Fillers hydrophobicity

Hydrophilic filler Hydrophobic plus (meth)acrylic acid Core hydrophobic Shell plus acrylic acid Water retention in plastics... 

As already alluded to, stearic acid is often added to aid filler handling during production. It is especially useful with very fine precipitated materials, such as calcium carbonate, where it reduces shrinkage on drying and thus improves dispersability. It also makes the filler hydrophobic, and reduces water adsorption, but can make some fillers considerably dustier. 

Two types of membranes may eontains fillers hydrophobic membranes (usually polydimethoxysiloxane filled with zoelites) and heterogeneous ion-exchange membranes which may contain various surface modified inorganic materials. 

Hydrophilic/hydrophobic properties. In water-based systems, the filler should be compatible with water because filler dispersion occurs in an aqueous medium before a polymer emulsion is added. In general, most fillers are hydropho-... 

The results of mechanical properties (presented later in this section) showed that up to 20 phr, the biofillers showed superior strength and elongation behavior than CB, cellulose being the best. After 30 phr the mechanical properties of biocomposites deteriorated because of the poor compatibility of hydrophilic biopolymers with hydrophobic natural rubber(results not shown). While increasing quantity of CB in composites leads to constant increase in the mechanical properties. Scanning electron micrographs revealed presence of polymer-filler adhesion in case of biocomposites at 20 phr. 

The interaction between polymer matrix and filler leads to the formation of a bound polymer in close proximity to the reinforcing filler, which restricts the solvent uptake [13]. The composites containing acetylated cellulose fillers exhibited higher uptake of toluene compared to water in accordance with their hydrophobic nature. 

The surface of silica is covered by a layer of acidic silanol and siloxane groups. This highly polar and hydrophilic character of the filler surface results in a low compatibihty with the rather apolar polymer. Besides, highly attractive forces between silica particles result in strong agglomeration forces. The formation of a hydrophobic shell around the silica particle by the sUica-sUane reaction prevents the formation of a filler-filler network by reduction of the specific surface energy [3]. 

An example of the effectiveness of this equation is given by an aqueous HEUR gel made up of a polymer with Mn = 20 x 103 Daltons at a concentration of 30kgm-3 filled with a poly(styrene) latex with a particle diameter of 0.2 pm at q> = 0.2. The unfilled gel had a network modulus of 0.4 kPa, whilst the modulus of the filled gel was 0.7 kPa. Equation (2.68) predicts a value of 0.728 kPa. The poly(styrene) particles act as a non-interactive filler because the surface is strongly hydrophobic as it consists mainly of benzene rings and adsorbs a monolayer of HEUR via the hydrophobic groups, resulting in a poly(ethylene oxide) coating that does not interact with the HEUR network. This latter point was... 

To illustrate how the effect of the adsorption on the modulus of the filled gel may be modelled we consider the interaction of the same HEUR polymer as described above but in this case filled with poly(ethylmetha-crylate) latex particles. In this case the particle surface is not so hydrophobic but adsorption of the poly (ethylene oxide) backbone is possible. Note that if a terminal hydrophobe of a chain is detached from a micellar cluster and is adsorbed onto the surface, there is no net change in the number of network links and hence the only change in modulus would be due to the volume fraction of the filler. It is only if the backbone is adsorbed that an increase in the number density of network links is produced. As the particles are relatively large compared to the chain dimensions, each adsorption site leads to one additional link. The situation is shown schematically in Figure 2.13. If the number density of additional network links is JVL, we may now write the relative modulus Gr — G/Gf as... 

An interesting point is that our original RTV 118 quenching data is due in part to the presence of hydrophobic silica filler in the polymer. 64 We have examined in considerable detail the effect of silica filler in polymer supports. 33 It is noteworthy that the hydrophobic filled polymers give much less hooked results than the hydrophilic silica. Indeed, the hydrophobic silica gives nearly ideal one-site quenching results. 64 ... 

Precipitated silica is used to produce molecular sieves, as an anti-caking agent, and as filler for paper and rubber. Hydrophobic silica is a defoaming agent. 

The anode layer of polymer electrolyte membrane fuel cells typically includes a catalyst and a binder, often a dispersion of poly(tetraflu-oroethylene) or other hydrophobic polymers, and may also include a filler, e.g., acetylene black carbon. Anode layers may also contain a mixture of a catalyst, ionomer and binder. The presence of a ionomer in the catalyst layer effectively increases the electrochemically active surface area of the catalyst, which requires a ionically conductive pathway to the cathode catalyst to generate electric current (16). 

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