Note: Descriptions are shown in the official language in which they were submitted.
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HIGHLY ORIENTED POLYOLEFIN FIBRE
The invention relates to a highly oriented
polyolefin fibre containing polyolefin with an
intrinsic viscosity of at least 5 dl/g, which fibre has
a tensile strength of at least 26 cN/dtex and a modulus
of tension of at least 700 cN/dtex, a process for the
preparation thereof and the use in ropes or anti-
ballistic shaped articles. The invention also relates
to improved ropes and anti-ballistic shaped articles.
The said highlv-oriented polyolefin fibres
are known from EP-A-0 205 960.' The highly oriented
polyolefin fibres described there have a very high
tensile strength and modulus of tension and a low creep
rate, making them particularly suitable for use in,
inter alia, ropes and anti-ballistic shaped articles.
The fibres are prepared by spinning a solution of a
polyolefin into a gel fibre, extracting the solvent
from the fibre, and drawing the extracted and dried
fibre in one or more steps.
However, there is an ongoing need for
further improvement of the quality of such fibres, or
at least for optimization of the properties of the
fibres to such an extent that the quality of the
products, such as ropes and anti-ballistic shaped
articles, made from these fibres can be improved. The
invention provides highly oriented polyolefin fibres
with improved properties in said applications.
Surprisingly, this is achieved in that
the fibre contains 0.05 - 5 wt.o of a solvent for the
polyolefin (relative to the fibre's total weight).
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In a product aspect, the invention provides a
highly oriented polyolefin fibre comprising a polyolefin
with an intrinsic viscosity, in decalin at 135 C, of at
least 5 dl/g, the fibre having a tensile strength of at
least 26 cN/dtex and a modulus of tension of at least
700 cN/dtex, wherein the fibre comprises 0.05 - 5 wt.% of a
solvent for the polyolefin, relative to the total fibre
weight, and wherein the fibre has a creep, defined as the
elongation as a percentage of the original length after
5 hours under a load of 8.11 gr/dtex at 50 C, of at most
10%.
In a process aspect, the invention provides a
process for the preparation of the highly oriented
polyolefin fibre of the invention, comprising: forming a
solution of the polyolefin in the solvent; forming a gel
fibre by extruding the solution through one or more spinning
apertures and subsequently cooling the extrudate to a gel
fibre; removing the solvent from the gel fibre and drawing
the fibre in one or more steps, wherein the solvent is not
fully removed from the gel fibre and a solvent-containing
precursor fibre is formed after one or more drawing steps;
and wherein the precursor fibre is subsequently drawn, at a
temperature above the equilibrium melting temperature of the
polyolefin and between 145 and 160 C, to a highly oriented
polyolefin fibre comprising 0.05 to 5 wt.% of the solvent
and having a creep of at most 10%.
In a further process aspect, the invention
provides a process for the preparation of a highly oriented
polyolefin fibre, comprising: forming a solution of the
polyolefin with an intrinsic viscosity of at least 5 dl/g,
in decalin at 135 C, in a solvent, wherein the solvent
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consists substantially of a mixture of a first solvent (A)
and a second solvent (B), with (A) being removed and
(B) remaining in the fibre; forming a gel fibre by extruding
the solution through one or more spinning apertures and
subsequently cooling the extrudate to a gel fibre; removing
solvent from the gel fibre and drawing the fibre in one or
more steps, wherein the solvent is not fully removed from
the gel fibre and a solvent-containing precursor fibre is
formed after one or more drawing steps; and wherein the
precursor fibre is subsequently drawn, at a temperature
above the equilibrium melting temperature of the polyolefin,
to a highly oriented polyolefin fibre having a tensile
strength of at least 26 cN/dtex and a modulus of tension of
at least 700 cN/dtex and comprising 0.05 to 5 wt.% of the
solvent, and wherein (B) has a higher melting temperature
than (A), and (A) is removed by extraction at a temperature
at which no or substantially no evaporation of (B) takes
place.
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It has been found that the fibres according
to the invention are eminently suitable for use in
anti-ballistic shaped articles since shaped articles on
the basis of these fibres have a high Specific Energy
Absorption (SEA), which means that less fibre, and
hence less weight, is needed to obtain the same level
of protection. It has also been found that the fibres
according to the invention are suitable for use in
ropes, inter alia because their compactness is better
without any loss in flexibility and because the
strength of the ropes is enhanced.
The improved quality of the fibres is
particularly surprising since up to now the presence of
a significant amount of solvent in the fibre has been
considered undesirable as this reduces the mechanical
properties of the fibre, in particularbecause the
fibre's creep rate is higher and its strength and
modulus are lower. It is also surprising that solvent-
containing fibres have a higher anti-ballistic quality
than "dry" fibres of comparable strength and modulus,
for in itself the solvent cannot contribute to the
level of protection, while it does increase the areal
density.
Fibres that contain solvent are known in
the state of the art. However, these fibres are not
highly oriented and they are unsuitable for the desired
applications as their mechanical properties are not
good enough. Within the context of the present
application, highly oriented is understood to mean that
the fibre has a modulus of tension of at least 700
cN/dtex and a tensile strength of at least 26 cN/dtex
(as determined according to the method specified
below) . The known solvent-containing fibres are
intermediates in a process in which the fibre is
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prepared from a solution. The description makes it
clear that the solvent is undesirable in the end
product and therefore still needs to be removed. US-A-
5,213,745, for example, describes optimum extraction
agents for the removal of mineral oil solvent from an
undrawn gel fibre. This publication does not describe
solvent-containing, highly oriented polyolefin fibres.
EP-A-0,115,192 describes fibres having a high solvent
content and a low tensile strength and modulus of
tension. These fibres, too, are intermediates, and as
such unsuitable for use in the said applications.
The tensile strength (or strength) and the
modulus of tension (or modulus) are defined and are
determined as specified in ASTM D885M, using a nominal
gauge length of the fibre of 500 mm, a crosshead speed
of 50%/min and Instron 2714 clamps. Before the
measurement the fibre is twisted at 31 turns per metre.
On the basis of the measured stress-strain curve the
modulus is determined as the gradient between 0.3 and
1% strain. For calculation of the modulus and strength,
the tensile forces measured are divided by the titer,
as determined by weighing 10 metres of fibre. Creep is
here and hereinafter understood to be the elongation as
a percentage of the original length after 5 hours under
a load of 8.11 gr/dtex at 50 C. The elongation includes
the elastic elongation.
A fibre is understood to be a continuous or
semi-continuous object such as a monofilament,
multifilament yarn, tapes or staple fibre yarn. In
principle, the filaments may have any cross-sectional
shape and thickness. Preferably, the filament titer is
at most 5, more preferably at most 3 denier per
filament. The advantage of such a low filament titer is
that the fibre has better anti-ballistic properties.
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Varying polyolefins can be used in the
.fibre according to the invention. Particularly suitable
polyolefins are homo- and copolymers of polyethylene
and polypropylene. In addition, the polyolefins used
may contain small amounts of one or more other
polymers, in particular other alkene-i-polymers. Good
results are achieved if linear polyethylene (PE) is
chosen as polyolefin. Linear polyethylene is here
understood to be polyethylene with fewer than one side
chain per 100 carbon atoms, and preferably fewer than
one side chain per 300 carbon atoms, which may moreover
contain up to 5 mol% of or more alkenes that can be
copolymerized with it, such as propylene, butene,
pentene, 4-methylpentene or octene. Besides the
polyolefin and the solvent the fibre may contain small
amounts of the additives that are customary for such
fibres, such as anti-oxidants, spinfinish, thermal
stabilizers, colourants, etc.
Preferably, the polyolefin fibre, in
particular the polyethylene fibre, has an intrinsic
viscosity (IV) of more than 5 dl/g. Because of their
long molecule chains, polyolefin fibres with such an IV
have very good mechanical properties, such as a high
tensile strength, modulus, energy absorption at break.
This is also the reason why even more preferably the
polyolefin is a polyethylene with an IV of more than 10
dl/g. The IV is determined according to method PTC-179
(Hercules Inc. Rev. Apr. 29, 1982) at 135 C in decalin,
the dissolution time being 16 hours, the anti-oxidant
is DBPC, in an amount of 2 g/1 solution, and the
viscosity at different concentrations is extrapolated
to zero concentration..
To ensure a good anti-ballistic effect, the
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tensile strength of the fibre is at least 26 cN/dtex
and the modulus at least 700 cN/dtex. Preferably, the
modulus is at least 880 cN/dtex, more preferably at
least 1060 cN/dtex, and most preferably at least 1235
cN/dtex. The strength is preferably at least 31
cN/dtex, more preferably at least 33 cN/dtex, and most
preferably at least 35 cN/dtex. Surprisingly, it has
been found that at relatively low, but for the purpose
of the invention effective, solvent concentrations, the
creep of such a highly oriented fibre is only to a very
low extent adversely affected by the solvent.
Preferably, the fibre according to the invention has a
tensile strength of at least 26 cN/dtex, a modulus of
at least 700 cN/dtex, a solvent content of 0.05 - 2
wt.% and a creep of at most 20%, more preferably at
most 15%, even more preferably at most 10%/h and most
preferably at most 5%. Such a low creep is favourable
in particular for use in ropes. When use is made of
copolymer with more than 2 short side chains per 1000
carbon atoms, the creep can be reduced further.
Preferably, the creep then is at most 10% and more
preferably at most 5%.
Solvent is here and hereinafter understood
to be a substance that is capable of dissolving the
polyolefin in question. Suitable solvents for
polyolefins are known to one skilled in the art. They
can, for example, be chosen from the 'Polymer Handbook'
by J. Brandrup and E.H. Immergut, third edition,
chapter VII, pages 379 - 402. Preferably, use is made
of a solvent with a chi-parameter for the polyolefin
used, in particular polyethylene, of less than 0.5,
more preferably less than 0.45, even more preferably
less than 0.4, and most preferably less than 0.35. Chi-
parameters of solvents are presented in Handbook of
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sol. parameters and other cohesion parameters, 2nd
edition, published by Allan Barton, p. 386. This has
the advantage that, at the same solvent content, the
quality improvement can be greater, mutatis mutandis,
that less solvent is needed to achieve the same
improvement in anti-ballistic properties. Examples of
suitable solvents for polyolefin, in particular for
polyethylene, are, separately or in combination:
decalin, tetralin, toluene, lower n-alkanes such as
hexane, (para-)xylene, paraffin oil, squalane, mineral
oil, paraffin wax, cyclooctane. For the reasons cited
above, the solvent is most preferably paraffin oil or
decalin.
Preferably, the solvent is a non-volatile
solvent, such as paraffin oil. This has the advantage
that the fibre has a better stability, which means that
the properties of the fibre, and of the products based
on it, do not deteriorate with time and that the useful
service life is longer. Another advantage is that the
fibre does not have such a bad smell and is not toxic
or detrimental to health, which is relevant in
particular in body protection applications. A non-
volatile solvent is understood to be a solvent that
virtually does not evaporate at a temperature below the
melting temperature of the polyolefin. Preferably,
these are solvents having a boiling temperature that is
substantially, preferably 50 to 100 C, higher than the
fibre's melting temperature.
The fibre according to the invention
contains 0.05 - 5 wt.% of a solvent for po.lyolefin.
Solvent contents below 0.05 wt.% have no or hardly any
effect. Contents higher than 5 wt.% have the
disadvantage that they no longer essentially contribute
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to the improvement in, or even impair, the anti-
ballistic properties. The SEA increases with the
solvent content up to a certain optimum solvent
content, at which the contribution to the energy
absorption no longer compensates for the increase in
areal density and above which the SEA drops again.
Although a higher than optimum solvent content in the
shaped article ultimately obtained may be advantageous,
since solvent is cheaper than fibres, the solvent
content is preferably chosen with a view to obtaining
the highest possible anti-ballistic quality. The
optimum solvent content also depends on the fibre
configuration, the quality of the solvent chosen, and
the compression conditions. On the basis of the
guidelines given here, one skilled in the art can
determine the optimum amount for each process
condition. For the above reasons, the solvent content
in the fibre is preferably from 0.1 to 3 wt.%, more
preferably 0.2 - 2 wt.%, even more preferably 0.2 - 1.2
wt.%, and most preferably 0.3 - 1.0 wt.%. Such low
solvent contents are preferably used for good solvents,
in particular solvents having a chi-parameter lower
than 0.5, and for use in uni-directional composites.
The solvent content of the fibres can be determined in
a known way, for example directly by means of infrared
techniques, C13 NMR, or indirectly by solvent removal,
for example by extraction or head-space chromatography
or combinations of said techniques.
The fibre according to the invention can be
prepared by contacting a highly oriented "dry"
polyolefin fibre with a solvent for the polyolefin,
with the fibre taking up 0.05 - 5 wt.% of the solvent.
The highly oriented "dry" polyolefin fibre may have
been prepared in a known way from the polyolefin
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polymer, for example by gel spinning (Smith and
Lemstra), by solid phase processing of virgin reactor
powder (Chanzy and Smith), by extrusion from the melt
(Ward) or by extrusion from powder recrystallized from
solution (Kanamoto) with one or more drawing steps to
increase the orientation.
Preferably, the fibre is prepared directly
in a gel spinning process. The invention also relates
to a process for the preparation of a highly oriented
polyolefin fibre according to the invention comprising:
forming a solution of a polyolefin in a solvent,
forming a gel fibre by extruding this solution through
one or more spinning apertures and subsequently cooling
it to obtain a gel fibre, removing the solvent from the
gel fibre and drawing the fibre in one or more steps.
Such a process is known from EP-A-0,205,960. For the
preparation of the fibre according to the invention
this process is adapted in that not all of the solvent
is removed from the gel fibre resulting, after one or
more drawing steps, in the formation of a solvent
containing precursor which is subsequently, at a
temperature above the equilibrium melting temperature
of the polyolefin, drawn to obtain the highly oriented
polyolefin fibre containing 0.05 to 5 wt.% of solvent.
An advantage of the process according to
the invention is that fewer steps are needed for the
preparation of the fibre and that the fibre obtained by
this process has better anti-ballistic properties than
a fibre of comparable strength and modulus to which a
similar amount of solvent has been added in another
manner. A further advantage of the process according to
the invention, for example compared with the processes
described in EP-A-0,205,960, is that less fibre
breakage occurs during drawing of the solvent-
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containing precursor fibre to a highly oriented fibre,
at otherwise unchanged conditions. As a result, there
are fewer production stops and a higher productivity
can be achieved.
The precursor fibre may have been formed in
a single step by simultaneous drawing and solvent
removal or by separate solvent removal and drawing
steps. The solvent content in the precursor fibre is
chosen so that the end product, the highly oriented
polyolefin fibre, contains the desired amount of
solvent, between 0.05 and 5 wt.%, after drawing. It is
possible that part of the solvent is removed during the
last drawing step. Preferably, however, a non-volatile
solvent is used, with the solvent content during
drawing of the precursor fibre in the last drawing step
being virtually constant. This has the advantage of
better drawing process control, resulting in better
drawability.
In one embodiment of the process the
solvent in the highly oriented polyolefin fibre is the
same as the solvent of the solution from which spinning
takes place. The solvent content of the precursor fibre
can be set by incomplete solvent removal, for example
by shortening the evaporation or extraction time or by
influencing the evaporation or extraction rate.
In a particularly preferred embodiment of
the process according to the invention, the solvent
substantially consists of a mixture of a first solvent
(A) and a second solvent (B), with (A) being removed
and (B) remaining in the fibre.
The physico-chemical properties of these
solvents (A) and (B) are so different that the solvent
removal technique employed results in removal of (A)
while solvent (B) substantially remains in the fibres.
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The advantage of this embodiment is that the amount of
solvent (B) in the precursor fibre can be set directly
and more accurately through the choice of the spinning
solvent composition, without major changes in the other
process parameters. In the light of the aim of the
invention it is not necessary for the amount of (B)
present in the solution during removal of (A) and/or
drawing to remain completely in the fibre, but for
process control purposes it is advantageous if the full
amount of (B) remains in the fibre, at any rate during
the removal of (A), so as to prevent contamination of
the process. For this reason, (B) preferably almost
entirely remains in the fibre also during drawing.
There is no need for (A) to be fully removed, but for
process control reasons (A) is preferably fully
removed. Preferably, the content of (A) in the fibre is
not higher than 0.5 wt.%, preferably lower than 0.3
wt.%, more preferably lower than 0.2 wt.%, and most
preferably lower than 0.1 wt.%.
In an embodiment of the said process (B)
has a higher boiling point than (A) and (A) is removed
by evaporation at a temperature at which no or hardly
any evaporation of (B) takes place. Preferably, the
boiling temperature of (B) is chosen so that no or
hardly any evaporation of (B) takes place at the
drawing temperature, either. This has the advantage
that drawing is better controlled since the fibre
composition does not change during drawing and since
the fibre is not cooled by withdrawal of the heat of
solvent evaporation.
In the most preferred embodiment (B) is a
non-volatile paraffin in the process and (A) a volatile
solvent, preferably decalin. An added advantage of this
embodiment over the embodiment mentioned earlier is
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that the solvent mixture is much less viscous than just
.paraffin as solvent, so that paraffin-containing fibres
with low filament titers, in particular filament titers
lower than 5 and preferably lower than 3, can more
readily be prepared.
In another embodiment of the process (B)
has a higher melting temperature than (A) and (A) is
removed by extraction at a temperature at which no or
hardly any extraction of (B) takes place. Preferably,
(B) then is a paraffin wax and (A) paraffin oil.
The solvent-containing precursor fibre is
drawn to a highly oriented polyolefin fibre at a
temperature above approximately the equilibrium melting
temperature of the polyolefin. The equilibrium melting
temperature of the polyolefin is understood to be the
peak temperature of the melting curve of the polyolefin
powder, measured using a DSC at a heating-up rate of
10 C a minute. For polyethylene fibres this is
preferably above about 140 C. Obviously, the drawing
temperature is not chosen so high that effective
drawing can no longer take place. Most preferably, the
drawing temperature is between 145 and 160 C and the
solvent content of the precursor fibre during the last
drawing step is everywhere between 0.05 and 5 wt.%.
This has the advantage of a good productivity in
combination with a very good strength and modulus.
The invention also relates to a highly
oriented polyolefin fibre obtainable by the process
described above. This fibre has better anti-ballistic
properties than a fibre with otherwise comparable
properties to which a similar amount of solvent has
been added in a different way.
The invention also relates to the use of
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highly oriented polyolefin fibres according to the
invention for the manufacture of ropes and to ropes
containing highly oriented polyolefin fibres according
to the invention. Compared with solvent-free fibres
with comparable properties, the solvent-containing
fibres can more easily be processed into ropes. The
ropes are more compact, their feel is less woolly, and
yet their flexibility is good. It has been found that
the rope is also stronger.
The invention also relates to the use of
the highly oriented fibres according to the invention
as described above for the manufacture of anti-
ballistic shaped articles. The advantage of the use of
these fibres is in particular that the customary
process for the preparation of these shaped articles
can be used without essential modifications. Such
processes are disclosed, for example, in W097/00766 and
W095/00318. A major added advantage is that, for
example in contrast with fibres or fibre layers that
are wetted afterwards, the process equipment is not
fouled with solvent.
The invention also relates to anti-
ballistic shaped articles that contain highly oriented
polyolefin fibres according to the invention. Compared
with shaped articles on the basis of solvent-free
fibres, these shaped articles have a higher anti-
ballistic protection level at a comparable areal
density. Preferably, the anti-ballistic shaped article
according to the invention has a specific energy
absorption (SEA) when hit by an AK47 MSC point of at
least 115 J/kg/m2, preferably more than 120 J/kg/m2,
even more preferably more than 135 J/kg/m2, and most
preferably more than 145 J/kg/mZ.
The invention will be elucidated on the
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basis of the following examples.
Woven fabric: Co=arative experimeata
SK76 Dyneema yarn without paraffin was woven into a
simple fabric with 8 yarns/cm in the warp and the weft.
The areal density of the woven fabric was 318 gr/m2.
Twenty layers of this fabric were compressed to form
flat panels with 60 micron Stamylex (LLDPE) film
between each layer. The pressure was 10 bar, the
temperature was 125 C and the compression time was 20
min. After this compression time the panels were cooled
while the pressure was maintained. The V50 was
determined according to the Stanag 2920 standard test
using 17 grain FSP. The V50 was 532 m/s, corresponding
to an energy absorption (SEA) of 21.4 J/kg/m2.
The properties of the SK76 yarn employed
are:
Strength: 36.0 cN/dtex
Modulus: 1180 cN/dtex
Creep: 4.1%,
Woven fabric: Example 1
SK76 Dyneema yarn with a particular
paraffin content was prepared by gel spinning under the
conditions usually used for SK76 yarns, from a solution
of UHMWPE and a volatile solvent to which a particular
amount of paraffin had been added. Dunflussig paraffin
from Merck having a dynamic viscosity of 25-80 MPa/.sec
and a density of 0.818-0.875 gr/cm3 was used as the
paraffin. The specified paraffin content was calculated
on the basis of the percentage of paraffin added to the
_._....~_....-___ _
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solvent at complete retention of the paraffin in the
fibre during the fibre production process.
A panel was produced and tested according
to Comparative Experiment A, except that SK76 yarns
containing approximately 0.8% paraffin solvent were
used. The strength, modulus and creep of the yarn we're
the same as those of the solvent-free yarn. The areal
density of the woven fabric was 302 g/m2. The resulting
V50 of the solvent-containing panel was 560 m/s,
corresponding to an energy absorption of 24 J/kg/m2
Twill woven fabric: Comnarative experiment B
Dyneema SK75 yarns without solvent were
woven to form a twill 3/1 style fabric with 3.75
yarns/cm in the warp and the weft and an AD of 276
g/m2.22 Layers of this fabric were compressed to form
panels with 30 micron Stamylex (LLDPE) film between the
layers and tested in a way as specified in Example 4.
The V50 was 534 m/s, corresponding to an SEA of 23.8
J/kg/m2.
The yarn properties of the SK75 yarn used
(measured as in comparative experiment A):
Strength: 35.1 CN/dtex
Modulus: 1130 CN/dtex
Twill woven fabric: Example
2
A twill woven fabric as in Comparative
Experiment B was produced, only now using SK75 fibres
containing approximately 2000 ppm decalin. Although the
yarn properties were the same, the V50 of the panels
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was higher, namely 600 m/s, corresponding to an SEA of
28 J/kg/m2.
UD composite: Comnarative Experiment C and Examoles 3-7
SK76 and SK75 Dyneema yarns with different
concentrations of paraffin, produced as described in-
Example 1, were processed to form monolayers of
unidirectionally oriented yarns bound in a Kraton
matrix (isopropene-styrene copolymer from Shell). Four
monolayers were formed into a UD stack in which the
fibre direction in each monolayer was at an angle of 90
degrees with respect to the fibre direction in the
neighbouring layer. 75 of such UD stacks were
compressed to form an anti-ballistic shaped article at
a temperature of 125 C and a pressure of 165 bar for
35 minutes. The shaped article was cooled with water
while the pressure was maintained. The shaped articles
were tested according to the Stanag 2920 standard using
AK47 MSC rounds. The yarn properties had not been
affected by the addition of the paraffin.
Fibre Paraffin(%) V50 (m/s)
C SK75 0 <710
3 SK75 0.4 730
4 SK75 0.8 780
5 SK76 0.4 750
6 SK76 0.8 780
17 SK76 1.2 810
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Rope: Examples 8, 9 and 10
From SK76 Dyneema yarns with different
paraffin contents (prepared according to Example 1, all
having a titer of 1760 dtex per yarn) three braids (vi,
v2 and v3) were made on a 16-position Herzog braiding
machine. The resulting braid had 2.75 stitches/cm. The
braids were very compact and yet very supple. The
braids were tested on a Zwick 1484 tensile tester with
Zwick 8465 type clamps and a crosshead speed of 150
mm/min. The nominal gauge length between these clamps
is 2600 mm (see table below).
% paraffin Tensile strength CN/dtex
0.4 21.7
0.8 21.9
1.2 22.1
