Plastics News - Show Daily - October 20, 2022 - 14

14 * Plastics News, October 20, 2022
SHOW DAILY
Blow molding is reaching its
limits for production of large
gas tank liners. Hexagon Purus photo
Tank winding at IKV's
Aachen-Melaten composites
laboratory. IKV photo
IKV seeks solutions to fi lament-wound
hydrogen gas tank challenges
By David Vink
Plastics News Correspondent
In a presentation at the September
International Colloquium Plastics
Technology 2022 organized by
the Aachen, Germany-based IKV
Institute of Plastics Processing,
Jörg Strohhäcker, head of R&D at
Kassel, Germany-based pressurized
gas container producer Hexagon
Purus GmbH, talked about
challenges in the development of
type 4 pressure vessels for high
density hydrogen gas storage.
Strohhäcker was referring to work
on the H2-HD project " plastic hybrid
high-pressure tank systems for high
density hydrogen storage " within
the network Hydrogen Power Storage
& Solutions East Germany e.V.
(HYPOS), which ran from December
2018 until March 2022. Project partners
also included Fraunhofer Institute
for Material Microstructures
and Systems (IMWS) (Hall 7, Booth
SC01), Fraunhofer Institute for Mechanics
of Materials (IWM) and RayScan
Technologies GmbH.
Type 4 tanks involve use of a
nylon 6 or polyethylene thermoplastic
liner for low gas permeability
that is wound over by epoxide
thermosetting resin impregnated
carbon (inner) and glass fi ber (outer)
fi laments and cured to provide
the needed high strength to withstand
350 bar in buses and trucks
and 700 bar in passenger cars. But
where research aims to reach 1,000
bar in future with " innovative concepts
for design and analysis. "
Specifi ed limits for present type
4 tanks play on the safe side by requiring
high safety factors, in view
of the effects of " manufacturing-related
imperfections, "
as these cannot yet be
entirely eliminated. This
need for over-engineering
results in heavy and
expensive gas transport
cylinders, so there is a
strong inventive to make
production more consistent and
reliable with designs bringing realized
strength closer theoretically
possible fi ber strength, so that
increased " lightweight effi ciency "
is obtained, namely higher hydrogen-to-container
weight ratios.
Techniques employed in the H2HD
project have included increasing
load-bearing high-angle helical
fi ber layers combined with circumferential
layers and using computer
tomography full-tank and closer
regions-of-interest (ROI) scans
to look for pores; voids, winding
faults and gaps between the
wound fi laments and the thermoplastic
liner.
Liners are usually blow molded,
but Strohhäcker said the " limits of
blow molding are already being exIKV
hausted
today " for large gas transport
containers due to
plastic melt strength limits
and size limits on the
blow molding process.
Hall 14,
Booth C16
In view of long lead
times and expensive
tools for blow molding,
the project also develtimes
and expensive
tools for blow molding,
the project also developed
a new way of preparing at least
prototypes in which two caprolactam
monomer components are
mixed while still uncured and the
highly fl uid material is cast into a
silicone rubber or aluminum form,
in which polymerization takes place
in-situ to form a 500-millimeter-long,
280-mm-diameter and 3-4 mm wall
thickness liner half in nylon 6 polymer
weighing 1.8 kg and with an integrated
metal boss, as needed for
the fi lament winding stage.
Despite sink effects, slight deformations
and 70-170 percent wall
thickness fl uctuation due to use of
a silicone rubber mold, liner halves
were welded together to form complete
liners and wrapped impregnated
carbon fi ber fi lament in the same
way as had already resulted in a 700
bar tank when using a conventionally
produced thermoplastic liner.
The heat-stabilized cast nylon 6 liner-based
tank withstood burst testing,
with the carbon fi ber laminate
failure resulting liner destruction.
Strohhäcker believes the brittle
behavior of the liner could be
overcome with addition of an impact
modifi er to the uncured components,
in order to raise Charpy
impact strength above the region
of 2 kJ/m2, which is too low for
type 4 tanks. But this could present
new challenges, he warned, on
account of higher viscosity and
possible incompatibility with the
silicone mold.
Future work will also focus on
transferring the successful optimized
laminate design improvement,
simulation and the new liner
prototyping technique on small
tanks to large transport tank applications.
Help
from optics
IKV researcher Jannick Fuchs
described in his paper how laser
triangulation and shadow casting
optical inline measurement and
monitoring techniques had been
developed and used at the institute
to detect laminate thickness
increases during " wet " fi lament
winding production of type 4 pressure
vessels.
While laser triangulation has
high resolution, low cost and easy
data evaluation, shadow casting
excels in larger area evaluation. As
with the H2-HD project, the aim was
to reduce over-engineering and associated
costs, this time, however,
through over-sizing prevention.
The techniques enabled orientation
changes due to sliding, as well
as overlaps and gaps due to fl uctuating
fi ber band widths to be identifi
ed and remedied leads. Fuchs
also looked at factors such as fi ber
band resin loading and tension,
and laminate lay-up in robot-based
fi lament winding.
Future work aims at translating
measured geometries via an algorithm
into a pressure vessel digital
twin in order to establish a correlation
between measured process
data and failures and pressure vessel
quality in terms of burst pressure
performance. Further part
design could be based on real fi ber
band geometry, Fuchs suggested.
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Plastics News - Show Daily - October 20, 2022

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