sinc oxide to develop eatiefsotorv reinforcement and the megns of WrrectiDg it. A prograsa report of thia worbis here preeented; the problm, ia not entirely solved, but it in hoped that this summary will coordinate. with other work and &tuaJly lead to the complete solution otnonblack compoundiqg,ofGR8. VAllIAnuN OF 0R.S
P r o w bas been retarded to a oomiderable degree by v+-
iiilHil
INDUSTRIAL AND ENGINEERING CHEMISTRY
642 00
-
D
-
- OAO
---
30 Figure 2.
45
A
I
150 PT. ZINC OXlDE,O.l5P
45.3 PT. SRF BLACK -TENSILE STRENGTH - MODULUS AT 300 % ELONGATION
60 90 39 45 60 CURE -MINUTES AT I53 "C.
90
Effect of Z i n c O x i d e and Carbon Black in AcetoneExtracted GR-S
Vol. 36. No. 7
than those of the normal polymer. The cut-growth resistance is definitely better in the case of the extracted rubber under these curing conditions. Later tests showed that, when acetone-extracted GR-S stock was cured to approximately the same state as the regular GR-S, pendulum resilience and heat build-up properties were essentially equal. Although cut-growth resistance of acetone-extracted GR-S was degraded somewhat by the longer cure, it was still superior to regular GR-S. I t IS inferred from these results that the wettability of zinc oxidc by the elastomer is enhanced by removal of acetone-extracted matcrial. The residue from the acetone extraction contains several ingredients-fatty acid, antioxidant, and, according to some investigators, a relatively small amount of unreacted and incompletely polymerized butadiene and styrene. Evidence in support of the latter claim is that the residue from the acetone extraction has an iodine number indicative of considerable unsaturation. If this is the case, it might be postulated that the removal of short-chain compounds contributes to improved stressstrain properties, and the relatively higher breaking elongation of the extracted elastomer might be considered evidence of a greater percentage of long-chain molecules in this product than in the normal elastomer. The data raise the question as to whether the improvement in stress-strain properties is associated with removal of short-chain polymers or extraction of fatty acid. A gum stock prepared from acetone-extracted GR-S had only slightly higher tensiles than the regular elastomer; indications are that the improvement in reinforcement with zinc oxide in extracted GR-S is associated with an improvement in pigment-torubber bond rather than an increase in the cohesive strength of thc polymer.
The zinc oxide results show that elastomers A, B, D, and G developed better stress-strain and pendulum rebound than the other elastomers. Since three of the four elastomers that developed the best stress-strain properties in these tests were elastomers low in fatty acid content, compounding experiments with acetone-extracted GR-S were FORMULA 808 0.15 CUT-GROWTH RESISTANCE indicated. ORIGINALS 24-HR.AGINGS ELASTOMER 100 RESIDUE FROM I t is worthy of note that, comSULFUR 2.5 ACETONE EXTN. 0-6.5 ~OOOOFLEXINGS5300 19000 120000 FLEXINGS 2 MT 0.75 ZINC OXIDE I50 pared to regular GR-S, certain of t,he modified polymers which gave 0- GR-S .74 .85 improved properties with zinc A - ACETONE-EXTRACTED GR-S 35 .08 .48 ACETONE-EXTRACTED GR- S oxide gave lowered physical propU--WITH 6.5 RESIDUE FROM ACETONE EXTNS. .49 .53 .79 erties when compounded with car.-SPECIAL ELASTOMER .21 32 .27 bon black. I n these cases it is prdsumed that there was insufi200c cient fatty acid to properly disperse the carbon black.
-
ACETONE-EXTRACTED GR-S
GR-S contains 6% or less of fatty acid as a result of the process used in its manufacture. To study the effect of this fatty acid, compounding experiments were conducted with GR-S that had been extracted with acetone. Figure 2 and Table I demonstrate that a considerable change takes place in the reinforcement of GR-S by zinc oxide when the elastomer has been acetone-extracted. Ultimate tensile strength, breaking elongation, and modulus are improved significantly by extraction of acetone-soluble materials from GR-S. The rate of cure is retarded by the extraction so that, a t the same time of cure, pend u l u m resilience a n d h e a t build-up properties on the Goodrich flexometer are poorer
150C
IOOC
50C
1 24-HR. AGlNGS AT IOO°C] c 15
30 Figure 3.
45
60
75 90 0 I5 30 CURE-MINUTES AT 144.5'C
Comparison of Regular and Refined GR-S Stocks
45
60
INDUSTRIAL AND ENGINEERING CHEMISTRY
July, 1944
Table
643
I. Effect of Zinc O x i d e and SR Furnace Black in Acetone-Extracted GR-S
Compression Fatigueb Load (Lb./Sq. Pendulum Tensile % Max. ~ i ~Lb./-'. % In.) for ElongaIndeninitial perma- tefnp. Dynamic of Cure Sq. Elongation of: Perma- tation, % cornnent rise compression tion 300% 500% nent Set mrn. rebound pression s e t C O C: Initial Final GR-Sa (153 O C.) In. Regular; l 5 0 p a r t s ZnO 30 850 550 285 605 0.13 . ... .. .. .. (0.15 P ) 45 715 470 295 0.09 ... 60 890. 510 285 765 0.11 7:ii 6i:5 zO:i 0.8 25:1 i3:6 ii:o 90 525 420 285 .. 0.07 . .. .. .. .. Regular: 1 5 0 p a r t s Z n O (0.40 d
Regular: 5 parts ZnO 45.3 parts S R F
+
Acetone-extd.;l50parts ZnO (0.15 p )
Acetone-extd.; 150parts ZnO (0.40 w )
. .
..
0.14 0.13 0.14 0.10
..
..
..
7:34
59',6
19:s
..
..
0.07 0.03 0.03 0.02
... ...
7:34
6i:5
0.9
..
.. ..
18:7
7.'20
58:6
19:s
30 45 60 90
455 545
580
420
530 490 500 400
210 250 290 295
375
30 45 60 90
1960 1790 1680 1560
425 340 320 280
1260 1500 1560
30 45 60 90
1580 1920 2160 2040
550 545 510 440
625 750 935 1160
1375 1750 2080
0.46 0.43 0.37 0.24
30 45 60 90
1330 1360 1290 1195
550 540 510 470
580 660 665 700
1160 1195 1210
0.22 0.20 0.18 0.14
580
.. ..
..
..
..
..
..
...
.. ..
..
.. ..
7:39
.,
.. ..
.. ..
60:s
..
30 1420 495 645 .. 0 11 .. .. 45 1600 470 845 , 0.10 0.09 7:34 57:7 60 1730 440 940 90 1760 395 1120 .. 0.07 .. a Formula! 100 parts elastomer, 3 sulfur, 1 Altax; zinc oxide and SRF black variable. b In Goodrloh flexometer; test conditions, 100- ound load and 0.15-inch stroke. C At running time of 15 minutes. d Complete Failure.
Acetone-extd.: 5 parta ZnO f 45.3 parts SRF
. ..
Two zinc oxides of the fast-curing type and S R F carbon black are compared in regular and acetone-extracted GR-S (Figure 2 and Table I). Zinc oxide compounded with acetone-extracted GR-S had higher physical properties than with the normal elastomer and the S R F black stock with acetone-extracted GR-S was slightly poorer than with the regular elastomer. The difference in particle size of the two zinc oxides tested was reflected to a greater degree in the case of the acetone-extracted GR-S. I n other words, the somewhat greater surface of the finer oxide was more readily wet, and reinforcement was enhanced by the acetone-extracted elastomer more than by regular GR-S. These results indicate that the acetone-extractable materials or some portion of them have a n adverse effect on the wetting of zinc oxide. Conversely, the acetone-soluble materials in the elastomer seem to have a favorable influence on the interfacial relation between carbon black and elastomer. Except in the aforementioned instance, a fine-particlesize, fast-curing zinc oxide was used in subsequent experiments. The improvement in reinforcement of acetone-extracted GR-S with zinc oxide is further shown in the compounding data of Figure 3. A regular elastomer is compared with a GR-S sample specially refined by the removal of unpolymerized and partially polymerized portion that did not completely react during manufacture. T h e refined GR-S had somewhat better stress-strain and cut-growth resistance properties than the regular elastomer. A substantial improvement in tensile strength was noted when regular GR-S was extracted, whereas ultimate tensiles of the refined GR-S were affected to a very slight extent. The outgrowth resistance of the regular elastomer was considerably poorer than the corresponding acetoneextracted GR-5. However, the cutFigure 4. ~
..
.. ..
..
...
...
... ...
2i:e
0.9
2317
2.1
..
o:ii
..
c;cles
.. 'd
a!
.. ..
... ...
.. ..
cycles
.d. .. ..
1.5
1.5
.. ..
..
Cut-Growth Resistance, I n . of Failure a t :
0:6S
.. .. 0:iS ..
...
... ...
..* ...
..
... ...
0:39
...
growth resistance of the refined copolymer was the same before and after acetone extraction. When the residue from t h e acetone extraction was added to the acetone-extracted GR-S, the state of cure was advanced, but tensiles and moduli were lower than before the addition of the residue. The most striking change in physical properties occurred when these stocks were aged for 24 hours in hot air a t 100" C. Stress-strain properties of the regular and refined elastomers were degraded by the aging exposure. Breaking elongations of these compounds in particular were reduced, whereas breaking elongations of the acetone-extracted rubbers were not affected by the aging test. Furthermore, the tensiles of the acetone-extracted elastomers were distinctly higher after the aging test than before. The compound with the residue from acetone extraction had tensiles about equal to the regular elastomer. Cut-growth
CURE-MINUTES AT 144.5'C
Effect of Stearic A c i d on Acetone-Extracted
GR-S
644
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
resistance of all compounds after aging was somewhat poorer than the unaged stocks, and the acetone-extracted elastomers were superior in this property to the unextracted samples. The aging data indicate a more far-reaching effect that might be ascribed to acetone extraction of synthetic elastomers. It has been stated that acetone extraction eliminates fatty acid, antioxidant, and some unpolymerized material. The aging results in Figure 3 indicate that materials which adversely influence aging properties have also been eliminated by the acetone treatment. T o promote polymerization during the manufacture of GR-S, oxidation catalysts are added. Some investigators have sugtested that the stiffening and reduction in breaking elongation of GR-S after aging are due to oxygen vulcanization. It seems reasonable to assume that any residual peroxides or oxidation catalysts present in the elastomer will accentuate oxygen vulcanization. The exceptional aging properties of acetone-extracted . elastomers, particularly the fact that breaking elongation was not reduced by aging exposure, indicate that perhaps the oxidation catalysts had been eliminated by the acetone treatment. Another possible explanation for the anomalous aging results is offered. Antioxidant is added to GR-S during manufacture to inhibit polymerization. When the antioxidant is removed by the acetone treatment, the elastomer polymerizes to a higher deglee in the 24-hour exposure a t 100" C. than the unextracted poiymer. The more complete polymerization of the acetoneextracted GR-S is associated with higher tensile properties. I n another set of experiments a sample of acetone-extracted GR-S (prepared for Office of Rubber Director) was compounded with and without stearic acid in a 25-volume zinc oxide stock (Figure 4). The addition of 201, stearic acid to acetone-extracted GR-S degraded ultimate tensile strength, modulus, and pendulum resilience, but heat build-up properties and cut-growth resistance were improved. The three foregoing sets of compounding data show somewhat contradictory evidence. Some of the results suggests that the impaired reinforcement with zinc oxide is due t o the presence of the fatty acid; other data indicate that the unpolymerized and partially polymerized material is responsible for the poor properties. Perhaps both the fatty acid and the short-chain polymers are the offenders. The experiments with the special polymer refined to remove unreacted and short-chain material indicate that poor reinforcement is caused by the partially polymerized materials eliminated in the refining operation. The rest of ,the data, however, indicate that the fatty acid in the elastomer has an adverse effect on zinc oxide reinforcement. Further compounding evidence pointing to the deleterious influence of fatty acids was observed in experiments with a GR-S type polymer in which a rosin soap wm substituted for a fatty acid soap in the emulsification stage. The rosin soap polymer was compared with GR-S in a 25-volume zinc oxide stock accelerated with organic (Figure 5) and inorganic (Figure 6, Table 11)type accelerators. I n the compounds accelerated with 2MT808 and litharge, the rosin soap polymer developed superior stress-strain, rebound, and heat build-up properties but slightly poorer cut-growth resistance than regular GR-S. The relatively high stress-strain properties of litharge-accelerated stocks were also found with magnesia as the inorganic accelerating agent for 25-volume zinc oxide compounds. EFFECT O F C O U M A R O N E - I N D E N E RESINS
Plasticizers and dispersing agents appear to have a greater effect on physical properties in GR-S than natural rubber. Some of the higher-melting-point coumarone-indene resins favorably influence the ultimate tensile strength of GR-S stocks pigmented with zinc oxide. A formulation with Santocure and 25 volumes of zinc oxide had a n ultimate tensile strength of 800-900 initially, and the addition of 10 parts of Cumar MH2-3 increased the optimum tensile to 1500-1600 pounds per square inch (Figure 7). The resin softened the modulus of the stock and increased the
Vol. 36, No. 7
..
..,...
. . . . . G
.. .. .. .. ..m. cu (0
.. .. .. . .. ?
....
c! .. .. .. .. ..n N
. . . . .c
. . . . .e L?
July, 1944
INDUSTRIAL AND ENGINEERING CHEMISTRY
645
b r e a k i n g elongation. Pend u l u m r e s i l i e n c e and heat build-up properties were degraded slightly by the coal tar '* resin. The coumarone-indene resin is believed to serve as a Retting agent for the zinc oxide in the elastomer, An improvement in tensile strength with the addition of coumaroneindene resin to a gum stock was r e p o r t e d b y K e m p ' . However, experiments in this laboratory with 10 parts of Cumar MH2-+ in a GR-S gum s t o c k d i d not significantly change ultimate tensile strength. MILLING OF GR-S
I n the roll milling of zinc oxide in natural rubber, a large portion of the pigment goes into the rubber in the form of d e n s i f i e d flakes or pellets. C o n t i n u e d milling reduces these pellets, and eventually the pigment is intimately mixed and dispersed in rubber. The pigment is not only well dispersed in this hydrocarbon but seems to be well wet so that it has a definite reinforcing effect on the rubber. I n the case of GR-S, however, the incorporation of zinc oxide is somewhat different. M o s t z i n c oxides incorporate in GR-S more readily than in natural rubber, but it is difficult to redisperse the densified pellets of zinc oxide if they incorporate in GR-8 in this form. Therefore, with zinc oxide as well as other pigmentrr, it is preferable t o add the pigment slowly to the mill batch instead of dumping the pigment on the mill in large quantities, as sometimes done in t h e case of natural rubber. In a private communication the Bell Telephone Laboratories mentioned that the ultimate tensile properties of a high-zinc-oxide GR-S stock could be improved by severe milling on a cold mill, and subsequent tests have confirmed this statement. When the propionic-acid-treated zinc oxide was inc o r p o r a t e d in the conventional manner, tensiles of approximately 800 pounds per square inch were developed. The same compound was subjected to refining treatments on a tight mill with ice water circulating through the rolls, and ultimate tensiles were increased to 1400-1600 pounds per square inch. Microscopic e x a m i n a t i o n of t h e s e stocks indicated a n equal degree of disper1 Kemp, A. R., Ingmsnson, J. H., Howard, J. B., and Wallder, V. T.,h e . ENQ.CHEU, 36,366 (1944).
CURE-MINUTES AT 144.5.C. Figure
5.
Rosin Soap Polymer with Organic Acceleration
CURE -MINUTES AT 154.4OC. Figure 6.
P
3
0
Rosin Soap Polymer with Inorganic Acceleration
4
0
5
0
a
m
20?a4050aox)
CURE-MINUTES AT 153%.
Figure 7.
Entocure Formulation with &mar MHP-1/2
INDUSTRIAL AND ENGINEERING CHEMISTRY
646
Table Ill. A h . of Cure Tensile, % (144.5 Lb./Sq. Elongation ’ C.) In.
Vol. 36, No. 7
Trimene-Base Compoundsa with Increasing Loadings of Zinc O x i d e Tear Resistance 24-Hr. Geer Oven Aging a t 100’ C. permsat: Tensile, I’oad (lb./sq. in.) forelongation o f : nent Room lb,/sq. elo$&Set temp. 100OC. in. tion 100% 200% 300% 400% 500% ~
Load (Lb./Sq. In.) Elongation of: for 200% 300% 400% 500%
Permanent set
4.5 Parts Sulfur, 30.1 P a r t s (5 Volumes) Zinc Oxide 7.5 15 30 45 GO
685 535 545 440 500
745 43 5 325 275 265
80 165 295 280 330
120 290 460
.. .. ..
200 455
285
,,,
, , , , , ,
. .. .. .
,
0.17 0.06 0.04 0.03 0.04
. ,. .
.
125 210 210 240 250
290 340 405 460
110 185 230 245 330
110 225 340 365 405
335 495 610 735
130 270 270 280 350
220 315 405 440 485
310 495 675 720 835
395 720 1210
675 515 410 370 355
165 260 300 355 370
2 50 380 470 555 575
370 590 730 830 905
535 880 1370
P a r t s ( 5 Volumes) Zinc Oxide 32 28 875 545 46 34 900 450 45 27 800 370 36 23 765 340 37 19 760 320
125 165 200 200 230
210 245 320 360 380
110 160 245 240 290
45 44 50 33 34
29 34 27 21 19
990
830 595 725 71.5
570 430 300 315 290
165
290 41.5 595 683 I
.
.
410 750
620
.. .. .. ...
0.16 0.12 0.07 0.08 0.07
260 485 915
335 785
...
...
0.30 0.23 0.18 0.13 0.10
... ... ...
...
4.6 Parts Sulfur, 60.2 Parts (10 Voiumes) Zinc Oxide 7.5 15 30 45 60
610 715 665 740 675
1135 645 460 380 335
40 80 195 250 315
40 120 315 455 515
80 240 470
., . . ..
120 355
0.47 0.17 0.13
...
0.11
... .. .
0.09
73 46 53 47 50
29 32 41 28 30
.
1455 1195 1110 935 880
790 550 420 355 325
185
...
.. .. ..
4.5 Parts Sulfur, 90.3 Parts (15 Volumes) Zinc Oxide 7.5 15 30 45 60
1080 1490 1330 1455 1290
945 660 480 440 390
85 195 260 330 345
125 315 470 500 GOO
165 430 770 1040
...
250 625
0.38 0.28 0.28 0.29 0.21
. ... ,,
...
80 62 50 52 55
36 42 35 32 30
1710 1480 1210 1120 1270
710 510 405 365 350
... ...
615 1350
.... ..
...
0.42 0.34 0.23 0.19 0.17
4.5 P a r t s Sulfur, 120.4 P a r t s (20 Voiumes) Zinc Oxide 7.5 15 30 45 GO
1485 1750 1665 1470 1400
986 655 490 420 400
115 230 315 365 415
150 340 515 570 715 125 300 405
7 5 15 30 45 60
820 795 685 530 GOO
770 540 380 330 340
80 190 240 285 300
485
7.5 15 30 45 60
1360 1370 905 1120 1080
910 GO0 425 450 390
80 160 290 280 320
160 285 455 480 520
450
265 495 830 1140 1400 205 460 ,
..
... ...
340 800
0.47 0.50 0.43 0.34 0.27
,..
...
...
4 Parts Sulfur, 30.1 290 0.16 715 0.08 .. . 0.06 .. . 0.03 ,.. 004
93 84 67 68 62
77 48 41 39 37
890 1640 1420 1350 1280
780 1600
...
... ... ...
0.52 0.44 0.31 0.27 0.23
290 365 520 526 610
455 655
705
0.12 0.11
215 320 365 400 455
290 445 570 GOO 740
395 765
610 1290
0.24 0.23 0.15 0.14 0.12
130 235 275 280 320
220 350 430 440 480
360 625 640 760
485 815 1175
790 1475
0.41 0.33 0.23 0.21 0.18
165 250 315 315 330
285 370 430 510 540
366 535 665 745 790
...
...
... t . .
... ... ...
...
0.08 0.09 0.06
4 Parts Sulfur, 60.2 Parts (10 Volumes) Zinc Oxide 200 485 740 880
. ..
280 765
0.28 0.21 0.12 0.12 0.12
..... ,
.,.
63 50 46 45 46
32 30 35 32 24
1190 1290 1060 1040 945
615 500 390 365 335
... ... ...
... ... ...
4 Parts Sulfur, 90.3 Parts (15 Volumes) Zinc Oxide 7.5 15 30 45 60
1500 1610 1280 1170 1290
480 420 425
910
125 236 320 320 360
165 350 480 485 960
010 800 970 1130
930 636 485 425 425
115 200 325 360 405
190 285 490 560 610
225 485 855 1040 1140
650
?50
330 785
0.38 0.38 0.24 0.20 0.19
... ,,. ...
82 65 64 51 55
36 41 36 33 23
1800 1515 1215 1160 1120
670 510 410 375 350
505
...
.
.
I
.,.
...
...
4 Parts Sulfur, 120.4 Parts (20 Volumes) Zinc Oxide 7.5 I5 30 45 60
1430 1490 1350 1280 1310
340 765 ,
0.45 0.44 0.34 0.26 0.24
,.
... ,..
00
83 72 68 72
46 43 40 35 37
1800 1520 1410 1335 1210
665 510 426 395 355
530 820 1215
.... ..
815 1440 ,
.,
...
...
0.49 0.41 0.32 0.29 0.22
Resilience and Heat Build-up at 60-Minute Cure Compression Fatigueb
Formulation Sulfur, parts 4.5 4.5 4.5 4.5 .4 , 0 4.0 4.0 4.0 3
b C
ZnO Parts
Val.
30.1 80.2 90.3
5 10 15
120.4 30.1 60.2 90.3 120.4
2: 10
15 20
Pendulum Inden70 tation, mm. rebound 8.70 75.7 70.6 8.32 7.73 68.0 7.15 65.1 74.6 9.02 8.23 70.9 7.77 67.3 7.43 64.1
%
% I
initial com- permanent pression setc 22.3 1.2 1.0 22.2 19.8 1.1 18.2 1.3 13.3 0.6 1.0 22.2 20.5 1.3 19.2 1.2
Max. temp. rise
c:
11.8
14.6 17.7 19.8 12.2 15.2 18.5 20.1
Dynamic compression Initial Final 14.0 14.1 13.8 13.9 10.9 11.0 9.3 9.4 14.5 14.6 13.0 13.0 11.8 12.0 10.3 10.4
Cut-Growth ,Resistance, In. of Failure at: 750 3000 Shore cycles cycles Hardnesa 1 .o 51 0.33 O:k3 54 0.19 0.33 57 0.15 0.24 61 0.69 50 0:$8 54 0.33 0.13 0.29 57 0.13 0.25 GO
Formula: 100 parts GR-S, 4.5-4 sulfur, 1.1 trimene base, 5 E L C magnesia, 7.5 Cumar MH2-1/%,zinc oxide variable, I n Goodrich flexometer a t 100-pound load a n d 0.15-inch stroke. At running time of 15 minutes.
sion. Although the portion of the compound subjected to noimal milling was well dispersed in the elastomer, the pigment mas not in intimate contwt ~ \ i t hthe GR-S such as a as made possible by the high shearing stresses imparted in the refining treatment applied to the second portion of the sample. O R G A N I C AND I N O R G A N I C A C C E L E R A T I O N
Early in the investigation of the butadiene-styrene copolymer, inorganic accelerators such as litharge, magnesia, and lime were
considered, but nere abandoned a t that time because of the high permanent set developed by these compounds when subjected to heavy loads on the Goodrich flexometer. More recent examination of these materials, especially in conjunction n i t h a small amount of fugitive organic accelerators as “kickers”, ha3 offered considerable promise. One of the drawbacks of GR-S is that it tends to stiffen oil aging, since this a a s believed to be associated with state of cure, milder forms of acceleration, such as that developed by metallic oxiden, n-ere investigated. I n Figure 8 a high-zinc-oxide compound accelerated with 10
INDUSTRIAL AND ENGINEERING CHEMISTRY
July, 1944
Table
Min. of Cure
,,
IV. Comparison of 16-Volume Zinc O x i d e and SRF Carbon Black Stocks
~ ~ ~ ~ % i lLoad ~ (Lb./Sq. , In.) Lb./Sq. Elonga- for Of: In. tion 300% 500%
715 15 30 45 60 90
560 880 720 780 790 690
1095 705 340 310 320 285
85 200 560 660 705
170 620
7.5
1130 1555 1455 1060 1150 1210
890 640 450 365 375 340
175 295 500 610 595 830
305 715
15
30 46 60 90
ZnOd cured a t 144.5' C.
a
b d
Tear Resistance
Pendulum % Indenta% initial :omtion, mm. rebound pression
.. ..
..
... ... ...
...
Permanent Set
S R F Carbon Black Formulationc Cured a t 138.1' C. 0.33 92 .. .. 0.12 112 .. 0.03 79 0.03 72 8:61 62:2 28:6 0.03 78 0.02 75 8134 63:4 2611
,..
Zinc Oxide Formulationd Cured a t 144.5' C. 0 46 102 *. 0 43 87 0 35 74 0 22 66 7:90 6i:4 l9:5 0.22 68 1615 0.17 70 7:32 65:4
.. ..
.. ..
...
... ... ... Teated a t
Formulation
647
.*
looo C.
Min. of cure
Tensile, Ib./sq. in,
% elon-
7.5 15 30 45 60 90
300 450 535 440 485 425
675 555 450 345 330 295
gation
Load (lb./sq. in.) for elongation O f : 300% 500%
85
216
330 205 370 445
405
...
... ... . . I
.
Tear resistance
42 53 40
.
26 31 35
Compression Fatiguea % Max. pertemp. compression Dynamjc manent rise, set* ' C. Initial Final
. . . . . ...
.....
.. ..
.. I .
1.5
22:8
26:l
20'.6
1.0
2i:g
i+:7
i+:g
.. .,
.. ..
...
.. .. .. .. .. i:j
1814
16:s
ii:4
0.g
l8:O
8:3
8:6
...
Cut-Growth Resistance, In. ofa t : Failure 7000 cycles
..
.. 0:
bo
..
..
0:53
24-Hr. Geer Oven Aging a t looa C. Load (lb./sq. in.) Tensile % elonfor of: Permanent lb./sq. ih. gation 300% 500% set
1830 1630 1320 1350 1150 1330
655 510 390 385 330 330
555 600 820 880 975 1080
1570 1590
..
0 39 0 27 0.25 0.21 0.16 0 16
In Goodrich flexometer, a t 100-pound load and 0.15-inch stroke. A t runnin time of 15 minutes 100 parts &R-S, 3 sulfur, 0.75 C a p t a x , 0.12 D P G , 5 Bardol, 5 Paraflux, 30 S R F black, 4 zinc oxide. 100 parts GR-S, 4.5 sulfur, 1.1 Trimene base, 5 E L C magnesia, 7.5 Cumar MH2-1/2, 96 zinc oxide.
larly advantageous in that it develops a snappy cure which parts of magnesia is compared with a similar stock accelpersists only until the accelerator decomposes; the milder inerafed with 2MT-808. The inorganic-accelerated compound organic metallic oxide acceleration functions as the persistent developed higher tensiles and breaking elongations than the accelerating agent. organic-accelerated stock. Although the permanent set of the More recent experiments with a Trimene-base formulation inmagnesia stock was much higher than the 2MT-808 compound, the resilience and heat build-up properties were in the same dicate that it is possible to develop a high degree of reinforcement with 10 volumes of zinc; up to this time it has been necessary to sange, a n indication that the magnesia compound was well cured. pigment with 25 volumes of zinc oxide for satisfactory properties The high permanent set and high tensiles after 24-hour aging at 100; C., however, are sug-gestive of insufficient cure. It is also noteworthy that the magnesia compound had a higher breaking elongationin the 100' C. tensile tests and after 24-hour aging at 100' C. than the 2MT-808 compound. These results are not consistent with the behavior of organic accelerators in GR-S, and it may mean that a *differenttype of vulcanization or .polymerization is occurring with magnesia. A Btudy of magnesia acceleration now in progress may result in better understanding of these observations. Formulations having a lower permanent set can be developed by using a small amount of fugitive accel-erator, such as tMuram or dithiocarbamate, in conjunction with magnesia t o set off the cure. Adjustments i n this direction . _. permit lower sulfur loadings. CURE-MINUTES AT 138.I"C. 'The use of a small amount of ,fugitive accderatw i s particuFigure 8. Comparison of Organic and Inorganic Acceleration in a Zinc O x i d e Compound
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
648 FORMULA
GR-S
t Z I v C z
o
o-TENSILE STRENGTH
4.5-4 ,-~-I_-+--MODULUS AT 300%
SULFUR
")tI
A
o
IO0
,
o
-
VARIABLE
I
1
A
3
- 5 VOLUMES
ZINC OXIDE
A -10
VOLUMES ZINC OXIDE
I -15
VOLUMES ZINC OXIDE
e-20 VOLUMES ZINC OXIDE
1400
I
E 1200
#
3
1000
m E U
a
Vol. 36, No. 7
The influence of the Cumar resin is probably to improve the wetting of zinc oxide by the elastomer. A better impression of the merits of Trimene-base formulation may be gained in comparison with the War Production Board carcass formulation containing 16 volumes of SR Furnace carbon black. The Trimenebase stock was compounded with 16 volumes of zinc: oxide (Figure 10, Table IV). The carbon black compound developed tensiles of 700-800 pounds as compared with 1400-1500 pounds for the zinc oxide compound. The pendulum resilience and heat huildup properties of the zinc oxide compound were superior to those of the carbon black compound, I n cut-growth and tear resistance the stocks were about equal. When tested a t 100' C., the zinc oxide stock had higher tensile and tear properties than the carbon black. compound. The zinc oxide formulation stiffened somewhat less than the carbon black stock after 24hour aging a t 100" C. CONCLUSIONS
1. The data indicate that the fatty acids derived from prime tallow soap used in the emulsification of' GR-S are mainly responsible for the failure to develop satisfactory tensile properties for high loadings of zinc oxide employing conventional compounds. 2. The addition of magnesia tends to irnprove'thcsc physical properties, suggesting the formation of insoluble magnesium soaps. 3. Physical properties of zinc oxide stocks are' 15 30 45 60 further improved by plasticizers such as coumaroneCURE -MINUTES AT 144.5'C. indene resins, which are believed to act as wetting Figure 9. Effect of Increasing Loadings of Z i n c O x i d e in a Trimene Base agents for zinc oxide in GR-5. Formula 4. T o secure optimum results with high loadings of zinc oxide, it is necessary to pay close attention to, accelerator and sulfur content. in GR-S. Figure 9 and Table I11 show that, with 4.5 parts of sulfur, 15 volumes of zinc oxide ar? required for good reinforceACKNOWLEDGMENT ment, but that tensiles of well over 1000 pounds are developed The helpful criticism and assistance of B. R. Silver and G. S. with 10 volumes of zinc oxide in a stock with 4 parts of sulfur. Haslam in this investigation is appreciated by the writer. Furthermore, better stress-strain properties were found with 4.5 parts of sulfur in the 2(rvo1ume stocks than parts Of the PRESENTED before t h e spring meeting of the Division of Rubber Chemistry, canization agent. These data emphasize the desirability of usAMERICANCHEMICAL SOCIETY,in New York, N. Y ing somewhat more sulfur with high - of zinc - loadings oxide than other reinforcing pigments. Aside from the high sulfur loading, it should also be pointed out that the Trimene formulation includes magnesia and Cumar MH2-3, both of which contribute to the improved properties with zinc oxide. The specific effect of these materials in GR-S is not clearly understood, but it seems likely that they tend to minimize the influence of fatty acids in so far as zinc oxide is concerned. Perhaps Trimene base and magn e s i a , b e c a u s e of t h e i r alkaline nature, convert the fatty acids into inert soaps and there by remove them from the sphere of activity.
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