Rubber-Modified Epoxy Adhesive Compositions

1. Field Of The Invention

This invention relates to epoxy adhesive compositions. More particularly, the invention relates to epoxy adhesive compositions which are suitable for use in bonding rigid fiber-reinforced plastic composite materials to other materials of like or dissimilar composition, including metal materials which are coated with corrosion protection coatings.

The use of rigid fiber-reinforced plastic composite materials in the form of sheet molding compounds (commonly referred to as SMC) as an alternative for steel automotive body panels in an effort to reduce weight and corrosion susceptibility is increasing. Sheet molding compound can be described as reinforced resin compositions typically comprising unsaturated liquid polyester resin, ethylenically-unsaturated monomer, low profile thermoplastic resin, inert filler, magnesium oxide or other maturation aids and shaped or milled glass fiber reinforcement. The molding of the SMC under heat and pressure results in the formation of a rigid, self-supporting, thermoset, fiber-reinforced SMC part.

The rigid SMC parts are typically bonded to other rigid SMC parts, which may or may not originate from the same manufacturer, or to metal panels and structural members, in lieu of mechanical attachments. Reactive multi-component thermoset adhesives such as peroxide-cured unsaturated polyester, polyol polyurethane and epoxy adhesive compositions have typically been the adhesives most commonly employed in bonding these fiber reinforced plastic composite SMC materials. However, there are no adhesives whose performance is sufficiently stellar as to preclude the further development of structural adhesives for bonding reinforced plastic composite SMC materials.

Moreover, automotive adhesives must perform over a wide range of operating conditions, including temperatures on the order of 200.degree. C. which are encountered during paint bake cycles to below 0.degree. C. in cold weather climates, and environmental conditions such as exposure, oils, greases, humidity and high salinity. For example, one specification requires that SMC/metal adhesives withstand a torsional shear impact loading of 17 in. lb. per sq. in. at -29.8.degree. C. Standard epoxy adhesives cured with polyamidoamine curatives barely meet this minimum value, with the failure mode being adhesive to the metal surface. If the metal is coated with an organic corrosion inhibiting coating, the failure is typically between the corrosion inhibiting coating and the metal, rather than the desired failure within the SMC adherent. Another specification commonly required of automotive adhesives is that they be capable of bonding SMC parts without the need to further prepare or treat the SMC surface prior to bonding.

Attempts to improve the low temperature impact performance of SMC/metal assemblies by increasing the flexibility of the adhesive have resulted in impact values per se. However, the failure mode remained adhesive at the metal interface or between the corrosion-inhibiting coating and the metal, when the metal was so coated. In addition, the flexibilized epoxy compositions suffer from a decrease in cohesive strength at higher temperature. These failures have been alleviated in large part by curing the epoxy resin with a combination of amine curing agents that include polyamines, polyamides and amine-terminated acrylonitrile-butadiene rubbers. However, the cure rate and hence the rate of initial lap shear strength development of these adhesive compositions is less than desired.

The present invention is directed to epoxy adhesive compositions which in addition to possessing properties desired for automotive assembly line bonding of rigid SMC part as, for example, high impact strength, environmental resistance, high and low temperature performance and flexibility, also exhibit an increased rate of cure and hence an increased rate of initial lap shear strength development. In addition, the epoxy adhesive compositions of the invention eliminate the need for surface preparation prior to bonding. Consequently, the adhesive compositions of the invention allow for faster production rates in the adhesive bonding of SMC parts.

In another aspect of the invention there are provided epoxy adhesive composition which effectively bond SMC to metal substrates, particularly phosphatized metals, but also to other SMC parts. In a second aspect of the invention there is provided a method of bonding SMC to substrates selected from the group consisting of SMC and metals, including metals coated with organic corrosion inhibiting compositions.

These and other objects of the invention are provided by an adhesive compositions comprised of a Component A comprising:

a) about 50 to 95, preferably about 60 to 90, parts by weight of at least one epoxy resin having more than one epoxy group, and

b) about 5 to 50, preferably 10 to 20, parts by weight of acrylate or methacrylate ester of aliphatic polyhydric alcohol, and as a curing agent a Component B comprising

c) about 10 to 95, preferably about 12 to 35, parts by weight of an amine functional (i.e., an amine terminal) butadiene-acrylonitrile rubber; and

d) about 5 to 90, preferably 35 to 60, parts by weight of at least one polyamidoamine, derived from dimerized fatty acids of 12-28 carbon atoms and aliphatic amines, said polyamidoamine having an amine (mg KOH/gram resin) of at least 500.

In a preferred embodiment, there is included in Component A about 1.0 to 5.0 parts of polyol per 100 parts Component A and in Component B about 100 to 400 parts of silica per 100 parts of polyol.

In yet another preferred embodiment of the invention Component B further includes about 50 to 70, about 55 to 60, weight percent of at least one hydroxy-substituted aromatic hydrocarbon based on the total weight of the hydroxy-substituted aromatic compound and polyamidoamine hardener.

The epoxy compounds which are suitable for use in the practice of the invention can be any monomeric or polymeric compound or mixtures of compounds having an epoxy equivalency greater than one, that is, wherein the average number of epoxy groups per molecule is greater than one, with monomeric epoxides having two epoxy groups being currently preferred. Epoxy compounds are well-known. See, for example, U.S. Pat. Nos. 2,467,171; 2,615,007; 2,716,123; 3,030,336; and 3,053,855. Useful epoxy compounds include the polyglycidyl ethers of polyhydric polyols, such as ethylene glycol, triethylene glycol, 1,2-propylene glycol, 1,5-pentanediol, 1,2,6-hexanetriol, glycerol and 2,2-bis(4-hydroxy cyclohexyl) propane; the polyglycidyl esters of aliphatic or aromatic polycarboxylic acids, such as oxalic acid, succinic acid, glutaric acid terephthalic acid, 2,6-naphthalene dicarboxylic acid and dimerized linoleic acid; the polyglycidyl ethers of polyphenols, such as bis-phenol A, 1,1-bis(4-hydroxyphenyl) ethane, 1,1-bis)4-hydroxyphenyl) isobutane, and 1,5-dihydroxynaphthalene; and novolak resins; with the aromatic and cycloaliphatic polyglycidyl compounds being currently preferred. Generally the preferred epoxy compounds are resins having an epoxide equivalent weight of about 100 to 2000, preferably about 110 to 500.

The acrylate and methacrylate esters of the invention contain more than one acrylate or methacrylate group and are the acrylic and methacrylic esters of aliphatic polyhydric alcohols such as, for example, the di- and polyacrylates and the di- and polymethacrylates of alkylene glycols, alkoxylene glycols, alicyclic glycols and higher polyols, such as ethylene glycol, triethylene glycol, tetraethylene glycol, tetramethylene glycol, hexanediol, trimethylolethane, trimethylolpropane, pentaerythritol, dipentaerythritol, tripentaerythritol and the like, or mixtures of these with each other or with their partially esterified analogs.

Illustrative of these esters are trimethylolpropane triacrylate, trimethylolethane triacrylate, trimethylolpropane timethyacrylate, trimethylolethane trimethacrylate, tetramethylene glycol dimethacrylate, ethylene glycol dimethacrylate, triethylene glycol dimethacrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate, 1,6-hexanediol diacrylate, 1,6-hexanediol dimethacrylate, dipentaerythritol tetraacrylate, dipentaerythritol pentaacrylate, and the like. Particularly preferred esters are 1,6-hexanediol diacrylate, trimethylolpropane triacrylate, pentaerythritol triacrylate, and pentaerythritol tetraacrylate.

Also suitable are the acrylate or methacrylate esters of epoxide resins, wherein epoxide resins as used herein are considered to be polyols. The epoxide resins useful in reacting with acrylic or methacrylic acid are those epoxide resins described hereinbefore. The procedures for preparing these acrylate and methacrylate esters of epoxide resins is described in U.S. Pat. No. 3,377,406 which is hereby incorporated by reference.

Component B is the curing agent component of the adhesion composition and is comprised of a combination of an amine-terminal butadiene-acrylonitrile rubber, and at least one polyamidoamine. The presence of each of these hardeners in the defined proportions is essential to the invention otherwise the properties and performance characteristics desired in the final adhesive composition are not achieved and maintained. For example, if an amine-terminal butadiene-acrylonitrile rubber curing agent is not included, not only does the adhesive lack sufficient flexibility at room temperature but the bond loses, adhesive strength. Also, the amine-terminal acrylonitrile-butadiene rubber curing agent enables SMC parts to be bonded without the necessity of preparing the surfaces prior to bonding. It has believed that the highly polar C N groups of this liquid rubber curing agent "grab" even unprepared SMC surfaces with a tenacity not experienced with other curing agents. The polyamine curing agent, on the other hand, is included in the adhesive composition so as to meet high temperature lap shear strength specifications. Lastly, the principle role of the polyamide curing agent in the adhesive composition is to impart flexibility.

The amine-terminated butadiene-acrylonitrile curing agents are liquid elastomers that normally contain an acrylonitrile content of about 5 to 40, preferably 12 to 26, percent by weight and a butadiene content of 95 to 60, preferably 78 to 74, percent by weight, based on the liquid butadiene-acrylonitrile copolymer. Generally, the amine-terminated butadiene-acrylonitrile copolymers contain an average from about 1.7 to about 3 primary and/or secondary amine groups per molecule, preferably about 1.7 to about 2.3 primary or secondary amine groups per molecule and may have Brookfield viscosities measured at 27.degree. C. of about 500 cps to about 2,500,000 cps, preferably about 500 cps to about 500,000 cps. The amine-terminated butadiene-acrylonitrile copolymers of the invention are well known articles of commerce and can be prepared by reacting a carboxyl-terminated or ester-terminated butadiene-acrylonitrile copolymer with at least one aliphatic, alicyclic or heterocyclic amine containing at least two primary and/or secondary amine groups. The amine-terminated liquid copolymers may also be prepared by reacting an acid chloride-terminated butadiene-acrylonitrile with at least one of the aforementioned amines. The methods of preparation are described in more detail in, for instance, U.S. Pat. No. 4,129,670, hereby incorporated by reference.

The polyamidoamine hardeners of the adhesive composition of the invention are typically reaction products of aliphatic amines with dimerized fatty acids of 12 to 28 carbon atoms having an amine value (mg KOH/gram resin) of at least 500, preferably at least 550 up to about 800. These polyamidoamines are well known and commerically available. A typical example is Genamid 2000.

When it is desired to enhance the non-sag characteristics of the adhesive compositions of the invention the compositions may optionally include a mixture of a polyol and activated silica such as fuming silica.

It has been found that the activated silica combines with the polyol to provide hydrogen-bonding that is reactive with the amine groups of the curing agents of Component B to provide the thixotropic properties necessary to enhance non-sag characteristics of the adhesive composition.

The polyols that may be included in combination with the activated silica are well-known articles of commerce and are commonly referred to as polyalkylene glycols or polyalkylene ether glycols. They may be represented by the formula HO(RO).sub.n H, in which R is an alkylene radical and n is at least 2. The alkylene radical can be a single chain or can consist of two or more alkylene chains separated from each other by an ether oxygen atom. Preferred poly(alkylene oxide) polyols have from 1 to 9, preferably 1 to 6 carbon atoms in the alkylene chain separating each pair of oxygen atoms and have a number average molecular weight in the range from about 100 to about 4000, preferably about 100 to about 2500. Not all the alkylene units need be the same. Poly(alkylene oxide) polyols formed by the copolymerization or condensation of mixtures of different cyclic ethers, glycols, or glycols and cyclic ethers can be used; as can poly(alkylene oxide) polyols derived from cyclic ethers such as dioxolane, which affords a polyol having the formula HO(CH.sub.2 OCH.sub.2 CH.sub.2 O).sub.n H where n is greater than 1. The alkylene unit can be a straight or a branched chain, as a poly(propylene oxide) polyol. In the case where the alkylene unit is ethylene, it can be advantageous to incorporate the unit into a copolymer, for example, as a copolymer of ethylene oxide and propylene oxide, with up to 80 percent of such copolymer comprising ethylene oxide. Representative poly(alkylene oxide) polyols include poly(ethylene oxide) polyols, poly (propylene oxide) polyols, poly(tetramethylene oxide) polyols, poly(nonamethylene oxide) polyols, poly(oxymethylene-ethylene) oxide) polyols, poly(ethylene oxide-propylene oxide copolymer) polyols, and poly(pentaerythritol-ethylene oxide) polyols. Thus the poly(alkylene oxide) polyols will generally have from 2 to 6 hydroxyl groups, with polyols having 2 hydroxyl groups being currently preferred. Particularly preferred is diethylene glycol.

When employed, the mixture of activated silica and polyol generally form part of Component A. The polyol is usually added in an amount of 1.0 to 5.0 parts per 100 parts of Component A and the activated silica in an amount of 100 to 400 parts per 100 parts of the polyol.

Although optional, it is preferred to include along with the polyamine hardener a hydroxy, ring-substituted, aromatic hydrocarbon such as phenol, polyphenol and the like. It has been found that the hydroxy-substituted aromatic compound increases the cure rate of the epoxy resin of Component A by bringing it in line with the cure rate of the polyacrylate or polymethacrylate ester of Component A. The mechanism by which it does so is not known for certain but it is believed that the hydroxy-substituted aromatic hydrocarbon reacts preferentially and more readily with the epoxy resin, opening its epoxy ring and readying it for reaction with the amine hardeners present. This reaction occurs substantially simultaneously with the reaction of the polyacrylate or polymethacrylate esters with the amines. When employed, the hydroxy-substituted aromatic compound is added in an amount ranging from about 50 to 70, preferably about 55 to 60 percent by weight, based on the total weight of the hydroxy-substituted aromatic compound and polyamine hardener.

The adhesive compositions of the invention can also contain conventional additives normally found in epoxy adhesives, such as talc, metal powders, titanium dioxide and the like. Such additives are incorporated in current ratios well known to practitioners in the art of epoxy adhesives.

In production, the adhesives are provided as two-part compositions, i.e., a Component A and a Component B. The parts are metered and mixed together immediately before use in a weight ratio of Component A:Component B ranging from 0.5:1 to 10:1, preferably 0.8:1 to 6:1. After mixing, the adhesive is sufficiently viscous to form a discrete bead when extruded onto a surface and has a pot life of at least 30 minutes at ambient temperature. The adhesives are curable at ambient temperatures but are preferably cured at temperatures in the range from 70.degree. C. to 190.degree. C. at which temperatures cure is effected in 5 minutes or less dependent on temperature.

The adhesives of this invention are especially suited to bonding fiber reinforced unsaturated resin sheet molding compound (SMC) parts to other SMC parts or metals. A bead of adhesive is applied to at least one of the surfaces which are to be bonded, the parts are mated together and the assembly is heated at a temperature in the range from 70.degree. C. to 190.degree. C. for 2 to 5 minutes. At times a post-bake at temperatures in the range from 100.degree. to 205.degree. C. for 5 to 30 minutes can be beneficial in enhancing properties such as heat and environmental resistance. While the adhesives can be applied by any conventional method as by roll coater, brush, curtain coater, extrusion and hand rollers, robotic dispensing machines are preferred.

The following example is illustrative of the invention.

Invention Adhesive was prepared by blending the Component A and Component B, in a ratio of 1:0.9 by weight. The formulations which made up Component A and Component B were as follows:

Invention Adhesive was employed to bond automotive grade fiberglass reinforced polyester sheet molding compound (SMC) parts to other similar SMC parts. Individual parts were wiped with a dry rag, otherwise none of the surfaces were prepared or treated in any manner before bonding. The Invention Adhesive was applied at a dry film thickness of 30 mil and the bonded assemblies were cured at 200.degree. for 10 minutes and at 300.degree. for 30 minutes. The bonded assemblies were then tested on an Instron testing machine at a pull rate of 0.5 inches per minute. The lap shear strength results under various pressures, temperatures, and water-soaking for seven days were as follows:

The bonded assemblies exhibited 100% of fiber tear (FT) in all instances except those tested at a temperature of 180.degree. F. in which case the assemblies exhibited 100% of fiber tear or a combination cohesion/fiber tear. For instance, 10 COH/FT indicates 10% cohesive/90% fiber tear.