Particulate fillers dispersion

The quality of particulate filler dispersion in the elastomer matrix is of primary importance for compound mechanical and use properties (Gotten, 1983 Funt, 1986 Gerspacher and O Farrell, 1993 Bomo andMorawski, 1983 Richmond etal., 1993). 

Aid in the uniform dispersion of additives. Make powdered solids (e.g. particulate fillers with high energy and hydrophilic surface) more compatible with polymers by coating their surfaces with an adsorbed layer of surfactant in the form of a dispersant. Surface coating reduces the surface energy of fillers, reduces polymer/filler interaction and assists dispersion. Filler coatings increase compound cost. Fatty acids, metal soaps, waxes and fatty alcohols are used as dispersants commonly in concentrations from 2 to 5 wt %. 

The surface tension of two thermoplastics and three fillers are listed in Table 2. Large differences can be observed both in the dispersion, but especially in the polar component. The surface tension of the majority of polymers is in the same range, in fact between that of PP and PMMA. Those listed in Table 2 represent the most important particulate fillers, and also reinforcements used in practice, since clean glass fibers possess similar surface tensions to Si02. Surface treatment lowers the surface tension of fillers significantly (see Sect. 6.1). 

Physical Effects of Filler. Dispersion of any hard particulate matter in a soft matrix will yield a composite with quite different properties. The two main causes for these effects are load sharing of the filler particles and strain dilatation of filled elastomers. 

Increasing the proportion of sulfur in the binder, as shown by mixes No. 3, 4, 6, and 7 in Tables II and III, increases Marshall stability. This is attributed to the excess particulate sulfur, dispersed throughout the asphalt phase, which performs similar to a mineral filler. The particular sulfur may be observed using a microscope and is visible as small yellowish specks along broken mix surfaces. 

Narlex. [Hart Chem. Ltd.] Dispersant superplasticizer for pigments, clays, particulates, fillers. 

These considerations apply both to mixing of the components of a polymer blend and to dispersion of particulate fillers. For blends the obtained size of the... 

Uses Dispersant for pigments, fillers, clay, silt, other suspended matter in water detergent assistant soap builder particulate soil dispersant sequestrant for calcium, magnesium, iron for textile use scale inhibitor, deposit control agent for water treatment, soil removal for laundry, dishwash, consumer/institutional cleaning prods. food pkg. adhesives, coatings, paper... 

It may be recalled from earlier discussions that an equation of this form corresponds to an upper bound to a composite modulus (Section 12.1.1.1). Thus, particulate fillers tend to yield lower-bound values, as predicted by relationships such as Kerner s (1956b), while long, oriented fibers tend to yield upper-bound values of modulus. Short, randomly oriented fibers tend to yield intermediate behavior, but, as pointed out by Brody and Ward (1971), usually lead to moduli closer to the lower than to the upper bound, depending on the modulus of the polymer. On the other hand, Lavengood and Gulbransen (1969) reported moduli closer to upper-bound values for short fibers dispersed in an epoxy resin. 

The embedded material should reinforce the matrix, e.g., with respect to flexural strength. Because this effect is absent with interpenetrating networks, these are often not classed as composites, although, according to structure, they are composites with a zero-dimensional dispersed phase. Polymers reinforced with active or inactive particulate fillers are also not classed as composites. 

There are three basic procedures for mixing rubber compounds in an internal mixer namely, the conventional method, the rapid oil addition method, and the upside-down mix method. Many variations of these three methods are also used to suit the special characteristics of individual formulations and equipment. It is, in general, necessary to add particulate fillers early in the mixing cycle, so that good dispersion is achieved as a result of the high shear stress and high viscosity at the lower temperatures then prevailing. Similarly, the oils and plasticizers which reduce viscosity should be added later. Upside-down procedures and variants of it are attempts to implement these ideas in practice. 

The first hydrodynamic regime consists of randomly dispersed particulate filler in a rubber matrix, which gives an elastic reinforcement of the form given by Equation (3.1) ... 

Dispersive mixing of two immiscible polymers or particulate fillers in a polymer benefits greatly from intermaterial area generation. The increased intermaterial area promotes wetting and homogenization of fillers by polymers and also leads to rapid deformation and breakup of minor component polymers. The interfadal reactions can still occur, but the reaction products - the molecules of the new polymer - stay at the interface and compatibilize the domains. 

Traditional particulate fillers tend to agglomerate already during manufacture, and once agglomerated, it is difficult to subsequently disperse them. Dispersion requires addition of a dispersant additive (e.g., stearic acid or alkysilanes) and the input of energy to overcome the attractive forces between particles [6]. The correct selection of dispersant type and application method is crucial. In contrast, POSS as synthesized, already possesses its own intrinsic dispersant in the form of covalently bonded organic groups and dispersion is easy. 

Strength retention in any elastomer over an extended range of temperature is usually ascribed to the presence of a dispersed phase which results from the presence of strain-induced crystallites or the presence of a small particle-size particulate filler (with a chemically active surface). It is the domain structures in a urethane elastomer that are considered to provide these functions. 

With the notable exception of carbon blacks, the natural surfaces of most particulate fillers are less than optimum for dispersion into, and interaction with, polymers. Surface modifiers can frequently improve this, and other filler properties. Sometimes materials which are effective surface modifiers will be found to have been added for filler manufacturing reasons. The main reasons for finding surface modifiers present are ... 

While described as coupling agents, the main benefits illustrated appear to be in dispersion and especially in producing low viscosity filler dispersions of excellent stability in liquid systems such as unsaturated polyester resin. Among the advantages claimed are the ability to produce hydrolytically stable coatings easily and rapidly with aqueous slurries of particulate fillers including calcium carbonate. The reader is referred to publications by Cohen [90-92] for more details. 

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