Thermal Decomposition and Synthetic Rubber Stocks .
*.
h e n B. Prettyman
The Firestone Tire
*
'
- ' - 'mpan)
A
M
rom a thermal viewpoint, the ability of a natu) Fwnthetic rubber stock to withstand ser+ wnditioi dependent on the rate of heat generation as sovcrncr
m. Kuooer F P-33 b&k,
stocks wntdining zinc oxide, channel clay, and b l c e n e are compared. A -...parison of Hevca rubber, reclaim rubber, guayule, synthetic rubbers, and Neoprene GN in gum and inch ne1 blackstock i s made. gefinite and sometimes large differences in the d a m m i t i o n temperature were found.' In particular, stocks w n tdining Captax and Santocure had a higher decomposition temperature than those containing guanidine and A-39. An increase in the sulfur content of tread stock from 2.5 to 3.5 parts on 100 parts of rubber decreased the decomposition temperature. P-33 black, followed by clay, was superior in the pigment series. Buna-Vpe and Neoprene GN stock had much higher decomposition temperatures khan natural rubber. The desirability of measuring the decomposition temperature of natural and synthetic rubber stocks in the light of service conditions i s indicated.
_,
Buna-Y+i
temperature on the chemical structure. chemical degeneration i s the development of porosity in the stock A laboratory method for determining the decomposition temperature, as evidenced by the formation of blowholes in cube samples immersed in liquid metal, i s described. Dew m m i t i o n temperature results are given of the cumulative addition of zint oxide, accelerator, and softener to a rubbersulfur stock. A comparison of various accelerators in tread, cdrcass, and gum rubber stocks is made. The effect of variation in the sulfur and black content of rubber stocks i s
HE factors responsible for failure of natural and syn-
T
r
thetic rnbber stocks in tires in service are primarily thermal in nature. They govern the temperature attained by the stock and the ability of the stock $o withstand this tempratme. The fadors which determine the temperature attained include t h m dependent upon the stock itself and those dependent upon attendant conditions. In the first group may be mentioned thermal conductivity, internal friction, and elastic modulus. In the w n d are included the construction and design of the tire, the speed at which it is run,the load to which it ia subjected,its in9ation pressure, and the temperature of the road and surrounding air. Therefore, aesuming a fixed set of conditions for the second group, the problem resolves itself into a determination of the amount of beat generated witbin the stock, the rate at which the generated heat is conducted away from it, and the temperature it may attain without undergoing chemical degeneration. These chsracteristics can be investigated independently in the laboratory. The lirst two cbsrscteristjos bave received considerable attention. William (ZO), Barnett (Z), and Frnmkin and Dubinker ( 4 8 ) rue among thw who have investigated the thermal conductivity. The problem of heat generation has been studied by Barnett and Mathews (2), Fielding (4, Jones and Pearce (II), and others, using the rebonnd technique tb determine energy absorpton; by Kosten (2% Naunton and Waring (16,26), €&el& (Z7), 8ebreU and Dinsmore (M), Gehman et d. (7,8), using high-frequency vibrations; and by Cooper (S), H a v d (ZO), Lessig (ZS), Mscksy, Anderson, and Gmdner (24), and Gongh and P a r b n (8) in apparatus involving the destructive working of rubber stocks. The latter method gave results which in-
cluded the combined d e c t of internal friction, elastic mcduIns, and thermal conductivity. The third characteristic, the chemical degeneration temperature, has received little attention in the literature. Its importance is apparent when we realize that the primary consideration is not how cool a stock may run but rather the margin between the runningtemperature of the stock and its degeneration temperature. Thus, it is entirely pwible for a given stock to run cooler than another stock and yet to degenerate more rapidly than the second stock becam of the merenee in the degeneration temperature of the two stocks. (Not considered are other materiala in closa proximity to the stock, such as cord fabric, which have their independent degeneration tempraturea but may generate little heat themselves. Such materials reach their thermal degeneration temperature by virtue of the temperature attained by the stock. The ability of the stock to resist degeneration, of couree, baa no bearing on their thermal failure, although the degeneration of the stock may tend to hasten their failure from other caw, such as increased stress or separation.) One definite indication of this chemical degeneration is the development of porosity in the stock. The point at which pomsity or blowhole formation occure in a stock may be de6ned as its decomposition temperature. It is readily conceded that some chemical degeneration may take place at a temperature below the decomposition temperature. In fact, any elevated temperature produce8 some change in both natural and synthetic rubber stocks. In a broad sense this cbange conld be considered chemical degeneration. However, where rapid and complete stock failure is at issue, it a p p s w n a b l e to mume that the dective degeneration temperature approaches the decomposition tempeoature as defined.
1294
N m m h r , 1942 THERMOSTAT
***
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Chemical degeneradon, as considered here, is an internnl phenomenon produced by the d e c t of heat alone u p n the stock. Failurea such as sun checking or flex crwkhg are not considered. Thus, in order to exclude any possible d e c t of oxidation through contact with the air at elevated tempersture,the technique of Bubmewion in a molten alloy waa employed. This had the added advantage of heating the aamplee more rapidly and uniformly. Porosity Method for Thermd Decomposition Temperature
The apparatus (Figure 1) consisted essentially of an insulated ste8l bar, 76.2 X 30.5 X 8.9 cm. (30 X 12 X 3.5 inches), c o n W g cylindrical cavities 2.9 om. in diameter and 6.35 cm. deep (1.14 X 2.5 inches) for the teat samples locsted in columns of six at ten regular intervals along ita length,heater elementa at one end, and a cooling chamber at the other. During actual testa it wna found unuecezary to use the cwling chamber. The cavitiea were partially filled with a low-melting alloy. At test temperatureabove200°C., an alloy of 60.0 parte tin, 32.0 parte lead, and 18.0 parte cadmium waa d. At lower temperatures it waa necesessy to add aa much aa 10 p r cent mercury to the alloy to keep it from ~ o l i d i f h . Cub test samples, 1.9 cm. (0.75 inch), were placed in the cavities and forced into the liquid alloy with W e d plugs. The plugs oontsioed vent holes to allow escape of the air above the alloy in c&98 of a blowout of the test samples. A bimetal thermal regulator waa inserted in the bar near the beater end. Iron-comttuitan therm& couples w m inserted into the bsse of alloy-filled cavities at pinta TC,, TC, and TC,. The bar waa insulated on the sidw, ends, and bottom with 7.6 om. (3 inches) of msgnesh block. Lids of magnesia block and t r d t e were provided, one for each pair of cavity columos. Corks were fitted into one of the lids 80 that the cavity pluga could b removed and test samples inserted with a minimum of heat loas. The apparatus waa cslibrated by meaeuring the temperature in each of the cavitiea by meam of thermocouples after equilibrium thermal conditiom were reached. Figure 2 shorn the temperature gradient obtained. Since the temperatures of the three permanemt points, TC,, T C , and TC,, were found to lie on the w e , these pinta were used to detmmhe the cavity temperaturea of actual testa by drawing a curve through them of the eame general shsp aa the calibration curve. Temperature testa in each column in no w e showed the temperature in the cavities of the two outside rorn to be more than 2.0° C. lower than the temprature in the cavities
1295
of the respective columns of the two center m. At the cooler end of the bar, this differmtialwas, in general, less tban 1.0' C. Because thew variatiom were considerablylees than the ditTerence from column to column, they were dhgarded in obtaining the cavity temperaturea of the testa. The thermostat waa adjusted to give the proper temperature at the heater end of the bar. After the bar r w h d thermal equilibrium, usually in 48 hours, the temperatum at The teat TC,, TC, and TCr were m&. samples ware then hertea one at a time with all cavitiescovered except the one being loaded. The samples were removed at the end of one
VENT
Figure 1. Thermal Decomposition Apparatus
******
no change in the temperature of blowhole formation as a function of time in the interval 0.5 to 2 hours. The temperaturewere remeasured just beforeremoval. The averagetemperature reading at each position waa uaed to obtain the tempersture curve. After cooling, the samples were cut open to determine the presence of blowholes, splits, or other signa of heat failure. With the exception of the reclaim s h h , the formation of blowboles with rise in temperature waa abrupt. An interval of 10' C. waa usually m5cient to separate a normal sample from one conteining many boles of the order of 1.5 mm. in diameter. In the cme of reclaim stoclrs about twice thin interval waa required. The temperature of the bloek waa adjusted,by means of trial teak, 80 that blowhole formation occurred in the region from the middle to the cool end of the bar. In thin region the temperature cbange from column to column averaged about 4 O C. Themfore, by uniformly selecting, aa the temperature of blowhole formation, that temperature at which d or pinholes 6mt appeared, the actual decompasition temperature was in no ~ 8 8 8more than about 4' C. lower than the recorded temperature of blowhole formation. Evaluation of Stocks
Tables I and I1 give the stock form& and the curing times and tmuperaturea. The curea dected were such 88 to overcure the stocks slightly rather than to undercure them. Thus any d e c t which might reault from completing the cure oPan
9
poslTDN CT M R l O C a P L E A m T m U R E CWTlEs
Figure Q.
T
ical Temperature Gradient 'Curves of Thermal Decomposition Bar
******
1296
Table 1. Bmoked ah& sulfur Zino oxide Channel blwk 8tmrio wid Pina t u PY.BUX Antioxidant D. 0. T. c ) .
Bmoked .boot sulfur mo oxide C b e lb W 8-0
.aid
Pina tu
Antioxidant Csptu % L E
P-33
Curingtime at 148.9' C..
&
A
B
C
100
100
100
3 3
a
a
a a
D
100 3
a
Formulas and Cures of Hww Rubber S t d n n F Q E I J K 100 1w 100 100 100 l? I? 3
... 0.6 ... ... ... 1.1 ... ... ...
... 0.6 ... ... ... ... 1.0 ... ...
... 0.6 ... ... ... ... ... 1.5 ...
... 0.6 ... ... ... ... ... ... 0.7
42.6
66
55
66
56
55
Q 1M) 5
R 100 6 10
B 100 5 10
T 100 6
U 100
... ...
... ... ... ... ... ... ...
... ... ... ... ... ...
...
... ... ... ...1 ... ... ...
10 ... 1 ... ...1 ... ... ...
90
w
55
66
...
3 42.5
Sto&.
Basrr MA-. In the gum typ stocks, the Buns tspes (butadiene copolymers) were found to have the bighest decomposition temperatur:,, followed by Neoprene QN, Hevea, reclslm and guayule rubber in that general order (Table VIII).
3
12.5
a.6 8.8
66
66
V
W 100
1w2.6
3.3
3 8.8
a
8.3
60
a
3.3 60 3.a
55
X
Y '1MI
Z 100
1wa
3.3
3
06.5
... ... ...
1 ... ... ...
1 ... ... ...
... ... ...
... a ... ... 1 ... ... ...
55
56
55
56
56
1
M)
3 3.8
50
66
40
3.3 8.3
60
8.3
N 100
55
MI
8.8 8.8 1
60
3.8
M 100
8.8 3.3 ... ... .1. . . 1. . . .3.8 . 4.2 4.2 1 1 .1.0. . .. .. .. .. ..1 .. ... 1.1 ... 1.0 ... ... ... ... . . . . .1.5. ... 0.7 ...
8.3
a.a
E4
3.3 3.3 1 1
8.8
L
1y
Vol.34,No. 11
... 1 1.1 ... ... ...
'
& v u RWBF~R STOCKS.In all types of stocks (tresd, w c w , and gum) Captax, followed by Santocure, was found to be superior to D. 0. T. G. and A32 (aldehydeaxnine type) accelerators (Table 111). No &&cant d8erence was found between pine tar and Partdux in tread stocks (Table 111). The introduction of zinc oxide to a rubber-sulfur mix reduced the decomposition temperature. The intrcduction of Captan and Captax plus stearic acid to the rubbersulfu~nincoxide mix produced a rise in the decomposition temperature to that of the rubber-sulfur mix (Table IV). Low-aulfur (2.5 parts by weight on 100 parts of rubber) tread stock w88 found to have a higher decomposition temperature tban medium (3.0 pa&) and high (3.5 parts) sulfur tread stocks (Table V). In the channel black series the stocks with the low loadings of black bad a slightly lower decomposition temperature tban the stocks with loadings of 40 and 50 parts by Weight on 100 park of rubber (Table VI). In the pigment series P33 black, followed by clay, bad the bigheat decomposition temperature, zino oxide and channel black were inkmediate, and Calcene was lowest (Table
3
42.6
.... .0.. 6. . . .0.6 . . .... 0.0 0.6 1.76 1.76 1.76 1.75 .1. . . . .1. . .... .... 1 1 .... 1.a .... .... .... 1.2 .... .. .. .. .. .. .. .. .. .... 1.8 .... .... 0.85
undercured stock without the curing preeaure was eliminated. The desirability of comparing series of stocks containing only one variable precluded the p d b i l i t y of compounding the most d e a b l e commercid stocka in all instanced. Tables III to VIII give the decomposition temperatures, as indicated by the temperature of blowhole formation, of the stocks studied. Table IX gives the elkt of time of cure on some of thee
MI.
***
INDUSTRIAL A N D ENQINEERING CHEMISTRY
26
3.8 3.3 1 1
3.a 8.8
3 3
18
...3 ... 1 ... ... ... 66
3.3 8.3
66
55
56
55
O 100
a
3.3
MI
P 100
s
a.3
60
3.a 3.3 .4.2 . . . .4.2 .
..1.. .. .. ..1..
... 1.5 .., 0.7 66
65
M B B C C 100 100 100 3 3 3 3 3 .a
...a ...3 ...3 ... ... ... ... ... ... 1 1 1 ... 26 ... ... 26 ... ... 18 1 . .
65
66
56
In the tread typ stocks, the Buns typa and Neoprene GN had the highest decomposition temperature, followed by Hem, reclaim, and guayule rubber in that gemerd order (Table VIII). Stocks containing deresinated guayule had B somewhat higher decomposition temperature than those containing untreated guayule (Table VIII). In the case of the guayule stocks, the decomposition tem-
perature a p p m h e d the curing temperahm (Table VIII).
In fact, in the c a s of the tread stock containing the untreated
Tabh II. Fonnulas and Cum of Stocks lor B a r M.(.rial b n w r i m DD
EE
FF
GO
... ... ... ... 1%' ... 1W ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... 2 ... 2 3 5 3 3 0.6 0.5 0.5 0.5 ... 1.6 1.5 1.5 2 ... ... ... ... ... ... o.a ... .... ... 4 ... ... ... 0.6
1... w
... 100 ... ... ... ... ... ... ... 2
40
40
40
40
MM 100
NN
00
PP
3 50
a
a 60 a
6
... ... ... ... 1W ... ... ... 100 ... ... ... 100 ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... 2 ... 2 2 a
6
50
8 6 1.6
6
1.5 ... ... ... ... ...
... ... ... ... ...
1.6 ... ... ... ... ...
45
45
46
a6
1 ... ...
2 0.6
4
0.76 2.6
46
HH
I1
JJ
.. .. .. .. .. .. .. .. .. . . . ........ .. .. 164' . . . . . . ... .. .. .. .176 .... .160... . .a . . . 3. . . ..a .. a
0.5
a
0.5
3 f.6
KK
... ... ... ... ... ... ...
LL
... ...
... ... ... ... ... ... 100 100 ... 3.6 a.6 a
0.5
1
a
0.6 1
.. .. .. .. .. .. .. .. .. ... ... ... ... ... .. .. .. .. .. .. .. .. .. ... ... ... 1
1
40
40
40
QQ
RB
88
... a
... 3
... ... 3.5 3
40
40
.. .. .. .. UU .. .. ... ... ... ... ... ... . . . . ... ... ... .. .. .. .. ... ... ... ... ... ... 100 ... lib' ... .. .. .. .. .. .. ... ... 150 ... ... ... ... 100 3 60 3 8.6 1
3 24
a
3.6 1
3
48
3 a.6
1
TT
3 60
100 8.6 3 Ml
.1. . . .1.
1 ... ... ... ... ... ... ...
1 ...... ... ... . ..... ... ... .. .. .. .. ....
46
45
1
t..
46
46
4S
Table
samples were tsken. It is doubtful if thew cracks would a p pesr in the case of extruded stock. This point was not
111. Elhd of Acalmkwa In Hcnr Rubbet stodu
investigated.
fi;.O.
D
S.ntooure
Tabh
177 188 177 191
A B C
E
F 0
H
I J K L
173 19s 173 186
V. Elhct of Sulfur Content
Put.sulfur 1 w P . N Rubber Wt.)
M 0 P
174
174 199 178 184
N
aoi
174 196
38
E
T
194
EaFnm OF C m . The &ect of variation of the time of cure is given for only thrw of the stocks studied (Table E)for the sake of brevity. The &ect was greatest for the Buns S ' tread stock, MM, wheress no &ect wna apparent for the guayule tread stock, W. Other stocks tested showed smaller &e& than that for stock MM. In all casea the temperature of blowhole formation became nearly constant for times at and above the optimum cure (ea determind by tensile stremgth). All cures used in Tables III to VIII were at or lightly over the optimum, exceptin one case noted in Table IV. Significance of Results
191
of Hcnr Rubbet %do Tsm of Blowhole &IMtion NO. 4 c.
&
a.6
V . I U
3.0 3.6
ala
199
am
guayule, the decomposition temperature was actually lqwer thsn the curing temperature. Apparently the curing p wea the only thing that blowholes during cure. wea not maintained a su5cient time after removal of the buring preaslue to allow blowhoh to form. In the ca?a of some of the Buns tme Btocks. semration in the form of cracks or splits occurrd at tem&a&ea ldwer thsn those necesary to produce decomposition ea & d d by blowholes. Thus,in the caw of the Buna S gum stock, the decomposition temperature ea reported wea 2w0C., &e splits o c c d at 238' C. In the case of the tread s Buns 8 decomposed at 23V, aplit at 216O C.; Butal decomposed at 244', split at 187" C.; Butawne composed at 233', split at 2G9O C. It is believed separation occurred between the plies introduced in the stock for the curing of the blocks fromThic
*
It is apparent that a wide range of decomposition temperatures exists betweem merent stocks. At present the practical significance of theae occasional large difference in decoinposition temperature is not entirely known. It is inheating to note, however, the implication of the reault that Captgxaccelerated natural rubber Btocks yield over 20" C. higher decomposition temperaturea than stocks accelerated with D. 0. T. G. This result may at least partially explain the superior resistance to abrasion imparted by Captax ea coinpared with D. 0. T. G., a result not completely obvious from phssical teats. Similsrl~.the much M e r decommsition &mperaturea shown by -the Buna-type &nthetic &d Neoprene GN ea compared to natural rubber appesrs consistent with their satisfactory performance in tires, even with the higher runningtemperatures whish they produce. Knowledge of the physical properties of synthetic rubber stocks hardly allow8 prediction of this result with certainty. This test is referred to the chemist, to be employed ea a compounding guide in the consideration of one factor governing the suitability of stocks in the light of the applications for which they are intended. Although most of the compounds stu& were of the type used in conventional tire manufacture, the possible applications of the study of thermal decomposi&n to stocks deigned for many other usea where coma a factor are evident.
*
Acknowledgment
e author wishea to thank 0. D. Cole for compounding .mka s e t e d in this paper and J. W. Lish for his many
P.
Teble
VI. Elhct
of Channel Block Loading on
Stock,
Hma R u e
Table VIII. Complrllon of B a r M.tuldr Temp. of Blowhole Fornution GumTypoBtook NO. C. DD 264
B u a Material 0
18
26 40
w
Table VII.
Effect of Pigments in
B
EE FF
198
e 101 X 198 w a a 4 J
248 aa7 198
KK LL
168 143 160
HH I1 JJ
a03
I-
Hwea R u k r Stock,
Table IX.
am
00
RadTmBtmk NO. C. MM asa *NN a44 00 288 183
E E as 167 138 163
TT
uu
EffKt d Cure on Typical Trud Uoeb
10 Par- Pipmrnt 1w Put. Rubber (by dol.) Y
e M
BB
cc
a01
26
mi m
lea ai9
46 66 86
I
106 126
187 197
ma
... ...
...
187
228
a3a
aaa
... ...
16s 166 1KS 166 166
1M
1298
******
INDUSTRIAL A N D ENGINBERING CHEMISTRY
helpful suggestiOne concerning the standardhtion of the apparatus. The guidance of J. H. Dillon and the continued intercat of J. N. Street in the course of this work is greatly appreciated. Tbe permission of the Firestone Tire and Rubber C o ~ ~ p a ntoy publiah this work is gratefully sclmowledped.
***
VoL 34, No. 11
(8) Gshman, 8. D.,Woodford. D. E.,and Btsmbeugh, R. B., -0.
h. 33. . 1032-8 (1941):
cbm T&..
IND. 14.
(11 (11) Jone 17
(12) K w h . C. W.,h o c . Rubber T& C d . , Lmda,1938, 9871wO; Rubber C h . Tadi.. 12,38143 (1939). (13) W,E.T.,b. EIP((. C m . . ANAL ED.,9, SSZ8 (1937). (14) Macby, J. Q., Anderaon, J. 0.. and Gardner. E. R.. !l+am. I d . R u b k I d . . 18. 123-37 (1840): Fhbber CFtem. T d . . 14.
Literature Cited
(1) B-ett. C.E., h. ENQ.C-., 28,303-6 (1934). (2) Barnett, C. E.,and Matbeas. W.C.. M., 28, 1292-6 (1934): R e C h . Tadi.,8, 138-49 (1935). m L. ~ A LE. ~ .. 5. . . a6o-i (ma: _,rMlur. ---..-. .,v.. IXD.EW. c-.. ~~~~~~~. . .. Rubber Chem Tadi.,7, 125-9 (1934). (1939). (4) Fielding, J. E.,IND.ENQ. CH.Y., 29, 880-5 (1937): Rubber (16) Naunton, W. J. 8.. and Waring, J. R. 8.. Tram. I d . R&r C h . Tadi.. 10,807-18 (1937). I d . . 14, 340-64 (1939): Ru6bsr C h . Tadi., 12. 816-80 ' , L. S., and Dubinker, Y. B.. Caoukhue & Rubbsr (5) (1939). (U. 8. 25-84: Rubbe C h . Tadi.. 13. -~8. RJ. ~ June. ~ , 1939. _ (17) RoeIig, E.. ploc. Rubber Tadi. emf., Ladas 1938. 821-9: 38-74 (1940). Rubbe C h . Tadi.,12,38c400 (1939). (6) FRunldn.L.8.. and Dubink. Y.B.. J . RuMwr I d . (U.8.8. R.), (18) 8sbm.U. L. B.. , and Dinamore. R. P.,India Rubbm W&, 104, ' 13, 132-40. 338 (1986); Rubber Chum. Tadi.. 11, 368-71 46-60 (April. 1941). (1938). 110) Williams, lm Tan. Elm. Cam. 15. 1 .54-7 (1923). (7)Gehman, 8. D., J . dppl*dPhys.. 13,40Z13 (1842).
..~
...
~~~
~~~~~
~~~
Effect of .. * * Petroleum Proc
NI
I
Y
EFFECT O F KEROSENES
*
w
r
* *T*
Vulcanizates
he use of the aniline point
has been advocated
as a means of predicting the swelling power of
Donald F. Frsrer E. 1. du Pant de Nemourr & Company, Inc., Wilmington, Del.
N PRECEDING papers of thia series (9, J)it w88 demonstrated that the ViscaSitJI-gravty constant or, preferably, the aniline point of an oil could be used as a criterionof the swelling &ect on neoprene of oils reprenenting a wide ran@ of commercisl pmducte. In the case of petroleum producta similar to kemsene or fuel oil, it hss been found that the aniline point alone is not indieative of the swelling deet. However, as this paper will show, the aniline point oombind with the gravity in the form of the Diesel index will predict the swelling &e& and thus may be used to specify standard immersionmediaofthelenetype. The neoprene cornpasition used was identical with that of
I
lubricating and hydraulic oils on synthetic rubber vulcanizates. This scheme allows a means of describing constant-swelling-effect oils for specikcation purposes and has been used to this end. With kerosenes and kerosene-benzene blends the aniline point does not predict the swelling power. However, the Diesel index, a simple constant involving aniline point and gravity, may be used for this purpose. This allows the replacement of kerosene-benzene blends by constant-swellingeffect kerosenes which will permit the use of higher test temperatures and will eliminate the necessity of reflux condensers during immersion tests.
the previous investigations: 1M) 4
as.8 a 6
ssmplea of this composition were prepared for the volume increaen test as previously d e a c r i i . The physical data for the kerwaea usedin the swelling testa are shownin Table I.
Ala0 shown am the volume inoresse reaulta after immemion of neoprene speoimene in the kemsenea at 27.8', 70°, and 100" C. (W, 168', and 212' F.). The reaulta am t h m of the maximum or equilibrium swelling. The immersions were oontinued for a total of 56 dap, but equilibria bad been esbbhhd, depending on the temperature, in 2 to 7 dap.
