HOW PVC RESINS CAN BE DIFFERENT: COMPARISON OF S-PVC RESINS, PART 1

Each S-PVC resin is designed and produced to meet the requirements for a specific application, be it for the manufacture of rigid or flexible products.

However, not all producers have the same know-how or commitment to optimize product’s properties, which is reflected in different performances among different S-PVC resins when using them in the same manufacturing process.

When a vinyl processor has only one supplier of S-PVC resin, it is normal for their process conditions and operational and management responses to "get used" to that resin, making it difficult to objectively compare with resins from other producers.

However, current globalized markets and high competition make it possible and necessary to take advantage of the opportunity of using S-PVC resins from different suppliers in order to maintain or increase company’s competitiveness.

Then it is necessary develop the knowledge and experience to properly compare S-PVC resins in such a way as to avoid buying any one that may negatively affect either process economy (by requiring more energy and additives to process it) or the Quality of finished products (by generating imperfections either aesthetic or the ones that compromise product’s mechanical properties).

Following are a series of properties that should be analyzed when comparing a resin sample (B) against the one being currently used in process (A):

1. MOLECULAR WEIGHT

2. PACKING OF PRIMARY PARTICLES

3. PACKING OF GRAINS

4. THERMAL STABILITY

By evaluating each property with the resources at hand, you can get an idea of how a new resin B will behave in your process and so make an informed decision that ensures the stability and competitiveness of your manufacturing process.

1.- MOLECULAR WEIGHT

This is the most important variable in the selection of S-PVC resins so all producers strive to comply with it.

The length of polymeric chains determines both processing (melt viscosity, melt strength, die swell, etc.) and mechanical performance of product (tensile strength, impact, low temperature behavior, etc.) for an application, as shorter chains flow better but have lower crystallinity per molecule that boost physical properties while longer chains flow with more difficulty due to entanglements and produce a stronger material due to its increased crystalline crosslinking network.

It is generally assumed that Molecular Weight is homogeneous and that a single value describes it, commonly expressed as "K-value", but in reality it is a distribution of polymer chain lengths that varies in time and space during vinyl chloride polymerization, being determined mainly by polymerization temperature (commonly intended as isothermal) and to a lesser extent by initiator loading and the conditions under which chain propagation takes place.

Molecular Weight is obtained by measuring the effects of hydrodynamic volume of polymer chains in dilute solutions, either by viscometry (such as ISO 1628-2) or chromatography (GPC / SEC).

Producers of S-PVC resins provide Technical Datasheets with generic properties of their products, as well as Quality Certificates of lots produced, to validate that their products meet offered specifications.

If you cannot directly verify the Molecular Weight or K-value of the sample, either in your own laboratory or with a third party, it is valid to accept the material with a Quality Certificate in case to receive it directly from producer.

However, if it is received from an intermediary, it is highly recommended to carry out said analysis until you can certify the reliability of said intermediary, to avoid confusions or falsifications of received S-PVC.

Comparison:  

- If Molecular Weight of sample B is higher, you should expect an improvement in mechanical properties of finished product but also a decrease in processability (higher mixing viscosity) that you will have to compensate with higher consumption of energy and / or additives to melt and make PVC flow.

2.- PACKING OF PRIMARY PARTICLES 

S-PVC resins with the same K-value from different producers frequently perform differently during processing as shown in the following graphs that compare 2 resins produced using different polymerization conditions but with the same K-value and almost identical Molecular Weight Distribution (left graph).

Both resins were formulated with the same recipe and using the same dry-blending conditions, so torque-time graph measured in a Torque Rheometry (right graph) shows the different processing behavior that would be attributed to differences in internal morphological structures (both graphs from Darvishi et al).

As commented before, the way in which Primary Particles are packed has a direct effect on Porosity since loose agglomerates contain larger spaces and high Porosity while more packed/fused agglomerates contain smaller spaces and low Porosity.

Porosity in S-PVC resins is measured by natural or forced absorption of a liquid that under test conditions does not swell (penetrate) polymer matrix but only occupies the volume of holes or "pores" inside grains. 

Standardized methods such as Centrifuge Plasticizer Absorption (ASTM D3367) assume that all grains are homogeneous and therefore a single value is sufficient to describe Porosity, while other methods such as Mercury Intrusion Porosimetry (ASTM D2873) shows a continuous penetration curve related to the internal structures formed within grains.

Whether you manufacture flexible or rigid articles, it is highly recommended to measure total Porosity (cm3 per PVC gram) directly and/or ask your provider to include this analysis in Quality certificates as it serves to have a general idea of how agglomerated Primary Particles are.

However, S-PVC resins always have a different levels of heterogeneity that depends on the conditions in which they were produced so different resins with the same total Porosity value could have different distributions of morphological structures.

Therefore, it is advisable to use methods to visualize heterogeneities such as Mercury Intrusion Porosimetry (MIP) or a modification of Centrifugal Plasticizer Absorption method at high temperature and different times, as shown in the following figure.

S-PVC resins with high heterogeneity are made up of grains ranging from very porous and easy to process grains to very compact grains that slowly absorb plasticizer and are difficult to melt (some do not even melt like "Fish Eyes" or gels ).

Comparison:  

- If Porosity of sample (B) is higher than that of resin A:

o For rigid applications, expect faster fusion and easier processing.

o For flexible applications, expect faster absorption and dispersion of plasticizer.

- If Heterogeneity of sample (B) is larger than that of resin A:

o For rigid applications, you should expect imperfect additive dispersion and melt heterogeneities that could result in mechanical failure in your final product, so you'll have to compensate with increased energy and additive consumption to homogenize molten plastic.

o For flexible applications, you should expect inhomogeneous plasticizer concentration and melt heterogeneities that could result in cosmetic flaws or even mechanical failures in your final product, so you'll have to compensate with higher consumption of energy and additives to homogenize molten plastic.

In the second part we will finish reviewing the properties necessary for the comparison between S-PVC resins.

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