Titanium – a real alternative to antimony catalysts in PET bottles

Plastic bottles, clothing, videotape, packaging film... all of these articles are made using polyethylene terephthalate (PET) which is commonly called "polyester". The most common route for the production of PET is the reaction of pure terephthalic acid (PTA) with ethylene glycol helped by a catalyst. For many years now the catalyst of choice in the polyester industry has been antimony. Some other metals have been used (notably germanium and titanium) but the vast majority of PET produced today is made with antimony. However, it would seem that the tide is beginning to change and more and more companies are realizing the benefits of titanium as a catalyst for the production of PET bottles, fibres and films.

At the recent PET Strategies 2006 conference in Atlanta, Jim Bruening of Wellman Inc. presented a paper on the production of polyester resin for bottles using titanium catalysts1. Wellman is a world leader in the production of PET resin for bottles. Mr. Bruening stated that Wellman have chosen to make titanium catalysts their chosen strategic technology platform, and he believes other companies in the market will follow suit. Wellman also launched their new grade of resin specifically for carbonated soft drinks and bottled water, Ti842, at the conference. This grade will compliment their existing offering for hot-fill beverage bottles, the titanium catalyzed PET resin, Ti818.

Traditional reluctance to use titanium was due in part to colour issues and a lower activity in the solid-state polymerization (SSP) process step. However, Mr Bruening presented data showing that Wellman had surmounted these problems and were thus able to achieve the full benefits of the titanium catalyst. In hot-fill applications, titanium PET was more resistant to shrinkage than the antimony-catalyzed bottle and Mr Bruening said that the titanium catalyst opened the door to light-weighing hot-fill bottles because of superior strength. Titanium also gives a higher clarity bottle and this has seen the adoption of the titanium-catalyzed resin for critical applications such as white grape juice bottles.

In carbonated soft drink (CSD) applications, Wellman's new resin grade Ti842, also demonstrated significant advantages over the traditional antimony catalyzed resin. It showed reduction in cycle times of 5% to 10% during the injection moulding process. This corresponds to an increase in output and efficiency for the producers of bottle pre-forms. A 25%-30% drop in acetaldehyde content was also observed in the Ti842. Acetaldehyde is a by-product of the PET process and can have an adverse effect on the sensory qualities of the packaged beverage. It is particularly important in bottles for water.

The issue of antimony supply was also raised in the presentation. Over 90% of the antimony used in the world today is supplied from China, and the price of antimony catalyst has risen sharply recently as supply has become constrained. A notable effect was seen when an accident closed down one of the main mines. By contrast, titanium pricing has remained stable driven by its abundance in the earth's crust and its widespread production in many countries2.

For a number of years now, Johnson Matthey Catalysts's VERTECTM business has been collaborating with Wellman in the incorporation of titanium catalysts into PET resins and continues to supply Wellman with commercial supplies of titanium products today.

Wellman have nailed their colours to the titanium mast. It now remains to be seen if the rest of the market will follow suit.