Vulcanization of Neoprene Type W - Effect of Metallic Oxides

the proper relation between the accelerator and the metallic ox- ides used. It has long been established that variations in metallic oxide ratios are ...
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VULCANIZATION OF NEOPRENE TYPE W Effect of Metallic Oxides F. H. FRITZ AND L. R. MAY0 E. I. du Pont de Nemours & Co., Inc., Wilmington, Del.

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OLLOWING the comThe effects of these mePermalux, Retarder W, and 2-MT in the presence of sulmercial introduction of tallic oxide variations are fur are efficient accelerators for the vulcanization oY NeoNeoprene Type W and the markedly influenced by the prene Type W. They are unique in that the processing early recognition of the need accelerator. T h e comsafety of stocks containing them is easily controlled by the for accelerators to develop pounds accelerated with the type and amount of metallic oxides used without affecting the most desirable vulcandi-ortho-tolylguanidine salt the properties of the vulcanizates. NA-22, Tonox, and izate properties (5-5), a deof dicatechol borate (PerDPG in the presence of sulfur are also efficient accelerators, tailed investigation of the malux), salicylic acid (Rebut stocks containina them cannot be controlled in a simifactors controlling its curing tarder W), and 2-mercaptolar manner, characteristics has been thiaaoline (2-MT)-sulfur made. In this work was are strongly affected by the included a study of the influence of varying the amount, type, and metallic oxide ratio used. These effects are similar to those observed by Forman et at. ( 4 ) in their study of unaccelerated stocks. ratio of metallic oxides used on the processing safety, rate and state of cure, and vulcanizate properties of accelerated Type W By reversing the amounts of magnesia and zinc oxide, a striking compositions. I t was established early in this work that effects improvement in scorch time is obtained. On the other hand, such commonly used neoprene accelerators as Zmercaptoimidaeoline due to variations in metallic oxides were greater in the presence (NA-22), diaminodiphenylmethane (Tonox) or diphenylguanidine of certain accelerators than had previously been recognized. It (DPG)-sulfur are essentially insensitive to such variations. was also apparent that the use of certain accelerators resulted in very fast curing stocks which displaced strong tendencies to vulFrom the &&,-mentioned “sensitive” group of accelerators, canize a t processing temperatures. . The problem of controlling Permalux was selected for a more comprehensive study of the this scorching tendency without undesirable impairment of vuleffects of metallic oxide variations. Comparable stocks containing a representative of the “nonsensitive” group, NA-22, were incanizate properties has emphasized the importance of obtaining the proper relation between the accelerator and the metallic oxcluded in this work for purposes of comparison. Previous exides used. perience has indicated that the unique properties of Type W vulIt has long been established that variations in metallic oxide canizates, notably low compression set, can be developed to a ratios are effective in changing the scorch characteristics of high degree with either of these accelerators (a, 5). Neoprene Type GN compounds (1, 8 ) . It has been shown that Effect of Metallic Oxide Concentration. The effect of a more the rate of cure increases with the amount of zinc oxide present complete range of metallic oxide concentrations on the processing and decreases as the magnesia concentration is raised. Forman safety of a stock accelerated witfi 0.35 part of Permalux is illusand coworkers ( 4 )showed that these same phenomena were found trated graphically by the solid lines in Figure 1. The base compound used was the same as that shown in Table I. ln Neoprene Type W stocks but limited their study to compounds containing metallic oxides as the only curing agents. Because such stocks are seldom used, their study has had limited practical value. Table I. Effect of Varying Metallic Oxide Ratios on ProcThe compounds discussed herein were prepared and tested essing Safety in the Presence of Various Accelerators according to procedures established by the American Society for Base Compound Parts Testing Materials. The following methods are applicable to the Neoprene Type W 100.0 Phenyl-a-naphthylamine various tests: 2.0

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Teste Modulus, tensile strength, and elongation Compression set Hardness Resilience Oxygen pressure aging Oven aging

ASTM Designation D 412-49T D 395-49T (Method B) 676-49T (Shore A Durometer) 945-49T 572-50 673-48

D D D D

The tendency of the compounds to scorch was measured by means of the Mooney shearing disk viscometer operating a t a temperature of 121.1’ C., using the small rotor. The time to scorch was selected as the time for the Mooney viscosity to increase 10 points from the minimum value.

Stearic acid Ma nesia SR# carbon black Zinc oxide Accelerator

Accelerator, Parts Permelux, 0.36 Retarder W, 2.0

NA-22, 0.25

April 1952

4

4 1 4

4

1

2-MT,-0.75-sulfur, 1.0

PROCESSING SAFETY

Effect of Varying Metallic Oxide Ratios. The first experiments undertaken covered the effect of three different zinc oxidemagnesia ratios on the processing safety of Neoprene Type W stocks containing a variety of different accelerators. The results obtained are summarized in Table I.

Magnesia, Parts

Tonox, 0.35

DPG, 0.75-sulfur, 1.0

INDUSTRIAL AND ENGINEERING CHEMISTRY

4 4 1 4

4 1 4 4

1 4 4 1

0.5 As shown 29.0 As shown As shown Zino Oxide, Parts 1 4 4 1 4 4 1 4 4 1 4 4 1 4 4 1 4 4

Mooney Scorch, Minutes 57 30 23 16 8 2

50 26 18 17 16 16

25

25 23 28 27 28

831

-ELASTOMERS-Compounding The trends established by the data in Table I are consistently followed over a range of concentrations (1 to 8 parts) for each of the metallic oxides. With zinc oxide held constant, the indicated processing safety with Permalux is more than doubled as the amount of magnesia is increased from 1 to 8 parts. The effect of varying zinc oxide with a constant amount of magnesia is even more striking. A reduction of zinc oxide concentration from 8 parts to 1part more than triples the resistance to scorch. Further evidence of the tolerance of stocks accelerated L with KA-22 to metallic oxide variations is shown in Figure PERMALUX1 by the dotted straight line parallel to the horizontal axis. All of the scorch times for the g" four different concentrations 2 4 6 8 of magnesia studied fall on 0 PARTS - Ziffi OXIDE this single line. Effect of Varying Grade of Figure 1. Effect of Metallic Oxide Concentration on Metallic Oxide. The unusual Processing Safety results which were obtained using various ratios and concentrations of metallic oxides indicated the desirability of studying different grades of these curing agents. These investigations were conducted using the same base formula shown in Table I. Four, parts each of magnesia and zinc oxide were used. I n the tests involving variations in type of magnesia, an American process zinc oxide was used and in the tests involving variations in type of zinc oxide, an extra light calcined grade of magnesia was used. The results obtained are shown in Figures 2 and 3. In Figure 2, hIooney scorch values are shown for the comea i i o * T - s . Y ~ i z 0 Lle*T.$AYPLE * pounds containing the four i types of magnesia tested. d,p These included two classified i20 as extra light, one as medium, and one as heavy, and thus provided examples of all the HI types normally used in neoprene stocks. A high degree Figure 2* Effect Of TYpa of Magnesia o n Processing of sensitivity in the Permalux Safety acdelerated compound to the type of magnesia used is shon-n by the decrease in scorch time as the heavier grades are used. This response to variations in magnesia is consistent with experience gained with the other types of neoprene, and is in contrast with the lack of senvitivit,y of the stocks containing NA-22. The effect of four different zinc oxides on the hiooney scorch values of compounds accelerated with Pernialux and NA-22 is I

YEO1""

shoiin in Figure 3. T n o French process grades, one having fine particle size and identified as F.P., and two American piocess grades, one of which has been washed and treated with propionic acid and is identified as W . and Y'., were studied. As with magnesia, the Permalux accelerated stocks are influenced by the type of zinc oxide used, resulting in a spread of from 12 to 27 minutes in scorch time, whereas the NA-22 stocks are relatively unaffected. This wide spread between the two French prorwh grades indicates that particle size may have considerable significance. The differences obtained with the two American prorem zinc oxides suggest that surface treatment may influence the effect of zinc oxide on the scorch behavior of neoprene ('ompounds. RATE AND STATE OF C U R E

Effect of Metallic Oxide Concentration. Having establishrtl that processing safety as indicated by the Mooney test may be varied widely by changing the type or amount of metallic oxide curing agents in the presence of a sensitive accelerator, the rwiilt of these changes on curing characteristics was next considel c ~ l Because the nonsensitive accelerators, such as KA-22, are not 111fluenced by variations in metallic oxides, they Rere not included. The effect of metallic oxide concentration on the idte and state of cure of Permalux stocks was examined using the w i i e formulations a3 those on which Figure 1 is based-e.g., combinations of 1, 2, 4, and 8 parts of both zinc oxide and magnesia Stress-strain results summarized in Table I1 show that on short, &minute cures a t 153" C., the rate of cure follows the pattern which would be predicted from scoich data. However, ~ i t h slightly longer cures, 10 to 20 minutes, a t 153" C , thew diffwences are essentially eliminated. That high states of cure are attainable for these stochi 13 ithin reasonable curing times is further substantiated by the coinpression set and resilirnce data shoqn in Table 111. I t will be noted that in spite of the tremendous variation in scorch resistance among these stocks the dlfferences in set and resilimre values are negligible with the exception of the stock containing fi parts of magnesia, which has substantially less-resistance to COIDpression set. Even this winpound is capable of resrhinp en r n i n c u 0 FRLHC" r P a state of cure comparabk to that of the others if thfl cul-n~g time is moderately rxtencirtf AGING CHARACTERISTICS

Figure 3. Effect of Types of Zinc Oxide on processins Safety

Effect of Metallic Oxide Concentration. It hab ~ W I I demonstrated repeatedly t h a t both zinc oxide and m a g n w h

Table 11. Effect of Metallic Oxide Concentration on R a t e of Cure MagMooney nemum Zino Scorch Accelerator, Oxide, Oxide, T h e , Parts Parts Parts Min. Permalux, 0 . 3 5 1 1 37 1 2 25 1 4 18 1 8 12 2 1 43 2 2 30 2 4 21 2 8 12 4 1 59 4 2 47 4 4 32 4 8 19 8 1 84 8 2 60 8 4 46 8 8 2J a h-ot sufficient cure t o test.

832

Stress a t 300% Strain, Lb./Sq. Inch a t 153' C., Min..> 10 20 40 923 1100 1175 1250 1100 1150 1250 1275 1150 1200 1275 1300 1150 1300 1325 1350 1025 1150 1250 1275 1000 1150 1200 1250 1150 1200 1250 1350 1250 1275 1325 1350 1200 1376 1450 a 1325 1425 1475 1450 1500 1550 1125 1500 1550 1600 a 1230 1350 1626 " 1225 1325 1675 1075 1350 1550 1100 1200 1450 1500

-Cure

Tensile Strength, Lb./Sq. Inch -Cure a t 153' C., Min.-5 10 20 40 3275 360(! 3700 3625 3600 3720 3600 3475 3625 3650 3475 3275 3400 3525 3525 3350 2025 3325 3400 3400 3050 3550 3525 3500 3400 3600 3400 3275 3525 3625 3476 3300 a 3200 3575 3550 3700 3600 3375 a 3700 3675 3525 3100 3675 3525 3500 a 2200 3300 3375 a 3800 3375 3675 3075 3450 3475 2300 3200 3600 3550

Elongation at Break '/c

,-Cure at 153O'C., h1in.5 10 20 40 700 680 640 610 660 650 640 580 660 650 610 570 640 630 620 600 620 610 600 600 700 610 620 600 670 620 600 580 630 610 580 560 610 580 560 a 640 570 560 600 590 560 700 600 570 560 * 680 650 310 a 680 580 540 a 600 610 530 770 690 600 570

INDUSTRIAL AND ENGINEERING CHEMISTRY

Shore A Hardnev --Cureat 153OC., 1 1 ~ 5 10 20 40 48 51 5 1 ? ' 5 3 52 52 51 52 53 53 52 52 53 5.4 40 5 1 .52 3 2 49 52 53 :3 62 53 53 5 4 52 33 54 55 a 5 3 55 ,55 a 34 55 ? ; 0 54 6 3 0,) 51 54 55 56 52 55 56 54 56 57 " 5.5 56 57 52 56 57 5 8

::

Vol. 44, No. 4

RLASTOMERS-Compounding In addition to these accelerated tests, vulcanizates of stocks, such as those listed in Table IV, have been exposed to outdoor weathering both unstretched and under 20% strain. After one (Specimens cured 25 minutes at 153' C.) year's exposure neither the appearance of the stretched samples Compresnor the physical properties of the unstretched specimens show difsion Seta Yerzlev MFgneWooney at Resilieice ferences attributable to variation in metallic oxides used. Since mum Zinc Scorch 30% at 20% Oxide, Oxide Time, Deforma- DeformaAccelerator, longer aging periods than one year are generally required to Parts Parts' p i n u t e s tion, % tion, % Parts evaluate adequately the weather resistance of neoprene vul4 1 62 23 68 Permalux,, 0 . 3 5 4 2 56 24 67 canizates of this type, these tests are being continued. 4 4 27 23 68 Considering all of the factors involved there is little technical 4 8 20 24 67 reason for reducing the magnesia below 4 parts. Reduction in 1 2 23 25 69 2 2 35 23 68 the amount of zinc oxide below generally accepted standards does 4 2 56 24 67 8 2 71 66 64 not appear to be deleterious to aging characteristics as long as no a 70 hours at 100" C. ASTM Method B. less than 2 parts is used. However, those compounds requiring the ultimate in heat aging should be carefully Table IV. Effect of Metallic Oxide Concentration on Aging Characteristics evaluated if lower than normal amounts of zinc oxide are employed. Original, Cured 20 A ed 60 Days in 0% Aged 30 Days in Air Oven Minutes at 153O C. ' at 3OO%b./Sq. In. and 70° C. at 100' C. Effect of Metallic Oxide ConcentraMagTensile Elong. Shore Tensile Elong. Shore ' Tensile Elong. Shore nesium Zinc strength, at A strength, at A strength, at A tion in the Presence of Various Fillers. Oxide, Oxide, hardThe effect of metallic oxides in the Parts Parte sq.lb./ inch break, % hardne88 sq.lb./ inch break, % hardness sq.1bJ inch break, % ness 2000 370 67 2175 160 79 presence of Permalux has been dem3600 580 54 4 1 4 2 3475 680 54 1925 460 66 2800 240 76 onstrated in a standard test recipe 3400 590 55 1950 450 67 2675 250 76 4 4 I 8 3450 580 55 1875 440 69 2625 260 77 containing 29 parts of SRF carbon 1 2 3676 650 54 1500 500 59 2600 230 73 black. I n order t o determine whether 2650 260 73 2 2 3325 590 54 1825 500 64 or not similar effects would be ob1 2 3475 580 54 1925 460 66 2800 240 76 8 2 2900 610 55 2125 460 66 2600 200 78 served in the more heavily loaded compounds common to the rubber industry, differTable V. Effect of Metallic Oxide Concentration in the Presence of Varioucr Fillers ent magnesia-zinc oxide ratios have been evaluated in Base Compound practical stocks such as those Parte used in mechanical goods. The Neoprene T y e W 100.0 Phenyl-=-napEthylamine 2.0 reaults are shown in Table Stearic acid 0.6 &fagnesia As shown V. It will be observed that Filler As shown marked improvements in procProcess oil 10.0 Zina oxide As shown essing safety accompany the Permalux 0.7 reduction in zinc oxide conCured 20 Minutes at 153O C. centration in the presence of Stress at Elongaall the fillers tested. Mag200% Tensile tion Shore Compression nesium Zi.nc Scorch strain, stren th, at A Seta (Cured I n the presence of the v d Oxide, Oxide, Time lb./ Ib.7 break, hard25 Min. ous types of carbon black Parts Parts Min.' aq. in. sq. In. % ness at 153" C.), % Filler Parts high states of cure are de4 5 32 1750 &IT carbon black 150 270 75 16 1950 4 2 45 1825 270 , 76 16 2050 veloped with both zinc ox4 5 19 2750 2760 200 SRF carbon black 80 73 17 ide concentrations. The strong 4 2 33 2800 2800 200 74 17 retarding effect of clay on 4 5 17 2350 HAF carbon blaak 50 2900 270 69 25 4 2 23 2550 3300 280 72 22 Permalux accelerated Neo4 5 25 1275 H e ~ dclay 140 1500 400 69 95 prene Type W compounds is 4 2 38 1125 1375 420 66 98 illustrated by the low modua ASTM Method B, 70 hours at 100e C : lus values and poor resistance to compression set. Metallic oxide variations do not contribute markedly to the good aging characteristics of neoprene appear to improve this condition. vulcanizates. For this reason it has been customary to include a minimumof 4 partsof each metallicoxide per 100 parts of neoprene LITERATURE CITED in most compounds. Since the data PreViOUS~ypresented herein ( I ) Catton, N. L.,Fraser, D. F., and Forman, D. E.,nu Pont CO., have shown certain advantages for lower oxide concentrations, Rubber Chemicals Division, R e p t . 40-2, 11 (1940). particularly in the case of zinc oxide, tests on the effectof metallic (2) E. I. du Pont de Nemours & Go., Rubber Chemicals Divlslon, oxide variations on accelerated and natural aging have been BL R e p t . 237 (1950). (3) Forman, n. B., Radcliff, R. R., and Mayo, L. R., n u Pant Co., made. The data shown in Table IV, which were obtained using Rubber Chemicals Division, R e p t . 49-3, 8 (1949). Permalux accelerated compounds such as those in Figure 1, indi(4) Forman, D. B.l R. R.* and L. IND.IZNG. cate that a serious loss in age resistance is not encountered unless CHEM.,42,686 (1950). the concentration of either metallic oxide is reduced below the 2(5) Neal, A. M., Rubber Age, 67, 569 (1950). part level' At a concentration Of part Of zinc Oxide the aging (6) Starkweather, H. W., and Walker, H. W., IND.ENG.CHEM.,29, properties after exposure in the oxygen bomb and circulating air 872 (1937). oven are adversely affected. Reducing the magnesia concentraACCEPTED January 18, 1951. RECEIVED for review September 17, 1952. to part impairs the resistance to loss Of strength after Contribution No. 87, E. I. du Pant de Nemours & Co., Inc., Rubber Laboxygen pressure aging. oratory.

Table 111. Effect of Metallic Oxide Concentration o n Compression Set and Resilience

Radcliffp

April 1952

INDUSTRIAL AND ENGINEERING CHEMISTRY

R*i

833