Elastomer blend approach to extend heat life of natural rubber based engine mounts.

Automotive underhood temperatures have been trending

upward due to pressure on fuel and space efficiency and

aerodynamic styling trends. One area of increasing concern is

engine mounts. Natural rubber (NR) has been used for many

years in both conventional and hydromount designs because

of favorable dynamic properties and low cost. However,

there have been some failures of NR mounts in recent years

as the underhood temperatures of certain car models have

risen.

Butyl rubber, although used widely for body and cradle

mounts, has unfavorable dynamic properties for engine

mounts. where vibration isolation is more important than low

frequency damping. A low dynamic spring rate is the

primary consideration when specifying an engine mount

compound. Previous work has shown that brominated

isobutylene-p-methylstyrene rubber (BIMS), when blended at

concentrations of 20-30 phr with natural rubber, result in

compounds with favorably low spring rates and significantly

improved hot air aging performance (ref. 1). This is due to

the outstanding heat aging resistance of the new

isobutylenebased copolymer (ref. 2). This copolymer is itself stable for

long periods of time. However, the natural rubber is

vulnerable to aging. The age resistance of the cured blends can be

improved further by the use of age-resistant additives which

are used in many NR based compounds.

This article will present some dynamic property data for

rubber compounds of varying compositions and the change

in these properties as the compounds are subjected to hot air

aging.

Experimental

Compounds containing NR and BIMS were prepared at the

following ratios of NR to BIMS; 100:0, 80:20, 70:30 and

60:40. The complete formulations are given in table 1. Note

that antioxidants were included only in the NR case. Table 2

includes compounds which contain an 80:20 ratio of NR to

BIMS. Some contain antioxidants and some do not, and two

types of BIMS compounds which vary in mole % benzylic

bromine were used. Other types of BIMS compounds were

studied, with wt.% of PMS levels from 5 to 13.5, and mole %

benzylic bromine levels from 0.5 to 1.2. Table 3 includes the

formulations which showed the best resistance to ozone

cracking and were therefore selected for further evaluation.

Accelerators and antioxidants were added on a two-roll

laboratory mill and tensile pads were cured by compression

molding according to ASTM D412-92. Dynamic specimens

were cured by compression molding into blocks with the

dimensions 20mm x 40mm x 40mm. An MTS 730

Elastomer Testing System with an MTS 830 load frame was used

to test dynamic properties. The sample was positioned

between two platens and a -45 N preload was applied. At the

beginning of the test -800 N was applied and after a 10

second delay the force was oscillated through the specimen at a

constant frequency and deflection. Two frequency and

amplitude conditions were run for every test: 15 Hz, 0.5 mm and

100 Hz, 0.05 mm. In some cases, a third condition of 200 Hz,

0.05 mm was run. The tests were run at 24 [degrees] C for all specimens.

In some cases, data at other temperatures were obtained.

Samples were aged at 150 [degrees] C or 125 [degrees] C as indicated.

Surface ozone cracking data were obtained using ASTM

method D1149-91 with some variations. Samples were not

cured using aluminum foil or polyester as a barrier.

Specimen B (bent loop) was used for this method. The

temperature was set at 38 [degrees] C and ozone concentration was 1 ppm.

Dynamic ozone cracking data were obtained using ASTM

method D3395-91 with some variations. A dumbbell shaped

specimen such as is used for tensile testing (ASTM D412-92,

method A) was used in place of a rectangular specimen.

Samples were not cured using aluminum foil or polyester as

a barrier. The temperature was set at 38 [degrees] C and ozone

concentration was 1 ppm. Physical properties were obtained

according to ASTM D412-92 for tensile, modulus and percent

elongation. Hardness was obtained according to ASTM

D2240-91 for shore A hardness.

[TABULAR DATA 1 TO 3 NOT REPRODUCIBLE IN ASCII]

Results and discussion

The compounds described in table 1 were tested for dynamic

properties after aging at 150 [degrees] C for 0, 70, 140, 210 and 280

hours. Figure 1 shows that the 100 NR sample showed a

large increase in damping after aging, but the samples with

BIMS showed less change in damping. Although the samples

with BIMS were initially higher, differences in damping

become minimal after 140 hours at 150 [degrees] C. Figure 2

compares spring rate at the same aging conditions. The sample

with 20 phr BIMS maintained the lowest spring rate. Another

set of the same samples 1 and 2 were aged at 125 [degrees] C for 0,

168, 336, 504 and 1,008 hours. Figure 3 shows that the NR

sample increased in damping to approximately the same level

as the 20 phr BIMS sample within the first week of aging.

Over the next five weeks at 125 [degrees] C, the samples had

approximately the same damping response. Figure 4 shows a large

increase in spring rate for the NR sample while the sample

with 20 phr BIMS increased slowly and at a much lower

spring rate than the NR sample.

[FIGURES 1 TO 4, GRAPHS OMITTED]

Aging resistance may be enhanced by using age inhibiting

additives in the formulation. Table 2 shows the formulations

for compounds with 20 phr BIMS. The base formulations

vary as do the cure packages with some including age

resistant additives. Figure 5 shows the spring rate of these

compounds after aging at 125 [degrees] C for 0, 70, 210, 280 and 500

hours. The samples that varied only in base formulation

changed very little with aging. The sample that contained age

inhibiting additives had a lesser increase in spring rate over

time than the formulations with no age inhibitors. Figure 6

shows the damping of these compounds under the same

aging conditions. Variations in base formulation resulted in

modest changes to initial damping. Differences between the

different formulations became insignificant when aging

reached 210 hours at 125 [degrees] C. Table 2 also shows some minor

variations in scorch/cure rate balance. For example,

compounds 1 and 2 are better than 3 and compound 5 is better

than 4. The additive package used in fully formulated motor

oils apparently improves both scorch safety and cure rate.

[FIGURES 5 & 6, GRAPH OMITTED]

Two formulations based on blends of NR with 30 phr of

different types of BIMS were selected. The formulations were

found to have optimum ozone resistance for 30 phr BIMS

blends. One resisted cracking in the static ozone test for 70

hours. The other resisted cracking in the dynamic ozone test

for 70 hours. Aging and ozone inhibiting additives were

added to improve aging properties. Table 3 gives the formulas

for 30 phr and 20 phr BIMS/NR blends with two types of

BIMS polymer. Figure 7 shows that all four compounds

exhibited good aging stability at 125 [degrees] C with regard to spring

rate. However, BIMS structure did appear to exert a

significant influence on the stiffening response as phr BIMS was

increased from 20 to 30. The BIMS with 1.0 mole % benzylic

bromine and 9.5 wt.% PMS showed less variation when the

level of BIMS was varied than did the BIMS with 1.2 mole %

benzylic bromine and 7.5 wt.% PMS. Figure 8 shows the

effect of BIMS composition and level on damping. There was

no significant change in damping when the composition of the

BIMS polymer was changed. There was, however, a

significant change in damping when the level of BIMS was varied.

[FIGURES 7 & 8, GRAPH OMITTED]

Conclusions

Heat aging resistance of natural rubber based dynamic

compounds can be improved by blending in BIMS elastomer.

The heat aging resistance of NR/BIMS blends can be further

improved by using age inhibiting additives to protect the

natural rubber part of the blend from aging. The heat aging

resistance can also be improved by using the optimum PMS

and BrPMS level for the BIMS in the blend. Figures 7 and 8

show that the compounds described in table 3 exhibit

excellent stability (less than 10% change) of dynamic properties

out to 210 hours at 125 [degrees] C. They should be of interest as

starting point formulations for design engineers who wish to have

the dynamic and strength properties of natural rubber but

want stability of those properties as engine mounts are

subjected to extended heat aging.

References

[1.] McElrath, K.O., Measmer, M.B. and Yamashita, S.,

"Dynamic properties of elastomer blends," presented to the

ACS Rubber Division Meeting in Montreal, Canada, May

4-8, 1996, paper No. 44.

[2.] Kruse, D.F. and Fusco, J.V., "A new isobutylene 

copolymer," Rubber & Plastics News, 1993 Technical Yearbook, p.

10-14.