Development of a Better Processing GR-S D . I,. Schoene, A . J . Green, E . R . Burns, and G . R . F’ila L3ITED STATES RUBBER COMPANY, NAUGATUCK, COXN.
new GR-S type synthetic rubber has been deieloped with impro-ed processing characteristics. Among i t s advantages are low shrinkage, improved rugosity and surface appearance, smooth tubing, reduced swelling a t extrusion dies, and improved aging. These improFements are obtained by t h e use of small amounts of divinylbenzene during polymerization, and t h e resulting polymer is highly cross-linked. Such a polymer has been produced as GR-S :X-285 for large scale experimentation b y t h e rubber industry. -4nother improved processing polymer from a latex blend is also described, b u t t h e X-285 type is more practical and better suited to a wide variety of uses. X-285 is best utilized by blending i n any proportion with
A
standard GR-S. This permits wid.e lulitude i n selecting t h e most fatorable balance of properties for a broad range of applications. Because of their altered structure the blends require compounding adjustments to realize optim u m physical properties. Sulfur should be reduced for best results, and this further accentuates their improi ed aging properties. The superior processing characteristic* of X-285 blends eliminate evcessite hot plastication and permit a reduction in pigment and softener loadings where these have been used in excessive quantities as processing adjuvants. These alterations offer t h e possibility t h a t an over-all improiement can be made i n quality as well as in processability.
P
chain scission. Table I gives data extracted from the report, showing these relations ( i ) and , Figure 1 pictures the correlatioii of swelling a t die and gel content with time of hot milling. TKO other reports ( 1 , Q ) extended this concept t o show a positive correlation between gel content and processability of GR-S. They also review much of the available literature on this subject. The met,hods for measuring processability have already been published ( 8 ) . The processing tests used as criteria in the present work include the determination of shrinkage and rugosity of calendered samples and tubing characteristics on egtrusion through a Garvey die ( 2 ) . Shrinkage is determined by measuring the final length of samples which had previously been marked a t definite intervals on a constant-temperature free-running calender roll; i t is expressed as a percent,age of the original length. Rugosity or roughness is determined on the same calendered samples, following the published procedure (5). It is a measure of the average hill height and is determined by the rate of air pa+ sage between the sample and a flat disk. Extrudnbility is rated by the swell a t the die, tubing rate, and surface appearance of extruded samples.
OOR processability hab long been recognized as one of the shortcomings of GR-S. Since the problem is of major importance, this deficiency has been the subject of a number of investigations. Freshly milled samples of GR-S shrink rapidly and become very rough. Samples extruded through a die swell excessively and give irregular and broken edges. These faults have, in many instances, nece-sitated excessive loading with com-
5 0 ,
.
LATEX BLENDllVG
I E
n
O
20 MINUTES
40
HOT MILLING
Figure 1. Iniproved Processing by Hot RIilling
Concurrent with the improvement in processing obtained with hot-milled polymers was a corresponding sacrifice in some physi-
pounding materials to permit satisfactory calendering and extrusion. Sacrifices in quality have often been tolerated t o obtain suitable processing. The present paper describes a new type of GR-S in which vastly improved processing properties have been imparted t o the polymer during the copolymerization reaction.
50 YOONEY STANDARD GR*S SO 40
30
PLASTICATION
20
An earlier report ( 7 ) on the plastication of GR-S revealed that breakdown of the raw polymer on a hot mill gave a subsequent compound which tubed more smoothly and exhibited less swelling a t the die. It \vas also observed that hot plastication reduced the Mooney viscosity but a t the same time decreased solubility of the polymer in benzene, These effects were attributed t o the formation of a cross-linked structure (gel phase) accompanied by
IO
0
-
% GEL
%SHRINKAGE
RUGOSITY x.300
% SWELL AT DIE
Figure 2. Improved Processing from Blending High and Law Mooney Latices
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INDUSTRIAL AND ENGINEERING CHEMISTRY
December, 1946
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TABLE I. IMPROVED PROCESSING BY HOTMILLING (7) 20
40
Minutes on hot mill Mooney viscosity
Unmilled 62 0.3
swell a t dien urface appearance
61.6
30 37.1 36.6
Rough
Smooth
f
24 44.8 30.7 Very smooth
5 Calculated as increase in diameter from the original d a t a which were reported as increase in area.
cal properties. Moreover, the hot plastication technique was time consuming and expensive. These facts led to the attempt t o duplicate and surpass the effects of plastication by exercising greater control over molecular structure of the polymer during polymerization. This goal was attained in part by blending latices of different molecular characteristics. One component of the blend was a polymer of very high Mooney viscosity and high gel content. The other component was gel-free and of very low viscosity. The resulting latex blend yielded a polymer selfplasticized by the low-molecular-weight component and yet containing the gel fraction necessary for low shrinkage. As expected, its molecular weight distribution was extremely broad. The processing properties of the polymer derived from this latex blend agreed well with theory. As Figure 2 shows, calender shrinkage was reduced from 46% for standard GR-S to GR-S 100 33% for the blend. Calender x-285 o rugosity was decreased from 0.15 to 0.10 unit. Swelling at the extrusion die was reduced from 50 to 36%. The gel content of the blend was 39%, the standard was gelfree. Figure 3 shows how tubing is improved with a blend as compared t o unplasticized and t o hotplasticized GR-S. It is apparent that the blend with its low swell maintained more clearly the image of the Garvey die and exhibited a smoother tubed surface. Physical properties of the blend were generally comparable to those of the hotplasticized GR-S. CONTROLLED CROSS LINKING
Although latex blending yielded a better processing polymer, it had the disadvantages of requiring the preparation, storage, and controlled blending of different latices. Accordingly a means was sought to produce the same effects in a single-step process. This was realized by the introduotion into the polymerization formula of small amounts of a crosslinking agent to tie the growing polymer chains together into a network or gel. Such a technique had already been applied to insolubilize
0.15
Figure 3. Improved Tubing with GR-S Made from High and Low Mooney Blends GR-S 75 x-285 25
0.09
GR-S 60 x-285 50
GR-S 0 X-285 100
0.08
0.02
ROuoHNEsa
I C . 54%
42 %
31% TUBING SWELL
Figure 4.
Improved Processing wi t h X-285 Type Blende
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INDUSTRIAL AND ENGINEERING CHEMISTRY
1248 1500
TABLE 11. COMPARISON OF x-285 MILL BLENDAND MODERATELY CROSS-LINKED POLYMER
2500
TENSILE
I? S.I.
n
1c
1500 1000 500 0
800
6oo
ELONGATION
400
%
2 00 0
OR-S
[ 2M 100
---
X-285 TYPE POLY MER
75
50
_--
25
SO
IO0
Figure 5. Aging of Cross-Linked Mill Blends, Compounded with 2 Parts Sulfur and Cured 60 Minutes
n
2500 ZOO0
TENSILE
1500
too0
600 MODULUS 0
15 CUT 5 GROWTH
100 90
BuHrEuup
50 40
~
,,
nnLl
0
::
Formula Standard GR-S 0.5 divinylbenzene polymer (X-285) 0.3 divinylbenaene polymer E P C carbon black Zinc oxide B R T iYo. 7 Sulfur Mercaptobenzothiazole Mooney viscosity. compounded Calender shrinkage, % Calender rugosity Swell a t die, Tubing rate, grams/min. Properties of unaged compounds* Modulus a t 300%, lb./sq. in. Tensile, lb./sq. in. Elongation Flex crack krowth, mils/kilocycle Tensile of compounds0 after aging 96 h r . a t 212' F., lb./sq. in. a
50 50
..
A
.. id0 50 5
50 5 5 2 1.5 77 22 0.05 31 75
5 2 1.5 74 27 0.07 27 68
900 2300 550 3.4
550 1720 620 5.6
1700
1800
All cures a t 292' F. for 60 minutes.
TABLE111.
COhIPARISOS O F
X-285
TYPE
HLEllrDS
WITH
G1t-S
Formula Standard GR-S 100 75 50 X-285 type polymer .. 25 50 io0 E P C carbon black 50 50 50 50 Zinc oxide 5 5 5 5 BRT xo. 7 6 5 5 5 Sulfur 2 2 2 2 Mercaptobenzothiazole 1.5 1.5 1.5 1.5 Slooney viscosity After breakdown 30 30 29 29 Compounded 58 65 77 121 Calender shrinkage, % 43 33 22 9 Calender rugosity 0.15 0.09 0.05 0.02 Swell a t die, yo 54 42 31 15 69 70 75 54 Tubing rate, gram/min. Resilience after BO-min. cure 37 37 36 36 Room temp. 212' F. 45 48 48 52 Properties of unaged compounds (cured a t 292' F.) Modulus a t 300G/,, lb./sq. in. 200 200 390 600 30-min. cure 900 1190 60-min. cure 600 700 1500 900 1000 1190 SO-min. cure Tensile, lb./sq. in. 1450 1000 1190 1190 30-min. cure 2520 2380 2300 1620 60-min. cure 2790 2300 2050 1700 90-min. cure Elongation, 875 820 700 465 30-min. cure 765 665 550 410 60-min. cure 635 505 465 355 90-min. cure Heat build-up in Goodrich flexometer, F. 156 119 86 30-min. cure 94 75 76 61 60-min. cure 77 70 70 51 90-min. cure Av. flex craqk growth, mils/kilocycle 1.2 2.4 4.0 13.9 Properties of compounds after aging 96 hr. at 212' F. (cured a t 292' F.) Modulus a t loo%, lb./sq. in. 910 300 780 750 30-min. cure 1080 1290 1150 1150 60-min. cure 1500 1400 910 1150 90-min. cure Tensile, lb./sq. in. 1450 1300 1490 1400 30-min. cure 1310 1700 1700 1600 60-min. cure 1500 1400 1710 1150 90-min. cure Elongation, % 150 150 140 100 30-min. cure 110 125 125 120 BO-min. cure 150 100 125 100 90-min. cure
..
10
70
Vol. 38, No. 12
OR-S XQelTYPE
100 0
75 25
n 50 50
0 100
POLYMER
Figure 6. Comparative Physical Properties of CrossLinked Polymer Mill Blends, Cured 60 Minutes resins ( 4 , 6) but the only reported work with dienes (6) utilized such high concentrations of cross-linking agent that the polymer was no longer rubberlike. Divinylbenzene was chosen as the most efficient cross-linking agent, and results obtained far exceeded expectations in that ultimate improvement in shrinkage and rugosity was much greater than had been achieved by any other method. As little as 0.5 part of cross-linking agent substantially eliminated shrinkage, while calendered stocks remained almost as smooth as the cal-
ender rolls. Polymer prepared in accordance piith this principle and containing 0.5 part divinylbenzene has been designated GR-S :X-285. MILL BLENDS
Picturrs of calendrred and extruded samples of standard GR-S, 5-285, and mill blends of the two in various proportions are shown in Fjgure 4, using the data of Table 111. Comparison of the samples in Figure 4 ahom that shrinkage is improved from 43 to 9%, rugosity from 0.15 to 0.02 unit, and tubing swell from 54 to 15% by substituting X-285 for standard GR-S. The mill blends of X-285 with standard GR-S give intermediate improvements.
December, 1946
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INDUSTfiIAL A N P ENGINEERING CHEMISTRY
TABLE IV. EFFECTOF REDUCED SULFURON AN X-285 TYPE BLEND
800
Formula Standard GR-8 X-286 type polymer EPC carbon black Zinc oxide BRT No. 7 Sulfur Mercaptobeniothiarole Mooney viscoeity, compounded Calender shrinkage, % ' Calender rugosity Swell at die yo Tubing rat:, grams/min. Rwilience at room temp. (60-min. owe)
600 MODULUS
200
64 47 0.15 63 74 40
Properties of unaged com ounds (cured at 292' F.) Modulus at 300%, lb.&. in. 220 30-min. cure 680 60-min. cirre 1000 90-min. cure Tensile, lb./sq. in. 1600 30-min. cure 2960 60-min. cure 2990 90-min. cure Elongation, % ' 866 30-mln. cure 720 60-min. cure 626 90-min. cure
Av. flex crack growth, mila/kilocycle
89 28 0.10 31 68 38
82 26 0.10 28
600 1100 1410
390 660 900
1950 2700 2620
I190 2160 2600
715 685 486
760 660 616
4.2
4.7
1.5
71 37
Properties of compounds after aging 96 hr. at 212O F. (cured at 292' F.) Modulus at loo%, lb./sq. in. 700 1000 30-min. cure 390 980 680 60-min. cure 390 460 1020 600 90-min. cure Tensile, lb./sq. in. 1400 1410 1960 30-min. cure 1800 1760 1850 60-min. cure 1760 1500 90-min. cure 1990 Elongation, % ' 166 150 300 30-min. cure 290 200 160 BO-min. cure 300 200 126 90-min. cure Av. flex crack growth, mils/kilocycle
1000
6.0
12.4
4.8
For a majority of uses such mill blends will be preferred, since in this way the altered physical properties of X-285 will be less apparent and will require less adjustment in compounding technique for any specific application. Polymers prepared with lower concentrations of cross-linking agents were compared with mill blends of the more highly crosslinked variety. At equivalent processing levels the mill blends gave superior unaged tensile properties and cut growth resistance as shown in Table 11. Moreover] mill blending to the desired processability level permits both the compounder and the polymerization plant to keep a low inventory of one cross-linked polymer rather than a number of polymers of varying processability. It was for these reasons that the high cross-linked GR-S modification was selected for future development and has been produced as GR-S:X-285 for large scale experimentation by the rubber industry. The slightly poorer unaged stress-strain properties of crosslinked stocks are balanced by a much better retentivity upon aging. Even without compounding adjustments, the crosslinked polymers become equal or superior to standard GR-S in tensile strength, modulus, and elongation after oven aging for 96 hours at 212 ' F. Figure 5 and Table I11 show these relations. Aged cut-growth data were too variable in this series to be significant and are not included. Indications are, however, that aging lessens the difference between the flexing properties of GR-S and X-285 blends. COMPOUNDING ADJUSTMENTS
X-285 and its blends are appreciably altered in structure from standard GR-S, and compounding adjustments are required t o secure the optimum properties for this type polymer. The crosslinking reaction is similar to vulcanization in that both iTsolubilize the polymer. This fact, coupled with the higher modulus throughout the curing range and the relation of low heat build-up
0
n
3000 2000
TENSILE
lo00
P. s.1.
0
B1
800 'O0
ELONOATION
400
%
200
2 100
OR-S X-28STYPE POLYMER
Figure 7.
B 2
0
SULFUR
--
so so
Effect of Reduced Sulfur on CrossLinked Blends
to high cut growth shown in Figure 6, suggests that a reduction in sulfur is the most logical compounding variation, with or without an accelerator adjustment. Figure 7 and Table IV compare the stress-strain properties of GR-S and a 50-50 blend of X-285 and GR-S at two sulfur levels in a typical carbon black recipe. The lower sulfur level, 1.25 parts, gives the expected improvement in modulus and elongation a t only a slight sacrifice in unaged tensile strength. After oven aging, however, the low sulfur blend becomes superior t o standard GR-S. Preliminary studies (not reported here) on X-285 blends with nonblack fillers indicate that a similar reduction in sulfur will give physical properties, unaged as well as aged, which are superior to standard GR-Scompounded normally. It is noteworthy that the processing improvements obtained with these nonblack loadings are fully equivalent to those prevailing in black compounds. I n addition t o sulfur and accelerator adjustments, the altered solubility characteristics of X-285 blends suggest that a study of different types of plasticizers might be beneficial. It is apparent also that the improved processability of X-285 blends will permit a reduction in filler and softener loadings where these have been excessive heretofore in order to achieve tolerable .processability. Such a reduction offers the possibility of an over-all improvement in physical properties as well as processability. LITERATURE CITED
(1) Baker, W. O.,Walker, R. W., and Pope, N. R., Office of Publication Board, P. B. 9687 (March 14, 1944). (2) Garvey, B. S., Jr., IND. ENQ.CHEM.,34,1309 (1942). (3) Mooney, M.,IWD. ENQ.CHEM., ANAL.ED.,17,514(1945). (4) Staudinger, Trans. Faraday Soo., 32,3?3(1936). (6) Staudinger and Houer, Ber., 67, 1164 (1934). (6) Tschinkus and Bock, U. S. Patent 1,938,931(Deo. 12,1933). (7) Vila, G.R.,IND. ENO.CHEM.,36, 1113 (1944). (8) White, L. M., Ebers, E. S., and Shriver, G. E., Ibid., 37, 767 (1945). (9) White, L. M., Ebers, E. S., Shriver, G. E., and Breck, S., Ibid., 37,770 (1945). PRESENTED before the Division of Rubber Chemistry at the 109th Meeting SOCIETY,Atlantic City, N. J. of the AMERICAN CHEMICAL
