Action of Organic Accelerators in Buna Y
George R.
S
Vila
Naugatuck Chemical rnvision, United States Rubber Company, Naugatuck, Conn.
*
.
f
. . .
he broad concl sion of this investigation is T t h a t the p o s s i b i h of &ng dithiocarbamates as primary accelerators should be given careful consideration becduse they offer a method for ovecroming slow curing, characteristic of Bum S, without resorting to the use of excessive acceleretor concentration which has been seen to affect tensile strength adversely and cause embrittlement. Low sulfur levels are indicated where maximum resistdnce to aging is essentidl. Nbrmal sulfur ratios of approximately 9 parts should be efficient and economical for the maiority of compounds in which l o w hysteresis is not essential. It is believed there i s sufficient evidence to justify a thorough evaluation of the possibility of resorting. to sullur concentrations somewhdt in excess of 9 per cent, with a corresponding reduction in acceleration, in order to achieve minimum hysteresis and minimum brittleness in the same compound.
UNA 8 is an unssturated polymer capable of undergoing vulcanization. This may be efIected by the application of heat in the p m c e of sulfur. The process is accelmated by certain organic bodies which are used widely for a similar purpose in natural rubber. F'reliminsry teak bave indicated that most of them are applicable to Buna 8 and appesr supdor to any other chemiosls yet investigated. This is fortunate in view of the prment emergency, aa p h t a for their manufmtwe already exist and the industry is familiar with their we. In spite of supw6ciaUy similsr de&, more careful study has indicated that it is an oJersimplificationto wume vruiouS dmtore will produce identical &e& in both polyinm. A wide lsnge of matmi& are now available to the industry, and the tssk of acouratdy evaluating all of them is complex. In eddition. an indefinite number of combinations theoretid y are possible. The p m t inve8tigation waa undertaken b determine what d e c t Wetant chemical types of ogsnio soedentors were likely to have on pmpestiss of f u n h t d import.nes
B
S and.Natural Rubber VUI.CANIZ~,COMPOUNDS.As a bsolr-
Comparison of Bun. PROpBRRlas .OF
ground it is nbceairy to q m i n e certain basic chare&&tica of the polymer undex investgation in order to determine the relative magnitude of thm pMpertiw likely to be modified by organic accelerators. In Table I Buna 8 and natural rubber are compared with this end in view. These data refer to tread-type compounds containing approximately 60 parts of carbon black and for the most part r e p m t valuae re ported in the literature. Table 1. Compuiron of Bun. Sand Natural Rubbr in Tnrd-Type Comwunds
B W8
a1100m
wo
It in evident that Buns 8 in poorer in tam resistsnce, heat buildup, and flexing, but superior in aging and reeietance to abrasive wear'. The ultimate tensile strength developed in a tresd-type stock IOWar tban that in n a t d rubber dthough the m a l character of the stresketrain owvea are eimilar. The low tensile strength of unloaded Buna 8 vulcanhtea is recognired aa a serious de6ciency for cerbin a p plications, and for this reason the p-t study will be confined to compounds containing appreciable lasdings of asrbon blaclr; this type of stock is considered to be of the wideat immediste inter&. The d u m in Table I for relative abraaion losses are undoubtedly wider than ditIerenws which are W y to be found under m i c e conditione. It seeme reasonable to conclude, however, from d t a already reported (6,") that Buna 8 treadsmay be expeDted to reeist abrasive wesr a t lesst aa wall if not better than narubk.
1270
k
* * * **
INDUSTRIAL A N D ENGINEERING CHEMISTRY
***
Val. 34, No. 11
There have beem codicting reports regarding the ability of Buns 8 to withstand dynamic k h g . Sebrell and bore reported d u e s (6)b d on laboratozy data which would indicate it to be relatively inferior. Stmet and Ebert ("), on
inrreaaea progressively with time of vulcanization, whewa that of natural rubber decreases after resching a maximum value. This phenomenon in natural rubber is referred to 88 reversionand is characteristi0 of long overcum. Buns S
the other hand, found it to be better if the pigmente were properly dispersed but stated that otherwise it WBB likely to be poorer. In thii hlmratory, reaulta from dynamic flexing testa on Buna S have been extremely erratic for the number of 0exurea required until the first crack appear. Thereafter, however, their rate of growth is extremely rapid and appears b be a reproducible phenomenon, which provides a hasie for wmp k g the &at of aceelerators. The low tear resistance of Buna 8 m y be a factor contributing to the rapid growth of c r a c k once they appear. For convenience in evaluation and disoussion, thm two propert hc rack on dynamic flexing and tear) will be ties ( g ~ ~ w of referred to collectively BB brittleness. Ram on Cum. The relative rates of cure exhibited by different accelerators are vitally important from the point of view of factory prooesSing and obviously must be considered on the same plane with properties produced in the W e d vulcanisata. Figure 1 oompares the curing rates of natural rubber and Buna S in tresd-type stocke which are identical in every reapect except for the bssic polymer. ,
does not revert in the manner characteristio of rubber but tends rather to become progressively s M e r with prolonged curinp. A similar phenomenon is characteristic of Buns 8 dcanisates upon accelerated aging. As will be eeen later, the rate of sMening has been found to be a more sensitiveaging index than the change in tensile strength which daressss at E reb tively slow rate. Thin is in marked w n t w t to vulcnnid ~ t u r s rubber l which tend8 to go to a d t h k y condition during the w l y stages of aging which in characteriaed by a decrease in modulus and tensile strength. An examination of the interior of samples which have blown out in the Schopper detrition test exhibit t h m characteristica 8trikingly. Buns 8 &em appear hard and dry, whereas natural rubber is d y soft and tacky. Smmna~OF Comarson. l h m published data (47,s) and from inspectionof Figure 1, it may be assumed that, in general, those characteristics in Buna 8,- cornpared with natural rubber, which are likely to &e& the evaluation of accelerators may be s t u n m a w 4 asfollows:
.
Banas In Equd or B u W r to Naturd Rubber u1: Ahion
Adw
Reversion on overcuring Tendency to wmmh during
umcsssins
Bunas Ia Likely to Be Infb rior to Naturd Rubber in: Dynamio &I proking Beat build-up
Brittleness Rata of k . tim
This table may m e 88 a guide, therefore, in investigating the action of organic accelerators. Inasmuch BB brittleness and heat build-up appear to be major de6ciencies in Buns 8,. the etlect of accelerators on t h e properties have been emphasized in the present investigation. Experimental Procedure VI 1000
ODULUS AT 400%
30
60
90
120
MINUTES CURE AT 292.F.
Figure 1. Comparison of Buna S and Natural Rubber Vulcanizater Under the ahove conditions n a t d rubber coma to ita optimum cure in approximately 30 minutes, w h e w Buns S requirea about 60 minutee. It slso vuloaniaes considerably more slowlv at 2 2 7 O to 239' F., which is an indication of greater pro-ksning safety. Them data show that, for practical purposes, Buns S is slower curing then natural rub&; con&u&tly it would s p pear advisable to investigate the possibility of overcoming thin either by using higher ratio8 of accelerators or more active t-, the limitig factors beihg proceasibility and adverse etieeta on quality in the vulcanisate. The slower rate of cure at prooesSing temperature is &set to mme extent by a tendency of Butla S to develop greater heat during mixing, calendering, and extruding operations. Evidence to date indicates, however, that the net d e e t will be in favop of Buna 9. which admits the wmibility of Using stronger cmtives. F~~VEESION. Figure 1illustrates an additionalfundamental difiarence b e t w m the heaic polymers which is I h l y to idumee the evaluation of acceleretore. The modulus of Buns 8 ~
Buns 8 is 8 new material in this nation, and procedures for testing it have not been standardized. Tberefore the following remarka on techniques followed in this laboratory in the present study undoubtedly will be of interest. It is well known that Buns S requires substantial loadings of a reinforcing pigment to develop physical properties comparable to those of ~ k vrubber; d hence i t is to be egpected that thorough pigment dispersion is of prime importance. It follows, therefore, that dii%rences in the relstive degm of pigment dispersion are likely to influence resulte obtained when evaluating other compounding impedient& &nee quently it is n~ce8881yto maintain rigid control of processing pmcedurea if reproducible results are to be achieved. -ma. The following prinuplea in re to *and cunng were d e r e d to after a t h o m a s t u e ! T o ? p d u m : Comwunds were mixed on an open mdl mth imtd Au hcchm were mu temperatuk conslant for eaeh mixed aoeordin to a rigid time rhedule; mmpounding io(ppdimta were addefin the -e order and at the m e rate each time. Specimens were vulcanized in'a pma as soon as possible after mixing; sll c u w were d e d out at 292' F. -car. P~~PEETIES OF Vacwmm. Modulus, tenrile atren@h, and elongation at break'were determined on a Bmtt machme acooldmg to A. 8.T. M. pmcedures. HBATB ~ L D - OAND P HmThem c h t m i s t i w wen, detarmined by the torsional bystmenis teat (4). with a Goodrich Flexometer J) operated at a load of bS.6 pound8 and a ntmke of 0.126 inch. $he torsional hysteresis test was quicker to p r f o m but the W n c h Flexometer ap to be more aenaitiva Dmaurc F-a. Ordinary e i c flexingtesta gavemati0 d t a which yere not re nnluci&e. .It wsd pceaible, however to obtain .connutent by followmg the rate. of mwth whch wen, preaent initially 88 maLs, one mch apart. i
&
~
Nwembq, 1942
f
*
INDUSTRIAL A N D ENGINEERING CHEMISTRY
******
lPl
Compounds were cured over a raoge of time at 292' F., but only thoee valuea which represent the maximum cure for each acoelerator variation are reported. A blauk wntsining no accelerator in included. The reduction in curing time resulting from the LONGWION addition of small amounte of accelerator i striking evidence of their economic value in manufaduring operations. The data reported TENIlll STRENGTW in Table I1 are shown g r a p h i d y in Figure 2. Increasing ratios of accelerator increase moduli and, in general, decrease bred& elongation& For each accelerator there is a specific wncentration range in which maximum tensile strength is developed. Aa this range is exceeded, tenaile strength becomes proBreB8iyey lower, accompanied by a w m sponding decrease in breaking elongation and PARTS OF ACCELERATOR increase in modulus. This is not an unexpected result. The ntrdtion on b t e of Cure and rapidity with which physical pmpertiea. d s cream when the optimum concentration of accelerator is e x d e d , however, appears to place a limitation on decreasing cu&g timea by resorting to excessive accelerator ratios. The data indicate that better phyEical properties are likely to result from using a more active type and staying witbin the Wta of ita optimum concentration. The data &ow a more drastic decrease in phyEiosl properties for the higher concentrations of the sulfamine and TMTMS than for MBT, because the two former materials are stronger on an equal weight basis; it is believed that higher concentrationn of the latter would result in a c o m p BooBoaero cI3ylA-a. It ha? beell Bt.%n-+rd able falling of€of tenaile &en&. in many laboratmea to eatmate s w m l q edenuea o natural Examination reveals no Biepifmt diiTerenoea between the rubber by the dative time required for maplent vulcsniration maximum tensile strength developed by eauh accelerator in a pram at a2o-F. Buna 8 compound8 resUirea several hours to show evidence of curing at thin tempershue. Consequent , at ita optimum wnmtration. There doea appear to be a dkthe teat waa aacelerated by inCreaaing the temperature to 239' tind diiTmce, bowever, in the modulus valuea between the This cbank!e did not alter the dative ratiuea of several accdem MBT and TMTMS stocks. This relation is brought out in tors inv€atipatea. Figure 3, where physical pmpertiea for two compounda 80A ~ ~ T Affmff E DTEm. The usual testa were rarried out in standard eqdlprnent. Buns 8 compoondn are much more re d e r a t e d with 1.5 per cent MBT and 0.35 per cent T M T M B , sistant to accelersted aginp than natural rubber. The inrespectively, are plottea aa a function of time of vul&ation. in modulus or deure4.w in dongation on pflonpl erpowre to air The MBT compound comes to ita optimum pmpertiea at 21P F. waa f q d to be the most eenatave mdex. more gradually and, being more persistent, eventually d e b B n s I O N !ham. where l l U l , . h h r y k t 8 showed no WUsinkllt difierencea between vanow d e r a t o r s : aonaeauentlv no d u e a are reported. lbtwFo~)rans.Alldatainthepresantinvedigationwere Table II. Meet on Phyriui Roprtlr of Varying Anoumb of obtained from a base compound of the following wmponition: Asseleraton
b
F
1.
-
-
100
60
a
6 6
V-bla Vdbh
The Buna 8 used WBLI a wpolymer of butadiene and styrene prepsredby an emubi6mtionprooern. The above compound WBL~&wen as a type which is W y to be of wide immdate inbeat. Strictly spaking, all d t a r e p o d here apply d y to thia compound and to the partimrlar polymer utilized in tbk dudy. Effeci 01 Amount of Sulfur COvPOmpDe COmWNING 2 h l W % OF 8ULTWE. Table II shows the &e& on physical pmpertiea of varying anmunta of mcaoaptobenrotlusro ' le (MBT), c y d o h e x y l ~ t h i a s ydl
amine (sulfamine), and t&cam&ylthiuram mowul6de (TlKTMS) ippompounds wnteiniagZpa& of8ulfur. They were chosen as typiad mpmentativea of the thiasole, dfunioe,and dithiocarkunateb.
0 0.76 0.86 1.00 1.60 3.00
Cyolohaqrlbonmthirryl Sulfamine 800 800 a m BO wo 3600 60 44l 80
20
1010 1100 1400 1660
3sBo 8My)
aaoo
1000
680
MI0 660
646 860 816
.
-
In2
* * * * * .k
INDUSTRIAL AND PNGINLPRING CHEMISTRY
wlop a higher modulus which approach an asgmptote. The TMTMS compound, by contrsst, rises to ita medmum pIoperties rather suddenly, starting later but coming to a full aure 8ooner. The modulw $so appear6 to remain conatant at a lower lewl once the maximum value has developed. The lower moduli am alno d e c t e d in higher breaking elongatiom throughout the curing range.
-M-M-:-)TMTMS a3
400 2 Y
x
200
**
VOr. 34, No. 11
The former may be described an &g more &tent and thus developing higher ultimate moduli and lower braaking elongatiom. The signi6mce of them obsm'ations in relation to other properties will be discussed later. The d a m i n e n and thiazolinea follow pattern similar to the thisnolesin the development of .phyxicsl propertiea 88 vulcanization progresses. The sulfamines lag mmewhat during the early stapes, and this h apparently re0ected in greater proceasing eafety. An outatanding feature of the t h i a ~ l i n e e in their tendency to develop higher moduli than any other Single accelerator which has been investigated by this labomtory. Table III lists the optimum ration found for sccelerators LY juaged by the development of maximum tensile stmngth. These data are reprcduced in order to eatablish the relative d v i t y of materials which are available commercially. Strictly speaking, the results am applicable only to the compound in which they were investigated. It in believed however, that the same relative ratings ahodd apply generslly. TMTMS
30 60 90 120 IM MINUTES CURE AT 292'F.
Fi ure 3. Comparative Effecb of dercaptobenzothidzole and Tetrdmethylthiuram Monosulfide on Physical Properties in Bund
s
It is intereating to nota that neither compound exhiits any tendency toward reveraion, and both appear to haw equally b r a d curing range%. In fact, the nonpersistent quality exhibited by the TMTMS wlcanizate might be interpreted an evidence of a comparatively broader curing range. This is in shsrp contrast to the performance of them two mb terials in natursl rubber where &thioc=wbamah usually exhibit narrow curing rangesin compound8 containing nmmd d u r ration. The foregoing cornpariaon deacribea a pattern which all thissolea and dithiocarbamates appear to follow in Buna 8. 1.M. 111. Relative Activity d Aeceluaton In BUM S bnbinlng 50 Part, by Weight d a R e i n k i n s G b n Black and P Part, d Sulfur
90
-Q20 0,275 PZTS OFACCELERATOR
0.3
Figure 4. Effect of Increasing Amounts of Accelera. tor dnd Time of Cure on Heat Build-up
The above ratios are approximate, and w l w h t i o n rote may be further adjusted by varying the concentration, within limits, of any particular accelerator. Care must be talwn, however, not to overacceleratebecause of a deleterioua pdiect on physical proprtiea. Some of the dithiocarbamates are more active than any membra of any other group, and the monosullide exhibits unusual eafety an shown by ita extremely low cureburn ratio. The &c d t of dimethyl dithiocarbamate h alno very active but likely to be relatively more scorchy at prcceshg tempersturea.
D. 0. T.G.
1.6 1.6 1.0
40 46
40
1.m 1.00 1.m
1.6
40
0.k
1.0
40
0.m
1.16
46
1.00
0.86
50
0.a
om
MI
0.m
0.40
a0
I .a
1.15
16
0.-
l.M)
Bo
1.60
90
1.0
76
1.9
The ddehydeamine and guanidine are comparatively weak and exhibit unfavorable cure-burn ratios. The low pH of the carbon black in the batch h undoubtedly a factor contributing to their low order of activity. This &odd not praclude further consideration for them in nonhlack compounds. In fact,there is evidence that some ddehyasamiaea may develop bigher tensile strengths than other types in compounds loaded w i t h sinc oxide or a h e particle siae whiting. Because of their relative inactivity in the bsae compound under inveatigation, they w i l l not be considered further in the pressnt study. INTEWCEDUT. ANXI Low saw^. Table IV show the phyxicsl properties readting from variow ration of MBT and TMTMS in the presemce of 1 per cent and 0.6 per cent of sulfur, rwpectively. The thiuram is more e5cient than the thiprole at low sulfur ratio& The ditieremcss bscome p m
-
These dab show a d u r content of approximately 2 parts to give the maximum temsile strength, although it is doubtful if these ditIeremces BIY) resl. As sulfur content is increased, curing range becomes progmnively m w e r as judged by the decrease in elongation at b d over a of nue%. It is interesting to note that no significant change takes p b in the time required toreach the maximum CUIB as j n d d by
Table IV. u.c( d Int.nmdi.1. and Low Sulfur Conknb on Phvtiul Pfowliir
one Put Euuur
physical properties. Aging
0.6 Put suuur
greater as the sulfur concentration decwases. It in ale0 evident that Buna 9 is still SemSitiva to exceea accelerator ratios even at low sulfur levels. V ~ C - A ~ O N m o m S m . Vulcsnisation in the absence of sulfur may be &ieoted by tatramethylthiuram disulfide. The prooess is analogous to the action of this msterial in natural rubber. A tensile strength (Table V) in exof 3oM) wunda per wnam inch may be achieved bv wing's conmiration i f 3 &a per 100 parts of poly^&; the time reqwed for vul&tion at 282' F. is approxia ~similar to those obmately 30 minutea. The p r o p ~ ~ &me tgineed with 0.5 part d u r and 2 psrts tetramethylthiuram @vely
monwulfide (Table IV).
-
P
Table
V. V u l u n l d i o n d Buna S d t h T.(r.m.(hylthlurm wlRde without Sulfur
Pub Td-
?e%'
DWhde 3
DI-
Lb. pr 80 In. Min. to Cure .t292'
I6 30
so
90
iao 16 80
6
Bo 90 120
-
F.
g=
M d u l l u et
W X laso I760
lSa0 IS80 1WO
.. ...... ..
Elytbn.
a m
733
3170 3180 3200 3wo
683
2180 2170 2 m
470 m 403
2L)80
1930
670
838 620
380
370
*
Concentrations of tetramethylthiuram disulfide in exceea of 3 parts pmduce progressively lower tensiles and exhibit the eame trend typical of overaccelerationin compounda containing 2 psrts of sulfur.
Effed of Sulfur-Accelerator Ratio on Physical Properties The detemmation of ,,hySical properties at differentiuifur content8 is complicated by the ratio of scceleratorpreaent. Thin must be adjusted for eacb sulfur level, otherwise an unbalanced compound will result. Table VI
.
ellom the t e d e Btrengths and elongations obtained a i various sullur levels for a sen- of bslsnoed compounda accelerated with appropriate quanti& of
TMTMB.
~
Table
EQQECE OP Av.xl5BAToRe. Typid result8 obtsined With a thiazole and two dithiocarbamates upon accelerated aging in an oxygen bomb and an air oven at 212' F.are recorded in Table VII. These data indicate tbat the dithiocarbamate resist aging somewhat bettar during $he early dages, wherean the thiazole appears to overcome these d8emce.q during the longer aging periods. In either instance the variations are comparatively small, and it 8 8 ~ m sreasonable to BBBume that no appreciable diflerences exist between the two types. Table VI. Variation in Phvtical Ropntln d t h Sulfur-Ac~lerato~ Retia sulfur. put. 0.6 1.16 2.0 a.o
"'"
Aderator. m t . .topbum%%!!!ptiitm-
Mia. to Optimum
Ib./m.in.
2.0 0.6 0.36 O.%I6 3200 3180 8470 31W 760 720 ello €4 30 30 SO 30
The aging oharact=ristics of dl the sccelerstore listed in Table 111were investigated, and no consistent or apparentlp aigni6cant Wmnce.q were found. It is concluded that a+ c e l e r a t o r s are not controlling in the agingofBunaSvuldes&, and ut for practical pnrposm t i i s factor may be n e g l e c t e d when selecting suitable types. B w S resist% oxidation better than natural rubher, and 9 the result8 obtained .o 20 areingoodagreement with those prev i o d y reported in 1 2 3 4 the literature (7). It PARTS OF SULFUR hsa been OM, however, that underFigure 5. Effect of Increasing c u d stooka do not Amounts of Sulfur on Hysteresis resist oxidation well. at Maximum Cures ~~
VII. M e s t d Different Acnln.(y oF-Fying d Bun. S V u l u n l u t u Conblnlng L r a m or JUI~UI.
-az
-24
-27 48 06 I44
I92 pH)
O v g m Bomb st 70. C. (168. -1.9 -11 -28 -a6 -38 -a8
-a5 -24
-16 -10
-39 -40
-3a -48
-31
-29
P.).nd
--2322P o d 300
-as
-16 -a4
-14 -12 -24
-11 4a -6e -61
-ai
si h e n at ala. F. -ao -ai
-41 -48
-a#
-83
-68
-
-31
-44
-m -66
-4.a
--w
-68 76
-
-74
1 4
******
INDUSTRIAL AND ENGINEERING CHEMISTRY
***
Vd. 34, N a 11
minimum heat build-up, although tensile properties would have decked in the m a m a charactexistic of overaccelerated compounds as sbown in Table II. E m OF S a p w CONC~WRUTION.Figure 5 shows the &e& of sulfur on heat build-up characteristics although them reaulta are reported as torsional bystemis. They are e x p d 88 K , the logarithmic decrement. Essentially this in a measure of the rate of damping by a rubber specimen which has been ddected in a suspended position by the application of a torque. Lower values of R correapond to lower hystewis. The testa were performed at 280' F., and a value of 0.20 corresponds rougbly to a AT of 100' F.in a Goodrich Flexometer. It has the advmutage of heing quick and usually is more satisfactory for compounds likely to exhibit very high heat build-up where the Goodrich FIexomeh might be too drmtic and c a w blowouta. The accelerator used with the various sulfur ratios for the compounds whose hysteresis values are charted in Figure 5 w88 tetramethylthiuram monosulfide. The quantity was witbin the optimum range for each sulfur level; the exact concentrationsand hysteresis values are shown in Table VIII. 05
2
I
-
3
PARTS OF SULFUR
Figure 6.
Relative Influence of Sulfur Content. and Actelerdtion on Hysteresis
_pc
Table VIII. Vnhtton in Hy~terealrwith Sulfur Content Put.of sulfur
P u t . of TMTMS
0.s
0.W
E!
OB
sawB
tsininn lower concentrations of d u r age relatively better. The &tion is comparable to the rea& o t x e r d (I) in natural rubber, although the VaristiOns are considerably d e r in magnitude.
4.0
Tabla IX.
1.0
EFFEm OF A C C E ~ T O RFigure S. 4 reveals the deet of cure and accelerator concentration on heat build-up. The resulk were obtained with a Goodrich Flexometer and are exp d as the maximum tempersture rise (AT)in ' F. at equilibrium. AU ssmples were brought to 100' F. prior to st91.ting the test. Samplea were built up from pieces died out of ten8ile slabs. Figure 4 shows that AT decresaes with progressive curing timea and reaches a practical minimum at a state of v u l d nation which comapads roughly to the one which gave maximum physical propertiea 88 indicated in Table 111. It is also evident from Figure 4 that AT becomes progressiVely lower with increasing concentrations of accelerator. Tbk would indicate that, other things heing equal, a vulcani&ate' containing an excassive amount of accelerator would exbibit
i:iE
2.0
8.0
V.riatlon in Tonlonal H stem11 with Acceleration at D i h n n t Sulfur L n t n t s
TMTMS MBT MBT MBT $!$:Ma NO-
MBT MBT Sull*mina MET MBT
2.0 2.0 1.0 2.0
a.aw
!$} _._, 1.0
o.aro o.am o.aza
2.0 2.0
a.so8
a.o 1.0
0.288 0.268 0.248
TMTMS
0.35% TMTMS
c t3 90
30
MINUTES CURE AT 292.P
60
90
Figure 7. Effect of Time of Cure dnd Increasing Amounts of Accelerator on Brittleness
NWmJ
JA" JA"
JAW
E.&-
.
0.212 0.197
o m
It in evident that hysteresis,and presumsbly heat build-up, decreaees perceptibly with incrensing d u r . The question srises as to the relative &e& on hysteresis of the quantity and type of accelerator at Merent sulfur levels.' As Table IX indicates, sulfur concentration is the controlling factor for hysteresis even in the prrsence of excessive concentratim of acceleration. It is interesting to nota that a compound containing 2 parta of sulfur and no accelera gave a hyderesk value at ita optimum cure (10 h p m ) w ch f ewthinthe upper range of the values reportea for that particular d f u r
%
0.4.0X
-
I
0.s
Heat Build-up and Hystenrir
.
Cun
Z.2-g
i:a
I."
prolonged overturing, on the other band, does not appear to have 811 adverse &e&.
at %We timum F.)
0.m 0.267
2.0
1.0 3 :
Emm
X
~
N,-
***
1942
* * ***It
INDUSTRIAL A N D ENGINEERING CHEMISTRY
12m
Brittleness pa up as the sulfur content i n m e s . The magnitude is not excessive, however, at the meelerator ration employed, which suggnb that the sulfur level is not the controlling factor in brittleness as it wan for hyrheresis. Table X givw the brittlmeas ratineg for a number of compounds containing wrious acesleratorn and sulfur concentrstiom. The redation between the actual accelerator content and the optimum concentration for that aceelerator at the indicated sulfur content is also shown. For the sslre of brevity this relation will be r d e r d to a~ the A factor and may be.de6ned as follows:
10 -
9E m m 7-
where C
e., LOW
I
Relative Influence of A Factor and Sulfur Content on Brittleness
that hynterwia ten& to rise Sometimes upon prolonged ing which may explain this anomaly. It is Meved tbat the quantity of accelerator is liMy to be more critical than the in determining heat build-up characteristics, dth& when Eompared at their optimum ratios at 2 prta of sulfur, the thiazoles generally a p p r to give lower values than the dithiocarhsmateu (Figure 4). This might be expected from the lower modulus exhibited by the dithiocarbamate as noted in Figure 3. Such diiTerencea as may exist, however, might be eradicated by an increase in sulfur contenk The development of low heat build-up by adding e x w i v e acceleration bas the disadvantages of producing low tensile strength and of causing undue embrittleme& Brittlencu E m m ow Acmmumw. ‘%Brittleness” is the term choaen to designate the tendenoy for nicks to grow and eventually to crack thmugh, under dynamio flexing. Samples were rated fmm 0 to 10, dter M),oM) fleaurn, according to the sire ofa crack presemt initidly 88 a nick in the pmve of 8 & X i i eample. The sngle of aexure in dl c88BB was 4 5 o a n d the sped, appmximately 260 cydr, per minute. Figure 7 &OWE the brittlam fm three d8-t WWItmtions of blBT and TMTMS over a rsngeofaures. It is elem that inmaaim the concentration of accelerator resulta in progressively hiej;, brittleness. Brittleness also increaam with time of cue, although this is lasa evident for thethiuram whioh ghibitm a 0at cluing CIWB. E m m ow 8C o r n . Sulfur levels were vsried in compounds containing ac&atom held at a oonstant Level or within the range of optimwn conmtrstion; this w a nefpb ~ aarp inordertoeegregataw mu& aepoeeible the&& of the a c h t o r . Tbe Variation in brittlene6n was as follows:
C
ooncentratioo of accelerator or ~eoelerstorsin a
compolind oonoentration of aderator or aderator cambination wbich develops max. tensile s w q t b in the compound
and overaO&ation, respectivdy.
9. Schematic Relation beHpteresisand Brittlenessfor s Values of A Factor dnd It is e.vident that brittlema inmasea progreeaively with inmeasing rsnges of the A factor. Any &tion hetween dfur content and brittlenm appears to be rmreked by the concatration of accelesator e r p r e c s e d w the A factor. me data fmm Table X 1v8chsrtdd on Figure 8 which may be looked upon 88 the counterpart of Figure 6. The A factor
Table X. W I t I n a and RaIdve Sulhrr and AmlaBrit* P = .#
PuadSollw
Pvt.dTMTM
Wtthushthu
ox.1
0.8s 0.80 0.90
2.6 a.1 O.P
4.6 4.3 0.a
The first two wiatirma give a direct compsrison of the &e& of sulfur when the acoeleration is kept wnhnt. It was neewary to reduca the TMTMS content in the lnst asm, otherwise the compound d d have been ovemcelernted. Results ara recordad for the msdmum OUIB inaachinstencs.
- r,
Tbua A factora of less than unity, equal to unity, or greater than unity denote underacceleration, n o d acoelerstion.
8.1 8.1
2.00 4.00
-
Afaotor
8.1
s.a
6.S 6.0 6.8
10 10
AFWtU k t L . 1
kthnl
23::
EWdtol EplvltO1 EWdtol Eslultol 1 to 1 1 1 to 1,s 1 to 1,s Onr 1.6 over 1.6
Pvt.d-tOZ TMTm MBT TMTMB TMTMS TMTlta MBT TMTM
Put. d 0Uuw
OJb
1
0.60 2.00
0370
9.00
TMTMS TMTMB MBT
2.W 0.1.60 0.200 1.00 0.m 0.40 I.W
Roll*& TMTMB
1.00
BollViU
Conk.(
1.6
0.60 2.00
a.00
4%
8.00 2.00 P.00 s.00 1.00
1W
******
INDUSTRIAL A N D ENGINEERING CHEMISTRY
imposes d a t e maximum and minimum limits on brittle ness which apparently cannot lp exceeded by variation in the
sulfur content. It seems reasonable to conclude, thedore, that the A factor, rather than d u r content, is the controlling factor in emhrittlement. It is interesting to recall that the antithesis of this relation was found to apply for bystereSia where relative sulfur content, rather than 8Cderatirn wra the controlling element.
It is clear fromthe foregoing Seotions that certain fsctors which tend to decrease hysteresis rue slso Likely to increase brittleness. For example, increasing sulfur ratios, time of sure, and accelerator concentration all tend within limits to decrease hysteresisbut with a simultaneous increase in brittle ness. This relation is shown in Figure 9, where relative values
V O 1 3 4 N o . 11
1. Otherthingsbeingeq~,hi~@ersulhvmn~tntionisthe mntrollig faetor in ppdu rrrrmmum hyaerosls. 2. Other thinm bang minimum accelerator mc?ntration is Wle eontrolling factor in pmdueing low brittlenee+ a. Thecornbiastion of the two contmbg factors-I. e., hi& d u r and low deration-m the same compound appears to produce the mast d c i e n t compromise between them propatips.
-3,
The uea of higher d u r and lower accelerator ratios tends to lenetben out curinr. cycles, and the relatively slow vulcanisation rate inherent to Buna S is
__
I
Relation between Hysteruir and Brittleness
***
alresdy considered
to be a
.24
edous
drawhack. It is g.22 pro& that this m i g h t be miniY mieed by using 6.20 0 w more active typed are plotted yhich might result from: (a) an overaccelerated "- .28 ofaccelerator. It I compound wth a high sulfur content, (b) an underaccelerated has been shown z compound with a low sulfur content, (e) a compound conthat certain mem,26 taining low sulfur and too much accelerat~r,(d) a compound bers of t h e dia 0 which repreaenta a comprnmiae consisting of n o d sulfur thiocarbamate d 24 snd aceelemtor ratios, and (e) one containing high sulfur and group are among e low accelerator, which presumably would give the begt r e the most active 22 sults under service conditions requiring low hysteresis and materials availLo t low brittleness in the m e compound. Actually the present able. Some memdata may be -bled on such a dwm. b m also exhibit 50 Figure 10 charte the relative hysteresis and brittleness for extreme proc8?athree rutios of MBT in compounds containing 2 parte of sul- ingsafety. Tbere Figure 10 (Above). Effect of Infur. AB the accelerator conmtration incresses the hysteresis is also the possicreasing Amounts of Accelerator is seen to decresse with a concomitant rise in brittleness. A bility of w-accelon Hysteresis and Brittleness senrice test would be required to pick the p r o p e r ratio in order eration, in which to determine how much hysteresis could be deedin order t h e dithiocarFigure 11 (Below). Relative Into achieve less brittleness or vice vem. bamate pradodfluence of A Factor and Sulfur The upper line of Figure 11 repments an attempt to renates, in order to Content on Hysteresis and Brittleduce hysteresis by using n high concentration of an active 80 reduce vulcaniaaness Eclerabr. If the ensuing brittleness csn be absorbed, this a p tion timed. pars to be one solution to the problem; otherwise a comAdmittedly SBVpromise must he effectedor another solution eought. era1 factorsremain to be evaluated for compounds containing Data in Table X indicated that a high A factor (exwaive higher sulfur and lower ratios of a dithioesrbte-type accelacceleration) WBB the primary factor in inducing brittleness, erator-for example, resistance to abrasive wear, blooming whereas its &ect on hysteresis WBB distinctly secondary to tendencies, and an exact appraisal of the ultimate &ect of t h e that exerted by the sulfur concentration. Conversely, higher poorer aging which is certain to result from higher sulfur ratios. sulfur contents, while capable of inducing low hysteresis, The foregoing remarks apply essentially to compounds were not accompanied by exwpsive brittleness if the accelerawhich must exhibit minimum hysteresis. Resmblyallrubtor content WBB kept within ita optimum range of concentm her pmducta should be as free BB possible from brittleness, hut tion. low hysteresis is not always eSaential. For this Latter claes of The two lower curve9 of Figure 11show the resulte obtained compounds the d i t h i o c a r b t e s der the advantage of prowith compounds contninhg 3 and 4 pnrts of sulfur, respecviding maximum v u l d t i o n ratea without resorting t m tively, and acceleration within the range of optimum concenexcesBive accelerator concentrations. Their lack of persi%t. tration for those sulfur levels. These compounds, containence reeulting in a broader curing ran@ may be a d d r a b l e ing the higher sulfur rntios with compondingly lower 4- feature. eration, have produced low hysteresis without the develop For those compounds whichqnire &um aging chanroment of exwpsive brittleness. W i w above all else, low sulfurratios obviously will be used. It has been shown that the ditbiocarbtea are more active than the thissoles for this 6ype of compound, and it hss a b Dimmion been established that they produce higher p h p i d pro@= (Table IV). Several references in the litemture (S,& 0 ) have indicate3 that Buna 8 cslls for somewhat lower mlfw and higher 80. celerator ratios than natural rubber. Two per cent of sulfur Literature Cited has been suggested ea approximstdy normal. During the course of this inveatigation an analpin of mme of the fundamental relations involved points to the conclusion that a shiftin the direction of higher sulfur and lower acceleratar ratios may be a more promising appmsoh to the problem of &ecting the most dcient compromiae betweem hysteresis and b r i t t l m . This is supporbd by the following observstim:
2
(0
-
'
