Kaolin surface treatment

Although kaolins are easily wetted by both oil and water, surface treatment may sometimes be used to enhance the oleophilic properties. The kaolin in paint may represent 20—30% of the pigment. 

Surface-treating agent PVA as, 25 618 Surface treatment(s) kaolins, 6 679... 

Special surface modifications are available to further improve reinforcement. The objective of the surface treatment is to increase filler loading and/or improve physical properties without loss of rheological characteristics. A variety of surface-modified kaolins have been introduced including clays treated with silane, titanate, polyester, and metal hydroxide. Silane-treated kaolin is used in applications requiring maximum aging characteristics in the service environment. 

Applications. The following uses of contact angle were reported in the literature surface energy of different sizes for fibers, correlation between contact angle of fiber and interlaminar shear strength of composite, effect of surface treatment of fillers for paints, the matrix-filler adhesion parameter for PS filled with CaCO, dispersion stability of PEO-modified kaolin particles, determination of contact angle of carbon fibers and its dependence on treatment, wettability of fiber sur-... 

The effect of fillers on creep phenomena (essentially the inverse of stress relaxation) is also of interest a detailed study by Nielsen (1969b) of creep in filled polyethylene is illuminating. Kaolin and wollastonite were used as fillers, both treated with a silane coupling agent and untreated. A major aim was to discover whether the major effect of a filler is due to its effect on elastic modulus, or whether a filler also changes the viscoelastic nature of the system. As reported in previous work by others, the presence of a filler did in fact reduce the creep, the relative effect being nearly independent of the applied stress. The nature of the filler and the surface treatment were also found to be important. In experiments at a constant volume fraction (0.2), kaolin was more effective than wollastonite. Silane treatment of the filler surface tended to decrease creep, especially if the specimens had been soaked in water. 

Surface treatment is another value-added step that can improve the performance of kaolin. Since the filler is naturally very hydrophilic due to its hydroxyl groups, a treatment can be applied to render its surface hydrophobic or organophilic. These surface-modified kaolins are useful especially in plastics and rubber industries, where they improve adhesion and dispersion and hence act more effectively as functional fillers. Silanes, titanates, and fatty adds as discussed in Chapters 4-6, respectively, may be used to modify the surface charaderistics of either hydrous or calcined kaolins, promoting dea lomeration, often lower viscosities, and improved mechanical and eledrical properties. 

Kaolin deposits are cored and analyzed before mining to determine quality. Mined clays are then either wet or dry processed by air floatation or water fractionation. Surface-modified clays can be made by treating standard, delaminated, and calcinated grades with surface modifiers. The treatment can be performed by either the supplier or the end user. These surface modifiers include silane, titanate, polyester, and metal hydroxide. The objective of these surface treatments is to increase filler loadings and/or improve physical properties such as melt viscosity, thermal stability, and modulus without loss of physical characteristics. Electrical applications represent the largest use of surface-modified kaolin in plastics. 

Keywords filler, aspect ratio, particle shape, calcium carbonate, talc, platelets, reinforcement, glass fiber, kaolin, particle size, particle size distribution, chemical composition, adhesion, interface, aggregation, specific surface area, flow-induced orientation, hardness, surface free energy, surface tension, surface treatment, mechanical properties, thermal properties. 

Both kaolin and calcined clays are available with surface treatments. Many treatments may be applied but few have any commercial importance. Kaolins are available treated with surfactants and pH adjusters, which produce kaolins that may be dispersed directly into water, (e.g., rubber latices), or with amines to enhance cure performance. Stearic acid treated products may provide ease of dispersion. The most important treatments, technically, are the organo-silanes. 

The particles are hexagonal platelets which tend to stack together producing larger particles. Surface treatment is needed to facilitate dispersion in a resin. The surface treatment can have side-effects, inhibiting reaction with epoxy and vinyl polymers. The particles are rendered hydrophobic by suitable treatment, for achieving low dielectric loss. Very fine kaolin particles can increase rather than decrease the strength of certain thermoplastics. 

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. 

The BET specific surface area values are listed in Table 2. Acid treatment at room temperature did not significantly modify the surface area of the metakaolin, thus confirming the low effectiveness of this treatment. Activation at 90°C during 6 h produces solids with relatively high surface area, 200 m g, when the kaolin is calcined from 600 to 800°C. These high values are due to the formation of an amorphous silica phase during this acid treatment. However, the solid obtained from MK-900 has a surface area of only 59 m g", due to the above mentioned sinterization of the kaolin at this temperature. 

Data in Table 3 show that after high temperature treatment the alumina/kaolin base matrix retains, 32 % of its original surface area, while the silico-aluminate of reference retains 58 %. The experimental prototypes treated under the same conditions, in addition to having a considerably greater area, after being deactivated, appear to be more stable, since they retain 78% on the average of their original surface area. 

Powders are commonly used as fillers for rubber mixes. The most popular are carbon black, silica, kaolin, or more modem like graphene, fullerenes and carbon nanotubes. The nature of their surface is the main attribute of fillers, as surface energy and specific area determine the compatibility of filler with mbber matrix and the affinity to other c ingredients. One of the major problems is the tendency of fillers to agglomeration - formation of bigger secondary stmctures, associated with lower level of filler dispersion, what is reflected by the decrease of mechanical properties of mbber vulcanizates [1]. Surface modification of powder can improve interaction between mbber matrix and filler. Application of low-temperature plasma treatment for this purpose has been drown increasing attention recently [2, 3]. 

Plasma treatment of kaolin (Fig. 12.3c) during 16 min results in an increase of SFE value and its polar component. After this time further changes are not observed. Hydrogen termination of the process (lasting 2 min) results in almost doubled the polar component of SFE, probably being the effect of surface present carbonyl groups reduction to the more stable carboxyl ones. 

The efficiency of the treatment depends on the filler. Changes to surface free energy and its components are observed for kaolin and wollastonite, whereas practically no eneigetic effect is present in the case of silica. 

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