From Science & Research
From Science & Research Stereoblock-PLA: Lab gimmick or competitive addition to the market? Although polylactides (PLA) entered the polymer market some time ago, the application fields are still restricted by a number of drawbacks. Two of the most severe of these are the limited thermal stability (HDT) and the high degree of brittleness of the material. The main type of PLA that is currently found on the market is PLLA, the enantiomer prepared from L-lactide and L-lactic acid, respectively, because of the natural abundance of the latter. By blending PLLA and PDLA – the most common path to increase the HDT – stereocomplex (sc) crystals are formed that have an approximately 50 K higher melting point than the homochiral (hc) crystals of PLLA and PDLA. However, in simple sc blends, both sc and hc crystals are formed, with the latter becoming more dominant as the molecular weight of the PLA chains increases. This leads to the deterioration of the thermal properties. Since high molecular weights are necessary to achieve satisfactory mechanical properties, a conflict of aims is virtually preprogrammed. A more efficient sc crystallisation takes place in the so-called stereoblock (sb) PLA. Here, blocks of PLLA are chemically linked with PDLA blocks [1]. Nowadays, the availability of D-lactic acid is increasing – a prerequisite for diversification of PLA by sb-PLA formation. Nevertheless, the challenge to develop a simple synthesis process for sb-PLA, which can be integrated into the implemented synthesis process of PLA, still remains. Recent studies performed by the authors have focussed on the development of a reactive extrusion process for synthesising sb-PLA with improved heat distortion temperature, good mechanical properties and good processability in injection moulding. Synthesis of sb-PLA Diisocyanates were chosen as suitable coupling agents for the isomeric PLA blocks due to their high reactivity with hydroxyl groups. Consequently, the PLA chains to be coupled had to be terminated with hydroxyl groups on both chain ends (PLA diols). Within the sb-PLA, the isomeric PLA blocks are then linked by urethane bonds (fig. 1). Different ratios of PLLA/PDLA are easy to adjust in this way. PLA-diols with different number average molecular weights were synthesised by ring opening polymerisation (ROP) of L- and D-lactide, respectively, in the mini plant of the Fraunhofer IAP. Molecular weights were adjusted by the quantity of the bifunctional initiator ethylene glycol used. A prepolymer method was applied for the polyaddition reaction. In the 1 st step, an NCO-terminated prepolymer was synthesised by reacting the PLLA isomer with a twofold excess of diisocyanate. Afterwards, in the 2 nd step, sb-PLA was produced by reaction with the isomeric PDLA-diol. Both reaction steps were performed by reactive extrusion in a Brabender DSE 12/36 twin-screw extruder. All heating zones were set to 170 °C during preparation of the NCOprepolymer; in the 2 nd step, a temperature gradient from 190 °C (feed) to 230 °C (nozzle) was applied. The feeding rate was set to 200 g/h. The extrusion technique is easily implementable into existing production units and is even already being used for finishing steps. Four different types of sb-PLA with varying molecular weights and compositions were synthesised, as is shown in the table (fig. 2). Fig. 1: Principle of sb-PLA synthesis Fig. 2: Synthesised sb-PLAs and their hydroxyl-telechelic prepolymers HO HO OCN OH OH NCO sb-PLA Mn L-PLA* Mn D-PLA* ratio L/D Mn** Mw** g/mol g/mol g/mol g/mol 1 17 16.8 50/50 45.7 126 2 15.5 13.5 50/50 25.5 56.5 3 17.9 16.8 80/20 44.8 106 4 17.9 9 80/20 50.2 132 * GPC: methylene chloride; universal calibration (viscosity); IAP ** GPC: hexafluoroisopropanol; PMMA standard; PSS Mainz 42 bioplastics MAGAZINE [06/15] Vol. 10
From Science & Research By Elke Mitzner 1 , André Gomoll 1 , Felix Reiche 2 , Antje Lieske 1 1 Fraunhofer Institute for Applied Polymer Research IAP Potsdam-Golm, Germany 2 CEO, hesco Kunststoffverarbeitung Luckenwalde, Germany Properties of sb-PLA with equivalent D/L ratio The thermophysical behaviour of the materials was determined by dynamic scanning calorimetry (DSC; heating and cooling with 10 K/min). A comparison of two different sb-PLAs (sb-PLA 1 and 2 in the table), both with an L/D-ratio of 50/50 but different molecular weights, with Ingeo ® 3251D (standard injection moulding grade) is given in figure 3. What is immediately noticeable is that the melting point of the sb-PLAs is about 40 K higher than that of the commercial PLLA. Furthermore, only sc crystals and no lower melting hc crystals are detected. It is worth mentioning that – in contrast to the commercial PLA – crystallisation of the sb-PLAs took place during cooling, indicating a clear acceleration of the crystallisation rate for sb-PLA. For injection moulding processes, for example, this represents a major benefit, as shorter cycle times become feasible. It can also be seen that the crystallisation behaviour is influenced by the molecular weight. sb-PLA 1, with the same composition but a higher molecular weight than sb-PLA 2, crystallised in a wider temperature range. The two peak maxima are probably an indication of the formation of two different crystal forms. On the other hand, the degree of crystallisation given by the melting enthalpies was not influenced and was calculated to be 58 %, which is a fairly high value compared to 27 % in Ingeo 3251D and the 35 % given as average value in literature [2]. The fast sc crystallisation of sb-PLA leads to higher heat distortion temperatures (HDT). Within our investigations, the lower molecular weight sb-PLA 50/50 reached the highest HDT (B) of 92 °C right after injection moulding of test specimens. This is approximately 30 K higher than that of commercial PLLA without additives. Heat flow [mW] 8 6 4 2 0 -2 -4 -6 -8 sb - PLA 1 sb - PLA 2 Ingeo 3 251D -10 20 40 60 80 100 120 140 160 180 200 220 Temperature [°C] Fig. 3: DSC traces of sb-PLA (50/50) in comparison with Ingeo 3251D (10 K/min, 2nd run) Fig. 4: Notched Charpy impact strength of sb-PLA (50/50) in comparison with Ingeo 3251D Charpy, notched [kJ/m²] 7 6 5 4 3 2 1 0 3251D 3251D + PBS sb-PLA 2 1 2 sb-PLA 2 + PBS bioplastics MAGAZINE [06/15] Vol. 10 43
