Fillers.pdf

FILLERS

Introduction

By deﬁnition, ﬁllers are used to extend a material and to reduce its cost. However,

few inexpensive ﬁllers, such as walnut shells, ﬂy ash, wood ﬂour, and wood cel-

lulose, are still being used purely for ﬁlling purposes; nearly all ﬁllers employed

provide more than space ﬁlling. Considering their relative higher stiffness com-

pared to the material matrix, they will always modify the mechanical properties

of the ﬁnal ﬁlled products, or composites. Fillers can constitute either a major or

a minor part of a composite. The structure of ﬁller particles ranges from precise

geometrical forms, such as spheres, hexagonal plates, or short ﬁbers, to irregu-

lar masses. Fillers are generally used for nondecorative purposes in contrast to

pigments, although they may incidentally impart color or opacity to a material.

Additives that supply bulk to drugs, cosmetics, and detergents, often referred to as

ﬁllers, are actually applied as diluents because their primary purpose is to adjust

the dose or concentration of a product, rather than modify its properties or reduce

cost. Fibers and whiskers are not discussed here because they are generally re-

garded as reinforcements, not ﬁllers, although a majority of the ﬁllers discussed

here have reinforcing effects (see R EINFORCEMENT ). Also, ﬁllers and additives that

primarily modify or impart electromagnetic properties, such as electrical conduc-

tivity, are not discussed (see C ONDUCTIVE P OLYMER C OMPOSITES ).

The ﬁrst manmade composites appeared in

∼

5000 BC in the Middle East

region, where pitch was used as a binder for reeds in building boats. Although

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glass ﬁber was known to be used by Phoenicians for bottle making, the use of

ﬁllers to modify the properties of a material started in early Roman times, when

artisans used ground marble in lime plaster, frescoes, and pozzolanic mortar.

It was not until the establishment of the modern polymer industry in the mid-

nineteenth century that the rapid development of commercial ﬁllers occurred.

The ﬁrst polyester resin was prepared by the Swedish chemist Berzelius in

1847, although the ﬁrst commercial plastic was not forthcoming until 1862 when

Parkes introduced a cellulose nitrate plastic. With the marketing of Bakelite

resin in 1909, a phenol–formaldehyde plastic ﬁlled with paper or cloth, along

with the usage of Carbon Black (qv) ﬁllers by B. F. Goodrich in natural rubbers,

the modern age of ﬁlled polymers was ushered in.

Fillers can be classiﬁed according to their source, function, composition,

and/or morphology. No single classiﬁcation scheme is entirely adequate due to the

overlap and ambiguity of these categories. Considering some examples of ﬁllers

used in modern polymers listed in Table 1 (1), the emphasis of this article is on

particulate ﬁllers. Extensive usage of particulate ﬁllers in many commerical poly-

mers is for the enhancement in stiffness, strength, dimensional stability, tough-

ness, heat distortion temperature, damping, impermeability, and cost reduction,

although not all of these desirable features are found in any single ﬁlled polymer.

The properties of particulate-ﬁlled polymers are determined by the properties of

the components, by the shape of the ﬁller phase, by the morphology of the system,

and by the polymer-ﬁller interfacial interactions.

Table 1. Fillers Used in Commercial Polymers

Particulate

Fibrous

Organic

Inorganic

Organic

Inorganic

Wood ﬂour

Glass

Cellulose

Whiskers

Cork

Calcium carbonate

Wool

Asbestos

Nutshell

Beryllium oxide

Carbon/graphite

Glass

Starch

Iron oxide

Aramid ﬁber

Mineral wool

Polymers

Magnesia

Nylons

Calcium sulphate

Carbon

Magnesium carbonate

Polyesters

Potassium titanate

Proteins

Titanium dioxide

Boron

Carbon nanotube

Zinc oxide

Alumina

Zirconia

Metals

Hydrated alumina

Antimony oxide

Metal powder

Silica

Silicates

Organo-nanoclays

Clays

Barium ferrite

Silicon carbide

Potassium titanate

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FILLERS 3

Physical Properties of Fillers

m, provide stronger en-

hancements in properties. Nanoparticles, with dimensions ranging up to 100 nm,

deliver the strongest enhancements when they all are properly dispersed. Due to

increases in both the surface area and the corresponding surface area to volume

ratio with a reduction in ﬁller particle size, ﬁner particles are prone to agglomerate

for the conservation of internal energy (2) and are more difﬁcult to be dispersed.

The shape of an individual particle has great impact on the ﬂexural modulus

(3), permeability (4), and ﬂow behavior (5) of a ﬁlled polymer. Although there are

many ways to measure the shape (6) of a ﬁller particle, the aspect ratio, or the

ratio of the longest length of particle to its thickness, is most commonly used.

A sphere, regardless of its size, has the lowest aspect ratio equal to 1.0. As a

ﬁller’s shape progresses from a sphere to a block, to a plate, or to a ﬂake, the

aspect ratio increases. The aspect ratio of a ﬁller affects its packing and, hence, its

loading level in a polymer. Theoretically, percolation, or ﬁller networking, occurs

when the volume fraction of monodispersed spherical particles reaches 0.156 (7).

The increase in the ﬁller’s aspect ratio lowers the percolation threshold and this

reduction, in turn, is of critical importance in loading electrical conductive ﬁllers

in a material to achieve electrical conductivity (8).

Almost all ﬁllers do not exist as the discrete individual particles of their

primary structure. They form aggregates, ie, secondary structure, which can ag-

glomerate into tertiary structures in the material to be ﬁlled. An aggregate is a

collection of primary particles that are chemically bonded together. The surface

area of an aggregate is less than the sum of surface areas of all primary particles

in that aggregate. In general, aggregates are extremely difﬁcult to be broken down

into individual primary particles by physical methods such as mechanical mixing.

The union of aggregates, although weakly associated through nonbonded physical

interactions, leads to an agglomerate. Filler materials often exist as agglomerates

in their natural state. The total surface area of an agglomerate is similar to the

sum of individual surface areas of aggregates in that agglomerate. The mixing

and dispersion of ﬁllers in a material involves primarily the incorporation and

distribution of ﬁller pellets or powders, and breakdown into agglomerates, and

then into aggregate structures.

In any commercial ﬁller grade, there exists a collection of multiple shapes

and sizes. The particle size and shape distributions of ﬁllers can be best mea-

sured by direct microscopic inspection together with image processing although

there are other methods to determine particle sizes and shapes. The size distribu-

tion of ﬁllers that are

m. Finer particles,

m and are also moderately spherical in shape could

The overall value of a ﬁller is a complex function of intrinsic material character-

istics, such as average particle size, particle shape, intrinsic strength, and chem-

ical composition; of process-dependent factors, such as particle-size distribution,

surface chemistry, particle agglomeration, and bulk density; and of cost. Abra-

sion and hardness properties are also important for their impact on the wear and

maintenance of processing and molding equipment.

Particle Morphology, Shape, Size, and Distribution. Filler particles

come in a variety of shapes and sizes. In general, for most polymer applications,

the ﬁller size required is

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Vol. 10

be determined conveniently by sieving in accordance to the ASTM E-11-70 sieve

designation. The Coulter technique could be used to measure ﬁllers in sizes rang-

ing form 4–40

m. Filler particle-size distributions

Most descriptions of particulate ﬁllers are given in terms of equivalent spheri-

cal diameter, ie, the diameter of a sphere having the same volume as the parti-

cle. Although microscopy can provide direct observation of ﬁller particles, two-

dimensional (2D), instead of the true three-dimensional (3D), size and shape

distributions are acquired in most cases. Statistical transformation of a 2D size

distribution into a 3D size distribution for near-spherical, randomly distributed,

particles can be performed (11).

Intrinsic Strength, Hardness, and Abrasivity. The intrinsic strengths

and moduli of some crystalline ﬁllers can be calculated along the crystallographic

axes using molecular simulation (12,13). For these crystalline ﬁllers, such as talc

and mica, the common fracture results from the delamination between the crys-

talline planes. As for the hardness, the primary measure is based on the Mohs

hardness scale, which is an empirical hardness measure according to the ability

of one material to scratch another (14) (see H ARDNESS ). The Mohs hardness goes

from 1, such as for talc, to 3, such as for calcite, to 7, such as for quartz, to 10, such

as for diamond, on a nonlinear scale. Abrasivity of a ﬁller particle depends on its

hardness, but also on its size and shape. Particles with sharp edges or rod shapes

are more abrasive than those of smooth and round particles, and large particles

are more abrasive than smaller particles of the same shape. Additionally, the coef-

ﬁcient of friction, surface treatment, surface energy, and purity of a ﬁller all affect

its abrasivity. Purity is important since one of the most common contaminants in

natural ﬁllers is the highly abrasive sand.

Surface Area, Chemistry, Wetting, and Coupling. Available surface

areas of ﬁllers include surfaces of ﬁller aggregates and agglomerates and surfaces

in their pores, crevices, and cracks. Measured values of surface area of ﬁllers

vary depending on the measurement methods. The direct method of surface area

summation from microscopic imaging of ﬁller particle size distribution typically

yields a lower surface area value because of the inability to measure surfaces in

pores and crevices by microscopy. In practice, surface area is determined from the

measured nitrogen adsorption assuming monolayer coverage on the ﬁller particle

surface (15) according to the BET theory (16) and is expressed in square meters

per gram (m 2 /g).

The chemical compatibility between the ﬁller surface and a polymer to be

ﬁlled is critically important in both the wetting and dispersion of this ﬁller by the

polymer and the ﬁnal physical performance of the resulting ﬁlled polymer. Filler

surfaces are commonly deﬁned, according to water afﬁnity, as hydrophilic, which

has a high afﬁnity for water, to hydrophobic. Many commercial ﬁllers, especially

mineral types, are surface coated or chemically treated with hydrophobic wetting

agents to modify their surface chemistry, to alter wetting characteristics, and to

aid their dispersion in organic polymers, particularly in nonpolar polymers. These

wetting agents also assist in deagglomerating ﬁller particles which, in turn, allow

for higher ﬁller loadings with lower viscosities during ﬁller incorporation. A ﬁller

particle’s oil or water absorption value provides an indirect measurement of ﬁller

m could be obtained

by sedimentation, permeametry, or light-scattering methods (9,10). Laser light-

scattering method can now analyze particles ranging from 0.05 to 2000

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FILLERS 5

relative wetting properties. The wettability can also be estimated by the contact

angle (17) measured between a drop of water or oil and the ﬁller surface. Com-

mercial wetting agents typically include polymeric esters, stearates, fatty-acid

esters, and organosilanes, among which the organosilanes, are the most widely

used.

Organosilane wetting agents are also called silane coupling agents (qv)

and, in general, consist of a trialkoxy group and a functional group having a

(RO) 3 Si R ∗ structure. The trialkoxy [(RO) 3 group provides chemical interactions

or reactions with functional groups on the ﬁller surface, and the R ∗ functional

group delivers compatibility with the polymer matrix across the silane coupling

bridge. This functional group can be selected from various chemical groups, such

as isobutyl, mercapto, aminopropyl, methacryl, vinyl, epoxy, or haloalkyl, in

accordance to the desirable interactions required with the polymer matrix (3).

However, the extent and uniformity of the alkoxide reaction with a ﬁller particle’s

surfaces vary depending on the treatment method. The pretreatment of ﬁllers

generally results in a more uniform reaction with a coupling agent than that ob-

tained by mixing all ingredients in an internal mixer, particularly under low shear

conditions.

Loading and Density. The amount of ﬁller in a ﬁlled polymer is termed

the loading and is always expressed quantitatively although the quantitative mea-

sures vary from industry to industry. In plastics and rubber industries, ﬁller load-

ing is formulated according to parts of ﬁller used per 100 parts of polymer (phr),

weight percent (wt%), or volume percent (vol%). In the paint industry, volume

percent pigment (ﬁller) in the dry paint ﬁlm or the volume ratio of ﬁller to binder

is commonly used. In the paper industry, ﬁller weight percent of sheet weight or

percent ash based on a loss-on-ignition method is applied.

The optimal loading of ﬁllers in a polymer is a balance between physical prop-

erty enhancement and trade-off, and processing and material cost over the ﬁller

loading range. A theoretical maximum ﬁller loading based on packing efﬁciency of

monodispersed particles sets the upper limit of ﬁller loading. The maximum vol-

ume fraction of spherical ﬁllers in a hexagonal close packing is 0.74, whereas the

maximum volume fractions achievable in a random close packing and in a cubic

packing are 0.64 and 0.52, respectively (18). In practice, the maximum packing

varies with particle shape, particle size distribution, and state of particle agglom-

eration. Agglomerates and nonspherical particles generally have smaller maxi-

mum packing than that of spheres (19). It is possible to achieve maximum packing

with minimum void volumes by having very wide particle-size distribution (20).

Except in a few cases, it is difﬁcult to predict the maximum volume fraction from

theory.

The average mass per unit volume of the individual particle is the true den-

sity or speciﬁc gravity of the ﬁller. It is used to calculate the volume fraction of

ﬁllers and is determined by a simple liquid displacement method for large, non-

porous, and spherical particles. Densities of ﬁnely divided, porous, and irregular

ﬁllers are typically measured by a gas pycnometer that ensures all pores and

crevices of ﬁller agglomerates are penetrated. Apparent, or bulk, ﬁller density

refers to the total amount of volume occupied by a given mass of dry ﬁllers and it

includes the void volumes in the ﬁller aggregates and agglomerates. Bulk density

is used in weighting ﬁllers during ﬁller purchasing, shipping, and storage.

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