Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02390273 2002-06-11
TWO-LAYERED SHEET MATERIAL FOR WRAPPING METAL
Field of the Inyention
The present invention is directed to wrapping materials for use in
packaging metal goods. More particularly, it is directed to a two-layer
sheet material that reduces corrosion of the wrapped metal goods during
handling, transportation and storage.
$ackground of the Invention
Most metals are susceptible to corrosion from ambient or atmo-
spheric conditions. The metal packaging industry has successfully
developed various methods and compounds for reducing corrosion of
metals during storage and use. However, the effectiveness and shelf life
of metal wrapping materials still requires improvement. Developing
new formulas for corrosion inhibitors and new methods of incorporating
them in the packaging material are two major areas of research.
Commonly, volatile corrosion inhibitors (also called vapor phase
corrosion inhibitors) are used in wrapping materials for metal items to
protect them from corroding. By vaporization from a solid or liquid
state, the volatile corrosion inhibitors reach the surface of the object to
be protected and form a stable bond with the metal surface. The limita-
tions for good efficiency are (1) the transfer of the inhibitors to the
metallic surface, (2) the rate of transfer, and (3) the effectiveness of the
packaging seal around the metal object, i.e. to prevent the inhibitor
from escaping into the atmosphere.
Contact corrosion inhibitors are transferred to the metal surface
by creeping of the inhibitors from the wrapping material directly to the
metal. This allows the corrosion inhibitor to remain with the metal even
if the package is not sealed properly.
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~u~ of Invention
The present invention provides a sheet material for packaging
metal items so as to protect them from corrosion during storage. It has
two layers, bonded together only at their edges along the length of the
material, in which the other layer is a woven scrim and the inner layer
is a polyolefin film having a combination of volarile corrosion inhibitors
and contact corrosion inhibitors. The outer layer may be a composite
layer in which one or more polyolefm films or coatings, or kraft paper,
is laminated to the scrim. The inner layer can slide freely across the
outer layer, permitting it to move independently of the outer layer and
cling to the wrapped metal items.
The unique combination of features provided by the sheet material
of the invention results in a mechanically strong and highly effective
wrapping material able to constantly and adequately supply corrosion
inhibitors to wrapped metal products for a substantial period of time,
effectively inhibiting corrosion.
Brief Description of Drawings_
Figure 1 is an enlarged cross-sectional view through the sheet
material according to the first embodiment of the invention.
Figure 2 is an enlarged cross-sectional view through the sheet
material according to the second embodiment of the invention.
Figure 3 is an enlarged cross-sectional view through the sheet
material according to the third embodiment of the invention.
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Figure 4 is an enlarged cross-sectional view through the sheet
material according to the fourth embodiment of the invention.
Detailed Description
Figures 1 to 4 show preferred embodiments of the sheet material.
In these drawings, the same or similar elements in the various embodi-
ments are indicated by like reference numbers.
Figure 1 illustrates a first embodiment of the invention. Sheet
material 2 comprises a scrim 4 and a polyolefm film 6 bonded along
opposite edges 8, 10 by strips of adhesive 12. In use for wrapping
metal goods, the film 6 is the inner layer, i.e. the layer that contacts the
wrapped metal goods, and the scrim is the outer layer.
It will be understood that the sheet material of the invention is
produced by feeding the layers off rolls and through conventional
fabricating equipment, including rollers or laminators. The direction
along the length of the material as processed through the production
equipment is referred to herein as the "machine direction." The direc-
tion perpendicular to the machine direction is referred to herein as the
"cross direction," being the direction along the width of the sheet
material between edges 8 and 10.
The scrim 4 is a woven structure made of polyolefm tapes which
are fabricated by methods well known in the art. The tapes are about
1.5 to 6 mm in width. The number of tapes in the machine direction is
preferably in the range of 4 to 16 tapes per inch. The number of tapes
in the cross direction is preferably in the range of 2 to 16 tapes per inch.
The dicetex of the tapes (grams per 10,000 meters) is preferably in the
range of 500 - 2, 500. The weight of the scrim is preferably in the range
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of 30 to 400 grams per square meter. The width of the scrim 4, and of
the sheet material 2, can be any convenient width that can be processed
on available production equipment and is suitable for a particular
application. It is preferably in the range of 30 to 160 inches.
Scrim 4 has high mechanical and tensile strength in both the
machine and cross directions. During production of the tapes, they are
stretched from three to five times their original length by passing
through hot pinch rolls to align polymer chains within the body of the
tapes to increase their strength. It is a function Qf the scrim to impart
mechanical strength to sheet material 2.
The scrim 4 may be fabricated of any suitable polyolefin, includ-
ing high density polyethylene, low density polyethylene and polypropy-
lene.
Film 6 is an extruded film made of any suitable polyolefin,
including polyethylene (low density or high density) or polypropylene.
The thickness is preferably in the range of 0.5 - 4 mils. The polyolefm
of film 6 can be the same as or different from the polyolefm of scrim 4.
Film 6 is impermeable to air and water, providing a barrier to protect
the wrapped goods from the atmosphere,
Polyolefm film 6 incorporates both a volatile corrosion inhibitor
and a contact corrosion inhibitor. Preferably, the volatile corrosion
inhibitor is present in concentrations of 0.1 to 5 percent by weight (i.e.
relative to the weight of the inner layer) . Preferably, the contact corro-
sion inhibitor is present in concentrations of 0.5 to 15 percent by
weight. The volatile and contact corrosion inhibitors can be selected
from ones well known in the art. For example, the volatile corrosion
inhibitor can be a nitrite compound such as dicyclohexylammonium
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nitrite; the contact corrosion inhibitor can be an alkali metal nitrite or
benzoate, such as sodium nitrite or sodium benzoate. Any of a wide
variety of volatile and contact corrosion inhibitors known in the art can
be selected for use in the invention. The volatile corrosion inhibitors
are transferred to the metal surface by volatilizing from the solid phase
and inhibit corrosion by forming a protective layer around the metal
surface. The contact corrosion inhibitors protect by creeping from the
film to the metal surface. The sheet material is preferably able to
supply corrosion inhibitors to the metal item for a period of up to two
years, effectively inhibiting corrosion.
The film 6, being substantially impermeable to gases, forms a
barrier that holds the volatile corrosion inhibitor, released from the film,
in proximity to the wrapped item, preventing the volatile corrosion
inhibitor from escaping into the atmosphere.
The scrim 4 is bonded to the film 6 by strips of adhesive 12 along
edges 8 and 10, i.e. along the edges in the machine direction. The
strips of adhesive are narrow relative to the total width of the sheet
material. They are preferably in the range of 0.5 - 3 inches. The layers
may alternatively be bonded by means of melt bonding or sonic bond-
ing, or other suitable means. The two layers of the sheet material are
accordingly unattached except at their edges 8 and 10, and are accord-
ingly free to slide relative to one another across their middle part and
thereby work independently have each other in protecting the wrapped
item. Not being laminated to the scrim, the inner layer is more free to
cling to the wrapped metal item. The natural tendency of a polyolefin
film to hold a static charge promotes static cling of the film to the metal
item. The close contact of the film 6 and the wrapped item is conducive
to transfer of the contact corrosion inhibitor to the metal item.
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The embodiments of the sheet material shown in Figs. 2 to 4
differ from that in Fig. 1 in having a composite outer layer. The inner
layer (film 6) and the bonding of the inner and outer layers along their
edges in the machine direction, remains the same.
Referring next to Figure 2, which illustrates a second embodiment
of the invention, the sheet material 20 differs from sheet material 2 in
having a film 22 laminated to the outer side of scrim 4. The film 22 is a
coating of polyolefin applied to the scrim to impart additional protection
against the transmission of air and water into contact with the wrapped
item. The film 22 is preferably from 0.5 to 3 mils in thickness and has
a weight of 12 to 72 grams per square meter. The polyolefm of film 22
may be any suitable polyolefm, including high and low density polyeth-
ylene and polypropylene. It can be the same as or different from the
polyolefin of scrim 4 and of film 6.
A third embodiment of the sheet material is shown in Fig. 3.
Sheet material 30 differs from sheet material 20 in having the film 32
laminated to the inner side of scrim 4. The film 32 is preferably from
0.5 to 3 mils in thickness and has a weight of 12-72 grams per square
meter. The polyolefin of film 32 which may be any suitable polyolefin,
including high and low density polyethylene and polypropylene. It can
be the same as or different from the scrim 4 and film 6.
The fourth embodiment is shown in Fig. 4. Sheet material 40
differs from sheet material 2 in that it includes a layer of kraft paper 42
laminated to the inner side of scrim 4 by means of laminating polyolefin
layer 44. The kraft paper 42 preferably has a weight in the range of
200 grams per square meter. It imparts strength and moisture-absor-
bency to the outer layer. The laminating layer 44 can be any suitable
polyolefin, including high and low density polyethylene and polypropy-
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CA 02390273 2002-06-11
lene. It can be the same as or different from the polyolefm of scrim 4
and film 6. It has a thickness in the range of 0.5 to 2 mils.
The corrosion inhibitors are incorporated into the resin used to
make the film of the sheet material by methods well known in the art.
Typically, the compositions are compounded with a carrier resin in a
master batch, which is then mixed and diluted with the base resin used
to make the film.
Although the invention has been described in terms of specific
embodiments, it is not intended that the invention be limited to these
embodiments. Various modifications within the scope of the invention
will be apparent to those skilled in the art. For example, instead of a
layer of kraft paper, a layer of non-woven fibers can be used. One or
more layers may include pigments or other additives, such as flame
retardants and UV-resistant compositions, making the product more
suitable for particular applications. The outer surface may be treated
with an anti-skid coating. The scope of the invention is defined by the
claims that follow.
