CA2126981A1

CA2126981A1 - Removable, low melt viscosity acrylic pressure sensitive adhesives

Info

Publication number: CA2126981A1
Application number: CA002126981A
Authority: CA
Inventors: Albert I. Everaerts; Jeffrey D. Malmer
Original assignee: Individual
Current assignee: 3M Co
Priority date: 1991-12-31
Filing date: 1992-12-21
Publication date: 1993-07-08
Also published as: EP0619830B1; US5552451A; NO942461D0; US5648425A; NO942460D0; NO942460L; WO1993013147A1; EP0619830A1; AU665606B2; WO1993013148A1; TW221061B; KR100270155B1; ES2092806T3; DE69214164T2; EP0619831A1; BR9206999A; NO942461L; CN1073962A; DE69214163D1; DE69214163T2

Abstract

The present invention relates to a permanently removable, low melt viscosity acrylic pressure-sensitive adhesive comprising a lower alkylacrylate, a higher alkylacrylate, at least one polar monomer and a crosslinker.

Description

2 1 2 ~
W~/t~14~ ` ~C~/~1S92/11156 REMOVABLE, LOW MELT VISCOSITY
ACRYLIC PRESSURE SENSITIVE ADHESIVES

Field of Invention The present invention relates to a permanently lO removable, low melt viscosity acrylic pressure-sensitive adhesi~e. In particular, the present invention relates to an adhesive comprising a lower --alkyl acrylate, a higher alkyl acrylate, a polar ~.
monomer and a crosslinker.
Backqround of the Invention Removable pressure sensitive adhesives (PS~) adhered to a backing, which p:redictably adhere, yet remain repe~tedly peelable from a variety of substrates over a long period of time without damaging or marring 20 the substrate, have many comm~ercial uses~ For example~ .
masking tapes, rPmovable labels or office notes, protective films and medical tapes all must quickly :~.
adhere to me~al, paper, plastics and skin, ~
respectively, but must also peel smoothly away ~rom ~ .-25 these ~aried substrates without leaving~beh;ind any ~.
adhesive residue on or damaging the surface of a ~ :
particular subs~rate.
: Ideallyf depending on the substrate,:the:removable i-adhesive must provide su~flcient t~ack (or quick stick) ~ -~
30 to quickly fix t~e;adhesive to the desired:substrate, adequate peel strength to:prevent damage of~the :surface ~ `
when the adhesive is removed, and have ~he appropria~e ~:
cohesive strength to~control~ the transfer of adhesive to the substrate~ ohesive strength must also be 35 controlled in order to lim.it the~cold:~f~ow of the adhesive on a surface,~a~process~whi~oh leads to an .1.
undesirable:building of peel ~trength over~time~
. Balancing the-e pressure sensltive adho~i/e:~ploperties~

W~ iR PCr/US92/11156 particularly in a removable adhesive, poses difficulties to the formulator.
U.S. Patent No. 3,691,140 (Silver) discloses the use of solid, inherently tacky, pressure-sensitive 5 adhesive microspheres which, when adhered to a first substrate, may when contacted to a second substrate be easily removed ~rom the second substrate without delaminating the first substrate or the second substrate. While exhibiting a high degre~ of 1~ removability, microsphere adhesives tend to transfer to contac~ surfaces such as the second substrate. -~
U.S. Patent No. 5,045,569 (Delgado) discloses hollow, inherently tacky pressure-sensitive adhesive acrylate microspheres which, due to their unique 15 morphology, provide enhanced peel and shear strength and less adhesive transfer to substrates than "repositionable" adhesives based on solid acrylate-based microspheres.
U.S. Pa*ent No. 4,599,265 (Esmay) discloses an 20 adhesive having a high degree of cohesive strenyth.
The adhesive disclosed is an acrylate, has low tack and maintains peelability from a variety of ordinary substrates. Esmay teaches that through the -~
crosslinking of the tapels adhesive layer and the use 25 of low levels of p~lar monomer (up to 3 mole percent of a strongly polar monomer, such as acrylic acid3 along with alkyl acrylates having sidP chains 4-12 carbons in length in the ~opolymeric adhesiYe, the required ~ ;
balance of low tack and high cohesive strength can be 30 imparted to the removable adhesive. One possiblie drawback associated wîth the Esmay adhesive is that its monomers may bleed through a subs~rate su~h as paper prior to curing. Such bleeding is undesirable because of the damage done to the substrate. ~ ~;
U.S~ Patent No~ 4,737,559 (Xellen et al~3 describes a PSA formulation in whi~h the ~iscous flow, ~;
and resultant adhesion buildup, is cont~olled through `~

W~ 14~ 21 ~ ~ 9 ~ 1 PC'r/US92/111~6 the ~ddition of a small amount of a free radically polymerizable photocrosslinker (a mono-ethylenically unsaturated aromatic ketone, most preferably 4-acryloyl-oxy-benzophenone) to at least one alkyl 5 acrylat~ or methacrylate. The Kellen et al. adhesive is formulated for adhesion to skin and at times, considered to be too aggressive for paper.
U.S. Patent No. 3,635,754 (Beede) discloses the use of a heat-activated, multi-phase pressure-sensitive --10 adhesive copolymer whose melting point is greater than --`-35C and whose glass transition temperature (Tg) is below 5C. At room temperature, the adhesive is non tacky. At that temperature, a crystalline or ordered phase and an amorphous or disordered phase co-exist.
15 At skin temperature ~approximately 35C), the copolymer -~
becomes sufficiently amorphous to become tacky. This phenomenon may be reversed by cooling the adhesive for removal from a substrate, with the crystallization of the side chains providing sufficient cohesive strength 20 to prevent any transfer of adhesive to the substrate.
World Patent Application Serial Number`90/13420 (Stewart et al.3 dlscloses a temperature activated, pressure-sensiti~e adhesive which is rapidly converted from non tacky to tacky over a predetermined 25 temperature range. Similar to the multi-phase adhesivè of Beede, this temperature dependent adhesive relies on the careful selection of side chain crystallizable monomers, particularly those monomers ~-which have linear aliphatic side chains of at least 10 30 carbon atoms, including Cl~-C22 al~yl acrylates or me~hacrylates. Stewart et al. specify that th~se composition~, when a mixture of copolymerizable monomers are used, must cont~in at leas~ 50% by weigh~ -monomers with crystallizable side chains. However, at ~-35 room temperature, these adhesives are substantially non-tacky.

~, .

., ., ~

WO~J1~14~ 2 ~ ~6~3:I P~r/uss~ fi U.S. Patent No. 4,038,454 (Lehmann et al.) discloses a pressure-sensitive adhesive which comprises either a primary alkyl acrylate or a mixture of primary alkyl acrylates and crosslinker. Lehmann et al.'s 5 alkyl acrylate contains at least four carbon atoms. I~
a mixture of primary alkyl acrylates is utilized, the primary alkyl acrylates can have carbon atoms whi~h ;
contain l to 14 carbon atoms. Half the carbon ~toms should contain at least four carb~n atoms in a mixture.
lOIn using PSAs, it is also desirable to combine these removable qualities with the procedural, economical and environmental advantages of a hot melt processible adhesive. To date, most attempts to prepare acrylic hot melt PSAs have been limited because 15 such materials have poor cohesive strength at room temperature or are too viscou~; even for use at high temperatures found in current melt processing equipment and practices. Maintaining sufficient cohesive strength, as noted above, is one critical factor in the 20 formulation of permanently r~movable PSAs.
Typically, the cohesive strength of acrylic polymeric PSAs, which are not microspheres, may be improved in a variety of ways, but each way has its disadvantages, especially when the formulator wishes to 25 maint~in a low melt viscosity for the PSA. The molecular weight of t~e acrylic pol~mer may be raised to improve cohesive strength, but an unavoidable and unacceptable rise in melt viscosity also results.
Polar monomer content, such as acrylic acid, may also i30 be increased to improve cohesive strength, a pr~perty which is use~ul in many adhesive ~pplica~iQns.
However1 increased polar monomer content has ~aused greater adhesion buildup, as described by U S. Patent No. 3,008,850 (Ulrich). Finally, cohesi~e strength may 35 be enhanced through the covalent crosslinking of the adhesive. Covalent crosslinking,~though providing the ~ ., Wl~ 14~ 2 l'~,G9 ~ 1 PCr/US9~/111~6 needed increase in cohesive strength, eliminat~s the potential for melt processing.
Physical crosslinking is described in U.S. Patent No. 4,554,324 (Husman et al.). Husman et al. disclose 5 a hot melt processible acrylate PSA which gains the needed balance of cohesive strength, high tack, and low melt viscosity through the chemical modification o~ the so~t acrylate backbone by grafting reinforcing high Tg polymeric moieties to the acrylate chain. These high Tg 10 moieties provide glassy domains which enhance the cohesive strength of the adhesive at lower temperatures without dramatically increasing the melt viscosity of the composition. The peel adhesion strengths of the exemplified compounds, however, tend to exceed those 15 values which are required for remo~ability from most substrates.
Thus, there currently exists a need for a ~;
removable acrylic press~re sensitive adhesive which displays a superior balance oE tack, peel strength and 20 cohesive strength at room temperature and which may be ;~
formulated for remo~al from a variety of substrates without damaging or depositing adhesive residue on the substrate and without building excessively in adhesion over time.
Summary of the Invention The present invention comprises a removable acrylic pressure-sensitive adhesive which displays a superior balance of tack, peel strength and cohesive strength at room temperature. The adhesives of the 30 present inYentlvn can be removed from a variety of substrates without damaging or depositing adhesive `~
residue on the substrate and without building excessively in adhesion over time.
The pressure-sensitive adhesive of the i~vention 35 comprises~

.: '''~
-'''''';''' W~9~ IB P~US~/11I$6 2 1~6~81 6 -a) from about 10 to about 50 percent by weight of at least one higher alkyl acrylate having an alkyl group from 12 to 26 carbon atoms;
b3 from a~out 50 to about 90 percent by weight S of at lea~t one lower alkyl acrylate having an alkyl group fxom 4 to 12 atoms wherein said upper and lower alkyl acrylates cannot simultaneously have alkyl chains of 12 carbon atoms; :
c) at least one polar monomer copolymerizable ~ -10 with said higher and lower alkyl acrylate;
d) sufficient crosslinker to impart enough cohes.ive strength to the adhesive in order to prevent substantial adhesive transfer.
The polar monomer of the pressure-sensitive 15 adhesive of the present invention can comprise either a strongly polar and/or a moderately polar monomer and can comprise up to about 5% by weight polar monomer i~
it is a strong polar monomer and up to about 30~ by weight polar monomer if it is a moderately polar 20 monomer.
In ~ preferred embodiment of the present invention, the higher alkyl acrylate has a car~on group which has from 13 to 26 carbon atoms.
This invention also provides for pressure-25 sensitive adheslve sheets and tapes ~oat2d wlth t~epermanently removable, acrylic adhesive composition.
Detailed Descri~tion of_the Invention The removable, low melt viscosity acrylic pressure-~ensltiv~ adhesi~e of the inven~ion comprises ~
30 four comp~onents~ at least one lower alkyl acrylàt~ ~.
having an a:lkyl group which ~comprises:~rom about 4 to 12 carbon atoms, ~2~ at least one higher alkyl acrylate having an alkyl~group which comprises from about 12 to 26 carbon atoms, (3) at least one polar 35 monomer and (4) sufficient crosslinker to impart cohesive strength to the adhesive~

: ~-''',~ ~

w~ 2 ¦ ~ ~i t) ~ I PCI/US~2/lll56 The linear or branched lower alkyl acrylate or methacrylate esters useful in preparing the removable, low melt viscosity pressure sensitive adhesiv~s of this invention are linear or branched monofunctional -~
5 unsaturated acrylate or methacrylate esters of non-tertiary alkyl alcohols, the alkyl groups of which have from about 4 to about 12 carbon atoms. These lower linear and branched acrylates, as is well known in the pressure sensitive adhesive art, provide the ;~
10 properties of low glass transition temperature and viscoelastic characteristics that result in materials which are tacky in nature. Examples of the shorter chain, lower alkyl acrylates and methacrylates used in the invention include, but are not limited to, n-butyl 15 acrylate, isobutyl acrylate, 2-ethylhexyl acrylate, isooctyl acrylate, n-octyl acrylate, n-octyl methacrylate, 2-methylbu~yl acrylate, isononyl acrylate, isoamyl acrylate, isodecyl acrylate, isodecyl methacrylate, 4-methyl-2-pentyl acrylate~ and mixtures -20 thereof. Preferred lower acrylate and methacrylate esters of the inYention inclu~e isooctyl acrylate, n-butyl acrylate, and 2-ethyIhexyl acrylate.
The higher alkyl acrylate or methacrylate monomers -;
useful in preparing the pressure sensitive adhesives of 25 this invention are monofunctional unsaturated acrylate or methacrylate esters of non-tertiary alkyl alcohols, the alkyl groups of which have from about 12 to about -~
26 carbon atoms. Preferably, the alkyl groups have from about 13 to about 26 carbon atoms. It is 30 theorized that the long side chains of these acrylate and methacrylate es~ers increase the entanglement molecular weight o~ the polymer and facilitate the melt processing of the polymer. At the roncentrations used in this invention t it is believed that the side chains ;~

3~ do not crystallize substantially at room temperature and that the resulting polymers are amorphous~ If -~
~here was substantial crystallization, the modulus of , W~ rC~`/US92/lllS~
2i26~$1 the material woul~ increase causing a loss of pressure sensitive adhesive tack. The long alkyl chains of these acrylate and methacrylate esters also increase the non-polar character of the adhesive, thus reducing 5 the specific inter-molecular interactions, such as hydrogen bonding, acid-base interaction, and the like, which lead to excessive adhesion build-up between the adhesives and substrates susceptible to such interactions.
Examples of the long side chain acrylate and methacrylate esters used in the present inv~ntion include, but are not limited to, lauryl acrylate, lauryl methacrylate, isotridecyl acrylate, n-tetradecyl acrylate, n-tetradecyl methacrylate, n-hexadecyl 15 acrylate, n-hexadecyl methacrylate, n-octadecyl acrylate, n-octadecyl methacrylate, eicosanyl acrylate, hexacosanyl acrylate and mixtures thereof. Preferred long side chain alkyl acrylates and methacrylates include laur~l acrylate, lauryl methacrylate, octadecyl 20 acrylate, octadecyl methacrylate, hexacosanyl acrylate, eicosanyl acrylate and isotridecyl acrylate.
The polar monomers used in the pressure-sensitive adhesive of the present invention can be either strongly polar and/or moderately polar. The polar 25 monomers can be used singularly or in combination with other polar monomers.
Strong polar monomers include mono-, di-, and multi-functional carboxylic acids and salts such as acrylic acid, metha~rylic acid, itaconic acid, crotonic 30 acîd, maleic acid and fumaric acid. Other suitable~ ;
monomers include cyanoalkyl acrylates, acrylamides and acrylonitriles.
Moderately polar monomers include, but are not limited to, N-vinyl lactams, such as N-vinyl 3S pyrolidone, vinyl halides, vinylidene chloride, vinyl toluene, styrene, hydroxyalkyl acrylates and/or methacrylates~ 2-hydroxyethyl acrylate and 3- :~
' ~

W~3~ lq~ 2 12 G 9 ~ 1 Pcr/us~2/l 1 lS~

hydroxypropyl methacrylate are examples of suitable moderately polar monomers.
Crosslinkers are addeid to enhancfe the cohesive strength of the pressure-sensitive adhesive 5 composition. Preferred crosslinkers usfPiful when carboxylic acid moiet.ies are utilized include multifunctional aziridine amides which have the genera~
formula:
.~ .-, .
/O H\ R'~
R~C N~c~

15 wherein R can ba either an alkylene or aromatic group and R' can be a hydrogen or alkyl group. A suitable multifunctional aziridine amide includes 1,1'-(1,3-phenylenedicarbonyl)bis[2-meithyl aziridine] which has the following formula:

I H 2`N~

N
`CH 3 Another suitable multifunctional aziridine amide is 2 r 2 f~ 4-trimethyladipoyl bis [2-ethyl aziridine] which ;~ ;
has the formula:

W~ I'tR PCI tlJS')2/11156 2 1 ~ o -C~[2~ CIH 3 CH2 I / N -C -Cl -CH2-1 -CH2-C-N I

c2~ ~C~ -CH3 An additional multifunctional aziridine amide which can be utilized in the present invention is 1,1'-Azelaoyl bis [2-methyl aziridine] which has the following formula:

CH 2\ 11 ,CH 2 CH N -C--(CH ~7-C-N~CH
C~ /
3 ~H3~-2,4,6-tris(2~ethyl-l-azi~idinyl)-1,3 t 5 triazine which 15 has ~he following formula is another exam~le of a suitable multifunctional aziridine-H2C~\ ,C~ -C2~s C~ 3 1 ~ N ~ Nj /CH 2 ~H 3 CH2- ~ - N - C~,C-N -CH-CH2 ~ -.

25 Another suitable group of crosslinkers ~employed when ~:
carboxylic acid moie~ies are employed include metal ion ~
crosslink~rs. Exampl2s o~ suitable metal ion ~;
crosslinkers include copper, zinc, zirconium and ;:
chro~ium. ~ ~ ~

: ''.

~ 2 ~ ~.G`)~3 I Pcr/us~2/l1 lSh Especially preferred.metal ion crosslinkers are chelated esters of ortho-titanic acid sold under the tradename Tyzor and commercially available from the E.I. du Pont de Numours company. An especially 5 preferred Tyzor is T~zor AA which is titanium acetyl acetonate.
When hydroxy functional moderately polar monomers such as 2-hydroxyethyl acrylate or 3-hydroxypropyl methacrylate are utilized, polyfunctional isocyanate ::
10 crosslinkers may be utilized. Useful polyfunctional crosslinkers include aromatic polyfunctional .
isocyanates such as toluene diisocyanate, aralkylene polyfunctional isocyanates such as bis (4-isocyanato phenyl) methane, cycloaliphatic polyfunctional 15 isocyanates such as bis (4-isocya~ato hexyl) methane and aliphatic polyfunctional isocyanates such as hexamethylene diisocyanate or. tetramethylene diisocyanate.
Chromophore-substituted halomethyl~s-triazines r 20 such as those described by U.. S. Patent Nos. 4,329,384 -:~
~Vesley et al.); 4,330t590 (~esley~;-and 4,379,201 (Vesley), al~ of which are incorporated herein py reference, are also preferred crosslinking agents for compositions or applications which require no further -:
25 melt processing`or solvent casting.
Other crosslinkers which enhance the cohesive strength of the remoYable pressure-sensitive adhesive composition include, but are not limited to, :~
multifunctional acryla~es and methacrylates, , and .. -~
! 30 benz~phenone- and acetophenone~d2rived~ ~ :
photocrosslinking compounds.
Preferred crosslinkers are selected according~to the processing method used in preparing the adhesives of this invention. : For example, multifun ional 35 aziridine amides and me~al ions are utilized when ~;
thermal curing is reguired or when the adhesive is solution oated. For compositions which require no ... .

~ 14~ ~/US~/11156 6!3~:~ 12 -further melt processing following the initial polymerization, multifunctional acrylates and methacrylates, such as 1,4-butanediol diacrylate and ~ hexanediol diacrylate, can be used as crosslinkers.
5 Additional polyfunctional acrylic monomers which can be used as crosslinkers in the present invention are disclosed in U.S. Patent No. 4,379,201 (Heilmann et al.), which is incorporated herein by reference.
Silanes can also be used as crosslinkers.
10 Suitable silanes include trialkoxy silanes haviny mono-ethylenically unsaturated functionality. Other suitable silanes include silanes having dialkoxy or monoalkoxy substituted silane groups having mono-ethylenically unsaturated functionality.
Compositions of the invention which require further processing and undergo crosslinking by exposure to ultraviolet radiation following the initial polymerization may employ several preferred classes of crosslinkers including: chromophore-substituted ~0 halomethyl-s-triazines, such as those in the Vesley Patents described above, all of which are incorporated herein by reference; and mono-ethylenically unsaturated aromatic ketones, particularly 4-acryloyl-oxy-benzophenone, as describ~d by Kellen et al. in U.S.
25 Patent No. 4,737,559, which is incorporated herein by reference. -The pressure sensitive adhesive of this invention should comprise from about 10 to about 50% by weight of ;~-at least one iinear or branchPd alkyl acrylate or ~;
30 methacryl~te having from 12 to 26 carbon atoms; from about 50 tc about 90% by weight of at least one lower alkyl acrylate; sufficient crosslinker to impart cohesive strength to the adhesive and up to about 5% by weight of at least one strong polar monomer or 35 alternatively up to about 30% by weight of a moderately ~`
polar monomer. Generally, the amount of crosslinker vari~s from about 0O05~ to about 1~0~ by weight of the ~ :.
. . . ~

W~ ~tl~ 2 1 2 ~ 3 1 r~cr/us~2/l 1l56 total composition. However, the amount of the crosslinker used depends upon the type of crosslinker used. Sufficient crosslinker should be utilized so that no adhesive transfer from a backing to a substrate can 5 be visually observed. For example, when multifunctional aziridine amide is utilized, the amount o~ crosslinker, varies from about 0.10 to about 0.5% by weight of the total compositon and for Tyzor A~ the amount r~nges from about 0~1 to about 0.75% by weight 10 of the total composition. The range imparts to the adhesive enough tack to adhere to a contact surface an~
su~ficient peel adhesion to allow a substrate coated with the adhesive to be easily removed from a contact surface.
The photocrosslinkable adhesiYes can be polymerized using suspension, emulsion and solution polymerization methods. ~In t:he preferred method, a solution p~lymerization method, the monomers along wit~
the solvent and a copolymeriz:able crosslinker are 20 charg~d into a f~ur-neck reaction vessel which is equipped with a stirrer, a thermometer, a condenser, addition funnel and a thermowatch. After the vessel is -~
charyed, a concentrated thermal initiator solution i5 added to the addition funnel. The whole reacti~n~
25 vessel and addition funnel with their contents are then purg~d with nitrogen to create an inert atmosphere.
Once purged, the solution within the vessel is stirred ~-and heated to a~out 55~C and the initiator is added. A
98 to 99% conversion should be obtained after about 20 30 hours. I ~
If khe solution is then coated onto a substrate and no further processing is desired, the coating can be cured by exposure to ultraviolet light. ~`~
If the solution is not cast, but ins~ead a hot ~5 melt coating is desired, the solvent i5 stripped~ The ;~
s~lvent is s~ripped by applying the mixture onto a silic~nized release liner which is then oven dried.

,~`,,"' WO~ t~l~ 2 I269~3 1 ~cr/l)ss2/l 1156 -- lq -- ~
Thus, an adhesive product is left in solid form. The adhesive is then heated from about 145C to about 170C
to reduce melt viscosity, coated onto a suitable substrate and then cured by exposure to an ultraviolet 5 light source.
Another method of applying the adhesive of the prasent inven~ion comprises W prepol~mPrization of the monomer mixture to a syrup-like consistency, followed by the addition of a crosslinker. Next, this syrup-10 like mixtur~ is knife-coated onto a substrate and W
polymerized in an inert atmosphere to yield the finished adhesive coating.
The aclhesive of the present invention can also be polymerizecl by charging the monomers with solvent, 15 initiator and a silane crossli.nker into a three neck reaction vessel equipped with a stirrer, a thermometer, a thermowat~h and condenser. Aft~r the monomer is charged into the reaction vessel, the whole reaction vessel with~its contents is purged with nitrogen and 20 heated to 55C~ After about 20 hours, a 97 to 98% ~--conversion should be obtained. The mat~rial is then -~
coated on a subskrate and dried in an oven. Typically, a catalyst can be added to the solution prior to coating and oven drying. In the presence ~f moisture, -~
25 a crosslinked adhesive is obtained and no further processing is r~quired.
The removable PSA of the present invention, depending upon its viscosity, can be coa~ed via any o~
a Yariety of conventional coating~methods, such as roll 30 coating, knife coating; hot melt coating, or extrusion.
The composition can be applied to at least a portion of at least one surface of suitable flexible or inflexible backing or sheet and cured to produce PSA-coated sheet materials. Fo~ example, it can be placed on a backing 3~ to form a tape. Or it can be applied to a sheet to form pressure sen5it~ive sheet5. For some purposes; it ~;`
is preferred that the adhesive be coated on both sides :" -.~
":

W~ X 2 12 6 9 ~ r/uss~

of a sheet. Those skilled in the art will recognize that the novel adhesive can be applied to a variety of sheets and backings and can be simultaneously coated on both sides of the sheet or backing. Useful flexible 5 backing materials include paper, plastic films such as poly(propylene), poly(ethylene), poly(vinyl chloride), poly(tetrafluoroethylene), polyester [e.g., poly(ethylene) terephthalate)~, polyimide film such as DuPont's Kapton~, cellulose acetate and ethy~
10 cellulose. Backings can also be of woven fabric formed of threads of synthetic or natural materials such as cotton, nyLon, rayon, glass, or ceramic material, or they can be of nonwoverl fabric such as air-laid webs of natural or synthetic fibers or blends of these~ In 15 addition, suitable backings can be formed o~ metal, metallized polymeric film or ceramic sheet material.
PSA-coated sheet materials carl take the form of any article conventionally known to be utilized with PSA
compositions, such as labels, tapes, transfer tapes (comprising a film of the PSA borne on at least one release iiner), signs, covers, marking indices, and the like. Primers can be utilized, but they are not always necessary. -~
Test Methods 25 Peel Adhesion Peel adhesion is the force required to remove a coated flexible sheet~ma~erial from a test substrate ~-~
measured at a specific angle and rate of removal. In the examples, this force i5 expressed in Newtons per 30 decimeter (dm) width of coated sheet~ The procedure -~
followed is:
A ~trip 0.127 dm~in width of the sheet coated with ;~
the adhesive to be tested is applied to the hori~ontal surface of ~aper test substrate with at least 1.27 ~5 li~eal dm in firm contact. Three passes in each ~-.
direction with a 2 kg hard rubber roller are used to ~;
apply the strip.~ If~air bubbles are entrapped between . .
.~

W~ 3~ 2 ~ 2 ~ g ~31 P~r/us~2/~ l ls~

the test substrate and the test strip, the sample is discarded. The free end of the coated strip is doubled back n~arly touching itself so the angle of removal will be 180. The free end is attached to an adhesion 5 tester load cell. The paper test substrate is adhered to the plate of an I-mass 3M-90 Peel Tester which is capable of moving the substrate away from the load cell at a constant rate of 0.3 meters per minute. The dwell time after roll down is specified in the tables belowO
l0 The scale reading in Newtons is recorded as the tape is peeled from the glass surface.
Abbreviations and Tradenames AA - acrylic acid ABP - 4-acryl--oxy-benzophenone 15 ACM - acrylamide ACN - acrylonitrile BAm - bis-aziridine amide or 1,l' - (l,3 -ph~n~lenedicarbonyl) bis [2 - met:hyl aziridine]
EtOAc - ethyl acetate 20 HEA - 2-hydroxy-ethylacrylate ;
HMDI - bis (4'isocyanato hexyl~ methane HPM~ - 3-hydroxy propylmethacrylate ~
IOA - isooctyl acryla~e --ITAA - itaconic acid 25 ODA - octadecyl acrylate ODMA - octadecyl methacrylate Tyz~r AA ~ titanium acetyl acetonate, available from the E.I. du Pont de Nemours Company VA~O~ 64 - 2,21-azobis(isobutyronitrile), available 30 from the E.I. du Pont de Nemours Company.
The percentages in this specification and appended claims are all percentages by weight unless o~herwise noted.
..

", W~ 3l4# 2 ~ ~ ~j 3 8 1 PCT/US92/11]56 In a 500 ml ~our-necked reaction vessel, equipped with a stirrer, thermometer, condenser, addition funnel and therm~watch, 68 grams of IOA, 62 grams of ODA (@
5 48% solids, in EtOAc), 2 grams acrylic acid, 107.5 grams of ethyl acetate, and 0.2 grams of ABP were charged. A solution of 0.3 grams VAZO~ 64 in lO grams of ethyl acetate was added to the addition funnel.
Both the solution in the reaction vessel and the lO materials in the addition funnel were then purged with argon (or r,itrogen). The solution in the reaction vessel wac; stirred while heated to 55~C and initiator was added. After about 20 hours, a 98-99% conversion was obtained. The mixture was then coated from 15 solution on twenty pound boncl paper from the Georgia-Pacific Company and oven dried for lO minutes at 65C.
An oven dried coating thickness of about l.O mil of the adhesive was obtained. T~e coating was passed three times under W light ~PPG W processor equipped with -~
20 medium pressure mercury lamps at 300 WattsJ~.5 cm) at 25 meters/min.
The coated paper was then applied to a sheet of -~-copy paper as described in the peel adhesion test. ---Peel adhesion was then measured for samples from the -~
25 test substrate after dwell of one day and~one week.
The composition, the weight percentage of the components and the test results are reported in Table ~;~

EXAMPLES 2-4 AND COMPARATI~E EXAMPLES_C-l ~ND C 2 Examples 2-4 and ~omparative~Examples C-liand~C-2 were prepared;in accordance with the procedure outIined ;;~
in ~xample l. The composition, the weight percentage of the component~ and the test~results are reported in Table 1.

.,:
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W~39~/l3l4R P~l'/US~2/lll56 2 i 2 ~ 18 -TABLE l Peel Adhesion (N/dm) _ , , Ex. C~mposition Wt. ~ (N/dm) 1 Day 1 Week _ 1IOA/ODA/AA/ 68/30/ 30.2 26.1 ABP 2/0.2 . _ .. .. I
2 IOA/ODA/AA/ 66/30/ 49.9 52.8 l ABP 4/0.2 _ ¦
C-l IOA/ODA/AA/ 64/30/ TEAR TE~R
ABP 6/0.2 I _ _ _ l . .
3 IOA/ODA/ 68/30/ 26.9 21.2 ITAAJABP 2/0.2 ¦¦
_~ l 4 IOA/ODA/ 66/30f 42.9 25.0 l ....
ITAA/ABP 4/0.2 l ~ 11 C-2 IOA/ODA/ 64/30/ 51.4 TEAR
ITAAIABP 6/0.2 -- _ _ _ . _ =! _ _ _ _ _ =-- --The test results indicate that good peel adhesion was obtained with the composition of the present invention.

1S EXAMPLES 5-10 :
Examples 5 10 examined the use of the multifunctional aziridine amide ~1,1'-(1,3 phenylenedicar~onyl bis [2-methyl aziridine] as a -~
crosslinkex. These examples were prepared in .~
20 accordance with the procedure outlined in Example 1 ~.
with ~he exception that the multifunctional aziridine amide was utilized instead of the ABPr ~he multifunctional aziridine amide was added just prior to :-coating. The composition, the weight percentage of the ~.
25 components and the test resolts are reported in Table , . ,.

, .
.
~ ., W~9~ 2 1 2 6 ~ 8 I Pcrr/~sg2/l 1 1S~

Peel Adhesion (N/dm) . ~= _ -Ex. Composition Wt. % Pa~ dm) c . 1 Day __ 1 Week . -IOA/ODA/AA/68/30/ 53.0 77.1 BAm 2/0.05 6 IOA/ODA/AA/68/30/ 51.2 65.5 BAm 2/0.1 l _ .
7 IOA/ODA/AA/68/30/ 27.8 54.3 i :
BAm 2/0.15 j ~
8 IOA/ODA/AA/68/30/ 11.5 18.6 ~ .
BAm 2/0.5 l I __ _ __ :
9 IOA/ODA/AA/68/30/ 5.17 9.79 ¦ _ BAm _ _/0.75 IOA/ODA/AA/68/30! 5.41 8.45 l BAm 2/1.O I
__ _ The test results indicate that good peel adhesion was :
obtained with the composition of the present invention.

Examples 11-22 examined the use of the moderately :~
15 polar monomers, 2-hydroxy-ethyl acrylate and 3-hydroxy-propyl methacrylate. These examples were prepared in accordance with the procedure outlined ln Example 1 except tha-t a total of 200 g of ethyl acetate was utilized. The composition, the weight percentage of ~-20 the components and the test results are reported in Table 3. .;

: -;
'', ~"`', W~ l4R PCr/US92/l1156 212~

Peel Adhes~on (N/dm) _ _ _ _ .
Ex Composition Wt. % Paper I ..
. _ .
1 Day 1 Week ¦ ~
. i .
11 IOA/ODA/HEA 64/30t 10.3 12.2 /ABP 6/0.2 _ 12 IOA/ODA/HEA 62/30/ 7.57 11.5 /ABP 8/0.2 _ 13 IOA~ODA~HEA 60~30/ 14.8 17.3 ¦ :~
/ABP 10~0.2 . I
_ . . I
14 IOA/ODA/HEA 58/30t 27.4 23.2 l /ABP 12/0.2 ¦ ~;
IOA/ODA/HEA 55/30/ 29.6 29.6 ¦
_ /ABP 15/0.2 I
16 IOA/ODA/HEA 40/3¢/ 5-9 5.0 I ..
. . /ABP 30/0.2 I .
17 IOA/ODA/ 64/30/ 3.83 3.79 HPMA/ABP 6/0.2 _. _ _ _ I
18 IOA/ODA/ 62/30/ 6.97 7.99 I .:
. ~ HPMA/ABP 8/0.2 I ~
I ._ . _ _ .
19 IOA/ODA/ 60/30/ 4.22 3.77 ~PMA~ABP 10~0~2 I ~ :
IOAfODA/ 58/30/ 14.4 9.6 HPMA/ABP 1210.2 I __.
21 IOA/O~A/ 55/30/ 150 8 19.3 _ HPMAfABP _ 15/0.2 _ :~
_ _ -~ ...
22 I~A/OD / 50/30/ 20.8 21.0 ~.
H~MA/ABP : 20/0.2 ~.
~ _ ~; _ _ .~
C-3 IOA/ODA/ 30/30/ TEAR _ `~
HE~/ABP~ 40/0.2 .;.
1, ~ - ~ - __ _ , ___ ~ , ~,~.. ' The test ~esults.indicate~that good peel adheslon was ob~ained with the composition of the presen~
20 invention. Comparativ:e~C-3 illus~rates the need to limit the amount of polar mon~mer in the present invention. ~ ~
EXAMPLES:23-34 ~: -s Examples 23-34 axamined~the use of~ariQus polar 25 monomers. These examples were prepared ln a~cordance ~' . ..
~;~. .-.-, :
:

. .
: . . .

w~ 2 :~ 2 ~ PCT/US92/11156 with,the procedure outlined in Example l. The composition, the weight percentage of the components and the test results are reported in Table 4.

Peel Adhesion (N/dm) _-_ ~
Ex.Composition Wt. % Paper ¦
l Day l Week _ :, 23 IOA/ODA/ACN 68/30/ 6.76 l0.7 /A~P 2/0.4 _ _ 24 IOA/ODA3ACN 66/30/ 2.17 2.4l /ABP 4/0.4 _ . _ IOA/ODA/ACN 64/30/ 0.63 .31 /ABP 6/0.4 _ _ l0 26 IOA/ODA/ACM 68/30/ 8.69 ll.2 -- -- IABP 2/0.4 _ 27 IOA/ODAIACM 66/30/ 17.2 26.5 A~P 4~0.4 . _ _ _ . _ I
28 IOA/ODA/AC~ 64/30/ 49.7 32.0 /ABP 6/0.4 , _ ___ 29 IOA/ODA~HEA 6~/30/ 5.08 5~9l /ABP 2/0.4 _ . .. _ _ . , IOA/ODA/HEA 66/30/ 5.63 6.35 :-/ABP 4/0.4 :2:
__ . ~: .,.
15 31 IOA/ODA/HEA 64/30/ l0.3 12.2 ~:
~ABP 6/0,4 _ _ .. _ _ ., ,:
32 IOA~ODA/ 68/30/ 8.03 9.63 ..
HP~A/ABP 2/0.4 . . ~ : .
33 IOA/ODA/ 66/30/ 8.08 l0~3 HPMA~ABP 4/0.4 I _ , ~ . _....... ~ . __ , 34 IOA/ODA/ 66/30/ 4.B~ 4.60 HPMAlABP 610.4 : .
l. _= ~ _ . . , .. -20The test results re~eal that good peel adhesion :~
was obtained with the composition of the present : invention.
: EX~MPLE$ 35-3g Examples 35~39 examined the use of the cros51inker ~' 25 Tyzor ~A~ These examples were prepared in accordance with the pro~dure outlined in Example l except that no W~ 'CI'/U~92/l~l56 2~38 1 ABP was utilized and that the Tyzor AA was added just prior to coating. The composition, the weight percentage of the co~ponents and the test r~sults are reported in Tabl~ 5.

Peel Adhesion (N/dm) _ -- l Ex. Composition Wt. ~ PaE ler 1 Day 1 Week __ IOA/ODA~AA/ 6B/30/ 21.7 24.3 Tyzor AA 2/0.1 _ _ 36 IOA/ODA~AA/ 68/30/ 37.2 25.2 Tyzor AA 2/0.25 I _ . I
37 IOA/ODA/AA/ 68/30/ 16.2 13.6 Tyzor AA ~l0.5 I I
38 IOA/ODA/AA/ 68/30/ 6.79 10.7 Tyzor AA 2/0.75 .
. _ _ _ 39 IOA/ODA/AA/ 68/30/ 2.87 4.82 Tyzor ~A 2/1.0 _ = _ The test results reveal t:hat good peel adhesion 15 was obtained with the comp~sit:ion of the pr.~sent invention. -~
EXAMPLES_40 42 Examples 40-42 examined the use of an isocyanate crosslinker~ These examples were prepared in 20 accordance with the procedure outlined in Example 1.
The composition, the weight percentage of the components are reported in Table 6.

Peel Adhesion (N/dmj ~-~ _ _ _ 25 Ex. Composition Wt. % Paper I ~.
. _ _~ .
1 Day 1 Week ¦ :
~ ~ -- ~1 4 0IOAlHEA/ODA/~MDI_ 65/5/3~/0-Q5 : 53 9 67.9 _ ;.
41~ / G~: / I~DI ~ 5 / 5 / 3_ 2 3 . 9 4 0 9 42 `IOAjHEA/ODA~HMDI 65/5/30J 0.25 :4.4 6.3 ~ ~ ~ -~ - ----~==' '',~.

~,:

w~ 2~6~S l pcr/vs~2/llls6 - ~3 -The test results reveal that good peel adhesion was obtained with the composition of the present invention.
In summary, novel adhesive compositions having a 5 low melt viiscosity have been described. Althou~h specific embodiments and examples of the present invention have been described herein, it should be borne in mind that these are by way of explanation and illustration and the present invention is not limited 10 thereby. Certainly modificationis which are within the ordinary skill in the art are considered.~o lie within :
the scope sf this invention as defined by the following claims including all equivalents.

.',.",.~`
'.' .'~
.:

' .':
', ' ' - ' .

Claims

We Claim:

1. An acrylic pressure-sensitive adhesive comprising:
a) from about 10 to about 50 percent by weight of a higher alkyl acrylate having an alkyl group from 12 to 26 carbon atoms;
b) from about 50 to about 90 percent by weight of a lower alkyl acrylate having an alkyl group from 4 to 12 atoms wherein said alkyl groups of said lower and higher alkyl acrylates do not simultaneously have 12 carbon atoms;
c) at least one polar monomer copolymerizable with said higher and lower alkyl acrylate; and d) sufficient crosslinker to impart enough cohesive strength to the adhesive in order to prevent substantial adhesive transfer wherein said pressure-sensitive adhesive is substantially tacky at room temperature.

2. The pressure-sensitive adhesive of Claim 1 wherein said polar monomer is strongly polar.

3. The pressure-sensitive adhesive of Claim 2 wherein said polar monomer is selected from the group consisting of a carboxylic acid, a cyanoalkyl acrylate, an acrylamide, an acrylonitrile and mixtures thereof.

4. The pressure-sensitive adhesive of Claim 1 wherein said polar monomer is moderately polar.

5. The pressure sensitive adhesive of Claim 4 wherein said polar monomer is selected from the group consisting of a hydroxy alkyl acrylate, a vinyl halide, a vinyl toluene, a styrene, a hydroxylalkyl methacrylate, a N-vinyl lactam and mixtures thereof.

6. The adhesive of Claim 1 wherein said alkyl group of said higher alkyl acrylate has from 13 to 26 carbon atoms.

7. The adhesive of Claim 1 wherein said crosslinker is selected from the group consisting of a multifunctional aziridine amide, a multifunctional acrylate, a metal ion, a multifunctional methacrylate, a chromophore-substituted halomethyl-s-triazine, a benzophenone, an acetophenone, a silane,a multifunctional isocyanate, and a mono-ethylenically unsaturated aromatic ketone.

8. An adhesive coated sheet material having a bearing on at least one major surface thereof an adhesive composition comprising:
a) from about 10 to about 50 percent by weight of a higher alkyl acrylate having an alkyl group from 12 to 26 carbon atoms;
b) from about 50 to about 90 percent by weight of a lower alkyl acrylate having an alkyl group from 4 to 12 atoms wherein said alkyl groups of said lower and higher alkyl acrylates do not simultaneously have 12 carbon atoms;
c) at least one polar monomer copolymerizable with said higher and lower alkyl acrylate; and d) sufficient crosslinker to impart enough cohesive strength to the adhesive in order to prevent substantial adhesive transfer.

9. The pressure-sensitive adhesive of Claim 8 wherein said polar monomer is strongly polar.

10. The pressure-sensitive adhesive of Claim 9 wherein said polar monomer is selected from the group consisting of a carboxylic acid, a cyanoalkyl acrylate, an acrylamide, an acrylonitrile and mixtures thereof.

11. The pressure-sensitive adhesive of Claim 10 wherein said polar monomer is moderately polar.

12. The pressure sensitive adhesive of Claim 11 wherein said polar monomer is selected from the group consisting of a hydroxy alkyl acrylate, a vinyl halide, a vinyl toluene, a styrene, a hydroxylalkyl methacrylate, a N-vinyl lactam and mixtures thereof.

13. The adhesive of Claim 8 wherein said alkyl group of said higher alkyl acrylate has from 13 to 26 carbon atoms.

14. The adhesive of Claim 8 wherein said crosslinker is selected from the group consisting of an multifunctional multifunctional aziridine amide, a multifunctional acrylate, a metal ion, a multifunctional methacrylate, a chromophore-substituted halomethyl-s-triazine, a benzophenone, an acetophenone, a silane, a multifunctional isocyanate and a mono-ethylenically unsaturated aromatic ketone.