BRIEF REVIEW OF PIPE-GRADE PVC RESINS

After World War II, the potential of PVC as a versatile and innovative material was recognized and efforts were made to solve the problems associated with the processing of unplasticized PVC.

As a result, great advances were made in extrusion technology (mainly in Europe), as well as in additive technologies (mainly in USA), that made possible the production of rigid PVC articles.

Late 1940s PVC pipes were first produced in Europe and early 1950s USA quickly followed, building a strong and fast-growing position in water pipe markets previously dominated by materials such as concrete, iron and other metals.

For this, PVC was evaluated by national organizations, first in the form of compounds and then as individual raw material, to verify that it can meet the mechanical properties both in the short-term and long-term required for water conduction application.

As the mechanical properties of polymers depend mainly on molecular weight, it was originally defined that PVC resins for pipe extrusion must have a K-value greater than 64.5 to guarantee mechanical integrity without failure. The increased melt viscosity, in contrast, limited K-value to a maximum of 68.5 for proper processing.

This initial definition led PVC producers to have a single grade of resin under the "one size fits all" principle, which is still maintained by several producers to this day.

However, as new rigid PVC applications have been developed with particular and even more demanding requirements, PVC processors have increasingly relied on the use of expensive additives to "force" processing and "squeeze" the maximum properties out of available PVC resins.

Knowing this, some PVC producers with enhanced process expertise have seen an opportunity and offer PVC resins with fine-tuned K-value for specific applications, helping processors in their quest for increasing process and product performance.

In this way, applications that require higher mechanical strength can use resins with K-value in the upper limit while applications that require higher flow can use resins with K-value in the lower limit.

Other producers have chosen a different way to address the more specific needs of their Customers, knowing that not all PVC resins have the same processing performance despite having the same K-value (as many processors have found out mainly through bad experiences).

This is because PVC morphology can affect processing, since loose structures are easier to destroy and melt, while compact ones require more energy (heat and shear). Since inhomogeneities caused by uneven fusion affect both impact resistance and long-term strength of extruded pipes, there are a few generic recommendations for PVC resins in the industry:

- Minimum coarse grains (retained in 40 mesh) to avoid hard-to-melt particles

- Low fine grains (thru 200 mesh) to minimize static charging

- Enough porosity to achieve the required residual VCM

- The higher bulk density, the better to maximize productivity in extrusion

- High consistency, both from grain-to-grain and batch-to-batch to form an homogeneous melt and maintain a stable process with consistent mechanical properties 

Such recommendations ensure adequate performance, but if outstanding performance is sought, more precise and purposeful control is necessary. 

Producers using differentiated control of morphological characteristics offer PVC resins with the same K-value but with tailored processing and performance for different rigid applications.

As mentioned, PVC resins with loose internal morphology are easier to process, which is advantageous for thin walled extrusion applications that would be more affected by inhomogeneities.

Denser structures, with enough porosity to obtain residual VCM, would be more adequate for thick-walled applications to maximize extrusion productivity.

The best option, as mentioned in a previous post, is to revamp PVC resins by optimizing both charge/operating conditions and the use of suitable polymerization additives to obtain the maximum possible benefit…

a. For PVC producers

o High consistency in resins, with minimum off-grade

o Tailored morphology of resins with…

 Optimized and consistent spatial distribution of molecular weights

 Optimized and consistent size and shape of agglomerated primary particles to get low residual VCM and fast diffusion of thermal stabilizers

 Optimized and consistent size and shape of grains to get high bulk density

o Increased loyalty from Clients (processors)

b. For PVC processors

o Lower gelation to reach required mechanical properties

o Improved long-term impact and strength

o Higher thermal stability

o Maximized extruder output

o Consistency in process, with minimum of adjustments between lots

o Consistency in final product, with minimum off-grade

o Lower consumption of processing additives and energy (lower carbon foot-print)

Someone once told me that Clients (processors) would not pay for better Quality and that they had to "learn to chew" the PVC resins offered to them.

I did not agree then and I do not agree today... It is not a question of costs or an issue of "us against them", but of improving performance and reducing the negative impacts of processes and products for the benefit of people of today and tomorrow.

caguilar063@hotmail.com