Study on the Curing of Phenol-Formaldehyde Resol Resins by

Ind. Eng. Chem. Res. 1991,30,798-803

798

Oles, P. J.; Siggia, S. Atomic Absorption Method for Determining Micromolar Quantities of Aldehydes. Anal. Chem. 1974, 46, 911-914. Reddy, K. T.; Cernansky, N. P.; Cohen, R. S. Modified Reaction Mechanism of Aerated n-Dodecane Liquid Flowing Over Heated Metal Tubes. Energy Fuels 1988,2, 205-213. Suresh, A. K.; Sridhar, T.; Potter, 0. E. Autocatalytic Oxidation of Cyclohexane-Modeling Reaction Kinetics. AZChE J . 1988, 34, 69-80.

Van Sickle, D. E. Oxidation of 2,4,6-Trimethylheptane. J. Org. Chem. 1972,37, 755-760. Van Sickle, D. E.; Mill, T.; Mayo, F. R.; Richardson,H.; Gould,C. W. Intramolecular Propagation in the Oxidation of n-Alkanes. Autoxidation of nBentane and n-Octane. J. Org. Chem. 1973,38, 4435-4440.

Received for review August 6 , 1990 Accepted October 23, 1990

Study on the Curing of Phenol-Formaldehyde Resol Resins by Dynamic Mechanical Analysis Moon G. Kim* and World L.-S. Nieh Forest Products Laboratory, Mississippi State University, Mississippi State, Mississippi 39762

Robert M. Meacham Chemical Division, Georgia-Pacific Corporation, Crossett, Arkansas 71635

Dynamic mechanical analyses (DMA) were carried out by using a Du Pont DMA 982 unit on phenol-formaldehyde (PF) resol resins a t 110-225 "C with a rapid initial heating schedule. The curing schedule adopted for simulating the pressing parameters used in the manufacture of wood composite boards afforded the observation of the rigidity increase and loss modulus changes within the short time span. The tan 6 curve maximum occurring during the rigidity increase was interpreted as the vitrification point of the resin. At low curing temperatures the vitrification occurred in the early part of rigidity curve with higher intensities, while a t higher curing temperatures it occurred in the later part with lower intensities. In summary, the DMA method as used in this study measured the cure time, vitrification time, and other useful cure parameters of PF resins as affected by the temperature and resin composition. The method appears to be useful in optimizing the synthesis and curing parameters of P F resin adhesives used in the manufacture of wood composite boards.

Introduction A dynamic mechanical analyzer (DMA) measures the stored and dissipated energies of a viscoelastic specimen put under oscillation at varying temperatures. The stored energy depends on the polymer type, temperature, and frequency of oscillation and is represented as the elastic modulus (bending) or rigidity (shear). The dissipated energy represented by the loss modulus is due to the molecular frictions occurring in the viscous flow. Thermoplastic polymers have small but significant loss modulus values that ordinarily change linearly with the rigidity except at transitions where the tan 6 value, the ratio of loss modulus to rigidity, shows a maximum. Similar tan 6 maxima occur when thermoset resins gel or vitrify. When the DMA frequency of oscillation is low, as with the Du Pont 980 dynamic mechanical analyzer (see below), the rigidity of glassy polymers below their glass transition temperatures and the rigidity of highly cross-linked polymers are relatively independent of the oscillation frequency and range from 1.0 X 1O'O dyn/cm2 to higher values. Their loss moduli vary in the 107-10g dyn/cm2 range with the values of the latter type polymers being in the lower side (Ferry, 1961). The loss moduli for low molecular weight solid polymers such as uncured thermosets are far lower in the 104-106 dyn/cm2 range, but since their rigidities are also low, the tan 6 values can be relatively high. The reason for the higher loss moduli for glassy or cross-linked polymers in comparison to those of the thermosets in precure stage is that the efficiency of energy dissipation increases due to the eatablished polymer networks. Lofthouse and Burroughs (1978) described the Du Pont 980 dynamic mechanical analyzer as an instrument of low loss factor. With ita low oscillation frequency

available in the fixed displacement mode of operation, the measurement can be done with low frequency dependency. According to Flory (19531, gelation of a thermosetting polymer occurs when an infinite molecular network is formed, and since it results in an incipient rigidity with resistance to flow, ita timing and the extent of reaction are thought to be the important parameters in the processing of thermosetting polymers. With the torsion braid analysis (TBA) method, Gillham (1979) described the curing of epoxy resins in terms of gelation and vitrification. Vitrification, the attainment of the glassy state, involves a further polymerization and cross-linkings. In epoxy resins gelation and vitrification were reported to occur separately, except when the cure temperatures were very high. Phenol-formaldehyde (PF) resol resins are used as binders in the manufacture of wood products and in other thermosetting applications. The degree of polymerization of PF resins has not yet been measured accurately, but it is generally represented in the resin's solids level and viscosity: resins with low solids levels are advanced more than resins with higher solids levels. Urea is often added in small amounts as diluent and as a means to capture the residual formaldehyde in wood adhesive PF resins (Kim et al., 1990). The curing step is base-catalyzed and involves the continuing formation of methylene bonds among the trifunctional phenolic rings. In their differential scanning calorimetry and isochronal TBA study of a plywood adhesive PF resin, Steiner and Warren (1981) reported that the curing process is completed near the second endotherm at about 145 "C, where the relative rigidity sharply increased and the relative damping showed a shoulder before dropping off to a low value. An earlier study of PF resin curing processes using Du Pont DMA 980 was published

0888-5885/91/2630-0798$02.50/00 1991 American Chemical Society

Ind. Eng. Chem. Res., Vol. 30,No. 4,1991 799 Sample : PF133 Size : 15.00 x 0.10 x 12.70 mm Method : WN HOLD AT 125C Comment : F i b ~ m cbth k~

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(Young et al., 1981), but the details were not reported. Detailed information on the curing process of PF resins has long been lacking.

Experimental Section A Du Pont DMA 982 unit was used with the fixed displacement mode where the natural oscillation frequency ranged from 2 to 25 Hz. A glass cloth with dimensions of 15 (1) X 12.7 (w) X 0.1mm was impregnated uniformly with 1620 mg of liquid resin and clamped horizontally between the two arms. The DMA run began with a temperature jump to 50 "C, and, thereafter, the temperature was increased at a rate of 25 "C/min until the cure temperatures were reached at 110,125, 150, 175,200, and 225 OC, respectively. This fast heat-up schedule was adopted to simulate the pressing conditions currently used in the manufacture of wood composite boards. The DMA run was continued for 30-50 min after reaching the cure temperature. After cooling, the cured specimen was weighed to obtain the cured resin weight. The triplicate runs generally gave similar results from which one was selected for use based on the range of the cured resin weight. From the rigidity and damping curves measured, the loss modulus and tan 6 curves were derived. In a typical DMA experiment the following values were definable (Figure 1): the times at loss modulus and tan 6 maxima, the times of incipient loss modulus or tan 6 increases, the slope of rigidity curve at the inflection point (cure speed), the intersecting point of the slopes of the rigidity curve (cure time), and the rigidity value at the time of tan 6 maximum ),6( with respect to the final rigidity value. The following three PF resol resins, useful as oriented strand board (OSB) binders, were prepared at 80 "C, at a 0.39 sodium hydroxide to phenol ratio and at a 50.0% resin solids level: a formaldehyde/phenol (F/P) ratio of 2.00 and a final viscosity of 700 CPfor resin A, a F/P ratio of 2.25 and a final viscosity of 500 CPfor resin B, and a F/P ratio of 2.50 and a final viscosity of 1100 CPfor resin C. During the synthesis of resin C, several samples were

taken: sample C1 after completion of formaldehyde addition, which took 1 h, sample C2 after 2.5 h of reaction, sample C3 after 3.0 h, and sample C4after 3.5 h. Resin C, the final sample, was taken at 4.0 h. Resins Au, Bu, and Cu were made by adding urea, 10% by weight based on the resin solids, to resins A, B, and C, respectively. Resin D, a plywood P F adhesive resin, was obtained from Georgia-Pacific Corp., Louisville, MS, and ita analysis values were as follows: resin solids level of 38.5%, sodium hydroxide content 5.0%, and viscosity of 650 cP.

Results and Discussion Typical DMA Results (Resin Cu). Results of the DMA experiment a t 125 OC for resin Cu are shown in Figure 1. The rigidity increased in 5 min to a low value of about 3.5 X 109 dyn/cm2, which probably represents the evaporation of water and gelation. Following this, the rigidity increased gradually, then rapidly, and then more gradually to reach a plateau at 3.5 X 1O'O dyn cm2. The beginning loss modulus of 2.8 X lo7 dyn/cm , a typical value at this stage (Ferry, 1961), did not change until about 8 min, and then it increased sharply to a maximum of 18.0 X lo7 dyn/cm2 before decreasing rapidly to a low value. The increasing loss modulus curve was followed behind by the increasing tan 6 curve, which reached a maximum a t The increases of these two curves a value of 6.5 X coincided with the rapid increasing phase of the rigidity curve and reflect the polymerization reactions progressing to form the networks that effected a more efficient energy dissipation. The decreasing concentration of the low molecular weight components appears to begin showing effect at the maximum point of the loss modulus curve. Furthermore, the maximum point of the tan 6 curve coming slightly later appears to be where the effect of rigidity increase (networks) on loss modulus becomes balanced by the (decreased) amount of the low molecular weight components. Thii considerationsuggests that the tan, 6 point is where the polymer molecules became extensively involved in the

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network structure, i.e., the vitrification point. The subsequent increase in rigidity is accompanied by the decrease in loss modulus, which reaches a very low value at the end. The fact that only one tan 6 peak was observed during the major development of the rigidity seems to indicate that the gelation of this system has occurred elsewhere with only a small effect on the rigidity. The early period in the cure showed little variations on the loss modulus but high varying values on the tan 6 curve. The gelation step appears to occur somewhere in this region, where, however, the tan 6 curves were found to be less satisfactorily reproducible, probably because of the rapid heating schedules used in this study. A slow heating schedule might allow the observation of the gelation step. Effect of Temperature on Vitrification and Cure. The increase of cure temperature increased the slope of the rigidity curve and shortened the vitrification and cure times for resin Au (Figure 2) and for other resins (Table I). As these trends were expected, the DMA method characterized the cure in terms of the rate of rigidity development and other cure time parameters. It is noted that the cure rate of PF resins became high a t above 150 "C but showed a wide spread between this temperature and 110 "C, wherein lies the prevailing temperatures af the core layers during the pressing of wood composite boards. The possible sensitivity of the core layer properties is indicated due to the temperature variations. Other subtle effects were observed for the increase of cure temperature: (a) a decrease of the area and height of the tan 6 curve and (b) a shift of the tan 6 peak toward the end of the rigidity curve (Table I). Although observation b may be simply the result of the increasing cure temperature that enhances the glass-transition temperature of the curing system, observation a indicates that as the cure temperature increases, the concentration of the components responsible for the loss modulus, Le., the low molecular weight components, became smaller a t the vitrification point. In other words, at higher cure temperatures the polymerization reactions appear to have gone further before reaching the vitrification point. The report

Table I. DMA Data for Resins A, B, and C with and without Ureao time at cure rigidity hbu, resin temp, "C time, min slope 8__ , min A 125 15.3 1.47 0.70 12.0 1.06 0.85 10.1 150 11.3 175 10.4 1.74 0.94 9.7 Au 110 25.8 2.01 0.41 13.3 125 18.4 3.21 0.48 11.6 150 14.6 3.61 0.68 11.7 175 11.2 7.25 0.85 10.7 200 10.1 7.25 0.84 9.5 225 9.3 6.63 0.89 9.0 B 125 14.8 0.94 0.71 12.0 150 12.8 1.94 0.85 11.2 175 9.8 1.01 0.98 10.0 Bu 110 25.4 1.57 0.36 12.7 125 19.0 2.75 0.55 13.6 150 13.0 3.76 0.80 12.1 175 11.3 7.89 0.76 10.1 200 9.8 8.28 0.92 9.8 225 9.2 9.80 0.92 9.0 C 110 19.6 0.56 0.69 12.0 125 16.9 0.62 0.73 11.8 150 11.6 1.81 0.86 10.8 175 9.9 1.05 0.87 10.0 cu 110 24.4 1.38 0.48 14.0 125 18.5 2.85 0.57 13.5 11.5 150 13.1 3.89 0.80 175 11.1 5.02 0.89 10.8 200 9.7 8.67 0.79 9.0 225 9.4 9.76 0.98 9.6 ~

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a Cure time: aa defined in the text. Rigidity slope: at the maximum rigidity increase (IOs dyn/cm*/min). Relative rigidity 8 , rigidity at tan 8 , as the percentage of the final rigidity.

of Carswell (1947) stating that PF resol resins cured at higher temperatures involve a larger number of colloidal particles than those cured at lower temperatures appears to be in accord with this observation. Furthermore, although the rigidity values observed at various temperatures were not normalized to the values at a standard temperature and, therefore, not comparable directly with each

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other, the cure temperature appears to affect the final rigidity by allowing a particular amount of material to take part in the vitrification point. Mole Ratio and Urea Addition Effect. The synthesis mole ratio (F/P) effected subtle differences in tan 6 and rigidity curves (Figure 3). The typical DMA results shown for cure at 125 "C indicate that the tan, 6 of resin A (F/P = 2.00) was slightly faster than that of resin B (F/P = 2.25), which was again slightly faster than that of resin B (F/P = 2.5). The tendency was also clearly shown after the addition of urea, especially for resin Au. The urea added at the end of the synthesis reacts quickly to form (hydroxymethy1)ureas with the unreacted formaldehyde whose concentration is proportionately higher for resins synthesized at higher F/P ratios (Kim et al., 1990). Since resin Au is expected to have more unreacted urea than the others, the observation of the faster tan, 6 suggests that P F polymers react faster with urea than with (hydroxymethy1)ureas. The mole ratio effect on rigidity is also significantly different for resin A or Au, which appears to be reflecting the higher molar masses of phenol in these resins. However, a detailed study with a more exact control of variables in the resin synthesis and DMA run is necessary. Resins with added urea behaved differently from the original resins (Figure 3). The urea addition effects were as follows: (a) the final rigidity values increased by more than 50%, (b) the tan, 6 peak heights increased, and (c) the ratio of the rigidity at the tan ,6 to the final rigidity decreased. These observations can be interpreted as the added urea, in unreacted or hydroxymethylated form, being present as low molecular weight components in the vitrification stage but bringing the system to a rigid network formation relatively early. This consideration and the higher rigidity of urea-added resins suggest that chemical reactions occur to a significant extent between PF resin polymers and urea molecules. Resin Samples C1-C, and C. The DMA profiles of resins C1-C4and C at 125 "C (Figure 4) show that the time to reach the tan 6 peak became shorter and the peak height

decreased as the results of longer cook times. The results of longer cooking times are in accord with the expectation of the higher degrees of polymerization attained and are, in a way, similar to the effects of the cure temperature increases discussed above. The high tan, 6 for resin C1 apparently reflects the presence of a larger amount of the low molecular weight components in the vitrification step. The rates of the incipient rigidity increase were also in accord with the cooking times. The varying final rigidity values appear mostly due to the resin load variations. Resin D. As a plywood adhesive type PF resin, resin D was a highly advanced resin, as represented by the analytical values: a low solids level and a high sodium hydroxide content at a reasonable viscosity. The DMA results (Figure 5) show that the rigidity of resin D developed in stages to high final values only at higher curing temperatures. The rigidity did not develop much at 110 "C and only to a medium value at 125 OC with somewhat erratic tan 6 curves (not shown). The early period (2-3 min) of the rigidity development shows fluctuating values for all cure temperatures, indicating the possibility of the involvement of melting phases. The initial curing of resin D at 110 "C, in contrast to that a t higher temperatures, apparently resulted in an intermediate composition that could not be cured adequately just by prolonging the heating. It is probably because the glass-transition temperature of (dried) resin D was above 110 "C to begin with and the polymerization reaction progressed without first melting to result in an unconsolidated structure. Resins with high degrees of advancement are thus seen to be cured adequately only above certain minimum temperatures, although the minimum temperatures could probably be affected by the presence of water, as it would be in practice. This observation appears to be related closely to the 'dry out" problem often encountered in the plywood adhesion processes normally occurring with long lay-up times because of the greater amounts of water lost from the adhesive layer. Applicability in Oriented Strandboard Pressing. The cure time as defined in this work would be a useful

802 Ind. Eng. Chem. Res., Vol. 30,No. 4, 1991 Sample Sue : 15.00 x 0.10 x 12.70 mm Flberglass cloth

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indicator of the adhesive bond development because the rigidity at this point has developed in most casea to almost 90% of the final value. In addition, the vitrification time and the relative rigidity at tan, , a would be informative indicators of bond quality. The temperature in OSB pressing operations reaches approximately 125 "C in the core layer, and the duration of this temperature is short, normally 3-4 min. At the press opening, the board will

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not increase in thickness or delaminate if a sufficient bond strength was reached with the attained glass-transition temperature of the adhesive higher than the board temperature. Normally OSBs are stacked together shortly after the manufacture, and the board temperature re& high for many hours. This post curing step is known to increase the bond strength significantly. Since the DMA results show the vitrification occurring in about 10 min at

Ind. Eng. Chem. Res. 1991,30, 803-804 125 O C , the adhesive vitrification in the core layer of OSB should occur during this post manufacture period and be responsible for the strength improvement. Therefore, the adhesive strength that holds the board together at press opening does not appear to involve vitrification, but probably involves the gelation, as mentioned above, that helps resist the board expansion. Our experimental method with the fast heat-up schedules did not give the details on this early curing period. Further study in this region would be fruitful. Overall the DMA method used in this study appears to be very useful for comparing the curing rates of different PF adhesive resins used in the manufacture of OSB.

Conclusion The DMA method provided a detailed observation of the rigidity increase and loss modulus changes during the curing of P F resol resins. Tan -,6 was assigned as the vitrification point, which occurred in the early part of rigidity development with higher intensities at low cure temperatures. At higher curing temperatures the tan, 6 occurred later with lower intensities. This observation was interpreted to indicate that at higher temperatures PF resins vitrify after a higher extent of polymerization than at lower temperatures. The DMA method was shown to be a useful analytical tool in that the curing process of PF resins are characterized in terms of the cure time, vitrification time, and other useful parameters. The characterization results in turn allowed the differentiation of resin compositions and the optimization of resin synthesis or curing parameters as used in the manufacture of wood composite boards.

803

Acknowledgment We appreciate the many helpful discussions with Prof. Terry Sellers, Jr., and Mr. C. R. Davis. This work was done in partial fulfillment for the requirement of the Ph.D. degree of W.L.-S.N. Financial support from the USDA Wood Utilization Research Grant Program and the Mississippi Forest Products Laboratory is gratefully acknowledged. Registry No. F/P (copolymer), 9003-35-4;(formaldehyde)(phenol)(urea) (copolymer), 25104-55-6.

Literature Cited Carswell, T. S. Phenoplasts; Interscience Publishers, Inc.: New York, 1947. Ferry, J D.Viscoehtic Properties of Polymers;John Wiley & Sone: New York, 1961. Flory, P. J. Polymer Chemistry; Cornel1 University Press: Ithaca, NY, 1953. Gillham, J. K. Formation and Properties of Network Polymeric Materials. Polym. Eng. Sci. 1979, 19, 676-82. Kim, M. G.; Amos, L. W.; Barnes, E. E. Study of the Reaction Rates and Structures of a Phenol-Formaldehyde Resol Resin by C-13 NMR and Gel Permeation Chromatography. Znd. Eng. Chem. Res. 1990, 29, 2032-7. Lofthouse, M. G.; Burroughs, P. Material Testing by Dynamic Mechanical Analysis. J. Therm. Anal. 1978, 13, 19-53. Steiner, P. R.; Warren, S. R. Rheology of Wood-Adhesive Cure by Torsion Braid Analysis. Holrforschung 1981, 35, 273-8. Young, R. H.;Kopf, P. W.; Salgado, 0. Curing Mechanism of Phenolic Resins. Tappi 1981, 64, 127-30.

Received for review April 3, 1990 Revised manuscript received October 17, 1990 Accepted November 11, 1990

CORRESPONDENCE Comments on “The Probability Distribution of Growth Rates of Anhydrous Sodium Sulfate Crystals” Sir: Recently, Klug and Pigford (1989) performed two types of crystallization experiments to study the growth behavior of anhydrous sodium sulfate crystals. In the first, many isolated single crystals were grown in a flow cell and growth rates determined from measured crystal size-time variations. In the second set of experiments, transient crystal size distributions (CSDs) measured from an isothermal batch crystallizer were used to determine the moments of probability distribution of growth rate activity. The like momenta of distributions in these two sets of experiments are different. The stochastic distribution of growth rate activities determined from a large number of single isolated crystals may not necessarily be applicable to an ensemble of growing and nucleating crystals as in an isothermal batch crystallizer. Although many other variables may contribute to this difference, it is necessary to point out that basic definitions of growth rates conventionally used in these two types of experiments are different. For a single individual crystal, the growth rate is usually determined from the gradient of crystal eize-time variation. Experimental evidence based on singlecrystal studies tends to suggest that a given

crystal grows at a constant intrinsic growth rate and different crystals can have different growth rates for the same global environmental conditions. Thus, the growth rate of a single crystal may be used as its property or be characterized by an additional independent variable as used by the authors and termed growth rate activity of an individual crystal. The average growth rate of many isolated single crystals is

where E N j is the total number of crystals used. The conslstent definition of the average growth rate used for an ensemble of crystals with changing population with time is

where fi,, is the average zeroth moment and assumed constant over a small time interval between t and t + At. From eq 1

Q888-5885/91/263Q-Q8Q3$Q2.5Q f Q 0 1991 American Chemical Society