WHY FILLERS FOR PVC ARE SURFACE TREATED?

Introduction

Fillers are inert materials used in PVC for two reasons –

1.      To reduce cost (Extenders)

2.      To impart certain properties (Functional)

Calcium carbonate is most widely used filler, whereas, talc, asbestos, wood flour, ATH are used for specific reasons in PVC. Calcium carbonate is available in two forms –

a.      Ground calcium carbonate (GCC) – Contains impurities, available in particle size 2 microns or less, have lower oil absorption and bulk density up to 1.2.

Dry ground fillers have irregular shape. Wet ground fillers have uniform and smooth shape.

b.     Precipitated calcium carbonate (PCC) –Purest form, particle size 5 microns or less, have higher oil absorption and bulk density 0.85.

Why surface treatment is necessary?

In order to improve processability, dispersability, dispensability, stability, wettability etc, fillers are surface treated with surface active agents.

Both GCC and PCC are coated for reasons -

1.      There should be adequate wetting of filler particles by PVC at processing temperature. Inefficient wetting and bonding reduces overall physical properties. Coating is done to improve wettability.

2.       Coating with surface active agents modifies the surface tension and brings it closer to the surface tension of PVC.

3.      CaCO3 is surface treated to make the product hydrophobic.

Such a hydrophobic surface is easily wetted by the polymer matrix.

4.      Treatment is generally designed to improve dispersion by displacing absorbed water.

5.      As the particle size gets smaller, the surface area increases. This improves the interaction between the polymer/mineral surfaces. However, fine size minerals with high surface area affect dispensability and stability leading to non homogeneous mixtures in the finished parts.

The toughness of PVC products is improved when filler particle size is lower. However, too fine particles are very difficult to disperse and can form agglomerate (large particles) that act as stress concentrate and hence reduce the strength and toughness. Filler particles are therefore coated to prevent agglomeration.

6.      Calcium carbonate is abrasive. Its hardness on Mohs scale is 3. Coating with surfactants reduces wear and tear.

Nature of surface treatment:

Surface treatment applies to any agent that alters the surface of the filler. CaCO3 is surface treated with stearic acid, stearates, titanates, zirconates and silanes.

Stearic acid or stearate treated GCC or PCC in fact have a mono layer of calcium stearate as explained below -

Treatment of GCC:

Usually GCC is treated with stearic acid that reacts with fine size CaCO3 to yield water, CO2 and calcium stearate that forms a mono layer film on filler particle and imparts lubrication during processing.

Level of stearic acid is 0.5 – 1% depending on the surface area of CaCO3 and wetting surface of the surface treatment.

Grams of CaCO3 x surface area of CaCO3 (m^2/g)

Grams of treatment = -----------------------------------------------------------------                                                    wetting surface of treatment (m^2/g)

1% by wt Stearic acid is required for 3.5 sq. m. surface. As the particle size reduces to 2 - 3 microns, stearic acid treatment level is as high as 3%. This provides lubrication or slip.

Treatment of PCC:

On the other hand, precipitated CaCO3 is reacted with sodium stearate solution to form a mono layer of Calcium Stearate film. Sodium carbonate formed is washed out. This is very efficient method due to very fine particle size.

Other effects of coating:

All stearic acid treated CaCO3 provide lower tensile strength than untreated counterparts.

Surface treated CaCO3 reduces the peak torque and melt viscosity vs. untreated CaCO3. Untreated calcium carbonate is used in plasticized PVC products like floor tiles.

Coupling agent

Zirconates & titanates offer reduction in viscosity with increase in physical properties like impact strength by forming covalent bonds between inorganic filler and organic polymer.

On one side they adhere to calcium carbonate and on the other side they adhere to PVC, improving bonding between filler and PVC.

However, such a product is prone to fail in pressure testing at 60 deg C or long term test.