Issue 04/2016

Basics
Do biopolymers need additives?
„Plastics without additives are not viable“. This pithy phrase
opens the “Plastics Additives Handbook” which is a reference
book in this field. Therefore, there should be reasons for this
valuation.
When talking about plastics people normally think about
a well-designed serviceable material, e. g. a plastic bag, a
detergent container or – from an industrial point of view – a
sealable food container, a heat resistant engine block cover
or an impact resistant smart phone housing. What we don´t
keep in mind is that the raw material, based on polymerised
monomers, as obtained from (bio)chemical reactors is not a
plastic. These synthetic materials have inherent properties,
based on their atomic linkages, molecular structures and
the associated interactions between the chains. This applies
to fossil based polymers as well as to biobased polymers.
Therefore, as an example, polyethylene will always be softer
than a polyester like PET due to fewer interactions between
the chains. However, the inherent mechanical properties of
materials are only one side of the medal. Synthetic polymers
alone are, in general, not a processable plastic. Processable
means that the materials must not stick at the surfaces of the
plastic processing machinery, must have the right viscosity
and melt strength for molding and should not be changed in
their molecular structure as a result of thermomechanical
treatment.
Therefore, additives reducing stick and slip effects, tailoring
melt flow and strength as well as stabilisers are a must
in processing. PVC, independent of the source (fossil or
biobased), is an outstanding example for heat stabilisation.
Due to self-catalyzed mechanisms which eliminate hydrogen
chloride from PVC well below processing temperatures,
resulting in a coloured material, heat stabilisers are needed
to interrupt the catalytic cycles. Often, the viscosity of the raw
material is too high leading to increased shear energy input.
Plasticisers may then be needed. Moreover, due to production
conditions, water (even in small quantities like moisture of the
surrounding atmosphere adsorbed on the material) and oxygen
can degrade the material by chemical reactions. This process
may be slow at ambient temperature, but not in thermoplastic
processing. Depending on atmospheric humidity and material
dryness, polyesters especially need a sophisticated temperature
control and additivation to maintain molecular weight and
structure and therefore properties.
Let us now move one step further, from processing to use.
Will we have a serviceable plastic after being able to control
the processing using appropriate additives? Not at all! The
materials will become exposed to the environment and a
serviceable plastic material should not be susceptible to rapid
degradation caused by ultraviolet and visible radiation, oxidation
or hydrolysis. Therefore, several classes of additives have been
developed which intervene in the underlying polymer-type
depending molecular processes.
Furthermore, a serviceable plastic should have the right
morphology which should not change during usage. This is very
important for semi-crystalline polymers like PE, PP, PET and
PLA as well as for blends, which comprise a large market share
in the field of biobased plastics. Therefore nucleating additives
and clarifiers as well as compatibilisers have been developed.
Permeability of gases through a PLA/PHBV (3:1) blend film (100 µm)
Permeability of gases through a PLA/PHBV (3:1) blend film (100 µm)
18
110
16
Compatibiliser (reducing surface tension)
Reactive compatibilisation
PLA/PHBV stat. blockcopolymer
100
H 2
O / gm -2 d -1 and N 2
/ cm 3 m -2 d -1 bar -1
14
12
10
8
6
4
90
80
70
60
50
40
O 2
and CO 2
/ cm 3 m -2 d -1 bar -1
Yellowing due to
sunlight from window
Original white
2
30
0
H 2
O
23 °C,
85 % humidity
N 2
O 2
23 °C,
0 % humidity
CO 2
20
Example for poor UV-stabilizer
Specific synthesis of the compatibilizer (PLA/PHBV stat Blockcopolymer) gives
(slightly) better results than general types of commercialised surface reducing agents
42 bioplastics MAGAZINE [04/16] Vol. 11