U.S. patent application number 09/971710 was filed with the patent office on 2002-08-01 for olefin polymers.
This patent application is currently assigned to BTG International Limited. Invention is credited to Bonner, Mark James, Hine, Peter John, Jones, Richard Albert, Ward, Ian MacMillan.
Application Number | 20020101009 09/971710 |
Document ID | / |
Family ID | 10800927 |
Filed Date | 2002-08-01 |
United States Patent
Application |
20020101009 |
Kind Code |
A1 |
Jones, Richard Albert ; et
al. |
August 1, 2002 |
Olefin polymers
Abstract
A polyolefin plaque is made by hot compaction of an assembly of
fibers of the oriented polymer. It has been found to be beneficial
to subject the fibers to a prior crosslinking process. Hot
compaction is then less temperature-sensitive and produces plaques
with excellent hot strength properties. Preferably the fibers have
been subjected to prior stages of irradiation and annealing, both
in a non-oxidising environment, for example acetylene.
Inventors: |
Jones, Richard Albert;
(Leeds, GB) ; Ward, Ian MacMillan; (Leeds, GB)
; Hine, Peter John; (Leeds, GB) ; Bonner, Mark
James; (Leeds, GB) |
Correspondence
Address: |
NIXON & VANDERHYE P.C.
8th Floor
1100 North Glebe Road
Arlington
VA
22201-4714
US
|
Assignee: |
BTG International Limited
|
Family ID: |
10800927 |
Appl. No.: |
09/971710 |
Filed: |
October 9, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09971710 |
Oct 9, 2001 |
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09285256 |
Apr 2, 1999 |
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09285256 |
Apr 2, 1999 |
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PCT/GB97/02675 |
Oct 6, 1997 |
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Current U.S.
Class: |
264/496 ;
264/258; 428/500 |
Current CPC
Class: |
D04H 1/54 20130101; B29B
13/08 20130101; D04H 1/44 20130101; B29C 43/006 20130101; Y10T
428/24826 20150115; B29C 2035/0827 20130101; Y10T 428/31855
20150401; B29C 2791/005 20130101; B29K 2105/25 20130101; Y10T
428/249942 20150401; Y10T 442/69 20150401; B29K 2023/12 20130101;
D04H 1/4291 20130101; B29K 2023/06 20130101; Y10T 428/24994
20150401; B29C 2035/085 20130101; B29K 2105/243 20130101; D04H
1/554 20130101; B29C 2035/0877 20130101; B29C 2071/022 20130101;
B29K 2223/06 20130101; B29K 2105/06 20130101; Y10T 428/249947
20150401 |
Class at
Publication: |
264/496 ;
264/258; 428/500 |
International
Class: |
B29C 031/00; B32B
031/04; B29C 035/08; B32B 027/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 4, 1996 |
GB |
9620692.5 |
Claims
1. A process for the production of a monolithic article in which
process an assembly of fibres of an polyolefin oriented polymer is
subjected to a compaction stage wherein the assembly of fibres is
maintained in intimate contact at an elevated temperature
sufficient to melt a proportion of the polymer, and is compressed,
characterised in that prior to the compaction stage the fibres have
been subjected to a crosslinking process.
2. A process as claimed in claim 1, wherein the compaction stage
comprises two distinct steps, namely a step of maintaining the
assembly of fibres in intimate contact at an elevated temperature
sufficient to melt a proportion of the fibre at a first, contact,
pressure, and a subsequent compression step wherein the assembly is
subjected to a second, compaction, pressure, higher than the
contact pressure.
3. A process as claimed in claim 1, wherein the compaction stage
comprises a single step of maintaining the assembly of fibres in
intimate contact at a given pressure and at an elevated temperature
sufficient to melt a proportion of the fibres.
4. A process as claimed in claim 1, wherein the crosslinking
process is an irradiation crosslinking process involving an
ionising step which comprises irradiating the fibres with an
ionising radiation, and then an annealing step comprising annealing
the irradiated polymer at an elevated temperature.
5. A process as claimed in claim 4, wherein the irradiation step is
carried out in an environment which is substantially free of oxygen
gas and which comprises a monomeric compound selected from alkynes,
and from alkenes having at least two double bonds.
6. A process as claimed in claim 5, wherein said environment
comprises acetylene.
7. A process as claimed in claim 4, wherein the annealing step
which follows irradiation is carried out in an environment which is
substantially free of oxygen gas and which comprises a monomeric
compound selected from alkynes, and from alkenes having at least
two double bonds.
8. A process as claimed in claim 7, wherein said environment
comprises acetylene.
9. A process as claimed in claim 1, wherein the fibres used in the
process as formed from molten polymer.
10. A process as claimed in claim 1, wherein the fibres have a
weight average molecular weight in the range 10,000 to 400,000.
11. A process as claimed in claim 10, wherein the fibres have a
weight average molecular weight in the range 50,000 to 200,000.
12. A process as claimed in claim 1, wherein the polyolefin polymer
is selected from the group comprising polypropylene homopolymer, a
copolymer containing a major proportion of polypropylene,
polyethylene homopolymer and a copolymer containing a major
proportion of polyethylene.
13. A process as claimed in claim 4, wherein the irradiation step
is effected at a temperature not exceeding 100.degree. C.
14. A process as claimed in claim 13, wherein the irradiation step
is effected at a temperature in the range 0-50.degree. C.
15. A process as claimed in claim 4, wherein the ionizing radiation
is selected from electron beam, ultra-violet and
.gamma.-radiation.
16. A process as claimed in claim 4, wherein the radiation dose is
in the range 0.5 to 100 MRads.
17. A process as claimed in claim 16, wherein the radiation dose is
in the range 2 to 20 MRads.
18. A process as claimed in claim 4, wherein the polyolefin polymer
is annealed at a temperature of at least 60.degree. C.
19. A process as claimed in claim 4, wherein the polyolefin polymer
after the irradiation and annealing steps has a gel fraction of at
least 0.4
20. A process as claimed in claim 19, wherein the polyolefin
polymer after the irradiation and annealing steps has a gel
fraction in the range 0.55 to 0.7.
21. A process as claimed in claim 2, wherein the contact pressure
is in the range 0.01 to 2 MPa, and the compaction pressure is in
the range 0.1 to 50 MPa, but is higher than the contact
pressure.
22. A process as claimed in claim 21, wherein the contact pressure
is in the range 0.3 to 0.7 MPa and the compaction pressure is in
the range 0.6 to 7 MPa, but is at least double the contact
pressure.
23. A process as claimed in claim 3, wherein the single pressure
applied is in the range 0.1 to 10 MPa.
24. A process as claimed in claim 1, wherein the proportion of the
polymer which melts during the compaction stage is 10 to 50% by
weight.
25. A process as claimed in claim 1, which process employs an
inorganic filler material, such that the filler is present in the
compacted monolithic article in an amount up to 60 vol % of the
article.
26. A polyolefin polymer monolith prepared in accordance with the
process of the invention, as claimed in claim 1.
Description
[0001] This invention relates to processes for the production of
polymer sheet materials from oriented olefin polymer fibres and to
the products of such processes.
[0002] GB 2253420B describes a process whereby an assembly of
fibres of an oriented polymer may be hot compacted to form a sheet
having good mechanical properties. The process involves an initial
processing step in which the fibres are brought to and held at the
compaction temperature whilst subject to a pressure sufficient to
maintain the fibres in contact, the contact pressure, and
thereafter compacted at a higher pressure for a few seconds, the
compaction pressure. In the process a proportion of the fibre
surfaces--most preferably from 5 to 10% by weight--melts and
subsequently recrystallises on cooling. This recrystallised phase
binds the fibres together. Preferred materials for use in this
process are homo- and co-polymers of polyethylene.
[0003] The process of GB 2253420B can be used to produce
complicated and precisely shaped monolithic articles having high
stiffness and strength, and good energy-absorbing properties.
However, a drawback of this process is the criticality of the
compaction temperature, especially for polyethylene. This is shown
by Comparative Example A in GB 2253420B.
[0004] In accordance with the present invention there is provided a
process for the production of a monolithic article in which process
an assembly of fibres of an oriented polyolefin polymer is
subjected to a compaction process wherein the assembly of fibres is
maintained in intimate contact at an elevated temperature
sufficient to melt a proportion of the polymer, and is compressed,
characterised in that prior to the compaction process the fibres
have been subjected to a crosslinking process.
[0005] In some embodiments (referred to herein as "2-step
compactions") the compaction process may comprise two distinct
steps, namely a step of maintaining the assembly of fibres in
intimate contact at an elevated temperature sufficient to melt a
proportion of the fibre at a first, contact, pressure, and a
subsequent compression step wherein the assembly is subjected to a
second, compaction, pressure, higher than the contact pressure--as
in GB 2253420B.
[0006] In some embodiments (referred to herein as "1-step
compactions") the compaction process may comprise a single step of
maintaining the assembly of fibres in intimate contact at an
elevated temperature sufficient to melt a proportion of the fibre,
and at a given pressure. In such embodiments there is no subsequent
step of applying a higher pressure.
[0007] Preferably the monolithic article is an article which is
shape stable under its own weight, such as a plaque.
[0008] The crosslinking process may be a chemical crosslinking
process, involving the use of a chemical reagent which forms
reactive radicals under predetermined initiation conditions.
Suitably the reagent may be a cumene compound, or a peroxide, for
example DMTBH or DCP, or a silane, for example a vinyl silane,
preferably vinylmethoxy silane.
[0009] The crosslinking process may be an irradiation crosslinking
process involving an ionising step comprising irradiating the
fibres with an ionising radiation, and then an annealing step
comprising annealing the irradiated polymer at an elevated
temperature.
[0010] For general information on known crosslinking processes,
reference may be made to Sultan & Palmlof, "Advances in
Crosslinking Technology", Plast. Rubb. and Comp. Process and Appl.,
21, 2, pp- 65-73 (1994), and to the references therein.
[0011] Irradiation crosslinking is believed to be particularly
suitable, for the process of the present application.
[0012] The pre-compaction process of crosslinking has been found to
increase the "temperature window" available for the subsequent
compaction stage, and thus to make the compaction stage much easier
to control. Further, compacted products produced by the process of
the present invention have exhibited superior hot strength
properties, compared with compacted products made from fibres which
have not been subject to prior crosslinking.
[0013] The term "fibres" is used herein in a broad sense to denote
strands of polyolefin polymer, however formed. The fibres subjected
to prior crosslinking may be non-woven fibres laid in a web, or may
be comprised within yarns, or constituted by bands or fibrillated
tapes, for example formed by slitting films. If comprised within
yarns or constituted by bands or fibrillated tapes, those yarns,
bands or fibrillated tapes may be laid together or they may be
formed into a fabric, for example by weaving or knitting.
[0014] Suitably the fibres used in the process of the invention are
formed from molten polymer, for example as melt spun filaments.
[0015] Preferably the fibres used in the present invention have a
weight average molecular weight in the range 10,000 to 400,000,
preferably 50,000 to 200,000.
[0016] The polyolefin polymer can be selected from polyethylene,
polypropylene or polybutylene, or copolymers comprising at least
one of those olefin polymers. The polyolefin polymer used in the
process of the present invention may suitably be a polypropylene
homopolymer or a copolymer containing a major proportion of
polypropylene. Advantageously it may be a polyethylene homopolymer
or a copolymer containing a major proportion of polyethylene.
[0017] A polyethylene copolymer comprising a major proportion of
polyethylene as defined herein is one comprising more than 50% by
weight of polyethylene. Preferably, it comprises more than 70% by
weight of polyethylene, most preferably, more than 85% by weight of
polyethylene.
[0018] A polyethylene polymer as defined herein may be
unsubstituted, or substituted, for example by halogen atoms,
preferably fluorine or chlorine atoms. Unsubstituted polyethylene
polymers are however preferred.
[0019] A polyethylene copolymer comprising a major proportion of
polyethylene may have one or more different copolymers, following
copolymerisation of ethylene with, for example, one or more of
propylene, butylene, butadiene, vinyl chloride, styrene or
tetrafluoroethylene. Such a polyethylene copolymer may be a random
copolymer, or a block or graft copolymer. A preferred polyethylene
copolymer is a ethylene-propylene copolymer, having a major
proportion of polyethylene and a minor proportion of
polypropylene.
[0020] A polypropylene copolymer comprising a major proportion of
polypropylene as defined herein is one comprising more than 50% by
weight of polypropylene. Preferably, it comprises more than 70% by
weight of polypropylene, most preferably, more than 85% by weight
of polypropylene.
[0021] A polypropylene polymer as defined herein may be
unsubstituted, or substituted, for example by halogen atoms,
preferably fluorine or chlorine atoms. Unsubstituted polypropylene
polymers are however preferred.
[0022] A polypropylene copolymer comprising a major proportion of
polypropylene may have one or more different copolymers, following
copolymerisation of propylene with, for example, one or more of
ethylene, butylene, butadiene, vinyl chloride, styrene or
tetrafluoroethylene. Such a polypropylene copolymer may be a random
copolymer, or a block or graft copolymer. A preferred polypropylene
copolymer is a propylene-ethylene copolymer, having a major
proportion of polypropylene and a minor proportion of
polyethylene.
[0023] It is essential in the practice of the present invention
that the process employs fibres which have been subjected to a
crosslinking process. However, the co-use of a polymer component
(not necessarily a polyolefin) which has not been subjected to a
crosslinking process, and/or of an inorganic filler material, is
not excluded.
[0024] A polymer which has not been subjected to a crosslinking
process may, when present, be present in an amount up to 50 vol %
of the total polymer content of the article. Preferably, however,
substantially the entire polymer content of the article derives
from polyolefin polymer which has been subject to a crosslinking
process.
[0025] An inorganic filler material may be present. An inorganic
filler, when present, may be present in an amount up to 60 vol % of
the article, preferably 20 to 50 vol %. An inorganic filler
material may, for example, be selected from silica, talc, mica,
graphite, metal oxides, carbonates and hydroxides and apatite, for
example hydroxyapatite, a biocompatible calcium phosphate
ceramic.
[0026] The preferred crosslinking process, involving irradiation
crosslinking, will now be further defined.
[0027] Preferably, the environment for the annealing step which
follows irradiation is a gaseous environment.
[0028] Preferably the irradiation step is effected in an
environment which is substantially free of oxygen gas. For example
it could be performed in vacuo or in the presence of an inert
liquid or gas. Preferably however the environment for the
irradiation step comprises a monomeric compound selected from
alkynes, and from alkenes having at least two double bonds.
[0029] Preferably, the annealing step which follows irradiation is
carried out in an environment which is substantially free of oxygen
gas but which comprises a monomeric compound selected from alkynes,
and from alkenes having at least two double bonds.
[0030] There is no necessity for the environments to be the same,
in the irradiation and annealing steps. Indeed there is no
necessity for said monomeric compound used during the irradiation
step to be the same as said monomeric compound used in the
annealing step; the monomeric compound used in the irradiation step
could be replaced in whole or in part by a different monomeric
compound for the annealing step. However it is believed that the
properties desired of said monomeric compound in the irradiation
step will generally be the same as those required in the annealing
step, so there will generally be no necessity to effect a whole or
partial replacement. Most conveniently, therefore, the monomeric
compound is the same throughout. In some cases however it may be
advantageous to supply a further charge of said monomeric compound,
as the process proceeds.
[0031] For either or both steps, a mixture of monomeric compounds
could be employed.
[0032] The environment employed for the irradiation and/or the
annealing steps is preferably constituted entirely by said
monomeric compound, but may also comprise a mixture being said
monomeric compound together with one or more other components, for
example an inert gas or liquid. Suitably the said monomeric
compound is gaseous at least under the treatment conditions
employed and is employed in the irradiation and/or annealing steps
at a pressure, or partial pressure in the case of a mixture, in the
range of 0.2-4 atmospheres (2.times.10.sup.4 Pa - 4.times.10.sup.5
Pa), preferably 0.5-2 atmospheres (5.times.10.sup.4 Pa -
2.times.10.sup.5 Pa), most preferably 0.3-1 atmospheres
(3.times.10.sup.4 Pa - 1.times.10.sup.5 Pa).
[0033] Preferred monomeric compounds for use in the present
invention, in either or both of the irradiation and annealing
steps, are alkynes, and alkenes having at least two double bonds,
which alkenes are not substituted by halogen atoms. They are
desirably gaseous under the treatment conditions employed and
should be able to diffuse into the polyolefin polymer under the
treatment conditions employed. Preferred are unsubstituted alkynes
or alkenes i.e. alkynes or alkenes made up substantially entirely
by hydrogen and carbon atoms. Examples are unsubstituted C.sub.2-6
alkynes, preferably having only one triple bond, for example
acetylene, methyl acetylene, dimethyl acetylene and ethyl acetylene
(of which species acetylene is preferred) and unsubstituted
C.sub.4-8 alkenes having at least two double bonds, preferably only
two double bonds, for example 1,3-butadiene, 1,3-pentadiene,
1,3-hexadiene, 1,4-hexadiene and 1,3,5-hexatriene (of which species
1,3-butadiene is preferred).
[0034] One preferred class of alkenes for use in the present
invention has at least two conjugated double bonds, thus including
1,3-butadiene, 1,3-pentadiene, 1,3-hexadiene and 1,3,5-hexatriene.
Preferably the conjugation extends throughout the length of the
compound, as is the case with 1,3-butadiene and
1,3,5-hexatriene.
[0035] Another preferred class of alkenes for use with the present
invention has double bonds at least as the terminal bonds in the
compounds, thus including 1,3-butadiene and 1,3,5-hexatriene.
[0036] A particularly preferred class of alkenes has at least two
conjugated double bonds, preferably with the conjugation extending
throughout the length of the compounds, and double bonds at least
as the terminal bonds of the compounds. Compounds of this type thus
include 1,3-butadiene and 1,3,5-hexatriene.
[0037] Preferably said alkyne or alkene having at least two double
bonds is the sole crosslinking agent employed in the irradiation
step and/or the annealing step.
[0038] Acetylene is an especially preferred monomeric compound for
use in the present invention. Preferably acetylene is used as
substantially the sole said monomeric compound both in the
irradiation step and in the annealing step.
[0039] Suitably the irradiation step is effected at a temperature
not exceeding 100.degree. C., preferably not exceeding 80.degree.
C. A preferred range is 0-50.degree. C., most preferably
15-30.degree. C. Conveniently the step is effected at ambient
temperature.
[0040] In carrying out the process of this invention, any ionizing
radiation can be employed. In practice, however, the types of
ionizing radiation which can be used with greatest practicality are
electron beams, ultra-violet radiation and, especially,
.gamma.-rays.
[0041] The radiation dose is suitably in the range 0.5 to 100 MRads
inclusive, preferably 1 to 50 MRads inclusive, most preferably 2 to
20 MRads inclusive. For many applications a radiation dose of 3 to
10 MRads inclusive appears very favourable.
[0042] Preferably the polyolefin polymer is annealed at a
temperature of at least 60.degree. C., preferably at a temperature
in the range 80 to 120.degree. C. inclusive.
[0043] Preferably the polyolefin polymer is annealed at an
annealing temperature at least 20.degree. C. below its melting
point, most preferably at an annealing temperature which is below
its melting point by a temperature differential in the range 20 to
50.degree. C., inclusive, most preferably, 30 to 40.degree. C.,
inclusive.
[0044] The period for which annealing is carried out is not thought
to be critical, provided that the time is sufficient for
substantially all of the polymer which has been irradiated to reach
the said annealing temperature and for substantially all of the
radicals formed to have reacted. This can be assessed by trial and
error following ESR or mechanical testing of irradiated and
annealed samples; the presence of unreacted radicals is believed to
lead to chain scission and diminution in mechanical properties.
[0045] Suitably the polyolefin polymer after the irradiation and
annealing steps has a gel fraction at least 0.4, preferably at
least 0.5. Preferably the polyolefin polymer after the irradiation
and annealing steps has a gel fraction no greater than 0.85,
preferably no greater than 0.75. A particularly preferred gel
fraction is in the range 0.55 to 0.7, most preferably 0.6 to
0.65.
[0046] In accordance with the present invention there is provided a
polyolefin polymer monolith prepared in accordance with the process
of the invention, as defined above.
[0047] In relation to the compaction stage which follows the steps
of irradiation and annealing, the description in GB 2253420B is in
general applicable to the modified process of the present
invention, for example in relation to treatment times, temperature,
proportion of material which is to melt, the assembly of the fibres
and molecular weights, and the description of GB 2253420B may be
regarded as incorporated into the present specification by
reference, insofar as it applies to the production of polyolefin
articles. However, the preferred pressure conditions are different,
and are set out below.
[0048] In 2-step compactions in accordance with the present
invention the contact pressure is suitably in the range 0.01 to 2
MPa, preferably 0.1 to 1 MPa, most preferably 0.3 to 0.7 MPa; and
the compaction pressure is suitably in the range 0.1 to 50 MPa,
preferably 0.2 to 10 MPa, most preferably 0.3 to 7 MPa. In such
2-stage processes the compaction pressure should be higher than the
contact pressure, suitably by a factor of at least 2, and
preferably by a factor of at least 4.
[0049] In 1-step compactions in accordance with the present
invention the (single) pressure applied is suitably in the range
0.1 to 10 MPa, preferably 0.2 to 5 MPa, most preferably 0.3 to 4
MPa.
[0050] It is especially preferred that the pressure, or maximum
pressure, is such that the process may be carried out in an
autoclave, or in a belt press or other apparatus in which the
assembly is fed through a compaction zone in which it is subjected
to said elevated temperature and said pressure.
[0051] A further difference between the present invention and that
of GB 2253420B is that in the process of the invention the
proportion of the polymer which melts is suitably at least 10% by
weight, preferably 10 to 50% by weight.
[0052] The invention will now be further described, by way of
example, with reference to the examples which follow.
Samples
[0053] In the first examples, the materials used were commercially
available oriented melt spun homopolymeric polyethylene, sold as a
1800 denier multifilament yarn under the Trade Mark TENFOR, and as
a 250 denier multifilament yarn, under the Trade Mark CERTRAN. The
polyethylene of TENFOR and CERTRAN is of the same grade, and is
characterised as follows:
1 Tensile Modulus Breaking Initial Molecular Weight Strength Secant
2% Mw Mn Process GPa GPa GPa 130,000 12,000 melt spun 1.3 58 43
Pre-treatment and Annealing
[0054] A bobbin of the TENFOR polyethylene was subjected to
.gamma.-radiation at a low dose rate (less than 0.1 MRad/hour) at
ambient temperature under an atmosphere of acetylene at a pressure
of about 5.times.10.sup.4 Pa above atmospheric. The total dose was
7.39 MRad. After irradiation the sample was annealed for 2 hours at
110.degree. C. under an atmosphere of acetylene at a pressure of
5.times.10.sup.4 Pa above atmospheric. The resultant gel content
was 80.9%. Physical properties of the resultant irradiated and
annealed fibre were as follows:
[0055] Density: 979.3.+-.0.1 kg/m.sup.3
[0056] Tensile modulus: 30.+-.1 GPa (fibre straight off the bobbin;
1%/min)
[0057] Tensile strength: 0.82 GPa.+-.0.01 GPa (fibre straight off
the bobbin; 1%/min)
[0058] The CERTRAN polyethylene yarns, ten 3 g hanks, were wound
onto glass tubes and were subjected to electron beam irradiation at
a fast dose rate (0.655 MRad/min) for 7 minutes at ambient
temperature under an atmosphere of acetylene at 3.times.10.sup.4 Pa
above atmospheric pressure. Because of the high dose rate the
temperature of the sample increased during irradiation. No external
temperature control was used. The total dose was about 4.6 MRad.
After irradiation the sample was annealed for 2 hours at 90.degree.
C. under an atmosphere of acetylene at 3.times.10.sup.4 Pa above
atmospheric pressure. Consequently eight samples with gel contents
between 69.4% and 71.8% were produced with the remaining two
samples having gel contents of 77% and 84%.
[0059] Preliminary DSC experiments were performed on the irradiated
and annealed TENFOR and CERTRAN yarns, in comparison to untreated
fibres, to assess melting thereof. Partial melting experiments were
carried out, in which the yarns were held for ten minutes at a
given SOAK temperature before being cooled and scanned. This
allowed a comparison of the normalised area of the DSC melting peak
for different amounts of partial melting, and hence gave an
indication of the amount of fibre which was destroyed by melting,
at that temperature. FIG. 1 is a graph in which the results for
TENFOR are plotted, and it will be seen that the unirradiated yarn
shows a rapid fall off in fibre content between 136.degree. C. and
138.degree. C. In contrast, the fall off in fibre content of the
irradiated and annealed fibre was gradual, with some material being
retained unmelted up to 144.degree. C. Corresponding testing was
carried out on CERTRAN polyethylene having the 77% gel content, and
this yielded a similar result, as shown in FIG. 2., The results
from these preliminary tests indicated that the irradiated and
annealed yarns should have a wider range of temperatures over which
they could be compacted, than the corresponding untreated yarns. In
particular, the lack of a sudden drop in fibre content indicated
that the compaction process should be much more controllable, with
the yarns crosslinked by irradiation and annealing.
[0060] Gel contents were measured by the method as described in the
Journal of Polymer Science: Part B: Polymer Physics, 1993, Vol. 31,
p. 809 (R. A. Jones, G. A. Salmon and I. M. Ward).
Compaction
[0061] Hot compacted samples were prepared from the TENFOR
irradiated and annealed yarns in the following way. The TENFOR was
wound around a U-shaped former 80 mm wide and with a traverse of 55
mm, as shown in FIG. 3. The winding of the yarn onto the former was
carefully controlled so that there were 100 windings of the yarn
over the 55 mm traverse, this being achieved by revolving the
former, and by a yarn translation mechanism, as shown schematically
in FIG. 4. The yarn was thereby wound neatly, uniformly and
unidirectionally. Several traverses were carried out so that there
were several layers of yarn, generally 6, about the former, this
number leading to compacted plaques approximately 2.25 mm thick.
The wound yarn was then laid over a mould 55 mm square, and having
two side walls and two open ends, as shown in FIGS. 5-7. A lid 55
mm square was then fitted over the mould, against the wound yarn,
and the former was then removed. The mould assembly was then
subjected to an elevated temperature at a first pressure, the
contact pressure, for a set period of time, the contact time, and
then subjected to a second, higher, pressure, the compaction
pressure, for a shorter period of time, at the same temperature.
For all samples cooling was initiated as soon as the compaction
pressure was applied, thus controlling a temperature rise of up to
2.degree. C. upon compaction. At 144.degree. C. and above the said
pressure increased naturally the contact time, and no attempt was
made to prevent this.
[0062] In interpreting the results of the later compaction tests it
should be noted that superheating effects due to constraint raise
the optimum temperature by about 4.degree. C., compared with the
DSC experiments described earlier.
[0063] Table 1 below sets out the processing regimes for the TENFOR
samples.
2TABLE 1 Compaction Methods For Crosslinked Tenfor Plaques Contact
N.sup.o Contact Compaction Temp Time of Layers Pressure Pressure
Pressure (.degree. C.) (mins) Of Fibre (MPa) (MPa) Build 140 15,
10, 5, 2 6 0.35 34 No 141 15, 5 6 0.35 34 No 143 15 6 0.35 34 No
145 15 6 0.35 17 Yes 148 15 6 0.35 17 Yes 150 15 4 0.35 8.4 Yes 152
15 6 0.35 3.5 Yes
[0064] In subsequent experiments it did not prove possible to
achieve compaction in tests performed with contact pressures
significantly above 0.35 MPa at a temperature of 140.degree. C. In
experiments performed at contact pressures of 0.52 and 0.7 MPa and
a temperature of 140.degree. C. the fibre in the plaques did not
appear to have to undergone any melting, the plaques could be
broken quite easily by hand along the yarn axis, and the yarn was
still clearly visible.
[0065] Samples were prepared in a similar manner with irradiated
and annealed CERTRAN yarn, but using 32 layers of windings, because
of the lower denier of CERTRAN.
[0066] Table 2 below sets out the compaction regimes for the
CERTRAN fibres.
3TABLE 2 Methods Used To Produce Compacted Plaques of Crosslinked
Certran Contact Contact Compaction Temp Time Number Pressure
Pressure (.degree. C.) (mins) Of Layers (MPa) (MPa) 141 10 32 0.35
37 145 10 32 0.35 17 150 10 32 0.35 10
Testing
[0067] The flexural modulus and flexural strength of the samples in
the yarn direction or perpendicular to the yarn direction, as
appropriate, were measured.
[0068] Due to the size limitation of the plaques the tests could
not be carried out exactly to ASTM standards. The equipment used
was compliant with ASTM D790, and three point bending was used as
described in method 1 of this standard. The dimensions of the
specimens tested were:
4 Longitudinal Span 40 mm Width 5 mm Thickness governed by plaque
Transverse Span 30 mm Width 10 mm Thickness governed by plaque
[0069] In general this means-that the ratio of thickness to span
was around 16 to 1. In all cases the rate of crosshead motion was 1
mm per minute.
[0070] Initial longitudinal flexural modulus was determined by
taking the slope of the initial part of the output curve and using
the following formula. 1 Modulus = ( Load Deflection ) .times. span
3 4 .times. thickness 3 .times. width
[0071] Flexural strength was determined by taking the peak load
before breaking and using the following formula. 2 Strength = 6
.times. load .times. span 4 .times. width .times. thickness 2
Properties of Compacted TENFOR
[0072] All of the measured mechanical properties are shown in Table
3 below and FIGS. 8, 9 or 10 set out the longitudinal flexural
modulus, longitudinal flexural strength and transverse flexural
strength of samples given a 15 period at the contact pressure at
the stated temperatures.
5TABLE 3 Mechanical Properties of Crosslinked Tenfor Plaques
Compaction Longitudinal Longitudinal Transverse Method Flexural
Flexural Flexural (Contact Modulus Strength Strength Density time;
.degree. C.) (GPa) (MPa) (MPa) (Kg/m.sup.3) 2 min 140 12.9 152 19
-- 5 min 140 19.7 139 20 -- 10 min 140 19.5 158 27 -- 15 min 140
19.5 146 11 973.3 15 min 141 17.0 124 18 973.1 15 min 143 13.1 151
12 972.4 15 min 145 11.8 141 18 965.9 15 min 148 7.0 -- -- 964.7 15
min 150 2.1 125 31.2 963.1 15 min 152 2.25 132 32.6 962.4
[0073] For a fixed contact time of 15 minutes the longitudinal
flexural modulus decreases steadily from 19.5 GPa at 140.degree. C.
to 2.5 GPa at 152.degree. C., FIG. 8. The longitudinal flexural
strengths remain relatively constant from 140.degree. C. to
152.degree. C., at around 140 MPa, FIG. 9. The transverse flexural
strengths are constant around 15 MPa from 140.degree. C. to
145.degree. C., and then increase with increasing temperature, FIG.
10.
[0074] Clearly, the properties of the plaques vary with temperature
and, based on the compactions at 140.degree. C., contact time. The
optimum contact time can be determined by trial and error. The
optimum temperature will depend on the properties required.
However, the results make it clear that, as anticipated by the DSC
work described earlier, there is a reasonable degree of latitude,
in terms of the temperature, in carrying out compaction. Within the
12.degree. C. range in the tests, there were no abrupt fall-offs in
properties of the plaques, as would be expected with corresponding
compaction of unirradiated yarns, using the method as described in
GB 2253420B.
Comparative Tests on TENFOR
[0075] Further experiments were carried out to compare the
properties of plaques moulded from crosslinked and normal
(non-crosslinked) TENFOR polyethylene.
[0076] The data for normal TENFOR was gathered using the same
method as for the crosslinked TENFOR. The only differences were
that the contact pressure was 0.7 MPa as compared to 0.35 MPa for
the crosslinked material, and the contact time was 10 minutes
instead of 15 minutes.
[0077] The results are set out in Table 4 below. This mentions
degrees Celsius above the onset of compaction, instead of giving
absolute temperature values. Temperature is indicated in this way
because the crosslinked and non-crosslinked materials started to
compact at different temperatures. We believe this is due to
crosslinking delaying the onset of melting.
6TABLE 4 Comparison of Mechanical Properties of Crosslinked and
Non-crosslinked TENFOR Degrees Celsius above onset of
Non-crosslinked Crosslinked compaction (GPa) (GPa) a) Longitudinal
Flexural Modulus Onset 17.5 19.5 1 15.5 17.0 2 16.7 3 completely
melted 13.1 5 11.8 8 7.0 10 2.1 Degrees Celsius above onset of
Non-crosslinked Crosslinked compaction (MPa) (MPa) b) Longitudinal
Flexural Strength Onset 139.5 146 1 108.6 124 2 117.8 3 completely
melted 151 5 141 8 10 125 c) Transverse Flexural Strength Onset
23.5 11.0 1 38.5 18.0 2 38.5 3 completely melted 12.0 5 18.0 8 10
31.2
Properties of Compacted CERTRAN
[0078] The mechanical properties of the plaques compacted from the
irradiated and annealed CERTRAN fibres were tested in the same way
as described above for plaques of TENFOR fibres. The results are
shown in Table 5 below.
7TABLE 5 Mechanical Properties of Irradiated CERTRAN Plaques
Longitudinal Longitudinal Transverse Compaction Flexural Flexural
Flexural Temperature Modulus Strength Strength (.degree. C.) (GPa)
(MPa) (MPa) 141 11.3 20.8 145 5.7 127 25.5 150 4.9 111 30.2
[0079] The lower than expected modulus results, when combined with
the relatively high transverse strengths and the observation that
more material extruded from the mould than in the case of the
TENFOR suggests that more fibre was destroyed by melting, than with
the TENFOR. Thus it was concluded that compaction conditions are
slightly different than for TENFOR, and a higher contact pressure
must be used.
[0080] Regrettably all the irradiated and annealed CERTRAN had been
used and no further compaction could be attempted. However, even
these preliminary results indicate that a wide "temperature window"
should be available for compaction.
Elevated Temperature Properties - CERTRAN
[0081] Plaques made from crosslinked and non-crosslinked CERTRAN
were tested for their mechanical properties at elevated
temperature.
[0082] A. Properties of 63% gel fraction materials.
[0083] A piece of woven CERTRAN cloth 51 cm long by 13 cm wide was
wound around a former. For irradiation it was placed in a glass
tube and irradiation was carried out by electron beam. The electron
beam irradiation was to a total dose of 4.5 MRad, in an acetylene
environment at a pressure of 3.times.10.sup.4 Pa above atmospheric,
at ambient temperature. The polymer was then annealed for 2 hours
at 90.degree. C. in acetylene, at a pressure of 3.times.10.sup.4 Pa
above atmospheric. The resultant gel fraction was 63%. The treated
cloth was then compacted using a two-stage process as described
above, at a temperature of 144.degree. C., with a contact
time/pressure of 10 minutes/0.35 MPa, followed by compaction at 7
MPa. Dumbbell-shaped samples of 26 mm gauge length were cut from it
for tensile testing. The results of these tensile tests, carried
out over a range of temperatures, and performed at a nominal strain
rate of 20% per minute, are shown in FIGS. 11 and 12. It will be
seen that the crosslinked CERTRAN plaques have better hot
temperature failure strength, than the non-crosslinked plaques.
FIGS. 13 and 14 show stress-strain curves of the 63% gel fraction
crosslinked material and the untreated fibre respectively over a
range of temperatures. It is seen that the cross linking
substantially changes the stress-strain behaviour.
[0084] B. Properties of 85% gel fraction materials.
[0085] FIG. 15 shows the tensile failure strengths of samples
prepared from 85% gel fraction CERTRAN yarn. The yarn had been
irradiated by .gamma. radiation to a total dose of 5 MRad, in
acetylene at a pressure of 5.times.10.sup.4 Pa atmospheric, at
ambient temperature; then annealed for 2 hours at 90.degree. C. in
acetylene, at the same pressure. The treated polymer was then
compacted using a two-stage process as described above, at a
temperature of 144.degree. C. with a contact time/pressure of 10
minutes/0.35 MPa, and a compaction pressure of 7 MPa. The plaques
thus formed were tested at different temperatures, and compared to
untreated samples. It will be seen that the pre-irradiated plaques
have substantially better failure strengths at elevated
temperatures. In this case those plaques had lower failure strength
at 20.degree. C. This is thought to be due to the fact that the gel
fraction, at 85%, is higher than is optimal.
Single Stage Compaction - CERTRAN
[0086] 10 kg of woven CERTRAN cloth, 107 cm wide, was placed in a
purpose built cylinder, which was first evacuated and then
pressurised to 6.times.10.sup.4 Pa with acetylene. Irradiation was
carried out using .gamma. radiation to a dose of 1.8 MRad at
ambient temperature. After irradiation the cylinder and cloth were
repressurised to 6.times.10.sup.4 Pa with acetylene and annealed at
90.degree. C. for 8 hours. After annealing the cloth was removed
from the cylinder and scoured to remove any by-products of the
crosslinking process. In addition it was found that optimum
compaction was achieved if the surface of the cloth was lightly
abraded over a sandpaper covered roller to further remove any
crosslinking by-products. The gel content of the crosslinked cloth
was measured as 62%.
[0087] Compaction experiments were carried out at 144.degree. C.
using a single stage process with a pressure of 0.7 MPa used
throughout. Dumbbell samples were cut from the compacted sheets and
tensile modulus and strength were determined at 20.degree. C. and
80.degree. C. Table 6 below shows a comparison of these results
with results for compacted crosslinked CERTRAN cloth using a two
stage process (compaction temperature 144.degree. C., contact
pressure 0.35 MPa and compaction pressure 7 MPa, also lightly
abraded before compaction) and results for compacted
non-crosslinked CERTRAN cloth (compaction temperature 139.degree.
C., contact pressure 0.7 MPa and compaction pressure 7 MPa,
non-abraded).
8TABLE 6 Non-crosslinked Crosslinked Crosslinked Compaction
Temperature 139.degree. C. 144.degree. C. 144.degree. C. Contact
pressure 0.7 MPa 0.35 MPa 0.7 MPa Compaction pressure 7 MPa 7 MPa
0.7 MPa Tensile Modulus (20.degree. C.) 10.0 GPa 9.8 GPa 10.1 GPa
Tensile Strength (20.degree. C.) 200 MPa 200 MPa 220 MPa Tensile
Modulus (80.degree. C.) 2.7 GPa 5.3 GPa not measured Tensile
Strength (80.degree. C.) 20 MPa 100 MPa 107 MPa
[0088] In general it may reasonably be concluded from the totality
of the experiments carried out, that the properties of compacted
plaques prepared from polyolefin given a pre-treatment of
irradiation and annealing in order to achieve crosslinking are more
controllable due to the wider melting range, and exhibit certain
improvements in mechanical properties, particularly failure
strength at elevated temperatures.
* * * * *