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DISPERSION OF CERAMIC PARTICLES IN POLYMER MELTS By DOW JOAN-HUEY A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 1992 UNIVERSin OF FLORIDA LIBRARIES ACKNOWLEDGEMENTS I am grateful to Dr. M. D. Sacks for his guidance and support on the research work. Support from the Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences (DE-FG05-85ER45202) is gratefully acknowledged. The advice from Dr. P. H. HoUoway is appreciated. Thanks should also go to Drs. C. D. Batich, E. D. Whitney, and G. B. Westermann-Clark for their suggestions on this dissertation. I would like to thank Dr. A. V. Shenoy, G. W. Scheiffele, C. Khadilkar, T.- S. Yeh, H. W. Lee, S. Vora, R. Raghunathan, M. Saleem, A. Bagwell, and Dr. A. Fradkin for their assistance in carrying out various experiments and editing this dissertation. This dissertation is dedicated to my parents for their love, understanding, and encouragement. u TABLE OF CONTENTS Page ACKNOWLEDGEMENTS ii ABSTRACT v CHAPTERS 1 INTRODUCTION 1 2 BACKGROUND 5 2.1 Evaluation of the State of Particulate Dispersion by Non-rheological Techniques 6 U 2.2 Rheology of Fluids and Particle/Fluid Mixtures H 2.2.1 Overview 2.2.2 Polymer Melts 16 2.2.3 Particle/Fluid Mixtures 17 2.3 Particle/Fluid Mixing 22 2.4 Particle/Fluid Wetting 25 2.5 Characteristics of Alumina Surfaces with Adsorbed Water and Hydroxyl Groups 30 2.6 Effects of Moisture on Ceramic/Polymer Composites 32 34 2.7 Chemical Additives 35 2.7.1 Structures 2.7.2 Effects of Chemical Additives on Rheological Properties . . 40 EXPERIMENTAL 42 3 3.1 Materials and Materials Preparation 42 42 3.1.1 Starting Materials 3.1.2 Treatment of Alumina Powder 46 3.2 Characterization of Ceramic Powders, Powder Compacts, and Polymers 49 3.2.1 Alumina Powder Characterization 49 3.2.2 Alumina Powder Compact Characterization 53 iii 11 3.2.3 Polymer Characterization 54 3.3 Mixing of Ceramic Powders and Polymers 54 3.4 Characterization of Ceramic Powder/Polymer Mixtures 57 57 3.4.1 Rheology 3.4.2 Quantitative Microscopy 62 3.4.3 Ceramic/Polymer Wetting Behavior 63 67 3.4.4 Elemental Analysis 3.4.5 Characterization via FTIR 68 3.4.6 Analysis for Iron Content 68 3.4.7 Microhardness Measurements 68 4 RESULTS AND DISCUSSION 69 4.1 Effects of Mixing Conditions 69 4.1.1 Single-Segment Mixing Schedules 69 4.1.1.1 Effects of mixing temperature on rheological and 70 wetting behavior 4.1.1.2 Quantitative microscopy 94 4.1.1.3 Effects of rotor speed 134 4.1.1.4 Effects of mixing time 136 4.1.2 Multi-Segment Mixing Schedules 136 4.1.2.1 Mixing with change in temperature 146 4.1.2.2 Mixing with change in rotor speed 154 4.2 Effects of Ceramic Powder Characteristics 159 4.2. Calcination Effect 160 4.2.2 Aging Phenomenon 210 4.3 Effects of Polymer Characteristics 227 4.3 Effects of Chemical Additives 248 4.4. Coupling Agents 248 275 4.4.2 Surfactants 288 4.4.3 Lubricants SUMMARY 322 5 REFERENCES 329 BIOGRAPfflCAL SKETCH 342 IV Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy DISPERSION OF CERAMIC PARTICLES IN POLYMER MELTS By Dow Joan-Huey May, 1992 Chairperson: Dr. Michael D. Sacks Major Department: Materials Science and Engineering The effects of mixing conditions, powder and polymer characteristics, and chemical additives on dispersion ofceramic particles in polymer melts were investigated. Fine-sized alumina powder and low-molecular-weight polyethylene (PE) were used in mostexperiments. Samples wereprepared using a high-shearbowl mixer and the mixing operation was monitored by torque rheometry. The state of dispersion was evaluated using Theological and quantitative microscopic measurements. Ceramic/polymer melt wetting behavior was evaluated by the sessile drop and polymer peneti^tion methods. Further understanding ofmixing and dispersion behaviorwas developed by using particle size and surface area measurements, infrared specti-oscopy, mercury porosimetry, microhardness measurements, gravimetric analysis, etc. Samples mixed at lower temperatures and higher rotor speeds had better particulate dispersion (i.e., due to increased agglomerate breakdown rates and decreased coagulation rates). Mixed batches prepared with heat-treated powders (1(X)-1000°C) showed relatively poor particulate dispersion. This was due to changes in the physicochemical characteristics ofthe heat-treated powders (i.e., due to removal ofwater and hydroxyl groups on powder surfaces at low temperatures and interparticle neck growth at higher temperatures). Samples prepared witii heat-treated powders were also highly susceptible to aging effects due to absorption of moisture from tiie ambient air atmosphere. Mixed batches prepared with polyetiiylene or ethylene-acrylic acid copolymer showed relatively good dispersion compared to batches prepared with ethylene-vinyl acetate copolymer. Further investigation is needed to understand the reason for this behavior. Particulate dispersion in mixed batches was also highly dependent upon the presence of chemical additives (i.e., coupling agents, a surfactant, and a lubricant). In some cases, it was possible to establish correlations between the state of dispersion in the suspensions used to coat powders witii additives, the peak torques generated during powder/polymer mixing, and the state of dispersion in the mixed batches. VI , CHAPTER 1 INTRODUCTION The state of particulate dispersion and the rheological properties of ceramic powder/polymer melt mixtures are important for ceramic shape forming processes such as injection molding [Edi86, Ino89, Man82, Man83, Schw49, Tay62]. The first major step in the process is to mix ceramic powder with polymer melt at an elevated temperature to form a plastic mass. The ceramic/polymer mixture should have suitable fluidity in order to fill the die completely and uniformly without leaving any defects in the shaped parts. Usually, chemical additives are used to improve the processibility of the mixtures. After the shape forming step, the parts are heated to remove polymer and then sintered to form the final products. The state of dispersion of the ceramic powder in the polymer melt, i.e., the distribution and packing of ceramic particles in the polymer, determines the fluidity of the ceramic/polymer mixture and thus controls the flow pattern of the mixture in the die during injection molding. Italso has a strong effect on the maximum solids loading (i.e. volume fraction of ceramic powder in the mixture) that can be achieved. In general, a high solids loading is desired in order to minimize the polymer amount to be removed and to reduce the amount of shrinkage during sintering. During the polymer burnout step, transport ofpolymer molecules is influenced by the pore size and size distribution formed by the packing arrangement of the ceramic particles and, thus, the polymer 1 2 removal process is indirectly dependent on the dispersion state. Furthermore, the densification rate and the grain growth rate during sintering are strongly affected by the particle packing arrangement in the ceramic powder compact. Therefore, it is important to examine the dispersion of ceramic powders in polymer melts since it strongly influences each step in the processing sequence and ultimately affects the microstructure and properties of the final product. Information aboutdispersion and rheology is also crucial in processing ofpolymer composites in which inorganic particles or fibers are incorporated into polymers, either to reduce the cost or to tailor the composite properties [Han74, HesW82, Utr82]. For example, the existence of porous particle agglomerates in a polymer matrix (i.e., poor dispersion) can significantly reduce the mechanical strength of the composites. In addition to affecting physical properties, the energy consumption during processing of the composites is much less for a well-dispersed mixture since the viscosity is lower. The state of dispersion in ceramic/polymer mixtures is dependent on the mixing conditions and the properties of the starting materials (i.e., ceramic powders, polymers, and chemical additives). The present study addresses the following four areas: Mixing conditions. The effect of mixing variables, including time, temperature, and rotor speed, on the dispersion of alumina in polyethylene was investigated. The study was confined to a simple two-phase ceramic/polymer mixturewithout any chemical additives. Rheological flow measurements, torque rheometry, ceramic/polymer melt wetting behavior, and quantitative microscopic analysis were used to evaluate the effect of mixing variables on the state of dispersion. 3 Ceramic powder characteristics Ceramic/polymer injection molding is affected . by ceramic powder properties, such as particle size, size distribution, particle shape, etc. To some extent, these variables have been studied [Big84b, Wil78]. However, the effect ofceramic surface hydroxylationand adsorbed molecular water onparticle dispersion has not been investigated. In this part of the study, alumina powders were calcined at temperatures in the range of 100-1000°C prior to mixing with polyethylene (i.e., in order to remove surface hydroxyl groups and adsorbed water). The effects of calcination on dispersion and aging behavior were evaluated using rheological flow measurements, torque rheometry, infrared spectroscopy, hardness tests, particle size and specific surface area measurements, ceramic/polymer melt wetting behavior, and qualitative and quantitative microscopic analysis. Polymercharacteristics Theceramic/polymerrheologicalbehaviorcan bealtered . significantly by varying polymer properties. In this part of the study, experiments were carried out using polymers with different functional groups: polyethylene (PE), ethylene- acrylic acid (EAA), and ethylene-vinyl acetate (EVA). The influence of polymer chemistiy on the ceramic/polymer mixture properties were investigated by rheological flow measurements of polymer melts and ceramic/polymer mixtures, ceramic/polymer melt wetting behavior, quantitative microscopic analysis, and torque rheometry. Chemical additives Chemical additives can be used to modify the . ceramic/polymer interface and thereby alter the state of dispersion and composite properties [Big83, Zha88]. In thispart ofthe study, several coupling agents, surfactants, and fatty acids were used to modify the alumina dispersion in polyethylene. The role of 4 these chemical additives was investigated using torque rheometry, rheological flow measurements, ceramic/polymer melt wetting behavior, and quantitative microscopic analysis.