WO2008080067A2

WO2008080067A2 - Curative systems useful in structural foams

Info

Publication number: WO2008080067A2
Application number: PCT/US2007/088570
Authority: WO
Inventors: Vettithara C. Koshy
Original assignee: Huntsman Advanced Materials Americas Inc.
Priority date: 2006-12-21
Filing date: 2007-12-21
Publication date: 2008-07-03
Also published as: WO2008080067A3

Abstract

The present invention provides a storage stable one-package curative system including (i) an epoxy resin (ii) an epoxy-amine adduct curing agent (iii) a hardener and optionally (iv) fillers. The one-package curative system is shelf stable at temperatures up to about 130°F, has an onset temperature of greater than 130°C and can be fully cured at temperatures greater than 150°C.

Description

IN THE UNITED STATES PATENT AND TRADEMARK OFFICE

CURATIVE SYSTEMS USEFUL IN STRUCTURAL FOAMS

INVENTOR:

Vettithara C. Koshy

The Woodlands, Texas

CURATIVE SYSTEMS USEFUL IN STRUCTURAL FOAMS

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Pat. App. No. 60/876,237 filed on December 15, 2006 which is incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable.

FIELD OF THE INVENTION

The present invention relates to one-package curative systems containing an epoxy resin, an epoxy-amine adduct curing agent, a hardener and optionally one or more additives. The curative systems, which have excellent storage stability and rapidly cure under heat, may be used as an adhesive or in the manufacture of structural reinforcement parts for automotive and aerospace applications.

BACKGROUND OF THE INVENTION

Cured articles prepared from an epoxy resin exhibit excellent adhesion, mechanical properties, thermal properties, chemical resistance and electrical properties and are therefore utilized in a number of applications including paints, adhesives, and electrical and electronic insulation materials. Epoxy resin compositions generally used in such applications are provided as a two-package or one-package curing system.

Two-package curing systems include separately stored epoxy resin and curing agent components. When needed, the separately stored components are weighed and mixed just prior to use. However, it is often difficult to both weigh the correct amount of each component and form a homogenous mixture. This is further compounded by the fact the reaction between the epoxy resin composition and curing agent begins as they are mixed.

In comparison, in one-package curing systems, the epoxy resin composition and latent curing agent are stored in the same package. The latent curing agent, which is non-reactive with the epoxy resin at room temperatures, upon heating reacts with the epoxy resin to effect curing. Amine compounds, and in particular epoxy-imidazole adducts, such as those disclosed in US Pat. Nos. 4,066,625 and 6,492,437, are widely used as latent curing agents. Because the potential for inaccurate weighing is eliminated and the variation in properties due to incomplete mixing is reduced, one-package curing systems are highly preferred.

In addition to the epoxy resin composition and latent curing agent, one-package curing systems may further be combined with a blowing agent and fillers and used in automotive applications as structural reinforcement foam. When used for this particular purpose, the curing system, blowing agent and filler are combined and (i) exposed first to prebake temperatures of about 130⁰C then (U) fully cured at bake temperatures of about 150°- 190⁰C. Examples of structural reinforcement foam formulations may be found in US Pat. No. 5,575,526, US Pat. No. 5,755,486, and US Pat. No. 6,376,564. For example, US Pat. No. 6,376,564 discloses the use of an azodicarbonamide blowing agent in combination with an epoxy-amine adduct and epoxy resin to provide a system which exhibits stability at storage conditions (i.e. temperatures up to 54⁰C for two weeks). However, a drawback to the use of these disclosed one-package curing systems in structural reinforcement foam applications is partial curing occurs at the prebake temperatures causing the foam to expand less than desired at the higher bake temperatures. This produces structural reinforcement parts having inferior reinforcement and sealing properties. Therefore, it would be desirable to produce a one- package curing system for use in automotive structural reinforcement applications that (a) is stable at typical storage conditions (b) does not react and is therefore stable at prebake temperatures and (c) fully cures at bake temperatures to provide superior structural reinforcement automotive parts.

SUMMARY OF THE INVENTION

In one aspect of the present invention a one-package curative system is provided comprising (1) an epoxy resin, (2) an epoxy-amine adduct curing agent which is the reaction product of (i) an imidazole compound having the following structure (I):

wherein each R group is independently selected from hydrogen, a linear or branched Ci-Cg alkyl group or a cycloalkyl group containing up to 8 carbon atoms and (ii) an epoxy compound, and (3) a hardener wherein the one-package curative system has an onset temperature above about 13O⁰C. The one-package curative system, which are storage stable at temperatures up to about 130⁰F, upon heating fully cure at temperatures greater than 150⁰C- In another aspect of the present invention, the above described curative system is further combined with a blowing agent and optional additives to provide a storage stable foamable composition. The foamable composition, when foamed and cured produces reduced density foams having excellent physical properties.

In an additional aspect of the present invention, a composite article is provided having a first surface and a second surface joined by an adhesive bond made with a cured mass of the above described one-package curative system. The curative system of the present invention can form high quality adhesive bonds between metallic components (e.g., iron, aluminum, titanium, magnesium, copper, etc. and alloys thereof), between non-metallic substrates (e.g., reinforced and unreinforced thermoplastic and thermoset polymers, as well as other organic materials or organic composite materials) and between metallic and non-metallic substrates.

In another aspect of the present invention, a method is provided for assembling the above described adhesively bonded composite article. The method comprises the steps of: (a) applying an uncured mass of the curative system to at least one of a first substrate and a second substrate; (b) sandwiching the uncured mass between the first substrate and the second substrate; and (c) curing the composition to form an adhesive bond so as to adhere the first substrate and the second substrate together, the adhesive bond comprising a thermally cured mass formed from the curative system.

DETAILED DESCRIPTION OF THE INVENTION

The present invention generally relates to one-package curative systems containing an epoxy resin, an epoxy-amine adduct curing agent, a hardener and optionally one or more additives. It has surprisingly been found that the curative system of the present invention is non-reactive, as measured by the rate of viscosity increase of the curative system, at temperatures up to about 13O⁰F. Equally surprising is the curative system will fully cure at temperatures greater than 15O⁰C and preferably greater than 15O⁰C up to 190⁰C. Thus, the curative systems will exhibit an onset temperature (Tj) of greater than about 130⁰C and a peak temperature (T_p) of greater than 150⁰C. Since the curative system provides excellent storage stability and processability, it may be combined with a blowing agent and optional additives and used in automotive structural reinforcement applications such as impregnating and casting applications. Epoxy Resin

As a first essential component, the curative system of the present invention includes from about 35-90% by weight, based on the total weight of the curative system, of an epoxy resin. Any epoxy resin having on average more than one epoxy group per molecule may be used. The epoxy resin generally has an epoxide equivalent weight of between about 300- 4000.

Examples of epoxy resins which may be used include a polyglycidyl ether obtained by reacting a polyphenol, such as bisphenol A, bisphenol F, bisphenol AD₃ catechol or resorcinol, or a polyalcohol, such as glycerin or polyethylene glycol, with epichlorohydrin. In another embodiment, the epoxy resin is a glycidyl ether ester obtained by reacting a hydroxycarboxylic acid, such as hydroxybenzoϊc acid or β-hydroxynaphthoic acid, with epichlorohydrin. In yet another embodiment, the epoxy resin is a polyglycidyl ester obtained from a polycarboxylic acid such as terephthalic acid. In another embodiment, the epoxy resin is a glycidylamine compound obtained from 4,4'-diaminodiphenylmethane and m- aminophenol. In yet another embodiment, the epoxy resin is an epoxidized novolak or epoxidized polyolefin. In yet another embodiment, the epoxy resin is a cyclohexene-oxide containing compound.

The curative system of the present invention may include one epoxy resin or a mixture of epoxy resins. Epoxy- Amine Adduct Curing Agent

As a second essential component, the curative system of the present invention includes from about 0.5-10% by weight, based on the total weight of the curative system, of an epoxy-amine adduct curing agent. The epoxy-amine adduct curing agent, which is the product of a reaction between (i) an amine compound and (ii) an epoxy compound, is solid and insoluble in the epoxy resin at room temperature. Upon heating, the epoxy-amine adduct becomes soluble in epoxy resins to thereby function as an accelerator.

In one embodiment, the amine compound used as a starting material for the production of the epoxy-amine adduct curing agent is an imidazole compound having the following structure (I):

wherein each R group is independently selected from hydrogen, a linear or branched Cj-Cg alkyl group or a cycloalkyl group containing up to 8 carbon atoms.

In one embodiment, each R group in the imidazole compound of formula (I) is independently selected from hydrogen and a branched C₃-Cg alkyl group. In another embodiment, each R group in the imidazole compound of formula (I) is independently selected from hydrogen and a branched C₃-C₅ alkyl group. In yet another embodiment, the R in the 2-position of the imidazole compound of formula (I) is a branched C3-C5 alkyl group and the remaining R's are hydrogen. In a further embodiment, the imidazole compound is 2- isopropyl imidazole or 2-isobutyl imidazole.

The epoxy compound used as a starting material for the production of the epoxy- amine adduct curing agent generally has an epoxide equivalent weight of about 170-2000 and may be a monofunctional epoxy compound such as n-butyl glycidyl ether, styrene oxide and phenylglycidyl ether; a bifunctional epoxy compound such as bisphenol A diglycidyl ether, bisphenol F diglycidyl ether, bisphenol S diglycidyl ether; trifunctional epoxy compounds such as triglycidyl isocyanurate, triglycidyl p-aminophenol; tetrafunctional epoxy compounds such as tetraglycidyl m-xylene diamine and tetraglycidyl diamindiphenlymethane; and compounds having more functional groups such as cresol novolac polyglycidyl ether, phenol novolac polyglycidyl ether.

The epoxy compounds may also be selected by considering the melting point of the epoxy-amine adduct that is formed and the compatibility in a molten state with respect to the epoxy resin to be cured. Thus, for example, if the epoxy resin to be cured includes bisphenol A diglycidyl ether, this compound is most typically used as the starting material in the preparation of the epoxy-amine adduct curing agent. In addition, the amount of epoxy resin utilized should be sufficient to provide a ratio of oxirane groups to imidazole compound molecules of between about 1:1 to 2:1.

Additionally, a solvent may be used to dissolve the amine compound and epoxy compound. Examples of such a solvent may include methyl isobutyl ketone, methyl isopropyl ketone, methyl ethyl ketone, acetone, butanone, acetic acid, isobutyl acetate, ethyl acetate, methyl acetate, tetrahydrofuran, 1,4-dioxane, cellosolve, ethyleneglycol monoethyl ether, diethyleneglycol dimethyl ether and toluene. The epoxy-amine adduct curing agents are easily obtained by mixing the above mentioned components (i) the amine compound and (ii) the epoxy compound and carrying out the reaction with heating at temperatures ranging from room temperature up to 200⁰C, then solidifying and grinding or pulverizing the reaction product; or by carrying out the reaction in the presence of a solvent such as butanone, removing the solvent by any number of processes such as by a trap and/or vacuum, and then grinding or pulverizing the resulting reaction product.

The crushed and pulverized epoxy-amine adduct curing agent may then be sieved to obtain particles of a desired size. The particle size of the epoxy-amine adduct powder may range from about 1000 - 2300 microns. In another embodiment, the particle size of the epoxy-amine adduct powder may range from 100 - 350 microns.

A commercially available example of the above-mentioned solid epoxy-amine adduct curing agents is Aradur® 3261 (Huntsman Advanced Materials Americas Inc.).

The curative system of the present invention may include one epoxy-amine adduct curing agent or a mixture of epoxy-amine curing agents described above. Hardener

As a third essential component, the curative system of the present invention includes from about 5-40% by weight, based on the total weight of the curative system, of a hardener. The hardener may be one or more phenolic compounds, mercaptans, polycarboxylic acids and their anhydrides, amino compounds, amine salts and quaternary ammonium salts. Examples of amines that may be used in combination with the epoxy-amine adduct curing agent are aliphatic monoamines such as dimethylethanol amine, methyldiethanol amine, morpholine, stearyldimethyl amine, tri-n-hexylamme; aliphatic polyfunctional amino compounds such as ethylene diamine, diethylenetriamine, N,N-dimethyl aminopropylamine, dicyandiamide, guanidine, and amidines; cycloaliphatic amines such as di(4- aminocyclohexyl)methane, di(3-methyl-4-aminocyclohexyl)methane, and 1-amino-e- aminomethyl-3,5,5-trimethyl cyclohexane (isophorone diamine); aromatic amines such as p,p^r-bis-(aminophenol)methane, p,p'-bis(aminophenyl)sulphone, m-phenylenediamine, and heterocyclic amino compounds such as melamine. Polycarboxylic acid anhydrides that can be used as a co-curing agent are: phthalic anhydride, tetrahydrophthalic, anhydride, hexahydrophthalic anhydride, bicyclo-2,2,l-heptene-2,3-dicarboxylic anhydride, methyl bicyclo-2,2,l-heptene-2,3-dicarboxylic anhydride isomers, 1,4,5,6,7,7-hexachloro-bicyclo 2,2,l-5-heptene-2-3-dicarboxylic anhydride, succinic anhydride, alkenyl succinic anhydrides, pyromeϋitic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, trimellitic anhydride and partial esters thereof with ethylene glycol and glycerol. Examples of phenolic compounds may include, for example, bisphenols such as, bisphenol A, phenolic/epoxy adducts and novolak compounds. In one embodiment, the hardener is dicyandiamide and is present in the curative system at amounts of up to 25 times the amount of the epoxy-amine adduct present in the curative system. In another embodiment, the weight ratio of dicyandiamide to epoxy-amine adduct curing agent in the curative system is from 0.04:1 to 25:1. In another embodiment, the weight ratio of dicyandiamide to epoxy-amine adduct curing agent is from 1:1 to 5:1. Additives

Optionally, the curative systems of the present invention may contain one or more additives. Examples of additives include: expanding or blowing agents such as azo compounds, hydrazides microspheres, and the like; tougheners; inorganic fillers such as alumina, silica, aerosϋ, calcium carbonate, aluminum hydroxide, magnesium hydroxide, talc, bentonite and barium sulfate; organic fillers such as nylon particles, fine polystyrene particles, fine polyethylene particles, cross-linked fine rubber particles, fine acrylic core-shell particles, fine silicone particles, and fine ethylene-acrylate copolymer particles; fluidity regulators such as acryl oligomers and silicone; surface modifiers and surface active agents; diluents; flame retardants; coloring agents; releasing agents; and, mixtures thereof.

The curative systems of the present invention may be obtained by uniformly blending or mixing the required amounts of the above components. Once formulated, the curative system of the present invention can be packaged in a variety of containers such as steel, tin, aluminum, plastic, glass or cardboard containers. Applications

In one embodiment of the invention the one-package curative system is used as an adhesive for gluing or adhering parts made of the same or different substrates to form a composite article. First, the curative system is formed and placed in contact with at least one of two or more substrates to be bonded in the manner desired for adhesion. The curative system may be applied to the substrates in a variety of ways, such as by extruding, spraying, gravure printing, or coating. Thus, in one embodiment, the curative system is sandwiched between a first and second substrate. The composite article is then cured by contacting the substrates and heating the substrates and curative system at a cure temperature of greater than 150⁰C. By applying heat, an adhesive bond is formed so as to adhere the first substrate and second substrate together. Substrates which can be coated or bonded include plastics, metals, ceramics, glass and cellulosic materials. In another embodiment, the curative system is applied as a matrix resin for fiber- reinforced composite materials. There are no particular restrictions on reinforcing fiber materials for molding of the fiber-reinforced composite material, and any reinforcing fiber material commonly used for fiber-reinforced composite materials, such as carbon fiber, glass fiber, high-stirring organic fiber, metal fiber or inorganic fiber may be used. There are also no particular restrictions on the form of the reinforcing fiber material, and for example, it may be a unidirectional material, cloth or mat, or a tow composed of several thousand or more filaments.

In another embodiment of the invention the curative system further contains one or more expanding agents to form a foam precursor material which may be utilized to form structural foams to stiffen and reinforce cavities, gaps, structural members and the like. The foam precursor material may be supported or contained within a carrier or receptacle or the like so as to position or orient the foam precursor material such that it expands in one or more particular directions when heated to induce curing and foaming. The foam precursor material thus is particularly useful in filling irregularly shaped spaces, as the material will expand so as to come into contact with a greater portion of the substrate surfaces in the vicinity of the foam precursor material than would occur if no expanding agent was present. The foamed, cured material stiffens and/or increases the energy absorption capacity of vehicle cavities and structural members. Although not wishing to be bound by theory, the expanding agent in the foam precursor material is understood to decompose during heating to form a porous structure and at the same time, the epoxy cures to produce a foam material. Because an epoxy resin is utilized as the resin component, the resulting foam material possesses the properties of the epoxy resin. Thus, a foam material can be made from a lightweight epoxy resin and have excellent mechanical properties, chemical properties and electrical insulating properties.

In one embodiment, a foam precursor material having an appropriate shape is placed into a vehicle part having a hollow section. Both the vehicle part and the foam precursor material can be passed through a heating furnace to melt, expand and cure the epoxy resin to completely fill the hollow section. Preferably, sufficient heat is applied to the foam precursor material to raise the temperature of the foam precursor material to at least the decomposition temperature of the expanding agent, at least the melting temperature of the epoxy resin in the curative system and at least the curing temperature of the epoxy-amine adduct curing agent. If other components have been included in the foam precursor material, the temperature of the foam precursor material is also raised to be at least melting or softening temperature of the other components. Appropriate temperature ranges are preferably between about 130° - 200° C, and more preferably between about 150° - 190° C.

Because of the properties of the foam precursor material, foam materials may be produced that are suitable as packing materials, sound-absorbing materials, soundproofing materials, anti-vibration materials, structural materials, flame retardant materials and other useful materials. Besides vehicle parts, these foam materials may also be used in homes, electrical appliances and any other situation in which the foam material is appropriate.

The present invention will be further illustrated in more detail with reference to the following examples, which are not however to be interpreted as limiting the invention thereto. EXAMPLES

Araldite® GY 6010 epoxy resin (a blsphenol A type epoxy resin produced by Huntsman Advanced Materials Americas Inc.), dicyandiamide and an epoxy-amine adduct were formulated in the amounts shown in Table 1.

TABLE l

Ajicure® PN 23 and 40 curing agents are epoxy-amine adducts which are commercially available from Ajionomoto, Inc.

Aradur® 3261 curing agent is an epoxy-isopropyl imidazole adduct commercially available from Huntsman Advanced Materials Americas, Inc.

The samples were then subjected to heat and their onset temperature (T₁) and peak temperature (T_p) were measured by Differential Scanning Calorimeter, DSC 2910 - DuPont Instruments. The results for each of the samples are shown in Table 2.

TABLE 2

Although making and using various embodiments of the present invention have been described in detail above, it should be appreciated that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention, and do not delimit the scope of the invention.

Claims

What is claimed is: 1. A curative system comprising:

(a) an epoxy resin;

(b) an epoxy-amine adduct curing agent which is the reaction product of (i) an imidazole compound having the following structure (I):

wherein each R group is independently selected from hydrogen, a linear or branched Ci-Cg alkyl group or a cycloalkyl group containing up to 8 carbon atoms and (ii) an epoxy compound; and (c) a hardener, wherein the curative system has an onset temperature above about 130⁰C.

2. The curative system wherein R is independently selected from the group consisting of hydrogen and a branched Cj-C₈ alkyl group.

3. The curative system of claim 2 wherein R is independently selected from the group consisting of hydrogen and a branched C₃-C₅ alkyl group.

4. The curative system of claim 3 wherein the hardener is dicyandiamide.

5. The curative system of claim 1, wherein the curative system fully cures at temperatures greater than 150°C up to 190⁰C.

6. The curative system of claiml wherein the imidazole compound is 2-isopropyl imidazole or 2-isobutyl imidazole.

7. A curative system comprising:

(a) about 35-95% by weight of an epoxy resin;

(b) about 0.5-10% by weight of an epoxy-amine adduct curing agent which is the reaction product of (i) an imidazole compound having the following structure (I):

wherein each R group is independently selected from hydrogen, a linear or branched Cj-Cg alkyl group or a cycloalkyl group containing up to 8 carbon atoms and (ii) an epoxy compound; and

(c) about 5-40% by weight of a hardener, wherein the percent by weight is based on the total weight of the curative system and wherein the one-package curative system has an onset temperature above about 130⁰C.

8. The curative system of claim 7 wherein each R is independently selected from hydrogen and a branched C₃-C₅ alkyl group.

9. The curative system of claim 7 wherein the imidazole compound is 2-isopropyl imidazole or 2-isobutyl imidazole.

10. The curative system of claim 9 wherein the hardener is dicyandiamide.

11, The curative system of claim 10 wherein the curative system fully cures at temperatures greater than 150⁰C up to 190⁰C.

12. A method of assembling a composite article comprising the steps of: i) forming a curative system comprising

(a) an epoxy resin;

wherein each R group is independently selected from hydrogen, a linear or branched Ci-Cs alkyl group or a cycloalkyl group containing up to 8 carbon atoms and (ii) an epoxy compound; and

(c) a hardener, wherein the curative system has an onset temperature above about 130⁰C; ii) applying the curative system to at least one of a first substrate and a second substrate; iii) sandwiching the curative system between the first substrate and second substrate; and iv) forming an adhesive bond so as to adhere the first substrate and second substrate together to form the composite article, the adhesive bond comprising a thermally cured mass formed from the curative system.

13. The method of claim 12 wherein the first and the second substrates are independently selected from plastic, metal, ceramic, glass and cellulosic materials.

14. A composite article formed according to the method of claim 12.

15. A one-package curative system comprising the curative system of claim 1 and a container.