Cellulose Ester Plastics Plasticising Action of Homologous Series of Esters of Sebacic and Phthalic Acids on Cellulose Acetate Butyrate L. W. A. RlEYER AND W. 31. GEARHART Tennessee Eastman Corporation, Kingsport, Tenn. T h e physical properties of plastics compounded from cellulose acetate butyrate and homologous series of phthalate and sebacate esters show fairly good correlation with solvent ability of the plasticizer for the polymer as determined by tiscosity relationships of dilute solution of the cellulose esters in a mixed sol-\ent consisting of plasticizer and acetone. The toughness of the plastics a t low temperatures shows some correlation with the viscositytemperature relationship of the plasticizers. More data on this and other polymer systems are needed to establish
more generally the relationships between the physical properties of the plastic and the solvent power of plasticizers over a temperature range. The primary object in presenting this paper a t this time is to make available to workers in this field additional physical property data for use in establishing these relationships.
HE physical properties of cellulose ester plastics are dependent to a great extent on the kind and amount of plasticizer present, This variable determines the ease of flow, the mechanical characteristics, and the dimensional stability at temperatures of normal use. I n this paper are discussed the physical properties of cellulose acetate butyrate plasticized with such typical organic materials as the esters of sebacic and phthalic acids. By holding the processing and molding conditions as constant as possible, the effect of lengthening the esterifying alcohol chain of the plasticizer may be observed. I n any cellulose ester-plasticizer system, three main types of intermolecular attraction exist: t h a t between plasticizer molecules, t h a t between polymer molecules, and t h a t between plasticizer and polymer molecules. We are here concerned primarily with the latter. The plasticizers chosen for study vary in solvent power (for cellulose ester) from very high to very low and inM N C IN GRAMS/LITER GONG INGRAMS/LITER clude examples of those often referred to as Figure 1 ( L e f t ) . Acetone-Phthalate Ester Viscosities solvent and nonsolvent types. The amount Figure 2 (Right). 4cetone-Sebacate Ester Viscosities of plasticizer present covers a range of almost none to approximately 30y0, and thus affords an opportunity to study the effect of concentration as &ell as type. The attraction between plasticizer and polymer has becn the subject of several recent papers by Frith ( 6 , 6 ) ,who pointed out that a comparative measure of conipatibility can be obtained from the specific viscosity, q s p , of dilute solution$ of polymer in mixed solvents consisting of plasticizei and active solvent. The slopcs of the curveb obtained \+hen specific viscosity over concentration M as plottcd against concentration wcrc shown to be dependent on the plasticizerpoll mcr interaction; the stceper t h e slope, the better was tho solvent power of the plasticizer. Doolittle (2, 3) has also measured this attraction in terms of the dilution ratio of nitrocellulose in variTEMPERATURE “C TEMP E R ATU R E “C ous plasticizers. The solvent ability is given as the mole concentration of solvent Figure 3. Icinernatic T’iscosities of l’lasticizers 1418
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The cellulose acetate butyrate was a medium viscosity cellulose ester of 12y0 acetll Evaporation 96-Hour and 37y0 butyryl. (The Rate, Boiling 24 Hr. a t Water methyl, ethyl, n-butyl, and 2100' C., Stability, .4cid ethyl hexyl esters of phthalic G./1000 7% acid were obtained commerSq. Cm./Hr. Hydrolysis No. cially and used aft,er analysis 0.47, 0.11 0.22 showed them t o be of a rea0.28 0.05 0.07 sonable degree of purity. The 0.05 0,05 . 0 , 0 6 n-hexyl ester was synthesized 0.02 0.04 0.33 in the authors' 1abora.tories. 0.02
1.0
9570 7200 5400 4170 2630 1550
-
0.2 0.2 0.3 0.6 1.o I ,1
2.4 1.9 1.8 1.7 1.6 1.6
171.6 159.6 148.3 138.3 131.1 121.1
1 0 7.8 11.5 15.2 19.3 23.6
%
1.1 1.2 1.2 1.2 1.8 1.1
*
1.0 4.8 9.1 13.0 16.6 20.0
160.0
72 Hr. a t 82O C..
0.0 0.0 0.1 0.1 0.1 0.2
20 0
4.8 11.7 15.2 19.3 24 2
4480 2900 2110 1430 930
14,155 12,770 10,660 9080 7400 6100
505q
lo ti^^
2.2 1.9 1.8 1.6 1.2 1.5
8.1 13 0 16 6
,
6170 5770 5620 5080 3990 3070
Lous on
Water Absorption __ Wt. gain Soluble soluble loss, Toss, % %
170.4 159.4 145.7 134.4 121.8 113.8
1.0 4 8
R116 R114 R108 R102 R9l R79
,
1.80 1.75 1.60 1.55 2.30 1.20
1481
__-_
0.2 0.4 0.7 1.1 1.3 2.1 0.1 0.1 0.1 0.2 0.2
0.1 Exuded slightly
--
0.0 0.0 0.0 0.0 0.0
0.1 0.1 0.1
0.1
Exuded
series. The slopes of the sebacate esters tend to be lower than the phthalates, thus showing decreased solvent, power. This is further substantiated by the fact that 2-ethyl hexyl sebacate is such a poor solvent that even with 50% acetone i t will not dissolve the base material; in the plast#ici t WRS found t,o he compatible t o the extent of only about 20%. The viscosities of the plasticizers a t various temperatures are given in Table 11. These data are shown graphically in Figure
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
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Figure 9.
Figure 10.
Tensile Strength
side chain of the cellulose ester acts somewhat as a n internal plasticizer. Thus it is possible t o process this material without any plasticizer, although a small amount is usually preferred for its lubricating action. The presence of the butyryl group also greatly increases the solvent ability of the ester, so t h a t all the plasticizers here considered are compatible. The methyl and ethyl esters are solvents for the base material at room temperature and below, whereas the hexyl and 2ethyl hexyl esters become solvents at higher trmperatures. The entire commercial flow range is obtained by addition of plasticizer in quantities of approximately 1 to 30y0 as shawn in Figure 4. Here the flow temperature as ordinate has been plotted against per cent plasticizer as abscissa. The flow temperature is the temperature at which the material attains a defined degree of flow when subjected to a prescribed pressure for a prescribed time. It has been determined by A.S.T.i\I. D56944T. Of these plasticizers methyl phthalate shows the greatest attraction for the polymer, as indicated by the small quantity required for a given flow. Great difficulty is encountered in attempting t o remove this plasticizer from the base material upon reprecipitation of an acetone dope of the plastic with isopropyl ether. Ethyl hexyl sebacate has the least attraction for the cellulose ester, as indicated by the relatively large quantity required for a given flow and by the fact t h a t this ester is not compatible in amounts greater than 20%. These data indicate that little difference in physical properties is to be expected of the harder formulations having flow temperatures above 150” C., as the plasticizers in these cases are not present in sufficient amount to asseit their specific characteristics. However, as the flow temperature decreases, more and more of the “active centers” on the polymer macromolecules become covered until a point is reached where the intermolecular forcer betTTeen plasticizer and polymer become significant. PHYSICAL PROPERTIESBearing in mind these considerations, we can now proceed with
Flexural Strength
3, where the viscosity has been plotted as ordinate and the temperature as abscissa. The relationship is shown on the basis of log log kinematic viscosity in centistokes, v , versus log absolute temperature, T . As this results in approximate linearity, the slopes, m, may be determined from the following equation : log log
Y
=
712
log 2‘
+c
The phthalates as a group have lower slopes than the sebacates and within each series of esters the higher molecular weight esters have lower slopes. This indicates that temperature sensitivity becomes less as t h e aliphatic nature of plasticizer becomes more pronounced and as the molecular weight increases. GEKERAL CONSIDERATIONS. The butyryl
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Figure 11. RIodulus of Elasticity
August 1948
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
a discussion of t h e physical properties of the plastics. I n each case, the property as abscissa has been plotted against thc flow temperature as ordinate, so that the latter will remain unchanged in the following figures. The plotted values represent a five-test average and are, therefore, not too rigorous. Variations in processing, molding, and shaping are unavoidable and the precision of the standard test methods is not too high. However, t h e authors feel t h a t the data are sufficiently accurate t o show the general behavior of the ,plasticizers (Table 111). Because the permanence properties are of such importance, this subject is considered first. Test data represented are on accelerated aging and water absorption which are prime factors in determining the dimensional stability of t h e plastic in its end use. The ability of a plastic t o withstand heat and moisture in its environment sets t h e limit of its general utility. The loss on heating of the phthalate series is shown in Figure 5 (left). These data represent percentage loss on heating specimens inch thick for 72 hours at 8 2 ” C. in a circulating air oven. The strongly solvated methyl and ethyl phthalates actually show lower loss on heating than the higher boiling butyl ester. The hexyl and octyl esters have sufficiently low vapor pressures to be retained in the plastic even though poorly solvated. The loss on heating of the sebacate series is shown in Figure 5 (right). These esters are not so strongly solvated as in the case of the phthalates and here t h e heat loss is considerably higher, even though the evaporation rates of the esters themselves are superior t o the phthalates. The methyl ester is retained slightly better than the ethyl, but both are considerably inferior t o the other esters. The composition containing 20% 2-ethyl hexyl sebacate was found t o exude plasticizerduringthe test. Thisis anexample of insufficient solvation for commercial use. T h e water absorption and soluble loss of the phthalate series are shown in Figure 6 . These data were determined by A.S.T.M. D57042, which is a 24-hour immersion test. The per cent gain decreases regularly with increasing molecular weight of the ester except in case of the methyl ester. Because methyl phthalate itself has higher water solubility. than the ethyl ester, the lower absorption of its plastic must be due to closer packing accompanying greater solvation of the cellulose ester. This is further indicated by the fact t h a t the methyl ester has the lowest soluble loss.’ The water absorption and soluble loss of the sebacate series are shown in Figure 7. Although the sebacates are, in general, less susceptible t o water than the phthalates, their plastics have higher water absorption. This, again, can be attributed t o their lower ability t o solvate t h e cellulose ester polymer. Methyl sebacate, unlike the phthalate, has appreciable soluble loss, which indicates its lower degree of attraction. One of the more outstanding properties of cellulose ester plastics is toughness. The standard’procedurc for measuring this property is t h e Izod impact test, A.S.T.M. D256-43T. Although this method is subject t o considerable error, it remains a fairly reliable means of evaluation, providing careful technique is followed. The impact strengths of t h e phthalate series are
Figure 12.
Tensile Strength
Figure 13.
Flexural Strength
1483 ’
given in Figure 8 (left). Data are reported at 25’ and at -40’ C. At 25” C. the impact strength increases regularly with increasing molecular weight of t h e plasticizer. At -40” C. all esters yield uniformly low strengths. The reason for this behavior is not dear, although it is found to be true generally in the case of aromatic type plasticizers with cellulose esters. The impact strengths of the sebacate series, shown in Figure 8 (right), are considerably higher than t h e phthalates, especially at -40” C. The impact strengths increase regularly with increasing side group except for the 2-ethyl hexyl ester. This ester is so close t o the compatibility limit t h a t it has a tendency t o gel, which results in “cheesy” structure. As the degree of solvation decreases u p the series, the intermolecular attractions decrease, and this allows the plastic t o absorb greater shock before fracturing. Spurlin (8) attributes the increase in toughness t o the ability of nonsolvent type plasticizers t o dissolve only t h e lower molecular weight portions of the cellulose ester. This creates so-called “plasticizer rich” and “plasticizer lean” phases and results in a more or less continuous network of polymer chains which enclose pockets consisting of plasticizer together with some of the shortelt, more
1484
INDUSTRIAL A N D E N G I N E E R I N G C H E M I S T R Y
Figure I .i. Hockwell Hardness
Vol. 40, No. 8
5oluble cellulose t i t t i chains. Such N scheme is no1 unbuited t o these data. Evi dently, the 2-ethyl hexyl ester is so limited in compatibility that it cannot solvate evt~tr the material of lo\\ molecular ucight. Thc.bc, data also shall coirelation wilh Jonrs postulation that plasticizers whose viscc i s Less affected by teniperaturcb c~hangcss greater toughness a t IOK trmpwat ures. ?'lit sebacate esters have lower sloprs than t tit phthalates and generally havr bvtt tit lov teniperitl ure impact strengths; in r a w oi the sebacatrs, the higher rpolecular weight esters h a r e lower slopes and better low temperatuie impact strengths than the lo\irr esters. Hov+evrr,in case of t h e phthalates, there is n o increase in 1on teniperature iinpact strength with i n c r e a h g moleculai weight. Although there is grntral a g e c merit R ith Joneq on the vi~rosity-tein~)r.iiIture irlationship and toughness at IOM ieiriperature, the data do riot rorrelate well with freezing point data. Thr sebacate csteia up to the n-hexyl ester beconie solids by the time the temperature falls to -10" C. (see TahIv I:. This loss of fluidity should result in a lobs of impart stitsugth a
