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Effect of Variation in S-pvc Particle Size, Shape, Porosity & K-value on Conveying, Processability and Product Quality

Abstract:PVC is the only thermoplastic that derives its mechanical strength from the fusion that takes place during processing.

Properties concerned with stiffness – such as tensile strength and resistance to internal pressure shows a continuing improvement with increase in fusion level, until a maximum value is reached.

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Introduction:

S-PVC is commercially available in the form of particles of 95-250 microns size. Now a day, narrow particle size PVC 140-180 microns is also available. PVC particle has popcorn like structure that has a hard porous skin and the inner structure of PVC consists of sub grains, agglomerates, primary particles, domains, crystallites etc. that is responsible for porosity.

S-PVC received by the processor has variations in terms of:

  1. Particle size and its distribution.
  2. Particle shape variation and distribution.
  3. Molecular weight distribution (K- Value),
  4. Extent of porosity and its distribution

All these variations affect conveying, processability of PVC as well as quality of finished product.

Selection of PVC is therefore very important vis a vis processability and specific quality of products.

Effect of particle size and distribution:

Lot-to- lot variation in particle size and particle size distribution is undesirable.

Such changes may affect bulk flow properties of the resin, its dry blending behaviour, processability of compound as well as surface finish of the product.

S-PVC with particles mesh size 100 to 80 [149 to 180 microns] is available from modern plants [Mesh x Microns ≈ 15000]. No more than a trace should remain on 75 mesh screen [200 microns].

Fine particle size is preferred in case of UPVC films.

On the other hand, 200 micron particles with high porosity are preferred for plasticized application.

Flow properties of PVC compounds are highly dependent on average particle size.                             

Larger the particle size better is the flow.

Effect of particle shape variation:

On the other hand, bulk density of compound is highly dependent on particle shape.

More the irregular shape, lower is the bulk density.

Fines cause lower bulk density and poorer flow rate.

Flow rates are slower for resins compacted by tapping or shaking.

Resin particles that have sharp and defined structure, cannot be worked out through shear mixing.

Knobbiness:

Knobbiness in PVC grain is judged from packing fraction (Pf).

Pf = BD [p + 1 / d]                                            

Where,

BD= Bulk density

 p = Internal porosity of the resin powder [cc/g], which is void space between primary particles.

d = Density of solid PVC [1.4 g/cc]

Lower the Pf, very knobby and poorly packed are the grains.

More the Pf, smoother and easily packed are the grains.

Knobby PVC resin grains have -

  1. Lower apparent bulk density, and
  2. Poor powder flow.

Effect of K - Value variation:

K- Value denotes extent of polymerization or polymerization degree (PD).

The variation given in the technical data sheet for S-PVC is usually +/- 1. This means that K-67 resin can contain anything between K- 66 to K- 68 resin. If we apply rounding off factor, then K value can vary from 65.6 to 68.4.

It is necessary to understand that K-value tested in laboratory indicates average K- value and not the K value distribution. 

K value decides the viscosity of PVC at processing temperature.

Viscosity α [Mole. Wt.]^3.4 or α [K value]^3.4.

If we calculate the viscosity difference between K-66 and K-68 resin, it would be around 10.70% and between 65.4 and 68.4 it is 15 %. To avoid this variation in viscosity, K – value needs to me measured by the processor in their QA laboratory and polar lubricant level needs to be adjusted to match the viscosity.

Effect of porosity variation:

High porosity in PVC grains is desired for plasticized application.

Low porosity in PVC grain is desired when –

  1. High bulk density is desired ( more the bulk density, more the output from twin screw extruder)
  2. Few liquid formulating additives are used [like U PVC pipe].

Particle porosity is controlled by the PVC manufacturers of PVC.As the K value increases, porosity increases, while bulk density reduces.

For example, K-66 suspension resin for pipe may have porosity around 0.25 cc/g, and apparent bulk density 0.55 to 0.60 g/cc.

On the other hand, K-70 suspension resin for soft PVC may have porosity around 0.41 cc/g and apparent bulk density 0.45 to 0.49 g/cc.

Powder flow:

Static electricity sometimes is a concern in powder flow.

Many PVC resins contain a small amount of antistatic agent to take care of –

  1. Resin repulsion,
  2. Caking, and
  3. Low bulk density.

Apart from this, compounding procedure some time can lead to particles with different electric charge and get attracted to each other, causing particles to be non-flowing. While carrying out sieve analysis of S-PVC, 0.05 to 0.2 % carbon black is added as an antistatic agent, to facilitate sieving.

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