Structural and Physical Changes in Phenol−Formaldehyde Resol

Sep 13, 2007 - Structural and Physical Changes in Phenol−Formaldehyde Resol Resin, as a Function of the Degree of Condensation of the Resol Solution...
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Structural and Physical Changes in Phenol-Formaldehyde Resol Resin, as a Function of the Degree of Condensation of the Resol Solution Janne Monni,* Leila Alvila, and Tuula T. Pakkanen Department of Chemistry, UniVersity of Joensuu, P.O. Box 111, FIN-80101 Joensuu, Finland

The structural and physical differences of phenol-formaldehyde resol resins collected during a resin synthesis were studied, as a function of the degree of condensation of the resin. The structural and physical properties of the resins were analyzed using nuclear magnetic resonance (NMR) spectroscopy, viscosity and refractive index measurements, gel permeation chromatography (GPC), differential scanning calorimetry (DSC), solids content analyses, and contact angle determinations on wood veneer. As a result of the condensation reactions, the amount of methylol groups and free reactive sites of phenolic species decreased, while the amount of methylene bridges increased, as a function of the degree of condensation. The changes in the resin structure resulted in an increase in the viscosity, refractive index, and average molar masses, and a decrease in reactivity, with increasing degrees of condensation. The wetting of the surface of the wood veneer was reduced as the viscosity and average molar masses of the resin solution each increased. Introduction Phenol-formaldehyde resol resin is the main component of exterior-grade plywood adhesives in the woodworking industry. These cured resol resins with additives generate durable, moisture- and weather-resistant adhesive joints between the sheets of veneer, and they improve the mechanical stability of the plywood board.1,2 Plywood resins are typically synthesized in batch reactors under accurate temperature control and feed of the starting materials.1,2 In industrial processes, an alkaline catalyst, such as NaOH, and formaldehyde (as an aqueous solution) (formalin) are commonly introduced into the reaction mixture, in multiple process steps, to improve the control and safety of the production. However, a multistage production process complicates the reaction system, increases production time, and could affect the structure and properties of the final products. The batch syntheses of the resol resins under various formaldehyde/phenol (F/P) molar ratios,3,4 catalysts,3,5 catalyst concentrations (alkalinity),6 reaction conditions (temperature, time),6-11 and the effects of these parameters on the properties of the resins, have been widely studied. In most studies, the process conditions, especially temperatures and alkalinities (pH), are mild and significantly differ from those generally applied in the production of resol resins. In addition, a large amount of synthesis parameters contribute to the progress of the process, thus affecting the final structure and properties of the resol resin. This type of case specificity makes a reliable comparison of resins processed with various reaction parameters in different processes more difficult. The objective of this study was to investigate differences between resins collected during one resol resin synthesis, during which the only changing synthesis parameter was the degree of condensation of the resin solution. The synthesis arrangement of the study allowed for a reliable and straightforward comparison of structural and physical properties between different resins. Another goal of the study was to examine the effects of the degree of condensation of the resol solution on the wetting behavior of the resin on wood veneer. In addition, the synthesis * To whom correspondence should be addressed. Tel.: +358-132513330. Fax: +358-13-2513390. E-mail: [email protected].

was performed under a gradual addition of formalin in a one process step, and the process conditions, such as molar ratios of starting materials, alkalinity, and temperature ranges in the prevailing synthesis stages, were selected to resemble the conditions utilized in the production of industrial-type resol resins. The viscosities and refractive indices of the resin solutions were measured and compared with the structural changes of the resins determined by 13C nuclear magnetic resonance (NMR) spectroscopy. Gel permeation chromatography (GPC) was used in the determinations of average molar masses, as a function of the degree of condensation of the resin. The effects of changing the degree of condensation on the reactivity and thermal behavior of the resol solutions were studied using differential scanning calorimetry (DSC). Finally, the wetting properties of the resol resins on wood veneer (silver birch) were investigated using contact angle measurements. Experimental Section Starting Materials and the Synthesis of a Resol Resin. The synthesis of a phenol-formaldehyde resol resin with a F/P molar ratio of 2.0 and alkalinity of 4.5 wt % was conducted in a batch reactor. The reactor system consisted of a glassy batch reactor (V ) 1 L) that was equipped with a thermostat-controlled (Lauda E 100) silicone-oil jacket, a mixer (Heidolph RZR 2051), a condenser, and a contact thermometer. The synthesis was started by pouring the molten phenol (319 g, Merck p.a.), additional water (150 g), and NaOH solution (50 wt %, 89 g), in separate steps, into the reactor (Toil ) 40 °C) while stirring. The selected starting time of the process was the beginning of the formalin addition (46.8 wt %, 431 g). Formalin was fed into the reactor dropwise with a burette in one synthesis step for 30 min, and the solution temperature was allowed to increase to 89-90 °C. After the addition of formalin was completed, the solution was condensed at 86-89 °C for 60 min, and at 75 °C for 140 min, after which point the process was finished, yielding a total reaction time of 230 min. The advancement of the process was followed by taking viscosity measurements. Resol resins with different degrees of condensation were collected during the run; the convention used to label each sample involved combining a capital “S” with the lapsed

10.1021/ie070297a CCC: $37.00 © 2007 American Chemical Society Published on Web 09/13/2007

Ind. Eng. Chem. Res., Vol. 46, No. 21, 2007 6917 Table 1. Synthesis Stages of the Phenol-Formaldehyde Resol Synthesis synthesis time (min)

Tsolution (°C)

synthesis stage

0 30 45 90 130 185 205 230

54 89 88 86 75 75 75 75

start of formalin addition end of formalin addition sampling of resin S45 decrease of condensation temperature sampling of resin S130 sampling of resin S185 sampling of resin S205 sampling of resin S230

time (in minutes) for which the resin was sampled (e.g., “S230” represents the resin sample that was sampled for 230 min; see Table 1). The resins were kept frozen until analysis. The effect of high alkalinity on the properties of the resin solution was examined by increasing the alkalinity of resin S230 (pH 10.3, using a Consort P600 pH meter) from 4.5 wt % to 6.0 wt % with a NaOH solution (concentration 50 wt %), yielding resin S230.60 (pH 11.0), and by increasing the alkalinity of resin S230 from 4.5 wt % to 7.5 wt % with the same NaOH solution, yielding resin S230.75 (pH 11.5) after the synthesis. The properties of the modified resins were analyzed and compared to those of resin S230. NMR Experiments. The structural changes in the resol resins, as a function of the degree of condensation, were characterized by a Bruker AMX-400 NMR spectrometer (300.5 K) observing 13C at an operating frequency of 100.623 MHz. Quantitative 13C NMR spectra were measured using an inverse gated proton decoupling technique. The acquisition parameters for 13C runs were a 90° pulse of 8 µs, a 120-300 s delay time, and a number of scans ranging between 650 and 1536. The 13C spectral scales were calibrated to the central resonance line of DMSO-d6 (δ ) 39.5 ppm), which was also used as a solvent and an agent to obtain a deuterium lock. The NMR samples were prepared by dissolving 1 mL of resin in 3 mL of pure DMSO-d6 (99.8% deuterated dimethylsulfoxide, Euriso-top), and by taking the sample into a 5-mm NMR tube (VWR, 5 mm × 178 mm, >400 MHz). An integral value of 1.000 in the 13C spectra was set for the phenoxy carbon region (150-165 ppm) and all the other integrated individual signals or signal groups were normalized relative to that value. The 13C NMR signals and the signal areas of the resol resins, including formaldehyde (formalin), were identified according to earlier literature.12-18 Viscosity. The degree of condensation and the state of the process was followed by measuring the viscosities of the resol resins with a Brookfield DV-II+ digital viscometer, including a small sample adapter and an SCN-31 spindle. The viscometer was calibrated with the viscosity standards (Brookfield) of 100, 500, 1000, and 5000 mPa s. Ten-milliliter resin samples were poured into the sample adapter and placed into the sample holder, which was surrounded by a circulated-water jacket (25 °C) in the viscometer. The samples were stabilized for ∼20 min before the viscosity values were determined. Refractive Index. The advancement of the synthesis was studied by measuring the refractive indices of collected resol resin solutions with an Abbe refractometer (Erma, No. 6525, Tokyo). The tempered (20 °C) resin samples were pipetted on the tempered (20 °C) prism of the refractometer, and the refractive indices were observed. The results were the average of three parallel determinations. GPC Experiments. GPC analyses were performed to study changes in the average molar masses of the resins, as a function of the degree of condensation of the resol solution. The GPC equipment consisted of a Waters 510 pump, a Rheodyne loop

injector, and four Styragel columns (HR 0.5, HR 1, HR 3, HR 4, 300 × 0.78 mm) in series at room temperature. The GPC system was calibrated with nine polystyrene standards (weightaverage molar mass of Mw ) 2 560 000, 841 700, 320 000, 148 000, 59 500, 28 500, 10 850, 2930, or 580 g/mol (EasiCal)), and one 1-phenylhexane standard (Mw ) 162 g/mol). The resol samples were diluted in 20 mg/mL tetrahydrofuran (THF), including some acetic acid and sulfur as an internal standard, and filtered before the injection. THF was used as an eluent at a flow rate of 1.0 mL/min. The resin components, which were separated during the GPC runs, were detected using a Waters 2487 UV detector operating at 254 nm. Differential Scanning Calorimetry (DSC). The reactivity changes and thermal behavior of the resol resins with different degrees of condensation were determined by a Mettler Toledo DSC instrument. The thermograms of the curing were analyzed by STARe thermal analysis software. The resin samples (9.011.0 mg) were pipetted into the high-pressure steel pans (Mettler Toledo) and sealed with gold-plated copper seals. The closed steel pans were then heated from 25 °C to 250 °C at a heating rate of 10 °C/min. The DSC results were the average of two parallel determinations that were made with the same steel pan. The repeatability and reproducibility standard deviations of the reaction heat measurements for the resol samples are 5% and 15%, respectively.19 Dry Solids Content. The dry solids contents of the resins were determined by weighing the resin (∼1 g) on a flat bottom steel dish, and heating the resin in a static convection oven with natural ventilation at 105 °C for 3 h. After the heating period, the sample was cooled to room temperature in a desiccator (∼20 min) and then weighed again, and the percentage residues were determined. The results were the average of three parallel determinations. Contact Angle Experiments. The wettability of wood veneer by the aqueous resol resin with different degrees of condensation was studied by contact angle measurements. Static contact angle determinations were performed with a KSV Cam 200 contact angle meter at room temperature (22-24 °C) and in ambient relative humidity (RH ) 30%-40%). Veneer material was silver birch (Betula pendula, age ≈ 70 years old) felled from a fresh heath (Myrtillus-type) in southern Finland, incubated at 3540 °C, and rotary-cut into 2-mm-thick veneer sheets (135 cm × 90 cm). Areal inhomogeneity of the veneer sheet was taken into account by dividing and cutting the sheets into six sections (with dimensions of 45 cm × 45 cm) and cutting each separated section into 45 pieces (with dimensions of 9 cm × 5 cm). In addition, the “tight side” of the veneer was marked on each test piece, and the veneer pieces were mixed prior to the drying stage. The veneers were dried in a ventilated oven at 105 °C for 24 h, stored in a desiccator, and randomly chosen for the contact angle measurements. The resin samples (5-8 µL) were dropped on the “tight sides” and the earlywood areas of the test pieces by a 1-mL gas-tight syringe (Hamilton 1001TPLT syringe, Hamilton 22/51/pst3 needle). The resin drops were photographed parallel to the grain direction of the wood with a charge-coupled device (CCD) camera once per second for 100 s. The contact angles were determined from the photos by fitting the Young-Laplace curve along the outlines of the drops. The contact angle of the resin drop at 100 s was presented as the final result of the measurements. The results were the average of 50 parallel determinations for each resin. Results and Discussion Structural Changes in Resol Resin, as a Function of the Degree of Condensation of the Resin Solution. The 13C NMR

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Table 2.

13C

NMR Integration Results of Samples (S45, S130, S185, S205, and S230) Collected during Resin Synthesisa group

S45, D1 ) 120 s

S130, D1 ) 120 s

S185, D1 ) 240 s

S205, D1 ) 240 s

S230, D1 ) 300 s

1.000 0.116 0.559 4.451

1.000 0.029 0.272 4.669

1.000 0.007 0.181 4.807

1.000 0.006 0.142 4.830

1.000 0.000 0.137 4.983

0.385 1.331 1.716

0.109 1.223 1.332

0.076 1.151 1.227

0.059 1.137 1.196

0.054 1.131 1.185

0.186 0.080 0.266 2.34

0.255 0.384 0.639 0.66

0.263 0.479 0.743 0.55

0.267 0.505 0.771 0.53

0.269 0.518 0.787 0.52

0.155

0.480

0.606

0.645

0.664

1.98

1.97

1.97

1.97

1.97

aromatic carbons phenoxy free para free ortho other methylol groups para ortho total methylols methylene bridges psp′ o-p total methylene bridges psp′/o-p cross-linking ratio (total methylene bridges, relative to total methylols) F/P molar ratio a

D1 denotes the delay time.

Table 3. Viscosity, Refractive Index (RI), Average Molar Mass (Mn and Mw), and Polydispersity (PD) Values of Resol Resins Collected during the Resin Synthesis, and Viscosity of Alkalinity-Modified Resins S230.60 and S230.75 Average Molar Mass (g/mol) sample

viscosity (mPa s)

refractive index, RI

S35 S45 S65 S73 S100 S120 S130 S150 S185 S205 S230 S230.60 S230.75

22 25 40 48 99 132 165 220 457 751 1215 804 493

1.4725 1.4732 1.4780 1.4791 1.4805 1.4808 1.4821 1.4827 1.4837 1.4845 1.4845

analyses were focused on the structural changes in the resol resins, as a function of the degree of condensation of the resol solution. The structures of five resinssS45, S130, S185, S205, and S230sthat were collected during the synthesis were analyzed by determining the amounts of free para and free ortho sites, ortho methylol and para methylol groups, and para-para and ortho-para methylene bridges (see Table 2). The quantitativeness of the NMR measurements of the resins with a high degree of condensation was improved by increasing the delay times (D1) of resins S185 and S205 to 240 s, and the delay time of resin S230 to 300 s from the delay time of 120 s used for resins S45 and S130. The first analyzed resin, S45, was collected after a synthesis of 45 and 10 min after the end of the formalin addition. Surprisingly, no free formaldehyde-based signals were observed in the spectrum of resin S45. This indicated that the gradual addition of formalin in one synthesis step (30 min), and the initial alkalinity of 4.5 wt %, consumed formaldehyde effectively in the methylolation reactions with free phenol and phenolic derivatives. In addition, it showed that, after the 45-min synthesis, the reactions that occurred in the reaction solution until the end of the resol process were condensation reactions that yielded methylene bridge structures between the phenolic units. The amount of free para and free ortho sites decreased as a function of the degree of condensation of the resol solution. A higher reactivity of free para sites20,21 toward the methylol groups of phenolic units in the NaOH-catalyzed resol systems

number-average, Mn

weight-average, Mw

polydispersity index, PD

469

567

1.21

939

1525

1.62

1159 1268 1410

2208 2630 3207

1.91 2.08 2.28

was observed as a lower relative amount of free para sites, compared to free ortho sites (two available ortho sites). In the spectrum of resin S185, the number of free para sites was already very low (0.007), and at the end of the synthesis (S230), free para sites were not observed. In addition, the number of para methylol and ortho methylol groups decreased with advancing synthesis that resulted from the condensation reactions of methylol groups. The high ortho/ para methylol ratio observed for the resins reflected the higher reactivity of the para methylol groups.20-22 The amount of para methylols clearly decreased from 0.385 (S45) to 0.054 (S230) within 185 min. It suggested that the resins with a high degree of condensation and very low amounts of free para sites and para methylol groups (e.g., S205 and S230) undergo curing reactions (condensation) mainly via the reactions of reactive ortho groups. Predictably, the decreased total amount of methylol groups with an increased degree of condensation was observed as an increased total amount of methylene bridges. In resin S45, the number of para-para (p-p′) methylene bridges was still higher than that of the ortho-para (o-p) ones, because of a reactivity difference between the para and ortho groups;20-22 however, in resin S130, the amount of o-p bridges exceeded that of the p-p′ bridges. The ratio of the p-p′ and o-p methylene bridges (p-p′/o-p) drastically decreased, from 2.34 for resin S45 to 0.66-0.52 for resin S130-S230, because of the very low amounts of free para sites and para methylol groups available in the condensation reactions. The change in the degree of

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condensation of the resins can also be observed as an increase in the cross-linking ratios of the resin solutions (the total amount of methylene bridges, relative to the total amount of methylols). The alkalinity modifications of resin S230 that produced resins S230.60 and S230.75 did not have an effect on the amount of structural groups observed in the 13C NMR spectra of the resins. Kim et al.23 synthesized a wood adhesive-type resol resin with properties similar to those of the resins of this study, and they investigated the structures of the completely cured resols resins with CP/MAS 13C NMR spectroscopy. They found the methylene group/phenol molar ratios to be 1.35-1.46 in the fully cured resol resins. In the S230 solution, the corresponding ratio was 0.79, which suggested that the condensation reactions that occur during the curing of S230 should almost double the number of methylene bridges detected in the liquid state, to result in a completely cured resin. However, the amount of free para sites (0.000) and para methylol groups (0.054) was very low in the NMR spectrum of S230, which strongly indicated the curing reactions of ortho methylol groups with free ortho sites and other ortho methylol groups forming ortho-ortho methylene bridges. These bridge structures were not detected in the 13C NMR spectra of resol solutions; however, the analyses of the reported CP/MAS 13C NMR spectra of the resols23-27 indicated that ortho-ortho methylene bridges could not be completely excluded from the structures of the cured resol resins. Changes in the Viscosity of the Resol Resin, as a Function of the Degree of Condensation of the Resin Solution. The advancement of the resol resin process and the degree of condensation of the resin solutions were studied by measuring the viscosities of 11 resins (S35, S45, S65, S73, S100, S120, S130, S150, S185, S205, and S230) collected during the synthesis (see Table 3). The first resin sample (S35) was taken 5 min after the end of formalin addition, and the last resin sample (S230) was obtained when the synthesis was finished after 230 min. The viscosities of the resins are shown in Table 3. The viscosities of the resins steadily increased from 22 mPa s (resin S35) to 220 mPa s (resin S150) during the first 150 min of the synthesis. During the following 35 min, the increase of viscosity was more evident, yielding a viscosity of 457 mPa s in resin S185, which was approximately double the value of resin S150. This fast development of the viscosity continued for the last 45 min until the synthesis was finished. In the end, the viscosity was 1152 mPa s (resin S230). This increase in the viscosity of the resol solution, as a function of progressing synthesis, is well-known;11,28,29 however, the effects of different structural groups on the development of viscosity was studied by presenting the amounts of structural groups of resins (S45, S130, S185, S205, and S230) and viscosity, as a function of synthesis time (see Figure 1). This figure clearly shows that changes in the structure of the resol solution were minor after 185 min of processing, because of a high degree of condensation. However, only a slight change in the total amount of methylene bridges caused a significant viscosity increase in the resin solution. This was observed by examining the total amounts of methylene bridges 0.743 (resin S185), 0.771 (resin S205), and 0.787 (resin S230) in Table 2 with their corresponding viscosities in Table 3: 457 mPa s (resin S185), 751 mPa s (resin S205) and 1215 mPa s (resin S230). The effect of alkalinity on the viscosity of the resol resin solution was studied in two experiments, in which the alkalinity of resin S230 was increased with a 50 wt % NaOH solution, from 4.5 wt % to 6.0 wt % (S230.60) and 7.5 wt % (S230.75).

Figure 1. Amount of structural groups in the resol resin (S45, S130, S185, S205, and S230) and the viscosity of the resin solution, as a function of synthesis time. Note a: The number of phenoxy carbons in the aromatic carbon region of 13C NMR spectra was set at 1.0.

Figure 2. Gel permeation chromatography (GPC) distribution curves of the resol resin solutions (S45, S130, S185, S205, and S230).

The viscosity of resin S230 substantially decreased from 1215 mPa s to 804 mPa s in S230.60, and to 493 mPa s in S230.75, showing the efficiency of the NaOH solution in viscosity adjustment of the resin solution (see Table 3). This decreased viscosity mainly resulted from the reactions of additional NaOH with the OH groups of phenolic derivatives improving the water solubility of the system. Changes in the Average Molar Masses (Mn and Mw), Polydispersity, and Refractive Index of the Resol Resin, as a Function of the Degree of Condensation of the Resin Solution. The effect of the degree of condensation of the resol solution (S45, S130, S185, S205, and S230) on the numberaverage molar mass (Mn), weight-average molar mass (Mw), and polydispersity index (PD) of the resin was determined using GPC (see Table 3). The quantitativeness of the molar mass determinations of the resol resins is limited by known error sources;11,30,31 however, GPC is a useful tool in the characterization of the different resol resins. Predictably, the Mn and Mw values of the resin solution increased as a function of synthesis time,11,28,29 indicating polycondensation reactions and the formation of methylene bridge structures between the phenolic derivatives, thus confirming the results of the analyses previously described. The Mw value of the resol solution increased faster than the Mn value during the synthesis. As a result, the PD value increased, which denoted a broadening of the molar mass distribution in the resin solution. The GPC distribution curves of resins S45, S130, S185, S205, and S230 are presented in Figure 2. A comparison of the shapes and retention times of the curves clearly shows the development of molar masses, as a function of the degree of condensation. The relative amount of high-molar-mass species with a short retention time increased as the degrees of condensation increased.

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Figure 3. Number-average molar mass (Mn), weight-average molar mass (Mw), and total amounts of methylol groups, methylene bridges, and free reactive sites of the resol resin solutions (S45, S130, S185, S205, and S230), as a function of synthesis time. Note a: The number of phenoxy carbons in the aromatic carbon region of 13C NMR spectra was set at 1.0.

The viscosity of the resin solution increases as the average molar masses (Mn and Mw) increase, if no resin modifications (i.e., addition of additives or evaporation of water) are performed. In this study, the relation between the viscosity and average molar masses was clearly observed by examining the viscosity, Mn, and Mw values of the resol resin solutions (S45, S130, S185, S205, and S230), as a function of synthesis time. At the end of the process, the viscosity of the resin solution increased almost exponentially, but the changes in Mn and Mw were minor. This indicates that the slight additional branching or lengthening of the polymer chain strongly affects the viscosity development of the resin solution. Figure 3 shows the average molar masses (Mn and Mw) and total amounts of methylol groups, methylene bridges, and free sites (S45, S130, S185, S205, and S230), as a function of synthesis time. The decrease in the amounts of the methylol groups and free sites, and the increase in the amount of methylene bridges due to the condensation reactions, increased both the Mn and Mw values. By the end of the synthesis, a minor increase in a total number of methylene bridges affected the value of Mw more than that of Mn. The refractive index (RI) of the resin increased as the viscosity and molar masses each increased during the synthesis (see Table 3). The RI value of 1.4725 in S35 could be considered typical for a resin at that stage of resol synthesis.1 In the viscosity range of 400-1215 mPa s, the changes in RI were minor, which indicated only slight structural changes in the resol solution. This was consistent with the results of the NMR analyses previously discussed. The RI values of the resol resins could deviate significantly from each other at the end of different production processes, because the synthesis parameters (such as the degree of condensation (structure), alkalinity, additives, and solids content) contribute to the formation of the RI value. However, RI analysis, along with the viscosity measurement, could be used to evaluate the advancement of the process and the degree of condensation of the resol resin solution during the synthesis. Changes in the Thermal Behavior of the Resol Resin, as a Function of the Degree of Condensation of the Resin Solution. The reactivity (∆H) of the resol resin is affected, for example, by the amount of catalyst (alkalinity),32-34 the F/P molar ratio12,33,35 and the degree of condensation36 of the solution, which complicates the comparison of thermal behavior in different resin systems. In this study, the degree of condensation of the resin was the only variable parameter that facilitated the interpretation of the DSC data.

Figure 4. Reaction heat (∆H) and total amounts of free sites, methylol groups, and methylene bridges of the resol resin solution (S45, S130, S185, S205, and S230), as a function of synthesis time. Note a: The number of phenoxy carbons in the aromatic carbon region of 13C NMR spectra was set at 1.0. Table 4. Reaction Enthalpies, Peak Temperatures, Onset, and End Temperatures of the Curing of Resol Resins Collected during the Resin Synthesis, and Those of Alkalinity-Modified Resins S230.60 and S230.75 Temperatures (°C) sample

reaction enthalpy, ∆H (J/g)

peak, T1

onset

end

S35 S45 S65 S73 S100 S120 S130 S150 S185 S205 S230 S230.60 S230.75

-286 -285 -238 -217 -208 -190 -185 -179 -167 -161 -134 -121 -114

146 146 147 147 147 147 146 147 146 146 147 150 154

130 130 130 130 128 128 128 128 126 126 127 117 121

166 166 166 166 165 166 165 166 166 165 167 175 180

The thermal behavior of 11 resol resin solutions (S35, S45, S65, S73, S100, S120, S130, S150, S185, S205, and S230) during the curing stage was studied by analyzing the resins with a DSC instrument at 25-250 °C. The DSC measurements were focused on the change in the reactivity (∆H), the curing peak temperature (T1), the start temperature (Onset), and the end temperature (End) of the curing, as a function of the degree of condensation of the resin solution (see Table 4). According to the NMR data, the highly exothermic methylolation reactions were completed (no free formaldehyde observed) when resin S45 was collected. Thus, the released reaction heats observed in the DSC measurements of the curing resulted from the formation of methylene bridges in the condensation reactions of the methylol groups with free sites of phenol and other phenolic species, and from the reactions of two methylol groups forming dibenzyl ether bridges.12,32,37 The reactivity of the resin decreased as a function of the degree of condensation of resin solution from -286 J/g for resin S35 to -134 J/g for resin S230. However, the peak temperatures (∼147 °C), onsets (∼128 °C), and end temperatures (∼166 °C) of the curing were not significantly affected by the changes in the degree of condensation. Figure 4 presents the reactivity of the resol solution (for resins S45, S130, S185, S205, and S230) and the total number of structural groups (free sites, methylol groups and methylene bridges), as a function of synthesis time. The reactivity of the resin decreased as a function of the decreasing amount of free sites and methylol groups, and as a function of the increasing

Ind. Eng. Chem. Res., Vol. 46, No. 21, 2007 6921 Table 5. Viscosities, Solids Contents, and Contact Angles of Resin Solutions (S45, S130, S185, S205, S230) and Those of Alkalinity-Modified Resins (S230.60 and S230.75) on Silver Birch Veneer at 100 s

Figure 5. Differential scanning calorimetry (DSC) curves of resins S35, S45, S65, S73, S100, S120, S130, S150, S185, S205, and S230 collected during the resol resin synthesis.

amount of methylene bridge structures. The structural differences between resins S185, S205, and S230 were minor; however, a decrease in the reactivity was more evident, as a result of the decrease in the number of reactive groups. The alkalinity modifications of resin S230, yielding resins S230.60 and S230.75, showed the effects of higher pH on the thermal behavior of the resin system (see Table 4). The reactivities of the modified resins decreased slightly with the addition of extra NaOH solution, and the peak temperatures (T1) and end temperatures of the curing shifted to temperatures that were higher than those of resin S230. In addition, lower onset temperatures, which also have been observed in an earlier study,27 and higher end temperatures of modified resins (compared with those of resin S230) resulted in a wider temperature range in the curing stage. The retardation effect of higher alkalinity on the reactivity of resol resin was also observed in earlier reports, in which the gel time of the resin increased with a higher NaOH/phenol molar ratio.29,35 This could be explained by the formation of intermolecular chelate structures that contain the hydroxyl group of the phenolic ring, a Na+ ion, and a hydroxyl group of ortho-methylol, which probably retard or hinder the condensation reactions of orthomethylol substituents.35 The DSC curves of the resins (S35, S45, S65, S73, S100, S120, S130, S150, S185, S205, and S230) are shown in Figure 5. The shapes of the exotherms changed slightly as a function of synthesis time. In the curves of low-degree-of-condensation resins S35 and S45, minor shoulder peaks next to the main curing peak were observed; however, other exotherms showed only one signal, which is common for high-alkalinity resol resins.32 The shoulder peaks of resins S35 and S45 could be due to reactions of methylol groups with free reactive sites of phenolic components still available and distinguishable in the DSC experiments. The second large and well-separated curing signal, occasionally observed in the exotherms of the resol resins at ∼190225 °C, is attributed to the condensation reactions of dibenzyl ether bridges forming methylene bridges via the release of formaldehyde. However, the formations of dibenzyl ether bridges are very unlikely in resin systems with high alkalinity,32,38 and these phenomena were not observed in this study. In the earlier studies, a second large exotherm at 190225 °C was observed when the F/P molar ratio of the resol resin was increased from 1.90-2.00 to 2.15-2.3012 and the alkalinity of the resin was low (1.5 wt %).32 In addition, it has been suggested that the changes in the Mw value affect the number of exotherms observed in the thermograms of DSC.35 In our study, the F/P molar ratio and the alkalinity were the same for

sample

viscosity (mPa s)

solids content (wt %)

contact angle, CA (°)

S45 S130 S185 S205 S230 S230.60 S230.75

25 165 457 751 1215 804 493

51.7 53.5 54.0 54.1 54.2

81.2 ( 5.6 119.9 ( 5.7 128.3 ( 5.5 130.7 ( 5.0 138.7 ( 6.3 137.8 ( 5.8 137.2 ( 5.5

all of the resins, but the Mw value increased as a function of the degree of condensation, yielding only one exotherm in the thermogram of the studied resins. Our results do not support the claim35 that the differences in the Mw values of the resins affect the number of exotherms observed in the DSC curves of the resol resins. Change in the Contact Angle of the Resol Resin on Silver Birch Veneer, as a Function of the Degree of Condensation of the Resin Solution. Adequate wetting of the wood veneer surfaces by an aqueous resin solution is a fundamental requirement for a generation of strong adhesive joints. The resins must wet the surfaces of the veneers well but not penetrate too deeply into the wood, which would result in poor adhesive joints39 and an increased consumption of resin. Several internal and external parameters affect the surface properties of the wood and complicate the study of the interaction (wetting) of the wood surfaces with the resin solutions. The inhomogeneity of the veneer surface is a sum of numerous factors, which must be taken into account in the study of wetting. The surface roughness of the veneer surface (and, furthermore, the wetting of the veneer) could be affected by different types of mechanical treatments.40-42 Surface defects, such as lathe checks and flaws, are caused by the rotary-cut conditions and affect the penetration and spreading of the liquids.43 The slicing blade of the lathe produces veneer sheets with two types of sidessa “tight” side and a “loose” sideswhich behave differently in contact with liquids.43,44 The porosity differences between the areas of earlywood and latewood, as a result of different diameters of wood cells, influence the penetration and spreading of the liquids into the veneers.44-46 Aging of the wood material results in the migration of the extractives on the surfaces of wood47,48 and loss of extractives from the surfaces, which affects the wetting of the wood. In addition, the wettability of wood is influenced by the moisture content49 and grain direction of the veneer.50 In this study, the effects of the inhomogeneity of the veneer surfaces on the contact angle measurements were minimized by drying the veneers completely in a ventilated oven51 and measuring the contact angles of the resol drops parallel to the grain direction on the earlywood region and “tight sides” of the silver birch veneers. One piece of veneer sheet was used for ∼5 contact angle measurements. The wetting of the veneer surfaces was studied by determining the contact angles (CA) of five aqueous resol resin (S45, S130, S185, S205, and S230) solutions with different degrees of condensation on silver birch veneer (see Table 5). In addition, the effects of the modification of the alkalinity and solids content of the resin on the contact angles were evaluated (Table 5). Because of the penetration and spreading of the resin solution on the wood, the apparent CA changed as a function of time.41,50 The decrease in the CA of the resin was very fast during the first 10 s of measurement, after which point the CA value slowly

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Figure 6. Contact angle (CA) of the resol resin solution (S45, S130, S185, S205, and S230) on silver birch veneer, as a function of the viscosity of the resin solution.

stabilized. The CA of the resin drop at 100 s was presented as a final result (50 parallel determinations) in Table 5. The CA values on silver birch veneer increased as a function of the degree of condensation of the resol resin solution (see Table 5). The inhomogeneity of the veneer surfaces was observed as standard deviations of ∼5°-6° in the CA value. The low-viscosity and low-molecular-weight resin S45 yielded the lowest CA (81.2°) and wetted the surface of the silver birch veneer best. The CA of resin S130 (119.9°) was ∼40° larger than that of resin S45 (81.2°), but the increase in the CA values of the resins collected after resin S130 slowed. Resin S230, with the highest degree of condensation, resulted in the highest CA (138.7°) that wetted the silver birch veneer poorly. The CA values of the resins observed in our study were higher compared with earlier reports,28,39,44,45 mainly because of differences in, for example, wood species, resins, moisture content, and the surface structure of the wood. Figure 6 presents the CA values of resins S45, S130, S185, S205, and S230; the bars denote the standard deviations, as a function of the viscosity of the resin solution. The CA values of the resin increased as the viscosity increased; however, the changes in the CA values were minor after a viscosity of ∼400 mPa s. The increase in the molar mass (Mn and Mw) of the resin caused an increase in the CA, reducing the wetting of the veneer. This was in accordance with the effects of the viscosity on the CA values of the resins and showed the importance of molar mass in the wetting behavior of resin. In this study, the reducing effects of the viscosity and molar mass of the resin on the wetting of the wood surface were consistent with the literature.28,40,46 The effect of higher alkalinity on the wetting of wood was studied by measuring the CA values of resins S230.60 and S230.75 on the silver birch veneer (see Table 5). Surprisingly, the CA values of resins S230.60 (137.8°) and S230.75 (137.2°) were similar to that of resin S230 (138.7°), despite the significant differences in the viscosities of the solutions (see Table 3). The lower viscosities of resin S230.60 (804 mPa s) and S230.75 (493 mPa s), compared with the viscosity of S230 (1215 mPa s), were expected to promote the wetting of the veneer surface. The surface tension of the resin solution increases as the alkalinity (NaOH/phenol molar ratio)28,39 and Mn value each increase.28 In addition, high-viscosity resol resins with lower alkalinity wet the surface of the wood (southern pine) better than high-viscosity resins with higher alkalinity.28 In our study, the average molar masses and structures of the resin S230, S230.60, and S230.75 solutions were the same, which indicated that the structure of the resin strongly contributed to the formation of the CA of the

resin on the wood veneer. In addition, the enhancing effect of the increased mobility (lower viscosity) on the wetting behavior of the modified systems was partially eliminated by the probable increase in the surface tensions of resins S230.60 and S230.75, resulting in CA values similar to that of resin S230. The solids contents of the resins (S45, S130, S185, S205, and S230), which were determined by heating the solutions at 105 °C for 3 h and weighing the residues, were almost the same (see Table 5). The solids content of resin S45 (51.7 wt %) with the lowest viscosity (25 mPa s) deviated slightly from the solids contents of other resins. However, the results of the solids content determinations indicated that slight differences in the solids contents of the resins did not have an effect on the CA values and wetting of the veneer surfaces, which was in accordance with the literature.45 The addition of extra NaOH solution into the alkalinity-modified resins S230.60 and S230.75 did not have a significant effect on the solids content of the resin solution, because of the 50 wt % NaOH solution used in the modifications. Conclusions The progress of separate resin syntheses, in which the molar ratios of phenol and formaldehyde, reaction temperatures, and catalyst concentrations are varied, is not necessarily similar, which complicates direct comparison of the products of these syntheses and causing much uncertainty in the conclusions of the results. The experimental arrangement of the present study, in which the degree of condensation of the resin solution was the only changing parameter during the batch synthesis, facilitated the interpretation of the analysis data, allowing straightforward and reliable comparison of different resins that have the same molar ratio of phenol to formaldehyde but different condensation properties. The structural characterization of the resins by 13C NMR indicated that the gradual addition of formalin and the initial alkalinity of 4.5 wt % effectively consumed the fed formaldehyde (F/P ) 2.0) in the methylolation reactions with phenol and phenolic derivatives. The addition of formalin in one process step could shorten the synthesis time required in the large-scale reactors and improve the quality of the final products, compared to the multistage processes. In addition, a very low concentration of free formaldehyde reduces the release of harmful formaldehyde fumes (for example, during the manufacture of plywood), and the amount of urea, which is usually needed to bind free formaldehyde in the resol resin solutions, also can be decreased. The number of free reactive sites and methylol groups decreased as the degree of condensation increased, yielding methylene bridge structures. By the end of the synthesis, the very low amounts of reactive free para sites and para methylol groups strongly suggested that the reactions that occurred during the curing stage could include the reactions of ortho methylol groups with free ortho sites and other methylol groups forming ortho-ortho methylene bridges not detected in the 13C NMR spectra of the resol solutions. The increase in the number of methylene bridges, as a function of the degree of condensation, denoted oligomerization of phenolic units, resulting in the increased viscosity, average molar masses (Mn and Mw), and refractive index value. The high degree of condensation decreased the reactivity of the resin, but the peak temperatures, onset temperatures, and end temperatures (∼166 °C) of the curing were not significantly affected by changes in the degree of condensation. This result clarifies and facilitates the understanding of the effects of different additives and resin modifications on thermal properties of the resol resins.

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The wetting behavior of the resol resin on wood veneer was greatly influenced by the structure (viscosity and average molar masses) of the resol solution, so that low viscosities and low average molar masses promoted the wetting of the veneer surfaces. The effects of the alkalinity modifications on the properties of the high-viscosity resin were slightly controversial. Additional NaOH solution effectively decreased the viscosity of the resin solution and reduced the reactivity of the resin system in the curing stage, but it did not affect the structure or the wetting behavior of the resin solution. This indicated that NaOH solution could be effectively used in those applications, in which the processability of the high-viscosity resins must be adjusted without affecting the wetting abilities of the resin system. Generally, the results of this study indicate that the structure of the resol resin fundamentally defines the physical properties of the resins observed in the analyses. By knowing the structure of the resin, rather specific evaluations and predictions concerning the properties of the resol resins can be performed, which facilitates the design and completion of the resin synthesis. Literature Cited (1) Knop, A.; Pilato, L. A. Phenolic Resins: Chemistry, Applications and Performance; Springer-Verlag: Berlin, 1985. (2) Gardziella, A.; Pilato, L. A.; Knop, A. Phenolic Resins: Chemistry, Application, Standardization, Safety and Ecology; Springer-Verlag: Berlin, 2000. (3) Bouajila, J.; Raffin, G.; Waton, H.; Sanglar, C.; Paisse, J. O.; GrenierLoustalot, M-F. Phenolic ResinssCharacterizations and Kinetic Studies of Different Resols Prepared with Different Catalysts and Formaldehyde/Phenol Ratios. Polym. Polym. Compos. 2002, 10, 341. (4) Astarloa-Aierbe, G.; Echeverria, J. M.; Martin, M. D.; Etxeberria, A. M.; Mondragon, I. Influence of the Initial Formaldehyde to Phenol Molar Ratio (F/P) on the Formation of a Phenolic Resol Resin Catalyzed with Amine. Polymer 2000, 41, 6797. (5) Grenier-Loustalot, M.-F.; Larroque, S.; Grande, D.; Grenier, P.; Bedel, D. Phenolic Resin: 2. Influence of Catalyst Type on Reaction Mechanism and Kinetics. Polymer 1996, 37, 1363. (6) Astarloa-Aierbe, G.; Echeverria, J. M.; Vazquez, A.; Mondragon, I. Influence of the Amount of Catalyst and Initial pH on the Phenolic Resol Resin Formation. Polymer 2000, 41, 3311. (7) Drumm, M. F.; LeBlanc, J. R. The Reactions of Formaldehyde with Phenols, Melamine, Aniline, and Urea. In Step-Growth Polymerizations; Solomon, D. H., Ed.; Marcel Dekker: New York, 1972; pp 227-246. (8) Manfredi, L. B.; Riccardi, C. C.; de la Osa, O.; Vazquez, A. Modelling of Resol Resin Polymerization with Various Formaldehyde/ Phenol Molar Ratios. Polym. Int. 2001, 50, 796. (9) Grenier-Loustalot, M-F.; Larroque, S.; Grenier, P. Phenolic Resins: 1. Mechanisms and Kinetics of Phenol and of the First Polycondensates Towards Formaldehyde in Solution. Polymer 1994, 35, 3046. (10) Astarloa-Aierbe, G.; Echeverria, J. M.; Riccardi, C. C.; Mondragon, I. Influence of the Temperature on the Formation of a Phenolic Resin Catalyzed with Amine. Polymer 2002, 43, 2239. (11) So, S.; Rudin, A. Analysis of the Formation and Curing Reactions of Resole Phenolics. J. Appl. Polym. Sci. 1990, 41, 205. (12) Holopainen, T.; Alvila, L.; Rainio, J.; Pakkanen, T. T. PhenolFormaldehyde Resol Resins Studied by 13C-NMR Spectroscopy, Gel Permeation Chromatography, and Differential Scanning Calorimetry. J. Appl. Polym. Sci. 1997, 66, 1183. (13) Luukko, P.; Alvila, L.; Holopainen, T.; Rainio, J.; Pakkanen, T. T. Optimizing the Conditions of Quantitative 13C-NMR Spectroscopy Analysis for Phenol-Formaldehyde Resol Resins. J. Appl. Polym. Sci. 1998, 69, 1805. (14) Fisher, T. H.; Chao, P.; Upton, C.; Day, A. J. One- and TwoDimensional NMR Study of Resol Phenol-Formaldehyde Prepolymer Resins. Magn. Reson. Chem. 1995, 33, 717. (15) Fisher, T. H.; Chao, P.; Upton, C.; Day, A. J. A 13C NMR Study of the Methylol Derivatives of 2,4′- and 4,4′-dihydroxydiphenylmethanes Found in Resol Phenol-Formaldehyde Resins. Magn. Reson. Chem. 2002, 40, 747. (16) Park, B.-D.; Riedl, B. 13C-NMR Study on Cure-Accelerated Phenol-Formaldehyde Resins with Carbonates. J. Appl. Polym. Sci. 2000, 77, 1284.

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(44) Schupe, T. E.; Hse, C. Y.; Choong, E. T.; Groom, L. H. Effect of Wood Grain and Veneer Side on Loblolly Pine Veneer Wettability. Forest Prod. J. 1998, 48, 95. (45) Hse, C.-Y. Wettability of Southern Pine Veneer by Phenol Formaldehyde Wood Adhesives. Forest Prod. J. 1972, 22, 51. (46) Scheikl, M.; Dunky, M. Measurement of Dynamic and Static Contact Angles on Wood for the Determination of its Surface Tension and the Penetration of Liquids into the Wood Surface. Holzforschung 1998, 52, 89. (47) Wålinder, M. E. P. Study of Lewis Acid-Base Properties of Wood by Contact Angle Analysis. Holzforchung 2002, 56, 363. (48) Nussbaum, R. M. Natural Surface Inactivation of Scots Pine and Norway Spruce Evaluated by Contact Angle Measurements. Holz Roh. Werkst. 1999, 57, 419.

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ReceiVed for reView February 27, 2007 ReVised manuscript receiVed July 24, 2007 Accepted July 25, 2007 IE070297A