Note: Descriptions are shown in the official language in which they were submitted.
CA 02535503 2012-02-23
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35016-0673
Title: HEAT SHRINKABLE LAMINATED COVERING
Field of Invention
[0001] The present invention relates to heat shrinkable coverings, and in
particular to heat shrinkable coverings used for the mechanical and corrosion
protection of oil, gas and water transmission pipelines and having improved
high
service temperature, and high temperature mechanical and chemical properties
such as penetration resistance, impact resistance, tensile strength,
stiffness,
thermal insulation, and permeability resistance.
Background
[0002] Heat shrinkable protective coverings are used in variety of
applications, including the sealing and protection of pipe weld joints,
electrical
cable splices, and the like, from adverse mechanical and environmental
conditions, such as impact, penetration, moisture, and corrosion.
[0003] There is increasing demand in the oil and gas industry for higher
performance heat-shrinkable coverings to protect and insulate steel
transmission
pipelines operating at temperatures of 120 C, or higher, in order to withstand
higher oil and gas temperatures resulting from deeper drilling, or to
facilitate
pumping efficiency. This imposes severe mechanical and chemical demands on
traditional thermoplastic polymer materials, such as polyolefins, used for
pipe
coating and field joint systems.
[0004] Current pipeline joint protection systems manufactured from
thermoplastic polyolefins, such as polyethylene and polypropylene, are
typically
deficient in mechanical performance, such as penetration and impact
resistance,
at temperatures above their softening point or crystalline melting point. This
can
be addressed to some degree by imparting some thermoset characteristic to the
polymer by crosslinking the thermoplastic. This renders the polymer resistant
to
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melting and flowing when it is heated to a temperature close to or above the
crystalline melting point but it does not totally prevent the polymer from
softening. Hence the polymer will still remain susceptible to damage by impact
or
penetration, for example, at elevated temperatures close to or above the
softening point of the polymer, namely those temperatures which are
experienced by high temperature pipelines. In addition, higher temperatures
render the polymer more susceptible to thermal degradation, chemical attack,
and permeation by moisture and gases.
Summary of Invention
[0005] This invention describes a method of overcoming the above
mentioned deficiencies through the use of a polymer-based laminated covering
wherein one or more of the polymer layers comprising the laminate provides
resistance to, or protection from, one or more of the adverse conditions noted
above, such as softening at the pipeline operating temperature, or overcomes a
mechanical or chemical deficiency, and wherein one or more of the polymer
layers provides the heat-shrinkable properties necessary to impart controlled
recovery of the laminated covering over the pipe or pipe joint on application
of
heat. The heat-shrinkable layer(s) is typically crosslinked to render the
material
thermoset and resistant to melting on application of the heat required to
effect
recovery of the laminated covering over the pipe or pipe joint. Crosslinking
of the
heat-shrinkable layer may be accomplished by a moisture cure or radiation cure
process.
[0006] The layers comprising the laminated covering are prepared using
suitable polymeric materials, including polyolefins such as polyethylene and
polypropylene, crosslinked polyolefins, polyolefins modified with the addition
of
reinforcing fillers and nanofillers, thermoplastics chosen for their low
permeability, such as ethylene vinyl alcohol and polyvinylidene fluoride, and
so-
called engineering thermoplastics having high softening points and superior
mechanical performance, such as nylons, polyesters, and polyurethanes.
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[0007] The present invention provides a laminated heat shrinkable covering
comprising: a) at least one heat shrinkable layer comprising a crosslinked
polymeric material; b) a functional layer having at least one property
superior to
said heat shrinkable layer, said at least one property selected from a group
consisting of: high temperature penetration resistance, softening point,
impact
resistance, long-term thermal stability, tensile strength, toughness,
stiffness,
thermal insulation, electrical conductivity or static dissipation, and
impermeability to gases and moisture; and c) an adhesive layer.
[0008] In an embodiment of the invention, the heat shrinkable layer
comprises a crosslinked polyolefin material.
[0009] In a further embodiment of the invention, the functional layer
comprises a polymeric material.
[00010] In a still further embodiment of the invention, the functional
layer
comprises a non-crosslinked polymeric material.
[00011] In another embodiment of the invention, the crosslinked polyolefin
material and/or the non-crosslinked polymeric material comprises propylene
homopolymer or copolymer of propylene with an olefin other than propylene,
such as ethylene. The propylene homopolymer or copolymer may further be
modified with a functional group selected from a group consisting of: silanes,
acrylic acids, alkyl acrylic acids, glycidyl acrylates, alkyl acrylates,
anhydrides
and combinations thereof.
[00012] In a still another embodiment of the invention, the crosslinked
polyolefin material and/or the non-crosslinked polymeric material comprises
propylene-ethylene copolymers.
[00013] In a further embodiment of the invention, the crosslinked
polyolefin
material and/or the non-crosslinked polymeric material comprises ethylene
copolymers. The copolymers may be selected from a group consisting of: vinyl
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acetate, vinyl alcohol, alkyl acrylates, and higher olefins, such as butane,
hexane, and octane.
[00014] In a still further embodiment of the invention, the crosslinked
polyolefin material and/or the non-crosslinked polymeric material comprises a
polyethylene homopolymer selected from a group consisting of: high density
polyethylene, medium density polyethylene, linear medium density polyethylene,
low density polyethylene, linear low density polyethylene, and combinations
thereof.
[00015] In another embodiment of the invention, the crosslinked polyolefin
material and/or the non-crosslinked polymeric material comprises an elastomer
selected from a group consisting of: ethylene-propylene diene elastomers;
crystalline propylene-ethylene elastomers, and thermoplastic polyolefin
elastomers.
[00016] In a further embodiment of the invention, the crosslinked
polyolefin
material and/or the non-crosslinked polymeric material comprises polymers
prepared using metallocene catalysts, also known as single-site, stereo-
specific,
or constrained geometry catalysts, and may also comprise a bimodal molecular
weight distribution.
[00017] In a further embodiment of the invention, the crosslinked
polyolefin
material and/or the non-crosslinked polymeric material comprises a blend of
two
or more of the polyolefin polymers described above.
[00018] In a still further embodiment of the invention, the crosslinked
polyolefin material and/or the non-crosslinked polymeric material comprises a
compound selected from a group consisting of: a filled polyolefin, a
polyolefin
nanocomposite, an engineering thermoplastic, a barrier polymer, a thermally
insulating polymer and an electrically conductive polymer.
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[00019] In an embodiment of the invention, the filler in the filled
polyolefin
is selected from the group consisting of: clay, mica, talc, silica,
wollastonite,
wood, glass fibres, and metal oxides.
[00020] In an embodiment of the invention, the polyolefin nanocomposite
comprises a polyolefin and an exfoliated clay additive.
[00021] In an embodiment of the invention, the engineering thermoplastic is
selected from the group consisting of: nylons, polyesters, and polyurethanes.
[00022] In an embodiment of the invention, the barrier polymer is selected
from the group consisting of: ethylene vinyl alcohol, polyvinyl alcohol and
polyvinylidene fluoride.
[00023] In an embodiment of the invention, the thermally insulating polymer
comprises a polyolefin and a low conductivity insulating filler selected from
the
group consisting of: hollow glass, ceramic and polymer microspheres.
[00024] In an embodiment of the invention, the electrically conductive
polymer comprises a polyolefin and an electrically conductive filler
comprising
carbon black or a metal powder.
[00025] In an embodiment of the invention, the at least one heat shrinkable
layer further comprises at least one additive selected from a group consisting
of:
cross-linking promoters, cornpatibilisers, modifiers, pigments, antioxidant
stabilizers, heat stabilizers, ultraviolet (UV) stabilizers, fillers, flame
retardants,
and process aids.
[00026] The said antioxidant stabilizer may be selected from a group
consisting of: hindered phenols, hindered amines, phosphites, bisphenols,
benzimidazoles, phenylenediamines, and dihydroquinolines.
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[00027] The said compatibiliser may be selected from one or more of
ethylene-propylene copolymers; ethylene-propylene diene elastomers;
crystalline
propylene-ethylene elastomers; thermoplastic polyolefin elastomers;
metallocene
polyolefins; copolymers of ethylene with vinyl acetate, vinyl alcohol, and/or
alkyl
acrylates; polybutenes; hydrogenated and non-hydrogenated polybutadienes;
butyl rubber; polyolefins modified with reactive functional groups selected
from
the group comprising silanes, alcohols, amines, acrylic acids, methacrylic
acids,
acrylates, methacrylates, glycidyl methacrylates, and anhydrides; polyolefin
ionomers; polyolefin nanocomposites; block copolymers selected from the group
comprising styrene-butadiene, styrene-butadiene-styrene, styrene-
ethylene/propylene and styrene-ethylene/butylene-styrene; thermoplastic
elastomers comprising polypropylene blended with an elastomer such as
ethylene propylene.
[00028] In an embodiment of the invention, the adhesive layer is bonded to
the heat shrinkable layer.
[00029] In another embodiment of the invention, the adhesive layer is
bonded to the functional layer.
[00030] In a further embodiment of the invention, the articles comprise a
first heat shrinkable layer and a second heat shrinkable layer.
[00031] In a further embodiment of the invention, the second heat-
shrinkable layer is crosslinked to a lesser degree than the first heat-
shrinkable
layer.
[00032] In a still further embodiment of the invention, the second heat
shrinkable layer is positioned between the first heat shrinkable layer and the
functional layer and wherein the second heat shrinkable layer is bonded to the
first heat shrinkable layer and the functional layer, and the functional layer
is
bonded to the adhesive layer.
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[00033] In yet a further embodiment of the invention, the second heat
shrinkable layer is positioned between the functional layer and the adhesive
layer
and wherein the second heat shrinkable layer is bonded to the functional layer
and the adhesive layer, and the functional layer is bonded to the first heat-
shrinkable layer.
Brief Description of the Figures
[00034] Preferred embodiments of the invention will now be described, by
way of example to the accompanying drawings, in which:
[00035] Figure 1 is a cross-section illustration of a heat shrinkable
covering
comprising a heat shrinkable, crosslinked polymeric, first layer; a polymeric,
second layer; and an adhesive/primer third layer;
[00036] Figure 2 is a cross-section illustration of a heat shrinkable
covering
comprising a polymeric, first layer; a heat shrinkable, crosslinked polymeric,
second layer; and an adhesive/primer third layer;
[00037] Figure 3 is a cross-section illustration of a heat shrinkable
covering
comprising a heat shrinkable, crosslinked polymeric, first layer; a heat
shrinkable, crosslinked polymeric, second layer; a polymeric third layer; and
an
adhesive/primer fourth layer; and
[00038] Figure 4 is a cross-section illustration of a heat shrinkable
covering
comprising a heat shrinkable, crosslinked polymeric, first layer; a polymeric
second layer; a heat shrinkable, crosslinked polymeric, third layer; and an
adhesive/primer fourth layer.
Detailed Description
[00039] The present invention provides a laminated heat shrinkable covering
comprising: (a) at least one heat shrinkable layer comprising a crosslinked
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,
polymeric material; (b) a functional layer having at least one property
superior to
said heat shrinkable layer, said at least one property selected from a group
consisting of: high temperature penetration resistance, softening point,
impact
resistance, long-term thermal stability, tensile strength, toughness,
stiffness,
thermal insulation, electrical conductivity or static dissipation, and
impermeability to and gases and moisture; and (c) an adhesive layer.
[00040] The laminated material possesses the advantage of heat
shrinkability while retaining the desired physical and other properties.
[00041] In a preferred embodiment, the heat shrinkable layer
comprises a
crosslinked polyolefin material and the functional layer comprises a polymeric
material and more preferably a non-crosslinked polymeric material.
[00042] In a further preferred embodiment, the at least one heat
shrinkable
layer is bonded to the functional layer. Depending on the intended
application,
either the heat shrinkable layer or the functional layer may constitute an
outer
lamina of the laminated heat shrinkable covering.
[00043] The laminated heat shrinkable coverings according to the
invention
can be manufactured in forms such as extruded tubing and sheets which may be
suitable for protective or insulative coverings for pipe weld joints,
electrical cable
splices, and the like.
[00044] The use of both crosslinked and non-crosslinked layers for
the
manufacture of laminated heat shrinkable articles such as protective
coverings,
provides numerous advantages over conventional heat shrinkable coverings. In
particular, the use of separate crosslinked and non-crosslinked layers of
different
composition and functionality allows the heat shrinkable covering to be
tailored
more specifically to the intended application.
[00045] The multilayer construction of the heat shrinkable covering
allows
for the use of non-crosslinkable materials which could not be used on their
own.
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The present invention allows for the utilization of non-crosslinkable
materials
having beneficial properties, for example polymers which have high softening
points for superior high temperature penetration resistance, by combining them
with a crosslinkable material. Multilayer construction allows the use of
functional
fillers such as reinforcing fillers, nanocomposites, and flame retardants
while
maintaining required levels of crosslinking and installation performance.
Multilayer construction also allows for custom tailoring the crosslinking
levels of
individual layers for optimization of installation performance, inter-layer
adhesion, and split resistance. Additionally, multilayer construction enables
the
provision of anisotropic shrinkage characteristics to the article through the
ability
to impart different stretch ratios to the individual layers.
[00046] As compared to prior art heat shrinkable protective coverings, the
present invention provides a cost effective solution for providing superior
performance under a diverse range of operating conditions. By combining layers
of different composition and functionality, optimum performance can be
achieved
at reduced cost. The present invention allows for the use of inner layers
comprising reduced cost materials for a cheaper overall system cost. For
example, the multilayer construction allows for the use of a thinner
crosslinked
outer layer and a subsequent reduction in material thickness of the generally
highest cost component.
[00047] Figure 1 shows a cross-section of one embodiment of the laminated
heat shrinkable covering 10 wherein the heat shrinkable layer 12 forms the
outer
lamina, the functional layer 14 the intermediate lamina and the adhesive layer
16, the inner lamina. The heat shrinkable layer 12 and the functional layer 14
are bonded to one another by laminating the two layers together (for methods
of
preparation, see further discussion set out below). Articles according to the
invention having the heat shrinkable layer on the outside and the functional
layer
on the inside are particularly suitable for applications requiring improved
high
temperature penetration resistance, resistance to permeation by gases and
moisture, and thermal insulation, for example.
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[00048] Figure 2 shows a cross-section of another embodiment of the
laminated heat shrinkable covering 20 wherein the functional layer 24 forms
the
outer lamina, the heat shrinkable layer 22 the intermediate lamina, and
adhesive
layer 26, the inner lamina. Articles according to the invention having the
heat
shrinkable layer on the inside and the functional layer on the outside are
particular suitable for applications requiring improved impact resistance,
surface
toughness, and chemical resistance, for example.
[00049] In a further embodiment of the invention, the laminated heat
shrinkable covering may comprise one or more heat shrinkable layers. As shown
in Figure 3, the laminated heat shrinkable covering 30 may comprises a first
and
second heat shrinkable layer 32, 34 bonded directly to one another. The
functional layer 36 is layered between the second heat shrinkable layer 34 and
the adhesive layer 38. Alternatively, as show in Figure 4, the laminated heat
shrinkable covering 40 may comprise a first and second heat shrinkable layer
42,
44 wherein the first and second heat shrinkable layers are separated by the
functional layer 46, and wherein the covering 40 further comprises an
adhesive/primer layer 48. In either of the embodiments illustrated in Figures
3
and 4, a heat shrinkable layer constitutes the outer lamina and the adhesive
layer constitutes the inner lamina.
[00050] Heat Shrinkable Layer(s)
[00051] The heat shrinkable layer comprises a polymeric material which has
been crosslinked and more preferably a crosslinked polyolefin material.
Crosslinking is the formation of permanent covalent bonds between individual
polymer chains which act to bind the polymer chains together and prevent them
from irreversibly separating during subsequent heating. It is this crosslinked
structure which, while retaining the elastomeric nature of the material,
renders
the material thermoset and resistant to melting which, in turn, is a desirable
property for producing heat-shrinkable articles. Crosslinking also provides
the
article with heat resistance, allowing it to maintain mechanical toughness and
integrity at high service temperatures.
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[00052] In some embodiments of the invention, the laminated heat
shrinkable covering may comprise more than one heat shrinkable layer. In
embodiments comprising a first and second heat shrinkable layer, each of the
heat shrinkable layers can be prepared using the same crosslinked polyolefin
material to provide layers having similar chemical and physical properties.
Alternatively, the first and second heat shrinkable layer can be prepared with
different polyolefin materials. Depending on the intended applications of the
heat shrinkable covering, the first and second heat shrinkable layers may be
prepared using polymeric materials having differing softening points and
performance properties such as mechanical strength and chemical resistance.
[00053] For example, the first heat shrinkable layer can be prepared with a
polyolefin material characterized by a high softening point and/or higher
performance properties, the second heat shrinkable layer can be prepared with
a
polyolefin material having a lower softening point and/or performance
properties.
In such instances, the second heat shrinkable layer can often be prepared
using
a lower cost version of the materials used to prepare the first heat
shrinkable
layer thereby improving the cost effectiveness of the heat shrinkable covering
while maintaining the desired level of performance and durability. The degree
of
crosslinking may also be varied between the first and second heat shrinkable
layer depending on the intended application. For example, it may be required
that the second heat shrinkable layer have a lower degree of crosslinking than
the first heat shrinkable layer in order that the second heat shrinkable layer
preferentially softens when heated, thereby providing improved adhesion to the
functional or adhesive layer beneath and ensuring an integrally bonded
composite structure. An additional benefit is the improvement in split
resistance
of the heat shrinkable covering during heat recovery through the mitigation of
the high recovery stresses of the highly crosslinked first heat shrinkable
layer by
the relatively more fluid second heat shrinkable layer.
[00054] The heat shrinkable layer can be prepared using any polyolefin
material which can be crosslinked. In one preferred embodiment, the heat
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shrinkable layer is prepared using polyolefin material comprising propylene
polymer, such as polypropylene homopolymer or copolymer of propylene with an
olefin other than propylene, such as ethylene. The propylene homopolymer or
copolymer may further be modified with a functional group selected from a
group
consisting of: silanes, acrylic acids, alkyl acrylic acids, glycidyl
acrylates, alkyl
acrylates, anhydrides and combinations thereof. Where the polypropylene is a
copolymer, it preferably contains at least about 80 percent by weight
propylene.
[00055] The polypropylene homopolymer or coplymer is preferably isotactic
in nature, having a density of about 0.85 to 0.91 g/cm3 and a melt flow index
of
about 0.1 to 10 dg/min. The crystalline melting point of the polypropylene
homopolymer or copolymer is usually in the range of about 160-170 C, with
about 165 C being typical.
[00056] In another embodiment of the invention, the heat shrinkable layer
may be prepared using a polyolefin material comprising ethylene homopolymers
or copolymers modified with reactive functional group such as, but not limited
to:
vinyl acetate; vinyl alcohol; and alkyl acrylates; or combinations thereof.
[00057] The heat shrinkable layer may be prepared using a polyethylene
such as high density polyethylene (HDPE), medium density polyethylene (MDPE),
linear medium density polyethylene (LMDPE), low density polyethylene (LDPE) or
linear low density polyethylene (LLDPE), or blends thereof. The terms HDPE,
MDPE and LDPE as used herein are defined in accordance with the American
Society for Testing and Materials (ASTM) D1248 standard definitions. LDPE is
defined to have a density from 0.910 to 0.925 g/cm3. MDPE have densities
ranging from 0.926 to 0.940 g/cm3 and HDPE has a density of at least 0.941
g/cm3. The polyethylene includes both ethylene homopolymers, and copolymers
of ethylene with higher alpha olefins such as butene, hexene and octene, and
may be of a predominantly linear molecular structure. The polyethylene may
preferably be manufactured using metallocene catalysts, also known as single-
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site, stereo-specific or constrained geometry catalysts, and may also comprise
a
bimodal molecular weight distribution.
[00058] In yet a further embodiment of the invention, the heat shrinkable
layer may be prepared using a polyolefin material comprising an elastomer such
as but not limited to: ethylene-propylene diene elastomers; crystalline
propylene-ethylene elastomers, or thermoplastic polyolefin elastomers.
[00059] The heat shrinkable layer may also be prepared using polyolefins
prepared with metallocene catalysts, also known as single-site, stereo-
specific,
or constrained geometry catalysts, and may also comprise a bimodal molecular
weight distribution.
[00060] The heat shrinkable layer may further be prepared using a blend of
any of the polymers and/or elastomers described above.
[00061] In a preferred embodiment, the heat shrinkable layer is prepared
using a blend of polypropylene and polyethylene. Preferably, the polypropylene
has a melt viscosity, as measured by melt flow index, which is similar to that
of
the polyethylene component at the same temperature and under the same shear
conditions required for processing the blend, to ensure optimum blend
compatibility.
[00062] The polyolefin material may further comprise one or more optional
additives and fillers known in the art in addition to the polymeric base.
Additives
useful for preparing the heat shrinkable layer include but are not limited to:
cross-linking promoters (also known as radiation sensitizers),
compatibilisers,
modifiers, pigments, antioxidant stabilizers, heat stabilizers, ultraviolet
(UV)
stabilizers, fillers, flame retardants, or process aids.
[00063] The polyolefin material may be prepared using polyolefin polymers
which have been modified with reactive functional group such as, but not
limited
to: silanes, acrylic acids; alkyl acrylic acids, such as methyl acrylic acid;
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acrylates; alkyl acrylates, such as methyl acrylates; anhydrides; or
combinations
thereof.
[00064] In a preferred embodiment, silane modified olefin polymers may be
employed to provide a polyolefin material which is moisture crosslinkable. In
one
embodiment, the polyolefin polymers are modified with an alkoxysilane such as
vinyltrimethoxy silane and vinyltriethoxy silane.
[00065] In another preferred embodiment, the polyolefin material may
further comprise a radiation sensitizer to promote crosslinking of the
polyolefin
material when it is subjected to electron beam or gamma radiation. The
radiation
sensitizer is preferably selected from the family of multifunctional monomers
typically used as crosslink promoters for polyolefin-based polymers. Preferred
monomers include trimethylol propane triacrylate, trimethylol propane
trimethacrylate, tetramethylol tetraacrylate, ethylene glycol dimethacrylate,
triallyl cyanurate and triallyl isocyanurate.
[00066] The polyolefin material may also further comprise an antioxidant
stabilizer. The antioxidant stabilizer may be chosen from any suitable
antioxidant or blend of antioxidants designed to prevent degradation of the
composition during melt processing and subsequent heat aging of the final
product. Examples of suitable antioxidants and heat stabilizers include those
classes of chemicals known as hindered phenols, hindered amines, phosphites,
bisphenols, benzimidazoles, phenylenediamines, and dihydroquinolines. These
are preferably added in an amount of about 0.1 to 5% by weight of the blend,
depending upon the aging properties required and the type and quantity of
optional destabilizing ingredients in the composition, for example halogenated
flame retardants or mineral fillers. It should also be noted that these
antioxidants
and stabilizers, if added in excessive amounts, may become "radiation
scavengers", acting to reduce the effectiveness of the radiation to induce the
desired crosslinking reaction and the resultant degree of crosslinking
obtainable
for a given radiation dosage.
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[00067] In embodiments wherein the polyolefin material comprises a blend
of two or more polymers, it may be desirable to include one or more
compatiblisers or modifiers. The addition of a particular compatibliser or
modifier
will depend on the particular polyolefin material used. The function of the
compatibiliser or modifier is to promote the miscibility of different
copolymers
when they are blended together. The compatibiliser is preferably added in the
amount from about 1 to 50% and most preferably from about 5 to 10% by
weight of the blend. Examples of possible compatibilisers and modifiers
include,
but are not limited to: ethylene-propylene copolymers; ethylene-propylene
diene
elastomers; crystalline propylene-ethylene elastomers; thermoplastic
polyolefin
elastomers; metallocene polyolefins; copolymers of ethylene with vinyl
acetate,
vinyl alcohol, and/or alkyl acrylates; polybutenes; hydrogenated and non-
hydrogenated polybutadienes; butyl rubber; polyolefins modified with reactive
functional groups selected from the group comprising silanes, alcohols,
amines,
acrylic acids, methacrylic acids, acrylates, methacrylates, glycidyl
methacrylates,
and anhydrides; polyolefin ionomers; polyolefin nanocomposites; block
copolymers selected from the group comprising styrene-butadiene, styrene-
butadiene-styrene, styrene-ethylene/propylene and styrene-ethylene/butylene-
styrene; thermoplastic elastomers comprising polypropylene blended with an
elastomer such as ethylene propylene.
[00068] The constituents of the blend comprising the polyolefin material
may
be melt blended either in-situ with forming of the final product during melt
processing, or prior to forming by melt mixing using a machine designed
specifically for that purpose, such as a continuous twin-screw compounder,
kneader, or internal batch mixer.
[00069] Functional Layer
[00070] The choice of material for preparing the functional layer will
depend
on the desired performance properties of the finished laminated article and
the
choice of the materials used to prepare the heat shrinkable layer(s). The
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functional layer is prepared with a material having at least one performance
property superior to the heat shrinkable layer(s). The performance property
may
be one or more of: high temperature penetration resistance, softening point,
impact resistance, long-term thermal stability, tensile strength, toughness,
stiffness, thermal insulation, electrical conductivity or static dissipation,
impermeability to gases and moisture, and reduced cost. Preferably, the
functional layer comprises a polymeric material and even more preferably, a
non-crosslinked polymeric material.
[00071] In the present specification, "high temperature" denotes operating
temperatures typically 120 C, or higher. Materials which are suitable for use
at
"high temperatures" are those with an expected thermal stability and service
life
of at least 20 years at said operating temperature, typically defined in terms
of
time to reach 50% retained elongation.
[00072] In the present specification, a functional layer having the
property
of "high temperature penetration" refers to a functional layer which is
characterized as having a penetration of less than 0.4mm at 130 C as
determined by DIN 30678.
[00073] In the present specification, a functional layer having the
property
of "softening point" refers to a functional layer which is characterized as
having a
Vicat softening point of at least 130 C as determined by ASTM D1525, or a heat
deflection temperature under load (DTUL) of at least 80 C at 0.45MPa as
determined by ASTM D648.
[00074] In the present specification, a functional layer having the
property
of "impact resistance" refers to a functional layer which is characterized as
withstanding an impact energy of at least 5 3 per mm. of coating thickness as
determined by DIN 30678.
[00075] In the present specification, a functional layer having the
property
of "long term thermal stability" refers to a functional layer which is
characterized
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as having a service life at the operating temperature of at least 20 years as
determined by Arrhenius extrapolation of thermal aging data as described in
ASTM D3045, or similar.
[00076] In the present specification, a functional layer having the
property
of "tensile strength" refers to a functional layer which is characterized as
having
a tensile strength of 20 MPa or greater as determined by ASTM D638.
[00077] In the present specification, a functional layer having the
property
of "stiffness" refers to a functional layer which is characterized as having a
flexural modulus of 1,000 MPa or greater as determined by ASTM D790.
[00078] In the present specification, a functional layer having the
property
of "thermal insulation" refers to a functional layer which is characterized as
having a thermal conductivity less than about 0.20 W/mK.
[00079] In the present specification, a functional layer having the
property
of "electrical conductivity or static dissipation" refers to a functional
layer which
is characterized as having a volume resistivity of less than about 1012 ohm.cm
as
determined by ASTM D257.
[00080] The polymeric material used to prepare the functional layer may
comprise any of the polyolefin polymers and elastomers described above with
respect to the heat shrinkable layer. As compared to the heat shrinkable
layer,
the functional layer is preferably not crosslinked prior to lamination.
[00081] The functional layer may be filled with one or more conventional
fillers such as, but not limited to: clay, mica, talc, silica, wollastonite,
wood, glass
fibres, and metal oxides. The propylene, ethylene, and other polyolefin
polymers
described above can be combined with any of these inorganic fillers to reduce
costs or improve performance.
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[00082] The functional layer may also comprise nanocomposites of any of
the polymers described above wherein the polymer contains an exfoliated clay
additive.
[00083] The functional layer may also comprise a lower cost polymer or
recycled polymer in order to reduce the overall cost of the finished laminated
article.
[00084] The polymeric material used to prepare the functional layer may be
an engineering thermoplastic, which as used herein, refers to any
thermoplastic
that exhibits higher temperature resistance, higher softening point and/or
superior mechanical properties, such as penetration resistance, impact
resistance, stiffness and toughness, as compared with the heat-shrinkable
layer(s). Examples of engineering thermoplastics useful for preparing the
invention include materials such as nylons, polyesters, and polyurethanes.
[00085] The polymeric material used to prepare the functional layer may be
a barrier resin having low moisture and/or gas permeability. Examples of
barrier
resins which can be used to practice the invention include, but are not
limited to:
ethylene vinyl alcohol, polyvinyl alcohol or polvinylidene fluoride. Choice of
a
suitable barrier resin will depend on the extent of allowable moisture and/or
gas
transmission of the intended application.
[00086] The polymeric material used to prepare the functional layer may be
a polymer or polymer composite having thermal insulation properties, namely
those having a lower thermal conductivity relative to the heat shrinkable
layer(s). Such materials include polymers incorporating insulating fillers
such as
wood and hollow glass, ceramic or polymer microspheres.
[00087] The polymeric material used to prepare the functional layer may be
a polymer or polymer composite having electrical conductive properties, namely
those having a lower volume resistivity relative to the heat shrinkable
layer(s).
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Such materials include polymers incorporating conductive fillers such as
carbon
black or metal powders, or intrinsically conductive polymers such as
polyaniline.
[00088] Adhesive Layer
[00089] The adhesive layer can be prepared using any suitable adhesive.
The choice of adhesive will depend on the intended application of the
laminated
heat shrinkable covering and specifically the operating temperature of the
application. It is also dependant upon the composition of the heat shrinkable
layer, the functional layer, and the substrate to which it needs to bond. For
example, the adhesive layer may be prepared using a mastic-based sealant for
lower temperature applications or that requiring superior corrosion
protection.
For higher temperature applications, hot melt or curable adhesives such as
those
based on modified polyolefins, or polyamides, may be used.
[00090] The adhesive layer may further comprise a primer layer, for
example a curable epoxy resin, in instances where the heat shrinkable covering
is required to bond to a metal substrate, such as a steel pipe.
[00091] Preparation
[00092] The laminated heat shrinkable covering according to the invention
may be prepared by extruding the heat shrinkable layer using conventional
methods well known in the art. In embodiments comprising more than one heat
shrinkable layers, the heat shrinkable layers can be combined, through
extrusion
lamination or direct co-extrusion of the multiple heat shrinkable layers, to
form a
laminate structure having discrete but intimately bonded layers. Each of the
heat shrinkable layers may be prepared having identical or different
functional
properties or cost.
[00093] Once extruded, the heat shrinkable layer or layers may be
crosslinked by irradiation, preferably with electron beam, gamma or UV
radiation
depending on its composition. Preferably, the heat shrinkable layer or layers
are
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irradiated by electron beam radiation at a dosage from about 1 to 20 megarads
in an electron beam accelerator, for example a "Dynamitron" manufactured by
Radiation Dynamics Inc. The desired dosage is dependent upon the desired
properties of the article. Too low a dosage will result in the article having
a low
degree of crosslinking, poor mechanical toughness and a tendency to
prematurely soften or melt at elevated temperatures. Too high a dosage may
result in degradation, especially with polypropylene, with a resultant
unacceptable deterioration in mechanical properties. Preferably the radiation
dosage is between about 5 and 10 megarads for the manufacture of laminated
heat-shrinkable articles according to the invention. The dosage of radiation
should be sufficient to provide the article with a level of crosslinking, as
measured by the gel fraction, of about 20 to 90 percent. Preferably, the gel
fraction of the crosslinked layer is from about 30 to 70 percent and more
preferably from about 40 to 70 percent.
[00094] Alternatively, if the heat shrinkable layer comprises a silane-
modified polymer, crosslinking is effected by subjecting the heat shrinkable
layer
to moisture, preferably at an elevated temperature, which 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, but
the rate of crosslinking may be increased by the use of an artificially
moistened
atmosphere, or by immersion in liquid water. Also, subjecting the composition
to
combined heat and moisture will accelerate the crosslinking reaction. Most
preferably, crosslinking is affected 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% for approximately 100 hours.
[00095] Articles produced according to the invention can be rendered heat-
shrinkable since they exhibit the property of not melting when heated to a
temperature close to or above the crystalline melting point of the highest
melting
point polymer component. This is important because the crosslinked structure
allows the article to be stretched with minimal force and without melting, and
to
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retain its mechanical integrity when heated to this temperature. The article
is
fixed in this stretched state by rapidly cooling it below the crystalline
melting
point while holding the article in its stretched position, the reformed rigid
crystalline regions of the polymeric components of the material preventing the
article from spontaneously recovering to its original dimensions.
[00096] Following crosslinking, the heat shrinkable layer is bonded to the
polymeric functional layer by laminating the layers together. In embodiments
comprising a first and second heat shrinkable layer, the heat shrinkable
layers
may be directly laminated to one another. Alternatively, the laminate may be
formed with the functional layer sandwiched between the first and second heat
shrinkable layers. The heat shrinkable and functional layers are laminated
together by heating the layers to a temperature at which the individual layers
will fuse intimately together and then applying pressure, using casting
rollers for
example, to ensure adequate bonding of the layers.
[00097] The laminated article thus formed is then stretched using
mechanical, pneumatic or hydraulic methods. Cooling the article in its
stretched
state may be accomplished by air, water or other heat-transfer medium.
Subsequent re-heating of the stretched article above the melting point of the
highest melting point component will cause the crystalline regions to re-melt
and
the structure to elastomerically recover to its original unstretched
dimensions.
The crosslinked structure provides the initial recovery force and ensures that
the
article does not melt and that it maintains its mechanical integrity.
[00098] The resulting laminated structure comprising at least one heat
shrinkable layer and functional layer is then further laminated with an
adhesive
layer by passing the stretched sheet beneath a hot adhesive coating applicator
or
doctor blade.
[00099] The following examples illustrate suitable combinations of
polymeric
materials for use in the invention and the production of the heat shrinkable
coverings therewith.
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[000100] Example One
[000101] A composition comprising a silane-grafted polypropylene-
polyethylene blend, a silanol condensation catalyst, an antioxidant
masterbatch
and black pigment was heated above the melt temperature of the resin
components in an extruder and formed into sheet of 0.045in. thickness. The
sheet was then crosslinked by conditioning it at a temperature of 95 C and a
relative humidity of 90% for approximately 100 hours.
[000102] The crosslinked sheet was then extrusion laminated to an
uncrosslinked polypropylene functional layer by melt extruding said
polypropylene onto the pre-heated crosslinked sheet and passing the layers
between heated casting rollers.
[000103] The laminated sheet was then re-heated to a temperature of
approximately 150 C, and stretched by approximately 30% using a machine-
direction (MDO) mechanical stretcher. Whilst in the stretched state, the sheet
was rapidly cooled by feeding it between water-cooled steel rollers to below
the
crystalline melting point of the composition to fix the sheet at the stretched
dimensions. The laminated sheet was subsequently extrusion laminated with a
layer of hot melt adhesive.
[000104] The laminated sheet was then tested to determine the degree of
crosslinking and for the mechanical properties as indicated below:
[000105]
Property Test Method Performance
UltimateTensile ASTM D638 35
Strength (MPa)
Ultimate Elongation ASTM D638 420
(%)
Flexural Modulus! ASTM D790 1080
Stiffness (MPa)
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Hardness (Shore D) Shore D Hardness 72
Hot Tensile Strength Internal Specification 5
@ 200 C (psi)*
Water Absorption ASTM D570 0.5
(%)
100 C Penetration DIN 30678 <0.1
(mm)
110 C Penetration DIN 30678 0.2
(mm)
130 C Penetration DIN 30678 0.3
(mm)
Adhesive Peel DIN 30672 243
Strength @ 23 C
(N/cm)
Adhesive Peel DIN 30672 77
Strength @ 110 C
(N/cm)
* Hot Tensile Strength determines the degree of crosslinking by
measuring the strength of the composition =
above the melting point and hence the strength of the crosslinked network
only.
[000106] Example Two
[000107] A composition comprising a polyethylene, a radiation sensitizer, an
antioxidant rnasterbatch and black pigment was heated above the melt
temperature of the resin components in an extruder and formed into sheet of
0.045in. thickness. The extruded sheet was then crosslinked at a dosage of
approximately 10 megarads using a Radiation Dynamics "Dynamitron" electron
beam accelerator.
[000108] The crosslinked sheet was then extrusion laminated to an
uncrosslinked polypropylene functional layer by melt extruding said
polypropylene onto the pre-heated crosslinked sheet and passing the layers
between heated casting rollers.
[000109] The laminated sheet was then re-heated to a temperature of
approximately 150 C, and stretched by approximately 30% using a machine-
direction (MDO) mechanical stretcher. Whilst in the stretched state, the sheet
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was rapidly cooled by feeding it between water-cooled steel rollers to below
the
crystalline melting point of the composition to fix the sheet at the stretched
dimensions. The laminated sheet was subsequently extrusion laminated with a
layer of hot melt adhesive.
[000110] The laminated sheet was then tested to determine the degree of
crosslinking and mechanical properties, as described above.
Although the invention has been described with reference to illustrative
embodiments, it is to be understood that the invention is not limited to these
precise embodiments.
