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Kaolin Clay is a Platy Alumino Silicate. Its continuous Sheet Structure produces thin Particles which exist in nature as overlapping flakes. These can occur as “Books” which inder magnification resembling stacks of paper. Kaolin Crystals are bound via hydrogen bonding of the Octahedral layer hydroxyl face of one plate to the tetrahedral layer oxygen face of adjacent plate. Separation into individual clay plates is therefore difficult, but can be accomplished by mechanical means to produce delaminated kaolin.
The Effects of Fillers Depend upon:
- Particle Size
- Particle Surface Area
- Particle Shape
- Particle Surface Activity (Compatability with adhesion to Matrix)
Surface Activity relates to the compatabilty of the filler with a specific elastomer and the ability of the elastomer to adhere to filler.
Functional Fillers transfer applied stress from the rubber matrix to the strong and stiff mineral. It seems that this stress transfer will be better effected if the mineral particles are smaller because greater surface is thereby exposed for a given mineral concentration. If these particles are needle like, fibrous or Platy in shape , they will better intercept the stress propagation through the matrix.
A compound’s physical/mechanical properties can be strongly influenced by the surface activity of the filler, which is the ability of the filler’s surface to bond with the matrix. For instance, an air gap between a filler particle size and the matrix represents a point of zero strength.
If the size of the filler particle greatly exceeds the polymer interchain distance, it introduces an area of localized stress. This can contribute to elastomer chain rupture on flexing or stretching.
Fillers with particle size greater than 10,000nm (10mcrons) are therefore generally avoided because they can reduce performance rather than extend or reinforce.
Fillers with Particle Size between 1000 to 10000nm (1 to 10microns) are primarily used as Diluents and usually have no significant effect, positive or negative on Rubber Properties.
Semi- reinforcing fillers ranging from 100 to 1000nm (0.1 to 1microns).
The Truly reinforcing fillers which range from 10nm to 100nm (0.01 to 1 micron) can significantly improve rubber properties.
Functional Fillers transfer applied stress from the rubber matrix to the strong and stiff mineral. It seems that this stress transfer will be better effected if the mineral particles are smaller because greater surface is thereby exposed for a given mineral concentration. If these particles are needle like, fibrous or Platy in shape , they will better intercept the stress propagation through the matrix.
A compound’s physical/mechanical properties can be strongly influenced by the surface activity of the filler, which is the ability of the filler’s surface to bond with the matrix. For instance, an air gap between a filler particle size and the matrix represents a point of zero strength.
If the size of the filler particle greatly exceeds the polymer interchain distance, it introduces an area of localized stress. This can contribute to elastomer chain rupture on flexing or stretching.
Fillers with particle size greater than 10,000nm (10mcrons) are therefore generally avoided because they can reduce performance rather than extend or reinforce.
Fillers with Particle Size between 1000 to 10000nm (1 to 10microns) are primarily used as Diluents and usually have no significant effect, positive or negative on Rubber Properties.
Semi- reinforcing fillers ranging from 100 to 1000nm (0.1 to 1microns).
The Truly reinforcing fillers which range from 10nm to 100nm (0.01 to 1 micron) can significantly improve rubber properties.
Application of Kaolin Clays in Rubber
Hard Clays :
Particle Size : 0.25 -0.5 Microns
Hard Clays :
Particle Size : 0.25 -0.5 Microns
- Very Fine grained
- Provides good Tensile Properties
- Stiffnesss
- Abrasion Resistance
- Improves Properties of Ground Calcium Carbonate Compounds
- Low Cost Substitute for Portion of Carbon Black/PPT Silica
Soft Clays:
Particle Size : 1 – 2microns
Particle Size : 1 – 2microns
- Larger Flakes
- Low Reinforcing Effect
- Higher Loadings
- Quicker Extrusions
Rubber Fller Clays are Classified as Hard or Soft Clays in relation to their particle size and stiffening effect in Rubber.
A Hard clay will have a median particle size of Approximately 0.25 to 0.5microns and will impart High Modulus, High Tensile Strength, Stiffness and good abrasion resistance to the Rubber compounds.
Soft Clay have a median particle size of 1 to 2 microns and is used where high loadings (for economy) and faster extrusion rates are more important then Strength.
A Hard clay will have a median particle size of Approximately 0.25 to 0.5microns and will impart High Modulus, High Tensile Strength, Stiffness and good abrasion resistance to the Rubber compounds.
Soft Clay have a median particle size of 1 to 2 microns and is used where high loadings (for economy) and faster extrusion rates are more important then Strength.
| Hard Clay | Soft Clay | GCC | |
| Median Particle Size (Microns) | 0.3 | 1.3 | 3.0 |
| BET Surface Area (m2/gms) | 23 | 16 | 3 |
| Hardness, Shore A | 61 | 62 | 53 |
| Tensile Strength (Mpa) | 12.8 | 8.8 | 1.6 |
| M300 (Mpa) | 3.2 | 2.8 | 1.2 |
| Die A Tear (KN/m) | 24.6 | 20.2 | 5.3 |
This comparison of Hard Clay, Soft Clay & GCC graphically Illustrates the effect of Particle Size, Surface Area & Shape on reinforecement of an SBR Compound.
More hard clay then soft is used in rubber because of its semi –reinforcing effect and its utility as a low cost complement to other fillers. It is used to improve the tensile strength and modulus of GCC compounds and will substitute for a portion of more expensive carbon black/ppt. Silica in certain compounds without sacrificing physical Properties. The Equal filler volume study demonstrates the advantage of hard clay as opposed to GCC in Replacing a portion of Precipitated Silica without compromising compound properties significantly.
More hard clay then soft is used in rubber because of its semi –reinforcing effect and its utility as a low cost complement to other fillers. It is used to improve the tensile strength and modulus of GCC compounds and will substitute for a portion of more expensive carbon black/ppt. Silica in certain compounds without sacrificing physical Properties. The Equal filler volume study demonstrates the advantage of hard clay as opposed to GCC in Replacing a portion of Precipitated Silica without compromising compound properties significantly.
Air Float Clay:
It is dry ground kaolin that has been air separated to minimize impurities and control Particle Size Distribution.
About 80% of the Kaolin used in Rubber is Air Float Hard Clay.
It is dry ground kaolin that has been air separated to minimize impurities and control Particle Size Distribution.
About 80% of the Kaolin used in Rubber is Air Float Hard Clay.
Calcined Clay:
The Kaolin is heated to partially remove surface hydroxyls. Its used in different industries such as
Wire & Cables for excellent Die electric and Water Resistant Properties
To Produce Calcined Clay for Filler uses, the Kaolin is water washed and heated partially to remove surface hydroxyl groups. The Partial or complete removal of surface hydroxyls provides a corresponding decrease in surface activity and thus reinforcement, but calcined clay is commonly used in wire and cable coverings because of excellent Dielectric & water resistance properties.
The Kaolin is heated to partially remove surface hydroxyls. Its used in different industries such as
Wire & Cables for excellent Die electric and Water Resistant Properties
To Produce Calcined Clay for Filler uses, the Kaolin is water washed and heated partially to remove surface hydroxyl groups. The Partial or complete removal of surface hydroxyls provides a corresponding decrease in surface activity and thus reinforcement, but calcined clay is commonly used in wire and cable coverings because of excellent Dielectric & water resistance properties.
