Characterization of Modified Phenol Formaldehyde Resole Resins

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Characterization of Modified Phenol Formaldehyde Resole Resins Synthesized in-situ with Various Boron Compounds Yubo Chai, Junliang Liu, Yong Zhao, and Ning Yan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02156 • Publication Date (Web): 01 Sep 2016 Downloaded from http://pubs.acs.org on September 5, 2016

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Industrial & Engineering Chemistry Research

Characterization of Modified Phenol Formaldehyde Resole Resins Synthesized in-situ with Various Boron Compounds Yubo Chai a, b Junliang Liu a, Yong Zhao b, Ning Yan b * a. Research Institute of Wood Industry, Chinese Academy of Forestry, Beijing, China 100091 b. Faculty of Forestry, University of Toronto, 33 Willcocks Street, Toronto, ON. Canada M5S 3B3 * Corresponding author: Tel:+1-416-946-8070, +1-416-978-3834, Email address: [email protected]

Abstract In this study, three different boron compounds were used together with alkaline catalyst to synthesize in-situ Phenol Formaldehyde (PF) resole resins. The resin curing behavior, molecular structure, bonding performance and properties of resin-impregnated wood were investigated. Results showed that boron compounds modified PF resins had a lower degree of polymerization than the control PF resin made in lab. The curing kinetics, molecular structure and functional groups of the modified resins varied depending on the type of boron compounds used. Thermal stability of cured modified PF resins was slightly lower than that of lab made control PF resin. Boron compounds modified PF resins exhibited comparable dry and wet bonding strength to the lab made control PF resin. Wood impregnated using modified PF resins had comparable dimensional stability, mechanical properties and improved fire resistance than the wood impregnated using lab made control PF resin regardless the type of boron compounds used. Key words: Phenol formaldehyde resin, boron compounds, liquid-state

13

C NMR,

physical and mechanical properties, wood and wood composites.

1. Introduction

1 ACS Paragon Plus Environment


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Phenol formaldehyde (PF) resole resins are widely used as wood adhesives for producing various wood-based products for exterior applications, such as oriented strand board (OSB), plywood, and laminated veneer lumber (LVL) etc. PF resole resins are obtained via phenol and formaldehyde undergoing addition and condensation polymerization reactions under alkaline conditions. PF resole resins demonstrated excellent bonding performance, water resistance and durability. 1,2 Compared with amino plastic resins such as urea-formaldehyde (UF) resin, PF resin showed a higher bonding efficiency but slower cure speed. Slow resin cure rate would lead to lower production efficiency and higher production costs. This could be a major factor limiting the application of PF resins as impregnation resins and in the production of particleboard and medium density board. 3 Many studies have reported on the attempts for acceleration of PF resin's curing rate for wood adhesive application. Bivalent metal oxides,

4, 5

carbonates,

6-9

and esters,

3, 10-14

were found to be effective in acceleration of PF resin curing. The acceleration mechanism was associated with the bivalent metallic ions forming complexes with phenols and formaldehyde. The acceleration effect was determined by the metal exchange rate in solution and the stability of the complexes.

4, 5

The cure acceleration mechanism of

carbonates and esters were found to be complex. So far, there was no conclusive mechanism presented in the literature. Organic or inorganic carbonates and esters accelerated the PF resin curing process differently. Organic carbonates both increased the curing speed and improved the strength of the cured resin, while inorganic carbonates accelerated the curing but contributed little to the hardened network strength. The reactivity of phenolic rings was reported to increase by adding carbonates or esters. Meanwhile, carbonates and esters also affected the resin structures, such as the distribution of ortho-para to para-para linkages, and resin curing kinetics. In addition, the mechanism involving the formation of an ortho carbonyl or carboxyl group by the phenate ion on the phenolic ring was reported. These groups rearranged and shifted to sites other than the ortho position. Anhydride-like bridges formed temporarily and contributed to oligomeric structure with or without methylene bridges. As a result, PF resin cure process was accelerated. 3, 6-14 2 ACS Paragon Plus Environment

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With the increasing stringent demand for fire safety, wood and wood composites are required to be treated with fire retardants, such as boron compounds15 to achieve the necessary flame resistant properties. In order to make wood composite panels with desirable termite and decay resistant properties, boron-based chemicals, such as boric acid, borax, zinc borate, ammonium pentaborate and mixture of boron compounds, are routinely added.

15-18

The boron-based chemicals are either directly applied to wood

composites or blended with wood strands or adhesives prior to hot press. Boron based chemicals affected the resins curing kinetics, and the adverse effect of borates on mechanical and physical properties of the wood composite panels bonded using PF resins was frequently reported including a lower internal bond (IB), an increased thickness swell (TS), and a decreased wood failure rate.

19-24

It was due to the resin started to gel

caused by the boron ion reacting with the methylol groups before an effective bond can be developed.. Meanwhile, the low solubility and poor dispersion of boron compounds in PF resol resin typically associated with post-blending methods tends to downgrade the mechanical properties of the resulting wood composites. Adding hydroxyl rich organic flow agents, like PEG to the PF resins, could potentially minimize the adverse effect caused by borates,

23, 24

however, it adds extra cost to the manufacturing of the PF resin

and wood composite panels. Instead of using boron compounds as additives or physically post-blending with PF adhesives, boron compounds could be chemically incorporated into phenolic resin structures and improve their physical, thermal and mechanical properties.

25-28

The

resulting arofene phenolic resins with superior heat, chemical and water resistance could be found in many applications including brake pad, metal plate, grinding wheel, laminations and so on. The reaction mechanism to produce PF resin is step-growth polymerization, therefore, subtle variations in reaction conditions could have profound effect on the resulting resins’ structure and performance.2 The recent structural investigation of boron modified phenol formaldehyde resins showed that the PF resin was cross-linked via tetragonally coordinated boron species (BIV). The boron-based crosslinks included monoesters and diesters complex represented by six-membered spirocyclic borate anions. During the thermal curing process, the monoester and diester borate complexes underwent additional transformation in which the spirocyclic borate anions 3 ACS Paragon Plus Environment


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were more tightly incorporated into the polymer matrix via additional N-type cross-links. A 11B−11B double-quantum correlation MAS NMR experiment showed that the majority of the monoester and diester borate complexes (ca. 80%) were uniformly distributed within and effectively isolated by the polymer matrix, with an average

11

B···11B

interatomic distance greater than 6 Å. The final properties of the boron modified PF resins were enhanced with increased cross-link density, which will be used as novel binders for soft magnetic composites. 28 However, there is few literature reported PF resol resin in-situ synthesis with boron compounds for wood adhesive applications. It is also unknown how the boron compounds affect the resin’s molecular structure and curing behavior. There are no previous studies investigated the properties of wood impregnated using boron compounds modified PF resins. Therefore, in this study, three different boron compounds, boric acid, borax and zinc borate were added with alkaline catalyst to synthesize in-situ PF resole resin for wood adhesive application, respectively. The resulting modified PF resins were used to impregnate low-density and low quality plantation poplar wood. The resin molecular structure, curing behavior, resin bonding performance as well as the properties of the impregnated wood were investigated. 2. Experimental section 2.1 Resin synthesis The solution mixture of phenol, 37% formaldehyde, and 40% sodium hydroxide was stirred at 40ºC for 15 min in a three-neck flask with a reflux condenser first. The mixture was then heated up to 65ºC in 30 min and was kept at this temperature for another 30 min. The mixture was then heated to 85ºC and stayed at this temperature for an hour after the remaining amount of sodium hydroxide was added. The resulting resin mixture was cooled to room temperature and used for subsequent analysis as the lab made control PF resin. PF resins with different boron compounds were synthesized following this procedure. Boric acid, borax and zinc borate were added with the 40% sodium hydroxide solution together upfront in resin synthesis respectively. The total addition amount of boron 4 ACS Paragon Plus Environment

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compounds was 2 wt.% of the final resins. The resulting resins were denoted as PF-BA, PF-BX and PF-ZB for boric acid modified PF, borax modified PF and zinc borate modified PF resins, respectively. All chemicals were supplied by Caledon laboratory Chemicals, Canada, and were used as received. 2.2 Resin characterization 2.2.1 pH, solids content, gel time and molecular weight Both pH and viscosity of the resins were tested at 25°C. A Brookfield rotary viscometer was used for the viscosity measurements. ASTM D 3529 was followed for determining the solids content of the resin. Gel time was measured similarly to what were reported in our previous studies. 35, 36 5 g of resin in a test tube (16 mm diameter) was heated in an oil bath at 100±1°C. Gel time was recorded as the time duration from the test tube entering the oil bath to the resin starting to gel by forming a string when lifting a glass rod from the resin. The gel time is reported as the average value from three replicate measurements. Matrix-Assisted Laser Desorption/Ionization-Time-of-Flight/Time-of-Flight spectrometer (MALDI-TOF/TOF, Applied Biosystems, Framingham, MA, USA) was used to evaluate the weighted average molecular weight (Mw), number average molecular weight (Mn) and polydispersity index (Mw/Mn) of the PF resins. Low linear mode with a mass range from 60 to 2000 Da and a laser intensity of 6000 arbitrary units were used for obtaining resin mass spectra. 0.1% trifluoroacetic acid and DHB(2, 5-dihydroxy benzoic acid) in 50% aqueous ethanol were mixed to form a 10mg/ml concentration of matrix solution. 1 µL of the premixed resin and matrix mixture at a ratio of 1:4 (v/v) was air-dried on the MALDI target and each spectrum was based on the total number of ions collected from 500 laser pulses. Mané et al.’s methods were used to calculate the average molecular weight and polydispersity index. 29 2.2.2 Resin curing behavior and curing kinetics Resin curing behavior was investigated using high-pressure pans (DSC-Q1000, TA Instruments, USA). Dynamic scans were conducted with multiple heating rates of 5, 10, 15 and 20°C/min, starting from room temperature to 250°C.



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Curing activation energy was calculated using the Kissinger method 31:  φ ln 2 T  p

  = − E 1 + ln RA   R Tp  E  

(1)

Where, φ = heating rate (K/s), Tp = peak temperature (in Kelvin) at the given heating rate, A = pre-exponential factor, R = ideal gas constant, and E is the activation energy. A randomly selected sample with three replicates was used for the testing. There was less than 1°C of variation amongst the replicated for the onset and peak temperature measurements. 2.2.3 Thermal gravimetric analysis (TGA) of the cured resins All the resins were first oven-cured at 80°C for 48h prior to be grounded into fine powders smaller than 100-mesh screen and tested in a thermogravimetric analyzer (TGA, Q500, TA, USA). A platinum pan holding 10 mg of each cured sample was heated from room temperature to 800 °C at 10°C/min under N2 atmosphere. 2.2.4 Evaluation of bonding Strength Wood strips with the dimension of 3 mm in thickness, 25.4 mm in width, and 108 mm in length were cut from Birch Veneer. The wood grain was parallel to the wood strips length direction. The two-layered birch wood veneer samples were glued together using the four different types of resins, i.e., lab made control PF resin, boric acid modified PF (PF-BA), borax modified PF (PF-BX) and zinc borate modified PF (PF-ZB) resins, respectively. The resins were applied on one side of the birch strip and covered an area of 25.4mm x 25.4mm (Figure 1) with the controlled spread rate of 0.025-0.035 g/cm2 on the solids basis. Another birch strip without resins was overlapped to the area of the birch strip coated with resins to form the two-layered lap shear sample. The lap shear sample was then subjected to hot press at 160C with thickness control of 4.5mm for 300 sec. After hot press, the samples were cooled and conditioned and tested for shear strength on a Zwick universal test machine (Zwick/Z100, Zwick Roell Group, Germany). The standard lap shear test methods in ASTM D5868 were followed. The crosshead speed was set at


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1.3mm/min. The average value of the shear strength is reported based on a minimum of 10 replicates.

Figure1. Lap shear specimen dimensions

2.2.5 Water resistance tests In order to evaluate the durability of adhesives, water-soaking-and-drying (WSAD) test and boiling water test (BWT) were conducted. The test procedures were based on the voluntary standard PSl-95 published by the US Department of Commerce through the Engineered Wood Association, Tacoma, WA. For the WSAD test, the lap shear samples were first soaked in water at room temperature for 24 h, and then air-dried with good circulation (fume hood) at room temperature for another 24 h, after that the samples were tested for shear strength. For the BWT test, the samples were first boiled in water for 4 h, and then oven-dried at 63 ± 2°C for 20 h. After that, the samples were boiled again in water for another 4 h, cooled using tap water, and then immediately tested for shear strength when they are still wet. The resulting shear strength was defined as BWT/W strength. 2.2.6 Resin molecular structure The sample preparation was conducted according to previous studies were first dissolved in D2O. The liquid-state

31

. The PF resins

13

C NMR spectra of these samples were

acquired using a Unity 500 spectrometer. The experimental conditions are: the pulse angle was 60 degrees (8.3 µs), the relaxation delay was 10s and with gated Waltz-16 1H decoupling during the acquisition period. Each spectrum was accumulated by about 40000 scans. Sodium 2,2-dimethylsilapentane-5-sulphonate (DSS) was used as the internal standard for the measurement of the 13C chemical shifts. Usually quantitative 13C 7 ACS Paragon Plus Environment


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NMR requires long relaxation delays to achieve sufficient signal-to-noise ratios and results in extremely long acquisition times. Gated decoupling 13C NMR using only short delay between pulses could be used for ratios determination of small molecules and for end group analysis of polymer. If certain precautions are taken, long delays will not be required to acquire meaningful ratios. However, differences in the nuclear overhauser effects (NOE) and relaxation time should be considered. For both polymer and small molecules, signal-to-noise ratio might affect the accuracy if a species or end group only has an exceptionally low concentration. 2.2.7 PF resin impregnated wood and their properties 2.2.7.1 Wood impregnation Samples with the dimension of 400mm×40mm×40mm (longitudinal direction×radial direction × tangential direction) was cut from low density plantation poplar wood (Populus ussuriensis). The lab made control PF resin, boric acid modified PF (PF-BA), borax modified PF (PF-BX) and zinc borate modified PF (PF-ZB) resins were diluted to 30% solids content for wood impregnation, respectively. Wood samples were treated using vacuum-pressure method. Each impregnation condition contained 15 wood samples as replicates. The treatment process was as follows: 0.095Mpa of vacuum was first applied on the wood samples for 40min, then the wood samples were impregnated with PF resin solutions under the vacuum condition. After releasing the vacuum, 0.8Mpa of pressure was applied and kept for 2hours. Once the treatment was completed, the impregnated wood samples were oven-dried at 60°C for 72 hours, then 80°C for 72 hours and 100°C for 48 hours. The weight percent gains (WPG) was calculated using equation (2) WPG =

G2 − G1 × 100% G1

(2)

Where, G1 is the weight of oven-dried sample before treatment, G2 is the weight of oven-dried sample after treatment. 2.2.7.2 Properties of impregnated wood 8 ACS Paragon Plus Environment

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Based on the requirements of different national test standards, the treated and untreated wood samples were cut into different dimensions. According to GB/T1934.2-2009, the dimensional stability of the samples was evaluated by measuring the moisture absorption (MA) and anti-swelling efficiency (ASE). The moisture absorption (MA) and antiswelling efficiency (ASE) were calculated using equation (3) and (4), respectively.

MA =

ASE =

m1 − m0 ×100% (3) m0

SC − ST × 100% SC

(4)

Where, MA is the moisture absorption rate, m1 is the weight of sample after moisture conditioning; m0 is the weight of oven-dried sample. ASE is anti-swelling efficiency of samples, Sc is the volume-swelling rate of untreated samples, ST is the volume-swelling rate of treated samples; In addition, the air density, modulus of elastic (MOE), modulus of rupture (MOR) and oxygen index of untreated and treated wood samples were tested according to GB/T 1933-2009, GB/T 1936.1-2009, GB/T 1936.2-2009 and GB/T 2406-2009, respectively. 3. Results and Discussion 3.1 Resin properties The basic properties of different resins are shown in Table 1. The resins solids content ranged from 52.26% to 52.82%. The pH value of resins ranged from 10.04 to 10.42. The viscosity of resins ranged from 401.2 cps to 586 cps. The lab made control PF resin had a slightly higher average molecular weight and polydispersity index than the boron compounds modified PF resins. Although all the resins were cooked following the same procedure, the average molecular weight of the boron compounds modified PF resins varied depending on the type of boron compounds. The gel time of PF resins with boron compounds was generally longer than the lab made control PF resin. The Boric acid modified PF (PF-BA) had the longest gel time, followed by the borax modified PF (PFBX), and then zinc borate modified PF (PF-ZB). 9 ACS Paragon Plus Environment


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Table 1. Properties of the resins Resin Type PF control PF-BA PF-BX PF-ZB

Solids content (%) 52.75 52.64 52.26 52.82

pH

Viscosity (cps)

Mw (Da)

Mn (Da)

PI

10.14 10.04 10.42 10.11

468.5 401.2 586.2 535.6

1347 1048 1178 1222

727 654 668 708

1.85 1.60 1.76 1.73

Gel time at 100°C 15 min 16 sec 20 min 53 sec 19 min 28 sec 16 min 25 sec

3.2 Resin curing behavior and curing kinetics All the resins demonstrated single exothermic peak at different heating rates in their DSC curing curves, indicating condensation reactions mainly dominated the resin curing process (Figure 2). The addition reactions associated with the formation of methylol phenol and oligomers may have been completed in large proportion during the resin synthesis stage.32-36

Figure 2. DSC curing curve of different resins (10°C/min)


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It can be found that the curing curves of the resin shifted to higher temperature when the heating rate increased (Figure 3). The onset temperature and peak temperatures of all the resins increased with the increasing heating rate, which was consistent with previous studies on PF resins.32-36 Since the heating rate did not affect the actual resin cure temperatures, for further comparison, the onset and peak temperatures obtained from dynamic DSC runs at different heating rates were extrapolated to the heating rate of zero. 32-36

The cure temperatures of different resins with variable heating rates are listed in

table 2.

Figure 3. DSC curing curve of lab made control PF resin at different heating rates

Boron compounds modified PF resins exhibited different cure temperatures from the lab made control PF resin. Boric acid modified PF (PF-BA), borax modified PF (PF-BX), and zinc borate modified PF (PF-ZB) resin had a lower onset temperature than the lab made control PF resin, suggesting the boron compounds modified PF resins will start to cure at a lower temperature than the lab made control PF resin without boron compounds.



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There was no significant difference in the onset temperature of PF-BA, PF-BX and PFZB. The peak temperature of boric acid modified PF, borax modified PF, and zinc borate modified PF resin was close to the lab made control PF resin.

Table 2. Curing characteristics of different resins Lab made control PF resin

Boric acid modified PF resin

Borax modified PF

Zinc borate

resin

modified PF resin

Heating rate (°C/min)

Onset temp. (°C)

Peak temp. (°C)

Onset temp. (°C)

Peak temp. (°C)

Onset temp. (°C)

Peak temp. (°C)

Onset temp. (°C)

Peak temp. (°C)

0

110.4

129.7

98.1

131.3

101.7

127.6

97.0

129.2

5

115.7

136.7

105.5

138.0

108.4

137.7

103.3

133.8

10

122.5

145.7

116.7

149.4

116.4

147.7

116.0

145.2

15

129.3

151.2

124.1

156.3

124.5

158.4

118.2

151.7

20

133.1

159.7

131.5

162.7

129.8

168.0

128.6

154.5

Curing activation energy

83.39

77.22

60.83

86.98

1.39×107

1.84×106

1.31×104

4.61×107

300.8

260.3

249.9

289.2

(KJ/mol) Pre-exponential factor

(/s) Enthalpy ∆H (J/g)

The curing activation energy, pre-exponential factor and enthalpy of different resins are listed in Table 2. Boric acid modified PF resin and borax modified PF resin had a lower curing activation energy than the lab made control PF resin. It suggested that boric acid modified PF and borax modified PF resin were thermodynamically favourable to cure than the lab made control PF resin. However, the pre-exponential factor of boric acid modified PF resin and borax modified PF resin was lower than the lab made control PF


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resin, indicating that the boric acid and borax retarded the resin curing reaction kinetic rate. Similar results were observed for PF resins post-blended with boron compounds. 1519

On the opposite side, zinc borate modified PF resin demonstrated a slightly higher

curing activation energy than the lab made control PF resin. However, the higher preexponential factor of zinc borate modified PF resin indicated that zinc borate slightly accelerated the resin curing reaction kinetic rate when there were sufficient thermal energy available. It is consistent with previous studies that the PF resin cure speed was accelerated by bivalent metallic ions.

4,5

In addition, boric acid modified PF, borax

modified PF resin and zinc borate modified PF resin demonstrated a lower curing reaction enthalpy than the lab made control PF resin. Similar trend was observed for PF resins post-blended with ammonium pentaborate and wood powder as additives.

16,17

Since the enthalpy of the resin curing reaction could be used as an indicator for the completion of reaction, our results suggesting that the curing degree of the boron compounds modified PF resins was lower than the lab made control PF resin, and the cross-linked structure of boron compounds modified PF resins would differ from the lab made control resin. Meanwhile, the endothermic effect from the boron compounds could inhibit the curing process of the resulting PF resins and lead to a unchanged or slightly higher curing peak temperature and lower curing reaction enthalpy compared to the lab made control PF resin. The detailed cross-linked structure of boron compounds modified PF resins are currently under investigation. 3.3 Liquid-state 13C NMR spectra of different PF resins The liquid-state 13C NMR spectra of the lab made control PF resins and boron modified PF resins are shown in Figure 4-7. The chemical shifts for the resins are assigned according to published literatures. 31, 37-40



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Figure 4. Liquid-state 13C NMR spectrum of the lab made control PF resin

Figure 5. Liquid-state 13C NMR spectrum of the boric acid modified PF resin


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Figure 6. Liquid-state 13C NMR spectrum of the borax modified PF resin

Figure 7. Liquid-state 13C NMR spectrum of the zinc borate modified PF resin



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The chemical shifts at 166~168 ppm representing carbonyl groups demonstrated similar intensity in the lab made control PF resin and boron compounds modified PF resins. The formation of carbonyl groups was attributed to the oxidation of the phenolic rings to quinone structures during and after the resin synthesis. Phenoxy carbons had the chemical shifts at 153.5-163.9 ppm. The changes of polarity of the phenoxy groups would cause the changes of resin's pH, as a result, the chemical shifts of phenoxy carbons could vary. The chemical shifts between 158.0 and 159.5 ppm were from the para-alkylated phenolic groups; their intensities were higher than those of chemical shifts of ortho-alkylated phenolic groups between 153.5 and 157.2 ppm. Due to the changes of polarity of the phenoxy groups, the chemical shifts of phenoxy carbons including both para and ortho carbons varied with the changes of the resin’s pH. The peaks at 114-119 ppm related to unsubstituted ortho phenolic carbon and the peaks at 120-124 ppm associated with unsubstituted para phenolic carbon were observed in the lab made control PF resin and boron compounds modified PF resins. The peaks of unsubstituted para phenolic carbon of boric acid modified PF resin shifted to downfield. The possible reason could be attributed to the lower pH value from the neutralization between boric acid and sodium hydroxide. The boron compounds modified PF resins had a different peak intensity of unsubstituted para phenolic carbon than the lab made control PF resin, indicating boron compounds affected the reaction between formaldehyde and para aromatic carbon of phenol. The chemical shift at 48.7 ppm was associated with methanol (CH3OH). Methanol is known as a stabilizer in formaldehyde solution. Meanwhile, Cannizzaro reaction of formaldehyde under strong alkaline condition could be the reason for the formation of methanol in PF resin synthesis. The methyl carbon in hemiformal (CH3OCH2OH) has the chemical shift at 55.2 ppm.. The chemical shift of methylene carbon was at 89.3 ppm (CH3OCH2OH). The unreacted formaldehyde with the chemical shift at 81.7 was absent in all the resins. The peak around 83-86 ppm was from formaldehyde oligomers. The chemical shifts of 92.1-93.7 ppm due to the ortho- and para-Ph-CH2OCH2OCH2OH were present in all the resins. There were no peaks found between 69 and 73 ppm, suggesting that no methylene ether bridges formation between the phenolic rings in the PF resins 16 ACS Paragon Plus Environment

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synthesized under the current experimental conditions with and without boron compounds. It was consistent with the results in previously published article.31 The chemical shifts at 29.2-29.6 ppm for the ortho-ortho linkage of methylene groups (Ph-CH2-Ph) between phenols were not observed for all the resins, while the ortho-para and para-para linkage of methylene groups with the chemical shifts at 34-38 ppm and 39-43 ppm were observed. The presence of sodium hydroxide in PF resin synthesis did not favor the formation of the ortho-ortho methylene linkages. The peaks at 60.0-67.0 ppm were associated with the methylol groups (Ph-CH2OH) in the PF resins, with the 60.0-63.0 ppm for ortho linkage and 64.0-67.0 ppm for para linkage. Methylol groups affected the resin stability and kept the polymeric molecules soluble in the aqueous medium. Although there were no significant difference in the chemical shifts between the lab made control PF resin and boron compounds modified PF resins, quantitative

13

C NMR

analysis indicated that the resin structure was indeed changed with the addition of different boron compounds in the resin synthesis. Table 3 shows the variation of the functional groups on the phenolic rings for the PF and boron compounds modified PF resins studied. Generally, the para position of phenolic ring has slightly higher relative reactivity toward formaldehyde than the ortho position. Compared with the lab made control PF resin, boron compounds modified PF resins had a lower ratio of para/ortho methylol groups, suggesting the addition of boron compounds in PF resin synthesis promoted the methylolation between formaldehyde and ortho position of phenolic ring. The decrease in the para/ortho ratio of methylol groups was caused by the decrease in the free ortho positions and increased reaction with para positions. Meanwhile, a higher ratio of para-para/ortho-para methylene linkage was observed for boron compounds modified PF resins compared to the lab made control PF resin. It indicated that boron compounds promote condensation between para methylol groups. The unsubstituted/substituted hydrogen (–H/–CH2OH) ratio of lab made control PF resin was lower than that of boron compounds modified PF resins. The possible reason could



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be the addition of boron compounds in the PF resin synthesis reduced the substituted hydrogen and retarded the reaction between phenol and formaldehyde. Meanwhile, the lab made control PF resin demonstrated a lower methylol/methylene (-CH2OH/-CH2-) ratio, which was used as an indicator for the degree of polymerization. The boron compounds modified PF resins had a lower degree of polymerization than the lab made control PF resin. These differences possibly explained the results of DSC that although boric acid and borax reduced the curing activation energy of the resulting PF resins, they still retarded the resins curing process according to a lower pre-exponential factor. The possible molecular structure existing in boron compounds modified PF resins is shown in Figure 8.

However, given the complexity of boron containing PF, more detailed

structural analysis in future is needed to be certain of the structural diagram of the resins. In future studies, it would be interesting in conducting the

11

B NMR analysis to better

understand the different boron compounds modified PF resins for wood adhesive applications.

Figure 8. Possible molecular structures existing in boron compounds modified PF resins.

Table 3. The ratios of the relevant functional groups relative to phenolic rings

Sample

Lab made

para/ortho

p-p/o-p link

Unsubstituted/substituted

Methylol/methylene-

(-CH2OH)

(-CH2-)

hydrogen (-H/-CH2OH)

(-CH2OH/-CH2-)

0.38

1.03

0.13

3.30


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control PF Boric acid

0.26

1.67

0.21

3.99

0.15

2.38

0.19

4.70

0.18

1.69

0.15

3.44

modified PF Borax modified PF Zinc borate modified PF

3.4 Thermal degradation of cured PF resins The structure and cross-linking density of cured PF resins highly determined its thermal stability.41-43 Thermal stability is an important property closely related with resin’s durability. The thermal degradation curves of different PF resins are shown in Figure 9. In general, the weight loss of the cured PF resins from room temperature to 200°C was mainly due to the moisture evaporation, dehydration, and the evaporation of small molecular weight compounds. The cured resin underwent further degradation from 200°C to 400°C. The major polymer decomposition took place when temperature increased from 400°C to 600°C. With the temperature further increased from 600°C to 800°C, collapse and carbonization will occur in the cured resin network along with the graphitization process.41-43



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Figure 9. Thermal stability of different post-cured resins In general, boron compounds modified PF resins had a slightly higher total weight loss than the lab made control PF resin up to 800 °C. Borax modified PF resin had the highest total weight loss, followed by boric acid modified PF resin and zinc borate modified PF resin (Table 4). Boric acid modified PF resin had a slightly lower weight loss than the borax modified PF, zinc borate modified PF and lab made control PF resins from room temperature to 200°C. All the resins had a similar weight loss at 200°C to 400°C. Boric acid modified PF resin and borax modified PF resin had a similar weight loss from 400°C to 600°C, which were higher than the lab made control PF resin and zinc borate modified PF resin. The weight loss of borax modified PF resin and zinc borate modified PF resin from 600°C to 800°C was higher than that of boric acid modified PF resin and lab made control PF resins. The addition of boron compounds in the PF resin in-situ synthesis did not significantly change the thermal degradation of cured PF resins at the temperature lower than 400°C. However, different types of boron compounds had different effects on the resulting cured PF resins thermal stability when the degradation temperature was higher than 400°C. From the previous DSC analysis, the curing reaction enthalpy of the 20 ACS Paragon Plus Environment

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boron compounds modified PF resins were lower than the lab made control PF resin, suggesting a lower curing degree of the boron compounds modified PF resins. Meanwhile, the cross-linked structure of cured boron compounds modified PF resins differed from the lab made control resin. Higher curing reaction enthalpy indicated a more stable cured resin structure, which explained a lower total weight loss for the lab made control PF resin from thermal degradation compared to boron compounds modified PF resins. Although boron compounds were widely used as fire retardant to post-blend with wood adhesives, the final cross-linked structure of cured resins will dominate their thermal degradation behavior and degradation kinetics. Meanwhile, the fire retardant effects of boron compounds under nitrogen atmosphere could be different from air environment. The detailed cross-linked structures, thermal degradation kinetics of the cured PF resins, resin’s structure changes during thermal degradation, as well as the effects of boron compounds on the cured PF resins’ thermal stability under both nitrogen and air atmosphere are currently under investigation. Table 4. Weight losses of different resins in thermal degradation process Curing reaction Weight loss at different temperature range

Total weight loss

enthalpy

(%)

(%)

(J/g)

RT~

200°C~

400°C~

600°C~

200°C

400°C

600°C

800°C

RT~800°C

Control PF

11.2

9.8

14.3

12.9

48.2

300.8

Boric acid modified PF

9.8

10.8

18.1

13.5

52.2

260.3

Borax modified PF

11.2

10.4

18.2

16.4

56.2

249.9

Zinc borate modified PF

11.9

10.2

13.1

16.5

51.7

289.2

Resin Type

3.5 Bonding strength 21 ACS Paragon Plus Environment


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The bonding strength of different resins is shown in Figure 10. The boron modified PF resins had comparable dry bonding strength to the lab made control PF resin. There was no significant difference in the dry bonding strength among the boric acid modified PF, borax modified PF and zinc borate modified PF resins. After the water-soaking-anddrying (WSAD) treatment and the boiling water treatment (BWT), no sample delamination was found. The wet bonding strength of all the resins was lower than the drying bonding strength. The lab made control PF resin had a similar wet bonding strength to the zinc borate modified PF resin and a slightly higher wet bonding strength than the boric acid modified PF resin and borax modified PF resin, however, the ANOVA analysis showed that there was no statistically significant difference in the wet bonding strength of all the resins.

Figure 10. Shear strength of lap shear samples bonded with different resins (Column shared the same letter is statistically insignificant different based on ANOVA and Tukey’s test (α=0.05) 3.6 Impregnated wood and their properties The properties of the treated and untreated wood are shown in Table 5. After PF resins impregnation, the treated wood gained about 45-50 percentage of weight on average (WPG%). The density of the treated wood also increased. The PF resins treated wood had a lower moisture absorption (MA) compared to the untreated wood. The anti-swelling


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efficiency (ASE) of the treated wood ranged from 47-53%. It indicated that the PF resin impregnated wood had a higher dimensional stability than the untreated wood. Meanwhile, the MOE and MOR of PF resin impregnated wood increased significantly compared to the untreated wood. The wood impregnated using boron compounds modified PF resins (PF-BA, PF-BX and PF-ZB) had comparable dimensional stability (MA and ASE) and mechanical properties (MOE and MOR) to the wood impregnated using the lab made control PF resin without boron compounds. In addition, lab made control PF resin impregnated wood had a higher oxygen index than the untreated wood, suggesting the improvement in the fire resistance after impregnation. Wood samples treated with boron compounds modified PF resins had higher oxygen index than those treated with the lab made control PF resin, indicating that the boron compounds modified PF resins had a higher fire resistance than the lab made control PF resin without boron compounds. There were no significant differences in the dimensional stability, mechanical properties and fire resistance of wood impregnated using different boron compounds modified PF resins. Table 5. Physical and mechanical properties of untreated and treated wood WPG (%)

Density

MA(%)

ASE(%)

MOE

MOR

(Gpa)

(Mpa)

8.57

56.84

(0.60)

(5.35)

50.72

12.92

92.36

(4.11)

(0.81)

(6.77)

53.51

13.64

96.58

(3.79)

(0.78)

(6.96)

47.64

11.75

89.45

(3.95)

(0.85)

(5.88)

3

(g/cm )

Untreated

0

0.38 (0.02)

index (%)

9.32 (0.55)

NA

wood Treated

47.6 (3.1)

0.54 (0.03)

Oxygen

5.83 (0.48)

wood by

24.5 (4.7)

37.8 (3.9)

lab made control PF Treated

49.3 (2.5)

0.57 (0.03)

5.62 (0.52)

wood by

47.9 (3.2)

PF-BA Treated wood by

44.2 (3.7)

0.51 (0.04)

6.47 (0.61)

48.6 (3.5)



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PF-BX Treated

45.7 (2.9)

0.52 (0.03)

6.15 (0.50)

wood by

48.65

12.26

91.22

(4.01)

(0.69)

(6.01)

50.2 (4.0)

PF-ZB Values in parentheses are standard deviation

4. Conclusion In this study, different boron compounds were used together with an alkaline catalyst to synthesize in-situ Phenol Formaldehyde (PF) resole resins for wood adhesive and wood impregnation applications. Compared to conventional post-blending PF resole resins with boron compounds, the in-situ synthesis of PF resin with boron compounds overcame the issues related to low solubility and poorer mechanical properties. The effects of boron compounds on the novel PF resins’ structure and performances as well as their application as modifier for low density and low quality wood impregnation are systematically investigated. Results showed that the modified PF resins had a slightly lower average molecular weight and polydispersity index and a longer gel time than the lab made control PF resin without boron compounds. Addition of boron compounds affected the curing kinetics of the resulting PF resins. Boron compounds modified PF resins started to cure at a lower temperature than the lab made control PF resin. Different boron compounds had different effects on the resin curing kinetics. Boric acid modified PF resin and borax modified PF resin retarded the resin curing process while zinc borated modified PF resin slightly accelerated the resin curing process compared to the lab made control PF resin. The liquid-state 13C NMR showed that the addition of boron compounds in PF resin synthesis promoted the methylolation between formaldehyde and ortho position of phenolic ring and condensation between para methylol groups. Boron compounds modified PF resins demonstrated a lower degree of polymerization than the lab made control PF resin. The resins molecular structure, functional groups as well as cross-linked structure after curing varied depending on the type of boron compounds. The post-curing thermal stability of boron modified PF resins was slightly lower than that of the lab made control PF resin. 24 ACS Paragon Plus Environment

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Lap shear tests showed that boron compounds modified PF resins had comparable dry and wet bonding strength to the lab made control PF resin. Wood impregnated using boron compounds modified PF resins (PF-BA, PF-BX and PF-ZB) had comparable dimensional stability, mechanical properties and improved fire resistance than the wood impregnated using the lab made control PF resin regardless the type of the boron compounds used. The boron compounds modified PF resins could be used as an effective and multi-functional modifier for making value-added wood products with higher performance and sustainability. In future studies, it would be interesting to conduct the

11

B NMR analysis to better

understand the different boron compounds modified PF resins for wood adhesive applications. Meanwhile, the detailed cross-linked structures, thermal degradation kinetics of the cured boron compounds modified PF resins, resin’s structure changes during thermal degradation, as well as the effects of boron compounds on the cured resins’ thermal stability under both nitrogen and air atmosphere are also worthy of further investigations.

Supporting information Assignments of chemical shifts for the liquid-state

13

C NMR spectra of different boron

compounds modified PF resins and lab made control PF resins are summarized in Table S1. The methodology to obtain the 13C NMR spectra of different PF resins is in Section 2.2.6. The detailed discussions for the chemical shifts and resin molecular structures are in Section 3.3. The supporting information is available at http://pubs.acs.org

Acknowledgement The authors would like to acknowledge the financial support from Special Fund for Forest Scientific Research in Public Welfare (201404501) provided by State Forestry Administration of the People’s Republic of China and NSERC-discovery, Natural Sciences and Engineering Research Council of Canada.



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formaldehyde resins, J. Appl. Polym. Sci. 1993, 50, 685.

For Table of Contents Only


Characterization of Modified Phenol Formaldehyde Resole Resins

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