April 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
effects of plasticizers on the aging of rubber compositions. A few have definite inhibitory effects on oxidation; some amines and phenols are outstanding in this respect. SWELLING IN HYDROCARBON FLUIDS.Swelling of the rubber composition in hydrocarbon fluids is increased by the presence of materials of high viscosity, which presumably cannot be extracted readily by the fluids. Resins and natural oils are outstanding examples of this class. Swelling is decreased in some instances by the presence of materials of low viscosity and high degree of efficiency, including certain nitriles, esters such as dibutyl sebacate and diisobutyl adipate, and certain coal tar distillates; these materials presumably are extracted by the fluid in question. Typical data illustrating these effects are shown in Table 111. WATERRESISTANCE. Although many plasticizers are relatively insoluble in water, practically all have a deleterious effect on water resistance of the vulcanized rubber composition. The worst in this respect are those which are water sensitive themselves, such as those containing ether or hydroxyl groups. An exception to this general effect has been noted in the case of neoprene (1%). EFFECTON STRESS-STRAIN PROPERTIES. A few plasticizers show reinforcing effects in the rubber compositions. These are chiefly resinous materials which function as processing aids and a part of their effect may be simply that of facilitating the fabrication of a more homogeneous composition. A few materials exhibit a weakening effect on the rubber; examples are acidic materials, halogen compounds, and materials of low compatibility. EFFECT ON RESILIENCE. Those materials which have the most outstanding effects in improving resilience of the rubber composition are chiefly low viscosity liquids, including some esters and ethers and a few nitro compounds. Low TEMPERATURE FLEXIBILITY. The most outstanding materials in improving this characteristic are those of low molecular weight and low viscosity, including especially some ethers, nitriles, and esters containing relatiyely ehort hydrocarbon chains. Poor low temperature flexibihty 1s Imparted by cyclic materials, many resins, halogenated compounds, and phenols. There are many other properties of interest and importance, but the available information is both limited and confusing. It can safely be stated that there is room in this field for extensive additional work. Many of the principles and general observations are similar to those of the plasticized thermoplastics and any advances in general knowledge will assist in elucidating some of the puzzling problems existent in t,he field of plasticized polar qmthetic rubbers.
675
LITERATURE CITED (1) Am. SOC.Testing Materials, Designation D 797-44T (1944). (2) Baker Castor Oil Co., “Baker Plasticirers for Synthetic Rub-
bers” (1945). (3) Boyer, R. F., and Spencer, R. S., J . Polymer Sci., 2, 157 (1947). (4) Conant, F. S., and Liska, 3. W., J. Applied Phys., 15, 767 (1944). (5) Crossley, R. H., and Cashion, C. G., Rubber A g e (Ar. Y.), 58,197 (1945). (6) Doolittle, A. K., J . Polymer Sci., 2, 121 (1947). (7) Doty, P., and Zable, H. S.,Ibid., 1 , 90 (1946). (8) du Pont, E. I. de Nemours and Co., Inc., Rept. BL-53, (Nov. 6, 1942). (9) Ibid., Rept. BL-79 (Feb. 10, 1943). (10) Farmer, E. H., and Shipley, F. W., J . PolymerSci.,.l, 293 (1946): Rubber Chem. and Technol., 20. 341 (1947). (11) Flory, P. J., IND. ENG.CHEM.,38,417 (1946): Rubber Chem, and Technol., 19, 552 (1946). (12) Fraser, D. F., Neoprene and Rubber Compounding Report 41-2. E. I. du Pont de Neinours & Co. (1941). (13) Frith, E. M., Trans. Faraday Soc., 41,90 (1947). (14) Goodrich, B. F., Chemical Co., Hycar Blue Book (1944). (16) Jones, H., Trans. Inst. Rubber I n d . , 21, 298 (1946); Rubbep Chem. and Technol., 20,184 (1947). (16) Jones, H., and Chadwiok, E. C., J. Oil & Coolour Chemials” Assoc., 30,199 (1947). (17) King, G. E., IND. ENG.CHEM.,35, 947 (1943). (18) Liska, J. W., Ibid., 36, 40 (1944). (19) Moll, R. A., Howlett, R. M., and Buckley, D. J., Ibid., 34, 1284 (1942). (20) Morris,’R. E., Hollister, J. W., and Seegman, I. P., Rubber Age ( N . Y.), 56, 163 (1944). (21) Patton, T. C., and Smith, M. K., I n d i a Rubber World, 116,643 (1947). (22) Patton, T. C., and Smith, M. K., Ibid., 115, 666 (1947). (23) Reed, M. C., IND.ENG.CHEM.,35, 896 (1943). (24) Stanco Distributors, Inc., Perbunan Compounding and Processing (1942), (25) Starkweather, H. W., and Walker, H. W., I n d i a Rubber World, 96,43 (1937). (26) Stickney, P. B., and Cheyney, L. E,,J . Polymer Sci., 3, 231 (1948). (27) Stiehler, R. D., and Wakelin, J. H., IND. ENQ.CHEX.,39, 1647 (1947). (28) Zhurko;, S. N., and Lerman, R. L., Compt. rend. acad. sci., U.R.S.S., 47, 106 (1945). RECIBIVEDJuly 8. 1948.
PLASTICIZERS IN VINYL CH LORIDE-ACETATE RESINS M. C. REED AND JAMES HARDING Bakelite Corporation, Bound Brook, N. J.
Results of the evaluation of 64 plasticizers in a vinyl chloride-acetate copolymer resin are presented. The test methods are discussed briefly. Tensile strength, elongation at break, extensibility at 25” C., brittle temperature, low temperature stiffening characteristics, volatile loss, oil and water resistance, and compatibility with the resin are tabulated. Supplier, molecular weight, specific gravity, refractive index, viscosity at two temperatures, boiling range under reduced pressure, and color are given for most of the plasticizers. The migration of plasticizers from vinyl sheetings into surface coatings is discussed, and a test method for evaluating this property is described. Test results for seven plasticizers and four types of surface coatings are given. The migrationof four plasticizers into polystyrene was studied. Polymeric plasticizers are surveyed briefly.
A
WIDE variety of nonvolatile or high boiling materials has
been used for the plasticization of vinyl chloride-acetate copolymers. The physical and chemical properties essential to satisfactory performance have been discussed at length in earlier papers in this series and by other writers. Sales promotion literature now frequently contains much useful information, such as chemical composition, boiling range or vapor pressure, and physical properties of vinyl compounds containing the plasticizers being offered for sale. One of the objectives of the present paper is to list a group of plasticizers, not previously discussed, together with the p r o p erties and behavior of these plasticizers when used in vinyl chloride-acetate copolymer resins. A resume of high molecular weight materials used for the plasticization of vinyl polymers fa also included.
676
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 41, No. 4
April 1949
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
I
I
6’17
PREPARATION OF TEST SPECIMENS The resin used in the preparation of test compounds was a copolymer of approximate!y 95 parts of vinyl chloride and 5 parts of vinyl acetate, known to the trade as Vinylite Resin VYKW. The following formulations were used in obtaining the data presented in the tables: Parts by Weight VYNW resin Lead stearate Litharae Plasticizer
Foririula A 67.0 1. o 2.0 30.0
100
n
62.2 0.9 1.9
57.4 0.8 1.8
33.0
40.0
1oo.n
100.0
64.0
59.0 0.5
Forinula R
VYNTV resin Lubricant Stabilizer Plasticizer
000
ww*
‘0000
emem
m . .. h n
. ’
3
m
’ .0O
(om
. *
..
.
. ,
.
.o d
69.0 0.5 0 . ,5
I VY XW resin Lead stearate Litharge Flexol plasticizer D O P Plasticizer
VYNW resin Lubricant Stabilizer Flexol plasticizer DOP Plasticizer
30.0
0.3 0.5 35.0
-_
40.0
100.0
100.0
100 ,,o
Formula C 67.0 1.0 2.0 20.0 10.0
62.2 0.9 l.D
20.0 15.0
57.4 0.6 1.8 20.0 20.0
100.0
100.0
100.0
64.0 0.5
59.0
20.0 15.0
20.0 20.0
100.0
100.0
1.0
62.2 0.9 1.9 20.0
57 4
2.0 20.0
10.0 --
15.0 _-
20.0 ___
100.0
100.0
100.0
63.3 1.2
58.4
35.0
40.0
Forinula D 69.0 0.6
0.5 20.0
10.0
100 0
VYNW resin
Lead stearate Litharge Tricresyl phosphate Plasticizer
VYNW resin Stabilizer Stearic acid Plasticizer
0.5
Formula E 67.0
Formula F 68.2
1.3 0.5
0.5
__
0.5
30.0
--
100.0
100.0
0.5 0.5
0.8 1.8 20 0
1.1 0.5
I
100.0
The ingredients of each batch were blended in a kitchen mixer. The batch was fluxed and milled on a two-roll mill heated with 50 pounds per square inch steam pressure. After ten endwise passes through the mill, the batch was sheeted off at 0 020 to 0.025 inch thickness. Test samples were molded in a 6 X 6 inch flash mold for all tests except volatility and extractions. For the latter, film 0.003 to 0.006 inch thick was made on the two-roll mill.
EXPLANATION
OF TABLES
For convenience, t,he plasticizers shown in Tables I and I1 are listed according to suppliers. The plasticizers submitted for test were technical products, and values obtained on the samples tested are not necessarily within the manufacturer’s present specifications. The data shown in Table I concern the sources, trade names, compositions, and properties of the plasticizers, independent of their use in resins. I n many cases, the information shown in this
678.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
681 table was obtained from the supplier of the material. This is true of the boiling range and chemical. composition. Minor corrections were made in some instances in temperature or pressure in order that all the plasti: cizers might be compared on the same basis. Table I1 presents the results obtained in the Bakelite Corporation Laboratories with these plasticizers in vinyl chloride-acetate resins. I n most cases, plasticizer concentrations of 30, 35, and 40% of the batch were tested. The final concentration shown for each series (the one yielding an extensibility of 75%), together with the properties listed for that concentration, was obtained by interpolation OP extrapolation. This plasticizer concentration i s used as a measure of plasticizing efficiency, and is used as a basis of comparison between plasticizers.
TENSILE STRENGTH, ELONGATION AT BREAK, AND EXTENSIBILITY I n the previous work (6,6)extensibility (elongation a t a loading of 1000 pounds per square inch) was measured on an autographic stress-strain tester operating a t a constant rate of loading of 800 pounds per square inch per minute ( 4 ) . PI as t i c i z i n g ef -.
682
INDUSTRIAL AND ENGINEERING CHEMISTRY
ficiency was expressed as the amount of plasticizer required to yield a n extensibility at 25' C. of loo%, based on the original cross section. I n the present paper, the Scott Serigraph, Model IP-4 (8)was used to measure extensibility. The carriage was loaded according to the dimensions of the specimen so as to obtain a constant rate of loading of 15,000 pounds per square inch per minute, This method has the advantage, in addition to employing the same rate of loading for all specimens, of providing a convenient autographic stress-strain chart from which the data can be read from any point of the curve without the necessity of calculation. Thus, tensile strength, elongation a t break, and extensibility were obtained directly from the stress-strain diagram. These values are included in the tables of data of this paper. Measurements of the effect of plasticizer-resin ratio on the extensibility determined by both methods indicated that, for practical purposes, 75% extensibility by the 15,QOO pounds per square inch per minute procedure is equivalent to 100% extensibility by the earlier method, which employed a rate df loading of 800 pounds per square inch per minute. The newer test procedure yields, in some cases, slightly different plasticizing efficiencies. T o facilitate comparisons, data by the newer method are included for several of the more common plasticizers listed in previous papers. All measurements of tensile strength, elongation a t break, and extensibility were made at 25" * 1 C. and values were based on original cross section. T o eliminate errors due to slip and the necessity of correcting for nonuniformity of cross section of the test specimen near the grips, a die-cut annular test specimen, 1.152 inches inside diameter and 1.403 inches outside diameter, was used. Parallel cylindrical pins or posts 0.125 inch in diameter and spaced 1.885 inches apart (center to center) were used in place of clamps to hold the specimen. The errors due to uneven stretching from the inside to the outside of the ring were found to be inconsequential at elongations above 25 %.
Vol. 41, No. 4
Losses by extraction, as shown in Table 11, are expressed, a? in the previous papers, as weight loss in per cent from 0.004 inch film immersed in the extractant for 10 days at 25' C. Solubility of the plasticizer in the extractant and rate of migration are thr principal rate governing factors. Limited experience with the A.S.T.M. Method D 543-43, which utilizes a specimen 0.125 inch thick, indicates that i t yields good precision.
LOW TEMPERATURE BEHAVIOR All plasticized vinyl resin compounds become stiffer and evmtually become brittle as the temperature is lowered. In Table 11, the brittle temperature is the lowest temperature a t which five successive specimens do not fracture when subjected to a sudden shearing action as defined by A.S.T.M. Method D 74644T. Td is the temperature at which the compound had a stiffness of 10,OOO pounds per square inch. It was determined graphically from a plot of the logarithm of the stiffness against the temperature as described by Clash and Berg ( I ) . This is believed to be a more useful concept than the flex temperature (T,) which was used in the previous papers of this series. S, is a measure of temperature sensitivity. It is the maximum slope of the log stiffness-temperature plot as described by Dienes and Dexter ( 2 ) . T , is the temperature a t which the log stiffnesstemperature curve has the maximum slope (S,) The amount by which the log stiffness-temperature curve is shifted along the temperature axis by changing plasticizer concentrations is shown in the column headed Slope. It is expressed as the change in T , resulting from a change of one yoof plasticizer, based OB the total batch weight
SWEAT-BUT Sweat-out or exudation was judged by visual examination of the milled sheets a t intervals up to tit, least 1 month.
PERMANENCE Since one half to one part of plasticizer is required for one part of vinyl resin to produce elastomeric or flexible materials, i t is essential that the plasticizer have good permanence. Loss of plasticizer from the plastic results in decreased flexibility, higher brittle temperature, and a shrinkage in volume proportjonal to amount of plasticizer lost. I n addition t o the effects of plasticizer loss on the properties of the plastic itself, plasticizer loss may cause contamination of foodstuffs, as in the case of paekaging applications, or may damage surface coatings or other plastics with which contact is made Methods of measurement of volatile loss of plasticizers were investigated by Rider and Sumner ( 7 ) . They used a specially designed oven maintained at 85" C., forced circulation of air, and test pieces 0.010 inch in thickness. A somewhat different method has been tentatively adopted by the American Society for Testing Materials. This uses an Abderhalden tube a t 99 C. with an air Aow of approximately 10 inches per minute over the specimen. Results shown in Table I1 and in previous papers (5, 6') were obtained using an oven at 60' C., an air velocity of 20 feet per minute without recirculation, and test .pieces averaging 0 004 inch thick. Results are expressed as weight loss in per cent for 10 days. The authors have adhered to this slower procedure in the belief that its results may be translated into field behavior with more confidence than those obtained by more rapid tests at higher temperatures Vapor pressure, ohemical stability, and rate of migration are the main factors which determine the relative rates of volatile loss of different plasticizers. I n a previous paper (6) the effects of temperature, time, and specimen thickness on the amount of plasticizer lost were discussed and an equation relating these factors was presented. O
TABLE111. ATTACK O B SURFACE COATINGSAT 40' C. PLASTICIZED VINYLSHEETINGS
BY
Condition of CoatingQ a t , Days 2 7 28
Plasticizer
Nitrocellulose Lacquer 45% 45% 40% 35% 35%
E2 45% 45% 40% 35% 359' 359 35d
Paraplex G-25 Paraplex G-40 Flcxol plasticizer R-1 Tricresyl phosphate Ylexol plasticizer DOP Flexol plasticizer T O F Flexol plasticizer 8N8 Paraplex G - 2 5 Paraplex (2-40 Flexol plasticizer R-1 Tricresyl phosphate Flexol plapticizer DOP Flexol plasticizer T O F Flexol plasticizer 8 N 8
S,PT N8 NS
NS NS NS S
VSI.
Si ST
9;
T
Linseed Oil Base Paint NS XS sS NR NS NS
NS S
j
SL
S
.
S
S'
a
Baked Alkyd Enamel
45YG Paraplex G-25 45% Paraplex G-40 40% Flexol plaEtioizer R - l 33% Tricresyl phosphate 35% Flexol plasticizer D O P 35% Flexol plasticizer T O F 35% Flexol plasticizer 8 N 8 Spar Varnish Paraplex G-25 Paraplex G-40 Flexol plasticizer R-1 'I'ricresyl phosphate Flexol plasticizer DOP Flexol plasticizer T O F 35% Flexol plasticizer 8x8
E$
h-8
SP SS NS
NS N8
NS NS
NS SL
S. A S
S,T SL
S SL
s: A
s,' 'r, ST, A
NS
NS
not softened; S softened; S L = slightly softened; 1'91, = very slightly softened; ST = stained; I, = softened t o viscous liquid; A = vinyl sheeting adhered strongly t o coating: T = sticky or tacky. Q
4
INDUSTRIAL AND ENGINEERING CHEMISTRY
April 1949
M I G R A T I O N OF PLASTICIZERS The damaging of surface coatings by migration of plasticizer has been mentioned as being an important aspect of plasticizer permanence. I n this case the loss of plasticizer is more damaging to the adjacent surface than to the elastomeric plastic itself. This effect of plasticizer migration, often referred to as lacquer lifting or marring, is a result of diffusion of the plasticizer into the surface coating. This results in softening of the coating which may be lifted or partially removed when the plastic is removed. The method used in this laboratory for studying the effect of plasticized vinyl resins on surface coatings consisted of placing plasticized sheeting in close contact with a coated panel, and observing a t intervals any softening or marring of the coating. The four types of coating used were: (1) a linseed oil base paint; (2) a spar varnish; (3) a nitrocellulose lacquer; and (4) a baked alkyd enamel. The plasticized sheeting had a glossy surface, and a pressure of 1pound per square inch, temperatures of 25' C. and 40" C., and test periods up to 60 days were used. Plasticizer concentrations were selected to yield approximately equal extensibilities. Some typical results are shown in Table 111. Although only a limited number of plasticizers were studied, a few generalizations were indicated. It was found that monomeric aliphatic esters are among the most active plasticizers in attacking eurface coatings. The aromatic plasticizers, such as tricresyl phosphate, are generally among the less active. Polymeric plasticizers are in a class by themselves, exhibiting little or no attack on surface coatings under ordinary conditions, It is also possible to predict, with fair success, the behavior of a plasticizer toward surface Finishes from B knowledge of the oil extraction of the plasticizer from the vinyl sheet. High oil extraction, whether it he due to high plasticizer concentration or to the chemical composition of the plasticizer, is usually associated with pronounced attack of surface coatings. Another important factor controlling rate of plasticizer migration into surface coatings is the concentration of plasticizer in the vinyl sheeting. A small change in plasticizer concentration frequently causes a large difference in rate of migration. Plasticizers rated as nonmarring in low concentrations may mar surface finishes when used in high concentrations.
TABLEIV.
MARRINGOF POLYSTYRENE AT 25" C. PLARTICIZED VINYLSHEETING
Plasticizer
BY
Appearance of Polystyrene 1 week 3 weeks
Plasticized compositions may also damage the surface of some rigid plastics such as polystyrene in a manner analogous to their action on surface coatings. A plasticiFer inert toward one material may be detrimental to another. Table IV summarizes results of a study of migration of plasticizers into polystyrene. The test method used for this study was to place specimens of plasticized vinyl sheeting 0.040 inch in thickness in contact with injection molded polystyrene plaques. Contact was maintained by moderate pressure in a clamp. Another example of plasticizer migration is the softening and weakening of cements used to attach plasticized vinyl sheetings in such articles as shoes and luggage.
683
demand for plasticized vinyl compounds that will not affect surface coatings, attack cements or other plastics, or contaminate foodstuffs. All these properties along with the requirements for good heat and light stability, good low temperature properties, processability, and reasonable cost comprise an imposing eet of specificationsfor the vinyl resin compounder to meet. The important polymeric plasticizers are esters of dibasic acids and dihydric alcohols, sucB as are described by Fligor and dumner (S), and the nitrile rubbers, copolymers of butadiene and acrylonitrile. These are the only high molecular weight materials which have achieved any commercial importance as plasticizers for vinyl chloride resins. The best of the polyester type plasticizers is still not completely resistant to extraction by all solvents, but the rate of extraction by mineral oil is extremely low. Compounds containing these plasticizers do not soften or lift surface coatings under ordinary conditions. However, since some small amounts of this type of plasticizer may be extracted by fats or oils, the use of these polyesters in films for food packaging should he preceded by animal feeding tests. Excellent resistance to extraction or swelling and to volatile loss as well as physiological inertness has been obtained by blending vinyl chloride-acetate resins with nitrile rubber. An acrylonitrile content of 35 to 40% in the nitrile rubber gives optimum compatibility. Better low temperature flexibility can be obtained with lower acrylonitrile-butadiene ratios but only a t a sacrifice in compatibility with the vinyl resin. Nitrile rubbers sometimes produce rough sheeting and extruded products. This behavior can be remedied to some extent by the addition of 0.5 to 1.0% of an antioxidant, based on the nitrile rubber content of the compound. Since many amines cause discoloration of the vinyl resin, phenolic type antioxidants are preferred. Proper choice of processing temperatures and the use of auxiliary liquid plasticizers also aid in obtaining smooth processing stocks. For a comprehensive discussion of the properties and processing of nitrile rubber-vinyl resin blends, the reader is referred t o the work of Young, Kewberg, and Howlett (9).
S U M M A R Y AND CONCLUSIONS Results of the evaluation of 64 plasticizers, from 23 suppliers, in a vinyl chloride-acetate copolymer resin are tabulated, and test methods are discussed briefly. The data shown in the tables have been used as a basis for selecting plasticizers for vinyl resins. The following conclusions relating properties of plasticizers are based on these data: 1. Volatile losses of plasticizers are closely related to their boiling points. A plasticizer should have a boiling point above 200' C. a t a pressure of 4 mm. of mercury t o have sufficient permanence for most applications. 2. The rate and degree of attack of surface coatings by plasticizers in vinyl sheetings depends, in part, on the chemical structure of the plasticizer. Open chain aliphatic plasticizers are among the most active in attacking finishes. High molecular weight of the plasticizer reduces the rate and degree of marring. 3. There is a correlation between the extractability by mineral oil and the marring activity of a plasticizer. Those plasticizers most eaHily extracted by mineral oil are generally the most active in marring. 4. The temperature a t which a compound has an apparent modulus of elasticity of 10,000pounds per square inch is suggested as a convenient measure of stiffening a t low temperatures. This temperature is shown in the tables under the designation of Td. 5. High molecular weight polyesters and nitrile rubbers are useful plasticizers for vinyl resins because of the outstanding resistance to volatile loss and extractions, and the low marring action of these plasticizers.
POLYMERIC PLASTICIZERS One of the most important trends in the field of plasticizers for vinyl resins has been the increasing use of polymeric plasticizers. The development of this type of plasticizer has been the result of the need for more permanent, compositions. There is a
ACKNOWLEDGMENT The authors wish t o express their appreciation to the Physics Division of the Development Department a t Bakelite Corporation for the determination of many of the physical properties
684
INDUSTRIAL AND ENGINEERING CHEMISTRY
shown in these tables. The cooperation of the suppliers in providing samples of plasticizers and information concerning them is gratefully acknowledged.
L I T E R A T U R E CITED (1) Clash, R. F., Jr , and Berg, R. M., M o d e r n Plastics, 21, 119--24 (1944). (2) Dienes, G. J., and Dexter, F. D., IND.ENG.CHEX, 40, 2319-25 (1948). (3) Fligor, K. K., and Sumner, J. K., Ibid., 37, 504-8 (1945).
Vol. 41, No. 4
(4) Reed, M. C., Ibid., 35, 429 (1943). ( 5 ) Ibid.t Pp* 896-90*. (6) Reed, hl. C., and Connor, L., Ibid., 40, 1414-22 (1948). (7) Rider, D. K., and Sumner, J. IC., IND. ENG.CHEM.,ANAL.ED.. 17,730-3 (1945). (8) Scott Testers, Inc. (formerly Henry L. Scott Co.), Providence, R. I., Bull., Scott Serigraph, Model IP-4. (9) Young, D. W., Newberg, R. G., and Howlett, R. 'AI.? IXD. ENO. C H E M . , 39, 1446-52 (1947). RECEIVED July 2 6 , 1948
Effect of Plasticizers G R E G O R Y M. M O E L T E R
AND
ERNEST SCHWEIZER
Celanese Corporation of America, Summit,
From measurements of the softening temperatures of a series of plasticized cellulose acetate films i t is shown that the softening temperatures of these films is a function of the fractional mole plasticizer content n of the film; this function is expressed by an equation of the form t = t g e - k 7 r where k is a constant termed the softening point depression coefficient. It is a measure of the extent of lowering of the softening temperature of the polymer by a mole fraction of plasticizer and characterizes a plasticizer in regard to its softening effect on cellulose acetate. Examples of softening temperature-plasticizer content graphs are given together with a table of k values and a table of plasticizer retentivities in cellulose acetate under heat softening conditions.
N THE study of polymer plasticizer systems numerable tests
I
have been developed for measuring efficiency, compatibilitv, and permanence of plasticizers, in order to measure and Understand the nature of the bonding forces between polymer and plasticizer and to classify plasticizers in regard to performance. It is evident that no single test procedure mill thoroughly evaluate a plasticizer but that a series of tests each possessing particular merits are needed to acquire a n over-all knowledge of a polymer-plasticizer system. I n the present paper a n additional test method, a creep test, is presented for measuring plasticizer efficiency and retentivity in thermoplastic films, specifically a t their heat softening temperature. By means of this test, data are given showing the effects of plasticizers on the heat softening and on the rheological properties of plasticized cellulose acetate films. Such data, in addition to throwing light on the interrelation betm een the chemical constitution and structure of plasticizers and their softening effect on plastics, are also of value for studying the flow characteristics of thermoplastic materials both for processing and for field use. The creep of a plastic film subjected to a low stress is a function of the temperature of the film and the time during which the stress is applied. By measuring the creep of a film against increasing temperature, temperature range is obtained a t which the rate of creep increases sharply. This is the softening range of the film. Plastic films soften ovcr a wide temperature interval. Most softening tests do not consider this temperature range, but rather determine that temperature at which the specimen reaches a certain degree of deformation or assumes a
N. J.
prescribed physical state. By the creep method a softening curve is obtained which portrays the change in state of the film over the entire softening range. From this curve an arbitrary point vhich has been designated as the softening point is derived. This point is reproducible within l o C. T h e i e s t is mainly applicable to Films which a t elevated temperature3 s h o ~ largely viscous rather than elastic deformation.
EXPERIMENTAL In the creep test for determining softening points, a strip of film in an air bath is subjected to a small load. The temperature of the sample is raised a t a uniform rate of 2' C. per minute and the elongation of the sample is measured against temperature. To compare films run at different stresses, the elongation readings are converted to strains which in turn are divided by the strers. The resultant value, called compliance, is plotted against temperature. From this plot a softening point is obtained.
THERMOMETER .
AME.5
DIAL
OIL BATH
5AMPl F FIXED PIN
It M
CLOSURE GAP Figure 1.
Apparatus for Measuring Film Softening Tern pera turer
