ENGINEERING RESEARCH INSTITUTE THE UNIVERSITY OF MICHIGAN ANN ARBOR Quarterly Report No. 2 INVESTIGATION OF ORGANIC CURING AGENTS FOR GR-S RUBBER L. M, Hobbs Supervisor and Associate Profewssor of Materials Engineering R. G. Craig Principal Investigator and Research Associate C. W. Burkhart Research Assistant Project 2376 DEPARTMENT OF THE ARMY ORDNANCE CORPS, DETROIT ORDNANCE DISTRICT CONTRACT NO. DA-20-018-508-ORD- (P)-42 PROJECT NO. 1180, RAD ORDER NO. 57180100-99-00604 September 1955

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN TABLE OF CONTENTS Page LIST OF TABLES iii ABSTRACT iv OBJECT v INTRODUCTION1 EXPERIMENTAL EQUIPMENT AND METHODS 1 MILLING 1 CURING 1 AGING 2 SPECIMEN TESTING 2 EXPERIMENTAL RESULTS 2 FUTURE WORK 9 ------------------- 1 —----------------

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN LIST OF TABLES Table Page I COMPOUND FORMULATIONS 3 II REPEATABILITY OF COMPOUNDING, CURING, AND AGING METHODS 4 III PHYSICAL PROPERTIES OF VARIOUS NONFREE SULFUR CURES IV PHYSICAL PROPERTIES OF SAMPLES USED IN CHECKING THE "CRUMBLING SCORCH TEST" 7 V SCORCH EVALUATION BY MOONEY VISCOMETER 8 iii

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN ABSTRACT Apparatus and methods used in the preparation of vulcanized GR-S rubber samples are described; aging and physical testing of these rubber vulcanizates are also discussed. Repeatability tests showed that our apparatus and methods were satisfactory for preparing experimental samples of cured GR-S rubbero GR-S cured with Methyl Tuads-El Sixty or Methyl Tuads —Captax had excellent aging properties. Optimum physical properties were obtained when 1.75 PER of Methyl Tuads was used with either 1 PER of El Sixty or Captax. The changes in the 300% modulus and the percent elongation on aging showed that the Methyl Tuads with El Sixty or Captax were superior to either the Santocure-Sulfur or the Tetrone A-Captax cures. GR-S cured with Di Cup 40 C had aging properties superior to any curing system investigated with little, if any, change in physical properties on aging at 212~F for 72 hours. However, the original percent elongation was not as high as with the other curing systems. The addition of one extra PHR of PBNA over that in the GR-S improved the physical properties of the vulcanizate substantially. The scorch tendencies of the nonfree sulfur systems were compared with Santocure-Sulfur using the "crumbling scorch test" and the Mooney viscometer. The latter method was shown to be preferable. The scorch of the Di Cup 40 C was nearly comparable to Santocure-Sulfur while Tetrone A-Captax was very poor and Methyl Tuads with Captax or El Sixty were intermediate. Addition of retarders to the Tetrone A-Captax or the Methyl Tuads-Captax systems did not show any marked improvement in scorcho iv

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN OBJECT The object of this study was to investigate nonfree sulfur curing systems for GRI-S rubber with the purpose of improving the aging properties of rubber without sacrificing any other important physical properties -- — v

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN INTRODUCTION Reproducibility of results and reasonable agreement with results from other laboratories were essential; therefore, check samples were prepared, tested, and compared with data from Rock Island Arsenal, Experimental rubber vulcanizates were prepared and subjected to various physical tests in order to evaluate their processing and aging properties. EXPERIMENTAL EQUIPMENT AND METHODS MILLING All batches were milled on a 3- x 8-in. Thropp mill using the following mixing procedure: (1) the GR-S 1500 was passed three times through a cold-tight mill (1 min); (2) the mill was opened to 0.055 in. and the rubber masticated, a 3/4 cut being made each half minute (10 min); (3) the ZnO was added (2 min); (4) one half of the carbon black was added,.one 3/4 cut being made from each side and then the remainder of the black added (10 min); (5) the stearic acid and antioxidant were added (2 min); (6) the accelerator and vulcanizer were added (2 min); (7) three 3/4 cuts were made from each side (2 min); (8) the batch was cut from the mill and passed endwise through the mill six times at a mill setting of 0.030 in. (2 min); and finally (9) the mill was opened and the batch run out to give two uncured pads of approximately 6W x 6- x 0.1-in. CURING An electric-heated Preco hydraulic press and a 6- x 6- x 0.080-in. chromium-plated- steel mold were used to cure the samples. The uncured pads were placed in the hot mold at 307~F, the pressure was raised tO 2200 psi, the sample cured for 30 minutes at 307~F, and the cured pads were quenched in cold water. 1

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN AGING A Precision-Freas mechanical convection oven was used for aging all specimens, The temperature control at 212~F was + 0.7~F and the air flow used was 100 linear feet per minute through the aging chamber. SPECIMEN TESTING An Instron tensile testing instrument was used to obtain tensile, modulus, and percent elongation measurements. Eight specimens were cut from two 6- x 6-in. cured pads using an ASTM Die C; four were used as originals and four for aging. A Shore durometer, Type "A", was used to determine specimen hardness. A Mooney viscometer with a 1-3/16-in. rotor was used for determining scorch of the uncured rubber. EXPERIMENTAL RESULTS The use of the 3- x 8-in. mill required that single batches be limited to approximately 160 g, from which two 6- x 6-in. cured pads could be obtained. Thus, it was of primary importance to be able to repeat batches and obtain satisfactory check values on physical properties. Separate batches 1 and 7 (see Table I for compositions of all samples), using Santocure-Sulfur, and 2 and 8, using Tetrone A-~Captax cures (Table II), showed that batches could be repeated and good check values obtained for the physical properties of the rubber vulcanizates. In addition, the values obtained for the physical properties were in satisfactory agreement with results on vulcanizates containing identical compositions reported by the Rock Island Arsenal staff. Therefore, we are satisfied that our methods are satisfactory for evaluating other vulcanizing systems. It has been reported that t'hiuram disulfides (e.g,1 Methyl Tuads.), used either alone or as accelerators, empart better aging properties to rubber vulcanizates than thiuram tetrasulfides (e.g., Tetrone A). The curing systems Methyl Tuads-El Sixty and Methyl Tuads-Captax were, therefore, investigated to determine their aging properties. Table III lists the physical properties of the rubber vulcanizates prepared using various PHR of Methyl Tuads with one PHR of either El Sixty or Captax. When the PRe of Methyl Tuads were increased in the Methyl Tuads-El Sixty curing system, both the original and aged tensiles were increased. How2 ---

m Z m TABLE I COMPOUND FORMULATIONS* Z I BatCh No,. topoun i g I 7 2, I 9 10 11 12 13 14 15 16,26 17 18,29 19 20 21 22 25 27 28 30 31 32 F I ngredient 123 24 70 GR-S 1500 100 100 1 00 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 Zinc Oxide 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 Stearic Acid 1 1 1 1 2 2 2 2 1 1 1 1 1 1 1 l. i 1 PBNA 2 2 2 2 1 1 1 1 "Philblack A 5 0 50 5050 50 50 505050 50 50 50 50 50 50 5050 50 50 50 50 50 Sulfur 1.75 1.75 1.75 1.75 Santocure 1 1 1 1 Tetrone A 1 1.5 1.5 1 Captax 1 1 1 1 1 1 1 1 1 Z Methyl Tuads 2 3 2 1.75 1.75 1.75 1.75 1.75 < El Sixt'y 1 1 1 Di Cup 40 C 2.5 5 2.5 5.0 2.5 5.0 Vultrol 1 1 1 1 1 Santoflex AW 1 Tenamene-2 n *All samples were cured at 307~F for 30 minutes,. I Z

m Z m TABLE II Z REPEATABILITY OF COMPOUNDING, CURING, AND AGING METHODS* m Tensile, psi' 200 Modulus psi 300% Modulus, psi % Elongation Shore A Batch No, Curing System I Batch No,. Curing Sys;tem:Orig. Aged Orig. Aged Orig. Aged Orig Aged Orig Aged 1 1 Santcur 2680 2920 - 2330 2140 - 400 250 68 74 _ 1 Santocure 1.75 Sulfur + 6 7 Saoure 2870 2980 1180 2260 2170 - 410 250 68 73 I Santocure 1 Tetrone A + Z 2 ~ Captax 2210 2380 -10 1090 1330 1830 470 380 66 67 1 Tetrone A + 8 Captax 2230 2460 700 1030 1340 1850 460 390 69 65 O *Aged specimens were air oven aged at 212~F for 72 hours. -n I

TABLE IIII PHYSICAL PROPERTIES OF VARIOUS NONFREE SULFUR CURES* Z C Batch No. Curng Sytem ensile psi 20 0% odus ps Modulus psi Eloation Shore A m B h N Curing S Orig. Aged mrig. Aged Orig. Aged Orig. Aged Orig. Aged 1..........dO 101 15 Mtyl Tuads 1970 2680 800 1270 650 610 6 653 1.75 Methyl Tuads 21 1875 Methyl Tuads 210 2650 950 1290 650 580 65 65 1 EI Sixty m 9 Methyl Tuads 2620 2830 1200 1620 610 320 64 69 1 El Sixty 32 [5 Methyl Tuads 10 E Sixty1 u2970 2890 1750 2070 500 270 67 72 100 1 Me l Tuads 970 2080 440 950 640 600 61 64 f 1.75 Methyl Tuads 25 { eth Tuad 2350 2725 1020 1440 610 550 66 66 1 Captax 15 l2 Methyl Tuadss 15 Captax 2580 2850 1200 1610 580 490 66 66 21 2.5 Di Cup 40 C 2220 2270 1370 1420 - - 280 290 67 67 22 5.0 Di Cup 40 C 1830 1940 -- -1 10 75 7 C 16 {2.5 Di up 40 C 2420 2300 950 900 1730 1680 390 390 65 65 f5.0 Di Cup 40 C 17 r1PBNA 40 2000 2060 - - - 190 190 72 71 1 PBNA 2.5 Di Cup 40 C 3 Zinc Oxide 1 Stearc Acid 2170 2292019 150 0 410 210 20 72 72 1 PBNA 5.0 Di Cup 40 C 0 Zinc Oxide F0 l~tearic Acid 2170 2350 1930 1970 -- 210 220 72 72 1 PBNA 5 *All cures were at 3070F for 30 minutes; all aged specimens were air oven aged at 212~F for 72 hours. l ~ ~ ~ ~ ~ s ImnlW~ lroe O US

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN ever, the original tensile increased faster than the aged tensile and thus the two values approached each other at 3 PHR. The original percent elongation (%E) decreased gradually with increasing PER of Methyl Tuads; however, the aged %E decreased very rapidly between 1.75 and 2 PHR of Methyl Tuads. The Methyl Tuads —Captax curing system was similar to the Methyl Tuads-El Sixty system when the original and aged tensiles were compared. The aged %E, however, did not decrease as fast between 1.75 and 2 PER of Methyl Tuads in the Captax as in the El Sixty system. The optimum concentration of Methyl Tuads was 1.75 PER in combination with 1 PHR of either El Sixty or Captax. Original and aged tensile values were about 2550 and 2700 psi, respectively, and original and aged %E values were about 630 and 570%, respectively. The optimum Methyl Tuads-El Sixty cure had slightly better physical properties than the optimum Methyl Tuads-.Captax cure but the former had a noticeable bloom while this property was not serious with the latter. When the physical properties of these two systems were compared with Tetrone A-Captax (Table II) they both showed superior aging properties. One of the most promising nonfree sulfur cures was the Di Cup 40 C cure (40% dicumyl peroxide on!iCaC03). Table II lists results obtained using 2.5 and 5.0 PER of Di Cup 40 C alone, plus 1 PER PBNA (GR-S 1500 contains 1.25 PER PBNA) and with 3 ZnO + 1 stearic acid + 1 PBNA PER. Although the original %E values were not as high as the thiuram cures no decrease in the %E occurred on aging for 72 hours at 212~F. The remaining physical properties were satisfactory and showed no change on aging. The results showed that 2.5 PHR of Di Cup 40 C was much better than 5.0 PER in all cases. It is somewhat surprising that 1 extra PHR of PBNA using 2.5 Di Cup 40 C increased the tensile by 200 psi and the %E by 100 percent. Although Hercules Power Company advised against the use of ZnO and stearic acid, formulations containing these materials showed a slight improvement in the %E. However, considerably more variation was observed in the testing results with these materials present. One objection to the nonfree sulfur cures has been that they are "scorchy." Efforts to use the "drumbling Scorch test" described in Rock Island Arsenal TR-54-4255 were not completely successful. Table IV lists the physical properties for increasing aging times before curing for Santocure-Sulfur and Tetrone A-Captax with and without Vultrol. The data, with the exception of the low tensile and 300% modulus values for 1 and 2 days' aging with SantocureSulfur-Vultrol, are in fair agreement with those listed in the above-,mentioned report. However, the "crumbling scorch tests" were inconclusive since in the Santocure-Sulfur combinations smearing on the mill rolls occurred. Even with the Tetrone A-Captax system without Vultrol, cracking, not crumbling, of the aged uncured pad occurred. The difficulty with the "crumbling scorch test" may have been with the size of the mill rolls. Satisfactory scorch evaluations were made using a Mooney viscometer. Table V lists results obtained on a number of nonfree sulfur systems compared 6

m Z TABLE IV Z PHYSICAL PROPERTIES OF SAMPLES USED IN CHECKING THE "CRIUMBLING SCORCH TEST" m 73 Time Aged at 212~F Tensile, 200 Modulus, 300% Modulus, Elongation Shore Batch No. Curing System Before Curing i Elongation 11 1.75 Sulfur + 0 days 5000 2020 420 68 1 Santocure 11 1 2350 - 1560 44 65 11 2 2170 -- 1310 500 11 3 2200 980 6oo 58 12 1.75 Sulfur + 0 2560 - 1480 430 66 1 Santocure + _ 1 Vultrol C 12 1 640 -- 310 640 61 12 2 670 -400 580 5 12 3 1270 - 6oo 610 56 13 1.5 Tetrone A + 0 min 2300 1200 2040 520 69 - 1 Captax Z 13 15 / 2170 1330 - 300 70 13 30 2040 - - 220 67 13 60 1860 970 - 250 65 14 1.5 Tetrone A + 0 2210 1130 1980 320 68 1 Captax + - 1 Vultrol O 14 30 2250 1130 190 330 67 14 90 2000 1120 - 10 66 z 14 120 1900 1050 -- 30 0 6 6Z

m m TABLE V m SCORCH EVALUATION BY MOONEY VISCOMETER (i) 1 PER Vultrol Added 1 PHR Santoflex AW 1 PHR Tenamene-2 Batch No. Curing System Moqoey Scorch, min Mooney Scorch, min Mooney Scorch, min Mooney Scorch, min IIti1_2 _ t2o(3) ti t2oI ti tzo,ti t2o Cf 23,27 1 Santocure 47 74 50 120 1.75 Sulfur Z 24,:28 1.5 Tetrone A 5 9 10 19 co 1 Captax C 25,30,31,32 1.75 Methyl Tuads 15 32 15 32 15 27 10 18 1 Captax C 29 1.75 Methyl Tuads 12 26 Z 1 El Sixty 26 2.5 Di Cup 40 C 25 94 1 PBNA 0 (1) All scorch runs were made at 250~F. -n (2) Time in minutes for initial rise, (3) Time in minutes for a 20-unit rise, Z

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN to Santocure-Sulfur. The time in minutes for an initial rise above the minimum, ti, and the time in minutes for a rise of 20 units above the minimum, to20 were recorded. A good criterion is usually ti if the rate of increase in viscosity of the two systems compared is nearly equal. However, if the rate of increase in viscosity of the two systems is very different it is also important to determine t2o. The Santocure-Sulfur system had very good scorch properties and was used as a standard for comparison. The systems listed in the order of increasing scorch tendency were: Santocure-Sulfur, Di Cup 40 C- PBNA, Methyl Tuads-Captax, Methyl Tuads-oEl Sixty, and Tetrone A —Captax. Although ti for Di Cup 40 C-PBNAwas 25 minutes compared to 47 minutes for Santocure-Sulfur, the viscosity increase with time was very slow for the former and t2o was 94 minutes compared to 74 minutes. Therefore,the scorch properties of Di Cup 40 C were almost as good as for Santocure-Sulfur. The addition of Vultrol to the Tetrone-Captax cure improved the scorch properties; however, the addition of retarders to the Methyl Tuads-*Captax cure at best did no good and with Tenamene-2 the scorch tendency was increased. FUTURE WORK Investigation of nonfree sulfur cures will continue with emphasis on the Di Cup 40 C system. It is planned to determine optimum compositions for this system and to study the action of various antioxidants since the concentration and type appear to be very important. Other physical tests than those listed in this report, such as flex life, will be investigated to determine whether other physical properties of nonfree sulfur vulcanizates are important. 9