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AD902238

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TOApproved for public release, distributionunlimited

FROMDistribution authorized to U.S. Gov't.agencies only; test and evaluation; 16 Aug1972. Other requests shall be referred toU.S. Army Weapons Command, Attn:AMSWE-RET, Rock Island, IL 61201.

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Per RIA ltr, 28 Jan 1977

THIS PAGE IS UNCLASSIFIED

THIS REPORT HAS BEEN DELIMITED

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UNDER DOD DIRECTIVE 5200,20 AND

NO RESTRICTIONS ARE IMPOSED UPON

ITS USE AND DISCLOSURE2

DISTRIBUTION STATEIMENT A

APPROVED FOR PUBLIC RELIASEF

DISTRIBUTION UNLIMITED,

RE TR 70-1.21

" DEVLOPMENT OF RUBBER PADS'FOR TRACKED VEHICLES

TECHNICAL REPORT

E. W. Bergstromand

J. ft. Cerny

February T970 E

'SCIENCE & TECHNOLOGY LABORATORY

RESEARCH & ENGINEERING DIRECTORATE

U.. S.. ARMY WEAPONS COMMAND

Each transmttta1of thies docunt Outs Ide the agencies ofthe U. S.Government must have prior Approval of the U. 5.; Army Weapons CommandScience and Technology Lf(or&!ory, AM8WE-RET / • .,o/

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The fi~ndings of thle report-are not to be construed asan officiai1 Department~of theý Army position u less sodesignated by other authorilzed documnbntB.

The citation oft commercial products in this repor.t does-' not constitute an official ipdorsome1nt-or aporoval of such

products.

!7

AD

SCIENCE AND TECHNOLOGY LABORATORY

RESEARCH AND ENGINEERING DIRECTORATE

U. S, ARMY WEAPONS COMMAND

TECHNICAL REPORT

RE TR 70-121

DEVELOPMENT OF RUBBER PADSFOR TRACKED VEH!CLES

E. W. Bergstrom

and

J. R. Cerny

February 1970

Distribution limited to U.S. Gov't. agencies only;Test and Evaluation; ,//IA/ /9/' Other requestsfor this document must be referred to •S, X•. ,,^y

DA :06210.A329 AMS Code 5025.11.295

iAch -ý.-ansmit,!al of this document outside the agencies of the'!s S. Government must have prior approval of the U. S, ArmyWea;ý-ns Ccwnand Science and Technology Laboratory, AMSWE-RET.

* - - - -- • P • ' ' " • .'T • " -*-"V' S * -" .' •'

ABSTRACT

Improvement in the wear resistance of rubber trackpads was sought through compounding studies and evaluationof rubber-to-metal bonding agent3. Correlation was soughtbetween laboratory tests on the rubber component of trackpads and service tests on the entire pad. Track pads fabri-cated from millable polyester urethanes provided improvedservice over that of commercial control pads. Smallamounts of calcium oxide in millable polyester urethanevulcanizates eliminates the internal porosity which leadsto early failure of track pads in high speed service testsbut has an adverse effect on hydrolytic stability and wearresistance. Initial correlation efforts using the DeMattiaand Firestone Flexometer tests are sufficiently interestingto warrant further examination. A satisfactory ruLber-to-metal bonding system for millable polyester urethanes hasbeen found.

I

iiL

CONTENTS

Page

Title Page i

Abstract ii

Table of Contents iii

Objective 1

Background 1

Approach 1

Results and Discussion 8

Track Pads Prepared from Millable Urethanes 8

Compounding Studies 18

Service Tests of Experimental Track Pads 20

Correlation of Laboratory Tests with 27Service Performance

Polyester Urethane Rubber-to-Metal Bond 32Studies

Conclusiors 37

R•commendat ions 37

Literature Cited 38

Dist-r ibution 42

DD Form 1473 {Document Control Data - R&D) 48

iii '

OBJECTIVE

The object of this work was to develop rubber trackpads having improved service life, to manufacture them inpilot lot quantities for service testing, and to attemptto correlate physical properties of the rubber with ser-vice performance of the pads.

BACKGROUND

The operating life of the T97E2 track used on theM48- and M60-series tanks averages only 2200 miles becauseof the limitations of both its rubber and metal components.Recent reports from Vietnam indicate that in some casespads have a service life of less than 800 miles.

The U. S. Army Tank-Automotive Command developed theT142 track with a detachable track pad to replace theT97E2 type. Tests on eight sets of T142 track at AberdeenProving Ground, Fort Knox, Yuma Proving Ground and theArctic Test Board indicated that it remained operationalfor 5000 miles or more. However, the average life of therubber pads ranged from only 1200 to 3600 miles and de-pended on the operating conditions and ambient temperatures. 1

Avarage life of the rubber pads was 1200 to 2200 miles atspeeds of 30 mph and 1885 to 3600 miles at 20 mph.

It has been estir.ted that $100,000,000 is spent eachyear for track suspension systems. Of this amount, approxi-mately $25,000,000 goes into rubber components - pads,blocks and road heels. Therefore, the development of animproved track pad not only would provide obvious tacticaland logistical advantages for modern high speed trackedvehicles but also would lead to savings in the cost of therubber components.

The ultimate objective of this work is to develop a5000 mile track pad that would match the operational lifeof the track itself. Previous reports 2 - 6 presented the re-suits of compounding studies performed on numerous elasto-mers as well as the results of service tests conducted on Iexperimental track pads. The results presented in thisreport Include compounding studies, rubber-to-metal bondevaluatAon studies, correlation between service tests andlaboratory tests, and environmental aging tests conductedsince issuance of the last report.

APPROACH

Service tests of T130 and T142 track pads wt e arrangedthrovgh the L. S. Army Tank-Automotive Command, Warren,Michigan (ATAC); and conducted at the FMC Corporation,

I!

San Jose, California, ATAC, and Aberdeen Proving Ground,Aberdeen, Maryland (APG). The following wear rating wasused to compare the performance of the rubber track padstested:

VolumeWear = Average volume loss of commercial SBR control pads X 100

Rating Average volume loss of experimental pads

Static exposure tests of T130 track pads and ASTMtest pads (6 by 6 by .080 inch) in Panama were arrangedthrough the cooperation of Dr. Leonard Teitell of thePitman-Dunn Research Laboratories, Frankford Arsenal.Arctic exposure tests were conducted at Fort Greely, Alaska,through arrangements made with the U. S. Army Test andEvaluation Command, Aberdeen Proving Ground, Maryland.Exposure tests were also conducted outdoors at Rock Island,Illinois.

A Number 1 Banbury mixer was used to mix compoundsselected for fabrication of track pads. The Banbury-mixedcompound was then transferred to a 30 inch mill for addi-tional mixing and sheeting-out. The cooled stock was latertransferred to an 18 inch mill for warmup and sheeting-outto the desired thickness for the preparation of track padpreforms from rolled stock.

The following surface preparations were performed onthe metal backup plates (inserts) prior to their vulcan-ization bonding to the rubber preforms: degreasing. glassbead-blasting, solvent wiping, brush application of bond-ing agent, and drying,

Tensile strength, elongation, and modulus were deter-mined at ambient and elevated temperatures by use of aScott Model L-6 rubber tensile tester equipped with aScott Model HTO hot tensile oven and autographic recorder-controller. Each tensile specimen was placed in the gripsof the tester and conditioned for a period of six minutesat temperature before being tested. All other physicalproperties were determined by ASTM procedures, where ap-plicable.

Compound formulations are given in Table I.

2

L~guis reinforced SHA received S~

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RESULTS AND DISCUSSION

Track Pads Prepared from Millable Urethanes

As previously reported, 6 track pads prepared frommillable polyester urethane (A) consistently exhibitedsignificantly improved wear and chunking resistance whencompared with SBR control pads. However, there are anumber of problems ass(,ciated with the use of millablepolyester urethanes in general and with the one mentionedhere in particular.

For example, urethane track pads have failed becauEsof adhesive bond failures, the rubber pads actually separa-ting from the metal insert during test and dropping offin many instances. The search for an improved rubber-to-metal bonding system has continued since that time, andsatisfactory vulcanization bonding systems have been found,as will be discussed later. No track pad failures due topoor bonds have occurred in service tests since thesesystems have been used.

Another problem associated with the use of the millablepolyester urethanes is that of their hydrolytic instability. 7

The degradation in physical properties caused by hydrolyticattack can be delayed by the use of additives such as adiisocyanate (TDI) or polycarbodiimide (PCD), but cannotbe eliminated. T130 track pads prepared from compoundswith and without an additive were exposed outdoors in Panama(open sun and rain forest) and at Rock Island, Illinois,with a planned exposure period of five years. Results ob-tained thus far after one year of exposure are given inTable II. Significant deterioration in physical proper-ties has occurred in the uninhibited pads, particularly atPanama. The PCD-inhibited pads show little or no changein tensile strength after exposure in the open sun atPanama and Rock Island, but the tensile strength of padsexposed in the rain forest indicates that the surface ofthe pads has been adversely affected by hydrolytic degra-dation.

Physical properties were also determined on T142 padsprepared from a polyester urethane (A) compound containingno hydrolysis inhibitor (compound 1, Table I) which were(1) molded in October 1965, tested at Yuma, and stored atATAC until sent to this Laboratory for evaluation inJanuary 1969; and (2) molded in June 1968, tes. 4 at APG,and stored at ATAC until sent to this Laboratory forevauation in January 1969. These results are shown inTable III. All pads exhibited severe tensile strength

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deterioration which points up again the need for incorpor-ation of a hydrolysis inhibitor in polyester uretL'ane trackpads. However, even polyester urethane compounds contain-ing hydrolybis inhibitors (TDI or PCD) are subject tosevere deterioration when aged in tropical environmentssuch as Panama, as shown in Figure 1. On the other hand,these same compounds withstand temperate (Rock Island,Illinois) an% arctic (Fort Greely, Alaska) environments.More detailed results of the hydrolytic stability of poly-urethane vulcanizates may be found in References 7, 8 and 9.

A recent service test at APG on T142 track pads pre-pared from polyester urethanes revealed failures whichappeared to be caused by internal gassing of the padsduring high-speed testing on paved and gravelled roads.This gassing was duplicated in this Laboratory by use ofthe Firestone Flexometer. Cross sections of samplestested are compared with a failed track pad in Figure 2.The gassing of the Flexometer specimens occurred whetheror not the specimens were conditioned for three days a- ,212°F prior to test. Thick section pieces (1 by 1 by1-1/2 inch) of Firestone Flexometer specimens were airoven aged to determine the effect of heat alone (no dynamicflexing) on the vulcanizates. Photographs of cross sec-tions of the specimens after aging are shown In FigLres 3-5,These tests revealed the following:

1. Certain polyester urethane specimens exhibitgassing when aged at 350OF and 4000 F, whereas SBR specimensshow no gassing when aged at temperatures up to 400°F(Figure 3).

2. Conditioning at 2120F for three days prior toaging at 350°F reduces the gassing of a polyester uvethanevulcanizate containing no hydrolysis inhibitor and com-pletely eliminates the gassing at 350°F in a polyesterurethane compound containing PCD (Figure 4)

3. Gassing was eliminated at 350°F in a polyesterurethane compound which contained PCD and 5 parts/1O0 rhccalcium oxide (Figure 5). Unfortunately. the inhibitingaction of PCD against hydrolytic attack is destroied asshown in Table IV. Less than 5 parts of calcium oxidewere ineffective in eliminating gassing at 3500 F.

Thin specimens (0.075 inch) of polyester urethane vulcani-zates exhibited no gassing when aged at temperatures up to4000 F. It is the opinion of this Laboratory that bhefallu-e of track pads at APG was due to a combination ofhigh sheartng stresses (due to pad design) and high-heat.bul!dup within the rubber itself during the htgh-speedtest.

11

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FIGURZ 3 AIR OVEN-AGING OF SBRAND POLYESTER URETHANE VULCANIZATES

14

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FIGURE 4 AIR OVEN-AGING OF POSTCURRD POLYESTERURETHANE VULCANIZATES

15

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FIGURE 5 AIR OVEN-AGING OF POLYESTER URETHANEVULCANIZATES WITH AND WITHOUT CALCIUM OXIDE

16

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Il

Another disadvantage connected wish the use of mill-able polyester urethanes is their high cost compared toSBR. illlable polyester urethane (A), for example, costs$1.50 per pound compared with $0.23 per pound for SBR 1500.

Compounding Studies

Compounding studies were carried out on numerouselastomers and blends of elastomers sin-e issuance of theprevious report 6 on this subject. Track pads were pre-pared for service testing from the most promising compounds.

The curing systems and fillers evaluated in SBR 1500are shown in Table I Compounds cured with sulfur/N-cyclo-hexyl-2-benzothiazole sulfenamide tCBS) and dicumvl per-oxide (DCP) have comparable physical properties at roomtemperature (compounds 5 and 6); but at 4000 F, the tensileof the DCP-cured compound is somewhat better.

Comparisons of DCP, DCP-sulfur, and DCP-CBS (compounds6-9) reveal that DCP-sulfur and DCP-CBS cured compoundshave lower tensile strengt hs at room temperature and at400OF than the compound cured with the use of DCP onlyIn addition, the compounds employing the DCP-sulfur cureexhibit lower modulus and higher elongation and tear re-sistance at both room temperature and 4000 F. The DCP-CBScured compound, on the other hand, exhibits higher modulusand hardness and lower elongatlon. An optimum cure resul-ted from a combination of DCP, sulfur and CBS (compound10). Further studies with this cure (compounds 11, 13and 14) revealed that the addi-ion of f:ne particle sizEsilica significantly improves the abrasion resis'tance andtear strength at room temperature and at 400OF .compound14). Vulcanizates based on --his cure had excellen°. a:rovGn-aging resistance.

A maleimide-peroxide curing system "compound 15) isequal to the DCP-sulfur-CBS system for impvcvea physicalproperties at ambient and elevated lemperatUres.

A lignin reinforced SBR compound ýNo. 16) suppld -dby the Defense Chemical Biological and Radiation Labora-tories was evaluated Tens:_• streng,:h measured at 400QFwas rather low.

A roll calendered coated f:bergIas -ire cord fabricwas also evaluated in ccmpound 12, bj s,.ress-strainproperties could not be obtained because af the aifficultyin molding ASTM test pads,

18

Comparisons of SBR/cis-polybutadione blends (compounds17-21) reveal the improvement in tensile strength at 400°Fwhen a peroxide-sulfur-CBS curing system is used Insteadof a conventional sulfur-accelerator system. It is alsoevident that a fine particle size silica significantly im-proved the tear strength of these vulcanizates at bothroom temperature and 4000 F. A compound suggested by acommercial manufacturer of SBR and polybutadiene (compound22) was also evaluated. An antiozonant was added to pro-vide 03 resistance (compound 23).

Compounding studies were conducted with SBR/EPDMblends to develop ozone-resistant vulcanizates with physi-cal properties suitable for track pad applications Onephase of this study revealed that excellent resistance toozone could be obtained by use of an 85/15 blend of SBR1500/ultra high Mooney EPDM (Mooney ML-4 0 212OF over 200,(HM-EPDM). The use of HM-IPDM rather than a conventionalEPDM, e.g. 70 Mooney, resulted in superior stress-strainproperties and abrasion resistance (com.pounds 24-27) Itwas desired to do more work with the HM-EPDM; however,this specialty polymer was no longer available 1 0 Mean-while a comprehensive study was being conducted to definethe extent of usefulness and limitations of ethylene-prcpy-lene copolymer and terpolymer rubbers as antiozonants fo:diene elastomers° This study1 1 included conventicnal EPMand ZPDM, HM-EPDM, and the new'y introduced fast-cu-IngEPDM elastomers (FC-EPDM). It was found that vulcaniza-.eswith excellent ozone resrstance and other physical p:zc.i:-ties could be obtained by use of 70/30 blends of SBR1500/FC-EPDM (compounds 28 and 29,, 11 was a~sc fcatdthat a vulcanizate (compound 30' prepared from a 85/15blend of SBR 1500/140 Mooney EPDM U.Mcon.',y ML-4 0 212°F -

140) had excellent physical properý:tes and ozone rexis-tance, but it has been learned hat -he 140 Mooney EPDMhas also been withdrawn from the market,

Stereospecific SBE %as investigated :n blends *.thbFC-EPDM (compounds 31 and 32). -riginal phys*cal proper-ties did not equal those of conven'.-Acnal SBR 1500/FC-EPDMblends although the tenslh strength weasured a. 4(0CFwas excellent. Tear resistance of SBR 1500/FC-EPDV andstereo SBR/FC-KPDM blends was poorer than that of v•lcar.;-zates prepared from SBR or FC-2PDM oulyv

SBR/cis-polybutadieve/EPDM blends .compounds 33 and34) are also of interest because of th6ei inherent. ozoneresistance Altho.zgh both compoundq have excelleni st•-ss-strain properties, the SBR/c4s-polybutadiene/HM-EPDM hasvery superior abrasion resistance. N- attempt was madeto fabricate track pads from this blend because t• omcound

19

is difficult to mix and has little or no building tack.

Formulationsbased on chlorobutyl only and chloro-butyl/FC-EPDM blends (compounds 35 and 36) were evaluated.Sufficient tensile strength could not be obtained withperoxide curable butyl compounds (37 and 38) to warrantfurther investigation.

Vulcanizates based on polyester urethane (B) andpolyester urethane (C) stocks received fully compoundedfrom the manufacturer were tested (compounds 39-41). Thesestocks were especially developed by the manufacturers fortrack pads. The vulcanizate prepared from the polyesterurethane (C) stock had poor physical properties at elevatedtemperatures.

Glass fiber-filled polyester and polyether urethanevulcanizates furnished by the manufacturer had poor tensilestrength at 2500 F. The retention of tensile strength ofthe polyester pads was also poor after aging over uaterfor 30 days at 158 0 F.

Service Tests of Experimental Track Pads

Since issuance of the previous report, 6 experimentaltrack pads were prepared for the following scheduled ser-vice tests:

Track Pad ScheduledTest Site Type Starting Date

1. General Motors Test Track T142 Spring 1967at Milford, Michigan

2. FMC Corporation - T130 Summer 1968San Jose, California

3. Aberdeen Proving Ground T142 Summer 1969

The test scheduled for the GM test track at Milfordwas delayed over a period of months (because of carburetortrouble on the test vehicle and broken track) and wasfinally cancelled. The pads were placed in storage atATAC where they are presently. The T142 track has beenmodified since the pads for the Milford test were preparedand will not accommodate the old style insert used inpreparation of the pads for the Milford test.

The next scheduled test in early summer 1968, was a6000-mile durability test to be performed on a Ml13A1Vehicle SIN SJ-136 by the Ordnance Engineering Divisionof the FMC Corporation, San Jose, California. The Mll3AL

20

Vehicle uses T130 track pads, and fifty-five T130 pads pre-pared from various formulations (all but one of which wereincluded in the Wilford test) were sent to FMC. In ad-dition, sixty-two T130 pads prepared from nine recipesfeaturing several polyester and polyether resins with sev-eral levels of fibrous glass reinforcement were injection-molded by the urethane manufacturer and also included.Service test results based on volume wear ratings of the55 pads prepared by this Laboratory are given in Table VI.These results show that one compound performed slightlybetter than the commercial controls. This was a millablepolyester urethane (A) compound containing 4 parts PCDhydrolysis inhibitor and 8 parts DCP curative. The com-pound that had demonstrated good wear resistance in pre-vious tests (compound 1) demonstrated very poor wear re-sistance in this particular test, ebpecially after 2000or more miles. It is to be noted that service test re-sults are given up to only 3000 miles, although the testwas of a 6000-mile duration. This is due to the fact thatmany of the pads were removed prior to 4000 miles becauseof chunking and delamination or need for shoe change which,reulted in an insufficient number of pads remaining tog.ve meaningful results. As reported by FMC, 1 2 however,several isolated pads remained on the test vehicle for5000 miles or more as reported below:

Compound(see

Tables Iand VI) Mileage Condition Reason for Removal

Coirecial

control 5000 Good Shoe changed6000 Fair Some chunking and de-

lamina-ion at completedtest

1 5500+ Fair Some chunking and de-lamination at 5500 m.les.lost during vehicleoperatlcn

13 5000 Good Shoe changed13 5500+ Good at Shoe changed

5600 miles44 6000 Fair Shoe changed - some

chunking12 6000 Fair Shoe changed se~e

chunkingk2 6000 Fair Some chunking, completed

test

21

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The injection molded glass reinforced polyurethanepads performed poorly in the service test. After only100 miles, the pads were worn flush with the shoe grousers,and had to be removed from test after 151 miles of oper-ation. It was reported by FMC 1 3 that the pads also lackedadequate traction.

In the summer of 1968, twelve T142 pads prepared frommillable polyester urethane (A) (compound 1) were testedfor 1171 miles, the first 100 miles at ATAC and the re-maknder at APG. After 1171 miles, the pads had a volumewear rating of 145. It was in this test that the severeinternal gassing due to high-heat buildup was noticed.

This Laboratory was afforded the opportunity to placeadditional T142 pads at APG in service tests scheduled forthe summer of 1969. Included were millabl^ polyester ure-thane pads prepared from stock received fally compoundedfrom two different commercial manufacturers kmillablepolyester urethanes (B) and (C)). The results of thesetests are g±ven in Table VII. The internal gassing andporosity noticed in the millable polyesttT (A) pads inthe previous test was again noted. It was found that theaddition of calcium oxide eliminated the gassing andpo.osity, but pads containing this add;-.ive 'compou.nd 4)had poor wear resistance. Test pads prepared frci m_11-able polyester urethanes 'B) and (C'. received f-j.'v coT-pounded from the manufacturers also 6.hlb'.ted poor wearresist:ance.

In a previous report on this subject,6 It was statedthat results up to that time had .nd'_caied that poor wt%:ýratings had been associated with the use of PCD in miliablepolyester urethanps. The volume wear ratz.ngs t•, dat-. oftrack pads wt.h and withoi:. hydrolysis inhib,&to-s 1PCD orTDI) are given in Table VIII. These results reveal a la-.gesfread in wear ratings no matter which hydrolysis ;nhibt'.cris used. However. compound No. I containing no Inhibi:orshows u better wear rating than those wit.h lnhib.•lo:s,

A surmary of volume wear-ratings -beEt to worst" ofal. compounds service-tested in this program, sinc~e tbe:.ssuance of the latest report 6 is given in Table IX. Only"*Ie mil able polyester urethane *A' pads gave volume-*arra-irgs greater than those of the commerclal cont.rol j.s.

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Correlation of Laboratory Tests with Service Performance

Since obvious advantages would exist if it were pos-sible to forecast the wear characteristics of a vulcani-mate without actually conducting a costly and time con-suming service test, attempts were made to correlateservice performance with an accelerated static cr dynamiclaboratory test. It has been previously stated 0 that, inthe case of the urethanes, good service durability wasassociated with good stress-strain properties at elevatedtemperatures. This appears to hold true in most cases forthe millable urethanes but not for the other elastomers,as shown in Table X.

Attempts were made to correlate service performancewith resistance to crack growth as measured on the DeMattiatester (ASTM D813-59). These results, given in Table XI,indicate that the crack growth test may predict how themillable polyester urethane vulcanizates will wear, butit does not appear to be accurate in predicting how theelastomers will perform in service when compared with oneanother. Some correlation does appear to exist, however,in cases where compounds exhibit extremely good (crackgrowth of 3/32 or less) or extremely poor (cracks acrossin less than 1500 cycles or so) crack growth resistanceon the DeMattia. Specimens of millable polyester urethaneA (compound 1) showed a crack growth cf only 2/32 after50,000 cycles, and the average volume-wear rating for allsrvx:e tests on this compound is 141. On the o-ber hand,

specimens based on carboxylic elastomer (compound 64)cracked complately across in leas than 600 cycles, and thevolume rating ts only 46. The d.fferpnce in crack growth7ate for the two compounds is very evident in Figure 7,which shows the specimens after 50,000 cycles in comparisonw*.th the Science and Technology Laboratory SBR controlcompound. Additional test results will have to be obtainedIn fut.ue service tests to verify this correlation.

In other attempts to obtain correlation, tear resis-.ance (ASTM D624-54, Die C), heat buildup (Firestone

Flexometer) and compression modulus (ASTM D575-67, MethodA) were Investigated. These results are shown in TableXII WIth regard to the millable polyester urethanes,sce correlation appears to exist b36¼een service perfor-mance and the properties measured. A good volume-wear:Va°.rg .on* greater than 100) appears to be directlyVropo!!'ional to good tear strength and inversely pro-•. bonal to low heat buildup and high compression modulus

27

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Again, however, additional data will have to be obtainedto verify these findings. At the present time, no evi-dence of correlation exists between service performanceand the properties measured for the other elastomers orblends of elastomers.

Polyester Urethane Rubber-to-Metal Bond Studies

A four-year study has been concluded which was de-signed to determine the effect of shelf-aging at ambienttemperature and storage at 100 percent relative humidityon the 90 degree peel strength of millable polyester ure-thane A and D vulcanizates bonded to 1020 steel with twodifferent bonding systems (Table XIII). These millablegums compounded with a polycarbodlimide hydrolysis in-hibitor (PCD) and bonded with Bonding System 2 were theonly formulations which exhibited no appreciable loss in90-degree peel strength after a 2-year period undereither environmental condition. The PCD inhibitor alsoappeared to improve the stability of Bonding System 1 tosome extent. The 4-year aging results reveal that (1)shelf-aged specimens bonded with System 2 exhibit excel-lent bond retention whereas specimens exposed to 100percent R.H. exhibit more than a 50 percent loss from theoriginal bond strength values, (2) 90-degree peel strengthvalues for Bonding System 1 are significantly lower thanthose for Bonding System 2, (3) the lower rubber-to-metalbond strength values for all specimens exposed to the100 percent R.H. atmosphere probably can be attributed tothe poor hydrolytic stability of the polyester urethaneelastomers rather than to the instability of the bondiagsystem employed, and (4) the PCD-inhibited gum A bondedwith System 2 appears to be adequate for tank track padapplication as far as the environmental stability of thebond is concerned.

It was postulated that high temperature bond reten-tion at tank track pad operating temperature (250 0 F) wasanother important criterion necessary for an acceptablerubber-to-metal bonding system. Accordingly, 90-degreepeel strengths were determined on candidate urethane bondsystems at 250OF and at ambient temperature and are shown inTable XIV. The importance of high-temperature bondstrength becomes quite evident when one examines adhesiveservice performance and laboratory test data at elevatedtemperature. For example, poor bond strength retentionof bond systems 1 and 3 correlates with numerous rubber-to-metal bond separations that occurred during earlysergice tests. Correspondingly, good bond strength at250 F for Bonding Systems 4, 5 and 6 further substantiate

32

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this claim in that no polyester urethane pads separatedfrom the metaý. :.nserts because of bond failure during thelatest FMC and Aberdeen siervice tests when these systemswere used.

Rubber to-vietas bond strength data obtained fromT130 polyester urethane track pads that were bonded withSystem 4 and subjected to outdoor exposure for I year atRock Island, .:.:.!n.. and at Panama Canal Zone are con-".ained In Table .V. The bond strengths determined onzhe aged pads :Lnd.•(-ate rhat (1) the pads fabricated fromthe PCD-inhib*.*;ed eLuLstor, er (compound 2) show little orno change J.n 90--de|;ree peel strength after aging and (2)the boric strerngthsi of! the uninhi.bited puds (cormpound 1)dropped signL ;.c:..v,

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CONCLUSIONS

Track pads fabricated from certain millable polyesterurethanes continue to demonstrate the best resistance towear when compared with commercial SBR pads. The highcost and the poor hydrolytic stability of the polyesterurethanes, even when hydrolysis inhibitors are incorpor-ated, may preclude their use in this application.

The addition of small amounts of calcium oxideeliminates the porosity caused by heat buildup in millablepolyester urethane track pads service tested at high speeds,but has an adverse effect on the hydrolytic stability andwear resistance of vulcanizates inhibited with PCD.

A satisfactory rubber-to-metal bonding system for

millable polyester urethanes has been found.

RECOMMENDATIONS

A concentrated effort should be made to have a servicetest performed on the T142 pads which were prepared for theservice test scheduled for the General Motors test track atMilford, Michigan. It is believed that much valuable datacould be obtained on these pads even though they are nowtwo to three years old.

Efforts should continue in the search for improvedtrack pads fabricated from elastomers other than themillable polyester urethanes.

The search should continue for a more effective hy-

drolysis inhibitor for the millable polyester urethanes.

ACKNOWLEDGEMENT

This work was done in cooperation with and partiallyfunded by the U. S. Army Tank-Automotive Command, Warren,Michigan. That agency also arranged for the service tests.

37

LITERATURE CITED

1. U. S. Army Materiel Command Technical InformationReport 30.7.7.1, "Double-Pin Track, T142,"April 1968, Prepared by University of PittsburghResearch Staff under Contract DA-49-186-AMC-214(D).

2. Rock Island Arsenal Laboratory Technical Report63-1242, "Synthesis and Zvaluation of PolyurethaneElastomers," 17 April 1963.

3. Rock Island Arsenal Laboratory Technical Report63-2900, "Polyurethane Elastomers for Track PadApplications," 10 September 1963.

4. Rock Island Arsenal Laboratory Technical Report64-2678, "The Development of Elastomeric Vul-canizates for Track Pad Applications," 10September 1964.

5. Rock Island Arsenal Laboratory Technical Report64-3579, "Polyurethane Vulcanizates for Track PadApplication," 21 December 1964.

6. Rock Island Arsenal R&E Div. Technical Report 66-2517,"Development and Service Testing of Rubber TrackPads," August 1966.

7. Ossefort, Z. T. and Testroet, F. B., "HydrolyticStability of Urethane Elastomers," Rubber Chemistryand Technology, Vol. 39, No. 4,' Part 2, September1966, pp. 1306-1327.

8. Army Weapons Command Science and Technology LaboratoryReport 68-3276, "Hydrolytic Stability of CommorcialPolyurethane Vulcanizates," November 1968.

9. Army Weapons Command Science and Technology LaboratoryReport 68-3462, "Five Year Aging of ElastomericVulcanizates in Panama, Alaska, and Illinois,"December 1968.

10. Private Communication, Uniroyal Chemical, 10 January1968.

38

LITERATURE CITED

11. Ossefort, Z. T. and Bergstrom, I. W., "Ethylene-Propy-lene Rubbers as Antiozonants," Rubber Age, Vol. 101,No. 9, September 1969, pp. 47-60.

j•utLter from FMC to ATAC, dated 10 February 196i.,Subject: Fifty-Five Polyurethane Track Shoe PadsSupplied to PEC by Rock Island Arsenal duringearly 1968.

13. Letter from FMC to ATAC, dated 10 February 1969,Subject: Test of Glass Reinforced Estane Poly-urethane, Injection Moulded Track Pads.

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DOCtMENT CONTROL DATA- R & D(UJ urimt eleae11allcesi of tIN.. .*0 or •e•ttret a nd aonp ine e ~ tlma must be ofnE whm me *"roll reoft is c,..leeelildj

1. @~ORIS G ACTIVITY (CIM 0 401640) loe. 1119090"? ISCUMITY CLASSSWICAsOM

U. S. Army Weapon. Command UnclassifiedResearch & Engineering Directorate i. GROUPRock Island, Illinois 61201 I

1- NiPONT TITSIK

DEVELOPMlENT OF RUBBER PADS FOR TRACKED VEHICLES (U)

4. 0199CRIPTIV9[ NOTI9$ (Trpe 01Jrelerl damd dmg/l dmere)

Interim report on a continuing program5. AVJ T"Oftlll) (Pies1 riame, milldle Mi~~lla. toot ftefte)

Edward W. Bergstrom andJohn R Cerny

4. 0CPONT OAT e a 11. TOTAL NO. OF PAGRI ]IL NO. OP 13

February 1970 49 13es. CONTRACT OR GRANT NO. oe OSI0MNAOR'S 51.ON5 MUMl*6 14

b. POJ•Cc 4O. RE TR 70-121DA IT062105A329

€. lOb. OTHLF4rst NtPOftT NO4re) (Any o~ther lars 4 &.t aW he "stated

AMS Code 5025.11.295 S. M O.dd.

10. O IlU~ iON ?ASMUNl

Each transmittal of this document outside the agencies of the U. S.Government must have prior approval of the U. S. Army Weapons CommandScience and Technology Laboratory, AMSWE-RETI. SUIPPLrEMTENYAV NOTCS I3. SPONSORIPNG MILITARY ACTIVI tY

U. S. Army Weapons Command

Is. ASS¶SACT

Improvement in the wear resistance of rubber track pads was soughtthrough compounding studies and evaluation of rubber-to-metal bondingagents. Correlation was sought between laboratory tests on the rubbercomponent of track pads and service tests on the entire pad. Trackpads fabricated from millable polyester urethanes provided improvedservice over that of commercial control pads. Small amounts of calciumoxide in millable polyester urethane vulcanizates eliminates the in-ternal porosity which leads to early failure of track pads in highspeed service tests but has an adverse effect on hydrolytic stabilityand wear resistance. Initial correlation efforts using the DeMattiaand Firestone Flexometer tests are sufficiently interesting to warrantfurther examination. A satisfactory rubber-to-metal bonding system formillable polyester urethanes has been found. (U) (Authors)

DD '00..1473 :11 '""'" @A Unclassified€l~crity Ce.ilediUoo

Unclassified

1,LINK A LINK 6 LINK C

. lstoners040,i TT LC Y

2. Tank Track Pads

3. Service Tests

4. Boniding Agents

5. Envi~ronment aging

6. Properties -General

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