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

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Clay Loading and Dispersion Effects on the Rheological Properties of Unsaturated Polyester Nanocomposites
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Nguyen, Anthony
Zaman, Abbas ( Mentor )
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Gainesville, Fla.
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University of Florida
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English

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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 exfoliatedd) 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 Cl. Cl has a surface area of 16m2/g, as measured with the Quanta Chrome NOVA 1200. Particle

size analysis was performed on Cl1 using a Coulter LS230 laser diffraction apparatus and

the experimentally measured volume average (d50) particle diameter is 4 pm. Figure 1 is an image of

the Cl1 clay particles at 50X objective captured with the Olympus BX60 Optical Microscope with SPOT

RT Digital Camera.


Figure 1. Delaminated, dispersed Cl 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 (25AC � 0.1AC) 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 the UPE/CI nanocomposite 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 nanocomposites, 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.







10000 *0 %wt

1D00D- 10 wt
20 %wt
0 1D00 30 %wt
H * 40 %wt



� 1 u* 1 I *

0.1
0.001 0.i 10 1000 100000

Shear Rate, 1/s
Figure 2. Viscosity as a function of shear rate for UPE/CI composites at different solid loadings

(25AC).




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 nanocomposites system as a function of clay solids loading at two

different shear rates. Limiting viscosities 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].







S10 1/
25 . 5000 1/s



10
. 15



5

0 _
ID


0 10 20 30 40 50

% Solids (wt)
Figure 3. Viscosity as a function of % solids loading at high and low shear rates for UPE/CI
composites (25AC).



10DODO--------------__
100000

1000D , . . . . . - * * ".
* " * 2%
1000 a
6**"** -10%

0 100 20%
10! 130%
0
I0. L 4%



0.1 lU 1 JO
Frequency, Hz
Figure 4. Effect of % solids loading on storage modulus at 25AC for UPE/CI 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 nanocomposites occur with increasing solids loading. Previous
research has shown that high storage modulus at low frequencies are exhibited for
intercalated nanocomposites 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 nanocomposites.


3.2 Surface Structure of UPE/Clay Nanocomposites








The surface of the nanocomposites with 5 wt% loading of Cl was observed via a scanning

electron microscope (SEM). Figures 5 and 6 show the SEM images of the nanocomposites 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 nanocomposites 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 theological and

tensile properties of the nanocomposite samples. A study of the rheology of polyethylene

oxide/organoclay nanocomposites showed that different surfactants adsorbed to the exterior surface

of the platelet domains mediate differences in the attractive interparticle interactions that give rise to

the nanocomposite structure [23]. Some methods commonly employed to obtain exfoliation

where dispersion is difficult include the addition of a compatibilizing agent and/or surface

treatment. Future work will attempt to address these possibilities.





Clay Laye s






- Frackure Line


Figure 5. SEM micrograph of Swt% UPE/Ci composites at 10,000 magnification.













Aggregated Clay






Polywier Matrix







Figure 6. SEM micrograph of 5wt% UPE/CI composites at 50,000 magnification.




SUMMARY AND CONCLUSION



In this work, nanocomposites with C1 clay and unsaturated polyester were prepared for

theological testing. Rheological tests show a shear thinning behavior for the pure polymer system and
for varying loadings of clay. SEM micrographs show non-uniform dispersion of the Cl1 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
theological behavior of the nanocomposites 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 nanocomposites made with Cl1 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 Siebein is also
greatly acknowledged. Any opinions, findings and conclusions or recommendations expressed in
this material are those of the authors) and do not necessarily reflect those of the National
Science Foundation.










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