An impact-modified polylactic acid (PLA) supplied by Nature Works is "very close", said Rasmussen, but fails Lego's demanding internal test requirements for clutching power. The PLA bricks press together OK, but fail to stay locked after a couple weeks. The company markets elaborate models that need to stay intact for a significant period of time. Rasmussen said that neither the company nor its customers, mostly young children, will tolerate any compromises.
The problem, however, seems to be addressable.
"We are aware of the issue of creep behavior with PLA, and we know the factors that cause that behavior," said a technical spokesman for Nature Works. The company is investigating solutions.
Rasmussen said that Lego is testing other bioplastics, but he did not identify them.
Lego first molded plastic bricks in 1949 using, ironically, a renewably sourced polymer, cellulose acetate. In the 1960s, the company shifted to ABS, which offered more rigidity and durability. Now the company uses several types of plastic, including polycarbonate, nylon, polyacetal, polyethylene and thermoplastic polyurethane.