Adhesive products having polymer-treated embrittled zones

Adhesive products such as label stock, labels, tapes, and the like having a facestock, an adhesive layer and a removable, fibrous backing are described. The backing is provided with integral embrittled zones which crack when the adhesive product is flexed and offer gripping points for removal of the backing. The embrittled zones contain brittle, film-forming organic polymer suffused among the fibers. The presence of the polymer in the embrittled zone helps to enhance the brittleness of the zones and to reduce the transfer of residual chemical embrittling agent from embrittled zones of the backing to portions of the facestock brought into contact therewith when the adhesive product is stacked or rolled. Embrittled zones are preferably formed by applying a thickened acidic solution comprising a polymer emulsion to the backing of the adhesive product and then curing and drying the solution by heating.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to adhesive products such as label stock, 
labels, tapes, and the like and more particularly to adhesive products 
having protective backings provided with embrittled zones that aid in 
removing the backing from the adhesive. 
2. Description of the Prior Art 
Many adhesive products, particularly those having pressure sensitive 
adhesives, are provided with a backing which is secured to the surface of 
the adhesive and which protects the adhesive before use. It is known to 
provide embrittled zones in the form of thin lines running across the 
backing of such adhesive products. When the adhesive product is sharply 
flexed along an embrittled line, the backing cracks along the line and the 
newly-formed edges of the backing lift up off of the adhesive layer, 
thereby offering gripping points for peeling the backing from the 
adhesive. Embrittled zones that are not sharply flexed have sufficient 
tensile strength to permit removal in one continuous piece of a portion of 
backing having one or more of such embrittled zones. Thus, when two or 
more embrittled zones are present in the backing of a label, one of the 
zones can be cracked selectively and the backing can be removed in only 
two pieces. Until the flexing operation occurs, however, the adhesive 
surface is protected and contained by the backing. The embrittled zones 
are integral with the backing and serve to protect and contain the 
adhesive until cracked. 
U.S. Pat. No. 3,035,957, incorporated herein by this reference, describes 
one such adhesive product having a fibrous backing with embrittled zones. 
The embrittled zones are produced by applying heat, a flame, a chemical 
substance such as sulfuric acid, ultraviolet radiation, or the like to 
partially degrade or decompose a portion of the fibrous backing. U.S. Pat. 
No. 3,006,793, incorporated herein by this reference, describes adhesive 
products in which embrittled zones extend diagonally across the backing 
from edge to edge. In one embodiment, the embrittled zones are produced by 
applying a chemical embrittling agent such as an aqueous solution of 
sulfuric acid with the use of a printing roll having raised helical 
printing ridges. 
Japanese Patent Application No. 1971-11392, which was filed on Mar. 5, 
1971, and laid open to public inspection on Nov. 14, 1972, and is 
incorporated herein by this reference, describes forming the embrittled 
zones by applying lines of viscous, aqueous solution of hydrochloric acid 
with the use of an applicator roller having helical grooves which receive 
the acid solution from a dip bath and transfer the solution to the 
backing. The applied lines of solution are dried and cured at temperatures 
of 300.degree. to 400.degree. F. The viscous solution comprises up to 
about 13 weight percent hydrogen chloride, sufficient organic thickener to 
provide a viscosity of 2,000 to 3,000 centripoises, and minor amounts, up 
to about 2 percent, of wetting agent and anti-foam agent. 
When an acidic solution is used to form the embrittled zones, a problem is 
encountered. Residual acid remains in the backing after the drying and 
curing operation even when a volatile acid, such as hydrochloric acid, is 
used. When label stock having these acid embrittled zones is rolled or 
stacked for transfer to a label printer, for example, the backing of one 
layer of label stock comes into contact with the face of another layer of 
label stock. Residual acid is transferred from embrittled zones of the 
backing to the face of the adjacent label stock and changes the printing 
characteristics of the face. When ink is subsequently applied to the label 
face in a printing operation, the portions of the label affected by the 
residual acid take on a color or intensity of color different from that of 
the surrounding regions. The effect is typically seen as a line of lighter 
or contrasting color running across the face of the printed label. The 
degree of this effect depends upon the chemical nature of the ink used. 
Some inks are so sensitive to the acid-caused changes that they cannot be 
used with the acid-embrittled adhesive products of the prior art. 
Another problem encountered in the manufacture of the prior art products is 
the need to balance carefully the degree of brittleness and the tensile 
strength of the embrittled zones to provide a balance between good 
cracking performance upon flexing and avoidance of unwanted cracking. 
SUMMARY OF THE INVENTION 
It has now been found that presence of a brittle, film-forming organic 
polymer suffused into the embrittled zones, that is, between and among the 
fibers of the zones and not merely on the surface of the zones, enhances 
the brittleness, the crackability of the embrittled zones, and retards the 
transfer from embrittled zones of an adhesive product backing to a surface 
of any residual chemical embrittling agent in contact therewith, e.g., 
from the backing of one sheet of label stock to the face of another in a 
stack. 
The brittle, film-forming organic polymers useful herein are sufficiently 
brittle for the backing to crack cleanly along the embrittled zones 
without any bridging. 
The invention provides an adhesive laminate comprising a facestock, an 
adhesive layer on the facestock, and a removable fibrous backing in 
contact with the adhesive layer, the backing having at least one narrow 
embrittled zone, integrally included in the backing and the embrittled 
zone comprising an organic, brittle film-forming polymer. When the 
embrittled zone comprises residual chemical embrittling agent, the 
brittle, film-forming polymer is effective to retard the transfer of 
residual chemical embrittling agent from an embrittled zone to a surface 
in contact with the embrittled zone. 
In the practice of a preferred embodiment of this invention, such adhesive 
products are made by (1) applying to a portion of a removable fibrous 
backing of the laminate, an aqueous embrittling solution comprising a 
chemical embrittling agent and a brittle, film-forming organic polymer 
resistant to the chemical embrittling agent, and (2) curing and drying the 
applied solution to form an embrittled zone comprising an amount of said 
polymer effective to retard transfer of residual chemical embrittling 
agent from the embrittled zone to a surface in contact therewith. In a 
more preferred embodiment, the chemical embrittling agent is hydrochloric 
acid and the polymer is acid resistant. 
The invention also provides a process for the production of an adhesive 
laminate comprising a facestock, an adhesive layer on the facestock, and a 
removable fibrous backing, with the fibrous backing having at least one 
embrittled zone, which process comprises: (1) applying to a portion of the 
fibrous backing an aqueous embrittling solution containing 1 to 15 weight 
percent hydrogen chloride, 0.5 to 3 weight percent organic thickener, and 
1 to 10 weight percent organic, brittle film-forming polymer in emulsion 
form; and (2) curing and drying the applied solution at 150.degree. to 
250.degree. C. to form an embrittled zone comprising said polymer. 
The invention also provides a process for the production of an adhesive 
laminate comprising a facestock, an adhesive layer on the facestock, and a 
removable fibrous backing in contact with the adhesive layer in which at 
least one integral embrittled zone is formed in the backing, and an 
organic, brittle film-forming polymer is introduced into the embrittled 
zone. 
The invention also provides an aqueous solution for providing embrittled 
zones on a removable fibrous backing of an adhesive laminate comprising 
also a facestock, and an adhesive layer on the facestock, the fibrous 
backing being in contact with the adhesive layer, which solution comprises 
(1) from 1 to 15 weight percent of a volatile chemical embrittling agent 
capable of embrittling zones of the backing when the solution is applied 
to the backing and is dried by heating; and (2) an organic, brittle 
film-forming polymer that remains in the embrittled zones after the 
solution is dried. 
The invention also provides an aqueous solution for providing embrittled 
zones on a removable fibrous backing of an adhesive laminate comprising 
also a facestock, and an adhesive layer on the facestock, the fibrous 
backing being in contact with the adhesive layer, which solution comprises 
from 1 to 15 weight percent of hydrogen chloride, from 0.5 to 3 weight 
percent of an organic thickener, from 0.4 to 1 weight percent of wetting 
agent, from 0.5 to 2 weight percent of anti-foam agent, and from 1 to 10 
weight percent of organic, brittle film-forming polymer; the solution 
being such that when applied to zones of the backing and dried by heating, 
embrittled zones are formed on the backing, and the polymer penetrates the 
backing and retards the transfer of residual hydrochloric acid from the 
embrittled zones.

DETAILED DESCRIPTION OF THE INVENTION 
The adhesive products of the present invention are laminated structures 
comprising a facestock, an adhesive layer on the facestock, and a 
removable fibrous backing in contact with the adhesive layer which 
protects the adhesive layer before the product is used. Any conventional 
facestock such as paper, plastic, cloth, and the like may be used. 
Similarly, any conventional adhesive, such as natural and synthetic rubber 
based adhesives, acrylics, and polyurethanes, may be used. When elevated 
curing and drying temperatures are encountered in the practice of this 
invention, the facestock and adhesive are chosen from materials that can 
withstand such temperatures. The backing comprises a fibrous material 
having integral zones that are embrittled by application of heat, a flame, 
a chemical substance, or radiation such as ultra-violet radiation so that 
the embrittled zones crack upon flexing but retain sufficient tensile 
strength to maintain the integrity of the backing during ordinary 
handling, such as removal of the backing from a label. Useful backing 
materials include paper, cloth, and the like. A fibrous backing of paper 
or a similar cellulosic material is preferred. The side of the backing 
that contacts the adhesive is typically coated with a release agent such 
as a silicone to prevent the adhesive from sticking to the removable 
backing. 
The adhesive laminate is first assembled by any convenient technique, many 
of which are known to those skilled in the art. Conveniently, a web of 
facestock is continuously coated with adhesive and then laminated to the 
release-coated side of a continuous web of backing; alternatively, the 
adhesive coating is applied to the backing web prior to lamination rather 
than to the facestock. Then, either in separate operations or in a 
combined operation, narrow embrittled zones are formed in the backing and 
an organic, brittle film-forming polymer is suffused into the embrittled 
zones. Preferably, substantially all of the polymer in the backing is 
within the embrittled zones. 
The embrittled zones may be formed by the application of heat, for example, 
by contacting the backing with a flame or hot wire or stylus; by directing 
an intense beam of radiation such as infrared radiation, visible light, or 
ultraviolet radiation, at the backing; or by applying a chemical 
embrittling agent, for example, a solution of a strong organic or mineral 
acid such as acetic acid, sulfuric acid or hydrochloric acid. The use of 
an aqueous solution of hydrochloric acid is preferred because, as 
described below, the polymer can be incorporated in the acid solution so 
that the embrittled zones are formed and the polymer is suffused into the 
zones in a single operation. 
The polymer can be introduced into the embrittled zones in a variety of 
ways. When the embrittled zones are formed by the application of intense 
heat or radiation, it is desirable to introduce the polymer after the 
embrittled zones are formed so as to avoid degradation of the polymer by 
heat or radiation. The polymer can be applied to the embrittled zones by a 
printing process, as a hot melt, or dissolved in an organic solvent, or 
preferably emulsified in water. The adhesive product with applied polymer 
is then passed through a heating zone where the polymer penetrates into 
the backing within the boundaries of the embrittled zones and where 
solvent or water is removed. 
The fibrous backings used in the adhesive products include a minor amount, 
typically less than about 10 volume percent, of void volume between and 
among the fibers of the backing. The polymer is suffused into the backing 
so as to occupy at least a portion of the void volumes within the 
boundaries of the embrittled zones, rather than merely lying on the 
surface. It is indeed preferred to avoid a raised line of polymer 
protruding above the surface of the backing, because such a raised line 
can interfere with stacking and printing operations. 
The entire backing can be suffused with polymer to assure that the 
embrittled zones are included, but the cost of materials may render this 
approach uneconomical. 
In a preferred embodiment of this invention, an embrittling solution 
comprising both a chemical embrittling agent and a polymer is used to form 
the embrittled zones and to incorporate the polymer into the zones in one 
operation. Particularly preferred is an aqueous embrittling solution 
comprising a volatile acid, notably hydrochloric acid, and a polymer in 
emulsion form. 
In the practice of this preferred embodiment, the assembled laminate is 
first dried. In particular, the laminate is dried to a moisture content 
below about 3 weight percent, e.g. between about 0.5 and 2 weight percent 
to avoid delamination in the subsequent steps. An aqueous, embrittling 
solution comprising hydrochloric acid and a polymer emulsion is applied to 
portions of the backing of the dried laminate and the laminate is heated 
to dry the embrittling solution and form the embrittled zones. The 
laminate is then passed through a humidification chamber to raise the 
moisture content of the laminate to prevent curling. 
The preferred chemical embrittling solution is an aqueous solution 
containing a volatile acid, notably hydrochloric acid, and an acid 
resistant polymer in emulsion form. To improve the handling properties of 
the solution, an organic thickening agent and minor amounts of a wetting 
agent and an anti-foam agent are usually added. The solution contains from 
1 to 15 weight percent, preferably from about 2 to about 5 percent acid. 
The lower concentrations are preferred in order to minimize problems of 
acid transfer. The polymer is present in the solution in amounts of from 
about 1 to about 10 percent by weight of solution, preferably from about 5 
to about 7 percent by weight of solution. Sufficient organic thickening 
agent is added to provide a solution having a desired viscosity that 
depends upon the method of application used. Between about 0.5 and about 3 
weight percent of thickener is added, preferably between about 1 and about 
2 weight percent. Minor amounts of antifoam agent and wetting agent are 
used. The anti-foam agent is included in amounts of between about 0.5 and 
about 2.0 percent, preferably about 1 percent. The wetting agent is 
included in amounts of between about 0.4 and about 1 percent, preferably 
about 0.5 percent. The thickener, wetting agent and anti-foam agents are 
added so that the solution can be applied in a controlled manner in the 
form of narrow zones or lines, preferably between about 0.2 and 2 
millimeters wide, on the backing of the laminate. Consequently, the 
relative proportions of these ingredients can be varied to provide optimum 
results with the particular method of application being employed. 
During the heating of the laminate to form the embrittled zones, not only 
the liquid content of the solution evaporates, but also there is a loss of 
the more volatile solids. Preferably the dry solids remaining in the 
embrittled zones after the heating are less than 8 percent by weight of 
the original solution. Higher residual solids in the embrittled zones can 
leave a raised line of solids above the surface of paper backing. The 
proportion of polymer solids in the total solids of the embrittling 
solution should be kept as high as possible to more fully realize the 
advantages of the present invention. 
The embrittling solution is normally prepared in a two stage operation. 
First, the required water, thickening agent, anti-foam agent, wetting 
agent, and polymer emulsion are combined as a master batch having a higher 
viscosity than is desired for the working solution. The master batch has a 
reasonable shelf life for storage. When a new batch of working solution is 
called for, the required amount of hydrochloric acid is added slowly with 
agitation. This reduces the viscosity to the desired level, for example, 
about 2,000 to 3,000 centipoises. The solution when so prepared has a 
normal service life of at least about four hours. If desired, the polymer 
can be added to the solution in the second stage of mixing. The acid is 
added to the solution shortly before use because it gradually hydrolyzes 
the organic thickener, causing the viscosity of the solution to eventually 
drop below a useful level. 
A wide variety of polymers can be used. Exemplary polymers include 
plasticized and unplasticized polyvinyl chlorides, polyvinyl alcohols, 
polyvinyl acetates, styrene acrylate copolymers, and thermoplastic and 
self cross-linking acrylic polymers. Acrylic polymers tend to be acid 
sensitive. Polyvinyl acetate polymers are preferred. The polymers are 
typically added to the embrittling solution in the form of an aqueous 
emulsion. Useful emulsions are sufficiently acid stable so that the 
polymer remains in emulsion in the embrittling solution for the normal 
service life of the solution, for example, at least about four hours. 
The polymers form films that are brittle at normal ambient temperatures, 
e.g., 15.degree. to 30.degree. C. The brittleness of the polymer film 
enhances the brittleness (the crackability) of embrittled zones containing 
the polymer. When the embrittled zones are formed chemically, for example 
with acid, the polymers are resistant to the embrittling agent; that is, 
the polymers are chemically stable in the presence of residual chemical 
embrittling agent in the embrittled zones and retard the transfer of 
residual embrittling agent to a surface in contact with the embrittled 
zones. 
It is believed, without intending to be bound by the theory, that the 
polymers form a film or coating on the fibers of the backing in the 
embrittled zone to trap residual embrittling agent, e.g., acid that is 
present in the fibers, thereby retarding the transfer of the residual acid 
out of the backing to another surface. The polymers are capable of forming 
a film at temperatures up to about 250.degree. C. The polymers do not 
exhibit any tackiness after drying or in the presence of high humidity. 
Any organic thickening agent that is sufficiently acid-stable to provide a 
solution having a working life of at least about four hours can be used in 
the embrittling solution of this invention. Examples of thickening agents 
include vegetable gums, such as xantham gum and gum arabic, ethylene oxide 
polymers, and various cellulose derivatives such as methylcellulose, 
hydroxypropyl methylcellulose, hydroxybutyl methylcellulose, hydroxyethyl 
cellulose, and the like. Hydroxypropyl methylcellulose is preferred and is 
normally present in an amount of 1 to 2 weight percent, preferably between 
about 1.2 and 1.4 weight percent. When hydroxypropyl methylcellulose is 
used as a thickener, a minor amount of ammonia, less than about 0.05 
weight percent, can be added to the solution in order to help dissolve the 
thickener. 
Examples of wetting agents include anionic sulphosuccinates, 
alkylarylpolyether alcohols such as isooctyl phenoxy polyethoxy ethanol, 
ethoxylated nomyl phenol, and naphthalene sulphonates, and conventional 
anti-foam agents such as silicone anti-foam agents. 
To establish the embrittled zone, the laminate is passed as a web 
preferably under tension in contact with a suitable applicator which 
applies the embrittling solution. Preferably the applicator is a roller 
having helical grooves. The grooves receive the solution from a dip bath 
with excess solution being wiped away from the surface of the roller by a 
doctor blade in firm contact with the surface of the applicator. Upon 
contact with the backing of the laminate, the grooves release the 
embrittling solution to the paper backing. 
The pattern produced is a series of parallel lines which traverse the web 
at some angle to its direction of travel. Spacing between the zones will 
vary depending upon the size of the label to be cut from the stock and is 
adjusted to assure that each label cut will have at least one embrittled 
zone on its backing. Following application of the embrittling solution the 
web is passed through a curing zone which is maintained at a temperature 
from about 150.degree. to about 250.degree. C. preferably from about 
180.degree. to about 200.degree. C. Upon heating, the zones are cured, the 
lines of embrittlement are established, and the polymer present in the 
embrittling solution forms a protective film on the fibers of the backing. 
The web is maintained at the curing temperature for a time between about 
0.5 and about 5 minutes, preferably from about 1 to 2 minutes. 
The minimum film-forming temperature of the polymers is, for example, not 
greater than 50.degree. C.; but since, as described above, during curing a 
temperature of up to 250.degree. C. may be achieved, polymers having 
minimum film-forming temperatures well in excess of 50.degree. C. can be 
feasible. 
After the embrittled zones are cured, the web is passed through a 
moisturizing zone. Generally in such a zone the moisture content of the 
paper is allowed to increase to a level of between about 2 and 10 percent 
and preferably to about 3 to 5 percent by weight of paper. The 
moisturizing is conveniently accomplished by passing the web through a 
chamber maintained at a relative himidity of 90 percent, such as with 
steam. The resultant moisturized label stock will lie flat for subsequent 
printing and cutting operations. 
Although this invention has been described in terms of particular detail 
and embodiments thereof, the particulars are not intended to limit the 
invention, the scope of which is defined by the following claims.