Hydrophilic acrylic pressure sensitive adhesives

Acrylic pressure sensitive adhesives which exhibit hydrophilicity and moisture permeability can be made by curing or crosslinking low molecular weight acrylic oligomers modified with substantial quantities of a hydroxyl containing monomer. The resultant modified pressure sensitive adhesive acrylic copolymers, upon crosslinking, will then have a hydrophilic hydroxy component incorporated into the copolymer backbone. Such adhesives are particularly useful in skin contact applications, such as the adhesives used on bandages or the adhesives used in transdermal drug delivery systems.

HYDROPHILIC ACRYLIC PRESSURE SENSITIVE ADHESIVES 
This invention relates to adhesives based on normally pressure sensitive 
polymers and copolymers made from C.sub.4 to C.sub.10 alkyl esters of 
acrylic acid wherein substantial quantities of hydroxyl groups have been 
incorporated through modification of prepolymers or oligomers of said 
acrylic alkyl esters with monomers such as hydroxy ethyl methacrylate 
(HEMA). Said hydroxyl addition renders the resultant copolymers 
hydrophilic and moisture permeable, features which are desirable, for 
example, in adhesives which are to be applied to skin for wound care 
purposes, and capable of enzyme immobilization and controlled release of 
bioactive agents. 
BACKGROUND OF THE INVENTION 
Diverse adhesive technologies have been used for skin contact applications. 
A review article by Pierson (TAPPI Journal, June 1990, pages 101 to 107) 
describes the broad range of approaches which have been taken to provide 
adhesive systems which will adhere to skin and have utility in a diverse 
number of areas. 
One major category of such skin contact adhesives are compositions based on 
physical blends of hydropholic additives into normally pressure sensitive 
materials. For example, Chen in U.S. Pat. No. 5,339,546, Osburn in U.S. 
Pat. No. 4,477,325 and Doyle et al. in U.S. Pat. No. 4,551,490, all teach 
variations of this art. While effective in some applications, such 
physical blends suffer in that the hydrophilic additives are not 
chemically bound into the polymeric backbone. As a result, when becoming 
hydrated by the exudate of body fluids, such as perspiration, phase 
separation can occur. This leads to the formation of weak boundary layers 
and loss of adhesives properties. Further embodiments of this same 
technique of physically blending hydrophilic additives, such as 
hydrocolloid powders as powdered sodium carboxymethylcellulose (NaCMC), 
into a polymeric pressure sensitive adhesives as binders have been 
proposed, including the use of acrylic pressure sensitive materials as the 
polymeric binder. Further, the addition of other water soluble, but not 
chemically bound agents, such as low molecular weight polyols, as 
glycerol, have been proposed to enhance the rates of hydration (as in 
International Application WO 91/09633). These too suffer from not having 
the added hydrophilic agent chemically bound into the polymer system. 
It has long been known that pressure sensitive adhesives can be produced 
from a variety of alkyl esters of acrylic acid. Polymers and copolymers 
based on C.sub.4 to C.sub.10 acrylic esters are inherently pressure 
sensitive and have a well established use in industry. The effects of 
monomer type and of the resultant molecular weight on pressure sensitive 
adhesive properties are well known to those skilled in the art. For 
instance, in the Handbook of Pressure Sensitive Adhesive Technology, Satas 
teaches how the chain length of a dependent alkyl group influences such 
properties as glass transition (T.sub.g) and resistance-to-peel (pages 397 
to 402). Numerous other publications can be found to further illustrate 
this well known technology. 
Attempts to enhance the moisture permeation of acrylic pressure sensitive 
adhesives themselves have involved such techniques as aerating the 
adhesive as it is applied to a web and dried, creating a micro structure 
resembling a polymeric foam. Other attempts to enhance the hydrophilicity 
of acrylic pressure sensitive adhesives have involved the addition of 
monomers, such as n-vinyl pyrrolidone or acrylamide, during synthesis of 
the pressure sensitive adhesive, as discussed by Lucast and Taylor (TAPPI 
Journal, June 1990, pages 159 to 163). However, the use of said monomers 
pose toxicity concerns during manufacture and are often used in only 
relatively minor quantities. Furthermore, polymerization must be carried 
out in solvent media in order to adequately incorporate these monomers, 
which contribute to hydrophilicity, into the polymer backbone. The 
resultant polymer must then be cast from a solvent onto a web in order to 
create useful products. Such solvent casting techniques now pose 
environmental concerns over volatile organic emissions and have inherent 
production inefficiencies. Kellen et al. in U.S. Pat. No. 4,737,559 
teaches that such solution polymerization techniques are common and well 
known in the art. 
Hydroxy containing acrylic monomers have long been used in the manufacture 
of medical products. For example, polymerizates of hydroxy ethyl 
methacrylate (HEMA) are used in the manufacture of contact lenses. The 
resultant products made from pure HEMA are hard, brittle plastics which 
require some plasticization in order to yield functional materials. 
However, because of their hydroxy functionality, said products are 
inherently moisture permeable and susceptible to moisture pick-up and are 
thus inherently hydrophilic. Polymerization of HEMA itself is exceedingly 
sensitive to reactor conditions (as taught by Bursky et al. in U.S. Pat. 
No. 4,904,749). Because of the high charge transfer of its --OH group, 
polymerization of HEMA readily results in gel or partially crosslinked 
polymer networks and when not gelled poly-HEMA must often be stored under 
cool or refrigerated conditions to prevent further autopolymerization. 
Nonetheless, Korol in U.S. Pat. No. 4,563,184 teaches the value of 
polymerized HEMA adhesive systems as being efficacious in the delivery of 
certain drugs or bioactive components and in wound healing applications. 
Attempts to incorporate HEMA or similar hydroxy containing monomers into 
acrylic pressure sensitives have, as a consequence, been limited to 
relatively small additions, say &lt;5%, to the monomer make up of an adhesive 
polymerizate. Even at these relatively low concentrations, difficulties 
are encountered in attempting to control the kinetics of synthesis. Often 
an undesirable polymer gel results when attempting to synthesize 
conventional, inherently pressure sensitive acrylic copolymers based on 
C.sub.4 to C.sub.8 acrylic esters to which hydroxy containing monomers, 
such as HEMA, have been added. Such gels cannot be further processed or 
applied to webs or substrates as adhesive materials. Thus, the use of 
hydroxy containing monomers, such as HEMA, has been limited in the use of 
pressure sensitive adhesives for applications in which the hydrophilicity 
of HEMA like substitutive groups would be desirable, as for skin contact 
adhesives. 
The chemical structures below are illustrative, but not necessarily 
inclusive, of the differences between conventional acrylic based pressure 
sensitive adhesives and the novel adhesives based on modifying a 
prepolymerized acrylic pressure sensitive precursor with hydroxy ethyl 
methacrylate. 
Conventional emulsion and solution random acrylic pressure sensitive 
copolymers 
##STR1## 
If y&gt;x (more 2-ethylhexyl acrylate or isooctyl acrylate than butyl 
acrylate), the greater the tack and the lower the cohesive strength. 
Acrylic acid, z, often ranges from 0 to 5% and creates hydrogen bonding 
between polymer chains to enhance cohesive strength and also promotes 
adhesion to polar substrates. 
Novel hydrophilic acrylic pressure sensitive adhesives: 
##STR2## 
For example, homopolymers or random copolymer of butyl and/or 2-ethylhexyl 
acrylate are prepolymerized and in the form of a viscous liquid oligomer, 
wherein the molecular weight, as indicated by a number of repeat units, n, 
can be in the range of 1,000 to 150,000. Hydroxyethyl methacrylate (HEMA), 
which enhances hydrophilicity, moisture permeability and skin contact 
adhesion, can be incorporated at as high as the 40% level and will 
copolymerize into the homopolymer or random copolymer pressure sensitive 
oligomer base upon crosslinking or curing using any free radical source, 
such as those resulting through photoinitiation or, alternatively, 
exposure to electron beam ionizing irradiation. 
SUMMARY OF THE INVENTION 
It has been discovered that blends of an acrylic pressure sensitive 
adhesive oligomeric base, made from C.sub.4 to C.sub.10, acrylic ester 
monomers having a molecular weight &gt;1,000, can be modified with 
substantial quantities, &gt;5%, but &lt;40%, of a hydroxy containing acrylic 
monomer, such as HEMA, to produce hydrophilic and moisture permeable 
pressure sensitive adhesives upon curing or crosslinking. Said cure can be 
initiated by conventional free radical sources, such as peroxide addition 
followed by thermal initiation, by photoinitiation via exposure to 
ultraviolet light or by exposure to ionizing irradiation from electron 
beams. 
Said blends of C.sub.4 to C.sub.10 oligomeric bases with HEMA monomer can 
be easily made and then processed and coated directly onto webs as 100% 
non-volatile liquids and finally subjected to ultraviolet light or 
electron beam initiated curing and copolymerization. Unlike other monomers 
heretofore selected to impart hydrophilicity to acrylic pressure 
sensitives, HEMA is itself relatively non-toxic. For example, the oral 
toxicity of HEMA LD.sub.50 is &gt;5,000 mg/kg for rodents, whereas the 
toxicity of either acrylamide monomer, as proposed by Krampe et al. in 
U.S. Pat. No. 4,693,776, is LD.sub.50 170 mg/kg or n-vinyl pyrrolidone is 
LD.sub.50 is 1.44 mg/kg. While suitable diluents for said acrylic pressure 
sensitive oligomer bases, neither acrylamide nor n-vinyl pyrrolidone could 
therefore be practically used in substantial quantities when such bases 
are designed to be applied as 100% non-volatile liquid precursors. Those 
skilled in the art will also realize that pendant hydroxyl groups pose 
less notable potential for skin irritation than the heretofore use of acid 
functionality which was achieved by incorporating acrylic acid into a 
polymer chain. 
Thus in accordance with this invention, there is provided an adhesive 
composition comprising: 
(a) an oligomer base made by the synthesis of C.sub.4 to C.sub.10 acrylic 
esters or similar acrylic esters having comparable pendant chain length 
having a molecular weight &gt;1000 Daltons, or preferably an oligomer made 
from either n-butyl acrylate (C.sub.4) or combinations with isooctyl or 
2-ethyl hexyl acrylate (C.sub.8) or mixtures thereof said oligomer base 
having a molecular weight &gt;1,000 but &lt;150,000 Daltons with said base being 
at most a viscous liquid at ambient temperatures; 
(b) a diluent monomer with hydroxy functionality, preferably either hydroxy 
ethyl acrylate or optimally hydroxy ethyl methacrylate (HEMA) with said 
monomer present in quantities of mixtures thereof &gt;5% but &lt;50%, preferably 
&gt;5% and &lt;40%, e.g. at least 5% up to 25% by weight optimally at least 5% 
to 20%; 
(c) optionally other diluent monomers, such as 2(2-ethoxyethoxy) 
ethylacrylate, a slightly water soluble monomer without hydroxy groups, or 
isodecyl acrylate but preferably either isooctyl acrylate or 2-ethyl hexyl 
acrylate which are used as diluents to both reduce application viscosity 
and to increase pressure sensitive tack after curing; 
(d) optionally multifunctional monomers, such as difunctional and 
trifunctional acrylic monomers, preferably trimetylol propane triacrylate, 
which can be incorporated at modest levels, say up to 5% to enhance the 
cure rate of the adhesive composition or its response to photoinitiation 
or electron beam curing conditions; 
(e) optionally a source for free radical initiation of cure by either the 
incorporation of peroxides which decompose on exposure to heat or 
photoinitiators which decompose on exposure to light, preferably in the 
ultraviolet range or by exposure to an electron beam, x-rays or gamma 
source. 
Furthermore, it is provided a process for manufacture of a hydrophilic 
pressure sensitive adhesive which consists of: 
(a) blending the aforementioned oligomers and monomers and, if desired, 
photoinitiators and/or other additives to form a liquid coating which is 
non-volatile at ambient temperatures; 
(b) coating said liquid onto a moving web or other object; 
(c) exposing said adhesive precursor to either ultraviolet light or to the 
ionizing irradiation from an electron beam to induce curing and 
copolymerization of the hydroxy moiety into the pressure sensitive 
adhesive. 
Finally, articles such as bandages or other products wherein an inherently 
hydrophilic pressure sensitive adhesive would be of commercial value are 
envisioned applications for the above art. Because of the benign nature of 
the pendant hydroxyl groups, these adhesives are particularly suited for 
drug delivery systems. Said hydroxy functionality can be used to 
immobilize enzymatic and/or other bioactive agents, providing for 
controlled release upon hydration from body fluids and the like.

DETAILED DESCRIPTION OF THE INVENTION 
Oligomer bases for coating onto webs or other objects and subsequently 
exposing such normally 100% non-volatile liquids to either ultraviolet 
light or electron beam irradiation are well known in the art. Of 
particular interest in the development of pressure sensitive adhesives are 
oligomeric bases produced by UCB Radcure. Specifically, UCB Radcure 
pressure sensitive adhesive bases IRR 84, IRR 85 and IRR 153, the former 
two being supplied with monomer diluents already present, the later being 
a non-diluted oligomer, i.e. a polymeric base of &gt;1000 Daltons in 
molecular weight. The oligomers used in these materials are proprietary 
but based on C.sub.4 to C.sub.8 acrylic esters. 
Hydroxy containing monomers suitable for use in these compositions are, for 
example, hydroxy ethyl methacrylate (HEMA), sold under the trade names of 
Sipomer CL-100 by Rhone-Poulenc or Rocryl 400 by Rohm and Haas. 
To enhance the cure rate of blends of the oligomer bases diluted with the 
HEMA monomer, small quantities of a multifunctional acrylate, such as 
trimethylolpropane triacrylate sold by the Sartomer Company as SR-351, can 
be used. 
When using ultraviolet light to promote cure and crosslinking, the 
preferred photoinitiator is 
2-(4-(2-hydroxy-2-methyl-1-oxopropyl)phenoxyethyl)-2-propenoate having a 
formula of C.sub.15 H.sub.18 O.sub.5 and sold under the designation of 
ZLI-3331, formerly by EM Industries, Inc. but now by the Ciba-Geigy 
Corporation. This photoinitiator is preferred since upon decomposition 
when exposed to ultraviolet light, its decomposition by-products graft 
into the polymer backbone, leaving no deleterious extractables. 
The combinations of oligomer, monomers and photoinitiator described below 
can be easily blended together in a low shear mixer or, for experimental 
purposes, even by hand stirring. Because of the higher viscosity of the 
IRR 153 a slight amount of heating is needed to facilitate mixing, up to 
50.degree. C. was adequate when using this oligomeric base. Because the 
ZLI-3331 photoinitiator comes as a dried powder, some heating may also be 
need to incorporate this additive. However, the ZLI-3331 was found to be 
directly soluble in some preferred monomeric diluents. 
These acrylic pressure sensitive adhesives can contain other ingredients as 
commonly used by those skilled in the art. For example, to enhance the 
tack of the cured or crosslinked adhesive it is preferred to add 2-ethyl 
hexyl (C.sub.8) and/or isodecyl (C.sub.10) acrylate monomers to the liquid 
blends. These too will cure or crosslink into the polymer matrix. 
Electron beam irradiation, as generated, for example, from linear filament 
accelerators having a voltage potential of 150,000 to 300,000 volts, is 
suitable for copolymerizing the HEMA monomer into the base acrylic 
pressure sensitive without the use of photoinitiators or other additives. 
When using photoinitiation, adequate cure could be obtained for 50 micron 
thick adhesive films with 300 watt/inch medium pressure mercury vapor 
ultraviolet lamps such as manufactured by American Ultraviolet. 
Electrodeless lamp systems, such as those manufactured by Fusion UV Curing 
Systems, were also found to be effective. In particular, tighter cure was 
obtained using the Fusion "D" bulb with a peak intensity in the near 
ultraviolet, 350 to 400 nm, than when using the Fusion "H" bulb, which has 
greater intensity in the far UV range, 200 to 325 nm range when running at 
comparable under lamp speeds. Good cure was achieved in air without the 
need for nitrogen inerting, as is sometimes practiced in the art. 
EXAMPLE 1 
The pressure sensitive oligomer composition IRR 84 from UCB Radcure, 
containing 47% of a C.sub.4 to C.sub.8 acrylic ester oligomeric copolymer 
and 53% of a mixture of monomeric diluents, such as 2-ethylhexyl acrylate, 
isobornyl acrylate and 4% n-vinyl-2-pyrrolidone was mixed with 5% of the 
ZLI-3331 photoinitiator as a control. A similar composition was made with 
20% HEMA monomer added to the mixture as below: 
TABLE I 
______________________________________ 
Control 1 
A 
______________________________________ 
IRR 84 acrylic oligomer 
95% 75% 
Sipomer CL-100 HEMA monomer 
-- 20% 
ZLI-3331 photoinitiator 
5% 5% 
______________________________________ 
Hand draw downs were made onto a release coated paper using a Meyer rod to 
achieve approximately 50 microns thick wet coating. These were then passed 
under an American Ultraviolet laboratory curing unit with the conveyor set 
at 25 feet/minute using a 300 watt/inch medium pressure mercury vapor 
ultraviolet source. Tactile evaluation indicated that the control sample, 
without any HEMA monomer, cured well on one pass and could be transferred 
from the release paper to a polyethylene or polyurethane film without 
difficulty. Sample A with the 20% HEMA content, required four passes to 
achieve the same cure. This was not unexpected since additional monomer 
often retards the cure rate of ultraviolet activated systems. 
When comparing the Control 1 and Compound A as skin contact adhesives, the 
HEMA containing adhesive remained in contact with the forearm without 
lifting over a several day period of wear. The non-HEMA UV cured acrylic 
Control 1 remained in contact with skin for only a few hours and lifted 
easily of its own accord. 
EXAMPLE 2 
Laminates of these adhesives were made to the breathable, high moisture 
vapor transmission rate (MVTR) Exxaire 10B04 film which is available from 
the Exxon Chemical Company. These were tested to demonstrate the effects 
of HEMA modified acrylic pressure sensitive on MVTR or the ability to pass 
moisture through them. Tests were run per ASTM E-96 using the inverted cup 
method. Several additional samples based on the background art are 
included by way of example. 
TABLE II 
______________________________________ 
g/m.sup.2 /24 
hrs 
Moisture Vapor Transmission Rate Data 
@ 40.degree. C. 
______________________________________ 
Exxaire 10B04 breathable film (control) 
5400 
Exxaire + Sample A = IRR 84 with 20% HEMA 
500 
Exxaire + 2 mils hot melt adhesive 
35 
Exxaire + 30 mils hydrocolloid containing 
25 
hydrophilic adhesive per US 4,551,490 
______________________________________ 
From this data it is apparent that the HEMA modified pressure sensitive 
acrylic is more moisture permeable, even than those adhesives containing 
hydrocolloids, which are well known in the trade for their hydrophilicity 
and their skin contact properties. 
EXAMPLE 3 
Of concern in the medical products field and in the manufacture of bandages 
is the effects of sterilization conditions on adhesive properties. The 
Control 1 and Compound A from Example 1 were cured using ultraviolet light 
per the example and again transfer laminated to the Exxaire breathable 
film. These were then subjected to electron beam irradiation at 2.5 Mrads, 
a typically prescribed sterilization dose, and to 5.0 Mrads using a 4.5 
MeV Dynamitron accelerator. 
Probe tack (ASTM D-2979) results confirmed that these systems remain 
adhesive even after electron beam sterilization. Using probe conditions of 
100 g/cm.sup.2 load, 1 second dwell and 1 cm/sec separation rate, the 
following results were obtained for these adhesives: 
TABLE III 
______________________________________ 
Adhesive Film EB dose Probe Tack 
______________________________________ 
Control 1 
Exxaire 10B04 none 30 g 
Control 1 
Exxaire 10B04 2.5 Mrad 70 g 
A Exxaire 10B04 none 350 g 
A Exxaire 10B04 2.5 Mrad 90 g 
A Exxaire 10B04 5.0 Mrad 60g 
______________________________________ 
The HEMA modified acrylic pressure sensitive retains a reasonable tack even 
after electron beam sterilization. The tack of composition A is comparable 
to the Control 1 prior to sterilization. 
EXAMPLE 4 
A pure acrylic pressure sensitive oligomer not containing monomer diluents 
based on C.sub.4 to C.sub.8 acrylic esters was evaluated, UCB Radcure's 
IRR 153. Comparisons were made to determine the uniqueness of the hydroxyl 
acrylic monomer addition, HEMA, and to determine the effects of 
polyfunctional acrylates on the rate of ultraviolet curing. The four 
compositions below were prepared by heating the viscous IRR 153 and then 
adding the powdered photoinitiator and then the monomeric diluents. 
2(2-ethoxyethoxy) ethylacrylate was used as a comparative, slightly water 
soluble monomer without hydroxy groups. This is available from the 
Sartomer Company as SR-256. 
TABLE IV 
______________________________________ 
Control 2 
B C D 
______________________________________ 
IRR 153 acrylic oligomer 
95% 75% 75% 75% 
Rocryl 400 HEMA monomer 
-- 20% 17% -- 
Sartomer TMPTA SR-351 
-- -- 3% -- 
Sartomer EOEOEA Sr-256 
-- -- -- 20% 
ZLI-3331 photoinitiator 
5% 5% 5% 5% 
______________________________________ 
These compositions were drawn down onto release coated polyester film using 
a Bird applicator to achieve approximately 50 microns wet film thickness. 
Samples were then passed under an American Ultraviolet laboratory curing 
unit with the conveyor set at 20 feet/minute using a 300 watt/inch medium 
pressure mercury vapor ultraviolet source. Tactile evaluation indicated 
that the Control 2 sample, without any HEMA monomer, cured well on one 
pass and could be transferred from the release paper to the breathable 
polyethylene Exxaire film without difficulty. Compound B with the 20% HEMA 
content, required but two passes to achieve the same relative cure. In 
contrast to Compound A in Example 1, this higher cure rate reflects the 
lower overall monomer content in this composition. As might be expected, 
Compound C, with the polyfunctional acrylate added, cured tightly in only 
one pass, even with a high content of HEMA monomer in it. Compound D also 
cured in only one pass at 20 feet/minute, since the EOEOEA monomer is 
known for its reactivity. 
Transfer laminates made to the breathable polyethylene film were applied to 
the forearm and evaluated for skin adhesion. Both Compound B and the 
faster curing Compound C, which contained substantial quantities of HEMA 
copolymerized into the cured pressure sensitive adhesive polymer, remained 
in contact for several days with no lifting or loss of adhesion. In 
contrast, Compound D, without pendant hydroxyl groups, lost adhesion to 
skin and lifted in a matter of hours. 
To those skilled in the art, embellishments of the above described acrylic 
pressure sensitive adhesives would be apparent. For example, isooctyl or 
2-ethyl hexyl acrylates (C.sub.8) or isodecyl acrylate (C.sub.10) can be 
added as diluents to enhance the pressure sensitive tack. Balances between 
the oligomer base, IRR 153, HEMA, other monofunctional monomers used to 
enhance tack and polyfunctional monomers used to increase cure rate and 
the levels and types of photoinitiators, if ultraviolet curing is to be 
used, can be tailored for specific end use applications. Because of the 
low temperature curing of both ultraviolet light and electron beam 
systems, temperature sensitive bioactive ingredients, such as enzymes, can 
be incorporated into these adhesive compositions prior to curing for 
subsequent release upon hydration by body fluids. Other bioactive 
materials can also be contained in the uncured, liquid adhesive precursor 
for release upon application to skin or body parts.