04 | 2010

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