Filler Size and Shape

Particle size effects maybe negligible at high shear rates since for filled systems high shear viscosity is often governed by the matrix characteristics and low shear viscosity 

---

Minimising this loss in toughness is a key feature in much development work and this subject has recently been reviewed by [34]. The subject is a very complex one and despite all the effort to date is still far from completely understood. The factors involved have been identified as including filler size and shape dis-... 

Tear Resistance. The resistance of an elastomer to tearing is affected by the particle size and shape of the filler it contains. Tear resistance generally increases with decreasing particle size and increasing sphericity of fillers. 

Effect of Filler Particle Size and Shape on the Rheological Properties of Composites... 

Although a majority of these composite thermistors are based upon carbon black as the conductive filler, it is difficult to control in terms of particle size, distribution, and morphology. One alternative is to use transition metal oxides such as TiO, VO2, and V2O3 as the filler. An advantage of using a ceramic material is that it is possible to easily control critical parameters such as particle size and shape. Typical polymer matrix materials include poly(methyl methacrylate) PMMA, epoxy, silicone elastomer, polyurethane, polycarbonate, and polystyrene. 

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. 

Many of the chemical and physical properties of mineral fillers are important in their application in thermoplastics. These include purity, specific gravity, hardness, electrical, thermal and optical properties, surface area, particle shape and size. The determination and importance of many of these has been covered in several reviews [65,66]. Only a brief coverage is given here for the less ambiguous properties such as specific gravity, hardness and standard thermal and optical properties, with most attention being concentrated on properties such as size and shape which have been found to give particular problems in measurement and interpretation. 

The production process is able to produce all three crystal modifications of calcium carbonate and a wide variety of particle sizes and shapes, including plates and acicular forms [107]. However, only the calcite form with a rhombic shape and a low aspect ratio seems to have found much commercial application in polymers. For filler applications the particles have an ultimate particle size of 50-100 nanometers, a specific surface area of 15-25 m g and a low aspect ratio. 

The hardness of the filler has a strong effect on the wear of the processing equipment, but this is also influenced by the size and shape of the particles, the composition, viscosity, and speed of processing etc. [16]. 

Yield stress values can depend strongly on filler concentration, the size and shape of the particles and the nature of the polymer medium. However, in filled polymer melts yield stress is generally considered to be independent of temperature and polymer molecular mass [1]. The method of determining yield stress from flow curves, for example from dynamic characterization undertaken at low frequency, or extrapolation of shear viscosity measurements to zero shear rate, may lead to differences in the magnitude of yield stress determined [35]. 

Precipitated CaCO. is produced in a number of chemical processes. Somclimcs it is economical In dry and calcine the byproduct to regenerate CaO or Cat OH) . Some precipitated CaCOt is made to specific particle size and shape, whiteness, and purity for use as functional filler for paper coatings, paint, and polymers. These products command a premium price as compared with pulverized limestone fillers. 

The carbonization of a syntactic foam opens up the previously closed cell structure. The size and proportion of micropores formed depend mainly on the size and shape homogeneity of the filler. Thus, there is quite a narrow size distribution of open pores in the carbonized material made from a novolac oligomer and Krecasphere carbon microspheres (Fig. 9)39). 

Particle size and shape will affect the degree of mixing required. Particles with large aspect ratios, such as fibrous fillers, and particles of large size are typically more difficult to disperse. In addition, high filler loading makes sufficient wetting of the filler more difficult because of the increased viscosities encountered. 

Another recent report (6) states that the filler particle size and shape also contributes to stress. In this case, a point of a large, irregularly shaped filler particle that contains the chip causes a "point stress" defect. The cure for this is the use of a filler having a smaller particle size or the use of a chip coat (to be discussed later). 

Silica-based nano- and microsized tubular stmctures have been Icnown since the mid-1990s [1]. The preparation using the sol-gel process is a low-temperature process at room tenqterature and offers scope for manipulation of, e.g., the size and shape of these tubes. Silica-based tubular structures have many advantages, such as easy accessibility, stability, and the possibility of surface functionalization. They can be used for catalysis, separation, reinforcing materials, and fillers for plastics and ceramics. 

Size and shape of one of the fillers (HDPE) were changed to measure the effect on the strength. It was theorized that a fine-graded filler like HDPEfine (FM=2.24) with a large unbonded surface area would have a different effect on the strength than a large-sized filler like HDPEflake (FM=4.89) with less surface area. The tests were designed to determine this effect. 

Slump was most affected by the size and shape of the plastic particles. Typically, small size and high angularity of the fillers caused a reduction in slump of the concrete batch. There were no noticeable effects on slump when 1 percent plastic was used regardless of the particle size and shape. A 4-percent plastic level, however, caused a reduction in... 

In another similar example nanocomposite was formed in a polyurethane matrix. Solvent soluble polyurethane had pyridine groups attached which formed complexes with metal salts. Films were then formed and subjected to a reducing agent in order to produce particulate metal filler. In this case the distribution of the filler which was formed was not uniform because the filler had tendency to aggregate (even though it was chemically attached to the matrix prior to the reduction). The following were factors controlling size and shape of these metal particles ... 

A recent paper by Cussler, et al., (9) using a regular array model also predicted quite high barrier improvements. Their equation (Equation 2) predicted that aspect ratio and volume fraction of filler would be the major variables. They incorporated a universal correction designated, /j, as a geometric factor to correct for the reality that available platelet fillers were not shaped like uniform rectangular parallelepipeds of uniform size and shape. 

Fillers in Rubber. Carbon black and calcium silicate are able to reinforce rubber. For example, the tensile strength of an SBR vulcanizate can be raised from 350 to 3500 Ib/in. by compounding with 50% of its weight of carbon black (54). The activity of the carbon black depends on particle size and shape, porosity, and number of active sites, which are less than 5% of the total surface. Elastomers of a polar nature, such as chloroprene or nitrile rubber, will interact more strongly with filler surfaces having dipoles, such as -OH and -CCX)H groups or chlorine atoms. 

The only requirement for an additive to qualify as filler for PTFE is that it should be able to withstand the sintering temperatures of polytetrafluoroethylene. Sintering involves exposure to temperatures close to 400°C for several hours, which excludes a great many materials. Characteristics of the filler such as particle size and shape and the chemical composition of the filler affect the properties of compound. A list of most common fillers and descriptions of their important characteristics can be found in Table 3.6. 

---