Thermal Analysis of the Polymerization of Phenol Formaldehyde Resins Eric W. Kendall Bruce R. Trethewey, Jr. Manager, Materials Testing Lab Senior Manager, Product Quality Wilsonart International, Inc. Wilsonart International, Inc. Temple, TX 76503 Temple, TX 76503 ABSTRACT

The polymerization of phenol formaldehyde (phenolic) resins was studied by three different thermal analysis methods. The analysis involved the use of High pressure differential scanning calorimetry (HPDSC), Rheology, and Dynamic mechanical thermal analysis (DMTA). Phenolic resin was evaluated both in neat resin form and as a saturated Kraft paper composite. All three thermal analytical methods were effective in measuring the increase in reaction rate (reaction kinetics) with increased catalyst concentration. INTRODUCTION Phenol Formaldehyde (phenolic) Resin Phenolic resins, which are the condensation product of phenol and formaldehyde, have been used commercially for over 90 years. Phenolic resins are used in such areas as adhesives (1), coatings (2), wood binders (3), and laminates (3). The extensive use of phenolic resin is due to such things as the heat resistance, water resistance, and the mechanical properties of the cured phenolic resins. Phenolic resins can be prepared by two types of chemistry. The first, termed Novolacs, are phenolic resins that are prepared under acid catalysis with an excess of phenol (4). The polymerization, or cure, of the Novolac resins requires the additions of further formaldehyde (4). The second class, Resoles, are phenolic resins prepared by the alkaline catalysis of phenol with an excess of formaldehyde (4). Resoles can be cured by heating without additional chemicals or monomers. In this paper we focus on the characterization of the polymerization, or cure, of a resole phenolic system. Resoles cure by a thermal condensation reaction of one phenolic monomer with another through reactive methylol groups. During the cure process, the methylol groups condense to form methylene bridges between the phenolic monomers (4). Cross linking (or cure) takes place through the multiple numbers of reactive sites on the each phenolic monomer. Since the phenolic resin monomer is prepared by the reaction of phenol and formaldehyde, the monomer is actually a mixture of several monomeric species. Because of this, the cure reaction is actually the thermal condensation of several monomer species. Therefore, a successful characterization of the cure chemistry requires an understanding of the cure reactions of several monomer species. This requires more analysis than most typical polymerization reactions. Analysis of phenolic cure chemistry has been reported to some extent by thermal analysis (5-10) and NMR (11-13) along with some limited chromatography studies (14-16). Here we report the characterization of the cure reaction of phenolic resins by three different thermal analysis techniques, High pressure differential scanning calorimetry (HPDSC) Rheology, and Dynamic mechanical thermal analysis (DMTA). High Pressure Differential Scanning Calorimetry (HPDSC) The cure reaction of phenolic resins is exothermic. This allows for analysis of the cure reaction by accurately measuring the heat of reaction. Differential scanning calorimetry (DSC) quantitatively measures the heat of reaction, which correlates with the reaction rate and percent reaction of the phenolic resin at any specific time in the cure process (10,17). Knowing this, the heat generated during the cure reaction can be used to define the extent of reaction as follows. Ap = Hp/Hult (1)

Where: Ap = the conversion or extent of reaction at a specific time and temperature. Hp = the heat generated by the reaction at a specific time and temperature. Hult = the total heat generated during the entire cure reaction. It has been shown previously that from equation (1) one can derive equation (2) (10,17). (dHp/dt)(1/Hult) = Ae-(E/RT)(1-Hp/Hult)n (2) Where: A = the pre-exponential constant (min-1) R = the gas constant (8.32 J/mol/K) E = the activation energy (J/mol) n = the reaction order T = the temperature (K) DSC can be used to determine the values for Hp, Hult, and dHp/dt for the cure of phenolic resin. This information can then be used to directly calculate the extent of reaction at a known temperature vs. time. This thermal analysis method requires a very accurate measure of the heat of reaction. Since the phenolic cure reaction liberates water, the use of HPDSC instead of conventional DSC is required. Volatilization of the reaction by products results in endothermic heat sinks at atmospheric pressure. The use of elevated pressures suppresses this phenomenon aiding the accurate measure of these heats of reaction. This suppression of volatiles is also taking place in the laminate manufacturing process when phenolic resin saturated papers are cured in heated presses under pressure. Rheology Thermal rheological analysis involves the application of an oscillatory force to a sample of defined geometry. In this study, a parallel plate geometry was used, resulting in a sinusoidal torsional stress applied to the sample. The applied stress, , will result in a measurable deformation or strain, . From this response, a complex modulus (G*) is measured as the materials response to the sine wave of applied force. From the complex modulus, the storage modulus (G) and the loss modulus (G) can be calculated (18). G* = G + iG (3) The storage modulus, G, is a measure of the materials ability to store and return energy, i.e. Hookean spring or elastic like behavior. The loss modulus, G, is a measure of materials ability to absorb energy, i.e. viscous behavior. The complex viscosity, *, of the sample is determined from the complex modulus and frequency of oscillation, *, by equation (4) (18). * = G*/* (4) For thermosetting resin systems (reactive or curative systems typical of the resin systems used in HPDL) rheology analysis can be used to measure such critical thermal transitions as the gel point of the resin, the onset of the cure reaction, and the onset of vitrification. The analysis can be done by both ramp heating and isothermal heating methods. Dynamic Mechanical Thermal Analysis (DMTA) DMTA analysis involves the application of an oscillatory force to a sample that will result in a sinusoidal stress applied to the sample in a tensile geometry (vertical movement) coupled with the measurement of the resultant response (both amplitude and deformation) to that stress. The applied stress, , will result in a measurable deformation or strain, . From this response, a complex modulus (E*) is measured as the materials response to the sine wave of applied force. From the complex modulus, the storage modulus (E) and the loss modulus (E) can be calculated (19).

E* = E + iE (5) The storage modulus, E, is a measure of the materials ability to store and return energy, i.e. Hookean spring or elastic like behavior. The loss modulus, E, is a measure of materials ability to absorb energy, i.e. viscous behavior. The ratio of the loss modulus to the storage modulus is known as tan (the damping factor, as shown in equation (6) (8). tan = E/E (6) For thermosetting resin systems DMTA analysis can be used to measure such critical thermal transitions as the Tg (the glass transition is commonly observed as the peak in the tan curve) of the resin both before and after the cure reaction. The Tg of the resin in a thermoset resin is an indicator of the extent of the cure reaction. Also, the DMTA analysis can be used to measure the gel point of the resin, the onset of cure reaction, and the onset of vitrification of the resin. These thermal transitions in the cure reaction can be observed as changes in the measured storage and loss moduli (19). The DMTA analysis technique is very similar in type to the rheology analysis, but the DMTA analysis requires a solid or self supporting sample (high viscosity sample). For this study, this requires the use of treated saturating Kraft paper with phenolic resin for the DMTA analysis. The inclusion of the paper matrix does not drastically interfere with the measurement of some of these resin thermal transitions and it allows for the study of thermal behavior within the actual composite matrix. It should be noted that in the DMTA analysis of these phenolic resin/Kraft paper composites a Tg was not observed. A Tg is normally observed for thermosetting resins in decorative papers that have a higher weight % resin content than in the phenolic/Kraft paper system. It is believed that the reduced resin content results in a damping of the observed tan signal making the identification of the Tg extremely difficult. EXPERIMENTAL Raw Materials and Sample Preparation A generic resole phenolic resin was utilized in this study. A catalyst, C, was added in normalized weight/weight % levels listed in Table 1. Saturating Kraft paper typical to the manufacture of High pressure decorative laminate (HPDL) was saturated with the phenolic resin on a laboratory scale treater with constant treating conditions for both the resin content and the volatile level as the catalyst levels were varied. HPDSC Analysis Samples of phenolic resin, with known catalyst levels, were analyzed on a Rheometrics HPDSC. Samples were enclosed in a sealed aluminum pan with a pierced lid with a hole of approximately 0.5 mm in diameter. The hole is required to equilibrate the test chamber pressure with the sample pressure. A pressure of 5.1 MPa (750 psi) was used. This pressure was sufficient to suppress any endothermic volatilization during the cure reaction. HPDSC calibration was done using indium (Tm = 156.66 C) and tin (Tm = 231.93 C) NIST standards. Each resin sample was analyzed per the ASTM E698 method to determine the pre-exponential factor, A, the activation energy, E, and the reaction constant, k (17). This was accomplished using three dynamic heating scans, from 25 C to 250 C at 5 C/min, 10 C/min, and 15 C/min. Then, using an assumed reaction order, n = 1, the percent reaction as a function of time at several set isothermal temperatures was calculated. Rheology Sample of phenolic resin, with known catalyst levels, werer analyzed on a Rheometrics ARES rheometer. Testing was done by both temperature ramp methods and isothermal methods. Temperature ramp testing was done from 30 C to 180 C at a ramp rate of 3 C/min. Isothermal testing was done at 115 C, 125 C, and 135 C. Testing was done using a 25 mm diameter parallel plate geometry with an applied oscillatory strain of 50% and a frequency of 1 Hz.

DMTA Analysis Samples of phenolic resin treated paper were analyzed using a Rheometrics Mk III DMTA. Analysis was done using both a temperature ramp method and an isothermal method. The temperature ramp method was performed from 25 C to 200 C at a ramp rate of 3 C/min. Testing was done in the tensile geometry at 1 Hz and 0.02% applied strain. RESULTS HPDSC Testing The HPDSC results are shown in Table 2. These results are reported as the activation energy, E, and the reaction constant, k, as a function of catalyst, C, concentration. From these results, the activation energy decreases with increasing catalyst concentration while the reaction constant increases with increasing catalyst concentration. This indicates that as the catalyst concentration is increased the rate of reaction also increases. From this HPDSC test data, the percent conversion vs. reaction time was calculated for each of the phenolic resin samples. To simplify a comparison of these results, the time to reach 50% conversion was calculated for isothermal cure temperatures at 115 C, 125 C, and 135 C. These results are listed in Table 3 and are shown in Figure 1. These results show that as the catalyst level increases the percent reaction increases as a function of time indicating the reaction rate has increased with increased catalyst level. Rheology Testing The temperature ramp rheology testing results are listed in Table 4. The results are reported as the crossover point of the storage modulus and the loss modulus (G = G). The crossover point is generally accepted as the gel point of gel temperature of the cure reaction. The results show a decrease in the gel point temperature with increasing catalyst concentration. This indicates that the cure rate is increasing with increasing catalyst levels. The isothermal rheology testing results are listed in Table 5. The results are reported as the onset in the increase in the storage modulus, G (the onset of the cure reaction, identified as the fist inflection point in the storage modulus), and the onset of the maximum in the storage modulus, G (onset of vitrification, identified as the second inflection point in storage modulus). The observed increasing the storage modulus is due to the start of polymerization of the phenolic resin resulting in a build of the elastic properties of the liquid resin as it becomes a cured solid. The plateau or maximum in the storage modulus is a measure of the resin reaching its ultimate properties, which occurs at vitrification. The results are reported in time (seconds) at the isothermal temperature of 115 C and 125 C. The cure reaction at 135 C proved to be too fast to get meaningful data. From Table 5 and Figures 3 and 4, the isothermal results show that as the catalyst level is increased, both the onset of reaction and the onset of vitrification occur at faster rates (the rate of the cure reaction increases). DMTA Testing The results of the temperature ramp testing are shown in Table 6. The results are reported as the peak in the tan , the peak in the loss modulus (E), and the maximum in the storage modulus (E). The peak in the tan is associated with the peak reaction temperature. The peak in the loss modulus is associated with the gel point of the resin. The maximum in the storage modulus is attributed to the vitrification or completion of the polymerization or cure process. These thermal transitions are plotted as a function of the catalyst concentration in Figure 5. It should be noted that the DMTA analysis of these phenolic resin/Kraft paper composites do not show the typical Tg normally observed for thermoplastic and thermoset resin systems. It is thought that the Kraft paper damping of the resin (a result from the % resin vs. % paper weight in these samples) obscures the Tg normally observed as a lower temperature peak in the tan curve than the peak reaction temperature peak in the tan .

From the results shown in Figure 5, it can be seen that as the catalyst concentration is increased the peak reaction temperature, the gel point, and the vitrifcation temperature all shift to lower temperatures. The most dramatic change is from zero catalyst to the fist additions of catalyst (2% by weight). This indicates that for this resin/catalyst system the treating process is essentially only drying the resin and not curing the resin. It should be noted that this observation is particular to this resin/catalyst system and does not necessarily mean that treating processes generally do not advance the cure of the resin systems involved. The results of the isothermal testing are listed in Table 7. The results are reported as the onset of cure, the gelation point, and the onset of vitrification. All of these results are reported in time (seconds). The onset of cure is observed as the first inflection point in the storage modulus, E. The gelation point is observed as the peak in the loss modulus, E. The onset of vitrification is observed as the second inflection point in the storage modulus. From the results, it can be seen that as the catalyst level is increased the time to reach the onset of cure, the gelation point, and the onset of vitrication all decrease (as shown in Figures 6 - 8) indicating an increase in the resin cure rate with catalyst concentration. CONCLUSIONS In this study we have used three different thermal analysis methods to study the cure of phenolic resin as a function of catalyst concentration. From the results of this study, it has been shown that both HPDSC and rheology testing are able to probe the thermal cure of phenolic resins in the neat form. But, DMTA allows for analysis of the thermal cure of phenolic resin within the paper composite matrix. REFERENCES

1. Pizzi, A; Mital, K.L., Handbook of Adhesive Technology, Marcel Dekker, Inc., New York, 1994. 2. Kopf, P., The Encyclopedia of Polymer Science, 1998, Vol. 11, 45. 3. Pizzi, A., Wood Adhesives Chemistry and Technology, Vol. 1, Marcel Dekker, New York, 1983. 4. Odian, G., Principles of Polymerization, John Wiley and Sons, New York, 1981. 5. Wang, X.M.; Riedl, B.; Christiansen, A.W; Geimer, R.L., Polymer, 35, 5685, 1994. 6. Kay, R.; Westwood, A.R., Eur. Polym. J., 11, 25, 1975. 7. Chow, S.; Steiner, P.R., J. Appl. Polym. Sci., 23, 1973, 1985. 8. Christiansen, A.W. et. Al., Holzforschung, 47, 76, 1993. 9. Kiran, E.; Iyer, R., J Appl Polym Sci, 41, 205, 1990. 10. Turi, E., ed., Thermal Characterization of Polymeric Materials, Academic Press, New York,

1981. 11. So, S.; Rudin, A., J Appl Polym Sci, 41, 205, 1990. 12. Bedel, D., Polymer, 37, 1363, 1996. 13. Pizzi, A.; Mercer, A.T., J Appl Polym Sci, 61, 1697, 1996. 14. Rudin, A.; Fyfe, C.A.; Vines, S.M., J Appl Polym Sci, 28, 2611, 1983. 15. King, P.W.; Mitchell, R.H.; Westwood, A.R., J Appl Polym Sci, 18, 117, 1993. 16. Kendall, E.K.; Frei, J.; Benton, L.D.; Trethewey, B.R., 1998 TAPPI Plastic Laminates Symposium

Proceedings, 81, 1998. 17. Kohl, W.S.; Trethewey, B.R.; Frei, J., 1996 TAPPI Plastic Laminates Symposium Proceedings,

159, 1996. 18. Rohn, C.L., Analytical Polymer Rheology, Hanser, New York, 1995. 19. Menard, K.P., Dynamic Mechanical Analysis, CRC Press, New York, 1999.

AKNOWLDGEMENT The authors would like to thank Wilsonart International, Inc., for supporting this research work and TAPPI for allowing us to present this work.

Table 1. Phenolic resin samples used in this study. The catalyst concentration is reported as the normalized weight/weight %.

Sample Catalyst concentration (%) A 0 B 2 C 4

Table 2. HPDSC results. Results are reported as the activation energy, E, and the reaction constant, k (125 C).

Sample Activation Energy, E k (125 C) A 162.9 0.023 B 129.5 0.048 C 82.1 0.085

Table 3. Time for 50% reaction conversion reported for temperatures, 115 C, 125 C, and 135 C. Results reported in units of time (minutes).

Sample 115 C 125 C 135 C A NM 74 22 B 58 21 8 C 17 8 5

NM = not measurable. Table 4. Rheology tremperature ramp results reported as the gel point (G = G) in C.

Sample Gel Point ( C) A 138 B 135 C 132

Table 5. Rheology isothermal temperature results reported as the onset of reaction and the onset of vitrification. Results are reported in time (seconds).

Sample Temp. ( C) Onset of Cure Onset of Vitrification A 115 1191 1422 A 125 549 714 B 115 1056 1509 B 125 400 460 C 115 616 1077 C 125 235 568

Table 6. DMTA temperature ramp results. Results are reported as the Tg of the resin, onset of reaction, the gel point, and the vitrification point. All of these results are reported in C.

Sample Onset of Reaction Gel Point Vit. Point A 141.5 167.6 174.8 B 131.2 144.3 153.3 C 124.6 137.6 146.9

Table 7. DMTA isothermal testing results. Results are reported as the onset of cure, the gel point, and the onset of vitrification. All of the results are reported in time (seconds).

Sample Temp. C Onset of Cure Gel Point Onset. Of Vit. A 115 1567 3415 4578 A 125 948 2156 2762 A 135 622 1445 1730 B 115 937 1506 2382 B 125 649 985 1522 B 135 499 715 1010 C 115 708 1066 1587 C 125 529 755 1195 C 135 436 567 816

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Figure 3. Isothermal Rheology results for the onset of reaction and the onset of vitrification vs. catalyst concentration at 115 C.

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Figure 4. Isothermal rheology results for the onset of reaction and onset of vitrification vs. catalyst concentration at 125 C.

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Figure 5. DMTA heat ramp test results for the onset of reaction, the gel point, and the onset of vitrification vs. catalyst concentration.

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Figure 6. DMTA 115 C isothermal results.

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Figure 7. DMTA 125 C isothermal results.

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Figure 8. DMTA 135 C isothermal results