Purac teams with Sulzer to grow European PLA

Netherlands-based lactic acid and lactides specialist Purac and Swiss company Sulzer Chemtech are together developing a PLA polymer production technology tailored to Purac's specific lactide raw materials that could lower the cost of manufacturing performance-optimised PLA resins.

PLA (polylactic acid or polylactide) is a biodegradeable polyester manufactured from renewable resources. The process involves fermenting sugar or starch to produce lactic acid, which is subsequently converted to lactide. This lactide is then polymerised into polylactic acid.

Current commercial PLA production technologies produce a high clarity and compostable resin, but thermal resistance is limited to around 130°C. Purac, which has been producing lactic acid and derivatives for more than 70 years for food and medical use and claims to have a 65% share of the global market, believes its technology could overcome this restriction, lifting melting points to 180°C and beyond.

The company is using Sulzer's static mixer technology in its new manufacturing route to PLA, which it claims is both cost-effective and faster than other production methods.

Purac marketing manager Hans van der Pol explained that the new process consists of two steps - polymerisation and devolatilisation, where residual monomers are removed from the polymer.

During polymerisation, there is a steep increase in the viscosity of the PLA.

The static mixer provides the ability to cool and mix at the same time, generating a homogeneous product after a comparably short residence time, according to Purac.

"Sulzer Chemtech's system offers a very mild process with good temperature control and a very efficient high vacuum devolatilisation process," said van der Pol.

The end product has a high molecular weight, a controlled polydispersity and low colour, says Purac. The process also enables it to keep racemisation (the chemical transformation of lactides between L and D isomer forms) at low levels, so relatively pure PLLA (L+) and PDLA (D+) variants can be obtained, the company claims.

This means the Purac process is able to make PLAs that display a higher melting point than current commercial grades. The company claims a melting point of around 180°C can be achieved with almost 100% pure L(+) lactic acid.

"By combining the lactides in new and creative ways, the improvement of the PLA heat-stability - one of its key issues - through stereocomplexation concepts can become a reality," said Hans van der Pol.

Purac mixes its lactide with additives from AkzoNobel, which allows its materials to be handled in commercial polymer processing equipment.

"Purac's Innovation Center has recently demonstrated the ability to produce cups with a heat-stability of over 100°C by injection moulding using less than 10% of PDLA," he said.

Looking forward, Purac anticipates melting points of 200°C if it can produce pure PLLA chains and pure PDLA chains and then convert them into stereo-block-copolymers by transesterification.

The first PLA producer to sign a contract with Purac to produce their own PLA is the Dutch company Synbra, which produces EPS (expanded polystyrene) foam. Synbra's goal is to find a biodegradable alternative based on renewable resources. It says its E-PLA foam will offer comparable or better properties than EPS.

Purac operates lactic acid plants in Brazil and in the US as well as in the European countries of the Netherlands and Spain. At the end of 2007, it opened a new plant for L(+) lactic acid in Thailand, enabling the firm to convert its plant in Spain from L(+) to a dedicated factory for the fermentation of D(-) lactic acid and lactides.

"This is now the first step, but we expect that by 2015 our partner model will have resulted in plants where PLA production from sugar is integrated with lactic acid fermentation and lactide production on a 100kt scale," said Hans van der Pol.

The firm sees a potential capacity for PLA of 500,000tpa by 2015, not all of which will be used for packaging.

"New value-added applications for PLA are, for example, phones or flat screens, heat-resistant fibres, special packaging, foamed products, construction, hot fill applications and even automotive applications, an area where we are seeing sustainability becoming an increasingly important trend," said van der Pol.