Mineral fillers hydrophilicity

Very low water uptake, by reduction in the polarity of mineral fillers in paints and hydrophilic building materials. 

Of the various mineral fillers used, calcium carbonate (CaCO ) is one of the most common, due mainly to its availability in readily usable form and low cost [76]. However, the incompatibility of its high energetic hydrophilic surface with the low-energy surface of hydrophobic polymers, e.g., polyethylene (PE) and polypropylene (PP), is a particular problem. For this and other reasons, the surface of calcite is often rendered organophilic by a variety of surface modifiers such as silanes, titanates, phosphates, and stearic acid. 

Surface modifiers change the physical chemistry of mineral and fiber surfaces. Hydrophilic surfaces, as commonly found with many mineral fillers, can be made hydrophobic by surface modification. Mineral and glass fibers can be treated so that the polymers they reinforce will wet their surfaces. Surface modifiers are chemically bound to one surface and physically attracted to another surface or material. They... 

A typical formulation of a photocurable composite resin contains four basic components a radical-type photo initiator, an acrylate functionalized oligomer, a reactive diluent and the clay mineral filler. The photoinitiator is usually an aromatic ketone which cleaves into two radical fragments upon UV exposure. The telechelic oligomer consists of a short polymer chain (polyurethane, polyether, polyester) end-capped by the very reactive acrylate double bond. An acrylate monomer is generally used as reactive diluent to reduce the resin viscosity. Figure 7.2 shows some typical compounds used in UV-curable acrylic resins. Different types of phyllosilicates were selected as mineral filler an organophilic clay (Nanomer I-30E from Nanocor), native hydrophilic clays (montmorillonite KIO and bentonite) and a synthetic clay (beidellite). 

Inorganic fillers are widely used to reinforce polymers to improve their properties. Organic fibres such as cellulose-based fibres have started to replace synthetic minerals [27-35]. The main disadvantage of organic fibre is its enhanced moisture absorption and the poor adhesion between hydrophilic fibres and the hydrophobic polymer matrix. 

Moisture content should be controlled to be as low as possible, bearing in mind the surface area and chemistry of the filler. Low surface area fillers with hydrophobic surfaces will naturally maintain a low moisture content once dried. High surface area fillers with hydrophilic surfaces, such as precipitated silica, will regain moisture from the atmosphere rapidly and relatively high levels of moisture (5-7%) are unavoidable. Other minerals such as kaolin are also sensitive to moisture regain, but to more acceptable levels of moisture (0.5-2.0%). Total absence of moisture, however, should be avoided as water plays a subtle role in many crosslinking reactions. 

Most fillers - calcium carbonate, kaolin, mica and wollastonite - have polar surfaces. Conversely, many polymers, such as the polyolefins, are hydrophobic. These will not readily wet hydrophilic fillers. It is therefore necessary to treat the filler surface to facilitate intimate polymer-mineral contact. Surface treatments also act as internal lubricants and improve the dispersion of the filler in the plastic matrix and the flow characteristics of the filled polymer. A further effect of some surface treatments is to improve the mechanical properties when exposed to water in vapor or liquid form, especially at high temperatures. These topics are discussed in detail in the chapter Mi era/ Surface Modification. 

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