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)
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
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
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 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 . So it was found that Ti-catalysts provide a low crystallization rate . 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 . 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 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 . 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 . 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 and providing a stable catalytic Ti-complex is the composition of phosphinic acid titanates that are combined with phosphoric acid . Advantages are high catalytic activity at low catalyst content, high clarity and lodiscoloration.
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 . 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. Because of the high catalytic activity of titanium and the possible acceleration of the esterification reaction, too, titanium will remain one of the main tools for developing new catalyst systems.
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 . Zeolites have been used 25 years ago as finely distributed additives to modify film polyester . 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 . 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 . 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 .
Beside titanium and aluminium only a few other elements and element combinations like samarium , tin , iron , molybdenum and tungsten , magnesium/potassium/phosphorus  and zirconium/ silicon/cobalt  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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