Grades of Talc
Many commercial talcs are available, with the main differences between talc grades being:
Purity Color Lamellarity Particle size Surface treatments
Two types of impurity can be distinguished: external and structural. External refers to the nontalc matter mixed with the talc. Structural refers to impurities in the talc crystals themselves. The first type is mainly minerals such as magnesite, chlorite, and tremolite, referred to earlier, and these vary with the ore origin and processing. They mainly dilute the special effects of talc. Structural impurities are substitution of silicon in the structure by metals such as aluminum and iron. Such substitutions don’t usually have much effect on the primary effects of talc in composites but can have profound effects on important secondary properties, such as color and stability.
While pure talc is white, commercial talcs can vary markedly and can even be brown or black. This is usually due to structural impurities as outlined above.
Lamellarity refers to the platiness of the particles. This can also be described as their aspect ratio and is the plate diameter divided by plate thickness. This can vary significantly and is a main differentiating feature between talcs. The ultimate thickness is fixed by the crystal structure, and so the aspect ratio is determined by the plate size and how well the particles are delaminated during processing (both during powder manufacture and mixing with the polymer).
The plate size (diameter) is a function of the geological deposit, the processing involved in separating and finishing the product for filler applications and lastly by the polymer processing conditions. The latter should not be ignored as much of the delamination often occurs during this stage.
Two extreme types of talc deposit can be recognized: microcrystalline and macrocrystalline. The microcrystalline type has the smallest diameter plates and will give rise to low aspect ratio particles. The macrocrystalline type has the potential to give much higher aspect ratios if properly processed.
Particle size mainly refers to plate diameter and is a function of the ore source and the processing conditions. Plate sizes in talc deposits can be as low as 1 pm and as high as 100 pm. While the largest size plates give the highest aspect ratio, they can have deleterious effects on other composite properties, and a top size of no more than about 12 pm with an average size of less than 5 pm is generally preferred.
Talcs for polymer use have aspect ratios in the range 5-20 in the powder form. The balance between particle size and aspect ratio is tailored to meet different applications.
Some talcs are also offered with added surface treatments. These range from stearate salts through to organo-silanes but currently have limited utility, and a true coupling agent does not seem to have been found. This is due to two factors. The first is the inertness of the plate faces, relegating chemical coupling to the more reactive edges. The second is the weak nature of the crystal stacks. Unless complete delamination and reaction occurs, these can fail in the composite.
The smallest size, high lamellarity talcs have low bulk densities (often less than 0.3), leading to difficulties in polymer compounding operations. Densified talcs (up to 0.9 bulk density) have been developed to overcome this.