In PVC products CaCO3 is added for two reasons- to reduce the cost and to provide functional properties. Viscosity of PVC compound is important from processing point of view. It increases with addition of filler because CaCO3 does not melt during processing. From product application point of view, modulus of elasticity is a function of particle shape, while impact strength is a function of particle size and compressive strength is a function of quantity of filler.
Then why there is a tendency among the manufacturers to add more filler?This is because they think that this reduces the cost and improves profit.
The question is to what extent in reality, cost is reduced vis-a-vis expectations?
or this, we must consider both visible and invisible costs: As calcium carbonate level increases, specific gravity of compound also increases, volume reduces and hence relative cost saving also reduces. Processors wrongly calculate cost reduction on the basis of weight. It should be calculated on volume basis. Thanks to Mr. YASHODHAN KANADE for his studies on this topic. We want to share with you this analysis: Let us say that for “X” Phr CaCO3, the specific gravity of compound is around 1.47. If 100 kg of material at specific gravity of 1.47 is required to fill certain volume, then quantity of compound having specific gravity of 1.54 (say “Y” Phr) required to fill the same volume =100*1.54/1.47 = 104.49 kg. This means, additional 4. 49 kg of compound must go into the system to maintain the same volume. Point to understand is it is not 4.49 Kg of CaCO3, but 4.49 Kg of compound. One has to find out; to what extent this reduces the cost. Suppose compound A contains 8 Phr of calcium carbonate and compound B contains 20 Phr of calcium carbonate. Then, cost of compound A = X Rs/kg, will be more than cost of compound B= Y Rs/kg If we manufacture same thickness of pipe from both compounds then the weight W1of pipe from compound A will be less than weight W2 of the pipe manufactured from compound B. Since calcium carbonate does not melt at processing temperature, viscosity of compound will be more and flow will be less. Therefore, number of pipes manufactured per hour from compound A =N1, will be more than number of pipes manufactured per hour from compound B = N2
Cost of pipes per hour from compound A & B will be:
Rs. X x W1 x N1 and Rs. Y x W2 x N2 respectively.
One can compare it and find the difference practically.
Then why should we add CaCO3 what are the Advantages?
- It offers surface smoothness, if particle size is finer.
- Reduces shrinkage and provides dimensional stability.
- Thermally stable during processing.
- Impact resistance if used to certain level [8 Phr].
- Improves flow ability [8phr].
- Refractive index 1.66 offers translucency at low level.
- Lower extrudate expansion & higher melt viscosity aid in downstream calibration of profile & cools faster.
- Used up to 20 % (wt) depending on requirement of end use [SWR or low pressure pipes]. 100-150 Phr used in floor tiles.
- mproves hardness and compressive strength.
Use of CaCO3 does not result in cost reduction without sacrificing quality and damage to process equipment.