REVAMPING PVC RESINS TO BETTER FACE CURRENT CHALLENGES

PVC produced by suspension polymerization is one of the simplest polymers from a chemical point of view but has complex structures (morphology) that allow it to be easily handled and mixed with the necessary additives for its modification and subsequent processing.

However, despite its importance and all the research that has been done to understand the phenomena involved in its formation and its relation with performance, industrially produced PVC resins have not changed significantly in the last 40 years.  

The last major change was the increase in porosity and the consequent improvement in morphological homogeneity that occurred as a consequence of the efforts to decrease residual VCM between late 70s and early 80s.

What is the reason for this? Let's explore this using the Materials Science approach, which states that the way a material is modified in a PROCESS directly defines its internal STRUCTURE, which in turn affects its measured PROPERTIES, which ultimately determine its PERFORMANCE in a specific application.

PROCESS → STRUCTURE → PROPERTIES → PERFORMANCE FOR PVC RESINS

Suspension polymerization is a PROCESS in which vinyl chloride (liquefied under pressure) is dispersed as droplets in water and then subject to polymerization reaction under specific conditions that includes water/VCM ratio, additives (types, amounts and ratios), temperature, pressure, agitation, etc.

The result are PVC resins formed with hierarchical STRUCTURES such as …

- Grains, formed by the controlled coalescence of subgrains at specific stages of polymerization

- Subgrains, which originally were vinyl chloride droplets that underwent polymerization

- Agglomerates, resulting from the fusion of primary particles during polymerization

- Primary particles, growing from the precipitation of micro-domains on domain’s surface

- Domains, particles resulting from aggregation of ~1000 micro-domains

- Micro-domains, formed by initiation and subsequent chain transfer and termination reactions from a single initiator molecule

- Crystallites, formed by the close interaction of stereoregular segments of nearby PVC chains

All such structures are not uniform but consist of distributions of sizes and characteristics that depends on PVC nature, molecular weight distribution, tacticity, mixing dynamics and several polymerization conditions.

Those structures make PVC resins to have specific PROPERTIES that are measured using standard methods, from which a single value is assumed to describe an entire batch/lot.

However, as it is obtained from small samples intended to be representative of a whole batch/lot, the value is really an "average" of the variations that PVC resins have both grain-to-grain inside a batch (reflecting process technology) and batch-to-batch within continuous production (reflecting process control).

PVC resin producers use these values as a process control measure, working under the assumption that their current technology (whether original or modified over the years) serves to meet the performance requirements of their customers, although some applications may not have existed when Plant’s base Technology was designed.

Most important properties includes:

- K-value.- Measure of reactor temperature control

- Bulk density.- Measure of recipe and drying control

- Porosity.- Measure of recipe control

- Fish-eyes.- Measure of reactor antifouling and cleaning control

- Residual VCM.- Measure of stripping control

- Volatiles.- Measure of drying control

- Sieve/size tails.- Measure of recipe control

- Thermal stability.- Measure of inhibition and stripping/drying control

The PERFORMANCE of PVC resins are also evaluated by standardized methods that use representative samples with simple formulations to minimize the contribution from additives, so they have the same limitations in qualifying both grain-to-grain variability within a batch and batch-to-batch variability in continuous production.

In general, the performance characteristics of PVC can be grouped into:

- Melt characteristics.- Gives an idea of how easily PVC morphological structures are destroyed during processing and make it flow, regardless of what processing additives are added.

- Degradation.- Gives an idea of the heat resistance of PVC to avoid coloration due to thermal degradation, regardless of what type and level of heat stabilizers may be added.

- Mechanical properties.- Give an idea of how strong PVC structure is after melting and being allowed to reform by cooling, regardless of what performance-enhancement additive may be added.

PROCESS → STRUCTURE → PROPERTIES → PERFORMANCE FOR PVC PRODUCTS

As mechanical and rheological properties are strongly influenced by PVC mass and molecular weight (for all applications) and plasticizer absorption (for flexible application), both resin producers and processors agree that performance is defined by K-value and bulk density for rigid applications, while for flexible applications porosity and fish-eyes are also considered.

PROCESSING of PVC resin begins with its physical mixing with solid and liquid additives of the type and quantity necessary to modify its properties according to the requirements from the final application. The resulting dryblend is then melted with an extruder through CDFE mechanism (compaction/ densification/ fusion/ elongation) to remove to some extent the original resin structures and form a new structure within the finished vinyl product. Additional processes such as injection, calendering or other may be included depending on the application.

The final STRUCTURE would be a homogeneous material at the appropriate scale to guarantee its performance in the final application but partially heterogeneous at smaller scales, with structures such as:

- Amorphous, continuous and homogeneous fraction with dispersion of solid fillers (if any)

- Crystalline fraction forming a continuous 3D network with PVC chains joining:

o Primary crystallites, remnants from the original primary particles

o Secondary crystallites, formed by the fusion of original crystallites and then a gradual recrystallization of stereoregular sequences that tend slowly to equilibrium

The ratio of secondary to total crystallinity is known as Gelation degree or Fusion level, and is used as a processing indicator since there is empirical evidence that a gelation degree of around 60%-70% is necessary to reach an optimum level of some mechanical properties.

Since PVC is used for a wide variety of end-use applications but processing temperature is limited by the onset of degradation, modifications of the type and/or amount of processing additives are favored to address any processability limitation that PVC resins might have.

This is why PVC processors are "used to" making modifications to their formulation and processing conditions to ensure the properties of their products, even if it means increasing both additive and energy costs.

As a result of the structure developed during processing, vinyl products have PROPERTIES that can be measured to evaluate its quality such as: Density, Hardness, Tensile strength, Elongation at break, Thermal stability, as well as other more directly related to its PERFORMANCE in the specific application.

For example, for pipe extrusion tests such as falling weight impact energy, notched Charpy impact strength, yield stress and hoop stress are valuable indicators of the performance of PVC pressure pipes.

CURRENT DYNAMICS OF IMPROVEMENT IN PVC INDUSTRY

Currently, PVC is considered a commodity so many producers and processors have in common the idea that meeting specifications is enough to accept the equivalence between resins.

This belief has made their improvement dynamics more focused on costs in such a way that:

- Producers, despite being very conservative in terms of process changes, seek both to increase their productivity for reducing unitary costs and to use the lowest-cost, average-to-barely-performing polymerization additives available.

- Processors are accustomed to making continual adjustments to their processes to compensate for any variation in their raw materials, so they seek to use the lowest-cost, average-to-barely-performing PVC resins and processing additives available.

This means that the incentives for performance improvement are minimized ("customers aren't going to pay for better quality" some managers say) and instead focus on reducing direct or indirect costs through incremental process adjustments looking for localized prompt benefits.

This has been advantageous over a long period as the 'responsibility' for meeting new PVC performance requirements has been delegated to the use of improved additives, leading to an impressive growth of such industry.

However, as we have seen, any change in a process will generate a change in product performance that can be positive or negative, significant or insignificant. This means that some changes will surely be counterproductive and generate increased costs (either direct, indirect or hidden!) if Process → Structure → Properties → Performance causalities are not fully considered.

For example, there are changes that a PVC producer can make that would be positive for its immediate results (for example, changing a polymerization additive for a cheaper one) but could result in decreased performance of its PVC resins with processing customers (for example, they would require more energy and hence more thermal stabilizer for proper processing), resulting in customer dissatisfaction.

In some cases, complaining processors would have to be "incentivized" by a PVC producer with price discounts to continue buying... at least until the change is backtracked or compensated, usually after a long and costly "trial and error" process.

NEED OF SUSPENSION PVC REDESIGN

In addition to the requirements of PVC for each of its applications, today there is a need to reduce PVC carbon footprint to increase its sustainability through different strategies such as:

1. Use of clean energy in production processes

2. Use of more environmentally friendly feedstock

3. Reduce energy consumption in production processes

4. Minimize waste and maximize yield

5. Increase the recycling of vinyl products

Strategies 1 and 2 have a greater effect on the carbon footprint, which is why they have attracted great interest, but they involve large resources and form close alliances and/or cooperation with different sectors and companies.

For PVC producers, Strategies 3 and 4 require improvements in their processes that would affect the structures of PVC resins, while for PVC processors a redesign of the structure of PVC resins is necessary to achieve both strategies. And such a redesign also is needed to maximize Strategy 5.

That is why a Performance-driven Design is necessary that includes not only the sustainability requirements but also the performance requirements for each particular application, in order to define the appropriate modifications to production PROCESS that generate the STRUCTURE of PVC resin with the right PROPERTIES to achieve the desired PERFORMANCE for the application.

For example, the initial performance requirements for the manufacture of PVC pipe, that would have to be agreed with the processors, could be something like:

1. Compliance with current applications.- Exceed the performance required by currently applicable standards and legislations for PVC pipe (eg ASTM D1785)

2. Compliance to reduce carbon footprint.- Minimize the thermal/mechanical energy needed to process PVC without negatively affecting the mechanical performance of pipes 

3. Compliance with sustainability and cost reduction.- Minimize the required amount of additives used in processing, mainly thermal stabilizers, without negatively affecting the performance in appearance or mechanical resistance of pipes.

4. Compliance with sustainability and cost reduction.- Maximize grain-to-grain and batch-to-batch consistency.

The following steps require strategic vision and development work so that, based on currently available knowledge, a suitable structure of PVC resin that meet performance requirements can be defined and then all changes to the process can be proposed and tested.

The final result would be a new generation of PVC resins with better performance in terms of sustainability, profitability and compliance with the requirements of each particular application.

Let me help you in this process of revamping your PVC resins.

Contact me at caguilar063@hotmail.com