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 a n 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 t o the point where today over 100 million pounds of plasticizer are required annually. Virtually thousands of materials have been offered t o vinyl technologists t o fill this great need. Thus, it is important t o 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 m w e 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 t o migration. These six characteristics are directly dependent on the plasticizer. Resistance t o 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 a n 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 t o 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 t o 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 .
INDUSTRIAL AND ENGINEERING CHEMISTRY
690
Vol. 41, No. 4
TABLE I. PROPERTIES OF POLYVINYL CHLORIDEWITH VARIOUSPLASTICIZERS (Apparent modulus of elasticity of each formulation is 1800 pounds per 8auare inch.) . \Tolatility, plasticizer Plasticizer Migration, Plasticizer Compatibility, Efficiency, Mg* of Plasticizer Viscosity Parts Plasti- P a r t s PlastiFlex. SC Plasticizer Extracted, Supplier of (30' c . ) , cizer per 100 cizer per 100 Lost from Plastic Temp., Loss (24 hr. % (24 hr. in Sample0 Centistokes P a r t s Resin Parts Resin a t 105' C.) Hz0 at 25OC.) Lacquer OC. Varnish BIonsanto 10.0 300C 46 5.5 71.3 0.89 360 680 300C Monsantn 41 55.2 0.76 8.7 , - 5.5 310 400 ..... 300C 13.4 42 48.0 0.56 - 3.J 190 270 Monsanto 300C 13.9 41 36.0 0.45 12 200 400 Eastman 300C 18.1 42 - 13 27.0 0.04 300C 33.3 14 50 11.9 0.15 iio 266 300C 0.15 33 .,6 48 -12.5 10.0 140 220 Ba%< 2ooc-3001 Solid 88 4.6 0.04 6 30 f 7 Ohio-Apex 300C 27.2 49 -29 0.02 1.9 , . . 300C 48.4 Carbide & Carbon 50 - 23 0.02 4.1 25 ZQ Resinous Products 300C 44.5 4.6 54 -24.5 15 0.04 5 300C 75 - 33 0.34 2.5 0Lib:kpex 2OOC-3001 42 -13.5 i:2 2.21 54.3 2?b 636 ..... 300C 46 21.4 23.1 9.5 0.26 100 250 300C 52 21.2 37.6 - 7 150 0.05 D - 5 300C 41.6 10.7 60 0.06 6 240 300C 27.7 0Lib:k;ex 17 46 9.7 120 80 0.15 Ohio-Apex 43.8 -17.5 52 75C-1001 14.6 470 540 0.94 21.3 46 300C - 14 17.1 220 360 0.24 300C M onsanto 38.8 10.1 46 - 12 35 120 0.09 32.0 300C Monsanto - 2 45 14.4 260 90 0.00 5 o c - 501 ..... Sol/d 60 4- 2 . 5 230 40 4.5 0.05 ..... 60C- 751 over 75 Solid .... ,. ... ... ~~
Phthalates Dimethyl Diethvl
-
-
Di-n-octyl Di-2-ethyl hexyl Dicapryl Dilauryl Di a11y 1 Dibutyl monochloro Dibutyl dichloro Dibutyl trichloro Dibutoxy ethyl Diearbitol Butyl hexyl Butyl glycollyl butyl Butyl benzyl Dibeneyl Diphenyl Sebacates Dimethyl Dibutyl Dihexyl Di-2-ethyl hexyl Dibutoxy ethyl Dibenzyl PhosDhatea
. I
-
I .
Hardesty Resinous Products Hardesty Resinous Products Resinous Products Resinous Products Carbide & Carbon Monsanto Monsanto Ohio-Apex
.
Miscellaneous Dioctyl maleate Dicapryl diglycollate Dioctyl thiodiglycollate Ethylene glycol dipelargon Dibutyl lauramide Ethyl ricinoleate Ethyl oleate Ethyl stearate Octyl laurate
4.9 7.8 13.0 17.7 Solid
ii:i
Solid 68 8.9 19 8 13.8
Deehy Produrtn Eastman Kessler KessleP
12:o 8.9 16.8
..
.. ..
Index of Suppliers: Barrett Division of dllied Chemical & Dye Corp. Carbide & Carbon Chemicals Corp. Deecy Products Co. Eastman Kodak Co. Emery Industries, Inc.
2OOC-3001 300C 300C 300C , 300C 2ooc-3001
35 37 54 54 44 42
-17.5 -34.5 - 47 -57.5 34 - 13
74.9 36.4 18.4 1.0 9.4 9.9
2.73 0.33 0.13 0.00 0.26 0.04
1ooc-2001 300C 50C- 601 300C 300C 25C- 401
38
-25.5 - 58 - 2.5 1.5 -31
61.5 5.1 5.8 1.5 21.4
2.25 0.02
300C 300C 300C 2OOC-3001 300C 25C- 401 25C- 401 25C- 401 40C- 501
47 46 48 50 43
56
52 67 44
..
-
+
....
- 3Y - 19
-35.5 - 56 -38.5
.... ....
...
, .
37 5 l5,6 4.7 22: 1 . I
0.19
0.04 2.82
..
0.24 0.18 0.20 0.25 0.10 I .
220 320 590 130 720 260
870 1150 450 50 49 0 840
>600 >600 >600 >600 >600
> 600
680 360 350
820 420 810
'90
>BOO
...
... ...
...
>600 >600 > 600 > 600
...
)
.
j
... ..
Hardesty Chemical Co Keesler Chemical Co. Monsanto Chenucal Co. Ohin-Apex Inc. R o h m & Haav Chenural C o
PREPARATION OF SAMPLES The plasticizer, resin, and st'abiliaer were first thoroughly blended by hand. Each batch was then fluxed on a hot' two-roll mill. Roll temperatures were maintained at 280" to 290" F. Mixing was continued for 5 minutes after a continuous band had been formed around the front roll. The stock !vas continually cut back and forth during the &minute interval. Roll clearance was set to permit removal of a 0.020-inch thick sheet from the Tolls. A sheet 0.250 inch thick was first molded to provide a specimen for the room temperature modulus measurement. Variation of more than 300 pounds per square inch in modulus was considered too large and warranted adjustment in the plasticizer content.. A new mix was then made and its modulus rechecked. A 0.075-inch sheet was molded for the low temperature flexibility test. This and the 0.250-inch sheet were pressed using a bminute heating cycle at 340" F. All other tests were determined on 0.020-inch sheets, molded 1 minute a t 340" F. TEST METHODS Compatibility. The film casting technique was used to determine the compatibility limit of the plasticizer. The desired plasticizer concentration was added t o a 570 solution of the polymer in cyclohexanone. This solution was cast onto glass to produce a thin film, which was dried and conditioned a t 25" C. Exudation of plasticizer, if present, was easily observed. Plasticizer Efficiency. As described previously, the plasticizer cont>ent necessary to impart an apparent modulus of elasticity
of 1800 pounds per square inch at 25 C. served as a measure of efficiency. Modulus was obtained by the torsional method of Clash and Berg (2)by which the deflection of the specimen under a specified torque is measured after 5 seconds. Low Temperature Flexibility. The torsional method was again used with the exception that the specimen used was only 0.075 inch thick. The flex t)emperature, which has been arbitrarily defined as that temperature a t which the elastic modulus is 135,000 pounds per square inch was used as a measure of the relative low temperature flexibility. Volatility. The relative volatility of the plasticizer was determined by measuring the extent of evaporation under controlled conditions of air flow and temperature from a sheet whose dimensions were 3 X 1 X 0.020 inch. Result,s reported in t.he work were determined a t 105' C. for 24 hours. A specially constructed oven in which air was continually exhausted to the atmosphere was used. Air flow in this oven was at the sate of 200 feet per minute. Water Sensitivity. The extent of water absorption and plasticizer leaching from a 0.020-inch thick specimen during 24 hours immersion at 25" C. was measured. The specimens were conditioned for 4 hours at 50 C. before and after immersion. Resistance to Migration. The work reported herein was concerned only with migration into varnished or lacquered surfaces, both typical furniture finishes. The nitrocellulose lacquer was a typical finish marketed by the Merrimac Division of Monsanto Chemical Company and designated as clear lacquer lt-641. This lacquer is plasticized with dibutyl phthalate and contains a substantial quantity of ester gum. The varnish was typical of furniture finishes and consisted of a linseed oil-p-phenyl phenol resin The lacquer or varnish was first brushed or sprayed onto the surface of a flat steel plate. The finish was permitted to air dry
April 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
-53
691
0
A
PHT~ALATCI SCDACATlS
-45Y
o
P U
E
-40-
z 0 Y
-wc Y
2
-30-
U L
/
J
-15
+++++!Ao C A R D O N S IN A L K Y L c n A i w
Figure 1. Effect of A l k y l Chain Length on Plasticizer Efficiency Appar ent modulus
of elasticity of each lormulation i s 1800 1 b . h in.
at least 7 days before use. A 0.020-inch thick plastic sheet 2 X 3
inches was then placed against the finish with a pressure of 0.33 pound per square inch. The effect of the plastic on the finish was observed after 7 and 30 days a t 25" C. After 30 days the change in weight of the plastic was measured
C O M M E N T S O N TEST M E T H O D S Lt is recognized that the casting method w e d for determining compatibility often gives higher limits than those observed on milled compounds. This may be caused by the presence of small amounts of tightly held solvent in cast films. The use of a standard modulus in measuring plasticizer efficiency permits a comparison based on the amount required to echieve a desired flexibility a t room temperature. The smaller the amount required to yield the standard modulus, the more efficient is the plasticizer. Results obtained in this manner are more meaningful than those obtained by comparison of a fixed quantity of plasticizer. Reed ( 3 ) has compared plasticizers in a concentration necessary to yield an elongation of 100% a t 25'. C. with a load of 1000 pounds per square inch. This method is only an approximation because no account is taken of the shape of the stress-strain curve, which truly defines the flexibility of the material. The ClashBerg equipment appears to yield a better measure of modulus. Even with the Clash-Berg there may be a wide divergence between the shapes of the curves prior to reaching the 5-second interval. This has been well illustrated by Alfrey, Janssen, Aiken, and Mark (I). Particularly outstanding were the variations in behavior between tricresyl and trioctyl phosphate. A slow but significant increase in elastic modulus occurs during the first 48 hours of conditioning. I n this work a standard conditioning time of 48 hours was used. Efficiency results are accurate to a t least plus or minus 1part per 100 of resin. The flex temperature, used as a measure of relative low temperature flexibility, is generally considered the point below which a material loses its elastomeric properties. In determining the rate of plasticizer volatility, the rate of air displacement is extremely important. This has been discussed by Rider and Sumner ( 4 ) and Reed ( 3 ) . Volatility serves as a
CARBONS IN ALKYL
Figure 2.
tn*in
Effect of A l k y l Chain Length on Low Temperature Flexibility
measure of the service life of the material. The permissible volatility is largely dependent on the thickness in which the material is to be used. Plasticizers whose volatility is less than 5% are preferred for most applications. Although resistance to migration is not a factor in many applications of vinyl resins, the lack of this property has been the cause of many complaints in recent years. An understanding of this phenomenon is essential to proper plasticization. The change in weight of a plastic after 30 days' contact with nitrocellulose lacquer or varnish was used as a measure of the migration resistance. Many observations have established a correlation between weight change and the final condition of the finish. The following table illustrates the effect: Plastio Weight Loss, Mg. 0- 25 25-100 100-200 200-400 Over 400
Condition of Finish Slight marring Very slight softening Slight softening Very soft Soft and sticky, lifting of finish in some cmes
A sample containing Paraplex G-25 showed a weight gain in thie test; Paraplex G-25 is considered one of the best nonmigratory plasticizers.
DISCUSSION OF RESULTS The results obtained in this investigation have been arranged to illustrate the effect of carbon chain length and functional groups in the plasticizer on the properties of the plasticized polymer. The actual data are given in Table I. Effect of Chain Length. A complete homologous series of phthalate esters ranging from the dimethyl t o the dilauryl ester was investigated. Several esters of sebacic acid and two phosphate esters are included. These represent three distinctly different chemical types, designated by the formulas:
0
-COOR
-COOR
ROOC=CaH16=COOR
/ OR O=P-OR \OR
INDUSTRIAL A N D ENGINEERING CHEMISTRY
692 I
CARBONS
Figure 3.
IN ALKYL
Effect of A l k y l Chain Length
105°C.)
group represents the major portion of the molecule, is inferior to the. di-2-ethyl hexyl ester! whew the alkyl group is predominant. The value of long syninietrica,l chains in improving low temperature flexibilit,y is demonstrated by the superiority of alkyl sebacates over alkyl phthalates. Figure 3 demonst,rates the rapid decrease in volatility with increasing chain length. With each of the three types, eight carbon alkyl groups are necessary to produce sufficiently low volatility for most end uses. Plasticizer extractionresultsalso indicate the value of long chain aliphatic groups (Figure 4). Correlation with the water solubility of the plasticizer itself is only fair. Based on .water solubility further improvement between dibutyl and di-2-ethyl hexyl phthalate would not' be expected. But this does not prove to be the case. Figures 5 and 6 show the effect of chain length on migration to surface finishes. Here again, migration rate diminishes with increasing chain length. Rate of migration is undoubtedly dependent on rate of diffusion of plasticizer through polymer and affinity of aurface finish for plasticizer
CHAIN
on Plasticizer Volatility (24 Hours at
00
1
e
3 4 CARBONS
ti 6 IN ALKYL
7
8
CHAIN
Figure 4. Effect of A l k y l Chain Length on Plasticizer Extraction (24 Hours in Water at 250 C.)
With all phthalates and sebacates in which R represents a n alkyl group, compstibility exceeded 300 parts based on 100 parts of resin. This is to be expected in view of the tm-o carboxylic ester groups in each molecule. I n the case of phos80O1phate plasticizers, the tribut'yl esker was compatible t.0 only 100 parts, whereas trioctyl phosphate was miscible at 300. 0 PHTHALATES A SEBACATES This would indicate an increase in compatibility with increasing chain length, probably going through a maximum. In general, plasticizing efficiency based on percentage by weight decreases with length of alkyl chain. An exception to this trend occurs wit'h phthalate esters with which efficiency is at. a maximum a t the diethyl ester. If each plasticizer series is compared on a molar basis, the variation reverses. On a molar basis, di-2-ethyl hexyl phthalate is the most efficient plasticizer of ... that series. Results are shown in Figure 1. An increase in the length of 100 alkyl chain improves the low ALL TRIALUYL PHOSPHATES ARE temperature characteristics in all - A B O V E S O O W O . IN H I O R A T I O N cases, as illustrat,ed by Figure 2. This can be explained, since long unbranched alkyl chains 00 1 h C A3R B O NbS I'N bA L K Y L CHAIN 7 8 enhance the mobility of the plasticizer molecule in the polyFigure 5. Effect of A l k y l Chain Length mer. For example, dimethyl on Plasticizer Migration into Lacquered pht'halate, in which the aryl Surfaces
z
Vol. 41, No. 4-
60Or
500
400
'00
PHTHALATES SEBACATES
1
100-
A L L TRIALKYL P H O S P H I T E S P k E A B O V E 6 0 0 NO. IN M I G R A T I O N
-
I\ '0
I
z
l-A.--L 5 6 7 IN A L K Y L CHAIN I
3
4
CARBON5
a
Figure 6. Effect of A l k y l Chain Length on Plasticizer Migration into Varnished Surfaces
INDUSTRIAL AND ENGINEERING CHEMISTRY
April 1949
693
which is inefficient and imparts extreme temperature sensitivity. However, because of its ring structure, this ester is quite different in form. Effect of Other Functional Groups. T h e effect of addition of chlorine into the benzene ring of the phthalate plasticizer was studied. This is considered beneficial due t o the flame retardant properties of the chlorine group. However, t h e results (Figure 10) indicated that addition of c h l o r i n e c a u s e d a corresponding decrease in efficiency and low temperature flexibility. Conversely, volatility and resistance to migration were improved. One example of a n unsaturated phthalate ester was studied. I n this case the unsaturated ester. diallyl phthalate, was superior t o its saturated counterpart, dipropyl phthalate, in its temperaFigure 7. Effect of Ether Linkage on Plasticization, ture sensitivity. This advantage was offset by inPhtholater Sebacatea Phosphates ferior water resistance. Little change was ob1 . Dihexyl 4. Dihexyl 6. Trioctyl served in migration resistance. 9 . Dibutoxy ethyl 5. Dibutoxy ethyl 7. Trlbutoxy ethyl Another interesting study was the comparison 3. Dicarbitol of butyl hexyl phthalate with butyl phthalyl butyl glycollate. Each contain a n equal number of carPhthalate esters show a gradual but significant improvement bons but the latter has a n additional carboxyl group. Strangely in migration resistance in going from the dimethyl to the di-2enough, this carboxyl group appeared t o impart considerable ethyl hexyl ester. Di-2-ethyl hexyl phthalate has almost no more resistance t o migration. No significant differences in other tendency t o soften either varnish or lacquer. Figure 5 shows that properties were noted. with sebacate esters the curve gives a maximum a t the dihexyl Several long chain esters of fatty acids, each containing twenty carbon atams, were also investigated. The results ester. I n general, sebacate esters are poorer than the correillustrated the necessity of having two carboxylic ester groups t o sponding phthalate. The two trialkyl phosphate esters have been omitted from effect compatibility. It was found also t h a t the ether linkage, Figures 5 and 6, as their migration is so severe that weight as present in dicapryl diglycollate, was detrimental t o low temperature flexibility. changes are impossible t o measure with accuracy. The phosphates tend t o soften the finishes t o the point where they become CONCLUSIONS extremely sticky and pull away from their supporting medium. Effect of Ether Linkage. Several ester plasticizers containing No correlation existed between viscosity and low temperature flexibility. ether linkages were investigated. These included dibutoxy ethyl Compatibility, wholly dependent on the plasticizer molecule, phthalate and sebacate, tributoxy ethyl phosphate and dicarbitol phthalate. can be controlled through the presence of adequate functional I n general, the primary effect was a n increase in water sensigroups, often called solubilizing groups. The carboxylic ester group is perhaps one of the strongest such groups, b u t two are tivity. This is well illustrated in Figure 7. Ether linkage appears to improve plasticizing efficiency but, conversely, required t o e n s u e satisfactory behavior. Thus, oleates and ricinmigration and low temperature flexibility are poorer. Dibutoxy oleates are incompatible but phthalates and sebacates are misethyl phthalate does not appear to show the extreme chanaes exhibited bv the sebacate and Dhosphate esters. Dibutoxyl "ethyl phthalate differs EFFICIENCY LOW TCYPERATURE VOLATILITY MIORATION FLEXtWLlTY from the dihexyl ester only in t h a t slightly improved efficiency is imparted. Effect of Aryl Groups. Figure 8 illustrates the effect of aryl groups on plasticization. The substitution of aryl groups for alkyl groups in the molecule tends t o improve both volatility and migration resistance, but at a serious sacrifice t o efficiency and low temperature flexibility. A notable exception to these trends is dibenzyl sebacate, which is considerably more efficient than dihexy1,sebacate. Water sensitivity, not shown on the graph, remains the same. However, a definite decrease in compatibility occurred. Effect of Branching. Only a limited number of esters (all phthalates), in which the alkyl groups consisted of branched chain alkyl groups, were investigated. As shown in Figure 9, it was found that branching tended t o harm efficiency, Figure 8. Effect of Aryl Groups on Plasticization low temperature flexibility, and volatilitv. Thus, t h e dilz-octyl was superior to both t h e di-2: Phthalate: Sebacater Phosphates ethyl hexyl and dicapryl esters. An extreme 1, Dlbutyl 4. Dibutyl 6. Trloctyl 2. Butyl benzyl 5. Dibenzyl 7. Tricresyi example waa that of dicyclohexyl phthalate, 3. Dibenzyl EFFICIENOY
LOW TEYPLRATURC FLEXIBILITY
WATER
EXTRAOTIOW
YIORATION
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
694 TFFICIEIICY
LOW TLYPERATURC FLEXIDILITY
VOLATlLlTl
R
: 2
2 0
>
I75
Y
Y
k I50 (r
H
2
:I25 -1
d
I P 3 4 6 f i
Figure 9.
Effect of Branching on Plasticization
Phthalates 1, Di-n-hexyl 4. Di-n-octyl 7. Di-%ethyl butyl 5. Di-2-ethyl hexvl 3. Disyclohexyl 6. Dicapryl
Vol. 41, No. 4
%ethyl hexyl and dicapryl phthalate are superior to all other ester plasticizers. Several more temperature sensitive plasticizers, such as tricresyl phosphate, are poorer. Phosphate esters, as a class, are migratory, although tricresyl is considerably better than trioctyl phosphate. It is known that different results are obtained with other type* of finishes. Each fuiish appears to offer a separate problem and must be considered individually. Migration into nitrocellulose lacquer was generally much more severe than t h a t into varnish. However, in a few instances the reverse was true. Plasticizere resistant t o varnish, but migratory into lacquer also occur. Volatility and water resistance need no further explanatiou Volatility correlates quite well with molecular weight for eacb homologous series, although small variations often are caused b> differences in solvent power, Water resistance, in general, is dependent on the water solubility of the particular plasticizer Water sensitive groups such as ether linkages should generally be avoided. Proper plasticization is always a problem of obtaining the besi compromise in a range of properties. The compromise is dependent on the application involved and the properties required for that application. Perhaps a t some future date, the so-called ideal plasticizer may be developed, making compromise methods no longer necessary. I
cible to high proportions of plasticizer concentration. The user should be cautioned that many plasticizers normally considered satisfactory may exude after exposure to ultraviolet light. This is probably due to degradation of the plasticizer molecule caused by light with or without heat, thus changing the compatibility behavior. Because the presence of suitable stabilizers will UPUally prevent this breakdown, this phase has been omitted from this discussion. Phosphate groups have been found to be strong solubilizing groups. One sample, dibutyl lauramide, also indicates the solvent power of the amide group. On the other hand, ether linkages and double bonds have little effect. Plasticizing efficiency is probably the most important single property of a plasticizer. Moreover, with nearly all plasticizers now selling a t 4 5 6 7 0 prices considerably above that of the resin, the compounder naturally seeks to kee6 plasticizer concentration a t a minimum. Greatest effiFigure 10. Effect of O t h e r Functional Groups o n Plasticization Phthalates oiency can be obtained with molecules whose func1 . Dibutyl 5. Dipropyl tional groups amount to the largest percentage 2. Dibutyl monochloro 6. Diallvl of the total molecule. Thus, dimethyl phthalate 3. Dibutyl dichloro 7. Butyl hexyl is considerably more efficient than the dioctyl 4, Dibutyl lrichloro 8. Butyl phthalyl butyl glycollate ester. Furthermore, aliphatic groups promote e5ciency better khan aromatic. LITERATURE CITED Low temperature flexibility can best be obtained by 101iy etraight chain aliphatic molecules. For example, dioctyl seba(1) Aiken, W., Alfrey, T., Janssen, A., and Mark, H., J . Polymer s'ci.. 2, 178 (April 1947). cate, dioctyl phthalate, and tricresyl phosphate rank in that order. (2) Clash, R. F., and Berg, R. hf., Modern Plastics, 21, 119 (Jut) Unfortunately, the same factors which lead toward the best low temperature flexibility also lead t o migration tendencies. Yet, 1944). (3) Reed, 31.C.. IND. ENG.CHEM.,37,604 (1945). the correlation is far from perfect. (4) Rider, D. K., and Sumner. J. K.. IND.ENG.CHEM..ANAL. E n . This paper has been concerned only n-ith migration into 17, 730 (1945) varnished and lacquered surfaces, probably the most widely used furniture finishes today. With these finishes, di-n-octyl, diR E V E ~ Y E ~ 28, 1949 ~
