Polystyrene plastics as high frequency dielectrics A. v o n Hippel and L . G . Wesson LABORATORY FOR INSULATION RESEARCH, MASSACHUSETI’S INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MASS.
T h e dielectric loss encountered in styrene monomer is analyzed, and the dipole moment of the styrene molecule is measured. h variety of polymerization conditions and the effect of moisture are investigated, and optical tests for the purity of the plastic are considered. The excellent polystyrene produced is further modified by cross linking, copolymerization, and hydrogen substitution ; finally special filler materials of low loss are introduced w-hich allow adjustment of the dielectric constant and thermal expansion coefficient over a wide range. Compositions of special usefulness are obtained by matching the thermal coefficient to that of metals (Polyglas plastics). Some applications of major interest are considered in this paper.
T
HE extensive literature t o date on styrene and its polymerization products is predominantly concerned with the kinetics of the polymerization reaction and with the structure and mechanical properties of the polymers. Their desirable electrical properties (6) %-erea more or less accidental by-product until war requirements placed new emphasis upon the fabrication of low loss materials. Polystyrene immediately gained an outstanding position in the high frequency field, but it became apparent that the conditions for producing material of the best electrical quality were not known. The dielectric loss of the plastic varied within relatively wide limit’s, subject to the manufacturer and the lot sample submitted. (“Dielectric loss” is used throughout this paper as a nonspecific term.) Measurement3 made in this laboratory indicated variations in tan 6 from 0..0008 to 0.0028 a t 3 X 109 cycles, and even the best material obtainable showed higher loss than might be anticipated for the pure plastic. Further shortcomings limited the usefulness of polystyrene in war equipment. Its low heat distortion temperature (about 80 O C.) made the material unsuitable under mechanical stress and heat; its lorr softening temperature (90 O t o 120 O C.) made polystyrene insulation impractical in joints subjected t o the heat of a soldering iron. In addition, particular applications called for a dielectric constant matching specified values, and for a thermal expansion coefficient reduced to that of metals. An investigation was therefore undertaken to establish the conditions under which polystyrene of the best electrical grade could be produced, and to find means of raising the heat distortion and softening temperatures and of modifying its dielectric constant’ and thermal expansion without impairing the electrical quality.
material. The best results, tan 6 = 0.0013 as listed below, were obtained by drying the liquid for 2 days over Drierite, followed by vacuum distillation at 25 O C. in the presence of the inhibitor. If the inhibitor is removed before drying and distilling, oxidation and increased electrical loss ensues (Table 111). Similar results are obtained if the liquid is overheated or if traces of air are admitted. To remove all traces of water completely is very difficult. The water content of samples of styrene dried in different ways‘ was determined by titration with Karl Fischer reagent (3) and compared with the dielectric loss of the material. The influence in the microwave range proved negligible (Table IV), but it was pronounced a t low frequencies (Figure 1). These facts may be interpreted by reference to the dielectric characteristics of pure water (Figure 2). The loss tangent of conductivity water a t 3 X log cycles, due to dipole loss, is 0.11 at 25” C. Therefore, liquid or dissolved water, present in a concentration of 624 parts per million, would be expected t o make only a minor contribution t o the t o t d loss a t this frequency. At 60 cycles, on the other hand, a condenser filled with water would exhibit a loss tangent of about 600 because of ionic conductivity. Hence, 624 p.p.m. would cause a loss tangent of 0.37 if H Aand OH- exist in the styrene monomer in a concentration governed by the ionization constant of water and with mobilities corresponding to those in pure water. The measured value is only 0.0082. On the assumption that the water is dispersed as droplets, its contribution to t h e loss, calculated on the basis of “interfacial” polarization according to the Maxwell-Wagner theory ( 7 ) would be zero for direct current, and would rise t o a maximum, too small to be detected, in the rangcl06 to l o 7 cycles. Therefore, it seems likely that the water is in solution with a reduced ionization constant or that it ionizes 1 Phosphorus pentoxide, an obvious choice as a drying agent, could not be employed, as styrene polymerizes violently in its presence.
TABLE I. DIELECTRIC Loss
STYRENE FROM VARIOUS
Monomer Supplied by: Company A, technical sample Company A, research sample Company B Company C Company D Lab. for Insulation Research Prepared from 8-phenethyl alcohol Prepared from cinnamic acid D o w 5-100,further purified
Preparation of low loss polystyrene STYRENE MONOMER.The dielectric loss of styrene may vary appreciably with the origin of the sample (Table I). These variations are attributable t o the presence of impurities; the inhibitor, p-tert-butylcatechol, which is added to retard polymerization, changes tan 6 only insignificantly in the small concentration (0.005’%) used commercially (Table 11). A number of methods were employed in preparing highly purified styrene, with Dow Chemical Company’s 3-100 as starting
OF
SOURCES T a n 6 a t 3 X 100 Cycles and 25’ C. 0.0018 0 0016 0 0017 0 0016 0 0020 0 0015 0.0029 0.0013
ON THE DIELECTRIC Loss TABLE 11. EFFECTOF INHIBITOR
OB
STYRENE Material Styrene, highly purified Same 1% p-tert-butylcatechol Effect of 0.005% p-tert-butylcatechol (calcd.)
+
1121
T a n 6 a t 3 X 100 Cycles and 25’ C.
0.0013 0.0074 0.000031
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
1122
DO0
Vol. 38, No. 11
TAN 6
LOO .IO0 .OlO
FREQUENCY IN CYCLES PER SECOND
Figure 1. Dielectric constant and loss of styrene as function of water content
.OOl
IO* lo3 lo4 os id IO' I FREQUENCY IN CYCLES PER SECOND CONWCTlVlTY RANGE
loi
e'/co designates the specific dielectric conmtant-i.e., the ratio of the measured dielectric conmtant t o that of vacuum-and is independent of the u n i t s and dimensions used.
Figure 2.
traces of impurities; the resultant ionic conductivity is the source of the dielectric loss at low frequency. The increase in the dielectric loss of the purified styrene a t high frequencies (Figure 1) is attributable t o the dipole moment of the molecules themselves. This dipole moment, PO, can be calculated from Debye's theory (8) if the static dielectric constant, e,, the dielectric constant at optical frequencies, e,, the number of molecules per cc., N , and the temperature, T, are kn0Wn:
(The dipole moment of the styrene molecule is so small that we have assumed the validity of the Clausius-Mosotti equation, as modified by Debye, Equation 1, for the monomer. The equations are expressed in the customary electrostatic units.) The loss tangent reaches a maximum, tan b,.*
=
e, es +
Id
Id
d
DIPOLE RANGE
Dielectric constant a n d loss of conductivity water
which must increase the dielectric constant E, above thesquare of the index of refraction for the D line ( n i l 5 = 1.565 == 0.001). the "effective" optical dielectric constant, e, = 2.464, was obtained from the experimental value of tan, , ,a (0.0044 * 0.0003). e,, and Equation 2. The value of the acting dipole moment of the styrene molecule in the liquid phase calculated from these data, PO = 0.12 * 0.03 X lo-'* e.s.u. (electrostatic unit), ia in accord with the limit set by direct measurementz, pa 5 0.2 X IO-'* e.s.u., in the gas phase. The dipole theory also predicts that the maximum in the l o s b tangent will appear a t a vacuum wave length: = 2m(3
where
T
x
IO'")
E
e,
t 2 2 cn'.
+
= relaxation time
2 C . P. Smyth, of Princeton LTniversity, kindly provided this informatmu based on his measurement of the vapor by the beat frequency method (4
€0.
€0.
a t 25' C. above the frequency range accessible to our present equipment (Figure 1) but is shifted sufficiently to lower f r q u e n c i a by operating at lower temperatures (Figure 3). Hence the values of the above quantities were determined at - 15' C. A measurement on the purified monomer a t loa cycles yielded a value of e, = 2.486 * 0.005. From the density (d;15 = 0.94) and molecular weight i t follows t h a t N = 5.46 X loz1. Since styrene exhibits absorption bands in the infrared (Figure 7)
2$0
fi
TAN .. ...6-
OF INHIBITOR IN DISTILLATION OF STYRENE TABLE 111. EFFECT
Tan 6 at 3 X 100 Cycles and 25' C. N-100 etyrene (Dow) 0.0017 Inhibitor ertd.. styrene dried and fractionated in vacuo 0,0022 Inhibitor not ertd., styrene dried and fractionated in v.ell0 0.0013
TABLEI IV.
EFFECTOF DISSOLVEDWATER ON DIELECTRIC Loss OF STYRENE
Condition of N-100 Styrene Intd. with water at 28" C. Dried with Desicchlora Dried with Drierite
Water Content, P.P.M. 624 130 32
Tan 6 at 3 X 100 Cycles and 2.5' C. 0,0017 0.0016 0.0016
% . I
-40 -20
0
20
40
~
60-80
TEMPERATURE,O C.
Figure 3. Dielectric constant and loss of styrene in the microwave range as a function of temperature
INDUSTRIAL AND ENGINEERING CHEMISTRY
November, 1946
TABLEV. DIELECTRIC LOSSOF POLYSTYRENE PREPARED UNDER VARIOUSCONDITIONS
T a n 6 a t 3 X 10' Cycles a n d 25' C.
A. Polymerized in evacuated a n d sealed tube 4 days a t 90' C., 2 days a t 120°, 1 d a y a t 150" Same a s A but tube sealed a t 1 atm. of oxygen-free
0.00041
C.
Same a s A but tube sealed a t 1 atm. of air
E.
Polymerized in evacuated and sealed tube 4 days a t 1100 c. Same as E but resealed in vacuo and further heated 4 days a t 150' C. From styrene prepared from 8-phenethyl alcohol, polymerized as A Same as G , reheated 24 hr. a t 100' C. under continuous H g pump vacuum
0.00049 0,00095 0.00161
E.
xr
D. Same as A but tube sealed a t 1 a t m . of 0,
F. 0.
a.
TABLEVI. DIELECTRIC LOSS
0.00089 0.00064 0.00053 0,00027
O F P O L Y S T Y R E N E FROM
VARIOUS
SOURCES T a n d a t 3 X 108 Cycles and 25' C. 0.00029 0.00024 0.00023 0.00026 0.00029 0.00022
Polynier Supplied by: Company A , sample 1 Company A, sample 2 Company B, sample 1 Company B. sample 2 Company D.sample 1 Company D,sample 2 Lab. for Insulation Research
0.00017-0.00024
The maximum a t - 15 ' C.was obtained at a wave length Xmaz = 3.2*cm., determining the relaxation time as 7 = 1.69 X 10-11 second for this temperature. A spherical molecule of radius a, rotating in a continuous medium of viscosity 7 a t temperature T , would have a relaxation time according to Stokes' law: 7
=
8r7aa/2kT
(4)
Despite the fact that the styrene molecule is essentially flat, an effective spherical radius may be calculated from Equation 4 on the basis of the known relaxation time and measured viscosity q - , , 1.36 X lo-* poise:
-
a =
LSA.
1123
selected. The inhibitor chosen was p-tert-butylcatechol (Eastman) purified further by recrystallization from benzene; the catalyst, benzoyl peroxide (Lucidol) was precipitated with methanol from chloroform. Commercially 0.005% of inhibitor and 0.1 to 0.2% of catalyst are added t o styrene; in the present experiments, a range from 0 to 2% was investigated (Table VII). The number-average molecular weights were calculated from the specific viscosities, nrp, of 0.3% by weight solutions in sodiumdried benzene by means of the modified Staudinger constants and formula as derived by Schulz and Husemann (1.2). (The viscometer was a calibrated Ostwald tube thermostated at 25 * 0.01 a C. The specific gravity of the 0.3% solution was 0.8765 at 2.5' C. I n the case of samples containing catalyst the constant found by Schulz and Huqemann t o hold for concentrations of was employed 0.1-20/, of benzoyl peroxide, 2k = 0.73 X in the calculation; whereas for samples containing inhibitor. since similar data were not available, the constant applying to pure polystyrene polymerized a t 90" C.,2k = 0.96 X lo-* was used.) The increase in the loss tangent with increasing concentratione of butylcatechol and benzoyl peroxide must be ascribed to the loss produced by these substances per se, since chain length and branching of the polystyrene chains is without perceptible effect on the microwave power factor (Table VIII).
TABLEVII. EFFECT OF INHIBITOR AND CATALYST O N MOLECULAR WEIGHTAND DIELECTRIC Loss OF POLYSTYRENE^
-
p-tert-Butylcatechol Tan 6 at 3 x 109 cycles vrp Mol. wt. a n d 25' C. 0.583 240,000 0.00043
Benioyl Peroxide Tan 6 at 3 x 10' cycles Per cent 7.p hlol. wt. and 25" C 0.583 240,000 0.00043 0 0.512 277,000 0.00049 0.05 0.00046 Q6:ddO 0.00051 0.468 254,000 0.1 0:233 0.339 54,000 184,000 0.00064 0.3 0.132 0.00078 0.317 41,000 0.00124 0.00127 172,000 0.00068 0.6 0.100 0.263 33,000 143,000 0.00068 0.7 0.081 0.147 27,000 0.00144 1.0 0.066 80,000 0.00079 0.132 18,000 0.0022 2.0 0,043 72,000 0.00113 0.00011 Av. increase per . I % 0.00006 I, D a t a obtained ,before highly purified styrene waa available; initial polymerization carried o u t a t 90 C
.....
Nitrobenzene, of similar molecular structure and size, has an efTABLEVIII. EFFECTOF TEMPERATURE OF POLYMERIZATION fective radius of approximately 1.3 A,, according to the measureON MOLECULAR WEIGHTAND DIELECTRIC Loss ments of Oncley and Williams (IO). T e m p . of T a n 6 a t 3 X IO$ PolymeriCycles and INFLUEKCE OF POLYMERIZATION CONDITIONS ON ELECTRICAL vsp .Mol. wt.0 nation, O C . 23' C. PROPERTIES. The dipole loss of styrene should shrink t o a small Et: 90 0.641 264,100 .~ 0.00024 fraction of its original value during the polymerization process be120 0.195 85.700 0.00021 150 0.102 48,000 9 0.00017 cause steric hindrance and van der Waals forces between neigh0,00025 170 0.094 boring chains tend t o immobilize the dipole groups. Consequently, a polystyrene of a very low power factor should result unless disturbing impurities are introduced in the course of the polymerization. Table V and Figure 4 show the electrical 4 Constante taken from S c h u h and Husemann (12) to calculate molecular properties of polystyrene prepared under various conditions. weight from 9.p. According t o these results, a moderately high vacuum or an inert atmosphere is required during the polymerization t o avoid oxidation and a high temperature aftertreatment is helpful in reducing the electrical loss. The improvement produced by the latter treatment is probably due to the polymerization of residual monomer and in some cases t o the elimination of volatile impurities by continuous pumping at elevated temperatures. The data of Table V were obtained before the final purification TAN6 002 method for the monomer had been worked out (Table 111). Starting with the purest monomer and employing the 7-day .001 polymerization treatment of Table V, the power factor may be reduced t o 0.00017. Similar low losses were measured on the 0 best materials submitted by several manufacturers (Table VI). 0' IO* 10' IO' lo5 ioe IO' on 10' IB EFFECTOF INHIBITOR AND CATALYST ON MOLECULAR WEIGHT FREQUENCY IN CYCLES PER SECOND AND DIELECTRIC Loss. T o study the effect of specific impurities, Figure 4. Effect of air during polymerization of ntyrene on dielectric loss of polymer the technically important case of inhibitor and catalyst was
TAN
Vol. 38, No. 11
INDUSTRIAL AND ENGINEERING CHEMISTRY
1124 S
T: 251;.
WATER ABSORBED
0.070%
4048% 4027%
IO
20
30
40
53
60
70 80
HOURS IN HUMIDITY TAN 6
I
10
20
53 60 HOURS DRYING OVER P205
3040
Figure 5 . Effect of moisture on dielectric loss of polystyrene a t 3 X 109 cycles
I
I
2
I
70
I 3
5
4
PER CENT PARAFFIN IN POLYSTYRENE
Figure 6 . Reduction of dielectric loss of polystyrene exposed to 9oC)”humidity a t 2.5” C. by addition of paraffin
WATER ABSORPTION. ~ N D MOISTUREPROOFING. Absorption of moisture is a factor of real importance in low loss polystyrene. Test samples, exposed in a closed vessel a t room temperature t o air of prescribed humidity, became saturated with moisture in about 75 hours, and a similar time was required to remove the water by drying over phosphorus pentoxide (Figure 5 ) . This moisture increased the microwave loss of the material strongly; 0.01% of water changed the loss tangent a t 3 X lo9 cycles by about 0.00008, whereas in accordance with the dipole effect of liquid water (Figure 2) an increase of only 0.00001 would be expected. In addition, it was found that, in contradistinction t o the effect of water on styrene monomer (Figure l), the influence of 0.07y0moisture upon the low frequency loss of the polymer was undetectable. These facts suggest that the moisture in
polystyrene is not, present as a liquid phase producing conductivity, but as single dipoles which can make their full contribution without being weakened by association. Figure 5 stresses the need of moistureproofing polystyrene if its low loss in the ultrahigh frequency range is t o be preserved. Such protection may be obtained, for example, by adding a small quantity of paraffin wax before or after polymerization. A polystyrene containing 0.1% of paraffin exhibited an increase in tan 6 of 0.0003 after exposure to air of 90% humidity for 175 hours, the loss tangent then undergoing no further change with time. Higher percentages of paraffin conferred only slightly greater degrees of moistureproofing (Figure 6). ISVESTIGATION BY OPTICALMETHODS. A brief attempt waa made to employ optical absorption spectra as a means of distinguishing between low and high loss polystyrene. For the detection of residual styrene in polystyrene the fact can be utilized that in the polymerized material the C=C bond disappears accompanied by characteristic changes in the vibration-rotation spectrum (infrared) and in the electronic states (ultraviolet). Tn-o bands in the near infrared are especially characteristic of
ABS33PTION COEFFICIENT
2700
2800-
2900 3000 3100 WAVELENGTH,
3200
3500
34 W
i.
Figure 7.
Infrared transmission of styrene and polystyrene
Figure 8.
Effect of impurities on ultraviolet absorption by polystyrene
November, 1946
INDUSTRIAL AND ENGINEERING CHEMISTRY
styrene: m e a t 1630 cm-' attributable to the C=C vibration, the other near 1410 cm.-' associated with the bending motion of the C-H groups next to the double bond. Since the absorption of polystyrene does not interfere a t these frequencies, the infrared absorption spectrum can be used to detect small quantities of monomer in polystyrene (Figure 7). However, the ultraviolet absorption constitutes a more sensitive method of analysis (f 1 ) . Figure 8 compares the absorption of purified polystyrene with that of polystyrene samples containing small quantities of foreign substances. The presence of 2% of the monomer alters the absorption curve completely, the polymer becoming more transparent to longer wave lengths and the characteristic double-band structure due to the monomer making its appearance. According to Figure 8 it should be possible to detect residual monomer of 2 0.1% in polystyrene by this method. Similarly, other contaminants of polystyrene such as inhibitors, accelerators, and oxidation products of styrene may be traced by their influence on the absorption in the near ultraviolet (Figure 8) and by their elimination of a transmission region of polystyrene near 2370 A. Thus, it may be possible to specify an optical acceptance test for polystyrene of the highest electrical quality in place of dielectric analysis.
Plastics related to polystyrene Raising the heat distortion temperature and softening point of polystyrene can be effected in a number of ways: (a)The chains of the polymer can be linked together by primary valence forces to form macromolecules (cross linking); ( b ) part of the styrene groups in the chains can be replaced by other groups which increase the attraction between the chains by secondary valence forces or block the mutual displacement of the chains by steric hindrance (copolymers); (e) the hydrogen atoms of styrene can be replaced by other atoms; and (d) filler materials can be introduced. All four approaches appear promising, and some plastics of each group are now commercially available. CROSS-LINKEDPOLYSTYRENES. That thermoplastics like polystyrene can be cross-linked was first discovered by Staudinger and Heuer (13). In the course of their preparation of styrene, some divinylbenaene was formed as an accidental by-product and produced a cross-linked copolymer detected by its insolubility in benzene. From the standpoint of obtaining a low loss material, it is obvious that p-divinylbenzene, an electrically balanced molecule, is a very desirable cross-linking agent. This compound, as well aa two other cross-linking agents (divinyl sulfide and divinylacetylene), were investigated. A number of other substances (divinyl ether, diallyl sulfide and disulfide, and sulfur itself) were also tested as to their cross-linking properties with negative results; linear copolymerization occurred instead. p-Divinylbenaene (CHZ==CH-CCH,-CH=CHZ) was prepared from p-xylene by the method of Morton and Donovan ("A). The material obtained in their synthesis was a colorless liquid, freezing a t 0" and remelting a t 0.5" to 1' C. Further work done by us has given preparations, the melting points of which have been 8' to 10 C. Because of the considerable labor involved in the preparation of even small amounts of the material and the large loss in purification, efforts to obtain a product of still higher purity were not undertaken. Cross-linked polymers with styrene were made in concentrations of 0.05, 0.1, 0.5, 1.0, 1.75, 2.5,5.0,and 10% of the crosslinking agent. Various temperatures from 40" to 120 ' C. were used, but 105" C. was adopted as most favorable for the initial polymerization. Complete linkage of all polystyrene chains was obtained even for the 0.05% mixture, as shown by their insolubility in benzene. The polymers obtained under favorable conditions were clear and colorless, but a slight. yellowness was observed in the higher concentrations. The electrical losses were low (i.e., a t 3 X lo9 cycles, tan 6 = 0.00045 for the 1% polymer).
1125
-".
-,-
e
Figure 9.
Cauliflow-er effect in styrene-divinylbenzene polymerization (XS)
However, the heat distortion temperature Fas not increased by the inclusion of 10% or less of p-divinylbenzene3. Divinyl sulfide (CHZ==CH--S-CH=CH,) was studied because its source, B,B'-dichlorodiethyl sulfide (mustard gas), would be cheap and plentiful, and divinyl sulfide is easily prepared by treatment with potassium hydroxide (1). The molecule has a dipole moment2 of 1.20 * 0.04 X 10-18, and the liquid a loss tangent of 0.0334 a t 3 X lo8 cycles and 25" C. On polymerization of 0.1,0.5, 1.0, 2.5, 3.5,and 5.0% mixtures with styrene at the optimum temperature of 105 C., tough colorless polymers were obtained with a disagreeable odor. The loss factor a t 3 X log cycles was as high as 0.00071 for the 1%mixture with styrene. We were not able to determine the heat distortion temperature because of the apparently unavoidable formation of bubbles in the castings needed for the standard test. Cross linking x+-ith divinylbenzene or divinyl sulfide leads to the formation of "cauliflower" unless a certain temperature is O
* A series of high softening, cross-linked polystyrenes are now produced commercially by The Dow Chemical Company (Q-200 series) using mixed a - , m-, and p-divinylbenaene free from other substances. For instance, their Q-200.7 which contains 770 divinylbenzene has a heat distortion temperature of 110-115° C. Our difficulty was, no doubt, caused by impurities in the available p-divinylbeneene, which probably acted as plasticizers.
1.0
TAN 6
.I
.01
,001
.0001 10'
102
10'
lo4
los
loB
FREOUENCY IN CYCLES
IO'
PER
lo8 SECOND
los
Figure 10. Dielectric constant a n d loss of poly2,5-dichlorostyrene and poly-3,4-dichlorostyrene
do
Vol. 38, No. 11
INDUSTRIAL AND ENGINEERING CHEMISTRY
1126 EXP. COEFF;
x'o;
THEORETICAL CHARACTERISTIC FOR IDEAL MOLECULAR MIXTURES o EXPER I MENTAL VALUES FOR ACTUAL MIXTURES
:I 2
I
O
0
I
IO
I 20
I 30
I 40
I
I
I
I
50
60
70
80
90
100
PERCENT GLASS BY WEGHTFigure 11. Thermal expansion of glass-polystyrene mixtures exceeded in the initial polymerization period. White spots of a granular appearance are formed in the styrene which tend to enlarge and to fill the entire tube in some cases (Figure 9). Staudinger and Husemann (Id), working with divinylbenzene, concluded that these granules consisted essentially of polydivinylbenzene. However, analysis of the granules produced in the 0.5% divinyl sulfide and styrene polymerization showed that the sulfur content (0.16%) is practically the same as that of the clear matrix (0.17%) and does not correspond to that of a polydivinyl sulfide. We believe, therefore, that the cauliflower effect is due to the greater rate of cross linking a t lower temperatures relative to that of linear copolymerization, and that the highly crosslinked polymers separate because of their low solubility in styrene and polystyrene. Divinylacetylene ( C H ~ H - C E C - C H ~ H ~ ) was prepared according to the procedure of X e u d a n d and co-workers ( 8 ) , and polymerized in 0.05, 0.1, 0.2, 0.3, 0.5, 1.0, and 10% concentrations in styrene. The main portion of the polymerization in each case was done a t 90" C. The polymers thus obtained were tough, almost colorless (except for the 10% sample which was quite yellow), and insoluble in benzene. The loss at 3 X 109 cycles in the case of the 10% product was 0.00063, and its heat distortion temperature, 77 O C. The ''flow" temperature was appreciably raised, however, by the addition of as little as 1% of the divinylacetylene. Introduction of larger quantities of crosslinking agent is rendered difficult by the inhibitory effect of divinylacetylene on the polymerization reaction.
It is clear from the preceding discussion that a large amount of cross-linking agent is required to raise the heat distortion temperature of polystyrene. However, small amounts suffice to reduce greatly the tendency of the plastic to flow a t elevated temperature. Our results with divinylacetylene are quite a t variance with those of Korrish and Brookman ( 9 ) , who described the copolymers with styrene containing more than 0.1% of this cross-linking agent as beiag rubbery in consistency. It would seem that, because of the inhibitory effect of divinylacetylene on the reaction, complete polymerization was not obtained with most of the concentrations that they prepared. We found long continued heating (for the 1%, 8 days a t 90"C.), followed by a finishing heating ( 2 days a t 120°, 1 day a t 150" C.), to be necessary for complete polymerization and the production of hard, tough, cross-linked material. Korrish and Brookman, after a study of eight divinyl compounds, concluded that the more effective the divinyl compound was as a cross-linking agent, the greater the inhibiting effect OD the polymerization of styrene. We found, as did Korrish and Brookman, that divinylacetylene is both effective and inhibitory; however, our findings as regard divinyl sulfide oppose Norrish and Brookman's conclusions in that divinyl sulfide, an effective cross-linking agent, actively promotes the polymerization of styrene. In fact, the catalytic action is so pronounced that samples of styrene containing 1% divinyl sulfide gel within 3 weeks in the dark in evacuated, sealed, glass tubes a t 7 " C. Divinylbenzene (not studied by Norrish and Brookman) likewise is an effective cross-linking agent (small amounts such as 0.05% will give a practically completely benzene-insoluble polystyrene) but will
TABLEIX.
VARIATION OF DIELECTRIC CONSTANT WITH FILLER MATERIAL
-
e'/m T-TanS102 IO' 10" 101 106 1010 cycles cycles cycles cycles cycles cycles Ti02 and Poly-2,5-dichlorostyrene 5.3 0.0031 0.0003 0.00086 41.9 6.2 5.3 5.3 65.3 10.3 10.2 10.2 10.2 0.0016 0.0003 0.0013 81.4 22.1 23.6 23.0 23.0 0.0080 0,0012 0.00158 SrTiOs and Poly-2,5-dichlorostyrene 37.0 4.9 6.20 6.18 4.90 0.0020 0.0003 0.0014 9.65 9.61 9.36 0.0041 0 0010 0.0023 59.5 9.6 0.025 0.0030 0.0064 74.8 18.0 18.0 16.6 15.2 20.2 20.2 0.090 0.0060 0.0060 80.6 28.5 25:O Silica Aerogel and Poly-2,5-dichlorostyrene 66.0 1.43 .... 0.0040 5 From the empirical equation for the dielectric constant of a mixture: log a log e'. b log t'b! where a and b are the volume fractions, and e'm and e'b are the respective dielectric constants (at 10'8 cycles in this case) of the components. The approximation begins to fail for powder-plastic mixtures containing a large volume percentage of the iller.
Filler
%
Calcd.'
-
. ..
...
...
....
+
3.5
3.0 ,004
TAN 6
,001
,002
,001
FREQUENCY IN CYCLES PER SECOND
Figure 12. Dielectric c o n s t a n t and loss of Polyglas Pf Dotted line represents technical Polyglaa Pc.
FREQUENCY IN CYCLES PER SECOND
Figure 13. Dielectric constant a n d loss of Polyglas Dotted line represents technical Polyglas D+.
Df
November, 1946
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 14. 1. Wave guide window8 2a. b. Beads for pickup probes 2c, d. Pressurized coaxial connectors 4.
Polyglas applications
Metal inserts 3e, f. Female portion of connector Same molded into Polyglas male 3g. h. New connector designs conparts of high voltage connector taining Polyglan molded inside Polyglas D+ sleeve, '/gz inch thick. on beryllium-copper rod 3a, b. 3c, d.
ttctivate instead of retard the styrene polymerization. As a converse of this, the rate of polymerization of styrene is greatly recarded by diallyl sulfide and disulfide and by sulfur, and unaffected by divinyl ether, all of which apparently copolymerize with styrene but do not cross link. COPOLYMERS OF STYREVE. The following substances were copolymerized with styrene: vinylnaphthalene, vinylcarhazole, and vinyl cyanide. The most promising of these was N-vinylCsHi
I
i
'
\
N-CH=CHZ,
1127
which was furnished by the
of poly-3,4-dichlorostyrene, furnished by the Monsanto Chemical Company (Figure 10). The former, with a heat distortion temperature of about 113" C., served successfully as a substitute for polystyrene when the improvement in mechanical stability was of predominant importance. Unfavorable is the fact that the surface arc resistance is greatly lowered by this substitution (9 seconds as against 95 seconds for polystyrene). ~~
I
~
~~
~~
MATERIALS
/
CsH4 General Aniline Works and purified by crystallization until it %-as white and sharp melting (86" C.). The polymerized So% mixture with styrene was found to be the most favorable when the heat distortion temperature, 127 C., is considered together with the toughness of the product. In order to obtain satisfactory plastics of this composition, i t was found necessary to carry out the polymerization very gradually, starting with 60" C. for 7 days, then 75" for 3 days, 105' for 2 days, 120"for 2 days, and 150" C. for 1 day. This procedure gives chains of greater length than would otherwise be formed and thus increases the toughness. The loss of the 80% vinylcarbazole material at 3 X log cycles was 0.00081. SUBSTITUTED POLYSTYREKES. Measurements were made on samples of poly-2,5-dichlorostyrene(-CH,CR-C6H3Cl2) ,, and
~
TABLE X. VARIATION OF DIELECTRIC Loss WITH FILLER to/-'
Filler,
5;
102 cycles
11 1 20.0
3.73 6.56
Filler,
70
X. Cm.
30
100
40 50
60
10 100 10 100 10 100 10
80 90
10 10
---Tan 108 10'0 101 cycles cycles cycles Carbon Black and Polyetyrene 3.50 3.22 0.0175 4.65 4.60 0.105 Magnetite and Polystyrene €'/eo
3.42 3.40 4.04 3.98 4.73 4.53 6.52 6.51 16.4 57.1
P'
1.17 1.09 1.27 1.19 1.38 1.17 1.60 1.30 1.56 1.71
Tanad 0.0047
....
0.0047 0.0055 0.0068
....
0.0156
....
0.0596 0.232
-8 108 cycles
10'0 cycles
0.0105 0.040
0.0051 0.028
Tandm 0,027 0.067 0.041 0.093 0.056 0,181 0 076 0.182 0.354 0.833
Total Tan6 0.032 0.067 0.045 0 099 0.062 0.181 0.092 0.182 0.414 0.865
INDUSTRIAL AND ENGINEERING CHEMISTRY
1128
POLYGLAS BEAD
POLYGLAS WINDOW I
Figure 1.5.
Type N connector ( A ) ; Polyglas window in rectangular wave guide ( E )
Chlorine atoms in the 2,j-position give a low loss product because their dipole moments approximately cancel each other; a high loss is given by the 3,4-isomer since the dipole moments add vectorally. Figure 10 shows that with increasing temperature the dipoles begin to follow the field more freely; a loss maximum appears and moves to higher frequencies as the viscosity decreases. The 3,4-compound seems to have only academic interest a t the present time because its heat distortion temperature is only 103" C. and its arc resistance is as inferior as that of the 2,5-isomer. POLYSTYRENE PLASTICS WITH FILLER MATERIALS. Although a high percentage of filler materials does not improve the heat distortion temperature of a plastic to a marked degree, it can impart sufficient stiffness to prevent flow a t elevated temperatures. In addition, other properties may be modified over a considerable range. The dielectric constant may be adjusted within wide limits by the proper choice and concentration of the filler without too great an increase of the dielectric loss. Table IX shows that €'/eo may be raised to 23 a t 1 0 1 0 cycles by the use of titanium dioxide or certain titanates, and that it may be lowered to 1.4 by the use of silica aerogel. Similarly the dielectric loss may be varied within wide limits by the use of more or less conducting materials in the proper concentration (Table X), and the thermal expansion coefficient may be lowered (Figure 11) from that of the plastic proper to values that match the expansion coefficient of metals. It is evident from Figure 11 that a t higher glass concentrations the actual thermal expansion coefficients are lower than predicted. This deviation is not due to appreciable macroscopic porosity of the samples since the density of the molded material corresponds fairly well with the theoretical values. The effect is caused by the microstructure of the sample, which depends on the size and shape of the glass particles. With increasing concentration, these particles contact each other, and thus interstitial voids are formed
TABLEXI. PROTECTIVE EFFECT OF PARAFFIN AND SILICONE GREASEIN POLYGLAS D+
Polyglaa D Dry After equilibrium in 90% relative humidity a t 25' C. Polyglas D Dry After equilibrium in 90% relative humidity a t 2 5 O C.
T a n 6 a t 25' C. 102 3 x 10 cycles cycles
0 0009
0 00072
0 0102
0 00102
0 0006
0 00079
0 0023
0 00085
+
Vol. 38, No. 11
into which some of the plastic expands with increasing temperature. The thermal expansion coefficient of the mixture is a very sensitive measure of such voids because the ratio of the thermal coefficient of the plastic to the glass is high (70 to 1). The density of the material, on the other hand, is an insensitive measure owing to the more nearly equal densities of the constituents (0.5 t o 1). The possibility of matching the thermal expansion of metals proved extremely useful during the war, and led to the development of the Polyglas plastics ( 5 ) . The first and most widely applied members of this group consist of the low loss Corning 790 glass (Vycor) as filler material and polystyrene or poly-2,5-dichlorostyrene as plastic binder. If each particle of the filler is coated properly' before molding, a thermoplastic composition results which can be handled without major difficulty in compression and injection molding with casehardened or chromium-plated molds. The moistureproofing of such compositions re-
4 Laboratory method is as follows: Add the powdered filler t o a benzene solution containing the requisite amount of polymer and moistureproofing agent. Evaporate the solvent with constant stirring until a very viscous mass results. Warm the dish and direct a blast of dry air on its center to speed the evaporation and prevent moist air from coming in contact with the liquid. Press the lumps of material into thin sheets, cut into pea size, and completely dry in a vacuum oven a t 50' C. The following commercial method is used by the hfonsanto Chemical Company for large batch preparations: Soften the plastic on differential mills operating a t about 150" C. Admix filler and moistureproofing additives with further milling until homogenization is achieved. C u t from the rolls, cool, grind, size, and pack.
TABLE XII.
CovPo~ImoxASD PROPERTIES OF POLYGLA~ P+ ASD P O L Y G L D+ A~ Polyglas P+
Composition, 70 Glass powder (Corning 790) dried a t 900' C. Pol! istyrene Pol3i-2,5-dichlorostyrene (Monsanto) Par;a 5 n wax Igni tion sealing compound No. 4 (Dow orning) Color Appearance Density a t 25' C. Impact resistance (CharDy unnotched), ft.-lb./in. Thermal coe5cient of ex ansion per O C. X 106 Approx. -20 t o +25'C. Approx. +25 to +5fjo0 C. Heat distortion temp., C. R a t e r va or permeability, from 100 to 0% humirfity, g./24 hr./sq. m. Burning rate Resistance t o : Weak acids Strong oxidizing acids i Alkalies Solvents for polystyrene Water Sunlight hfoisture absorption (90% humidity), % Surface arc resistance, sec. Surface 307 humidity resistivity, ohms, after equilibrium in:
c
60% humidity 9Oy humidity Vol. rlsistivity, ohm-cm.. after equilibrium in: 30% humidity 60% humidity 90y0 humidity Breakdown strength, volts/mil Compression molding Temperature, C. Pressure, Ib./sq. in. Injection molding, Temperature, C. Pressure, Ib./sq. in. Bulk ratio hIolding powder (mfd. by Monsanto)a Present price per Ib. in 100-lb. lots Availability a
Polyqlas D
0.1 0.1 White or gray Opaque 1.80 1.82
0.46
0.45
1.65 1.69 86.6
1.72 1.74 112.7
0.43 0.17 Self-extinguishing
0.07 182
Good Poor Excellent Poor Excellent Good
0.06 6-181
> 6 X 1016 > 5 x 10'6 > 5 x 1014
> 5 X 1016 > 5 x IO"
1 x 1010 1 X 1016
> 5 x 10'6 > 5 X 1010 > 5 X 1016 940
8 X 1016 1050
>5
x
1016
185 5000
200 5000
......
290 25,000 2 to 1
...... 2 to 1
$1 .60 $5.00 Production quantities
Compression-molded b y Plax Corp., injection-molded by Arnold Hril-
h a r t , Ltd.
INDUSTRIAL AND ENGINEERING CHEMISTRY
November, 1946
TABLESIIT. DIELECTRIC COXSTANT ISD Loss AT 3 x 10’ C Y C L E S , A D L I S E 4R THERVAL EXP~PI~ CO ~EIFO F I CK I E N T PER c. OF V IRIOC? POLYGLAS TIPES Polyglas P ; Polyglas D Polyg1asFe D Polyglas 11 Polyglas V Polyglas s Titanium dioxide Polyglas D Strontium titanate Polyglas D Glass-titanium dioxide Polyglas D Glass-silica aerogel Polyglas n-
Composition
e’/q
3.35 3.22 3.32 *4.69 3.43 3.40
23.0 19 6
8.96
Thermal Expansion X 106 1.69 1 74 1.14 1.55 1.40 1 46 1.80 1.73
Tan 6 0,00078 0,00079 0.00071 0,033 0.0013 0 0014 0.0015 0,0045 0.0010
1.68
u 2.12 0.0046 1.83 Alumina Polyglas S 5 61 0 ‘0144 1.80 Carbon PolyglasFe P 37.71 7.71 1.17 a Table XI1 and Fig.:12 and 13 give composition and other properties. b Glass powder, Cornlux 790 (74.4%),poly-2,5-dichlorostyrene( 2 5 . 6 % ) . C Glass fiber, Corning I: (49.8701,melamine-formaldehyde reqin (50.2%). d Glass powder, Corning 790 (80.8’%), polyrinylcarbazole (19 0 % ) lIonsanto HB-40 0 1 , ( 0 . 2 % ) . 6 Glass, Corning 76, special, 80-120 mesh ( S O % ) , silicone resin, Do% Corning 2103 ( 2 0 5 ) ’J TiOz, Titanium Alloy Company Ticon T-I, hea7.y grade ( 7 8 5 ) , poly2,5-dich!orostYren.e (22%). GrTiOs, Titanium Alloy Ticon 34290-C (80.6%), poly-2,5-dichloro3tprene
h Glass powder, Corning 790 (31.0rc). TiOz, Ticun TL heavy grade (49.6%),poly-2,5-dichlorofS o . 714 glass fibeh and S o . 790 glass ponder of large mesh sizi’. S. L. Bass and 0. D. Blessing, of the Dow Corning Corporation, provided silicone resins aqd greases. C. L. Jones of the Plastics Division, Monsanto Chemical Company, informed us about the preparation of Polyglas by milling; J. F. White of the Everett Division, llonsanto Chemical Company, prepared some aerogel of the highest purit,y. H. .I.Hutchinson of the Hood Rubber Company together with A. E. Giesler, Group 56, Radiation Laboratory, Ll.I.T., worked out the manufacturing process for Polyglas A l ; C. F. Galehouse of Arnold Brilhart, Ltd., kstablished the proper conditions for the injection molding of Polyglas. E. 11.Purcell of the Radiation Laboratory drew our attention to Santocel, and W. R. Purcell, Chemistry Department, University of Michigan, gave us samples and information of special moistureproofed aerogel materials.
Literature cited (1) Bales, S. H., and Nickelson, S. A., J. Chem.
Soc., 121, 2137
(1922); 123, 2486 (1923). (2)
Debye, P., “Polar Molecules”, New York, Chemical Catalog
(3)
Fischer, K., Z. angezo. Chem., 48, 394 (1935); Almy, E. G., Griffin, W. C., and Wilcox, C. S.,IND.ENG.C H ~ Y .ANAL. ,
Co., 1929.
ED., 12, 392 (1940). (4) Hannay, S . B., and Smyth, C. P., J . Am. Chem. SOC., 68,244, 1005 (1946).
( 5 ) Hippel, A. yon, Kingsbury, S. XI., and TTesson, L. G., Natl. Defense Research Comm., Div. 14, Rcpt. 539 (1945). (61 . , Matheson. L. h.,and Goggin, W. C., IND.EXG.CHEM.,31, 334 (1939) : Xarner, J., Elec. Communication, 21, 180 A
