Elastomeric polyurethane polymers are commonly used as adhesives for bonding to a variety of materials. Such polyurethane polymers are often prepared to have terminal isocyanate groups, which, on exposure to atmospheric moisture, cure and bond to the surface or substrate. For example, U. S. Patent 4,780,520 describes such a moisture curable adhesive comprising a poiyurethane prepolymer having an isocyanate functionality between 2.3 and 3.0, and a catalyst of dimorpholinodiethyl ether (incorporated herein by reference).

One important use of polyurethane adhesives is in the automotive industry for adhering automotive parts made of glass, such as windshields, rear windows, to a substrate.

However, adhesion to a substrate is difficult when the substrate is painted.

Consequently, primers are typically applied to the painted surface before an adhesive is applied, in order for the adhesive to adhere to the painted substrate effectively. For example, U. S. Patent No. 4,525,511 describes a composition which is applied to a painted surface to improve the adhesion of an adhesive composition to the surface (incorporated herein by reference). In addition, U. S. Patent No. 4,857,366 (incorporated herein by reference) teaches applying a solution to a painted surface and thereafter contacting the painted surface with an uncured adhesive.

However, a problem with the art described above is that using a primer requires an added step and additional materials. It would be an advance in the art of adhesives to provide a primerless adhesive which could effectiveiy adhere to a painted substrate.

In one aspect, the present invention is a one-part moisture curable adhesive comprising an isocyanate-terminated prepolymer and an organic monosulfonic acid.

In a second aspect, the present invention is a method for adhering a glass surface to a coated surface, comprising contacting an adhesive to a glass surface and a coated surface such that the adhesive is disposed between the glass surface and the coated surface, the adhesive comprising an isocyanate-terminated prepolymer and an organic monosulfonic acid; and thereafter allowing the adhesive to cure so as to bind the glass to the coated surface.

In a third aspect, the present invention is a method of bonding glass into an automobile comprising contacting an adhesive to a windshield and a painted automobile surface such that the adhesive is disposed between the windshield and the painted surface.

The adhesive comprises an isocyanate-terminated prepolymer and an organic monosulfonic acid. Thereafter, the adhesive is allowed to cure so as to bind the windshield to the painted surface.

The adhesive compositions of the present invention have an acid incorporated directly therein. Such adhesives are useful in bonding glass to plastic, metal, fiberglass and composite substrates which may or may not be painted. The adhesive compositions of the present invention give unexpectedly high lap shear strength when no primer compositions have previously been applied to the substrate. In particular, the adhesives of the present invention enable adhesion to acid-resistant paints, including melamine carbamate and acrylic melamine, which have previously been especially difficult to adhere to.

The present invention is a one-part moisture curable adhesive composition comprising an isocyanate-terminated prepolymer and an acid. Suitable isocyanate- terminated prepolymers include any compound having an average isocyanate functionality of at least 2.0 and a molecular weight of at least 2,000. Preferably, the average isocyanate functionality of the prepolymer is at least 2.2, and is more preferably at least 2.4. Preferably the isocyanate functionality is no greater than 4.0, more preferably, no greater than 3.5 and most preferably, no greater than 3.0. Preferably, the weight average molecular weight of the prepolymer is at least 2,500, and is more preferably at least 3,000; and is preferably no greater than 40,000, even more preferably, no greater than 20,000, more preferably, no greater than 15,000, and is most preferably, no greater than 10,000.

The prepolymer may be prepared by any suitable method, such as by reacting an isocyanate-reactive compound containing at least two isocyanate-reactive groups with an excess over stoichiometry of a polyisocyanate under reaction conditions sufficient to form the corresponding prepolymer.

Suitable polyisocyanates for use in preparing the prepolymer include any aliphatic, cycloaliphatic, araliphatic, heterocyclic or aromatic polyisocyanate, or mixture thereof.

Preferably the polyisocyanates used have an average isocyanate functionality of at least 2.0 and an equivalent weight of at least 80. Preferably, the isocyanate functionality of the polyisocyanate is at least 2.0, more preferably at least 2.2, and is most preferably at least 2.4; and is preferably no greater than 4.0, more preferably no greater than 3.5, and is most preferably no greater than 3.0. Higher functionalities may also be used, but may cause

excessive cross-linking, and result in an adhesive which is too viscous to handle and apply easily, and can cause the cured adhesive to be too brittle. Preferably, the equivalent weight of the polyisocyanate is at least 100, more preferably at least 110, and is most preferably at least 120; and is preferably no greater than 300, more preferably no greater than 250, and is most preferably no greater than 200.

Preferred polyisocyanates include aromatic isocyanates, including diphenylmethane- 4,4'-diisocyanate and polymeric derivatives thereof, isophorone diisocyanate, tetramethylxylene diisocyanate, 1,6-hexamethylene diisocyante and polymeric derivatives thereof, bis (4-isocyanatocylohexyl) methane, and trimethyl hexamethylene diisocyanate.

More preferably the isocyanate is diphenyl methane diisocyanate.

To form the prepolymer, the isocyanate is reacted with an isocyanate-reactive compound. The term"isocyanate-reactive compound"as used herein inclues any organic compound having at least two isocyanate-reactive moieties, such as a compound containing an active hydrogen moiety, or an iminofunctional compound. Illustrative of active hydrogen moieties are-COOH,-OH,-NH2,-NH-,-CONH2,-SH, and-CONH-.

Preferable isocyanate-reactive compounds include polyamines and polyols, particularly polyether polyols. In the preferred embodiments, the polyamines will form urea linkages on the prepolymer, while the polyols will form urethane linkages. Thus, the resulting prepolymer can have urea linkages, urethane linkages, or a combination thereof.

The choice of the linkages in the prepolymer depends upon the type of substrate.

That is, bonding will be enhanced between a glass surface and a particular substrate if certain linkages are present in the adhesive composition. For example, if the substrate is coated with melamine carbamate, then either urea linkages, urethane linkages, or a combination thereof can be present in the prepolymer. On the other hand, if the substrate is coated with acrylic melamine, then both urea linkages and urethane linkages are preferably present in the prepolymer to obtain optimal adhesion between a glass surface and the acrylic melamine.

Suitable polyamines useful in the preparation of the prepolymers containing urea linkages include any primary or secondary, aromatic or alkyl, amines or diamines. Preferred polyamines include oligomeric diamines such as polypropylene ethers terminated with aromatic amines. Examples of commercially available polyamines include those marketed under the VERSALINK brand name, available from Air Products and Chemicals, Inc.

Suitable polyols useful in the preparation of the prepolymers include, for example, polyether polyols. Polyether polyols are well-known in the art and include, for example, polyoxyethylene, polyoxypropylene, polyoxybutylene, and polytetramethylene ether diols and triols.

Preferably, the functionality of the isocyanate-reactive compound is at least 1.5, more preferably at least 1.8, and most preferably at least 2.0; and is preferably no greater than 4.0, more preferably no greater than 3.5, and most preferably no greater than 3.0.

Preferably, the equivalent weight of the isocyanate-reactive compound is at least 200, more preferably at least 500, and is more preferably at least 1,000; and is preferably no greater than 5,000, more preferably no greater than 3,000, and is most preferably no greater than 2,500.

The prepolymer may be prepared by any suitable method, such as bulk polymerization and solution polymerization. The reaction to prepare the prepolymer is carried out under anhydrous conditions, preferably under an inert atmosphere such as a nitrogen blanket, to prevent cross-linking of the isocyanate groups by atmospheric moisture.

The reaction is preferably carried out at a temperature between 0°C and 150°C, more preferably between 25°C and 80°C, until the residual isocyanate content determined by titration of a sample is very close to the desired theoretical value. The weight of the isocyanate groups in the prepolymer is preferably in the range of 0.1 percent to 10 percent of the total weight of the prepolymer, more preferably in the range of 0.5 percent to 5.0 percent and most preferably in the range of 1.0 percent to 2.0 percent.

Preferably, the prepolymer is present in an amount of 20 percent by weight or greater based on the weight of the adhesive, even more preferably 40 percent by weight or greater and most preferably 70 percent by weight or greater. More preferably the prepolymer is present in an amount of 99.8 percent by weight or less based on the weight of the adhesive and most preferably 85 percent by weight or less.

The reactions to prepare the prepolymer may be carried out in the presence of urethane catalysts. Examples of such include the stannous salts of carboxylic acids, such as stannous octoate, stannous oleate, stannous acetate, and stannous laurate. Also, dialkyl tin dicarboxylates such as dibutyl tin dilaurate and dibutyl tin diacetate are known in the art as urethane catalysts, as are tertiary amines and tin mercaptides. Preferably, the reaction to prepare the prepolymer is catalyzed by stannous octoate. The amount of catalyst employed

is generally between 0.005 and 5 percent by weight of the mixture catalyzed, depending on the nature of the isocyanate.

For formulating adhesive compositions of the present invention, the prepolymer is blended with an acid to improve adhesion to the substrate to which the adhesive is applied.

Preferably, the adhesive compositions also include catalysts, fillers and other additives, as will be described hereinbelow. The prepolymer and acid can be blended before these other materials are added, or the acid can be blended with the prepolymer at the same time that the catalysts, fillers and other additives are blended into the prepolymer.

Preferably, the amount of acid in the adhesive composition is a minimum of 0.1 weight percent (by weight of the adhesive composition), more preferably a minimum of 0.25 weight percent and even more preferably a minimum of 0.4 weight percent. Preferably the amount of acid in the adhesive compositions is a maximum concentration of 10 weight percent, by weight of the adhesive, more preferably a maximum of 8 weight percent, and even more preferably a maximum of 6 weight percent.

Suitable acids preferably have a pKa of 2.0 or less, more preferably 1.0 or less, and even more preferably 0.7 or less. Preferably, the pKa of the acid is 0.5 or greater.

Acids suitable for the adhesive compositions of the present invention include organic monosulfonic acids such as paratoluenesulfonic acid, dodecylbenzenesulfonic acid, and dinonylnapthalenesulfonic acid. The superiority of such acids may be attributed to their solubility in the adhesive composition, as well as to their acid strength. An acid is considered to be soluble in the prepolymer when there is no visual phase separation when a mixture of acid and prepolymer is allowed to stand overnight. Organic sulfonic acids are additionally desirable from the point of view of their relative lack of corrosiveness on surfaces to which they are applied.

For the formulation of an adhesive composition according to the present invention, the acid is preferably in liquid form so as to ease dispersion throughout the prepolymer.

However, if desired, an acid in non-liquid form can first be dissolved in a solvent or a plasticizer, and subsequently dispersed throughout the adhesive composition. Suitable solvents are those organic liquids which are volatile at room temperature, such as toluene or methylethylketone, for example.

The formulation may further comprise a catalyst which promotes the curing of isocyanate moieties by atmospheric moisture. Such catalysts are well known in the art.

Preferred catalyst include organo tin compounds such as dialkyl tin dimercaptides, tin

carboxylates, organo silicontitinates, alkyl titinates, bis carboxylates, tertiary amines, tin mercaptides, napthenates or alkanoate salts of lead, cobalt manganese, bismuth or iron, dimorpholinodialkyl ethers and di ((dialkylmorpholino) alkyl) ether. Catalysts useful are well known to those skilled in the art and many examples may be found, for example, in the Polyurethane Handbook, Chapter 3, §3.4.1 on pages 90-95; and in Polyurethane Chemistry And Technology, in Chapter IV, pages 129-217.

Preferred organo tin compounds include tin (li) salt of organic carboxylic acids, such as tin (II) diacetate, tin (II) dioctanoate, tin (II) diethylhexanoate and tin (II) dilaurate; and dialkyl tin (IV) salts of organic carboxylic acids, (dialkyl tin dicaboxylates); stannous salts of carboxylic acids, such as stannous octoate, stannous oleate, stannous acetate, and stannous laurate.

The dialkyl tin dicarboxylates preferably correspond to the formula (R2OC (O)) 2-Sn- (R3) 2 wherein R2 and R3 are independently in each occurrence a C1, 0 alkyl, preferably a C1 3 alkyl and most preferably a methyl. Dialkyl tin dicarboxylates with lower total carbon atoms are preferred as there are more active catalyst species in the compositions. The preferred dialkyl dicarboxylates include 1,1-dimethyl tin dilaurate, dibutyl tin dilaurate, 1,1-dibutyl tin diacetate, dioctyl tin diacetate 1,1-dimethyl tin dimaleate and dibutyl tin dimaleate. A preferred dimorpholinodialkyl ether is dimorpholinodiethyl ether. A preferred di ((dialkylmorpholino) alkyl) ether is (di- (2- (3, 5dimethylmorpholino) ethyl) ether).

In one embodiment a preferred catalyst composition is an active hydrogen free, glycol acid salt of a tertiary amine and an organometallic compound. Preferred tertiary amines include triethylene diamine and 1,8 diazabicyclo [5,4,0] undecene. The organometallic compound can be any organometallic compound which is known as a catalyst in polyurethane reactions. Preferred organometallic compounds include dialkyl tin dicarboxylates. A preferred catalyst is an active hydrogen free glycol salt of triethylenediamine and 1,1-dibutyl tin diacetate. The glycol salt of triethyienediamine and 1,1-dibutyl tin diacetate is available from Air Products as DABCO DC2 catalyst. The active hydrogen moieties of the catalyst are contacted with a compound which reacts with the active hydrogen moieties. In a preferred embodiment, active hydrogen moieties are reacted with isocyanate moieties. This catalyst composition may be used in an amount of 0.05 percent by weight or greater based on the weight of the adhesive and preferably 0.2 percent by weight or greater. This catalyst may preferably be used in an amount of 4.0 percent by weight or less, based on the weight of the adhesive, more preferably 1.0 percent by weight and most preferably 0.4 percent by weight or less. This adhesive composition preferably

contains a second catalyst which has good stability in the absence of atmospheric moisture, but which has a rapid cure rate in the presence of atmospheric moisture, such as an organo tin catalyst, a dimorpholinodialkyl ether, a di ( (dialkylmorpholino) alkyl) ether or a mixture thereof.

The dimorpholinodialkyl ether or di ((dialkylmorpholino) alkyl) ether, when employed, are preferably employed in an amount, based on the weight of the adhesive, of 0.01 percent by weight or greater based on the adhesive, more preferably 0.05 percent by weight or greater, even more preferably 0.1 percent by weight or greater, and most preferably 0.2 percent by weight or greater; and 2.0 percent by weight or less, more preferably 1.75 percent by weight or less, even more preferably 1.0 percent by weight or less, and most preferably 0.4 percent by weight or less. The organo tin catalyst is present in an amount of 60 parts per million or greater based on the weight of the adhesive, and more preferably 120 parts by million or greater. The organo tin catalyst is present in an amount of 1.0 percent or less based on the weight of the adhesive, more preferably 0.5 percent by weight or less, and most preferably 0.1 percent by weight or less.

In another embodiment of the invention the catalyst is a mixture of an organo tin catalyst and a dimorpholinodialkyl ether or a di ( (dialkylmorpholino) alkyl) ether. Most preferably the catalyst is a dimorpholinodiethyl ether, dibutyl tin dilaurate, stannous octoate, bismuth octoate or a mixture thereof.

For formulating adhesive compositions, the prepolymer and/or catalyst composition is preferably combined with fillers and additives known in the prior art for use in elastomeric compositions. By the addition of such materials, physical properties such as viscosity, flow rate, sag, can be modified. However, to prevent premature hydrolysis of the moisture sensitive groups of the polymer, the fillers and additives should be thoroughly dried before admixture therewith. Exemplary filler materials and additives include materials such as carbon black, titanium dioxide, clays, calcium carbonate, surface treated silicas, ultraviolet stabilizers, antioxidants. This list, however, is not comprehensive and is given merely as illustrative. The fillers are preferably present in an amount of 5 percent by weight or greater based on the amount of the adhesive, more preferably 10 percent or greater, and even more preferably 15 percent or greater. The fillers are preferably present in an amount of 60 percent by weight or less based on the weight of the adhesive, more preferably 45 percent by weight or less and even more preferably 30 percent by weight or less.

The adhesive composition also preferably contains one or more plasticizers or solvents to modify rheological properties to a desired consistency. Such materials should be free of water, inert to isocyanate groups, and compatible with the polymer. Such material may be added to the reaction mixtures for preparing the prepolymer, or to the mixture for preparing the final adhesive composition. Suitable plasticizers and solvents are well-known in the art and include dioctyl phthalate, dibutyl phthalate, a partially hydrogenated terpene commercially available as"HB-40", trioctyl phosphate, epoxy plasticizers, toluene-sulfamide, chloroparaffins, adipic acid esters, castor oil, xylene, 1-methyl-2-pyrrolidinone and toluene.

The amount of plasticizer used is that amount sufficient to give the desired rheological properties and disperse the components in the adhesive composition. Preferably the plasticizer is present in an amount of 0 percent by weight or greater, more preferably 5 percent by weight or greater and most preferably 10 percent by weight or greater. The plasticizer is preferably present in an amount of 60 percent by weight or less, more preferably 40 percent by weight or less and most preferably 20 percent by weight or less.

It is desirable to have one or more adhesion promoters present in some form.

Suitable adhesion promoters include silanes. Preferred silanes include mercapto-silane or an amino-silane, and more preferably, the silane is a mercapto-trialkoxy-silane or an amino- trialkoxy silane. The silane can be pre-reacted with the isocyanate-reactive compound, or the silane can be blended with the prepolymer or reacted into the backbone of the prepolymer. The amount of silane present is that amount which enhances the adhesion of the adhesive to the painted surface without the need for a primer. The amount of silane is preferably 0.1 percent by weight or greater based on the weight of the adhesive and most preferably, 1.0 percent by weight or greater. The amount of silane used is preferably 10 percent by weight or less and most preferably, 2.0 percent by weight or less.

The adhesive composition of this invention may be formulated by blending the components together using means well-known in the art. Generally the components are blended in a suitable mixer. Such blending is preferably conducted in an inert atmosphere and in the absence of atmospheric moisture to prevent premature reaction. It may be advantageous to add any plasticizers to the reaction mixture for preparing the isocyanate containing prepolymer so that such mixture may be easily mixed and handled. Alternatively, the plasticizers can be added during blending of all the components. Once the adhesive composition is formulated, it is packaged in a suitable container such that it is protected from atmospheric moisture. Contact with atmospheric moisture could result in premature cross- linking of the prepolymer containing isocyanate groups.

The adhesive composition is applied to a first substrate and is thereafter contacted with a second substrate such that the adhesive composition is disposed between the two substrates. Thereafter the adhesive is exposed to curing conditions, such as moisture, heat, or a chain extender/cross-linker. in a preferred embodiment the first substrate is glass and the second substrate is a plastic, metal, fiberglass or composite substrate which may optionally be painted. This method does not require the use of a primer or a priming step.

This method is especially effective for substrates painted with an acid resistant paint such as automobile bodies. In preferred embodiments, the surfaces to which the adhesive is applied are cleaned prior to application, see for example U. S. Patents 4,525,511,3,707,521 and 3,779,794.

Generally, the adhesives of the invention are applied at ambient temperature in the presence of atmospheric moisture. Exposure to atmospheric moisture is sufficient to result in curing of the adhesive. Curing may be further accelerated by applying heat to the adhesive by means of convection heat, or microwave heating. Preferably the adhesive of the invention is formulated to provide a working time of 6 minutes or greater more preferably 10 minutes or greater. Preferably the working time is 15 minutes or less and more preferably 12 minutes or less.

Molecular weights as described herein are determined according to the following procedure: determined using the Waters Model 590 Gel Permeation Chromatograph. This unit is connected to a multiwavelength detector and a differential refractometer to measure the elution volume. A column of styrogel is used for the size exclusion and it can determine molecular weights from 250 to 50,000. The molecular weight of the prepolymer is then determined by measuring the elution volume through this column using tetrahydrofuran as the eluting solvent. The molecular weight is then calculated from a calibration curve of molecular weight vs. elution volume obtained from a polystyrene polyethylene glycol column.

The quoted molecular weights are weight average molecular weights unless otherwise specified.

In reference to polyurethane prepolymers, average isocyanate functionality is defined as being the average number of isocyanate groups per molecule, which can be determined using the functionality of the raw materials and the molar ratios of the raw materials.

Functionality of the raw material is generally disclosed by the raw material supplier. It can be determined empirically by means of titrating the polyol or isocyanate to determine the

average number functional group per molecule. One skilled in the art knows how to determine the functionality based on data developed by titration.

The theoretical average molecular weight of the prepolymer is calculated as equal to the average isocyanate functionality times the isocyanate equivalent weight of the prepolymer.

The following are tests used for the prepared adhesives: A 6.3 mm (width) x 6.3 mm (height) x 76.2 mm (length) size adhesive bead is placed on 101.6 mm x 101.6 mm piece of an acid resistant paint panel and the assembly is cured for a specific time in the condition of 23°C and 50 percent relative humidity. The cured bead is then cut with a razor blade through to the painted surface at 45 angle while pulling back the end of the bead at 180 angle. Notches are cut every 3 mm on the painted surface. The degree of adhesion is evaluated as adhesive failure (AF) and/or cohesive failure (CF). In case of adhesive failure, the cured bead can be separated from the painted surface, while in cohesive failure, separation occurs within the adhesive bead as a result of cutting and pulling. The tested paint substrate can be used as supplied, or treated by wiping with isopropanol (IPA) or naphtha (NP). For the adhesive of the invention, adhesion of a adhesive develops sooner to the treated substrate than to the untreated one.

The following examples are not meant to limit the scope of the invention. All parts and percentages are by weight unless otherwise indicated.

Examples For each of the following examples, an isocyanate-terminated prepolymer was prepared by reacting 386 parts of a polyoxypropylene diol having an average molecular weight of 2000, and 559 parts of polyoxypropylene triol having an average molecular weight of 4500, in a 2-liter resin kettle equipped with a mechanical agitator, a nitrogen inlet adapter and a thermometer. Under nitrogen purge, the mixture was heated to 50°C. 170 grams of molten diphenylene methane 4,4'diisocyanate were added to the mixture and the mixture thoroughly mixed. The resulting isocyanate-terminated prepolymer has an isocyanate content of 1.47 percent by weight.

Comparative Example 930 parts of the isocyanate terminated prepolymer were first degassed under agitation in a planetary mixer for 30 minutes. Then, to the above prepolymer 420 parts of dry carbon black were added and mixed for 25 minutes. Finally, 50 parts of a mixture of 6 parts of stannous octoate, 24 parts of bismuth octoate and 56 parts of phthalate plasticizer, were added and mixed for 10 minutes.

Example 1 930 parts of the isocyanate terminated prepolymer were first degassed under agitation in a planetary mixer for 30 minutes. 420 parts of dry carbon black were then added and mixed for 25 minutes. Finally, 65 grams of a mixture containing 25 parts of para-toluene sulfonic acid, 40 grams of y-butyro lactone, and 2 parts of dibutyl tin dilaurate were added and mixed for 10 minutes.

Example 2 930 parts of the isocyanate terminated prepolymer were first degassed under agitation in a planetary mixer for 30 minutes. 420 parts of dry carbon black were then added and mixed for 25 minutes. Finally, 50 parts of a mixture of 14 parts of dodecylbenzene sulfonic acid, 3 parts of stannous octoate, 12 parts of bismuth octoate and 28 parts of phthalate plasticizer, were added and mixed for 10 minutes.

Example 3 An isocyanate-terminated prepolymer having polyurea linkages was prepared by reacting 355 parts of a polyoxypropylene diol having an average molecular weight of 2000, and 514 parts of polyoxypropylene triol having an average molecular weight of 4500, and 87 parts of Versalink P-1000G having an average molecular weight of 1000, in a 2-liter resin kettle equipped with a mechanical agitator, a nitrogen inlet adapter and a thermometer.

Under nitrogen purge, the mixture is heated to 50°C. 170 grams of molten diphenylene methane 4,4'diisocyanate and 524 g of Palatinol 711 p plasticizer were added to the mixture and the mixture thoroughly mixed. The resulting isocyanate-terminated prepolymer has an isocyanate content of 1.47 by weight.

930 Parts of the isocyanate terminated prepolymer having polyurea linkages, prepared previously, were first degassed under agitation in a planetary mixer for 30 minutes.

420 Parts of dry carbon black were then added and mixed for 25 minutes. Finally, 50 parts of a mixture of 14 parts of dodecylbenzene sulfonic acid, 3 parts of stannous octoate, 12

parts of bismuth octoate and 28 parts of phthalate plasticizer, were added and mixed for 10 minutes.

In all of the examples, except the Comparative Example, adhesive compositions were formulated by blending the acid indicated into the prepolymer. A 6.3 mm (width) by 6.3 mm (height) by 76.2 mm (length) size bead of adhesive was placed on 101.6 mm by 101.6 mm piece of melamine carbamate cured paint on a 0.8 to 1.0 mil (0.020mm to 0.025mm) thick film panel, and allowed to cure for 72 hours at 23° C and 50 percent humidity.

Adhesion of the cured adhesive was determined by cutting the edge of the cured beads and observing the mode of bond failure. For the Comparative Example, no acid was added to the prepolymer. Table I reports the failure modes.

Table I Adhesive Mode of Failure Comparative Example (no acid) 0% CF Example 1 100% CF Example 2 100% CF Example 3 100% CF The mode of failure can be described as either adhesive failure (AF) or cohesive failure (CF). AF indicates that the bead is separated from the painted surface, which is undesirable. CF indicates that the separation occurs within the bead, as a result of cutting and pulling. Thus, in the Comparative Example, no cohesive failure occurred; therefore, all of the failure was due to adhesive failure, indicating that the bead did not adhere to the painted surface. By contrast, the formulations of the present invention both show 100 percent CF, which means that all of the failure is within the bead itself rather than resulting from separation from the substrate.