Additives/Masterbatches
Additives/Masterbatches Natural resin meets bio-polyester Impact resistant biobased polyesters modified with native resin systems Chemically novel biobased polyesters, which include polylactic acids and polyhydroxyalkanoates, account for a not inconsiderable 15 % or so of the total quantity of bioplastics produced annually [1]. Of this group of biopolyesters, the most prominent is PLA, with a share of some 14 %. PLA is primarily used as a packaging material, for disposable solutions or mechanical components that are not exposed to much mechanical stress. Due to its brittleness, unmodified PLA is unsuitable for use in numerous applications. Often petrochemical additives can help here. However, while these are biodegradable, they must be added to the polymer at high loadings. As a result, the compound or blend has only a small proportion of renewable raw materials. Special resin as special solution In the context of several research projects supported by the German Federal Ministry for Economic Affairs and Energy (BMWI, AiF) and the German Federal Ministry of Food and Agriculture (BMEL, FNR), Robert Kraemer GmbH & Co. KG, together with the Chemnitz University of Technology, has developed innovative additives based on native resins that enable a significant improvement in the impact strength of biobased polyesters, in particular PLA. The resin systems consist mainly of renewable raw materials. The most important component is a rosin (Fig. 1). These raw materials, originally used in the paint, adhesive and printing ink industries, are boiled together with other biobased basic chemicals such as fatty acids, oils, or other resin acids. The result is a completely new special resin system for the modification of biobased polyesters. Figure 1: Rosin as the basic raw material: From tree to additive As simple as the idea of modifying biobased polyesters using the resin systems may seem, the practical realisation is quite challenging. In addition to the large number of possible raw materials, the reaction control during synthesis is decisive for the macroscopic properties (e.g. rheology or polarity) and the chemical properties (e.g. molecular weight or functionality). In addition, the esterification of the resin acid carboxyl groups with suitable alcohols to resin esters determines the viscosity of the resin systems. The range extends from solid to low-viscosity additives, which plays a key role in process engineering for the subsequent combination with the thermoplastic bio-polyesters. Rosin meets biobased thermoplastic The biobased polyesters and resin systems were combined in a compounding process on a Noris Plastics ZSC 25/40D twin-screw extruder in the Technology Centre of the Chair of Lightweight Structures and Polymer Technology at the Chemnitz University of Technology. Solid resin systems were added to the biobased polyester by gravimetric dosing using a side-feeder. A heatable gravimetric metering pump was used for the addition of liquid resin systems (Fig. 2). The dosing unit, which works on the principle of a gear pump, was connected to the first atmospheric degassing of the compounder. Not only could the feed and dosing rate be precisely adjusted, the heated tank and hose of the pump allowed the viscosity of the resin system to be varied during metering. In this case, improving the flowability was of secondary importance to an effective delivery. The temperature, and hence the viscosity of the liquid resin system in contact with the polymer melt mainly determines the homogeneity of the compound. Together with the adjustable process parameters during compounding (speed, temperature, etc.) and the screw geometry, this affects the resulting mechanical properties of the compound. The abundance of influencing factors in compounding, the broad selection of raw materials in the production of the resin systems as well as the choice of biobased polyesters and the ratio of quantities led to an enormous variety of compounds. liquid resin solid resin biobased polyester Figure 2: Addition points during compounding A4 V A3 A2 A1 M SF4 SF3 SF2 SF1 M-Main feeder A -Atmospheric degassing SF -Side feeder V -Vacuum degassing 28 bioplastics MAGAZINE [03/20] Vol. 15
Additives/Masterbatches By: Sebastian Buschbeck, Claudia Reichelt, Roman Rinberg, and Lothar Kroll Chemnitz University of Technology Chemnitz, Germany Torsten Germer Robert Kraemer GmbH & Co. KG Rastede, Germany Next to improving the properties of the various commercially available biobased polyesters, another important goal was to ensure that the resin systems developed were compatible with these polyesters. Finally, a marketable solution was to be developed that would enable product realisation. To achieve this goal, different PLA grades (Ingeo 3251D for injection moulding and Ingeo 2003D for extrusion) from NatureWorks are used. The polyhydroxyalkanoates tests were conducted using a PHA type from Metabolix (Mirel F1006) – unfortunately, no longer commercially available. For each resin system, there were a number of settings regarding the process parameters. Here, the specific properties of the resin must be taken into account. With regard to the dosing, for example, this meant that resin systems that were viscoplastic at room temperature were added at a higher temperature than those with a low viscosity at RT. In order to find the optimum dosing temperature, the viscosity curves of the resin systems were determined. Much the same holds for other relevant process parameters. The screw rotation speed, which influences the shearing in the compounding process and consequently the viscosity of the melt, is an example of this. This is also directly related to the residence time of the material in the extruder, which has an effect on the homogeneity of the compounds. Natural resin systems and compound properties The research resulted in the development of new biobased resin systems, such as Bremar RK 7103, which, with a low loading in the compound, leads to a specific enhancement of the impact strength. In the PLAs studied, the Charpy impact strength (unnotched) was increased by up to 150 % to 40 kJ/m² compared to the original material. Other mechanical properties under consideration, such as tensile modulus or tensile strength, were only marginally reduced, by 4 % and 16 % respectively. These values were achieved by adding 5 wt. % of the biobased additive, at a dosing temperature of 80 °C and a screw speed of 400 rpm. At higher loadings, migration effects occurred during injection moulding, meaning that the additive was no longer completely bound in the polymer. No further reduction in brittleness was seen. Lower proportions led to correspondingly lower improvements in Charpy impact strength. At less than 2 wt. %, no changes in the mechanical properties were observed. Varying the metering temperature and/or the screw speed during compounding, also reduced the effect on the characteristic values investigated (Fig. 3). To gain a better understanding of the mixing and hence of the obtained results, the compounds were examined under a scanning electron microscope. The resin systems were clearly visible as finely distributed droplets in the solidified PLA (Fig. 4). As expected, there was no chemical bond between the polymer and the additive. Change in mechanical properties (%) 160 — 140 — 120 — 100 — 80 — 60 — 40 — 20 — 0 — -20 — Tensile strength Bending modulus Flexural Strength Charpy impact strength (unnotched) 153 Figure 3: Change in mechanical properties by varying the dosing temperature 112 112 -4 -3 -5 -2 -0 -3 -4 -2 -4 -10 -14 -14 Dosage at 400 rpm and 80°C Dosage at 400 rpm and 100°C Dosage at 400 rpm and 120°C Figure 4: SEM image of the compounds (left - 2 wt. % resin system; right - 5 wt. % resin system) In the PHA type studied, a 5 wt. % resin system produced a lower percentage improvement in Charpy impact strength of up to 75 % compared to the starting material. In absolute terms, the value was 45 kJ/m². The reduction in other mechanical properties was more significant than for PLA, with -29 % in the tensile modulus and -19 % in the tensile strength. The PHA series showed minor changes in mechanical properties compared to PLA. From research to the product: Resin systems as additives The theoretical effectiveness of the biobased resin systems has been proven on the basis of the studies conducted. To convert the developed resin systems into a marketable product, further practical questions needed to be solved. Only a few plastics manufacturers are able to produce their own compounds, especially with the aid of gravimetric dosing pumps. The use of liquid additives, with bioplastics MAGAZINE [03/20] Vol. 15 29
