So far, PLA has been available from one major supplier, viz, NatureWorks. However, there appear to be few new sources who are preparing new PLA copolymers. A large European molder of EPS beads has opted to polymerize its own PLA and make the world's first expanded PLA (EPLA) bead foams, making an improved form of PLA with superior strength and heat resistance.
Synbra Group in the Netherlands already polymerizes its own EPS beads in a batch process, but wanted to develop an expandable biopolymer packaging material suited to its requirements. Hence it developed the world's first EPLA Synbra Technology, acquiring the first license for a new PLA reactor technology developed by Purac in the Netherlands and Sulzer Chemtech AG in Switzerland. Synbra's EPLA, trademarked BioFoam, will be colored light green to distinguish it from conventional white EPS. BioFoam is aimed at specialty packaging for consumer goods and at bio-derived cushion filling. EPLA beads contain no VOCs and have no shelf-life limitation. Purac, which produces lactide monomers, the precursors of PLA, as well as medical grades of PLA, is providing Synbra with polymerization know-how, catalysts and monomer. Sulzer has long had a static-mixer/reactor for polymerizing PS, and recently adapted it to production of PLA. Sulzer has made PLA in a lab-scale version of the new process. Synbra has also modified its existing EPS shape molding equipment to handle EPLA. Synbra has started molding the first foam samples after procuring PLA from Sulzer, and plans to have an 11 mln-lb/yr semi-works PLA reactor running in the Netherlands by late 2009. Once that reactor is on line, Synbra expects to build a full scale 100 mln lb/yr PLA plant.
Sulzer's PLA process is a continuous, two-reactor series occupying a space about 40 sq. ft. The first reactor is an SMR (Sulzer Mixer Reactor) in a loop configuration with static mixers inside. The second reactor is a straight-through SMR reactor. Both reactors run at relatively high temperatures, heated by the catalyst reaction. Polymerization occurs along the inside shell of the SMR cylinder. The static mixers inside the reactors are a series of bent tubes through which a me­dium travels to remove excess heat and control the reaction rate of polymerization. After the two reactors comes a devolatilization unit, which strips off any unpo­lymerized monomer. Polymer then goes directly into a pelletizer�there is no extruder. Sulzer's PLA process is only available for use with Purac's lactides. Lactide is shipped as an inert flake in dry bulk containers. The only difficulty transporting and storing lactide is that it must be protected from moisture by special packaging, or it reverts to lactic acid again.
So far, PLA could not be used for expanded bead foam because it was too brittle and too heat-sensitive for the reheat. Sulzer Chemtech's static-mixer reactor for PS has been adapted to make PLA with lactide monomers from Purac-the first process capable of making high-temperature stereocomplex PLA. Synbra's solution is to use a stereo-complex PLA made with a new D-lactide monomer that can be polymerized into a PDLA homopolymer. This can then be alloyed with standard commercial PLA (which is mostly PLLA with small amounts of PDLA impurities) to make new stereo-complex PLA. Self-nucleating stereocomplex PLA has the highest strength, crystallinity, and heat resistance in the PLA spectrum (see figure). Its melt-temperature range is 428 to 446 F, vs. 300 to 320 F for standard PLA. HDT is 320 to 330 F for 50/50 PDLA/PLLA vs. 212 to 300 F for highly crystalline PLLA and 130 to 140 F for standard PLA.