Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
0
B~CKGROUND TO THE INVENTION
This invention relates to heat-recoverable articles.
Heat-recoverable articles, especially heat-shrinkable
articles, are now widely used in many areas where insulation,
sealing and encapsulation are required. Usually these
articles recover, on heating, towards an original shape from
which they have been previously deformed, but the term
"heat-recoverable" as used herein also includes an article
which, on heating, adopts a new configuration, even if it has
not been previously deformed.
Heat-recoverahle articles are typically made from
polymeric materials exhibiting the property of plastic
or elastic memory as described, for example, in U.S. Patents
2,027,962, 3,086,242 and 3,957,372. In other articles, as
described, for example, in British Patent No. 1,440,524, an
elastomeric member such as an outer tubular member is held
in a stretched state by a second member, such as an inner
tubular member, which, upon heating, weakens and thus allows
the elastomeric member to recover.
Such heat-recoverable articles are frequently in the form
of heat-shrinkable tubes, caps or boots or other articles having
a closed cross-section and one or more open ends. One wi~ely
used method of making such articles is to mould a molten,
crystalline, thermoplastic polymer into a shape which approx-
imates to the shape of the final covering required, cross-link
the shaped polymer, heat the cross-linked, shaped polymer to a
temperature above its crystalline melting point, expand the
hot, cross-linked, shaped polymer, e.g. by a mandrel or
,~
. ~.
.
internal pressure, and cool the polymer in its expanded shape.
The present invention especially relates to such heat-shrinkable
articles, including in particular (but not limited to) those
made by the process described above.
One serious limitation of such heat-shrinkable articles
has been that if too high an expansion ratio (i.e. the ratio
of the dimension of the article after expansion to the corres-
ponding dimension before expansion) is used in making the
article, the article tends to split on expansion or during
shrinkage, and we have observed that this problem is partic-
ularly noticeable at the open ends of such articles. The
problem is especially acute if the article has been cut so as
to leave a notch or nick which may propagate on recovery and/
or when the article is caused to recover about a large
substrate leaving a large degree of unresolved recovery. In
accordance with the present invention, we have discovered
that it is advantageous if the or each open end of the article
is formed by a section such that, when the article is fully
shrunk, the open end is larger than the adjacent portion of
the article. In particular we have found that the adjacent
portion can then be given a higher expansion ratio than would
otherwise be possible without danger of splitting.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a heat-shrinkable
tubular cap which comprises an open end section having a
first expansion ratio, an intermediate section having a
second expansion ratio and an unexpanded closed end section,
the second expansion ratio being greater than the first
expansion ratio by at least 0.5~
DETAILED DESCRIPTION OF THE INVENTION
As mentioned above, the present invention is espec-
ially applicable to heat-shrinkable articles made from
materials, especially plastics materials, possessing the
property of elastic or plastic memory.
Amongst such heat-shrinkable articles there may
especially be mentioned those with a closed cross-section.
The articles are commonly tubular, the term "tubular"
including both hollow cylindrical members and members of
irregular and/or varying cross-section as well as those of
more cornplex configuration such as ~-shaped, T-shaped and
X-shaped members having 3 or 4 ends.
In most cases the open end section will have a larger
cross-section after recovery than the adjacent intermediate
section which has a substantially greater degree of recovery.
The open end section may have a finite length, for example
it may extend over a length of from 1 to 10cm., but, in some
cases the article may be given a differential expansion so
as to produce an increasing expansion ratio along its length
away from the open end and the present invention is, therefore,
not limited to heat-recoverable articles having a discrete
end region.
It will be appreciated that the end region may be
non-recoverable, i.e. it may have an expansion ratio of 1,
,
11~11~70
but, in many applications, it will be advantageous for it
to exhibit a small degree of heat-recovery. However, in
such cases the recovery is preferably less than 100%, more
preferably less than 25% (corresponding to expansion ratios
of 2 and 1.25, respectively).
On the other hand, in order fully to utilise the
advantages of the present invention, the intermediate region
generally exhibits a high degree of available recovery.
The expansicn ratios which will, in the absence of end
sections as required by the present invention lead to
splitting on shrinkage are dependent inter alia on the
polymer employed and the thickness. There is seldom a problem
with expansion ratios below 2.5, and usually expansion ratios
of up to 3.0 can safely be used. Howevever, at expansion
ratios of 3.5 or higher splitting is usually a problem and
this applies more so at ratios of 4.0, 4.5 and 5Ø Thus
the present invention is especially applicable at ratios of
from 4 to 6 and above, for example up to 10, i.e. where the
amount of avaiLable recovery is from 400% to 1000%, usually
from 400% to 600%, especially when the percentage of
unresolved recovery after shrinkage is high.
Another factor which is relevant to the problem
is the difference between the expansion ratios of the
open end region and the intermediate region. Generally
speaking, the higher the expansion ratios, the greater should
this difference be, but in most cases a difference of
170
at least 1.0 and, most preferably, at least 1.5, will be
desirable. The difference should preferably be at least
0.~ times, especially at least 0.5 times, the expansion
ratio of the lntermediate region.
It will be appreciated that the heat-shrinkable
articles of the present invention may have one or more
ends which are closed but which are intended to be
opened, for example, by cutting, at a later stage, the
invention may also advantageously be applied to such
temporarily closed ends.
The preferred articles of the present invention
may be prepared from any of the polymers and mixtures of
polymers known from the prior art to be useful for the
production of heat-shrinkable articles. Generally
the articles will be monolithic and of constant com-
position throughout, however, laminates of two heat-
shrinkable polymers can be used, and the articles can be
constructed by joining together, e.g. with adhesive, two
or more different parts. Suitable polymers include polyolefins,
especially polyethylene, copolymers of ethylene and vinyl
acetate, (which are particularly valuable when flexibility
is required), copolymers of ethylene and ethyl acrylate,
chlorinated or fluorinated polymers, especially polyvinyl
chloride, polyvinylidene fluoride and polymers incorporating
units derived from polyvinylidene fluoride, hexafluoro-
ethyIene and chlorotrifluoroethylene, and rubbers such
- 6 -
7~
such as ethylenepropylerle rubber, chlorinated rubbers
(~eoprene) and silicone rubbers which may be used in a blend
with a crystalline or glassy polymer such as an olefin polymer~
The internal surface of the articles may be provided with
a sealant such as a hot-melt adhesive or a mastic. Especially
suitable hot-melt adhesives include for example polyamide
materials, ethylene/vinyl acetate copolymers and terpolymers
(with or without incorporated waxes) and polyesters. Such
materials are described, for example, in British Patent
~o. 1,440,810 and German OS 2,709,717. Also suitable are
curable adhesives which also melt and flow upon heating, but
which will not afterwards be hot-meltable. There may also be
mentioned epoxy resins and conventional mastics such as, for
example, those based on butyl and isobutylene rubbers modified
with suitable materials known in the art. Obviously the type
of sealant employed and the method of its application will
depend on the particular requirements in any given case.
Bearing in mind the above discussion it will be
appreciated that the shape and design characteristics of
the heat-recoverable articles of the present invention will
depend on the use intended for them. In some cases, it may
be preferable for the open end regions to be thicker than the
intermediate high-recovery regions in which case it may be
convenient to prepare a moulded article which has (prior to
expansion) the same wall thickness throughout so that the
ratio of the thickness of the open end section(s) to the
thickness of the intermediate section(s) after expansion will be
7~11
substantially the same as the inverse ra-tio of their expansion
ratios.
Similarly, the shape, after shrinkage, of the open end
region and the intermediate region will be dependent of the
relative sizes of the article and of the substrate as well as
the expansion ratios of the two regions. Thus both regions
may shrink into contact with the substrate along their entire
length. More usually, however, the open end section will
spontaneously recover on heating to a size larger than the
substrate and will, therefore, form a collar at the end of the
article which flares out of contact with the substrate.
Depending upon the application concerned, this collar may be
left in place or may subsequently cut off.
In general, the tubular cap will be of circular cross-
section, although it may have another cross-section, and it
is often preferable that it should have a substantially
constant cross-section.
The articles are preferably monolithic, i.e. are moulded
or otherwise formed as one part, but they may be made from
two or more parts bonded together, for example with an adhesive,
or otherwise attached on to the other~
The heat-recoverable articles of the present invention
are especially useful when the degree of recovery required in
a given application is, or may be, very high because the
articles are capable of providing a high degree of recovery
or accommodating a high degree of unrecovered strain without
splitting,
~r --8--
7(;)
A tubular cap according to the present invention
is particularly useful for providing a sealed end closure
of an elongate substrate having a terminal portion of size
greater than the main part of the substrates, especially a
telephone cable comprising a plurality of wires to which
connectors have been attached. To an ever increasing extent,
connectors are being installed on telephone cables in the
factory, in order to avoid some of the expense and incon
venience of working at the installation site, and it is
vitally important to protect the end section, having the
connectors attached thereto, while the cable is taken from
factory to site. To install the cap, it is placed over the
substrate so that the closed end section extends over
terminal portion as far as the main part of the substrate,
and the intermediate section of the article is then heated
to cause shrinkage thereof into contact with the main part of
the cable, but not into contact with the terminal portion
of the substrate. It is to be noted that the presence of the
open end section of lower expansion ratio means that the inter-
mediate section can be given a higher expansion ratio, andcan therefore pass over a larger terminal portion, than
would otherwise be the case. This improvement is of particular
importance in the case of telephone cables as described above,
since the multitude of connectors which must be attached
to the individual wires (often 400 or more in number)
occupy a considerable space, whose circumference is often
.;. ~
17~D
at least 4 times, for example about 6 times, the circum-
ference of the main part of the cable, to which the inter-
mediate section of the cap must shrink and seal.
The dimensions and expansion ratios of a cap
according to this second embodiment of the invention will of
course be dependent on the substrate, but typically the open
end section is 1 to 7.5 cm., preferably l.to 5 cm. long,
the intermediate section is 2 to 15 cm., preferably 7 to 12 cm.,
long and the closed end section is 20 to 100 cm. long, and the
cap has a diameter of 10 to 25 cm~
It will be appreciated that the heat-recoverable
articles of the invention may be made by various methods and
that the invention is not limited to any particular method
for their manufacture. However, in a preferred embodiment,
the present invention also provides a method of making a
heat-shrinkable tubular cap which comprises the steps of
(a) moulding a molten, thermoplastic, crystalline
polymer into a shaped article having an open end,
an intermediate section and a closed end, the
open end being defined by an end section whose
circumference is greater than the intermediate
section of the article adjacent thereto,
(b) cross-linking the moulded article;
(c) heating the cross-linked article above the
crystalline melting point of the polymer,
(d) expanding at least the intermediate section
of the heated article, and
- 10 --
. ~ ' .
7~
(e) cooling the article whilst maintaining it in
the expanded condition.
In its third aspect the invention provides a method
of covering a part of an elongate substrate which comprises
placing around said part of the substrate at least part of
a heat-shrinkable tubular cap as described above and heating
at least part of the article to cause shrinkage thereof
into contact with the substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
1~ Figure 1 is a cross-section through a heat-recoverable
article according to the present invention
formed as an end cap, and
Figure 2 is a cross-section showing the end cap of
Figure ~ installed about a telephone cable
having connectors attached to the individual
wires thereof.
DETAILED DESCRIPTION OF THE DRAWINGS
Referring now to the drawings, in Figure 1 there is
shown a tubular end cap 31 which has an open end section 32
having an expansion ratio of about 2 at the open end thereof,
an intermediate section 33 which has an expansion ratio of
about 5, and a closed end portion 34 which is not heat-
shrin~able. The cap is made from high density polyethylene.
The dotted lines in Figure 1 show sections 32' and 33' of
the article prior to expansion which become sections 32 and
33. Typical dimensions, in inches, of three such end caps
are as follows:
,~ - 11 -
a b c d e f g h
1st 0.80 4.0 0.075 2.80 0.15 1 3 4
2nd 1.50 6.0 0.105 3.50 0.15 1 3 4
3rd 2.50 8.0 0.135 4.50 0.15 1 3 4
All three caps have a total length of 34 inches and the cap of
Figure 1 is shown in truncated form for convenience.
Referring now to Figure 2, telephone cable 40 comprises
a plurality of wires 41 encased by a protective sheath 42 which
has been removed from the end of the cable. Connectors have
been attached to the ends of the wires 41, and the mass of
wires and connectors is designated generally by 43. An end
cap 31 as shown in Figure 1 has been passed over the mass of
wires and connectors 43 and the sections 32 and 33 have been
heated so that they shrink to provide shrunken sections 32a
and 33a respectively. Section 33a is in contact with the
cable while section 32a, having a lower expansion ratio, forms
a flared collar.
Attention is drawn to copending Application Serial No.
290,309, title "Heat-Recoverable Article", inventors
Francis De Blauwe and Frank Selleslags, filed 7th ~ovember 1977
~ , .
.f`'~"
