Glycerol and Citric Acid Treatment of Lodgepole Pine

Keywords: Impregnation, Biopolymer, Glycerol, Citric acid, Esterification, Wood, Dimensional stability, Hardness, Roughness Diane Schorra,b, Pierre Blancheta,b , Gatien Geraud Essoua Essouaa,b

a Département des sciences du bois et de la forêt, Université Laval, G1V 0A6, 2325 Rue de l’Université, Québec, QC, Canada; b CIRCERB (Chaire industrielle de recherche sur la construction écoresponsable en bois), Université Laval, G1V 0A6, 2325 Rue de l’Université, Québec, QC, Canada;

Abstract

As part of a research program to expand the potential of wood products in exterior applications, lodgepole pine wood was impregnated with a solution of glycerol and citric acid to improve its dimensional stability and other relevant properties. Two catalysts were studied: one liquid (hydrochloric acid, HCl) and one solid (nanoclay, Cloisite 30B). After impregnation, the specimens were polymerized at two temperatures (160°C and 180°C). The nanoclay dispersion and reticulation analysis were performed by XRD and TEM methods. Other analysis methods such as SEM, DSC, TGA, FT-IR were used to characterize the treatment solutions, polymers and treated specimens. The dimensional stability, hardness, adhesion strength and roughness of the specimens were measured and analyzed. Cloisite 30B proved to be a good catalyst, as it contributed to improving the dimensional stability and hardness of the wood while outperforming the HCl catalyst in counteracting the adhesion strength loss attributed to the wood-polymer composite. The polymerization temperature also affected the results. At 180°C, dimensional stability and adhesion strength were higher than at 160°C but the loss of treatment solution was also higher. Impregnating wood with a glycerol ester could drastically improve its physical properties, including dimensional stability and hardness, particularly when Cloisite 30B nanoclay is used as a catalyst. More work is required to better quantify reticulation in the polymer and optimize the polymerization temperature (properties versus loss of material).

Introduction

World demand for exterior siding was estimated to be 4.8 billion square meters in 2014. Of this total, 4.3% was for wood products [1]. However, wood siding is expected to lose market share because of a higher price and maintenance requirements compared to the other materials (concrete, metals and polymers). Exterior conditions do affect the performance of wood siding more than that of competing materials. Dimensional stability, for example, can be a problem with wood based products [2, 3]. It is therefore important for the wood siding industry to find solutions to these problems, as well as lower selling prices and maintenance requirements. Wood is constituted of cellulose, lignin, hemicelluloses and extractives. These macromolecules have hydroxyl groups that attract water by hydrogen bonds. As a result of variations in moisture content, wood swells or shrinks, with negative effects on physical and mechanical properties. Various treatments have been applied to counter these negative effects. Heat treatment was tested as a means of improving wood resistance to decay [4] as well as dimensional stability and performance in the presence of moisture [5]. Many studies have focused on the chemistry of wood. For example, research has shown that wood esterification does improve not only dimensional stability [6] but also other properties such as thermal stability [7, 8]. Studies on wood impregnation with a polymer such as polymethyl metacrylate (PMMA) [9, 10], a glyoxal aqueous solution [11] and glycerol [12] have demonstrated that impregnation could yield better

1

dimensional stability, better protection against fungi and also better mechanical properties than for untreated wood. The combination of the chemical reaction and cell wall bulking clearly achieve great results for wood properties. In the case of ester bonds, for example, Despot et al observed that wood impregnated with a solution containing citric acid was more resistant to fungi [13]. Impregnating wood with a mixture of glyoxal/glycerol and boric acid [14] or with a mixture of glycerol and maleic acid [15] also improved dimensional stability and resistance to fungi. In another study, Essoua Essoua et al studied the impregnation of white pine (Pinus strobus L.) and lodgepole pine (Pinus Contorta D.) wood with glycerol and citric acid using hydrochloric acid (HCl) as the catalyst. They observed an improvement in many physical properties such as dimensional stability, hardness and resistance to fungi. However, the polymerization temperature (180°C) may have been too high, causing swelling or cracks that damaged the wood. Adhesion strength was also reduced. In a new approach, nanoclay additions have been tested with some success in coatings [16-18], composites [19, 20] and impregnated wood [21-23] to improve mechanical properties, thermal properties, hydrophobicity and dimensional stability. In this study, the wood was impregnated with glycerol and citric acid and a comparison of two catalysts (HCl and nanoclay) was studied to find the best properties as well as the best curing temperature (160°C and 180°C). The dispersion of nanoclay in the polymer was studied. The resulting impregnated wood was tested for several physical and mechanical properties, including adhesion strength, a significant factor in the manufacture of wood siding.

Materials and Method

Lodgepole pine (Pinus contorta D.) wood from Western Canada was used for this study. Wood planks were cut into specimens of different sizes for the different standard tests (Table 1).

Table 1: Wood specimen dimensions for the different tests

The wood specimens were chosen without knots, resin pockets and sapwood. Specimens were prepared in longitudinal, tangential and radial direction. They were conditioned at 20 ± 2°C and 40 ± 4% relative humidity (RH) to constant mass. Citric acid (CA) and hydrochloric acid (HCl) were obtained from Sigma-Aldrich Co., St Louis, USA. Glycerol (G), a by-product of biodiesel production, was obtained from Rothsay Biodiesel, Ville Sainte-Catherine, Quebec, Canada. A chemical characterization of the raw glycerol showed that it contained 89% of glycerol, 7.9% of water, 2.4% of ashes and less than 0.5% of non-glycerol organic matter. Its density was 1.214 g/cm-3. Finally, Cloisite 30B (30B) organoclay produced by Southern Clay Products (USA) was chosen as a reinforcing agent and catalyst. 30B is a commercial natural montmorillonite (MMT) modified with a quaternary ammonium salt (Table 2).

2

Test Dimensions (mm)Dimensional stability 20 x 20 x 10Water sorption 3 x 3 x 3Pull-off test 60 x 60 x 10Hardness test 50 x 20 x 20Density 50 x 50 x 20

Table 2: Characteristics of Cloisite 30B organoclay (MT2EtOH: methyl, tallow, bis-2-hydroxyethyl, quaternary ammonium)

  Organic modified Modified concentration

% moisture

% weight loss on ignition

Density g/cm3

d001 (Å)

Cloisite 30B

MT2EtOH,

90 meq/100 g < 2% 30% 1,98 g/cm3

18,5 (Å)

Preparation of glycerol/citric acid/water (G/CA/W) solution

For 1 mole of the G/CA/W solution, 0.1325 mole of citric acid and 0.6771 mole of water (W) were mixed during 5 minutes. Then, 0.1904 mole of raw glycerol was added to the solution. It was stirred until complete solubility of the citric acid in the solution at 70°C (approximately 45 minutes). After the solution had cooled down, 2% by weight of the catalyst (HCl or Cloisite 30B nanoclay) were added. HCl was added to the solution and then mixed during 5 minutes. Cloisite 30B was dispersed in the solution ().

Figure 1: Preparation of G/CA/W solution with the two different catalysts (HCl and Cloisite 30B nanoclay)

Dispersion of nanoclay

Two percent (2%) by weight of Cloisite 30B were pre-mixed in 20% v/v of the total solution during 5 minutes. Then, the rest of the solution was added. During 5 minutes, the solution was mixed for homogenization. The nanoclay was dispersed with a high intensity ultrasonic horn. The parameters were fixed as follows: 4 X 10 minutes of ultrasound with 40 kV power. The solution was kept under 60°C in a cooling bath set at 25°C.

Characterization of nanoclay dispersion

3

X-ray diffraction analysis (XRD)

Small-angle X-ray scattering was used to study the efficiency of the ultrasonication technique in dispersing the nanoclay platelets in the polymer solution. XRD experiments were conducted by an X-ray diffractometer (Siemens/Bruker, USA). Cu was used as the radiation source. The instrumentation included a Kristalloflex 760 generator, a 3-circle goniometer and a Hi-Star area detector. The polymer containing the nanoclay was cured at 180°C for 2 hours and then ground into a fine powder. For the nanoclay powder and the admixture, each was inserted into thin-walled (0.01 mm) glass capillary tubes (1.0 mm diameter). They were examined separately using Cu kα radiation (𝜆 = 1.5406 ˚A) at 40 kV and 40 mA. The interlayer spacing between the nanoclay platelets was calculated according to Bragg’s law: 𝑛𝜆 = 2𝑑001 sin 𝜃, with 𝜆 being 0.15406 nm and d001 the interlayer platelet distance.

Transmission electron microscopy (TEM)

Transmission electron microscopy observation was carried out to determine the internal structure, spatial distribution and dispersion of the nanoclay within the ester polymer. Analyses were performed on a JEOPL JEM-1230 (JEOL Corporation, Japan), transmission electron microscope at 80kV. After polymerization at 180°C or 160°C during 2 hours, the polymer was crushed into a fine powder. Then it was mixed and dispersed in methanol using an ultrasound bath during 15 minutes. A volume was taken and put on a carbon grid. High-magnification images (x 40 000) were recorded at an acceleration voltage of 80 kV.

Characterization of solution polymerization at different temperatures

FT-IR spectroscopy

FT-IR measurements were performed with a Fourier Transform Infrared spectrometer (ATR-FT-IR/FT-NIR Perkin Elmer Spectrum 400, UK). A resolution of 4 cm-1 was chosen for each analysis, 64 scans being recorded to decrease noise. All the spectra were recorded between the wavenumbers 4000 and 650 cm-1. A background test was performed before every analysis. Finally, all the ATR FT-IR spectra were baseline-corrected.

Differential scanning calorimeter (DSC)

To characterize the glass transition temperature of the polymer solution cured at different temperatures (100°C, 160°C and 180°C) during 1 h, DSC was performed on a Perkin Elmer Pyris series DSC 8000 (UK). The program was chosen as follows: 5 minutes at - 80°C, then the temperature increased from - 80°C to 130°C at a rate of 20°C/min. A nitrogen gas purge was used at 20 ml/min. The analysis was conducted three times for each specimen.

Thermogravimetric analysis (TGA)

To study the thermal degradation of the polymer solution cured at different temperatures (23°C, 100°C, 160°C and 180°C) during 1 h, thermal gravimetric analyses were performed on a Mettler Toledo TGA/DTA 851e (UK). Four (4) mg of polymer were used to determine mass loss during heating. The specimen was heated from 25°C to 700°C at a rate of 10°C/min. Nitrogen was used as specimen purge at a flow of 35ml/minand balance gas was 50 ml/min. For each TGA analysis, derivation was performed to DTG curves. Thanks to these DTG and TGA curves, the water, citric acid, ester and ash content were determined. Two repetitions per specimen were analyzed.

4

Wood impregnation

Lodgepole pine wood specimens were impregnated with either of the test solutions. The specimens were placed in a pressure reactor vessel, model 4522, from PARR Instrument Company, Illinois, USA. The HCl-G/AC/W solution (256 cp at 23°C) or the 30B-G/AC/W solution (360 cp at 23°C) was then introduced into the reactor vessel and a vacuum was applied at 720 kPa for 45 min. The pressure and reaction time varied with specimen dimensions ( ). At the end of the impregnation process, the specimens were removed from the reactor and any excess of solution was wiped off the surfaces. This procedure was similar to that described in the Essoua Essoua et al, 2016 study [24]. In this study, however, the clean impregnated specimens were pre-cured at 90°C for 24 h before high-temperature curing at 160°C or 180°C in an oven for 2 hours (). After the specimens had cooled down at ambient temperature, their surfaces were cleaned with a saw. All dimensions were checked to ensure that they met the tests requirements. The cured specimens were maintained under constant conditions at 20 ± 2°C and 40 ± 4% RH until all the tests had been completed. Table 3: Pressure and reaction time parameters for the impregnation of lodgepole pine wood specimens Test Dimensions (mm) Pressure (kPa) Time (h)Dimensional stability 20 x 20 x 10 2068 2Pull-off 60 x 60 x 10 2758 3.5Hardness 50 x 20 x 20 2758 3.5Density 50 x 50 x 20 2758 4.5

Figure 2: Summary of the impregnation and polymer curing process in lodgepole pine wood

Density profiles

5

To determine density profiles, an X-ray densitometer system QDP-01X (Quintek Measurement Systems Inc, USA) was used on the same specimens before and after treatment. The test was performed in general accordance with the ASTM D2395 (2014) test method. Six specimens of 50 x 50 x 20 mm dimensions (length x width x thickness) of each wood species were selected, measured and studied with the densitometer.

Dimensional stability

The determination of swelling and shrinkage was performed according to ISO standards 4469 and 4859 respectively. The results were used to calculate the anti-swelling efficiency (ASE sw) and anti-shrinkage efficiency (ASEsh) of the treatments. Two hundred and twenty specimens, with dimensions 20 x 20 x 10 mm, were subjected to three cycles of water immersion (at ambient temperature during 72h) and oven dried (24h at 103°C ± 2°C). Radial and tangential dimensions were measured on each specimen after each step. Anti-swelling efficiency was calculated as per Equation 1:

Sw (%) = [(R1 - R0/R0 x 100] + [(T1 - T0)/T0 x 100] (1)

And shrinkage efficiency was calculated as per Equation 2:

Sh (%) = [(R1 - R0)/R1 x 100] + [(T1 - T0)/T1 x 100] (2)

With Sw and Sh being the swelling and shrinkage coefficients; R0 and T0 the radial and tangential dimensions before the swelling test; R1 and T1 the radial and tangential dimensions after the swelling test.To characterize dimensional stability, the anti-swelling efficiency (ASE sw) and anti-shrinkage efficiency (ASEsh) factors were calculated for the treated specimens relative to those of the untreated specimens as indicated in Equations 3 and 4 below.

ASEsw (%) = (Sw0 - Swt)/Sw0 x 100 (3) ASEsh (%) = (Sh0 - Sht)/Sh0 x 100 (4)

Where Sw0 and Swt are respectively the swelling coefficients of untreated and treated specimens, and Sh0 and Sht the shrinkage coefficients of untreated and treated specimens. Water vapor sorption

To study the effect of the different impregnation solutions and esterification temperatures on the equilibrium moisture content (EMC) of the treated wood specimens, a VTI-SA and vapor sorption analyzer (TA Instrument, USA) was used. Small specimens of 3 x 3 x 3 mm were used to characterize the different test conditions (controls without impregnation or heating; wood that was only heated at 160°C or 180°C, and wood treated with HCl solution, 30B solution at the two esterification temperatures of 160°C and 180°C). Relative humidity (RH) in the test room was ramped up from 0 to 95%. Room temperature was constant at 25°C. The reduced equilibrium moisture contents (EMCr) were calculated according to Equation 5 below: EMCr = (M2 - M1)/M0*100 (5)

With M0 the weight of the dried untreated wood; M1 the weight of dried treated wood, and M2 the weight of the treated wood at equilibrium with the ambient air at a given relative humidity.

6

Hardness

A Qtest/5KN (MTS, USA) system was used to study the specimens’ surface hardness. These tests were performed in general accordance with ISO standard 1534 (2000) but the procedure had to be somewhat modified. The Brinell hardness is normally obtained by measuring the diameter (in millimeters) of the residual indentation with a digital caliper. This measurement is not very accurate. Instead of using the Brinell equation, three parameters were set: the surface detection load at 10 N; the high extension limit, 1 mm, and the extension speed at 2 mm/min. The ball diameter was 10 mm. After impregnation, the wood specimens were sawed, cleaned and then conditioned at 20 ± 2°C and 60 ± 4% RH. Two indentations were performed per specimen, one on the tangential face and the other on the radial face. The load applied to achieve the indentation criteria was the factor considered to compare the different treatments. For impregnated wood, 30 specimens were tested per condition, and 20 specimens were tested for the untreated wood. During the hardness test, the room was kept at 20 ± 2°C and 60 ± 4% RH.

Wood morphology

The morphology of the untreated and treated wood specimens was studied by SEM with a JSML-6360 LV device (JEOL corporation- Japan). The wood specimens were cut into 10 x 10 x 10-mm cubes, and then coated with gold for specimen preparation. Parameters for the analysis were as follows: the acceleration voltage was 15 kV and the images were taken with 1000X magnification.

Surface roughness

The topography and the surface roughness of untreated and impregnated wood specimens were analyzed using a 3D non-contact profilometer from Bruker (USA) and a contour GT-K 3D profilometer. Light interferometry is used to quantify surface roughness. The magnification was 5X with a multiplier of 1X. For average roughness determination, each surface was measured twice on a 3 x 3-mm surface area of every specimen, three different specimens being measured per treatment. The average roughness of each treatment specimen was thus based on six different areas of 9 mm2.

Adhesion strength

All the impregnated specimens were sanded with sandpaper (aluminum oxide type, Sia abrasives Inc., Switzerland) grit 150, mounted on a Costa belt sander (model 36CCK 1150 Costa and Grisson Machinery CO Inc., Italy). The feed speed was 8 m/min.. A white opaque coating (self-crosslinking water-based acrylic for exterior applications manufactured by Cabot, USA) was applied with a high-volume low-pressure (HVLP) spray gun. Three layers were applied onto one of the specimen surfaces. The first and second film layers were found to be 2 mils and the last layer was 3 mils of wet paint. The coated specimens were then dried at room temperature and 24% RH for 24 h. For the pull-off strength test, a 20-mm aluminum dolly from DeFelsko Corporation, Ogdensburg, NY, USA was cleaned, with dust removed. A marine epoxy adhesive (from Henkel Canada Corporation, Mississauga, ON) was used. The two adhesive components were first mixed without air bubbles during one minute before being applied to the dolly surface in a 1-2 mm layer. The dolly was pasted at the center of each specimen surface with low manual pressure. The excess of adhesive was cleaned. Each coated specimen had one dolly placed at the center of its coated surface. The specimens were then cured in a conditioning chamber at 20 ± 2°C and 60 ± 4% RH for 24h. The pull-off test machine was a Material Testing System (MTS),

7

QTest/5kN model from MTS systems Corporation, Eden Prairie, MN, USA. The dollies were placed in the machine and a load was applied perpendicularly to the dolly at a specimen displacement rate of 2 mm/min.; 30 untreated and treated specimens were tested. During the test, the room was kept at 20 ± 2°C and 60 ± 4% RH. Adhesion strength was calculated with Equation 6 below:

Adhesion strength (MPa) = 4Fmax/πd2 (6)

Where “Fmax” is the maximal load applied (N); and “d” is the dolly diameter (mm)

Statistical analyses

The Statistical Analysis System (SAS) 9.3 was used to analyze the density, hardness, pull-off strength and roughness results. An analysis of variance (ANOVA) was performed on this experimental plan (two factors: catalyst: HCl and Cloisite 30B; and temperature: 160°C and 180°C).

Results and Discussion

Nanoclay dispersion

Impregnation mixture homogeneity is important for a good impregnation process. To assess this, the quality of the dispersion and exfoliation of the nanoclay in the formulation was characterized by X-ray diffraction (XRD), a technique that allows for the distance between nanoclay layers to be determined withBragg’s law. Any intercalated clay shows up on the diagram. The shift of a characteristic peak towards a smaller angle indicates a higher distance between the layers. Figure3 shows the diagram of the pure Cloisite 30B and that of the G-CA-W solution with 2% of 30B. The d001 peak at 4.6° in the pure Cloisite 30B diagram shows that the distance between the layers according to Bragg’s law was 1.85 nm. This result was in accordance with several studies that used this same clay [17, 18]. For the solution, the disappearance of the d001 peak could have two explanations. One is that the clay may have been totally exfoliated in the solution. The polymer penetrated between the platelets. The layers were thereby independent from one another, so the distance between the layer was infinite (no peak) [25]. A second explanation could be that the Cloisite 30B content (2%) in the solution was too low for a peak to be observed (too small) (Landry et al., 2008). The test was therefore inadequate to confirm the total exfoliation of the clay and other techniques were required to analyze the results, such as transmission electron microscopy (TEM).

8

1.6 2.6 3.6 4.6 5.6 6.6 7.6 8.6 9.60

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

G-CA-W-2% 30B30B

2 Theta (°)

Inte

nsity

(cou

nts)

Figure 3: XRD patterns for the pure nanoclay 30B (dotted curve) and for the polymer G-CA-W with 2% of Cloisite 30B (solid curve) polymerized at 180°C

The TEM technique has been used extensively to study the exfoliation of nanoclay in polymers [16,18, 26]. As shown in Figure 4, two polymers were studied by TEM with high magnification (scale 200 nm). Both polymers contained 2% of 30B, however one was polymerized at 160°C (Figure 4 a and b) and the other at 180°C (Figure 4 c and d). In all the images, single clay platelets were observed (black arrows). Aggregates were still observed, however, (Figure 4 b and d white arrows) for both polymerization temperatures. These results show that the clay was partially, but not completely, exfoliated in both polymers, even though no peaks were observed on the XRD diagram. No differences were observed between the 160°C and 180°C treatments. It was actually quite difficult to observe the Cloisite 30B in the polymer cured at 160°C, possibly because of a lower degree of reticulation than with the 180°C treatment.

9

Figure 4: Dispersion of Cloisite 30B in the polymer: TEM images of (a and b)-polymer with 2% of 30B polymerized at 160°C at high magnification (scale: 200 nm) and (c and d) polymer with 2% of 30B polymerized at 180°C at high magnification (scale: 200 nm). Black arrow: single platelet and white arrow: aggregates

Study of the solution and its polymerization

What happen inside impregnated wood structure at polymerization time? To have an information about impregnation mixture polymerization, mixture without specimen was studied. Several analytical technics were performed.

FT-IR spectroscopy

The citric acid, glycerol and water in the mix form new chemical bonds through esterification reactions generating a crosslinked polymer [27]. The esterification reaction is affected by the curing temperature and the catalysts used. The effects of these parameters were assessed with FT-IR spectra, and the results are shown in Figure 5. Citric acid and glycerol were studied alone.

10

b

c d

a

Their spectra were then compared with those of the G/CA/W solution in relation to the catalysts used (without, with HCl and with Cloisite 30B) and to the temperature (23°C, 160°C and 180°C). The citric acid spectrum showed two important peaks at 1741 and 1688 cm -1. These peaks correspond respectively to the anhydride form of anhydrous citric acid and to the free carboxylic acid C=O stretching vibration. For the glycerol, the most important bands were the stretching vibration of the hydroxyl groups (O-H), a large band near 3280 cm -1; two bands at 1105 and 1032 cm-1 which respectively represent the O-H deformation of secondary and primary hydroxyls in the glycerol; and the stretching vibration of C-O at 1642 cm -1. Analysis of the FT-IR spectra for the G/CA/W mix at different temperatures and with different catalysts indicated that esterification had occurred in all the specimens. The shift of the peak at 1688 cm -1 of citric acid to 1709-1721 cm-1 in all the G/CA/W solution conditions confirmed the formation of ester bonds [28, 29]. The peak observed at 1200-1166 cm-1, corresponding to the ester C-O stretching vibration, also indicated the formation of ester bonds. The occurrence of esterification was further confirmed by a decrease of the bands characteristic of the stretching vibration and deformation of O-H hydroxyl bonds, and a decrease of the stretching vibration of C-O glycerol bonds at 1032 cm -1. However, some differences were observed between these spectra. The broad band at 3400 cm-1 was more pronounced at 23°C than at 160°C or 180°C because high temperatures favor esterification at the expense of O-H hydroxyl groups. Besides, the characteristic peak of the C=O stretching vibration in ester and carboxylic acid bonds (1709-1721 cm-1) revealed important information. Shifting of the peak to a higher wavenumber is indicative of more ester bonds and fewer carboxylic acid groups from free citric acid. This means that more ester bonds were formed at 180°C (1721 cm -1) than at 160°C (1715 cm-1) and even more than at 23°C (1709 cm -1). This was also confirmed by the band at 1645 cm-1, characteristic of the glycerol, observed on the spectrum at 23°C, which declined sharply at the higher temperatures. No major differences were observed between 160°C and 180°C and also between the two catalysts used except for a shift of the peak corresponding to the C=O ester stretching vibration, which increased with the curing temperature at 180°C, and was higher with the 30B catalyst than with HCl. Deconvolution of this peak would provide useful information on ester quantification and excess citric acid in the polymer [30].

11

Figure 5: FT-IR spectra of the polymer solution (CA/G/E) without and with catalysts (HCl and Cloisite 30B) at three different curing temperatures (a- Glycerol and citric acid at 23°C; and G/CA/W solutions with and without catalysts at b- 23°C, c- 160°C and d- 180°C)

Differential scanning calorimetry (DSC)

Figure 6 shows the glass transition temperature (Tg) of the polymer cured at three different temperatures (100°C; 160°C and 180°C during 1h). The Tg of raw glycerol was -34°C. The Tg of the glycerol-citric acid-water solution varied with the catalyst used. At 100°C, the Tg was higher (8.7°C) with HCl than with nanoclay (-2.5°C) and than without a catalyst (-2.2°C). Increasing the polymerization temperature from 160°C to 180°C had no effect on Tg regardless of the catalyst used, but a lower value was observed in the absence of a catalyst. With the highest temperature

12

d-c-

b-a-

used (180°C) the Tg rose to 52°C; 62°C and 59°C respectively without a catalyst, with HCl and with 30B. These results may suggest that the reaction was initially faster with HCl than with the other catalyst (30B) because of its higher Tg value, while the differences became smaller at higher temperatures. The increase of Tg may be explained by the crosslinking created by esterification in the polymer at high temperature. Furthermore, when the polymer is more reticulated, the movements of the molecular chains are constrained so it takes more heat for the chain to move and break out from a rigid glassy state to a soft rubbery state [31]. So, the most reticulated polymer, with the highest Tg, seems to be the polymer at 180°C with HCl.

without

cata-lyst HCl 30B

-100

102030405060

Tg (°

C)

Figure 6: Glass transition temperature from DSC curves for polymers with different catalysts at different polymerization temperatures

Thermogravimetric analyses (TGA)

Thermogravimetric analyses performed on the different polymers (without catalyst, with HCl and with 30B) at different temperatures (23°C, 100°C, 160°C and 180°C) provided an assessment of the polymerization process in relation to the curing temperature. Examples of TGA and Derivative thermo-gravimetric (DTG), curves are shown in . The DTG curve shows three peaks of rate degradation. The first one is under 125°C, which is mainly attributed to the water content. Another peak is present around 210°C, which could be associated with the degradation rate of free citric acid still present in the polymer. Holser (2008) confirmed the presence of free citric acid in the TGA/DTG curves of their formulations of starch, citric acid and glycerol around 200°C [32]. It could also be related to glycerol degradation, which reaches its maximum near 245°C. The last large peak, observed at 370°C, results from the degradation of the rest of the polymer. Finally, the ash content was obtained from the TGA residue at the end of the experiment (under nitrogen). Ashes are considered to be the char resulting from the cross linkage process occurring at high temperature under inert atmosphere.

13

25 125 225 325 425 525 6250

20

40

60

80

100

-0.001

-0.0009

-0.0008

-0.0007

-0.0006

-0.0005

-0.0004

-0.0003

-0.0002

-0.0000999999999999999

0

TGA-30B-2%-23C DTG-30B-2%_23 °CTemperature (°C)

Wei

ght (

%)

DTG

(%/m

in)Wa-

ter

Figure 7: TGA and DTG curves for the citric acid-glycerol-water-2% 30B polymer at 23°C

The TGA curves yielded information on water, citric acid and glycerol, glycerol ester and ash contents (Figure 8). At 23°C, the water content of the solution with nanoclay proved lower than that of the other solutions, this being due to the use of ultrasounds to disperse the nanoclay, which increased water evaporation. For the three polymers, the water content decreased with increasing polymerization temperature up to 160°C, with no difference between the 180°C and 160°C conditions. Nor were differences observed between the catalysts. The citric acid and glycerol contents decreased with increasing polymerization temperature, especially with the Cloisite 30B catalyst at 180°C. The ester glycerol content increased with the increase in temperature. The glycerol ester content also seemed higher with Cloisite 30B than with HCl or without a catalyst. This could be explained by differences in the way the catalysts operate, one hypothesis being that, with Cloisite 30B, mono esterification could be privileged over di or tri esterification, so more citric acid molecules are used. On the other hand, the HCl catalyst may have promoted multi esterification, hence fewer citric acid molecules being used with more acid function of the same molecule reacting. This effect may create a more reticulated polymer, a result that would be in line with those presented above with DSC. Finally, the ash content also increased with increasing polymerization temperatures, especially with the catalysts at high temperature (160°C-180°C). At high temperature under inert atmosphere, rearrangement of the polymer causes the formation of a char product that is difficult to degrade. Different studies have shown that the more a polymer is reticulated, the higher its ash content during a TGA analysis under inert atmosphere [27]. This study confirmed that a higher curing temperature generates a more reticulated polymer. The analyses focusing on the catalysts suggests that the hypothesis proposed above on the effect of catalyst on the type of esterification could also be valid. With HCl favoring the formation of multi esterification more than Cloisite 30B catalyst, the resulting polymer contained more ashes, and could have been more reticulated than with the Cloisite 30B, a result that would agree with the DSC results. To confirm these hypotheses, it will be necessary to quantify mono, di and tri esters in the different polymers in relation to temperature and the catalysts used. This could be done with high-performance liquid chromatography (HPLC), gas chromatography (GC) or even by

14

Ester of glycerol

Citric acid

Ashesr

titration. Some studies have already focused on such analyses for other esters of glycerol [15, 33-35]

without cat HCl 30B 2%0

5

10

15

20

25Water content

23°C 100C 160°C

Cont

ent (

%)

without cat HCl 30B 2%0

5

10

15

20

25

30

35

40Citric acid content and glycerol

content

23°C 100 °C160°C 180°C

without cat HCl 30B 2%0

10

20

30

40

50

60

70Ester glycerol content

23°C 100C 160°C

Cont

ent (

%)

without cat HCl 30B 2%0

5

10

15

20

25Ashes content

23°C 100C160°C 180°C

Figure 8: Water, citric acid, ester and ash content of polymers in relation to catalysts used and polymerization temperature

Study of impregnated wood

Color and aspect of the impregnated wood

After impregnation and polymerization, the specimens were examined for color, deformation and cracks. The loss of impregnating material was also calculated on 55 specimens per formulation. For specimens polymerized at 160°C, no loss of material was observed. No deformation or cracks were observed on all the 160°C specimens (with HCl or Cloisite 30B). With the 180°C specimens however, some loss was observed. With HCl as a catalyst, 14.5% of the specimens were deformed or displayed some cracks. With Cloisite 30B nanoclay as a catalyst, the material loss was only 7.25%, i.e 50% lower. The loss of material (due to deformation or cracks) is a serious problem for the industrial process, since it increases production costs. The loss material came out because of specimens shrinkage under heating effect. Figure 9 illustrates treated specimens produced under different conditions at the end of the process. In terms of esthetics, color can be

15

critical if the wood is not coated. The Cloisite 30B catalyst seemed to yield better results in this respect, the corresponding specimens having a more natural color than those produced with HCl, which presented a grey or black color (Figure 9).

Figure 9: Pictures of impregnated and cured specimens in relation to the catalyst used (HCl or Cloisite 30B) and to the esterification temperature (160°C and 180°C)

Density of untreated and treated wood specimens

The weight percent gain of wood specimens after treatment is an important parameter in the industrial system. This affect greatly the product transport and truck load. Figure 10 shows specimen density before and after treatment for each catalyst and esterification temperature used. The density of the untreated lodgepole pine specimens averageds 394 g/cm3. Specimen density increases with polymer impregnation as the solution is stored in the voids present in the wood (tracheids for example). After polymerization at 160°C or 180°C, the polymer is reticulated by esterification reaction and becomes solid and permanent. For wood impregnated with the solution containing HCl, the density was 572 g/cm3 and 562 g/cm3 depending on the polymerization temperature (respectively 160°C and 180°C). With the 30B catalyst, the corresponding densities were 621 g/cm3 and 623 g/cm3. According to the statistical analysis of the results, the density was not significantly affected by the catalyst (HCl or 30B nanoclay) or by the polymerization temperature (160°C or 180°C).

16

Before treatment

G/CA/W with HCl at

160

Before treatment

G/CA/W with HCl at

180

Before treatment

G/CA/W with 30B at

160

Before treatment

G/CA/W with 30B at

180

391

572

399

561

393

621

394

623DE

NSIT

Y

Figure 10: Density of untreated and treated specimens. For each set, values followed by the same letter are not significantly different, while values followed by different letters are significantly different at a 0.05 probability level

Dimensional stability

Improved dimensional stability is a major advantage for wood used as exterior siding. displays the ASEsw-sh (Anti swelling efficiency and Anti shrinkage efficiency) values of the various specimen conditions as a function of the swelling and shrinkage cycle. For the 180°C polymerization temperature, the ASEsw-sh was the same for both catalysts, averaging 49%. Essoua Essoua et al, 2016 found an ASE of 60% for swelling and 59% for shrinkage for the impregnation of lodgepole pine wood with the same solution using HCl [24]. The difference with this study was the 24h pre-drying step at 90°C, which decreased the ASE sw-sh average while allowing for fewer failures in the esterification phase of the process. For the 160°C temperature, differences were observed in connection with the catalysts. The ASEsw-sh of the specimens produced with Cloisite 30B as a catalyst showed a 10% advantage over those based on HCl (38% on average against 28% with a standard deviation of 2%). Other authors have observed dimensional stability improvements with the addition of nanoclay various polymers used for wood impregnation [21, 36]. For example, Leemon et al., (2015) observed a dimensional stability improvement ranging from 6% to 17% according to nanoclay concentration in a thermoset polymer (low molecular weight phenol formaldehyde) used to impregnate wood [21]. In the current study, the differences observed with respect to the temperature of polymerization (180°C and 160°C) could be explained by the fact that, at 160°C, the Cloisite 30B nanoclay catalyst may have acted as an additional water barrier by comparison with the HCl catalyst. The nanoclay would therefore reinforce the effect of the reticulated polymer in the treated specimens. At 160°C, the ASEsw-sh values were higher with Cloisite 30B than with HCl. At the 180°C level however, dimensional stability may only be due to the better reticulation of the polymer, the natural water barrier provided by the clay having no significant effect on the results. The Cloisite 30B catalyst therefore seems more useful at a 160°C polymerization level as it acts as a catalyst as well as a water repellent.

17

Swelling I Shrinkage I Swelling II Shrinkage II Swelling III Shrinkage III

48.2%

45.6%

49.1%51.7%

49.4%45.4%47.6% 45.8% 48.3%

51.5% 49.8%

44.2%41.3%

35.8%38.9%

40.8%

37.9%

35.6%36.3%

29.6% 28.9%31.5% 31.4%

24.0%

HCL 180C 30B 180C 30B -160C

ASE

(%)

Figure 11: Average ASEsw-sh for 3 cycles of swelling and shrinkage in relation to catalyst and polymerization temperature

Vapor sorption

shows the sorption isotherms of the controls (untreated wood at 23°C) and the untreated and treated wood heated to 160°C and 180°C. The same results were observed for both polymerization temperatures. The EMC at 95% RH of untreated wood processed at 160°C or 180°C was 19.6% and 18.3% respectively, slightly higher than that of the unheated wood at 23°C (17.8%). By comparison, the specimens treated at 160°C displayed EMCs of 13.8% and 14.3% respectively for the HCl and 30B catalysts, the corresponding numbers being 9.8% and 10.5% at 180°C. A higher polymerization temperature led to lower EMCs, which may be explained by the higher reticulation of the biopolymer occurring at 180°C. This result is of much significance in the improvement of wood products designed for non-soil contact application. Given that dimensional stability is governed by moisture uptake, the lower EMCs would provide a real benefit. This observation correlates with the previous results associating the highest ASE sw-sh

values to the 180°C polymerization temperature. No major differences were observed between the catalysts.

18

Cycle sequences

0 10 20 30 40 50 60 70 80 90 10002468

101214161820

Untreated wood at 23 °CUntreated wood at 160°CTreated wood-30B-160°CTreated wood-HCl-160°C

RH (%)

EMC

(%)

0 10 20 30 40 50 60 70 80 90 10002468

101214161820

Untreated wood at 23 °CUntreated wood-180°CTreated wood-HCl-180°CTreated wood-30B-180°C

RH (%)EM

C (%

)

b)

Figure 12: Adsorption and desorption isotherms of untreated wood at 23°C and untreated and treated specimens (with HCl or Cloisite 30B catalysts) at 160°C (a) and at 180°C (b)

Specimen microstructure (SEM assessment)

shows scanning electron microscopy (SEM) images of the microstructure of untreated and treated (HCl or 30B at 160°C or 180°C) wood cells. In the untreated specimens, the cells were empty except for some that contained natural resin. For the treated specimens (images b- to e-), each SEM image shows cells filled with a polymer, which explains why the treated specimens were denser than the untreated ones, as observed in Figure 10. The wood cells not filled with a polymer already contained natural resin or exhibited a defect that had blocked polymer penetration into the cell, as observed in image e. However, some differences were observed in relation to the catalysts and the polymerization temperature. With both catalysts (HCl or 30B) the polymer was smoother and contained fewer cracks when cured at 160°C than at 180°C. An explanation for this could be that the more reticulated polymer obtained at 180°C (as confirmed by the DSC results) became more rigid, thus more likely to develop cracks or irregularities. Differences were also noted with respect to the catalysts. The HCl catalyst seemed to produce a polymer with a smoother surface than did the Cloisite 30B, so much so that it even proved difficult to locate cell limits. This may be due to better polymer penetration resulting from the lower viscosity of the treating solution or from the use of a liquid catalyst (HCl) rather than a solid catalyst (Cloisite 30B).

19

a)

Figure 13: SEM images of untreated specimens, treated specimens with HCl at 160°C and 180°C and treated specimens with 30B at 160°C and 180°C

Hardness

Figure 14 displays the loads applied to measure the hardness of the untreated and treated wood specimens with the two temperatures (160°C and 180°C) the two catalysts (HCl and Cloisite 30B) under study. Hardness was generally greater in the tangential direction than in the radial direction, which was to be expected given the orthotropic material properties (longitudinal, tangential and radial) of wood [37].

Radial direction Tangential direction

383456

604

893

519

721730

1047

649

925

Untreated specimen HCl160 HCl 180 30B 160 30B 180

Load

(N)

Figure 14: Average load values applied to the untreated and treated specimens in the radial and tangential directions. Specimens identified with the same letter are not significantly different,

20

b*d*c*b*bdcb a*a

e-d-

c-b-a-

Natural wood resin

Natural wood resin

while identification with different letters indicates significant differences at the 0.05 probability level.

The polymer-impregnated specimens showed better hardness than the untreated ones. This confirms that wood impregnation with the glycerol/citric acid/ water solution is effective in increasing the hardness of wood. , the polymer (with HCl or 30B) filled up the empty cell cavity, and became solid during the heating process, which consequently increased the material’s hardness. The gains varied, however, with the curing temperature and the catalyst, the best hardness performance being obtained with the nanoclay catalyst. Several studies have demonstrated improvements in hardness and mechanical properties with the addition of various nanoclay particles in composites (Wang et al., 2014; [19]; and also in impregnated wood [21, 22]. As for temperatures, the lower polymerization temperature (160°C) yielded better hardness results regardless of the catalyst used. This observation was similar to those reported in Ates et al 2009 for a study on Calabrian pine wood treated at different temperatures (130°C, 180°C and 230°C) and over different times (2 and 8 h). The authors observed that a heat treatment of wood had a beneficial effect on hardness, especially on the tangential and radial directions. However, too high a temperature (180°C and 230°C) combined with exposure over 2h could curtail hardness gains, and even bring it lower than that of non-heat-treated wood [38]. In this study with lodgepole pine, the best hardness results were obtained with specimens treated with a solution using Cloisite 30B, and polymerized at 160°C. With the Cloisite 30B, the hardness results had improved by 191% and 230% respectively in the radial and tangential directions over those of the untreated specimens, as compared with 121% and 117% with the HCl catalyst. The superior performance obtained with the nanoclay catalyst may be due to a combination of the nanoclay and the polymer creating another reticulation between the wood and the polymer, particularly at the surface of the treated wood [21, 36, 39]. As regards the better hardness results obtained at the lower temperature, the SEM images shown in revealed a smoother polymer in the wood cells with the 160°C curing temperature than with 180°C, and this with both catalysts. As already suggested, the polymer resulting from the higher temperature was more reticulated and seemed more rigid, thus more fragile and prone to cracking, which could have a negative effect on hardness.

Surface roughness (3D non-contact optical profilometry)

shows the three-dimensional surface profiles obtained from untreated and treated specimens cured with two different catalysts (HCl and Cloisite 30B) at two different polymerization temperatures (160°C and 180°C). For each specimen, roughness was measured on two different areas of 9 mm2. Two different specimens were tested for each condition.

21

Figure 15: 3D optical profilometer images of the surface roughness of untreated and treated specimens (with HCl and 30B catalysts) at 160°C and 180°C

provides the surface roughness results obtained in this study, which indicated that the polymerization temperature did not affect surface roughness but the catalyst did. According to the statistical analyses (multiple comparison), the specimens cured with the 30B catalyst exhibited a higher degree of surface roughness than those cured with HCl or not treated. This is in accordance with the observations made from the SEM images, and it could also be explained by the presence of nanoclay particles at the treated wood surface.

Untreated wood HCl 160 HCl 180 30B 160 30B 180

4136

41

4648

Roug

hnes

s (µm

)

Figure 16: Surface roughness of untreated and treated specimens (with HCl and 30B) at 160°C and 180°C

22

Different treatments

Adhesion strength

displays the adhesion strength results obtained for all the specimens studied. The untreated wood specimens yielded the highest coating adhesion strength. As regards the treated specimens, some differences were observed in relation to the catalysts and the temperatures. For both catalysts, the higher polymerization temperature led to an increase in adhesion strength: from 2.18 MPa (at 160°C) to 2.35 MPa (at 180°C) for HCl, and from 2.57 to 3.12 MPa for Cloisite 30B. The better performance achieved with Cloisite 30B could be explained by the presence of solid clay particles at the surface of the treated wood, making it rougher as suggested in the 3D optical profilometer analysis. The SEM analysis () had also shown how the catalyst could have an effect on coating adhesion. It appeared, however, that the roughness advantage provided by the Cloisite 30B catalyst in terms of coating adhesion was not sufficient to deliver the adhesion strength level of the untreated wood.

untreated wood HCl160 HCl 180 30B 160 30B 180

3.39

2.18 2.35 2.573.12

Adhe

sion

stre

ngth

(Mpa

)

Figure 17: Average adhesion strength values of the untreated and different treated wood specimens. Specimens identified with the same letter are not significantly different, while identification with different letters indicates significant differences at the 0.05 probability level.

The fracture types described in Table 4 were based on visual observations of the dolly and wood surfaces in accordance with the ASTM D 4549 (2009) test method. There are three types of fracture: wood failure, wood-coating fracture and coating layers fracture. As can be seen, the untreated and treated specimens with Cloisite 30B at 160°C showed all three types of fracture but wood failure was prevalent. The other treated specimens showed only two types of fracture (wood failure and wood-layer fracture). These results show that the weakness affecting adhesion strength lies between the treated wood and the coating because of the impregnation treatment. With the polymer filling up cell cavities, penetration of the coating into the wood structure is inhibited and mechanical anchorage greatly reduced.

23

Different treatments

Table 4: Type of fracture between the wood and dolly surfaces

Type of fracture

Wood specimens wood failure

wood-coating failure

Coating layers failure

Untreated wood 78% 9% 13%HCl 160°C 72% 28% 0%HCl 180°C 75% 25% 0%30B 160°C 79% 14% 7%30B 180°C 90% 10% 0%

Conclusions

With the aim of improving the material properties of wood for exterior applications, lodgepole pine wood specimens were impregnated with a glycerol-citric acid-water (G-CA-W) mix. The study covered two different catalysts (HCl and Cloisite 30B) and two polymerization temperatures (160°C and 180°C). The initial phase of the study indicated that the Cloisite 30B nanoclay was dispersed in the polymer solution (G-CA-W) but partially exfoliated. The solutions containing a catalyst (HCl or 30B) performed better, in terms of esterification and polymerization, than those without a catalyst. HCl may have created a more reticulated polymer than Cloisite 30B whereas Cloisite 30B may have increased the mono ester content in the polymer. More research is required on this subject to better quantify reticulation in the polymer (mono, di and tri esterification content). In the second phase of the study, lodgepole pine wood specimens were impregnated with the G-CA-W solution with the two different catalysts, and polymerized in situ at 160°C or 180°C. Different observations were obtained during this study:

- There was more treating material loss with the HCl catalyst at the higher curing temperature (180°C).

- Wood treated with the Cloisite 30B catalyst was closer to the color of untreated wood than was the HCl-treated wood, whose color was grey.

- Impregnated wood had a higher density than untreated wood because of the presence of the polymer in the cell cavities, as confirmed by SEM images. The polymerization temperature had no effect on density, and nor did the catalysts.

- Impregnated wood proved significantly more dimensionally stable than untreated wood, especially at the 180°C curing temperature, where the choice of a catalyst made no difference. At the 160°C level, however, the Cloisite 30B catalyst led to better dimensional stability than did the HCl catalyst.

- The best hardness results were obtained with impregnated wood using the Cloisite 30B catalyst and cured at 160°C, while the highest adhesion strength was achieved with Cloisite 30B and a 180°C curing temperature.

Impregnating wood with a glycerol ester could drastically improve its physical properties, including dimensional stability and hardness. Moreover, the use of Cloisite 30B nanoclay as a catalyst in the formulation offers great potential for enhancing the adhesion strength of coatings and the aesthetical aspect of the treated wood. Impregnated wood treated as described in this research should prove attractive for outdoor applications such as exterior siding, where improved durability and resistance to weathering are critical.

24

Bibliography

1. Freedonia Group inc. World Siding, Freedonia Focus Reports World Collection. Cleveland, Ohio, USA. 2015, p. 1-32.

2. Hill, C.A.S. Wood Modification chemical thermal and other processes. Chichester, UK: John Wiley and sons, Ltd. 2006,

3. Nicholas, D.D. Wood deterioration and its prevention by preservative treatments Volume 1: Degradation and protection of wood. Syracuse, NY: Darrel D. Nicholas Editors. 1973.

4. Mohareb, A.; Sirmah, P.; Pétrissans, M.; Gérardin, P. Effect of heat treatment intensity on wood chemical composition and decay durability of Pinus patula. Eur. J. Wood and Wood Prod. 2012, 70(4), 519-524.

5. Li, X.; Cai, Z.; Mou, Q.; Wu, Y.; Liu, Y. Effect of heat treatment on some physical properties of Douglas fir (Pseudotsuga menziesii) wood. Adv. Mat. Res., 2011, 197-198 (2011), 90-95.

6. Salla, J.; Pandey, K. K.; Prakash, G. K.; Mohadevan, K. M. Photobleaching and Dimensional Stability of Rubber Wood Esterified by Fatty Acid Chlorides. J. Wood Chem. Technol., 2012, 32(2), 121-136.

7. Thiebaud, S.; Borredon, M. E.; Baziard, G.; Senocq, F. Properties of wood esterified by fatty-acid chlorides. Bioresource Technol., 1997, 59(2–3), 103-107.

8. Gardea-Hernández, G.; Ibarra-Gomez, R.; Flores-Gallardo, S. G.; Hernandez-Escobar, C. A.; Perez-Romo, P.; Zaragoza-Contreras, E. A. Fast wood fiber esterification. I. Reaction with oxalic acid and cetyl alcohol. Carbohyd. Polym., 2008, 71(1), 1-8.

9. Kumar, B.; Zaidi, M. G. H.; Rathore, S.; Rai, A. K.; Thakur, I. S.; Sah, P. L. Optical, Morphological, Thermal, Mechanical, and Fungal Characterization of Wood Polymethyl Methacrylate Composites. Instrum. Sci. Technol., 2006, 34(1-2), 67-83.

10. Soulounganga, P.; Loubinoux, B.; Wozniak, E.; Lemor, A.; Gérardin, P. Improvement of wood properties by impregnation with polyglycerol methacrylate. Eur. J. Wood and Wood Prod., 2004, 62(4), 281-285.

11. Yan, Y.; Dong, Y.; Li, C.; Chen, H.; Zhang, S. Optimization of reaction parameters and characterization of glyoxal-treated poplar sapwood. Wood Sci Technol, 2015, 49(2), 241-256.

12. Ghazali, H.; Yusoff , R. Effets of glycerol impregnation on compressive strength parallel to grain of acacia auriculiformis upon drying Born. Sci., 2011, 29, 1-9.

13. Despot, R.; Hasan, M.; Jug, M.; Sefc, B. Biological durability of wood modified by citric acid. Drvna industrija, 2008, 59(2), 55-59.

14. Toussaint-Dauvergne, E.; Soulounganga, P.; Géraldin, P.; Loubinoux, B. Glycerol/Glyoxal: A New Boron Fixation System for Wood Preservation and Dimensional Stabilization. Holzforschung. 2000, 54, 123-126.

15. Essoua Essoua, G.G.; Blanchet, P.; Landry, V.; Beauregard, R Maleic Anhydride Treated Wood: Effects of Drying Time and Esterification Temperature on Properties. Bioresources, 2015, 10(4), 6830-6860.

16. Kaboorani, A.; Riedl, B.; Blanchet, P. Ultrasonication Technique: A Method for Dispersing Nanoclay in Wood Adhesives. Journal of Nanomaterials, 2013, (2013), 1-9 .

17. Landry, V.; Riedl, B.; Blanchet, P. Nanoclay dispersion effects on {UV} coatings curing. Prog. Org. Coat., 2008, 62(4), 400-408.

18. Nkeuwa, W.N.; Riedl, B.; Landry, V. UV-cured clay/based nanocomposite topcoats for wood furniture: Part I: Morphological study, water vapor transmission rate and optical clarity. Prog. Org. Coat., 2014, 77(1), 1-11.

19. Khanjanzadeh, H.; Tabarsa, T.; Shakeri, A. Morphology, dimensional stability and mechanical properties of polypropylene–wood flour composites with and without nanoclay. J. Reinf. Plast. Comp., 2012, 31(5), 341-350.

25

20. Subasri, R.; Hima, H. Investigations on the use of nanoclay for generation of superhydrophobic coatings. Surf. Coat. Technol., 2015, 264, 121-126.

21. Leemon, N. F.; Ashaari, Z.; Anwar U., M. K.; Bakar, E. S.; Tahir, P. Md.; Ridzuan S., M. A.; Ghani, M. A.; Lee, S. H. Characterisation of phenolic resin and nanoclay admixture and its effect on impreg wood. Wood Sci Technol, 2015, 1-16.

22. Devi, R. R.; Maji T. K. In situ polymerized wood polymer composite: effect of additives and nanoclay on the thermal, mechanical properties. Mater. Res, 2013, 16, 954-963.

23. Rahman, R. Md; Hamdan, H.; Islam, Md. S.; Ahmed, A. S. Influence of Nanoclay/Phenol Formaldehyde Resin on Wood Polymer Nanocomposites. Journal of Applied Sciences, 2012, 12(14), 1481-1487.

24. Essoua Essoua, G.G.; Blanchet, P.; Landry, V.; Beauregard, R. Pine Wood Treated with a Citric Acid and Glycerol Mixture: Biomaterial Performance Improved by a Bio-byproduct. Bioresources, 2016, 11(2), 3049-3072.

25. Zhou, X.; Pizzi, A.; Du, G. The effect of nanoclay on melamine-urea-formaldehyde wood adhesives. J. Adhes. Sci. Technol., 2012, 26, 1341-1348.

26. Agubra, V.; Owuor, P.; Hosur, M. Influence of Nanoclay Dispersion Methods on the Mechanical Behavior of E-Glass/Epoxy Nanocomposites. Nanomater., 2013, 3(3), 550.

27. Halpern, J.M.; Urbanski, R.; Weinstock, A.K.; Iwig, D. F.; Mathers, R. T.; Von R., H. A. A Biodegradable Thermoset Polymer Made by Esterification of Citric Acid and Glycerol. J. Biomed. Mater. Res. Part A, 2014, 102(5), 1467-1477.

28. Shi, R.; Bi, J.; Zhang, Z.; Zhu, A.; Chen, D.; Zhou, X.; Zhang, L.; Tian, W. The effect of citric acid on the structural properties and cytotoxicity of the polyvinyl alcohol/starch films when molding at high temperature. Carbohyd. Polym., 2008, 74(4), 763-770.

29. Garcia, P. S.; Grossmann E, M. V.; Yamashita, F.; Mali, S.; Luiz Henrique Dall'Antonia, L. H.; Barreto, W. J. Citric acid as multifunctional agent in blowing films of starch/PBAT. Química Nova, 2011, 34, 1507-1510.

30. Budhavaram, N.; Barone, J. Simple esters of crude glycerol and citric acid. ASABE Annual International Meeting. 2008 Providence, Rhode Island, USA.

31. Schiraldi, A. Kinetics of polymerization of epoxy adhesives and composites. Thermochim. Acta, 1985, 96(2), 283-297.

32. Holser, R. A. Thermal analysis of glycerol citrate/starch blends. J. Appl. Polym. Sci., 2008, 110(3), 1498-1501.

33. Silva, L.N.; Gonçalves, V. L. C.; Mota, C. J. A. Catalytic acetylation of glycerol with acetic anhydride. Cat. Commun., 2010, 11(12), 1036-1039.

34. Zhou, L.; Al-Zaini, E.; Adesina, A. A. Catalytic characteristics and parameters optimization of the glycerol acetylation over solid acid catalysts. Fuel, 2013, 103, 617-625.

35. Zhou, C. H.; Zhao, H.; Tong, D. S.; Wu, L. M.; Yu, W. H.Recent Advances in Catalytic Conversion of Glycerol. Catalysis Reviews, 2013, 55(4), 369-453.

36. Cai, X.; Riedl, B.; Zhang, S. Y.; Wan, H. Effects of nanofillers on water resistance and dimensional stability of solid wood modified by melamine urea formaldehyde resin. Wood Fib. Sci., 2007, 39(2), 307-318.

37. Kretschmann, D. E. Chapter 05: Mechanical properties of Wood, in Wood Handbook, Wood as an Engineering Material; General Technical Report FPL-GTR-190, F.P. Laboratory, Editor. 2010: Madison, Wisconsin, USA; 5-1 to 5-46.

38. Ates, S.; Akyildiz, M. H.; Ozdemir, H. Effects of heat treatment on calabrian pine (pinus brutia ten.) wood. Bioresources, 2009, 4(3), 1032-1043.

39. Cai, X.; Riedl, B.; Zhang, S. Y.; Wan, H. The impact of the nature of nanofillers on the performance of wood polymer nanocomposites. Compos. Part A: Appl. Sci., 2008, 39(5), 727-737.

26