Foam Transient
Foam Transient elongational viscosity η E + (t) in Pas 10 6 Temperature = 160 °C Strain rate = 0.5 s -1 Strain hardening 10 5 10 4 PLA ex 10 3 PLA1 PLA2 PLA3 PLA5 10 2 10 -1 10 0 10 1 10 -2 Time in s Figure 5: Transient elongational viscosities Table 3: Characteristic foam properties of the PLA foams PLA PLA1 PLA2 PLA3 PLA4 PLA5 ρ in g/cm 3 0.39 0.11 0,21 0.43 0.17 0.11 d in µm 19 ± 6 99 ± 28 8 ± 3 38 ± 12 35 ± 11 37 ± 17 Figure 6: SEM images of modified PLA samples prepared in a batch foam process at 157 °C, 180 bar and a saturation time of 0.5 h The foam density and the cell size were also determined (tab. 3). The right choice of modification foam properties can be improved. In general foams with fine, uniform and closed cell structure in a density range of 0.11 to 0.43 g/ cm 3 were achieved. A density reduction up to ~ 25 % was possible. The experiments further showed, that the foamed materials have haptic properties that are similar to the well-known polystyrene. Besides PLA2, all other modifications lead to an increased expansion ratio and larger cell sizes when foamed. Compared with the neat PLA foam a further reduction of the density was possible except for PLA3. As a result of the high molecular weight and weight distribution, good transient elongational flow properties such as high extensional viscosity and a well-pronounced strain hardening PLA1 shows the largest cell growth. PLA1 and PLA5 have the lowest density. This expanded PLA could be appropriate as insulation or packaging material. Contrary to expectations, PLA2 does not show much better properties than the other modifications although it shows good shear and elongational flow properties. Furthermore, PLA2 showed smaller cells as well as bigger voids and a degree cell rupture. Conclusion and future works The aim of generating bio-foams with standard and cost effective PLA was achieved by using appropriate modifiers. It was shown that the molecular weight could be increased by the use of the modifiers. Furthermore, some of the investigated modifications (PLA1 and PLA2) showed an increased elongational viscosity. Strain hardening was noticed for all materials. It was noticed that most of the modified PLAs possess a reduced foam density but larger cells. With the previously examined modifiers a first start was made but further studies will be conducted. The future work will focus on the foam extrusion process, on the addition of various nucleating agents to improve the cell morphology, and on the use of different standard PLA types with lower molecular weight, lower zero shear viscosity and more reactive end groups, probably leading to a higher reaction rate. www.ikt.uni-stuttgart.de www.polymer-engineering.de 40 bioplastics MAGAZINE [04/15] Vol. 10
Basics Plastics foaming By Michael Thielen Lightweight materials with improved cushioning, insulating, structural performances, and other characteristics are the reasons for an increasing demand for plastic foams [1]. Applications for foamed plastics include cushioning materials (e. g. as protection of electronic devices during transport), air filters, furniture, toys, thermal insulation, sponges, plastics boats, panels for buildings, even lightweight beams and much more [2]. Plastics foaming is a plastics processing technology that involves the uses of blowing agents, and sometimes other additives such as nucleating agents, to generate cellular structures in a polymer matrix [1]. Plastic foams and their processing Plastic foams possess cellular structures within the solid plastics matrices. The properties of the final foams are derived from the properties of the polymer matrix and the retained gas, as well as the foam morphology. Therefore, the choices of the base polymers, the blowing agents, and the controls of the cell structures will influence the applications of the foamed plastics. In general, foamed plastics can be classified in different ways: by stiffness as flexible, semi-flexible, and rigid foams, by density as low- and high-density foams and by structure as open- or closed-cell foams (fig. 1 and 2) [1]. Plastic foams can be produced by processes such as batch foaming, foam extrusion, and injection foam moulding. The cellular structure in plastics may be produced mechanically, chemically, or physically [3]. Regardless of the methods, the material to be foamed is in viscous state during the process. Mechanical foaming produces a cellular structure by mechanically whipping or frothing of gases into a polymeric melt, suspension, or solution. As the material solidifies, it encapsules gas bubbles in the polymer matrix, and thereby yields the cellular structure. In chemical foaming processes, the decomposition of a chemical blowing agent is used to produce gas within the viscous plastics and generate the cellular structure. For example, an organic nitrogen compound decomposes and liberates nitrogen gas to foam some types of PVC. The physical foaming process is another popular method to produce plastic foams. Here, the necessary gas is dissolved in the viscous melt and expanded by physical changes, i. e. by pressure reduction or expansion of a low-boiling liquid by further heat. A special form of chemical foaming is the so called particle foam, best known from EPS (expanded Polystyrene; one of the brand names is Styropor ® ; fig. 3) In a first step, small Polystyrene beads take up a blowing agent by means of diffusion processes and get filled into a mould. By means of hot steam, the beads are getting softer and the reacting blowing agent expands them to bigger spheres. Due to the limited space in the mould, the expanding beads squeeze and weld together and form a rigid foam structure. For processes like film blowing, thermoforming and foaming, the used polymers need a certain molecular structure, which allows an entanglement of the side chains for a so-called strain hardening. For this reason, foaming does not work with any plastics type on the market. Bioplastic foams Just like conventional thermoplastics, some types of biopolymers can be foamed as well. For example there are blends on the market, based on thermoplastic starch (TPS), cellulose derivatives (cellulose acetate CA, cellulose acetobutyrate CAB, cellulose propionate CP) or Polylactidacid (PLA) with Polybutyleneadipateterephtalate (PBAT) which can be used for chemical or physical foaming. Particle foams are commercially available made from PLA (fig. 3), but also PHA and cellulose acetate can be converted into particle foams. And last but not least, (partly) biobased polyurethanes have been commercially applied for example for car seats and cushions. For more details on foamed bioplastics, please refer to the individual issues of bioplastics MAGAZINE (usually issue 01 of each year) [1] Leung, S.: Mechanisms of cell nucleation, growth, and coarsening in plastic foaming: theory, simulation, and experiment, PhD Thesis, Univ.Toronto, 2009 [2] http://www.britannica.com/technology/foamed-plastic [3] Nawaby, A. V. and Zhang, E., “Thermoplastic Foam Processing: Principles and Development,” Gendron, R. (Eds), CRC Press, Boca Raton, FL, pp. 1 – 42, 2004 (cited in [1]) [4] Bonten C.: Kunststofftechnik, Einführung und Grundlagen, Hanser, 2014 Fig. 3: PLA particle foam (Synbra) Fig. 1: Open cell foam: Good acoustic insulation [4] Fig. 2: Closed cell foam: Good thermal insulation [4] bioplastics MAGAZINE [04/15] Vol. 10 41
