Computational Approach to Modelling Fracture Behaviour of Polypropylene/Talc Composites

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Journal of Minerals and Materials Characterization and Engineering, 2012, 11, 841-847

Published Online August 2012 (http://www.SciRP.org/journal/jmmce)

Computational Approach to Modelling Fracture

Behaviour of Polypropylen e/Talc Composites

Chinedum Ogonna Mgbemena1*, Obuora Anozie Okoye2

1National Engineering Design Development Institute, Nnewi, Nigeria

2Nnamdi Azikiwe University, Awka, Nigeria

Email: *[email protected]

Received June 20, 2012; revised July 30, 2012; accepted August 14, 2012

ABSTRACT

Fracture represents one of the major problems associated with the selection and use of engineering materials for high

temperature applications. The fracture toughness is of special relevance on the design of components. In this work, the

fracture behavior of Polypropylene/Talc composites was studied. From the results of the tensile and flexural tests con-

ducted on the composite, scatter diagrams were made using Microsoft Excel to evaluate and show the effect of the addi-

tion of the talc filler as it affects the tensile strength, percentage elongation at break, flexural strength and modulus. In

order to give additional analysis, the talc filler content effect was presented mathematically to further describe explicitly

the various equations associated with each scatter diagram earlier developed using Microsoft Excel. The mathematical

expression developed shows the actual talc filler content on the fracture mechanical properties of the sample composite.

Keywords: Fracture; Tensile Strength; Talc; Polypropylene; Flexural Strength; Tensile Test

1. Introduction

Polypropylene (PP) composite is one of the most exten-

sively produced polymers, widely used especially as

automotive parts [1]. This is attributed to their high im-

pact strength and toughness when fillers are incorpo-

rated.

Polypropylene is isotactic, notch sensitive and brittle

under severe conditions of deformation, such as low

temperatures or high temperatures. This makes limited its

wider range of usage for manufacturing processes. It is a

versatile material widely used for automotive compo-

nents, home appliances, and industrial applications.

Polypropylene (PP) filled with particulate fillers is of

great interest in both research and industry. It is well

known that polypropylene has good processability and

accepts different types of natural and synthetic filers.

Mica, kaolin, calcium carbonate, and talc are the most

often used fillers to reduce both the production costs and

to improve the properties of the thermoplastics, such as

rigidity, strength, hardness, flexural modulus, dimen-

sional stability, crystallinity, electrical and thermal con-

ductivity. However, fillers have a detrimental effect on

other properties such as the impact property and de-

formability. The filler type, content and size, interfacial

adhesion and bond strength between PP matrix and filler

and surface characteristics of the composite can greatly

influence the filled system.

In a highly filled polymer system, non-uniformity of

properties can exist because of poor dispersion of the

filler in the matrix. A good interfacial adhesion between

matrix and filler may improve the mechanical strength

[1-8]. Due to the high transient temperature and in par-

ticular, at low stress temperatures, notch toughness of

polypropylene is not sufficient and limits its applications.

Introduction of fillers or reinforcements into PP often

alters the crystalline structure and morphology of PP and

consequently results in property changes [5,9]. Although

toughness strength is improved by blending elastomer,

this causes a decrease in strength and stiffness [10]. In

order to overcome these limitations numerous studies

have been performed to improve the toughness, stiffness,

and strength balance. Therefore, polypropylene has been

modified by different fillers and elastomers to produce

ternary composites. The mechanical properties of such

ternary composites are determined not only by their

composition and the characteristics of the components

but also by the phase morphology, and in particular, the

relative dispersion of additive components [11].

To meet demanding engineering and structural speci-

fications, PP is rarely used in its original state and is of-

ten transformed into composites by the inclusion of fill-

ers or reinforcements.

There are a number of inorganic mineral fillers used

with Polypropylene. The most common of these fillers

*Corresponding author.

C. O. MGBEMENA, O. A. OKOYE

842

are talc, calcium carbonate and barium sulphate; other

mineral fillers used are calcium silicate (wollastonite)

and mica (aluminosilicates).

Mineral fillers are generally much less expensive than

polypropylene resin itself. Mineral fillers reduce the

costs of the compound formed with polypropylene and

also increase the stiffness. Mineral fillers also provide

reinforcement to the polymer matrix as well. Some min-

eral fillers are surface treated to improve their handling

and performance characteristics. Silanes, glycols, and

stearates are used commercially to improve dispersion,

processing, and also to react with impurities [10].

In this work, the mineral filler used is talc. Hydrated

magnesium silicate [Mg3Si4O10(OH)2], or better known

as talc occurs as the alteration products of magnesium

carbonate rock by the natural action of hydrothermal

solutions. The purer forms are called steatite talc. The

advantages of talc are: Good stiffness, hardness, dimen-

sional stability and reduced creep compared with un-

modified PP [12].

Talc can resist temperatures up to 900˚C. It is unaf-

fected by chemicals and will not harm living tissue. Talc

can be utilized as a medium filler of average whiteness in

thermosetting as well as thermoplastic resins where im-

provements in electrical insulation, heat, and moisture

resistance, chemical inertness and good machinability are

needed. Talc has low absorption rate and because of its

plate like structure, certain grades can improve flexural

properties of mouldings.

In PP, talc gives a good balance of rigidity and impact

strength [13]. Advanced milling technology can be used

to obtain the finest talc without reducing the reinforcing

power of the lamellar structure. Talc filled composites

are also easier to colour with reduced pigment require-

ment due to its whiteness and low yellow index. Al-

though investigations on talc-filled PP have been done

since the 1981, there are still many works that involve

the characterization of talc filled PP, which are still on-

going until today [8].

2. Materials and Methods

2.1. Materials

The grade of Polypropylene used in this work was

SEETEC Homo polymer PP by LG Chem Korea. This

acts as the matrix. The homo polymer PP has a density of

0.90 g/cm3 and a melt flow rate of 14 g/10 minutes (2.16

kg at 230˚C). The nano filler used in this work was Talc

(Zeta talc EW 20) manufactured by Eral, Turkey, with

particle size of around 2 μm. All the materials were pur-

chased from the local chemical market at Ajasa Ose,

Onitsha. The mould release agent used was Petroleum

jelly (Vaseline). The characteristic properties of the ma-

terials are shown in Table 1.

The nanocomposites were prepared by melting the PP

in a mixer and melt compounding it with talc respec-

tively at filler loadings of 0%, 5%, 10%, 15%, 20%, 25%,

30%, 35% and 40% volume fractions. The tensile sam-

ples were cast in an aluminum mould in accordance with

ASTM standard D638 for tensile tests, D790 for flexural

tests and D256 for impact tests.

2.2. Method of Preparation

Manual Mixing and Compounding

The PP was melted from its pelletized form at a tem-

perature exceeding 180˚C in a mixing chamber. Meas-

ured amounts of Talc were added to the melted PP by

volume fractions and stirred continuously for 10 minutes

to ensure a uniform dispersion of the mixture. The com-

pounded mixture was cast in an aluminum mould that has

been treated with a mould releasing agent and dried. The

composite was allowed to cure for 72 hours and was later

de-moulded. The composition of composites taken for

mechanical tests is shown in Table 2.

2.3. Tensile Testing of the Samples

The tensile experiment was performed on ABBA Uni-

versal Testing Machine at a laboratory temperature of

25˚C. The testing machine has rectangular upper and

lower grip equipped with centre marking to facilitate the

correct positioning of the test specimen when mounted

vertically. Tensile testing was performed to determine

elastic modulus, ultimate stress, and ultimate strain for

all samples. The specimen was prepared and tested in

accordance with ASTM D 638.

A minimum of seven samples were tested in each

specimen at their various volume fractions. The speci-

men subjected to tensile test has the dimension 50 × 30 ×

20 mm.

Table 1. Characteristic properties of the materials used for the composites.

Material Trade Name Supplier Melt Flow IndexDensity g/cm3 Melting Temperature ˚C Shape

Polypropylene SEETEC

Homo Polymer

LG Chem

Korea

230˚C,

2.16 kg/10 min 0.90 170 Pellets

Talc Zeta talc EW 20 Eral Turkey - 2.7 - Powder

C. O. MGBEMENA, O. A. OKOYE 843

Table 2. Combination of composites taken for mechanical

tests by volume fractions.

Specimen Code PP % Talc %

PPT-0

PPT-1

PPT-2

PPT-3

PPT-4

PPT-5

PPT-6

PPT-7

PPT-8

100

In order to analyze the data, load was converted to

stress, σ, from Equation (1)



 (1)

where F is the force applied as reported from the tensile

testing equipment, and A0 is the original cross sectional

area calculated from the average of the sample’s neck

measurements.

Displacement was converted to millimeters and strain,

ε, was calculated using







l

(2)

where the change in length of the sample as ob-

tained from the extensometer data, and is the original

extensometer gage length.

The natural strain (or True fracture ductility) is ex-

pressed as



 (3)

where A0 is the original cross sectional area calculated

from the average of the sample’s neck measurements and

Af is the Area of the fractured surface.

The Ultimate strength is expressed as

max





(4)

where Fmax is the maximum applied force to break the

specimen.

Ultimate stress and strain were taken as the maximum

values at the sample fracture point, as determined in the

data. Results from multiple tests were averaged for each

system.

2.4. Flexural Testing of the Samples

Flexural modulus and strength were measured according

to ASTM D790 test method with three point bending and

was carried out using Instron Universal testing Machine

Series XI and support span length was adjusted to 50

mm.

The Flexural Modulus values () were calculated

using the following equation:

Ebd

 (5)

where, m is the slope of the tangent to the initial straight

line portion of the load-deflection curve.

The Flexural strength (S) in the units of MPa was cal-

culated using the following equation:

Sbd

 (6)

where P is the applied load at the deflection point, L is

the span length, d and b are the thickness and width of

the specimen respectively.

3. Results and Discussion

From the results of the tensile and flexural tests con-

ducted on the composite, scatter diagrams were made

using Microsoft Excel to evaluate/show the effect of the

addition of the talc filler as it affects the tensile strength,

percentage elongation at break, flexural strength and

modulus. In order to give additional analysis, the talc

filler content effect was presented mathematically to fur-

ther describe explicitly the various equations associated

with each scatter diagram earlier developed using Mi-

crosoft Excel. This mathematical expression shows the

actual talc filler content on the fracture mechanical prop-

erties of the sample composite.

Furthermore, the experimental data was used to search

for a relationship amongst the fracture mechanical prop-

erties in order to predict the point of convergence of the

product material properties in terms of its fracture me-

chanical properties.

3.1. Tensile Test Results

Figure 1 depicts the linearity of the stress-strain proper-

ties obtained from the tensile tests. It explains ductile to

brittle transition of the PP matrix as talc filler particles

are incorporated. At lower talc filler concentrations of

5% - 20%, extensive plastic deformations (molecular

chain breakage) took place before fracture of the Talc/PP

composite when the tensile test was performed. The line-

arity of the initial part is consistent with the Principles of

Hooke’s Law for elastic deformation and the Young’s

Modulus of Elasticity can be obtained from the straight

line. Also, at filler concentration higher than 20%, the

Talc/PP composite exhibited little and no plastic defor-

mation before fracture. This is because the Talc/PP com-

posite displays less ductile characteristics and miniature

C. O. MGBEMENA, O. A. OKOYE

844

(visible) signs of brittle characteristics such as restriction

of the polymer chains.

Equation describing the curve is

1042 18.75







8.75 1042

(7)





 (8)

1042

118.75

18.75















(9)



18.75 155.573





 (10)

where 0



is the ultimate strength at zero strain and is

equal to 18.75 MPa and Equation (10) is in the form of

equation developed by [14]. This becomes



1 55.573







55.573

(11)



From Equation (11), the talc filler content can be

mathematically represented as

(12)

3.2. Flexural Test Results

Figure 2 shows the effect of talc filler on the flexural

force applied on the nanocomposite. The highest applied

force is at 10% talc filler concentration because of the

high ductility characteristics exhibited by the PP matrix

and at higher concentrations, the flexural force applied

declines. The ductile characteristics start to reduce at

higher concentrations till the brittleness starts taking ef-

fect on the composite.

Equation describing the curve is

7277 184.19.3090.108F (13)





184.1 9.3090.108

7277 17277 72777277







  









253

7277 10.02530.00121.4810F

(14)







(15)





Figure 1. Plot of ultimate stress on tensile strain.

Figure 2. Plot of flexural force on volume fraction.

C. O. MGBEMENA, O. A. OKOYE 845

where 0

is the ultimate flexural force at zero volume

fractions and is equal to 7277N and Equation (15) is in

the form of equation developed by [14]. This becomes

253

1.48 10

 

253

1.48 10

 

0.663 0.009

1 0.02530.0012

F   (16)

From Equation (16), the talc filler content can be

mathematically represented as

0.0253 0.0012  (17)

Figure 3 shows the effect of talc filler on the deflec-

tion of the sample composite when the flexural tests were

conducted. The deflection displays a steady reduction or

decline in response to increasing talc filler concentrations

and as such, describing the flexural characteristics of the

sample composite at each volume fraction.

Equation describing the curve is



 (18)

0.009

10.663











0.663



(19)





0.663



1 0.0136

(20)

where δ0 is the ultimate defection at zero volume frac-

tions and is equal to 0.663 and Equation (20) is in the

form of equation developed by [14]. This becomes



1 0.0136 

0.0136

46.91 0.5340.017S

(21)

From Equation (21), the talc filler content can be

mathematically represented as

(22)

From Figure 4, it was found that the flexural strength

of the neat PP is 46 MPa and for talc filler concentrations

of 10% to 15%, there was a steady increase in the flex-

ural strength as shown above. The maximum flexural

strength was obtained for talc concentration of 10%. Also,

talc concentration of 20% and more led to the increase of

brittleness which caused the decline of the flexural

strength of the PP/Talc composite [15].

Equation describing the curve is



 (23)

0.534 0.017

46.91 146.91 46.91

















46.91 10.0113.6210S

(24)







 (25)





where 0 is the ultimate flexural strength at zero vol-

ume fractions and is 46.91 MPa and Equation (25) is in

the form of equation developed by [14]. This becomes

10.0113.62 10







0.0113.62 10

(26)

From Equation (26), the talc filler content can be

mathematically represented as





1381 69.841.9030.025

(27)

Figure 5 shows that the addition of talc greatly in-

creased the flexural modulus at all levels of the composite.

This continuous increase is as a result of the Talc filler

particles mixed into the polypropylene matrix making the

composite more brittle. The most significant increase is

seen in talc filler concentrations of 10% - 30%.

Equation describing the curve is





(28)

69.84 1.9030.025

1381 11381 13811381

















253

1381 10.0510.00141.8110

E

(29)







 (30)





where Εb0 is the ultimate flexural modulus at zero vol-

ume fractions and is 1381 MPa and Equation (30) is in

the form of equation developed by [14]. This becomes

253

10.0510.00141.81 10





  (31)

Figure 3. Plot of deflec tion on volume fraction.

C. O. MGBEMENA, O. A. OKOYE

846

Figure 4. Effect of filler concentrations on flexural strength of the composite.

Figure 5. Effect of filler concentrations on flexural modulus of the composite.

From Equation (31), the talc filler content can be

mathematically represented as

253

1.81 10

 0.051 0.0014  (32)

4. Conclusions

The results of the study showed that the addition of talc

nano-filler has resulted in some improvement in the ten-

sile mechanical properties of the Homo Polypropylene.

The flexural modulus increases with increase in the talc

filler concentrations of 0% to 20%. From the result of the

experiments conducted, it was observed that 10% of talc

filler concentration led to an optimum value of the flex-

ural strength. The tensile strength showed no consider-

able change up till nano-filler concentration of 10%.

According to Liang et al., the strength of particulate-

filled polymer composites depends, to a great extent, on

the interfacial adhesion between the matrix and the filler

which will facilitate the transfer of a small section of

stress to the filler particle during deformation. Tensile

mechanical properties seem to be affected by the disper-

sion/distribution of the nanoparticles.

Fracture represents one of the major problems associ-

ated with the selection and use of engineering materials

for high temperature applications. The fracture toughness

is of special relevance on the design of components. It is

one of the mechanical characteristics that have more dif-

ficulty in its determination and analysis as there are nu-

merous factors affecting it such as: Temperature, Strain

rate, Specimen dimension and the testing geometry.

The fracture behavior/toughness of polymers is

strongly affected by the addition of rigid particles [16-

18]. Several features of the particles have a decisive in-

fluence on the values of the fracture toughness: shape

and size, chemical nature, surface nature, concentration

by volume, and orientation. Among those of thermoplas-

tic matrix, polypropylene (PP) composites are the most

industrially employed for many different application

fields.

From the foregoing, it is clear that the strength of a

nano-filled composite depends to a great extent on the

interfacial adhesion between the polymer matrix and the

C. O. MGBEMENA, O. A. OKOYE 847

nano-filler which aids the transfer of a small section of

stress to the filler particle during deformation.

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