me

, P.O

ffecatioR) wt atropiones licts

NBR) hforcinginationcal proby a cigh-enecan be

cross-ldevelo

rubber (BR), and acrylonitrile butadiene rubber (NBR), contain avarying degree of unsaturation in the polymer backbone or in pen-dant positions. Peroxide radical could potentially react by additionto a double bond or by abstraction of an allylic hydrogen, and bothmechanisms occur concurrently in the vulcanization of unsatu-rated elastomers [4].

tomers has been common practice in the rubber industry for many

adversely affected by the high crosslinking dose required. To over-come this problem, several authors [9] have reported that poly-functional monomers PFMs (coagents) such as multifunctionalacrylates and methacrylates are useful to obtain optimummechan-ical properties at lower dose levels. These PFMs form a networkstructure with polymeric materials at a lower dose because of itshigher reactivity [10] and the resulting structure is useful for theimprovement of mechanical properties as well as thermal stability[11] .

This article is a comparative study between the different tech-niques to vulcanize NBR rubber.

National Center for Radiation Research and Technology (NCRRT), 3 AhmedEl-Zomor St., P.O. Box 29, Nasr City, Cairo, Egypt. Tel.: +20 124114780; fax: +20222749298.

Materials and Design 32 (2011) 33613369

Contents lists availab

Materials an

elsE-mail address: khaledfarouk65@eaea.org.egrate is controlled essentially by the decomposition of the peroxideat a given temperature [3]. Compared with sulfur vulcanization,crosslinking by peroxides is a relatively simple process, with phys-ical properties such as high modulus, low compression set and heatageing properties superior to sulfur cure systems. On the otherhand, the peroxide crosslinking has many disadvantages, such aslow tensile and tear strength, and ex resistance, which have re-stricted their use in diene rubbers. Many unsaturated rubbers, suchas natural rubber (NR), styrenebutadiene rubber (SBR), butadiene

crosslinking type rubbers when exposed to high-energy radiation[6]. Compared with the conventional chemical processes such asperoxide [7] or sulfur [8] induced vulcanization used for crosslink-ing rubber, radiation crosslinking has the advantages of being fas-ter and being more versatile, leads to more uniform crosslinking,consumes less energy, and occupies less oor space for processing.

Inherently waste free nature of the technology makes it lesspolluting than the conventional technologies. The disadvantage isthat the physical properties of radiation vulcanized rubber wereAcrylonitrile butadiene rubber (tance. However, shows no self-reincrystallinity, but when used in combvulcanizates with excellent mechanifrom NBR [1]. Vulcanization occurssulfur or peroxide. Alternatively, helectron beam or gamma radiationbers [2].

The use of organic peroxide as afree radical process is also largely0261-3069/$ - see front matter 2011 Elsevier Ltd. Adoi:10.1016/j.matdes.2011.02.010as excellent oil resis-effect, as there is nowith reinforcing llers,perties can be obtainedhemical agent, such asrgy radiation, such asused to vulcanize rub-

inking agent through aped. The vulcanization

years. Coagents are typically multifunctional vinyl monomers thatare highly reactive toward free radicals and readily graft to elasto-mer chains to form a complex crosslinked network. These coagentswith peroxide are used to improve the physical properties and pro-cessability of peroxide-cured elastomers. Also, they increase notonly the crosslinking efciency of the vulcanization process butthe cross-link density as well [5].

During this last decade, the crosslinking of rubbers by means ofelectron beams has strongly developed in place of the use of cross-linking agents, such as sulfur or peroxides. NBR belongs to the1. Introduction The use of coagents in conjunction with peroxides to cure elas-Effect of different curing systems on theproperties of acrylonitrile butadiene rubb

Khaled F. El-Nemr Radiation Chemistry Dept., National Center for Radiation Research and Technology, AEA

a r t i c l e i n f o

Article history:Received 6 October 2010Accepted 4 February 2011Available online 26 February 2011

Keywords:Acrylonitrile butadiene rubberCuring systemMechanical properties

a b s t r a c t

In the present study, the eperoxide/coagent and raditrile butadiene rubber (NBcomparison was carried ouvulcanizates. Mechanical ptearing strength and abrasphysico-chemical propertiOn the other hand, the effe

journal homepage: www.ll rights reserved.echanical and physico-chemicalr vulcanizates

. Box 29, Nasr City, Cairo, Egypt

t of different curing systems including sulfur, dicumyl peroxide, dicumyln/coagent on the mechanical and physico-chemical properties of acryloni-as studied. In order to correlate, the effect of curing systems on rubber, thecomparable value of volume fraction of rubber in swollen gel (Vr) for NBRerties like tensile strength, elongation at break, modulus, Youngs modulus,loss of vulcanizates have been followed up for comparison. In addition,ke swelling ratio, soluble fraction, and cross-link density were investigated.of fuel, thermogravimetric analysis, and thermal ageing have been studied.

2011 Elsevier Ltd. All rights reserved.

le at ScienceDirect

d Design

evier .com/locate /matdes

2. Experimental

2.1. Materials

Acrylonitrile butadiene rubber (NBR) of Europrene N3345 fromEnichem company Inc, Italy having (acrylonitrile content-34%,Mooney viscosity (ML (1 + 4) at 100 C-46). Zinc oxide was sup-plied from Shijiazhuang Golden Color Chemical Co., Ltd., China,its concentration 99.7% in appearance of white powder. Stearic acidobtained from Hebei Liancheng Chemical Co., Ltd., China, it hassmall akes shape and melting point 56 C. Dioctyl phthalate(DOP) was supplied by Henan Tianfu Chemical Co., Ltd., China; ithas acidity 0.01 maximum and ester value 99.5%. Sulfur was sup-plied by standard chemical company private Ltd., Madras.

accelerator (1.5 MeV, 3 mA) facility installed at the National Centerfor Radiation Research and Technology, Cairo, Egypt. The irradia-tion was performed to give a total dose of 25 kilo gray (kGy) foreach pass. Multiple passes obtained the total irradiation doses25, 50, 75, 100 and 150 kGy for different measurements.

2.3. Measurements

2.3.1. Mechanical propertiesThe tensile strength was measured using dumbbell shaped test

pieces at a crosshead speed of 500 mm/min at 25 2 C using ten-sile testing machine HOUNS FILD, England. The tearing strength(load at failure/thickness) of the samples was determined usingunnicked 90 C angle test pieces according to ASTM D 624-81.

mined by means of equilibrium swelling in acetone laboratorygrade at 25 C. The equilibrium swelling was used to calculate

P1

15216315

3362 K.F. El-Nemr /Materials and Design 32 (2011) 33613369Mercapto benzothiazyl disulphide (MBTS) was obtained fromBayer India Ltd., Bombay. Tetra methyl thiuram disulphide (TMTD)was supplied by NOCIL, Bombay, India. 1,2-dihydro.2,2,4-trimethylquinoline (TMQ) as antioxidant was obtained from IntatradeChemicales (GmbH), Germany.Carbon black (N 375). The peroxidecross-linking agent was dicumyl peroxide [DCP] from Aldrich(Germany), its purity 99%. The polyfunctional monomer PFM(coagent) was penta erythritol triacrylate PETRA from Aldrich witha molecular weight of 298.29 g/mol and a density of 1.167 g/cm3.

2.2. Compounding and curing

The compounding recipes of mixes are given in Table 1, thecodes letters S, P, P-PFM and R-PFM represent the sulfur, dicumylperoxide, dicumyl peroxide coupled with coagent and radiationvulcanized system respectively. The subscript indicates the weightin phr (part per hundred part of rubber) for sulfur, and dicumylperoxide corresponding to each designation code; while subscriptin case of radiation vulcanization indicate the radiation dose.

Mixing was carried out at room temperature using a two-rollmixing rubber mill having a friction ratio 1:1.4. The compoundedmixes as follow

2.2.1. Sulfur and peroxide curingThe compounded mixes with different amounts of curing sys-

tem were compression molded using an electrically heatedhydraulic press at 160 C under pressure 120 kg/cm2 for their opti-mum curing times i.e. 5 min. for sulfur cured system and 20 minfor peroxide and peroxide coupled with coagent cured systems.

2.2.2. Radiation curingThe compounded mixes were compression molded between

aluminum foil at 160 C under pressure 120 kg/cm2. Irradiationwas carried out in air at ambient temperature on the electron beam

Table 1Formulation of the mixes.

Code formulation (phr)* S1 S1.5 S2 P1 P1.5 P2 P1-PFM

NBR 100 100 100 100 100 100 100ZnO 5 5 5 5 5 5 5Stearic acid 2 2 2 2 2 2 2TMQ 1 1 1 1 1 1 1DOP 6 6 6 6 6 6 6MBTS 1 1 1 TMTD 0.5 0.5 0.5 Sulfur 1 1.5 2 HAF 30 30 30 30 30 30 30Peroxide (DCP) 1 1.5 2 1PETRA 5

Irradiation dose (kGy)

* phr = part per hundred part of rubber.the cross-link density, which is the number of network chain den-sity by applying the FloryRehner equation [12] as follow:

m 1=Vs ln1 Vr Vr v1V2r

V1=3r Vr=2

" #2

where, m = cross-link density; v1 = polymersolvent interactionparameter; Vs = molar volume of solvent; Vr = volume fraction ofrubber in the swollen gel; Vr was calculated using the relation

Vr Ds Ff Awqr1

Ds Ff Awqr1 Asqs1 3

where Vr, Ds, Ff, Aw, As, qr and qs are volume fraction of rubber,deswollen weight of the sample, fraction of insoluble, sample

.5-PFM P2-PFM R25-PFM R50-PFM R75-PFM R100-PFM R150-PFM

00 100 100 100 100 100 1005 5 5 5 5 52 2 2 2 2 21 1 1 1 1 16 6 6 6 6 6

0 30 30 30 30 30 30.5 2

5 5 5 5 5 5The experimental conditions for the tear measurements were thesame as that of the tensile testing. The hardness of the sampleswas measured according to ASTM D 2240-2000 using durometertype A, and the units of hardness was expressed in shore A.

2.3.2. Physico-chemical measurements2.3.2.1. Soluble fraction. Measurements of soluble fraction were car-ried out as follow, the cured samples, about 0.2 g were accuratelyweighed (Wo) and placed in a special stainless grids. The grids con-taining samples are transferred to special round ask 2/3 lledwith acetone. The heating was carried out under reux for 24 h.After extraction, the samples were dried to constant weight (W1)in dry oven at 50 C. The soluble fraction was calculated as follow:

Soluble fraction Wo W1=Wo 1

2.3.2.2. Determination of cross-link density (m). The volume fractionof rubber in swollen network of the vulcanizates Vr, was deter-25 50 75 100 150

2.4. Thermogravimetric analysis (TGA)

Wi Wf

3. Results and discussion

3.1. Sulfur vulcanized NBR

The physical properties of NBR composites vulcanized byvarious ratios of sulfur, keeping accelerator level xed, are givenin Table 2. The results obtained for the modulus at 100% elongationor tensile modulus (M100), Youngs modulus (Eo) and hardnessshow an increasing trend with increase of sulfur to acceleratorratio, while elongation at break (Eb) as well as tensile strength(TS) decrease. With increasing sulfur level the number of CASxACbonds increases, and is reected in the values of Vr. Moreover,the results of Q,Mc, and cross-link density conrmed the formationof crosslink network structure. However, increasing crosslinks overan optimum level cause localized stresses, which results in lower

3.2. Peroxide vulcanized NBR

3.3. Peroxide/coagent cured NBR

Design 32 (2011) 33613369 3363Mass loss mg per revolution n

1000 7

where Wi is the original mass of sample (gm), Wf the nal mass ofsample (gm), and n is the number of revolutions (84).

2.7. Thermal ageing measurements

The ageing of samples was carried out by keeping the samplesin a hot air oven for 24 h at 100 C. Then the samples were condi-The thermogravimetric analysis (TGA) technique was appliedusing TG-50 instrument from Shmadzu (Japan). The heating wascarried out at temperature to 600 C with a heating rate of 10 C/min under nitrogen gas atmosphere.

2.5. Effect of fuel

Effect of fuel has been carried out specically for testing the ef-fect of fuels on polymeric materials. The samples in triplicateswere weighed to nearest mg (M1). They were then transferred toask 2/3 lled with fuel A (commercial gasoline). Heating has beencarried out at 100 C for 22 h. After that, the ask is allowed to coolat room temperature. The samples were then blotted lightly withlter paper to remove the remaining liquid fuel. They were then re-weighed again (M2) .The percentage change in weight was thencalculated using the following equation:

Change in weight; % M2 M1M1

100 6

2.6. Abrasion measurement

Abrasion loss measurement was performed on samples by usingan abrasion tester of the type AP.40 (MASCHINEBAU GmbHRAUENSTEIN THURINGEN, Germany).

The testes were carried out under the following conditions:Load = 1 kg.Abrasion path = 40 meter which equivalent to 84 revolutions.The loss in the mass was calculated according to following

equation:weight, weight of the absorbed solvent corrected for swelling incre-ment, density of rubber and density of solvent respectively.

2.3.2.3. Number average molecular weight between crosslinks(MC). The number average molecular weight between crosslinks(MC) was determined according to FloryRehner equationas follow

m 1=2MC g1 mol 4

2.3.2.4. Swelling ratio (Q). The swelling ratio (Q) was calculated andexpressed in terms of the reciprocal of volume fraction of rubber inswollen gel as follow [13,14]:

Q V1=Vo 1=Vr 5where V1 is the volume of swollen rubber, Vo the volume of unswol-len rubber, and Vr is the volume fraction of rubber in the swollengel.

K.F. El-Nemr /Materials andtioned at ambient temperature for at least 16 h before mechanicaltesting was carried out on tensile testing machine HOUNS FILD,England.NBR composites with different contents of DCP at a xed con-centration of PETRA, namely, 5 phr was shown in Table 4. The ob-tained results showed that as DCP increases theM100, hardness andcross-link density signically increase. On the other hand, coagentsare typically multifunctional vinyl monomers that are highly

Table 2The physical properties of NBR vulcanized by sulfur.

Formulation

S1 S1.5 S2

Mechanical propertiesModulus at 100% elongation (M100), MPa 2.12 2.30 2.42Tensile strength (TS), MPa 17.44 16.00 15.85Elongation at break (Eb), % 1070 739 659Youngs modulus (Eo) 0.92 1.24 1.37Hardness, shore A 58 60 60

Physico-chemical propertiesSoluble fraction (SF) 0.093 0.081 0.07Volume fraction of rubber in swollen gel (Vr) 0.24 0.28 0.31Swelling ratio (Q) 4.17 3.56 3.24The physical properties of NBR composites cured by differentconcentrations of DCP, namely, 0.5, 1 and 1.5 phr, are given in Ta-ble 3. The data have shown that the TS pass through a maximumand then decrease with the increase of DCP content. While, Ebdiminishes with DCP concentration. Shore A hardness and M100values increase with the increase of DCP indicating the increasein cross-link density of the vulcanizates. It is thought that the in-crease of DCP level leads to an increase in the number of CACbonds [15], and this is reected in the values of Vr, Mc and cross-link density. Decreasing the values of TS by increasing cross-linkdensity, can be explained by the fact that on using DCP the perox-ide radicals can act through abstraction and addition mechanisms.The over-crosslinked domains (or clusters) may experience thehighest forces where the network strand will reach a critical forceand break [16].values of TS which we can explained as follow. Tensile strength,unlike the tensile stress at a given elongation and hardness, doesnot rise continuously with the number of crosslinks. Instead it riseswith the number of crosslinks until an optimum is reached, afterwhich if crosslinking is continued (in which case over-crosslinkingtakes place), it initially falls steeply.Molecular weight between crosslinks (Mc),g/mol

1154.5 794.33 636.04

Cross-link density 104 (m = 1/2MC), mol/cm3 4.37 6.3 7.86

sulfur and peroxide. The mechanism consists of reactions in theelastomer matrix, in the coagent domains, and between the elast-mer and the coagent.

The polymerized coagent radicals are incorporated into the net-work structure by a reaction with the elastomer molecules [20].

3.4. Radiation/coagent cured NBR

An electron beam accelerator was used to generate free radicalson the carbon atoms of the NBR and coagent. The radicals mayeither be reacting with each other or grafting on the coagent form-ing crosslinks. Table 5 shows the results for radiation curing ofNBR by different radiation doses up to 150 kGy in the presenceof 5 phr of PETRA. The M was found to increase linearly with

Table 3The physical properties of NBR vulcanized by dicumyl peroxide.

Formulation

P1 P1.5 P2

Mechanical propertiesModulus at 100% (M100), MPa 2.18 2.53 3.22Tensile strength (TS), MPa 13.41 16.09 13.57Elongation at break (Eb), % 541 481 331Youngs modulus (Eo) 1.23 1.56 2.29Hardness, shore A 60 63 65

Physico-chemical propertiesSoluble fraction (SF) 0.089 0.079 0.072Volume fraction of rubber in swollen gel (Vr) 0.27 0.31 0.34Swelling ratio (Q) 3.73 3.26 2.94Molecular weight between crosslinks (Mc), 885.01 646.82 500.07

3364 K.F. El-Nemr /Materials and Design 32 (2011) 33613369g/molCross-link density 104 (m = 1/2MC), mol/cm3 5.62 7.73 9.98

Table 4The physical properties of NBR cured by dicumyl peroxide/coagent.

Formulation

P1 PFM P1.5 PFM P2 PFMMechanical propertiesModulus at 100% elongation (M ), MPa 4.15 4.82 5.90reactive toward free radicals and graft to elastomer chains to formcomplex crosslinked network [17,18]. Because of all common coa-gents contain terminal unsaturations, it can be concluded thataddition reaction is the principal mechanism by which they reactin the compound; this has been conrmed by observed loss of coa-gent unsaturation during peroxide curing [19]. The formation ofcomplex crosslink network has a major impact on decreasing val-ues of TS observed at maximum concentration of DCP. The data ob-tained for Vr, Q and Mc indicated the increase of cross-link densityby values much more when compared by other cured systems like

100

Tensile strength (TS), MPa 11.89 12.73 8.84Elongation at break (Eb), % 200 190 133Youngs modulus (Eo) 3.3 3.76 4.88Hardness, shore A 70 72 75

Physico-chemical propertiesSoluble fraction (SF) 0.066 0.064 0.06Volume fraction of rubber in swollen gel

(Vr)0.365 0.39 0.42

Swelling ratio (Q) 2.75 2.56 2.38Molecular weight between crosslinks

(Mc), g/mol420.55 354.17 289.84

Cross-link density 104 (m = 1/2MC),mol/cm3

11.89 14.22 17.25

Table 5The physical properties of NBR cured by radiation/coagent.

Formulation

R25 PFMMechanical propertiesModulus at 100% elongation (M100), MPa 2.18Tensile strength (TS), MPa 11.2Elongation at break (Eb), % 1041Youngs modulus (Eo) 0.55Hardness, shore A 52

Physico-chemical propertiesSoluble fraction (SF) 0.174Volume fraction of rubber in swollen gel (Vr) 0.183Swelling ratio (Q) 5.47Molecular weight between crosslinks (Mc), g/mol 2073Cross-link density 104 (m = 1/2MC), mol/cm3 2.41100

radiation dose indicating the linear relationship of cross-link den-sity with radiation dose, whereas the latter inversely lowers thevalues of Mc. The increase in values of Vr, decrease in Eb, Q andin solubility were considerable by increasing radiation dose indi-cating the increase of cross-link density of vulcanizate. Also thereis an increase in TS up to 100 kGy, beyond which there is a veryslight decrease in TS values dose. The leveling off or decrease inTS by radiation may be explained as follows: TS of a polymer isa function of cross-link density and energy dissipation, at highercrosslinking density, i.e. at 150 kGy, the segments of the macro-molecule become immobile, the system becomes stiffer and theelasticity decreases [21]. Also, it can be assumed that the rate oftwo processes, namely crosslinking and degradation may then oc-cur with the same rate, hindered mobility of macromolecules dueto increased rate of crosslinking may contribute also to decreasingthe values of TS.

3.5. Physical properties comparison of NBR cured by sulfur, peroxideand radiation/coagent (100 kGy) at a comparable value of Vr

3.5.1. Mechanical propertiesThe Vr can be taken as an indicator of cross-link density and

reinforcing ability of ller [22,23]. The data listed in the perviousTables 14, show that the values of Vr 0.31, are comparable forNBR cured by 2 phr sulfur, 1.5 phr DCP and radiation/coagent at(100 kGy). For comparison Table 6 represents the respective phys-ical data. Obviously the crosslink structure and cross-link densityaffect the mechanical properties of NBR vulcanizates dependingon the type of curing system. The tear strength, TS and Eb valuesare comparatively the highest in the case of sulfur cured system.The Eb data can be explained on the basis that the system canaccommodate more stress and elongation via the exible and labileS-S linkage [24]. The lower tear strength and Eb values of DCP curedsystem are attributed to the short and rigid CAC crosslinks

R50 PFM R75 PFM R100 PFM R150 PFM

2.68 3.5 4.0 5.211.6 13.4 14 13.6633 540 359 2521.06 1.48 2.37 3.4955 60 65 70

0.14 0.11 0.08 0.060.235 0.285 0.313 0.374.251 3.513 3.13 2.7

1196.5 776.2 584 402.74.18 6.48 8.48 12.4

volatile matter, consists of the amount of oil added as a processingagent and a small amount 12% of decomposed fragments of thevulcanizing agent (sulfur or peroxide) vulcanization acceleratorand antioxidant [25]. The second fraction 350550 C repre-sented the medium volatile matter, the amount of decomposedpolymer. Evidently, the radiation curing system exhibits higherthermal stability than the others as conrmed by the data shownin Table 7.

The differential thermal gravimetric (DTG) curves demonstratefor the three curative systems, only one peak indicating a singlestage decomposition in the NBR composite, Fig. 3, with a consistent

Table 6The physical properties of NBR cured by sulfur, peroxide and radiation/coagent.

Formulation

S2 P1.5 R100-PFM

Mechanical propertiesModulus at 300% elongation (M300), MPa 5.81 7.82 10.57Modulus at 100% elongation (M100), MPa 2.42 2.53 4.00Tensile strength (TS), MPa 15.85 16.09 14Elongation at break (Eb), % 659 481 359Youngs modulus (Eo) 1.37 1.56 2.37Hardness, shore A 60 63 65Tearing strength, N/mm 43 38 27

Physico-chemical propertiesSoluble fraction (SF) 0.07 0.079 0.08Volume fraction of rubber in swollen gel (Vr) 0.31 0.31 0.313Swelling ratio (Q) 3.24 3.26 3.13Molecular weight between crosslinks (Mc),

g/mol636.04 646.82 584

Cross-link density 104 (m = 1/2MC),mol/cm3

7.86 7.73 8.48

Fig. 2. TGA curves of NBR cured by sulfur, peroxide and radiation/coagent.

K.F. El-Nemr /Materials and Design 32 (2011) 33613369 3365between macromolecules chains [24]. The hardness results showslightly higher values in the case of radiation curing system thanthe others as a result of the associated higher cross-link density.Meanwhile, the lowest tensile strength values compared to thoseof the peroxide and sulfur cured systems can be related to the lossand isomerization of less relaxed network. On the other hand, thehighest reported values of M100 and M300 for the radiation/coagentcured system compared to the other systems can be attributed tothe cross-link density.

3.5.2. Stressstrain behaviorThe stressstrain behavior of NBR demonstrates a remarkable

dependence on the applied curing system as shown in Fig. 1. Theinitial modulus is almost the same in sulfur and DCP cured sys-tems, however, the DCP system exhibits higher tensile modulusthan sulfur though TS values are comparable. Radiation/coagentcured system can accommodate less strength at rupture but exhib-its higher modulus values at any given elongation as a result of theestablished cross-link density.

3.5.3. Thermal analysisFig. 2 shows the TGA thermograms of the NBR composites curedby sulfur, S2, peroxide, P1.5, and radiation/coagent, R100-PFM. Therst weight loss stage 150350 C is due to the loss of a highly

Fig. 1. Stressstrain curves of NBR cured by sulfur, peroxide and radiation/coagent.Table 7Decomposition temperature of NBR cured by sulfur, peroxide and radiation/coagentat different weight losses.

Composition Temperature (C)

Weight loss, % S2 P1.5 R100-PFM

T10 415.14 427.00 428.70T30 460.00 465.50 461.00T50 484.00 486.00 484.00T60 497.00 506.00 560.00T65 563.00 569.00 580.00T70 590.00 590.00 590.00

DTG 484.00 484.00 484.50

Fig. 3. DTG of NBR cured by sulfur, peroxide and radiation/coagent.

(Tmax) located at 484 C. The relatively higher thermal stabilityexhibited by the radiation/coagent and peroxide systems is dueto the formation of CAC linkages which are more thermally stablethan CASAC and CASxAC linkages in sulfur vulcanization [26]. Fur-thermore, the CAC linkage in both cured systems possesses a high-er bond energy of 85 k cal mol1 whereas the bond energy of theSAS crosslink is only 57 k cal mol1 [27].

3.5.4. Effect of fuelNBR rubber is applied specically as fuel and oil resistant. The

resistance can be followed up by measuring the change of weight%,by immersing in fuel A (commercial gasoline) and then plotted as afunction of different curing systems, as shown in Fig. 4. It can beseen that the NBR cured by radiation/coagent gave the best resultsfor swelling resistance in commercial gasoline compared to thesulfur and peroxide systems at the same Vr value. The resistanceto swelling can be argued on the basis of the chemical crosslinkingmagnitude induced in the rubber component by radiation with re-spect to the other methods of curing.

3.5.5. Abrasion resistanceFig. 5 illustrates the dependence of the abrasion loss in the NBR

composite samples on the implemented curing system. Generally,it can be seen that the abrasion loss decreases by the increase incross-link density. These data indicate that the abrasion resistance

slightly enhanced in case of radiation/coagent followed by sulfurthen peroxide cured systems.

3.6. Physical properties comparison of NBR cured by peroxide/coagentand radiation/coagent (150 kGy) at a comparable value of Vr

3.6.1. Mechanical propertiesDepending on the data obtained before, Tables 14, comparison

can be established between NBR cured by DCP/coagent, (P1 PFM)with that cured by electron beam radiation/coagent, (R150 PFM)at comparable values of Vr 0.37. The physical properties of thevulcanizates are illustrated in Table 8. Comparable values for hard-ness, Eb, tearing strength, soluble fraction SF, Q, Mc and cross-linkdensity were recorded for these two vulcanizates, whereas the vul-canizate samples cured by radiation/coagent gave higher values ofTS and tensile modulus. The behavior of the latter is due to the in-crease in cross-link density.

3.6.2. Stressstrain behaviorFig. 6 shows the stress against strain for NBR cured by DCP/coa-

gent and radiation/coagent systems. It can be observed that theradiation/coagent cured samples revealed higher TS and tensilemodulus values, the increase in values of TS can be related to theincrease in cross-link density.

Table 8The physical properties of NBR cured by peroxide/coagent and radiation/coagent.

Formulation

P1-PFM R150-PFM

Mechanical propertiesModulus at 100% elongation (M100), MPa 4.15 5.2Tensile strength (TS), MPa 11.89 13.6Elongation at break (Eb), % 200 252Youngs modulus (Eo) 3.3 3.49Hardness, shore A 70 70

3366 K.F. El-Nemr /Materials and Design 32 (2011) 33613369Fig. 4. Effect of fuel A on the NBR cured by sulfur, peroxide and radiation/coagent.Fig. 5. Abrasion loss of NBR cured by sulfur, peroxide and radiation/coagent.Tearing strength, N/mm 29.2 30

Physico-chemical propertiesSoluble fraction (SF) 0.066 0.06Volume fraction of rubber in swollen gel (Vr) 0.365 0.37Swelling ratio (Q) 2.75 2.7Molecular weight between crosslinks (Mc), g/mol 420.55 402.7Cross-link density 104 (m = 1/2MC), mol/cm3 11.89 12.4Fig. 6. Effect of peroxide/coagent and radiation/coagent curing systems on stressstrain behavior of NBR vulcanizates.

Fig. 8. DTG curves for NBR cured by peroxide/coagent and radiation/coagent.

DeFig. 7. TGA curves for NBR cured by peroxide/coagent and radiation/coagent.

Table 9Decomposition temperature of NBR cured by peroxide/coagent and radiation/coagent.

Composition Temperature (C)

Weight loss, % P1-PFM R150-PFM

T10 428.00 428.00T30 466.00 463.35T50 488.00 487.00T60 526.00 566.75T65 593.00 593.00K.F. El-Nemr /Materials and3.6.3. Thermal analysisFig. 7 and Table 9 shows the TGA thermograms and data,

respectively, of DCP/coagent and radiation/coagent cured NBR vul-canizates. At temperature up to 500 C both vulcanizates demon-strated similar thermal stability. It can be observed that thedecomposition temperatures are nearly comparable for both twocured systems except at 60% decomposition (T60). The temperatureof degradation obtained for radiation/coagent cured system ishigher than that of the DCP/coagent cured system, 566.75 C and526 C respectively. Meanwhile, both cured systems exhibitedthe same decomposition temperature for 50% loss (T50) which isfairly close to Tmax of both vulcanizates, 485 C, as shown bythe DTG curves, Fig. 8.

The slight enhancement in thermal stability of radiation/coa-gent cured vulcanizates may be related to a slight increase incross-link density which is a factor affecting the thermal stabilitythat leads to higher values of activation energy needed for thermaldecomposition of vulcanizates [28,29].

3.6.4. Effect of fuelThe impact of the two different curing systems, peroxide/coa-

gent and radiation/coagent (150 kGy) on the change in weight%of NBR samples soaked in commercial gasoline is illustrated inFig. 9. Evidently, the nal weight of the samples cured by radia-tion/coagent is slightly lesser than that of the other ones curedby peroxide/coagent. The results are clearly in good agreementwith the results of cross-link density and Mc, i.e. the greater thecross-link density, the higher the fuel resistance.

3.6.5. Abrasion resistanceAbrasion resistance of NBR rubber cured by peroxide/coagent

and radiation/coagent (150 kGy) is shown in Fig. 10. It can be

T66 600.00 600.00DTG 486.80 485.00sign 32 (2011) 33613369 3367noticed that the abrasion resistance of radiation/coagent curedNBR composites is relatively higher than that of those cured byperoxide/coagent system and this behavior may be attributed tothe increase in crosslinks induced by radiation.

Fig. 9. Effect of fuel A on the NBR cured by peroxide/coagent and radiation/coagent.

Fig. 10. Abrasion loss of NBR cured by peroxide/coagent and radiation/coagent.

3.7. Effect of thermal ageing

The effect of thermal ageing on the mechanical properties,namely TS, Eb and hardness of NBR vulcanizates cured by sulfur,peroxide, peroxide/coagent and radiation/coagent is shown in Figs.1113, respectively. The values of TS for sulfur and radiation/coa-gent (150 kGy) cured systems at 150 kGy were found slightly re-duced on ageing while the other cured systems were notaffected. On the other hand, only the values of Eb for sulfur curedsystem decreased on ageing. Meanwhile, the hardness values forall NBR vulcanizates encountered a slight alteration on ageing.

The decrease in values of TS and Eb in case of sulfur vulcanizedsamples may be attributed to the fact that the polysuldic cross-links cleave and rearrange into shorter crosslinks [30]. Also, the de-crease in values of TS of NBR vulcanizates cured by radiation/coagent may be attributed to chain scission by thermal ageing.Moreover, compared to sulfur vulcanized samples, radiation/coa-gent vulcanized samples are more resistant to heating due to thenature of networks, as CAC linkage is more stable possessing high-er bond energy than CASAC linkage. It is also well known thatpolysuldic linkage is of relatively low bond strength, i.e. poorerthermal ageing resistant.

peroxide/coagent and radiation/coagent (150 kGy) gave compara-ble values of Vr, 0.37, and similar values for the physcio-chemicaland mechanical properties, except tensile strength reportedslightly higher values as a result of irradiation.

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