Remoistenable adhesive compositions are prepared by adding to ethylene vinyl acetate resin latices which have been polymerized utilizing dextrin as the protective colloid sufficient phosphoric acid to lower the pH to about 2.

The present invention is directed to remoistenable adhesive compositions, 
particularly remoistenable adhesives for use as front-seals in the 
manufacture of envelopes. The adhesive compositions comprise a dextrin 
emulsified ethylene vinyl acetate copolymer to which phosphoric acid and 
conventional components are post-added. 
Conventional remoistenable adhesives for use as "front-seal" adhesives on 
envelopes have generally been prepared from either of two adhesive 
systems. The first class of adhesives are those prepared by the addition 
of dextrin, plasticizer and other additives to dextrin emulsified vinyl 
acetate homopolymers; the second class are based primarily on homo- or 
copolymers of polyvinyl acetate which has been emulsified with polyvinyl 
alcohol and to which additional polyvinyl alcohol and plasticizer are 
post-added. Recently, in U.S. Pat. No. 4,575,525 issued Mar. 11, 1986, 
there were disclosed remoistenable adhesive compositions prepared by 
post-addition of conventional additives to ethylene vinyl acetate 
copolymers which had been polymerized using dextrin as the emulsifier or 
protective colloid. The latter adhesives were prepared using relatively 
high levels of the expensive ethylene vinyl acetate copolymer since 
adequate adhesion properties were not obtained with adhesives containing 
less than about 55% by weight of the copolymer. 
I have now found that the addition to the adhesives formulated with the 
dextrin emulsified ethylene vinyl copolymers of sufficient phosphoric acid 
to lower the pH to about 2 substantially improves the adhesion properties 
thereof and permits formulation of satisfactory remoistenable adhesive 
compositions using as little as about 15% by weight of the dextrin 
emulsified ethylene vinyl acetate copolymer. Moreover, the resultant 
adhesives which generally contain phosphoric acid in amounts of less than 
about 1% exhibit superior remoistening tack, i.e., they rapidly build up 
tack strength on remoistening. 
Thus the present invention is directed to remoistenable adhesive 
compositions comprising at least 15% by weight of a dextrin emulsified 
vinyl acetate resin latex having a T.sub.g of at least about 0.degree. C.; 
0.5-5% humectant; and sufficient phosphoric acid to lower the pH to about 
2; with the remainder of the composition (to 100% by weight) comprising 
dextrin and water. 
The dextrin emulsified ethylene vinyl acetate copolymers utilized herein 
are those containing ethylene in amounts which provide a T.sub.g (glass 
transition temperature) of at least about 0.degree. C. It is generally 
accepted that polyvinyl acetate has a T.sub.g of 30.degree. C. By addition 
of increasing amounts of ethylene, it is possible to lower the T.sub.g 
value to the desired level. When ethylene is added to a level whereby the 
T.sub.g is reduced below about 0.degree. C., blocking becomes a problem. 
Blocking is an undesirable phenonmenon which results when the coated 
substrates, such as the envelopes, are stored and exposed to varying 
degrees of humidity resulting in the tendency of the adjacent surfaces of 
the stacked substrates to adhere to one another. The choice of the 
particular T.sub.g to be used in the adhesive formulation will depend, in 
part, on the desired end use (i.e. the stock to which the adhesive is to 
be applied) as well as on whether or not dextrin is post-added to the 
adhesive system. Thus, if no additional dextrin is added, softer adhesive 
bases (i.e. those having T.sub.g values at the lower end of the range) may 
be desired in order to obtain adequate adhesion. 
As to the dextrin utilized herein, the dextrin may be derived from any of 
the available starch bases, including, but not limited to, waxy maize, 
waxy sorghum, sago, tapioca, potato, corn, sorghum, rice and wheat as well 
as the derivatives thereof. In all instances, however, the applicable 
starch base should be in ungelatinized form and shoul remain in that form 
throughout the subsequent dextrinization process. 
In converting these starch bases into dextrins, one may employ any of the 
usual dextrinization procedures well known to those skilled in the art, 
including treatment of starch with either heat or acid or by any other 
means desired by the practitioner. It should be noted that when reference 
is made to "dextrins" in the process of this invention, we also 
contemplate as included therein the degraded starch products prepared 
either by means of a process wherein the applicable starches are converted 
with acids and/or oxidizing agents, in the presence of water, at 
superatmospheric pressures and temperatures in excess of about 212.degree. 
F., or, by means of an enzyme conversion procedure utilizing such enzymes 
as alpha-amylase. Additional information relating to the dextrinization of 
starches, may also be obtained by reference to chapters XII-XIII of 
"Chemistry and Industry of Starch" edited by R. W. Kerr, published in 1950 
by the Academic Press of New York, N.Y. 
While the choice of the particular dextrin to be used is left to the 
practitioner, it is preferred that the same dextrin base used as the 
protective colloid in the polymerization process be used as the post-added 
dextrin component in formulating the adhesives of the preferred 
embodiment. 
The adhesive polymer base is prepared by conventional ethylene vinyl 
acetate polymerization procedures with the one difference being the use of 
an aqueous solution of dextrin as the emulsifier or protective colloid. 
The polymerization is then carried out in an aqueous medium under 
pressures less than about 130 atmospheres in the presence of a catalyst 
with the pH being maintained by a suitable buffering agent, if necessary, 
at a pH of 2 to 6. The polymerization is performed at conventional 
temperatures from about 70.degree. to 225.degree. F., preferably from 
120.degree. to 175.degree. F. for sufficient time to achieve a low monomer 
content, e.g. from about 0.5 to 120 hours, preferably from 2 to 8 hours, 
to produce a latex having less than 1.5, preferably less than 0.5 weight 
percent free monomer. Conventional batch, semi-continuous or continuous 
polymerization procedures may be employed and are taught, for example, in 
U.S. Pat. Nos. 3,708,388 and 4,164,488. 
More specifically, a solution of the dextrin in water, typically at a 
concentration of 30 to 50% solids, is prepared in a polymerization vessel 
and a portion of the vinyl acetate monomer charged therein. After purging 
the reactor with nitrogen, the polymerization is then initiated by water 
soluble free radical initiator such as water soluble peracid or salt 
thereof, e.g. hydrogen peroxide, sodium peroxide, lithium peroxide, 
peracetic acid, persulfuric acid or the ammonium and alkali metal salts 
thereof, e.g. ammonium persulfate, sodium peracetate, lithium persulfate, 
potassium persulfate, sodium persulfate, etc. A suitable concentration of 
the initiator is from 0.05 to 5.0 weight percent and preferably from 0.1 
to 3 percent based on the weight of the monomer. 
The free radical initiator can be used alone and thermally decomposed to 
release the free radical initiating species or can be used in combination 
with a suitable reducing agent in a redox couple. The reducing agent is 
typically an oxidizable sulfur compound such as an alkali metal 
metabisulfite and pyrosulfite, e.g. sodiuum metabisulfite, potassium 
metabisulfite, sodium pyrosulfite, etc. The reducing agent is generally 
added towards the end of the polymerized and is used in an amount of from 
about 0.1 to 3 weight percent of the amount of polymer. 
The ethylene content of the copolymer is determined by controlling the 
ethylene content of the aqueous polymerization medium. This may be 
accomplished by precharging the desired amount of ethylene into the 
reactor and allowing the reaction to proceed in the closed reaction vessel 
or by regulating the partial pressure of ethylene in the vapor phase over 
the medium, the temperature of polymerization and the degree of mixing 
between the vapor phase and the liquid medium. The polymerization is 
generally performed at temperatures from 120.degree. to 175.degree. F. 
and, at these temperatures if the ethylene was not precharged, ethylene 
partial pressures from 50 to 1,000 preferably from 250 to 750 psig. are 
sufficient to incorporate from 1 to 30, preferably from 5 to 25, weight 
percent ethylene in the polymer. The reaction medium is preferably 
agitated with a stirrer, however, other agitation can be used. The 
preferred copolymerization procedure is a batch process wherein the 
appropriate amount of ethylene is precharged into the reactor, the vessel 
closed and the reaction allowed to proceed over the polymerization period 
which can be from 0.5 to about 120 hours, preferably from about 2 to about 
8 hours. 
In addition to the required dextrin solution used as a protective colloid, 
other emulsifiers, generally of the nonionic and anionic oil-in-water 
variety may also be used in the polymerization reaction. When used, it is 
generally present in amounts of 0.1 to 1 percent of the monomers used in 
the polymerization and is added either entirely to the initial charge or 
continously or intermittently during polymerization or as a post-reaction 
stabilizer. 
The latices are produced and used at relatively high solids contents, e.g. 
between 35 and 75% although they may be diluted with water if desired. The 
preferred contents of total solids are from 40 to 70, and, most preferred, 
from 50 to about 68 weight percent. When used herein the term "solids" 
refers to the combined amounts of ethylene vinyl acetate resin, dextrin 
and other non-violatiles present in the latex. 
The particle size of the latex can be regulated by the quantity and type of 
the emulsifying agent or agents employed. To obtain smaller particles 
sizes, greater amounts of emulsifying agents are used. As a general rule, 
the greater the amount of the emulsifying agent employed, the smaller 
average particle size. 
The humectant used herein may be any of those conventionally used in 
formulating remoistenable "front seal" adhesives. Typically included in 
this class are sugars; sorbitol; glycerine and related derivatives; 
carbowaxes; propylene glycol and similar related glycols; and the glycol 
ethers. These humectants are used in the adhesive formulations at levels 
of about 0.5 to 5% by weight of the total adhesive formulation. 
The phosphoric acid component is added to the adhesive composition in a 
quantity sufficient to lower the pH to about 2. In general, this pH level 
can be obtained using concentrated phosphoric acid in an amount of 1% by 
weight of the adhesive fonmulation. pH values substantially less than 
about 2 are not desired while at pH values substantially greater than 2 
adequate adhesion may not be obtained in those compositions containing 
lower levels of the dextrin emulsified ethylene vinyl acetate. While it 
will be recognized that other acids may be used to lower the pH, the 
relative acid strength and lack of volatility make phosphoric acid the 
most preferred choice. 
In the preferred embodiment where dextrin is post-added to the latex, the 
dextrin may be added at levels up to about 60% by weight, preferably about 
30 to 55% by weight of the adhesive formulation. 
In preparing the adhesive composition, an aqueous solution of the dextrin 
may be prepared and added to the ethylene vinyl acetate latex or the dry 
dextrin added directly to the latex. The adhesive composition is then 
heated and maintained at a temperature of about 160.degree.-180.degree. F. 
with agitation for a period sufficient to ensure complete dissolution. The 
phosphoric acid and any other additives which are to be employed would be 
added at this point and the resulting mixture then diluted with additional 
water, if necessary, to the desired viscosity, generally in the range of 
about 2,000 to 15,000 cps., preferably about 6000 cps. In the embodiment 
wherein dextrin is not post-added, it may be necessary to add a thickener 
(e.g. polyacrylamide, carboxymethyl cellulose, hydroxyethylcellulose, 
etc.) in order to obtain a viscosity within these limits. Various optional 
additives, such as plasticizers, preservatives, thickeners, bleaching 
agents, etc. may also be present in the adhesive compositions in order to 
modify certain characteristics thereof. 
Although the humectant component and the optional additional dextrin have 
been referred to as being "post-added", it should be recognized that the 
post-addition is merely the most convenient and generally accepted method 
of formulating "front" seal adhesives and that it is possible to add the 
humectant and the additional dextrin directly to the monomer charge prior 
to the actual polymerization.

In the examples that follow, an adhesive was prepared from ethylene vinyl 
acetate base which had been polymerized using dextrin as a protective 
colloid in accordance with the following basic procedure. In the examples, 
the amount of ethylene to achieve a desired Tg of 6.5 was used. (T.sub.g 
values are determined by differential thermal analysis using a DuPont 
Model 910 Differential Scanning Calorimeter.) 
A premix cooker was charged with 1200 parts water and 778 parts dextrin 
dissolved therein with agitation. Then 5.23 parts Aerosol OT (surfactant), 
12.1 parts Colloid 681F (defoamer), 2.4 parts sodium bicarbonate and 6.2 
parts sodium persulfate were added with 729 parts of vinyl acetate and 
this initial charge transferred to a reactor together with 73 parts water. 
While at room temperature, the pressure was brought to 600 pounds with 
ethylene, held for one half hour to equilibrate and then the temperature 
was raised to 75.degree. C. Polymerization was initiated and a slow 
addition of solutions of 16.7 parts sodium bicarbonate in 175 parts water 
and 15.6 parts sodium persulfate in 109 parts water was begun. The 
temperature was held at 80.degree. C. and the reaction allowed to continue 
to a solids level of 45% at which time a delayed addition of 1460 parts 
vinyl acetate was begun and continued over a four hour period. The 
catalyst and activator solution slow additions continued for an additional 
one half hour to achieve a 65% solids level. The latex was stripped of 
residual ethylene and degassed. The resultant latex had a total solids 
content of 64%, a T.sub.g of 6.5, a pH of 4.6, residual monomer content of 
1.5 and a viscosity of 2050 cps. 
In this example, a variety of moistenable adhesive compositions shown in 
Table I were prepared with and without phosphoric acid using varying 
amounts of ethylene vinyl acetate polymers prepared as above. As a 
control, a similar adhesive was prepared using 65% of dextrin emulsified 
polyvinyl acetate following the teachings of copending patent application 
Ser. No. 668,603. In all cases, additional water was added in an amount 
sufficient to dilute the samples to a viscosity of 6,000 cps. 
TABLE I 
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A B C D E F G H I J K 
__________________________________________________________________________ 
EVA 65 55 55 45 45 35 35 25 25 15 15 
Water 6 8 8 10 10 12 12 15 15 18 18 
Defoamer 0.3 
0.3 
0.3 0.3 
0.3 
0.3 
0.3 0.3 0.3 0.3 
0.3 
Phosphoric Acid (86%) 
-- -- 0.8 -- 0.8 
-- 0.8 -- 0.8 -- 0.8 
Amioca Dextrin 
27 37 37 42 42 47 47 52 52 57 57 
Formaldehyde 
0.1 
0.1 
0.1 0.1 
0.1 
0.1 
0.1 0.1 0.1 0.1 
0.1 
Propylene Glycol 
3.1 
3.1 
3.1 3.1 
3.1 
3.1 
3.1 3.1 3.1 3.1 
3.1 
Solids (Cenco) 
70% 
67% 
65.4% 
66% 
66% 
64% 
65.6% 
64.8% 
64.8% 
62% 
64.2% 
pH 3.9 
3.9 
2.1 3.8 
2.02 
4.0 
2.0 3.5 2.0 3.35 
2.0 
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When tested for adhesion on texturized stocks and rag stocks, the samples 
containing phosphoric acid exhibited improved adhesion to the substrates 
when contrasted with those containing the same levels of ethylene vinyl 
acetate but lacking the phosphoric acid. The improved effects on adhesion 
imparted by the addition of phosphoric acid were especially pronounced 
after heating the samples at 225.degree. F. for 2 hours and 300.degree. F. 
for 15 minutes (to simulate aging). In these cases, the adhesion results 
observed even for Sample K (containing only 15% ethylene vinyl acetate) 
were superior to those for Sample A (containing 65% ethylene vinyl 
acetate). When tested for blocking resistance under high humidity 
conditions, the Control and Samples B-K exhibited satisfactory 
non-blocking. 
A sample similar to that designated Sample G was tested on white wove and 
other stocks for relative remoistening tack strength using a Hunter-String 
guage. In this test, the adhesive samples were coated onto test paper at 
1/2 inch width applications, dried for 72 hours, and cut into 11/2 inch 
pieces. The strips were then remoistened with a dispenser that was 
modified with a pressure device to obtain consistent remoistening. Open 
time was set at 5 seconds and the remoistened strip comfined to a plain 
strip of the same type paper and ccmpressed. Strips were attached to the 
guage and peeled at predetermined time intervals to determine the relative 
tack development. The results indicated that the speed of tack development 
on these substrates was dramatically faster than that of the conventional 
dextrin emulsified vinyl acetate homopolymer based adhesives against which 
it was tested. 
Now that the preferred embodiments of the present invention have been 
described in detail, various modifications and improvements thereon will 
become readily apparent to those skilled in the art. Accordingly, the 
spirit and scope of the present invention is to be limited only by the 
appended claims, and not by the foregoing disclosure.