Feb., 1942
ELECTRICAL PROPERTIES OF PLASTICIZED POLYVINYL CHLORIDE [CONTRIBUTION FROM
THE
283
RESEARCH LABORATORY OF THE GENERALELECTRIC Co.]
Electrical Properties of Solids. XII. Plasticized Polyvinyl Chloride1 BY DARWINJ. MEAD,ROBERTL. TICHENOR AND RAYMOND M. Fuoss
Introduction Most linear polymers of high molecular weight are hard brittle solids a t room temperatures. On heating, they soften and gradually become more and more flexible and plastic. This property is, of course, the one which makes them useful for molding and extrusion. The flexibility a t room temperatures can be increased by the addition of materials of relatively low molecular weight, called plasticizers. With increasing amount of plasticizer, a given polymer becomes softer and more flexible and elastic, going through a rubber-like stage. As more and more plasticizer is added, the material becomes weaker mechanically, soft gel-like substances being produced, and finally (assuming complete miscibility), viscous liquids are obtained. The plasticization of a polymer by a given compound is as specific as solubility relationships among compounds in the ordinary range of molecular weights; as a matter of fact, the two processes of solution and plasticization have many other points of similarity. In covering the whole range of compositions in the system polymer-plasticizer, many decades of viscosity are spanned, going all the way from the enormous viscosities characteristic of glasses to the relatively low ones of liquids. If polar constituents are present in the plasticized system, we are provided with a tool for investigating the internal dynamics of the system, through its response to an electrical field of variable frequency. Eventually a correlation should be found between the internal dynamics measured through molecular response and the bulk mechanical properties such ;ts elasticity, plasticity, tensile strength and the like. A knowledge of the generalized dielectric constant e
=
,r
- ie’f
(1)
where E’ is the dielectric constant and E” the loss factor, furnishes a complete description of the electrical properties of a system. Considered as a vector, E has two components, e’ which is 90” out of phase with the field, and measures the capacity of a unit cube, and e” which is in-phase with the field, and measures the energy dissipated as heat (1) Paper XI, THISJOURNAL, 68, 2832 (1941).
per cycle in a unit cube. For the 2-component polymer-plasticizer system, we may write E
= e(f,T,c; a l , . .an;bl,. . .,bn)
(2)
where f is the frequency of the applied field, T the temperature, c the composition, and the ai’s and the bi’s are molecular parameters characteristic of polymer and plasticizer. In previous papers of this series, we have presented the results of systematic investigations of the dependence of e on f r e q ~ e n c y ”and ~ temperatureL6 for several systems, and a few data on variable c o m p o s i t i ~ n ~ J > ~ , ~ have been given. I t is the purpose of this paper to present the results of an investigation in which polyvinyl chloride was plasticized with a fairly wide variety of compounds : that is, we are primarily interested here in the variables c and bl, . . ., b, of Eq. (2). Measurements were made a t 40 and 60’ and at 60, 600 and 6000 cycles, over the concentration range 8-30 weight per cent. plasticizer. The measuring temperatures were kept fairly low in order to minimize the uncertain corrections due to direct current conductance. In the pure polymers, a t ordinary temperatures, probably crystallized regions8 and amorphous regions are both present. On account of the high viscosity, the time of mechanical relaxation is very great, and hardly any readjustment or further crystallization in quenched polymers is possible. At low or room temperdtures, the electrical response of linear polymers like polyvinyl chloride, acetate and chloroacetate‘ is very similar to that reportedBfor crystallized polyesters. As the temperature is lowered, the similarity of the audiofrequency properties of the linear polymers discussed here to the high frequency properties of the crystalline polyesters becomes steadily more marked. In the pure polymers or in those containing only a small amount of plasticizer, the (2) Fuoss, i b i d . , 61, 3257 (1939). (3) Fuoss, ibid., 68, 369 (1941). (4) Fuoss, ibid., 63, 378 (1941). (5) Fuoss and Kirkwood, i b i d . , 68, 385 (1941). (6) Fuoss, i b i d . , 68, 2401 (1941). (7) Fuoss, ibid., 68, 2410 (1941). (8) Mark, J . Phys. Chcm., 44, 764 (1940); Bekkedahl and Wood, J . Chcm. Phys., 9, 193 (1941). (9) Yager and Baker, “Symposium on Electrical Insulation Materials,” Atlantic City meeting of the American Chemical Society, Sept. 11, 1941.
individual dipoles are I~UL free to ~iiove,and the lmml low absorption band is probably due to the clectrical response of the crj-stallites. As t h e temperature is raised, or as the plasticizer cviiteiit I,: increased, the c-rystalline regioiis disappear (,melt or clissoh-t"j a r i d with iricreasing internal iiiobility, the relatively sharp high peak in absorption characteristic of the dipoles on thc polymeric chain appears. With further iiicreasc of plasti.. cizer, the dipole iiiaxiriiuni shifts to higher frtl-yuencies. corresponding to tlic decreased time of relasation of the polar groups. 'These considerations dictated thr choice of our conceiitration range; less than about plasticizer gives plastics which a t con-\7eni working teniperatures art" largely mixtures oi amorphous and rystalline phases: arid at more than about plasticizer, the dipole peak in which we are sent particularly interested, has shifted he.yond our working frequericy range (,6D pcles to I O kilocyclesj. 'Thr p o l y i n y i c.hlorrde wa.s 3 saniplr froiri which the low iiiolecular weight coiuponents were ex tracted with warni acetone, by the same procidurc which 1, miiple A5 for soinc of the earlier wor intrinsic viscosity, t h e wr.c4ght.averagi, ruolecular weight iahout 98,Of)O.~'' Some of the coriipotiricls used for plasticizer.; were re&,ure or recrystallized, but niust ( i f good grade c . I>. chcinicals. As thc will bvar out. tlie clertrical propcrlies of plastics are not very sensitivr t o small quantities of impurities (,other than clcctmlytic, of coiirw;i in thr plas ticizer. 'The plasticizers were disholved in petroleurn ether or i l l diethyl ether, and added ta Lhe polyrric%r. ( I t was rather interesting to nottk that diethyl cthrr swells polyvinyl chloride, while petroleuiii ether merely wets it..) Aftcr 3tirring until nearly all the solvent Itad evaporated, the drying was completed in a vacuum oveii at 60-100', dcpending on the vapor pressure of the plasticizer. Then the powders were placed in a mold at. 120', and after standing closed for two minutes under contact prcssure to heat the cctnpound, full pressure was applied for three minutes. after which the mold was coolcd (two LO three minutes) t o woni temperature. For the harder samples, ti000 lb. /sq. in. were used; this was decreased to 3000 as the plasticizer f]!J')
19.3 14.8 14.3 13.7 12.6 13.4 10.4
. .
24.7 22.9
i13.9 30.5
28.2 24.8
..
..
..
21.0 21.5 17.9
23.7 24.3 21.2
28.5 29.2 34.1
..
. .
10.3 18.7
21,s 21.5 20.4
25.1 ZLO
18.0 24.8 22.4 19.3
x3.4 32.7 31.4 24.8
27.4 25.4 21.9
siARI,F;
A-\'ALUES .,1;
16.7 11.2 10.9 10.4 15.17 13.4 11.3
600 GOfI
23.3 17.7 17.2 14.9 15.9 12.5 18.4 1.4.0 13.2 12.4 18,s 16.;3 13.7
b
FOR DIIPFERENT P L A S T I C I Z E R 5 'loo tjOO
3.95 3.65 4.25 4.10 .. . , 3.95 4.25 3.45 3.50 3.70 3.70 4.50 4.50 4.10 '4.25 3.50 3.40 3.85 3.90 2.45 2.60 3.90 3.63 3.80 3.80 (40' and 1000
ttii(ill
u ii
..
3 . C)O
:LG
5.86 2.95 2.65 3.80 2.70 3.05 3.10 3.25 2.90 2.90 2.15 3.05 3.15
.'44t5
33.83 3.10 3.i;O
3.90 3.05 3.76 2.45 3.15 3.65 -.)
= 3
7
Typical test plots of Eq. 5 are shown in Fig. .i. using dibenzyl as an example, where log z is plotted against log $. It will be seen that straight lines average the points quite satisfac torily. The value of p,,, the concentration for the maximum, is determined by the intersection of the line with the axis a t log z = 0. A few of the figures of Table IV represent extrapolated values. The slopes of the lines of Fig. 5 are measures of tlie rate of change of viscosity with concentration of plasticizer, the latter reckoned as ratios to the concentration at which the maximum loss appears for a given temperature frequency. These
Feb., 1942
-
ELECTRICAL PROPERTIES
289
OF PLASTICIZED POLYVINYL CHLOIUDE
unknown, is exactly specified by the frequency and temperature. That is, a characteristic electrical response, such as maximum loss factor at fixed frequency and temperature, provides a quantitative basis for comparing the effects of different substances on the internal mechanics of the plastics. If we start with undiluted polymer, we presumably have a close-packed structure which may be almost entirely polycrystalline" or, depending on its chemical structure, it may contain crystallized regions separated by amorphous regions in which the chains have random orientation. In any case, there is practically no freedom for motion of chain segments. If we add plasticizer, we are inserting small molecules between chains, and in so doing, break the chain to chain coupling,* either by dilution of the amorphous phase or by solution of the crystalline phase. This leads to two effects: first, the absorption due to the pure polymer structure disappears (the low temperature t"-maximum) and, second, the internal viscosity decreases, allowing the dipole absorption to begin (the high temperature €"-maximum). Now the larger the number of small molecules added, the greater will be the number of places where the chain structure is opened up. Consequently we expect and find that a smaller weight per cent. of a low molecular weight plasticizer is needed to give a given electrical response than of a plasticizer of high molecular weight. This is shown graphically in Fig. 6a, where p m for 60' and 60 0.06
0.04
i
2011.-.-----I
0
0.02
& 10
0 100
0
0
0
-
300
200
M.
400
-
Fig. 6.-Dependence of plasticizing ability on molecular weight M: (a) weight per cent. for 60" - 60 maximum; (b) mole ratio for 60" 60 maximum.
-
-
(17) Mooney, THISJ O V X N ~ L ,63, 2828 (1941).
DARWIN J.
"0
ROBERTL.
MEAD,
TICHENOR AND
cycles is plotted against M , the molecular weight the plasticizer. For example, 10.4% tetralin i_ll = 132) gives the same effective viscosity as 1!4.3% dibenzyl phthalate (31 = 346). The points of Fig. 6 scatter quite a bit, of course, because the number of molecules is only one of the variables involved. Two compounds, dibenzyl wbacate and dioctyl phlhalate, are markedly different from the average. If the former is plotted a t one-half i t s molecular weight (black points in Fig. 6a), however, it conforms to the general pattern. in other words, one molecule 01 C~HsCHzCOz(CTIz) kc: 02CH2CsH6 acts like two independent molecules of plasticizers, on account of the long flexible methylene chain between the two polar (plasticizing) ends. Dioctyl phthalate is, as has been mentioned before, exceptional in many ways, and is very different in structurr from the other compounds of T,Lble I li we calculate compositions on a mole ratio bitsis, the regularities discussed above appear very clearly. and further specific differences due to structure appear. If nierely number of particles vere all that was iri\wh ed, the mole ratios would be the same for all plasticixrs for a given electrical response; if we ronsider the mechanism oi plasticization, hovvevw, .vr.e see that the shape a b well as the size oi t h t plasticizer molecule must also i-te taken mto itccount. Iri I'able VI, the plasticizers arc arrangcd in sequence oi rzlfJkcda1 weights, which ;are given in the second L-olumn, m[: i i i the tiiirri rt!rtl ioiirtli columns are given ~ 6 , , L ' 11d YGIJ[) t h c nl\,lc t if
J,
1 .%:[LE >,-i
i) i : w w m C E
'0 > I
i 11
PRO
1
FLASTI 90";
x?
I 154
1 i :t
Ll
:i: 11
1
1'
td4'X
1 1 1
hjp?
8 40
ri714
s 40 h 3(J 8 13 ";:](I
I:
12
I
5 I
4 J
' I
:i
14
-
(18) The mole ratio IS defined here as I = ( P / M ) ( 6 2 . 5 / ( 1 P ) ) ; that is, the mule ratio is reckoned in terms of monomoles of polyvinyl chloride ( m = 6 2 . 3 ) because plasticizer-polymer interaction is local, and on:y a small part of the polymeric chain is affected by a ,i*,aIt ~,1,4,ti&t.,-
m
