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 “inline” 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.

                                                          Toughness        

Figure 1: Polypropylene properties.

source:www.nexant.com/products/csresports/index.asp?

Recently, many nanometer-sized types of filler have been commercially produced and they represent a new class of alternative fillers for polymers. Among the promising nano fillers that have stirred much interest among researchers include organo clay, nano silica, carbon nano tube and nano calcium carbonate. 

Studies have shown that the large surface area possessed by these nano fillers promotes better interfacial interactions with the polymer matrix compared to conventional micrometer sized particles, leading to better property enhancement

(Goa, 2004).

1.1 Mineral Filled Polypropylene (PP)

There are a number of inorganic mineral fillers used in Polypropylene. The most common of these fillers are talc, calcium carbonate and barium sulphate; other mineral fillers used are wollastonite and mica. 

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 mineral fillers are surface treated to improve their handling and performance characteristics. Sudhin and David (1998) Silanes, glycols, and stearates are used commercially to improve dispersion, processing, and also to react with impurities.

For the purpose of this thesis, the mineral filler used is calcium carbonate.

Calcium Carbonate (CaCO₃) can be classified as:-

Mineral ground or Natural

Precipitated or Synthetic

Naturally occurring CaCO₃ is found as chalk, limestone, marble and is the preferred variety for filler incorporation into PP.

A typical composition of filler grade CaCO₃ is shown below:

CaCO₃                   : 98.5 – 99.5%

MgCO₃         : up to 0.5%

Fe₂O₃                     : up to 0.2%

Source: Material safety data sheet

Other impurities include Silica, Alumina and Aluminum Silicate, depending on location and source of the ore.

Loadings of CaCO₃ in PP typically run from 10 to 50%, although concentration as high as 80% has been produced Karger-Kosis (1995). 

CaCO₃ is usually selected as filler when a moderate increase in stiffness is desired. It also increases the density of the PP compound; reduces shrinkage, which can be helpful in terms of part distortion and the ability to mould in tools designed for other polymers. At typical levels of 10 to 50%, the CaCO₃ does not significantly affect the viscosity of the compound. The main secondary additive employed in CaCO₃ is a stearate. The stearic acid acts as a processing aid. It helps to disperse the finer-particle size CaCO_3. It also helps to prevent the absorption of stabilizers into the filler. Finally, as an added benefit, it acts to cushion the system, resulting in improved impact. The dispersion qualities of CaCO₃ particles play a crucial role in its toughening efficiency.

1.2 Motivation

Nanocomposites have attracted attention in recent years because of improved mechanical, thermal, rheological, solvent resistant and fire retardant properties compared to the pure or conventional composite materials. Therefore, much work has focused on developing PP/CaCO₃ nanocomposites with tailored mechanical and morphological properties.This has received much attention from academia and industry globally. 

Engineers and polymer scientists are working hard to produce lightweight materials as suitable replacement for metals.

1.3 Objectives of the Study

The present work aims to: