The document summarizes a study that used three thermal analysis methods - high pressure differential scanning calorimetry (HPDSC), rheology, and dynamic mechanical thermal analysis (DMTA) - to analyze the polymerization of phenol formaldehyde resins with varying catalyst concentrations. The results showed that increasing the catalyst concentration decreased the activation energy and increased the reaction rate for all three methods. Higher catalyst levels led to lower gelation and vitrification temperatures by DMTA and faster onset of curing, gelation, and vitrification by rheology and DMTA. The three methods were effective in measuring how catalyst concentration impacts the reaction kinetics of phenolic resin curing.

1. 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)

2. 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, σ, will result in a measurable deformation or strain, γ. From this
response, a complex modulus (G*) is measured as the material’s 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 material’s ability to store and return energy, i.e. Hookean
spring or elastic like behavior. The loss modulus, G”, is a measure of material’s ability to absorb energy,
i.e. viscous behavior. The complex viscosity, η*, of the sample is determined from the complex modulus
and frequency of oscillation, ω*, by equation (4) (18).
η* = G*/ω* (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, σ, will result in a
measurable deformation or strain, γ. From this response, a complex modulus (E*) is measured as the
material’s 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).

3. E* = E’ + iE” (5)
The storage modulus, E’, is a measure of the material’s ability to store and return energy, i.e. Hookean
spring or elastic like behavior. The loss modulus, E”, is a measure of material’s ability to absorb energy,
i.e. viscous behavior. The ratio of the loss modulus to the storage modulus is known as tan δ (the damping
factor, as shown in equation (6) (8).
tan δ = 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 δ 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 δ 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.

4. 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 δ, the peak in the loss modulus (E”), and the maximum in the storage modulus (E’). The peak in the
tan δ 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 δ curve than the peak
reaction temperature peak in the tan δ.

5. 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.
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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.
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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.

6. 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

7. 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

8. 0
10
20
30
40
50
60
70
80
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
Catalyst Conc.
Time(min.)
115 C
125 C
135 C
Figure 1. HPDSC results, the calculated time to reach 50% cure at 115 °C, 125
°C, and 135 °C.

9. 131
132
133
134
135
136
137
138
139
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
Catalyst Conc.
Temp.(C)
G'=G"
Figure 2. Rheology crossover temperature (G’ = G”) vs. catalyst concentration.

10. 0
200
400
600
800
1000
1200
1400
1600
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
Catalyst Conc.
Time(sec.)
Onset of Cure
Onset of Vit.
Figure 3. Isothermal Rheology results for the onset of reaction and the onset of
vitrification vs. catalyst concentration at 115 °C.

11. 0
100
200
300
400
500
600
700
800
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
Catalyst Conc.
Time(sec.)
Onset of Cure
Onset of Vit.
Figure 4. Isothermal rheology results for the onset of reaction and onset of
vitrification vs. catalyst concentration at 125 °C.

12. 0
20
40
60
80
100
120
140
160
180
200
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
Catalyst Conc.
Templ.(C)
Onset of Reaction
Gel Point
Vit. Point
Figure 5. DMTA heat ramp test results for the onset of reaction, the gel point,
and the onset of vitrification vs. catalyst concentration.

13. 0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
Catalyst Conc.
Time(sec.)
Onset of cure
Gel Point
Onset of Vit.
Figure 6. DMTA 115 °C isothermal results.

14. 0
500
1000
1500
2000
2500
3000
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
Catalyst Conc.
Time(sec.)
Onset of cure
Gel Point
Onset of Vit.
Figure 7. DMTA 125 °C isothermal results.

15. 0
200
400
600
800
1000
1200
1400
1600
1800
2000
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
Catalyst Conc.
Time(sec.)
Onset of cure
Gel Point
Onset of Vit.
Figure 8. DMTA 135 °C isothermal results