WO1993009152A1

WO1993009152A1 - Uv-cured hot melt pressure-sensitive adhesives

Info

Publication number: WO1993009152A1
Application number: PCT/US1992/009493
Authority: WO
Inventors: Tjoan Liem Ang
Original assignee: Avery Dennison Corporation
Priority date: 1991-11-05
Filing date: 1992-11-04
Publication date: 1993-05-13
Also published as: CA2122921A1; AU3126993A

Abstract

The present invention relates to a pressure-sensitive-adhesive copolymer comprising copolymerizable photoinitiator monomers such as benzoin compounds, acyloin compounds, and mixtures thereof; low-glass-transition-temperature monomers; and multifunctional monomers. The invention also provides a process for the preparation of such copolymers. The actinic radiation cure of the polymer is performed with light at about 240 to about 410 nm wavelength, a light intensity of about 6 to about 10 kWatts/m2, and at a temperature of about 45 °C to about 125 °C, under aerobic conditions. The resultant pressure-sensitive adhesives exhibit cohesive strength and high-temperature shear strength which, in a static shear test, does not fail after several thousand minutes; a tack performance of about 300 to about 500 N/m; a peel adhesion performance of about 550 to about 650 N/m; a shelf-life at room temperature of more than about 10 months, with little or substantially no increase in the viscosity of the polymer; and a hot melt pot-life at 100 °C of at least 10 days, with little increase in the viscosity of the polymer.

Description

UV-CURED HOT MELT PRESSURE-SENSITIVE ADHESIVES

Field of the Invention

This invention is related to photo-curable, pressure-sensitive adhesives.

Background of the Invention

High-performance pressure-sensitive adhesives (PSAs) are normally characterized by the ability of the adhesive to withstand creep or shear deformation at high loadings and/or high temperatures while exhibiting adequate tack and peel adhesion proper¬ ties.

This balance of properties is derived from the polymer structure of the PSA. Such polymers are characterized by a high-molecular weight to provide the necessary resistance to shear deformation and by a low modulus of elasticity of the polymer backbone to allow the polymer to conform to a substrate surface upon contact. Certain surface energy and polarity requirements must also be satisfied for a good bond to form between the adhesive and the substrate.

The desired high molecular weights can be obtained either by primary polymerization of monomers to form long-chain-length backbones, or by crosslink- ing of pre-existing primary polymer chains. The PSA polymer backbone should consist of material of low enough modulus or high enough compli¬ ance to make a good contact with the adherent at the application temperature. This condition is normally satisfied by the use of material of low glass-transi¬ tion temperatures (Tg's) , such as polymers or copoly- mers of 2-ethyl-hexylacrylate, iso-octylacrylate, butylacrylate, ethylacrylate, methylacrylate, or the like. Frequently, small amounts of other types of monomers are incorporated to modify the polarity, the rheological and surface properties of the polymer, such as acrylic acid, maleic acid, /3-carboxyethylacr- ylate, esters of ethacrylic acid, laurylacrylate, stearylacrylate, styrene, dibutylfumarate, dioctylmaleate, dioctylfumarate, vinylacetate, or the like.

Resistance to creep or shear deformation in a high-performance PSA is normally the result of a high internal cohesive strength. This condition can usually be obtained from a high primary polymer molecular weight. However, if the internal cohesive strength imparted by the polymer is insufficient, high internal cohesive strength can also be obtained by incorporating, into the polymer body, a high degree of interchain hydrogen bonding, ionic associa¬ tion, or covalent crosslinking.

With hot melt PSAs, crosslinking the adhesive by radiation after coating is one of the preferred methods of incorporating high cohesive strength. The conversion of a fluid which flows and coats out well, into a non-flowing viscoelastic material can be done with better control by radiation crosslinking than by thermochemical means. In order for a polymer to be crosslinkable by UV radiation, the polymer needs to carry vinyl unsaturation which, in the presence of small amounts of added multi-vinyl-functional monomers and a photoinitiator, would respond to vinyl addition polymerization upon UV radiation. The combination of vinyl unsaturation on the polymer and multi-vinyl- functional monomers leads to very poor melt pot lives; polymer gelation in only an hour at the melt temperature is not uncommon with hot melt PSA formu¬ lations.

Crosslinking by UV radiation of a polymer without vinyl unsaturation is also possible when a peroxy compound is present in addition to the multi- vinyl-functional monomer and photoinitiator. Peroxy compounds which have been used are benzoylperoxide and t-butylperbenzoate. However, once again, the pot lives of the unfunctionalized hot melt polymer in the presence of the peroxy compound and multi-vinyl- functional monomers are very short, i.e., of the order of an hour at the melt temperature.

U.S. Patent No. 4,181,752 discloses the manufac- ture of PSAs in web polymerization of a polymer- monomer syrup by UV irradiation. To form polymer molecules of sufficiently high molecular weights, the UV energy of 300 and 400 nm wavelength is supplied at low intensities, 0.001 to 0.070 k atts/m² while maintaining the web temperature below 35°C. In an on-web-syrup polymerization, the low UV intensity requirement necessitates that cure takes place at relatively low web speeds of about 2 inches per minute to 2 feet per minute. However, this irradia- tion is preferably carried out under anaerobic conditions thus requiring an atmosphere control syste .

U.S. Patent No. 4,052,527, discloses a method of production of UV crosslinkable hot melt pressure- sensitive adhesives which contain photoinitiators of the 3-(chlorinated aryloxy)-2-hydroxypropyl acrylates or methacrylates. The photoinitiators were copoly- merized into acrylic polymers and which contained other modifying monomers such as vinylics, and were made in solution, emulsion, or in bulk. When not made in bulk, the solvent medium is vacuum-stripped so that the polymer can be coated as a melt between 120°C and 180°C. UV cure is obtained from commercial medium-pressure lamps of 200 Watts per inch intensi¬ ty. However, multiple passes under the UV lamp are required to obtain moderate increases in shear strengths, e.g., improvement of room-temperature shears from 4 minutes before UV cure to only 0.5 hour shear after passing 15 times under the UV lamp, 1.5 hour shear after 25 passes, and 3.5 hours shear after 35 passes under the UV lamp. U.S. Patent No. 4,144,157 and DE 27-43-979 discloses the UV crosslinkability of acrylic pres¬ sure-sensitive adhesives which were exemplified by polymers which are made in and coated out of solu¬ tion, or when made in bulk are to be coated out of solution. The pressure-sensitive-adhesive materials are UV-cross- linkable to high shear strengths, because of copolymerizable photoinitiators in the pressure-sensitive adhesive polymer which belong to a class of (2-alkoxy,2-phenyl-2-benzoyl)ethyl 2- acrylates or methacrylates and the acrylate or methacrylate of benzoin or its derivative. The UV cure is obtained by radiation from high-pressure mercury lamps or burners, with very-high-intensity outputs of from one lamp of 900 Watts to four lamps of 2000 Watts each. Despite the use of high-intensi¬ ty, high-pressure mercury lamps, the pressure-sensi¬ tive-adhesive material needs relatively high UV dosages of at least about 10 kJ/m² before high shear strengths can be obtained in the adhesive. The admixture of reactive crosslinking agents such as multifunctional acrylic monomers to the pressure- sensitive-adhesive material prior to coating and UV cure is explicitly discouraged to prevent poor shelf stability of the coater-ready material.

U.S. Patent No. 4,281,152 describes the synthe¬ sis of benzophenone derivatives which are copolymeri- zable into a polymer backbone by way of acrylic functional groups. The benzophenone derivatives are useful as a component in UV-curable resin composi¬ tions and in anaerobic thermosetting adhesive compo¬ sitions. U.S. Patent No. 4,672,079 describes a method of manufacturing polystyrene derivatives with UV- activatable functional groups, and the use of these compounds in the photopolymerization of monomer or oligomer mixtures with ethylenic unsaturation for use in wood .paints, paper lacquers, printing inks, engraving printed and integrated circuits, fabric printing, and the like, but not for pressure-sensi¬ tive adhesives.

European Patent Application No. 87-304396.2 and U.S. Patent No. 4,737,559 describe the synthesis and the use of benzophenone and acetophenone photo¬ initiator derivatives, which are coplymerizable into a copolymer backbone by acrylic or acrylamide func¬ tional groups for use in adhesive compositions for human skin contact application. A pressure-sensitive adhesive is described which is a copolymer of acrylic monomers and the ketone derivative, by means of which the copolymer can be UV-crosslinked. The amount of crosslinking ketone comono er and the degree of polymerization of the copolymer are controlled, to obtain the right creep compliance value for an enhanced level of initial adhesion when applied to the skin, but resist objectionable adhesion build-up over time. DE 35-12-179 and DE 35-34-645 describe the conversion of phenyl(2-hydroxy-2-propyl)ketone (DAROCUR 1173) , and similar acyloin compounds into copolymerizable acrylated or methacrylated deriva¬ tives. The use of these copolymerizable photoinitiators in coatings and ink compositions give products, upon UV curing, which are free of odors and other low molecular weight photoinitiation material. Baeumer et al. from E. Merck, in a marketing publication, described the usefulness of the above functionalized photoinitiators in UV-cured hard coating compositions which do not give off solvent extractables or low-molecular-weight photochemical products, which ooze to the surface of the cured coatings. No application of these photoinitiators is claimed in the UV cure of pressure-sensitive adhe¬ sives. J. P. Guarino and J. P. Ravijst, Journal of Radiation Curing. July 1988, disclose the use of copolymerizable photoinitiators in coating composi¬ tions which emit very low levels of odor upon UV curing. No application is mentioned for UV-curable pressure-sensitive adhesives.

Therefore, there exists a need for a PSA which is UV-curable and which has a long shelf-life and a long hot melt pot life, even in the presence of multifunctional monomers, but which retains cohesive strength, high-temperature shear strength, and tack and peel adhesion performance. It is also desirable that the UV curing be performed using conventional lamps and in an atmosphere of air.

Summary of the Invention

The present invention relates to a pressure- sensitive-adhesive copolymer comprising at least one copolymerizable photoinitiator monomer, at least one low-glass-transition-temperature monomer, and at least one multifunctional monomer and to a process for preparing such polymers.

The actinic radiation cure of the polymer is performed with light at about 240 to about 410 nm wavelength, light intensity of about 6 to about 10 kWatts/m², and temperature of about 45°C to about 125°C, under aerobic conditions. The resultant pressure-sensitive adhesives exhibit cohesive strength and high-temperature shear strength which, when put to a static shear test, do not fail after several thousand minutes; tack performance of about 300 to about 500 N/m; peel adhesion performance of about 550 to about 650 N/m; a shelf-life at room temperature of more than about 10 months, with no increase in the viscosity of the polymer; and a hot- melt pot-life at 100°C of at least 10 days, with little or substantially no increase in the viscosity of the polymer.

The coplymerizable photoinitiator monomers for use in the present invention are selected from the group consisting of benzoin compounds, acyloin compounds, and mixtures thereof, and, in a preferred embodiment of the invention, are selected from the group consisting of DAROCUR ZLI 3331 and DAROCUR 2959.

The multifunctional monomers for use in the present invention are selected from the group con¬ sisting of diethy leneglyco ldiacry late , triethyleneglycoldiacrylate , tr ipropyleneglycol- diacrylate, 1 , 6 -hexaned i o Id i a cry 1 a t e , trimethylolpropanetriacrylate , tr imethylolpropanetr i- methacrylate, pentaerythritoltriacrylate, and mix¬ tures thereof.

The low-glass-transition-temperature monomers for use in the present invention are selected from the group consisting of 2-ethylhexylacrylate, iso- octylacrylate, butylacrylate, and mixtures thereof.

Other compounds such as acrylic acid, maleic acid, /3-carboxyethylacrylate, esters of methacrylic acid, laurylacrylate, stearylacrylate, styrene, dibutyl- fumarate, dioctylmaleate, dioctylfumarate, and vinylacetate are added to the polymer to modify the properties of the polymer.

Brief Description of the Drawings

These and other features and advantages of the present invention will become better understood with reference to the following description, appended claims, and accompanying drawings, wherein:

FIG. 1 is a graph of looptrack vs UV dosage for polymer-2.

FIG. 2 is a graph of peel adhesion vs. UV dosage for polymer-2. FIG. 3 is a graph of melt viscosity at 100°C vs heating period at 100°C for polymer-4.

Detailed Description

The present invention relates to high-perfor¬ mance, UV-cured, essentially acrylic-based, pressure- sensitive adhesives which can be in the form of hot melts, solution adhesive compositions, or emulsion adhesives, and to methods for their manufacture.

The adhesive polymer compositions comprise low Tgmonomers, such as 2-ethylhexylacrylate, iso-octyl- acrylate, butylacrylate, or the like. Preferably, the low Tg monomer is present in an amount of from about 40% to about 60% by weight.

Other types of monomers are incorporated to modify the polarity and the rheological and surface properties of the polymer, such as acrylic acid, methacrylic acid, maleic acid, jS-carboxyethyl- acrylate, esters of methacrylic acid, laurylacrylate, stearylacrylate, styrene, vinyltoluene, dibutylfumarate, dioctylmaleate, dioctylfumarate, vinylacetate, or the like. These monomers are present in an amount of from about 10% to about 40% by weight.

The adhesive polymer contains a copolymerizable photoinitiator sold under the trade name "DAROCUR ZLI 3331" by E. Merck of Darmstadt, Germany, and which is 4 (2-acryloyloxyethoxy) phenyl- (2-hydroxy-2- « propyl)ketone. The acryloyl group makes the molecule copolymerizable with other acrylic and vinylic monomers which make up the body of the adhesive polymer. The photoinitiator activity of DAROCUR ZLI 3331 resides in the phenyl(2-hydroxy-2-propyl) etone moiety of the molecule. A compound with this latter structure is sold under the trade name "DAROCUR 1173" by E. Merck. An additional, related photoinitiator sold under the trade name "DAROCUR 2959" is also sold by E. Merck. This compound is 4(2- hydroxyethoxy) henyl-(2-hydroxy-2-propyl)ketone. DAROCUR 2959 can also be made copolymerizable through the use of methacrylate ester and maleate halfester compounds. The maleate halfester can be formed .in situ during the copolymerization of the acrylic and vinylic monomers and maleic anhydride in the presence of DAROCUR 2959. The halfester forma¬ tion can be "pushed" to completion by the addition of a tertiary amine such as N,N,-dimethylbenzylamine at the termination of the polymerization reaction. The copolymerizable photoinitiator monomer is present in the range of from 0.1% to 10% by weight, and preferably from 0.5% to 5% by weight, in the adhesive copolymer composition.

Another monomeric component which may be added to the polymer composition is one or more vinyl esters present in a total amount of 0 up to about 20% by weight based on the total weight of the monomers. Vinyl esters contain from 2 to about 16 carbon atoms in the alkyl group of the acid. Representative of the vinyl esters are vinyl acetate, vinyl butyrate, vinyl propionate, vinyl isobutyrate, vinyl valerate, vinyl versitate, and the like. Vinyl acetate is preferred.

The monomer proportions are adjusted in such a way that the adhesive has a glass-transition tempera¬ ture of about -30°C or less and at least about 50°C below the use temperature, giving a good balance of adhesion and tack at room temperature and low temper¬ atures. The present invention is especially suitable for making hot melt PSAs, which have low melt viscosi¬ ties, allowing them to be easily coated out on a web as a thin film, even by a printing process. Usually, low melt viscosities are obtained by dilution of the PSA polymer with relatively large quantities of reactive diluents. However, in the present inven- tion, low melt viscosities are achieved by relatively low initial polymer molecular weight.

The PSA polymers of the present invention are compounded with small amounts of multifunctional acrylates or methacrylates, such as diethyleneglycol- diacrylate (DEGDA) , triethyleneglycoldiacrylate (TEGDA) , tripropyleneglycoldiacrylate (TPGDA) , 1,6- hexanedioldiacrylate(HDODA) ,trimethylolpropanetri- acrylate (TMPTA) , trimethylolpropanetrimethacrylate (TMPTMA) , pentaerythritoltriacrylate (PETA) , or the like. The multifunctional acrylates or methacrylates are present in the range of from 0.1% to 10% by weight, preferably from 1% to 5% by weight.

After coating the compounded polymer as a thin film, it can be UV-cured very rapidly with small UV dosages of as low as 3 kJ/m², to produce a PSA of very high cohesive strength and high-temperature shear strength, which retains a good balance of tack and peel performance. The UV radiation in this invention is preferably by conventional, medium-pressure mercury lamps, with emission bands ranging from 240 nm to 410 nm, light intensities from 6 to 10 kWatts/m², and temperatures from 45°C to 125°C at the web surface. Unlike other UV cures, the UV cures of the present invention are performed in an aerobic atmosphere.

Also unlike other PSAs, which contain multifunctional acrylic monomers, the PSAs of the present invention respond readily to UV crosslinking, and the coater-ready hot melt PSA composition has an excellent shelf-life as well as an excellent melt pot-life. Storage of the polymer at room temperature of more than about 10 months results in no increase in the viscosity of the polymer. Similarly, exposing the polymer to 100°C for 10 days results in only a slight increase in the viscosity of the polymer, from its initial viscosity of 16 Pascal seconds to only 28 Pascal seconds after 10 days at 100°C.

The tack and peel adhesion performance of the PSAs of the present invention are relatively insensitive to wide variations in the UV irradiation dosage needed for cure, to variations in the polymers' initial viscosity, and to levels of added multifunctional acrylic monomers. The decrease in tack and peel adhesion upon accelerated aging is also smaller than in conventional UV-cured PSAs.

Polymer Synthesis

In a 1000-5000 ml jacketed resin kettle, which is equipped with a strong mechanical stirrer, thermocouple, water-cooled condenser, and nitrogen blanketing, is polymerized a mixture of acrylic and other vinylic monomers; copolymerizable photoinitiator; polymerization initiator, which can be a peroxide or other polymerization initiator such as those sold under the trade name "VAZO" by DuPont of Wilmington, Delaware; and chain transfer modulators.

About 10% of the mixture is placed in the reactor, and the polymerization is allowed to initiate. Heating of the reacting mix is provided through the jacket from a circulating oil bath. After the reaction of the initial mixture has subsided, the remainder of the mix is added to the reactor, via a metering pump, over a period of 1 to 3 hours, while maintaining the reaction temperature in the range of 80°C to 110°C. The reaction temperature varies depending on the halflife temperature of the initiators used.

The polymerization product is sampled for percent conversion and for melt viscosity measurements. The polymerization product is immediately compounded with the multifunctional acrylic monomer, inside the reactor, before the product is discharged as coater-ready material. Adhesive Coating and UV Cure The compounded polymer is direct-coated on about 50 μm polyester film at a coating weight ranging from about 30 to about 40 g/m², and is cured by UV radiation from two conventional medium-pressure mercury lamps which have spectral emissions from 240 nm to 410 nm, and light intensities from 6 to 10 kWatts/m². The coated polyester film is carried on a moving web under the UV radiation source, and the web temperature is from about 45°C to about 125°C. The UV dosage which the coated adhesive film receives is controlled by the UV lamp-intensity setting and web speed. All UV cure is performed in an aerobic atmosphere, and no special precautions to exclude air are required.

After the adhesive film is cured, it is laminated with a sheet of release liner, for protection, until it is ready for application. When solution adhesives or emulsion adhesives are to be coated and cured, the major portion of the low volatiles are removed from the coated material, before UV cure. The volatiles are removed by placing the coated material in a forced-air oven at about 70°C for about 5 minutes before cure, and, to remove the last traces of low volatiles, for another 15 minutes after cure.

Adhesive Performance Testing Looptack

Samples of the coated adhesive construction are cut in 2.54 cm x about 20 cm test strips. The strips are formed into a loop, which is brought, in an Instron tensile tester, into contact with a standard stainless steel test panel at a rate of about 30 cm/minute, with no other force than the weight of the strip itself. After a brief contact period, the loop is peeled from the test panel at about 30 cm/minute.

The force to remove the adhesive loop is measured, in newtons (N)/m. Tests are performed in triplicate.

The possible adhesion failure modes are: panel (p) = no visible stain on panel; panel staining (ps) = visible stain on panel, but no sticky residue; cohesive (c) = adhesive film splits during the test, leaving residue films both on panel and on facestock; facestock failure (f) = adhesive delaminates completely from facestock; mixed (p/c or p/f) = failure with mixed p/c or p/f.

180° Peel Adhesion

Samples were cut in 2.54 cm x about 20 cm test strips, which were rolled down on standard stainless steel test panels with about a 2 kg rubber-clad steel roller, back and forth once at a rate of about 30 cm/minute. After a dwell time of 20 minutes at standard testing laboratory conditions, the test strips were peeled away from the test panel in an Instron tensile tester at 180° to the test panel, i.e., folded back on itself and parallel to the surface of the panel, at a rate of about 30 cm/minute. The force to remove the adhesive test strip from the test panel was measured in N/m. Tests were performed in triplicate. Possible modes of adhesion failures are the same as above.

Static Shear Samples were cut in 1.27 cm x about 5 cm and 2.54 cm x about 6 cm test strips. When the shear test was performed at room temperature (RTS) , the 1.27 cm x 5 cm test strips were applied on a standard stainless steel test panel, making a sample-overlap of 1.27 cm x 1.27 cm with the test panel. The sample portion on the test panel was rolled down with about a 2 kg rubber-clad steel roller, back and forth once at a rate of 30 cm/minute. The free end of the test strip was folded back on itself, and a small hole was made at the free end, where a load of 500 g could be attached during the test.

The sample's free end can be reinforced with aluminum foil, cardboard, or the like, to prevent the sample from tearing when the load is applied. After a dwell time of at least 20 minutes at standard laboratory testing conditions, the test panels, with the test strips on it, were placed on a rack in a vertical position, and a load of 500 g was attached to the hole at the test strips' free end. The time, in minutes, for the sample to fail and fall off the panel was measured by means of a timing device. The modes of adhesive failure are recorded as above. Tests were performed in triplicate.

When a sample does not fail after several thousand minutes, without any creeping of the test strip down the test panel surface, it is recorded either as "infinite, NC" or, e.g., "4000+, NC" (NC = no creep) .

When the shear test was performed at an elevated temperature (ETS) , either at 70°C, 100°C, or at any higher temperature, the 2.54 cm x 6 cm test strips were used, and were applied on the test panels to make a sample-overlap of 2.54 cm x 2.54 cm with the test panel. The rack with panels, test strips, 500 g load and timing device are placed in an oven, equipped with a constant-temperature-control mechanism.

Example 1 Polymer Synthesis PSA polymer-1 (P-l) was a control and comprised 58% by weight 2-ethylhexylacrylate, 24% methyl- acrylate, 10% 3-carboxyethylacrylate, and 8% dibutyl- fumarate. P-l had a weight-averaged molecular weight (M-,) of 385,000 and a number-averaged molecular weight (M of 66,000, as determined by size exclusion chromatography (SEC) . After compounding with 2% by weight of HDODA, P-l had a 100°C Brookfield melt viscosity of 46.5 pascal seconds (Pa.s.), using the #29 spindle at 10 rpm. It was also compounded with 2% by weight of photoinitiator DAROCUR 1173.

PSA P-2 was the test material, made according to the present invention, and comprised 2% by weight of the copolymerizable photoinitiator DAROCUR ZLI 3331, 56% 2-ethylhexylacrylate, 24% ethylacrylate, 10% β- carboxyethylacrylate, and 8% dibutylfumarate. P-2 had a M„ 55,000, a M. = 20,000 by SEC, and a 100°C Brookfield melt viscosity of 10 Pa.s. after compounding with 2% by weight of HDODA. The compositions of the P-l and P-2 are summarized in Table I.

Table I

P-2 developed infinite shear (4000+ minutes) after only 3 kJ/m² of UV exposure, whereas the control, P-l, with the conventional photoinitiator, did not form cohesive strength, even with 9 kJ/m² of UV. The results obtained with P-l and P-2 are summarized in Table II.

Table II

.000 c 7 c 0 c

4 c 0 c

560 p 4000+,NC

450 p 4000+,NC

c = cohesive, adhesive film splits during the test, leaving residue films both on panel and facestock p = no visible stain on panel

NC = no creep

P-2, which was UV crosslinkable by a mechanism using a copolymerizable photoinitiator, was not only unique in its rapid response to UV cure in developing high cohesive strength, but the adhesive performance in looptack and 180° peel adhesion remained essentially the same over a wide range of UV radiation dosage, from 3 to 9 kJ/m². This corresponds to a web speed variation of about 50 to about 15 m/minute on a conventional UV printing press, and indicates that UV overcure is improbable under these conditions. FIGs. 1 and 2 illustrate a relatively flat response of looptack and 180° peel adhesion of P-2 to UV radiation dosage. Example 2

Polymer synthesis

Adhesive performance of PSAs which are made according to the present invention is characterized by a low sensitivity to variations in the starting polymer viscosity, its formulation, and the UV irradiation dosage.

The synthesis of P-2 was repeated twice more (P- 2a and P-2b) . Due to slightly different polymeriza¬ tion conditions, the resultant products exhibited different 100°C melt viscosities — 35 Pa.s. for the first repeat, and 65 Pa.s. for the second repeat — indicating different polymer molecular weights in the products. The polymers were subsequently compounded with 1.0% by weight, 1.5% by weight, 2.0% by weight, and 5.0% by weight of HDODA, coated and cured with 3 to 9 kJ/m² of UV. The compositions of the polymers are summarized in Table III.

Table III

2-ethylhexylacry methylacrylate jS-carboxyethylac dibutylfumarate DAROCUR ZLI 3331

HDODA, compounded after polymerization 1 1.5 2 5

All samples gave infinite shears, and, despite the differences in initial melt viscosities, different levels of HDODA, and different UV dosages, the looptack and peel adhesion performance were about the same over the range of these differences. The results are summarized in Table IV.

Table IV

Low Dependence of Adhesive Performance on Starting Polymer Viscosity, Formulation_r and UV Irradiation Dosage

(Adhesive coating weight 35-40 g/m² on 2 mil

PET film; tack, peel, and shear tests on stainless steel panels)

P-2a

35 Pa.s. at 100°C, 2.0% HDODA

3 519 p 566 p Infinite 70°C

ETS

6 453 p 455 p Infinite 70°C

ETS P-2b

65 Pa.s. at 100°C, 2.0% HDODA

3 568 p 628 p Infinite 70°C ETS

6 484 p 458 p Infinite 70°C ETS ETS = Elevated Temperature Shear; RTS = Room Temperature Shear; p = All sample failures were panel failure

Example 3

Post Cure Coated and UV-cured adhesives of the present invention have better aging stability than adhesives which are made by conventional UV curing methods.

Drawdowns of the PSAs of the present invention were subjected to an accelerated aging at 60°C over a period of one week. Table V shows that the drop in the adhesive's looptack and 180° peel is only about 10%-20% from the original values. In conventional UV-cured PSAs, losses in excess of 50% in these adhesion properties are very common.

Table V

Looptack and Peel Performance after One Week's Aging at 60°C

(Adhesive coating weight 35-40 g/m² on 2 mil

PET film; tack, peel, and shear tests on stainless steel panels)

Polymer-3 was made according to the present invention. The photoinitiator material used was the alcohol DAROCUR 2959 or 4(2-hydroxyethoxy)phenyl-(2- hydroxy-2-propyl)ketone, which was made copolymerizable through the maleate halfester compound. The maleate halfester, however, was not made in a separate synthesis step, but was made in situ during the polymerization of the monomers which comprise the PSA material. The following monomers: 57% by weight of butylacrylate, 4% acrylic acid, 35% dioctylfumarate, and 2% maleic anhydride, were copolymerized in the presence of 2% by weight DAROCUR 2959, at 90°-95°C, using Vazo 52/64 as polymerization initiators. At the end of the polymerization step, the maleate halfester formation with the photoinitiator alcohol DAROCUR 2959 was completed by the addition of 0.25% of N,N-dimethylbenzylamine.

The composition of P-3 is summarized in Table VI.

Table VI butylacrylate 57 acrylic acid 4 dioctylfumarate 35 maleic anhydride 2

DAROCUR 2959 2

N,N-dimethylbenzylamine 0.25 Vazo 52/64 polymerization initiator HDODA, compounded after polymerization 4

The product has a 100°C melt viscosity of 30.4 Pa.s. before compounding. It was subsequently compounded with 4% by weight of HDODA and after coating on 50 μ poly-(ethylene terephthalate) (PET) at a coating weight of 30 g/m² and UV-cured. The adhesive performance of the PSA is summarized in Table VII.

Table VII

Loop- 180° 70°C UV tack Peel ETS kJ/m² fN/m) (N/m) (Min.)

P-3 4 407 p/ps 638 C 4000+,NC

6 315 p 554 c/ps 4000+,NC 8 402 p 546 p 4000+,NC c = cohesive, adhesive film splits during the test, leaving residue films both on panel and facestock p = no visible stain on panel ps = visible stain on panel, but no sticky residue NC = no creep

Example 5 Resistance to Liouid Immersion

Coated and UV-cured adhesives of the present invention exhibit good immersion resistance to a number of liquids, provided when the facestock material is not affected by the liquids in the immersion tests.

Polymer P-3 was evaluated for an under-the-hood automotive labeling adhesive application, where the labels might be exposed to a number of automotive liquids. In this evaluation, 2.54 cm x about 20 cm test samples from the previous illustration were applied to standard stainless steel test panels. After a dwell time of 24 hours, the panels and applied test samples were immersed in the automotive liquids. After the specified times of immersion, the panels and samples were taken out. The adhering liquids were carefully blotted with filter paper, and after a 1-hour recovery at ambient, the test strips were examined for liquid edge penetration and for any changes to the facestocks. Maximum acceptable edge penetration is normally no more than 5 mm. Finally, the test strips were peeled away in a 180° peel angle from the test panels. The results are summarized in

Table VIII.

Table VIII

Changes to Facestock, Edge Penetration and Peel Measurements after Exposure of Applied Labels on Stainless Steel Panels

Change to face- Cure UV stock and edge 180°Peel kJ/m² penetration N^m

1 hour in gasoline 4 No change to 764 c at ambient temp. 6 facestock, 2 mm 741 c

8 edge penetration 646

c = cohesive, adhesive film splits during the test, leaving residue films both on panel and facestock

Example 6

Stable Pot-Lives of Compounded Hot Melt PSA Formulations

Polymer-4 was a scale-up of P-3 from 800 g to

6000 g. It had a 100°C melt viscosity of 27.9 Pa.s. before compounding. It was subsequently compounded with 4% by weight of HDODA and after coating on 50 μm

PET at a coating weight of 30 g/m², was UV-cured. The adhesive performance of P-4 is summarized in Table

IX.

Table IX

Loop- 180° 70°C UV tack Peel ETS kJ/m² (N/m) (N/m) (Min.)

P-4 4 462 p 701 c 4000+,NC

6 391 p 548 p 4000+,NC 8 295 p 430 p 4000+,NC c = cohesive, adhesive film splits during the test,

10 leaving residue films both on panel and facestock. p = no visible stain on panel. NC = no creep.

Compounded polymers which are made according to

_ the present invention have very good pot-lives, even when kept at elevated temperatures.

The coater-ready P-4 which contained 4% by weight of HDODA was heated continuously at 100°C in the Thermocell of a Brookfield viscometer, and the

₂₀ melt viscosity followed over several days. The melt viscosity rose from about 16 Pa.s. to only about 22

Pa.s. after 8 days of continuous heating, after which time it rose quicker to about 29 Pa.s. after 11 days of heating. The results are presented in FIG. 3.

_₅ Example 7

Polymer-5 was made according to the present invention. It contained 2% by weight of the copolymerizable photoinitiator DAROCUR ZLI 3331, and consisted further of 56% 2-ethylhexylacrylate, 24%

₃₀ methylacrylate, 3% acrylic acid, 7% β- carboxyethylacrylate, and 8% dioctylmaleate. It was subsequently scaled up to a 100 kilogram batch run, which, after compounding with 4% by weight of HDODA, had a 100°C melt viscosity of 12.0 Pa.s. The

_₅ composition of P-5 is summarized in Table X. Table X

2-ethylhexylacrylate 56 methylacrylate 24

/3-carboxyethylacrylate 7 acrylic acid 3 dioctylmaleate 8

DAROCUR ZLI 3331 2 HDODA, compounded after polymerization 4

The scaled-up batch had the following adhesive performance on 50 μm PET facestock at a coating weight of 30 g/m² and after UV cure. The properties of P-5 are summarized in Table XI.

Table XI

c = cohesive, adhesive film splits during the test, leaving residue films both on panel and facestock p = no visible stain on panel NC = no creep

The scaled-up, P-5 coater-ready material was stored in closed, 19-liter metal pails, and was retrieved after 10 months of storage under uncontrolled warehouse conditions. The material had an excellent shelf-life. The 100°C melt viscosity rose only to 12.3 Pa.s. after the 10-month storage period. The material was still very UV-reactive; after coating on 50 μm PET at 30 g/m² coating weight and UV cure with only 4 kJ/m², it gave infinite 70°C ETS, without any creep.

Example 8 Solution- and Emulsion-Based PSAs

The formation of high cohesive strength in PSAs by UV cure of the present invention is also 1 applicable to solution-based as well as emulsion- based adhesives.

Polymer-6 was made in ethylacetate solution from 52.5% by weight of 2-ethylhexylacrylate, 18% methyl- 5 acrylate, 18% vinylacetate, 8% dioctylmaleate, 3% acrylic acid, and 0.5% DAROCUR ZLI 3331, using Vazo 52 as the polymerization initiator. The composition of P-6 is summarized in Table XII.

Table XII

10 2-ethylhexylacrylate 52.5 methylacrylate 18 vinylacetate 18 acrylic acid 3 dioctylmaleate 8

DAROCUR ZLI 3331 0.5 HDODA, compounded- l -o._ after polymerization 4

A portion of the product was evaporated to dryness; it had a 100°C melt viscosity of 136.0 Pa.s.

The remainder of the product was subsequently compounded with 4% by weight of HDODA based on the 0 polymer solids. UV cure developed high cohesive strength in the adhesive coating on 50 μm PET at a dry coating weight of 30 g/m². The properties of P-6 are summarized in Table XIII.

Table XIII 5 Loop- 180° 70°C UV tack Peel ETS ^• kJ/m² (N/m) (N/m) (Min.)

P-6 0 770 p 666 C 0 c

4 424 p 710 ps 1200+,NC 0 6 358 p 501 p/c 1200+,NC

8 380 p 381 p 1200+,NC c = cohesive, adhesive film splits during the test, leaving residue films both on panel and facestock p = no visible stain on panel 5 ps = visible stain on panel, but no sticky residue NC = no creep Example 9

Polymer-7 was made in emulsion and consisted of 55% by weight of 2-ethylhexylacrylate, 20% methyl- acrylate, 8% dioctylmaleate, 5% acrylic acid, and 1% Of DAROCUR ZLI 3331, using Alipal CO-436 (manufactured by Rhone Poulenc/Triton X-165 (manufactured by Union Carbide) as the surfactant, and t-butylhydroperoxide-FeEDTA as the redox initiator system. The product was compounded with 4% by weight of HDODA based on polymer solids. The composition of P-7 is summarized in Table XIV.

Table XIV

2-ethylhexylacrylate 66 methylacrylate 20 acrylic acid 5 dioctylmaleate 8

DAROCUR ZLI 3331 1

HDODA, compounded after polymerization 4

UV cure developed high cohesive strength in the adhesive coating, on 50 μm PET at a coating weight of 30 g/m². The properties of P-7 are summarized in

Table XV.

Table XV

Loop- UV tack kJ/m² (N/m)

P-7 0 244 p 267 c/ps 0 c

6 135 p 50 p 1200+,NC 8 137 p 49 p 1200+,NC c = cohesive, adhesive film splits during the test, leaving residue films both on panel and facestock p = no visible stain on panel ps = visible stain on panel, but no sticky residue

NC = no creep

Claims

WHAT IS CLAIMED IS:

1. A pressure-sensitive-adhesive copolymer suitable for the preparation of hot melts, comprising: a) at least one copolymerizable photoinitiator monomer selected from the group consisting of benzoin compounds, acyloin compounds, their derivatives and mixtures thereof; b) at least one low-glass-transition- temperature monomer sufficient to provide a polymer having a Tg as low as about 50°C below the use temperature; and c) at least one multifunctional monomer, wherein the actinic radiation cure is performed with light at about 240 to about 410 nm wavelength, a light intensity of about 6 to about 10 kWatts/m², and at a temperature of about 45°C to about 125°C, and wherein the resultant pressure-sensitive adhesive has a cohesive strength and high-temperature shear strength that does not fail after several thousand minutes; a tack performance of about 300 to about 500 N/m; a peel adhesion performance of about 550 to about 650 N/m; a shelf-life at room temperature of more than about 10 months, with no increase in the viscosity of the polymer; and a hot melt pot-life at 100°C of at least 10 days, with little or substantially no increase in the viscosity of the polymer.

2. A pressure-sensitive-adhesive copolymer as recited in claim 1 wherein the low-glass-transition- temperature monomers are selected from the group consisting of 2-ethylhexyacrylate, iso-octylacrylate, butylacrylate, and mixtures thereof. _

-33- 3. A pressure-sensitive-adhesive copolymer as recited in claim 1 wherein the coplymerizabie photoinitiator monomers are selected from the group consisting of 4(2-acryloyloxyethoxy)phenyl-(2- hydroxy-2-propyl)ketoneand4(2-hydroxyethoxy)phenyl- (2-hydroxy-2-propyl)ketone.

4. A pressure-sensitive-adhesive copolymer as recited in claim 1 wherein the coplymerizabie photoinitiator monomers are present at a concentration of about 0.1% to about 10% by weight.

5. A pressure-sensitive-adhesive copolymer as recited in claim 1 wherein the coplymerizabie photoinitiator monomers are present at a concentration of about 0.5% to about 5% by weight.

6. A pressure-sensitive-adhesive copolymer as recited in claim 1 wherein the multifunctional monomers are selected from the group consisting of diethylene-glycoldiacrylate, triethyleneglycol- diacrylate, tripropyleneglycoldiacrylate, 1,6- hexanedioldiacrylate, trimethylolpropanetriacrylate, trimethylolpropanetrimethacrylate, pentaeryth- ritoltriacrylate, and mixtures thereof.

7. A pressure-sensitive-adhesive copolymer as recited in claim 1 wherein the multifunctional monomers are present at a concentration of about 0.1% to about 10% by weight.

8. A pressure-sensitive-adhesive copolymer as recited in claim 1 wherein the multifunctional monomers are present at a concentration of about 1% to about 5% by weight.

9. A pressure-sensitive-adhesive copolymer as recited in claim 1 wherein the light is supplied by medium-pressure mercury lamps.

10. A pressure-sensitive-adhesive copolymer as recited in claim 1 wherein the cure is performed in an aerobic environment.

11. A pressure-sensitive-adhesive copolymer comprising: a) at least one copolymerizable photoinitiator monomer; b) at least one low-glass-transition- temperature monomer sufficient to provide a polymer having a Tg as low as about 50°C below the use temperature; and c) at least one multifunctional monomer, wherein the actinic radiation cure is performed under aerobic conditions.

12. A pressure-sensitive-adhesive copolymer as recited in claim 11 wherein the low-glass-transition- temperature monomers are selected from the group consistingof 2-ethylhexyacrylate, iso-octylacrylate, butylacrylate, and mixtures thereof.

13. A pressure-sensitive-adhesive copolymer as recited in claim 11 wherein the coplymerizabie photoinitiator monomers are selected from the group consisting of benzoin compounds, acyloin compounds, and mixtures thereof.

14. A pressure-sensitive-adhesive copolymer as recited in claim 13 wherein the coplymerizabie photoinitiator monomers are selected from the group consisting of 4(2-acryloyloxyethoxy)phenyl-(2- 14. A pressure-sensitive-adhesive copolymer as recited in claim 13 wherein the coplymerizabie photoinitiator monomers are selected from the group consisting of 4(2-acryloyloxyethoxy)phenyl-(2- hydroxy-2-propyl)ketoneand4 (2-hydroxyethoxy)phenyl- (2-hydroxy-2-propyl)ketone.

15. A pressure-sensitive-adhesive copolymer as recited in claim 11 wherein the coplymerizabie photoinitiator monomers are present at a concentration of about 0.1% to about 10% by weight.

16. A pressure-sensitive-adhesive copolymer as recited in claim 11 wherein the coplymerizabie photoinitiator monomers are present at a concentration of about 0.5% to about 5% by weight.

17. A pressure-sensitive-adhesive copolymer as recited in claim 11 wherein the multifunctional monomers are selected from the group consisting of diethyleneglycoldiacrylate, triethyleneglycoldi- acrylate, tripropyleneglycoldiacrylate, 1,6- hexanedioldiacrylate, trimethylolpropanetriacrylate, trimethylolpropanetri ethacrylate, pentaeryth- ritoltriacrylate and mixtures thereof.

18. A pressure-sensitive-adhesive copolymer as recited in claim 11 wherein the multifunctional monomers are present at a concentration of about 0.1% to about 10% by weight.

19. A pressure-sensitive-adhesive copolymer as recited in claim 11 wherein the multifunctional monomers are present at a concentration of about 1% to about 5% by weight.

20. A pressure-sensitive-adhesive copolymer as recited in claim 11 wherein the light is supplied by medium-pressure mercury lamps.

21. A pressure-sensitive-adhesive copolymer as recited in claim 11 wherein the copolymer is suitable for use as a hot melt.

22. A process for preparing a pressure- sensitive-adhesive copolymer suitable for the preparation of hot melts comprising: a) copolymerizing at least one low-glass- transition-temperature monomer in an amount sufficient to provide a polymer having a Tg as low as about 50°C below the use temperature, at least one coplymerizabie photoinitiator monomer, and a polymerization initiator; b) heating the polymerization mixture to maintain the polymerization reaction; c) adding at least one multifunctional monomer to the copolymer; d) coating the copolymer onto a suitable surface; and e) curing the coated copolymer with light at about 240 to about 410 nm wavelength, a light intensity of about 6 to about 10 kWatts/m², and at a temperature of about 45°C to about 125°C under aerobic conditions.

23. The process as recited in claim 22 wherein the low-glass-transition-temperature monomers are selected from the group consisting of 2- ethylhexyacrylate, iso-octyl- acrylate, butylacrylate, and mixtures thereof.

24. The process as recited in claim 22 wherein the coplymerizabie photoinitiator monomers are selected from the group consisting of benzoin compounds, acyloin compounds, and mixtures thereof.

25. The process as recited in claim 24 wherein the coplymerizabie photoinitiator monomers are selected from the group consisting of 4(2- acryloyloxyethoxy)phenyl-(2-hydroxy-2-propyl)ketone and 4 (2-hydroxyethoxy)phenyl- (2-hydroxy-2- propy1)ketone.

26. The process as recited in claim 22 wherein the coplymerizabie photoinitiator monomers are present at a concentration of about 0.1% to about 10% by weight.

27. The process as recited in claim 22 wherein the coplymerizabie photoinitiator monomers are present at a concentration of about 0.5% to about 5% by weight.

28. The process as recited in claim 22 wherein the multifunctional monomers are selected from the group consisting of diethyleneglycoldiacrylate, triethylene-glycoldiacrylate, tripropyleneglycol- diacrylate, 1 , 6-hexanedio1diacry1ate , trimethylolpropanetriacrylate, trimethylolpro- panetrimethacrylate, pentaerythritoltriacrylate, and mixtures thereof.

29. The process as recited in claim 22 wherein the multifunctional monomers are present at a concentration of about 0.1% to about 10% by weight.

30. The process as recited in claim 22 wherein the multifunctional monomers are present at a concentration of about 1% to about 5% by weight.

31. The process as recited in claim 22 wherein the light is supplied by medium-pressure mercury lamps.