A bit of isosorbide makes a big change

Copolyesters are formed when modifications are made to polyesters. These modifications can considerably modify the performance profile of the polyester in question. Case in point: PEIT.

Polyethylene terephthalate, or PET, as this polyester is more familiarly called consists of the two main building blocs ethylene glycol and terephthalic acid, which are then combined to form a polymer chain. Strong and food-safe, PET has become one of the mainstay materials of the packaging industry and is used in everything from bottles and trays to blisters and clamshells.
There are, however, areas where the material performs less well. A good example is in hot-fill applications. Marked improvement can be achieved through the incorporation of an alternative comonomer, which modifies the performance profile of the material. More specifically, it has long been known that replacing the monoethylene glycol as  a diol with isosorbide benefi- cially modifies the properties of PET, boosting, among other things, its glass transition temperature and the optical properties of finished packaging. Ethylene glycol is still predominantly a petrochemical feedstock so (partial) substitution by plant-based isosorbide is also potentially appealing from the perspective of the material’s environmental impact.

Isosorbide – what is it?

“Isosorbide is a really versatile, safe, environmentally benign performance chemical with great potential as a co-monomer,” said Bruno Plancke, head of Global BU Industry, at Roquette, the France-based leading producer worldwide of plant-based ingredients such as isosorbide for the performance plastics and chemistry markets.
Isosorbide is produced from annually renewable plant feedstocks by a process of sequential hydrolysis and hydrogenation. Hydrolysis converts plant starches to glucose which are subsequently converted to sorbitol; the hydrogenation of sorbitol to isosorbide is the final step. Isosorbide production technology has been optimised over the last two decades, to minimise carbon footprint and deliver consistently high purity.

Since starting production at a modest scale in 2002 in Lestrem, France, Roquette has continued to expand its isosorbide activities. Today, the company operates a large-scale production facility at its Lestrem site, using its patented sorbitol-based process to produce isosorbide which is marketed under the brand name Poly- sorb. The different purification steps in the production process produce isosorbide grades of very high purity—over 99.5% isosorbide content—which enables Roquette to comply with all quality requirements of its customers, as well as to support the development of innovative high-performance solutions for the global plastics and chemistry industry. Its Polysorb is a REACH compliant, non-toxic, non-endocrine disrupting, valuable feedstock for multiple applications; moreover, it is suitable for food contact, and compatible with cosmetic and pharmaceutical requirements.
Previously, the availability of a consistent supply of high-purity isosorbide tended to be uncertain, but now, having successfully put a reliable, high purity industrial-scale are free to exploit its full potential, said Plancke.

Leading the way – SK chemicals

South Korean green chemistry and life science company SK chemicals is one such polymer manufacturer. Early on, the company saw the potential of isosorbide to enhance the properties of PET and realised how this could even, in some in- stances, open up new areas of use, such as hot fill.

“We’ve found that incorporating isosorbide can help PET to perform perfectly in areas where it currently fails,” said Eung-Soo Kim, head of the Copolyester business at SK chemicals.

“Hot fill for food and beverage is a prime example. Isosorbide is highly effective at increasing Tg and allows us to produce a stretchable, trans- parent, heat-resistant plastic that can be directly substituted into existing blow moulding processes.”

The company launched its first polyethylene-co-isosorbide terephthalate - PEIT - products under the Ecozen brand name in 2012, based on isosorbide from Roquette. SK chemicals has since continued to advance the development of its Ecozen portfolio, in cooperation with Roquette.

According to Jiae Kim, Global Market Manager for Performance Materials division at Roquette in South Korea, this illustrates precisely the kind of long-term, collaborative relationships Roquette maintains with its customers.

“Roquette is not simply a manufacturer of a plant-based monomers; we are experts in the chemistry of these monomers,” she said. “Developing new polymers for specific applications is a demanding task and the insight we bring to the table is valuable to this process.”

Roquette considers itself therefore far more than as just a supplier, said Jiae Kim.

”We are very much an active partner in the development of new PEIT grades,” she stresed.

The development process was one that demanded patience and hard work to achieve the quality and performance attributes required by the market, while at the same time delivering an improved environmental profile.

“Since 2012, the Ecozen range has been progressively segmented, refined and expanded to closely match customer requirements,” Jiae Kim explained. “For example, the Ecozen T polymers all offer chemical resistance, heat resistance and transparency but different grades are tailored to provide different levels of heat resistance, for defined applications. In its entirety this range is now suitable for use in the manufacture of products ranging from food containers and baby care products to construction materials and household appliances.”

She added that PEIT can typically be processed using existing equipment and that many grades can be directly recycled into the PET waste stream. “In other words, switching to PEIT does not incur additional costs in these areas,” she stressed.

Latest developments

SK chemicals has now developed two new PEIT polymers tailored to the requirements of hot-fill applications. Semi-crystalline and offering high thermal stability, these polymers have a glass transition temperature of 90 or 95oC depending on grade, in contrast to that of PET - 70oC - and PETG’s 70 –80oC. PETG, a transparent copolyester, is a glycol-modified class of PET; an example is Eastman’s Tritan material.

Both grades have undergone extensive testing to establish their suitability for hot-fill applications. These tests revealed, for example, that the intrinsic viscosity (IV) of the new polymers compares with a range of 0.6 to 0.85 for PET, depending on the application; PET for bottles typically has an IV of around 0.73 to 0.80. As IV is indicative of the flow properties of the molten polymer, these values suggest that the new polymers will work efficiently in existing manufacturing processes; stretch ratios are also comparable with PET. All the new grades are suitable for use with food in accordance with EU 10/2011.

A bottle shrinkage test, where a bottle filled with hot water (75 – 90oC) is allowed to stand for 2 hours, after which any change in volume is measured as the water is discarded demonstrated substantially enhanced performance relative to PET for both grades. Moreover, the bottles made with the new polymers had a burst pressure of 27 bar – most applications require a maximum of 20 bar – and exhibited good colour and transparency.

The first commercial applications for the new materials have also been launched, said SK chemicals. Morning Recovery, made by More Labs, Los Angeles, USA, a supplement touted as a hangover cure consisting of a blend of electrolytes and other ingredients for rehydration and replenishment, now uses a PEIT-based polymer in place of glass for export applications.

Produced on an existing injection moulding line, the resulting packaging is light- weight and hot filled at 85oC to confer good shelf-life.

Next to hot-fill applications, the company is also exploring the use of PEIT grades containing low levels of isosorbide in cosmetics packaging.

“We also see great potential for incorporating isosorbide to improve the properties of PET for cosmetic packaging ,” said Eung-soo Kim, of SK Chemicals. “We’ve developed several grades for this application that combine excellent optical properties, good chemical resistance and the high flowability required for easy injection moulding.”

The new polymers have semi-crystalline properties but perform well across a wide processing window, exhibiting slow crystallisation rates during processing. This means flexibility with respect to mould temperature, making it possible to produce parts up to 8mm thickness with relative ease and no hazing.

Fit for the future

As the transition towards more circularity for plastics gains momentum, the need for a consistent supply of polymers that fit within this development has grown. The transition is being driven by consumer demand, as well as by new legislation that will come into effect in the EU in a few short years. Under the new laws, among other things, all EU countries must achieve a recycling target of 50% for plastic packaging by 2025 and PET bottles will need to contain at least 25% recycled plastic from that same year, rising to 30% by 2030. Commitments made by the industry to incorporate more recycled content, especially PCR - the declared goal is 10 million tonnes – must also be met by 2025.

The PEIT grades developed by SK chemicals fit well with these goals. Completely com- patible and miscible with PET, they carry resin identification code 1, the code reserved for PET. This means that after use the polymers can be disposed of and reused or recycled via existing PET-handling channels. Furthermore, tests show that the inclusion of PEIT in a blend with PET enhances the properties of the recyclate - an encouraging result for the use of PCR materials.

SK chemicals has capitalised on the potential of isosorbide to commercialise a new polymer family. As a green chemicals company, it has identified four core areas of business - bio- materials, composites, high-performance materials, and energy-saving materials - in which, going forward, it will focus its development work. Collaboration with Roquette on the further optimisation of its Ecozen portfolio is certain to be part of that.

Note: POLYSORB® isosorbide is a registered trademark of Roquette. ECOZEN® is a registered trademark of SK chemicals

A full version of this article appeared in the March/April issue of Sustainable Plastics.