Date of Award

August 2017

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Engineering

First Advisor

Nidal H. Abu-Zahra

Committee Members

Benjamin C. Church, Chang Soo Soo Kim, Hong Chang, Wilkistar A. Otieno

Keywords

Composites, Fly Ash, Interfacial Interaction, Polymer Foam, Polyvinyl Chloride (pvc), Properties

Abstract

Fly ash, a byproduct of coal combustion process in power plants, consists of fine, powdery particles that are predominantly spherical in shape, either solid or hollow, and mostly glassy (amorphous) in nature. It is capable of being recovered and used as a low-cost reinforcing filler. Adding fly ash particles to a thermoplastic foam poses many challenges to understanding the physical, mechanical, viscoelastic, thermal, and morphological changes to the composite. The effects of fly ash particles on the foaming process; e.g. nucleating and growth steps, cell types and size, and microstructure need to be evaluated in order to develop a commercial composite material. The main goal of this work is to evaluate the use of fly ash as a cost-effective and reinforcing filler in polymer foam composites. PVC foam is selected as the base polymer and it is expected that the research findings can be generalized to other polymer foam systems.

Initially, two classes of fly ash, class-C and class-F (two different particle sizes) are well characterized and studied. Their elemental, chemical, structural, thermal and morphological properties are evaluated. Although both classes of fly ash have almost the same surface area, Class-C contains more quartz and lime content than class-F; whereas class-F is highly concentrated with iron oxides. In addition, 25 micron sized class-F fly ash possesses different chemical composition compared with 50 micron class-F fly ash. Structural analysis showed the presence of hydroxyl functional groups in both types of fly ash particles.

In the second phase, PVC foam/fly ash composites containing various levels of fly ash are prepared using extrusion as a processing method. To understand all performance aspects of fly ash, a wide range of characterizing methods was used to evaluate the physical, mechanical, thermal, and morphological properties of the composites. The experimental results showed that the density increases with increasing the amount of fly ash, while the cell size decreases. Tensile and flexural properties increased by adding fly ash, which indicates that fly ash particles are properly incorporated into the polymer. However, the elongation and impact strength of the composites decreased with increasing fly ash due to the higher rigidity of the polymer composites. Thermal analysis shows that the glass transition temperature of the composites is not significantly affected by the addition of fly ash. Thermal decomposition studies show that the dehydrochlorination of PVC is accelerated in the presence of fly ash, while the main backbone crack is enhanced.

Kinetic studies were carried out on the loaded and unloaded fly ash composites and the activation energies of both decomposition steps were estimated using Flynn-Wall model. A higher activation energy in the first step of decomposition of the pure PVC foam is noticed compared with that of fly ash loaded, while the estimated activation energy of the second step in the composites is significantly higher. Dynamic mechanical analysis confirmed the increase in stiffness as fly ash content increased in the composites due to good interfacial adhesion between the filler and the matrix. Dimensional stability of the composites improved considerably with the addition of fly ash at high loadings and morphological studies confirmed good dispersion, distribution, and interaction between fly ash and PVC matrix.

The effect of the chemical composition of fly ash on the final properties of the foam composites are studied using both classes of fly ash, C and F. The experimental results show that class-C fly ash interacts better with the polymer matrix and improves the mechanical and thermal properties significantly when compared to class-F. This is attributed to a higher SiO2 and CaO content in class-C fly ash. Interfacial interaction of the composites reinforced with both classes of fly ash was estimated using Pukanszky model to confirm. Structural analysis confirmed the presence of hydroxyl (–OH) functional groups on the surface of SiO2, which plays a significant role in the formation of physical bonding and therefore interfacial interactions between fly ash particles and the polymer matrix. Morphological studies of the fracture surfaces using SEM/EDS line-scan also confirmed the effectiveness of silicon and calcium elements in the properties improvement due to their high concentration in the well-bonded particles.

The effect of the particle size of fly ash on the composites structure and performance was also evaluated. Two pre-sieved fly ash particles in the size range of 25 micron and 50 micron were used to prepare rigid PVC/fly ash composites and examine the dependency of the composites performance on the filler size and composition. It was found that at the same fly ash loadings (10wt%), 25 micron sized loaded composites show better mechanical properties due to their higher surface area and the amount of hydroxyl groups. The interfacial interaction between the two different fly ash particle sizes with rigid PVC was evaluated experimentally, using nanoindentor, and quantitatively, using models developed by Pukanszky and Kubat.

Recyclability and reprocess-ability of the fly ash filled composites were also investigated. The experimental results achieved by processing measurements showed that the maximum and minimum torque values increase by adding more regrind. Whereas, increasing fly ash content decreases the melt viscosity and improves processability. It was also observed that the mechanical properties of the reprocessed composites improve by adding more regrind, which indicates a good gelation in the composites containing both virgin and regrind of PVC/fly ash foam. SEM images confirmed a good level of mixing and gelation between virgin and regrind foam matrix up to 40wt% regrind content.

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