The U.S. Army is currently spending 300 million dollars per year replacing rubber track pads. An experimental rubber compound has been developed which exhibits 2 to 3 times greater service life than standard production pad compounds. To improve the service life of the tank track pads various aspects of rubber chemistry were explored including polymer, curing and reinforcing systems. Compounds that exhibited superior physical properties based on laboratory data were then fabricated into tank pads and field tested. This paper will discuss the compounding studies, laboratory data and field testing that led to the high wear elastomer compound

Crawford, D.

Feuer, H.

Flanagan, D.

Rodriguez, G.

Teets, A.

Touchet, P.

English

NASA Technical Reports Server

N93-25607 
ELASTOMER COMPOUND DEVELOPED FOR HIGH WEAR APPLICATIONS 
D. Crawlord, H. Feuer, D. Flanagan, G. Rodriguez, ATeets, & P. Touchet 
Engineering Materials & Coatings Division 
Materials Directorate 
Army Research Laboratory 
Ft. Belvoir, VA 22060 
ABSTRACT 
The U. S. Army is currently spending 300 million dollars per year replacing rubber track pads. An 
eqerbeaenatd rubber compound has been developed which exhibits 2 to 3 times greater service life than 
standad production pad compounds. To improve the service life of the tank track pads various aspects of 
rubber chemistry were explored including polymer, curing and reinforcing systems. Compounds that 
efibited superior physical properties based on laboratory data were then fabricated into tank pads and field 
tested, This paper will discuss the compounding studies, laboratory data and field testing that led to the high 
wear elastomer compound. 
BACKGROUND 
Track laying vehicles, wherein a continuous track is constantly laid down in the direction of 
movement sf the associated vehicle, are well known. Examples of such track laying devices are the military 
tanks and personnel carriers. Such devices have an endless track with a plurality of linked metal track shoes. 
These waibtary tracked vehicles are equipped with rubber track pads, rubber blocks or endless-band rubber 
track to reduce shock, noise, wear and damage to road surfaces. The endless tracks render the vehicles 
operational in rough, uneven terrain when necessary under military conditions. The vehicle, however, also 
travels over roads and hard surfaces, therefore, the elastomeric components of the endless track should be of 
the type that wears well under abrasion and rough terrain. 
Historically, field performance of these elastomeric components have been poor, especially for the 
medium to heavy tonnage tracked vehicles. The problem is further complicated with off-the-road service 
csn&tions where pads fail at a much faster rate. The severity of the wear is more pronounced on the M-1 
main battle tank than on the older M-60 due to an increase in weight and acceleration, while using a pad 
with a smdler footprint. This produces higher stresses resulting in higher heat build-up. Therefore, costly 
and frequent replacement is necessary to keep the tanks operational. The cost of maintaining and replacing 
track pads is consuming about 25% of the total U.S. Army Operational Maintenance Budget or about 300 
million dolla~rs per year. 
The service life of these tank pads is affected not only by the terrain conditions but also by the 
speed, weight of the vehicle and track design. While the operational life of the metal components is 
appro~~mately 5000 miles for heavy vehicles, the average life of the rubber pads is seldom more than 1 5 0  
miles under the best circumstances and is usually less than 550 miles under the severest conditions. During 
service, the elastorneric pad components are adversely affected in several ways. Most common effects of 
wear include cuts, tears, heat build-up, flex fatigue, and abrasion. 
Conventional track pads, based on styrene-butadiene rubber (SBR), usually fail prematurely in 
sewice because of excessive wear, blow-out, which then leads to chunks of material leaving the track pad, 
and rubber-to-metal bond failure. The widespread use of SBR rubber in tank pad applications is primarily 
due 90 the relative low cost of the base SBR polymer along with a U.S. Government policy that requires 
makerials used for military applications to have a domestic source. This policy resulted from the non- 
availability sf natural rubber during World War 11. performance specification MIL-T-11891D was approved 
in 1984 dio&wing the use of polymers other than SBR for tank track applications. This option provided 
https://ntrs.nasa.gov/search.jsp?R=19930016418 2020-03-24T07:27:14+00:00Z
infinite alternatives to the compounder to approach the optimization of specific material properties. Untd 
this point, previous efforts since the mid 1960's had only provided incremental improvements inm s e ~ c e  Ue of 
tank pads of about 5%. 
APPROACH 
To improve field performance of tank pads, one must fvst identify those properties that are cr~i t id  
and then optimize them. This is by no means a trivial task. Any rubber compounder would agree that to 
improve performance of tank pads, properties such as cutting and chipping resistance, tear and tensile 
strength, crack initiation and growth resistance, abrasion resistance, hysteresis and retention of properties at 
elevated temperatures would have to be improved. To achieve this tremendous task the U.S. Army 
sponsored a series of investigations involving industry, government and academia. In 1983 the Rubber and 
Coated Fabrics Research Group, Belvoir RD&E Center (currently the Engineering Materials md Coatbe 
Division of the Army Research Laboratory) was tasked to conduct a series of studies to improve the sehce 
life of tank pads. 
Compounding Studies 
Comprehensive compounding and processing studies were performed to determine what 
combinations of formulation ingredients and/or mixing variables affect physical properties. VaPrious paallper 
systems were evaluated including chloroprene, nitrile (NBR), highly saturated nitrile (HNBR), wethme, 
natural and synthetic polyisoprene, carboxylated nitrile, polybutadiene, SBR and blends of the above 
polymers. 
Selection of the base polymer is critical to obtain specific characteristics of the frnal product. For 
example, natural rubber compounds will most likely exhibit superior resistance to tear and lower Ingisteresis 
when compared with polybutadiene compounds which in turn provide excellent flexibiity, superior resistmce 
to abrasion, crack growth and heat build-up. Nitriles and neoprenes have high resistance to 03s aaad 
chemicals. 
Various fillers and curing systems were explored to enhance physical properties and m 
reversion. Carbon black and novel non-black fders were used to improve dispersion, increase tou&ess and 
abrasion resistance. 
Laboratorv Phvsical Testing 
The experimental materials underwent extensive physical testing in the laboratory. Physid tests 
included tensile strength and tear strength at ambient and elevated temperatures, abrasion, cuttbg md 
chipping and dynamic tests such as blow-out and flex crack growth. Compounds that exhibited superior 
physical properties based on laboratory data were then fabricated into tank pads by Caterpillar Tractor 
Company. These tank pads were subjected to field testing on the Counter Obstacle Vehicle (CoiV) m d  the 
M-60 tank. Simultaneous to the field testing, samples were taken from the fabricated tank pads for 
additional laboratory testing. In addition to the experimental formulations, standard production SBR pads 
were also included in the laboratory and field tests. The laboratory data generated from the fabsimted pads 
is shown in Tables I and 11. Table I11 lists the test methods that were used. 
Vehicle Field Testing 
Two types of vehicles were employed to carry out the field testing. The COV is an en@eenhg: type 
tracked vehicle that weighs about 72 tons. The other vehicle used was the M-60 battle tank w e i m g  a b u t  
45 tons. The testing of the T-107 (COV) pads was performed over a severe course designed to comb"me a91 
possible operational and terrain factors. The COV testing was conducted at the Engineering P r o ~ g  
Grounds at Ft. Belvoir, Virginia. The testing was performed from November 1986 through April 198'7. A 
total of BtXM miles was accumulated on the T-107 pads. 
The test plan for the M-60 (T-142 pads) field test was designed to include three phases consisting of 
mile paved surface course, a 900 mrle hilly cross-country course and a 1000 mile combination course. 
The M-60 test was conducted at the U.S. Proving Grounds in Yuma, Arizona. The testing began in October 
19% and was completed in May 1988. 
RESULTS AND DISCUSSION 
MR-12, an experimental compound based on a hi$- r saturated nitrile elastomer and a novel filler 
and curing system exhibited superior physical properties based on laboratory data as shown in Tables I and 
11. The tensile strength of NBR-12 is about 30% higher than that of the commercial pads. The NBR-12 
materid ret&ed 100% of its original tensile strength after heat aging, compared to about 50% tensile 
retention for the standard materials. This material also exhibited higher hardness and load bearing capability 
(see 40% compressibility). SBR rubber with an equivalent hardness is too difficult to mix and process into 
tarad;: pad c ~ ~ p a t i o n s .  Another significant improvement was achieved on tear resistance. Improvements in 
t e a  strength of about 50% were observed at ambient temperature and in some cases tear strength of NBR- 
12 d~oubled that of the standard material at elevated temperatures. 
Abrasion resistance, a critical tank pad property, was measured by the Tabor and Pico methods. A 
24 fold increase in the Tabor abrasion resistance was exhibited by the NBR-12 material. The increase in 
resistance to abrasion as measured by the Pico method ranged from 300% on COV pads to as much as 
600% on the M-60 pads. 
A common mode of failure for pads during cross-country operations is cutting and chipping. This 
property was measured with the Goodrich Cutting and Chipping Machine and both specimen diameter and 
wei&t loss were recorded. Test results for both measurements were in excellent agreement and the NBR-12 
material provided a 75% improvement for COV pads and over 30% for M-60 pads. 
'The NBR-12 elastomeric tank pad materials exhibited higher heat build- up but the unique 
combinatiorn of the highly saturated nitrile polymer (HNBR) with the novel curing and reinforcement system 
produced an unparalleled retention of physical and dynamic properties at high operating temperatures, thus 
reducing premature failures. 
Crack growth resistance relates to the ability of the rubber to deter crack propagation once the 
rubber has been cut, typical of operation over cross-country terrain. Crack propagation further deteriorates 
inhato tear and eventually chunks of rubber can be removed from the pad. Crack growth was measured using 
a Dematkia Rex Tester. The crack growth resistance of NBR-12 showed greater than 400% improvement 
over the standard COV pad material at ambient temperature while providing at least a 60 fold improvement 
in crack g o w h  resistance over the standard COV pad material at elevated temperature (250 O F ) .  NBR-12 
efibited a 300% improvement in crack growth resistance at room temperature compared to the standard M- 
64 pad material and greater than a 30 fold improvement at elevated temperature. 
MR-12 showed exceptional resistance to wear during field testing, extending the service life 2 to 3 
times that of standard production pads during the M-60 field test. During the paved course portion of the 
M-60 fielid test, standard production pads failed on the average at 1200 miles. The pads with the NBR-12 
mateaid were tested to 2000 miles (the maximum duration allowed under the test plan) with out failures. 
The h i t e d  wear of these pads indicated a projected life of 3400 miles, a service level never before achieved 
by any commercial or other experimental material. Subsequent field tests on M-60 vehicles confirmed the 
o r i e d  projections of the service life of the NBR-12 material by exhibiting serviceability beyond 3700 miles 
on the combhation course at Yuma, AZ. Table IV and Figure I, show the improved service life of NBR-12 
over the staodard production pads. 
CONCLUSIONS 
The superior physical properties of NBR-12 have been demonstrated in the laboratory as well as in 
the field. A U.S. patent has been awarded for this material and patents have been filed in 7 foreip 
countries. Although initial fabrication cost using NBR-12 is approximately 2.5 times as expensi: as s t a n d ~ d  
production pads, a preliminary economic analysis indicates a possible savings on future "higher Ue" tracks 
(with 6000 mile life expectancy) currently being designed. Further changes in track design using this 
improved pad material could contribute significantly to a more reliable and dependable fleet of battle tanks 
for the U.S. Army while reducing track operating and support costs. 
Future research on this high wear elastomer compound is aimed at improving processibBQ and 
reducing cost by blending it with other polymers. There is an on-going program to coat NBR-12 m d  Mends 
of this material onto nylon fabric to be field tested on the Army's Lighter Air Cushion Vehicle (LCPICV-a). 
This is a challenging application from the processing standpoint as well as field performance where abrasion 
resistance and high frequency dynamic flexing or flagellation are critical. 
As discussed earlier, NBR-12 is the result of research and development aimed to improve the life of 
rubber pads used on military vehicles. Applications which require a high resistance to abrasion, resistmce to 
chemicals and fuels, or retention of physical properties at elevated temperatures could benefit from mR-12's 
outstanding wear properties. Potential commercial applications for this material include conveyor and "V" 
belts, treads for off the road tires, gaskets, o-rings and seals for the oil industry, fenders and bumpers on 
loading docks, shock and vibration pads, and rubber covered rolls for paving equipment. 
FIGURE I: Field Performance of T-142 Track Pads 
COMBINATION COURSE 
PAD WEIGHT (grams) 
0 500 1000 1500 2000 2500 3000 4000 
MILES 
PAVED COURSE 
PAD WElOTW (grams) 
MILES 
HILLY CROSS-COUNTRY 
TABLE I: Phvsieal Prowrties 
WTERLAL I. D. CODE 
PROPERTIES 
ORIGINAL 
TENSILE STRENGTH, psi 
200% MODULUS, psi 
ELONGATION, % 
SHORE A HARDNESS, POINTS 
BASHORE REBOUND, % 
SPECIFIC GRAVITY 
40% COMPRESSIBILITY, psi 
TEAR STRENGTH, DIE C 
ROOM TEMP, lb/in 
AT 250 OF, lb/in 
OVEN AGED 70 HRS AT 250°F 
ELONGATION RETENTION, % 
TENSILE RETENTION, % 
ABRASION 
S/1000 CYCLES 
PIC0 RATING 
GOODRICH CUTTING AND 
CHIPPING 
ETER LOSS, cm 
WEIGHT LOSS, GRAMS 
DYNAMIC PROPERTIES 
GOODRICH FLEX 
BLOW OUT TIME, MINUTES 
TEMPERATURE RISE 
INTERNAL, "C 
EXTERNAL SURFACE, "C 
DEWTTIA FLEX 
CRACK GROWTH 
UNAGED, in/MINUTE 
20 HR @ 250 "F, in/MIN. 
CRACK INITIATION, 
1000 CYCLES 
NBR-12 - Experimental pad material based on 
STD PAD - Commercial pad material based on 
TP-150 - Experimental pad material based on I 
NN-9 - Experimental pad material based on a n 
of Materials From T-107 (COW Pads 
NBR-12 STD. NAT-150 
PAD I TP-AF 
INBR polymer. 
jBR rubber. 
~tural rubber. 
t./HNBR blend. 
TABLE 11: Physical ~ r o ~ e r t i e s  of Materials From T-142 (MdOIPads 
m T E R W  I. D. CODE 
PROPERTIES 
ORIGINU 
TENSILE STRENGTH, psi 
200% MODULUS, psi 
ELONGATION, % 
SIHORE A DNESS, POINTS 
BASHORE REBOUND, % 
SlPECIIFllC GRAVITY 
44% COMPRESSIBILITY, psi 
T STRENGTH, DIE C 
ROOM TEMP, Ib/in 
AT 250 O F ,  Ib/in 
OVEN AGED 70 HRS AT 250°F 
ELONGATION RETENTION, % 
TENSILE RETENTION, % 
ABMSION 
S/lOOO CYCLES 
PlCO MTING 
GOODRICH CUTTING AND 
CHIPPING 
DMMETER LOSS, cm 
MEIGHT LOSS, GRAMS 
DmAMIC PROPERTIES 
GOODRICH FLEX 
BLOW OUT TIME, MINUTES 
TEMPEHPATURE RISE 
ImELrPNAL, "C 
EXTERNAL SURFACE, "C 
DIEM7nLA FLEX 
CMCBC GROWTH 
UNAGED, in/MINUTE 
22 HHR @ 250 OF, in/MIN. 
C U C K  BNITMTION, 
B W  CYCLES 
NOTES: 
NBIR-12 - Experimental pad material based on HNBR polymer. 
STD PAD, TP-A - Commercial pad material based on SBR rubber. 
TP-II - Eqerirnental pad material based on chloroprene rub. 
TI?-K - Eqerirnental blend pad material from industry. 
TABLE 111: Test Methods 
PROPERTY TESTED TEST METHOD 
ORIGINAL PROPERTIES 
SPECIFIC GRAVITY D 792, P A M  15 
TENSILE, ELONGATION & MODULUS 
HARDNESS, SHORE A 
RESILIENCE, BASHORE REBOUND 
TEAR STRENGTH, DIE C 
ABRASION, TABOR 
ABRASION, PIC0 
PROPERTIES RUN AT 250 "F 
TEAR STRENGTH, DIE C D 573 & D 624 
TENSILE AND ELONGATION RETENTION D 573 & D 412 
FLEX FATIGUE TESTS 
DEMATTLA 
CRACK INITIATION D 430, METH B 
CRACK GROWTH, UNAGED 
CRACK GROWTH, AGED 20 HR @ 250 OF 
GOODRICH FLEX, BLOW OUT & TEMP. RISE 
TABLE IV: Field Performance of Tank Pads 
MATERIAL I D M-60 PADS TESTED IN YUMA 
PAVED HILLY 
ROAD X-COUNTRY COMBINATION 
(Miles) (Miles) (Miles) 
3402 1251 3302 
1237 530 1351 
2002 470 1451 
1590 380 1321 
920 710 1526 
1237 681 1701 
BELVOBR 
COURSE 
(Miles) 
2802 
1305 
---- 
---- 
210 h 
2052 


Elastomer Compound Developed for High Wear Applications

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