PLIOLITE-RUBBER MIXTURES H. R. THIES
Pliolite is a cyclized rubber whose composition is the same as that of rubber but which is physically different in many of its properties. Mixtures of Pliolite and rubber can be made, and the commercia1 value of some of these mixtures is pointed out. Mixtures of Pliolite and synthetic elastomers can be made and may have some usefulness.
The Goodyear Tire & Rubber Company, Akron, Ohio
YCLIZED rubbers have attracted the attention of chemists for a long time. When pure, all of these derivatives of rubber consist of a hydrocarbon or a mixture of hydrocarbons ( C ~ H Eand ) ~ , all have less unsaturation than does the parent' substance. Their physical properties, however, vary from rubbery to hard shellaclike types, and their formation is attributed to internal cyclic formation, since there is a change in the unsaturation but no change in the composition when compared to the original rubber.
C
#
I
JELL
PLlOLlTE CYCLIZATION CURVL
800
>
t
::
600
P
DISTORTION POINTS
>Io0
investigated by Staudinger and Bondy (18)showed that up to 150" C. the number of double bonds remained the same but a t higher temperatures cyclization occurred. As early as 1781 Leonhardi referred to. a tough elastic product which was obtained by treating rubber with sulfuric acid. In 1851 Macintosh obtained a patent for a process in which the extruded or molded articles of gutta-percha werb subjected to superficial hardening by immersion in coneen: trated sulfuric acid. However, the sulfuric- or sulfonic-acidcyclized rubber of today is based upon the investigations of Fisher and eo-workers (3, 4). These materials, known as thermoprenes, were shown to be cyclic isomers of the rubber hydrocarbon, in agreement with the observations of Kirchhof (12)and Staudinger (17). Rubber isomers from halides of amphoteric metals, such as stannic and stannous chlorides, aluminum chloride, boron fluoride, and chlorostannic acid, embrace some of the most valuable chemical derivatives of rubber. A comprehensive study of these reactions was made by Bruson, Sebrell, and Calvert ( 1 ) . The properties of amphoteric metal halide derivatives of rubber which render them specially valuable as molding materials were described by Thies and Clifford (22) and by Jones and Winkelmann (11).
FIGURE 1
u)ou
Several different methods have been utilized for the preparation of a cyclized rubber. Harries (6) obtained a white inelastic solid by treating a rubber hydrogen halide with an organic base. When rubber is heated under conditions which preclude complete breakdown, a diminution in the number of double bonds is effected together with increase in density. Cyclization by pyrogenetic decomposition was accomplished as early as 1838 by Himly (8).
so00
PLIOLITE-RUBBER MIXTURES BEFORE AUO AFTER
4 00Mg AGING
.Numerous other methods for isomerizing rubber include the silent electric discharge (6, Q), heating in the presence of surface-active substances (IO),phosphorus oxychloride (16, 23, 24), and hydrogen fluoride (7,IS,14).
FIGURE 2. COMPARISON OF SMALLER
DUMBBELL WITH THE CONVENTIONAL TYPE
The heat-cyclized rubber obtained by Staudinger and Geiger (19) was a white powder, the solutions of which were of low viscosity. The effect of heat on rubber in solution as
Pliolite When rubber is cyclized with agents such as tin tetrachloride or chlorostannic acid under proper conditions, some interesting physical changes take place, especially if this phenomenon is carried out with heat in a solvent (2, 2%')(Figure 1). The first indication of a change taking place in the rubber is an immediate build-up in the viscosity of the solution.
INDUSTRIAL AND ENGINEERING CHEMISTRY
390
Rubber unstretched
Pliulite unstretched
Rubber elongated 500y0
Pliolite stretched hot
.io rubber-50
Vol. 33. No. 3
80 ru!iber-20
Pliolite
50 r u b b e r 4 0 Pliolite elongated 500%
Pliolite
80 rubber-20 P h i i t , ? elongated 50070
FIGURE 4
This is followed by a gradual decrease in viscosity, as heating progresses, until a stage is reached where further heating does little in further reducing the viscosity. At this point the final viscosity is only about one thousandth of the starting viscosity, and the cyclization has been completed. Further, the density of the product is now 1.06 instead of 0.93, the specific gravity of the original rubber. This reaction can be followed on the curve of Figure 1. Chemically the reaction product analyzes about 96 per cent (CsHs),, and a whole family of resins can be produced, depending upon where on the reaction curve the process is stopped. The group of materials prepared by this method has been given the trade name “Pliolite”. The general rule
TOO Oh
OF PLIOLITE-RUBBER MIXTURES
600
z 0
ELONGATION AT BREAK
is that the longer the reaction is allowed to run, the harder and more brittle the resulting resin becomes and the higher is its distortion temperature. It has also been proved that as the cyclization progresses, the unsaturation of the original rubber is decreased until, with a fully reacted product, the chemical unsaturation is only about half of what it is with the original material. Thiscalls for a structural change from the GH3
conventional
I
(HzC-C=CH-CHz),
of rubber to a more satu-
rated formula (15) such as:
L
I
CH3
I
CHa
Commercial applications have been made (20, 21) of all the derivatives shown on the reaction curve, but in this discussion we will attempt to cover some of the work that has been done on mixing Pliolite derivatives with rubber and rubberlike materials.
200
tu wo t
4hw 300
Pliolite-Rubber Mixtures
$200 106 0 0
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96 PUOLITE 1 0 2 0 3 0 4 0 s o 6 0 1 0 1 0 9 0 l 0 0 w eo 70 CD 50 +o so ao m o % Ruemira FIGURE
5
I n milling Pliolite alone, a marked difference in the performance on the mill as compared to rubber is observed. The usual power consumption of rubber during the milling procedure is approximately 1 horsepower per inch of mill length. I n the case of Pliolite this power consumption is 5 horsepower per inch of mill length. This means that on a 60-inch mill, for example, rubber requires 60 horsepower for milling, whereas
March, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
Pliolite requires approximately 300 horsepower for the same batch. I n milling, Pliolite powder, a h e white material, is massed by one pass through a tight mill and is then banded slowly on the mill until the thermoplastic mass forms a bank. To make rubber-Pliolite mixtures, it is necessary to put the Pliolite on first and add the rubber slowly in the beginning until approximately 10 to 15 per cent of the rubber is in the batch; then the relative speed of the rubber ‘addition can be increased. The reverse method of adding Pliolite to rubber is unsuccessful; the Pliolite does not soften and simply floats in the rubber matrix more or less as a solid pigment.
PLlOLlTE RUBBER MIXTURES
FIQURE6
Mixtures of Pliolite and rubber partake of the characteristics of the two mixed ingredients. The rubber tends to function as a solid, nontacky plasticizer for the Pliolite; and as the rubber content is increased, the mixture takes on more of the characteristics of rubber and here Pliolite functions as a stiffening and hardening agent for rubber.
Compounding Data As an example of the characteristics of mixtures of Pliolite and rubber, we have obtained some data on the followlng test formula : Rubber and/or Pliolite Zinc pxidq Stearic acid Laurio acid Zino dimethyl dithiocarbamate (Zimate) Mercaptobenzothiaaole (Captsx) Di-8-naphthyl-p-phenylene diamine (Agerite white)
100.0 6.0
1.5
1.6
391
tallites. Then, as the curve rises on the high Pliolite side, it is possible for the mixture to begin to partake of the tensile properties of the Pliolite resin rather than the discontinuous properties of the equal-part mixtures. Figure 4 shows x-ray diagrams of Pliolite, rubber, and mixtures of the two, unstretched and stretched; they clearly point out the development of crystallites during stretching. The characteristic elongation a t break of Pliolite-rubber mixtures is shown in Figure 5. As the Pliolite content of the mixtures increases, the elongation decreases. Figure 6 gives the modulus a t 300 per cent on the range up to BO Pliolite-40 rubber. The higher concentrations of Pliolite broke a t less than 300 per cent elongation. This curve shows that as the Pliolite is added, the modulus is decidedly increased; examination of these stocks by hand brings out this difference even more strongly than the curve shows. The stocks become stiffer as the amount of Pliolite is increased, and they possess the ability to stand under their own weight in sheet form after about 30 parts Pliolite-70 parts rubber has been reached. Figure 7 shows their performance under their own weights. They also possess another interesting property which could be called “resistance to cutting” or “resistance to shear”. As the Pliolite content of the stock is increased, the shear resistance as measured by a pair of hand shears is markedly increased (Figure 8). It is difficult to cut a ‘/s inch slab of the 70 Pliolite-30 rubber mixture with shears. Figure 8 also shows the rebound of Pliolite-rubber mixtures as measured by the pendulum method. As the Pliolite is increased, the rebound decreases until a flat portion of the curve is reached toward the higher loading range. Figure 8 also shows the hardness of these mixtures when measured by Shore durometer, types A and D. The type A durometer is conventionally used for measuring the hardness of rubber stocks, while the type D durometer is for hard rubber or plastics. The type A was capable of reading only the first two ranges of the Pliolite-rubber mixtures, and a marked increase in hardness was noticeable. On the type D curve increasing additions of Pliolite to rubber gave increasing hardness; these data seem to line up in the same relative order that measurements on shear resistance would show. I n Figure 8 Grasselli abrasions on Pliolite-rubber mix-
0.2 1.5 2.0
This is a relatively fast-curing formula, and a range of cures was obtained running from 1 to 10 minutes a t 300”F. 1 It was found impractical to test these stocks using the usual centimeter dumbbell at the usual gage thickness of approximately 1/17 inch. It was necessary to reduce the gage to approximately 0.035 inch with a width of l/g inch. A comparison of the smaller dumbbell with the conventional type is given in Figure 2. The maximum tensile strength of the various mixtures over this range of cures gives the curve shown in Figure 3. It is typical of these mixtures in that it shows high values at the ends where one ingredient predominated (either Pliolite or rubber) with a lower plateau in the center where the two ingredients are mixed in more nearly equal proportions. We were somewhat at a loss to explain this phenomenon until some light was shed upon it through the x-ray diffraction diagrams. From these studies it seems that, for rubber to develop its maximum strength, a considerable number of crystallites must be formed upon stretching. This crystallization is induced by strain and begins to take place at elongations somewhere in excess of 300 per cent, intensifying as elongation increases. The theory is that the low plateau effect in these mixtures is due to the Pfiolite preventing the rubber from stretching enough for the formation of crys-
FIQURE 7. RELATIVESTIFFNESS OF PLIOLITE-RUBBER MIXTURES
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
392
/
6000
000 000
TENSILE STRENGTH PLlOLlTE-D.P. RUBBE' MIXTURES
5006
'\ BUITTLf
b
060
-
SWEAR RESISTANCE
400
Vol. 33, No. 3
PLIOLITE RUB0ER HlXTURES
,000
0
0 IO ZO 100 90 90
30 70
40 50 60 70
80
40 30
20
60
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0
%RUBBE1
300% MODULUS AND ELOWCATION A T O R E 6 K OF PLlOLlTE -0.P. RUBBER RIXTURES
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PERCENT REBOUND
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OF PLIOLITC-RUBBER
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MIXTURL
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% RUBBER HD
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40
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60
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TYPE A
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SHORE DUROMETER
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HARDNESS
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/
PLIOLITE-RUBBER MIXTURES
TYPE
D
WATER ABSORPTION CURVE
.oox PLIOLIl'E-iP. RUBBER MIXTURES
DUROMETER
sb
0
IO
100
90
tO 90
30 70
PLioLiir P 66 SO 40 60 sb RUBBER
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1m
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CRASS€ LLI ABRASIONS FIGURE 9
ON
P L IO LITE- RUBBER
MIXTURCS
tures are also plotted. A mixture of 20 Pliolite-80 rubber gives a much lower loss per horsepower hour than the allrubber stock, and these values flatten off as the Pliolite content is increased. Some interest has been evidenced in mixtures of Pliolite and deproteinized rubber for electrical insulation. A tensile strength curve over a range with deproteinized rubber is shown in Figure 9, as well as the modulus and elongation a t break and the water absorption of these mixtures.
0
% PLIOLITE 0 */Q RUBBER 100
,
20 80
30
40
70
60
FIGURE 8
50 50
60 40
70 30
60 %O
Pliolite with Synthetic Elastomers Pliolite was compared as a component mixed with synthetic' elastomers. For this work neoprene, Thiokol, Vistanex, and Chemigum were used, and thc ratios tried are shown in Figtwa 10. These data indicate but do not show that higher tensile'
I
March, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
NlXTURES OF PLIOLITE WITH SYNTHETIC
ELASTOMERS
f y
1ow
NEOPRENE
THIOKOL
VI5TANEX
CHElllCOPl
FIGURE 10
strengths are obtained from neoprene, Thiokol, and Chemigum mixtures if the stock is cured, whereas curing has no particular effect upon the tensile strength of Vistanex mixtures. The usual stiffening and increased resistance to cutting is obtained in these stocks as well as in rubber.
Commercial Uses of Pliolite-Rubber Mixtures The so-called synthetic balata mix D-30-T14 for high Pliolite-low rubber ratio is used widely as a golf ball cover stock; it is employed as a basic ingredient, and more balata OT rubber is added as required. The resiliency of such mixtures-for example, when tried in driving tests on golf balls covered with this stock-is somewhat greater than the resiliency of balata, and a tough noncutting cover stock can be obtained. Another use is in the field of molding protective helmets, which have high impact resistance and are quite light. Still another application is in molded goods which need to be stiff, light, and shock resistant. Other uses of Pliolite-rubber mixtures have been made in the field of coated fabrics; Pliolite has been utilized in the dough for coating various types of fabrics to make a nontacky, abrasion-resistant, glossy coating which embosses sharply and performs exceptionally well in usage. Fabrics €or quarter linings, raincoats, golf jackets, and other sportswear are on the market with these mixtures. The Pliolite content of such mixtures generally ranges from 10 to 25 per cent. The rubber is taken care of by the usual vulcanizing, and the methods of making the dough are of two types: I n the first application the Pliolite resin powder is cut in a solvent, and this solution is blended with the necessary rubber cement which has the compounding and curing ingredients added to it. The second method is to cut the 50-50 Pliolite-rubber master batch 29902H in a solvent along with the rubber and make up the dough in one operation.
393
Another growing field for Pliolite-rubber mixtures is in the field of wire insulation. It has been found that Pliolite functions as a stiffening ingredient of excellent dielectric properties, and such mixtures are highly adaptable for the extrusion of thin-wall insulation. It has always been a problem with the wire industry to get conventional rubber insulating compounds to a gage thickness which can be maintained when the gage is thin. The addition of 10 to 25 per cent of Pliolite as a 50-50 rubber-Pliolite nonproductive to a correctly formulated insulating compound practically overcomes all tubing difficulty and makes it possible to run this insulation a t high speed. It also gives a compound which possesses excellent insulating values; further, such a material meets the underwriters’ specification as a pure gum compound. These compounds have been found to give insulation resistance in excess of 10,000megohms per 1000 feet of wire a t 15.5”C., (59.Q0F.), power factors of less than 0.8 after immersion for one week in water a t 70” C. (158” F.), and maximum dielectric constant of 2.65.
Aclmowledgmen t The writer wishes to acknowledge the helpful assistance of M. J. DeFrance for the compounding data and of S. D. Gehman and J. E. Field for the x-ray data discussed.
Literature Cited (1) Bruson, Sebrell, and Calvert, IND. ENG.CHEM.,19, 1033 (1927). (2) Endres, H. A. (to Wingfoot Corp.), U. S. Patent 2,052,391 (Aug. 25, 1936). (3) Fisher, H.L.,Chem. Rea., 7,51 (1930). (4) Fisher, H.L. (to B. F. Goodrich Co.). U. S. Patent 1.605.180 (Nov. 2, 1926). (5) Fromandi, G., Kautschuk, 4, 185 (1928). (6) Harries, C. D.,Ber., 46, 733 (1913). (7) Harries, C. D.,“Untersuchungen uber die natiirlichen und ktinstlichen Kautschukarten”, Berlin, Julius Springer, 1919. ( 8 ) Himlv. Ann.. 27. 40 (1838). (9) Hock; L.,2. EZektrochenz.,’34, 664 (1928). (IO) I. G. Farbenindustrie, Brit. Patent 382,755 (1932). (11) Jones, W. A.,and Winkelmann, H A. (to B. F. Goodrich Co.), U. 9. Patent 1,751,817(March 25, 1930). (12) Kirchhof, F., Kolloid-Z., 27,311 (1920). (13) Lawson, W. E. (to E. I. du Pont de Nemours & Go.), U. S. Pab ent 2,018,676(Oct. 29, 1935). (14) Nielsen, A., Kautschuk, 9, 167 (1933). ENG.CHEM.,19, 1033 (1927). (15) Sebrell, Bruson, and Calvert, IND. (16) Shadbolt, F. S. (to Walpamur Co.), U. S. Patent 2,052,672 (Sept. 1, 1936). (17) Staudinger, H.,HeZv. Chirn. Acta, 9,529 (1926). (18) Staudinger and Bondy, Ann., 468,1 (1929). (19) Staudinger and Geiger, Helv. Chim. Acta, 9,549 (1926). (20) Thies, H.R.,Packaoino Parade, 8, 16 (1940). (21) Thies, H.R.,Paper Trade J., 108,79(1939). (22) Thies, H. R.,and Clifford, A. M., IND.ENG. CHEM.,26, 123 (1934). (23) Walpamur Go., French Patent 752,502(Sept. 23,1933). (24) Walz, E. (to I. G. Farbenindustrie), U. 5 . Patent 1,987,171 (Jan. 8, 1935). PRESENTED before t h e Division of Rubber Chemistry a t the 100th Meeting of t h e American Chemical Society, Detroit, Mich.
