INDUSTRIAL AND ENGINEERING CHEMISTRY
April 1949
701
energies of flow in the increasing order: phthalate, sebacate, adipate, indicating as they do, increasing temperature variation of properties in the same order, seem t o be the reverse of the order of temperature dependence of the physical properties a t lower temperatures. This may be a consequence of marked differences in t h e course of polymer-plasticizer interaction in the transition region. Since such physical properties are usually measured at 25' C., little correlation might have been expected between them and plastometer data taken at temperatures ranging from 120" t o 195' C. At these high temperatures the plasticizers would undergo changes in solvent power. Since diethyl phthalate has high solvent power a t 25" C., the higher temperatures would not be expected t o increase this power t o any great extent. Dibutyl sebacate would be enabled t o increase its solvent power to become a good solvent. Di-2-ethyl hexyl adipate, especially in the higher temperatures above 170"C., becomes a rather good solvent.
ACKNOWLEDGMENT The authors wish to express their appreciation for the assistance of James Stranch, who carried out t h e experiments and many of the calculations. PERCENT
Figure 13.
Log
72
PLASTICIZER
vs. Per Cent Plasticizer at Constant, 162" C.
T
LITERATURE CITED =
lation between the plasticizing efficiency of the three plasticizers and these physical properties might be found. Whereas, at a fixed percentage of plasticizer, the diethyl phthalate was found to give viscosities generally higher than those of the other two, a t corresponding temperatures, the flow temperatures indicate the reverse is true as was pointed out above. The greater activation
(1) Dienes, G. J., J . Colloid Sci., 2,131 (1947). (2) Dienes, G. J., and Dexter, F. D., IND.ENQ.CHDM.,40, 2319 (1948). (3) Dienes, G. J., and Dexter, F. D., J . Colloid Sci., 3, 181 (1948). (4) Dienes, G. J., and Klemm, H. F., J . Applied Phue., 17,458(1946). (5) Meyer, L. W. A., and Gearhart, W. M., IND.ENO.CHEM.,40, 1478 (1948). (6) Piech, F.E.,and Gloor, W. E., A.S.T.M. Bull. 151 (Maroh 1948). (7) Spencer, R.9.. and Dillon, R. E., J. Colloid Sci., 3, 163 (1948). R E C E I ~ EFebruary D 2, 1949.
PLASTICIZED POLYVINYL CHLORIDE Structure and Mechanical Behavior TURNER ALFREY, JR., A N D N O R M A N WIEDERHORN, Polytechnic Institute of Brooklyn, Brooklyn, N. Y. RICHARD STEIN
AND
ARTHUR TOBOLSKY, Princeton University, Princeton, N. J.
T h e existence of a three-dimensional network system in plasticized polyvinyl chIoride and Vinylite VYNW compositions has been established on the basis of creep, stress relaxation, dilatometry, birefringence, and x-ray measurements. X-ray diffraction diagrams of unoriented, plasticized VYNW and Geon 101 films yield patterns which are crystalline in nature. The mechanical properties of a given formulation can be altered by recrystallization procedures.
N A previous publication ( 1 ) the creep behavior of plasticized Vinylite VYNW was described. At that time a n attempt was
I
made t o relate this behavior to the structure of plasticized polymer compositions. One of the conclusions arrived at was t h a t a three-dimensional gel network of great permanence must be present. This was necessary t o explain the combination of the small amount of long-time creep, associated wilh large short-time compliance and the essentially compIete recovery of films which had been subjected to stresses for weeks.
The nature of the gel structure now has been studied more thoroughly with the aid of numerous experimental techniques. The results of this work not only confirm the existence of a threedimensional network system in plasticized polyvinyl chloride and W N W compositions, b u t also lead t o a fairly definite picture of its details. It has been established on the basis of creep, stress r e laxation, dilatometry, birefringence, and x-ray measurements, that these materials are partially crystalline in nature. The relative amounts of crystalline and amorphous polymer in any one composition has not been established, but there is no doubt that the crystalline regions comprise but a small fraction of the total composition.
X - R A Y DIFFRACTION STUDIES The most direct evidence for the existence of crystallization in plastic polyvinyl chloride is probably supplied by the x-ray diffraction results. These results may be summarized as follows: 1.
Unoriented, plasticized VYNW and Geon 101 films yield
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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
Figure 1. X-Ray Diffraction Diagram of Vinylite VYNW with 3570 Trioctyl Phosphate, Unoriented
Figure 2. X-Ray Diffraction Diagram of Geon 101 with 28,570 PG-16, Unoriented
patterns which are crystalline in nature. A considerable number of diffraction diagrams were obtained. Figures 1 and 2 are typical. 2. Vinylite compositions oriented by heating to 90.0' C. under stress, cooling to 35.0" C., and then removing the stress a t 35.0' C. ve fiberlike patterns. Figure 3, showing the orientation of eon 101, is typical of the results obtained with oriented polvvinyl chloride and Vinylite VYNW.
G
These diagrams give conclusive evidence for the following facts: first, that plasticized VYSW and Geon 101 compositions .we partially crystalline in nature; secondly, that it is possible, by suitable means, to orient the crystallites. Concerning the distances between repeat units, these distancet are independent of the resin, as well as of the plasticizer type and concentration. From this i t may be concluded that the crystallites consist of only vinyl chloride segments. Other&e, if plasticizer or vinyl acetate segments were included in the unit cell, the dimensions would be altered. In a. few instances it was observed that a particular formulation was stiffer (exhibited less creep) than would be expected from the properties of other siinilar formulations. For example, a formulation consisting of Vinylite VYKW plus 357, trioctyl phosphate vas found t o be less pliable in certain temperature ranges than a formulation containing only 25% trioctyl phosphate. These anomalous mechanical properties occurred infrequently, and foi reasons which were not ascertained. IIowever, in each such case the abnormally stiff formulation was found t o exhibit an unusually sharp x-ray diffraction diagram. Thus it can be concluded that the structural origin of the unusual stiffness lies in the excessively large extent of crystallization, although the reasons rvhy ccrtairi samples exhibited this effect is not known, Auxiliary studies, substantiating the existence of a polycrystalline structure in plasticized polyvinyl chloride and Vinylitr VYNW, are presented in another paper ( 2 ) . The role of crystallization in forming the gel network helps t u explain several facts. It has been shown ( 1 ) that vinyl chloridevinyl acetate copolymers containing fairly large amounts-for exvinyl acetate do not exhibit the pronounced ample, X%-of long-time stability against small stresses which is characteri3tic of polyvinyl chloride and VYNW (5% vinyl acetate). It is reasonable to expect that excessive amounts of comonomer break up the sequences of vinyl chloride units, and oppose crystallization. Furthermore, in spite of numerous attempts, the authors were unsuccessful in producing a vinyl chloride copolymer, plasticized only internally by a suitable comonomer, which duplicates the mechanical properties of a polyvinyl chloride formulation prepared with a liquid plasticizer Invariably, whenever sufficient comonomer was present t o impart flexibility, there was no indication of a gel structure being present. Over long periods of time, the inter-
Figure 3.
Vol. 41, No. 4
X-Ray
Diffraction Dia ram
of Geon 101 with 16,
Oriented
16.9
to Final of 7570
%G-
Ekgation
nally plasticized copolymers exhibited unlimited deforniatiorr even for small stresses.
ALTERATION OF POLYCRYSTALLINE CHARACTER BY RECRYSTALLIZATION One of the most important consequences of the polycrystalliric character of polyvinyl chloride is the fact that the mechanical properties of a given formulation can be altered markedly by recrystallization under various conditions of temperature and loading. For example, it has been found possible to orient polgvinyl chloride films. I n this extreme case of altering the normal crystal structure of the composition, the creep properties of the resultant films are changed markedly. I t has been found, for t h k rase, that: Plasticized vinylite compositions may be oriented by heating t u a high temperature, stressing at this temperature, and then cooling under stress. On removal of the load a t a lon-er temperature a n elastic recovery takes place but there is a residual elongation. Films treated in this manner are dimensionally stable a t temperatures below that a t which the load was removed. On heating above this temperature, the films undergo a contraction along the axis of the previously applied stress. Varying degrees of orientation mav be obtained bv varying either the time, the temperature, or the applied stress during the orientation cycle. The mechanical properties of these oriented films are notcdl! different from those of the original unoriented film. They are stiffer as measured by tensile creep measurements. Furthermore, as the extent of the orientation increases, the compliance of the film decreases.
_ _ _ _ ~
30
Figure 4.
40
50 60 70 TEMPERATURE,%.
Compliance-Temperature
BO
90
Relation
06 Geon 101 with 48.570 Tricresyl Phosphate
April 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
The films, after orientation, are anisotropic with regard t o their mechanical properties. ,4n oriented film has a higher modulus of elasticity along the axis of orientation than perpendicular to this axis. At an intermediate angle, the modulus has a n intermediate value.
It is, of course, well known that plasticized polyvinyl chloride becomes somewhat anisotropic in certain forming operations such as calendering, extrusion, etc. However, in such operatfons there is ordinarily an opportunity for a considerable amount of the orientation to relax before the structure is frozen in, so that only a residual effect is observed. Much larger variations in mechanical properties can be achieved by temperature and stress sequences such as those used here. It is not unlikely that such recrystallization operations could find industrial application. A set of experiments was carried out to study the recrystallization under constant load in some detail. The plasticized films were first subjected to a stress a t 30.0" C. After 60 minutes the samples were slowly heated to 90.0" C.; the heating cycle was 2 hours. On reaching 90.0' C., the samples were cooled to 35.0" C. This cycle being completed, the samples were reheated to 90.0 " C. and the resulting contraction, due to melting of the crystalline Jtructure, was followed. A typical diagram of the compliance (strain per unit stress) as a function of temperature, is presented i n Figure 4. Here it is noted that process A, the initial upheat, is
703
nonreversible. On cooling, curve B results. Process B, the cool. ing process, on the other hand, is essentially reversible, since, on reheating, curve C results. There is some hysteresis during this process, but i t is small. This curve is typical of the curves obtained with all the samples examined. If the load is removed a t a low temperature-for example, 30" or 40" C.-only a small amount of recovery occurs. The samples retain their elongated, oriented form permanently, as far asthese experiments can show. (Samples stored for 6 weeks showed no signs of further recovery.) Of course, if the oriented samples are heated to a temperature of 90' C., slow recovery to the original, unstressed length will take place, and the anisotropic properties will disappear.
SUMMARY AND CONCLUSIONS Plasticized polyvinyl chloride and plasticized Vinylite V Y S W exhibit a three-dimensional gel structure, the cross links of which are crystallites. The mechanical properties of a given formulation can be markedly a1tered by suitable recrystallization procedures. LITERATURE CITED (1) Aiken, Alfrey, Janssen, and Mark, J. Polymer Sci., 2 , 178 (1917). (2) Alfrey, Stein, Tobolsky, and Wiederhorn, J . Colloid Sci., in press. RECEIVEDJuly 30, 1948.
[END OF S Y M P O S I U M ON PLASTICIZERS]
LACTOPRENE EV ELASTOMER Effect of Plasticise rs W. C. MAST AND C. H. FISHER Ihstern Regional Research Laboratory, Philadelphia 18, Pa.
The
properties of Lactoprene EV vulcanizates compounded with a number of plasticizers were studied to find plasticized elastomers having relatively l o w brittle points in addition to the desirable Characteristics of unplasticized Lactoprene EV.
T(
HE preparation and certain properties of Lactoprene E V
copolymer of 95% ethyl acrylate and 5 % 2-chloroethyl vinyl ether) and its vulcanixates were described in recent papers ( 7 , 1 6 ) . Although the vulcanizates have exceptional flex life and resistance to high temperature, oils, oxidation, and sunlight, the relatively high brittle point (about -15' C. or 5" F.) is objectional for some applications. I n view of the several outstanding properties of Lactoprene EV and the paucity of information on this type of elastomer, the investigation of acrylic vulcanizates was extended to include plasticized E V compositions. The purpose of this study was to find plasticized elastomers having relatively low brittle points in addition to the desirable characteristics of the unplasticixed material and t o improve resilience, water resistance, and other rubberlike characteristics. METHODS AND MATERIALS
The samples were compounded by the master batch technique previously described ( 7 ) . The following standard recipe was used in most experiments. Brittle points, determined by the method of Selker, Winspear, and Kemp (91) were used as LL screening test, although brittleness
tests and flexibility tests are no longer recognized as adequate for complete evaluation of low temperature properties. The cooling medium used was a mixture of low boiling paraffins; tests were made after the specimen had been immersed for 2 minutes. Resilience measurements were made with a Bashore resiliometer; pieces of the standard test slab (6 X 6 X 0.075 inch) were grouped so t h a t the total thickness was 0.5 inch. Since this instrument is designed for measuring rebound at small deformations, the data would not be expected t o correlate with those of the GoodyearHealy rebound resiliometer. Swelling tests were run by American Society for Testing Materials, Method B, Designation D 47143T. Heat resistance was determined by suspending specimens in a mechanical convection oven a t 300" F. for 3 days, followed by examination of the aged specimens a t room temperature; other tests were according to A.S.T.M. specifioation D 412-41.
STANDARD RECIPE Lactoprene E V
Stearic acid Tetramethylthiuram monosulfide Trimene base Sulfur SRF black Plasticizer
Parts by weight 100 1 1 1
2
60
10
dome variety in chemical structure of the plasticizers (Table I ) was obtained by employing phosphates, sebacates, a fat acid ester, a polyether formal, various ether-esters, a sulfonamide, chlorinated diphenyl, a phthalate, silicon derivatives, and a carbonate. Plasticizers known to be efficient in lowering the brittle point of other elastomers and resins were used in most instances.
