Thermal degradation performance of bark based phenol formaldehyde adhesives

Journal of Analytical and Applied Pyrolysis 115 (2015) 184–193

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Thermal degradation performance of bark based phenol formaldehyde adhesives 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 28 March 2015 Received in revised form 19 July 2015 Accepted 20 July 2015 Available online 26 July 2015 Keywords: Bark based PF adhesives TGA/FTIR couple system Thermal stability Evolved gaseous products PF adhesive thermal degradation

a b s t r a c t Neat phenol formaldehyde adhesive (neat PF), bark bio-crude synthetic bio-based phenol formaldehyde adhesive (bio-crude synthetic BPF) in which 50% phenol was substituted by bio-crude during synthesis, bark bio-crude formulated bio-based phenol formaldehyde adhesive (bio-crude formulated BPF) that was prepared through direct blending of aqueous bio-crude with neat PF at the weight ratio of 50:50, and bark extractives synthetic bio-based phenol formaldehyde adhesive (extractive synthetic BPF) in which 50% phenol was substituted by alkali extractives during synthesis were characterized by thermogravimetry analyzer coupled to Fourier transform infrared spectroscopy (TGA/FTIR), to investigate the stability, thermal degradation kinetics, structure change and the evolved gaseous products. It was found neat PF and BPFs all went through typical three stages degradation. Introduction of bio-crude into BPF reduced the thermal stability for the resulted BPF to a greater extent than extractive introduction. With respect to residual weight after degradation at 800 ◦ C, adhesives thermal stability followed the order: neat PF > extractives synthetic BPF  bio-crude formulated BPF > bio-crude synthetic BPF, which is consistent with the activation energy for the adhesives degradation in the main degradation stage. Auto-oxidation occurred during all the adhesive thermal degradation. But bio-crude based BPFs are more auto-oxidation resistant than neat PF or extractive synthetic BPF. Gaseous products evolved from neat PF degradation mainly consisted of water, phenol, cresol, CO2 , CH4 , CO and carboxylic acids. During BPFs degradation, CH4 , phenol, cresol were yielded at lower temperatures than neat PF degradation. Besides, xylenol not yielded during neat PF degradation was evolved from BPF degradation. © 2015 Elsevier B.V. All rights reserved.

1. Introduction To date, many studies have been conducted to investigate the thermal degradation performance of phenol formaldehyde (PF) resins. A prevenient three stage degradation mechanism has been proposed for PF resin degradation [1]. In the first stage, additional cross-linkages are formed due to the further condensations among the un-reacted functional groups, such as reaction between two methylols resulting in ether crosslink, or reaction between a phenol hydroxyl and a methylene forming a carbon–hydrogen crosslink. Besides, removal of small exposed groups in PF resin also occurs. The second stage refers to the break of methylene bridges into methyl groups, evolving methane, hydrogen, carbon monoxide, small oligomers and water. Meanwhile, methylene scission contributing to volatiles such as phenol and methyl derivatives takes place in the second stage. In the third stage, removal of hydro-

∗ Corresponding author. E-mail address: [email protected] (C. Xu). http://dx.doi.org/10.1016/j.jaap.2015.07.015 0165-2370/© 2015 Elsevier B.V. All rights reserved.

gen atoms from the ring structure, evolution of hydrogen gas, degradation of phenol groups, and the collapse, carbonization and graphitization of PF resin network occur [2–4]. Specifically as results of temperature elevation, PF resin structure changes significantly. It has been evidenced that below 300 ◦ C, PF resin structure does not change too much. Around 300 ◦ C, the amount of hydroxyl group begins to decrease. Above 350 ◦ C, water and unreacted oligomers are formed due to the condensation between residual methylol groups and phenolic hydroxyl groups, or between phenolic hydroxyl groups [4]. Above 400 ◦ C, condensation between phenolic hydroxyl groups and methylene bridges yielding diphenylmethane structure occurs [5], and the diphenyl ether type linkage decomposes rapidly into benzylphenyl ethers, then xanthene or diphenylene. In addition, methylene can be also oxidized or scissored out of polymer chain backbone. It has been found that phenol and its methyl derivatives are always the majority volatiles at the temperature range of 450–750 ◦ C, indicating the scission of methylene is mainly responsible for PF resin degradation. At 500 ◦ C, the polymer network remains essentially intact, but carbonyl groups in the residue are detected. PF resin itself

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can be oxidized as hydroxyl radicals yielded during the degradation provide an oxidative atmosphere for methylene oxidization, contributing to the formation of oxidized terminal groups such as aldehydes and carboxylic acids, as well as CO2 [2,4]. Above 500 ◦ C, PF resin structure changes dramatically due to the collapse of the network in polyaromatic domains. From 600 ◦ C, the aromatic hydrogen also begins to be eliminated and PF resins become structureless likely due to the condensation of aromatic nuclei. The oxygen left may be present as inner-ring oxygen, or in diphenylene oxide [6,7]. Above 650 ◦ C, benzene and its methyl derivatives can be identified, due to the strip off of phenolic side groups [2]. Over the past few decades, a lot of studies have been reported on value-added products from bark. Bio-phenol production from bark has been achieved through alkaline extraction, phenolysis, and pyrolysis. The introduction of bark bio-phenols into PF resin as phenol substitution changes the thermal properties of the resulted BPF resins. It has been reported that thermal stability of bark extractives based BPF resole is better than that of a lab synthesized neat PF resole, while phenolated bark based BPF resole and bark pyrolysis bio-oil based BPF resole are less thermally stable than neat PF resole [8,9]. During PF resin degradation, many gases such as CO2 , CH4 , CO and C2 H6 [10], as well as low boiling point volatiles such as water, phenol, cresol and other oligomers are released. However, gas or low boiling point volatiles evolved from polymer degradation cannot be characterized by TGA, and study on the gases or volatiles evolved from bio-based BPF resin degradation have not been reported yet to date. Thermogravimetry analyzer coupled to Fourier transform infrared spectroscopy (TGA/FTIR) has been considered as a very useful instrument for polymer degradation analysis, in that it can analyze the thermal stability of polymer, as well as the evolved gas and volatiles in situ. This study aims at investigating BPFs thermal degradation performance, and clarifying the reactions involved in the thermal degradation of BPFs. Three bark based BPFs, namely extractives synthetic BPF, bio-crude synthetic BPF and bio-crude formulated BPF were prepared and pre-cured at 125 ◦ C for 105 min. Then TGA, FTIR and TGA/FTIR coupled system were employed to analyze the thermal stability, structure change of BPFs, and the gaseous products evolved from BPFs thermal degradation. 2. Materials and methodology 2.1. Materials White birch bark was obtained from a local wood mill in Thunder Bay, Ontario, Canada. The bark was air dried and ground into particles of 20 mesh. Anhydrous ethyl alcohol (ethanol) and acetone (≥99.5%) were supplied by Caledon Laboratory Chemicals, Canada. Phenol (A.C.S. reagent, ≥99.0%) was provided by Sigma–Aldrich, while formaldehyde (reagent grade, 37% aqueous formaldehyde) and sodium hydroxide solution (A.C.S. reagent, 50%) were purchased from EMD, Germany. 2.2. Methodology 2.2.1. Preparation of extractives and bio-crude Extractives were prepared through reflux of white birch bark in 3% sodium hydroxide solution. In one extraction run, 30 g air dried white birch bark was charged with 180 mL 3% sodium hydroxide solution into a 500 mL three neck glass reactor, then refluxed for 180 min. The reactor was then cooled down to room temperature and the slurry was filtered. The filtrate was collected and the water was removed through rotary evaporation at 60 ◦ C under reduced

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pressure, then vacuum-dried at 60 ◦ C for 24 h. The dried products were ground using a pestle and a mortar, designated as extractives with a yield of 62.4%. Bio-crude was produced through bark liquefaction in hotcompressed water/ethanol (50:50, v/v) mixture. In one typical bio-crude production process, 25 g oven dried white birch bark and 250 mL water/ethanol (50:50, v/v) mixture were fed into a 500 mL Parr batch reactor and mechanically stirred at 175 rpm. The residual air in the reactor was removed by alternative vacuumingN2 purge for three times, the reactor was finally pressurized with 2.0 MPa N2 and heated to 300 ◦ C (pressure at this temperature was 12.0 MPa) at 10 ◦ C/min. After 15 min liquefaction at 300 ◦ C, the liquefaction was stopped by quenching in a water/ice bath. After the reactor was cooled down to room temperature, gas in the reactor was vented. The reactor was opened and rinsed with acetone, the slurry and rinsing acetone were then collected and filtered. The filtrate was later evaporated under reduced pressure to remove acetone, ethanol and water at 45 ◦ C, 60 ◦ C and 75 ◦ C, respectively. The left dark viscous product was vacuum-dried at 60 ◦ C for 24 h and designated as bio-crude with a yield of 66.5%. 2.2.2. Preparation of Neat PF and BPFs As the reference, a neat PF adhesive with an formaldehyde/phenol (F/P) molar ratio of 1.8 was synthesized. Specifically, 40 g phenol, 12 g 50% aqueous sodium hydroxide solution and 40 g water were charged into a 250 mL three neck-flask connecting to a condenser, then magnetically stirred. The reactor was then heated to 84 ◦ C. During the heating process, 62.1 g 37% formalin was fed into the reactor dropwise. The synthesis reaction was hold at 84 ◦ C for 180 min. The synthesis reaction was stopped by quenching in a water/ice bath, and the viscous liquid in the reactor was designated neat PF. As for extractive synthetic BPF adhesive synthesis, 20 g phenol, 20 g extractive powder, and 40 g water were charged into a 250 mL three-neck flask equipped with a condenser, and magnetically stirred. The reactor was heated at 80 ◦ C for 60 min to get a homogeneous solution, then 62.1 g 37% formalin was fed into the reactor dropwise. The reactor was further heated to 84 ◦ C and hold at 84 ◦ C for 180 min. The synthesis reaction was stopped through quenching the reactor in a water/ice bath. The product was designated as extractive synthetic BPF. Bio-crude based PF adhesives were prepared through two pathways. One was to replace 50 wt% phenol during PF resin synthesis, the synthesis process was similar to that for extractive synthetic BPF adhesive preparation, the only difference is that 12 g 50% aqueous sodium hydroxide solution was charged together with phenol, bio-crude and water. This bio-crude based PF resin was designated as bio-crude synthetic BPF. The other bio-crude based PF adhesive was prepared through post-mixing aqueous bio-crude and neat PF adhesive. The process is briefed as the followings: 20 g bio-crude, 20 g water and 6 g 50% NaOH were fed into a 250 mL three neck reactor and heated at 80 ◦ C for 60 min under magnetic stirring of 210 rpm to get a homogeneous bio-crude solution, then the aqueous bio-crude solution was cooled down and transferred into a 250 mL beaker, to which 46 g neat PF adhesive was added. The beaker containing both aqueous bio-crude and neat PF adhesive was then sealed with para-film and the mixture was magnetically stirred for 15 min. The obtained homogenous aqueous mixture was designated as bio-crude formulated BPF. Some basic properties of the neat PF and the three bio-based BPFs are as displayed in Table 1. 2.2.3. TGA/FTIR tests on neat PF and BPFs Before thermal characterization, neat PF and BPFs were all precured at 125 ◦ C for 105 min, then the pre-cured adhesives were ground into powder using a mortar and a pestle. A TGA/FTIR

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Table 1 Basic properties of the neat PF and the BPFs.

pH Solid content (%) Viscositya (cP) a

Neat PF

Extractive synthetic BPF

Bio-crude synthetic BPF

Bio-crude formulated BPF

11.32 36.7 (0.12) 32.1 (2.45)

11.28 39.4 (0.22) 65.5 (4.13)

10.85 38.6 (0.08) 32.6 (3.5)

11.11 36.7 (0.07) 17.8 (1.95)

Tested at 50 ◦ C.

coupled system was employed to evaluate the thermal degradation properties of all the adhesives and to simultaneously analyze the gaseous products evolved from adhesives degradation. The TGA/FTIR coupled system consists of a thermogravimetry analyzer (Pyris 1 TGA, Perkin Elemer) and a frontier infrared spectrometer (Frontier FTIR, Perkin Elemer). TGA and FTIR were connected with a TL 8000 transfer line for on-line transferring the gaseous products from the TGA furnace to the FTIR with a N2 flow rate of 20 mL/min. The coupled system continuously monitored the weight change of the adhesive sample being tested, as well as the gaseous products evolution vs. temperature. FTIR spectra in the region of 4000–500 cm−1 with a resolution of 4 cm−1 were obtained continuously at a temperature interval of 1 ◦ C. Specifically in one typical run of TGA/FTIR coupled system, the transfer line as well as the FTIR cell were firstly heated to 300 ◦ C and the pump was switched on. The FTIR gas cell was then fed with liquid N2 for cooling down and concentrating the gaseous products (gaseous products herein are defined as chemical components with a boiling point lower than 300 ◦ C) for detection by FTIR equipped with a wide band mercury cadmium telluride (MCT) detector. Meanwhile, 10 mg pre-cured adhesive powder was loaded into a platinum pan and the pan was suspended on the balance in the furnace. Then the adhesive sample was heated from 50 ◦ C to 800 ◦ C at 10 ◦ C/min under N2 atmosphere. At the moment of heating from 50 ◦ C, the online FTIR scanning on the evolved gaseous products was automatically triggered. The FTIR continuously scanned the evolved gaseous products till the cooling process of TGA started at 800 ◦ C. After each TGA run, the solid residue left in the platinum pan was collected for a lithiun tantalate (LiTaO3 ) detector equipped FTIR scan to investigate the functional group change during thermal degradation. Furthermore, individual TGA run on each adhesive from 50 ◦ C to various temperatures (200 ◦ C, 300 ◦ C, 400 ◦ C, 500 ◦ C, 600 ◦ C, 700 ◦ C and 800 ◦ C) at 10 ◦ C/min was conducted and the solid residue was scanned with the FTIR as well. Adhesive thermal degradation kinetics were analyzed using a multiple heating rates method. Adhesive was heated at the rate of 5 ◦ C/min, 15 ◦ C/min, and 20 ◦ C/min to 800 ◦ C under N2 atmosphere. The degradation kinetic parameters were calculated from the following Kissinger equation: d ln(ˇ/Tp2 ) d(1/Tp )

=−

E R

affecting its resistance to thermal degradation [11,12]. TGA and the derivative thermal degradation (DTG) profiles of neat PF and BPFs are as shown in Fig. 1. It is clear that the residual weight of all adhesives constantly reduces during the temperature elevation from 50 ◦ C to 800 ◦ C. Even though the adhesives were pre-cured at 125 ◦ C for 105 min before TGA tests, there are still typical weight losses at the temperatures lower than 125 ◦ C, due to the evaporation of small molecular weight components and absorbed moisture, as well as the further condensation in the adhesives. At temperatures below 150 ◦ C, the neat PF loses weight faster than all the BPFs likely due to the fact that the introduction of big molecular weight bio-crude or extractives results in less curing by-products such as water and thus less weight loss from the further curing of the bio-crude synthetic BPF or extractives synthetic BPF. The second DTG peak of neat PF is indistinct, while for bark-based BPFs the DTG peaks are all distinct and the peak temperature is higher than that of neat PF. At 180 ◦ C, the weight loss rate of bio-crude synthetic BPF exceeds that of neat PF, while the degradation rates of extractive synthetic BPF and bio-crude formulated BPF exceed that of the neat PF at round 230 ◦ C. Neat PF degradation rate is almost constant till 350 ◦ C. The degradation rates of bio-crude synthetic BPF, extractive synthetic BPF and bio-crude formulated BPF further increase, and peak at 257 ◦ C, 262 ◦ C and 297 ◦ C, respectively, then the degradations slow down. The more weight loss of neat PF could be because on one hand, the neat PF contains reactive sites which can condensate with each other or with methylol; on the other hand, the bio-crude synthetic BPF, extractive synthetic BPF and bio-crude formulated BPF contain less reactive functional groups, leading to less reaction by-products at 250–300 ◦ C. Above 350 ◦ C, degradation of all the adhesives accelerate and peak again at 411 ◦ C, 403 ◦ C, 410 ◦ C and 408 ◦ C, respectively. Adhesive degradation at this temperature range is due to the break of methylene bridges into methyl groups, and the evolution of small oligomers and water from cross-linkages reduction, as well as volatiles such as phenol and methyl derivatives. The last DTG peak was due to methylene scission involving the removal of hydrogen atoms/molecules from the ring structure, evolution of hydrogen gas, degradation of phenol group. In this temperature range above 500 ◦ C, collapse, carbonization and graphitization of the cured adhesive network take place [2,3]. The peak degradation temperatures in this

(1)

where ˇ is the heating rate (◦ C/min), Tp (K) is the largest peak temperature obtained from the derivative thermal degradation (DTG) curves, E is the activation energy (kJ/mol), R is the gas constant. E is calculated from the slope of the linear plots of ln(ˇ/Tp 2 ) vs. −1000/Tp . 3. Results and discussions 3.1. Thermal stability and thermal kinetics of neat PF and BPFs 3.1.1. Thermal stability Thermal stability of a thermoset polymer is highly dependent on its structure and cross-linking density. For PF resin, nature and proportion of catalyst, synthesis procedure, additives incorporated into the pre-polymer and final cross-linking degree are key factors

Fig. 1. TG and DTG profiles of neat PF and BPFs degradation at the heating rate of 10 ◦ C/min.

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Fig. 2. DTG profiles of neat PF (A) and bio-crude synthetic BPF (B) degradation at various heating rates.

degradation stage for bio-crude synthetic BPF, extractive synthetic BPF and bio-crude formulated BPF are 508 ◦ C, 504 ◦ C, 522 ◦ C, respectively. For neat PF, it is 563 ◦ C, much higher than that of all the three BPFs. Judged from the DTG peak temperatures, the two biocrude based BPFs thermally degrade faster than the neat PF and extractive synthetic BPF. During the whole process, the maximum degradation rate and the peak temperature of neat PF, extractive synthetic BPF, bio-crude formulated BPF and bio-crude synthetic BPF is 1.29%/min at 562.8 ◦ C, 1.16%/min at 519.1 ◦ C, 2.09%/min at 410.6 ◦ C, and 2.02%/min at 406.9 ◦ C, respectively. At 800 ◦ C, the weight residue of neat PF and extractive synthetic BPF is 65.76% and 63.50%, respectively; while for the bio-crude synthetic and formulated BPFs, the residual weight is only 52.83% and 49.53%, from which it can be inferred that the introduction of bio-crude into BPF decreases the thermal stability of the resulted BPFs, regardless of the introduction stage for the bio-crude, while extractive introduction reduces thermal stability of the resulted BPF to a much smaller extent. 3.1.2. Activation energy for neat PF and BPF degradation To characterize the thermal degradation kinetics of the adhesives, a multiple heating rates method was employed. Thermal kinetics of PF resin degradation are dependent on the PF resin type, F/P ratio [11,16]. DTG profiles of neat PF and bio-crude synthetic BPF at various heating rates are as shown in Fig. 2. A higher heating rate leads to a faster thermal degradation rate and a higher degradation peak temperature. Similar observations were obtained on DTG profiles for the extractive synthetic BPF and bio-crude formulated BPF (figures not shown here). The activation energy for adhesive thermal degradation was obtained from the slope of linear plots of ln(ˇ/Tp 2 ) and −1000/Tp , as listed in Table 2. The activation energy at the first degradation peak (P1 ) for the degradation of neat PF and extractive synthetic BPF is very close, and are both smaller than that for the degradation of bio-crude synthetic BPF and bio-crude formulated BPF. At the second degradation peak (P2 ), the activation energy Table 2 Activation energy for neat PF and BPFs degradation. Adhesives

Neat PF Extractive synthetic BPF Bio-crude synthetic BPF Bio-crude formulated BPF

Activation energy (kJ/mol) P1

P2

P3

106.5 106.7 136.3 146.4

220.5 174.5 142.7 157.9

245.1 241.0 221.7 213.0

follows the order of neat PF (220.5 kJ/mol) > extractive synthetic BPF (174.5 kJ/mol) > bio-crude formulated BPF (157.9 kJ/mol) > biocrude synthetic BPF (142.7 kJ/mol). As the break of methylene bridge is the main reason for the mass loss at the P2 temperature range, and the amount of methylene bridge in neat PF is believed to be more than that in BPFs due to the more reactive sites of phenol to formaldehyde and smaller molecular of phenol than bio-crude/extractives. Thus, dissociating methylene links in the neat PF requires a higher activation energy than those for BPFs methylene links break. At the third degradation peak (P3 ), the activation energy for the degradation of various adhesives follows the similar sequence as above for P2 , i.e., neat PF (245.1 kJ/mol) > extractive synthetic BPF (241.0 kJ/mol) > biocrude synthetic BPF (221.7 kJ/mol) > bio-crude formulated BPF (213.0 kJ/mol). As descripted in previous part, bark alkali extractives were prepared through bark reflux in 3% NaOH for 180 min, and the produced extractives mainly contain flavonoid oligomers and polymers, waxes, suberin degradation products, a significant amount of lignin derivatives, with a Mw over 2000 [13,14,17], while bark bio-crude was obtained from bark liquefaction in water/ethanol mixture at 300 ◦ C under 12.0 MPa for 15 min, and bio-crude mainly consists of derivatives from lignin and cellulose, which are highly decomposed products with a Mw of 870 [18]. Thermal stability of extractives is much better than that of biocrude, leading to a better thermals stability of extractives synthetic BPF than bio-crude based BPF. On the other hand, the less stability of bio-crude based BPF could be due to the presence of molecules with numerous thermally unstable side chains in the bio-crude [15], and thus less thermal stability for bio-crude based BPF. 3.2. Structure changes of neat PF and BPFs during thermal degradation Adhesive structure change when being heated to various temperatures was monitored by FTIR equipped with a lithium tantalate (LiTaO3 ) detector. The intensities and absorbance bands of the four adhesive residues at various temperatures are as displayed in Fig. 3. For neat PF pre-cured at 125 ◦ C (Fig. 3(A)), its FTIR spectrum displays strong absorbance assigned to OH at 3600–3200 cm−1 . Two bands at 2935 cm−1 and 2850 cm−1 attributed to the in-plane and out of plane stretching of aliphatic CH. C C stretch in aromatic ring appears at 1612 cm−1 and 1424 cm−1 , respectively. The strong absorbance at 1464 cm−1 is assigned to the stretching of methylene bridge ( CH2 ) in neat PF. Peaks at 1270 cm−1 , 1200 cm−1 and 1150 cm−1 confirm the existence of C O C stretch (dialkyl). Absorbance at 1033–100 cm−1

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Fig. 3. FTIR spectra of neat PF and BPFs residues after being heated to various temperatures (A: neat PF, B: bio-crude synthetic BPF, C: extractive synthetic BPF, D: bio-crude formulated BPF).

indicates the existence of methylol ( CH2 OH) groups. Absorbance at 880 cm−1 , 820 cm−1 and 770 cm−1 is attributed to the unsubstituted CH bend in aromatics. Above 200 ◦ C, the absorbance due to OH stretch begins to be indistinct and disappears at 700 ◦ C. This is on one hand due to the additional crosslinking reaction between un-reacted phenolic hydroxyl and residual methylol groups [4], on the other hand, it could be due to the stripping off of phenolic hydroxyls at high temperatures with the formation of hydroxyl radical and the reaction between the hydroxyl radicals with methylene and hydroxylmethyl, releasing

carbon dioxide and carbon monoxide. During this process, methylene and phenolic hydroxyl are both active groups, where hydroxyl radicals provide an oxidative atmosphere for methylene oxidation. The oxidation reaction at 300 ◦ C is also confirmed by the appearance of carbonyl groups (C O, 1742 cm−1 ), which contributes to the formation of carboxylic acids, and eventually CO and CO2 in the evolved gaseous products [2]. At the temperature range of 125–600 ◦ C, absorbance at 1270 cm−1 , 1200 cm−1 and 1150 cm−1 always exist, indicating the existence of diphenyl ether type linkages. These linkages could be resulted from the reaction between

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Table 3 Functional groups identified by FTIR spectra in neat PF and BPFs residues during thermal degradation. Wavelength (cm−1 )

Functional groups

3600–3200 2935, 2850 1742 1612 1464 1424, 880 1270–1200, 1150 1033–1000 770

OH Aliphatic CH2 Carbonyl group C C in benzene ring Methylene group Benzene ring C O C (dialkyl) Methylol Benzene ring

Temperatures where absorbance appears (◦ C) Neat PF

Extractive synthetic BPF

Bio-crude synthetic BPF

Bio-crude formulated BPF

125–600 125–400 500–600 125–600 125–600 125–800 125–600 125 125–600

125–500 125–500 500–600 125–600 125–400 125–800 125–400 125–300 125–800

125–500 125–500 700 125–600 125–500 125–800 125–500 125–500 125–500

125–500 125–500 700 125–600 125–400 125–800 125–600 125–400 125–700

benzene nuclei with the evolved water [6]. It has been found that methylol can be still produced at 200–400 ◦ C while alkyl phenols are generated at 400–600 ◦ C [12]. Above 600 ◦ C, the collapse of the PF resin network to form polyaromatic domains [7] evolves aromatic hydrogen and leads to structureless IR spectra as evidenced in Fig. 3(A). Dramatic structure changes can be evidenced by the FTIR spectra in Fig. 3(A): the absorbance at 1464 cm−1 assigned to the bending of methylene ( CH2 ) becomes weaker and finally disappears at temperatures higher than 600 ◦ C due to the complete break of methylene bridge. Similar observations were obtained on all the BPFs. At such high temperatures, the absorbance at 770 cm−1 assigned to aromatic CH (ortho) bending disappears due to the removal of hydrogen atoms from aromatic ring structure. At 700 ◦ C or 800 ◦ C, only benzene ring is detected, suggesting that the collapse, carbonization and graphitization of adhesive network take place [2,3]. PF adhesive can be progressively transformed into amorphous carbon through polycyclic reactions at high temperatures [2]. Structure change of BPFs during thermal degradation is similar to that of neat PF, except for some differences in the temperatures at which some functional groups appear or disappear as shown in Table 2. Different from neat PF, bio-crude synthetic BPF (Fig. 3(B)), extractives synthetic BPF (Fig. 3(C)) and bio-crude formulated BPF (Fig. 3(D)) show absorbance of hydroxyl group and aliphatic CH2 at the highest temperature of 500 ◦ C. During the whole degradation process, an additional crosslink is firstly formed between methylene and phenolic hydroxyl, then some methylene are scissored, generating such volatiles as phenol and methyl derivatives. The triggering and ending temperature for this reaction is dependent on the nature of PF adhesives, such as number of residual methylol groups, dimethylene ether [5,19]. As bark extractives or bio-crude is not as reactive as phenol, BPFs contains a smaller number of methylols than neat PF. Thus, the incomplete condensation between OH and methylol would cause the disappearing of the residual OH through scission of phenolic rings at 400–500 ◦ C. Another degradation difference between BPFs and neat PF is the temperature at which methylene bridges disappear. It has been widely accepted that methylene bridge break is the main reaction for PF resin degradation. As shown in Table 3, methylene bridges in neat PF disappear at a higher temperature than that in the BPFs, confirming the better thermal resistance of neat PF. This is due to the fact that the cured neat PF contains high content of methylene links, resulting in higher energy for methylene dissociation. At 600 ◦ C, aromatic CH bending absorbance at 770 cm−1 disappears in IR spectrum of bio-crude synthetic BPF, while this happens to the extractive synthetic BPF and bio-crude formulated BPF at 700 ◦ C, the same temperature as neat PF. In addition, both biocrude synthetic BPF and bio-crude formulated BPF display carbonyl group absorbance at 1742 cm−1 at higher temperature (700 ◦ C) than neat PF or extractive synthetic BPF (500–600 ◦ C). Carbonyl groups (C O) was proposed to be formed from methylene autooxidation [2], from which it is inferred that bio-crude based BPFs are

more auto-oxidation resistant than neat PF or extractive synthetic BPF. 3.3. Analysis on the gaseous products evolved from BPFs degradation In addition to the structure analysis of adhesives residue after thermal degradation, the evolved gaseous products from adhesive thermal degradation were also analyzed in situ with the online FTIR equipped with a MCT detector. Fig. 4 displays the Gram–Schmidt (GS) profiles for evolved gaseous products from the adhesives thermal degradation at the heating rate of 10 ◦ C/min from 50 ◦ C to 800 ◦ C in N2 . The GS profiles show the total change of the IR signal during heating process. As seen from the profiles, the total absorbance intensity of neat PF evolved gaseous products is much weaker than that of BPFs, confirming the much smaller amount of detectable gaseous products evolved during degradation. The absorbance intensity of neat PF derived gaseous products peaks at 290 ◦ C, 400 ◦ C, 560 ◦ C, and 730 ◦ C, respectively. For gaseous products evolved from BPFs, their absorbance intensity varies with temperature and adhesive types. For example, the absorbance intensity of gaseous products evolved from the biocrude synthetic BPF peaks at 285 ◦ C, 383 ◦ C, 460 ◦ C and 710 ◦ C, while for the bio-crude formulated BPF derived gaseous products, the absorbance peaks at 280 ◦ C, 406 ◦ C, 510 ◦ C, and 739 ◦ C. Peaks for the IR absorbance at 155 ◦ C, 310 ◦ C, 400 ◦ C and 570 ◦ C were observed for the gaseous products from extractive synthetic BPF degradation, and the absorbance intensity keeps increasing above 630 ◦ C. To further investigate the composition of the evolved volatiles from the adhesives thermal degradation, IR spectra of the gaseous products at the peak temperatures and some other typical temperatures (150 ◦ C, 200 ◦ C, 300 ◦ C, 400 ◦ C, 500 ◦ C, 600 ◦ C, 700 ◦ C and 800 ◦ C) were extracted and collected in Fig. 5. For IR spectra of neat PF derived gaseous products (Fig. 5(A)), the bands at 3940–3550 cm−1 assigned for OH can be observed at 125–800 ◦ C

Fig. 4. Gram–Schmidt (GS) profiles of gaseous products from neat PF and BPFs degradation.

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Fig. 5. FTIR spectra of gaseous products from neat PF and BPFs degradation at the heating rate of 10 ◦ C/min (A: neat PF, B: bio-crude synthetic BPF, C: extractive synthetic BPF, D: bio-crude formulated BPF. Red highlighted profiles are the FTIR spectra of gaseous products evolved at the peak temperatures as determined in Fig. 4). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

and the absorbance intensities increase with temperature elevation. OH exists mainly in the form of water which can be formed from further condensation between ortho CH and residual methylols (reaction 1 Fig. 6) at low temperatures, or phenolic OH–OH condensation (reaction 2 Fig. 6) starting at about 300 ◦ C and yields diphenyl ether linkages. At higher temperatures (300–500 ◦ C), water can also be produced from the reaction between phenolic OH and methylene bridge (reaction 3 Fig. 6). At 560 ◦ C and 600 ◦ C,

absorbance at 3177–3050 cm−1 , together with the absorbance at 1590 cm−1 , 1492 cm−1 and 1279 cm−1 confirms the existence of cresol. It should be noted that only o-cresol or p-cresol can be formed during PF degradation, because during the neat PF synthesis, formaldehyde can only react with the CH at ortho or para positions of phenol, forming ortho or para methylol, then methylols may react with phenolic hydroxyl during TGA tests. C H on phenolic structure reacts with methylol through ortho/ortho,

S. Feng et al. / Journal of Analytical and Applied Pyrolysis 115 (2015) 184–193

OH

CH2OH

OH

OH H2 OH C

+

+

OH + HO

H2O

O

HO

(1)

H2 O

+

CH

HO

(3)

H2O

+

HO OH

H2 C

(2)

HO

OH + H2C

OH

191

OH

CH3

OH H2 C

(4) OH

H2 C

H C

CH2

H C

+

H2 C

H2

H2 C

CH3

H3CO

OH

+

CH2OH

OH

CH4

H2

(5)

(6)

CHO

(7)

COOH

+

H2

+

CH2

OCH3

+

+ CO2

H2

+ HO

OH

2CH4

(8)

OH

Fig. 6. Proposed reactions for neat PF and BPFs thermal degradation.

para/para, ortho/para, forming methylene linkage during the curing process, and at a higher temperature, these linkages between aromatic structure would break to form phenolic methyl. The formation pathway of cresol is as displayed in reaction 4 in Fig. 6. Interestingly, absorbance at 2966–2880 cm−1 also evidences phenol formation from the neat PF at 560 ◦ C and 600 ◦ C, which could occur via reaction 4 in Fig. 6 [20]. Meanwhile, at 500 ◦ C, an weak band assigned to the CH stretch in CH4 appears at 3016 cm−1 , and the intensity increases at 560 ◦ C, then weakens at 600 ◦ C and

700 ◦ C, and even disappears at 800 ◦ C, indicating CH4 is mainly produced at 500–700 ◦ C and CH4 evolution maximizes at 560 ◦ C. The evolution of CH4 can be illustrated as reactions 5 and 6 in Fig. 6. The existence of CO2 gas in the volatiles is evidenced by the absorbance at 2400–2385 cm−1 and 792–630 cm−1 . The bands at 2240–2050 cm−1 assigned to the existence of CO appear at 500 ◦ C, then its intensity peaks at 560 ◦ C and increases further with elevated temperature till 800 ◦ C. As commonly observed, carbon dioxide and carbon monoxide are formed through the

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oxidation of methylene bridge by hydroxyl radicals at a high temperature (as reaction 7 in Fig. 6). Above 500 ◦ C, some phenolic hydroxyls are stripped off, resulting in the formation of hydroxyl radical and the formed hydroxyl radicals reacts with methylene and hydroxylmethyl, releasing carbon dioxide and carbon monoxide [2,4]. Moreover, the absorbance of C O stretch at 1728 cm−1 and 1086 cm−1 for C O stretch confirms the existence of carboxylic acid, which can be formed through reaction 7 in Fig. 6. Actually, PF adhesive itself can yield either OH radicals or water, acting as an oxidizing agent for the formation of carboxylic acids during the thermal degradation [4]. IR spectra of gaseous products from the thermal degradation of BPF are also as shown in Fig. 5(B–D). The chemical components of volatiles evolved from BPFs degradation are qualitatively similar to those from the neat PF degradation except for the xylenol formation during BPFs degradation, as confirmed by the absorbance at 1200–1120 cm−1 . Xylenol can be derived from the phenol–formaldehyde structures, or from the lignin derivatives. As well known, formaldehyde can be added onto three reactive sites (two ortho sites, one para site) of one phenol structure during the PF synthesis. It should be noted that the bio-crude synthetic BPF or extractives synthetic BPF was synthesized by replacing 50% phenol with bio-phenols (extractives or bio-crude). However, molecular weight of extractives or bio-crude is much higher than phenol, and they both have less reactive sites than phenol, resulting in more extra formaldehyde available to react with phenol and the bio-phenols [14,21]. These contribute to more xylenol structure during the synthetic BPF synthesis. In addition, some lignin derivatives contain non-reactive xylenol structure, which explains the appearance of xylenol at 300 ◦ C (Fig. 5(C)). Some other lignin derivatives have cresol structure, which reacts with formaldehyde during the BPF synthesis to form xylenol structure. Also as evidenced by Fig. 5(B–D), some gases were formed from BPF degradation at lower temperatures than those of neat PF. Phenol was identified during the thermal degradation of bio-crude synthetic BPF, extractive synthetic BPF and bio-crude formulated BPF at 460–600 ◦ C, 500–600 ◦ C and 406–700 ◦ C, respectively, lower than that for phenol release from the neat PF degradation. CH4 was yielded when heating the neat PF at 500–800 ◦ C, peaking at 560 ◦ C. While heating the BPF, CH4 formed at 460–600 ◦ C, 500–600 ◦ C and 400–600 ◦ C, with a peak at 500 ◦ C, 570 ◦ C and 510 ◦ C, respectively, in good agreement with the literature work [20]. It can thus be concluded that CH4 can be formed more easily during the thermal degradation of BPF than neat PF. The easier release of CH4 can be due to stripping-off of OCH3 from lignin derivatives in BPF and the conversion of OCH3 into CH4 (reaction 8 Fig. 6). CO is formed from the thermal degradation of bio-crude synthetic BPF, extractives synthetic BPF and bio-crude formulated BPF at 460–800 ◦ C, 800 ◦ C and 500–800 ◦ C, respectively. Attention needs to be paid to the phenomenon that CO is formed from the extractive synthetic BPF degradation at 800 ◦ C, much higher than neat PF and other BPF. Interestingly, for the gaseous products evolved from the extractives synthetic BPF degradation at 500 ◦ C, the absorbance signal at 2400–2300 cm−1 becomes very strong, implying intense auto-oxidation of the BPF at this temperature, which is consistent with the GS profile in Fig. 4.

4. Conclusions Thermal stability, thermal degradation kinetics, structure change and the evolved gaseous products during neat PF and BPFs degradation were extensively analyzed through TGA/FTIR. Key conclusions were summarized below:

(1) Upon being heated to a high temperature in inert atmosphere, neat PF and BPFs all went through typical three stage degradation. (2) Introduction of bio-crude into BPF reduced the thermal stability for the resulted BPFs to a greater extent than extractive introduction. Adhesives thermal stability with respect to the residual weight 800 ◦ C followed the order: neat PF > extractives synthetic BPF  bio-crude formulated BPF > bio-crude synthetic BPF, which is consistent with the activation energy for the adhesives degradation in the main degradation stage. (3) Auto-oxidation occurred during all the adhesive thermal degradation. But bio-crude based BPFs are more auto-oxidation resistant than neat PF or extractive synthetic BPF. Aqueous products evolved from neat PF degradation mainly consist of water, phenol, cresol, CO2 , CH4 , CO and carboxylic acids, among which water and CO2 are the main components. Xylenol not yielded neat PF degradation was produced during BPF degradation. Other gaseous products such as CH4 , phenol, cresol were formed from BPFs degradation at lower temperatures compared with those from neat PF degradation. 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 [1] K. Ouchi, H. Hond, Pyrolysis of coal I – Thermal cracking of phenol formaldehyde resins taken as coal models, Fuel 38 (1959) 429–443. [2] H.Y. Jiang, J.G. Wang, S.Q. Wu, Z.Q. Yuan, Z.L. Hu, R.M. Wu, Q.L. Liu, The pyrolysis mechanism of phenol formaldehyde resin, Polym. Degrad. Stab. 97 (2012) 1527–1533. [3] Y.F. Chen, Z.Q. Chen, S.Y. Xiao, H.B. Liu, A novel thermal degradation mechanism of phenol–formaldehyde type resins, Thermochim. Acta 476 (2008) 39–43. [4] L.L. Costa, R.D. Montelera, G. Camino, E.D. Weil, E.M. Pearce, Structure–charring relationship in phenol formaldehyde type resins, Polym. Degrad. Stab. 56 (1997) 23–35. [5] E. Fitzer, W. Shaefer, The effect of crosslinking on the formation of glasslike carbons from thermosetting resins, Carbon 8 (3) (1970) 353–364. [6] K. Ouchi, Infra-red study of structural changes during the pyrolysis of a phenol–formaldehyde resin, Carbon 4 (1966) 59–66. [7] C. Morterra, M.J.D. Low, I.R. studies of carbons – VII. The pyrolysis of a phenol–formaldehyde resin, Carbon 23 (5) (1985) 525–530. [8] Y. Zhao, N. Yan, W.M. Feng, Thermal degradation characteristics of phenol–formaldehyde resins derived from beetle infested pine barks, Thermochim. Acta 555 (2013) 46–52. [9] C. Amen-Chen, B. Riedl, C. Roy, Softwood bark pyrolysis oil – PF resols. Part 2. Thermal analysis by DSC and TG, Holzforschung 56 (2002) 273–280. [10] J. Li, S.J. Li, Pyrolysis of medium density fiberboard impregnated with phenol–formaldehyde resin, J. Wood Sci. 52 (2006) 331–336. [11] M.R. Rao, S. Alwan, K.J. Scariah, K.S. Sastri, Thermochemical characterization of phenolic resins: thermogravimetric and pyrolysis–GC studies, J. Thermal. Anal. 49 (1) (1997) 261–268. [12] J.E. Shafizadeh, S. Guionnet, M.S. Tillman, J.C. Seferis, Synthesis and characterization of phenolic resole resins for composite applications, J. Appl. Polym. Sci. 73 (1999) 505–514. [13] N. Gierlinger, D. Jacques, M. Grabner, R. Wimmer, M. Schwanninger, P. Rozenberg, E.L. Paques, Colour of larch hearwood and relationships to extractives and brown-rot decay resistance, Trees 18 (2004) 102–108. [14] Y. Zhao, N. Yan, W.M. Feng, Biobased phenol formaldehyde resins derived from beetle-infested pine barks – structure and composition, ACS Sustain. Chem. Eng. 1 (2013) 91–101. [15] S.N. Cheng, I. D’Cruz, Z.S. Yuan, M.C. Wang, M. Anderson, M. Leitch, C.C. Xu, Use of biocrude derived from woody biomass to substitute phenol at a highsubstitution level for production of biobased phenolic resole resins, J. Appl. Polym. Sci. 21 (2011) 2743–2751.

S. Feng et al. / Journal of Analytical and Applied Pyrolysis 115 (2015) 184–193 [16] Y.K. Lee, D.J. Kim, H.J. Kim, T.S. Hwang, M. Rafailovich, J. Sokolov, Activation energy and curing behavior of resole- and novolac-type phenolic resins by differential scanning calorimetry and thermogravimetric analysis, J. Appl. Polym. Sci. 89 (2003) 2589–2596. [17] Y. Zhao, N. Yan, M.W. Feng, Bark extractives-based phenol–formaldehyde resins from bettle-infested lodgepole pine, J. Adhes. Sci. Technol. 27 (18–19) (2013) 2112–2126. [18] S.H. Feng, Z.S. Yuan, M. Leitch, C.C. Xu, Hydrothermal liquefaction of barks into bio-crude – effects of species and ash content/composition, Fuel 116 (2014) 214–220.

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[19] G.M. Jenkins, K. Kawamura, L.L. Ban, Formation and structure of polymeric carbons, Proc. R. Soc. Lond. (A) 327 (1972) 501–507. [20] A.K. Trick, E.T. Saliba, Mechanism of the pyrolysis of phenolic resin in a carbon/phenolic composite, Carbon 33 (11) (1995) 1509–1515. [21] S.N. Cheng, I. D’cruz, M.C. Wang, M. Leitch, C.C. Xu, Highly efficient liquefaction of woody biomass in hot-compressed alcohol–water mixture, Energy Fuels 24 (2010) 4659–4667.