By Cynthia Flanigan, Laura Beyer,David Klekamp, David Rohweder and

Dan HaakensonFord Motor Co.

Organic plasticizers, such as poly-cyclic aromatic hydrocarbons (PAH) andtreated distillate aromatic extracts(TDAE) are important ingredients inrubber compound formulations.

These extender oils are used to improvethe processing of rubber compounds dur-ing the mixing, extruding and moldingstages. Plasticizers also are used to im-prove the physical properties of elas-tomers by increasing elongation, de-creasing hardness, increasing tack andimproving low temperature elasticity.By modifying the ratios of oil, filler and

elastomer content, the physical proper-ties of the rubber compound can be ad-justed according to the targeted applica-tion.

Plasticizers are selected according totheir chemical compatibility with theelastomer matrix and glass transitiontemperature, Tg, which affects the com-pounded rubber’s dynamic mechanicalproperties. In tire tread formulations,oils such as MES (mild extraction sol-vate), TDAE, RAE (residual aromaticextract) or naphthenic extenders com-monly are used. These oils are com-pounded with rubber matrices such asstyrene-butadiene rubber (SBR), polybu-tadiene rubber (BR), natural rubber (NR),or blends of these types.

In addition to petroleum-derived plas-ticizers, naturally occurring oils fromagricultural sources such as plants,

TECHNICAL NOTEBOOKEdited by Harold Herzlichh

Rubber & Plastics News ● February 11, 2013 15www.rubbernews.com

Technical

Using bio-based plasticizers, alternative rubber

nuts, seeds and fruit have been investi-gated for several years1. Alternativesources to petroleum-based chemicalsprovide an opportunity to improve envi-ronmental impact and increase renew-able content of industrial products2. Re-newable oils currently are used withinthe chemical industry for applicationsincluding paints, coatings and cosmet-ics.

Investigation of these eco-oils has ex-panded into the rubber industry as well,with recent reports of limited usage incommercial tire applications3,4.

Researchers have studied a wide vari-ety of natural oils, including soy, sun-flower, corn, canola and castor oils forcompatibility and performance in rubber

compounds5,6. With diverse options ofnatural sources available, sustainableoils may vary in degree of polarity, mo-lecular weight, transition temperaturesand unsaturation sites7. Further re-search is necessary to develop compoundformulations and oil-rubber compatibili-ty for widespread usage in high perform-ance rubber applications.

A previous study by the authors com-paring sustainable oils in SBR tread com-pounds indicated that bio-oils showedpromising results for both physical anddynamic mechanical properties8. In this

Executive summaryPlasticizers such as treated distillate aromatic extracts and aromatic oils

commonly are used in tread compounds because of their chemical compatibilityand ability to improve rubber processing. Alternative sources to these petrole-um-derived plasticizers, such as oils extracted from crops, were evaluated inmodel tread compounds.

Select bio-based oils with varying fatty acid compositions and functionalitywere used to replace part of the petroleum oil and compounded into silica-basedtread formulations. These bio-based plasticizers were evaluated in rubber com-positions with 80/20 blends of solution styrene-butadiene rubber and naturalrubber, as well as blends with an alternative bio-derived rubber, guayule.

Effects of these bio-based plasticizers and rubber on the material’s cure ki-netics and key physical properties are discussed. Dynamic mechanical thermalanalysis results are correlated to on-vehicle performance predictors for rollingresistance and traction by assessing tangent delta in various temperature re-gions.

This study shows that use of bio-based materials provides a promising alter-native to standard oil and rubber matrices, as supported by comparable me-chanical properties and performance predictors. Key technical challenges andthe potential future outlook for using sustainable raw materials in tread com-pounds are discussed.

Table I. Fatty acid profile of select bio-oils.

Table III. Mixing protocol.

Table II. Recipe formulation, parts perhundred rubber (phr), by weight.

Table IV. Rubber compound cure kinetics.

paper, four additional bio-plasticizerswere selected for evaluation in modeltread rubber with a blend of SBR/NR:palm oil, flaxseed oil, cashew nut shellliquid and low saturated soybean oil.

Additionally, an alternative, bio-de-rived rubber called guayule was used toreplace polyisoprene in the rubber blendsto increase the potential of sustainablecontent in the compounded tread.

Guayule is a shrub grown in arid cli-mates of Mexico and the southwest re-gion of the U.S. By extracting rubberparticles from its bark, producers areable to collect high molecular weightrubber that provides similar character-istics to natural rubber from Heveabrasiliensis (para rubber tree)9.

According to a 2011 report on alterna-tive rubber and latex sources, an esti-mated 10 million tons of natural rubberand latex are produced per year, with 70percent of the supply supporting the tireindustry10. With high demands and pricespikes of up to $6/kg for natural rubber,alternatives to imported latex and natu-ral rubber from Hevea trees have gainedattention.

This paper explores the technical mer-its and challenges of using guayule as asubstitution for Hevea natural rubberand in combination with bio-derivedplasticizers in silica-based, model treadcompounds.

ExperimentalMaterials selectionDry guayule natural rubber (GNR) in

bale form was supplied by the OhioAgricultural Research and DevelopmentCenter) in Wooster, Ohio. The notedelasticity and softness of guayule rubberhas made it an interesting choice for re-placing the cis-1,4-polyisoprene rubberin this study.

Guayule (Parthenium Argentatum), aperennial shrub grown in desert re-gions, accumulates latex rubber parti-cles in its bark.

By drying or coagulating these natu-ral latex particles, GNR is produced.Fig. 1 shows a photograph comparingchopped bales of SMR natural rubberand GNR used in this study.

The bio-plasticizers used in the studyinclude palm oil, flaxseed oil, cashewnut shell liquid (CNSL) and low-saturat-ed soybean oil (SBO), as shown in Fig.2. Palm oil was supplied by Cargill In-dustrial Oils & Lubricants Inc. Nulinflaxseed oil was provided by Viterra Inc.and is bred with higher levels of lin-olenic acid than traditional flaxseed andis expeller crushed.

Modified cashew nut shell liquid wassupplied by Composite Technical Ser-vices, under the product name XFN-50.The low-saturated soybean oil is a prod-uct of Zeeland Food Services Inc., andhas been refined, bleached and deodor-ized.

These bio-plasticizers were selectedSee Plasticizers, page 16

aoil extendedbbis(triethoxysilylpropyl) tetrasulfide

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based upon their range in saturation orunique chemical structure.

In contrast to the combination of aro-matic rings characterizing PAHs, manyof the plant-based oils are triglycerides,containing fatty acids comprised of longunbranched aliphatic tails attached to acarboxyl group.

Plant oils can be characterized bytheir fatty acid distributions, which de-termine the relative level of unsatura-tion in the oil. Fig. 3 shows the chemicalstructures of five of the prominent fattyacids: palmitic acid, stearic acid, oleicacid, linoleic acid and linolenic acid.

These plant-based oils are triglyc-eride-based esters of glycerol and threefatty acid chains, as shown in Fig. 4.The composition of fatty acids will varythe number of alkane groups (single car-bon bonds), alkene groups (double car-bon bonds) and alkyne groups (triplecarbon bonds) in the oil. Palm oil, for ex-ample, has a high concentration ofpalmitic and oleic acid, whereas low-sat-urated soybean oil has over 60 percentlinoleic acid in its fatty acid distribution.

The distribution of fatty acids in theseoils also varies based upon the cropsource and processing methods. Table Iprovides a comparison of the percentageof main fatty acid constituents for eachoil type. Low saturated soybean oil wasselected based upon promising results ofdegummed soybean oil used in a previ-ous study8. The low-saturated soybeanoil has a level of saturation of about 7percent, compared to traditional soy-bean oil with a saturation level of ap-proximately 15 percent. The flaxseed oilthat was selected for this study has aunique fatty acid profile compared totraditional bulk flaxseed oil and is froma specialized breeding program whichyields oils with approximately 82 per-cent polyunsaturated fatty acids com-pared to 72 percent for traditional

flaxseed oil11.While triglyceride based oils are often

extracted from plants or seeds, addition-al agricultural based plasticizers mayinclude other chemical structures. Es-sential oils such as d-limonene, com-monly known as orange oil, have cyclicterpene structures and cashew nut shellliquids are based upon aromatic ringswith long aliphatic hydrocarbon tails, asillustrated in Fig. 5.

The modified cashew nut shell liquidin this study is a polymeric materialproduced from the natural resins ofcashew shells, which are byproducts ofthe nut industry.

Iodine number can be used as an indi-cator of available unsaturation or sitesfor bonding opportunities within thechemical. Reported iodine numbers forthese bio-oils provide a ranking fromhigh unsaturation to high saturation:CNSL, flaxseed oil, low saturated SBO,palm. Two of the four bio-plasticizers,palm oil and low saturated soybean oil,were selected for evaluation with theguayule rubber due to their anticipatedcompatibility with the rubber matrix.

Characterization of plasticizersOils were evaluated using differential

scanning calorimetry (DSC), with a TA-3000 Mettler thermo-analytical instru-ment. Bio-oil samples of 10 mg wereplaced in standard Al crucibles and com-pared to the heat flow in empty cruciblereference cells at a heating and coolingrate of 0.17°C/second. Melting thermo-grams were obtained by cooling samplesto -50°C, holding temperature for 10minutes, and then heating samples to100°C. Heat flow was recorded in W/gduring the segments.

Compound formulations and mixingThe rubber compound used in this

study is a model passenger tire treadformulation with 80/20 blend of styrene-butadiene rubber and natural rubber.Table II provides the compound formu-lation, expressed in parts per hundredrubber (phr) by weight. Solution styrene-

Fig. 1. (a) Para rubber tree natural rubber; (b) guayule natural rubber.

Fig. 2. Selected bio-plasticizers used in model tire tread compounds: (a)palm oil; (b) high linolenic flaxseed oil; (c) cashew nut shell liquid; and (d)low-saturated soybean oil.

Fig. 3. Fatty acid chemical structures ofnatural oils.

Fig. 4. Chemical structure of triglyc-eride-based oils.

Fig. 5. Chemical structure of cashew nut shell liquid.

Table V. Physical properties of bio-oil compounds and guayule compounds.

Table VI. Heat-aged physical properties of bio-oil compounds and guayule com-pounds.

butadiene rubber was a blend of oil ex-tended and clear grade rubber fromLanxess, Buna VSL 5025-2 HM SBR(50% Vinyl/25% Styrene) and Buna VSL5025-0 HM SBR (50% Vinyl/25%Styrene), respectively. The natural rub-ber used in the rubber compounds was acontrolled viscosity NR from para rub-ber trees, CV-60, or when indicated, 20phr of guayule derived natural rubberwas used.

Precipitated silica, Zeosil 1165MPwith BET of 165 m2/g, was supplied byRhodia and used as the primary filler inaddition to 10 phr of N234 carbon blackfiller. Processing oil in the control rubberwas TDAE, Tudalen 4192 by Hansen-Rosenthal Oils. In the experimental rub-ber compounds, bio-plasticizers wereused to replace TDAE oil at 10 phr load-ing levels. Additional chemicals includ-ed standard rubber formulation ingredi-ents such as antioxidants, antiozonants,activators, accelerators and sulfur.

Two distinct sets of rubber compoundswere formulated and compounded: natu-ral rubber with alternative bio-oils andguayule rubber with alternative bio-oils.In total, eight tire tread formulationswere compounded in a Farrel Model 2.6BR Banbury mixer using a three passmixing approach. Table III details themixing protocol. In the first stage, theelastomers, fillers, processing oil, silanecoupling agent and antidegradants weremixed, followed by the addition of cureactivators and processing aid in the sec-ond pass. During the first two mixingstages, rotor speed was adjusted afterthe addition of each set of ingredients toraise the batch rubber temperature to160°C for completion of the silanizationreaction. In the final, productive pass,the sulfur and accelerators were mixedwith the rubber. The Banbury mixing

process used a 68-70% fill factor withram pressure ranging from 40-50 psi asnoted. Rubber compounds were sheetedout on a Farrel two-roll mill after eachBanbury mixing stage.

Processing propertiesAn oscillating disc rheometer (Mon-

santo Rheometer ODR 2000) was usedto determine cure kinetics of the rubbercompounds, following ASTM D 208412.An oscillating shearing action of con-stant amplitude was applied while thesample was held under pressure at160°C. As is typical for SBR vulcaniza-tion curves, the torque plateaued atmaximum torque and no significant re-version was evident. From the torqueversus time graphs, the time needed toachieve 90% maximum torque, tc90, wasnoted. Rubber charges were compres-sion molded at 160°C with moldingtimes equal to tc90+5 minutes.

Physical and heat-aged propertiesAn Instron dual column testing sys-

tem equipped with a 5-kN load cell anda long-travel extensometer was used toevaluate key physical properties: tensilestrength, elongation and tear resistance.Under ASTM D 41213 guidelines, TestMethod A, Die C, five dumbbell-shapedtensile specimens per sample were die-cut from a 2-mm thick test plaque usinga hydraulic die press. Tensile sampleswere tested at 500 mm/min cross-headspeed with gage length of 25 mm. Tearresistance was evaluated according toASTM D 624 Die B, razor-nickedmethod14. Hardness was measured as di-rected in ASTM D 224015 using a ShoreA Durometer.

Aged rubber physical properties wereobtained after conditioning samples inan air oven at 70°C for 168 hours, fol-

Technical

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lowed by room temperature exposure for12 hours on a flat surface. Per ASTM D57316, five tensile, tear and durometerspecimens from each set were tested fortensile strength, tensile modulus, elon-gation at break and Shore A hardness.

Dynamic viscoelastic propertiesViscoelastic rubber properties and re-

sponses were characterized using aRheometrics RSA II machine, with 1 Hzfrequency, 0.25% strain and tensilegeometry. Dynamic mechanical analysis(DMA) was used to evaluate storagemodulus (E’), loss modulus (E”), complexmodulus (E*) and tan (ratio of loss tostorage modulus) with temperaturesweeps from -100°C to 100°C.

Results and discussionCharacterization of plasticizersBio-plasticizers were selected primari-

ly according to their range in saturation,from low saturation of CNSL and chosengrades of soybean oil and flaxseed oil tohigh saturation in the palm oil. Fig. 6compares the DSC melting thermo-grams of five processing oils used in thestudy: TDAE, palm, flaxseed, cashewnut shell liquid (CNSL), and low satu-rated soybean (SBO)

The low saturated soybean oil exhibit-ed a transition temperature on heatingaround -29.6°C, whereas two transitionpeaks were noted in the palm oil, at 6°Cand 32°C range. Unsaturated fatty acidsare characterized by carbon chains hav-ing one or more double bonds, whichprovide opportunities for bonding withthe rubber or other additives. Low satu-rated soybean oil is liquid at room tem-perature, whereas the palm oil is in asolid form. In palm oil, the high satura-tion indicates structures with long chaincarboxylic acids that are saturated withhydrogen with limited or no carbon-car-bon double bonds. The two noted peakscorrespond to the low and high solidifi-cation segments within the palm oil,caused by fractions of Olein and Stearin.

The degree of saturation and indica-tive transition temperatures can affect

Fig. 6. Melting thermograms of bio-pro-cessing oils at a heating rate of0.17°C/sec.

Fig. 7. Rheological curves of a) bio-oil compounds; b) guayule compounds (meas-ured torque while curing rubber at 160°C).

Fig. 8. Comparison of stress-strain behavior of select bio-oil and guayulecompounds.

Fig. 9. (a) Normalized physical properties of bio-oil compounds; (b) normalizedphysical properties of guayule compounds.

Fig. 10. (a) Tangent delta, E’ and E” versus temperature of bio-oil compounds; (b)tangent delta, E’ and E” versus temperature of guayule compounds.

the oils’ compatibility with the rubbermatrix. Highly polar polymers are moremiscible with processing oils with ahigher level of unsaturation. For lowerpolarity polymers, compound miscibilityis likely to be improved by choosing oilswith lower polarity which is expressedby fewer areas of unsaturation.

Compound processingRheological curves are compared for

the two sets of samples using either bio-oils or guayule derived natural rubber,shown in Fig. 7. As compared to controlrubber with TDAE oil and 20 phr natu-ral rubber (Fig. 7a), substitution ofTDAE oil with bio-oils accelerated therubber cure slightly from 17.96 minutesto 16.03 minutes with SBO. A similarresult was noted for guayule-based com-pounds using bio-oils as compared toguayule control sample, in Fig. 7b. Con-trol rubber had the highest maximumtorque, indicating potential for highercross-linking and hardness of the rubbercompound. Compounds using bio-oilsplateaued to maximum torque valuesthat were lower than the control butsimilar to each other.

Table IV provides a summary of thescorch time (ts2), cure times at 50% and90% maximum torque, tc50 and tc90, re-

spectively, as well as the maximumtorque of the rubber compound aftercuring. One noted advantage of theguayule compound compared to the con-trol rubber was the increase in scorchtime with slight decrease in time forcuring the rubber formulation.

The formulation containing 20 phrnatural rubber and 10 phr SBO (com-pound 5), exhibited enhanced processingability compared to the other com-pounds. Formulations with 20 phr guay-ule (compounds 6-8) experienced hightack in the Banbury and on the two-rollmill, making it difficult to obtain asmooth sheet. Of the three compoundscontaining guayule, the addition of palmoil (compound 7) reduced tack and im-proved the ability to process the master-batch. The CNSL formulation bagged onthe two-roll mill after the second andthird mixing passes. Overall, the SBOand palm oil compounds provided pro-cessing advantages compared to the con-trol formulation. Blooming of the pro-cessing oil after cure was not observedin any of the compounds in this study.

Physical and heat-aged propertiesPhysical properties of the molded

tread compounds were compared fortensile performance, tear resistance and

hardness. Use of guayule rubber inplace of 20 phr natural rubber providedcompounds with similar physical attrib-utes including tensile strength, modulusat 100% elongation and Shore A durom-eter. Fig. 8 compares the stress-strainbehavior of this alternative rubber tothe control as well as a comparison ofpalm oil and low saturated soybean oilas plasticizers. In these samples, use ofSBO with guayule increased the percentelongation of the molded rubber com-pound from 401% to 526%, while in-creasing the tensile strength from19.54MPa in the control compound to23.87MPa in guayule/SBO compound.

An overview of the key physical prop-erties is presented in Table V, whereaverage values and standard deviationsare reported for the eight tread com-pounds. Comparison of rubber tensilestrengths indicate that use of bio-basedplasticizers increased the percent elon-gation of the rubber, while maintainingtensile strength. In particular, cashewnut shell liquid and low saturated soy-bean oil significantly increased ultimateelongation of the rubber. Modulus at300% elongation was highest for TDAE/-natural rubber compounds (control) anddecreased significantly for SBO based

Technical

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rubber. Tear strength resistance was similar

for all eight compounds, within one stand-ard deviation of the reported average.Shore A durometer ranged from approx-imately 59 to 65, with control samplesand guayule samples having the samehardness value. Use of soybean oil de-creased the hardness to 58.75 Shore A.

Table VI compares the percentchange of heat aged physical propertiesof the eight compounds using bio-oilplasticizers and/or guayule derived rub-ber, versus the as-molded compounds.As expected, control rubber specimensshowed a slight increase in modulus,tensile strength and elongation to breakafter heat aging. Cashew nut shell liq-uid samples and soybean oil samples ex-hibited 17-28% change in either modu-lus at 100% elongation or modulus at300% elongation. These changes wereconsistent with the increase in durome-ter for each of the compounds. Due tothe increased number of carbon-carbondouble bonds in highly unsaturated oils,rubber using low saturated processingoils is more likely to be susceptible to ox-idative aging.

Normalized values of key physicalproperties are compared in Figs. 9a and9b, with the control compound used asthe baseline. Graphs are presented solarger values are preferred for each at-tribute. Fig. 9a shows that CNSL andSBO affected the rubber compound pro-perties to a similar degree, in whichdurometer was decreased slightly and asignificant improvement in tensilestrength, tear strength and elongationwere noted. Use of palm oil and flaxseedoil also showed interesting performancein terms of increased elongation andtensile strength, while maintaining mo-dulus at 300% elongation.

This selection of bio-oils has a range ofreactive carbon-carbon double bonds that

may influence the curing process, andtherefore, the reactivity of sulfur be-tween elastomer chains. This mecha-nism is consistent with the noted de-crease in durometer and increase inelongation. For example, low saturatedsoybean oil increased the rubber’s elon-gation from 400% in the control sampleto over 540%, but decreased Shore Adurometer by 6 points. One of the highlysaturated oils, palm oil, provided an in-crease in elongation of 57% compared tothe control with only a 3 point drop inhardness. For SBO and CNSL, the in-crease in elongation is reflected in a re-duction in tensile modulus at 300% ex-tension.

In Fig. 9b, guayule rubber had verysimilar overall physical performancecharacteristics to the natural rubbercontrol formulation. Combinations ofguayule and soybean oil or palm oil ex-tended the elongation and tensilestrength, while decreasing the modulusat 300% elongation.

Dynamic viscoelastic properties andtire performance predictors

Evaluating the viscoelastic responseof rubber through different temperatureregimes has been used extensively inrubber compound development to pre-dict its performance in tire tread appli-cations17, 18, 19. Dynamic mechanical anal-ysis (DMA) provides an opportunity tocompare the effects of different chemi-cals and rubber, such as plasticizers orbase compounds, on resulting perform-ance properties. Using this approach,Figs. 10a and 10b provide temperaturesweeps from -80°C to 80°C versus E’(storage modulus), E” (loss modulus)and tangent delta, ratio of loss modulusto storage modulus for the two sets ofsamples.

In Fig. 10a, use of the bio-plasticizersin the model compounds resulted in anincrease in the tan delta curve peakheight and a shift to a slightly lowerglass transition temperature. Compari-son of the guayule to natural rubbercontrol compound in Fig. 10b showed

Fig. 11. Normalized laboratory predictors for key tire performance attributes usingDMA data from (a) bio-oil compounds; (b) guayule compounds.

Fig. 12. DMA performance predictors for rolling resistance and wet traction predictors.

Fig. 13. Normalized laboratory predic-tors for rolling resistance, wet tractionand wear resistance of select bio-oiland guayule compounds.

Appendix A. Tire tread formulation expressed as phr (parts per hundred rubber, byweight).

higher tan delta for the guayule com-pound relative to the control in the tem-perature regime of 5°C to 80°C, withlower values for the guayule/bio-oil com-binations.

It may be noted that the S-SBR inthese formulations had a glass transi-tion temperature of -22°C for the cleargrade and -29°C for the oil extendedgrade. The glass transitions of the com-pounded rubber ranged from -17.6°C forthe guayule formulation to -21.7°C forthe SBO formulation. The relativelyhigh compound Tg makes this particularrecipe more suited for a summer tire ap-plication instead of a tire exposed to coldweather conditions. In future studies,base polymers with lower glass transi-tion temperatures may be chosen in or-der to provide a formulation more suitedto all-season use.

As reported previously20, tangent deltaand storage moduli values at a givenstrain and frequency may be used aspredictors for tire tread performance fordifferent conditions. Fig. 11 providesnormalized radar graphs of performancepredictors of tread compounds comparedto the control rubber, presented withhigher values indicating desired per-formance. In this figure, tangent deltavalues of the tread compounds at 10°Cand 60°C are used to assess the predict-ed performance for wet traction androlling resistance, respectively. Lowertan delta values are preferred as indica-tors for lower rolling resistance, where-as higher tan delta values reflect predic-tion of improved wet traction of thetread compounds. Dry handling is as-sessed by measuring the storage modu-lus, E’, at 30°C in tension and snow trac-tion is predicted by comparing tan deltavalues at -10°C. Tan delta values at30°C are used as performance predictorsfor dry traction.

The rolling resistance indicators sug-gest that use of guayule/palm oil,guayule/SBO, SBO, palm oil and flax-seed oil may be desirable in tread com-pounds where fuel economy is a key at-tribute. However, performance predic-tors for dry handling, dry traction andwet traction indicate that this benefitmay result in a tradeoff for these key at-tributes. Fig. 11a compares the predict-ed performance of the influence of bio-oils on compound properties and Fig.

11b compares DMA predictive responsefor guayule compounds. As shown inFig. 11a, use of cashew nut shell liquidas processing oil provided a compoundwith similar performance predictors asthe control rubber, but with less desir-able rolling resistance. Compounds us-ing flaxseed oil showed an improvementin predictors for snow traction, slightimprovement in rolling resistance anddetriment to dry handling.

For closer examination of the influ-ence of bio-plasticizers and alternativenatural rubber on tread properties, Fig.12 provides a comparison of the typicaltrade-off performance attributes for roll-ing resistance and wet traction. In thisgraph, lower rolling resistance predic-tors (tan delta at 60°C) and higher wettraction predictors (tan delta at 10°C)are preferred. Compared to the controlrubber, guayule rubber performanceshows the typical trade-off of improve-ment in wet traction at the detriment ofrolling resistance. Use of low saturatedsoybean oil and flaxseed oil decreasedrolling resistance but reduced wet trac-tion, according to the DMA predictors.Combinations of guayule and palm oil orSBO showed an optimization of theproperties indicated by each of thechemical constituents.

Tire researchers often refer to the“magic triangle” of tire performance, in-cluding impact on fuel economy, all sea-son traction and wear resistance. As a

Technical

PlasticizersContinued from page 17

See Plasticizers, page 19

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VMI Group Extrusion is marketing its Shark90 multisystem that provides final mixing, strain-ing, preform and profile capabilities, all coupledwith optional vacuum.

The company said its gear pump systems arecomprised of a single screw extruder in conjunctionwith a gear pump. The extruder is used to achieve

effective plasticization of the compounding whileensuring sufficient feeding to the gear pump, VMIsaid.

More information is available at the firm’s web-site, www.vmi-az.com.

ContiTech A.G. has introduced its Conti Secur

Premium adhesive for cold bonding on conveyorbelts.

The single-component adhesive—unlike typicaltwo-component products—is ready for immediateuse and will provide long service life, ContiTechclaims. The product, which was created in collabo-ration with H.B. Fuller Co., is based on poly-chloroprene. The company said it can be used inrubber-to-rubber and rubber-to-metal bonding.

Visit www.contitech.de for more information.

Cabot Corp. has released a fumed silica tailoredtoward applications requiring transparency andmechanical reinforcement.

Cab-O-Sil Duramold 2150 is a high-temperaturevulcanizing silica that can be useful for clear HTVsilicone applications such as medical tubing andconsumer goods, Cabot said. With 30 percent less

measured grit, it delivers levels of performanceachieved through higher structure products, ac-cording to the company.

The new silica is aimed at such markets as auto-motive seals, gaskets, electrical insulating tubingand cable seals, industrial seals, bushings andhoses. Cabot provides more data on the product atits website, www.cabot-corp.com.

Wacker Chemie A.G. is offering a tin-free cata-lyst for two-part, room-temperature-curing siliconerubber mold-making compounds.

Elastosil Catalyst NEO is formulated to provide along pot life and ensure it cures within 24 hours,the company said. The catalyst has been tailoredwith condensation-curing components of ElastosilM rubber grades. Visit www.wacker.com for moreinformation on the product.

PRODUCTS

Rubber & Plastics News ● February 11, 2013 19www.rubbernews.com

laboratory predictor for wear resistance,DIN abrasion was measured for thetread compounds. Fig. 13 provides anormalized plot of laboratory predictorsfor rolling resistance, wet traction andabrasive wear for selected bio-based com-pounds. Selection of oils and naturalrubber type influences the laboratorypredictors for these three key attributes.Use of soybean oil predicted improve-ment in rolling resistance and treadlifebut with reduction of wet traction. Thecompound using guayule rubber to sub-stitute Hevea natural rubber in the80/20 SBR/NR blend showed an increasein wet traction with degradation in pre-dicted tread wear and rolling resistance.

Replacing the Hevea natural rubberwith guayule in conjunction with theuse of sustainable processing oils helpedto mitigate some of the predicted per-formance trade-offs. In particular, use ofguayule rubber with soybean oil con-taining low saturation or highly saturat-ed palm oil provided well balanced at-tributes for predictors of rolling resis-tance, treadlife and wet traction. Gua-yule/palm oil based rubber and gua-yule/SBO based rubber showed an im-provement of 8% and 10% in rolling re-sistance compared to the control rubber,respectively. This result is promisinggiven that the reduction of wet tractionwas only 2% in guayule/palm oil and 3%in guayule/SBO based rubber comparedto the control.

While combinations of guayule rubberand bio-oil were evaluated for SBO andpalm oils, it would be interesting to eval-uate oils with increased number of po-tential reactive sites, such as flaxseed orCNSL. However, it should be noted thatheat aged physical properties suggestoxidative stability issues with com-pounds using highly unsaturated oils.For example, percent change in elongationand durometer were greatest for rubbercompounds using CNSL, followed byflaxseed oil. SBO and palm oil had loweroverall percent change of these properties.

This study examined the DMA per-formance predictors for use of bio-plasti-cizers and alternative natural rubber inmodel, silica-based tread compounds.While physical and viscoelastic proper-ties are useful in the development ofnew material systems, they are not in-tended to be a substitute for tire testingor on-vehicle evaluation. Additionally,other attributes including noise, wethandling and wear need to be assessedprior to further developments in this area.

Conclusions

This study evaluated the usage of sus-tainable raw materials in high perform-ance, silica-based tread compounds. Thescope of this experimental evaluation in-cluded agricultural-based processing oils,guayule rubber obtained from desertshrubs, and combinations of bio-oilswith this alternative source to Heveanatural rubber. The purpose of the pa-per was to evaluate the potential of us-ing sustainable materials in model treadcompounds by determining key process-ing parameters, physical properties andtire performance predictors.

Selected bio-plasticizers were chosenbased upon their range of saturationand miscibility with the base rubbercompounds. Low saturated soybean oil,palm oil, cashew nut shell liquid andflaxseed oil were evaluated in silica-based tread compounds. Since oils de-rived from agricultural resources varyin their chemical structures, molecularweight and compatibility with differentrubber matrices, the use of selected bio-oils influences the resulting rubber pro-perties.

Experimental evaluations indicatedthat these bio-oils provided ease in rub-ber processing while demonstrating sim-ilar physical properties to the TDAEcontrol rubber. Each of these bio-oils in-creased the compound’s elongation butin some cases, decreased durometer andreduced modulus at higher extensions.

Dynamic mechanical analysis was usedto predict tire tread performance in vari-ous conditions, including rolling resist-ance, wet traction and dry handling.These results showed that selected bio-plasticizers influenced rubber perform-ance, with typical trade-offs betweenrolling resistance and wet traction.

Rubber using cashew nut shell oil hadsimilar, overall performance predictorsto that of the control with TDAE pro-cessing oil. Similarly, use of flaxseed oiland palm oil provided tread compoundswith well balanced attributes but with apredicted decrease in dry handling. Lowsaturated SBO provided a low hystereticrubber compound, but with significanttrade-offs for dry handling and dry trac-tion predictors.

Additionally, guayule rubber was as-sessed as an alternative to Hevea natu-ral rubber for 20 parts in blends of 80/20styrene butadiene rubber/natural rub-ber. Use of guayule rubber in the treadcompound resulted in similar propertiesto that of natural rubber, with the bene-fit of increased tear resistance. Perfor-mance property indicators from DMAanalysis showed that guayule rubber en-hanced wet traction, dry traction, anddry handling performance of the treadcompounds, with equivalent predictedsnow performance to the control rubber.The trade-off to these performance bene-

fits was a reduction in predicted fueleconomy, as determined through DMApredictors for rolling resistance.

By combining low saturated soybeanoil or palm oil with guayule rubber, thebalance between fuel economy and wettraction was adjusted to provide per-formance attributes similar to the con-trol rubber. Blending two or more sus-tainable processing oils in guayuleformulations in order to further refinecompound chemistry may yield addition-al performance improvements. Based uponprocessing, physical and performancepredictor results, future studies are rec-ommended to further optimize use ofpalm oil, soybean oil, blends of sustain-able oils and guayule rubber in SBR/NRor NR compounds.

Overall, the use of bio-plasticizers andalternative natural rubber provides apromising material choice compared tostandard oil and rubber matrices, assupported by comparable mechanicalproperties and performance predictors.These materials provide opportunitiesto reduce the environmental footprint ofrubber products and in some cases, in-crease domestic availability of raw in-gredients. The sustainable plasticizersshow promise in replacing part or all ofpetroleum derived processing oils suchas TDAE oil for use in low rolling resist-ance tread compounds. Potential futureoutlook for using these types of sustain-able raw materials in tread compoundsis dependent upon economies of scale,availability and further evaluations foroptimizing tire performance during ve-hicle usage. The recent expansion of re-search into agricultural, raw materialsfor rubber compounds provides newchoices for developing high performanceproducts with increased sustainable con-tent.

AcknowledgementsThe authors would like to thank the

United Soybean Board, New Uses Com-mittee, for its financial support of thisproject. Katrina Cornish, Ohio StateUniversity, and OARDC are thanked forproviding guayule rubber used in thisstudy. We appreciate the technical dis-cussions and samples provided by Sol-vay-Rhodia.

Also, chemicals provided by AkrochemCorp., Cargill Inc., Composite TechnicalServices L.L.C., Hansen-Rosenthal Oils,Lanxess Corp. and Viterra Inc. are muchappreciated.

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