Laminated pressure sensitive adhesive strip for use in plastic bags

An improved adhesive system for forming resealable channel closures for flexible bags such as thermoplastic bags made from polyethylene, e.g. sandwich bags. The adhesive system comprises a layer of hot melt adhesive over which a thin second layer of a liquid-based adhesive such as a water-based pressure sensitive acrylate is applied.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an improved adhesive system for use in sealing 
reclosable flexible bags such as thermoplastic bags made from 
polyethylene. Related applications assigned to the same assignee as this 
application include the following copending and concurrently filed 
applications: "Adhesive Bag Closure That Opens Easily By Hand But Resists 
Opening By Contents," Ser. No. 335,800, filed Dec. 30, 1981; "Protective 
Strip for Z-Fold Bag Closure," Ser. No. 335,955, filed Dec. 30, 1981, now 
U.S. Pat. No. 4,410,130; "Manufacturing Process For Channel Seal," Ser. 
No. 365,814, filed Apr. 5, 1982, now U.S. Pat. No. 4,392,897; and 
"Adhesive Channel Closure For Flexible Bags," Ser. No. 335,798, filed Dec. 
30, 1981, the last application being incorporated by reference herein. 
2. Description of the Prior Art 
Plastic bags are used in a variety of household applications, especially 
those involving packaging and storing food items, e.g., sandwiches. The 
use of these bags for food packing and storing appears to have added an 
impetus to the search for better means of sealing such bags in such a way 
that the bags may be opened and then resealed. One area of development has 
included the so-called profile bags, an example of which may be seen by 
referring to U.S. Pat. Re. No. 28,969 to Naito. These profile bags have at 
least one set of mating channels which must be placed in registration 
before sealing can be effected. Such profile structures may be used to 
construct bags which are openable and resealable. As seen in U.S. Pat. No. 
4,186,786 to Kirkpatrick, colored channels may allow the user to more 
easily detect complete occlusion of profiled bag openings. In order to 
form a seal, a profile channel must be placed in registration with its 
mating member, but this may be inconvenient for the user. 
A second area of development is the use of adhesives to form resealable 
closures for bags. Usually one or more strips of adhesive are applied to 
one surface of a bag opening and sealing is effected by contacting the 
surface having the adhesive strip with a second surface on the opposite 
side of the bag opening. For example, U.S. Pat. No. 3,670,876 to Davis 
discloses a bag for household use including an improved closure comprising 
a plurality of pressure sensitive adhesive strips located in spaced apart 
relationship on an exterior portion of the bag. An offset flap portion is 
adapted to be folded over and attached to one or more of the adhesive 
strips to tightly close the bag. 
While bags with adhesive closures have an advantage of being adjustable to 
the size of the object contained in the bag, it has been difficult to find 
a satisfactory adhesive which is suitable for use with reclosable plastic 
bags, such as those made of polyethylene film, which are packaged in a 
carton or on a roll. More particularly, an adhesive strip which adheres 
well to a polyethylene surface may form a seal that is not easily 
reopenable without destruction of the closure or a portion of the bag, 
while an adhesive that forms a resealable closure may not adhere well to a 
base film or closure strip. One test which is generally applied to 
products such as this is that the product must not deteriorate when 
subjected to a temperature of 140.degree. F. for 24 hours. 
Previous attempts at finding adhesives suitable for use in constructing 
resealable closures for flexible bags have involved two particular 
problems. First, while hot melt adhesives exhibit exceptionally high bond 
strengths when applied hot and are easy to apply at high speed without 
need of a drying step, the adhesive strength of a hot melt adhesive may 
build up on standing which leads to destruction of the closure upon 
opening. Also, the strength of a hot melt adhesive may be reduced after 
aging at 140.degree. F. while in contact with polyethylene film (e.g. in a 
sealed position with an opposing surface of polyethylene film). Second, 
while liquid-based adhesives such as water-dispersions of acrylates do not 
suffer the disabilities of strength build up or deterioration with aging 
at 140.degree. F. while in contact with polyethylene film, these water 
dispersions of adhesives do not form a satisfactorily permanent bond to 
polyethylene film; thus sections of an adhesive strip may be removed 
during opening of the bag leading to a deterioration of the seal after a 
number of openings and resealings. Other types of liquid-based adhesives 
such as those comprising organic solvents are difficult to use in a 
manufacturing process because of the requirement of a separate drying step 
and means to safely remove vapors generated thereby. 
Processes have been tried to provide a closure in which the surface to 
which the adhesive is to be applied is pretreated so that the adhesive 
will more firmly adhere to the pretreated section to create an effective 
seal which is openable and resealable without deterioration of the seal or 
destruction of the closure. Pretreatment of polyolefins such as 
polyethylene has often involved an oxidizing step. U.S. Pat. No. 3,348,762 
to Kasinkas, discloses a household bag on which an oxidized strip is 
formed across the upper closure flap on the surface. The oxidized strip is 
formed by a process in which a gap voltage generated in the air between an 
electrode and an area forming strip on a bag ionizes the ambient proximate 
thereto. The ionized ozone thus formed acts as an oxidizing agent, and 
oxidizes a strip across the upper closure flap on the surface beneath the 
electrode. A pressure sensitive adhesive is then applied to the oxidized 
area and dried. This type of oxidizing step, however, may be difficult to 
control from a manufacturing standpoint because undesired adhesion of film 
layers may occur, and it may be difficult to localize the area exposed to 
such an oxidizing step. 
In the pressure sensitive adhesive art, the solution to the problem of 
sticking during shipment and storage has been to provide a release layer 
over the pressure sensitive adhesive. For example, pressure sensitive 
plastic tapes have a release layer which impedes the sticking of the 
adhesive to the plastic of the adjacent layer in the roll. This approach 
is not desirable for use on closures for plastic bags because of the 
expense and difficulty in manufacture. 
Thus, it is an object of this invention to provide an adhesive system for a 
flexible bag which selectively adheres to one side of a closure structure. 
It is another object of this invention to provide a closure for a flexible 
bag which forms an effective seal but which is openable and resealable. It 
is yet another object of this invention to provide a closure for a 
flexible bag which closure comprises an easily manufacturable closure 
having an effective adhesive system. It is a further object to provide a 
resealable closure for a flexible bag comprising polyethylene in which the 
closure comprises a hot melt/acrylate adhesive system which is effective 
with a very thin layer of the acrylate component. These and other objects 
of the invention will be apparent from the following explanation. 
SUMMARY OF THE INVENTION 
The present invention provides an adhesive system suitable for use in 
forming resealable closures for flexible bags wherein the adhesive system 
comprises a first layer of a hot melt adhesive affixed to a base surface 
of the closure and a thin second layer of a liquid-based pressure 
sensitive adhesive, such as a water-based acrylate or a solvent-based 
adhesive, affixed to the first layer. A closure formed with the adhesive 
system of this invention provides a protected adhesive strip which 
selectively adheres more firmly to the base surface to which it was 
originally applied, thus providing a reopenable closure which can be 
sealed, opened and resealed without substantial loss of adhesive to the 
surface contacted by the adhesive portion of the closure. The protective 
channel structure of the closure keeps the outer layer of adhesive out of 
contact with other surfaces until sealing is effected. 
In a preferred embodiment a polyethylene bag has applied thereto a 
separately extruded polyethylene channel strip having a layer of hot melt 
adhesive therein. A layer of water-based acrylate pressure sensitive 
adhesive in the form of a dispersion is applied over the layer of hot melt 
adhesive in the channel strip at a preselected thickness such that the 
thickness of the dried film is from 0.01 mil to about 0.05 mil.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIG. 1 shows a perspective view of a bag having a closure constructed with 
an adhesive system of this invention. The bag body has first wall 17 
(shown in cutaway section) having a top edge 18, two sides, and a bottom 
portion; and a second wall 19 having a top edge 10, two sides, and a 
bottom portion, which is sealingly connected to the first wall along a 
major portion of the side edges and bottom portions shown as seal lines 
11a and 11b to form an open topped bag body. Optionally, the bag body may 
be constructed with an internal fold of film 14 or a gusset 13 (shown in 
phantom). The top edge 18 of the first wall 17 extends beyond top 10 of 
the second wall 19 to define a closure flap 12. A channel strip 15 is 
positioned on flap 12. The channel strip may be formed as a separate 
structure and affixed to flap 12 or formed as an integral portion of flap 
12, e.g. by a heat deformation process. A top view of the adhesive system 
16 placed interior to channel 15 is also shown. 
FIG. 2 is an enlarged cross-section of a sectional view of a film upon 
which a closure constructed with an adhesive system of this invention has 
been positioned. Film 21 which may be the flap of a bag body (not shown) 
has attached thereto a channel strip 24 having a trough, with channel 
strip 24 comprising a base portion 26, and channel walls 25, each of which 
walls has an interior height indicated as z. A first layer 27 of hot melt 
adhesive is applied within the trough and over base 26 with a thickness 
indicated by x. Preferably first layer 27 is a substantially continuous 
layer. A thin second layer 28 of a pressure sensitive liquid-based 
adhesive, e.g., a water based acrylate dispersion or a solution comprising 
toluene and an acrylate, is applied over first layer 27. The thickness of 
second layer 28 is indicated by y. Preferably second layer 28 is also 
substantially continuous. As will be explained below, it is also preferred 
that the liquid adhesive used to form second layer 28 be a water-based 
adhesive, e.g. a colloidal dispersion of an acrylate, to minimize 
manufacturing problems in disposing of vapors from organic solvents. The 
adhesive system of this invention allows a much reduced thickness of the 
second layer to be used and still achieve a satisfactory seal. Channel 
walls 25 keep the top outer layer of the adhesive system out of contact 
with other surfaces until sealing is effected. Sealing of the closure is 
effected by contacting the channel strip having the adhesive system 
therein with an opposite surface such as by closing the flap over the bag 
opening. By exerting pressure along the channel strip the adhesive is 
contacted with the surface and sealing is accomplished. 
FIG. 3 is an enlarged cross-section of an alternate embodiment of a closure 
having an adhesive system of this invention. In this embodiment channel 
strip 34 has been constructed with a trough structure by affixing two 
beads 35 a preselected distance apart on a film 31. Each bead has a height 
indicated generally as z', although the height along the length of the 
bead may vary somewhat. A first layer 37 of a hot melt adhesive is applied 
within the trough to a portion of film 31, which portion is between beads 
35. The thickness of first layer 37 is indicated as x'. Preferably first 
layer 37 is substantially continuous and covers the entire portion of film 
31 which lies between beads 35. A second layer 38 of a liquid adhesive is 
applied on top of first layer 37 and with a thickness indicated as y'. 
While a range of thickness may be used for x, y, x' and y' it is preferred 
that thicknesses be chosen such that x+y&lt;z and x'+y'&lt;z'. In other words, 
the combined thickness of the first and second layers should be less than 
the depth of the channel strip or the interior height of its walls. This 
allows the ribs or walls of the channel structure to keep the surface of 
the adhesive strip out of contact with other surfaces until sealing is 
desired. By constructing a bag closure in accordance with the adhesive 
system of this invention, it is also possible to use very thin thicknesses 
of the liquid-based adhesive such as from 0.01 to about 0.05 mils for the 
dried film of the second layer. 
Liquid adhesives suitable for forming adhesive systems of this invention 
include solvent-based adhesives (an adhesive substantially dissolved in a 
solvent) and water-based dispersions. Examples of such adhesives include 
water dispersions of acrylates, solutions of aromatic hydrocarbons (e.g. 
toluene) and acrylates, and rubber latex. 
The adhesive system should be selected according to the material comprising 
the bag. For a bag comprising polyester an adhesive system comprising a 
first layer of hot melt adhesive and a second layer comprising rubber 
latex is suitable. For a flexible bag comprising polyethylene it is 
preferred to use a water-based pressure sensitive adhesive applied in the 
form of a colloidal dispersion. The use of this water-based dispersion 
reduces or eliminates the problems attendant to evaporating and removing 
organic solvent vapors, and drying may be allowed to occur in the packaged 
product. 
One of the important features of this invention is that it allows a 
water-based acrylate pressure sensitive adhesive to be used with 
polyethylene film. While such adhesives have very desirable properties and 
are easier to use from a processing standpoint, heretofore such 
water-based adhesives have not been very effective in forming seals for 
polyethylene bags in which a lasting and resealable closure was desired. 
In a particularly preferred embodiment for a bag and channel strip 
comprising polyethylene, an acrylate adhesive is used as the pressure 
sensitive adhesive second layer and is applied as a water-based dispersion 
having about 10% solids over the hot melt adhesive first layer. This 
preferred adhesive system may be formed by extruding a hot melt pressure 
sensitive adhesive (IP84008 from Swift Adhesives and Coatings) through a 
slot die and applying the adhesive within an extruded channel strip of the 
type seen in FIG. 2 to a trough surface to form a first layer about 1.5 
mils thick. This first layer should be immediately overcoated with a 
second layer comprising a 10% solids water-based acrylate adhesive (made 
by diluting Adhesive 72-9292 from National Adhesives with water) applied 
in a thickness of about 0.016 mils (e.g. at a rate of about 0.4 grams per 
60 feet over an area about 1/8 inch wide). 
Various compositions comprising polyethylene, copolymers and terpolymers 
comprising polyethylene, and additives thereto may also be used, e.g., 
additives to enhance the stickiness of a polyethylene channel to a 
polyethylene film bag body. A preferred channel strip for forming a 
closure in the above explained, particularly preferred embodiment having 
the adhesive system of this invention comprises 83% polyethylene and 17% 
ethylene vinyl acetate, and has a width of about 3/16 inch and a height of 
about 50 mils. It is also preferred to add a coloring agent to the strip 
so that the closure may be more easily located visually. 
While various embodiments of this invention have been described other 
variations may be developed by those skilled in the art which are within 
the spirit and scope of the invention.