Commercially Important Fillers

There are also synthetic fillers such as hollow, solid and expandable microspheres made of glass or polymer (several are available in coated or metalled form). 

Only a few fillers will be considered here individually. Others are discussed under the heading of flame retardants or antistatic agents. 

Calcium carbonate is readily available in all continents and its use in the plastics industry is much greater than that of any other filler. (Quantities used are discussed in Chapter 6.) Important sources of calcium carbonate include limestone, chalk and marble. Ground chalk is sometimes called whiting. 

The reasons for the popularity of calcium carbonate in plastics are its ready availability and low cost, together with its favourable particle shape which does not increase the polymer viscosity excessively, nor lower the strength and impact resistance by too much. 

The majority of calcium carbonate used in plastics is produced by grinding rocks to form particles. The purity of calcium carbonate is often low because of contamination with metallic ions, many of which introduce imwanted colour. Chalk can then appear cream coloured, and marble and limestone are both frequently contaminated by iron and other transition metals, whereas pure marble should be white. The removal of the impurities is not usually worthwhile, so they are a hindrance to the wider usage of calcium carbonate in plastics. 

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It is common practice in the siHcone mbber industry to prepare specific or custom mixtures of polymer, fillers, and cure catalysts for particular appHcations. The number of potential combinations is enormous. In general, the mixture is selected to achieve some special operating or processing requirement, and the formulations are classified accordingly. Table 6 Hsts some of the commercially important types. 

Thus, a brief survey of the current understanding of the molecular and super-molecular structures of common thermoplastics is presented first. This review starts with a brief description of the current state-of-the-art knowledge of the constitution, configuration, conformation and supermolecular structure of common glassy and semicrystalline thermoplastics. Later in this chapter, specific features of the structure-property relationships are discussed in greater detail for the most frequently filled thermoplastics. Effects of fillers on the structural variables in polypropylene, considered the most commercially important matrix, are especially emphasized. 

It is estimated that over one million tons of mineral fillers were used in thermoplastic applications in western Europe in 1986 [2], and the figure is doubtless much greater today. Mineral fillers are used to some extent in virtually all the commercially important thermoplastic polymers but, in volume terms, the principal markets are in PVC and polyolefins, where calcium carbonate dominates the filler types with over 80% of the volume consumption [2]. 

The incorporation of carbon black into elastomeric systems is a process of significant commercial importance. However, the additional stiffness of the sample imparted by the reinforcement effect of fillers is not favourable in terms of the experimental conditions for high-resolution NMR spectroscopy. Electric conductivity of the carbon black may also interfere to some extent. Under these circumstances, filled formulations are not widely used for the study of elastomer vulcanisations where high resolution and signal-to-noise ratios are required to detect small amounts of vulcanisation products. 

TABLE 4.33 Some important properties of commercial mineral fillers used in plastics industry and/or in WPC materials (adapted from Ref. [1])... 

Although many kinds of fillers and fibers have been added to POs over the years, and new ones continue to be developed, the sections below cover the most used and most commercially important materials. These fillers and fibers continue to draw the greatest efforts from industry and academia for further development and improvement. Some newer kinds, such as nanofillers and plant-based fibers, are included here mainly because of their potential future importance. As in other chapters of this book, here the focus is more on materials that can be added in a typical compounding operation or "at the press"—rather than modifiers that are added more upstream by the resin producer, or hybrid combinations of materials, such as glass-mat composites or laminates, where the reinforcing material is not added during screw processing. 

The interaction between the surface of particulate and fibrous fillers and a polymer matrix plays a key role in deterrnining the processability and properties of filled composites. Unmodified filler surfaces often give poor interactions and this has led to the growth of an industry based on the use of additives to modify filler surfaces and improve their interactions with polymers. Several types of additives have been evolved for this purpose. Two of these, organofunctional silanes and titanates, have been described in Chapters 4 and 5. This chapter covers other approaches that have been studied, although only a few of these, notably fatty and unsaturated carboxylic acids and functionalized polymers, have achieved much commercial importance. 

Ethylene/propylene copolymerization is of significant commercial importance. Elastomers formed from random ethylene/propylene copolymers (EP) possess a number of valuable properties including a high plateau modulus ( 1.6 MPa), which permits a higher filler loading and more cost-effective compounding. Furthermore, of the major hydrocarbon-based rubbers, EP is by far the least reactive with oxygen and ozone. 

A vast theoretical and experimental material was devoted to the reinforcement of the polymer materials with short fibres. Thus, a new class of engineering materials was obtained, which are characterised by improved stiffness and strength, heat distorsion temperature, comparatively to the analogous without fillers, and which gained a peculiar commercial importance in the last 2-3 decades [1253-1256]. 

Whilst many of these areas fall outside the scope of this chapter, particulate polymer composites are becoming increasingly complex and commonly require more than just inclusion of a filler or particle additive in order to achieve optimum properties. For example, rubber modification of mineral-filled thermoplastics to yield a balance of enhanced toughness and stiffness, is an area of commercial importance. In these ternary-phase systems, there is not only a requirement to attain good dispersion of the filler component, but also a need for breakdown of the rubbery inclusion to yield the most effective size and spatial location within the composition. Whilst this may depend to a large extent on characteristics of the material s formulation, it can also be influenced by the material s compounding route. 

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