ABSTRACT
The mechanical properties of Polypropylene and
Polypropylene/Calcium Carbonate nanocomposites were evaluated. Data on the
influence of Calcium Carbonate on the tensile strength, young’s modulus,
elongation and creep modulus were obtained for the nanocomposite by conducting
a tensile test for the coated and uncoated samples and creep test for the
coated samples at different Calcium Carbonate loadings by varying the stresses
and temperatures. It was found that the resistance to creep was high for the
nanocomposite as compared to the neat Polypropylene. The Young’s modulus of the
nanocomposite showed some improvements with the incorporation of the Calcium
Carbonate nano-filler while the tensile strength deteriorated. The Creep
modulus decreases with increase in temperature and time. Above all, the
Polypropylene and Polypropylene/Calcium Carbonate creep responses showed a non-linear
response for the properties evaluated revealing viscoelasticity of the polymer
matrix materials.
CHAPTER ONE
INTRODUCTION
Nanocomposites refer to
materials consisting of at least two phases with one dispersed in another that
is called matrix and forms a three-dimensional network. It can be defined as a
multi-phase solid materials where one of the phases has one, two or three
dimensions of less than 100 nano metres (nm) or structures having nano-scale
repeat distances between the different phases that make up the
material(Manias,2007).
Nanocomposites differ from
conventional composite materials mechanically due to the exceptional high
surface to volume ratio of the reinforcing phase and/or its exceptional high aspect
ratio. The reinforcing material can be made up of particles (e.g. minerals),
sheets (e.g. exfoliated clay sticks) or fibres (e.g. carbon nanotubes or
electro spun fibres). The area of the interface between the matrix and the
reinforcement phase(s) is typically an order of magnitude greater than for
conventional composite materials.
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 components, home appliances, and industrial applications. This is
attributed to their high impact strength and toughness when filler is
incorporated.
To meet demanding engineering
and structural specifications, PP is rarely used in its original state and is
often transformed into composites by the inclusion of fillers or
reinforcements.
Introduction of fillers or
reinforcements into PP often alters the crystalline structure and morphology of
PP and consequently results in property changes (Karger-Kosis, 1995).
Polypropylene is an exceedingly
versatile polymer, made from a widely available, low cost feedstock in a
relatively straightforward and inexpensive process. Polypropylene has good
mechanical properties, chemical resistance, accepts fillers and other selected
additives very well, and is easy to fabricate by a variety of methods. In
addition, it is quite easy to incorporate small amounts of other copolymers,
such as ethylene, to yield Polypropylene copolymers with different and
commercially desirable properties. Overall, the combination of low cost, ease
of fabrication, ability to tailor the resin with co-monomers, and its acceptance
of high levels of fillers and other additives make Polypropylene a material of
choice in many cost-sensitive application.
However, the levels of fillers
and other additives that must be incorporated to achieve the desired properties
are difficult or even impossible to incorporate “in-line” either in the
polymerization process or in the fabrication step.
These fillers generally target
specific property improvement, such as stiffness and elastomeric properties, as
shown in figure 1, or to meet service requirements such as flame retardant
specifications.
The common materials compounded
into Polypropylene are mineral fillers (e.g. calcium carbonate, talc or barium
sulphate), glass fibre, elastomers such as polyolefin elastomers or Ethylene-Propylene-Diene
Rubber, and high levels of colourants or other additives.
The incorporation of fillers
and additives by compounding serves to extend the performance envelope of
Polypropylene to compete with engineering plastics or against thermoset or thermoplastic
elastomers.
For the purpose of this thesis,
a composite is defined as a mixture of Polypropylene and ingredient(s) in
specific proportion to give a defined result or product. The production of
Polypropylene materials containing high levels of additives, most notably
fillers, is considered as compounding.The resultant composite formed using nano
filler is called a nanocomposite.
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