The Current
Status of Catalysis and Catalyst Development for the Industrial Process
of
PoIy(ethylene terephthalate) Polycondensation
ULRICH K. THIELE
PUBLISHED and COPYRIGHT: International Journal Polymeric Materials, 2001, Vol. 50, pp. 387-394,
(Received 4 January 2001; In final form 19 January
2001)
SUMMARY
In this survey of the industry, it is shown that despite
the partial dissatisfaction with antimony-based polycondensation catalysts these
catalysts arc expected to remain the mainstay of industrial PET polycondensation
catalysis. This is despite the intensive efforts invested in the search of
other, stable and inexpensive non-antimony catalysts, such as those based on
titanium, aluminium, and several transition metals such as molybdenum, cobalt
and zirconium.
Keywords:
Poly(ethylene terephthalate); PET; Polycondensation;
Catalysis; Antimony; Titanium
INTRODUCTION
Since the first
polyester patents of H. W. Carothers, P. Schlack, J. R. Winfield and J. T.
Dickson, who provided the basic knowledge of our current industrial polyester
process, there were many polymer scientists trying to develop new catalysts. The
metal catalysts based on antimony and germanium dominate the industrial
production process since that time. Meanwhile nearly all elements of the
periodic System have been tried as catalysts in the polyester polycondensation.
The main driving force of catalyst research during the 19501970 period was to
protect or circumvent existing polyester process patents. The scientific work at
that time was concentrated in describing the catalytic mechanism of the metal
catalysts [1 6]. A large number of metals and non-metals show a significant
catalytic effect but the replacement of antimony and germanium as
polycondensation catalysts in an industrial scale has not succeeded until today.
Between 1970 and 1990 the main emphasis of the development work was the
optimisation of the catalyst recipe, feeding and preparation techniques and
polymer stabilization to meet the requirements of polyester processing such as
spinning and film making. With the fast growing packaging industry where PET is
used to make bottles and other packages, a new wave of catalyst development was
observed starting in 1990. The main target of this still ongoing development is
on one hand the replacement of the antimony which will be shown below to have a
negative environmental impact, and, on the other hand, the replacement of
germanium because of its incredibly high price. Besides, until today there is no
scientific evidence regarding any negative health impact of antimony used as
polycondensation catalyst and no governmental regulation of the many companies
searching for new catalytic systems. lt seems that the new generation of
chemists like in general idea of replacing antimony by a more powerful and
efficient catalyst. The aim of this paper is to give an overview about the
industrial catalyst knowledge, the driving forces for new developments and a
summary of the recent catalyst patents.
THE CURRENT
INDUSTRIAL CATALYST KNOWLEDGE
Antimony
Today, three different antimony compounds are used as polycondensation catalysts. The majority of the polyester is catalysed by antimony trioxide originating from a wide variety of sources. More than 12 companies are competing in this market. Antimony triactate which is produced by a few companies is used for about 30% of the
CATALYSIS FOR PET
total polyester production. As a niche product also the
highly purified antimony g]ycolate/glycoxide is in some cases applied. important
for the success for all this commercial antimony catalysts is the correlation
between price and product performance. Because of the current highly increased
line capacity of continuous plants, up to 400 t/d and more, the catalyst
business functions in a very sensitive network of experience and trust. in
addition to the kind and source of the antimony catalysts, the preparation and
feeding technology play a major role regarding the final product quality.
Comparing the basic quality and performance data of polyester, like colour,
clarity, DEG-content, COOH-groups, filterability and spinnability out of well
optimised and operated plants, one can find no significant quality differences
between polyester polymers catalysed by oxide, acetate or glycolate of antimony.
Germanium
Germanium
catalysed polyester were mainly used during the early years in film applications
because of its high clarity. Today the main portion of germanium catalyst is
still consumed by Japanese polyester producers who like the high brilliancy of
the polymer for bottle applications. Because of its tendency to support the
oxidative degradation germanium catalysts arc used together with stabilizer
based on phosphorus. Germanium is applied as germanium dioxide whereas
the main issue is to provide an oxide with high solubility. Meanwhile catalyst
producers who specialize in germanium catalysts are offering stable solutions of
germanium dioxide. The price of about 500 US$ per kg pure germanium dioxide is
the main driving force to gradually replace this catalyst.
Recent Catalyst
Developments
With the
booming growth of the polyester bottle resin production during the late 1980s
and the early 1990s the development of antimony free catalyst Systems was
supported by different driving forces and ideas. One driving force was the
sometimes hysteric discussion of environmental issues in the public, which
scared the polyester producers lest they be subjected to public pressure.
TABLE 1
Improvable properties of antimony-based catalysts
Property
Effect to
product or processor
1. Catalytic
efficiency 150 - 300 ppm Sb depending on
equipment and technology,
delta IV
0,15 - 0,25 dl/g per hour
2. Precipitation and
discoloration Sb metal as fine black particles,
Sb-oxidhydrates, insoluble
Sb2O5,
SbPbO4 and other Sb-P species
3. Reaction products of
antimony Sb-oligomers occurring as spinning smoke deposited
as "egg hells" around spinnerets
Another driving force was the always present desire to
improve the reactivity. lt is still the target today to have a catalyst System
which is able to increase the plant capacity significantly and which provides
the same or better product quality obtained from antimony.
Development
Targets for New PET Catalysts
To define
development targets for a new catalyst the properties of antimony witch could be
improved are colleted in Table 1.
The catalytic
efficiency is related to the polymer quality: the less catalyst the higher the
purity of the polyester in general, and related to production cost: the less
catalyst the lower its cost. As one can see, major points are precipitation and
discoloration. This is associated with a variety of secondary effects such a
black spots created by antimony metal deposits at pipe and reactor walls,
greenish to greyish colour tone and white to grey haziness of the polymer. Also
the crystallization rate and level of polyester is significantly influenced by
the kind and concentration of catalyst [7]. So it was found that Ti-catalysts
provide a low crystallization rate [8]. One important process impact of antimony
is caused by the reaction of oxygen with glycol or glycolates during
esterification and prepolycondensation followed by the creation of CO which
reduces the Sb + 3 present in the reaction mixture to metallic greyish
precipitation [9]. Out of this collection one can summarize the following
development targets for a new polyester catalyst:
à
Higher catalyst efficiency, high reaction speed in melt phase and solid phase
polycondensation, lo
catalyst
concentration, low catalyst cost.
à
High polymer purity reflected in brilliancy, white colour, high transparency,
excellent polymer
filterability, low oligomer content.
à
High thermal and oxidative polymer stability, low acetaldehyde generation.
Non toxic,
environmentally neutral, easy handling and easy application.
à
Added values such as catalytic activity during esterification, process
functional like improved IR
absorption
or significantly changed crystallinity.
à
No negative impact to downstream processing like spinning, bottle blowing or
film making.
Titanium
Titanium
alkoxides arc the state of the art catalysts to produce
poly(butyleneterephthalate) (PBT). Until today simple Ti-alkoxides play no role
as catalysts for the production of PET. Because of their high catalytic activity
titanium catalysts are the basis of most new developments concentrated on
modification of titanium catalysts to find new formulations which fit the above
mentioned development targets. The well-known negative properties of simple
Ti-alkoxides are the yellow discoloration, the precipitation of Ti02
by hydrolysis connected with low reproducibility of the catalytic activity and
the thermal instability of the polyester. To reach a neutral colour tone,
titanium is mostly combined with cobalt [10,11] or with an organic blue toner
[12]. Additionally phosphorus compounds arc used to improve the thermal
stability. To prevent against an early catalyst destruction by hydrolysis
different strategies arc applied. One path is the creation of stable
Ti-complexes by adding before or during the reaction complexation compounds like
2-hydroxy carboxylic acid such as citric acid or tartaric acid [13, 14] or
2-hydroxy-ketones like
a-tropolone
[15]. Another way is the application of finely dispersed
Tioxides/hydroxides or mixed Si/Ti precipitate which becomes partially resolved
under the polycondensation conditions. Here, the idea is to preserve a certain
solubility of the solid precipitate and use the precipitate particles as
Ti-donor during the whole polycondensation reaction [16 18]. Another approach to
preventing the discoloration
To protect the
catalyst metal like titanium against hydrolysis during the process a new
principle is suggest by absorbing the catalyst before the reaction at the inner
surface of finely dispersed charcoal or silica [20]. To prepare a polymer of
high clarity and brilliancy with such kind of semi soluble catalyst the particle
size must be significantly less than 500 nm.
Aluminium and
Zeolites
During the last
decade catalysts based on aluminium came into the focus of the +technical
development. The first patents suggesting aluminium as catalyst date back to the
1950s [21]. Zeolites have been used 25 years ago as finely distributed additives
to modify film polyester [22]. lt might be a matter of practical experience that
shortly before the mentioned additive patent expired, the catalytic activity of
zeolites was discovered. The minimum zeolite concentration useful to get a
sufficient polycondensation reaction was reported as 900 ppm. The commercial
zeolites recommended for use as catalysts are dried and contain 1,52,5% water
[23]. lt was later discovered that the drying of the zeolites after their
synthesis provides bower catalytic activity compared to zeolites commercially
available with their original water content of approx. 30 weight %. The amount
of aluminium dissolved in the reaction mass of the polycondensation was found to
be the catalytic active component. The sufficient concentration of these semi
soluble zeolites could be reduced to 400 ppm. Colour and thermal stability are
adjusted by the addition of small amounts of cobalt and phosphorus compounds
[24]. Finally a combination of aluminium trichboride and glycol soluble cobalt
compounds like cobalt acetate is suggested as polycondensation catalyst using
15 ppm Co and 55 ppm Ab [25].
SUMMARY
Beside titanium
and aluminium only a few other elements and element combinations like samarium
[26], tin [27], iron [28], molybdenum and tungsten [29],
magnesium/potassium/phosphorus [30] and zirconium/ silicon/cobalt [31] have been
suggested as polycondensation catalysts during the last decade. Until today
there is no commercial substitute to antimony in sight, even though some of the
described development catalysts were tried on an industrial scale and are also
available in semi commercial amount. The risk to change the catalyst of a large
continuously running polyester plant is high because of the very close
connection between kind of catalyst and the downstream processing performance in
textile and bottle processing. Therefore we may see a prolonged development
phase until one could say that a new polyester catalyst was successfully
introduced to the industry.
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