Cure of Phenol-Formaldehyde Resin PROGRESS DETERMINED BY ULTRASONIC WAVE PROPAGATION G. A. SOFER, A. G. H. DIETZ, AND E. A. HAUSER Massachusetts Institute of Technology, Cambridge, Mass.
. ‘T s
can be determined from the velocity of propagation and the attenuatioii of ultrasonic waves. The latter technique had been applied t o several thermosetting resins for the purpose of observing the progress of polymerization and characterizing the extent of cure as cure progresses. A strong dependence of the velocity and attenuation of longitudinal ultrasonic waves on the time, and hence the extent, of cure was established for several three-dimensional polymer systems, including a commercial polyester resin, a three-dimensional polymethyl methacrylate, and a resorcinol-formaldehyde resin ( 7 ) . In the present investigation, the pulse-propagation technique was extended to a phenol-formaldehyde molding resin.
HE manufacturers of the thermosetting resins have long recognized the handicap resulting from a lack of a simple and reliable method for determining the degree of polymerization or cure of such resins, as cure progresses. The measurement of dynamic properties of high polymers has gained wide interest recently, particularly since the development of a pulse-propagation technique whereby dynamic properties
DYNAMIC PROPERTIES OF HIGH POLYMERS
7
L-liiGZJ
Figure 1.
RXCD muI\nw
Functional Block Diagram of Ultrasonic Pulse System
Figure 2.
Several mechanical models have been proposed to describe the response of viscoelastic materials to application of stress, A widely accepted treatment of the problem is independent of any model, and merely assumes the existence of an elastic and a viscous component. An imaginary modulus is used to describe damping losses in a manner analogous to the treatmknt of damping in electrical oscillations. The equation of motion of a vibrating system of mass M is written:
where E, and E2 are, respectively, the elastic and viscous moduli and F ( t ) is the external force as a function of time.
Ultrasonic Pulse-Propagation Apparatus and Mold
2743
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
2744
but a distance x teger.
Vol, 45, No. 12
+ nX, where X is the wave length and n an in-
Taking logarithms, ALIGNMENT P I N
Thus fi is the reciprocal of the distance of wave travel corresponding to a decrease of vibrational amplitude by a factor l/e. Equation 5 represents the attenuation in t e r m of nepers. A more common expression of attenuation, in terms of decibels, is:
-.
I
KNOCKOUT PLATE
Figure 3.
Here, J stands for wave intensity and A for the amplitude, It can be shown that a neper is equal to 8.686 decibels. To relate the elastic and viscous moduli to the parameters p , 0 , and v, Equation 3 is substituted in Equation 2. After separating real and imaginary parts and rearranging, we have:
EJECTION PIN
Cross Section of Ultrasonic Mold Assembly
and When the period of vibration is short enough to approach the time required for propagation of a wave through the medium, a wave equation may be used: For the range, of ultrasonic variables encountered with high polymers, ~ 2 v 2 < < w 2 ; hence, Equations 7 and 8 can be simplified to:
y = yo e-wei[ot
- (w/u)zl
(3)
where p is an attenuation factor whose dimensions are reciprocal length, w is the angular frequency, v is the velocity of wave travel, t is the time, y is the displacement from equilibrium position, and x is the distance in the direction of wave travel. The significance of p becomes apparent if the displacement a t time t and distance x is compared to the displacement at the same time
RUN
&
,
,
,
,
,
0
616
99
*e20
loo
(9)
Substituting for E X ,and recognizing that a neper equals 8.686 decibels,
E2 =
pv3L ~
4.343
where L is the attenuation in decibels per unit length. The above derivations permit the evaluation of the elastic and
60
99.5
X 619
p ~ 2
PREHEAT FINAL TEMP. *C.
TEMP,
*C. 7
=
E1
where p is the density of the propagation medium. The solution of this equation may be written in the form:
90
10
0
.e
A
a 6
.E
1.0
12
1.4
1.6
1.8
0.0
2.2
2.4 2.6
2.8
3.0 3 1
Figure 4. Relation of Longitudinal Ultrasonic Attenuation of Pure Phenolic Resin to Cure Time Pressure 2000 Ib./sq. inch.
Frequency 2.1 Mc.
Figure 5. Relation of Longitudinal Ultrasonic Attenuation of Pure Phenolic Resin to Cure Time Pressure 2000 Ib./sq. inch.
Frequency 2.1 ME.
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
December 1953
2745
EXPERIMENTAL
.
Figure 6. Relation of Longitudinal UltrasonicAttenuation of Pure Phenolic Resin to Cure Time Pressure 2OOO lb./aq. inch.
Frequency 2.1 Mc.
viscous moduli from a measurement of the propagational velocity and the attenuation of the ultrasonic wave. They apply equally to longitudinal and shear waves.
The essential features of a pulse-propagation system are: the generation of a high-frequency electrical pulse, conversion of this pulse to mechanical vibrations, propagation of the mechanical pulse through the test specimen, conversion of the mechanical pulse back to an electrical pulse, and detection of the received pulse and measurement of its time of arrival and its amplitude on an oscilloscope screen. The delay resulting from the travel of the ultrasonic pulse through the specimen is in the order of a few microseconds; special provisions are therefore required for its measurement. A pulse-modulated ultrasonic signal is generated intermittently about 1000 times per second, the duration of each individual pulse being about 10 microseconds. Each pulse travels through the specimen and is converted into a pulse-modulated electrical signal a t the receiving transducer. The electrical signal is fed to a cathode-ray oscilloscope where an electron beam scans the oscilloscope screen once for each individual pulse generated. As the delay changes, so does the point in the electron sweep cycle where the received pulse begins. The initial point of the electron sweep can be advanced or retarded by manipulating a potentiometer dial calibrated in microseconds. If the initial point of the electron sweep is made to coincide with the point of arrival of the received pulse-modulated signal, the delay of ultrasonic wave through the specimen can be obtained by comparing the delay dial readings for several specimens of different thickness and extrapolating to zero thickness (Figure 1). The attenuation of the ultrasonic wave through the specimen is determined by comparing the voltage of the received pulse to the voltage corresponding t o a specimen of zero thickness. AE the voltage of the received pulse is proportional to the amplitude of the ultrasonic wave a t the outer surface of the receiving transducer, we have
LX = 20 logio(Ao/A)
= 20 logio(Eo/E)
(12)
where Lx is the attenuation of the specimen in decibels, A represents the amplitude of the ultrasonic wave, and E is the voltage.
RUN
IO0
.e
.4
.6 CURE
.8 TIME
-
TEYP.'G. 153.5
I HOURS
Figure 7. Relation of Longitudinal Ultrasonic Attenuation of Pure Phenolic Resin to Cure Time Pressure 2000 lb./sq. inch.
.
e629
Frequency 2.1 Mc.
Figure 8. Relation of Longitudinal Ultrasonic Attenuation of Pure Phenolic Resin to Cure Time Pressure 2000 lb./sq. inch.
Frequency 2.1 Me.
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Figure 3 shows the detailed construction of the mold and transducers. The m d d was electrically heated, continuously variable transformers being employed to regulate the temperature. Mold temperature variation of less than 2 " F. could be maintained for several hours.
8.C
0620
.2
.4
.6
.8
1.0
I2
1.4
100
1.6
1.8 2.0
2.2
2.4 2.6 2 8 3.0
3.2
Figure 9. Relation of Longitudinal Ultrasonic Velocity of Pure Phenolic Resin to Cure Time Pressure 2000 lb./sq. inch.
,4n unfilled phenol-formaldehyde molding compound vas used as the subject of exhaustive tests in this study. This resin was prepared from 6 parts by weight of 3775 aqueous formaldehyde solution and 5 parts of "natural" phenol, the latter being a mixture of 800/, (by weight) phenol and 20% "cresol," where the cresol is predominantly the ortho isomer but contains also some of the ot,her two isomers and a small amount of xylenols. The molal ratio of formaldehyde (CHzO) to phenolics was therefore 1.43. The initial polymerization reaction v a s carried out in the presence of 1.25% barium hydroxide, and 4.2 % hexamethylenetetramine (based on the final weight of the molding resin) was reacted into it (1, 2 ) . The resin was supplied by the Bakelite Co. with the designation BR 17840. An auxiliary hand-operated mold was eniplo? ed t o preheat the molding resin to 85' C. and to densify the charge by subjecting it t o a pressure of 3000 pounds per square inch. Molding pressure was kept constant at 2000 pounds per square inch.
Frequency 2.1 >IC. RESULTS
The dependence of velocitj and attenuation on the cure time and temperature of the phenolic resin (BR 178.10) is illustrated in Figures 4 to 12. i l l 1 curves indicate a rise in the longitudinal attenuation, followed by a notable dpcrease and a leveling off region as cure progresses. The longitudinal velocity increased nith cure, reaching an asj mptotic value. Figure 13 indicates a very similar dependence on cure time for a commercial filled phenolic resin (Monsanto Resinox S o . 2000 TAN). Figure 14 illustrates the fact that optimum tensile strength occurs long before the asymptotic portion of the attenuation curve is reached. Although pronounced variation of ultrasonic velocity and attenuation was observed at molding temperatures, variation at room temperature with the evtent of cure rias rather small (Figure 15).
t 0
s 5
DISCUSSION A'VD DEVELOPMENT OF RESULTS
.o
2
.4
.6
.a
LO
I,2
1.4
Figure 10. Relation of Longitudinal Ultrasonic Yelocity of Pure Phenolic Resin to Cure Time Pressure 2000 lh./sq. inch.
The variation of lorigitudiiial velocity and atxenuatioii %ith cure of phenolic molding resins is in line with the general type of
Frequency 2.1 RIc.
The subscript zero refers to conditions corresponding to a specimen of zero thickness, obtained by measurement of three or more samples of diflerent thickness and extrapolating to zero thickness. By this method any error due to beam reflection a t the interface between specimen and transducer is eliminated. The attenuation per unit length, L, is arrived a t upon division of the attenuation of the specimen by its thickness, z. For pulse-propagation measurements under niolding conditions, ultrasonic transducers were incorporated into the force and cavity of a representative compression mold. This mold and the ultrasonic pulse propagation apparatus are shown in Figure 2. A quartz crystal is employed as a source of longitudinal ultrasonic vibrations. A steel plug, ultrasonically insulated by a Teflon ring, conducts the ultrasonic pulse from the crystal to the test specimen and vice versa. A frequency of 2 megarycles was chosen so as to utilize fully the attenuation range measurable by the apparatus used. A much higher frequency results in a highly attenuated pulse, while a much lower frequency does not yield sufficient change in attenuation with cure, and hence results in reduced accuracy. Large quartz crrstals (1 inch in diameter) n ere used and the specimen thickness was kept a t about 0.25 inch, in order to minimize any error resulting from diversion of the ultrasonic beam. KOremarkable distortion of the received pulse was noted, except a t the tail end where the pulse seemed to taper off gradually beyond 10 microseconds from the head of the pulse.
.I
.3
.S
.I
.B
IJ
Figure 11. Relation of Longitudinal Ultrasonic Velocity of Pure Phenolic Resin to Cure Time Pressure 2000 lb./sq. inch.
Frequency 2.T Mor
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INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1953
10.0
kl
00
?
.r
3O
to
0 X
616 612
6.0
?r
x
610
b P
*
139 137 I39
60
W
' 7.0 50
0
.P
.6 .a CURE TIME- HOURS
.4
m .
1.0
Figure 12. Relation of Longitudinal Ultrasonic Velocity of Pure Phenolic Resin to Cure Time Pressure 2000 lb./sq. inch.
Frequency 2.1 Mc.
40
10.00
so 9.00
changes observed with several three-dimensional polymers ( 7 ) . Furthermore, the dependence of ultrasonic parameters on cure is strikingly similar to the changes resulting from temperature variation alone reported by Nolle and Mowry ( 6 ) , Nielson (6),and Mikhailov and Gurevich (4). In all cases the attenuation passes through a sharp maximum, and the propagation velocity increases as the polymer acquires greater and greater rigidity. The maximum rate of increase of the velocity does not always coincide with the point of maximum attenuation, but generally lies close to it. It may be well to assume that these variations are directly related to the physical state of the polymer, and that their dependence on the extent of cure or the temperature is a consequence of the resulting modification in physical state. Examination of Figures 4 to 12 suggests that the plots of velocity and attenuation may be useful in determining the rate of reaction. The greatest slope of the curve of attenuation us. cure, beyond the point of peak attenuation, and the maximum slope of the curve of velocity us. cure, appear to be useful empirical param-
20
8.00
IO 7.00 0
J
1
,I
A
.S
-6
J
-8
.O
1.0
Figure 13. Relation of Longitudinal Velocity and Attenuation of Resinox (Monsanto Resin 2800 TAN) to Cure Cured at 134O C. and 2000 lb./s
.inch pressure. 8un 521
Frequency 2.05 Mc.
eters in this connection. In Figure 16 the logarithm of the greatest attenuation slope is plotted against the reciprocal of the absolute temperature. A straight line is obtained, having a slope corresponding to an activation energy of 20,400 calories per gram-mole. A similar straight line is obtained from the greatest slope of the velocity curve, having an activation energy of 20,800 calories per gram-mole. The activation energies of the polymerization reaction of a wide variety of phenolic resins were determined by the Frankford Laboratory ( 3 )and found t o lie between 20,400 and 26,200 calories per gram-mole. These data were ob-
m
4.
0
UWC
'5i
70
L.0 $ 0
IS
1.
11.0
10.0
90
m
$
E : x
w€
eo
CURE
TIME
IN
HOW8
Figure 14. Dependence of Certain Properties of Pure Phenolic Resin on Cure Time Attenuation measured at 2.1 ME. and mold temperature 126.5O ko 128.5O C. Runs 603 to 609. Superficial hardness, Rockwell 15-T scale
w)
i
.e
A
.0 HWRS
8 Of
ID
Lz
u
t
CURE
Figure 15. Relation of Longitudinal Ultrasonic Velocity and Attenuation of Pure Phenolic Resin to Cure Time Pressure 2000 Ib./sq. inch.
Frequency 2.1 Me.
Runs 603 to 609
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Vol. 45, No. 12
CONCLUSIONS
The strong dependence of ultrasonic propagation parameters on the extent of cure of phenolic molding resins suggests that they may be used as criteria for the extent of polymerization of phenolic and other thermosetting molding resins. Measurement of these parameters can be made while the resin is undergoing cure in a typical compression mold, thus providing instantaneous information regarding the progress of cure. The technique also seems capable of providing useful indication of the rate of cure. This method, however, will not furnish absolute information relative to the extent or rate of reaction, but may be calibrated against any technique which might yield such information. LITERATURE CITED
(1) Adams, J. H., Bakelite Co., Union Carbide &- Carbon Corp.,
personal communication, July 24, 1952. Auerbach, V., Bakelite Co., Union Carbide & C a r b o n Corp., personal communication, Dee. 3, 1952. (3) Green, R. B., Barrett. Division, Allied Chemical & Dye Corp., personal communication, Dee. 22, 1952. (4)Mikhailov, I. G., and Gurevich, S. R., Zhur. Eksptl. i Teort. Fiz., 19, 193-201 (1949). (5) Nielson, L. E., Am. SOC.Testing Materials Bull., 165, 48-52 (1950). ( 6 ) Xolle, A. W,, and Rlowry, S. L., J . Acoust. SOC.Am., 20, 432 (1948). ( 7 ) Sofer, G . A., and Hauser, E. A., J . P o l y n w Sci.. 8, 611 (1952).
(2)
1000/T*K
Figure 16.
Relation of Maximum Slope of Attenuation t o Cure Temperature
Data plotted against reciprocal of absolute temperature
tained from the Arrhenius equation, using the rate of change of viscosity of polymer solutions, reacting under alkaline conditions (pH 7.0 to 8.5) a t a series of different temperatures, as an index for the rate of reaction.
RECEIVF for ~ reriew March 30, 1953. ACCEPTED August 15, 1963. Work descrihed is one aspect of the research which constit,uted a doctor of science thesis in the Department of Chemical Enginearing a t the Massachusetts Institute of Technology, Cambridge, llass., under the supervision of E . A. Hauser. Work sponsored by the Plastics Group, Manufacturing Chemists’ .4ssociation, Washington, D. C., under the general direction of Albert G. H. Dietz. professor of structural engineering and director of the Plastics Research Laboratory, RIassachusetts Institute of Technology, Cambridge, Mass.
Viscosity Changes in Thermosetting Resins I
D. I. MARSHALL Development Laboratories, Bakelite Co., Division of Union Carbide 6% Carbon Corp., Bound Brook, N. J .
S
OLID thermosetting resins have been manufactured and
used for many years for molding material and bonding applications in which performance is related to viscosity and reaction speed. Therefore, viscosity-temperature relations up to fabricating temperatures and viscosity-time curves in the fabricating temperature range are of primary concern to the thermosetting plastics industry. However, serious attempts to measure the properties effectively have been announced only recently. The standard test methods used by the industry, such as the hot-plate gel time test, have been useful for production control but have not yielded fundamental data. Recently published work on flow properties of phenolic resins has included three papers dealing with the viscosity of the Novolak (nonsetting) type of resin (2, 4,&). The work included no results on reactive phenolic resins, however. Two methods have been announced for measuring the viscosity of thermosetting resins and following viscosit,y change during reaction, but data had not appeared a t this writing. The methods include a rotating shearing disk viscosity method by Sontag ( 7 ) and an ultrasonic viscosity method by Roth and Rich (6). This paper describes the procedure and presents results obtained using a third method of measuring rapid viscosity changes -namely, the parallel plate plastometer method. Results ob-
tained on several phenolic resins and a silicone resin are presented. The method is not continuous and is not applicable to materials departing appreciably from Newtonian behavior. However, it has the very desirable features of using layers of resin thin enough for rapid temperature equilibrium, no cleaning problem, and an extremely wide viscosity range. THEORY OF METHOD
The theory of the parallel plate plastometer and the application of the device to the measurement of the viscosity of Novolak resins have been described by Dienes (3, 3 ) . Briefly, a test specimen is squeezed between two parallel plane surfaces, causing radial flow, and the distance between the planes, h, is measured as a function of time. I n accordance with the theory, a plot of l / h 4 versus time, equivalent to a deformation-time curve, is constructed and the slope determined. The equation for viscosity is 1 7 =
8.21 X 1 0 W mV2
where 7 = viscosity, poises; W = load on sample, kg.; V = volume of sample, cc.; ?n = slope of the plot, cm.? sec.-l
