Note: Descriptions are shown in the official language in which they were submitted.
23
This invention relates to heat-shrinkable polymeric articles and
is a division of Canadian Application Serial No. 325,913, allowed
November 3, 1981.
Heat-recoverable polymeric articles are well known in the art, and
heat-shrinkable articles have been extensively used for covering substrates,
for example the ends of electrical cables and splices between electrical cab-
les. Reference may be made, for example,to United States Patents Nos.
2,027,692, 3,086,242, 3,243,211 and 3,396,455. The heat-recoverable article
is commonly employed in the form of a heat-shrinkable tubular sleeve or wrap-
around sleeve as described, for example, in United States Patents Nos.
3,243,211, 3,297,819 and 3,379,218, and in British Patents Nos. 1,155,470,
1,211,988 and 1,346,479.
In wrap-around sleeves a heat-recoverable article having a closed
cross-section is made by wrapping a sheet of heat-recoverable material around
a substrate to be covered and securing the edges of the sheet together. A
number of ways of securing the edges together have been disclosed, for
example, in United States Patents Nos. 3,379,218, and 3,455,336.
When a hermetic seàl is needed between the heat-recovered article
and the substrate and/or the seal must withstand high separation forces, as
for example when the substrate is a pressurised telephone cable, it is common
practice to coat the interior surface of the heat-recoverable article with a
layer of a fusible material such as a hot-melt adhesive or a mastic to
improve the seal between the recovered article and the substrate. Heat-
shrinkable articles having a solder insert have been extensively used to pro-
vide insulated soldered joints between electrical components.
Recovery of heat-recoverable articles is usually effected by hot
air or a flame. However, the combination of convection and conduction heat-
~'''~
23
ing which these methods involve can be unsatisfactory when a fusible insert
is employed, since the continued heating which is required to fuse the
insert, after the article has recovered, can damage the article. This prob-
lem is considered in United States Patent No. 3,396,455, which describes the
use of infra~red radiation to heat a heat-shrinkable polymeric sleeve and a
fusible insert, typically a solder insert, positioned within the sleeve, the
sleeve transmitting a fraction of the infra-red radiation to the insert so
that the insert is heated to its fusing temperature more rapidly than it
would be if it was heated by convection and conduction; preferably the sleeve
is heated to its recovery temperature before the insert reaches its fusing
temperature.
The present invention may be defined as a generally tubular heat-
shrinkable article which comprises at least one heat-shrinkable band portion
whose interior surface is composed of a cross-linked polymeric composition,
which composition comprises ~a) a cross-linked crystalline olefin polymer
having a gel fraction of at least 0.3, a density of at least 0.95 and a modu-
lus at 175C of at least 40 psi, and (b) has an absorptivity such that said
band portion, when exposed to radiation having a wavelength of 1.15 microns~
transmits 5 to 50% of the radiation.
The term "generally tubular" as used herein includes~ for example,
tubes of circular and non-circular cross-section as well as Y-shaped, T-shaped,
and X-shaped members, which tubes may, for example, have at least one closed
end. Thus, amongst covering members which are included within the scope of
the present invention there may be mentioned, for example, heat-shrinkable
sleeves, wrap-around sleeves, udders, boots and end-caps.
In preferred embodiments of the present invention the second poly-
meric composition from which at least the interior wall portion of the cover-
--2--
Z3
ing member is made comprises an absorptive material dispersed therein and,
for convenience, the invention will be described with reference to the pres-
ence of such an absorptive material. It will be appreciated, however, that,
although in preferred embodiments the absorptivities of the substrate and
covering member will be governed by the nature and amounts of the infra-red
absorptive fillers dispersed therein, in some cases the absorptivity of the
second polymeric composition may be such that the incorporation within it
of a dispersed absorptive material is unnecessary.
The radiation used to heat the substrate and recoverable article
is preferably infra-red radiation, in which case the absorptive materials
in the substrate and the covering member (which materials can be the same or
different) should, of course, be ones which absorb infra-red radiation, e.g.
carbon black, which is usually preferred. The amount of absorptive material
in the second crystalline polymeric composition is preferably such that the
heat-shrinkable band portion transmits from 5 to 50%, particularly from 5 to
40%, especially from 5 to 20% of~the radiation used to effect heating, for
example, in the case of infra-red radiation, radiation of wavelength 1.15
microns. When using carbon black as the absorptive material, the amount
thereo in the second polymeric composition is generally from 0.01 to 0.04%
by weight, preferably from 0.025 to 0.035% by weight. The amount of absorp-
tive material in the exterior wall portion of the substrate is preferably
such that it will absorb substan~ially all the radiation transmitted by the
heat-shrinkable article, for example, in the case of carbon black, from 0.25
to 2%, preferably from 0.25 to 1%. However, some reflection and transmission
of the radiation can take place providing that the desired heating effect is
obtained.
The exterior of exterior wall portion of the substrate is preferably
--3--
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heated to a temperature of at least Tl before the heat-shrinkable band por-
tion is heated to its shrinkage temperature. In order to obtain the best
sealing, the exterior wall portion should be heated to a temperature of at
least Tl to a depth of at least 0.0005 inch, e.g. from 0.0005 to 0.005 inch.
However, if the wall portion is heated excessively, this can lead to rela-
tively less satisfactory results, and it is therefore preferred that at a
depth of 0.075 inch, especially at a depth of 0.050 inch, the temperature of
the wall portion should not exceed (Tl - 5)C.
The first crystalline polymeric composition is at most lightly
cross-linked, and is preferably substantially free from cross-linking. Its
gel fraction is below 0.3, preferably below 0.2, especially below 0.1. The
polymer generally has a crystallinity o at least 15%, preferably at least
20%. Suitable polymers include olefin polymers (including copolymer of ole-
fins with other comonomers, generally in amount less than 50% by weight),
especially ethylene polymers containing at least 90% by weight of ethylene,
e.g. polyethylene and copolymers of ethylene with one or more comonomers
such as vinyl acetate and alkyl esters of acrylic and methacrylic esters.
The polymer in the second crystalline polymeric composition is
cross-linked to a gel fraction of at least 0.3, preferably at least 0.4 and
has a crystalline melting point which is at least ~Tl + 5), preferably at
least ~Tl ~ 10) especially at least ~Tl ~ 15)C. Polymers having a modulus
at 175~ of at least 40 psi, e.g. 40 to 75 psi, are preferred. It is theor-
ised that when using such polymers, the band portion, as it shrinks, exerts
a hoop stress on the substrate which helps to ensure a good seal. The modu-
lus at 175C is measured at 100% extension using a specimen 0.125 inch wide
and 0.075 inch thick on an Instron tensile tester with the jaws 1 inch apar~
and a separation speed of 2 inch/minute.
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23
The heat-shrinkable band portion of the covering member will gener-
ally be of uniform composition, in which case the interior wall portion ref-
erred to above will constitute the whole of the band portion, but the inven-
tion includes covering members in which the band portion includes one or more
layers on the outside of the interior wall portion, which layers may ~but
need not be~ independently heat-recoverable.
The method of the parent application is of particular value for
applying an end cap to an air-core telephone cable. However, it is to be
understood that that method is useful in a wide variety of other situations
which make use of the combination of parameters which we have found to be
critical to providing a high quality seal by direct contact between a heat-
shrinkable article and a substrate. While the invention does not rely upon
the use of a fusible material such as a hot-melt adhesive or a mastic to pro-
vide the seal, it is to be understood that the invention does not exclude
the possibility of using a fusible insert at some other point in the assem-
bly, for example a solder insert to provide or improve an electrical connec-
tion between cables within a tubular heat-recovered splice case, or an
intermediate layer of a hot-melt adhesive or mastic in an area adjacent to
the area of direct contact between the substrate and the article.
One form of embodiment of the present invention will now be des-
cribed, by way of example only, with reference to the accompanying drawings,
in which
Figure 1 is a cross-sectional view of an assembly of an air-filled
telephone cable, a heat-shrinkable end cap placed around the cable, and an
infra-red heater placed around the end cap;
Figure ~ is a cross-sectional view of the cable having a heat-
recovered end cap sealed thereto obtained from the assembly of Figure 1.
--5--
Referring now to the drawings, Figure 1 shows heat-shrinkable end
cap 1 disposed about an air-core telephone cable containing telephone wires
3 and insulating jacket 2 composed of an ethylene/vinyl acetate copolymer
containing about 4% by weight of vinyl acetate and having about 3.0% of car-
bon black dispersed therein. End cap 1 is composed of cross-linked high
density polyethylene containing about 0.03% by weight of carbon black.
End cap I is formed with a heat-recoverable protuberance 5 in the
end thereof, for example as described and claimed in German Gebrauchsmuster
7,62S,978. Also shown in Figure 1 is cap-heater 6 containing twelve 6 inch
long quartz tungsten lamps 7. The cap-heater is switched on for 30 seconds
and effects recovery of ~he end cap around the cable to give the structure
shown in Figure 2.
The invention is further illustrated in the following Example.
EXAMPLE
__
High density polyethylene of density 0.96 ("Alathon 7030"), 0.03%
of carbon black and 1% of zinc stearate (a mould release agent) were blended
together. The blend was injection moulded at 6000 psi into a cylindrical
cap having an internal diameter of 0.70 inch, a length of 3 inch and a
thickness of 0.09 inch. The cap was irradiated to a dosage of about 10
Mrads, and then heated to about 163C in a mixture of glycerine and water.
The heated cap was expanded by means of air at 90 psi into water cooled
mould so that it has an internal diameter of about 1.75 inch. The polymeric
composition of the heat-recoverable cap had a gel content greater than 0.5
and a melting point of about 125C.
The cap was placed over the end of an air-core telephone cable of
outer diameter about 1.4 inch in an assembly as shown in Figure 1, and
recovered by switching on the cap hea~er for 30 seconds. The cable jacket
--6--
923
was composed of an ethylene/vinyl acetate copolymer containing about 4% vinyl
acetate as described above and had a gel fraction of zero and a melting
point between 90 and 95C. The seal between the cable and the cap withstood
at least 120 lb of shear load per square inch of sealed area.
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