Filler production

The principal mineral fillers used in thermoplastics and the reasons for using them are identified, together with those features that have to be controlled in order to achieve the optimum results and to avoid associated deleterious effects. General methods of filler production are outlined in the light of these requirements and their application to the fillers in most use is described. Attention is given to the use of surface modification methods where these are part of the production process. 

Although there is an extensive scientific literature dealing with the complex subject of the effects of mineral fillers on the properties of thermoplastics, there is much less dealing with the equally important area of filler production, much of the information in this area being regarded by producers as proprietary. Useful information on some aspects can be found in a number of works [3-8], but much of what is published is in trade rather than scientific literature and, although some of this is of a high standard, it is not readily accessible. 

Of the various synthetic processes that are available, two are of most relevance in the present context - precipitation from aqueous solution and melt forming. These methods are used where it is not possible to produce adequate products directly from natural sources. This will be because there is no suitable mineral, due to the chemical nature of the product, of particle size and shape requirements, or to purity considerations. The other principal synthetic method in use for filler production is pyrolysis/combustion. This type of process in which the particles are formed in the gas phase is used where very small particles are required, such as with carbon blacks and some silicas. This type of filler is not widely used in thermoplastics and so these processes are not discussed in any detail here, although some information specific to the production of antimony oxide will be found later. 

Like many filler production processes, there is no overall chemical change, the reactions being used to reduce particle size and to allow a high level of purification by going through a solution phase. 

Filling techniques can also be used immediately before ETCA, as the ETCA post-peel cream soon stops inflammation and oxidation. The cream can also absorb the free radicals that are released by the peel and that could damage the three-dimensional structure of dermal fillers, such as hyaluronic acid. In fact, the instructions for most dermal filler products specify not to apply a peel after the filler, but... 

The first edition of this book was written in 1992. At that time it was not obvious that the pace of filler development was accelerating. In the intervening 6 years, much has been done and there are many new filler products on the market and under development. These have opened new and exciting business opportunities which formulators and marketing managers have exploited in a wide range of new products. The new edition of the book covers many of these developments and discusses the potential for future research and development. What was dealt with only as a passing reference in the fust edition now requires a chapter to do it justice. 

Because PCC is most often produced in close proximity to a host paper mill, filler products are usually delivered to a mill storage tank via pipeline at low solids. This eliminates the need for dispersing agents, which almost always interfere with papermaking retention chemistries. 

Hydrous kaolins, standard GCC, rhombohedral PCC, talc and titanium dioxide may be considered products that comprise discrete particles. These filler products typically range from plates to spheres to blocks of varying uniformity. 

Solvay products are used for the manufacture of products at considerably lower temperatures than needed for PVC, processed by calendering, extrusion, and thermoforming. Highly transparent finished products and highly filled (up to 75 wt% filler) products can be manufactured. 

Improved filler production. Thus, they can be used as milling aids and to improve filtration and reduce hardness development during drying. 

Similar solution coating procedures have been widely used in much of the scientific work on silane treatment of particulate fillers. Unfortunately such procedures do not lend themselves readily to most commercial filler production processes, where some form of direct reaction between the silane and filler powder is frequently used. In many instances the filler coating is actually carried out in situ during the compounding process, essentially utilising the polymer matrix as the solvent. These distinctions must be borne in mind when trying to relate laboratory studies to results achieved with commercial products. 

Likewise, the density of a filler is a property that is rarely influenced by the filler production process and therefore should not form part of a production specification. It is of great importance, however, to know the density, if for no other reason than for costing purposes. 

An experimental glass-ceramic filler production line with capacity of 1000 tons per year was constructed and successfully operated at the Experimental Glass Making Plant (Tula, Russia). This production line included an SCM (Fig. 7) designed by the Gas Institute NASU, a crusher and a rotating furnace for crystallization. Figure 8 shows a newer design of the two-phase thermosyphon vault of the SCM. 

Figure 7. Schematic of SCM for glass-ceramic filler production 1 - charge loading, 2 - feeder, 3 -melt, 4 - SCM body (water jackets), 5 - burners, 6 and 7 - cooling water inlet and outlet, 8 - tap-hole for melt draining, and 9 - flue-gases duct.

The production, shipping and application of GCC fillers in wet (slurry) form has become by far the most preferred practice. GCC filler slurries exhibit a soHds content of 65-72% (by weight) and are usually stabilized by using an anionic grinding and dispersing agent. Specifically, cationically stabilized GCC filler products are also available. 

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