No-Aldehydes Glucose/Sucrose-Triacetin-Diamine Wood Adhesives for
Particleboard

A three reagents adhesive system for wood particleboards not containing any aldehyde was developed by the reaction of glucose or sucrose with triacetin (glycerin triacetate) and with hexamethylene diamine. The system was found to be based on the mix of three reactions, namely the reaction of (i) glucose with triacetin, (ii) of the diamine with triacetin, and (iii) of glucose with the diamine. The chemical species formed were identified by Matrix Assisted Laser Desorption Ionization Time of Flight (MALDI-ToF) mass spectrometry. Wood particleboard panels were prepared with this adhesive system and gave good internal bond (IB) strength results suitable for interior grade panels and with extremely low formaldehyde emission.


Introduction
In the search for alternative adhesives for wood panels not using formaldehyde a large number of different approaches have been taken by a considerable number of research groups. These span from substituting formaldehyde by non-or less toxic non-volatile aldehydes in traditional synthetic adhesives [1,2] to a variety of bio-based adhesives of many different types, from traditional to even rather unconventional approaches [3][4][5][6][7][8][9].
The use of unconventional approaches to wood adhesives is slowly mounting in interest, by use of different polymeric materials or unusual reactions. In this paper a totally new approach, never tried before, partially bio-based, but compounded by non-toxic synthetic materials and their unusual reactions is presented. The adhesive resin is based on the reaction of simple carbohydrates such as glucose and sucrose with triacetin (glycerine triacetate, also a biosourced material) and hexamethylene diamine.
reason that triacetin, an inexpensive chemical, was chosen as it presented characteristics rendering it capable to react with both glucose and a diamine. The systems was tried and the bonding results obtained were sufficiently encouraging to decide to explore what really happened at the molecular level for such an adhesive to work.
The approach can be extended to other diamines or multiamines even biosourced or of natural origin, diamine having been chosen only for the simplicity of demonstrating the concept of this new adhesive. The internal bond (IB) strength results of particleboard bonded with this new approach and the distribution and structure of the complex oligomers formed leading to the final, hardened cross-linked network have also been investigated.
Since its introduction by Karas et al. [11], Matrix-Assisted Laser Desorption/Ionization (MALDI) mass spectrometry (MS) has greatly expanded the use of mass spectrometry towards large molecules and has revealed itself to be a powerful method for the characterization of both synthetic and natural polymers [12][13][14][15][16][17]. Fragmentation of analyte molecules upon laser irradiation can be substantially reduced by embedding them in a light absorbing matrix. As a result intact analyte molecules are desorbed and ionized along with the matrix and can be analysed in a mass spectrometer. The technique is then particularly useful to separate and identify a sequence of oligomers of progressive molecular mass issued from a reaction, without the need of mixture separation. It has been used successfully for such a type of application on wood adhesive resins of several different types [2][3][4]9,10,[18][19][20][21]. This soft ionization technique is mostly combined with time-of-flight (TOF) mass analysers. This is so as TOF + MS present the advantage of being capable to provide a complete mass spectrum per event, for its virtually unlimited mass range, for the small amount of analyte necessary.

Reaction of Glucose with Triacetin and Hexamethylene Diamine
The adhesive system was prepared as follows: 100 g of glucose were mixed with 81 g of triacetin and 100.02 g of water, and heated to 60°C for 60 min in a glass vessel equipped of reflux condenser and mechanical stirring. 162.96 g hexamethylene diamine were then added to the mixture and heated to 90°C for 60 min, then cooled to room temperature. The glucose-triacetin-diamine resin obtained had a pH of 8.9, a viscosity at 25°C of 205 mPa.s and a density of 1.032 g/ml. The sucrose-triacetin-resin obtained had a pH of 9.1, a viscosity of 148 mPa.s and a density of 1.025 g/ml.

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

Wood Particleboard Preparation and Testing
Duplicate one layer particleboard panels of 350 × 350 × 14 mm dimensions were prepared by adding 10% of the resin solids mixture on dry wood particles, for a percentage wood moisture content of the resinated particles of 13% and pressed at a maximum pressure of 28 kg.cm 2 followed by a pressuredecreasing pressing cycle, at 220°C for a 10 minutes pressing cycle. The target density of the panels was of 0.75 g/cm 3 .The panels, after light surface sanding, were tested for dry internal bond (IB) strength (EN 312) [22], 24 h cold water swelling and for formaldehyde emission (EN 120) [23].

Results and Discussion
The possibility of a mixed complex reaction between different materials was originally due to the search for monosaccharides-based non-polyurethane (NIPU) for adhesives, coatings and foams [3,4,10,24]. During such a work awareness arose that, even without producing NIPUs, alternative reactions occurred to reach a hardened network that can be used as an adhesive. One of these reactions, leading to a type of polyamides, was first explored as a binder for wood particleboard with more than encouraging results [9]. The second set of reactions discovered for the formation of alternative types of adhesives is the one described here. It is based on three concomitant reactions, namely (i) the reaction of glucose or sucrose with triacetin (glycerine triacetate), and (ii) the reaction of a diamine with triacetin, and (iii) the reaction of a diamine with monosaccharides, and their interactions leading to system cross-linking. These three reactions can be schematically represented as follows: Reaction (1) Leading to the two products shown indicated and to other compounds too.
Reaction (2) Reaction (3) To show this, the MALDI ToF mass spectrometry gives a good insight in the compounds formed. The assignment for the peaks in Fig. 1 are listed in Tab. 1.
Among these three categories of products are noticed according to the three reactions given above. Thus the peaks at 279 Da, 442 Da and 643 Da represents the progressive substitution of the acetate ion by glucose on the glicerine structure of the triacetin as described in reaction (1) above. The peaks at 297 Da, 351 Da and 407 Da are the first products of the progressive substitution of the acetate ion by the diamine as per reaction (2)  Without and with Na+, connecting three triacetin molecules in which the acetate ion is partially or totally substituted by three diamines. This reaction appearing to be fairly easy to occur. Second observation is that compounds having a structure of two glucoses linked by a diamine do not occur whatsoever. This means that polymerization to higher oligomers and cross-linking cannot occur through this reaction alone, contrary to reactions (1) and (2) where the reaction can proceed to higher oligomers at suitable molar ratios of the reagents. As the system proceeds further to higher structures in which more glucose hydroxyls have reacted with the diamine, and both glucose and diamine have reacted with triacetin, the point of gelling and hardening is reached.
Having thus defined the chemical reactions occurring in this three reagents mixture, one has to check if the system can be used as a wood panel adhesive. Laboratory single layer particleboards were prepared and then tested according to EN 312 [10] for internal bond (IB) strength, 24 hours thickness swelling and formaldehyde emission (EN 120) [11]. The results show in Tab. 2 indicate that for the adhesive system the IB strength satisfies well the requirements for interior grade panels of relevant standards, and moreover that the formaldehyde emission is extremely low, being limited to just the level of what is emitted by the heated wood furnish as no aldehydes are present in the adhesive system itself.

Conclusions
The adhesive system based on the unusual set of concomitant reactions between a monoasaccharide (such as glucose or sucrose ) with triacetin and with a diamine yielded mixed product species based on the reaction of glucose with triacetin, of glucose with hexamethylene diamine ad of the diamine with triacetin. It led to polymerization and cross-linking through the formation of chemical species comprising all the three reagents linked together through the three different reactions. Confirmation of the capacity of this novel, no-aldehyde, wood adhesive system to bond particleboard panels was  obtained by the good IB strength results suitable for interior grade particleboard and the extremely low formaldehyde emission obtained.
Funding Statement: The author(s) received no specific funding for this study.
Conflicts of Interest: The authors declare that they have no conflicts of interest to report regarding the present study.