Adhesives formulated from bark bio-crude and phenol formaldehyde resole

Industrial Crops and Products 76 (2015) 258–268

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Adhesives formulated from bark bio-crude and phenol formaldehyde resole Shanghuan Feng a,b , Zhongshun Yuan a , Matthew Leitch b , Chunbao Charles Xu a,∗ a b

Institute for Chemicals and Fuels from Alternative Resources, Western University, London N6A 5B9, Canada Faculty of Natural Resources Management, Lakehead University, 955 Oliver Road, Thunder Bay P7B 5E1, Canada

a r t i c l e

i n f o

Article history: Received 4 March 2015 Received in revised form 23 June 2015 Accepted 25 June 2015 Keywords: Bark liquefaction Bio-crude PF adhesive Thermal stability Free formaldehyde emission level Tension shear strength

a b s t r a c t Rather than replacing phenol in the synthesis of PF resole, bark-derived bio-crudes were used as an adhesive constituent by simply formulation with neat PF adhesive in this study. Bio-crude derived from white birch bark (WBB)—a typical hardwood bark and white spruce bark (WSP)—a typical softwood bark were blended with neat PF adhesive to prepare bark bio-crude formulated bio-phenol formaldehyde (BPF) adhesive (designated as bio-crude formulated BPF adhesive). It was discovered that the viable formulation ratio between bark bio-crude and neat PF adhesive depended on the bark species. WBB biocrude could be formulated with neat PF adhesive at a formulation ratio of 50:50 (wt/wt), while WSP biocrude can be formulated with neat PF adhesive at a higher formulation ratio up to 75:25 (wt/wt). The peak curing temperatures of the bio-crude formulated BPF adhesives were lower than that of neat PF adhesive. Activation energy for the curing of WBB bio-crude formulated BPF adhesives was higher than that for the curing of neat PF adhesive, while WSP bio-crude formulated BPF adhesive required less activation energy for curing than neat PF adhesive. Condensation between bio-crude and neat PF adhesive occured during the pre-curing process at 125 ◦ C and was confirmed by FTIR spectra. Interestingly, the introduction of biocrude enhanced the thermal stability of the bio-crude formulated BPF adhesives at low temperatures, but as expected, thermal stability of the bio-crude formulated BPF adhesives reduced at higher temperatures and a higher formulation ratio led to lower thermal stability. At a same formulation ratio, WSP bio-crude formulated BPF adhesives showed better thermal stability than WBB bio-crude. More interestingly, the free formaldehyde emission level of 3-ply plywood bonded by bio-crude formulated BPF adhesives, in particular with the WSP bio-crude, was lower than that of the neat PF adhesive bonded plywood. Free formaldehyde emission levels from 3-ply plywood bonded with WSP bio-crude formulated BPF adhesives at the formulation ratios of 50:50 (wt/wt) and 75:25 (wt/wt) reached JIS F*** level. However, bio-crude formulated BPF adhesives reduced the tension shear strength of bonded 3-ply plywood, in particular at higher formulation ratios. At the same bio-crude formulation ratio, WSP bio-crude formulated BPF adhesives gave better tension shear strength for 3-ply plywood than the WBB bio-crude formulated BPF adhesives. Nevertheless, 3-ply plywoods bonded with bark bio-crude formulated BPF adhesives at a ratio up to 50:50 (wt/wt) still met the JIS standards with respect to tension shear strength. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Phenol formaldehyde (PF) resoles are the alkali catalyzed polycondensation products from phenol and formaldehyde, and have been widely used as adhesives for the manufacture of plywood, OSB, particleboard, etc. However, the high price of phenol and formaldehyde emission from PF adhesive bonded wooden panels are challenging PF resin industry. Lots of literatures have reported

∗ Corresponding author. Fax: +1 519 661 4016. E-mail address: [email protected] (C.C. Xu). http://dx.doi.org/10.1016/j.indcrop.2015.06.056 0926-6690/© 2015 Elsevier B.V. All rights reserved.

on utilizing renewable phenol alternatives for the production of PF resins (Effendi et al., 2008). Renewable phenol alternatives can be obtained through two pathways, one is to extract chemical components such as lignin, alkaline extractives from lignocellulosic biomass such wood and barks; The other is to convert lignocellulosic biomass via various thermochemical processes (such as phenolysis, liquefaction, and pyrolysis etc.) into bio-crude or bio-oil or pyrolysis oil, which may be collectively called “bio-phenols”. In addition to substitution for phenol in PF resin synthesis, modification of PF resin precursors with some other chemicals such as urea, lignin, tannin, starch, etc., could also produce adhesives of great

S. Feng et al. / Industrial Crops and Products 76 (2015) 258–268

performances without deteriorating the bonding capability of the resulted PF resin (Gangi et al., 2013). Plywood bonded with cornstarch/quebracho tannin/PF formulated resin (15:5:80, w/w/w) even exhibits better mechanical properties than that bonded with a commercial PF resin, and the introduction of cornstarch and quebracho tannin into PF resin by formulation improves the water resistance of the resulted adhesive and contributes to a lower formaldehyde emission level from the bonded plywood (Moubarik et al., 2009). Turunen et al. (2003) investigated the effects of urea, lignin and corn starch, and their adding stage to the PF resin precursor on the resulted adhesives. It was found that Mw of the resulted PF resin increased when starch, lignin, and urea were added during the PF resin synthesis, and most exothermic curing reactions occurred in the wheat–starch modified PF resin. Compared with neat PF resin, almost the same or even slightly higher exotherms were reached with the lignosulfonate or urea modified PF resins. Wheat–starch addition stage had a significant effect on modified resin’s reactivity, and the curing heat reduced when the addition point of wheat starch was deferred. The later the modifier was added during the resin synthesis, the more the methylene bridges were formed. Soy protein has also been widely applied in PF resins formulation. Before the formulation with PF resins, native soy protein needs to be hydrolyzed to break the internal bonds and uncoil the poplar protein molecules, which helps to expose the functional groups in protein complex and increase the protein’s solubility. The hydrolyzed soy protein can then be readily mixed and formulated with PF resins to facilitate the curing reaction as well as molecular entanglement of soy protein with PF resins to form a highly cross-linked thermoset matrix (Hse et al., 2000). In one case, low-fat soybean flour and peanut flour were hydrolyzed in 10% NaOH solution at 140 ◦ C for 2 h. The hydrolyzed protein was then respectively formulated with a lab synthesized PF resin through vigorous stirring for 20 min at the protein/PF resin ratio (wt/wt) of 70:30, 60:40 and 50:50. Properties of the formulated adhesives were found to be dependent on the quality and quantity of protein. Formulated adhesive containing 70% protein performed very well in gluing medium density fiberboard (MDF) and flakeboard. Flakeboards bonded with soy protein formulated PF resins met the Canadian Standard Association (CSA) requirement for OSB and had superior aging durability to that bonded with neat PF resin. Furthermore, 2 h boiling and sixcycle aging tests on the boards bonded with soy protein formulated PF resins demonstrated excellent co-polymerization between the PF resin and the protein (Yang et al., 2006). In addition, formulation of tannin with formaldehyde based adhesives is very common. Condensation reactions between tannin and methylol groups is the main mechanism for the co-polymerization and curing of tannin/PF resin formulated adhesives (Pizzi and Merlin, 1981). Condensed tannins have complex poly-phenolic structure with several hydroxylation sites giving higher reactivity to formaldehyde than phenol. This higher reactivity to formaldehyde makes tannin very suitable for wood adhesives, particularly it can effectively reduce formaldehyde emission after curing (Bisanda et al., 2003). Tannin addition also reduces the curing temperature for the formulated adhesives. Phenol–urea–formaldehyde (PUF) resins formulated with aqueous Pinus pinaster bark tannin (extracted by aqueous NaOH at 90 ◦ C for 30 min) can cure at 75–85 ◦ C, and PUF adhesive formulated with 29 wt% tannin reached the European Standard EN 622-5 exterior grade requirements for MDF, and thickness swelling of the bonded MDF was only half of that of a commercial PF resin glued MDF (Suevos and Riedl, 2003). Addition of tannins extracted from chestnut wood into PF resins shortened the pressing times by 33% without deteriorating the gluing quality of plywood (Kulvik, 1977). Hoong et al. (2011) extracted tannin from Acacia mangium tree bark in water at 75 ◦ C, then prepared a tannin solution of 40%

259

solid content, and formulated the tannin solution with a commercial PF resole at the weight ratio of 50:50, 70:30 and 90:10, to which wheat flour with 3% paraformaldehyde was added as the harder. Results showed the formulated tannin-paraformaldehydePF (90 parts tannin, 10 parts commercial PF, 3% paraformaldehyde) adhesive contributed to shear strength meeting the requirement for European norms EN 314-1 and EN 314-2:1993 with respect to dry tests, cold water and even boiling tests on the bonded plywood. Furthermore, for 3-ply plywood bonded with tannin-PF resin formulated adhesive (50:50, wt/wt), the hot press time and formaldehyde emission level both reduced, and the shear strength met the minimum requirements of EN-314-1 and EN-314-2:1993 with respect to interior and exterior applications (Hoong et al., 2010). For Maritime Pine bark tannin obtained by extraction at 100 ◦ C in 1% NaOH for 30 min, the incorporation of its solution into PF adhesive did not adversely affect the physical properties or bonding capability of the resulted adhesive (Jorge et al., 2002). Pine bark tannin formulated PF adhesives not only required a shorter pressing time, but also displayed better veneer moisture content tolerance, good spreading as well as satisfactory bonding capability (Vazquez et al., 1996). Furthermore, bio-oil or bio-crude produced from thermochemical conversion of lignocellulosic biomass have also been used to formulate with PF adhesives. In the study (Ugovsek et al., 2010), black poplar sawdust was liquefied in glycerol at a biomass/solvent ratio of 1:3 (wt/wt) with sulphuric acid at 180 ◦ C for 90 min, then the slurry was diluted with 1,4-dioxane/water (4:1, v/v) mixture and filtered to remove the insoluble parts of wood. The filtrate was then evaporated under reduced pressure to recover the liquefied wood, containing glycerol. The liquefied wood was then directly used to partially substitute for a commercial PF adhesifve with a replacement up to 100 wt%. The formulated bio-crude PF adhesive was spread on beech wood lamellas at a rate of 200 g/m2 and pressed at 180 ◦ C under 1 MPa. The introduction of liquefied black poplar into PF resin at a ratio up to 25% was found to increase the dry shear strength of bonded plywoods, while further increasing the formulation ratio would rapidly decrease the dry strength of the bonded plywoods. However, the inferior bonding capability of the formulated adhesive at a high formulation ratio can be improved by prolonging the hot press time. Kunaver et al. (2010) liquefied spruce wood at 180 ◦ C under atmospheric pressure in glycerol/diethylene glycol (4:1) co-solvents at a biomass/solvent ratio of 1:3 (wt/wt) with the catalysis of p-toluenesulfonic acid for 3 h. Then the liquefied spruce wood was formulated with melamine formaldehyde resin or melamine urea formaldehyde resin precursors with 0.3 wt% ammonium chloride and 15–20 wt% water. Formulation of liquefied spruce wood with the resin at a ratio of 50:50 (wt/wt) led to a 40% decrease of free formaldehyde release without significantly compromising the mechanical properties of the bonded particleboards. In addition, the hot press temperature for particleboards manufacture can be lowered by 20 ◦ C from 180 ◦ C to 160 ◦ C without significantly deteriorating the bonding properties of the formulated adhesives. Formulated PF resins also found application in wood preservation. Mourant et al. (2007) mixed pyrolysis biooil from softwood bark (70% balsam fir, 28% white spruce and 2% larch) with PF resole (F/P molar ratio 2.5:1) at a formulation ratio of 50 wt%, 75 wt%, 85 wt%, and 100 wt%. The results showed the addition of softwood bark pyrolysis bio-oil increased the viscosity and gel times of the resulted adhesives, but the impregnation of pyrolysis bark oil/PF Resole mixture into wood blocks that were previously treated with copper chloride or copper chloride–sodium borate mixture improved the decay resistance without negatively impacting the mechanical properties of the treated wood. In our previous study, efficient liquefaction of bark was demonstrated in water/ethanol (50:50, v/v) mixture, and the obtained bio-crude has been successfully applied in synthesis of PF resole

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Table 1 Elemental composition and chemical contents of white birch bark and white spruce bark.

WBB WSP a b c d e

Elemental composition/wt%, d.b.a C H N

Ob

Chemical components/wt%, d.b.a Extractivesc Lignind

Cellulosed

Hemi-cellulosed

Ashe

53.47 47.35

37.89 44.29

11.18 29.89

37.98 35.1

28 38.72

2.78 3.07

7.87 7.15

0.1 0.14

34.02 27.18

On a dry basis. Determined by the difference between 100% and total carbon/hydrogen/nitrogen/ash content. Determined by 70 vol% aqueous acetone extraction under stirring at room temperature for 3 h, based on oven dried bark weight. Determined in accordance with NREL/TP-510-42718, based on extractive and ash-free dry bark. Determined by bark combustion at 575 ◦ C for 3 h.

through substituting for phenol at a ratio of 50%. However, barkderived bio-crude has not yet been reported as a component to formulate with PF resole precursor. In this study, bark-derived biocrude was first produced through liquefaction in water/ethanol (50:50, v/v) mixture, then the obtained bio-crude was mixed with a neat PF resole to prepare bark bio-crude formulated BPF adhesives. Characterizations of the formulated BPF adhesives were carried out to analyze the curing properties, thermal stability, functional groups, as well as free formaldehyde emission level and mechanical properties of the bonded 3-ply plywoods.

and white spruce bark was designated as WBB bio-crude and WSP bio-crude with a yield of 66.5% and 58.4%, respectively. The obtained bio-crudes were characterized with a Waters Breeze GPC-HPLC instrument (1525 binary lamp, UV detector set at 270 nm, Waters Styragel HR1 column at 40 ◦ C) to measure the molecular weights and distribution, using THF as the eluent at a flow rate of 1 mL/min. The equipment had been calibrated with polystyrene standards. Hydroxyl number was determined in accordance with Ueno’s method (Ueno et al., 2002). The molecular weights and distribution, and hydroxyl number of the two biocrudes are as shown in Table 2. below.

2. Materials and methods 2.3. Preparation of bio-crude formulated BPF adhesives 2.1. Materials White birch bark (WBB) and white spruce bark (WSP) were obtained from a local sawmill in Thunder Bay, Ontario, Canada. The barks were oven dried at 105 ◦ C for 24 h and ground into particles of 20 mesh. Elemental composition and chemical contents of the barks are shown in Table 1. Anhydrous ethyl alcohol (ethanol) and acetone (≥99.5%) were A.C.S. reagent, supplied by Caledon Laboratory Chemicals, Canada. A.C.S. reagent grade phenol (≥99.0%) was provided by Sigma–Aldrich. 37% aqueous formaldehyde and sodium hydroxide solution (50 wt% water solution) were purchased from EMD, Germany. 2.2. Bio-crude production and characterizations Bio-crude was produced from two barks, respectively, i.e., a hardwood bark—white birch bark (WBB) and a softwood bark—white spruce bark (WSP), through liquefaction in water/ethanol (50:50, v/v) mixture at 300 ◦ C for 15 min in a 500 mL Parr batch reactor. For a typical liquefaction run, 25 g oven dried bark and 250 mL water/ethanol (50:50, v/v) mixture were fed into the reactor. Air in the reactor was purged by alternate vacuuming-N2 purge for 3 times. Then the reactor was pressurized with 2.0 MPa N2 , and heated to 300 ◦ C (pressure at this temperature is 12.0 MPa) at the rate of 10 ◦ C/min under continuous stirring (175 rpm). After 15 min liquefaction at 300 ◦ C, the reactor was cooled down at room temperature through quenching in a water/ice bath. Gas in the reactor was vented, then the reactor was opened and rinsed with acetone. The slurry and rinsing acetone were collected and filtered. The filtrate was evaporated under reduced pressure to remove acetone, ethanol and water, respectively at 45 ◦ C, 60 ◦ C and 75 ◦ C. The left dark viscous liquid product was then vacuum-dried at 60 ◦ C for 24 h, and designated as “bio-crude”. Bio-crude from the liquefaction of white birch bark

A neat PF resole was synthesized at the formaldehyde/phenol (F/P) molar ratio of 1.8. The synthesis process is briefly described as followings: 40 g phenol, 12 g 50% NaOH solution and 40 g water were charged into a 250 mL three-neck flask reactor equipped with a reflux condenser, thermometer, and magnetic stirring bar. The reactor was heated up from room temperature to 84 ◦ C under 210 rpm stirring. 62.1 g 37% formaldehyde was fed into the reactor dropwise during the heating process. After 180 min reaction at 84 ◦ C, the synthesis reaction was stopped through quenching the reactor in a water/ice bath. The neat PF adhesive was then formulated with a bark bio-crude to produce formulated BPF adhesive. Firstly, a homogenous aqueous bio-crude solution was obtained by heating the mixture of 40 g bio-crude, 40 g water and 12 g 50 wt% NaOH in a three-neck flask reactor at 80 ◦ C for 60 min under magnetic stirring. The targeted adhesives were then formulated by directly blending the aqueous bio-crude solution with neat PF adhesive under magnetic stirring for 15 min at room temperature, ensuring a homogeneous mixture of the bio-crude formulated BPF adhesive. In the bio-crude formulated BPF adhesives preparation, bio-crude/neat PF adhesive weight ratio was set at 25:75, 50:50 and 75:25, respectively. 2.4. Characterizations of bio-crude formulated BPF adhesives pH values of bio-crude formulated BPF adhesives were measured with a pH meter (Thermo Scientific, Orion2 Star pH Benchtop). Solid content was determined by drying the adhesives at 125 ◦ C for 105 min. Viscosities of all adhesives were measured by Brookfield CAP 2000+ viscometer (Brookfield Engineering Laboratories, Middleboro, MA) at 50 ◦ C. Curing properties of neat PF adhesive and bio-crude formulated BPF adhesives were tested in a differential scanning calorimetry (DSC, Mettler Toledo, Stare System). In a typical DSC test, 10 mg

Table 2 Molecular weights and hydroxyl number of WBB bio-crude and WSB bio-crude. Molecular weight (g/mol)

WBB bio-crude WSP bio-crude

PDI

Mn

Mw

(−)

360 340

940 860

2.61 2.53

Hydroxyl number (mg KOH/g)

4.37 5.56

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Table 3 pH values, solid contents and viscosities of bio-crude formulated BPF adhesives.

Neat PF adhesive WBB bio-crude/PF (25:75, w/w) adhesive WBB bio-crude/PF (50:50, w/w) adhesive WSP bio-crude/PF (25:75, w/w) adhesive WSP bio-crude/PF (50:50, w/w) adhesive WSP bio-crude/PF (75:25, w/w) adhesive

pH

Solid content (wt%)

Viscosity (cP)

11.32 10.89 11.11 10.87 11.00 11.20

36.7 (0.09) 36.9 (0.03) 36.7 (0.07) 36.4 (0.24) 36.4 (0.29) 36.0 (0.10)

32.1 (2.45) 20.9 (3.39) 17.8 (1.95) 18.5 (2.45) 17.4 (2.73) 17.8 (2.60)

adhesive was load into an aluminum crucible, then heated from 40 ◦ C to 250 ◦ C at 10 ◦ C/min under 50 mL/min N2 flow. Besides, functional groups and thermal stability of neat PF adhesive and the bio-crude formulated BPF adhesives were characterized by FTIR and TGA, respectively. Before these tests, neat PF adhesive and bio-crude formulated BPF adhesives were pre-cured at 125 ◦ C for 105 min, and then ground into powder using a pestle and a mortar. For the functional groups characterization, a frontier infrared spectrometer (Frontier FTIR, Perkin Elmer) was employed at the scanning resolution of 4 cm−1 from 4000 cm−1 to 500 cm−1 . As for the TGA test, 10 mg adhesive was heated in a N2 flow of 20 mL/min from 50 ◦ C to 800 ◦ C at 10 ◦ C/min using a thermogravimetric analyzer (Pyrolysis 1 TGA, Perkin Elmer). Finally, 3-ply plywood was produced using the bio-crude formulated BPF adhesives to determine the free formaldehyde emission level and bonding capability of the adhesives. Yellow birch veneers (11 × 11 × 1/16 inch3 ) were firstly conditioned at 20 ◦ C and 65% relative humidity for 15 days. Adhesive to be tested was first mixed with wheat flour (10 wt% based on adhesive), and then brushed on the surface of the conditioned veneers at a rate of 250 g/m2 . The face and center veneer were then bonded in directions perpendicular to each other by hot-pressing at 3.0 MPa at 140 ◦ C for 4 min. Free formaldehyde emission levels from 3-ply plywood were determined in accordance with a 24 h desiccator method in accordance to the Japanese JIS A 1460 Standard (JIS, 2001), using a 10 L glass desiccator. In each run for the determination of free formaldehyde emission level from the 3-ply plywood, ten plywood specimens with a dimension of 150 mm × 50 mm × 4.5 mm were placed in the desiccator for 24 h. The formaldehyde released from the test pieces during the 24 h period was absorbed by 300 mL distilled water in a petri dish. The absorbed formaldehyde was then determined photometrically at 412 nm in a UV spectrophotometer (Evolution 220, Thermal Scientific). 3 replicate were carried out for 3-ply plywood bonded by one adhesive to ensure good reproducibility of the measurements. Plywood specimens for mechanical tests were cut from each panel in accordance with ASTM D 906-98 (ASTM, Reapproved 2011). The specimens were tested for shear stress by tension loaded with a Bench-top universal testing machine (Model H10K-T UTM, Tinus Olsen Material Testing Machine Co., Horsham, PA) at a loading rate of 3 mm/min till failure. Half of the 3-ply plywood specimens were tested after being conditioned at 20 ◦ C and 65% relative humidity for 7 days, while the other half were cooked in boiling water for 3 h, then cooled down in fume hood and used for

the wet tension shear strength tests. It should be noted that 3-ply plywood bonded with WBB bio-crude/PF (75:25) bio-crude formulated BPF adhesive delaminated after hot press, thus, this adhesive will not be further discussed in the following sections.

3. Results and discussion 3.1. Basic properties of bio-crude formulated BPF adhesives Some basic properties of neat PF adhesive and bio-crude formulated BPF adhesives are displayed in Table 3. pH value of the neat PF resole is 11.32, while the pH values of bio-crude formulated BPF adhesives range from 10.87 to 11.44. Solid content of all adhesives vary very little, from 36.0 wt% to 37.2 wt%. The viscosity of the neat PF adhesive is 32.1 cP at 50 ◦ C, while the viscosities of the bio-crude formulated BPF adhesives range from 17.4 cP to 20.9 cP, which are much lower than that of the neat PF adhesive, likely due to the dilution effect from the aqueous bio-crude solution. Reverse to tannin-PF formulated adhesives, the formulation of PF with aqueous bio-crude solution reduces the viscosities of the resulted BPF adhesives. This implies that formulation of bark biocrude with PF resole eases the spreading of the adhesives onto the wooden materials for panel production.

3.2. Curing properties of neat PF adhesive and bio-crude formulated BPF adhesives During the curing process of PF resole, a variety of exothermic reactions may occur, such as condensation between methylol groups with phenol to form methylene bridges, and polymerization of two methylol groups forming dibenzyl ether bridges and thus methylene bridges. Peak curing temperatures of neat PF adhesive and various bark bio-crude formulated BPF adhesives are shown in Table 4. Compared with that of the neat PF adhesive, the peak curing temperatures of all bark bio-crude formulated BPF adhesives are lower, regardless of bark species or formulation ratios, and a higher bio-crude/PF ratio contributes to a lower peak curing temperature. The curing kinetics of all adhesives were studied using a multiple heating rate method. Specifically, the neat PF adhesive or biocrude formulated BPF adhesives was heated at the rate of 5 ◦ C/min, 10 ◦ C/min, 15 ◦ C/min and 20 ◦ C/min, respectively. The activation

Table 4 Peak curing temperatures and activation energies for the curing of bio-crude formulated BPF adhesives. Heating rate (◦ C/min)

Neat PF adhesive WBB bio-crude/PF (25:75 w/w) adhesive WBB bio-crude/PF (50:50 w/w) adhesive WSP bio-crude/PF (25:75 w/w) adhesive WSP bio-crude/PF (50:50 w/w) adhesive WSP bio-crude/PF (75:25 w/w) adhesive a

By linear extrapolation.

Peak temperature (◦ C)

Activation energy (kJ/mol)

0a

5

10

15

20

122.3 121.9 119.9 116.7 115.7 114.2

128.1 125.9 123.5 126.3 124.5 123.1

141.2 133.6 134.8 127.8 126.1 129.6

149.8 138.8 135.9 141.9 142.8 143.1

152.8 141.7 142.1 147.0 143.7 147.4

69.79 112.55 97.00 67.33 64.59 64.23

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energy for the curing of an adhesive is then calculated from the Kissinger equation as Eq. (1) (Markovic et al., 2001):

Kissinger equation : dln

ˇ Tp 2



  =− 1 Tp

E R

where ˇ is the heating rate (◦ C/min), Tp (K) is the peak temperature, E is the activation energy (kJ/mol), R is the gas constant. The value of E/R is obtained as the slope of a linear plot of ln(␤/Tp 2 ) vs. −1000/Tp . As shown in the Table 4, the activation energy for the curing of bio-crude formulated BPF adhesives depends on the bark species and formulation ratios between bio-crude and neat PF adhesive. For the curing of WBB bio-crude formulated BPF adhesives at the formulation ratio of 25:75, 50:50, the activation energy is 112.55 kJ/mol and 97.00 kJ/mol, respectively, much higher than 69.79 kJ/mol for the curing of neat PF adhesive. However, the activation energy for curing of WSP bio-crude formulated BPF adhesives are all lower than that of neat PF adhesive, which is 67.33 kJ/mol, 64.59 kJ/mol and 64.23 kJ/mol, respectively, at the WSP/PF ratio of 25:75, 50:50, 75:25, suggesting that the introduction of WSP bio-crude into PF adhesive favors the curing reactions of the resulted WSP bio-crude formulated BPF adhesives. This could on one hand because white birch bark contains a higher content nonreactive betulin, which retard the condensation reaction between WBB bio-crude and PF adhesive; on the other hand, white spruce bark contains more reactive substances such as tannin which can accelerate the condensation/cross-linking reactions between WSP bio-crude and PF adhesive, and hence reduce the activation energy (Liiri et al., 1982). More importantly, as given previously in Table 2, hydroxyl number of WSP bio-crude is bigger than that of WBB biocrude, the reactions between hydroxyl group and methylol are an important pathway for curing of bio-crude formulated BPF adhesives. Thus, WSP bio-crude of a higher hydroxyl number could account for its favored curing than WBB bio-crude when blended with a PF resole. 3.3. Functional groups in neat PF adhesive, bio-crude and bio-crude formulated BPF adhesives FTIR spectra of WBB and WSP bio-crudes, pre-cured neat PF adhesive and bio-crude formulated BPF adhesives are as shown in Fig. 1. FTIR spectra of WBB bio-crude and WSP bio-crude are very similar, except that the spectrum of WSP has a stronger absorbance peak at 1089 cm−1 ascribed to catechin, a tannin precursor. For both bio-crudes, the broad and strong absorbance peak at 3600–3200 cm−1 is attributed to the existence of aliphatic and aromatic OH. Absorbance at 2935 cm−1 and 2850 cm−1 are due to the symmetrical and asymmetrical C H stretching vibrations of alkanes. Absorbance at 1717 cm−1 is ascribed to C O probably in the form of aldehyde (Lievens et al., 2011). Absorbance peaks at 1612 cm−1 , 1424 cm−1 result from the aromatic ring breathing vibrations. The sharp peak at 1510 cm−1 may be assigned to lignin and derivatives. Absorbance at 1464 cm−1 is assigned to the CH2 bend. Absorbance at 1373 cm−1 is due to phenol OH from lignin derivatives in the bio-crudes. Peaks at 1270 cm−1 and 1200 cm−1 are due to the C O C stretch in dialkyl, while absorbance at 1110 cm−1 and 1039 cm−1 are due to C O stretching in alcohols. Absorbance peak at 880 cm−1 is attributed to meta CH bend in lignin, while the peak at 820 cm−1 can be assigned to the para CH bend in lignin. Detailed peaks and functional groups assignment for the spectra of the WBB and WSP bio-crudes are shown in Table 5. IR spectra of neat PF adhesive and bark bio-crude formulated BPF adhesives are very similar to each other, regardless of bark species or formulation ratios. A striking difference between the spectra of adhesives and those of the bio-crudes is that the absorbance

C=O catechin

-OH

(1)

lignin derivatives

WSP bio-crude

Absorbance



dln

WSP bio-crude/PF (50:50) formulated BPF adhesive

Neat PF adhesive -CH2WBB bio-crude/PF (50:50) formulated BPF adhesive

WBB bio-crude

Ar-OH -CH2OH

Neat PF adhesive

4000

3500

3000

2500 2000 1500 Wavelength (cm-1)

1000

500

Fig. 1. FTIR spectra of neat PF adhesive, bio-crudes and bio-crude formulated BPF adhesives.

peaks in the spectra of all adhesives are much weaker than those of the spectra of bio-crudes. This may be mainly due to the fact that the all the adhesives samples were pre-cured at 125 ◦ C for 105 min when condensation among the functional groups occurred, weakening the IR absorbance peaks of most functional groups. For instance, the weak absorbance at the wavelength range of 3600–3200 cm−1 assigned to OH is due to the condensation reaction between OH and methylol during the pre-curing process. The absorbance in the IR spectrum of neat PF adhesive and biocrude formulated BPF adhesives at 1200 cm−1 can be ascribed to the ether ( C O C ), formed through the condensation reaction between OH and methylol, or between condensation of methylol and methylol. After formulation, the absorbance at 1510 cm−1 (typical of lignin and derivatives) disappears, indicating the reaction between some lignin derivatives and neat PF adhesive during pre-curing at 125 ◦ C. Peaks at 1464 cm−1 are assigned to methylene bridge in the adhesives. It is worth-noting that the absorbance at 1373 cm−1 assigned to phenol OH only exist in the spectra of biocrudes and the neat PF adhesive while this peak is not detectable in the spectra of the bio-crude/PF bio-crude formulated BPF adhesives, regardless of bark species. Meanwhile, the absorbance at 1000 cm−1 due to the existence of CH2 OH is weaker in the biocrude formulated BPF adhesives, compared with that of the neat PF adhesive, which is believed to result from the condensation reactions between phenol OH and CH2 OH or phenol OH or even CH2 −, eliminating water. Meanwhile, the IR peak at 820 cm−1 assigned to the para CH bend in lignin from bio-crude disappears in the pre-cured bio-crude formulated BPF adhesives. It thus implies that after formulation and pre-curing, H in para-position of aromatics has been substituted, confirming the reaction between bio-crude and neat PF adhesives. Detailed peaks and functional groups assignment for the spectra of the neat PF adhesive and the bio-crude/PF bio-crude formulated BPF adhesives are shown in Table 5. 3.4. Thermal stability of neat PF adhesive, bio-crudes and bio-crude formulated BPF adhesives TGA and derivative thermogravimetric (DTG) profiles of neat PF adhesive, bio-crude and bio-crude formulated BPF adhesives with various formulation ratios are displayed in Fig. 2. The neat

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Table 5 Functional groups in bio-crudes, neat PF adhesive and bio-crude formulated BPF adhesives. Wavelength (cm−1 )

Functional groups

WBB bio-crude

WSP bio-crude

Neat PF adhesive

WSP bio-crude/PF adhesive

WBB bio-crude/PF adhesive

3600–3200 2935, 2850 1717 – 1612, 1464

3600–3200 2935, 2850 1717 – 1612 1464 1510 1424 1373 1270, 1200

3600–3200 2935, 2850 – 1650 1612, 1464

3600–3200 2935, 2850 – 1650 1612, 1464

3600–3200 2935, 2850 – 1650 1612, 1464

OH Aliphatic CH2 C O C O C C in aromatics

– 1424 1373 1270, 1200 1150 – – – 1000 880 – 770,700

– 1424 – 1270, 1200 1150 – – – 100 880 – 770, 700

– 1424 – 1270, 1200 1150 – – – 1000 880 – 770, 700

Lignin derivatives Methylene group Phenol OH C O C (dialkyl) C O C O (ester) C O in tannin C O CH2 OH Meta C H bend in aromatics Para C H bend in aromatics Mono C H bend in aromatics

1510 1424 1373 1270, 1200 – 1110 – 1039 – 880 820 –

1110 1089 1039 – 880 820 –

PF adhesive shows three thermal degradation temperature stages, with peaks at 199.7 ◦ C, 411.2 ◦ C, 562.8 ◦ C, respectively. The first thermal degradation stage may result from formation of additional cross-linkages due to the extra condensation between the un-reacted functional groups, such as reaction among methylols, reactions among two phenolic hydroxyls resulting in an ether cross-link, or even a phenol hydroxyl and a methylene forming a carbon-hydrogen crosslink. The second thermal degradation stage is related to the break of methylene (ether) bridges into methyl groups, evolving methane, hydrogen, carbon monoxide, small oligomers or water. Meanwhile, methylene scission into such volatiles as phenol and methyl derivatives would take place in this temperature range (Jiang et al., 2012; Chen et al., 2008; Costa et al., 1997). The third thermal degradation stage is believed to be attributed to the degradation of phenol groups, and the collapse, carbonization and graphitization of PF resin network (Jiang et al., 2012; Chen et al., 2008; Costa et al., 1997). Also as shown in Fig. 2, compared with the neat PF adhesives, both WBB and WSP bio-crudes show smaller residual weights. At 800 ◦ C, the residual weight of WBB bio-crude and WSP bio-crude dropped to 24.08% and 30.79%, respectively, while the residual weight of neat PF adhesive is 65.76%. The introduction of bio-crude into PF resole also reduces the residual weight for the bio-crude

formulated BPF adhesives. When heated to 800 ◦ C, the residual weight of WBB bio-crude/PF formulated adhesives at the weight ratios of 25:75 and 50:50 is 63.5% and 52.8%, respectively. For WSP bio-crude formulated BPF adhesives, the 800 ◦ C residual weight was measured to be 64.4% (25:75 w/w formulation ratio), 59.1% (50:50 w/w formulation ratio) and 54.5% (75:25 w/w formulation ratio), respectively. The residual weights of bio-crude formulated BPF adhesives at 800 ◦ C, although lower than that of neat PF adhesive, are still satisfactorily high, e.g., the residual weight is higher than 50% for the WSP bio-crude/PF (75:25 w/w) formulated BPF adhesive. As clearly shown in Fig. 2, the bio-crude formulated BPF adhesives also exhibits three typical decomposition peak temperatures. As expected, the thermal degradation peak shift to lower temperatures compared with those of the neat PF resole. The maximum decomposition rates of neat PF adhesive, bark bio-crudes, and bio-crude formulated BPF adhesives, and the corresponding temperatures are summarized in Table 6. The maximum degradation rate of neat PF adhesive during the whole degradation process is 1.29%/min at 562.8 ◦ C, while it is 3.12%/min at 356.6 ◦ C for WBB bio-crude, and 1.73%/min at 522.3 ◦ C and 2.09%/min at 410.6 ◦ C, respectively, for the WBB bio-crude formulated BPF adhesives at the formulation ratios of 25:75 and 50:50 (wt/wt). Compared with the WBB bio-crude and WBB bio-crude formulated

Fig. 2. TGA and DTG profiles of bio-crude, neat PF adhesive, and bio-crude formulated BPF adhesives.

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Table 6 Maximum decomposition rates of neat PF adhesive, bio-crude, bio-crude formulated BPF adhesives. Neat PF adhesive

◦

Peak temperature ( C) Decomposition rate (%/min)

WBB bio-crude

WBB bio-crude formulated adhesive

562.8 1.29

25:75

50:50

522.3 1.73

410.6 2.09

WSP bio-crude formulated adhesive

356.6 3.12

WSP bio-crude

25:75

50:50

75:25

523.6 1.23

419.2 1.68

394.1 2.16

341.3 3.04

Fig. 3. Free formaldehyde emission levels from 3-ply plywood bonded by bio-crude formulated BPF adhesives.

BPF adhesives, the maximum degradation rates of WSP bio-crude and WSP bio-crude formulated BPF adhesives are all smaller, correspondingly. The maximum degradation rates of WSP bio-crude formulated BPF adhesives with the ratios of 25:75, 50:50, and 75:25 (wt/wt) are 1.23%/min, 1.68%/min and 2.16%/min, respectively at 523.6 ◦ C, 419.2 ◦ C and 394 ◦ C. 3.5. Free formaldehyde emission from plywood bonded with neat PF adhesive and bio-crude formulated BPF adhesives Formaldehyde has been classified as “carcinogenic to humans” by the International Agency for Research on Cancer (IARC) of the World Health Organization (WHO) (IARC, 2006). The acceptable levels of free formaldehyde emission from wooden panel have been continuously reduced over the past decades, due to the increased public awareness and the consumer demand for non-hazardous products, as well as the environmental regulations. Formaldehyde emission from wooden panels is affected by wood species, panel types, adhesive types and loading rate, humidity, air exchange rate, temperature, additives, the local formaldehyde concentration within the space where wooden panel is placed, and the surface treatment (Yu and Crump, 1999; Bohm et al., 2012; Salem et al.,

Fig. 4. Tension shear strength of 3-ply plywood bonded by neat PF adhesive and bio-crude formulated BPF adhesives.

2012; Uchiyama et al., 2007). Two formaldehyde sources including un-reacted free formaldehyde and formaldehyde from degradation of the resin are responsible for the free formaldehyde emission from wooden panels bonded by formaldehyde based adhesives. Free formaldehyde emission levels from 3-ply plywood bonded by neat PF adhesive and bio-crude formulated BPF adhesives are displayed in Fig. 3. The free formaldehyde emission level from plywood bonded by neat PF resole is 1.90 mg/L. After being formulated with bio-crude, the free formaldehyde emission level from the bonded 3-ply plywood reduces. For 3-ply plywood bonded with WBB and WSB bio-crude formulated BPF adhesive at the

CHOH

CHOH

CHOH

H3CO

H3CO

OH

OH

Hydroxyphylpropane (H)

Guaiacylpropane (G) Fig. 5. Lignin monomer structures.

OCH3 OH

Syringylpropane (S)

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OH

OH H2 C

HOH2C

OH

265

CH2OH

CHOH

A OH

HOH2C

OH

H2 C

H2 C

H2 C

OH

OH

OH

H2 C

CH2OH

CHOH

B OH HOH2C

OH

H2 C

OH H2 C

OH H2 C

CH2

OH CH2OH

CH

HOH2C

C OH H2

OH HOH2C

OH

CH2OH

OH CH2OH

HCHO

C CH2OH

CH2OH HOH2C

OH H2 C

OH CH2OH

D OH HOH2C

H2 C

OH CH2

OH H2 C

OH

H2 C

OH CH2OH

CH

HOH2C

C OH H2

OH

CH2OH

Fig. 6. Proposed curing reaction of bark bio-crude formulated BPF adhesives.

ratio of 25:75 (wt/wt), the free formaldehyde emission level is 1.37 mg/L and 1.03 mg/L, respectively, meeting the requirement for F** level in accordance with Japanese Industrial Standards (JIS) for plywood free formaldehyde emission. For bio-crude formulated BPF adhesive at the ratio of 50:50 (wt/wt), the free formaldehyde emission level from the bonded plywood further decreases

to 0.54 mg/L (with WBB bio-crude) and 0.45 mg/L (with WSP biocrude), reaching the JIS F** and F*** level, respectively. For 3-ply plywood bonded by WSP bio-crude formulated BPF adhesive at a high formulation ratio of 75:25 (w/w), the free formaldehyde emission level drops to as low as 0.32 mg/L, reaching JIS F*** level. It is thus clear that at the same bio-crude/PF formulation ratio, ply-

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OH HOH2C

H3CO

OH H2 C

OH

OH

CH2OH H3CO

OH

OH H2 C

H2 C

CH2OH

A

CHOH

CHOH

B OH

H3CO

HOH2C

H3CO

C OH H2

OH

OH H2 OH CH2OH C

HOH2C

OH

OH H2 C

H2 C

CH2OH

CH2OH

OH

OH

H3CO

CH2OH

HCHO

C CH2OH

CH2OH

D

H3CO

HOH2C

C OH H2

OH

OH

H2 C

OH H2 C

HOH2C

OH

H2 C

OH

CH2OH

OH CH2OH

CH2OH

Fig. 7. Proposed curing reaction of WSP bio-crude formulated BPF adhesives.

H3CO

OH

OCH3 HOH2C

OH H2 C

OH

OH CH2OH

HOH2C

CHOH

H2 C

OH CH2OH

CH

H3CO

OH

OCH3

Fig. 8. Proposed curing reaction of WBB bio-crude formulated BPF adhesives.

wood bonded with WSP bio-crude formulated BPF adhesives gives a lower free formaldehyde emission level than those bonded with WBB bio-crude formulated PF adhesives, which could be attributed to existence of more tannin or tannin derivatives in the WSP biocrude as confirmed by FTIR and its higher hydroxyl content as shown in Table 2. The lower free formaldehyde emission levels from bio-crude formulated BPF adhesives-bonded 3-plywood are likely due to two main reasons: (1) formulation of bark bio-crude with neat PF adhesive reduces the amount of PF resole applied in the plywood; (2) bark bio-crude contains a lot of hydroxyl groups (confirmed by FTIR), which can react with methylol, free formalde-

hyde originally present in the PF resole or the free formaldehyde released during the hot pressing (Myers, 1983). 3.6. Tension shear strength of 3-ply plywood bonded by neat PF adhesive and bio-crude formulated BPF adhesives Fig. 4. shows the bonding strength of 3-ply plywoods bonded by the neat PF adhesive and bio-crude formulated BPF adhesives. The neat PF adhesivebonded plywood contributes to 2.64 MPa and 2.35 MPa tension shear strength at dry and wet condition, respectively. Formulation of bio-crude with the neat PF adhesive

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reduces the tension shear strength of the bonded 3-ply plywood at both dry and wet condition, and a higher formulation ratio leads to lower tension shear strength level. It is worthy-noting that at the formulation ratio of 50:50 (wt/wt), the bonded 3-ply plywood still meets the JIS standards regardless of bark species or dry/wet conditions.For WBB bio-crude formulated BPF adhesivesat the formulation ratios of 25:75 (wt/wt) and 50:50 (wt/wt), the tension shear strength of plywood reduces to 1.48 MPa and 1.33 MPa, respectively at dry condition, and 1.33 MPa and 1.31 MPa at wet condition. For WSP bio-crude formulated BPF adhesivesat the formulation ratios of 25:75 (wt/wt) and 50:50 (wt/wt), the dry tension shear strengths of bonded 3-ply plywood are 1.98 MPa and 1.80 MPa, respectively; while the wetstrengths are 1.76 MPa and 1.50 MPa, respectively. Further increase of formulation ratio to 75:25 (wt/wt) reduces the dry/wet bonding strengths drops to 1.19 MPa and 1.00 MPa, respectively for the plywood.It is obvious that WSP bio-crude formulated BPF adhesives contribute to a better bonding strength than WBB bio-crude formulated BPF adhesives. This could be accounted for by the higher hydroxyl value of WSP bio-crude and the larger concentration of tannin derivatives that contribute to more cross-links during the hot pressing. 3.7. Proposed reaction for bio-crude formulated BPF adhesive curing As confirmed by FTIR spectra, bark bio-crudes contain an abundant amount of aromatic structures, which mainly derive from lignin during the liquefaction. Lignin is composed of three monomers as shown in Fig. 5. White spruce bark lignin of softwood species is mainly based on guaiacylpropane units (G) that crosslink with one another to form an extensive polymer molecule with a minor amounts of p-hydroxyphenylpropane (H) and syringylpropane units (S), while white birch bark lignin of hardwood contains both guaiacylpropane (G) and syringylpropane (S) units and smaller amounts of phydroxyphenylpropane (H) (Walker, 2006). Methylols in PF adhesive precursor can react with lignin at orth-C in bio-crudes. Each hydroxyphenylpropane (H) structure unit has two reactive orth-H to methylols of PF adhesive precursors, as displayed in Fig. 6(A), which favors the condensation between bio-crude and PF adhesive precursors during hot press. Both WSP bio-crude and WBB bio-crude contain hydroxyphenylpropane (H) structure. In addition, para-H in PF adhesive precursors also condense with ␣-OH in the propyl side chains of lignin derivatives in both WBB bio-crude and WSP bio-crude via the loss of water to form ether linkage as shown in Fig. 6(B). Furthermore, WSP bio-crude is rich in guaiacylpropane (G) having one reactive orth-H and one and ␣-OH in the propyl side chains, the curing reactions of some WSP bio-crude formulated BPF adhesives are proposed as shown in Fig. 7(A and B), while for WBB bio-crude containing syringylpropane units (S), ␣-OH in propyl side chain is the only reactive site to para-H in neat PF adhesive, and the reaction is as proposed in Fig. 8. The more reactives sites in WSP bio-crude formulated BPF adhsives than that in WBB bio-crude formulated BPF adhesives contributed to a higher crossing linkage density, and thus explains why WSP bio-crude formulated BPF adhesives own better thermal stability, better bonding strength than WBB bio-crude formulated BPF adhesives at the same formulation rates. In addition, hydroxyphenylpropane (H) and guaiacylpropane (G) can react with free formaldehyde and further condense during the hot press process, which are illustrated in Figs. 6–8(C and D). 4. Conclusions Bark bio-crude and neat PF adhesive were successfully blended to produce novel formulated BPF adhesives. The viable formula-

267

tions ratio between bark bio-crude and neat PF adhesive depend on the bark species. WBB bio-crude can be formulated with neat PF adhesive at the ratio of 50:50 (wt/wt), while WSP bio-crude can be formulated with neat PF adhesive at a formulation ratio up to 75:25 (wt/wt). Some key conclusions are made below: 1) Compared with that of neat PF adhesive, the peak curing temperatures of all bark bio-crude formulated BPF adhesives were lower, regardless of bark species or formulation ratios. A higher bio-crude/PF formulation ratio contributed to a lower peak curing temperature. Activation energy for the curing of bark biocrude formulated BPF adhesives depended on the bark species: activation energy for the curing of WBB bio-crude formulated BPF adhesives was higher than that for the curing of neat PF adhesive, while the activation energy for curing of WSP biocrude formulated BPF adhesives was all lower than that for the curing of neat PF adhesive. 2) Condensation between bio-crude and neat PF adhesive was confirmed by FTIR spectra. Thermal stability of bark bio-crude formulated BPF adhesive was lower than that of neat PF adhesive, and higher bark bio-crude/PF ratios contributed to a lower thermal stability. WSP bio-crude formulated BPF adhesive displayed better thermal stability than WBB bio-crude formulated BPF adhesive at the same formulation ratios. 3) Bio-crude formulated BPF adhesives led to a lower free formaldehyde emission level from the bonded 3-ply plywood than neat PF adhesive. 3-ply plywood bonded with WBB biocrude formulated BPF adhesive at the weight ratios of 25:75 and 50:50 (w/w) released a free formaldehyde to JIS F** level. While for 3-ply plywood bonded with WSP bio-crude formulated BPF adhesives at the weight ratios of 50:50 and 75:25, the free formaldehyde emission level reached JIS F*** level. 4) Formulation of bio-crude with neat PF adhesive also reduces tension shear strength of the bonded 3-ply plywood at both dry and wet condition, and a higher formulation ratio resulted in a lower strength. However, bark bio-crude formulated BPF adhesives at the formulation ratio up to 50: 50 (wt/wt) still met the JIS standards in terms of dry and wet strength requirements. At the same bio-crude formulation ratio, WSP bio-crude formulated BPF adhesives resulted in a better bonding strength than WBB bio-crude formulated BPF adhesives on 3-ply plywoods. Acknowledgements The authors are grateful for the financial support from the Natural Science and Engineering Research Council of Canada (NSERC) through the Discovery Grant, and from Ontario Ministry of Economic Development and Innovation via the Ontario Research Fund (ORF) for Bark Biorefinery project led by Dr. Ning Yan at University of Toronto. The authors would also like to acknowledge the support from the industry partners: FP innovation, The Woodbridge Group, Huntsman, Arclin, Tembec, Resolute, St. Marys Paper Corp, Sault STE and MARIE Innovation centre, etc. References ASTM. D906-98. Reapproved 2011. Standard test method for strength properties of adhesives in plywood type construction in shear by tension loading. Bisanda, E.T.N., Ogola, W.O., Tesha, J.V., 2003. Characterization of tannin resin blends for particle board applications. Cem. Concr. Compos. 25, 593–598. Bohm, M., Salem, Z.M.M., Srba, J., 2012. Formaldehyde emission monitoring from a variety of solid wood, plywood, blockboard and flooring products manufactured for building and furnishing materials. J. Hazard. Mater. 221–222, 68–69. Chen, Y.F., Chen, Z.Q., Xiao, S.Y., Liu, H.B., 2008. A novel thermal degradation mechanism of phenol-formaldehyde type resins. Thermochim. Acta 476, 39–43.

268

S. Feng et al. / Industrial Crops and Products 76 (2015) 258–268

Costa, L.L., Montelera, R.D., Camino, G., Weil, E.D., Pearce, E.M., 1997. Structure-charring relationship in phenol formaldehyde type resins. Polym. Degrad. Stab. 56, 23–35. Effendi, A., Gerhauser, H., Bridgwater, A.V., 2008. Production of renewable phenolic resins by thermochemical conversion of biomass: a review. Renew. Sust. Energ. Rev. 12, 2092–2116. Gangi, M., Tabarsa, T., Sepahvand, S., Asghari, J., 2013. Reduction of formaldehyde emission from plywood. J. Adhes. Sci. Technol. 27 (13), 1407–1417. Hoong, Y.B., Paridah, M.T., Loh, Y.F., Jalaluddin, H., Chuah, H.A., 2011. A new source of natural adhesive: Acacia mangium bark extracts co-polymerized with phenol-formaldehyde (PF) for bonding Mempisang (Annonaceae spp.) veneers. Int. J. Adhes. Adhes. 31, 164–167. Hoong, Y.B., Paridah, M.T., Loh, Y.F., Koh, M.P., Luqman, C.A., Zaidon, A., 2010. Acacia mangium tannin as formaldehyde scavenger for low molecular weight phenol formaldehyde resin in bonding tropical plywood. J. Adhes. Sci. Technol. 24, 1653–1664. Hse, C.Y., Fu, F., Bryant, S.B., 2000. Development of formaldehyde-based wood adhesives with co -reacted phenol/soybean flour. Green Chem. Adhes., 13–19. IARC, 2006. Formaldehyde, 2-butoxyethanol and 1-tert-butoxypropan-2-ol. Monographs, Vol. 88. IARC Monographs, Lyon, France. Japanese Industrial Standard (JIS). A 1460, 2001. Building boards determination of formaldehyde emission. Desiccator method. Jiang, H.Y., Wang, J., Wu, S.Q., Yuan, Z.Q., Hu, Z.L., Wu, R.M., Liu, Q.L., 2012. The pyrolysis mechanism of phenol formaldehyde resin. Polym. Degrad. Stab. 97, 1527–1533. Jorge, F.C., Neto, C.P., Irle, M.A., Gil, M.H., Jesus, P.D.J., 2002. Wood adhesives derived from alkaline extracts of maritime pine bark: preparation, physical characteristics and bonding efficacy. Holz als Roh- und Werkstoff 60, 303–310. Kulvik, E., 1977. Chestnut wood tannins extract as cure accelerator for phenol-formaldehyde wood adhesive. Adhes. Age 3, 33–34. Kunaver, M., Medved, S., Cuk, N., Jasiukaityte, E., Poljansek, I., Strnad, T., 2010. Application of liquefied wood as a new particle board adhesive system. Bioresour. Technol. 101, 1361–1368. Lievens, C., Mourant, D., He, M., Gunawan, R., Li, X., Li, C.Z., 2011. A FT-IR spectroscopic study of carbonyl functionalities in bio-oils. 90, 3417-3423. Liiri, O., Sairanen, H., Kilpelainen, H., Kivisto, A., 1982. Bark extractives from spruce as constituents of plywood bonding agents. Holz als Roz- und Werkstoff 40, 51–60. Markovic, S., Dunjic, B., Zlatanic, A., Djonlagic, J.J., 2001. Dynamic mechanical analysis study of the curing of phenol-formaldehyde novolac resins. J. Appl. Polym. Sci. 81 (8), 1902–1913.

Moubarik, A., Pizzi, A., Allal, A., Charrier, F., Charrier, B., 2009. Cornstarch and tannin in phenol-formaldehyde resins for plywood production. Ind. Crops Prod. 30, 188–193. Mourant, D., Riedl, B., Rodrigue, D., Yang, D.Q., Roy, C., 2007. Phenol-fomaldehyde-pyrolytic oil resins for wood preservation: a rheological study. J. Appl. Polym. Sci. 106, 1087–1094. Myers, E.G., 1983. Formaldehyde emission from particleboard and plywood paneling: measurement, mechanism, and product standards. For. Prod. 33 (5), 27–37. Pizzi, A., Merlin, M., 1981. A new class of tannins adhesives for exterior particleboard. Int. J. Adhes. Adhes (July), 261–264. Salem, Z.M.M., Bohm, M., Srba, J., Berankova, J., 2012. Evaluation of formaldehyde emission from different types of wood-based panels and flooring materials using different standard test methods. Build. Environ. 49, 86–96. Suevos, L.F., Riedl, B., 2003. Effects of Pinus pinaster bark extracts content on the cure properties of tannin modified adhesives and on bonding of exterior grade MDF. J. Adhes. Sci. Technol. 17 (11), 1507–1522. Turunen, M., Alvila, L., Pakkanen, T.T., Rainio, J., 2003. Modification of phenol-formaldehyde resole resins by lignin, starch and urea. J. Appl. Polym. Sci. 88, 582–588. Uchiyama, S., Matsushima, E., Kitao, N., Tokunaga, H., Ando, M., Otsubo, Y., 2007. Effect of natural compounds on reducing formaldehyde emission from plywood. Atmos. Environ. 41, 8825–8830. Ueno, T., Ashitani, T., Sakai, K., 2002. New method to determine the hydroxyl value in liquefied bark as polyurethane material. J. Wood Sci. 48, 348–351. Ugovsek, A., Kariz, M., Sernek, M., 2010. Bonding of beech wood with an adhesive mixture made of liquefied wood and phenolic resin. Hardwood science and technology. The 4th Conference on Hardwood Research and Utilization in Europe, 6 pages. Vazquez, G., Antorrena, G., Gonzalez, J., Alvarez, J.C., 1996. Tannin based adhesives for bonding high moisture eucalyptus veneers: influence of tannin extraction and press conditions. Holz als Roh- und Werkstoff 54, 93–97. Walker, J., 2006. Chapter 2. Basic wood chemistry and cell wall ultrastructure. In: Walker, J. (Ed.), Primary Wood Processing: Principles and Practice. , 2nd Edition. Springer, Dordrecht, The Netherlands, pp. 23–67. Yang, I., Kuo, M.L., Myers, J.D., Pu, A.B., 2006. Comparison of protein-based adhesive resins for wood composites. J. Wood Sci. 52, 503–508. Yu, C.W.F., Crump, D.R., 1999. Testing for formaldehyde emission from wood-based products—a review. Indoor Built Environ. 8, 280–286.