C 1 Clay Particles example essay topic

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 delaminate d 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 micrograph's 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 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 " [1]. Polymer-layered silicate containing low levels of exfoliated clays, such as 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 scale. To influence the interactions that govern the mechanical properties of polymers, specific scale reinforcement is efficient and beneficial. For example, clay provides such reinforcement through the interaction of polymer chains with the charged surfaced of clay lamellae [2]. The use of as precursors to formation has been extended into various polymer systems including epoxies, polyurethanes, poly imides, nitrile rubber, polyesters, polypropylene, polystyrene and poly siloxanes, 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 arise as the clay are uniformly dispersed (exfoliated) in the polymer matrix, as opposed to being aggregated or phase separated as or simply intercalated. As 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, among others [11-12]. The complete dispersion of clay 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 ordinarily improve rigidity, but they often sacrifice strength, elongation and toughness. However, exfoliated clay, have to the contrary shown improvements in all aspects of their mechanical performance [3]. 1.3 Preparation and Synthesis The preparation of requires extensive delamination of the layered clay structure and complete dispersal of the resulting platelets throughout the polymer matrix. Nanocomposites 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 poly olefin by melt compounding considerably more difficult [13].

Several studies exist for examining behavior of polymer / clay with weak adsorbing parts [14]. Common methods to synthesize polymer 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 C 1.

C 1 has a surface area of 16 m 2/g, as measured with the Quanta Chrome NOVA 1200. Particle size analysis was performed on C 1 using a Coulter LS 230 laser diffraction apparatus and the experimentally measured volume average (d 50) particle diameter is 4 μ m. Figure 1 is an image of the C 1 clay particles at 50 X objective captured with the Olympus BX 60 Optical Microscope with SPOT RT Digital Camera. Figure 1. Delaminate d, dispersed C 1 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 (25 C +/- 0.1 C) using water as the heat transfer fluid. SEM micrograph's with a J EOL JSM 6330 F 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 the UPE / C 1 system, Figure 2 shows that viscosity increases with solids loading, and decreases with shear rate.

The pure polymer system (0 wt% clay) has much lower viscosity than the, indicating a lack of matrix reinforcement that would exist with the presence of clay. At low shear rates, the dependency of viscosity on solids loading is more significant. There is an indication of Newtonian behavior at low shear rates, a shear thinning region at intermediate shear rates, and a second Newtonian plateau at higher shear rates. Figure 2. Viscosity as a function of shear rate for UPE / C 1 composites at different solid loadings (25 C).

For all clay loading weight percentages, the data shows decreasing viscosity with increasing shear rate. There is significant decrease in the viscosity at high shear rates for all clay loading percentages, including the pure polymer. With increasing shear rate the conformations of the intercalated chains are expected to change as silicate layers align parallel to the flow field, thus showing a shear thinning effect, especially for higher shear rates [20, 21]. Figure 3 shows the viscosity behavior of the system as a function of clay solids loading at two different shear rates.

Limiting are significantly affected by solids fraction at low shear rates. With increasing clay loading the viscosity increases and the point at which the viscosity approaches infinity may be considered the point of maximum packing fraction. With increasing shear rate, the conformations of the intercalated chains are expected to change as silicate layers align parallel to the flow field, and therefore transition from liquid-like behavior to solid-like behavior occurs at significantly higher solids loadings [13]. Figure 3. Viscosity as a function of % solids loading at high and low shear rates for UPE / C 1 composites (25 C). Figure 4.

Effect of % solids loading on storage modulus at 25 C for UPE / C 1 composites. Figure 4 represent plot of storage modulus as a function of clay loadings and frequency for the samples used in this study. It can be observed that the storage modulus increases as a function of frequency and solids loading. This is evidence that improvements in terms of enhanced reinforcement potential of the occur with increasing solids loading. Previous research has shown that high storage modulus at low frequencies are exhibited for intercalated due to the reinforcement effect of a well-dispersed, or exfoliated clay in the polymer matrix [22]. Enhanced moduli over the entire frequency range are expected for exfoliated.

3.2 Surface Structure of UPE / Clay Nanocomposites The surface of the with 5 wt% loading of C 1 was observed via a scanning electron microscope (SEM). Figures 5 and 6 show the SEM images of the at 5 wt%. The dark entities are regions of polymer matrix and the light colored shapes are surface fractures, clay particles, or areas of agglomerated clay layers. From the surface of the the clay particles appear to be not uniformly dispersed throughout the polymer matrix. The clay particles are coagulated together like conventional fibers. This is likely to affect the rheological and tensile properties of the samples.

A study of the rheology of polyethylene oxide / showed that different surfactants adsorbed to the exterior surface of the platelet domains mediate differences in the attractive inter particle interactions that give rise to the structure [23]. Some methods commonly employed to obtain exfoliation where dispersion is difficult include the addition of a agent and / or surface treatment. Future work will attempt to address these possibilities. Figure 5. SEM micrograph of 5 wt% UPE / C 1 composites at 10,000 magnification. Figure 6.

SEM micrograph of 5 wt% UPE / C 1 composites at 50,000 magnification. SUMMARY AND CONCLUSION In this work, with C 1 clay and unsaturated polyester were prepared for rheological testing. Rheological tests show a shear thinning behavior for the pure polymer system and for varying loadings of clay. SEM micrograph's show non-uniform dispersion of the C 1 clay in the UPE polymer matrix. Viscosity versus shear rate data show a shear thinning effect at high shear rates and also a convergence to a similar viscosity which is attributed to the alignment and orientation of the clay particles to the flow field at high shear rates. There is strong indication that rheological behavior of the is related to clay loading and the extent of clay dispersion in the polymer matrix.

Surface treatment may be employed to bring about exfoliation of the particles in the polymer matrix. Further testing to be conducted on the made with C 1 clay are XRD and tensile stress / strain tests. ACKNOWLEDGEMENTS The authors are grateful for the financial support provided by the University of Florida Particle Engineering Research Center (NSF Grant No. EEC-94-02989) and the industrial partners of the PERC. Useful discussions with Professor C.L. Beatty and his graduate student Mr. Ajit Bhaskar is greatly acknowledged.

Assistance from Ms. Kerry Sie bein is also greatly acknowledged. Any opinions, findings and conclusions or recommendations expressed in this material are those of the author (s) and do not necessarily reflect those of the National Science Foundation.

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