Processing Behavior of High' Polvrners -
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EFFECT OF PLASTICIZER TYPE A. M. GESSLER AND A. F. SAYKO Esso Laboratories, Standard Oil Development Company, Elisabeth B , iY. J . T h e processing behavior of synthetic rubbers can be greatly improved by properly chosen immiscible plasticizers. The latter are preferably polymeric materials, selected in an intermediate and closely defined molecular weight range which obviates exuding tendencies and volatility, and at the same time provides optimum processing properties and intimate dispersion. With such materials milling behavior, rate of incorporation of compounding materials and pigments, and extrusion and calender behavior surpass results usually obtainable with conventional plasticizers. The experiments were carried out under laboratory conditions that closely simulated factory practice. Specific examples of immiscible plasticizers are given, and the data are discussed for Perbunan, GR-S, Rutyl rubber, and neoprene.
S
YKTHETIC rubberlike polymers are more difficult to process than natural rubber. This difficulty is particularly pronounced with gum or near-gum compounds, and is evidenced under normal processing conditions by tendencies which appear to arise from a high degree of elastic behavior. The purpose of this paper is to show that the solubility relations between the polymer and the plasticizer modify this elastic effect and the processing performance of the resulting high polymer systems. Plasticizers may be classified as miscible or immiscible with the matrix polymer. Some workers (9) have chosen to consider both the polymer and the plasticizer as liquids, and, under this condition, miscibility a t any temperature becomes a function of the concentration of each component in the system. The work reported here was limited t o those cases where the concentration of the plasticizer was taken in the range of 5 to 40 weight 7 0 based on the polymer. The classification of materials as miscible or immiscible thus may be made, depending on whether one-phase or two-phase structures result from the mixtures so prepared. T o illustrate this point, Figure I shows photomicrographs of thin films prepared from a few typical mixtures of 20 parts of plasticizer per 100 parts of polymer. The systems Perbunan 26 with dibutyl phthalate and Butyl rubber (GR-I) with hydrocarbon oil (Zerice 42, a mineral oil of naphthenic base from Columbian crude) represent the case of miscibility. A single, homogeneous phase is shown in both instances, the scattered objects in the pictures being bits of foreign matter on which the microscope was focused. For the mixtures of Perbunan 26 with 12,000 polyisobutylene (12,000 molecular weight, Staudinger, from [ T ] = K M , where M = molecular weight, K = 3.18 X 10-6, and [ T ] = intrinsic viscosity at 20' C . in diisobutylene as solvent) and Butyl with 10,000 Perbunan (l0,OOOmolecular weight from [ T ] = KM0.e4,where K = 4.9 X 10-4 and [?I = intrinsic viscosity a t 20" C. in benzene &s solvent) immiscibility is evidenced in Figure 1 by the characteristic speckled arrangement of two distinct phases. As Jones (6) pointed out, limited miscibility is undesirable because the plasticizer will exude or sweat oat from the compound on standing. This is true only when the fluidity of the plasticizer is
great enough to allow its migration, by diffusion, through the molecular network of the high polymer system. If the viscosity of the plasticizer is increased, a point is reached, depending on the molecular size and structure of both polymer and plasticizer, where this motion is reduced to such a low order of magnitude that the material ceases to be migratory. Thus a fluidity range is obtained over which the immiscible plasticizer may be used. As its viscosity is taken above this range, severe incompatibility results, the two phases no longer exhibit cohesive tendencies, and the mass, on mechanical working a t elevated temperatures, disintegrates into a nonhomogeneous crumb. EVALUATION OF PROCESSIYG PERFORiMANCE
The difficulties (14) encountered in correlating the processing behavior of high polymers with results obtained from standard plasticity methods (7,1$?)are well known. It was possible to avoid some of these difficulties by employing actual processing equipment (No. '/z Royle extruder) as an instrument for rapid deformation experiments. With a coastapt worm speed of 80 revolutions per minute and a t a temperature (barrel and head) of 220" F. the machine was fixed with die and pin to form a tube (0.4-inch outside diameter and 0.05-inch wall thickness). Constant stock temperature was assured by cycling a given volume (150 cc.) of material twice through the extruder. On the third pass, tube scctions representing 30 seconds of running time were collected, taken directly to an air circulating oven at 220' F., and allowed to rest for 10 minutes on a liberally talcked base. The specimens were cooled for 5 minutes a t room temperature after the heating period, and their lengths and weights measured. From the specific gravity of the stock and from the measurements of weight and length taken, the unit volume, in cubic centimeters per inch, was calculated for each tube. If a material were purely plastic, i t would extrude to die diinensions and would have, under the conditions employed here, a unit volume of 0.90 cc. per inch. This value, therefore, may be taken as the ideal value representing the case of purely plastic behavior. Elastic tendencies of high polymer systems result in tube volumes which are greater than the ideal, the difference (between the actual and ideal tube volumes) being proportional to the degree of elasticity present. It is thus possible to relate plasticity to the stock swell a t the die of the extruder, and to express the results in terms o f either the unit tube volume or per cent swell, based on the ideal value. In the figures which follow, both methods of expressing results w'ere used. In addition, thin, cross-sectional pieces sliced from each extruded tube have been superimposed and subsequently photographed on the ourves to add visual significance tQ the relations treated. The areas of these cross sections are, of course, proportional to the swell a t the die, since in every case the volumes were taken at constant (unit) tube length. The distortion of an extruded form as it issues from the die of the extruder is dependent on the tendency of the stock to recover from the deformation induced-i.e., on the subsequent development of the reversible, high elastic component of total deforma1751
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Perbunan 26-dibutyl phthalate
Perbunan 26-X2,000 polyisobutylene
Butyl rubber-10,000 Perbunan
Butyl rubber-hydrocarbon oil
Figure 1.
Vol. 41, No. 8
Photomicrographs of ‘Thin Films of Typical Polymer-Blas ticizer S y s terns
tion. This component, which has often been discussed ( I , 3 , 6 , Q - d I ) , attains full development slowly if thc tube is allowed to cool. To provide for the completion of this high elasticity (vihich manifests itself in a lateral svelling and longitudinal shrinkage of the tube section), and thus to assure equilibrium epeciinen dimcnsions, the 10-minute heat treatment was employed before any measurements were taken, The test, x-hich has the advantage oi being carried out under actual processing conditions, is capable oi excellent precision, results being repi oducible viitliin 1 or 2Yc. PREPARATION OF POLYMER SYPTEUS.On a 6 X 12 inch mill, plasticizer was added to 200 grams of polymer as rapidly as possible, consistent 79ith good mixing techniques. The starting temperature for each batch was held a t 90 O to 95 O F., and cooling n-ater was circulated through the rolls during the mixing. No attempt was made to maintain a constant mill setting for the preparation of all the compounds studied; instead, the mill was set in each case to allox- for an active, rolling bank. To minimize polymer breakdown differences which might result horn variations in the extent of mechanical treatment, mastication of each of the seveial batches in a series was continued, after incorporation of the plasticizer, for varying intrrvals until for parh a ronstant total mixing period v, as obtained.
cizer types are given in Table 1. The results obtained from the extrusion testing of these mixtures are shown in Figure 2, where processing performance, judged by the elastic swell of the extruded tube, is expressed as a function of the plasticizer concentration. Plasticizers function to reduce the viscosity of the polymer mass and to render i t more mobile and extensible (6). With miscible plasticizers this is accomplijhcd by the separation of polymer chains, and the swollen, gellilie structure which results is characterized by enhanced elasticity. Thus the processing behavior of the polymers in Figure 2, a t least in so far as it is relatcd to those operations which require high plasticity, is adverjely influcnced by
TABLE I.
PLASTICIZER
MATRIX POLYXER Perbunen 2 6 (Fig. 2)
PLASTICIZER Dibutyl phthalate 10 000 perbunan (BK-10) 12:OOO Polyisobutylene (B-12)
Butyl (Fig. 2 )
Hydrocarbon oil (Zerice 42) 12,000 po!yisobutylene (B-12) 10,000 Perbunan (.%X-l0)
Misriblc Misoible Immiscible
GR-S (Fig. 2 )
Dibutyl phthalate 1 2 000 polyisohutylene !B-12) l0:OOO Perbunan (AN-10)
Miscible Miscible Immiscible
Keoprene (Fig. 2)
Dibutyl phthalate 12,000 polyisobutylene (B-12) 10,000 Perbunan (Ah-10)
Rljacible iVIiscible Imniiscible
APPARENT ELASTICITY
Although a large number of polymer-plasticizer systems has been investigated, only a few typical cases are considered here. The components of these systems and the classification of plasti-
POLYMER-PLASTICIZER SYSTBXS TPPS
Miscible Miscible Immiscible
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10 20 30 40 PARTS 8Y #EIGHT OF PLASTICIZER FER 100 PARTS OF PERBUNAN 26
I
0
3.5
5 IC 15 20 PARTS BY WEIGHT OF P L A S T I C I Z E R PER IO0 PARTS O f BUTYL
i ---l__i
4l
25
I
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I
1
GR-5-PLASTICIZER
G'EUTYL PHTHALATE
-288
233
178
5c Y
sc
123
PARTS BY WEIGHT OF PLASTICIZER PER 100 PARTS OF GR S
a
5
10
5
20
PARTS BY WEIGHT Of PLASTICIZER PER 1 0 0 PARTS OF NEOPRENE
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the addition of miscible plasticizers. The pentagonal centers and thick walls of the corresponding tube cross sections attest the high degree of elasticity which is realized. The above discussion is not intended to infer that miscible plasticizers are not processing aids. The softening of the rubber mass achieved by adding them is justifiably associated with easier handling. On the open mill, for example, some polymers will not form hole-free bands without plasticizer. The activity of the bank is generally increased when plasticizer is added to the rubber mass. The incorporation of fillers and other compounding ingredients into the batch is facilitated by plasticizer addition, and power requirements for Banbury, open mill, calender, and extrusion operations are reduced. These facts and wany others not mentioned refer to plasticizers in general, and are not limited by considerations of plasticizer type. The immiscible plasticizers m-ork as well as the miscible and, as will be shownlater,arc employed in the viscosity range which allows them to be mixed with the rubber much more rapidly than is true of the great majority of conventional miscible plasticizers used by the industry today. The magnitude of t h r forces required to deform a rubbery substance is dependent to a large extent on the viscosity or toughness of the material being deformed. When the viscosity of the plasticizer is high, the viscosity of the corresponding polymer masg is high, and the greater forces required to accomplish the extrusion process may result in a less reversible deformation. For miscible systems this argument tends to explain the fact that greater elasticity is realized with the liquid, monomeric plasticizers than with the solid or semisolid, polymeric substances. Figure 2 shows that the elastic behavior of the mixtures with immiscible plasticizers decreases sharply in every case; the resulting tubes, as indicated by the photographed cross sections, soon approached the range of excellent extrusion performance. This fact, coupled with knowledge garnered from Figure 1, leads to the speculation that plasticization in these instances is not accomplished by the separation of single molecular chains but rather by the separation of groups or agglomerates of polymer molecules. On deformation of the polymer mass, these agglomerates might be rolled past one another in a manner simulating the action of so many tiny ball bearings, a phenomenon which would produce the decreased elastic behavior observed. The experimental work reported here is limited to extrusion considerations only. The writers wish, however, that the data be taken in a broader sense to embrace all those processing steps in which elasticity is undesirable. These steps include a host of prevulcanization fabrication prqcedures in which the rubber mass is deformed under a given set of conditions with the objective that the mass assumes permanently some imposed shape or form. Rapid deformations of the type found in extrusion are most frequently encountered. I n the authors' laboratories volume measurements of mill and calendered sheeting have shown the same relations for miscible and immiscible systems as were given for extruded tubing. An advantage in the ease of handling as well as in the appearance of friction and skim coated fabric*hasbeen demonstrated for systems containing immiscible plasticizers. Test pieces have been molded without vulcanization from Perbunanpolyisobutylene systems, whereas they could never be SO molded from similar Perbunan-dibutyl phthalate systems. This suggests the important role immiscible plasticizers might play in the plastics industry. While the foregoing represent only a few processing operations, the results support the view that the use of immiscible plasticizers leads to general processing improvements which can be correlated with the data obtained from the extrusion test as the authors have used it. The reduction in viscosity obtained with immiscible systems is
+-Figure 2.
Elastic Swell of Extruded Tubes of Four Polymer-Plasticizer Systems
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picture already mentioned. The ease of deformation of a rubberlike polymer mass will depend on the intermolecular forces present in the system. With miscible systems it is necessary to slide one chain molecule past the other so that the total force required €or the deformation is the product of the van der Taals forces tying two chains together and the number of chains in the mass. With immiscible systems whole groups of chains may be rolled pa,st one another in an agglomerate a-ithout appreciably altering the relative positions of the chains in the agglomerate. Thus the number of chains moved with respect tjo neighboring chains would be greatly reduced, and the force required to complete a given deformation of the mass would be correspondingly less. This type of speculation seems to fit, the extrusion data obtained. IMMISCIR1.E PLASTICIZERS
Figure 3 .
K a t e of Tuhe Exit
often many times greater than that obtained with miscible systems. Experimental evidence has shown that tKo-phase mixtures have unusual rheological properties suggesting the existence of thixotropic and/or yield value structures. The folloiring Afooney viscosity data illustrate this point: Time, Min. 0.5 2 4
6
8 A Mooney
hlooney Viscosity a t 100' C GK-I, inn; GE-I, in0 Zerice 42, 10 AN-10, 10 5J 35 30 50 33 17 47 32 13 46 31 11 46 31 10
C:R-I
.
8
4
20
T h e high A Mooney (differerice between Mooney reading a t 0.5 and 8 minutes) obtained with the GR-I-AX-10 blend is real, for if the Mooney machine is stopped and the material allo\wd l o rest, the curve can be reproduced on the second run. RATE OF EXTRUSIOA
The exit rate of material from the extruder a t constant \\orin speed is influenced by many lactors. Although i t is not possible at this time to evaluate all the variables, i t can be demonstrated (Figure 3) that immiscible systems are characterized by comparatively rapid rates of material throughput. I n Figure 3 the rate of t u b exit per minute, based on the equilibrium specimen dimensions, is expressed as a function of plasticizer concentration For the polymers considered, sharp increases in extrusion rate are obtained only with the immiscible plasticizers. These increases, which, like the reductions in elasticity observed previously, ale linear in the first approuimation. fall in line with the ball-hcaring
Given the polymer, any number of materials can be selected as immiscible plasticizers on the basis of chemical dissimilarity. A description of some of those tried in this laboratory and a few observations made during these trials are given be lo^. ACRYLOXITRILE COPOLYMERS.Rather extensive investigations of Perbunan-polyisobutylene systems established for the first time that plasticizers of limited miscibility could be used. Although polyisobutylene of 3000 Staudinger molecular weight' (viscosity 650 centistokes a t 100' C.) produced desirable processing effects, i t exuded rapidly from Perbunan compounds, raw or vulcanized, prepared with or without reinforcing fillers. Polyisobutylenes in the molecular weight range 10,000 to 20,000 (viscosity a t 100" C. greater than 8000 centist.okes) were excellent processing aids for Perbunan 26 (Figure 2) and exhibited little or no migrat,ory tendencies. (Other nitrile polymers tested behaved similarly, including Perbunan 1s and 35 and IIycar OR-15 and OR-25.) As the molecular weight of the added hydrocarbon polymer was increased from 20,000 to 40,000, the magnitude of t h r x desirable processing effects diminished. Perbunan mixtures containing polyisobutylenes in the range 40,000 to 100,000 molecular weight failed to exhibit cohesive strength, and crumb of varying heterogeneity resulted from the attempted extrusions. The experiments carried out, with Perbunan-polyisobutylenc blends have been described in some detail to demonstrate the role that viscosity considerations must play in the proper selection of thc iiiimiscible plasticizer. For nitrile rubbers results similar to those obtained with B-12 were obtained aith other aliphatic hydrocarbon polymers of lox niolecular Feight, such as polvbutadiene, polyisoprene, polypropylene, copolymers of isohutylene with Ca diolefins, and low molocular weight copolyrnei~sof diolefins n-ith styrene. Ko general rule has been found to allow the conipounder t,o select, without previous knowledge or experimentation, iinrniscible plasticizers which are suitable for use. Each case appears to be unique, and he must det.ermine the viscosity range of the plasticizer which will yield the optimum effects. For examplc, a propylene polymer of about t,he same viscosity as the 3000 molecular weight polyisobutylene already mentioned was found not l o exude From Perbunan admixtures as did t,he lat,tcr, but to be so immiscible .ivit,h the matrix polymer that extrusion a t 220" F. produced the type of nonhomogeneous crumb obtained with polyisolsutylenes whose molecular weights were 40,000 or more. The effect of temperature is also an important consideration. Irnmiacible systems which appear to exhibit cohesive strength a t low temperatures-for example, during preparation on a cool mill--may crumble or dissociate under stress at higher temperatures. The Perbunan-pol?-propylene mixtures appeared quite usable under cool mixing condit,ions. The same stock fed into the extruder a t 220" F. crumbled so that the preparation of a tube was not possible. With cooling water on the head and barrcl of t,he extruder, a rapid extrusion of the Perbunan-polypropylene systems was realized, and the tubes, which were formed a t approximately 110" F., showed little or no tendency to swell a t the die. These same temperature effects were obtained to a some-
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the result is to cancel out some of the desirable processing effects obtained with the first plasticizer. Thus, from the standpoint of increased polymer plasticity, these two plasticizers may be regarded as antagonistic. If, instead of dibutyl phthalate, we add the immiscible plasticizer diethylene glycol phthalate ( a rather viscous liquid), the processing performance of the system continues to improve aa it did for polyisobutylene addition done, and the two plasticizers may be said to be complementary. These facts are summarized in Figure 4 where the results from the extrusion test are shown for the mixtures just discussed. The case of mixed plasticizer types (miscible and immiseible) is interesting because it represents a method of obtaining vqrying degrees of immiscibility in the same system. I
1
I
I
5 10 I5 20 ?ARTS BY WEIGHT OF PLASTICIZER PER 100 PARTS
Figure 4.
I 25 OF PERBUNAN 26
I
i 5 6
I
I
Elastic Swell of Extrusion Systems Containing Two Plasticizers
what lesser extent from Perbunan blends prepared with polyisobutylenes of greater than 40,000 molecular weight. PREDOMINANTLY HYDROCARBON POLYMERS. For hydrocarbon polymers such as Butyl and GR-S, immiscible plasticizers were used such as polymethyl, polyethyl, and polyoctyl acrylates, polymethyl methacrylate, polyvinyl ethers such as polyvinyl nbutyl and isobutyl, and selected condensation products of polyhydric alcohols and polybasic acids. Certain polymerized vegetable oil gels, such as those prepared commercially from linseed, soybean, and castor oils, are immiscible with both the hydrocarbon and more polar acrylonitrile polymers, and were found to be cxcellent processing aids for Perbunan, Butyl, and GR-S. The processing improvements observed were much more pronounced than those obtained from the vulcanized vegetable oils, the commercial factices. adding polyisobutglene B-12 to Butyl rubber is much like adding low molecular weight species of the same polymer. The effect is to change the molecular weight distribution to the mass, as when “low ends” were added. The increased elastic tendency which results is evidenced by the swollen tube volumes shown in Figure 2 (Butyl) and is in agreement with the work of Zapp and Baldwin (1.9). A very low molecular weight Perbunan oil (viscosity 247.6 centistokes a t 100’ C.) was found to exude from Butyl, GR-S, and neoprene admixtures. This did not occur with the 10,000 Perbunan used in connection with the data of Figure 2 for these three polymers (viscosity of 6700 centistokes a t 100” C.). With standard Perbunan 26 as the plasticizer, no desirable processing effects were obtained, and the resulting blmds were characterized by rather severe incompatibility. As indicated by Figure 2, 12,000polyisobutylene was not effective in reducing the nerve of prepared GR-S mixtures. It was, however, rather effective in reducing the nerve of similar mixtures prepared with the German I.G. Buna S. The latter contains an appreciable quantity of insoluble gel which, it is believed, produced with polyisobutylene a two-phase structure similar to those described here. These results seem to agree with recent work on GR-SBO (8),a better-processing GR-S prepared, by treatment with divinylbenzene or by hot milling, to contain a gel fraction. NIXED P LA STlCI ZER S
To obtain specific vulcanizate qualities, it is often desirable to use more than one plasticizer. The effect that a second plasticizer may have on the processing quality of the system is important. An example is a blend of Perbunan 26 with 10% by weight of 12,000 polyisobutylene (Figure 2). If we add dibutyl phthalate which, unlike polyisobutylene, is miscible with the matrix polymer,
EFFECT OF RElNFORClNG FILLER
Figure 5 shows the processing behavior, as judged by the extrusion test, for two systems as a function of the concentration of channel black added as reinforcing filler (MPC, Kosmobile 66). In the first system, where 20 parts of the miscible plasticizer (dibutyl phthalate) were used per 100 parts of Perbunan 26, the elastic swell of the extruded tubes decreases linearly as the concentration of black is increased. I n the second system with 12,000 polyisobutylene as immiscible plasticizer, the elastic behavior of the resulting black mixtures declines initially and then remains unchanged by the further addition of filler. These experiments were repeated with other types of carbon blacks and clay, and the same general results were obtained. It is evident, then, t h a t the differences in procwsing quality which have been shown for the miscible and immiscible gum systems are reduced by the addition of filler and to an extent proportional to the quantity of filler employed. As Figure 5 shows, the improved processing qualities realized with immiscible systems persist through the pigmen% loading range used for most rubbery products. r
,
0
IO
20
30
40
PAPTS BY WEIG*T Of CYANNEL BLACK PER 100 ? A R B OF PERBUNAN 26
Figure 5 .
50 I
Elastic Swell of Channel Black-Plasticizer Extrusion Systems EASB Op \liXlhG
The ease with which plasticizer may be added to an elastomer on the open mill depends, to a great extent, on the viscosity of the plasticizer. With very fluid liquids, addition is difficult and must be made intermittently and slowly to prevent the breaking apart or “lacing” of the polymer band formed around the mill roll. A s the viscosity of the plasticizer is increased beyond the consistency of honey or heavy molasses, addition becomes somewhat easier, although considerable time and care are still required to avoid the difficulties just mentioned. When a t room temperature the plasticizer is semisolid, like molding putty or heavy asphslt, addition on the mill may be made rapidly eves] without permasti-
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ring than does honey; its plastic viscosity, 6.3 poises, is much less than that 5 cl*er iecositya of Plasti- Time t o Add 50 G. Plaaticizer at loOO c., to 200 of thelatter, 11O.Opoises(C). But honey, Polymer Plasticizer Type Centistokes G. Polymer, Min. because i t is a true liauid with no yield Perbunan 26 Dibutyl phthalate Miscible 2 34 (1’) 8 value, flows under its own Keight; mayPerbunan 26 12,000 polyisobutylene Immiscible 20,000 (S) 2 onnaise does not and is reported (4) to GR-S Dibutyl phthalate XIiscible 2 34 91/4 have a yield value of 850 dynes per sq. GR-S 10,000 Perbunan 1 miniscible 247.6 (H) 20 em. [The coefficient of plastic viscosity GR-S 10,000 Perbunan I inmiscible 6,690 (S) 2 GR-S 3,000 polyisobutylene Miscible 6 is defined ( 4 ) as the tangential force, in 650 (H) GR-S 12,000 polyisobutylene hliscible 20,000 1 I/¶ excess of the yield value, that >.illinduce GR-I Hydrocarbon oil Miscible 4.72 (V) 13 a unit velocity gradient.] GR-I 10,000 Perbunan Immiscible 247 6 30 6,690 1‘/z With Perbunan many colorless, inexGR-I 10,000 Perbunan Immiscible GR-I 12,000 uolvisobutslene Miscible . . 20,000 a pensive plasticizers derived from petroa V = very fluid, S = semisolid, H = heavy molasses. leum cannot be used because they are immiscible with the rubber and have such high fluidity that they will exude from cation or breakdown of the niatrix polymer, axid incorporation is prepared compounds. The hydrocarbon oil Zerice 42, for exalmost instantaneous. I n order to prevent “bleeding,” the visample, “bleeds” rapidly from Perbunan admixtures. It has been cosity of the immiscible plasticizer must be high. I t is fortunate found that these plasticizers may be employed with nitrile rubber that the minimum viscosity to assure ease of addition with this if a small portion of an all-hydrocarbon polymer (Butyl rubber, type of plasticizer has, in almost every case, coincided with the polyisobutylene, butadiene-styrene polymer, polybutadiene, minimum viscosity required to suppress the bleeding tendency. polyisoprene, etc.) is also included in the compound t o act as the Plasticizer type, though not so important as viscosity, also insolute which imbibes the otherwise migratory plasticizer. The fluences the ease of mill addition. At high fluidity levels, inirapid increase in viscosity of a solvent on the addition of soluble miscible plasticizers are more difficult to incorporate with the polymer is well known. This action, which apparently takes place polymer than miscible plasticizers. -4t the semisolid level theie is as the compound is prepared and is subsequently allowed to rest no significant difference in the rate a t which the two types of before vulcanization, destroys the fluidity of the plasticizer and plasticizer can be mixed with the polymer. The foregoing results produces a nonmigratory mass. Interesting compositions have are summarized in Table 11; the data were compiled from work been prepared from Perbunan 26 using G R - I 4 0 (Mooney viswith a 6 X 12 inch mill. cosity 31-40) in combination with all-hydrocarbon oils. This technique of utilizing fluid immiscible plasticizers may be applied FLOW to other synthetic rubbers simply by selecting the proper poly‘The flow properties of two-phase polymer systems differ from meric solute; it is of particular interest with Perbunan because inthose of single-phase systems. T o illustrate, tube sections were expensive, colorless plasticizers which are miscible with the extruded from the following blends: (1) Butyl 100 hydrocarpolymer are not presently available. 10,000 Perbunan 20. The tubes bon oil 20 and (2) Butyl 100 Except in Butyl compounds where materials of high unsaturawere cooled in water immediately after exit from the extruder tion must not be used, satisfactory vulcanizates may be prepared and allowed to rest a t room temperature on a flat surface. with immiscible plasticizers. In general, higher tensile strength The stock from blend 1 flowed rapidly; despite the fact that the and modulus are realized with the latter than with the more contube was thick-walled, its cylindrical shape was destroyed by ventional plasticizers. These points are illustrated in Table 111 collapse within a few hours. The tube from blend 2, on the other where the vulcanizate properties are compared for a typical hand, showed little or no tendency to f l o under ~ its own weight Perbunan compound prepared with the immiscible plast3icizer and preserved its original shape for weeks. This rheological behavior was not predicted since each of the components of TABLE111. EFFECTO F PLASTICIZER T Y P E ON VULCANIZATE PROPERTIES blend 2 was characterized by the ability (Base recipe: Perbunan 26, 100; zinc oxide, 5 ; SRF black, 75; sulfur, 1.5; benzothiazyl disulfide, to flow more or less rapidly into the shape 1.0; cured a t 287O F.) of its container. The action of the imPolyisobutylmiscible plasticizer here appears, as in the ene B-12 0 10 .., 30 40 . . 20 Dibutyl case of the extrusion results, to be like phthalate 0 .~~ 20 30 ... 40 10 ... t h a t of reinforcing filler, but n-ith this im15-min. cureu portant difference: The retardation of Tensile 3180 560 1960 2155 2565 1180 2255 1540 2520 Elongation 455 825 595 605 785 510 515 680 575 polymer flow obtained with filler depends Modulus 2235 526 1240 1305 710 385 965 1010 655 on an increase in the viscosity of the system (as measured by either the Williams or &looney tests); in the case of the immiscible plasticizer, the retardation occurs in systems of reduced viscosity. 43-niin. cure 1355 2105 2325 Tensile 3370 2665 1925 2880 2765 2390 These results suggest for immiscible 655 615 395 Elongation 360 495 455 525 400 475 1015 1215 790 Modulus 1495 2820 2130 2030 1420 1790 mixtures the possibility of yield value. Yield value, t h e shearing force necessary 00-min. cure 1605 2280 1930 2365 2535 2415 Tensile 3435 3015 2760 to produce an infinitely slow rate of 660 585 395 Elongation 415 420 445 420 455 390 905 1135 1320 Modulus 2925 2320 1965 2015 1595 1565 flow, has been observed in many twophase systems and attributed to floc90-min. cure 2185 2355 1600 2245 1980 2770 2530 Tensile 3565 3160 culation. It occurs a t low viscosity in 340 590 535 385 385 435 415 Elongation 350 420 materials which cannot flow until a 1440 985 1335 1670 1725 2090 2140 Modulus 2500 3000 certain minimum force is applied to cause a Tensile strength, in pounds per square inch; elongation, in per cent, modulus of elasticity a t 300% them to do so. Mayonnaise, for exelongation, in pounds per square inch. ample, offers much less resistance to stir-
TABLE11. ErrccT
+
OF
PLASTICIZER TYPE ON MIXING
+
...
INDUSTRIAL A N D ENGINEERING CHEMISTRY
August 1949
polyisobutylene and with the miscible plasticizer dibutyl phthalate. Special problems of vulcanizate quality demand special treatment and cannot be considered in this paper. Work is in progress in this laboratory to investigate thoroughly the effect of immiscible plasticizers on the resulting vulcanizates. ACKNOWLEDGMENT
The writers are indebted to the late Henry Green of the Interchemical Corporation for the preparation of the photomicrographs. LITERATURE CITED
(1) Alexandrov and Luzurken, Acta Physiochim. U.R.S.S., 12, 647 (1940). (2) Frith, Trans. FaradaU Soc., 40,90 (1945).
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(3) Guth and Mack, Naturwissenschaften, 25,353 (1937). (4) Hoagland, Stewart, “Rheology of Surface Coatings,” Bound Brook, N. J., R. B. H.Dispersions, Inc., 1947. (5) Jones, Trans. Inst. Rubber I n d . , 21,298 (1946). (6) Kuhn, Naturwissenschaften, 24,346 (1936). (7) Mooney, IND.ENG.CHEM., ANAL.ED.,6, 147 (1934). (8) Schoene, Green, Burns, and Vila, INP. ENG.CHEM.,38, 124& (1946). (9) Tuckett, Chemistry & I n d u s t r y , 62,430 (1943). (IO) Tuckett, Trans. Furaclay SOC.,38,310 (1942). (11) Ibid., 39,158 (1943). (12) Williams, IND.Ex*. CHEM.,16,362 (1924). (13) Zapp andBaldwin, Ibid., 38,948 (1946). (14) Zapp and Gessler, Ibid., 36,656 (1944). RECEIVED March 16, 1948. Presented before t h e meeting of the Division of Rubber Chemistry, AXERICANCHEXICALSOCIETY, in Cleveland, Ohio May 1947.
Vapor-Liquid Equilibria of Hvdrocarbon Svstems above Atmospheric Pressure J
d
EDWARD GELUS, STANLEY MARPLE, JR., AND EM. E. MILLER’ Shell Oil Company, Inc., Houston, Tex.
A
steel recirculation apparatus equipped with automatic pressure control has been constructed which is suitable for obtaining equilibrium vapor and liquid compositions up to 500 pounds per square inch gage. Data are presented for the system 2,2,4-trimethylpentanetoluene at 14.7, 29.7, and 59.7 pounds per square inch absolute. This system is useful for testing continuous fractionating columns over a wide range of liquid-vapor ratios. Data at atmospheric pressure are also presented for 2,2,4-trimethylpentane-methylcyclohexane,useful for testing columns of high theoretical plate number.
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UNDAMENTAL. in the design and testing of efficient fractionating equipment is a knowledge of the volatility characteristics of the materials being fractionated. The available body of data for hydrocarbon s y s t e m such as are encountered in petroleum refining is highly inadequate, particularly above atmospheric pressure. The work reported here was undertaken for the purpose of developing a suitable apparatus and obtaining necessary data, particularly for use above atmospheric pressure. Data are reported for the system 2,2,4-trimethy1pentane-to1uene at 14,7,29.7, and 59.7 pounds per square inch absolute and for the system 2,2,4-trimethylpentane-methylcyclohexane at atmospheric pressure. The first system is useful for testing fractionating columns of normal plate numbers, particularly at molar liquid-vapor ratios other than 1.0. The latter system is useful for testing fractionating columns of high theoretical plate numbers. The recirculation still technique was chosen for the investigation after careful consideration of other methods. The type of apparatus used is relatively simple, the accurate controlling of the pressure being the main problem. Temperature control in this technique is less critical than in other methods, and with hydrocarbon s y s t e m well below critical conditions, the compositions are not sensitive to small errors in temperature measurement.
* Present address, 5411 Auatin,
Houston, Tex.
The difficulties encountered by earlier investigators in the, operation (especially under pressure) of recirculation stills aye as follows: ( a ) entrainment of liquid droplets in the vapor stream, ( b ) disappearance of a liquid phase, (c) refluxing on the walls of the boiler with subsequent enrichment] ( d ) flashing of the condensate as it is returned to the boiler, ( e ) backflow surging from boiler to condensate receiver, (f) solution of inert gases in the liquid under pressure, and ( 9 ) accurate pressure control. Entrainment may be eliminated by allowing proper disengaging space in the boiler. Gilliland and Scheeline ( 2 ) were able to guard against difficulties b and c by building the still of glass, which permitted visual observation. Supplementary heat could be applied to the top section if refluxing started; however, the apparatus was limited by the high temperature strength of glass and by the thermal and chemical stability of the neoprene sealing gasket. I n the Gilliland apparatus the condensate was returned by means of a tube extending from the top of the boiler to a point below the liquid surface. The authors feared that, since the cold condensate would be a t a lower temperature than the vapors in the still, i t might be difficult to prevent refluxing on the surface of the return line. Inert gases may be removed from a closed system by evacuation; this, however, necessitates a system of pressure control without the convenient blanketing effect provided b y the presence of a manostated inert gas beyond the condenser. Griswold, Andres, and Klein (3) balanced variable heat input against variable condensing surface, and achieved a manual pressure control. Gilliland and Scheeline accomplished a similar balance automatically by means of a mercury switch installed between the condensate trap and a tank of inert gas a t the desired pressure. Othmer ( 5 ) constructed an apparatus which utilizes direct venting with an inert gas, believing that no difficulty would be encountered with inert gas dissolving in the liquid under the ditions used. cted by the present authors is metal ause of the difficulty of obtaining large ocarbons, the minimum charge is 150 ml., rather than the 600-ml. charge of the Griswold apparatusa. A pressure control system based on the same principle as that of
