Extenders_Fillers
Fillers and extenders are added to adhesive formulations in order to reduce cost, improve processing and performance properties. The use of fillers and extenders can also impair certain properties, hence the formulator must balance the expected improvements against possible property decline.
Fillers and extenders generally have different functions which may overlap and then they can be thought of as serving the same purpose depending on the application. Make your selection process easy by getting in-depth knowledge about fillers and extenders, their effects, applications and formulation examples.
Fillers and extenders are used in adhesive and sealant formulations to improve properties and lower cost, extenders or minerals are solid powdery material, white to off-white, natural or synthetic and inorganic often characterized by-
- low refractive index, lack the pigmentary properties e.g. colour, opacity , R.I. falls within a narrow range 1.55 to 1.65, transparent in oil and resin but opaque in water.
- inexpensive (economical considerations).
- coarser texture, low to high specific gravity, high bulking value, different particle shape.
- broad range of functional properties.
Introduction & Synthesis of extenders
Fillers and extenders are used in epoxy adhesive formulations to improve properties and to lower cost. Properties that can be selectively improved include both the processing properties of the adhesive as well as its performance properties in a cured joint. However, the use of fillers can also impair certain properties. Typically, the formulator has to balance the improvements against property decline.
please watch the given lecture to know more—>
Extenders are normally classified according to group
watch the given lacture to know more about following extenders
Carbonate, Silica & Sulphate Groups
The natural extenders are obtained from mines and some are synthesis by precipitation or calcination or vaporization process.
Proper choice of extenders may improve certain properties such as consistency, leveling, sagging, tooting, and settling in paints. certain extenders show mechanical properties e.g. reinforces the structure, while others increase resistance to the transmission of moisture in the coating. extenders can be used, some time, with pigment, replacing some of the opaque pigment to lower the cost without affecting the coating performance.
Hardness of extender or pigment is measured on Mohs scale where talc is the softest extender having moh scale 1 and diamon is the hardest material having 10 reading.
The alkaline extenders such as calcium carbonate etc. are having good buffer properties so used preferably in water based paint and also its use retard the growth of mold while the extenders with high hardness values are preferred in floor paint or high abrasion resistance coating. Light or fluffy extenders impart anti-settling character and extenders having high Oil absorption value arrest the sagging of coating. Such properties would be discussed by taking individual extender.
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
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 | Increase weight (depending on density) | Increase thermal and electrical conductivity. |
| Reduce shrinkage on drying or curing. | Loss of transparency, change in optical properties. | Reduce thermal expansion coefficient. |
| Decrease exotherm temperature on curing. | Difficulty in machining, slitting, or die-cutting adhesives with hard fillers. | Hardness change depending on filler. |
| Improve cohesive properties. | Reduce flexibility and elongation. | Change in water absorption depending on the type of filler. |
| Improve abrasion resistance. | Increase abrasion and wear of mixing equipment | Generally, increase viscosity, reduction in cold flow. |
| Increase modulus and heat distortion temp. | increase or decrease shelf-life, gel time, cure rate. | |
| Increase compressive strength. | Permeability control | |
| Increase electrical insulation strength. | Improve biodegradation with natural fillers | |
| Improve toughness if fibrous fillers are used. | ||
| Improve flame and smoke suppression. | ||
| Improve moisture, chemical, and / or corrosion resistance |
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) |