Concentrated Dispersions - American Chemical Society

Chapter 4

Downloaded by UNIV OF MISSOURI COLUMBIA on March 31, 2013 | http://pubs.acs.org Publication Date: April 20, 2004 | doi: 10.1021/bk-2004-0878.ch004

A New Wetting Polymer for Magnetic Dispersions: Methyl Acrylate and 2-HydroxyI Ethyl Acrylate Copolymers 1

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Meihua Piao , Shukendu Hait , David M. Nikles , and Alan M. Lane 1

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MINT Center and Departments of Chemical Engineering and Chemistry, The University of Alabama, Tuscaloosa, AL 35487 Department of Chemistry and Geochemistry, Colorado School of Mines, 1500 Illinois Street, Golden, CO 80401 2

Abstract A new wetting binder, methyl acrylate and 2-hydroxyl ethyl acrylate copolymer was synthesized and characterized for its ability to disperse magnetic particles in an organic solvent and compared to the commercial binder MR110. The effects of the amount of functional group and the binder molecular weight were studied. For this new binder, the functional group is a hydroxyl group that attaches to the particle surface. It is observed that the optimum amount o f hydroxyl group is 5 mol%, and the molecular weight of the polymer has less significant effects on dispersion quality than the amount o f functional group. The results suggest that the hydroxyl group is not such a strong functional group as the sulfonic group in MR110.

© 2004 American Chemical Society In Concentrated Dispersions; Somasundaran, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

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Introduction The stability of a magnetic dispersion is determined by the adsorption of polymer molecules from solution onto particle surfaces and their conformations in the adsorbed state. The importance of the conformation of the adsorbed polymer wasfirstrecognised in 1951 by Jenckel and Rumbach, who suggested a " loops and trains* model to describe the adsorption behavior. It is also recognized that the segment density distribution is very important for steric stabilization. In other words, the distribution of trains, loops and tails is important for dispersion stability (1,2). It is generally accepted that the interaction between polymer and particle surface is primarily a base and acid interaction (3-5). Therefore, in order to obtain a good dispersion, it is necessary to introduce functional groups in polymer molecules. A binder should have an optimum amount of functional groups. If the concentration of functional group is low, the train density is low resulting in low surface coverage. Consequently, this reduces the stability of magnetic dispersions. If the concentration of functional group is high, the functional group not only acts as an anchor group, but also exists in the loop and tail part of the adsorbed polymers, which causes gelation because of the strong hydrogen bonding interaction between the functional groups in the loops and tails (6). According to the results of Kim et al. (7) and Sumiya et al. (8), a medium molecular weight polymer results in an improved and more stable dispersion. If the polymer molecular weight is low, the molecule is too short to provide good steric barrier since the loops and tails are reduced in this case. If the molecular weight is high, the long molecular chains cause entanglements or bridging effects hindering the orientation of particles, which is harmful to magnetic properties of the dispersion. Therefore, there exists an optimal molecular weight for binders. MR110 is the most commonly used binder in commercial magnetic coatings. It has 0.7 wt% sulfonic acid groups, 0.6 wt% hydroxyl groups, and 3.0 wt% epoxy components. And the active functional group is sulfonic acid and hydroxyl which interact with the surfaces of the magnetic particles by acidic and basic interaction (9). From the point view of environment protection, MR110 is hazardous because it contains chloride. Our aim is to find an environmentally friendly polymer to replace the PVC copolymer as the binder in a magnetic dispersion system. Arcylate polymers are chosen because they do no harm to the environment and they dissolve in green solvents, such as ethyl lactate. In order to introduce functional groups, we used methyl arcylate and 2-hydroxyl ethyl acrylate (CPA), in which hydroxyl group is the anchor group. We regard MR110 ink sample as the model system to compare the quality of dispersions made by CPA polymers.


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In Concentrated Dispersions; Somasundaran, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

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Experimental 2.1 Dispersion The magnetic particles in this study were Co-y-Fe 03, with length 350 nm, density 4.8 g/cm and aspect ratio 6. The binders were C P A polymers synthesized and characterized in our lab. The only difference between these C P A polymers is the monomer ratio, which leads to different concentrations of functional groups as shown in Table 1. Cyclohexanone was chosen as the solvent for its low volatility and its good solubility to C P A polymers. A l l the dispersions were milled in a ball mill for 48 hours. There are two series of samples. The first series is used to investigate the optimum amount of functional group, including CPA1.5, C PA 3 , CPAS, C P A 8 , in which the numbers refer to the mole concentrations of functional groups. A n d the second series is for studying effects of molecular weight, including CPA5-01, CPA5-02, CPA5-03, CPA5-04, corresponding to molecular weight from low to high. 2


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2.2 Characterization Rheological measurements including linear viscoelasticity and steady shear viscosity were done with an A R E S cotrolled shear strain rheometer with a truncated cone and plate geometry (50 mm diameter, 0.0392 rad cone angle). It was equipped with a solvent cover to minimize evaporation. The magnetic measurements were made using an A C susceptometer built in our laboratory.

Results 3.1 Optimize the concentration of hydroxyl group Figure 1 shows the frequency dependence of storage modulus for the first series. A l l the samples are relatively independent of frequency. Storage modulus represents the elasticity of the system. The independence of G ' on frequency means solid like behavior. For magnetic dispersions, this behavior results from network structure formed by the magnetic interaction between aggregates or floes, which is counteracted by the steric barrier provided by polymer adsorption (10,11). The increase of polymer adsorption enhances the steric barrier that weakens the structure. If the polymer adsorption does not


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Table I. Functionality and inherent viscosity of CPA polymers.


Sample

Functionality (mol%)

CPA1.5 CPA3 CPA8 CPA5 (CPA5-02) CPA5-01 CPA5-03 CPA5-04 MR110

inherent Viscosity (dl/g) 0.49 0.27 0.39 0.57 0.46 0.66 0.80 0.37

1.5 3.0 8.0 5.1 5.7 5.1 5.5

5

k

ιυ

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A CPA1.5 • CPA3 Τ CPA5 • CPA8 • MR110

ω CL

ID 10

3

, in* 0.01

4 é

. . 111111 : : :\\\\[! ι—

0.1

ι —

1

—

10

100

ω (rad/s)

Figure 1. Frequency dependence of storage modulus of magnetic inks in the first series.


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provide sufficient surface coverage, the particles form big aggregates. When the adsorption get worse, the dispersion become poorer or even not dispersed at all, leading to closely packed aggregates. In this situation, the G ' value is expected to be extremely high due to its closeness to a solid. As one observed that MR110 ink has the smallest G value resulted from relatively good polymer adsorption. CPAS has the closest G to MR110 ink. CPA1.5 has a higher G ' than that of C P A 5 , which may come from the relatively big aggregates because of insufficiency of functional group. For CAP8, its excessive amount of functional group strengthens the network structure by strong hydrogen bonding between functional groups in loops and tails, which is responsible for its high G ' value. The extremely high G value of CPA3 could result from the closely packed aggregates, since it has both low functionality and molecular weight. Steady shear viscosity data is represented in Fig. 2 and Fig. 3. It is observed that all C P A samples but CPAS have shear thickening behaviors at relatively high shear rate. Especially, C P A 8 has two shear thickening regions while others have one. MR110 shows typical shear thinning behavior. The shear thickening results from big aggregates in the system, which can be explained by the order to disorder transition mechanism (12). In the case of C P A 8 , the second shear thickening may be from the shear induced gelation due to its excessive amount of functional group. When the distance between aggregates becomes close, the opportunity for collision of the functional group in loop and tail increases, leading to gelation. Transverse susceptibility characterizes the dispersion quality by probing the way the material responds to the external magnetic fields. Individual particles or small aggregates correspond to the change of field easily, which increases the magnitude o f transverse susceptibility and enhances the height of peak. If the particle loading is similar, which means the impact from neighboring particles is similar, the high transverse susceptibility means more individual particles or small aggregates, which in turn means good dispersion. The details of this equipment can be found in (13). In Fig. 4, the transverse susceptibility result is illustrated, where one can see that the magnitude of transverse susceptibility of MR110 is the highest. Then come CPAS, C P A 8 , CPA1.5 and CPA3 respectively. It is reasonable to say that CPAS has the best dispersion among C P A inks although it is worse than MR110 ink. One thing needs to be paid attention is that CPA3 has no peak, which is similar to the profile o f dry magnetic powder. Thus, CPA3 has the worst dispersion quality, which is consist with the rheological results. From the results above, we can conclude that CPAS is the best binder among all these C P A polymers, although it is inferior to MR110. This result is consistent with the result obtained by Nakamae et al.(6). They found that at the optimum value of the magnetic properties, the amount of functional group was 4-6 mol% for O H group. f


f

f


46 10= A CPA1.5 • CPA3 • CPAS • CPA8 • MR110

10' 10

J

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Concentrated Dispersions - American Chemical Society

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