Indian Journal of ChemicalTechnology

Vol. 4, May 1997, pp. 135-140

Modified carbon black as reinforcing filler for SBR

Amp Kumar Ghosh, SukumarMaiti & Basudam Adhikari*

Materials ScienceCentre, Indian Instituteof Technology,Kharagpur721 302, India

Received 18 July 1996;accepted 16December 1996

Carbon black HAF (N330) was treated with an unsaturated hydroxy organic fatty acid in presence orabsence of dicumyl peroxide for the attachment onto carbon black as a flexible· bridge between fillersurface and elastomer chain to improve some of its reinforcement behaviours in SBR vulcanizates. Thetreated black was chracterized and evaluated for its processing behaviour and the level of reinforcement inSBR. It was found that the surface modification with the organic fatty acid resulted in a considerableimprovement of processing as well as vu1canizate properties, such as, tensile strength, abrasion resistanceand flex crack growth resistance.

Modification of carbon black filler and subsequentevaluation of its reinforcing action in differentsynthetic and natural rubbers have long been asubject of study of many researchers. Grafting ofselected oligomers and polymers onto the blacksurface, surface oxidation, use of differentpromoters or coupling agents, and reaction ofvarious reactive compounds with carbon black areworth noticing among the methods developed tilldate. Donnet and collaboratorsl-7grafted polymersonto carbon black with an objective to make it moreefficient as a filler. But the polymer grafted-carbonblack could not bring a marked improvement in thereinforcing behaviour in most of the cases.

89Dannenberg and coworkers' proposed the conceptof deactivation of the surface of carbon black when

grafted with polymers. In some recent studies, Vidaland associates'O grafted alkyl chains to carbon blackand reported a weak filler polymer interaction due tothe grafting. Use of silane compounds for graftingalso resulted in a decrease of reinforcing effect ofcarbon black'i .

Many chemical promoters were developed duringthe last fifty years for use as coupling agentsbetween the carbon black surface and elastomers

and for higher reinforcement in products based onnatural, styrene-butadiene, butyl and nitrile rubbers.In most cases only little improvement in someproperties at the cost of some others was observed.Rivin and Dannenberg'Z attached such organicfunctional groups as -NHz, -NHCH3, -N(CH3)Z'

*For correspondence

-NHCHzCH=CHz, and -oOH to the carbon blacksurface as a means of chemical modification. Such

modifications resulted relatively small changes inthe bound rubber reactivity, and in moduli andtensile properties in vulcanizates of SBR, cis­polybutadiene and butyl rubber.

However, some patents were published for use ofmodified carbon black in SBR vulcanizates.Furukawa and coworkers 13 obtained enhanced

reinforcing properties in SBR after increasing theactive hydrogen content of carbon black bymodifying with diethylene glycol, ethanolamine,hexamethylene diamine, 1,2-dimercaptopropylalcohol or phenylisocyanate. Some improvements inproperties were obtained by fatty acid modificationsof carbon black in two patents'4,15. Gajewski andprot'6 have shown increased crosslink density andimproved strength properties in SBR using carbonblack modified by treatment with S8, CSz, SOz andHzS. But the cause for improvement of thevulcanizate properties by such modifications wasnot understood properly. Recently, the presentauthors have reported some improvements in thereinforcing behaviour of some modified carbonblacks in SBR17,18. The major objectives of thestudies reported in the present communication havebeen to achieve better processing behaviour andimproved vulcanizate properties with an organicfatty acid modified carbon black.

Experimental ProcedureMaterials-SBR 1502 (Synthetics and Chemicals

Ltd), carbon black N 330 (Philips Carbon BlackLtd), dicumyl peroxide (DCP) (BDH, England),

136INDIAN 1. CHEM. TECHNOL., MAY 1997

I·· Ii

CBS (ICI), unsaturated hydroxy organic fatty acid(proprietary agent), zinc oxide (BDH). Otherreagents/compounds such as acetone and toluene all

AR grade chemicals, and commercial grades of

stearic acid, sulphur and spindle oil were used asreceived.

A solution of the organic fatty acid and theperoxide in acetone were mixed with carbon black ,air dried at room temperature for 24 h and thenheated at 140-l50°C.

Carbon black characterization-Fatty acid

treated carbon black samples were extracted byacetone in a soxhlet for 30-40 h. The extracted

carbon blacks were dried to constant weight and

characterized for surface area (by ASTM D 1510

procedure) and structure (by ASTM D2414

method). The pH of the carbon black samples was

also measured with an aqueous slurry containing 5 gblack in 25 mL deaerated distilled water and a few

drops of acetone by using a pH meter, model EC­5652 ECL, India. Measurement time was allowedupto constant pH.

Rubber compounding and vulcanizationcharacteristics--Compounding of SBR was done ina two-roll mixing mill at a friction ratio 1.2. Mixingtemperature of the compound was recorded with athermocouple at a constant time interval. Toestimate the bound rubber content the rubber

compounds were kept at room temperature for sevendays for conditioning. Approx. 0.5 g of such samplewas immersed in 250 mL of toluene in a stainless

steel wire cage (320 mesh). The solvent wasrenewed every 24 h and after 5 days the residue wastaken out of the solvent and vacuum dried to

constant weight. The bound rubber content wasexpressed as the weight per cent of the polymer left

19

as carbon gel .The cure characteristics of the compounds were

analyzed using a Monsanto Rheometer (R-100) at150°C as per ASTM D2084-81.

Testing-Samples for physical testing were curedin an electrically heated hydraulic press (Carver,model 2518) at 150°C and 8 ton pressure (ram dia2.562") for the respective optimum cure times.Tensile properties were measured by a tensiletesting machine (KMl, model 1.3 D) followingASTM D412-51 T method. Flex crack growthresistance was tested with a De Mattia Flex Tester

(Prolific Engineer) as per ASTM D813-59. Abrasionloss was evaluated by a Du Pont abrasion tester(Prolific Engineer). Heat build up test was carried

Table l-F onnulation of carbon black (CB) for modification

CB designation Carbon black, g Acid, g DCP, gCB Ia 100

CB lIb 100

CB IlIa 100 lO

CB IV' 100 10

CB Vb 100 10 1.0"No heat

treatment .

~eat treated under mild operation

out with the cylindrical samples by GoodrichFlexometer as per ASTM D623-78 method. For the

determination of swelling value (Q), the curedsamples were immersed in 200 mL toluene to

equilibrium swelling. The samples were taken out of

the solvent and weighed immediately. The swellingvalue (Q) of cured samples was calculated by thefollowing equation:

Swollen weight - dried weight

Q = (original weight of cured sample x 100) / formula weight

where, formula weight is the total weight of rubberplus compounding ingredients based on 100 parts ofrubber2o•

Results and Discussion

Carbon black modification-Rubber grade carbonblack has got reactive organic functional groups andfree radicals on its surface and it is chemicallyactive2J.22• Such carbon black was modified with the

unsaturated organic fatty acid which is likely to bechemically linked to the carbon black through freeradicals or through functional groups. In the actualmodification, carbon black was mixed with theunsaturated fatty acid followed by heating inpresence or absence of DCP. Compositions ofcarbon black samples are shown in Table 1. CB Iand CB II were used as control where CB I was not

heated to compare the effect of heating onmodification of others (CB II, CB IV, CB V). CB IVwas modified with the fatty acid only whereas CB Vwas modified with the fatty acid along with DCP tofacilitate grafting reaction. In CB III, carbon blackwas soaked with the acid to see the improvement inreinforcement characteristics.

Characterization of modified carbonblack-Table 2 describes the characterization of

modified carbon black along with the control. Theacetone extracted carbon blacks (unmodified andmodified) were characterized by gravimetric

•

I P \

GHOSH et al MODIFIED CARBON BLACK AS REINFORCING FIll.ER FOR SBR 137

75

Fig. I---Plot of temperature liS time of mixing of different blackfiIled formulations

blacks without process oil, whereas formulations 1to 4 were taken as control containing unmodifiedcarbon blacks. Formulations 1 and 2 do not contain

any process oil. Formulation 5 was made to examinethe processing aid function of the fatty acid if any.Formulation 6 was made to test the effect of the

organic acid present in the physical mixture with thecarbon black on the reinforcing properties of thevulcanizate.

Processing behaviour-To compare theprocessing behaviour of modified carbon black thetemperature rise of the carbon black rubber mix wasmonitored during milling. Viscosity averagemolecular weight of the compounded rubber wasmeasured to evaluate the extent of chain scission on

milling and the variance (d) for test results using 4to 5 specimens in each case for their tensile strengthand modulus values was also estimated to assess the

dispersion of carbon black. During carbon blackincorporation and dispersion high shearing action

242016

1. Untreated CB 1\51. CB I2 Healed CBI4S). CB IIJ. CB III41»4ISpindle Dill4. CB 11411-4ISp,ndle Dill5. CB I {i.ll + 4 {Organic Acid I6. CB + Organic Acid (No Heall. CB III7 CB· Organic ACid (Heated I, C B lV8. CB· Org_ Acid + DCP (Healed I, C8 V

12

Tim' of Mixing, rnin

35o

60

;' SS

701- --<>---<>--- 1

-A-----!>--- 1--(}-----{}- 3-.-1(-:4

65~ ~,5----6---6- - . 6_______ m ,------ .

CB Bound12surfaceDBPApH of

designation

org. acidarea,value,ag.on CB, %

m2/gmUIOO gslurry

CBI

-80.2102.17.2

CBIV

18.0275.396.65.6

CBV

25.3674.394.85.2

Table 2--Characteristics of modified carbon black

analysis, estimation of the surface area by 12­

adsorption, determination of the structure bydibutylphthalate adsorption (DBP A) and

measurement of pH of the aqueous slurry (Table 2).Gravimetric analysis of the soxhlet extractedsamples shows the percentage attachment of theorganic fatty acid with carbon black onmodification. There is 18.02% attachment of acid

with the carbon black (CB IV) by simple heating at140-150°C whereas the bound acid contentincreases to 25.36% (CB V) on heating in presenceofDCP. So, this result shows a positive contributionof peroxide in forming some type of covalentlinkages between the fatty acid and the carbonblack. Both 12 and DBP adsorption values getmeasurably decreased due to the modificationoutlined. Reduction in surface area and structure of

the carbon black is merely due to the action of themodifying agents (acid, peroxide) which formstrong linkages with carbon black. pH of theaqueous slurry of the modified carbon black ismeasurably lower than that produced by the parentcarbon black. This is due to the increase in acidcontent of carbon black after modification with an

acidic compound.Rubber compounding--The modified carbon

blacks were evaluated in SBR vulcanizates for their

roles as processing aid and reinforcing agents. Table3 shows the formulations of the rubber compounds.Formulations 7 and 8 contain the modified carbon

Table 3--Mix formulation of rubber compounds

Ingredients, phr

Formulation numberI

234567CBI

45--4141

CB II

-4541

CB III

-----45

CBIV

------45

CBV Organic acid

----4(as processing aid) Spindle oil

--44(as processing aid)

8

45

Base formulation (phr): SBR 1502: 100, ZnO: 5, Stearicacid: 2, CBS: 1, Sulphur: 1.75.

138 INDIAN 1. CHEM. TECHNOL., MAY 1997

Table 4-Effect of mixing on polymer molecular weight and carbon black dispersion

Formulation number

'If...

1

2345678 ..MyxlO-5

1.231.25 1.47--1.701-75

Variance ((i), MPa 200% Modulus

0.1350.1460.0980.0890.0360.1070.0270.031

300% Modulus

0.1810.2020.1360.1480.0840.1070.0750.065

Tensile strength

0.5320.7450.2820.2870.3820.4870.2460.263

Table 5-Effect of modified carbon black on curing behaviour in SBR vulcanizates

Monsanto rheometer

R-IOO data at 150°C

Optimum cure time, minScorch time, minExtent of cure, dNm

Cure rate index, minot •

I27

7

585.0

2

27

7

60

5.0

3

27

7

565.0

Formulation number

4 527 31

7.5 7.5

54 575.1 4.3

6

32

7.5

52

4.1

7

32

8.0

56

4.2

8

32

8.0

59.54.2

Table 6----Effect of modified carbon black on mechanical properties ofSBR vulcanizates

Vulcanizate properties Formulation number

1

2345678

200% Modulus, MPa

5.95.84.44.53.53.54.14.2

300% Modulus, MPa

11.311.88.68.58.97.58.88.6

Ten. strength, MPa

19.419.718.018.919.018.020.320.4

Elong. at break, %

420400450460465510505500

Hardness, Shore A

7272666668666767

Bound rubber, %

30.931026.526.625023.225.025.2

Heat build up, 0('

2828262627292930

Flex-cracking resistance,

2830444344558284

k cycles Abrasion loss, % In

10001231141.J21.361271.470.960.89

cycles Swelling value, 02932.953.323.333.413.533.823.89

1""...

causes polymer chain sciSSIOn and increases thetemperature of the mix23. The decrease of viscosityof the mix due to chain scission and temperaturerise, causes plasticization around carbon blackagglomerates and thus retards the dispersivemixing24.26. Moreover, use of process oil for thedispersive mixing affects the tensile properties ingeneral27. The temperature rise during mixing isshown in Fig. 1. The results show the highertemperature rise in the formulations 1 and 2compared to formulations 7 and 8. The formulations3 and 4 containing spindle oil as processing aid andthe formulations 5 and 6 showed intermediatetemperature rise.

Formulations containing modified carbon black(formulations 7 and 8) showed the lowest chainscission and the formulations 1 and 2 showed the

highest chain scission as evident by viscosityaverage molecular weight (Table 4). Formulation 4with process oil showed intermediate chain scission.

The relatively lower value of variance (d) formodi tied black formulations supported theunifOlmity of carbon black dispersion in the rubber

phase. These observations indicate better processingbehaviours and uniformity in ultimate propertiesoffered by modified carbon blacks over those ofcontrol.

Cure characteristics---(:-:ure characteristics are

presented in Table 5. From the results it is evidentthat either the presence of acid as processing aid(formulation 5) and as simple physical mixture withcarbon black or the acid modified carbon blacks

(formulations 7 and 8) has marginally increased theoptimum cure time and scorch time which are quitenormal observations. since cure rate decreases inacidic environment. However, modification does notatTect the extent of cure to a considerable extent.

Tensile properties--Table 6 shows the tensileproperties of all the formulations containingmodified and unmodified carbon blacks. Both 200%

and 300% moduli have changed a little due to themodification. In the formulations 7 and 8 containingmodified carbon blacks, the moduli values remainedalmost in the same level as those of the formulations3 and 4. But the formulations 1 and 2 where no oil

was used during mixing show the comparatively

I Ii1'1 II II , '111' ,!"! i1111'llj'

GHOSH et af. MODIFIED CARBON BLACK AS REINFORCING FILLER FOR SBR 139

higher moduli. The relative higher value of 200%and 300% moduli in these formulations are due to

the higher effective vo~ume of loading (45 phr) and

without process oil. These make the sample morestiff, which is reflected in the enhanced value of

hardness28. The tensile strength values offormulations 7 and 8 containing modified carbonblacks are improved over that of the controlformulations. But the formulation 5 where the acid

(modifying agent) is used as processing aid and theformulation 6 where the acid is present in a simplephysical mixture do not show significantimprovements of the above mechanical properties.However, it is worth mentioning that modificationof carbon black with the organic acid is associatedwith the improvement of the ultimate properties.

The value of percentage elongation at break(Eb%) is also presented in Table 6. Formulations 7and 8 show higher values of Eb% than formulationsI to 5. Higher values of Eb% in formulations 7 and8 indicate higher flex fatigue life of the sample29.Formulation 6 (modifying acid is in simple physicalmixture with carbon black) also shows an enhancedEb% value. These results are also the indication of

the higher flexibility offered by the modifying acidpresent as a simple physical mixture with carbonblack.

Bound rubber-The effect of modification of

carbon black with the organic fatty acid was alsotested by measuring the bound rubber content. Theresults are included in Table 6. It is evident from theresults that the modified carbon blacks

(formulations 7 and 8) slightly lower the boundrubber content compared to the control compounds(formulations 3 and 4). This lowering of boundrubber is probably due to the decrease of surfacearea and DBP structure of carbon black after

modification with the organic fatty acid. Since thebound rubber gives a measure of reinforcemeneoand is a function of surface area and structure3l it is

prominent that formulations 1 and 2 without processoil show around 5% higher bound rubber contentthan that of the formulations 3 and 4 with processoil. This phenomenon again can be understood onthe basis of higher carbon black content informulations 1 and 2 having no process oil.

De Mattia crack growth and abrasionresistance--!3ince carbon black filler plays animportant role on fatigue flex life32 De Mattia crackgrowth resistance of both modified and unmodifiedblack filled stocks was measured (Table 6). It is

interesting to observe that the crack growth

resistance value is quite higher in the formulations 7

and 8 containing modified black compared to all

other formulations (Table 6). Although the flex

cracking resistance is also a function of crosslinking3334 .

system used ' , the hIgher value of. crack growthresistance in formulations 7 and 8 is believed to bedue to the attachment of the flexible chain of the

fatty acid to the carbon black surface.This modified carbon black is assumed to provide

a flexible bridge between the filler surface and theelastomer matrix thereby relieving the localizedstresses generated during flexing operation, thusresulting in an improvement of flex life35,36. Themoderately higher values of flex cracking resistancein formulations 3 to 6 compared to the formulations1 and 2, are due to the presence of process oil andmodifying agent as physical mixture with carbonblack.

In respect of abrasion resistance modified carbonblack filled elastomer shows much improved resultsof abrasion resistance compared to unmodifiedcontrol stocks (compare formulations 7 and 8 withformulations I to 4). Many workers have shownbetter abrasion resistance after modification ofcarbon black with various agents in differentI . 153738 he astomenc systems' , . In our study t e

improvement of abrasion resistance is probably dueto the presence of chemically bound organic acid asa bridge between the carbon black and the elastomerwhich resists the polymer to be abraded out of thesystem. The higher abrasion losses in theformulations 3 to 6 are belived to be due to the

presence of oil which may be responsible forpolymer dilution and thus increases the loss duringabrasion39•

Goodrich heat build up--Goodrich heat build upvalues (in terms of temperature rise in degreecentigrade) is presented in Table 6. It is seen that theformulations 7 and 8 show a little higher heat buildup than that shown by other unmodified black filled(control) stocks, (formulations 1 to 4). Heat build upis related to bound rubber characteristics which isalso related to the carbon black rubber interaction

density. In the present investigation, vu1canizatescontaining modified carbon black (formulations 7and 8) showed lower bound rubber content andhigher heat build up characteristics. This is in goodagreement with the findings of Fabre and Bertrand40

who showed decrease of heat build up with increaseof bound rubber by attaching chemical promoters to

140

",~ .. ,-.. _, .r., .. ",,- "",- .-' . ., -""""1-0'----

INDIAN 1. CHEM. TECHNOL., MAY 1997

.- - U!fflllfI It-,----, ------ HI

carbon black. The relation of heat build up and

bound rubber may be explained as follows. Lower

polymer carbon interaction and hence lower bound

rubber may be regarded as longer folded/coiled

polymer chain segments between two points of

interaction. Application of cyclic stress generates

frictional heat due to unfolding/folding of such

coiled segments and consequently longer chainsegments will generate more heat. Moreover, it is

known that heat generation is associated with the

crosslinking system 41. Decrease in the crosslink

density causes higher heat generation 42. The

swelling value (Q) of the cured samples (Table 6) is

taken as a measure of the total crosslink density of

the vulcanizates. Higher values of Q in case offormulations 7 and 8 indicate establishment of lower

crosslink densities (interaction density) in them. The

loosely crosslinked system is partially responsiblefor relatively higher heat build up in the modifiedblack formulations. Slightly higher swelling values(Q) for formulations 3 and 4 compared to those forfommlations I and 2 are due to spindle oil used asprocessing aid. The most densely cured samplesshowing the lowest Q value, formulations 1 and 2,showed heat build up that is higher than that forfomlUlations 3 and 4. This can be attributed to the

effective higher loading without the use of processoil.

Conclusion

The present investigation on the behaviour ofmodified carbon black to the tensile and dynamicproperties of rubber vulcanizates indicates betterflex crack growth resistance and abrasion resistancewith improved balance of tensile properties thusmaking the vulcanizates much suited for tyreapplications.

AcknowledgementThe authors thankfully acknowledge Birla Tyres,

India for providing the facilities of heat build upmeasurement of rubber samples.References1 Donnet J B & Henrich G, J Polym Sci, 46 (1960) 277.2 Donnet J B, Henrich G & Riess G, Rev Gen Caout, 39

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I II'I " "1'1' If 'I fill III ~"II !!!,U!I,J .... !.! :.,!, ;!•. 1 jILU,1 J I;