Selecting Fillers and Extenders for Adhesives and Sealants

What are Fillers and Extenders?

Fillers and extenders are used in adhesive and sealant formulations to improve properties and lower cost.

There is a wide variety of fillers and extenders in adhesives and sealants. They can be both organic and inorganic. Besides, in a filler family, grades vary in:

Size (mean and distribution)
Aspect ratio
Surface area, and
Surface chemistry

Their loading can vary from just a few percent to where the filler is the major component by weight. It depends on the primary resin, the other ingredients in the formulation, and the targeted end-properties.

Fillers can have a deep effect on cost, compounding characteristics, and final adhesive properties. So, you should select them with care. Fillers are available as fibrous and non-fibrous forms. Some common forms used with adhesives include:

Non-fibrous fillers - Powders, Spheres, Granules, Fibers, Whiskers, Needles, Flakes
Fibrous fillers - Fibers, Continuous or chopped strands, Yarn, Spun and woven roving, fabric and mats

It should be noted that adhesive film carriers such as fabrics or mats can be considered as a type of filler.

In general, fillers are broadly defined as particulate or fibrous materials. They are chemically inert in nature. When added to adhesive formulations, fillers help in improving:

Working properties
Strength
Permanence
Adhesion
Flow
Chemical and weather resistance
Adhesive's rheological behavior
Mechanical properties
Thermal and optical properties

Definition of Extenders

Some fillers may act as extenders. When fillers are added to reduce the concentration of more expensive adhesive components with an aim to reduce formulation costs. Extenders may also have positive value in modifying the physical properties of the adhesive, although their primary purpose is to reduce cost.

Common extenders are inorganic minerals & metals, flours, asphalt, and pulverized partly cured synthetic resins.

Today, there is growing focus on extenders that are produced from common waste products. Polymeric waste is, perhaps, the ideal extender. Recycling of industrial wastes is very interesting from an ecological and safety point of view. In addition, the resulting materials have useful physical and mechanical properties.

For example: Recycled fillers (powdered rubber, tire rubbers, micronized tire fibers, and milled electrical cable waste) have been used to formulate polymeric mortars.¹

One should note, however, that the cost variations can be very different if considered per volume or per weight. In the first case, filling can lead to cost increase instead of cost saving. Thus, it is essential to consider the right basis (per weight or per volume) of the price for the targeted application.

Advantages & Disadvantages of Fillers

The use of fillers can improve both the processing properties of the adhesive as well as the performance properties in the final product. Yet, the use of fillers can also result in certain negative features. In short, the formulator must balance the expected improvements against possible regression.

Examples of the possible benefits and limitations of filler addition are as follow:

Potential Advantages (Depending on Filler)	Potential Disadvantages (Depending on Filler)	Advantage or Disadvantage (Depending on Application)
Lower cost Reduce shrinkage on drying or curing Decrease exotherm temperature on curing Improve cohesive properties Improve abrasion resistance Increase modulus and heat distortion temp Increase compressive strength Increase electrical insulation strength Improve toughness if fibrous fillers are used Improve flame and smoke suppression Improve moisture, chemical, and / or corrosion resistance	Increase weight (depending on density) Loss of transparency, change in optical properties Difficulty in machining, slitting, or die-cutting adhesives with hard fillers Reduce flexibility and elongation Increase abrasion and wear of mixing equipment	Increase thermal and electrical conductivity Reduce thermal expansion coefficient Hardness change depending on filler Change in water absorption depending on the type of filler Generally increase viscosity, reduction in cold-flow increase or decrease shelf-life, gel time, cure rate Permeability control Improve biodegradation with natural fillers

Advantages and Disadvantages of Filler Addition

Types of Fillers and Extenders Used in Adhesives & Sealants

Inorganic Fillers & Extenders

The most common fillers used in formulations are inorganic components because they are inexpensive than organic components. They are commercially available in particle sizes greater than 0.1 micron and in various grades.

The most common chemical families used in this category are oxides, hydroxides, silicates, salts of calcium carbonate, barium sulfate, calcium sulfate, etc.

Calcium Carbonate

Calcium Carbonate is the most commonly used extender. It is widely available, low in cost, and provides for improvements in certain performance properties.

Calcium Carbonate is a mineral that is mined throughout the world. Common forms of calcium carbonate include limestone, marble, calcite, chalk, and dolomite. It is manufactured by precipitation processes and is commercially available from a number of sources. Calcium carbonate is available in many different particle sizes and in various grades. To improve dispersion in certain resins the filler is often coated with calcium stearate or stearic acid.

Silica

Silica is also often used as an extender in adhesive formulations. Similar to calcium carbonate, silica is an abundant mineral found in crystalline form (quartz) and amorphous form (diatomaceous silica).

Diatomaceous silica is used more extensively than quartz because it is a softer material providing less machining and abrasive problems. There is also concern over respiratory problems possibly associated with inhalation of finely divided quartz.

Although it is an excellent additive for increasing viscosity, diatomaceous silica has a low oil adsorption and very large surface area so that it is not a particularly good extender since it cannot be easily incorporated into the formulation in high concentrations.

Kaolin

Kaolin is commonly called clay, is another naturally occurring mineral that is often used as an extender in adhesive formulations. It is a hydrated aluminum silicate with a hexagonal plate-like crystal structure. In addition to reducing cost, kaolin clay provides shrinkage control, dimensional stability, viscosity increase, and pigmentation. Certain clays, such as bentonite, are highly alkaline and can accelerate or even catalyze an epoxy curing reaction. Therefore, their reactivity must be taken into consideration when calculating stoichiometric mix ratio of resin and curing agent.

Talc

Talc is a hydrated magnesium silicate that is composed of thin platelets primarily white in color. Talc is useful for lowering the cost of the formulation with minimal effect on physical properties. Because of its platy structure and aspect ratio, these extenders are also considered reinforcement. Polymers filled with platy talc exhibit higher stiffness, tensile strength and creep resistance, at ambient as well as elevated temperatures, than do polymer filled with particulate fillers.

Talc is inert to most chemical reagents and acids. The actual chemical composition for commercial talc varies and is highly dependent on the location of its mining site.

Check out the improvements provided by certain fillers & extenders below.

Filler	Resulting Improvement
Aluminum	Machineability
Alumina	Abrasion resistance, electrical
Aluminum silicate	Extender
Aluminum trioxide	Flame retardant
Barium sulfate	Extender
Calcium carbonate	Extender
Calcium sulfate	Extender
Carbon black	Pigment, reinforcement
Copper	Machineability, electrical conductivity
Glass fiber	Reinforcement
Graphite	Lubricity
Iron	Abrasion resistance
Kaolin clay	Extender
Lead	Radiation shielding
Mica	Electrical resistance
Phenolic or glass microspheres	Decreased density
Silica sand	Abrasion resistance, electrical properties
Silicon carbide	Abrasion resistance
Silver	Electrical conductivity
Titanium dioxide	Pigment
Zinc Oxide	Adhesion, corrosion resistance
Zirconium silicate	Arc resistance

Inorganic Fillers for Common Adhesive Formulations

Organic Fillers & Extenders

Naturally occurring organic materials such as cellulose, wood fiber, cotton fiber, etc. are also sometimes used as fillers and extenders in adhesive formulations.Organic extenders are primarily of two types:

Fillers derived from organic materials - Like wood flour, shell flour, and other cellulosic fillers are the most common types. They also provide a margin of mechanical property reinforcement because of their relatively high aspect ratio.

Low cost naturally occurring or synthetic resins - Like petroleum-based derivatives as well as soluble lignin and scrap synthetic resins.

Coal tar pitch is the most widely used resinous extender for epoxy resins.

It is primarily used in surface coating formulations but can also be used as a cost reducer and flexibilizer in epoxy adhesives and sealants.
In addition to the increase in flexibility (and reduction in thermal and chemical resistance), coal tar pitch extenders provide excellent water resistance.
Their primary applications, therefore, are often in the marine, pipe, tank, and general industrial maintenance areas.
Petroleum derived bitumens are also used as extenders in certain adhesive formulations such as epoxy.

High boiling petroleum distillates can also serve as low cost extenders, but a compatibilizer such as an alkylphenol must be present in the mix to achieve compatibility between the resin and the extender.

Furfural resins can also be used with epoxies, phenolics, and other resins to reduce cost and also to obtain increased resistance to acids.

As one might expect with extenders, the extender must be lower in cost than the base resin. In order to meet this criterion, many agricultural-based (non-petroleum) extenders are used. These include wood flour, walnut, coconut, and pecan shell flour, and soya bean flour.

Cheap inorganic minerals such as gypsum, powdered chalk, clays and a number of other oxides and silicates are also used. There really is almost no limit to the list of insoluble substances that could be pulverized and used.

Synthetic Resins for Adhesives & Sealants

Synthetic resins, such as thermoplastic powders, may also be used as fillers; however, they are more often considered as a resinous modifier or toughening agent. Several common fillers for adhesives are shown below.

Type	Chemical Family	Common Examples
Inorganics	Oxides	Glass (fibers, spheres, microballons, flakes), magnesium oxide, aluminum oxide, antimony oxide, beryllium oxide, titanium dioxide, other metal oxides
	Hydroxides	Aluminum hydroxide, calcium hydroxide, magnesium hydroxide
	Salts	Calcium carbonate, barium sulfate, calcium sulfate
	Silicates	Talc, mica, clays, calcium metasilicate, silica, magnesium silicate, potassium silicate
	Metals	Aluminum, gold, silver, copper, nickel
Organics	Carbon	Carbon black, carbon fibers, graphite
	Natural polymers	Asphalt, cellulose fibers, wood flour
	Synthetic polymers	Resins, fibers (polyester, nylon, aramid)

Fillers for Common Adhesive / Sealant Formulations

Effects of Fillers on Adhesive Properties

The type of filler, and
Its properties (particle size, shape, size distribution, and concentration), surface chemistry, dispersion characteristics, dryness, and compatibility with the other components in the formulation.

Property	Advantages	Drawbacks
Raw material cost per kg	+++++
Viscosity		+++++
Processing ease		-----
Processing cost		+++++
Hardness	+++++	+++++
Tensile strength		-----
Elongation at break		-----
Rigidity	+++++	+++++

Various Effects of Filler Addition²

By selective use of fillers, the properties of an adhesive can be changed significantly. Few properties that can be modified by use of fillers are - Flow, Bond Line Thickness, Coefficient of Thermal Expansion, Shrinkage, Conductivity (Electrical and Thermal), Electrical Properties, Specific Gravity, Cohesive Mechanical Properties, Heat and Chemical Resistance, Working Life and Exotherm, Fire Resistance, Color.

Control of Flow (Viscosity, Thixotropy, Etc.)

Controlling flow is an important part of the adhesive formulation process. Control of flow is important for several reasons.

It allows easy and reproducible metering and mixing prior to applications.
It provides for certain application characteristics of the adhesive (brush, spray, trowel, penetration, etc.).
It can provide sag resistance (thixotropy) for adhesives that are applied to vertical surfaces.
It can provide for a practical and reproducible bond line thickness in the final joint.

The first three factors are generally controlled by the rheological properties of the liquid adhesive through the application of fillers in the formulation. The final factor can be controlled through the viscosity; however, other methods are also possible to control the bondline thickness such as the use of mechanical shims in a joint design.

To ensure that adhesives and sealants function well during their application and end-use is that the formulator must be able to control the flow properties of the product. The challenge that the formulator faces is that the adhesive or sealant may need different flow characteristics at different times.

Related Read: Rheology Modifiers Selection for Adhesives and Sealants

For example, adhesives must flow readily so that they can be evenly applied to a substrate and wet-out the surface. Yet, there should not be an excess of penetration into porous substrates, nor should the adhesive “run” or “bleed” to create a starved joint.

Certain adhesives and sealants must also be capable of convenient flow application by trowel or extrusion, but they must also exhibit sag and slump resistance, once applied. Therefore, the flow properties, or rheology, of the material must fit the desired method of application.

The primary rheological property of concern is viscosity, which can be increased by the addition of fillers.

Fibrous fillers cause a larger viscosity increase than particulate fillers.
Finer filler particle size having a larger surface area will generally but not always result in higher viscosity than equal concentration of larger particle sizes.

Increased viscosity provides a method to control the flow characteristics of the adhesive. However, too high a viscosity can yield undesirable processing properties. The maximum filler loading for any system is frequently set by the maximum viscosity allowable for its method of application.

The table below shows the maximum amount of certain fillers that can be tolerated in a liquid epoxy for pouring.

Filler	Concentration (pph) at Maximum Pourable Viscosity
Black iron oxide	300
Tabular aluminum oxide	200
Atomized aluminum	150
Graphite	50
Titanium dioxide	50
Calcium carbonate	30
Silica flour	30

Maximum Amount of Filler for a Pourable Epoxy Resin Mixture

Although most fillers provide adhesive systems with viscosities that are unaffected by shear rate, certain fillers can provide thixotropy which results in an adhesive that will not flow under low levels of stress (e.g. under its own weight when applied to vertical surfaces). Yet the compound will exhibit lower viscosities when under higher levels of stress such as when being dispensed or applied to a substrate.

The thixotropic fillers work by forming a temporary “structure” in the mixture, which can be broken down at high rates of shear. Thixotropy can be obtained at fairly low loading concentrations with colloidal silica, bentonite, and several types of fibers like cellulose, polyolefin, & aramid types.

Today colloidal silica (fumed silica) is the most common thixotropic agent in epoxy resins. Because of its high surface area to weight ratio, formulations generally require only a little fumed silica (1-5% by weight) to achieve thixotropic properties. Other common thixotropic fillers are described below.

Filler	Characteristics
Colloidal (fumed) silica	Fumed silica is typically available with sizes in the 7-40 nanometer range and surface areas ranging from 50 to 380 m²/g. Unlike precipitated silica, fumed silica has no internal surface area. The specific gravity of fumed silica is approximately 2.2.
Precipitated calcium carbonate (CaCO₃)	This filler functions as a thixotrope in sealant and adhesive formulations as well as being a low cost extender (often used in the 40-50% by weight range) and reinforcement. Ultrafine (< 100 nanometer) precipitated calcium carbonate provides the greatest efficiency. These have surface areas from 15 to 30 m²/g. Precipitated calcium carbonates are generally surface treated to render them hydrophobic and to improve their dispensability in hydrophobic systems.
Kaolin clay	Kaolin is a commonly used inexpensive filler used primarily as an extender in adhesive and sealant formulations. The cost of the adhesive is reduced because the kaolin addition increases the product’s volume. Depending on the grade, kaolin can also be used to prevent drip or sag, provide reinforcement, and reduce shrinkage.
Bentonite clay	Bentonite is a colloidal clay that is both hydrophilic and organophilic. It is water swelling with some types of clay absorbing as much as five times its own weight in water. It is used in emulsions, adhesives, and sealants. It is a gritty, abrasive white particle filler.
Talc	Talc is also often used as an extender, but it also has flow control properties. Talc is used in higher solids, high viscosity applications such as caulking compounds, automotive putties, mastics and sealants. Talcs are either plate-like or needle-like in shape. Thin platelet particles have aspect ratios varying from 20:1 to 5:1. Coarse particle sizes (10-75 microns) are commonly used in these applications at loading levels of 5-30%. Fine talcs (1-10 microns) are more expensive and require intensive dispersion processes. Platy grades enhance barrier properties and air, water, and chemical resistance.
Attapulgite	Attapulgite is a very cost effective thixotrope. However, unlike kaolin or talc it is not used at high loadings and, thus, is not considered as an extender. Attapulgite consists of acicular shaped particles with a size of about 0.1 micron. Typical usage levels range from 2-8% by weight. Attapulgite particles are easy to disperse and commonly added to the formulation with other dry components.

Common Fillers Used to Control Flow in Adhesive Systems

Control of Bond Line Thickness

If the adhesive has a propensity to flow easily before and during cure, then one risks the possibility of a final joint that is starved of adhesive material. If the adhesive flows only with the application of a great amount of external pressure, then one risks the possibility of entrapping air at the interface and too thick of a bond line. These factors could result in localized high stress areas within the joint and reduction of the ultimate joint strength .

Flow characteristics can be regulated by the incorporation of fillers of the types noted above. The type and amount of fillers are chosen so that a practical bond line thickness will result after application of the necessary pressure (usually only contact pressure, approximately 5 psi). Ordinarily, the objective is a bond line thickness of 2-10 mils.

Glass, nylon, polyester, and cotton fabric or mat are also often used as an adhesive carrier to maintain bondline thickness. The strands of the fabric offer an “internal shim” so that the bond line cannot be thinner than the thickness of these strands.

Glass or polymeric microballoons, incorporated directly into the adhesive formulation can also provide the shimming function. Here the diameter of the microballoons is the positive stop that will prevent too thin a bond-line.

Coefficient of Thermal Expansion

Depending on the substrate, the curing temperatures, and the service temperatures that are expected, the adhesive formulator may want to adjust the coefficient of thermal expansion of the adhesive system. This will lessen internal stresses that occur due to differences in thermal expansion between the substrate and the adhesive. These stresses act to degrade the joint strength.

There are several possible solutions to this problem.

One is to use a resilient adhesive that deforms with the substrate during temperature change. The penalty here is possible creep of the adhesives, and highly deformable adhesives usually have low cohesive strength.
Another approach is to adjust the expansion coefficient of the adhesive to a value that is nearer to that of the substrate. This is generally accomplished by formulating the adhesive with specific fillers to "tailor" the thermal expansion coefficient.

The general effect of most fillers is to reduce the coefficient of thermal expansion in proportion to the degree of filler loading. Ideally, the coefficient of thermal expansion should be lowered (or raised) to match that of the material being bonded.

With two different substrate materials, the adhesive's coefficient of thermal expansion should be adjusted to a value between those of the two substrates. This is generally done by using fillers as shown in below. It is usually not possible to employ a sufficiently large filler loading to accomplish the degree of thermal expansion modification required to match the substrate.

The Coefficients of Thermal Expansion of Filled Epoxy Resins Compared With Those of Common Metals

Shrinkage

Nearly all polymeric materials shrink during solidification (drying or cure). Sometimes they shrink because of escaping solvent, leaving less mass in the bondline. Even 100% reactive adhesives, such as epoxies and urethanes, experience some shrinkage because their solid polymerized mass occupies less volume than the liquid reactants.

The typical percentage cure shrinkage for various reactive adhesive systems are shown. The result of such shrinkage is internal stresses at the adhesive substrate surface and the possible formation of cracks and voids within the bondline itself.

Adhesive Type	Shrinkage (%)
Acrylics	5-10
Anaerobic	6-9
Epoxies	4-5
Urethanes	3-5
Polyamide hot melts	1-2
Silicones	< 1

Shrinkage of Common Types of Adhesives

Depending on the primary base resin, the adhesive formulator may need to reduce the amount of shrinkage when the adhesive hardens. This can be accomplished in several ways. Elastic adhesives deform when exposed to such internal stress and are less affected by shrinkage.

Related Read: Test Methods to Measure Impact Strength of Adhesive Joints

Fillers also reduce the rate of shrinkage by bulk displacement of the resin in the adhesive formulation. This results in an increase in the inherent bond strength of the adhesive. Fillers may improve operational bond strength by 50 to 100%.

Conductivity (Electrical and Thermal)

In certain applications like electrical & electronic industries, adhesive systems must have a degree of electrical and / or thermal conductivity. Electrical conductivity is, of course, important in electrically conductive adhesives, and in adhesives that must provide electromagnetic or radio frequency interference (EMI and RFI) functions.

Thermal conductivity is also important in highly integrated electronic applications where the heat generated by components must be transferred to a heat pipe or by some other means outside the electronic package. Thermal conductivity within adhesive systems is also a means of reducing exotherm and stresses that could develop during the curing cycle or other excursions to elevated temperatures.

For optimum electrical or thermal conductivity in an adhesive, the metal particles used as fillers must be so concentrated that they come into contact with each other. This generally requires such a high level of filler loading that other properties such as flexibility and tensile-shear strength are significantly degraded.

Appropriate fillers have been used to produce adhesives with high electrical conductivity. It should be noted that, regardless of the adhesive system itself, electrical conductivity is improved by minimizing the adhesive bond line and by minimizing the organic or non-conductive part of the adhesive.

Electrically conductive adhesives owe their conductivity as well as their high cost to the incorporation of high loadings of metal powders or other special fillers of the types shown in the table below.

Virtually all high performance conductive products today are based on flake or powdered silver. Silver offers an advantage in conductivity stability that cannot be matched by copper or other lower cost metal powders.

Conductive carbon (amorphous carbon or fine graphite) can also be used in conductive adhesive formulations if the degree of conductivity can be sacrificed for a lower cost adhesive.

Material	Specific Gravity (g/cm³)	Volume Resistivity (ohm-cm)
Silver	10.5	1.6 X 10^-6
Copper	8.9	1.8 X 10^-6
Gold	19.3	2.3 X 10^-6
Aluminum	2.7	2.9 X 10^-6
Best Silver Filled Inks & Coatings		1 X 10^-4
Best Silver Filled Epoxy Adhesives		1 X 10^-3
Unfilled Epoxy Adhesives	1.1	10¹⁴- 10¹⁵

Volume Resistivity of Metals, Conductive Plastics and Various Insulation Materials at 25°C

Metal powder filled adhesives, such as those described above for electrically conductive adhesives can conduct both heat and electricity. Some applications, however, must conduct heat but not electricity. In these applications the adhesive must permit high transfer of heat plus a degree of electrical insulation.

Fillers used for achieving thermal conductivity alone include aluminum oxide, beryllium oxide, boron nitride, and silica. The thermal conductivity values for several metals as well as for beryllium oxide, aluminum oxide, and several filled & unfilled resins are listed below.

Material	Thermal Conductivity, BTU (h·°F·ft²/ft)
Silver	240
Copper	220
Beryllium Oxide	130
Aluminum	110
Aluminum Oxide	20
Best Silver Filled Epoxy Adhesives	1-4
Aluminum Filled Epoxy (50%)	1-2
Unfilled Epoxies	0.1 - 0.15

Thermal Conductivity of Metals, Oxides and Conductive Adhesives at 25°C

Electrically and Thermally Conductive Adhesives Formulation Strategies

Electrical Properties

Non-conductive fillers are employed with electrical-grade epoxy adhesive formulations to provide assembled components with specific electrical properties. Metallic fillers generally degrade electrical resistance values although they could be used to provide a degree of conductivity as discussed above.

The effect of electrical grade fillers (e.g. silica) on the electrical properties of the adhesive is usually marginal. Generally, fillers are not used to improve electrical resistance characteristics such as dielectric strength. The unfilled adhesive is usually optimal as an insulator. Also, under conditions of high humidity, fillers may tend to wick moisture and considerably degrade the electrical resistance properties of the adhesive.

The one exception where certain fillers can provide improvements is in arc resistance. Here, hydrated aluminum oxide and hydrated calcium sulfates will improve arc resistance if the application or cure temperatures are sufficiently low to prevent dehydration of the filler particles.

Specific Gravity

The majority of mineral fillers have a higher specific gravity than adhesive resins and will therefore increase the specific gravity of the formulated product. The increase in specific gravity is proportional to the loading volume of the filler. Note the effect of the fillers’ specific gravity on cost calculation basis (weight or volume) which has been described above.

Fillers with a density lower than the base resin can be used to provide reduced specific gravity. These are usually glass or plastic microballoons. Although they generally bring about a significant increase in viscosity, the microballoon filled products (sometimes called syntactic foam adhesives) are often used in marine applications where low density and buoyancy are important criteria.

Cohesive Mechanical Properties

Only certain fillers can be used to increase the cohesive strength of the cured adhesive formulations. Generally fibrous or flake fillers such as talc, glass fiber, or mica, will bring about a certain degree of increase in strength; however to notice a significant increase in physical strength these fillers must be used at a relatively high concentration.

Tensile-shear strength and modulus are generally increased in proportion to the amount of filler when tested at room temperature. The addition of particulate fillers generally decreases compression fatigue, but increases ultimate compressive modulus and compressive yield strength, because of a stiffening effect.

The impact strength, elongation, and peel strength are generally adversely affected by particulate fillers.

The improvements in adhesive strength of structural adhesives that are attributable to fillers are not as much related to the cohesive characteristics of the adhesive as the reduction in internal stress due to modification of coefficient of thermal expansion, shrinkage, etc.

Today fillers are undergoing a paradigm shift; their former primary function to lower the production costs is changing towards a distinct tuning of material properties such as compression strength, processability, and flame retardancy. This is especially true for ultra-fine grades or so called nanofillers which provide an improvement in properties such as material reinforcement due to their larger surface area. In general, a smaller filler particle size will have a greater impact on the material properties when the particles are properly dispersed.

Heat and Chemical Resistance

Fillers improve the thermal properties of the cured epoxy formulation by bulk displacement of organic components. This reduces long term shrinkage and thermally induced weight loss. However, the glass transition temperature is not significantly affected and, in some cases, may be lowered. The main effect of fillers on thermal properties is through improvement in thermal shock resistance. This is achieved via modification of the thermal expansion coefficient.

Differences in particulate fillers have been noticed with regard to thermal shock resistance. Silica, for example, provides relatively poor thermal shock improvement. Mica and fibrous fillers used in relatively high loading provide very good improvement.

Fillers having high thermal conductivity will also increase the thermal conductivity. Therefore, their addition will improve heat dissipation.

Aluminum powder, in particular, is frequently employed at relatively high concentrations in high temperature structural adhesive formulations.

The filler provides improvement in both tensile strength and heat resistance, and it also increases the thermal conductivity of the adhesive.
It also reduces undercut corrosion and, hence, improves adhesion and durability of adhesive between bare steel substrates.
It is believed that this is accomplished by the aluminum filler providing a sacrificial electrochemical mechanism.

Fillers often have a significant effect on the moisture resistance, the moisture – vapor transmission rate, and the solvent and chemical resistance of the cured adhesive bond. The effect, however, can be in either direction. Some fillers such as calcium carbonate tend to lower acid resistance, where others, such as silica or aluminum may tend to lower alkali resistance.

Many fibrous fillers exhibit a wicking action for moisture, and this is particularly true of glass fibers and especially when the fibers are exposed as when the cured epoxy is machined or a crack is formed in the adhesive. For this reason, glass fibers that are used in adhesives are often treated with a coupling agent such as an organosilane to improve the bond between the fiber and the epoxy matrix.

Particulate fillers, on the other hand, are believed to extend the pathway along which the water must diffuse resulting in a reduction in the rate of water absorption. For example, silica flour (1 to 75 microns) has been reported as improving the boiling water resistance of epoxy films.

Get unique technical features into your adhesive or sealant by formulating organofunctional silane oligomers. Take this free webinar now >>

Working Life and Exotherm

Fillers generally lower the curing reaction rate and reduce the degree of exotherm. This is primarily due to their diluting effect and the resulting increase in thermal conductivity. The effect of various fillers on working life and peak exotherm is presented below. The more filler added, the less will be the heat evolved during cure. However, even with highly filled epoxy systems, the filled resin is still a poor conductor of heat.

Filler	Viscosity at 25°C, cps	Peak Exotherm, °C	Working Life, min	Modulus of Rupture, psi	Shrinkage, %
None	1100	223	48	18,174	0.91
Silica	51,500	53	95	16,875	0.77
Mica	54,500	51	94	12,358	0.66
Limestone	10,000	59	90	12,433	0.47
Atomized aluminum 101	4,600	47	100	13,710	0.8
Barytes	4,200	83	84	17,417	0.71

Effect of Fillers on Viscosity, Exotherm, Modulus of Rupture, and Shrinkage of an Epoxy Resin

Fire Resistance

Flame retardant additives work by acting chemically and/or physically in the condensed phase or gas phase. The types of flame retardant additives and their operating characteristics are described below.

Char formers: Usually phosphorus compounds, which remove the carbon fuel source and provide an insulation layer against the fire’s heat.
Heat absorbers: Usually metal hydrates such as aluminum trihydrate (ATH) or magnesium hydroxide, remove heat by using it to evaporate water in their structure.
Flame quenchers: Usually bromine- or chlorine-based halogen systems which interfere with the reactions in a flame.
Synergists: Usually antimony compounds, which enhance performance of the flame quencher.

There are many families of flame retardants each with advantages and disadvantages.

It is common to formulate polymers with multiple flame retardants, typically a primary flame retardant plus a synergist such as antimony oxide, to enhance overall flame resistance at the lowest cost. Several hundred different flame retardant systems are used by the polymer industry because of these formulation practices.

Color

Fillers can be used as pigments to provide color to an adhesive or sealant formulation. In sealants, coloring is used to match the color of substrates. Both pigments and dyes have been used successfully as colorants. With most structural adhesives and sealants, inorganic pigments seem to provide optimal properties. Organic pigments are generally less effective.

Pigments are dispersed into the resins formulation at relativity low percentages (1-3%) to provide the required color. Titanium dioxide is frequently employed in connection with the colorant to provide a whiting agent with the necessary hiding power suitable for tinting.

Reducing Formulation Cost

Fillers are often used for the sole purpose of reducing the raw material cost of the adhesive system. They do this by replacing relatively expensive synthetic organic components with inexpensive inorganic components, generally naturally occurring minerals.Fillers that are used for this purpose are often referred to as “extenders”.

However, material cost is only part of a filler’s contribution to the overall cost. Fillers and extenders can also either improve or degrade the compounding characteristics of the formulation providing a consequence on the energy cost and elapsed compounding time.

There are also costs related to the inventory, storage, safety and health characteristics, and waste removal of each additional material used in a formulation. Thus, fillers and extender can have a significant positive or negative effect on overall costs.

One should note also that filler selection based cost calculations can be very different if considered on a volume or weight basis. In the first case, filling can lead to cost increase instead of cost saving. The table below shows examples for two fillers with the same price (0% or 50% of the polymer price) and of different densities (2 and 5 respectively).

Filler Density	Cost per weight, %	Saving per volume, %
Cheap filler is equal to 50% of the polymer cost
Filler density = 2	17	0
Filler density = 5	17	Cost increase
Cheap filler is equal to 10% of the polymer cost
Filler density = 2	30	16
Filler density = 5	30	5

Improvements in Processing and End-use Properties

The selection of the proper filler or extender is based on a number of factors. The most important, of course, are the improvements in processing and end-properties that they will provide (shown in the table below).

Processing Properties Affected	End-Use Properties Affected
Flow properties Viscosity, Thixotropy Working life Exotherm Drying and / or curing conditions Bondline thickness control Shrinkage	Adhesion Cohesion Specific gravity Coefficient of thermal expansion Toughness Conductivity (electrical and thermal) Electrical resistance Heat and chemical resistance Fire resistance Color

Processing and End-Use Properties Affected by the Addition of Fillers

Selecting Fillers for Adhesives - Tips That Can Be Useful

Before selecting a functional filler / extender, you must identify the key characteristics that are required in your final product. Since there are a great number of fillers available with considerable variations within any one family, you should focus on the materials that give the most leverage in helping to achieve the desired results.

Selection of the proper filler is based on a number of factors. The most important, of course, is its cost and the improvements in processing and end-properties. However, other important considerations when selecting a filler include:

Availability
Surface chemistry
Water adsorption
Oil adsorption
Density

Dry Filler - A filler should be dry or with a neutral or only slightly basic pH. Adsorbed water, which is present in some degree in most fillers, inhibits dispersion. Thus, most fillers must be dried before being added to the adhesive formulation. The drying process will drive off adsorbed moisture and gases from the surface of the filler.

Non-Reactive Filler - The filler should generally be non-reactive with the base resin or curing agents that are used in the formulation. If there is reactivity, stoichiometric considerations must be observed. Some hydroxyl bearing fillers are reactive and can be used advantageously since they provide crosslinks to the resin matrix in the adhesive system. Certain types of fillers, even though unreactive, will affect the pot life and exotherm of the adhesive system.

Generally, modifications of cure or reactivity are not the prime functions of the filler. However, these effects need to be considered especially when the curing process is critical.

So generally speaking, fillers can be selected according to - Properties Affected, Primary Functions, Base Polymer, Ease of Compounding & Key Characteristics. let's discuss each factor in detail.

Select Fillers According to Properties Affected

As we have already discussed the effect of fillers in detail, here you will find a comprehensive review to make the right selection.

Property Affected	Mechanism
Reduce cost	Fillers / extenders displace higher cost materials (generally polymers).
Dimensional control	Bondline thickness, coefficient of thermal expansion, shrinkage on cure, etc. can be modified by the filler.
Rheology control	Fillers generally result in increased viscosity. Thixotropy, anti-sag, surface smoothing, etc. can be achieved by certain fillers.
Reinforcement	Generally, fillers result in increased cohesive strength (tensile, compressive strength and modulus depending on aspect ratio); whereas, flexibility and elongation are generally decreased.
Conductivity	Metal fillers can increase electrical and / or thermal conductivity.
Electrical insulation improvement	Electrical insulation characteristics (volume resistivity, breakdown strength, etc.) can be modified by certain fillers.
Flame and smoke suppression	Hydrated fillers reduce the spread of flame and smoke.
Pigmentation	Metal oxides are often used to impart color.
Exotherm control	Due to increased thermal conductivity, many fillers reduce exotherm on curing.
Cure and pot life control	Metal oxide fillers are used to catalyze certain reactions. Fillers can also extend cure time and pot life.
Specific gravity	Fillers can be used to increase specific gravity (e.g., minerals and metals) or decrease (e.g., microballoons) specific gravity.
Shielding	Certain fillers will shield the base polymer from radiation (UV or other).
Abrasion resistance	Hard fillers will provide abrasion resistance.
Corrosion resistance	Certain fillers will act as a corrosion inhibitor at the metal/adhesive interface by reducing galvanic corrosion potential.
Moisture and chemical resistance	Certain (e.g., hydrophobic) fillers will reduce the absorption of moisture and some (e.g., ceramic) can provide improved chemical resistance.
Heat resistance	Fillers generally improve the heat resistance of the adhesive joint (measured by heat deflection temperature and not glass transition temperature).
Modification of surface properties	Improvement in blocking resistance, lubricity, and overall speed of machinability (e.g., slitting and die cutting) can be achieved with certain fillers.
Permeability control	Certain fillers (flake, platelets) can be used to reduce permeability and others (porous) can be used to increase permeability.
Degradability	Bio-based fillers (e.g. wood flour, cellulose) can provide biodegradability.
Optical properties	Fillers can be used to modify optical properties such as index of refraction.

Properties Affected by Filler in Adhesive and Sealant Formulations

Select Fillers According to Primary Functions

Fillers possess certain properties which are imparted to the adhesive formulation when added as additives. A particular filler can offer many properties. Selection of a filler on the basis of its primary function can help the formulator achieve the desired results.

For example, to formulate electrically conductive adhesives, the formulator will have to choose appropriate fillers that can provide this desired quality. The table below list some common fillers used in adhesive formulation and their primary functions.

Filler	Primary Function	Notes
Aluminum	Dimensional control, Reinforcement, Exotherm control, Corrosion resistance	Excellent corrosion resistance.
Alumina	Electrical insulation improvement, Abrasion resistance	Good electrical insulator.
Alumina trihydrate (aluminum hydroxide)	Reduce cost, Flame and smoke suppression	Decomposes at ~180 °C, absorbing heat and giving off water. Low absorption of UV suitable for UV cure systems.
Aluminum oxides	Reduce cost, Electrical insulation improvement	Electrical insulator with high thermal conductivity, chemically inert and white.
Antimony oxides	Flame and smoke suppression	Used with halogen containing polymers.
Asphalt (bitumen)	Reduce cost, Rheology control	Plasticizer.
Barium sulfate (barite)	Electrical insulation improvement,Pigmentation	High specific gravity (4.4), sound deadening, good chemical resistance.
Beryllium oxide	Electrical insulation improvement	High thermal conductivity and electrical insulation.
Boron nitride	Electrical insulation improvement	High thermal conductivity and electrical insulation.
Calcium carbonate (limestone) - ground	Reduce cost, Rheology control	Most widely used filler / extender. Soft.
Calcium carbonate - precipitated	Reduce cost, Rheology control, Reinforcement	Finer particle size than ground CaCO_3.
Calcium hydroxide (hydrated lime)	Reduce cost, Rheology control, Reinforcement	Same benefit as CaCO₃ (but including functionality, lower abrasion, and lower density).
Calcium metasilicate (wollastonite)	Reinforcement, Electrical insulation improvement, Pigmentation	High aspect ratio.
Calcium sulfate (gypsum)	Reduce cost, Reinforcement	Hydrous and anhydrous forms available.
Carbon black	Electrical insulation improvement, Pigmentation, Shielding	Many types-acetylene black best for electrical conductivity. Good UV barrier.
Cellulosic derivatives (particles and fibers)	Reduce cost, Rheology control, Reinforcement, Degradability	Natural product from plants.
Clays (kaolin, fuller’s earth, bentonite, etc.)	Reduce cost, Rheology control	Soft extender. Kaolin is 2^nd most often used extender.
Copper	Conductivity	Provides good electrical and thermal conductivity.
Fibers (glass, aramid, cellulose, etc.)	Rheology control, Reinforcement	Large aspect ratio provides reinforcement.
Gold	Conductivity	Provides good electrical and thermal conductivity.
Metal oxides (e.g., iron, lead, zinc)	Pigmentation, Cure and pot life control	Pigment. Sometimes use as a catalyst.
Magnesium hydroxide	Flame and smoke suppression	Has a higher decomposition temperature than aluminum trihydroxide.
Mica (sheet and ground)	Dimensional control, Electrical insulation improvement, Moisture and chemical resistance, Permeability control	High aspect ratio, platelets provide impermeability, low coefficient of expansion, good electrical resistivity.
Microspheres (ceramic or plastic)	Specific gravity	Very low specific gravity useful for syntactic foams.
Nickel	Electrical insulation improvement	Provides good electrical and thermal conductivity.
Potassium silicate and sodium silicate	Reduce cost	Water soluble extender for waterborne adhesives and sealants.
Resin (natural and synthetic)	Reduce cost, Rheology control, Specific gravity, Modification of surface properties	Many types are available, generally in particle or solution form.
Sand (ground silica)	Reduce cost, Dimensional control, Rheology control , Reinforcement	Inexpensive, chemically inert.
Silica (precipitated and fumed)		Hydroxyl groups and very high surface area provide hydrogen bonding and strong thixotropy properties.
Silver	Electrical insulation improvement	Provides optimum electrical and good thermal conductivity.
Talc (magnesium silicate)	Reduce cost, Rheology control, Reinforcement, Modification of surface properties	Soft inert filler, good electrical insulator.
Titanium dioxide	Pigmentation, Shielding, Optical properties	White pigment. UV absorber.
Wood flour	Reduce cost, Rheology control, Reinforcement, Permeability control, Degradability	Fibrous reinforcing agent. Porous nature increases permeability.
Zeolite	Cure and pot life control	Moisture scavenging molecular sieve.
Zinc sulfide	Pigmentation, Optical properties	Next highest refractive index to TiO₂.

Common Fillers and Their Properties

Get detailed knowledge about sodium silicates used in adhesive formulations - their manufacturing process, forms, properties, uses and safety information.

Select Fillers According to the Base Polymer

Once candidate fillers are chosen based on the intended function, the formulator then will need to match the filler to the base polymer in the formulation. This will depend on a number of factors including chemical and physical compatibility and history of successful use in similar applications.

The table below provides an indication of fillers that are well matched to certain base polymers used in adhesive formulations or sealant formulations.

Filler	Most Suitable Polymer Base (see key below)	Adhesive	Sealant
Aluminum	C, D, J	X
Alumina	A, D, I, K, M, N	X
Alumina trihydrate (aluminum hydroxide)	A, C, D, E, I, H, K, M	X	X
Aluminum oxides	C, D, J	X
Antimony oxides	D, H, I, K, Q	X
Asphalt (bitumen)	D, M, Q	X	X
Barium sulfate (barite)	C, H, I, M	X
Beryllium oxide	D, M, O	X
Boron nitride	D, M, O	X	X
Calcium carbonate (limestone) - ground	General	X	X
Calcium carbonate - precipitated	General	X	X
Calcium hydroxide (hydrated lime)	E, H, K, P	X	X
Calcium metasilicate (wollastonite)	A, C, D, F, G, I, K M, R	X	X
Calcium sulfate (gypsum)	H, I, M	X	X
Carbon black	A, D, E, F, K, M, O, Q	X	X
Cellulosic derivatives (particles and fibers)	A, C, D, L, M, Q	X
Clays (kaolin, fuller’s earth, bentonite, etc.)	General	X	X
Copper	D, M, O	X
Fibers (glass, aramid, cellulose, etc.)	A, C, D, L, M, Q	X
Gold	D, M, O	X
Metal oxides (e.g., iron, lead, zinc)	D, E, F, R	X	X
Magnesium hydroxide	D, E, G, H, I, K, Q, R	X
Mica (sheet and ground)	A, D, G, H, I	X
Microspheres (ceramic or plastic)	C, D, M, O	X
Nickel	D, M, O	X
Potassium silicate and sodium silicate	A, F, N	X	X
Resin (natural and synthetic)	General	X	X
Sand (ground silica)	A, D, H, I, K M	X	X
Silica (precipitated and fumed)	A, C, D, E, F, I, O, Q, R	X	X
Silver	D, M, O	X
Talc (magnesium silicate)	A, F, K, Q	X	X
Titanium dioxide	General	X	X
Wood flour	D, E, G, M,	X
Zeolite	M	X	X
Zinc sulfide	A, D, G, H, I, K	X
Base Polymer Key
A. Acrylic copolymers B. Amines C. Aminoplasts and phenoplasts (phenolic, UF, MUF) D. Epoxies E. Ethylene co- and terpolymers	F. Natural rubber G. Polyamides H. Polychlorovinyls I. Polyesters J. Polyimides K. Polyolefins	L. Polysulfides M. Polyurethanes N. Polyvinyl acetate emulsions and derivatives O. Silicones P. Silyl-modified polymers	Q. Styrene copolymers R. Synthetic elastomers

Commonly Used Combinations of Fillers and Base Polymers

Select Fillers According to Ease of Compounding

Within a specific family of fillers there are a number of forms including particles, fibers, and platelets. The table lists common forms of fillers.

It should be noted that adhesive carriers such as fabrics or mats can be considered as a type of filler. However, particulate fillers are more common in adhesive and sealant formulations due to their easy of compounding.

Shape	Sphere	Cube	Block	Flake	Fiber
Description	Spheroid	Cubic, prismatic, rhombohedral	Tabular, prismatic, irregular	Platy, flaky	Fibrous, elongated
Shape ratios (length:width:thickness)	1 : 1 : 1	1 : 1 : 1	1 to 4 : 1 : <1	1 : <1 : 0.25 to 0.01	1 : <0.1 : <0.1
Surface area equivalence	1	1.24	1.26 to 1.5	1.5 to 9.9	1.87 to 2.3
Examples	Glass spheres, microballoons	Calcium carbonate	Silica, barite	Kaolin clay, talc, mica	Calcium silicate, wood fiber, glass fiber

Filler Characteristics

Select Fillers According to Key Characteristics

The key properties of particulate fillers can be differentiated by their physical characteristics such as - Density, Particle shape, Particle size and distribution, Surface area, Aspect ratio, Surface energy and Moisture content.

The affect that these filler characteristics have on adhesive and sealant formulations are summarized below.

Property	Characteristic
Density	Most fillers have a density between 2 and 3. The addition of filler increases the free volume of the polymer, and generally there is a critical concentration of filler at which the density of the formulation increases.
Particle Shape	The interactions between the filler surface and the polymer depend on the filler shape and functional groups on the filler surface that may react with the polymer. Surface roughness is also important in the development of polymer-to-filler adhesion. Platelets (e.g., mica) can provide good barrier properties.
Particle Size and Distribution	In general fillers should have a particle size smaller than 5µm to avoid settlement. Small particle size filler provides transparency and better reinforcement, but they are more difficult to disperse. Narrow particle size distribution is recommended for optimum properties.
Surface Area	Specific surface area is an important property of fillers. Particles with higher surface area show better adhesion to the polymer matrix and provide better properties.
Aspect Ratio	Aspect ratio is the length of the particle divided by its diameter. A high aspect ratio provides better reinforcement.
Moisture Content	For some applications fillers must be completely dried to exhibit adequate performance. Moisture adsorbed on the surface of fillers impacts the rate and extent of curing in certain adhesives.

Physical Property Characteristics of Particulate Fillers

Formulating with Fillers

Fillers generally represent one of the major components by weight in an adhesive formulation. However, their concentration is quite often limited by viscosity constraints, cost, and negative effects on certain properties.

The degree of improvement provided by a filler in a formulation will heavily depend on the type of filler and its properties (particle size, shape, size distribution, and concentration), surface chemistry, dispersion characteristics, dryness, and compatibility with the other components in the formulation. The table summarizers the properties of selected fillers in epoxy adhesive formulation.

Property	Calcium Carbonate	Kaolin	Talc	Mica	Glass Microspheres	Hydrous Alumina	Silica	Wood Flour
Thermal conductivity, W/m-K	2.3	2.0	2.1	2.5	0.008	0.08	2.9	0.3
*CLTE, 10^-6/K**	10	8	8	8	8.8	4	10	5 - 50
Hardness, Mohs	2.5 - 3	2	1	2.5 - 3	5	2	6.5 - 7	2
Density, g/cm³	2.71	2.58	1.8	2.82	0.15 - 0.3	2.4 - 2.42	2.65	0.5-0.7
Dielectric constant	2.71	2.58	2.8	2.82	1.5	7	4.3	5

Selected Properties of Fillers Used in Epoxy Adhesive Formulations² *CLTE: Coefficient of thermal expansion

Filler Loading - What is the best use level?

The filler concentration in any adhesive formulation will depend mainly on the following factors:

Handling and viscosity characteristics of the formulated adhesive,
Wetting and compatibility characteristics of the filler with other components in the formulation,
Ultimate processing and end-use properties desired,
Cost of the filler or extender

The particle size, density, and oil absorptivity will determine the maximum weight loading in a specific formulation.

Lightweight fillers, such as diatomaceous silicas, can greatly increase the viscosity at lower filler loadings, even at concentrations of several pph (parts by weight per hundred parts of base polymer resin).
Medium weight granular filler, like talc, powdered aluminum, and alumina, can be used at loadings up to 200 pph.
The heavier, nonporous fillers, such as aluminum oxide, silica, and the calcium carbonates, can be used at levels as high as 700-800 pph without causing the viscosity to be unworkable. Fillers with fine particle size will tend to settle-out less than those with larger filler sizes.

Dispersion of Fillers & Viscosity Issues

Particle interactions resulting in aggregates of particles will adversely affect dispersion. Special surface treatments help achieve higher loading with less effect on viscosity. They reduce aggregation forces and improve suspension stability. These chemically functional surface treatments can be applied directly to the filler. Many grades of treated fillers are commercially available.

Some functional fillers can create strong covalent bonds to the resin matrix. It results in improved performance.

Formulators often use wetting agents along with fillers. It helps achieve good filler dispersion, settling, and adhesion to the resin matrix. Often, the desire is to incorporate as large an amount of filler as possible into the system. The goal is to optimize a specific technical property as much as possible. Unfortunately, large amounts of filler also create very high viscosities. It could result in unworkable adhesive products. Wetting and dispersing additives aid in achieving significantly lower viscosity (see the figure below).

Increased Viscosity with Increased Filler Content and Effect of Wetting Agent Additive

Particle Size & Shape - What are the key considerations?

Particle size and shape will affect the degree of mixing required. Particles with large aspect ratios (such as fibrous fillers), and particles with large size are in general more difficult to disperse. Also, high filler loading makes the wetting of the filler more difficult because of the increased viscosities encountered. In certain circumstances, one can use blends of different size fillers. With heavier or larger size fillers, there is a greater tendency of the filler to settle. Lightweight secondary fillers can be used as anti-settling agents.

An alternative is to use a broad size distribution of a singular type of filler. It is also an effective way of achieving anti-settling and good dispersion properties.

High shear mixing is generally required for compounding of most formulations. A vacuum equipment is sometimes needed to eliminate the possibility of air from being beaten into the adhesive formulation. Often the formulation is warmed to a moderately elevated temperature to ease mixing.

For very viscous formulations, roll-milling equipment may be needed to achieve efficient mixing and dispersion of the filler. In some cases, it may be necessary to use a grinding device to accomplish the mixing with enough thoroughness. Without complete and efficient mixing, the final formulation will not have the desired properties.

Solvent / Diluent Addition - Can this impact crosslinking & other properties?

Solvent addition or the addition of low molecular weight diluents to the base resin is another method of lowering the viscosity. But, in these cases, the formulator must address the high vapor pressures of the solvent or diluent (as well as various health, safety, and environmental issues). Adding diluent can impact the crosslinking density and thermal or chemical properties.

The viscosity of a filled and unfilled resin as a function of percentage of the diluent phenol glycidyl ether in filled epoxy systems is illustrated below.

Effect of Reactive Diluent Concentration on the Viscosity of a Filled Epoxy Resin³

In two-part adhesive systems, the filler can generally be incorporated into either the base-resin or the curing agent component. Prevailing factors will be the characteristics mentioned above (viscosity of each component, dispersion characteristics, reactivity, etc.). However, the effect on mix ratio and on the ease of mixing the two components prior to application must also be considered.

Fillers/Extender – Available Grades for Adhesives and Sealants

View a wide range of fillers and extenders available today, analyze technical data of each product, get technical assistance or request samples.

References

Bignozzi, M.C., et. al., "New Polymer Mortars Containing Polymeric Wastes", Composites Part A: Applied Science and Engineering, Vol. 31, No. 2, February 2000, pp. 97-106.
Katz, H.S. and Milewski, eds., Handbook of Fillers for Plastics, van Nostrand Reinhold, New York, 1987.
Lee, H. and Neville, K., Epoxy Resins, McGraw-Hill, New York, 1957, p. 148.

Suppliers

Brands

Fillers and Extenders for Adhesives & Sealants: Selection and Formulation Tips