The Catalyst Review June 2020 - 7

SPECIAL FEATURE

Polymerization Activators, Stereo-Regulators and Supports
By Salvatore Ali, PhD
Over the past 70 years or so, the plastics industry has built an impressive and relatively consistent record of growth accomplishment.
Synthetic polymers are an industry with a worldwide production of about 425 million tons in 2018 (PlasticsEurope, 2019), composed
of functional and structural polymers, elastomers/rubbers and synthetic fibers (Textile World, 2015). The plastics industry is a key
enabler of innovation of many downstream sectors of the economy such as healthcare, energy generation, aerospace, automotive,
construction, electronics, packaging or textile. None of these sectors would innovate and grow as much as they do without plastic
materials and solutions; the unique characteristics of plastics allow them to make a strong contribution to a more sustainable and
resource efficient world. The major market needs that revolve around plastics improvements and innovation are:
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Stronger materials that enable down gauging (lightweighting)
Materials with rheological features that make it possible to increase fabrication rates (faster)
Materials with less catalyst residuals, lower volatile organic compounds content, lower taste and odor by avoiding peroxide
cracking and non-phthalate-based catalysts (cleaner)
Materials with higher transparency (clearer)

These themes together with sustainability are the dominant innovation drivers. Market drivers are a key part of the innovation
process, but catalyst technology capability is critical to being able to provide solutions that enable polymer producers, converters
and brand owners to cope with market needs. In general, the catalyst function is to activate and accelerate the polymerization
reaction enabling the economical scale-up of the related polymer production process from the lab to the industrial plant.
The catalyst productivity or yield is a key driver of the polymer process efficiency and product quality (a high yield makes the
catalyst residues negligibly low producing high purity polymers). The catalyst controls the polymer microstructure, the polymer
molecular weight (MW) and the polymer macrostructure (i.e., morphology, content and phase distribution of co-monomers); the
catalyst therefore effectively determines which polymers can be produced and how efficiently they can be produced (Ali, 2015).
Coordination catalysts as Ziegler-Natta (ZN) and chromium oxide systems must be used in the polymerization together with cocatalysts activators (aluminum alkyls) that destroy catalyst poisons and increase catalytic productivity. For isotactic polypropylene
(PP), also stereo-regulators, internal (e.g., phthalates, others) and external (e.g., silanes) electron donors, have to be added to the
system for tuning the polymer chain stereo-regularity, the molecular mass distribution and the co-monomer incorporation.
With the advent of metallocene and single site catalysts (SSC), new and more complex types of activators such as aluminoxanes
(MAO), fluorinated boron compounds and similar complex structures are used. Even though a few processes for ethylene
polymerization use homogeneous catalysts in solution reactors, most olefin polymerization processes operate with heterogeneous
catalysts in two-phase or three-phase reactors. Commercial heterogeneous catalysts are then supported, i.e. bound to a solid with
a high surface area. The support in the majority of the ZN catalysts is MgCl2, one of the reactant species of ZN catalyst system.
Another popular support for most catalysts is micro-porous spheres of amorphous silica. In special polymerization cases (i.e., PP
nano-composites), also clay microspheres are used as special catalyst support.
Coordination catalyst systems (ZN, Chromium oxide, SSC and metallocene) enable
the production of large polymer families as linear polyethylene − PE (including
high density PE or HDPE, linear low density PE or LLDPE), PP and some rubbers
(ethylene propylene rubber or EPR/EPDM, polybutadiene rubber or BR and so on).
As seen, the catalyst really is a catalytic system comprising different components
as activators, stereo-regulators and supports. The selection of the catalyst
components has a substantial influence on polymer products that can be made
(in terms of molecular flow rate or MFR, molecular weight distribution or MWD,
isotactic index or II for PP, copolymer distribution, rubber content and so on) and
on the operability and the cost in use of the polymer-manufacturing units. See in
Figure 1 a summary of such various catalyst components and their influence. The
catalyst key performances, affecting polymer properties, offer advantages to all the
related resin industry stakeholders (resin producers, converters, brands and enduse consumers), having however each its own diverse objectives, as follows:
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Figure 1: PP Catalyst System Components Influence
Product/Process Capability

Source: Author

Producers want to optimize the production efficiency, polymer capability to universal application as well as efficient use of raw
materials like catalysts.
Converters emphasize optimum polymer processability as well as ever-increasing production efficiency and polymer property
advantages.

The Catalyst Review 										

	

June 2020

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The Catalyst Review June 2020

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