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I N D U S T R I A L A N D ENGINEERING CHEMISTRY
Vol. 21, s o .
s
Effect of Stearic Acid on Reclaimed Rubber H. A. Winkelmann and E. R. Busenburg PHILADELPHIA RUBBERWORKSCOMPANY, AKROS,OHIO
I
T IS the purpose of this paper t o discuss the effect of practically all converted to sodium stearate in the alkali stearic acid on the process of manufacture of reclaimed process. This has no softening action and under these rubber and its effect when used with reclaimed rubber conditions has a drying action and removes all trace of "tack." in vulcanized rubber goods. The uses of stearic acid in This effect is obtained quite generally with the fatty acid reclaimed rubber can best be grouped as follows: (1) in the soaps. I n the alkali process of reclaiming, stearic acid does digester or heater as a plasticizing agent to speed up the disag- not have an opportunity of functioning as a softener because gregation of the rubber; ( 2 ) when added to the plasticized it is immediately converted t o a soap. I n the preparation of rubber on the mill or in an internal mixer, to facilitate addi- heater or digester reclaims in which no caustic soda is used, tion of a pigment or to impart some desirable property or stearic acid functions as a softener. consistency; (3) in containing Table I-Effect of Softeners in Alkali Whole-Tire Reclaim-Digester Process tial amounts of reclaimed rubber. N O OIL 4% PINETAR 4 % STEARIC ACID In most reclaiming processes softeners are used (1) (2) (1) (2) (1) (2) % % 7 0 7 , 7 0 % in the devulcanizer in conjunction with heat, to aid Acetone 8.25 8.38 8 . 6 3 9.64 7.23 7.41 in plasticizing the rubber. They may also be added Ash 18.14 18.26 17.76 17.90 16.83 16.80 29.82 28.07 25.98 25.15 on a warm-up mill or in an internal mixer to give a :fi$;mK 25.4:,7i4.90 4.68 6.01 reclaim with desirable properties for tubing machine Reclaim 100, sulfur 57, and calender operations. It may be desirable to ct'e Tensile Tensile have a definite amount or a complete absence of 141.6' Tensile strength Elong. Set strength Elong. Set C. strength Elong.Set tack. It is therefore essential that the right softener K g . < L,hs;/ Kg.< cm. Absi/ .En. 7, % Kg.; cm. Lbs./ in.2 % 7, is used and that it is added a t the proper time. M i n . cm. rn. % % 435 22 844 4 4 . 5 634 387 15 Factory conditions such as type of equipment, ii 4"::; giz fi 55 95 .. 40 780 410 16 3 9 . 6 564 325 11 4 3 . 8 623 340 13 3 7 . 8 538 350 12 method, and speed of processing and cooling con25 3 7 . 7 536 367 12 340 12 30 3 7 . 1 528 360 12 4 6 . 0 654 3 5 . 4 503 317 11 ditions determine the limits of firmness, softness, 35 4 0 . 5 576 370 IO 4 8 . 2 655 370 15 3 6 . 8 524 320 10 STRESS-STRAIN tackiness, plasticity, milling properties, and nerve f-.,( Lbs .( K g . i Lbsil of the reclaimed rubber required by the rubber E ' g g . ,",g.;/ :$/ rn. cm. In. goods manufacturer. 100 1 2 . 0 171 1 0 . 4 148 1 0 . 4 148 2 0 . 4 291 1 7 . 4 248 200 1 6 . 8 239 Effect of Softeners in Alkali Whole-Tire Reclaim 300 2 7 . 0 384 3 6 . 6 520 3 1 . 4 446
:$
DIGESTER PRocEss-Table I shows some of the chemical and physical tests ordinarily run on samples of reclaim. Three experimental reclaims were prepared from the same lot of whole-tire scrap by the alkali process. I n one case no oil was used for softening purposes, in the second 4 per cent of pine tar was added, and in the third 4 per cent stearic acid. The same amount of 6 per cent sodium hydroxide solution was used in each experiment, The samples were &vulcanized 14 hours a t 160 pounds (4.2 kg. per sq. cm.) steam pressure. As indicated by the comparative chloro-
HEATERPRocEss-Table 11 shows physical test data on three reclaims prepared by the heater process. The fiber-free ground treads were heated for 16 hours at. 130 pounds (9.1 kg. per sq. cm.) steam pressure without softener and with 5 per cent of pine tar and 5 per cent of stearic acid. The stock softened with pine tar approached proper consistency while the reclaim made without softener and the reclaim made with stearic acid were quite harsh. The reclaim using stearic acid was more plastic in its milling properties than the reclaim using no softener. It was smoother and more compact
form extracts and plasticity coefficients, the first reclaim was dry and harsh. It was very slow to break down on the mill and difficult to keep around the roll until broken down. The second reclaim using pine tar was much softer. Its consistency was about the same as that of an average highgrade whole-tire reclaim. The curing properties are improved. The third reclaim using stearic acid was also dry and harsh. On the mill it broke down slightly better than the reclaim containing no softener. The stearic acid is
with less tack than the sample using pine tar although not so plastic. Effect of Stearic Acid Added to Plasticized Rubber The second use for stearic acid is for incorporation into the reclaimed rubber after devulcanization. After treatment in the digester, the plasticized rubber is washed, dried, and sent to the mill room for the refining, straining, and leaf sheeting operations. The softener may be added a t any convenient
I X D U S T R I A L A N D ENGIXEERING CHEMISTRY
August, 1929
T a b l e 11-Effect of Softeners i n a Heater Process Tire R e c l a i m No SOFTENER 57, PINE TAR 57, STEARXC ACID Devulcanized 16 hours at 130 Ibs. (9.1 kg. per sq. cm.)
%
% Acetone 12.20 Ash 16.04 Chloroform 18.39 Snecific rravitv 1.132 Cure at Tensile 141 6’ C. strength Elong. Lbs;/ M i n . cm. rn. I 290 2 5 . 7 366 15 250 20 26 0 369 243 28 6 407 25 215 2 9 . 9 425 30 187 40 2 9 . 4 418 205 3 1 . 4 446 50 5 62 K Reco very, per cent 4 00 I
_
ml/
%
12.76 15.24 24.60 1,127
12.86 14.93 22.07 1,125 Reclaim 100, sulfur, 57,
Set
5%
Tensile strength Elong. Set Kg.{ Lbs./ cm. m. % %
Tensile strength
Ke.( Lbs.1 cn. cm.
4.94 5.74
of Stearic Acid i n R e c l a i m for Activation Purposes i n a Compound STEARIC ACID STEARIC ACID PRESENT I N RECLAIM ADDSDTO COMPOUND Smoked sheets 15 15 Reclaim, 2 per cent ... 50 stearic acid Reclaim no stearic acid 49 .. 5 Zinc oxide 5 27.8 Whiting 27.8 2 Sulfur 2 0.2 Captax 0.2 , . . Stearic acid 1 Table 111-Value
-_
---
100.0
100.0
Tensile Streneth Elone. Min. Kg./cm.z Lbs.lin.2 ’%480 1007 15 70.7 480 1074 20 75.5 460 76.2 1085 25 1142 480 30 80.3 1062 440 35 74 6 983 400 40 69.1
%
7c 10 6 9 6 5 3
8 4 4 2 4 4
time after washing. Prior to refining a softener may be added in an internal mixer to effect further plasticization or to facilitate dispersion of a pigment which is to be refined into the reclaim. The reclaim now has a low alkali content and the stearic acid in this case acts as such and imparts to the reclaim some very desirable properties. Approximately four volumes of various pigments were refined into a wholetire reclaim with and without addition of stearic acid to determine the effect of stearic acid on physical properties, plasticity, etc.
Cure at 141.6’ C.
Elong. Set
Set
% 33 35 33 36 33 29
Tensile Streneth Elona. K g . / c m . l Lbs.lin.1 % 1075 513 75.5 1105 503 77.6 1130 500 79.4 1172 480 82.4 1094 450 76.9 1082 437 76.0
Set
% 36 34 36 37 32 33
Figure I illustrates the curing properties of two reclaims made from whole tires by the alkali process. I n one case the reclaim was refined with no addition of pigment or softener on the warm-up mill and in the other case with 2 per cent of stearic acid added. The stearic acid improves the curing properties. The tensile strength is improved and the stressstrain curve indicates better reenforcing properties. The per cent alkalinity as KaOH after 48 hours’ extraction was 0.30 per cent on the control and 0.24 per cent when stearic acid was added. Figure I1 shows two reclaims containing 10 per cent of clay added prior to refining on the warm-up mill, with 2 per cent of stearic acid added in one case. The same improvement is noted as in Figure I. Figure I11 gives the same comparison as Figure 11, except that whiting was added instead of clay. Figure IV shows the comparison with 7.5 per cent of carbon black added. Figure V shows a comparison of plasticity coefficients of the eight reclaims, four with and four without stearic acid. Each stock containing stearic acid is more plastic than the one without stearic acid, the difference being less marked in the stocks containing carbon black. These plasticities were run on the day following the refining operation. Table I11 shows a comparison of plasticity coefficients taken a week later as against plasticities taken the day after the refining operation. The recovery results were ob-
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tained by measurement of the increase in thickness of the plasticity disk during the 10 minutes following its removal from the Williams plastometer. It is expressed as percentage increase in thickness. This result is a measure of the nerve of the reclaim. The recovery or nerve is less in the stocks in which stearic acid was used. The reduction of nerve in reclaimed rubber improves the tubing and calendering properties of compounds in which it is used. Stearic acid is valuable for this purpose because it accomplishes this result without the production of further “tack.” Effect of Stearic Acid i n Compounds Containing Reclaimed Rubber
Table IV gives a comparison of physical test data of two compounds containing stearic acid. In one case stearic acid was present in the reclaim and in the other case it was added in the compound. Physical tests indicate that s t e a r i c acid p r e s e n t i n reclaimed rubber is valuable for activation of Captax. Table IV-Effect of Stearic Acid on Plasticity of R e c l a i m e d Rubber STEARIC ACIDA N D PIGMENT ADDED PLASTICITY PLASTICITY K TO WHOLE-TIRE RECLAIM 100 K AFTER 1 WEEX RECOVERY 4.32 4.86 4.17 A-one 3.62 4.49 3.70 Stearic acid 2 Clay 10 Clay 10, stearic acid 2
4.67 4.24
5.05 5.19
4.57 2.60
Whiting 10 Whiting 10, stearic acid 2
4.45 3.86
4.57 4.71
3.85 1.22
Carbon black 7.5 Carbon black 7 . 5 , stearic acid 2
6.41 5.46
6.88 6.04
3.08 2.35
After plastizing, it is often desirable to add a softener on the mill to reduce excessive “tack” to metal rolls. Stearic acid reduces tack in plastic reclaims and gives stocks which have
They break down readily with little tendency to go to the back roll or become mushy and difficult t o remove from the mill. The improvement in curing properties of reclaimed rubber produced by the addition of stearic acid indicates that the use of stearic acid in compounds containing reclaim would produce a similar im-
a minimum amount of nerve.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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provement. This has been found to be true. In compounds containing substantial amounts of alkali process reclaim, a low sulfur and a high accelerator ratio, there is sometimes a rapid reversion causing blowing after the proper cure has been reached. Compounds containing stearic acid show less tendency to reversion under these conditions than compounds containing other softeners. Conclusions
1-Stearic acid does not compare with other softeners in plasticizing efficiency when used in contact with vulcanized rubber scrap during devulcanization.
VOl. 21, KO.8
2-Stearic acid when added as a softener to devulcanized scrap on the mill prior to refining imparts properties which are very desirable. (a) Makes the reclaim batch more plastic, ( b ) improves tubing and calendering properties, (c) reduces nerve without production of excess tack, (d) improves curing properties (higher tensile strength, higher modulus, and improved molding properties) of reclaimed rubber, ( e ) when pigments are added it gives better dispersion with improved physical properties. 3-Stearic acid when used in compounds containing reclaimed rubber improves the curing properties of these compounds.
Stearic Acid in Litharge-Cured Rubber Compounds J. R. Sheppard THE EAGLE-PICHER LEADCOMPANY, JOPLIN, Mo.
I
T HAS been known at least since 1912 that the resins present in crude rubber are essential to vulcanization with litharge. I n that year Weber ( 4 ) reported that deresinated rubber mill not cure and concluded that “resins play an active part in the vulcanization;” but he formulated no theory of the way in which they function. At that time the composition of the rubber resins was not so well known as now. I n 1916 Stevens (3) verified Weber’s conclusions, but believed his findings supported the Esch and Auerbach ( 2 ) theory that litharge accelerated vulcanization through the rise in temperature occasioned by an exothermal reaction. It is now well understood that resins promote vulcanization through their content of organic acid. It was proposed by Bedford and Winkelmann (1) in 1924 that litharge vulcanization proreeds by the following steps: (a) reaction of lead oxide with an organic acid (naturally present in the resin or added during the mixing) to form a rubber-soluble soap; (b) reaction of the lead soap with hydrogen sulfide to form a hydrosulfide salt or a hydrosulfide; ( c ) reaction of the latter with sulfur t o form a disulfide and then a polysulfide; (d) decomposition of the (unstable) polysulfide to yield a very active form of sulfur-the ultimate vulcanizer. The polysulfide theory has been generally accepted as accounting adequately for the known facts. It is clear, then, that the efficiency of litharge as an accelerator is absolutely dependent upon the existence of an organic acid in the rubber as mixed. The mere statement of this general principle, however, by no means gives the compounder all the information he desires in the matter of organic acids as related t o litharge curing. Various questions of practical importance arise. I n the present paper an attempt will be made not comprehensively or systematically to survey all these numerous questions, but only to illustrate a number of scattered but specific cases. It is advisable not to generalize too broadly, but to confine the conclusions to the particular conditions applying to each case. Low-Grade and High-Grade Crude Rubbers-High Oxide Stock
Zinc
Several years ago the writer made a study of various softeners in a high zinc oxide litharge compound, using the formula: Rubber Sulfur Litharge Zinc oxide Softener
100 8 20
100 5
Two series of stocks were made up, one using a typical “highgrade” crude (smoked sheet) and the other a “low-grade”
wild rubber (Lapori). Each series was master-batched up to the point of adding the various softeners, which were added to portions of the master batches on the laboratory mill. The stocks were vulcanized in the press a t 141’ C. (40 pounds steam) over a series of cures in order adequately to develop the optimum cure and to show the effect of the various softeners on the two grades of rubber. Stressstrains were determined on all the vulcanizates. The results were entirely different for the two crudes, as will be seen from Tables I and 11. I n Table I the physical properties of the stocks of the Lapori series a t their optimum cures (defined by maximum energy) are shown. While the control mix (without softener) came to an optimum in 150 minutes with a tensile of 2115 pounds per square inch, most of the softeners reduced the time to reach optimum and raised the properties a t optimum. This table should not be used to draw fine distinctions between two given softeners, particularly where they are close together in the table. But the outstanding fact is that those softeners which are either acids or salts, or contain an acid, and which therefore can bring the lead into solution in the rubber as a salt, are the ones which significantly aided the cure. On the other hand, in the corresponding data for the smoked sheet series we find no enhancement of cure by any of the softeners. As measured by time to reach optimum and (or) properties a t optimum, most of the softeners inflicted some damage (slight or moderate). It should be noted, however, that this damage in the case of smoked sheet is distinctly less than the benefit conferred by the acid class of softeners on Lapori. A further insight into the results themselves is furnished by Table 11, giving the stress-strains for all the cures on both Lapori and smoked sheet compounded without and with 5 per cent stearic acid. The data for Lapori emphasize how far apart in their properties the control stock and the stearic acid stock are a t a given cure. For example, on this basis stearic acid about doubled the modulus of Lapori. The presence of bloom up to 180 minutes without stearic and its disappearance between 60 and 90 minutes with stearic further confirm the accelerating effect of this acid. I n sharp contrast to these are the results on smoked sheet. Here there is, if anything, a retardation of cure by stearic and a t a given cure the modulus is greatly lowered, although the tensile is not much affected. This serves to emphasize that the observed stiffening of the Lapori vulcanizate is the net result of two opposing tendencies: (a) a stiffening due t o the enhancement of the cure, consequent in turn upon the acid bringing more lead into the available, soluble form;
