I
1. 0. AMBERG and A. E. ROBINSON Research Center, Hercules Powder
Co., Wilmington,
Del.
Ethylene- Propylene Rubber Uses of ethylene-propylene rubber are based on its superiority in
v' v' v'
ozone and weathering resistance chemical inertness lightness of color
A saturated rubber is inherently very resistant to deterioration, and particularly resistant to cracking induced by ozone. Until recently, however, it has been necessary to accept unsaturation in general-purpose hydrocarbon rubbers since a base must be provided for vulcanization with sulfur and accelerators. I n the past few years, development of new peroxides has provided a practical route for effecting cross-linking \vhich is applicable to saturated hydrocarbon polymers. Thus, it is now possible to utilize the inherent stability of such polymers in typical applications for vulcanized elastomers. Work in these laboratories and those of Farbwerke Hoechst AG. has shown that ethylene and propylene can be copolymerized with Ziegler-type catalysts (5, 6) over the complete composition range. By varying both composition and molecular weight, copolymers can be made which range from buttery or balsamic salves, through typical elastomeric rubbers, to tough semirigid products. Xatta (3') has also published in this field. Vulcanization is difficult at the high propylene end of the composition range. Very high ethylene copolymers tend to be boardy because they crystallize. Thus, preferred polymers for elastomeric use have a moderately high ethylene content for ease of vulcanization with enough propylene to provide elastomeric properties.
V'
resistance to soiling resistance
v' a brasion
omer distribution, with low chain branching and residual unsaturation. Ethylene-propylene rubber (EPR) does not break down appreciably on milling. Also, it does not appear to respond to conventional peptizers. As a result of this stability, the Mooney viscosity of the raw polymer must be lower than that of styrene-butadiene rubber (SBR) to arrive at the same compound viscosity. I n general, EPR with a polymer Mooney viscosity of about 30 to 40 gives the same compound viscosity as SBR with a polymer Mooney viscosity of 55 to 65. The poor building tack of EPR at room temperature makes fabrication of some products difficult. Certain resins, such as Staybelite (Hercules PoTvder Co.) resin and Amber01 ST 137X, are partially effective in improving building ldck. EPR does not bond directly to conventional rubbers with enough strength
to resist delamination on flexing thick sections such as tire treads. Butyl rubber is an exception. A direct bond is obtained with butyl which should be strong enough to allow manufacture of butyl-lined EPR tires. Indirect bonds can be obtained with halogenated butyl tie gums. These tie gums are particularly effective in retreading SBR tires lvhere a bond to cured SBR surfaces is required.
Vulcanization of EPR Peroxides have been the most promising curing agents for EPR. Dicumyl peroxide (Hercules Di-cup), alone or in combination with sulfur or other coagents, gives high quality vulcanizates. This curing system does not use additives to change vulcanizarion time at a given temperature because the peroxide normally forms free radicals at a rate deter-
+-y-T E N S I L E S T R E N G T H ,
P. S , I .
-9-
2860 60
Composition and General Properties Typical copolymers selected for this evaluation contain 60 to 70 mole 70 ethylene and have Mooney viscosities (ibfL4, 212' F.) of 30 to 45. Their specific gravity is about 0.87. The polymers show little crystallinity and evidently have essentially random mon-
368
INDUSTRIAL AND ENGINEERING CHEMISTRY
20
0
20
40
60
80
PRESS C U R E T I M E A T 3100 F. , M I N U T E S
Figure 1 .
EPR is very flat curing
The composifions contained 50 ports HAF and were cured with 4 parts Di-cup Rand 1 part sulfur
mined by temperature only ( I ) . Figure 1 shows the effect of cure time from about 50% undercure (20 minutes) to about 10070 overcure (80 minutes) on physical properties. EPR is very "flat curing" ; vulcanizate properties change very little on overcure. Experience with peroxide vulcanization of rubbers, polyethylene, and silicones has shown that in development of optimum formulations it is necessary to take into account possible nonproductive ionic reactions of the peroxide, and reactions between free radicals formed from the peroxides and the compounding ingredients. EPR is more difficult to cure than SBR, and as expected the effects of the compounding ingredients are greater in the saturated polymer. To avoid complications introduced by additives, most of the following EPR data were obtained by testing compounds containing only the polymer, 50 p.h.r. HAF black, and the curing ingredients. Table I shows some typ,ical properties obtained with three curing systems: dicumyl peroxide ; dicumyl peroxide and sulfur; and dicumyl peroxide, sulfur, quinone dioxime, and red lead. O n a basis of over-all properties, the 4 parts of dicumyl peroxide and 1 part of sulfur curing combination was chosen for general use in EPR work. However, modulus (and therefore dynamic properties) can be maintained while simultaneously varying dicumyl peroxide and sulfur. Lowering both dicumyl peroxide and sulfur gives improved odor (particularly in eliminating an organic sulfide odor), but there may be some sacrifice in tensile and tear strength.
Physical Properties and Environmental Response EPR is similar to SBR in depending on reinforcing pigments to give physical properties high enough for satisfactory performance. As shown in Figure 2, a gum stock had a tensile strength of only 435 p.s.i. When reinforced with FEF, HAF, or SAF blacks, strength was increased to the 2000 to 3300 p.s.i. range. Tear strength of EPR is lower than that of SBR. T h e significance of the low tear strength is yet to be determined. Probably the outstanding property of EPR is its ozone resistance. Samples have not cracked after more than a year of exposure, either static outdoors, or on flexing in the laboratory a t the usual 50 parts-per-hundred-million ozone concentration. Resistance of E P R to oven aging a t 250°, 400°, and 50O0 F. is shown in Table 11. T h e test compound contained only a small amount of antioxidant. Elongation a t break was 30070 after 168 hours a t 250' F. After 72
Cure Systems for EPR
Table I.
50 p.h.r. HAF
Cure Systems
Di-cup R 4
Di-cup R 4 Sulfur 1
30 320 2690 1310 320
40 3 10 3320 1135 420
Cure Minutes Temperature, 'F. Tensile strength, p.s.i. Modulus at 2OOyo, p.s.i. Elongation at break, %
Di-cup R
4
Sulfur GMF Red lead
2
2 10
40 3 10 3340 1375 415
700
I bo0
300 -
200- ELONGATION
i
100
-0 0
50
100
-
;
O
-
-
AT BREAK
0
O
CARSON BLACK,RHR.
Figure 2.
Carbon blacks reinforce EPR as with conventional rubbers
The compounds were cured with 4 parts Di-cup R and 1 part sulfur
hours a t 400' F., the same composition had lost most of its strength and was brittle on the surface. However, softening and embrittlement had progressed inward from the surface, indicating that the deterioration probably is oxidative and might be protected by properly selected antioxidants. At 500" F. deterioration was rapid, as was expected with a hydrocarbon rubber. After 15 minutes aging modulus had decreased markedly and elongation increased somewhat, whereas tensile strength decreased to about one third of the original value. There was no apparent surface resinification. Properly chosen antioxidants added during compounding or as a surface coating after cure give further improvements in aging resistance. T h e resistance of EPR to gasoline and other solvents is about the same as that
Table II.
of other hydrocarbon rubbers. However: as shown in Table 111, vulcanized SBR usually is slightly more resistant to aliphatic hydrocarbons than is EPR, as might be expected, whereas EPR is m-ore resistant to aromatic hydrocarbon and aromatic ester solvents. Although these differences are small, they could be important to some applications, such as automotive mechanical goods. Dynamic Properties Heat build-up of EPR tread-type compounds i s a function of modulus, as shown in Figure 3. This curve resulted from testing polymers of varying viscosity with the same curing combination. A similar curve results when the modulus of a single polymer is varied by changing the curatives.
Effect of Aging Conditions on Resistance of EPR 50 p.h.r. HAF;
Temperature, O F. Time, hr. Modulus at 300%, p.s.i. Tensile strength, p.s.i. Elongation at break, yo Hardness, Shore A2 Resilience, Bashore, yo Break set, yo
tested at room temperature
Control 1560 2940 480 55 37
5
250 168 2200 2200 300 60 38 10
VOL. 53,
400 72 Brittle 48 5 0 83 27 0
NO. 5
500 0.25 360 1040 630 46 34 55
MAY 1961
369
Table 111.
Tear
4 0 hours at room temperature
Plasticizer
Volume Increase, yo ERPa SBRa
Liquid Iso-octane Toluene Carbon tetrachloride Acetone Ethyl alcohol Dibutyl phthalate Tricresylphosphate
140-1 70 155-195 195-210
75 240 245 15
