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Gel spun high-performance polyethylene fibres
1.
2. • Gel-spun polyethylene fibres are ultra-strong, high-
modulus fibres that are based on the simple and flexible
polyethylene molecule.
• They are called high-performance polyethylene (HPPE)
fibres, high-modulus polyethylene (HMPE) fibres or
sometimes extended chain polyethylene (ECPE) fibres.
• The gel-spinning process uses physical processes to
make available the high potential mechanical properties
of the molecule.
3. • The chemical nature of polyethylene remains in the gel-
spun fibre and this can both be positive and a limitation:
abrasion, flexlife, etc. are very high but the melting point
is sometimes too low for certain applications.
4. • Gel-spun high-performance polyethylene fibres are
produced from poly-ethylene with a very high molecular
weight (UHMW-PE).
• This material is chemically identical to normal high-
density polyethylene (HDPE)
5. •
• In aramids and comparable fibres, the molecules tend to
form rod-like structures and these need only be oriented
in one direction to form a strong fibre.
6. • Polyethylene has much longer and flexible molecules and
only by physical treatments can the molecules be forced
to assume the straight (extended) conformation and
orientation in the direction of the fibre.
8. • High performance polyethylene fibres are commercially
produced under the trade names Dyneema by DSM High
Performance Fibers in the Nether-lands and by the
Toyobo/DSM joint venture in Japan, and Spectra by
Honeywell (formerly Allied Signal or Allied Fibers) in the
USA.
9. • To make strong fibres, the molecular chains must be
stretched, oriented and crystallised in the direction of the
fibre.
• Furthermore, the molecular chains must be long to have
sufficient interaction and for this reason polyethylene with
an ultra-high molecular weight (UHMW-PE) is used as
the starting material.
10. • spinning these fibres from the melt is almost impossible
due to the extremely high melt viscosity.
• Furthermore, the drawing of a melt-processed UHMW-PE
is only possible to a very limited extent owing to the very
high degree of entanglement of the molecular chains.
• Melt-processed UHMW-PE can be drawn up to five times
only.
14. Dyneema •
• The production of Dyneema fibres demands relatively
little energy and uses no aggressive chemicals. The
product can easily be recycled so environmental pollution
from product and process is minimal.
15.
16. • PE with Mw = 1.5 • 106 was stabilized with 0.5% by
weight di-t-butyl-p-cresol (DBPK) at 150 0C, then
dissolved with 2% by weight in decaline at 130 0C, and
spun through spinneret holes of 0.5 mm diameter
according to the air gap process into a bath of decaline
and water at 10 0C, extracted over a long distance, and
drawn at 140 0C in a hot air oven.
17. • After a draw ratio of 1:32 the multifilament has an elastic
modulus of 90 GPa, a tenacity of 3.0 GPa (=36.5 g/dtex)
and 6% breaking elongation.
18. • The draw ratio is also dependent on the temperature and
the type of drawing: Wet drawing is limited to about 1:60,
while hot air drawing has achieved ratios of 1:90.
19. • While the better solvent is decaline, paraffin oil is
technically preferred, especially for working around 180..
.200 0C. Then the drawing is divided into a first stage of
1: (2. . .4) directly following the coagulation bath, a
second stage of up to 1 :12 after extraction, and a third
stage after drying at 180.. .200 0C in hot air with drawing
over a long heated plate at > 130 0C.
20. In the autoclave (10) the solvent
(11) and the stabilizer (12) are
mixed and heated for the paraffin
oil to 140.. .170 0C. In the high
shear mixer (15) the PE powder is
pasted with this and homogenized
in the twin screw extruder (18).
This gel is then extruded by the
spin pump through the spin head
(23) into an air gap before the
filaments enter into the coagulation
bath (30). During take-up there is a
hot air drawing zone between the
godets (54) and (59). A long
distance extraction (37) follows and
so does hot air drying (45). This
yarn can either be wound (52) and
taken up from this bobbin, or it can
be hot drawn
23. •
• these two factors together result in a solution that has a
viscosity that is far too high to spin.
• Already at 2% PE concentration in paraffin oil viscosities
of up to 20,000 Pa- s can occur.
25. • The solvent used in the polyethylene gel-spinning
process should be a good solvent at high temperatures
(>100 °C) but at lower temperatures (<80 °C) the polymer
should easily crystallize from the solution.
26. • After the spinneret, the solution is cooled in the quench,
the solvent is removed and a gel fibre is formed. This can
be done by evaporation or by extraction of the solvent.
27. • From a diluted solution, polyethylene crystallizes in the
form of flat crystals of about 20 nm thickness, in which
the chains are neatly folded.
• In these crystals the C-axis or chain axis is perpendicular
to the crystal (lamella) surface.
• The crystal structure is orthorhombic, which implies that
the crystal axes are at right angles, two by two.
28.
29. • Dyneema and Spectra are produced as a multifilament
yarn.
• The titre of the monofilaments varies from about 0.3
denier per filament (dpf) (0.44 dtex) to almost 10 dpf (11
dtex).
• Tenacity of one filament may well be over 5 N/tex, and
the modulus can be over 120 N/tex.
• Most fibre grades have a more or less circular cross-
section. The fibre skin is smooth.
32. •
-
PBO
• In contrast to the high tensile strength, the gel-spun fibre
has a low compressive yield strength, approximately 0.1
N/tex.
33.
34.
35.
36. • HPPE fibres are the first high-performance fibres that not
only have a high tenacity but that also have tension and
bending fatigue properties comparable with the
commonly used polyamide and polyester grades in
ropes.
• Fatigue is very important in, for example, rope
applications.
37. • Carbon fibres and glass fibres have a high modulus and
a brittle breaking mode, but Dyneema and Spectra fibres
have a high modulus but still are flexible and have a long
flex life,
38.
39. • This fiber has a very low coefficient of friction; is self-
lubricating; and is highly resistant to abrasion, in some
forms being 15 times more resistant to abrasion than
carbon steel.
• Its coefficient of friction is significantly lower than that of
nylon and acetal, and is comparable to that of
polytetrafluoroethylene (PTFE, Teflon), but UHMWPE
has better abrasion resistance than PTFE.
40. • It is odorless, tasteless, and nontoxic.
• HPPE fibres are produced from polyethylene and do not
contain any aromatic rings or any amide, hydroxylic or
other chemical groups that are susceptible to attack by
aggressive agents.
• The result is that polyethylene and especially highly
crystalline, high molecular weight polyethylene is very
resistant against chemicals.
