Basics Basics of
Basics Basics of Compounding by Michael Thielen Fig. 1: Co-Rotating Twin screw extrusion [2] What is compounding? Wikipedia says, “Compounding consists of preparing plastic formulations by mixing or/and blending polymers and additives in a molten state. There are different critical criteria to achieve a homogenous blend of the different raw materials“ [1]. And why do we need compounding? Like most thermoplastics, pure bioplastics (starch, PLA, PHA, PBS and others), when they come out of the (bio) reactor or straight from nature resp., are not ready to use, i.e. they cannot be directly processed with standard converting equipment such as extrusion or injection moulding machines. In most cases the properties and characteristics of ‘pure‘ bioplastics are not directly and completely suitable for certain processing techniques and/or applications and thus need to be tailored according to the specific needs [2, 3]. Therefore plastics, and especially bioplastics, are blended or mixed with fillers and additives. Sometimes even different types of bioplastic (and sometimes conventional plastics) are blended with each other. They are ‘compounded‘ into more functional, and in many cases applicationtailored and process-tailored materials. The aim especially is to adapt the mechanical property profile of the polymer to the required properties in the final product, such as flexibility, UV stability, impact resistance, etc. Native starch, for instant is not even thermoplastic. It has to be ‘cooked‘ or plasticized using liquid plasticizers [2]. A further very important aspect of the compounding step is getting rid of excess water that is either a natural component of the biopolymer or moisture that is absorbed from the atmosphere but causes heavy hydrolysis/depolymerisation. Water in the final product will cause, for example, major problems in blow film processes [2]. Furthermore, it is possible to adjust lifetime and degradation characteristics via a compounding step [2] and in some cases cheaper ingredients such as certain natural fibers or wood flour is used simply to reduce the cost of a compound [5]. The compounding of raw bioplastics requires special knowledge in the field of additives and a smooth compounding process. The Ingredients So what is being mixed or compounded? On the one hand, different raw bioplastics are mixing partners (phases) themselves, for example PLA and PBAT (polybutylene adipate-co-terephthalate). On the other hand coupling agents or compatibilitizers between these phases are needed. Sometimes, coupling is done by cross linking by means of intelligent reactive extrusion (e.g. for transesterfications). Furthermore there are some intelligent fillers [3]. 34 bioplastics MAGAZINE [04/10] Vol. 5
Basics Bioplastics Fillers can be for instance chalk, talc or wood flour. Another compounding ingredient can be fibres. Even if traditional reinforcing fibres, such as glass fibres could be used without compromising the ‘biobased carbon‘ and the biodegradability, natural fibres like hemp, kenaf, abaca, bamboo, cellulose or wood are preferable for use with bioplastics (see bM 03/2010). If flexibility is needed, fibres can be counterproductive. If however a higher strength and stiffness is required, fibres could be helpful [3]. And then there are the mystical additives. It would be too much to go into detail here (please see separate articles in this issue). Such additives are, for example, compatibalizers to improve the blendability or mixability of different plastics, i.e. to handle the liquid dispersion of different immiscible phases (an alloy between hydrophobic and hydrophilic phases, like an emulsion) [2]. Coupling agents can be applied to enhance the adhesion between fibres and the plastic itself. Plasticizers help make plastics softer, more elastic or flexible. And then there are UV stabilizers, impact modifiers, chain extenders (to improve processability in film blowing), crystallisation aids and - of course - colours! [2, 3, 4, 5] Fig. 2: Screw elements on multi-profile screw-shaft [7] Process and equipment Again, we first ask Wikipedia: “Dispersive and distributive mixing as well as heat are important factors. Co-kneaders and twin screws (co-rotating and counter-rotating) as well internal mixers are the most commonly used compounders in the plastics industry“ [1]. Bioplastics tend to be heat and shear sensitive, and often they need high torque. Good mixing is needed, so co-kneaders, internal mixers, single screw and counter-rotating extruders are almost never used for compounding of bioplastics. The machine type of choice is a co-rotating twin screw extruder [2, 5]. These screws are in most cases individually ‘built‘ to purpose from different kinds of screw elements assembled on the two screw-shafts (Fig. 1 and 2) Co-rotating twin-screw extruders offer the following characteristics [4]: • Intensive material and heat exchange thanks to the permanent melt transfer from one screw to the other • Almost perfect self-cleaning effect and narrow residence time profile produced by the closely intermeshing screws • Efficient melt degassing by axially open screw channels • Output rate independent of speed • Optimum adaptation to the given process task by the modular barrel and screw elements • Highly flexible adaptation to different process tasks thanks to the modular design of the processing section. • Long service life of the processing section bioplastics MAGAZINE [04/10] Vol. 5 35
