GEL AS A DEFINITIVE PROPERTY IN GR-S TECHNOLOGY La Ma WHITE, E. S. EBERS, G. E. SHRIVER, AND S . BRECK’ General Laboratories, United States Rubber Company, Bassoic, N. J.
A
The principal objective of t h i r investigation has been to LONG with the many benzene cat room temperature. determine the significance of gcl fraction in GR-S in relaother f u n d a m e n t a l P w M y bemuse of thehigher tion to its processing and phydcal properties. Data are properties of GR-S type extraction temperature in the presented from which it is coudaded that the presenee of polymers-molecular weight, former method (4,some difIow-BwelIing (i.e., tight) gel Matmiany improves certain molecular weight distribuferences between the resdts important types of processing behavior, but adversely tion, branching, space isomfrom the two methods have influences many physical properties of the vulcanizate. erism, and other structural been neted on occasionalsamA theory is advanced to explain the obstfled effects of gel, i r r e g u l a r i ti es-the signifiples (7). However, data obbased on the exclusion of filler by the gel, and is supported cance of the presence of a tained in the present study inby a limited amount of direct evidence. benzene-insoluble specie8 dicate thltt the two methods with an extremely high may be used interchangeabty molecular weight on the for gel studies of this type. general utility of the elastomer has been the concern of many inThe mechanical setup of the Baker and Mullen method permits melasurementof the equilibrium “swelling index” of the gelvestigators in the field (S, 8, fo). This insoluble fraction, comnamely, the ratio of the weight of the swollen gel to the weight of monly termed “gel”, swells in benzene or similar solvents and the dry gel. The swelling index is related to the density of points tends to break down under shear, either to soluble molecules or to dispersible units, aa is shown plainly by enhanced soluof immobility (crom links, entanglements) between and within chains, high values of the swelling index being indicative of low bility and increased swelling volume. The structure of gel in CfRS has not been demonstrated conclusively. Although most density (i.e,, loose gel) and vice versa. A second method of discriminating between loose and tight investigators favor a three-dimensional cross-linked structure, gels, now to be described, involves determining whether the gels the gel may consist of substantially linear molecules with exwill become dispersible as the result of shear during normal factremely high molecular weight. tory fabrication. To simulate such shear in the laboratory, a treatment of ten cold mill refmings (a refining is one pass 1 I I I through rolls of clearance of 0.010 inch) wm adopted. Gel which f BOTH ONLY ONLY remains insoluble after this treatment is here termed I‘B gel”; I I 5 A I A a 8 I that which disperses is called “A gel”. I Investigation of the connection between swelling index and A GEL I GEL GEL I and B gel contents revealed (Figure 1) that B gel rarely occurs in I I I B) has a swelling index above G R S in which the total gel (A 90 7b 50 30 70. If the swelling index of the total gel is below 50, it is found to SWELLING INDEX be largely B gel. The total gel in the intervening region contains Figure 1. Relation of A and B Gel with both A and B gel. In all, approximately, one hundred samples Swelling Index have been studied, and only one sample has been found which does not fit this classification. The abnormal sample was shown Gel may be formed in GR-S during polymer-manufacturing t o contain B gel even though the swelling index of the total gel operations or during plastication and mixing of the finished polyW a s 85. mer. In the dry polymer the gelation or “network-forming” The behavior of a tight gel during continued shear is exemplireaotions become appreciable a t temperatures above 250’ F. ( I 1). fied in Figure 2 in which a polymer, initially containing 60% gel Prwence of a small amount of oxygen catalyzes the gelation. On with swelling index of 25, is repeatedly cold-refined. The imthe other hand, oxygen also causes a simultaneous scission reaction, the rate of which is proportional to oxygen concentration; hence, large amounts of oxygen reduce the net rate of gelation. The purpose of this paper is to report and discuss the results of an investigation to determine the significance of the gel portion in relation to several important practical properties of GR-S.
I
,
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DETERMINATION AND CHARACTERIZATION OF GEL
Methods for the determination of gel content have been evolved over a period of two or three years. The background work for this paper was based on gel dabs from the method of Ewrtrt and Tmgey (9) which involves a continuous extraction of a 0.20-gram sample by benaene of about 80’ C. Later this was superseded by the Baker and Mullen method (3) and its m o a catien, whiGh is now more widely used in the industry. The latter method employs static extraction of a 0.20-gram sample in a
NUMBER OF REFINIWS
Figure 2 . Effect of Refining on Gel Content )-t( and Swelling Index (- -01
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Deercued Sbptsmber 8, 1944.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
August, 1945
portant features are that the swelqng index increases with the number of refinings, an indication of the gradual disintegration of points of immobility between chains, and that gel content does not decrease appreciably until the swelling index has risen to 70. It is to be emphasised that A and B gel are believed to differ only in degree, not in kind. In other words, B gel is thought to be A gel in which the density of the points of immobility has been greatly increased. In fact, the higher the k gel content of the polymer, the earlier will occur the development of B gel. For example, in a cornpasison of GR-S polymers 1 and 2, containing 34% A gel (swelling index of 96) and no gel, respectively, polymer 1 showed a considerably advanced development of 3 gel during Banbury plastication at 320"F. as Figure 3 shows. GEL AND TECHNICAL PROPERTIES
This investigation has been concerned primarily with the correlation of gel content with the important processing properties of GR-S (filler stiffening, length shrinkage, rugosity, and compound viscosity) and with the significant physical properties of its vuiankates (tensile, elongation, modulus, and cut growth). Methods of measuring the specific processing properties and their implieations in factory-scale production are discussed in another paper (16). The conventional Scott testing equipment was used in determining stress-strain properties of the vulcanizate. Cut growth life waa measured by determining the number of bending cycles required to propagate an initiated crack to a length of one inch. the bending cycle being 0"to 90"to 0". Early attempts to correlate technical properties with total gel content, without further characterization, did not yield conclusive evidence. Not infrequently, samples of G R S containing 40-!50'% gel exhibited unusual properties, but other lot4 having similar gel contents possessed normal properties. This anomalous behavior prompted investigation of the possible existence of different kinds of gel, which led to the conclusion cited above that gels could differ in degree (density of cross linking) but probably not in kind. OF GR-S TABLEI. EFFECTOF B GEL ON PBOPERTIES
GR-6 No.
2
B gel, %
..0
Swehng index of B gel
3 46 27
Proaeseing roperties 40 05 Filler stitening 45 81 Length shrinkage, 7 66 27 Rugosity. mm. X. 16, 83 67 Compound woos?ty Vuloanriate propertxes 1300 700 Stress' at 3009' Ib./sq. in. 2860 2060 Tensile strengtg:, lb./sq. in. 370 460 Elongation' Cut growth iig kiloaycles/in. 110 330 At 2% oombined 8 170 290 At 600 lb./sq. in., 300% strainb At 2% combined sulfur (cured 240 minute.8 at 292' .I?.). b Values estimated for a fixed modulus by interpolation from a range of cures. Q
EFFECTSOF LOOSE(A) GEL. Prom a re-examination of our preliminary data, taking into consideration the degree of cross linking, it was found that gels of swelling indices greater than 70 (A gel) exerted no detectable influence on properties. Presumably the A gel was destroyed (or dispersed) during the low-temperature laboratory mixing cycles. As Figure 3 shows, if processing temperatures are high, high initial content of A gel leads to early appearance of B gel. However, Gross,Tierney, and Vila (10) showed and Baker (2) confirmed in a large sample study that even loose gel (A gel) exerts a small influence on processing and physical properties, although it is probably dispersed during mixing. If the gel is reduced to normal molecular ekes, there may be a different distribution of chain lengths and changes in the branching which should lead to a different material.
7?1
E m m OF TIQHT (B) GEL. Once the importance of density of cross links in relation to persistency of gel during compounding was recognized, the correlation between tight gel and properties became more clear-cut. Preliminary data, obtained on several commercial lots of G M manufactured in such a manner as to contain tight gel, indicated that the presence of a substantial quantity of B gel in a tread compound improved (Le., lowered) the length shrinkage and rugosity, but adversely influenced other properties, such as increasing the filler stiffening, compound viscosity, and modulus, and lowering the elongation and cut growth life. A typical comparison is that of GR-5 2 and 3 of Table I, Generally speaking, B gel improved calendering and extrusion qualities but placed properties of the vuloanieate a t a disadvantage. Of course, the high modulus and low elongation could be corrected by proper compounding (reduction of combined sulfur, etc.), but the disadvantage in cut growth life would persist aa shown by the comparison made at constant modulus.
3
m
'st.
MINUTES PLASTICATION AT 320" F. Development of B Gel in High A Gel ( - - - 0 - - - ) and Low A Gel (-0-) Polymers
Figure 3.
Such comparisons are open to the objection, however, that the observed difference8 might not be due to gel but to other changes made in the polymer during polymerization. Probably a more reliible method of ascertaining the importance of gel is studying the properties of a single lot of CRS aa its gel content is increased progressively-for example, by heat plasticizing GR-Sat about 320"F. in a Banbury mixer. Individual samples of GRS 2 and GRS 4, both gel-free and representing normal production of two plants, were heat plaeticized 0, 5, 10, 15, 20, 30, 45, and 60 minutes a t 320' F. (stock temperatures) in a laboratory Banbury mixer of 1000-cc. capacity. As controls, separate samples of each lot were plasticized on cold mill rolls (stock temperatures of 100-120' F.) for 0,20, 40, and 00 minutes, conditions under which experience has shown that no gel is formed. The use of cold-masticated polymers as controls was believed necessary to compensate for structural and moleeular weight changes resulting from mastication. The pertinent data on the plasticized samples, after compounding in a conventional tread test recipe, are given in Table I1 as the averages of the values obtained on the two samples for each plastication time. Most of the data are plotted in Figure 4, in which the soIid line represents the changes for the samples plasticated under gel forming conditions (320O F.) and the broken line for the samples plasticated under nongel-forming conditions (120"F.). No effort has been made to indicate the reproducibility of the value by the size of the circles. However, a test of the significance of the differences within a given series can be made with the following coefficients of variation (ratio of standard deviation to mean): gel, 4%; swelling index, 7%; uncompounded viscosity, 4%; m e r stitrening, 10%; percentage length shrinkage, 2%; rugosity, 14%; compound viscosity, 4%; streaa at Zoo%, 14%; tensile strength, 10%; pewentage elongation, 5%; cut growth life, 24%.
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Figure 4.
Effects of Plastication under Nongel-Forming (-
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August, 1945
INDUSTRIAL AND ENGINEERING CHEMISTRY
The replication errors between series are slightly larger as B consequence of day-today uncontrolled variations in mixing, curing, and testing oonditiona, as indicated by the initial (0minutes plasticizexi) values oi Figure 4 and of Figure 7. These graphs show that increasing the B gel content of e. polymer: Increases filler stiffening Decreases length shrinkege Decreases rugosity
Increases modulus Decreases tensile Decrease out growth life
n3
mostly the soluble iraetions of higher moleoular weight. This lstter point is amply supported by experiments on gelation of individusl polymers in which the intrinsic viscosity of the soluble fraction i8 shown to drop rapidly as gel formation begins (6. 9, 1 c 14).
Some of the properties studied are 80 interreletad that one property will vary simply beearm the other is inEuenced by gel. For example, since modluus is increased by gel, either tensile or elongation or both must reflect this change in ~
TABLE 11. C ~ A N QOF E PROPERTIES OF GRS*
___-Senbury Ple4tioalion s t 320. P l u t i u t i o " time. mi".
0
Gel oharaotsrhtioa Total gel, % Swelling index
Bsd %
Inm1,'with 50 hl,aokb. % ProasMing pro r e l a
u".c.mpo"ngd
viaooeityr
~ i stlncmog i i ~ ~ Length shrinkage. % Rugosity. mm. X, 10,
5
10
Compound vbaaa!ty' v"loanl.ats pro errt,fa Streas nt 20084 lb./as. in. Tenailad lb./c '' Elongntibnd I"' Cut groxth iifl, kilayole/in. .At 2% combined 8 .At600Ib./s~.in., 2 ~ % a t r a i n i
9
30
34 38 42
39
41
"
47 38
40 27 47
29 63
27 68
I7
24 n5
24
IS50 28
0
32
*Y 28 22
30
50 38
50
46
3E
88
dP
41 4s
35
70
65
900
2700
2700 400
35:
110
850 380
75'
100.
43 30 MI
2550 310 30' 90"
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20
0
13
F.
15
0
37
10
67 l7
e50
56
35 8 64
1100
28 7 E6
2200 360
2200 310
1200 2050 290
30'
15.
10.
60'
40'
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TIGHT(B) GET.COWPENT
WITW
30'
$5
5 20 66
MOO
1950
270
60
0
44
0
Cold -Roll Plaatioatian (iOO-iZO' F.) 20 40 0
23 46
70
I7 5
64 1350 lY00 250
..
0
..
51
27
0
60
50
44 45 3E
42
37 42 41 21
66
43 43 31 63
BW
800
1000
2400 380
2200 320
35 43 40 15 60
61
2200
330
5. 6' E7 58 56 20. i s 90 85 YO mercaplobsn.othia.ola. 1.5; suilur. 2.
BW 2500 370 61 126
* Uecip y t a by weight: GR-8, 100; EPC blaok. 50. iiauid m*l-tar soltensr, 5 : d n o oxide, 5 , 9 0 PO ymer !hat Y insolubis ~n the 50 EPC lileck i i t ~ t r hatch, i !&en~y vuo"~ties e.1212' F. with isrgs m ~ r . I ) At 2% sombmed aulfur,(eured 240 minates Y! 2Y2' F.) * Averages of true mill rnzxes on same '~mpIe4bnrtead 01 &heusus1 aingle mixes on two diffennt RemP~e4. Valuea utimatsd for D fired modulus by mierpoialioo from B range 01 cares.
*
TABLE 111. M O ~ N E V~scosrrr V AND S O L ~ I O Vrscosmr N
AS shown in the diseugsiion oi TRble I, t,he pceeaenee of large Bmounta of tight gel ie particularly helpful ior the processing operations of calendering and extrusion. The magnitude of the improvement in length shriekage is shown in Figure 5 ; the samples portrayed the calender mat apecimens from one oi the series'oi polymers used in t.lii~investigstioa. The original (""shrunk) length of those specimens (IO inehsa) is indicated by the bracketed BROWS at the sides oi Figure 5 . Not sll oi the length shrink- and rugosity improvement can be ascribed directly to gel. From comparison with the control (120" P.) curves, some of the improvement must be due to the lower Mwney viscosity of the uncompounded polymer. Furthermore the intrinsic viscosity (a function of an average molecular weight close to the weight average) of the soluble portion in gelled polymers is much lower than would be expected from Mwney viscosity measurements (Table 111). The soluble mobile portions of these partially gelled GR-6 samples have average molecular weights fully 50% lower than would be snticipatad from the Mwney viscosities of the total polymer. Of course, the effects we still due indirectly to gel, because gel fomat,ioo remove^
modulus. These data point to a decrease in tensile rather than in elongation, although the correlation i8 not tw elenr-cut. An&her case in point is that of rugosity and length shrinkage. Part ai the drop in rugosity is unquestionably due to reduced Length shrinkage. Further, cut growth life is susceptible to modulus changes, inoreased modulus resulting in reduced cut growth life. In this case, however, the interaction with modulus can be removed conveniently, as was done in the tabulation and plot of cut growth life at constant stress of 6W pounds per B Q U inch ~ s t ZOO% strain. TIXWRY OF EFFECTS O F G E L
x
.-
Ibu
Figure 5. Changes in Length Shrinkage with Plastiutien at 320. F.
The manner and degm in which the several properties afiected by inoreasing amounts of tight gel wggest the poasible explanation that the filler (e.g., carbon black) emnot be dispersed on the tight (B)gel portion and therefore is dispermd only in the soluble portion. This results in an increased oarban black loading on the soluble portion and should. as shown by experience on completely soluble polymers, inorease filler stiffening and modulus and decrease length shrinkage and rugosity. T h e deereased ten.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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80
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4020 0
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PARTS EPC BLACK
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plastication at 320" F. for black master batches (solid line) is much less than for the pure gum polymer of Table I1 (broken line). The 5-10% decrease in solubility is not sufficient to lend conclusive support to the proposed theory. With that reservation in mind, however, the behavior of most of the properties is in line with expectations based on the following theory: Length shrinkage increases rather than decreases, rugosity does not change more than might be expected from viscosity reduction, compound viscosity drops rapidly instead of remaining substantially constant, and modulus decreases rather than increases. The reductions in tensile, elongation, and cut growth life are not easily explained on the basis of the theory although they may have resulted from larger reduction in molecular weight than was obtained for equal times of plastication under nongel-forming Conditions (compare compound viscosities of this series with the series of Table 11,plasticated at 120" F.). Apropos of the 20-250/, insoluble polymer in the carbon black master batches plasticated 0 minutes, it should be pointed out that with GR-5, aa with Hevea ( l a ) , channel black partially insolubilizes the polymer in the form of so-called bound rubber, The figure of 20-25% bound rubber, using 50 parts of EPC blank in a 100% soluble GR-S, is in agreement with the observationa of Baker and Walker (6)and of Lawrence and Hobson (18). As to why the presence of the EPC black during plastioation should reduce gel formation, it is believed that at least two factors may be operative. First, channel black, because of its great adsorptive capacity and the presence of surface impurities, may retard gelation by adsorption and inactivation of the intermediates that cause gelation. Second, the relatively higher and more localized shearing stresses during mastication of the black master batch, contrasted to those during mastication of the raw polymer, may under the conditions in question destroy or disperse the gel nearly as fast as it is formed.
k,..% '\
1
0
voi. 37, N ~ a.
9.6
17.5
24.1
I 80
IO0
31.9
34.6
I
% BLACK BY VOLUME
- -
Figure 6. Variation of Shrinkage and Rugosity with Black h a d i n g for Gel (+)-) and Nongel (- -0- -) Polymers of the Same Viscosity
sile and cut growth life may result from the poor tear resistance and general weakness of the black-free, tightly cross-linked, microscopic gelled areas (the properties of such areaa should not be greatly different from those of a vulcanized gum stock of GR-S). Support for the above exclusion theory was obtained through calculations kindly furnished by Baker (8). He found that the average elemental volume in a gel of swelling index of 100 (well above the tight gel region) is probably less than the volume of the average EPC black particle which, therefore, cannot enter it without rupture. Experimental support for the theory was obtained from a study of the influence of EPC black loading on length shrinkage and rugosity (Figure 6) for a 100% soluble polymer (No. 13) and a 45% B gel polymer (No. 14) of approximately equal Mooney viscosities. For the latter there are earlier reductions in both properties as the black loading is increased. This behavior suggests an apparent increased filler loading which may be a combination of the contribution of gel as a filler and increased black loading in'the soluble polymer. Also, if one considers the relative values at low loadings (less than 50 parts by weight) for which the curves approximate linearity, a loading of black in polymer 14 of approximately half of that in polymer 13 results in even better properties. At extremely high loadings (60-100 of black) this relation does not hold. Here, however, as a result of longer milling times and higher shearing stresses, the gel is probably being more and more disintegrated and its effect necessarily reduced. Evidence that the gel acts as a filler may also be obtained from extrapolation of the curves to zero loading, where the gelled polymer undoubtedly is superior. Another method for checking the validity of the exclusion theory is to determine the effects of gel when it is formed only after the added filler has been normally dispersed. Gel so formed would not be expected to give a final compound with areas in which filler waa concentrated and other areas in which filler was absent. On the basm of the theory the effects ascribable to gel present in the stock before compounding would not be noted. Attempts to build up substantial quantities of gel with 50 parts of EPC black present were only partially successful. AS Table IV and Figure 7 show, the decrease in solubility during
TABLE I v . BANBURY PLASTICATION AT 320" F., WER GELFOMNGCONDITIONS, IN THE PRESENCE OF BLACK Plastication time, min. 0 5 10 15 20 30 45 60 IPsolublewith 50black". % 22 28 29 32 30 30 34 34 Proceasing roperties .Length s&iokage o/ 44 49 49 47 47 50 50 50 Rugosity, mm. X: 51 41 25 26 is 21 21 15 Compound vlscosityb 70 62 55 56 51 4s 52 44 Vulcanisate ro erties Stress at 2 8 0 4 : !b./sq. in. 700 750 750 760 750 650 650 650 Tensilea lb ," m. 2250 2250 2200 2150 2450 2250 1900 1850 Elongati6na 380 350 330 350 340 330 310 300 Cutg rowth llfe, kiloc clea/in. At 2% combined 85 85 90 70 70 65 50 55 At 600 Ib./sq. in., 200% 95 straind 115 75 75 80 45 70 50 0 of polymer insoluble in 50 EPC black master batch, rtrl ooney viscosities at 212O F. with large rotor. e At 2% true combined sulfur (owed 240 rmnutea at 29Z0 FJ. d Values estimated for a 5xed modulus by interpoltabon from a range of curen.
18%
d
The practical implications of this comparison of plasticizing of GR-S at 320" F. in the absence and in the presence of black should not be overlooked. For short heat-treatment periods (up to 15-20 minutes at 320" F.), which are common to many factory processes, the advantages and disadvantages of having black present are almost insignificant. For extended heat treatment periods (30-60minutes at 320" FJ,however, the presence of the filler is particularly an advantage for protection of final properties of the vulcanizate (modulus and cut growth life). On the other hand, the presence of black is a disadvantage in that the processing improvements are not nearly so great (particularly as to length shrinkage and rugosity). SUMMARY AND CONCLUSIONS
1. Of the gel fraction that may occur in GR-5, only that por-
tion which has low s w e l ! i y o l u m e in b e n z e n e t i g h t ( g ) gelmarkedly affects procesmng ehavior and product quality.
INDUSTRIAL AND ENGINEERING CHEMISTRY
August, 1945
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