1.93
8.8
6.6
been boiled for 150 hours in 60% HzS04 without loss in strength or change in dimensions. The properties of the laminates produced from blue asbestos felts depend on the pressing conditions and the type of lay-up employed. The physical properties to be expected from typical hand lay-up and pressure-molded laminates are shown in Tables V, VI, and V I I . Blue Asbestos Rovings. Finally, when the maximum possible strength is required, preimpregnated blue asbestos rovings are recommended. These materials are characterized by high wet strength and ease of fabrication and can be readily arranged into laminates having flexural strengths of 80,000 to 90,000 p.s.i. and tensile strengths of 60,000 p.s.i. I t will be realized that because of their high strength, coupled with excellent chemical resistance, preimpregnated blue asbestos felts, papers, and rovings are particularly suitable for the manufacture of a wide range of articles to be used in the chemical industry. .Inumber of manufacturers are producing chemically resistant reinforced plastic pipes based on blue asbestos, and the demand for this is steadily rising. Shafts, pump impellors, acid trays, and various parts of chemical plants are other typical uses.
0.71
6.8
5.48
Literature Cited
0.54
0.22
0.18
0.031
0.015
0.011
Table VI.
Mechanical Properties of Pressure laminates Processed with Crocidolite Asbestos Felt Epoxy Polyester Phenolic Resin/ Resin / Resin/ Crocidolite Crocidolite Crocidolite Felt Felt Felt 1000 210 300 Molding pressure, p.s.i.
320
Molding temp., OF. Molding time, min. Tensile strength, p.s.i.
26,00028,000 3.25
212 60 20.00022,000 42,00043,000 30,00031,000 2.8
250 30 24,00025,000 44,00045,000 24,00025,000 3.06
1.86
1.6
1.7
30
28,00031,000 57,000-
Flexural strength, p.s.i.
58:OOO Compressive strength, p.s.i. Young's modulus in tension! p.s.i. X 10-6 Specific gravity
Table VII.
Electrical Properties of Laminates Reinforced with Crocidolite Asbestos Felt EPOXY Polyester Phenolic Laminate Laminate Laminate
Dielectric constant at at 1 kc./sec. Dielectric constant at 10 kc./sec. Power factor at 1 kc./ sec. Power factor at 10 kc.jsec. Volume resistivity, megohm cm.
H-FILM-A
3.89 X 105 1.66 X 106 5.75 X
lo5
(1) Broutman, L. J., McGarry, F. J., SOC.Plastics Ind. Conference, Reinforced Plastics Division, Chicago, Ill., February 1958. (2) Gaze, R., Zukowski, R., ,Vature 183, 35-7 (1959). RECEIVED for review April 4, 1963 July 12, 1963 ACCEPTED 18th Reinforced Plastics Division Conference, Society of the Plastics Industry, Chicago, Ill., February 1963.
NEW HIGH TEMPERATURE
D I ELECTRIC LEONARD E. A M B O R S K I Yerkes Research and Development Laboratory, E. I. du Pont de .Vemours @ Co., Inc., Buj'alo,
.V. Y .
A polyimide film resulting from the polycondensation reaction between pyromellitic dianhydride and an aromatic diamine has been characterized as a high temperature dielectric. It has outstanding mechanical properties from liquid helium temperature to 500" to 600" C. Polypyromellitimide film possesses excellent electrical properties-high resistivity, high dielectric strength, and low loss. The dielectric properties are relatively constant with temperature and frequency. Both the mechanical and electrical properties remain a t a high level after prolonged exposure to elevated temperatures. This flameproof film which has no known solvent i s also infusible. Above 800" C., it begins to char. A high degree of radiation resistance has also been observed. The unique stability of this organic polymer i s attributed to its cyclic structure, possessing both aromatic and heterocyclic rings. HE RAPID DEVELOPMENT in recent years of electrical and Telectronic devices particularly for defense and space projects has necessitated a myriad of dielectric materials to operate under extreme conditions of temperature, radiation, chemical environments, and electrical stress (2, 4-6). T o fill these needs, a variety of organic polymers has been developed as electrical insulating materials. However, one of the limitations of organic insulating materials has been the maximum use temperature. At the other end of the thermal scale many organic insulating materials embrittle a t sub-zero temperatures thus eliminating them from consideration in many cryogenic areas.
Extensive research to s) nthesize tough, high temperature organic polymers ith good electrical properties has resulted in the development by Du Pont of polyimide polymers which exceed class H (180" C.) insulation requirements. Resulting from this reccarch are a varnish and a n enamel known by the tradename Pbre-hfL. I\ hich are fast gaining acceptance as high temperature insulation for \tire and cables. S o w available in experimental quantities is another polvimide product, a film designated as H-film. Currently made in gages from 1 mil (0,001 inch) to 5 mils, this film is a polypyromellitimide, resulting from the polycondensation reaction between pyromellitic dianhydride and an aromatic diamine as shown in Figure 1. VOL
2
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SEPTEMBER 1963
189
Table II.
Comparative Dielectric Properties l o 3 c.P.s., 23' C., 50% RH
Material 0
Pyromellitic Dianhydride
Polypyromellitimide Polyethylene terephthalate (Poly )tetrafluoroethylene/ hexafluoropropylene copolymer Polyethylene Polystyrene Cellulose acetate Poly(vinyl fluoride)
Aromatic Diamine
Polyamide Acid
Die Iectric Constant
Dissipation Factor
3.4 3.2 2.1
0.0020 0.0021 0.0002
2.3 2.5 5.0 7.5
0.0002 0.00015 0.0250 0.050
Polyimide
Figure 1 . Condensation reaction of pyromellitic dianhydride and an aromatic diamine
polyester film. H-film does not melt or burn but will char at temperatures above 800" C. The stiffness of polypyromellitimide film is similar to that of polyethylene terephthalate film. Electrical Properties
The highly aromatic character of this polymer contributes to its extremely good thermal and radiation resistance (no changes after l O Q r. dosage of gamma rays) and results in a fairly stiff but extremely tough film. Combining these attributes with high electrical resistance, low dielectric loss, and outstanding electrical breakdown strength, this flameproof film which has a zero strength temperature (maximum temperature a t which it sustains a stress of 20 p.s,i.) in excess of 800' C., offers the electrical insulation engineer a new dimension in the design and use of high temperature electrical and electronic components. Furthermore, the over-all useful temperature range begins at the cryogenic level, where H-film maintains flexibility (elongation of 50%) below liquid nitrogen temperature. The polyimide film has a density of 1.42 grams per cc. and is characterized by a light golden color in thin gages. Comparative values of pertinent properties are given for polyimide film and other dielectric films in Table I ( 7 > 3, 8 ) . As seen from these data, H-film possesses the toughness of polyethylene terephthalate a t room temperature, and retains it to a greater degree at temperatures far above the melting point of the
Table 1.
Electrical insulating materials are used in two general ways : as insulation against electrical stresses where a high resistivity and high breakdown strength are important, and to provide capacitance, that is, essentially a circuit element in which the dielectric material exhibits a capacity to store electrical energy. It is in the latter category in which the dielectric constant (specific inductive capacity) and the dissipation factor, a measure of the dielectric loss or electrical energy lost as heat, are important. Polypyromellitimide film has electrical properties which suggest its utility in both areas--Le., as a high temperature insulation for motors, coils, wire, cable, and transformers, as well as a capacitor dielectric. The presence of a large number of carbonyl groups would normally result in a high dielectric constant but the symmetry of the polypyromellitimide molecule coupled with the highly aromatic character restricts somewhat the polarizability, resulting in a dielectric constant intermediate between the cellulosics and the hydrocarbons. The moderately high dielectric constant and low dissipation factor suggest its utility as a high temperature dielectric material.
Comparative Properties of Dielectric Films Thickness, 0.001 inch
Tenrile Modulusb P.S.I. X
Film
Density 230 c,,' Tensile Strengthb Gram/ P.S.I. x 10' 108 Cc. 23' 200' 500" 23O 200'
PneumaticC Zero Ultimate I m p , Strength,c Elon ation, 23 C., Meltingd C. Kg.-Cm./ P:int, (Sustains 23' 200' Mil c. 20 P.S.I.)
k
Glass Tranritione Temp.,
' c.
Shrinkage -~ % "C.
H-poly imide
1.42
25
15
5
400
250
70
90
6
None
815
>500
0 . 5 300
Polyethylene terephthalatef (Po1y)tetrafluoroethylcne/hexafluoropropylene copolymer0 Poly( vinvl fluoride)&
1.39
23
7
..
500
50
100
125
6
260
250
70
1 . 5 150
2.15
3
0.4
..
43
2
300
...
4
290
250
85
...
1.37
15
1
..
300
1
150 200
5
...
300
55
A S T M D 7 5 0 5 - 5 7 7 . b A S T M 0-638. fluorocarbon film. h Tedlar P V F f i l m .
190
D u Pont test.
d
Fisher-Johns.
I L E C PRODUCT RESEARCH A N D DEVELOPMENT
4
Modulus-temperature.
f
Resistante to Organic Solvents No known solvent Good
...
Excellent
5 130
Excellent
Mylar polyester film.
9
TeJon FEP
Dielectric Properties
The dielectric constant and dissipation factor of H-film are compared with other dielectric materials in Table 11. These were measured according to ASTM Method D150-59, using silver-painted electrodes. As shown in Figures 2 and 3 the dielectric constant and dissipation factor are relatively constant with temperature from room conditions to 220" C. However, below room temperature, a dielectric loss peak appears, shifting to lower temperature with decreasing test frequency. The peaks bear a strong similarity to loss peaks observed for oriented polyethylene terephthalate polyester film in this same temperature region. However, where another set of loss peaks is found for the polyester film just above its glass transition temperature ( S O o C.), corresponding peaks do not exist for polyimide film. Actually, there is a tlvofold decrease in the dissipation factor of the polyimide in going from room temperature to 220' C. The enhanced mobility of the dipoles in polyethylene terephthalate in the amorphous regions when above the glass transition has no counterpart in polypyromellitimide. To date, in investigations u p to 500" C., no glass transition temperature has been found for this polyimide polymer. As is commonly found for polymers, the dielectric absorption process can be described by the molecular theory of Debye in which the relaxation time of the dipolar molecules depends upon the hindrance to free rotation. Plotting the frequency, fm, of the loss maximum against the temperature a n Arrehenius-type plot is obtained : f m = A,,, (-E/'RT). The activation energy for this low temperature dielectric process in H-film is 15.4 kcal. per mole as calculated from this plot (Figure 4). Corresponding to the dielectric loss peaks, the dielectric constant has its greatest change in the same region as shown in Figure 2. An unusual and unexplained sharp decrease occurs just above room temperature. The origin of the dielectric loss peaks has not been determined, but may be due to carboxyl or amine end groups. Since these peaks bear a strong resemblance to the low temperature peaks for polyethylene terephthalate, which have been shown to be due to terminal OH groups (7), it may be the OH of the carboxyl end groups. However, the amine end groups are also strong dipoles. Future work will be aimed a t finding the origin of this dielectric dispersion. The symmetry of the four carbonyl groups on each pyromellitic moiety tends to cancel the polarity of each normally highly polar carbonyl group. Ordinarily, the dipoles in the chain lead to a dispersion in the glass transition regions, but since no transition exists for this polyimide there is no such dispersion. As shown in Figure 5: the dielectric constant remains relatively unchanged over the frequency range 100 to 100,000 cycles per second (c.P.s.) for all temperatures from -40' to +250° C. Measurements out to 10 Mc. a t room temperature follow the same pattern, changing only 37, over this range of 5 decades of frequency. On the other hand, the dissipation factor a t room temperature increases \vith frequency quite sharply above 10,000 c.P.s., but is relatively constant a t the elevated temperatures as sho\vn in Figure 6. This is desirable for the high temperature applications of this film. The increase in the room temperature dissipation factor continues upward a t frequencies up to 10 Mc. being 0.0120 at this frequency. From both measurements on single sheets of film, and capacitors wound from single layers of 1-mil poly-imide film between aluminum foil electrodes to a size of 0.06 pf., the temperature coefficient of capacitance is negative as shown in Figure 7. Thus? in high temperature applications of capacitors
29 I
,
-60
l
-40
,
/
-20
I
o
,
I 20
I
I 40
I
I 60
I I 80
1 IOO
l
~ I , 120
, 40
.
I
.
[
ao
160
I
1
200
I
1
220
T E M P E R A T U R E , -C
Figure 2.
Dielectric constant vs. temperature
Frequency 1 O2 to 1 O 5 c.P.s., 0,001 -inch H-film
- 009 - 008 007
006
\ 2
004
0
003
4005
\
003
01
-60 - 4 0
-20
20
0
40
60
80
100
I20
40
160
80
200
220
TEMPERATURE. ' C
Figure 3.
Dissipation factor vs. temperature
Frequency 1 O2 l o 1 O5 c.P.s., 0.001 -inch H-film
from polypyromellitimide film, the problem of "run away" due to heat losses is not likely. Insulation Resistance
The electrical insulating qualities of a material are determined by the volume resistivity and the surface resistivity, the former being of greater significance. Measurements of the volume resistivity were made according to ASTM Method D257-52T, using 2-inch diameter painted silver electrodes. Volume Resistivity
Dry polypyromellitimide film has a volume resistivity a t room temperature in the range of lo1@ ohm-cm. (125 volts d.c.. 6-minute time) to rate with the best of dielectric materials. Comparative values ( 9 ) of volume resistivity are given in Table 111. However, it is a t elevated temperature where other dielectrics are not usable, that the polyimides such as ML and H-film exhibit exceptional mechanical integrity and a high volume resistivity. As seen in Figure 8. at 300" C . the volume resistivity is 10" ohm-cm. The activation energy of the conduction procrss in polyimide is 21.7 kcal. per mole. The high electrical resistance a t these elevated temperatures is a reflection of the retention of structural rigidity \\hich provides a high internal viscosity, thus restricting the mobility of the current carriers. The absence of a glass transition and the highly VOL. 2
NO. 3 S E P T E M B E R 1 9 6 3
191
0:3
I
2827,
38
4 2
50
46
f x IO-'
/
,
I
I
54
,
1
,
I
, 10'
10)
102
10'
FREQUENCY
OK
Figure 6. Dissipation factor vs. frequency (cycles per second)
Figure 4. Frequency of dielectric loss maximum vs. reciprocal absolute temperature
-70'
C. to 1-200'
C.,
0.001-inch
H-film
Activation energy of 15.4 kcal. per mole, 0.001 -inch H-film
-4
20
40
60
80
100
I20
140
160
I80
lo''
200
1 50
100
150
200
250
300
Figure 7. Capacitance change vs. ternpe ra ture
Figure 8. Volume resistivity vs. temperature
1 -mil films
0.001-inch H-film, 100 Mylar C polyester film, activation energy of 2 1.7 kcal.. per mole
I
2 3 4 THICKNESS, mils
5
Figure 10. A.C. dielectric strength vs. film thickness
1000
2000
3000 4000 5000
I&EC
6000
Figure 1 1 . voltage
Dielectric life vs. a x .
'/d-inch electrodes, 23' C., 60% RH
P R O D U C T RESEARCH A N D D E V E L O P M E N T
I00 125 $50 TEMPERATURE, ' C
175
200
Figure 9. Insulation resistance
of capacitors vs. temperature. Capacitors wound with 0.001 inch H-film, 100 Mylar C polyester film, and paper
25
50
75
100
,
,
1
I
I
125
150
175
200
225
TEMPERATURE,
PIC V O L T S
l/d-inch electrodes, 500 volts per sec., rate of voltage rise, 23' C. 60% RH, H-film 192
75
TEM?ERATJRE. 'C
TEMPERATURE, 'C
Figure 12. ture
1 1
'c
A.C. dielectric strength vs. tempera-
0.001 -inch H-film; '/r-inch rate of voltage rise
electrodes; 500 volts per sec.
Table 111.
Comparative Volume Films
Resistivity of Dielectric
Table IV.
Surface Resistivity of Dielectric Films 50% RH, 2 3 " C.
Lo'$
Reszstwity, at 25' C.. 50% R H 18 18-1 9 19 16 17
Material Polypyromellitimide Polyethylene terephthalate Polystyrene Polyethylene (Poly)tetrafluoroethylene/ hexafluoropropylene copolymer
Ohm-Cm. at 200" c. 13-14 11
Mica Polyhexamethylene adipate Poly(viny1fluoride)
. . ..
Current too small to measure.
17
16 15-1 6 13-1 4 13
Polychlorotrifluoroethylene
Film Polypyromellitimide Polyethylene terephthalate (Poly )tetrafluoroethylene/ hexafluoropropylene copolymerPolycarbonate Cellophane Polystyrene
7
Surface Resistivity
The surface resistivity of H-film is given in Table IV along \\ith other dielectric films. Measurements were made with a Keithley 210 Electrometer using A S T M Method D257-52T. Electrical Breakdown Strength
An insulator fails when it can no longer sustain the electrical stress applied to it. Each material has a n intrinsic breakdown strength but in practice this is subject to mechanical flaws or defects in the material. or contaminants which lead to conductive paths and eventual failure. it'hile there has been much discussion as to the true significance of breakdown tests because of the many variables affecting the results, they still are useful in comparing the relative insulating value of materials. As is typical of insulating materials the dielectric breakdown strength of polypyromellitimide film on a unit thickness basis increases with decreasing thickness as shown in Figure 10. The value of 1-mil film under a.c. stress is 7000 volts per mil. comparable to
c
15-1 6 10-1 1 16
.. ..
16
A.C. Breakdown of Dielectric Films
Table V.
aromatic symmetrical character contribute to this low conductivity. The insulation resistance of 0.06-pf. capacitors wound from 1-mil polyimide film bears out this superior electrical resistance over a temperature range. The figure of merit is the product of the resistance and capacitance. This is expressed as megohms X microfarads. As seen in Figure 9, polyimide film is a t least one order of magnitude higher than polyethylene terephthalate. and in addition is still useful a t temperatures \vel1 above the melting point of polyester films.
Log Resistivity, Ohms 500 V. 7000 v. 17 16 16 16-1 7 16
1-mil thickness, 2 3 ' C., 60 c.P.s., volt/mil
Polypyromellitimide Polyethylene terephthalate (Poly)tetrafluoroethylene/
7000 7000 4000
hexafluoropropylene - _. copolymer Poly(viny1fluoride) Polyethylene Polystyrene
4100 5000 6000
polyethylene terephthalate as shown in Table V along with other insulating materials( 70). Measurements were made according to ASTM D149-59. using '/,-inch electrodes with a voltage rise of 500 volts per second. Exposure to various a.c. electrical stress levels until failure is shown in Figure 11 as compared with equivalent thicknesses of polyester and fluorinated copolymer. The temperature dependence of the a.c. and d.c. breakdown is shown in Figures 12 and 13. At temperatures above 150" C., the superiority of polyimide becomes apparent. It surpasses all known organic insulators in high temperature electrical breakdoxbn. The d.c. values are higher than the a.c. values apparently due to the absence of corona. Polyimide film maintains its mechanical and electrical properties after prolonged exposure to elevated temperatures. Extrapolation of data on films aged a t 300' to 400' C. predicts a useful life at 250' C. of several years. For example, after aging a t 300" C.. for 2 months. the electrical breakdown strength of 1-mil film is still 6000 volts per mil. and the volume resistivity a t 200' C. is l O I 4 ohm-cm., a value actually higher than the initial value. This may be due to crystallization or cross-linking which reduces the carrier mobilit! . At the low end of the temperature scale, electrical properties generally improve. LVhat is more significant is the retention of mechanical properties. Measurements of elongation of 507, have been made on H-film a t liquid nitiogen temperature and qualitative tests indicate that the film is still flexible a t liquid helium temperatures (4' K.), being able to be bent around a 'is-inch rod a t this temperature ~ \ i t h o u cracking. t Acknowledgment
20
40
60
80
100
120
140
160
180
TEMPERATURE, ' C
Figure 13.
D.C. breakdown vs. temperature
'/l-inch electrodes, 500 volts per sec., rate of voltage rise; 0.001 2-inch H-film; 100 Mylar C polyester fllrns
The assistance of T. D . Mecca of the Yerkes Research and Development Laboratory for many of the physical and electrical measurements and G. D . Patterson. Jr.. of the Experimental Station Laboratory of the Film Department for the low temperature dielectric measurements is gratefully ackno\vledged. VOL.
2
NO. 3
SEPTEMBER
1963
193
Reddish, W., Trans. Faraday Soc. 4 6 , 459 (1950). Simril, V. L., Curry, B. .4.,M o d . Plastics 36, 121 (1959). Warfield, R. W.? Petree, M. C., SPE Trans. 1, 80 (1961). IYhitehead, S.,“Ditlectric Breakdown of Solids,” Oxford Clarendon Press, 1951.
literature Cited
(1) Amborski, L. E., Burton, R. L., Elec. Mfg. 53, 124 (1954). (2) Birks, J. B., “Modern Dielectric Materials,” Academic Press, Inc., N. Y . , 1960. (3) Du Pont de Nemours, E. I., & Co., Teflon FEP Fluorocarbon Film Tech. Rept., 1961. (4) Jackson, W., “The Insulation of Electrical Equipment,” Chapman & Hall Ltd., 1954. (5) Javits, A. E., Elec. M f g . 65, 60 (1960). (6) Mathes, K. N., Electro-Technol. 66, 149 (1960).
RECEIVED for review June 3, 1963 ACCEPTEDJuly 10: 1963 Division of Polymer Chemistry, 144th Meeting, .4CS, Los Angelrs, Calif., April 1963.
EFFECT OF MOLECULAR STRUCTURE ON PROPERTIES OF HIGHLY CROSS-LINKED URETHANE POLYMERS W . C. D A R R , P. G . G E M E I N H A R D T , A N D J.
H. SAUNDERS
Mobay Chemical Co., Pittsburgh, Pa.
Physical properties of highly cross-linked urethanes were evaluated by examining Vicat softening data of solid polymers prepared from a variety of polyol and polyisocyanate reactants. The complete curves of Vicat penetration vs. temperature were plotted and interpreted as the modulus-temperature relationship of the polymers, thus indicating the property behavior of the polymers over a wide temperature range. Interesting relationships with molecular structures of the reactants are shown. for rapidly screening the many urethane reactants available for rigid foam.
HE tremendous growth of the urethane chemical industry Tduring the past few years has been tied very closely to the flexible foam market and the use of di- and trifunctional reactants. Because of several recent economical improvements, including lower cost polyether resins and less costly. improved isocyanates. interest has been increased in the production of more highly cross-linked polymers, especially in the form of rigid foam. Many widely variant raw materials are now available for study. These materials, both polyols and polyisocyanates. differ widely in chemical composition, functionality. and molecular configuration. These variations and the resulting effects on physical properties of the polymers are being investigated in this work. The pol yether polyols used had branched aliphatic. heterocyclic, or aromatic nuclei. The polyols also varied in functionality from three to eight. Aromatic isocyanates were used primarily in this study, and a n aliphatic diisocyanate was included for theoretical interest. The effects of structure of the
The study offers promise
reactants on certain of the final properties of the polvmers \\‘ere much greater than expected. In addition. techniques used in this work offered a new procedure for screening the many possible polymer combinations which may be of interest for rigid foams. Experimental
This work was primarily concerned \\it11 stud>ing the different types of polyols and polyisocvanates now commercially available. No attempt was made to compare the many similar products of each type. The raw materials and their analytical data are shown in Tables I and 11. Polyols are given a code designation showing the nature of the initiator to which propylene oxide \vas added, the functionalitv. and the equivalent weight.
Polyols. ALIPHATIC.3(150), polyether triol. equivalent weight about 150, Pluracol T P 440. based on trimethylolpropane (Wyandotte Chemicals Corp.).
Typical Chemical Properties of Various Polyether Polyols .4liphatic Heterocyclic 4( 725) 4( 750) 6( 72.5) 6( 750) 4( 725) ?( 750) 8(125)
Table 1. 3( 750)
Hydroxyl No.
400
450
P . 0 ner hrancha
1 6 7.0 c .03
1 6
r--
xiid
NO.^ Viscosity, c.P.s., 25’ C.
615
6.6
0.027
1550
375 2 0 6.9
0.018
1150
490 1 4 5 2 0.15 10,000
380 2 0 5.1 0.16
460 1 2 0.17
3000
>100,000
7 1
370 1 8 7.5 0 16 22,500
430 1 4 10.0
