a4
*
*
+
ROSS E. MORRIS, JOSEPH W. HOLLISTER, AND PAUL A. MALLARD
f
Sr
Rubbm Lobwetwy, Navy
A GR-S h a d r(0dr men nl.ll*.ly f r t n h n Hwea bead I~O& in thlek &In. The hrkr r a k d cum i 8 due to ch. hlghn t h u d d W t y of OR-S &&. Thr t.mpnetan et the cater d thlek arclcla made holk GR-S r i m highr h n the wbide swing tempwt!m, .I a rnvlt of exothrnnic rudlon of v u l u n i u t i o n . The ternp.ratum codficlent d rul-
* + * *
uniutionlor1O0F.intarel~,ulcul~trdhommodulvrd~1., i s 1.45 foraGRS a d dock accelerated with meruptobrnrothkzole and d i p h r n y l # w n i d i m .
NFORMATION on tberelativeratesofeureuf(;RS &kJ and eimilar Hevea &,ocks in thick &ions is of interest to mang rubber manufacturers. Sines curing conditions for thick arrieles from Hevea stocks have been eatsblished, they would like to know how thew condition8 must he altered when G R 8 stacks are used in the same applicntions. They could develop a G R 8 stock with the Bame rate of oure as the Heves stock which it replaces according to lahatory teats on comparatively thin sheets, hut this agreement does not mean necessarily that thick sections will cure at the enme rate. The respective rate6 of
Tablr 1.
/A
,.-.-
Recipes
A
-
&
heat Bow through the rubberti must be ronaiderpd. Only if the ram of heat Eow as well as the curiug races of thin sections are in agreement, will the curing ratM of the thick seetiom he equal. Juve and Garvey (S) found that C R 8 tread st& cure faster in the canter of thick sections than War Hevea tread stocks. They were unable to explain thia behavior beoause, aocording to their measurements, the t h l conductivity of the G R 8 tread stock was less and ita specisc heat was greater than the corresponding value8 for the Hevea tread stock. They concluded that the diflerenca may he due to an exothermic reaction. Incidental to the development of cures for thick articles such 88 miaeon &eta, the authors have investigated the relative curing rates of G R 8 and Hevea tread stoeke. The rates of cure on the outside and in the canter of 'le and %inch thick round dabs were found at 275', 290'. and 305' F., and temperature measuTBmenta were made in %inch thick round slabs during and after cure. The recipes for the tread stocks are given in Table I. The tensile properties of theas stocks, m d on 0.Winch thick sheets cured for various times at 290' F. are graphed in Figum 1. The GR-8 Btock cured more slowly than the Hevea stuck in thin sheeta.
1'
1
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T F W Y U SIRENCT"
CRS
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Y d , Mnr Idmd, Calif.
A
,
-
-
?-om
6
A -
'J
/A
Q--IM
DElERMINAnON OF RATE OF CURE
The modulus a t 3M)% elongation was used for establishing state of cure on the I/,- and %inch slabs. The curing timee required to reach a modulus of loo0 pounds per square inch for the G R 8 Btock and of 12M) pounds for the Hevea stock were regardd as a measure of the respective rates of cure. Theea moduli cmreapnded to about the he& technical curw from the standpoint of teIldlepropertiss. The I/,- and 2-inch slab were built up before cure by plying together0.Winchlaysrsofstock. T h e l a y e r o n t h e bottom and the layer in the middle of
i5 \[ np+---&-r* .dr'" Y
UODULUS *T 300% CLONC
1
"EvfA
--Ma
"CKA
'\,
*/ I
c
l
ULTliMTE ELONGATION
/* MINUTES CURE AT
I
1
I
rsdr
,m
8
I
-.
- 50s
I
Figw 1. R . n d u n Dab at 290. F. on T w d Stosb Molded m 0.05-lnsh Shwb
e4
thethat BO stack they were could dusted he separated with soapltone easily from the rest of the slab after cure. F i i e 2 illustrates the laminations after cure on a 1-inch slab which WBB used in preliminary tests. The direction of the grain was marked cure.
After cure the layers we10 sepa-
INDUSTRIAL AND ENGINEERING CHEMISTRY
650
Figure 2.
Vol. 36, No. 7
Laminations after Cure in 1-Inch Slab, Used in Preliminary Tests
rated and eight tensile specimens, A.S.T.M. type C, were died out with the grain from the bottom layer and also from the middle layer. The modulus values for each layer were averaged. The diameter of the I / h - and 2-inch slabs was 13 inches. The eight tensile specimens were died out so that none was closer than . 2 inches t o the edge of the slab. Thus, all of the heat for the vulcanization of these specimens came from the end plates, since the end plates were closer than the wall of the circular mold to the locations from which the tensile specimens were taken.
A graphical method was used for finding the times of cure to reach the critical moduli. A typical graph is shown in Figure 3. At least two points were obtained below and two points above t h e critical modulus for every curve so that the point of intersection of the curve with the modulus line could be established accurately. Figure 3 shows that the bottom layer and the rniddle layer of the I/a-inch slab cured apparently a t the same rate; this held true for the l/d-inch slabs from both stocks a t the three temperatures. The times t o reach critical moduli in the l / 4 - and 2-inch slabs a t the three temperatures are listed in Table 11. D E G R E E O F O V E R C U R E ON S U R F A C E
l,'-INCH
SLAB
/I
/ 2 - I N C H SLAB, CENTER LAYER/
The data in Table I1 are in agreement with the observations by Juve and Garvey regarding the relative rates of cure of GR-S and Hevea stocks in thick articles. The times to reach best cure in the center of the 2-inch slabs were slightly less for the GR-5 stock even though the Hevea stock cured faster on the outside layer of these slabs and also in the */a-inch slabs. The data demonstrate that thick articles of this GR-S stock can be properly cured a-ith less overcure for the stock located near the surface than in the case of thick articles of the Hevea stock.
x.
2-INCH SLAB
Figure 3. Typical Relations between Time of Cure and M o d u l u s in Bottom and Center Layers of H e v e a TreadStock Slabs Cured at 275 ' F.
The important timc intervals obseived in this testing program follow: not less than 16 hours b e h e e n mixing and calendering for both stocks, 4-24 hours between Calendering and curing for the GR-S stock, 16-30 hours between calendering and curing for the Hevea stock, 72-96 hours between curing and testing foi both stocks. The stocks were conditioned a t 82" F. during t h r intervals between operations. Before curing, the hydraulic press platens were adjusted t o the desired temperature within *0.5' F. by means of a thermometer block. Tensile testing was done a t 82" * 5" F.
Table
Tread Stock
II.
Curing Times to Reach Critical M o d u l i (Best Cure) Curing Temp.,
F.
GR-S
275 290 305
Hevea
275 290
305
Minutes to Reach Critical TvIodulua 1/ein. slab, 2-in. slab av. of bottom & Bottom Center center layers layer layer 52.5 60 118.5 30.5 39 96 17.5 23.5 80 41.5
23.5 14.5
49
29.5
20
120.5 99 85
IO0
i I
CURING TEMPERATURE
Figure 4.
OF.
Effect of Curing Temperature o n Degree of Overcure of a 2-Inch Slab
Figure 4 shows this more clearly. The curves are based on the assumption that the actual time of cure beyond thc best cure may be regarded as a measure of the extent of overcure. The curve for the Hevea stock is above that for the GR-S stock; this shows that the outer layers of the Hevea stock were more overcured than the outer layers of the GR-S stock when the respective center layers reached the respective critical moduli.
July, 1944
INDUSTRIAL AND ENGINEERING CHEMISTRY
651
TEMPERATURE MEASUREMENTS
side of the mold so that the temperature of the thermocouple would not b? affected appreciably by heat flowing along the wire. Moreover, i t w a s considered that the temperature of the thermocouple would not be affected by heat flowing in from the side of the mold through the rubber because this distance was twice the distance t o the faces of the mold. It could be safely assumed, therefore, that all of the heat reaching the center thermocouple came through the rubber from the faces of the mold. The procedures for preparation and test were as follows: The coId mold was placed on the cold bottom plate with the bottom iron-constantan thermocouple in position. A dis4 of raw stock, slightly over 1 inch in thickness and slightly under 4 inches in diameter, was placed in the mold. The center iron-constantan thermocouple was placed in position on top of this disk, and another disk of raw stock with similar dimensions was placed on top of the thermocouple. The cold top plate was placed in position, and the entire assembly put into the hydraulic press which had previously been brought to the correct temperaDure. The press was closed at once, and the time measurement with a stop watch was started. The rise in temperature of the thermocouples was followed with a potentiometer. After the temperature of both thermocouples had leveled off, the cure was discontinued and the rubber was cooled. The rubber specimen was then cut open to determine the actual location of the center thermocouple. The I center thermocouples in all slabs were found BO+TOM THERMOCOUPLE CENTER THERMOCOUPLE to be within 0.02 inch of vertical center. Figure 5. Cross Section of Assembly Used in H e a t Flow Experiments It was assumed that the cold aluminum plates attained the curing temperature almost immediately, and that the cold mold had little or no effect on the temperature rise of the thermocouple because the intermediate rubber 240provided adequate insulation. ,'-HEVEA T R E A D STOCK The temperature rise data for cures at 275" and 305" F. are plotted in Figure 6. The data for the cures a t 290" F. gave similar curves. These curves show that the center of the slabs reached the curing temperature much more slowly than did the outer surfaces. Of more AT 275°F signifkance, they also show that the rates of temperature rise at the centers of the GR-S slabs were faster than the rates at the centers of the Hevea slabs; and that the temperatures at t h e 1 I centers of all slabs rose above the respective I40 I60 180 320 curing temperatures.
The next problem was to find why a GR-S tread stock cures relatively faster in a thick section than a similar Hevea tread stock. The explanation offered by Juve and Garvey can be easily tested by making temperature measurements in the rubber under suitable conditions. It was decided to conductexperiment8 along this line. A 4-inch diameter, 2-inch thick mold was made with holes drilled at two diametrically opposite points through the side to accommodate No. 26 thermocouple wire. The holes were centered with respect to the faces of the mold. Notches to accommodate the wire were made at diametrically opposite points on one face of the mold. The appearance of the assembly during cure is shown in Figure 5. The center thermocouple was located far enough in from the
I
EXOTHERMIC REACTIONS
240
The difference in rates of temperature rise explains why thick articles of GR-S stocks cure relatively faster than thick articles of similar Hevea stocks. It does not appear that the faster rate of temperature rise of the GR-S stock could be due t o an exceptional heat generation in this stock, as suggested by Juve and Garvey, because the center temperature3 of both stocks exceeded the respective bottom temperatures by about the same amount. The peak values of the differential temperatures follow:
-
A T 305°F
Curing Temp., I
I40
I
160
O
180
F
275 290 305
Max. Temp. Rise, F. GR-8 Hevea 5 5 5
5 7 7
The curves for both stocks are similar. The bottom thermocouples soon attained curing temperature a t both heatings; the center thermocouples surpassed the curing temperature duiing the original cure but leveled off a t the curing temperature during the second heating, due to the absence of further exothermic reaction. There was no evidence that an exceptional exothermic reaction occurred in the GR-S stock. The relatively faster cure of thick GR-S articles must be due simply to the faster thermal diffusivity in these stocks.
28
BOTTOM TH~RMOCOUPLE B O T H C O N D I .e'T
,
I
CENTER THERMOCOUPLE O DURING N S DURING ~ HEATING CURE OF CURED S L A B
24
20
16
T H E R M A L DlFFUSlVlTlES
GR-S TREAD STOCK 12
a
- /
i l
The curves may be used for calculating the thermal diffusivities of the stocks. Data were obtained according to the method described by Williams (6) and are presented in Table 111. In every case the thermal diffusivity increases as the temperature of the rubber rises. The average diffusivities of the GR-S stock are higher than the average diffusivities of the Hevea stock. The diffusivities during actual cure are higher than the diffusivities during reheat of the cured slabs, probably as a result of heat generated in the exothermic reactions. Both cure and reheat data indicate that the GR-S stock has higher diffusivity than the Hevea stock. The last relation contradicts the diffusivity data given by Juve and Garvey, calculated from thermal conductivity and specific heat measurements. Their data show that the diffusivity of a Hevea tread stock is 19% greater than that of a GR-S tread stock. The data obtained in this work indicate that the diffusivity of the GR-S tread stock is 16% greater than that of the Hevea tread stock. No explanation is offered for this difference.
T I M E OF CURE I N MINUTES
I
20
I
60
40
I
80
I
100
I
120
1
I
140
160
I
I 160
/
28
24
20
I6
12
Vol. 36, No. T
INDUSTRIAL AND ENGINEERING CHEMISTRY
652
- I/
HEVEA T R E A D S T O C K
T I M E OF CURE I N M I N U T E
B
I
I
20
40
Figure 7.
60
I
100
80
120
140
Temperature Rise i n 2-Inch Slabs at 290"
F.
These differential temperatures were due to exothermic reactions between sulfur and the respective rubber hydrocarbons which have been investigated (1, 2 ) . That no exceptional exothermic reaction occurs during the vulcanization of the GR-S tread stock was definitely proved by the following experiments: A 2-inch slab of the GR-S tread stock was vulcanized at 290" F. in the assembly depicted in Figure 5. The temperature rises during vulcanization were followed, and the. cure was continued until the temperature of the center thermocouple dropped to the temperature of the bottom thermocouple, an indication that exothermic reaction had stopped. The total curing time was 220 minutes. The assembly was then withdrawn from the hydraulic press and cooled. When the slab had reached a uniform temperature of about 80" F. throughout, the assembly was again placed in the hydraulic press at 290' F. and the rise in temperature of the two thermocouples was followed. The temperature rises during cure and during the second heating are plotted in Figure 7 for both stocks. The time for the Hevea tread stock to pass through the exothermic reaction was 370 minutes.
Table
At 120' F. At 260' F. Av., 80-290'
F.
111.
TEMPERATURE COEFFICIENTS
From the data on modulus us. time of cure for the bottom layer of the l/r-inch slabs, as exemplified in Figure 3, it was possible t o calculate values for the respecthe temperature coefficients of vul-
Thermal Diffuoivity of Tread Stocks -Diffuaivity, 290" F. Cure GR-S Hevea 0 . 0140 0,0124 0,0169 0.0144 0.0152 0.0131
Calcd. by Juve & G a r v e y , , , ,
.
, ,,
Bq. In./Min.
........., ., ..
290' F. Reheat GR-S Hevea 0.0138 0.0117 0.0162 0.0140 0.0149 0.0128 0,0118
0.014
I
I
275
280
Figure 8.
TEMPERATUREOF.
I
285
290
295
6
I
I
300
305
Curing Time vs. Temperature Curing Time Equals Time to Reach Critical M o ulus)
INDUSTRIAL AND ENGINEERING CHEMISTRY
FACTORS INfLUEWNG THE
Y
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