Fracture Behavior of Dynamically VulcanizedThermoplastic Elastomers

ALAN J. LESSER, NATHAN A. JONES

Polymer Science and Engineering Department, Silvio O. Conte Research Center, University of Massachusetts,Amherst, Massachusetts 01003

Received 21 December 1998; accepted 22 June 1999

ABSTRACT: The energetics and micromechanisms of fracture in model dynamicallyvulcanized thermoplastic elastomers have been studied. Their fracture toughness val-ues have been quantified under mode 1 loading conditions using both the criticalJ-integral approach and an essential work-of-fracture method. Additional studies eval-uating the effect of specimen geometry are reported. For these studies it was found thatcenter-notched and double edge-notched test geometries were equivalent under J-integral test conditions. The effect of elastomer composition was also studied withregard to fracture resistance. Increasing the weight percentage of both elastomer andprocessing oil caused a considerable decrease in both the material’s resistance to bothfracture initiation and fracture propagation. Increasing the molecular weight of thethermoplastic phase caused a smaller reduction in fracture resistance. The phasemorphology of one model compound, TPE6114, consists of an isotactic polypropylene-rich matrix containing discrete elastomer-rich domains of a diameter of 1–3 mm. Aprocess zone was associated with fracture in this material. The process zone consists ofan array of voids and crazes that were 10–30 mm in diameter, an order of magnitudelarger than the elastomer-rich domains. These were characterized by scanning electronmicroscopy (SEM) and optical microscopy. The crazes were found to grow at an angleoblique to the overall crack growth direction. Ruthenium stained SEM samples showedthat these crazes and voids occur in both the polypropylene and elastomer domains, andthat at least some of the craze fibrils are composed of the elastomeric phase. © 2000 JohnWiley & Sons, Inc. J Appl Polym Sci 76: 763–770, 2000

Key words: thermoplastic elastomer; fracture toughness; process zone; J-integral,method of essential work

INTRODUCTION

Over the recent years, dynamically vulcanizedethylene propylene diene monomer rubber(EPDM)–polypropylene ThermoPlastic Elastomers(EPTPEs) have gained significant interest by thepolymer community due to their complex mor-

phologies and unique properties. Excellent dis-cussions showing the range of morphologies andbasic properties that can be obtained with EPTPEalloys have been reported by others.1,2

The purpose of this communication is to de-scribe the energetics and micromechanisms offracture in model EPTPEs subjected to Mode Iloading conditions on relatively thin specimens.The difficulties inherent in conducting these frac-ture studies are twofold. First, the large revers-ible deformation that occurs in the vicinity of thecrack, due to the elastomeric nature of the mate-rial, significantly alters the stress singularity

Correspondence to: A. J. Lesser.Contract grant sponsors: Advanced Elastomer Systems

and Exxon Chemical Company.Journal of Applied Polymer Science, Vol. 76, 763–770 (2000)© 2000 John Wiley & Sons, Inc.

763

during loading. Second, the large irreversible de-formation that occurs as a consequence of boththe material behavior and specimen thickness ne-gates the assumption of small-scale yielding andrequires that nonlinear fracture studies be con-ducted on these materials.

Early studies have been conducted by Thomasto develop a characteristic energy criterion forthin vulcanized elastomers.3,4 In their work,Thomas and coworkers proposed a Griffith-typecriterion as well as three different test geometriesfor this class of crosslinked materials. However, itis well established that a Griffith-type criterioncannot be applied to materials that dissipate sig-nificant energy during the fracture process, whichis typical for thermoplastics. Hence, alternativemethods must be sought.

For ductile materials, two approaches havebeen adopted to characterize their fracture behav-ior. The most widely used parameter for charac-terizing fracture in ductile materials is the J-integral approach proposed by Rice.5 Tradition-ally, fracture characterized by this methodrequires that the specimen must meet certain sizeconstraints to generate a plane-strain condition.6

A second approach used to characterize the frac-ture of ductile materials is referred to as the Es-sential Work.7 In this method, the total work offracture is considered to be made of two compo-nents; one associated with the initiation of theinstability (essential part), and the other associ-ated with the plastic deformation in the plane-stress condition (inessential part). More recentstudies by Paton and Hashemi studied the equiv-alence of both of these approaches to characterizethe plane-stress fracture behavior.8

This article investigates the use of both J-inte-gral and essential work methods to characterizethe fracture behavior of these materials underMode I loading conditions. The single-specimenJ-integral approach is considered for the compar-ison because this method, if validated, reducesthe sample volume requirements necessary forscreening studies. The effect of specimen geome-try is also investigated with regard to the J-inte-gral tests.

Finally, the effect of elastomer morphology andcomposition are investigated. The J-integral frac-ture resistance is measured for a range of elas-tomer compositions. We also investigate the dam-age that occurs in the process zone of the crack tipof one of the compositions, and discuss its scaleand form with relation to the morphology of theEPTPEs.

EXPERIMENTAL

Materials

Three formulations designated as TPE6101,TPE6112, and TPE6114 were provided to us byAdvanced Elastomer Systems, L. P. Details ofthese model EPTPE alloys are given in Table I.Samples were compression molded into plaques3-mm thick, 200-mm long and 105-mm wide in ahot press. The polymer was melted at 200°C,pressed to 3.5 MPa, and cooled to room tempera-ture by flowing cold water through pipes in thehot press while maintaining the pressure at 3.5MPa.

J-Integral Tests

Either center-notched or double edge-notched testspecimens were fabricated for the J-integral tests.The center-notched specimens were fabricatedfrom the compression-molded plaques by cuttinga 38-mm notch parallel to the plaques length us-ing a new razor blade. Similarly, the double edge-notched test specimens were fabricated by cuttingtwo 19-mm notches on opposing sides of theplaque parallel to its length. Each specimen wasthen clamped along the entire specimen lengthusing specially designed clamping fixtures. Afterclamping the specimen, the gauge width reducedto 64 mm.

The fracture tests were conducted on an In-stron Model 1123. The sample was cyclicallyloaded between its initial position and a maxi-mum extension, which was increased at a rate of2 mm per cycle. All tests were conducted at acrosshead speed of 25.5 mm/min and the cracklengths were measured optically with a Zeiss ste-reomicroscope. All crack lengths were measuredat the minimum load of each cycle. A typical load–

Table I Weight Percentage Composition of theVarious Thermoplastic Elastomers Discussed inthis Article

FormulationiPP

ContentElastomerContent

ProcessingOil

Content

MFRof

iPP

TPE6114 50% 25% 25% 20TPE6112 50% 25% 25% 0.7TPE6101 20% 40% 40% 20

MFR, melt flow rate.

764 LESSER AND JONES

displacement curve for a center-notched test spec-imen is presented in Figure 1(a).

The fracture energies were calculated using asingle-specimen test method close to that de-scribed in the ASTM E813.4 The nonlinear energyrelease rate, J, was calculated by

Ji 5nUi

BL (1)

where Ui is the energy consumed in the propaga-tion of the crack (area under the load–deflection

curve), B is the specimen thickness, and L is theligament length (W 2 a, where W is the plaquelength and a is the total crack length). h is adimensionless constant reflecting the geometry ofthe specimen. For both the center-notchedand double-edge notched fracture specimens hequals 1.

Method of Essential Work

Center-notched samples were prepared in thesame manner as for the J-integral tests exceptthat the crack lengths were varied between 45and 164 mm (corresponding to total ligamentlengths of between 153 and 34 mm). As before,fracture tests were conducted on an InstronModel 1123. Unlike the J-Integral tests, thesesamples were monotonically loaded to failure at acrosshead speed of 25.5 mm/min. The crossheadposition and load were recorded digitally. Fromthis data the total work of fracture was calcu-lated. The total work of fracture was then decom-posed into essential and inessential componentsas described by eq. (2).5

wf 5 we 1 bwi L (2)

In eq. (2) the total work of fracture wf is de-scribed in terms of we is the essential work offracture (deemed to be a material property in thistheory), b is the shape factor, wi is the inessentialwork of fracture, and L is the ligament length.Tests were conducted on center-notched speci-mens where the total work of fracture was mea-sured from a series of different specimens con-taining a range of ligament lengths. The essentialwork was then calculated by extrapolating themeasured values of specific work to a zero liga-ment length.

RESULTS AND DISCUSSION

Effect of Test Method

A series of experiments were conducted to evalu-ate the equivalence between various methods ofevaluating the fracture energy and isolate anygeometric effects. Center-notched specimens ofthe TPE6114 elastomer were tested in accordancewith a single specimen J-integral approach andby the essential work of the fracture method,which is a multiple specimen approach. The re-sults from the center-notched J-integral tests are

Figure 1 (A) Load deflection curve for the TPE6114J-integral center notched test. (B) Plot of Energy Re-lease Rate (J) vs. crack extension for TPE6114 fabri-cated into a center-notched test sample. J1c was calcu-lated from the intercept of the extrapolated data withthe J-integral value axis.

DYNAMICALLY VULCANIZED THERMOPLASTIC ELASTOMERS 765

shown in Figure 1, with Figure 1(a) showing theload deflection curves for nine cycles of crack ex-tension, and Figure 1(b) showing the correspond-ing values of fracture energy. Note in Figure 1(b)that the critical fracture energy measured on thecenter-notched specimen of TPE6114 was 46 kJ/M2, with a slope in the J 2 Da curve of 5.8MJ/M3. This indicates that this formulation isrelatively tough for an elastomeric material andthat after fracture (tearing) initiates the processis stable over the entire range of observed exten-sions. It should be noted that no offset was used inthis estimate because the crack extensions weremeasured optically and no blunting line is neces-sary.

The results from the essential work experi-ments for elastomer formulation TPE6114 aresummarized in Figure 2. Inspection of the resultsin Figure 2 indicates that two regimes arepresent. One regime indicates that the specificwork of fracture is independent of ligamentlength. Note that this regime occurs at relativelylong ligament lengths, which indicates that thefailure of the elastomer is dominated by grossyielding of the material, and is not significantlyaltered by the presence of the flaw. In the secondregime (i.e., at shorter ligament lengths) the flawdoes affect the total work of fracture. In this re-gime, the data are extrapolated to a zero ligamentlength to isolate the essential work of fracture.Results from this analysis indicate an essential

work of fracture for this material is 54 kJ/M2,which is somewhat higher than that estimatedfrom the single-specimen J-integral values. How-ever, given the significant differences in bothanalysis and test details between these two meth-ods, the authors consider this to be a reasonableagreement between these methods. Other re-searchers have argued the equivalence of thesetwo methods from analytical viewpoints,9 includ-ing constitutive effects for elastomeric materials,which were not considered here. Because bothmethods produce similar results for the criticalfracture energy, additional tests utilized the J-integral single specimen test method because lessmaterial is required.

Effect of Specimen Geometry

Another series of experiments were conducted toevaluate the invariance of the fracture energy onspecimen geometry. Again, the TPE6114 elas-tomer formulation was used for this comparison.For this study, a second set of J-integral fracturestudies were conducted on a double edge-notchedtest specimen. Results from the double edge-notched specimen produce a fracture energy of 48kJ/M2, which is essentially the same for the cen-ter-notched specimens. This further supports theview that the single-specimen J-integral ap-proach can be used, and that either center- ordouble edge-notched specimens can be used.

Effect of Material Composition on Fracture

To study the effects of elastomer composition onthe fracture energy, the authors adopted the cen-ter notched geometry, and have compared thecritical J-integral value of the three TPEs (seeTable II). The major factor affecting fracturetoughness appears to be the weight percentage ofthe three blend components. TPE6101 consists of40% elastomer, compared with 25% for TPE6114,

Table II Comparison of the Fracture Behaviorof the Elastomers Investigated Herein

FormulationJ1C

(kJ/m2)Slope of “J–Da” Curve

(MJ/m3)

TPE6114 46 5.8TPE6112 42 5.2TPE6101 24 1.6

Results were obtained from J-integral tests on center-notched samples.

Figure 2 Work of fracture for TPE6114 samples witha range of ligament lengths. The essential work offracture is given by the extrapolated work of fracturefor zero ligament length. Note that for ligament lengthslonger than 80 mm the work of fracture becomes inde-pendent of the ligament length.

766 LESSER AND JONES

and subsequently has a markedly reduced resis-tance to fracture in terms of both the criticalJ-integral value (24 kJ/m2 compared with 46 kJ/m2) and in slope of the J-integral vs. crack exten-sion graph (1.6 MJ/m3 compared with 5.8 MJ/m3).The former represents the resistance to crack ini-tiation, and the latter, the resistance to crackgrowth. It is unclear whether this is due to highelastomer content blends being innately weak orwhether this is due to the large amount of pro-cessing oil required to process high elastomer con-tent blends. By comparing the fracture ofTPE6112 and TPE6114, it is apparent thatchanging the molecular weight of the iPP hadonly a marginal affect on the fracture properties.Both the critical J-integral value and the slope ofthe J-integral vs. crack extension graph areslightly reduced by implementing a more viscous(higher molecular weight) grade of iPP.

Micromechanisms of Fracture

The previous section showed that the fractureenergy and stability are highly dependent on thecomposition and morphology of the EPTPE blend.Different compositions damage to different ex-tents as the composition is altered. It is well es-tablished that the damage that proceeds and sur-rounds the crack consumes a significant amountof the fracture energy, and often accounts for amajor part of the material’s resistance to fracture.Consequently, it is useful to identify the micro-mechanisms associated with the fracture process

to help guide development of these materials. Thefollowing discussion of the morphological changesresulting from fracture and micromechanisms offracture is based solely on results observed inTPE6114.

During the fracture tests, a process zone ofdamage was observed in the optical stereomicro-scope by the scattering of white light transmittedthrough the sample. To isolate the micromecha-nisms associated with this damage, optical sec-tions were cut perpendicular to the fracture sur-face through the damaged material. These sec-tions were cryo cut using liquid nitrogen coolingand a glass knife; they could then be examined bytransmission optical microscopy (TOM). Similarlyoriented block faces were microtomed smooth in aliquid nitrogen cooled microtome, and then coatedin gold and examined using scanning electronmicroscopy (SEM).

Figure 3 shows a composite of TOM and SEMmicrographs from a section cut through the dam-age area of a J-integral test sample (note that thefracture surface is on the far left). The whiteningmacroscopically visible is evident here as a seriesof vertical dark streaks perpendicular to thestress direction. The horizontal streaks are mic-rotome knife marks and the V-shape in the lowerhalf of the image is a second smaller section—both should be ignored. Closer inspection of thedamage features by SEM showed that these de-fects are, in fact, both crazes and voids (see in-serts on right). The size of these flaws varies from

Figure 3 Combined TOM micrograph and SEM micrographs of the process zoneassociated with crack growth of the J-integral test for TPE6114. The crack growthdirection is perpendicularly out of the page, and the fracture surface is indicated. Theflaws that make up the process zone are both voids and crazes (see inserts). Thedimensions of these flaws are generally between 10 and 30 mm.

DYNAMICALLY VULCANIZED THERMOPLASTIC ELASTOMERS 767

10 up to 30 mm. Note that these flaws are largerthan the discrete elastomer domain size (1–3mm)4 within the continuous polypropylene matrixby an order of magnitude. From these sections thecrazes appeared to be, in general, perpendicularto the applied load direction, although the shapeof the crazes is often tortuous. It is evident fromFigure 3 that the density of crazes is greatestnear the fracture surface and gradually decreaseswith increasing distance from the fracture sur-face. This is in accord with the diffuse nature ofthe damage zone in Mode I fracture tests.

Further information can be gathered from ex-amination of damage in sections cut parallel toboth the crack growth direction and the directionof applied load (see Fig. 4). Again, both voids andcrazes are observed, but in these sections thecrazes are seen to grow obliquely to both the ap-plied load and the crack growth direction. Theorientation of these oblique crazes to the directionof crack growth clearly indicates that theseoblique crazes are not shear bands. It is knownthat at the crack tip the stress state is triaxial,and hence, it is not surprising that the stressdirection in the proximity of the crack tip isoblique to the applied load. This local stress state

could account for these crazes being oriented ob-liquely to the overall crack growth direction.These oblique crazes could account for the natureof the highly textured fracture surface (see the topof Fig. 4).

Microscopic examination of the process zoneassociated with crack growth in the essentialwork samples reveals a similar scheme of dam-age. The process zone consists of an array of voidsand crazes that are an order of magnitude largerthan elastomer phase domains. The crazes arealigned obliquely to the overall direction of thecrack growth, and there is a highly textured frac-ture surface, possibly due to the oblique crazes.

A block face cut from within the process zone,perpendicular to the crack growth direction, wascryo-microtomed, stained using ruthenium vapor,and then examined using SEM (see Fig. 5). Theruthenium preferentially stains the elastomer-rich regions; these become electron dense, emitmore secondary electrons, and hence, appear asthe bright domains in Figure 5. Figure 5(a) showsmany interesting features of the damage mecha-nisms in these polymer systems. For the crazesand voids to be many times the size of the discreteelastomer domains it is necessary that these

Figure 4 SEM micrograph of the process associated with crack growth in a J-integraltest on the TPE6114 samples. The crack growth direction is from left to right, and thefracture surface is indicated at the top of the image. As in Figure 3, the process zoneflaws can be seen to be both voids and crazes. However, from this orientation it can beseen that many of the crazes are aligned obliquely to the overall crack growth direction,and that the texture of the fracture surface is parallel to these oblique crazes. Twooblique crazes have been highlighted.

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crazes and voids exist within the polypropylenedomain and possibly the elastomer domains aswell. In the top middle of Figure 5(a) there is anexample of a craze that has passed through boththe polypropylene domain and an unusually large(8 mm) elastomer domain. It is apparent that the

material between the craze faces within this elas-tomeric domain is highly fibrillated. Further-more, the craze at the middle left of Figure 5(a)has grown through the lower part of an elastomerdomain, and an elastomeric fibril has extendedbetween the two faces of the craze. The elastomer

Figure 5 SEM micrographs of a cryomicrotomed and then ruthenium stained blockface of the process zone around crack growth in TPE6114. The loading direction isvertical and the overall crack growth direction is out of the page. (A) Shows details ofa crack growing through both polymer and elastomer domains (top–middle) and a crazein which at least one fibril consists of elastomeric material. (B) Illustrates a voidedelastomer-rich domain.

DYNAMICALLY VULCANIZED THERMOPLASTIC ELASTOMERS 769

domain shown in Figure 5(b) has become voidedin a manner reminiscent of that in many otherelastomer toughened polymeric systems.

CONCLUSIONS

The fracture behavior of three model EPTPEsillustrates that these are tough materials. For themodel EPTPE KW6114, we confirm that mea-surements of critical energy release rate (46 kJ/m2) and essential work (54 kJ/m2) are equivalent.Furthermore, changes to the sample geometrywere not found to affect the critical energy releaserate (double edged notched, 48 kJ/m2, and centernotched, 46 kJ/m2). Increasing both the elastomerand processing oil components of these blendswas found to drastically reduce the resistance tofracture initiation and propagation. Increasingthe molecular weight of the thermoplastic phasewas found to marginally reduce the material’sresistance to fracture initiation and propagation.Moreover, these materials fracture in a stablefashion, and no evidence of instability was found.

The micromechanisms of damage occur as anarray of crazes and voids, which increase in den-sity as the fracture surface is approached. Fur-ther, these flaws are of a scale that is approxi-mately one order of magnitude larger than the

heterogeneous texture of the alloy (i.e., the do-main size of the elastomer phase). It was foundthat these flaws grow through both the polymerand elastomer domains, and that at least some ofthe craze fibrils consist of the elastomer phase.

The authors gratefully acknowledge Advanced Elas-tomer Systems and Exxon Chemical Company for theirfinancial support and materials provided for this study.Also, the authors would like to thank Dr. M. D. Ellul forher many thoughtful comments and suggestionsthroughout this study.

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