Benoit Ndiwe

Antonio Pizzi
email: antonio.pizzi@univ-lorraine.fr

Beda Tibi

Raidandi Danwe

Noel Konai

Siham Amirou

African tree bark exudate extracts as biohardeners of fully biosourced thermoset tannin adhesives for wood panels

Keywords: "bio" hardeners, bioadhesives, wood adhesives, Vachellia nilotica, Senegalia senegal seyal, exudates, 2, 5-dihydroxymethyl furan, carbohydrate oligomers

A totally "bio" hardener derived from the exudate extract of the African trees Vachellia nilotica and Senegalia senegal was investigated. It hardened a maritime pine tannin resin without any obvious presence of an aldehyde thus obtaining a completely "bio" adhesive for wood panels. MALDI-ToF and CP MAS 13 C NMR analysis indicated that the hardeners cross-linking species was 2,5-dihydroxymethyl furan linked onto carbohydrate chains. The hydroxymethyl furan moieties reacted both after detachment from the carbohydrate chains but also with carbohydrate chains still attached. The carbohydrates too were then involved as part of the binder. The gel times were slower than what was obtainable with paraformaldehyde but sufficiently fast to possibly be of industrial significance.

INTRODUCTION

Tannin adhesives for wood panels and their derivatives have now been used industrially for many years in a number of countries especially from the southern emisphere [START_REF] Plomley | Tannin-formaldehyde adhesives for wood. II, wattle tannin adhesives[END_REF][START_REF] Pizzi | Wattle-based adhesives for exterior grade particleboard[END_REF][START_REF] Pizzi | Pine tannin adhesives for particleboard[END_REF][START_REF] Pichelin | Structural beams from thick wood panels bonded industrially with formaldehyde free tannin adhesives[END_REF][START_REF] Pizzi | Bioadhesives for wood and fibres[END_REF][START_REF] Pizzi | Wood products and green chemistry[END_REF][START_REF] Pizzi | Natural phenolic adhesives from tannins and lignin[END_REF][START_REF] Sealy-Fisher | Increased pine tannins extraction and wood adhesives development by phlobaphenes minimization[END_REF][START_REF] Pizzi | Low-formaldehyde emission, fast pressing, pine and pecan tannin adhesives for exterior particleboard[END_REF] 2 [START_REF] Pichelin | Structural beams from thick wood panels bonded industrially with formaldehyde free tannin adhesives[END_REF][START_REF] Moubarik | Development and optimisation of a new formaldehyde-free cornstarch and tannin adhesive[END_REF][START_REF] Ping | Condensed tannins extraction from grape pomace : characterization and utilization as wood adhesives for wood particleboard[END_REF][START_REF] Valenzuela | Industrial production of pine tannin-bonded particleboard and MDF[END_REF][START_REF] Navarrete | Low formaldehyde emitting biobased wood adhesives manufactured from mixtures of tannin and glyoxalated lignin[END_REF][START_REF] Abdullah | Tannin-Furfuryl alcohol wood panel adhesives without formaldehyde[END_REF][START_REF] Zhou | Lightweight tannin foam/composites sandwich panels and the coldset tannin adhesive to assemble them[END_REF][START_REF] Zhou | A novel fiber-veneerlaminated composite based on tannin resin[END_REF][START_REF] Santoso | Characterization of Merbau Wood Extract Used as an Adhesive in Glued Laminated Lumber[END_REF]Santiago-Medina et al 2016;[START_REF] Santos-Ucha | Environmentally friendly wood adhesives based on chestnut (Castanea sativa) shell tannins[END_REF]Shirmohammadi et al 2018;Ghahri and Pizzi 2018a,b) but also in some Asian countries [START_REF] Pichelin | Structural beams from thick wood panels bonded industrially with formaldehyde free tannin adhesives[END_REF] and even occasionally in European countries. Over the years, starting from their early commercial utilization, in the search for more effective or more environment friendly systems, a number of different hardeners have been tried and used plus or minus successfully for these adhesives. Among these paraformaldehyde [START_REF] Pizzi | Wattle-based adhesives for exterior grade particleboard[END_REF][START_REF] Pizzi | Pine tannin adhesives for particleboard[END_REF], formalin solution (Carter Holt Harvey 1974-1989;[START_REF] Pizzi | Liquid formaldehyde concentrates as hardener for tannin-based adhesives[END_REF], resorcinol or phenol-formaldehyde resols [START_REF] Plomley | Westralian Plywoods, private communication[END_REF], hexamine [START_REF] Pizzi | Advanced Wood Adhesives Technology[END_REF][START_REF] Pichelin | Structural beams from thick wood panels bonded industrially with formaldehyde free tannin adhesives[END_REF], trishydroxymethylnitromethane (known also as 2nitro-2-(hydroxymethyl)-1,3-propanediol) [START_REF] Trosa | A no-aldehyde emission hardener for tannin-based wood adhesives[END_REF]), 2-amino-2-(hydroxymethyl)-1,3-propanediol [START_REF] Grigsby | Adhesives[END_REF], glyoxal [START_REF] Ballerini | Non-toxic, zero-emission tannin-glyoxal adhesives for wood panels[END_REF]Navarrete et al 2010a), and a variety of different aldehydes [START_REF] Pizzi | Wood Adhesives: Chemistry and technology[END_REF], Pizzi et al 1980;[START_REF] Rossouw | The kinetics of condensation of phenolics polyflavonoid tannins with aldehydes[END_REF]Bohm et al 2016), and even tannin autocondensation [START_REF] Meikleham | Induced accelerated autocondensation of polyflavonoid tannins for phenolic polycondensates, Part 1: 13C NMR, 29Si NMR, X-ray and polarimetry studies and mechanism[END_REF][START_REF] Pizzi | Autocondensation-based, zeroemission, tannin adhesives for particleboard[END_REF][START_REF] Pichelin | Structural beams from thick wood panels bonded industrially with formaldehyde free tannin adhesives[END_REF] have been tried and used. Only in three cases, and quite recently, fully biohardeners have been used to yield 100% bioadhesives from tannins, with some degree of success, these being hydroxymethyl furfural [START_REF] Santiago-Medina | Hydroxymethylfurfural hardening of pine tannin wood adhesives[END_REF], furfuryl alcohol [START_REF] Trosa | Industrial hardboard and other panels binder from tannin/furfuryl alcohol in absence of formaldehyde[END_REF][START_REF] Abdullah | Tannin-Furfuryl alcohol wood panel adhesives without formaldehyde[END_REF]) and vanillin and its derivatives [START_REF] Santiago | Lignin-derived non-toxic aldehydes for ecofriendly tannin adhesives for wood panels[END_REF]. The former gave particleboards that although not reaching the internal bond (IB) strength requirements of relevant standards showed nonetheless some promise for the future.

The second reached the requirements of the standards but marginally, the furfuryl alcohol behaving more as a copolymerization partner than just a hardener. The last one instead gave acceptable IB strength results when using a fast reacting procyanidin tannin, such as pine tannin, but at this stage is too expensive for industrial optimization, although increased production for other purposes may decrease its cost in future.

Considering the above, the search is still on for a biohardner which can be effective for tannin adhesives while presenting an acceptably low cost. Recently, extracts from some African tree barks, fruits and sap have shown promise for such an application. Senegalia senegal (known also as Acacia senegal) is a small tree known by different popular names, such as Gum acacia, Gum arabic tree, Sudan gum and Sudan gum arabic. In India, It is known as Kher or Khor in India [START_REF] Pervez | Food habits of the Indian crested porcupine, Hystrix indica in Sindk, Pakistan, Pakistan[END_REF]. It is native to semi-desert regions of Sub-Saharan Africa, as well as Oman, Pakistan, west coastal India. It grows to a height of 5-12 m, with a trunk up to 30 cm in diameter. Vachellia seyal (also known as Vachellia nilotica, Acacia seyal or Acacia nilotica) as Senegalia senegal, is a tropical african thorny tree of the Fabaceae family, Mimosoideae subfamily, which can reach 10 meters of height and parts of which are traditionally used in the countries of origin for a number of applications. Recently, it was noticed that extracts of the bark, exudates and fruits from these and similar species were capable of reacting with local tannins.

As these materials are relatively inexpensive the work presented here addresses first of all the interest in trying to use them as hardeners of tannin extracts under different conditions.

Secondly, it aims to analyse them to identify what could be the active chemical and its mechanism of reaction that could allow them to be used as tannin adhesives biohardners. The third aim was to prepare fully natural thermosetting wood panel adhesives, and to evaluate their effectiveness.

EXPERIMENTAL

Extraction

For both Vachellia nilotica and Senegalia senegal from the Dacheka forest in the extreme northern region of Cameroun the bark of the tree was hurt by incisions and from the wound came out a complex organic solution of high viscosity called an exudate.

The locality of Dacheka is located in the Extreme-North Cameroon region, a semi-arid Sahelian tropical region with an aridity index between 0.20 and 0.50 [START_REF] Onana | Cartographie des écosystèmes du Cameroun[END_REF] where the ambient temperature is very high, reaching more than 40 ° C in March. This type of ecosystem is characterized by the non-availability of soil water with low rainfall, the annual rainfall being 811mm. The harmattan wind blows from October to March. The heart of the dry season is in January. Southwest winds appear timidly in June, before bringing rain in August and especially in September. This climate favors the cultivation of Vachelia nilotica and Senegalia senegal, cotton and cereals. Depending on the season, the landscape is characterized by savannah, grassland and prickly steppe.

The exudates were collected and dried at 37°C for 21 days. Finally, the dried exudates were crushed to obtain a soluble whitish powder easier to stock and to use.

The pine (Pinus pinaster) tannin bark extract used as resin was a pure tannin obtained from DRT (Derivés Resiniques et Terpeniques, Dax, France) and characterized previously (Navarrete et al 2010b[START_REF] Navarrete | Characterisation of two maritime pine tannins as wood adhesives[END_REF].

Gel Time

The gel test for a tannin resin is performed as follows: 10 g of the reaction mixture composed of pine tannin or a tropical Acacia tannin as a 50% solution in water to which the hardener has been added in proportion by weight Tannin solids : Hardner = 5:1.

MALDI-TOF analysis

Samples for Matrix assisted laser desorption ionization time-of-flight (MALDI-TOF) analysis were prepared first dissolving 5mg of sample powder in 1mL of a 50:50 v/v acetone/water solution. Then 10 mg of this solution is added to 10µL of a 2,5-dihydroxy benzoic acid (DHB) matrix. The locations dedicated to the samples on the analysis plaque were first covered with 2µL of a NaCl solution 0.1M in 2:1 v/v methanol/water, and predried. Then 1µL 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 2 shots accumulated per profile. The spectrum precision is of +1Da.

CP-MAS 13C NMR Spectra

The exudates extracts were analyzed by solid state CP MAS 13 C NMR. Spectra were obtained on a Brüker AVANCE 400 MHz (Brüker, Billerica, MA, USA) spectrometer with a 4 mm probe at a frequency of 12 kHz. The pulse duration at 90° was 4.1 µs, with a contact time of 2 ms and a recycling delay of 4s. Chemical shifts were determined relative to tetramethyl silane (TMS) used as control. The spectra were accurate to 1 ppm. The spectra were run with suppression of spinning side bands.

Thermomechanical Analysis (TMA)

The resins were tested by thermomechanical analysis. The samples were prepared by applying each adhesive at a surface spread of 200 g/m 2 of liquid adhesive between two beech wood decorative plies of average density 0.750 g/cm 3 , of moisture content of 11%, and of with dimensions of 21mm × 6mm × 1.1mm. The beech wood plies were very smooth as these were cut from decorative veneers of 0.5 mm thickness. These beech-resin-beech sandwiches were tested in non-isothermal mode between 25°C and 250°C at a heating rate of 10°C/minute with a Mettler Toledo 40 TMA equipment (Mettler-Toledo, Zurich, Switzerland). They were tested in three-point bending on a span of 18 mm exercising a force cycle of 0.1/0.5 N on the specimens, with each force cycle of 12 seconds (6s/6s). The classical mechanics relationship between force and deflection

E = [L 3 /(4bh 3 )][F/(Δfwood -∆fadhesive)]
where E is Young's modulus, L = the length of the span tested, b and h are respectively the width and the thickness of the specimen, F is the force exerted on the tested assembly, Δfwood and ∆fadhesive are the deflections that have been proven to be constant and reproducible [START_REF] Pizzi | On the correlation of some theoretical and experimental parameters in polycondensation cross-linked networks[END_REF][START_REF] Pizzi | Molecular mechanics modelling of interfacial energy and flexibility[END_REF]. The formula allows the calculation of the Young's modulus E by measuring the change in deflection amplitude of the tested sandwich between the initial and final equilibrium deflections [START_REF] Pizzi | On the correlation of some theoretical and experimental parameters in polycondensation cross-linked networks[END_REF][START_REF] Pizzi | Molecular mechanics modelling of interfacial energy and flexibility[END_REF], for each case tested. Such a measuring system has been introduced and is used to follow the progressive hardening of the adhesive with the increase of temperature and to indicate comparatively if an adhesive system is faster or slower hardening and if it gives stronger joints than another one.

Particleboards preparation and testing

Three identical monolayer particleboards were prepared using the following glue mix: 100 g of purified maritime pine (Pinus pinaster) tannin was dissolved in water to yield a concentration of 40%-45% solids. The pH was corrected to 7 with NaOH solution at 33% concentration. To this were added 10 g (10% solids on solids) Vachellia nilotica exudate extract powder. The glue mix was applied at a level of 10% by weight of adhesive solids content calculated on dry industrial pine wood chips. The wood chips were at a moisture content of 2%. The panels were hot pressed at 220°C, for 7.5 minutes with a pressure cycle of 2.7MPa/1.47MPa/0.5MPa for 2 min/2.5 min/3 min respectively. The panels prepared were 35x35x1.4 cm size. After cooling each panel was conditioned at a temperature of 20°C at 65% relative humidity to reach an equilibrium moisture content of 12% and then cut and 5 samples each of 5 x 5 cm size for each panel were tested for dry internal bond (IB) strength according to European Norm EN319 (1993). Proof of bonding performance is known to be best measured in particleboards by measuring the internal bond (IB) strength. The IB strength is measured by a tension test perpendicular to the plane of the panel according at 2 seconds per mm separation rate and according to European Norm EN319. Control panels using an industrial urea-formaldehyde (UF) resin (Georgia-Pacific LEAFC2 resin) of molar ratio urea to formaldehyde of 1:1.08 , solids content of 66%, viscosity at 20°C of 450 mPa.s and pH of 7.8 were also pressed under the identical conditions. The adhesive was applied at the same resin load level, so that the results could be compared. Ammonium sulphate hardener solution was added to the UF adhesive at the weight ratio of 2% solid ammonium sulphate on total adhesive solids content.

RESULTS AND DISCUSSION

The series of gel time results shown in Figs. 1 indicate that addition of the exudates of Vachellia nilotica or Senegalia senegal to either an African extracted bark tannin or to a well codified pure maritime pine tannin extract yield rapid gel times, well in the range usable for wood panel adhesives.

Please insert here Figure 1 Of particular interest are first of all the comparison of the shape of the gel time curves obtained using these hardeners to cure a pure pine tannin extract Their trend is characteristic of the gel time trend of phenolic materials, markedly accelerating as one passes from pH 4 to pH 9. Moreover, the gel times at pH 9 is around 50 seconds at 100°C for pure maritime pine tannin hardened with paraformaldehyde and around 350 seconds for the same tannin hardened with the raw exudate from either Vachellia nilotica or Senegalia senegal (Fig. 1). Even much more marked differences can be observed at the other pHs tested, with the difference increasing as the pH used shifts towards the pH of mimimum reactivity of the pine tannin.

Thus, comparing the results and the shape of the get time curves at different pHs the mechanism of hardening caused by the exudate extract on pure pine tannin appears to be different from that of an aldehyde (Fig. 1), one main characteristic being that the exudatecaused gel time is much slower than the paraformaldehyde one at all pHs but particularly at the pH of lower reactivity (Fig. 1). This already indicates that the conceiled hardener involved in the exudate is not likely to be an aldehyde as its chemical behavior is rather different..

I

For species I the furan nuclei C3 shift is calculated at 108 ppm (experimental 109 ppm) and its C4 is calculated at 80 ppm (experimental 82 ppm) [START_REF] Pretsch | Spectral data for structure determination of organic compounds, 13 C NMR, 1 H NMR, IR[END_REF]. The shifts between 72 and 98 ppm are the region of carbohydrates (the central peak characteristic of all sugars), with the considerable width of the peaks indicating that these are carbohydrate oligomers of some sort [START_REF] Perlin | Characterization of Carbohydrate Polymers by Carbon-13 NMR Spectroscopy[END_REF]Idstrom et al. 2016). The C=O of an ester is at a shift calculated at 173 ppm (experimental 176 ppm) (Pretsch et al 1989, Wehrli andWirtlin 1978).

The C-OH attached to the furan ring C2 is calculated at 99 ppm (experimental = 104 ppm), meaning that there is another molecule possibly linked to this (Species II) (BioRad Laboratories 1999). It must be pointed out that species I and II are observed in the MALDI spectra (Fig. 3a,b,c,d,e; peaks structure assignements in Table 1), the first at 304 Da (calculated 306 Da) and the second at respectively a calculated and experimental value of 495 Da. (Figs. 3) and the equivalent peak at 522 Da.

Please insert here Figure 3a,b,c,d,e and Table 1 However different species could occur with the same NMR shifts but presenting a different structure, namely in the form of -CH-(OH)2 this does not appear to be the case because the specimen was dry and there is no aldehyde group shift observed in the NMR of the compound indicating that structure IV is highly unlikely. Thus, it is structure III that is the one that is most probable to occur.

A species of type V could also be immagined

+ +

Carbohydrate chains with and without furan moieties attached

The oligomer shown above also occurs in Fig. 3e at 882 +23 (Na + ) = 905 Da (903 Da exp)

and equally with an -OH substituted by an -OCH3 thus at 917 Da (with Na + ) (experimental 918 Da) and with two -OH groups substituted by -OCH3 is represented by the peak at 932.8

Da.

In Fig. 3a are also noticeable oxidized hexoses, thus carrying a -COOH group, such as the one at , and its equivalent 196+23 (Na + ) = 219 Da (experimental 218 Da) and also the unit minus an -OH = 180 Da, this being a gluconic acid (Fengel and Wegener 1989) repeating unit. It occurs at 177.9-178.9 Da (exp.) in Fig. 3a. Please insert here Figure 4a,b,c,d and Table 2 In the condensation product the peak at 138 Da of the 5-Hydroxy-2-hydroxymethyl furan is present as well as the peaks at 128 and 150 Da for the 2,5-dihydroxymethyl furan, together with the peaks at 178 Da and 199 Da characteristic of some oxidized monocarbohydrates (Fig. 4a). From the condensed flavonoid tannin are present in Fig. 4b There are also flavonoid oligomers that have both reacted with 2,5-dihydroxymethyl furan, such as at 601 Da a dimer of catechin + Na + , and at 857 Da a fisetinidin-fidetinidin-catechin trimer +Na + (exp 858 Da).

(exp. 697) a fisetinidin-catechin dimer reacted with one 2,5-dihydroxymethyl furan and of structure XXVI, and at 711 Da (with Na+) (exp 711-713) an equally reacted catechin dimer of structure XXVII (Fig. 4d,e).

XXVIII

A very small proportion of species in which the furan moiety is still linked to a carbohydrate also appear to occur, such as fisetinidin-catechin dimer at 848 Da (exp. 847 Da), the latter presenting structure XXIX (Fig. 4e).

XXIX

Thus, the real hardener present in the exudate extract of the African tree species Vachellia nilotica and Senegalia senegal seyal is 2,5-hydroxymethyl furan both detached from the sugar chains carrying it as well as still attached to the carbohydrate residues carrying it.

After defining the molecular species involved it is wise to test the bonding capability of the combination of a pine tannin resin with the biohardeners. First the systems were tested by thermomechanical analysis. Thermomechanical analysis methods have been developed over many years to determine more rapidly the potential effectiveness of different adhesives systems, experimental or not [START_REF] Pizzi | On the correlation of some theoretical and experimental parameters in polycondensation cross-linked networks[END_REF][START_REF] Kamoun | The effect of humidity on cross-linked and entanglement networking of formaldehyde-based wood adhesives[END_REF][START_REF] Lecourt | Comparison of TMA and ABES as forecasting systems of wood bonding effectiveness[END_REF]. Thus, thermomechanical analysis was first carried out to initially explore the potential of such new adhesive systems. Wood particleboard panels bonded with the combination of biohardener and bioresin were then prepared. The results obtained are shown in Fig. 5 for the TMA and in Table 3 for the internal bond (IB) strength of the particleboard.

Please insert here Figure 5 and Table 3 In Fig. 5 the TMA results indicate that the best performance judged by the highest value of MOE obtained [START_REF] Kamoun | The effect of humidity on cross-linked and entanglement networking of formaldehyde-based wood adhesives[END_REF][START_REF] Lecourt | Comparison of TMA and ABES as forecasting systems of wood bonding effectiveness[END_REF] is when the tannin is used at pH 7.

Pine tannin is so reactive that it is used at pHs near to its lower reactivity (pH 4.2-4.5) when using a fast hardener such as formaldehyde [START_REF] Pizzi | Advanced Wood Adhesives Technology[END_REF]). Thus, it appears logical that its curing will be possibly slower with the biohardners. As the pH is higher pine tannin becomes considerably more reactive, thus a pH of 7 is needed with the biohardeners. Equally, at pH 9 the reaction may start to be slightly too fast to yield optimal results. It is of interest to note from Fig. 1 that the gel times at pHs 7 and 9 are not too different, with the one at pH 7 being slightly slower, while at pH at 3.5 the gel time is very much slower. This notwithstanding, the TMA results in Fig. 5 are rather good at all the three pHs tested. This is likely to be due to different effects: at pH 3.5 the 2,5-dihydroxymethyl furan is the reactive species reacting with the tannin, that instead is near to its minimal reactivity. At pH 9 instead the tannin is very reactive possibly driving the reaction with the hardener. This is so because previous work has indicated that furan hydroxymethyl functions react particularly well with tannin phenolic nuclei at the low pH and high pH [START_REF] Abdullah | Tannin-Furfuryl alcohol wood panel adhesives without formaldehyde[END_REF]. In Fig. 5 it is noticed that reactivity depends from the temperature. At constant temperature increase as the TMA test is done it can be noticed that (i) the slope of the curves, hence the rate of increase of the MOE is more rapid both at pH 7 and pH 9, while it is slower for pH 3.5 and (ii) that the maximum value of the MOE is reached when the adhesive system is at pH 7. At pH 9 the cross-linking reaction being too fast engenders a too rapid immobilization of the hardened network hence a less cross-linked one. At pH 3.5 the reaction being slower, cross-linking is slower hence the network is weaker too.

Having thus determined by TMA the best pH conditions for using the adhesive this was tested by preparing laboratory particleboards. As proof of bonding performance is best measured by the internal bond (IB) strength test by European Norm EN319 the dry IB strength results are shown in Table 1. In Table 1 are also reported the results obtained by bonding wood panels under the same conditions with a synthetic, industrial urea-formaldehyde (UF) adhesive. The IB strength obtained using the experimental adhesive is comparable to that obtained with the control.

CONCLUSIONS

Totally "bio" wood particleboard panels have been prepared with a totally "bio" adhesive using as hardener of a maritime pine tannin adhesive the extract of exudates of the African trees Vachellia nilotica and Senegalia senegal. The analysis of these showed that the reactive species of these hardeners were furan hydroxymethyl species linked to carbohydrate oligomers. These gave 2-hydroxy-5-hydroxymethyl furan and especially 2,5-dihydroxymethyl furan, these second being the reactive species leading to cross-linking. The hydroxymethyl furan moieties were shown to react both after detachment from the carbohydrate chains but also with carbohydrate chains still attached, thus involving the carbohydrates too as part of the binder. The gel times were slower than what obtainable with paraformaldehyde but sufficiently fast to possibly be of industrial significance. exudates with a flavonoid tannin (Fig. 4).

  Please insert here Figure2a,bThe first indication of what can be the structure of the exudate is obtained by CP MAS 13 C NMR (Fig.2a,b). There are different species that respond to the NMR signals present but the most likely one is species I.

  species II the branching carbon attached to the furan C2 has a calculated shift at 82.6 ppm (experimental 82.6 ppm), with the C attached to the next sugar through a glycosidic linkage being foreseen at a calculated 60 ppm (experimental 62 ppm) (BioRad Laboratories 1999).

  species III being confirmed by the MALDI peak at 325 Da (exp 326 Da) thus 302 MW+ 23 Na + . Species IV could exists too but at 300+23 = 323 Da is more far from the experimental 326 Da in Fig 3b. While in presence of moisture often the aldehyde groups are

  would equally present the C3 and C4 of the furan ring both at a calculated shift of 80 ppm (experimental 82 ppm), and the CH2 of the ether linkage at 70 ppm (exp. 71 ppm)[START_REF] Pretsch | Spectral data for structure determination of organic compounds, 13 C NMR, 1 H NMR, IR[END_REF]. However, no species of this type is observed in the MALDI spectra, thus its presence is really unlikely. A species of type VI can also be present, C3 = 108 ppm (exp 109 ppm) C4 80 ppm (exp 82 ppm) C=O of ester 173 ppm (exp 176 ppm)[START_REF] Pretsch | Spectral data for structure determination of organic compounds, 13 C NMR, 1 H NMR, IR[END_REF], sugar carbons between 72 and 98 ppm (the central peak characteristic of all sugars) (Idstrom et al 2016;[START_REF] Lagel | Automotive brake pads made with a bioresin matrix[END_REF]. The CH2 of the furanic hydroxymethyl function is foreseen at 57 ppm (experimental 62 ppm) for both furan and sugars, species VI being observed also in the MALDI as 290+23 (Na + ) = at a calculated 313 Da (experimental 313 Da).What starts to become clear from the above is then that the biohardeners are composed of a carbohydrate chains, for example short glucose chains, possibly hemicellulose fragments, onto which are linked furanic moieties that hydrolyze under certain conditions to obtain a hydroxyl furfuryl alcohol analogues, such as 5-hydroxy-2-hydroxymethyl furan (VII) and 2,5dihydroxymethyl furan (VIII). This species presents C3 shift at 80 ppm (experimental 82 ppm), C4 shift at 108 ppm (experimental 109 ppm), the CH2 of the hydroxymethyl function being foreseen at 57 ppm (experimental 62 ppm) for both furan and sugars[START_REF] Pretsch | Spectral data for structure determination of organic compounds, 13 C NMR, 1 H NMR, IR[END_REF] BioRad laboratories 1999; Idstrom et al 2016;[START_REF] Lagel | Automotive brake pads made with a bioresin matrix[END_REF]. The species is also found at 137 Da (138 Da experimental) with Na + in the MALDI analysis(Fig 3a). the species at 151 Da (experimental 150 Da) with Na + , the species without Na+ also being noticed at 128 Da (calculated and experimental) (Fig.3a).O OH HOVIIIOther two species could occur but their MW with and without Na + MALDI enhancer are further away from the values present that are 128 Da and 151 Da (exp 150 Da). These are Which present respectively 124 Da (IX) and 126 Da (X) without Na+ and 147 and 149 Da with Na+. The first is definitely not occurring, but the second at 126 and 149 Da might also occur although is not noticed in further analysis.Several oligomers presenting the same 13 C NMR shifts but at higher molecular weight can be identified by MALDI ToF. Thus as the species XI to XV.Thus by MALDI at 496 Da (exp 495 Da) in Fig.3cthe species XI responding to the same NMR shifts can be identified (without Na + ) species XIII at 622 Da (exp 619 Da) without Na + (Fig.3d) 785 Da (calculated and 786 Da experimental) without Na + is visible in Fig.3d. This is oligomer XIV. structure (XIV) is a mix of β-D glucuronic acid (the two higher units) with methylglucuronic acid as the lower unit. These type of linkages alternate with glucose to glucose linkages in short chains. The equivalent structure XV due to a mix of just three β-D glucuronic acid units is also possible at the same 785 Da (Fig.3d). structure XVI at 799 Da calculated (exp. 801 Da) without Na + and at 847 Da calculated (exp 845 Da) (Fig.3d,e). at 882 MW+23 = 905 Da (Exp. 903 Da) and equally with an -OH substituted by an -OCH3 thus at 917 Da (with Na+) (experimental 918 Da) and with two -OH groups substituted by -OCH3 is represented by the peak at 932 Da (Fig.3e). also as a repeating tract of oligomer at 910 MW+23 = 933 Da (experimental and calculated) (Fig.3e). species XVIII at 896 MW+23 (Na + ) = 919 Da (exp 919 Da) as well as species XIX with two hydroxymethyl furan reactive function at 911 MW+23 = 934 Da exp and calculated (Fig.3e). to indicate what is the mechanism of action of the biohardeners tried. Thus, there are oligomers of different types all yielding 5-hydroxy-2-hydroxymethyl furan and 2,5dihydroxymethyl furan which are then the real hardening molecules. It cannot be excluded however that the free, reactive hydroxymethyl furan functions of furan moieties linked to the carbohydrate chains may be able to react as such, bringing the attached carbohydrate chain in the reaction as well.The reactions being schematically, for example

  Fengel and Wegener 1989) MW 194, minus one -OH = 178 D as a glucuronic acid repeating unit. It occurs at 193 Da experimental, and 178-179 Da exp.acid (Fengel and Wegener 1989), MW = 210 Da minus one -OH = 194 Da; it occurs at 193 Da (exp). It exists also at 210+23 = 233 Da , (exp 233 Da), as shown in Fig. primary interest to also determine if the 5-hydroxy-2-hydroxymethyl furan and the 2,5dihydroxymethyl furan, obtained by hydrolysis from the sugar oligomers carrying it, are the active hardeners of a tannin-based adhesive. To this purpose MALDI examination of pine tannin hardened by the biosourced Vachelia nilotica exudate extract was carried out to determine the structure of the oligomers and of the cross-linking that takes place on hardening.

  Fig.4e. Structures XXII, XXIII, XXIV show that cross-linking has occurred and that 2,5dihydroxymethyl furan is a rather reactive cross-linker for the pine tannin resin.

  dimers reacted with 2 molecules of 2,5-dihydroxymethyl furan are also present. For example, the peak at 789 Da (with Na + ) (exp 787 Da) (Fig.4d,e) could corresponds a structure of the type

Figure 1 .

 1 FIGURE LEGENDS

Figure 2 .

 2 Figure 2. CP MAS 13 C NMR of (a) the exudate extract of Vachellia nilotica, and (b) of the exudate extract of Senegalia Senegal seyal.

Figure 3 .

 3 Figure 3. MALDI ToF of the exudate extract of Vachellia nilotica: (a) 20 Da-200 Da range. (b) 200Da-400 Da range. (c) 400 Da -600 Da range. (d) 600 Da -800 Da range. (e) 800 Da -1000 Da range.

Figure 4 .

 4 Figure 4. MALDI ToF of the condensation of a maritime pine tannin resin with the exudate extract of Vachellia nilotica: (a) 20 Da-200 Da range. (b) 200Da-400 Da range. (c) 400 Da -600 Da range. (d) 600 Da -800 Da range. (e) 600 Da -900 Da range.

Figure 5 .

 5 Figure 5. Thermomechanical analysis results at different pHs of the maritime pine bark tannin resin during its cure with the exudate of Vachellia nilotica as hardener.

  Figure 1

  Fig. 2b.

  Fig. 4e

  Fig. 5

  The solution is placed in a test tube and this is placed in a water bath, maintained at boiling temperature (100ºC) at normal atmospheric pressure. A wire spring is inserted in the test tube and rapidly moved up and down and the time to gelling is measured by stopwatch. The test is done in duplicate and the average value is reported. This test is a standard FESYP (European Federation of Panels Manufacturers) test and is usely extensively in Europe for wood adhesives(FESYP 1979).

Table 1 .

 1 MALDI ToF peaks. Oligomers structures assignements for the exudates MALDI spectra shown in Fig.3.

	128 Da	HO	OH
		O
	138-139 Da

  K7[c] 25 Oct 2018 12:09 Cal: REF 25 Oct 2018 11:55 Shimadzu Biotech Axima Performance 2.9.3.20110624: Mode Linear, Power: 80, P.Ext. @ 2300 (bin 78) sum= 2625 mV] Profiles 1-200 Smooth Av 50 -Baseline 150 Data: 67B10001.K7[c] 25 Oct 2018 12:09 Cal: REF 25 Oct 2018 11:55 Shimadzu Biotech Axima Performance 2.9.3.20110624: Mode Linear, Power: 80, P.Ext. @ 2300 (bin 78) Profiles 1-200 Smooth Av 50 -Baseline 150 Data: 67B10001.K7[c] 25 Oct 2018 12:09 Cal: REF 25 Oct 2018 11:55 Shimadzu Biotech Axima Performance 2.9.3.20110624: Mode Linear, Power: 80, P.Ext. @ 2300 (bin 78) sum= 75881 mV] Profiles 1-200 Smooth Av 50 -Baseline 150 Data: MA10001.I7[c] 25 Oct 2018 11:57 Cal: REF 25 Oct 2018 11:55 Shimadzu Biotech Axima Performance 2.9.3.20110624: Mode Linear, Power: 83, P.Ext. @ 2300 (bin 78) Profiles 1-200 Smooth Av 50 -Baseline 150 Data: MA10001.I7[c] 25 Oct 2018 11:57 Cal: REF 25 Oct 2018 11:55 Shimadzu Biotech Axima Performance 2.9.3.20110624: Mode Linear, Power: 83, P.Ext. @ 2300 (bin 78) sum= 48471 mV] Profiles 1-200 Smooth Av 50 -Baseline 150 Data: MA10001.I7[c] 25 Oct 2018 11:57 Cal: REF 25 Oct 2018 11:55 Shimadzu Biotech Axima Performance 2.9.3.20110624: Mode Linear, Power: 83, P.Ext. @ 2300 (bin 78)

	Data: 67B10001.K7[c] 25 Oct 2018 12:09 Cal: REF 25 Oct 2018 11:55
	Shimadzu Biotech Axima Performance 2.9.3.20110624: Mode Linear, Power: 80, P.Ext. @ 2300 (bin 78)
	%Int. %Int. %Int. %Int.	170 mV[sum= 33964 mV] Profiles 1-200 Smooth Av 50 -Baseline 150 273.8 13 mV[611.3 379 mV[138.0 242 mV[441.6
	100														156.0
				419.4
															786.8
				617.0											177.9
															349.7
				619.2
				420.5		442.6 655.2	274.8			304.9
	640 650 641 649 0 653	200 200.9 Fig. 3b Fig. 4a 601.2 20 23.7 215.8 220 217.7 627.0 40 231.8 240 39.8 58.0 251.9 260 663.1 60 457.4 72.0 443.6 458.4 427.6 405.4 400 420 440 460 600 620 640 660	280 276.8 86.1 80 679.2 480 680	300 m/z 293.8 294.8 m/z 699.0 713.1 313.7 320 326.8 327.8 335.7 340 128.1 742.9 726.7 732.5 751.0 350.7 757.1 360 367.7 375.5 380 385.6 157.0 760.4 100 120 140 160 180 176.9 150.0 128.5 178.9 114.0 522.5 550.5 494.6 697.0 764.6 711.2 m/z 500 520 540 560 580 700 720 740 760 780 800 400 399.6 199.9 1[c].K7 1[c].I7 200 600 1[c].I7 1[c].K7 m/z
		Up Fig 4c, Down Fig. 4d
	%Int. Fig. 3d 519 mV[sum= 103801 mV] Profiles 1-200 Smooth Av 50 -Baseline 150 Data: 67B10001.441.5 %Int. 78 mV[sum= 15625 mV] 273.7
	%Int.		16 mV[sum= 3274 mV] 858.6
	100
				419.5
															138.0	349.6
							442.6
															156.0
				420.6			274.8
															177.9
												293.8
		200.9	215.8			457.4 251.9	276.8	290.7	309.7	326.9	350.6	375.5	391.4
	651	200 801.0	220 405.6	240 427.7	260 458.5	280		300 494.5 m/z	522.5 320	340	550.5	360 567.1	380	400	1[c].I7
			400	420	440 845.0	460	480	500	520	540	560	580	600	1[c].K7
	643 0 642	23.7 Fig. 4b. 800 Fig. 3c	39.7 43.9 830.8	58.0 850	73.0 72.0 872.7 86.1	114.1 m/z 100.0 900 m/z 888.9 902.5	128.1 128.6 932.8 139.0 918.7	176.9 178.9 168.0 962.4 157.0 150.0 950	198.9 192.9 1000 1[c].K7 199.9
		Fig. 3e
		Fig. 3a	_________________________________________________________________
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