Clay Loading And Dispersion Effects On The Rheological Properties Of Unsaturated Polyester Nanocomposites

Clay Loading and Dispersion Effects on the Rheological Properties of Unsaturated Polyester Nanocomposites

Tony Nguyen

(Mentor: Abbas A. Zaman, College of Engineering)

ABSTRACT

The objective of this work is to characterize the influence of clay loading and dispersion effects on the rheological properties of unsaturated polyester composites. Toughened unsaturated polyester (UPE) composites were synthesized by the blending of delaminated clay with unsaturated polyester. Rheological behavior is shown to be strongly influenced by clay loading and the extent of clay dispersion in the polymer matrix. Transition from liquid-like behavior to solid-like behavior shifts to significantly higher solids loading at higher shear rates which may be due to the alignment of the particles in the direction of flow at high shear rates. SEM micrographs are used to display the extent of intercalation and dispersion of the clay within the polymer matrix.

INTRODUCTION AND BACKGROUND

1.1 Definition

Polymer/clay nanocomposites display a change in composition and structure over a nanometer length scale and have been shown to present considerable property enhancements relative to conventionally scaled composites. Layered silicates dispersed as a reinforcing phase in an engineering polymer matrix are one of the most important of such "hybrid organic-inorganic nanocomposites" [1]. Polymer-layered silicate nanocomposites containing low levels of exfoliated clays, such as montmorillonite and vermiculite have a structure consisting of platelets with at least one dimension in the nanometer range. One of the most important features of polymeric materials is the possibility of controlling their macroscopic physical properties by tailored manipulation of their structures at a nanoscopic scale. To influence the interactions that govern the mechanical properties of polymers, specific nanoscopic scale reinforcement is efficient and beneficial. For example, montmorillonite clay provides such reinforcement through the interaction of polymer chains with the charged surfaced of clay lamellae [2].

The use of organoclays as precursors to nanocomposite formation has been extended into various polymer systems including epoxies, polyurethanes, polyimides, nitrile rubber, polyesters, polypropylene, polystyrene and polysiloxanes, among others. Even a variety of inorganic materials, such as glass fibers, talc, calcium carbonate, and clay minerals, have been successfully used as additives or reinforcements to improve the various properties of polymers [3-10].

1.2 Structure

The optimal properties of nanocomposites arise as the clay nanolayers are uniformly dispersed (exfoliated) in the polymer matrix, as opposed to being aggregated or phase separated as tactoids or simply intercalated. As nanolayer exfoliation becomes achieved, there is a trend in the improvement in desired properties that is manifested as an increase in tensile properties, enhancement of barrier properties, a decrease in solvent uptake, an increase in thermal stability and flame retardance, among others [11-12]. The complete dispersion of clay nanolayers in a polymer optimizes the number of available reinforcing elements for carrying an applied load and deflecting cracks. The coupling between the and the polymer matrix facilitates stress transfer to the reinforcement phase, allowing for tensile and toughening improvements. Conventional polymer-clay composites containing aggregated nanolayers tactoids ordinarily improve rigidity, but they often sacrifice strength, elongation and toughness. However, exfoliated clay nanocomposites, have to the contrary shown improvements in all aspects of their mechanical performance [3].

1.3 Preparation and Synthesis

The preparation of nanocomposites requires extensive delamination of the layered clay structure and complete dispersal of the resulting platelets throughout the polymer matrix. Nanocomposite synthesis by conventional polymer processing operations therefore requires strong interfacial interaction between the polymer matrix and the clay in order to generate shear forces of sufficient strength. This is readily achieved with high surface energy polymers such as polyamides, where polarity and hydrogen-bonding capacity generates considerable adhesion between the polymer and clay phases. However, low-energy materials such as polyethylene and polypropylene interact only weakly with mineral surfaces, making the synthesis of polyolefin nanocomposites by melt compounding considerably more difficult [13]. Several studies exist for examining behavior of polymer/clay nanocomposites with weak adsorbing parts [14]. Common methods to synthesize polymer nanocomposites are: 1) intercalation of a suitable monomer followed by polymerization, 2) polymer intercalation from solution, 3) and direct polymer melt intercalation [14-19].

EXPERIMENTAL PROCEDURE

2.1 Material and Methods

The polymer used in this study was unsaturated polyester (UPE). The silicate clays used is referred to as C1. C1 has a surface area of 16m2/g, as measured with the Quanta Chrome NOVA 1200. Particle size analysis was performed on C1 using a Coulter LS230 laser diffraction apparatus and the experimentally measured volume average (d50) particle diameter is 4 μm. Figure 1 is an image of the C1 clay particles at 50X objective captured with the Olympus BX60 Optical Microscope with SPOT RT Digital Camera.

Figure 1. Delaminated, dispersed C1 clay particles.

Measured quantities of UPE were mixed with the clay in a custom-built high/low shear blender. After sufficient mixing of the polymer and clay, an initiator was added to induce polymerization and further blending was provided. While in the melt state, data for steady-shear viscosity and storage modulus were obtained using parallel plate geometry on a Paar Physica UDS 200 rheometer. The diameter of the upper disk was 50 mm, and the gap distance between the two plates was 0.3 mm. The sample temperature was kept constant at room temperature (25žC ± 0.1žC) using water as the heat transfer fluid. SEM micrographs with a JEOL JSM6330F cold field emission scanning electron microscope were taken and is used to visually evaluate the surface dispersion of the clay within polymer matrix.

RESULTS AND DISCUSSION

3.1 Rheological Analysis of UPE/Clay Nanocomposites

For