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Urea-formaldehyde resins and preparation of UF/nanoclay composites

Three different UF resins were used. For particleboard a commercial UF resin of urea: formaldehyde molar ratio 1:1.1 was used. For plywood both a commercial UF resin and a laboratory manufactured resin both of urea:formaldehyde molar ratio 1:1.5. The laboratory UF resin was prepared according to the following commercial resin procedure: 478.5 g formurea (a liquid concentrate composed of 57% formaldehyde, 23%urea and 20% water obtained from Dynea, Krems, Austria) were charged under continuous mechanical stirring in a glass reactor equipped with a reflux condenser, thermometer and pH electrode and the temperature raised to 50 °C. Urea (114.1 g) in water (111.5 g) was then charged while the temperature was maintained at 50 °C.
The pH was then adjusted to 4.8-5.0 with a 22% NaOH water solution and 8.4 g melamine were charged in the reactor. The temperature was increased to 70 °C and the pH adjusted to 7.2-7.6. The temperature was then raised as fast as possible to 90 °C. The pH now dropped by itself to 6.3-6.6. Once 90 °C had been reached the pH is adjusted to 5.4-5.6. The temperature was increased to 95 °C and 85.1 g second urea added to the reacting mixture. When the viscosity, measured at 50 °C, reached 220-250 centipoises the pH was adjusted in the 7.0-7.5 range with a 22% NaOH water solution.
Immediately after adjusting the pH, 0.6 g borax was added, followed by 36.4 g third urea, the temperature decreased rapidly to 65-70 °C and maintained at thislevel for 25 min. The resin is then cooled down to 25-30°C and the pH adjusted to 8.0-8.5 with a 22% NaOH solution. The characteristics of the finished resin where pH 8.0-8.5; solids content 64%-66%, gel time of 50-80s at 100 °C; U:F molar ratio 1:1.5. The UF resin was mixed with the montmorillonite clay by mechanically stirring for 5 min at room temperature and then leaving the mixture overnight.

Preparation of PF/nanocaly composites

This study is the only one in the literature so far, involving liquid phase mixing of the wood adhesive phenol formaldehyde resin with clay. Therefore, two different preparation routs of PF/nanoclay composites were tried.
The first route is based on adding montmorillonite during the resin cooking. The phenol-formaldehyde (PF) resins were prepared at molar ratio P:F 1:1.76. The preparation procedure was as follows: 1.5 moles of phenol were mixed with 2.65 moles formaldehyde (as a 37% formalin solution) in a glass reactor equipped with mechanical stirring, thermometer and reflux condenser. NaMMT was then added at room temperature and the whole mixture was stirred overnight (12 hours). The temperature was then increased to reflux (95°C) in 30 min under continuous mechanical stirring. Once 95 °C reached, 0.25 moles NaOH (as a 30% water solution) were then added in 5 lots with each lot at 10 minutes interval. The mixture was maintained at reflux until the viscosity of resin reached 400-500 mPa·s, measured at 25 °C. The resin was then cooled and stored.
The second route, mixing after resin cooking, was as following: The PF resin was prepared exactly as the same as the first route only without NaMMT being used. The desired amount of clay was then mixed with the PF resin by a thoroughly stirring for 1 h at 45-50 °C.


Thermomechanical analysis

The hardening reaction of the glues mixes can be evaluated by TMA by studying the rigidity of a wood-resin joint as a function of temperature. Thus, different glues mixes as indicated in the figures and tables were analyzed by TMA in bending according to a technique already reported[7]. Triplicate samples of two beech wood plys (sliced decorative beech wood (Fagus sylvatica) veneers) of 0.6 mm thickness bonded with the test resins as a layer of 350 m , for a total samples dimension of 21×6×1.2 mm were tested with a Mettler 40 TMA apparatus (Mettler-Toledo, Giessen, Germany). In three points 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 (6 s/ 6 s). The classical mechanics relation between force and deflection in bending E = [L3/(4bh3)][F/( f)] , where L is the span, b the width, h the thickness of the specimen, F the force exercised on it and f the resultant deflection, allows the calculation of the Young’s modulus E for each case tested. All TMA tests were conducted under the same conditions: heat rate = 10°C / min, 30mg of resin system, temperature range is 25-250°C. The software used for data treatment is STARe (France). Deflection curves that permit MOE determination have been obtained by three point bending mode. The MOE of a wood-resin system gives a good indication of the end strength of the final application of the glue tested. The MOE max value and its increase as a function of time or temperature for wood-resin systems give a good indication of the possible end performance of the adhesive system tested.

13C CP-MAS NMR spectra

The hardened soy specimens, lignin specimens, and the original lignin and soy flour were analysed by solid state 13C CP-MAS NMR. Spectra were obtained on a Bruker MSL 300 FT-NMR spectrometer at a frequency of 75.47 MHz and at sample spin of 4.0 kHz. The impulse duration at 90 °C was 4.2 ms, contact time was 1 ms, number of transients was about 1000, and the decoupling field was 59.5 kHz. 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.

FT-IR analysis

Solid state FT-IR spectra of the original lignin, lignin after treatment and the products for them to react with glyoxal were obtained by preparing KBr pills on a Shimadzu FT-IR 8200 infrared spectrophotometer (Champs sur Marne, France).

X-ray diffraction

Wide angle X-ray analysis (XRD) was carried out to investigate the effectiveness of the clay intercalation and if any change in crystalline structure of the UF resin and phenolic resin occurred. XRD samples of UF resins hardened at 103 °C in an oven after mixing with it 0%, 4% and 9% NaMMT were powdered and mounted on a Phillips XRD powder diffractometer for analysis. A 2 angle range from 2° to 100° in reflection mode was scanned at 2°/min. A computer controlled wide angle goniometer coupled to a sealed tube Cu source K radiation ( = 1.54056 Å ) was used. The Cu K radiation was filtered electronically with a thin Ni filter.
The interlayer could be calculated when possible from the (001) lattice plane diffraction peak using Bragg’s equation[8]. Some consideration on the cristallinity level of UF resins and phenolic resins with and without Na-MMT could be obtained from the XRD investigation.

Differential Scanning Calorimetry

Differential scanning calorimetry (DSC) was done on UF and phenolic resins. To UF, 2% ammonium sulphate hardener was added as a 30% solution in water and 0% and 4% Na-MMT nanoclay was added. This was done to study the influence of the nanoclay on the hardening rate of the UF resin. To PF resins, 0% and 6% Na-MMT nanoclay was added. DSC scans were carried out with a A Perkin-Elmer DSC spectrometer. PYRISTM Version 4.0 software was used for data acquisition and processing. The DSC was calibrated with a standard sample before analysis. Large volume capsules containing samples were heated from 25 to 250 °C at a heating rate of 10 °C/min to obtain the exothermic curing reaction curves. Samples were cooled and reheated to establish the baseline.

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MALDI-TOF-MS analysis

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-TOF-MS) sample preparation are as follow: the sample was dissolved in water (4 mg/mL). The sample solution was mixed with an acetone solution (10 mg/mL acetone) of the matrix. As the matrix, dithranol was used. NaCl was not added to the matrix. The solutions of the sample and the matrix were mixed in equal amounts, and 0.5 μL of the resulting solution was place on the MALDI target. After the evaporation of the solvent, the MALDI target was introduced into the spectrometer.
The MALDI-TOF mass spectra were recorded on a Kratos Kompact MALDI 4 instrument (Shimadzu Corporation, Kyoto, Japan). The Irradiation source was a pulsed nitrogen laser with a wavelength of 337 nm. The length of one laser pulse was 3 ns. The measurements were carried out under the following conditions: a positive polarity, a linear flight path, a high mass (20-kv acceleration voltage), and 100-150 pulse per spectrum. The delayed extraction technique was with delay times of 200-800 ns. The mass peaks corresponded to M + Na (from natural abundance) and M + H attached cations.

Particleboard manufacture and testing

For natural adhesive, one-layer laboratory particleboard with dimensions of 350×300×14 mm were prepared with only core particles of beech (Fagus sylvatica) and Norway spruce (Picea abies) wood mixture at a maximum pressure of 28 kg/cm2 and a press temperature of 195°C. The resin solids load was maintained at 10% of the total mix of lignin or soy-based adhesive. The total pressing time was maintained at 7.5min. But a series of boards prepared progressively reducing the pressing times was also done for soy-based adhesives. All particleboard were tested for dry internal bond (IB) strength.
For The performance of the UF/nanoclay composites was tested by preparing laboratory plywood and wood particleboard and evaluating respectively their tensile and internal bond (IB) strengths.
For UF/nanoclay composites, duplicate one layer laboratory particleboard of 350×310×16 mm dimensions were prepared by adding 10% total resin solids content on dry wood particles. 28kg/cm2 was the maximum pressure used, followed by a decreasing pressure cycle. Press temperature at 195 °C and a total pressing time of 5 min were used. The panels’ constant thickness was fixed by placing metal stops between the pressing platens. The aimed average density for all the panels was 700 kg/ m3. The panels, after light surface sanding, were tested for dry internal bond (I.B.) strength. Formaldehyde emission was measured by the dessicator test method according to standard specification AS/NZS 1859.1 2004[9].

Plywood manufacture and testing

Duplicate three-layer laboratory plywood panels of 450×450×6 mm were prepared using two urea-formaldehyde (UF) adhesives of U:F molar ratio = 1:1.5 and beech (Fagus sylvatica) veneers. To all these glue mixes were added (a) 2% ammonium sulphate hardener, solids on UF resin solids, the ammonium sulphate being predissolved to a 30% solution in water, and (b) either wheat flour or sodium montmorillonite (Na-MMT) by weight on the UF resin solids content used, as indicated in the tables and the figures. The glue-spread used was of 300-320 g/m2 of liquid glue-mix double glue line (dgl). Pressing time was 5 minutes at 120°C and 11 kg/cm2 pressure. The plywood panels were cut according to EN 314. After being tested for dry tensile strength other specimens were placed in boiling water for 15 and 25 minutes and several of them tested for residual tensile strength. Moisture resistance was evaluated by immersing in boiling water tensile strength specimens for 15 or 25 minutes followed by drying them overnight at room temperature.

Table of contents :

I.2.1. Tannin structure and composition[1-3]
I.2.2. Tannin reaction with aldehydes
I.2.3. Acid and alkaline hydrolysis and autocondensation
I.2.4. Sulfonation
I.2.5. Chemistry and technology of industrial tannin adhesive formulations
I.3.1. Chemical background
I.3.2. Utilization of lignin in phenol-formaldehyde (PF) wood adhesives
I.3.3. Chemical modification of lignin
I.4.1. Characteristics of soy-based adhesive
I.4.2. Modification of soy protein
I.4.3. Application of soy-based adhesive in the wood panels industry
I.4.4. Conclusions
I.5.1. Activable fillers
I.5.2. Inert fillers
I.6.1. Main objectives and main work
I.6.2. Main characteristic and innovation of this work
II.1.1. Phenol-formaldehyde resin preparation and tannin extract used
II.1.2. Lignin and glyoxalation of lignin
II.1.3. Heat and pressure treatment of lignin
II.1.4. Soy flour –formaldehyde resin preparation[4-5]
II.1.5. Soy flour-formaldehyde-lignin (or phenol) preparation
II.1.6. Soy flour-glyoxal preparation
II.1.7. Blending of glyoxalated glyoxalated lignin and/or soy flour with tannin or phenolic resins and pMDI
II.1.8. PUF resin preparation
II.1.9. Montmorillonite nanoclays
II.1.10. Urea-formaldehyde resins and preparation of UF/nanoclay composites
II.1.11. Preparation of PF/nanocaly composites
II.2.1. Thermomechanical analysis
II.2.2. 13C CP-MAS NMR spectra
II.2.3. FT-IR analysis
II.2.4. X-ray diffraction
II.2.5. Differential Scanning Calorimetry
II.2.6. MALDI-TOF-MS analysis
II.2.7. Particleboard manufacture and testing
II.2.8. Plywood manufacture and testing
III.2.1 Study on the performance of different glyoxalated lignin formulations
III.2.2. Effects of glyoxal on the glyoxalated lignin formulation
III.2.3. Effects of type of lignin on the performance of glyoxalated lignin formulation
III.2.4. Effects of pMDI on the performance of glyoxalated lignin formulation
III.2.5. Strength results of glyoxalated lignin-based particleboard
III.2.6. Effects of heat treatment on the glyoxalated lignin
IV.2.1. Performance analysis of soy-formaldehyde resins
IV.2.2. Performance of glyoxalated soy-based adhesives
IV.2.3. Structure analysis of soy-based adhesives
IV.2.4. Effect of preparation process of particleboard on its performance
IV.2.5. Effects of glyoxal on the structure of lignin
IV.2.6. Effects of glyoxalated lignin on the performance of mix-glue
V.2.1. Select of MMT
V.2.2. XRD results of MMT/UF resin system
V.2.3. DSC analysis of MMT/UF resin system
V.2.4. Influence of addition percentages of Na-MMT on resin’s performance
V.2.5. Curing characteristic study on MMT/UF resin
V.2.6. Effects of Na-MMT on the performance of plywood
V.2.7. Effects of Na-MMT on performance of particleboard
V.2.8. Comparation study of influence of Na-MMT and wheat flour on UF resin
V.2.9. Determination of addition amount of Na-MMT for UF resins
VI.2.1. Effects of mixing methods of Na-MMT with PF resins on the performance of resin system
VI.2.2. XRD results of Na-MMT/PF resin system
VI.2.3. Plywood performance boned with Na-MMT/PF resin
VI.2.4. Influence of Na-MMT with different percentage on the performance of PUF resin
VI.2.5. XRD analysis of Na-MMT/PUF resin system
VI.2.6. Performance of Na-MMT/PUF resin
VI.2.7. DSC analysis of Na-MMT/PF resin
VI.2.8. Effects of MMT types on performance of PF-based plywood


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