THE USE OF EPDM IN BUTYL INNER TUBES FOR IMPROVED PERFORMANCE
Thomas L. Jablonowski and Julian M. Mitchell Uniroyal Chemical Co., Inc.
Naugatuck, Connecticut, USA
and
Dr. B. Suryanarayanan Herdillia Chemicals
Limited Bombay, India
INTRODUCTION
Inner tubes for automotive, bus, and truck tires are typically made of butyl rubber because of butyl
rubber's outstanding resistance to air permeability.
In India, consumption of butyl rubber has .increased
steadily and is expected to reach a level of 16,000 tons/yr. by the year 1990.
At present, India's entire
requirement of butyl rubber is met by imports and there are no definite plans to manufacture it indigenously
in the near future.
The foreign exchange outgo due to butyl imports in 1990 will be on the order of USD 30
million at the current price level.
Butyl rubber suffers from certain disadvantages which are generally tolerated in the manufacture
and use of inner tubes, such as marginal green strength, and fair ozone, heat, and reversion resistance.
These disadvantages can generally be overcome or minimized by the partial replacement of butyl rubber with
EPDM (ethylene-propylene-diene rubber).
Heat softening due to reversion is of particular importance in
the severe service conditions encountered in India.
For example, in high speed, heavy-load truck
and bus tire service where the tube temperature can exceed 130° С.,
butyl inner tubes on prolonged exposure tend to soften due to reversion.
This tendency can be minimized
by blending EPDM with the butyl rubber.
Although EPDM is imported into India at present, Herdillia Chemicals Limited plans to manufacture
it by 1990 and is in the process of setting up a plant to manufacture 10,000 tons/year of EFR and EPDM-
Thus,
it is very appropriate to study the partial substitution of butyl rubber in inner tubes with EPDM.
The
advantages arising out of such blending are as follows:
1.
Improvements in green strength, heat and reversion resistance, and ozone resistance.
2.
Raw material cost reduction, since the cost of EPDM is generally lower than that of butyl
rubber.
3.
Compound cost reduction due to the higher filler and oil loadings possible in butyl/EPDM
blends.
4.
Savings in foreign exchange.
5.
Indigenous availability.
Although the air permeability of the EPDM/butyl blends is generally increased somewhat compared to
the 100% butyl compounds, the increase in permeability does not have an effect on the practical use of these
blends for truck, bus, and automobile tubes.
EXPERIMENTAL
The compounds discussed in this paper were mixed according to the formula in Table I.
The formula
is a typical of inner tube formulas found in the literature 2'3'4. It uses a medium unsaturation butyl rubber
which gives a good balance of fast cure rate and good heat resistance.
The EPDM has a low level of ENB
(ethylidene norbornene) unsaturation which gives a cure rate similar to the butyl and gives excellent heat
resistance.
EPDM, due to its saturated molecular backbone, has excellent resistance to the
effect of ozone.
As seen in Table II, the 100% butyl compound shows the first signs
of ozone cracking within 24 hours in an ozone chamber, while the compounds with 25 and
35 parts of EPDM last 11 times longer before the ozone shows an effect. Fifteen parts
of EPDM seems to offer little, if any, improvement in the ozone resistance over the
100% butyl compound and 35 parts of EPDM offers little additional ozone resistance over
the 25 part compound.
Green Strength
Green strength is an important parameter for tube manufacture as far as preventing
thinning during handling and curing.
Blending EPDM with butyl generally improves the
maximum green strength and the resistance to thinning as seen by the improvement in
Tmax and Tz' values in Table II and Figure 3.
Die Swell
Die swell is a measure of the increase in size of an extrusion after the compound
exits the extruder die.
Blends of EPDM with butyl generally have slightly lower die
swell than 100% butyl compounds, as seen in Table III.
The lower swell allows wider
die openings which can reduce scorching tendencies of the compound.
Building Tack
The tack of butyl tube compounds is important to their manufacture because of
its effect in allowing strong cured splices. As seen in Table II, the Monsanto Tel-Tak
of 100% butyl and 85/15 butyl/EPDM blends is greater than 0.20 MPa.
This is due to
the fact that the tack exceeds the green strength of the compounds.
The tack of the
75/25 and 65/35 butyl/EPDM compounds is 0.27 MPa and 0.20 MPa, respectively, indicating
that the tack does drop off at higher levels of EPDM but the tack values are still
excellent.
Of interest is the fact that the cured splice adhesion of all the compounds,
determined by peeling apart press-cured plies of the compounds, showed only stock
failure, indicating excellent cured splice strength.
Air Permeability
Butyl rubber is one of the best elastomers for low permeability to gases.
for widespread use in the inner tube application.
This property is its reason
The air permeability of butyl/EPDM blends increases as
the level of EPDM increases, as shown in Table III. Although the air retention properties of the butyl/EPDM
blends are poorer than that of 100% butyl compounds, they have proven to be satisfactory for inner tube service.
The differences in air loss are generally insignificant in light of the trend in trucking industry regulations
that tire air pressures be checked daily.
Actual air pressure losses for truck tubes of 100% butyl and 80/20
butyl/EPDM compounds, shown in Appendix 2, show minor differences in air loss on a daily basis.
Low Temperature Resistance
EPDM polymers are excellent for low temperature characteristics as typically measured by low
temperature brittleness or other tests. Butyl rubber is also good in this respect but not at good as EPDM.
Resistance to low temperatures is especially important for service in cold regions of the world.
Blends of butyl/EPDM with as little as 15 parts of EPDM have shown to make an appreciable difference
in the low temperature brittleness point of inner tube compounds 5.
The reference gives typical data
as follows:
100%
85/15 BUTYL
BUTYL/EPDM
BRITTLENESS TEMPERATURE, deg C.
-47
-54
CONCLUSIONS
Although in most cases butyl rubber is the polymer of choice for inner tube applications because
of its outstanding resistance to air permeability, blends with EPDM as the minor component have shown to
offer benefits in improving several of the properties deficient in all butyl compounds.
The properties that
EPDM blends can generally improve are ozone resistance, heat and reversion resistance, green strength, and
lower die swell.
The increase in air permeability seen with the addition of EPDM to butyl, although measurably
significant, has little effect on the practical use of these blends for truck, bus, and automotive inner
tubes.
The benefits listed above, plus the benefit of lower compound costs, are the reasons many inner tube
manufacturers (including most of the United States tube manufacturers) have gone to butyl/EPDM blends for
this application.
REFERENCES
1
1. Pliskin, Rubber Chemistry and Technology, V46 (1973),
2
p. 1218-1233.
Exxon Butyl Rubber Formulary (1974). 3
R.L. Zapp and P. Hous, Chapter 10, Rubber Technology,
2nd
edition, M. Morton, editor (1974). 4
Polysar Butyl Handbook (7/77). 5
Garilov, and V.A. Sapronov, International
Polymer Science and Technology, V3, No.4 (1976), p. Т/45-47.
N.P. Zueva, V.M.
TABLE II PROPERTIES OF BUTYL/EPDM BLENDS
1A
BUTYL 268
EPDM
PRESS CURE 10'/175 deg. С.
HARDNESS, SHORE A 100% MODULUS,
2J
3F
100
85
75
65
0
15
25
35
52
56
56
57
1.4
5.1
12.4
630
35.0
35.9
1.7
6.3
13.2
590
31.5
35.0
1.8
6.7
14.8
620
35.0
38.5
1.7
6.9
14.7
610
26.3
40.3
4.0
-68
630
0
15.8
-55
17.5
4.3
-67
280
-53
18.4
-42
22.8
5.6
-62
240
-61
21.0
-40
24.5
6.5
-56
220
-64
21.9
-17
26.3
-51
-35
-36
-35
2.1
-84
120
-80
10.0
-68
8.9
-75
3.3
-78
120
-81
12.1
-66
10.0
-74
4.1
-72
120
-80
15.4
-41
12.4
-69
MPa 300% MODULUS, MPa TENSILE
STRENGTH, MPa ELONGATION, % TEAR,
DIE B, kN/m TEAR, DIE C, kN/m
OVEN AGED 24 HR/150 deg. C.
TENSILE STRENGTH, MPa
% CHANGE ELONGATION, %
% CHANGE TEAR, DIE B,
kN/m
% CHANGE TEAR, DIE C,
kN/m
% CHANGE
OVEN AGED 166 HR/150 deg. C.
TENSILE STRENGTH, MPa
% CHANGE ELONGATION, %
(TACKY)
% CHANGE TEAR, DIE B,
(V TACKY)
kN/m
% CHANGE TEAR, DIE C,
kN/m
% CHANGE
(ТОО)
(POOR)
(ТО)
(TEST)
4.4
-88
3.8
-90
4K
TABLE II (cont'd) PROPERTIES OF BUTYL/EPDM BLENDS
1A
BUTYL 268
0
100
15
25
2J
85
3F
75
4K
65 EPDM
35
OZONE RESISTANCE -- BENT LOOP, 50 MPa, 38deg. С.
HOURS TO VVS CRACKING
24
24
264
264
GREEN STRENGTH
Tmax, MPa
8
12
33
17
28 % RETAINED
50
72
24
24
0.20+
0.25+
148 T2', MPa
24
19
MONSANTO TEL-TAK
MPa
STOCK-TO STOCK ADHESION
--ALL WITH 100% STOCK FAILURE—
0.27
0.20
TABLE III
DIE SWELL
BUTYL
A
В
100
75
25
EPDM
CAPILLARY DIE SWELL AT 116 deg. С.
190%
150%
TABLE I
INNER TUBE FORMULATION
BUTYL1
AS SHOWN
EPDM2
AS SHOWN
N660 CARBON BLACK
70
SUNPAR 110 OIL3
28
ZINC OXIDE
5
MBTS
0.5
TMTD
1.0
SULFUR
2.0
TOTAL
206.5
1
ML(1+8)/125 deg. C. = 50, 1.5% mole unsaturation
2
ML(1+4)/125 deg. C. = 58, Ethylene/propylene ratio = 60/40, % ENB
= 2.0
3
Paraffinic oil from Sun Oil Co.
TABLE IV
AIR PERMEABILITY
А
С
BUTYL
0
100
20
В
80
70 EPDM
30
AIR PERMEABILITY, FT^/MIL/DAY AT STP
0.0005
0.0014
0.0017 RELATIVE AIR PERMEABILITY
1
2.8
3.4
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APPENDIX 1
GREEN STRENGTH TEST FOR INNER
TUBE STOCK
This test simulates the forming operation in innertube manufacture and has been used
successfully in predicting stock performance.
SAMPLE PREPARATION:
Use plant tubed stock or lab milled sheet prepared as follows:
a.
Mill compound on back roll of hot (70-80^ С.} mill.
Adjust to 2.5-3.8 mm
(.100"-.150") gauge.
b.
Add sufficient stock to give small "pencil" bank and mill until stock is smooth
and air-free.
c.
d.
e.
Stop mill-remove sheet.
Mark grain direction and talc both sides of sheet
Allow to rest at least four hours before testing. Die out 12.7 mm (0.5") wide
tensile dumbbell samples with the grain.
Gauge sample.
PROCEDURE:
INSTRON TESTER, 4.5 kg (10#) HEAD, 50 cm/min. (20"/MIN.) Set jaws 7.6 cm {3") apart. Clamp
sample in jaws - use minimum air pressure. Run out to 15.2 cm (6") - approximately 100%
extension. (Extension time approximately 0.015 minutes) Hold extended for 1.85 minutes
(total time 2.0 minutes). Remove from jaws.
Allow to relax.
Mount sample. Calculate Tmax,
T2', % retained tensile.
CALCULATION OF RESULTS:
Tensile (maximum), Tmax, MPa*= kgf * 0.77 gauge, mm
Tensile at 2', T2', MPa*
= kgf (at 2') x 0.77 gauge, mm
% retained tensile
= T2' x 100/Tmax
* For values in psi, tensile = Ibf x 2
gauge, inches
APPENDIX 2 Air Pressure Loss in Inner Tubes
The calculation for air pressure after time t is given by the equation
Pt = Рое- ^t
=^ = PoA^ VH
For a 10.00 x 20 tube (0.125
ft^ and A = 16.7 ft^. At 30° С., a
air permeability value of 5.6 x 10-^
will have an air permeability value
Pt = Pressure at time t Po =
Initial Pressure t = Time (days)
A = Surface area of tube ^ = Air
Permeability H = Thickness of
tube
in. thick) inflated to 90 psi in a tire, V = 3.9
typical butyl inner tube compound will have an
ft^/mil/day while an 80/20 butyl/EPDM compound
of 14.1 x 10'^ ft^/mil/day.
The calculations are as follows (after 1 day):
(90 x 16.7 x 5.6 10-" x 1 Butyl:
89.8 psi
P(i day) = 90e-{3.9 x 125)
<90 x 16.7 x 14.1 10-" x 1
Butyl/EPDM: P(i o.y) = 90e-(3.9 x 125)
=
== 89.6 psi