Y
*
*
Y
*
*
Y
Y
* * * *
rrocessing Characteristics of Synthetic Tire Rubber
e processing characteristics ot synthetic tire rubber such as is being made in the government program are reviewed. Laboratory tests for evaluating these properties of crude rubbers and of compounded batches are given. The results of Mooney viscosity of the crude rubbers, Goodrich plasticity of the batch stocks, and a special laboratory tubing test are discussed in relation to factory mixing and tread tubing results.
Th
r
p d g characteri8tice of synthetic tire rubber, such as is being made in the government program, are diciently difterent from those of natural rubber to muire Wemt laboratory and f&ry h t m e n t . Since the laboratory tests used for the evaluation of proceasing characteris tica were dewloped for natural rubber, it is not surprising that they are not 80 satisfactory for synthetic rubber. The purpoea of this paper is to diacuas some of the characteristica of aynthetic tire rubber and to describe Oertain tests which have heen found useful for evaluating difterent tm of synthetic rubber or difterent compounds of the same syntheticrubber. In the c o u w of the development it has been neceaeQTg to evaluate rubbers with a wide Variety of prooeaSing properties. An production has increae8d, there has been less variation in the output from any one plant. As the newer and larger planta come into full production, we can expect greater uniformity from eaoh plant and between Werent planta. We can slso look forward to gradual improvemente in processing charscteristios. This development will be enenred by adequate laboratorg methds for eyluating proceesing properties. This discusionis 6psed in@ on variations encountered in development work and in part on the-pmp&ies OS %he present production from one plant. Processing Chrracterirtia MILLINQ.Synthetic rubber difters from natural rubber in sweral important mpecta. It is tough& and consequently tends to become hotter on the mill. Also it is ahortar and lees thermoplastic. These dBerenw show up as a marked difterence in milling behavior. When crude natural rubber is put on a cold mill, it is tough and knotty until well broken down. If the mill is heated to 180-200° F., the ruhber quickly smooths out, becomes soft, and is egsily pulled out to a considerable extent. Under similar conditions the synthetic
B. S. Garvey, Jr., M.H. Whitlock, and 1. A. Freese, Jr. The 8. F. Goodrich Company, Akron,
Ohio
material smooths out somewhat more quickly on the cold mill but remain8 rather tough and nervy. A t 18O-aOO” F. it becomes considerably softer, but alee b m e a short and weak and is rougher than on 8 cold mill. gome of the experimental rubbers become so short and weak that they crack on the edges or become lacy, and in extreme c88e8 fall off the mill. HEATSOZCENINO. With regsrd to oxidative or “heat” softening, tire-type synthetic rubber is intermediate between natural rubber and the oil-reaktmt butadiene oopdymera. Under proper conditions it can be softened in hot air, but unless the conditions are carefully controlled, the operation m y actually harden the synthetic rubber. This is due to the fact that the oxidation has a double &e&, a breakdown and a sort of oxidation vulcaniaation. In the first 8 t a W of heating, breakdown seema to be predominant and the rubber 8Oftene. Subsequently, however, the rmlCaniaing action becomes p r e dominant and the rubber ~ ~ s u m easSoorChad appesrsnce, becoming grainy, short,and hard. B~KDOWN F’robably . dated to the heat softeningaction is the mill breakdown. The eynthetic rubber does not breakdown m much as natural rubber doea. Considerably more milling is neoe8gsrJT for adequate breakdown .wid the breakdown is lass obvious. Nevertheless it is important in Bubsepuent operations. It is hard to describe accurately and should be observed on a laboratory mill. The breakdowm can be obtained either More or after pigment8 are ad& There is mme indieation that the addition of pigmenh facilitgtea breakdown. The important thing is that the o m . dl milling must be dequate and tbat -down is favored by a cool mill and a tight one. Trrsme. The fact that the hot stock is short and weak sometimes lads to trouble in tubing operations. When the extruded cmas section is thicker in the middle than on the edges, as in a complete tread and side wall, the stock flows faster in the thick sedion than at the thin edge. As 8 result, the edges must be pulled out. When the hot stock ia too short and weak, this stretching results in cracked edgw. If the stock is properly compounded and CALENDERINO. mixed, it will friction and calender without parthlar di6iculty in moat c w . A calendered sheet may haw more shrinksge than natural rubber and hence more tendency to check.
Not run 11 (2. 3.3.3
98 98
86
84
sa
77 76 75 73 74 73 71 70
70
69 70 67 66 84
63
..
57
56
40 43 40
ii
....
71
..
.. ....
60 71 84
..
66
.... ..
6
.... ......
54
..
214
.,
211
'9
14
11 11 7 11
P:
8.6 10 10 7 X.6
7.6
'i 7
8 3
2
.. .. .. as
..
ai
8
....
a7
..
9 .. ..
.e5
a4 3.5 30
6
ii
.... ....
ii
24
38
..
33
6 7
88 36
...... 1.
is
6i
....
46 47
* 6 pounds st 85' C.
TACK. Synthetic tire rubber is much less tacky than natural rubber. The stocka are moderatdy responsive to freshening with gasoline or benzene. A cement of the synthetic mbber stock is better. At present the best solution a p p m to be a cement of a natural rubber compounded aimih l y to the synthetic compound. LABOahToBY MIXIWQ WOCSDURE. The choice Of a lab* ratory mixing procedure will depend on the object of the experiment. If one is trying to duplicate factory conditions, he should use a hot mill. On the other hand, if the object is to get the best batch for comparative evaluation. it is baet to use a cold mill. In one p d u r e for genwd laboratory use, the rubber should be well broken down More pigments or softenersare added, although sulfur and activation zinc oxide may be added during the breakdown period. The softener should be added before the pigments, and the latter should not be added rapidly enough to c a w the batch to break. This is especially true in the 0888 of the hard blacks. The accelerator is added last. If the batch c o n h i i hard black, it should be thoroughly oooled and than remilled for a short time on a cold mill. On a lZineh mill it is best to uge not more than 30&400 grams of synthetic rubber in the batoh. It is obvious that various modilications are possible and that this discussion should be used primarily as a guide. FACTORY MWNQ h O C E D U R E . F d O r y p r O ~ is g eeentidy Laboratory p r o c d g on a Large scale, as modified by limitations of equipment snd demands of production. The best breakdown and mixing are obtained on cool tight mills. This requires smaller batches and better cooling than is usually practical in the factory. N o d y a factory batch will run hotter and in a thicker band on themill thanalaboratory batch. This results in lea breakdown. Addition of black wm to facilitate breakdown under these conditions, and it may be advisable to keep the mill tigbt, add the black as soon as the rubber is muring smoothly, and add other materials later. In a Banbury mixer normal4ae batches should be used to ensure adequate ram pressure. In general, more total milling is necessary with synthetic rubber than with natural rubber. At the same time "heat history" should be kept at a minimum. A well mixed compound w i l l process better in subsequent operations' and have better vulcanized properties than one which is not well mixed. Trials must be conducted on the equipment available in each mill mom to determine the beat balance between quality and output.
Laboratory Tests for Processing
It is often deeirable to evaluate proceasing characterietics in the laboratory,either for the 6eleotion of rubbem or for the selection of compounding materials. No single teat has been found eatisfactory. For determining the processing characteristicsof a series of experimental rubbem, the teate found most useful in this laboratory are: Mooney viscosity, Y value, and milling behavior on the-crude rubber, and Goodrich plasticity, milling behavior, and tubing index of a batch mixed from the rubber. For comparison of compounding materiala the mixing time, Goodrich plasticity, milling behavior, and tubing index are
used. TEST. Some indication of processing character-
istics can be obtained from the 10-gram milling teat. When an experimentalrubber becomes too short and especially when it becomes lacy on E hot mill, it will probably give trouble in the factory. DilTerencea among compounded batches can also be observed on the mill.
% t;
E@
d23 Z vl 5 TIME LIIWES
IN FLASTWETER ( ~ ~ e - u i w rWUUI r
mm)
Figure 1. Shear Force vs. Time
MOONEYVISCO~~TY. For measuremente on the crude rubbers, the Mooney plastometer appesrs to be the most Satisfactorytype. In general,a 4minute viscosity at 185O F. between 60 and 75 seems desirable, although some rubbera with higher viscosity have given good resulk and some with lower viscosity have given poor results.
Nobm.br, IS42
* *
INDUSTRIAL ANDd$NQI NmRCNQ CHEMISTRY
, 7
sccrmtv cw
+fF-A Figure
AA
-
9.
1311
This batoh ia mixed on a 12-inch mill, cooled for 24 hours, d e d , and again cooled. To simulate operatingconditions, it ia then warmed up on a mill at about 180' F. and tubed h u g h a No. '/* Royle tube machine, like* heated to about 180' F. Under them conditions the stack extrudes at about !Me F. The die has the shape and dimensions ahown in Figure W . As the stock is druded, it is pulled straight out from the die so that the narrow edge ia pulled out. A small piece ia cut from the best part of the strip and graded 88 follows:
II*wLyD
4
******
Details of Die Opening
If omtical. it is orobablv more desirable to run v i ~ c o ~ i t i a at zizo or 2260 F. Y VALUE. If the shear force reading at 185' F. (or higher temrperatum) on the U. S. plastometer is plotted against time at half-minute intervals, a curve of the type shown in Figure 1 ia obtained. The diflerence between the minimum and the maximum readings is called the Y value. To expedite t d n g for factory contml work, the first &minute portion of the ahear force curve m y be used. Under these conditions the 4-minute viscoSity and the Y value for the initial &minute portion of the viscosity curve m e as indication~of the procesesbility of the rubber. A 10-minute determination is sometbum advantageous. The Y value, r n r aebve, is&measurement of the ce both mastication shape of the time oslty curve. and heat sof ' g alter the shape of the timeviscosity curve, the Y value%%umte,d applications. It be wed 88 a produced in synthetic
1. The mntoursbwld be tbeshspeofthadie end not tm mollenorporous. T h e M i a thewmstl. 2. The edge (Figure 2) &o$d be amooth and continuous rather than rough, ragged, or h h n . The best in 4, the worat 1. 3. The Bat aurfacsa should be smcoth and shiny, r a t h than wavy, dull, or lumpy. The rating is from1 to 4. 4. The cornera should be sharp and smooth rather than rounded or rough. The rating in fmm 1to 4.
The tubing index is the %urn of the four figures and is followed in parentheses by the figures them8elvea in the order given. Thus the beat tubing stocks would have an index of 16 (4444)and the worst an index of 4 (1111). The Becond figure, that for tlle edge, 888038 to be mhet critical and should be at least 3.
A
U41.40
C
O ( U . 2 ~ ) IZfJJS8) l6(+$+4) 13(+44+)
Figure 3. Type Samples Showing Evaluation of Tubing Properties
At present we feel that an index of at least 12 (-, 3, -, -) is necessary to give reaaombly good processing in the factory. A set of st9ndards, such as is shown in Figure 3, should be kept for compmipns. Discunion of Processing T&
plastometer ap to give more u d u l ' ormation than the other typs. U s a 5-pund weight at%' F.; a Goodrich plasticity of 26 or high-8 jprable for a tread type
stock. To determine the d e c t of "heat history",the laboratorymixed stock without sulfur and socelerator can be heated in a p m for several intervsls and the plasticity determined. Indicationa are that after 80 minutea at 300' F.the plasticity should still be at least half what it wae before and pderably should be over 20. ~ I N TEST. O A laboratory tubing test =ma to be the mod satidsotory single method of evaluation. For this purpose a batch containing 200 grama of ruhberia mixed on the following recipe:
A high vismsity reading indicah a harder rubber and a high Y value seema to indicate a more nervy one. Both of these characteristics make it harder to get proper mixing in the factory and caw the stock to run hotter with resultant poorer mixing and tubing. A high Goodrich plasticity on the batch stock indicates gocd milling but not n d y good tubing. Tubing c ~ ~ t i arec indicated s by the tubing index. Since laboratory mills can he run cooler and tighter than factory mills, it is often poasible to obtain better mixing in the laboratory. This is especially true with harder and more nervy rubbers. When this OCCUTB, the laboratory-mixed stocks will tube better than the fsetory-mixed stocks. Based on the laboratory compnthn of a considerable variety and number of experimentd ruhbem, the following conclusions have been drawn. When the Mooney viscosity and Y value of the crude rubber are low and the Goodrich plasticity and tubing index of the mixed stack are high, the rubber will pmcess well in the factory. If the revem is true, it will not. When Mooney viscosity and Y value are high and Goodrich plasticity and tubing index are slso high, there m y
1312
* *****
INDUSTRIAL A N D BNQINEERINQ CHEMISTRY
***
Vd. 34, N a 11
Data obtained in the LaboratorJr and on the factory-mixed stocks are given in Table I. The Y value correlsteswell with the degree of black dispersion (determined mimmopidy) and the exhaion temperature. There h also good correlation between faobry tubing d t a and predictions based on a combination of viscosity, Y value,and tub= index.
be trouble in getting a gmd factory mix but, if such a mix is obtained, subsequent processing will be eatiaf.tactry.
A series of chargea made in regular factory operations were checked in the laboratmy for viscosity, Y value, and tubing i n k . The same chargea w e v ~then mixed into tresd Btoclrs in the factory and treade were tubed t h u g h a plate die.
Ebonitp r *from Hycar
OR45 B. S. Garvey, Jr., and D. V. Srbach The 8. F. Goodrich Company, Akron,
Ohio
1
_ _ .je#om Hkar Impact and tensile strengths are as high ahhose of ebonites m d e from natural rubber. The sdkning *mperatures are considerably higher. The optimum amount of sulfur is 35 parts per hundred parts OF rubber. Accelerators may be used. This paper reports the effect of various amounts of sulfur, of accelerators, and of a variety of softeners and pigments on the tensile strength, impact strength, elongation, hardness, and softening temperature of ebonites made from Hycdr OR-I 5. Hi$-quaIit:
... .. ..
OR-15.
4s
n
T
HIS study waa underbhn to determine some of the important c Y m a c M c s of ebonites made from EIycar OR-IS and to obtain fun& mental compoundingdata which could be used for the development of commeMial stocks. The effecta of variations in amount of sulfur and the &e& of some acceleratorn, softeners, and pigmenta have. bean investigated. The multa of the i n d g a t i o n have shown that ebonites from Eyw ORA5 have phyaical properties equal to thoea of natural rubber ebonites and in addition have comidmblyhigher softening temperature. Theee nynthetic ebonites also take a high polish and may be made in varioun colorn by using colored inorganic pigmenta such 88 iron and chromic oxidea, antimony sulfides, asdmium de
w.
m.LIyu
I
nide, etc.
M&od All compounde were mixed on lzinch Lsborstory milla in total batcb ai.esrsnging from 600 to IO00 grame. Hard rubber strips, 0.26 X 1 X 6 inch-, were p d , aswed, and ground to &a for the vtuioun teata. The trannverae impact tests were run on cut and b d e d pieces measuring 2.5 X 0.5 X 0.25 inch. The teet pieces were packed,in crushed ice for one hour prior to teating to ~ a 8 u t ‘ ~
Figure 1. Effect of Sulfur Ratio
I .
1
