Green Carbon for Polymer Composites

INTRODUCTION Char originating from biomass can be used as a sustainable carbon additive in the production of polymer compounds with enhanced characteristics.1,2 The advantages are manifold: firstly, char addition might improve the mechanical, thermal and dielectric properties of the composites and secondly, the substitution of fossil carbon sources reduces the CO2-footprint of the composite production. Furthermore, utilizing organic residues, e.g. from biomass gasification-based CHP-plants, can improve the overall plant economy by generating a valuable side-product. Chars differ considerably depending on their origin. Indeed, initial feedstock, conversion technology, reactive agents and operating conditions strongly affect the final characteristics of the char. In order to assess the suitability of char in polymer composite applications, the investigation of the char, the composite production processes itself and the detailed characterization of the final products are fundamental. One objective of this work is to identify correlations between char characteristics and properties of the final composite to determine requirements on char as an BPC additive to achieve enhanced product qualities.

METHODOLOGY Within the project, chars of different origin, i.e., 1) the solid residue from biomass gasification processes, 2) pyrolytic biochar, are sampled and analysed. Threshold values such as moisture content, volatile content, maximum particle size, enabling subsequent polymer compounding processes are identified. Char parameters like ash content, fixed carbon, elemental composition, particle density, particle size distribution, specific surface area, pore size distributions, structural constitution via Raman spectroscopy are determined to investigate possible correlations to polymer composite properties. Gasifier process conditions are varied in order to assess possibilities to improve the properties of the char residues considering their application as carbon additive in polymers. During the subsequent compounding process char is blended with various thermoplastic polymers resulting in different char-polymer composites. These polymer composite samples are characterized regarding their applicability for specific assignments. Two fields of application are targeted: 1) production of char-polymer composites with adequate mechanical and thermal properties for 3D-printing applications and 2) functionalized utilisation in high voltage (HV) technology as a semiconducting dielectric or a dielectric with high permittivity for targeted field grading in HV-cables and cast-resin isolators.

RESULTS First screening tests showed that the general processability of char as polymer additive producing filaments for 3D-printing can be achieved, adhering boundary conditions, e.g., regarding the char properties, such as content of volatiles, particle size, and limits of char to polymer ratios. Manufactured polymer composite samples have been analysed on their dielectric properties and showed comparable behaviour to fossil-based carbon sources, such as carbon black, employed as reference additive. Systematic investigations on the influences of operating conditions during biomass conversion and subsequent char treatment steps on the material properties of char itself and the final polymer composite-product are currently in progress. Results will be shown in this contribution.