Section-II: Nanocomposite polymers and elastomers...

Chapter 2 Present Theories and Practices

23

Section-II:

Nanocomposite polymers and elastomers- Processing and

properties

2.7 Rubber and the concept of nano-reinforcement

2.7.1 Natural and synthetic rubber

Although the use of rubber is proved since Olmec civilization (circa 1300-300

BC) [127], the term “rubber” was coined in 1770 by Joseph Priestley (1733-1804)

[128], a British chemist who discovered oxygen. Natural Rubber (NR), the only non-

synthetic rubber that accounts for almost 40 % of the world’s rubber consumption, is

a biopolymer found in the latex that comes from Hevea brasiliensis (botanical name)

trees originally from Amazon River valley [129-138]. Rubber trees grow to a height

of about 60 feet tall, in hot damp climates. Latex, extremely sticky and viscous fluid,

is collected in a cup mounted on each tree, by slashing the bark to reach the latex

vessels, which are like blood veins of the tree. The extracted liquid is 30-40% rubber

and may contain relatively high levels of organic and inorganic impurities. The latex

is dried and we have natural rubber [139]. NR consists of cis-1, 4-polyisoprene

molecule biosynthesized by carbon dioxide, making it carbon neutral and not

contributing in global warming [138]. At present 99 % of NR produced worldwide is

obtained from the domesticated rubber trees cultivation in Southeast Asia [138],

primarily in Malaysia, Indonesia, and Thailand. In India, rubber trees are found in the

southern coastal belt of Kerala.

In 1860, Greville Williams (1829-1910) obtained a liquid by distilling rubber

[140-141] with an empirical formula (C5H8) shown by Michael Faraday (1791-1867)

for rubber in 1829 [142]; he called it "isoprene". Gustave Bouchardat (1842-1918)

was the first to obtained synthetic rubber in 1879 but from natural rubber [143-

144], when he observed that heating isoprene with hydrochloric acid produced a

rubberlike polymer. However, the credit of first truly obtained synthetic rubber goes

to William Tilden (1842-1926), when he obtained isoprene by cracking turpentine in

1882 [145-146]. Synthetic rubber industry was grown by US during World War-II,

when the natural rubber supply from Southeast Asia was cut off. United States was

using about half the world's supply of natural rubber, most of it coming from

Southeast Asia. The construction of a military airplane, a tank and a battleship

requires half ton, one ton and 75 tons of rubber respectively, whereas each military


24

person requires 32 pounds (14.5 kg) of rubber for footwear, clothing, and equipments.

The U.S. government joined armed forces with the rubber companies, petrochemicals

industries, and academic research laboratories which results in an engineering

achievement. The partnership of the government, industry, and academe expanded the

U.S. synthetic rubber industry output from 231 tons of rubber per annum in 1941 to

70,000 tons a month in 1945 [147]. The commonly used synthetic rubbers for making

footwear, adhesives, tires, treads, technical goods etc. are shown in Table 2.2.

2.7.2 Types of reinforcement

Rubber, technically elastomer, is a unique material which is both elastic and

viscous [148] and consists of innumerable microsized spring-mass dashpot systems.

The spring justifies the elasticity which obeys Hooke’s law and increases with

increase in stress whereas the dashpot exhibits viscosity which follows the Newton’s

law of viscosity and increases with the strain rate.

Table 2.2 Synthetic rubber- commercial and chemical names with ASTM classification

Sr.

No. Common names Chemical definition

ASTM D-2000

Classification

1 Buna-N, Nitrile, NBR Butadiene Acrylonitrile BF, BG, BK

2 Neoprene® Polychloroprene BC, BE

3 EPR, EPT, EPDM Ethylene Propylene CA

4 Silicone Polysiloxane FC, FE, GE

5 Fluoro-silicone Fluoro-silicone FK

6 Fluoro Elastomers Fluorinated Hydrocarbon HK

7 SBR, GRS Styrene Butadiene AA, BA

8 Urethane, Polyurethane Polyester/ Polyether Urethane BG

9 Butyl Isobutylene Isoprene AA, BA

10 Hydrin Epichloro-hydrin CH, DK, DJ

11 Hypalon Chlorosul-fonated/ polyethylene CE

Some of these elastomers strain crystallizes and exhibit high tensile strength,

flexibility, and tear strength with outstanding resistance to fatigue [149] and the others

require reinforcement at microstructural level to obtain the adequate static and

dynamic characteristics. Polymer and elastomer nanocomposites reinforced by

relatively small amounts of ultrafine nano-particles proved exceptionally promising

engineered materials with unexpectedly amplified properties [150]. The composition

of nanofillers in polymer matrix may vary from 1 % to 10 % (in mass) or more and


25

are incorporated in addition to traditional additives and vulcanization agents. The

addition of the nanoparticles at very low concentrations (~0.2 weight %) in polymers /

elastomers significantly improves flame retardancy [151] and compressive and

flexural mechanical properties [152-153].

2.7.2.1 Reinforcement based on structure of filler

The structure of nanofillers, based on the shape and size, may be of three

types: zero dimensional (nanoparticles/ quantum dots/ nanocrystals), one dimensional

(nanorods/ quantum wire/ nanofibers/ whiskers) or two dimensional (nanofilms/ thin

films/ quantum well/ nanolayers/ nanoclays/ nanosheets/ nanoplatelets). Figure 2.6

illustrates the three types of nanofillers.

Among zero dimensional nanofiller are the metal and metal oxide nanoparticles such

as silver [154-156], gold [157-158] and zinc [159] fillers, titania (TiO2) [160-161],

alumina (Al2O3) [162], zirconia (ZrO2) [163] fillers etc. SEM and TEM images of

silver nanoparticles reinforced in elastomeric matrix are shown in Figure 2.7 [156].

Each of these fillers has its own features, advantages and limitations and the

choice of a specific type depends upon the application defined. Polyurethane (PU)–

Nanosize dimension

0 Dimensional 1 Dimensional 2 Dimensional

Figure 2.6 Three types of nanofillers

Figure 2.7 a) SEM images of silver nanoparticles filled elastomer; b) TEM images. Reproduced

with the permission of Ref. [156] © Nature


26

titania NCs, for example, show excellent piezoelectric behavior [160]; whereas the

refractive index of TiO2 nanoparticles filled polyglycidyl methacrylate (PGMA)

increases linearly from 1.5 to 1.8, depending on nanosized titania composition [161].

Similarly Jose and Thomas [162] have reported alumina-clay- polyethylene ternary

hybrid nanocomposite in which 1:1 ratio of Al2O3 and clay exhibits 100% and 208%

increase in tensile strength and Young's modulus respectively but the nanocomposite

containing Al2O3-clay ratio 2 : 1 shifts the properties to the negative hybrid effect

region due to the steric effect of alumina clusters.

One-dimensional (1-D) nanometer-sized materials, such as nanorods,

nanowires, carbon nanotubes (CNTs) etc., have attracted considerable attention of

researchers because of their great potential for modification in fundamental physical

properties as well as for applications as fillers in functional materials. Cadmium

selenide (CdSe) semiconductor nanorods reinforced in conjugated polymer poly-

3(hexylthiophene) can be used as efficient hybrid solar cells [164]. Many researchers

have reported reinforcement of CNTs in natural / synthetic rubber [165-168] and other

polymer matrix [169]. Multi-walled carbon nanotube filled silicone rubber composite

was suggested for pressure sensing applications [168]. The Young’s modulus and

tensile strength of the CNTs reinforced epoxy nanocomposites increases upto 716%

and 160% respectively compared to pure epoxy [169].

High-performance lightweight composites could be developed and tailored to

specific applications through reinforcement of ultrathin films of strong and highly

stiff materials in a polymer matrix. Poly (methyl methacrylate) reinforced with

graphene sheet shows a considerable uptrend in modulus, ultimate strength and

thermal stability and shifts the glass transition temperature over 40 °C [170].

2.7.2.2 Reinforcement based on morphology of filler

The final performance and properties of nanocomposite depends on various

factors such as selection and design of nanofillers, their shape, size and modification,

nanostructure morphology, processing method etc. The microstructural morphology

of nanocomposites can be classified in the following three types, as shown in Figure

2.5 depending upon the dispersion and geometrical orientation of fillers relative to the

elastomeric chains:

i) Randomly dispersed structure [171]

ii) Intercalated structure [172-173]


27

iii) Exfoliated structure [172, 174]

When the reinforced phase is dispersed in the form of individual particle or

aggregates in the form of clusters in the matrix then it is phase separated or randomly

dispersed structure. Intercalated structure consists of the polymer molecular chain

separated by nanoclay layers and exfoliated morphology has randomly dispersed

fillers between the polymer chains. When the degree of intercalation is much

enhanced, the structure becomes exfoliated.

Natural Rubber (NR) nanocomposites with graphene [175] and TEGO (Thermally

Exfoliated Graphite Oxide) [176] increase the electrical conductivity and cause an

important enhancement on the mechanical behavior of NR. Styrene–butadiene

rubber/clay nanocomposite can enhance the modulus eight times at 300% strain and

tensile strength seven times compared with cured pure rubber [177].

Intercalated structure

Randomly dispersed structure

Exfoliated structure

Exfoliated

nanoclay

Intercalated

nanoclay

PDMS surface

Cross linking

Dispersed NPs

NPs cluster

Rubber molecular chain

Gold NPs

a

f e

d c

b

Figure 2.8 Dispersed, intercalated and exfoliated morphological structure of nanocomposites.

a) and b) Schematic and TEM micrograph of randomly dispersed NPs [171], © 2009 IEEE; c) and

e) Schematic of intercalated and exfoliated structures [172]; d) TEM micrograph of intercalated

structure [173] © John Wiley & Sons ;f) TEM micrograph of exfoliated structure [174] © RSC


28

2.7.2.3 Reinforcement of composite nanofillers

Till date few researchers have practiced composite nanofillers with core-shell

structure in polymer / elastomer matrix for engineering applications [178-185] as

illustrated in Figure 2.9 [186].

Core–shell silver nanoparticles coated with Ni0.5Zn0.5Fe2O4 spinel ferrites at different

ferrite/silver ratio were synthesized and reinforced in polyurethane matrix and applied

as microwave absorber [178]. In a similar study, core shell rubber (CSR) and CNTs

were reinforced in benzoxazine–epoxy–phenolic (BEP) to form a ternary system and

observed that the addition of CNT and CSR nano-fillers increased the toughness and

flexural strength of the BEP system by 160% and 30% respectively with slight

increase in glass transition temperature [179]. ZrTa core-shell nanofillers dispersed in

a Nafion® were found thermally stable up to 170 °C and used for low relative

humidity fuel cells [180].

2.8 Preparation, compounding, vulcanization and

reinforcement in elastomers:

Rubber, as commercially available, is a complex system with multiple

components besides the rubber molecules of the latex, such as curing agents,

coagulants, processing aids, accelerators, reinforcements and fillers etc. The special

and complicated behavior of rubber shows so many uncertainties and contradictions

in the literature on the part of vulcanization [187-188], since the discovery of sulfur

vulcanization by Goodyear [129], perhaps being a multicomponent system. In its

original form, natural rubber has limited usefulness being extremely sticky at elevated

temperature and brittle in cold condition. The rubber molecules must be cross-linked

or vulcanized termed as elastomer after processing, to develop the toughness and

strength normally associated with it. From the point of collection as latex to its

Figure 2.9 TEM image of Fe-SiO2 core-shell NPs filled Polyurethane. Reprinted with permission

from Ref. [186]. © (2011) American Chemical Society


29

commercial application NR undergoes a complex processing, known as vulcanization

derived from Roman word ‘Vulcan’ means ‘the God of Fire’ [189], to enhance its

usefulness. The following terminology is commonly associated with this procedure.

Vulcanization: Vulcanization is a chemical process for converting natural rubber

or related polymers into more durable materials via the addition of sulfur [190] or

other equivalent curatives or accelerators. These additives modify the polymer by

forming cross-links (bridges) between individual polymer chains [191].

Cross-linking: A cross-link is a bond that links one polymer chain to another.

They can be covalent bonds or ionic bonds. "Polymer chains" can refer to synthetic

polymers or natural polymers (such as proteins or NR) [192].

Scorch and Curing: These are the two transient states in the vulcanization process

of rubber, based on the ingredients and process parameters, which finally decides the

rheological, mechanical, thermal and chemical properties of rubber [193-194].

Vulcanization agents: These are the compounding ingredients added to activate,

accelerate and decide the degree of vulcanization process and enhance the properties

of rubber by curing [193].

Fillers: These are generally carbon black, clay, or calcium carbonate powders in

the form of micro or nanosize particles added to rubber in order to improve its

properties and reduce formulation costs. They can increase tensile strength, hardness,

and resistance to tear and abrasion.

Cross-linking is a chemical process of inter-linking the separated long polymer

molecular chains, generally with the addition of Sulfur as curing and cross-linking

agent, by applying heat and pressure. After cross-linking the separated chains become

a single unit and the vulcanization ingredients makes the rubber applicable for various

engineering applications. The accelerators and activators speed up the vulcanization

process to optimize compound properties with minimum cure time. This processing

decreases hot stickiness, cold brittleness, increases elasticity, and makes rubber less

sensitive to changes in temperature. When ultrafine nanosize fillers are specially

prepared and reinforced in rubber during vulcanization to develop some specific

properties then it becomes a nanocomposite rubber. In nanocomposite rubber, the

nanofillers are intercalated or exfoliated in the intergallery spacing of elastomeric

matrix as shown schematically in Figure 2.8-a, c and e.

http://en.wikipedia.org/wiki/Polymer

http://en.wikipedia.org/wiki/Covalent_bond


30

In a text edited by Thomas and Stephen [194], Gatos and Kocsis have

discussed in a chapter that there are three methods most commonly applied to prepare

rubber nanocomposites.

Solution intercalation / mixing (solvent method)

Latex compounding / In situ intercalation (water-assisted technique)

Melt mixing (direct compounding method)

In the following sections, these methods of preparing nanocomposite elastomers are

briefly discussed from the reports of the pioneers of the field.

2.8.1 Solution intercalation / mixing (solvent method)

This method is based on a solvent solution system in which the elastomer is

soluble. The nanofillers are dispersed in the solvent by stirring, when rubber is

dissolved in the solvent. The solvent is removed by evaporation and the rubber

structure reinforced with the nanofillers is obtained [195-198]. In this method, the

curatives agents are generally added in the compound after the evaporation of the

solvent and the compound is mixed on an open mill [199-200] or in an internal mixer

[201-202]. Addition of the curatives during the solvent solution mixing can also be

practiced by few researchers [203-204]. Methyl ethyl ketone was used to blend the

NBR with the clay modified with dimethyl dehydrogenated tallow quaternary

ammonium salt and mixed on two-roll open mill after evaporation of the solvent. The

obtained NBR/organoclay nanocomposite after vulcanization shows increased

mechanical performance and decreased water and methanol permeability up to 85%

and 42%, respectively [198]. Lo´pez-Manchado et al. [199] have applied solution and

melt compounding both methods to formulate NR/organophilic MMT nanocomposite

and compared the structure and properties of NC produced by two procedures. MMT

was intercalated initially with octadecylamine (MMT-ODA) and toluene was used as

solvent. It was observed that the dynamic mechanical properties, compression set and

hardness in NC produced by the solution technique is superior; however the structure

of NC in both cases were found similar. Ethylene propylene diene rubber (EPDM)

reinforced with 10 phr montmorillonite modified with octadecylamine (MMT–ODA)

was prepared on internal mixer and compared by varying the process parameters such

as mixer type, rubber recipe, temperature etc [200-201]. Similarly NR/ Onium ion-

modified montmorillonite nanocomposite was prepared with different nanofiller

loadings by melt compounding in internal mixer and structural, rheological and


31

mechanical properties were compared [202]. Natural rubber was dissolved in toluene

with MMT modified with primary or quaternary intercalants bearing different

hydrocarbon alkyl tails and subsequently the curing ingredients were mixed in the

solution. After drying the solvent the compound was homogenized on an open mill

and predicted that the nanocomposites produced by primary amines show better

mechanical performance than quaternary amine intercalants [203]. Lu et al. [204]

have practiced a thermal activation of the solvent-mixed compound when the solvent

evaporation was not complete and its molecules were still present within the silicate

galleries to form exfoliated structure.

2.8.2 Latex compounding / In situ intercalation

(Water-assisted technique)

The submicronsized rubber particles of latex are dispersed in water, which is

used as host medium. The nanofillers can be added directly to the rubber latex or in its

aqueous dispersion (slurry). Most of the nanofillers, such as layered silicates are

strongly hydrophilic and adsorb water molecules, which is associated with an

expansion of their intergallery spacing. Polymerization can be initiated either by heat

or radiation, by the diffusion of a suitable initiator. The compound is mixed by

stirring, then cast in a mould and left to dry. The rubber curatives may or may not be

added during latex blending. After casting and drying the nanocomposite can be cured

accordingly. NR/layered silicate vulcanized nanocomposites were prepared by

compounding the dispersions of clays and other latex chemicals necessary for

vulcanization. The nanocomposites were subjected to structural, mechanical, thermal

and swelling tests [205]. SBR-modified clay (MMT) nanocomposite was prepared by

mixing SBR latex and the curatives were added to the dried rubber mix in a melt

mixing step followed by vulcanization [206]. A similar SBR / MMT with pristine NC

was formulated through latex mixing method but with a minor modification. SBR

latex with pristine MMT (cation: Na+) mix was dried and subsequently the melt was

compounded along with curatives with a possible intercalant for the MMT (hexadecyl

trimethyl ammonium bromide) and coupling agent (3-aminopropyl triethoxy silane)

[207]. Similarly, Polyurethane (PUR) prevulcanized NR latex with clay [208] and

NR/PUR/layered silicate [209-210] nanocomposites were practiced by many

researchers through latex compounding method.


32

2.8.3 Melt mixing (direct compounding method)

This method involves equipment like internal mixers and open mills alongwith

annealing, statically or under shear, a mixture of the elastomer, curatives and

nanofillers above the softening point of the elastomer. The mixed compound is

homogenized through melt and mold by using heat and pressure. This method has

remarkable advantages as below over the other two methods discussed in the previous

sections.

Since the organic solvents are not involved, this method is environmentally

benign.

It can be performed with current industrial process, such as extrusion and

injection molding.

This process can be applied for the elastomers which were previously not

suitable for solution mixing or latex compounding.

The process is simple and controlling process parameters is comparatively

easy, hence economically feasible.

The intercalation / exfoliation phenomena are possibly governed by the chemistry

involved during compounding and curing. Intercalation is promoted when the

curatives in a mix are introduced prior to curing. The vulcanization curatives migrate

into the intergallery space during mixing, which widens the basal spacing and thus

supports the nanocomposite formation [211]. High shearing should be provided by an

internal mixer in order to obtained intercalated / exfoliated nanocomposites with

outstanding performance [212]. Many scholars have followed the melt mixing and

compounding technique to prepare rubber / organoclay nanocomposites [213-218]

and agreed that the final dispersion state of the nanofillers in the rubber matrix

depends upon the choice of rubber matrix, the type of curatives and modification

treatment of the fillers.

2.9 Characteristics of nanocomposite rubber

Nanocomposites consisting of an elastomer and nanofillers (modified or not)

frequently exhibit remarkably enhanced mechanical and materials properties when

compared to those of without nanofillers. Higher modulus, increased strength, heat

resistance and flame retardancy, increased biodegradability and decreased gas

permeability are few of the common properties trend reported for the rubber


33

nanocomposites. The major cause of these improved properties in nanocomposites is

the stronger interfacial interaction between the matrix and reinforced nanofillers,

compared with conventional filler-reinforced systems.

2.9.1 Cross-linking and rheology

Cross-linking agent or compatibilizer is introduced in a nanocomposite

purposely to improve the surface adhesion of filler as well as to reduce the surface

tension between immiscible polymers and filler. The cross-linking agents make the

rubber rigid, strong and hard and prevent segregation of the elastomeric molecular

chains. This is done to obtain the optimum filler dispersion which will enhance the

properties of nanocomposite [219-221]. The property enhancements include increased

operating temperature, improved mechanical properties, and increased chemical and

solvent resistance. Chain cross-linking and scission are the two reactions that occur

during curing and vulcanization of rubber which are influenced by the molecular

structure of rubber. Elastomers may typically undergo simultaneous scission and

cross-linking, or one or the other clearly predominating [222]. Electron beam

irradiation was used to enhance the mechanical properties of cross-linked rubber. The

degree of cross-linking is proportional to the radiation dose [223]. Crosslinked natural

rubber (NR) nanocomposites were prepared using cellulose nanowhiskers

(CNWs) using two-roll mill and subsequently cured to introduce crosslinks in the NR

phase [224]. Mechanical, tribological and rheological properties of rubber mainly

depend on cross-link density [225].

The crosslink density is experimentally determined by torque rheometry in which

the difference between maximum torque and minimum torque is the decision criteria.

It is known that the torque difference can be indirectly related to the crosslink density

of the blends. Consequently the degree of crosslinking [226] in the rubber blends is

determined using the rheometric data. Crosslink density is a significant structural

parameter for cured rubbers. Natural and synthetic rubber vulcanizates with different

crosslink densities are obtained by varying sulfur and accelerator amounts and

accelerator types. The crosslink density was characterized by an H-NMR (Hydrogen-

Nuclear Magnetic Resonance) technique and its influence on mechanical properties,

such as Shore A hardness, 300% modulus, tensile strength, and elongation at break, of

NR vulcanizates was investigated by Zhao et al. [227].


34

2.9.2 Mechanical properties

One of the main advantages of rubber nanocomposites is the amplification in

the mechanical properties of the rubber matrix at relatively low filler content. Thus,

nanoparticles, NcPs and layered silicate can be considered as potential substitutes of

carbon black and silica, which have to be mixed at high concentrations in order to

reach the same performance [228]. The mechanical properties needed to decide the

practical applicability of nanocomposite rubber can be classifies into the following

two categories.

Static mechanical properties

Dynamic mechanical properties

2.9.2.1 Static mechanical properties

The static mechanical properties of elastomers are measured by plotting load–

deformation or stress–strain relationships applying a uniaxial force on a dumbbell or

any other standardized shaped specimen. The nature of applied force may be tensile,

shear, torsional, compressive or bending. The result is plotted in the form of

characteristic stress–strain curve or stress relaxation, which is very important in

determining the applications and limitations of an elastomer. There are number of

tests an elastomer undergoes depending on the application, few are discussed below

with this study orientation.

Tensile test: Tensile test is generally conducted on universal testing machine by

cutting the specimen in dumbbell shape. An extensometer attachment is required due

to very large elongation of elastomer prior to fracture. In the standard (ASTM: D-412-

68) [229] procedure the speed of ram is defined alongwith the shape, size and gauge

length of the specimen. The stress-strain behavior of polymers is nonlinear and

exhibit curvature at higher stresses. The initial slope of the stress–strain curve gives

the idea about the modulus of the material. The area under the curve indicates the

strain-energy stored prior to fracture the sample and is directly related to the

toughness of the material. A good elastomer is capable of exhibiting the maximum

elongations as much as 1000% before failure. Microscopically the mechanical

properties are a direct reflection of the mobility of the rubber molecular chains [230-

231]. The characteristics which can be estimated from tensile test are modulus, %

elongation, tensile strength, resilience, toughness etc.


35

Hardness test: The Hardness of an elastomer or polymeric material can influence

the overall performance in its ultimate product use. Hardness measurement provides

an idea about the ability of the material to recover after being indented through the

applied force. There are various test methods commonly applied to evaluate and

benchmark these results. Durometer is the instrument used for determining the

hardness of elastomers also known as durometer hardness. The hardness of an

elastomer is indicated as Shore A, Shore D and Shore M measurements. Hardness is

measured when a force is applied to the sample with an indentor under specific

conditions. The indentation hardness is then determined based on the elastic modulus

and viscoelastic behavior of the sample.

Abrasion and wear test: Abrasion resistance is the ability of a material to resist

mechanical action such as rubbing, scraping, or erosion that tends progressively to

remove material from its surface. In abrasion testing, the rubber specimen is kept in

contact with a rotating ceramic wheel and the wear is measured from the difference in

the thickness of the specimen before and after 10 hours of abrasion [232]. Abrasion

may be measured in terms of the percentage of material lost, either by mass or by

volume, between the start and end of the test [233]. In spite of its practical

importance, amongst the various types of failures of rubber abrasion is perhaps the

least understood phenomena, as it is difficult to predict the abrasion behavior from

other rubber properties [234].

Creep test: The creep test is simple but time dependent; a constant force is applied

to the rubber and the transient change in deformation is monitored. The data available

for creep of filled natural rubber is less, which indicates less interest of researchers in

investigating this characteristic. However, for specific applications such as bridge

bearings a considerable amount of data is generated for unfilled rubber [235].

Fatigue test: Fatigue may be defined as change in the properties of a material

caused by prolonged action of cyclic stress. Still there is no standard available for

fatigue testing of rubber. Few researchers have even practiced fatigue life estimation

of filled rubbers.

2.9.2.2 Dynamic mechanical analysis

Dynamic mechanical analysis (DMA) or Dynamic mechanical thermal

analysis (DMTA) measures the response of a given material subjected to a sinusoidal

strain as a function of temperature and frequency. DMA results are analyzed by three


36

parameters, (a) the storage modulus (E’) (b) the loss modulus (E”) and (c) the loss

factor tan δ; the ratio E”/E’; useful for determining the occurrence of molecular

mobility transitions, such as the glass transition temperature Tg. DMA has been used

to study temperature dependency of storage modulus of nanocomposite under

different experimental conditions. The modulus of an elastomer in glass and rubbery

states can be evaluated and the temperature dependency of these parameters can be

quantified by DMA. This technique is ideally suited for the detection of phase

separation in polymers, for which two values of Tg may be observed. These tests are

highly sensitive to morphology, crystallinity, orientation, degree of cure in

thermosets, dependence on molar mass in thermoplastics and miscibility of

multiphase systems including composites and polymer blends [236-237].

2.9.3 Thermal properties

Thermal performance of elastomers and polymers are determined commonly

by two tests; Thermogravimetry analysis (TGA) and Differential scanning calorimetry

(DSC).

Thermogravimetry analysis (TGA): TGA is the study of the relationship between

mass of the sample with temperature, which assists in determination of temperature

dependent physical and chemical characteristics, such as evaporation, thermal

degradation etc. Polymers thermal stability is identified and distinguished in terms of

temperature range, extent and kinetics of decomposition by using very minute

quantity of the sample [238].

Differential scanning calorimetry (DSC): Some application oriented processes do

not result in change in mass of polymers, in such cases simultaneous measurement by

DSC is useful. In DSC, glass transition temperature (Tg) is a measure of the thermal

energy required to allow polymer motion involving 10 to 15 monomeric units and

corresponds to the softening of a polymer. Volatile decomposition products may be

detected and identified in order to elucidate the mechanism of mass changes [237].

Thermal analysis is used for quantitative compositional analysis of polymers and their

kinetics, to predict the behavior and performance in all stages of polymer

development, fabrication and component testing. A comparative data is illustrated

about different types of elastomers in Table 2.3 [239].


37

Hyp

alo

n

CE

Chlo

rosu

l-

fonat

ed/

poly

ethyle

ne

45 -

100

1000 -

3000

500

Fai

r

Fai

r

E

xce

llen

t

Good

Fai

r to

Good

Fai

r to

Good

-30°

to -

60

°

to 2

25°

Exce

llen

t

Exce

llen

t

Hyd

rin

CH

, D

K,

DJ

E

pic

hlo

ro-

hydri

n

40 -

90

500

-250

0

350

Good

Good

Good

Good

Good

Exce

llen

t

-15°

to -

50

°

to 2

25°

Good

Good

Bu

tyl

AA

, B

A

Isobuty

lene

Isopre

ne

40 -

90

500 -

3000

850

Fai

r to

Good

Fai

r

Fai

r

Good

Poor

Poor

-10°

to -

60

°

to 2

50°

Exce

llen

t

Good

Ureth

an

e,

Poly

ureth

an

e

BG

Poly

este

r/

Poly

ether

Ure

than

e

35 -

100

500 -

6000

750

Poor

Good

Exce

llen

t

Exce

llen

t

Poor

Good

-10°

to -

30

°

to 1

75°

Exce

llen

t

Fai

r to

Good

SB

R,

GR

S

AA

, B

A

Sty

rene

Buta

die

n

e

30 -

100

500

-300

0

600

Good

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38

Active nanoparticles can form physical or chemical bonding with rubber

matrix and enhance exceptionally the mechanical properties of the nanocomposites.

The chemical bonding may develop a solid solution like strengthening mechanism by

creating a solute-solvent relationship between the reinforced nanoparticles and rubber

base [240]. The development and characterization of these outstanding properties are

among the major objectives of the materials science in the current millennium.

2.10 Nanocomposite NR as vibration dampers and isolators

2.10.1 Damping theory and determination methods

Vibration is any kind of repeated motion about a mean position which

involves mass, motion and elasticity [241], and damping is a mechanism by which

kinetic energy of vibration is converted into sound or heat [242]. In damping basically

the energy is absorbed in the damper, stored or lost, hence the intensity (amplitude) of

the oscillating object diminishes. Damping is always present in all real systems [243].

Vibration damping can take place due to variation in either microscopic or

macroscopic parameters of the system. At microscopic level every material consists of

atomic arrangement and each atom itself occupies multiples of empty space as

compared to its actual dimensions to cause structural or hysteresis damping. At

macroscopic level the system elements in contact can provide the vibration damping

because of frictional forces or viscous forces. The vibration damping characteristics of

nanocomposite elastomers can be determined by various methods described in the

following sections with the related terminology.

Loss Factor (tanδ): It is defined as the ratio of Loss (Damping/ Viscous-𝐸")

modulus to the Storage (Elastic-𝐸′) modulus. The higher the loss factor, the higher is

the damping a material can provide [244].

𝑡𝑎𝑛𝛿 =𝐸"

𝐸′

where;

𝐸" =𝐹

𝐴⁄

𝛿𝐿𝐿⁄

𝑠𝑖𝑛𝛿 = 𝐼𝑚𝑎𝑔.𝐹

𝛿𝐿∗

𝐿

𝐴

and 𝐸′ =𝐹

𝐴⁄

𝛿𝐿𝐿⁄

𝑐𝑜𝑠𝛿 = 𝑅𝑒𝑎𝑙.𝐹

𝛿𝐿∗

𝐿

𝐴

Dynamic Stiffness 𝐹

𝛿𝐿= 𝑘

Hence, the above equations are modified as;


39

𝐸" = 𝑘 ∗𝐿

𝐴∗ 𝑠𝑖𝑛𝛿

𝐸′ = 𝑘 ∗𝐿

𝐴∗ 𝑐𝑜𝑠𝛿

The two different moduli are due to visco-elastic nature of rubber. The in-phase

response of the material describes elastic behavior and indicated by storage modulus,

whereas the out-of-phase response describes viscous behavior and indicated by loss

modulus. The overall response is expressed by Complex modulus (𝐸∗) of rubber as

shown by Eq. (2.1);

𝐸∗ = 𝐸′ + 𝑖𝐸"

𝐸∗ = 𝐸′(1 + 𝑖𝑡𝑎𝑛𝛿) (2.1)

In the above relations F, A, δL and L are respectively force applied, cross-sectional

area, change in length and original length.

Stiffness (k): In general, stiffness is the load required for unit deflection, but for

rubbery materials it becomes a complicated phenomenon to describe. Visco-elastic

property of the material defines two types of stiffness as static and dynamic. The

static stiffness corresponds to the load deflection relationship, which is approximately

linear in tension as well as in compression for low values of strains. The energy

dissipated during loading and unloading is represented by the area under hysteresis

curve. The dynamic stiffness of rubber is a function of forcing frequency and slowly

increases with increase in frequency.

Damping coefficient (c): Damping coefficient is an inherent property of any

material and characterized by viscosity and / or velocity (frequency). Interestingly, in

most of the vibration related literature even if the value and behavior of damping

coefficient is investigated, its exact definition is not expressed. In this study, it may be

defined as the inherent property of a material to dissipate the energy at atomic and

molecular level. It is expressed in the equation of motion by c and consists of

variables depending upon type of the system and damping, few of them are

exemplified as under;

𝑐 = 𝜇𝐴

𝑡 Viscous damping

𝑐 = 𝜇′𝑅𝑁 Coulomb damping

𝑐 =𝛼

𝜋𝜔 Structural damping


40

In the above equations μ, A and t are viscosity, area in contact and thickness of

viscous layer; μ, and RN are coefficient of friction and normal reaction and α and ω are

material constant and forcing frequency respectively. In case of visco-elastic materials

such as rubber, the damping coefficient is temperature and frequency dependent. It

varies very slowly with the temperature within the working temperature range. Unlike

other properties of rubber the exact value or range of damping coefficient is not

generally expressed in the literature, but most of the researchers have expressed in the

form of temperature dependent characteristic curves determined through DMA test.

Damping Factor (Damping Ratio) (ζ): Damping factor is a mathematical term to

describe the level of damping of the system and defined as the ratio of damping

coefficient to the critical damping coefficient.

ζ=𝑐

𝑐𝑐

Critical damping coefficient cc is given by;

𝑐𝑐 = 2𝑚𝜔𝑛

In case of rubber, most of the researchers have discussed the damping capacity with

respect to loss factor because of its moduli complexity and not by damping factor. Its

value also varies with temperature and forcing frequency. The numeric values of the

above four vibration damping properties of natural rubber at normal temperature are

shown in Table 2.4 with the respective references cited.

Table 2.4 Vibration damping properties of Natural Rubber

Sr.

No. Name of the Property Unit

Value

Reported References

1 Loss Factor (tanδ) No unit 0.1-1.0 [245-246]

2 Static Stiffness (ks) N/m (.103) 21.198

689.23

[247]

[248]

3 Dynamic Stiffness (kd) N/m (.106) 0.391

2.156

[247]

[248]

4 Damping Coefficient (c) N-sec/m 2803.15 [249]

5 Damping Factor (ζ) No unit 0.05 [250]

Variations in the data reflect uncertainties and the reason of non-standardizing these

parameters. Following methods are used for calculation of vibration damping

properties of filled or unfilled rubbers.


41

2.10.1.1 Energy Method

In this method the damping parameter tanδ is defined as ratio of the amount of

energy dissipated by the system at a certain frequency to the amount of the vibrational

energy that remains in this system at the same frequency [251] and given by;

𝑡𝑎𝑛𝛿 =𝐸𝑠

𝑈

where; 𝐸𝑠 = 𝜋𝑐𝜔𝑋2 (2.2)

𝑈 = 1

2𝑚𝜔2𝑋2 (2.3)

Es = Energy dissipated per cycle

X = Maximum deflection (Amplitude)

U = Maximum strain energy of vibration

c = Damping coefficient

Hence the loss factor is given by;

𝑡𝑎𝑛𝛿 =2𝜋𝑐

𝑚𝜔 (2.4)

The load-deflection curve for rubber shows that the behavior as it is loaded is not the

same as when it is unloaded. The curves form a hysteresis loop as shown in Figure

2.10. A load deflection hysteresis loop for one cycle is drawn to find the energy

consumed and lost during the cycle. The respective area under the diagram represents

the total vibrational energy and the energy absorbed due to damping [245]. The

hysteresis curve for general stress-strain behavior, filled rubber and shape memory

alloys are found to follow different shapes [242].

Lo

ad

Deflection

Figure 2.10 Hysteresis loop form during loading-unloading cycle of rubber


42

2.10.1.2 Logarithmic Decrement Method

Damping factor (ζ) is the measure of the damping level. The successive

amplitudes from the transient output response of the system are measured to find the

rate at which the amplitude is decaying. The damping factor is directly related with

logarithmic decrement δ as given below [252];

ζ=𝛿

√(4𝜋2+𝛿2) (2.5)

where; δ =1

nloge (

x

xn+1) (2.6)

2.10.1.3 Half-Power Bandwidth Method

The loss factor of a damping system is also determined by half-power

bandwidth method [253] as specified below with the help of Figure 2.12.

𝑡𝑎𝑛𝛿 =∆𝜔

𝜔𝑛 (2.7)

∆𝜔 = 𝜔2 − 𝜔1 (2.8)

xn+1

xn

Time

Figure 2.11 Logarithmic decrement of amplitude

Frequency (Hz)

M a

g n

i t u

d e

n

2

1

Amax

0.707 Amax

Figure 2.12 Half-Power Bandwidth method


43

𝜔1 and 𝜔2 are the frequescies corresponding to the amplitude with respect to resonant

amplitude as;

𝐴 =𝐴𝑚𝑎𝑥

√2 (2.9)

𝐴𝑚𝑎𝑥 = Resonant Amplitude

𝜔𝑛 = Resonant Frequency

The relation between loss factor and damping factor (ζ) is given by;

𝑡𝑎𝑛𝛿 = 2ζ (2.10)

In the context of the above discussion if the recent literature is overviewed, then it is

realized that the data available for vibration damping properties of filled elastomers is

regarding tanδ, E” and E’ determined directly from dynamic mechanical analyzer and

not through the experiments based on the vibration equipment set-up. Even less data

is available for vibration damping characteristics of NcNR. In the following lines, the

work reported by the pioneers and early adapters of this field are summarized.

Natural rubber is a unique and extensively researched material with high

mechanical performance, which can provide excellent vibration damping effect under

different loading and environmental conditions. In its original uncured form NR is

practically of limited use due to low strength, softening in hot environmental

conditions and brittleness in cold weather [254-255]. For improving its practical and

commercial uses, the thermo-mechanical and dynamic properties of NR can be

excellently improved by reinforcing phases like silica particles [256-257] and carbon

black [258]. Vibration damping properties of a structure can be significantly improved

by reinforcing the structure at molecular level to avoid dangerous oscillating loadings

[259]. The complex viscoelastic characteristics of elastomers are excellent in

vibrational energy dissipation but their performance is temperature and frequency

dependent [260]. Factors such as size distribution, shape, volume fraction,

permittivity, and conductivity of the particles and the host solvent, have been well

known to affect the electrical and rheological properties of polyisoprene based blends

or composites [261-264]. CNTs reinforced NR was characterized for vibration

damping properties by Khan et al. [265]. Acoustical efficiencies of natural rubber

with the addition of sodium bicarbonate were studied, and the results show a

significant influence of viscoelastic and acoustic damping properties of the base

matrix. Both of these properties are found to be governed by the average cell size,

relative density, crosslink density and number of cells per unit volume [266]. Rubber


44

nanocomposites filled with layered silicates as well as nano calcium carbonate

composite for various tire applications has been studied by Chandra and Bhandari

[267]. A positive impact on the tensile strength, E-modulus, storage modulus, tanδ

peak position and thermal stability of the cross-linked NR were reported by Visakh et

al. formulating NR nanocomposite on two-roll mill using cellulose nanowhiskers

(CNWs) extracted from bamboo pulp residue of newspaper production, as the

reinforcing phase [268]. The natural rubber nanocomposites with Cloisite 15A®, a

commercial organoclay were prepared and characterized for their rheological,

morphological, thermal and mechanical properties by Carli et al. [269]. High tensile

strength (15.8 MPa), high saturation magnetization (22.9 emu/g) and high loss factor

of the rubber composites over a wide frequency range (0–100 Hz) was obtained

through the doping of SrFe12O19 nanoparticles coated with silane coupling agents (Si-

69) into nitrile butadiene rubber (NBR) matrix [270]. The modal damping values of a

composite beam through longitudinal, flexural, and torsional vibration responses were

analyzed by short-time Fourier Transform (STFT) and Q-factor approximation

methods by Yesilyurt and Gursoy [271]. For the products made of filled rubber

compounds that operate under dynamic loads, such as the tire and air-spring [272] we

still do not have a complete data to understand its mechanical and dynamic behavior.

2.11 Structure and morphology

The research at nanoscale synthesis and manipulation is suddenly enhanced

due to high resolution scanning probe microscopy after the invention of scanning

tunneling microscope (STM) in 1981 [273]. The spatial resolution of these

microscopes is in the range ~1nm and facilitates the observation of structural images

with ultrafine details, hence applied for investigation in the characterization of

nanostructures. Optical and electron microscopy involve the diffraction, reflection, or

refraction of electromagnetic radiation/electron beams interacting with the specimen,

and the subsequent collection of this scattered radiation or another signal in order to

create an image [274]. The following characterizations are generally carried out to

illustrate the nanoscopic details of structural morphology.

Scanning Electron Microscopy (SEM): The scanning electron microscopy

involves a focused beam of high-energy electrons to reveal the information about

the sample such as surface morphology, crystalline structure, chemical

composition and orientation of phases making up the sample. A two-dimensional


45

image is generated to display the spatial variation of these properties over the

area ranging from 1 cm to 5 μm with magnification from 20x to 30,000x. SEM

facilitates rapid data acquisition in digital formats but suitable for solid samples

only. It requires vacuum environment for scanning [275].

Atomic force microscopy (AFM): The AFM uses various forces occurred when

two objects are brought in close proximity of nanometers with each other. It has a

probe to draw the surface profile of the sample being in contact with the surface

causing a repulsive force or being a few nanometers away causing an attractive

force. Piezoelectric elements that facilitate minute but accurate and precise

movements on command enable the very precise scanning to draw the three

dimensional scanned images. AFM samples do not require any special

treatments, such as metal/carbon coatings, that may irreversibly change or

damage the sample. Vacuum environment is not necessary for AFM and it can

work in ambient air or even in a liquid environment. In principle, AFM can

provide higher resolution than SEM [276].

Transmission Electron Microscopy (TEM): A beam of electrons is transmitted

through the specimen which is an ultrathin film. The electron beam interacts with

the inner structure of the sample and an image is formed from this interaction of

transmitted electrons. The focused and magnified image is obtained on a

fluorescent screen or recorded by using a digital camera. TEM is capable of

resolving as fine details as a single column of atoms. A vacuum of about 10-4

Pa

for general TEM and 10-7

to 10-9

Pa for high voltage TEM is required in the

closed chamber of the microscope. A complex specimen preparation process is

required to produce a thin sample to transmit the electrons [277-278].

X-ray diffractometry (XRD): An incident beam of X-rays strikes the specimen

and produces scattered beam which makes a diffracted pattern. The diffraction

pattern of spots, the strengths and angles of these beams are recorded as the

crystal is gradually rotated. X-ray diffraction data can determine the mean

chemical bond lengths within a few thousandths of an angstrom and angles

within a few tenths of a degree [279].

Fourier transform infrared spectroscopy (FTIR): A mathematic operation,

Fourier transform, is required to convert the recorded data into spectrum. FTIR

spectrometer acquires the spectrum of light emitted by the sample induce by


46

luminescence or Raman scattering. This technique is equally useful for solid,

liquid or gas specimen to determine the absorption, emission and

photoconductivity [280].

2.12 Modeling and simulation of nanocomposite rubber

The development of nanoscale fillers has lead to researchers in the field of

modeling of the microstructure-mechanical properties relationship. Modeling of a

nanocomposite elastomer is based on developing mathematical description of the

behavior of a small volume of the nanocomposite, exhibiting all the structural features

of the complete material. Elastomers are unique systems with viscoelastic behavior at

macroscopic scale governed by the relaxation process at molecular scale. Modeling of

elastomer nanocomposite dampers differs significantly from the modeling and

simulation of bulk material with and without conventional fillers, which uses

rheological properties of the polymer, the filler volume fraction and the filler shape

(aspect ratio). It is frequently accepted that no single technique offers the range

necessary to overcome the broadly varying nature in filled viscoelastic material

properties [281]. Kim et al. [282] have discussed the following three methods of

modeling the filled polymers on nanoscopic and molecular scale.

2.12.1 Potential energy function

The Hamiltonian gives the overall energy of the molecular system as below;

𝐻 = ∑ 𝑉𝑖

𝑁

𝑖=1

+ ∑ 𝑈2𝑏

𝑁−1

𝑖=1

(𝑟𝑖,𝑖+1) + ∑ 𝑈3𝑏

𝑁−2

𝑖=1

(𝜃𝑖,𝑖+1,𝑖+2) + ∑ 𝑈4𝑏

𝑁−3

𝑖=1

(𝜏𝑖,𝑖+1,𝑖+2,𝑖+3) + ∑ ∑ 𝑈𝑁𝑏

𝑁

𝑗≥𝑖+3

𝑁−1

𝑖=1

(𝑟𝑖𝑗)

In the above relation the terms to right hand side indicate respectively kinetic energy,

2-body potential (stretching), 3-body potential (bending) and 4-body potential

(torsional). V, U and N are kinetic energy, potential energy and number of atoms in

the system, whereas r, θ and τ represent bond length, bond angle and torsional angle

respectively. The individual terms are given by;

𝑉𝑖 =𝑝𝑖

2𝑚

where pi is the momentum and m is mass of ith

atom during polymer flow.

The energy stored when two adjacent atoms bonded to each other undergoes relative

displacement is considered as 2-body potential. It is determined by the principle of a


47

simple harmonic oscillator consisting of a mass m attached to a rigid wall by a spring

of stiffness kr.

𝑈2𝑏 =1

2𝑘𝑟(𝑟𝑖𝑗 − 𝑟𝑖𝑗,𝑒)2

where rij is the bond length between ith

and jth

atom with subscript e referring to

equilibrium bond length, and rij - rij,e = r [283].

When the bond angle θ formed by three atoms stores the potential energy at molecular

scale as a consequence of relative displacement is considered 3-body potential and

given by;

𝑈3𝑏 =1

2𝑘𝜃(𝜃𝑖𝑗𝑘 − 𝜃𝑖𝑗𝑘,𝑒)2

where kθ refers to the bending stiffness by the angle formed by atom i, j and k.

When a series of four atoms undergoes a torsional displacement about its own axis,

the amount of energy stored is 4-body potential given as follows [284-285].

𝑈4𝑏 =1

2𝑘𝜏[1 + 𝑠𝑐𝑜𝑠(𝑛𝜋)]

where 𝑘𝜏, n and s indicates torsional stiffness, periodicity of the potential and the

barrier to rotation (or phase factor).

Most of the researchers have introduced simplifying assumptions, such as neglecting

bond stretching or torsional displacement of bond due to involvement of enormous

number of atoms and strain rate dependent nature of elastomers [286].

2.12.2 Molecular dynamics

In this method Newton’s second law of motion is applied to derive the

equation of motion as shown by the following equation and finite difference

numerical approach is used to solve it.

𝑚𝑖

𝑑2𝑟𝑖

𝑑𝑡2= −

𝑑𝑈

𝑑𝑟𝑖

The positions and velocities of each atom at time t lead to the prediction of properties

at the time t+δt.

Verlet algorithm [287] is used to approximate the parameters of individual atom in

which the position of each atom ri(t), the corresponding atom’s acceleration ai(t) and

the position of atom from previous step ri(t - δt) is required. Taylor’s expansions for

ri(t ± dt) about ri(t) will give;


48

𝑟𝑖(𝑡 + 𝛿𝑡) = 𝑟𝑖(𝑡) + 𝑣𝑖(𝑡)𝛿𝑡 +1

2𝑎𝑖(𝑡)(𝛿𝑡)2 + ⋯ (X)

𝑟𝑖(𝑡 − 𝛿𝑡) = 𝑟𝑖(𝑡) − 𝑣𝑖(𝑡)𝛿𝑡 +1

2𝑎𝑖(𝑡)(𝛿𝑡)2 − ⋯ (Y)

From the above two Equations;

𝑟𝑖(𝑡 + 𝛿𝑡) = 2𝑟𝑖(𝑡) − 𝑟𝑖(𝑡 − 𝛿𝑡) + 𝑎𝑖(𝑡)(𝛿𝑡)2 + ⋯

Subtracting (Y) from (X) will give the atomic velocity by the following equation;

𝑣𝑖(𝑡) =𝑟𝑖(𝑡 + 𝛿𝑡) − 𝑟𝑖(𝑡 − 𝛿𝑡)

2𝛿𝑡

2.12.3 Monte Carlo method

The displacement of an atom is determined using random numbers in Monte

Carlo method. An atom is randomly picked and displaced from rim

to rin in any

random direction such that the maximum displacement is an adjustable parameter

δrmax. The change in potential energy of the atom during this displacement for an

assumed form of the interatomic potential is given by;

𝛿𝑈𝑚𝑛 = 𝑈𝑚 − 𝑈𝑛

As a consequence, if this interatomic displacement gives rise to decreased energy

(δUmn < 0), then the new position is unconditionally accepted. However, if it results to

increase in energy (δUmn > 0), then this approximated atomic movement is accepted

only conditionally checking through Boltzmann probability factor, exp(-δUmn/kBT).

The elastic constants are determined through the fluctuations in the stress tensor with

sufficiently large number of samples configured. This technique is equally suitable for

liquid as well as solid samples [288].

2.13 Methodology of this study

In the context of the reports of pioneers discussed above it can be concluded

that there is much scope of research in the field of rubber nanocomposite to

understand its behavior by constructing a satisfactory model to demonstrate the

nanoscale mechanism. The vibration damping characterization of NcNR based on

experimental approach is rarely reported in the literature and also with controversies

of data; hence this study is an effort to describe the behavior of alumina-zirconia NcPs

filled NR for vibration damping properties. Structural, thermal and mechanical

characterization is additionally carried out to take the relevant data for vibration

analysis as well as to describe the nano-effect and micromechanics of the proposed


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nanocomposite. The method followed in this study is illustrated with the help of

following flow chart in Figure 2.13.

It can be concluded from the above discussion that the rapid and successful

applications of NcPs are implemented in anti-cancerous drug delivery systems as well

as in few engineering applications. On the part of processing, NcPs can be

synthesized by various routes, but sol-gel synthesis method is simplest and less costly

alongwith a control over process parameters. Alumina-zirconia NcPs are practiced by

various researchers for mechanical properties evaluation. Reinforced nanocomposite

elastomers can be formulated through various processes, but most commonly by using

two-roll mill, with various compounding ingredients and vulcanized at high

temperature and pressure conditions. Most of the researchers have characterized

nanocomposite elastomers for microstructural, thermal and mechanical properties.

Vibration damping properties are reported mostly through the results of DMA as a

function of frequency, temperature and strain. In the methodology of this study

experimental approach is followed to determine the damping characteristics.

Synthesis Al2O3-ZrO2

Nanoparticles

Nanocomposite

NR+Al2O3-ZrO2

Modeling Macroscopic and

Microscopic

Characterization Structural, Thermal,

Mechanical

Characterization

Vibration damping

SEM, TEM, XRD, FTIR

Composition and sample

preparation

Simply supported beam (Macro) Damping (Micro)

SEM, XRD, DSC, TGA, Tensile,

Rheology

Modal analysis, Tanδ, ζ,

Transmissibility

Size, morphology

Two-roll mill

Decision parameters

Basic analysis

Mathematical, FEM

Figure 2.13 Flow chart for methodology of this study

Section-II: Nanocomposite polymers and elastomers...

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