514
INDUSTRIAL AND ENGINEERING CHEMISTRY
figures reveals a nunilwr of intercssting relations: (1) The straight line character of the duromcxter hardness, elongation, and 1\Ioonciy value C U ~ V C ' Hindicatils that these propc1rtit.s may be governid t i ? thtx same factors. 12, The modulus and the dynamic coiiipresare similar in shapr and approach a straight line relation. (3) Tht. T-50 curvcs shon- the effect of compounding ingredients upon this determination; the conflict with the compression set data indicates that the T-50 value is not aln-ays a true nieasurc of thil state of cure of a compound but rather a comhination nieasurtmient of state of cure and of the relative amount of cwmpountling ingredients which undergo hardening or stiffening as a result of exposure to the low temperature of the test. ( 4 1 T h e compression set d a t a come closest t o indicating the actual rate and state of cure since they show quantitatively the activating c+fect of furnace black upon the rate of cure. The activating cffect of inereasid thick and softener quantities, shown by the, roniplrtt~rtwss-strain and hystcresis propc,rtirxs, is evidfxnt only at the shortvr curing tinics and, consequently, cannot be seen i n t h t w curves which us(' the 75-minute curt' values. T h e comprcw sion set data do shox t h a t the increased black loading results in a hig1ir.r state of curt' whereas the increased softener results i r i a lower state of cure. ( 5 ) The heat build-up tends t o inrreaw rapidly as the black quantity is inrreased and is relatively unaffvctcvi by the softener loading, whereas the resilit,nce decreases as hot11 the hlack and softener quantities are increased. The fart that the softener quantity has little effect upon thrs hear huild-up and considerable effect upon the resilience is particularly noteworthy. This phenomenon suggests that the black and softi,ner exercise a damping effect, upon the resilient rubber molecule, and the hysteresis of the rubber is affected by the impact velocity and the frequency with which such impacts occur. This partially explains the fact t h a t resilience values determined a t low frequencies do not always correlate with heat build-up values determined at higher frequencies. (6) The abrasion 103s curves show hoiv critically this propert>- is affectcd hy thcx
Vol. 39, No. 4
L T I L I T Y 01.' ( : I - R V E S
r-standing nix,(i for a rcyitlily it( hensive JOUI'CC of compounding rcscipcx Indiv the corresponding physical propt:rtiw art' vcilutninou* 1111t irtmquentlj- fail t o offer the completcl infoini:ltion d c ~ ~ i r ~ , ~I tt l I,S felt that the method of data prosc~ntationi n this ~ I L J I O ~I,. I 1 ) : ~ ticularly advantageous friini the standpoint of cornplcat ('JN brevity and that the system may be rctadily c~stontl(~d 11y i~c~-c,vduation in a similar manner at tlifferent acrc~lr~ratioli I t ~ \ o I - . I.'t~om these curves it is possible to srlwt the. twut 1)lack-softc~rir~r c'olntiin:ttioti.* t o uw for certain application, \r.Iwi,e thct Y I i c q S i f i c . iiropt,t,t icas d w i r r d have b w n previously spt~cifietl. .A typicd esarnple of holy these curvw ('an 1)c u-cvl in ( Y I I I I poutiding niay be cited. -4 tlcsircd conijiountl must n i t v s t t h n follon.ing specifications: h w t liuild-up of not nior~bthan 85" F,, minimum tc~nsilt~ stwngth of 2400 pound.: i x ~ rs:iu:ir,', iiicll, niiiliti:irtlitcw if : I ~ ~ I ~ < I Y I niuni chig:lttion o f 450(';, arid rluromc~tc~r~ niatcly 60 unite. Thih hcat tiuiltl-up limits the black loadiilg t o ahout 50 1'IlI< (parts p i ' i ' 100 of rubher), and the elongation limits t:itx >oftc.ric>r loading to 10 PHIt a t tho 50 black loading. .4t thew- 1)lacL atitl softenrr levels the tensile strength is 2500 pounds p t ~ rsquare' inch, the durometer hardness 59, the elongation 450Tc, and thr heat build-up 84" F. The 0tht.r p r o p d r s may lie rmdily dctermined from the appropriatc graphs. It is obvious that, if the, specifications arcs not v ~ r ytlxarting, a large number of hlacksoftcner (.omtiinations may tw siJlwtcd which will mwt the requircnient s.
Device for Evaluating Surface
Cracking of GR-S 31. C. THRODAHL .Ifonsunto Chemicul Conipuny, .Vitro,
A dynamic flexing device for studying the cracking characteristics of GR-S vulcanizates is shown and described. Specimens cracking during dynamic flexing while exposed to ultraiiolet light illustrate the effecti\eness of certain organic compounds in minimizing this deficiency of GR-S iulcanizates. It was observed that 0.25-1.0570 concentrations of certain ketone-amine condensations minimized greatly the tendency to crack at the surface. Correlation of results of exposure to summer sunshine with exposure to the artificially generated ultraviolet light is shown. The flexing mechanism is based on the premise that a dynamicall) stretched rubber surface increases the probability of formation of cracks and increases the rate of growth of those already formed. 4 double turntable fixed to a double cam provides angular motion which ensures uniformity of radiation and also causes linear di9placement, w hich is necessary to stretch the surface of the specimens. Ordinary dumbbell tensile speciniens are stretched oier specially designed rods, arranged concenrrically near the outer diameter of the turntable.
W'. P'u.
THE
scope of this paper is limited t o :t study o f ~ I I Prurfiicc. cracking on a GR-S vu1e:mizate dyn:rniic:illy stretched a t periodic intervals. .ibundant literature ( 2 , 4,6 , 7 ) on experimental techniques and interpretation of the mechanism of surface cr:tcking indicates marked variance of opinion, liut most :ruthor< agree t h a t t h e prwence of ozone is necessary t o hurface cracking. However, Killiams (9) showed t h a t ultr:tviolet light activated f'ormntion of a skin on the surface of stretchid rubber which pi,otected t h e interior from continued crack g i ~ ~ ~ t111h . thc eva1u:rtion of material- designed to niininiize 01' retard thr m t e of crnck growth, the .it:ttie .A.S.T.lI. t m t ( 2 ) iiitlic:ited that t1io.e bhoTving blooming tendencies apparently provided ii surfacia film, n-hich in turn inhibited detrt~iomtionbeneath t!ir s x f a c e . .llthough it has apparently been estatilished tti:it ultraviolet light itself does not cause surface cracking ( 6 ) , GR-Svulcariizates exposed t o periodic surface changes under strain crack h d l y iri the presence of i t , Although this mechimism is not thorouglily UIIder,>tood, it wiiq thought worthy of further investigation, erpecinlly in view of the fact t h a t most studies on surface cr:icIiing \yere made on .st:itici11Iy ~ x p o s r dspecimens.
April 1947
INDUSTRIAL AND ENGINElERING CHEMISTRY
515
CONCENTRIC RING SPEC1MEN 1r.1 HOLDER
1-1 II 1 1
-a& STIRRUP ,, ,
---------
;-r -----------I
CAM
SECTION A-A
CONCENTRIC
R ING 1 EN
IER
Figure 2.
Figure 1.
Dynamic Flexing Apparatus, with Turntable (below) Showing Specimens in Place
FLEXING DEVICE
The flexing mechanism shown in Figures 1, 2, and 3 is designed l r ~ the i assumption that a dynamically stretched surface increases the probability of crack formation and increases the rate of growth of those already formed. -idouble turntable fixed to a rlouble cam provides angular motion nhich ensures uniformity oi' radiation; it also causes linear displacement, Ivhich is neces- : ~ r yto stretch the surface of the specimens. Ordinary dum1)hell tpnsile specimens (I) are stretched over specially designed pins :md stirrups (Figure 3) arranged concentricully (Figure 2) near the outer diamet,er of a turntable consisting of two 17-inch diami,ter circular plates. The top turntable rotates in a fixed plane. .I study of the variables of the flexing device involved the f o l h v i n g factors: angular velocity of turntable, linear diqplacemexit or per cent strain of specimen, intensity of radi:ition, and time of exposure. Angular velocity was arbitrarily selected at 24 revolutionr per nririute because its function was only t o provide conqtant rat? of
Schematic Diagrams of Flexing Mechanism
change of pohitioii to alloiv each specimen >I constnnt exposure. Reduction in angular velocity was obtained with a verticaldrive Boston gear reducer with a 43 to 1 capacity. Linear displacement was varied by means of removable pins holding small stirrups over which the specimens flexed. The length of these pins determined the amount of strain or linear displacement, which was finally established a t 6 l l 6 inch, causing a 35y0 strain in the specimens. Ultraviolet light was generated by a type S-1 sun lamp manufactured by the General Electric Company. Data are reported (5, 8) which show the wave length distribution of the S-1 lamp to be nearly equivalent to t h a t of natural sunlight. The reflector was mounted on a boom held vertically over a distance of 100 em. from the turntable top. Exposure time for a selected fixed height of the light source varied, but definite cracking was observed after 50 hours of dynamic exposure. hlost test data, however, were taken a t 100 hours of exposure, this period wa.s sufficient to cause surface cracking on the best of stocks. Figure 4 shows the effect of the method on the cracking characteristics of GR-S tread stocks containing typical additives for reducing the rate of surface cracking. Typical stock compositions
OF GR-S STOCKS CLRED90 MINUTES AT TABLE I. COMPOSITIOX
142"
c.
Base Formula
Parts
Channel black (Kosmobile 7 7 ) Zinc oxide Softener (Paraflux) Sulfur .~.-Cyclohesyl-2-benzothiazole sulfenaiiiide
100.0 40.0 3.0 8.0 1.75 1.2
GR-S
Additive Benzidine-acetone condensation 4-te~t-Butyl-m-cresolmonosulfide p-Phenetidine-acetone cbndensation Protecrix-e n-as
-
Suiiiber--3 7 8
,---Stock 1 2
1 0
... ... ...
...
1 0
..,
...
. .
.. ...
1 0
,
,..
1 0
,.. , . .
., ... ,
516
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 TABLE11.
Vol. 39, No. 4
DYSAMIC SrRFACE GR-S TREAD STOCKS".
OBsERVATIOX O F
CRACKISG OF
cU1l,~~r;l~~~i,~~
Stock 1 '2
3
7 8
a
Condition of Suriace c r a c k i n g a f t e r ~ y naniic Exposure t o S - 1 Lanip Dynamic Expo-ure in 50 hours 100 hours J u l y .itinu-ptiere Sone T.. sl. to13 s u r f . , r. SI.o n both .urf. P I . hot. surf. Deep, wide on top Severe cracking on Severe o n b o t h biiri. surf. above stirrup; both surf. -1. nn b u t . surf. Sone Y . 4. tori s u r f . , nnne SI. on t o n surf , v . v . on b i t . surf. 21. o n h o t . s u r f . $1 t o p s u r f . , =I.hot. Ileep, fine cracking Deep. un but11 iiirf .surf. on both surf. Deep, n i d e o n t o p Deep, coarse on top Deep, coarre OLI both - i d . ah,ive stirrup: surf.: coarse o n surf.: v .
