FROM PRIMORDIAL PVC TO PRESENT (AND FUTURE) PVC RESINS

The history of PVC since the beginning of its production in early 20th century has been a long path of improvements and innovations to correct some weaknesses that still need to be addressed today:

• PVC requires both heat and shear to melt and develop good mechanical properties, similar to what is required for other thermoplastics
• However, PVC undergoes thermal degradation by dehydrochlorination at temperatures similar to those needed to melt and flow (processing).  PVC also degrades faster as shear increases
• PVC therefore has a narrow processing window, meaning that it is not possible to melt fully without starting degradation that quickly deteriorates first its aesthetic appearance (coloration) and then its mechanical properties
• Molten PVC has high melt viscosity because processing temperature cannot be increased too much

• Poorly melted PVC contains heterogeneities, due to the incomplete destruction of the original morphological structures, that have limited anchoring with entanglements and crystallites to molten chains, resulting in products with inferior long-term mechanical properties

Early 20th century the problem was so serious that primordial PVC was considered an intractable, fragile and unstable material with very limited applications.

As experiences and knowledge of the complex physical-chemical phenomena involved in polymerization and processing were generated, improvements and innovations emerged to mitigate such weaknesses, allowing the industrial production of vinyl products.

To put into context and understand the different adaptations made to PVC resins that have allowed the explosion of vinyl products in our world today, let's group them into:

1. Changes in composition (mixture).- They include mixing with compatible chemical compounds (additives), solids (fillers) or polymers (blends/alloys) to form a quasi-stable matrix with PVC chains that improves mechanical properties by decreasing or increasing interchain interactions and/or to improve melting and processability by reducing glass transition or melt viscosity
2.  Changes in macromolecular structure (morphology).- They include differences in size, shape and stability of structures/particles formed when PVC chains precipitate during polymerization to improve mixing performance by reducing diffusion length and/or to improve melting and processability by improving heat/shear transfer
3.  Changes in molecular structure.- They include copolymerizations, functionalizations and any other form of chemical modification of PVC chains to either improve thermal stability, mixing performance and/or to improve melting and processability

The first group define what is known as “formulation” for compounds, while the others are the routes to design different PVC resins, from General Purpose (GP) resins to Specialty resins.

GP resins are those that are produced in large volumes in markets with well-established requirements accepted by PVC processors for mature applications, and where different resin producers with similar technologies and resins compete based on low prices.

On the other hand, Specialty resins are produced in limited quantities based on specific requirements of a few consumers, who are willing to pay the higher price of an evolving technology to obtain some advantage(s) that such resin provides, mainly in performance in a specific application.

General Purpose rigid-grade resins

It is currently made up of two main grades:

-          extrusion, with higher molecular weight to improve mechanical properties

-          injection, with lower molecular weight to improve melt flow

To reach optimal characteristics for each application, it was necessary to make changes to primordial PVC both in its molecular structure and in its macromolecular structure.

The most important change was the introduction of low amounts of chemical compounds currently known as “thermal stabilizers” that allow the chemical substitution, at lower temperatures than processing temperatures, of labile sites on PVC chains that can initiate degradation such as allylic and tertiary chlorines by more thermally stable functional groups.

To ensure the correct distribution of thermal stabilizers, it was also necessary to optimize the polymerization processes (both suspension and bulk) to obtain free-flowing powders that could absorb said thermal stabilizers.

Further morphological adjustments were necessary to ensure the reduction of residual VCM in PVC resins required by the promulgation of new exposure regulations following the discovery of VCM carcinogenicity in late 1970s.

General Purpose flexible-grade resins

In late 1920s, Waldo Semon of B.F. Goodrich Company discovered that the characteristics of PVC could be drastically modified from a rigid solid to a flexible material by adding chemical compounds of specific compatibility, today known as “Plasticizers”.

To ensure the rapid and homogeneous absorption of liquid plasticizers, modifications were made to polymerization processes (both suspension and bulk) to obtain resins that could absorb increasingly higher amounts of plasticizers for new flexible applications and remain as free-flowing powders for proper handling before processing.

In addition, morphological adjustments had to be made to comply with the new regulations for residual VCM in PVC resins.

CPVC resins

The functionalization of PVC by chlorination arose from the search to improve the dimensional stability of vinyl products exposed to temperatures higher than their Tg (80°C).

There are two main applications: extrusion and injection, which can vary in the degree of chlorination depending on the desired properties and the limitations of the chlorination process.

Chlorination process depends strongly on its conditions (there is a “dry” process and a “wet” process) but also on the morphological characteristics of the PVC resin used as a base for chlorination.

For this reason, modifications are made to PVC polymerization processes (both in suspension and bulk) to obtain base resins with suitable molecular weight that can absorb chlorine quickly and homogeneously in chlorination conditions to obtain improved CPVC resins.

Although CPVC resins can still be considered Specialty, there is currently the entry of many new producers that will eventually turn into General Purpose resins.

Paste-grade resins

An option adopted for specific applications was to modify the supermolecular structure of PVC to form very small particles that can remain dispersed at low temperatures in plasticizer (“plastisol”), so they can be applied to surfaces in several ways (coating, dipping, spraying, molding) depending on the rheological behavior determined by particle size distribution.

When the plastisol is subjected to heating, plasticizer diffuses rapidly into PVC particles due to their small size and without the need to apply shear stress, which implies less exposure to conditions that cause thermal degradation.

Some paste resins also include copolymerization with initial or continuous addition of one or more comonomers during reaction to improve specific mechanical properties in the finished product.

Ultra-high molecular weight resins

General Purpose flexible-grade resins have disadvantages in applications with extreme temperatures, since the impossibility of completely melting the original morphological structures produces inhomogeneities that rapidly deteriorate the mechanical properties under such conditions (fast ageing).

However, using PVC resins with much higher molecular weights it is possible to mitigate such limitation by increasing the amount of entanglements and crystallites that attach said inhomogeneities with the longer molten chains.

The result is a flexible material with greater toughness that can be used as a replacement of elastomers from refrigeration seals at low temperatures to under-the-hood automotive components at high temperatures.

Polymerization conditions (in suspension) must be adjusted not only to obtain the required molecular weight, but also the morphology that allows a homogeneous absorption of plasticizers and other additives for compounding.

Low-gloss and cross-linked resins

PVC resins with low degree of crosslinking by copolymerization with a divinyl comonomer were developed to provide a matte surface finish to vinyl products, unlike the glossy aspect obtained with conventional PVC.

Resins with higher degrees of cross-linking have been designed and used in very specific applications to increase the mechanical properties of the final product.

It is obvious to point out that it is very important to take care of the homogeneity of crosslinking reaction as well as its effect on the formation of morphological structures to guarantee adequate processing.

There are several vinyl resins that are either used in small volumes or have been discontinued in favor of other materials with better performance in the specific application.  It is not the purpose of this short article to provide an exhaustive review of these resins.

The important thing is to point out that it is possible, with the experiences and knowledge generated in 100 years of PVC production …

• Design and produce resins suitable for currently booming applications such as Additive Manufacturing (3D printing)
• Revamp General Purpose resins to improve their performance in their current applications and possibly make their way into applications currently restricted due to their limited performance

Let me know if you are interested in 21th century PVC.

caguilar063@hotmail.com