Issue 04/2023

46 bioplastics MAGAZINE [04/23] Vol. 18
From Science & Research
Biodegradable water-soluble
support structures for
additive manufacturing
In the AquaLoes project, researchers from the Institut
für Kunststofftechnik (IKT) at the University of Stuttgart
(Germany) have developed biodegradable support structures
for 3D printing. They are made mainly of polyhydroxybutyrateco-valerate
(PHBV) and table salt and detach easily from the
components in a water bath. The dissolved components
can either be extracted from the water bath and recycled
or disposed of in wastewater without creating microplastics.
PHBV, which comes entirely from biological sources, is
biodegradable in fresh water and seawater.
In order to further develop the material concept to market
maturity, the IKT is currently looking for industrial partners.
Biodegradable plastics offer the great advantage of
an alternative disposal option compared to conventional
plastics. Critical voices believe that the use of biodegradable
plastics encourages littering. However, there are specific
applications where biodegradability offers reasonable
advantages. The best-known examples are products such
as mulch films and plant clips from the agricultural sector,
which are usually difficult to collect, clean, and recycle if
made from conventional plastics.
Another application field of bioplastics offers additive
manufacturing. Due to the almost limitless possibility of
producing various products such as toys, decorations, tools,
or household helpers, 3D printers are meeting with a great
deal of enthusiasm, especially in home applications.
The widely used fused filament fabrication (FFF) process,
describes the continuous, layer-by-layer deposition of a fused
strand to form three-dimensional geometries and is referred
to below as 3D printing. In this process, support structures
must be printed in many cases to prevent the component
from sagging in the event of overhangs or undercuts. These
structures must be removed from the component after
the printing process is completed. A common process
is the mechanical removal of these structures after the
component has solidified, which often leads to damage to
the component surface.
For this reason, the use of soluble materials for support
structures has become established in recent years. In this
process, the support structure is printed from a soluble
material using multi-material printing with the aid of a
second print head. After printing, the component and its
support structure are immersed in a suitable solvent, which
dissolves the support structure. This results in high-quality
surfaces even after the support structure has been removed.
A frequently encountered material for such soluble
support materials is, for example, polyvinyl alcohol (PVA),
which, however, does not easily biodegrade. Without further
purification steps, the support structures remain in the
wastewater of private and also industrial users in the form
of individual dissolved polymer chains and thus inevitably end
up in the environment. In this application, the use of a fully
biodegradable material, even in wastewater, would reduce
the environmental impact of the materials used.
Biodegradation depends not only on the type of material
used but also on the prevailing environmental conditions
such as temperature and microbial activity. Therefore, only
polymers that are biodegradable in a freshwater environment
can be considered for the application presented here. At the
same time, not only biodegradability determines the choice of
the appropriate polymer, but the printing process also requires
a certain strength of the substrate to support the structure.
These requirements speak in favour of plastics such as PHBV
as the ideal choice for biodegradable support structures.
The polymer PHBV is obtained biotechnologically. Certain
bacterial strains produce PHBV as a storage substance,
mostly from plant nutrients such as sugar, starch, or residual
and waste materials. This metabolic product can be extracted
from the bacteria and processed into plastics.
The IKT came up with the idea of developing a new material
for support structures based on PHBV. It has the advantage
of being completely biologically metabolized to water and
CO 2
even in seawater, which is a harsh environment for
many bacteria. Since PHBV itself is biodegradable in water
but not water-soluble, the researchers wanted to achieve a
quick fragmentation of the supporting material by mixing
it with table salt, which dissolves in water rather fast. The
PHBV fragments could then either be filtered out or, if they
remain in the wastewater, biodegrade by bacteria within a
manageable period of time.
In this project, more than 30 different formulations
were developed for the printing and dissolution tests.
For this purpose, a PHBV from TianAn Biologic Material
(Ningbo, China) was used as the base polymer. In addition,
due to its brittleness, a short-chain polyethylene glycol
from Sigma-Aldrich (Taufkirchen, Germany) was used
as a plasticizer, which does not adversely affect the key
property of biodegradability. To achieve dissolution of
the supporting structure, a very fine table salt from the
European Salt Company (Hannover, Germany) with a particle
size < 0.15 mm was used.