Note: Descriptions are shown in the official language in which they were submitted.
WO91/17203 2 0 81918 PCT/NL91/00072
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CRO5SLINKED ORIENTED HIGH MOLECULAR WEI~H~
POLYETHYLENE AND A PROCESS FOR PREPARING
ARTICLES FROM SUCH POLYETHYLENE
~he invention relates to crosslinked oriented high
molecular weight polyethylene and to a process for preparing
articles from such polyethylene.
Oriented high molecular weight polyethylene is
known, inter alia, from US-A-4,344,908, in which is
described the production of polyethylene fibres with a high
tensile strength at break and a high modulus.
~he disadvantage of these polyethylenes however, is
that their resistance against high temperatures is poor. If
a clamped article-from the known oriented high molecular
weight polyethylene is exposed for some time to a
temperature of 145C, the tensile strength and the elastic
modulus will show a very strong decrease. At a temperature
lS higher than 155C, the article will break. Attempts have
been made to improve the resistance against high
temperatures by crosslinking the polyethylene. For instance,
from D.J. Dijkstra, A.J. Pennings, Pol~mer Bulletin l7, 507
(1987) it is known to crosslink oriented high molecular
weight polyethylene fibres by radiation with high-energy
electrons.
Howèver, the fibres thus obtained do not have
sufficient resistance against high temperatures, while the
tensile strength at break decreases in consequence of the
2S radiation.
Wosl/l72o3 2 0 81918 PCT/NL91/00072
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An object of the invention is oriented high
molecular weight polyethylene with great resistance against
high temperatures, a high tensile strength at break (o) and
a high elastic modulus (E).
This object is achieved in that the polyethylene
contains up to 30% (wt) crosslinked poly-1,4-butadiene.
Articles from polyethylene according to the invention do not
break when they are exposed under stress to a temperature
even as high as 200C. The articles have been found to have
a high initial tensile strength at break (~) and a high
elastic modulus (E), which continues to be high after
exposure to a temperature of 200C.
The poly-1,4-butadiene preferably consists for at
least 90% (mole) of poly-trans-1,4-butadiene.
Poly-trans-1,4-butadiene is known per se from G.
Natta, M. Pegoraro and P. Cremonesi, Chim. e Industria, ~7,
No. 7, 722, (1965).
Articles with a high tensile strength and E-modulus
cannot be obtained from it, because articles from
poly-trans-1,4-butadiene cannot be properly oriented. For
one thing, the maximum drawability of an article from
poly-trans-1,4-butadiene is only about 5 (see S. Iwayanagi,
I. Sakurai, T. Sakurai and T. Seto in Journal of
Macromolecular Science-Physics, B2, 163, (1968).
An article according to the invention, containing
high molecular weight polyethylene and up to 30% (wt)
poly-1,4-butadiene, has a high drawability at temperatures
ranging from 70-140C. At 100C this drawability is at least
10 (for instance 100).
High molecular weight polyethylene is understood to
mean, according to the invention, a polyethylene having a
average molecular weight of at least 5~105 kg/mole. The
weight-average molecular weight (Mw) is determined by
applying the methods known for this purpose, such as Gel
Permeation Chromatography (GPC) and Light Scattering. The
number-average molecular weight (Mn)~ too, can be determined
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WO91/17203 2 0 81918 PCT/NL91/00072
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by applying GPC. In the case of polyethylene with a
weight-average molecular weight (Mw) of at least 5*105
kg/kmole, the Mw is calculated from the Intrinsic Viscosity
'IV) determined in decalin at 135C. The said wei~ht average
molecular weights of 0.5 and 1.0*106 kg/kmole correspond
with an IV in decalin at 135C of 5.1 resp. 8.5 dl/g
according to the empirical relation:
Mw Y 5.37 x 104 [IV]1 37.
Preference is given to the use of a very high
molecular weight polyethylene (ultra high molecular weight
polyethylane, or UHMWPE). Of such a polyethylene the
weight-average molecular weight Mw is, for instance, between
1*106 and 10*106 kg/kmole.
High molecular weight polyethylene is in this
connection further understood to mean linear polyethylene
with fewer than 10 side chains per 1000 carbon atoms and
preferably with fewer than 3 side chains per 1000 carbon
atoms, or such a polyethylene containing also minor amounts,
preferably smaller than 5% (mole), of one or more other
alkenes copolymerized therewith, such as propylene,
butylene, pentene, hexene, 4-methyl-pentene, octene, etc.
The polyethylene may further contain minor amounts,
preferably 25% (wt) at most, of one or more other polymers,
particularly an alkene-l-polymer, such as polypropylene,
polybutylene or a copolymer of propylene with a minor amount
of ethylene.
The poly-1,4-butadiene used according to the
invention is prepared according to processes known in the
art. In this connection see, for instance, G. Natta, M.
Pegoraro and P. Cremonesi, Chim. e Industria, 47, No. 7,
722 (1965). Generally, poly-1,4-butadiene with a
viscosity-average molecular weight (Mv) of at least 1 x 104
is used. The Mv preferably amounts to at least 3 x 104,
particularly at least 6 x 10 . In the polymerization process
a high poly-trans-1,4-butadiene content is preferably aimed
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W O 91/17Z03 ~' 2 0 819 ~ 8 PC~rINL91/00072
at. The poly-1,4-butadiene consists of, for instance, at
least 90% (mole) poly-trans-1,4-butadiene. The
poly-trans-1,4-butadiene content is particularly at least
95~ ~mole~, more particularly at least 98~ (mole).
The article according to the invention contains up
to 30% (wt) crosslinked poly-1,4-butadiene. It preferably
contains up to 0.5-10% (wt) poly-1,4-butadiene, more
particularly 0.5-5% (wt). The degree of crosslinking of the
poly-1,4-butadiene must at least be 50%, particularly at
least 90%, more particularly at least 95%.
The article according to the invention may also
contain non-polymeric materials, such as solvents,
colourants, stabilizers, anti-oxydants, waxes and fillers.
The amount of these materials may total up to 60% (vol) in
respect of the polymer.
The preparation of articles according to the
invention is carried out according to processes known in the
art for the preparation of articles from oriented high
molecular weight polyethylene. Preferably a process can be
applied in which a solution of high molecular weight
polyethylene and poly-1,4-butadiene in a suitable solvent is
converted into a gel article by thermally reversible
gellation, upon which the resulting gel article is drawn,
with orientation of the polymer molecules. This
last-mentioned process, the so-called gel route, will be
elucidated extensively helow.
In applying the gel route according to the present
invention various solvents can be used. Suitable solvents
include halogenated or non-halogenated hydrocarbons, such as
paraffins, paraffinic waxes, toluene, xylene, tetraline,
decalin, monochlorobenzene, nonane, decane or petroleum
fractions. Of course, mixtures of solvents can be used also.
The polyethylene and poly-1,4-butadiene
concentrations in the solution may vary. Of importance in
this connection are, inter alia, the nature of the solvent
and the molecular weight of the polyethylene. Solutions in
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which the total concentration of pol~mer with a very high
molecular weight IMw for instance higher than 1 x 106) is
higher than 50% (wt) are difficult to handle on account of
the prevailing high viscosity. For the viscosity of the
solution the molecular weight of the poly-1,4-butadiene is
of minor significance, because it is relatively low
molecular in respect of the polyethylene. The disadvantages
of the use of solutions with a total concentration of, for
instance, less than 0.5% (wt) are a loss of yield and an
increase in the costs for the separation and recovery of
solvent. Therefore, a solution will generally be started
from having a total high molecular weight polymer
concentration of between 1 and 40% (wt), particularly 5-30%
(wt).
The solutions to be applied can be prepared in
various ways, for instance by suspension of solid
particle-shaped poly-1,4-butadiene and polyethylene in a
solvent, followed by stirring at elevated temperature, or by
converting the suspension into a solution in an extruder,
for instance a twin-screw extruder provided with mixing and
conveying devices.
The conversion of the solution into a shaped,
solvent-containing article can be effected in the present
invention in different ways, for instance by spinning, via a
spinning head with a circular die or a slit die, into a
filament or ribbon, or by extrusion via an extruder, usually
with a profiled extruder head.
The temperature during the shaping must be chosen
above the dissolving point. This dissolving point is
determined by the solvent, the polyethylene and
poly-1,4-butadiene concentrations, the molecular weights of
3~ these polymers and the pressure applied.
This temperature is preferably at least 90C,
particularly at least 100C. Of course, this temperature
must be chosen below the decomposition temperature of the
poly-1,4-butadiene and of the high-molecular polyethylene.
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The shaped, solvent-containing article is
subsequently cooled to below the gelation temperature in
such a manner that a gel article with a homogeneous gel
structure is obtained, in which process the article is
cooled rapidly using air and/or a liquid cooling medium, for
instance water. The gelation temperature depends on, inter
alia, the solvent and generally virtually corresponds with
the said dissolving temperature. The article is preferably
cooled to about ambient temperature.
The gel article thus obtained can successively be
drawn. It is also possible for at least part of the solvent
to be removed, before the drawing, by, for instance,
extraction. ~he drawing can be carried out also under such
conditions that the solvent still present is wholly or
partly removed, for instance by means of a gas, or by
drawing in an extraction bath.
The drawing must be carried out according to the
invention at a temperature higher than the temperature of
the first order solid transition of poly-trans-1,4-
butadiene. This is the temperature at which a change occurs
in the crystal structure (monoclinic to pseudo-hexagonal) of
the poly-trans-1,4-butadiene.
In this connection see ~. Moller, Makromol. Chem.
Rapid Comm., 9, 107 (1988). This temperature, like other
phase transition temperatures mentioned herein, such as
melting temperatures, is determined by means of Differential
Scanning Calorimetry (DSC). Using this technique an
endothermic peak corresponding with the first order solid
transition temperature of poly-trans-1,4-butadiene is found
at about 65-70~C and an endothermic peak corresponding with
the melting temperature of the high molecular weight
polyethylene is found at about 140-145C. The endothermic
peak corresponding with the melting point of trans-1,4-
polybutadiene also occurs at 140-145~C and can therefore not
be observed separately from the endothermic peak of the
polyethylene.
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Wo91/17203 ~ 0 81 9 1 ~ PC~/NL91/00072
According to the invention DSC measurements are
carried out in the following manner. Thermograms are made
using a calorimeter of the DSC-7 type of the firm of
Perkin-Elmer. The heating rate applied is lO~C/min. The
standard for temperature calibration is Indium, which has a
melting temperature (Tm) of 156.6C and a melt enthalpy
(~Hm) of 28.4 J/g. The samples weigh lO mg. A drop of
silicone oil is added to the samples for proper heat
conduction.
In the drawing process, high draw ratios can be
applied according to the invention. Generally, a draw ratio
lS of at least lO is applied, preferably at least 20, and
particularly at least 40.
Otherwise, in addition to the gelling process
mentioned hereinbefore, other processes for the preparation
of oriented polyethylene articles can be used also, for
instance the processes known in the art for the processing
of virgin high molecular weight polyethylene. In this
connection see W0-87/03288. Instead of pure virgin
polyethylene, a mixture of virgin polyethylene and
poly-l,4-butadiene is used in the process according to the
invention.
In order to obtain a good resistance against high
temperatures, an article according to the invention must be
crosslinked at least in part. The crosslinking can take
place before, during or after the orientation of the
molecules in the article. So, if the 'gel route' is used,
the gel, for instance, can be crosslinked, or crosslinking
may take place during or after the drawing. Crosslinking
preferably takes place after the orientation of the
polyethylene and poly-l,4-butadiene molecules.
The crosslinking takes place according to processes
known in the art for the crosslinking of materials. This can
be done by radiation with r-rays or high-energy electrons,
or by the addition of crosslinking agents. Preference is
given to the use of electron radiation. The radiation dose
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WO91/17203 2 ~ 8191~ PCT/NL91/00072
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is preferably 1-150 kGy (Kilo Gray), particularly 10-100
kGy. The temperature at which radiation takes place is
important. At higher temperatures a higher degree of
crosslinking takes place. This temperature is -10 to 150Cr
preferably this temperature is 80-140C.
As the polyethylene (fibres, films, profiles, etc.)
according to the invention is crosslinked, it can not only
be used in known applications of oriented polyethylene, as
well as in such applications where resistance against creep,
compression strength, resistance against high temperatures
and fibrillation resistance are important. This is the case,
for instance, in load-bearing composites, for which fibres
according to the invention are highly suited as
reinforcement.
The invention will be elucidated below by means of
the following examples.
In examples I to III inclusive and in comparative
example A, the effect of the degree of radiation is
examined.
Example I:
The high molecular weight polyethylene used in this
example has a Mw of 1.5 x 106 g/mole and a Mn of 2 x 105
g/mole and is of the Hostalen ~ur-412r type of the firm of
Hoechst Ruhrchemie. Poly-trans-1,4-butadiene is prepared
according to G. Natta, M. Pegoraro and P. Cremonesi, Chim. e
Industria, 47, No. 7, 722 (1965). The poly-1,4-butadiene
thus prepared has a viscosity-average molecular weight Mv f
7.5 x 104 g/mole and a vinyl content of 0.9-1.2% (mole) and
a trans-1,4-butadiene content of 99.1-98.8% (mole).
1~ g high molecular weight polyethylene and 3 g
poly-1,4-butadiene are suspended in 1 dm3 xylene, with
di-t-butyl-p-cresol (DBPC) added as stabilizer in an amount
or 0.5% (wt) calculated on the high-molecular weight
polyethylene. The resulting suspension is devolatilized in
vacuum, subsequently saturated with nitrogen gas and heated
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WO91/17203 2 0 8191 8 PCT/NL91/00072
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in a silicone bath to about 120C. During the heating the
dispersion is stirred and a homogeneous dispersion is
formed. After some time the stirring is discontinued, upon
which the dispersion is kept at 130C for about 4 hours
until a homogeneous solution is obtained. This solution is
poured out into an aluminium dish, upon which the solution
lO is cooled to room temperature, at which gelling takes place.
The resulting film is air-dried and the stabilizer is
extracted at 23C using n~hexane. Subsequently the film
obtained is compressed for l hour at a pressure of 3 x 107
Pa.
Drawing
The film is cut into ribbons measuring 25 x 8 mm
and drawn at 100C over a hot plate. The draw ratio is
determined by marking a non-drawn sample at every other
20 millimeter into the direction of drawing and measuring the
distances between the marks before and after the drawing.
The draw ratio (~) is the quotient of the distances between
the marks after the drawing and the distances between these
marks before the drawing. If the drawing is to be
25 homogeneous, the distances between all marks, after the
drawing, must be about the same. In example I, drawing takes
place up to a draw ratio (~) of 40. The tensile strength
at break (o) of the (unradiated) ribbon obtained is l.0 GPa
and the elastic modulus (E) is 41 GPa.
Radiation
Electron radiation is carried out using a Van de
Graaf generator. The drawn sample is radiated with a bundle
of electrons with an eneryy of 3 MeV, with an amperage of
35 150 ~A, resulting in a radiation dose of 0.855 kGy
for every passage through the bundle. The sample is radiated
with a total dose of 60 kGy (70 passages) in a N2 atmosphere
at a temperature of 30C.
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WO91/17203 PCT/NL91/00072
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Determination of the high-temperature resistance
The followin~ temperature treatment is carried out.
The radiated sample is clamped in a framework at constant
length, upon which the framework is placed in an oven with a
temperature of 200~C. After 30 seconds, the framework is
removed from the oven. The sample does not break during this
treatment and no change in the cutward appearance of the
sample can be observed.
The results are shown in table 1.
Example II
In the same way as in example I a film is prepared,
but this time starting from 15 g high molecular weight
polyethylene as used in example I and 0.75 g
poly-1,4-butadiene. The film is cut into ribbons, the
ribbons are drawn and radiated as in example I. The results
are shown in table I.
Comparative example A
In the same way as in example I a film is prepared,
but this time starting only from 15 g high molecular weight
polyethylene. The film is cut into ribbons and the ribbons
are drawn as in example I. The tensile strength at break (~)
of the resulting (unradiated) ribbons is 1.5 GPa and the
modulus of elasticity (E) is 62 GPa. The ribbons are
subsequently radiated as in example I. After radiation, the
tensile strength at break ~a) is 0.9 GPa and the elastic
modulus (E) 62 GPa. In the temperature treatment as
described under example I, the ribbon shrinks and breaks
within a few seconds. The results are shown in table 1.
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WO91/17203 ~ PCT/NL91/00072
Table l: Effect of the amount of poly-l,4-butadiene
(1,4-PB) on the tensile strength at break (a) and
elastic modulus ~E) of radiated ribbons before and
after the temperature treatment (Ttr).
Example l,4-PB before Ttr a~ter Ttr
percentage o E a E
(%) (GPa) (GPa) (GPa) (GPa)
~ 1.1 42 0.6 15
II 5 1.1 49 0 . 8 13
A 0 0 . 9 62 - -*
~ The article breaks.
In examples III, IV and comparative example B the
effect of the radiation dose on the mechanical properties of
the ribbons is examined (no temperature treatment).
Example III
In the same way as in example I a film is prepared.
The film is cut into ribbons, the ribbons are drawn and
radiated as in example I, but the radiation dose is 20 kGy.
The mechanical properties of the ribbons obtained are shown
in table 2.
Example IV
In the same way as in example I a film is prepared.
The film is cut into ribbons, the ribbons are drawn and
radiated as in example I, but the radiation dose is lO0 kGy.
The mechanical properties of the ribbons obtained are shown
in table 2.
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WO91/17203 : 2 0 8 1 9 1 8 PCT/NL91/00072
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comparative example B
In the same way as in example I a film is prepared.
The film is cut into ribbons and drawn as in example I, but
no radiation takes place. The mechanical properties of the
ribbons obtained are shown in table 2.
Table 2: Effect of the radiation dose on the mechanical
properties of the ribbons obtained. The amount of
1,4-PB is 20% (wt), the draw ratio (~) is 40.
Example Radiation a E
dose
(kGy) (GPa) (GPa)
B 0 l.0 41
III 20 l.l 38
I 60 l.l 42
IV lO0 0.9 39
No real effect of the radiation dose on the
mechanical properties can be demonstrated.
In examples V to VIII inclusive and in comparative
example C the effect of the amount of poly-l,4-butadiene on
the mechanical properties of the ribbons is examined. The
samples are not radiated.
Exam~le V
In the same way as in example I a film is prepared,
but starting from 14.8; g polyethylene and 0.15 g
poly-1,4-butadiene. The film is cut into ribbons, the
ribbons are drawn as in example I. The mechanical properties
of the ribbons obtained are shown in table 3.
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WO91/17203 2~191 8 PCT/NL91/00072
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Example VI
In the same way as in example I a film is prepared,
but starting from 14.25 g polyethylene and 0.75 g
poly-1,4-butadiene. The film is cut into ribbons, the
ribbons are drawn as in example I. The mechanical properties
of the ribbons obtained are shown in table 3.
Example VII
In the same way as in example I a film is prepared,
but starting from 13.5 g polyethylene and 1.5 g
poly-1,4-butadiene. The film is cut into ribbons, the
lS ribbons are drawn as in example I. The mechanical properties
of the ribbons obtained are shown in table 3.
Example VIII
In the same way as in example I a film is prepared,
but starting from 10.05 g polyethylene and 4.95 g
poly-1,4-butadiene. The film is cut into ribbons, the
ribbons are drawn as in example I to a ratio (~) of 40.
Ribbons with a draw ratio (~) of 1 and 11 are prepared also.
The degree of orientation of poly-1,4-butadiene and
polyethylene molecules in these ribbons (~ = 1, 11
and 40) is determined by means of X-ray diffraction.
X-ray diffraction measurements (Wide Angle X-ray
Scattering (WA~S)) are carried out using a camera of the
Statton type. Ni-filtered CuK~-radiation was generated at
a voltage of 50 kV and an amperage of 30 mA. The distance
between sample and film was 50 mm. The W~XS photos are shown
in figure 1. A draw ratio ~ of 1 is marked (a), a draw ratio
of 11 is marked (b) and a draw ratio ~ of 40 is marked
(c). A point pattern as in figures lb and lc is indicative ~'
of a high degree of orientation of both polyethylene and
poly-1,4-butadie~e molecules. Without drawing, there is
hardly any orientation (figure la). The mechanical v/
properties of the ribbons obtained (at ~ = 40) are shown in
table 3.
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WO91/17203 2 0 819118 PCT/NL91/00072
Comparative example C
In the same way as in example I a film is prepared,
but starting from 7.5 g polyethylene and 7.5 g
poly-1,4-butadiene. The film is cut into ribbons, the
ribbons are drawn as in example I. However, with a draw
ratio (~) of 25, the article breaks. The results are shown
in table 3.
Table 3: The effect of the amount of poly-1,4-butadiene
(1,4-PB) on the properties (u and E) of the
non-radiated ribbons; the draw ratio (~) is 40.
Example Amount of ~ E
1,4-P~
(% (wt)) (GPa) (GPa)
A 0 1.5 52
V 1 1.4 52
VI S 1.2 49
VII 10 1.1 45
I 20 l.0 41
VIII 33 0.8 30
C 50 - *
* At ~ = 25, the article breaks.
Com~arative example D_and examples IX-XI
Preparation of fllaments
The high molecular weight polyethylene used in this
example has an intrinsic viscosity determined in decaline at
135C of 17 dl/g~ The poly-1,4-tubadienen is the same as
used in example 1.
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WO91/17203 2 0 81918 PCT/~L91/00072
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A blend of lO parts by weight poly-l,4-butadiene and 90
parts by weight polyethylene was suspended as decalin to a
lO weight % suspension. 0,25 weight % of di-t-butyl-p-cresol
(DBPC) was added to the suspension as a stabilizer. The
suspension was kneaded in a twin screw extruder having a
temperature of 210C producing a uniform and clear solution.
This solution was spun through an orifice having a diameter
of 0.8 mm. The spun filaments were cooled to 65C and
subsequently drawn in two stages in an air heated oven.
During the first stage the filaments were drawn 20 times (by
length) at 110C, during the seçond stage 4 times at 147C.
The decalin evaporated during the drawing procedure. The
filaments obtained have an elastic modulus E of 76,1 GPa and
a tensile strength o of 2,4 GPa at an elongation at break
of 3.5%.
Radiation
Electron radiation is carried out as in example l.
In comparative example D the filaments are not radiated (0
kGy). In example IX the radiation dose is 20 kGy, in example
X 50 kGy and in example XI lO0 kGy.
The mechanical properties of the irradiated
filaments are shown in table 4.
Table 4: mechanical properties of irradiated filaments
Example ~osis E (GPa) a (GPa) ~ (%)
(kGy)
D 0 76,1 2,4 3,5
IX 20 77,9 2,1 3,5
X 50 75,8 l,9 ~,l
XI lO0 81,1 1,3 3,2
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Determination of the_high-temperature resistance
I
The filaments were clamped in a framework upon
which the framework was placed in an oven with a temperature
of 170C. After 90 secunds, the framework is removed from
the oven. The filaments of comparative example D shrink and
break within a few seconds. The filaments of examples IX to
XI do not break and no change in appearance occured during
the heating treatment. The mechanical properties of the heat
treated filaments are shown in table 5.
Table 5: mechanical properties of inadiated filaments after
heating treatment
Example dosis E (GPa) a (GPa) ~ (%)
(kGy)
D 0 * _ _
IX 20 9,9 0,68 15,1
X 50 12,8 0,86 17,6
XI 100 13,3 0,75 13,4
* filaments were broken during the heating treatment
It is found that the elongation at break increased
by the heating treatment.
Retractive stress measurement
The retractive stress upon heating was determined
by clamping the filaments at constant length in a framework
of an apparatus for measuring tensile strength. The
framework was placed in an oven. The temperature was
increased from 20C to 200C. During the increase of
temperature the stress in the filaments was measured.
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The results are shown in figure 2. On the
horizontal (I) axis the temperature (C) is shown, on the
vertical (II) axis the retractive stress (MPa). Comparative
example D is marked ~, Example IX is marked O, Example X is
marked O and Example XI is marked ~ in this figure.
Comparative example D shows a sharp increase in retractive
stress. ~he filaments break at about 145C. The irradiated
filaments (examples IX-XI) show a moderate increase in
retractive stress and show a retractive stress at
temperatures of 150-200C that is comparable to the
retractive stress at 20-,110DC.
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