34

CHAPTER II

LITERATURE REVIEW

Early works on EPDM have been published since 1950s, but a significant

progress was made only during the last decade with respect to the preparation, processing

techniques, characterization, morphology, mechanical, thermal and electrical properties,

along with the EPDM composites and their nanocomposites. Background knowledge

relevant to polymer rubber nanocomposites and a general review of related literature are

provided in this chapter.

Polar adhesives are reported to have changed the polarity in rubber systems and

there is a parallel relationship between the hydrophilic properties of the surface and their

subsequent polarity [1, 2].

Surface treatment of polymers, especially rubber (both vulcanized and

nonvulcanized) by chemical modification with reagents such as acids and oxidizers,

increases the surface polarity which causes an increase in molecular forces between

substrates and hence an increase in adhesion strength [3-6].

Epoxidation of ethylene propylene diene rubber in situ generated performic acid

in which the conversion of double bonds increased rapidly within the first 1 h, and then

gradually over the next 2 h with only a slight increase. The maximum conversion ratio of

double bonds is about 70% whereas the relative content of epoxy groups has a top value

at about 7 h [7].

Ying et al. epoxidised EPDM dissolved in toluene using tert-butyl hydroperoxide

(TBHP) as oxidant and molybdenum oxide as catalyst and have revealed that no

35

significant side reactions take place in this system even at high epoxidation levels (90%)

[8].

Mir et al. have developed a method for epoxidation of EPDM with in-situ

generated dimethyldioxirane (DMD)/MoO3 complex to a controlled increase in the

functionality [9].

A cast film of EPDM-ethylidene norbornene terpolymer containing C60 kept in

the dark is reported to remain soluble but exposure to light has resulted in a crosslinked

rubber [10] and finally a highly uniform photoconductive film was obtained from the

solution.

Ceni et al. have studied the processing, cure characteristics, and mechanical

properties of EPDM rubber containing ground EPDM vulcanizate of known composition

and reported an enormous increase in stress–strain behavior, in addition to marginal

changes in heat buildup, resilience, and abrasion resistance. The interplay between the

filler effect of the ground EPDM and the crosslink density changes of the EPDM matrix,

could be the reason for the variation in mechanical properties and it is believed that sulfur

migration occurs from the raw EPDM matrix (R-EPDM) to the ground waste EPDM (W-

EPDM) particle while accelerator migration occurs from W-EPDM to R-EPDM [11].

They have further modified the work by substituting virgin EPDM rubber in a

thermoplastic elastomeric blend of Propylene (PP) and EPDM rubber by ground EPDM

vulcanizate of known composition to show that the mechanical properties showed

improvement with intermediate W-EPDM loadings and the R-EPDM/W–EPDM/PP

blends are reprocessable [12].

36

Sulfonation of previously vulcanized EPDM membranes was developed by

Barroso-Bujans et al. in a swelling solvent with acetyl sulfate which avoids the need to

pre-dissolve the raw polymer and yields a degree of sulfonation comparable to the

chlorosulfonic acid procedure. The mechanical properties, tensile strength and Young’s

modulus of sulfonated EPDM increased 5–6 times compared to the non-sulfonated one,

while elongation at break did not change. The thermal stability of sulfonated EPDM is

decreased by the presence of sulfonic groups in the polymer backbone, which induces

cleavage of the polymer at a lower temperature than in pure EPDM [13].

Verbruggen et al. have reported that the behavior of EPDM vulcanizates during

pure thermal devulcanization depends on the EPDM third monomers like 2-ethylidene-5-

norbornene (ENB), dicyclopentadiene (DCPD), and 1, 4-hexadiene (HD) and the

crosslinker used, among which peroxide vulcanizates of ENB-EPDM devulcanize only to

a small extent and predominantly by random scission and in contrast, sulfur vulcanizates

of ENB-EPDM, devulcanize mainly by crosslink scission [14].

Ginic-Markovic et al. suggested that when EPDM is treated with maleic

anhydride-grafted rubber, there is increase in adhesion strength [15].

Al-Malaika et al. functionalized EPDM with glycidyl methacrylate (GMA) during

melt processing by free radical grafting with peroxide initiation when increasing the

peroxide concentration resulted in an increase in the GMA grafting yield. The overall

grafting level was low, accompanied by higher degree of crosslinking of EPDM which

was found to be the major competing reaction. The mechanical properties of the blends

are strongly influenced by the performance of the graft copolymer, which in turn, is

determined by the level of functionality, molecular structure of the functionalized rubber

37

and the interfacial concentration of the graft copolymer across the interface. It was

shown that binary and ternary blends prepared with functionalized EPDM in the absence

of Tris with lower levels of g-GMA effected a significant improvement in mechanical

properties especially in elongation to break, which could be accounted for by the

occurrence of a reaction between the epoxy groups of GMA and the hydroxyl/carboxyl

end groups of PET resulting in a graft copolymer. This co-polymer most probably,

preferentially located at the interface, thereby acting as an ‘emulsifier’ is responsible for

decreasing the interfacial tension between the otherwise two immiscible phases. This

resulted in lower mechanical properties due to the difference in the microstructure of the

graft and the level of functionality in these samples resulting in less favorable structure

responsible for the coarser dispersion of the rubber phase, the lower extent of Tg shift of

the PET phase, the lower height of the torque curve during reactive blending and a lower

extent of the interfacial chemical reaction between the phases in this Tris-containing

blend sample [16].

Kim et al. have modified the polarity of EPDM using a brominated EPDM which

was produced by reacting EPDM solution in hexane that was sampled directly from

commercial plant with N-bromosuccinimide, and then further reacted with sodium

acetate or sodium chlorodifluoroacetate in the presence of phase transfer catalyst to yield

the functionalized EPDM which showed increase of the work of adhesion and lap shear

strength [17].

Qingjun et al. prepared crosslinking networks in the EPDM matrix with calcium

carbonate particles (20% wt), which were chemically treated with acrylic acid (AA) in

which the tensile strength and the elongation at break of the composites were improved

38

significantly, and the maximum tensile properties were achieved. The composites have an

evident crosslinking structure with good interfacial adhesion between CaCO3 and

copolymer [18].

Susanta et al. have observed that the presence of an oxidizing agent has a

pronounced effect on the surface alteration of EPDM, where the effect of high molar

mass is found to be relatively significant on the macro-molecular scale particularly in

coupling reactions of the limited number of oxygenated species generated through

surface degradation process. Apparently, presence of long branching has no effect in

chemical degradation and the chemical reactions are believed to take place at the ENB

moiety while the backbone of the EPDM rubber remains unaffected under the studied

exposure conditions. The surface degradation is found strong enough to affect the

macroscopic behavior of pure EPDM to a significant extent as revealed by substantial

increase in gel fraction upon exposure [19].

Zhang et al. have used N-Chlorothiosulfonamides to modify EPDM to enhance

the compatibility of EPDM in NR/butadiene rubber (BR)/EPDM blends for ozone

resistance in which ethylidene norbornene (ENB)-EPDM, hexadiene (HD)-EPDM, and

dicyclopentadiene (DCPD)-EPDM, HD-EPDM are very effective towards N-Chlorothio-

N-butyl-benzenesulfonamide (CTBBS)-modification. The DCPD-EPDM showed very

low reactivity so that no modification could be realized but CTBBS added to ENB-

EPDM caused gelation resulting in poor modification. With the limited modification

efficiency for ENB-EPDM, there is no significant improvement when applying the

modified ENB-EPDM into NR/BR/EPDM blends. Moreover, tensile strength and

39

elongation at break showed only limited improvement by applying modified EPDM into

the blends [20].

Snijders et al. have established linear trends for the relative carbonyl absorbance

and the oxygen absorption rate as a function of the third monomer content for

uncompounded, non-crosslinked EPDM elastomers [21].

Melt-laminates of wood/natural rubber (wood/NR) composite and (EPDM foam

were prepared by Yamsaengsung et al. using compression molding technique in which

two different forms of 4, 4-oxybis (benzenesulfonylhydrazide) (OBSH) blowing agents

were used. They found that the EPR–b-OBSH gave EPDM foam with greater number of

cell structures, higher porosities and resistance to water penetration on the foam surface.

However, the EPDM with EPR–b-OBSH agent had worse elastic recovery as compared

to that with OBSH due to deformation of cell structures after prolonged compression

loading [22].

According to Younan et al., addition of neoprene to EPDM decreases the tensile

strength and increases the elongation at break. They have concluded that the two rubbers

are incompatible but can be made compatible by addition of some compatibilisers and

even white and black fillers [23]. Botros et al. also obtained single Tg value for

EPDM/neoprene blend but only by adding good percentage of compatibilizer [24].

Dubey et al. have confirmed that EPDM and neoprene are incompatible when

dissolved in toluene but have shown indications of improvement in interaction between

the two rubbers at higher EPDM fraction in chloroform. They have also shown that

increase in concentration of EPDM increases the level of mixing between EPDM and

neoprene [25].

40

Janczak et al. modified EPDM rubber using low density Polyethylene (PE) or

isotactic Polypropylene (PP) with peroxide crosslinking and found symptoms of

interfacial grafting of EPDM on the particles of plastomers. They have also indicated

that, with the addition of PE or PP to EPDM, its tear strength, hardness and coefficient of

kinetic friction are found to increase. When similar values of the solubility parameters are

considered, little difference in polymer adhesion to the steel track is expected. Therefore,

the micro-heterogeneous structure of the mixtures and their hysteretical properties are

responsible for the high values of co-efficient of friction [26].

A study of the morphology of immiscible and highly incompatible blends of

Soronaw polymer [Poly (trimethylene terephthalate)] and EPDM blends shows a two-

phase morphology, narrow interphase, and poor physico-chemical interactions across the

phase boundaries. A reactive route was alternatively employed to compatibilize these

blends by the addition of EPM-g-MA using an internal mixer in which the EPDM phase

was preferentially dispersed as domains in the continuous Soronaw matrix up to 30% of

its concentration. A co-continuous morphology was observed above 30 wt% of EPDM

content followed by a phase inversion beyond 60 wt% of EPDM. The addition of EPM-g-

MA reduces the domain size of the dispersed phase followed by a leveling off at higher

concentrations of the compatibilizer indicating interfacial saturation and decrease in free

volume [27].

The thermal studies of blend obtained from the mixture of 4-

dodecylbenzenesulfonic acid doped polyaniline (PANI (DBSA)) and a terpolymer of

ethylene-propylene-5-ethylidene-2-norbornene (EPDM) by casting from organic solvents

studied by Schmidt et al. under nitrogen atmosphere showed more than one stage of

41

degradation, of which the second (temperature range 380–520 oC) was observed to be

more significant. Apparently in this stage, the simultaneous degradation of the bound

dopant, the polyaniline (PANI) and the elastomer occurred. The activation energy (E) for

the crosslinked blends was higher (180–250 kJ mol-1

) than those of pure components and

non crosslinked blends (150–210 kJ mol-1

), which is not due to the PANI (DBSA)

blended with maleated EPDM, and hence the mechanism of degradation of both systems

is associated with a random scission of the chain [28].

Mauro et al. showed that conductive composites based on

dodecylbenzenesulfonate doped polyaniline/organoclay nanocomposites and EPDM–

ethylidene norbornene rubber prepared by melt-blending using an internal mixer, had

cryofractured surfaces exhibiting high conductivities, up to 10−3

Scm−1

for 40 wt% of

conducting nanocomposite, and good mechanical properties. They also presented high

microwave attenuation values, in the frequency range of 8–12 GHz and this property

depends on the concentration of the conductive nanocomposite and on the film thickness.

The composites can be used for antistatic coatings or for electromagnetic shielding [29].

Kratofil et al. have prepared SAN/EPDM polymer blends by reactive extrusion,

and observed that, elongation at break increases with EPDM content whereas the tensile

strength varies inversely [30].

Linear low-density polyethylene (LLDPE) and EPDM blends, with scrap rubber

tire (SRT) prepared by Helson et al. in a single-screw extruder ended in a composite of

SRT–EPDM particles which resulted in expressive diminution in the crystallization

temperature (Tc), crystallinity (%w), heat of crystallization (DHc) and melting

temperature (Tm) values. The SRT also does not impose substantial increase in the elastic

42

modulus (G0), viscous modulus (G00) and complex viscosity (Z*) values of

LLDPE/EPDM/SRT mixtures [31].

In composites of EPDM, high density polyethylene (HDPE) and ground tire

rubber powder (GTR) at different ratios, gamma irradiation of various doses up to

250kGy led to a significant improvement in the properties for all blend compositions but

the improvement was inversely proportional to the substituted ratio of GTR, attributed to

the development of an interfacial adhesion between GTR and blend components.

Abou et al. have reporteds non-compatibility between EPDM, HDPE and GTR and

decrease in tensile and hardness properties with the introduction of GTR [32].

In a study of compatibilizers for EPDM/plasticized PVC blends, Maria et al. have

revealed that the most efficient crosslinking system is the phenolic resin system closely

succeeded by the benzoyl peroxide and trimethylpropane trimethacrylate (TMPT) [33].

EPDM rubber is found to be an effective substrate for the thermal insulation of

case-bonded solid rocket motors (SRMs) due to the advantages such as low specific

gravity, improved ageing properties and longer shelf life. In spite of these advantages,

EPDM, a non-polar rubber, lacks sufficient bonding with the propellant matrix. Bonding

properties are found to improve when EPDM is blended with other polar rubbers like

polychloroprene (neoprene), chlorosulphonated polyethylene (CSE), etc. This type of

polar polymer when blended with EPDM rubber enhances the insulator-to-propellant

interface bonding. Sureshkumar et al. have further shown that the tensile strength

decreases with increase in HTPB concentration in the EPDM–CSE based insulator

prepared by incorporating hydroxy terminated polybutadiene (HTPB), a polar polymer as

well as a polymeric binder, as an additive [34].

43

Masoud et al. have reported that solvent blending of PP and EPDM in a

composition of 50:50 formed two-phase morphology in which EPDM appeared as

dispersed phase with irregular shape and the size of dispersed phase reduced significantly

to almost spherical domains by addition of the nanoclay. Addition of compatibilizer

provided a good dispersion of nanoclay and exhibited a very ductile surface indicating a

good compatibility of PP and EPDM rubber and also, a possible contribution of

nanoparticles to deformation mechanisms, such as extensive shear yielding in the

polymer blend and a decrease in crystallinity up to 27%, suggesting a reduction in

spherulites growth. However, the melting temperature remained unchanged. The increase

in barrier property of the blend, despite a decrease in crystallinity, indicated the dominant

role of organoclay platelets in barrier improvement. According to the permeability model,

very high barrier property could be obtained if the aspect ratio of the flakes or platelets of

the organoclay could be significantly increased in the blend [35].

EPDM–clay nanocomposites (EPDM–CNs) with organoclay that was intercalated

with maleic anhydride grafted EPDM (MAH–g-EPDM) and EPDM–clay composites

(EPDM–CCs) with pristine clay, have been prepared through melt intercalation

technique. An indirect method by Seyed et al., resulted in complete exfoliation of

organoclay in the EPDM matrix and showed significant improvement in the mechanical,

thermal and chemical properties of nanocomposites with the addition of organoclay.

Another simple and cheaper direct method was examined to prepare EPDM–CNs which

also showed similar properties as in the indirect method [36].

Based on the theory of polymer melt intercalation in organo-modified clay

(OMC) [37], it should be thermodynamically possible to obtain highly filled polymer

44

clay nanocomposites (PCN) by melt blending. However, there are very few experimental

studies on highly filled PCNs due to processing difficulties caused by the high viscosity

of polymer melt filled with large amounts of nano-silicates. To prepare highly filled

PCNs with homogeneous dispersion of silicate platelets, both long processing time and

high mixing torque are required. Hence, for thermoplastic polymer matrices, long

processing times under high-temperature would result in polymer degradation and

reduction of the resultant nanocomposite properties and also, the processing equipment

for plastics (i.e., screw extruder) hardly sustains such large mixing torques [38-40].

The study on the effects of crosslink density and crosslink type on the tensile and

tear strengths of gum Natural Rubber (NR), Styrene-Butadiene Rubber (SBR) and

Ethylene Propylene Diene Monomer (EPDM) vulcanizates by Kok et al. have showed

that sulphur vulcanizates for the rubbers have higher strengths than the peroxide

vulcanizates and it hints that crystallizability of the rubbers is not an important factor in

producing separate curves in the strength vs crosslink density plots. Tear strengths appear

to be more sensitive to crosslink structures than tensile strengths and the composite plot

denotes that tensile strengths are approximately proportional to tear strengths for all three

rubbers [41].

EPDM blends, one vulcanized (micro-7) and the other peroxide-cured and then

post-cured (micro-8) were prepared by Virgilia et al. among which micro-8 showed

diffuse globular structures measuring 0.5mm but the surface microstructure of micro-7

exhibited considerable irregularity, with several globules, microtroughs measuring about

150-200 nm in diameter, and even craters and globules of up to 4 mm thus exhibited the

45

most irregular surface. The spectroscopic patterns of micro-7 and micro-8 rubbers were

almost identical whereas micro-8 has good resistance to degradation [42].

Emile et al. found that crosslinking of ethylene-2-norbornene (ENB)-containing

EPDM with dicumyl peroxide dramatically influenced the UV ageing. In weather-O-

meter (WOM) ageing at fixed ageing time, linear relationships were found between

relative carbonyl absorbance and third monomer content as well as between relative

carbonyl absorbance and peroxide concentration. The microhardness of the various

crosslinked EPDMs showed a maximum, although the samples were crosslinked before

UV ageing, but decreases after the maximum as it was linearly dependent on the third

monomer content. The crosslink density showed that the relationship between the soluble

part of the sample and the microhardness throughout the WOM ageing was linear which

indeed shows that upon UV ageing crosslinking and chain-scission reactions compete

[43].

Abdel-Aziz et al. observed that in γ-radiated vulcanized EPDM rubber/Al2O3

composites, the addition of Al2O3 leads to increase in thermal diffusivity coefficient and

the dielectric constant and decrease in specific heat for composites. The thermal and

electrical properties of the composites depend on the particle size of the incorporated

filler beside the irradiation dose of vulcanization and also are slightly changed with

temperature [44].

An improvement in thermal resistance of EPDM by exposure to γ-radiation (137

Cs

source) in the presence of divinyl benzene, was studied by means of oxidation resistance

according to the oxygen uptake procedure in which a low dose of around 90 kGy is

enough to attain high stability. At this dose the oxidation induction times (OIT) for

46

EPDM/ divinyl benzene specimens are 2-3 times longer than the OIT of control films.

The increased thermal stability is due to increased crosslinking and of course, as a

capability of radiochemical procedures to improve the durability of materials [45].

Rubber blend of acrylonitrile butadiene rubber (NBR) and EPDM rubber (50/50)

loaded with increasing contents (up to 100 phr) of high abrasion furnace (HAF)-carbon

black when subjected to gamma radiation doses (up to 250kGy) showed better

mechanical properties, tensile strength (TS), tensile modulus at 100% elongation (M100),

and hardness as a direct function of irradiation dose and degree of loading with filler. The

blend with higher content of HAF exhibits a better electrical conductivity whereas the

thermal stability of the blend was improved by both degree of loading with HAF and

crosslinking induced by gamma irradiation [46].

Quanlin Zhao et al. have showed that oxygenated functions such as C–O–C, C=O

and O–C=O groups were generated in EPDM rubber when exposed to an artificial

weathering environment (produced by fluorescent UV/condensation weathering device).

Moreover, aging occurred from EPDM surface and propagated to EPDM inner body.

They have also reported an increase in surface energy but the thermal stability of EPDM

predominantly remained unaffected [47].

Studies on PCNs with low organically modified clay (OMC) contents have

disclosed that melt intercalation of polymer in OMC is an enthalpy-driven process, and

polymer chains with high polarity are easily intercalated into silicate galleries due to their

strong interactions with silicate layers [48].

In silicane-EPDM composites the addition of polar SiO2 was found to increase the

thermal diffusivity, whilst it decreases upon the addition of non-polar SiO2. Degradation

47

is found to be the predominant process for all EPDM loaded samples at g-irradiation dose

>15kGy. The addition of ethylene/glycol/dimethacrylate (EGDM) as coagent increases

the value of the dielectric constant for EPDM containing non-polar SiO2 whilst, it

decreases for the other white fillers [49].

There are two idealized morphologies that can be developed using nano-silicate

fillers: exfoliated and intercalated. The spatial distribution of clay particles in exfoliated

PCNs is much more homogeneous than that in intercalated nanocomposites. In the

exfoliated PCNs, the individual clay layer acts as the basic reinforcing unit, but in the

intercalated PCNs, the reinforcement unit is the intercalated clay stacks, the equivalent

aspect ratio of which is much lower than that of an individual clay layer [50, 51].

Usuki et al. showed that in EPDM/clay hybrids with montmorillonite (MMT), the

tensile strength and storage modulus improved with MMT content and the permeability

decreased 30% compared to neat EPDM [52].

Arroyo et al. have reported that the mechanical properties of natural rubber (NR)

filled with 10 phr organoclay were comparable to those of the compound with 40 phr

carbon black. Moreover, the organoclay improved the strength of the NR without any

reduction in the elasticity of the material [53].

There are reports of EPDM/organoclay nanocomposites showing a strong increase

in the stress behavior at the high strain approaching break which resulted from the

macromolecular chain orientation and the resultant orientation of the clay mineral layers

brought about by the rubber macromolecular orientation [54].

Morlat et al. established that the EPDM/MMT nanocomposites degrade faster

than the pristine polymers because of the degradation of the alkyl-ammonium cation

48

exchanged in MMT and the catalytic effect of iron impurities of the organo-

montmorillonite. Iron (Fe3+

) could catalyze the decomposition of the primary

hydroperoxide formed by photo-oxidation of polymers [55].

Yong-Lai et al. have showed that the highly filled RCNs (up to 60wt %) have

intercalated silicate structures and the dispersion homogeneity of clay layers improves

with increasing content of organically modified clay (OMC). It was shown for the first

time that the melt-like thermal transition of alkyl chains of the surfactant of OMC still

occurs in the intercalated OMC. Addition of large amount OMC to rubber greatly

improves the modulus of material and they possess outstanding gas barrier properties

compared to neat rubbers. They have also stated that when compared with thermoplastics,

the processing temperature of rubbers is relatively lower (i.e., less than 60oC), thus

allowing long processing times. Moreover, the processing facility for rubbers (i.e., inner

mixer and two-roll mill) can provide and bear large mixing torques. This class of highly

filled nanocomposites is used as high-performance sealant materials in aerospace

applications [56].

Zheng et al. showed that the montmorillonite (MMT) modified with

trimethyloctadecylamine or dimethylbenzyloctadecylamine existed in the form of an

intercalated layer structure and the MMT modified with methlybis(2-

hydroxyethyl)cocoalkylamine was fully exfoliated in the EPDM matrix. The expansion

of the distance between the silicate layers first took place after the HAAKE mixing and

then the silicate layers were exfoliated in the EPDM matrix after the EPDM/OMMT

composite was cured. The EPDM composite containing 15 phr OMMT which was

modified with the alkylamine containing hydroxyl groups showed high tensile strength of

49

25 MPa and greater glass transition temperature (Tg). The OMMTs had delaying effects

on the vulcanization reaction and decreased the crosslink density of the EPDM/OMMT

composites [57].

Incorporation of MMT modified with octadecylamine (MMT-PRIM) in EPDM

and its more polar version (contains EPDM-g-MA) resulted in intercalated and exfoliated

structures, respectively and deintercalation of the clay (collapse of the layers) in both

EPDM and EPDM-g-MA during vulcanization was attributed to the reactivity of the

PRIM and to its ability to participate in complex formation with the curatives. When the

less reactive modifier, viz. octadecyltrimethylamine served as MMT intercalant (MMT-

QUAT) in which the corresponding nanocomposite contained mostly intercalated clay

layers, it developed no tendency to collapse [58].

The clay-reinforced EPDM nanocomposites with organo-montmorillonite

(OMMT) and EPDM–clay conventional composites with pristine MMT prepared by

Seyed et al. via melt intercalation process with maleic anhydride-grafted EPDM (MAH-

g-EPDM) as the compatibilizer, have exfoliated nanocomposites and showed superior

mechanical properties and solvent resistance compared to that of the conventional

composites. These are attributed to the more uniformly dispersed nanoparticles of

organoclay (OMMT) in the polymer matrix [59].

Sunil et al. have concluded that the antioxidants added during the preparation of

EPDM–clay nanocomposites via melt processing are usually incorporated with clay,

which allows phenolic antioxidant molecules to get adsorbed into acidic clay platelets

and their interaction with metallic impurities reduces the stabilizing efficiency of the

antioxidant. In the nanocomposites obtained by solution-dispersion followed by melt

50

compounding of EPDM and organophilic montmorillonite (OMMT), it was found that,

upon photo-irradiation (l > 290 nm) studies photo-degradation was lowered by the

antioxidant and the efficiency of the antioxidant could be improved by initial

incorporation of antioxidant in the EPDM matrix. However, in EPDM–clay

nanocomposites, a stabilizing activity of the antioxidant was observed above some

threshold concentration of the antioxidant [60].

The effect of gamma irradiation on EPDM/clay nanocomposites reported by

Seyed et al. confirms intercalation of nanolayer silicates, shift in α-relaxation peaks to

higher temperature and increase in storage modulus with irradiation dose. They have also

suggested that the exposure of EPDM hybrids to gamma rays improves the tensile

strength of samples at lower irradiation doses due to cross-linking effect and the

nanocomposites exhibit superior irradiation-resistance compared to unfilled EPDM and

conventional composites [61].

Sandrine et al. have shown that in photooxidation of a vulcanized

EPDM/montmorillonite nanocomposite under accelerated UV-light irradiation, the

presence of MMT dramatically enhances the rate of photooxidation of EPDM with a

shortening of the oxidation induction time, leading to a decrease in the durability of the

nanocomposites. But in EPDM/nanocomposite with stabilizers, the addition of stabilizers,

either Tinuvin P or 2-mercaptobenzimidazole inhibits the degradative effect of MMT

[62].

Peiyao et al. have reported exfoliation of the organoclay particles for

EPDM/organoclay (bentonite) nanocomposites prepared by melt extrusion in a twin-

screw extruder. The tensile strength at 3phr clay had five-fold increase compared to pure

51

EPDM and 2.8 times compared to that prepared by direction blending; furthermore it was

above that of carbon black composites with 15.0 phr. The addition of clay also increases

the thermal properties and improved the processability of the terpolymer as a result of the

decrease of mooney viscosity [63].

The directional and parallel arrangement of kaolinite sheets bring about

improvements in the tensile properties and heat resistance performance [64-66].

The increase in t10 (temperature at 10% weight loss) of EPDM/nanokaolin blends

is of benefit with regard to mixing, rolling, extrusion, molding, and filling into molds and

it also improved the productive efficiency, which is good with regard to prophase

vulcanizing operation. The elasticity and tensile strength were sharply improved, and

facilitated the curing up to the optimal integrated properties in a short sulfuration time,

which advanced the productive efficiency and saved processing energy [67, 68].

A novel flame retardant system composed of nano-kaolin and nano-HAO (nano-

sized hydroxyl aluminum oxalate) was proposed by Zhi-Hong et al. who has established

that adding nano-kaolin enhanced the thermal stability of the composites. The synergistic

effect of nano-kaolin and nano-HAO on flame retarding LDPE/EPDM composites was

attributed to the improvement on rebuilding compact char barrier [69].

Qinfu Liu et al. have shown that Nano Kaolin (NK) can greatly improve the

vulcanizing process by shortening the time to optimum cure (t90) and prolonging the

setting-up time (t10) of cross-linked rubbers, which improves production efficiency and

operational security. NK sheets were well dispersed in the rubber matrix in the parallel

arrangement providing good interfacial interactions between the NK and the rubber

chains giving these rubber composites excellent processability, mechanical properties,

52

thermal stability, and elastomeric properties. The tensile strengths of the rubber/NK

composites are close to those of rubber/precipitated silica (PS) composites, but the tear

strength and modulus are not encouraging [70].

Wake has demonstrated that the intrinsic adhesion between fibre and rubber arises

from primary forces, chemical or van der Waals forces and in order to maximize

adhesion it was found that the fibre needed to be embedded, before the interfacial shear

strength exceeded the tensile strength [71, 72].

Rajeeva et al. have shown that the introduction of melamine fibre improves the

thermal and ablative properties of rubber-fibre composites based on EPDM, maleated

EPDM and nitrile rubber composites [73].

Formulations of EPDM rubber containing mixtures of aluminum hydroxide

(ATH) and carbon black (HAF) as fillers were developed by Cristine et al. in which the

addition of this filler leads to deterioration in both electric and flammability properties.

Only the blend with 7.5phr carbon black meets the requirements of flame resistance and

good electrical properties, along with an adequate mechanical performance for

application as insulation in electric wires and cables [74].

EPDM rubber-fullerene (EPDM/C60) composite, partially crosslinked by

ultraviolet (UV) radiation, prepared by Wonseop et al. showed unsaturation of EPDM,

along with formation of oxidation products of C60, such as epoxide, keto, aldehyde and

carboxylic groups and also an increase in the glass transition temperature peak of UV-

cured EPDM. The UV exposure reduced the thermal decomposition temperature of

EPDM/C60, pristine EPDM and dicumyl peroxide (DCP)-cured EPDM. The UV-

irradiated EPDM/C60 composite showed higher tensile strength and elongation at break

53

than that of DCP-cured EPDM but oxidation of C60 during longer exposure time reduced

the tensile strength and elongation at break [75].

Lorenz et al. have showed that when ethanol is used as dispersion agent, the

electrical percolation threshold was found to decrease, compared to ‘‘dry” mixing,

correlating with improved optical dispersion. The mechanical reinforcement in rubbers is

relatively high for 3phr of CNT which can partly be attributed to hydrodynamic

reinforcement by the presence of CNT agglomerates. For all systems they observed

significantly steeper stress–strain curves by addition of 1.6 vol% CNT to the systems

with conventional fillers. In EPDM, the increase in stress by silica or Carbon black (CB)

approximately adds to that of the CNT and the failure strain decreases by additional CNT

which was attributed to an insufficient dispersion of CNT agglomerates, i.e. crack

initiation at larger CNT agglomerates. The electrical conductivity of the hybrid filler

systems was found to be generally larger, compared to the pure CNT or CB composites

due to higher shear forces in the hybrid systems [76].

El-Tantawy et al. have prepared EPDM/TiC composites as thermistors, with new

double negative and positive temperature coefficients of conductivity (NTCC/PTCC), but

the electrical properties of EPDM composites are strongly affected by TiC content, and

exhibit hopping conductivity and P-type semiconductor behavior. The dielectric constant

increases linearly with temperature, without any remarkable change in behavior. TiC

improves the thermal stability and microstructure core of the rubber matrix i.e. gave more

difference in the sample temperature for the same power. It also improves the thermal

cycles for a long time (about one day) at constant electric power and reproducibility core

of rubber composites [77].

54

Acharya et al. who synthesized partially exfoliated EPDM/Mg–Al layered double

hydroxide (LDH) nanocomposites by solution-intercalation using organically modified

LDH (DS-LDH) as nanofiller have reported molecular level dispersion of LDH

nanolayers and significant improvement of mechanical properties in EPDM/DS-LDH

nanocomposites (2–8wt %) with respect to neat EPDM. The thermal stability of

nanocomposites containing 3 wt% DS-LDH improved by ≈40oC when 10% weight loss

was selected as point of comparison. The first step in the decomposition process induces

the charring and can account for better fire safety of the nanocomposites. Optical studies

showed better clarity at lower DS-LDH loading in EPDM/LDH nanocomposites [78].

Bueche et al. have pointed out that tensile strength and tear strength are not

necessarily proportional to each other [79].

Saha et al. reported that asbestos fibre increases the thermal stability of the

respective composite as it increases the endothermic decomposition temperature and the

energy of activation for degradation, whereas Fe2O3 catalyses the degradation and cork

has a marginal effect [80].

The anodic properties of conductive polymeric composites based on the EPDM

and designated for cathodic protection depend on the content of admixed carbon black

because it affects the resistivity of the composite and simultaneously undergo gradual

oxidation during long-lasting polarization. For low carbon black content, a significant

increase in the anode current flow resistance occurs in time and is beneficial to introduce

larger amounts of conductive carbon material but, too high a content will damage the

processing properties of the polymeric mixture [81, 82].

55

Conductivity of conductive rubber composites changes significantly when

subjected to mechanical stress and strain as discussed in light of breakdown and

formation of the carbon black-rubber structure. It’s found that electrical resistivity of

strained samples depends on strain amplitude (% elongation) and frequency of the stress-

strain cycles. Moreover, there is similarity in the change of modulus and electrical

resistivity against degree of strain and frequency of strain for different samples [83].

Carbon black and short carbon fibre (SCF)-filled conductive composites were

prepared from EPDM rubber by Das et al. in which the resistivity of the composites

increases with increase in mixing time and rotor speed of the internal mixer whereas there

is a marginal decrease in resistivity with increasing mixing temperature. Moreover,

electrical resistivity decreases with increasing applied pressure upto a certain level for all

carbon black-filled composites, except EPDM-based carbon black composites. In

contrast, the electrical resistivity of short carbon fibre-filled composites gradually

increases with an increase in applied pressure [84].

The observations and findings of the various researchers who have done

pioneering work in EPDM-based polymeric composites, have formed the basis of the

present work.

56

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