CA2108260A1

CA2108260A1 - Epoxide-based adhesive

Info

Publication number: CA2108260A1
Application number: CA002108260A
Authority: CA
Inventors: Kam Chu Wu; Stefan Weigl
Original assignee: Individual
Current assignee: 3M Co
Priority date: 1991-05-16
Filing date: 1992-03-12
Publication date: 1992-11-17
Also published as: WO1992020754A1; EP0585360A1; JPH06507664A

Abstract

Photopolymerizable epoxide-based adhesives that are tacky and pressure-sensitive at room temperature and that provide high increased bond strength when cured. Also, transparent overlays comprising protective cover sheets and a layer of such adhesives.

Description

,~ W092/20754 PC~/~S92/01')74 l~lY~S~
Field of Invention The present invention relates to epoxide-based adhesives, particularly adhesives whose hond ~trength is i~?~ ~ enhanced through photopolymerization. The invention also relates to transparent overlays with such adhesive which can be applied to documents to deter tampering.
Backqround Pressure-sensitive adhesives ("PSAs") exhibiting tackiness at room temperature have been made in a variety of compositions. Such adhesives offer advantages of easy application, with successful bonding typically being achieved upon application of limited ~;~ pressure, e.g., hand pressure. In many cases, such adhesives tend to exhibit reduced bond strength when subjected to elevated temperatures or organic solvents.
Although the bond strength of pressure-sensitive adhesives is acceptable for many ; applications, in some instances, an adhesive that provides useful room temperature tackiness as well as resistance to adhesive failure under elevated temperatures or upon exposure to organic solvents is desired.
, one technique sometimes used to improve the performance, e.g., increase the shear strength and peel resistance, o~ certain pressure-sensitive adhesives at elevated temperatures or in the presence of organic solvents is to further crosslink the adhesive after its application to a substrate. Such crosslinking can be induced photochemically or thermally, or via chemical reaction, e.g., 8uch as with moisture or encapsulated ~ 35 reactants. It has been observed, however, that in some ; instances crosslinking a pressure-sensitive adhesive results in a reduction in adhesiveness. For example, U.S. Patent No. 4,286,047 (Bennett et al.) discloses a - normally tacky, pressure-sensitive adhesive having an F~ 'f`~

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epoxy equivalent value of about 400 to about 900 which is susceptible to ultraviolet-induced detackification.
U.S. Patent No. 4,690,957 (Fujioka et al.);
x Chemical Patents Index, Basic Abstracts ~ournal, Section Ch, r~eek 8519, 3 July 1985, Derwent Publications Ltd., GB, Class A, AN 85-113545/19 & JP A 60 055 069 (Mitsubishi Electric Corp.), 29 March 1985; and Chemical Patents Index, Basic Abstracts Journal, Section Ch, Week 8518, 26 June 1985, Derwent Publications Ltd., London, GB, Class A, AN 85-107731/18 ~ JP A 60 051 717 (Mitsubishi Electric Corp.), 23 March 1985 disclose ultraviolet radiation-c~rable resin compositions which each comprise:
a) 100 parts by weight of an epoxy resin having two or more oxirane groups in each molecule thereof;
b) 0.5 to 20 parts by weight of a phenoxy resin having a molecular weight of from 10,000 to 60,000;
c) 0.5 to 20 parts by weight of a polyol; and d) O.S to 10 parts by weight of a photo-initiator.
U.S. Patent No. 4,2S6,828 (Smith) discloses photocopolymerizable compositions which contain epoxides, organic material with hydroxyl functionality, and a photosensitive aromatic sulfonium or iodonium salt of a halogen-containing complex ion.
U.S. Patent Nos. 4,173,476 and 4,231,951 (~oth Smith et al.) disclose photopolymerizable epoxide compositions containing triarylsulfonium complex salts ~` as photoinitiators.
U.S. Patent No. 4,218,531 (Carlson) discloses photopolymerizable compositions containing epoxides, `-aromatic sulfonium complex salts, and a material containing at least one non-aromatic carbon-carbon ethylenic unsaturation. The reference teaches that such compositions can be cured without generation of offensive odors.

i' :` 2 1 ~ ,~ 2 ~ 0 U.S. Patent No. 4,593,051 (Koleske) disclose~
photocopolymerizable compositions comprising an epoxide, organic compound having two or more active hydrogens per molecule, and photoinitiator.
Summary of Invention The present invention provides a normally tacky, pressure-sensitive adhesive which, upon exposure to ultraviolet radiation, undergoes crosslinking to result in increased bond strength and bond durability.
Bonds made with the adhesive compositions provided herein typically exhibit resistance to failure, e.g., shear release, peel release, or cohesive failure, under elevated temperatures (e.g., about 100C) and/or exposure to organic solvents. Furthermore, the compositions provided herein exhibit high transparency to visible light both before and a~ter being cured.
Coupled with the bond strength characteristics mentioned above, this makes adhesive compositions of the invention well suited for use on tamper-resistant overlays for documents.
In brief summary, the novel adhesives provided comprise, and if desired may consist essentially of, epoxide resin, polymeric film former, photoinitiator, and polyol. Adhesives of the invention contain between about 20 and about 80 parts by weight of film former per 100 parts by weight of epoxide component, between about l and about 10 parts by weight of photoinitiator per 100 parts by weight of epoxide component, and between about 30 2 and about 20 parts by weight of polyol per 100 parts - ~ ` by weight of epoxide component. Adhesives of the invention may also optionally contain one or more of the following: reactive diluent, solvent, toughening agent, and other additive.
Adhesives of the present invention typically exhibit an initial tackiness sufficient to provide an initial peel strength of about 0.28 kilograms/centimeter-P,~S~t i~/~ ~e.3~

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bisphenol A ("DGEHBA"), and bisphenol F ~'IDGEBFI');
cycloaliphatic epoxides; epoxide cre~ol novolac re~ins;
epoxide phenol novolac resins; and the like. Such epoxide resins may be used singly or in combinations of two or more.
. DGEBA resins are commonly available, e.g., from Shell Chemical Company, Dow Chemical Company, and Ciba Geigy Company. Typically, low molecular weight DGEBA
resins with an epoxy equivalent weight of below about 280 are preferred. DGEBA resins with epoxy equivalent weights substantially above that range, e.g., above about 300 or more, typically tend to be more solid in character and less liquid in character at room temperature, thereby reducing the desired tackiness of the composition.
i Illustrative examples of cycloaliphatic epoxide resins useful herein include those having an average of two or more epoxide groups per molecule. The preferred cycloaliphatic epoxide is 3,4-epoxide cyclohexylmethyl -3,4-epoxidecyclohexane carboxylate, e.g., available commercially as EPOXIDE ERL 4221 from Union Carbide Company. Readily available, this is a fast reacting, low viscosity resin. Bis~(3,4-epoxidecyclo-hexylmethyl) adipate is an illustrative example of a cycloaliphatic epoxide resin which is useful herein.
Phenol-novolac and cresol-novolac epoxide resins are multifunctional resins derived from epichlorohydrin and polynuclear phenolia or cresol novolac. They are typically characterized by low amounts of ionic and hydrolyzable chlorine impurities and high chemical and thermal resistance. Illustrative examples of commercially available supplies thereof include Ciba-Geigy's EPN and ECN Epoxide Resin series.
Adhesive compositions o~ the present invention may contain one or more of the epoxide resins described above. Preferably, the room temperature viscosity of the epoxide resin mixture is between about 1,000 and about 30,000 centipoise, preferably between about 3,000 and about ~0,000 centipoise. Adhesive compositions made .. . . ~
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-3/~-width (25 ounces/inch-width) when laminated to a stainless steel panel and rolled once with a ~ 0 kilogram (4.5 pound) roller. Prior to being cured, the adhesives provide a low bond strength, i.e., an overlap shear strength of less than about 7.0 kilograms/centimeter2 (100 pounds/inch2) when laminated between chromic acid etched aluminum panels. Upon full cure of the adhesive with ultraviolet radiation activation, sometimes in conjunction with heat, a bond strength sufficient to provide an overlap shear strength of about 141 kilograms/centimeter2 (2000 pounds/inch2) on chromic acid etched aluminum may be achieved.
, .
Detailed Description of Illustrative Embodiments Epoxide resins which may be used herein include those containing two or more epoxide groups having the formula:
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-Cl I , and which are liquid at room temperature. The polymeric film formers discussed below are typically relatively non-tacky. In order to impart desired room temperature tacky, pressure-sensitive characteristics to the overall composition, the epoxide resin is preferably a liquid at room temperature. Illustrative examples of useful epoxide resins include diglycidyl ethers of various phenol8, e.g., bisphenol A ("DGEBA"), hydrogenated W092t207s4 PC~/US92/01974 ~ 5~
with epoxide resins having viscosities which are too high may tend to exhibit undesirably low tack, whereas those made with epoxide resins having visco~it~e~ which are too low may tend to be subject to resin migration and phase separation.
c- The polymeric film former is a pol~meric material having a molecular weight which is higher than that of the epoxide resin component. The film former should be compatible with the epoxide resin, i.e., they should not undergo phase separation such as during storage of the resultant adhesive composition or of an article comprising same. Polymeric materials which can form a tough film at room temperature are preferred.
The film former preferably has a weight average molecular weight between about lO,O00 and about 30,000.
~dhesive compositions made with film formers having molecular weights which are too low tend to provide poorer peel strength prior to photopolymerization.
Adhesive compositions made with film formers having molecular weights which are too high may be harder to make as the film former is typically more difficult to handle, to dissolve, etc., and may be too viscous.
Illustrative examples of polymeric film formers which are useful herein include polyacrylates, polymethacrylates, phenoxy resins, polysulfones, and polyether sulfones.
The proportion of film former incorporated in adhesives of the invention will depend in part upon the nature o~ the epoxide component, the particular film former, and the degree of room temperature tackiness desired. Typically, adhesives of the invention will contain between about 15 and about lO0, preferably between about 20 and about B0, and most preferably between about 40 and about 60, parts by weight of film former per lO0 parts by weight of epoxide component.
Adhesive compositions of the invention typically contain a polyol component to increase the initial tackiness, post cure flexibility, and open time of the adhesive composition. Illustrative examples of 4 ~cr/Us92/0l974 f~ 2 6 ~3 -6- ~
poly~ls which are useful in adhesive compositions o~ the invention include the ~ollowing: polyether polyol, polyester polyol, polycaprolactone polyol. The aforementioned U.S. Patent Nos. 4,256,82~ and 4,593,051 disclose pol~ols which are useful in photopolymerizable epoxide compositions. , Adhesive compositions of the invention typically contain between about 2 and about 20 parts, preferably between about 3 and about 15 parts, and more lo preferably between about 4 and about lo parts, by weight of polyol per loo parts by weight of epoxide component.
Adhesive compositions containing too much polyol tend to undergo ultraviolet curing more slowly and, once cured, exhibit lower peel strengths. Adhesive compositions containing too little polyol may not cure completely, even with thermal post treatment.
Many photoinitiators known for use in photopolymerizable epoxide compositions may be used in adhesive compositions of the invention. Illustrative examples are disclosed in the aforementioned U.S. Patent Nos. 4,231,951 and 4,256,828 and U.S. Patent No.
4,058,401 (Crivello). Preferred photoinitiators include triaryl sulfonium salts and diaryl iodonium complex salts. These photoinitiators are typically thermally stable and readily available commercially and exhibit good ultraviolet reactivity.
Adhesive compositions of the invention typically contain between about 1 and about 10, preferably between about 1.5 and about 6, and more preferably between about 2 and about 4, parts by weight of photoinitiator per 100 parts by weight of epoxide component. Use of excessive amounts of photoinitiator may act as a screen, thereby impairing the desired photopolymerization of the adhesive. Also, large residual amounts of unused initiator may tend to impair the bonding characteristics of the cured adhesive.
If desired, adhesive compositions of the invention may also contain one or more of such additives as metal particles, metal-coated particles, coloring ...
, W092/207s4 PC~r/US~2/0l974 -7~
agents, processing aids, adhesion promoters, wett~ng agents, and coupling agents.
Metal particles and metal-coated particles can be incorporated to render the adhesive composition conductive including anisotropically conductive if desired. Illustrative examples include nickel particles between about 6 and about 25 microns in diameter which can be randomly dispersed in the composition.
The adhesive composition may be substantially clear and colorless or may be colored by use of such coloring agents as dyes and pigments. It will be understood, however, that coloring agents may tend to increase the opacity of the adhesive composition so as to affect the photocuring properties thereof, particularly when the adhesive is used in thick layers.
Accordingly, coloring agents used herein should be selected and used in such quantities as will not undesirably inhibit the photocuring properties of the adhesive composition.
Adhesive compositions o~ the invention may contain processing aids and modifiers, e.g., rheology controlling agents such as fumed silica. Illustrative examples of adhesion promoters and coupling agents include silanes and titanates. Fluorochemical and other surface energy controlling agents may be used. The various components used in compositions of the invention are preferably compatible with each other such that the cured adhesive is substantially transparent, thereby permitting the composition to be used in overlays, e.g., over information on documents.
Adhesives of the invention may be supplied in film form, e.g., between release liners as a transfer tape, to be laminated to a substrate.
Photopolymerization of the composition may be performed after the various elements are assembled, e.g., by exposure to ultraviolet radiation through a substrate which is sufficiently transparent to such wavelengths, or the adhesive may be exposed prior to assembly. Under typical conditions, e.g., temperature of about 25C, WO92/20754 PCT/~IS92/0~974 2 ~ 2 6~ -8-relative humidity of about 60 percent, and W exposure of about 0.1 Joule/centimeter2, adhesive compositions o~
the invention typically exhibit an open time of about 1 to 2 minutes. --Adhesive compositions of the invention can be used to bond substrates that previously known thermally , cured epoxide adhesives bonded, simultaneously providing the advantages discussed herein. Subsequent to curing by ultraviolet radiation, adhesives of the invention may optionally be heated, i.e., thermally postcured or thermal post treatment, to enhance the structural bond strength of the bond. Typically, such treatment involves heating the adhesive to a temperature of about 100C to about 120C for several minutes.
In some instances, adhesives of the invention can be used to make transparent overlays for making documents more tamper-resistant. One typical embodiment of such overlays compriseæ a protective cover sheet such as disclosed in U.S. Patent No. 3,170,183 (Sevelin et al.), incorporated herein by reference, with a layer of the adhesive provided herein on the back side. In an inexpensive embodiment, the cover sheet may simply be single or multi-ply polymer film, e.g., polyester, which is typically preferably flexible and abrasion resistant.
Exa~p~e~
The invention will be further explained by the ~ollowing illustrative examples which are intended to be nonlimiting. Unless otherwise indicated, all amounts are expressed in parts by weight.
Unless otherwise indicated, the following test method wa5 uged.

Ovçrlap Shear Strength Overlap Shear Strength of indicated adhesives was determined using aluminum panels having an overlap ^ region of about 1.0 inch by about 0.5 inch (2.5 by 1.3 centimeters). The aluminum panels were conditioned in chromic acid at a temperature of about 65C to about ;

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, 70C for about 15 minutes and then rinsQd with tap water. The panels were then allowed to dry at room temperature for about 30 minutes and then at about 60C
to about 65C for 30 minutes. The sub~ect adhesive wa~
applied to a test panel at a thickne~s o~ about 1 to . about 2 mils (25 to 50 microns), activated with ultraviolet radiation as indicated, and then the ~econd panel laminated thereto. Unless otherwise indicated, the bond was then thermally postcured at about 110C to lo about 115C ~or 20 minutes.

Examples 1-4 Adhesives were made using the following components:
15 Notation Explanation EPON 828 DGEBA resin from Shell Chemical Company;
DER 684-EK40 Phenoxy resin in MEK, 40 percent by .
volume, from Dow Chemical company;
20 UCAR PKHH Solid phenoxy resin from Union Carbide Company;
TASHFA Triarylsulfonium he~xafluorantimonate;
NIAX PPG 4025 Polypropylene glycol from Union Carbide Company;
25 BTA-IIIF Plastic toughening agent from Rohm and Haas Company;
CABOSIL M-5 Fumed silica, surface area 200 ~ 25 meters2/gram, from Cabot Corporation;
CABOSIL N70-TS Fumed silica, surface area 100 + 20 meters2/gram, from Cabot Corpor~tion;
MEK Methyl ethyl ketone, reagent grade;
Ni-CP Nickel-coated particles, 10 micron average diameter.
The composition in each Example was as fol~ows:
Component Example _ l _ _2 3 4 EPON 828 35.0 35.0 35.0 30.0 - .

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DER 684-EK40 21.0 0.0 24.0 36.0 UCAR P~H 0. 0 9 . O
TASHFA 1.70 1.70 0.73 0.75 NIAX PPG 4025 1.50 1.50 1.70 2.0 BTA-IIIF 4.0 0.0 0.0 0.0 CABOSIL M-5 1.3 1.3 0.0 0.0 CABOSIL N70-TS 0~0 0.0 1.25 1.3 MEK 2.0 15.0 0.0 4.0 Ni-CP 0.0 0.0 2.5 0.0 In Examples 1 and 2, the adhesive compositions were cured with 6 seconds exposure to a 100 watt W -A
mercury bulb providing a W input of about 0.2 Joules/centimeter2 at room temperature. In Examples 3 ~. .1 and 4, the adhesive compositions were cured with a 200 Watts/inch (79 Watts/centimeter) electrodeless mercury lamp in a W processor. The exposure was measured with a W-A radiometer to be about 0.050 Joules/centimeter2.
The joints were thermally postcured under slight compression pressure using small binder clips. In each sample, the adhesive composition was substantially transparent before and after cure.
The Overlap Shear Strength of the adhesive in each Example in kilograms/centimeter2 (in pounds/inch2) was as follows:
Example_1 _ _2 _ ___ _4 _ Overlap Shear211 120 127 127 (3000) (1700~ (1800) (1800) Example 5 Adhesive of the same composition as in Example ~ ~ 3 was placed in a thickness of between about 1.5 and about 2 A 0~ mils (37 and 50 microns) onto two substrates:
(1) a gold-plated circuit board and (2) a glass slide.
The adhesive was activated with exposure to a 200 Watts/inch (79 Watts/centimeter) mercury lamp with total energy output of about 0.050 Joules/centimeter2. A
flexible printed circuit was laminated to the adhesive under about 30 kilograms/centimeter2 pressure at about R~TITI I~ C c~ u ~eT

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110C for 10 seconds. The laminates were postcllred at 110C for 20 minutes.
The resistance,between the flexible printed circuit leads and gold circuit board was 0.2 ohms or less. The resistance between the adjacent leads o~ the printed circuit was greater than 20 mega ohms, the maximum value of the ohm meter used. This demonstrates the highly anisotropic conductivity properties which may be obtained with adhesives of the invention.

Example 6 An adhesive was made with the following components:
Amount Components 1549.3 EPONEX DRH 1510, DGEHBA epoxide resin from Shell Chemical Company;
31.-2 DER 684-EK40;
1.7 NIAX PPG 4025;
2.9 TASHFA;
2014.0 MEK;
0.9 CABOSIL M-5.
The adhesive was coated with a knife coater onto the bottom side of 3M CONFIRM Brand Retroreflective Security Laminate from which the commercially-shipped adhesive had been removed to provide a dry thickness between about 1 and about 2 mils (25 to 50 micrometers).
After evaporation of the MEX solvent, several samples of the laminate were cut from the sheet and applied to a document.
Each sample was exposed through the security laminate using a 100 Watt WA mercury bulb placed at between about 3 and about 5 inches (between about 7.6 and about 12.7 centimeters) from the document to cure the adhesive. Irradiation for between about 0.5 and about 3 minutes provided substantially equivalent performance. The text on the doc~ment was easily readable through the overlay in each sample.

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. .. : . , , Different samples were treated with high temperature, low temperature, water immersion, high humidity, and immersion in organic solvents. Even after ,, .~ ;

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, ' , WO92/207s4 PCI/US92/01974 treat~e~ 6 Qersion for the indicated periods, none of the samples could be removed from the document without so severely damaging the document or the overlay as to render them unusable. Samples were exposed to the indicated conditions or immersed in the indicated solvents for the indicated period shown in Table I.
Unless otherwise indicated, test was performed at room temperature of about 23C.
Table I
Condition Duration Exposure to 150C 30 minutes Exposure to -300C 30 minutes Immersion in water at 23C 30 days Exposure to lO0 percent 30 days relative humidity at 60C
" , .
. Immersion in Heptane 24 hours Immersion in Turpentine 24 hours Immersion in Acetonitrile l hour*
Immersion in Toluene 1 hour*
Immersion in Methanol 1 hour*
Immersion in Isopropanol 1 hour*
Immersion in Acetone 1 minute*
Immersion in Dimethyl formamide 1 minute*
Immersion in Ethyl acetate l minute*
Immersion in Chloroform 1 minute*
--------------______________________ * Substrate was destroyed when tested for longer durations. Adhesive not observed to have degraded. , .
~ Various modifications and alterations of this : invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention.

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Claims

Claims:

1. An adhesive that is tacky at room temperature, characterized in that said adhesive comprises:
a) at least one epoxide resin which contains two or more epoxide groups having the formula:

;

b) at least one polymeric film former, said adhesive containing between about 20 and about 80 parts by weight of film former per loo parts by weight of epoxide component;
c) at least one photoinitiator, said adhesive containing between about 1 and about 10 parts by weight of photoinitiator per 100 parts by weight of epoxide component; and d) at least one polyol, said adhesive containing between about 2 and about 20 parts by weight of polyol per 100 parts by weight of epoxide component;
wherein said adhesive exhibits an increase in bond strength upon exposure to ultraviolet radiation.

2. The adhesive of claim 1 further characterized in at least one of the following:
a) said epoxide resin comprises one or more of the following: diglycidyl ethers of phenol, cycloaliphatic epoxides, epoxide cresol novolac resins, and epoxide phenol novolac resins; or b) said epoxide resin is a diglycidyl ether of bisphenol A having an epoxy equivalent weight of below about 280; or c) said epoxide resin is 3,4-epoxide cyclohexymethyl-3,4-epoxidecyclo- hexane carboxylate; or d) the room temperature viscosity of said epoxide resin component is between about 1,000 and about 30,000 centipoise.

3. The adhesive of claim 1 further characterized in at least one of the following:
a) said polymeric film former has a weight average molecular weight of between about 10,000 and about 30,000; or b) said polymeric film former comprises one or more of the following: polyacrylate, polymethacrylate, phenoxy resin, polysulfone, and polyether sulfone.

4. The adhesive of claim 1 further characterized in that said adhesive contains between about 40 and about 60 parts by weight of film former per 100 parts by weight of epoxide component.

5. The adhesive of claim 1 further characterized in at least one of the following:
a) said polyol component includes one or more of the following: polyether polyol, polyester polyol, polycaprolactone polyol;
b) said adhesive contains between about 3 and about 15 parts by weight of polyol per 100 parts by weight of epoxide component; or c) said adhesive contains between about 4 and about 10 parts by weight of polyol per 100 parts by weight of epoxide component.

6. The adhesive of claim 1 further characterized in at least one of the following:
a) said photoinitiator component contains one or more of the following: triaryl sulfonium salts and diaryl iodonium complex salts; or b) said adhesive contains between about 1.5 and about 6 parts by weight of photoinitiator per 100 parts by weight of epoxide component; or c) said adhesive contains between about 2 and about 4 parts by weight of photoinitiator per 100 parts by weight of epoxide component.

7. The adhesive of claim 1 characterized in that said adhesive further comprises one or more of the following: metal particles, metal-coated particles, coloring agents, processing aids, adhesion promoters, wetting agents, and coupling agents.

8. A transparent overlay comprising a protective cover sheet and a layer of the adhesive of claim 1 on one side thereof.