Filler particle

Filler particle si2e distribution (psd) and shape affect rheology and loading limits of filled compositions and generally are the primary selection criteria. On a theoretical level the influence of particle si2e is understood by contribution to the total energy of a system (2) which can be expressed on a unit volume basis as ... 

Because of the diversity of filler particle shapes, it is difficult to clearly express particle size values in terms of a particle dimension such as length or diameter. Therefore, the particle size of fillers is usually expressed as a theoretical dimension, the equivalent spherical diameter (esd), ie, the diameter of a sphere having the same volume as the particle. An estimate of regularity may be made by comparing the surface area of the equivalent sphere to the actual measured surface area of the particle. The greater the deviation, the more irregular the particle. 

For large amounts of fillers, the maximum theoretical loading with known filler particle size distributions can be estimated. This method (8) assumes efficient packing, ie, the voids between particles are occupied by smaller particles and the voids between the smaller particles are occupied by stiH smaller particles. Thus a very wide filler psd results in a minimum void volume or maximum packing. To get from maximum packing to maximum loading, it is only necessary to express the maximum loading in terms of the minimum amount of binder that fills the interstitial voids and becomes adsorbed on the surface of the filler. 

Bulk Density. Bulk density, or the apparent density, refers to the total amount of space or volume occupied by a given mass of dry powder. It includes the volume taken up by the filler particles themselves and the void volume between the particles. A functional property of fillers in one sense, bulk density is also a key factor in the economics of shipping and storing fillers. 

The abihty of fillers to improve paper brightness increases with their intrinsic brightness, surface area, and refractive index. According to the Mie theory, this abiUty is maximum at an optimum filler particle size, about 0.25 pm in most cases, where the filler particle size is roughly one-half the wavelength of light used for the observation. 

The retention of fillers in the sheet during the forming process is important. Both hydrodynamic mechanisms and colloidal or coflocculation phenomena are significant in determining filler retention (7). Polymeric retention aids are used to bridge between filler particles and fibers. Talc is sometimes used with mechanical pulp furnishes in order to reduce the deposition of pitch-like materials onto paper machinery. 

Typical papers processed using wash deinking are 100% old newspaper and sorted office paper from which toner ink-printed paper has been removed. The effluent from washers is heavily laden with ink, mineral coating and filler particles, and small cellulose fibers. As a result, it can be difficult to clarify. 

Reverse cleaners operate on the same principles as forward cleaners (20). Contaminants less dense than water migrate toward the center of the cleaner and exit as a separate (reject) stream from the pulp slurry. Reverse cleaners are used to remove adhesive and plastic particles as well as paper filler particles and lightweight particles formed from paper coatings. 

Composite Resins. Many composite restorative resins have incorporated fluoride into the filler particles. One commonly used material, yttrium trifluoride [13709-49-4] is incorporated as a radiopaque filler to aid in radiographic diagnosis, and is also responsible for slow release of fluoride from the composites (280). This same effect is achieved with a barium—alumina—fluoro-siUcate glass filler in composite filling and lining materials. Sodium fluoride [7681-49-4] has also been used in composites by incorporating it into the resin matrix material where it provides long-term low level release (281-283). 

This article addresses the synthesis, properties, and appHcations of redox dopable electronically conducting polymers and presents an overview of the field, drawing on specific examples to illustrate general concepts. There have been a number of excellent review articles (1—13). Metal particle-filled polymers, where electrical conductivity is the result of percolation of conducting filler particles in an insulating matrix (14) and ionically conducting polymers, where charge-transport is the result of the motion of ions and is thus a problem of mass transport (15), are not discussed. 

A filler cannot be used to best advantage in a polymer unless there is good adhesion between them. In particular the filler particle-polymer interface will not be stress-bearing and therefore provides a point of mechanical weakness. 

One way of improving the adhesion between polymer and filler is to improve the level of wetting of the filler by the polymer. One approach, which has been used for many years, is to coat the filler with an additive that may be considered to have two active parts. One part is compatible with the filler, the other with the polymer. Probably the best known example is the coating of calcium carbonate with stearic acid. Such coated or activated whitings have been used particularly with hydrocarbon rubbers. It is generally believed that the polar end attaches itself to the filler particle whilst the aliphatic hydrocarbon end is compatible with the rubbery matrix. In a similar manner clays have been treated with amines. 

Another interesting innovation is that developed by the Malaysian Rubber Producers Research Association. In this case the coupling agent is first joined to a natural rubber molecule involving an ene molecular reaction. The complex group added contains a silane portion which subsequently couples to filler particles when these are mixed into the rubber. 

Fig. 7. Microstructures of the three primary graphites used in this work (a) H-451, (b) IG-I I, and (c) AXF-5Q. [F]-filler particles, [P]-pores and [C] cracks.

This is the probability that failure will occur due to the propagation of one tip of the initial defect c under stress o, where is the critical stress intensity factor of the filler particle and a is the filler particle size. 

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