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Fillers in PVC products – why, when, where and how to add?
1
Why fillers are used in PVC products?
- Talc, CaCO3 and carbon black are fillers of choice used in plastic products.
- Carbon black has Sp. Gr. of 1.8, while talc and CaCO3 has a Sp. Gr. of 2.7.
- Generally, CaCO3 in different forms are used in PVC products.
- CaCO3 has aspect ratio approaching 1, due to its near spherical shape.
- Level of CaCO3 is restricted in India for pipes, due to two limitations as per IS 4985:2021, namely, Sp. Gr. should not be more that 1.46 and the sulphated ash content should not be more that 11%.
- For SWR pipes, being non (low) pressure application, CaCO3 is used upto 20 phr level, as above restrictions are not there.
- In calendared or T die extruded sheets, Sp. Gr. of product is as per agreement with the customer.
- For floor tiles however, CaCO3 is used at as high as 150 phr level to provide dimensional stability and compressive strength.
- Finer the particle size, more will be the falling weight impact resistance for UPVC pipe.
- Talc has an aspect ratio of 4. It will be useful in increasing the flexural strength. But it will reduce impact strength, and enhances degradation during processing, needing addition of impact modifiers and stabilizers.
- In summary, from product application point of view, modulus of elasticity [stiffness] is a function of particle shape, impact strength [toughness] is a function of particle size and compressive strength is a function of quantity of filler.
2
Type of CaCO3 used in PVC products:
- Fillers are used in PVC products for two purposes:
- To reduce the product cost, (extender fillers), and
- To improve certain properties, (functional fillers).
- Calcium carbonate is known by different names, e.g., Chalk, Calcite, Dolomite etc.
- Both, ground calcium carbonate (GCC) and precipitated calcium carbonate (PCC) are used in PVC products.
- Again, coated as well as uncoated versions are available.
- Precipitated CaCO3 has low bulk density and absorb plasticizers.
- Hence for the plasticized products, ground CaCO3 will be a functional filler.
- PCC is highly pure version of Calcium carbonate.
- The Moh hardness of calcium carbonate is 3.
- Dolomitic CaCO3 has more level of MgCO3 and have Moh hardness of 3.5.
- Talc and carbon black has Moh hardness of 1.
- Obviously, by adding calcium carbonate, the compound will be more abrasive, resulting in wear and tear of the processing equipment.
- Both GCC and PCC are therefore available in coated version for UPVC products. For plasticized products however, uncoated CaCO3 is preferred.
- GCC contains silica and silicates as an impurity.
- Silica/Silicates have Moh hardness up to 6, and cause more abrasion.
- One can check the silica or silicate contents in the laboratory.
- To save cost, today, processors tend to use even 70-90 phr of CaCO3.
3
Advantages and disadvantages of adding filler to PVC:
Advantages:
- Filler offers surface smoothness, if particle size is finer.
- Reduces shrinkage and provides dimensional stability.
- CaCO3 is 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 and higher melt viscosity aid in downstream calibration of pipe and especially, profile.
- Improves hardness and compressive strength.
- Improves VST, as it resists the needle to penetrate.
- Thermal conductivity of Ground CaCO3 is 2.7 W/mK, as compared to that of PVC, 0.146 W/mk. Hence CaCO3 acts as a heat sink. It heats and cools faster than PVC.
- Coating Ground Calcium Carbonate (GCC) with stearic acid or stearate can reduce its thermal conductivity, because S.A. acts as an insulator.
Disadvantages:
- Increases compound viscosity during processing, as CaCO3 does not melt during processing. More the filler, more the viscosity.
- Hence, during extrusion, linear speed of pipe will be reduced.
- Product weight/m increases or for the same weight, thickness reduces.
- Tensile strength and modulus of elasticity will reduce.
- In injection moulding of fittings, to achieve injectability of fluxed PVC, CaCO3 cannot be increased beyond certain level.
4
To what extent, cost will be reduced by incorporating CaCO3?
- Suppose compound A contains 8 phr calcium carbonate and compound B contains 20 phr filler. Then, cost of compound A = X Rs/kg, will be more than cost of compound B= Y Rs/kg
- If we manufacture same thickness pipe (profile / sheet) from both compounds, what could happen? Weight W1 of 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.
- Let the number of 6m long pipes manufactured per hour from compound A =N1.
- This will be more than number of pipes manufactured per hour from compound B = N2.
- Cost of pipes per hour based on compound A & B will be
Rs. (X x W1 x N1) and Rs. (Y x W2 x N2), respectively. - Multiply this cost by direct overheads like power, manpower, and other direct overheads.
- Also, consider wear and tear of process equipment that will further reduce the linear output and require more stabilizer due to increased residence time.
- One can compare it and find the difference practically.
- We must understand that the increased weight does not belong to CaCO3, but belongs to the compound B.
5
When CaCO3 should be added during high-speed mixing?
- During high-speed mixing, additives are added to modify PVC.
- Coated CaCO3 does not absorb any additives.
- If quantity of CaCO3 is less, say 8 phr, then it can be added along with PVC in high-speed mixer.
- But for larger quantity, adding CaCO3 along with PVC, is not logical.
Problems associated with larger quantity of CaCO3 during mixing are:
- The volume occupied in the mixer crosses the required 66% filling level, and the vortex and in turn homogeneity of dry blend is not achieved.
- Moh hardness of CaCO3 is 3 and TiO2 is 6. This will contribute to the wear and tear of the mixer blades that eventually increase the cycle time.
- Most importantly, the emery coming out of the blades due to wear and tear, (especially when blades are replaced from local cheaper source), gets mixed with the compound and eventually enters in the extruder.
- This can be seen as a metal powder adhering to the magnetic grill placed in the hopper. It can harm the screw and barrel surface.
- It is therefore advisable to add filler and TiO2 towards the end of mixing cycle that gets distributed uniformly in a cooler mixer.
Expansion contraction aspects:
- Coefficient of thermal expansion for CaCO₃ is around 4-5 × 10⁻⁶ /°C and for PVC it is typically in the range of 50-150 × 10⁻⁶ /°C, depending on the formulation and whether it is rigid or flexible.
- On an average, UPVC expands approximately 16-17 times more than CaCO₃ /°C of temperature rise. This is why CaCO3 provides dimensional stability.
6
How to improve mechanical properties with high filler content?
- On one side, processors want to add more filler to reduce cost, on the other side, tensile strength and impact properties get compromised.
Filler particles shown in the figure with red dots in fused PVC, reduces fusion when its particle size or phr level is increased beyond limit.- If the product justifies more filler content, then we need to:
- Improve impact strength by using finer particle size filler,
- Improve tensile strength by improving binding between CaCO3 and PVC matrix, by using titanate or Silane based coupling agent coated CaCO3.
- Improve impact strength by using a multifunctional impact modifier.
- One such agent that can be added to the high-speed mixer is Chlorinated Poly Ethylene (CPE) containing 25-48% chlorine, in powder form (Tg16°C).
- CPE is not a pre-defined particle sized impact modifier. It demands that the necessary morphology must be developed during processing.
- CPE coats unfused PVC primary particles, creating a network like structure.
- With the rise in temperature and shear, phase inversion occurs, giving discrete CPE particles in a continuous matrix of PVC.
- Hence, desired morphology for optimum fusion occurs, at low processing temperature for desired impact strength.
- This situation favours increasing CaCO3 content, increasing impact strength and tensile strength, with medium molecular weight CPE at 5 phr level.
Ref: Don A. Ventresca, increasing filler concentration in PVC compounds using CPE, J. Vinyl & Additive Technology, 1997, Vol.3, No.4, Pg. 274
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