I N D U S T R I A L A N D ENGINEERING CHEMISTRY
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Vol. 16, No. 9
Influence of Accelerators on Plasticity of Uncured Rubber Stocks' By Stanley &all FIRESTONE TIRE& RUBBERCo., AKRON, OHIO
I
Several investigators have N T H E commercial appThe tendency of raw rubber stocks to set-up or scorch during procpublished methods for measlication of organic accelessing has been determined in the laboratory by means of plasticity uring this physical change erators, the workability measurements. Data are gioen for some well-known organic accelerain raw stocks. Cadwell and of the raw rubber stocks contors. Smith3 heated films of the taining them is of major imA t a temperature of 90" C.. thiocarbanilide and aldehyde raw'stocks in an atmosphere portance. The stocks must ammonia are the only ones of those tested that produce appreciable of carbon dioxide at a fixed be plastic, but not too soft set-up, when the amounts used are within the range generally employed; temperature and then deterand sticky or they will adquick curing stocks containing a high concentration of accelerator mined the stiffness of the here to the liners and books are not included. fiIms, at stated intervals of in which they are stored. The influence of the concentration of accelerator oaries with the time, by a hand examinaOn the other hand, if they accelerator. The concentration of diphenylguanidine has appreciation. The method suffices are not soft enough, they ble influence, while that of thiocarbanilide or the condensation prodfor qualitative comparisons will require a higher temuct of aniline with acetaldehyde and formaldehyde has only a slight only. Marzetti4 devised a perature to make them run effect. plastometer for this purwith smooth surfaces durThe effect of the concentration of sulfur is small indicating that the pose. E s s e n t i a l l y t h e ing processing, and a t the concentration of sulfur is much less important in connection with the method consists in placing same time, as a result of factory workability of stocks than either the kind or concentration of the sample in a cylinder, their stiffness, the temperaaccelerator. with a conical bottom protures developed may be sufZinc oxide decreases the influence of aldehyde ammonia in provided with an outlet at its ficiently high to set-up or ducing premature oulcanization; it increases that of diphenylapex, and forcing the sample scorch them owing to preguanidine slightly and of thiocarbanilide tremendously. through the orifice, at a mature vulcanization. Lime and calcined magnesia in the presence of zinc oxide increase stated temperature, by The majority of organic the setting-up tendency of stocks containing aldehyde ammonia and means of a compressed gas. accelerators have a softendiphenylguanidine. I n some cases the actions appear to be simply The stock extruded after ing action on raw rubber, additioe. in others, more than additiue. successive periods of time but in the presence of sulfur This method is not adapted to the study of most superaccelerators, is measured, and the dethis softening action may be because when they begin to set-up, the action is too rapid to permit crease in amount extruded entirely masked by the tendetermining the rate of change. during the successive interdency of the accelerator to Sulfur, eoen u p to 12 per cent on the rubber, does not act as a vals is taken as a measure bring about premature vulsoftener of raw rubber, at least so f a r as can be determined by this of the rate of premature canization during milling, method. vulcanization at that temcalendering, etc., when temperatures of 80" to 100" C. are attained. If the temperature perature. Owing to the time required to bring the relatively is kept low enough during mixing, the softening action of the large volume of rubber to the temperature of the bath, Maraccelerator will predominate, even in the presence of sulfur. zetti recommends that the portion extruded during the first For example, when the compound (rubber 100, zinc oxide 3, half hour be ignored. Of these two methods the latter appears sulfur 4) containing 0.75 per cent of diphenylguanidine, or even the more promising, but the type of plastometer used is such 1.25 per cent of aldehyde ammonia, was mixed a t a suffi- that the method is long and laborious. The plastometer designed by Williams2 permits of a much ciently low temperature (55" C.), plasticity values, K,2 of 2.02 and 1.92, respectively, were obtained while the same simpler procedure. The sample used is small, the time stock in the absence of any accelerator had a value of 2.27. required for a single determination is only 25 to 30 minutes, To be able to predict, in the laboratory, the behavior of and the manipulation is very simple. I n the work recorded an accelerated stock in the factory is of considerable impor- herein this plastometer has been employed in studying the tance and in the past has been a thing of considerable difficulty. influence of both organic accelerators and compounding inAn instance of this is the situation which arose at the time thio- gredients on the premature vulcanization of raw stocks. cafbanilide was introduced in the rubber industry, when enorPROCEDURE mous quantitiee of "thio" stocks were ruined by scorching. A pellet of 2-cc. volume is placed in the press at a fixed Premature vulcanization, scorching or setting-up produces temperature, and readings are taken at stated intervals, changes in raw stocks that are thought of in physical rather preferably 5-minute intervals, up to 30 minutes. These than chemical terms. Without doubt, premature vulcaniza- readings follow the true plasticity curve ( K = YXO.lg8) until tion ik accompanied by a chemical combination of rubber and setting-up begins. When this occurs, the curve deviates from sulfur, but the extent of the reaction is so small that the determination of combined sulfur is impractical. The change the normal and quickly approaches a parallel to the time axis. The true plasticity value, K , from the first portion of in the physical properties of the raw stocks, however, is suffi- the curve, being known, the deviation or increase in K may be ciently marked to be measurable. expressed as percentage increase at any time interval. The 1 Presented under the tltle "The Influence of Organic Accelerators on percentage increase in K is plotted against the time of heatthe Plasticity of Uncured Rubber Stockk" before the Division of Rubber ing to obtain a setting-up curve for the stock. The true Ch'emistry a t the 67th Meeting of the American Chemical Society, Was& ington, D . C., April 21 t o 26, 1924. * THISJOURNAL, 16, 362 (1924).
a India Rubber W o u l d , 68, 782 (1923). 4 I b i d . , 68, 776 (1923).
I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
September, 1924
plasticity value is attained after 5 t o 10 minutes of heating, but in instances where the stock starts to set-up almost immediately upon placing in the press, the 5-minute plasticity value is taken as the normal value, although in such cases it may be slightly high.
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other hand, when information concerning the tendency for premature vulcanization only is desired, it is necessary to maintain only approximately the same conditions, as in this case small variations in the absolute,plasticity have no apparent effect.
24
22 20 18 I6
14 12 IO
8 6
4
2 0
T i m e of
heoting
- minutes
T,me o f h e o t i n g - m i n u t e s
This method measures change in plasticity, and therefore cannot be employed after the plasticity curve becomes flat or parallel to the time axis. The test should be completed within 30 minutes, as the actual decrease in thickness after that time is generally so small that the experimental error becomes too large for satisfactory results. This is especially true when the stock shows appreciable tendency to set-up, but is ~ I R the O case when the sample does not begin to set-up within 30 minutes. It is not advisable to rely on a single reading after the stock begins to set-up, as the experimental error is sufficiently large to give erroneous conclusions. T h e , curves approach straight lines, but they are invariably somewhat irregular, as is apparent from the curves in this paper. Aside from information on the premature vulcanization occurring a t the temperatures developed during factory processing, a knowledge of the plasticity changes which take place a t ordinary or slightly elevated temperatures is also quite essential. For such work, the original plasticity value, K , is determined and then a portion of the stock is heated, in the absence of air and preferably in a mold, a t the desired 1,emperature for any suitable period of time. The
The concentration of accelerator is an important factor; hence equivalent quantities should be used in all comparisons. Almost without exception, the quant'ity of accelerator employed in the present work was the amount necessary to produce the maximum modulus or stiffness when the stock was cured for 60 to 75 minutes at 143.3" C. (290' F.). The base formula used was as follows: Smoked sheets.. . . . . . . Zinc o x i d e . , , , . . . , . . . S u l f u r . .. . . . . . . . . . .
.
100
3 4
DATA The setting-up curves for some well-known accelerators are shown in Figs. 1 and 2. Over a period of 30 minutes a t 80" C., there is no setting-up of this type of stock with any of these accelerators. Some oi these accelerators, however, will cause premature vulcanization a t this temperature if heated for a longer period of time, but the rate of setting-up is insufficient to be serious from a factory handling standpoint, provided time is allowed for the proper cooling of the stocks prior to storage. At 90" C . the thio-
24 22
Zinc Oxide
-
3
20
18 16
14
10
8 6
4
2
f i m e of hea4ng-m/nufes
T;ma of h c a t ~ n p - m i n u t e s
plasticity of the treated sample is determined and the increase in K is expressed in percentage. When a comparison of the absolute plasticity of stocks is required, it is essential that all mixes compared be given uniform treatment during the milling operation. The conditions to be maintained identical are size of batch, distance between rolls, temperature of rolls, and time of milling. Obviously, the stocks compared must be run on the same mill or on mills of the same size, speed, and gear ratio. On the
/5
20
Time of heohng-rnrnutes
2.5
carbanilide stock sets up appreciably, the aldehyde ammonia stock only slightly, and the remaining stocks not at all. At 100" C. the influence of the accelerators is in the following order, the one with the greatest effect heading the list, and the last three showing no effect : thiocarbanilide, aldehydeammonia, diphenylguanidine, di-o-tolylguanidine, accelerator A (condensation product of aniline with acetaldehyde and formaldehyde, melting point 73 O C.), triphenylguanidine, hexamethylenetetramine, ethylidine aniline (condensation
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
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product of aniline with acetaldehyde, melting point 78" C.), and p-nitrosodimethylaniline. The effect of the concentration of accelerator varies with
1s PO Time of h e d n g - m , n u f e s
the accelerator, as is shown in Figs. 3, 4,and 5. The concentration of diphenylguanidine has an appreciable influence, while that of thiocarbanilide and accelerator A has only a slight effect. The influence of the concentration of sulfur on premature vulcanization is shown by the curves in Figs. 6, 7, and 8. The mass action effect is noted when the concentration of sulfur is increased. The increase in rate of setting-up, however, is nominal, indicating that the concentration of sulfur is much less important, in connection with the factory workability of stocks, then either the kind or concentration of accelerator. The influence of zinc oxide on the action of several accelerators is demonstrated by the curves in Fig. 9. The addition of zinc oxide decreases the influence of aldehyde ammonia in producing premature vulcanization, increases slightly the influence of diphenylguanidine, and increase s t r e m e n -
dously the influence of thiocarbanilide. I n the absence of zinc oxide, thiocar b a n i lide stocks do not set up at all, even up to 30 minutes a t 100" C. The F m o of h e o t m g - m , n u t e s action of zinc oxide with respect to premature vulcanization is similar to its action in ordinary vuloanization-viz., zinc oxide has practically on influence on the activity of aldehyde ammonia, it does increase the activity of diphenylguanidine, and is absolutely essential in conjunction with thiocarbanilide, in the absence of other basic oxides. TABLE I-INFLUENCE OF THIURAM DISULFIDE ACCELERATORS ON
Minutes in Dress 5 10 15
20
I n Figs. 10 and 11 are plotted data showing the effect of hydrated lime and calcined magnesia on the setting-up properties of stocks containing aldehyde ammonia and diphenyl-
is 20 fime d h e o t i n q -minutes
25
SCORCHING AT looo C. AFTER16 HOURS AT NORMAL 60' C. Thickness Thickness Mm. K Mm. K 1 28 1.75 1.07 1.67 1166 1:fk 0.97 0.915 1165 h .
..
....
Vol. 16, No. 9
25
IO
5
30
15 20 T,me of h e o f i n p - m t n u t e s
25
guanidine. I n the presence of zinc oxide, lime and calcined magnesia increase the setting-up tendency of stocks containing the above-mentioned accelerators. Lime alone causes premature vulcanization to an appreciable extent, while the effect of calcined magnesia is only slight. I n some cases the actions appear to be additive, in othersmore than additive. TABLE11-EFFECT OF Rubber Sulfur Minutes in press
SULRUR ON T H E PLASTICITY OP R A W
FORMULAS
100 100 4 8 Thickness readings (mm.) at 80' C. 1.48 1.47 1.50 1.34 1.35 1.37 Plasticity value, K ,a t SOo C. 2.32 2.31 2.33 2.28 2.29 2.33 2.30 2.30 * 2.33
100
..
10 15 10
15 Average
RUBBER 100 12 1.46 1.34 2.30 2.28 2.29
When a stock containing the thiuram disulfide class of super accelerators starts to scorch, the action is so rapid that this method cannot be employed for following the rate of change. This is clearly demonstrated by the data obtained a t 100" C. (Table I ) ; at the end of 40 minutes the base stock containing 0.1 per cent tetramethylthiuram disulfide showed no indication of premature vulcanization, but after 50 minutes it had a fair c o m m e r c i a l cure. I t is possible, however, to determine the approximate time such a stock will withstand a certain temperature Erne of h e d , n g - r n i n u t e s without change. This is done by determining the normal plasticity and then heating a portion of the sample a t the desired temperature and again determining the plasticity a t stated intervals of time. The data presented indicate that this type of stock can be heated without change for approximately 40 minutes at 100" C. and for more than 16 hours at 60" C. Many rubber technologists are of the opinion that sulfur has a softening action on raw rubber. Data are given in Table I1 indicating that sulfur, even up to 12 per cent on the rubber, does not act as a softener for raw rubber, at, least so far as can be determined by this method. ~
