Note: Descriptions are shown in the official language in which they were submitted.
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TEMPORARILY BONDED CONSTRUCTIONS
This invention relates to temporarily bonded construction utilizing
a non-pressure sensitive hot melt adhesive which comprises specific
A-B-A block copolymers and relatively high levels of plasticizing oil.
There has been an increasing need in recent years for adhesives to
be used in applications requiring temporary bonding properties, i.e.,
adhesives which will bond an item to a substrate for a unlimited
period, which bond may be easily broken when desired. One of the
largest applications for such adhesives is for the attachment of
plastic cards (e.g., credit cards) to paper stock for mailing to a
customer. Upon receipt, the card can be readily removed by peeling it
from the paper stock.
Previous attempts to provide temporary bonding involved the use
of non-pressure sensitive rubber latex adhesives or hot melt pressure
sensitive adhesives, i.e., adhesives which are permanently tacky at
room temperature and which adhere on mere contact with the surfaces to
which they are applied. In the case of both types of adhesive, removal
of the card from the paper stock was often accompanied by some tearing
of the paper and it was necessary for the customer to scrape or
otherwise remove the residual tacky adhesive, and often some paper,
from the card prior to use. While some of these problems could be
overcome using specially treated release paper, this coated stock is
costly and generally cannot be printed upon.
There is therefore a continuir,g need for an adhesive capable of
forming temporary bonds, and in particular, capable of forming tempor-
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ary bonds between plastic and preprinted stock. The latter requirementis especially important in applications where, after removal of the
card, the customer is to sign and return the preprinted paper substrate
for further processing either by hand or on automated equipment and
wherein tearing of the paper or the presence of residual tacky adhesive
either on the card or the paper stock would be unacceptable.
A hot melt adhesive suitable for temporary bonding applications
may be prepared from specific A-B-A type block copolymers and oil. The
resulting non-pressure sensitive adhesives are resistant to shear but
have low tensile strength, have little or no residual tack and are not
subject to cohesive failure. As such, they are especially suited for
temporary bonding applications where easy releasability and clean
strippability are critical.
The present invention is directed to temporarily bonded con-
structions comprising: a plastic, paper, glass, ceramic or metal
substrate attached to a base stock substrate utilizing a non-pressure
sensitive hot melt adhesive, the adhesive comprising:
a) 5-40% by weight of at least one A-B-A block copolymer wherein
the A blocks are non-elastomeric polymer blocks which, as homopolymers
have glass transition temperatures above 20C., while the elastomeric
polymer blocks B are isoprene or butadiene which is partially or sub-
stantially hydrogenated;
b) 95-60% by weight plasticizing oil, and
c) 0.1 to 2% by weight of a stabilizer.
While temporarily bonded constructions formed from paper base stock
sheets and plastic, e.g., credit cards, are described in detail herein,
the invention is not intended to be limited thereto and other construc-
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tions requiring temporary bonding are also contemplated to be within
the scope of the invention. In particular, such other uses may include
the attachment of a plastic or glass vial containiny a sample perfume
fragrance or the like to an explanatory card; the insertion of multiple
containers within a preformed package for safe transport; the attach-
ment of reply cards to magazine stock; collating of paper to paper in
business forms; and the attachment of a coin to a printed survey form.
Hot melt adhesives are 100% solid materials which do not contain
or require any solvents. They are solid materials at room temperature
but, on application of heat, melt to a liquid or fluid state in which
form they are applied to a substrate. On cooling, the adhesive regains
its solid form and gains its cohesive strength. In this regard, hot
melt adhesives differ from other types of adhesives which achieve the
solid state through evaporation or removal of solvents or by polymeri-
zation. Hot melt adhesives may be formulated to be pressure sensitive,i.e., aggressively tacky at room temperature so that a bond may be
found on mere contact between the two surfaces or, in contrast, to be
relatively free of tack at room temperature as required herein.
In formulating the non-pressure sensitive adhesives used in the
present invention, the rubber copolymers employed are block or
multi-block copolymers having the general configuration:
A-B-A or A-B-A-B-A-B
wherein the polymer blocks A are non-elastomeric polymer blocks which,
as homopolymers have glass transition temperatures above 20~C., while
the elastomeric polymer blocks B are isoprene, or butadiene which is
partially or substantially hydrogenated or mixtures thereof. Further,
the copolymers may be linear or branched. Typical branched structures
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contain an elastomeric portion with at least three branches which can
radiate out from a central hub or can be otherwise coupled together.
The non-elastomeric blocks which make up 14 to 30% by weight of
the block copolymer may comprise homopolymers or copolymers of vinyl
monomers such as vinyl arenes, vinyl pyridines, vinyl halides and vinyl
carboxylates, as well as acrylic monomers such as acrylonitrile,
methacrylonitrile, esters of acrylic acids, etc. Monovinyl aromatic
hydrocarbons include particularly those of the benzene series such as
styrene, vinyl toluene, vinyl xylene, ethyl vinyl benzene as well as
dicyclic monovinyl compounds such as vinyl naphthalene and the like.
Other non-elastomeric polymer blocks may be derived from alpha olefins,
alkylene oxides, acetals, urethanes, etc. Styrene is preferred.
The elastomeric block component making up the remainder of the
copolymer is isoprene or butadiene which is hydrogenated as taught,
for example, in U.S. Pat. No. 3,700,633 issued October 24, 1972 to
M. M. Wald et al. This hydrogenation of butadiene may be either
partial or substantially complete. Selected conditions may be employed
for example to hydrogenate the elastomeric butadiene block while not so
modifying the vinyl arene polymer blocks. Other conditions may be
chosen to hydrogenate substantially uniformly along the polymer chain,
both the elastomeric and non-elastomeric blocks thereof being
hydrogenated to practically the same extent, which may be either
partial or substantially complete.
Typical of the rubber block copolymers useful herein are the poly-
styrene-polyisoprene-polystyrene and polystyrene-poly(ethylene-buty-
lene)-polystyrene. These copolymers may be prepared using methods
taught, for example, U.S. Pat. Nos. 3,239,478 issued March 8, 1966
to J.T. Harlan; 3,427,269 issued Feb. 11, 1969 to F.C. Davis et al.;
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3,700,633 issued Feb. 11, 1969 to M. M. Wald e-t al.; 3,753,936 issued
August 21, 1973 to 0. L. Marrs; and 3,932,327 issued Jan. 13, 1976 to
F. E. Naylor. Alternatively, some may be obtained from Shell Chemical
Co. under the trademarks Kraton D1107, D1111, D1117, G1650, G1652 and
G1657 and from Phillips Chemical Co. under the trademarks Solprene 418
and 423. Also useful herein is Kraton GX 1726 which comprises a 30/70
blend of a polystyrene-poly(ethylene-butylene)polystyrene tri-block
copolymer and a polystyrene-poly(ethylene-butylene) diblock copolymer.
Most preferred for use herein are the block copolymers containing the
hydrogenated butadiene midblock, in particular those available from
Shell under the trademarks Kraton G1650, G1652 and G1657.
The plasticizing (extending) oils are used in the adhesive in
amounts of 60% to about 95%, preferably 70 to 80%, by weight. The
above broadly includes not only the usual plasticizing oils as the
petroleum derived hydrocarbon oils (primarily mineral oil), but also
contemplates the use of the olefin oligomers and low molecular weight
polymers. The petroleum derived oils which may be employed are
relatively high boiling materials containing only a minor portion of
aromatic hydrocarbons (preferably less than 30% and, more particularly,
less than 15% by weight of the oil). Alternatively, the oil may be
totally non-aromatic. The oligomers may be polypropylenes,
polybutenes, hydrogenated polyisoprene, hydrogenated polybutadiene, or
the like having average molecular weights between about 350 and about
10 ,000 .
Among the applicable stabilizers or antioxidants utilized herein
are included high molecular weight hindered phenols and multifunctional
phenols as sulfur and phosphorus-containing phenols. Hindered phenols
are well known to those skilled in the art and may be characterized
as phenolic compounds which also contain sterically bulky radicals
in close proximity to the phenolic hydroxyl group hereof. In particu-
lar, tertiary butyl groups generally are substituted on-to the benzene
ring in at least one of the ortho positions relative to the phenolic
hydroxy group. The presence of these sterically bulky substituted
radicals in the vicinity of the hydroxyl group serves to retard its
stretching frequency and, correspondingly, its reactivity; this steric
hindrance thus providing the phenolic compound with its stabilizing
properties. Representative hindered phenols include: 1,3,5-trimethyl
2,4,6-tris (3,5-di-tert-butyl-4-hydroxybenzyl)benzene; pentaerythrityl
tetrakis-3(3,5-di-tert-butyl-4-hydroxyphenyl)propionate; n-octadecyl-
3,(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate; 4,4'-methylenebis (2,
6-tert-butylphenol); 4,4'-thiobis (6-tert-butyl-o-cresol); 2,6-di-tert-
butylphenol; 6-(4-hydroxyphenoxy)-2,4-bis(n-octylthio)-1,2,5-triazine;
di-n-octadecyl 3,5-di-tert-butyl-4-hydroxybenzylphosphonate, 2-(n-
octylthio)ethyl 3,5-di-tert-butyl-4-hydroxybenzoate; and sorbitol
hexa[3-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate].
The performance of these antioxidants may be further enhanced by
utilizing, in conjunction therewith known synergists for example,
thiodipropionate esters and phosphites, preferably distearylthiodi-
propionate.
These stabilizers are generally present in amounts of about 0.1 to
1.5%, preferably 0.25 to 1.0%, by weight.
Other additives such as plasticizers, pigments, dystuffs, etc.,
conventionally added to hot melt adhesives for the various end uses
contemplated may also be incorporated in minor amounts into the
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formulations of the present invention.
Additionally, it may be desirable to replace a portion of the oil
with small amounts of a tackifier to further plasticize the adhesive.
When present, the tackifier is used in amounts to replace up to about
45% of the oil, a quantity not sufficient to render the adhesive
pressure-sensitive. Suitable tackifiers include liquid tackifying
resins such as Wingtak 10 or conventional solid tackifiers such as
hydrocarbon resins, synthetic polyterpenes, rosin esters, natural
terpenes, and the like. More particularly, the useful tackifying
resins include any compatible resins or mixtures thereof such as (1)
natural or modiFied rosins, for example, gum rosin, wood rosin,
tall-oil rosin, distilled rosin, hydrogenated rosin, dimerized rosin,
and polymerized rosin; (2) glycerol and pentaerythritol esters of
natural and modified rosins, for example, glycerol ester of
pale, wood rosin, glycerol ester of hydrogenated rosin, glycerol ester
of polymerized rosin, pentaerythritol ester of hydrogenated rosin, and
phenolic-modified pentaerythritol ester of rosin; (3) copolymers and
terpolymers of natured terpenes, e.g., styrene/terpene and alpha methyl
styrene/terpene; (4) polyterpene resins having a softening point, as
determined by ASTM method E28,58T, of from 80 to 150C; the
latter polyterpene resins generally resulting from the polymerization
of terpene hydrocarbons, such as the bicyclic monoterpene known as
pinene, in the presence of Friedel-Crafts catalysts at moderately low
temperatures; also included are the hydrogenated polyterpene resins;
(5) phenolic modified terpene resins and hydrogenated derivatives
thereof, for example, the resin product resulting from the
condensation, in an acidic medium, of a bicyclic terpene and a phenol;
1~ 8()~
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(6) aliphatic petroleum hydrocarbon resins haviny a Ball and Ring
softening point of from 70 to 135C.; the latter resins resulting
from the polymerization of monomers consisting of primarily of
olefins and diolefins; also included are the hydrogenated aliphatic
petroleum hydrocarbon resins; (7) aromatic petroleum hydrocarbon
resins and the hydrogenated derivatives thereof; and (8) alicyclic
petroleum hydrocarbon resins and the hydrogenated derivatives
thereof; and (a) aliphatic - aromatic copolymers and their hydrogenated
derivatives, for example, coumarone-indene resins.
The desirability and selection of the particular tackifying agent is,
in large part, dependent upon the specific block copolymer employed.
In the case of adhesives containing a copolymer with an isoprene
mid-block, the use of a tackifier is generally not desirable; however
those containing copolymers of hydrogenated butadiene will be improved
by the use of small amounts of a tackifier.
The adhesive compositions are prepared by blending the components
at a ter,lperature of 130-180C until a homogeneous blend is obtained,
approximately 2 hours. Various methods of blending are known in the
art and any method that produces a homogeneous blend is satisfactory.
An exemplary procedure involves placing the block copolymer, stabilizer
or antioxidant, and any optional additives whose presence may be desired,
together with approximately half the oil in a jacketed mixing kettle,
preferably in a low shear stainless steel kettle which is equipped with
rotors and thereupon raising the temperature to a range of from 120 to
170C. When the blend has dissolved, stirriny is continued and the
remainder of the oil and any tackifying resin which may be employed are
added over a short period in order to avoid the formation of lumps.
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Mixing and heating are continued until a smooth, homogeneous mass is
obtained whereupon the remainder of the tackifying resin and the oil
are thoroughly and uniformly admixed therewith. The resultant hot melt
adhesives are generally produced in bulk form and packaged in release
coated containers.
The base substrate employed in forming the constructions of the
invention may be paper; plastic film, sheets and foils; textiles; glass;
ceramic; and metal foils and sheets. As described herein, the substrate
is most commonly paper which may or may not be preprinted on one or
both sides.
The other surface in the construction is generally plastic,
although the invention also contemplates use of metal, glass or paper
as the item to be temporarily bonded to the base substrate.
The hot melt adhesives are applied to the base substrate in thin
film form in a molten state at a temperature greater than about 130C.
The adhesives may be applied using conventional techniques as by use of
a roller, dauber plus doctor blade, printed dots via heated offset
rollers, extrusion gun or the like. Since the specific hot melt
adhesives used are not pressure sensitive and therefore lack sufficient
tack to form a bond at room temperature, the item to be attached to the
base substrate is generally affixed while the adhesive is still in its
molten or partially molten form, i.e., at temperatures of at least
about 100C. Depending, however, on the degree of plasticization of
the block copolymers chosen and the nature of the substrates to be
bonded, a relatively weak bond, sufficient for some temporarily bonded
constructions, may be formed at room temperature.
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In the examples of preferred embodiments which follow, all parts
and percentages are given by weight and all temperatures in degress
Celsius unless otherwise noted.
EXAMPLES
The following examples show the various block copolymers and
relative amounts that can be used in preparing non-pressure sensitive
adhesives for use in the temporary bonded constructions of the present
invention. Also noted in Table I are the adhesive properties observed
for each of the compositions. As those skilled in the art will recog-
nize, the adhesive requirements will vary depending upon the polarity
of the substrates to be bonded. It will be seen from the following
results that the type of polymer and the molecular weight of the polymer
as well as the amount of the plasticizing oil used will all have an
influence on the adhesive wetting ability of the composition and it is
left for the artisan to select the appropriate formulation or blend
thereof for the intended end use application.
In preparing the samples described in Table I, the block copolymer,
oil, and any optional components were mixed in a low shear stainless
steel vessel at a temperature of 150C until a homogeneous mass was
obtained.
Small quantities of the samples designated 2, 5, 7, 8 and 11 were
heated until molten and applied in a bead form at 180C to paper board
stock. A plastic credit card was immediately affixed thereto with
application of slight pressure. The construction was held overnight
under ambient conditions. The card was then easily removed from the
stock with a clean release of the adhesive from the credit card. The
adhesive was then readily removable from the paper board.
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