The Catalyst Review June 2020 - 10

SPECIAL FEATURE
modern industrial PP manufacturing process technologies. For ZN catalysts, the
Figure 2: Recent disclosure of internal donors.
internal and external electron donors are very critical in tuning the chain structure
(isotacticiy range), MWD of the polymer, polymerization decay characteristics,
oligomer (low MW polymer) content and hydrogen response. Internal electron
donors are added during catalyst preparation while external electron donors and
activators Al-Alkyls are added in the process of polymerization. For a given system,
changing the internal donor/Ti ratio and/or Al/external electron donor might lead
to a dramatic difference in the polymer structure. The search for further catalyst
improvements brought to shed light on various novel internal donors. In recent
decades, numerous researches focused on finding more potent non-phthalate (NP)
internal electron donors for the 5th generation ZN PP catalysts. As shown in Figure 2,
there are several types of NP internal electron donors that are commercially used or
proposed (Severn, 2018). The diether compounds developed and used by Basell, in
particular 2,2-disubstituted-1,3-dimethoxypropanes, show high stereospecificity even
without using an external donor. The diether compounds exhibit particularly high
polymerization activity, typically giving yields exceeding 100 tons PP/Kg cat, good high
hydrogen sensitivity, and relatively narrow MWD. Aliphatic dicarboxylic ester-based
internal donor and, in particular, succinates and polyol esters have been employed
Source: Wang 2019
by Basell and BRICI/Sinopec. The succinate internal donors developed by Basell
produce PP with much broader MWD using an alkoxysilane as an external donor. Further, the succinate-based catalysts also generate
ethylene-propylene copolymers with lower glass transition temperature, which enables the production of heterophasic copolymers
having great balance of stiffness and toughness (Wang, 2019). In addition to this, mixed donors are also employed in many cases, for
example, Basell uses a mixture of succinate and diether donors for its general-purpose offering of NP PP catalysts.
Figure 3: Last generations of internal donors in-use.
Grace with its
non-phthalate
PP catalyst
solution
(Consista C
601), using a
substituted
1,2-phenylene
dibenzoate
internal donor,
is undoubtedly
Source: Author
a leader in the
developments of phthalate catalyst replacement. See in Figure 3 the
structures of phthalate, diethers, succinates, and phenylene dibenzoate
internal donors. Consista C 601 PP catalyst is a drop-in NP catalyst system
with improved product properties, including high melt flow grades that
avoid visbreaking, higher yield, and higher hydrogen response, high
selectivity/low xylene extractables, excellent impact/stiffness balance and a
broader MWD. Despite the fact that the phthalate based catalysts produce
PP with far lower phthalate content below the 0.3 wt% concentration limit
in the REACH Regulation in European Union, a phthalate-free solution is
highly motivated and becomes a competitive advantage. The phthalate 4th
generation PP catalyst is still the market global workhorse with 80% global
use; however, the market penetration of NP PP catalyst in Europe is now
close to 70% (see Figure 4) demonstrating a strong increase year by year.
Americas market is following the increasing trend in non-phthalate catalyst
usage driven by brand owners and converters; also in Asia and ME, these
catalysts start to be requested for suited application as food contact and so
on.

Figure 4: Non-phthalate PP catalyst market europe.

Source: Davis 2019 modified by the Author
Figure 5: Typical external donors in-use.

Source: Wang 2019

Typically, alkoxysilane compounds are used as external donors and added into the polymerization reactor. Figure 5 shows some of
the structures of such external donors. The stereospecificity of the ZN catalysts could be modified by changing the substituents of
the alkoxysilanes containing relatively bulky groups. Silanes containing hydrocarbon substituents and oxygen atom with larger size
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The Catalyst Review

June 2020

The Catalyst Review June 2020

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