins. The gel accelerated with increasing catalyst content and with the decreasing of the pH in the UG resin. The panels prepared with IL had higher mechanical strength and dimensional stability compared to those made from UG resins containing NH4Cl. Scanning electron microscope (SEM) micrographs showed that the panels prepared with ionic liquid presented low porous. DSC analysis showed that the addition of IL to the UG resin decrease the energy of activation of the curing reaction to render possible cross-linking. The MALDI TOF results indicated a preponderant linearity of the oligomers formed, implying a high energy of activation of curing for UG resins.

Keywords: acidic ionic liquid; urea-glyoxal resin; catalyst; MALDI TOF; 13C NMR; DSC

1. Introduction With the introduction of a non-toxic aldehyde such as glyoxal as a substitute of formaldehyde, urea-glyoxal (UG) resins can be produced as green wood adhesives. Contrary to urea formaldehyde (UF) resins, UG resins are eco-friendly and present lower water absorption and thickness swelling [1,2]. Hence, UG as green adhesive seems to have good potential in wood-based panels manufacturing in the near future. UG resins as other thermoset aminoplastic resins need an acid medium to polymerize and cross-link [1]. It is for this reason that, in the previous studies, ammonium chloride (NH4Cl) was used as the usual acid catalyst in the preparation of panels bonded with UG resins [1,2]. So far, several research works were carried out on the importance of the use of catalysts in aminoplastic thermoset resins [3,4]. Among several types of catalysts, ammonium salts (such as NH4Cl and NH3SO4) are one of the most important and effective types of catalysts used for accelerating resin curing. Compared to other types of catalysts, NH4Cl is cheap, effective, free of annoying byproducts, and easy to use. The

Polymers 2016, 8, 57; doi:10.3390/polym8030057 www.mdpi.com/journal/polymers Polymers 2016, 8, 57 2 of 14

influence of NH4Cl on resin curing involves the release of hydrochloric acid (HCl), which brings the pH to very low values and speeds up the resin cure rate. In spite of the several advantages of NH4Cl as a thermoset resin catalyst, it also presents some disadvantages: (I) several bubbles form in the glue line due to the decomposition of NH4Cl in the hot press and consequent emission of ammonia gas, which reduces the mechanical strength of the resin and of the panels bonded with it; (II) furthermore, chlorine (Cl) is a toxic element and raises concerns of environmental pollution, especially in the case of green adhesives such as UG resins; (III) the residual acid left in the glue line as a consequence of the addition of an acid catalyst can increase the degradation of the cured resin in particleboards [3] as well as causing degradation by acid hydrolysis of the wood carbohydrates at the interface wood/adhesive. Thus, due to the defects of NH4Cl, and the green nature and low mechanical strength of the UG resins, it is imperative to find a nontoxic and eco-friendly high-quality catalyst. Conversely, ionic liquids (ILs), which have been widely promoted as “green solvents,” are attracting much attention for applications in many fields of chemistry and industry due to their chemical stability, thermal stability, low vapor pressure and high ionic conductivity properties [5]. Over the last few years, ILs have been popularly used as solvents for organic synthesis and also have been used as media for extraction processes [5]. Previous studies have indicated that ILs can also be used as an efficient catalyst in the synthesis of various materials. Among different types of ionic liquids used as catalysts, Brønsted acidic ionic liquids such as N-methyl-2-pyrrolidone hydrogen ´ sulfate ([NMP]+HSO4 ) has been reported as being most effective in terms of short reaction time and ´ with excellent yields of the desired product [6]. Wang et al. (2011) indicated that ([NMP]+HSO4 ) can be used as a substitute of various types of homogenous and heterogeneous catalysts due to its special properties such as being green, of low viscosity, non-volatile, recyclable and inducing no corrosion [7]. Additionally, Brønsted acidic ionic liquids are emerging as most promising alternatives for a wide variety of acid-catalyzed reactions, as they have useful characteristics of both solid acids and mineral liquid acids, and they can be designed to replace customary mineral liquid acids such as sulfuric acid and hydrochloric acid in chemical processes [6]. Regarding the type and the content of the catalyst directly affecting resin curing and the performances of final products [3], the aim of this study was the investigation of the effect of Brønsted acidic ionic liquids as a new resin catalyst on the physical and mechanical properties of the wood-based panels bonded with UG resins. It is important to note that the acid part of the IL obtained by its hydrolysis due to the increased temperature during hot pressing is the accelerator of resin hardening. The added advantage is that, as the hardened glue line cools, since none of the two parts of the IL are volatile, the almost neutral IL forms again, bringing the pH of the hardened adhesive to almost neutral again. The period of acid contact with the wood surface is very short, thus minimizing any hydrolysis damage to the wood surface carbohydrates [8]. The resultant strength of the panel is then unaffected.

2. Materials and Methods

2.1. Preparation of UG Resin The UG resin was prepared according to the method of Deng et al. (2014) [1]. Glyoxal (40%) was placed in the reactor, and the reaction pH was adjusted to 4–5 with a 30% NaOH solution. Subsequently, the ﬁrst urea (U1) was added, and the mixture was then heated to 75 ˝C for 2 h. Then, the second urea (U2) was added, and the reaction mixture became a weak acid at pH 4–5 for 1 h at 75 ˝C. Lastly, the reaction mixture became a weak alkaline at pH 7.5 and cooled to room temperature ready for use. The U1/U2 weight ratio was 2:1. UG resins used for this study were prepared at G/U molar ratio of 1.4:1.

2.2. [HNMP][HSO4] Synthesis

[HNMP] HSO4 was synthesized according to the method of Wang et al. 2011 [7]. 1.0 mol N-methyl pyrrolidone was weighed and placed into a 250 mL three-neck ﬂask, and 1.0 mol sulfuric acid was added dropwise into the 250 mL three-neck ﬂask with a dropping funnel under continuous mechanical Polymers 2016, 8, 57 3 of 14

Polymers 2016, 8, 57 3 of 14 stirring by magnetic stirrer at room temperature. A considerable amount of white smoke appeared appearedduring the during dropwise the dropwise addition addition of acid. Theof acid. mixture The mixture became became a sticky, a lightsticky, yellow light yellow transparent transparent liquid ˝ liquidafter stirring after stirring at 80 Cat for80 °C 4 h. for The 4 h. resultant The resultant liquid liquid [HNMP] [HNMP] HSO4 HSOwas4 washed was washed three three times times with etherwith etherand ethyl and acetate,ethyl acetate, respectively, respectively, and was and dried was underdried vacuumunder vacuum to remove to remove the ether the and ether ethyl and acetate. ethyl acetate.The schematic The schematic synthesis synthesis of [HNMP] of [HNMP] HSO4 is HSO shown4 is inshown Figure in1 .Figure 1.

H 80°C N CH3 - + H 2 SO 4 N CH3 HSO4

O O

Figure 1. Synthesis of [HNMP] HSO4..

2.3. Addition of Catalysts to UG Resin

Different proportions of [HNMP] HSO4 (0, 1, 2, 3 wt %) were added to prepare the UG resin at Different proportions of [HNMP] HSO4 (0, 1, 2, 3 wt %) were added to prepare the UG resin at room temperature. To compare the performance of the acidic ionic liquid with NH4Cl, a UG resin room temperature. To compare the performance of the acidic ionic liquid with NH4Cl, a UG resin containing 2 wt % NH4Cl (20% solution) was prepared as a control sample. These resins were then containing 2 wt % NH4Cl (20% solution) was prepared as a control sample. These resins were then immediately used to prepare the woodwood panels.panels.

2.4. Measurements of the pH and Gel Time The pH values values of of the the resins resins at at both both ambient ambient an andd elevated elevated temperatures temperatures were were measured measured with with a Hannaa Hanna pH pH meter meter (Limena, (Limena, Italy). Italy). Before Before each each me measurement,asurement, the the pH pH meter meter was was calibrated with standardized buffer solutions at pHs of 4 and 7. After calibration, 200 g of a UG sample was pipetted into a 250-mL beaker, and the initial pH values of the resin and the solution, adjusted by the gradual addition of [HNMP] HSO4 fromfrom 0% 0% to to 3% 3% (solids (solids on on solids), solids), we werere recorded as ambient pH after 5 min of magneticmagnetic stirringstirring at at 20 20˝C. °C. For For measuring measuring pH atpH hot at condition, hot condition, the temperature the temperature of UG resin of UG increased resin increasedto about 50 to˝ aboutC (before 50 °C gelation (before state gelation occurred), state occu andrred), ILs were and then ILs were added then to the added resin. to pHthe valueresin. waspH valuemeasured was measured at this temperature at this temperature as hot pH. as In hot order pH. to In determine order to determine the gel time, the fivegel time, grams five of grams the resin of werethe resin introduced were introduced into a dry into glass a dry beaker. glass Then, beaker. aqueous Then, acidic aqueous ionic acidic liquid ionic was liquid added was to the added resin. to Thethe resin.beaker The was beaker then immersed was then inimmersed boiling water, in boiling and thewater, time and until the hardening time until was hardening measured was according measured to accordingthe Younesi-Kordkheili to the Younesi-Kordkheiliet al. (2015) method et al. (2015) [9]. Two method replicates [9]. forTwo each repl sampleicates for were each made. sample were made. 2.5. DSC Analysis 2.5. DSCThe Analysis changes in enthalpy and curing temperature of the resins were determined using the thermal analyzerThe NETZSCHchanges in DSCenthalpy 200 F3 and model. curing The temperature DSC scans were of the recorded resins at were a heating determined rate of 10using˝C/min the thermalunder nitrogen analyzer atmosphere NETZSCH with DSC a flow200 F3 rate model. of 60 cmThe3/min. DSC scans To determine were recorded the curing at atemperature heating rate ofofthe 10 °C/minresins, about under 5 mgnitrogen of freeze-dried atmosphere sample with was a addedflow rate to the of aluminum60 cm3/min. pan. To Additionally, determine 2%the ILscuring was temperatureadded to the preparedof the resins, resins about (based 5 onmg dry of weight)freeze-dried before sample measuring was the added curing to temperaturethe aluminum of resins. pan. Additionally,Then, the samples 2% ILs were was heated added from to ambientthe prepared temperature resins (based (25 ˝C) on to dry 200 ˝weight)C under before nitrogen measuring atmosphere. the curing temperature of resins. Then, the samples were heated from ambient temperature (25 °C) to 13 2002.6. °C CP-MAS under nitrogenC NMR atmosphere. Analysis Solid-state cross-polarization/magic angle spinning 13C nuclear magnetic resonance (CP-MAS 13 132.6.C CP-MAS NMR spectrum C NMR of Analysis the hardened resin after grinding to a very fine powder was recorded on a BrükerSolid-state MSL 300 cross-polarization/magic spectrometer at a frequency angle of 75.47spinning MHz. 13C Chemical nuclear shiftsmagnetic were resonance assigned relative(CP-MAS to 13tetramethylC NMR spectrum silane (TMS). of the Thehardened rotor was resin spun after at grinding 4 kHz on to a doublea very bearingfine powder 7-mm was Brüker recorded probe. on The a ˝ Brükerspectrum MSL was 300 acquired spectrometer with 5 at s recyclea frequency delay, of a 75.47 90 pulse MHz. of Chemical 5 µs, and shifts a contact were time assigned of 1 ms, relative and the to tetramethylnumber of transients silane (TMS). was 3000.The rotor was spun at 4 kHz on a double bearing 7-mm Brüker probe. The spectrum was acquired with 5 s recycle delay, a 90° pulse of 5 µs, and a contact time of 1 ms, and the number of transients was 3000.

2.7. MALDI-TOF-MS Analysis The samples were dissolved in acetone (5 mg/mL). 2,5-dihydroxy benzoic acid was used as the matrix. For the enhancement of ion formation, 0.1 M NaCl was added to the matrix. The solutions of

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2.7. MALDI TOF-MS Analysis The samples were dissolved in acetone (5 mg/mL). 2,5-dihydroxy benzoic acid was used as the matrix. For the enhancement of ion formation, 0.1 M NaCl was added to the matrix. The solutions of the sample and matrix were mixed in equal amounts, and 1.5 µL of the resulting solution was placed on the MALDI target. After evaporation of the solvent, the MALDI target was introduced into the spectrometer. The spectra were recorded on a MALDI TOF instrument (AXIMA Performance, Shimadzu, Manchester, UK). The irradiation source was a pulsed nitrogen laser with a wavelength of 337 nm. The duration of a single laser pulse was 3 ns. The measurements were carried out using the following conditions: polarity-positive, ﬂight path-linear, mass-high (20 kV acceleration voltage), and 100–150 pulses per spectrum. The delayed extraction technique was used by applying delay times of 200–800 ns.

2.8. Particleboard Manufacturing The manufacturing of the particleboards was performed according to the method of Younesi-Kordkheili et al. (2015) [10]. A forming mold frame (35 cm ˆ 35 cm ˆ 1 cm) was placed on a stainless steel caul plate to control thickness. The surfaces of each plate were sprayed with a mineral oil releasing agent to ease demolding of the panel after hot pressing. Dried particles were then blended with the prepared resins in a rotating drum-type mixer ﬁtted with a pneumatic spray gun. Then, compounded material was poured into the frame and spread to ﬁll the frame evenly. When forming was complete, the top caul plate was placed on the top of the mat, and the entire assembly was placed into an oil-heated press that was used for compression molding. The temperature of the press plates was maintained at 180 ˝C at a maximum pressure of 25 bar pressure. The pressing of the compound material was carried out respectively in two stages by hot and cold presses for 5 min. A 5 cm wide edge of each board was trimmed to remove the low density and poor bonding areas of the boards [11]. To ensure reproducibility, three panels were manufactured for each formulation. The nominal thickness and density of manufactured panels was 16 mm and 0.8 g/cm3, respectively. Preparation of the samples for mechanical tests and water absorption measurements was started 24 h after pressing.

2.9. Panel Testing All physical and mechanical tests of the particleboards prepared were carried out to the appropriate standard methods. The tests performed on the specimens were internal bond strength (IB-tensile strength perpendicular to the panel plane) [12], static bending [bending strength and modulus of elasticity (MOE)] [13] and water absorption and thickness swelling [14]. The samples were conditioned at a temperature of 23 ˘ 2 ˝C and a relative humidity of 60% ˘ 5% for two weeks. Five specimens were tested for each panel.

2.10. Scanning Electron Microscopy (SEM) The morphology of the particleboards was examined using a scanning electron microscope (XL30) supplied by Semtech Limited (Neuchatel, Switzerland). The fracture surfaces of the specimens after the bending test were sputter-coated with gold before analysis. All images were taken at an accelerating voltage of 17 kV.

3. Results and Discussion

3.1. Effect of the [HNMP] HSO4 Content on the pH and Gel Time of the UG Resin

The pH value of the UG resin decreased with increasing [HNMP] HSO4 content from 0% to 3%, as shown in Figure2. Figure2a showed that decrease in the pH value was initially very quick, and the slope of curve then became less sharp with increasing catalyst content. The effect of [HNMP] HSO4 on Polymers 2016, 8, 57 5 of 14

+ 2´ 2´ UG-resin curing is to release H and HSO4 . With an increasing HSO4 concentration in the system, 2´ the rate of HSO4 release is retarded. Thus, the pH decreases very quickly in the beginning and then morePolymers slowly 2016, 8, (Figure 57 2a). 5 of 14

(a)

(b)

Figure 2. pH value of the UG resin versus the acidic ionic catalyst content at (a) ambient temperature (b) hothot temperature.temperature.

According to Figure 2a, the curve of the UG resin containing 0%, 1%, 2% and 3% catalyst shows According to Figure2a, the curve of the UG resin containing 0%, 1%, 2% and 3% catalyst shows remarkable differences in pH values. This seems to contradict the previous findings about the remarkable differences in pH values. This seems to contradict the previous findings about the influence influence of NH4Cl on UF resins. Previous studies have indicated that the pH value of UF resin of NH Cl on UF resins. Previous studies have indicated that the pH value of UF resin decreases with decreases4 with catalyst addition very quickly at the beginning, but the changes in the pH value catalyst addition very quickly at the beginning, but the changes in the pH value become smaller with become smaller with increases in the catalyst content [4]. This finding indicates that, compared to increases in the catalyst content [4]. This finding indicates that, compared to NH4Cl, the use of acidic NH4Cl, the use of acidic ionic liquid as a catalyst can decrease the pH value uniformly and ionic liquid as a catalyst can decrease the pH value uniformly and continuously. Additionally, Figure2b continuously. Additionally, Figure 2b indicated that increasing ILs from 0% to 3% decreased the pH indicated that increasing ILs from 0% to 3% decreased the pH of resin at a hot temperature. This finding of resin at a hot temperature. This finding gives important information about the behavior of UG gives important information about the behavior of UG resin under a hot pressing condition for wood resin under a hot pressing condition for wood panel manufacturing. According to Figure 2b, the pH panel manufacturing. According to Figure2b, the pH of the resin is still higher than 3.8, even with the of the resin is still higher than 3.8, even with the addition of 3% catalyst. This is comparable to the pH addition of 3% catalyst. This is comparable to the pH obtained by catalyzed urea-formaldehyde resins, obtained by catalyzed urea-formaldehyde resins, and it ensures that the pH reached on curing by the and it ensures that the pH reached on curing by the UG resin catalyzed with IL is not damaging to the UG resin catalyzed with IL is not damaging to the lignocellulose of the wood substrate. lignocellulose of the wood substrate. One of the most important resin parameters affecting the processing of wood-based panels is the One of the most important resin parameters affecting the processing of wood-based panels is the resin gel time. The gel time is the time from when the material begins to soften to when gelation resin gel time. The gel time is the time from when the material begins to soften to when gelation occurs, occurs, where gelation is the irreversible transformation from a viscous liquid to an elastic gel. The where gelation is the irreversible transformation from a viscous liquid to an elastic gel. The effect of effect of the acidic ionic liquid content on gel time is shown in Figure 3. It can be seen that the addition of ILs from 1% to 3% significantly reduces the gel time of UG resins. The faster gel time of catalyzed UG resins indicates that their cross-linking rate becomes faster by adding [HNMP] HSO4. The shortening of the resin gel time with the addition of the catalyst is surely related to the decreasing pH value. The advantages of shorter gel times of thermoset resins with the addition of catalysts have been reported by several researchers [15–17].

Polymers 2016, 8, 57 6 of 14 the acidic ionic liquid content on gel time is shown in Figure3. It can be seen that the addition of ILs from 1% to 3% signiﬁcantly reduces the gel time of UG resins. The faster gel time of catalyzed UG resins indicates that their cross-linking rate becomes faster by adding [HNMP] HSO4. The shortening of the resin gel time with the addition of the catalyst is surely related to the decreasing pH value. The advantages of shorter gel times of thermoset resins with the addition of catalysts have been reported by several researchersPolymers 2016, 8 [, 1557 –17]. 6 of 14

Polymers 2016, 8, 57 6 of 14

Figure 3. Effect of acidic ionic catalyst content on gel time of UG resin. Figure 3. Effect of acidic ionic catalyst content on gel time of UG resin. 3.2. DSC Results 3.2. DSC Results The influence of ILs on thermal curing behavior of UG resins was investigated by DSC. The DSC curves of the UG resin without catalyst and UG resin with 2% ILs are shown in Figure 4. The curing The influenceof the UG of resin ILs is on an thermal exothermic curing reaction behavior similar to other of UG formaldehyde-based resins was investigated thermoset resins. by The DSC. The DSC curves of theexothermic UG resin peak without ofFigure the UG catalyst3. Effectresin ofis acidicaffected and ionic UG by catalyst the resin presence content with ofon 2% the gel ionic ILstime of areliqu UGid. shown resin. Adding in the Figure IL caused4. The curing of the UG resinthe ispeak an of exothermic temperature (T reactionpeak) to be reached similar much to otherearlier and formaldehyde-based at a lower peak temperature. thermoset For the resins. The 3.2.pure DSC UG Resultsresin, the average Tpeak was about 103 °C. Adding the ILs did change the peak temperature exothermic peakto 77The of°C. theinThefluence decrease UG of resin ILs in on peak is thermal affected temperature curing by behavior of the thepresence UG of resinUG resins containing of was the investigated ionic the IL liquid.catalyst by DSC.indicated Adding The DSC that the IL caused the peak of temperaturecurvesILs accelerates of the UG the (T resin peakhardening without) to be of catalyst a reached UG resin. and muchUGThe resiDSCn earlier withanalysis 2% and alsoILs are showed at shown a lower that in FigureT peakonset increased 4. temperature.The curing from For the of35 theto 45 UG °C resin with is the an additionexothermic of 2%reaction ionic similaliquid.˝r Thto eother clear formaldehyde-based indications of all these thermoset are the additionresins. The of pure UG resin, the average Tpeak was about 103 C. Adding the ILs did change the peak temperature exothermicIL to the UG peak resin of decreasing the UG resin the isenergy affected of activati by the onpresence of the curingof the ionicreaction, liqu id.rendering Adding possible the IL caused cross- to 77 ˝C. The decrease in peak temperature of the UG resin containing the IL catalyst indicated that thelinking. peak This of temperature was confirmed (Tpeak by) to the be DSC reached results much showing earlier that and the at aenthalpy lower peak of the temperature. cure reaction For (Δ theH) (the area under the DSC exotherm curves) of the UG resin containing ILs was lower than that of pure ILs acceleratespure the UG hardening resin, the average of a UGTpeak was resin. about The 103 DSC°C. Adding analysis the ILs also did change showed the peak that temperatureTonset increased from 35 to 45 ˝C withtoUG, 77 indicating °C. the The addition decrease also that in of low peak 2% heat temperature ionic was generated liquid. of the byThe UG this resin clearmix. containing indications the IL catalyst of all indicated these are that the addition ILs accelerates the hardening of a UG resin. The DSC analysis also showed that Tonset increased from of IL to the UG35 to resin 45 °C with decreasing the addition the of energy2% ionic liquid. of activation The clear indications of the curing of all these reaction, are the addition rendering of possible cross-linking.IL This to the wasUG resin confirmed decreasing bythe energy the DSC of activati resultson of showing the curing reaction, that the rendering enthalpy possible of thecross- cure reaction (∆H) (the arealinking. under This the was DSC confirmed exotherm by the DSC curves) results of showing the UG that resin the enthalpy containing of the cure ILs reaction was lower (ΔH) than that of (the area under the DSC exotherm curves) of the UG resin containing ILs was lower than that of pure pure UG, indicatingUG, indicating also also that that low low heat heat was was generated generated by this by mix. this mix.

Figure 4. DSC curve of the UG resin and UG resin + 2% ILs.

Figure 4. DSC curve of the UG resin and UG resin + 2% ILs. Figure 4. DSC curve of the UG resin and UG resin + 2% ILs.

Polymers 2016, 8, 57 7 of 14 Polymers 2016, 8, 57 77 ofof 1414

13 3.3. 13C NMR of UG Resin 3.3.Polymers13C NMR 2016, 8 of, 57 UG Resin 7 of 14 Previous studies have reported that the structure and property of thermoset resins vary as the synthesis conditions change [2]. Nuclear magnetic resonance spectroscopy (NMR) is one of the most synthesis3.3.Previous 13C NMR conditions studiesof UG Resinchange have reported [2]. Nuclear that magnetic the structure resonance and property spectroscopy of thermoset (NMR) is resins one of vary the as most the 13 effectivesynthesis methods conditions of changestudying [2]. resin Nuclear structure. magnetic So resonancefar, 13C NMR spectroscopy has been (NMR)used to is characterize one of the most the Previous studies have reported that the structure and property of thermoset resins13 vary as the structureseffective methods of various of resins, studying such resin as UF, structure. PF and PUF So far, resins.13C The NMR solid has phase been CP used MAS to 13 characterizeCNMR spectra the ofsynthesis a hardened conditions UG resin change is shown [2]. inNuclear Figure magnetic 5. The NMR resonance of the spectroscopyhardened UG (NMR) resin shows is one13 ofsome the featuresmost ofstructures a hardened of various UG resin resins, is shown such in as Figure UF, PF 5. and The PUF NMR resins. of13 the The hardened solid phase UG resin CP MAS showsCNMR some features spectra ofeffective interest. methodsThe shift of at studying 163 ppm resin is indicative structure. of So the far, C=O C ofNMR urea has disubstituted been used tofor characterize reaction with the the of ainterest. hardened The UG shift resin at is163 shown ppm inis Figureindicative5. The of NMRthe C=O of the of hardenedurea disubstituted UG resin for shows13 reaction some with features the glyoxal,structures and of the various shift resins,at 154 suchppm as indicates UF, PF and a urea PUF more resins. multisubstituted, The solid phase CP indicating MAS CNMR tridimensional spectra glyoxal,ofof interest. a hardened and The the UG shift shift resin at at 163 is154 shown ppm ppm in is indicates Figure indicative 5. aThe urea of NMR the more C=O of the multisubstituted, of hardened urea disubstituted UG resin indicating shows for reaction some tridimensional features with the cross-linking of the hardened resin. The shifts at 87, 77 and 61 ppm are indicative of the carbon shifts cross-linkingglyoxal,of interest. and The theof the shift hardened at 163 154 ppm ppm resin. is indicates indicativeThe shifts a ureaofat the87 more, C=O77 and multisubstituted,of 61urea ppm disubstituted are indicative indicating for reactionof the tridimensional carbon with the shifts of the following repeating unit, ofcross-linking glyoxal,the following and of the therepeating shift hardened at 154 unit, ppm resin. indicates The shifts a urea at 87, more 77 and multisubstituted, 61 ppm are indicative indicating of tridimensional the carbon shifts of the following repeating unit, cross-linking of the hardened resin. The shifts at 871, 77 and3 61 ppm are indicative of the carbon shifts 1 3 of the following repeating unit, HHNHCONH CH O CH 2 OH4 OH 21 34 ,, HHNHCONH CH O CH OH OH with thethe 8787 ppmppm shift shift belonging belonging to to the the C1 C1 carbon carbon directly2 directly linked4 linked, to the to –NH–the –NH– group group of urea, of urea,the 77 the ppm 77 ppmshifts shifts to the to C3 the not C3 linked not linked to the to urea, the andurea, the and wide the but wide small but 61–62 small ppm61–62 peak ppm belonging peak belonging to the C2 to andthe C2C4with and of the the C4 repeating 87of ppmthe repeating shift unit belonging above. unit above. to the C1 carbon directly linked to the –NH– group of urea, the 77 ppmThe shifts peak to at the 100 C3 ppm not linked belongs to the to the urea, C linkedand the to wide twotwo but ureaurea small molecules,molecules, 61–62 ppm asas inin peak thethe belonging structure:structure: to the C2 and C4 of the repeating unit above. NHCONH2 The peak at 100 ppm belongs to the C linkedNHCONH to two2 urea molecules, as in the structure: NHCONH CH CH NHCONH NHCONH CH CH NHCONH OH NHCONHOH 2 . NHCONH CH CH NHCONH . OH .

13 Figure 5. 13C NMR spectrum of UG resin. 13 FigureFigure 5.5. 13CC NMR NMR spectrum spectrum of of UG UG resin. resin. 3.4. MALDI TOF Results 3.4. MALDI TOF Results 3.4. MALDIThe MALDI TOF ResultsTOF spectrum in Figure 6 shows a number of peaks. There are clearly two repeating TheThe MALDI MALDI TOF TOF spectrum spectrum inin FigureFigure 6 shows aa numb numberer of of peaks. peaks. There There are are clearly clearly two two repeating repeating motives in the spectrum, one of 175 + 1 Da and a second one of 161 + 1 Da. motivesmotivesThe in MALDI in the the spectrum, spectrum, TOF spectrum one one of of 175in175 Figure ++ 1 Da6 showsand aa secondsecond a number one one of of peaks.161 161 + +1 1 ThereDa. Da. are clearly two repeating motives in the spectrum, one of 175 + 1 Da and a second one of 161 + 1 Da.

Polymers 2016, 8, 57 8 of 14

Polymers 2016, 8, 57 8 of 14 Performance PolymersData: 6-MODIFIED0001.D18[c] 2016, 8, 57 23 Jan 2014 14:25 Cal: 26 Mar 2013 11:25 8 of 14 PolymersPolymersShimadzu 2016 Biotech2016, 8, ,57 Axima8, 57 Performance 2.9.3.20110624: Mode Linear, Power: 85, P.Ext. @ 2094 (bin 76) 8 of8 14 of 14 Performance PolymersData:%Int. 6-MODIFIED0001.D18[c]2016, 815, 57 mV[sum= 14883 23 JanmV] 2014 Profiles 14:25 1-1000 Cal: 26 Smooth Mar 2013 Av 50 11:25 -Baseline 150 8 of 14 PerformanceShimadzu Biotech Polymers Axima Performance 2016, 8, 57 2.9.3.20110624: Mode Linear, Power: 85, P.Ext. @ 2094 (bin 76) 8 of 14 PerformanceData: 6-MODIFIED0001.D18[c] 23 Jan 2014 14:25 Cal: 26 Mar 2013 11:25 537.86 Data:Shimadzu 6-MODIFIED0001.D18[c]%Int. Biotech15 AximamV[sum= Performance 23 14883 Jan 2014 mV] 2.9.3.20110624: 14:25 Profiles Cal: 1-1000 26 Mar Mode Smooth 2013 Linear, 11:25 Av 50 Power: -Baseline 85, P.Ext.150 @ 2094 (bin 76) 100 PerformanceShimadzu Biotech AximaPerformance Performance 2.9.3.20110624: Mode Linear, Power: 85, P.Ext. @ 2094 (bin 76) Data: 6-MODIFIED0001.D18[c] 23 Jan 2014 14:25 Cal: 26 Mar 2013 11:25 Data: 6-MODIFIED0001.D18[c]%Int. 15 mV[sum= 23 14883 Jan 2014 mV] 14:25 Profiles Cal: 1-1000 26 Mar Smooth 2013 11:25 Av 50 -Baseline 150 %Int. 15 mV[sum=Shimadzu537.86 14883 Biotech mV] Axima Profiles Performance 1-1000 2.9.3.20110624: Smooth Av Mode 50 -Baseline Linear, Power: 150 85, P.Ext. @ 2094 (bin 76) Shimadzu100 Biotech Axima Performance 2.9.3.20110624: Mode Linear, Power: 85, P.Ext. @ 2094 (bin 76) 90 %Int.537.86 15 mV[sum= 14883 mV] Profiles 1-1000 Smooth Av 50 -Baseline 150 %Int.100 15 mV[sum=537.86 14883 mV] Profiles 1-1000 Smooth Av 50 -Baseline 150 100 537.86 90 100 537.86 80 100 90 699.91 90 90 80 90 70 699.91 80 80 80 699.91699.91 70 699.91 80 60 70 70 551.63 699.91 70 60 531.8460 70 50 551.63 551.63 60 531.84 60 531.84551.63 713.74 551.6350 50 553.85 40 531.84 60 531.84 713.74 50 551.63 553.85 713.74 50 40 875.61 553.85 40 531.84 713.74 875.61 503.82 713.74 1037.86 50 30 553.85503.82 30 875.61 1037.86 40 516.78553.85 575.80 40 516.78 575.80 503.82 713.74 513.80 513.80 875.61 1037.86 30 553.85 875.61 40 20 516.7820 575.80 503.82 582.88 582.88 1037.86 30 503.82 727.58727.58 30 513.80516.78 875.61 1052.111037.861052.11 516.78 575.80 20 575.80 834.10 1192.2 503.82 10 677.87 775.93 903.96 1142.04 1192.2 513.80 582.88 677.87 834.10 1005.99 1037.861066.27 30 10 513.80 727.58 775.93 903.96939.26 1142.04 20 516.78 1005.99 1052.111066.27 20 575.80582.88 939.26 0 727.58 1192.2 513.80 582.88 834.10 1[c].D 10 500 550 600 677.87650727.58700 750775.93 800 850 900903.96950 1000 1050 1052.111100 1150 12001142.04 0 1005.991052.111066.27 20 m/z 939.26 1192.21[c].D 10 582.88 677.87 775.93 834.10 903.96 500 550 600 650 727.58700 750 800834.10 850 900 950 1000 1050 1100 1142.041150 1192.12002 10 677.87 775.93 903.96 1005.991052.111066.27 1142.04 0 m/z 939.26 1005.99 1066.27 Figure 6. MALDI TOF spectrum of the939.26 UG resin. 1192.2 1[c].D 677.87 834.10 10 0 500 550 600 650 700 775.93750 800 850 903.96900 950 1000 1050 11001142.041150 1200 Figure 6. MALDI TOF spectrum of the UG resin.1005.99 1066.27 0 m/z 939.26 1[c].D 500 550 600 650 700 750 800 850 900 950 1000 1050 1100 1150 12001[c].D 500 550 600 650 Figure700 6. MALDI750 800 TOF spectrum850 900 of the950 UG resin.1000 1050 1100 1150 1200 These are representative of the following two repeatingm/z units, namely for the 174–176 Da: 0 m/z 1[c].D 500 550 600 650 Figure700 6. MALDI750 800 TOFOH spectrum850 900 of the950 UG resin.1000 1050 1100 1150 1200 These are representative of the followingfollowing twotwom/z repeatingrepeating units,units, namelynamely forfor thethe 174–176174–176 Da:Da: FigureFigure 6. MALDI 6. MALDINHCONH TOF TOF spectrum CH spectrum CH NHCONH of theof the UG UG resin. resin. OH These are representativeFigure of 6.the MALDI following TOF spectrumOHtwo repeating of the UGunits,. resin. namely for the 174–176 Da: These areThe representative second repeating of mass the followingintervalNHCONH at 162 tw CH Dao CHisrepeating indicativeNHCONH ofunits, the following namely repeating for the unit:174–176 Da: These are representative of the following twoOH repeating units, namely for the 174–176 Da: NHCONH CHOH O CH These are representative of the followingNHCONH twoOH CH repeating CH NHCONH units, namely. for the 174–176 Da: OH OH OH NHCONH CH CH NHCONH. The second repeating mass intervalNHCONH at 162OH CH Da CH OHisNHCONH indicative of. the following repeating unit: The secondThe repeating two units are mass forced interval to alternate at 162to form Da a isOH repeating indicative unit that of the is: following repeating unit: The second repeating mass intervalNHCONHNHCONH at 162 CH DaOH CH CH isNHCONH indicative O CH . of. the following repeating unit: OH The second repeating mass interval at 162 DaOH isOH indicative OH . of the following repeating unit: The second repeating mass intervalNHCONH at CHNHCONH 162 O CHDa isNHCONH CH indicative O CH CH CH ofNHCONH. the following repeating unit: The second repeating mass interval atNHCONH 162OH Da OH is CH indicative O CHOH of the following repeating unit: NHCONH CH OOH CH OH . . The two units are forced to alternate to form a repeatingOH OH unit that is: This alternance give rise to theNHCONH series of peaks CHOH Oat 699–728, CH OH . 875, . 1037 and 1192 Da. The two units are forced to alternate to form a repeatingOH unit that is: The two unitsThe following are forced one in to the alternate series is: to formOH a repeating OH . unit that is: The two units are forcedNHCONH to alternate CH O to CH formNHCONH a repeating CH CH unitNHCONH that is: The two units are forced to alternate to form a repeatingOH unit that is: OH OH OH The two units are forced NHCONHto alternate CH to O form CH a repeatingOH unitOH that is: H NHCONH CH O CH NHCONHNHCONH CH CHOHNHCONH CH CH CHNHCONH O CH NHCONH. H NHCONH CH O CH NHCONH CH CH NHCONH NHCONH CHOH OOH OH CH NHCONH OH OHOH CH CHOHNHCONHOH OH. , This alternance give rise to the seriesOH of peaks OH at 699–728,OH 875, 1037 and 1192 Da. NHCONH CHOH O CH OHNHCONH CH CHOH NHCONH . The followingand so on. Theone peak in the at 538 series Da is is: derived from the above oligomer species by. the loss of a –CH2OH This alternance give rise to the seriesOH of OH peaks at 699–728,OH 875, 1037 and 1192 Da. This alternancegroup and added give to rise the Na+to the used series as a matrix of peaks enhancer. at 699–728, Thus, for 875,example, 1037 the .and peak 1192 at 728 Da. Da is the ThisThe alternance following give one rise in the to theseries series is: of peaks at 699–728, 875,+ 1037 and 1192 Da. This alternancefollowing molecule giverise deprotonated to the series and added of peaks to OH23 Da at 699–728,of the Na used 875, as 1037 enhancer and of 1192 the matrix Da. ThisTheThe followingalternance following one give one in rise thein the toseries theseries seriesis: is: of peaks at 699–728, 875, 1037 and 1192 Da. The followingH NHCONH one in theCH series O CH is: OH CH O CH NHCONH H NHCONHOH CH CH NHCONH The followingH oneNHCONH in the CH series O CH is:NHCONH CH CH NHCONH CH O CH NHCONH CH O CH NHCONH H OH OH OH OH OH OH H NHCONH CHOH O OH CH NHCONHOHOH CH CH NHCONHOH OH CH OOHCH OHNHCONH, H, H NHCONH CH O CH NHCONH CH CH NHCONH CH O CH NHCONH H H NHCONH CH OOH CH NHCONH OH OH CH CHOHNHCONH CH OOHCH NHCONHOH H 728 Da, deprotonated +Na+. , 2 and so on. The peak at 538 DaOH is derived OH from the aboveOH oligomer speciesOH by OHthe loss of a –CH OH H NHCONHThe peaks at CH respectivelyOH O CH OH699NHCONH and 713 Da CH are obtainedOH CH NHCONH from the aboveCHOH OmoleculeCH OHNHCONH by the loss of H an, , groupand so and on. addedThe peak to theat 538 Na+ Da used is derived as a matrix from enhancer. the above Thus, oligomer for example, species bythe the peak loss at of728 a –CHDa is2 OHthe –CH2OH and –OH group,OH respectively. OH The peakOH at 875 Da is derived OHfrom theOH addition of a 176 followingand so on. molecule The peak deprotonated at 538 Da is derivedand added from to the23 Daabove of the oligomer Na+used species as enhancer by the ofloss the of matrix, a –CH 2OH andgroup so on. and Themotive added peak to totheat the538713 peak Na+Da istoused formderived as an aoligomer matrixfrom theof enhancer. the above type oligomer Thus, for species example, by the peakloss of at a728 –CH Da2OH is the andgroup so and on. Theadded peak to the at 538 Na+ Da used is derived as a matrix from enhancer. the above Thus, oligomer for+ example, species bythe the peak loss at of 728 a –CHDa isOH the andgroupfollowing so andon. Theadded molecule peak to theat deprotonated 538 Na+ Da used is derived as anda matrix added OHfrom enhancer. the to 23above Da Thus,of oligomer the forNa example, usedspecies as enhancer bythe the peak loss ofat of the728 a matrix–CHDa is2OH the 2 + groupfollowinggroupfollowing and andH molecule addedNHCONH molecule added to deprotonated the to CH deprotonated the Na+ O Na+ CH used usedNHCONH andas anda as addedmatrix a added CHmatrix to CHenhancer. 23to enhancer.NHCONH 23Da Da of Thus,theof CH Thus,the Na for ONa+used forCHexample,used example, asNHCONH asenhancer enhancer the the CHpeak of peak O theofatCH the728 atmatrixNHCONH 728 matrixDa Dais the is H the OH + following molecule deprotonatedOH OH and addedOH OH to 23 Da of theOH Na+ usedOH as enhancerOH of theOH matrix following HmoleculeNHCONH deprotonated CH O CH NHCONH and addedOH CH to CH 23NHCONH Da of theCH Na OusedCH asNHCONH enhancer CH of O theCH matrixNHCONH H, H NHCONH CHOH O CH OHNHCONH CHOH CH NHCONH CHOH O CHOHNHCONH CHOH O CH OHNHCONH H H NHCONH CH O CH + NHCONHOH CH CH NHCONH CH O CH NHCONH CH O CH NHCONH H , 728 Da, deprotonatedOH +Na . OH OH OH OH OH OH H NHCONH CHOH O CH OHNHCONH CH CHOH NHCONH CHOH O CH OHNHCONH CHOH O CH OHNHCONH H , , 728 Da,The deprotonated peaks at respectively +Na+. 699 and 713 Da are obtained from the above molecule by the loss of an OH OH OH OH OH OH OH + , 728–CH728 Da, Da,2 TheOHdeprotonated deprotonated peaksand –OH at respectively +Nagroup, +Na+. . respectively. 699 and 713 The Da peakare obtained at 875 Da from is derivedthe above from molecule the addition by the loss of aof 176 an 728motive–CH Da,2 ThedeprotonatedOH to peaks andthe 713–OH at peakrespectively +Nagroup, to+. form respectively. 699 an oligomerand 713 The Da of peak arethe obtainedtype at 875 Da from is thederived above from molecule the addition by the loss of aof 176 an The peaks at respectively+ 699 and 713 Da are obtained from the above molecule by the loss of an 728–CH Da,2OH deprotonated and –OH group, +Na .respectively. The peak at 875 Da is derived from the addition of a 176 –CH motive2TheOH peaks andto the –OH at 713 respectively group,peak to respectively. form 699 anand oligomer 713 The Da peak areof the obtained at type 875 Da from is thederived above from molecule the addition by the loss of aof 176 an motive to the 713 peak to form an oligomer of the type –CHmotive 2OH to andthe 713–OH peak group, to form respectively. an oligomer The of peak the type at 875 Da is derived from the addition of a 176 motive to the 713 peak to form an oligomer of the type

Polymers 2016, 8, 57 9 of 14

The peaks at respectively 699 and 713 Da are obtained from the above molecule by the loss of an –CH2OH and –OH group, respectively. The peak at 875 Da is derived from the addition of a 176 motive Polymersto the 2016 713, peak8, 57 to form an oligomer of the type 9 of 14

Polymers 2016, 8, 57 OH 9 of 14 H NHCONH CH O CH NHCONH CH CH NHCONH CH O CH NHCONH CH O CH NHCONH H OH OH OH OH OH OH OH NHCONH CH CH NHCONH2 H NHCONH CH O CH NHCONH CH CH NHCONH CH O CH NHCONH CH O CH NHCONH H OH , OH OH OH OH OH OH NHCONH CH CH NHCONH2 OH which, added to the Na+ used as a matrix enhancer, gives 875 Da. The following peak at 1037–1038, which, added to the Na+ used as a matrix enhancer, gives 875 Da. The following peak at 1037–1038 Da Da iswhich, the oligomer added to derivedthe Na+ byused adding as a matrix a further enhancer, 162 Da gives fragment 875 Da. to The the following875 Da oligomer, peak at 1037–1038 giving the is the oligomer derived by adding a further 162 Da fragment to the 875 Da oligomer, giving the followingDa is the type oligomer of oligomer: derived by adding a further 162 Da fragment to the 875 Da oligomer, giving the following type of oligomer: following type of oligomer: OH H NHCONH CH O CH NHCONH CH O CH NHCONH CH CH NHCONH CH O CH NHCONH CH O CH NHCONH H OH OH OH OH OH OH OH OH OH NHCONH CH CH NHCONH2 H NHCONH CH O CH NHCONH CH O CH NHCONH CH CH NHCONH CH O CH NHCONH CH O CH NHCONH H OH OH OH OH OH OH OH OH OH NHCONH CH CH NHCONH2 . OH . Three more findings result from the MALDI TOF analysis, the first of which being that the –CH–O–CH–Three ethermore bridges findings are result formed from by the glyoxal MALDI resins TOF just analysis, as easily the as first methylene of which ether being bridges that the are Three more findings result from the MALDI TOF analysis, the first of which being that the formed–CH–O–CH– in UF resins. ether These bridges types are formedof bridges, by glyoxal howeve resinsr, appear just as to easily be more as methylene stable than ether the bridges methylene are –CH–O–CH– ether bridges are formed by glyoxal resins just as easily as methylene ether bridges are etherformed bridges in UFof resins.UF resins, These where types ofrearrangements bridges, howeve withr, appear elimination to be more of stableformaldehyde than the methylene are rather formed in UF resins. These types of bridges, however, appear to be more stable than the methylene common.ether bridgesHowever, of rearrangementsUF resins, where of rearrangements such bridges with with elimination elimination of of glyoxal formaldehyde cannot be are excluded rather ethercommon. bridges However, of UF resins, rearrangements where rearrangements of such bridges with with elimination elimination of formaldehyde of glyoxal cannot are rather be excluded common. at this stage. These glyoxal ether bridges present in the 162-Da repeating unit alternate with definite However,at this stage. rearrangements These glyoxal of ether such bridges present with elimination in the 162-Da of glyoxal repeating cannot unit bealternate excluded with at definite this stage. –CH–CH– bridges characteristics of the 176-Da repeating unit. One last thing evident from the These–CH–CH– glyoxal bridges ether bridgescharacteristics present of in the the 176-Da 162-Da repeating repeating unit. unit One alternate last thing with evident definite from –CH–CH– the MALDI analysis is that the oligomers formed are mainly linear and appear to present little bridgesMALDI characteristics analysis is ofthat the the 176-Da oligomers repeating formed unit. are One mainly last thing linear evident and fromappear the to MALDI present analysis little is ramification, a fact that should definitely have a bearing on the gelling and curing time and behaviors thatramification, the oligomers a fact formed that should are mainly definitely linear have and a appearbearing toon present the gelling little and ramification, curing time a and fact behaviors that should of such adhesives. One of the bearings of these characteristics is most likely to be the higher energy definitelyof such adhesives. have a bearing One of on the the bearings gelling andof these curing char timeacteristics and behaviors is most likely of such to be adhesives. the higher One energy of the activation for curing UG resins than that for UF resins. That is why ILs, and the lower pHs reached bearingsactivation of thesefor curing characteristics UG resins isthan most that likely for UF to beresins. the higherThat is energywhy ILs, activation and the lower for curing pHs reached UG resins during hot-pressing, are necessary for proper curing. thanduring that forhot-pressing, UF resins. are That necessary is why ILs,for proper and the curing. lower pHs reached during hot-pressing, are necessary As regards to the lower pH reached on curing, this has been shown to do little damage to the for properAs regards curing. to the lower pH reached on curing, this has been shown to do little damage to the lignocellulosic substrate due to a very short period of time the acid is free and in contact with it, lignocellulosicAs regards tosubstrate the lower due pH to a reached very short on curing,period of this time has the been acid shown is free to and do in little contact damage with to it, the exclusively during the short hot pressing time, before being again neutralized on the cooling of the lignocellulosicexclusively during substrate the short due hot to a pr veryessing short time, period before of being time again the acidneutralized is free andon the in cooling contact of with the it, panelpanel and and the thereforming reforming of ofthe the IL ILin inthe the cured cured resin resin [8]. [8]. Moreover, Moreover, thethe pHspHs reached withwith the the adhesive adhesive exclusively during the short hot pressing time, before being again neutralized on the cooling of the systemsystem UG+IL UG+IL are are not not lower lower than than what what was was achi achievedeved by by a a UF UF resin resin andand a standard hardener.hardener. panel and the reforming of the IL in the cured resin [8]. Moreover, the pHs reached with the adhesive system UG+IL are not lower than what was achieved by a UF resin and a standard hardener. 3.5. 3.5.Properties Properties of Particleboards of Particleboards 3.5.Water PropertiesWater absorption absorption of Particleboards and and thickness thickness swelling swelling of of wood wood-based-based panelspanels isis one of thethe mainmain factors factors that that limit their outdoors application. As regards to this issue, several researchers have focused on finding limit theirWater outdoors absorption application. and thickness As regards swelling to this of issue, wood-based several panelsresearchers is one have of the focused main on factors finding that new methods to reduce water absorption of wood-based panels [9,18]. Short-term water absorption newlimit methods their outdoors to reduce application. water absorption As regards of wood-b to this issue,ased panels several [9,18]. researchers Short-term have focusedwater absorption on finding and thickness swelling of the panels prepared are shown in Figures 7 and 8, respectively. andnew thickness methods swelling to reduce of waterthe panels absorption prepared of wood-basedare shown in panels Figures [9 ,718 and]. Short-term 8, respectively. water absorption and thickness swelling of the panels prepared are shown in Figures7 and8 respectively. These figures (Figures7 and8) show that water absorption and thickness swelling increase with increasing immersion time. The panels prepared without any catalysts exhibit the highest water absorption (23%) and thickness swelling (16%). Conversely, the panels containing acidic ionic liquid exhibit lower water absorption and thickness swelling compared to those catalyzed with NH4Cl. The results also show that the lower water absorption and thickness swelling content can be achieved by increasing the ionic liquid proportion from 0% to 3%, as shown by the panels containing 3% [HNMP] HSO4 exhibiting lower water absorption and thickness swelling. Higher water absorption and thickness swelling of the panels prepared with NH4Cl compared to those containing [HNMP] HSO4 can be explained by the SEM micrographs. SEM micrographs shown later indicate that, contrary to NH4Cl, the fracture surface of the panels prepared with the ionic liquid, produces no porosity and voids in glue line. These voids are the place where water molecules accumulate and increase the panel’s water absorption and thickness swelling. Conversely, it appears that the glue lines will be

Figure 7. Water absorption of the manufactured panels made from synthesized resins. Figure 7. Water absorption of the manufactured panels made from synthesized resins.

Polymers 2016, 8, 57 9 of 14

OH H NHCONH CH O CH NHCONH CH CH NHCONH CH O CH NHCONH CH O CH NHCONH H

OH OH OH OH OH OH NHCONH CH CH NHCONH2 OH ,

which, added to the Na+ used as a matrix enhancer, gives 875 Da. The following peak at 1037–1038 Da is the oligomer derived by adding a further 162 Da fragment to the 875 Da oligomer, giving the following type of oligomer:

OH H NHCONH CH O CH NHCONH CH O CH NHCONH CH CH NHCONH CH O CH NHCONH CH O CH NHCONH H

OH OH OH OH OH OH OH OH NHCONH CH CH NHCONH2 OH . Three more findings result from the MALDI TOF analysis, the first of which being that the –CH–O–CH– ether bridges are formed by glyoxal resins just as easily as methylene ether bridges are formed in UF resins. These types of bridges, however, appear to be more stable than the methylene ether bridges of UF resins, where rearrangements with elimination of formaldehyde are rather common. However, rearrangements of such bridges with elimination of glyoxal cannot be excluded at this stage. These glyoxal ether bridges present in the 162-Da repeating unit alternate with definite –CH–CH– bridges characteristics of the 176-Da repeating unit. One last thing evident from the MALDI analysis is that the oligomers formed are mainly linear and appear to present little ramification, a fact that should definitely have a bearing on the gelling and curing time and behaviors of such adhesives. One of the bearings of these characteristics is most likely to be the higher energy activation for curing UG resins than that for UF resins. That is why ILs, and the lower pHs reached during hot-pressing, are necessary for proper curing. As regards to the lower pH reached on curing, this has been shown to do little damage to the lignocellulosic substrate due to a very short period of time the acid is free and in contact with it, exclusively during the short hot pressing time, before being again neutralized on the cooling of the panel and the reforming of the IL in the cured resin [8]. Moreover, the pHs reached with the adhesive system UG+IL are not lower than what was achieved by a UF resin and a standard hardener.

Polymers3.5. Properties2016, 8, 57of Particleboards 10 of 14 Water absorption and thickness swelling of wood-based panels is one of the main factors that smootherlimit their andoutdoors therefore application. less capable As regards of absorbing to this issue, water several molecules researchers in this region have focused by increasing on finding the acidicnew methods ionic liquid to reduce content. water Kardos absorptionet al. (1986) of wood-b alsoased indicated panels that [9,18]. some Short-term microcracks water and absorption voids are createdand thickness during swelling the UF resins of the curing panels process prepared [19 are]. shown in Figures 7 and 8, respectively.

Polymers 2016Figure, 8, 57 7. Water absorption of the manufactured panels made from synthesized resins. 10 of 14 Figure 7. Water absorption of the manufactured panels made from synthesized resins.

FigureFigure 8. Thickness8. Thickness swelling swelling of theof the manufactured manufactured panels panels made made from from synthesized synthesized resins. resins.

TheThese flexural figures modulus (Figures and 7 strength and 8) show of the that panels water containing absorption 0%, and 1%, thickness 2% and 3% swelling acidic ionicincrease liquid with comparedincreasing to thoseimmersion had NH time.4Cl areThe illustrated panels prepared in Figures without9 and 10 any respectively. catalysts exhibit It can be the observed highest that water panelsabsorption with acidic (23%) ionic and liquidthickness exhibited swelling higher (16%). flexural Conv modulusersely, the and panels strength containing values acidic than those ionic thatliquid usedexhibit ammonium lower water chloride absorption as a catalyst. and thickness swelling compared to those catalyzed with NH4Cl. The resultsAs shown also show in Figures that the9 and lower 10 increasing water absorption acidic ionic and liquidthickness content swelling from content 0% to 3% can into be UGachieved resin by improvedincreasing flexural the ionic properties liquid proportion of the panels. from The 0% highest to 3%, as flexural shown modulus by the panels and strengthcontaining values 3% [HNMP] were 4 relatedHSO to exhibiting UG resin containlower water 3% [HNMP] absorption HSO 4andas athic catalyst,kness whereasswelling. the Higher UG resin water without absorption catalysts and exhibitedthickness the swelling lowest values.of the panels In spite prepared of the fact with that NH glue4Cl compared line can act to as those a stress containing concentration [HNMP] point HSO4 in bendingcan be explained strength, by the the higher SEM MOEmicrographs. and MOR SEM of themicr panelsographs made shown from later UG indicate resin with that, acidic contrary ionic to liquidNH4 canCl, the be attributedfracture surface to increasing of the panels mechanical prepared properties with the of theionic resins liquid, with produces the addition no porosity of acidic and ionicvoids liquid. in glue In addition, line. These in hotvoids press are condition,the place where the voids water were molecules produced accumulate after resin and shrinkages increase or the evaporationpanel’s water HCL absorption gas in the UGand resin thickness structures. swelling. The voidsConversely, in the glueit appears line can that reduce the glue the mechanical lines will be propertiessmoother of and the resintherefore as well less as capable the panels. of absorbing So far, several water researchers molecules have in this shown region that by a high increasing amount the of additionacidic ionic of NHliquid4Cl content. decreases Kardos the mechanical et al. (1986) strength also indicated of the resin that and some associated microcracks panels and [20 ].voids are created during the UF resins curing process [19]. The flexural modulus and strength of the panels containing 0%, 1%, 2% and 3% acidic ionic liquid compared to those had NH4Cl are illustrated in Figures 9 and 10, respectively. It can be observed that panels with acidic ionic liquid exhibited higher flexural modulus and strength values than those that used ammonium chloride as a catalyst.

Figure 9. Flexural modulus of the manufactured panels made from synthesized resins.

Polymers 2016, 8, 57 10 of 14

Figure 8. Thickness swelling of the manufactured panels made from synthesized resins.

These figures (Figures 7 and 8) show that water absorption and thickness swelling increase with increasing immersion time. The panels prepared without any catalysts exhibit the highest water absorption (23%) and thickness swelling (16%). Conversely, the panels containing acidic ionic liquid exhibit lower water absorption and thickness swelling compared to those catalyzed with NH4Cl. The results also show that the lower water absorption and thickness swelling content can be achieved by increasing the ionic liquid proportion from 0% to 3%, as shown by the panels containing 3% [HNMP] HSO4 exhibiting lower water absorption and thickness swelling. Higher water absorption and thickness swelling of the panels prepared with NH4Cl compared to those containing [HNMP] HSO4 can be explained by the SEM micrographs. SEM micrographs shown later indicate that, contrary to NH4Cl, the fracture surface of the panels prepared with the ionic liquid, produces no porosity and voids in glue line. These voids are the place where water molecules accumulate and increase the panel’s water absorption and thickness swelling. Conversely, it appears that the glue lines will be smoother and therefore less capable of absorbing water molecules in this region by increasing the acidic ionic liquid content. Kardos et al. (1986) also indicated that some microcracks and voids are created during the UF resins curing process [19]. The flexural modulus and strength of the panels containing 0%, 1%, 2% and 3% acidic ionic liquid compared to those had NH4Cl are illustrated in Figures 9 and 10, respectively. It can be observed that panels with acidic ionic liquid exhibited higher flexural modulus and strength values Polymersthan those2016 ,that8, 57 used ammonium chloride as a catalyst. 11 of 14

Polymers 2016, 8, 57 11 of 14 Figure 9. Flexural modulus of the manufactured panels mademade fromfrom synthesizedsynthesized resins.resins.

Figure 10. Flexural Strength of the manufactured panels made from synthesized resins.

InternalAs shown bond in Figures (IB) of the9 and manufactured 10, increasing panels acidic is io shownnic liquid in Figure content 11. from It can 0% be to seen 3% thatinto greater UG resin IB valueimproved can beflexural achieved properties for the panelsof the panels. madefrom The hi UGghest resin flexural with the modulus addition and of strength catalysts. values The panels were maderelated with to UG 1%, resin 2% andcontain 3% ionic3% [HNMP] liquid had HSO 27%,4 as 62%a catalyst, and 130% whereas higher the IB UG strength resin without than the catalysts control exhibited the lowest values. In spite of the fact that glue line can act as a stress concentration point in samples (NH4Cl), respectively. The greater IB strength can be attributed to higher heat conductivity bending strength, the higher MOE and MOR of the panels made from UG resin with acidic ionic of the ionic liquid compared to wood particles and NH4Cl. This causes better polymerization of theliquid resin can in be the attributed board core to increasing layer. So far, mechanical several researchers properties haveof the indicated resins with that the the addition use of materials of acidic withionic higherliquid. heatIn addition, transfer in properties hot presscan condition, reduce the voids hot press were time produced and increase after resin IB strength shrinkages of the or panelevaporation [21]. Wang HCLet gas al. in(2014) the UG investigated resin structures. the inﬂuence The voids of various in the typesglue line of catalysts can reduce on thethe synthesismechanical of properties of the resin as well as the panels. So far, several researchers have shown that a high amount different materials [7]. They indicated that [NMP]+HSO4 exhibited the best catalyst results among of addition of NH4Cl decreases the mechanical strength of the resin and associated panels [20]. various acidic ionic liquids because it has several advantages: (1) the IL [NMP]+HSO4 shows better catalyticInternal activity bond and (IB) high of the yields; manufactured (2) the halogen panels free is shown IL is more in Figure advantageous 11. It can andbe seen the solventthat greater free IB value can be achieved for the panels made from UG resin with the addition of catalysts. The panels conditions for its use meet the greener aspects of catalysis; and (3) [NMP]+HSO4 can be easily recycled aftermade separation. with 1%, 2% and 3% ionic liquid had 27%, 62% and 130% higher IB strength than the control samples (NH4Cl), respectively. The greater IB strength can be attributed to higher heat conductivity 3.6.of the Morphology ionic liquid Characteristics compared to wood particles and NH4Cl. This causes better polymerization of the resin in the board core layer. So far, several researchers have indicated that the use of materials with Scanning electron microscopy is an effective media for the morphological investigations of higher heat transfer properties can reduce the hot press time and increase IB strength of the panel wood-based panels. Figure 12a corresponds to the panels hardened with NH4Cl. As can be seen, there [21]. Wang et al. (2014) investigated the influence of various types of catalysts on the synthesis of different materials [7]. They indicated that [NMP]+HSO4 exhibited the best catalyst results among various acidic ionic liquids because it has several advantages: (1) the IL [NMP]+HSO4 shows better catalytic activity and high yields; (2) the halogen free IL is more advantageous and the solvent free conditions for its use meet the greener aspects of catalysis; and (3) [NMP]+HSO4 can be easily recycled after separation.

3.6. Morphology Characteristics Scanning electron microscopy is an effective media for the morphological investigations of wood-based panels. Figure 12a corresponds to the panels hardened with NH4Cl. As can be seen, there are bubbles in the fracture surface that cause weakness sites in the panels. Therefore, when stress is applied, it causes fractures to occur faster in this region. Contrary to the panels contain NH4Cl, there are no voids and bubbles in the particleboards with acidic ionic liquid hardeners (Figure 12b). Previous researchers have indicated that the existence of cavities in the wood-based panels helps the panels to absorb water and reduce mechanical properties. The panels, hardened with the green catalyst, present a smooth and monotonous matrix, and water penetration in the panels’ deeper holes and cavities is thus prevented. Previous studies have also shown that the high strength of the glue line directly influences the panels’ mechanical strength.

Polymers 2016, 8, 57 12 of 14 are bubbles in the fracture surface that cause weakness sites in the panels. Therefore, when stress is applied, it causes fractures to occur faster in this region. Contrary to the panels contain NH4Cl, there are no voids and bubbles in the particleboards with acidic ionic liquid hardeners (Figure 12b). Previous researchers have indicated that the existence of cavities in the wood-based panels helps the panels to absorb water and reduce mechanical properties. The panels, hardened with the green catalyst, present a smooth and monotonous matrix, and water penetration in the panels’ deeper holes and cavities is thus prevented. Previous studies have also shown that the high strength of the glue line directly PolymersinﬂuencesPolymers 20162016 the,, 88,, 5757 panels’ mechanical strength. 1212 ofof 1414

FigureFigure 11.11.11. Internal InternalInternal bond bondbond strength strengthstrength (IB) (IB)(IB) of theofof the manufacturedthe manufamanufacturedctured particleboard particleboardparticleboard made mademade from synthesizedfromfrom synthesizedsynthesized resins. resins.resins.

((aa)) ((bb))

FigureFigure 12.12. SEMSEM micrograghsmicrograghs ofof thethe partparticleboardsicleboards preparedprepared withwith ((aa)) UGUG resin+resin+ NHNH44Cl;Cl; ((bb)) UGUG resin+resin+ Figure 12. SEM micrograghs of the particleboards prepared with (a) UG resin+ NH4Cl; (b) UG resin+ ionicionic liquid. liquid.

4.4. ConclusionsConclusions Conclusions

InIn thisthis work,work, thethe effecteffect ofof anan acidicacidic ionicionic liquidliquid ([HNMP]([HNMP] HSOHSO44)) asas aa newnew catalystcatalyst onon physicalphysical In this work, the effect of an acidic ionic liquid ([HNMP] HSO4) as a new catalyst on physical and andmechanicaland mechanicalmechanical properties propertiesproperties of wood-based ofof wood-basedwood-based panels bondedpanelspanels withbondedbonded urea-glyoxal withwith urea-glyoxalurea-glyoxal (UG) resin was(UG)(UG) investigated. resinresin waswas investigated.Theinvestigated. following TheThe conclusions followingfollowing can conclusionsconclusions be drawn cancan from bebe the drawndrawn results: fromfrom thethe results:results: •• ‚ AcidicAcidic ionicionicionic liquids, liquids,liquids, such suchsuch as asas a aa halogen-free halogen-freehalogen-free and andand ecofriendly ececofriendlyofriendly ionic ionicionic liquid, liquid,liquid, is anisis an effectivean effectiveeffective catalyst catalystcatalyst for urea-glyoxalforfor urea-glyoxalurea-glyoxal resins. resins.resins. •• SEMSEM micrographsmicrographs showedshowed thatthat thethe wood-basedwood-based panelspanels containingcontaining anan acidicacidic ionicionic liquidliquid hadhad aa smoothsmooth andand monotonousmonotonous matrix.matrix. •• TheThe particleboardparticleboard panelspanels preparedprepared withwith anan ionicionic liquidliquid hardenerhardener hadhad higherhigher mechanicalmechanical strengthstrength andand betterbetter dimensionaldimensional stabilitystability comparedcompared toto thosethose mademade fromfrom UGUG resinresin containingcontaining NHNH44Cl.Cl. •• TheThe gelgel timetime ofof thethe UGUG resinresin waswas acceacceleratedlerated withwith increasingincreasing [HNMP][HNMP] HSOHSO44 content.content. •• MALDIMALDI TOFTOF massmass spectroscopyspectroscopy indicatedindicated thatthat UGUG resinsresins needneed toto overcomeovercome aa highhigh energyenergy ofof activationactivation onon curing.curing. •• DSCDSC analysisanalysis showedshowed thatthat thethe additionaddition ofof ILIL toto ththee UGUG resinresin decreasedecrease thethe energyenergy ofof activationactivation ofof thethe curingcuring reactionreaction toto renderrender possiblepossible cross-linking.cross-linking.

Polymers 2016, 8, 57 13 of 14

‚ SEM micrographs showed that the wood-based panels containing an acidic ionic liquid had a smooth and monotonous matrix. ‚ The particleboard panels prepared with an ionic liquid hardener had higher mechanical strength and better dimensional stability compared to those made from UG resin containing NH4Cl. ‚ The gel time of the UG resin was accelerated with increasing [HNMP] HSO4 content. ‚ MALDI TOF mass spectroscopy indicated that UG resins need to overcome a high energy of activation on curing. ‚ DSC analysis showed that the addition of IL to the UG resin decrease the energy of activation of the curing reaction to render possible cross-linking.

Author Contributions: Hamed Younesi-Kordkheili contributed the UG resin manufacture, the ionic liquid technology, the DSC and the board making and testing. Antonio Pizzi contributed the UG resin making technology, the 13C NMR and the MALDI TOF analysis and their interpretation Conﬂicts of Interest: The authors declare no conﬂict of interest.

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