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. CHEM.,
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
.April 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
Apparatus. The creep tester, the piece of equipment in which ,the softening test is carried out, consists of a rectangular tube 14.5 X 1.25 X 0.3125 inch (inside dimensions), constructed of sheet brass 0.0625 inch thick with a brass cap or rubber stopper for sealing the bottom of the tube. An Ames dial with indicator and shaft provided with a grooved pulley is mounted on top of the brass tube so that the groove of the pulley is in the center line of the tube. A side arm carrying a grooved pulley is also mounted on top of the tube, TWObrass pins are provided for holding the sample; the lomer of these pins is held stationary during a test in two.recesses located a t the bottom of the tube (Figure 1). I n making a run the creep tester is placed in an oil bath (mineral oil) which is t t t e d with a stirrer, a Centigrade thermometer calibrated in 0.2 , and sufficient heaters to raise the temperature of the bath a t a rate of 2 " C. per minute. The heaters are connected through a Variac so that the rate of temperature rise can be controlled. Specimens. Hand-cast cellulose acetate films 0.003 inch thick were prepared; these contained various plasticizers a t plasticizer contents from 0 to 50 grams of plasticizer per 100 grams of cellulose acetate. The cellulose acetate employed was a commercial film grade flake having an acetyl value of 38.9% while the plasticizers were all first grade commercial materials and were not treated prior to use. All films were cast on glass plates from solutions containing 400 grams of solvent (80% acetone, 20% ethyl alcohol) per 100 grams of cellulose acetate. The films were dried free of solvent and then were conditioned 24 hours a t 0% relative humidity and 25' C. Specimens 10.25 inches long were cut from the films with a paper cutter having parallel knives set 15 mm. apart. A specimen 10.25 inches long by 15 mm. wide was wrapped around an aluminum plate (5 X 2 X 0.0625 inch) in the lengthwise direction. The ends of the specimen were lapped 0.25 inch and sealed with a small amount of acetone. A loop, the perimeter of which is 10 inches, was thus obtained. I n order to remove retained solvents and moisture, the prepared specimens were seasoned 2 hours a t 140' F. and then 24 hours a t 0% relative humidity and 77" F. Test Procedure. The specimen was measured for thickness a t several points and an average value calculated. The specimen with the brass pin inserted was lowered into the brass tube by means of the string attached to the pin until the lower end of the loop was below the brass tube. The other pin was then placed i n the loop and the specimen raised until this pin touched the two stops. The string was laid over the two pulleys and a known weight attached. A weight was chosen so that the applied stress was between 1.5 X 106 and 4 X lo6 dynes per s uare cm. The tester was closed and placed in the oil bath whichxad been brought to a temperature a t least 20" C. below the expected softening point of the specimen. The Ames dial was set a t zero reading. The temperature of the oil bath was raised a t a rate of 2' C. per minute. The reading of the Ames dial and the bath temperature were recorded every minute until the sample had elongated approximately 40%. Curve Drawing. The readings of the Ames dial were converted
t o strains. These strains were divided by the stress on the sample giving values of compliance in units of square cm. per dyne. A smooth curve was plotted on rectangular coordinate paper of compliance values against temperature in degrees centigrade. T h e softening point was obtained by drawing a straight line through the two points on the curve whose compliance values The intersection were 60 and 100 square cm. per dyne X of this line with the temperature axis passing through the 0compliance value gave the softening temperature. By drawing a straight line through two ordinates of the softening curve the rate of softening was taken partially into account in deriving the softening temperature and less error was incurred than would have been the case if the softening point dere taken as that temperature a t which the film reached a predesignated value of compliance or apparent viscosity. The ordinates of 60 and 100 square cm. per dyne X were selected since the softening .curves generally approach a straight line through this range of compliance. By calculating the viscosity of the film at the softening temperature by a treatment similar to that employed by Wiley (8) it was found that a t the softening range the apparent viscosity of the film decreased sharply (about a hundredfold in a 15 C. temperature interval), the rate of viscosity decrease varying with the elastomeric properties of the polymer. At the softening temperature, determined by the method outlined, the
f Q
-
685
3 =0.57MlN. =I.O"/MIN. de d t =Z.O"/MIN.
TEMPERATURE"c
Figure 2. Effect of Temperature Rate on Softening Temperature of Plasticized Cellulose Acetate
apparent viscosity of the polymer is between 10 X 108 and 30 X 1 0 8 poises. Above the softening temperature, the viecosity of the polymer decreases less rapidly. In Figure 2 softening curves are represented for the same plasticized cellulose acetate film run at three different rates of temperature rise The softening temperature increases slightly with increase in tke rate of temperature rise. In order to shorten tests as much as possible a rate of temperature increase
dt
of d0
2' C. per minute was selected as a standard rate. An experienced operator can run softening tests a t a rate of temperature rise constant within 1 0 . 1 ' C. per minute. Thus, a constant error in the rate of temperature rise of 0.1' C. per minute would result in a maximum error in the softening temperature of 0.15' C. In Figure 3 softening curves are presented for the same cellulose acetate film run a t different stresses. These tests were made to determine whether for small and moderate stresses, the strain at any time is directly proportional to the stress. The softening temperature rises slightly with increase in stress. However, If
60 rr)
JE MPfRATUFZ€ "C
O
Figure 3.
Effect of Stress on Softening Temperature of Plasticized Cellulose Acetate
.
686
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 41, No. 4
O F P L A S T I C I Z E D CELLULOSE ACETATE (38.9% ACETYL) FILMS TABLE I. SOFTENIR'G TEMPERATURES
Plasticizer Dimethyl phthalate Diethyl phthalate Dipropyl p h t h a l a t e Dibutyl phthalate Diamyl p h t h a l a t e hfethyl phthalyl e t h y l glycollate E t h y l phthalyl ethyl glycollate Butyl phthalyl butyl glycollate Dimethoxy ethvl p h t h a l a t e Diethoxy ethyl'phthalate Dibutoxy ethyl phthalate Trimethyl citrate Trie thy1 citrate Tripropyl citrate T r i b u t y l citrate Acetyl triethyl citrate hcetyl tripropyl citrate Acetyl tributyl citrate Triacetin Tripropionin Tributvrtn Diethylene glycol diacetate Diethylene glycol dipropionate Trjphenyl phosphate Tricresyl phosphate 0 - a n d p-Toluene ethyl sulfonami dP 5- and p-Toluene sulfonamide 1-Cresyl-p-toluene sulfonate Dibiityl t a r t i a t e Pontaerythrttol tetracetate
;\lolecular Weight 194 222 250 278 306 966 280 336 982 3 10 366 234 276 318 360 318 360 402 218 260 302 190 218 326 368 199 171 262 262 304
5
10
174.0 174.0 175.7 177.7 180.2 172,O 172.0 176.0 177.5 179.8 180.0 176,7 177.6 178.8 180.9 180.1 180.2 181.2 175.0 174.5 176.8 169.1 171.2 178.3 182.1 173.2 176.3 179.8 174.8 180.4
158.5 158.5 161.5 163.3 169.6 159.5 160.8 163.8 161.9 164.9 170.5 161.5 163.6 165.1 168.5 168.1 170.1 170.5 167.9 157.6 159.4 147.3 158.8 167.1 172.6 158 1 160.1 165.2 164.9 165.4
the load is chosen so that the applied s t r e s is between 1.5 X loe and 4 X 108 dynes per square cm., the maximum possible error in the softening point from this cause is 0 . 3 " C. The maximum total error in softening point determinations by this method will be approximately 0 . 5 " C. This limit in error applies only to tests on film of identical composi;Cion. Variations in the mixing and casting of film samples frequently result in larger differences in the softening point determined on samples of the same formulation but prepared a t different times. In Figure 4 are shown three representative softening temperature curves determined by the creep method for three celluLose acetate films containing 10, 20, and 30 grams of dimethyl phthalate per 100 grams of cellulose acetate. The softening temperatures of these films are respectively 119.7', 134.2", and 161 O C.
DISCUSSION OF DATA As stated, softening temperatures vere determined for cellulose acetate films containing various plasticizers a t plasticizer contents from 0 to 50 grams of plasticizer per 100 grams of cellulose acetate. These data are given in Table I. The softening temperature of unplasticized cellulose acetate (38.9% acetyl) film was found to be 193" C. The softening temperatures were plotted against plasticizer content of the films. The softening temperature-plasticizer content curves for an homologous series of plasticizers were plotted on the same graph. A smooth curve was obtained for each plasticizer. The curves in Figures 5, 6, and 7 are examples of these softening temperature-plasticizer content plots. The following conclusions may be drawn from these curves:
The softening temperature of plasticized cellulose acetate film depends on the quantity and type of plasticizer used and the type of specific groups present in the plasticizer. By comparing softening point determinations with other methods of measuring the activity and compatibility of plasticizers in films, it was found that the softening temperature of the film depends to some extent on the activity and Compatibility of the plasticizer; the more active the plasticizer the lower the aoftening temperature of the film. Plasticizer activity and compatibility may be judged from solution temperature and other data reported in the literature ( 1 , 2 ) . I n films plasticized with equal amounts of plasticizers belonging to an homologous series, the softening point depression increases with decrease in the molecular weight of the plasticizer.
G r a m s Plasticizer per 100 Grama Cellulohe Acetate ____~__ 15 20 25 30 35 40 50 Softening Temp., O C . 145.5 125.5 134.8 121.2 105.8 100.0 147.0 129.5 137.7 124.5 111.8 100.5 150.0 141,O 130.3 135.0 123.8 121.8 154.7 148.2 147.0 147.0 150.5 ., 165.5 166.0 164.0 ... 146.5 127.2 136.0 ii9:o 104:S 93.5 147.6 129.1 137.0 121.1 108.5 99.8 164.4 140.8 146.3 140.0 138.8 ... 136,2 146.6 129.0 116.3 102.0 90.0 141.1 160.8 136.4 128.1 121.8 118.2 165.1 163.1 163.3 163.2 . . 145.9 122.5 134.4 113.5 97.1 80:6 148.5 127. I 136.8 117.1 104.8 91.7 151.7 141.7 124.5 130.6 115.0 105.3 160.0 155.2 152.3 152.0 133.7 ... 145. 6 15238 129.9 136.0 117.6 98. I 157.2 142.6 151.9 134.9 134. I 128.4 164.7 166.3 163.6 166.3 143.5 122.3 130.5 113.2 102:3 95.0 141.2 124.4 132.6 117.0 110.0 98.5 146.5 129. I 136.4 122.4 112.5 109.3 136.2 128.5 119.0 112.0 101.2 96.9 141.5 114.6 127 1 121.8 98.1 107.4 156.4 139.3 133.2 147.1 117.5 123.9 164.2 165.6 162.4 ... ... 143.7 131.0 117.3 108: 9 91.5 80.0 142.9 135.5 121.1 111.4 96.5 83.5 156.2 144.8 135.5 129.7 122.2 119.8 131.5 125.4 142.9 115.7 99.2 9G. 4 152.2 143.0 129.3 120.0 105.0 88.3
-
j
.
.
Since the softening temperature appeared to be a function of the molecular weight of the plasticizer, the softening temperatureplasticizer data were replotted on a mole basis. The logarithm of the softening temperature was plotted against the mole frnrtion of plasticizer calculated from: Moles of piasticizer Mole fraction of -_ plasticizer in film - Moles of plasticizer moles of acetylated glucose anhydride units
+
The molecular weight of a single acetylated glucose anhydride unit of cellulose acetate of 38.9% acetyl value is 261.2. This molecular weight was used in the above formula. I n Figuree 8, 9, and 10 the logarithm of the softening temperature of films plasticized with phthalates, glycollates, and citrates, respectjively, has been plotted against mole fractions of plasticizers. The experimental softening points obtained for a series of samples plasticized with plasticizers belonging to an homologous wries lie approximately on one straight line in the log softening
i
I
P
2 80
6 s c,
\
6
d3
40
z
5 .A
n
z
8 120
140
I60
TEMPERATURE " C Figure 4. Softening Temperature of Cellulose Acetate Plasticized with Dimethyl Phthalate
682
INDUSTRIAL AND ENGINEERING CHEMISTRY
April 1949
0 DIMETHYL
PHTHALATE t DIETHYL PHTHALATE DIFROPYL PHTHALAT€ DIBUTYL PHTHALATE DJAMYL PHTHALATE
0 METHYL PHTHALYL ETHYL GLYCOLLATE f
ETHYL PHTHALYL ETHYL GLYCOLLATE
8 BUTYL PHTHALYL BUTYL GLYCOLLATE
hi
a 5
w t-
?! I
I
I
J
30 qo 90 60 0 IO EO CRAMS PLASTICIZER PER I00 GKAMS CELLULOSE ACETATE Figure 5. Softening Temperature of Cellulose Acetate Plasticized with Dialkyl Phthalates
k
temperature-mole fraction plasticizer content diagram, except that a deviation from the straight line may occur for the higher molecular members of a series at high plasticizer contents. The plasticizer contents as shown in the graphs are the contents of the original mixtures. If there is no loss of plasticizer during the creep test, the indicated plasticizer content will be present also in the specimen a t its softening temperature. However, in the specimens which correspond to points not falling on the straight lines of Figures 8, 9, and 10, it was found by analysis before and after the test that there was a loss of plasticizer during the test. In these cases the softening points shown belong actually to films of lower plasticizer content than shown in the graphs. For example, plasticizer analyses of films which initially were plasticized with 30, 35, and 40 grams of dibutyl phthalate per 100 grams of cellulose acetate showed that after the softening test, these films contained respectively 21, 19, and 22 parts of dibutyl phthalate; this agrees approximately with the 19 grams (0.151 mole fraction) plasticizer content indicated by the softening temperature-plasticizer content curve (Figure 8 ) . The plasticizers which show a deviation from the straight line, therefore, are not held as tightly to the cellulose acetate as those which show no such deviation. The plasticizer concentration or the softening temperature depression a t which the deviation from the straight line occurs can be taken as a measure of the retentivity of the cellulose acetate for a particular plasticizer (Table 11).
T A B L11. ~ PLASTICIZER RETENTION IN CELLULOSE ACETATE
Plasticizer Dibutoxy ethyl phthalate Diamyl phthalate Tributyl citrate Trieresyl phos hate Tributyl aoetyf oitrate Butyl phthalyl butyl glycollate Dibutyl phthalate Diethoxy eth 1 phthalate Dipropyl phtgalate o-Cresyl-p-toluene sulfonate Triphenyl phosphate Tripropyl acetyl citrate
Mole fraction of plasticizer in film 0.056
0,078 0,072 0.076 0.078 0.12
0 , iSi
0.176 0.210 0.232 0.196 0.182
Grams of plasticizer per 100 grams cellulose acetate 8.3 9.9
10.7 11.6 13.0 17.6 19.0 25.4 26.2 30.4 30.5 30.6
0 IO 20 30 40 50 60 GRAMS PLASTICIZER PER 100 GRAMS CELLULOSE ACETATE Figure 6.
Softening Temperature of Cellulose Acetate Plasticized with A l k y l Glycollates
0
TRIMETHYL CITRATP
+ TRIETHYL CITRATE 180
@
TRIPROF'YL CITRATE TRIBUTYL CITRATE
~
20 30 40 50 GRAMS P1.ASTICIZER PER 100 GRAMS CELLULOSE ACETATE Figure 7. Softening Temperature of Cellulose Acetate Plasticized 0
IO
with Trialkyl Citrates
The curves for several of the higher molecular weight plastioizers portrayed a minimum-that is, the softening temperature of the fiilms increased slightly a t high plasticizer contents. It is believed that this was due to the fact that a t these high plasticizer contents a point was' reached a t which homogeneous frlms were no longer obtained. This was evidenced by slight blushing of these films a t these plasticizer contents which probably resulted in a change in the structure of the films causing an increase in softening temperature. The softening temperature-fractional mole plasticizer content curves fit equations of the form
where t equals the softening temperature in degrees centigrade of a cellulose acetate film containing n mole fractions of plasticizer, to is the softening temperature in degrees centigrade of the un-
688
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
--
0 DIJVE-THYL PHTHALATE + D I E T H Y L PHTHALATE DiPROPYL F H T H A l A 7 P D I B U T Y L PHTHALATE ' i l ?HTHAlAT€
*
PriTrlkiYl ETHYL GLYCOLLATC + ETHYL PHTUALYL ETUYL GLYCOLLATP PrlTHALYL BUTYL GLYCOLLATE w
rr
z
4
t 2.1 0
J
201 0
I
1
2
I
Figure 8. Softening Temperature of Cellulose Acetate Plasticized with Dialkyl Phthalates
2.3 CITRATE + TRIETHYL CITRATE TFIIYROPYL ClTRATe
0 TRIM€THYL
u
2 2
@
TRIEUTYL.
CITK4Tf
5 z,2
4
s
2
plasticized cellulose acetate flake, and IC is a constant termed the softening point depression coefficient of a plasticizer. The constant IC is a measure of. the extent of lowering of the softening temperature of the polymer by a mole fraction of plasticizer. High values of k indicate that the plasticizer lowers the softening temperature of the polymer to a great extent while the reverse is true for low values of /2. It is evideut that a relation of the Bind given above is purely empirical, since it is based on the arbitrary centigrade scale. However, the degree centigrade data approximate a straight line better than degree kelvin data, which would be preferred from a theoretical point of view. On the basis of the data given in Table I the k values for a number of homologous series of plasticizers and for a fev single plasticizers were calculated and are shown in Table 111. Table 111shows no definite relation between the structure of a group of plasticizers and their softening point depression coefficient. The active groups in an homologous series of plas-
.
0
2
0.I 0.2 Q3 0.4 MOLE FRACTION OF PLASTICIZER Figure 9. Softening Temperature of Cellulose Acetate
a3 0.4 MOLE FRACTION OF PLASTlGlZER
0.2
0./
Vol. 41, No. 4
0
Plasticized with A l k y l Phthalyl Glycollates
ticizers have the same arrangement and apparently satisfy the forces betx-een adjacent cellulose acetate chains to the same extent thus producing the same degree of loosening of the polymer structure. The k value can be considered as a measure of the reduction in bonding forces (dipole effects, etc.) produced by 1 mole of plasticizer. The size of side groups determines how many molecules can be accommodated close to a cellulose acetate chain. The larger these side groups, the fewer the molecules n hich can be accommodated and, therefore, the lower the retentivity for the plasticizer. Since k is determined by the forces between cellulose acetate chains and plasticizer molecules, it is to be expected that it varies a i t h the acetyl value of the cellulose acetate. fa, the softening temperature of the unplasticized cellulose acetate, also will vary Kith the acetyl value of the acetate. Runs for a cellulose acetate of 38.9y0 acetyl result in values of t o = 193" C. and IC = 2.14 (for n-alkyl phthalyl glycollates as plasticizers) whereas for a cellulose acetate of 40.2% acetyl, to = 197' C. and IC = 1.83 (same plasticizers). The increase in acetyl value from 38.9% to 40.2'%'0, therefore, gives a decrease in softening point depression for this particular group of plasticizers. The creep softening curves also can be used to study the flow characteristics of cellulose acetate in the softening range. By a treatment similar to that presented by Wiley (8)the apparent viscosity of the film can be calculated over the softening range from the rate of change in slope of the creep curve. This treat-
COEFPICIENTS OF 111. SOFTENIKG POINT DEPRESSION PLASTICIZERS FOR CELLULOSE ACETATE(38.9% ACETYL)FILMS
TABLE
Plasticizer Type Dialkoxyalkyl phthalates Tri-n-alkyl citrates Glyceryl triesters Di-n-alkyl tartrates Pentaerythritol tetracetate n-Alkyl phthalyl glycollates Diethylene glycol esters Acetyl tri-n-alkyl citrates Triphenyl phosphate 0- and p-Toluene ethyl sulfonamide Di-n-alkyl phthalates o-Cresyl-p-toluene sulfonate 0 - and p-Toluene sulfonamide Tricresyl phosphate
k (Softening Point Deprwsion Coeffioient) 2.25 2.20 2.19 2.18 2.18 2.14 2.03 1.98 1.96 1.93 1.71 1.09 1.04 1.61
April 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
ment could be carried further and the apparent activation energy of flow determined. A treatment of this type might provide additional information regarding the softening characteristics of plasticized polymers. No attempt was made to correlate other physical properties of cellulose acetate-plasticizer mixtures with mole fractional plasticizer contents. It appears probable that physical properties besides the heat sbftening characteristics of plasticized polymers also may be related to mole fractional plasticizer content.
689
ticizer content fits the equation, t = toewkn, where to ( t and $0 in O C.) is the softening temperature of the unplasticized cellulose acetate, n is the mole fraction of plasticizer in the film, and k is a constant termed the softening point depression coefficient. k is dependent on the nature of the plasticizer. Plasticizers belonging to an homelogous series have the same k value. The plasticizer content at which a deviation from the straight line of the log softkning temperature-mole fraction plasticizer content plot occurred was taken as a measure of the retentivity of the plasticizer by the cellulose acetate.
SUMMARY
LiTERATURE CITED
By means of a creep test, softening temperatures were determined of cellulose acetate films plasticized with various plasticizers over a range of plasticizer contents. Softening temperature-plasticizer content curves were plotted from these data. at was found that the relation between softening point and plas-
(1) Fordyce, C. R., and Meyer, L. W., IND.END.CHEM.,32, 1053
(1940). (2) Gloor, W., and Gilbert, C., Ibid., 33,597, (1941). (3) Wiley, F.R.,Ibid., p. 1377.
RECEIVED September 8, 1948.
POLYVINYL CHLORIDE COMPOUNDS Effect of Plasticizer Structure on Properties ROBERT R. LAWRENCE AND ELIZABETH B. MCINTYRE Monsanto Chemical Company, Springfield, Mass.
T h e properties a plasticizer imparts to polyvinyl chloride are primarily dependent on the plasticizer's functional groups. * This paper discusses the properties imparted by carboxylic ester groups, ether linkages, aryl groups as compared to aliphatic, and branched chains. Results indicate that carboxylic ester groups are one of the most efficient solubilizing groups, but two are required to ensure satisfactory behavior. Phosphate groups are also effective, but ether linkages and double bonds have little value. Plasticizer migration into surface finishes has attained increasing importance. In general, aryl groups are superior to alkyl groups in resisting migration. High molecular weight esters are also less prone to give trouble. Phthalate esters, as a class, are better than sebacates or phosphates. This investigation served to emphasize the fact that the choice of plasticizer for any specific application is a matter of compromise. Improvements in properties imparted by each functional group are generally at a sacrifice to other properties.
HE use of polyvinyl chloride and vinyl chloride copolymers in elastomeric applications has grown to the point where today over 100 million pounds of plasticizer are required annually. Virtually thousands of materials have been offered to vinyl technologists to fill this great need. Thus, it is important to have some basis for choosing those plasticizers which impart the properties desired for any specific application. It is one purpose of this paper to aid in supplying the background necessary to make proper selections. An adequate understanding of plasticization requires a knowledge of two factors: First, the properties which are desirable in the best plasticizer; and secondly, how changes in the plasticizers' molecular configuration or functional groups affect these properties. A plasticizer must meet several mwe or lesa fundamental re-
T
quirements for each application in which it is to be used. Included are: compatibility, plasticizing efficiency, low temperature flexibility, water sensitivity, volatility, and resistance to migration. These six characteristics are directly dependent on the plasticizer. Resistance to heat and light have not been included as they are dependent on the stabilizer used. Many other properties are often of importance but were considered secondary in this investigation. Among these are flammability, odor, color, electrical properties, and toxicity. Their degree of importance is largely dependent on the particular end use concerned. These properties are also dependent on other compounding ingredients and impurities present in the plasticizer itself.
PROCEDURE The polyvinyl chloride used in this investigation had a specific viscosity of 0.55 as determined on a 0.4Oj, solution in cyclohexanone at 25" C. It was compounded into the following proportions (by weight) : Polyvinyl chloride Plasticizer Basic lead carbonate
100 Varies 3
The proportion of plasticizer was varied to obtain an apparent modulus of elasticity of 1800 pounds per square inch a t 25" C. The plasticizer concentration then served as a measure of plasticizer efficiency, which is defined as the relative ability of the plasticizer to soften the polymer at room temperature. The elastic modulus of 1800 pounds per square inch corresponds to that obtained with 50 parts of di-2-ethyl hexyl phthalate. The plasticizers used were either obtained from various manufacturers or were synthesized in this laboratory. All are believed to be substantially pure and as represented. Basic lead carbonate, used as the stabilizer in this investigation, was added in the same proportion in all w e e .
