Note: Descriptions are shown in the official language in which they were submitted.
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LIQUID-PERMEABLE FILM FROM THERMOPLASTIC MATERIAL
The invention relates to a liquid-permeable film from
thermoplastic material~ which is particularly provided
for use as covering layer for articles having liquid-
absorbing inner layers, such as e.g. bandages and napkins.
The permeability of the film for liquids when the film is
used as covering layer for an absorbent material should
essentially only be present in the direction of this
material.
A liquid-permeable film has already been described in
United States patent 3,929,135 dated December 30, 1975
in the name H.A. Thompson as an inner layer of a disposable
napkin, The film proposed here has tapered capillaries
which have a conical shape and an apex, the apex containing
an opening.
It is an object of the present invention to provide a per-
forated film of the above-mentioned type, which not only
has a good capillary action but is also more simple to
manufacture, and for which primarily a uniform size of the
openings is better guaranteed.
In the films according to the invention the walls of the
embossments are so shaped as to be at right angles to the
base surface of the film and the inside cross-section of
these embossments is equal over their depth and corresponds
with the cross-section of the openings at the end of the
embossments. The embossments and openings can have any
given shape and they may e.g. be circular, rectangular,
polygonal or star-shaped. The minimum dimension of the
inside cross-section depends on the demands made on the
film. Minimum dimensions up to 0.1
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m~ are possible. In the case of polygonal or star-shaped cross-
sections of the embossments this minirnum measurement of the dia-
meter is an inner circleimagineable in the embossments. The depth
of the embossments is at least 0.3 mm and can also be 0.5 mm and
even up to about 1 mnl. It is likewise dependent on the ~emands made
on the film. In a preferred embodiment the cross-section of the
embossments is slit-shaped and has dimensions of 0.2 by 6.0 mm,
preferably 0.3 by 2.0 mm. These slit-shaped embossments are
preferably arranged in the film in rows staggered with respect
to each other and from row to row they have an alternating direction
with respect to the axis and/or the machine direction of the film.
Preferably they are inclined over 45 with respect to the axis.
In a further embodiment of this film the edge of the openings at
the ends of the embodiments is reinforced. This reinforcement is
effected by a bead. Advantageously, the bead lies on the outside
of the side walls of the embossments, so that the cross-section
here is not reduced.
Some embodiments of the embossments and openings in the film
according to the invention are illustrated in the drawings:
Fig. 1 and 2 are sections through two different embossmentsi
Fig. 3 to 5 show various arrangements of embossments and
openings in the film.
The embossmer,ts are made in such a way that the inner surfaces
of the side walls 2 of the embossments are formed at an approx-
imately right angle to the basic surface 1 of the film.
Deviations from the right angle of up to 5, as they may be
useful for the preparation and take-off of the film on an
engraved cylinder, are regarded as insignificant, similarly as
small rounding radii between the film 1 and the side walls 2.
The side walls 2 of the embossments have at their open ends a
head 3, which preferably only lies on the outside and does not
confirle the cross-se(tiorl of the capillaries 4 (Fig. 1). If the
3~ mininlurrl dimensiorl o-f the inside cross-section is sufficiently
large, the beads can, however, project a little into the
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capillaries, as shown in Fig. 2.
Fig. 3 shows a part of a film 11 with triangular embossments and
openings 12, and
Fig. 4 shows another film 13 with circular embossments and
openings 14.
The size, shape and depth of the embossments and the openings can
in any given case be adapted to the requirements. It is advan-
tageous however to leave a sufficiently wide, flat base surfacebetween the individual embossments, so that the film still
possesses an adequate firmness and is not deformed by the web
tension on winding and further processing.
Fig. 5 shows a preferred embodiment~of the film according to
the invention, in which the embossments and openings have the
shape of rectangular slits. The embossments and openings 16 in
the film 15 are arranged in rows, staggered with respect to
each other, and in these rows alternately inclined, preferably
at about 4~ to opposite sides, with respect to the axis.
In this preferred embodiment the embossments and the slit-shaped
openings 16 at the ends of the embossments have an inside
cross-section of 0.3 by 2.0 mm. The distance between the end
of a slit and an adjacent slit in this embodiment is about
1.7 to 1.8 mm. The flat surfaces in the film 15 remaining
between the slits could be embossed with a very fine pearl struc-
ture. When using this film in a thickness of about 0.025 mm as
covering film for an absorbent intermediate layer, such as the
one used in disposable napkins, it appeared that the film was
pleasant to the skin, that it could rapidly draw liquid from
the skin and that also in tne case of a more thoroughly wetted
interlllediate layer and pressure on the film, there was no
flowing back of the 1i4uid. At normal pressure on the combi-
3h nation no appreciable deformation of the slit-shaped embossments
and openings occurred. Only if a larger pressure was applied
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and a deflection of the film took place, did the dimensions in
the centre of the slits change, whereas at their ends capillaries
were retained which had about the sarne cross-section over their
length. In more thoroughly wetted intermediate layers this decrease
s in cross-section, which acts approximately as a "lip valve", was
evidently favourable in order to prevent the liquid from flowing
back.
As the cross-sections of the embossments are equal over their
entire depth, all openings in a film have the same inside cross-
section, also if owing to mechanical or thermical influences
during manufacturing the depth of the embossments in the finished
film should be different. The constant cross-section of the em-
bossments, moreover, results in an optimum capillary action. By
compressing the article which is prbvided with an absorbent inter-
mediate layer and is covered by the film according to the invention
on the side from which the liquid is to penetrate, opposite walls
of the embossments can be compressed, thus causing a change in the
free cross-section. However, no complete shutting can occur and -~
also as a result of the beads there will still be left free passages
at least at the ends or corners of the embossments.
A substantial closure of the openings by pressure on the material
will become possible if the depth of the embossments is larger
than the minimum dimension of their cross-section. In the case of
an approximately rectangular shape and a corresponding slit the
value chosen for the depth of the embossment will be larger than
that of its width. For certain films however, e.g. for a
"breathing" film, it may also be desirable that the slit, and the
opening, are not shut by the compression to so large an extent,
in order ~ still retain a sufficient passage of vapours, if a
reductiorl occurs in the passage of liquid possible. Slit and em-
bosslllerlt will then be so shaped as to ensure that the depth of
the embosslllent is e.g. smaller than its minimum dimension.
3'j
If beads 3 are made, e.g. by shrinking the ends of the embossments
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on forming the openings, the beads reinforce the edges of the
openings and thus prevent tearing of the film. The embossments
are usually so arranged as to penetrate into the inner layer,
consisting of a fibrous material. Since the beads are located on
the outside of the ends of the embossments, they promote an
anchoring between the covering film and the intermediate layer,
so that the film cannot simply be removed from the intermediate
layer and also any shifting of parts of this intermediate layer with
respect to the film is impeded. Any unevennesses in the formation
of the beads can be advantageous for the connection with the inter-
mediate layer.
The film can be provided with embossments and openings on its
entire surface or only in specific areas.
The liquid-permeable film is formed from a thermoplastic material
which in itself is imperrneable, such as polyethylene, polypropylene,
polyvinylchloride or their copolymers. The starting film has a
thickness of 0.02 to about 0.06 mm. According to a preferred
~0 process, which is not an object of this invention, first the em-
bossments are formed in an embossing nip, or by vacuum-deformation,
which embossments are still closed at their ends. By means of heat-
ing the end surfaces of the embossments are caused to shrink, as a
result of which the openings are formed as well as the beads on the
outside of the side walls of the embossments.
