1.3 FILLER PROPERTIES AFFECTING PERFORMANCE

1.3.1 PARTICLE SIZE, SIZE DISTRIBUTION AND SHAPE:

Particle Size, Size Distribution and Shape of fillers are key variables affecting the performance. These filler properties strongly influence the mechanical properties and play a central role in the properties of the formulation. These variables control the rheology and resulting processibility also[1,8].

PARTICLE SIZE:

The surface area of the filler increases with decrease in particle size. Due to increase in the surface area, the area available for the matrix to interact with the filler is high and hence the strength of the bond between matrix and the filler increases. This leads to an increase in the mechanical properties.

PARTICLE SIZE DISTRIBUTION:

The particle size distribution of the fillers should be a narrow distribution; this ensures that all the filler particles are having identical particle sizes and hence the bonding between the filler and the matrix will be uniform throughout and because of this, there is no region, which can act as stress raisers.

PARTICLE SHAPE:

The particle shape of the fillers should be more regular in order to provide better properties to the formulation. This is because; when the particle shape is more regular, the particle size distribution automatically becomes narrow facilitating better mechanical properties.

The particle size, shape and the size distribution can be determined by using test methods such as sieve analyses, gravitational sedimentation, centrifugal sedimentation, and microscopy.

1.3.2 SURFACE AREA AND POROSITY:

In addition to the particle size, size distribution and shape, surface area and porosity of the fillers are important factors affecting the performance of the filled system. Surface area is the area available for the matrix to bond with the filler incorporated into it. Porosity indicates the presence of surface flaws or pores on the surface of the filler. It can be characterized; by the pore size, pore volume, pore area and pore shape. The effects of these properties are explained as follows.

SURFACE AREA:

If the surface area available for interacting with the filler is high then the number of bonds formed between the matrix and the filler will increase, this leads to an increase in the mechanical properties of the formulation.

POROSITY:

The porosity of the filler should be as low as possible because if the porosity is high then the matrix penetrates into the pores and accumulates there. This results in higher filler to matrix ratio thereby resulting in inferior properties.

1.3.3 SURFACE TREATMENT:

Surface treatment is the process used to alter the filler surface chemistry, the energy and the degree of interaction at the filler – matrix interface. This leads to enhancement of the mechanical properties. Apart from these benefits, the surface treatment of the fillers increases the compatibility, dispersibility and processibility. Surface treatment also ensures that the filler does not deactivate the other additives that are required for optimum performance. By using an appropriate coupling agent along with surface treated filler, the degree of filler - matrix adhesion increases. The most common chemical agents used for surface treatment are fatty acids.

1.3.4 DISPERSIBILITY:

Proper dispersion of the filler is essential to the performance and ultimate success of the filled system. Dispersibility of a filler is established primarily by the chemical nature of the filler, its surface characteristics, its particle size and the process by which it is made.

Particle size of the filler is also important to its ease of dispersion. In general, the surface energies of the particles greater than 1µm are such that the driving force for aggregation is minimal and the aggregates that do form are generally weak. However, as the particle size drops below 0.1 µm the tendency of the particles to aggregate in large clusters increases progressively. In this region, protective colloids or surfactants must be employed to stabilize the particles from agglomerating.

1.3.5 ABRASION AND HARDNESS:

The abrasivity of mineral fillers is generally recognized to depend on three key factors; the filler hardness, particle size and particle shape. The hardness of mineral fillers is established by their comparative ratings on the Mohs scale. The higher the Mohs hardness value, the harder the mineral. Abrasion of mineral filler samples shows a linear increase in abrasion with particle size i.e. the finer the particle size the lower the abrasion of the product. The particle shape also tends to affect the abrasivity of the filled compound. Spherical shapes and smooth surfaces decrease the overall abrasivity of the product. Particles with sharp crystal points or edges are capable of more abrasion damage. Hence, abrasivity should be assessed early in the development of filled formulations.

1.4 CALCIUM CARBONATE

The carbonate mineral calcite is a calcium carbonate corresponding to the formula CaCO₃ and is one of the most widely distributed minerals on the Earth's surface. It is a common constituent of sedimentary rocks, limestone in particular. It is also the primary mineral in metamorphic marble. It also occurs as a vein mineral in deposits from hot springs, and also occurs in caverns as stalactites and stalagmites. Calcite is often the primary constituent of the shells of marine organisms (e.g. plankton, bivalves, etc.). Calcite represents the stable form of calcium carbonate; aragonite will change to calcite at 470°C.

Calcite is transparent to opaque and may occasionally show phosphorescence or fluorescence. It is perhaps best known because of its power to produce strong double refraction of light, such that objects viewed through a clear piece of calcite appear doubled in all of their parts[8,14,15,].

1.4.1 DIFFERENT FORMS OF CALCIUM CARBONATE

Aragonite:

Aragonite is a common carbonate mineral. Aragonite is an interesting and attractive mineral. Aragonite is a polymorph of calcite, which means that it has the same chemistry as calcite but it has a different structure, and more importantly, different symmetry and crystal shapes. Aragonite's structure that is more compact is

composed of triangular carbonate ion groups (CO3), with a carbon at the center of the triangle and the three oxygen atoms at each corner. Unlike in calcite, the carbonate ions do not lie in a single plane pointing in the same direction. Instead they lie in two planes that point in opposite directions; destroying the trigonal symmetry that is characteristic of calcite's structure. Aragonite has an orthorhombic symmetry when compared to that of calcite's "higher" trigonal symmetry. Aragonite is technically unstable at normal surface temperatures and pressures. It is stable at higher pressures, but not at higher temperatures, such that in order to keep aragonite stable with increasing temperature, the pressure must also increase. If aragonite is heated to 400 degrees C, it will spontaneously convert to calcite at constant pressure. Aragonite is a constituent of many sea creatures' shell structures; most bivalve animals and corals secrete aragonite for their shells and pearls are composed of mostly aragonite. Other environments of formation include hot springs deposits, cavities in volcanic rocks, caves and mines[16].

Limestone:

Limestone (CaCO₃) is a sedimentary rock composed of the mineral calcite (calcium carbonate). The primary source of this calcite is most commonly marine organisms. These organisms secrete shells that settle out of the water column and are deposited on ocean floors as pelagic ooze. Secondary calcite may also be deposited by supersaturated meteoric waters (groundwater that precipitates the material in caves). A further form is composed of oolites (Oolitic Limestone) and can be recognised by its granular appearance. Limestone makes up about 10 percent of the total volume of all sedimentary rocks.

Pure limestones are white or almost white. Because of impurities, such as clay, sand, organic remains, iron oxide and other materials, many limestones exhibit different colors, especially on weathered surfaces. Limestone may be crystalline, clastic, granular, or dense, depending on the method of formation. Crystals of calcite, quartz, dolomite or barite may line small cavities in the rock. Though the limestone used for construction is good for humid climates, it is vulnerable to acids, making acid rain a problem when it occurs in places where limestone is used extensively.

Chalk:

Chalk is a soft, white, porous form of limestone composed of the mineral calcium carbonate. It is relatively resistant to erosion and slumping compared to the clays that it is usually associated with. Chalk is formed in shallow waters by the gradual accumulation of the calcite mineral remains of micro-organisms over millions of years. Embedded flint nodules are commonly found in chalk beds. Because chalk is porous, chalk downland usually holds a large water table, providing a natural reservoir that releases water slowly through dry seasons.

1.4.2 PREPARATION OF CALCIUM CARBONATE:

Calcium carbonate can be manufactured from lime by three basic technologies, which are as follows[8]:

· Lime / CO₂ :

In this process, the calcium hydroxide is treated with carbon di oxide gas, which reacts to form calcium carbonate and water.

Ca (OH)₂ + CO₂ → CaCO3 + H₂O

· Lime / Soda:

In this process, the calcium hydroxide is made to react with sodium carbonate, which reacts to form calcium carbonate and sodium hydroxide.

Ca (OH)₂ + Na₂CO₃ → CaCO₃ + 2NaOH

· Solvay Process:

This is the most commonly used chemical process for the manufacture of calcium carbonate. In this process, ammonia is treated with water carbon – di - oxide and sodium chloride. These react to form sodium bi carbonate and ammonium chloride. The ammonium chloride formed is then treated with calcium hydroxide and these two react to form calcium chloride and ammonium hydroxide. The calcium chloride formed is then treated with sodium carbonate, which gives calcium carbonate and sodium chloride.

NH₃ + H₂O + CO₂ + NaCl → NaHCO₃ + NH₄Cl

Ca(OH)₂ + 2NH₄Cl → CaCl₂ + 2NH₄OH

CaCl₂ + Na₂CO₃ → CaCO3 + 2NaCl

1.4.3 PROPERTIES OF CALCIUM CARBONATE:

DOUBLE REFRACTION:

Double refraction occurs when a ray of light enters the crystal and due to calcite's unique optical properties, the ray is split into fast and slow beams. As these, two beams exit the crystal they are bent into two different angles because the angle is affected by the speed of the beams. The extremely high index of refraction of calcite that causes the easily seen double refraction is also responsible for the interference colors that are seen in calcites that have small fractures[16,17,18].

TRIBOLUMINESCENCE:

Triboluminescence is supposedly a property that should occur in most specimens, but is not easily demonstrated. It occurs when the specimen is struck or put under pressure; in a dark room, the specimen should glow when this happens.

ACID TEST:

The best property of calcite is the acid test. Because calcite always will effervesce (bubble) when even cold weak acids are placed on specimens. Even the cement in sandstones will effervesce assuring the geologist of identification of the cementing mineral. The reason for the bubbling is in the formula below:

CaCO₃ + 2H (+1) -------> Ca (+2) + H₂O + CO₂ (a gas)

The carbon dioxide gas (CO₂) is given off as bubbles and the calcium dissolves in the residual water. Any acid, just about, can produce these results, but dilute hydrochloric acid or vinegar is the two recommended acids for this test.

1.4.4 PHYSICAL CHARACTERISTICS OF CALCITE:

• Color is extremely variable but generally white or colorless or with light shades of yellow, orange, blue, pink, red, brown, green, black and gray.
• Luster is vitreous to resinous to dull in massive forms.
• Crystals are transparent to translucent and crystal system is trigonal.
• Crystal Habits are extremely variable with almost any trigonal form possible. Common among calcite crystals are the scalenohedron, rhombohedron, hexagonal prism, and pinacoid. Combinations of these and over three hundred other forms can make a multitude of crystal shapes, but always trigonal or pseudo-hexagonal.
• Twinning is often seen and results in crystals with blocky chevrons, right angled prisms, heart shapes or dipyramidal shapes. A notch in the middle of a doubly terminated scalenohedron is a sure sign of a twinned crystal lamellar twinning also seen resulting in striated cleavage surfaces.
• Cleavage is perfect in three directions, forming rhombohedrons.
• Hardness is 3 (only on the basal pinacoidal faces, calcite has a hardness of less than 2.5 and can be scratched by a fingernail).
• Specific Gravity is approximately 2.7 (average)