Synthesis and Characterization of Biobased Melamine Formaldehyde

Jun 18, 2014 - Application of Biobased Phenol Formaldehyde Novolac Resin Derived from Beetle Infested Lodgepole Pine Barks for Thermal Molding of ...
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Synthesis and Characterization of Biobased Melamine Formaldehyde Resins from Bark Extractives Yubo Chai,†,‡ Yong Zhao,‡ and Ning Yan*,‡ †

Research Institute of Wood Industry, Chinese Academy of Forestry, Beijing, China 100091 Faculty of Forestry, University of Toronto, 33 Willcocks Street, Toronto, Ontario, Canada M5S 3B3

‡

S Supporting Information *

ABSTRACT: In this study, bark alkaline extractives from the mountain pine beetle (Dendroctonus ponderosae Hopkins) infested lodgepole pine (Pinus contorta Dougl.) was used to partially replace 30 wt % of melamine in formulating the biobased bark extractive−melamine formaldehyde (MF) resin. Results showed that the addition of the bark extractives and the type of solvent system used for resin formulation significantly affected the initial molecular weight, molecular structure, viscosity, curing behavior, postcuring thermal stability, and bonding performance of the resulting resins. The bark extractive−MF resins exhibited similar dry and wet bonding strengths to the laboratory made control MF resins formulated in the same type of solvent system. The liquid-state 13C NMR study showed that bark extractives were reactants and incorporated into the resulting biobased MF resin structures. Bark extractives obtained from the mountain pine beetle infested lodgepole pine showed promise as a suitable partial replacement for melamine in MF resin formulations.

1. INTRODUCTION Melamine formaldehyde (MF) resins, obtained from addition and condensation reactions between melamine (1,3,5-triamino2,4,6-triazine) and formaldehyde, have been widely used as adhesives for manufacturing exterior or semiexterior wood panels, wood flooring, and decorative laminates and overlay products. Compared with urea formaldehyde (UF) resin, MF resins have a higher resistance to water attack and weather conditions. Also, MF resins have some other advantages, such as good hardness and thermal stability, transparency, scratch and abrasion resistance, flame retardancy, and surface smoothness.1 Typically, MF resins are used in products that require good toughness and ease of manufacture. It has been shown previously that resin synthesis conditions, such as molar ratio of the reactants, pH conditions of the reactions, temperature profile during resin cooking, and solvent system, have significant effects on the characteristics of the resulting adhesives.1,2 Meanwhile, curing behavior and degree of crosslinking of the MF resins determine the properties of the end products. Insufficient cure will result in weak mechanical strength, low hardness, poor durability, and resistance to hydrolysis and chemicals.3,4 Moreover, urea is usually added in the MF formulation to reduce the consumption of melamine and to make lower cost melamine−urea−formaldehyde (MUF) resins. Up to 50 wt % of melamine could be replaced by urea without significantly compromising the resistance to water attack and weather conditions.2 For both MF and MUF, the formulation procedure, cure mechanism, cross-linking pathways, and chemical structures were found to be complex. The resin formulation was found to be a series of complex equilibriums. Fourier transform infrared (FTIR) spectroscopy, 13C NMR, and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry were useful analytical tools to follow the reaction © 2014 American Chemical Society

progress during the MF/MUF resin synthesis. The distribution and structures of various resin molecular species at different stages during the resin formulation, and the rearrangements of chemical bonds such as from methylene ether bridges to methylene bridges, could be identified using the above techniques.5,6 The resins were reported to undergo several transitions when changing from liquid to cross-linked glassy state.1,2 Two key events, gelation and vitrification, will occur during the curing process. In fact, curing transitions, such as gelation and vitrification, will significantly influence the structure and properties of the MF resins, including rheological properties, resin curing rate, morphology, density, hardness, internal stresses, and resistance to hydrolysis and chemical agents of the cured resins.3,4 Various studies investigated the curing mechanism of MF and MUF resins. Previous work usually reported the presence of two exothermic peaks in the differential scanning calorimetry (DSC) measurements, indicating a two-step cross-linking reaction in the curing process.2,7−12 It was generally believed that it consisted of an initial methylolation with the subsequent thermally induced condensation reaction forming the methylene links and ether links with the latter decomposing into methylene links at temperatures above 135 °C. The reversible demethylolation reaction which releases free amine was also observed to be dominant at the cure temperature range of 140−160 °C. When the cure temperature was higher than 160 °C, the cross-linking reaction dominated the curing process.7 Formaldehyde trapped within the cross-linked matrix could further diffuse and react with Received: Revised: Accepted: Published: 11228

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complicated curing mechanism. No coreaction between the MUF and the liquefied wood was found. Bark is a renewable biomass feedstock available in large quantities as waste residues in forest mills during conversion of wood logs to various forest products. In our previous studies, bark was successfully applied to partially substitute petroleumbased phenol in phenol formaldehyde resin formulations in the form of either phenol-liquefied bark or bark alkaline extractives.21−25 No previous studies have reported the application of bark or bark components in MF or MUF resin formulations. Tannin, a natural polyphenolic compound with flavonoid units presenting in a relatively large quantity in coniferous tree barks, could react rapidly with formaldehyde and be used as an efficient bioscavenger for free formaldehyde emitted from MF resins.26 Addition of 5 wt % tannins to MF resin could significantly reduce the formaldehyde emission of the engineering floorboards bonded by the MF resin. Tannin also improved the thermal stability of the resulting MF resins in the low temperature range from 100 to 300 °C. However, commercial tannin significantly differs in composition from bark alkaline extractives;23,24 the incorporation of bark components into MF resin formulation and its effects on the resin properties such as curing behavior, resin storage stability, and bonding performance are still largely unknown. In this study, bark alkaline extractives from the mountain pine beetle (Dendroctonus ponderosae Hopkins) infested lodgepole pine (Pinus contorta Dougl.) were used to partially replace 30 wt % of melamine in formulating the biobased bark extractive−melamine formaldehyde resins. According to our previous study, the bark alkaline extractives to be used in this study are complex mixtures containing tannins, degraded lignin, and degraded hemicellulose in their composition.24 The tannins and degraded lignin could participate in the reaction with formaldehyde during the MF resin synthesis. MF resins with and without bark extractives should be expected to have differences in structure and performance. Due to the extremely fast growth in viscosity, the bark extractive−MF resins cannot be successfully formulated by simply replacing melamine with the bark extractives. A small amount of urea (10 wt % of melamine) was added in the bark extractive−MF resin formulation as the viscosity stabilizer. Meanwhile, water and methanol were used as the solvent systems for making storagestable MF resin formulations. The bark extractive−MF resins were successfully formulated both in water and in methanol. Properties of the resulting biobased bark extractive−MF resins were investigated and compared with the laboratory made control MF resins.

terminal amino and methylol groups, which were much less mobile at lower temperatures.2,7,11 Shelf life, i.e., storage time, is another important factor for MF resins. The limited storage stability of MF solution usually hampers their applications.13,14 During storage, the transparent MF resin solution starts to develop turbidity accompanied by a rise in viscosity which gradually becomes a solid. Colloidal particle formation followed by their clustering was found to be associated with the aging of aminoplastic resins. The aging of the MF/MUF resins of different formulation procedures could be summarized in three stages: clean resin (molecular colloidal aggregation), superclusters of a whitened heavily thixotropic resin (the beginning of physical gelation), and liquid/cluster separation (terminal stage of physical gelation). Some clustering appeared rather early even when the great majority of the resin was not in the colloidal state and even when the resin was still transparent.15 The physical gelation could be classified into reversible physical gelation and irreversible physical gelation. The reversible physical gelation occurred when the resin average molecular mass was low; while the irreversible physical gelation occurred when the average molecular mass of the resin was high and the resin can never again behave as a liquid.16 The star-like structures visible in the colloidal globules were the light interference patterns of the early colloidal structures in the MF resins. It became more complex with resin aging to more complex aggregates, and eventually filament-like and rod-like structures started to appear. The formation of globular masses indicated the true start of physical gelation.17 Previous studies also showed that the storage stability of MF resins was governed by a combination of different physical processes including precipitation of low molecular weight (methylol)melamines, supramolecular aggregation and physical gelation, coexistence of physical gelation and liquid−liquid phase separation, and liquid−liquid phase separation. It is also found that the chemical structure of the fresh and aged MF resins did not change significantly during storage at room temperature. The methylene bridges of MF resins were found to have a greater tendency toward precipitation than ether bridges. MF resins were also found to have a higher storage stability when stored at 50 °C instead of room temperature.13,14 In addition, stabilizers were also used to improve the storage stability of MF resins. Urea was one of the commonly used stabilizers in MF resin formulation to improve their storage stability by breaking the hydrogen bonds. The addition of molecules such as free urea and acetals could give prolonged resin shelf life and improve the performance in hardening by disrupting the colloidal aggregation mechanisms.15 Etherification of MF resins with alcohols was another common way to improve their storage stability. The MF resins with extremely high storage stability could be successfully formulated by adding excessive methanol during resin synthesis.18,19 With the growing concerns for the rapid depletion of nonrenewable fossil fuel resources and the rising price of petroleum-based products, there is strong interest in exploring renewable biomass materials as an alternative feedstock to replace petroleum-derived chemicals. The curing kinetics of melamine−urea−formaldehyde (MUF) resin/liquefied wood was investigated using DSC previously.20 The liquefied wood was added to the MUF resin precursor to reduce the extent of formaldehyde emission and cost. The MUF/liquefied wood resin exhibited a very

2. EXPERIMENTAL SECTION 2.1. Bark Extractives. Air-dried bark powder sample of mountain pine beetle infested lodgepole pine that had passed through 35-mesh sieve was extracted by 1% NaOH aqueous solution. The bark to solvent solution ratio was 1:10 (weight/ volume). The extraction was conducted at 85 °C for 2 h under atmosphere condition. After extraction, the mixture was filtered on a 2000 mL glass Büchner funnel with a coarse porosity fritted disk with suction. The bark extractive solution was then spray-dried directly using a lab-scale spray dryer with the inlet temperature of 160 °C and outlet temperature of 70−80 °C (ADL311SA, Yamato Scientific America, Inc., USA). The obtained bark extractives were in the form of brown powders. 11229

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Table 1. Properties of the Resins

a

resin type

solids content (%)

pH

initial viscosity (cP)

Mw (Da)

Mn (Da)

PDIa

gel time (s)

BEMF M BEMF W Lab MF M Lab MF W

41.35 36.38 43.11 36.34

11.33 10.67 12.31 12.07

75 65 25 11

1178 1048 847 622

748 554 527 348

1.58 1.89 1.61 1.79

1959 3612 1785 2543

PDI, polydispersity index.

placed in a platinum pan and heated from room temperature to 800 °C at the rate of 10 °C/min under a N2 atmosphere using a thermal gravimetric analyzer (TGA-Q500, TA Instruments, USA). 2.3.4. Bonding Strength. Birch 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 birch wood veneer specimens were bonded with the four different types of resins, i.e., two bark extractive−MF resins (BEMF M and BEMF W) and two lab made MF resins (Lab MF M and Lab MF W). The adhesives were applied to one side of the birch strip over an area of 25.4 mm × 25.4 mm. The spread rate of the adhesives was 0.025−0.035 g/cm2 on the solids basis. The adhesive-coated area of the birch strip was then overlapped with an uncoated birch strip. The resulting two-layered lap shear specimen was hot pressed at 160 °C under the thickness control of 4.5 mm for 3 min. 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. 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 watersoaking-and-drying (WSAD) test and a boiling water test (BWT), according to voluntary standard PSl-95 published by the U.S. Department of Commerce through the Engineered Wood Association, Tacoma, WA. For the WSAD test, the specimens were soaked in water at room temperature for 24 h, dried in a fume hood at room temperature for another 24 h, and then subjected to shear strength measurements. For the BWT test, the specimens were boiled in water for 4 h and then dried for 20 h at 63 ± 2 °C. After drying, the specimens were boiled in water again for 4 h, 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. 2.3.5. Bonding Development of Biobased MF Resins. The bonding development of the MF resins was measured using dynamic mechanical analysis (DMA) (DMA-Q800, TA Instruments, USA) from room temperature to 220 °C at a heating rate of 10 °C/min. The samples were prepared by bonding two pieces of oak veneer with dimensions of 50 mm × 13 mm × 0.65 mm using bark extractive−MF resins and lab made MF resins. The resin load was approximately 100 mg per sample. The grain direction of the veneers was parallel to the length direction and perpendicular to the loading force. Samples were prepared immediately before the tests. All samples were tested under dual cantilever mode. The clamp torque for the sample installation was set at 0.9 N m. The frequency used for the test was 1 Hz, and the strain of the sample was controlled at 0.01%. At least six repetitions were carried out for each sample. 2.3.6. Liquid-State 13C NMR Study of Biobased MF Resins. The bark extractive−MF resin and lab made control MF resin were first dissolved in DMSO-d6. The liquid-state 13C NMR

The bark extractives have a Stiasny number (an indication of formaldehyde-condensable polyphenol content) of 37.93.23,24 2.2. Resin Formulation. Two types of lab made melamine formaldehyde resins were formulated as control resins. For the lab made melamine formaldehyde resin Lab MF W, 12 parts of melamine and 18 parts of 37% formaldehyde aqueous solution were first charged into a three-neck flask, and the pH value of the reactants was adjusted to 10 using 10% sodium hydroxide solution. The reactants were then heated to 80 °C and kept there for 30 min. After adjustment of the pH value of the reactants to 12, 20 parts of distilled water and a calculated amount of urea (10 wt % of melamine used for resin formulation) were added to the reaction system. The reaction temperature was kept at 70 °C for 1 h and then cooled to room temperature. The lab made melamine formaldehyde resin Lab MF M was formulated following exactly the same recipe and procedure for synthesizing Lab MF W except the 20 parts of distilled water was replaced by methanol. Two types of bark extractive−melamine formaldehyde resins (BEMF W and BEMF M) were formulated with 30 wt % of melamine substituted by the dried bark extractives. The synthesis recipe and cooking procedure for the bark extractive−melamine formaldehyde resins were kept exactly the same as for the lab made control melamine formaldehyde resins. BEMF W indicated the resin formulated in water, and BEMF M signified the resin synthesized in methanol. 2.3. Resin Characterization. 2.3.1. Resin Properties. The pH values of the resins were measured at 25 °C. The viscosities of the lab made MF resins and bark extractive−MF resins were measured using a Brookfield rotary viscometer. The procedure described in the ASTM D 3529 standard was used for the measurements of the solids content. 2.3.2. Resin Curing Behavior and Curing Kinetics. Highpressure pans (DSC-Q1000, TA Instruments, USA) were used for investigating the resin curing behavior. Dynamic scans were carried out at heating rates of 5, 10, 15, and 20 °C/min, starting from room temperature and increasing to 250 °C. The Kissinger method27 was used to calculate the activation energy as ⎛ ϕ ⎞ ⎛ RA ⎞ E 1 ⎟ + ln⎜ ln⎜⎜ 2 ⎟⎟ = − ⎝ E ⎠ R Tp ⎝ Tp ⎠

(1)

where ϕ is the heating rate (K/s), Tp is the peak temperature (in kelvin) at the given heating rate, A is the pre-exponential factor, R is the ideal gas constant, and E is the activation energy. A sample was randomly chosen for testing, and three replicates were done for each sample. The maximum variation in the onset and peak temperatures was found to be less than 1 °C. 2.3.3. Thermal Stability. Two types of bark extractive−MF resins and two types of lab made MF resins were cured in an oven at 105 °C for 24 h. The cured resins were ground into fine powders that were able to pass through a 0.149 mm sieve (100mesh screen). About 10 mg of each cured resin sample was 11230

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spectra of these samples were recorded using a Unity 500 spectrometer under the following conditions: a pulse angle of 60° (8.3 μs), a relaxation delay of 10 s, and with gated Waltz-16 1H decoupling during the acquisition period. About 40 000 scans were accumulated for each spectrum. The 13C chemical shifts were measured using tetramethylsilane (TMS) as the internal standard.

3. RESULTS AND DISCUSSION 3.1. Properties of the Resins. The basic properties of different resins are shown in Table 1. Bark extractive−MF resins had a higher solids content than the lab made control MF resins. The solids content of the two types of bark extractive−MF resins were 43.11% (BEMF W) and 41.35% (BEMF M), while the solids content of the two types of lab made MF resins were 36.34% (Lab MF W) and 36.38% (Lab MF M), respectively. The pH value of the MF resins ranged from 10.67 to 12.31. The bark extractive−PF resins had a slightly lower pH value than the lab made MF resins formulated in the same solvent system. The pH value of the same type of MF resins formulated in water was slightly lower than that formulated in methanol. The bark extractive−MF resins had a higher molecular weight and a higher initial viscosity than the lab made MF resins formulated in the same solvent system, and the MF resins formulated in methanol exhibited a higher molecular weight and a higher initial viscosity than the resins synthesized in water. Different from water, methanol not only served as a solvent, but also acted as a reactant in the MF resin formulation through a reaction shown in Figure 1. The formaldehyde released from

Figure 2. Changes in viscosity during resin storage at room temperature.

more efficient stabilizer for the bark extractive−MF resins, in contrast to the lab made MF resins. For the MF resins formulated in water, the viscosity of the lab made MF resin was lower than that of the bark extractive− MF resin within the first 5 days after the resin formulation. From the sixth day on, the viscosity of the lab made MF resin increased rapidly and reached 591 cP after 2 weeks. While the bark extractive−MF resin with the initial viscosity of 64.8 cP had a relatively more stable viscosity than the lab made MF resin during storage, its viscosity was only 146.4 cP after 2 weeks. The bark extractive−MF resin formulated in water was more stable than the lab made MF resin formulated in water during storage at room temperature. For the MF resins formulated in methanol, the viscosity of the bark extractive−MF resin increased from 126.6 to 2320 cP from shortly after the resin formulation to the seventh day after, while the viscosity of the lab made MF resin increased from 25.2 to 128 cP, which was much slower than that of the bark extractive−MF resin. The viscosity of the lab made MF resin was 333.8 cP at the 14th day after the resin formulation. This indicated that the lab made MF resin formulated in methanol had a more stable viscosity than the bark extractive−MF resin formulated in methanol. 3.2. Curing Behavior of the Resins. Previous studies indicated that the reaction mechanism and pathways involved in the cross-linking of MF resins as well as the resulting chemical structures were complex.2,7−12 Therefore, it is expected that the introduction of bark extractives in the MF resin would significantly affect the curing behavior of the resulting resins. The curing process of the lab made MF resins and bark extractive−MF resins as measured by DSC are shown in Figures 3−Figure 6. The lab made MF resin formulated in water (Lab MF W) and bark extractive−MF resin formulated in methanol (BEMF M) exhibited a single exothermic peak in the DSC curing profile. Two exothermic peaks were observed for the lab made MF resin formulated in methanol (Lab MF M) and bark extractive−MF resin formulated in water (BEMF W). The single exothermic peak indicated that the curing process of resins was mainly dominated by the condensation reactions. Two exothermic peaks implied that the cross-linking process of resins involved a two-step reaction. The first exothermic peak could be attributed to the methylolation reaction or the reversible reaction of methylolmelamine, while the second

Figure 1. Possible conversion reaction of methanol to formaldehyde.

the methanol during the resin formulation could then react with melamine or bark extractives and result in the differences in the pH values, initial viscosities, and molecular weights of the resins. It is generally believed that MF resins are unstable and have a short pot life due to their rapidly rising viscosities.1,13,14 The variations in viscosity of different resins were monitored for 14 days, and results are shown in Figure 2. It is evident that the viscosities of different resins increased with the prolonged storage time. The changes in viscosity of the lab made MF resin formulated in water (Lab MF W) were similar to those of that formulated in methanol (Lab MF M) until the seventh day. Then the increase in viscosity of the Lab MF W was higher than that of the Lab MF M. It indicated that methanol could serve as a better stabilizer for lab MF resins when stored at room temperature. The viscosity of bark extractive−MF resin formulated in water (BEMF W) was relatively stable and ranged from 64.8 to 92.4 cP in the first 7 days after the resin synthesis. After 10 days, the viscosity of BEMF W increased to 112.8 cP and further increased to 146.4 cP on the 14th day. For the bark extractive−MF resin formulated in methanol (BEMF M), the viscosity was higher than that of the BEMF W and it increased quickly from the initial viscosity of 75.2 cP to 774 cP at the fifth day. The viscosity was too high to be measured by the viscometer at the 13th day. This indicated that water was a 11231

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Figure 3. Dynamic DSC curves of the bark extractive−MF resin formulated in methanol (BEMF M).

Figure 6. Dynamic DSC curves of the lab made MF resin formulated in water (Lab MF W).

Table 2. Cure Temperature of Bark Extractive−MF Resin Formulated in Methanol (BEMF M) heating rate (°C/min)

onset temp (°C)

peak temp (°C)

0 5 10 15 20

132.9 140.3 148.7 157.2 163.4

162.3 170.4 180.6 189.7 196.9

Table 3. Cure Temperature of Bark Extractive−MF Resin Formulated in Water (BEMF W) heating rate (°C/min)

onset temp (°C)

peak temp 1 (°C)

peak temp 2 (°C)

0 5 10 15 20

127.4 134.3 142.9 150.4 156.6

143.1 151.4 163.9 172.4 180.6

171.5 178.1 190.8 197.2 203.8

Figure 4. Dynamic DSC curves of the bark extractive−MF resin formulated in water (BEMF W).

Table 4. Cure Temperature of Lab Made MF Resin Formulated in Methanol (Lab MF M) heat rate (°C/min) 0 5 10 15 20

onset temp 1 onset temp 2 (°C) (°C) 100.1 102.7 106.0 108.7 111.3

peak temp 1 (°C)

peak temp 2 (°C)

101.5 108.2 114.5 120.0 127.9

158.7 167.4 177.8 186.0 195.2

121.0 129.7 141.7 147.6 159.1

Table 5. Cure Temperature of Lab Made MF Resin Formulated in Water (Lab MF W) Figure 5. Dynamic DSC curves of the lab made MF resin formulated in methanol (Lab MF M).

exothermic peak was generally believed to be corresponding to the cross-linking of methylol to methylene bridges or methylene ether bridges during the thermal curing process.2,7−12 The cure temperatures of different resins with variable heating rates are listed in Tables 2−5. It is found that the onset temperature and peak temperatures of all the resins increased with increasing heating rate, which was consistent with previous

heating rate (°C/min)

onset temp (°C/min)

peak temp (°C/min)

0 5 10 15 20

117.2 123.9 134.9 142.2 148.3

134.4 143.0 153.1 164.7 170.3

studies on MF resins.7,11 Since the actual cure temperatures are expected to be independent of the heating rates, the onset and peak temperatures were extrapolated to the heating rate of zero for further comparison. 11232

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curing process, which was reflected by the pre-exponential factor. Compared with BEMF M, BEMF W had a relatively low curing activation energy calculated from the first exothermic peak, i.e., methylolation and reversible reaction of methylolmelamine, indicating the BEMF W was easier to cure at lower temperature. However, the higher curing activation energy calculated from the second peak for the condensation reaction of BEMF W could also support that BEMF W had a relatively stable viscosity during storage. Further detailed studies on the synergistic effects of bark extractives and the solvent system used for resin formulation are currently under investigation. 3.3. Bonding Development. The bonding development of the bark extractive−MF resin formulated in water (BEMF W) during the curing process was measured by DMA (shown in Figure 7). The storage modulus (E′) increased with increasing

For the same type of resins, the lab made MF resin formulated in water exhibited a higher onset temperature than the lab made MF resin formulated in methanol, indicating that the lab made MF resin formulated in methanol was more reactive at low temperatures. On the contrary, the bark extractive−MF resin formulated in water had a lower onset temperature than the bark extractive−MF resin formulated in methanol, indicating that the bark extractive−MF resin formulated in water was more reactive at low temperatures instead. For the resins formulated in the same solvent system, the bark extractive−MF resins had a higher onset temperature than the lab made MF resins, implying that the bark extractive−MF resins were less reactive than the lab made MF resins at low temperatures. It is hard to compare the peak temperatures of the bark extractive−MF resins and lab made MF resins formulated in the same solvent system, since some had two exothermic peaks and others had only a single exothermic peak. The curing activation energies and pre-exponential factors are listed in Table 6. For the resin formulated in water, the lab Table 6. Curing Activation Energies and Pre-Exponential Factors of MF Resins resin BEMF M BEMF W Lab MF M Lab MF W

E1 (kJ/mol)

A1 (s−1)

82.54 69.50 82.22 67.93

× × × ×

1.43 8.66 7.25 8.60

9

10 107 1010 107

E2 (kJ/mol)

A2 (s−1)

NA 89.76 78.39 NA

NA 6.69 × 109 5.26 × 108 NA

made MF resin (Lab MF W) had a curing activation energy of 67.93 kJ/mol and pre-exponential factor of 8.6 × 107/s. The curing activation energy and pre-exponential factor of the bark extractive−MF resin (BEMF W) calculated from the first exothermic peak were 69.50 kJ/mol and 8.66 × 107/s, and the curing activation energy and pre-exponential factor calculated from the second exothermic peak were 89.76 kJ/mol and 6.69 × 109/s. For the resin formulated in methanol, the curing activation energy and pre-exponential factor of the bark extractive−MF resin (BEMF M) were 82.54 kJ/mol and 1.43 × 109/s. The curing activation energy and pre-exponential factor of the lab made MF resin (Lab MF M) calculated from the first exothermic peak were 82.22 kJ/mol and 7.25 × 1010/s, and the curing activation energy and pre-exponential factor calculated from the second exothermic peak were 78.39 kJ/ mol and 5.26 × 108/s. It is clear that the introduction of the bark extractives to the MF resin formulation affected the curing behavior and curing kinetics of the resulting MF resin formulated in the same solvent system. Meanwhile, the solvent system used for synthesizing the same type of MF resins also affected the resins’ curing behavior and curing kinetics. The curing activation energies of the Lab MF M resin calculated from the first and second exothermic peaks were generally higher than that of the Lab MF W, indicating that the Lab MF M was harder to cure than the lab MF W. The higher curing activation energy also supported that methanol could serve as a stabilizer of the MF resin, which contributed to the relatively stable viscosity of the Lab MF M during storage at room temperature. However, the release of formaldehyde from methanol at higher cure temperatures could accelerate the

Figure 7. DMA parameters determined in the curing process of BEMF W resin.

temperature, due to the cross-linking induced by the resin curing reaction. The increase in E′ from the flat area of the curve following the evaporation of water to the maximum value was caused by the consolidation of the resin network. This difference (ΔE′) could be used to characterize the rigidity of the cured resins. The gel point (Tgel) and Ttan δ were used to characterize the resin curing process.10,11,26,28 The tan δ value increased sharply to a maximum value with increasing cure temperature, which was usually attributed to the increase in resin viscosity during the curing process. Then the tan δ value decreased as temperature further increased; it finally increased again after the curing reaction was completed. These observed changes in our study are consistent with those reported in previous studies on MF resins.10,11,26 The gel points of the bark extractive−MF resins were lower than those of the lab made MF resins formulated in the same solvent system. Also, the same type of MF resins formulated in methanol had a lower gel point than that formulated in water (Figure 8). These differences could be attributed to the different molecular weights of the resins.28 As discussed earlier, the bark extractive−MF resins had higher molecular weights than the lab made MF resin formulated in the same solvent system, and the same type of MF resins formulated in methanol had higher molecular weights than those formulated in water. Higher molecular weight resins with a higher degree of conversion could reach the gel state more easily than the low molecular weight resins. 11233

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Figure 8. Tgel values of MF resins.

Figure 10. Rigidity of MF resins.

Ttan δ values of bark extractive−MF resins were higher than those of the lab made MF resins formulated in the same solvent system. The Ttan δ of the same type of MF resins formulated in methanol was higher than that of the resins formulated in water (Figure 9). The possible reasons could be the resins with higher

MF resins are shown in Figure 11. Based on the total weight loss from room temperature to 800 °C, bark extractive−MF

Figure 11. Thermal stabilities of cured MF resins. Figure 9. Ttan δ of MF resins.

resin formulated in water (BEMF W) had the highest thermal stability, followed by the bark extractive−MF resin formulated in methanol (BEMF M) and lab made MF resin formulated in water (Lab MF W), while the lab made MF resin formulated in methanol (Lab MF M) had the lowest thermal stability. The weight loss of resins at different temperature ranges is shown in Table 7. The major weight loss of the cured MF

molecular weights had less mobility and were hard to cure at relatively high temperature. However, other studies reported that Ttan δ of a MF resin with 5 wt % addition of commercial tannin as a bioscavenger was lower than the control MF resin without tannin.26 This difference could be attributed to the bark extractives used in this study, which had different composition and reactivity toward formaldehyde from the commercial tannin. The rigidity could be related to the bonding strength of cured resins. It could also reflect the degree of cross-linking of the cured resins.26,28 The rigidities of the bark extractive−MF resins were higher than that of the lab made MF resin formulated in the same solvent system, and the same type of MF resins formulated in the methanol had higher rigidities than that formulated in the water (Figure 10). The possible reasons could be that the tannins or phenolic structures with rigid structure and large molecular weight in the bark extractives contributed to the higher rigidity of the cured resins. Besides, methanol could decompose into formaldehyde during the resin curing process, which could further react with melamine or bark extractives to form cross-linking and increase the rigidity of the cured resins. 3.4. Thermal Stability of the Cured Resins. The bark extractives and solvent systems used for the MF resin formulation were expected to affect the thermal stability of the resulting cured resins. Thermal stabilities of different cured

Table 7. Weight Loss of Different MF Resins during Thermal Degradation Process weight loss (%) temp (°C)

BEMF W

BEMF M

Lab MF W

Lab MF M

RT−200 200−400 400−600 600−800 total

6.71 50.93 17.45 8.03 83.12

8.30 52.75 17.39 8.76 87.20

5.02 60.30 13.56 12.17 91.05

7.72 43.94 23.37 17.98 93.01

resins occurred in the range 200−400 °C, which could be corresponding to structural decomposition of the cured resins. The lab made MF resin formulated in methanol (Lab MF M) exhibited the lowest weight loss, while the lab made MF resin formulated in water (Lab MF W) had the highest weight loss. The bark extractive−MF resins formulated in water and methanol had similar weight losses from 200 to 400 °C. 11234

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14. The chemical shifts for the resins are assigned according to published literature.3,5,29−32 The mono-, di-, and trisubstituted melamines (158−167 ppm) were formed in both bark extractive−MF resin and lab made control MF resin. Methylene ether (68−77 ppm) and methylene links (48−54 ppm) formed between melamine and formaldehyde or urea and formaldehyde also existed in the bark extractive−MF resin and lab made control MF resin. The difference in the liquid-state 13C NMR spectra between the bark extractive−MF resin and lab made control MF resin could be clearly observed. The incorporation of bark extractives in the MF resin formulation introduced new peaks. The chemical shift at 176.2 ppm attributed to the carbonyl group in the quinone structure from bark tannin phenolic rings was observed in the bark extractive−MF resin. The chemical shifts between 107 and 150 ppm associated with bark tannin phenolic rings (C5, C7, C3′, C4′, C5′) and the interflavonoid bonds (C4−C6 and C4−C8) were also found in the bark extractive−MF resin. The appearance of the new chemical shifts at 33.1 and 31.5 ppm could be attributed to the methylene links between bark tannin phenolic A rings and melamine and between bark tannin phenolic A rings and urea, respectively. The chemical shifts at 36.7 and 34.9 ppm were associated with methylene links between bark tannin phenolic B rings and melamine and between bark tannin phenolic B rings and urea, respectively. Meanwhile, the methylene links formed by bark extractives themselves were also observed, i.e., methylene link between bark tannin phenolic A rings (17.7 ppm), methylene link between bark tannin phenolic B rings (26.7 ppm), and methylene link between bark tannin phenolic A rings and B rings (21.9 ppm). These new methylene peaks indicated that the bark extractives reacted with formaldehyde during the MF resin formulation. Possible resin structures are shown in Figure 15. Our previous study reported that the bark extractives were reactants in the bark extractive−PF resin formulation and contributed to the acceleration of the curing rates of the resulting resins.24 Similarly, the bark extractives were also reactants in the bark extractive−MF resin formulation and the incorporation of bark extractives in the MF resin formulation contributed to the differences in the properties of the resulting resins including the curing rate. A more detailed study on the relationship between the MF resins’ molecular structure and resin performance is currently under investigation.

For the lab made MF resins without bark components, the application of methanol as the solvent system for resin formulation improved the thermal stability of the resulting cured lab made MF resins in the range 200−400 °C, while for the bark extractive−MF resins, the solvent systems used for resin formulation did not significantly affect the thermal stability of the resulting cured bark extractive−MF resins at 200−400 °C. For the MF resins formulated in water, the addition of bark components improved the thermal stability of the resulting cured bark extractive−MF resins at 200−400 °C, which was opposite to the MF resins formulated in methanol. The possible structures of the cured resins and their structural changes during thermal degradation are currently under investigation. 3.5. Bonding Strength. The bonding strength of different resins is shown in Figure 12. The same type of MF resins

Figure 12. Shear strength of lap shear specimens bonded with different MF resins.

formulated in methanol exhibited a higher dry and wet bonding strength than that formulated in water. The bark extractive− MF resin had similar dry and wet bonding strengths to the lab made MF resin formulated in the same solvent system. This is consistent with the results on resin rigidities obtained from DMA. It is interesting to note that the water-soaking-and-drying (WSAD) bonding strengths of all tested resins were similar to the dry bonding strengths. The considerable low solubility of melamine in water contributed to the higher resistance of MF resins to attack by water. However, after boiling water treatment (BWT), the bonding strengths of all tested resins degraded significantly. The reason for that could be attributed to the hydrolysis of the aminoplastic or aminomethylenic bonds of the MF resins in the hot water.1 Meanwhile, the addition of urea in the MF resin formulation could also negatively affect the bonding strength of the boiling water test, although only a small amount of urea was used in this study. It is different from the case of commercial PF resins which also generally contain 10−15 wt % urea in their composition and exhibited satisfactory bonding strength for the boiling water tests.21,23 3.6. Liquid-State 13C NMR Spectra of the Lab Control and Biobased MF Resins. The liquid-state 13C NMR spectra of the lab made control MF resin (Lab MF W) and bark extractive−MF resin (BEMF W) are shown in Figures 13 and

4. CONCLUSIONS Two types of biobased bark extractive−MF resins with 30 wt % of melamine substitution have been successfully formulated with either water (BEMF W) or methanol (BEMF M) as the solvent. It is found that both bark extractives and solvent systems used for resin formulation affected the properties of the resulting resins. Bark extractive−MF resins had higher solids contents, higher pH values, higher initial molecular weights, and higher viscosities than the lab made MF resins formulated using the same solvent system. The same type of MF resins formulated in methanol exhibited a higher molecular weight and a higher initial viscosity than the resins synthesized in water. The bark extractive−MF resin formulated in water (BEMF W) had the most stable viscosity and the bark extractive−MF resin formulated in methanol (BEMF M) has the least stable viscosity during the 14-day storage period 11235

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Figure 13. Liquid-state 13C NMR spectrum of lab made MF resin.

Figure 14. Liquid-state 13C NMR spectrum of bark extractive−MF resin.

among all the resins. The viscosity of the Lab MF M was more stable than that of the Lab MF W during storage. Lab MF M had a lower onset temperature and generally higher curing activation energy than Lab MF W. The addition of bark extractives in the MF resin formulation brought complexity to the resin’s curing behavior and curing kinetics.

The bark extractive−MF resins had a higher onset temperature than the lab made MF resins formulated in the same solvent system. Two exothermic peaks were observed for BEMF W, while only one exothermic peak was observed for BEMF M during the curing process. 11236

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Figure 15. Possible structure of bark extractive−MF resin.

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Bark extractive−MF resins had a lower gel point but higher Ttan δ and rigidity than the lab made MF resins formulated in the same solvent system. The same type of MF resins formulated in methanol had a lower gel point but a higher Ttan δ and rigidity than those formulated in water. The addition of the bark extractives and solvent systems used for resin formulation also affected the thermal stability of the resulting cured MF resins. The same type of MF resins formulated in methanol exhibited higher dry and wet bonding strengths than that formulated in water. The bark extractive−MF resin had similar dry and wet bonding strengths to those of the lab made MF resin formulated in the same solvent system. The bark alkaline extractives were reactants in the bark extractive−MF resin formulation and were incorporated into the molecular structures of the resulting resins.

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(1) Pizzi, A. Wood Adhesives Chemistry and Technology; Marcel Dekker Inc.: New York, 1983. (2) Likozar, B.; Korošec, R. C.; Poljanšek, I.; Ogorelec, P.; Bukovec, P. Curing kinetics of study of melamine-urea-formaldehyde resin. J. Therm. Anal. Calorim. 2012, 109, 1413 DOI: 10.1007/s10973-0111883-0. (3) Mercer, A. T.; Pizzi, A. A 13C-NMR analysis method for MF and MUF resin strength and formaldehyde emission from wood particleboard. II. MF resin. J. Appl. Polym. Sci. 1996, 61, 1697. (4) Kandelbauer, A.; Teischinger, A. On the warping behavior of particleboards coated with melamine formaldehyde resin impregnated papers. Eur. J. Wood Wood Prod. 2009, 67, 367 DOI: 10.1007/s00107009-0327-z. (5) Despres, A.; Pizzi, A.; Pasch, H.; Kandelbauer, A. Comparative 13 C NMR and matrix-assisted laser desorption/ionization time-offlight analysis of species variation and structure maintenance during melamine-urea-formaldehyde resins preparation. J. Appl. Polym. Sci. 2007, 106, 1106. (6) Kandelbauer, A.; Despres, A.; Pizzi, A.; Taudes, I. Testing by Fourier transform infrared species variation during melamine-ureaformaldehyde resins preparation. J. Appl. Polym. Sci. 2007, 106, 2192. (7) Merline, D. J.; Vukusic, S.; Abdala, A. A. Melamine formaldehyde: curing studies and reaction mechanism. Polym. J. (Tokyo, Jpn.). 2013, 45, 413. (8) Kandelbauer, A.; Wuzella, G.; Mahendran, A.; Taudes, I.; Widsten, P. Model-free kinetic analysis of melamine-formaldehyde resin cure. Chem. Eng. J. 2009, 152, 556. (9) Kandelbauer, A.; Wuzella, G.; Mahendran, A.; Taudes, I.; Widsten, P. Using isoconversional kinetic analysis of liquid melamineformaldehyde resin curing to predict laminate surface properties. J. Appl. Polym. Sci. 2009, 113, 2649. (10) Kim, S.; Kim, H. J.; Kim, H. S.; Lee, Y. K.; Yang, H. S. Thermal analysis study of viscoelastic properties and activation energy of melamine-modified urea-formaldehyde resins. J. Adhes. Sci. Technol. 2006, 20, 803. (11) Cai, X. L.; Riedl, B.; Wan, H.; Zhang, S. Y.; Wang, X. M. A study on the curing and viscoelastic characteristics of melamine-ureaformaldehyde resin in the presence of aluminum silicate nanoclays. Composites, Part A 2010, 41, 604. (12) Park, B. D.; Jeong, H. W. Cure kinetics of melamineformaldehyde resin/clay/cellulose nancomposites. J. Ind. Eng. Chem. 2010, 16, 375.

ASSOCIATED CONTENT

S Supporting Information *

Assignments of chemical shifts for the liquid-state 13C NMR spectra of the lab control and biobased MF resins are summarized in Table S1. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: (416) 946-8070. Fax: (416) 978-3834. Notes

The authors declare no competing financial interest.

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REFERENCES

ACKNOWLEDGMENTS

Financial support from the public and private partners of the Ontario Research Fund-Research Excellence project: Bark Biorefinery is highly acknowledged. Dr. Darcy Burns of the NMR center, Department of Chemistry, University of Toronto, is highly appreciated for his help on the NMR measurements. 11237

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