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Fillers and extender 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 are mentioned below: 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. Definition of Fillers Fillers are broadly defined as particulate or fibrous materials. They 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. View All the Commercially Available Fillers and Extenders Here! This adhesives database is available to all, free of charge. You can filter down your options by suitable resin, system or application (adhesives, sealants...), supplier and regional availability. Let's understand the common inorganic, organic & synthetic fillers and extenders in detail... Common Fillers and Extenders for 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. 1. Calcium Carbonate It is the most commonly used extender. It is widely available, low in cost, and provides for improvements in certain performance properties. The material 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. 2. Silica It 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. 3. Kaolin It 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. 4. Talc It 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 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 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 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 has to 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 Effects of Adding 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 an adhesive 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. 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 Effects of Fillers on Adhesive Properties 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 #1. 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 insure 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. 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 products 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 #2. 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. #3. 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 #4. 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. 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%. #5. 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 25C 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 (hFft2/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 25C #6. 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. #7. 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. #8. 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. #9. 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. #10. 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 25C, 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 #11. 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 fires 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. #12. 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 fillers 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 Saving per weight% Cost 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 Before selecting a functional filler / extender, the formulator must identify the key characteristics that are required of the final product. Since there are a great number of fillers available with considerable variations within any one family, formulators should focus on the materials that give the most leverage in helping to achieve the desired results. Fillers can be selected according to: Properties Affected Primary Functions Base Polymer Ease of Compounding Key Characteristics 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 Use a 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. Use a 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. #1. 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 #2. 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 CaCO3. 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, fullers earth, bentonite, etc.) Reduce cost,Rheology control Soft extender. Kaolin is 2nd 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 #3. 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, fullers 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 #4. 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 #5. 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 thefree 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 5m 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 formulations. 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 Coefficient of thermal expansion, 10-6/K 10 8 8 8 8.8 4-4 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 Filler Loading 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 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 make 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 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. Commercially Available Fillers and Extenders Check the Latest News About Fillers and Extenders 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.