Clay linked Gels : Mechanical Enhancement of Hydrogels by Incorporation of Clay Minerals

Superabsorbent polymers are found in numerous industrial applications due to their outstanding swelling ability for water. Besides swelling capacity, the performance of absorbent gel materials (AGM) is critically determined by the gel strength in the swollen state. If the gel strength of the hydrogel is low, it is easily deformable, which allows the occurrence of gel blocking. Gel blocking hampers homogeneous swelling of the AGM and limits the effective total capacity. The primarily swollen particles gain volume upon swelling and built up a sealing layer that avoids wetting of the lower AGM layers. For application of AGM in hygiene products it is therefore highly desirable to minimize gel blocking while not sacrificing any swelling capacity. In order to strengthen AGM particles, either the gel strength of the bulk polymer may be improved or the surface of AGM particles may be stiffened by additional crosslinking, which results in core – shell particles. This work explores the incorporation of stiff inorganic fillers - clay minerals - into the AGM matrix in order to improve the gel strength of bulk AGM or Young’s -modulus of surface shells of AGM particles. Enhancement of the gel strength and Young’s -modulus is thus achieved by designing AGM composites. The high ionic strength in combination with low pH of the partially neutralized acrylic acid mixtures used to polymerize AGM represents the major obstacle on the way to such AGM composites. Clays tend to agglomerate in such environments and resulting agglomerates might even weaken the gels. In order to exploit the full potential of clay minerals as fillers in AGM matrices it is essential to first disperse the clay platelets homogeneously in the suspension of the partially neutralized monomers. We were able to obtain stable suspensions of clay minerals by modifying them with a commercial cationic dentrimer, ODD. ODD carries an ethylene glycol brush on a poly (ethylene imine) core and is electrostatically bound to the clay surface by intrinsic protonation. A successful dispersion of modified clays allowed to explore the influence of critical parameters like the aspect ratio of the clay platelets on the mechanical performance of the AGM composites by comparing synthetic lithium hectorite (aspect ratio approx. 10000) and montmorillonite PGV® (aspect ratio approx. 30). While the hydrogels could be reinforced by incorporating the different fillers, the tradeoff between swelling capacity and gel strength of the AGM in fully swollen state could be improved only with montmorillonite PGV®, however not with lithium hectorite. Interestingly, for the high aspect ratio lithium hectorite a crossing of the trade off curve could be observed with better performance of the AGM at low swelling (< 5.5 g/g), whereas the PGV® AGM composites showed superior properties at any swelling degree compared to the reference. With the core – shell approach high swelling capacity and high gel strength are spatially separated. While the core of the particle provides the high swelling ability, the shell will grant pressure resistance of shape reducing gel blocking. To optimize this concept, mechanical strengthening of the shell has to be achieved while preserving maximum flexibility expressed by a high stress at break beyond certain threshold values for strain (> 400 %) and Young’s -modulus (> 100 kPa). Moreover, core and shell have to be kept chemically compatible. We considered commercial bulk AGM for the core, while the shell should be built up by an AGM composite material incorporating different clay minerals as filler. The type of clay, the filler content, the amount of organic crosslinker, the acrylic acid content, and the degree of neutralization were varied to optimize the mechanical properties of the composite shells. For the large aspect ratio lithium hectorite both strain at break and Young’s -modulus were found to be below the threshold values. For composites of modified montmorillonite PGV®, however, a significant improvement in performance of the shells could be achieved. For the best composite shells 800 % strain at break in combination with 600 kPa stress at break and a Young’s -modulus of 140 kPA were observed.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2014

Author dc:creator

• Stirner, Manuela

Contributors dc:contributor

• Breu, Josef

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/1746/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:1746