sbr 1502 stress strain

Properties of EPDM/SBR BlendsCured with Peroxide and SulfurCoagentJ. Zhao, G. Ghebremeskel and J. PeaselyPort Neches (USA)

EPDM SBR Blends Peroxidecure Sulfur cure Coagent

The objective of this study is to ad-dress the replacement of up to 30parts of EPDM with SBR to reducethe cost of finished products and toimprove selected blend propertiesusing more efficient cure systems.Effects of sulfur, peroxide, and coa-gent curing systems on blend prop-erties were studied. It was foundthat the addition of a small amountsof sulfur as a coagent to the perox-ide cure system in EPDM/SBR com-pounds remarkably improved me-chanical properties of the blends.Important compound properties in-cluding: mechanical properties,compression set, heat aging, andozone resistance of the EPDM/SBRblends are also compared withthose of EPDM compounds.

Eigenschaften von EPDM/SBRMischungen vernetzt mit Per-oxid und Schwefelcoagenzien

EPDM SBR Mischungen Per-oxidvernetzung Schwefelvernet-zung Coagentien

Die Untersuchung zielt auf den Er-satz von 30 Teilen EPDM durch SBR,um die Kosten von Fertigproduktenzu senken und die Wirkung eineseffizienten Vernetzungssystems aufdie Eigenschaftsverbesserung zubewerten. Die Wirkungen von Ver-netzungssystemen wie Schwefel,Peroxid und Coagentien wurden un-tersucht. Es wurde gefunden, dadie Zugabe geringer MengenSchwefel als Coagens von Peroxid-systemen zu einer beachtenswertenVerbesserung der mechanischen Ei-genschaften von EPDM/SBR-Com-pounds fu hrt. Letztlich wird ein Ver-gleich der wichtigen Vulkanisatei-genschaften wie Druckverfor-mungsrest, Hitzealterung undOzonbesta ndigkeit von EPDM/SBR-und EPDM-Compounds vorgestellt.

Crosslinking with peroxides has beenknown for a long time [1]. It became com-mercially important only with the develop-ment of the saturated and highly satu-rated polymers, such as EPM andEPDM. Curing of EPDM (EPM) rubbershave been accomplished largely by theuse of peroxide alone or in conjunctionwith co-curing agents [24]. Peroxidecuring of EPDM (EPM) elastomers notonly can improve performance and long-er service life, but can also improve high-temperature resistance, and reducecompression set. Peroxide curing, how-ever, has been confined to special appli-cations because of the limited compoundand processing flexibility and typicallyhigher cost relative to sulfur cure systems.

EPDM like SBR can be vulcanized withsulfur cure systems. However, differencesin the solubility of sulfur and the level ofunsaturation in the two elastomers createcure incompatibility in blends of theseelastomers [2, 57].

The purpose of this study is to investi-gate the effects of peroxide and coagenton the curing behavior and mechanicalproperties of 70/30 EPDM/SBR blends.The effect of cure system on importantcompound properties including: me-chanical properties, compression set,

heat aging, and ozone resistance were in-vestigated.

Experimental

Materials

The SBR used in this study is SBR 1502type from the Ameripol Synpol Copora-tion. EPDM with differing levels of dieneand ethylene content were obtainedfrom various suppliers. Carbon Black,N330, was obtained from EngineeredCarbons, Inc. (ECI). Oil (Sunpar 2280)was obtained from Sun Company. Dicu-myl peroxide, ZnO, sulfur, accelerators,stearic acid, and stabilizers used in thisstudy were of commmercial grade andsources.

Formulations and mixing

Table 1 and Table 2 show the general rec-ipe used in this study. Compounding wascarried out in a small-scale laboratoryBrabender Plasti-corder or a laboratorysize Banbury mixer. The speed of the ro-tor in the Brabender and Banbury mixerswas set at 80 rpm. The total volume ofeach mix in the Brabender mixer waskept constant at about 60 cm3. Recipes

Table 1. General formulation

Sample # P (phr) S (phr) C (phr)

EPDM 70 70 70SBR1502 30 30 30CB (N330) 80 80 80Sunpar 2280 50 50 50ZnO 5 5 5Stearic Acid 1 1Sulfur 1.5 0.3TMTD 1.0 0.10MBT 0.5 0.12DCP40 2.0 3.0

KGK Kautschuk Gummi Kunststoffe 54. Jahrgang, Nr. 5/2001 223

ELASTOMERE UND KUNSTSTOFFE

ELASTOMERS AND PLASTICS

in the Banbury mixer were adjusted togive equal mixing volumes of 1200 cm3

times the specific gravity for each com-pound. Mixing was done in two stages.All ingredients except for curing agentswere mixed in the Banbury or Brabender.Curatives were added to the batch on amill.

Cure characterization was carried outwith a Monsanto ODR 2000E Rheometerin accordance with ASTM 2084. Sampleswere compression molded at 160 8C foran optimum curing condition. The proce-dure used to determine tensile, hardness,tear, compression set, and heat aginghas been described in detail in previouswork [8]. The procedure used to deter-mine brittleness temperature and ozoneresistance is discribed below.

Brittle point measurement

Brittleness temperature was measuredby a Scott Tester according toASTM D 746-79. A modified T-50 speci-men was die punched. A solution ofmixed solid carbon dioxide with acetonewas used to achieve low temperaturesand an electric immersion heater wasused for raising the temperature. Whenthe test temperature reached equilibrium,the specimens were installed and im-mersed into the bath for 3 minutes atthe test temperature. After immersingspecimens for 3 minutes, the tempera-ture was recorded and a single impactblow was delivered to the specimens.Brittleness temperature was calculatedas follows:

Tb Th DTS

100 1

2

1

where Tb is the brittleness temperature;Th is the highest temperature at whichfailure of all the specimens occurs; DTis a temperature increment; and S isthe sum of the percentage of the brokenspecimens at each temperature.

Ozone resistance

Ozone resistance was carried out underboth static and dynamic conditions. Inthe static test, ozone resistance wasperformed in accordance withASTM D 1149-91.

The cracking resistance of sampleswas determined by exposing the samplesto an atmosphere containing 50 pphm

ozone at 40 8C. The specimens werekept under a surface tensile strain. Thetime elapsed for visible cracking to occurwas determined using a magnificationX10.

Dynamic ozone resistance wasperformed in accordance with ASTMD 3395-86 method A using die C dumb-bell specimens. The cracking resistanceof samples was estimated by exposingtest samples to 50 pphm of ozone at40 8C under dynamic strain conditions(from 0 to 25 % strain at a rate of0.5 Hz). The time elapsed for the first visi-ble crack to appear and the changes inthe stress-strain curves due to the ozoneexposure were monitored.

Results and discussions

The purpose of this study was to evaluatethe effects of sulfur, peroxide, and coa-gent on the curing, mechanical and phy-sical properties of EPDM/SBR blends.Performance of the end-products curedwith peroxide and sulfur coagent as de-termined by mechanical properties,ozone resistance, heat aging, and com-pression set was also investigated.

Cure systems

In this section, a comparative study ofsulfur, peroxide and coagent cure sys-tems is presented.

Curing behavior

Table 3 shows the cure characteristics ofthe EPDM/SBR blends cured with di-cumyl peroxide (P), sulfur (S), and coa-gent (C). Scorch time, T50, T90, andthe cure rate showed the followingtrends:

Scorch time, T50, and T90: P > C > SCure rate: P < C < S

The compound cured with the coagentsystem showed higher maximum torque(34 dNm) than those cured with peroxideor sulfur cure system (26 dNm). The ad-dition of a relatively small amounts of sul-fur (0.26 mole of sulfur/mole of peroxide)to the peroxide cure system improved theperoxide efficiency significantly.

Mechanical properties

Figure 1 shows the stress-strain curves ofEPDM/SBR compounds cured with per-oxide cure system (PCS), sulfur cure sys-tem (SCS) and coagent cure systems(CCS). Mechanical properties and hard-ness of the compounds are given in Ta-ble 4. Comparison of the stress-straincurves of PCS, SCS, and CCS showthat there is no significant difference inthe modulus of CCS and PCS at the low-er strain region (up to 200 %). The mod-ulus of SCS was slightly higher than thatof CCS and PCS. This observation canbe explained by the fact that the crosslink

Table 2. General formulation of EPDM and EPDM/SBR blends

Sample # EPDM (phr) EPDM/SBR (phr)

EPDM 100 70SBR1502 30CB (N330) 80 80Sunpar 2280 50 50ZnO 5 5Stearic Acid 01 01Sulfur 0.30.5 0.30.5TMTD 0.180.3 0.180.3MBT 0.120.2 0.120.2DCP40 35 35

Table 3. Cure characterization of the SBR/EPDM blends with varying cure systems

Sample # P S C

Max. Torque, dNm 26.7 25.9 33.9Min. Torque, dNm 5.9 6.2 6.7Delta Torque, dNm 20.80 19.68 27.24Scorch time, minutes 2.46 1.90 2.05T50, minutes 7.12 3.10 5.03T90, minutes 18.17 7.62 15.42Cure rate index, 1/min 6.37 17.48 7.48

P: peroxide system, S: sulfur system, C: coagent system

224 KGK Kautschuk Gummi Kunststoffe 54. Jahrgang, Nr. 5/2001

Properties of EPDM/SBR Blends . . .

density in the SBR domains is signifi-cantly higher than of the EPDM domainin the SCS. This behavior is due to the dif-fusion of the accelerator into the more po-lar and/or faster curing phase of the elas-tomer blend. In support of this conclu-sion, Woods and Davidon [9] have foundsulfur and accelerators from the EPDMphase diffuse to the NBR phase duringthe vulcanization process.

The modulus at higher strain for thePCS was higher than SCS because theEPDM matrix of the blends cured withsulfur did not have a high enough cross-link density to support the higher stress.The accelerator loss from the EPDMphase results in lower crosslink densityin this phase resulting in decrease of elon-gation at high the stress.

The tensile strength and the energy atbreak of the CCS was about 18 MPa and12.2 J, while the tensile strength and theenergy at break of the compound PCSwas 15 MPa and 10.7 J. The tensilestrength and the energy at break of the

SCS were found about 11.5 MPa and9.18 J, respectively. The ultimate elonga-tion of CCS, PCS, and SCS were found tobe almost identical. The higher tensilestrength and the higher energy at breakfor CCS indicates that the compoundscured with peroxide and sulfur havestronger and better network properties.The addition of a relatively small amountof sulfur as a coagent to the peroxide curesystem (0.26 moles of sulfur/mole of per-oxide) showed a remarkable influence onthe mechanical properties of the blends.Tensile strength, energy at break, elonga-tion at break and modulus at high strainincreased significantly (Figure 2). Thecoagent cure system (combination of sul-fur and peroxide cure system) covulcan-ized the continuous phase (EPDM), thedispersed phase (SBR), and phaseboundary of the elastomers. This resultin increase in the interfacial strength sothat the blend properties are similar tothose attributed to a single polymer.The polysulfidic crosslinks are generally

stronger than the C-C or monosulfidecrosslinks [1015]. Vulcanizates withthe appropriate mix of crosslink typeshave superior strength and fatigue resis-tance compared to networks containingonly the stronger monosulfide or C-Cbonds. Polysulfidic bonds are weakerand more readily broken than C-C bonds,and thus, high stresses in the molecularnetwork are relieved by fracture of at leastsome of these crosslinks before back-bone chains are broken. The broken S-S- bonds may either reform again un-der load [16] or link up with the carbonblack to form either chemical carbon-gel bonds or physical carbon-gel link-ages. The reformed bonds can continueto support stress and to generate moreenergy dissipation due to chain slippageas the deformation is increased.

Hardness and the modulus at lowstrain of the compounds cured with thecoagent were not affected by the additionof the sulfur. Since the amount of sulfuradded to the peroxide cure package isvery small, only the C-C bonds (peroxidecure) play the major role in determiningthe hardness and modulus at low strain.In addition, the coagents improve the ef-ficiency of peroxide crosslinking by sup-pressing unwanted side reactions ofpolymer radicals [17].

The cure behavior and mechanicalproperties of the blends cured with coa-gent and sulfur cure systems are dis-cussed below in further detail.

Table 4. Mechanical Properties of the EPDM/SBR compounds

Sample # P S C

Tensile Strength (MPa) 15.0 11.5 18.0Elongation at Break (%) 428 428 432100 % Modulus (MPa) 2.69 3.43 2.61200 % Modulus (MPa) 6.58 6.37 6.77300 % Modulus (MPa) 11.3 9.22 12.3Hardness Shore A 68 72 67Energy at Break (J) 10.7 9.18 12.2

Figure 1. Stress-strain curves of EPDM/SBR (70/30) blends with dif-ferent cure systems

Figure 2. Stress-strain curves of EPDM and EPDM/SBR blends withperoxide and sulfur coagent



Peroxide and sulfur coagent

In the previous section, it was reportedthat EPDM/SBR blends cured with per-oxide and sulfur coagent have better cur-ing and mechanical properties. Com-pression set, heat aging, mechanicalproperties and ozone resistance of theblend compounds were also comparedto those of the EPDM compounds. Theresults are summarized below.

Curing behavior

Table 5 above shows the cure character-istics of EPDM and EPDM/SBR (70/30)compounds cured with peroxide and sul-fur coagent. Two levels of curatives wereinvestigated (Table 2). The curing beha-vior of the blend compounds was not sig-nificantly different than that of the EPDMcompounds. Due to the higher Mooneyviscosity of the EPDM relative to that ofthe SBR used in this study, the maximumtorque of the EPDM compounds washigher than that of the EPDM/SBR com-pounds.

Mechanical properties

Figure 2 shows the stress-strain curves ofthe EPDM and the EPDM/SBR com-pounds cured with peroxide and sulfurcoagent. The mechanical and physicalproperties of the compounds are givenin Table 6. Replacing 30 parts of theEPDM by SBR decreased the tensilestrength and ultimate elongation by about18 % and 15 % respectively. The tearstrength decreased, while the modulusincreased at the low strain region dueto the increase in the crosslink densityin the SBR domains. The SBR domainsin the blends played a reinforcing role.

Compression set

The compression set of the EPDM andEPDM/SBR compounds cured with per-oxide and sulfur coagent were measuredaccording to Method B of ASTM D395-89. The compression set of the EPDM/SBR compounds was not significantlydifferent from that of the EPDM com-pounds (Table 7).

The compression set of EPDM andEPDM/SBR compounds was higher atroom temperature than at 100 8C.When a sample is subjected to compres-

sion, both physical and chemical stressrelaxation can occur simultaneously. Atroom temperature, the physical stress re-laxation dominates over the chemicalstress relaxation, while at a higher tem-perature the relaxation process is domi-nated by chemical reactions.

Heat aging

Changes in the mechanical properties ofEPDM and EPDM/SBR compoundscured with peroxide and sulfur coagentafter 7 days aging at 100 and 140 8Care given in Table 8 and Table 9. At100 8C, tensile strength and ultimateelongation of both compounds did notchange significantly. Modulus and hard-ness increased slightly.

At 140 8C, the mechanical propertiesof the EPDM/SBR compounds werefound to be significantly worse than thoseof the EPDM compounds. This is due tothe fact that the heat resistance of EPDM

vulcanizates is better than that of the SBRvulcanizates.

Ozone resistance

The EPDM and EPDM/SBR blends wereexposed to ozone under dynamic strainconditions (from 0 to 25 % at a rate of0.5 Hz) in an atmosphere containing50 pphm of ozone at 40 8C. Optical mi-croscope analysis of the surface of allthe samples after 12 days of dynamicand 14 days of static ozone agingshowed no cracking.

Other properties

Table 10 shows that replacing 30 parts ofEPDM with SBR decreased the com-pound Mooney by up to 10 points. Thisis a definite plus in the production of ex-truded goods. Other physical properties,such as brittleness, hardness, abrasionloss, rebound and heat build-up were

Table 5. Cure behavior of EPDM and EPDM/SBR (70/30) blends with peroxide and sulfurcoagent cure system

Sample # EPDM1 EPDM/SBR1 EPDM2 EPDM/SBR2

Cure system High level Low levelMax. Torque, dNm 46.1 41.9 38.8 34.8Min. Torque, dNm 7.1 7.1 6.7 6.6Delta Torque, dNm 39.0 34.9 32.1 28.2Scorch time, minutes 1.72 1.68 2.20 1.98T50, minutes 3.89 3.88 5.37 4.77T90, minutes 12.4 13.2 15.5 14.9Cure rate index, 1/min 9.35 8.67 7.55 7.73

The compounds contain 80 phr N330 and 50 phr Oil.

Table 6. Mechanical properties


Cure system High level Low level

Stress-Strain

Tensile Strength, (MPa) 23.7 19.2 22.2 18.4Elongation at Break, (%) 391 338 529 443100 % Modulus, (MPa) 3.03 4.15 2.54 3.04200 % Modulus, (MPa) 9.21 10.5 6.34 7.49300% Modulus, (MPa) 17.3 17.1 11.6 12.6

Die C-Tear

Tear Strength, (kN/m) 49.2 44.3 54.8 44.9

Table 7. Compression set



Test Conditions

23 8C for 70 hours, % 35.8 31.9 42.4 37.3100 8C for 70 hours, % 20.5 20.9 30.6 27.3



not significantly affected by the presenceof the SBR.

Summary

The purpose of this study was to find asuitable cure system for replacing up to30 parts of EPDM in some products bythe lower cost emulsion-SBR without de-terioration in the mechanical and physicalproperties of the end-products. The re-

sults of our study show that the additionof a small amount of sulfur as a coagentto the peroxide cure system in the EPDM/SBR compounds have a remarkable po-sitive influence on all the mechanicalproperties.

When 30 parts of the EPDM was re-placed by the lower cost emulsion SBRcured with the peroxide and sulfur coa-gent, compression set, 100 % modulusand 300 % modulus were improved by

10 %, 20 %, and 9 % respectively. No sig-nificant change in hardness, brittlenesstemperature, heat aging, and ozone re-sistance properties were observed.

The blend compounds, however,showed slight reduction in tensilestrength, elongation and tear strengthcompared to the EPDM compounds.Also, the high temperature (140 8C)heat aging properties of these blendswas also not comparable to the EPDMcompounds.

Acknowledgment

The authors wish to acknowledge the support andencouragement given for this work by the AmeripolSynpol Corporation.

References

[1] I.I. Ostromyslenski, J. Russ. Phys. Chem. Soc.,47 (1915) 1467.

[2] J. Brooke Gardener, Rubber Chem. Technol. 41(1968) 1312; 42 (1969) 1058; 43 (1970) 370.

[3] A. Robinson, J. Moore and L. Amberg, RubberWorld, 144, No.4 (1961) 86.

[4] P. Wei and J. Rehner, Jr., Rubber Chem. Tech-nol., 35 (1962) 133.

[5] F.-X. Gullaumond, Rubber Chem. Technol., 49(1976) 105.

[6] O. Olabisi, L.M. Robeson and M.T. Shaw, Poly-mer-Polymer Miscibility, Academic Press Inc.,New York (1979) Chp 2.

[7] C.M. Roland, Rubber Chem. Technol. 62 (1989)456.

[8] J. Zhao, G.N. Ghebremeskel and J. Peasey,Rubber World, 219, No. 3 (1998) 37.

[9] M.E. Woods and J.A. Davidson, Rubber Chem.Technol., 49 (1976) 112.

[10] G.J. Lake and P.B. Lindley, J. Appl. Polym. Sci. 9(1965) 1233.

[11] R.J. Chang and A.N. Gent, J. Polym. Sci.,Polym. Phys. Ed., 19 (1981) 1619.

[12] L. Yanyo, Int. J. Tract. 39 (1989) 103.[13] A.N. Gent and S.-M. Lai, J. Polym. Sci., Part B:

Polym. Phys. 32 (1994) 1543[14] A.N. Gent and S.-M. Lai, C. Nah and C. Wong,

Rubber Chem. Technol. 67 (1994) 649.[15] E. Southern, in Elastomers: Criteria for Engineer-

ing Design, C. Hepburn and R.J.W. Reynolds,Eds., Applied Science Publishers, London(1979) 273

[16] A.G. Thomas, J. Polym. Sci., 31 (1958) 467.[17] L.D. Loan, Rubber Chem. Technol., 40 (1967)

149.

The authors

All the authors are employees of the Ameripol SynpolCorporation, Research & Development Department.Dr. Zhao is a Materials Research Scientist, Dr. Gheb-remeskel is the manger of the Materials and AnalylicalDivision, and J. Peasely is a technican in the MaterialsGroup.

Corresponding authorJunling ZhaoR + D Ameriol Synpol CorporationP. O. Box 6671215 Main StratPort Neches USA Texas 77651

Table 8. Heat aging* studies



Stress-Strain

Tensile Strength Retention, (%) 2.53 0.52 5.41 1.63Strain at Break Retention, (%) 1.28 2.37 4.54 8.80100 % Modulus Retention, (%) 10.23 21.20 17.7 22.0200 % Modulus Retention, (%) 5.97 8.57 21.8 11.8300 % Modulus Retention, (%) 3.47 3.51 15.5 10.3

Hardness

Hardness Retention, (%) 2.86 4.17 1.39 2.82* The compounds were aged at 100 8C for 7 days.

Table 9. Heat aging* studies



Stress-Strain

Tensile Strength Retention, (%) 4.22 39.58 7.21 41.85Strain at Break Retention, (%) 16.1 60.1 18.9 62.510 % Modulus Retention, (%) 38.0 153 50.0 212200 % Modulus Retention, (%)300 % Modulus Retention, (%)

Hardness

Hardness Retention, (%) 7.14 30.5 4.17 29.6* The compounds were aged at 140 8C for 7 days.

Table 10. Comparison of compound properties



Compound Mooney

Mooney (1 4min) 100 8C 57.3 53.9 65.4 56.8Hardness

Hardness Shore A 70 72 72 71Brittleness Temperature

Temperature, 8C > 60* > 60* > 60* > 60*DIN Abrasion

Vol. Loss, mm3 79.9 79.9 80.1 86

Zwick Rebound

23 8C, % 54.1 47.5 52.3 48.370 8C, % 62.3 54.3 56.1 50.9

Goodrich Flexometer

Delta T, 8C 47.5 59.7 70.8 80.2Permanent Set, % 2.1 2.8 6.6 6.3

*At that temperature (60 8C), zero specimen was failed. No further tested.



sbr 1502 stress strain

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