Relation between Gel Content, Plasticity, and Dilute Solution of Elastomers Viscositv u, AAROS L. BACK' National Synthetic Rubber Corporation, Louisville, K y . Three important properties of elastomers-gel content, plasticity, and dilute solution viscosity-have been found to be interrelated. The relations between these variables apparently are not dependent upon the nature and ratio of comonomers nor upon the nature and amounts of the other ingredients in the polymerization recipes; furthermore, the relations hold for standard GR-S as w-ell as for many types of experimental polymers.
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HIS paper presents an observed relation between three important properties of elastomers-gel content, dilute solution viscosity, and Mooney plasticity. For a variety of emulsion copolymers (including GR-S) the values of the three variables seem to depend largely upon each other and not primarily upon chemical composition per se. The values of the variables discussed in this paper depend in part upon their definitions; therefore certain details of test procedure are presented in order to define the properties. Plasticity (do), expressed in terms of hIooney plastometer units, is defined as the 4-minute reading in the Mooney plastometer (38) using the large rotor, both platens of the plastometer being maintained a t 212' F. The milling procedure prior to plasticity measurement is standardized as follon-s: The sample is given one refining pass, ten milling passes (sheeting out on the last pass), and a rest period of at least 30 minutes. The mill temperature is maintained at less than 130"F. A 1-minute warm-up is given in the plastometer. Gel content, expressed in weight per cent based on whole elastomer, is the portion of elastomer sample insoluble in benzene under specified test conditions. The procedure (40) utilized to measure gel is based on the static method of Baker and Xullen ( 6 ) . A 0.2-gram sample (accurately weighed) is subjected to extraction in 25 or 50 ml. of benzene in a special extraction cell called a Baker cell (6); t,he cell consists of a weighing bottle equipped with a cover (ground-glass joint), a drain equipped with a stopcock, and a set of horizontal 50- or 100-mesh stainless steel screens, 0.5 inch apart. The .screens serve to retain the insoluble (gel) portion. (Rubber and elast'omers consist of two components, a sol portion, soluble in a specific solvent under given test conditions, and a gel portion insoluble under the same conditions.) Extraction is carried out over a 24-hour period a t room temperature; care is taken that minimum mechanical action and no heating occur during the extraction. After determining the solids content of the sol solution resulting after extraction, the gel content is determined by difference. (Before test, the polymer sample is subjected to no heating or mechanicalaction that may alter its structure in any way.) Dilute solution viscosity, expressed in the units 100 ml. per gram, is the inherent kinematic viscosity obtained on a dilute solution of the sol portion of elastomer sample. [The term "inherent" is used in accordance with a system of nomenclature for 1 Present address, Devoe and Raynolds Company, Inc., Louisville, Ky. The National Synthetic Rubber Corporation was a n agent for the Rubbex Reserve Company, Reconstruction Finance Corporation.
expressing viscosity measurements suggested by I,. H. Cragg(lB).] Measurement of this property (40)is part of the static method for determining gel content (6). Aliquot portions of benzene extract of the sol portion of elastomer drained from the Baker cell are taken for viscosity and concentration measurements. Viscosity is measured in terms of time of efflux of solution nf sol, relative to time of efflux of the solvent, benzene; an Ostmald-type viscometer is employed. Concentration is determined upon evaporating an aliquot portion of the sol solution to dryness. The dilute solution viscosity (DSV) is calculated by means of the equation'
where t , t o = times of efflux of solution and solvent, respective13 C = concentration of solution, g./lOO nil. s o h . SIGNIFICANCE OF VARIABLES
MOONEYPLASTICITY. This property is taken to be proportional to the mean absolute viscosity of elastomw sample (8); it is widely measured in all phases of research, development, and control work. GELCOSTENT.This is a property of fundamental significance. It is not a new entity, having been known and studied in natural rubber technology for many years (3, 16, 25, S i , 37, 42, 46, 61, 66) ; in fact, an investigator commented on rubber gels as early as 1807 ( 1 ) . S a t u r a l rubber contains 20 to 45% grl (so-called beta rubber), depending on previous treatment ( 5 6 ) . S o t until the advent of the recent mar was its true significance understood or appreciated. It remained for Baker and associates, Flory, and others to point out the theoretical and practiral implications. Apparently the relative chemical stability of butadiene residues to chain scission by oxidation was the prime factor which altered the emphasis on the study of gel content of synthetic compared to natural rubber. Impetus was given the study of this entity by the development of a suitable method of measurement ( 6 ) ,and by standardization of the method through the combined efforts of the Rubber Reserve Company (32) and constituent rompanies in the rubber program. The gel component differs from the sol portion by virtue of the fact that the latter is taken to be that part of the elastomer hydrocarbon soluble in a specified solvent under prescribed condition8 whereas the gel portion is the insoluble part. The difference in solubility is apparently a consequence of diff crence in molecular structure. It is considered that highly branched molecules as well as linear molecules are soluble in such solvents as benzene (1) and that the formation of a sufficient number of primary valence bonds between linear chains is responsible for insolubilization. On the basis of Flory's statistical work ( 2 7 ) relatively few crosB linkages (one per 32,000 chain atoms) are expected to produce a. gel (insoluble) structure. Gel, then, is consideled to consist of molecular structural units (linear and/or branched), cross-linked to each other through the agency of primary valence bonds ( 1 , 9 , 49). The quantitative theory of gel has been developed largely
1339
1340
INDUSTRIAL A N D ENGINEERING CHEMISTRY
3'3m
II 0
;
3 . o t GELCONTENT: ZERO
---
I
RAW 80
80
MOONEY PLASTICITY
100
110
120
130
Figure 1. Relation between Dilute Solution Viscosity and JIooney Plasticity for Gel-Free Polymers
by Flory, who has given a theoretical treatrnent of the InsolubiIitj of netted polymers in liquids which act as solvents for the sol (26$
ar). An entire gamut of gel types exists, ranging from a loose and fragile (highly swollen) type to a dense and compact variety. The type of gel present, as well as amount, has been found to affect processability markedly; in fact, the nature of the gel seems to have a greater effect than amount on processability ( 2 1 , 2 2 ,25, 67, 58, 50, 60). Gel may be characterized by its swelling index, its swelling volume, or by milling studies of whole elastomer. The swelling index, measured in terms of the relative weight of solvent imbibed by the gel residue after extraction, is obtained in the course of the Bakerand ?\lullen static method (1,6). .4ctual swelling volume of the gel may be determined (20,61). I17hite,Ebers, Shriver, and Breck have devised a method for classifying gel as loose (type A) or tight (types B and C) by the cold milling of whole elastomer followed by solubility measurements (21, 22, 23, 50). These characterization methods serve as indices of over-all looseness or tightness (or density) of cross linkage. Gel has been found to exist either as macrogel or microgel ( 1 , s ) . The former gel structurerepresents a conditioninwhichtheboundariej of cross linkage extend throughout relatively large portions of the sample; the micro structure represents a condition in which cross linkage is restricted to extremely small volumes-for example, of diameter comparable to that of a latex particle ( 5 ) . Microgel is considered to form during polymerization and other operations carried out with elastomer in the latex form; macrogel is believed to form after coagulation has been effected, through such agencies as heat and oxidation (5). In fact, dense microgel may form-for example, in latex, under conditions in which shortstopping agent fails to reach the polymerizing particle ( 5 ) . Presence of microgel colloidally dispersed in solution may be shown by means of light-scattering measurements ( 2 , 20). Gel content is not a constant factor in a given elastomer sample; its value may be varied (reduced or increased) by thermal, oxidative, and mechanical action ( 6 , 13, 16, 17, 19, 21, 22, 35, 41, 43, 65, 63). Moreover, its initial value depends upon manufacturing operations, such as polymerization ( 4 , 5 ,I S , sa), completeness of short stopping (4), coagulation procedure (35), drying operations ( 1 , 5 , 13, $4,%), and conditions of storage ( 2 2 ) . Oxidation of GR-S has been found to initiate or propagate gel formation; oxidation of natural rubber also brings about such an increase (3, 60). Further, there is evidence from hardening and swelling volumes that gel formation goes on in vulcanized stocks ( 5 ) .
Vol. 39, No. 10
The effects of gel on processability ( 1 , 4 , 6 , 12, 23, 33,44, 5 4 , 5 ? , 58, 60) and on properties of the vulcanizate (1, d S , S l , 57) have been investigated recently. Gel tends to improve calendering and extrusion operations (SO, 59). However, resistance to cut growth on flexing is less, the higher the gel content (1,2S, 52, 57), and increase in gel content effects an increase in modulus ( 1 1 , Si. 36, 57). Gel formation in GR-S is apparently responsible for thermal shortening and hardening; the proportion of this structure has been reported as high as 99.8%, the material in this case being a hard resin ( 6 ) . Stiffening or toughness increases progressively with gel formation ( 5 ) . To explain pel propagation, a free radical mechanism has been proposed (33. DILLTE SOLUTIO^ VISCOSITY.This property I > an index for characterizing the portion of the polymerized hydrocarbon other than the gel portion-that is, the sol portion ( 7 ) . I t is a measure of the molecular weight of the sol, the molecular weight being expressed by an equation (8,SZ) of the type:
where a, b = constants li?v = viscosity average molecular weight DSV = dilute solution viscosity of sol portion
For GR-S, the constants a and b are reported to be of the order I., Ebers, E. S., and Shriver, G. E., ISD. ESG. CHEV.. ogy of
37, 767-9 (1945). [,59) White, L. X i . , Ebers, E.
S.,and Shriver, G. E., U. S. Rubber Co., private communication (Nov. 29, 19-13). i60) T h i t e , L. >I., Ebers, E. 9.. Shriver. G . E.. and Breck, S., IXD. ESG. CHEM.,37, 770-5 (1945) ; Rubbe: Chem. Tech., 18, 83344 (1945). ( t i l ) Wijk, D. J. van, Kuutschuk, 9, 18 (1933). (62) Yanko, J. A., B. F. Goodrich Co., private coinrriunication (Oct 16, 1943). (63) I'anko, J. A , , and Pfau, E. S., I h i d . (April 3 , '1946).
PRESESTED before the Division of Rubber Chemistry at the 110th Meeting of t h e AXERICANCHEXICAL SOCIETY, Chicago. Ill. This investigation was carried out under t h e sponsorship of t h e Reconstruction Finance Corporation, Office of Rubber Reserve, in connection with t 3e government's synthetic rubber program
Distribution in Hvdrocarbon-Solvent Systems J
J
T. G. HUWTER AND T. BROWN University of Birmingham, England Liquid-liquid equilibria in ternary systems consisting of aniline and two hydrocarbons have been investigated and are described in this paper. After discussing the methods of investigation employed, equilibria data at 25' C. for the following s j stems are recorded : aniline-cetane-benzene, aniline-cetane-cyclohexane, aniline-heptane-cyclohexane. and aniline-cetane-n-heptane.
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AKY ternary systems of pure components have been re-
ported in the literature, but these systems deal almost exclusively with mixtures of components other than hydrocarbons. So far as is known, only three ternary hydrocarbon-solvent systems have been previously recorded-namely, the systems of aniline-n-heptane-methyl cyclohexane ( 3 ) , aniline-n-hexanemethyl cyclopentane, and n-heptane-toluene-acetic acid (2). During an investigation into t h e separation by solvent extraction of aromatic, naphthenic, and paraffin hydrocarbon mixtures, several such ternary hydrocarbon-solvent systems were determined experimentally and are reported here.
As the present investigation was primarily designed t o determine the effect of a solvent on the separation (of aromatic-paraffin, naphthene-paraffin, and paraffin-parafin hydrocarbons, cptane-that is, n-hexadecane (CleH14)-and n-heptane were chosen, because they were readily available io a high state of purity, for the paraffin hydrocarbons. At the selected temperature of equilibrium (25' C.) cetane (melting point 18' C.) is in a liquid state. Cyclohexane was selected as a representative naphthene, and benzene as a representative aromatic hydrocarbon. The sources and properties of all the hydrocarbons used are s h o m in Table I. Aniline was employed as the solvent, and thaL used mas Analar standard, dried over sodium hydroxide for 24 hours and distilled; the fraction boiling between 181-182" C. was used. SIZETHODS OF INVESTIGATIOX
I n order to define liquid-liquid isothermal-isobaric systems of pure components graphically on ternary coordinates, it is neces-
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