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Filler (Materials)
Filler materials are particles added to resin or binders (Plastics, Composites, Concrete) that can improve specific properties, make the product cheaper or a mixture of both. The two largest segments for filler material use is elastomers and plastics. Worldwide, more than 53 million tons of fillers (with a total sum of approximately EUR16 billion) are used every year in different application areas, such as paper, plastics, rubber, paints, coatings, adhesives and sealants. As such, fillers, produced by more than 700 companies, rank among the world's major raw materials and are contained in a variety of goods for daily consumer needs. The top filler materials used are ground calcium carbonate (GCC), precipitated calcium carbonate (PCC), kaolin, talc, and carbon black. Filler materials can affect the tensile strength, toughness, heat resistance, color, clarity etc. A good example of this is the addition of talc to polypropylene. Most of the filler materials used in plastics are mineral or glass based filler materials. There are two main subgroups of filler materials being particulates and fibers. Particulates are small particulates mixed in the matrix where particle size and aspect ratio are important. Fibers are small circular strands that can be very long and have very high aspect ratios.

Calcium Carbonate (CaCO3)
Referred to as "chalk" in the plastic industry, is derived from limestone and marble. It is used in many applications including PVC's and unsaturated polyesters. As much as 90% CaCO3 can be used to make a composite. These additions can improve molding productivity by decreasing cooling rate. They can also increase operating temperatures of materials and provide insulation for electrical wiring.

Kaolin
Kaolin is derived from kaolinite and was discovered about 150 million years ago. If kaolinite is heated above 500°C it changes into metakaolinite which can withstand temperature upwards of 960°C. It is mainly used in plastics for its anti blocking characteristics as well as an infra red absorber in laser marking and agriculture field. It increase properties such as impact strength and heat resistance. Metakolinite is used in PVC to remove harmful ions from the matrix. Kaolin has also been shown to increase the abrasion resistance and can replace carbon black as a filler material and improve flow properties of glass reinforced substances.

Magnesium Hydroxide (talc)
Talc is most known for its use in babypowder and as a thickening agent. It is the softest mineral known and generally more expensive than calcium carbonate. It is derived from layering sheets of magnesium hydroxide with silica. In the plastic industry it is used for packaging and food application due to its long term thermal stability.

Wollastonite (CaSiO3)
Wollastonite has an acicular crystal structure with a relatively high specific gravity and high hardness. Additions of this filler can improve moisture content, wear resistance, thermal stability and has a high dielectric strength. Wollastonite competes with platy filler substances like mica and talc and also can be used to replace glass fibers when creating thermoplastics and thermosets.

Glass:
Glass filler materials come in a few different forms like glass beads, short glass fibers, long glass fibers. Even though there are many different fiber additions glass is the most used fiber in plastics by tonnage. Glass fibers are used to increase the mechanical properties of the thermoplastic or thermoset such as flexual modulus and tensile strength, There is normally not an economic benefit for adding glass as a filler material. Some disadvantages of having glass in the matrix is low surface quality, very viscus when melted, low weldability and warpage. The addition of glass beads will help with oil absorption and chemical resistance.

Nanofillers
Nanofiller must have a particle size less than 100 nanometers. They have a high aspect ratio and are mainly used as scratch resistant and fire resistant fillers. Nanofillers can be broken out into 3 different groups nanoplates, nanofibers and nanoparticles. Nanoparticles are more widely used than nanoplates and nanofibers but nanoplates are starting be become more widely used. Nanoplates are like conventional platy fillers like talc and mica except the thickness is much smaller. Advantages of this are creating a gas barrier and flame retardant properties.

Tensile Strength
Tensile strength is the most used method to evaluate filler materials. The tensile strength of the composite can be calculated using the equation

σc= σp(1-aΦbf +cΦfd)

σc - tensile strength of composite

σb - tensile strength of polymer matrix

Φf - volume fraction of filler

a, b, c, d are constants depending of the type of filler. "a" relates to stress concentration and is based off of adhesion characteristics of the filler material. "b" is normally 0.67. c and d are constants that are inversely related to particle size.

Impact resistance
In general fillers will increase impact resistance. The contributing factors that improve impact resistance is particle size, particle shape and particle rigidity. Particle shape has the biggest influence and fibers improve impact resistance the most due to there large aspect ratio. Low hardness fillers will decrease impact strength. Particle size increases impact strength when the size is in a specific range that changes based on what filler type is used.

Wear resistance
The wear volume for plastic materials can be calculated using the equation below.

Ws=KμPDW/(EIs)

Where Ws= Wear Volume, K = Proportionality constant, P = force, E = Modulus, D = Sliding distance, W = load, Is= Interlaminar shear strength.

Matrix and filler both contribute to wear resistance. In general you want to choose a filler metal that will decrease the friction coefficient of the material. Particle size and shape also can affect the wear resistance. Smaller particle size will increase wear resistance because they cause less debris. silica, alumina, molybdenum disulfide and graphite powder are common fillers that improve wear resistance.

Fatigue resistance
Filler can have a negative or positive effect on fatigue resistance depending on the filler type and shape. In general fillers create small discontinuities in the matrix. This can contribute to crack initiation point. If the filler is brittle fatigue resistance will be low, where as if the filler is very ductile the composite will be fatigue resistant. Adhesion is also a important factor influencing fatigue resistance. If stress is higher than the particles adhesion a crack will form/propagate. Fiber ends are areas where cracks initiate most often due to the high stress on fiber ends with lower adhesion. Talc is a filler that can be used to increase fatigue resistance.

Thermal Deformation
Filler have a large influence on thermal deformation in crystalline polymers. Amorphous polymers are negligibly affect by filler material. Glass fiber additions are used the most to deflect the most heat. Carbon fibers have been shown to do better than glass in some base materials. In general fibrous materials are better at deflecting heat than particle fillers.

Creep
Creep majorly affected by filler materials. The equation below will show the creep strain of a filled material:

εc(t)/εm(t) = Em/Ec

Where εc(t) is strain of filled polymer, εm(t) is strain of matrix or unfilled polymer, Em is Young's Modulus of matrix, Ec is the Young's Modulus of filled polymer.

The better the filler bonds with the matrix the better creep resistance will be. Lots of interactions will have a positive influence. Glass beads and fibers both have been shown to improve creep resistance in some materials. Aluminum oxide also has a positive effect on creep resistance. Water absorption will decrease the creep resistance of a filled material.

Weldability
Additions of filler materials can drastically effect the weldability of the plastic. This also depends on the type of welding process used. For ultrasonic welding, fillers like calcium carbonate and kaolin can increase the resins ability to transmit ultrasonic waves. For electromagnetic welding additions of talc and glass will reduce the weld strength by as much as 32%. This is also seen during hot plate welding. They strength of the plastic after welding would decrease with increasing amount of fillers in the matrix compared to the bulk material. Use of abrasive fillers can have an effect on the tool used for welding. Abrasive fillers will degrade the tool of welding faster for example the surface of the ultrasonic horn in contact with the plastic. The best way to test the weldability of a filler material is to compare weld strength to resin strength. This can be hard to do since many filler materials contain different level of additives that change the mechanical behavior.