Fillers for Polymer Applications

Fillers have been incorporated into all polymer types, thermoplastics, elastomers, and thermosets, from the very beginning, and it is probably true to say that the development of many polymers would not have been possible without them. They continue to play a vital role today.

One of the original purposes was simply to reduce cost, but this is less important today, as polymers have become less expensive and more of a commodity. Fillers have distinctly different properties to polymers, and by their judicious selection one can produce composite materials with enhanced properties for a given use. It is important to recognize that while benefitting some properties, a filler may be detrimental to others and considerable skill is needed to arrive at the best compromise for any application.

The main reasons for using particulate fillers today vary with the polymer type but include improved processing, increased stiffness, heat distortion temperature and creep resistance, better abrasion and tear resistance, and flame retardancy.

Key properties include cost, specific gravity (density), hardness, purity, particle size and shape, surface chemistry, and thermal stability. Optical, thermal, and electrical properties can also be critical in some applications.

The interface between a particulate filler and the polymer matrix plays an important role in determining the processability and properties of composites. The surface of most commonly used particulate fillers, with the exception of carbon blacks, is suboptimal for this purpose and surface modification is widely used, both to enhance wetting and to improve interaction with the polymer. This modification is generally achieved by chemically reacting a suitable organic modifier with the filler surface.

Two distinct types of surface modifier can be recognized, noncoupling and coupling, depending on the type of organic group introduced. Both types have strong anchorage to the filler surface; but only the coupling type has strong interaction with the polymer. Fatty acids are the most common noncoupling treatment, while organo-silanes are most often used as coupling treatments.

Reaction of the filler surface can be carried out before addition to the polymer or during the polymer/filler mixing process. Both methods have advantages and limitations and are widely used.

Although particulate-filled polymer composites are mature materials with a long history of application, their structure–property correlations are more complicated than usually assumed. The characteristics of all heterogeneous polymer systems including composites containing micro- or nanofillers are determined by four factors: component properties, composition, structure, and interfacial interactions. Several filler characteristics influence composite properties, but the most important ones are particle size, size distribution, specific surface area, and particle shape. The main matrix property is stiffness. Composite properties usually depend nonlinearly on composition, thus they must be always determined as a function of filler content. The structure of particulate-filled polymers is often more complicated than expected. Although segregation rarely occurs in practice, aggregation and the orientation of anisotropic particles take place quite frequently. The former usually deteriorates properties, while the latter determines reinforcement. Interfacial interactions invariably develop in composites; they lead to the formation of a stiff interphase considerably influencing properties. Interactions can be modified by surface treatment, which must be always system specific and selected according to the goal of the modification. Nonreactive coating helps dispersion and beneficial for impact strength, while coupling is needed for reinforcement. Particulate-filled polymers are heterogeneous materials in which inhomogeneous stress distribution and stress concentration develop under the effect of external load. These initiate local micromechanical deformation processes, which determine the macroscopic properties of the composites. Usually debonding is the dominating deformation mechanism in particulate-filled polymers. Although the number of reliable models to predict properties is relatively small, they offer valuable information about structure and interactions in particulate-filled composites. High compounding prices require the thorough consideration and utilization of all benefits of particulate fillers including large stiffness, strength, dimensional stability, increased productivity, etc.

The properties of filled thermoplastics critically depend on how the filler is presented in the polymer, especially its degree of interaction with the host matrix and the nature and extent of mixing achieved. Furthermore, the nature of the filler has a profound influence on the compounding methodology employed. Heat- and shear-sensitive fillers need a very different approach to fillers which have a strong tendency to agglomerate. Different technologies will also be required to produce highly loaded filled compounds to those containing small amounts of filler. The method used to combine filler and polymer defines the microstructure developed, principally through exposure to the shear and elongational flow fields encountered during melt compounding. This is influenced by the rheology of the formulation and the constructional design and operational parameters used, which, in turn, define the extent and mode of mixing achieved. Additional functional stages may be required, for example, to extract volatiles from the compound or undertake reactive steps in the process. A key aspect in many filled compounding operations is the need for dedicated ancillary equipment, which often includes pre-blending components in the formulation, controlled feeding of filler and polymer into the compounder, and downstream cooling and pelletizing of the ultimate compound. In order to monitor compound consistency, a variety of in-process characterization techniques are being used or are under development. More energy-efficient filled compounding procedures are available which combine the mixing requirements with end product formation by extrusion or injection molding.

This entry will explore these issues, highlighting specific compounding requirements for differing filled polymer combinations, the principles of mixing particulate fillers into thermoplastic melts, and how this knowledge influences the engineering design and effective operation of industrial compounding plant.

Although thermoset polymers are significant users of particulate fillers, the scientific literature is much scarcer than for other polymer types. This is because they are usually used in combination with glass fibers, which then dominate the properties of the composite.

The processing of thermosets is different to that for the other two polymer types, with the filler being added to a relatively low viscosity, often liquid, phase; where the high shear that helps dispersion in other polymer types is missing. On the other hand, there is less damage to the filler particles, which means that it is easier to use fillers such as mica, wollastonite, and glass fibers. It is also easier to incorporate hard fillers, such as crystalline silicas, and temperature-sensitive ones like cellulosics, than it is with other polymers. Thermoset polymers are also able to tolerate larger particle size fillers. Because they are polymerized during molding, thermosets can also exhibit higher mold shrinkage and controlling this is an important role of particulate fillers. Fillers are also important in reducing polymerization exotherms, which can otherwise cause problems.

The main general purpose filler used in thermosets is calcium carbonate in various forms. This is mainly employed for cost reduction, shrinkage, and exotherm control. Large quantities of aluminum hydroxides are also used for low smoke and fume flame-retardant and aesthetic purposes. Epoxy printed circuit boards use fillers such as alumina to impart high thermal conductivity while retaining low electrical conductivity. Thermosets make more use of hard fillers such as crystalline silicas than other polymer composites. These are used to improve abrasion resistance in flooring and solid surface applications.

Most thermosets are polar, and this means that they can wet and interact well with many types of fillers, especially minerals like carbonates. This reduces the need for surface-modifying species, but dispersants and coupling agents may still be utilized, especially with siliceous fillers. Coupling agents are also often used to help property retention under adverse environmental conditions rather than to improve initial properties.

Fine particulate fillers are widely used in elastomers (indeed are essential for many applications) and are small hard particles usually of carbon or inorganic origin. The main fillers used in general purpose elastomers are carbon black, precipitated silicas, clays, and natural carbonates. There is also significant use of precipitated calcium carbonates and of synthetic flame retardant fillers such as aluminum hydroxide. Fumed silica plays a key role in silicone elastomers while finely ground crystalline silicas are important in specialist high temperature elastomers.

Filler reinforcing effects depend very much on the type of elastomer. They are most obvious in noncrystallizing elastomers, such as most synthetic types. Here, the fine filler particles are able to act as tiny crystallites and markedly improve the tensile strength, abrasion resistance, and higher extension modulus. In the crystallizing types, such as natural rubber, the elastomer crystallites which develop on stretching often hide filler effects in laboratory tests, but they are still present and important in many applications.

Size, shape, and surface activity are all important factors determining reinforcing ability. Small size, high structure, and high surface activity generally give the best blend of properties. Carbon blacks owe their preeminence to the natural ability of their surface to form strong interaction with hydrocarbon polymers. Other fillers generally require the addition of coupling agents to achieve the same effect.

Calcium carbonate fillers have ideal properties for many polymer applications and the world consumption is over ten million tonnes annually. Both natural (ground (natural) calcium carbonates, GCC) and synthetic forms are in use and can be derived from abundant and widely occurring natural deposits. The natural forms are available with a wide particle-size range (0.5 to over 100 μm), are less expensive to manufacture, and have the bulk of the market. They are widely used in thermoplastics, thermosets, and elastomers. The synthetic (precipitated) forms used in polymer applications fall into the nano-size range (under 100 nm) and are used where such small particles are beneficial. Their main use is in elastomers and PVC. Fatty acid surface coatings are often added and there are also specialized coupling agents available.

Ground (natural) calcium carbonates (GCC) is a critical and functional mineral for plastics, and there is no doubt that it will continue to be one of the principal fillers used by the polymer industry. While GCC fillers are long established, they continue to evolve. Developments in grinding are likely to result in even finer forms and increased opportunities. Examples of recent developments include an innovative mineral additive for fibers and nonwovens from Imerys known as FiberLink®. FiberLink increases softness, reduces luster, improves opacity, and improves tensile strength of the fabric/web.

Clays of various sorts are widely available throughout the world and have been used in polymer composites, especially those based on elastomers, since the early days of their industrial application. While there are a large number of clay materials, the main ones used in polymers are based on the mineral kaolinite and are often referred to as kaolin or china clay.

The china clays themselves have limited application in thermoplastic and thermoset applications. This is due to a number of factors, such as poor color and heat aging, especially in polypropylene. The water of crystallization is also an issue for water-sensitive polymers such as nylon and thermoplastic polyesters, as it can be released during processing. Many of the problems are overcome by calcination, and calcined forms are more widely used for these polymers.

On the other hand, china clay is a widely used white filler in the rubber industry. Depending on particle size, it can be used as a semi-reinforcing filler (hard clay) or a non-reinforcing filler (soft clay). Common elastomer applications include chemical liners, bicycle tires, conveyor belts, shoe soles, gaskets, and flooring.

Mica (i.e., muscovite and phlogopit) are platelet-shaped reinforcing fillers used for increasing heat deflection temperature and reducing warpage in polymers. Mineralogically the mica group consists of phyllosilicates all with monoclinic crystal structure and perfect basal cleavage. Micas are muscovite (typically white to gray color), phlogopite (brownish depending on iron concentration), biotite (dark brown to black), lepidolite (lithium-mica, white to gray), and others. Industrially in general and especially in polymers, the most commonly used are muscovite, phlogopite, and to some extent also biotite.

Main use is in polyamide for under the bonnet applications in automotive industry. They are truly versatile functional fillers. Whereas sheet and scrap mica minerals are used for larger thermal and electrical insulating shields, ground mica minerals (mostly muscovite) are used as fillers in lacquers, exterior and interior paints, plasterboards, joint compounds, adhesives, and sealants as well as in polymers. Most of the properties conveyed to the matrix are due to the crystal structure, chemical composition, fracture and cleavage behavior, as well as processing parameters used to produce the products. As functional fillers, mica mineral flours are making a crucial contribution to improve the properties of modern plastics.

In polypropylene muscovite mica is commonly used for reinforcing purposes due to the good equilibrium between modulus and IZOD strength as well as good HDT. Due to the brownish color, phlogopite mica is not that common in polypropylene although the reinforcing properties due to the good equilibrium between modulus and IZOD strength as well as a high HDT reveal the benefits for the compound even more than muscovite mica.

Muscovite and phlogopite mica are commonly used in polyamide 6 because of their equilibrated mechanical behavior but even more due to their ability to reduce isotropically the shrinkage of the compound and increase the HDT both resulting in extremely low warping materials. This is one of the prerequisites for using PA6 compounds as functional parts in environments being subjected to high temperature changes.

Talc is a naturally occurring magnesium silicate mineral, widely used as a polymer filler. Its main differentiating features in this context are its softness (it is the softest mineral known) and lamellarity (platiness). Useful deposits are widespread and processed by conventional mining operations to produce filler grade material. The platiness depends on the source and manufacturing methods. The main use for talc is in thermoplastics, such as polypropylene, where it gives a useful balance of strength and stiffness, together with increased heat distortion temperature and creep resistance. The platy nature also provides better gas barrier properties than blocky fillers like calcium carbonate. Significant issues with talcs in composites are the limited response to coupling agents such as organo-silanes and accelerated heat aging due to impurities which varies with the origin of the deposit.

Wollastonite is a metasilicate having an inosilicate (chain silicate) structure. It belongs to the pyroxenoid group of minerals. It is the only white acicular mineral and became famous and widely used in the industry in the 1970s 1980s as a nonhazardous replacement for asbestos fibers. During crushing and grinding, it forms acicular particles having an aspect ratio (length vs. diameter ratio) in the range of 1:3 up to 1:15. The structure of the wollastonite particles depends partly on geological conditions of genesis but also to a large extent on the grinding technology. The applications are widely spread reaching from additive in metallurgy (protective slag, welding electrodes) over ceramics and ceramic boards to reinforcement in thermoplastics and thermosets (car interiors, fenders, trims) and corrosion inhibitor as well as reinforcement in paints and coatings.

Especially in cost-efficient PP systems, high-aspect-ratio (HAR) wollastonite can confer the compound properties necessary to replace other high-price polymeric systems, e.g., in automotive industry. Properties asked for by automotive applications are low thermal expansion (CLTE), zero-gap bumpers, painting ability, high flow, mold in color, high stiffness, reduced shrinkage, and improved resistance to UV and petrol. Besides the fibrous structure of the mineral and its size distribution, the surface treatment plays a major role when looking at dispersing ability, scratch resistance, and mechanical properties.

LAR, aminosilane-coated wollastonite flours have been successfully used for many years as functional fillers for polyamides. With HAR wollastonite the rigidity of the compound is even higher than with LAR wollastonites. Polyamides reinforced in this way offer a range of different possibilities for the design of low warpage, rigid moldings/parts. They are used in, e.g., wheel covers, air filter housings, and electrical appliance parts.

Polyurethane for reaction injection molding (PUR RIM) is a microcellular (integral) hard foam for thin-walled moldings exposed to compressive and flexural stress. The PUR R-RIM-reinforced variants with HAR wollastonite are used when a higher elastic memory is required.

In comparison to other fillers in mid-voltage insulation applications based on epoxy resin molding materials, wollastonite is used due to its reinforcing properties when wall thickness in technical parts shall be reduced or resistance toward cracking – i.e., better flexural modulus – should be improved. The coefficient of linear thermal expansion is comparably low.

In recent years the development of brake pads/linings based on phenolic resins has led to the use of environmentally friendly raw materials like wollastonite, which is critical to health.

LAR, silane-coated wollastonite flours are successfully used for many years as functional fillers in shaft seals based on fluoroelastomers in order to adjust the hardness. Apart from the good reinforcement in fluoroelastomers, LAR wollastonite (due to its bright and neutral color) provides also the advantage of colored bright mixtures. These optimized fluoroelastomers are ideally suited for the applications under special conditions, e.g., when high mechanical load and high temperatures apply and whenever chemical resistance is crucial.

Feldspars are a group of tectosilicate minerals being the most important ones for rock formation of the Earth’s crust having a share of 51% on it.

Syenite is a group of plutonic (or intrusive) rocks being rich on feldspars – nepheline syenite is the most important member of that group. In ceramics and glass, it is used like feldspar as a source of alumina and as a flux.

In polymer-bond systems, the feldspars and nepheline syenite are common as fillers in paints and lacquers as well as in pigment pastes. Refractive indices are in the region of 1.50–1.55 with little to no birefringence, so they match refractive indices of several resins very well to form highly transparent films with high abrasion and scratch resistance.

The use in thermoplastics is – due to their high Mohs hardness – limited to few applications like antiblocking in polyolefin films and lately also as a filler for dental composites. Apart from their use in ceramics and glass making, the most common use in polymer or resin bond systems is in paints and coatings where they provide excellent chalking and weathering resistance in exterior paints, high UV transparency in radiation-curable films (parquet coatings), and scratch resistance as well as high transparency for visible light in resins matching their refractive index due to their low birefringence.

In thermoplastics feldspars are used for antiblocking. Polyolefin films (mostly blown or cast polyethylene (LLDPE) and polypropylene (PP) – but also PVC and to some extent PET) tend to adhere to each other due to strong van der Waals interaction or electrostatic charges when being in close contact. To avoid the adherence of layers due to a close contact, particulate matter is introduced into the film in a highly diluted concentration. By that measure the contact area of film layers is minimized and adherence suppressed. Due to their optical properties, feldspars are preferably used in this respect.

Another application is light and thermal management in agricultural films. Greenhouses made from polyolefin films (LDPE) offer cost-effective solutions for intensified agriculture. Transmission and reflection of the solar spectrum are important key parameters for the growth of plants and the heat management in the greenhouse. The biologically active UV/VIS radiation for plant growth should pass the film as completely as possible, but heat should be reflected during the day to avoid overheating during the day but should be kept inside during cold nights to avoid cooling during the night. Feldspars are used to make those LDPE films suitable for thermal management. Advantages are early and high-quality crops, protection against cold/frost, and higher yields.

Apart from their use in veneers, highly pure feldspars become more and more interesting to dental applications like dental fillings, inlays, or artificial teeth in polymer compounds. Besides that these applications do not represent mass markets, they use similar principles in optical appearance and transparency and color like the applications mentioned before.

Magnetite, Fe₃O₄, is a naturally occurring, safe iron oxide with an unusual and useful combination of properties compared to the better-known functional fillers for plastics. It is used to make high-density compounds for sound damping, particularly in automobiles, and also to lend heft, that is, the impression of quality. It is able to block x-rays and other types of radiation, making it useful as a nonhazardous replacement for lead-based blocking materials. A very high volumetric specific heat capacity allows it to absorb and release large amounts of heat energy, which is useful for green buildings. It is microwave and induction heatable, suggesting potential in cooking applications. Thermal conductivity is useful for heat dissipation in electrical devices and electricity conductivity allows it to be used as a permanent antistatic additive. Natural magnetite is composed of hard, angular particles that can enhance slip resistance of polymer flooring. Last, but not least, it is strongly attracted to magnets, so it can be used to impart magnetic properties to plastics. Applications of this multifaceted and relatively unknown specialty filler continue to expand.

Carbon black is the generic name for a family of small-size, mostly amorphous, or paracrystalline carbon particles grown together to form aggregates of different sizes and shapes. Carbon black is formed in the gas phase by the thermal decomposition of hydrocarbons in the absence or presence of oxygen in substoichiometric quantities and is industrially manufactured in the form of hundreds of defined commercial grades that vary in their primary particle size, aggregate size and shape, porosity, surface area, and chemistry.

Carbon blacks are mainly used as reinforcing fillers in tires and other rubber products. The reinforcement effect is influenced by the interaction between the elastomer molecules, between the carbon black particles themselves, and between the carbon black particles and the elastomer matrix. For elastomer reinforcement, the primary particle size (specific BET surface area) and surface activity of the carbon black types are important as well as their carbon black structure. In addition, the degree of carbon black dispersion achieved and the carbon black loading used in the elastomer composite play a role. The type of carbon black can significantly influence the properties of the resulting rubber compounds. This explains the existence of many different standardized industrial carbon black grades being used in rubber compounds for the body and tread of tires.

Carbon blacks are expected to continue to dominate the rubber market for the foreseeable future, but they are coming under considerable pressure from precipitated silica in some important tire applications. This is because the silica offers lower rolling resistance properties and hence improved fuel economy and lower emissions. This trend is expected to continue to grow.

Specialty carbon black grades are used as black color pigments in plastics, paints, and inks, as ultraviolet (UV) stabilizers in polymers to avoid their degradation under the influence of visible and UV light, and as fillers to impart electrical conductivity to polymers for electrostatic dissipative and conductive applications.

Precipitated and fumed silicas are important, high-value, effect fillers mainly used in elastomers, sealants, adhesives, and surface coatings. They are amorphous materials, with primary particles in the nano-range and with high specific surface areas. The precipitated products are lower in cost and have the majority of the market in terms of volumes used. One of their most important applications is in fuel-efficient, energy, or green tires. The fumed silicas are an essential component of many silicone elastomer formulations. Both forms are excellent thixotropic agents for liquid systems such as surface coatings, sealants, and adhesives, with the fumed product being the most effective. Silica fume, a by-product from the manufacture of silicon and alloys, is a totally different product to fumed silica, although it is often confused with it and has very limited polymer applications today. Silica gels, which have much in common with precipitated silica, are used in a number of specialist polymer applications, including coatings and films. Surface treatments play a key role in many of the applications, with both functional and nonfunctional organo-silanes being widely used.

While the growing interest in sustainable sourcing has led to a revival of interest in particulate fillers derived from biomaterial sources, this is nothing new. Wood flour was one of the first fillers used in plastics, and rice hull-derived silica was being promoted for use in place of some carbon blacks in the 1970s.

In addition to their sustainability credentials, many of these fillers offer other advantages, particularly weight saving, over minerals. On the other hand, they frequently have a number of limitations including higher cost, poorer thermal stability, color issues, and moisture sensitivity which have hindered their acceptance. Even so, renewed effort is going into their development, especially that of nano-sized particles.

Wood flour is the standout commercial example today, with large volume use in wood polymer composites. Starch, cellulose, lignin, and even proteins are being explored as nanoparticles, especially for tire use. Although their use as a carbon black replacement failed to develop, rice hulls are still being developed for other applications and even as a raw material for precipitated silica filler manufacture.

Mineral filler fire retardants are one of the most important classes of fire retardant, and one of the most important classes of polymer additives. In addition to reducing the flammability of the polymer to within acceptable limits, they can also provide structural integrity and reinforcement to the polymer composite. Mineral filler fire retardants operate through endothermic decomposition with the release of an inert gas or vapor. Four fire retardant effects have been quantified: heat capacity of the filler, decomposition endotherm, heat capacity of the gas or vapor, and heat capacity of the residue. In specific fire scenarios, other factors, such as shielding from radiant heat, may also play a critical role. Unfortunately, the screening techniques for assessment of fire retardant performance do not adequately capture real fire behavior. The common techniques, and their deficiencies, in relation to mineral filler fire retardants are reviewed.

Metals are by far still the most important materials in heat management with extremely high intrinsic heat conductivities (aluminum metal in the range of 200 W/m K). Their severe drawbacks on the other side are their high price, their high density (weight), and their high electrical conductivities.

Thermoplastics and thermosets are widely used in E&E (electric and electronic) applications. Their low thermal conductivity in the range of only 0,2–0,4 W/(m K) is unfortunately linked to their chemical structure.

Materials for thermal management in direct contact with heat sources have to transport heat by conduction as fast and effectively as possible away from them. Literature and experiments reveal that for effective removal of heat, heat conductivities exceeding 1–2 W/m K already drastically reduce temperature at the contact area between heat source and heat sink.

Since polymers offer complete freedom of design (but offering only heat conductivities in the range of 0,2–0,4 W/m K) in contrast to metal, using appropriate additives to increase their insufficient heat conductivity will be the future in integrated heat sink/housing design for LEDs, electric motors/engines, as well as for luminaires and other electronic parts. To achieve this, new rules of design, recipes, compounding, and molding have to be applied to the polymers.

Heat conductive additives for polymers differ heavily in price and performance. Performance must never be measured by heat conductivity alone but especially for housings, parts under mechanical stress or heat sinks also mechanical properties have to be taken into account. For electrically insulating solutions, alumosilicate shows equilibrated properties; heat conductivities of 2–3 W/m K can be achieved. Adding hexagonal boron nitride to an alumosilicate/PA compound can increase heat conductivities even further without deterioration of mechanical properties and with only moderate effect on the price.

The use of thermally conductive plastics creates a whole series of important advantages. Besides the benefits of lightweight construction, the use of plastics offers the possibility of producing complex geometries quickly and cost efficiently by means of injection molding or casting technologies. This new development will influence heavily forthcoming solutions in electronics, electrical devices, automotive industry, e-mobility, and like trends.

Carbon black is incorporated into polymers for permanent electrostatic discharge protection, explosion prevention, and polymer applications that require electrical volume resistivities between 1 and 10⁶ Ω cm. Typically, the so-called conductive carbon black is used since grades that belong to this specialty carbon black family impart electrical conductivity to polymers at lower critical volume fractions than conventional carbon black. Hence, conductive carbon black materials influence to a lower degree the mechanical properties of the resulting conducting polymer compound.

Conductive carbon black grades are produced by furnace black processes and by specially designed processes like the ENSACO^® process or are obtained as by-products from the gasification of hydrocarbons; these processes are based on the thermal-oxidative decomposition of hydrocarbons. In contrast, acetylene black being another conductive carbon black is formed during the exothermic decomposition of acetylene to carbon black and hydrogen occurring above 800 °C in the absence of oxygen.

Conductive carbon black grades show a large carbon black structure indicated by a high void volume. The void volume can be characterized by the oil absorption number (OAN) being above 170 mL/100 g of carbon for typical conductive carbon black. The oil absorption number at a given compression state (COAN) is attributed to the difference in sensitiveness of the carbon black structure toward compression observed between different carbon black grades. Therefore, the COAN indirectly indicates the resistance of the carbon black structure toward shear stress as well as the ability of carbon black to form a conductive network and maintain it in the polymer compound.

Usually the critical carbon black volume fraction at which the polymer compound becomes electrically conductive is decreasing with increasing COAN. The steplike transition from the insulating to the conducting state, which occurs at the critical carbon black volume fraction when incorporating carbon black into the polymer, can be described by a percolation mechanism. The amount of carbon black required to make a polymer compound conductive is, besides the carbon black type, influenced by the polymer type and polymer properties like crystallinity, viscosity, and surface tension. Due to the occurrence of shear stress during the dispersion of the carbon black in the compounding process as well as during the finishing process to the final polymer article, both compounding and finishing have to be considered as well when determining the amount carbon black required for a conductive polymer compound. Statistical, thermodynamic, and structure-oriented percolation models are the best applicable to describe at a theoretical scientific level the formation of the conductive carbon black network in the polymer matrix and to calculate the percolation from the insulating to the conducting state.

Carbon is used as filler in multifunctional polymer compounds. Carbon is present in nature or can be synthetized in different forms. Due to its valency, carbon is capable of forming many allotropes. Well-known forms of carbon include diamond and graphite. In recent decades many more allotropes and forms of carbon have been discovered and researched including ball shapes such as buckminsterfullerene and sheets such as graphene. More extended structures of carbon include nanotubes, nanobuds, and nanoribbons. Other unusual forms of carbon exist at very high temperature or extreme pressures. Graphite is the most common allotrope and is characterized by good electrical, thermal, and lubricating properties. The term “graphitic carbon” includes various types of carbon powders with different levels of crystallinity like natural and synthetic graphite. Natural graphite from ore deposits occurs in three main forms: flake graphite, lump or vein graphite, and amorphous graphite. Synthetic graphite is manufactured from natural or petroleum carbon precursors in high-temperature processes that transfer amorphous carbon to carbon of higher structural order.

The electrically and thermally insulating character of most polymers can be changed by the addition of electrically and thermally conductive fillers like graphite powders. Graphitic carbon powders especially represent a valid filler solution for thermally conductive polymer compounds in the case that electrical insulation is not a prerequisite. For applications where high electrical resistivity is required, graphite can still be used at low concentration when the resulting graphite polymer composite has not percolated to the electrically conductive state but already shows significant thermal conductivity. The two-dimensional crystal structure and anisometric particle shape of graphite lead to anisotropic properties of the final polymer compound. The degree of anisotropy can be influenced by the graphite type, polymer type, and processing conditions. Graphitic carbon powders can also be used as solid lubricant, infrared shielding filler, and gas barriers to reduce the gas penetration through polymer films. Graphitic carbons have been poorly considered in the past as fillers for electrically conductive polymers due to the, compared to carbon black, higher impact on the mechanical properties of the resulting polymer compounds. However, the recent search for metal-free polymer compounds with good thermal conductivity and light weight has offered new opportunities to further exploit the potential of graphite-filled polymer composites in various applications. We will review the main properties of graphitic carbon powders, alongside with processing and properties of the resulting graphite-filled polymer composites and the related final applications as well as the mechanisms for lubrication, electrical, and thermal conduction obtained in graphite-filled polymer compounds.

Anti-blocking is a term used to describe measures to prevent film sheets to stick together. Polyolefin films tend to adhere to each other due to strong van der Waals interaction or electrostatic charges when being in close contact (adjacent layers). The higher the temperature, pressure, and contact/processing time, the higher the tendency to stick to each other.

To avoid the adherence of layers due to a close contact, particulate matter is introduced into the film in a highly diluted concentration. By that measure a micro-rough surface is created, and contact area of film layers is minimized; the distance between the layers is maximized and adherence suppressed. Minerals used for that application should have little to no impact on the mechanical properties of the film; must not deteriorate transparency, haze, color, gloss of the film; and should be compatible with the film-processing process. Several minerals are used for this purpose: talc, calcined kaolin, cristobalite, precipitated silica, diatomaceous earth, mica, calcium carbonates, calcium sulfate (anhydrite), magnesium carbonate, magnesium sulfate, and feldspars.

Organic alternatives used for anti-blocking or anti-stick are amides, fatty acid amides, fatty acids, salts of fatty acids, silicones, or others. They work with different mechanisms compared to the inorganic anti-blocking additives, migrating to the film surface upon cooling and forming a release layer. Sometimes combinations of inorganic and organic anti-blocking additives are used. Organic additives typically have lower anti-blocking ability in comparison to the inorganic ones but better slip effect.

This entry summarizes the mechanisms of anti-blocking and subsumes results from literature on that subject with an emphasis on the mineral anti-blocking additives.

Environmental issues, such as life cycle impact and sustainability, are becoming important concerns for most industrial activities. Because of their scale of operation, composite materials are particularly affected, and this is filtering down to their major components, such as fillers.

In defense of using fillers at all, it has been shown that at least in some cases, replacing part of a synthetic polymer by fillers can reduce the environmental impact of a composite. While this is true, more could be done, and the main thrusts are for recycling and the use of renewables. The industry recognizes this; but, despite significant commercial activity, there are difficult obstacles to overcome and only limited success has been achieved so far. Recovery and recycling of fillers themselves is hampered by the low cost of many virgin products and has made little headway. Recycling of waste products from other industries has also made little progress, often due to competition from more valuable alternative applications for the wastes.

More progress has been made where the composite itself is recycled, examples being talc/polyolefin composites in automotive applications. This is much harder to accomplish in cross-linked polymers (thermosets and elastomers), which are inherently more difficult to recycle. The majority of fillers are currently from nonrenewable (although often vast) mineral resources. The exceptions are natural fibers, such as wood, cotton, and flax, and these are considered to be more sustainable.

Despite much effort from the tire companies on products such as starch, renewable particulate fillers have so far made little progress. It has, however, been demonstrated that carbon black can be made from biomass rather than petroleum oil, and this may develop as a significant commercial option in the future.

Despite the difficulties, pressure on the major volume products is only going to increase and we can expect more recycling and use of renewables in the future. This will mainly be in large volume products used in applications such as vehicles.

Nanofillers, especially those based on clays and carbon nanotubes, have received a great deal of attention recently. The accepted definition of nanoparticles is that they should have at least one dimension in the range 1–100 nm and that the others should be greater than 100 nm. This can be further divided; when three dimensions are in the nano range we have nanoparticles, when it is two then nanofibers, and when one then nanoplates.

The first thing to notice is that, despite all the recent publicity, nanofillers are nothing new, and nanoparticle fillers already have a very significant market presence. These include the carbon blacks, precipitated and fumed silicas, and precipitated calcium carbonates. More recently interest in using synthetic polymer nanoparticles has been revived.

Where the principal novelty lies today is with the nanoplates and nanofibers, which are more recent arrivals on the scene. Mineral derived nanoplate fillers are the more promising for volume applications, due to a relatively low cost compared to the fibers. Color is also a limiting factor for the nanocarbon fillers.

While a large number of layer minerals exist with the potential for delamination and dispersion as nanoplates, clays and especially those of the montmorillonite family have shown most promise and have been focused on commercially. Nanoclays offer the potential for high stiffness at low loadings, excellent barrier properties, and some useful flame retardant properties. However, after more than two decades of effort and some limited successes, their market penetration is well short of initial hopes and commercial interest is declining. This is due to the practical difficulties in fully delaminating and dispersing them, and thus realizing their potential benefits, in all but a few cases.

Among other nanoplates, graphite and ultimately graphene have promise for some advanced composite applications. Cost (and color) are likely to be limiting factors. The same factors will affect graphene fiber analogues, such as carbon nanofibers and tubes.

Extraction of nanocrystals such as starch and cellulose from abundant plant materials is also of interest, especially from the sustainability angle. Despite some promise, the current production processes are costly and far from truly “green.” They also face similar difficulties in redispersion into polymer matrices as the nanoclays.