Evaluation of the mechanical properties of polypropylene/calcium carbonate nanocomposite at various creep conditions

Thesis Overview

<p> </p><p>INTRODUCTION<br>Nanocomposites refer to materials consisting of at least two phases with one<br>dispersed in another that is called matrix and forms a three-dimensional network.<br>It can be defined as a multi-phase solid materials where one of the phases has one,<br>two or three dimensions of less than 100 nano metres (nm) or structures having<br>nano-scale repeat distances between the different phases that make up the<br>material(Manias,2007).</p><p>Nanocomposites differ from conventional composite materials mechanically due<br>to the exceptional high surface to volume ratio of the reinforcing phase and/or its<br>exceptional high aspect ratio. The reinforcing material can be made up of particles<br>(e.g. minerals), sheets (e.g. exfoliated clay sticks) or fibres (e.g. carbon nanotubes<br>or electro spun fibres). The area of the interface between the matrix and the<br>reinforcement phase(s) is typically an order of magnitude greater than for<br>conventional composite materials.</p><p>Polypropylene is isotactic, notch sensitive and brittle under severe conditions of<br>deformation, such as low temperatures or high temperatures. This makes limited<br>its wider range of usage for manufacturing processes. It is a versatile material<br>widely used for automotive components, home appliances, and industrial<br>applications. This is attributed to their high impact strength and toughness when<br>filler is incorporated.</p><p>2<br>To meet demanding engineering and structural specifications, PP is rarely used in<br>its original state and is often transformed into composites by the inclusion of<br>fillers or reinforcements.</p><p>Introduction of fillers or reinforcements into PP often alters the crystalline<br>structure and morphology of PP and consequently results in property changes<br>(Karger-Kosis, 1995).</p><p>Polypropylene is an exceedingly versatile polymer, made from a widely available,<br>low cost feedstock in a relatively straightforward and inexpensive process.<br>Polypropylene has good mechanical properties, chemical resistance, accepts fillers<br>and other selected additives very well, and is easy to fabricate by a variety of<br>methods. In addition, it is quite easy to incorporate small amounts of other<br>copolymers, such as ethylene, to yield Polypropylene copolymers with different<br>and commercially desirable properties. Overall, the combination of low cost, ease<br>of fabrication, ability to tailor the resin with co-monomers, and its acceptance of<br>high levels of fillers and other additives make Polypropylene a material of choice<br>in many cost-sensitive application.</p><p>However, the levels of fillers and other additives that must be incorporated to<br>achieve the desired properties are difficult or even impossible to incorporate “in<br>line” either in the polymerization process or in the fabrication step.</p><p>3<br>These fillers generally target specific property improvement, such as stiffness and<br>elastomeric properties, as shown in figure 1, or to meet service requirements such<br>as flame retardant specifications.<br>The common materials compounded into Polypropylene are mineral fillers (e.g.<br>calcium carbonate, talc or barium sulphate), glass fibre, elastomers such as<br>polyolefin elastomers or Ethylene-Propylene-Diene Rubber, and high levels of<br>colourants or other additives.</p><p>The incorporation of fillers and additives by compounding serves to extend the<br>performance envelope of Polypropylene to compete with engineering plastics or<br>against thermoset or thermoplastic elastomers.</p><p>For the purpose of this thesis, a composite is defined as a mixture of<br>Polypropylene and ingredient(s) in specific proportion to give a defined result or<br>product. The production of Polypropylene materials containing high levels of<br>additives, most notably fillers, is considered as compounding.The resultant<br>composite formed using nano filler is called a nanocomposite.</p><p>4<br>Stiffness/ HDT<br>Toughness<br>Glass filled coupled PP<br>Glass filled PP<br>Mineral filled PP<br>Homo PP<br>Co PP</p><p>Figure 1: Polypropylene properties.<br>source:<a target="_blank" rel="nofollow" href="http://www.nexant.com/products/csresports/index.asp">www.nexant.com/products/csresports/index.asp</a>?<br>Recently, many nanometer-sized types of filler have been commercially produced<br>and they represent a new class of alternative fillers for polymers. Among the<br>promising nano fillers that have stirred much interest among researchers include<br>organo clay, nano silica, carbon nano tube and nano calcium carbonate.<br>Studies have shown that the large surface area possessed by these nano fillers<br>promotes better interfacial interactions with the polymer matrix compared to<br>conventional micrometer sized particles, leading to better property enhancement<br>(Goa, 2004).</p><p>1.1 Mineral Filled Polypropylene (PP)<br>There are a number of inorganic mineral fillers used in Polypropylene. The most<br>common of these fillers are talc, calcium carbonate and barium sulphate; other<br>mineral fillers used are wollastonite and mica.</p><p>5<br>Mineral fillers are generally much less expensive than Polypropylene resin itself.<br>Mineral fillers reduce the costs of the compound formed with Polypropylene and<br>also increase the stiffness. Mineral fillers also provide reinforcement to the<br>polymer matrix as well. Some mineral fillers are surface treated to improve their<br>handling and performance characteristics. Sudhin and David (1998) Silanes,<br>glycols, and stearates are used commercially to improve dispersion, processing,<br>and also to react with impurities.</p><p>For the purpose of this thesis, the mineral filler used is calcium carbonate.<br>Calcium Carbonate (CaCO3) can be classified as:-<br>Mineral ground or Natural<br>Precipitated or Synthetic<br>Naturally occurring CaCO3 is found as chalk, limestone, marble and is the<br>preferred variety for filler incorporation into PP.<br>A typical composition of filler grade CaCO3 is shown below:<br>CaCO3 : 98.5 – 99.5%<br>MgCO3 : up to 0.5%<br>Fe2O3 : up to 0.2%<br>Source: Material safety data sheet<br>Other impurities include Silica, Alumina and Aluminum Silicate, depending on<br>location and source of the ore.</p><p>6<br>Loadings of CaCO3 in PP typically run from 10 to 50%, although concentration as<br>high as 80% has been produced Karger-Kosis (1995).<br>CaCO3 is usually selected as filler when a moderate increase in stiffness is desired.<br>It also increases the density of the PP compound; reduces shrinkage, which can be<br>helpful in terms of part distortion and the ability to mould in tools designed for<br>other polymers. At typical levels of 10 to 50%, the CaCO3 does not significantly<br>affect the viscosity of the compound. The main secondary additive employed in<br>CaCO3 is a stearate. The stearic acid acts as a processing aid. It helps to disperse<br>the finer-particle size CaCO3. It also helps to prevent the absorption of stabilizers<br>into the filler. Finally, as an added benefit, it acts to cushion the system, resulting<br>in improved impact. The dispersion qualities of CaCO3 particles play a crucial role<br>in its toughening efficiency.</p><p>1.2 Motivation<br>Nanocomposites have attracted attention in recent years because of improved<br>mechanical, thermal, rheological, solvent resistant and fire retardant properties<br>compared to the pure or conventional composite materials. Therefore, much work<br>has focused on developing PP/CaCO3 nanocomposites with tailored mechanical<br>and morphological properties.This has received much attention from academia and<br>industry globally.<br>Engineers and polymer scientists are working hard to produce lightweight<br>materials as suitable replacement for metals.</p><p>7<br>1.3 Objectives of the Study<br>The present work aims to:<br>ï‚· Evaluate the Tensile properties of coated and uncoated calcium carbonate<br>nano-filler with Polypropylene as the host polymer for different volume<br>fractions.<br>ï‚· Evaluate creep behavior of the coated nanocomposite, for each volume<br>fraction of the fillers.<br>ï‚· Examine the effects of the fillers on the mechanical behavior of the<br>nanocomposite. This examination can be utilized for processability and in<br>developing optimum morphology to maximize products performance.<br>ï‚· This work is an attempt at nano structure fabrication and to get into the<br>main stream of composite technology of the 21st century.</p> <br><p></p>