0

o OFFICE OF NAVAL RESEARCHContract NO0014-76-C-0408

eProject NR 092-555

Technical Report Noe 27

TEAR STRENGTH OF ELASTOMERS UNDER THRESHOLD CONDITIONS

by

A. K. Bhowmick, A. N. Gent and C. T. R. Pulford

Institute of Polymer Science D T CThe University of Akron

Akron, Ohio 44325 APR 1 8 198

April, 1983 "' B

Reproduction in whole or in part is permitted

for any purpose of the United States Government

Approved for Public Release; Distribution Unrestricted

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Technical Report No. 27 rqD hJ7 _________(___04. TITLE (Mod S""doN) L TYPE OF REPORT & PRIaOD COVERED

Tear Strength of Elastouiers under Threshold Technical ReportConditions

_______________________Zupan

7. AUmo401j 4. CONTRACT OR GRANT numUER(si

A. K. Bhowinick, A. N. Gent and C. T. R. Pulford N00014-76-C-0408

9. PaRFORMING ORGANI ZATION N AME AMC ADDRESS 14- PROGRAM ELEMENT. PROJECT. TASKCInstitite of Polymner Science ARCA 6 WORK UNIT NUMBERSThe University of Akron NR 092-555Akron, Ohio 44325 _____________

11. CONTRtOLLING OFFICE NAME AND ORES9 IIL REPORT DATSOffici of Naval Research April, 1983Power Program .NUEROA sArlington, VA 22217 _____________

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ISSUPPLEMENTARY NOTELS

Submitted for publication in: Rubber Chemistry and Technology

15.y KEYi WOW Cimie,. d Iueeear mEd""ow"& Mee& 5

Crack propagation, Crosslinking, Elastame'rs, Fracture, Reinforc ement,Rubber, Strength, Tear resistance.

-Measurements have been made of the tear strength of several elastamericmaterials under threshold conditions, i.e., in the swollen state and athigh temperatures when dissipative contributions to the work of fractureare minimized. The materials studied were Neoprene WRT, PB, SBR, EPDM anda castable polysulfide material, LP-32. They were crosslinked to differentdegrees, using a free-radical source, or sulfur, or a metal oxide as the-.crosslinking reagent. Values obtained for the threshold tear strength ".

DD I AM7 1473 EITION Oir I NOV $a 19 OSOLCTEtS/N 0 102. LP.O 0 14601 SECURITY CLASSIFICATION. OF THIS PAGE9 (When. Des SalinE)

="~MTV CL.AW~PICAIWWH OF T0111 PASSZ fUWO DO* hemW

20. (continued) /.aprewere similar in all casesl'ying in the range 50 -200 Jl.They apaeto be approximately proportional to E i,. where E is the tensile (Young's)modulus of the elastomer, as predicted by the theoretical treatment ofLake and Thomas. Values for networks with polysulfidic crosslinks weresignificantly higher than with monosulfide or C-C crosslinks, by about a-factor of 2. Values for carbon-black-filled materials were higher also,by about the same factor, in comparison with the equivalent unfilledmaterials. These'effects are attributed to rupture of labile crosslinks.,or bonds to carbon black, prior to main-chain rupture.-

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2

Introduction

The tear strength or fracture energy of a material, denoted here

Gc , represents the amount of mechanical energy required to propagate a

tear through a unit area of the material. Vulcanized rubber compounds

have a minimum tear strength under certain conditions, when visco-

elastic and other dissipative processes are minimized. This minimum

strength, denoted here the threshold strength, has been measured

previously for a number of elastomeric networks (1-4). The results

have been found to agree satisfactorily with a theoretical treatment

due to Lake and Thomas (5) both in absolute magnitude and in the

dependence upon the degree of crosslinking (3,4). In accordance with the

theory, the threshold tear strength is found to increase with increasing

length of the molecular strands comprising the network, approximately

in proportion to Mc where Mc is the average molecular weight of a

network strand. Thus, the relationship for the threshold tear strength

Gco is

Gco KM (

The coefficient K relating the threshold tear strength to the

molecular weight of network strands is itself dependent upon the

chemical structure of the molecules comprising the network. It

depends upon the length of the main-chain bonds, the average mass

per main-chain atom, the flexibility of the chain and the dissociation

energy of the weakest main-chain bond. This dependence has been shown

to account for the particularly low threshold strengths of silicone

rubber and phosphonitrilic fluoroelastomer compared to polybutadlene

and polyisoprene (3,4). The principal factor responsible is

- .i

3

the larger average mass per main-chain atom for the former two elastomers

compared to the latter.

Equation 1 can be reformulated in terms of the tensile (Young's)

modulus E of elasticity of the material, using the theoretical relation (6)

M a 3pRT/E

where R is the gas constant and T is absolute temperature. Equation I

then becomes

G co = K I E" , (2)

and predicts that the threshold tear strength is a slowly decreasing

function of Young's modulus E.

Measurements have now been made of the threshold tear strengths for

several other elastomeric materials: a representative copolymer of

styrene and butadiene (25:75, SBR 1502, Firestone Tire and Rubber Company;

polychloroprene (Neoprene WRT, E. I. Du Pont de Nemours & Co.); a poly-

sulfide sealant material (Thiokol LP-32); and a copolymer of ethylene

and propylene (EPDM, Vistalon 4608, Exxon Chemical Company). These

materials represent a wide variety of chemical structures.

In addition, measurements have been made of the threshold strength

of molecular networks prepared by free-radical crosslinking, when the

molecules are interlinked by C-C bonds, and by sulphur crosslinking

systems of various kinds, in order to study the effect of crosslink type

upon the threshold tear strength.

Finally, some preliminary measurements are reported of the threshold

fracture energies for a carbon-black-reinforced elastomer.

i4

Experimental

(a) Preparation of test strips

Mix formulations and vulcanization conditions are given in Table 1.

In all cases, vulcanized rubber sheets were prepared, about 1 - 2 mm

thick. Test strips were then cut from the sheets, about 2 cm wide and

6 cm long.

(b) Measurement of tear strength

The threshold tear strength is attained when dissipative processes

are minimized. Measure r the tear strength were therefore carried

out at high temperatures, 80 C2- 150°C, at low rates of tearing,

I - 10 um/s, and using samples swollen with mobile liquids. Test strips

were scored along a central line to a depth of about one-half of the

thickness, leaving the other half to be torn through, as shown schematically

in Figure 1. The tear strength Gc was calculated from the tear force F

as follows

Gc = 2 Xs2 F/w

where As is the linear swelling ratio of the sample and w is the measured

width of the tear path (Figure lb). The factor As2 takes into account

the reduced number of network strands crossing a unit area in the swollen

material. For unswollen specimens Xs = 1.

Polybutadiene (PB), SBR and EPDM materials were swollen with liquid

paraffin oil. Neoprene WRT and Thiokol LP-32 vulcanizates were swollen

with 1, 2, 4-trichlorobenzene (TCB).

(c) Measurement of E

Values of E were determined from tensile stress-strain relations at

small strains, using unswollen samples.

5

Results and discussion

Experimentally-determined values of the threshold tear strength

Gco are given in Table 2 for all of the materials examined. They

show several important features, as discussed below.

Polychloroprene materials

It proved possible to crosslink the polychloroprene elastomer

with either an oxide crosslinking system or a sulfur crosslinking

system, although in neither case was it found possible to obtain a

high degree of crosslinking. Values of Young's modulus E were con-

sequently rather low, Table 2. When conventional formulations were

used, capable of forming both types of crosslink, substantially

higher values of E were obtained.

Difficulties were experienced in all cases in determining the

threshold values of tear energy, Gco. As shown in Figure 2, when

the tear strength Gc of unswollen samples is plotted as a function

of temperature, it appears to reach a lower limit at temperatures of

about 1200C or higher, but these values are unexpectedly high,

150 - 500 J/m2 . Moreover, at temperatures of 1500C or higher the

samples showed signs of rapid decomposition. Similar behavior was

shown previously by cis-polyisoprene materials (4). It is attributed

to the occurrence of strain-induced crystallinity at the tip of the

propagating tear, even at temperatures as high as 1500C, which

enhances the tear strength markedly.

Values of the tear strength of highly-swollen samples were found

to be much lower, comparable to those for other elastomeric materials,

and they did not vary significantly with the test temperature over

6

the range 800C to 140°C or with the nature of the swelling liquid.

These values have therefore been taken as threshold values, although

no independent evidence of having reached threshold conditions was

obtained.

Polychloroprene, SBR, polysulfide, PB and EPDM

As the results given in Table 2 show, these materials all gave

roughly similar values of threshold tear strength, about 50-100 J/m2,

when crosslinked with oxide, C-C, or simple sulfur crosslinks. Thus,

there is no obvious effect of the varied chemical structure of the

polymer molecules, or of the crosslinks that connect them, upon the

tear strength.

When the values of threshold tear strength for these materials

were plotted against the corresponding values of Young's modulus E,

Figure 3, using logarithmic scales for both axes, the results were

found to be consistent with a linear relationship having a slope

of -1/2, in accordance with equation 2. Thus, the variations found

in the threshold tear strength for these different materials are

apparently largely accounted for by variations in the degree of cross-

linking. Neither the chemical structure of the polymer chain nor

of the crosslinks between them appears to be significant. This is

probably because of the close similarity in chain flexibility, bond

length, mass per main-chain atom, and bond dissociation energy for

these varied systems. However, other systems were found to be

significantly stronger, as discussed below. It has previously been

shown that polydimethylsiloxanes and phosphonitrilic fluoroelastomers

are substantially weaker, because of their greater mass per main-

chain atom (4).

7

Sulfur crosslinking

In Table 2, the symbol -Sn- is used to denote crosslinking systems

which yield predominately polysulfidic crosslinks, having more than

two sulfur atoms per crosslink. (The symbol -Sx- is used to denote

systems which yield crosslinks of unknown structure, but probably

with two or fewer sulfur atoms per crosslink.)

The "inefficiently" crosslinked materials, with polysulphidic

crosslinks, were found to be significantly stronger than those with

monosulfidic, disulfidic or C-C crosslinks. At the same general level

of crosslinking, and therefore at similar values of Young's modulus,

the threshold tear strength was approximately twice as high.

It has been generally accepted that polysulfidic crosslinks give

stronger vulcanizates than monosulfidic or C-C crosslinks (7), but

the present measurements are the first ones to be carried out under

threshold conditions, as far as the authors are aware. Explanations

in terms of enhanced energy dissipation are thus ruled out. Instead,

it seems probable that crosslink rearrangement or scission before

rupture of the main chain is responsible for the observed strengthening

feature (7). In effect, the small-strain elastic properties are those

of relatively highly-crosslinked, short-chain, vulcanizates while the

tear strengths arise from relatively lightly-crosslinked, long-chain

structures, as a result of rearrangement of the original crosslinks

under high stresses.

Carbon black reinforcement

The determination of the threshold tear strength for carbon-black-

reinforced materials was found to be more difficult than for the

corresponding unfilled materials. Low and constant values of tear

A___m* _ _ _ _ _ _ "l- - - - - -

8

strength were only attained in the swollen state and at elevated

temperatures. Although these values have been taken to be threshold

values, there was no clear proof that threshold conditions had, indeed,

been achieved.

The values obtained in this way were found to be considerably higher,

about twice as high, as for the corresponding unfilled materials

(Table 2), being generally around 200 J/m2 . Because the reinforced

compounds were based on sulfur crosslinking systems yielding poly-

sulfidic crosslinks (as is customary) the corresponding unfilled

materials were stronger than the simplest vulcanizates, by another

factor of about 2, as discussed previously. Thus, the carbon-black-

reinforced materials were 3-4 times stronger than the simplest

unreinforced vulcanizate of the same elastomer under threshold conditions.

The large reinforcing effect of carbon black under normal conditions

is well-known. It is interesting to note that a substantial degree

of reinforcement remains under threshold conditions, when dissipative

effects are minimized. It must therefore be ascribed to structural

effects. One possible mechanism of reinforcement, analogous to that

put forward in connection with polysulfidic crosslinking, would be

the detachment of adhering polymer molecules from particles of carbon

black at forces somewhat below those causing main-chain fracture.

Thus, again, the effective network strand would be initially short,

giving a stiff material, but at high stresses it would become longer,

and hence give a strong, tear-resistant, material.

9

Acknowledgements

This work was supported by research grants from General Dynamics

Corporation, the Office of Naval Research (Contract N00014-76-C-0408),

the Naval Research Laboratory (Contract NOOO14-82-K-2015), Cabot

Corporation, and Lord Kinematics Division of Lord Corporation. The authors

are also indebted to J. A. Hartwell, H. J. Kim and R. W. Niebaum for

experimental assistance at various times.

References

1. H. K. Mueller and W. G. Knauss, Trans. Soc. Rheol. 15,

217-233 (1971).

2. A. Ahagon and A. N. Gent, J. Polymer Sci. Polymer Phys. Ed. 13,

1903-1911 (1975).

3. A. N. Gent and R. H. Tobias, in "Elastomers and Rubber Elasticity,"

ed. by J. E. Mark and J. Lal, ACS Symposium Series 193,

American Chemical Society, Washington, D.C., 1982, pp. 367-376.

4. A. N. Gent and R. H. Tobias, J. Polymer Sci. Polymer Phys. Ed.,

in press.

5. G. J. Lake and A. G. Thomas, Proc. Roy. Soc. (London) A300,

108-119 (1967).

6. L. R. G. Treloar, Physics of Rubber Elasticity, 2nd ed.,

Clarendon Press, Oxford, 1958.

7. E. Southern in "Elastomers: Criteria for Engineering Design,"

ed. by C. Hepburn and R. J. W. Reynolds, Applied Science

Publishers, London, 1979, pp. 273-310.

10

Table 1. Mix formulations, in parts by weight, and vulcaniza-

tion conditions employed for preparing test specimens.

Polychloroprene, oxide crosslinks:

Neoprene WRT, 100; magnesium oxide, 2; zinc oxide, 5.

Vulcanized at 150 0C for 30, 40 and 90 min.

Polychloroprene, sulfur crosslinks:

Neoprene WRT, 100; sulfur, 2; diorthotolyl guanidine (DOTG),

0.5; tetramethylthiuram disulfide, 1.5. Vulcanized at

1400C for 40 and 80 min.

Polychloroprene, oxide and sulfur crosslinks:

(i) Neoprene WRT, 100; sulfur, 1; tetramethylthiuram

monosulfide (TMTM), 1; DOTG, 0.5; magnesium oxide, 4;

zinc oxide, 5; stearic acid, 1.

(ii) Neoprene WRT, 100; sulfur 2; TMTM, 1; DOTG, 1;

magnesium oxide, 8; zinc oxide, 10.

Both compounds were vulcanized at 1500C for 2 h.

Polysulfide:

(i) Thiokol LP-32, 100; manganese dioxide, 4.

(ii) Thiokol LP-32, 100; manganese dioxide, 10.

Both compounds were vulcanized at 100°C for 1 h.

I ______"_ ____.. ..... . . . .....________

11

Table 1 (continued)

SBR:

FR-S 1502, 100; dicumyl peroxide, 0.5. Vulcanized at 1500C

for 2 h.

Polybutadiene (PB), carbon-carbon crosslinks:

Diene 35 NFA, 100; dicumyl peroxide, 0.2. Vulcanized at

1500C for 1 h.

Polybutadiene, EV formulation:

Oiene 35 NFA, 100; zinc oxide, 5; sulfur, 0.6; zinc 2-euhyl

hexanoate, 2; 2-morpholinothiobenzthiazole (Santocure MOR),

1.44; tetrabutylthiuram disulfide, 0.6; Agerite Resin D, 1.

Vulcanized at 1400C for 40 min.

Polybutadiene, conventional sulfur recipe:

Diene 35 NFA, 100; zinc oxide, 3.5; stearic acid, 2.5;

sulfur, 2; Santocure MOR, 0.6; Philrich oil HA5, 5;

phenyl-2-naphthylamine (PBNA); 1. Vulcanized at 1500C for

1 h.

Polybutadiene, carbon-black-reinforced, conventional sulfur recipe:

(i) As the preceding unfilled recipe, plus:

N330 carbon black (Cabot Corporation, Vulcan 3), 50.

(ii) As the preceding unfilled recipe, plus:

N765 carbon black (Cabot Corporation, Sterling

black), 50.

Both compounds were vulcanized at 1500C for 1 h.

12

Table 1 (continued)

Ethylene-propylene copolymer (EPOM), carbon-carbon crosslinks:

Vistalon 4608, 100; dicumyl peroxide, 2. Vulcanized at 160 0C

for 2 h.

EPDO, sulfur crosslinks:

Vistalon 4608, 100; sulfur, 1.5; mercaptobenzothiazole, 0.6;

methyl ethyl tuads, 1.5; zinc oxide, 4; stearic acid, 1.

Vulcanized at 150 0C for 40 min.

EPOM, carbon-black-reinforced, sulfur crosslinks:

As the preceding unfilled recipe, plus:

N330 carbon black (Cabot Corporation, Vulcan 3), 50.

Vulcanized at 1500 for 40 min.

I

13

Table 2. Threshold tear strength Gco for various compounds. (The

mix formulations and vulcanization conditions are given

in the Appendix.)Linear

Young's swellingCrosslink modulus ratio

Elastomer type E (kPa) X s Gco (J/m2)

Swollen withTCB

Neoprene WRT -0- 370 2.58 120

" 610 2.32 110

850 2.17 66

Neoprene WRT -Sx- 600 2.39 88" 850 2.17 70

Neoprene WRT -0- and -Sx- 2050 1.6 48

3200 1.4 65

Thiokol LP-32 -S2- 640 1.71 96" " 1010 1.67 66

Swollen withparaffin oil

SBR 1502 -C- 2250 1.20 60

PB (Dlene 35 NFA) -C- 2900 1.30 62" -S-(EV) - 1.32 56

mSn - - 1.34 106

PB + 50 phrN330 carbon black -Sn- - 1.2 200

PB + 50 phrN765 carbon black -Sn- - 1.36 ca 200

EPDM -C- 1.38 56(Vistalon 4608)

-Sn- 1.44 ca 160

EPDM + 50 phrN330 carbon black -Sn- 1.3 210

Figure captions 1

Figure 1. Method of measuring tear strength.

Figure 2. Tear strength G~ cVS temperature for polychloroprene andSBR vulcanizates. Polychioroprene oxide crossi inks :A,Q;sulfur crosslinks: +,0C. SBR, C-C crosslinks: 0

Figure 3. Threshold tear strength G covs Young's modulus E.

Polychloroprene materials, 0; polysulfide, 0; polybutadiene,

0; SBR,. The line is drawn with a slope of-.

( 15

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