Note: Descriptions are shown in the official language in which they were submitted.
This invention is directed toward a process for
use in making a bag. The invention is also directed toward
a sleeve for use in making a bag, and to the bag itself.
Many bags, particularly those used in the food
industry, are now made from layered, coextruded films or
sheets to provide certain desirable characteristics. The
outer layer of the film normally has good heat resistance
for example, while the inner layer of the film has good heat
seal characteristics so that the ends of the bag can~be
sealed together. Because of these characteristics, however,
the longitudinal seam in a bag made from such two-layer co-
extruded film must be a fin seal. In a fin seal the inner
layer of the film abuts on itself and since the inner layer
is made of material which has good heat seal characteristics,
the fin seal produced is strong.
It would, however, be preferable to use a lap seal
for the longitudinal seam. A lap seal requires 4 to 7% less
film material than a in seal thus resulting in a substantial
saving. In addition, existing bag filling machines would
not have to be changed over from their lap seal configuration,
which configuration is most common for bags made from waxed
glassine and cellophane film, to a fin seal configuration
needed when using layered, coextruded film. A lap seal is
not used, however, with layered, coextruded film since the
outer layer, which has good heat resistance, normally does
not seal to the inner layer at the low heat-sealing tempera-
tures used commercially.
It has now been discovered that the layered,
coextruded film can be treated in such a manner that the film
can produce commercially satisfactory lap seals. More
. 1 ~
~l~ 3~ Y
particularly, it has been found that if layered, coextruded
film is subjected to a fairly high corona treatment on both
sides, the heat-resistant outer layer of Eilm will bond to
the inner layer of film at relatively low heat-sealing tem-
peratures. Thus, lap seals where the inner layer of film
abuts the outer layer of film can be successfully used with
their attendant advantages instead of fin seals where -the
inner layer of film abuts itself.
The invention is particularly directed toward a
process for use in making a bag. The process comprises
providing a layered, coextruded film with one outer layer of
the film made from a first type of thermoplastic material
and the other outer layer of the film made from a second
type o~thermoplastic material, the layers of the first and
second types of thermoplastic material normally not heat-
sealable tcgether at low sealing temperatures. A corona
treatment is applied to both sides of the film to at least
slightly oxidize the outer surfaces of the film. Side
edges of the treated film are then overlapped to form a lap
joint and the lap joint is heat-sealed together at a low
temperature to form a sleeve for use in making a bag.
The invention is also directed toward the sleeve
or bag made from layered, coextruded film with a lap joint.
The invention will now be described in detail
haviny reference to the accompanying drawings, in which:
FIGURE 1 is a perspective view of a sleeve;
FIGURE 2 is a cross-sectional view of the sleeve
taken along line 2-2 of Figure l;
FIGURE 3 is a graph of heat-sealing temperature
vs. joint strength for various joined
materials;
FIGURE 4 is another graph of hea-t-sealing
temperature vs. joint strength for
various joined materials; and
FIGURE 5 is a further graph of heat~sealing
temperature vs. joint strength for
polypropylene and EVA.
A sleeve 1 is manufactured in accordance with
the present invention, for use in making a bag. A sheet 3 of
layered thermoplastic film has its ed~es 5, 7 overlapped to
form a lap joint 9. The lap joint 9 is then haat-sealed
together as shown at 11. The outer film layer 13 of the
sheet is made from one type of thermoplastic material and
the inner film layer 15 is made from a second type of t:hermo-
plastic material. More particularlyr the outer Eilm
layer 13 is made Erom suitable thermoplastic material which
is more heat-resistant than the inner film layer 15 while
the inner film layer is made from thermoplastic material -
which has good heat seal characteristics in comparison to the
outer layer. The outer film layer 13 is preferably made
from HDPE, although it could also be made from PP r or
copolymers of PP in which the propylene is usually present
as a major component. The inner film layer 15 is preferably
made from EVA although it can also be made from EEA, EMA or
~AA or other similar polymers or copolymers. The two layers
are coextruded together to form a one-piece film.
Normally, the HDPE outer layer 13 does not pro-
perly heat seal to the EVA inner layer 15 at low temperatures.
In accordance with the present invention the sheet 3 is given
a fairly high corona treatment on both sides prior to forming
the sleeve. The corona treatment, in the order of 40 dynes/cm.
(which is a measurement of the property of the material), has
the effect of oxidizing the surfaces of the layers 13, 15, thus
appearing to make them more chemically compatible. The
measurements may be ~rom a reading of 36 dynes and up,
e.g., up to 55 dynes, but problems may be encountered at
higher levels. When the lap joint 9 is heat sealed together,
the bond in the joint appears to be more of a chemical type
than of a melt fusion type commonly associated with conven-
tional heat sealing.
The corona treatment thus permits the overlapped
HDPE and EVA surfaces of layers 13, 15 to bond tightly to-
gether at low heat-sealing temperatures, whereas without the
corona treatment the surfaces would not bond tightly to-
gether, particularly at low heat-sealing temperatures.
The improvement in bond strength following corona
treatment i9 more clearly shown having reference to Figure
3. Samplas of different ~ilms have been heat-sealed to-
gethar in a laboratory at fixed conditions of sealing
pressure, sealing bar jaw width, and dwell time. The tem-
perature of heat-sealing has, however, been varied to
provide the only variable. The seals obtained, at the dif-
ferent temperatures, are then pulled apart on a laboratory
tensile tester at a fixed separation rate. The maximum force
required to pull the seals apart is noted. A plot was then
made of force required vs. the heat-sealing temperature for
the different seals tested as shown in Figure 3.
The first sample seal tested involved sealing
untreated EVA film to EVA film. This sample generally
duplicates the condition in actual bag making where normally
a two-layered coextruded sheet is used having a HDPE outer
~layer and an EVA inner layer. The two edges of the sheet
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are brought together with the EVA inner layer abutting to
form a fin seal. From Figure 3 it will be seen that the
EVA to EVA sample has a great deal of strength at relatively
low heat-sealing temperatures.
The second sample seal tested was that of HDPE
film to EVA film. Both films are untreated and the seal
generally duplicates the case in bag making where a lap seal
is ~ormed from a two-layered coextruded sheet with the
inner EVA surface at one edge, lying on top of the o~ter
HDPE surface at the other edge. As seen by the plot of
"EVA to HDPE (untreated)" in Figure 3, the strength of the
seal is very low in comparison to the strength of the "EVA
to EVA (untreated)" seal, particularly in the commercially
used sealing temperature range of 200F to 280F.
When the films have been given a fairly high corona
treatment, of about 40 dynes/cm., on both sides and then
sealed together, the strength of the joint is markedly im-
proved. When the treated film is used to duplicate a fin
seal, with the EVA surfaces abutting, greater strength
is obtained at low temperatures than with untreated ab~tting
EVA surfaces, as shown by the plot of "EVA to EVA (treated)"
in Figure 3. A more marked difference, however, is shown
by the plot of "EVA to HDPE ~treated)". With this plot, it
is seen that adequate seal strength is obtained, even at low
sealing temperatures, after the normally incompatible HDPE
and EVA film layers have been corona treated.
The plots in Figure 4 are similar to those in
Figure 3, showing the marked improvement in the strength of
the seal ai low temperatures for "EVA to EVA" when treated
as compared to untrea-ted, and for "EVA to HDPE" when treated.
~3~
The plots of "EVA to HDPE (treated)" in both
Figures 3 and 4 show that a lap seal can be satisfactorily
used in sleeve or bag manufacture when using co-extruded film
consisting of a HDPE layer on an EVA layer provided both
sides of the film have been corona treatecl.
With reference to Figure 5, the improved
results using polypropylene to treated EVA will be readily
evident compared to the results obtained f-or untreated PP
to EVA and thus, very significant increases have been
obtained,.as is evident from Figure 5.
While a two-layer film has been the only film
discussed, a three-layer film, having HDPE as one outer
layer, and one of EVA, or a copolymer thereof, EEA, EM~
or EAA as the other outer layer can also be used.
As used herein, the.preceding abbreviations
have their standard meanings in this art, namely, ethylene
vinyl acetate, high density polypropylene, ethylene maleic
anhydride, etc.
