Ind. Eng. Chem. Res. 1994,33, 693-697
693
Characterizations of Phenol-Formaldehyde Resol Resins Robert A. Hauptt and Terry Sellers, Jr.' Mississippi Forest Products Laboratory, Mississippi State University, Mississippi State, Mississippi 39762-5724
A series of phenol-formaldehyde resol (plywood type) resins was made with various formaldehyde t o phenol molar ratios (1.70,2.00, and 2.30) and sodium hydroxide to phenol molar ratios (0.40 and 0.60). A broad viscosity range (20-1000 mPa.s) was sampled from each resin cook and characterized for pH, gel time, viscosity, alkalinity, nonvolatiles, density, molecular weight, surface tension, and contact angle. The results showed that viscosity and gel time depended on weight-average molecular weight. Condensation rate varied with formaldehyde to phenol ratio. The Cannizzaro reaction reduced alkalinity and indirectly affected viscosity and gel time. Cook procedure affected molecular weight distribution and accelerated gel times.
Introduction Phenol-formaldehyde (PF) resins are used as adhesives for wood compositessuch as plywood, oriented strandboard (OSB), waferboard, hardboard, particleboard, and laminated veneer lumber. Worldwide, approximately 700 kt of PF resin solids are used for these purposes, and about half of that volume is used in the United States (Sellers, 1992). The principal PF resins useful for wood binders are alkaline-catalyzed resols and, to a lesser extent, acidcatalyzed novolaks. Novolaks are made with formaldehyde to phenol (F:P) molar ratios of less than 1.0 (typically 0.75-0.85), while resols have theoretical F:P molar ratios between 1.0 and 3.0 (Knop and Pilato, 1985). In practice for wood adhesives, resols usually have F:P molar ratios between 1.6 and 2.6. Resols are thermosetting polymers which, when used as wood adhesives, will normally cure adequately without the addition of any catalyst other than heat. The three resol polymerization stages are: addition, condensation, and curing (Knop and Pilato, 1985). Addition of formaldehyde to phenol forms methylolphenols. Methylolphenols condense to diphenylmethanes and low molecular weight polymers. With the application of heat, the various molecular weight polymers condense further to form a rigid cross-linked network. Gel permeation chromatography (GPC) is very useful for characterizing the molecular weight distributions (MWD)of PFresins used as wood binders (Armonas,1970). To investigate the effects of reaction parameters on resol composition, tetrahydrofuran (THF) mobile phase and cross-linked polystyrene gels have been used (Duval et al., 1972). Wellons and Gollob (1980)described problems with THF solvent systems used for GPC with cross-linked polystyrene gels and low-angle laser light scattering (LALLS) detection, This work demonstrated the difficulty of dissolving resols in THF, especially with precipitated or dried resins. The LALLS procedure was judged to be useful for determining absolute weight average molecular weight (M,)but limited to molecular weights greater than 1000with variation depending on the solventpolymer compatibility. Bain and Wagner (1984)developed a method for dissolving resols in THF with 10% trichloroacetic acid and ran GPC on a cross-linked polystyrene gel system. King et al. (1974) demonstrated the utility of GPC in relating F:P molar ratios, catalyst type, and reaction time to MWD and nuclear magnetic resonance (NMR) analysis. + Present address:
Dyno Industrier A.S.,Lillestrom,Norway.
0000-5S05/94/2633-0693$04.50/0
Heat curing further condenses and cross links the P F prepolymers to the actual wood-bonding polymer. The curing reactions, however, may involve other mechanisms that differ from condensation reaction mechanisms (Knop and Pilato, 1985). Christiansen and Gollob (1985) used differential scanning calorimetry to study the effects of resin formulation parameters on thermal curing characteristics. One conclusion of this work was that addition occurred between 98 and 129 "C, while condensation occurred between 139 and 151 OC. The work associated the higher temperatures of the condensation reaction (around 150 "C) with lower F:P ratios and higher sodium hydroxide to phenol (Na0H:P) ratios. By using differential thermal analysis, Chow et al. (1975) related the thermal curing properties of resols to the F:P ratios and to the plywood bond qualities. Thermal softening temperatures and methylol content increased with increasing F:P molar ratios. One conclusion of this work was that higher Na0H:P levels required higher temperatures to cure. In this study, plywood bond quality diminished below F:P = 1.4, and certain minimum temperatures were required to achieve complete cure. Dynamic mechanical measurements characterize the viscoelastic properties of the curing prepolymer resin. Kelley et al. (1986)applied torsional braid analysis to study the dynamic mechanical properties of curing resols as a function of formulation variables. This method allowed study of the progression of reaction from the liquid polymer through gelation and on to vitrification, the point when the polymer becomes a molecularly immobilized amorphous glass. Initial rigidity of the uncured polymer increased with M , and viscosity. Final rigidity of the cured polymer was highest with high F:P ratios and high Na0H:P ratios. Young (1986) employed dynamic mechanical analysis (DMA) to determine rate and degree of cure for P F resins. DMA degree of cure correlated with solidstate NMR measurements of the extent of polymerization and cross linking. There was evidence of a correlation between oriented strandboard internal bond strengths with the DMA results. Kim and Nieh (1991) examined temperature and resin composition effects on resol cure and vitrification with DMA. This study showed that vitrification occurs after a greater degree of polymerization for higher cure temperatures than for lower ones. The objective of this work was to characterize some of the interactions of the physical and chemical properties of a series of PF resol resins used to bond wood. Resin interaction with southern pine wood and the effect of wood surface treatments on wetting were also studied and reported elsewhere (Haupt and Sellers, 1993). 0 1994 American Chemical Society
694 Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994
Experimental Methods Resin Synthesis. A P F resin series was synthesized with varying F:P and Na0H:P molar ratios. The resin F:P ratios were 1.70, 2.00, and 2.30, while the Na0H:P ratios were 0.40 and 0.60. Two replicates of the six variable combinations were synthesized for a total of 12 resins. All formulations had 50 % theoretical solids. The materials used were 100% phenol, 43 % formalin, and 50 % NaOH; all reagents were technical grade. The cooks were conducted in a round bottom glass reactor equipped with a mechanical agitator, a reflux condenser, a thermometer, and a sample port. Temperature was controlled with an electric heating mantle and an ice water cooling bath. Large samples (200-500 mL) were drawn by vacuum aspiration and small samples (10-20 mL) by pipette. The initial charge consisted of all the phenol, enough NaOH for a Na0H:P ratio of 0.20, and 200 g of water; the initial temperature was between 45 and 50 "C. Formalin was charged over 30 min, along with 150-200 g of water. The reaction was allowed to heat to 60 "C and controlled at that temperature through 60 min of the cook. Then the remaining NaOH and water were charged, and the temperature was increased to 85 "C. In order to separate the effects of sodium hydroxide molecular weight interactions on viscosity, samples were taken during each synthesis at representative viscosities of about 20, 100, 400, and 1000 mPa.s (cP). Once the final viscosity was reached, the resins were rapidly cooled. Resin C haracterization. Four samples from each cook were characterized for gel time, viscosity, density, pH, nonvolatile solids, alkalinity and molecular weight. Thus, resin formulation characteristics could be compared with graphical plots of properties against molecular weight. Lotus Freelance Plus software was used to fit the X-Y data with the appropriate linear, logarithmic, power, or exponential equation to give the highest r2value possible. Other details can be obtained from Haupt (1992). The sample run order was randomized for each test. Small test portions were removed, frozen to minimize polymerization during storage, and thawed prior to testing. Gel Time. Gel time was characterized with a Techne Gelation Timer using 10-mLresin samples in 17-mm-wide test tubes in a boiling water bath (100 "C). The gel time was recorded when a metal plunger ceased moving through the resin and tripped a timer. Viscosity. Viscosity was measured with a viscometer (Viscometers UK Ltd.) that had a small sample adapter. The sample adapter was connected to a constant temperature bath (MGW Lauda C6) set at 25 OC. Samples were equilibrated to constant temperature for 5 min before reading the viscosity. Low viscosities (less than 150mPa.s) were taken with a number 5 spindle at 1.25rad/s (12 rpm). High viscosities were taken with a number 7 spindle at 1.25 rad/s (12 rpm). Samples were 8 and 10 mL, respectively. Density. Density (g/cm3) was measured with a Paar DMA 46 Calculating Digital Density Meter at 25/4 O C and converted to 25/25 "C density by multiplying by a conversion factor of 1.00294. Measurements were made after 4 min equilibration. pH. The pH was measured with a Radiometer Copenhagen PHM Standard pH meter. The electrode was calibrated with pH 7 and 11 standards. Measurements were taken after 2 min of equilibration. Samples were at ambient temperature. Nonvolatile Solids. Two 1-g resin samples were dispensed in two tared metal pans and weighed to 0.0001 g on a Mettler analytical balance. The samples were placed
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Figure 1. Plot of pH against number-average molecular weight. Table 1. Acid Dissociation Constants for Methylolphenols compound
exptl pKa value
thermodyn pK. value
phenol 2-methylolphenol 4-methylolphenol 2,4-dimethylolphenol 2,6-dimethylolphenol 2,4,64rimethylolphenol
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Table 2. Theoretical Percent Alkalinity of the Formulations ~
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F:P ratio
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4.96% 4.70 % 4.47%
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in a forced air convection oven and heated at 120 OC for 2 h. After removal, the samples were placed in a desiccator to cool prior to weighing. The weight was measured, and percent nonvolatile resin solids (NV) were calculated. The two NV values were averaged. Alkalinity. Alkalinity samples were prepared by diluting 1g of resin solution with 75 mL of distilled water. The resin solution was weighed to the nearest 0.01 g in a 150-mLflask. A magnetic stir bar was placed in the flask, which was then put on a magnetic stirring plate. A pH electrode was placed in the solution and was titrated with 0.1 mol/L hydrochloric acid (HC1) to a pH 4.5 end point. Percent alkalinity was calculated from the resin weight added and the HC1 volume consumed. Molecular Weight. Weight-average molecular weight (AI,), number-average molecular weight (AIn), and polydispersity (Mw/Mn)were determined by GPC on 1000-, 50-, and 10-nm p-Styragel columns in series. The 30-cmlong columns had 7-mm internal diameter. The mobile phase was THF and a proprietary additive of Dyno Industrier. Flow rate was 1mL/min. The columns were calibrated with 34500-, 5000-, 1250-, 446-, and 162molecular weight (MW) standards. The 34 OOO-, 5000-, and 1250-MWstandards were polystyrenes, while the 446MW standard was n-decyl phthalate, and the 162-MW standard was 1-phenylhexane. A Waters 481 UV detector was set at 254 nm. The pump was a Waters 510 with a Rheodyne injector. Samples were diluted to 20 mg/mL in the mobile phase and filtered, and 50 p L was injected. The resulting chromatograms were analyzed for M,, Mn, and MwIMn. Results and Discussion pH. Resin pH increased with increasing Na0H:P and Mn but decreased with higher F:P mole ratios (Figure 1). Accordingly, the methylolphenol composition should become more acidic as addition progresses until conden-
Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 695 1rn.
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Figure2. Plot of gel time against log weight-averagemolecular weight extrapolated to 0 min gel time. 5000
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Figure 4. Plot of percent alkalinity against log number-average molecular weight.
sation begins, when the more acidic methylolphenols convert to dihydroxydiphenylmethanes (DPM). DPM structures probably favor higher pKa values since they have fewer electron-withdrawing methylol groups. Methylolphenol pKa values as determined by Sprengling and Lewis (1953) are shown in Table 1. These data indicate that higher F:P ratios will favor higher methylolation and therefore lower PKa values. M,, probably affected pH indirectly, since the level of methylol saturation would increase and affect the composition of methylolphenol pKavalues. Two factors could contribute to the lower pH associated with higher F:P levels. One, the high F P ratio formulations had less NaOH as a percentage of total material at 50 5% theoretical solids (Table 2). Two, high F:P mole ratios favor production of the most acidic methylolphenols, which would also reduce PH. Gel Time. Gel time decreased with higher F:P and M , but increased with higher Na0H:P. Tohmura et al. (1991) concluded that PF condensation is second order with respect to methylol concentration. Chow et al. (1975)found increasing methylol content with F:P ratios from 0.75 to
43.0 200
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0.40
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Figure 7. Plot of percent nonvolatilee against log number-average molecular weight.
2.40 for similar MW. Consequently, higher F:P ratios should have higher methylol content, more cross linking, faster condensation, and quicker gel times. Gelation occurs more rapidly with increasingM , because the polymer is closer to infinite network formation, which is one definition of gelation (Rodriguez 1989). Full gel conversion of resin may predictably be incomplete and unnecessary for most uses; minor quantities of unreacted monomers may be present within the network (Encyclopedia of Polymer Sci. & Tech., 1966). Extrapolation of the gel time resulta to 0 min suggested that gelation occurred between a M , of 10 000 and one of 50 000 g/mol (Figure 2). Assuming the validity of the extrapolation, the gel could be a high molecular weight colloidal association. Colloidal materials of finite M , are known to form such associative gels (Encyclopedia of Polymer Sci. & Tech., 1966). Hydrogen bonds could enhance this association. High Na0H:P may have slowed the condensation reaction by diluting the PF concentration. The higher phenoxide concentration could also inhibit associative
696 Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994
15
20
Retention Time (min)
25
30
Figure 8. GPC chromatograms: molecular weight distribution alterations.
gelation by reducing phenolic hydroxyls available for hydrogen bonding. Viscosity. The results showed that viscosity increased exponentially withM, (Figure 3). According to Rodriguez (1989),this relationship occurs when a polymer molecule entangles and drags along its neighbors under viscous shear forces. Larger, bulkier molecules would make more contact and entangle more readily. High Na0H:P lowered viscosity for the same M,. Increased ionization of phenolic hydroxyl groups which could increase intermolecular ionic repulsions can explain the viscosity reduction at high Na0H:P. The divergence of the plots at high Na0H:P may be caused by formic acid from the Cannizzaro reaction which would reduce alkalinity and ionization, resulting in a higher viscosity. Alkalinity. Alkalinity dropped markedly within resin cooks having F:P = 2.0 and 2.3, Na0H:P = 0.60, confirming the likelihood of the Cannizzaro reaction (Figure 4). The Cannizzaro reaction is the base catalyzed oxidationreduction of formaldehyde (or any other aldehyde) by itself with two formaldehyde molecules yielding one molecule of formic acid and methanol (Walker, 1964). Since the side reaction depends on hydroxide ion and formaldehyde concentrations, the higher F:P and Na0H:P levels would favor this reaction. This reaction is undesirable because the formic acid would neutralize the catalyst and increase the resin viscosity. Also, sodium formate may be harmful to the durability of water-resistant PF adhesive bonds. CondensationRate. A plot of M,, against elapsed cook time showed the fastest reaction at high F:P and low Na0H:P (Figure 5). Higher methylolation increased the reaction rate. Since condensation depends on methylol concentration: condensation rate = k [-CH20Hl (Tohmura et al., 1991). The slower rates associated with high Na0H:P had several potential causes. Formaldehyde loss from the Cannizzaro reaction could lower the methylol concentration. The additional NaOH could also reduce the concentration of reactive methylolated species in maintaining 50 % theoretical nonvolatiles and inhibit the reaction. Density. The resin densities increased during the course of polymerization (Figure 6). Polymers are, as a rule, more dense than their monomers, with shrinkage as high as 20% (Rodriquez, 1989). The polymer volume decreases because of reduced interatomic distances.
The high Na0H:P level had increased resin densities. Aqueous NaOH density increases with concentration. Literature values for the 20120 “C density of 4.50, 6.50, and 40.00% NaOH solutions (w/w) in water are 1.0502, 1.0667,and 1.4324 g/cm3,respectively (CRC Handbook of Chemistry and Physics, 1990). The effect of F:P on density is unclear. However, methylol side groups may alter the ability of molecules to pack closely. Nonvolatile Solids. Percent NV increased with increasing M,, and NaOH:P, but decreased with higher F:P (Figure 7). Nonvolatile measurements showed the greatest variability within individual formulations of all the characterizations performed. Although the theoretical NV target was 5096, the measured values ranged between 43.5 % and 46 ?4 . The values are not unlike other laboratory work for plywood resols (Tahir, 1989). This variation is typical and due to the varying raw materials and the resol condensation reaction of phenol and formaldehyde (Sellers 1985). The extent to which these condensation reactions take place depends on the ratio of formaldehyde and phenol, the reaction temperature, the reaction time, the pH, the concentration, and the catalyst (Sellers 1985). The higher F:P level resins, which had the odor of free formaldehyde, probably released formaldehyde during drying and lowered the final NV. The cause of the Na0H:P effect is uncertain except for the proportional relationship of the NaOH to the total resin solids. Resin Reactivity Properties. Gluing high-moisture wood (>8% moisture content) will require faster gel times to prevent overpenetration (Wellons1988). An alternative synthesis (two phenol charges versus one phenol charge) was attempted to gain quicker gel times at equal viscosities. While the resulting resin had an altered molecular weight distribution with greater proportions of low and high molecular weights, some of the low molecular weight materials appeared to be unreacted phenol (Figure 8).This MWD was more polydisperse. The resultingresin gel times were faster than those for resin from the earlier procedure (Figure 9). Plots of viscosity against M, showed that the altered MWD resin had lower viscosity over the same M, ranges. The low molecular weight portion reduced viscosity, while the high molecular weight portion sped gelation.
Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 697
Literature Cited
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No Second.P>enol Charge
Figure 9. Log-log plot of gel time against viscosity: molecular weight distribution effects.
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Figure 10. Plot of gel time against logviscosity: Cannizzaroreaction simulation.
The alternative cook procedure had less alkalinity loss than the original one (Figure 8). Formic acid may have inhibited the gel times of the original cooks. To eliminate alkalinity reduction by formic acid as the cause of the slower original gel times, formic acid and methanol were added to the resin to simulate Cannizzaro reaction effects. The formic acid decreased alkalinity and increased viscosity. Gel times, however, decreased or remained the same in comparison to similarly formulated resins (Figure 10). The lower alkalinity due to formic acid accelerated gelation, consistent with the previous results. The cook procedure, and not alkalinity, produced a faster resin, since the altered MWD caused the quicker gel times.
Summary and Conclusions This research examined the relationship between phenol-formaldehyde resin properties, including cure speed modifications. The results showed that viscosity increased withM, and decreased with Na0H:P. Gel time decreased with increasing F:P and M , but increased with higher Na0H:P. The results also indicated that PF gelation may have occurred at finite molecular weights and may have involved colloidal aggregation of hydrogen-bonded macromolecules. Cook procedure alterations (multiple vs single phenol charges) gave a more polydisperse resin with faster gel times for the same viscosities. Acknowledgment This work was performed with the financial assistance and at the facilities of Dyno Industrier A.S. of Lillestrom, Norway, and the Mississippi Forest Products Laboratory of Mississippi State, Mississippi. Appreciation for professionaltechnical assistance is expressed to Eva Mokastet of Dyno Industrier AS. for GPC work.
Armonas, J. E. Gel Permeation Chromatography and Ita Use in the Development of Resins. For. Prod. J. 1970,20,22-27. Bain, D. R.; Wagner, J. D. Molecular Weight Distribution of PhenolFormaldehyde Resols by High Performance Gel Permeation Chromatography. Polymer 1984,25,403-404. Chow, S.;Steiner, P. R.; Troughton, G. E. Thermal Reactions of Phenol-Formaldehyde Resins in Relation to Molar Ratio and Bond Quality. Wood Sci. 1975,8,343-349. Christiansen, A. W.; Gollob, L. Differential Scanning Calorimetry of Phenol-Formaldehyde Resols. J. Appl. Polym. Sci. 1985,30,22792289. Weast, R. C., Lide, D. R., Astle, M. J., Bayer, W. H., Eds. In CRC Handbook of Chemistry and Physics; CRC Press: Boca Raton, FL, 1990. Duval, M.; Bloch, B.; Kohn, S. Analysis of Phenol-Formaldehyde Resols by Gel Permeation Chromatography. J. Appl. Polym. Sci. 1972,16,1585-1602. Encyclopedia of Polymer Science and Technology; John Wiley & Sons: New York, 1966;Vol. 4, pp 18,64. Haupt, R. A. Phenol-Formaldehyde Resin Interaction with Wood. M. of Sci. Thesis, Mississippi State University, 1992,University Microfilms International: Ann Arbor, MI. Haupt, R. A.; Sellers, T., Jr. Phenol-Formaldehyde Resin Interaction of Wood. For. Prod. J . 1994,44,69-73. Kelley, S. S.;Gollob, L.; Wellons, J. D. The Effects of Resin Formulation Variables on the Dynamic Mechanical Properties of Alkaline Curing Phenolic Resins. Holzforschung 1986,40,303308. Kim, M. G.; Nieh, W. L. S. A Study on the Curing of PhenolFormaldehyde Resins by Dynamic Mechanical Analysis. Ind. Eng. Chem. Res. 1991,30,798-803. King, P. W.; Mitchell, R. H.; Westwood, A. R. Structural Analysis of Phenolic Resole Resins. J. Appl. Polym. Sci. 1974,18,11171130. Knop, A.; Pilato, L. A. Phenolic Resins, Chemistry, Applications and Performance, Future Directions; Springer-Verlag: Berlin, 1985. Rodriguez, F. Principles of Polymer Systems, 3rd ed.; Hemisphere Publishing Corporation: New York, 1989. Sellers, T., Jr. Plywood and Adhesive Technology; Marcel Dekker, Inc.: New York, 1985. Sellers, T., Jr. World Adhesive Market Builds a Promising Future on a Profitable Past. Adhes. Age, 1992,35,22-25. Sprengling, G. R.; Lewis, C. W. Dissociation Constants of Methylol 1953,75,5709-5710. Phenols. J. Am. Chem. SOC. Tahir,P. M. Synthesis and Evaluation of OrganosolvLignin-Modified Phenolic Resins for Bonding Southern Pine Plywood. M. of Sci. Thesis, Mississippi State University, 1989,University Microfilms International: Ann Arbor, MI. Tohmura, S. I.; Higuchi, M.; Sakata, I. Kinetics of the Curing of Phenolic Resin Adhesives. Presented at the 1991 Adhesives and Bonded Wood ProductsSpposium, Nov., Seattle, WA; Forest Prod. Res. SOC.:Madison, WI, in press. Walker, J. F. Formaldehyde, 3rd ed.; Robert E. Krieger Publishing ComDanv: Malabar. FL, 1964. Wellons, J. D. Approaches to High Moisture Gluing. Plywood Panel World 1988,29, 27. Wellons, J. D.; Gollob, L. GPC and Light Scattering of Phenolic Resins-Problems in Determining Molecular Weights. Wood Sci. 1980,13,68-74. Young, R. H. Adhesive Cure as Determined by Dynamic Mechanical Analysis and Its Effect on Wood Composite Performance. In Wood and Adhesivesin 1985;Christiansen,A.W., Gillespie,R. H., Myers, G. H., Rivers, B. H., Eds.; Forest Prod. Res. SOC.:Madison, WI, 1986;pp 267-276. Received for review May 24, 1993 Accepted November 9,1993' Abstract published in Advance ACS Abstracts, January 1, 1994. @
