Patent Description:
Degree of cure indicators (i.e., cure indicators) have been devised that change color during curing. For example, <NUM> Impact Resistant Structural Adhesive, <NUM>, a commercially available two-part epoxy adhesive, includes phenolphthalein as a cure indicator. During cure, the color changes from silver-grey to silver-purple. This enables a user to verify that the adhesive has cured but provides no other useful information on the kinetics of cure.

Similarly, PCT Patent Application No.<CIT>, discloses a two-part epoxy adhesive that includes mix and cure indicator systems. While this may improve a user's confidence in the final adhesive bond strength, these indicator systems do not provide the user with specific guidance on when an adhesive bond can be formed (i.e., closed) and still achieve a suitable ultimate bond strength. Us <NUM><NUM><NUM> discloses s a cold-curing two-component-epoxy adhesive, comprising (a) an epoxy-resin-component and (b) a curing agent-component, comprising a mercaptan curing agent, in which the epoxy-resin-component comprises a color change indicator. Also described is a method for monitoring the curing progress of cold-curing two-component-epoxy adhesive via color change of the adhesive.

A practical visual indicator system that monitors working life has not been developed to date. As used herein, the term "working life" refers to the period of time that a curable adhesive remains low enough in viscosity that it can still be easily applied to a part or substrate in a particular application and still provide an effective overlap shear strength.

Advantageously, the present disclosure provides just such a visual working life indicator for two-part epoxy resins. Using two-part epoxy adhesives as described hereinbelow, a user can have confidence that the adhesive is usable for bonding two substrates and achieve at least a predetermined level of overlap shear adhesion.

In a first aspect, the present disclosure provides a two-part epoxy adhesive comprising:.

In many preferred embodiments, wherein at least one of Part A or Part B further comprises reflective pigment.

In another aspect, the present disclosure provides a reaction product preparable by mixing Part A with Part B.

In yet another aspect, the present disclosure provides a method of bonding two substrates, the method comprising:.

As used herein:
the phrase " exhibiting an abrupt color change from colorless to a color" refers to color as detected by a normal human eye having <NUM>/<NUM> vision using D<NUM> spectral power distribution illumination.

Two-part epoxy adhesives according to the present disclosure comprise a Part A and a Part B. In use, Part A and Part B are mixed together, which typically initiates a curing process in which the curable epoxy resin and amines react to form a crosslinked polymer network. Part A includes components a) and b).

Based on total weight of components a) and b), Part A includes <NUM> to <NUM> weight percent, preferably <NUM> to <NUM> weight percent, of at least one curable epoxy resin (i.e., the total amount of all curable epoxy resin(s) in Part A). Examples of suitable epoxy resins include those derived from epoxy-functionalized monomers such as monomers containing one or more multifunctional glycidyl ethers.

Typical monomers in curable epoxy resins include: glycidyl ethers of dihydric arenes, aliphatic diols or cycloaliphatic diols. Glycidyl ethers of aliphatic diols include linear or branched polymeric epoxides having one or more terminal epoxy groups such as, e.g., diglycidyl ethers of polyoxyalkylene glycols.

Examples of aromatic glycidyl ethers include those that can be prepared by reacting a dihydric arene with an excess of epichlorohydrin. Dihydric arenes, as referred to herein, are arenes having two hydrogen atoms available for a reaction with epichlorohydrin. Examples of useful dihydric arenes include resorcinol, catechol, hydroquinone, and the polynuclear phenols including p,p'-dihydroxydibenzyl, p,p'-di(hydroxyphenyl)sulfone, p,p'-dihydroxybenzophenone, <NUM>,<NUM>'-di(hydroxyphenyl)sulfone, <NUM>,<NUM>-dihydroxy-<NUM>,<NUM>-dinaphthylmethane, and the <NUM>,<NUM>'-, <NUM>,<NUM>'-, <NUM>,<NUM>'-, <NUM>,<NUM>'-, <NUM>,<NUM>'-, and <NUM>,<NUM>'-isomers of di(hydroxyphenyl)methane, <NUM>,<NUM>-di(hydroxyphenyl)propane, <NUM>,<NUM>-di(hydroxyphenyl)butane, <NUM>,<NUM>-di(hydroxyphenyl)pentane, <NUM>,<NUM>-di(hydroxyphenyl)-<NUM>-phenylpropane, <NUM>,<NUM>-di(hydroxyphenyl)-<NUM>-phenylbutane, <NUM>,<NUM>-di(hydroxyphenyl)-<NUM>-phenylpentane, di(hydroxyphenyl)tolylmethane, <NUM>,<NUM>-di(hydroxyphenyl)-<NUM>-tolylethane, di(hydroxyphenyl)cyclohexylmethane, and <NUM>,<NUM>-di(hydroxyphenyl)cyclohexane.

Some preferred examples of curable epoxy resins include those epoxy resins having one or more units derivable from bisphenol A, bisphenol F or both. Other preferred examples of epoxy resins include those that can be prepared by bisphenol A, bisphenol F or both with epichlorohydrin. The epoxy resins may have a molecular weight in the range of from about <NUM> to about <NUM>,<NUM>/mol, preferably from about <NUM> to about <NUM>,<NUM>/mol. The average epoxy functionality in the resin is typically greater than <NUM> or <NUM> and less than <NUM>. Epoxy-novolac resins may also be used.

Examples of commercially available aromatic and aliphatic epoxides useful in the disclosure include: diglycidyl ether of bisphenol A (e.g., available as EPON <NUM>, EPON <NUM>, EPON <NUM>, and EPON <NUM> from Hexion Specialty Chemicals GmbH, Rosbach, Germany; DER-<NUM>, DER-<NUM>, and DER-<NUM> available from Olin Corp. , Niagara Falls, New York; diglycidyl ether of bisphenol F (e.g., EPICLON <NUM>) available from Dainippon Ink and Chemicals, Inc. ); and flame retardant epoxy resins (e.g., DER <NUM>, a brominated bisphenol type epoxy resin available from Olin Corp.

Based on total weight of components a) and b), Part A includes <NUM> to <NUM> weight percent, preferably <NUM> to <NUM> weight percent, of at least one pH indicator dye exhibiting an abrupt color change from colorless to a color in the pH range of <NUM>-<NUM> (i.e., the total amount of all pH indicator dye(s) in Part A exhibiting an abrupt color change from colorless to a color in the pH range of <NUM>-<NUM>). Examples of suitable indicator dyes include phenolphthalein, o-cresolphthalein, ethyl bis(<NUM>,<NUM>-dimethylphenyl)acetate, and thymolphthalein. Of these, phenolphthalein, o-cresolphthalein, and ethyl bis(<NUM>,<NUM>-dimethylphenyl)acetate are more preferred.

In some embodiments, bromothymol blue (<NUM>,<NUM>-bis[<NUM>-bromo-<NUM>-hydroxy-<NUM>-methyl-<NUM>-(propan-<NUM>-yl)phenyl]-<NUM>, 1λ<NUM>-benzoxathiole-<NUM>,<NUM>(<NUM>H)-dione) may be beneficially added in addition to the indicator dye(s). Addition of the bromothymol blue may enhance the appearance of the two-part epoxy adhesive during curing, thereby facilitating the determination of color change. If present, the amount of bromothymol blue is typically in a range of <NUM> to <NUM> percent of the total weight of the at least one pH indicator dye, although his is not a requirement.

Part B includes components c) - f) (i.e., components c), d), e), and f)).

Based on total weight of components c) - f), Part B includes <NUM> to <NUM> weight percent, preferably <NUM> to <NUM> weight percent, of at least one aliphatic primary polyamine (i.e., the total amount of aliphatic polyamine(s) wherein the amino groups are primary).

In one embodiment, useful aliphatic primary polyamines have a molecular weight of greater than about <NUM>/mol and less than less than <NUM>/mol, for example, between <NUM> and <NUM>/mol; however other molecular weights may also be used.

Examples of suitable aliphatic primary polyamines include aliphatic primary diamines and aliphatic primary triamines. Examples of suitable aliphatic primary diamines include those according to the general formula.

R<NUM>R<NUM>N-R<NUM>-NR<NUM><NUM>     (I).

wherein R<NUM>, R<NUM> and R<NUM> independently represent hydrogen, a linear or branched alkyl or a linear or branched poly(oxyalkyl) moiety.

The residues R<NUM>, R<NUM>, R<NUM> may independently comprise an aliphatic hydrocarbyl group containing about <NUM> to <NUM> carbon atoms or an aliphatic polyether containing from <NUM> to <NUM> carbon atoms. Preferably, one, more preferably two, and more preferably all three of R<NUM>, R<NUM>, and R<NUM> are hydrogen.

R<NUM> represents a divalent linear or branched alkylene, aminoalkylene, poly(aminoalkylene), poly(amidoalkylene), poly(alkyleneoxy) residue having at least <NUM> carbon atoms. R<NUM> may also be a divalent residue of a polyamidoamine or a polyamidodiamine that can be derived by reacting a dimer or trimer carboxylic acid with a polyetheramine.

Preferably, the aliphatic primary polyamine is a polyetherdiamine or polyethertriamine including those polyetherdiamines and polyethertriamines that can be derived from polypropylene oxide or polyethylene oxide. In some preferred embodiments, the aliphatic primary polyamine(s) contain exactly two nitrogen atoms.

Exemplary suitable polyetherdiamines include those corresponding to any of the general formulas:.

H<NUM>NCH<NUM>CH<NUM>CH<NUM>O[CH<NUM>CH<NUM>O]nCH<NUM>CH<NUM>CH<NUM>NH<NUM>     (II).

H<NUM>NCH<NUM>CH<NUM>CH<NUM>O [CH<NUM>CH<NUM>CH<NUM>O]nCH<NUM>CH<NUM>CH<NUM>NH<NUM>     (III).

H<NUM>NC(CH<NUM>)HCH<NUM>O[CH<NUM>CH(CH<NUM>)O]nCH<NUM>CH(CH<NUM>)NH<NUM>     (IV).

with n being a positive real number (e.g., an average number) within the range of <NUM> and <NUM> such as, for example, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or between <NUM> and <NUM> (e.g., <NUM> or <NUM>), between <NUM> and <NUM> (e.g., <NUM> or <NUM>), between <NUM> and <NUM> (e.g., <NUM> or <NUM>), between <NUM> and <NUM> (e.g., <NUM> or <NUM>), or n being <NUM>, <NUM>, <NUM>, or between <NUM> and <NUM>.

Suitable aliphatic primary diamines are commercially available as PC AMINE DA from Performance Chemicals Handels GmbH, Hamburg, Germany, or under the trade designation JEFFAMINE from Huntsman Corp. , The Woodlands, Texas (e.g., JEFFAMINE D-<NUM> or JEFFAMINE D-<NUM>).

Exemplary aliphatic primary triamines can be represented by formula (V):
<CHM>
wherein m, x, y, and z are each independently positive real numbers positive real number (e.g., an average number) within the range of <NUM> and <NUM> such as, for example, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or between <NUM> and <NUM> (e.g., <NUM> or <NUM>), between <NUM> and <NUM> (e.g., <NUM> or <NUM>), between <NUM> and <NUM> (e.g., <NUM> or <NUM>), between <NUM> and <NUM> (e.g., <NUM> or <NUM>). R<NUM> represents H or alkyl having from <NUM> to <NUM> carbon atoms (e.g., methyl, ethyl, propyl, or butyl).

Compounds of this type are commercially available, for example, under the trade designation Jeffamine T (e.g., Jeffamine T-<NUM>, Jeffaine T-<NUM>, and Jeffamine T-<NUM>) from Huntsman Corp.

Based on total weight of components c) - f), Part B includes <NUM> to <NUM> weight percent, preferably <NUM> to <NUM> weight percent, of at least one tris(<NUM>,<NUM>,<NUM>-dialkylaminoalkyl)phenol having from <NUM> to <NUM> carbon atoms (i.e., the total amount of tris(<NUM>,<NUM>,<NUM>-dialkylaminoalkyl)phenol(s)). Examples include tris(<NUM>,<NUM>,<NUM>-dimethylaminoethyl)phenol, tris(<NUM>,<NUM>,<NUM>-diethylaminoethyl)phenol, tris(<NUM>,<NUM>,<NUM>-dimethylaminopropyl)phenol, tris(<NUM>,<NUM>,<NUM>-dimethylaminomethyl)phenol, tris(<NUM>,<NUM>,<NUM>-diethylaminopropyl)phenol. One preferred compound is tris(<NUM>,<NUM>,<NUM>-dimethylaminomethyl)phenol, which is available as ANCAMINE K54 from Evonik Industries, Essen, Germany.

Based on total weight of components c) - f), Part B includes <NUM> to <NUM> weight percent, preferably <NUM> to <NUM> weight percent, of at least one aliphatic tertiary <NUM>,<NUM>-bis(N,N-dialkylaminoalkyl)urea having from <NUM> to <NUM> carbon atoms (i.e., the total amount of aliphatic tertiary <NUM>,<NUM>-bis(N,N-dialkylaminoalkyl)urea(s)). In some embodiments, the aliphatic tertiary <NUM>,<NUM>-bis(N,N-dialkylaminoalkyl)urea is represented by formula (VI):
<CHM>
wherein each R<NUM> independently represents a divalent alkylene group having from <NUM> to <NUM> carbon atoms (e.g., methylene, ethylene, propylene, or butylene) and each R<NUM> independently represents an alkyl group having <NUM> to <NUM> carbon atoms (e.g., methyl, ethyl, propyl, or butyl). Examples include <NUM>,<NUM>-bis(<NUM>-dimethylaminobutyl)urea, <NUM>,<NUM>-bis(<NUM>-dimethylaminopropyl)urea, <NUM>,<NUM>-bis(<NUM>-dimethylaminoethyl)urea, <NUM>,<NUM>-bis(<NUM>-diethylaminobutyl)urea, <NUM>,<NUM>-bis(<NUM>-diethylaminopropyl)urea, and <NUM>,<NUM>-bis(<NUM>-diethylaminoethyl)urea. One preferred compound is <NUM>,<NUM>-bis(<NUM>-dimethylaminopropyl)urea, which is commercially available as VERSAMID EH-<NUM> from Huntsman Corporation, The Woodlands, Texas.

Based on total weight of components c) - f), Part B includes <NUM> to <NUM> weight percent, preferably <NUM> to <NUM> weight percent, of at least one an alkali metal nitrate or an alkaline earth nitrate (i.e., the total amount of alkali metal nitrate(s) and alkaline earth nitrate(s)). Examples of suitable alkali metal nitrates and alkaline earth nitrates include lithium nitrate, sodium nitrate, potassium nitrate, rubidium nitrate, cesium nitrate, beryllium nitrate, magnesium nitrate, calcium nitrate, strontium nitrate, barium nitrate. Of these, calcium nitrate, magnesium nitrate, sodium nitrate and potassium nitrate are often preferred.

The relative amounts of Part A and Part B typically will depend on the stoichiometry of the reactive species contained in each part. Often the molar ratio of active amine hydrogens to epoxy functionalities is greater than <NUM>; for example, from <NUM> to <NUM>, although other amounts may also be used. Selection of an appropriate ratio is within the capability of those skilled in the art.

Likewise, the ratio of the total combined weight of components a) and b) to the total combined weight of components c) - f) is in the range <NUM> to <NUM>, preferably <NUM> to <NUM>, and more preferably <NUM> to <NUM>.

While not required, Part A and/or Part B of two-part adhesive compositions according to the present disclosure preferably include one or more reflective pigments to enhance visualization of color. Examples of suitable reflective pigments include reflective metal flakes and powders (e.g., aluminum flakes and powders, silver flakes and powders, or copper flakes and powders) and white pigments (e.g., titanium dioxide, white pearl mica, porcelain mica, and zinc oxide). Any useful amount of reflective pigment may be selected. Typical amounts range from <NUM> to <NUM> weight percent of the total combined weight of Part A and Part B.

While not required, Part A and/or Part B of two-part adhesive compositions according to the present disclosure may further include optional additive(s) such as, for example, toughening agent(s), reactive diluent(s), rheology modifier(s), adhesion promoter(s), flame retardant(s), antioxidant(s), UV-protecting agent(s), non-reflective pigment and/or dyes, solvent, and combinations thereof. Any desired additive and amount may be used provided that it does not substantially interfere with the function of the visual working life indicator in the two-part epoxy adhesive composition. The optimum amounts of additive(s) typically will depend on the amounts and characteristics of the other ingredients present in the curing agent and/or the curable epoxy resin or in the total curable composition. Optimum amounts can be identified through routine experiments for example, by measuring the Brookfield viscosity of the composition, or the characteristics of the cured composition.

Exemplary toughening agents include butadiene-(acrylo)nitrile rubbers (BNR). BNRs are copolymers comprising repeating units derived from <NUM>,<NUM>-butadiene and/or <NUM>,<NUM>-butadiene and a nitrile-function containing olefin, such as e.g., <NUM>-propenenitrile (acrylonitrile).

Typical BNRs have a Brookfield viscosity (at <NUM>. ) of greater <NUM> and less than <NUM> Pa·s (pascal-seconds). Preferably the BNRs have a low viscosity (such as a Brookfield viscosity of from about <NUM><NUM> to about <NUM><NUM> Pa·s at <NUM>. The BNRs may be solid or liquid at ambient conditions. The butadiene acrylonitrile rubber is preferably amine-terminated. Suitable BNRs are commercially available, for example, under the trade designation HYCAR from Emerald Performance Materials, Vancouver, Washington. Core-shell rubber particles may also be used as tougheners. Such materials include core-shell rubber (CSR) particles dispersed in epoxy resin that is available as KANE ACE MX-<NUM> and KANE ACE MX-<NUM> from Kaneka Corporation, Tokyo, Japan.

Reactive diluent(s) may be added to control the flow characteristics of the two-part epoxy adhesive composition (e.g., before or after mixing Part A with Part B). Preferably, the diluent(s) is/are part of Part A. Suitable diluents can have at least one reactive terminal group and, preferably, a saturated or unsaturated cyclic backbone. Preferred reactive terminal groups include glycidoxy groups. Examples of suitable diluents include the diglycidyl ether of resorcinol, diglycidyl ether of cyclohexanedimethanol, diglycidyl ether of neopentyl glycol, diglycidyl ether of <NUM>,<NUM> butanediol, diglycidyl ether of <NUM>,<NUM> hexanediol, triglycidyl ether of trimethylolpropane. Commercially available reactive diluents include, for example, those available as HELOXY MODIFIER <NUM> from Hexion (Columbus, Ohio), EPODIL <NUM> from Air Products (Allentown, Pennsylvania), and EPILOX P13-<NUM> from Leuna-Harze (Leuna, Germany).

The optimum amounts of reactive diluent(s) typically will depend on the amounts and characteristics of the other ingredients present in the curing agent or in the total composition. Optimum amounts can be identified through routine experiments for example, by measuring the Brookfield viscosity of the curing agent.

Examples of rheology modifiers include, but are not limited to, silica-gels, Ca-silicates, phosphates, molybdates, fumed silica, clays such as bentonite or wollastonite, organoclays, aluminum-trihydrates, hollow-glass-microspheres; hollow-polymeric microspheres and calciumcarbonate. Commercially available rheology controlling agents, include, for example: a synthetic amorphous silica/calcium hydroxide mixture available as SHIELDEX AC5 from Grace Davison (Columbia, Maryland); CAB-O-SIL TS <NUM> (Cabot GmbH, Hanau, Germany); hydrophobic fumed silica-treated with polydimethyl-siloxane-polymer; glass-beads class IV (<NUM>-<NUM> microns) available as Micro-billes de verre <NUM>/<NUM> (CVP S. , France); glass bubbles K37 from <NUM> Company (Maplewood, Minnesota); amorphous silica available as MINSIL SF <NUM> (<NUM> Company); amorphous fumed silica available as APYRAL <NUM> ESF from Nabaltec GmbH (Schwandorf, Germany); treated fumed silica available as AEROSIL R. <NUM> from Evonik (Essen, Germany); potassium magnesium aluminum silicate; polydimethylsilane-surface-functionalized fumed silica; and combinations thereof.

Examples of flame retardants include, but are not limited to, aluminum trihydrates, or magnesium hydroxides, red phosphorous and ammonium polyphosphate. Examples of commercially available products include Portaflame SG40 (Ankerpoort, the Netherlands), aluminum trihydrate, epoxysilane-functionalized (<NUM> wt %) aluminum trihydrate.

Pigments may include inorganic or organic pigments. Typical examples include but are not limited to ferric oxide, brick dust, carbon black, titanium oxide, and combinations thereof.

Adhesion promoters for example, silane-containing compounds may or may not be included. An example of a commercially available adhesion promoter includes SILANE Z-<NUM> γ-glycidoxypropyltrimethoxysilane from DOW-Corning (Midland, Michigan).

When mixed to together Part A and Part B begin to react, forming a reaction product, and for a period of time (i.e., the working time) may be handled and used for bonding two substrates together. Once cure advances beyond a certain point (e.g., the working time) reliable bonding may not be achieved. The present disclosure provides two-part epoxy adhesives that, when mixed together, change color at a point during curing at or before working time thereby indicating to the user that the material should no longer be used for bonding.

Exemplary substrates include metals (e.g., steel, galvanized steel), graphite composite, fiberglass, thermoplastics, thermosets, glass, and rubber. Dissimilar substrates or similar substrates may be used.

Mixing may be active (e.g., using an agitator mixer) or passive (e.g., using a static mixing nozzle). In some preferred embodiments, the reaction product is flowable and curable under ambient conditions (e.g., <NUM>, <NUM> atm (<NUM> kPa)). Application of the mixed adhesive (i.e., typically a reaction product) to one or both of the substrates to be bonded may be done according to any of numerous well-known methods (e.g., a glue gun having a static mixing nozzle or a robotic dispenser).

In some preferred embodiments, the reaction product ultimately cures to the point that according to ASTM test method <NPL>", the reaction product has an overlap shear strength of at least <NUM> megapascals when bonding steel (e.g., including hot-dipped galvanized steel) specimens.

While the reaction product is capable of undergoing an abrupt color change visible to the unaided human eye that indicates curing has advanced to a predetermined working life endpoint of the reaction product, in some embodiments it may be desirable to compare color of the reaction product against a working time endpoint reference color standard.

The materials in Table <NUM>, below, were used in the Base Resin formulations in the Examples.

The materials in Table <NUM>, below, were used in the Accelerator Resin formulations in the Examples.

Base resins corresponding to the weight fractions listed in Table <NUM> were prepared by measuring the components into a DAC mixing cup. The components of Compositions BR-A and BR-B were mixed using a DAC <NUM> FVZ SPEEDMIXER (FlackTek Inc. , Landrum, South Carolina) at <NUM> revolutions per minute (RPM) for <NUM> minutes. The cup was briefly checked, and the mixing process was repeated until the mixture was homogeneous.

To make each composition in Table <NUM>, CAN, if required, was first dissolved into JD400 and JD230 with the aid of mild heating (<NUM>). MX257 and MX154 were weighed out according to the weight fractions in Table <NUM> and mixed using a DAC <NUM> FVZ SPEEDMIXER at <NUM> (RPM) for <NUM> minutes. CAN/amine solutions were further added into corresponding mixing cups and mixed at <NUM>,<NUM> RPM for another <NUM> minutes. After mixing, each mixture was placed into an oven held at a temperature of <NUM> for <NUM> minutes. They were then removed from the oven and allowed to cool to room temperature.

Accelerator resins corresponding to the weight fractions listed in Table <NUM> were prepared by first dissolving CAN into JD400 and JD230 with the aid of mild heating (<NUM>). This mixture was then poured into a DAC mixing cup. The remaining components of Compositions AR-A to AR-AA were mixed using a DAC <NUM> FVZ SPEEDMIXER. Each composition was first mixed at <NUM> rpm for <NUM> minutes. This mixing procedure was repeated for at least one other time until the mixture was homogeneous. R208 was added last and mixed at <NUM> rpm for <NUM> minutes after all other components were fully mixed.

For the following Examples and Comparative Examples, <NUM> parts by weight of a base resin composition and <NUM> part by weight of an accelerator resin were measured into DAC mixing cups, hand mixed for <NUM> minute, and speed mixed using a DAC <NUM> VAC-LR SPEEDMIXER (FlackTek Inc. ) for <NUM> minutes under vacuum program (<NUM> rpm, <NUM> sec, atmospheric pressure; <NUM> rpm, <NUM> sec, <NUM> mbar; <NUM> rpm, <NUM> sec, <NUM> mbar; <NUM> rpm, <NUM> sec, <NUM> mbar) to form an adhesive composition. For the following adhesive compositions, time to purple was recorded when the sample changed color from metallic silver to purple. The cured adhesive compositions were checked after <NUM> hours to see if the purple color persisted. Results for the color change experiments are collected in the Table below.

For the following Examples and Comparative Examples, <NUM> parts by weight of a base resin composition and <NUM> part by weight of an accelerator resin were measured into DAC mixing cups, hand mixed for <NUM> minute, and speed mixed using a DAC <NUM> VAC-LR SPEEDMIXER for <NUM> minutes under vacuum program (<NUM> RPM, <NUM> sec, atm; <NUM> RPM, <NUM> sec, <NUM> mbar; <NUM> RPM, <NUM> sec, <NUM> mbar; <NUM> rpm, <NUM> sec, <NUM> mbar) to form an adhesive composition. Hot-dipped galvanized steel laps (<NUM> inch by <NUM> inch by <NUM> inch) were cleaned, labelled, abraded, and assembled in a controlled temperature and humidity room (<NUM>, <NUM>% relative humidity) or at a lab bench. A thick line of the adhesive composition was applied over the abraded area and assembled to form a <NUM> wide bond line. All sides of the bond were checked for any excess adhesive to be cleaned off as well as for missing adhesive from any of the points of contact between the two laps. The assembled lap joints were clamped with small or medium sized binder clips and placed back in the specified conditioning space. After <NUM> hours from mixing the adhesive composition and forming the lap shear bonds, the bonds were pulled in shear mode on an Instron at a pull rate of <NUM>/min. The resulting cohesive failure and peak measured stress were recorded for analysis. Results for the overlap shear testing are collected in Table <NUM>, below.

For the following Examples and Comparative Examples, <NUM> parts by weight of a base resin composition and <NUM> part by weight of an accelerator resin were measured into DAC mixing cups, hand mixed for <NUM> minute, and speed mixed using a DAC <NUM> VAC-LR SPEEDMIXER for <NUM> minutes under vacuum program (<NUM> RPM, <NUM> sec, atm; <NUM> RPM, <NUM> sec, <NUM> mbar; <NUM> RPM, <NUM> sec, <NUM> mbar; <NUM> rpm, <NUM> sec, <NUM> mbar) to form an adhesive composition. To measure the LAB and delta E values of the samples, a small blob of adhesive, <NUM> in diameter, was dispensed onto a microscope slide, and a second slide was used to squish the adhesive, capturing it between the two slides.

A multi-angle spectrophotometer (Model MA9X, X0RITE Incorporated) was used to measure the L*a*b* color coordinates (<NUM> degree angle) of the adhesive between the two slides. The L*a*b* color coordinates were measured initially after the adhesive composition was formed, when the sample looked purple, and <NUM> hours after the sample was prepared. The ΔEL*a*b* values between these increments of time were calculated using the below equation: <MAT> L*a*b* and ΔEL*a*b* values for the Examples and Comparative Examples are reported in Table <NUM>, below.

Adhesive working life was checked using T-Peel testing. For the following Examples and Comparative Examples, <NUM> parts by weight of a base resin composition and <NUM> part by weight of an accelerator resin were measured into DAC mixing cups, hand mixed for <NUM> minute, and mixed using a DAC <NUM> VAC-LR SPEEDMIXER for <NUM> minutes (Step <NUM> - <NUM> revolutions per minute (rpm), <NUM> sec, <NUM><NUM> kPa; Step <NUM> - <NUM> rpm, <NUM> sec, <NUM> kPa; Step <NUM> - <NUM> rpm, <NUM> sec, <NUM> kPa; Step <NUM> - <NUM> rpm, <NUM> sec, <NUM> kPa) to form an adhesive composition. The start for the work time/open time began when the adhesive was dispensed on the metal T-Peel laps. A glass slide of the adhesive was also made to monitor and measure the LAB values at the specified times and track when the adhesive turned purple. The T-Peels were then assembled after the specified work time/open time ended, or in this case, when the adhesive turned purple per the LAB measurement. All sides of the T-Peel bond were checked for any excess adhesive to be cleaned off as well as for missing adhesive from any of the points of contact between the two laps. The assembled T-Peel bonds were placed back in a controlled temperature and humidity room (<NUM>, <NUM>% relative humidity). <NUM> hours after the adhesive compositions were mixed, the T-Peels were placed in an oven held at <NUM> for <NUM> minutes. After this process, the T-Peels were placed back in a controlled temperature and humidity room (<NUM>, <NUM>% relative humidity). and pulled after <NUM> days from assembly. The bonds were pulled on an Instron at a pull rate of <NUM> in/min. The resulting cohesive failure was recorded for analysis. Cohesive failure was determined following ISO <NUM>. <NUM>% cohesive failure was determined when no bare metal was found either side of the laps; <NUM>% cohesive failure is when bare metal is showing on all of one lap, and all other values is the portion of bare metal compared to the total combined bond area. Bare metal appears shiny; thin film adhesion does not count for cohesive failure. The adhesive was determined to have been bonded past the adhesive working life if the T-Peel sample failed with less than <NUM>% cohesive failure mode.

Claim 1:
A two-part epoxy adhesive comprising:
a Part A comprising, based on the total weight of components a) and b):
a) <NUM> to <NUM> weight percent of at least one curable epoxy resin;
b) <NUM> to <NUM> weight percent of at least one pH indicator dye exhibiting an abrupt color change from colorless to a color in the pH range of <NUM>-<NUM>; and
a Part B comprising, based on the total weight of components c) - f):
c) <NUM> to <NUM> weight percent of at least one aliphatic primary polyamine;
d) <NUM> to <NUM> weight percent of at least one tris(<NUM>,<NUM>,<NUM>-dialkylaminoalkyl)phenol having from <NUM> to <NUM> carbon atoms;
e) <NUM> to <NUM> weight percent of at least one aliphatic tertiary <NUM>,<NUM>-bis(N,N-dialkylaminoalkyl)urea having from <NUM> to <NUM> carbon atoms; and
f) <NUM> to <NUM> weight percent at least one of an alkali metal nitrate or an alkaline earth nitrate,
wherein the ratio of the total combined weight of components a) and b) to the total combined weight of components c) - f) is in the range <NUM> to <NUM>.