Irradiation of Polymers - American Chemical Society

Chapter 16

Electron-Beam Manufacturing of Tank Track Pads Downloaded by UNIV MASSACHUSETTS AMHERST on October 14, 2012 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0620.ch016

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Byron J. Lambert , Ahmed A. Basfar , Walter J. Chappas, and Joseph Silverman Laboratory for Radiation and Polymer Science, Department of Materials and Nuclear Engineering, University of Maryland, College Park, MD 20742-2115

The US Army's fleet of tracked vehicles operates on replaceable rubber pads which fail through wear, chunking, and chipping. The standard elastomer is styrene butadiene rubber (SBR). Evaluations in Germany of the Army's main battle tank show that the pad consumes 73% of all operating and service costs, with a replacement cost of $25 per mile. This paper describes the development of a high-wear EB cured SBR formulation and the industrial manufacture of 250 pads. The production process, including formulation, compounding, molding, and irradiation is described. Laboratory data collected at the University of Maryland and Fort Belvoir, as well as field results conducted by the Army at the Yuma Proving Grounds, demonstrate that the new pads are substantially superior to any previous sulfur-cured SBR formulation. In addition, the new formulation exhibits superior ozone resistance.

This work was funded by the US Army Tank and Automotive Command to develop a new rubber pad for the Army's fleet of tracked vehicles. The styrene butadiene SBR pads for the Army's M60 Battle Tank survive about 2,000 km (1,200 miles) on-road. Off-road, on rugged terrain, the pads last less than 900 km (500 miles). More importantly, the cost and complications of supporting the vehicle are excessive, about $15/km ($25/mile). An SBR tank pad, while in use, experiences large cyclic deformations, both compression and extension at high temperatures. The result is cracking and loss of

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Current address: Guidant Corporation, 26531 Ynez Road, P.O. Box 9810, Temecula, CA 92591-4628 Current address: Atomic Energy Research Institute, King Abdulaziz City for Science and Technology, Riyadh 11442, Saudi Arabia

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0097--6156/96/0620-0206$12.00/0 © 1996 American Chemical Society In Irradiation of Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

Downloaded by UNIV MASSACHUSETTS AMHERST on October 14, 2012 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0620.ch016

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Electron-Beam Manufacturing of Tank Track Pads

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large sections of the pad. Elemental analysis of the failure interface (1) detected a high concentration of chemical curing agents, suggesting that a non-uniform crosslink distribution caused by inadequate mixing might be, in part, responsible for the pad's short life. Unfortunately, mixing of the additives cannot be improved without degrading the molecular weight distribution of the base rubber. A novel formulation and curing procedure for SBR was developed in this laboratory: a sulfur precure with about 25% of that in the normal formulation followed by electron beam irradiation to full cure (2). Field tests on the new SBR pads and laboratory tests on specimens of the new SBR demonstrated the marked superiority of the elastomer. Also, early tests demonstrated a remarkable resistance of sulfur-radiation cured SBR to ozone. This, in part, was surprising since the irradiated rubber's high unsaturation content should have led to lower ozone resistance. In this work, the previous work is summarized and new imformation is presented on the mechanism for the unexpected large increase in ozone resistance. Experimental Various SBR formulations were developed, blended, pressed, and cut at the University of Maryland and at the Army's Ft. Belvoir Research Development and Engineering Center (Ft. Belvoir, VA). SBR was obtained from the Firestone Rubber Company. Laboratory test samples were irradiated at the University of Maryland and Irradiation Industries, Inc. (Gaithersburg, MD); the tank pads used for field tests were irradiated with 12 MeV electrons at the IRT Corporation (San Diego, CA). Based on our laboratory tests for mechanical properties, one of the most promising formulations was blended and molded at Firestone's Noblesville, IN, plant into standard T-142 tank pads. The pads were molded as follows: the bolt and backing plate were degreased, cleaned, coated with an adhesive (Chemlock 205), air-dried, coated with a second adhesive (Chemlock 233), assembled, and placed in the mold. The SBR was mixed in a Banbury mixer and hot-extruded into strips, followed by hot extrusion into 15 mm x 15 mm x 20 mm blocks. The blocks were water cooled and placed in the molds. The mold top plate was held at 422 K (300 °F) and the backing plate face at 416 K (290 °F). They were bumped three times to remove air pockets, cured for 70 minutes, and removed. The specific gravity was measured to be 1.15. Trimming was the only other treatment performed. The ingredients for the standard SBR formulation and our formulation are given in Table I. The principal difference is in the sulfur content. The vulcanization and molding procedure for the conventional SBR, and the precure of our high performance SBR was the same. The precure provided dimensional stability during demolding and during the transportation to IRT for the post-molding radiation cure of 100 or 150 kGy. The chemical accelerators were provided by Firestone and were not identified. A crosslink sensitizer (3,9-divinyl-2,4,8,10-tetraoxaspiro [5,5] undecane or DTUD) was added to reduce radiation requirments but its concentration (0.1 pphr), while sufficient for a small reduction (25 kGy) of the dose for full cure, did not modify the almost conventional composition (except for sulfur content) in any meaningful way. Also, higher D T U D concentration tended to reduce the hot tear strength.

In Irradiation of Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

208 Table I.

IRRADIATION OF POLYMERS

Standard and U M Formulations


Ingredients

Sulfur Cured System (pphr)

EB Cured System (pphr)

SBR-1500

100

100

Carbon Black N-110

45

45

Zinc Oxide

4

4

Stearic Acid

2

2

Sulfur

2

0.5

Sensitizer (DTUD)

-

0.1

Accelerators (Firstone proprietary compounds)

2

2

Antioxidants

2

2

Antiozonant

3

3

An adiabatic temperature increase caused by 100 kGy to raw SBR is greater than 63 K and even more to the steal backing plate to which the pad is cemented. As a result, doses were delivered to each side in 25 kGy passes beneath the radiation beam followed by cooling so as to reduce overheating of the pad and the interface between the pad and the steel backing. Attenuated Total Reflection (ATR) Fourier Transform Infrared (FTIR) spectroscopy was used to measure changes in the rubber before curing, after thermal and radiation curing, and as a function of aging as simulated by exposure to ozone. The method minimizes the effects of specimen thickness and orientation and reduces the error propagation among samples. The gripper sample clamp ensured reproducible contact between the sample and the A T R crystal. Since the sample compartment is protected, a nitrogen purge to remove atmospheric water vapor and carbon dioxide was not used. The absorption of the transvinylene (965 cm ) and vinyl (910 cm" ) groups were the focus of interest in this study. Dose depth profiles were theoretically calculated using E D M U L T (3) and experimentally measured (Figure 1). The measurements were made by slicing a commercially produced T-142 pad, parallel to the metal backing plate. Dosimeters were then positioned between each of the slices and the pad was reaSwSembled. Each dosimeter package contained three Far West Technology dosimeters that were calibrated at the University of Maryland's High-Dose Secondary Standards Laboratory in accordance with procedures that are fully traceable to the National Institute of Standards and Technology. _1

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The pads were then irradiated at IRT Corporation (San Diego, CA). Since 12 MeV electrons possess insufficient energy to fully penetrate a pads thickness from one side, the pad was irradiated on each side in separate passes beneath the beam, each time receiving a surface dose of 25 kGy. Results The results of laboratory tests (Table II) for this elastomer are for a formulation with hot-tear (ASTM D624, Die C) properties 40-80% above those of comparable conventional rubber formulations (Figure 2), greater than 10 times the resistance to crack initiation (DeMattia Crack Initiation Test, unaged), and less than one-third the crack growth (DeMattia Crack Growth Test, unaged). The new formulation was used at the Firestone Rubber Company to manufacture 250 tank pads. In field testing by the Table II. Comparative Properties of tank pad rubber

Sulfur Cured SBR

EB Cured SBR

Tensile Strength

3,300 psi

3,390 psi

200 % Modulus

650 psi

655 psi

Hot Tear

130 lb/in

182 lb/in

515 %

610%

20,000 cycles

>200,000 cycles

25 mil/min

9.4 mil/min

Mechanical Properties

Elongation at Break Crack Initiation Crack Growth

U.S. Army on 200 pads, the pads showed a 60% wear improvement for on-road tests and a 30% improvement for the mixed on-road/off-road test (4). Even more remarkable was the rubber's resistance to aging. In ozone tests performed by the U.S. Army (according to A S T M D-1149 Bent Loop Test, using the specimen B bent loop test, at a temperature of 377 K (215 °F)), and at an ozone concentration of 50 pphm) the rubber survived 36 days "without any sign of cracks" while the standard formula failed within 5 days (5). A more severe aging test (according to A S T M D - l 149 Bent Loop Test at 377 K (215 °F) and with an ozone concentration of about 400 mPa) performed at the National Institute for Standards and Technology showed no sign of cracking after 5 days (6) while the standard Army formula failed within 3 hours. These initial results demonstrate that the combination sulfur-radiation curing of rubber offers a new technology to the Army, in particular, and to industry, in general, for the manufacture of SBR materials with properties thus far unattainable by traditional chemical curing techniques. The pads manufactured at Firestone and IRT were evaluated at the US Army's Yuma Proving Ground. The results of tests performed on a M-60 tank with a gross weight of 96,100 pounds is summarized in Table III. With the exception of three pads


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0

100

200

300

400

500

600

Dose (kGy) Figure 2: Dose profile through support structure and pads with 12 MeV electrons. Solid line are computer simulations and the open circles are experimental meas urements.



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that were lost due to adhesive failure at the steel backing plate-rubber interface, all pads exceeded the test limit of 2,000 miles. Evaluations of the weight loss indicate that the pads were approaching their life limit at 2,000 miles. In order to characterize the effect of unsaturation on ozone resistance, sulfur-radiation cured SBR samples with a partial sulfur concentration of 0.5 pphr were irradiated at several doses. The optical densities of the transvinylene and vinyl groups decrease as the dose increases up to 1 MGy (Figures 3 and 4). Also, the IR spectra of sulfur cured samples at two sulfur concentrations (i.e. 1.5 pphr and 2.0 pphr sulfur) were obtained.. The optical density of the unsaturation for partial sulfur cured samples at zero dose are 130 % higher than that of 1.5 pphr sulfur cured samples, and 900 % higher than that 2.0 pphr sulfur cured samples. Of particular interest is the content of the vinyl group in the above mentioned systems as it relates to their ozone resistance. At a dose of 1 MGy, the vinyl content of the sulfur-radiation samples is as low as that of sulfur cured samples. Additional experiments with formulations with a partial sulfur level of 1.0 pphr were carried out in the same manner as the above. In these experiments, the same trend is observed with the exception that a lower dose is needed to reduce the vinyl content to the same low dose as that of sulfur cured systems (i.e. 200 kGy versus 1 MGy). Discussion Speculation concerning how radiation can improve mechanical properties for tank pad applications can be understood in light of tank pad failure modes (1). For off-road and gravel surfaces, "cutting and chunking" are reported to be the principal failures. Cutting is caused when the pad hits a "road hazard", a sharp rock or other shaip object able to produce a high point stress, with enough force to penetrate or cut the surface. Chunking can then follow by "scrubbing" the pad over rough or sharp objects. It should be noted that pad operating temperatures are high due to hysteretic heat production. Under such failure conditions, a network with high tear strength at elevated temperatures, high elongation, and high energy at break seems more to have optimum properties. The optimization of these properties is balanced by the fact that a network with very high energy at break leads to high hysteretic temperature increases and another failure mode, blowout. A rubber with a high point tear strength might be expected from a uniform crosslink density spatial distribution of bonds of high strength at elevated temperatures. It is clear that the crosslink density distribution of sulfur crosslinked systems is dependent on the microscopic dispersion level of sulfur in the complex rubber formulation. However, dispersion is not uniform on the microscopic scale. A radiation cured system, on the other hand, is expected to have a crosslink density spatial distribution of carbon-carbon bonds that closely follows the spatial distribution of the dose. This is one potentially advantageous property of radiation which offers promise for overcoming tank-pad failures and hence improving the mechanical properties. Another is that radiation curing can be used to provide a wide range of specified spatial distributions of crosslinking density designed to overcome specific failure modes.


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o 5 0.04

400

600 Dose (kGy)

800

1,000

1,200

Figure 3: Transvinylene absorbance v. dose in U M formulation.

0.50

0)

u S 0-25

•s

°

o 0.20

Irradiation of Polymers - American Chemical Society

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