Injection Moulding The
Injection Moulding The blend makes the difference Selectively optimizing the material properties of bioplastics PLA used: Ingeo 4043 D by Nature Works LLC; PBS used: GS Pla FZ 91 PD by Mitsubishi Chemical; binder used: Vinnex 2504 and Vinnex 2510 by Wacker Chemie AG With the exception of niche applications, bioplastics have so far failed to make a breakthrough on mass markets – often due to their unsatisfactory material properties or the lack of cost-effective production processes. Using sophisticated chemical techniques, the Munich (Germany) based chemical company WACKER has developed a solution for eliminating the inherent weaknesses of bioplastics. The improved physical properties of these materials mean they can now be processed like standard thermoplastics, using methods such as injection molding, extrusion or thermoforming. New Polymers Must Be Compatible with Polymer Industry Processes In order to expand their potential, bioplastics must possess properties that justify their use over traditional plastics. In addition to that requirement, however, bioplastics also have to be compatible with processes commonly used in the polymer industry, such as injection molding. A material that meets some of these requirements is polylactic acid (PLA), which is similar to traditional thermoplastics, and can easily be processed in existing plants. An inherent disadvantage of pure PLA, however, is that it is very rigid and its impact strength is low. Attempts have already been made to compensate this drawback through the use of suitable blends. One US patent, for instance, identifies a variety of aliphatic polyesters that can be blended with PLA to increase the impact strength of the material or make it more flexible [1]. Vicat A [°C] 0 10 20 30 40 50 60 70 80 90 PLA PBS PBS / PLA / VINNEX PBS / PLA / VINNEX / Talc Fig. 1: Thermostability of PBS/PLA/Vinnex blends compared to PLA and PBS Tensile Strength [MPa] Elongation [%] Vicat A [°C] 15,84 Fig. 2: Long-term stability of the thermal and mechanical properties of a PBS/PLA/Vinnex blend 3,78 99,6 15,63 3,55 99,3 16,78 3,75 98,3 0 10 20 30 40 50 60 70 80 90 100 0 4 weeks 8 weeks 14 bioplastics MAGAZINE [02/14] Vol. 9
Injection Moulding Another disadvantage of PLA is its poor resistance to heat, as amorphous PLA begins to soften at temperatures of approximately +60°C, making the material unsuitable for wide range of applications Crystallization generally improves PLA‘s thermostability. However, crystallization results in long processing times, which reduce the cost-effectiveness of the process. Therefore, the goal of development was to avoid costly thermal posttreatment. Eliminating Poor Heat Resistance and the Miscibility Gap Wacker developers discovered that the heat resistance of PLA increases from +58°C to +65°C when blended with polybutylene succinate (PBS). Further research at Mitsubishi Chemicals (an important PBS manufacturer) demonstrated that this effect can be magnified – increasing heat resistance to +100°C – through the use of a different grade of PBS (see Fig. 1). Making use of this effect, however, meant overcoming yet another hurdle. One study showed that PLA/PBS miscibility is limited and that a miscibility gap arises when PBS is blended with PLA at a concentration of 20% [2]. Researchers also found that the amount of PBS required to produce the desired properties falls within that miscibility gap. A solution to this problem is provided by VINNEX ® , a Wacker binder system based on polyvinyl acetate. Studies have demonstrated that Vinnex is compatible with both PLA and PBS, and that the addition of 15 to 20% Vinnex eliminates the miscibility gap. This results in visibly homogeneous polymer blends in which both polymers can be combined in any mixing ratio and essentially adjusted to the application at hand. The resulting blend combines the advantages of both components. Also, a partially crystalline PBS grade was used with a largely amorphous grade of PLA, which meant that another issue had to be resolved: long-term stability. Studies showed that the properties of PLA/PBS blends containing Vinnex had not changed within eight weeks, and that Vinnex apparently suppresses effectively post-crystallization of the PBS portion of the blend (see Fig. 2). Unlike PLA, PBS is not yet available on the market in large quantities, consequently making it expensive. That situation is set to change in the near future, however. The PTT Public Company Limited of Thailand and Mitsubishi Chemicals are already planning a joint venture (PTT MCC) involving the construction of a production facility in southeast Asia for manufacturing PBS from renewable raw materials. Improving Cost-Effectiveness with the Right Fillers In order to optimize the cost-effectiveness of the process, studies were performed with the aim of maximizing the PLA content of blends without affecting the thermostability achieved with PBS. One other project involved diluting costs by adding fillers such as calcium carbonate (chalk) or talc [N/mm 2 ] 0 500 1000 1500 2000 2500 3000 3500 4000 4500 0% Filler 10% CaCO3 20% CaCO3 30% CaCO3 10% Talc 20% Talc 30% Talc Fig. 3a: Effects of chalk and talc on the elastic modulus of PBS / PLA / Vinnex blends Fig. 3b: Effects of chalk and talc on the impact strength of PBS / PLA / Vinnex blends 0% Filler 10% CaCO3 20% CaCO3 30% CaCO3 10% Talc 20% Talc 30% Talc 0 10 20 30 40 50 60 70 80 90 [kJ/m²] Tensile E-Modulus Charpy impact strength bioplastics MAGAZINE [04/14] Vol. 9 15
