HOW PVC RESINS CAN BE DIFFERENT: COMPARISON OF S-PVC RESINS (PART 2)

In the first part of this article we began discussing some properties that are the origin of different performances between S-PVC resins. We will continue to review these properties as they are used to compare a resin sample (B) with the one currently used in process (A).

3.- PACKING OF GRAINS

This property is not usually considered and instead more attention is given to Bulk Density (total mass of PVC per volume unity) which is typically measured by methods such as ASTM D1895 Method B. In this type of methods a quantity of resin is flowed through a funnel into a cylinder both standardized, so test conditions resemble the packing of grains that would be expected by a gravitational flow in storage conditions and feeding to an extruder.

Bulk density is considered an important property in extrusion of rigid products such as pipes and profiles, since Productivity of extruder is defined by the amount of material that can be fed by gravity and although PVC resin is mixed with several additives before processing, resins with high Bulk density will flow more easily resulting in a higher density dry-blend and therefore higher productivity.

Bulk density is not considered a very important variable in flexible applications as high internal Porosity is required to absorb plasticizers and then it is expected that Bulk density will be lower as Porosity and Bulk Density have a reciprocal relationship.

However, as total volume is occupied by the volume of solid PVC plus the volume of internal pores (Porosity) plus the volume of voids between grains, said Bulk density does not measure how well S-PVC grains pack so we have to subtract the contribution of internal Porosity.

The following equation allows to calculate the percentage of volume occupied by porous grains in a sample of S-PVC, which gives an idea of how easy such PVC resin flows and packs by gravity:

% Packing= ( (1/ρPVC) + PTU) / (1/ρB )

… where ρPVC is density of solid PVC (1.39 g/cc), PTU is total Porosity of grains in cc/gr and ρB is Bulk density in gr/cc measured using ASTM D1895 method B or similar.

For that reason, whether you manufacture rigid or flexible products, it is highly recommended to measure Bulk Density (gram of PVC per cm3) directly and/or ask your provider to include this analysis in Quality certificates so you can calculate the degree of grains packing.

Grains packing depends on several factors such as:

- Shape: Round grains flow better

- Surface roughness: Smooth grains flow better

- Size distribution: Wide size distributions pack better

All these factors are the result of polymerization conditions used by S-PVC manufacturers and these generally do not change over time, making Grain packing an important indicator to compare resins from different suppliers.

Denser packing of grains is achieved when S-PVC producers have good polymerization process control and optimization, so that monomer droplets are homogeneous and form grains with homogeneous morphological properties. 

On the other hand, looser packing of grains is formed with unstable and not-optimized polymerization conditions, which result in heterogeneous monomer droplets and grains with inhomogeneous morphological structures.

Comparison:

- If Packing of sample B is higher:

o For rigid applications, expect improvement in melt homogeneity and increased extruder output with reduction of unitary consumption of processing energy.

o For flexible applications, expect an improvement in plasticizer dispersion and melt homogeneity with increased extruder output with reduction of unitary consumption of processing energy.

4.- THERMAL STABILITY 

PVC is a plastic with limited thermal stability since its polymeric chains are not perfect but contain small amounts of labile functional groups that can initiate a degradation reaction called dehydrochlorination that propagates by a unzipping mechanism (releasing hydrochloric acid) throughout length of the chains to form a product with poor physical properties and color that can range from yellow to brown or black.

Degradation occurs when polymer chains increase their mobility due to heat transfer by conduction and heat generation due to viscoelastic dissipation of mechanical stresses, both processes that occur during processing.

However, because PVC is a thermal insulator, temperature is not completely homogeneous when it is heated and melted so Thermal Stability depend not only on the nature of PVC chains (concentration of labile sites) but also of how much energy is used to melt its grains (molecular weight and internal morphology).  

To limit the negative effects of degradation, producers of S-PVC resins choose their recipes and polymerization conditions to minimize the formation of labile functional groups or to slow down degradation propagation.

In any case, vinyl processors must add chemical compounds called "Thermal Stabilizers" whose function is to penetrate PVC matrix and substitute labile functional groups to make them more stable and prevent the unzipping degradation reaction from continuing to avoid deterioration of final product. So Thermal Stabilizers must be added in sufficient quantity and must be adequately dispersed to reach labile sites in a timely manner to achieve their required performance.

Thermal Stability can be measured by methods that use conductive heat transmission alone (Static Thermal Stability) or in conjunction with mechanical processing (Dynamic Thermal Stability) to melt PVC and expose it to degradation conditions.

Static Thermal Stability can be evaluated either by comparing onset of degradation (change in color with forced convection heating) or measuring the kinetics of degradation (neutralization of released hydrochloric acid or with thermogravimetric analysis).

Dynamic Thermal Stability can be evaluated by measuring the strong change in rheological properties resulting from advanced degradation using a torque rheometer (ASTM D2538) or other processing devices at varying conditions of temperature and rotational speed.

All methods for measuring Thermal Stability have different contributions of heat transmitted by conduction and self-generated heat by mechanical stress. And they also offer a different vision of the degradation process, whether measuring its beginning, development or end point.

For this reason, it is highly recommended that you identify which method is most suitable for your needs and have it installed in your Quality Laboratory, so that you can compare S-PVC resins with the same base.

Comparison:  

- If you use Static Thermal Stability method and sample (B) has a lower Thermal Stability:

o If sample (B) has a similar or more compact internal structure than your current resin (A), expect to significantly increase the amount of Thermal Stabilizer.

o If sample (B) has a looser internal structure than your current resin (A), expect to make a mild to negligible increase in the amount of Thermal Stabilizer.

- If you use Dynamic Thermal Stability method and sample (B) has a lower Thermal Stability, 

o Expect to increase the amount of Thermal Stabilizer accordingly.

In the final part of this article, I will discuss a final aspect to be consider when comparing S-PVC resins: Consistency

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