Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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CROSSLINKED COMPOSITIONS CONTAINING SILANE-MODIFIED POLYOLEFIN
BLENDS
FIELD OF THE INVENTION
[0001] The present invention relates to polymeric compositions and theiruses,
and more
particularly to crosslinked compositions of silane-modified blends of
polyolefins, more
specifically polyolefins such as polyethylene with homopolymers and/or
copolymers of
propylene and other higher olefins, and their uses as heat shrinkable coating
and insulating
materials, and as wire and cable irisulation materials, but not necessarily
restricted thereto.
BACKGROUND OF THE INVENTION
[0002] Polyolefins derived from propylene and other higher olefins are ideally
suited to the
preparation of coatings and insulations designed for use at operating
temperatures in excess
of those that can be withstood by other polyolefins, for example,
polyethylene, which exhibit
lower softening and melting temperatures, or do not retain suitable physical
properties at
highertemperatures. Other attractive features are their high rigidity and
toughness, low cost
and relatively low density. Applications for these coatings and insulations
would include
polymeric insulation for electrical wires and cables, and heat-shrinkable
protective sleeves for
high-temperature transmission pipelines, or for applications requiring greater
heat distortion
resistance, toughness and rigidity than is afforded by polyethylene-based
systems. For
example, heat-shrinkable sleeves used for the corrosion protection of high
temperature
pipeline joints are required to maintain dimensional stability, toughness and
integrity at the
operating temperature of the pipeline._ Hence it is necessary to use a
material, such as
polypropylene, with a softening ternperature or melting point higher than the
continuous
operating temperature of the pipeline to prevent creeping or sagging of the
sleeve from the
pipe at this temperature.
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[0003] Also, in orderto maximise heat-resistance, hot deformation resistance,
and physical
properties, such as is required for high temperature electrical insulations,
it is necessary to
impart some thermoset characteristic to the material. This is done by
crosslinking the polymer
to some required degree. Crosslinking is also necessaryforthe production of
heat-shrinkable
articles to impart controlled shrinkage characteristics. The aim of this
invention is to provide
a means of preparing crosslinked, predominantly polypropylene-based materials,
which can
be used in the applications described, but not necessarily restricted thereto.
[0004] Polymers in which the predominant chain units comprise propylene or
higherolefrns,
such as butene, are known to preferentially depolymerise when exposed to free
radicals
required to effect crosslinking. Hence, unlike similar materials, namely
polyolefins such as
polyethylene and copolymers of polyethylene, it is not possible to directly
crosslink these
materials to satisfactory levels, as is required, for example, in the
production of wire and cable
insulations, and heat-shrinkable articles, such as tubing, sheet, and moulded
shapes, by using
standard free-radical methods of crosslinking, such as electron beam
irradiation or peroxide
initiated crosslinking.
[0005] It is also a well known process to produce crosslinked polyolefins, and
articles made
-therefrom, by grafting a vinyl silane onto an olefin homopolymer or copolymer
such as is
described in US Patent No. 3,646,155. Alternatively, the vinyl silane may be
copolymerised
directly with olefin monomers as described in US Patent No. 4,413,066, for
example.
However, since these methods require a free-radical generatorto initiate the
grafting reaction,
the polypropylene or higher polyolefin by itself is unsuited to this method of
crosslinking.
Hence, it is therefore necessary to resort to alternative strategies to create
crosslinked
compositions consisting predominantly of polypropylene or higher polyolefins
of the type that
preferentially degrade in the presence of free-radical generators.
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SUMMARY OF THE INVENTION
[0006] The present invention overcomes the above-discussed problems of the
prior art by
providing moisture crosslinkable blends of predominantly polypropylene, or
higher polyolefins
such as polybutene or polymethylpentene, with silane-modified polyolefins;
more specifically
of silane-modified blends of polyethylenes or polyethylene copolymers with
polypropylene
copolymers or homopolymers, with or without an additional material added as a
compatibilising agent for the polyolefin and polypropylene.
[0007] In the method of the present invention, one or more polypropylene
homopolymers,
or copolymers of polypropylene with an olefin other than polypropylene, are
blended with one
or more polyolefins otherthan polypropylene (hereinafter referred to as
polyolefins), preferably
polyethylenes or copolymers of polyethylene, the blend being then grafted with
a suitable
silane to produce the desired silane-modified polypropylene composition.
[0008] Suitable polyolefins in this invention would include those materials
known in the
industryas low density polyethylene, high density polyethylene, linear low
density polyethylene;
copolymers of polyethylene, including those based on ethylene-butene, ethylene-
hexene,
ethylene-octene, ethylene-vinyl-acetate, ethyl e n e-m eth yi-acryl ate,
ethylene-ethyl-acrylate,
ethylene-butyl-acrylate, and similar materials; and ethylene-propylene
orethyiene-propylene
ciiene elastomers; and those of the above prepared using so-called metallocene
catalysts.
[0009] In addition, one or more additional materials may be incorporated to
act as
compatibilising agents for the polyolefin and polypropylene blend. Such
materials would
include the polypropylenes, higher polyolefins, and polyolefin materials
described above,
including those modified with reactive functional groups, such as acrylic
acids, methacrylic
acids, acrylates, methacrylates and anhydrides; and block copolymers, such as
styrene-
butadiene, styrene-butadiene-styrene, styrene-ethylene/propylene and styrene-
ethylene/butylene-styrene. These compatibilising agents may be incorporated
prior to the
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silane grafting reaction, but may also be added to the silane-modified
composition during
subsequent melt processing.
[0010] The silane-modified blend is then formed into the desired article by
melt processing
techniques such as extrusion and moulding, including multi-layer processing,
for example co-
extrusion of the blend with another material to form discrete but intimately
bonded layers. The
article thus formed is cross-linked in the presence of a silanol condensation
catalyst under
suitable conditions of heat and moisture, the catalyst being either blended
with the
composition during melt processing or added subsequently by coating the formed
article, for
example. The crosslinking thus performed stabilises the physical structure of
the blend of
silane-modified polypropylene and polyolefin through the formation of an
interpenetrating
network.
[0011] Accordingly, in one aspect, the present invention provides a heat
shrinkable coating
material of a crosslinked composition consisting of a silane-modified
polypropylene -
polyolefin blend, said material being formed by a process comprising: (a)
reacting a pre-
blended mixture of polypropylene and polyolefin resins with an appropriate
silane and silane-
grafting initiator to produce a silane-modified polypropylene - polyolefin
composition; (b)
forming the coating or insulating material by melt processing the composition
produced in
step (a) with a silanol condensation catalyst; and (c) crosslinking the formed
coating or
insulating material by exposing it to a combined regimen of heat and moisture.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0012] The crosslinking of polymers, in particular polyolefins, by the
combined process of
chemical grafting of silane molecules onto the polymerto form a silane-grafted
resin, followed
by catalysed hydrolysis and condensation of the silane, is a well known and
established
process, such as is described in US Patent No. 3,646,155.
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[0013] According to the present invention, a blended mixture is formed from a
polyolefin
which primarily crosslinks in the presence of free radicals; and a
polypropylene, or higher
polyolefin, which primarily degrades, undergoes chain scission and/or becomes
reduced in
molecular weight in the presence of free radicals.
[0014] The polyolefin which primarily crosslinks in the presence of free
radicals is preferably
selected from polyethylene and co-polymers of ethylene prepared by
polymerising ethylene
with one or more of an unsaturated olefin monomer having from 3 to 20 carbon
atoms,
preferably propylene, butylene, hexene oroctene, a substituted olefin such as
vinyl acetate,
imethyl acrylate, ethyl acrylate or butyl acrylate, or a diene monomer, such
as ethylidene
norbomene. The polyolefin is preferentially present in the form of high-
density polyethylene,
linear low-density polyethylene or an ethylene copolymer polymerised using a
so-called
metallocene catalyst. Preferably, the polyolefin resin comprises about 50 to
100% by weight
ethylene, more preferably about 60 to 90% by weight ethylene, and most
preferably about 80
to 95% by weight ethylene. Preferably, the density of the polyethylene or the
ethylene co-
polymer is in the range from about 0.85 to about 0.97 g/cm3.
[0015] The polypropylene or polypropylenes in this invention may be selected
from
tiomopolymers or copolymers of polypropylene, being preferentially isotactic
in nature, with
a melt viscosity chosen to be similar and comparable in value to the
polyolefin described
above for maximum process compatibility, and being added in an amount of from
about 10
to about 90 percent by weight of the composition. In orderto provide a
polymeric composition
having suitable physical properties at high temperatures, it is preferred that
the polypropylene
content of the composition be maximized. For example, it is preferred thatthe
polypropylene
content of the composition be greater than 50% by weight, and more preferably
greaterthan
60% by weight. In particularly preferred embodiments of the present invention,
the content of
polypropylene in the composition is from about 50 to about 80% by weight, more
preferably
from about 55 to about 70% by weight, and even more preferably from about 60
to about 70%
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by weight. Preferably, the polypropylene has a density in the range from about
0.86 to about
0.92 g/cm3.
[0016] The higher polyolefins wtiich primarily degrade in the presence of free
radicals
comprise polyolefins containing tertiary or quaternary carbon atoms and of the
general formula
==(CH2CR'R2 CH2),-, where R' is an alkyl group and R2 is an alkyl group or a
hydrogen atom.
Specific examples of such higher olefins include polybutene,
polymethylpentene,
polyisobutylene and butyl rubber. In embodiments of the inventions where such
higher
polyolefins are utilized, they are preferably added in the amounts described
in the preceding
paragraph for preferred amounts of polypropylene. Similarty, a combination of
polypropylenes
and such higher polymers may be utilized, with the combined amounts of the
polypropylenes
and higher polymers being as described in the preceding paragraph.
[0017] In addition, it is generally preferred that a compatibilising agent,
orcompatibiliser,
is also incorporated to provide enhanced blending of the polypropylene, or
higher polyolefin,
resin with the polyolefin resin, these resins being typically relatively
incompatible with one
another, and thus prone to phase separation which leads ultimately to the
integrity and
rnechanical properties of the mixture being compromised. A suitable
compatibiliser will
increase the miscibility and hence compatibility of the mixture by decreasing
the scale of
segregation of the polypropylene, or higher polyolefin, and polyolefin resin
components. The
compatibiliser may also be added at the melt processing, or forming, stage
after grafting of
the blend.
[0018] The compatibiliser may be selected from any of the polypropylenes,
polyolefins and
higher polyolefins resins described above, or these resins modified with
reactive functional
groups, such as acrylic acids, methacrylic acids, acrylates, methacrylates and
anhydrides; and
block copolymers, such as styrene-butadiene, styrene-butadiene-styrene,
styrene-
ethylene/propylene and styrene-ethylene/butylene-styrene. Preferred
compatibilisers include
polyolefins, including polypropylenes, grafted with maleic anhydride, acrylic
acid or other
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suitable functional groups, ethylene-propylene elastomers, and metallocene-
catalysed
polyolefin resins. The compatibilising agent is preferably added in an
amountfrom about 1 to
50 percent by weight.
[0019] The polypropylene and/or higher polyolefin resin, and polyolefin resin
described
above, with or without a compatibiliser, are mixed together in the solid state
by tumble
blending or similar method, the admixed resins then being reacted in the
molten state with an
organic silane having the general formula RR'SiY2, wherein R represents a
monovalent
olefinically unsaturated hydrocarbon radical, Y represents a hydrolysable
organic radical and
R' represents an R radical or a Y radical. The monovalent olefinically
unsaturated hydrocarbon
radical preferably comprises vinyl, allyl, butenyl, cyclohexenyl,
cyclopentadienyl, or
cyclohexadienyl radicals. The group Y may represent any hydrolysable organic
radical, for
example an alkoxy radical such as niethoxy, ethoxy and butoxy radicals; an
acyloxy radical,
for example the formyloxy, acetoxy or propionoxy radicals; oximo radicals such
as -
ON=C(CH3)2, -ON=CCH3C2H5 and ON=C(C6H5)2; or substituted amino radicals such
as
alkylamino and arylamino radicals, examples of which are -NHCH3, -NHC2H5 and -
NH(CsH5)z.
[0020] Preferably, the silane has general formula RSiY3, with the most
preferred group R
being the vinyl radical, and the most preferred Y group being the methoxy and
ethoxy radical.
Accordingly, the most preferred silanes for use in the present invention are
vinyltriethoxysilane
and vinyltrimethoxysilane.
[0021] The amount of silane reacted with the polyolefin depends in part upon
the reaction
conditions and the degree of modification desired in the polyolefin. The
proportion mayvary
from about 0.1 to about 50% by weight based on the total weight of the silane-
grafted resin,
rnore preferably from about 0.5 to 10% by weight, and most preferably from
about 1.0 to 2.5%
by weight.
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[0022] Afree-radical initiator is also incorporated into the molten mixture to
initiate the graft
polymerization reaction. Preferred free-radical initiators are organic
peroxides such as
benzoyl peroxide, dichlorobenzoyl peroxide, dicumyl peroxide, di-tertiarybutyl
peroxide. The
most preferred free-radical initiator for use in the compositions of the
present invention is
dicumyl peroxide. The criteria for choosing an appropriate free-radical
initiator are known
to persons skilled in the art and are described in the above-mentioned U.S.
Patent No.
3,646,155 and will not be repeated here.
[0023] Preferably, the organic peroxide free-radical initiator is added in an
amount of from
about 0.1 to about 1.0% by weight of the silane-grafted resin, more preferably
from about 0.05
to 0.25% by weight.
[0024] The silane and peroxide are reacted with the resin mixture above the
melting point
of the highest melting point resin component under conditions in which all the
components are
subjected to dispersive and distributive mixing, using processes known to
those skilled in the
art.
[0025] The silane-modified resin blend so produced is subsequently melt-
processed as
described previously, with a suitable amount of silanol condensation catalyst,
and optionally
with one or more of a number of other ingredients, such as pigmenting agents,
mineral fillers,
flame-retardant additives, antioxidants, stabilisers, lubricants,
compatibilisers and the like, to
form a composition according to the invention.
[0026] The silanol condensation catalyst is typically selected from the group
comprising
organic bases, carboxylic acids and organometallic compounds including organic
titanates
and complexes or carboxylates of lead, cobalt, iron, nickel, zinc and tin.
Preferably, the
catalyst is selected from dibutyltin dilaurate, dibutyltin diacetate,
dibutyltin octanoate, dioctyltin
maleate, dibutyltin oxide and titanium compounds such as titanium-2-
ethylhexoxide. The most
preferred silanol condensation catalyst is dibutyltin dilaurate, though any
material which will
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catalyse the silane condensation reaction is suitable for the invention. The
condensation
catalyst is preferably added in an amount of from about 0.01 to about 1
percent by weight of
the coating material, more preferably about 0.05 to about 0.5 percent by
weight, and most
preferably about 0.1 to 0.2 percent by weight.
[0027] Subjecting the composition thus produced to moisture, preferably at an
elevated
temperature, will induce crosslinking of the silane groups via a combined
hydrolysis and
condensation reaction. Atmospheric moisture is usually sufficient to permit
the crosslinking
to occur, butthe rate of crosslinking may be increased bythe use of an
artificially moistened
atmosphere, or by immersion in liquid water. Also, subjecting the composition
to combined
heat and moisture will further accelerate the crosslinking reaction. Most
preferably,
crosslinking is effected at a temperature above 50 C and most preferably by
exposing the
composition to a temperature of 85 C and a relative humidity of 90% or greater
for
approximately 100 hours.
(0028] A particularly preferred process for producing and forming a
composition of the
present invention will now be described below:
(0029] The required quantities of polypropylene, polyolefin, silane, peroxide
free-radical
initiator, and optional compatibiliser and processing stabiliser, are pre-
blended, and then
passed through a suitable mixing device, such as an extruder or continuous
compounding
machine, at a temperature above the melting point of the highest melting point
resin
component and the decomposition temperature of the peroxide, in order to
accomplish
grafting of the silane onto the polyolefin. The silane may be added with or
without a resin or
mineral carrier or binder for the silane, in order to facilitate handling of
said silane. The
specific conditions of temperature and mixing efficiency dictate the extent of
grafting and the
quality of silane-grafted material thus produced, and are of prime
consideration. The silane
grafted material is then passed through a multi-strand dye into a trough of
cooled water, or
through a die-face pelletizing unit, then chopped into pellets and dried.
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[0030] The grafted polymer pellets are then combined with the desired
quantities of a silane
condensation catalyst, an antioxidant stabiliser, and any number of optional
ingredients,
including colorants, mineral fillers, flame retardants, compatibilisers, and
processing aids,
individually or in pre-masterbatched form, this mixture then being homogenised
by melt
processing, for example, by extruding, co-extruding or moulding, and then
formed whilst in this
state into the desired shape of the article required, for example sheet,
tubing, electrical
insulation or moulded shape.
[0031] Alternatively, the independent silane grafting, and extrusion mixing
and forming
processes described above may be combined into a single step operation wherein
the
extrusion mixing and forming occurs sequentially and in rapid succession with
the silane
grafting of the resin blend. This may be accomplished in a single extruder
with a dual stage
screw, orscrews, the first stage designed for silane grafting and the second
stage designed
for subsequent incorporation and melt mixing of the additional ingredients
into the silane
grafted melt. Another option would be a "piggy-back" configuration, wherein a
separate silane
grafting reactor or extruder feeds directly into a second extruderforthe
subsequent mixing and
forming operation.
[0032] The formed material thus produced is then exposed to moisture,
preferably at an
elevated temperature, to induce crosslinking of the material. This may be
accomplished in a
hotwater bath or, more preferably, in a steam chamber, for example. The
material can also
be made to crosslink at ambient conditions of temperature and humidity, but
the time to effect
complete crosslinking of the material will necessarily be longer. Preferably,
the degree of
crosslinking is such that the materials according to the invention have a gel
fraction of greater
than about 20%, more preferably greater than about 25%.
[0033] The composition of the invention thus produced exhibits the property of
softening but
not melting when re-heated to a temperature which is close to or above its
softening point or
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crystalline melting point. This is desirable forthe manufacture of heat-
shrinkable articles since
the formed material may be stretched beyond the original extruded or moulded
dimensions
without rupture using relatively low forces, and can then be frozen in the
stretched state by
cooling it rapidly to below the melting point. Stretching can be accomplished
by mechanical,
pneumatic or hydraulic means. At this point the stretched crosslinks are held
in a stable state
by the re-formed, solid crystalline regions. Subsequent re-heating of the
stretched article
above the melting point will cause the crystalline regions to re-melt and the
structure to revert
to its original extruded or moulded dimensions. The crosslinking also prevents
the article from
becoming liquid during this shrinking process, and provides the heat-
distortion resistance and
hot-set properties required for high temperature coating, electrical
insulation, and jacketing
applications, for example.
[0034] The invention is further illustrated by the following Examples:
EXAMPLES 1, 2, 3, 4
[0035] The polypropylene-based compositions listed as Examples 1,2,3, and 4 in
Table
1, below, were prepared as silane- grafted resin blends by tumble blending the
resin and
silane components, these then being reactiveiy processed through a 30:1 UD
extruder
designed to provide the required mixing efficiency and residence time for
effective grafting,
at a melt temperature above the melting point of the polypropylene component.
Examples 3
and 4 contain compatibiliser.
TABLE I
Silane-Grafted Polypropylene Blends
[ Ingredient Percent By Weight
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Example Example Example Example
1 2 3 4
Polypropylene 68.63 68.63 58.82 58.82
Density 0.900 /cm3, Melt Flow Rate 0.45 /10min.
Linear Low Density Polyethylene 29.41 29.41
Density 0.919 /cm3, Melt Index 6.0 /10min.
High Density Polyethylene 29.41 29.41
Density 0.960 /cm3, Melt Index 4.9 /10rnin.
Maleic Anhydride Modified Polypropylene 9.81 9.81
Density 0.91 g/cm3, Melt Flow Rate 7.0, g/10min. 0.2%
MA
Vinyl Triethoxysilane 1.96 1.96 1.96 1.96
EXAMPLES 5, 6, 7, 8
[0036] These examples describe the production of a crosslinked, extruded sheet
according
-to the present invention.
1[0037] The grafted pellets produced according to Examples 1, 2, and 3 were
blended with
ithe ingredients indicated in Table 2. The combined ingredients were fed at a
melt
temperature of approximately 180''C through a 24:1 L/D single screw extruder
equipped with
a single layer sheet die. The extruded sheet was fixed to the required
dimensions of width
and thickness by passing itthrough a cooled, 3-roll calendering stack, then
wound onto reels.
[0038] The sheet was crosslinked by conditioning the reeled sheet at a
temperature of
85 C and a relative humidityof 90%forapproximately 100 hours. This crosslinked
sheetwas
then tested to determine the degree of crosslinking and for mechanical
properties as
indicated in Table 3.
TABLE 2
Moisture-Crosslinkable, Polypropylene Compositions
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Silane Grafted Pol , rQ lene Bland Percent B Weight
F.xample Example Example Example
6 7 8
Example 1 95.24
Melt Flow Index 2.3 /10min.
Example 2 95.24 85.72
Melt Flow Index 1.2 g/10/min
Example 3 95.24
Melt Flow Index 1.3g/lOmin.
Maleic Anhydride Modified Pol ro lene 9.52
Dibutyltin dilaurate Catalyst* 1.42 1.42 1.42 1.42
Antioxidant Stabiliser** 2.86 2.86 2.86 2.86
Carbon Black*** 0.48 0.48 0.48 0.48
kAdded as a 2% masterbatch in polyethylene
'k*B1end of hindered phenol and phosphite stabiliser added as a 15%
masterbatch in polyethylene.
k*''Added as a 25% masterbatch in polyethylene
TABLE 3
Properties of Crosslinked Sheet
-value
Pr0p@rty
Example5 ifx*mpie6 E4fnplo~7 ~em e8 i
Gel Fraction (% degree of crosslinking) 24 23 23 26
Hot Tensile Strength @ 200C, 100% ^ 4.3 5.6 5.5 6.5
Elongation (psi) Ultimate Hot Elongation 200C (%) _ 140 >200 >200 160
Ultimate Tensile Strength @ 23C (psi) 4400 4300 3500 2500
Ultimate Elongation @ 23C (%) 630 700 550 500
! Flexural Modulus (psi) `^` ~_ 3 000 48000 _48000 58000
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EXAMPLES 9, 10, 11, 12
[0039] This example describes the production of a heat-shrinkable, extruded
sheet product
according to the present invention.
[0040] The crosslinked sheets prepared in Example 5, 6, 7 and 8 were re-heated
to a
temperature close to or above the softening point or crystalline melting point
of the
composition and then stretched by a factor of approximately 2:1 whilst at this
temperature
using either a machine-direction ortransverse-direction mechanical stretcher.
Whilst in the
stretched state, the sheetwas rapidly cooled, using air, wateror some
othersuitable medium,
-to below the softening or crystalline melting point of the composition in
order to fix the sheet
at the stretched dimensions. The stieet, either prior to or after stretching,
may be extrusion
laminated orcoated with an additional layerof material having different
functional properties,
such as a heat-activated adhesive. The stretched sheet was mechanically tested
for suitability
as a heat-shrink coating for high-teniperature pipeline joints to
specifications common to the
industry.
EXAMPLE 13
[0041] In another example, a heat-shrinkable tubing product was produced by
extruding the
formulation in Example 5 into a tubular cross-section, collecting the extruded
tubing on a reel,
crosslinking the reeled tubing using the conditions noted above for Examples
5, 6, 7 and 8,
heating the thus crosslinked tube to a temperature close to or above the
softening point or
crystalline melting point of the composition, stretching the heated tube by
mechanical or
pneumatic means whilst at this temperature, and then finally rapidly cooling
the tube with air
or water, or some other suitable means, to below the softening point or
crystalline melting
point of the composition whilst maintaining the tubing in the stretched state.
The tubing, either
prior to or after stretching, may also be internally extrusion coated with an
additional layer of
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material having different functional properties, such as a heat-activated
adhesive. The
stretched tubing was mechanically tested for suitability as a heat-shrink
insulation for electrical
and electronic connections, splices, and terminations in accordance with
specifications
common to the industry.
[0042] Although the invention has been described in relation to certain
preferred
embodiments, it will be appreciated that it is not intended to be limited
thereto. Rather, the
invention is intended to encompass all embodiments which fall within the scope
of the
following claims.
