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Synthesis and characterization of bio-based phenol-formaldehyde resol resins from bark autoclave extractives.

Abstract

With the growing concern for fossil fuel depletion and environmental carbon footprint, there is a strong interest in exploring the renewable biomass materials as substitutes for petroleum-based feedstock. In this study, bark autoclave extractives from the mountain pine beetle (Dendroctonus ponderosae Hopkins)-infested lodgepole pine (Pinus contorta Dougl.) were used for partially replacing petroleum-based phenol in the phenol-formaldehyde (PF) resol resin synthesis. The structural characteristics of the bark autoclave extractives were examined using liquid-state [sup.13]C nuclear magnetic resonance (NMR). The curing behavior and curing kinetics, bonding strength, and bond development of the resulting bio-based bark extractive-PF resol resins were investigated using differential scanning calorimetry (DSC), lap shear, and dynamic mechanical analysis (DMA) tests, respectively. Results showed that bark autoclave extractives were a complicated mixture containing tannin, degraded hemicellulose, and degraded lignin components. The bark extractive-PF resins exhibited a higher molecular weight, higher viscosity, shorter gel time, and faster curing rate than the laboratory-made PF resin without bark components. The bark extractive-PF resins had comparable bonding strength to a commercial PF resin even when the phenol replacement rate was as high as 50 percent by weight. Bark autoclave extractives obtained from the beetle-infested lodgepole pine are suitable as a partial replacement of petroleum-based phenol in making PF resol adhesives.

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Phenol-formaldehyde (PF) resins, made from the addition and condensation reaction between phenol and formaldehyde under either acid or alkaline conditions, have been widely used in the adhesives, moldings, insulation, construction materials, and electronic devices for a long time due to their excellent bonding performance, water resistance, and durability (Pizzi 1993). The raw materials for making PF resins are usually derived from fossil fuel resources (petroleum and coal).

In recent decades, growing concerns about the depletion of fossil fuel, environmental carbon footprint, and the ever-rising price of petroleum-based products, such as phenol, have created a strong interest in exploring renewable biomass materials as alternative feedstock to fully or partially replace petroleum-based phenol for producing PF resins. PF resins containing tannins, lignin, bark, etc., are some examples of bio-based PF resins (Pizzi and Scharfetter 1978; Grigoriou 1997; Pizzi 1997; Santana et al. 1997; Alma et al. 1998; Vazquez 2002; Gao et al. 2007; Lei et al. 2008; Zhao et al. 2010, 2013a, 2013b, 2013c, 2013d).

Bark, a waste material generally available in large quantities from the forest industry, has potential to be used as an alternative feedstock to phenol because bark is rich in phenolic compounds (Harkin and Rowe 1971, Hon and Shiraishi 2000). In our previous studies, both phenol-liquefied bark-PF resol resins and alkaline bark extractivePF resol resins were successfully formulated using the mountain pine beetle (Dendroctonus ponderosae Hopkins)--infested lodgepole pine barks (Pinus contorta Dougl.; Zhao et al. 2010, 2013b). The bark-based PF resol resins exhibited comparable bonding performance to commercial PF resins for oriented strand board (OSB) face board at 30 percent (by weight) phenol substitution level. The bark components, either in the form of liquefied bark or alkaline extractives, affected the properties and performance of the resulting bark-based PF resins, such as the molecular structures, curing behavior and curing kinetics, thermal stability, and bond strength compared with a control PF resin (Zhao et al. 2010, 2013b, 2013d). Liquefaction and alkaline extractions were found to be two effective and suitable methods to obtain phenol substitutes from bark, which could be used as alternative feedstock of phenol for PF resin formulation (Gao et al. 2007; Zhao et al. 2010, 2013c).

Autoclave extraction has been widely used for improving the extraction procedure of different materials. It includes the extraction of chemical elements in soil and water-soluble vitamins in milk. Meanwhile, autoclave extraction is also used for improving the tannin extraction from bark, and the commonly used solutions are water and sodium sulfite (Deschamps and Leulliette 1985, Wina et al. 2010). To our knowledge, there are no previous studies reporting bark alkaline extraction by using the autoclave method with 1 percent sodium hydroxide, especially for the mountain pine beetle-infested lodgepole pine bark. The differences of the bark extractives obtained from the autoclave extraction and regular extraction method are unknown. The influence of the bark alkaline extractives from autoclave extraction on the performance and properties of the resulting bark-based PF resin is unclear.

Therefore, in this study, the lodgepole pine bark with mountain pine beetle infestation was used as the raw material for the phenolic compound extraction. The extraction method was autoclave extraction, and the extraction solvent was 1 percent NaOH alkaline solution. The extractives were used to partially replace petroleum-based phenol for formulating bark extractive-PF resins. The properties of the resulting bark extractive-PF resins with different phenol substitution levels were investigated.

Experimental

Bark extractives

An air-dried bark powder sample of mountain pine beetle--infested lodgepole pine passing through a 35-mesh sieve was extracted using 1 percent NaOH aqueous solution in an autoclave set at 120[degrees]C for 30 minutes. The solvent-to-bark ratio was 5:1 (vol/wt). The same extraction method was repeated two more times with fresh caustic solution. Each extract was then poured through Whatman filter paper. The resulting alkaline filtrates from the three-stage extractions were combined for further drying.

The alkaline insoluble bark residue was washed with 1 percent acetic acid aqueous solution and distilled water until pH 7 and then air-dried to a constant weight. The alkaline soluble fraction was separated into two portions: one was kept at alkaline conditions and the other portion was neutralized using 1 M HCl to pH 7. Both fractions were then oven-dried at 60[degrees]C to constant weights. The resulting solid extractives were ground by mortar and pestle into powders for resin formulation.

The yield of the alkaline extractives was quantified using Equation 1:

Extractives (%) = [W.sub.0] - [W.sub.r]/[W.sub.0] x 100% (1)

where [W.sub.0] is the air-dried weight of the bark before extraction and [W.sub.r] is the air-dried weight of the residues after extraction.

Extractive characterization

Stiasny number of the bark extractives.--The Stiasny number is an indicator of the content of formaldehyde-condensable polyphenols in the bark extractives. It is measured according to published procedures (Hillis and Urbach 1959, Hillis and Yazaki 1980, Garro Galvez et al. 1997). About 100 mg of bark extractives ([W.sub.s]) were first dissolved in 10 mL of distilled water. Then 1 mL of 10 M HCl and 2 mL of formaldehyde solution (37%) were added to the bark extractive samples, and reactants were heated and refluxed for 30 minutes. The reaction mixtures were filtered, and the precipitates were washed with an excessive amount of hot water and oven-dried to a constant weight ([W.sub.p]). The Stiasny number was calculated using Equation 2.

Stiasny number (%) = [W.sub.p]/[W.sub.s] x 100% (2)

[sup.13]C NMR of bark extractives.--Liquid-state [sup.13]C nuclear magnetic resonance (NMR) was used to investigate the bark extractives. The analytical method was referred to our previous study (Zhao et al. 2013a). Bark extractives were dissolved in dimethyl sulfoxide-[d.sub.6]. The liquid-state [sup.13]C NMR spectrum of the sample was recorded using a Unity 500 spectrometer under the following conditions: a pulse angle of 60[degrees] (8.3 [micro]s), a relaxation delay of 10 seconds, and gated Waltz-16 1H decoupling during the acquisition period. At least 400 scans were accumulated for each spectrum. The [sup.13]C chemical shifts were measured using tetramethylsilane as the internal standard.

Resin formulation

The resin formulation was conducted based on our previous studies (Gao et al. 2007; Zhao et al. 2010, 2013b). Calculated amounts of bark extractives from autoclave extraction (either dried under neutral or alkaline conditions), phenol (crystal form), 37 percent formaldehyde, and 40 percent sodium hydroxide (1/3 of total NaOH weight) were used in the resin formulation. All the chemicals were purchased from Caledon Laboratory Chemicals, Canada, and used without further purification.

Two types of bark extractive-PF resins were formulated. The bark extractive--PF resins made with 30, 50, and 70 percent (wt/wt) phenol replacement by the autoclave extractives dried under alkaline conditions were denoted as 30 BEA-PF, 50 BEA-PF, and 70 BEA-PF, respectively, while the bark extractive--PF resins made with 30, 50, and 70 percent (wt/wt) phenol replacement by autoclave extractives dried under neutral conditions were denoted as 30 BEA-PF(N), 50 BEA-PF(N), and 70 BEA-PF(N), respectively. The bark extractive--PF resins were viscous liquids with dark brown color. These bark extractive--PF resins were subjected to various tests without further drying.

The laboratory-made (lab-made) PF resin without bark extractives was prepared by following exactly the same reaction steps used for the bark extractive--PF resins formulation. A commercial PF resin for the face layers of OSB production was used for comparison.

Resin characterization

Resin properties.--The pH values of the resins were measured at 25[degrees]C. The viscosity of the lab-made PF resin and bark extractive--PF resins was measured using a Brookfield rotary viscometer. The procedure described in the ASTM D3529 (ASTM International 2010) standard was used for the measurements of solids contents. The molecular weight of resins was measured using a matrix-assisted laser desorption/ionization-time-of-flight/time-of-flight spectrometer (MALDI-TOF/TOF; Applied Biosystems, Framingham, Massachusetts). The detailed method for molecular weight calculation can be found in the previous studies (Zhao et al. 2010, 2013b)

Resin curing behavior and curing kinetics.--High-pressure pans (DSC-Q1000; TA Instruments, USA) were used for investigating the resin curing behavior. Dynamic scans were carried out at heating rates of 5[degrees]C, 10[degrees]C, 15[degrees]C, and 20[degrees]C per min, starting from room temperature and increasing to 250[degrees]C.

The Kissinger method (Kissinger 1957) was used to calculate the activation energy as

ln([phi]/[T.sup.2.sub.p] = - [E/R] 1/[T.sub.p] + (RA/E) (3)

where [phi] is the heating rate (K/s), [T.sub.p] is the peak temperature (in Kelvins) at the given heating rate, A is the preexponential factor, R is the ideal gas constant, and E is the activation energy. A sample was randomly chosen for testing, and three replicates were performed for each sample. The maximum variation in the onset and peak temperatures was less than 1[degrees]C.

Thermal stability.--Two types of bark extractive--PF resins with different phenol substitution rates, lab-made control PF resin and commercial PF resin, were first cured in an oven at 105[degrees]C for 24 hours. Then the cured resins were ground into fine powders that were able to pass through a 0.149-mm sieve (100-mesh screen). About 10 mg of each cured resin sample was placed in a platinum pan and heated from room temperature to 700[degrees]C at the rate of 10[degrees]C/min under a [N.sub.2] atmosphere using a thermal gravimetric analyzer (TGA-Q500; TA Instruments).

Bonding strength.--Poplar veneer was cut into strips (3 mm thick, 25.4 mm wide, and 108 mm long) with the length direction parallel to the wood grain. The two-layered poplar wood veneer specimens were bonded with the different resins. The adhesives were applied to one side of the poplar strip over an area of 25.4 by 25.4 mm. The spread rate of the adhesives was 0.025 to 0.035 g/[cm.sup.2] on a solid base. The adhesive-coated area of the poplar strip was then overlapped with an uncoated poplar strip. The resulting two-layered lap shear specimen was hot pressed at 160[degrees]C under the thickness control of 4.5 mm for 3 minutes. After cooling and conditioning, the specimens were tested for shear strength on a Zwick universal test machine (Zwick/Z100; Zwick Roell Group, Germany) following the standard lap shear test methods as described in ASTM D5868 (ASTM International 2014). The crosshead speed was 1.3 mm/min. The average value based on a minimum of 10 replicates is reported.

The lap shear specimens were subjected to both a water-soaking-and-drying (WSAD) test and a boiling water test (BWT), according to voluntary standard PS-1-95 published by the US Department of Commerce through the Engineered Wood Association, Tacoma, Washington. For the WSAD test, the specimens were soaked in water at room temperature for 24 hours, dried in a fume hood at room temperature for another 24 hours, and then subjected to shear strength measurements. For the BWT test, the specimens were boiled in water for 4 hours and then dried for 20 hours at 63[degrees]C [+ or -] 2[degrees]C. After drying, the specimens were boiled in water again for 4 hours, cooled with tap water, and then tested for shear strength while still wet. The shear strength obtained using this method was defined as BWT/W strength.

Results and Discussion

Properties of bark extractives

Molecular weight and Stiasny number.--The average yield of bark alkaline extractives obtained from autoclave extraction was 55.32 percent. Table 1 shows the molecular weight and the content of formaldehyde-condensable phenolic compounds of the bark alkaline extractives obtained from autoclave extraction measured by the Stiasny number. The molecular weight of the extractives decreased after drying under alkaline conditions, indicating the occurrence of the degradation or the hydrolysis of extractives during the drying process. For the extractives dried under neutral conditions, the molecular weight increased after drying, suggesting that there might be condensation reactions of the extractives during the drying process under pH-neutral conditions. The Stiasny number of the extractives dried under alkaline conditions and neutral conditions was 43.11 and 42.96 percent, respectively. It indicated that the neutralization of extractives before the drying process did not affect the reactivity of the extractives toward formaldehyde. Similar trends were observed in our previous studies on bark alkaline extractives obtained using the regular extraction method (Zhao et al. 2013b). Compared with the bark alkaline extractives obtained from regular extraction methods, bark alkaline extractives obtained from autoclave extraction had a slightly higher Stiasny number and a lower molecular weight. It indicated that the bark alkaline extractives underwent more degradation under autoclave extraction than regular extraction. The reactivity of bark alkaline extractives from autoclave extraction toward formaldehyde was higher than the bark alkaline extractives from the regular extraction method.

Structure and composition.--The liquid-state [sup.13]C NMR spectrum of bark alkaline extractives obtained from autoclave extraction is shown in Figure 1. The assignment of the chemical shifts was conducted according to previous studies (Vazquez et al. 1997, Grigsby et al. 2003, Sun et al. 2005, Navarrete et al. 2010, Zhao et al. 2013a).

The [sup.13]C NMR spectrum of bark alkaline extractives obtained from autoclave extraction resembled that of bark alkaline extractives obtained from the regular extraction method. The chemical shift at 181 ppm was assigned to the carbonyl group in the quinone structure; it came from the oxidation of phenolic hydroxyl groups. The chemical shift at 174 ppm could be assigned to either catechin or epicatechin gallate, suggesting the C=O bond of a gallic residue linked to catechin or epicatechin gallate.

The tannin flavonoid structure was found in the bark alkaline extractives obtained from the autoclave extraction. The chemical shifts at about 150 ppm representing C5 and C7 attached to the phenolic -OH group on the flavonoid A-ring; the shifts at 140- to 145 ppm representing the C3' and C4' on the B-ring; and shifts at 131 ppm, 116- to 118 ppm, 110- to 115 ppm, and 105 ppm indicating the C1', C5' and C2', C4-C8, and C4-C6 interflavonoid bonds, respectively. Less C4-C8 and C4-C6 interflavonoid bonds were observed in the bark alkaline extractives obtained from autoclave extraction compared with the bark alkaline extractives obtained from the regular extraction method reported in our previous study (Zhao et al. 2013a). It also indicated that the bark alkaline extractives obtained from autoclave extraction underwent more degradation than the bark alkaline extractives obtained from the regular extraction method.

The intensity of the chemical shift at 110 to 115 ppm was higher than that at 105 ppm, indicating the tannins in the bark alkaline extractives obtained from the mountain pine beetle-infested lodgepole pine through autoclave extraction were mainly procyanidin type.

The existence of the pyrogallol-type B-ring and catechol B-ring in the tannins from the alkaline pine bark extractives was proved by the chemical shift at 116 ppm. The chemical shift of C1' at 130 to 132 ppm for the catechol B-rings and 132 to 135 ppm for the pyrogallol B-rings was also observed, indicating the coexistence of the catechol B-rings and pyrogallol B-rings in the tannins from the bark alkaline extractives obtained from autoclave extraction.

In addition, the lignin fragments and degraded hemicellulose were also observed in the bark alkaline extractives from autoclave extraction. The chemical shift at 55.6 ppm belonged to the methoxyl groups in the lignin. The chemical shift at 62.9 ppm was attributed to Cy in the [beta]-O-4 structure. The chemical shift at 122 ppm was assigned to the C6 in the guaiacyl units. The chemical shift at 69.6 ppm indicated the existence of CH-4 in xylose non-reducing end unit.

The liquid-state [sup.13]C NMR spectrum has confirmed that the bark alkaline extractives from mountain pine beetle-infested lodgepole pine through autoclave extraction contained tannin, degraded lignin, and degraded hemicellulose. The tannin structures of the extractives are mainly procyanidin type, consisting of phloroglucinol A-ring, catechol B-ring, and pyrogallol B-ring. Less C4-C8 and C4-C6 interflavonoid bonds and more degraded lignin fragments and hemicellulose were found in the bark alkaline extractives obtained from autoclave extraction. Overall, the bark alkaline extractives obtained from autoclave extraction had similar chemical compositions and characteristics to the bark alkaline extractives obtained from the regular extraction method reported in our previous study (Zhao et al. 2013a); however, the bark alkaline extractives obtained from autoclave extraction underwent more degradation than the bark alkaline extractives obtained from the regular extraction method.

Bark extractive-PF resins

Properties of bark extractive--PF resins.--The basic properties of the bark extractive--PF resins are shown in Table 2. The solids content of the bark extractive--PF resins ranged from 47.98 to 50.91 percent. Bark extractive--PF resins were found to have a higher molecular weight, higher viscosity, and shorter gel time than the lab-made control PF resin without bark components. The bark extractive-PF resins made using autoclave extractives dried under alkaline conditions exhibited a lower molecular weight, longer gel time, and lower viscosity than the bark extractive--PF resins made using autoclave extractives dried under neutral conditions.

Curing behavior and curing kinetics of bark extractivePF resins.--The differential scanning calorimetry (DSC) curves of the bark extractive--PF resins (BEA-PF and BEA-PF(N)) with 30 percent phenol substitution level are shown in Figure 2. Two exothermic peaks were observed, one is attributed to the addition reaction, and the other is the condensation reaction (Lei et al. 2006). It is different from the bark extractive--PF resins made using extractives from the regular extraction method (Zhao et al. 2013b).

The cure temperatures of the resins are summarized in Table 3. The actual cure temperatures should be independent of the heating rate; thus, peak and onset temperatures were extrapolated to zero heating rate for comparison (Zhao et al. 2010, 2013b).

The onset temperature was higher than that of the labmade PF resin without bark components (95[degrees]C), and peak temperature of the resin was lower than that of the lab-made PF resin (136[degrees]C). The onset temperature and peak temperature of the bark extractive--PF resins with 30 percent phenol substitution level by autoclave extractives were not affected by the neutralization of the extractives before the drying process.

The curing behavior of the bark extractive--PF resins was affected by the phenol substitution rate. Only one exothermic peak was observed in the DSC curves of the curing process of bark extractive--PF resins with 50 and 70 percent phenol substitution levels (shown in Figs. 3 and 4). This peak indicated that only condensation reactions occurred during the resin curing process, and the addition reactions finished during resin synthesis.

The cure temperatures for the bark extractive--PF resins with 50 and 70 percent phenol substitution levels are shown in Table 4. The 50 BEA-PF, 50 BEA-PF(N), and 70 BEAPF exhibited higher onset temperatures and lower peak temperatures than that of the lab-made PF resin, while the 70 BEA-PF(N) had a higher onset and peak temperature than that of the lab-made PF resin.

The resins made with the extractives dried under neutral conditions had higher onset and peak temperatures than the resins made with extractives dried under alkaline conditions. The resin made with 70 percent phenol substitution level had higher onset and peak temperatures than those made with 50 percent phenol substitution rate. The presence of more neutralized extractives with a high molecular weight would reduce the reactivity of the resulting bark extractive--PF resins, possibly due to the immobility and inaccessible reactive sites on the extractive molecules (Zhao et al. 2010, 2013b).

The two important parameters (curing activation energy E and preexponential factor A) for the curing kinetics of the bark extractive--PF resins are shown in Table 5.

The introduction of bark components in the PF resol resin formulation affected the resin's curing activation energy and curing rates. When the phenol substitution level was 30 percent, the resin made with extractives dried under neutral conditions had lower curing activation energy and pre-exponential factor for the addition reaction than the resin made with extractives dried under alkaline conditions. When the phenol substitution level was 50 and 70 percent, resins made with extractives dried under neutral conditions exhibited higher curing activation energy and higher pre-exponential factors than resins made with extractives dried under alkaline conditions.

Bonding development of different resins.--The bonding development of bark extractive-PF resins with 30 percent phenol substitution rate during the curing process was measured by dynamic mechanical analysis (DMA; shown in Fig. 5). Gel point ([T.sub.gel]) and [T.sub.tan] [delta] were used to characterize the resin curing process; the difference of the storage modulus was used to characterize the rigidity of the resin (He and Yan 2005).

From the DMA tests, it is found that the gel point and [T.sub.tan] [delta] of the bark extractive--PF resins decreased slightly with the increasing phenol substitution rates in resin formulation. There was no significant difference in the rigidity of the resins made with different phenol substitution rates (shown in Fig. 6).

Thermal stability.--Thermal stability of the cured bark extractive--PF resins is shown in Figures 7 and 8. The thermal stability of the cured bark extractive-PF resins made with autoclave extractives dried under alkaline and neutral conditions was lower than the lab-made control PF resin without bark components and higher than the commercial PF resin based on the remaining weight of the resins. Similar trends were observed for the bark extractive--PF resins made with extractives obtained from the regular extraction methods (Zhao et al. 2013b).

Bonding strength.--The bonding strength of the bark extractive--PF resins is shown in Figure 9. Based on the statistical t test, at the level of P = 0.05, bark extractive--PF resins with the 30 percent phenol substitution level by autoclave extractives dried under alkaline and neutral conditions and bark extractive-PF resin with the 50 percent phenol substitution level by autoclave extractives dried under alkaline conditions had comparable dry and wet bonding strengths to a commercial PF resin for OSB face board. The bark extractive--PF resin with the 50 percent phenol substitution level by autoclave extractives dried under neutral conditions exhibited similar dry and wet bonding strengths to the lab-made PF resin without bark components. Neutralization of extractives before the drying process negatively affected the bond strengths of the resulting bark extractive--PF resins when the phenol substitution level was higher than 50 percent. No delamination was observed in any of the specimens after the WSAD treatment and the boiling water treatment.

Conclusions

Bark alkaline extractives from the mountain pine beetle (Dendroctonus ponderosae Hopkins)--infested lodgepole pine (Pinus contorta Dougl.) were obtained using autoclave extraction. The extractives were found to contain tannin, degraded lignin, and degraded hemicellulose in their composition. The tannin structures of the extractives are mainly procyanidin type, consisting of phloroglucinol A-ring, catechol B-ring, and pyrogallol B-ring. Compared with the bark alkaline extractives obtained using the regular extraction method, less C4 to C8 and C4 to C6 interflavonoid bonds and more degraded lignin fragments and hemicellulose were found in these autoclave extractives. The PF resins made using bark alkaline extractives from autoclave extraction had higher molecular weight, higher viscosity, shorter gel times, and generally faster curing rates than the lab-made PF resin without bark components. Neutralization of extractives before the drying process affected the extractives' molecular weight, curing behavior, and the bonding strength of the resulting bark extractive-PF resins. Bark alkaline extractives obtained from autoclave extractions are suitable to partially replace petroleum-based phenol for PF resin formulation.

Acknowledgment

Financial support from partners of Ontario Research Fund-Research Excellence project: Bark Biorefinery is highly acknowledged.

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The authors are, respectively, Postdoctoral Fellow and Professor, Faculty of Forestry, Univ. of Toronto, Toronto, Ontario, Canada (zhaoyong17@yahoo.com, ning.yan@utoronto.ca [corresponding author]; and Senior Scientist, FPInnovations, Vancouver, British Columbia, Canada (Martin.Feng@fpinnovations.ca). This article was received for publication in January 2015. Article no. 15-00001.

doi: 10.13073/FPJ-D-15-00001

Table 1.--Properties of the bark alkaline extractives obtained from
autoclave extraction. (a)

                                      Mn (Da)             Mw (Da)

Alkaline extractives (wet)       1.46 x [10.sup.3]   2.45 x [10.sup.3]
Alkaline extractives (dry)       1.07 x [10.sup.3]   1.96 x [10.sup.3]
Neutralized extractives (wet)    1.66 x [10.sup.3]   3.01 x [10.sup.3]
Neutralized extractives (dry)    2.53 x [10.sup.3]   5.85 x [10.sup.3]

                                 Mw/Mn   Stiasny no. (%)

Alkaline extractives (wet)       1.67          --
Alkaline extractives (dry)       1.83         43.11
Neutralized extractives (wet)    1.81          --
Neutralized extractives (dry)    2.31         42.96

(a) Mn = number average of molecular weight; Mw = weight average of
molecular weight.

Table 2.--Properties of bark extractive-phenol-formaldehyde resins
made using autoclave extractives. (a)

                          Solids         Viscosity at
                 pH     content (%)   25[degrees]C (cps)

30% BEA-PF      12.42      50.91             440
50% BEA-PF      11.74      48.53             370
70% BEA-PF      11.25      50.84             325
30% BEA-PF(N)   12.11      48.81             580
50% BEA-PF(N)   11.90      48.72             520
70% BEA-PF(N)   10.91      47.98             495
Laboratory PF   11.93      48.87              25
Commercial PF   11.16      59.00             200

                   Gel time at
                120[degrees]C (s)   Mn (Da)   Mw (Da)   Mw/Mn

30% BEA-PF              78           481.6     960.5    1.99
50% BEA-PF              89           452.8    1132.2    2.50
70% BEA-PF             105           557.8    1243.8    2.23
30% BEA-PF(N)           59           801.2    1261.4    1.57
50% BEA-PF(N)           67           905.1    1385.9    1.53
70% BEA-PF(N)           88           698.4    1337.4    1.91
Laboratory PF          173          258.95    327.27    1.25
Commercial PF          172          212.09    386.48    1.82

(a) Mn = number average of molecular weight; Mw = weight average of
molecular weight; BEA-PF = bark extractive-phenol-formaldehyde resins
made with autoclave extractives dried under alkaline conditions;
BEA-PF(N) = bark extractive-phenol-formaldehyde resins made with
autoclave extractives dried under neutral conditions.

Table 3.--Cure temperature of bark extractive-phenol-formaldehyde
resins with 30 percent phenol substitution rate. (a)

                                   30 BEA-PF

Pleating rate       Onset temp     Peak tempi     Peak temp2
([degrees]C/min)   ([degrees]C)   ([degrees]C)   ([degrees]C)

0                      101            104            132
5                      102            108            138
10                     112            118            152
15                     114            123            158
20                     115            126            163

                                  30 BEA-PF(N)

Pleating rate       Onset temp     Peak tempi     Peak temp2
([degrees]C/min)   ([degrees]C)   ([degrees]C)   ([degrees]C)

0                       99            104            132
5                      103            109            139
10                     110            118            151
15                     114            124            160
20                     118            127            165

(a) BEA-PF = bark extractive-phenol-formaldehyde resins made with
autoclave extractives dried under alkaline conditions; BEA-PF(N) =
bark extractive-phenol-formaldehyde resins made with autoclave
extractives dried under neutral conditions.

Table 4.--Cure temperature of bark extractive-phenol-formaldehyde
resins with 50 and 70 percent phenol substitution level. (a)

                                 50 BEA-PF

Heating rate             Onset               Peak
([degrees]C/min)   temp ([degrees]C)   temp ([degrees]C)

0                         112                 116
5                         117                 122
10                        122                 128
15                        129                 134
20                        133                 140

                               50 BEA-PF(N)

Heating rate             Onset               Peak
([degrees]C/min)   temp ([degrees]C)   temp ([degrees]C)

0                         117                 122
5                         122                 126
10                        124                 133
15                        128                 136
20                        132                 141

                                 70 BEA-PF

Heating rate             Onset               Peak
([degrees]C/min)   temp ([degrees]C)   temp ([degrees]C)

0                         124                 129
5                         128                 133
10                        134                 140
15                        137                 146
20                        141                 149

                               70 BEA-PF(N)

Heating rate             Onset               Peak
([degrees]C/min)   temp ([degrees]C)   temp ([degrees]C)

0                         132                 142
5                         136                 146
10                        137                 149
15                        140                 152
20                        144                 156

(a) BEA-PF = bark extractive-phenol-formaldehyde resins made with
autoclave extractives dried under alkaline conditions; BEA-PF(N) =
bark extractive-phenol-formaldehyde resins made with autoclave
extractives dried under neutral conditions.

Table 5.--Parameters of curing kinetics of different resins. (a)

                El (KJ/mol)    Al ([S.sup.-1])

30 BEA-PF          90.91      1.06 x [10.sup.12]
30 BEA-PF(N)       87.76      3.76 x [10.sup.11]
50 BEA-PF          95.17      1.59 x [10.sup.12]
50 BEA-PF(N)      128.44      3.29 x [10.sup.16]
70 BEA-PF         115.18      2.91 x [10.sup.14]
70 BEA-PF(N)      190.36      4.20 x [10.sup.23]
Laboratory PF      70.22      1.59 x [10.sup.8]
Commercial PF      82.23      1.18 x [10.sup.10]

                E2 (KJ/mol)    A2 ([S.sup.-1])

30 BEA-PF          74.15      7.05 x [10.sup.8]
30 BEA-PF(N)       70.92      2.56 x [10.sup.8]
50 BEA-PF           --               --
50 BEA-PF(N)        --               --
70 BEA-PF           --               --
70 BEA-PF(N)        --               --
Laboratory PF       --               --
Commercial PF       --               --

(a) BEA-PF = bark extractive-phenol-formaldehyde resins made with
autoclave extractives dried under alkaline conditions; BEA-PF(N) =
bark extractive-phenol-formaldehyde resins made with autoclave
extractives dried under neutral conditions.
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Author:Zhao, Yong; Yan, Ning; Feng, Martin
Publication:Forest Products Journal
Date:Jan 1, 2016
Words:5728
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