THERMOSETTING RESINS
Ultrasonic Waves as a Measure of Cure N TWO previous papers the use of ultrasonic waves to Ilated follow the cure of polyesters, phenol-formaldehyde, and reresins has been described. By means of barium titanate or (1, 2)
quartz transducers, pulses of high-frequency pressure waves were set up in the material being investigated. Velocity of transmission and attenuation of the energy of the waves were measured as the material underwent cure. For cast polyesters, barium titanate transducers were placed in the walls of a simple container. For phenol-formaldehyde resins quartz transducers were built into the force and cavity of a heated compression mold, The frequencies employed were 2.2 to 2.4 megacycles. The pulse length was 10 microseconds. Velocities were measured to 1 0 . 5 % and attenuations to hO.5 decibel per inch. A noticeable rise in velocity of transmission occurred as the materials passed from the uncured through the gel stage to the fully cured condition. Attenuation rose to as much as 90 to 95 decibels per inch during the gel stage and then dropped in some instances to less than 20 decibels per inch in the fully cured materid. Acetone extractions, hardness tests, and strength tests were made on specimens of varying cure. Additional tests, utilizing the same compression mold as was used in the phenolic series, have been made on both pure and filled melamine resins at a frequency of 2.1 megacycles per second. The behavior in most respects is similar to that of phenolic and polyester. As shown in Figure 1, the velocity increases during the gel stage and levels off at a maximum value in the cured material. The final attenuation is considerably lower than the maximum attained in the gel stage. Increasing the cure temperature from 136" to 153" C. increases the final velocity and decreases both the dip in the attenuation curve and the final attenuation. The dip noted in the melamine-formaldehyde attenuation curves was not found in the earlier work with phenolics and polyesters. The reason for it is not clear. Figure 2 show that the maximum attenuation is reached
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1.5 WATER ABSORPTION ( 3 0 MIN. B O I L ) I .o
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Figure 2.
Flow and cure of unfilled melamine resin with cure time Temperature, 155'
C.
after the setting point of the plastic and minimum water absorption is attained shortly after maximum attenuation. Because the variation of ultrasonic velocity and attenuation is similar for the several three-dimensional resins investigated, it may be reasonable 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 of physical state. Similar tests carried out over a wider range of frequencies would be desirable because such data might indicate structural relaxations within the materials as a function of cure. Acknowledgment
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The information on melamine resins was supplied by W. C. Schneider, I. H. Updegraff, H. P. Wohnsiedler, and C. R. Stock of American Cyanamid Go., part of the work having been done in collaboration with the Plastics Research Laboratory a t MIT, sponsored by the Manufacturing Chemists' Association.
0
9.6
literature Cited (1) Sofer, G. A., and Hauser, E. A., J. Polymer Sci. 8, 611 (June 1952),
9.4
(2) Sofer, G . A., Dietz, A. G. H., and Hauser, E. A,, IND.ENG. CHEM.45, 2743 (1953). 9.2
RECEIVED for review July 7, 1955.
ACCEPTED November 4, 1955.
A. G. H. DIETZ, E. A. HAUSER, F. 1. MCGARRY, AND G. A. SOFER* 0.032
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0.096
0.128
CURE T I M E
Figure 1.
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Velocity and attenuation of melamine resin
Pressure, 2000 Ib./sq. inch; frequency, 2.1 mc.
January 1956
0.256
HOURS
Massachuseffs Instifufe o f Technology Cambridge, Mass. 1
Present address, Netiond Research Corp., Newton, Mass.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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