Epoxide and rubber based curable compositions having good adhesion direct to metal

Abstract: A one package, stable curable composition contains: (a) a polydiene; (b) a polyepoxide; (c) a sulfur and zinc containing vulcanization system adapted to cure the polydiene and the polyepoxide; and (d) an anhydride containing material effective to promote adhesion of the curable composition to metal. The aforesaid curable composition is suitable for use as an adhesive or sealant.

Description: Cross Reference to Related Applications This application is related to EP Serial Number 88115552 .7 (EP-A-309904)filed even date herewith entitled EPOXIDE AND RUBBER BASED CURABLE COMPOSITIONS. Background of the Invention The present invention relates to curable compositions suitable for use as adhesives and sealants. In the manufacture and assembly of automobiles, adhesives and sealants are used for a variety of different purposes. As a consequence, depending upon the mode of use, each adhesive or sealant has different physical properties requirements, such as a certain threshold lap shear strength at a particular temperature, a wide latitude of cure temperatures with the ability to cure both at very low and at very high temperatures, resiliency, good elongation and good adhesion to differing substrates. Heretofore, because of the diversity and disparity in requirements, a different adhesive or sealant has been necessary for each of the different applications in automobile manufacture. For example, separate materials have been used as structural adhesives, as gap filling sealants, or as anti-flutter adhesives. From US-A-3,113,036 coating compositions are known which comprise a polymer of a conjugated diethylenically unsaturated hydrocarbon, a polyepoxide and an organic unsaturated cyclic anhydride to react with the addition product of the afore described hydrocarbons as well as with the epoxy groups of the polyepoxide. In US-A-4,501,853 a curable epoxy resin composition is disclosed comprising a conventional liquid epoxy resin and vulcanized rubber particles which are obtained by vulcanizing a dispersion of fine particles of a liquid rubber imcompatible with the epoxy resin in the epoxy resin with a vulcanizing agent. The curable compositions are useful as adhesives, coating material or molding material. There is a need, therefore, for a single curable composition which can be used for a variety of different purposes and with the capability to meet all of the physical properties differing requirements. This object is attained by a one-package stable curable composition comprising (a) a non-functional polydiene, (b) a polyepoxide having at least two 1,2 epoxy groups, (c) a carboxylic acid anhydride to promote adhesion of the curable composition to metal, characterized by comprising (d) a sulfur and zinc containing vulcanization system for the non-functional polydiene; and the polyepoxide, and wherein the carboxylic acid anhydride is pretreated by heating it in the presence of a diepoxide for at least five minutes at a temperature ranging from 60° to 150°C to an apparent dissolution of the carboxylic acid anhydride in the diepoxide and the carboxylic acid anhydride remains essentially unreacted with the diepoxide. Also provided is a method for preparing an adhesive bond between two surfaces to form a bonded structure which comprises: A. applying to at least one of the surfaces a one package, stable curable composition as defined above; B. placing the second surface in contact with the applied curable composition; C. at least partially curing the curable composition. The curable composition of the present invention comprises as one of its principal constituents a non-functional polydiene. The non-functional polydiene polymers include polymers of 1.3-dienes containing from 4 to 12 and preferably from 4 to 6 carbon atoms. Typical dienes include 1,3-butadiene which is preferred 2,3-dimethyl-1,3-butadiene. isoprene, and piperylene. Also, copolymers of 1,3-butadiene and a monomer copolymerizable with 1,3-butadiene such as isoprene, piperylene can be used. Other polymerizable monomers such as styrene can also be used. Preferably the polydiene polymer is 1,4-polybutadiene. Additionally a 1,4-polybutadiene acrylonitrile copolymer may be present. If desired, a variety of vulcanizable or non-vulcanizable synthetic rubbers can be used as inert fillers in conjunction with the polydiene. Examples of such synthetic rubbers include butyl rubber, ethylene propolyene terpolymer, silicone rubbers, polysulfides, polyacrylate rubbers and chlorinated polyethylene rubbers. Copolymers of many of the aforelisted synthetic rubbers with styrene can also be utilized. It should be understood that the polydiene polymer of the present invention is non-functional, that is, it does not contain functional groups such as, for example, hydroxyl, amino, carboxyl or mercapto. Another principle constituent of the claimed curable compositions is a polyepoxide. The polyepoxides are those materials having at least two 1,2 epoxide group present in the molecule. Hydroxyl groups may also be present and often are. In general, the epoxide equivalent weight can range from about 289 to about 4,000. These polyepoxides are saturated or unsaturated, cyclic or acyclic, aliphatic, alicyclic, aromatic or heterocyclic. They can contain substituents such as halogen, hydroxyl and ether groups. One useful class of poiyepoxides comprises the epoxy polyethers obtained by reacting an epihalohydrin (such as epichlorohydrin or epibromohydrin) with a polyphenol in the presence of an alkali. Suitable polyphenols include resorcinol, catechol, hydroquinone, bis(4-hydroxyphenyl)-2,2-propane. i.e., bisphenol A; bis(4-hydroxyphenyl)-1,1-isobutane; 4.4-dihydroxybenzophenone; bis(4-hydroxyphenyl)-1,1-ethane; bis(2-hydroxynaphenyl)-methane; and 1,5-hydroxynaphthalene. One very common polyepoxide is a polyglycidyl ether of a polyphenol, such as bisphenol A. More preferably the polyepoxide is a diglycidyl ether of bisphenol A. Another class of polyepoxides are the polyglycidyl ethers of polyhydric alcohols. These compounds may be derived from such polyhydric alcohols as ethylene glycol. diethylene glycol, triethylene glycol, 1,2-propylene glycol, 1,4-butylene glycol, triethylene glycol, 1,2-propylene glycol, 1,4-butylene glycol. 1,5-pentanediol, 1,2,6-hexanetriol, glycerol, trimethylolpropane, and bis(4-hydroxycyclohexyl)-2.2-propane. Another class of polyepoxides are the polyglycidyl esters of polycarboxylic acids. These compounds are produced by the reaction of epichlorohydrin or a similar epoxy compound with an aliphatic or aromatic polycarboxylic acid such as oxalic acid, succinic acid, glutaric acid, terephthalic acid. 2.6-naphthalene dicarboxylic acid and dimerized linoleic acid. Still another class of polyepoxides are derived from the epoxidation of an olefinically unsaturated alicyclic compound. These polyepoxides are non-phenolic and are obtained by epoxidation of alicyclic olefins, for example, by oxygen and selected metal catalysts, by perbenzoic acid, by acid-aldehyde monoperacetate or by peracetic acid. Among such polyepoxides are the epoxy alicyclic ethers and esters well known in the art. Useful polyepoxides also include those containing oxyalkylene groups in the epoxy molecule. Another class of polyepoxides consists of the epoxy novalac resins. These resins are obtained by reacting an epihalohydrin with the condensation product of aldehyde and monohydric or epichlorohydrin with a phenol formaldehyde condensate. Another group of epoxide containing materials includes acrylic copolymers containing copolymerized glycidyl acrylate or methacrylate units. These acrylic copolymers can be prepared by the reaction of alkyl esters of alpha.beta unsaturated mono- or dicarboxylic acid with either glycidyl acrylate or methacrylate. Other glycidyl containing copolymerizable monomers such as diglycidyl itaconate and diglycidyl maleate also can be used. These monomers can be optionally copolymerized in the presence of other copolymerizable monomers such as vinyl aromatic compounds, such as styrene or vinyl toluene, and also acrylonitrile or methacrylonitrile. Preferably the polyepoxide is a diepoxide. Preferably a polyglycidyl ether of bisphenol A is used, more preferably a diglycidyl ether. Examples of suitable materials are the EPON® epoxy resins which are commercially available from Shell Chemical. such as EPON® 828. It should be understood that mixtures of the aforedescribed polyepoxides can be used herein. In preferred embodiments of the present invention an epoxy-rubber adduct is utilized as an additional additive in order to achieve optimum adhesion of the curable composition to oily metal. A preferred adduct is that which is prepared from an excess amount of the diglycidyl ether of bisphenol A. e.g.. EPON® 828 from Shell Chemical and a carboxyl terminated polybutadiene acrylonitrile copolymer, e.g.. CTBN from B. F Goodrich. The resultant epoxy functional adduct is free of carboxyl functionality. The curable composition may additionally comprise an acrylonitrile butadien copolymer. A further principle constituent of the claimed curable compositions is a sulfur and zinc containing vulcanization system to cure the non-functional polydiene and the polyepoxide components. As used in this specification, vulcanization is the physicochemical change resulting from crosslinking of the polydiene with sulfur, generally with application of heat. The precise mechanism which produces the network structure during the cure of the claimed compositions is still not completely known. However, it is theorized that a physical incorporation of the epoxide into the polydiene rubber lattice may be in effect. The vulcanization system comprises a material or mixture of materials which is adapted to effect cure of the polydiene and the polyepoxide. Preferably the vulcanization system comprises a lower alkyl dithiocarbamate and a disulfide. A number of lower alkyl dithiocarbamates are useful herein, particularly those having from 1 to 10, preferably 1 to 5 carbon atoms in the alkyl portion. Examples of suitable dithiocarbamates include methyl, ethyl, propyl, butyl and amyl dithiocarbamate. Preferably the dibutyl dithiocarbamate is utilized herein. These materials are commercially available in association with zinc in salt form, namely zinc dibutyl dithiocarbamate, zinc dimethyl dithiocarbamate, zinc diethyi dithiocarbamate and zinc diamyl dithiocarbamate. These materials can be commercially obtained from Vanderbilt Chemical Company under the trade designation ZIMATE. The zinc which is part of the vulcanization system can be introduced in different ways. One example has been given above in connection with the vulcanization system: that is, the zinc can be associated with the lower alkyl dithiocarbamate in salt form. The zinc can also be introduced as zinc oxide. It should be understood that a variety of grades of zinc oxide are available and can be utilized for this purpose. The amount of zinc in the vulcanization system can vary widely, generally from about 0.1 percent by weight to about 10 percent by weight based on the total weight of the curable composition. The precise mechanism is not understood; however, it is believed that the zinc functions as an accelerator for the sulfur vulcanization. The disulfide component of the vulcanization system can also be selected from a variety of materials. Examples of suitable disulfides include 4-morpholinyl-2-benzothiazole disulfide; 4,4-dithiobismorpholine and benzothiazyl disulfide. Preferably the disulfide is benzothiazyl disulfide. It is believed that the dithiocarbamate and the disulfide components of the vulcanization system function as primary and secondary accelerators, respectively, for the vulcanization reaction. Moreover. in preferred embodiments of the present invention it is believed that the disulfide species functions as a retarder for premature vulcanization. It is believed that the thiocarbamate operates in conjunction with the sulfur in order to effect the crosslinked. cured system. It is also believed that the dithiocarbamate functions not only to assist in vulcanization of the rubber component of the composition but in addition functions to incorporate the epoxide component into the crosslinked network. As has been mentioned above, the precise mechanism this is not understood. A further principle constituent of the claimed curable compositions is a pretreated carboxylic acid anhydride to promote adhesion of the curable composition to metal. A variety of carboxylic acid anhydrides are contemplated to be within the scope of the present invention so long as they are capable of promoting adhesion of the curable composition direct to metal. The pretreated anhydride is a carboxylic acid anhydride selected from the group consisting of maleic anhydride, itaconic anhydride and phthalic anhydride. Preferably the carboxylic acid anhydride is maleic anhydride. If desired, mixtures of the aforesaid carboxylic anhydrides can be utilized. Preferably, the claimed curable compositions are prepared as a moisture free system. The presence of water is not preferred because it opens the anhydride ring structure and interferes with the desired cure. Therefore, dessicant materials are typically added in order to remove water from the system. The carboxylic acid anhydride is pretreated by heating it in the presence of a diepoxide for a period of time of at least about five minutes at a temperature ranging from about 60°C to about 150 C. It has been observed that when this pretreatment is conducted the carboxylic acid anhydride remains essentially unreacted. The pretreatment is an apparent dissolution of anhydride in epoxide. This result has been confirmed by infrared spectroscopy, acid number determinations and gel permeation chromatography. It has been observed that the carboxylic acid anhydride provides optimum stability and also performance in promoting adhesion to metal when pretreated this way. In preferred embodiments of the present invention, the claimed curable composition is essentially free of amino group containing materials. The presence of amine is not preferred because it detracts from the resiliency of the resultant cured composition. The amounts of each of the constituents of the claimed curable composition can vary widely depending upon the particular properties desired in the curable composition. For example, by varying the amount of polyepoxide and dithiocarbamate which is utilized in the curable composition, one can tailor the hardness of the ultimate cured composition. The more polyepoxide and dithiocarbamate which is utilized, the harder the stronger the polydiene polyepoxide cured material becomes. Generally, the amount of polydiene which is utilized in the claimed curable composition can vary with the range of from 2 percent by weight to 80 percent by weight. Preferably, the amount of non functional polydiene which is utilized varies from 5 percent by weight to 50 percent by weight, and more preferably from 10 percent by weight to 15 percent by weight. The amount of polyepoxide generally can vary within the range of from 1 percent by weight to 75 percent by weight, preferably from 5 percent by weight to 40 percent by weight, and more preferably from 10 percent by weight to 20 percent by weight, the percentages based on the total weight of the curable composition. The sulfur and zinc containing vulcanization system is utilized in amounts varying from 0.5 to 25 percent by weight based on the total weight of the curable composition. In preferred embodiments the dithiocarbamate primary accelerator can be present in an amount ranging from 0.1 percent by weight to 7 percent by weight, preferably from 0.5 percent by weight to 3 percent by weight and more preferably from 1 percent by weight to 2 percent by weight. The disulfide secondary accelerator can be present in an amount ranging from 0.2 percent by weight to 14 percent by weight, preferably from 1 percent by weight to 6 percent by weight and more preferably from 2 percent by weight to 4 percent by weight. All the percentages are based on the total weight of the curable composition. The amount of sulfur which is part of the vulcanization system can also vary widely. Generally the amount of sulfur varies from 0.1 percent by weight to 15 percent by weight, preferably from 0.2 percent by weight to 5 percent by weight and more preferably from 0.5 percent by weight to 1.5 percent by weight, the percentages based on the total weight of the curable composition. The sulfur can be utilized in a variety of forms but typically it is elemental sulfur and it is used as a solid oil-treated powder. For example, suitable sources of sulfur for the vulcanization system are the CRYSTEX brand sulfurs which are commercially available from the Stauffer Chemical Company. It should be understood that the acceierator materials discussed above can contribute a minor amount of the required sulfur in the vulcanisation system. The amount of pretreated anhydride can also vary widely depending upon the particular choice of material. When the pretreated carboxylic acid anhydride material is an unreacted carboxylic acid anhydride the amount can vary generally from 0.1 percent by weight to 10 percent by weight, preferably from 0.2 percent by weight to 5 percent by weight and more preferably from 0.3 percent by weight to 1.5 percent by weight. All of the percentages enumerated above are based on the total weight of the curable composition. The claimed curable compositions can comprise a variety of other optional additives in addition to the principle constituents which have been detailed above. Examples of additives include fillers such as calcium carbonate, stearic acid treated calcium carbonate, polybutadiene treated calcium carbonate, barium sulfate, calcium and magnesium oxide, carbon blacks, hydrocarbon tackifiers and various phthalate and adipate plasticizers and antioxidants. Examples of suitable antioxidants are butylated hydroxytoluene, butylated and styrenated phenols and cresols, alkylated quinones and hydroquinones and butylated hydroxy benzyl isocyanates. The claimed one package, stable curable compositions have a very wide latitude of cure temperatures. The claimed curable compositions can be cured by baking at a temperature within the range of from 104°C to 288°C (220°F to 550°F) for a period of time ranging from about 10 minutes to 60 minutes. Preferably, the claimed curable compositions are cured by baking within a temperature of from 163°C to 204°C (325°F to 400°F) in a period of time ranging from 15 minutes to 30 minutes. One very unexpected advantage of the claimed curable compositions is their ability to cure both at very high temperatures and at very low temperatures while achieving comparable physical properties at both extremes. This capability reduces problems associated with underbaking and overbaking and permits the tailoring of compositions to suit a variety of application conditions. In addition, the claimed curable compositions have outstanding adhesion direct to metal, particularly to oily metal. Moreover, the claimed curable compositions are capable of adhering to a wide variety of other substrates such as aluminum, primed metal, plastic, wood, and other substrates. It is believed that the anhydride containing material is very important in achieving the adhesion of the claimed curable compositions direct to metal. The claimed curable compositions can be applied by conventional means although typically they are applied by extrusion. The claimed one package. stable curable compositions also demonstrate a wide variety of other advantageous properties such as solvent resistance, heat resistance, good elongation, resiliency, good lap shear strength at high temperatures and in addition they demonstrate good overall strength at ambient temperature. The claimed curable compositions also exhibit good cohesive failure. Also. they have excellent package stability for prolonged periods of up to three months. The following example is intended to be illustrative of the invention and are not intended to be limiting. EXAMPLE I This Example illustrates the preparation of a curable composition according to the claimed invention. Ingredients Parts by Weight (grams) HYCAR 1312 LV 12.85 EPON® 828 13.22 MULTIFLEX SC 23.10 butylated hydroxytoluene 1.92 butyl zimate 0.86 ALTAX 1.72 POLIOL 130 11.26 calcium carbonate 17.75 calcium oxide 5.17 sulfur 1.72 carbon black 1.25 plasticizer 0.96 EPOXY-CTBN adduct 5.75 IDMA/STEREON 840 A 1.00 maleic anhydride 0.50 The adhesive composition was prepared by combining the ingredients together with mild agitation. The composition was tested for physical properties as follows. Lap Shear Strength : Lap shear bonds for testing were prepared using two strips of cold rolled steel 2.54cm x 10.16cm x 0.158cm (1 inch x 4 inches x 0.062 inch). A 2.95 millimeters (118 mil) thick film of a composition was applied onto one of the metal strips and then a second strip was placed over top of the first strip so that only a 3.16 square centimeter (one-half square inch) strip overlapped. The composition was cured at 160°C for 30 minutes. 190°C for 30 minutes and at 205°C for 120 minutes (three different bonds were prepared, one for each temperature). The lap shear strength of the bond in newtons per square millimeter (pounds per square inch (psi)) was determined according to ASTM D-1002-65 The data presented for each temperature was an average of three separate determinations. (The ends of the strips were pulled with an INSTRON TESTER device and the lap shear strength of the bond measured.) Tensile Strength : A 0.254cm (0.100 inch) thick layer of a composition was applied onto a TEFLON treated glass panel measuring 30cm × 30cm × 0.254cm (12 inches × 12 inches × 0.100 inch). The composition was cured by baking at 190°C for 30 minutes and then the panel was cooled to room temperature The free films were prepared and evaluated for tensile strength according to ASTM D 638. Each value in newtons per square millimeter (psi) is an average of three separate determinations. The percent elongation was also determined according to this ASTM test. Shore A Hardness: A 0.254cm, (0.100 inch) thick layer of a composition was applied onto a metal panel. The composition was cured by baking at 190°C for 30 minutes and then cooled to room temperature. The Shore A hardness was determined using a Shore Durometer Hardness Type A-2 instrument according to ASTM D676. T-peel Strength : The composition was evaluated for T-peel strength according to ASTM D1876. T-Peel bonds for evaluation were prepared as follows. Two strips of cold rolled steel measuring 2.54 centimeters X 15.24 centimeters X 0.079 centimeters (1 inch X 6 inches X 0.031 inch) were used. A 2 95 millimeters (118 mil) thick film of adhesive composition was applied onto one of the metal strips and then a second metal strip was placed overtop the first strip so that a 25.81 square centimeter (4 square inch) section was bonded Then the two ends of the panels which were not bonded were bent to form a T-shape. The load for the T-peel strength determination was applied at (12.7 centimeters) (50 inch) per minute The T-peel strength is measured in kilonewtons per meter (pounds per linear inch (lbs/in). The results are set out below: Lap Shear Strength at 190°C/30 min: 2.8 newtons mm 2 (410 psi) Lap Shear Strength at 110°C/30 min: 1.9 newtons.mm 2 (280 psi) Lap Shear Strength at 205°C/120 min: 2.5 newtons mm 2 (360 psi) T-peel Strength: 2.6 kilonewtons/meter (38 psi) Tensile Strength: 3.5 newtons/mm 2 (508 psi) Elongation: 180 percent Shore A Hardness: 68

Claims: A one-package stable curable composition comprising (a) a non-functional polydiene, (b) a polyepoxide having at least two 1,2 epoxy groups, (c) a carboxylic acid anhydride to promote adhesion of the curable composition to metal, characterized by comprising (d) a sulfur and zinc containing vulcanization system for the non-functional polydiene and the polyepoxide, and wherein the carboxylic acid anhydride is pretreated by heating it in the presence of a diepoxide for at least five minutes at a temperature ranging from 60° to 150°C to an apparent dissolution of the carboxylic acid anhydride in the diepoxide and the carboxylic acid anhydride remains essentially unreacted with the diepoxide. The curable composition of claim 1, wherein the curable composition is essentially free of amino group containing material. The curable composition of claim 1, wherein the curable composition is essentially moisture free. The curable composition of claim 1, additionally comprising an acrylonitrile butadiene copolymer. The curable composition of claim 1, wherein the non-functional polydiene is 1,4-polybutadiene. The curable composition of claim 1, wherein the polyepoxide is the diglycidyl ether of bisphenol A. The curable composition of claim 1, wherein the sulfur and zinc containing vulcanization system comprises a C 1-10 alkyl dithiocarbamate and a disulfide. The curable composition of claim 7 wherein the alkyl dithiocarbamate is zinc dibutyl dithiocarbamate and the disulfide is benzothiazyl disulfide. The curable composition of claim 1, wherein the carboxylic acid anhydride is selected from maleic anhydride, itaconic anhydride, phthalic anhydride and mixtures thereof. The curable composition of claim 1, wherein the non-functional polydiene is present in an amount ranging from 2 percent by weight to 80 percent by weight, the polyepoxide is present in an amount ranging from 1 percent by weight to 75 percent by weight, the sulfur and zinc containing vulcanization system is present in an amount ranging from 0.5 to 25 percent by weight, and the treated carboxylic acid anhydride is present in an amount ranging from 0.1 percent by weight to 10 percent by weight, the percentages based on the total weight of the composition. The curable composition of claim 1, wherein the nonfunctional polydiene is a non-functional polybutadiene, the polyepoxide is a diepoxide, the treated carboxylic acid anhydride is selected from maleic anhydride, itaconic anhydride, phthalic anhydride, and mixtures thereof, and the sulfur and zinc containing vulcanization system for the polybutadiene and the polyepoxide comprises a C 1-10 alkyl dithiocarbamate in combination with benzothiazyl disulfide. The curable composition of claim 11 wherein the alkyl dithiocarbamate is zinc dibutyl dithiocarbamate. A method for preparing an adhesive bond between two surfaces to form a bonded structure by (A) applying to at least one of the surfaces a onepackage stable curable composition according to any of claims 1 to 12 (B) placing the second surface in contact with the applied curable composition and (C) at least partially curing the curable composition.

Inventors: CHASSER ANTHONY MICHAEL, MAKHLOUF JOSEPH MICHAEL, CHASSER, ANTHONY MICHAEL, MAKHLOUF, JOSEPH MICHAEL
Applicants: PPG INDUSTRIES INC, PPG INDUSTRIES, INC.
Application year: 1988
Main classification: C08L21
IPC codes: C09J163, C08L21, C08G59, C08L63, C09K3