INDUSTRIAL AND ENGINEERING CHEMISTRY
AUGUST, 1939
accelerators, especially of the aldehyde amine and mercaptan classes, are effective plasticizing agents. Several of the materials are as effective as the guanidines as plasticizers but give somewhat slower curing stocks. Most of the samples for the 30-minute cure had a Shore Durometer Type A hardness of 58-60. Several common rubber chemicals are classified according to their action as chemical plasticizers of Neoprene Type G in Table IV. This classification is only approximate, since there are appreciable variations in effectiveness among the different members of each class. The data show that Neoprene Type GCX may be plasticized to a greater extent than Neoprene Type GCR by either water washing or chemical means. Advantage of this is taken in the preparation of soft neoprene for the manufacture of sponge and in the compounding of soft raw mixes of ex-
939
cellent flow characteristics, where permanent softness such as results from the use of oils, factice, etc., cannot be tolerated. In certain cases it is advantageous to combine chemical plasticization with softening by water washing. For example, the time required to soften Neoprene Type GCR by water washing may be materially shortened by use of a small aniount of chemical agent.
Literature Cited (1) “Compounding Ingredients for Rubber,” New York, Bill Bros. Publishing Corp., 1936. (2) Starkweather and Wagner, IND.EXG.CHEM.,31, 961 (1939). (3) Williams, IND.ENQ.CHEM.,16, 362 (1924). (4) Williams and Smith, U. 5.Patents 2,018,643-4 (1935); 2,064,580 (1936); 2,132,505 (1938). CONTRIBUTION No. 41 from Jackson Laboratory, E. I. du Pont de Nemours & Company, Ino.
Effect of Modifying Agents on Vulcanized Neoprene Compounds The term “modifying agent” is used to designate materials which in rubber are normally considered as activators, accelerators, or vulcanizing agents. Their effect on the modulus, tensile strength, resistance to tear, hysteresis properties, and rate of cure in a neoprene tread type of compound are discussed. Included in the list of materials tested are various concentrations of catechol, @-naphthoquinone, Captax, triethanolamine, di-o-tolylguanidine, butyraldehyde monobutylamine condensate, zinc chloride, sulfur, and stearic acid.
-
T
HE term “modifying agent” is used to designate ingredients which when added to neoprene compounds before vulcanization change the rate of cure or alter the physical properties of the vulcanizate. In rubber technology, compounding ingredients having such effects would be termed accelerators, activators, retarders, or vulcanizing agents. The use of such specific terms has been avoided because it is not certain how the materials investigated should be classified. The effects of various modifying agents on the properties of Keoprene Type GW vulcanizates were studied in the following formula : Neoprene Type GW Extra light calcined magnesia Channel black Cottonseed oil Zino oxide
100 4 36 3 5
Modifying agents (some of which are shown in Table I) were added in varying amounts to this compound. Stress-strain properties were determined on a standard Scott tensile testing machine using dumbbell test specimens
MAYNARD F. TORRENCE AND DONALD F. FRASER Organic Chemicals Department, Rubber Chemicals Division, E. I. du Pont de Nemours L Company, Inc., Wilrnington, Del. (Die C, A. 8. T. M. Designation D412-36T) died from slabs, 3 X 6 x 0.085 inch in size, which had been press-cured 30, 45, 70, and 90 minutes a t 130.5’ C. (267’ F.). Tear resistance was determined by the Winkelmann method (3) with the ends of the test specimens modified to fit the ’ grips of a Scott testing machine. Heat build-up tests were run on a Goodrich Flexometer ( 2 ); a stroke of 0.125 inch, a load on the sample of 155pounds per square inch, and a frequency of 1800 cycles per minute were used on cylindrical pellets 0.75 inch in diameter and 1.00 inch high, press-cured 60, 90, 120, and 180 minutes at 130.5’ C. The results of these tests are shown by curves in Figure 1. Resistance to flex cracking was determined on a du Pont flexing machine (A. S. T. M. Designation D430-35T, Method C). It was found that all the neoprene vulcanizates tested had a resistance to flex cracking approximately ten times that of the best rubber vulcanizates of a tire tread type. In the interests of brevity the results of these tests are not reported. The results of the stress-strain and tear tests are shown in Table I. To show more clearly the effects of the modifying agents, the base neoprene compound contains 4 parts by weight (on 100 parts of neoprene by weight) of extra light calcined magnesia, rather than the 7 parts generally recommended in Neoprene Type GW compounds. The base compound contains 20 volumes of channel black per 100 parts by weight of neoprene (16 volumes channel black per 100 volumes of neoprene). The effects on the neoprene vulcanizate of various amines, including hexamethylenetetramine, triethanolamine, di-
INDUSTRIAL AND ENGINEERING CHEMISTRY
940
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VOL. 31. NO. 8
TABLE I. EFFECT O F MODIFYING AGENTSON THE STRESS-STRAIN PROPERTIES O F A NEOPRENE TYPEGW COMPOUND Modifying Agent
Parts per 100 Parts 7 - 3 0 minutesM T E of Neoprene by Weight
Time of Cure a t 130.5O C. (26T0 F . ) : O minutes--70 mlnutes-
--45
minutes-
T
E
TT
M
T
E
TT
2025 2350 2600 3100 3425 2225 2326 2400 2075 2225 2350 2025 2000 1800 1775 2250 2325
3600 3475 3500 3675 3525 3600 3700 3550 3275 3750 3300 3575 3625 3775 3700 3925 3525
700 600 540 490 420 700 680 600 660 680 600 660 700 800 740 680 660
48 49 35 24 25 51 44 43 54 58 59 51 54 55 56 49 41
2125 2375 2625 3125 3525 2325 2450 2675 2200 2300 2425 2275 2075 1875 1800 2425 2525
3700 3550 3475 3825 3650 3625 3800 3675 3350 3700 3575 3825 3725 3725 3675 3750 3600
660 600 520 490 420 690 670 600 640 680 620 660 700 760 740 670 600
47 45 31 27 31 48 44 41 51 47 48 53 46 53 47 41 41
Catechol 2000 3576 770 56 2300 3850 690 53 Sulfur Hexamethylenetetramine 2375 3325 590 56 2825 3400 490 30 Sulfur a M modulus a t 400% elongation, lb. per sq. in.; T = tensile strength a t break, lb. per sq. in.; E tear test, lb. per 0.10-inch thickness.
2600
3775
640
49
2725
3875
620
48
3325
3625
470
26
3400
3600
420
20
. Catechol
1775 1900 2100 2250 2700 1825 1975 2175 1650 1750 1850 1775 1775 1475 1425 1750 1725
0.10 0.25 0.50 1.00
0.10
0.25 0.50 0.10 0.50 1.00 0.10 0.25 0.75 1.00 1 00 3.00
@-Naphthoquinone Mercaptobenzothiazole
Sulfur
3400 3350 3275 3375 3275 3525 3400 3450 3175 3350 3125 3775 3625 3275 3150 3550 3425
750 53 720 56 660 47 600 40 500 32 800 58 700 51 690 46 700 59 760 47 600 56 760 57 780 54 850 58 840 56 800 56 800 60
T
M
1825 2125 2375 2700 3225 1975 2150 2375 1800 2050 2200 1975 1825 1600 1575 2000 2000
3450 3350 3450 3523 3500 3525 3550 3525 3200 3750 3475 3575 3600 3350 3450 3625 3575
E
--90
M
740 54 650 48 580 41 540 32 480 26 740 54 700 52 660 45 690 57 700 50 660 54 710 58 730 60 820 60 830 62 800 52 770 47
....
None Hexamethylenetetramine
TT
TT
::;;1
:.a
o-tolylguanidine and butyraldehyde-monobutylamine condensate ("Du Pont Accelerator 833"), were studied when the amines were added in proportions up to one per cent by weight on the weight of the neoprene. The effects of these amines were of the same general trend, but hexamethylenetetramine appeared to be the most active, the effectiveness of the other amines being in the order shown above. In general, the effect of the amines is to increase the modulus with a consequent decrease in the elongation a t break, while the tensile strength is affected very little if a t all. The organic basic accelerators in general improve the tear resistance on the short cures but shorten the range of cure as evidenced by the rapid decrease in tear resistance with continued curing. On the Goodrich Flexometer the vulcanizates containing hexamethylenetetramine showed a lower heat build-up than any of the other amines or any other single modifying agent tested. The heat build-up after 15 minutes of flexing of the vulcanizates containing the various modifying agents
NEOPRENE TYPE O W 100. EX. LT. CALC. MOO 4.
P
CYIMUCl
I
Y v)
3
f
*
mI
LFY
COTTONSEED OIL ZINC OXIDE
-
IP
3
STROL
nin FREQ,- 1800 CYCLES PER MIN.
!
CURE AT 267O F.- AS SHOWN
C
c
I-CCoNTROL+JI)O$SULFUR 0.60% H E X A
+
RUBBER CONTROL
-
per cent elongation a t break; TT = Winkelmann
is shown graphically in Figure 1. Included in the figure is the heat build-up curve for a rubber tread stock compounded as follows: Smoked sheets Zinc oxide Channel black Stearic acid Pine tar
100 5 50 2 1 5
Phenyl-@-naphthylamine Thermoflex A Zenite B Sulfur
0 6
1 . 4 (1) 1 . 2 (1) 3
Catechol and /?-naphthoquinone were selected as representatives of hydroxy and quinone compounds, respectively. These materials produce the same general effects as hexamethylenetetramine, but it is necessary to use about twice the amount of catechol and about four times the amount of @-naphthoquinoneto produce vulcanizates having the same moduli as^ is obtained with hexamethylenetetramine. These agents, particularly /?-naphthoquinone, do not decrease the tear resistance on the longer cures as much as even smaller amounts of hexamethylenetetramine. Catechol and @-naphthoquinone decrease the heat build-up. Mercaptans (exemplified by mercaptobenzothiazole in Table I) retard the cure as shown by a decrease in modulus and an increase in elongation a t break, and a t the same time improve the resistance to tear. This retardation effect is more noticeable when larger amounts of modifying agents of this type are used. Sulfur activates the cure when used in amounts up to 3.0 per cent. The principal advantage derived from the use of sulfur is the marked decrease in heat build-up obtained both by its use alone and in combination with hexamethylenetetramine or catechol. The neoprene vulcanizate containing a combination of sulfur (3 per cent) and catechol (0.25 per cent) has a better resistance to tear and a higher modulus than vulcanizates containing the same amount of either modifying agent alone. The heat build-up is noticeably lower and approaches that of the rubber control. When 0.5 per cent hexamethylenetetramine replaces the catechol in the combination, the heat build-up is still lower (Figure 1). However, this advantage is somewhat offset by the rapid decrease in tear resistance on the long cures that results from using this combination. Neoprene Type GW compounds are sensitive to modification by certain chemical compounds, and the properties of a given vulcanizate can be appreciably varied to suit definite requirements by an intelligent selection of modifying agents. As an example it is reasonable to expect that the vulcanieate modified by the incorporation of 3 per cent sulfur and 0.25 per
AUGUST, 1939
INDUSTRIAL AND ENGINEERIKG CHEMISTRY
cent catechol would prove more serviceable for a particular application such as a tire tread or a conveyor belt cover than the same vulcanizate without these modifying agents. However, for a complete evaluation of such a compound it would be necessary to study the effect of the modifying agents on the stability of the unvulcanized compound and the effect on the physical properties of the vulcanizate a t elevated temperatures. The fact that the properties of a given vulcanizate can be so varied opens up a new field of compounding research since it is obvious that other combinations of these or other chemicals may be expected to produce similar 01more desirable results.
941
The carbon black in the compounds is a type commonly used in the rubber industry, but it should not be inferred that the varieties of carbon black that are found to be best in the compounding of rubber to meet definite physical requirements are necessarily best for the compounding of neoprene to meet the same requirements.
Literature Cited (1) “Compounding Ingredients for Rubber,” New York, Bill Bros. Publishing Corp., 1936. ( 2 ) Lessig, E. T., IND. EKG.cHEM., Anal. Ed., 9, 582 (1937). (3) Winkelmann, Proc. Am. SOC.Testing Materials, 31, 897 (1931).
Nomenclature of Synthetic Rubbers HARRY L. FISHER U. S. Industrial Alcohol Company, Stamford, Conn.
A brief history of synthetic rubbers and of synthetic rubberlike substances is given, together with an outline of the reasons for the continued use of the term “synthetic rubber,” especially for the rubberlike polymers of butadiene and its derivatives, including chloroprene. Recent and new class terms for rubberlike substances are given and their uses discussed. The new term “elastomers” is suggested to cover all rubberlike substances, “elastoprenes” for the diene polymers, “elastolenes” for the polyisobutylenes, and “elastothiomers” for the polyethylene sulfides (Thiolrol). It is also suggested that Stevens’ term “elastoplastics” be used for the growing class of rubberlike plastics such as plasticized vinyl chloride (Koroseal), certain polyacrylic esters, etc. “Plastomer” is used for the true and the thermosetting thermoplastics. A classification of all rubberlike substances is tabulated.
IXTY years ago, synthetic indigo, a dream of organic chemists, became a reality. Thirty years ago the increase in the knowledge of the chemistry of rubber made organic chemists dream also of synthesizing that important natural product. However, rubber or, more specifically, the rubber hydrocarbon has not yet been synthesized (8) in spite of the tremendous amount of time and effort already expended on this intriguing problem. But in scientific journals, books, newspapers, and advertisements there is much that is published about “synthetic rubbers.” Indigo was synthesized and there is no question about speaking of synthetic indigo. Since rubber has not been synthesized, why do we hear so much about synthetic rubbers? Are there any other names which could be used to describe the various “synthetic rubbers” and other rubberlike
S
products? These are the questions which will be discussed in this paper. Indigo is crystalline and can be purified easily; the rubber hydrocarbon can be crystallized only under very special conditions, cannot be distilled, and therefore is not easy to purify. Indigo has a comparatively low molecular weight which can easily be determined; the rubber hydrocarbon is an elastic polymer, the molecular weight of which is very high and cannot be determined with precision. A determination of the identity of the natural and synthetic samples of indigo can readily be made, but with rubber it is almost impossible with our present methods to show the absolute identity of two specimens, although it is possible for all practical purposes to do so.
History of Synthetic Rubber
As long ago as 1860 Williams (42) separated isoprene as a definite compound among the products of the destructive distillation of rubber. Fifteen years later Bouchardat (4) recognized the probable relation of isoprene to rubber and actually converted it to a rubberlike solid. I n 1882 Tilden (38) discussed the possible industrial significance of the polymerizability of isoprene, provided it could be obtained commercially from a source other than natural rubber; in 1892 he reported (39) that isoprene which had been prepared from turpentine had spontaneously polymerized to a rubberlike product-he said “rubber.” Early in the present century Kondakoff (20) found that 2,3-dimethylbutadiene, a homolog of isoprene, slowly polymerizes to a rubberlike product, and similar observations were made concerning piperylene or 1methylbutadiene (16, 37), and butadiene itself (14, 21). Commercial quantities of “synthetic rubber” were manufactured in Germany during the World War from 2,3-dimethylbutadiene; the product was known as methyl rubber (13,30,41). There was a lull in synthetic rubber research until the rise in the price of rubber in 1925 furnished a stimulus to further work. In the present decade wonderful strides have been made, and now large quantities of “synthetic rubber” are being made from butadiene in Germany and Russia, and from a chloro derivative of butadiene (chloroprene) in this country. These “synthetic rubbers” are all prepared by processes of polymerization, and they bear a strong resemblance to natural rubber. They differ chemically from natural rubber,
